NIST Special Publication 1055

Technical Digest: SOFM 2006

A NIST Symposiumfor Photonic

and Fiber Measurements

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

National Institute of Standards and Technology • Technology Administration • U.S. Department of Commerce
rheNational Institute of Standards and Technology was established in 1988 by Congress to "assist industry in the development of technology ... needed to improve product quality, to modernize manufacturing processes, to ensure product reliability ... and to facilitate rapid commercialization ... of products based on new scientific discoveries." NIST, originally founded as the National Bureau of Standards in 1901, works to strengthen U.S. industry's

competitiveness; advance science and engineering; and improve public health, safety, and the environment. One of the agency's basic functions is to develop, maintain, and retain custody of the national standards of measurement, and provide the means and methods for comparing standards used in science, engineering, manufacturing, commerce, industry, and education

with the standards adopted or recognized by the Federal Government.

As an agency of the U.S. Commerce Department's Technology Administration, NIST conducts basic and applied

research in the physical sciences and engineering, and develops measurement techniques, test methods, standards, and related

services. The Institute does generic and precompetitive work on new and advanced technologies. NIST's research facilities are located at Gaithersburg, MD 20899, and at Boulder, CO 80305. Major technical operating units and their principal

activities are listed below. For more information visit the NIST Website at http://www.nist.gov, or contact the Publications

and Program Inquiries Desk, 301-975-3058.

Office of the Director

•National Quality Program

International and Academic Affairs Technology Services

•Standards Services

Technology Partnerships

Measurement Services

Information Services

Weights and Measures

Advanced Technology Program

•Economic Assessment

Information Technology and Applications

Chemistry and Life Sciences

Electronics and Photonics Technology

Manufacturing Extension Partnership

Program

•Regional Programs

•National Programs

Program Development

Electronics and Electrical Engineering

Laboratory

•Microelectronics

•Law Enforcement Standards

•Electricity

•Semiconductor Electronics

•Radio-Frequency Technology'

•Electromagnetic Technology'

•Optoelectronics'

•Magnetic Technology'

Materials Science and Engineering

Laboratory

•Intelligent Processing of Materials

•Ceramics

•Materials Reliability'

•Polymers

•Metallurgy

•NIST Center for Neutron Research

Chemical Science and Technology

Laboratory

•Biotechnology

•Process Measurements

•Surface and Microanalysis Science

•Physical and Chemical Properties"

•Analytical Chemistry

Physics Laboratory

•Electron and Optical Physics

•Atomic Physics

•Optical Technology

•Ionizing Radiation

•Time and Frequency'

•Quantum Physics'

Manufacturing Engineering

Laboratory

•Precision Engineering

•Manufacturing Metrology

•Intelligent Systems

•Fabrication Technology

•Manufacturing Systems Integration

Building and Fire Research Laboratory

•Applied Economics

•Materials and Construction Research

•Building Environment

•Fire Research

Information Technology Laboratory

•Mathematical and Computational Sciences"

•Advanced Network Technologies

•Computer Security

•Information Access

•Convergent Information Systems

•Information Services and Computing

•Software Diagnostics and Conformance Testing

•Statistical Engineering

'At Boulder, CO 80305 'Some elements at Boulder, CO
NIST Special Publication 1055

Technical Digest: SOFM 2006

A NIST Symposium for Photonic

and Fiber Measurements

Nisr 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

LASEHS S CLBCTM OFTICS SOCIETY OSA Optical ScKiety of America

September 19-20, 2006

National Institute of Standards and Technology

Boulder, Colorado 80305

Edited by

T. J. Drapela

P. A. Williams

September 2006

U.S. Department of Commerce

Carlos M. Gutierrez. Secretary

Technology Administration

Robert Cresanti, Under Secretajyfor Technology

National Institute of Standards and Technology

William Jeffrey, Director
National Institute of Standards and Technology Special Publication 1055

Natl. Inst. Stand. Technol. Spec. Publ. 1055, 148 pages (September 2006)

CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 2006

For sale by the Superintendent of Documents, U.S. Government Printing Office

Internet: bookstore.gpo.gov Phone: (202) 512-1800 Fax: (202) 512-2250

Mail: Stop SSOP, Washington, DC 20402-0001
PREFACE

^

Welcome to SOFM 2006, the fourteenth installment of NIST's biennial symposium dealing with photonic

and optical fiber measurements.

In the Fall of 1980, Jimmy Carter was the U.S. President and disco dancing was just past its peak. TV

remote controls were just becoming commonplace. Cable, for those lucky few who had it, meant a dozen

or so channels—when it was working. The PC was still a couple of years away, while cell phones and the

internet were more than a decade away from becoming part of the common vernacular. The prefix "giga" was, at most, a rarely used curiosity, even among scientists, and was virtually unknown to the general

public.

At this same time the fiberoptic industry was young and in critical need of metrology development. The

National Bureau of Standards (NIST's precursor) speculated that maybe a conference devoted to fiber

measurements could be of some utility to this developing industry. It is unlikely that any of the organizers or attendees of that first SOFM could have envisioned this utility to span 26 years! During

these years the photonic industry has been through its share of ups and downs, including the exhilarating

highs and lows of the last six or so years.

As the fiberoptic industry now seems to have reached a certain level of maturity, the demand for cuttingedge metrology-related research relating to fimdamental fiber measurements—the traditional core focus

area for SOFM—seems to be leveling off This is in no way meant, however, to imply that our work as metrologists is done. Rather, crucial metrology needs are now springing up from a wider variety of other

related or derived fields of optoelectronics. This is evidenced in the SOFM 2006 technical program,

which is as diverse as any in SOFM history, including sessions on fiber sensors, dispersion metrology,

power-related metrology, "last-mile" issues, spectroscopic and frequency metrology, systems metrology,

and specialty fibers. This diversity is what inspired us to conclude this year's symposium with a "roadmapping" workshop on emerging and anticipated photonic metrology needs. SOFM remains

geographically diverse as well, with just over 60 % of this year's accepted papers originating outside the

U.S.

It has been a thrilling ride, and we are sincerely glad that you have decided to join us for this iteration. Please enjoy your visit, including SOFM 2006, itself, and all that our Rocky Mountain area has to offer.

Happy reading,

Tim Drapela

Paul Williams

Boulder, Colorado

September 2006
SYMPOSIUM COMMITTEE

P. A. Williams, NIST, Symposium Co-Chair

T.J. Drapela, NIST, Symposium Co-Chair

A. Barlow, PerkinEhner

A. Champavere, JDSU

G. W.Day, NIST

S.C. Fleming, Sydney University

D. Franzen, A^^r

N. Gisin, University of Geneva

M. Hackert, U.S. Navy

T.A. Hanson, Corning

D. Humphreys, NPL

J. Jackel, Telcordia

D.Y. Kim, GIST

P. Kolesar, SYSTIMAX Solutions

D. Krohn, Light Wave Venture

G. Kuyt, Draka Comteq .

A. McCurdy, OFS

G.W. Schinn, EXFO

C. Shaar, Photon Kinetics

H. Takara, NTT
CONTENTS

PREFACE iii SYMPOSIUM COMMITTEE iv

Photonic Sensor Market Evolution. Opportunities and Metrology Needs (invited)

David Krohn, Light Wave Venture LLC 1

Modulation Function Study of Coupling Based Intensity Modulated Fiber-Optic Sensors

Johan Jason.' " Hans-Erik Nilsson.' Bertil Arvidsson,' "^ Anders Larsson." 'Department of

Information Technology and Media, Mid-Sweden University, "Fiberson AB, "Ericsson Network

Technologies AB 6

Characteristics of Spun Fiber Grating Sensors

Yong Wang, Chang-Qing Xu, Department of Engineering Physics, McMaster University, Canada 10

Metrology of Integrated Waveguide Devices and Systems Using Near-Field Scanning Optical

Microscopy

Guangwei Yuan, Kevin L. Lear, Electrical and Computer Engineering Department, Colorado

State University 14

Return Loss Measurement in the Presence of Variable Insertion Loss Using Optical Frequency

Domain Reflectometry

Stephen Kreger, Mark Froggatt, Dawn Gifford. Matthew Wolfe, Brian Soller. Luna

Technologies 18

High-Power Nonlinearity of Optical Fiber Power Meters for Pump Lasers at 980 and 1480 nm

I. Vayshenker, S. Yang, NIST, R. Swafford, OZ Optics Ltd. 22

Novel Approach for Non-Intrusive "Clip-On" Fiber Monitoring

Gang He, Daniel Gariepy, Hongxin Chen, Jean Theberge, Gregory W. Schinn, EXFO ElectroOptical Engineering, Inc. 26

Measurement Technique for Stimulated Brillouin Scattering Threshold

Per Olof Hedekvist, Anne Andersson, SP, Swedish National Testing and Research Institute 30

Butt-Joint Splice with Refractive Index Matching Material for High Power Light in Optical Fiber

Communications

Ikutaro Ogushi, Hisashi Izumita, Kuniaki Tanaka, Fumihiko Ito, Masahito Arii, NTT Access

Network Service Systems Laboratories, NTT Corp., Japan 33

High Speed PRBS for Detection of Defects in Fibers and Fiber Connectors

S. Rangaswamy, D. Lao, E. van Doom, Intelligent Automation Inc. 37

Experimental Investigation on Special Shaped Fibers Coupling with InP-Based Planar Lightwave

Circuit Chips

Xu Liu, Jinbiao Xiao, Xiaohan Sun, Department of Electronic Engineering, Southeast

University, P.R. China 41

V
Fiber Frequency Combs: Development and Applications (invited)

N. R. Newbury, W. C. Swann, I. Coddington, J. J. McFerran, NIST

Generation of a Frequency Comb by FWM in SOAs Enhanced by Raman

Ana Carrasco-Sanz,^ Sonia Martin-Lopez/ Miguel Gonzalez-Herraez/ '^ Pedro Corredera,^ Maria

Luisa Hemanz,^ Laura Abrardi,^ '^Departamento de Metroiogia, Instituto de Fisica Aplicada,

(CSIC) Madrid, ''Departamento de Electronica, Universidad de Alcala, Spain

Comparison of Saturated Absorption Spectra of Acetylene Gas Inside Photonic Bandgap Fibers

K. Knabe, R. Thapa, O. L. Weaver, B. R. Washburn, K. L. Corwin, Department of Physics,

Kansas State University

CW Supercontinuum Generation in Standard Optical Fiber over the O, E, S and C Communication

Bands

Laura Abrardi,^ Sonia Martin-Lopez," Ana Carrasco-Sanz," Miguel Gonzalez-Herraez," '' Pedro

Corredera," Maria Luisa Hemanz" ''Departamento de Metroiogia, Instituto de Fisica Aplicada,

(CSIC), Madrid, ''Departamento de Electronica, Universidad de Alcala, Spain

A C-Band Optical Frequency Synthesizer Using 1.5 Micron NICE OHMS Frequency Standards and

an Optical Frequency Comb Generator

C. S. Edwards,'-^ H.S. MargoHs,' G.P. Barwood,' P. Gill,' ' W.R.C. Rowley,' 'National Physical

Laboratory, "Blackett Laboratory, Imperial College, UK

PMD: Measurement Method Linkages (invited)

T.A. Hanson, Coming Inc. PMD Measurements Using Fixed-Analyzer Technique on a 1000km Amplified Link that Includes

ROADMs

Vincent Lecceche,' Fabien Sauron,' Andre Champavere,' Alexey Turukhin,^ Enrico Gonzales,^

'JDSU Commtest Division, 'JDSU Optical Network Research

Chromatic Dispersion Measurement Technique Utilizing an Unstable Supercontinuum Pulse Source

Sucbei Moon, Dug Y. Kim, Gwangju Institute of Science Technology (GIST), Republic of

Korea

Accurate Measurement of Chromatic Dispersion Distribution along Dispersion Managed

Transmission System

Mirza Imran Baig, Tohru Miyauchi, Masaaki Imai, Department of Electrical and Electronic

Engineering, Muroran Institute of Technology, Japan

Assessment of the Potential Accuracy of a Four Wavelength OTDR to Measure Chromatic

Dispersion

David Ives, Joan Hall, Christian Hart, National Physical Laboratory, UK

Critical Issues Relating to FTTH/PON Testing (invited)

Andre Girard, EXFO E.O. Engineering, Inc. WDM-PON for FTTx (invited)

Wayne V. Sorin,' Chang-Hee Lee,^ Byoung Y. Kim,' 'Novera Optics, Inc., ^Korea Advanced

Institute of Science and Technology

vi
FTTx Deployment Issues (invited)

Jimmy Salinas, AT&T 104

Special Requirements for Measurement Techniques Used for POP and Thick Glass Fibers (invited)

Olaf Ziemann, Polymer Optical Fiber Application Center, University of Applied Sciences

Niimberg, Germany 107

Characterization of Hole-Assisted Fibers as Smgle Polarization Fibers and Polarization Maintaining

Fibers (invited)

Xin Chen, Joohyun Koh, Ming-Jun Li, Daniel A. Nolan, Coming Inc. 113

Micro-Tomographic Reconstruction for Arbitrary Refractive Index Profiling of Optical Fibers

N.M. Dragomir,^ X. M. Goh,' G. Baxter,' A. Roberts,' 'School of Physics, The University of

Melbourne, 'School of Electrical Engineering, Victoria University of Technology, Australia 119

Fictive Temperature Distribution in Highly Ge-Doped Multimode Optical Fibers

M. Lancry,' I. Flammer,' D. Simons,^ B. Poumellec,' C. Depecker,"* P. Nouchi,^ M. Douay,^

'ICMMO/LPCES, Universite Paris Sud (XI), "Draka Comteq France, ^Draka Comteq Fiber BV,

Netherlands, "^LSPES, Universite des Sciences et Technologies de Lille, France, ""PhLAM,

Universite des Sciences et Technologies de Lille, France 123

Monitoring OSNR in Dynamic WDM Network (invited)

Y.C. Chung, Korea Advanced Institute of Science and Technology, Department of Electrical

Engineering and Computer Science 127

Optical Signal Quality Monitoring Based on Asynchronous Amplitude Histogram for DPSK

Systems

H. Takara,' T. Ohara,' B. Kozicki,' 'NTT Network Innovation Laboratories, NTT Corp., "Osaka

University, Japan 129

New Methods for Investigating Mode Coupling in Multimode Fiber: Impact on High-Speed Links

and Channel Equalization (invited)

Stephen E. Ralph, Amp Policy, Georgia Institute of Technology, K. D. Pedrotti, R. P. Dahlgren,

J. A. Wysocki, Department of Electrical Engineering, University of Califomia, Santa Cmz 133

A Differential Mode Delay Measurement for a Multimode Optical Fiber with Fourier-Domain LowCoherence Interferometry

J.Y. Lee, T-J. Ahn, D.Y. Kim, Department of Information and Communications, Gwangju

Institute of Science and Technology, Republic of Korea 1 39

AUTHOR INDEX 145

APPENDIX I - PROGRAM 1 47

vii

Photonic Sensor Market Evolution, Opportunities and

Metrology Needs

David Krohn

Light Wave Venture LLC

7 Cobblestorte Drive Hamden. CT 06518

dkrohn&Jishtwaveventure. com

Abstract: The fiber optic sensor market has the potential to exceed $1 billion in 2010. Needs in the oil and gas industry, as well as homeland security, will be the primary market drivers. Photonic biochemical

sensors have the potential to expand the market potential fivefold. A key part of establishing standards is having metrology procedures in place to allow for confonnance to defined specifications. Standards efforts are moving forward with activity by the Fiber Optic Sensor Consortium (OIDA), ASTM and the Petroleum

Open Standards Consortium. However, better industry coordination is required to prevent the lack of

standards from being a barrier to industry acceptance and growth.

1. Opportunity

There are a broad range of military, homeland security and industrial sensing applications that can be

facilitated by distributed fiber optic sensors. These applications include:

• Oil well monitoring

• Pipeline monitoring

• Pipeline security

• Shipboard and avionic health monitoring and damage control

Industrial processes Smart structures (Most applications will be for strain monitoring. However, traffic flow,

temperature, toxic gases and icing can also be monitored) - Bridges / tunnels - Dams

- Buildings - Machines - Power lines (Temperature and strain) - Transportation

• Oil and gas exploration (seismic)

• Homeland Security - Chemical / biological - Security / intrusion - Wide area surveillance

One of the key drivers in fiber optic sensor market evolution is homeland security [1]. In earlier market

studies the need and opportunity for wide areas surveillance was not fiilly recognized with a much smaller market potential defined. Under current world conditions that need has changed. Many vital assets which

cover wide areas, such as pipelines and borders, are under constant threat of being attacked or breached. There is a rapidly emerging need to be able to provide identification of intrusion threats to such vital assets. Similar problems exit for monitoring the basic infrastructure such as water supply, power utilities, communications systems as well as transportation. There is a need to develop a coordinated and integrated solution for the detection of threats. From a sensor standpoint, consideration must not be limited to

1
detection, but how does detection lead to intervention and deterrence. Also, fiber optic sensor technology

must be compatible with integration with other surveillance technologies such as wireless mote technology.

Another key market driver is the oil and gas industry. With the current price of oil up over 300% in the last 5 years and limited supply, smart oil / gas wells as well as reservoir management are very important. A

typical well is only 30% efficient. With distributed fiber optic sensors to monitor well health and

perforaiance, well efficiencies can be drastically improved providing enhanced oil recovery with potential extraction from reactivation of old oil wells. In addition, seismic sensing systems can better map oil reserves and enhance extraction.

Fiber optic sensors are limited in their ability to monitor biochemical agents. By expanding fiber optic sensors to photonic sensors, the ability to monitor biological agents and chemicals is greatly enhanced [3]. Key market segments include:

• Environmental monitoring

• Homeland security / military

• Industrial process

• Medical diagnostics

• Agriculture /

•

• Food processing • . Dmg discovery and processing

2, Market Forecast Potential

Market infonnation sources include: OlDA data base

• Supplier, integrator, end user intei"views

• Geophysical industry surveys

• Government contracts monitoring

The OIDA (Optoelectronics Industry Development Association) data base was generated through three workshops on sensors that were held in 2005 [2, 3, 4]. Over 125 contacts were made with suppliers and end

users from December, 2005 to May, 2006. Significant government funding will be invested in sensor technology. However, markets are in early development stage, fragmented and difficult to forecast.

The fiber optic sensor market potential is shown in figure 1 [5, 6]. Fiber Optic Sensor Market Potential

Industrial Smart Structures Earthquake Monitoring

! Homeland Security

IVIilitary Shipboard/Avionic

Military Surveillance

Pipelines

Oil & Gas - Wells

E] Oil & Gas - Seismic

2005 2006 2007 2008 2009 2010 Year

Figure 1 Distributed Fiber Optic Sensor Market Potential

2
The market analysis for fiber optic sensors indicates tliat the key market drivers are: Smart oil and gas wells

• Seismic sensors for oil well reservoir management and exploration

• Pipeline monitoring

• Homeland security

Military surveillance and smart structures also have good growth potential. The forecast is based on

systems cost which includes the sensing elements, packaging, cabling and interrogation. Installation is not

included.

The estimate is that approximately 45% of the fiber optic distributed sensing market will use

interferometric sensors due to accuracy and speed requirements in seismic applications. About 20% of the distributed sensing market will use Bragg gratings or some form of spectral shift sensors such as FabryPerot devices, fluorescence devices or microresonators. Distributed temperature sensing systems (DTS)

based on Raman scattering or Brillouin scattering will account for about 1 5% of the market. Speckle

pattern and intensity modulated sensors will have about 10% market share with 5% going to other technologies.

Depending upon the sensing technology used, the inten-ogation interface represents about 50% to 60% of

the sensing system cost (installation not included)

High resolution systems are too costly for many applications. As a result, they will likely tier into two

levels Higher cost - high resolution systems

• Low cost - reduced resolution systems (Only need to provide an alarm condition in many

applications.)

Potentially, more than 60% of the wide areas surveillance applications will be for alarms.

The photonic biochemical sensing market expansion is shown in figure 2 [5, 7]. Clearly, the transition from

fiber optic sensors to photonic sensors has a dynamic impact increasing the market potential by a factor of

five.

Fiber Optic Distributed

Sensors

Biophotonic

Photonic Cliemical Sensors Sensors

Figure 2 Photonic Sensor Market Potential 2010

3
3. Barriers

There have been a number of barriers for the acceptance and broad use of smart fiber optic sensors. Compared to telecommunications the volume is low. This fact coupled with proprietary and custom

specifications has kept the price of fiber optic sensors high. There is a general lack of a manufacturing

infrastructure and lack of standards for packaging and reliability. Also there are several competing

technologies; some photonic based and other approaches based on conventional non-photonic technologies.

Photonic biochemical sensors face additional barriers. They may need to be functional from UV to mid IR. The industry is plagued with false alanns from various sensing technologies. Compliance requirements

may require qualification with several different organizations.

4. Standards and Metrology

There are several groups now involved with standards. The fiber optics Sensor Consortium in conjuction

with OIDA and NIST has begun preliminary work on fiber optic sensor metrology and standards. NIST

defined the need for standards [8]. Standards facilitate the use of technology as outlined in the following:

• Improve measurement accuracy Enable cost reduction -

• Promote adoption and market growth

• Allow interchangeability and nuilti-sourcing

• Foster innovation

Non-standardized items tend to be custom and expensive and slow industry growth.

The Fiber Optic Sensor Consortium has developed a preliminary standard for Bragg grating sensors, a Bragg grating sensor inteiTogator and an interferometric interrogator. The ASTM Subcommittee El 3.09 on

Fiber Optics, Waveguides, and Optical Sensors is focused on the formation and development of methods

for testing, specifying, and using fiber optics, integrated optics, and other advanced optical and guided- wave techniques for chemical sensing. Currently under development are standards for distributed fiber optic chemical sensors. The Petroleum Open Standards Consortium has drafted a communications standard

for DTS (distributed temperature sensing system).

Metrology will play a key role in standards. As an example, the OIDA Fiber Optic Sensor Consortium has

prepared a preliminary standard for a Bragg grating sensor interrogator. The interrogator is required to have

sensitivity for wavelength shift of ! picrometer. Standard measurement techniques to ensure compliance

need to be defined. Other metrology needs include:

• Bragg grating characterization

• Dynamic strain versus static strain measurements

• Temperature strain discrimination (Bragg grating and Brillouin Scattering)

• Long term drift . o Strain .

.

o Vibration -

• Biochemical detection threshold

5. Summary

The fiber optic sensor market has the potential to exceed $1 billion in 2010. Needs in the oil and gas

industry, as well as homeland security, will be the primary market drivers. Photonic biochemical sensors have the potential to expand the market potential fivefold. Standards efforts are moving forward; but better industry coordination is required to prevent the lack of standards from being a ban^ier to industry

acceptance and growth. A key part of establishing standards is having metrology procedures in place to allow for confomiance to defined specifications.

4
6. References

[1] David Krohn. "Photonic Sensor Opportunities for Homeland Security

Applications", PliAST Conference, May, 2006

[2] OlDA, Fiber Optic Sensor Workshop, March 22-23, 2005, The Photonics Center

at Boston University, Boston, MA

[3] OlDA, Biophotonic Sensors and Smart Fiber Optic Sensor Networks, August 17-

18, 2005, Rochester, NY

[4] OlDA, Wide Area Surveillance Workshop, December 13-14, 2005 - Washington

DC

[5] David Krohn, "Biophotonic Sensors and Fiber Optic Sensor Networks",

OpticsEast, Conference 5594, October 25, 2005

[6] David Krohn, "Market Opportunities and Standards Activities for Fiber Optic

Sensors", 18* International Conference on Fiber Optic Sensors, to be published

[7] David Krohn, 2005 OlDA Annual Forum, Washington DC, November 29, 2205

[8] Kent Rochford, Biophotonic Sensors and Smart Fiber Optic Sensor Networks,

August 17- 18, 2005, Rochester, NY

5
Modulation Function Study of Coupling Based

Intensity Modulated Fiber-Optic Sensors

Johan Jason'"^, Hans-Erik Nilsson', Bertil Arvidsson' '\ Anders Larsson^

' Department ofInformation Technology and Media, Mid-Sweden University, SE-851 70 Simdsvall, Sweden

-Fiberson AB, P.O. Box 1044. SE-824 11 HudiksvaU, Sweden

^Ericsson Network Technologies AB, SE-824 82 HudiksvaU. Sweden

johan. iason{cp,miun.se

Abstract: The modulation flinction for fiber-optic sensors based on coupling between fiber ends is studied for different fiber types and fiber configurations, including multiple fiber passes.

Sensitivity aspects are evaluated experimentally and theoretically. It is shown that multiple passes

in reality do not result in a significant sensitivity increase due to coupling losses related to fiber alignment and reflection loss.

1. Introduction

Intensity modulated sensors based on coupling between fibers enable simple and cheap displacement sensor

solutions (Fig. 1). Such sensors can be used for measurement of position, displacement or vibration [1-5]. Also other parameters can be measured if they have the ability to cause displacement in some way, e.g. temperature changing

the deviation of a bimetal strip [6]. In the field of vibration measurement applications, the intensity modulation may

be produced by micro-structure cantilevers in the light path [1] or by letting the transmitting fiber itself act as a cantilever [2-3]. Similar sensor designs based on, or combined with, integrated optics have also been realized [4-5]. Some of the most important parameters to consider in the optical design of the sensor are the signal-to-noise

ratio, the sensitivity and the linearity of the sensor in the intended measurement range. These parameters are improved, in turn, if the received optical power, the slope (index) of the modulation curve and the linearity of the modulation over the total displacement range are as high as possible. In terms of received optical power, multimode

fibers are to be preferred to single-mode fibers. Radial (transverse) displacement gives the most significant change

in coupled power [7], and should therefore be considered for sensitivity reasons. Furthermore, the choice of fiber type also affects the sensitivity and the linearity of the sensor. A way of further improving the sensitivity could be to arrange multiple passes over a connection according to Fig. 2, thus giving a multiple change in coupled power for a certain displacement. The possibility of realizing this, and the practical use, is investigated and analyzed experimentally and theoretically below.

Fig. 1 . Principal coupling-based vibration sensor. Fig. 2. Multiple pass configuration using fiber ribbons.

2. Experimental pre-study

Measurements of the transmission ratio, i.e. received/transmitted power (Pr/Pj), versus the transverse displacement were performed for 62.5 )im core graded-index (GI) multimode fibers and 4-fiber arrays of the same fiber type, see

fiber data in Table 1. The 4-fiber arrays were in the form of encapsulated ribbons (denoted R4), MT-connectors (MT4) and ribbon fibers temporarily put into V-groove arrays (V4). Transmitting and receiving fibers of 5 m length

each were coupled to a 1300 nm LED and a HPS 152 IB detector with multimode lens HP81050BL, respectively. An

xyz-translator was used to optimize fiber-to-fiber coupling ( 1 mm free ends with a 1 0 |Lim gap), and a transverse scan

in 1-2 |am steps was made from +50 to -50 jim. In Fig. 3 the measured modulation curves are shown. It can be noted that no significant sensitivity improvement

is achieved for multiple, in this case four, passes compared with a single one. Here the degree of fiber alignment seems to play an important role: the better alignment, the steeper is the modulation curve, i.e. the higher is the

sensitivity. Also the reflection loss at each fiber interface should be considered. A theoretical analysis of the case is performed in the next section.

6
1

0.8

0.6

0.4

0.2

GI-62.5-R4

GI-62.5-V4 , GI-62.5-MT4 »* GI-62.5

-25 0 25 50 Transverse offset (pm)

Fig. 3. Experimental modulation cunes for 62.5nm core fiber with single pass and for multiple (four) passes with different alignment techniques.

1

0.8

0.6

.T(j=0.85, n=4 .V1,n=1

-50 -25 0 25 50 Transverse offset (pm)

Fig. 5. Theoretical modulation curves for single-pass coupling (n=l

)

and four-pass coupling (n=4). comparing ideal (To=l) and

experimental (To=0.85) transmission ratio for a single fiber pair.

1

0.8

0.6

0.4

0.2

GI-62.5-R4

GI-62.5

Fitted 62.5

Est. 62.5-R4 ,/

-25 0 25 50 Transverse offset (pm)

Fig. 4. Fitted modulation curv e to experimental data for single pass, and estimated and experimental modulation

curves for multiple (four) passes.

X

«a

cco

3

o

oE

><

Fi2.

0,1

0.05

— n=1 n=2

1 n=12

r

i

i

./>

0 0.2 0.4 0.6 0.8 1 Transmission ratio at zero offset

6. Maximum modulation index vs. transmission ratio To

for a=0.00I6 lim-.

3. Theoretical analysis

There exist a number of models for calculation of the splice loss between two multimode fibers following radial offset [7-9]. The applicability of the models often depends on the actual case, and the models are more or less based on experimental data. For the case presented in this paper, we need a simple expression for the modeling of the modulation curv e and for the derivation of an expression for the sensitivity. A simplified way of describing the modulation function is by using a Gaussian expression for the transmission ratio T for n multiple passes as a function of radial (trans\ erse) offset x:

n-- n -nax~ To -e (1)

where To is the transmission ratio per pass at zero offset. Alternatively, instead of using the a parameter, a

characteristic width w can be assigned to the function T(x). such that a=l /w". The slope S(x) of the modulation

cur\'e, also called the modulation index, is given by

S{x) = dT

dx

(2)

with a maximum absolute value S^ax for x=±l/V(2na). With 0<To<l and n>l. the maximum achiex able modulation

index Sn,a.x can be expressed as a function of n and Tq:

(3)

7
With a=0.0016 \im'' and To=0.85, T(x) approximates experimental data of a single pass for graded index

62.5|im core fibers, as shown in Fig.4. The fitted curve has been used to estimate the modulation curve for a four- pass connection, provided that To is the same for each pass. This estimated cui"ve is plotted in Fig. 4 together with

experimental data for a 4-fiber ribbon coupling.

