NIST Special Publication 953

Technical Digest

Symposium on Optical

Fiber Measurements, 2000

Sponsored by the National Institute of Standards and Technology

in cooperation with the IEEE Lasers and Electro-Optics Society

and the Optical Society of America

QC

100

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m.953

2000 C.^

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N I St C E HiiU N I A L

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NIST Special Publication 953

Technical Digest

Symposium on Optical Fiber

Measurements, 2000

Digest of a symposium sponsored by the

National Institute of Standards and Technology

in cooperation with the

IEEE Lasers and Electro-Optics Society

and the Optical Society of America

September 26-28, 2000

National Institute of Standards and Technology

Boulder, Colorado 80303

Edited by

P.A. Williams

Optical Society of America G- W, Day

September 2000

Nisr

OSA

f> 111

U.S. Department of Commerce

Norman Y. Mineta, Secretary

Technology Administration

Dr. Cheryl L. Shavers, Under Secretary ofCommercefor Technology

National Institute of Standards and Technology

Raymond G. Kammer, Director
Certain commercial entities, equipment, or materials may be identified in this document in order to describe an experimental procedure or concept adequately.

Such identification is not mtended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the

entities, materials, or equipment are necessarily the best available for the purpose.

National Institute of Standards and Technology Special Publication 953

Natl. Inst. Stand. Technol. Spec. Publ. 953, 195 pages (September 2000)

CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 2000

For sale by the Superintendent of Documents

U.S. Government Printing Office

Washington, DC 20402-9325
Preface

This year, SOFM is two decades old. After all these years, fiberoptic metrology is not

only still important, but we are seeing a rise in metrology needs for the technologies

resulting from fiber optics - components, integrated optics and fiber systems.

Accordingly, we are pleased to see a significant number of component measurement

papers in this year's Symposium.

The majority of this year's papers fall into one of three categories: dispersion,

components and nonlinear optics. Dispersion is the largest category, containing two

sessions of polarization-mode dispersion, a session of chromatic dispersion and a session

for group-delay measurements on fiber Bragg gratings. Component papers include

grating metrology as well as receiver and amplifier characterization. Nonlinear

measurements fill two sessions with measurements of nonlinear coefficient and effective

area. And, with all of this, it is nice to see original work still going on in fibers - we find a full session of papers relating to novel (engineered) fibers.

This year we have 44 papers (9 invited and 35 contributed), with strong international

participation (slightly more than half originating outside the United States).

Happy reading.

Paul Williams

Gordon Day

Boulder, Colorado

September 2000

Except where attributed to NIST authors, the content of individual sections of this volume has not been reviewed or

edited by the National Institute of Standards and Technology. NIST therefore accepts no responsibility for comments or recommendations therein. The mention of trade names in this volume is in no sense an endorsement or recommendation by the National Institute of Standards and Technology.

iii
SYMPOSIUM COMMITTEE

G.W. Day, NISI, General Chair

P.A. Williams, NIST, Program Chair

M. Artiglia, CSELT

A.J. Barlow, PerkinElmer Optoelectronics

J. Benson, NPL

S.C. Fleming, University of Sydney

D.L. Franzen, NIST

N. Gisin, University of Geneva

M. Hackert, Coming, Inc.

T.A. Hanson, Coming, Inc.

J. Jackel, Telcordia Technologies

H. Nagai, Amitsu

W.A. Reed, Lucent Technologies

G.W. Schinn, EXFO

C. Shaar,GNNettest

K.K. Wong, Codeon Corporation

iv
CONTENTS

PREFACE iii SYMPOSIUM COMMITTEE iv

Characterisation of Photonic Crystal Fibres (invited)

T.A. Birks, J.C. Knight, B.J. Mangan, A. Ortigosa-Blanch, P. St.J. Russell,

University of Bath 1

Characterization of the Guiding Properties of a Bragg Type Photonic-Band-Gap Fiber

P. Roy, F. Brechet, P. Leproux, J. Marcou, D. Pagnoux, Institut de Recherche

en Communications Optiques et Microondes 7

Ultra-Broad Supercontinuum Generation in Tapered Telecommunications Fibres

T.A. Birks, W.J. Wadsworth, P. St.J. Russell, University of Bath 11 A Simple Servo Controlled Scanning Reflectance Measurement Method for Determining the

Refractive Index Profile of a Waveguide

Y. Park, N.H. Seong, Y.C. Youk, U.C. Peak, D.Y. Kim, Kwangju Institute

of Science and Technology 15

Characterizing the Filter Response of Optical Routers (invited)

Chellappan Narayanan, Gail Bogert, Lucent Technologies . 19

Saturated Optogalvanic Transition in Krypton at 1 564 nm

David A. Humphreys, National Physical Laboratory 25

Characterization of Multimode Fiber for 10 Gbps Operation (invited)

John S. Abbott, Coming, Inc 29

Short Pulse, Small Spot Size, Central Launch in Multimode Fibres: Comparison Between

Measurement and Computation

M.J.N, van Stralen, Plasma Optical Fibre B.V.; B.P. de Hon, M. Bingle, Eindhoven

University of Technology; F.J. Achten, Plasma Optical Fibre B.V. . 35

Measurement of the Non-Linear Index of Refraction, Nj (invited)

Dan L. Philen, Lucent Technologies, Bell Laboratories 39

Measurement Conditions of Nonlinear Refractive Index nj for Various Single-Mode

Fibers by CW Dual-Frequency Method

Kazuhide Nakajima, Takuya Omae, Masaharu Ohashi, NTT Access Network

Service Systems Laboratories 45

V
KDD Nonlinear Coefficient Round Robin Measurements for Various Dispersion

Shifted Fibers in Japan and UK

Yoshinori Namihira, KDD R&D Laboratories, Inc 49

Kerr Coefficient for Chaotic Light in Optical Fiber

S.M. Pietralunga, P. Martelli, M. Ferrario, CoreCom; M. Martinelli,

Politecnico di Milano 53

Estimation of the Polarization Coupling Length in Standard Telecom Fibers from

Measurements of Nonlinear Polarization Rotation

C. Vinegoni, M. Wegmiiller, N. Gisin, University of Geneva , 57

A Comparison of Three Techniques for Effective Area Measurement of Single-Mode

Optical Fibres

David A. Humphreys, Robert S. Billington, Andrew Parker, Brian Walker,

Dominic S. Wells, National Physical Laboratory; Andrew G. Hallam, GN Nettest;

Isabelle Bongrand, Universite de Nice 61

Model for Calculation of Mode Field Diameter Along the Fibre Using Bi-directional

OTDR Measurements

J. Mezger, G. v. Broekhoven, P. Pleunis, R. v. Laere, Plasma Optical Fibre B.V 65

A Novel Far-Field Scanning Technique for Rapid Measurement of Optical Fiber Parameters

Jeffrey L. Guttman, Razvan Chirita, Carmen D. Palsan, Photon, Inc 69

The Status of International Fiber Measurement Standards

W.B. Gardner, Q. 15/15 ITU-T; A.H. Cherin, Working Group 1, lEC SC86A 73

Measurement Issues in PMD Compensators (invited)

Dipak Chowhury, Corning, Inc 77

A Poincare Sphere Method for Measuring Polarization-Mode Dispersion

Using Four-Wave Mixing (FWM) in Single-Mode Optical Fiber

Shuxian Song, CIENA Corporation; Kenneth Demarest, Christopher Allen,

University of Kansas 79

Modulation Phase Shift Measurement of PMD: Towards an Engineered Solution

A.J. Barlow, J. Henstock, C. Mackechnie, PerkinElmer Optoelectronics 83

Interpretation of PMD Measurements in Components: A Case Study

Normand Cyr, EXFO Electro-Optical Engineering Inc 87

An Intercomparison of Polarisation Mode Dispersion Measurements and Calibration Artefacts

D.J. Ives, National Physical Laboratory, UK 91

vi
High Speed Soliton Transmission on Fiber Links with High PMD (invited)

Peter A. Andrekson, Chalmers University of Technology 95

Characterization of High-Order PMD

A. Eyal, Y. Li, A. Yariv, California Institute of Technology 101

Measurements of Enhanced BER Fluctuations Due to Combined PMD and PDL Effects

in Optical Systems

N. Gisin, M. Wegmiiller, University of Geneva; A. Bessa dos Santos,

J.P. von der Weid, Pontificia Universidade Catolica 105

Reflectometric Characterization of Distributed Birefringence in Installed Single-Mode Fibers

Massimo Artiglia, Marco Schiano, Tiziana Tambosso, CSELT; Andrea Galtarossa,

Luca Palmieri, Anna Pizzinat, Universita di Padova 1 09

Accurate Spectral Characterization of Polarization Dependant Loss

R.M. Craig, NIST 113

Heterodyne System at 850 nm for Measuring Photoreceiver Frequency Response

Paul D. Hale, CM. Wang, NIST 117

Time-Domain Measurement of the Frequency Response of High-Speed

Photoreceivers to 50 Ghz

Tracy S. Clement, Paul D. Hale, Kevin C. Coakley, CM. Wang, NIST 121

Optical Amplifier Measurement Issues: Doped Fiber, Semiconductor and Raman (invited)

Jack Dupre, Agilent Technologies 125

Comparison of Erbium Amplifier Characterisation Using an ITU Grid v/ith Varying

Channel Separations

M.E. Bray, R.T. Elliot, K.P. Jones, Nortel Networks 131

Gamma and Proton Radiation Effects in Erbium-Doped Fiber Amplifiers:

Active and Passive Measurements

Todd S. Rose, Deana Gunn, George C. Valley, The Aerospace Corporation 135

Issues in Raman Gain Measurements

R.H. Stolen, Virginia Tech 139

Broadband Raman Gain Characterization in Various Optical Fibers

F. Koch, S.A.E. Lewis, S.V. Chemikov, J.R. Taylor, Imperial College 143

Raman Spectra of Line Fibres Measured over 30 THz

D. Hamoir, N. Torabi, A. Bergonzo, S. Borne, D. Bayart, Alcatel 147

vii
Raman Gain Measurements in Optical Fibers

Stuart Gray, Coming, Inc 151

Overview of Coherent Reflectometry Techniques: Characterization of Components

and Small Systems (invited)

M. Wegmiiller, P. Oberson, J.P. von der Weid, O. Guinnard, L. Guinnard,

C. Vinegoni, M. Legre, N. Gisin, University of Geneva 155

Fiber Bragg Grating Metrology Round Robin: Telecom Group

A.H. Rose, CM. Wang, S.D. Dyer, NIST 161

Measurements of Dense Group Delay Ripple Using the Phase Shift Method: Effect

of Modulation Frequency

T. Niemi, M. Uusimaa, H. Ludvigsen, Helsinki University of Technology 165

A Fast and Accurate Measurement of Both Transmission and Reflection Group Delay

in Fiber Bragg Gratings

Shellee D. Dyer, Kent B. Rochford, NIST 169

Differential Group Delay Measurements of Chirped Fiber Bragg Gratings Using

Photon Counting

Hugues de Riedmatten, Mark Wegmiiller, Hugo Zbinden, Nicolas Gisin,

University of Geneva 173

Measurement Issues for Dispersion Compensation (invited)

R.M. Jopson, Lucent Technologies 177

Chromatic Dispersion Measurement Using Phase-Shift Technique for Installed Long Haul

Optical Submarine Cable

Toshio Kawazawa, Takuyuki Yamaguchi, Koji Goto, KDD 181

A Novel Method for Measuring Chromatic Dispersion of Long Optical Fibers by Means

of High Resolution Optical Ranging System Using a Frequency-Shifted Feedback Laser

Koichi liyama, Takahiro Maeda, Saburo Takamiya, Kanazawa University 185

VIII
Characterisation of photonic crystal fibres

T. A. Birks, J. C. Knight, B. J. Mangan, A. Ortigosa-Blanch and P. StJ. Russell

Optoelectronics Group, Department ofPhysics, University ofBath, Claverton Down, Bath BA2 7A Y, United Kingdom

pystab @bath. ac. uk

Abstract: Light is confined to the core of a photonic crystal fibre by an array of air holes in the cladding. Although the methods used to measure their characteristics are broadly similar to those applied to conventional fibres, the results of such measurements are often strikingly different.

Photonic crystal fibres

In 1996 we first reported an entirely

new class of optical fibre, the

photonic crystal fibre[l] (PCF, also

known as holey fibre or microstructure fibre). Whereas

conventional fibres confine light by

total internal reflection between a

high index core and a lower index

cladding, a PCF has an £irray of air

Fig. 1. The fabrication of a simple PCF from a stack of silica tubes with a central solid rod. In practice the stack would have around 10

tubes along one side. holes in the cladding that run along the length of the fibre. It is the presence of these holes that confines

light to the fibre's core, so the solid part of the fibre (core and cladding) can be made from a single

material, undoped fused silica. Fabrication begins with a number of silica tubes and rods, with an outer diameter of 1 mm or a little

less. These are painstakingly stacked into a close-packed array. The distribution of tubes and rods in the

stack determines the type of PCF. For example, the simplest is made entirely from tubes, except for a

single solid rod in the middle to form the core of the eventual fibre. Fig. 1. The stack is the fibre preform,

to be drawn into fibre using a standard drawing tower[l,2]. A solid silica jacket can be added around the

fibre to enlarge it and help strip cladding modes.

We have demonstrated a wide range of PCF designs, and developed the conceptual tools needed to understand their unique properties[l-12]. Other research groups, some in collaboration with us, have also

reported theoretical and experimental advances[13-18]. In most of these studies, the core is a region of

higher average index - usually the site of a "missing" hole. Such an "index guiding" fibre guides light by

total internal reflection from a cladding that (because of the holes) has a lower effective refractive index.

More recently, we demonstrated PCFs with a low-index core, such as an enlarged hole. Unlike previous hollow core fibres,

which are leaky waveguides, these "band gap guiding" PCF can

be true low-loss single-mode waveguides.

The characterisation of PCFs can be challenging at times,

though the techniques themselves are essentially the same as those used to characterise conventional fibres. However, the

results of such measurements are often strikingly different.

Single-mode propagation in index-guiding PCFs

Our first PCF[1,2] had a solid core, and the diameter d of the

holes was small compared to their pitch (centre-centre spacing)

A, Fig. 2. This was the simplest structure to attempt. Our

immediate question was: will the fibre guide light at all? Light

from a He-Ne laser was coupled into one end of the fibre, and

Fig. 2. SEM image of a PCF with A = 2.3 /jm and d/A = 0.15. The central site with

the "missing " air hole acts as the core.

1
the output light was projected onto a screen to examine the far field pattern. Fig. 3. A coupling efficiency of 50 % was readily obtained. As

well as confirming that the fibre was indeed a waveguide (and that physics is beautiful), this simplest of measurements indicated that the

fibre was single-mode. Although clearly not gaussian, the far field

pattern shared the full symmetry of the fibre, and did not change in distribution as the fibre was bent or the launch position and angle were

varied.

This experiment was repeated with every laser source we could lay our hands on. Surprisingly, we found that the fibre remained singlemode from 337 nm to 1550 nm. A measurement of the near field

pattern at the fibre's endface. Fig. 4, turned out to be the key to developing our understanding of this behaviour. At short wavelengths, the

light avoided the holes. In contrast, at longer wavelengths the light did not image the holes so well, and more of the

light propagated in air. As a result, the

light was "seeing" a lower effective

cladding index at long wavelengths than

at short wavelengths.

This insight led to our effective index

model for the PCF[3], by which the

effective V-value

/ 2 _

2 ^l^core ^cladding ' V^)

Fig. 3. Farfield pattern at X = 633

nm.

v =

Fig. 4. Contour plots of near field at 633 nm and 1550 nm, superimposed to scale on an SEM image of the endface.

can be found from a calculation of a rigorously-defined cladding effective index riciaddmg- Unlike a

conventional fibre, where V rises without limit as X —> 0, V in a PCF has an upper bound. This is because,

in the short wavelength limit, riciadding approaches ricore in such a way as to cancel the explicit X

dependence in eq. (1), Fig. 5. For sufficiently small relative hole sizes dlh, V will always be less than the

second-mode cutoff value. Hence a PCF with small enough holes will be endlessly single-mode, at all wavelengths.

One remarkable consequence is that an endlessly single-mode PCF must also be single-mode for all scales A, for a given dlh. We confirmed this by making a PCF like that of Fig. 2 but with A = 10 ^im and

a core about 22 |im across. The fibre was single-mode even for a wavelength as short as 458 nm[4]. This

directly shows that such a fibre with a core diameter of 75 |im would be single-mode at A, = 1550 nm!

1.46

Bend loss

The useful single-mode range of a conventional fibre is bounded at long wavelengths by a bend edge, beyond

which the fibre suffers unacceptably high losses due to unavoidable bending. It turns out that the useful singlemode range of an endlessly single-mode PCF is bounded at short wavelengths by a second bend edge.

The fibre was subjected to a single turn bend of a given radius. The transmission spectrum of the bent

fibre was measured using a white light source and

optical spectrum analyser. The measurement was

I 1.458

1.456

1.454

1.452

pure silica core

F-doped core

holey

cladding

0 400 800 1200 1600 2000

wavelength (nm)

Fig. 5. The effective cladding index of a PCF,

along with the index of pure silica and F-doped

silica cores. Material dispersionn has been omitted

for the sake of clarity.

2
800

Qi

COW5

700

600 -

500 -

400

4 5 6

bend radius (mm)

Fig. 6. The wavelength of 3 dB loss at the short- wavelength bend edge, versus bend radius, for the PCF of Fig. 2. The line is a fit to A oc r

'^' Inset: a

typical loss spectrum.

repeated for a number of radii of curvature[3]. A typical

loss spectrum is shown in Fig. 6, along with the

variation of the short-wavelength bend edge with radius.

Firstly, note how tightly the fibre must be bent to induce

significant loss. The existence of the new bend edge is a

less restrictive limit on the useful range of a PCF than

the second-mode cutoff of a conventional fibre. Secondly, the effective index model predicts that the

critical radius of curvature Rc, tighter than which

unacceptable bend loss occurs, varies as

The close fit to this dependence confirms the validity of

the model. The strong dependence on A indicates that large core PCFs are more susceptible to bend loss, and this is indeed a practical limit on the usefulness of

large core PCFs.

Interestingly, a simple model for the stripping of cladding modes in a PCF predicts that the length of

fibre required has the same dependence on A and X as Rc in eq.(2). Hence large core PCFs are also harder

to strip cladding modes from. This was a real experimental headache for us, because long lengths of our uncoated fibres needed to be immersed in index-matching fluid to observe the guided mode. Our present

ability to surround the photonic crystal region of such fibres with a solid silica jacket (see, for example,

Fig. 7) has greatly helped.

Depressed core PCFs

We made another PCF like that of Fig. 2, but the solid core rod was made of fluorine-doped silica[5]. Fig. 7. This depressed the core index by 0.0012. Fig. 5 is a plot of ricore and riciaddmg for this

fibre. The cladding index exceeds the core index at short

wavelengths, where the fibre becomes an antiguide. It resembles

the "dispersive fibres" reported by Morishita et al, and like them

has a short wavelength cutoff[19]. Note that this is a wavelength

below which no mode is guided, unlike the conventional "cutoff

wavelength" below which two modes are guided.

Transmission and bend loss measurements confirm the

existence of this cutoff, which is more graphically illustrated by

the near-field images in Fig. 8. These show how the field spreads out from the core as the wavelength decreases, in contrast with the more familiar behaviour in conventional fibres. This property is expected to be valuable in evanscent field devices with stronger

interactions at shorter wavelengths.

Multiple core PCFs

The original PCF was made from a stack of

tubes and a single solid rod. A PCF with more

than one core can be made just as easily, by „. „ „ ,

, . ... , ... , . , Fig. 8. Near field images at the including more than one solid rod m the depressed core PCF for three different laser wavelengths.

array[6]. The strength of coupling between At 458 nm, the light is not guided at all.

Fig. 7. SEM image of the depressed core PCF, showing the photonic crystal region

surrounded by a solid jacket.

3
adjacent cores is represented by the coupling beat

length Lb, the distance along which light couples from

one core to the other and back again. Attempts to measure this directly were unsuccessful: scattering from

the holes confounded our ability to transversely observe

laser light in each core. We contented ourselves with measurements of

spectral beating, from which some idea of the coupling

length can be deduced. Light from a tunable diode laser was focused into one core of a two-core PCF using a high power microscope objective, and the near field

pattern at the other end was imaged. The division of

power between the two cores was recorded as a function of wavelength. Fig. 9. The measurement was

repeated for similar fibres with different pitches A. The

spectral period Xfsr is plotted against A in Fig. 10, where

an exponential dependence is evident. (The normalised

quantity plotted is Lb/X if it is assumed that Lb is directly or inversely proportional to X.)

High birefringence PCFs

There are two sources of birefringence in optical fibres, stress birefringence (due to transverse stresses) and

form birefringence (due to the shape of the core). In conventional fibres with only a small core-cladding

index difference to play with, high birefringence (HiBi)

fibres are made by inducing stresses across the core, giving birefringence beat lengths no shorter than a few

153C

Fig. 9. The power transmitted in each core of a two- core PCF, versus wavelength.

10*

4

10^

10^

10=^

1 2 3 4 5

Fig. 10. The normalised spectral period of a two- core PCF versus pitch, for core separations of (a) ^/3A and(b) V/A

mm at ^ = 1550 nm. In contrast, the available index difference in a PCF is that between silica and air, and

very large form birefringence becomes possible. We made a HiBi PCF by varying the sizes of the air holes near the core[7], Fig. 11. The measurement challenge here is the determination of the birefringence beat length Lb. Again, it was not possible to measure this, and we had to resort to spectral measurements. Both eigenpolarisations were equally excited at the input of the straight PCF by the tunable laser. The light emerging from the

other end of the fibre was passed through a polariser at 45° to the principal axes and then detected. The

spectral beats measured are shown in Fig. 12. An estimate of Lb can be found from this measurement with the help of some numerical modelling,

which enabled us to fix the wavelength dependence of A,. The resulting estimated beat length of 0.43 mm

is an order of

magnitude smaller KaM"^^^^^^^^^^"^ io than the typical value

for conventional HiBi

fibres at this wavelength,

indicates

potentially

polarisation

properties

PCFs.

and

the

superior

holding

of such

Fig. 11. SEM image ofa HiBi PCF.

1534 1536 1538 1540 Wavelength (nm)

Fig. 12. Transmission spectrum through a HiBi

PCFfollowed by a 45° polariser.

4
Group velocity dispersion measurements

With our improved control over the fabrication process, we were

able to make PCFs with larger air holes relative to their separations.

These were predicted to have unusual dispersion properties,

including strong normal dispersion at 1550 nm[8] (for dispersion

compensation) and anomalous net dispersion at wavelengths shorter

than 1270 nm[9] (for nonlinear interactions[18], including soliton

propagation at visible and near infrared wavelengths! 10]).

Group velocity dispersion (GVD) in PCFs has been measured

using white light interferometry[l 1]. The PCFs were large-hole

structures, the most extreme of which is shown in Fig. 13. The

measured GVD is shown in Fig. 14. The measurement compares well

with the calculated GVD of a rod of silica in air, with the same

diameter as the core of the PCF. This is not surprising; the

thin webs of silica supporting the core do not contribute

much to the properties of the fundamental mode.

Fig. 13. SEM image of an extreme

small core PCF with anomalous

dispersion at visible wavelengths.

300

200

Band gap guiding PCFs

All of the measurements described above were made on

index-guiding PCFs. Despite their unusual structures these

all guide light by total internal reflection, and so despite

their distinctive properties they have many similarities with

conventional fibres. In contrast, band gap guiding PCFs (Fig.

15) are radically different[12]. Because the core is hollow,

light cannot be confined by total internal reflection - the

100

-100

-200

-300

Silica stiand computed

PCF measured

Bulk Silica

0.9 1.0

Wavelength - mm

Fig. 14. GVD of the PCF of Fig. 14 (two

polarisations), together with the GVD of the corresponding silica rod and of bulk silica. cladding index (however defined) will always be greater. Instead, light is confined by a generalised

Bragg scattering from the periodic array of large air holes[20].

Near field images of the fibre output, for white light input, clearly show a guided mode in the hollow

core. However, the light is invariably strongly coloured. This is because of the wavelength dependence of

the photonic band gap that confines the light. Transmission spectra of the fibre. Fig. 16, show passbands

separated by wavelength ranges within which no light is guided. The transmission of this fibre is far in excess of that seen in previous "hollow core fibres", which are in reality antiguides and therefore leaky.

One remarkable property of the fibre is that there is very little Fresnel reflection at the ends. Whereas

conventional fibres (and indeed index-guiding PCFs) have an index mismatch between the fibre and the

air at the ends, the light in the hollow core PCF sees no such mismatch.

-123 THz -1231Hz

Fig. 15. SEM image of a band gap guiding PCF

with a hollow core.

wavelength (nm) Fig. 16. Transmission spectrum of two similar hollow core PCFs with slightly different sizes.

5
Conclusions

We have reviewed some of the properties of photonic crystal fibres, and described how well-known

measurement techniques can be applied to these remarkable structures. Although PCFs behave in many

ways like ordinary fibres, some of their characteristics are strikingly different. These differences lead to a

variety of applications for which PCFs out-perform conventional fibres. We are in the process of setting

up a company to exploit these fibres[21].

Acknowledgements

Besides the authors, whole team of researchers has contributed to the many results described here,

including R. F. Cregan, W. J. Wadsworth, J. Arriaga and D. Mogilevtsev. The work was supported by

funding from EPSRC, DERA and Coming Inc, and our fibre drawing tower was donated by BT Labs. T. A. Birks is a Royal Society University Research Fellow.

References

[I] J. C. Knight, T. A. Birks, D. M. Atkin, P. St.J. Russell "Pure silica single-mode fibre with hexagonal photonic crystal cladding", Proc. OFC (1996) postdeadline paper PD3.

[2] J. C. Knight, T. A. Birks, P. St.J. Russell, D. M. Atkin "All-silica single-mode optical fiber with photonic crystal cladding" Opt. Lett. 21 (1996) 1547; errata ibid22 (1997) 484.

[3] T. A. Birks, J. C. Knight, P. St.J. Russell "Endlessly single-mode photonic crystal fiber" Opt. Lett. 22 (1997) 961.

[4] J. C. Knight, T. A. Birks, R. F. Cregan, P. St.J. Russell, J. P. DeSandro "Large mode area photonic crystal fibre" Electron.

Lett. 34(1998) 1347.

[5] B. J. Mangan, J. Arriaga, T. A. Birks, J. C. Knight, P. St.J. Russell "Photonic crystal fibre with a short wavelength cutoff

Proc. CLEO (2000) 606.

[6] J. C. Knight, T. A. Birks, B. J. Mangan, P. St.J. Russell, G. G. Vienne, J.P. DeSandro "Multicore photonic crystal fibres" Proc. OFS (1997) postdeadline paper PD5.

[7] A. Ortigosa-Blanch, J. C. Knight, W. J. Wadsworth, J. Arriaga, B. J. Mangan, T. A. Birks, P. St.J. Russell "Highly

birefringent photonic crystal fibers" to appear in Opt. Lett.

[8] T. A. Birks, D. Mogilevtsev, J. C. Knight, P. St.J. Russell "Dispersion compensafion using single material fibers" IEEE

Photon. Technol. Lett. 11 (1999) 674.

[9] D. Mogilevtsev, T. A. Birks, P. St.J. Russell "Group-velocity dispersion in photonic crystal fibers" Opt. Lett. 23 (1998)

1662.

[10] W. J. Wadsworth, J. C. Knight, A. Ortigosa-Blanch, J. Arriaga, E. Silvestre, P. St.J. Russell "Soliton effects in photonic

crystal fibres at 850 nm" Electron. Lett. 36 (2000) 53.

[II] J. C. Knight, J. Arriaga, T. A. Birks, A. Ortigosa-Blanch, W. J. Wadsworth, P. St.J. Russell "Anomalous dispersion in photonic crystal fiber" to appear in IEEE Photon. Technol. Lett. [12] R. F. Cregan, B. J. Mangan, J. C. Knight, T. A. Birks, P. St.J. Russell, P. J. Roberts, D. C. Allen "Single-mode photonic

band gap guidance of light in air" Science 285 (1999) 1537.

[13] T. M. Monro, D. J. Richardson, N. G. R. Broderick, P. J. Bennett "Holey optical fibers: an efficient modal model" IEEE

J. Lightwave Technol. 17 (1999) 1093.

[14] M. J. Gander, R. McBride, J. D. C. Jones, T. A. Birks, J. C. Knight, P. St.J. Russell, P. M. Blanchard, J. G.Bumett, A. H. Greenaway "Measurement of the wavelength dependence of beam divergence for photonic crystal fiber" Opt. Lett. 24

(1999) 1017.

[15] B. J. Eggleton, P. S. Westbrook, R. S. Windeler, S. Spalter, T. A. Strasser "Grating resonances in air-silica microstructured optical fibers" Opt. Lett. 24 (1999) 1460.

[16] S. E. Barkou, J. Broeng, A. Bjarklev "Silica-air photonic crystal fiber design that permits waveguiding by a true photonic

bandgap effect" Opt. Lett. 24 (1999) 46.

[17] A. Ferrando, E. Silvestre, J. J. Miret, P. Andres, M. V. Andres "Full-vector analysis of a realistic photonic crystal fiber" Opt. Lett. 24 (1999) 276.

[18] J. K. Ranka, R. S. Windeler and A. J. Stentz "Visible confinuum generation in air-silica microstructure optical fibers with anomalous dispersion at 800 nm" Opt. Lett. 25, 25 (2000).

[19] J. Nishimura, K. Morishita "Changing multimode dispersive fibers into single-mode fibers by annealing and guided mode

analysis opf annealed fibers" IEEE J. Lightwave Technol. 16 (1998) 990.

[20] T. A. Birks, P. J. Roberts, P. St.J. Russell, D. M. Atkin, T. J. Shepherd "Full 2-D photonic bandgaps in silica/air structures" Electron. Lett. 31 (1995) 1941.

[21 ] Blaze Photonics Ltd, http://www.blazephotonics.com

6
CARACTERIZATION OF THE GUIDING PROPERTIES

OF A BRAGG TYPE PHOTONIC-BAND-GAP FIBER

P. ROY, F. BRECHET, P. LEPROUX, J. MARCOU and D. PAGNOUX

Institut de Recherche en Communications Optiques et Microondes

UMR CNRS n°6615

123 Avenue Albert THOMAS - 87060 LIMOGES cedex - FRANCE

Abstract: A novel BRAGG type photonic band gap fiber, having a cladding made of alternate

high and low index layers, is presented. Single mode propagation has been experimentally

observed The propagation of light into a BRAGG type photonic band gap fiber is strongly

dependent on the wavelength. The transmission spectrum is experimentally measured

Introduction: In this paper, a BRAGG type photonic-band-gap (PBG) fiber is presented. It consists in a low-index cylindrical core surrounded by a cladding made of layers with

alternating high and low indices (respectively ni and ns), whose values are higher than that of

the core. In the cladding, the radial quasi-periodical multilayer structure acts as a Bragg reflector

and the light is kept confined into the antiguiding core. Theoretical studies have predicted

single-mode propagation of light at an operating wavelength \q = 1060nm and the ability to achieve zero-dispersion at short wavelengths [1],[2]. Thanks to these novel characteristics, the

fiber is very attractive for applications in non linear optics : soliton propagation, parametric

amplification and oscillators. In this communication, the very first propagation of a single-mode

pattern into a BRAGG type PBG fiber is reported at = 1053nm and its transfer function is measured.

Manufacture and test of a single-mode PBG fiber: A preform for drawing a PBG fiber has

been manufactured by means of the classical MCVD technique. The fiber has been drawn to reach a 125|im diameter. Its index profile has been measured with a YORK FCmlOOO device.

This profile, superimposed with the theoretical electric field of the guided mode, is presented on

Refractive Electric field index (A.U.)

• • ^

1.460'

1.458

1.456

1.454

1.452

1.450

40 30 20 10 0 10 20 30 40

Radius (|im)

Figure 1 : Measured profile index and associated computed electric field of the

manufactured PBG fiber.

7
figure 1. The fluctuations of the values of nz are due to unexpected variations of the

concentration of Ge blended with silica to make high index layers during the MCVD process.

The fluctuations of ns are attributed to the measurement device [3].

A lOOmW pattern from a CW YLF solid state laser operating at Xo= 1.053nm was

launched into a Im length of the fiber. Figure 2.a is the near field observed at the output when

the input pattern is properly focused on to the central core. The guiding is made possible into the core thanks to the transverse photonic band gap effect. When the input pattern is focused on to the cladding, the light only propagates in the high index layers, as shown on figure 2.b.

Figure 2 : Near field observed at the output of a PBG fiber, (a) when the input pattern

is properly focused on to the central core and (b) when the input pattern is focused on

to the cladding

On the index profile of figure 1, Di and D2 are respectively the diameter of the middle of the

first high index layer and the diameter of the first transition from nz to na. On figure 2, D'l and

D'2 are two dimensions measured on the output near field pattern : D'l is the diameter of the first ring of light when launched into the cladding (figure 2.b) and D'2 is the diameter of the first ring

of the single-mode pattern obtained with centered launching conditions (figure 2.a). According

to the predicted conditions of propagation on the field [3], Di and D2 must respectively

correspond to D'l and D'2. The ratios D1/D2 and D'i/D'2 are respectively equal to 1.12 and 1.13.

These two ratios are very close to each other, showing that experimental results are in good

agreement with theoretical predictions.

The measurement of the modulus of the transfer function of the fiber (called for

convenience iTF|) as a function of wavelength is achieved with the experimental setup described

chopper

(frequency Q

halogen lamp gratmg

monochromator

single-mode step index fibers

PBG fiber under test \. .. . /

micro positionmg system

cooled Ge detector

spectrum analyser

Figure 3 : Experimental measurement setup

8
on figure 3. The monochromatic beam from a grating monochromator chopped at the frequency

fc = 80Hz is first launched into a short length of 850nm cutoff wavelength single-mode fiber

(SMFl). The gaussian beam (wo = 3.8nm) from this fiber is then carefully launched into the core of a 40cm length of the PBG fiber. A second length of the same single-mode fiber (SMF2)

is finely aligned at output end of the PBG fiber. Its role is to spatially filter the output beam

from the upstream fiber and to collect the only power from its core. This power is detected at the

output end of SMF2 by a cooled Ge-detector connected to an electrical spectrum analyser. The

power P1(X) in the fc spectrum line of the detected optical signal is measured and registered,

from 900nm to 1300nm. Then, SMFl is directly connected to SMF2 and the reference power

F2(k) detected at the fc frequency is measured versus wavelength and registered. Finally, the

P2(A,)/P1(X)

ratio R(X) = provides the normalised ITFI of the tested PBG fiber. R{k) is (P2/Pl),i„

computed and plotted in dB on figure 4. The central wavelength is lOSOnm. The measured 3dB

Normalised |TF|

-20 •

-40

-60 -

950

^ ^.rrrrrr.^ ^ -3dB

-lOdB

^

1050 1150

Wavelength (nm)

1250

Figure 4 : Normalised modulus of the transfer function (|TF|) measured on the PBG fibre.

and lOdB bandwidths are respectively SOnm and 140nm. Out of the transmission bandwidth, the

1TF| dramatically decreases from -lOdB to the minimum accurately measurable value (-50dB)

within about 40nm (i.e. the slope reaches IdB/nm). The measured values of the transmission

bandwidth are approximately two times narrower than that we compute using a 3D BPM

algorithm. In accordance with theoretical predictions, this narrowing is attributed to the

unexpected irregularities of the actual index profile. However, the bandwidth of the

manufactured fiber remains large enough for many non linear applications. In spite of the noted

discrepancy, the spatial filtering behaviour of the fiber around 1060nm is clearly pointed out. Further measurements are planed with new fibers just manufactured.

9
Conclusion: A depressed core index photonic band gap fiber lias been manufactured and the

propagation of light into the central core has been observed at Xo = LOSSjim, thanks to the

transverse photonic band gap effect The modulus of the transfer function (|TF|) of a PBG fiber

has been measured. Experimental measurements with a 40cm piece of manufactured PBG fiber

exhibit a 3dB bandwidth of 80nm around lOSOnm. The slope of the |TF| reaches IdB/nm out of

the transmission bandwidth. The bandwidth is large enough for non linear applications such as parametric amplification into this fiber that is supposed to exhibit a zero dispersion at the central

wavelength of the bandwidth.

Acknowledgements

The authors are grateful to B. Dussardier, G. Monnon and F. Ubaldi from LPMC Nice for

manufacturing the preform and to J.M. Blondy and G. Boutinaud from IRCOM Limoges for

drawing the fiber. This work is supported by the French CNRS under grant n° TL98006.

References

HI P. YEH, A. YARIV, E. MAROM, "Theory of Bragg fiber", Journal of Optical Society ofAmerica, Vol 68,

n°9, 1978.

12] J. MARCOU, P. ROY, "Monomode photonic band gap fibers for dispersion shifting towards short wavelengths", proceeding oi European Conference on Optical Communication, 1999 Nice, France.

[3] F. BRECHET, P. ROY, J. MARCOU, D. PAGNOUX, " Single mode propagation into a depressed-coreindex photonic-band-gap fiber designed for zero-dispersion propagation at short wavelengths ", Electronics Letters, vol 36. n°6 pp.514-515, 2000.

10
Ultra-broad supercontinuum generation in tapered

telecommunications fibres

T. A. Birks, W. J. Wadsworth, P. StJ. Russell

Optoelectronics Group, Department ofPhysics, University ofBath, Claverton Down, BathBAl 7AY, United Kingdom

pystab @bath.ac. uk

Abstract: Optical fibres tapered to about 2 microns diameter over a length of 90 mm were used to generate supercontinuum light from the 76 MHz femtosecond pulses emitted by an unamplified

Ti: sapphire laser. Output spectra were broad enough to fill the wavelength range 370 - 1545 nm

(over two octaves). This source of high-intensity single-mode "white" light should find widespread

applications in optical fibre measurements, especially since it does not require any unconventional

fibres for its operation.

Introduction

We report the generation of ultra-broad supercontinuum

light in tapered telecommunications fibres. Powered by

femtosecond pulses from a common unamplified

Ti: sapphire laser, the generated light was guided in the

fundamental mode of the fibre. Its spectrum could span

over two octaves from the ultraviolet to the third

telecommunications window (370 - 1545 nm at the 20 dB

level). With an average spectral density of 0.25 mW nm"',

this "white" light source has the brightness of a laser with

the bandwidth of a light bulb.

Generation of supercontinuum light in a "microstructure

untapered

fiber

taper waist

untapered

fiber

Fig. 1. A schematic diagram of a tapered fibre,

showing how light in the core of the untapered

fibre expands to fill the whole fibre in the taper

waist.

(a)

2 3 4

diameter (microns)

400

200 -

(b)

I

p.

§ -200

CD -400

400 800 1200

wavelength (nm)

1600

Fig. 2. Calculated GVD of a narrow tapered fibre waist in

air: (a) versus waist diameter for A = 850 nm (the broken

line is an asymptote, the GVD of bulk silica); (b) versus wavelength for (left to right) waist diameters of 1.0, 1.5 and

2.5 pm, and for bulk silica. Positive values represent

anomalous dispersion.

photonic crystal fibre (PCF) using lowenergy fs pulses has already been

demonstrated[l,2]. The fibre was an undoped

silica-air structure, in which a small core

(1-2 \im across) was surrounded by large air holes. The waveguide therefore resembled a thread of silica in air[3]. Unfortunately the

need for the special fibre means that the

technique is not widely available. The PCF

work has however identified a number of

applications for such sources. For example, the

retention of the longitudinal mode structure of

the modelocked laser has led to stunning recent advances in optical frequency metrology[4]. For optical fibre measurements, some obvious

applications include transmission spectroscopy

(of fibres and devices) and white light

interferometry (for measuring fibre dispersion,

for example[5]), which suffer from the low

spectral density and spatial coherence of

conventional broadband sources. We tapered our conventional

telecommunications fibre by heating and

stretching it in a flame, to reduce its diameter

11
to about 2 |im over a length of 90 mm. Fig. 1 depicts the resulting

structure, with a narrow taper waist connected to untapered fibre at both ends by taper transitions. The residual fibre core is so small

that the waist is effectively a simple thread of silica surrounded by

air, light being guided at the outer boundary. In this respect it resembles the PCF described above.

The large refractive index step between fibre and air allows light

to be funnelled into a very small area, increasing its intensity and Fig. 3. Typical visible output far-field promoting nonlinear effects. Furthermore, the group velocity pattern from one of the taperedfibres. dispersion (GVD) can be reduced or even made anomalous at visible and near infrared wavelengths

(Fig. 2). For a waist of diameter 1.8 ^m, the calculated GVD and nonlinear effective area are +122 ps nm ' km ' and 1.21 |jm^ giving nonlinear and dispersion lengths of 0.6 mm and 150 mm

respectively[6] (for laser pulses typical of our experiments). Significant self phase modulation is therefore expected despite the strong dispersion. Indeed, the anomalous dispersion helps to counteract the

normal dispersion suffered by the pulses as they propagate through an isolator, a launch objective and some untapered fibre en route to the taper waist.

Results

The telecommunications fibre used was Coming SMF-28 (cutoff wavelength 1250 nm, NA = 0.1), which

is widely available and costs a few cents per metre. The fibre was tapered using a travelling flame, which

allowed us to form waists that were uniform in diameter and long but with relatively short transitions[7].

The tapered fibres reported here all had waists about 90 mm long, with 35 mm transitions. The optical

insertion loss for light propagating through the structures was typically -0.2 dB. Each tapered fibre was

promptly glued at both ends in an enclosed housing after being made, in such a way that the waist and

transitions remained surrounded by air. The unamplified modelocked Ti: sapphire laser (Coherent MIRA) emitted 70-100 fs pulses with a

repetition rate of 76 MHz and a wavelength of 850 nm. The light passed through an isolator and launch optics

(spreading the pulses to 200-500 fs) and into the

tapered fibre. The taper's input fibre was kept short to minimise further normal dispersion of the pulses; the

output fibre was about 1 m long.

In most cases the output wave was visible as a very

intense far-field spot. Fig. 3. Although both the

untapered fibre (core-cladding waveguide) and the

taper waist (cladding-air waveguide) were multimode at visible and Ti: sapphire wavelengths, the

supercontinu' m light was always generated in the

fundamental mode.

The output spectra from two fibres identically

tapered to 1.8 |jm diameter are plotted in Fig. 4, for a time-averaged laser power of 300 mW (pulse energy

3.9 nJ). The spectra cover over two octaves in frequency, with Fig. 4(b) within 20 dB of the peak

value throughout the range 370 - 1545 nm. Unlike all the other spectra reported here. Fig. 4(b) is for a fibre

with a cutoff wavelength of 735 nm instead of

1250 nm, which was therefore single-mode at the laser wavelength. However, Figs. 4(a) and (b) are very

400 800 1200 1600

wavelength (nm)

Fig. 4. Output spectra from tapered fibres with

waists of 1.8 /.im diameter, for an average laser power of 300 mW. The cutoff wavelengths of the fibres that were tapered were (a) 1250 nm (a

standard telecoms fibre) and (b) 735 nm. The

broken line is the spectrum of the input light, scaled

vertically for comparison.

12
similar because in the taper waist the two structures

differ only in the size of the insignificant residual

fibre core. Output spectra from another tapered fibre are

plotted in Fig. 5 for different laser powers (and

hence pulse energies). Increasing the power

broadens the spectrum without increasing the peak

value.

Spectra for four different waist diameters, two

being simply copied from Figs. 4 and 5, are compared in Fig. 6. (Recall that waist diameter

determines the GVD, Fig. 2, as well as the effective

area.) The meaning of such a comparison for

different laser powers is limited, but smaller

diameters not only require less power, they also

produce spectra that are less flat. A "bite" is progressively taken out at visible wavelengths, leaving

decrease of the mean photon energy seems to indicate

to the spectral broadening.

No spectral broadening was observed (or expected)

400 800 1200

wavelength (nm)

1600

Fig. 5. Output spectra from a tapered fibre with a

waist of 2 iim diameter, for (top to bottom) average

powers of 380, 210 and 60 mW, together with the spectrum of the input light. The curves are scaled

vertically for comparison; the three output spectra all had approximately the same peak value. an asymmetric spectrum. The resulting systematic an increasing contribution from Raman scattering

in untapered fibre.

Discussion

Our results show that tapered fibres are as effective as PCFs for generating ultra-broad high-intensity

single-mode spectra. Although our technique does have some disadvantages (including the limited

interaction length and the need to protect the tapered fibre from dust and damage), definite advantages can be identified. Most significantly, no special fibre is required. A standard widely-available

telecommunications fibre is all that is needed. That the fibre is multimode is no drawback because the

supercontinuum is generated in the fundamental mode. The output is guided in a standard fibre and so is readily compatible with the familiar techniques and devices (fusion splices, connectors, directional

couplers, polarisers, etc) of conventional fibre optics.

Furthermore, although the nonlinear interaction occurs in the narrow waist, the input laser pulses are coupled into a standard fibre core with a large diameter of about 9 nm. Hence the input light is not tightly

focused, simplifying alignment and making the fibre's input endface relatively durable. The tapered fibre

itself is surprisingly robust. None of our samples deteriorated over time (over the several days available),

and indeed one tapered fibre in its housing was accidentally dropped onto a hard floor from a height of

1 m without being destroyed.

Tapering is a well-known technique, low in cost and quick to implement, which is widely used to make fibre directional couplers. (Indeed, the durability of such couplers gives confidence that tapers like ours would also be durable.) It takes only a few minutes to taper a fibre, and a few more to fix it in its housing. We could therefore generate samples very quickly and easily. It is also straightforward to form

non-uniform taper waists with a given diameter profile. From Fig. 2, such a diameter profile entails a

dispersion profile, a degree of freedom that is hard to implement in PCF but which should give greater

control over the shape of the supercontinuum spectrum.

By providing a way to generate ultra-broadband light without special fibres, our results show that exciting nonlinear effects in silica do not require long lengths, high powers or complex structures. Tapered fibres should be effective hosts for a wealth of other nonlinear effects besides supercontinuum

generation.

13
Conclusion

We have demonstrated the generation of an ultra-broad

supercontinuum in standard teleconiinunications fibre

that has been tapered to a diameter of ~2 \xm.

Femtosecond pulses from an unamplified Ti: sapphire

laser are sufficient to give a spectrum over two octaves broad at the 20 dB level. The generated light was guided

in the fundamental mode of the fibre, and its spectrum

spanned the range 370 - 1545 nm. Such a light source

will greatly simplify many optical fibre measurements.

References

[1] J. K. Ranka, R. S. Windeler and A. J. Stentz "Efficient

visible continuum generation in air-silica microstructure optical fibers with anomalous dispersion

at 800 nm" in Conference on Lasers and Electro- Optics (Optical Society of America, Washington DC,

1999), paper CPD8; J. K. Ranka, R. S. Windeler and A.

J. Stentz "Visible continuum generation in air-silica microstructure optical fibers with anomalous dispersion

at 800 nm" Opt. Lett. 25, 25 (2000).

[2] W. J. Wadsworth, J. C. Knight, A. Ortigosa-Blanch, J. Arriaga, E. Silvestre, B. J. Mangan and P. St.J. Russell

"Soliton effects and supercontinuum generation in photonic crystal fibres at 850 nm" in IEEE Lasers and

Electro-Optics Society Annual Meeting (1999), paper PDL5; W. J. Wadsworth, J. C. Knight, A. OrtigosaBlanch, J. Arriaga, E. Silvestre and P. St.J. Russell

"Soliton effects in photonic crystal fibres at 850 nm"

Electron. Lett. 36, 53 (2000).

[3] T. A. Birks, D. Mogilevtsev, J. C. Knight and P. St.J. Russell "Dispersion compensation using single-material

fibers" IEEE Photon. Technol. Lett. 11, 674 (1999).

[4] see, for example, T. W. Haensch, R. Holzwarth, J. Reichert and T. Udem "Precision spectroscopy with

femtosecond light pulses" in Quantum Electronics and

Laser Science Conference (Optical Society of America,

Washington DC, 2000), 109.

10 r 1 0

-

a. -10 - o

1

-20 -

-30 -400 800 1200

wavelength (nm)

1600

Fig. 6. Output spectra from tapered fibres with a

waist diameter and average power respectively of

(a) 2.5 pm and 430 mW, (b) 2.0 pm and 380 mW,

(c) L8 pm and 300 mW, and (d) L5 pm and

70 mW. The broken line is the vertically scaled input

spectrum.

[5] M. Tateda, N. Shibata and S. Seikai "Interferometric method for chromatic dispersion measurement in a single-mode optical fiber" IEEE J. Quantum Electron. 17, 404 (1981).

[6] G. P. Agrawal, Nonlinear Fiber Optics (Academic Press, San Diego CA, 2nd Ed. 1995).

[7] T. A. Birks and Y. W. Li "The shape of fiber tapers" IEEE J. Lightwave Technol. 10, 432 (1992).

14
A simple servo controlled scanning reflectance measurement method for

determining the refractive index profile of a waveguide

Y. Park, N. H. Seong, Y. C. Youk, U. C. Peak, and D. Y. Kim

Department ofInformation and Communications, Kwangju Institute ofScience and Technology,

Oryong-dong 1 Puk-gu Kwangju, Korea.

A simple scanning method is described for determining the refractive index profile of an optical fiber by

measuring reflectance of the end surface of an optical fiber. Instead of an objective lens, a lensed fiber is used

for focusing probe light on a surface to be measured, and a dithering servo control is employed to maintain a

constant distance between the lensed fiber and the surface. It is shown that this method is applicable to measure refractive index structures of fibers and planar waveguides.

INTRODUCTION

The most widely used technique for measuring the refractive index profile of an optical fiber is

the refracted near field technique owing to its simplicity in principle. However, this technique

has relatively low spatial resolution and needs complicated sample preparation such as immerging a fiber into an index matching fluid. It may not be used for determining the refractive

index profile of a waveguide if its structure is not axially symmetric.

In this paper, a simple scanning reflectance measurement technique with an auto-focusing servo

control is demonstrated for determining the refractive index profile of a cross section of a waveguide. Michelson interferometer type optical setup is used with a 3dB fiber directional

coupler replacing a bulk beam splitter which splits the input and the output beams. In order to

simplify the experimental setup, a lensed fiber is used for focusing probe light on a sample

surface. A dithering servo control system is combined with the reflectance measurement setup by

attaching the lensed fiber on a PZT stack. It is to apply small vibration to the lensed fiber along

the vertical direction. The reflected light is coupled maximum into the lensed fiber when the

sample surface is at the focusing point of the input beam. By detecting the power of the coupled

light into the lensed fiber with a lock-in amplifier, the offset of the sample surface from the

focusing point of the input beam is monitored. A feedback loop is constructed with a voltage

15
adder to compensate this offset and the system makes the sample surface to be located at the

focusing point of the input light all the time during scanning. As reflectivity is a function of the

refractive index of the sample surface, the index profile can be obtained from the scanned

reflectance profile measurement.

EXPERIMENTAL SETUP

A schematic diagram of refractive index profiling setup is shown in Figure 1 . In this experiment,

ASE(amplified spontaneous emission) noise from an EDF(Erbium doped fiber) is used as input

optical light source to eliminate unwanted interference patterns. The half of the source power is measured by detector! through a 3dB coupler to monitor fluctuations of input optical power and

the rest is focused by a lensed fiber on the end surface of a sample surface. The reflected light

from the sample surface is coupled back into the lensed fiber again and propagates backward

direction. The power of this backward propagating light is measured by detector! through the

3dB coupler. The transmitted light toward the detector 1 and the reflected light toward the

detector2 are stored while scanning the sample surface along the X and Y directions. The

reflectance profile along the transverse direction of the sample is obtained by the ratio of power

by the detectorl and the detector2. The Fresnel equation in Equation (1) is used to convert the

obtained reflectance profile R(x,y) into the refractive index profile n(x,y) by assuming a normal

incidence of the focused light onto the sample surface.

RC filter

Figure I A schematic diagram of the refractive index profiler with auto-focusing apparatus.

16
At a fixed scan point, the power detected by the detector! is the maximum when the sample

surface is at the focusing point of the input beam. If the lensed fiber is dithered by a PZT along

the vertical direction at a few kHz fi-equency, the optical power at the detector! will oscillate

with the same fi-equency. A lock-in amplifier is used to measure the phase as well as the power

of the detected signal by the detector!. As the phase of the oscillating signal obtained by the

detector! changes by 180 degrees when the surface of the sample passes through the focal point

of the input beam, the output signal of the lock-in amplifier changes its sign at the focal point of

the input beam. Figure 1 show how the output voltage of the lock-in amplifier is applied to the

PZT driver with a voltage adder as a feed back signal. This auto-focusing system makes the spot

size of the input beam minimum at the sample surface and it brings out the maximum spatial

resolution. It also keeps the distance between the sample surface and the lensed fiber constant

while scanning the sample along the transverse direction.

RESULTS

Figure ! shows the reflected optical power at the detector! while the distance between the lensed

fiber and the sample surface is changed by varying the voltage applied to the PZT when the

sample surface is nearby the focal point of the input beam. Periodic oscillations are due to the

interference between the reflected light from the surface of the lensed fiber and the reflected light

from the sample surface.

0-

30 40 50 60 70 80 Applied Voltage

Figure 2. Detected optical power at detector2 as a function of the voltage applied to the PZT in Figure 1.

17
The period corresponds to the half of the center wavelength of the light source in space, which is about 1.54 f^m in this case. The linewidth of the ASE noise source from an EDF is about 40 nm. The envelope of the fast oscillating signal is maximum when the voltage applied to the PZT is about 53 volt. The full width half maximum of the focused beam is estimated to be 2 ± 0.2 \xm

by using a knife-edge. Figure 3 shows the measured reflectance distribution and refractive index

profile of a commercial graded index fiber around the core area. The refractive index profile is

calculated from the reflectance profile by using the Fresnel equation. Total scanned length is 60

|j.m with 0.4 |im step size. 300 samples were acquired at 15k samples per second and averaged to each step. The maximum variation of the measured reflected light power is about 1 50mV.

8

c

TO

0.0360-

0.0355 -

o

0)

0.0350

0.0345

—•— Refractive irxJex ——Refiedance V

1.470

1 465

1 460

73%

Q_<

CD5'

Q.

CDX

1 455

-0.03 -0.02 -0.01 000 0.01 0.02 0.03 Scan length [mm]

Figure 3. The measured reflectance and refractive index profile of a commercial graded index fiber

This work was supported in part by the KOSEF through the Ultra-Fast Fiber-Optic Networks

Research Center and by the Korean Ministry ofEducation through the BK21 Program.

[1] W. J. Stewart, ''Optical Fiber and Preform Profiling Technology," J. of Quantum Electron.,

vol. QE-18, no. 10, pp. 1451-1466. 1982.

[2] W. Eickhoff and E. Wiedel, ''Measuring method for the refractive index profile of optical

glass fibres." Opt. and Quantum Electron., vol. 7, no. 2, pp. 109-113. 1975.

[3] Timothy R. Corle. Gordon S. Kino. Confocal scanning optical microscopy and related

imaging systems, San Diego: Academic Press. 1996.

18
Characterizing the filter response of Optical Routers

Chellappan Narayanan, Gail Bogert

Lucent Technologies,

Microelectronics Group,

9999 Hamilton Blvd.,

Breinigsville, PA 18031.

Introduction

Although silica based planar waveguide technology has been around for almost two

decades, it is only in the last few years that we have seen a growth in the demand for

planar waveguide components in the marketplace. The reasons for this change is the

emergence of WDM technology, and the timely development of three key enabling

technologies, the wavelength multiplexer/de-multiplexer, the Erbium-doped optical

amplifier and wavelength tunable laser sources.

There are various ways of building an optical de-multiplexer. One promising device that can be realized using the silica-on-silicon planar waveguide technology is the waveguide

router (WGR)[1]. It is also referred to by other name, such as phased array, arrayed

waveguide grating, and Phasar. Silica planar waveguide technology provides a platform

for low-cost mass fabrication of high quality DWDM components. The advantages of

WGR include precise channel spacing, very good environmental stability, and negligible

chromatic dispersion. The other attractive feature of this technology is that it offers the

lowest cost approach to scaling to higher channel counts, because the manufacturing

process steps are transparent to design type and number of channels.

Testing at various steps of the planar waveguide manufacturing process plays an

important role in delivering a reliable and cost-effective product. First we will explaia the

terms used to describe the response of optical filters. Then we wUl focus on the

characterization of optical routers. Specifically, we wiU cover testing at two different

stages of the manufacturing process - Chip Level testing (fi-ont-end wafer testing) and

final packaged device testing. Although most of what is discussed in this paper is applicable to any optical filter technology, there are certain aspects that are very specific to planar waveguide DWDM.