In Fig. 5, the modeled modulation function T(x) with a=0.0016 \xm' and To=0.85 is plotted for n=l and n=4,

also for the ideal case when To=l. Fig. 5 gives a picture of what could be expected from an ideal multiple pass case, compared with the experimental result of Fig. 3: a higher slope (higher sensitivity) and a narrower measurable

displacement range (smaller measurement range). However, in order to reach the theoretical limit, besides having a good fiber alignment, reflection losses at the fiber ends must be eliminated. This is discussed in the next section. The

reflection loss can be modeled by replacing To with T|:-To in the expression for T(x), with Th=0.96"=0.92 being the Fresnel transmission factor per fiber pair, corresponding an approximate 4% Fresnel reflection loss at each glass/air

interface. Fig. 6 shows the corresponding maximum modulation index S^ax plotted versus To for some values of n, using

a=0.0016 |im From Fig. 6 it is clear that a veiy high To is needed to take benefit from multiple passes. In the case of n=4, a To=0.85 just about gives a modulation index higher than for n=l. The more passes, the higher To is needed

per fiber pair. This fact flags for problems with end reflections and with fiber alignment as experienced in the experimental pre-study.

Fiber sample NA Pt(hW)

SI 8.3/125 lam 0.11 0.37 GI 50/125 |im 0.20 13.0 GI 62.5/125 urn 0.275 31.2 GI 100/140 |.u-n 0.29 36.0*' GI 4x62.5/125 urn (ribbon) 0.275 31.9 GI 4x62.5/125 (V-groove) 0.275 35.8 GI 4x62.5/125 (MT-con.) 0.275 17.7 8.3-50-62.5-100 (hybrid rib.) 0.11-0.29 0.32

*' Fiber connected to source with 62.5/1 25|.im fiber

4. Further experimental work

1

0.8

0.6

0.4

0.2

MT4, SOjjm gap

« -indexmatch ^ o1x, lOpm gap o

* -indexmatch ^*p"

0<X>o

o 00

o

50

Fig.

-50 -25 0 25 Transverse offset (|jm)

7. Effect of index-matching liquid on the transmission ratio.

In order to maximize the transmission ratio further, the practical effect of index matching liquid, in this case glycerol

(refractive index «1.47), was investigated. As stated above, the Fresnel reflection contribution to the coupling loss theoretically results in a transmission factor Tp of 0.92 per fiber-to-fiber coupling with an air gap. In Fig. 7 the modulation curve for a single 62.5|im core fiber pair with a 10 \im end gap is shown with and without the use of

index-matching liquid in the gap. A raise of transmission ratio Tp-To from 0.85 to about 0.95 is seen, which gives a transmission factor Tf=0. 85/0. 95=0. 89 if the reflection loss is assumed to be totally eliminated (Tf=1) by the index- matching liquid. The effect of index-matching was also tested on the 4-fiber MT-connector airangement, here with an end gap of 50 ^m. In this case the total change in transmission ratio changed from about 0.45 to roughly 0.8, which would resuh in a transmission factor Tf=0.87 since Tf''=0.45/0.8=0.56 under the same assumption as above.

Clearly the reflection loss plays an important role when using multiple passes. Another way of increasing the transmission ratio is to couple the light into a fiber with a higher numerical

aperture (NA) and a larger core than the transmitting fiber. With the same experimental conditions, modulation

curves were first recorded for other fiber types (see Table 1 ). The results of these basic measurements are shown in Fig. 8. It is clear from the figure that different fiber types result in different values of a, or characteristic width w, in the modeled T(x). In the next step, hybrid fiber pairs, where the receiving fiber has a larger core diameter and

numerical aperture (NA) than the transmitting fiber, were investigated. Finally a hybrid, three-pass fiber array, aligned with V-grooves, was measured on. The hybrid array consisted of a transmitting standard single-mode fiber coupled to a GI 50|im core fiber, further to a GI 62.5|am core fiber and finishing with a receiving GI 100|rm core

fiber. The result, seen in Fig. 9, is an increase in To for the complete array, and overall more relaxed alignment

demands on the fiber pairs. Also it can be noted that the fiber pair with the highest modulation index Smax determines

Smax for the hybrid array, i.e. the same modulation index could be achieved with just a fiber pair, in this case a single-mode fiber to multimode 50|am core fiber coupling.
-50 -25 0 25 Transverse offset {pm)

50

Fig. 8. Experimental modulation curves for single pass (n=l) coupling of some different fiber types.

-25 0 25

Transverse offset (|im) 50

Fig. 9. Experimental modulation curves for hybrid fiber pairs and hybrid fiber array.

5. Discussion

When designing coupling-based intensity modulated sensors a decision must often be made whether sensitivity or measurement range, i.e. measurable range of displacement, should be the priority. Also the coupled power in the system might be an important parameter as well as the fiber alignment possibilities. All these parameters finally

determine the most suitable fiber type and fiber configuration of the sensor. From the experiments (Fig. 8) it can be noted that different fiber types have different modulation indices and

therefore result in different sensitivities, but they also have different measurement ranges. By using multiple passes

(Fig.3-5), the same effect on the modulation curve is achieved (at its best) as if a fiber with smaller core diameter

and numerical aperture was chosen instead: a narrower, steeper curve. Despite possible ways of improving coupling

and partly overcoming alignment problems (Fig. 7), increased sensifivity is more easily realized by changing fiber type. In the absence of suitable fiber types though, a multiple pass arrangement may be considered, provided

alignment and reflection loss elimination demands are met.

6. Conclusions

We have shown theoretically and experimentally that multiple passes, due to almost unreachable alignment and

coupling demands, do not significantly increase sensitivity in practice. Precision alignment and elimination of

reflection losses are needed in order to reach a significant sensitivity enhancement compared to a standard single

fiber pair arrangement. Also, the multiple pass effect on sensitivity could be realized with the single fiber pair by

changing to a fiber type with suitable core diameter and numerical aperture. In reality, fiber types in such a pair should be chosen depending on the displacement range of interest in each sensing application.

7. References

[I] E.Peiner et.al. "A micromachined vibration sensor based on the control of power transmitted between optical fibres", Sensors and Actuators A 65, .23-29 (1998).

[2] J.M. Lopez-Higuera et.al. "New low cost fiber optic accelerometer system for stator winding monitoring of hydroelectric generating machines", Proc. of SPIE Vol. 2868 (SPIE, 1996), pp. 510-515.

[3] M. Kimura and K. Toshima, "A New Type Optical Fiber Vibration-Sensor", in Transducers '97 . 1997 International Conference on Solid-State Sensors and Actuators, Chicago (IEEE, 1997), pp. 1225-1228.

[4] K.E. Burcham et.al, "Micromachined Silicon Cantilever Beam Accelerometer Incorporating an Integrated Optical Waveguide", in Integrated Optics and Microstrucfares , Proc, of SPIE Vol. 1793 (SPIE, 1992), pp. 12-18.

[5] E. Oilier et.al, "Integrated micro-opto-mechanical vibration sensor connected to optical fibres". Electronics Letters 33, 525-526 (1997).

[6] J. Jason et.al, "Fiber-Optic Temperature Monitoring in Pulp Production", to be published in Proceedings of the 55'" IWCS/Focus Conference

(International Wire & Cable Symposium, Inc.. Eatontown, NJ, 2006)

[7] CM. Miller, S.C. Mettler and I. A. White, Opticalfiber splices and connectors (Marcel Dekker, Inc., 1986).

[8] G, He and F.W. Cuomo, "A Light Intensity Function Suitable for Multimode Fiber-Optic Sensors", Journal of Lightwave Technology 9, 545-551 (1991).

[9] A. Cobo et.al, "More Accurate Coupling Function Approach for Optical Transducers Based on Power Coupling Between Multimode Fibers", Proc. of SPIE Vol. 3746 (SPIE, 1999), pp.442-445.

9
Characteristics of spun fiber grating sensors

Yong Wang and Chang-Qing Xu

Department ofEngineering Physics, McMaster University, Hamilton, Ontario, Canada

wangyong_x(a),yahoo.com

Abstract: Novel characteristics of spun fiber grating-based sensors are presented in this paper. These gratings were written into special spun fibers with certain intrinsic linear birefringence and

short spinning periods. Due to the modal conversion and averaging effect in spun fibers, the introduced linear birefringence enables the spun fibers and spun fiber gratings to have certain resistance to external perturbations, such as transverse pressure and bending. Furthermore,

short-period fiber Bragg gratings (FBGs) always exhibit multiple side peaks near the main Bragg

peak in their reflection spectra. In term of the main Bragg peak, these spun FBG sensors have

similar strain and temperature coefficients to those of traditional FBGs, but weaker polarization

dependence. On the other hand, those side reflection peaks have strong polarization dependences,

which can be used to simultaneously monitor the change of input polarization state.

1. Introduction

Optical fiber gratings play important roles in both optical fiber communications and sensing [1]. Besides the

traditional single-mode fibers, gratings have been successfully imprinted into a variety of optical fibers, in which they

have different features, and lead to different applications. Fiber-Bragg-grating (FBG)-based sensors, whose reflected wavelengths vary with the measurands such as temperature and strain, are simple intrinsic sensing elements, and have

all the advantages attributed to fiber-optic sensors. In addition, these sensors can also be easily multiplexed in a serial fashion along a single strand of fiber. This type of sensors is highly sensitive and free from the normal fiber transmission losses. Among various fiber gratings, short-period FBGs and long-period gratings (LPGs) are dominant

in the sensing field.

It is well known that the core of a traditional optical fiber is not ideally symmetric in its cross section; meanwhile, an

external perturbation, such as lateral pressure and macro-bending, usually impacts more on one polarization than the

other. As a result, an optical fiber always exhibits a certain polarization dependence on external effects. Furthermore

in a fiber grating, nonuniform refractive index change caused by the side UV exposure in a conventional fabrication process can fonn a polarization-dependent loss (PDL). These factors make traditional FBG sensors polarizationsensitive to some extent. In many high-precision sensing applications, where polarized tunable lasers are adopted as scanning sources, a strong PDL of FBG may deteriorate the interrogation accuracy, and can even cause a failure of

measurement due to severe distortions of FBG reflection spectra. Rotating fibers in the manufacturing process, known

as spun fibers, can effectively reduce the polarization-mode dispersion and birefringence of fiber [2, 3]. As a result, spun fibers have attracted considerable attention in the fields of both optical communications and fiber sensing [2-4].

It has been found that the spun fibers with millimeter spin periods and certain intrinsic birefringence have many

unique characteristics, and the FBGs imprinted in these spun fibers exhibit distinctive properties from those made of

the conventional single-mode fibers [5, 6]. In this paper, more characteristics of spun fiber gratings are presented, and

their sensing applications are also discussed.

2. Spun fibers and spun fiber gratings

The investigated spun fibers were drawn from an asymmetric preform, which had a D-shaped cladding and a round

core in the center. After the drawing process, they have a symmetric structure - a round cladding and a round core at the center - due to the presence of surface tension in the fiber manufacturing process at a high temperature of above

1000 °C. However, non-uniform stress in the fiber cross section can introduce some intrinsic linear birefringence into the fiber. In these spun fibers, the intrinsic stress has a certain spiral structure at a spin rate of a in the longitudinal

direction as schematically shown in Fig. 1(a), where (.v, v) is the fixed laboratory coordinate system, and.v corresponds

to the slow axis, i^and ^'represent the local rotating coordinates with respect to the slow and fast fiber axes. All these

fibers have core diameters of -8.0 |am and cladding diameters of -125 |am. To form FBGs and LPGs, these fibers with

similar Germanium concentration to the standard fibers were side-exposed to uniform UV light (Excimer laser at 248 nm, -60 mJ/cm") through either a holographic phase mask with a pitch of 1 064.82 nm or long-period amplitude masks

after two-week hydrogen loading at 1500 Psi. In Fig. 1(b), the stress distribution in the fiber cross section is shown

with respect to a fixed exposure direction (.v axis).

10
(a)

Fig. 1. (a) Schematic of residual stress in a spun fiber, (b) Stress distribution in the cross section against UV exposure. A variety of FBGs and LPGs have been fabricated in our spun fibers with spin periods of 1-4 mm. Figure 3 shows

typical reflection and transmission spectra of a 15-mm long FBG imprinted in the spun fiber with a 2.25-mm spin

period, and a transmission spectrum of a 30-mm long LPG in the same kind of spun fiber. The spectra were measured

by using a broadband light source and an optical spectrum analyzer ( ANDO AQ63 1 7). The UV-induced index change

is about 3.0xlO '^. The LPG period is 300 i^m. We can see in Fig. 2(a) that the Bragg peak and four side peaks are dominant with a 0.37-nm spacing between two neighboring peaks in the reflection spectrum, and only one notch at the main Bragg wavelength is apparent in the transmission spectrum. The 3-dB notch width of the LPG is about 13 nm.

For spun FBGs, the spectral spacing between two neighboring side peaks is inversely proportional to the spin period.

Similar to FBGs in SMF-28, at the wavelength of 1 .55 |im, these spun FBGs have a strain coefficient of ~1 pm/jLte, and

a temperature coefficient of 1 0 pm/°C for all reflection peaks.

1539 1541 1543 Wavelength (nm)1545 1500 1520 1540 1560 Wavelength (nm)

1580

Fig. 2. (a) Transmission and reflection spectra of a spun FBG. (b) Transmission spectrum of a spun LPG.

3. Evolution of state of polarization

These spun FBGs have distinct spectral and polarization characteristics from those imprinted in traditional single-mode fibers, which is attributed to different evolutions of states of polarization (SOPs) in the fibers. This can be

verified theoretically. The x- and v -polarized modes that are mutually orthogonal in an unspun fiber can be coupled to each other in a spun fiber. Regardless of the transmission loss, the modal coupling can be described by [7] d

dz = -J

y5„ +0.5A/?^cos(2cc)

0.5Ay5, sin(2cc)

0.5A/?, sin(2oz)

/?, -0.5A/?, cos(2cc)

(1)

where and Ey are the amplitudes of two coupled modes polarized in the x and v directions, respectively, fio is the average propagation constant of the two normal modes, and A/?i corresponds to the intrinsic linear birefringence { Ani).

Due to the presence of strong modal coupling in the spun fibers with millimeter spin periods, these spun fibers cannot be regarded uniformly birefringent [5]. Instead, the two polarization modes can interact and exchange power in a spun fiber. This can be understood from the evolutions of three typical SOPs shown in Fig. 3, where Aiii = 0.5 x 10"'', Ps = 2.0 mm were adopted in the simulation. Despite different higher-frequency components, the three SOP evolutions

exhibit the same major spatial period, denoted by the conversion length Iconv, at which the optical power converts

alternatively between two orthogonal modes. Based on the Fourier transform, the conversion length L^om was then

11
calculated as a function of spin period and fiber birefringence, and is plotted in Fig. 4. We can see that Lconv rapidly

drops from hundreds to tens of millimeters with an increase of P, or Ani. This property enables a spun fiber to have a

certain resistance to external perturbations, such as lateral pressure or bending, as well as non-uniform refractive index

change in a spun FBG. It is often observed in experiments that when a lateral pressure is applied onto a traditional FBG

strain or temperature sensor, the FBG spectrum may suffer from a severe distortion. In the case of a split spectrum for the two polarization modes, the measurement may be failed if an interrogation system with a simple peak- or mean-wavelength-tracing method is adopted. Therefore, for those embedded FBG sensors, particularly in civil structural health monitoring, a special design of FBG packaging is usually required [8]. However, for a spun fiber grating, if the fiber section suffering from a lateral perturbation is longer than the aforementioned conversion length,

the two orthogonal modes may experience the same perturbation, and consequently weaken the influence of external

perturbation. In the experiment, we placed a few bare spun FBGs and other FBGs (SMF-28) in a V-groove, and

compared their resistances to lateral forces applied under a certain tension. It was observed that the former can resist a higher lateral pressure than the latter approximately by a factor of 3~10, depending on FBG length and spin period, before a significant distortion of FBG reflection spectrum occurred. This verifies that the spun FBG sensors can take advantage of higher resistance to external perturbations, as compared to the traditional FBG sensors.

200 300 400 500 2 (mm)

Fig. 3. Evolutions of SOP in spun fibers for different input SOPs.

(a) 0° linearly polarized input, (b) 45° linearly polarized input,

(c) right-circularly polarized input.

Fig. 4. Polarization conversion length versus linear birefringence and spin period.

4, Polarization dependent loss To characterize the PDL of spun FBGs, the output SOP was measured using a lightwave polarization analyzer (LPA,

Agilent 8509C). Since FBGs may be used in both the reflection and transmission manners, the transmitted and

reflected outputs were investigated respectively. The experimental setup is schematically shown in Fig. 5(a) and (b) for the transmission and reflection measurements, respectively. The output from a tunable laser source (TLS, HP

8164A) is linearly polarized, and the SOP test in the reflection manner is obtained through a three-port circulator. The

input SOP of an FBG under test is modified through a fiber polarization controller (PC). Note that during the test, only

the PC was adjusted to select the required output SOP, and the spun FBGs and their lead fibers were kept unchanged.

TLS

PC

FBG under test

LPA

FBG under test

(a) (b) Fig. 5. SOP test setup for (a) transmission and (b) reflection characteristics of polarization. TLS: tunable laser source, LPA: lightwave polarization analyzer, PC: polarization controller.

For the previous spun FBG shown in Fig. 2(a), the reflection spectra under five typical SOPs, i.e. three linear SOPs

(0 and ±45"), left-handed circular SOP and an elliptical SOP, were compared in Fig. 6(a). Different variafions in amplitude at those reflection peaks can be seen. In particular, the PDL is about 0. 1 dB for Pg, 1 .5 dB for P±i, 4.0-6.0 dB

for P±2. Correspondingly from the transmission spectra as shown in Fig. 6(b), the PDL is 0.5 dB for Pb, and < 0.03 dB

for other wavelengths out of the FBG stop-band. From the Fourier transform theory, we know that P±i and P±2 correspond to the fundamental spin period and its second hannonic, respectively. Since the UV-induced index change

is related to the spiral stress and polarization-dependent, the amplitudes of P±i and P±2 have different sensitivities to the

12
input SOP. The low-polarization dependence of Pb is due to an averaging effect along the grating. Such strong PDLs at the side peaks and a low PDL at the main Bragg peak in the reflection spectrum provides an outstanding property to spun FBG sensors. In particular, when a spun FBG is adopted as a temperature or strain sensor, temperature or strain information can be read out from the shift of Ab, while polarization information may be indicated from the intensity variations of the side peaks, and used to monitor the input SOP of the FBG. It is found that for this kind of sensing

application, a spin period in the range 0.8 ~ 8.0 mm and an intrinsic birefringence of > 0.3x 10"* are desired.

m

T3

1540.9 1541.3 1541.7 1542.1 Wavelength (nm)

1542.5

1541.5 1541.6 1 541.7 1 541.8 1 541.9 1542.0 Wavelength (nm)

Fig. 6. FBG spectra in (a) reflection and (b) transmission manners for different SOPs. Solid, dashed, dotted, dash-dotted and

short-dashed lines correspond to three linear SOPs (0 and ±45°), left-handed circular SOP and an elliptical SOP, respectively.

5. Conclusion

The novel properties of FBGs imprinted into spun fibers with certain intrinsic birefringence and millimeter spin

periods have been presented. Due to the modal conversion and averaging effect in spun fibers, the introduced linear birefringence enables the spun fibers and spun FBGs to have higher resistance to external perturbations, such as transverse pressure and bending, than the traditional FBG sensors. Due to the presence of the superstructure in a spun

FBG, the multiple peaks appear near the Bragg wavelength in its reflection spectrum. The main Bragg reflection peak

has similar temperature and strain coefficients to traditional FBGs, but weaker polarization dependence on input SOP,

while the side reflection peaks have strong polarization dependences. These render spun FBG sensors feasible in simultaneous monitoring of the input SOP together with ordinary temperature or strain measurement.

Acknowledgment

This work is supported by the Ontario Photonics Consortium (OPC), the Photonics Research Ontario (PRO), the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Canada Foundation for Innovation

(CFI) under New Opportunities program. We would like to thank Dr. Viatcheslav Izraelian for useful discussions.

References

1. A. Othonos, and K. Halli, Fiber Bragg Gratings: Fundamentals and Applications in Telecommunications and

Sensing, Artech House, Boston, London, 1999.

2. D. N. Payne, A. J. Barlow, and J. J. Ramskov-Hansen, "Development of low- and high- birefringence optical

fibers;' IEEE J. Quantum Electron., 18, 477-486, 1982.

3. D. A. Nolan, X. Chen, and M. J. Li, "Fibers with low polarization-mode dispersion", J. Lightwave Technol., 22,

1066-1077,2004.

4. J. R. Qian, Q. Guo, and L. Li, "Spun linear birefringence fibres and their sensing mechanism in cuirent sensors with temperature compensation", lEE Proc-Optoelectron., 141, 373-380, 1994.

5. Y. Wang, C.-Q. Xu, and V. Izraelian, "Characterization of spun fibers with millimeter spin periods". Opt. E.xp. , 13,

3841-3851,2005.

6. Y. Wang, C.-Q. Xu, and V. Izraelian, "Bragg gratings in spun fibers", IEEE Photon. Technol. Lett., 17, 1220-1222,

2005.

7. J. Sakai and T. Kimura, "Birefringence and polarization characteristics of single-mode optical fibers under elastic deformations," IEEE J. Quantum Electron. 17, 1041-1051, 1981.

8. S. C. Tjin, Y. Wang, X. Sun, P. Moyo, and J. M. W. Brownjohn, "Application of quasi-distributed FBG sensors in reinforced concrete structures". Measurement Science and Technology, 13, 583-589, 2002.

13
Metrology of integrated waveguide devices and systems

using near-field scanning optical microscopy

Guangwei Yuan and Kevin L. Lear

Electrical and Computer Engineering Department, Colorado State University, Fort Collins, CO, 80523-1317, USA

gwyuan@engr.colostate.edu, kllear@engr.colostate.edu

Abstract: Near-field scanning optical microscopy (NSOM) is a valuable tool for measuring • modal propagation parameters including loss, effective index, and coupling coefficients of a

variety of waveguide structures. Examples are provided of simple waveguide sections and bends

as well as an on-chip optical clock distribution system using an H-tree containing splitters and

leaky-mode polysilicon photodiodes.

1. INTRODUCTION

1.1 Motivation

Optical waveguide devices are essential components for photonic integrated circuits, on-chip optical

interconnects, and integrated optical biosensors. Near-field scanning optical microscopy (NSOM) is a high

resolution alternative to more traditional scatter imaging methods or cumbersome cutback studies for determining

waveguide loss. It can easily resolve modal interference signals that can be used to determine propagation constants and related parameters. Several examples drawn from recently published work [1-3] are compiled to illustrate the power of this technique for analysis of integrated waveguide devices and systems.

1.2 Bacltground on Near-field Scanning Optical Microscopy

NSOM is able to simultaneously measure surface topology and the optical intensity on the surface of optical

systems. In particular in waveguide systems, the evanescent field at the interface to the air above the waveguide

core or upper cladding is readily measured and related to the optical field intensity propagating in the waveguide.

The measurements reported here were made using a commercial Alpha-NSOM system from WiTec with

simultaneous optical and topological profiling capability. The experimental setup is shown in Fig. 1(a). The

combination atomic force microscopy (AFM) and NSOM tip is a hollow aluminum pyramid integrated on a Si cantilever that converts the evanescent field to a propagating wave by locally frustrated total internal reflection. A

photomultiplier tube is aligned to a subwavelength diameter pinhole centered in the tip to detect the wave emanating

from the tip that is proportional to the evanescent field. A reference laser beam reflecting off the cantilever is directed to a quadrant detector by a dichroic beam splitter and is used to sense tip deflection in AFM mode. As the

tip is moved toward the top surface of the waveguide, an exponentially increasing optical field, such as

gutter

cantilever

:;:r800

c

3

O

600

CO 400

CD 200

.H

_i 0

100 200 300

Tip to core distance (nm)

(b)

400

Fig. 1. (a) Schematic illustration of Witec Alpha-SNOM using optical detection of cantilever deflection, after [1]. The sample is scanned

laterally, (b) An exponentially increasing optical field as the tip moving away from the top surface of the waveguide.

14
the measured data plotted in Fig. 1(b), is observed in addition to a constant background signal due to reference beam

scattering. During in-plane optical profiling, the NSOM was operated in contact mode so that the field at the top

surface of the waveguide was measured [1]. All of the measurements discussed below were performed by coupling

a 654 nm laser diode to the waveguides under test using a visible single mode fiber with 4 |am core diameter.

2. MULTIMODE INTERFERENCE IN WAVEGUIDES

2.1 Single mode, straight waveguide

Prior to presentation of modal interference characterization in a few waveguide structures, it is useful to examine

the measured topology and optical intensity distribution associated with a segment of straight, single-mode

waveguides. The waveguide core was fabricated from a 105-nm-thick SiN film sputtered onto a ~2 ^im thick Si02

lower cladding supported on a Si substrate. No solid upper cladding was deposited above the SiN to maximize the evanescent field interaction with the region above the core for biosensing purposes. This approach also provides the NSOM ready access to the evanescent field in the air above the core which serves as the upper cladding. Dry

etching 21 nm of the SiN outside a ~2 |im-wide ridge defined the lateral extent of the core. The surface topology of

the ridge, shown in Fig. 2(a), and the lateral intensity distribution of the evanescent field immediately above the

core, illustrated in Fig. 2(b). were simultaneously acquired with the NSOM. The ridge appears wider than the nominal wavguide width due to the blunt nature of the probe tip around the optical aperture. The optical intensity

pattern reveals a smooth, fundamental mode distribufion without unduladons associated with mode beating.

Fig. 2. NSOM measurements of (a) surface topography and (b) optical intensity at the surface. Single mode observation is observed.

2.2 Multimode straight waveguides

Investigations of waveguides with larger dimensions revealed modal interference in waveguides. The surface

intensity distribution of a ~4 |im wide waveguide, otherwise identical to the waveguide described above, resulted in multiple horizontally transverse modes seen in Fig. 3(a). The waveguide only supports two TE modes, TEqo and

TEio, according to modal calculations. The NSOM data in the figure shows the intensity maxima shifting back and

forth laterally as the modes travel down the waveguide with different propagation constants. Another waveguide,

500 nm thick with a 250 nm ridge height, displayed interference between multiple orders of vertically transverse modes as seen in Fig. 3(b). The NSOM sees periodic extinction of the surface intensity when the modes are out of

phase at the top of the waveguide. The beat distance is much smaller than in Fig. 3(a), indicating a larger difference

in effective indices for the two modes.

(a) (b)

Fig. 3. Surface optical intensity shows modal interference between the schematically illustrated modes for (a) 88 nm thick x ~4 nm wide waveguide core and (b) 500 nm thick x -10 |J.m wide waveguide core.

15
2.3 Nearly single mode waveguide bend

More complicated structures, such as waveguide bends, also present the opportunity to study modal evolution

and position and to extract propagation constant information the NSOM measurements [2]. The optical distribution

in a ~4 |im wide waveguide similar to that described in Section 2. 1 but with a shorter ridge was investigated in a 90° waveguide bend with a centerline radius of 400 |im shown in Fig. 4(a). Two bound modes in the bend determined

from conformal mapping techniques matched those observed with NSOM. Simultaneous topography and optical measurements established the shift of the modes to the outside of the waveguide bend as seen in Fig. 4(b). Fitting of

the transverse intensity profile at various angles along the bend allowed the determination of the relative intensity

and phase of the two modes. The fitting procedure gave attenuation losses of the fundamental and the first order

mode as 0.46 dB/90° and 15.1 dB/90°, respectively, demonstrating the effectiveness of planar waveguide bends for mode stripping. Interesting scattering patterns were observed at the exit of the bend to a straight waveguide where

the radius of curvature and thus lateral eigenmode peak position shifts abruptly.

(a)

' (b)

Fig. 4. (a) Schematic diagram (not to scale) of a 90" waveguide bend connected to straight input and output sections, (b) NSOM images of

optical intensity at (a) 20°, (b) 40°, (c) 60°, and (d) 80° from the start of the waveguide bend. Dotted lines denote the waveguide core boundaries determined from simultaneous topographical scans. Reproduced from [2].

3. ON-CHIP OPTICAL INTERCONNECTS

A more complicated CMOS based, integrated waveguide optical system for on-chip clock distribution provided an environment for further NSOM studies [3]. The chip contains a series of waveguide straight sections, splitters, and bends in the form of an H-tree as illustrated in Fig. 5(b) to deliver an optical clock signal to 16 nodes where a leaky-mode detector converts the optical signal to photocurrent. Fig. 5(a) depicts the structure of the leaky-mode

detector in an initial single metal level chip fabricated in Avago's 0.35 )im CMOS line in Fort Collins, Colorado.

Scatter from the waveguides and detectors associated with four nodes is seen in Fig. 5(c). The SiN core waveguides

are all 350 nm thick, with the width after the input taper decreases from to 8 )im down to 0.5|J.m. In contrast to the waveguides described in Section 2, this system did include a thin upper SiOi cladding whose minimal 250 nm

thickness still provided sufficient evanescent field strength at the air interface to allow NSOM measurements.

Optical intensity recorded along the H-tree was used to extract the loss and performance of the waveguides,

splitters, and detectors. NSOM provides a higher resolution alternative to scatter imaging more commonly used for such studies. The first generation splitters had a high excess loss of 2dB or higher with forward scatter, mode

beating in the outputs, and undesired unequal branching ratios as seen in Fig. 5(d). Revised designs for a second

generation chip now being fabricated are aimed at addressing these issues. NSOM also proved valuable for investigating the coupling of light from the waveguide into the leaky mode detector and in particular determining the

effective absorption coefficient of the polysilicon metal-semiconductor-metal detectors. Fig. 5(e) is the NSOM

image of the detector region and the waveguide feeding it. Data taken from the centerline intensity is plotted in Fig.

5(f) where the detector portion has been fit to determine the effective absorption coefficient. The measured value of

0.67 dB/jim agrees very well with a calculation of 0.68 dB/|a,m and is 25 times higher than previously recorded

values for leaky mode coupling to Si detectors [3]. Mode beating between the leaky mode centered in the SiN core

16
and radiation modes generated by the mode mismatch result in a beat signal seen in the decaying optical intensity on

the surface above the detector seen in Fig. 5(f).

luigBtenviasfcofTtacte vJT^^

n=1.45 250mi

n=1^ 350 rm

pol^iBiiNeXH n=3,9 300 TMti Sa IFQX] *• 10 an ^

sjllcoi substrate

via. --fHTI

(a) (b)

• /

Lighnnput

Detection

region /

Modal mismalch

loss \

(c)

• (d)

10

, Leaftbdiiu)

(e)

Fig. 5. (a) Cross-sectional (left) and top view

(right) diagrams of the waveguide and

photodetector. (b) Diagram showing 16 node waveguide H-tree. (c) Microphotograph of the scattered light from a portion of the H-tree

(indicated by the dashed line in part (b)). NSOM

measurements of the light intensity at (d) one Tsplitter of the H-tree and (e) waveguide and

detector region. Note that the horizontal and

vertical scales are different, (f) Intensity distribution on the waveguide and detector

centerline from measurements (diamonds), fit to leaky and radiation modes (dashed), and fit to only

the leaky mode (solid). Some portions reproduced from [3].

10 00

^. 100

Oj67(lB/pmi

(f) 4

NSOM

Fitted leaiy mo>de ouly

s Fit ted Leakv and radiation mO'dss

10 15

In addition to the systems described above, NSOM has been used to characterize the performance of a novel

integrated waveguide biosensor that relies on local detection of evanescent fields [1], which is very similar to the mechanism of NSOM itself. Further, 1-D effective index models that are in good agreement with wide-angle beam

propagation method (BPM) simulations have been developed that offer insight into the function of the leaky mode

detectors as well as the new biosensor.

In summary, NSOM is a valuable tool for measuring modal propagation parameters in integrated waveguide

structures. Further development of this technique including investigation of methods for accurately relating the surface intensity to the waveguide power in heterogeneous structures is wan'anted in view of the expected continued

development of photonic and optoelectronic integrated circuits. The authors thank the National Science Foundation for supporting this work under contract number ECS0323493 and the National Institute of Health for supporting this work under contract number EB00726.

REFERENCES

1. G. W. Yuan, M.D. Stephens, D.S. Dandy, K. L. Lear, "Direct imaging of transient interference in a single-mode

waveguide using near-field scanning optical microscopy," IEEE Photonics Technology Letters, vol. 17 (no. 11), pp.2382 - 2384, Nov. 2005 and references therein.

2. G. W. Yuan, M.D. Stephens, D.S. Dandy, K. L. Lear, "Characterization of a 90 degree waveguide bend using

near-field scanning optical microscopy," Applied Physics Letters, vol. 87 (no. 19), Art. No. 191 107 Oct. 2005.

3. G. W. Yuan, et al "Characterization of CMOS compatible, waveguide coupled leaky-mode photodetectors,"

IEEE Photonics Technology Letters, in press.

17
Return Loss Measurement in the Presence of Variable Insertion

Loss Using Optical Frequency Domain Reflectometry

Stephen Kreger, Mark Froggatt, Dawn Gifford, Matthew Wolfe, and Brian Soller

Luna Technologies, 2020 Kraft Dr., Suite 2000, Blacksburg, VA 24060

Abstract: The high spatial resolution and high sensitivity inherent to optical frequency domain reflectometery enables

precise measurements of distributed insertion loss and return loss events. The ability to compensate return loss for variable insertion loss greatly adds to the accuracy and practicality of measurements. Further, the capability of measuring the Rayleigh backscatter internal to the instrument provides a stable power calibration artifact.

Introduction

Excessive system return loss (RL) negatively impacts source stability and contributes to loss in high-speed

fiber optic telecommunication systems. In order to meet system performance goals, precise measurement of return

loss in individual components as well as in installed networks is required, especially when temporary mechanical

connections are used. Most current return loss measurements are made using optical continuous wave reflectometry

(OCWR) and optical time domain reflectometry (OTDR) [1]. One of the primary error sources for RL

measurements using these devices is the error induced by variable insertion loss at the connection to the test equipment optical interface. While this error source may be minimized by splicing the device under test (DUT) to the instrument, this procedure is not practical in high volume manufacturing environments or for installed networks.

Both OTDR and optical frequency domain reflectometery (OFDR) are well suited for characterizing networks with some degree of spatial resolution. Both techniques typically have enough sensitivity to monitor the fiber Rayleigh

backscatter level which can, in turn, be used to measure distributed loss and gain [2,3]. Typically OTDRs lack

sufficient spatial resolution to be useful at the component and module level where one might be interested in, for example, locating a spurious reflection among a concatenation of several components each with multiple elements. OFDR is a tunable laser-based frequency domain technique that has several distinct advantages over time domain

and low coherence techniques when the optical systems under test are several tens of meters in length [4,5]. These

advantages include sub-millimeter resolution measurements over a few hundred meters of optical length, high

sensitivity, and high dynamic range.

The capability of measuring localized insertion loss using OFDR presents a unique opportunity to provide consistent measurements of device RL even in the presence of variable connector loss, even for short lead lengths. Further,

the lack of a dead zone and high sensitivity allows our OFDR-based instrument to calibrate return power levels to the Rayleigh backscatter level of fiber within the instrument. This onboard calibration capability provides a highly

stable and reproducible reference for RL measurements. This paper outlines the methodology used to establish a value for the scatter in optical fiber, and how this Rayleigh scatter level is used to maintain consistent reflection measurements.

Measurement Apparatus

The optical network used to implement OFDR is shown in Fig. 1 . Light from a tunable laser source is split into measurement and reference optical paths. In the measurement path, the light is further split by a 50/50 coupler.

A third coupler is used to recombine the light fi-om the measurement path with the light from the reference path. After recombination, the light is split by a polarization beam splitter. Interference is detected at two PIN

photodiodes that are connected via amplification circuitry to a data acquisition card. This polarization diverse

detection scheme ensures that an interference signal will be present on at least one of the detectors irrespective of the

polarization state of the field reflected from the device under test (DUT). Not shown in Fig. 1 is an auxiliary

interferometer used to monitor phase error during laser tuning. This technique is called triggered acquisition and is cormnonly used in OFDR systems to remove laser tuning errors from the data [4]. Also not shown is a portion of

the network wherein a Hydrogen Cyanide gas-cell is used to monitor the instantaneous wavelength of the scanning

laser.

The network shown in Fig. 1 is used to measure reflected power as a function of wavelength. The back-reflected power as a function of length is obtained via the Fourier transform of the raw data (see reference [6] for details). The maximum measurable length for this instrument is determined by the sampling resolution in the optical

18
uouplar ToDUT

u

<

0

Q. coupler polarization controller

coupler Beam

Splitter

Eo

o

p

time

Fig. 1. Optical network used to perform polarization diverse measurements of Rayleigh backscatter.

frequency domain which is in turn determined by the physical delay difference of the auxiliary interferometer used

for data triggering. In this paper, the instrument used had a maximum scan range of 30 m with -20 |im spatial resolution.

To calibrate the measured back-reflection to an absolute RL, the response of a set of polished flat end face connectors was recorded. The expected value of the RL can be calculated using the Fresnel equation:

According to the fiber manufacturer the value for the effective index of refraction n for these connectors is 1 .4682 at 1550 nm, resuhing in an expected RL of 14.44 dB. Although the above equation is only an approximation and the RL of such connectors is dependent on the surface polish quality and cleanliness, we have observed that the consistency for such connectors manufactured in-house is better than that of most commercially available metal-film

fiber reflectors. After the reference set of reflectors was used to scale the return power, the Rayleigh backscatter

level for a segment of fiber within the instrument close to the front panel connector was recorded. Upon any

subsequent recalibration of the instrument, any drift in detector responsivity or amplifier gain can be corrected for by comparing the measured backscatter level of the fiber segment to the recorded value. Although we have not yet

completed an extensive survey, we have found that the repeatability of the Rayleigh backscatter level for the fiber used in our instruments. Coming SMF-28e, is excellent, with a standard deviation less than 0.05 dB for a 1 m

integration width.

Making Insertion Loss-Independent Return Loss Measurements

Any measurement of RL involves making a connection to the device under test. If there is loss in the connection, this loss will add directly to the apparent RL. As a practical matter, the insertion loss of the connector

to the calibration artifact, component or network is not controllable and typically varies by several tenths of a dB

every time a new connection is made. Since OFDR provides a means of measuring this connector loss, reproducible RL measurements are possible even in the presence of variable connector loss.

To demonstrate the repeatability of a RL measurement in the presence of loss, we measured the RL of a polished ST

connector at the end of a 1.9 m lead with a FC-APC connection to the measurement instrument. To induce a varying

amount of loss, the fiber was mandrel wrapped at roughly 0.3 m after the connection to the instrument. Fig. 2 shows

two example scans of this DUT and one scan with no device connected. Because the ST connector has a high RL,

two measurements are required to determine the RL. First, the DUT is measured with no alteration. Second, a measurement is taken with the fiber immediately before the ST connector pinched off to reduce the RL from the connector. This reduces the effect of the tails from the reflection peak on the measured data and, thus, enables a measurement of the Rayleigh scatter immediately before the reflection.

Four measurements of the reflected power along the measured traces are required to determine the RL with best accuracy. These measurements are obtained by measuring the integrated normalized power over a defined length centered at a specified location. The first power measurement, Pq, is recorded over a span of fiber inside the instrument and will be the sum of the Rayleigh back scatter Pra and the background noise level Pb. The second

power measurement Pj is recorded over a span of fiber just prior to the reflection event and is given by the sum of

Pb and the Rayleigh backscatter level divided by the double pass attenuation factor A. This factor. A, is the

(1)

19
-40 Front Panel Connector

Induced Loss

Mandrel Wrap

_Flat End Face

Connector

Fig. 2

Position (m) Sample traces for determining the return loss of a reflection event in the presence of high insertion loss. The

four dark lines beneath the traces indicate measurement integration regions for the four measurements.

attenuation caused by the double-pass insertion loss (from the front panel connection and the mandrel-wrap)

between Poand Pj. Both Poand P| are measured using the pinched-off trace. If the attenuation is large, it is also necessary to record a value of the background noise level, Pb, by placing measuring the power in the background

trace near the locations where P, and Pt are recorded. The expressions describing Poand P] are.

and

^0 - ^RA'^ (2)

(3)

The power returned from the reflection event Pt is recorded using the unaltered trace over a span of data centered on

the reflection event. In addition to the power from the ST reflection Pst, this measurement will also contain

components due to Pra and Pb. Pi is, then, expressed as: P2=(Pst+Pra/^)^A + Pb

Using these equations, the IL of the loss events and the RL of the ST connector can be calculated as

/L = 10log(/\)/2 = 5log

kP-\~Pb J

and

= -1

0

\og{PsT ) = -1 0 logf^^-^(P2 - Pe ) - (Po - Pe )/

2

(4)

(5)

(6)

To demonstrate that the return loss value is repeatable even with high insertion loss, the RL was calculated for the ST connector described above with varying numbers of wraps around a mandrel. The recorded reflection traces without the pinch off applied for each attenuation level are shown in Fig. 3. The results of the measurements are shown in Table 1. The resulting standard deviation for the RL calculation over a range of insertion losses from 0.79

to 14.65 dB was only 0.05 dB.

To demonstrate the repeatability between instruments, RL measurements were taken on a set of seven carefully

polished flat end face connectors using four different instruments. The insertion loss at the front panel connection

for these measurements was generally less than 0.5 dB. The results are presented in Table 2. The standard deviation

is a measure of the combined variations of the RL measurement and the instrument calibration procedure and is equal to or less than 0.15 dB.

20
0.0 0.5 1.0 1.5 2.0 2.5

Position (m) Fig. 3. Traces of a reflection event with varying amounts of insertion loss.

Table 1 . Return Loss measurements for the same component for a wide range of Insertion Loss.

Relative Power Level Measurements Results

Mandrel Wraps

Po

(dB)

D

(dB)

P2

(dB) Pb(dB)

Insertion Loss

(dB)

Return Loss

(dB) 0 -76.45 -78.01 -16.09 -96.31 0.79 14.51

0.3 -76.43 -81.20 -19.42 -96.31 2.43 14.57

0.5 -76.44 -84.88 -23.19 -96.31 4.36 14.47

1 -76.41 -88.82 -27.76 -96.21 6.62 14.52

2 -76.48 -93.74 -35.29 -96.31 10.36 14.57

3 -76.48 -95.85 -43.91 -96.31 14.65 14.61

Table 2. Return loss values for a set of flat end face connectors for 4 different instruments.

Return Loss (dB)

Instrument Con 1 Con 2 Con 3 Con 4 Con 5 Con 6 Con 7 OBR 7022 14.21 14.19 14.19 14.37 14.10 14.82 14.36 OBR 7071 14.35 14.39 14.48 14.63 14.45 14.97 14.69 OBR 7078 14.37 14.31 14,30 14.56 14.23 15.01 14.59 OBR 7083 14.55 14.46 14.44 14.70 14.37 15.10 14.66

Std. Dev. 0.14 0.12 0.13 0.14 0.15 0.12 0.15

Summary

We have demonstrated that an OFDR-based instrument is capable of producing repeatable RL

measurements even for wildly different values of the insertion loss in close proximity to the reflection event. This

capability greatly adds to the utility of the measurement, as the uncertainty due to the insertion loss at the connection

to the instrument is eliminated. Additionally, we have shown that our instrument calibration procedure, which ties the RL values for a reference set of flat end face fiber reflectors to the Rayleigh backscatter level for a fiber segment

inside the instrument, produces consistent results between instruments. Establishing the consistency of the precise

level of the fiber Rayleigh scatter for fiber manufactured under very tight tolerances may eventually allow its use as a widely available, inexpensive, and stable return loss calibration artifact. These are preliminary resuhs and are expected to improve as we continue to refine the calibration process for absolute return-loss measurement.

References

[1] lEC 61300-3-6, Second Edition, "Basic test and measurement procedures - Part 3-6: Examinations and Measurements - Return Loss," 2002.

[2] J. P. von der Weid, R. Passy, and N. Gisin, "Coherent reflectometry of Optical Fiber Amplifiers," IEEE Photon. Tech. Lett. 9, 1253-1255

(1997).

[3] M. Froggatt, B. SoUer, D. Gifford, and M. Wolfe, "Correlation and keying of Rayleigh scatter for loss and temperature sensmg in parallel optical networks," OFC Technical Digest, Los Angeles, March, 2004, paper PDF 17.

[4] U. Glombitza and E. Brinkmeyer, "Optical frequency domain reflectometry for characterization of single-mode integrated optical waveguides," J. Lightwave Tech. 11, 1377-1384 ( 1993).

[5] J. P. von der Weid, R. Passy, G. Mussi, and N. Gisin, "On the characterization of optical fiber network components with optical frequency domain reflectometry," J. Lightwave Tech. 15, 1 131-1 141 (1997).

[6] B. J. Soller, M. Wolfe, M. E. Froggatt, "Polarization resolved measurement of Rayleigh backscatter in fiber-optic components," OFC

Technical Digest, Los Angeles, March, 2005, paper NWD3.

21
High-Power Nonlinearity of Optical Fiber Power Meters for Pump Lasers at 980 and 1480 nm

I. Vayshenker and S. Yang

Optoelectronics Division, National Institute ofStandards and Technology, 325 Broadway,

Boulder, Colorado 80305

R. Swafford

OZ Optics Ltd, 219 WestbrookRd, Carp, Ontario, KOA ILOOZ Optics, Canada

Abstract

We describe the nonlinearity of optical fiber power meters at wavelengths of 980 and 1480 nm

up to 2 W. We also describe two measurement systems that are based on the triplet superposition

method.

1. Introduction

Erbium-Doped Fiber Amplifiers (EDFAs) have revolutionized the field of optical

telecommunications. An EDFA uses high-power pump lasers at 980 and 1480 nm. These pump

lasers require an accurate measurement of optical power. Current optical fiber power meters

(OFPMs) are capable of measuring powers exceeding several watts in an optical fiber. As the

demand for higher powers increases, the measurement of OFPM nonlinearity becomes very

important. Optical power meter nonlinearity is defined as the relative difference between the

response at an arbitrary power and the response at the reference power [1]. While the calibration

of an OFPM gives the true input power from the OFPM output reading at the calibration point,

the measurement of nonlinearity together with calibration provides the input vs. output

relationship at any power for the whole dynamic range of the OFPM. The nonlinearity of some

OFPMs could have spectral dependence [2]. This paper also discusses this issue in Section 2.

1.1. Correction factor for nonlinearity and range discontinuity

The true input power P is obtained from an OFPM reading Vhy

f;CF(P)' (1)

where Fc=Vc/Pc is the calibration factor and CF(P) is a correction factor due to nonlinearity and

range discontinuity of the OFPM.

2. Measurement systems and results

To measure the nonlinearity of OFPMs we have built two measurement systems at 980 and

1480 nm. We have based the operation of our systems on the triplet superposition method [3,4],

which relies on the principle that, for a linear power meter, the sum of meter outputs

corresponding to inputs from two single beams should equal the output when both beams are combined and incident on the meter at the same time. A basic measurement system for both

980 and 1480 nm is depicted in Fig. 1 and described below. Each laser is a laser diode in a

butterfly package with an integrated thermoelectric cooler, monitor photodiode, and thermistor.

The laser diode is used in a constant-power regime. The output fiber of the first laser diode is aligned with a high-power collimator that produces a beam diameter of approximately 6 mm

22
(for 980 nm) and 3 mm (for 1480 nm). This beam then propagates through a high-power optical

shutter. The shutter is a rotary solenoid with a beam blocker mounted onto the shaft. When the

shutter is activated, the beam blocker is removed from the optical path, enabling the beam to pass

through.

Fig. 1 . Measurement system.

The beam then passes through a linear, variable neutral density (ND) filter. The filter density is variable from 0 to approximately 30 dB (for 980 nm) and 40 dB (for 1480 nm). The filter is mounted on a linear translation stage and is driven by a combination of servo motor and encoder.

The beam then enters the beam splitter and propagates to the high-power output coupler where

the beam is coupled into the output fiber. This fiber is routed to the outside of the case via a

3 mm stainless-steel armored cable (for safety) and is terminated with an FC/APC connector.

The second laser transmits, via an identical system, to the beam splitter. The measurement

system at 980 nm described in [3] does not use polarization maintaining (PM) fibers; the

measurement system at 1480 nm uses lasers with PM fiber pigtails.

The measurements are performed by taking sets of three power readings from the test OFPM: (1)

shutter 1 is open and shutter 2 is closed, (2) shutter 1 is closed and shutter 2 is open, and (3) both

shutters are open. This sequence is then repeated at different powers. The high-power

nonlinearity systems described here and in [3] use two lasers whose center wavelengths are separated by several tenths of a nanometer. Two lasers produce enough power to calibrate highpower OFPMs and compensate for the insertion loss of the system. In contrast, the low-power

nonlinearity system in [4,5] uses only one laser whose radiation is divided into two paths.

23
Fig. 2 and 3 depict correction factors obtained for two commercially available high-power

OFPMs, labeled #1 and #2 at 980 and 1480 nm. Note: the reference power (where the correction

factor is 1) is chosen to be 100 mW (at 980 nm) and 1 mW (at 1480 nm). Each data group (line)

represents a separate power range of the meter. At 980 nm, the total nonlinearity of 0FPM#2 is approximately 3.5 %, while the total nonlinearity of the 0FPM#1 is about 0.5 % for powers from

several milliwatts to several watts. At 1480 nm, the total nonlinearity of the 0FPM#2 is approximately 1.4 % while the total nonlinearity of the 0FPM#1 is less than 0.25 % for powers

from 0.2 mW to 0.6 W. Figs. 2 and 3 show that the 0FPM#1 is more linear (approximately

7 times) than the 0FPM#2 at both 980 and 1480 nm.

0 I ,000

/

7

/

Meter 1 \ \

^Ref

lE+l lE+2 OUTPUT POWER (niW)

Fig. 2. Correction factor vs. output power at 980 nm.

/

/

/

Meter 2 ^ ^ Metei 1

^Ref

O 1008 <

u.z

0 i.006

OUTPUT POWER (mW)

Fig. 3. Correction factor vs. output power at 1480 nm.

24
3. Conclusions

We have designed and built two nonlinearity systems that allow obtaining correction factors for

OFPMs at high powers at 980 and 1480 nm. We showed that the nonlinearity for a given OFPM

has spectral dependence at 980 and 1480 nm. Therefore, a user should not assume that if an OFPM were linear at one wavelength, the OFPM would be linear at a different wavelength. The

nonlinearity systems are important to characterize OFPMs at high powers. Similar systems could

be built at other telecommunication wavelengths and higher powers.

4. References

1. International standard, lEC 61315, Calibration of fibre optic power meters, Ed. 2.0,

hitemational Electrotechnical Commission, Geneva, Switzerland, to be released.

2. Boivin, L.P, "Automated absolute and relative spectral linearity measurements on photovoltaic

detectors," Metrologia 1993, 30, 355-360.

3. I. Vayshenker, R. Swafford, and S. Yang, "High-power nonlinearity of optical fiber power

meters," Natl. Inst. Stand. Technol. Spec. Publ. 1024, 145-148, 2004.

4. I. Vayshenker, S. Yang, X. Li, T. R. Scott, and C. L. Cromer, "Optical fiber power meter

nonlinearity calibrations at NIST," NIST Special Publication 250-56, 2000.

5. I. Vayshenker, S. Yang, X. Li, and T.R. Scott, "Automated measurement of nonlinearity of

optical fiber power meters," Proc. SPIE, Vol. 2550, pp. 12-19, San Diego, CA, July 1 1-12, 1995.

25
Novel Approach for Non-Intrusive "Clip-on" Fiber Monitoring

Gang HE, Daniel GARIEPY, Hongxin CHEN, Jean THEBERGE,

and Gregory W. SCHINN

EXFO Electro-Optical Engineering, Inc; 400 av Godin, Quebec QC GIM 2K2

CANADA

Abstract: We describe a previously unappreciated "universal" behavior of macrobend-induced

loss in singlemode optical fiber. We propose that this could be exploited in a "clip-on "-type

instrument that can detect qualitatively and quantitatively the presence ofpower propagating in an optical fiber or cable, while at the same time essentially guaranteeing that the induced

insertion loss on the propagating signal will be a small, known value, independent of signal

wavelength orfiber type.

It is often necessary to detect a signal in an optical fiber by extracting a portion of the guided

light. For example, one often wishes to be able to distinguish between live (signal carrying) and

dark (no signal) fibers, to extract a signal for sampling (e.g. to detect a modulation tone for identification purposes), to perfonn an approximate measurement of the guided optical power, to perform accurate differential measurement of the guided optical power before and after a possible

localized loss such as a splice, and to detennine the direction of traffic. It is desirable to perform

these operations without disrupting, overly attenuating or distorting the signal, especially in telecommunications systems, as the margins with which many such systems operate sometimes

do not permit more than about 1 dB of additional attenuation without the risk of inducing errors on the signal.

It is well known that light can be extracted from an optical fiber by inducing a "macrobend" (i.e. a bend whose radius of curvature is much larger than the fiber diameter) so that a fraction of the

propagating light escapes through the cladding [1-4]. The fraction of this extracted light with

respect to the total light in the fiber core is known as the ''insertion loss" (IL) or "bending loss". When such a macrobend is applied to a fiber, a non-negligible fraction of light may leak out and

be detected even if the fiber is cabled, since most buffer and jacket materials are partially

transparent at the near-infrared wavelengths used in telecommunications applications. Typical

cables found in central offices and outside plant applications frequently use 3-mm- or 1.6-mmdiameter jackets, filler material and a nominal 900-)j.m buffer about the optical fiber, or simply a 900-|am tight buffer. In addition, one frequently also needs to measure light in 250-|um-diameter

aerylate coated fibers.

For a given fiber, the degree of macrobending required to detect a certain amount of leaked light

is strongly wavelength dependent: Less bending is required for longer wavelengths and more

bending is required for shorter wavelengths. As well, if one wishes to detennine the absolute

power propagating in the fiber, it is necessary to "know" the induced insertion loss. Moreover,

for a given wavelength and macrobend angle, the required detection sensitivity depends upon the

fiber type (e.g. standard step-index, depressed clad, etc.) and the types and colors of the coating

and jackets protecting the fiber. Clearly, if the propagating signal power is low, and/or if the

absorption of the coatings and/or jackets is high, it is desirable to bend the fiber to a relatively

tight radius so as to extract as much signal as possible for detection, while remaining well below

the loss margin of the system. It is desirable, therefore, to control the bending radius carefully.

A number of commercial clip-on devices exist today that are based upon a fixed-bending

stnicture, i.e. where the bend radius is not changed during the measurement. A disadvantage of

this approach is that the insertion loss caused by this fixed macrobending is strongly dependent

26
upon wavelength, making it impractical to optimize both the IL and sensitivity when the

wavelength of the propagating light could be, say, 1310 or 1550nm. Generally, commercial

devices choose a bending radius such that the loss at 1310nm is "just" detectable, and the loss at 1550nm is "not too high". However, this induced insertion loss can vary significantly, even at the same wavelength and jacket type, if the fiber type is different. We report here a new approach that can be applied to a fiber clip-on device that induces a small,

knovm insertion loss, largely independent of signal wavelength, fiber type, or jacket and buffer

material. This approach is based upon what appears to be a hitherto little appreciated property of

all singlemode optical fibers, namely, that curves representing the rate of change of IL with

respect to the macrobend radius R [i.e. d(IL)/dR)], when plotted as a function of IL, are

essentially the same for any singlemode fiber type and wavelength.

The measurement setup used to confirm this behavior is shown conceptually in Fig. 1 . An optical

fiber (or cable) is held in appropriate clamps and the fiber (or cable) is bent in a controlled

fashion from an angle of 180 degrees (i.e. no bend, or R = oo) to about 120 degrees, or until the IL became excessive. (Note that in practice, it is generally more convenient to measure a bending

angle than the bending radius, although the two are of course directly related.) Care is taken to ensure that the "interaction length", that is the portion of the fiber that is bent, is held

approximately constant as the angle is changed. The optical power launched into the fiber,

and the optical power propagating through the macrobend and subsequently exiting the fiber

(after a short distance, so that the inherent fiber attenuation is negligible). Pout, are measured with

a standard power meter, as a function of macrobend angle. The insertion loss is defined as IL

(dB)=101og (Pou,/ P,n).

Figure 2 shows curves representing IL as a function of angle, for a wide variety of fiber and cable

types, and at different wavelengths. One observes that, for a given IL (e.g. -0.7 dB), the slopes of

all the curves are approximately constant. This can be seen more explicitly in Figure 3, where the

derivatives of these same curves with respect to angle are presented as a function of IL Although

there is some spread in the curves according to fiber type, etc., they exhibit a remarkably similar

dependence.

From Fig. 3, it can be seen that if one were to monitor only the slope d(IL)/d9 as the macrobend

angle 9 were varied, one could ensure that the macrobend-induced loss could be set to fall into, for instance, the range -0.7 ±0.2 dB, independent of the fiber type, wavelength, etc. (Note that in Fig. 3, the independent variable is plotted along the ordinate, whereas the dependent variable is along the abscissa.)

Of course, the underlying motivation for using a clip-on device is to avoid having to measure Pin and Pout , since these are not normally accessible parameters in practical measurement situations. Only the light escaping from the fiber core, Pe, or a fraction thereof, is available for the

measurement.

There is a well-defined relationship between the IL and the power escaping from the fiber core, Pe , which obtains from Pe = Pin - Pout- Expressing Pe and Pin in dBm units, and IL in dB, one has

Pe = EF + Pin

where

EF= 10*log(l- 10*'^''°^)

.

This can be recast as:

APe - EF'AIL,

where EF' is the derivative of EF with respect to IL.

27
If one could measure Pe as a function of macrobend angle, one could then plot dPc/d9 versus IL

and obtain another "universal" curve analogous to that in Fig. 3. Although in practice the

photodetector Di only detects a fraction of the total escaping light and hence one cannot directly measure Pe, the fact that we are dealing with derivatives of logarithmic quantities renders the

results insensitive to collection efficiency to first order. Hence, if the detector in a clip-on device

monitors a portion of the escaping light as the bend angle is adjusted (while maintaining a

constant interaction length), one can still determine the derivative dPe/d0 as a function of bend

angle. (Conveniently, a small dithering of the bend angle could be applied to obtain this derivative directly.)

Figure 4 shows the dPe/d6 vs IL curves for a series of selected fibers with different fiber types

and jacket types on both 13 lOnm and 1 550nm. For example, if one wishes to maintain a constant

insertion loss of -0.7 ± 0.2 dB, one sets the macrobend angle such that dPg/dG = -0.9 dB/deg.

In conclusion, we have presented a principle that could serve as the basis for a new generation of

"non-intrusive" clip-on instruments for signal monitoring or power measurement. These

instruments would be able to essentially guarantee that an excessive bending loss would never be

applied to the fiber, independent of its fiber type, jacket, or the propagating wavelength.

References: -

1. B.D. Campbell et al. US Patent 4,586,783: "Signal coupler for buffered optical fibers",

issued May 6, 1986.

2. S.M. James, "Non-intrusive optical fibre identification using a high-efficiency

macrobending 'clip-on' optical component", Elec. Lett. 24, 1221-22 (1988).

3. Larry R. Cox, European Patent EP 0639762B1: "Fiber identifier", issued Feb. 22, 1995.

4. Vincent C. So et al, US Patent 4,671,653: "Test instrument for an optical fiber", issued

Jun. 9, 1987. ;

Fig. 1. Schematic illustrating the

"clip-on" principle.

28
Insertion loss vs Angle

Fig. 2. Measurement data of

insertion loss (in dB) versus macrobend angle for a variety

of fiber types, jacket and buffer

types, and wavelengths of 13 1

1

and 1550nm. (DCSMF:

Depressed cladding singlemode

fiber; DSF: Dispersion shifted

fiber.) This insertion loss was

determined by directly

measuring Pout and Pin as a function of angle.