Waveguide Router: Principle of Operation

An arrayed waveguide router (Figure 1) consists of two star couplers connected by an

array of waveguides of constant linear path length difference. This device structure forms

a 1:1 optical imaging system. For each wavelength at the input waveguide an image is formed at a different point in the image (output) plane. In other words, as the wavelength

is swept at the input the image sweeps across the output plane. By placing waveguides

appropriately in the output plane a wavelength de-multiplexer is formed.

19
Waveguide Array

Image or Output plane

Figure 1 : Layout of a waveguide grating Router

DWDM Optical Performance Parameters

Parameters specifying the optical performance of a DWDM are summarized in Table 1

Parameter Symbol Comments

Number of Input Ports N, Usually 1 Number of Output Ports No 8,16,32, etc Wavelength Grid ^1, Nominal Wavelength for all outputs

Passband Center Wavelength A-Cb^C2--" Actual wavelengths for all outputs

Passband Wavelength Offset AA,ci,AA,c2 AA.ci= A,ci- A-j

Channel Spacing CS 1 Xi\ or 1 Vi+i- vi|;

100GHZ~0.8nm

Free Spectral Range FSR Spectrum periodicity

Insertion Loss L Peak or within a band

Insertion Loss Uniformity AL Difference between maximum and

minimum across all output ports

Passband width B At a given insertion loss value

Passband Ripple (Flatness) RorF In a given wavelength window

Adjacent Channel crosstalk AX Window width? Relative to A.ci or A,i? Relative to peak loss or passband loss

Non-adjacent channel crosstalk NX Same as AX

Total Crosstalk TX Same as AX

Polarization Dependent Loss PDL Max variation of L over all polarization states Polarization Dependent Wavelength PDW,PDA, Max variation of A,ci for all

20
polarization states Return Loss RL -55 to -35 dB

Chromatic Dispersion CD Negligible for silica WGR

Polarization mode dispersion PMD Negligible for sUica WGR

Table 1 : DWDM Optical Performance Parameters

The definition of insertion loss (L) and crosstalk is sho\\'n in Figure 3. The vertical

dotted lines represent the wavelength grid (X,i,A,2, A,3....) which specifies the desired

nominal center wavelength of the passband of each port. There is an industrial standard

for the wavelength grid set by the ITU and is generally followed by the system vendors.

The grid points are usually equally spaced in fi-equency. The distance between the grid

points is the channel spacing (CS, e.g., lOOGHZ ~ 0.8nm). A wavelength window of

width (WB), which is 40 to 50% of the channel spacing (usually less) and centered at each wavelength grid (shown as a shadowed area in Figure 3), is used to measure the

crosstalk, insertion loss and other parameters.

The crosstalk parameter specifies how much power is received from adjacent channels. It

is generally described by a matrix [Xjj] where - Xjj (in dB) is the maximum power

transmission of port I within the wavelength window around the wavelength grid j relative to the peak transmission (Lpeak) or the passband transmission (Lband)-

Figure 2: Insertion Loss (L) and Crosstalk (X)

The crosstalk is defined as a positive number in dB. The adjacent channel crosstalk of

port i is the minimum of Xjj.i and Xij+i. The non-adjacent channel crosstalk is the

minimum of Xg with |i-j|>l. The absolute value of the sum of the power values of - Xg

21
with j not equal to i, is the total accumulated crosstalk (TX). From a system perspective,

it is meaningful to measure the crosstalk relative the ITU grid wavelengths since the

lasers are set to be operating at the ITU wavelengths. However, the measured results are dependent on the channel ojBfset (difference between the passband center and the nominal

center wavelength, A>.ci= ^a- h)- For a channel-ojffset-independent characterization of

the wavelength isolation, which is more meaningful to component designers, the

crosstalk can be measured relative to the passband center, A.ci.

The passband width (B) is the width of the filter at a specified level below the peak (as

shown in Figure 4). Passband widths at several insertion loss values (e.g., IdB, 2dB, 3dB,

30dB) are specified to quantify the passband shape. The 30dB width is a measure of the

steepness of the filter slopes and an indication for the crosstalk. Another parameter that

specifies the passband shape is the passband ripple (R) or the flatness (F), which is the

maximum insertion loss variation in the wavelength window.

The center wavelength of a given port (A-ci) is usually measured at the center of the

passband at -3dB transmission. The channel offset (AX,ci) of the port is the difference

between the center wavelength and the nominal wavelength (ITU grid wavelength).

The parameters described above can be measured using an unpolarized light source. The

polarization dependent loss (PDL) and wavelength (PDW, FDX) are the maximum

variation in insertion loss and center wavelength over all polarization states.

Wavelength

Figure 3: Spectral Plots showing the Passband Width (B), Ripple (R), Center Wavelength

(A-c), and Offset (AA-c), Polarization dependent Loss (PDL), and Polarization dependent

wavelength (PDW)

22
Chip-Level Testing

The inadequacy of the wafer level inspection to predict the optical conformance of a

device to customer specifications necessitates the introduction of a screening step before

the device is packaged and Final Tested. At this step, only critical parameters that

contribute to low yield and other parameters that will assist in the packaging of the device

need to be measured.

WGR devices are extremely sensitive to processing variations and the current

technologies cannot achieve with high yield the channel accuracy (<0.05nm) that is required by system vendors. Therefore, WGR devices generally have extra inputs and

outputs so that the channel wavelength can be tuned after wafer processing. The input

waveguide spacing at the interface of the input star coupler is different fi-om the output

waveguide spacing at the interface of the output star coupler. By moving to a different

input and correspondingly a different set of outputs the channel wavelength can be

coarsely tuned so as to match the wavelength-comb of the device to that of the ITU

wavelength grid. The proper input and output combinations are determined at this stage

of testing, and this information is later used for fiber attach.

Final Testing

The last area of testing before the product is shipped to the customer is called the "Final

Testing." The product is tested according to a customer specification to determine if it is in conformance. The conforming device yield performance fi"om this area is used to drive a change in the device design and to establish control limits for the optical parameters at Chip-level testing.

The channel wavelength of WGR devices are temperature sensitive (0.012nm/deg C), and

therefore have to be temperature stabilized. The devices are generally operated using a

heater/sensor combination, which requires that the operating temperature of the device be

above maximum ambient temperature that the device is going to see in operation. The

maximum operating temperature is generally dictated by reliability concerns. So, there is a temperature range of operation, which can be used to fine-tune the wavelength. The

operating temperature of the device is determined by minimizing the wavelength offsets

for aU the channels.

The next section discusses the two types of measurement schemes that are available to characterize the wavelength performance of DWDM filters.

Measurement Schemes

There are basically two ways to configure a test system for the characterization of a DWDM demultiplexer. The first configuration uses a broadband light source and an

optical spectrum analyzer, which acts as the wavelength selective receiver. The

polarization controller serves to characterize the polarization dependencies of the device

under test, such as center wavelength shift or the PDL. The channels are tested one at a

time and the optical switches allow the automation of the testing. The required

23
wavelength accuracy by system vendors (10 pm) has pushed the capability of the

spectrum analyzers to its limit. Any variation in pressure, temperature, or index of the

medium in which the spectrum analyzers operate can cause deviations in the

measurement of the wavelength that are unacceptable. Therefore, the spectrum analyzers

need to be calibrated around the wavelength window of operation before measurement of

each device, using a more accurate instrument consisting of a wavelength meter and a

tunable laser source. The wavelength of the tunable laser source is precisely measured by

the wavelength meter and then again by the OSA. The delta between both the values is recorded as a correction factor in a look-up table. This method of calibrating the OSA

requires expensive equipment and adds to the cost. Furthermore, to accurately

characterize the wavelength dependence of a DWDM device using the above test system

requires time- consuming measurements. For example, to characterize a device over a 30nm range with 0.1 nm resolution requires 300 measurements, which takes few hours of

testing time. To increase throughput, more testing stations are required - which is an

expensive proposition

An alternative setup that speeds up the measurement time uses a swept wavelength

selective tunable laser and a broadband power meter as the receiver. Each channel has its own dedicated power meter. With this approach in one sweep of the laser the spectral

data of aU the channels is available at once. For this system to be useful for DWDM

measurements the tunable laser has to have a wavelength accuracy of 5pm and a dynamic

range (or isolation) of better than 60 dB. The fine wavelength accuracy is achieved by

using a built-in wavelength meter that is able to read the wavelength of the laser source

accurately in a matter of milliseconds. Unlike the previous method which tests each

channel one at a time, this configuration allows all channels to be tested simultaneously.

Using this configuration the measurement time is cut down from a few hours to less than an hour per device.

Conclusion

DWDM components are described by a new and complex set of parameters. By carefiilly

selecting and combining the right set of instruments, both measurement accuracy and

throughput can be optimized. Because of its high throughput and reasonable accuracy, the

swept laser system configuration is replacing old test systems that use the OSA.

References

[1] Y.P.Li and C.H.Henry, "Silicon Optical Bench Waveguide Technology," Chapter 8 in

Optical Fiber Telecommunications IIIB, edited by I.P.Kaminow and T.L.Koch, 1997,

Academic Press.

[2] C. Dragone, "A NxN optical multiplexer using a planar arrangement of two star couplers," IEEE Photonics Technology Letters, vol. 3, pp 812-814 (1991)

[3] C. Dragone, "Frequency Routing Device having a wide and substantially flat passband," patent number 5412744.

24
Saturated Optogalvanic transition in Krypton at 1564 nm

David A Humphreys: National Physical Laboratory, Queens Road, Teddington, Middlesex, TWl 1 OLW.

Introduction

The rapid increase in demand for bandwidth in WDM systems brings a need for higher accuracy wavelength

reference standards. These can be realised through saturated transitions where the natural linewidth can be

measured by selecting atoms or molecules that have no velocity component along the axis of the optical beam.

However, to obtain saturation in a gas such as acetylene requires significant levels of optical power. Previous

workers have identified a number of optogalvanic excited-state transitions in noble gasses at wavelengths used for

optical fibre communications'. Saturation of these transitions has been demonstrated at wavelengths up to 1547.8 nm" '". In this paper we present the first measurements of a Krypton saturated optogalvanic transition in the L- band. As these are excited-state transitions they have a broader linewidth than the saturated molecular transitions but are more readily achievable at modest optical powers.

Saturated optogalvanic system

The saturated optogalvanic system is shown in Figure 1. A chopper in the return beam discriminates between the saturated and unsaturated signals. A lock-in amplifier (1) operating at the chopper frequency recovers the saturated

signal and this is passed to a second lock-in amplifier (2) to recover the error signal. Lock-in amplifier (3) operates at twice the dither frequency and monitors the quality of lock that can be achieved. The cascaded lock-in amplifiers ( 1 &

2) give excellent discrimination of the saturated signal, compared with previous results.

External cavity tuneable laser

Optical output

Ctiopper

20/80%

splitter

Optical

amplifier Optogalvanic cell andxlO amplifier

Dither and _ lock signals

Combiner

Control

signal

To lock-In amplifiers

Filter

Lock-in amplifier (1

1 X f1 (saturation signal)

Oscillator f2

-f1-

Mirror

-Demodulated return sig na l

Lock-in amplifier (2)

1 X f2 (saturated error signal)

Monitor output

Lock-in amplifier (3) 2 X f2 (Saturated signal

lock monitor)

Analogue Integrator DVM

Figure 1 Instrument layout for Saturated optogalvanic system

25
A commercial hollow cathode cell, filled with Krypton to a pressure of 130 Pa and operating at a discharge current of

about 100 |i,A was used for the measurements. Saturation has also been successfully obtained in Cells with a higher

pressure (600 Pa). A lens was used to collimate the beam, giving a diameter of approximately 2.4 mm, estimated from

the fibre NA and the lens focal length. The filter transmission is 64%, reducing the intensity of the returned signal to 40% of the unchopped pump power. The chopper was operated over the frequency range (1200 - 1650) Hz and the

external cavity laser was modulated at 123 Hz to give a peak to peak modulation of 127 MHz.

Results

Measurements of the power dependence of the saturation have been made for a range of pump powers (1 - 7.8) mW

in the collimated beam. The results obtained at an optical power level of 7.8 mW and with no dither signal are shown

in Figure 2. The full width of the saturated dip is (67 - 81) MHz measured at (6.9 - 7.8) mW. This compares with the Doppler broadened linewidth, (410-427) MHz measured at (1 - 2) mW.

15

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 Frequency, GHz

Figure 2 Saturated response at 7.8 mW showing markers at the 50% points of the saturated and Dopplerbroadened transition to indicate the accuracy improvement that can be realised

The integral of the optogalvanic trace was used to estimate the saturation. The saturation threshold has been defined

as the point where the integral of the measured optogalvanic signal, normalised to the optical power, has been

reduced by a factor of 1/V2. This occurs at 680 W/m^ , corresponding to an optical power of 5.7 mW, as shown in Figure 3.

26
1.5

0.5

100

Optical power density W/square metre 1*10

Figure 3 Power density dependence of optogalvanic signal

Measurements against a ^^CaHa unsaturated reference

The wavelength of the saturated lock signal was measured against acetylene reference lines using a wavemeter. The

absorption cell contained '^C2H2 at 1.07 kPa with an optical path length of 300 mm . An integrating lock loop was used

to stabilise the external-cavity laser to the maximum absorption. The measurement system is shown in Figure 4. Three

lines (R25, R26 and R27) were measured to quantify the contribution due to offsets in the laser lock.

Germanium

Photodiode

a

Corner-cube

mirror Photocell

amplifier

1 5 cm path length

Acetylene Cell

Optical Output Fndicator of lock

Output

Laser

source

Oscillator

ac modulation

Referencp Lock-in

amplifier No 2 (2f)

Signal

dc feedback

Integrator

Lock-in

amplifier No 1 (0

Reference

Signal

Output

Figure 4 Acetylene unsaturated locit system used as a local reference

Each of the reference lines was measured for a 60 minute period and the saturated Krypton reference was measured

for a 120 minute period. The results for the Krypton reference measured over the period are shown Figure 5. The mean value for the Krypton saturated transition is estimated as 1 563.97644 (14) nm at 95% confidence.

27
0.2

0.1

-0.1 -u

-0.2'0 . 20 40 60 80 100 120 Time minutes

Figure 5 Stability of the saturated lock over a 120 minute period.

Summary

Saturated optogalvanic standards offer the potential for higher accuracy, compared to unsaturated transitions, and

the benefit of modest optical power requirements (<10 mW). Measured results for the saturated transition in Krypton

at have been reported for the first time at 1563.97644 (14) nm, using '^C2H2 reference wavelengths. Stable locking in excess of two hours was achieved.

Acknowledgements

I wish to thank The UK department of Trade and Industry for providing financial support for this work and

Mr F Wonnacott of Anritsu UK for providing the wavemeter. References

' Y. C. Chung: 'Frequency locked 1 .3 and 1 .5 |im semiconductor lasers for lightwave systems applications',

J. Lightvawe Tech., vol. 8, p. 869. 1990.

" A. J. Lucero, Y. C. Chung, S. Reilly, and R. W. Tkach: 'Saturation measurements of excited-state transitions in noble

gases using the optogalvanic effect', Optics Lett., vol. 16, no. 1 1, pp. 849-852, 1991.

"' U. H. P. Fischer and C. v. Helmolt: 'Saturation and Isotopic Shift of the Kr84 Excited-State Transition at 1547.825 nm', IEEE Photonics Technology Letters, vol. 7, No. 1, January 1995, pp.65-67.

P

28
Characterization of Multimode Fiber for

10 Gbps Operation

(Invited)

John S. Abbott

Coming Inc. HP-ME

Coming, NY 14831

Introduction

The demand for ever-increasing data transmission rates is driving continuous development in the data communication marketplace. Sources, fiber, receivers, and connectors are all being re-engineered to provide low cost solutions to fill the growing need. At the 1998 Optical Fiber Measurement Symposium,

the technical developments for 1 Gigabit per sec (Gbps) Multimode Mode (MM) applications were

described [1]. This paper summarizes recent work extending the capability to 10 Gbps systems and the

fiber characteristics (and their measurement) which make this practical.

The Telecommunication Industry Association (TIA) supports industry's need to develop standard

approaches for meeting the requirements of high-speed applications. Technical considerations requiring

resolution to develop the MM 10 Gbps systems are being addressed at this time through the TIA FO-2.2.1

Task Group on Modal Dependence of Bandwidth [2], which earlier dealt with characterization of fibers and

lasers for 1 Gbps applications. The results reported herein draw extensively fi-om the work of this group.

Currently, 10 Gbps 850nm vertical cavity surface emitting lasers (VCSELs) are just reaching

commercialization and they have not been extensively characterized. However, there is a demand for optical fiber that can be installed today for 1 Gps applications and upgraded in the near future to use at 10 Gbps when such lasers are available. Although this paper focuses on the characterization of the fiber, the

characterization of the lasers will be of equal importance.

Light Propagation in MM Optical Fibers; Bandwidth and DMD Fundamentals

Multimode fiber information carrying capacity is characterized using a bandwidth length product (MHzkm), which is a key factor determining how far a system can operate at a given bit rate [3]. In simple terms

the bandwidth is measured by taking the fast Fourier transform (FFT) of an output pulse and comparing the amplitude versus fi-equency to the FFT of the input pulse. The ratio of the two FFTs represents the broadening occurring as the pulse propagates down the fiber, and the -3dB bandwidth used in system

models is defined as the fi-equency where the ratio has dropped to 0.5 of the zero fi-equency value. Methods

exist for measuring the bandwidth in the fi^equency domain without using an actual pulse [4], but this paper

will emphasize pulse-based methods which give a better intuitive understanding of the multimode light

propagation.

The pulse spreading as the pulse propagates dovm the muUimode fiber depends primarily on intermodal

dispersion - the relative delays of the individual modes - and the chromatic dispersion of the glass, which

broadens each mode depending on the wavelength of the laser and its spectral width. At 850nm the chromatic dispersion can be the limiting factor in how narrow a pulse can be propagated, if a broad spectral source like an LED is used.

In modem high bandwidth multimode fibers, the index profile is approximately parabolic, and there is strong coupling between individual modes that have the same propagation parameter p. The light

propagation can be understood as the propagation of a relatively small number of degenerate mode groups

travelling at different speeds. The absolute delay Xm of each individual mode is related to the propagation

parameter by = (1/c) d/dk , and the relative delay 6Tm of each mode compared to that in a 'perfect'

profile is given by 6Tm = (1/c) 1 6n(r) d/dk(k r dr, where 5n(r) is the index perturbation [5], [6]

29
and v|/n,^(r) is the solution to the scalar wave equation [7], [8]. The output pulse depends on the relative mode delays bx^ and the modal power P^. An example making this clear is given in Figure 1 below. The

relative mode group delays in Fig 1(a) are calculated using an index profile measured with the refractive near field (RNF) [4]. The modal power distribution (MPD) is given in Fig 1(b) for an "overfilled launch"

Mode delay Modal Power W eighted Modes

5 10 15 mode group

Figure 1 . Example showing how the output pulse depends on both mode delays and modal power distribution.

(OFL) somewhat typical of an LED and the standard bandwidth measurement before the Gigabit Ethernet

(GbE) work. In an OFL bandwidth measurement an equal amount of power is launched into each

individual mode; the shape of the MPD in Fig lb is determined by the increasing number of individual modes in the higher order mode groups and the increasing attenuation of the highest order mode groups.

Finally in Fig 1(c) the weighted mode group delays are plotted, along with the resulting output pulse if a small amount of chromatic dispersion is assumed (if no chromatic dispersion is assumed, the impulse

response would be a sum of delta fiinctions I ?„, 6(t - dx^) )• Agreement between the measured and

predicted pulse shape for a variety of index profiles is used to verify the profile data, the calculation of the mode delays, and the assumed MPD.

This example shows that the output pulse, and hence the bandwidth, depends on both the properties of the

fiber, which determines the mode delays, and the laser, which determines the MPD. Generally the 1 Gbps

and the expected 10 Gbps lasers put dramatically less power into the outer modes than the curve in Fig

1(b), so that even with the same mode delays as in Fig 1(a), the output pulse in Fig 1(c) can change

significantly.

The relative mode delays can be analyzed if the MPD is tightly controlled and only a small subset of the mode groups carry significant power. This is the basis for the differential mode delay (DMD) measurement

[4] used as a tool for process tuning. By scanning a diffraction-limited Gaussian spot across the core diameter and recording the centroid of the output pulse as a function of launch position, it is possible to estimate the mode delays. Accurate frequencybased methods for the DMD measurement have

been developed at NIST [9], and an example from one of the round robin cables from IGbE

development is given in figure 2. In this example,

the profile is approximately optimized for a wavelength between 850nm and 1300nm, so that at 1300nm the outer modes arrive later than the center modes while at 850nm the inner modes arrive later. If the profile were optimized for 1300nm, the DMD

curve at that wavelength would be flat and all mode

groups would arrive at the same time.

Offset (microns)

Figure 2 - Differential Mode Delay for TIA TG 2.2 Round Robih

Cable (Fiber 7)

When a pulse-based method is used for the DMD measurement, it is possible to obtain additional information about the mode delays and MPD and more completely characterize the fiber. Figure 3 is a contour plot of the DMD measurement pulses for a fiber manufactured in the late 1980's, with a mean

30
DMD shape somewhat similar to that in Figure 2. This example (which is not typical) shows a complex

pulse structure which is different at 850nm than 1300nm because the detailed modal structure is different.

Figure 4 gives an example of a 1300nm-optimized fiber with a DMD which is flat and with little change in the pulses as the launch position is shifted.

1300nm DMD Pulse shapes @0, 2, 4, 6, 9um offsets

Time nsec/km

Figure 4. DMD of fiber optimized for a particular wavelength

Bandwidth Requirements for 1- and 10 Gbps Systems; Encircled Flux & MPD

As mentioned above, the multimode bandwidth is a key factor in determining the bit rate a given length of

fiber can support. A detailed "link model" was developed and documented by engineers at Agilent and

used in 1 GbE development [10]. The model, though necessarily approximate, is extremely useful and

provides a systematic tool for evaluating the effect of different system parameters on the length which can

be supported. For 1 Gbps transmission at 850nm over 62.5 ^m fiber, a bandwidth of 385MHz-km will support a 500m length [11]. For 10 Gbps transmission at 850nm over 50 f^m fiber, the estimated

bandwidth for 300m is 1800-2200 MHz-km[12]. The exact value depends on assumptions in the link model. Lab tests have demonstrated transmission on longer lengths[13]. In these examples, the fiber, transmitters, and/or other system components presumably do better than one or more of the worst case assumptions used in the model. When measured with an actual laser, the bandwidth is customarily referred

to as

"

effective modal bandwidth" (EMB), to make it clear that the measurement depends on the particular

laser and modal power distribution.

31
The next step in characterizing the fiber for 1 Gbps or 10 Gbps operation is to determine when the fiber will meet the requirctl EMB with available lasers. In the course of the work of the TIA FO 2.2.1 Task

Group on Modal Bandwidth, there have thus far been two experiments of extensive round-robin testing of

Gigabit Ethernet fiber at various laboratories to establish the level of repeatability and the sensitivity of bandwidth

to launch conditions. NIST has been central to all the work. The first Round Robin is reviewed in reference

[1]. The second round robin, the so-called "Validation

Cable", was completed in late 1999 and the results summarized at the June 2000 TIA meeting in Quebec [2]. This round robin involved some 95 different fibers each

measured with 60+ different lasers at various labs. At

850nm, the standard link model [9] indicates that a bandwidth of 385MHz-km is needed to go 500m at 1 Gbps over 62.5 |^m fiber. The fibers included 1/3 expected to fail with some lasers, 1/3 expected to pass with most lasers, and 1/3 to be intermediate. The results established the use of an "encircled flux" criterion to characterize the launch power distribution of Gigabit lasers. Figure 5 indicates by shading how the "effective modal

bandwidth" (EMB) for many fibers shifts above or below

385MHz-km depending on the particular laser used and the resulting modal power distribution. In this diagram the columns correspond to different fibers and the rows to different lasers. This diagram has all lasers but only the 62.5|j.m fibers from one cable. The lasers are ranked by the encircled flux (rows at the

top have smaller spots) The larger spot lasers tend to have more cases where a fiber measured below

385MHz-km.

Figure 5. Validation Cable results. White cells have EMB BW>385MHz-km

The encircled flux EF(r) is defined as the fraction of the total intensity within a radius r. It was found

necessary to characterize the lasers by the fraction inside a small radius like 4.5|j.m and the fraction inside a

larger radius like 15|j,m. A transition in the bandwidth was seen between lasers with an encircled flux of

0.65 at 15(j,m and those with an encircled flux of 0.85.

Encircled Flux - 1Gb and 10Gb Sources

Figure 6 plots the encircled flux at 4.5|im versus the flux at 15|im radius for the lasers in the Validation

Experiment. The recommendation by FO-2.2.1 for enhanced IGbE defines spots with less than 75% of the

encircled flux inside of a 15fj.m radius as 'too large'

for IGbE, and spots with more than 25% of the

encircled flux inside of a 4.5|a.m radius as 'too small'. Some initial results presented by Picolight [14] for a 10Gbit laser are included with this data. For that particular laser the spot is much smaller than the

typical 1 Gbit lasers. Therefore, the fiber profile needs

to be better controlled near the center

E

If)

In general the requirements for both fiber and laser must be considerably tighter for lOGbE applications.

In order to achieve anything near a 2000MHz-km

effective modal bandwidth with a variety of 10 Gbps

lasers, the fiber index profile needs to be optimized

for 850nm. The fiber DMD must look something like figure 4a and there can be little change in the output

pulse as the DMD input is varied, similar to figure 4b.

-a

o

c

LU

Oo ° 0

0 a

°O 0

' ^ 00 0

O 0

0 V 0 0

0

0.1 0.15 0.2 0,25 0.3 Encircled Flux @4.5um

Figure 6. Encircled Flux from Validation Expt. Lasers (Oum

offset) and a 1 0 Gb/sec laser (0, 6, lOum ofifeet). In the enhanced IGbE recommendation made by TIA

FO 2.2.1, the fiber is characterized for 500m IGbE (385 MHz-km EMB) applications by measuring the

32
bandwidth with a special 23.5|im core MM fiber. The resulting MPD is similar enough to the IGbE

sources whose EF is near the 75% 15um threshold that the validation experiment showed this to

successftilly screen the fiber. The 'restricted mode launch' (RML) has a repeatable MPD analogous to the repeatable MPD generated by an overfilled launch used successfully for systems with LEDs.

The purpose of an RML bandwidth is to generate a relevant MPD. Until 10 Gbps lasers are better characterized, it is difficult to prescribe an RML launch, and there is not a consensus on even whether the same idea can be successfully applied, because the significant change in the MPD with slight offsets of a small spot laser launch. What can be done initially is to test the fiber with MPDs that appear plausible,

and verify that the bandwidth meets the 300m 10 Gbps requirement. One way to do this is to use the

pulses generated by the DMD offset launches, with the input pulse deconvolved, to construct output pulses

corresponding to different but plausible MPDs. Assuming that some 10 Gbps lasers have a spot analogous

to the DMD spot, for example, the DMD pulses themselves serve as one 'surrogate'. The 'bandwidth' at each offset position (out to a radius determined by the connector tolerance) needs to be above 2000MHzkm or whatever is dictated by the system requirements. The effect of other MPDs can be generated by

taking a weighted average of the DMD pulses. A. J. Ritger has shown [15] the OFL pulse and bandwidth

can be estimated by using a weighting of r", as long as the outer pulses are deweighted because of modal

attenuation (this is analogous to the curve in figure lb, if one substitutes (r/25|j.m)" for m/mmax )• Weightings consistent with a physically realizable laser would have a maximum weighting for the DMD

pulse at some offset position 'r' and taper down for offsets less than and greater than this position.

In the limit that every weighting must be considered, one arrives at the idea of a template [15] [16] , where

the earliest mode for any offset position x, 0 < x < Xn^x, and the latest mode for any offset position are

specified to lie within the 'bit rate window'. The maximum 8

offset considered (15|im in reference [15] and [16]) is determined based on connector tolerances and the laser spots | ^

being considered. Figure 7 [16] gives an example of the |

template. This is a sufficient condition for the fiber meeting 5 2

the 10 Gbps requirement, but is more stringent than

necessary. It overestimates how much power the modes

excited with 0 offset can have simultaneously with those

excited with 15 jam offset. It would be better to have a °

smaller template (like 5 to 10|im) and have it shift over the -o.e -0.4 -0.2 0.0 02

whole region. Then the "latest - earliest' modes are Time (ns oven km) calculated from 0 to 5|j.m, 1 to 6|am, 2 to 7|im, etc. (if the template is 5p,m long, for example) out to 10 to 15|am.

Current Opportunities and Limits

Figure 7. Example DMD mask from Oum to 1 Sum

from reference [16]

If we are be able to bracket the MPDs generated by actual 10 Gbps lasers, we may be able to devise an RML launch which can efficiently screen most of the fiber, so that the DMD can be used for sampling

rather than the direct measurement of each fiber. In the case of IGbE VCSEL sources, it was found that the nearfield and farfield distributions tended to stay in proportion, so that a specification on the encircled

flux (measured in the near field) also restricted the far field. In the case of lOGbE sources, this needs to be checked once again. The output pulses from lOGbps lasers need to be compared to the RML pulses or pseudo-pulses generated fi-om DMD data, and this is complicated by the VCSEL lasers being modulated

rather than pulsed sources. The temporal aspects of the VCSEL modulation may cause the effective spot

shape to shift during the modulation cycle, which could also complicate development of a surrogate RML

measurement.

However, the fundamental issues are relatively simple. To achieve 10 Gbps over 300m, a certain effective modal bandwidth is required. This bandwidth depends on the relative mode delays inherent in the fiber and

the modal power distribution due to the laser and its deployment. The mode delays must be controlled (by

controlling the index profile) so that the required EMB is reached for the variety of MPDs which occur in

practice.

33
Acknowledgments

The author expresses his appreciation to all of the individuals and organizations contributing to the work of

the TIA FO 2.2.1 task group and its work on 1Gbit and 10Gbit systems. The work of NIST, and John

Schlager in particular, should be noted. The contributions and insights of John Ritger, Paul Kolesar, and

their colleagues at Lucent are also especially appreciated. The ideas, results, comments, and enthusiastic support of colleagues at Coming are gratefully acknowledged.

References

[I] Hackert, M.J., 'Characterizing Multimode Fiber Bandwidth for Gigabit Ethernet Applications',

Symposium on Optical Fiber Measurements, Boulder, Sept. 1998, pp.1 13-1 18

[2] Hackert, M.J., 'TIA FO-2.2.1 June 12, 2000 Update', presentation at TIA plenary, Quebec, June 2000.

[3]Bandwidth model developed for IEEE P802.3z Specification for 802.3 Full Duplex Operation

Supplement to Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and

Physical Layer Specification for 1000 Mb/s Operation.

[41 Marcuse, P., Optical Fiber Measurements. New York: Academic Press, 1981. p.287

[5] Olshansky, R., 'Pulse broadening caused by deviation from the optimal index profile'. Applied Optics

15(1976) 2, p.782

[6] Abbott, J.S., 'Light Propagation in Gbit LANs', presentation at IMA workshop on Analysis and

Modeling of Optical Devices, Minneapolis,September 9-10, 1999.

[7] Snyder, A.W., and Love, J.D., Optical Waveguide Theory. New York: Chapman and Hall, 1983.

[8] Mammel, W.L., & Cohen, L.G., 'Numerical Prediction of fiber transmission characteristics from

arbitrary refractive-index profiles', Appl. Optics 21 (1982) 4 pp 699-703.

[9] Mechels, S.E., Schlager, J.B., and Franzen, D.L., 'High-Resolution Differential Mode Delay

Measurement in Optical Fibers using a Frequency-Domain Phase-Shift Technique,' Photonics Technology

Letters. 9 (1997) 6, pp. 794-796.

[10] Cunningham, D.C., and Lane, W.G., Gigabit Ethernet Networking. Indianapolis: Macmillan

Technical Publishing, 1999.

[II] Pondillo, P.L., 'TIA FO-2.2.1 Validation Analysis Results for Recommendation of Enhanced

Performance over 62. Sum Fiber' presentation at TIA FO-2.2.1 meeting. Boulder, December 1999.

[12] Kolesar, P., and Swanson, S. 'Next Generation Multimode Fiber Objectives', presentation for TIA

TR42.8 at TIA interim meeting Greensboro March 24, 1999.

[13] 'Multimode optical fiber transmits lOGbit/s over 2.8 km', Laser Focus World, 36 (June 2000) 6, p. 11. [14] Jewell, J, et al., '850nm Serial PMD Experimental Update', presentation at TIA meeting, Ottawa June

2000.

[15] Ritger, A.J., 'Modal Dispersion Measurements', presentation to TIA FO-2.2.1 meeting. Savannah,

January 2000.

[16] Golowich, S.E., et al.,'50um Multimode Serial 10Gb Simulations', presentation at TIA Plenary, Quebec, June 2000.

34
Short Pulse, Small Spot Size, Central Launch in Multimode Fibres:

Comparison between Measurement and Computation

M.J.N. van StralenS B.P. de Hon^ M. Bingle^ F.J. Achten^

^Plasma Optical Fibre B.V., P.O. Box 1136, 5602 BC Eindhoven,

The Netherlands

^Eindhoven University of Technology, Faculty of Electrical Engineering,

P.O. Box 513, 5600 MB Eindhoven, The Netherlands

Introduction

Until recently, most optical networks based upon transmission through multimode fibres

operate at bit rates of 100 Mb/s or less. Light emitting diodes (LED's) are often used because

of their low cost and the ease of connection. The spot size of LED's exceeds the core size of

the fibre, and hence the fibre is fully illuminated. For systems that require higher data rates,

such as the Gigabit Ethernet networks, commercially available LED's are too slow. Diode

lasers such as 850 nm vertical cavity surface emitting lasers (VCSEL's), can be used in these

systems and have smaller spot sizes. The resulting partial illumination of a multimode fibre

changes the performance as far as the bandwidth or pulse broadening [1] are concerned.

We have conducted experiments at 1300 nm to investigate the propagation of short-pulse

power signals, and their excitation by a source with a small spot size. In this paper, we present our first results concerning the comparison between the measured and computed results.

Experimental Set-up

The experimental set-up is depicted in Figure 1. A semiconductor laser is connected to a single-mode patch cord, while the detector is connected to a multimode patch cord with a 62.5

\im core diameter. The fibre under test is mounted between the two patch cords using fibre

coupling devices. A symmetric launch is obtained at the interface when the fibre axes are

aligned.

Laser _Q Q Detector

Singlemode Fibre Multimode Fibre Multimode Fibre

under test

Figure 1: Experimental set-up.

The laser is internally triggered at a fixed repetition rate. The centre wavelength, the

wavelength spacing and full width of the distribution of impulses at half amplitude were measured to be 1297.5 nm, 0.7345 nm and 8.84 nm, respectively, as shown in Figure 2a. The

35
temporal response of the laser was measured by using a short sample of the test fibre (1 m) in

the set-up. This pulse is depicted in Figure 2b. Its full width at half amplitude is about 70 ps.

'^centre "^1297.6 nm; AA,=8.84 nm; resolution=0.1nm

(a) (b)

Figure 2: Measured spectral characteristics (a) and temporal response (b) of the laser.

The detector is mounted inside an optical-to-electrical converter. The conversion from mW

light power to mV electric voltage is linear. The electrical pulse is sampled with a sampling

head. The trigger signal of the laser is used to display the pulse.

Numerical Multimode Fibre Model

As part of the collaboration between Plasma Optical Fibre and the Eindhoven University of

Technology, a full-wave multimode fibre model has been developed. In its present form, the

HE,n and EH,n modes are calculated. The propagation coefficients and modal field

distributions are calculated at multiple frequencies. To a good approximation the illumination

by the single-mode fibre has a Gaussian field distribution. In the model it is decomposed into

the modal constituents of the multimode fibre to obtain the modal amplitudes. The observed

pulse at the receiving end of the fibre is the time-averaged instantaneous power, calculated

with the aid of analytic signals.

The Sellmeier model is employed to determine the refractive index profile in the frequency

range of interest from the preform measurements at 632.8 nm. The Sellmeier coefficients used

are those for bulk glass [2]. Profile changes due to the drawing process have not been taken

into account. The modal attenuation coefficients due to material losses are assumed constant over the pertaining frequency band about 1300 nm. These attenuation factors were not known

a priori for the fibres under test.

Experimental and numerical results

Two multimode fibres have been investigated. Each fibre has a core diameter of 50 |im and a

length of 8.9 km. One fibre is optimised for an overfilled launch in the 850 nm window, while

the other fibre is optimised in the 1 300 nm window.

From preliminary computations using a single carrier frequency, the resulting pulses showed

virtually no pulse broadening. After inclusion of the ensemble of carrier frequencies according

36
to the measured distribution and spacing, pulse broadening increased significantly. These

results still showed insufficient pulse broadening. However, the calculated the zero-dispersion

wavelengths of most modes lie between 1300 nm and 1310 nm, while the measured zero- dispersion wavelength is about 1320 nm. This discrepancy of about 15 nm is probably due to the deviation from the assumed refractive index model. To compensate for this discrepancy

the cenfre wavelength of the laser was shifted downward by 1 5 nm in the computation. The

attenuation of the pulse has been accounted for through power normalisation. Figure 3 shows

the measured and computed output pulses in both fibres.

(a) (b)

Figure 3: Measured (solid) and computed (dashed) output pulses for the 850 run optimised

fibre (a) and the 1300 nm optimised fibre (b).

From a comparison between the measurements and the computed results, it appears that the

fundamental mode dominates the pulse propagation. The measured output pulse of the 850 nm

optimised fibre appear to also contain power of other mode groups on both sides of the pulse.

This is confirmed by moving the single-mode patch cord away from the central axis.

5

1275 1280 1285 1290

wavelength [nm]

Figure 4: Difference in group slowness for modes n=\ (HE,, solid; EH,, dashed) and n>\

(dash-dot) with respect to HE,, mode at the central wavelength. The modes near

the cladding are not included in the figure. (1300 nm optimised fibre)

37
From the numerical model, extra information about the pulse transmission can be extracted.

For example, the group slownesses (group delay per metre) of the HE,„ and EHi^ modes have

been plotted in Figure 4. Non-dispersive pulse propagation would indicate that all the group

slownesses are equal and constant. Similarly, the changes with frequency of the group

slownesses form a measure for intramodal dispersion, while differences between mode group

slownesses are a measure for intennodal dispersion.

Conclusions

In Gigabit Ethernet systems and other high data-rate networks, multimode fibres are underfilled launched since the fast lasers used have smaller spot sizes than ordinary LED's. The

overall bandwidth differs from the standardised, overfilled, launch bandwidth. In order to analyse the pulse broadening effects using under-filled launch conditions, a numerical method

for pulse transmission through multimode glass fibres has been developed and compared with

experimental results. In the rotationally symmetric experimental set-up, the pulse propagation

was shown to be dominated by the fundamental mode. Although the intramodal dispersion is small, its effect is considerably enhanced due to the spectral width of the laser, leading to significant pulse broadening.

The advantage of comparing experimental results with numerical simulation is that underlying

physics of wave propagation become clear. As an example, the group slowness (group delay

per metre) is discussed.

In conclusion, comparison between calculations and measurements pertaining to a rotationally

symmetric experimental set-up shows good agreement.

Future work

In the near future, off-set launch conditions will be experimentally studied. The numerical

model will be extended to include the other mode groups. The refractive index model will be

refined.

References

[1] L. Raddatz, I.H. White, D.G. Cunningham and M.C. Nowel, "Influence of Restricted

Mode Excitation on Bandwidth of Multimode Fiber Links", IEEE Photonics Technology

Letters, Vol. 10, No. 4, 1998, p. 534.

[2] W. Hermann and D.U. Wiechert, "Refractive Index of Doped and Undoped PCVD Bulk

Silica", Mat. Res. Bull., Vol. 24, 1989, p. 1083.

38
Measurement of the Non-Linear Index of Refraction, N2

Dan L. Philen

Lucent Technologies, Bell Laboratories, 2000 Northeast Expressway, Norcross, Ga. USA 30071

770-798-3910, fax 770-798-4655, philen@lucent.com

Introduction: Concern about non-linear effects in optical fibers has become more important

because of the advent of optical amplifiers, long system spans, and high power divided among

many closely spaced channels. This is also more apparent in trans-oceanic systems where

distances range fi^om 6,000 to 10,000 km and there is no regeneration of the signal, only optical

amplification, for the entire distance. Knowledge of the non-linear refi-active index is necessary for the initial system design of these advanced systems.

The refi*active index of a material is a function of the fi-equency and strength of the incident

electric field. For silica, it can be accurately represented by the first two terms in a power

series: a linear term, which is fimction of frequency (wavelength) only, and a non-linear term,

which is a fiinction of the electric field strength (intensity).

N(©, |Ep) = Ni(co) + N2 |Ep

The intensity dependence of the refractive index leads to the non-linear effects of Self-Phase

Modulation (SPM) and Cross-Phase Modulation (XPM). SPM leads to a broadening in

frequency of an optical pulse causing the frequency components of one signal channel to overlap those of another. The magnitude of SPM is obtained by noting that the phase of the

optical field changes by:

O = N(2 71 L) = (Ni + N2 |Ep) 2 7rL

X X

One can use the self-phase modulation phenomenon for measuring N2. Here, the phase shift at the peak of an optical pulse is measured as a function of peak power and fiber length. The

relationship between the SPM phase shift, the fiber properties and the incident light is: 50= 2 7rL N2 Pm

X Aeff (1)

Here, the dependence of N2 on the field strength has been replaced by the power density,

Pm/Aeff, which is the peak power divided by the effective area of the fiber.

Experimental Methods: There are basically two techniques involving the use of self-phase

modulation. One uses relatively low average power, but very short pulses, while the other uses

relatively high power continuous wave (CW) sources.

39
Short pulse SPM method:

This technique uses a Gaussian pulse of approximately 35-70 ps width at a very high repetition

rate. (See for instance, Ref 1,2) Various researchers have used pulse rates varying from 100

MHz to 2 Ghz. The pulse width seems to be critical in this measurement so as to avoid

electrostrictive effects. Electro strictive effects come into play when an initial pulse width is equal to or greater than about 1 ns. Generally, a pulse of less than 100 ps is deemed

acceptable. In addition, while the average power may be in the ten to hundreds of milliwatts

range, the pulse width needs to be very small in order to get reasonably high peak power to see the effect. Long fiber lengths - on the order of several kilometers - are also needed to observe

the effect clearly.

A typical experimental set-up can be described in the following figure.

Ffter Under Test EDFA

Mode Locked

PvlsedLD

Optical Aiieitttiior

Pulse Width

Measurement

Oscilloscope (Pulse Width Measurement)

Optical Spectrum Analyzer

Power Meter

A pulse of approximately 50 ps in duration is launched through the first EDFA and then a 2 nm

band pass filter. It is then again amplified through the final EDFA to the desired launch power.

The optical attenuator is used to vary the power launched into the test fiber. Initial and exit

pulse widths are measured in order to estimate the average pulse width within the fiber. The

phase shift is determined using the number of peaks viewed on the optical spectrum analyzer,

i.e., phase shift, Ospm, is [Number of peaks - V2]n.

A typical phase shift spectrum of 8.5 pi is shown in the following figure.

Self Phase M odulation - Pulse Technique

6

2

0

1650 1660 1661 1561 1652 1552 1563 Wavelength - nm

40
Once the desired phase shift is achieved, the power at the input of the test fiber is measured

using a power meter. The end of the input is cut and cleaved and measured using a large area

detector. The peak power is then calculated using the average power measured, the pulse

width, and the repetition rate. The N2 is then calculated using the SPM equation shown in eq.

(1) above, where L is the effective length, Pm is the peak power, Aeff is the effective area, and

k is the operating wavelength.

The ease of this measurement is the fact that SPM is measured directly. The phase shift is easily determined fi*om the display on an optical spectrum analyser (OSA) and then power

measured. One disadvantage of this technique is the extremely high power required to acquire an appropriate phase shift observable on the OSA and the overall sensitivity of the method to the dispersion characteristics of the test fiber.

The Continuous Wave (CW) Method:

The measurement of the phase shift due to SPM is done using the technique described in

Reference 3. The technique uses two tunable, external cavity lasers (ECL), that are tuned

closely together, to generate a beat signal fi*om the two optical fi*equencies. At high power

levels, SPM side lobes are generated and the side lobe phase relative to the original signal

phase is measured as a function of power.

The method is very simple, in that the lasers are operated CW, thus no intricate timing or short

pulse generation is needed. The two ECLs are combined through fiber couplers, and their

polarization aligned using fiber polarization rotators. The combined signals are then amplified

by a high power erbium amplifier and injected into the test fiber. The output fi"om the fiber

under test is monitored using an Optical Spectrum Analyzer (OSA). The experimental

arrangement is shown the following figure.

Nj/Agff M easurem ent

E C L 1 00 B

P o la riza tio n con tro lie rs

Power Meter

41
The method is also very versatile in that very long lengths of fiber are not required to observe

the SPM effect. Thus, short fiber lengths are used, allowing measurement of fibers with very

high dispersion, e.g., dispersion compensating fiber (DCF). The phase shifl: is calculated for

various input powers, and a plot of phase shift vs. optical power gives the value of Ni/Aeff.

Non-Linear Phase Shift vs. Input Power

5 0.10

c

= 0.08

OZ 0.06

0.00 0.10 0.20 0.30

Average Power in Watts

0.40

Discussion of the Methods:

The entire problem of measuring N2 with either method can be reduced to the problem of

managing the dispersion of the test fiber. Historically, the short pulse method was first used

because one could easily generate short pulses of a few 10s of ps, even if high powers were not

readily available. Thus, very long lengths of fiber were used, sometimes on the order of 10 to 20 km. In order to keep the total dispersion low in the test fiber, one had to operate near lambda zero or the pulse would broaden sufficiently in the fiber and reduce the observed effect.

In the short pulse method, as long as there are no appreciable dispersive effects, the phase shift

is easily seen on the OSA and a plot of phase shift vs. power gives the desired slope of N2/Aeff • However, if there are any dispersive effects, then sophisticated computer modeling needs to be

done to recreate the observed spectrum to determine the non-linear conditions. The recent

availability of high power erbium amplifiers has eased this problem considerably, and allowed

researchers to measure fibers far away firom lambda zero (such as in dispersion compensating

fibers), and use shorter lengths in order to manage the total dispersion.

42
In the CW method, reasonably high power tunable CW sources are readily available. Coupling

these with a high power erbium amplifier makes the set-up very straightforward. The

difference in the CW technique compared to the short pulse technique is that one does not

determine the phase shift directly, but measures the phase of the side lobes generated from the

beat signal.

The ratio of the side lobe intensity to the fimdamental (Iq/Ii) is related to the non- linear phase

shift, Ospm through Bessel fimctions of the nth order Jn*

Several efficient algorithms exist for performing the inversion of this fiinction that enables the

phase shift to be calculated fi"om the measured values of Iq/Ii.

To manage the dispersion in the CW method, the following condition should be satisfied:

2 71 C (AXo/Xf Ospm L D « 1

where L is the fiber length in km, and D is the fiber chromatic dispersion in ps/nm-km and

AA,o = (kh - Xa), or the separation between the two CW lasers. In practice one may check this

condition by measuring the ratio of the side lobe to the fiindamental while varying the

separation between the two lasers. When that intensity ratio is constant, one is inside the

dispersion envelope, i.e. when the separation between the two CW lasers becomes too large,

the ratio of the side lope to the fiindamental will "faU off' as the separation is increased.

One of the most important parameters to determine in either method is the measurement of the

optical power. While the CW method does not directly measure the phase shift, the short pulse

method does not directly measure the peak power. Both techniques measure the average

power, and an accurate and calibrated power meter is essential for both techniques. In the short

pulse method, the peak power is determined from the measured average power, the pulse

repetition rate, and the measured pulse width. Even with fast detectors available today, the

short (10s of ps) pulses require that the detector response (rise and fall times) be de-convolved

from the measured pulse width to get an accurate determination of the true pulse width in order

to calculate the peak power.

Another consideration in the CW method is the need to avoid stimulated Brillouin scattering at high power levels. This limits the about of power that can be injected into the fiber and can

easily be monitored using a 1 % coupler at the input.

Finally, one sees that it is the value of Nz/Aeff that is directly measured, and that if one wants

knowledge of N2 itself, one needs an independent measurement of the effective area, Aeff. The

effective area is usually determined from the far field scanning method or the variable aperture

method.

43
For completeness, it should be noted that the non-linear index has also been measured using

four wave mixing (4) and Cross Phase Modulation (5) but are not covered in detail in this

paper.

References:

1) "Measurement of the nonlinear index of silica-core and dispersion shifted fibers," K.S.

Kim, R. H. Stolen, W. A. Reed, K.W. Quoi, Optics Letters, Vol. 19 pp-257-259, Feb. 1994.

2) "Non Linear Index Measurement by SPM at 1.55 )Llm," R. H. Stolen, K.S. Kim, S. E.

Evangelides, G.T. Harvey, W. A. Reed, OFC '95, San Diego, CA, Feb 1995, paper FDl.

3) "Direct continuous-wave measurement of ni in various types of telecommunication fiber at 1.55 um", A. Boskovic, S. V. Chemikov, J.R. Taylor, L. Gruner-Nielsen, and O.A.

Levring, Optics Letters, Vol 21, No. 24, Dec. 15, 1996 pp. 1966-1968.

4) "Measurement of fiber nonlinear Kerr coefficient by four-wave mixing", L. Prigent and

IP. Hamide, IEEE Phot. Tech Lett, Vol. 5 pp 1092-1095, 1995

5) "Measurement of nonlinear refi^active index in optical fibers by cross phase modulation

method using depolarized pump light," Optics Letters, Vol 20, pp 988-990, May 1995

Acknowledgements:

I would like to acknowledge helpfiil discussions and guidance fi-om Alexander Robayo, of

Tyco Submarine Systems Labs, Jerry Cline of Coming Inc., Igal Brener of Tellium Co., and

Roger Stolen, of Virginia Tech.

44
Measurement Conditions of Nonlinear Refractive Index nj for Various

Single-Mode Fibers by CW Dual-Frequency Method

Kazuhide Nakajima, Takuya Omae and Masaharu Ohashi

NTT Access Network Service Systems Laboratories

Tokai, Ibaraki, 319-1193 Japan

Abstract

The measurement conditions of the cw dual-frequency method based on self- phase modulation (SPM) is investigated theoretically for single-mode fibers, dispersionshifted fibers, and dispersion compensating fibers by solving the nonlinear Schrodinger

equation. The effect of the chromatic dispersion on the estimation of nonlinear refractive

index 112 is clarified. It is also shown that suitable fiber length depends on the dynamic

range in the measurement system.

1. Introduction

Optical fiber nonlinearities can represent a serious drawback in dense WDM

systems with optical amplifiers. The nonlinear refractive index of the fiber is one of the very important parameters in designing such transmission systems. Until now, there

have been many reports on test methods of nonlinear refractive index Uj using nonlinear

effects such as self-phase modulation (SPM)[l]-[2], cross-phase modulation (XPM) [3]

and modulation instabiUty (MI)[4]. SPM method is based on the phase shift due to SPM

by using continuous wave (cw) [1] or short pulse [2]. In particular, cw dual-frequency

method based on self-phase modulation is simple in estimating Uj. However,

measurement conditions such as fiber length and input power have not been clarified.

In this paper, we investigate the measurement conditions of the cw dual-frequency

method for various single-mode fibers by solving the nonlinear Schrodinger equation.

Moreover, we clarify the effect of chromatic dispersion on the estimation of the nonlinear

refractive index n^.

2. Theoretical Background

When two intense optical signals at frequencies (O^ and o\ propagate in a singlemode fiber, self phase modulation (SPM) in the fiber acts on the beat envelope to create sidebands in the frequency domain. The ratio of intensities (Iq/Ii) of the fundamental

frequencies and the sidebands is related to the nonlinear phase shift, which allows the

nonlinear coefficients (n2/Agff) to be calculated. The ratio I,/!, is related to the nonlinear

phase shift (|)spM through Bessel functions as [1]

^1 -^1 (4^PM/2)+^'((t)spM/2)

The nonlinear phase shift is expressed as

(1)

2 X A =—^k,P, (2)

eff where 'h=(k^+'k^l2, P is the average input power, and L^^ is the effective length given by

45
(3)

where L is the fiber length and a is the optical loss at wavelength A,.

In this measurement principle, the effect of chromatic dispersion is neglected and

the following condition must be satisfied [1]: 2nc(A(xi^/(af^^p^LD«l, (4)

where Aa)o=(co3-(0b).

It is difficult to estimate the suitable measurement conditions from Eq. (4), in

particular, suitable fiber length, for various single-mode fibers.

3. Simulation Results

In order to clarify the measurement conditions for various kinds of single-mode

fibers, we solved the generalized nonlinear Schrodinger (NLS) equation describing a propagation of signals in the optical fiber using split-step Fourier method. [5] In simulations, measurement conditions for the conventional single-mode fibers

(SMF), dispersion-shifted fibers (DSF) and dispersion-compensating fibers were

investigated. Their fiber parameters used in the calculations are listed in Table 1 . The

channel spacing of two signals was chosen to be 0.3 nm. Table 1 Fiber parameters

Parameter SMF DSF DCF

Aeff(lim') 80 45 20

n2 X 10^° (mAV) 2.28 2.4 2.7

Chromatic dispersion (ps/km/nm)*'^'"^' 16 •Note 2 -80.0

Dispersion slope (ps/km/nm^) 0.057 0.071 -1.0 Optical loss (dB/km) 0.21 0.21 0.4

Notel: Wavelength is 1550 nm. Note 2: Chromatic dispersion ranges from 0 to lOps/km/nm.

Figure 1 shows the relationship between input power P and power ratio Iq/I, for the

DSF with a dispersion of 5 ps/km/nm. It is found that the power ratio I^/I, decreases as the input power increases. The phase shift due to SPM was calculated by solving the

equation (1), in which we substitute the power ratio Iq/I, as shown in Fig. 1. The relationship between the input power and the phase shift is shown in Fig. 2.

It is found that the phase shift due to a small power change increases as the input power P

increases for longer fibers. On the other hand, for shorters fiber, the phase shift is in

proportion to the input power P.

Figure 3 shows the relationship between the input power P and relative error of

the nonlinear refractive index Uj. It is seen that for the higher input power or lower input

power, the relative error increases. This is because the modulation instability occurs at higher power. It is also found that the relative error increases with increasing fiber length

L.

Next, we calculated the suitable fiber length to estimate the nonlinear refractive

index with a relative error of less than 5 % as shown by the solid curve in Fig.4. Hei-e,

46
we define the dynamic range as the power range, which means the power difference

between signals and sidebands. We can not estimate the nonlinear refractive index

accurately if the dynamic range of the measurement system is smaller than the power

ratio. The dynamic range is critical for the estimation of Uj. The relationship between

input power and fiber length is also shown in Fig. 4. It is seen that the required fiber

length depends on the dynamic range in the measurement system.

Figure 5 shows the relationship between input power P and fiber length for the

DSFs with various dispersion values. It is found that the required fiber length depends on

the dispersion value. As the dispersion increases, the required fiber length decreases. It is also seen that the appropriate fiber length with a dispersion of less than 10 ps/km/nm is from 1 to 2 km for the dynamic range of 40 dB.

Figure 6 shows the fiber length plotted as a function of input power P for the SMF

and DCF. Both fibers have large dispersion values compared with the DSF. It is seen from Fig. 6 that the measurable area of DCF is amaller that that of SMF.

As a result, the appropriate conditions for Uj estimation are summarized in Table 2.

Table 2 Measurement conditions of estimation for SMF, DSF and DCF

Fiber AA, (nm) Fiber length

(km)

Average input

power P (mW)

Dynamic range

(dB)

SMF 0.3 0.5- 1 20 40

DSF 0.3 1-2 20 40

DCF 0.3 0.1-0.15 20 40

4. Conclusions

The measurement conditions of the cw dual-frequency method based on SPM

have been investigated theoretically by solving the nonlinear Schrodinger equation using

split-step Fourier method. It has been clarified that the measurement conditions can be

determined from the fiber length, average power and dynamic range.

Acknowledgments The authors thank K. Tajima and K. Kurokawa for fhiitful discussion and T. Horiguchi for continuous encouragement.

References

[1] A. Boskovic, S. V. Chemikov, J. R. taylor, L. Gruner-Nielsen and O. A. Levring,

Opt. Lett., vol. 21, pp. 1966-1968, 1996.

[2] R. H. Stolen and Chinlon Lin,Phy. Rev., vol. 17, pp.1448-1453, 1978.

[3] A. Wada, T. O. Tsun, and R. Yamauchi, Proc. ECOC'93, paper MoBl .2, Berlin

1993.