Fig. 3. Derivative d(IL)/de of

data in Fig. 2, but plotted

against IL.

Fig. 4. The derivative dPe/dO vs

IL, determined by measuring the

escaping power (for each of the

data points in Fig. 2) with

detector Di and plotted against

the directly measured IL.

S 15 8 •— Haid-J3ckel_3mm (1S50)

Ha(d-jackct_3mtii (1311

)

* So(t-Ja(*el„3nim (1550) Son-Jac*el_3mm (1311) SMF-28 (A)_3mm (1550) «— SMF-28 (A)_3mm (1311) SMF-28_0 9mm (1550) SMF-28_0 9mm (1311) OCSMF_0.9mm (1550) (3CSMF_D 9mm (1311) i SMF-28_0 25mm (1550) SMF-28_D 25mm (1311) *- DCSMF_0 25mm (1550) » DCSMF_0 25mm (1311) i +-OSF_0.25mm(1550) I -^DSF_0.25mm(1311)

150

Angto (degree)

dtlL)/d(Angle) vs IL

1 -t 2-«

3 . 4 . .

- H3td-Jacliel_3mm (1550) -Haid-j3<Ael_3mm(1311)

Sofl-Jackel_3mm (1550) S0lt-J3ckel_3mm(1311) 5 -•- SMF.28 (A)_3mm (1550) 6 —•-SMF-2e(A)_3mm(13t1)

7 SMF-28_0 9mm (1550) 8 SMF-28_09mm(1311) 9 DCSMF_09mm(1550)

OCSMF_0,9mm(1311)

SMF-2e_0,25mm(1550) SMF-28_025mm(1311) DCSMF_0,25mm (1550) tX;SMF_0 25mm (1311) OSF_0.2Smm(1550)

OSF_0.25mm(1311)

1

d(Pe)/d(Angle) vs IL

-2.S 23

-2 1

-1.9

-1.7 15

1"

° -0.7

O.S

0,1

i

«

Y

1

. /'

11

2

i

8

7

j

~ 16

1 Hard-Jackel_3mm (15501 2 Hard-jacl(el_3mm (1311) 3 Sod-Jacket 3mm (1550) 4 Soft-j3Ckol_3mm(1311) 5 -- SMF-28 (A)_3mm (1550) 6 SMF-28 (A)_3mm (1311) 7 SMF-28_0 9mm (1550) e SMF-28.0 9mm (1311) 9 OCSMF_0.9mm(1550) 10 DCSMF_0 9mm (1311) 11 SMF-23_0 25mm (1550) 12 SMF-28_0.25mm (1311) 13 DCSMF_0 25mm (1550) 14 OCSMF_0 25mm (1311) IS OSF_0 25mm (1550) 16 OSF_0 25mm (1311)

0.1 -0.1 -0.3 O.S -0.7 -0.9 -1.1 IS -1.5 1.7 -1.9 -2.1

IL (dB)

-2,3 -2.5 -2.7 -2.9 -3.1 -3.3 -3.5

29
Measurement technique for Stimulated Brillouin Scattering threshold

Per OlofHedekvist and Anne Andersson

SP, Swedish National Testing and Research Institute

' " Box 857, SE 50115 Boras, Sweden

• Per.olof.hedekvist@sp.se

Abstract

We propose a new technique to measure the SBS critical power. Preliminary results

show anticipated effects, and accuracy and repeatability is under investigation. The

technique is easy to automate, for simplified multiple fiber testing.

Introduction

The evolving technique for developing optical fibers with tailored properties arise a request

for straight-forward characterization methods. Several fibers are manufactured with decreased

nonlinearity for transmission of data at high power levels, e.g. the Large Effective Area Fiber

(LEAF) [1]. In addition new highly nonlinear fibers, to be used in all-optical processing, are developed with suppressed SBS critical power [2]. There is a huge benefit if this power level

could be easily characterized for a batch of fibers, to enable quality control as well as optimized choice of fibers for an installation. This paper addresses this issue, and present

results fi^om an ongoing project on measurement technique for accurate estimation of the SBS

critical power.

Technique description

The fundamental requirement for the technique is to be reproducible and easily automated.

The traditional technique includes cw light launched into the fiber, and the backscattered light

measured vs the input power level. This technique was used to present the appearance of SBS

in 1972, and is still widely used in scientific work [3]. Even though this technique may be

adequate with some addition of measurement analysis, it will be influenced e.g. by the

thermal noise of the detectors. Furthermore, there are no parameters to adjust for increased

inteipolation accuracy.

If the input light is modulated with a square wave at small modulation depth, the thermal

background can be filtered off The modulation frequency must be faster than the

recombination time of the Erbium doped amplifier, if this is used, but slower than the

Brillouin frequency (50 MHz). The modulation depth should be as small as possible, however

large enough to still achieve a distinct readout of the critical power.

The fiber under test, FUT, should be connected to the modulated light source, and both

launched and reflected light is detected in detectors with a bandwidth exceeding the

modulation frequency. For a traceable accuracy, the launched light must be detected in a calibrated device. The important parameters are the high-level of the launched light, and the

modulation energy in the back-scattered light. At lower levels, Rayleigh scattering will

dominate, but when the high-level reaches the SBS-threshold, the modulation energy will

increase. From the second derivative of the measured trace of modulation energy vs launched

high-level, the SBS-threshold appears as a peak.

30
Experiment and results

EDFA FUT

B

Figure 1: Experimental setup

Figure 1 shows the experimental setup. A narrowband laser is amplified in an Erbium Doped

Fiber Amplifier (EDFA) with variable output power, and a calibrated power monitor (A). The

solution with a circulator for reflection to (B) is used to avoid influence of reflection in port

A, as would be the case with a 2x2 power splitter. Nevertheless, port C at the output of the

fiber under t est, FUT. must be prepared to avoid reflections, especially if the fiber length is less than the effective length of the fiber (~ 20 km for standard single mode fiber).

a

The first preliminary results of the technique are shown in figure 2. where the FUT was a 25

km standard SMF. The modulation depth is 10% and the oscilloscope trace shows the

backscattered light when the high-level is above the SBS-threshold and the low-level is below. The signal will then be filtered in a narrow bandpass filter, and analyzed with respect

to the high-level of the launched light. The launched power level into the fiber, calculated

from the readout of the photodetector at position A, was 12.5 dBm, indicating an SBS critcal

power between 16.9 and 18.7 mW. Through further measurements, including smaller

modulation depth and power scanning, in addition to analysis with second derivative of the

backscattered power, the accuracy can be improved substantially.

1,2 n 1

1

-

c5 0,6

6 0,4 - c

0,2 -

0 J ——i

0 0,0005 0,001 0,0015 0,002

Time (ms)

Figure 2: Measured return response of the fiber

The detected trace resembles the response detected by Ippen et al [4], with the difference that

the modulation depth is used as a variable, while the reference used 100 % modulation

31
Conclusion

A new technique for automated fiber testing is proposed, quantifying the SBS critical power.

The technique differs from previously presented work through the modulation of the input

sugnal, and is easy to automate. It will therefore be suitable for production testing.

The paper describes work in progress, and further results will be presented regarding analysis

algorithms and measurement accuracy.

References:

1. "Dispersion-shifted large-effective-area fiber for amplified high-capacity longdistance systems", Liu, Y.-M.; Antos, A. J., Conference on Optical Fiber

Communication. OFC 97, pp. 69-70, 16-21 Feb. 1997.

2. "New dispersion decreasing fiber with high SBS threshold for nonlinear signal

processing", Li, M.-J.; Li, S.; Nolan, D.A.; Achmetshin, U.G.; Bubnov, M.M.;

Guryanov, A.N.; Dianov, E.M.; Khopin, V.F.; Sysoliatin, A.A. Optical Fiber

Communication Conference, OFC 2005. Volume 5, 6-11 March 2005 Page(s):3

3. "Observation of Stimulated Brillouin Scattering in low-loss silica fibre at 1.3 jam",

D.Cotter, Electron. Lett. Volume 18, pp. 495-496, 1982.

4. "Stimulated Brillouin Scattering in optical fibers", E.P. Ippen, and R.H. Stolen, Appl.

Phys. Lett. Volume 21. pp. 539-541, 1972

32
Butt-joint splice with refractive index matching material for

high power light in optical fiber communications

Ikutaro Ogushi, Hisashi Izumita, Kuniaki Tanaka, Fumihiko Ito, and Masahito Arii NTT Access Network Service Systems Laboratories, NTT Corporation. Hanabatake 1-7-1, Tsukuba, Ibaraki 305-0805, Japan

Tel:+81-29-868-6350. Fax:+81-29-868-6360, E-mail:ogiishi@ansl.ntt.co.jp

Abstract

We demonstrated the temporal loss increase of a butt-joint splice with refractive index matching material when

using a high power light for optical fiber communication systems. The temporal degradation at the splice as a function of the high launched power arises from the refractive index change caused by the thermo-optic effect. We also proposed the design of a butt-joint splice for high power light by diffusing the heat. Our method

reduced the loss increase by 2.3 dB for c/= 100 \im and P,,, = 1 W.

1. Introduction

Recently, the optical power in optical communication systems has been increasing rapidly through the use of

wavelength division multiplexmg (WDM) and distributed Raman amplification (DRA) technologies. When

DRA is applied to WDM systems, a high power light of as much as several watts is launched into optical fibers and devices. This makes it important to know how these optical fibers and devices behave under such high

power light input conditions.

In the optical communication systems using high power light, we have to undertake safety engineering to avoid the hazards that the optical power may induce. We must take account of various problems caused by

high power light. For example, there is the fiber fuse phenomenon [1], the high power performance of singlemode connectors [2], the infiuence of a high power light launched into optical fibers in an MT connector [3], the reliability of optical fiber [4, 5] and the optical fiber cord destruction process [6]. The light from optical transmitters is launched into the optical fiber distribution systems in a central office, and is hardly attenuated. So, we must clarify the performance and design of the optical devices of the optical distribution system in a

central office for high power light input. The devices are fiber selectors, which are installed in fiber line testing

systems, or MT connectors and mechanical splices, which are employed in connection splices. These devices

have a butt-joint splice connection mechanism, which often has a function for reducing Fresnel reflection and

suppressing optical connection loss by using refractive index matching material [7, 8], for example, silicone oil, silicone resin, or glycerin. In a previous work, we described the optical loss and temperature increase at a butt- joint splice with refractive index matching material when there was a large gap between the two fiber endfaces

and a high power light input, and we reported that air bubbles (or oxygen) in the refractive index matching

material may cause catastrophic damage [9]. In this paper, we launch a high power light into a butt-joint splice with refractive index matching material

and demonstrate its characteristics of the splice. We found temporal degradation at the splice as a function of

the launched high power arose from the refractive index change caused by the thermo-optic. We also proposed

the design of the butt-joint splice by diffusing the heat.

2. Characteristics of butt-joint splice with refractive index matching material inducing the high

power light

Figure 1 shows our experimental setup for exposing a butt-joint splice connection to high power light. We used

a laser operated at a wavelength of 1.55 |am and in the CW mode and an EDFA as the high power optical source. We used a V-groove to connect the fibers. The gaps {d) between the two fiber endfaces on the Vgroove were 10, 30, 50 and 100 ^m. We employed fusion splices for all the splice connections other than the

butt-joint splice connection to avoid a fiber fuse and to reduce the connection loss and reflection. Figure 2 shows the relationship between input power (P,„) and exposure time. We launched a constant high power light for 10 minutes and changed the optical power at a rate of ±500 mW/min. The constant optical

powers were 250, 500, 1000, 1500, and 2000 m.W. In order to investigate the change in the characteristics

33
against input power at the splice, we measured the optical loss, optical loss fluctuations and maximum

temperature changes with and without refractive index matching material on high power input. The optical loss fluctuations were measured by using a digital oscilloscope with an 0/E converter via 99: 1 couplers as the input

power was changed. The temperature change and distribution of the tested butt-joint splice connection were

measured with infrared thennography. We used silicone oil, which is commonly used in fiber selectors and

MT connectors, as the refractive index matching material. Figure 3 shows the measured optical loss in the splice with refractive index matching material. In Fig. 3, at 2000 mW and d = \00 ^im, the optical loss reached its maximum value of 1 1.6 dB. The difference between

the maximum and minimum optical loss values at d = 10 |im was small (less than 0.3 dB), but at d = 100 i^m,

it was strongly dependent on the input power P,„. We found that the optical losses with the refractive index

matching material depended on d and the input power P,„ and their characteristics were reversible. The

measured output power at the splice with refractive index matching material was saturated against the increased optical power when the optical power was increased at the larger gaps (d). These measured optical losses were larger than those calculated using the conventional formula for optical coimection loss [10]. Figure 4 shows temperature distribution at the splice with d = 50 |im (a) at 250 mW and (b) 2000 mW.

Compared with Fig. 4 (a), there was the clear temperature rise at the splice in Fig. 4 (b). The maximum value

of temperature rise was 15.3 °C, and the measured optical loss was 7.0 dB, respectively. The temperature of

the refractive index matching material rose as the optical loss increased. As with the optical loss characteristics, the temperature characteristics are reversible. It should be noted that the characteristics of the optical loss increase and temperature rise were reversible and were related each other.

3. Reduction of the optical loss increase due to the thermo-optic effect The optical loss increase was caused by the temperature increase induced by the high power light. Heat

generated by the energy absorbed by the refractive index matching material resulted in a radial thermal

gradient, and this led to the distribution of the refractive index in the medium. Using the thermal coefficient 5n/^, a radial refractive index can be derived from the temperature distribution according to

n(r) = „,+^[T(r)-T(Q)\

oi

where no is the refractive index of silicone oil at r = 0, and T(r) is the radial temperature when the light is launched into refractive index matching material. The thermal coefficient 5n/5T of silicone oil is negative [8] and the temperature distribution plotted in a temperature curve becomes T{r) > T(0). The refractive index

distribution of silicone oil has a radial refractive index gradient. This phenomenon is the thermo-optic effect

[11]. The refractive index matching material with the radial refractive index gradient behaves like an irradiated

lens. As a result, the light propagating in the refractive index matching material degraded, and optical power

input into the output fiber was limited as gap (d) increased. Here, we propose ways of suppressing the thermo-optic effect by cooling refractive index matching

material. One measure involves keeping the temperature constant at the refractive index matching material

while the splice is exposed to high power light. We conducted the same experiments at d = 10, 30, 50 and 100

|.xm while cooling the refractive index matching material exposed to 1 W light in order to suppress the temperature increase. The values for the suppression of the increase in the optical loss were 0.1 dB at 10 i^m,

0.1 dB at 30 jam, 1.8 dB at 50 |im and 2.3 dB at 100 |j.m respectively. These values differ with the calculated values according to the conventional foiTnula for optical connection loss [10], and the optical loss increase can be reduced theoretically. However, we need consider appropriate method and conduct quantitative measurements. There are several ways of using the technique to maintain a refractive index without a radial gradient. One example is to diffuse the heat with convection.

4. Conclusion

We demonstrated the characteristics of a butt-joint splice that employed refractive index matching material. We found that the optical loss and temperature increased temporally and reversibly at the butt-joint splice with

34
refractive index matching material as the input optical power increased. This degradation was mainly caused

by the thermo-optic effect of the radial refractive index gradient dependent temperature. We proposed a simple

approach for suppressing any optical loss increase, which involved keeping the temperature constant. This

work will contribute to the design of butt-joint splices with refractive index matching material for optical

communication systems that operate with high power light.

References

[I] R. Kashyap and K. J. Blow, "Observation of catastrophic self-propelled self-focusing in optical fibres," Electron.

Lett., 24, 47-49,(1988).

[2] M. D. Rosa, J. Carrberry, V. Bhagavatula, K.Wagner and T. Uenoya, "High-power performance of single-mode

fiber-optic connectors," J. Lightwave Technol., 20, 879-885, (2002).

[3] K. Hogari, K. Kurokawa and I. Sankawa, "Influence of high-optical power light launched into optical fibers in MT connector," J. Lightwave Technol., 21, 3344-3348, (2003).

[4] K. Kurokawa, C. Fukai, J. Zhou, K. Tajima, K. Nakajima, N. Yoshizawa and 1. Sankawa, "Long-tenr. reliability

of pure silica core single-mode fiber when exposed to high-power laser light," Photon. Technol. Lett., 16, 1110-

1112, (2004).

[5] R. M. Percival, E. S. R. Sikora and R. Wyatt, "Catastrophic damage and accelerated ageing in bent fibres caused

by high optical powers," Electron. Lett., 36, 41-1-416, (2000).

[6] I. Ogushi, H. Izumita, F. Ito and M. Arii, "Catastrophic damage and destructive process induced in optical fiber cord by 33 dBm pump light at 1480 nm in WDM systems," OFC/NFOEC, 0ME6 (2005).

[7] C. M. Melliar-Smith, P. D. Lazay, G. A. Pasteur, E. A. Chandross and D. L. Wood, "Index matching fluids for long wavelength (1.2-1.6 jam ) fiber optic applications," Electron. Lett., 16, 403-404, (1980).

[8] M. Kihara, S. Nagasawa and T. Tanifuji, "Return loss characteristics of optical fiber connectors," J. Lightwave

Technol., 14, 1986-1991, (1996).

[9] I. Ogushi, H. Izumita, K. Tanaka, F. Ito and M. Arii, "Observation of ignition induced by a high power light

input at a butt-joint splice with refractive index matching material, " OFC/NFOEC, 0FK6 (2006).

[10] D. Marcuse, "Loss analysis of single-mode fiber splices," Bell Sys. Tech. J, 56, 718, (1977).

[I I] M. D. Rosa, and S. Logunov, "Photothermal behavior of an optical path adhesive used for photonics

applications at 1550 nm," Applied Optics., 40, 661 1-6617, (2001).

Laser

a: 1550 nm

High power EDFA

Infrared themiography

Power

Max power 2000 mW

Butt-joint splice connection

Refractive index Digital

oscilloscope

Gap between two fiber endfaces c/= 10- 100 |im

Fig. 1 . Experimental setup

35
Exposure time (sec)

Fig. 2. Input power against exposure time in experiments

A d= 100 jun d= 50 ijm d = 30 nm m 10 |.im ^ 10

0

I Exposure time

Fig. 3. Measured optical loss in high power light exposure experiments

(a) At 250 mW (b) At 2000 mW

Fig. 4. Temperature distribution induced by high power light launched into the butt-joint splice with t/ = 50 urn

36
High speed PRBS for detection of defects in fibers and fiber connectors

Rangaswamy. S., Lao, D., and van Doom, E

Intelligent Automation Inc. 15400 Calhoun Dr, Suite 400

Rockville, MD 20855

Abstract

We present here a high performance OTDR based on Pseudo Random Binary Sequences (PRBS)

for solving resolution and dead zone problem associated with fiber and fiber connector

inspections. The presented system has the capability of spatial resolution and dynamic range that are compatible with the application and will allow efficient measurement of defects in fiber

connectors, independent of the measurement of the airgap reflection. The dynamic range of the

system used is 41dB and using components of higher frequency (10 GHz) it could be modified

for resolution of 2 cm with atleast 76 dB of dynamic range in a 40ft cable. This capability will

allow for independent measurement of airgaps inside connectors, and defect at the backend of

the same connectors in aircrafts, and increase the readiness of its fleet at lower cost of ownership.

Introduction

We present here a high performance OTDR based on Pseudo Random Binary Sequence (PRBS)

for solving inspection of fibers in dead zones and fiber connectors. In conventional optical time

domain reflectometer (OTDR), the impulse response is equivalent to the response of the fiber to a sudden pulse of light. But rather than using a single pulse followed by no injected signal (to

listen for echoes), we use a specific binary sequence to detect the reflections from return losses on the fiber. This use of this Pseudo Random Binary Sequence (PRBS) greatly increases the duty

cycle of the injected signal, and therefore the ability to average, and achieve great sensitivity.

This effect increases with increasing fiber (and sequence) length, but is about a factor of 500 for

30ft fibers inspected with 10Gbit sequences or lOOps pulses!

Rather than injecting a single pulse, and detecting the reflections, we inject a continuous

sequence of pulses, and measure the cross correlation between the injected and reflected signals.

The impulse response is equivalent to the response of the fiber to a sudden pulse of light, as is the case in conventional OTDR. To do this we first consider the result of a cross-coiTclation

operation between the input and output of a linear system. We then examine this in the case of

applying pure Gaussian noise (white noise) to the system as the input signal.

Let (^^ be the cross-correlation between the input Y(t) and the output Z(t) of the system, so

that (^) ~ 1 /

I

" ~ ^)^^- Let n{t) be a random noise function added by the system,

so that from the development in the preceding section we can represent the output of the system

with system noise as: Z{t)-

J" l{p)Y{t - ju)dju + n{t). That is, we represent the output at t as the

convolution of the input with the impulse response function plus the value of the noise at time t

.

Substituting this expression for Z(r) into the cross-correlation equation, we have:

^^{T) = \IT^l^[l{M)Y{t-ju)dM + n{t) Y{t-T)dt

37
from which, = ^

l(ju) 1 / 7;„ Y(t - ju)Y{t - T)dtdjU + (r), that can be written as

Assuming that the system noise is uncorrelated with the

input, over many trials ^,,j,(r)^0, and we have finally ^;,) (r)= (r-/^). Now

consider the case where the input signal Y{t) is random white noise. The autocorrelation

function of random white noise is by definition an impulse over sufficient trials. This may be

seen by considering that its correlation with itself is by definition zero except at r = 0 where it is unity. Therefore we may write ^jy(r) = AS{t) whevQ A is some constant. Then, substituting

this in the previous development^^^yir) = ^l{ju) AS{t - ju)djU, which may be written

^'^yir) = A^ i{ju)S{t - ju)djU, which is exactly the convolution of l{ju) by Sir) overr. But we

know already that S{t) = 0 if r ^ 0 and 1 if r = 0 . Therefore, ^^i^ ~ AI{t). Hence the cross- correlation of system input with system output will approximate the system's impulse response

function ever more closely over time if the system's input is Gaussian white noise.

Conventional OTDR can also be regarded as a special case of PRBS with only one pulse. In the

absence of noise, the performance of PRBS and OTDR would be same. In the presence of

random noise and the peak transmit power remaining same PRBS will reduce the noise level.

Following the derivation we consider the noise contribution to the correlated output

^'

y (r) = l/r,„

I' n{ty{t - T)dt, The noise level is represented by its variance

£|^,„(r)f =l/7;;£|('"n(0r(r-r)./r

^\IT:;l'-'E\n[u)\-Y'[t,-T)du -

Since E[n{tMh)yS{u-u), (r)f = l/r,; f£|n(0|' (r, - r)./r,

I

1

2

We can assume that ^ ^(ri)

' is independent of time; and since binary pulses are transmitted,

F" (/, -r) is constant. Hence £^|^„i (r)|" = E\ii{t^ )|" Y' IT^^^ . Assuming the pulse width is T, and the

number of pulses is A^, then we have T,„=NT. We can see the variance of the noise decrease with

the increase ofN.

Comparison of measurements using PRBS and single pulse

The test setup is shown in Figure 1 consists of Anritsu MP 1632C digital data analyzer to generate the PRBS and the signal was sent to the laser that was connected to one end of the 2x1

splitter. The reflected signal was received by the photodiode and amplified and fed to the

Tektronix TDS7404B/4M Digital Phosphor oscilloscope. There is no air gap at the connectors.

The detector we are using can detect received power as high as 20dBm.

38
PRBS/pulse

Generator (MP1632C)

Trigger

Computer

Oscilloscope

(TDS7404B)

Resistor

Laser diode

Avalanche

Photodetector

Fiber optic cable with connector

Splitter

Figure 1: Diagram of tlie system using PRBS/pulse generator

We compare the performance of using PRBS and

a single pulse by measuring the reflection from

the end of fiber without reflector. The results are shown in Figure 2. Both curves are obtained by

averaging over 64 measurements. We can see the

noise floor for results using PRBS is much lower

than that using single pulse. This is due to the

higher duty cycle of the PRBS.

Dynamic Range and Sensitivity of PRBS-based

OTDR

To understand what the noise figures mean for

sensitivity and dynamic range we performed

further measurements on a 62.5/125 jacketed 10 ft fiber cable with D38999 mil connector from

Glenair, Inc. Since the cable is long, 511 bit

PRBS is used. If we use 127 bit PRBS, we can

achieve the same SNR by averaging over 4

continuous sequences. With a laser output of 6.29

dBm and the attenuation from the splitter, the

possible maximal received power at the detector

achieved when there is a 100% reflector at the end

0.84-(-40.55) = 41.39 dB.

z

0

1 — 2'-l fKBS [

j

Delay (ns)

a

1 — Pulse

(

0 20 40 60 80 100 120 Delay (ns)

Figure 2: The noise floor using PRBS

(top) is much lower than using single

pulse (bottom)

is 6.29-3.34-3.79=-0.84 dBm (which can be

of the splitter). Therefore the maximal SNR is

Combine results using multiple PRBS's

Since we have found that SNR improves with lower frequency, but that spatial resolution

decreases simultaneously, one possible scheme is using multiple PRBS's for detection and

localization. In this scheme, we would use a low frequency PRBS (with good SNR) for

detection. Once we detect and roughly localize a reflection, we would "zoom in" with a higher

frequency PRBS to pinpoint the location of the defect. This scheme gives low false alarm rate for

detection with accurate localization of defects. We show results for 1, 2, and 4ns pulse widths

Figure 3 in a fiber connected having airgaps at the connectors.

39
0 50 100 150 200 250 300 350 400 450 'm 168 170 172 174 176 178 180 Delay (ns) Delay (ns)

(a) Overall view (b) Close up view

Figure 3: Detection using multiple PRBS

Tests in single mode optical fiber

We tested a single mode optical fiber which has a core diameter of 9|im (Figure 4). Since the

receptacle laser diode modules we have right now is not manufactured for single mode cable, we can only

transmit at most -4.6 dBm power into the single

mode fiber.

Low cost High speed PRBS-based OTDR

In previous sections, we have shown that systems

using PRBS can achieve excellent performance. We

should point out that the results are obtained using

expensive oscilloscope with high sampling rate.

To achieve high spatial resolution, it is essential to use a narrow pulse and for even better resolution, we need even faster sampling. To detect the narrow pulse, a high speed digitizer is required for both conventional OTDR systems and

systems using PRBS. We are working on a low cost lOGHz PRBS generator that achieves high

spatial resolution of 2 cm. -

-

Acknowledgement: We thank the Naval Air Warfare Center (NAVAIR). Lakehurst, NJ, for

supporting the work under SBIR phase I 2005 - Fiber optic cables

Length: 0 m. Delay: 33.46 ns

Length 1 m. Delay: 44,2 ns

i

Lencjin 2~ Df-s/ r-1.4 ns

Lenglh 3 m.

!i j| Delay 65.02 ns,

Delay (ns) Figure 4: Detection of end of fiber on

single mode fiber using multiple PRBS

40
Experimental Investigation on Special Shaped Fibers

Coupling with InP-based Planar Lightwave Circuit Chips

Xu Liu , Jinbiao Xiao, Xiaohan Sun**

Lab ofPhotonics and Optical Communications, Department oj Electronic Engineering,

Southeast University, Nanjing 210096. P.R.China

Abstract: Special shaped fibers, including tapered and lensed fiber (TLF) and wedge-shaped fiber (WSF), play an important role in planar lightwave circuit (PLC) technology to attain high efficient coupling with the elliptical

optical field of InP based multiple quantum well (MQW) waveguide chips. Output optical field of MQW-PLC, TLF,

WSF were numerically analyzed using beam propagation method (BPM) and staircase concatenafion method (SCM),

respectively. The optimum coupling conditions between WSF and PLC were achieved with wedge angle of 45°,

cylindrical endface lens curvature radius of 2.5|am, aspect ratio of PLC ellipdcal optical spot of 5, and misalignment

tolerance of 5. 5jam. The focused optical spot diameter of TLF and WSF were measured and calculated based on the transformation relationship between object and image Gaussian beams, which differed less than 3.2% from those of

the simulation. 1.55|am wavelength signal was injected into PLC-based coupler chip from SMF, TLF, WSF as input

fiber separately with the same SMF as output fiber. The coupling efficiency of WSF-PLC-SMF linkage was

24.827dB and 16.22dB higher than that of SMF-PLC-SMF and TLF-PLC-SMF schemes, and therefore an WSF-PLC-SMF experimental prototype was established for pigtailed PLC packaging technique.

Keywords: planar lightwave circuit (PLC) ; InP; multiple quantum well (MQW); coupling and packaging; special

shaped fiber

\. Introduction

Planar lightwave circuit (PLC) promises to be not only a solution to infonnation access and optical signal processing,

but it also directly addresses the issues of bandwidth, pincount, reliability and complexity and hence provides

foundation for photonic network. As a microphotonics platfoiTn PLC technology is being driven by the needs of cost,

coupling loss and footprint reduction with increasing functionality [1]. Smart nanopositioning and precision

alignment is vitally important in high-efficient connectivity between PLC chips and fiber arrays [2, 3]. Unfortunately refractive-index mismatching between fused silica and III-V compound, as well as mode field mismatching induced by the output elliptical spot size from PLC, are most serious handicaps. On one hand the

tapered spot size converter (SSC) [4] can be added at the ends of the PLC chip to enlarge the output spot laterally so

as to partially match the core size of SMF, and in other hand the special shaped fibers, such as TLF [5] and WSF [6], should be used to get the similar output spot as one from the chip through the endface lens.

Applicafion of TLF and WSF in coupling with the elliptical optical field of InP-based MQW-PLC chips was

demonstrated and compared to decide the better candidate in this paper. Firstly, we optimized the structin es of both

TLF and WSF, and discussed their coupling conditions with the chip, respectively. Secondly we observed the compressed spot size of TLF and WSF. Finally the experiment results on the bar state produced by PLC-based

coupler chip were obtained using different special shaped fibers and SMF, from which an optimum coupling scheme was devised.