[4] M. Artiglia, B. Ciaramella, and B. Sordo, Electron. Lett., vol. 31, pp. 1012-1013,

1995.

[5] G. P. Agrawal, Nonlinear Fiber Optics, T"^ Edition. New York: Academic Press,

1995.

47
40

30

20

DSF

D = 5ps/nm/km AX = 0.3nm

Ikm

• ....2km

4km

L = 6km

•a

100 Average input power (mW)

Fig. 1 Relationship between input power and

power ratio (Iq/Ii)

20 DSF

D = 5ps/nm/km = 0.3nni

L = 6km

/

/ /

/ /

I / 4km

2km.

1km

c

100 Average input power (mW)

Fig. 3 Relationship between input power and

relative error

3.5

3

2.5

2

1.5

1

0.5

0

DSF

D = 5ps/nm/km AX = 0.3nm

20 40 60 80 Average input power (mW)

Fig. 2 Relationship betv/een input

power and phase shift

0.1

40dB

JOdB

100 Average input power (mW)

Fig. 4 Relationship between input

power and fiber length

0.1

Ips/nm/km

5ps/nm/km

D= lOps/nm/km

DSF

AX, = 0.3nm

E

So

10 100 Average input power (mW)

Fig. 5 Meaurement conditions for DSF with

various dispersion values

0.1 r

0.01

smfvJ

'. D = 1 6ps/nm/km

DCF^ Relative error

f D = -80ps/nm/km of5%

Dynamic range 40dB

AA. = O.Bnm

100 Average input power (mW)

Fig. 6 Meaurement conditions for SMF and DCF

48
*Y. Namihira, SOFM2000(NL)*

KDD Nonlinear Coefficient Round Robin Measurements for

various Dispersion Shifted Fibers in Japan and UK

Yoshinori Namihira

KDD R&D Laboratories Inc. 2-1-15 Ohara, Kamifukuoka, Saitama 356-8502, Japan

Introduction: Optical nonlinearites limit the maximum transmitted power through optical fibers and

they have become a serious problem in ultra-long distance transmission systems using Er-doped fiber amplifiers (EDFAs) such as trans-oceanic optical fiber undersea cable systems. Therefore, accurate determination of the nonlinear coefficient {{nilKeff}', "2 is the nonlinear refi-active index, A^/y is the

effective area [1,2]} of optical fibers is required for the amplified optical transmission systems. To date, the nonlinear coefficient of the optical fibers have been measured by using the self-phase modulation

(SPM) method with a pulsed laser diode (LD) [3-8], and the SPM method with dual continuous wave (CW)-LDs [9-11], the cross-phase modulation (XPM) method [12,13] and the four-wave mixing (FWM)

method [14] at 1550nm.

Heretofore, intercomparison of {njlA-eff) measurements in various dispersion shifted fibers (DSFs) at 1550nm have been reported by COST-241[15], and KDD and Imperial college [16]. Recently, KDD round robin {nilA^ff) measurements in Japan and UK for various DSFs coordinated by

KDD R&D Labs. (Y. Namihira) have been successfully performed [17,18].

This paper presents KDD {nilAejj) round robin measurements in Japan and UK for DSFs, non-zero DSF (NZDSF) and large effective area DSF (LEDSF) using two kinds of SPM methods with pulsed-LD

[3-8] and dual CW-LDs [9-1 1], and XPM method [12,13] at 1550nm regions.

Experiments: The experimental set-ups of the (n2/Ae^) measurements for the various DSFs are shown in Fig.l. In Fig.l, (a), (b) and (c) are SPM method with a pulsed-LD, SPM method with dual CW-LDs, and XPM method, respectively. Here, n2 can be estimated by {njlAeg) multiplying the A^^. The Aj^was measured by the far-field scan (FFS) technique [19]. The five test samples prepared by KDD as shown in Table 1 were circulated to the five Laboratories ; KDD R&D Labs., Imperial College (UK), Fujikura, Furukawa, and Sumitomo. The DSFl, DSF2 and

LEDSF were fabricated by the VAD technique in Japan, and the DSF3 was fabricated by the MCVD

technique in USA, and the NZDSF was fabricated by the OVD technique in USA.

SPM method with a Pulsed-LD: In Fig. 1(a), as a pulsed-LD, mode locked-LD[3,4], gain

switched DFB-LD[5-6], and transform limited (TL) Gaussian pulse-LD[7,8] were used. The output optical pulse due to SPM was measured by the optical spectrum analyzer (OSA). As the

input optical power increases, the maximum phase shift increases in proportion to the input peak power. The (n2/A^j^) can be obtained fi-om the numbers of peaks in the SPM broadened spectra [1]. SPM method with dual CW-LDs : In Fig. 1(b), the optical beat signal was derived fi-om dual CWLDs operating at around 1550nm [9-11]. The beat signal was then amplified in a preamplifier (EDFAl)

and transmitted through a optical band pass filter (OBPF) to suppress the amplified stimulated emission

and a polarizer to a following high power erbium amplifier (EDFA2). The polarization controllers (PC) was used to ensure identical polarization and equal intensities of the two fi-equencies. The amplified beat

signal was then directed into fiber under test (FUT) via a variable optical attenuator (OATT), which was

used to control the input power. Both the launched power and the spectra were monitored with an

accurately calibrated optical power meter (0PM) and an OSA.

XPM method: In Fig. 1(c), the probe signal power is set relatively weak so that (rij/Aef^ in FUT is dominantly caused by amplified strong pump CW-LD through XPM and that the effect of SPM is negligible [12,13]. When pump CW-LD or CW-SLD [13] is modulated in its intensity, probe CW-LD is modulated in this phase through XPM [12,13]. Therefore, (nj/A^/;) can be determined by measuring

fi-equency components induced by phase modulation with the self-delayed heterodyne detection

49
Y. Namihira, SOFM2000(NL)*

technique while changing pump power [12,13].

Results and Discussions: The KDD nonlinear coefficient round robin measurements in Japan and

UK measurements at random polarization states (RP) using SPM method with a pulsed-LD, SPM

method with dual CW-LDs, and XPM method for DSFs, NZDSF and LEDSF at 1550nm are summarized in Table 1. Here, DL [ps/nm] is the product of chromaric dispersion D [ps/nm-km] and

fiber length L [km] [8] Also, n2(RP) =Tin2(LP), ti=8/9 for SPM[20,21], and the correction factor ti=2/3

for XPM[13] were used. LP represents linear polarization states. Fig.2 shows the one standard deviation (a) spread among the optical fiber types used in the KDD

round robin measurements. The offsets from average value (%) and nominal average values of {nilA^jj)

and n2 are shown in Tables 2 and 3, respectively. From Fig.2, Tables 1 and 2, it was found that measurement spreads of (n^/A^^^) values of conventional DSFs were less than +/- 8 %. On the other hand, the values of NZDSF and LEDSF were larger than that of the DSFs. It can be considered that DL

of DSF is less than 4.0 [ps/km], but that of NZDSF and LEDSF are greater than 7.0 [ps/km] and 1 1.0 [ps/km].

Fig. 3 indicates the one standard deviation (a) spread among the measurement methods (participants).

From Fig. 3 it was found that measurement spreads of (n2/Ae^) values of XPM method were less than +/- 7 %. Meanwhile, the values of SPM methods with a pulse-LD (P-SPM) and dual CW-LDs (CW-SPM)

were larger than that of the XPM method. It can be considered that there is no effect of chromatic

dispersion in fiber length (DL) in the XPM, but two SPM methods were found to be strongly affected by

the DL [8]. From Table 1, it is confirmed that DL of DSF is less than 3.6 [ps/km], but that of NZDSF

and LEDSF are greater than 6.5 [ps/km] and 10.5 [ps/km], respectively.

From Tables 2 and 3, it was found that the averaged values of (n2/Ag^) and n2 at RP of DSFs were

-10 -20 2 -4.71 X 10 [1/W] and -2.27 x 10 [m /W], respectively. Also, the averaged values of {nilK^fj) and n2

at RP ofNZDSF and LEDSF were -3.95 x

10"'° [1/W], -2.18 x10'^° [mW], and -2.75 x 10''°[1/W],

-20 2 -20 2 -2.05 X 10 [m /W], respectively. The averaged n2 values of -2.27 x 10 [m /W] of DSFs at RP is in

-20 2 good agreement with that of 2.1 - 2.3 x 10 [m /W] range of published results of the SPM method with

a pulsed-LD [5-8,16-18], and SPM method with dual CW-LDs [9-11,16-18] and FWM [14] method,

respectively.

Conclusions: From the KDD nonlinear coefficient round robin measurements in Japan and UK for various dispersion shifted fibers (DSFs), the measured values of (n2/A<,^) and n2 using two SPM methods

and one XPM method were found to be in good interlaboratory agreement within the limits of chromatic

dispersion in fiber length DL is less than - 4 [ps/nm]. It was confirmed that the averaged values of

-10 {njIKff) for DSFs, NZDSF, and LEDSF at random polarization state were - 4.7, - 4.0 and - 2.8 x 10 [1/W], respectively.

Acknowledgement: The author express special thanks to the following organizations; KDD R&D

Labs., Imperial College (UK), Fujikura, Furukawa and Sumitomo for testing in support of the KDD

Round Robin fWi/^eff* measurements in Japan and UK.

References:

[I] R. H. Stolen et al., Phys. Rev. A14, pp.1448-1453, 1948.

[2] Y. Namihira, Electron. Lett., vol.30, no.3, pp.262- 263, 1994.

[3] K. S. Kim et al., Optics Lett., vol.19, no. 14, pp.257-259, 1994.

[4] R. H. Stolen et al., JLT, vol.16, no.6, pp.1006-1012, 1998.

[5] Y. Namihira, Electron. Lett., vol.30, no.l4, p.l 171, 1994.

[6] H. Nakamura et al., lElCE, Tech. Rep. OCS95-81, p.29, 1995.

[7] Y. Namihira, OFMC'97, NPL, Teddington, UK, pp.50- 53, 1997.

[8] Y. Namihira, SOFM'98, NIST, Boulder, Co. USA, pp.83-86, 1998.

[9] A. Boskovic et al.. Optics Lett., vol.21, no.24, pp.988-990, 1996. [10]A. Nihonyanagi et al., lEICE, Nat.Conv.Rec.B-10-164,p.525, 1999.

[I I] T. Ajchareeyasoontorn et al.,IEICE, N.C. R , B-13-19,p.623, 2000.

[12] A. Wada et al., ECOC'92, Berlin, Germany, pp.45-48, 1992.

[13] T. Kato et al.. Optics Lett., vol. 20, no. 9, pp.988-990, 1995.

[14] L. Prigent et al., PTL, vol.5, no.9, pp.1092-1095, 1993.

50
*Y. Namihira, SOFM2000(NL)*

[15] A. Fellegara et al. Electron. Lett., vol.33, no. 13, p.l 168, 1997.

[16] Y. Namihira et al., OFMC'99, Nantes, France, p.66, 1999.

[17] Y. Namihira et al., OECC2000, 14C4-5, Makuhari Messe, Chiba, Japan, July, 2000.

[17] Y. Namihira, ECOC2000, Session 8.2.5, Munich, Germany, September, 2000.

[19] Y. Namihira et al., lEICE, Tech. Rep. OFT 99-4, 1999.

[20] S. G. Evangelides et al., JLT, vol., 10, no. 28, pp. 28-35, 1992.

[21] S. V. Chemikov et al.. Optics Lett., vol., 21, no. 19, p. 1559, 1996.

Pulsed-LD Mode Locked-LD

Gain Swilched-LD

Trajisform Limited Gaussian Pulse-LD

Er-Doped

Fiber Amplirier (EDFA)

OpHcal Fiber llndei Attenuator T„t (piin OBPF (OATT)

(a) SPM method with a pulsed-LD

Optical Power Meter (OPM)

Optical Pulsewidth Measurement

Optical Spectrum

Anal (oruyzer

Optical Pulsewidth Measurement

Polarization Controller rump Q H

CW-LDI

PC

Q H

CW-LD2

Er-Doped Optical Fiber BandPath

Amplifier Filler (EDFA) (OBPF)

Optical Fiber Under

Test(FUT) 0

(()) .. Optical Spectrum

Analyzer (OSA)

Optical Power Meter (OPM)

Q:X/4(7i/2), U:X/2{n)

(b) SPM method with dual CW-LDs

Depolarizer

Probe Q H CW-LD Of

(c) XPM method

Q : X/4 (It/2), H : X;2 (7t)

Optical Fiber Under BandPath

Test (FUT) Filter (OBPF)

Optical Power Meter (OPM)

Self-Delayed Heterodyne

Detection

Electrical Spectrum

Analyzer (ESA)

Fig.l Experimental set-ups for nonlinear coefficient (m/Aeff) measurements.

Table 1 Fiber parameters of KDD nonlinear coefficient round robin measurements

Fiber

L

Ikml

Aeff

lunfl (SilSSO nm

D

Ips/k m-nm|

@ 1 550 nm

|nm| So

Ips/km-nm^l DL

Ips/n ii4 @1550 nm

Measuremt

Methods

DSFl 2.46 47.10 -0.59 15582 0.072

-1.45 P-SPM CW-SPM

6.99 •4.47 XPM

DSF2

2.93 45.00 -124 1566.4 0.075

-3.63 P-SPM CW-SPM

6.98 -9.28 XPM

DSF3

327

52.10 -0.79 1560.6 0.075

-2.58 P-SPM CW-SPM

6.99 -5.52 XPM

NZDSF

2.92 54.60 -223 1578.1 0.079

-6.51 P-SPM CW-SPM

15.0 -33.9 XPM

LEDSF

2.95 73.67 -3.58 1576.1 0.133

-10.56 P-SPM CW-SPM

5.97 -24.84 XPM

51
*Y. Namihira, SOFM2000(NL)='

40

30 - <

20 -

10 -

0 «

> -10 -

o -20 -

-30 o -40 -

n2/Aef/(Vo) @Xsl550nm

<+/- 8%

• P^PMl O P-SPM2 CW-SPMl A CW-SPM2 CW-SPM3 + XPMl X XPM2

DSFl DSF2 DSF3 NZDSF LEDSF

Optical Fibers Fig.2 Measurement spread among the various optical fiber types of

offsets from the average values of (ni/Aeff)

Table 2 Offsets from average values (%) and nominal values of (m/Aeff)

Fiber

Offsets from Average ni/Aeff (%) -40-l»(l/W|

P-SPMl

11=1.0 P-SPM3

tl= 1.0 cwSPMl

11=8/9 SP%'2

tl=8/9

Ov- SPM3

r|=8/9 XPMl

11=2/3 XPM211=2/3 Average 0(%)

DSFl -2.40 8.79 -2.40 3.20 -1.98 -6.34 1.12 4.83 0.24 (4.9)

DSF2 -1.83 3.61 -6.89 -0.47 7.69 -6.69 4.58 5.14 0.29 (5.6

DSF3 -3.70 3.74 -1.78 3.26 1.82 0.86 -4.19 4.16 0.14 (3.2

NZDSF -4.09 3.00 -13.96 2.24 16.67 -4.34 0.47 3.95 0.37 (9.3

LEDSF -27.31 19.94 -22.59 7.94 8.67 5.76 7.58 2.75 0.49 (17.7

— • n I p , RF : Random Polarization State, LP : Linear Poiarii^tioo Stale Table 3 Offsets from average values (%) and nominal values of m

Fiber

Offsets from Average 02 ("/o) n2

IM0-20|m2/Wl

P-SPMl

n=i.o P-SPM21 = 1.0 cwSPMl

11=8/9 S?Sl"2

11=8/9

cw- SPM 3 1=8/9 XPMl

1=2/3 XPM21=2/3 Average O (%)

DSFl -2.50 8.87 -2.94 3.19 -0.75 -6.87 1.00 2.29 0.12 (5.0

DSF2 -0.61 3.24 -7.47 -0.61 8.81 -7.47 4.10 2.33 0.14 (6.0

DSF3 -3.80 3.53 -2.42 3.08 3.08 0.33 -3.80 2.18 0.07 (3.3

NZDSF -3.08 2.89 -14.57 1.97 18.04 -5.38 0.13 2.18 0.22 (9.9

LEDSF -26.24 19.68 -23.31 7.47 10.40 4.54 7.47 2.05 0.36 (17.6

'^RP~^*'^LP' : Random Polarization State, LP : Linear Polarization Slate ^ 40 ^ 30

:S 20 O

I

10

73

on2

« -10 <

1 -20 ^0 -

-40

n2/A^//(%) @A,=1550nm

i i

A <+/- 7%

0

A

A

DSFl 0DSF2 DSF3

A NZDSF A LEDSF

P-SPMl P-SPM2 CW-SPMl CW-SPM2 CW-SPM3 XPMl XPM2

Measurement Methods

Fig. 3 Measurement spread among the participants (Measurement

methods) of offsets from the average values of (ni/Aeff)

52
Kerr coefficient for chaotic light in optical fiber

S. M. Pietralunga, P. Martelli, M. Ferrario, and M. Martinelli*

CoreCom, Via Ampere 30, 20131 Milano, Italy

*Politecnico di Milano, Dipartimento di Elettronica e Informazione,

P.zza L. da Vinci, 32, 20133 Milano, Italy

Nonlinear effects are becoming a central issue in optical communications, relying on the

transmission of wideband amplified optical signals. In particular, the Kerr optical coefficient

«2 is the key parameter in a variety of fiber-optic nonlinear phenomena like soliton

transmission, self-phase modulation (SPM) and cross-phase modulation (XPM) in WDM

systems. In recent years, several detailed studies have been published, aimed at improving the

knowledge of the physics governing the Kerr effect in optical fibers The role of different

mechanisms, contributing to the effective n2, in dependence of the wavelength and modulation

bandwidth of the optical signal has been investigated. Namely, an ultra-fast electronic and a

fast Raman contributions have been isolated ^, and the effect of the electrostriction has been

evaluated The above quoted characterizations have always been performed using laser

beams, which can be satisfactorily described as coherent light. No quantitative evaluation of

the Kerr coefficient for light of chaotic statistics has ever been reported.

The aim of this work is to communicate the results of the direct measurement of «2 in optical

fiber for chaotic light at X = 1.55 |im and to detail the dependence of the Kerr response on

light statistics. It results a value of Ujp"^ = 4.1 -10'^^ cm^ /W for polarized chaotic light,

almost two times as large as the value commonly measured in the case of coherent beams.

It is well known that the dispersive and absorptive properties of the materials are related

respectively to real and imaginary parts of the complex optical susceptibility function. In

particular, the Kerr effect is connected to nonlinear photon absorption processes by nonlinear

X^'^^(w,-a),(jL)) susceptibility Since x^^^-related nonlinear absorption processes are ruled by

the second-order degree of coherence ^^^(0) of light ^, one can expect that the photon

statistics, which affects the ^'^'(0) value, also determine the Kerr coefficient «2. When

considering transformations between real and imaginary parts of x^^^ nonlinear susceptibility,

it is seen that nonlinear absorption contributions are linearly transformed into their dispersive

counterparts. In particular, if Two-Photon-Absorption (TPA) is made to vary, the same

scaling factor must be found in the corresponding electronic contribution to the optical Keneffect. In fact, since the TPA rates for coherent and chaotic light are different as a consequence of differences in ^^^^(0), corresponding variations on the amount of Kerr effect

should be measured.

The intensity dependent optical nonlinearity, which affects the propagation of an intense

single light beam in optical fiber through SPM, is generally expressed as the superposition of

three different contributions:

n2=nl+n2+n^\ (1)

53
representing respectively the ultrafast Kerr effect of electronic origin, the Raman contribution

and the electrostrictive effect The electronic contribution is the dispersive counterpart

of the TPA. In dependence on the nature of light, the single beam degree of second-order

coherence g{f(0) assumes different expressions. Since g{f(0) linearly affects the TPA rate^,

different values of second-order coherence imply different TPA absorption probabilities. In

the limit of negligible TPA, the values guloii(^) = ^ Snliia^^)-'^ assumed to be

valid along all the path of propagation. In turn, the electronic component of is expected to

be enhanced by the photon statistics contribution to TPA. When focusing on chaotic Hght, it can be demonstrated that the properties of second-order coherence are also affected by the

Degree Of Polarization (DOP) of the beam. In particular the value of gulhai^) decreases

from 2 to 3/2, as the DOP degenerates in passing from completely polarized to unpolarized

light^. As what concerns the other two parts of the Kerr coefficient, the Raman-related effect can be considered to be independent on g{i\0)^ and no electrostrictive contribution to the

nonlinear dephasing is expected for chaotic beams, no matter how intense, because of ultrafast

amplitude fluctuations characteristic of thermal light.

Therefore, the following relation between the nonlinear Kerr effect for respectively chaotic

and coherent light beams is expected to hold: „ -nf-l =r4\ +n^>4\ +4=4'"-\ . (2)

Moreover, independently on the nature of the light beam, the DOP of Ught directly affects

the overall SPM mechanism. In particular, if unpolarized light is used, the Kerr effect is weighted by a numerical coefficient 5/6, while a factor 8/9 must be accounted for in case of

polarized light, the state of polarization of which becomes randomized with propagation .

In order to measure the Kerr effect in chaotic light, SPM on a CW ASE (Amplified

Spontaneous Emission) chaotic beam has been evaluated. The ASE from the cascade of two

Erbium Doped Fiber Amplifiers (EDFA) has been used. The input spectrum is filtered by a bandpass filter of gaussian spectral shape and 1/e-bandwidth of 1.4 nm, centered at 1550 nm. The ASE emission is depolarized in itself and readily suitable for characterization of Kerr

effect in unpolarized light. A linear optical polarizer can be inserted in order to proceed with

characterizations using polarized light. The chaotic light, is then amplified by a third EDFA,

sent through a variable attenuator and launched into a 8.8 Km-long Dispersion Shifted fiber

coil of effective area Aeff=54 )a,m^, with zero -dispersion at 1550 nm. Since measurements are performed at zero dispersion wavelength, no dispersion-related phenomena are expected to

seriously affect the results. The effective broadening of the ASE spectrum is evaluated using an optical spectrum analyzer which resolves AA, = 0.05 nm. An example of directly measured spectral broadening is reported in Fig. 1 , where the original

gaussian-filtered spectrum is compared to nonlinearly broadened ones, respectively for

polarized and unpolarized light.

The experimental broadening of the ASE spectrum has been recorded at different input

powers and corresponding values of the 1/e bandwidth are reported in Fig. 2. In particular,

Figure 2(a) and Figure 2(b) refer respectively to unpolarized and polarized light.

Experimental resuUs have been analyzed within the theoretical frame elaborated by

Manassah which gives the formal mathematical description of nonlinear SPM occurring for

low-coherence optical pulses propagating in optical dispersionless fibers, characterized by a

54
nonlinear refraction index n2, without actually making any reference to the physical nature of

this parameter, or to the dependence of /?2 in itself on light statistics. Nor any quantitative

evaluation has ever been given up to now.

Nonlinear spectral broadening data are fitted by the Fourier transform of the following

correlation function:

K(T,L) = Al cxpi-icoJh / (3)

where Aj is the intensity of the propagating field, cOq is the spectral central wavelength,

f = exp(-r / 7^ ) is the input gaussian correlation function for a light of initial coherence

time Tc and the phase shift parameter a is related to fiber optic Kerr coefficient by

a = {a)ji2Al I c)L, where c is the speed of light.

The parameter «2 to be considered is the effective nonlinear coefficient, taking into account

the normalization factors 8/9 and 5/6 respectively for polarized and unpolarized light.

From fitting procedure, the values n^'J' =3.65 -IQ'^^ err? I W and n^'J'' = A.l -IQ-^^ cm^ IW

are deduced, respectively for unpolarized and polarized chaotic light. In Fig. 2 the

corresponding results are drawn as solid curves. This must be compared to the calculated

dependence of expected SPM-induced broadening on beam power, as obtained by substituting

the value n^'"' = 2.5^- cm^ I W for the Kerr coefficient m DS fibers, derived from

coherent hght measurements This corresponds to the dashed curves in Fig. 2, which

evidently do not match experimental data.

The role played by second-order degree of coherence in defining the results can be clarified

when the different contributions to the Kerr coefficient are isolated and discussed. By making

reference to Eq. (2), the expression for «2 can be detailed as: n^'''=n{+4=n2{g\\\Q) + li). (4)

In Eq. (4), ^2 =^2 '^if (^) is the electronic contribution, scaled by the dependence of twophoton absorption on second-order photon statistics, and = ^2 " is the Raman

contribution, where \i = 0.22 ^. Eq. (4) matches experimental results by assuming

g|f(0) = 3/2 and g[f(0) = 2 respectively for chaotic unpolarized and polarized light, and

^2(1 + = 2.59 • 10~'^cm^ / W which corresponds to the commonly estimated value of Kencoefficient in Dispersion-Shifted optical fibers

In conclusion, the fiber optic nonlinear Kerr effect for chaotic light has been experimentally

evaluated, using a CW ASE beam at X = 1.5 [im and its dependence on second-order photon

statistics has been demonstrated. In particular, only the electronic contribution to the Kencoefficient is enhanced by the increased second-order degree of coherence characteristic of

chaotic light. This however results in an enhancement of the overall Kerr coefficient of almost

a factor of two, when comparing measurements performed on coherent and polarized chaotic

light. Such an enhancement is certainly significant and we think that it might contribute to

refine the modeling of transmission in long-haul amplified optical fiber links.

REFERENCES

1. R. W. Boyd and E. L. Buckland, J. Nonl. Opt. Phys. Mat. 7, 105 (1998).

2. E. L. Buckland and R. W. Boyd, Opt. Lett. 21, 1117 (1996).

55
3. E.L. Buckland, Origin of the Third-order Nonlinear Optical Response in Silica Fibers - PhD

Thesis, (UMI Co. MI Ann Arbor, 1997)

4. Melloni, M. Frasca, A. Garavaglia, A. Tonini and M. Martinelli, Opt. Lett. 23 691 (1998).

5. S. Smolorz, F. Wise and N. F. Borrelli, Opt. Lett., 24 1 103-5 (1999)

6. D.C. Hutchings, M. Sheik-Bahae, D. J. Hagan, E. W. Van Stryland, Opt. and Quant. Electron.,

24, 1(1992)

7. P.N. Butcher and D. Cotter, The elements of Nonlinear Optics, (Cambridge University Press

1993), pp.174-175.

8. M. Weissbluth, Photon-Atom Interactions, (Academic Press, London, 1989) p. 392.

9. R. Loudon, The Quantum Theory of Light, 2"^^ Ed. (Oxford Univ. Press, Oxford, 1983).

10. J. T. Manassah, "Self-phase modulation of incoherent light revisited", Opt. Lett., 16, 1638

(1991).

IL K. S. Kim, R. H. Stolen, W. A. Reed and K.W. Quoi, Opt. Lett., 19, 257 (1994); A. Boskovic

S.V. Chernikov, J.R. Taylor, L. Gruner-Nielsen and O.A. Levring, Opt. Lett., 21,

1966(1996).

1

i '[ i^^N^M^ 1

\ i

1547.50 1550.50 1552.50 (nm)

Fig. 1. Measured SPM spectral broadening of chaotic light, (a): input spectrum;

(b): output spectrum in case of unpolarized ASE; (c): output spectrum in case of polarized ASE.

Fig. 2. Dependence of 1/e bandwidth for SPM-broadened spectra on input beam

power. Experimental results are represented by isolated dots. Solid line represent theoretical previsions from Eq.(3), where the Kerr coefficient is scaled with second-order degree of coherence; dashed line represent the

theoretical result in the absence of scaling, (a): unpolarized ASE light; (b):

polarized ASE light.

56
Estimation of the Polarization Coupling Length in Standard Telecom Fibers

from Measurements of Nonlinear Polarization Rotation

C. Vinegoni, M. Wegmuller, N. Gisin

Group ofApplied Physics, University ofGeneva,

20 Ecole-de-Medecine, CH-1211 Geneve 4, Switzerland

Abstract:

An analytical solution for the influence and strength of nonlinear polarization rotation (NPR) in an

optical fiber is presented. It agrees well with our measurements of NPR in a polarization maintaining

fiber, made possible by removing the much larger, fluctuating linear birefringence with a Faraday

mirror. The same technique is then employed to measure NPR in standardfibers, where its dependence

on the polarization mode coupling length can be exploited to get a direct estimate for this importantfiber

parameter.

INTRODUCTION

The potential of NPR to build ultrafast devices has been recognized a long time ago and received

considerable attention since then. It has been proposed to exploit it for optical switches, logic

gates, multiplexers, intensity discriminators, nonlinear filters, or pulse shapers. An inherent

problem to all this applications is the stability of the output state of polarization (SOP), generally

subjected to fluctuations of the linear birefringence caused by temperature changes and drafts in the fiber environment. We recently proposed a method [1] for removing the overall linear

birefringence, and therefore also its fluctuations, in a passive way by employing a Faraday mirror

(FM) [2] and a double pass of the fiber under test. It was also demonstrated that the nonlinear

birefringence, leading to NPR, remains unaffected, i.e. the NPR of the forward and backward

path add up. This allows to measure NPR both in polarization maintaining (PM) fibers and in standard fibers. However, the random variations of the intrinsic local birefringence axes in a standard fiber reduce the NPR. This reduction depends on the polarization mode coupling length

h, and can be intuitively understood by examining the extreme case of a fiber with a coupling

length close to zero. All orientations of the SOP with respect to the local birefringence axes will be encountered with equal probability, and as the sense of rotation of the NPR depends on

exactly this parameter [1], it averages out to zero. Consequently, it should be possible to derive

information about the coupling length from the amount ofNPR reduction.

The paper is organized as follows: First, the analytical model developed in [1], valid for PM

fibers, is reviewed and generalized for standard fibers. We then present results for NPR

measurements in a PM and 2 different standard fibers, and compare them with the simulations to obtain information about the coupling length h. THEORY

In a dielectric medium, an intense elliptical input pulse induces birefringence - via the optical

Kerr effect - due to the different amounts of intensity along the major and minor axis of the

57
polarization ellipse. It is well known that in isotropic media, this self-induced birefringence leads

to a rotation of the polarization ellipse while propagating in the medium. In an optical fiber

however, the situation becomes more complicated as there is also the local intrinsic birefringence

to be considered. Generally, the polarization ellipse changes are hard to predict in that case as the

linear and nonlinear birefringence interact in a complicated manner. Starting from the nonlinear

Schroedinger equation written in the form as given by Menyuk and assuming that the linear

birefringence is much larger than the nonlinear one, it is possible to show that the evolution of

the polarization vector \|/ is given by [1]:

(1) dz\\f ~ i(oB^ffOe\|/

where Oe accounts for the linear birefringence with axis 0. The amount of linear birefringence B

is replaced by an effective birefringence Beff, which includes the action of the nonlinear

birefringence. The solution for Eq.l is straightforward, \\f^ = exp(iQ)BgffOeZ)vj/o, and corresponds

to a rotation of the input polarization vector around the linear birefringence axis Oe. with a

rotation angle P given by

(2) p = coB^^ z = Q){B-^m, (0))z

where a=n2P/(3cAeff), ni is the nonlinear Kerr coefficient, P the power, and Aeff the effective

area. me(0) is the projection of the input SOP on the birefringent axis. In principle the NPR can now be measured by varying the input power P and observing the

corresponding change in the output SOP. However, the contribution of the nonlinear term is negligibly small compared to the linear one, and the slightest change in B will completely cover

the NPR. A way out of this problem is to make a double pass of the fiber under test by means of

a Faraday mirror. The linear birefringence accumulated during the forward path is thereby

automatically removed on the return path. The nonlinear birefringence on the other hand does

not cancel out but adds up [1]. This is because after reflection at the FM, which transforms the

SOP to its orthogonal counterpart, the sense of rotation of the NPR remains the same as during

the forward path.

As mentioned before, the situation is more complex in a standard fiber, and the NPR depends on

the polarization mode coupling length h. To calculate the NPR for this case, we therefore resort

to numerical simulations. The fiber is modeled as a concatenation of linearly birefringent trunks

with length h and random birefringence axis orientation. For each of these trunks, Eq.l still holds

as for typical standard fibers (beatlength of less than 100 m), the linear birefringence is still much larger than the nonlinear one. The SOP can therefore be calculated piece by piece, with the

projection me being different for each new trunk. The final SOP will obviously depend on the

choice of the (random) birefringence axis orientations, with variations being larger for large

coupling lengths h. We therefore made 100 runs for each specific coupling length to get a mean

value of the NPR.

EXPERIMENT

The experimental setup is shown in Fig.l. The light source consists of a distributed feedback

laser (DFB), operated in pulsed mode at a wavelength of 1559 nm. Typically, pulses with a duration of 30 ns, a repetition rate of 1 kHz, and a peak power of up to 6 W (after amplification

by an EDFA) are used. The light is then launched into the fiber under test (FUT) via a 90/10

58
coupler and a polarization controller (PCI). The coupler is inserted for the detection of the

backward traveling light after the double pass of the FUT, with its 90% output port connected to the source in order to maintain high launch powers. The polarization controller, PCI, allows to adjust the polarization of the light launched into the FUT, which is important for the strength of

the NPR as demonstrated by Eq.2. Note that for low launch powers (negligible NPR), the action

of PC 1 is removed by the Faraday mirror, and its setting is therefore of no importance in that case. The output SOP is examined by an analyzer consisting of a polarization controller PC2 and

a polarizing beam splitter (PBS). To achieve a good sensitivity of the analyzer, it is calibrated for equal power in the two PBS output arms for low power launch signals where no NPR occurs. The two PBS output channels were monitored by a fast photodiode (200 ps response time) and a sampling scope. The measurements were then performed in the following way: for a given

launch power, the polarization launched into the FUT was adjusted (PCI) to give the smallest

possible output power at the monitored PBS channel. Consequently, the difference between the

two PBS output channels is maximized, corresponding to a maximum value of the NPR.

RESULTS

First, a PM fiber with a length of 200 m was measured. The results are shown in Fig.2. One can

see that for this fiber NPR starts to be important for launch powers above 1 W, and that in spite

of the linear increase that would be experienced in the absence of NPR (straight line), the output

power actually starts to decrease for input powers above 2.5 W. The measured data (squares) are then compared to the calculated values (solid curve) using Eq. 1 , where me(0) was varied in order

to give a minimum output power from the PBS channel like in the experiment. As Fig.2 shows,

the agreement between measurement and model is good.

The same type of measurement was then performed for different standard fibers and compared to the corresponding simulations. The fiber lengths were typically 1 to 1.5 km, so that a large

number of couplings is obtained, allowing for comparison with the mean simulation values. An

example of the numerical results for a fiber length of 1.5 km is shown in Fig.3. One can see that the mean output power becomes indeed larger for smaller coupling lengths, indicating a reduced

NPR.

Fig.4 shows the results for 2 standard fibers with large and small PMD. Note that for ease of

comparison, the figure gives the strength of NPR, i.e. the reduction of the output power from the

value without NPR. As expected, the two standard fibers exhibit an NPR that is much lower than

for a PM fiber (note that the results for the PM fiber had to be rescaled to account for its different

length and ni/Aeff parameters). However, the amount of NPR for the two standard fibers is also

distinctively different. From comparison of the experimental results with the numerically

obtained data (lines), the coupling lengths can be estimated to about 75 m for the small PMD and

625 m for the large PMD fiber. A value of h<100 m is quite reasonable for a good (i.e. low

PMD) standard fiber. The coupling length of h~600 m for the high PMD 'standard' fiber is large,

but its high PMD value of 1.9ps/Vkm could indeed indicate that there might be well defined

birefringent axes in that fiber. Moreover, a different estimation for the coupling length of these

fibers from PMD and beat length measurements using Optical Frequency Domain Reflectometry

[3] is in good qualitative agreement with the results presented here.

59
CONCLUSIONS

A model and measurements of N^R in optical fibers were presented. As the amount of NPR

depends on the polarization mode coupling length, we were able to retrieve this important fiber

parameter by comparing the measured and numerical data.

Acknowledgements: Financial supportfrom the Swiss OFES in the frame of the COST 265 project is acknowledged

REFERENCES

[1] C. Vinegoni, M. Wegmuller, B. Huttoer, and N. Gisin "Measurement of nonlinear polarization rotation in a highly birefringent optical fiber using a Faraday mirror", /. Optics A, accepted for publication in 2000

[2] M. Martinelli, "A universal compensator for polarization changes induced by birefringence on a reti-acing beam", Opt. Comm. 72, 341-344 (1989).

[3] B.Huttner, J.Reecht, N.Gisin, R.Passy, and J. P. von der Weid, "Local birefringence measurements in single-mode fibers with coherent optical frequency-domain reflectometry", IEEE Photon. Technol. Lett., 10

10, 1458-1460, 1998

Figure 1: Experimental setup Figure 2: Minimum output power of PBS

channel 1 for a 200 m long PM fiber

Launched Peak Power (W)

Figure 3: Numerical results for a fiber length of Figure 4: Experimental and numerical results of NPR

1 .5 km with different polarization mode coupling strength in different fibers lengths h

60
A comparison of three techniques for effective area

measurement of single-mode optical fibres

David A Humphreys, Robert S Billington , Andrew Parker', Brian Walker, Dominic S

'Wells. National Physical Laboratory, Teddington, Middlesex. UK, TWll OLW, UK. Andrew G Hallam^ GN Nettest Ltd., York House, School Lane, Chandlers Ford. Hampshire S0534DG,

UK

Isabelle Bongrand Laboratoire de Physique de la Matiere Condensee (L.P.M.C), Universite de Nice, Faculte des sciences. Pare Valrose, 06108 NICE CEDEX 2, FRANCE

Introduction

Non-linear effects such as four-wave mixing (FWM), self-phase modulation (SPM) and stimulated Brillouin

scattering (SBS) are important considerations where intense electric fields are present in optical fibre. The

effective area (Aeff) is a crucial parameter in predicting the degree of non-linear behaviour as it can be used

to convert between aggregate optical power in the fibre and the intensity of the optical field.

Unlike mode-field diameter (MFD), effective area is defined only in terms of the near-field distribution of the

fibre. Any technique must therefore be capable of measuring this distribution, either directly or through the use of transforms from other measurements. We compared three different techniques for measuring Aeff that have previously been used to measure MFD. These were: the direct far-field (DFF) scan, variable aperture in the far-field (VAFF) and near-field (NF) scan. We also investigated the transverse offset method (TO) but no

results are presented owing to technical limitations of the measurement system.

Definition of Effective Area, Aeff

The effective area of an optical fibre with mode-field amplitude distribution is defined' as:

27C rdr In ll{r)rdr

jlE^rfrdr j /'{rydr

0 0 where I( r) is the intensity of the electric field of the fiindamental mode at radius r from the axis of the fibre.

Near-Field

Intensity,

E(r)'

Figure 1 Relationship between the techniques used to measure the effective area

' A Parker is now with Nortel Networks, Brixham Road, Paignton, Devon, UK TQ4 7BE (parkera@nortelnetworks.com).

' A Hallam is now with Halcyon Optical Services, Sandridge', Winchester Road, Waltham Chase, Hants UK S032 2LG

(ahallam@halcyonos.fsnet.co.uk).

61
Techniques for Measuring the Mode-Field Distribution

The inter-relationship between the methods used to measure Aeff is shown in Figure 1. Clearly the near-field scan method is the most direct since the effective area can be calculated directly from the measured data. The other techniques all require some degree of numerical processing, which can introduce errors. However,

once the practicalities of the measurements are taken into account, the most direct method, i.e. the near-field scan is not necessarily the best option.

Results

The effective areas of six fibres^ were measured near 1550 nm and 1310 nm. The results are shown in Tables

1, 2 and 3 for the far-field, near-field and variable aperture methods respectively.

Table 1 - Far-Field Scan Method

1549-nm 1309-nm

Aeff Std. Deviation. Aeff Std. Deviation.

AllWave® 80.94 0.23 66.39 0.13 TrueWave RS® 53.84 0.20 38.47 0.14 Matched Cladding 83.39 0.31 67.79 0.27 Dispersion Shifted 46.05 0.09 29.75 0.16 LEAFTM -Ve Dispersion 67.25 0.30 34.11 0.54 LEAFTM +Ve Dispersion 68.40 0.58 38.84 0.26

Table 2 - Near-Field Scan Method (NF)

1549-nm 1309-nm

Aeff Std. Deviation. Aeff Std. Deviation. (Hm^) (Hm^) (pm^)

AllWave® 80.63 0.23 66.18 0.13 TmeWave RS® 38.26 0.15 Matched Cladding 83.04 0.31 67.57 0.27 Dispersion Shifted 45.72 0.09 29.56 0.16 LEAFTM -Ve Dispersion 66.73 0.31 33.88 0.53 LEAFTM +'ve Dispersion 67.89 0.57 38.57 0.26

Table 3 -Variable Aperture in the Far-Field Method (VAFF)

1550-nm 1310-nm

Aeff Std. Deviation. Aeff Std. Deviation. (nm') (pm^) (pm^)

AllWave® 79.44 0.70 63.68 0.53 TrueWave RS® 54.3 2.0 37.72 0.23 Matched Cladding 81.94 0.80 65.41 0.34 Dispersion Shifted 45.8 1.0 29.52 0.57 LEAFTM -'ve Dispersion 67.38 0.72 34.16 0.31 LEAFTM +'ve Dispersion 68.1 1.3 39.00 0.60

The results from these three methods can be seen to be generally in good agreement. The notable exceptions to this are the VAFF results for the AllWave® and matched cladding fibres, which are

^ AllWave® and TrueWave RS® are registered marks of Lucent Technology and LEAFTM is a trademark of Corning Inc.

62
consistently lower than the values from the other two methods. The reason for this is the inversion of side- lobes in the far-field intensity distribution.

Not all fibre types exhibit side-lobes. However, where they appear, the amplitude of the far-field must be

given the correct sign before using the Hankel transform to calculate the near-field. Failure to do so can lead

to an underestimate of the effective area of typically (2 - 3) %. This is the origin of the discrepancies between

the VAFF results and the DFF and NF results for the attenuation flattened and matched cladding fibres. It can be seen from Figiu-e 3 that for VAFF measurements, the scarcity of points and the small differences in the signal level at high angles values make side-lobe identification difficult.

Angle (Degrees)

Figure 2 Example of a measured far-field intensity distribution including side-lobes.

Matched cladding fibre at 1551 nm.

Summary

Three techniques have been used to measure the effective area of single-mode optical fibre. The near-field scan method is the most direct but requires an imaging system, which can introduce uncertainty through

aberrations. The variable aperture method is quick and convenient but may not permit side-lobe inversion. The far-field scan method has significant advantages due to its ease of operation, repeatability and facility

to invert side-lobes.

63
Angle (Degrees) — Measured Far-Field at 1 549-nm

00 0 Measured VAFF at 1550-nm

Figure 3 DFF and VAFF measurements of the far-field distribution from an AllWave®

fibre.

Acknowledgements

I wish to thank The UK department of Trade and Industry for providing financial support for this work.

References

ITU COM 1 5-273-E "Definition and Test Methods for the Relevant Parameters of Single-Mode Fibres -

Appendix on Nonhnearities for G.650", (1996).

J

64
Model for calculation of Mode Field Diameter along the fibre

using bi-directional OTDR measurements

J. Mezger, G. v. Broekhoven, P. Pleunis, R. v. Laere

Plasma Optical Fibre BV

PO Box 1136, 5602 BC Eindhoven, The Netherlands

Tel.: (+31) 40 292 38 33

E-mail: h.mezger@plasma.nl

Abstract: A new modelfor the calculation ofthe Mode Field Diameter ofSingle-Modefibres from

OTDR measurements is presented. The model gives a high accuracy (s = 0.002 jum) and

good information about MFD uniformity along the fibre. The model is insensitive for MFD

variations ofthe reference fibre near the coupling to the fibre under test.

Introduction

The OTDR is becoming one of the most important measurement devices for qualifying Single-Mode

fibres. Especially when the OTDR is used in combination with an optical switch, two reference fibres

and two coupling devices as indicated in figure 1

Reference

Fibre Top

Coupling

Device Top

u Coupling

Device

Reference Bottom

Fibre Bottom

Bottom side

figure 1 , OTDR set-up

From the OTDR trace several important parameters can be derived, not only length, attenuation and

homogeneity but also parameters like Mode Field Diameter (MFD), Cut-off wavelength and even Chromatic Dispersion can be determined.

In the literature a number of contributions are presented, showing models for the calculation of the

MFD using bi-directional OTDR measurements [1, 2, 3, 4, 5, 6, 7, 8, 9]. Here the MFD at the end of

the reference fibres, sometimes indicated as dead-zone fibres, is used as reference.

In this contribution the model for calculation of the MFD along the fibre under test is adjusted in such

a way that the reference, needed for the calculation, is placed inside one of the reference fibres at a position several hundred meters from the coupling position. As a result the used reference value is no

longer dependent on re-cleaving of the reference fibre and the relatively inaccurate determination of

the splice loss at the coupling is no longer necessary, which determinations are used in the methods

described by ITU [8] and lEC [9]. The model calculations are directly based on differences in the OTDR signals.

The MFD of the reference fibre at the reference position is determined using the MFD model and a

large number of comparative measurements between the MFD determined by the Variable Aperture

65
Far Field method (VAFF) and the OTDR set-up for adjustment of the parameters used in the model.

These comparative measurements also guarantee a good agreement between both methods.

Using the model it appears that the repeatability of the calculated MFD at the ends of the fibres is about a factor of ten better then using commercially available VAFF measurement equipment,

respectively s = 0.002 \im and s = 0.02 |im.

The result of the described technique is a useful and accurate measurement method for the MFD of

Single-Mode fibres.

Theory

The MFD can be determined from the difference in bi-directional backscatter signal across a splice

with a reference fibre and the fibre under test [1, 2, 3, 4, 5, 6, 7, 8, 9]. The relation can be easily adjusted to make it applicable for any position in the fibre under test and

for using a reference position at a different position than the end of the reference fibre top:

g(A7--Afl)+/

MFD, = MFD^ • 10 20

where:

MFD^ : MFD at reference point (inside reference fibre top) [|im]

MFD, : MFD at point i [jim]

Ay- : difference in backscatter signal [dB] between the reference position and

the position at which the MFD is to be determined when measured from

T(op) to B(ottom)

A^ : difference in backscatter signal [dB] between the reference position and

the position at which the MFD is to be determined when measured from

B(ottom) to T(op)

g : adjustment factor (wavelength and fibre design dependent)

/ : adjustment factor (wavelength and fibre design dependent)

To reduce the influence of noise on the OTDR trace the OTDR signal at the reference position is obtained by averaging the OTDR signal over a distance of several hundred meter around the reference

position.

Set-up

The results are obtained using commercially available OTDR's which show a good linearity.

(According to manufacturer: < .04dB/dB.) The linearity can be checked by performing a bi-directional

OTDR measurement on a fibre with good uniformity. There should not be any deviations from

straight line visible larger than 0.01 dB which change with the measurement direction.

To avoid cross-talk between the two branches, blocking switches are added in the set-up. In this way

the isolation is increased to above 120 dB. The coupling devices used are selected for their stability.

Results

The figures 2 and 3 show the comparison of the MFD obtained by a VAFF set-up and MFD calculated

from OTDR measurements. Regular production measurements have been used for the comparison.

To avoid the influence of erroneous measurements, the difference between both methods should be

within the limits of repeatability of the VAFF method, ± 0.1 jum. These measurements are indicated in the graph with a solid dot. 66
ITU and EEC propose a validation check [8, 9]. In the graphs the following information is indicated: • the Bias (averaged difference between both methods),

• the Std. Dev. Bias (standard deviation of the differences: cr^ ), • the equivalence level or empirical accuracy (B ^\Bias\ + 2 ct^ /V« , with n being the sample size), • regression coefficient, r^. For both wavelengths a value of 0.005 \xm is obtained for B, which is much smaller than the upper

limit given by ITU and lEC, B < 0. 1 |Lim.

The correlation between both methods is good, as is indicated by the regression coefficients, > 0.9. The scatter in the graphs is mostly caused by the measurement error of the VAFF method

Figure 4 shows an example of the MFD determination along the fibre for a SM fibre using an OTDR

and the MFD model. The calculated MFD along the reference fibres is also indicated. It should be

noted that the MFD cannot be calculated close to the splice positions (indicated by the dotted lines) as a result of the dead-zone of the OTDR.

The noise on the MFD traces is a result of the noise on the OTDR traces. The undulation in the MFD trace is caused by small variations in fibre parameters which are well

within fibre specifications.

Mode Field Diameter

10.4

1"

Top Bottom

1550 nm

1310 nm

-5 0 5 10 15 20 25 30 35 40 45 50

Fibre Position [km]

Figure 4

67
Conclusions

The presented model for the calculation of the MFD from a bi-directional OTDR measurement gives an accurate prediction of the local MFD.

An excellent relationship between the model and the VAFF measurement for both wavelengths is obtained.

The repeatability of the MFD at the ends of a fibre determined with the model is about a factor of ten

better then using commercially available VAFM measurement equipment.

References

1 . D. Marcuse; Loss Analysis of Single-Mode Fiber Splices, Bell System Technical Journal, MayJune (1977).

2. P. Divita, V. Rossi; Backscattering Measurements in Optical Fibers; Separation of Power Decay

From Imperfections Contributions, Electronic Letters 15. P.467, (1979).

3. M.S. O'SuUivan, R.S. Lowe; Interpretation of SM Fiber OTDR Signatures. Proceedings SPIE'86

Optical Testing and Metrology (1986).

4. K.W. Kowaliuk, J. Ferner; A Technique to Estimate the Cutoff Wavelength Profile in SingleMode Fibers Using a Switchable Dual Wavelength OTDR, Proceedings NIST '88

Symposium on Optical Fiber Measurements (1988).

5. J. Warder, C. Saravanos; Mode Field Diameter and Cutoff Wavelength Profile Measurement

using Dual Wavelength OTDR, Proceedings NIST '92 Symposium on Optical Fiber

Measurements (1992).

6. W.T. Anderson, Consistency of Measurement Methods for the Mode Field Radius in a SingleMode Fiber, Journal of Lightwave Technology, Vol. LT-2 No. 2, April 1984.

7. M.J. Hackert, G.W. Loughran, Development of mode field diameter and cut-off Wavelength

measurements by optical Reflectometry, Proceedings Optical Fibre Measurements Conference

(1995).

8. ITU G650 - Definition and test methods for the relevant parameters of Single-Mode fibers. Clause 2.1.4. Third alternative test method for MFD. (Draft revised version, agreed April 2000)

9. lEC test method 60793-1-45, method D. (Committee draft under vote: 86A/533/CDV).

68
A Novel Far-Field Scanning Technique for Rapid Measurement

of Optical Fiber Parameters

Jeffrey L. Guttman, Razvan Chirita, and Carmen D. Palsan

PHOTON, inc., 6860 Santa Teresa Blvd., San Jose, CA 95119

phone:408-226-l OOG, fax: 408-226-1025, e-mail: jguttman@photon-inc.com

Abstract: A new far-field scanning technique allows for accurate measurement of Mode-Field Diameter

and Effective Area of single-mode fibers in less than 20 seconds with greater than 64 dB dynamic range. The Numerical Aperture is measured in real-time. An instrument based on this technique is described,

and measurement results are presented.

1. Introduction

A new technique for rapidly scanning the far-field of optical fibers is described. The far-field data is used for computing parameters of single-mode and multi-mode fibers in accordance with the

Telecommunication Industry Association/Electronic Industries Association (TIA/EIA) Standard Direct

Far-Field (DFF) Methods. The Mode-Field Diameter and Effecfive Area of single-mode fibers is obtained in less than 20 seconds with greater than 70 dB dynamic range. The Numerical Aperture of

multi-mode fibers can be obtained in real-time with up to 24 dB dynamic range. The speed of the measurement makes it possible to test greater numbers of fibers economically.

It also provides the ability to acquire statistical data, and potentially eliminate variations in measured

parameters due to source fluctuations. Finally, the technique lends itself to the design of compact

instruments suitable for portable field use.

2. Measurement Technique

An instrumentation system was constructed based on the measurement technique. The principle of

operation of the system [1] is as follows: The end of the optical fiber under test is positioned at the axis

of rotation of an optical fiber collector. The plane of rotation of the optical fiber collector is the far-field measurement plane. The collected light then propagates to a stationary InGaAs detector. The detector

signal is amplified by a transimpedance amplifier followed by a voltage amplifier with a total programmable gain range of 140 dB. The amplified signal is input to an automated PC based data

acquisition system. An optical encoder on the rotating collector and a phase-lock circuit provide motion

control and precise angular sampling.

The distance between the end of the fiber under test and the optical collector is 6.62cm. A 500

|im diameter pinhole at the entrance to the optical collector yields a 0.43° detector field-of-view at the source. The data is acquired with an angular sampling resolution of 0.055°. The scan rate is 10 Hz, so

single far-field scans are obtained in 50 ms, with updates obtained every 100ms. The system operates

under control of MS Windows-based software. The scan unit dimensions are 16.5 cm x 16.5 cm x 20.3

cm (6.5 in. x 6.5 in. x 8 in.). The far-field data is obtained by acquiring multiple high speed scans at different incremental gain

settings. The data from these multiple scans are then reassembled to provide the far-field pattern in less than 20 seconds. This method makes it possible to obtain far-field profiles with dynamic range

comparable to that obtained using conventional lock-in amplifier techniques, but at much higher speed.

In the present system the gain ranging method allows far-field profiles to be obtained with dynamic range

69
of 64 dB for a single-mode fiber source operating at a power level of ImW. For a 1 Watt fiber source, the obtainable dynamic range is 94 dB.

During data acquisition, to obtain adequate signal-to-noise ratio (SNR) at each gain setting, an

appropriate number of averages are used. When the individual scan data is reassembled to obtain the

final profile, an algorithm is used for detection of amplifier saturation and rejection of invalid data.

3. Measurement Results

Measurements were made using a Fabry-Perot laser diode source operating at a nominal wavelength of

1300 nm and output power of 2 mW. A typical far field profile obtained for a sample single-mode fiber

is shown in Figure 1 . The data shows the complete scan range from -90° to +90° with signal amplitude

ranging over 9 decades. Two sets of cusps are clearly defined, and two more cusps can be seen where the

signal approaches the background noise level at the 0.1 amplitude level.

-100 -80 -60 -40 -20 0 20 40 60 80 100 Degrees

Fig. 1 . Far-field profile of a single-mode optical fiber obtained with the new instrument. The Mode-Field Diameter (MFD) is calculated using the Petermann n Integral in accordance with the

TIA/EIA Standard DFF Method given in FOTP-191 [2], with slight modification. In particular, the data is translated in angle by the centroid of the measured distribution. Also, the data is not folded. Instead, the

integral is calculated for both negative and positive theta angles. The Effective Area, (A^ff) is calculated

according to the TIA/EIA Standard DFF Method of FOTP-132 [3], using a threshold algorithm to eliminate noise from the computation and to more accurately determine the locations of the cusps, necessary for sign reversal of the data. Comparative measurements of 2 different fibers were obtained to assess the accuracy of the

technique. These fibers were measured independently by different laboratories using their far-field goniometric measurement systems, and then using the Photon inc. system. The far-field profiles obtained

from these measurements are shown in Figure 2 and Figure 3. The numerical values for MFD are given

in Table 1 . The agreement is excellent.

Table 1. MlFD values for the Comparative Measurements with Lab #1 and Lab #2

Lab#l Lab#2 Photon Inc. A

Fiber #1 9.099 |im 9.092)im 0.007|im

Fiber #2 9.515)am 9.490 0.015|im

70
FIBER #1

-10 10 ANGLE (Degrees)

30 50

LAB #1 PHOTON

Figure 2. Comparative far-field measurements: ST single-mode fiber measured at Lab #1 and at Photon, inc.

FIBER #2

-10 10 ANGLE (Degrees)

• LAB #2

PHOTON

50

Figure 3. Comparative far-field measurements: bare single-mode fiber measured at Lab #2 and at Photon, inc.

Measurements were performed to assess the "push-button" repeatability of the instrument. Table

2 summarizes the results for 3 different series of measurements of MFD, and Table 3 summarizes the

corresponding results for A^ff. The results for series "A", with 250, measurements, had a 3o repeatability

of 0.004 fim for MFD and 0.463 fj,m^ for A«ff. Similarly, the series "B" measurement had a 3(7

repeatability of 0.0054 fim for MFD and 0.485 fim^ for Aem and for series 'C the values are 0.0095 \im

for MFD and 0.5 12 fj,m' for A^ff.

Table 2. Mode-Field Diameter "Push-Button" Repeatability for 3 Series of Measurements

Series (# of

Measurements)

MODE-FIELD DL\METER (^m)

MIN MAX MEAN 3 X STANDARD DEVL\TION

A (250) 9.3886 9.3954 9.3920 0.0040

B (250) 9.3860 9.3953 9.3901 0.0054

C (1000) 9.4367 9.4588 9.4463 0.0095

71
Table3. Effective Area "Push-Button" Repeatability for the 3 Series of Measurements of Table 2

Series (# oi

Measurements)

EFFECTIVE AREA (fam^)

MIN MAX MEAN 3 X STANDARD DEVIATION

A (250) 69.603 70.099 69.770 0.463

B (250) 69.444 70.035 69.727 0.4846

C(IOOO) 69.736 71.384 71.013 0.512

The results show the 3a push-button repeatability of the instrument is better than 0.01 |Lim for MFD and approximately 0.5 for Aeff. These values are of the same order of uncertainty due alone to problems associated with the computation algorithms for MFD and Aefr- Some of the variation in parameter values is also attributed to fluctuations in the source amplitude and wavelength during the

measurement, as evidenced by the broader range of MFD and Aeff values for the longer series "C"

measurements. Finally based on numerous series of measurements, the overall accuracy of the

instrument is conservatively specified at ±0.5% for MFD and approximately ±1% for Aeff. The measurement of Numerical Aperture (NA) is measured in accord with the TIA/EIA Standard

using the Far-Field Method described in FOTP-177 [4]. Specifically, the NA is obtained from the

inverse sine of the 5% intensity half angle. Angular width measurements are obtained with a standard

deviation of approximately 0.05°, which gives a 3a variation of approximately ±0.001 in the NA value

for typical single-mode optical fibers, which is better than ±1%.

4. Summary

A new technique has been developed for rapid measurement of the far-field profile of optical fibers. It provides far-field profile data with dynamic range comparable to that obtained using conventional lock-in

amplifier techniques, but at a fraction of the time. The far-field data is used for computing parameters of

single-mode and multi-mode fibers in accordance with the TIA/EIA Standard DFF Methods. The DFF

method for MFD is referred to as "the reference method" [2]. The Mode-Field Diameter and Effective

Area of single-mode fibers is obtained in less than 20 seconds with greater than 64 dB dynamic range for a OdBm source. The NA of multi-mode fibers can be obtained in real-time with up to 24 dB dynamic

range. An instrument based on the new technique has measurement accuracy specified at ±0.5% for MFD and approximately ±1% for Agff. The accuracy for NA measurement is better than ±1%. The speed

of the measurement makes it possible to test greater numbers of fibers economically. It also provides the

ability to acquire statistical data, and potentially eliminate variations in measured parameters due to source fluctuations. Finally, the compact size of the scan unit makes it suitable for portable field use.

> 5. References

1. Patent # 5,949,534

2. "Measurement of Mode-Field Diameter of Single-Mode Optical Fiber", Fiberoptic Test Procedure FOTP-191,

Telecommunications Industry Association, Standards and Technology Department, 2500 Wilson Blvd., Suite 300, Arlington,

VA, 22201 (1998).

3. . "Measurement of the Effective Area of Single-Mode Optical Fiber", Fiberoptic Test Procedure FOTP-132,

Telecommunications Industry Association, Standards and Technology Department 2500 Wilson Blvd., Suite 300, Arlington, VA,

22201 (1998).

4. "Numerical Aperture Measurement of Graded-Index Optical Fibers", Fiberoptic Test Procedure FOTP-177,

Telecommunications Industry Association, 2001 Pennsylvania Ave. N.W., Washington, D.C. 20006 (1992).

72
The Status of International Fiber Measurement Standards

W. B. Gardner

Rapporteur for Q. 15/15, ITU-T

2000 NE Expressway

Norcross, GA, 30071, USA

A. H. Cherin

Convener, Working Group 1, lEC SC86A

2000 NE Expressway

Norcross, GA 30071, USA

Abstract Fiber test methods which have been approved by the international standards

bodies ITU-T and lEC are summarized. Current directions of the standardization efforts

in these two organizations are described, including the recent restructuring of the fiber

standards documents.

Work in the International Telecommunication Union (ITU-T) Working Party 4 of Study

Group 15, and the International Electrotechnical Commission (lEC) Working Group 1 of

SC86A over the past two years has culminated in significant milestones being reached at the April 2000 meetings of these organizations:

• The rrU-T "determined" key fiber Recommendations, including G.650, which

contains the single-mode fiber test methods. G.650 has been submitted to the World

Telecommunication Standardization Assembly for decision in October 2000.

• The lEC measurement standards have been restructured and modernized to reflect

current industry capabilities and system requirements.

Multiple test methods have been adopted over the past decade for most of the

conventional transmission, geometrical, and mechanical parameters of single-mode

fibers. Recent standards-writing efforts in both organizations have been directed mostly

toward polarization mode dispersion (PMD) and such nonlinear attributes as effective area (Agff) and the nonlinear coefficient (n2/Aeff) of single-mode fibers.

Table I lists all of the test methods contained in ITU-T's Recommendation G.650, with

the Reference Test Method indicated in each case. The latter is the test method selected

for resolving any disagreements that may arise in testing.

73
Table I Single-mode fiber test methods in ITU-T Recommendation G.650

5.1 Test methods for the mode field diameter

5.1.1 Reference test method: The far-field scan

5. 1 .2 First alternative test method: The variable aperture technique

5.1.3 Second alternative test method: The near-field scan

5.1.4 Third alternative test method: Bidirectional backscatter difference

5.2 Test methods for the cladding diameter, core concentricity error and cladding

non-circularity

5.2. 1 Reference test method: The near-field image technique

5.2.2 First alternative test method: The refracted near-field technique

5.2.3 Second alternative test method: The side-view technique

5.2.4 Third alternative test method: The transmitted near field technique

5 .3 Test methods for the cut-off vi^avelength

5.3. 1 Reference test method for the cut-off wavelength (X^) of the primary coated

fibre and reference test method for the cut-off wavelength (Xcj) of jumper

cables: The transmitted power technique

5.3.2 Alternative test method for Xc'. The split-mandrel technique

5.3.3 Reference test method for the cut-off wavelength (A,cc) of the cabled fibre: The transmitted power technique

5.3.4 Alternative test method for the cut-off wavelength (^cc) of the cabled fibre

5.4 Test methods for the attenuation

5.4.1 Reference test method: The cut-back technique

5.4.2 First alternative test method: The backscattering technique

5.4.3 Second alternative test method: The insertion loss technique

5.5 Test methods for the chromatic dispersion

5.5. 1 Reference test method: The phase-shift technique

5.5.2 First alternative test method: The interferometric technique

5.5.3 Second alternative test method: The pulse delay technique

5.6 Test methods for prooftesting

5.6. 1 Reference test method: Longitudinal tension

5.7 Test methods for polarization mode dispersion

5.7.1 Reference test method: The Stokes parameter evaluation technique

5.7.2 First alternative test method: State of polarization method

5.7.3 Second alternative test method: Interferometric technique

5.7.4 Third alternative test method: The fixed analyzer technique

74
The Reference Test Method for PMD in Table I is a consoUdation of the Jones Matrix

Eigenanalysis and Poincare Sphere methods; the combination of the two has been given

the name "Stokes parameter evaluation technique". The fixed analyzer technique is sometimes referred to as the wavelength scanning technique.

Recommendation G.650 also contains two test methods in (non-normative) Appendices:

Appendix IV Test method for measuring chromatic dispersion uniformity based on the

backscattering technique

Appendix V Test methods for effective area (Aeff)

The attributes which these techniques measure are not yet specified in any of the ITU~T

Recommendations. In the absence of any requirement to measure these attributes, their

test methods appear in Appendices for information only.

The ITU-T has drafted both pulsed and cw test methods for na/Aeff , but these methods

have so far been withheld from the Recommendations pending completion of the round

robin evaluation of the methods.

As shown in Table II, the new organization of fiber measurement standards in the lEC is by fiber attributes. Continuous liaison between ITU-T and lEC is used to maintain

harmonization between the test methods of the two bodies, including the choice of

Reference Test Methods.

Current work in lEC SC86A WGl includes the development of a technical report

describing the measurement of a fiber's nonlinear coefficient, the revision of lEC

60793-1-41 (multimode fiber bandwidth measurement) to include a launching condition

needed for fibers used in high bit-rate applications, and the conversion of the polarization

mode dispersion technical report to a standard test (EC 60793-1-48).

Both the ITU-T and lEC have recently restructured their fiber standards documents as

well. These new specification structures make it easier to incorporate new fiber design

variations that evolve when the performance of each fiber type is optimized for a variety

of applications. Examples of this are new non-zero dispersion-shifted fibers for

multichannel transmission, and an extended wavelength range low water peak fiber.

Procedures for obtaining standards documents from these organizations can be found on

their respective websites, www.itu.int and www.iec.ch.

75
1 able II (Organization of Fiber Measurement Standards in lEC 60793-1

Attribute lEC Document Number Measurement methocl(s) included

Fiber geometry 60793-1-20

Refracted near-field Transverse interference Grey-scale, near-field Mechanical diameter

Coating geometry 60793-1-21 Coating geometry

Mechanical

Length 60793-1-22 Delay measuring

Fibre elongation

Backscattering

Rber proof test 60793-1-30

Constant stress Constant longitudinal strain Constant bending strain

Tensile strength 60793-1-31 Short length Long length

Strippability 60793-1-32 Mechanical

Stress corrosion

susceptibility

60793-1-33

Dynamic fatigue, axial tension Dynamic fatigue, 2-point bending

Static fatigue, axial tension

Static fatigue, 2-point bending

Static fatigue, uniform bending

Fiber curl 60793-1-34 (Under consideration)

Attenuation 60793-1-40

Cut-back

Insertion loss Backscattering

Spectral attenuation modeUing

Bandwidth 60793-1-41 Impulse response Frequency response

Chromatic Dispersion 60793-1-42

Phase-shift

Spectral group delay, time domain

Interferometric

Numerical aperture 60793-1-43 Far-field light distribution

Cut-off Wavelength 60793-1-44 Fibre & Cable cut-off

Mode field diameter 60793-1-45

Direct far-field scanning

Variable aperture in the far field OTDR

Near-field scan

Monitoring 60793-1-46 Transmitted power

Backscattering

Optical continuity

Macrobending

sensitivity

f.fYJQ'X 1^7 1 -*t/ Power monitoring Cut-back

Polarization Mode

Dispersion 60793-1-48 Conversion of Technical Report is Under Consideration

Damp heat exposure Dry heat Change of temperature Water immersion

60793-1-50

60793-1-51 60793-1-52

60793-1-53

Damp heat exposure Dry heat Change of temperature Water immersion

Nuclear Radiation 60793-1-54 Nuclear Radiation |

76
Measurement Issues in PMD Compensators

Dipak Chowdhury

Corning Incorporated

Coming, NY 1483 1, Phone: (607) 974-3593, Fax: (607) 974-3405 e-mail: dipak@corning.com

With increasing bit-rate, polarization mode dispersion (PMD) is becoming a major system

impairment [1-8]. PDM is defined as the statistical ensemble average of differential group delay (DGD)

between the two principal states of polarization (PSP) [9-11]. The fact that PMD is a statistical parameter

of the system, depending sensitively on environmental condition and deployment [11-13], mitigation of

PMD induced impairment is not trivial. With increasing need of higher bit-rate (40 Gb/s and higher) and

longer reach (1000-3200 Km) PMD is becoming an even larger issue than it was perceived to be a few

years ago. Consequently a lot of effort is being given to mitigate PMD induced impairment in recent years

[14-20]. Instead of addressing various methods of mitigating PMD induced impairments, in this paper we

discuss some of the critical issues related to the real-time measurement of PMD and performance of PMD

compensators (PMDC).

System Polarization

Controller

1

Control

Svstem

Impairment ^* Detector

Receiver

Figure 1: Basic block diagram of a PMDC. The thick lines are optical fiber. Thin lines are electronic signal

path. The content inside the gray box is the PMDC.

The basic block diagram of a PMDC is shown in Fig. 1 . Various implementations of each of the blocks within the PMDC are proposed and are still being worked on. However, for our discussion we will concentrate on issues related to two aspects of PMDC related measurements: (1) measurement of the impairment detector and; (2) measurement of the performance of the PMDC as a whole.

Two basic class of impairment detectors (ID) are proposed: (1) optical and; (2) electronic. Optical IDs depend on some type of dense spectral measurement of the states of polarization (SOP) over the signal

spectrum. Whereas, the electronic measurement of PMD relies on a few spectral samples at specific

fi"equencies over the signal spectra capturing the change in electrical spectrum induced by DGD. In

addition, the ID requirements for mitigating the 1^ and higher order PMD are significantly different. In the

presentation we will discuss and compare various aspects of these two ID methods.

Two types of performance evaluation of PMDC are possible: (1) In the laboratory environment

with controlled hardware emulating actual system induced DGD impairment, and; (2) In the field where a PMD impaired link is used to measure the performance of a PMDC [21]. Key issue for a laboratory

environment performance test is the design of the emulator, which mimics the system induced PMD

impairment. In this case the induced DGD is easily available by various types of measurement that are commercially available. Also, in the laboratory environment, it is easy to induce controlled but random

fluctuations in DGD at any desired speed to test the response time of a given PMDC. Whereas the experimental setup for a field evaluation of the DGD has to be such that the impact of a given PMDC can

be evaluated before and after the introduction of a PMDC. Also, in the field environment controlled

variation in DGD with given speed is not trivial. Above all, availability of PMD impaired link for a given

bit-rate might be difficult. In the presentation we will discuss these issues in detail with examples.

77
[I] C. D. Pool and J. Nagel, "Polarization effect in lightwave systems," m Optical Fiber

Telecommunications IIIA, I. P. Kaminow and T. L. Koch eds. (Academic Press, New York, 1997).

[2] P. R. Morkel, V. Syngal, D. J. Butler, and R. Newman, "PMD-induced BER penalties in optically

amplified IM/DD lightwave systems," Electron. Lett. 30, 806-807 (1994).

[3] P. Ciprut et al. "Second-order polarization mode dispersion: impact on analog and digital

transmissions," ZE^^ J. ofLightwave Technol. 16, 151-111 (1998).

[4] H. Bullow, "System outage probability due to first- and second-order PMD," IEEE Photonocs

Tecnol. Lett. 10, 696-698 (1998).

[5] M. Karlsson, "Polarization mode dispersion-induced pulse broadening in optical fibers," Opt. Lett. 23, 688-690(1998).

[6] C. D. Poole and C. R. Giles, "Polarization-dependent pulse compression and broadening due to polarization dispersion in dispersion-shifted fiber," Opt. Lett. 13, 155-157 (1988).

[7] C. Vassallo, "PMD pulse deformation," Electron. Lett. 31, 1 597- 1 598 ( 1 995).

[8] F. Bruyere, "Impact of first- and second-order PMD in optical digital transmission systems," Opt.

Fiber Technol 2, 269-280 (1996).

[9] N. Gisin, R. Passy, and J. P. Von der Weid, "Definition and measurement of polarization mode

dispersion: interferometric versus fixed analyzer methods," LEEE Photon. Technol. Lett. 6, 730-732

(1994).

[10] C. D. Poole and R. E. Wagner, "Phenomenological approach to polarization dispersion in long

single-mode fibers," Electron. Lett. 22, 1029-1030 (1986).

[II] G. J. Foschini and C. D. Poole, "Statistical theory of polarization dispersion in single mode fibers,"

IEEE J. of Lightwave Technol. 9, 1439-1456 (1991).

[12] C. D. Poole, "Measurement of polarization-mode dispersion in single-mode fibers with random

mode coupling," Optics Lett. 14, 523-524 (1989).

[13] F. Curti, B. Daino, G. De Marchis, and D. Matera, ''Statistical treatment ofthe evolution ofprincipal

states of polarization in single-mode fibers," IEEE J. ofLightwave Technol. 8, 1 162—1 165 (1990).

[14] D. Mahgerefteh and C. R. menyuk, "Effect of first-order PMD compensation on the statistics of

pulse broadaning in a fiber with randomly varying birefi"ingence," IEEE Photonocs Tecnol. Lett. 11,

340-342 (1999).

[15] R. Noe et. al., "Polarization mode dispersion compensation at 10, 20, and 40 Gb/s with various

optical equalizers," J. ofLightwave Technol. 17, 1602-1616 (1999).

[16] H Rosenfeldt et. al. "First order PMD-compensation in a 10 Gbit/s NRZ field experiment using

polarimetric feedback-signal," ECOC'99.

[17] S. Hinz et. al. "Distributed fiberoptic PMD compensation of a 60 ps differential group delay at 40

Gbit/s," ECOC'99.

[18] A. Meccozzi et. al., "A simple compensator for high order polarization mode dispersion effect," WL2-1 OFC'2000.

[19] H. Billow, "PMD mitigation techniques and their effectiveness in installed fiber," THl-1

OFC'2000.

[20] T. Takahashi et. al. "Automatic compensation technique for timewise fluctuating polarization mode

dispersion in in-line amplifier systems," Electronics Letter 30, 38-349 (1994).

[21] D. A. Watley et. al. "Field evaluation of an optical PMD compensator using an installed 10 Gbit/s

system," ThB6-2 OFC'2000.

78
A Poincare Sphere Method for Measuring Polarization-Mode Dispersion

Using Four-Wave Mixing (FWM) in Single-Model Optical Fiber

Shuxian Song

Lightwave System, CIENA Corporation, ssonH@ciena.com

Kenneth Demarest, Christopher Allen

ITTC, Department of Electrical Engineering and Computer Science, University of Kansas

Abstract: A nonlinear Poincare sphere method for measuring polarization-mode dispersion (PMD) using the four-wave mixing (FWM) effect in single-mode optical fibers is presented. This method is based on the FWM-power

transfer function on the polarization states of the input signals. Average PMD can be obtained by using this method

with measurements in just a narrow optical bandwidth. Key words: Polarization Mode Dispersion, PMD measurement, Poincare-Sphere Method, Four-wave mixing

(FWM)

I. Introduction

Polarization-mode dispersion (PMD) is one of the major limiting factors of ultrahigh-bit-rate

optical fiber communication systems. Currently, PMD is a big concern when upgrading legacy

networks of installed fiber to 10 Gb/s (OC-192) rates and higher. For example, at 10 Gb/s (OC192), the limit on PMD is about 10 ps [1]. Knowledge of the PMD in the fiber plant is becoming

very critical for system designs, evaluations and installations. Measuring PMD for installed fiber still is not an easy task. The time-domain methods are degraded by polarization state fluctuations, caused by polarization mode coupling in the fiber [2]. On the other hand, the frequency-domain method is limited by the motion-less condition. Any

motion of the measurement apparatus, especially at the ends the fibers, can totally destroy the

measured results [2, 3]. Maintaining a motionless condition is often difficult, especially with field measurements.

Recently, a novel nonlinear method was presented for measuring PMD using four-wave

mixing (FWM) [4]. It alleviated the strict requirement for a motion-less condition. This paper

presents a new technique for measuring PMD that is based on the same nonlinear effect (four- wave mixing, FWM) as in [4], but uses the Poincare sphere method to calculate PMD. In this method, two signals at different frequencies are launched into the fiber under tested. The output

signals are then input to a dispersion-shifted fiber (DSF) to produce FWM. The FWM power in the DSF is related to the arc length on the Poincare sphere and is measured to determine the PMD

value of the fiber under test. Theoretically, this method can produce a PMD value from each

measurement point on FWM power, thus it greatly reduce the signal wavelength scanning range

in the optical spectrum, though multiple measurements may be still needed to reduce the

influence of FWM power fluctuation.

II. The theory of the method

FWM is a nonlinear process induced by Kerr effect in optical fiber. If three signals at frequencies fj , fj and fk co-propagate through a single-mode fiber, the new frequency generated

through FWM would be fi = fi + fj - fk. For the partially degenerate case, fj = fj , the newly

generated frequency is fi = 2fj - fk. The generated FWM power depends not only on the signal

frequency separations, input signal power and fiber loss and nonlinear characteristics [5, 6, 7], but

also on the input signal polarization states [4, 8]. The FWM power transfer function on the state of polarization (SOP) of the input pump signals can be written as [4]

79
F{SOP, , SOP, ) = -[1 + J, (OJ, )

• 52 (co^ )]

=i[i+5;"5;^»+4'>5f +5;'^5f'

]

Where J", (gj, ) = [5,"* 5^" 5*'*]^ ancG^, ) = [5,'"' 5;-' s^'^Y are the normalized

vectors representing the two polarization states of the two input signals at frequencies coi and (O2, repectively. (1) is valid when the PMD in the fiber (measurement fiber) is small. This transfer function has been verified by both simulation and experiments in [4]. On the surface of Poincare sphere, we can write (1) as

F (s) = i[l + cos((/))]

1 (2) = -[1 + cos( s)]

where (f) is the angle between the two polarization vectors and s is the arc length between the two

end points of the two polarization vectors on the Poincare sphere. From (2), we can write 5 as a

function of F, that is

5 = cos "'(2F - 1) (3)

By definition, the first-order PMD is calculated by [9]

ds PMD =\ 1 (A)

dO)

where cois the signal angular frequency. Substituting (3) into (4), we get

1 dF

PMD = ,I 1 (5) ^F{\-F) d(0

If we use wavelength instead of frequency, (5) becomes

dF

PMD =

,

I 1 (6) 2kc^F{\-F) dX

where X is the wavelength and c is the speed of light.

The FWM transfer function, F, is obtained by measuring the FWM efficiency as a function of

the signal wavelength separation. In real measurements, F needs to be calibrated with the zero- PMD case to reduce the effects of chromatic dispersion in measurement fiber. The derivative in

(6) should also be replaced by a difference equation and we get

X^ AF

PMD = I 1 n\ lKc^F{\-F) AA ^'^

where AF is the change in FWM efficiency inside the small wavelength window AA,. To reduce

the influence of FWM power fluctuation, multiple measurements are needed either at one

wavelength or at multiple wavelengths.

80
III. Experimental results

An experimental setup for measuring PMD using FWM is shown in Fig. 1 . Here, a PMD

emulator was used to generate a known amount of PMD in the system and served as the fiber

under test. A 17.5-km dispersion shifted fiber (DSF) with a zero-dispersion wavelength of 1551

nm was used as the measurement fiber to produce FWM. The FWM power was measured by an

optical spectrum analyzer. In our measurements, the wavelength of one input signal was fixed at 1554.0 nm, and the other signal wavelength was varied, where the range of variation chosen

depended on the expected PMD values. During measurements, the PMD emulator was first set to zero PMD and the FWM efficiency was measured and recorded, where the FWM efficiency is defined as the FWM power normalized by its maximum value which occurs when the

polarization states of t.wo input signals are aligned. This data was used for calibrating the FWM

efficiencies for the non-zero PMD cases. Fig. 2(a) and (b) show the measured FWM efficiency

vs. the signal wavelength separation for two PMD values, 10 ps and 20 ps, after calibration. The

PMD-induced periodic variations on FWM power is clearly observed when signal wavelength is swept. The minimum measurable FWM level was hmited by ASE noise.

PMD

,—o Emulator

Splitter\ EDFA DSF

OSA

Tunable 000 ^ Laser PC

3-dB

Coupler

Isolator Power

Meter

Fig. 1 Experimental setup for measuring PMD using FWM.

LD-Laser diode; PC--Polarization controller; EDFA-Erbium-doped fiber amplifier; DSF—

Dispersion-shifted fiber; OSA-Optical spectrum analyzer

5 -2

>*u

c

I ^

u

S

-8

1

o -10

(a)

\ \

t

2

0.4 0.6 o.e 1 \2 1.4 1.6 1.8 Wavelengh separation (nm)

0

2. -2 >.o

c

I ^

S

-8

"«E5 -10

•••\

(b)!

i

0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6

Wavelengh separation (nm)

Fig. 2 Normalized FWM efficiency for PMD =10 ps and PMD = 20 ps after calibrated

with zero-PMD case, (a) PMD = 10 ps, (b) PMD = 20 ps.

Fig. 3 shows measured average PMD for different given PMD. The measured mean PMD

values were obtained by averaging the measurements in different wavelength ranges, represented

by the periods of variations of FWM efficiency on wavelength, as show in Fig. 2. Four cases in Fig. 3 correspond to 0.5, 1, 1.5 and 2 periods. The measured results for all four cases follow well

with the given values, but are mostly a little bit lower than the true PMD. This is due to measurement errors around the notch areas of the measured FWM efficiency curve. Theoretically,

the notches of the FWM efficiency curve should have approached zero, but in experiments these

points are non-zero due to the amplified spontaneous emission in the erbium-doped fiber

amplifier (EDFA).

To estimate the optical bandwidth needed for this method. Table 1 gives the calculated

width in nanometer for each period of the FWM power variations with different PMD values. It agrees well with measured FWM data in Fig. 2. For high PMD (> 10 ps), the period is quite

81
narrow. Thus the nonlinear Poincare sphere method can measure PMD without scanning

measurement signals in a wide optical bandwidth.

120 1

100

80

'»a

Q 60 S

a. 40

20

0

Measured PMD in 0.5 period Measured PMD in 1 period Measured PMD in 1.5 periods Measured PMD in 2 periods

1 fe--^'"-

Table 1 The wavelength width of one period of FWM

power variations for different PMD values

PMD

(ps) 5 10 20 50 100

Wavelength

width (nm)

per FWM period

1.6 0.8 0.4 0.16 0.08

Measurements Fig. 3 Measured mean PMD and the given PMD.

IV. Conclusion and discussion

In summary, we have presented a new Poincare sphere method for measuring PMD by using

FWM generation in single-mode optical fiber. It is based on the FWM power transfer function vs. the polarization states of the pump signals. Compared to the traditional Poincare sphere method,

this method does not require measurement of the Stokes vectors and is less insensitive to mechanical vibration of measurement apparatus. Compared to the nonlinear fixed polarization

analyzer method, this method does not need to scan a wide optical bandwidth and thus has fewer

requirements on the measurement fiber. Similar to the nonlinear method in [4], this technique may also be used as an in situ PMD

measurement or monitoring method on dense wavelength-division multiplexed (DWM), traffic- carrying fibers. If the polarization states of the transmitted signals are fixed, the FWM products in the measurement fiber generated by wavelength channels may provide an estimate of the PMD.

This can be done either span-by-span or over several spans.

References:

[1] Kapron, F., A. Dori, J. Peters, H. Knehr, "Polarization-mode dispersion: should you be concerned," Proceedings of NFOEC'96, Vol. 3, pp. 757-768, Sept. 1996.

[2] Namihira, Y., Maeda, J., "Comparison of various polarization mode dispersion measurement methods in optical

fibers," Electronics Letters, Vol. 28, No. 25, pp. 2265-2266, Dec. 1992.

[3] Galtarossa, A., G. Gianello, C. G. Someda, M. Schiano, "In-field comparison among polarization-mode-dispersion measurement techniques," IEEE J. of Lightwave Technology, Vol. 14, No. 1, pp. 42-48, Jan. 1996.

[4] Song, S., C. Allen, K. Demarest, R. Hui, "A novel nonlinear method for measuring polarization mode dispersion,"

IEEE J. of Lightwave Technology, Vol. 17, No. 12, pp. 2530-2533, Dec. 1999.

[5] Hill, K. O., D. C. Johnson, B. S. Kawasaki, and R. I. MacDonald, " CW three-wave mixing in single-mode fibers,"

J. Appl. Phys., 49(10), pp. 5098-5106, Oct. 1978.

[6] Shibata, N., R. P. Braun, and R. G. Wants, "Phase-mismatch dependence of efficiency of wave generation through four-wave mixing in a single-mode fiber," IEEE J. of Quantum Electronics, Vol. QE-23, No. 7, pp. 1205-1211,

July 1987.

[7] Song, S., C. Allen, K. Demarest, R. Hui, "Intensity effect on four-wave in single-mode fiber," IEEE J. of Lightwave

Technology, Vol. 17, No. 11, pp. 2285-2290, Nov. 1999.

[8] Inoue, K., "Polarization effect on four-wave mixing efficiency in a single-mode fiber," IEEE J. of Quantum

Electronics, Vol. 28, No. 4, pp. 883-895, April 1992.

[9] Poole, D. C, and D. L. Favin, "Polarization-mode dispersion measurement based on transmission

spectra through a polarizer," J. Lightwave Technol., vol. 12, pp. 917-929, June 1994.

82
Modulation Phase Shift measurement of PMD: towards an

engineered solution

A J Barlow, J Henstock, C Mackechnie

PerkinEimer Optoelectronics, Wokingham, U.K.

Abstract: This paper reports the development of the Modulation Phase Shift method

for measuring PMD and DGD in fibers and components. In particular we examine

practical issues for the implementation of the method.

Introduction: Polarisation Mode Dispersion (PMD) remains important for optical

communications, despite the impending deployment of PMD compensation methods.

These methods depend on a full understanding of the PMD phenomenon in fibers and

components. In particular, the compensation of first order as well as second order PMD

require a detailed verification of that behaviour within DWDM channels or in components and devices that have very narrow transmission bands.

Some but not all, of the traditional PMD measurement techniques developed for fibers

will operate over narrow wavelength bands and/or give full PSP information [1], [2], [3].

Included in these is the long-established modulation phase shift (MPS) method [4],

which is a direct time domain measurement of differential group delay (DGD). It effectively uses so little optical bandwidth [5] that it may be considered as a "spot"

measurement of DGD. By inference the DGD can be determined without any form of

polarisation differentiation with respect to wavelength [6]. Therefore, second order PMD

vectors can be obtained from the data with only one stage of differentiation.

The MPS method has received increasing attention recently, particularly for the study

of DGD in high PMD fibers, and for second order PMD effects [7]. It has also been

used in the study of PMD in fiber Bragg gratings [8], where the narrow band nature of

the technique is particularly advantageous in exploring regions of high ripple.

Background: Implementation of MPS is generally straightforward, but does pose some

difficult and/or expensive engineering issues. In particular, the method requires

repeatable polarisation states to be generated before entering the test fiber/device or to be sensed leaving the test fiber/device. The most obvious method is to use polarisation

controllers and look for minimum and maximum delays over all possible polarisation

states, but this involves a lengthy "search and measure" algorithm for each DGD point.

Other methods using polarisation modulation [9] and demodulation scheme after

polarisation searching to remove thermal drift effects have been proposed [10].

A simpler four-state launch set up was proposed by Williams [11]. This allows the full DGD and PSP [6] information to be obtained from four input states and four

corresponding delay/phase measurements. This is additionally attractive since

polarisation dependent loss (PDL) can be detected from power readings at the same

polarisation states. Finally, if two of the chosen states are mutually orthogonal, it becomes possible to measure chromatic delay (and dispersion) without error when

there is very large PMD and hence second order PMD [6]. In this paper, we examine the Williams four-state method in some detail. This

technique has the attraction of simple implementation and excellent compatibility with

83
current dispersion measurement hardware. Two aspects were examined: a) the

robustness of the analysis in the presence of measurement noise, b) the detailed

design of a practical hardware solution.

A) Data analysis: In the final equation used to calculate DGD, both Williams' equation

9 [11] and Jopson's formulation [6] involve the squares of the phase delays measured

for the four input polarisation states. Therefore, any noise in the phase due to instrumentation will undergo a non-linear transformation and "rectification" (i.e. loss of

sign of noise). The noise would no longer be averaged to zero and so would show up

as a bias. This bias is a direct result of the noise level.

STANDARD DEVIATION AND BIAS (ps)

.

1 —I

Figure 1:

Simulation

Result

showing the

effect of

phase noise

in the MPS

method.

TRLtE DGD VALUE (ps) STANDARD DEVIATION (ps) BIAS(ps)

To test this hypothesis, random bipolar phase noise was added to nominal phase

values and these were applied to Williams' formula to calculate DGD. The calculation

was repeated for different DGD levels and over 10000 simulation runs. The result is shown in figure 1 is for an equivalent delay noise level of 0.15 ps- a value typical of

some MPS measurement systems.

Clearly we can see three features:

1) The standard deviation (sigma) of the results is approximately twice the 0.15 ps

noise level of one individual delay reading. This is logical as there are four delay

readings, so overall noise increases as V4=2 times.

2) There is indeed a bias that is always positive, which increases as DGD decreases

to ~0.3 ps at 0.5 ps true DGD level. This bias is indeed indicative of the bipolar

noise being "rectified" by the action of the DGD calculation equation.

3) The bias only exceeds the noise level at very low DGD values.

Therefore, while the bias is clearly non-negligible, it is generally only significant for

DGDs below 1 ps, where it becomes comparable to the measurement noise.

Furthermore, because the bias may be fitted easily to an equation and depends only

upon the system noise level, it is therefore possible to correct the bias.

84
B) Design: We have implemented the MPS method using an adapted chromatic

dispersion test set [12], employing a tuneable laser source. A simple mechanical

switch/polarizer assembly, which provides the four switched polarisation states, was

designed. This system was capable of being highly automated and allows rapid and

routine measurements of DGD and PMD in fibers and components, along with

dispersion and delay. The rapid switching eliminated thermal drift effects.

Figure 2 shows the results obtained on a telecommunications fiber selected for its high

PMD.

Figure 2:

DGD and

Principal

States

measured

in a high

PMD fiber

spool.

PMD~20ps

1530 1535 1540 1545 1550 1555

Wavelength (nm)

DGD I Omega 1 —a— Omega 2 Omega 3

Also shown are the PSP components of the fiber over this wavelength range. This fiber

was a standard telecommunications single mode fiber wound on a standard shipping

spool. This fiber has a PMD in the region of 20ps, yet is only 4.2 km long.

Figure 3:

DGD and

Principal

States

measured

in a Hi-Bi

fiber

artefact.

DGD-15 ps 1530 1535 1540 1545

Wavelength (nm)

1550 1555

DGD —I—Omega 1 •Omega 2 Omega 3

85
Figure 3 shows a PMD measurement in a simple length of Hi-Bi fiber, showing a steady

DGD variation with wavelength. This is indicative of low mode-coupling behaviour.

Slight drift in the DGD/Principal axis positions was attributed to the effects of the fiber

lead birefringence and to thermal drift in the fiber leads. The latter can be eliminated by

altering the polarisation-switching pattern and will be presented later.

Conclusions: We have examined two issues related to correct engineering of the

Modulation Phase Shift method, each related to the four-state excitation technique. We

have shown that while in general instrumentation noise can give rise to a bias, that this

bias is easily corrected and is generally much lower than the DGD repeatability level.

Similarly, we have shown that it is possible to implement the four-state scheme with

elimination of thermal drifts using simple and inexpensive hardware.

The modulation phase shift method is therefore an extremely potent method for DGD

investigations in fibers and components, allowing ultra-narrow band DGD

measurements, for PSP and PMD2 information to be derived, while simultaneously

allowing full compatibility with existing methods/hardware for chromatic

delay/dispersion and PDL measurement.

Acknowledgements: The authors thank P Williams and R Jopson for helpful

discussions over the course of this work.

References:

[I] TIA FOTP-196 "Guideline for Polarization- IVIode Dispersion Measurement in Single-Mode Fiber

Optic Components and Devices" 1 999.

[2] HEFFNER, B.L., "Automated Measurement of Polarization Mode Dispersion Using Jones Matrix

Eigenanalysis", IEEE Photonics Techno!. Lett., 1992, 4(9), pp1066 - 1069.

[3] JOPSON, R. M., NELSON, L E., AND KOGELNIK, H., " Measurement of Second-order

Polarization Mode Dispersion vectors in Optical Fibers", IEEE Photon. Technol. Lett., 11,1999,

pp. 1153-1155.

[4] SANO, K. KUDOU, T. and OZEKI, T., "Simultaneous measurement of Group Delay Time

Dispersion and Polarization Mode Dispersion", Proceedings of 22"^* ECOC, 1996. Paper TuP.09;

also MONERIE M, "Polarisation Mode Dispersion measurements in long Single Mode Fibers" El. Lett. 16, 1980, pp. 907-908.

[5] WILLIAMS, P.A, BARLOW, A.J., MACKECHNIE, C, AND SCHLAGER, J.B.; "Narrowband

measurements of polarization-mode dispersion using the modulation phase shift technique",

NIST SOFM, 98, Sept 98.

[6] JOPSON, R. M., NELSON, L.E., KOGELNIK, H. and GORDON, J.P.; "Vector Measurement of Polarization-Mode Dispersion using the Polarization-Dependent Signal Delay Method", LEOS99,

post-deadline paper, Nov 99.

[7] DAL FORNO, A., PARADISI, A., PASSY, R., and VON DER WEID, J.; "Experimental and

Theoretical Modeling of Polarization-Mode Dispersion in Single-Mode Fibers", IEEE Jnl. Photonics Technol. Lett., 12, 2000, pp 296- 298.

[8] BERCHELOT L., GOURHANT, J, RIANT, I, SANSONETTI, P, "Vectorial Model of Bragg

Gratings", El. Lett, 36, 2000, pp 744-745.

[9] US patent 4,750,833.

[10] TIA FOTP-197 "Differential group delay measurement of single-mode components and devices

by the differential phase shift method" ,2000

[II] WILLIAMS, P. "Modulation Phase Shift Measurement of PMD using only four launched

polarisation states: a new algorithm". El. Lett, 35, 199, pp. 1578-1579.

[12] PERKINELMER CD400.

86
Interpretation of PMD Measurements in Components:

A Case Study

Normand Cyr

EXFO Electro-Optical Engineering, Inc., 465 Godin Ave, Vanier, Quebec, Canada, GIM 3G7.

Abstract: Peculiar PMD behavior that may arise in optical components are highlighted through a

case study, the arrayed-waveguide-grating. The emphasis is put on interpretations ofPMD measurements that can be misleading. Descriptions are given and appropriate interpretations proposed.

1. Introduction

The usual, and simple interpretation of polarization mode dispersion (PMD) in strongly-coupled,

single-mode fibers is based on the following premise: in principle, the probability density function (pdf) of the PMD-induced pulse spreading, including higher-order PMD, is deducible from a

single parameter, the root-mean-square differential group delay (rms DGD). From this pdf, one

can predict the pdf of BER fluctuations at a given bitrate. Also, it is assumed that the polarization dispersion vector varies little within the bandwidth of a telecom signal, so that only the local

DGD value, or the smallest orders of PMD are considered. This holds in principle, at least.

hi contrast, optical components may show, owing to their very nature and variety, hard to predict

combinations of PMD, PDL, muhipath interference (MPI), and filter effects in the case of nar- row band components. We illustrate some peculiar PMD behavior that may arise through a particular case study, the arrayed-waveguide-grating (AWG) filter. The aim is to discuss the rele- vance and significance of PMD measurements for this particular case, which raises such questions as: does the measurement yields the correct result, does the familiar relation between the

measured DGD and the expected pulse spreading still hold, how should the results be delivered

or interpreted if not, and have the relevant parameters been measured here in the first place?

2. Simulation of the Device Under Test

The AWG is modeled as a multipath interferometer. The incident "beam" is split into multiple

paths of different lengths, and the output field is the sum of the output fields of all paths. With

this simple model, we can easily take into account the birefringence of the waveguides, small

reflections at interfaces on both sides of the waveguides, and off-centering of the power weighting function (uneven splitting of the power between waveguides). The main features are that:

• Two sets of waveguides are interlaced, with slightly shifted peak-transmission wavelengths.

• Any two consecutive waveguides of the same set have the same path-length difference.

• Each waveguide is represented by a (2x2) Jones matrix. The Jones matrix of the device is the

sum of all these. It is used to compute the DGD, DAS, principal states of polarization (PSPs),

PDL, and transmission, all as a function of optical fi-equency. The differential attenuation

slope (DAS) /I / is the difference of the co-derivatives of the PSPs attenuation coefficients.

• The birefringence is assumed to originate from the waveguide geometry. Accordingly, the

birefringence axes of all waveguides are assumed to be in the same direction.

• We take into account small reflections that may occur at interfaces on both sides of the device, creating what might be termed "false PMD". To analyze this effect, the output field of

each waveguide is the sum of the fields from the first pass and from one double reflection.

87
3. Results and interpretation

The AWG simulated in this study was assumed to be characterized by the following key parameters, representative of realistic devices. The parameters common to all results are the mean index

no=1.5, the birefringence An/no=5.l7xlO"^ the DGD of the mid-waveguide 5to=16.8 fs, the aver- age length L=6.5 cm, and the FSR=2.512 THz . Also, the number of waveguides is 87 per set, the

power weighting function (hereafter named the window) is gaussian with a standard deviation of

30 (in number of guides), and the filter bandwidth is FWHM ~ 78 GHz (0.626 nm). Parameters

that are varied are the reflection coefficient, R, equal on both sides and independent of SOP, and

the window offsetting parameter, a, expressed in terms of the number of guides.

3.1 No Reflection, Centered Window

The results with R = 0 and a = 0 are shown if Figure 1. The DGD have a constant value of 16.8

fs (Fig. Ic). The polarization dependent shift of the transmission is ±5 GHz. The combination of

birefringence and filter effect generates a large PDL with steep variations outside the flat portion

of the filter (Fig. lb). The usual relation between DGD and PMD-induced pulse spreading

(maximum of DGD/2) holds in this case, as expected. No surprise up to this point.

However, even in this "ideal" case, attention should be paid to the PMD-filter combination. It gives a very large (up to 4 ps within FWHM), steeply varying DAS (Fig. Id), which may contribute to pulse spreading /1,2/. This happens when PMD and significant PDL coexist, so that the

two PSPs are no longer orthogonal, resulting in interference between them instead of a simple

addition of power 121. In this simulation, it is assumed for simplicity that waveguides are uniform, that they all have the same birefiingence axes, and that no PDL exists other that the one

created by the birefi*ingence-filter interaction. The PSPs then remain strictly orthogonal, so that

the DAS, even large, has no significant effect on pulse spreading. However, a less perfect uniformity or some additional PDL would result in some overlap of the PSPs. The consequence can

be a PMD-induced spreading significantly larger than inferred from the DGD value. In such an

eventuality, the most relevant parameters to be measured should be DAS and PSP overlap.

3.2 Double Reflection and "False PMD"

We now turn to a more striking effect. With R = -30 dB, the DGD oscillates, from 0 to a maximum value of 1 ps (Fig 2a), with a period FSR^r = l/x^r (53 periods within FWHM), where Xdr is the round-trip time through the waveguide section ("dr" stands for double reflection). The

maximum is approximately given by DGD^g^ = Stq + R |sin<|)| , where ^ is the retardance of

the mid-waveguide at peak-transmission wavelength (1550 nm). Parameters have been set such

that sin([)=l. The pulse spreading in such case is simply due to a double reflection. Apart fi-om a

negligible residual, it remains the same regardless of the PMD, be it DGD = 0 (with An=0), or DGD = 16.8 fs (if sin(|)=0), or a DGD that oscillates up to 1 ps, with a mean value of 338 fs. The

double reflection can be "imaged" in a large, fast oscillating DGD, but the pulse spreading is not

caused by PMD, and should not be interpreted as being. In that sense, the reflection-induced

DGD can be viewed as "false PMD". We also point out that in any case, it is not the mean value

of such a rapidly oscillating DGD that is the primary determinant of the pulse spreading. Rather,

the assumption that the dispersion vector varies little within the bandwidth of the signal fails

completely here, for a bitrate of, say, 10 Gb/s. Higher orders of PMD are dominant. In short, the

mean-DGD is not relevant in this case. Moreover, in a DGD measurement using a polarimetric

88
-80 -40 0 40 80 -80 -40 0 40 80 Figure 1: PMD-PDL behavior of the AWG, no reflection (R = 0), centered window (a = 0).

a) Filter transmission: input-SOP aligned with fast(—) or slow(—) birefringence axis (An/no =

S-nxlO"*, PDW=10GHz). b) PDL due to filter-birefringence combination: mean PDL = O.SldB

within ~80GHz filter bandwidth (FWHM, unpolarized input), c) Constant DGD=16.8fs. d) Differential Attenuation Slope (DAS) up to 4 ps within the 3dB bandwidth.

DGD with Double Reflection, or Off-Center Window

Optical Frequency Offset [GHz] Optical Frequency Offset [GHz]

Figure 2: DGD behavior, a) True DGD: with a = 0, R= -30 dB (black), and R = 0, a = 2

(white). Note: L=6.5cm => FSR<ir=1.46GHz. b) With R= -30 dB and a = 0, DGD that would be

obtained through polarimetric measurement using a tunable laser, with 0.02 nm steps, (Stokes

parameters analysis; JME or Generalized Poincare Sphere Analysis /3/ (GPSA)).

89
method, it is likely that the wavelength step will be too large. With a small step such as 0.02 mn,

a DGD curve would be obtained, but not showing fast oscillations (under-sampling), and giving

underestimated values (see Fig 2.b). A solution to this is discussed in section 3.4.

3.3 Off-Center Window

Another unfamiliar PMD behavior is illustrated in Fig 2. a (white curve). It occurs with an offcentering of the window, here as small as a = 2. Such steep variations, right in the middle of the

filter bandwidth, demand careful investigation, because high-order PMD is likely to be a major,

if not dominant contributor to PMD-induced spreading. This is but one example of what may

occur in narrow band components, as a result of the interplay between PMD and phase variations

occurring within the bandwidth of a filter.

3.4 PMD Measurements with a Large Resolution Bandwidth

Considering that small reflections with large delays are likely to occur in a number of optical

components, it seems reasonable that, when PMD measurement are made, one should routinely

proceed with the following steps. First, detect double reflections with large delays (e.g. with an OLCR). If a non negligible reflection exists, at least it will not be interpreted as PMD, and the

real problem will be identified, i.e. double reflection, not PMD. Then, use a technique capable of

measuring the intrinsic PMD, independent of reflection (as the 16.8 fs of the above case study).

This can be done through standard polarimetric methods, by measuring the output-SOP as a

function of optical frequency, but with a resolution bandwidth larger than FSR^r . Then, the fast

reflection-induced oscillations of the output-SOP are simply filtered out. In practical situations,

FSRdr should be smaller than the bandwidth of the DUT, and in the case of wideband DUTs, the

round-trip delay will be very much larger than any reasonable intrinsic DGD. If, say, 0 to 100 fs are to be measured in wideband DUTs, a resolution bandwidth of even as much as 1 THz should

be small enough. Moreover, such a large resolution bandwidth can significantly suppress measurement noise, an advantage that is lost if a narrowband, tunable laser is used.

As an example of implementation, a complete device characterization can be performed with a wideband source, measuring the output-SOP as a function of frequency with a polarimetric interferometer (used as Fourier-transform spectrum analyzer). Before proceeding to PMD measurement, the same interferometer can be used to detect reflections, measure round-trip delays, and

select a scanning range with a maximum delay smaller than the round-trip delays (equivalent to a

resolution bandwidth larger than FSRdr in the frequency domain).

References:

' III N.Gisin, B.Huttner, N.Cyr, "Influence of polarization dependent loss on birefringent optical fiber networks,"

Technical Digest of the 25th Optical Fiber Conference (OFC'2000), Baltimore, Maryland, March 2000, session

TuGl-l,p.86 .

Ill N.Gisin, B.Huttner, "Combined effects of polarization mode dispersion and polarization dependent losses in optical fibers," Optics Communications, 142, 1 October 1997, p.l 19.

73/ To be published: A generalization of the Poincare Sphere Analysis that includes DUTs with large PDL.

90
An Intercomparison of Polarisation Mode Dispersion

Measurements and Calibration Artefacts

D J Ives, National Physical Laboratory UK,

UKFOToN Group'.

Abstract: This paper reports the results of an intercomparison of Polarisation Mode

Dispersion: (PMD) measurements on a set of six artefacts circulated within the UK

FOToN group. The purpose of the intercomparison was to provide:- a snapshot of

current PMD measurement capability, a comparison of possible PMD calibration

artefacts and a chance to highlight possible future PMD measurement problems.

Introduction: Polarisation Mode Dispersion (PMD) is an important parameter in the specification of

optical fibre and optical fibre components for use in the new high bit rate systems. The measurement of

PMD has become routine, however the statistical nature of the parameter has denied the area of a stable

calibration artefact. This intercomparison was set up to provide a snapshot of current PMD measurements

and a comparison of possible PMD measurement calibration artefacts.

DeHnitions: For the purposes of this intercomparison polarisation mode dispersion (PMD) is defined as the RMS differential group delay (DGD). PMD, At, is given by:

where 5t is the DGD and the sum is over all wavelengths and/or temperatures.

Artefacts: Six artefacts were chosen for the intercomparison to give a range of PMD values. These were

divided into two groups, those with weak polarisation mode coupling and those with strong polarisation

mode coupling. The weak mode coupled artefacts were:

1) A temperature stabilised single quartz waveplate. This provided a PMD of -0.3 ps. 2) A ~6 m HiBi patch cord providing a PMD of -12.5 ps. 3) A WDM device constructed from a WDM filter and a length of HiBi fibre to provide a device with

approximately 1 nm of spectral bandwidth and a PMD of -1.5 ps.

The strong mode coupled artefacts were:

4) A 13 km length of dispersion shifted fibre on a shipping reel. This provided a PMD of -0.3 ps and a

real fibre against which to assess the other artefacts. 5) A temperature stabilised 32 quartz waveplate stack. The plates being stacked randomly to give strong

polarisation mode coupling. The device has a proven stability and a PMD of -0.5 ps. 6) A high PMD emulator based around a number of HiBi fibre segments joined at random angles. This

provided a PMD of -8 ps.

Measurements: Each participant was given basic information about each artefact including the type of

polarisation mode coupling regime and the approximate PMD value to allow the correct setting of the

' FOToN, Fibre Optic Technology Network is a pan European network aimed at promoting the exchange of best measurement practice and facilitating commercial exploitation within the areas of optical fibre, optical fibre component and optical fibre instrumentation technology.

91
measuring instruments. The participants were asked to measure the PMD of each artefact and provide

basic information about the test method, wavelength range, temperature, accuracy and traceability.

Methods: The following PMD measurement methods were employed which cover both time and

frequency domain techniques''^.

1.8

T3

S!3

(0«

c

(0

a

S

Ol

•o

S>3

(0

(QV

s

1.6 --

1.4

1.2

0.8

0.6

1 -'^^^A* ctn^ •

1^1ft

JMBPS

I A WSFT

• WSCC

•

3 4 5

Artefact Number

Figure 1 The normahsed measured PMD of each artefact in the 1550nm window.

JME/PS) Jones Matrix Eigenanalysis or Poincare sphere analysis of polarimeter based measurement

methods^^'^''*l A tunable narrow band source and a polarimeter is used to measure the output polarisation

state as a function of optical frequency for a number of different input polarisation states. The change of

the output polarisation state with optical frequency is used to calculate the PMD by either Jones Matrix

Eigenanalysis or Stokes vector rotation of arc on the Poincare sphere analysis.

I) Interferometric methods^^^ using the limited coherence length of a broadband optical source to probe

the PMD. For no polarisation mode coupled artefacts the interferogram is a number of peaks the

separation of which gives the PMD. For high polarisation mode coupled artefacts the FWHM of the

interferogram gives the PMD.

WS FT) Wavelength scan with fourier analysis'^^. This method is based on the transmission of a polarised

broadband optical source through the artefact. The output is passed through a fixed analyser to an optical

spectrum analyser or equivalent wavelength scanning device. The fourier transform of the scan is equivalent to an interferometric measurement and can be analysed similarly.

WS CC) Wavelength scan with cycle counting analysis'*'. The fixed analyser wavelength scan trace is analysed by counting the extrema to calculate the PMD. A factor of k=0.82 was used where there was known to be strong polarisation mode coupling.

92
Results: The results shown in Figure 1 represent the PMD measurements in the 1550 nm window (except

wavelength scan fourier transform (WS FT) technique which are for the dual 1310 and 1550 nm

windows). The measurements were all performed between 18°C and 25°C. The measured PMD for each

artefact was normalised by dividing by the mean measured PMD for that artefact in order to highlight the

relative variations between the different artefacts. Artefact Approx

PMD

Inter- laboratory

Repeatability

(la)

Typical

Laboratory

Repeatability

(la)

Expected

Statistical Variation'^'

(la)

1) Quartz Waveplate 0.3ps 1% 1% 0%

2) ~6m HiBi patch cord 12.5 ps 1.5% 0.2% 0%

3) WDM Filter + HiBi Fibre 1.5 ps 41% (2.5%) 4% 0%

4) 13km DS fibre on shipping reel 0.3 ps 11% 6% 18%

5) 32 quartz waveplate stack 0.5 ps 8.5% 2% 14%

6) High PMD emulator Bps 7% 1% 4%

The table above shows the repeatability of PMD measurements for each artefact. The interlaboratory

repeatability is the standard deviation of the all the available measurements from all the laboratories

except for the WDM device (3) where the figure in brackets corresponds to the standard deviation of the

JME/PS measurements alone. The typical laboratory repeatability is the RMS average of each

laboratory's repeatability and gives an indication of the repeatability of individual participants. The

expected statistical variation in the final column is the variation expected due to the statistical nature of

PMD in mode coupled fibre for a single PMD measurement with a wavelength range of lOOnm. It does

not strictly apply to the fixed mode coupled artefacts (5) and (6) but is an indication of the variation

expected for real fibre with a similar PMD. The variation would be observed for artefacts (5) and (6) if each participant measured a different lOOnm wavelength range.

Conclusions: The following conclusions for each artefact have been drawn from this intercomparison:

1 ) Non mode coupled devices with a broad spectral transmission can be routinely measured by all the

techniques with an interlaboratory repeatability of 1%.

2) The high value of PMD with the ~6m HiBi patch cord and the high PMD emulator were outside the

PMD measurement range of some of the instruments.

3) The WDM device gives significant measurement problems due to the narrow spectral transmission

bandwidth. The JME/PS techniques were able to perform the measurement with a repeatability of

2.5%. The restricted spectral bandwidth lead the Inteferometric and wavelength scan techniques to have a minimum measurable PMD which is much greater than the actual PMD. WDM devices with a lower PMD will probably need a new measurement technique.

4) The real fibre shows the expected statistical variation. All the laboratories were able to measure the

PMD to within the expected statistical accuracy.

5) The 32 quartz waveplate stack shows considerable variation for the interferometric method. This is probably due to different launch and receive polarisation states and different wavelength ranges. The

JME/PS method shows better interlaboratory repeatability where there are similar wavelength ranges.

6) The high PMD emulator shows interlaboratory repeatability larger than the statistical expectation of

real fibre for a lOOnm wavelength scan. This may be as the statistically independent wavelength

ranges are closer together for high PMD. This leads to a smaller wavelength difference giving a larger

PMD variation. Also some of the participants used smaller wavelength ranges, as the higher PMD

requires a smaller wavelength step and this will lead to a larger expected statistical variation.

93
These results are in broad agreement with previous PMD measurement intercomparisons'* but also

highlight the future problem of measuring PMD within restricted WDM bandwidths.

Acknowledgements: I would like to acknowledge Perkin Elmer for providing the single quartz

waveplate, Nortel Networks for the WDM filter and Fibercore for the high PMD emulator. NPL provided

the 32 quartz waveplate stack artefact.

I would like to acknowledge the participants from the UK FOToN group:

R Martin Anritsu Ltd

L Gleeson BT Labs

M Harrop Coming Cables Systems

THart Fibercore

R Brunt Lambda Photometric Ltd

D Watley Nortel Networks

A Barlow Perkin Elmer

C Wallace Pirelli Cables

A S Siddiqui University of Essex

References:

[1] NAMIHIRA Y., MAEDA J. "Polarisation Mode Dispersion Measurements in Optical Fibres.", SOFM

Boulder 92, 1992, NIST Special Publication 839, ppl45 - 150.

[2] HEFFNER B.L., "Automated Measurement of Polarization Mode Dispersion Using Jones Matrix

Eigenanalysis.", IEEE Photonics Tech. Lett., 1992, 4(9), ppl066 - 1069.

[3] POOLE CD., BERGANO N.S., WAGNER R.E., SCHULTE H.J., "Polarisation Dispersion and Principle

States in a 147km Undersea Lightwave Cable.", J. of Light. Tech., 1988, 6, ppl 185 - 1 190.

[4] NAMIHIRA Y., KAWAZAWA T., WAKABAYASHI H., "Polarisation Mode Dispersion Measurements in 1520km EDFA System.", Elect. Lett., 1992, 28(9), pp881 - 883.

[5] GISIN N., VON DER WEID J-P., PELLAUX J-P., "Polarisation Mode Dispersion of Short and Long

Single-Mode Fibers.", y o/L/g/zr. Tech., 1991, 9, pp821 - 827.

[6] BARLOW A.J., "Polarisation Mode Dispersion Measurement using Fourier Analysis: Investigation of

Mode Coupling Behaviour in Fibers.", SOFM Boulder 94, 1994, NIST Special Publication 864, ppl67 -

170.

[7] GISIN N., GISIN B., VON DER WEID J.P., PASSY R., "How accurately can one measure a statistical quantity like Polarization Mode Dispersion.", SOFM Boulder 96, 1996, NIST Special Publication 905,

ppl31 - 134.