2. Design Considerations

XuLiu, presenting author, email: liuare@seu.edu.cn. ** Xiaohan Sun, corresponding author, email: xhsim@seu.edu.cn

41
2.1 Output opticalfield ofMQW-PLC, TLF, WSF

Fig.l showed the optical fields of MQW-PLC with conventional rib width of 2.5|im (Fig. 1(a)) and SSC-MQW-PLC

with rib width of 8[im (Fig. lb). For the former, the semimajor axis was coox = 1 . lum, while the semiminor axis was

cooy = 0.8um. For the latter, coqx = 3.7um, cooy = O.Vum. Obviously the aspect ratio (AR) of elliptical field ranged from

1.38 to 5.29. Compared with the MQW rib waveguide, the output spot from SSC-MQW-PLC chip matched much

better with the core size of SMF along rib width direction.

lixivttt^KfUtpxiiatlbw: - nb wjvtpuds oolpul spol size ote nm

(a) • (b)

Fig.l (a) MQW and (b) SSC-MQW PLC output optical field by SCM simulation

(a) (b)

Fig. 2(a) Upper half of TLF (b) lightwave propagation along TLF by BPM simulation

The structure of TLF can ben seen in Fig. 2a where Zj ,a , represent cladding layer radius, core

layer radius, taper length, half core taper angle, thin lens curvature radius [7]. The BPM simulation showed from Fig.

2b that the compressed optical spot diameter for the TLF with certain parameters was 4.5|.un.

(a) (b)

Fig. 3(a) Illustration for WSF (b) WSF output optical field by SCM simulation

Fig. 3 describes the structure and output spot of WSF simulated by SCM [8], where R, 6, a stand for cylindrical

endface lens curvature, wedged angle, core radius of SMF section. The numeric result showed coox = 3.2um, cooy =

1.25um. Therefore, the elliptical field of WSF was closer to the output field distribution from the chip than that of

TLF.

2.2 Optimization of TLF and WSF

42
In general TLF is made from standard SMF (G.652) and taper length varies from 300|im to 400|.im. , « , R, were

optimized to 300|im, 0.733°, 13.485|im respectively, and consequently the optical spot diameter was solved to be

4.45 |im using a set of empirical equations [9] , which differed 1.11% from that of the BPM simulation. The coupling circumstance changes with the aspect ratio of PLC elliptical field, the structure of WSF and the

axial displacement (D) relationship. The optimum coupling conditions with 6=45°, R=2.5|im, coox=3.5|am, AR=5,

D=5.5^m were ascertained by calculating the coupling loss between WSF and SSC-MQW-PLC. The vertical displacement tolerance was smaller than that of horizontal. These results supplied a basement for the fabrication of

WSF and PLC and the precision adjustment of nanopositioner.

3. Experiment

3.1 Obsen'ation onfocusing performance

An experimental setup in Fig. 4(a) was established to investigate focusing performance of TLF and WSF. Gaussian

beam emitting from He-Ne laser was amplified by 40x object lens and injected into special shaped fibers, digital PC

camera received the output optical field. Fig. 4(b) and Fig.4(c) showed computer bitmaps of the TLF and WSF

samples when their tips were 20mm apart from PC camera lens. Knowing the percentage the red image bitmap

accounted for the blue background and the effective image size of CMOS sensor chip, the actual image size focused on the sensor chip was achieved, subsequently the output optical spot size in object space was calculated out. The

measurement result for TLF output spot diameter was 4.31|am. Compared with the theoretical prediction (4.45|am),

the error was 3.15%. The values for WSF was 6.2|imx2.5|am (2coox^2cooy), differing from the simulation

(6.4|imx2.5|am) within 3.125%).

(a) (b) (c)

Fig.4. (a) Output optical field measurement setup , camera acquisition bitmap for (b) TLF and (c) WSF output optical field

3.2 Coupling Experiment

Bar state of InP SSC-MQW-PLC direcfional coupler chip (Fig. 5(b)) was tested using different input fibers with the same SMF as output fiber. A WSF (wedged angle 45°) stimulated the upper left port of PLC chip 5.5jim apart (Fig.

5(a)), while a SMF received optical field close to the upper right port (Fig. 5(c)). Agilent 86142B Optical Spectrum

Analyzer embedded EELED light sources verified the whole transmission power at 1550nm peak to be -45.615dBm

(Fig. 6(c)). The output power of SMF-PLC-SMF and TLF-PLC-SMF was -70.442dBm (Fig. 6(a)) and -61.835dBm

(Fig. 6(b)). The coupling efficiency of WSF-PLC-SMF linkage was 24.827dB and 16.22dB higher than the above

two schemes.

4. Conclusions

We thoroughly investigated the implementation and application of TLF and WSF in PLC chip coupling technology.

The optimum coupling conditions were achieved for WSF wedge angle, cylindrical endface lens curvature radius,

semimajor axis of PLC elliptical field, aspect ratio of PLC elliptical field, misalignment tolerance between WSF and

PLC to be 45°, 2.5)am, 3.5jim, 5, 5.5^m individually. In comparison with the theorefical results, the measurement

error of focused spot size of TLF and WSF fell within 3.15% and 3.125%. The coupling efficiency of

WSF-PLC-SMF linkage was 24.827dB and 16.22dB higher than that of SMF-PLC-SMF and TLF-PLC-SMF

43
schemes, and therefore an experimental prototype of WSF-PLC-SMF couphng Hnkage was validated for pigtailed

PLC packaging technique.

(a) (b) (c)

Fig5. WSF-PLC-SMF experiment photograph

(a) WSF as input fiber (b) directional coupler chip (c) SMF as output fiber

(a) (b) (c)

Fig6. PLC chip bar port test result for different inputs (a) SMF (b) TLF (c)WSF

5. Acknowledgements

The research was partially supported by the National "863" Project of China (under grant 2002AA31230) and the Jiangsu Hi-Tech Project of China (under grant 7706008020).

6. Reference

[1]. tC. Kato and Y. Tohmori, "PLC hybrid integration technology and its application to photonic components," IEEE J. Select. Topics

Quantum Electron. 6, 4-13 (2000).

[2]. David W. Vemooy, Joel S. Paslaski, Heniy A. Blauvelt et al., " Alignment-insensitive coupling for PLC-based surface mount photonics,"

IEEE Photonics Technology Letters 16 , 269-271 (2004).

[3]. Rong Zhang and Frank G. Shi, "Manufacturing of Laser diode modules: integration and automation of Laser diode-fiber alignment and RIN characterization," IEEE Transactions on Advanced Packaging 26, 128-132 (2003).

[4]. T. Mizuno, T. Kitoh, M. Itoh et al., "Optical spotsize converter using narrow laterally tapered waveguide for planar lightwave circuits,"

Journal of Lightwave Teclinology,22, 833-839 (2004).

[5]. T.Alder, A.Stohr, R.Heinzemann et al., "High-efficiency fiber-to-chip coupling using low loss tapered single-mode fiber," Photonic

Technology Letter, 12, 1016-1018(2000).

[6]. Szu-Ming Yeh, Sun-Yuan Jfuang, Wood-Hi Cheng, "A new scheme of conical-wedge-shaped fiber endface for coupling between

high-power Laser diodes and single-mode fibers," Journal of Lightwave Technology, 23, 1781-1786 (2005).

[7]. J.D.Love, W.M.Henry, W.J.Stewart et a!., "Tapered single-mode fibers and devices Parti: Adiabaticity criteria," lEE Proc. 138, 343-354

(1991). . " .

[8]. Katsunari Okamoto, Fundamentals ofoptica! waveguides (Academic Press, 2000), 323-339.

[9]. R.Keil, E.Klement, K.Mathyssek et al., "Experimental investigarion of the beam spot size radius in single-mode fiber tapers," Electron.

Lett. 20, 621-622 (1984).

44
Fiber Frequency Combs: Development and Applications

N. R. Newbury, W. C. Swann, I. Coddington, and J. J. McFerran

National Institute ofStandards and Technology. 325 Broadway, Boulder, CO 80305

email: nnewbury@boulder.nist.gov, phone: 303-497-4227, fax: 303-497-3387

Abstract

The output of a femtosecond fiber laser provides a comb of lines in frequency space that can be

phase-locked to either a microwave or optical reference to form a stable frequency comb. We

discuss the basic configuration of fiber laser-based frequency combs, the underlying physics

behind the stabilization of the comb, and some applications of fiber-laser frequency combs to remote sensing and to measurements of the stability and phase noise of optical sources at 1550 nm.

Introduction

The development of frequency combs has led to a revolution in optical frequency

metrology.'"^ Figure 1 shows the basic picture of a frequency comb. It is a set of well defined,

evenly-spaced, lines in optical frequency space. The frequency of each line is defined directly in terms of the frequency of a reference oscillator that can be either a microwave oscillator or an

optical oscillator. If a microwave reference oscillator is used, then the comb provides the basic,

but previously extremely challenging, function of linking optical frequencies directly to microwave frequencies. If an optical reference oscillator is used, the comb provides the basic

function of linking that optical frequency directly to other optical frequencies or to microwave

frequencies. The importance of frequency combs to atomic clocks and precision spectroscopy is an important factor in the award of the Nobel prize to J. Hall and T. Hansch in 2005.

The initial development of frequency combs was based on use of femtosecond, solid-state

Ti:Sapphire lasers,'""^ and these systems continue to exhibit the highest levels of performance."^'

^ However, in recent years, frequency combs using femtosecond, fiber lasers have been

developed.^

' ^ As they mature, their performance is approaching that of Ti:Sapphire based

systems.'^ The interest in fiber laser frequency combs stems from several aspects. First, they can be relatively inexpensive. As a result they are much more attractive for widespread use and there

are several commercial products available. Second, they can be relatively compact and robust,

since "all-fiber" systems are possible. Third, they cover a wavelength range of 1 to 2 \im, which

covers the transparent window of fiber optics. As a result, fiber-laser frequency combs are compatible with the wide range of telecommunication components and Er-based amplifiers.

Moreover, the output can be transmitted long distances over optical fiber (although not without

impairment of the optical phase stability.)

Fig 1 : Frequency comb output from a femtosecond fiber laser. The laser output spans -80 nm

directly or 1000 nm after broadening in nonlinear fiber. The comb teeth are separated by the

repetition rate off,,p ~ 50 - 375 MHz, depending on the fiber laser. The position of the nth mode

is given by /, = nfep+f, where n is an integer that is -4x10* for the comb line near 1550 nm.

45
The main challenge in stabihzing the frequency comb from a femtosecond fiber laser (or

any femtosecond laser) is the detection of the offset frequency, fo. (In contrast, the repetition

frequency, /ep, is easily detected by shining the output onto a photodetector.) The basic method

for detecting the offset frequency is through "self-reference" detection with an f-to-2f

interferometer," although other methods are possible.'"* This self-referenced detection requires

broadening the laser output to form a supercontinuum that spans an octave, i.e., factor of two, in

frequency. The long-wavelength end is then doubled and beat against the short-wavelength end,

yielding a beatnote at/o The difficulty lies in generating from the laser an octave of bandwidth

that can provide a fo beat signal with sufficient signal-to-noise ratio. The first attempt at stabilizing the frequency comb output of a fiber laser essentially circumvented this problem by

using a second Ti: Sapphire frequency comb.'" However, since the first observations of the offset

beat, all current fiber laser frequency combs are based on this self-referencing technique. ^ '^ Below, we first describe the fiber-laser frequency comb and then briefly discuss some of

the applications, including basic frequency metrology in support of optical clocks, metrology of

narrow-linewidth lasers, and remote sensing.

Configuration

Fiber laser frequency combs are based on passively mode-locked fiber lasers. A number

of different laser designs have been successfully used including figure-8 laser^, soliton'° or

16 8 stretched-pulse fiber ring laser . and Fabry-Perot laser using a saturable absorber . The exact

configuration of the laser does not appear to be important, although it may affect the feedback

bandwidth. All these lasers put out -100 fs pulses of laser light at -0.1 nJ of energy and

repetition rates of -100 MHz. The spectral width of the pulses can range from 20 nm to 80 nm

depending on the laser configuration. As discussed above, a full octave of bandwidth is needed

to detect the offset frequency; therefore, this output must be externally spectrally broadened.

Because the pulses are so short, the peak powers are significant and one would expect significant

spectral broadening in optical fiber due to nonlinear effects. However, for current nonlinear

fibers the pulse peak powers are not sufficient to generate a full octave of bandwidth. Therefore,

the laser output is first amplified in a dispersion-managed erbium fiber amplifier. The output of

the amplifier is then injected into a special highly nonlinear optical fiber that generates an

octave-spanning supercontinuum. The development of this optical fiber was a critical step

17 18 toward achieving fiber laser frequency combs. '

-stabilized Comb

Fig 2: Basic schematic of a fiber laser frequency comb. The femtosecond fiber laser output is amplified and spectrally broadened in highly nonlinear fiber (HNLF). The offset frequency is phase-locked to a microwave reference. The remaining degree of freedom of the comb can be

stabilized through (1) phase-locking the repetition rate to a microwave reference or (2) phaselocking one tooth of the comb to an optical reference (shown in gray). Solid lines represent fiber optic paths and dashed lines represent electronic signals.

46
Once the supercontinuum is generated, the offset frequency /r; is detected by doubhng the

~2 |im portion of the comb and heterodyning it against the ~1 jum end of the comb to generate an

rf beat signal at fo. This signal is phase-locked to an rf synthesizer by feeding back to the pump

laser power. In initial efforts, the offset beats signal, fo, was extremely broad - on the order of

100 kHz or more. While the phase-locking ensured that the average value offo remained fixed

with respect to the reference, the broad linewidth indicated that on short time scales there was

significant jitter on fo. It turns out that this jitter arose from intrinsic amplitude noise on the

diode lasers that pump the femtosecond fiber laser. In principle, the feedback to stabilize the

offset frequency ought to remove this amplitude noise by applying the appropriate correction to the current supply driving the pump diodes. However, the amplitude noise was too large to be

totally removed by the simple proportional-integral feedback. Recently we found that by

operating the pump diode at high currents, its amplitude noise is considerably reduced.

Moreover, by using phase-lead compensation (i.e., derivative feedback) in the feedback loop the

feedback bandwidth can be extended considerably further.'^ The end result is a tightly phaselocked offset frequency with ~ 1 radian phase excursions from the reference oscillator

(corresponding to an instrument-limited linewidth).

The stabilization of the offset frequency holds the comb fixed in terms of translational

motion (see Fig. 1); however the spacing between the comb teeth is still free to change. There

are two alternatives to fully stabilizing the comb. First, one can phase-lock the repetition rate to a microwave reference by feeding back to the cavity length (see Fig. 2) through piezoelectric

fiber stretchers. In that case, the comb frequencies are given by f„ij) = nfepit) + fo{t), where we

explicitly allow the frequencies to have some slow variation with time, t. This allows one to define the optical frequencies in terms of the microwave reference and is therefore useful for

frequency metrology. However, the instantaneous noise on the comb lines can then be quite

large. The corresponding frequency noise power spectral density (PSD) as a function of Fourier

frequency, v (conjugate to t), is Sn{v) = n'S,ep{v) + So{v). Here, the PSD, S„{v), is defined as the

Fourier transform of the slow time variations in the squared fit). The frequency noise PSD Srep

and So in terms of the variations in the squared/,,^(0 andfo{t), respectively, are defined similarly.

The frequency noise on the offset frequency and repetition frequency, Syep and So, can include

both the noise on the microwave reference and noise resulting from imperfect phase-locking of

the comb to the reference. Since n ~ (200 THz/ 50 MHz) ~ 4x10^, any frequency noise on the

^ 1 ^ repetition frequency is "multiplied up" by ~ 10 , which is a very large number. As a result,

the optical comb lines will be significantly broadened.

Alternatively, the second method of phase-locking the comb involves stabilizing one

tooth of the comb to an optical reference, i.e., a narrow stabilized laser.^" ~^ This approach is shown as the grey line in Fig. 2. The optical reference laser might be stabilized to an optical

cavity if only short-term stability is important. It might also be the output of an optical atomic

clock if absolute stability is important. If the comb tooth n = riref, is locked to the reference laser

with frequency fREp with an offset of fb, the optical comb frequencies are given by f{t) =

{n/nref)ifREF{t)-fb{t)) + {\-n/n,-ef)fo{t). The phase noise (again ignoring correlations between fref

and fo) is now S„{v)= {nMreffSREfiy) + {\-{n/nref)')So{y), where Sref{v) represents any frequency

noise on the optical reference or on the phase-locking to the optical reference. The coefficients

multiplying Sref{ v) and So{ v) are now on the order of unity. For a sub-hertz stabilized optical

reference, SREpiv) is low, provided that a sufficiently high bandwidth piezo-electric fiber

stretcher is used to phase-lock the fiber laser to the optical reference. In that case, very narrow

lines can be observed from a fiber laser frequency comb. With IMRA America, we have

47
J.

demonstrated instrument-limited residual linewidths of less than 100 mHz across the comb

spectrum. In essence, by phase-locking the comb to an optical reference, we can transfer the

stability and low phase noise of a single optical reference all the way across an optical span of 1 to 2 microns." Additionally, the repetition frequency of the comb is now extremely stable with a

projected residual timing jitter of ~ 1 fs or less. This high phase coherence and low timing jitter can be exploited in a number of applications.

Metrology Applications:

The basic application of the fiber-laser frequency comb to metrology involves

heterodyning the laser-under-test with the frequency comb. The resulting beat signal is a measure of the frequency difference between the laser frequency, flaser-, and the frequency of the

nth comb tooth, /„. Assuming n is known (and there are methods for determining «), the absolute

frequency and stability of the laser can be determined with an uncertainty matching that of the

comb tooth. If the comb is phase-locked to a microwave reference that is in turn stabilized to a time standard, then the absolute laser frequency can be determined to hertz level accuracy. This

accuracy is seven orders of magnitude better than what can be achieved with a state-of-the-art

wavelength meter. Experiments have demonstrated that very high levels of stability cah be

achieved with fiber laser frequency combs.'"'

In some cases, the absolute laser frequency is of less interest than the linewidth or phase

noise on the laser under test. In that case, the frequency comb can be phase-locked to a narrow

stable optical reference. The beat signal then gives a measure of the absolute linewidth of the

laser under test over the given observation time. By demodulating the beat signal with a fast

Fourier transform (FFT) device, one can quantify the phase (or frequency) noise spectrum of the

laser under test. This approach, which is only just now being developed at NIST, would permit

the characterization of the phase noise of optical sources in a quantitative way that is consistent

with the current characterization of microwave sources.

1(f)

Fiber laser frequency comb

Laserunder-test

fhcai ^ Frequency Counter

s s

s

s

Rf Spectrum Analyzer

Fast Fourier Transform Spectrum Analyzer

Fig 3: Metrology with a fiber-laser frequency comb. The heterodyne beat note between the laser under test and the frequency comb can be "counted" to measure the absolute frequency stability on

long time scales. It can also be directed to an RF spectrum analyzer to measure the linewidth or to

a fast Fourier transform spectrum analyzer to demodulate the signal in order to measure the phase

noise or frequency noise spectrum.

Remote Sensing Applications:

One advantage of the fiber-laser frequency combs is that the output can be amplified with

standard, commercially-available Er-doped fiber amplifiers. The possibility then arises of

generating a high power, highly phase-coherent beam useful for remote sensing applications.

The high bandwidth of the source implies a very high potential range resolution, while the narrow linewidths imply good Doppler, or vibrational, sensitivity at long ranges. In order to achieve the most sensitivity for ranging experiments, and to achieve any sensitivity for Doppler

measurements, one is driven toward a coherent LIDAR system. In coherent LIDAR a strong

48
laser beam is scattered by a surface. A small fraction of the scattered signal is then collected and

heterodyned with a local oscillator (LO), which is typically a frequency-shifted, delayed copy of

the original transmitted pulse. For ranging applications with a pulsed transmitter, a range profile

of the target is measured from the strength of the heterodyne signal as a function of the LO

frequency. For vibrometry applications, the relative longitudinal velocity of the source and

target is monitored from the frequency shift of the heterodyne signal.

The difficulties with developing a coherent LIDAR with the femtosecond fiber laser are

threefold. First, as with any coherent LIDAR system, the return signal will suffer from speckle,

which limits the signal-to-noise ratio for both ranging and vibrometry measurements. Second,

the short pulse length of the transmitted signal will provide excellent range resolution, but only if the delay of the LO is precisely controlled. Third, any differential dispersion in the signal arm

will degrade the range resolution accordingly. Recently, we devised a frequency-resolved

coherent LIDAR (FReCL) system that effectively deals with these issues."^' The basic setup is shown in Fig. 4 below. By spectrally resolving the returning heterodyne signal we achieve three

things. First, we can mitigate the effects of speckle on vibration measurements by incoherently

averaging the signals from the N distinct detectors. This speckle-averaging reduces the variance

in the Doppler measurement by a factor of N. Second, we can use a fixed delay since the

effective delay of the LO can be scanned in post-detection software by applying the appropriate

phase shift to the N signals. Third, we can remove any differential dispersion in the signal arm

by applying the appropriate phase correction to the N signal channels.

Process N-channels: Coherently for range

Incoherently for Doppler

Fig 4: Frequency-Resolved Coherent LIDAR setup. For our laboratory setup, we used a rotating

aluminum disk as the "target". The frequency comb in this case was free-running. The return signal was spectrally resolved with an arrayed waveguide grating (AWG). We used six discrete detectors at a 4.8 nm spacing in order to span the 25 nm-wide output pulse.

An example of the range images are given in Fig. 5 below. Fig. 5 compares the results

obtained with a conventional setup (without spectrally resolving the return signal) to the results

obtained with the FReCL approach. The images are identical, except that the FReCL images are acquired N-times faster, and without physically scanning the delay in the LO path. In these

experiments, a single femtosecond fiber laser was used as the transmitter and local oscillator. As

a result, the round-trip time to the target needed to be very close to an integer multiple of the

laser repetition rate. (In other words, the software scanning of the delay can cover a range of

only ~ 1.5 mm, corresponding to the bandwidth of the individual AWG channels.) However,

with the recent results in phase-locking to fiber frequency combs to a single optical reference, one can envision a separate fiber laser frequency comb for a transmitter and local oscillator. In

that case, arbitrary ranges could be accommodated.

49
Fig. 5. Range image of the wobbling, rotating disk for one full rotation (0.12 sec) with a 10 ms averaging

time, (a) Truth data from conventional system at 20 LO delays, (b) Conventional data for a single delay,

(c) FReCL data at the same LO delay. Ta is the total acquisition time.

Conclusion

The output of femtosecond fiber lasers can be stabilized to either a microwave or an

optical reference to fonn a frequency comb. The compatibility of fiber-laser frequency combs

with telecommunication equipment should lead to a broad range of applications including

frequency metrology, fiber transport of time and frequency, and remote sensing.

References:

1 . T. Udem, R. Holzwarth, T. W. Hansch, Nature, 416, 233 (2002).

2. D. J. Jones, S. A. Diddams, J. K. Ranka, A. Stentz, R. S. Windeler, J. L. Hall, S. T. Cundiff, Science, 288,

635-9 (2000).

3. S. A. Diddams, Th. Udem, J. C. Bergquist, E. A. Curtis, R. E. Drullinger, L. Hollberg, W. M. Itano, W. D.

Lee, C. W. Gates, K. R. Vogel, D. J. Wineland, Science, 293, 825 (2001 ). 4. J. Ye, S. Cundiff, "Femtosecond optical frequency comb technology : principle, operation and application."

New York, NY: Springer, 2005.

5. L.-S. Ma, Z. Bi, A. Bartels, L. Robersson, M. Zucco, R. S. Windeler, G. Wilpers, C. Oates, L. Hollberg, S. A. Diddams, Science, 303, 1843-1845 (2004).

6. A. Bartels, C. W. Oates, L. Hollberg, S. A. Diddams. Opt. Lett., 29, 1081-1083 (2004).

7. B. R. Washburn, S. A. Diddams, N. R. Newbury, J. W. Nicholson, M. F. Yan, C. G. Jorgensen, Opt. Lett., 29, 250-252 (2004).

8. T. R. Schibli, K. Minoshima, F.-L. Hong, H. Inaba, A. Onae, H. Matsumoto, 1. Hartl, M. E. Fermann, Opt.

Lett., 29, 2467-2469 (2004).

9. H. Hundertmark, D. Wandt, N. Haverkamp, H. R. Telle, Opt. Express, 12, 770-775 (2004).

10. B. R. Washburn, R. Fox, N. R. Newbury, J. W. Nicholson, K. Feder, P. S. Westbrook, C. G. Jorgensen,

Opt. Express, 12, 4999-5004 (2004).

11. F. Adler, K. Moutzouris, A. Leitenstorfer, H. Schnatz, B. Lipphardt, G. Grosche, F. Tauser, Opt. Express,

12, 5872-5880(2004).

12. P. Kubina, P. Adel, F. Adler, G. Grosche, T. W. Hansch, R. Holzwarth, A. Leitenstorfer, B. Lipphardt, H.

Schnatz, Opt. Express, 13, 904-909 (2005).

13. H. Schnatz, B. Lipphardt, G. Grosche, in proceedings ofLasers and Electro-Optics/Qiiantiim Electronics

and Laser Science Conference (CLEO), p. CTuHl, 2006)

14. H. R. Telle, G. Steinmeyer, A. E. Dunlop, J. Stenger, D. H. Sutter, U. Keller, Appl. Phys. B, 69, 327-32

(1999).

15. J. Rauschenberger, T. M. Fortier, D. J. Jones, J. Ye, S. T. Cundiff, Opt. Express, 10, 1404-1410 (2002).

16. J. J. McFeiTan, W. C. Swann, B. R. Washburn, N. R. Newbuiy, Opt. Lett., in press (2006).

17. J. W. Nicholson, M. F. Yan, P. Wisk, J. Fleming, F. DiMarcello, E. Monberg, A. Yablon, C. G. Jorgensen,

T. Veng, Opt. Lett., 28, 643 (2003).

18. J. W. Nicholson, P. S. Westbrook, K. S. Feder, A. D. Yablon, Opt. Lett., 29, 2363-2365 (2004).

19. N. R. Newbury, B. R. Washburn, IEEE J. of Quantum Electron., 41, 1388-1402 (2005).

20. W. C. Swann, J. J. McFerran, I. Coddington, N. R. Newbury, I. Hartl, M. E. Fermann, P. S. Westbrook, J. W. Nicholson, K. S. Feder, C. Langrock, M. M. Fejer, submitted, (2006).

21. W.C. Swann, N.R.Newbury, Opt. Lett., 31, 826-828 (2006).

50
GENERATION OF A FREQUENCY COMB BY FWM IN SOAs

ENHANCED BY RAMAN

Ana Carrasco-Sanz'', Sonia Martin-Lopez'', Miguel Gonzalez-Herraez^'*', Pedro Corredera^

Maria Luisa Hernanz^ and Laura Abrardi''

^ Departamento de Metrologia, Institute de Fisica Aplicada. (CSIC). Serrano, 144. 28006 Madrid, Spain.

carrasco(S;ifa. cetef.csic.es

^ Departamento de Electronica. Escuela Politecnica Superior, Universidad de Alcala. Campus

Universitario, 28871 Alcala de Henares, Madrid, Spain.

Abstract: This paper describes and demonstrates afrequency-spacing comb based on fourwave mixing in t\vo semiconductor optical amplifiers enhanced by Raman amplification in a dispersion-shifted opticalfiber. As result we have achieved a stable frequency comb with

36 spacing constant channels locked in two absorption lines ofthe acetylene ('~C2H2).

Introducrion

In Dense Wavelength Division Multiplexing (DWDM) systems, the data is transmitted over a single optical fiber using multiple optical frequency channels following a frequency grid centered

at 193.1 THz (1552.525 nm) in which the channels are separated by 100, 50 or 25 GHz and

specified by the ITU. With such a narrow spacing, accurate frequency stabilization of the optical

carrier frequencies is required to ensure a robust transmission system and avoid possible

crosstalk between channels and losses in the filtering systems. For this reason, new frequency

standards are necessary to calibrate the filters, wavelength meters and optical spectrum analyzers

used in the maintenance and characterization of DWDM systems.

An interesting method to generate reference frequencies in the optical fiber

communications bands is the stabilization of lasers in the molecular or atomic absorption lines of

some specific gases. The absorption lines of gases such as '"C2H2, ^^C2H2, H^'^C^'^N, CH4,

'"C^^O, or HI provide frequency references that are very stable and have very well understood

physical behavior along the telecommunication transmission bands range from about 1 84 THz to 237 THz (1625-1260 nm) [1], [2], [3] and [4]. Unfortunately with this method the generation is limited to those frequencies at which these gases have well-resolved absorption lines. If we want

to obtain reference lines that cover all the ITU's bands it is necessary to use many cells with

different gases.

The generation of new frequencies based on standards laser lines using Four-Wave

Mixing (FWM) in fiber optic [5] and semiconductor optical amplifier [6] is an interesting method

to extend the frequency range.

Other methods have been developed to generate new reference frequencies. One of the

most attractive is the creation of a spectrum of discrete, equally spaced and known frequency

emissions over the bandwidth of interest, using an optical frequency comb generator. Other

systems developed are based on the modulation of a CW laser and extended by the generation of

FWM in zero dispersion fiber [7], [8], the modulation of a supercontinuum source [9] and the

FWM generation in multicore photonic crystal fibers [10]. In this paper we demonstrate experimentally a frequency-spacing comb based on

degenerate four-wave mixing in two semiconductor optical amplifiers (SOAs) enhanced by

Raman in a dispersion-shifted optical fiber. The comb is generated by locking two lasers into

different absorption lines of acetylene, centered in the zero dispersion region of the optical fiber.

51
Therefore, the frequency difference between the selected absorption Hnes sets the spacing of the

frequency comb. To assure a good FWM efficiency we use a SOA as nonhnear generation media.