[8] WILLIAMS P. A., "TIA round robin for the measurement of PMD", SOFM Boulder 96, 1996, NIST

Special Publication 905, ppl55 - 158.

[9] GISIN N., et al, "Definition of polarization mode dispersion and first results of the COST 241

round-robin measurements". Pure Appl. Opt., 1995, 4, pp511 - 522.

94
High Speed Soliton Transmission on Fiber Links with High PMD

Peter A. Andrekson

Department of Microelectronics, Photonics Laboratory

Chalmers University of Technology

Gothenburg, Sweden

Polarization-mode dispersion (PMD) is considered an increasingly important obstacle for realizing high capacity

lightwave transmission systems. Several solutions to this problem have been proposed, and the most obvious one is to simply boost the aggregate fiber capacity by inserting more and more Wavelength-Division-Multiplexed (WDM)

channels, as has been done commercially with great success over the past years. There are, however, clear cost advantages in increasing the per channel data rate rather than increasing the number of wavelength channels. Thus,

10 Gb/s line rates are now commercial and 40 Gb/s line rates are currently seriously being considered, while 160

Gb/s line rates are still at a very early research stage. When considering upgrading old fiber lines to 10 Gb/s, PMD is likely to be one of the key limiting factors, as PMD values for older fibers typically may be in the range of 0.5 - 1 ps/Vkm. If we consider, somewhat arbitrarily, that the Differential Group Delay (DGD) of the fiber link must be

smaller than 10% of the bit-slot duration for an acceptably small outage probability, the maximum PMD limited

propagation distance would, in the above example at 10 Gb/s, be in the range 100 - 400 km.

Much effort and progress has resulted in new, very low PMD fibers with PMD values around 0.1 ps/Vkm and 10 Gb/s data could thus be transmitted over oceanic distances (=10 Mm). However, as line rates soon are expected to reach 40 Gb/s, the best available fiber would then reach only about 600 km (while at 160 Gb/s, the maximum

distance would be as short as 40 km). It should be pointed out that in the above examples, with the allowable DGD

being 10% of the bit-slot, relies on using the NRZ format [1]. Other formats, such as the RZ or the phase-shaped

binary transmission format [2] may be more resilient to PMD, albeit perhaps at the expense of other drawbacks, e.g.

spectral efficiency.

As the line-rates go up and the PMD is not expected to be significantly reduced in the near-term, other techniques to combat the impact of PMD have been proposed and evaluated. These technique all essentially rely on PMD

compensating sub-systems that adaptively track and compensate the PMD induced changes dynamically, either using

optical solutions or electrical solutions after detection [3].

In this presentation, we will focus on the features of soliton pulse propagation in fiber links corrupted by PMD.

Solitons are nonlinear pulses that have been proven to be robust in presence of various perturbations. Their resilience

to PMD was first observed in a polarization-maintaining fiber several years ago [4]. It was, however, not until

recently this feature was first observed in a real transmission experiment using conventional fiber [5].

Much of our soliton and PMD related research was conducted on installed optical Dispersions-Shifted Fiber (DSF).

The fiber was installed in the central part of Sweden in 1991 and had a link average PMD value of about 0.3 ps/Vkm.

A maximum of 400 km of fiber was available during most of 1998. Our experiments were conducted as part of the European Union ACTS program in a projects named MIDAS. Fig 1 shows schematically the field site venue and

experimental setup for propagating single-wavelength 40 Gb/s solitons.

95
Fig. 1 Soliton field transmission venue and experimental setup.

A Mode-Locked Fiber Ring Laser (MLFRL) was used as a wavelength tunable, chirp-free pulse source providing

width adjustable pulses at 10 GHz. After data encoding, passive fiber multiplexing yielded a 40 Gb/s stream of data

in which intentional timing alteration among the pulses (i.e. a slight deviation from the nominal 25 ps) was

introduced to easily recover a 10 GHz clock at the receiver side. All active optics, including optical amplifiers

located every 57 km, were conveniently located in one of the hubs. On the receiver side, a Polarization-Insensitive Nonlinear Optical Loop Mirror (PI-NOLM) served to demultiplex the data to 10 Gb/s.

As the accumulated DGD over the available 400 km was as large as 6 ps (24% of the bit-slot), the PMD was

expected to be the key limiting factor in this experiments. This was also confirmed through numerical simulations.

Nevertheless, as we used solitons and their PMD robustness, we were able to achieve error-free performance over the

entire link as indicated in Fig. 2, which illustrates the bit-error rate (BER) vs. received optical power both before and

after transmission over the link for all four time-multiplexed channels. The pulses were 10 ps wide and the link average Group-Velocity Dispersion (GVD) was 0.2 ps/(km nm), while the average link loss was 0.33 dB/km.

10-13| 1 1 1 1 -40 -38 -36 -34 -32 -30 Average received optical power [dBm]

Fig. 2 BER vs received power before and after transmission for all four channels of the 40 Gb/s soliton data over 400 km of installed fiber. The word length was 2'^-l.

96
We studied both parallel polarized and polarization-division multiplexed (PDM) soliton transmission [6]. Fig. 2

represents the results with PDM solitons, which resulted in a transmission penalty of about 0.5 dB for each channel. The penalty was larger for the parallel polarized solitons (about 1-3 dB). This was contrary to simulated results, which clearly showed that for the high amount of PMD in this case, PDM solitons suffer much more as there is no mechanism to stabilize the relative temporal position of the orthogonal pulses. The reason for the experimental

observation, on the other hand, was found to be caused by less interference (not included in the numerical model) in

the case of PDM pulses arising from non-ideal pulse extinction in the formation of the high bit-rate data in the multiplexer.

We also carried out single-wavelength PDM soliton transmission at 80 Gb/s by simply reducing the pulse width to about 5 ps, adding one section to the multiplexer and modifying the demultiplexer for 80 Gb/s operation. The GVD

was approximately 0.1 ps/km nm. Error-free operation was achieved for all eight TDM channels over 172 km (3 spans) [7]. This represents the first (and still only) optical transmission field experiment at single wavelength bitrates beyond 40 Gb/s. Fig. 3 shows the sensitivity (@ BER = 10"', 2'^-l) of the eight TDM channels at 0 km and at 172 km as well as a typical demultiplexed eye diagram at the receiver. The penalty was in the range 0.6 -1.6 dB.

Fig. 3 Sensitivity before and after transmission over 172 km installed fiber with 80 Gb/s PDM solitons and

corresponding demultiplexed eye diagram.

As it was clear from the transmission experiments, that PMD was the most important limiting factor, we conducted

extensive measurements of PMD both in the laboratory and in the field link. Of particular interest/concern is the

temporal evolution of the PMD. We conducted continuous long-term DGD measurements (36 days) simultaneously

in two independent 1 14 km long spans of the installed link [8]. The wavelength range was 1505 - 1565 nm (0.1 nm

step size) which was sufficient to yield a constant average DGD over time (corresponding to 0.26 ps/Vkm). Fig. 4

shows an example of such a DGD measurement. As the fibers were located within the same cable there was a strong

correlation between the fibers in terms of DGD change over time (~ 10% /day) as well as of the change of the angle

of the principal axis (~ 20° /day). The DGD probability distribution was found to be approximately Maxwellian.

97
1510 1515 1520 1525 1530 1535 1540 1545 1550 1555 15«0 Wavelength [nm] Fig. 4 Measured DGD of 114 km installedfiber over 36 days in the wavelength range 1505 - 1565 nm

We, furthermore, studied the local accumulation of the DGD along the fibers with a novel P-OTDR technique [9]. By studying the back-scattered signal originating at a certain distance versus the wavelength of the input pulse, one

obtains a "fixed-analyzer" spectrum (see left side in Fig. 5) from which one can calculate the accumulated average

DGD by counting the number of extrema. The right hand side of Fig. 5 shows an example of the evolution of the

local DGD in a 57 km long leg of the installed link. (The total DGD was identical to that measured with conventional

techniques). This technique is very promising to localize poor, i.e. high PMD, sections in fiber optic transmission

links.

Fig. 5 Left: P-OTDR traces in the wavelength-distance space. Right: Corresponding accumulation of DGD

along the fiber section. The shaded areas correspond to portions of the fiber where the DGD is rapidly

increasing, i.e. the local PMD is large.

In order to substantiate more rigorously the soliton PMD robustness that we observed in the field fiber link [5], and

where this was evidenced in a few given cases, we recently conducted similar tests in a more controlled laboratory

environment [10]. Fig. 6 illustrates the setup and the pulse behavior when linear pulses are being propagated through

a fiber link corrupted by PMD. First the actual DGD was measured at the wavelength of interest. Then the pulse

width at the link output was measured about ten times with different, random input state of polarizations, before the whole procedure was repeated over many hours such that the actual DGD was allowed to drift due to environmental

98
changes. The input pulse width was originating from a MLFRL providing essentially chirp-free 12.5 ps wide pulses.

The link was made up of a 150 km of DSF and the link average DGD was 5 ps.

T= 12.5 ps

Polarization

.rotator GVD = 1 ps/km-nm [Zq = 60 km], <DGD> = 5 ps

detector

4 8 Time (hours)

4 8 DGD (ps)

Fig. 6 Experimental setup (top) and behavior of linear pulses to study pulse-width variations caused by PMD in a 150 km long DSF link. The dashed lines are guides to the eye.

In Fig. 6, where results with linear pulses are depicted, a short piece of Dispersion-Compensating Fiber (DCF) was

used to compensate for the link GVD. By combining the DGD-trace and the pulse-width trace as on the right hand

side in Fig. 6, the results are intuitively making sense: the larger the DGD the larger is both the average pulse width

and the pulse width variation. When using solitons, on the other hand, the situation differs substantially as illustrated

in Fig. 7.

Fig. 7 Output pulse width vs link DGDfor linear pulses (grey dots - from Fig. 6) andfor solitons (black dots)

99
With the solitons, we increased the power (N-value ~ 1.2) and omitted the DCF. This resulted in an average output

pulse width slightly shorter (10.5 ps) than at the input, but more importantly, also in a reduction of the pulse width

variation, particularly at high DGD values. Thus, it is clearly expected that the outage probability can be

substantially reduced by the use of solitons. The observed behavior was also confirmed through numerical

simulations, which indicated a reduction of rms-pulse width variation of about 50% relative to that with linear pulses.

We have very recently also conducted similar experiments with Dispersion-Managed (DM) solitons [11]. We used a 500 km long link with dispersion maps consisting of alternating sections of about 27 km standard SMF (GVD = 16 ps/(km nm)) and 4 km of DCF (GVD = -104 ps/(km nm)). Link average GVDs of 0.1 and 1.0 ps/(km nm) were

studied. EDFAs were placed at every second map (L^^p = 62 km). We studied the PMD robustness for different pulse

widths ranging from 11 ps to 22 ps and found in all cases significant PMD robustness similar to the constant GVD

case described in Figures 6-7.

In conclusion, it now is clearly evidenced that solitons have, from a systems viewpoint, a very interesting feature in

its robustness to PMD. It applies to conventional solitons as well as to DM solitons. This feature may be very useful

to combine with other techniques to compensate for the effects of PMD to enhance system performance further. We

believe part of the success of our 40 Gb/s and 80 Gb/s soliton field transmission experiments relies on this feature.

Acknowledgements

The author wishes to acknowledge the contributions from, P.O. Hedekvist, B.E. Olsson, B. Bakhshi, J. Hansryd, E.

Kolltveit, J. Brentel, A. Boyle, M. Karlsson, H. Sunnerud, X. Zhang, J. Li, and C. Xie. This work was financially

supported by the ACTS project MIDAS, the following Swedish councils; SSF, NUTEK TFR, and Chalmers Center

for High-Speed Technology. Telia is acknowledged for supplying the installed fiber lines, the ORC of Southampton

University for supplying the DS-PM fiber in the ring laser, and Ericsson for supplying DFB-EA chips.

References

1. H. Biilow, "Polarization mode dispersion (PMD) sensitivity of a 10 Gb/s transmission system," Proc. ECOC 96,

TuD3.6, Oslo, 1999

2. S. Lanne et al., "Extension of polarization-mode dispersion limit using optical mitigation and phase-shaped

binary transmission," Proc OFC 2000, ThH3, Baltimore, 2000

3. see e.g. H. Biilow, "PMD mitigation tecnhiques and their effectiveness in installed fiber," Proc OFC 2000,

ThHl, Baltimore, 2000

4. M.N. Islam et al., "Soliton trapping in birefringent optical fibers'. Opt. Lett., v. 14, pp. 101 1-1013, 1989

5. B. Bakhshi et al., "Experimental observation of soliton robustness to polarization dispersion pulse broadening,"

Electron. Lett., v. 35, pp. 65-66, 1999

6. E. Kolltveit et al., "Single-wavelength 40 Gb/s soliton field transmission experiment over 400 km installed

fibre," Electron. Lett., v. 35, pp. 75-76, 1999

7. I. Hansryd et al., "80 Gbit/s single wavelength soliton transmission over 172 km of installed fibre," Electron.

Lett., V. 35, pp. 313-315, 1999

8. M. Karlsson et al., "Long-term measurement of PMD and polarization drift in installed fibers," accepted for publication in J. Lightwave Technol., 2000

9. H. Sunnerud et al., "Technique for characterization of polarization mode dispersion accumulation along optical

fibers," Electron. Lett., vol.34, pp. 397-398, 1998

10. H. Sunnerud et al., "Experimental quantification of soliton robustness to polarization-mode dispersion," to be

presented at ECOC2000, Munich, Germany, September 2000

11. H. Sunnerud etal., "Experimental quantification of soliton robustness to polarization-mode dispersion in dispersion-managed systems," submitted to IEEE Photonics Technol., Lett., 2000

100
Characterization of high-order PMD

A. Eyal, Y. Li and A. Yariv

Dept. ofApplied Physics, California Institute of Technology, M.S. 128-95,

Pasadena, CA, 91125 USA,

Polarization Mode Dispersion (PMD) is currently one of the primary obstacles to ultrahigh speed optical

communication systems. Traditionally, the effects of PMD were described in terms of three descriptors: a

pair of Principal Polarization States (PSPs) and the Differential Group Delay between them (DGD) [1]. At low communication speeds the values of these descriptors at mid-channel are sufficient to fiilly describe the

effects of PMD over the transmitted signal. As the communication speed increases these descriptors can no

longer be assumed constant wdthin the modulation bandwidth. Commonly termed as high order PMD, this phenomenon necessitates the development of analytical tools for its description. Several approaches appear

in the literature for the description of high order PMD. The most common approach is based on the PMD

vector (PMDV), Q , which in the absence of polarization dependent loss (PDL) is a real, three dimensional

vector whose magnitude is equal to the DGD and its orientation is parallel to the Poincare sphere

representation of the fast PSP. According to this approach the descriptors of high-order PMD are the

derivatives of the PMDV with respect to frequency [2]. Another approach is based on a Taylor-expansion

of the Jones matrix of the medium. This approach was used to describe PMD-induced pulse broadening and

compression near the zero-dispersion frequency [3]. According to a third approach the second-order effects

of PMD are studied by constructing a Jones matrix with principal states and DGD that depends linearly on

the frequency [4]. Recently, we introduced a new approach for describing high-order PMD which is based

on an exponential expansion of the Jones matrix of the medium [5]. While this variety of descriptions is

helpful in giving insight to different aspects of the phenomenon it may lead to confusions. In this paper we

define a set of easy-to-measure descriptors that fully characterize high-order PMD. Our goal is to provide a

consistent system of terms which can be adopted by the optical community for characterization and

measurement of high order PMD. The method we use is a generalization of Poole and Wagner

'phenomenological approach' [1]. It is based on a recursive procedure in which the descriptors of the n +

order are defined by using the descriptors of lower orders and the « + l frequency derivative of the Jones

matrix of the medium. The first-order descriptors are the well known PSPs and the DGD between them and

the second order descriptors corresponds in a simple manner to the frequency derivative of the PMDV, .

In each recursion stage, e.g. the k + \ stage, we first analytically cancel the effects of the ^'th -order PMD

by mentally introducing a compensation system after the transmission medium. The compensation system

is defined in terms of the A:' th -order descriptors. The ^ + 1 descriptors are then defined by identifying two

PSPs which satisfy the condition that when the frequency is changed they show no variation up to the k'th

order in frequency. The third descriptor of this order is the difference between the propagation parameters

of these PSPs.

101
Let the field at the input of a linear transmission medium be described by: = ^e""e,„ where e,„ is a unit

Jones vector representing the input state of polarization (SOP), co is the optical frequency and A is the

complex amplitude of the field. Using Jones' formulation, the output field is given by: = T((o)£,„

where T(co) is a fi-equency-dependent 2x2 complex matrix. It is convenient to choose a fi-ame of reference

in which T(a)o) = I where cOq is the frequency at mid-range and I is the identity matrix. In this frame of

reference the variations of the output field as fiinction of fi-equency near cOq are described by:

E'J =Tl^4=TX (1)

The first-order PSPs are derived fi-om the requirement that the SOP at the output of the medium will be

independent of frequency to first order in i2 = (co-cOq). Namely, it is required that the first order variation

(with fi-equency) of the output field will be parallel to the output field. This is described by the following

eigen-value equation:

Where a, is scalar. The first order PSPs, e,^ , and their corresponding propagation parameters, a,+ , are

then respectively the eigen vectors and eigen values of N, = . Usually, we are interested in the

difference between the propagation parameters, Aa, = a,^ - a,_ , the imaginary part of which is the DGD

and the real part corresponds to the differential loss/gain dispersion of the medium. It is possible to cancel

the first order PMD by breaking the output-signal into two components which are aligned with the PSPs,

propagate them in media with propagation parameters which are equal and opposite to a,^ and then

recombine the components. Analytically, such a compensation system is described by:

C, (q) = P,Q, (a)"' ?,-' = exp(- QN,

)

where P, and Q, are 2x2 matrices defined by:

0 (4) 0

The compensated medium is described by T,''(o)) = C,(co-cOo)t(co). It can be easily verified that

= O (O is the zero matrix). In such a medium any polarization state is a first-order PSP,

however there are, in general, only two polarization states that are stationary to second-order in frequency,

i.e their corresponding Jones vectors satisfy: E"] = o.2E^^, . Like the first order PSPs these second-order

l(D|l

PSPs satisfy an eigen-value equation:

d'X r - i

a(0

102
^2=1^2. £2-] . Q2 =

0 (7)

The descriptors of second order PMD are thus the eigen vectors of = d^X {(o)/ d(0^\ and the difference

between the eigen values, Attj = a^^ - a^, . Again we can ehminate the second-order effects by

transmitting the output of T,''(cl)) through a compensating medium whose Jones matrix is given by:

C,(Q)= P,Q,(Qr P,-' =exp(-lQ^Nj

where:

0 e'

It has been shown in [6] that the Poincare sphere representations of the second-order PSPs are parallel to

the derivative of the PMDV, Q' , and ImjAaj} is equal to |Qj . In general, this correspondence does not

extend to higher orders.

The second-order compensated medium is described by T^ (to) = 02(00 -cOq)T,^(co) and satisfies

d^"h:H(o)/doi^"^ =0 for n -\,2 . The recursive procedure of descriptors-definition can be summarized

as follows: The k'th -order PSPs and their corresponding dispersion parameters are respectively the eigenvectors and eigen-values of the characteristic matrices, s (/'**Tj''_,(co)/c?co'*' , where T^(cl)) , the k'th -

order compensated medium, is defined by:

To^(co) = T(co)

T;(co)=exp

1 / u

• —(cO-COq) 7- kr °' dd)'

t;_,(co)

(8)

Using this definitions it is straight forward to show that the compensated media satisfy:

T;((Oo)=I foralU's

dai = 0 for l<j<it

Thus,

(9)

(10)

(11)

T;(cOo+Q)-To-'e-"^'e-^"''\..e-""'"'T(a)„+f2) = I + o{Q*^^').

which leads to exponential expansion of T:

A method for measurement of the characteristics matrices is described in [7]. It is based on the fact that

can be expressed in terms of the derivatives of T(co) at co„ [3]. Thus the measurement of or

equivalently the high order PSPs and the corresponding difference in propagation parameters is equivalent

to measuring the derivatives of the Jones matrix.

103
It is important to notice that this method of characterization is not unique. One can obtain infinitely many

sets of descriptors in a similar manner. Each set of descriptors corresponds to a different compensation

protocol. For example, placing the compensating systems before the medium instead of after it leads to a

different set of descriptors. The non-uniqueness of the descriptors is a result of the fact that the

transmission matrix of the medium and its derivatives are non-commuting operators. This is a fundamental

characteristic of PMD that can not be avoided. The most simple example of this characteristic appears

already in the case of first-order PMD: in general, a polarization dispersive medium posses different input

PSPs and output PSPs (in this paper we avoided this issue by defining T(a)o)= I ). Given that no unique set

of descriptors exist one is free to choose the set that is most suitable for the application. The set that we

chose is favorable since it leads to an insightful description of high order PMD in the time-domain [3]. The

set is especially useful in quantifying the distortion of pulses due to second-order PMD.

In summary, a generalization of Pooles and Wagner 'Phenomenological approach' was used to define highorder PSPs and the corresponding propagation parameters. The measurement of these descriptors is a

straight forward procedure which is equivalent to the measurement of a Jones matrix and its derivatives. It

can be performed using standard, commercially available polarization analysis equipment. There is a

simple correspondence between the proposed descriptors of the first two orders of PMD and the widely

used PMDV and its frequency derivative. This correspondence does not extend to higher orders. The

defined system of descriptors can be used to fully characterize the high order effects of PMD.

Reference:

1. CD. Poole and R. E. Wagner. Phenomenological approach to polarization dispersion in long singlemode fibers. Electronics Letters, vol. 22 (no. 19): pp. 1029-1030, 1986.

2. P. Ciprut, B. Gisin, N. Gisin, R. Passy, J. P. Von der Weid, F. Prieto, and C. W. Zimmer. Second order

polarization mode dispersion: Impact on analog and digital transmissions. Journal of Lightwave

Technology, vol. 16 (no. 5), pp. 151-111, 1998.

3. CD. Poole and C. R. Giles, Polarization dependent pulse compression and broadening due to

polarization dispersion in dispersion shifted fibers. Opt. Lett. vol. 13, no. 2, pp. 155-157, 1988.

4. C Francia, F. Bruyere, D. Penninckx and M. Chbat. PMD second order effects on pulse propagation in

single mode optical fibers. IEEE Photonics Technology Letters, vol. 10 (no. 12), pp. 1739-1741, 1998.

5. A. Eyal, W.K. Marshall, M. Tur and A. Yariv, Representation of Second Order Polarization Mode

Dispersion, £/ec.Le«. vol. 35 no. 19, pp. 1658-1659 1999

6. A. Eyal, Y. Li, W.K. Marshall, M. Tur and A. Yariv. Statistical determination of the length

dependence of high-order PMD. Accepted for publication in Optics Letters, July 2000.

7. Y. Li, A. Eyal, P-0 Hedekvist and A. Yariv. Measurement of high order polarization mode dispersion.

Accepted for publication in IEEE Photonics Technology Letters, July 2000.

104
Measurements of Enhanced BER fluctuations due to Combined PMD and

PDL Effects in Optical Systems

N. Gisin and M. Wegmuller

Group of Applied Physics, University of Geneva, 1211 Geneva 4, Switzerland

A. Bessa dos Santos, J. P. von der Weid

Centre for Telecommunications Studies

Pontificia Universidade Catolica do Rio de Janeiro, 22453-900, Rio de Janeiro,

Brazil

Abstract

Polarisation effects that can't be explained neither by PMD nor by

PDL alone have been predicted. Moreover, theoretical models and

simulations suggest that these combined PMD+PDL effects may

significantly increase the fluctuations that come along with

polarisation effects. We present measurements of BER fluctuations

in a concatenation of PMD and PDL elements. The result confirms

that combined PMD+PDL effects may introduce fluctuations larger

than those estimated by simple rms rules.

The fact that single mode fibres actually support two propagating modes, the

polarisation modes, is nowadays well known [1-3]. Polarisation effects have been

classified in two categories, the Polarisation Mode Dispersion (PMD) and the

Polarisation Dependent Losses (PDL). The first one causes differential group

delays, while the second one causes differential attenuation. Both could limit the

transmission bandwidth, like any dispersion or attenuation phenomena. PMD has

been widely studied. But with future networks incorporating many components

like amplifiers, add-drops, multiplexers, etc, with non-zero PDL, the study of the

latter is timely [4, 5]. The interplay between PMD and PDL is subtle, much more than usually thought.

Linear attenuation and chromatic dispersion can well be described independently,

but this is not the case of their polarisation-relatives! Indeed, as is well known,

PMD results not only from birefringence, but also from polarisation mode

coupling [6]. The latter strongly depends on the relative orientation between the

Polarisation State at the output of one fiber section and the birefringence axis of

the next one. But components with PDL do change the polarisation states, hence

modify the polarisation coupling. Actually, the phenomenon is made more

complex by the following fact: PDL does not merely rotate the polarisation

states, it transforms them in a more complicated way [7]. Consequently, there is a need to study, understand, measure and master combined PMD + PDL effects. According to previous studies [8-12], these combined effects are not important in today's communication systems but could significantly affect future optical

networks. In [8] a model of PMD + PDL that extends the well-accepted principal

polarisation state model of PMD was developed. In [9, 10], this model has been

105
tested in some special situations (admittedly, the values used for these tests do

not correspond to values expected in telecom systems, but the purpose of these

experiments was simply to test the theoretical model). In [11], numerical

simulations based on this model predicted significant BER penalties due to combined PMD + PDL effects. In this letter we present BER measurement

results on fibre artefacts with realistic values comparing experimental situations

with PMD only, PDL only, and with both PMD and PDL.

The experimental set-up is shown in Fig. 1. The PMD devices were pieces of

HiBi fibre with 22 ps PM delay each, both immersed in a thermal bath in order to maintain their birefringence stable. The PDL device was a thin (100 \im) glass

plate inserted with a variable angle in a collimated beam. The variable attenuator

adjusts the optical power transmitted through the test and reference paths to be

the same for each measurement. The optical switches SI and S2 commute the

measuring system between the test and reference optical paths. The third switch

commutes the signal between the power meter and the BER receiver. APC

connectors were used to avoid interferometric noise. For each test path

measurement a new reference point was measured. A PRBS sequence of 2^^- 1 bits at 2.488 Gb/s was used and the BER values were determined after counting

200 errors. The various Polarisation Controllers (PC) allow to randomly varying

the polarisation mode couplings. For each configuration of the test path the

statistics of the BER while the PCs are randomly varied is recorded and the

standard deviation a computed. In order to make sure that we measure

polarisation effects and not effects due to power fluctuations, for each

configuration of the PCs we continuously recorded the power immediately before

the BER-meter and adjusted the source power to maintain this power at a given

fixed level. The level was chosen to obtain BER values close to 3x10"^ at the

reference path. In order to evaluate the effects of the PMD and PDL combination we first measured the BER statistics for pure PMD and pure PDL cases and then

introduced all components in the link to measure the statistics due to the

combination. Figure 2 presents the BER statistics for the isolated PMD and PDL

(1 dB) cases and for the combined PMD and PDL, together with the reference

path measurements. The reference statistics is sharply peaked, with its width

<^REF ^ 0.07 determined essentially by the number of counted errors. The same

behaviour (Gpdl = 0.08) was observed for the isolated PDL case, meaning that no

distortion is induced by PDL in the absence of PMD. Pure PMD indeed

introduces a distortion which is polarisation dependent, as it is clear by the wider

distribution of the BER values (Gpmd = 0.27). This distortion is given by the

variable delay due to the combination of two HiBi fibres with their axes randomly oriented, plus the random input state of polarisation.

Figure 2 (c) displays the BER statistics for the combined PML and PDL case. A

significantly wider distribution is now observed: Cpmd+pdl 0.50. Note that it is also much larger than the RMS value: c^pmd+pdl > <S^pmd + ^^pdl which is approximately 0.28. This confirms that PDL distributed along a birefringent fibre link can produce abnormal effects on the BER! The effects are abnormal in the sense that they are larger than expected from simple addition or RMS rules. Note

that the abnormal effect may result in larger as well as in smaller BER values [8]. Hence, the mean value of the BER is not affected, but the fluctuations are

significantly increased, in good agreement with our numerical simulation [11].

106
Moreover, the magnitude of the effects is already relevant even for PMD values

as low as the usually admitted 10% of the bit slot. In conclusion, we tested the model predictions in a simple combination of PMD

and PDL devices with values close to common Telecom optical components. The

observation of important effects indicate that the safety margins of future systems may have to be adjusted to take into account the combined PMD + PDL

effects that will affect future networks incorporating large numbers of

components, each with a small PDL.

Acknowledgements

This work was partially supported by the Swiss OFES within the European

COST 265 project, and by CPqD Foundation (contract CTO/P&D/067/98),

Brazilian National Research Council (CNPq) and FAPERJ.

References

1. CD. Poole, R. W. Tkach, A. R. Chraplyvy e D. A. Fishman, "Fading in lightwave systems due to polarization-mode dispersion", IEEE Photon. Technol. Lett., vol 3, 1, pp 68-70, 1991.

2. E. lannone, F. Matera, A. Galtarosa, G. Gianello and M. Schiano, "Effect of Polarization Mode Dispersion on the performance of IM-DD Communication Systems" IEEE Photonics

Technol. Lett., vol. 5, 10, pp 1247-1249, 1993.

3. P. Ciprut, B. Gisin, N. Gisin, R.Passy, J.P. von der Weid, F. Prieto and C. Zimmer, "Second

order polarization mode dispersion: impact on analog and digital transmission", J. Lightwave

Tech. 16, pp.757-771, 1998.

4. A. Eyal and M. Tur, "Measurement of polarization mode dispersion in systems having

polarization dependent loss or gain", IEEE Photon. Techno. Letts. 9, pp. 1256- 1258, 1997.

5. A. Elamari, N. Gisin, B. Pemy, H. Zbinden and C. Zimmer, "Statistical prediction and

experimental verification of concatenations of fiber optic components with polarization

dependent loss" , J. Lightwave Tech. 16, pp.332-339, 1998.

6. CD. Poole and R.E. Wagner, "Phenomenological approach to polarisation dispersion in long

single-mode fibres". Electron. Letts. 22, pp. 1029- 1030, 1986.

7. N. Gisin, "Statistics of PDL", Opt. Comm. 1 14, pp.399-405, 1995.

8. N. Gisin and B. Huttner, "Combined effects of polarisation mode dispersion and polarisation

dependent losses in optical fibers", Opt. Comm. 142, pp.1 19-125, 1997.

9. B. Huttner and N. Gisin, "Anomalous pulse spreading in birefiingent optical fibers with

polarisation-dependent losses", Opt. Letts. 22, pp.504-506, 1997.

10. B. Huttaer, C. De Barros, B. Gisin and N. Gisin, "Polarisation-induced pulse spreading in birefiingent optical fibres with zero differential group delay". Opt. Letts. 24, pp.370-372,

1999.

1 1 . B. Huttner, C. Geiser and N. Gisin, "Polarization-induced distortions in optical fibre networks with polarization mode dispersion and polarization dependent losses", J. Quantum

Electron., in press, 2000.

12. L. Chen, J. Cameron, X. Bao, "Statistics of PMD in presence of PDL in single mode

fibers", Optics Commun. 169, 69-73, 1999.

107
Reference

Optical ' - -

Attenuator S1 — HiBi — PDL

Test

Figure 1

Experimental set-up for BER measurements.

-9 -8 Log[BER]

Figure 2

BER statistics for the pure PDL (a), pure PMD (b) and for the combined

PMD+PDL (c) cases. Black bar graphs correspond to reference measurements.

108
Reflectometric characterization of distributed

birefringence in installed single-mode fibers

Massimo Artiglia (1), Marco Schiano (1), Tiziana Tambosso (1)

Andrea Galtarossa (2), Luca Palmieri (2), Anna Pizzinat (2),

(1) CSELT, Via G. Reiss Romoli 274, 10148 Torino, Italy (massimo.artiglia@cselt.it)

(2) DEI, Universita di Padova, Via G. Gradenigo 6/A, 35131 Padova, Italy

Abstract

We describe local birefringence measurennents perfornned with a polarization sensitive OTDR on installed singlemode fibers. Results indicate that G.652 fibers are, on average, less birefringent than G.653 fibers.

Nowadays research activities on PMD are mainly focused on two mayor topics. The first one is the analysis of PMD

impairment on system performance [1] and includes the design of PMD compensators and emulators [2, 3]. The

second topic is the local characterization of the fiber random birefringence and is aimed to a deeper understanding

of the relationship between fiber birefringence and PMD behavior.

Distributed fiber birefringence has been studied and described by means of several theoretical models, but only

recently it has become the subject of experimental characterizations, based on a polarization-sensitive OTDR

technique (POTDR) [4]. Using this technique, the local beat length (i. e. the inverse of the birefringence vector modulus) of long single-mode

fibers has been measured for the first time. The measurement of local beat-length is very important especially for

fibre and optical cable manufacturers, since it may improve the control of fibre production or the control of cabling process in order to limit the effect of PMD. This aspect has become essential for new fiber installations due to the

stringent requirements of next generation high-speed transmission systems.

Results of laboratory tests performed on fibers wound on drums were reported in ref. [4]. In this paper, we present

the results of the first beat length measurements performed on installed optical fibers.

Measurement principle

The general feature of a POTDR equipment is a polarization analysis of the backscattered light from the fibre under

test. When an optical pulse is launched into an optical fiber, its state of polarization (SOP) evolves randomly as a

function of the pulse position inside the fiber. Consequently, also the backscattered field SOP varies at random.

Thus, if a fixed polarizer is placed in front of the POTDR photodiode, the SOP random evolution produces

fluctuations in the detected power. The basic idea, described in ref. [5], is that the fiber local beat length can be

calculated from the level crossing rate (LCR) of the backscattered power passing through a polarizer, exploiting a

further refinement of the statistical theory already developed for the "fixed-polarizer" PMD measurement technique.

In ref. [5] it is shown that the signal T{z) , representing the normalized backscattered power passing through a

polarizer, can be expressed as:

where R{z) is the Muller matrix of the fiber, M = diag{^,^-^) , and Sg{z) , Sq and p are the Stokes unit-vectors representing the SOP of the backscattered field, the input SOP, and the fixed polarizer, respectively. The following

step is to assume p = Sq (i.e. the input SOP and the polarizer SOP are the same), so that eq. (1) simplifies to:

Introduction

(1)

(2)

109
where Sf{z) = R{z)Sq is the Stokes unit-vector of the forward propagating field SOP. Since the statistical properties of Sf{z) are well known, we can calculate the statistical properties of T{z) . In Particular, the spatial

frequency of the backscattered power fluctuations is related to the beat length of the fiber by the following

expression [5]:

(3)

where v represents a generic level between 0 and 1, and n{v) is the mean number of times T{z) crosses the

level V per unit fiber length.

The condition p = Sq can be simply attained by placing a linear (bi-directional) polarizer just before the fiber input.

In spite of its simplicity, this solution suffers from a mayor drawback: the measured backscattered power

fluctuations are originated not only by SOP variations, but also by fiber local loss and fading noise due to the pulse

coherence. Since it is impossible to separate or compensate for these noise contributions, a more sophisticated

experimental set-up is needed. As shown in fig. 1, an effective solution is to measure the complete SOP of the backscattered field, but this requires some further theoretical considerations. Actually, we have to take into account

the optical splitter connected to the fiber under test (it can be a circulator as well as a directional coupler) which

separates the forward path from the backward one. Referring to fig. 1, we can define two useful Muller matrices. R^2 is the Mulier matrix representing the optical path from the input of the splitter (point 1 in fig. 1) to the point

where forward and backward path separate (point 2). is the Muller matrix of the path from point 2 to the input

of the polarization analyzer (point 3). The SOP measured by the analyzer can be expressed as follows: Sg^(z) = R23RB{z)R]2^in . where Rb{z) is the matrix representing the round-trip propagation in the fiber [5], and

s,n is the SOP at the input of the splitter (point 1). It can be noted that there is no control on the launched SOP,

since, as we will show, the knowledge of is not necessary. It is well known that the backscattered signal keeps

memory of the initial SOP, and as a matter of facts ^f?g(z)^ = (M/3) , where ^-^ represents the ensemble average.

If we assume that Rgiz) is ergodic (as can be easily numerically verified), then the ergodic average is equal to the ensemble one, and we can write:

where (^ j^ represents the ergodic average. Now it is easy to show that the signal T{z) can be calculated from

Sgfj^ only (i.e. from the SOP measured by the polarization analyzer only). It should be noted that /-?i2^;n is fhe SOP of the field at the fiber input (i.e. point 2): /?i2^/n = ^o • Moreover Rgiz) = MR^ {z)MR{z) , and recalling that R23R23 is fhe identity matrix (as for any Muller matrix), it results:

3(se„(z))^ Sb^{z) = {R23MR,2S;n)- (^aaf^e^iaS/n) = {R23MS0V R23{MR^ MR)So = s,(z) [Ms,(z)]

Finally, the complete procedure for local beat length measurement is:

1 . to measure the SOP of the backscattered field Sb^{z) ;

2. to calculate T{z) accordingly to eq. (4);

3. to perform the LOR analysis of T{z) according to eq. (3).

Experimental set-up

The POTDR experimental set-up is shown in fig. 1. A high-resolution (64,000 sampling points) commercial OTDR

was used to process and record the evolution of the backscattered power. Since the OTDR source optical

bandwidth is too wide for POTDR measurements, the optical pulses were reformed by using a directly modulated

external cavity laser followed by an erbium-doped fiber amplifier (EDFA). An acousto-optic modulator, placed after

Now we can state that the signal T{z) can be calculated as follows (the z dependence is omitted):

110
the EDFA, is necesary to block the spontaneous emission of the EDFA when the OTDR pulse is off. The

backscattered field coming from the fiber under test through an optical circulator, is sent to an adjustable

polarization analyzer and then to the OTDR detector. The polarization analyzer is composed by a rotating 'kj^ waveplate followed by a fixed analyzer. To perform the backscattered field polarization analysis, 6 POTDR traces

are acquired corresponding to 6 different positions of the rotating waveplate. Than the polarization ellipse is calculated for each point of the fiber, using a least square approximation algorithm. The interpolation error, averaged over the entire fiber length, is of the order of 2%.

This set-up lets to launch into the fiber under test pulses with a peak power sufficiently high to allow measurements

on fibers more than 30 km long. The operating wavelength was 1532 nm. The pulse width was set to 5 ns, in order

to achieve a spatial resolution of about 0.5 m that is consistent with the sampling step (0.6 m for the 40 km

distance range). Based on our experience, this is highest resolution compatible with reliable measurements of the beat length (down to a few meters) that can be found in some high PMD telecommunication fibers.

Measurement results

Measurements were performed on a 23.6 km long cable, installed about 5 years ago between the ISCTI laboratory

(Istituto Superiore delle Comunicazioni e delle Tecnologie dell'lnformazione) located in Rome, and a shelter located

in Pomezia. The optical link is composed of 10 cable sections spliced together. The first 9 sections are about 2.15 km long, the last one is 4 km long. The cable contains 80 single-mode fibers of three different types: 30 ITU-T G.652 standard step-index fibres, 30 ITU-T G.653 dispersion shifted fibres, and 20 ITU-T G.655 nonzero dispersion

fibres. Out of these, seven G.652, seven G.653 and four G.655 fibers were characterized in terms of beat length.

We have applied the LCR analysis separately on each section of the link, because the sections show nori uniform

characteristics. For example, the upper graph of fig. 2 shows the evolution of the normalized power measured on a G.652 fiber (random fluctuations are evident). The lower graph shows the corresponding evolution of the beat

length.

More results regarding G.652 fibers are shown in fig. 3. Each line refers to one fiber, and represents the evolution of the beat length averaged over the length of each section. Similar results are shown in fig. 4, referring to G.653

fibers, and in fig. 5, referring to G.655 fibers.

It can be seen that G.652 fibers are characterized by beat length values ranging between 10 and 110 meters. On

the contrary, the beat length of G.653 (with the exception of section 4) and G.655 fibers assumes values between

10 and 30 meters. Hence, these two kinds of fiber are more birefringent. This could have been foreseen since

G.653 and G.655 fibers have a more complicated index profile, which may induce a higher intrinsic birefringence.

Furthermore, G.652 fibers have lower intrinsic birefringence and consequently they should be more sensitive to

extrinsic random perturbation. This may be the reason why they show larger fluctuations in the beat length values,

compared with the other two kinds of fiber. As far as the anomalous behavior of fibers in section 4 is concerned, it is unlikely to ascribe it to some sort of external perturbation. In fact it is hard to believe that a perturbation can at the same time increase the birefringence

of G.652 fibers, decrease that of G.653 fibers and leave unaffected the G.655 ones. Most likely, section 4 of the cable behaves in a different way simply because its fibers may come from different production batches; however, at the time being we do not have any information supporting this hypothesis.

We have also measured the DGD of the fibers under test. On average, the PMD coefficient of G.652 and G.653

fibers is 0.032 and 0.231 ps/km^'^, respectively. Such a result agrees with the larger birefringence of G.653 fibers. On the contrary, even if G.655 fibers have almost the same birefringence as G.653 fibers, their average PMD

coefficient is as low as 0.061 ps/km"^ This result may be explained assuming that G.655 fibers have shorter

correlation lengths with respect to G.653 fibers.

Acknowledgements

Authors are indebted to ISCTI for permission to use the Roma-Pomezia link and to S. Cascelli, M. Guglielmucci and

L. Lattanzi for technical help during measurements.

A. Galtarossa, L. Palmieri and A. Pizzinat also thank IST/ATLAS project and MURST 40% project (prot. 9909198913) for supporting their activity.

References

[1] W. Weiershausen, R. Leppla, F. Kuppers and H. Scholl, 25"" ECOC (invited paper), Nice, (1999).

[2] F. Heissman, 24'^ ECOC (tutorial paper), Madrid, Spain (1998).

[3] C. R. Menyuk, 25'" OFC (tutorial paper), Baltimore, Maryland (2000).

[4] A. Galtarossa, L. Palmieri, M. Schiano and T. Tambosso, Opt. Lett., vol. 26, 384 (2000).

[5] F. Corsi, A. Galtarossa, L. Palmieri, Journal of Lightwave Technology, Vol. 17, 1999.

111
2.2 4.4 6.7 8.7 10.9 13 15.2 17.4 19.5 z [km]

23.6:

2.2 4.4 6.7 8.7 10.9 13 15.2 17.4 19.5 z [km]

23.6

Figure 2: Evolution of the normalized power (upper window) and the corresponding beat length profile

(lower window) of a G.652 fiber link.

Exlerna]

cavity laser

Pulse generator

Photodiode

P.G. EDFA

Optical

amplifier

Acusio- optic modulator

OTDR Polarization ^

,©

To nber-crible under test anahv.er Figure 1 : Scheme of the experimental set-up Figure 4: Beat length evolution along seven

installed G.653 fibers.

112
Accurate Spectral Characterization of Polarization Dependent Loss

R. M. Craig

National Institute of Standards and Technology, 325 Broadway, Boulder, CO 80303

reference are presented. This artifact is intended

to become a NIST Standard Reference Material

(SRM) with a nominal PDL of 0.1 dB for the

calibration of commercial instrumentation.

Concept

This method is a further variation of a matrix

technique developed by Favin et al. [4], and relies solely on power ratio measurements over a wavelength range using specific polarization

states. As in reference [2], four well defined

polarization states are necessary to determine the

first-row Mueller matrix elements of a component.

The global polarization dependence of

transmittance can be determined from these

(wavelength dependent) matrix elements.

Abstract - Building on previous work, a rapid,

automated, non-mechanical measurement system

for spectral characterization of polarizationdependent loss is presented. A deterministic

fixed-states method in conjunction with real-time

calibrated spectral information is used to infer

wavelength-dependent Mueller matrix elements

from power ratios at specific polarization states. Voltage-modulated liquid-crystal variable

retarders set the input polarization states. A

narrow, voltage-tuned filter generates a wavelength sweep from a broadband source. The

sweep is calibrated in real-time by hydrogen

cyanide reference lines. Actively balanced

photodetection is employed to achieve low noise

signals. I present polarization-dependent loss measurements from 0.0003 to 0.30 dB at 1550 nm

to verify performance and also present results from artifact calibration standards of an all-fiber design.

Introduction

Until the advent of dense wavelength-division

multiplexing (DWDM) systems, polarization

dependent loss (PDL) was usually characterized

only at a specific wavelength [1]. More recently,

however, the wavelength dependence of PDL

(WPDL) has assumed greater importance. A

modification of a previously reported,

deterministic fixed-states technique [2] has been

implemented that allows the simultaneous

characterization of wavelength dependence. The

goal is to establish a capability for rapid

measurement of WPDL to a resolution finer than

0.005 dB without sacrificing

accuracy. This approach uses a non-mechanical technique

employing ratio detection and

simultaneous spectral calibration

that is faster than most tunablelaser systems and more accurate than optical spectrum analyzers

[3]. In addition, new results from

an all-fiber WPDL artifact Figure 2: Diagram of swept wavelength source with HCN caHbrator (Sweep'Source)

Figure lA: Initial Poincare Figure IB: LCVR trajectories

trajectories of the liquid- following birefringent

crystal variable retarder displacement

(LCVR) pair. Because the measurement depends only on the

relative coordinates of the four states, the only

requirements on the set are that they maintain

relative angular separations of 90° about the origin

of the polarization (Poincare) sphere, cf. Fig 1

The four polarization states are produced by

FFP Tunable Filter io/90 Splitter 10/90 Splitter Sweep Output

Grating Tuned

Double-Pass ASE

Master Synch. Input

HCN Gas Cell Reference Calibration Signal

113
two liquid-crystal variable-retarder (LCVR) units

in series that hold the state during a wavelength

sweep of a low-coherence polarized beam. The

beam is generated by a double-pass amplifiedspontaneous-emission (ASE) source followed by a fiber-Fabry-Perot tunable-filter (FFP-TF) driven

by a ramp generator. The voltage ramp drives the

filter across a selectable portion of the ASE

spectrum in a repetitive sweep. This swept output

passes through splitters and a NIST hydrogen

cyanide (HCN) optical wavelength reference cell (SRM 2519), as shown in Fig. 2, which produces

intensity modulated output. The HCN absorption

0.4

0.2

-0.2-

• HCN Spectra Samples — Peak vs. Sample Fit

' 1546

200

1551

1541

1536 0 100 200 300 400 Sample Na

Figure 3: Typical P-branch HCN absorption signature from

1539 to 1551 nm with a quadratic calibration fit

spectrum is measured by an amplified differential

photodetector capable of 50 dB common-moderejection through active gain balancing. The

absorption peaks are fit by a least squares

quadratic to known

peak wavelengths

following each scan, providing real-time

spectral calibration

of each sample

point. An example

of the absorption

signature and the

subsequent peak fit are shown in Fig. 3. Following the

LCVR units, the

light is transmitted

by a single mode

fiber with arbitrary but relatively stable

birefringence through a splitter to both the deviceunder-test (DUT) and to a second actively

balanced photodetector. This detector is operated

in a logarithmic ratio mode and senses the four

output power ratios (I\(X)...h(X)). Input power

ratios (/a(A,).../d(A,)) are measured in the same way

with the DUT removed to establish a baseline

system response. Following wavelength

registration to account for drift, the four first-row

Mueller matrix elements (mii(A,) ... mi4(X,)) are calculated from these power ratios as in reference

[2]. Information about the global transmittance

extrema are contained in the first row matrix

elements. The transmittance extrema, TmniX) and

TmaxiX) are

Tm,. {^) = "hl {^)-4^2 {^f +^^3 {^)^ +"hA {^f

SO WPDL = PDL(A) = 101og[7_ (A)/r^„ (A)]

which applies over the entire polarization-state

space and wavelength range. Implementation

Figure 4 shows a schematic of the

measurement system. The system consists of five

major sections: ASE sweep source, cf. Fig. 2,

LCVR polarization state generator, measurement

and calibration section, clock, and control

computer. The polarization-modulated light is connected to the DUT through a spectrally

flattened 80/20 splitter and the power ratio is sampled through single-mode fiber and fiber connectors. Prior to any measurement, the empty

Open Beam State Generator

ASE Sweep Source LCVR 1 .451.

Out

LCVR 2 Measurement and Calibration Section

GRIN

80/20 Splitter

Signal 20% Ref Open Beam

Calibrator (Fresnel ref. inserted)

DUT Baseline Switch

Log Ratio Detector

Computer Figure 4: Simplified diagram of the measurement system

114
calibration cavity is switched directly to the

detector for an initial baseline after which the

Fresnel reference block is inserted at the

appropriate angle to generate a calibration signal.

Anti-reflection coated aspherical lenses are used

to collimate the open beam and a polished BK7

glass cube acts as the Fresnel reference. The

calibration signal is compared to the calculated

value to give a calibration multiplier for all subsequent measurements during that session.

Following calibration, the Fresnel reference is removed and another baseline is run for the DUT.

The DUT is then inserted and switched in. The

DUT output is depolarized with 40 m of high NA

(0.37) multimode fiber to reduce the impact of

PDL in the detector. Finally, the DUT WPDL

signal is sampled at 450 points for each 12 nm

wavelength scan. A scan period of 285 ms is required for each polarization state, after which

the state is changed to the next in the series. An

entire WPDL scan over the range of 1539 - 1551

nm takes less than 5 seconds including data

processing time, which is currently the limiting

factor.

Uncertainties

The design of this measurement system is both

simpler and more sophisticated than that described

in ref. [2] so that most uncertainties are now

reduced by taking ratios. Items 1 and 2 below

outline the two major systematic uncertainties in polarization and power that, together, comprise

over 90% of the total systematic contributions.

Discussion of minor effects will be deferred to a forthcoming publication. Values listed correspond

to lo deviations in either signal power or

polarization at 1545 nm.

1. Polarization state accuracy. Polarization

state accuracy depends on both the measuring

polarimeter and the LCVR bias calibration. I find that linear states exhibit an uncertainty of

0.69 % while the circular states have a 2.0 %

uncertainty.

2. System internal PDL variation. The system

retardance elements contribute their own PDL

(0.02 dB) to that of the DUT. Given a maximum observed drift of the internal PDL

over 5 minute intervals, this yields an

uncertainty of 0.025 %.

3. Propagated measurement system uncertainty.

Using the uncertainties above inherent to the

measurement system, an error propagation

model, based on the defining equations,

calculates 0.0035 dB at 0.1 dB of PDL.

4. PDL measurement repeatability. Ten

undisturbed measurements of a 0.12 dB BK7

WPDL were made following the initial baseline measurement. The random la

uncertainty averaged over all wavelengths

yields 0.0012 dB as the effective single-sweep

system noise and incorporates all effects that average down such as optical interference and

detector noise. This component does not incorporate residual systematic wavelength

dependence.

5. Combined standard uncertainty. Items 3 and

4 above are assumed uncorrelated, so the root- square-sum (RSS) at the 0.1 dB level is (0.0012 dB + 0.0035 dB) RSS = 0.0037 dB.

6. Connector uncertainty. As in reference [2], a

significant source of uncertainty, which is not

inherent to the LCVR system, is variations in measured PDL of DUT artifacts with

connectorized pigtails following

disconnection and reconnection. This could

be explained by connector alignment (core

offset) errors. For those artifacts, we add an

additional 0.0029 dB uncertainty.

Stress and thermal drifts in pigtail

birefringence must be minimized over the course

of a measurement. The measurement method is sensitive to small errors in the matrix elements,

I

I I I \ \ 1 u

Incidence Angle (deg.)

0.3 -

I III _ "" 'it^'-^n n

• Calibrated Measurement

Calculated Ret. Value

0.1 - - —— ,15

... .

.. . in

I -- - ^ 5

0.0 - '

'

' ' "'"

0

1538 1540 1542 1544 1546 1548 1550 1552 Wavelength (nm)

Figure 5: Open-beam BK7 artifact data calibrated at 30°

incidence. Centerline is the calculated WPDL value. Residual wavelength slope falls within 2a uncertainty.

115
such as those due to rotated, and therefore

uncompensated, PDL between the baseline and

DUT measurement. For this reason, one should

calibrate the system prior to a measurement series.

Results

The primary artifact chosen to test the

performance of the system provides a calculable

PDL of moderate accuracy. The artifact consists

of an open beam launcher/coUimator followed by

a 2.54 cm, polished BK7 glass cube. BK7 is convenient to use and has a well-known index of

refraction. The cube is mounted on a rotation

stage with 5' resolution. The beam pickup asphere

is translated to compensate for beam displacement

following rotation. Calculated (Fresnel) and

measured WPDL values as a function of input

angle are presented in Fig. 5 and are in good

agreement. The residual system PDL was

measured at 0° (normal incidence) as a 0.0014 ±

0.0006 dB offset over all wavelengths. A residual

slope indicates a systematic uncertainty.

All-Fiber PDL Artifact Results

As mentioned in [5], a series of all-fiber PDL

artifacts based on fusion-spliced sections of

single-mode, polarizing, and multimode fiber was

constructed. These artifacts form the basis of a new calibration transfer standard SRM at 0.1 dB.

The artifact section is a few millimeters of

polarizing fiber (18 dB/m extinction) that provides

relatively stable PDL over the range of 0 - 40 °C.

Some recent WPDL measurement results are presented in Fig. 6 using three SRM prototypes.

A nearly constant positive slope is typical of

^0,10 -

1541 1543 1545 1547 Wavelength (nm)

polarizing fiber and is evident, with structure, in the data. This measurement system will be used

to characterize WPDL in SRM artifacts. Conclusions

An accurate, rapid, non-mechanical technique

of characterizing the wavelength dependence of

polarization-dependent loss for both single-mode

and bulk-optic devices has been developed that offers advantages over more traditional methods.

The technique demonstrates a single measurement

uncertainty of 0.0037 dB and 0.0014 ± 0.0006 dB

offset from PDL values calculated for an openbeam artifact. Additional measurement sweeps

can be quickly averaged to improve random

uncertainty by l/ -Jn . Overall accuracy is dependent on calibration to a primary artifact. Three WPDL artifact standard candidates

have been tested and show the expected positive

slope. Measurement resolution is sufficient to discern the wavelength dependent structure.

References

[1] M. Gadonna and A. Mabrouki "Polarization

Sensitivity Measurements Methods for Passive

Optical Components," Conference on European

Fibre Optic Communications and Networks,

EFOC&N '93, The Hague, Netherlands, Technical

Digest, pp. 65-67.

[2] R. M. Craig, S. L. Gilbert and P. D. Hale, "High

Resolution, Nonmechanical Approach to Polarization Dependent Transmission

Measurements," IEEE J. Lightwave Technol., vol.

16, pp. 1285-1294, July 1998.

[3] D. Derickson, ed., Fiber Optic Test and

Measurement. Prentice Hall PTR, 1998, chap 9.5.

[4] D. L. Favin, B. M. Nyman, and G. Wolter, "System

and Method for Measuring Polarization Dependent

Loss," U. S. Patent 5371597, Dec. 6 1994; B. M.

Nyman, D. L. Favin, and G. Wolter, "Automated

System for Measuring Polarization Dependent

Loss," OFC '94, San Jose, California, U.S.A.,

Technical Digest, pp. 230-231.

[5] R. M. Craig, S. L. Gilbert and P. D. Hale, "Accurate Polarization Dependent Loss Measurement and Calibration Standard

Development," SOFM '98, Boulder, Colorado,

Technical Digest, pp. 5-8 (1998).

Figure 6: Wavelength dependent PDL for three SRM

orototvoes. A least-square fit line indicates trend.

116
Heterodyne system at 850 nm for measuring photoreceiver frequency

response

Paul D. Hale^ and C. M. Wang

National Institute of Standards and Technology

325 Broadway

Boulder, Colorado 80303

Abstract

We describe an 850 nm hetCTodyne system for measuring frequency response, that uses commercially

available tunable external-cavity diode lasers. The system's frequency resolution and reproducibility are

characterized. The advantage of a frequency response transfer standard consisting of a photodiode

combined with an rf power-sensor is reviewed. The expanded uncertainty (k=2) for calibration of an 850

nm transfer standard between 5 MHz and 25 GHz is found to be about 1.1 % (0.05 dB).

Introduction

Because of the high demand for test equipment for short-wavelength applications, such as Gigabit Ethernet and Fibre Channel, NIST has constructed a heterodyne frequency response measurement system operating

at 850 nm. Components, such as reference receivers for eye-pattern testing require frequency response

characterized with uncertainties of 0.3 dB or less'. Customers of standards laboratories often prefer that fransfer standard receivers be calibrated with uncertainties that are a factor of 3 to 4 smaller. The use of the heterodyne method for photoreceiver frequency response measurement is certainly not new^' ^' ^' ^' ^; however, description of work applicable to the short-wavelength telecommunications window has been

limited, and reported with an associated uncertainty larger than 0.3 dB^' In this work we report the

calibration of a combined photodiode/rf power-sensor fransfer standard and its expanded uncertainty of

0.05 dB. We estimate the expanded uncertainty for calibration of a stand-alone photodiode to be about 0.1 dB.

Heterodyne principle

The underlying concepts of precision heterodyne photoreceiver frequency response measurements have

been described in detail previously^' ^. The heterodyne beat note at frequency/is generated by combining

beams from two "single-frequency" lasers in a free-space beamsplitter and mixing them on a photodiode.

The photodiode is connected to an external bias network and dc blocking capacitor. We assume the laser beams have the same polarization and spatial mode after passing through the beamsplitter. A dc return path

must be provided for the photodiode, either through a bias T or through a matching resistor or attenuator. The photocurrent generated by the photodiode due to the incident optical power is given by

i,{f,t) = (Po,+Po2)RiO) + 2y[^;P~R(f)cos(2;rft), (1) where R(f) is the responsivity of the detector (in A/W) at frequency / and Pok is the power from the kth

laser. The first term on the right side is the dc photocurrent that flows through the bias supply, and the second term is the rf photocurrent that flows through the rf load (the rf power-sensor) through a dc

blocking capacitor. The mean squared photocurrents generated by the photodiode are

(ii) = iPo^+Po2fi^\0)

(2) (4) = 2/'^.V(/)-

If Pqi is nearly equal to P02, then 2(PoiPo2 )

is, to first order, equal to 0.5(Poi+Po2f- The normalized

frequency response which is defined as R^(f)/R^(0), can then be found by taking the ratio of the rf

electrical power P^f to halfthe dc elecfrical power delivered to a load Zi ,

Supported in part by the Office of Naval Research and the Space and Naval Warfare Systems Center.

117
R'(0) (ii)z,'

(3)

Prf is a function of frequency and includes corrections for the sensor's calibration factor and electrical mismatch, and is the power that would be delivered to an ideal load Zf,=50 Q'*^ The normalized frequency

response is expressed in decibels as 201og[9t(/)].

The ratio on the right side of eq (3) is insensitive to variations in optical power and simplifies the measurement apparatus because only the total photocurrent in the photodiode need be monitored, instead of

the power coupled to the detector from each laser. 1R^( / ) is an unbiased metric for comparing

measurements made with different systems. A common practice when comparing measurements of

frequency response is to add an offset to each curve to make the responses equal at some frequency or over some range of frequencies. This practice can be misleading since it assumes that both systems impart

negligible error at that frequency.

Combined photodiode/rf power-sensor transfer standard

Uncertainty in the measurement of a photodiode's frequency response using eq (3) includes the calibration uncertainty of the power-sensor used to measure Prf Combined with other user uncertainties, this may

give an unacceptable uncertainty for the intended application. The uncertainty can be reduced by

calibrating the frequency response of the photodiode combined with the rf power-sensor and bias T or attenuator used for dc return. This method totally eliminates uncertainties due to power-sensor calibration and impedance mismatch. The combined frequency response

calibration factor and impedance mismatch, and is given by

C

(/) includes the power-sensor's

(4)

hi eq (4), P^ is the indication of the power meter after zeroing and calibration against the 50 MHz reference

signal (from the power meter) using a calibration factor of 100 %. C is a frequency-dependent factor that Open loop

frequency

control

ECL#1

VA P L 6 SMF

Detector

under test I

RF powersensor

Figure 1. Simplified schematic diagram of 850 nm heterodyne system. Frequency confrol and data

acquisition is performed by a personal computer. Labeled components are external cavity laser (ECL),

anamorphic prism pair (AP), isolator (I), beamsplitter (BS), variable attenuator (VA), variable beamsplitter (VB), polarizer (P), lens (L), angled fiber end (AF), Fabry-Perot spectrum analyzer

(FPSA), single-mode fiber (SMF), and electrical spectrum analyzer (ESA).

118
includes power-sensor calibration and mismatch correction factors but does not need to be known. The

relative optical modulation depth of an arbitrary source can be found using a transfer standard with

calibrated M^(/). Uncertainty can be reduced further, at the expense of functionality^, by normalizing

R^(/) to M^(50 MHz) . This normalization eliminates uncertainty due to the power meter's 50 MHz

reference source.

850 nm heterodyne measurement system

A simplified schematic diagram of the heterodyne system is shown in figure 1. The system uses two singlemode semiconductor lasers that are grating-tuned near 850 nm. The lasers are adjusted so that nearly equal

power from each laser is delivered to the device under test (DUT) as measured by the bias-current monitor.

The combined laser beams pass through a polarizer so that the signal from each laser is in the same

polarization state when it reaches the DUT, ensuring nearly 100% modulation depth. The frequency of each

laser can be tuned by tilting the grating with a piezoelectric fransducer to give beat frequencies between a few megahertz and several hundred gigahertz. The voltage applied to the fransducer is controlled by a computer and digital-to-analog converter. The beat frequency is measured with an electrical spectrum

analyzer since the frequency jitter is too large to properly trigger an rf counter. As the frequency is scanned,

data are acquired automatically by the computer.

Resolution

To characterize the frequency resolution of the measurement system, we measured electrical "interference

fringes." The electrical output of a photoreceiver was split in a three-resistor power divider. One arm of

the divider was connected to a long coaxial line terminated by a 45 Q resistor. The other arm of the divider went to an rf power-sensor. The frequency of one of the lasers was swept and the rf power and dc

photocurrent were recorded for four different scans. The variance of the measured signal can be expressed

in a Taylor-series expansion as

<^ToJ « + , (5) where Ojotd is the total measured signal variance, is the additive noise variance, cr/ is the frequency

jitter variance, and is the derivative of the ideal response curve. To estimate the quantities in eq (5), we first selected an appropriate set of frequencies f, where / = 1, 2, ...n. A cubic B-spline was fit to the 4

combined scans. The result is shown in figure 2. For each scan, the frequency response at f, and the first- order derivative ^re estimated using the

B-spline. The sample variance s] at f, is then

calculated from the response of each of the 4

scans. The jitter is the square root of the slope h

of the simple linear fit: sf = a + b{g'(f^)Y . The highest resolution achievable is about 0.48

MHz, and is limited by the scan rate, laser frequency jitter, and time constants of the data

acquisition equipment.

Environmental efTects (reproducibility)

The combined response of a 50 GHz bandwidth

rf power-sensor, adapter, and matched

photodiode with 12 GHz nominal bandwidth was

measured three times over a span of seven months. Each measurement consisted of three or

four scans that were then kernel smoothed^' ^ to estimate the response in 50 MHz increments from

50 MHz to 25 GHz. At the beginning of each

scan, the power-sensor was connected to the power meter's reference signal and calibrated

-13

0.1 0.2 0.3 0.4 0.5 Frequency, GHz

Figure 2. Results of four separate high-resolution scans (°) and B-spline fit to data (solid line) to estimate frequency jitter.

119
using a calibration factor of 100 %. The dc

photocurrent due to each laser was matched at about 50 \iA., giving about -42 dB(re:l mW) rf power at low frequencies. The typical response and reproducibility with and without normalization

is shown in figure 3. The uncertainty after normalization is similar to that observed on the 1319 nm heterodyne system, about 0.12 % (0.005

dB) below 20 GHz. Uncertainty in the un- normalized measurements is typically 0.3 %

(0.018 dB).

Uncertainty analysis

The measurement uncertainty of the normalized

response of a combined photodiode/rf powersensor transfer standard in the 850 nm heterodyne

measurement system is similar to that previously

described for the NIST 1319 nm heterodyne

system^. The dominant terms are due to the power

meter's range-scaling uncertainty (0.5 %) and

reproducibility (0.12 %). The combined

uncertainty (root sum of squares) is then 0.52 %

and the expanded uncertainty, v/ith coverage factor k=2, is 1.1 % (0.05 dB). Uncertainty for measurement of the photodiode response alone would include the un-normalized reproducibility and the power-sensor's calibration uncertainty (about 1 % , Ict, below 20 GHz), giving a combined uncertainty of

about 1.1 % and expanded uncertainty of 2.3 % (0.1 dB). Also, mismatch uncertainty or mismatch

correction uncertainty would need to be included. Although using a combined photodiode/rf power-sensor

transfer standard may not be suitable for all applications, the uncertainty in its calibration is more tiian a

factor of 2 smaller than that for a photodiode alone.

' Optical interfacesfor equipments and systems relating to the synchronous digital hierarchy, CCITT (now

ITU-TS) recommendation G.957, Geneva, 1990.

^ L. Piccari and P. Spano, "New method for measuring ultrawide frequency response of optical detectors," Electron. Lett. 18, 116-118 (1982).

^ S. Kawanishi, A. Takada, M. Saruwatari, "Wide-band frequency-response measurement of optical receivers using optical heterodyne detection," J. Lightwave Technol. 7, 92-98 (1989).

T. S. Tan, R. J. Jungerman, S. S. Elliot, "Optical receiver and modulator frequency response measurement

with a Nd:YAG ring laser heterodyne technique," IEEE Trans. Microwave Theory Tech. 37, 1217-1222

(1989).

^ R. T. Hawkins, M. D. Jones, S. H. Pepper, and J. H. Goll, "Comparison of fast photodetector response measurements by optical heterodyne and pulse response techniques," J. Lightwave Technol. 9, 1289-1294

(1991).

^ D. A. Humphreys, "Measurement of high-speed photodiodes using DFB heterodyne system with

microwave reflectometer," SPIE Proceedings Vol. 1680, High-speed electronics and optoelectronics

conference, 138-152 (1992).

^ P. D. Hale, C. M. Wang, R. Park, and W. Y. Lau, "A fransfer standard for measuring photoreceiver

frequency response," J Lightwave Technol, 14 2457-2466, 1996.

^ New Focus Inc., 1999/00 Catalog, Vol.10, p. 86.

^ P. D. Hale and C. M. Wang, "Calibration service of optoelectronic frequency response at 1319 imi for combined photodiode/rf power-sensor fransfer standards," Natl. Inst. Stand. Technol. Spec. Publ. 250-51,

Dec. 1999. Fundamentals ofrfand microwave power measurements, Hewlett-Packard Application Note 64-1 A,

1997.

0.06

0 5 10 15 20 25 Frequency, GHz

Figure 3. Standard deviation of the three measurements with and without normalization to the 50 MHz value. Inset shows nominal

normalized response of photodiode combined

with power-sensor.

120
Time-domain measurement of the frequency response of high-speed

photoreceivers to 50 GHz*

Tracy S. Clement, Paul D. Hale, Kevin C. Coakley, and C. M. Wang

NaticMial Institute of Standards and Technology

325 Broadway, Boulder, CO 80303

cIementt@boulder.nist.gov

Abstract

140-

120-

100- >

E,c

'to C

'toE

oo

d>E

80-

60-

40-

20^

-20-

Ti:sapphire

laser Fiber (

trigger Detector a

We determine the frequency response of high-speed photoreceivers from time-domain measurements collected with a 50 GHz sampling oscilloscope. In order to obtain accurate results, it is important to correct the signals for timebase distortion in the oscilloscope, electrical mismatch, oscilloscope jitter, laser pulse width, oscilloscope frequency

response, and nonlinear response of the detector. Results are compared to the response obtained from a calibrated hetCTodyne system, and they differ by less than ±0.1 dB up to 25 GHz, and by less than ±1.0 dB at 50 GHz.

Introduction

Accurate measuremait of the response of high-speed

photoreceivers is necessary for applications in highspeed optoelectronic systems such as Gigabit Ethernet

and Fibre Channel. We have developed a method for measuring the response of high-speed photoreceivers at frequencies up to 50 GHz by use of a sampling

oscilloscope. Historically, both impulse and CW

excitatiwi have been widely used to characterize highspeed photoreceivers. Because of the extreme

differences in peak powers used in these two families of

measurement methods, it has been suggested that the measurement method should depend on the intended

application of the photoreceiver'. Heterodyne methods

have been shown to be extremely accurate^, but can be

very slow and give only the magnitude of the response. Time-domain measvirOTients can be performed very

quickly and can give both the magnitude and phase of

the response. Work has been done previously by

Hawkins et al. to compare heterodyne measurements and

oscilloscope-based impulse-response measurements^. In our work we correct the impulse response measurements

for the effects of oscilloscope frequency response, jitter, and laser pulse width previously considered by the Hawkins group, and we extend their work by correcting

for time-base distortion, electrical mismatch, and

measurement nonlinearity.

Detector Under Test

Sampling

Oscilloscope

31.0 31.2 31.4 31.6 Time (ns) —1

31.8 32.0

Figure 1. Time-domain photodiode signal. (Inset, schematic of time-domain measurement system.)

Experimental configuration

The inset of figure 1 shows a schematic of the time-domain measurement system. A mode-locked, Ti:sapphire laser provides pulses with a fiill-width at half-maximum (FWHM) less than 100 fe at 800 ran, with a repetition rate of

approximately 80 MHz. The laser beam is split to provide an optical pulse to the detector under test in one arm and a

trigger signal for the oscilloscope in the other arm. Reflective neutral-density filters provide attenuation in each arm. The sample arm contains 15 cm of single-mode fiber, which is connected to the detector under test. Dispersion in the

optical fiber broadens the optical pulse to Tp«180 fe FWHM. The opfical pulse is convolved with the photoreceiver response in the time domain. We correct for the measured pulse width in the frequency domain (assuming a Gaussian pulse shape).

F(C0): observed (CO)

exp<

81n(2)J

(1)

* Publication ofthe U.S. Government. Not subject to copyright in the United States.

121
We make every effort to ensure that the optical pulse is as short as possible, which results in a very small correction

for the convolution with the finite width pulse (3.5 x 1 0"^ dB at 50 GHz).

The measurmients reported here are performed with a commercially available photodiode having a 25 GHz nominal

electrical bandwidth. The photodiode is packaged with an internal 50 Q matching resistor and has an external responsivity of about 0.2 A/W. The photodiode is connected to a 50 GHz sampling oscilloscope, where the response

to the ultrashort optical pulses is sampled over a 10 ns time interval, just under the repetition period of the laser. Figure 1 shows a portion of a typical time-domain signal. The pulse duration is 15.7 ps FWHM. An electrical reflection is clearly visible at 750 ps after the peak of the pulse.

In order to improve the signal-to-noise ratio, 100 waveforms are acquired from the oscilloscope in just over two

minutes. If the waveforms are averaged internally by the oscilloscope, drift acts to broaden the pulse, and the response in the frequency-domain is reduced by 1.4 dB at 50 GHz. To compensate for the drift, we align the

individual waveforms before averaging (using an algorithm based on cross-correlation of all possible pairs of

waveforms'*). Although it is marginally slower to acquire individual waveforms from the oscilloscope, the timedomain technique still has the advantage of

increased speed of data acquisition over that of a

traditional heterodyne method. For example, it takes less than fifteen minutes to obtain 100 data

sets at each of six different laser powers with the time-domain method, while the heterodyne

method takes several hours. —no corrections —TBD and mismatch

correction

m

;a

inc

o

Q.

V)

<S>

\0)O

-oO

sz

CL

10-

-15-

Time-base distortion and jitter correction

Time-base distortion (TBD) is a deterministic deviation in the sample times of the oscilloscope

from ideal, evenly spaced sample times. An

estimate of the TBD during the same time

window as in the photodiode measurements is determined by acquiring multiple sine waves

with the oscilloscope and analyzing the data with an efficient least-squares algorithm^. The time

axis of the averaged waveform is adjusted by the TBD estimate, and the resulting signal is interpolated onto an evenly spaced time grid

using a regression spline model^. The signal is then transformed to the frequency domain using a

fast-Fourier transform. In figure 2, the black curve shows this frequency-domain response of

the photodiode up to 50 GHz. The ripple on the

signal comes from multiple reflections between

the photodiode and the oscilloscope, and must be

corrected to determine the frequency response. The effect of jitter on an averaged signal is that of a lowpass filter^. The variance of the measured signal can be

expressed in a Taylor-series expansion as

-20- —r10 —

I—

20 —

r—

30 —

r40 50 Frequency (GHz)

Figure 2. Photodiode frequency response with and without TBD and mismatch corrections

Total

2 , 1 -o-., +cr,

dt J

' (2)

where (Trotai'' is the total measured signal variance, <jn is the additive noise variance, cr/ is the jitter variance, and

dV/dt is the derivative of the ideal time-domain waveform. Typically cr ywl. 1-1.2 ps for our work. F((o) is then

multiplied by exp(cr/co^/2) to deconvolve the jitter effects. Mismatch and oscilloscope response correction

Electrical mismatch between the photoreceiver and the oscilloscope causes multiple reflections and dispersion of the time-domain signal as seen in figure 1. Reflection coefficients for both the photoreceiver and the oscilloscope are

122
measured from 100 MHz to 50 GHz with a vector network analyzer. The measured signal in the frequency-domain,

F((o), can be corrected for mismatdi to give the actual signal, F(a))pd, generated by the photodiode.

Fioy)^=Fiay)x(l-T,T^), (3)

where Ts is the electrical reflection coefficient of the oscilloscope, and Tpd is the electrical reflection coefficient of

the photodiode.

We choose not to window the data in the time-domain^ to remove the reflections because the initial impulse

response and its reflections overlap. Thus, windowing would remove an unknown portion of the pulse and introduce an unknown amount of error in the frequency response. Insating a delay line between the photodiode and the

oscilloscope could reduce the overlap of the signal with the reflection, but the effects of the delay line would also need to be measured and accoimted for. In addition, windowing the data can introduce incorrect information at low

frequencies.

The gray curve in figure 2 diows the frequency-dcHnain response of the photodiode after mismatch and TBD

correction. The magnitude of the ripples is greatly reduced by the mismatch correction, although there is still some

small effea remaining, especially above 30 GHz. The importance ofthe TBD correction is especially apparent when

combined with the mismatch correction. Without TBD correction, the ripples due to electrical mismatch are only

partially reduced because the spacing of the electrical reflections is distorted by the TBD.

The frequaicy-draiain photodiode response must be corrected for the response of the 50 GHz oscilloscope. The

magnitude response ofthe oscilloscope is measured by direct comparison to a caUbrated power meter, and the phase

response ofthe sampling oscilloscope is determined with a "nose-to-nose" measurement*.

Laser power dependence

Although the average power incidrait on the photodiode is relatively low, the peak power during the femtosecond

lasCT pulse can reach tais of kilowatts, {M-oducing vay high instantaneous carrier densities and peak curraits. At

very higji currents, space-charge effects and carriCT-recombination noilinearities may significantly degrade the photoreceiver performance*''. In addition,

nonlinearity in the vertical response of the

oscilloscope may cause changes in the photodiode response. We take data at many different input powers

ranging from 5 to 100 jiW. Figure 3 graphs the frequency response of the photodiode at some

representative input powers. (We actually take

data at more powers on the lower end, but they

become hard to distinguish in such a graph.)

The response decreases as a fimctiwi of

increasing power. Fot tiie lowest powers, this decrease can be approximated as a linear fimction of increasing power and increasing

frequency. At higho" powers (above -30 |iW),

significant nonlinear effects are also evident.

In ordw to correct for the power-dependent

effects, we observe the response as a fimction

of power at many frequencies. Assuming that the effect is linear at low powers, we obtain a

correction fector in dB/(|iW*GHz), and use this to effectively extr^olate the data back to zero average power. After this correction, the responses for all of the relatively low-power

inputs (below -30 nW average power) agree

within the noise of the measuremait.

Frequency (GHz)

Figure 3. Frequency response for various input powers

123
Comparison to heterodyne experiment

We have measured the frequency response of

the same photodiode with a heterodyne

technique"'. Figure 4 shows the difference

between a time-domain measurement with all of the above corrections applied and the measured heterodyne data. The disagreement is less than 0.1 dB from 100 MHz to 25 GHz. At

higher frequencies (inset) the difference

increases to an average value of -0.6 dB at 50

GHz, with excursions of about ±0.5 dB,

probably due to mismatch that is not corrected

in the time-domain measurement. We have also used the time-domain technique described here

to obtain the frequency response of several other photoreceivers, and comparisons with

heterodyne measurements show similar agreement. Although we have not focused on it in this paper, the time-domain technique

determines the phase as well as the magnitude

of the frequency response of a photoreceiver,

while the heterodyne method can determine

only the magnitude.

Acknowledgements: Part of this work was supported by the Office of Naval Research and the Space and Naval

Warfare Systems Center. We thank Don Larson for early contributions to this work.

References:

' J. E. Bowers and C. A. Burrus, "Ultrawide-band long-wavelength p-i-n photodetectors," J. Lightwave TechnoL, 5 1339-1350

(1987).

^ A. D. GifFord, D. A Humphreys, and P. D. Hale, "Comparison of photodiode frequency response measurements to 40 GHz

between NPL and NIST," Electron. Lett., 31 397-398 (1995).

^ R. T. Hawkins, M. D. Jones, S. H. Pepper, and J. H. Goll, "Comparison of fast photodetector response measurements by optical

heterodyne and pulse response techniques," J. Lightwave TechnoL 9, 1289-1294(1991).

K. J. Coakley and P. D. Hale, "Alignment of noisy signals," submitted to IEEE Trans. Instrum. Meas.

* C. M. Wang, P. D. Hale, and K. J. Coakley, "Least-squares estimation of time-base distortion of sampling oscilloscopes," IEEE

Trans. Instrum. Meas.,4S, 1324-1332 (1999). Y. Rolan, J. Schoukens, and G. Vandersteen, "Signal reconstruction for nonequidistant finite length sample sets: a KIS

approach," IEEE Trans. Instrum. Meas., 47, 1046-1052 (1998).

^ W. L. Gans, "The measurement and deconvolution of time jitter in equivalent-time waveform samplers," IEEE

Instrum. Meas., vol. 32, pp 126-133, 1983.

* P. D. Hale, T. S. Clement, K. J. Coakley, C. M .Wang, D. C. DeGroot, and A. P. Verdoni, "Estimating the magnitude and phase

response of a 50 GHz sampling oscilloscope using the 'nose-to-nose' method," 55'^ ARFTG Conference Digest, (June 2000).

' K. J. Williams and R. D. Esman, "Photodiode DC and microwave nonlinearity at high currents due to carrier recombination

nonlinearities," IEEE Photon. Tech Lett, 10, 1015-1017(1998).

P. D. Hale and C. M. Wang, "Heterodyne system at 850 nm for measuring photoreceiver frequency response," submitted to Symposium on opticalfiber measurement. See also P. D. Hale and C. M. Wang, "Calibration service of optoelectronic frequency response at 13 19 nm for combined photodiode/rf power-sensor transfer standards," Natl. Inst. Stand. Technol. Spec. Publ. 250-

51. Dec. 1999.

10 15 Frequency (GHz)

25

Figure 4. Difference between heterodyne and

time-domain resDonse.

124
Optical amplifier measurement issues:

doped fiber, semiconductor and Raman

Jack Dupre

Agilent Technologies

Santa Rosa, CA

1 Introduction

Optical amplification is critical to the continued explosive growth of optical fiber telecommunications. Increased transmission capacity through higher channel density and wider optical

bandwidth per fiber has placed stringent performance requirements on optical amplifiers used in booster,

in-line, and preamplifier applications. Additional requirements are imposed by all-optical networking

where channels are dynamically added and dropped. While the preponderance of optical amplifiers today

is of the erbium-doped fiber (EDFA) type, Raman and semiconductor (SOA) amplifiers are also commonly used. Each amplifier type has unique measurement issues.

This paper addresses some of the key measurement issues for each type of optical amplifier.

2. Erbium-doped fiber amplifiers for dense WDM applications

Gain and noise figure characterization for amplifiers with multichannel stimulus is complex

because the parameters are dependent on the power level in all of the input channels. To simplify the test apparatus, signal- spontaneous noise figure [1], which is derived from amplified spontaneous emission

(ASE) measurements on an optical spectrum analyzer, is commonly used. To measure ASE in the presence of the spontaneous emission fi-om the laser sources, methods using direct interpolation and timedomain extinction (TDE) are used. [2,3]. A typical configuration for TDE is shown in Figure 1. To

precisely characterize gain and noise figure for a particular multicharmel plan, the multichannel test source must duplicate the multichannel plan in wavelength and power level. This is problematic for tesfing amplifiers that are used with a number of channel plans as the source is very complex and must be

to sources

X

a.

Source measurement

path

Fig. 1 Typical setup for measuring gain ^nd noise figure using time-domain

extinction (TDE).

125
reconfigured for alternative channel plans. However, using the

largely homogeneous saturation characteristics of the EDFA, a reduced set of laser sources may be used with a minor compromise

in measurement accuracy [4]. A limitation on the reduction in the number of lasers is imposed by spectral hole burning (SHB). As

shown in Fig. 2, SHB is a wavelength-localized depression in gain

that is signal power dependent. It is important that in regions where

SHB is significant, at least one test laser must be placed within a

spectral hole width. Careful measurements of SHB [5] show that hole depth is deepest and width the narrowest in the 1535-nm region

and is small above 1550 nm. Simulations suggest that 16 to 32

equally spaced lasers are required to eliminate errors due to SHB [6]. Judicious wavelength placement (closer spacing in the 1535-nm

region) could further reduce this number. For L-band designs, it has

been reported that the localized gain depression caused by SHB is not a factor [7]. A further refinement of the reduced source approach is to combine a small-signal probe with the

saturating multichannel source. The probe may be

either a tunable laser or broadband noise. The

broadband noise probe method, often called noise gain

profile (NGP), is generally preferred because of its measurement speed. It uses a noise source such as an

edge-emitting LED to simultaneously probe the entire EDFA band. It is often implemented with modulated

saturating laser sources and in conjunction with time

domain extinction. Fig. 4 shows a typical

implementation of NGP. Compared to the direct TDE

approach of Fig. 1, NGP can have the following

advantages:

saturating signal wavelength

T / ^ depth

«—widti*-*'

1540 1545 1550 Wavelength (nm)

1560

Fig. 2 Spectral hole burning is a

localized signal-dependent

depression in gain.

CQCO

c

'<7>

E

hole

I Center of spectral I

't' Saturating signal wavelength

1520 1530 1560 1570 • Resolution of the gain and noise figure data is limited only by the optical spectrum analyzer's display

resolution—not by the number of lasers in the multichannel source.

1540 1550 Wavelength, nm

Fig. 3. SHB measurements on a 1480-nm single

pump silica-fiber based EDFA.

• By keeping the total input power constant, the gain and noise figure are largely immune to power

level variation of the individual lasers.

• Measurement speed is independent of the number of lasers so can be faster for large channel count.

Mult-X Source

Booster Optical Saturating signals EDFA switch (modulated 25 kHz

to 1 MHz for TDE)

mod

Mten

Broadband

Noise Source

Combiner

Fast

Polz

Ctrl

DUT

EDFA

Trigger from source mod

Optical Spectrum

Analyzer

Control signal from OSA

Source Control signal measurement path to broadband

noise source

' Probe signal

(modulated at 8 kHz

rate and < 10% duty

cycle)

Fig. 4. Typical setup for measuring gain and noise figure using noise gain profile (NGP).

126
While TDE and NGP are inherently different measurement methods, it is important tO that they measure the same gain and noise figure parameters. To confirm this, measurements made with three methods were compared and results for gain and noise figure are shown in Fig. 5. In addition to results for TDE and NGP, measurements were also made using the interpolation with source subtraction (ISS)

method. For TDE and ISS, data is only available at the wavelengths of the multichannel source while, for NGP the data is continuous fi-om 1528 to 1563 nm. The measurements for all three methods agree to within ±0.125 dB.

30

29

28

27

2625

24

6

5.8

5.6

5.4

5.2

e"3a 5

s 4.8 i

4.8

4.4

4.2

4

Gain

wavelength ngp Iss Iss-ngp tde tde-ngp

1532 29 211 29,111 -0 100 29.188 -0,023 1536 26,599 26 551 -0 04B 26.488 -0,1 1

1540 23.386 23,329 -0.057 23 295 -0.091 1544 24.553 24.532 -0 021 24 483 -0 070

1548 24 688 24,766 0 078 24 651 -0.037 1552 24,381 24 437 0.056 24 327 -0 054

1556 24 405 24 416 0 Oil 24 289 -0 116

IfiRTl 23.423 23.403 -0 020 23 311 -0.1 12

* wdm iss

• wdm tde

- —wdm ngp 1520 1530 i40 1550 15i Wavelength, nm

Noise Figure

wavelenqth nqp Iss Iss-nqp tde tde-nqp

1532 5 087 5 197 0 110 5 177 0 090

1536 5.008 5 049 0.041 S.107 0 099

1540 4.990 5,058 0.068 5 052 0 062

1544 4,945 4.973 0 029 5 002 0 058

1548 4.957 4 987 0.030 4.994 0.037 1552 4 923 5 015 0 092 4 993 0 070

1556 4.867 4 967 0 101 4 960 0.093 1560 4.854 4 976 0.122 4 931 0077

wdm iss wdm tde -wdm ngp

1530 1540 1550 1560 Wavelength, nm

FiR. 5 . Gain and noise figure comparisons for multichannel ISS, TDE and NGP methods.

A further simplification in the measurement method for variable and complex source environments it to use a single tunable laser and characterize the SHB effect vs. wavelength [8]. While

additional correlation studies are necessary to qualify the accuracy of the modeling of SHB, early results are promising as shown in Fig. 6. Single laser gain measurements correlate with multichannel

measurements to within 0.3 dB.

1525 1530 1535 1540 1545 1550 1555 1560 1565

W avelength, nm

Fig. 6. Predicted multichannel gain from SHB modeling and a homogeneous model are compared to direct measurements with a multichannel source.

127
Another important issue in testing EDFAs is the effect of polarization. The test setups in Fig. 1 and Fig. 2 both incorporate polarization controllers to either:

(1) Scramble the state of polarization (SOP) in order to measure amplifier

parameters for the average SOP , or

(2) Vary the SOP and make measurements to collect the polarization dependency of

the amplifier parameters.

When testing with a multichannel source, the polarization hole burning (PHB) phenomenon is of

particular concern. To understand the effect of PHB, consider two signals in the same wavelength vicinity - a large signal and a small signal. The large signal sets the inversion level. An additional laser provides a small signal probe. Because of PHB, the gain in the polarization state of the large signal is depressed.

This gain depression also will impact the small signal if it is in the same polarization state. However, if the small signal is in the polarization state orthogonal to the large signal, it does not experience the gain

depression. The difference between the gain in the polarization state aligned with the large signal and that

for the orthogonal state is usually referred to as polarization-dependent gain (PDG). With a multichannel

source, it is usually desirable to measure gain with randomly aligned source polarization. This can be

accomplished in three ways as shown in Fig. 7. The first method is to individually scramble each source. The second is to slowly scramble the combined sources followed by a birefringent element to decorrelate

the SOP of adjacent channels. The third method is to utilize a fast polarization scrambler after the combiner. In this context, slow and fast scrambling are with respect to the erbium-ion relaxation time.

polarization scramblers -J^—

(a)

"slow"

polarization birefringent scrambler element

"fast" polarization scrambler

(b) (c)

Fig. 7. When using a multichannel source, errors due to PHB are eliminated by (a) scrambling the polarization of

each channel, (b) scrambling the combined source and using a birefringent element, or (3) using a fast polarization scrambler.

3. Raman amplifiers

Raman amplification is increasingly being considered to extend the wavelength range for a transmission system or to extend the distance between repeaters. The active element that is pumped to achieve gain may be the actual link transmission fiber itself or, like a conventional EDFA, the active fiber may be packaged with the pump laser. In the case of Raman-pumped transmission fiber, significant

performance advantages can be realized over EDFA-only systems [9]. In addition, WDM pumping can

provide a 100-nm operating band with very flat gain characteristics [10]. Test issues result primarily

from a unique noise generation mechanism and much faster time dynamics when compared to the EDFA.

The important noise mechanisms in a Raman amplifier are amplified spontaneous emission (ASE)

and double Rayleigh scatter noise. As with an EDFA, ASE can be measured with an optical spectrum

analyzer and the signal-spontaneous noise figure, Fsigspon, calculated. TDE, as discussed above for the EDFA, cannot be used because of the significantly faster gain relaxation time for Raman gain.

Interpolation must be used. Another significant issue is that double Rayleigh scattering cannot be

128
observed as an increase in ASE because it results in heterodyne noise in an optical

receiver. The total noise figure -- which

includes the double Rayleigh backscatter

noise component— must be measured with

an electrical spectrum analyzer method.

The electrical spectrum analyzer test method (Fig. 8) for measuring F,oiai was

once commonly used for EDFA

measurements but was abandoned because

of the difficulty to obtain repeatable and

accurate results and because, for the EDFA,

Fsig-spon measured with an optical spectrum

electrical

In

optical amplifier n. electrical out

^total

SNR,input SNRoutput

Fig. 8. Total noise figure is defined in terms of the

electrical signal-to-noise ratios at the input and output of

ideal e/o and o/e converters. F,ou,i includes the effect of

double-Rayleigh backscatter. analyzer was sufficient. One of the major

sources of measurement uncertainty was

the calibration of the optical receiver. By using a RIN standard [11], this major source of uncertainty can

be greatly reduced. Calibration of an optical receiver with a RIN standard is shown in Fig. 9. Consisting

of an ASE source and a stable bandpass filter, the optical power is measured on a well-calibrated optical

power meter. The intensity noise spectral density is then known precisely and used as the calibration

standard for the optical receiver.

RIN Transfer Standard

ASE

Source (EDFA)

o

Optical Bandpass

Filter

piiotodetector and

electrical spectmm

analyzer ^

Optical Receiver

Optical Power Meter

Fig. 9. The accuracy of an electrical-method noise figure measuring setup in improved by calibrating the intensity noise frequency response with a RIN standard.

Through NIST, a Measurement Assurance Program (MAP) or Calibration Service is available for the spectral density of relative intensity noise [12].

4. Semiconductor optical amplifiers

While the EDFA has become the prevalent amplifier type and Raman is emerging to provide better performance, the semiconductor optical amplifier (SOA) continues to progress as a potentially more

compact and lower cost alternative. While not usually considered as a commercially viable option for 1550-nm transmission systems because of the refinement of the EDFA, 32 channel X 10 Gbit/s over 160 km using SOAs has been reported [13].

The SOA also has applications in optical switching and as a non-linear device in applications such

as wavelength conversion. The SOA may also be applied in the 1310-nm window where doped-fiber amplifiers have not proven to be viable.

129
Test methods for gain and noise figure of the SOA can leverage greatly from the EDFA

experience. Signal-spontaneous noise figure using the optical spectrum analyzer interpolation method

may be used. Like the Raman amplifier, time-domain extinction cannot be used because of the short

carrier lifetime. Additional test issues relate to properties that are unique to the SOA. These include

polarization dependence, gain ripple, and gain clamping laser artifacts. In an EDFA polarization

dependencies are so small that they may be ignored in many applications. This is not true with the SOW

in that it is inherently polarization dependent and excellent design is required to minimize this effect. Gain ripple with sub-nanometer period is also inherent due to reflections between the gain medium facets. Active gain clamping with a control laser is commonly employed to achieve automatic gain control. The

gain clamping laser leakage needs to be measured.

5. Acknowledgements

The author wishes to acknowledge Douglas Baney of Agilent Technologies Laboratories and Jim

Stimple of Agilent Technologies, Lightwave Division for their many contributions to this paper.

6. References

[I] lEC Publication 61290-3, Basic specification for optical fibre amplifiers test methods - Part 3: Test methods for noise figure parameters.

[2] J. Aspell, J.E. Federici, B.M. Nyman, D.L. Wilson, and D.S. Shenk, "Accurate noise figure

measurements of erbium-doped fiber amplifiers in saturation conditions" Optical Fiber

Communications, Vol. 5, 1992 OSA Technical Digest Series (Optical Society of America,

Washington, DC, 1992), paper ThA4.

[3] D. Baney, J. Dupre, "Pulsed source technique for optical amplifier noise measurement," European

Conference on Communications, paper WeP2.1 1, Berlin, 1993.

[4] D. Baney, J. Stimple, "WDM EDFA gain characterization with a reduced set of saturating channels,"

IEEE, Photon. Technol. Lett. 8(12), pp. 1615-1617.

[5] E. R Rudkevich, D.M. Baney, J. Stimple, D. Derickson, and G. Wang, "Nonresonant spectral -hole

burning in erbium-doped fiber amplifiers," IEEE Photon. Technol. Lett., vol. 1 1, no. 5, 542 (1999).

[6] P. Wysocki, "Measurement of Wide-Bandwidth Gain-Flat Amplifiers," Symposium on Optical Fiber

Measurements, pp. 9-14 (1998).

[7] F. A. Flood, "Inhomogeneous Gain Saturation Behavior in L-Band EDFAs," IEEE, Photon. Technol.

Lett. 12(8), August, 2000.

[8] J. Stimple and J. Dupre, "A method to predict multichannel gain fi-om single laser measurements,"

Optical Amplifiers and Their Application, Tech. Dig. 2000.

[9] C. Fludger, A. Maroney, and N. Jolley, "An analysis of the improvements in OSNR from distributed Raman amplifiers using modem transmission fibres" Optical Fiber Communications, 2000, paper

FF2-2, OSA Technical Digest Series, Optical Society of America, Washington, DC, 2000.

[10] Y. Emori and S. Namiki, "100 nm bandwidth flat gain Raman amplifiers pumped and gain equalized

by 12-wavelength-channel WDM high power laser diodes," Optical Fiber Communications, 1999

OSA Technical Digest Series, Optical Society of America, Washington, DC, 1999, paper PD-19.

[I I] D. M. Baney and R.L. Jungerman, "Optical noise standard for the electrical method of optical

amplifier noise figure measurement. Optical Amplifiers and Their Application, paper MB3, OAA'97,

Optical Society of America, Washington, DC, 1997.

[12] G.E. Obarski and J. D. Splett, "Measurement Assurance Program for the Spectral Density of

Relative Intensity Noise of Optical Fiber Sources Near 1550 nm," NIST Special Publication Series,

SP-250.

[13] L.H. Spiekman, A.H. Gnauck, J.M. Wiesenfeld, and L.D. Garrett, "DWDM transmission of thirtytwo lOGbit/s channels through 160km link using semiconductor optical amplifiers," Electron. Lett., 2000,36, pp. 1046-1047.

130
COMPARISON OF ERBIUM AMPLIFIER

CHARACTERISATION USING AN ITU GRID WITH

VARYING CHANNEL SEPARATIONS

M.E. Bray, R.T. Elliott and K.P. Jones

Nortel Networks, Brixham Road, Paignton, Devon, UK, TQ4 7BE (MBrav@norteInetworks.com)

Abstract: lOOGHz spacing ITU grid source measurements ofgain and noise figure of EDFAs are presented. Channel numbers can be halved or quartered giving deltas to the lOOGHz measurements of <~0.15dB and <~0.3dB

respectively.

Introduction

Recently wavelength division multiplexed erbium doped

fibre amplifier (EDFA) designs have progressed from "red band" (~ 1540-1 5 60nm) to "full band" (~I530-1560nm) and

eventually "extended band" (~1570-1600nm). As this happened problems have become apparent with existing methods of EDFA measurement such as the noise-gain

profile technique'. These methods suffer because they

concentrate the power saturating the amplifier into one or a very small number of channels. Spectral hole burning, and

the dependence of the amplifier's saturation on the wavelength of the saturating signal, lead to the results obtained to these techniques being unrepresentative of EDFA system performance^. For accurate measurements the test source used must

accurately represent the multi-channel loading used within

the system for which the amplifier is designed. Here we

present gain and noise figure results for an amplifier tested with lasers separated by 1 OOGHz at the wavelengths defined by the ITU^. Fully populating an ITU grid is expensive. Results have also been obtained using sources with

separations of 200, 400 and 800 GHz. These are compared

here to the 1 00 GHz benchmark to investigate cost reduction by the use of sub-populated grids. To the authors' 'mowledge

previous work has only modelled sub-populated grids".

It should be noted, that the gains and noise figures are only

calculated at the source wavelengths in this technique, so some of the separations may be prohibited by their reduced

spectral resolution

Test

The method used is the time domain extinction technique. In

this method, the sources are on-off modulated before the device under test at a sufficient frequency that the amplifier

saturation does not significantly alter during the off phase. Measurements on the optical spectrum analyser are gated so

that they occur when the sources are on (measuring signals and ASE noise) and also when they are off (measuring ASE

noise only). Back to back signal measurements are also made

to characterise the input to the amplifier. A schematic of the test equipment is shown in Fig. 1. The ITU sources are coupled together and fed into a boost EDFA.

This increases the input power to the device under test. The ITU sources are also monitored on a wavemeter. The boosted

signals are modulated by an acousto-optic modulator, which

operates with a 50% duty cycle. After passing through a

variable optical attenuator which controls the input power to the device under test, the polarisation of the signals is randomised by a polarisation scrambler. The signals then pass through the device under test (or bypass it for back to back measurements), and then through an attenuator to condition the power into the optical spectrum analyser. Another acousto-optic modulator with a 25% duty cycle then

gates the signals before measurement on an optical spectrum

analyser. By varying the phase of the second modulator with

respect to the first the optical spectrum analyser can measure

signal or ASE traces. The timing diagram for the acoustooptic modulators is shown in Fig. 2. Figure 1 Schematic of test equipment

1 1 1 1 1 M 1 1 1

ITU Grid Wavemeter

Pol'n scrambler ooo

1 DUT J>-^

131
Figure 2 Timing diagram of acousto-optic modulators

AOM on output arm, measuiing signal and ASE

On

I—^ I—11—11—II

OfT 1 I 1

I 1I 1I 1 - AOM on output arm, measuring ASE

On

I—I I—I I—I I—I I—

Off J I 1

I 1 I 1 I 1

Time

Results

Input power, output power and ASE spectra were collected from a flattened full band amplifier operating at its nominal

gain condition. An example set of measured spectra is shown

in Fig. 3. The input spectrum in this graph has been offset by

-lOdBm to clarify the results. The data was then manipulated

to obtain gain and noise figure spectra. This process was

repeated for several source grid wavelength plans with

separations of 100, 200, 400 and 800 GHz.

Figure 3 Measured input,output and ASE spectra

•70.00 J 1 , , , , , . . 1520.00 1525.00 1530.00 1S35O0 1540.00 1545.00 1550.00 1555.00 1560.00 1565.00 1570 00

Wavel«ng11i (nm)

Figure 4 Gain spectra measured using varying channel

separations

21

17 5

17 —

16.5 1^

16 i . . . .

! 1525.00 1S3O00 1535.00 1540.00 1545.00 1550 00 1555,00 1560.00 1565.00 Wavelength (nm)

Figure 5 Noise figure spectra measured using varying channel separations

6,50

3.00 J . . . , , , 1525.00 153000 1535.00 1540.00 1545.00 1550.00 1555.00 1560.00 1565.00

tWavelengtli (nm)

Figures 4 and 5 show the gain and noise figure results calculated from the measurements. The graphs for all the channel separations show very similar spectral profiles. The

200 and 400 GHz measurements are close in absolute value

to the 100 GHz benchmark, however the 800 GHz

measurements, while retaining the same profile, displays an

offset to the other measurements in both gain and noise

figure. This may be explained by spectral hole burning. The

greater intensity in each of the channels leads to deeper

holes being burnt around each channel, reducing the gain and increasing the noise figure. Close inspection of the figures shows that the 100 GHz and

200 GHz measurements are in very good agreement for both

noise figure and gain. The difference in gain and noise figure measurements are less than 0.1 dB and 0.1 5dB respectively. While the "average" values for the 400 GHz gain and noise

figures follow the 100 GHz values well, the individual values oscillate about the 100 GHz trace. The differences of

gain and noise figure measurements between the 100 and 400 GHz traces are both up to 0.3 dB. This is larger than the

error predicted by previously published modelling [4].

Conclusion

The expansion of the erbium amplifier spectrum used in wavelength division multiplexed optical systems has placed

additional requirements on test methods. Single channel

sources are no longer sufficient to set amplifiers in the

coiTect saturation condition. In addition they lead to errors due to spectral hole burning.

Ideally a source with the same channels and powers as the system for which the amplifier is designed should be used. However the capital requirement of such a source for full band amplifiers and beyond is severe.

It is shown here that a source with 40 lOOGHz spacing charmels can be very accurately represented by a source with

half the channels at twice the spacing. This represents a

source cost saving of up to 50%. If a slightly lower accuracy of the measurements can be accepted, then the spacing can

be increased by another factor of 2 to 400 GHz, reducing the number of sources by a factor of 2 again. Increasing the spacing to 800GHz reduces the accuracy unacceptably.

It should be noted however that reducing the number of

sources reduces the spectral resolution of both gain and noise

figure, and this should be considered while designing a test system.

132
References

1 The noise gain profile technique explained in "The

1996 Lightwave Transmission Seminar & Exhibition", W. Benger, and J. Vobis, Hewlett-Packard, pp64-65, 1996

2 C. Hentschel and J. Dupre, "EDFA measurements for WDM applications", "The 1998 Lightwave Symposium",

Hewlett-Packard, pp63-80, 1998

3 ITU Draft G.mcs "Optical interfaces for multichannel systems optical amplifiers", Question 16/15, Appendix rv

4 P. Wysocki, "Measurement of wide-bandwidth gain-flat amplifiers". Symposium on optical fiber measurements,

1998, pp9-14

133

Gamma and Proton Radiation Effects in Erbium-Doped Fiber Amplifiers: Active and

Passive Measurements

Todd S. Rose, Deana Gunn* and George C. Valley

Photonics Technology Department, Electronics and Photonics Laboratory

The Aerospace Corporation

2350 East El Segundo Blvd., El Segundo, CA 90245

ABSTRACT

Commercially available Er-doped fibers were irradiated with 5.6- and 28-MeV protons and ^''Co gamma rays, up to levels of 50 krad. Transmission spectra under passive conditions (no pump or signal) were measured at several radiation levels for the six types of the fiber tested. Comparison between the damage caused by the gammas and

protons were made for two fiber types. Samples from three of the fiber types were configured as optical amplifiers

using a 980-nm pump and a 1550-nm signal. In situ measurements of the gain, noise figure and ASE were made as

a function of pump power at several levels of radiation. A computer code, based on a conventional fiber amplifier

model, was written to simulate performance using input data provided by the fiber vendor and anchored to measurements made prior to radiation. Comparison between the simulations and experimental data show that the radiation-induced loss determined from the amplifier measurements can be a factor of two to three smaller than that determined from the passive transmission spectra.

1. INTRODUCTION

Because of the potential use of erbium-doped fiber amplifiers in space for optical intersatellite links and other

applications, the effects of radiation on these components has attracted interest by many groups over the past several years [1-5]. Although high-energy protons are the primary threat to components within a typical spacecraft

enclosure, most radiation tests to date have been conducted with ^'^Co gamma emission sources since they are more

accessible and less expensive to operate than high-energy particle accelerators. For this reason, only a limited number of tests involving erbium fibers and protons are discussed in the literature [4,6]. Equivalent doses of

gammas and protons are expected to cause the same amount of damage in a glassy system. It is important to note, however, that only the dose at the core is relevant to a fiber's performance, a point that seems to be neglected in the

literature. In all but the most extreme radiation condifions, color center formation in the outer claddings of fibers is not only significantly less than in the core, but inconsequential as well. Furthermore, traditional passive radiation

tests, or transmission measurements, are not adequate in predicting the performance of a fiber as an amplifier. In the absence of optically induced annealing and other unknown processes, the measured gain and noise figure (NF) of

the amplifier should relate to the measured passive loss. However, any quanutafive relation between the passive loss and noise figure derived from experiment must meticulously consider the effects of other "components" such as

filters, polarizers, splices and the method in which the NFs were determined.

2. EXPERIIVIENTAL SET-UP AND RESULTS

Radiation-induced transmission losses were measured for six commercially available fibers from Lucent

Technologies. Of the six fiber types, four are primarily used as amplifiers (HE980, HP980, MP980, and LP980),

while two are used as super-luminescent sources (HG980 and R37003). In all transmission measurements, 3-m Erfiber samples were fusion spliced to 3-m SMF-28 fiber leads at each end. Each Er section was concentrically

spooled into a flat annular geometry with a diameter on the order of 7.5 to 10 cm and mounted to a sheet of

cardboard, which was placed inside the ^''Co chamber. (This mounting geometry ensured that the Er fibers were

irradiated uniformly, which is particularly important for comparing the gamma and low-energy proton results discussed below.) The undoped fiber leads were connected to a broadband white light source and to an optical

spectrum analyzer (OSA). The ganmia tests were conducted over a 10-hour period at a rate of 1.4 rad/sec. In situ

' Present address: Hughes Space and Communications, El Segundo, CA

135
transmission spectra for the fibers were acquired at 10-krad intervals up to a total dose of 50 krad. A representative

example is shown for HP980 in Fig. 1 . The induced loss spectra for all the fibers at 50 krad are displayed in Fig. 2, which were obtained by normalizing the 50-krad transmission spectra to the pre- irradiation levels.

Proton tests for HE980 and HP980 fibers were conducted at the Lawrence Berkeley 88-in. cyclotron facility. The

samples received up to 50 krad (at the fiber core) of 5.6- and 28-MeV protons. The fluences required to deliver 50- krad doses to the fiber cores at 5.6- and 28-MeV were 4.3 x lO'^'/cm^ and 1.9 x lO'Vcm^, respectively. On average,

the fiber samples received the entire proton dose over 1 hr (i.e. 14 rad/sec). A comparison of the 5.6- and 28-MeV

protons and ^ Co gamma results, displayed in Fig. 3, show that for each fiber type, equivalent radiation doses cause comparable amounts of damage in the fibers. While there is little variation in the HE980 data (which is consistent with other work [4]), there is some spread in the HP980 data, which could arise from the doping variations known to occur in this fiber.

The effects of gamma radiation on the gain and noise figure of 10-meter samples of MP980, HE980 and HP980

fibers were investigated using the ^''Co chamber and irradiated up to 50 krad at a dose rate of ~3 rad/sec. The fibers were forward-pumped at 980 nm with input powers ranging between 5 and 50 mW. The input -40 dBm signal at 1550 nm was obtained from a tunable laser source with a 150-kHz linewidth and a side-mode suppression ratio > 45

dB. For the HP980 and MP980 fibers, an isolator was used at the output facet to eliminate the amplified

spontaneous emission (ASE) backscatter, which caused lasing at the higher pump powers. The gain vs. pump power

and the noise figure vs. pump power are shown at 10-krad intervals, up to 50 krad, in Fig. 4 for the three fibers. The

solid lines through the data points are the theoretical fits to the amplifier performance, which will be discussed in the following section.

3. MODELING

A computer code was written that solves the conventional EDFA equations [7,8] allowing for arbitrary

modifications, such as the addition of a saturable distributed loss, which are necessary for simulating radiation

effects. (Although a commercial program, OASIX, [7] is available to model certain fibers, it cannot be modified to evaluate radiation effects, since the source code is unavailable.) Our model consists of standard equations for the pump at 980 nm, the co-propagating signal in the 1500-1600 nm band, and the forward and backward amplified

spontaneous emission, all of which are coupled through the ground and excited state erbium number density. Input

parameters to the code include the overlap integrals for the gain and absorption spectra [8] and the saturation parameter [7], which are provided by the fiber vendor (Lucent Technologies).

Modeling the gain and noise figure of an LNA requires knowledge of three optical loss parameters: the loss at the pump and signal wavelengths entering the LNA fiber and the loss from the LNA fiber to the OSA at the signal

wavelength. These three parameters, which range from 0 to a few dB, were determined for each LNA fiber prior to radiation damage and are held fixed for all ensuing calculations. The only variables used to fit all the subsequent

radiation data are the radiation-induced losses at the pump and signal wavelengths. Our fitting results for the MP980, HE980, and HP980 fibers in Fig. 4 show that we are able to model the effects of radiation quite well using our prescribed approach. Interestingly, we observe that while the gain continues to degrade with radiation, that NFs

are unaffected at high pump powers. We further note that the radiation-induced losses at the pump and signal

wavelengths obtained by fitting the amplifier results are factors of 2-3 lower than those obtained from passive

transmission measurements. Thus, operation of the amplifier reduces the effect of the radiation, either by optical

annealing or bleaching.

4. CONCLUSION

We have discussed our current investigation into the radiation sensitivity of erbium-doped fibers used as low noise

amplifier fibers for space applications. Our comparative tests show that it is valid to use gamma radiation to simulate the energetic protons found in the space environment. We have measured the passive transmission through

these fibers, as well as the gain and NFs of the fibers in active amplifier configurations. We attribute the degradation in performance primarily to radiation-induced absorption of the 980-nm pump light. If high enough

pump levels are used in the amplifiers (achieving inversion saturafion levels), this problem may be alleviated. Furthermore, having the amplifier in a pumped operating state will allow continuous photo-annealing and recovery

to occur, decreasing the amount of degradation. Design parameters such as fiber composition and fiber length are

136
extremely critical to the performance of these components in a radiation environment. Further work is necessary to model the degradation and recovery mechanisms.

5. ACKNOWLEDGEMENTS

The authors would like to thank W.A. Kolasinski of the Aerospace Corporation for his invaluable assistance with the proton measurements. This work was funded by Aerospace IR&D.

REFERENCES

[1] G. M. Williams, M. A. Putnam, C. G. Askins, M. E. Gingerich, and E. J. Friebele, "Radiation effects in erbiumdoped optical fibers," Electron. Lett., vol. 28, 10 Sept. 1992, p. 1816-1818.

[2] W. C. Goltsos, "Radiation-induced loss studies in Er-doped fiber amplifier systems," SPIE, vol.2699. May 1996,

p. 304-309.

[3] G. M. Williams and E. J. Friebele, "Space radiation effects on erbium-doped fiber devices: sources, amplifiers,

and passive measurements," IEEE Trans, on Nuc. Sci., vol. 45, no. 3, June 1998, p. 1531-1536.

[4] G. M. Williams, M. A. Putnam, and E. J. Friebele, "Space radiation effects on erbium doped fibers," SPIE vol. 2811, May 1996, p. 30-37.

[5] E. W. Taylor, S. J. McKinney, A. D. Sanchez, A. H. Paxton, D. M. Craig, J. E. Winter, R. Ewart, K. Miller, T. O'Connor, R. Kaliski, "Gamma-ray induced effects in erbium-doped fiber optic amplifiers," SPIE conference on Photonics for Space Environments VI, July 1998, p. 16-23.

[6] R.H. Boucher, W.F. Woodward, T.S. Lomheim, R.M. Shima, D.J. Asman, K.M. Lilian, J. LeGrand, and G.J. Goellner, "Proton-induced degradation in interferometric fiber optic gyroscopes," Opt. Eng, vol 35, no. 4, pp. 955-976, 1996.

[7] P.C. Becker, N.A. Olsson and J.R. Simpson, Erbium-doped Fiber Amplifiers: Fundamentals and Technology, Academic Press, Boston, 1999.

[8] C.R. Giles and E. Desurvire, "Modeling erbium-doped fiber ampifiers", J.Ligthwave Tech., vol. 9, no. 2, pp. 271-283, 1991.

137
1000 1 100 1200 1300 1400 1500 1600 wavelength (nm)

Fig. 3 Transmission loss spectra for 3-m HP980 and HE980 fibers after 50 krad exposure to ^Co gammas and 5.6 and 28 MeV protons. The spread in the HP980 data is attributed to variations in the dooant concentrations.

MP980, OtoSOkRad MP 980, 0 to 50 kRad

20

-10

• 0 krad

10 krad 20 krad

A 30 krad

A 40 krad

0 50 krad

10 20 30 Pump power, mW

(a) HE 980, OtoSOkRad

40

20 30

Puirp power, mW

(b) HP 980, OtoSOkRad

20 30 Pump power, mW

(e)

so

50

50

• 0 krad

10 krad 20 krad

^ 30 krad

A 40 krad

o 50 krad t—» i10

20 30 Pump power, mW

(b) HE98O,0-S0kRad

40

20 30 Pump power, mW

(d) HP 980,0-50 kRad

40

20 30 Pump power, mW

(f)

50

• 0 krad 10 krad

i

20 krad

A 30 krad

A 40 krad ^ 0 SO krad

n

50

• 0 krad

10 krad 20 krad A 30 krad

A 40 krad 0 50 krad

• y if B H ~ —

Fig.4 Gain and Noise Figure as a function of pump power for 5 levels of radiation damage. The solid lines represent calculations while the symbols represent the measurements, (a) and (b) MP980. (c) and (d) HE980. (e) and (0 HP980.

138
ISSUES IN RAMAN GAIN MEASUREMENTS

R.H. Stolen

Virginia Tech., Dept. of Electrical and Computer Engineering

Blacksburg, VA 24060, stolen@vt.edu

I. INTRODUCTION

Applications of fiber Raman amplifiers as well as undesirable Raman interactions in dense WDM

transmission systems have increased the importance of accurate measurements of Raman gain. Fortunately

this need for increased accuracy comes at a time of increased precision and ease of fiber measurements.