Furthermore, since the SOAs are insensitive to phase matching condition, we obtain a wider

bandwidth over which the conversion efficiency is significant [6]. We incorporate a Raman

enhancement obtained by two Raman fiber lasers getting a comb of 36 frequencies. The spaced

of the comb is in our case defined by the separation between P23 (194.743 THz) and P25

(194.566 THz) absorption lines of the '-C2H2 (Af=176.636 GHz).

Stabilization of the comb in the acetylene C2H2 absorptions

Figure 1 shows the experimental setup for the proposed frequency comb that operates stably at room temperature.

1 Khz

Filter

Lock-in

Detector —H

TL1 OOP

TL2

SOA SOA Isolator Fiber

RFL1

RFL2

WDM

Att. OSA

Filter

Acetylene

cell

Detector —N

1 Khz

Figure 1.- Experimental setup

The experimental setup consists of two tunable lasers of 150 KHz width, two SOAs with a

saturation power of 15 dBm, an isolator, two Raman fiber laser at wavelength 1455 nm and 1427

nm, a 5 Km dispersion-shifted fiber, an absorption acetylene cell (traceable to NIST SRM 2517a

[11]), two detectors and two lock-in amplifiers. Two copolarized laser wavelengths are coupled

into a SOA with highly non-linear behavior, generating a four-wave product that will be the seed

of our comb. The output is injected in a second SOA with higher saturation power in order to enhance the FWM product. Figure 2 shows the waves generated by the first SOA and the mixing

product enhanced in the second.

Once we stabilized the lasers at P23 and P25 absorption lines of ''C2H2 respectively, at the output of the first SOA we get six equispaced frequencies (the two original lines and four

extra frequencies), and after the second SOA we obtain nine frequencies. The lasers (TLl and

TL2) are locked to the absoiption lines using a frequency modulation with a sinusoidal signal of

0.1 V at 1 KHz. The peak-to-peak amplitude of this signal produces a change of 30 MHz in the

frequency of the lasers. The new mixing is introduced in a 5 Km long fiber with a zero-dispersion wavelength

Xo = 1540 nm (just between the two original laser lines - R23 = 1539.43 nm and R25 = 1540.83

nm -). This fiber was selected in order to improve the FWM generation and to increase the

Raman gain.

52
Figure 2 - FWM generated in a high non-linear SOA and

enhanced in a high output saturation power SOA.

Figure 3.- Frequency comb generation: One Raman pump.

In order to enhance the comb generated we amphfied the mixing product using a Raman

fiber laser (RFLl) pumping at A^i = 1455 nm and output power Ppi= 1 W. We chose a co- propagating pump configuration for pumping our mixing product since it was more efficient than

the counter-propagating pump configuration as Figure 3 shows.

Using one Raman pump, the frequency comb generated consists of 28 equally spaced

frequencies (1.4 nm, 176.636 GHz) and covers a wavelength range from 1522 nm to 1561 nm. As we were increasing the output power, we could not enhance the number of frequencies

generated. As Figure 4 shows, pump powers higher than 1 W increase the output power of the

comb but diminish the OSNR of the frequencies generated by increasing the noise of the system.

V\feve!enc|th (nm)

1580 1585

m 195 196 Frequency (THz)

E

COo -loci. -25 O

V\fevelength (nm)

1550 1540 1530

Tv« feman fiber lasers Cne Rarran fiber la

Frequency (THz)

Figure 4.- Effect of the pump power in the frequency comb Figure 5 .- Frequency comb generation: Two Raman pumps,

generation In order to increase the number of frequencies generated we introduced another Raman

Fiber Laser, RFL2, pumping at Xp2 = 1427 nm and output power of Pp2 = IW through a 50/50

coupler. Figure 5 shows the frequency comb generation of the frequency comb obtained from the

amplification of the FWM product inside the fiber due to the double Raman amplification. The

new frequency comb generated consists of 36 equally spaced frequencies (1.4 nm) covering the

wavelength range between 1515 nm and 1565 nm.

53
The main difficulty found in the generation of this frequency comb arrives from the

competition between non-Hnear effects inside the dispersion-shifted fiber. The hnewidth of the

Hnes emitted by our lasers is very narrow (150 kHz bandwidth). This event contributes to generate a strong Brillouin scattering signal. The Brillouin scattering subtracts energy to the

system reducing the FWM generation. It would be necessary a dithering of the initial wavelength

emitted from the lasers at 10-20 KHz in order to enlarge the linewidth of the lasers and reduce

the Brillouin scattering.

The generation of new frequencies by FWM in our comb is limited by the chromatic

dispersion of the fiber. Far away from the zero dispersion wavelengths the chromatic dispersion

of the fiber introduces a large phase delay that avoids complying with the phase matching

condition necessary for the FWM generation.

Conclusions

We have experimentally demonstrated a frequency-spacing comb based on degenerate four-wave

mixing in two semiconductor optical amplifiers (SOAs) enhanced by Raman in a dispersionshifted optical fiber. The new frequency comb generated consists of 36 equally spaced

frequencies (176.636 GHz) covering a range from 1515 nm to 1565 nm. The absorption lines of

the acetylene set the spacing of the frequency comb. This frequency comb is useful for

wavelength metrology to calibrate DWDM systems such as wavelength meters or optical

spectrum analyzers, the generation of precise multiwavelength sources employed in DWDM and

synchronization of different fiber laser-based frequency comb.

Acknowledgements

This work was supported by Spanish Commision InteiTtiinisterial de Ciencia y Tecnologia, project TIC2003-01867

and by Comunidad Autonoma de Madrid through project FACTOTEM CM ref n. S-0505/ESP/0417. A. Carrasco acknowledges support from the CSIC, through the OP grant program co-supported by the Social European Fund.

References

[I] T. Dennis et al.," Wavelength References for 1300-nm Wavelength-Division Multiplexing". J. Ligthwave

Tech., 20, 776-782, (2002)

[2] S.L.Gilbert et al, "Hydrogen Cyanide H13C14N Absorption Reference for 1530-1560 nm. Wavelength

calibration- SRM 2519", NIST special publication 260-137

[3] "Carbon Monoxide Absorption References for 1560 nm to 1630 nm. Wavelength calibration-SRM 2514

(12C160) and SRM 2515 (13C160r, NIST special publication

[4] W.C. Swann et al., "Pressure-induced shift and broadening of 1510-1540-nm acetylene wavelength calibration

lines" J. Opt. Soc. Am. B, 17, pp. 1263-1270. (2000)

[5] D. A. Humphreys, "Synthesis of optical reference frequencies by four-wave mixing in optical fiber", 6th Optical Fiber Measurement Conference (OFMC'Ol) Proceedings. National Physical Laboratory, Teddington,

Middlesex, UK, pp. 69-73. (2001)

[6] A. CaiTasco-Sanz et al. "Synthesis of optical standard frequencies in the S, C and L telecommunication bands

by use of four-wave mixing in semiconductor optical amplifiers", in press, Opt. Communications

[7] G. A. Sefler, "Frequency comb Generation by Four-Wave Mixing and the Role of the Fiber Dispersion", J. Ligth. Tech., vol.16, No.9, (1998)

[8] Y. Han et al,"Wavelength-spacing tunable multiwavelength erbium-doped fiber laser cased on four-wave

mixing of dispersion-shifter fiber", Opt. Lett., vol.31. No. 6, (2006)

[9] H. Takara, "Multiple Optical Carrier Generation from a Supercontinuum Source", Optics & Photonics News

(2002)

[10] A. V. Husakou et al., "Frequency comb generation by Four-Wave Mixing in a multicore photonic crystal

fiber", App. Phy. Lett., vol.83. No. 19, (2003)

[II] Standard Reference Material 2517a, s/n NP013U08. NIST

54
Comparison of Saturated Absorption Spectra of Acetylene

Gas Inside Photonic Bandgap Fibers

K. Knabe, R. Thapa, O. L. Weaver, B. R. Washburn, and K. L. Corwin

Kansas State University, Dept. ofPhysics, 116 Cardwell Hall, Manhattan, KS 66506

email: convin&.phvs.ksu.edu, phone: 785-532-2263. fax: 785-532-6606

Abstract: Saturated absorption spectroscopy is performed inside hollow photonic bandgap fibers, with both 10 i^m and 20 i^m core diameters. Line width and signal quality depend on the fiber core diameter, as well as pressure and optical power. These parameters are optimized toward the creation of an optical reference. The effect of splicing the fiber to a single-mode fiber is also investigated.

The development of optical frequency standards in the near-infrared spectral region has been motivated in part by

the optical telecommunications industry. Acetylene gas offers a series of well-spaced spectral features in the 1 .5 |am

region, sparming the C band. By pressure-broadening these features to about 1 GHz in width, the National Institute of Standards and Technology (NIST) has created portable standards of moderate uncertainties, between 13 MHz and

130 MHz, that can be buih into commercial devices [1, 2]. Meanwhile, sub-Doppler spectroscopy of molecular

gases provides high-accuracy infrared optical frequency references. Typically the weak molecular overtone

transitions employed at these wavelengths require high powers for saturation, and therefore to date all high-accuracy

references have been based on power build-up cavities [3,4] , which provide power amplification and long effective

interaction lengths but are not readily portable. A series of measurements of lines in the V] +V3 band [4-6] led the Comitte International des Poids et Mesures (CIPM) to adopt a value of the P(16) line in '^C2H2 with an uncertainty

of 100 kHz [7]. With the advent of frequency comb technology, groups at the National Institute of Advanced

Industrial Science and Technology (AIST) in Japan [8, 9], National Physical Laboratory (NPL) in the UK [10], and

the National Research Council (NRC) in Canada [11, 12] have measured these lines with greatly increased

precision. The CIPM recently reduced the uncertainty of the P( 16) transition to 10 kHz, and most recently, 61 lines in the band have been realized with a width of 600 kHz and measured to an uncertainty of 1.4 kHz [13]. However,

there are many advantages to performing saturation spectroscopy independent of a power build-up cavity, as described in Ref. [14], where signals with widths ~1 MHz have been observed inside a 1 m long glass cell. While the science of frequency metrology has been transfomied, optical fiber technology has simultaneously been revolufionized by the advent of hollow, low-loss photonic bandgap (PBG) fibers. These optical

waveguides allows light to be confined at high intensities in a hollow air or gas-filled region with very low loss [15]. These fibers are vastly superior to capillary fibers for small core diameters [16], and have therefore been used in many recent demonstrations of nonlinear light-gas interactions. Examples include gas sensors [17], Raman

scattering in hydrogen-filled fiber as a tunable light source [18], and electromagnetically induced transparency (HIT)

in PBG fibers filled with acetylene and rubidium [19-21]. Lasers have been locked to the side of Doppler-broadened

transitions in sealed acetylene-filled fibers, toward the development of portable frequency references [22]. Finally, saturated absorption has been observed inside photonic bandgap fibers. Initial observations indicated large

background noise arising from interference and mode beating [23, 24], even in an all-fiber configuration [25]. A

more complete discussion of these issues appears in Ref [26]. For PBG fiber cells to be employed as frequency references, the saturated absorption spectrum must be

optimized in magnitude and minimized in width. Also, modulation of the transmitted light due to spurious

interference or mode beating must be suppressed. In the effort to develop optical frequency references using

acetylene filled PBG fiber cells, we have examined the dependence of signal size and width on gas pressure and

fiber diameter, among other factors. Furthermore, when the PBG fiber is sealed by splicing SMF solid-core fiber to either side, etalon effects and changes in mode coupling may give rise to spurious signals that must be minimized.

Thus we characterize saturated absorption inside a PBG fiber, as a function of gas and fiber dimensions. We also investigate the effects of splicing the fiber on the signal quality. In Fig. la, the experimental setup is shown for measurements of acetylene saturated absorption spectra in an isolated photonic bandgap fiber. Modifications to the experiment made to accoirmiodate a single splice between

a step-index single-mode fiber and a photonic bandgap fiber are shown in Fig. lb. The details of the splicing

technique for splicing hollow PBG fiber to step-index fiber are given in Ref [27]. A 5 mW beam from an extended

cavity diode laser (ECDL) is split, and 10% is amplified by an IPG Photonics'"' Erbium-doped fiber amplifier

(EDFA) to up to 500 mW. The remaining power is sent to diagnostics, which include an acetylene-filled glass cell

55
(as in Ref. [1]), in order to locate the relevant transitions, and a Michelson interferometer, used to monitor the laser frequency as it is swept across the transition under study. The output from the EDFA is split into pump and probe

beams, and the probe passes through an acousto-optic modulator (AOM) in order to shift the frequency between the pump and probe beams, minimizing the interference between the probe and any stray pump light. To compensate

for polarization rotation inside the fibers, half-wave and quarter-wave plates are used to cleanly separate the pump

and probe beams. Also shown are isolators (Iso.), polarization controllers (PC), polarizing beam splitters (PBS), the vacuum chambers (VC), and a photodetector (PD). In Fig. lb, the second vacuum chamber is obviated by a flision splice which seals one end of the PBG fiber. Two different PBG fiber types are invesfigated, as indicated in Fig. Ic and d, with 10 |im and 20 |im nominal core diameters, respectively.

®PD \ splice

Diagnostics;

Fig. 1. a) Schematic of the experimental setup, for unspliced fibers, b) Same as a, but with one end of fiber spliced to SMF. c) 10 pm core PBG fiber, made by Crytal Fibre HC-1550-02. d) 20 |am core PBG fiber, made by Crytal Fibre HCI 9-1 550-0 1 . Images of fiber cross-sections provided by Crystal Fibre A/S, www.c17stal-fibre.com.

The dependence of the observed signal size inside the 20 |im diameter fiber is shown in Fig. 2a. The central dip in transmitted power is due to the presence of the pump beam, which bums a Bennett hole in the ground-state

population of the acetylene molecules. When the probe and the pump are resonant with the same velocity class of

molecules, the probe light is less strongly absorbed. A laser may be locked to this feature and used as a frequency

reference. Both the strength of the absorption and the width of the saturated absorption feature increase with

pressure. Off resonance, the wings are very flat. In contrast. Fig. 2b is taken in a 10 |Lim fiber, and reveals large interference in the wings, indicative of mode beating. The difference between the two spectra is likely due to a reduced number of surface modes [28] present in the larger-core fiber.

-500 0 500

Frequency (MHz)

1000

10-,

0.8

0.6

0.4

0.2^

0.0

0.31 torr — - 0.48 torr 0.60 torr - - - • 0.84 torr

-1000 -500 0 500

Frequency (MHz)

1000

Fig. 2. a) Fracrional transmission versus optical frequency detuning at different gas pressures, for the '"C2H2 P (1 1) transidon with a pump power of 29 mW incident on the 0.78 m long fiber, b) Same as a), but in the 10 ^m with length 0.90 m long and 30 mW of pump power.

56
The above spectra are interpreted in terms of Beer's law [29], which describes the transmission of light through a medium as P = Pq e""'*"", where Pq is the incident power, P is the transmitted power, and / is the length of the medium. From Pq and P is calculated / as(v), as shown in Fig. 3a. Then / a,{v) is fit to an appropriate function,

taking into account the Gaussian nature of the Doppler broadened signal and the Lorentzian saturation dip [26]. The

full-width-half-maximum w of this Lorentzian feature is plotted as a function of gas pressure in Fig. 3b, for two

different pump powers, in the 20 [im fiber with length 0.78 m. Figure 3c shows the frequency discrimination of the saturated absorption feature, which is the signal height in fractional absorption divided by the signal width in MHz.

Larger values indicate increased suitability for an optical frequency reference. From Figs. 2 and 3, it is clear that the 20 |jm fiber provides cleaner signals and narrower signals than the 10 fiber. Thus the 20 [xm fiber is more

suitable for a frequency reference.

Fig. 3. a) aj calculated for the data shown in Fig. 2a, from top to bottom in order of decreasing pressure. Solid lines represent fits to the above equation, using the function described above, b) Width vs. pressure resulting from fits. Circles represent widths of Lorentzian features shown at left. Squares and asterisks represent different transitions interrogated within the same fiber, at the pump powers shown. Triangles represent the widths found in a 1 0 |im fiber with length 0.90 m. c) Frequency discrimination of the saturation dip, indicating the suitability of the standard for a frequency

reference.

In order to employ these PBG fibers as frequency references, they must be sealed by splicing solid-core fiber to each end. Typically this is done with more costly filament heater splicers, but we have demonstrated low-loss

splices using a conventional arc splicer [27]. In Fig. 4, spectra taken in spliced fibers are compared to those from

unspliced fibers. While the background oscillations in the 10 jam fibers are similar with and without splices, the very flat background offered in the 20 |am fiber is degraded in the presence of the splice. This is most likely due to reflections in the fiber, but may also be due to different coupling into the surface modes of the fiber when using

spliced fiber instead of free-space coupling. Further investigation is warranted. ^ 1.0 c

^ 0.8 n

^0.6

1 0.4

c/i E 0.2

(J) 2 0.0

;—1

1

1 1'

11 1

' 1

1 ' 1

' 1' 1' 1

%\

„i.TII

\\ '/ \* '/ \ \

\\

•/'

\» ti \» '/ \» . '/ *\

\\ //' \« t*i

vV

Fiber 1, Fusion arc splice

: b) Fiber 4, Fusion arc splice

Fiber 2, Filament heater splice Fiber 5, No splice

Fibers, No splice^

1 . 1 . 1 . 1 -1000 -500 0 500 1000 -1000 -500 0 500 1000

Frequency (MHz) Frequency (MHz)

Figure 4. Samrated absorption spectra in a) 10 |im and b) 20 |am diameter PBG fibers. Fiber 1 is 0.78 m long, spliced to SMF using a conventional arc splicer, as described in Ref [27], and the P( 1 1) spectrum was taken at 29 mW and 0.86 torr. Fiber 2 is 2.0 m long, spliced to SMF by Crystal Fibre A/S using a filament heating splicer, and the spectrum is taken of

the weaker P( 12) transition at 17 mW and 0.81 torr. Fiber 3 is the unspliced 10 |am fiber of Fig. 2b, of P( 11 ) at 0.69 torr. Fiber 4 is 43.5 cm long, spliced with an arc splicer to SMF, taken of P(l 1 ) at 43 mW and 0.8 torr. Fiber 5 is the 20 |im

fiber of Fig. 2a, at ~ 0.7 torr.

57
We gratefully acknowledge funding from AFOSR, and NSF EPSCoR and CAREER. We thank Sarah Gilbert, Dirk

Miiller, Ahmer Naweed, Bill Swann, and Kurt Vogel for helpful discussions, and Mike Wells and the J. R. Macdonald laboratory staff for technical support.

References

[I] W. C. Swann and S. L. Gilbert, "Pressure-induced shift and broadening of 1 5 10 - 1 540-nm acetylene wavelength calibration lines," J. Opt.

Soc. Am. B 17, 1263 - 1270 (2000).

[2] S. L. Gilbert and W. C. Swann, "Acetylene '"CiHi absorption reference for 1510 nm to 1540 nm Wavelength Calibration - SRM 2517a," 2001.

[3] M. d. Labachelerie, K. Nakagawa, and M. Ohtsu, "Ultranarrow ''CiHi saturated-absorption lines at 1.5 |im," Opt. Lett. 19, 840 - 842 (1994).

[4] K. Nakagawa, M. d. Labachelerie, Y. Awaji, and M. Kourogi, "Accurate optical frequency atlas of the 1 .5-\im bands of acetylene." J. Opt.

Soc. Am. B 13, 2718 - 2714 (1996).

[5] A. Onae, T. Ikegami, K. Sugiyama, F. L. Hong, K. Minoshiina, H. Matsumoto, K. Nakagawa, M. Yoshida, and S. Harada, "Optical frequency

link between an acetylene stabilized laser at 1542 nm and an Rb stabilized laser at 778 nm using a two-color mode-locked fiber laser," Optics Communications 183, 181-187 (2000), <Go to ISI>://000089 153500021.

[6] A. Onae, K. Okumura, K. Sugiyama, F.-L. Hong, H. Matsumoto, K. Nakagawa, R. Felder, and O. Acef, "Opfical Frequency Standard at 1.5 (im based on Doppler-free acetylene absoiption," in Proceedings of 6th Sypni on Frequency Standards and Metrology, P. Gill, (World

Scientific, Singapore, 2002) 445.

[7] T. J. Quinn, "Practical realizarion of the definition of the metre, including recommended radiations of other optical frequency standards

(2001)," Metrologia 40, 103 (2003).

[8] F.-L. Hong, A. Onae, J. Jiang, R. Guo, H. Inaba, K. Minoshima, T. R. Schibli, H. Matsumoto, and K. Nakagawa, "Absolute frequency measurement of an acetylene-stabilized laser at 1542 nm," Opt. Lett. 28, 2324-2326 (2003).

[9] J. Jiang, A. Onae, H. Matsumoto, and F. L. Hong, "Frequency measurement of acetylene-stabilized lasers using a femtosecond optical comb

without caiTier-envelope offset frequency control," Opt. Express 13, 1958-1965 (2005).

[10] C. S. Edwards, H. S. Margolis, G. P. Barwood, S. N. Lea, P. Gill, G. Huang, and W. R. C. Rowley, "Absolute frequency measurement of a 1.5-|am acetylene standard by use of a combined frequency chain and femtosecond comb," Opt. Lett. 29, 566-568 (2004).

[II] A. Czajkowski, J. E. Bernard, A. A. Madej, and R. S. Windeler, "Absolute frequency measurement of acetylene transitions in the region of 1540 nm," Appl. Phys. B 79, 45-50 (2004).

[12] A. Czajkowski, A. A. Madej, and P. Dube, "Development and study of a 1.5 \im optical frequency standard referenced to the P(16) saturated absorption line in the (v( 1 )+v(3)) overtone band of "CiHi," Optics Communications 234, 259-268 (2004).

[13] C. S. Edwards, H. S. Margolis, G. P. Bawood, S. N. Lea, P. Gill, and W. R. C. Rowley, "High-accuracy frequency atlas of "CiH: in the 1 .5 \im region," Appl. Phys. B 80, 977 (2005).

[14] A. Onae, K. Okumura, Y. Miki, T. Kurosawa, E. Sakuma, J. Yoda, and K. Nakagawa, "Saturation spectroscopy of an acetylene molecule in the 1550 nm region using an erbium doped fiber amplifier," Opt. Comm. 142,41 (1997).

[15] R. F. Cregan, B. J. Mangan, J. C. Knight, T. A. Birks, P. S. Russell, P. J. Roberts, and D. C. Allan, "Single-mode photonic band gap

guidance of light in air," Science 285, 1537-1539 (1999), <Go to ISI>://000082359500056.

[16] F. Benabid, J. C. Knight, G. Antonopoulos, and P. S. J. Russell, "Srimulated Raman Scattering in Hydrogen-Filled Hollow-Core Photonic

Crystal Fiber," Science 298, 399 (2002).

[17] T. Ritari, J. Tuominen, H. Ludvigsen, J. C. Petersen, T. Sorensen, T. P. Hansen, and H. R. Simonsen, "Gas sensing using air-guiding photonic bandgap fibers," Opt. Express 12, 4081 (2004).

[18] F. Benabid, G. Bouwmans, J. C. Knight, P. S. J. Russell, and F. Couny, "Ultrahigh Efficiency Laser Wavelength Conversion in a Gas-Filled Hollow Core Photonic Crystal Fiber by Pure Srimulated Rotational Raman Scattering in Molecular Hydrogen," Phys. Rev. Lett. 93, 123903-1 (2004).

[19] S. Ghosh, J. E. Sharping, D. G. Ouzounov, and A. L. Gaeta, "Resonant Optical Interactions with Molecules Confined in Photonic Band-Gap

Fibers," Phys. Rev. Lett. 94, 093902 (2005).

[20] F. Benabid, P. S. Light, F. Couny, and P. S. Russell, "Electromagnetically-induced transparency grid in acetylene-filled hollow-core PCF,"

Opt. Express 13, 5694-5703 (2005).

[21] S. Ghosh, A. R. Bhagwat, C. K. Renshaw, S. Goh, A. L. Gaeta, and B. J. Kirby, "Nonlinear Optical Interactions with Rubidium Atoms

Confined in a Hollow-Core Photonic Crystal Fiber," in Proceedings of Conference on Lasers and Electro-Optics (CLEO), (Optical Society of America, 2006)

[22] F. Benabid, F. Couny, J. C. Knight, T. A. Birks, and P. S. J. Russell, "Compact, stable and efficient all-fibre gas cells using hollow-core

photonic ciystal fibres," Nature 434, 488-491 (2005).

[23] M. Faheem, R. Thapa, and K. L. Corwin, "Spectral Hole Burning of Acetylene Gas inside a Photonic Bandgap Optical Fiber," in Proceedings of Conference on Lasers and Electro-optics (CLEO), (Optical Society of America, 2005)

[24] J. Henningsen, J. Hald, and J. C. Petersen, "Saturated absorption in acetylene and hydrogen cyanide in hollow-core photonic bandgap

fibers," Oprics Express 13, 10475 (2005).

[25] P. S. Light, F. Couny, F. Benabid, and P. S. J. Russell, "Electromagnetically-induced Transparency and Saturable Absorption in All-Fiber Devices Based on Acetylene-Filled Hollow-Core PCF," in Proceedings ofEuropean Conference on Optical Communication 2005

proceedings, 6, 2005) 59. [26] R. Thapa, K. Knabe, M. Faheem, A. Naweed, O. L. Weaver, and K. L. Corwin, "Saturated absorption spectroscopy of acetylene gas inside large-core photonic bandgap fiber," Opt. Lett, (in press) (2006).

[27] R. Thapa, K. L. Corwin, and B. R. Washburn, "Splicing Hollow-Core Photonic Bandgap Fiber to Step-Index Fibers Using an Arc Fusion

Splicer," in Proceedings of Conference on Lasers and Electro-Optics (CLEO), (Optical Society of America, 2006)

[28] J. A. West, C. M. Smith, N. F. Borrelli, D. C. Allan, and K. W. Koch, "Surface modes in air-core photonic band-gap fibers," Opt. Express

12, 1485-1496 (2004).

[29] W. Y3emtT6Ae.v, Laser Spectroscopy: Springer, 1996.

58
CW SUPERCONTINUUM GENERATION IN STANDARD OPTICAL

FIBER OVER THE O, E, S AND C COMMUNICATION BANDS

Laura Abrardi^ Sonia Mart^n-L6pez^ Ana Carrasco-Sanz% Miguel Gonzalez-Herraez^'*',

Pedro Corredera" and Maria Luisa Hernanz"

^ Departamento de Metrologia, Institute de Fisica Aplicada, (CSIC). Serrano, 144. 28006 Madrid, Spain.

Departamento de Electronica, Escuela Politecnica Superior, Universidad de Alcala. Campus

Universitario, 28871 Alcala de Henares, Madrid, Spain.

abrardi(a)ifa.cetef.csic.es

Abstract: The generation ofa broad CWsupercontimium source spanningfrom 1286 to 1538 nm is described. The device makes use ofa tunable Yb-dopedfiber laser, several silicafibers ofadequate

dispersion and a cascade of FBG mirrors that seed supercontimium emission at 1318 nm. The

source has an output power of3 W and exhibits >0 dBm/nm spectral density over a range ofmore

than 240 nm.

1. Introduction

Supercontinuum (SC) generation in optical fibers and waveguides has attracted considerable

research interest in the last years [1-12]. Applications of SC generation in optical fibers can now

be found in fields such as optical frequency metrology [1,2], optical coherence tomography [3,4],

and multiple optical carrier generation for wavelength-division multiplexing [5,6]. This renewed

research interest in SC light sources has been made possible thanks to the recent development of

special fibers with high nonlinearity, namely photonic crystal fibers [7] and dispersion-tapered

fibers [8]. SC is typically obtained in those fibers by pumping them with high peak-power input

pulses with durations ranging from several nanoseconds to several tens of femtoseconds. In these

cases, SC results from the combined effects of self-phase modulation (SPM), four-wave mixing

(FWM), and stimulated Raman scattering (SRS), that together lead to multisoliton generation and

collision processes. It is important to realize however that pulsed laser sources are in principle not necessary to generate a broadband SC. SC generation under cw pumping was only achieved

recently in optical fibers [9-12]. Of course, since cw sources do not benefit from the high peakto-average power ratio of pulsed lasers, longer interaction lengths are required. The characteristic

fiber lengths used in cw SC generation are in the km range even when highly nonlinear fibers are used [13]. Also, for a given average input power, short pulses produce much wider SC spectra

than cw pumps. However, few pulsed lasers are able to generate the multi-watt average output

powers that are typical of modem cw lasers such as cascaded Raman fiber lasers. Accordingly,

for similar spectral widths, cw SC has a higher average power and exhibits a higher power

spectral density than pulsed SC. Mean power densities of typical pulsed-pump SC usually range

from -20 to -10 dBm/nm [1,7]. In comparison, we have demonstrated in a previous work a cw SC

source with a peak power spectral density in excess of 8 dBm/nm, which represents an improvement of nearly two orders of magnitude [11]. CW SC generation in optical fibers is generally caused by the combined effect of modulation instability (MI) and stimulated Raman

scattering (SRS). Efficient generation of a broad modulation instability spectrum can only be

achieved in the region of very small anomalous dispersion (D>0 or (32<0). The use of a long fiber with low loss ensures strong power transfer from pump to MI and a low SRS threshold (~1 W).

In these conditions, SC generation is possible with low peak power sources (and even cw

as in this case) while allowing strong pump depletion and high power densities [12] (in the order

of mW/nm in our case). In this paper we present a new design of cw-pumped SC source spamiing

over the O, E, S and C communication bands. The source makes use of a tunable Yb-doped fiber

laser, several silica fibers of adequate dispersion and a cascade of FBG mirrors that seed

supercontinuum emission at 1318 nm. The source has an output power of 3.0 W and exhibits >0

dBm/nm spectral density over a range of more than 240 nm.

59
2. Supercontinuum generation

To obtain the new cw SC centered at 1318 nm we use the experimental setup depicted in Fig. 1

.