Now details of Raman gain measurements that had previously been lost in the measurement uncertainties are becoming significant.

Here these issues of Raman measurements are divided up into: the basic fused silica Raman spectrum, the

effects of Germanium doping, the effective area, frequency dependence of Raman gain, polarization

dependence, and the influence of four-wave mixing. In all of these areas there is need for both improved

understandmg and better measurements.

II. BASIC FUSED SILICA SPECTRUM

Raman gain measurements in both silica-core and Ge-doped fibers look very much like the fused silica Raman gain spectrum shown in Fig. 1 . This gain spectrum is a composite of measurements by several

different authors of spontaneous Raman scattering m bulk silica samples and silica-core fibers. [1] The two

curves are for scattering of light polarized parallel and perpendicular to the polarization of the exciting

light. The spontaneous spectrum differs from the gain spectrum by the Bose-Einstein population factor,

n+1, where n=l/[exp(hvc /ksT)-!]; h is Planck's constant, v is the phonon fi-equency in cm"', ks is the

Bohzmann constant, T is the temperature in degrees Kelvin, and c is the vacuum velocity of light.

1.2 T

^

1

0 200 400 600 800 1000 1200 1400

frequency (cm'^)

Fig. 1 . Parallel and perpendicular Raman gain spectrum of fused silica as compiled from various

sources. [1]

139
The absolute value of the peak gain usually comes from one of two sources. In the first, a gain coefficient

of 1.86 X 10-" cm/W was obtained for a pump at 526 nm in a fiber short enough to maintain linear polarization; the results agreed with bulk measurements in fijsed silica. [2] Including the wavelength

dependence of the gain coefficient leads to a convenient gain coefficient of approximately 1.0 x 10"" cm/W

for a pump wavelength of 1.0 microns. A gain of 2.46 x 10"" cm/W was obtained from spontaneous

scattering from a 488 nm source in bulk silica samples. [3] This translates to a gain of 1.2 X 10"" cm/W at

1 .0 microns. This difference between gain measurements is not yet resolved.

A discussion of the spectrum itself can be conveniently divided into three regimes:

A. 100-500 cm'': This is the regime of interest for fiber Raman amplifiers. Over this range of frequency

shifts the gain is highly polarized. The perpendicular contribution to the gain is small and gain

measurements agree fairly well with measurements of spontaneous scattermg. As discussed later, the gain

for randomly polarized light is half that for pump and signal light of the same polarization state.

B. 0-100 cm'': This low frequency regime is of concern for Raman interactions of channels in dense WDM

systems. Here bulk Raman studies show that the gain is depolarized with the perpendicular gain about 1/3

of the parallel gain. Note that a measurement of perpendicular and parallel Raman gains in a silica-core

polarization-maintaining fiber found a ratio of perpendicular to parallel gains of 45%. The discrepancy

between bulk and fiber measurements is not understood. [4] This is a regime where more work is needed.

C. >500 cm'': For frequency shifts greater than 500 cm"' the Raman gain is small and generally not of

importance in silica-based optical fibers. There are two exceptions in which a strong sharp Raman line appears when certain dopants are incorporated into the glass. These lines are useful for producing large

frequency shifts in fiber Raman lasers. Phosphorus doping leads to a line at 1330 cm"' [5] and H2 or D2 in the glass produces lines with extremely large shifts of 4139 and 2975 cm '[6] Note that the weak Raman

line at 1 070 cm"' in pure fused silica is the strongly infrared-active Si-0 stretching vibration whose

overtones produce the long wavelength transmission cutoff around 1.7 microns in silica fibers.

III. GERMANIUM DOPING

Germanium doping raises the Raman gain coefficient. The effect is small in standard fibers, which contain

about 4 mole % Ge02 but can raise the gain by about 50% in dispersion-shifted fibers. The gain is more

than doubled in dispersion-compensating fibers. A careftil series of measurements has been reported for spontaneous Raman scattering vs Ge02 concentration in a Ge-doped MCVD preform. [7] The maximum

increase is at a shift of 430 cm"' where 20 mole % Ge02 raises the gain by a factor of 2.6 over pure silica glass. For shifts less than 200 cm"' the increase is approximately a factor of 2.

In a fiber the mode field penetrates into the cladding so the effective Raman gain coefficient will be some

average over the doping profile, which will be less than the increase in gain from the maximum at the core

center. The proper way to include the increase in Raman gain from Ge-doping is to include it in the

calculation of the overlap integral. Similar issues arise in calculating the increase in nonlinear index from

Ge-doping. [8]

IV. EFFECTIVE AREA

Most of the increase in Raman gain in highly Ge-doped single-mode fibers comes from a decrease in effective area. Usually the uncertainty in measurement of Aeff is greater than most of the details discussed

here.

140
Note that the effective area for Raman gain is actually:

j^p^rdrj^^s^rdr

AefF = In

\ ^p' ^s' rdr

where 4^p and are the mode fields of the pump and signal. So far it has made little difference whether

AefF is calculated using only ^p or Ts. In some fiber designs AefF changes rapidly with wavelength and more care will be necessary.

V. WAVELENGTH DEPENDENCE

Raman gain is usually taken at the pump wavelength and the gain varies as 1/ ?ip. Actually the gain will fall off slightly faster with wavelength because the polarizibility of the ions in the glass decreases slightly with

wavelength. This is the same decrease in polarizibility that causes the wavelength dependence of both the

linear and nonlinear refractive indices. If we make some rather drastic assumptions we can argue that the

gain coefficient should vary as: (n^ -\fl n^ . This is not a large effect. If we calculate n using standard

Sellmeier coefficients [9], it amounts to only 2% between 1.0 and 1.5 microns. Including this factor,

however, means that the gain measurement at 526 nm of Ref2 translates to 0.96 x

10"'' cm/W at 1 micron.

This may explain a some lower values of gain coefficient obtained at 1.5 microns.[10]

There are some other approximations made in the relation of the gain coefficient to the spontaneous Raman

cross section. The small difference between pump and signal frequencies is neglected as well as the

difference in refractive indices at pump and signal wavelengths.

VI. POLARIZATION DEPENDENCE

Most Raman gain measurements are made in non-PM fibers where polarization is scrambled. It is usually a good approximation to take the gain with scrambled polarizaions to be 54 that for linear polarization of

pump and signal waves. This, however, neglects the perpendicular contribution to the gain. In the range

100-500 cm ' the perpendicular contribution is small. In the range 0-100 cm ' the perpendicular

contribution needs to be included. As a first approximation the simplest procedure for randomly polarized

light is to add the parallel and perpendicular gains and divide by two.

In low PMD fibers the measured gain may actually be closer to the polarized gain even though the state of

polarization is not maintained. [1 1] This is because the gain is maximum as long as the states of

polarization of the pump and signal waves are the same - even for elliptical polarizations. Eventually the

gain will drop when the polarization states get out of step because of the wavelength difference between

pump and signal. For example, this polarization length will be 2 km in a low PMD fiber with effective

birefringence of 10"* for a wavelength difference between pump and signal of 100 nm in the 1500 nm

wavelength range. At 1 0 nm pump-signal separation the polarization length is 20 km, which is the

nonlinear effective length for typical fibers in this wavelength range. This means that for Raman interaction

between dense WDM channels the gain should often be the polarized gain.

There is still the additional factor of the perpendicular gain in the 0-100 cm'' range. How this contributes

when the polarization state varies is still not well understood. Treatment of the total parallel and

perpendicular gains in this region should probably start with a general treatment using both the parallel and

perpendicular Raman response functions. [12].

141
VII. FOUR-WAVE MIXING

An additional complication for Raman gain measurements with small frequency separation between pump

and signal is four-wave mixing which can also amplify the signal. This problem can be overcome if the

dispersion is high enough that the coherence length for four-wave mixing is short.

The best way to avoid problems with four-wave mixing as well as from uncertainties over the polarization

length is to use a counterpropagating pump. This is the configuration for most Raman amplifiers to avoid

introducing noise from pump fluctuations. The real problems are likely to arise with dense WDM channels

VIII. CONCLUSION

So far, the major problems with Raman gain measurements seem to be in accurate measurement of the

effective area and an understanding of the effect of randomly varying polarization for small wavelength

separation between pump and signal. Of the additional corrections to the properties of the gain coefficient,

the next of importance is probably the modification to the IfK, dependence.

REFERENCES

1. R.H. Stolen, C.Lee, and R.K. Jain, "Development of the stimulated Raman spectrum in single-mode

fibers," J.Opt.Soc.Am., 652 (1984). The low frequency data includes subsequent measurements of

scattering at small frequency shift and low temperature. The perpendicular spectrum was compiled from

several published sources. Both curves are available as data files from the author.

2. R.H. Stolen and E.P. Ippen, "Raman gain in glass optical waveguides," Appl. Phys. Lett., 22, 276

(1973).

3. R.Hellwarth, J.Cherlow, and T-T. Yang, "Origin and frequency dependence of nonlinear optical

susceptibilities of glasses," Phys.Rev. B 11, 964 (1975).

4. D.J.Dougherty, F.X.Kartner, H.A.Haus, E.P. Ippen, " Measurement of the Raman gain spectrum of

optical fibers," Opt. Lett., 20, 31 (1995).

5. V.l.Karpov, E.M.Dianov, A.S.Kurkov, V.M.Paramonov, V.N.Protopopov, M.P.Bachynski,

W.R.L.Clements, LD-pumped 1.48-fim laser based on Yb-doped double-clad fiber and phosphorosilicatefiber Raman converter," OFC'99, San Diego, CA Feb.21-26, 1999: paper WM3.

6. A.R. Chraplyvy and J. Stone, "Single-pass mode-locked or Q-switched pump operation of D2 gas-inglass fiber Raman lasers operating at 1.56 ^m wavelength," Opt. Lett., 10, 344 (1985).

7. S.T.Davey, D.L.Williams, B.J.Ainslie, W.J.M.Rothwell, B.Wakefield, "Optical gain spectrum of GeOaSiOa Raman fibre amplifiers," lEE Proceedings, 136, 301 (1989).

8. D.L.Philon, D.W.Peckham, I.Brener, "Measurement of the non-linear index of refraction N2, for various

fiber types," OFC'2000, Baltimore, MD March 7-10 (2000); paper ThL5.

9. G.P. Agrawal "Fiber-Optic Communication Systems," (John Wiley, New York, 1997) p.41.

10. V. L DaSilva and J.R.Simpson, "Comparison of Raman efficiencies in optical fiber," OFC'94, San

Jose, CA, Feb.20-25 (1994): p. 136.

1 1 . R.H. Stolen, "Polarization effects in fiber Raman and Brillouin lasers," IEEE J.Quantum Electron.,

QE-15, 1157 (1979).

12. C-J.Chen, C.R.Menyuk, M.N.Islam, R.H.Stolen, "Numerical study of the Raman effect and its impact on soliton-dragging logic gates," Opt. Lett., 16, 1647 (1991).

142
Broadband Raman Gain Characterization in Various

Optical Fibers

F. Koch', S.A.E. Lewis'"^, S.V. Chernikov*% and J. R . Taylor'

*Femtosecond Optics Group, Physics Department, Imperial College,

Prince Consort Road, London SW72BW, UK

*IP Fibre Optics Ltd, Hoppingwood Farm, Robin Hood Way, London SW20 OAB

Email: fkoch @ic.ac. uk

Raman scattering in optical fibers currently attracts considerable research interest.

The use of stimulated Raman scattering as a gain mechanism gives certain advantages

over Erbium doped Fiber Amplifiers (EDFA) in that gain can be provided aDywhere

within the v^avelength range of transparency in optical fibers. The use as distributed

amplifiers as a way of upgrading older systems and for soliton transmission is also

advantageous compared to EDFAs. Additional Raman scattering is a source of noise in

wavelength division multiplexing (WDM) systems where the Raman gain is a source of

cross-talk between channels. For accurate modeling and estimation of these effects the

Raman gain coefficient needs to be known accurately. The effect of Germanium doping

has been investigated previously [1], however here we show the fine structural differences

for different dopant levels and provide an estimation of the relation between maximum

gain and dopant level. The technique for the determination of the Raman gain coefficient is based on the

measurement of the amplified spontaneous emission (ASE) [2] along the test fiber

pumped by a narrow Unewidth high-power fiber Raman laser. Figure 1 shows the

experimental set up with a circulator used to multiplex and de-multiplex the pump and

ASE signal from the fiber under test. The ASE power can be determined similar to the

technique in reference [3], but in Equation 1 we distinguished between the loss at the

pump wavelength (Op) and the loss at the signal wavelength (used in the calculation for

the Gain G). Also included is a thermal factor <l> [4]. Vs is the signal frequency, Af the

frequency difference between pump and signal and h is Planks constant. For every pump

power P the ASE power in the backwards direction can be calculated from Equation 1 . In

143
the experiment we measured the dependency of the ASE power with an optical spectrum

analyser (OSA) as a function of the pump power. All measurements had to take the losses

between point A (see Figure 1) inside the test fiber and the spectrum analyser into

account. A least square fitting algorithm fitted the measured data to Equation 1 with the

Raman gain coefficient Cr=Gr/Aeff as variable. For higher pump powers Rayleigh

scattering of the forward ASE into the backward direction also contributed to the

measured signal and increased the error of the fitting. By Umiting the pump power this

error was minimized and no additional terms for the Rayleigh scattering were required to

be added.

Figure 2 shows the normalized gain coefficient for three different fibers with

different Germanium concentrations. The pure siUca core fiber (PSCF) shows more fine

structure and the secondary peak (at 490 cm'^) is higher than the first (at 444 cm'^). Also

the peaks at longer wavelength shifts are narrower. The dispersion-shifted fiber (DSF)

with a medium Germanium content (-5-6 %) still shows the second peak (at 480 cm'')

but it is much lower now. In the high Germanium content dispersion compensating fiber

(DCF) (16% to 18%)all the features in the gain spectrum are washed out as a result of the

Germanium doping.

In Figure 3 the peak Raman gain coefficient for all measured fibers is shown as a

function of Germanium content. A clear connection between the Raman gain coefficient

and the Germanium indicates that the dispersion compensating fiber gives the highest

Raman coefficient.

This figure only indicates the Raman gain coefficient Cr, however the maximum

gain achievable and the performance of an amplifier depends on many more factors such

as the losses, effective area of the fiber, Rayleigh scattering and noise figure of the

amplifier [5]. For minimum cross talk in WDM systems other factors such as the

dispersion have to be taken into account.

In conclusion we have measured the Raman gain coefficient for fibers with

different Germanium dopant content and showed structural differences for the different

dopant levels. A clear relationship between the Germanium content and the maximum

Raman gain coefficient was shown.

144
exp(-ap-^)

2hVgAfO

Equation 1

Fibre Under Test (ID Splice

Circulator

7r\

Point A

Figure 1 "Experimental Set-Up"

3

<

c

o

"ooO

c

CO CDc

CDE

CO

Splice

Splice

Pump in @1455nm

Raman Shift [cm ]

OSA

Figure 2 "Normalized Gain of Various Fibers"

145
^ 1.5- E 2*DCF

2 * DSF

6

(0 O

c

E 0.5-1

PSCF + STF

0.0 -T10

—I—'—r- 12 14 —

r16 —

I 18 20

Germanium Content %

Figure 3 "Gain for Different Germanium Contents"

References:

(1) "Optical gain spectrum of Ge02-Si02 Raman fibre amplifiers", S.T. Davey, D.L.

Williams, B.J. Ainslie, W.J.M. Rothwell, B. Wakefield-B, lEE-Proceedings-J-

(Optoelectronics), Vol.136, No.6; Dec. 1989; p.301-6

(2) "Technique For Measurement Of The Raman Gain Coefficient In Optical Fibers", D.

Mahgerefteh, D.L. Butler, J. Goldhar, B. Rosenberg, G.L. Burdge, Optics Letters, 1996,

Vol.21, No.24, pp.2026-2028

(3) "Theory And Implementation Of A Raman Active Fiber Delay-Line", E. Desurvire,

M.J.F. Digonnet, H.J. Shaw, Journal Of Lightwave Technology, 1986, Vol.4, No.4,

pp.426-443

(4) "Temperature-dependent gain and noise in fiber Raman amplifiers", S.A.E. Lewis,

S.V. Chemikov, J. R . Taylor, Optics Letters, 1999, Vol.24, No.24, pp. 1823- 1825

(5) "An Analysis Of The Improvements In OSNR From Distributed Raman Amplifiers

Using Modem Transmission Fibres", C. Fludger, A. Maroney, N. JoUey, R. Mears,

OFC2000, Paper FF2

146
Raman spectra of line fibres

measured over 30 THz

D. Hamoir, N. Torabi, A. Bergonzo, S. Borne and D. Bayart

Alcatel, Corporate Research Center, Route de Nozay, 91460 Marcoussis, France

E-mail : dominique.hamoir@alcatel.fr

Abstract: The Raman efficiency ofline fibres is measured over a total range of 30 THz. The effect of tuning

the pump from 1400 to 1486 nm is shown. The information presented is essential for designing future wide

band transmissions with distributed Raman amplification.

3 6 9 12 Frequency Shift (THz)

15 1812 15 18 21 24 Frequency Shift (THz)

1400-nm pump

PSCF

\V

^ SMF _

\JeralLight

NZDSF-

\^NZDSF+_

27 30

Figure 1 : Raman gain coefficient in W'.km'', with a scale of 0.4 W'.km'Vdiv. For each fibre, the zero line is the division below the fibre name.

Introduction The transmission capacity of optical fibres is limited by the

signal-to-noise ratio required at reception and by non-linear

effects. Distributed amplification is one of the solutions that allow to further push the performance of transmission

systems. Stimulated Raman scattering (SRS) is a privileged way of obtaining a distributed amplification because it is intrinsic to every silica-based glass. This allows using

standard line fibres as amplification media /I/. Although it is beneficial for distributed amplification, SRS is a major limitation in very-dense, wide-band transmission systems /2,3/. This impairment is known as Raman tilt. The Raman spectra of line fibres have been measured in the past /I -4/. However these measurements were often relatively limited in bandwidth. They frequently disregarded Ramanshifts exceeding 18 THz and those focusing on amplification considerations usually neglected small shifts. However the design of wide-band systems with distributed Raman

amplification requires the prediction of pump-pump and

signal-signal interactions with small frequency differences, as well as the interactions between the higher-frequency pumps

and the lower-frequency signals. Hence it is fundamental to know the Raman gain coefficients of the line fibres for shifts ranging from virtually 0 THz up to 26 THz or more. To answer this need we measured the Raman efficiency of

standard line fibres from 120 GHz up to 30 THz. We also got valuable information on the effects of changing

the wavelength of the pump inside the 1.4-nm band when

considering amplificafion in the 1.5-(im band.

147
Residual pump Calibrated power

control Pump in

Signal In o^

1487-1629 nm ""° calibrated qSA

Figure 2 : Set-up for precise on/off gain measurement

Experiments

Indeed we pumped the fibres with wavelengths sitting on

both sides of the pumping-band used for Raman

amplification in the C+L band : 1486 nm for a first series of measurements and 1400 nm for a second. The signal was

tuned fi-om 1487 to 1629 rim, centred on the C+L band. The

experimental set-up is shown in figure 2. We choose to

inject 200 to 300 mW of pump into the fibre under test. This choice ensured minimal measurement errors along

with low amplified spontaneous emission and amplified

Rayleigh scattering. The pumping power was measured

thanks to a calibrated coupler. The splice to the fibre was

calibrated by cut-back. Hence the pumping power injected

into each fibre was known with a good precision. Since Raman gain is rather low, even very low for certain frequency shifts, an extreme accuracy is required in the gain measurement. We choose to perform "on/off' gain measurements to cancel out every inaccuracies on the (spectrally-dependent) attenuation of components, of

connectors, of splices and of the fibre itself For each fibre we acquired a reference spectrum at the "signal out" port, for which the pump was off, and a second spectrum in the exact same conditions, except that the pump was turned on

and its power precisely controlled. The precision of the gain curve obtained after subtracting the two spectra was

limited only by the power-repeatability of the signal laser and by the relative accuracy of the optical spectrum

analyser (OSA). The power of the tunable signal source was adjusted before measuring the spectra. A small-signal is required so that the "on"-spectrum is not affected by saturation or by stimulated

Brillouin scattering in the high-gain regions. However the

signal should be powerful enough to get a good precision vs integration-time compromise and to ensure that the contribution of amplified spontaneous emission to the "on"- spectrum is negligible. As a consequence we checked the gain as a function of the signal power and then choose a

signal power in the upper part of the linear regime.

Results The pump lasers were not polarised ; the Raman efficiency measured (figure 1) is the exact mean value of the

efficiencies obtained when the polarisafion of the signal is maintained either parallel or orthogonal to that of a polarised pump. The Raman efficiency of a given fibre is defined as Cr = Gor^ofr- (n(10)/10)/(Pp>^.Lefr), where go„/off is the on/off gain measured with a pumping power Ppun^, injected

into the fibre of effective length L^ff.

In figure 1 the fibres are ranked by their peak gaincoefficient at 13.3-THz, when pumped at 1486 nm

(figure 1, left). The non-zero-dispersion shifted fibres with

positive and negative dispersion at 1.55 ^lm presented the highest peak coefficients (0.84 and 0.77 W'.km''). Then came the TeraLight (0.57 W '.km"'). The lowest peak

coefficients were obtained with a standard single-mode

fibre (0.45 W '.km"') and with a pure silica core fibre (0.43 w '.km"'). All these fibres are respecfively referred to

as NZDSF+, NZDSF-, TeraLight, SMF and PSCF.

Table 1 : Fibres under test

Length Effective area Chromatic Dispersion

(km) @ 1558 nm dispersion slope (Mm=) (ps.nm''.km'') (ps.nm" .Ian'') NZDSF+ 24.7 55 3.2 0.07 NZDSF- 23.1 55 -2.3 0.08 TeraLight 50.0 67 8.0 0.058 SMF 23.3 83 17.0 0.058 PSCF 18.4 78 18.0 0.055 Measured Nominal at 1.55 ^m

If the pump and signal modes were not altered, changing

the wavelength of the pump should not modify the peak

efficiency of a given fibre when expressed in (photons/s)"'.km"'. In such a case the peak efficiency expressed in W '.km ' should only scale with the pump

wavelength because the energy per pump photon changes.

In particular the Raman efficiency should be reduced by

6 % when pumping at 1400 nm instead of 1486 nm. On the contrary we found much higher peak efficiencies with the 1400-nm pump (figure 1, right) than with the 1486-nm pump (figure 1, left). Indeed the peak efficiencies obtained with the 1400-nm pump were L25, 1.29, 1.01, 0.65 and 0.65 W '. km"' for respectively the NZDSF-i-,

NZDSF-, TeraLight, SMF and PSCF. This strong improvement in efficiency can be explained as follows : with lower pump and signal wavelengths the pump and

signal modes are more confined into the Ge-doped core, in addition to what the pump-to-signal mode overlap is improved.

Figure 3, next page, compares the Raman efficiencies of the

fibres for small frequency shifts, corresponding to signal- signal and to pump-pump interactions in C+L band systems

with distributed amplificafion. Our results are in good

agreement with /2/. The gain spectra can be exploited for

shifts beyond 300 to 500 GHz, the lower part of the spectra being affected by back-reflections of the pump into the

signal path. As a general rule, fibres that offer higher peak

gain coefficients unfortunately induce more tilt.

148
0 3 6 9 Frequency Sh'rft (THz)

Figure 3 : Raman gain coefficient in W'.km"'.

This is confirmed by figure 4 where each gain curve has been normalised with respect to its peak value. Figure 4,

left shows that the normalised gain curves measured are somewhat similar in shape for irequency-shifts up to

1 8 THz, despite differences to be expected in their composition. However some slight differences can be observed fi^om one

fibre to the other. For fi-equency shifts involved in Raman

tilt, below 9 THz, the PSCF presents higher normalised

gain coefficients than the other fibres. This is in agreement

with reference 151. Indeed, the spontaneous Raman spectra of vitreous Si02 and Ge02 in reference 151 permit to derive

that the amplitude of the so-called boson-peak, at about

3 THz, is higher with Si02 than with Ge02 when

normalised with respect to the main, 13.3-THz peak. Additional information can be derived fi'om figures 1 and 4. The broad 13.3-THz peak is strongly affected by Ge doping

(figure 1 ). This main peak extends up to beyond 20 THz. The sharp secondary peaks at about 14.5 THz and 18 THz

seem to be specific to pure silica as their absolute amplitudes are much less affected by Ge doping (figure 1), which turns into very sharp peaks for the PSCF when

normalised (figure 4). These deductions are confirmed by

the data of reference 151 on pure Si02 and Ge02 glasses.

Conclusion We have measured and compared the Raman efficiencies of

standard line fibres for fi-equency shifts ranging from a few

hundreds of GHz up to 30 THz, allowing to record even the 18-THz and 24-THz peaks of the Raman spectra. We

observed that the 13.3-THz-peak gain-coefficient of NZDSF+ was twice as large as that of PSCF. We also showed that the peak efficiency was much higher for a 1400-nm pump than for a 1486-nm one, due to the important change in pump/signal/core overlap. We also saw

that Raman tilt unfortunately increased with the peak gain

coefficient. Finally the normalised gain curves were found

to be rather similar from one fibre to the other. These new data are essential for designing future transmission systems based on distributed Raman

amplification.

In addition we would like to underline that the attenuation

at the pump wavelength is also paramount when designing

a system with Raman amplification. Aspects such as the attenuation at the signal wavelength, the resistance to other non-linear effects and PMD are as well of great importance. The best fibre for a given application is the best compromise.

Acknowledgement

The authors wish to thank P. Guenot, F. Leplingard and

S. Bigo for fioiitflil discussions.

References

III C. Fludger et al., paper FF2, OFC'OO (2000).

72/ S. Bigo et al., paper WJ7, OFC'99 ( 1 999).

/3/ G. Shaulov et al, paper TuA4, OFC'OO (2000).

74/ V. L. da Silva et al., paper WK13, OFC'94 (1994).

75/ F. L. Galeener, 8"^ International Conf on Raman

Spectroscopy, p. 529 (1982).

149
I

ir
Raman Gain Measurements in Optical Fibers

Stuart Gray

Coming Incorporated, SP-AR-0 1-1,Coming, NY, 14831

The use of distributed Raman amplification has recently been demonstrated to be a key

technology in upgrading the capacity or extending the reach of optical fiber communication

systems [1]. The design of systems including Raman amplification requires a detailed

understanding of how the Raman gain varies between different commercially available fiber

types and also the variation within the manufacturing tolerances of a particular fiber design. In

this paper I will describe a simple technique for measuring the spectrum of the Raman gain

coefficient in an optical fiber and discuss measurements made on fibers with different effective

areas. I will also present data on the gain and noise figure characteristics of distributed Raman

amplification in different fiber designs.

The technique for measuring Raman gain coefficients is illustrated in Figure 1 . The output of a broadband LED source covering the spectral range from 1300-1700 nm is transmitted through

the fiber under test and the light at the output is collected on an optical spectrum analyzer (OSA).

The length of fiber measured is typically 5-20 km. The pump source is a cascaded Raman

resonator with an output wavelength of 1486 nm pumped at 1 100 nm by an Ytterbium double

clad fiber laser. In all measurements the pump and signal are counter-propagated.

To calculate the Raman gain coefficient three spectra are recorded. First the spectrum of the un- amplified LED signal is taken. Second, the pump is turned on and the launched power is monitored through a calibrated tap coupler. The spectrum recorded at the output contains both

the amplified signal and amplified spontaneous emission (ASE). Finally, the LED is turned off

while leaving the pump on and a spectrum of the ASE is recorded and subtracted from the

previous spectrum to obtain the spectrum ofjust the amplified signal. The pump power is then

calibrated and the Raman gain coefficient calculated. This technique works because in the small

signal (un-depleted pump) regime the amplified signal and ASE are independent of each other

(see, for example, [2]). The Raman gain coefficient is then calculated using the following

equation.

( ^ P ^ G = exp ^^(l-exp(-aL)) \Aa )

Here, G is the signal gain, a is the fiber attenuation at the pump wavelength, L is the fiber length,

P is the pump power launched into the fiber, gR is the Raman coefficient of the fiber material and

A is the effective area. The quantity gR/aA can be used to compare the Raman efficiency of

different fibers and will be referred to below as the Raman figure of merit. The use of a broadband signal source for these measurements makes the procedure very fast compared to measuring the gain coefficient at discrete points using a tunable laser and also provides an

essentially continuous spectrum.

The value of gR/A measured using the above technique is shown in figure 2 as a function of

frequency offset from the pump for two commercially available fiber types, standard single151
mode fiber (SMF) and a non-zero dispersion shifted fiber (NZ-DSF). The Raman coefficient is plotted in units of W ' km"' for an unpolarized pump. The properties of these fibers are summarized in Table 1 where the attenuation and mode field diameter are given at the pump

wavelength. The NZ-DSF has a Raman coefficient 20% higher than SMF. This difference results

from the smaller effective area ofNZ-DSF combined with a higher GeOi content than exists in

SMF. The figure of merit for the NZ-DSF fiber is also correspondingly larger than in SMF and

demonstrates the higher Raman pump efficiency of the NZ-DSF fiber.

The design of a product for the distributed Raman amplification market has to take into account

variations in the Raman efficiency caused by the manufacturing distribution. This has been

studied for NZ-DSF by measuring the Raman figure of merit of several fibers with varying mode

field diameters. The properties of these fibers are summarized in table 2. The figure of merit of

these fibers varied from 8.7 to 9.4, a difference of 8%. This directly leads to an 8% variation in

the pump power required to achieve a fixed amount of gain in these fibers and has to be included

in the design of an appropriate pump module.

The design of a system incorporating distributed Raman amplification has to include the noise

properties of the distributed amplification process and its interaction with the lumped erbium

doped fiber amplifiers (EDFAs). Therefore the noise figure of the distributed Raman amplifier

has to be characterized in different fibers as a fiinction of the distributed gain. The gain and noise

figure of the distributed amplifier are defined by equating the gain and ASE to be equivalent to

that generated by a discrete amplifier placed at the end ofthe transmission span. This amplifier is in general not physically realizable and often gives noise figures that are below zero.

The gain and equivalent noise figure (ENF) are measured using a setup similar to that shown in

Figure 1 but with the LED replaced by a DFB laser and the test fiber replaced by typically 75-90

km of transmission fiber to represent a complete system span. A signal wavelength of 1590 nm

was chosen for these measurements which corresponds to the peak of the Raman gain curve for

the 1486 nm pump wavelength. The noise figure is calculated using ASE interpolation method

on the OSA and includes corrections for the coupling loss of the circulator.

Measured values of gain and noise figure for SMF and NZ-DSF fibers are shown in Figure 3. In

each case, a power of -10 dBm was used at the input. The behavior of the two fibers is very

similar. Initially the ENF decreases almost linearly with increasing Raman gain to a minimum at gains of -30 dB. Minimum equivalent noise figures of —4 dB were observed in both fibers. As

the gain is increased ftirther the measured ENF increases rapidly due to the effect ofdouble

Rayleigh scattering of the ASE. Identical behavior has been seen in both fibers for input powers

in the range -^0 to 0 dBm. Gains greater than 37 dB were not observed in SMF due to the

limited pump power available. The impact of double Rayleigh scattering, a cause of multiplepath interference, is expected to be different for different fiber types because the capture

coefficient of a fiber for scattered light varies as the inverse of effective area [3]. This is likely to place an upper limit on the amount of distributed Raman gain which can be used in a particular

fiber type before double Rayleigh scattering noise degrades system performance.

152
In conclusion, I have described a technique for measuring the Raman gain spectrum of an optical

fiber using a broadband signal source and have used this setup to measure the variation of

Raman gain efficiency in fibers with different mode field diameters. The distribution of Raman

properties requires a similar variation in the levels ofpump power required to achieve a

particular level of Raman gain. The noise figure as a function of distributed Raman gain has also

been measured for two different fiber types. The behavior of both fibers is similar in the regime

where double Rayleigh scattering is negligible. When double Rayleigh scattering becomes

important the noise figure is expected to increase most rapidly in the fiber with the smaller

effective area and higher capture coefficient.

LED

Circulator

Pump

Pump monitor

I Unabsorbed

Pump

OSA Figure 1. Raman coe£Bcient measuring system

0.6

0.5

0.4 <

0.3 C

u

'ou

o

Q

c

S 0.1

0.2

0.0

-A

7^

/

/ / —V

/

7

// /

\\

\\ —

SMF

NZ-DSF

0 2 4 6 8 10 12 14 16 18 20

Frequency (THz)

Figure 2 . Raman spectra of transmission fibers

153
Fiber Type MFD @ 1480 nm

(lam)

gR/A

(km"' W')

a @ 1480 nm

(km-')

Figure OfMerit

SMF 9.78 0.380 0.05 7.6

NZDSF 8.85 0.457 0.05 9.1

Table 1. Parameters of fibers in figure 2

Fiber MFD (1550 mn) a (1550 mn) Figure OfMerit

(dB/km) (pump =1486 nm)

1 9.77 0.2 9.38

2 9.68 0.2 9.04

3 9.78 0.2 9.22

4 9.78 0.2 9.12

5 9.77 0.2 9.04

6 9.74 0.2 9.23

7 9.74 0.2 9.35

8 9.77 0.2 8.69

Table 2. Properties of several NZ-DS fibers

- NZ-DSF

SMF

0 10 20 30 40

Gain (dB)

Figure 3. Gain v NT for SMF and NZ-DSF

1. P. B. Hansen, L Eskildsen, S.G. Grubb, A.J. Stentz, T.A. Strasser, J. Judkins, J.J. DeMarco, R.

Pedrazzani and D.J. DiGiovanni, Photonics Technology Letters, vol. 9 pp 262-264 (1997).

2. S.R. Chinn, Electronics Letters, vol 33, pp 607-608 (1997).

3. E. Brinkmeyer, Electronics Letters, vol 16, pp 329-330 (1997).

154
Overview of coherent reflectometry techniques:

characterization of components and small systems

M. Wegmiiller, P. Oberson, J.P. von der Weid*, O. Guinnard, L. Guinnard,

C. Vinegoni, M. Legre, N. Gisin

Group of Applied Physics, University of Geneva, 20 Ecole-de-Medecine, CH-121 1 Geneve 4,

Switzerland, mark. wegmuller@physics. unige. ch

* Center for Telecommunications Studies, PUC-Rio, Rio de Janeiro, 22453-900 Brazil

Abstract

Coherent reflectometry techniques have important advantages over direct detection techniques:

larger sensitivity, larger dynamic range, better resolution, and spurious light suppression. The

measurement range is typically limited to <1 m for Optical Low Coherence Reflectometry, and

to <~1 km for Coherent Optical Frequency Domain Reflectometry. Especially the second

technique is therefore well suited for measurements of optical modules with extended

components andfor small systems.

Introduction

The advantages of coherent reflectometry techniques are well known. In fact, they are still the only techniques giving two point resolutions in the mm range. Of the two major coherent

reflectometry techniques Optical Low Coherence Reflectometry (OLCR) and Coherent Optical

Frequency Domain Reflectometry (OFDR), the first is well established and widely used for component characterization [1]. In typical OLCR implementations, the measurement range is however limited to below 1 m. None of the proposals to extend this value have been retained on

a large basis so far, mainly for the sake of having robust and compact OLCR devices.

Although a sub-metric range is well adapted for the characterization of single, compact

components, it is too small for larger, more complicated optical modules including several

composites as couplers, WDMs, taps, modulators, etc. The path length in such devices can easily

reach some tens of meters, and OFDR has to be employed. Although this technique is well

known for several years, it has not seen broad commercial implementation so far. The main

reason is that the constraints on the source are heavy, and a simple, non-expensive yet reliable source giving the desired performance was lacking. In this paper, a novel OFDR device based on

a simple, mechanically tunable fiber laser [2] is presented. After giving the characteristics of this device, several applications relevant for small systems metrology are presented.

155
Principles of OFDR

To avoid all confusion, let us first stress that only the technique of coherent OFDR is considered

here (there exists also a 'normal', non-coherent OFDR technique which is the frequency domain

analogous of OTDR and measures the modulation transfer function [1]). The coherent OFDR

technique (Fig.l) is based on the detection of a beat signal between the distributed reflections

fi-om the fiber under test (Rayleigh backscatter, connectors, etc.) and a fixed Fresnel reflection

(local oscillator). Using a linear frequency sweep of the laser, one can straightforwardly map the

measured beat fi-equencies on a distance scale, whereas the normed square power for a given beat

frequency gives the reflectivity at the corresponding distance. For a good general overview of the

OFDR principles and limits, the reader is referred to [3]. In the OFDR used for the measurements

presented here, several important improvements have been implemented described in the

following along with the specifications of the present device.

Range: The measurement range of OFDR is limited by the coherence length of the source and by

its phase noise. The first one leads to signal fading, the second one reduces the dynamic range

[3]. The fiber laser employed presently has a very large coherence length of about 3.3 km

(FWHM bandwidth of 10 kHz) [1]. Fig.2 demonstrates the corresponding signal fading of about

5 dB/km, and the reduction in the dynamic range, limiting the useful range to about 2 km. Using more sophisticated (and complicated) sources or signal corrections, an order of magnitude larger

ranges have been demonstrated [4]. However, the spatial resolution drops below the one

achievable with OTDR, so the interest of such methods is questionable from an applications

point of view. Fig. 3 demonstrates that the 2 km range of our OFDR device is sufficiently large to assure a smooth transition between OFDR and OTDR applications.

Resolution: The maximum two-point resolution is given by the frequency excursion of the

source. However, this value can be seriously deteriorated by nonlinearities in the frequency

sweep. In our device, this is corrected for by using the signal from a reference interferometer

from which the instantaneous frequency is calculated. For the fiber laser used, a maximum

detuning of 0.3 nm is employed (41 GHz), giving a resolution of 5 mm as illustrated by Fig.4.

The two point resolution also depends on the measurement range and on the number of points

used. This is because the slope of the frequency excursion has to be reduced for a larger range so

that no aliasing in the sampling is encountered (i.e. maximum beat frequency<Nyquistfrequency). The two point resolution and the measurement range become

Al, , L .Aii, 2p ^ max ' max

respectively, where N is the number of sampling points (typically 8192), 5v the maximum

frequency excursion, c the speed of light, and n the group index. Consequently, for a given

range, the resolution is given by 0.08% of the distance range. For the maximum 5 mm resolution,

the range is 6.6 m. The importance of having a good resolution is clear from Fig.5. A line of 6 pigtails connected

with FC-APCs was measured using different resolutions. This was achieved by changing the

number of points N of the OFDR acquisition. Using 512 points (20 cm resolution, curve at top),

the high quality connector at 8 m is covered by the Rayleigh light, and the fiber break after the

156
last connector can not be discerned from the connector reflection (see inset). With 8192 points

(12 mm resolution), the Rayleigh level drops by about 12 dB, and the small reflection from the

high quality connector can be easily detected. Further, one clearly sees that the fiber break is after the last connector and not in front of it, an important feature for applications like fault detection in optical modules.

A consequence of having a high spatial resolution is that the Rayleigh backscattering (RBS)

becomes very noisy due to the coherent speckle. Note that this is a physical limit (and not a

technological one) affecting all high-resolution reflectometry methods. As less scatterers take

part in the interference, the RBS becomes more peaked. This can somewhat be reduced by

dithering the center frequency and averaging. In our device, the possible change in center frequency is however restricted by mechanical constraints of the fiber laser, and the smoothed

RBS still varies somewhat. If one can average over sufficiently large distances, small losses can

still be identified, as is demonsfrated by Fig.6 which shows a measurement of a bad splice with

0.4 dB loss.

Polarization dependence, sensitivity, and dynamic range: As for all coherent techniques, the

signal will depend on the polarization state of the reflected light. This can be advantageous to get

information about the beatlength distribution (see below), but is typically an unwanted effect in

standard appUcations. We therefore included a polarization diversity detection scheme in our OFDR device, reducing the polarization dependence of the reflected light to less than 0.5 dB.

The sensitivity and dynamic range of the present device is -110 dB and 80 dB, illustrated in Fig.4. As was mentioned before, the dynamic range decreases with distance (see Fig.2) because

of the source phase noise. Using time domain averaging, a sensitivity of -152dB with a dynamic

range of 106 dB has been demonstrated in earlier work [5]. This technique however requires a very stable phase of the reflected signal, which is only achievable for short measurement

distances.

Some measurement examples using OFDR

Due to its high sensitivity, dynamic range, resolution, and accuracy, OFDR is usefiil for a vast range of different applications in fiber metrology, like e.g. measurements of WDM components

[6], distributed gain measurements in EDFAs [7,8], or local birefiingence [9. 10].

Measurement ofdistributed gain in EDFAs: The coherent detection scheme of OFDR leads to a

strong rejection of background light from ASE or residual pump light. OFDR is therefore ideally

suited for non-destructive, distributed gain measurements in EDFA. Fig.7 shows a measurement

of the gain distribution in a 12 m long, 500ppm Er-doped fiber pumped by 1480 nm light. The

set of curves was obtained by gradually increasing the pump power from no pump at all (lowest

curve) to +15 dBm (top curve). Clearly, pump saturation led to gain clamping, with a maximum

gain reached at 4.5 m. After that distance, the Er ions were no longer inverted due to a lack of

pump power, and the signal is reabsorbed. Cut-back measurements were in very good agreement

with the OFDR gain measurements, typically to within the OFDR precision of 0.5dB.

Note that it is difficult to exactly model the gain distribution, as the necessary model parameters

are hard to measure with sufficient precision. This is demonsfrated in Fig.7 by the solid and

dashed lines, where two sets of model parameters obtained in different ways were used [8]. The

157
dotted line on the other hand was obtained by varying the model parameters for good agreement

with the measurement.

Measurement of distributed birefringence: The polarization dependence of the coherent

detection used in the OFDR can be exploited to get information about the evolution of the

polarization state along the fiber [9,10]. Due to the enhanced range of the OFDR at hand, we can

get a much better statistics of the distributed fiber birefiingence. Moreover, the polarization

independent signal fi"om the polarization diversity detection is used to subtract the (polarization

independent) Rayleigh structure, thereby removing the fi^equencies that are not related with the

fiber birefiingence. Fig. 8 shows the polarization dependent reflections fi-om a 1km long low

PMD fiber afl;er removal of the Rayleigh noise and normalization to zero mean power. This curve is then Fourier transformed to get information about the rotation period of the polarization

vector, i.e. the beatlength. The figure shows that there is not one specific, well-defined

beatlength period, but a distribution of such values. While the structure of the peaks changes

somewhat for different laimch polarizations, the mean value of the distribution is fairly constant

(variations of about 0.5m, i.e. 5%, were observed for the present example). An interesting feature

is that the value of birefiingence is found to vary along the fiber. From the normalized return

signal, it can be seen that the polarization changes slower towards the end of the fiber. Indeed, by

Fourier transforming only a section of the curve in Fig.8, mean values for the beatlength of 9.4m

and 13.1m were found for fiber sections fi-om 100-300 and 700-900ni, respectively. Note that when we inverted the fiber, we found the beatlength to be decreasing, demonstrating that this feature is not an artefect of the measurement technique, but indeed reflects the polarization

properties of the fiber. For comparison, we performed a similar measurement on a high PMD

fiber (1.9 ps/Vkm). Using the same range of 2km as before, the polarization dependent signal was

found to be often flat and 3dB lower than the polarization independent one. This happens if the

polarization is rapidly varying on a distance scale smaller than the OFDR resolution, so that only an averaged value is obtained. Looking at these flat sections with an increased resolution indeed

revealed fast polarization changes. The measurement was therefore done with a much higher

OFDR resolution on a 100m section of the fiber. As Fig.9 shows, two distinctive peaks can be

discemed as is expected fi-om the theory in [9] for a fiber with well-defined birefiingence. The

(mean) beatlength accounts to 55 cm.

Conclusions

For the measurement of optical modules and small systems, the range of typical OLCR devices,

very valuable for component characterization, is not sufficient. OTDRs on the other hand lack

the advantages of a coherent detection scheme. With the progress of technology, coherent OFDR

devices can now be realized that are capable of making the transition between the two

techniques.

Such a novel OFDR device, based on a mechanically tuned fiber laser, was presented here. With

its range (6 m to 1 km), resolution (0.08% of range), sensitivity (-110 dB), and dynamic range

(80 dB), it is well adapted for a vast range of different applications in fiber metrology, like e.g. measurements of all kinds of components and their combinations (connectors, WDMs, couplers,

taps, modulators, and to some degree even isolators or circulators), distributed gain

measurements in EDFAs, or distributed local birefiingence measurements.

158
Acknowledgements: Support by the Swiss Departmentfor Education and Research (OFES) in the framework ofthe European Project COST 265 is acknowledged.

References

1 W.V. Sorin, "Optical Reflectometry for Component Characterization", in Fiber Optic Test and Measurement,

7Hewlett-Packard, Ed. D.Derickson, Prentice Hall, 1998

2 P.Oberson, B.Hutmer, O.Guinnard, L.Guinnard, G.Ribordy, N.Gisin, "OFDR with a narrow linewidth fiber

laser", appears in the June issue of IEEE Photon. Techno!. Lett., 2000

3 J.P.von der Weid, R.Passy, G.Miissy, N.Gisin, "On the characterization of optical fiber network components

with OFDR", J. Lightwave TechnoL, vol. 15, no. 7, 1997, p. 1 131-1141 4 K. Tsuji, K.Shimizu, T.Horiguchi, Y.Koyamada, "Coherent OFDR using phase-decorrelated reflected and

reference lightwaves", J. Lightwave TechnoL, vol. 15, no. 7, 1997, p 1 102-1 109

5 G.Mussi, N.Gisin, R.Passy, J.P. von der Weid, "152. 5dB sensitivity high dynamic range OFDR", Electron. Lett., vol. 32, 1996, p. 926-927

6 J.P. von der Weid, R.Passy, A.O.Dal Fomo, B.Huttner, N.Gisin, "Return loss measurements of WDM filters with tunable coherent OFDR", /££•£ PAorow. TechnoL Lett., \o\. 9, no. 11, 1997, p. 1508-1510

7 M.Wegmuller, P.Oberson, O.Guinnard, B.Huttner, L.Guinnard, N.Gisin, "Non-destructive measurements of the

distributed gain in Er doped fibers with improved accuracy using OFDR", appears in Tech. Digest.

CLEO/Europe 2000

8 M.Wegmuller, P.Oberson, O.Guinnard, B.Huttner, L.Guinnard, C.Vinegoni, N.Gisin, submitted to J. Lightvi'ave

TechnoL

9 B.Huttner, J.Reecht, N.Gisin, R.Passy, J.P. von der Weid, "Local birefringence measurements in single-mode

fibers with coherent OFDR", IEEE Photon. TechnoL Lett., vol. 10, no. 10, 1998, p.1458-1460

10 M.Wegmuller, J.P. von der Weid, P.Oberson, N.Gisin, "High resolution fiber distributed measurements with

coherent OFDR", appears in Tech. Digest, ECOC 2000

Optical fiequenqr "AAV

Local nsciUatnr

Device under test

Fig.l: Set-up of the OFDR Fig.2: Fresnel reflections at different distances. The two pedestals are from acoustic permrbations

ISO wo 9» 1000 lOSO 1100

Fig.3: Measurements of a 10m jumper after a 1km SMF using OFDR and OTDR

distance [m]

Fig.4: Short range measurement of a series of

fibers with FC/PC and FC/APC connectors

159
8. '

dislance [m]

D&rq, 1.8 m

a beatleugth: 55 c

frequency [ra' ]

Fig.9: OFDR measurement of distributed birefringence of a large PMD fibre

160
Fiber Bragg Grating Metrology Round Robin: Telecom Group

A. H. Rose, C.-M. Wang, and S. D. Dyer

NIST, Boulder, CO 80303

Abstract

We briefly discuss the measurement of center wavelength, bandwidth, minimum transmittance, and

relative group delay ripple from a fiber Bragg grating round robin among a group of telecom companies.

We find that the state of fiber Bragg grating metrology in industry, needs improvement in most areas

(transmittance, reflectance, wavelength, and relative group delay). Source amplified spontaneous

emission can be a limiting effect in the minimum transmittance measurements, and wavelength accuracy

and step size are critical for measurement of bandwidth and relative group delay ripple.

1. Introduction

In this paper we report on some of the results from a fiber Bragg grating (FBG) round robin conducted by

NIST. FBGs are extremely important for telecommunication and sensor applications. In new

wavelength-division multiplexed (WDM) optical-fiber communication systems, FBGs are used as wavelength filters and dispersion compensators. Also, FBGs make excellent strain sensors that can be

networked to obtain distributed strain measurements of large structures, such as bridges and ships. In

spite of the numerous and growing commercial applications of FBGs, there are no standard measurement

procedures, and a variety of definitions are being used for important parameters.

Two parallel round robins were organized to assist industry in evaluating FBGs for telecommunication

and sensor applications. The round robin participants in the two groups were, ADC, Agilent, Coming,

Perkin Elmer, GNnet-test, NPL, and 3M in the telecom group, and Blue Road Research, CiDRA, EXFO,

Micron Optics, NRL in the sensors group. Because no formal methods for analyzing the spectral or

relative group delay (RGD) data existed only raw data, from the participants, were sent to NIST. In this paper we discuss only selected measurements from the telecom group on the center wavelength,

bandwidth, and transmittance of a FBG at 1552.526 nm (corresponding to the International

Telecommunication Union (ITU) channel 0) with a 50 GHz bandwidth. Also from the telecom group, we

discuss the RGD measurements on a chirped FBG that was 16 nm wide.

2. Measurement Techniques

Measurement techniques varied among the

participants. The telecom group used primarily

a grating-tuned diode laser, power meter, and

wavelength meter system for spectral

measurements. The system used by NIST to make spectral measurements is shown in Fig. 1

A tunable fiber Fabry-Perot (FFP) filter was

used to filter amplified spontaneous emission

(ASE) from a grating-tuned diode laser. A

Timable

Laser 3

Tunable FFP Filter

Wavelengdi

ITirtBT

Fiber n

Grating

-v^iii nffi n

Lock-in

Amplifiers

Figure 1 A schematic of NIST' s spectral measurement

system.

wavelength meter provided the wavelength scale to an uncertainty of 0.2 pm. The detectors and lock-in

amplifiers are linear to within 1% over the 60 dB measurement range. However, only relative reflectance

and transmittance data are reported because losses in each of the participant's measurement systems are not known. Source power fluctuations are removed by monitoring the power at the second coupler port.

The coupler splitting ratio has a weak wavelength dependence of about 0.01 dB/nm over the 1540 to 1560 nm range.

161
The effect of the tunable FFP filter on the laser can be

seen in Figs. 2 and 3. Fig. 2 shows the unfiltered and

filtered output of the tunable laser over the 1350 to 1650

nm band. These data were taken with a, 1 nm resolution,

optical spectrum analyzer. The laser peak power at 1553

nm was about 308 flW and the integrated ASE across the

measured spectra in Fig. 2 was about 1 .7 \iW. The ratio of these powers is about 23 dB and is a good measure of

the dynamic range of the system without the tunable FFP

filter. With the FFP filter the ASE is suppressed so that the transmitted laser power is about 1 74 |i W and the

integrated ASE power is about 76 pW, for a power ratio of about 64 dB.

Fig. 3 shows the effect of the ASE on a measurement of a FBG's relative transmittance. Without the FFP filter the

minimum relative transmittance is only about -25 dB.

With the FFP filter the minimum relative transmittance

is about -65 dB. -

The RGD of the chirped FBG was determined by

various rf-phase-shift techniques. A detailed

description of the NIST system can be found in reference [1]. The repeatability (2a) for NIST

measurements on the RGD linear slope using the

phase-shift system is about 0.25 ps/nm, which could be

improved by reducing drift in the rf-modulator. NIST

also employed a new low-coherence interferometer

method to determine the RGD of the chirped grating.

Details of this system are described in the literature [2, 3]. The low-coherence system has a RGD linear slope

repeatability of about 0.02 ps/nm.

Source Spectral Purity

1350 1400 1450 1500 1550 1600 1650 Wavelength (nm)

Figure 2 ASE fi-om a tunable laser diode with

and without a tunable FFP filter.

CO

o

0^

1552 1553 1554 Wavelength (nm)1555

3. Analysis Methods

To determine the center wavelength and bandwidth of a Figure 3The effect of ASE on the measurement

grating from the reflectance data, NIST used the of transmittance in a FBG.

following methods. First, the maximum reflectance in the plateau region is determined. Then, wavelengths at reflectance values of -3 dB and -0.5 dB from the

maximum plateau reflectance were found by interpolating between data. The center wavelength is defined as (X+ - A,.)/2, and the bandwidth is defined as c/(X+ - where X+,. are the wavelengths at data

values -X dB (x = -0.5 or 3) on each side of the plateau region, and c is the speed of light. The minimum

relative transmittance is determined from relative transmittance data by fitting a spline function to the

data and locating the minimum. From the RGD data, the linear slope of the chirped grating is determined

using a least squares fit from data within the -3 dB reflectance bandwidth. The residual RGD is found by

subtracting the linear slope from the RGD reflectance data.

4. Round Robin Results

Both FBGs -3 dB center wavelength was monitored throughout the round robin to correct for any

changes that might occur due to shipping damage or thermal shock. The ITU and chirped gratings did

show measurable changes, 37 pm for the ITU, and 75 pm for the chirped, and were attributed to a

162
mechanical shock that damaged other fibers in the

grating box. Appropriate corrections were added to each

participant's data. The gratings were place on a thermalelectric cooler and athermally packaged so temperature

variations from lab to lab would not affect the FBG

center wavelength.

The measurements of the minimum relative

transmittance of the ITU grating, taken from the relative

transmittance data showed the participants measuring

about a 23 dB minimum; NIST measured a 27.5 dB

minimum. As discussed earlier, for strong gratings, the

minimum relative transmittance measurement is quite

sensitive to the spectral purity of the laser light source. NIST's FFP filtered laser shows the lowest minimum

transmittance, an indication that the participants

probably had some amount of ASE in their measurement

systems.

55

50

45

40

35

30

Telecom Group. ITU Grating

I I J —i—I

I

-3 dB

-0.5 dB

NIST A B Round Robin Participants

Figure 4 Bandwidth of the ITU grating using

two band-edge criteria. Participant's error bars

The ITU grating determined from the round robin

uncertainty,

reflectance data sets with a -3 dB criterion had a 27.5 pm range of values, with a standard deviation a of

8 pm and a mean of 1552.521 nm. With a -0.5 dB criterion the values had a 42 pm range, with a a of

about 12 pm and a mean of about 1552.515 nm. Thus, no significant change in was found between the

-0.5 and -3 dB criteria and the O for each evaluation is adequate for WDM applications at present, but

may need to be improved as system requirements increase.

Fig. 4 shows the results of the ITU grating bandwidth determined with the -3 dB and -0.5 dB band-edge

criteria. The error bars for NIST uncertainty are 748 MHz while the participants error bars are just the fit uncertainty. For the -3 dB criterion, the mean bandwidth is 5 1 .2 GHz with a range of 9.5 GHz and a o of

3 GHz. In most cases the participants would pass this as a 50 GHz ITU grating using the -3 dB criterion

[4].

For the -0.5 dB criterion the mean is about 39.7 GHz with a range of about 12 GHz and a O of about 4

GHz. In some cases, where the bandwidth must fill the full channel spacing, the round robin participants

would reject this grating as a 50 GHz ITU grating. The difference in the mean bandwidth, between the

-0.5 and -3 dB criteria, is -11. 5 GHz.

In some cases the participants data intervals were large enough to effect the determination of the

bandwidth shown in Fig. 4. Participants C, E, and F all had coarse data sets. Participant D may not have

normalized the reflectance to source power fluctuations. The -0.5 dB bandwidth is very sensitive to the

shape of the ITU grating reflectance data.

The RGD linear slope of the chirped FBG was found by using a linear least squares fit to the RGD

reflectance data, over the chirped grating's -3 dB bandwidth. From the round robin data the chirped FBG

has a mean RGD linear slope of -6.81 ps/nm. Phase shift systems measure the RGD of all the fiber

between the modulator and detector. Thus, to remove the system-fiber contribution to the FBG RGD

linear slope the grating can be measured from both directions. The range on the mean of both directions

of the RGD linear slope is 0.1 ps/nm with a a of 0.04 ps/nm. If the system RGD is not removed, the

range on the RGD linear slope is 1 . 1 ps/nm, a factor of ten worse.

163
on

GO

Fig. 5 shows a portion of the residual RGD spectra taken with the NIST rf and lowcoherence systems and with participant's A, B, E, and G systems. For most cases the

ripple measurements agree, but wavelength accuracy, measurement uncertainty, and rf sideband averaging can lead to several ps differences [5]. The difference between the

low-coherence system and rf phase shift systems is still being investigated, but no

major differences have been observed [2]. The other round robin participant's RGD data

could not be used to compare the ripple

because of coarse wavelength steps. Fig. 5

illustrates the need for precision RGD ripple

measurements, because over a 0.5 nm

wavelength span the RGD changes rapidly

from +5 to - 4 ps. Chirped gratings with larger

RGD linear slopes will have larger RGD

ripple amplitudes, increasing the need for more precision in RGD ripple measurements

for WDM systems.

— T -- A

*—-B

E

- - •- - G — -NISTrf

- Lo-Coh

5. Conclusions

Metrology for WDM components, such as FBGs, must improve to meet the demands of

current and future WDM networks. From this sampling of the round robin results we can draw the following conclusions. The source

spectral purity is critical, ASE from diode

sources must be substantially reduced.

Wavelength accuracy <1 pm and <10 pm step

GO

1545.3 1545.4 1545.5 1545.6 1545.7 Wavelength (nm)

Figure 5 A portion of the RGD ripple from the chirped

FBG as measured by various participants and NIST

systems.

sizes are necessary for bandwidth and RGD ripple measurements. Source spectral power fluctuation

removal is necessary for bandwidth measurements. When using rf-phase-shift systems the best way to remove the system bias is by taking the mean of both directions on the grating. Stabilizing rf-phase-shift

measurement systems and working at rf frequencies that do not average over >10 pm is necessary for

RGD ripple measurements. Also, RGD resolution <1 ps will be required for most WDM components as data rates increase. RGD measurements with the low-coherence system compare well with phase-shift

systems and may be preferred for rapid component evaluation.

6. References

[1] S. E. Mechels, J. B. Schlager, and D. L. Franzen, "Accurate Measurements of the ZeroDispersion Wavelength in Optica! Fibers," J. Res. Natl. Inst. Stand Technol., vol. 102, May-Jun.

1997, pp. 333-347.

[2] S. D. Dyer, K. B. Rochford, A. H. Rose, "Fast and accurate low-coherence interferometric

measurements of fiber Bragg grating dispersion and reflectance," Optics Express, vol. 5, Nov.

1999, pp. 262-266.

[3] S. D. Dyer and K. B. Rochford, "Low-coherence interferometric measurements of fiber Bragg

grating dispersion," Electron. Lett., vol. 35, 1999, pp. 1485-1486.

[4] Generic Requirements for Fiber Optic Branching Components, GR- 1 209-CORE, Issue 2, Feb.

1998, Telcordia.

[5] R. M. Fortenberry, "Enhanced Wavelength Resolution Chromatic Dispersion Measurements

using Fixed Sideband Technique," Tech. Digest OFC 2000, TuG8, Mar. 2000, pp. 1 07- 1 09.

164
Measurements of dense group delay ripple using the phase shift

method: Effect of modulation frequency

T. Niemi, M. Uusimaa, and H. Ludvigsen

Metrology Research Institute, Helsinki University of Technology, P. O. Box 3000,

FIN-02015 HUT, Finland.

Abstract

The group delay ripple of a dispersion compensatmg grating has been measured and analyzed

using the phase shift method. The dependence of modulation frequency has been investigated.

Introduction

Characterization of group delay and amplitude properties of various fiber-optic components is important in designing advanced optical networks. The phase shift method is an established

technique for performing high-accuracy measurements of these parameters [1,2]. The phase and

amplitude measurements are conventionally made with a vector voltmeter or a network analyzer.

These devices typically have a resolution in the phase measurement on the order of 0.1 degree,

which is enough for resolving the changes in the group delay with sub-picosecond accuracy. The

accuracy of the measurement method to resolve the amplitude of the dense ripple in the group

delay, however, depends critically on the selected modulation frequency [3,4]. In this work, the

effect of the modulation frequency on the measured amplitude of the group delay ripple of

chirped fiber Bragg gratings is investigated. It is shown that the choice of the modulation

frequency is essential for obtaining reliable results.

Group delay measurements

We have characterized a 20-cm long fiber Bragg grating with a bandwidth of 2 nm and a nominal dispersion of -660 ps/nm. The experimental setup based on the phase shift method is outlined in Fig la. The light source is a wavelength-timable laser (Photonetics TUNICS-PRI)

whose wavelength is measured with a wavemeter (HP 86120B). The output of the laser was

sinusoidally intensity modulated at a frequency of cOm with a Mach-Zehnder modulator. The

polarization of the light entering the modulator is tuned using a polarization controller. After the

modulator the light is amplified with a fiber amplifier and divided into two paths one of which is the reference path and the other includes the component under test. The two optical signals are detected and amplified before they are fed to a two-channel digital sampling oscilloscope

(Tektronix TDS820). The stored time traces of the detected signals are transferred to a computer

for analysis of their relative phase and amplitude. The computer controls the wavelength of the

tunable laser, the data acquisition and the wavelength measurement.

The measured group delay for three modulation frequencies are given in Fig. lb. Each curve is an average of five measurements. In the wavelength range between 1554.01-1550.03 nm, the

traces have almost sinusoidal ripple. This ripple has a period of ca. 1 .9 GHz and an amplitude of

ca. 130 ps when the modulation frequency is 250 MHz. The fast variations of the group delay are smoothed out as the modulation frequency was increased.

165
Tunable

laser

Polarisation controller {

Component

Oscilloscope

PC

Figure la. The experimental setup based on

the phase shift method.

J5 -50

1554.00 1554.02 1554.04 1554.06 1554.08 1554.10 Wavelength [nm]

Figure lb. Traces of the group delay

ripple measured with three different

modulationfrequencies.

Analysis

The relation between the measured group delay Tgd((o) of the component at the optical carrier

frequency o) and the electrical phase shift 6(o))oi\hQ intensity modulated signal is given as

GD {CD) =

e{CD)

(1)

The group delay induced phase shift between the modulation sidebands in the component is usually approximated as a linear ftmction of frequency in a narrow frequency range around the

optical carrier. This approximation leads to accurate results when the variation of the group delay

changes slowly with wavelength. However, in certain fiber optic components, such as chirped

fiber Bragg gratings the group delay may fluctuate very rapidly.

By assuming that the variations in the group delay are sinusoidal (see Fig. 2a) we may write

r(^y) = ^sin(—), (2)

P

where Ap is the peak amplitude of the ripple and p indicates the period of the ripple. The phase

shift of the optical sidebands can be obtained by integrating the group delay over the modulation

bandwidth. The amplitude of the sinusoidal ripple as a function of the modulation frequency cOm

is then

^K,) = ^,sinc(^)

P

(3)

Equation (3) shows that the measured amplitude is zero if the modulation frequency is a multiple

of half the ripple period.

A comparison between the amplitude calculated from Eq. (3) and the measured data is given in

Fig. 2b. The horizontal axis gives the modulation frequency normalized to the period of the

measured ripple and the vertical axis presents the normalized amplitude of the ripple. The solid

line plots Eq. (3) with p^\ and the squares are the measured ripple amplitudes. The overall

agreement between the measured data and the model is good.

166
1,2 -

<0 a 1.0 - I

Optical frequency 0.0 0.2 0.4 0.6 0.8 Normalized modulation frequency

1.0

Figure 2a. Variation ofthe group delay

and the optical signal.

Figure 2b. Comparison between the model

and the measured group delay amplitude.

The model predicts that the ampHtude of the ripple should be inverted for normalized modulation

frequency values in the range from 0.5 to 1. However, with the present setup it was not possible

to verify this behavior.

In this paper, the effect of modulation frequency on the accuracy of the measured amplitude of

the group delay ripple with the phase shift method was investigated. It was shown that the choice

of the modulation frequency critically affects the measurement results. A model assuming

sinusoidal variation of the ripple amplitude with optical frequency was used to describe the

results. The model predicts that the amplitude of the ripples falls off as a sinc-fimction with

modulation frequency. This was confirmed with measurements of the group delay ripple of a

dispersion compensating grating. Accurate verification of the ripple is important since it has a degrading effect on the system performance if fiber Bragg gratings are used for dispersion

compensation.

[1] M. L. Rocha and R. Kashyap, "Characterisation of fibre Bragg gratings: A study on

accuracy and repeatability", in Digest of the 4''' Optical Fibre Measurement Conference,

Teddington, United Kingdom, pp. 14-17, 1997.

[2] S. Ryu, Y. Horiuchi, and K. Mochizuki, "Novel chromatic dispersion measurement method

over continuous gigahertz tuning range", J. Lightwave Technol., Vol. 7, pp. 1177-1180,

[3] C. Clark, M. Parries, K. Visvanatha, and A. Tager, "Measuring chromatic dispersion of fiber

gratings". Lightwave, pp. 70-77, February 1999.

[4] Y. Li, D. Way, N. Robinson, and S. Liu, "Impact of dispersion compensation gratings on

OC-192 systems", in proceedings of Conference on optical amplifiers and their

applications, Monterey, CA, paper TuB5, 1998.

Conclusion

References

1989.

167

A fast and accurate measurement of both transmission and

reflection group delay in fiber Bragg gratings

Shellee D. Dyer and Kent B. Rochford

National Institute ofStandards and Technology

Optoelectronics Division

325 Broadway, Boulder, CO 80303

We show that both the reflection and transmission group delay of fiber Bragg gratings can be

accurately obtained from one low-coherence interferometric measurement. The spectral reflectance

and transmittance of the grating can also be obtained from that same measurement. Both the

reflection and transmission group delay can be obtained in less than 60 seconds. This technique can

also be applied to the simultaneous measurement of the group delay of multiple fiber Bragg gratings

in series.

1. Introduction

Low-coherence interferometry has several advantages over conventional techniques such as the

modulation-phase shift method [1] for the characterization of fiber Bragg gratings (FBGs). A key

advantage is the rapidity with which a measurement of group delay and reflectance can be obtained.

The interferogram is obtained in less than a second, and processing the interferogram to obtain group

delay or reflectance takes less than 60 seconds [2], compared with the conventional modulationphase shift measurement, which can take several hours [1]. This measurement technique has low uncertainty (less than 1.5 ps) and, because of its speed, is immune to errors caused by thermal variations and instrument drift. Low-coherence interferometry

is also immune to the ripple washout problems that can occur with the modulation-phase shift measurement [3].

2. Measurement Method

A diagram of the low-coherence interferometric system is shown in Fig. 1 . A broadband Er

superfluorescent fiber source (SFS) provides the input signal. This source is directed into a pair of

fiber couplers. Fiber coupler 1 provides a comparison signal for the difference-over-sum (A/Z)

amplifier, as explained below. Fiber coupler 2 is part of a fiber-optic Michelson interferometer. The

Fiber Coupler 2 FBG

A/D TRIG Zero HeNe

Board Crossing

Detector Detector

1

Computer

BS M

HeNe

Laser

Figure 1. Diagram oflow-coherence interferometric systemfor measurement ofthe transmission

and reflection group delay ofFBGs. AR: anti-reflection coating, PC: polarization controller, GL:

grin lens, CE: cleaved endface, M: mirror, TS: translation stage, BS: beamsplitter, DBS: dichroic

beamsplitter, A/Z: difference over sum.

169
FBG under test is spliced onto the test arm of the interferometer. The far end of the grating's fiber

pigtail is cleaved to produce a Fresnel reflection. This reflection facilitates the measurement of

transmission group delay, as explained below.

The reference arm of the interferometer contains a variable-length air path so that the total optical path difference (OPD) of the interferometer can be varied. A frequency-stabilized HeNe

laser interferometer monitors the position of the reference arm mirror, and a zero-crossing detector

triggers sampling of the IR signal on positive-sloped zero crossings of the HeNe signal.

The light from the reference arm is recombined with the light from the test arm at fiber coupler

2, and the recombined light is directed onto the two IR detectors. The signals at the two IR detectors

have similar source excess-noise characteristics, while the interference terms are 180° out of phase.

Therefore, using a difference-over-sum amplifier will reduce excess noise from the SFS, which is the

dominant noise source. This improves the interferogram's signal-to-noise ratio (SNR) by a factor of

3, and yields a corresponding improvement in the group delay SNR.

If the spacing between the FBG and the cleaved endface exceeds the width of the individual

coherence functions, then the signal at the detector as a function of the OPD consists of two distinct

signatures. One of these signatures represents the interference of light reflected from the FBG with

the light reflected from the reference arm mirror. The other signature is created by the interference

of light reflected from the test arm's cleaved endface with the light from the reference arm. The shape and extent of the first signature is determined by the reflection characteristics of the

FBG. Neglecting background dispersion, the complex degree of coherence y(^) of this interferogram

is given by [4]

7(4) = r Gio-yicr) Qxpijip^ (or)) Qxp{j27ra4)da , (1)

J- 00

where £, is the OPD, a is the wavenumber, G(a) is the power spectral density of the Er SFS, and

r(a)exp(j(j)r(a)) is the complex field reflection coefficient of the FBG. The reflection group delay (tg) of this grating is calculated from the phase of the Fourier transform of the interferogram as follows:

Similarly, the second interferogram is related to the complex field transmission coefficient of the

FBG as follows:

, 7(<^) = r G(a)[r(c7) QxpiJ^, (a))Y cxpij27ra^)da , (3)

J-OO

where t(a)exp(j(j),(a)) is the complex field transmission coefficient of the FBG. The equations above

are derived assuming that the only dispersion difference between the two arms arises from the

dispersion of the FBG. In reality, there is a small background dispersion from the difference in the

length of the test and reference arm fibers, but the slope of the background group delay is approximately 1 1 fs/nm, which is negligible compared with the dispersion of a typical FBG [5].

3. Data Processing

The reflection group delay was calculated from the FBG reflection interferogram. We truncated

the interferogram at the points where the SNR was approximately unity. Next, we appended zeros to the interferogram array (zero padding) to obtain a total array length of 2*^. The choice of N

determines the wavelength resolution of the group delay results. Larger values of N give better resolution, but if N is too large, computational errors such as roundoff error affect the accuracy of the

results. For the results shown in this paper, we used N^l 8, giving a wavelength resolution of 14 pm.

•:•( 170
To obtain the group delay, we took the Fourier transform of the truncated and padded interferogram.

The magnitude of the Fourier transform is proportional to the magnitude of the field reflection

coefficient of the FBG. The relative group delay of the FBG is determined by differentiating the

phase of the Fourier transform.

The transmission group delay is calculated from the cleaved endface interferogram in a manner

similar to the calculation of the reflection group delay. We truncate and zero pad the interferogram.

take the Fourier transform, and then calculate group delay by differentiating the phase of the Fourier

transform.

4. Experimental Results

We used our interferometric measurement system to determine the transmission and reflection

group delay of a chirped FBG with a 5.4 nm bandwidth and a center wavelength of 1555.5 nm

(grating A). We cleaved the end of the FBG fiber at a distance of approximately 2.7 cm from the

grating. We then measured the interference pattern as a function of OPD and obtained two distinct

signatures. From the second interferogram, we calculated the double-pass transmission group delay

of this grating, which is shown in Fig. 2. Also shown in Fig. 2 is the relative power transmitted by

the grating. The transmission group delay results are valid only in regions where the transmitted

power is appreciable; in all other spectral regions there is insufficient signal for an accurate measurement. From Fig. 2, we see that the transmission group delay of this grating has features that extend beyond the 1 OdB

reflection bandwidth of the

grating.

We also used our

interferometric system to measure the group delay of

two gratings in series. We

spliced a second grating

approximately 6.5 cm after grating A. This second

grating (grating B) had a center wavelength of 1 548

nm and a 1.7 nm bandwidth

The endface of the test arm was immersed in index

matching fluid; therefore,

the interferometer output

consisted of two separate

interterograms, one tor each

of the tv.'o gratings. The

shape of the interferogram from grating A's reflection is determined by the complex field reflection

coefficient of that grating as given by Eq. 1 . The light transmitted by grating A sees the effects of

both gratings, and that interferogram is related to both gratings as follows:

ri^) = £ Gicr)[t^ (a) exp(7^, , (a))fr, (a) expC;^,, (a)) exipU27ra^)da , (4)

where tA(a)exp(j(t),A(cj)) is the complex field transmission coefficient of grating A, and

rB(a)expG(t),B(cT)) is the complex field reflection coefficient of grating B. We processed each

interferogram separately by truncating each near the points where the SNR was equal to one. We

then zero padded each interferogram to create arrays of length 2'^ and calculated the Fourier

transform of each padded interferogram. From the phase of the Fourier transform of grating A's

Q.

J3Q

a.3

O

O

1535 1540 1545 1550 1555 1560 Wavelength (nm)

Figure 2. Double-pass transmission group delay and relative

transmittedpower ofgrating A. Results are shown from two repeated

171
interferogram, we obtained the reflection group delay of that grating, which is shown in Fig. 3. From

the second interferogram, we obtained a group delay function, also shown in Fig. 3, which is the

product of the reflection group delay of grating B with the double-pass transmission group delay of

grating A. These results are relative group delay curves, and therefore each curve has its own

arbitrary additive constant; the relative group delay difference between the gratings is not shown in this graph. Comparing this group delay product at wavelengths near 1548 nm with the transmission

group delay of the grating A shown in Fig. 2, it is clear that the transmission group delay of grating

A is very small at wavelengths near 1548 nm, and can be neglected compared with the reflection

group delay of grating B. We also measured the

group delay difference

arising from the fiber

separation between the

two gratings. To obtain

this, we calculated the

central fringe index of

each individual

interferogram [6]. Then

we determined the group

delay difference from the

separation between the

interferogram s' central

fringes. We calculated a

total single-pass delay

between the gratings of

315 ps, which is too large to show in Fig. 3.

40 -

20

<u Q

D.3

O

a

-20

-40

1546 1548 1550 1552 1554

Wavelength (nm)

1556 1558 1560

Figure 3. Plot ofrelative reflection group delay measured oftwo

gratings in series. Results are shownfrom two repeated measurements.

The relative group delay difference between the two gratings is not

shown.

5. Conclusions

We have demonstrated the measurement of transmission group delay in FBGs, as well as the

measurement of the group delay of multiple gratings in series using low-coherence interferometry.

The key advantage of the low-coherence technique is speed, and it is possible to obtain the group

delay measurements in less than 60 seconds.

References

[1] S.E. Mechels, J.B. Schlager, and D.L. Franzen, "Accurate measurements of the zero-dispersion wavelength

in optical fibers," J. Res. Natl. Inst. Stand. Technol., vol. 102, pp. 333-347, 1997.

[2] S.D. Dyer, K.B. Rochford, and A.H. Rose, "Fast and accurate low-coherence interferometric measurements of fiber Bragg grating dispersion and reflectance," Optics Express, vol. 5, pp. 262-266,

1999.

[3] R.M. Fortenberry, "Enhanced wavelength resolution chromatic dispersion measurements using fixed

sideband technique," in Optical Fiber Communications Conference, OSA Technical Digest (Optical

Society of America, Washington DC, 2000), TuG8, pp. 107-109.

[4] U. Wiedmann, P. Gallion, and G.-H. Duan, "A generalized approach to optical low-coherence

reflectometry including spectral filtering effects," Journal ofLightwave Technology, vol. 16, pp. 1343-

1347, 1998.

[5] S.D Dyer and K.B. Rochford, "Low-coherence interferometric measurements of fibre Bragg grating

dispersion," Electronics Letters, vol. 35, pp. 1485-1486, 1999.

[6] B.L. Danielson and C.Y. Boisrobert, "Absolute optical ranging using low-coherence interferometry,"

Applied Optics, vol. 30, pp. 2975-2979, 1991.

172
Differential group delay measurements of chirped fiber

Bragg gratings using photon counting

Hugues de Riedmatten, Mark Wegmuller, Hugo Zbinden, Nicolas Gisin

Group of Applied Physics

University of Geneva

Rue de I'ecole de medecine 20

1204 Geneva, Switzerland

E-mail: hugues.deriedmatten@physics.unige.ch

Abstract

We present a novel measurement methodfor the differential group delay of chirped fiber Bragg

gratings, based on the time-of-fiight ofsingle photons. In contrast to existing techniques, the light source is a pulsed broadband LED. This simple and non-expensive source allows for

measurements over a large spectral range with a high spectral resolution (8 pm) obtainedfrom a

tunable Fabry-Perot filter. The photons are detected by an InGaAs avalanche photodiode

operated in photon counting mode, resulting in a precision for the differential group delay of

15 ps.

Introduction

Chirped fiber Bragg gratings are an attractive solution for chromatic dispersion compensation in

all-fiber applications. They allow compensating for a large amount of differential group delay

(DGD) in a very compact way. Further, by tailoring the dispersion slope, they can be ideally

matched to the specific situation of deployment. Obviously, the DGD of the grating has to be

exactly known for optimum compensation. Sufficiently accurate information can only be obtained

from measurements of the DGD, and several methods have therefore been reported. They are

either based on optical interferometry [1], or on direct phase-delay detection [2]. In both cases, a narrow band tunable laser was used to obtain a sufficiently good spectral resolution.

Here, we suggest to use a broadband source (pulsed LED) and a narrow band tunable filter instead. The advantage of using an LED is that one can cover a large spectral range. However, the

intensity after the filter is very low and therefore impossible to detect with classical techniques.

Consequently, one has to employ a photon counting detection. Intensive research and

improvements in detector engineering have made this technique - well known in fields like astronomy or spectroscopy - also available at telecommunication wavelengths.

173
Measurement method

The measurement method is a time-of-flight technique giving the relative pulse delays

experienced in the grating at different wavelengths. The setup is an adaptation of [3] and is shown

in figure 1

Bandwidlli:8pm Bandwi(ttli:0.3 nm //

LED

FPtunabN

filte

Figure 1 : Setup ofthe measurement technique

The light source is a standard LED for telecom applications, with a bandwidth of 60 nm (FWHM)

centered around 1550 nm. Driven by a fast pulse generator, it produces pulses of 300 ps duration

(FWHM) with peak power of 400 )LiW or 10^ photons/pulse. The wavelength is selected with a

tunable Fabry Perot filter with a bandwidth of 8 pm (FWHM). To increase the stability of the

piezoelectrically tuned crystal, the filter is stabilized in temperature. After the filter, a pulse

contains approximately 100 photons. The light is then injected onto the chirped Bragg grating

through a 50/50 fiber coupler, so that the reflected photons can be detected at the second coupler

output. The detector is an InGaAs avalanche photodiode (APD) used in the so-called Geiger

mode. To reduce the noise, it is active only during a gate window triggered by the LED pulser.

For a more detailed description ofthe detection scheme, the reader is referred to [4]. The time-of-flight is measured with a time to digital converter (TDC). The start is synchronized

with the LED pulse, and the stop is given by the avalanche generated by the arrival of the

reflected photon in the detector. The single measurements are stored in a histogram, and

eventually a reflection peak with a FWHM of 500 ps emerges. The shape and width of this peak

are due to a convolution of the LED pulse and of the time response of the detector. The

integration time used is about 3 s for each wavelength, for a repetition rate of 10 kHz and a

detection efficiency of 10 %. Note that this integration time can be drastically reduced by using a higher repetition rate and higher detection efficiency. The standard deviation of the peak position

is about 15 ps.

Results for two chirped Bragg gratings

The method was tested with two chirped Bragg gratings (Photonics), with reflection bandwidths

of 1.1 and 0.3 nm, respectively. Figures 2 and 3 show the measured group delays for the

respective gratings, with the set of curves giving the results for measurements done from either

J. 174
side of the gratings. Looking at the first grating, we see that the DGD is quite linear. The spectral

resolution of 8 pm is not sufficient to resolve the gain ripple at present. As our signal power is far above the detection limit, this could however be improved upon by simply using a filter with a narrower bandwidth. As expected, the two DGD curves are symmetric.

This is not the case for the second grating, where a distinctive asymmetry can be observed. In

fact, this grating is defective and also exhibits an uncommon reflectivity spectrum, obtained by

monitoring the number of reflected photons as a fimction of wavelength. It is not yet fully

understood if a defect in the index profile or blazing could lead to the observed asymmetry.

Interestingly, similar asymmetries have been found with a different measurement technique for different gratings [5], indicating that this might be an important problem for production.

DiagggrMfig

1552.60 1552.65 1552.70 1552.75 1552.80 1552JI5 1552.90 1552.95 1553.00 1553.05 1553.10

Figure 2: DGD measurements oftwo gratings with different reflective bandwidths.

The stability and precision of our photon counting DGD measurement set-up were tested by

performing repeated measurements of time-of-flight with a fixed voltage applied to the FP filter, corresponding - in principle - to a fixed wavelength. The results are presented in figure 3. The

variations in the time-of-flight are mainly due to electronic jitter. The standard deviation of these

values of typically 10-20 ps gives a good estimation of the temporal precision of our method.

175
500

4S0

X=l 552.87 nm

Figure 3 : Stability and precision of the measurement technique

Conclusions

A novel method for DGD measurements in chirped fiber Bragg gratings was presented. Its core elements are a pulsed LED followed by a high resolution tunable FP filter for the light source, and

a photon counting detection. This allows for measurements over a spectral range as large as 60 nm

with a DGD precision of 15 ps. The spectral resolution, essentially given by the employed filter bandwidth, was 8 pm, which could however be easily increased for ripple detection.

Acknowledgements

The authors would like to thank Jurgen Brendel for useful discussions and Christian Zimmer

(Swisscora) for the gratings. Support by the Swiss Department for Education and Research

(OFES) in the fi-amework of the European Project COST 265 is acknowledged.

References

1 M.M Ohn, SY Huang, S.Sandgren, R.Measures, T.Alavie, Measurements offiber gratings

using an interferometric and Fourier-Transform based technique, OFC'97, Technical Digest,

pp 154-155, 1997

2 R.Kashyap, M de Lacerda Rocha, On the group delay characteristics ofchirpedfibre Bragg

gratings, Opt.Comm. 153 (1998) 19-22

3 J.Brendel, H. Zbinden, N.Gisin, Single photon time of flight chromatic dispersion

measurements, OFMC 99, pp 75-78

4 G. Ribordy, J.D. Gauthier, H. Zbinden and N. Gisin, Performance of InGaAs/InP

avalanehephotodiodes as gated-mode photon counters, Appl. Opt. 37, 2272 (1998)

5 private communication COST 265 members

176
Measurement Issues for Dispersion Compensation

R. M. Jopson

Crawford Hill Laboratory

Bell Labs, Lucent Technologies

Abstract

The introduction of chromatic dispersion compensation to lightwave systems provides an

opportunity to measure properties of the compensation. We describe the importance of some of these properties. We also discuss the interplay between the measurement of

chromatic dispersion and the presence of polarization-mode dispersion.

Introduction

Most long-distance lightwave systems operating at bit rates of 10 Gb/s or higher require

compensation of chromatic dispersion. Obviously, accurate measurement of chromatic dispersion

is necessary for the successfiil implementation of compensation; however, other properties of the compensators are also of crucial miportance. The relative importance of various characteristics depends on the design of the system in which the dispersion compensation is used. Optical

nonlinearity can be of crucial importance in a compensator containing high optical powers but be

negligible in compensators containing low optical powers. Polarization-mode dispersion (PMD)

is a phenomenon that not only affects the measurement of chromatic dispersion but also invalidates a common definition of chromatic dispersion. We will describe the influence of PMD

on chromatic dispersion and describe a method for measuring the PMD vector and chromatic

dispersion simultaneously.

Dispersion Compensation

The chromatic dispersion of an object is obtained formally from a Taylor expansion of the propagation constant, P(0)), with respect to the angular fi-equency, co:

^(Gj) = i8o+A(fi>-<»o) + ij32(a)-6Jo)'+ii33(fi)-a)o)' + ... .

The constants Pn are the n*-order to derivatives of P(aj) evaluated at cdq. The first-order constant.

Pi is the reciprocal of the group velocity and is therefore a measure of pulse propagation delay

when multiplied by the propagation distance. One convenient measure of chromatic dispersion,

D, is the wavelength derivative of the propagation delay divided by the propagation distance. Using the speed of light, c, and the wavelength, A , D can be expressed as

and is often stated in units of ps/nm/km. Most long-haul lightwave systems operating at bit rates, B, of 10 Gb/s or higher use modulators to provide a signal optical bandwidth near the transform

limit. For these systems, a 1 dB penalty is observed in a system of length L when the total chromatic dispersion, DL, is about IOVb^ {Ghlsf ps/nm. Thus, for standard fiber, the dispersion

177
limit is 60 km for 10 Gb/s and less than 4 km for 40 Gb/s. As transmission rates increase, these

limits fuel a strong demand for dispersion compensation and a strong need for measurement of

the properties of the dispersion compensation.

Many methods for dispersion compensation have been proposed and demonstrated. Four

examples are dispersion compensating fiber, virtually-imaged phase-arrayed compensators, LPn

mode compensators, and chirped fiber gratings. For all of these methods, the chromatic

dispersion is the primary parameter to measure. The importance of secondary parameters

depends on the technology used for compensation. For instance, group delay ripple is a major

concern for the chirped fiber gratings while the quality of a compensator employing the LPn

mode depends on reducing the multi-path interference to tolerable levels. The relative importance of various parameters also depends on the design of the compensated

system. When compensator nonlinearity is not an issue, important parameters are chromatic

dispersion, optical loss and polarization-mode dispersion (PMD). For wavelength-division

multiplexed systems, the spectra of these parameters are needed. However, the PMD spectrum

usually evolves too rapidly to be usefiil so an understanding of PMD statistics is needed instead. The accuracy required for dispersion measurements depends on the application. In one study[l]

of a 10 Gb/s, 8x80 km system operating with nonlinear transmission fiber, but linear dispersion

compensation, Essiambre found that to limit the system penalty to 1 dB, the dispersion of the compensating fiber (and the transmission fiber) has to be known to within ±2.5%. In this study, it was assumed that the final dispersion compensation at the receiver could be optimized by trial and error or by dynamic dispersion compensation. If this fi-eedom is removed, then the total system dispersion-length product must be known to within the IOVb^ (Gh/sf ps/nm limit mentioned above. For a 1000 km, 40 Gb/s system over standard fiber, this requires a dispersion

measurement accuracy of ±0.2%.

Optical loss in the dispersion compensation will reduce the optical signal to noise ratio (OSNR) at the receiver and thereby impair performance. In most systems, an extra dB of loss in the dispersion compensation contributes much less than one dB reduction in the receiver OSNR.

Therefore, an accuracy of several tenths of a dB in the loss measurement should suffice for predicting the effect of the compensation on the system OSNR. The spectrum of optical loss in the dispersion compensation also contributes to the overall gain shape of the system. A tenth of a dB accuracy in the loss spectrum would be needed for some system designs.

A common requirement for the mean PMD of a system is that it be less than one tenth of the bit period. On average, such a system having a Maxwellian PMD distribution can be expected to experience a penalty greater than 3 dB for no more than one second per year. The PMD

specification for a system is an upper limit rather than a targeted value. Therefore, the PMD

measurement need not be very accurate so long as the inaccuracy of the measurement is included

when determining the acceptable values of mean system PMD. An important concern is the

statistics of the PMD. The Maxwellian distribution results fi-om the sum of many randomly

coupled sources of birefi"ingence. If the system PMD arises primarily from a small number of

sources, the distribution will not be Maxwellian.

In addition to adding impairment to systems, PMD also adds uncertainty to the measurement of

chromatic dispersion. Consider the apparatus shown in Figure 1. It is similar to equipment

commonly used to measure chromatic dispersion except that polarization control and

measurement components have been added. A network analyzer measures the phase delay of a

1 GHz amplitude modulation imposed on laser light that steps through a wavelength range. The

chromatic dispersion is calculated from the slope of the measured delay versus wavelength curve.

178
Polarizer

PC 2 eg

Polarizers

Figure 1. Apparatus used to measure chromatic dispersion and polarization-mode dispersion.

MOD: modulator. A: optical amplifier, PC: polarization controller, Rx: receiver

However, in the presence of PMD, this delay depends on the dot product between the Stokes

vectors representing the launch polarization, J , and the PMD vector at the fiber input, x . For

pulsed input light, Mollenauer, et al. [2] showed that.

The mean signal delay, , is the propagation-induced change in the first moment of the pulse

envelope in the time domain [2-5], while t„ is a polarization-independent delay. Similar

relations can be derived for sinusoidal modulation [6,7]. They form the basis of the new PSD

(polarization-dependent signal delay) method that measures PMD vectors and chromatic

dispersion simultaneously [7,8].

Since PMD provides polarization dependent delay, it is not surprising that chromafic dispersion

measurements can be affected by PMD. Figure 2 shows spectra of the delays that would be seen

for special launch polarization conditions. The upper and lower solid curves would be observed

if the launch polarizations were always aligned with the PMD vector, x , and its negative, -f as

the wavelength changed. A measured curve for light launched with fixed polarization, shown by

the markers, wanders between these two envelope curves as predicted by the equation above. By

launching four states of polarization, one can obtain , shovioi by the central solid curve, which

is the relative delay that would be obtained in the absence of PMD. The chromatic dispersion can

be determined firom the slope of this curve, which is not influenced by PMD.

Summary

Signals propagating though dispersion compensation are subject to many of the same

impairments that exist in other components in the transmission path. Therefore, the important

measurement issues in dispersion compensators are similar to those in other parts of the system.

However, polarization-mode dispersion is of special interest, not only because some

compensation technologies tend to exhibit PMD, but also because PMD impairs the measurement

of the most important property of compensators: chromatic dispersion.

179
100

COQ.

>s

JO

CD Q

1541.5 1542.0

Wavelength (nm)

1542.5

dec30dispF1.ep

Figure 2. The effect of PMD on the fiber delay spectra. The central solid curve shows the relative delay

that would be measured if the fiber had no PMD. The markers show measured delay for a fixed launch

polarization. The outer solid curves show the delay that would be measured if the launch polarization was

varied with wavelength so as to always launch along the slow principal state (upper curve) or the fast principal state (lower curve). A large, common-mode delay has been subtracted from all the curves.

References

[1] R-J. Essiambre, private communication.

[2] L. F. Mollenauer, and J. P. Gordon, "Birefringence-mediated timing jitter in soliton transmission," Optics Lett. 19, 375-377 (1994).

[3] M. Karlsson, "Polarization mode dispersion-induced pulse broadening in optical fibers," Optics Lett. 23, 688-690 (1998).

[4] W. Shieh, "Principal states of polarization for an optical pulse," Photon. Technol. Lett. 11, 677-679

(1999) .

[5] J. P. Gordon and H. Kogelnik, "PMD Fundamentals: Polarization mode dispersion in optical fibers,"

Proceedings ofthe National Academy of Sciences, 97, 4541-4550 (2000).

[6] P. A. Williams, "Modulation phase-shift measurement of PMD using only four launched polarisation

states: anew algorithm," Electron. Lett. 35, 1578-1579 (1999).

[7] L. E. Nelson, R. M. Jopson, H. Kogelnik, and J. P. Gordon, "Measurement of polarization mode

dispersion vectors using the polarization-dependent signal delay method", Optics Express, 6, 158-167

(2000).

[8] R. M. Jopson, L. E. Nelson, H. Kogelnik, and J. P. Gordon, "Polarization-dependent signal delay

method for measuring polarization mode dispersion vectors," LEOS'99 Postdeadline paper, PDl.l, San

Francisco (1999).

180
Chromatic Dispersion Measurement using Phase-shift technique

For Installed Long Haul Optical Submarine Cable

Toshio KAWAZAWA, Takuyuki YAMAGUCHI, Koji GOTO

757, Kawamuko-cyo, Tsuzuki-ku, Yokohama-shi, Kanagawa 224-0044, JAPAN

TEL:+81-45-476-7122, FAX:+81-45-476-7203

E-mail:kawazawa@kddscs. co.jp

Abstract

This is the first paper to report the behavior of chromatic dispersion (CD) for the installed long haul submarine cable system by Far-end CD measurement equipment. The system CD stability was

within 5.6ps/nm and the system length was 0.7m elongation during one-day measurement.

1. Introduction

Fiber-optic communication systems require the use of dense wavelength division multiplexing

(DWDM) and high power to meet the demands of data traffic growth. It is well known that in addition

to linear penalties the transmission capacity of DWDM systems is limited by transmission fiber non-linearlities and chromatic dispersion (CD). In the recent long haul optical submarine cable system, the precise management of system CD is required to minimize the interaction between

accumulated system CD and fiber nonlinearly. The submarine cable system consists of many fiber pieces and repeaters. The whole installed system CD could not be accurately charactenzed by

individual piece of cable and repeater measurement cause of cumulative measurement error. Even if the CD is characterized by parts data, it is hard to accurately estimate due to replacement or insertion

of cable or repeater. In addition, the system CD has a characteristic of environment dependency

such as temperature, stress and tension in the installed condition. The evaluation of system CD has

to be confirmed for such installation and cable repair in order to obtain the system performance by

adjust the pre and post chromatic dispersion compensation.

In this paper, we have conducted high accurate CD measurement for long haul submarine cable system. The stability of system CD in installed 8,000km submarine cable system was evaluated and

the variation of CD was stable within 5.6ps/nm that gives negligibly small impact for lOGbit/s

transmission performance.

2. Measurement configuration

2.1 CD IVleasurement Equipment

The CD measurement configuration is shown in figure2. The CD test equipment is using phase-shift

technique by measurement of the relative group delay experienced by the various wavelengths. The

optical reference channel as a suitable time delay generator was interposed between optical source and detector in order to compensate the fiber elongation [2]. The optical source was a wavelength

tunable laser diode (WTL) with an external modulation (450MHz) as a signal channel. The signal

channel was launched into the system under test. The spectral scan of the group delay is performed

from 1547 to 1553nm with 0.1 2nm step. The wavelength meter is used to obtain the accurate wavelength of tunable laser light source. The optical source of reference channel (1550nm) is a laser diode (LD) and modulated by external modulator with same frequency as the channel signal, and the

light launch to another fiber. At receiver side, the signal channel and reference channel phase shift is measured by the vector voltmeter as a delay detector. The time group delay was deduced from the corresponding phase shift (J) through the relation x=<^l2nf, f being the modulation frequency. The

measured group delay was fitted by the quadratic expression, since the majority fiber of the installed submarine cable system was DSF. The chromatic dispersion D(A,)=dT/d X can be determined from the

differentiated quadratic expression.

The system length L is calculated from the following formula L=(c/n) (^I2nf, where L is a system length, c is light velocity in vacuum, n is refractive index of the fiber. The average refractive index is assumed 1 .4738. The measured length is twice of real system length, because the end of the system

under test was connected to another fiber pair in order to compare with the absolute phase value of reference channel phase and signal channel phase at same frequency generator. For far end CD

measurement, the CD test equipment communicated on another fiber pair of the optical submarine

cable system with reference channel of data modulation. The CD measurement time is about 15min

for one time measurement.

2.2 System under test The measured optical submarine cable system is designed for Trans-ocean pacific systems with

181
4subsystem, 16WDMx10Gbit/s (1545.5nm to 1554.5nm) and RZ pulse format. The output of repeater is +9.5dBm that is pumped with 980nm erbium-doped fiber amplifier, spaced by 45km. The

cable spans are constructed usi.ig two types DSF (Dispersion Shift Fiber) and the -2ps/nm/km at 1550nm respectively. One is LCF (Large Core Fiber) to reduce optical power density at output of

amplifier, and the other one is NZDSF (Non-zero Dispersion Shift Fiber) to reduce CD coefficient (Slope) at input of amplifier [1]. The nominal CD management block is nine DSF and one DCF span. The DCF (Chromatic Dispersion Compensation Fiber) cable is conventional single mode fiber (SMF)

with 1.3um zero-dispersion wavelength (ZDW) to obtain a system ZDW of 1550nm. The maximum

and average sea depth is approximately 6000m and 4800m, respectively.

3. Results

3.1 Stability of CD for installed submarine cable system

To confirm the installed optical submarine cable system stability, the continuously measurement was

conducted for one day including measurement interruption. The number of spectral scan is 32points

for each CD measurement. Figure 2 shows the group delay time and CD at beginning of the continuously measurement. The system consist of two different CD fiber type, but the quadratic

equation well meet the measurement data, since the fitting error is less than 2.0E-11 during

measurement period. Therefor the calculated CD slope is constant at measurement range. The summary of the continuously measurement results and data are shown in tablel and figures

respectively. The some lack of measurement data caused of measurement controller error. 0.7m

(0.06m/day) cable elongation for system length was observed during measurement period. The trend

of length variation was considered not only elongation but also shrink because the measurement

period was shorter than the cycle of cable length variation. The average ZDW, CD slope and CD

(@1550nm) were 1549.846nm, 0.08337ps/nm /km and 102.0ps/nm respectively. The variation of ZDW, CD slope and CD was 0.009nm, 0.00084ps/nm^/km and 5.6ps/nm. The CD characteristics were not observation of increase and decrease tendency. From the point of system design, the CD

fluctuation of 5.6ps/nm is satisfy the system requirement of the lOGbit/s-transmission system.

Because the robustness of submarine cable systems for CD is more than several ten ps/nm.

3.2 Accuracy of cumulative individual data

We compared to the cumulative individual CD data with CD measurement by using above test equipment during the system assembly test to confirm the system CD design. The individual data

consists of the cable and repeater data that were measured at cable and repeater factory respectively.

The difference of CD between the measurement data and the cumulative individual data is 1.2~6.4ps/nm for 1,128km length and 8 sample (4 fiber pairs). The results show that the cumulative

individual data is good agreement with measurement results. Therefor, the cumulative individual CD

data is reference to adjust the terminal CD equalization.

4. Conclusion

We have developed Far-end CD measurement equipment for the installed long-haul submarine cable

system. The system CD stability was within 5.6ps/nm and the system length was 0.7m elongation

during one-day measurement. We also confirmed that the individual data could calculate the system

CD, but it is difficult to evaluate after the system repair including cable replacement, because there is no data of the cut cable CD. In such a case, the Far-end CD test equipment can evaluate the system

CD for the installation and repair system.

5. Acknowledgement

The authors would like to thank Y.Horiuchi of KDD R&D Labs for his supports, and M.lmamura and

E.Kimura of Advantest Co. for their co-operation.

6. References

[1] M.Suzuki, et al, OFC'98 San Jose, PD17, 1998.

[2] Y.Horiuchi, et al. Electronics Letters, 1993, vol.29, No.1 pp.4-5.

182
System Under Test Reference CD Measurement Equipment

Fig.1 Chromatic dispersion Measurement for far end set up configuration

Table 1 Summary for Length and CD measurement of long haul optical

Statistic Length [km] ZDW [nm]

CD slope

[ps/nm^/km]

CD @1550nm

[ps/nm]

Average 7963.1421 1549.846 0.08337 102.0 Maximum 7963.1424 1549.851 0.08377 104.8 Minimum 7963.1417 1549.842 0.08293 99.2 Dynamic Range 0.0007 0.009 0.00084 5.6

Standard deviation 0.0003 0.002 0.00021 1.4

183
4000 —I—I—

I—I—I—I—I—

I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—[- System Length : 7963.1417km (n : 1.4738)

CD Slope : 0.08327ps/nm^/km (666.727ps/nm ^)

1.547 1.548 1.549 1.550 1.551 Wavelength [nm]

1.552

4000

- 3000 o3"o

3

-4000

1.553

Fig. 2 The results of Group delay time and Chromatic dispersion

for long haul optical submarine cable system

7963.1430

7963.1420

7963.1410 0.080 Time [3hour/div] Time [3hour/div]

1549.900

g: 1549.860

IS

g 1549.840

g. 1549.820

(A O

g 1549.800 N

120

If

li) o

Time [3hour/div] Time [3hour/dlv]

Fig. 3 The continuously measurement results of system length and chromatic

dispersion for long haul optical submarine cable system

184
A Novel Method for Measuring Chromatic Dispersion of Long Optical Fibers by Means of

High Resolution Optical Ranging System Using a Frequency-Shifted Feedback Laser

Koichi liyama, Takahiro Maeda, and Saburo Takamiya

Department of Electrical and Electronic Engineering

Faculty of Engineering, Kanazawa University

1 Introduction

Recently, a new type of laser, a frequency-shifted feedback laser, has been actively studied [1-3]. The unique

feature of the FSF laser is that an optical frequency shifter is incorporated in the laser cavity, which means

that the optical beam in the laser cavity is frequency-shifted while recirculating in the laser cavity, and as a result, the optical frequency of the FSF laser is linearly chirped in time. The FSF laser is applicable to

optical rauiging system £md optical frequency domain reflectometry (OFDR) by utilizing the characteristic

feature of the frequency-chirped optical output. In the optical ranging system using a FSF laser, the spatial

resolution of about 1 cm is achieved over 1 km measurement range. [4] In this paper, we describe a novel measurement system of chromatic dispersion of long optical fibers by means of the optical ranging system using a FSF laser. The advantage of the proposed system is that no

high-speed electronics and optics are necessary.

2 Principle

Figure 1 shows the system configuration for measuring the chromatic dispersion of optical fibers. The FSF

laser consists of an erbium-doped optical fiber amplifier (EDFA), a tunable filter, an acoust-optic modulator

(AOM) driven by 80 MHz, which was used as an optical frequency shifter, a polarization controller (PC), a polarizer and an output coupler. The light from the FSF laser is sent to the Michelson interferometer, in which an optical fiber under test (FUT) is connected in an arm of the interferometer, and the beat signal

between the reflected light from the short arm (referred to as the reference light) and the reflected light at the fax end of the fiber (referred to as the delayed light) is measured by the spectrum analyzer. Since the

light from the FSF laser is frequency-chirped, the beat signal has a beat frequency proportional to the optical

path difference between the reference and the delayed lights. If the optical fiber under test has chromatic

dispersion, the beat frequency changes according to the wavelength of the FSF laser because the group

velocity in the fiber depends on the wavelength. As a result, the chromatic dispersion can be estimated from

the beat frequency change.

Now, we estimate the required spatial resolution in the proposed system. If the fiber length, the dispersion

vedue, the wavelength separation, and the group velocity in the fiber are denoted by L (km), D (ps/nm/km),

AA (nm), and Vg (m/s), respectively, then the required spatial resolution Sz is given by

Sz = DxLxAXxvgX 10'^^ (m) (1)

AOM

Frequency-Shifted Feedback Laser

reference FUT

7

^. go

delayed

PD Amp.

IH2- Spectrum

Analyzer

Figure 1: System configuration for measuring the chromatic dispersion.

185
where Vg = c/ug, c is the Ught speed in vacuum, and Ug is the

group refractive index of the fiber. The estimated spatial reso- lution for Z) = 1 ps/nm/km (a dispersion shifted fiber (DSF) is assumed), c = 3 x 10^ m/s, and Ug — 1.46 is tabulated in Table

1. It is found that the spatial resolution of about 1 cm is required

for measuring the chromatic dispersion of long DSF's. Such a

spatial resolution cannot be achieved in the conventional optical

time domain reflectometry (OTDR) and OFDR. As can be seen

in the next section, such a spatial resolution can be achieved in the optical ranging system using a FSF laser.

Table 1: Required spatial resolution in the proposed system.

L AA Sz

100 km 1 nm < 20.55 mm

10 nm < 20.55 cm

10 km 1 nm < 2.055 mm

id nm < 20.55 mm

3 Optical Ranging using a FSF laser

First, we investigated the performance of the optical ranging system using a FSF leiser. Figure 2 shows

the measured beat spectrum for (a) short optical fibers and (b) a long optical fiber. The frequency chirp

range was about 20 GHz. It is found from Fig. 2(a) that the beat spectrum consist of two beat signals

within the free spectral range (FSR) of the laser cavity, and the two beat signals approach each other with

increasing the length of the optical fiber under test. The spatial resolution is about 1 cm. As is shown in Fig. 2(b), even if the optical fiber is long, we can obtain the beat spectrum similar to that for short optical

fibers. The reason of such a beat spectrum is described as follows. The output of the FSF laser consists of a chirped frequency-comb, which means that many oscillating mode separated by the FSR of the FSF laser are simultaneously frequency-chirped. The number of the oscillating mode M is given by AF/FSR, where

AF is the chirp range, and M = 3,400 in our system. This is expressed as

TRT TRT

(2)

where v{t) is the instantaneous optical frequency, vps is the intracavity frequency shift caused by the AOM,

TRT = 1/FSR is the round-trip time of the laser cavity, and q is an integer. As a result, the beat signal

between the different oscillating modes can also been measured. The beat frequency /b is expressed as

^Fs ^ q_

TRT TRT

-—{t-Tg) - -—

TRT TRT

l^FS TRT

m

TRT

(3)

>m

•D

0

8.

CO

15 cm

41 cm

-I L J 1 I 1 d

2 4 6

Frequency ( MHz

)

(a) For short optical fibers.

>mIDo

o

o.c

O)

CO

-i ' r

0 2 4 6

Frequency ( MHz

)

(b) For a 5 km-long optical fiber.

Figure 2: Measured beat spectrum for short and long optical fibers.

186
0)

03o

wm

T3

g

8.

tocw

2 3 4 5 Frequency ( MHz

1530 1540 1550

Wavelength ( nm

1560

Figure 3: Measured beat spectrum for an

80 km-long DSF.

Figure 4: Measured beat frequency change for an 80 km-long DSF.

This work

Phase method .

1530 1540 1550 1560

Wavelength ( nm

Figure 5: Estimated chromatic dispersion for an 80 km-long DSF.

>

CQ

•DO

I

a

(0c

CO 2 4 6

Frequency ( MHz

Figure 6: Beat spectrum for connected two

SMF's.

where Tg is the delay time between the reference and the delayed lights and m = q — q' is a, beat order. The

beat order of the beat spectrum shown in Fig. 2(b) is m « 2,000. The spatial resolution is 1 cm, which is the same with that for short optical fibers.

4 Chromatic Dispersion Measurements

The chromatic dispersion of an 80 km-long DSF was estimated. Figure 3 shows the measured beat spectrum

for different lasing wavelength of the FSF laser. The beat frequency depends on the wavelength. The beat

frequency change against the lasing wavelength is shown in Fig. 4. The measured beat frequency change

is well fitted with a. parabolic function as shown in soUd lines. The zero-dispersion wavelength Ao is the

wavelength at which the beat frequency change is bottom (or peak) to be Aq = 1552.86 nm. From eq. (3), the change in the delay time Svg can be calculated from the beat frequency change Sfs to be

STg = ^<5/b. (4)

Therefore the chromatic dispersion can be estimated by differentiating the fitting curve shown in Fig. 3

with respect to the wavelength after converting Sfs to Sxg by using eq. (4). Figure 5 shows the estimated

chromatic dispersion against the wavelength. The solid line is the result by this work and the crosses are the result by the phase method. The chromatic dispersion can be well estimated by the proposed system.

Next, a 5 km-long single-mode fiber (SMF) and a 6 km-long SMF was connected by an FC/PC connector and were used as the optical fiber under test. In this experiment, the beat signals generated by the Fresnel

187
1530 1540 1550

Wavelength ( nm

)

1560

E

c

co

0)Q. b

12

10

8

64

2

0

-2

6 /cm frber

5 km fiber ,

,

, -i

This work (5km)

+ Phase method (5km)

This work (6km)

o Phase method (6km)

I I I 1 l_ 1530 1540 1550

Wavelength ( nm

)

1560

Figure 7: Measured beat frequency change for connected two SMF's.

Figure 8: Estimated chromatic dispersion for 5 km-long and 6 km-long SMF's.

reflections at the connector and the far end of the fiber are simultaneously measured as shown in Fig. 6, and the chromatic dispersion of the individual fiber can be simultaneously estimated. The measured beat

frequency change is shown in Fig. 7. The beat frequency change for the 6 km-long SMF is derived by

subtracting the beat frequency change at the connector from that at the far end of the fiber. The estimated

chromatic dispersion is shown in Fig. 8. The solid line is the result by this work and the crosses are the

result by the phase method. The individual chromatic dispersion of the 5 km-long and the 6 km-long SMF's

are simultaneously estimated.

5 Conclusion

We have proposed a novel method for measuring the chromatic dispersion of long optical fibers by means

of optical ranging system using a FSF laser. The advantage of this system is that no high-speed electronics

and optics is required. The chromatic dispersion for an 80 km-long DSF was estimated. And the individual

chromatic dispersion of the connected two optical fibers could be simultaneously estimated. The proposed

method is very useful technique for measuring the chromatic dispersion of the fibers as well as its length.

Acknowledgements

This work was carried out under the Cooperative Research Project Program of the Research Institute of

Electrical and Communication (RIEC), Tohoku University, and the authors would like to thank to Prof. H.

Ito, Dr. K. Nakamura and Mr. M. Yoshida of RIEC, Tohoku University, for their fruitful discussion. A part

of this work was supported by the Telecommunications Advancement Foundation.

References

[1] K. Nakamura. et. ah: "Spectral characteristics of an all solid-state frequency-shifted feedback laser",

IEEE J. Quantum Electron., vol. 33, pp. 103 (1997).

[2] K. Nakamura, F. Kowalski, and H. Ito: "Chirped-frequency generation in a translated-grating-type

frequency-shifted feedback laser". Opt. Lett, vol. 22, pp.889 (1997).

[3] K. Kasahara, et. al.: "Dynamic properties of an all solid-state frequency-shifted feedback laser", IEEE

J. Quantum Electron., vol. 34, pp. 190 (1998).

[4] K. Nakamura, et. al.: "A new technique of optical ranging by a frequency-shifted feedback laser", IEEE

Photon. Technol. Lett, vol. = 10, pp. 1772 (1998).

188
AUTHOR INDEX

Abbott, John S 29

Achten, F.J 35

Allen, Christopher 79

Andrekson, Peter A 95

Artiglia, Massimo 109

Barlow, A.J 83

Bayart, D 147

Bergonzo, A 147

Bessa dos Santos, A 105

Billington, Robert S 61

Bingle, M 35

Birks, T.A 1, 11 Bogert, Gail 19

Bongrand, Isabelle 61

Borne, S 147

Bray,M.E 131

Brechet, F 7 Broekhoven, G. v 65

Cherin, A.H 73

Chemikov, S.V 143

Chirita, Razvan 69

Chowhury, Dipak 77

Clement, Tracy S 121

Coakley, Kevin C 121

Craig, R.M 113

Cyr, Normand 87

de Hon, B.P 35

Demarest, Kenneth 79

de Riedmatten, Hugues ... 173

Dupre, Jack 125

Dyer, ShelleeD 161, 169

Elliot, R.T 131

Eyal, A 101

Ferrario, M 53

Galtarossa, Andrea 109

Gardner, W.B 73

Gisin,N 57, 105, 155, 173

Goto, Koji 181

Gray, Stuart 151

Guinnard, L 155

Guinnard, 0 155

Gunn, Deana 135

Guttman, Jeffrey L 69

Hale, Paul D 117, 121

Hallam, Andrew G 61

Hamoir, D 147

Henstock, J 83

Humphreys, David A. . . 25, 61

liyama, Koichi 185

Ives, D.J 91

Jones, K.P 131

Jopson, R.M 177

Kawazawa, Toshio 181

Kim, D.Y 15

Knight, J.C 1 Koch, F 143

Laere, R. v 65

Legre, M 155

Leproux, P 7

Lewis, S.A.E 143

Li, Y 101

Ludvigsen, H 165

Mackechnie, C 83

Maeda, Takahiro 1 85

Mangan, B.J 1 Marcou, J 7

Martelli, P 53

Martinelli, M 53

Mezger, J 65

Nakajima, Kazuhide 45

Namihira, Yoshinori 49

Narayanan, Chellappan .... 19

Niemi, T 165

Oberson, P 155

Ohashi, Masaharu 45

Omae, Takuya 45

Ortigosa-Blanch, A 1

Pagnoux, D 7

Palmieri, Luca 109

Palsan, Carmen D 69

Park, Y 15

Parker, Andrew 61

Peak, U.C 15

Philen, Dan L 39

Pietralunga, S.M 53

Pizzinat, Anna 109

Pleunis, P 65

Rochford, Kent B 169

Rose, A.H 161

Rose, Todd S 135

Roy, P 7

Russell, P.St.J 1, 11

Schiano, Marco 109

Seong,N.H 15 Song, Shuxian 79

Stolen, R.H 139

Takamiya, Saburo 1 85

Tambosso, Tiziana 109

Taylor, J.R 143

Torabi, N 147

Uusimaa, M 165

Valley, George C 135 van Stralen, M.J.N 35

Vinegoni, C 57, 155 von der Weid, J.P. ... 105, 155

Wadsworth, W.J 11 Walker, Brian 61

Wang, CM 117, 121, 161

Wegmuller, M 57, 105,

155, 173

Wells, Dominic S 61

Yamaguchi, Takuyuki .... 181

Yariv, A 101

Youk, Y.C 15

Zbinden, Hugo 173

AU.S. GOVERNMENT PRINTING OFnCE:2000-460-746/3645 1 189



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