The pump used in our experiment was a cw Yb-doped fiber laser tuned at 1 104.4 nm. The output

is single-mode and the power can be tuned in the range from 0.57 to 20 W. An isolator at the

laser output prevents damage in the laser by back-reflected light. To obtain spectral broadening

in a conventional fiber, one needs to have the pump tuned at wavelengths in small anomalous

dispersion regimes. We seed the spectral broadening process in conventional fibers at 1318 nm

by the use of a cascade of Fiber Bragg Grating mirrors (FBG) centered at 1318.0 nm, 1231.8 nm,

and 1 161.8 nm. These miiTors have reflection bandwidths of 3.12 nm, 1.73 nm, and 1.47 nm and

reflectivity of 37.1%, 99.0%, and 99.4% respectively. A 1/99 coupler is inserted between the

isolator and the FBG mirrors to monitor the power inserted into the system and the backreflected light power. The nonlinear medium is a cascade of optical fibers of increasing zero- dispersion wavelengths, as shown in table 1. Finally the output spectmm is analyzed through an

optical spectrum analyzer (OSA) after passing through a 10/90 and a 5/95 couplers that only act as fixed attenuators.

^•399 optical fibers gratings ^»

coupler „„| |„ coupler >^

Fig. 1. Experimental setup. FBG: fiber Bragg grating: OSA: optical spectrum analyzer.

The zero-dispersion wavelength A^o, dispersion slope So, loss coefficient a and length L

are reported, for each fiber, in the table 1

:

So a y L

[nm] [ps nm'^km''] [dB km-'] [W'km-'] [km]

Fiber 1 1302 0.085 0.35 0.4 6.0 Fiber 2 1307 0.087 0.33 0.4 8.0 Fiber 3 1311 0.083 0.34 0.4 2.0 Fiber 4 1312 0.085 0.34 0.4 2.0 Fiber 5 1416.9 0.085 0.34 0.4 2.0

Table 1. Properties ofthe cascade ofopticalfibers used in the experiment.

As it has been shown in earlier results [14], the non-linear broadening process that leads

to supercontinuum generation is mainly caused by MI and stimulated Raman scattering by

pumping high power cw light in small anomalous dispersion regimes. As we have said, the highpower output of our Yb-laser is tuned at 1104.4 nm. Hence, it lies well within the normal

dispersion regime of standard fibers. Our experiment has, therefore, two parts: one is to shift the

wavelength of the laser emission to around 1318 nm, where the standard fibers have small

anomalous dispersion. Then, as the power is increased, the broadening caused by MI becomes more efficient, and the supercontinuum spectrum grows. To seed the emission in the anomalous

dispersion regime, we use the cascade of Fiber Bragg Grating mirrors (FBG) described above.

The process of wavelength shift can be qualitatively described as follows: when we

increase the laser pump power and we pass the Raman threshold we observe strong stimulated

60
Raman scattering (SRS). Backward SRS appearing at 1 162 nm is reflected back into the fiber by

FBG3, and is subsequently amplified as it propagates in the forward direction. The emission at 1162 nm acts as a laser and plays a similar role to the pump laser. The same principle with FBG2

and FBGl gives rise to cascaded Raman frequency generation. In Fig. 2 we can see the spectrum

obtained at the fiber output for different pump powers. The input powers have been measured at the input of FBGl. The isolator losses are approximately 1 dB.

At 3.1 W the first Raman peak at 1 162 nm is clearly visible and the next Stokes order

appears. At 5.7 W the second Raman peak at 1232 nm is also present, a new Raman peak appears

at 1318 nm and depletion of the spectral lines at 1 162 and 1232 nm begins to occur. In Fig. 2(b)

the blue line represents the output spectrum for a pump power of 7.2 W. We can observe how for

this pump power level there is enough power around 1318 nm to generate its own Raman gain curve at 1397 nm. We can see that at power levels higher than 7 W the spectral broadening starts to take place.

1232.0 nm 1232.0 nm 1316.4 nm

—,

^

, . , -30-L-i .

,

.

1

.

1 1100 1200 1300 1100 1200 1300 1400 wavelength [nm] wavelength [nm]

(a) (b)

Fig. 2. Normalized output spectrum showing the Raman peaks at the output ofthefiberfor different input powers, (a) Spectrafor input powers of1.5, 3. 1 and 5. 7 W. (b) Spectrafor input powers of 6. 1, 6. 6 and 7.2 W; we can observe the pump broadening around 1318 nm.

When the Raman gain curve centered at 1318 nm reaches certain power level the second

part of our experiment starts. The first four fibers that we are using as amplifying medium have

been chosen with their Xq next to 1318 nm as shown in table 1 . In fact, the Raman peak at 1318

nm falls in the spectral region of small anomalous dispersion regime. In these condifions the

process of supercontinuum generation starts [16,17]. As we can observe in Fig. 2(b), for 7.2 W

of pump power, a Raman peak appears at 1397 nm. Ml-induced soliton fission and Raman shift

is also evident from the appearance of smooth red-shifted tails in the pump spectrum. Soliton

generation is done mainly in the first kilometers of the fibers. The fibers are organized so as to have increasing Xo order, hence decreasing values of chromatic dispersion at 1318 nm. This

organization enhances soliton compression, hence favours a broader supercontinuum. As the

pump power grows SRS starts to play a more important role in the broadening process. The

components of the Raman peak at 1397 nm are amplified too. The fifth fiber has its A,oat 1416.9 nm. We use it to try to reproduce the broadening process of 1318 nm at 1398 nm. In Fig. 3 the

two output spectra for four and five fibers are depicted. We can observe how the presence of the

fifth fiber improves the spectral width of our SC. The best results show a SC source in the 1300

nm spectral region with >260 nm spectral width when the input pump power grows up to 1 1 W.

This new SC covers the O, E and S bands completely, and partially the C band.

61
1397.8 nm

U-| .

1 1

1

1

, 1.

1 ,

1

1250 1300 1350 1400 1450 1500 1550 wavelength [nm]

Fig. 3. Normalized supercontimiim spectrum at thefiber output for tM>o different experimental configurations (4 and 5

fibers). Pump broadening covering the O, E, S and C bands is generated over >260 nm as measured 20-dB down

from the highest peak.

Conclusions

We have demonstrated pump spectral broadening and supercontinuum generation spanning more

than 260 nm over the O, E, S and C bands. This has been achieved by pumping a concatenation

of conventional fibers with an Yb-doped fiber laser and a frequency-selective reflective structure

that seeds supercontinuum emission at 1318 nm, in the regime of small anomalous dispersion of

the fibers used in the experiment.

Acknowledgements

We acknowledge financial support from the Ministerio de Educacion y Ciencia through the project TIC2003-01869

and the corresponding FPI contract, the support from the Comunidad Autonoma de Madrid through project

FACTOTEM_CM ref n. S-0505/ESP/0417 and the support from the CSIC, through the I3P grant program co- supported by the Social European Fund.

References

[I] S. A. Diddams, D. J. Jones, J. Ye, S. T. Cundiff, J. L. Hall, J. K. Ranka, R. S. Windeler, R, Holzwarth, T. Udem,

and T. W. Hansch, Phys. Rev. Lett. ( 84) 5102-5105 (2000)

[2] D. J. Jones, S. A. Diddams, J. K. Ranka, A. Stentz, R. S. Windeler, J. L. Hall and S. T. Cundiff, Science. ( 288)

635-639(2000)

[3] D. L. Marks, A. L. Oldenburg, J. J. Reynolds and S. A. Boppart, Opt. Lett., (bf 27) 2010-2012 (2002)

[4] Hartl, X. Li, C. Chudoba, R. Ghanta, T. Ko, J. G. Fujimoto, J. K. Ranka and R. S. Windeler, Opt. Lett. (26) 608- 610 (2001)

[5] T. Morioka, K. Mori and M. Saruwatari, Electron. Lett. ( 29) 862-864 (1993)

[6] H. Takara, T. Ohara and K. Sato, Electron. Lett. ( 39) 1078-1079 (2003)

[7] S. Coen, A. Hing Lun, R. Leonhardt, J. D. Harvey, J. C. Knight, W. J. Wadsworth and P. St. J. Russell, J. Opt.

Soc. Am B ( 19) 753-763 (2002)

[8] T. A. Birks, W. J. Wadsworth and P. St. J. Russell, Opt. Lett. ( 25) 1415-1417 (2000)

[9] M. Prabhu, N. S. Kim and K. Ueda, Jpn. J. Appl. Phys. ( 39) L291-L294 (2000)

[10] J. W. Nicholson, A. K. Abeeluck, C. Headley, M. F. Yan and C. G. Jorgensen, Appl. Phys. B, (77) 211-218

(2003)

[II] M. Gonzalez-Herraez, S. Martin-Lopez, P. Corredera, M. L. Hemanz and P. R. Horche, Opt. Commun. ( 226) 323-328 (2003)

[12] A. V. Advokhin, S. V. Popov and J. R. Taylor, Opt. Lett. ( 28) 1353-1355 (2003)

[13] A. K. Abeeluck, C. Headley, and C. G. Jorgensen, Opt. Lett. (29), 2163-2165 (2004).

[14] F. Vanholsbeeck, S. Martin-Lopez, M. Gonzalez-Herraez and S. Coen, Opt. Exp. (13) 6615-6625 (2005)

[15] S.V. Smimov, J.D. Ania-Castanon, T.J. Ellingham, S.M. Kobtsev, S. Kukarin, S.K. Turitsyn, Opt. Fiber Techn.

(12) 122 (2006)

[16] G. P. Agrawal, Applications of Nonlinear Fiber Optics, Academic Press, San Diego (1991)

62
A C-band optical frequency synthesizer using 1.5 micron NICE OHMS

frequency standards and an optical frequency comb generator

C. S. Edwards' -, H S. Margolis', G. P. Barwood', P. Gill' " and W. R. C. Rowley'

'National Physical Laboratory,

Hampton Road, Teddington, Middlesex TW1 1 OLW United Kingdom

E-mail christopher.edwards@npl.co.uk

"Blackett Laboratory, Imperial College London,

South Kensington Campus, London SW7 2BZ, United Kingdom

Abstract: Two acetylene frequency standards, which make use of the NICE OHMS technique, have been

developed in the 1.5 \im region. A summary of their performance will be presented, along with some

recently measured frequency values for lines in the vi + V3 combination band of '"^C^Ht. It is planned to use the standards in conjunction with an optical frequency comb generator to demonstrate arbitrary

frequency synthesis with kHz absolute accuracy in the spectral region 1.52 to 1.56 |im.

1. Introduction

There is considerable interest in the use of semiconductor lasers as frequency standards for optical

telecommunications in the 1.5 |im region using Doppler-free acetylene spectra [1-7]. The most commonly

used experimental arrangement utilises conventional cavity-enhanced saturation absorption

spectroscopy [1-6], where an acetylene cell is placed inside an enhancement cavity. Relative stabilities of

a few parts in lO'" at 1 s have been reported using this method [4-6], falling to a floor of a few parts

in lO'"* for a few thousand seconds averaging time [5, 6]. The relative frequency stability of these systems

is limited by the residual differential frequency variation between the laser and the enhancement cavity resonance, which introduces amplitude noise on the transmitted light, thus degrading the signal-to-noise

ratio of the acetylene features. The technique of Noise Immune Cavity-Enhanced Optical Heterodyne

Molecular Spectroscopy (NICE OHMS) was developed to address this problem [8]. Although this technique has been used before for acetylene spectroscopy at 1.54 \im [3], the performance achieved was

not superior to that demonstrated with conventional cavity-enhanced spectroscopy. This paper describes

work aimed at improving on the best relative frequency stability results reported to date [4-6]. It is planned to use the NICE OHMS systems in conjunction with an optical frequency comb generator

(OFCG) [6] to demonstrate arbitrary frequency synthesis in the spectral region 1.52 to 1.56 \xm, with the

potential for long-range dissemination of reference frequencies via fiber-optic network to end users in industry. In addition, as transportable local oscillators with a relative stability superior to that of a GPS-disciplined quartz oscillator, the NICE OHMS standards are candidates for use in a transportable

fiber laser femtosecond comb system for in situ frequency metrology and other applications.

2. NICE OHiMS Standards

The experimental arrangement of one of the NICE OHMS standards is shown in Fig.l. The output of the

diode laser is directed through a Faraday isolator and via a polarizing beamsplitter through two

electro-optic phase modulators. One of these adds a pair of 8 MHz sidebands to the carrier frequency

(with modulation index p = 0.7) which are used for Pound-Drever-Hall (PDH) locking to the

enhancement cavity. The PDH locking narrows the laser linewidth from 2 MHz to 50 kHz

full-width-at-half-maximum (FWHM). The enhancement cavity has a finesse of - 150 and contains a

63
sealed 200-mm Brewster-windowed cell containing 1.0 (3) Pa of '''C2H2 controlled at a temperature

of 25.0 ± 0.5°C. The one-way intra-cavity power density is typically 0.20 Wmm"'. The cavity length is dithered via a piezo-electric transducer (PZT) at 800 Hz, and this modulates the laser frequency to a depth

of 1 MHz peak-to-peak. The other electro-optic modulator adds another pair of sidebands at 490 MHz

0.3), this frequency corresponding to the free-spectral range of the enhancement cavity. These

sidebands are transmitted through the cavity and are used in the NICE OHMS detection scheme [8]. Interference between the carrier and these sidebands gives rise to an acetylene signal at 490 MHz which is demodulated using a double-balanced mixer (DBM) and subsequently demodulated at the second

hanuonic (1.6 kHz) of the cavity dither frequency to give a locking discriminant. Any differential frequency noise between the cavity resonance and the carrier frequency affects the transmitted sidebands

in the same way, and so there is a high degree of common mode cancellation of this noise on the observed

acetylene signal. This "noise immune" character is the principal advantage of the NICE OHMS technique.

A summary of the systems' relative stability and parametric sensitivities will be presented in this paper, as well as an assessment of their longer term reproducibility.

1.5 |Lim o/p

DO

PBS PBS

Acetylene Enhancement Cavity

"6

Signal i/p Servo Correction

490 MHz o/p

DBM/Amplifier

1.6 kHz

jSOO Hz o/p

PSD/Integrator

Diode Controller

Servo Correction

DBM/lntegrator

Signal i/p

8 MHz o/p

Fig. 1: Experimental arrangement of a NICE OHMS acetylene standard. Key:- DL = diode laser,

PBS = polarizing beamsplitter, EOM = electro-optic modulator, PR = Faraday rotator, D =

photodiode, DBM = double-balanced mixer, PSD = phase-sensitive detector.

64
3. Absolute Frequency Measurements and Synthesis

Three experimental techniques are used to measure the acetylene transition frequencies. In the first, a combined frequency chain and self-referenced TiS femtosecond comb system [5] have been used to determine a few reference frequencies. Using these reference frequencies, an acetylene standard is used to drive an OFCG, resulting in a comb of known frequencies which can be used to measure some unknown

acetylene frequencies [6]. Thirdly, it is planned to use a fiber femtosecond comb [9] to make direct

absolute frequency measurements. The measurements to date have resulted in a set of acetylene frequency

values with fractional uncertainties in the range 10"" to 10''', limited only by the reproducibility of the

standards.

The experimental arrangement for optical frequency synthesis is shown in Fig. 2.

RF Offset Lock

10 MHz

RF Synthesizer 2

IF DBM

Hydrogen Maser RF Synthesizer 1

NICE OHMS Standard

NPL-ECL2

10 GHz

OFCG

1 .55 jiiTi Output Laser

NPL-ECL1

RF

D

- Optical

Synthesizer

Output

Fig. 2: Optical Synthesizer Set-Up.

Key:- D = photodiode, DBM = double-balanced mixer

The OFCG used in this work has been described in detail elsewhere [6]. An electro-optic modulator is placed in a build-up cavity whose free spectral range is a sub-hannonic of the applied rf drive frequency

65
of 10 GHz. An applied rf power of 1 W gives a modulation index ~ 0.3. This results in a comb of modes

with a usable span of ~ 800 GHz. The comb is driven by NPL-ECL2 operated as a NICE OHMS

acetylene standard, and the output of the comb is mixed with light from NPL-ECLl, which provides the

optical synthesizer output. The detected beat frequency is mixed with the output of rf synthesizer 2 to give

a residual beat frequency which is stabilized at 50 MHz using the rf offset lock electronics. As the

absolute frequency of the NICE OHMS standard is known, together with the comb order number, the rf synthesizer frequency and the offset beat frequency, the optical synthesizer frequency of the output laser

is determined by the choice of acetylene line and the frequency of rf synthesizer 2, and so the optical

output frequency can be arbitrarily chosen in the range 1 93 to 1 97 THz.

Summary

A pair of NICE OHMS acetylene standards have been developed for use as frequency standards in the 1.5 |im region. The frequencies of various acetylene transitions have been measured with uncertainties in the

range 10"" to 10"'', coiresponding to an uncertainty range 0.02 to 0.002 fm in vacuum wavelength. When

combined with an OFCG, they will fonn an optical frequency synthesizer capable of sub-kilohertz

accuracy.

Acknowledgements

This work was funded by the National Measurement System Length Programme. C S Edwards would like to acknowledge useful discussions with Prof R C Thompson of Imperial College London (UK).

References

[1] . K. Nakagawa, M. de Labachelerie, Y. Awaji and M. Kourougi, "Accurate optical frequency atlas of the 1.5 |im bands of acetylene," 7. Opt. Soc. Am. B, vol. 13, pp. 2708-2714, 1996.

[2] A. Onae, K. Okumura, J. Yoda, K. Nakagawa, A. Yamaguchi, M. Kourogi, K. Imai and B. Widiyatomoko, "Toward an Accurate Frequency Standard at 1.5 )j.m Based on the Acetylene

Overtone Band Transition," IEEE Trans. Instnim. Meas., vol. 48, pp. 563-566, 1999.

[3] T. Kurosu and U Sterr, "Frequency-stabilization of a 1.54 micrometer DFB-laser diode to Doppler-free absorption lines of acetylene," Proc. SPIE, vol. 4269, pp. 248-254, 2001.

[4] A. Czajkowski, J. E. Bernard, A. A. Madej and R. S. Windeler, "Absolute frequency

measurement of acetylene transitions in the region of 1540 nm," Appl. Phys. B, vol. 79, pp. 45-50,

2004.

[5] C. S. Edwards, H. S. Margolis, G. P. Barwood, S. N. Lea, P. Gill, G. Huang and W. R. C. Rowley, "Absolute frequency measurement of a 1.5-p,m acetylene standard by use of a combined

frequency chain and femtosecond comb," Opt. Lett., vol. 29 , pp. 566-568 , 2004.

[6] C. S. Edwards, H. S. Margolis, G. P. Barwood, S. N. Lea, P. Gill and W. R. C. Rowley, "Highaccuracy frequency atlas of '^C2H2 in the 1.5 \\m region," Appl Phys. B, vol. 80, pp. 977-983,

2005.

[7] P. Balling, M. Fischer, P. Kubina and R. Holzwarth, "Absolute frequency measurement of

wavelength standard at 1542 nm: acetylene stabilized DFB laser," Opt. Express, vol. 13, pp. 9196-9201,2005.

[8] J. Ye, L.-S. Ma and J. L. Hall, "Ultrasensitive detections in atomic and molecular physics:

demonstration in molecular overtone spectroscopy," J. Opt. Soc. Am. B, vol. 15 pp. 6-15, 1998.

[9] P. Kubina, P. Adel, F. Adler, G. Grosche, T. W. Hansch, R. Holzwarth, A. Leitenstorfer, B. Lipphardt and H. Schnatz, "Long term comparison of two fiber based frequency comb systems,"

Opt. Express, vol. 13, pp. 904-909, 2005.

66
PMD: Measurement method linkages

T.A. Hanson, Corning, Inc. 27-Mar-06

Introduction

The study of polarization mode dispersion (PMD) in terms of both measurements and

system impairments has gone on for over fifteen years in Standards developments as well

as other venues. Some open issues remain, but many were closed with the Consent of

ITU-T Recommendation G.650.2 in December, 2004. This Recommendation clarified

the definitions and relationships of the different measurement methods for first order

PMD. Since then, the lEC Standards have been in an update process. See for example,

60793-1-48 for measurement of fiber and cable, or the general theory and guidance

document, 61282-9. This paper will try to illuminate the main mathematical aspects of

the learnings obtained in this convergence process. Although numerous references exist,

only a few will be cited here.

Background

PMD measurements can be considered as falling into two broad categories: the frequency

domain methods such as Stokes Parameter Evaluation (JME and PSA) and the time

domain methods such as Interferometric or the Fixed Analyzer FFT methods. As a parameter, the PMD value is representative of a wavelength or optical frequency range.

For the frequency domain methods, a parameter called differential group delay (DGD) is determined for individual optical frequencies in the range. The PMD value is reported as

either the linear average or root-mean-square (RMS) of these values. For the time

domain measurements, the aggregate of PMD effects across the frequency range is measured all together and the relationship to RMS PMD value for the frequency range of

the source is deduced.

System impairments are most easily considered in the frequency domain. The field of the

signal that would be obtained without PMD, but with the inclusion of other effects such

as chromatic dispersion can be considered in the time domain as f(t). The Fourier

transform of this complex scalar function can be considered as F(co) where co is the

angular frequency deviation about some central frequency, coo. This is multiplied with

the two dimensional complex output Jones vector, 7(<2;), representing PMD induced

variance in the neighborhood of coo- Inverse Fourier transformation yields the composite

time domain image of the signal as a complex two dimensional vector. Conversion to power yields the time domain PMD adjusted signal. Differences in the result, with or without PMD effects, gives an indication of the impairment. This impairaient is due only

to variation in the output Jones vector with optical frequency and power variations that

could arise from polarization dependent loss, which will not be discussed in this paper.

Many aspects of PMD can be simplified by considering coordinate transfomiations. The

situation for first order PMD can be stated as: There is a coordinate system where the

67
output Jones vector can be expressed as: j{coY = [c^e c^.e'^''"\ , where Cx and Cy

represent the complex output Jones vector at coq.

When this vector is muhiphed with the signal in the frequency domain and then the

results are converted back to the time domain one obtains the vector function,

fpMD (0 ~ \f.x fpMDx ) fpMDy )] •

The functions fpMDx/y(t) are shifted up and down by +T' from the center which would be

seen without PMD. The time domain power function, P{t) = f*pMD{^)fpMD{^)^ the

weighted total of the two split signals (where * represents the transpose complex

conjugate). The constants can be adjusted so one or another of the split signals

disappears - or so that both have equal power. In the latter case, the impairment is maximal. The difference between the centers of these two split signals is called the

differential group delay, Ar = 2/' . The reason for keeping a separate parameter, Y', is to

facilitate a more general description given in terms of a rotation angle.

The variance of the output Jones vector can include higher order effects - which can be

described as more complicated output variance functions which might be needed for a consideration of a broad signal spectrum. The signal bit rate and format can therefore

affect the range of frequencies that are important and therefore the importance of higher

order PMD. This paper will focus on first order PMD.

Stokes and Jones relationships

Poole [1] described PMD in tenns of the nomialized output Stokes vector, s[co), a

rotation matrix, R(co), and an input normalized Stokes vector, Sj^ , assumed to be

frequency invariant. Jones [2] described PMD in ternis of an output Jones vector, j{co),

a transfer matrix, T(od), and an input Jones vector, j,^ . One may wonder why the Stokes

representation is important. Reasons include:

• One does not measure Jones vectors, one measures Stokes vectors

• The Stokes vector representation will be important for the development of the

interferometric method

The nomialized Stokes vector is a three dimensional vector that moves on the Poincare

sphere. The values are obtained by measuring the power through a polarizer set at three

pairs of orthogonal settings. The three values are the differences in power between the

two orthogonal settings for each pair. The relationship between the Stokes and Jones

vectors can be expressed in tenns of a polar coordinate system defined as: cos2^?

J

e-"'-cosd

s = sin 26^ cos //

sin 2^ sin /j

(la)

68
The three polarizer pairs used to measure the Stokes vectors are: "6'1_|r0l k/2ll Jk/4] r-^/4]l Jk/4l r ttIA

'

// ~

1 0

' 0 M 0 ' 0 r 1

' -7cl2

(lb)

The first two pairs are called linear states and the last are circular states.

The parallelism of Jones and Stokes are shown as the following pairs, which are also

defined in terms of the output vectors at the central frequency, j(co^^ ) and s{cO(^ )

:

j{co) = T{co)jj^ S(cd) = R(co)s,^ (2a)

do) dco dco dco

(2b)

For first order PMD, these can be further expressed as the following, which will be

further elaborated.

CI,

dco K) <iy(ft>)|

dco

Q.y~s{co^) (3)

Qx are the elements of the three dimensional polarization dispersion vector (pdv), Q

,

which is related to the differential group delay, Ax, and principle state of polarization

(PSP) vector, p , as Q = At • p . If the output state is aligned with the PSP, there is, to the first order, no change with respect to frequency.

The jump from (2b) to (3) is done by representing T and R with polarimetric coordinates,

Gj, |LiT, rotation angles 2Yj = Tr, and rotation vector, y. The matrix, T, is represented as an orthogonal decomposition where the elements of S are the eigenvalues:

"cos6',e-'^^''

sin6'^e'^^''

T = VSV* where V =

R = yy^ {\- cos/ 1 cos/If +[vx]sin7^ ,

(4)

T =

cos/J - iy^ sin/J. - {y^ + zy, )sin/

(^3 -(V2)sin75r cos/J- +/y, sin/

(5)

(6)

Using the Jones/Stokes association defined in (la), T and R are equivalent in that: The

Stokes vectors given by the conversion of Jones input and output vectors derived from T

are also given by R. When the first column of the matrix, V, is converted to a Stokes

vector using (la), the result is the rotation vector, y.

69
Expansion of (2b) results in the following differential equation, which can also be written

out in polarimetric coordinates to yield both expressions in (3). This differential equation

follows from (2b) and allows writing (3).

Q. = 2 y + sin Ijj ^— + 2 sin " Yj

dco dco

VX

dco

(7)

If one sets At/2 = Y', one can write the left part of (3) as an orthogonal decomposition:

dj(co) \ dco = V,DV;j{co,),V^ =

cos 0^6

sin 0^^e

-sin d^e

, D =

"-/>' 0"

0 />'

(8)

The first column of Vp converts to the PSP vector, p . Assuming that this vector is locally fixed, one has a local solution for the output Jones vector given as:

j(co) =VSj (co)V^J-(co, ) = J(co)j(co, ), where Sj (co) =

-I/O)

0

0

(9)

Equation 9 is in the same form as what is used to describe the first order PMD

impairment. The change in coordinates can be done by pre and post multiplying J by Vp

and Vp. This concludes the connection between the Stokes and Jones formulations for

the first order PMD and differential group delay. This equation is also associated with a Stokes vector rotation which increases as the frequency deviation increases. The rotation

angle on the Poincare is equal to 2T'od. The rotation vector is the PSP.

Stokes parameter analvsis measurements

Both these methods, Jones Matrix Eigenanalysis (JME) and Poincare Sphere Analysis

(PSA), are based on measuring the normalized Stokes vectors at multiple optical

frequencies, incremented with Aco. For each frequency, three orthogonal input states are sampled to obtain three orthogonal output states. The states are nonnally given by the

(9,|_i) pairs: (0,0), {n/4,Q) and (K/4,n/2), but others are possible.

For JME, the output states are converted to Jones vectors using (la) and an assumption

that 0< 0< 71. The ratio of the x and y elements is taken to eliminate the one ti: ambiguity

and the application of the left part of (2a) yields estimates of the T matrix at the different

frequencies. Using (9), one can then write:

T(a>^+Aco) = j(Aco)T(co^) •

' (10)

The argument of the ratio of the eigenvalues of T(coo+Aco)T"'(coo), divided by Aco, yields

2Y' = At for frequency coq. The eigenvalues are the elements of Sj(Aco).

For the PSA method, the rotation on the Poincare sphere is considered. From (9) and a consideration that this relates to such a rotation, we have that the rotation angle swept out

70
is equal to AtAco. The change in output Stokes vectors from one frequency to the next is related to the angle, a, of the output state to the PSP, which is the frequency-domain

rotation vector. The following output difference applies to each input state:

\As\ = 2 sin a sin V 2

If the three orthogonal input/output states are designated as h , q and c for both coo and

coo+Aco they can be combined to yield:

Ah + A^f +|Ac|'' = 4sin" [sin- {a,, ) + sm-{a^ ) + sin' {a^ )] (12)

The sum of sin' terms on the right side expression is equal to two because of the

orthogonality of the vectors. The arcsine formula follows.

Interferometric measurements

Gisan [3] and later Cyr [4] elaborated on the interferometric measurement method. Most

of what is given in this section is a paraphrasing of the work of Cyr.

From Chamberlain [5], equation 4.17 gives the formula for the measured interferogram

for a polychromatic source, S((jo). It is the sum of a constant and the Fourier cosine

transform of the source.

The measurement of a single interferogram involves setting up a polarizer at the input

and one at the output and usually uses a broadband source. The field that emerges from

the output can be considered as {a))P{PpQi^ )T{co)jj^ , where P(9pol) is the output

polarizer with setting GpoL, T is the Jones transfer matrix, and So(co) is the power

spectrum before the output polarizer. The output spectrum going into the inteferometer,

S(co), is equal to this vector multiplied with its transpose complex conjugate. The nonconstant part of the interferogram, R(t), and this spectrum are related as:

R(t) = ^— ^S(co)cos{coT)dco and S[co) = ^— ^R{T)cos{coT)dT (13)

71

Invoking the Parseval theorum, it can be shown that (13) is equivalent to:

'dS[(o)

'

^S-[co)dQ) = 2\R-(T)dT and

dco

dcD= jr'i?'(r)i/r (14)

The spectrum out of the polarizer can also be written as the sum of the input and a normalized Stokes vector product, of the output Stokes vector and the Stokes vector

associated with polarizer, x{co) = s{co)»Sf'POL

71
S{a,) = ^

[^0 (^) + {co)] = i [S, (co) + A-(^)^„ {co)] (15)

The definition of So(co) and Sx(co) allow the definition of their Fourier cosine tranform

pairs, Ro(t) and R.x(t).

Rotation of the output polarizer to 90° from initial results in a new set of variables

designated here with subscript _L as x_l(co), R±(t), Si(ca), Sxi(co), and Rx±(x). Because

xi(co) = - x(co):

i(fi(r) + RAr)) = R,M \ («(r) - (r)) = (r) (16)

The RMS relationships of these function are given as the following, where the expected

value operator is with respect to random input and output polarizer settings, which can be

assured by polarization scrambling:

r I ll(dSico)\-

)dco

0 ,

- 0 (17)

• , dSico) dxico) ^ /X I ^dSAco) , , , From equation 1 5, we have — = —^

^q\<^) + xyx)

)

— , which leads

dco dco dco

to:

dSA(o)

dco

dx(co)\\ \ — , ^/ / ^dx(co)\ / \dSAco) i JdS^(coY ^

)S,[co)+2{x[co)^-^)S,[co)——+ [coy

dco dco dco dco

(18)

A consideration of the random relationship of the polarizer to the output Stokes vector

lead to (19) and then to (20), which is a simplification of (18):

l(dx(co)\ ds{co)

\l dco ) n dco

and (S;(^y)) = ^5o'(^^) (19)

'dSAco)\'\ 1 dco

CO, ds{co)

dco +

dS^ {co)

dco

(20)

The relationship to the DGD is based on the following:

Q(<u) = Ar((i»)p((i)) and

-^^^^ = D{co)y. s{q)) ^ 1!^t{co\p{co)>< s{co)) = u{co)lS,T{co)?,m0

dco

(21a)

(21b)

72
where 9 is the angle between the PSP and output Stokes vector, and u is a vector

orthogonal to both.

The random relative orientations of the PSP and output Stokes vector lead to the expected

value relationship:

The square root of (23) is the spectrally (squared) weighted RMS of the DGD, which is one way to represent the PMD. Simulations have shown that the statistics resulting from

this method are equivalent to those from using the JME in the case of random mode

coupling, where Ax is a random variable that is independent of frequency.

One of the key results of [4] is the segregation of the autocorrelation function from the

cross-correlation functions that is done with the orthogonal polarizer setting and (16).

Another is the linkage to DGD that is seen in (22).

Fixed analyzer method (FFT)

This method is conceptually very similar to the interferometric method. Instead of a broadband source, however, wavelength range is sampled discretely using a monochromator or tunable filter. The spectra, Sof(<jo) and Sf(co), before and after the

output polarizer are measured at discrete frequencies for a fixed output polarizer

(analyzer) setting. The resulting ratios, SF(ft))/SoF(o)), are Fourier transformed. The power

spectrum of the Fourier transform is analyzed to report an RMS value that would possibly

be present if the autocorrelation function were not included. The Fourier transfonn is normally done after multiplying the ratio with a windowing function, W(co), to reduce the

ringing associated with a finite sampling window. For this method, the spectrum before

the output polarizer is nominally constant.

If, instead of doing a Fourier transform, a Fourier cosine transform were done, the result

would be equivalent to the interferometric method using a source that is constant across

the frequency range. The multiplication with the windowing function plays the same role

as the shape of the power spectrum, So(co), for the interferometric method.

Make the following associations to the functions defined in the interferometric method:

(22)

The results of (17), (18), (20), and (22) are combined to yield:

(23)

0

73
S, (o)) = W(co) and S(co) = W(co)Si. S,^ (o)) (24)

Application of the Fourier cosine transform will yield the associated Ro(t) and R(i)

functions - although these are discrete vs. optical. Making the measurement with the

polarizer at a 90° offset will allow the additions and subtractions defined in the

interferometric method to remove the autocorrelation function. After this, the equations

defining ao and Qx can be applied to obtain the result from (23).

Conclusions

The results leading up to (9) are key to understanding the main PMD measurement

methods and the relationship to system impairments. Methods such as the phase shift

method are also easily understood in this context.

Understanding the results of [4] are key to understanding the linkage between frequency

domain methods and time domain methods.

The combination of a modified form of (9) and (10) can be used to characterize second

order PMD using matrix that maps not only to T(coo+Aco) but also back, to T(coo-Aco), by

using a quadratic function for Y(o3) and, for example, linear functions for Op and ^ip.

Given a measured or simulated pdv function, all orders can be investigated with a

solution for T(co) derived from (7).

References

[1] Poole, CD. and Wagner, R.E., "Phenomenological approach to polarization

dispersion in long single-mode fibres". Electron. Lett. 22( 1 9), 1 029- 1 030 ( 1 986)

[2] Jones, R.C., "A new calculus for the treatment of optical systems. VI:

Experimental determination of the matrix", J. Opt. Soc. Am. 37, 110-112 (1947)

[3] Gisin, N., Passy, R., and Von der Weid, J. P., "Defmitions and measurements of

polarization mode dispersion: Interferometric versus fixed analyzer methods", J. Lightwave Tech. 6, 730-732 (1994)

[4] Cyr, N., "Polarization-mode dispersion measurement: generalization of the

interferometric method to any coupling regime", J. Lightwave Tech., 22 (3), 794-

805 (2004)

[5] J. Chamberlain, The principles of interferometric spectroscopy, John Wiley &

'

Sons, 1979.

74
PMD measurements using Fixed-Analyzer technique on a 1000km

ampliHed link that includes ROADMs

Vincent Lecoeuche, Fabien Sauron and Andre Champavere

JDSU Commtest Division, Saint-Etienne, France

Alexey Turukhin and Enrico Gonzales

JDSU Optical Network Research, Eatontown, New Jersey

Abstract : We report PMD measurements, performed on an amplified network with Reconfigurable

Optical Add Drop Multiplexers (ROADMs) and a total fiber length exceeding 1000km, using a commercially available setup based on the Fixed Analyzer Fourier Transform (FA-FT) technique. We

have implemented straightforward numerical filtering that removes the inter-channel spectral data as well as dropped channels and demonstrated the improvement brought to the SNR. We show that this enhancement would allow one to maintain reliable PMD readings in Agile Optical Networks with

transmission pass-bands affected by ROADMs.

Introduction

Measurement of the polarization mode dispersion of Agile Optical Network represents new challenges

for Test & Measurement equipments. Not only do the signal paths depend on wavelength and time, but

the wavelength span available for actually performing the PMD measurement into channels may get so limited that measurement accuracy is compromised. This latter fiindamental limitation, associated with

the statistical nature of PMD, applies to any of the available measurement techniques [1]. Some

methods would indeed allow a precise measurement of the instantaneous DGD at the exact channel

wavelength, but this information alone is not really valuable as the DGD varies over time, even with

relatively short spans of buried cables [2]. In this context, we focused our attention on the only truly

representative parameter that can be measured, i.e. the most stable over time, which is the average

DGD over the available wavelength span. We have conducted laboratory experiments on an amplified network that includes commercial

ROADMs, using the FA-FT technique implemented into our field test mainframe. We showed that this method provides a meaningful reading of the mean DGD over the available wavelength span, provided

that a sufficient number of channels are passing through the link under test. In section 1, we will first briefly describe our experimental setup. In section 2, we will describe our

experimental results, observed both on a "regular" amplified link and one that does include ROADMs.

1 - Experimental arrangement

The FA-FT technique is a well documented measurement tool and JDSU's implementafion of it strictly follows the recommendations of the standards on PMD measurements [3]. The measurement

block diagram is shown in Figure 1. A broadband polarised source (BBS) is launched into the Device

Under Test (DUT), and the transmitted signal is analysed spectrally through a polarizer, integrated into

the OSA and commutable. Manual polarisation controllers are added before and after the DUT in order to improve measurement accuracy. Note that a reference spectrum is first performed without the polarizer, and the signal through the

polarizer is normalized to this reference signal before perfonning the FFT, so that the FA-FT is intrinsically immune to any source/amplifier spectral shape features. This direct normalization

procedure in the frequency domain is mathematically equivalent to other means that have been

proposed in order to compensate for the corresponding auto-correlation peak in the time domain, as observed on interferometric setups. We will see hereafter that the direct access to the spectral data also

allows the analysis of the ROADM passing channels information, yielding an improvement on the

measurement accuracy.

75
BBS ooo —+—^

ooo

Figure 1 : Measurement equipment

The link under test is described in Figure 2. This C-band link is fully functional and normally

populated with 40 WDM channels with lOOGHz spacing, modulated at lOGb/s using NRZ fonnat. The

Multi-port Wavelength Switches, used as ROADMs have been configured to allow traffic for all channels through the express port. Measurements have been performed both on the whole link, i.e. from launch point A to detection point C, including the 3 ROADMs to imitate a fragment of AON, and

from point B to C to consider the case of a regular point-to-point amplified network.

r

5 dB

Att.

80 km SMF

DCF i

^2 O) ^ 80 km SMF

Att.

80 km SMF

"S

80 km SMF V

DCF DCF

Att.

75 km SMF

1

"'^'^ 1 5 dB c

Figure 2 : Link under test, including 3 ROADMs (MWS 1X6+2), 14 EDFAs (OA3500), 940kms of SMF

and 140kms of Dispersion Compensating Fibre

2 - Measurements

a) PMD measurements on a regular amplified link The first test was performed on the link from point B to C. The value of the mean DGD, averaged over 50 successive measurements while manually changing the input/output polarisation launch conditions

between successive scans, was found to be 1.32ps with a standard deviation of 0.1 6ps. Figure 3-a)

shows the measurement distribution, while Figure 3-b) shows a typical Fourier transform.

76
a) b)

Figure 3 : a) Histogram of the 50 measurements recorded wliile changing input/output polarisation launch

conditions and b) typical FFT measured from point B to C.

For comparison, we used a commercial bench top instrument based on the Jones Matrix Eigenanalysis

(JME) method. The use of a JME on WDM amplified network brings some difficulties associated with

transmitting a single test channel, which is using the total power of the amplifiers. To avoid nonlinear

issues, the amplifiers must somehow be loaded with an additional signal. To do so we used a broadband ASE source combined with the tunable laser. While the degree of polarisation at the output

of the network was of only 10%, which is low to conduct reliable polarimetric measurements, the JME

instrument was robust enough to take a reading. We ran 3 successive measurements without changing

the launch conditions (the JME method being mostly insensitive to these changes), and the mean DGD

over 1527-1563 band was found to be 1.33, 1.23 and 1.38 ps.

b) PMD measurements through a ROADM

The second experiment was perfonued on the total link represented in figure 2 from point A to C. The

3 ROADMs are set to let all the channels pass through. The spectrum of the BBS transmitted by the network is shown in Figure 4.

1 , r , 1 1

1 1 1 1 1 11 1 1 1 1530 1540 1550 1580 Wavelength (nm)

Figure 4 : spectral transmission of the BBS from A to C. with ROADMs set in passing mode for all 45

channels.

The mean DGD was found to be 1 .63ps, averaged over 50 successive measurements while manually

changing the input/output polarisation launch conditions between successive scans, with a standard

deviation of 0.1 8ps. With the JME setup, we have obtained a mean DGD of 1.71ps after manually

removing the inter-channel DGD points that were totally wrong as one could expect. Figure 5-a)

shows the measurement distribution, and Figure 5-b) shows a typical Fourier transform.

We would like to emphasize that our standard test tool without any modification was able to provide

reliable PMD measurements despite the peculiar shape of the transmission associated with ROADM's pass-bands. However, we must consider that conditions were quite favourable, and in particular the value of the PMD of the link was quite advantageous here. A close look at Figure 5-b) reveals the

77
presence of signals at several haraionics of lOps which directly arises from the 100 GHz spacing of

the ROADM channels. If the PMD of the link had been slightly higher with components close or above lOps, these perturbations would have been integrated into the width calculation and the result would probably have been overestimated. The signal to noise ratio would also decrease if some

channels are dropped on the line or if the output Degree Of Polarization decreases.

0,0 OS Measured PMD (ps)

1339 \SSi IS40 1S4S 13» 15SS IS60 Wavelwigth fnm)

Delay (ps)

"1 tW

Figure 5 : a) Histogram of the 50 measurements recorded while changing input/output polarisation launch

conditions, b) typical FFT measured from point A to C. c) Nonnalized spectrum through polarizer (dotted

line), with inter-channel filtering applied (solid line), d) zoom on the ROADM perturbations in the FFT

(dotted line) removed with the inter-channel filtering applied (solid line).

As the FA-FT does a reference signal similar to the one shown in Figure 4) at every scan, we can use

this information to locate the passing channels and simply remove the inter-channel signals. Channels

are on the ITU grid, and we have located the passing bands with a power drop criteria of 0.3dB

compared to the channel centre value. The second step is to replace the inter-channel signals with

straight lines as shown in Figure 5-c). By doing so the lOps component and its harmonics are clearly

removed on the FFT as can be seen in Figure 5-d). Without fdtering, the reading on this scan was

1.54ps, with fdtering, the reading was 1.48ps, confimiing that the ROADM influence on our measurement remains minor in this experiment.

Conclusions

We have demonstrated PMD measurement capability on an amplified network using the FA-FT

technique. We showed that for this particular setup the measurements were not significantly affected

by the presence of ROADMs in-line, and we proposed a software enhancement that would allow one

to maintain valuable readings in more critical conditions. While this method is not meant to replace

polarimetric methods that allow high-resolution DGD readings, it provides a simple, cost effective and

field dedicated alternative when a sufficient number of passing channels are available.

References:

[ 1 ] Gisin et al, IEEE Photonics Technology Letters 8, n° 1 2, p. 1 67 1 ( 1 996)

[2] Allen et al. Journal of Lightwave Technology 21, n°l, p. 79 (2003)

[3] see for example lEC 60793-1-48, IEC61280-4-4. ITU-T G.650.2 or TIA 455-1 1

3

78
Chromatic dispersion measurement technique utilizing

an unstable supercontinuum pulse source

Sucbei Moon and Dug Y. Kim

Gwangju Institute of Science Technology (GIST)

Buk-gu, Gwangju 500-712 Republic of Korea

dykim@gist.ac.kr

Abstract: We report a time-domain chromatic dispersion measurement scheme that can analyze a very

short (down to 10 meters) optical fiber by using a supercontinuum pulse source. An ultra-wideband

supercontinuum pulse source that spans the entire range of the communication wavelength bands from

1,250 nm to 1,700 nm were generated by using an amplified diode-laser pulse source combined with a dispersion-shifted fiber. Wavelength dependent temporal delays for a spectrum-sliced superconfinuum

pulses were measured after transmitting through a sample fiber with a differential reflectometric configuration. A monochromator was used to do spectrum slicing, and a high-speed photodetector with an

oscilloscope were used to measure temporal delays. In order to increase the measurement precision, the measurement system used reflecting configuration and a differential measurement technique. This

enabled us to measure the dispersion of a 10-m-long single-mode fiber with good precision.

1. Introduction

Chromadc dispersion (CD) or group-velocity dispersion (GVD) is a key characteristic of a single-mode

fiber. In fiber-optic communications, it determines the transmission capacity and should be carefully

measured before the fiber is installed. In nonlinear-optic applications such as supercontinuum generation

and parametric amplification, zero-GVD wavelength is a crucial parameter that determines the efficiency

of the process. Time-of-flight measurements and modulation phase-shift methods are standardized and

commonly used for evaluating fibers longer than hundreds of meters.[l] The measurement wavelength

ranges for these methods are normally limited within 100 nanometers around C+L-band due to the limited

availability of suitable light sources outside this band. In analyzing specialty fibers like photonic-crystal

fibers (PCFs) where short fibers need to be measured outside the band, interferometric methods have been

usually used with white light sources.[l] However, those methods are not able to measure fibers longer

than a few meters because it is hard to construct a long reference ami in an interferometer. Thus the fiber

should be cut in order to be analyzed.

Supercontinuum (SC) generation is a powerful method to obtain high-power ultra-wideband pulses.

As the output spectrum of an SC source can cover the entire wavelength range of single mode fiber's (SMF) applications, time-of-flight measurement technique with a pulsed SC source has been studied so

far for wide-band capability and simplicity in dispersion measurements. [2] However, this technique

traditionally has been considered to be less precise because the relative temporal delay of a pulse is

usually hard to be determined when the required precision is much less than the measured pulse-width of

an output pulse. In principle, the temporal precision of the time-of-flight measurement method can be

much better than the output pulse-width because it does not have to resolve pulses but only measures the

pulse position. However, many superconfinuum sources generate pulses with randomly varying pulse

79
shape and unstable pulse position in time-domain when they are spectrally sliced. This problem has made

the GVD measurement schemes based on supercontinuum sources difficult to be utilized in the real-world

applications.

hi this paper, we report on an ultra-wideband time-of-flight measurement scheme that can measure the GVD of a fiber with high precision and accuracy even with an relatively unstable SC source. Because the spectrum of the supercontinuum source used in the experiment covered the entire transmission band of an

ordinary single-mode fiber, the scheme could measure the full GVD characteristics. Well-known

differential measurement technique was used, in which difference in temporal delay between two

different wavelengths is detected simultaneously. This alleviates the problem of the group velocity

variations induced by possible temperature changes. In addition, reflectometric configuration was used in

order to minimize effects induced by the instability of an SC source. As in OTDRs, back-reflected pulses

from both ends of a sample fiber are detected by a high-speed photodetector. And measured temporal

delay between these two pulses in arrival time coiTcsponds to the group delay of the sample fiber at the

wavelength. So, absolute accuracy can be obtained without calibration. Furthennore, this temporal delay

is independent with the stability of an SC source. As the standard deviation of this differential temporal

delay was only 0.35 ps in our measurement, even a 10 m long single-mode fiber sample can be measured

successfully. It proves that this measurement scheme exhibits very high accuracy such that it can be used

for the GVD measurement of a very short length fiber sample, which is considered to be measurable only

by an interferometric measurement method.

Fig. 1 . Schematic diagram of the supercontinuum source based on an ADP source and

an HN-DSF. The inset graph shows the output spectrum.

2. Supercontinuum

We had demonstrated a supercontinuum generation method based on an amplified diode-laser pulse

(ADP) source and a dispersion-shifted fiber in our previous report. [3] The original pulse was generated

by gain-switching a distributed-feedback laser diode (DFB-LD) at 1,550-nm wavelength and was

amplified by 3 concatenated erbium-doped fiber amplifiers (EDFAs) in order to obtain a high peak power. The amplified high-power pulse was launched into a dispersion-shifted fiber (DSF) for superconfinuum

generation. The same kind of the supercontinuum source had been used in this experiment. In order to get

higher efficiency, a highly-nonlinear DSF (HN-DSF) was used as a supercontinuum fiber inside the

80
source. Fig. 1 shows the schematic diagram of the supercontinuum source based on the ADP source and

the HN-DSF. The inset graph shows the output spectrum that covers the entire communication band from

1,200 to 1.700 nm.

Since the pulse was generated by an electric triggering signal, the pulse repetition rate can be

controlled freely in this source. The repetition rate was set to be 12.00 MHz in our experiments. The

spectral densities were above -13 dBm/nm or 50 |a W/nm for a wavelength band between 1,250 nm and

1,700 nm. The pulse-width of the spectral slice with 5-nm-bandwidth was about 200 ps. Therefore, the estimated peak power is over 100 mW for every 5-nm-bandwidth pulse. Significant pulse-to-pulse power

and shape variations had been obsei-ved. Because the supercontinuum generation process was based on

the cascaded effects of the S^'-order nonlinearity, small changes in the pump pulse properties such as power, shape and polarization state would result in significant variations of the output. This SC source is composed of the low-cost fiber-optic components only and is easy to be constmcted and maintained.

Trigger

SC

Monochromater

PD-TIA

BPF

PD

Oscilloscope

Gap -3tATO)

AT(/.„)

Fig. 2. Experimental setup of GVD measurement.

3. Measurement setup

We have built a wavelength-dependent group delay (GD) measurement system based on a differential

reflectometric configuration. Fig. 2 shows the experimental setup of our GD measurement system. Ultra- wideband pulses from the SC source were stretched in time-domain by a 200 m single-mode fiber (SMF)

in order to reduce the peak power and minimize possible nonlinear effects in the following fibers. After a 1x4 fiber-optic splitter, the pulse was launched into a fiber under test (FUT). A signitlcant amount of

back-reflection was made at the input end of the FUT after making a slight gap between two fiber

connectors used for the FUT. Another 4% back-reflection was made naturally at the other cleaved end of

the FUT. Lights from these two reflected points were split into four parts by the fiber splitter. In one

channel of the splitter, the reflected lights were spectrum-sliced by a monochromator with a 5-nm

bandwidth. The monochromator produced ~20-dB insertion loss. And an InGaAS p-i-n photodetector plus

transimpedance amplifier (PIN-TIA) converted the spectrum-sliced optical pulses to electric signals. This

signal was acquired by Ch A of an oscilloscope. In another channel of the 1x4 fiber splitter, the reflected wideband pulse from the FUT were also spectrum-sliced by a fixed band-pass filter whose center

wavelength and bandwidth were 1,550 nm and 2 nm, respectively. A p-i-n photodetector and Ch B of the

81
oscilloscope were used to measure these spectrum-sliced pulses. This channel works as a reference for

our GD measurement system. The bandwidth of the photodetection system used in our experiment was ~2

GHz. By changing the center wavelength of the monochromator, GDs were measured as a function of

wavelengths.

4. Results and discussion

Fig. 3 shows a typical set of oscilloscope traces captured simultaneously by Ch A and Ch B in an

equivalent-time sampling mode. Ch A detected the pulses spectrum-sliced by the monochromator with

center wavelength of 1,500 nm while Ch B did those of the reference at 1,550 nm. Each trace has 32,768

data points in a 50-ns interval after averaging 64 waveforms. In order to increase the signal-to-noise ratio (SNR), the acquired data were smoothened by adjacent averaging (N = 128), which con-esponds to digital

low-pass filtering. A 10-m-long single-mode fiber was used as an FUT. The position of a pulse was

determined as the center of two half maximum points within the pulse duration. The first pulses on the

left-hand side of a trace are due to the reflected light from the connector gap, and the second pulses on the

right-hand side are due to the reflected light from the cleaved fiber end. Hence, the group delays are the

time differences between the first and second pulses. The differential group delay is the difference between the group delays of Ch A and Ch B.

^ 0.2 Q.E 0.0 <^ 0.8

=! 0.6

.to. O) 0.4 a3 0.2

.*-* Q. 0.0 E< -0.2

i rh A

29 Time [ns]

Ch B

Time [ns]

Fig. 3. Measured oscilloscope traces for X = 1,500 nm (Ch A) and A,n = 1,550 nm (Ch B). The data were smoothened by adjacent averaging (N=128) for a better SNR.

When we have repeatedly measured in the same condition, we have obtained a standard deviation of

0.35 ps for differential group delay. Note that the pulse generation period exceeded the time range of the

measurement. Thus the actual absolute delay for 1,550 nm was the sum of the observed delay (13.238 ns)

and a period of the pulse generadon (83.333 ns) and was calculated to be 96.571 ns. The delay

corresponded to 19.72 meters, assuming the group index at the wavelength is 1.4682. Thus the fiber

length was measured to be the half i.e. 9.86 meters. The standard deviation of the length determination was 0.015 mm i.e. 1.5 ppm. These results demonstrate that relative group delays as well as absolute

temporal delays could be measured precisely with this scheme.

GVD of a 10-m-long single-mode fiber that complies ITU-T G.652 standard (Coming SMF-28TM) was

measured. Only 9 group delays were acquired with a ~50 nm wavelength interval. Fig. 4 shows these 9

82
measured group delays, their 3"^-order polynomial fitting curve. The calculated GVD from the fitting curve is also plotted on the same graph. The GVD measurement was repeated twice. The measured

chromatic dispersion and slope at 1,550 nm wavelength were 0.3094 ps/nm and 0.0016 ps/nm",

respectively in the first measurement. In the second measurement, they were 0.3112 ps/nm and 0.0015

ps/nm^, respectively. The deviation was only 0.6% for the measured chromatic dispersion values. The

GVD coefficient for this fiber was calculated to be 15.7 ps/nm.km at this wavelength. The zero-GVD

wavelengths were calculated to be 1,316 nm and 1,319 nm.

1300 1400 1500 1600

Wavelength [nm]

Fig. 4. Measured group delays (rectangular spots), their 3"'-order polynomial fit curve

(black line) and the calculated GVD from the curve (gray line).

5. Conclusion

A new time-domain GVD analysis method with a pulsed supercontinuum source was proposed and

demonstrated with a differential reflectometric-configuration for better accuracy and precision. The full

dispersion characteristic of a sample fiber in the entire transmission band of an SMF from 1,250 to 1,700 nm was analyzed with a single pulsed source. The differential reflectometric configuration guarantees the

accuracy of the measurement as well as the precision. A fiber as short as 10 meters could be analyzed

successfully with the scheme. This method must be useful in characterizing short-to-medium-length fibers

of which dispersion characteristics outside the C+L band are of prime interest.

References

[1] D. Derickson, Fiber Optic Test and Measurement, Prentice Hall PTR, 1998.

[2] K. Mori, T. Morioka and M. Saruwatari, "Group velocity dispersion measurement using supercontinuum

picosecond pulses generated in an optical fibre," Electronics Letters, vol. 29, 987 - 989 (1993).

[3] S. Moon and D. Y. Kim, "Generation of octave-spanning supercontinuum with 1550-nm amplified diode-laser

pulses and a dispersion-shifted fiber," Optics Express, vol. 14, 270-278 (2006).

83
Accurate Measurement of Chromatic Dispersion

Distribution along Dispersion Managed

Transmission System

Mirza IMRAN Baig* Tohru MIYAUCHI and Masaaki IMAI**

Dept. of Electrical & Electronic Engineering, Muroran Institute of Technology

27-1 Mizumoto-cho, Muroran, Hokkaido, 050-8585 Japan

sl261027@.mmm.muroran-it.ac.ip .* mimai@mmm.muroran-it.ac.ip **

Abstract: In this paper, a nondestructive

measurement technique, which uses a four

wavelength bidirectional optical time domain

reflectometer (OTDR) to measure the chromatic

dispersion distribution along a dispersion managed

transmission system, has been presented. A new

formulation has been developed to enhance the

correction factor, which leads to an improvement in the measurement results of a pre-existing method

and the obtained experimental results are in good

agreement with those obtained by the other techniques.

I. INTRODUCTION

The advent of optical repeaters based on erbium-doped

fiber amplifiers (EDFAs) has opened the new era of

optical transmission technologies, allowing us to use wavelength-division-multiplexing (WDM

)

technologies with simple, compact, and economical

approaches. Infact, the demonstrated capacity for long-haul optical transmission has been growing

remarkably, and more than a thousand fold increase in capacity has been achieved. The price we have to pay

for such success is to combat with the accumulated

impact of fiber nonlinearity, interplay with the chromatic dispersion (CD) of the transmission tlber, which grows with the transmission distance and

therefore becomes significant for ultra-long-haul

system. Dispersion management technologies have

been invented to overcome such inherent problems in optically amplified transmission systems [1].

Chromatic dispersion results from the difference in propagation velocity associated with different wavelengths of light traveling down the fiber. In a transmission system, it gives rise to pulse broadening.

The need for the distributed measurement of chromatic

dispersion along an optical fiber transmission system comes from the increasing diffusion of wavelength

division multiplexing (WDM) transmission systems.

Nonlinear effects, such as four wave mixing or

cross -phase modulation seriously affect WDM

transmission. For this reason, not only an average

estimation of CD along the fiber is required, but a

distributed evaluation is also essential. The OTDR

based techniques have been established in these

respects [2], which are based on the analysis of OTDR

traces acquired bi-directionally at different wavelengths.

II. THEORETICAL BACKGROUND

A. Backscattered Power:

For an optical fiber supporting single-mode with a Gaussian field distribution the optical power P(?,z), of

backscattered light from an input pulse of power Po and

duration W launched at one end of optical fiber at time

t=0 can be expressed as [3], [4].

P{k,z) = P,aXzWB{Kz)j^o^\^2\Y{x)dx\^ (1)

where Po is the input power,

l^^is the pulse width,

as(z) is the local scattering coefficient, B(Xz) is the backscatter capture fraction, c is the speed of light, A^is the group refractive index,

y(x) is local attenuation coefficient

.

The backscattered signal has two components. The first

is an exponential decay due to scattering and

absorption in the glass and is insensitive to the fiber end from which the pulse is launched. When it is displayed on a semi -log scale, this decay with distance

is usually a straight line whose slope represents the

fiber attenuation in dB/km. The second signal

component is sensitive to local imperfections, which

result from changes in MFD and ^. This portion of

backscattered signal does depend on the launching end. The OTDR semi log trace is the sum of these two

contributions. A reliable means of separating the effects of optical power decay and waveguide imperfections

from OTDR signals has been reported in [3] and [5].

84
B. MFD Evaluation: can be evaluated as [2]

Si(z) and ^(L-z) are the backscattering traces read, respectively, from the origin, and from the end of the

fiber, which can be obtained as follows:

5,(A,.-) = 101og[/^(A,r)], (2)

(A, L~z) = 10 \og[p^ (A, L-z)], (3) By using above two equations, the waveguide

imperfection contribution I(?,z) can be expressed as

5,(A,--) + 5,(A,£-.-)

2

/(A,.-)= 101og[a (.-)fi(A,.-)]+fl . (5) where Og, which is a constant independent of distance z,

is expressed as

/(A,r) = - (4)

a, =51og(P„,P„J-10log exp jy(^)a^ (6)

B(?,z) is expressed in terms of refractive index of the core n and the mode field diameter 2W(?,z) as [2] A

5(A,_-): (7) 2 \27tnW{X,z]

The variation in the local scattering coefficient is negligible compared to that in the mode field diameter

2W(?,z) [2], [6]. Therefore, the imperfectbn

contribution In(?,z) normalized by the value at z = ^ is

/„ (A,r ) s /(A, r) - /(A, = 20 log

W{X,z)

(8)

When the mode field diameter 2W(?,^) at z = Zo is given, the mode field diameter distribution 2W(?,z) can be obtained as [6]

W {X,z)= W^(A,z )lO

' /„fA,2 20

(9)

C. MFD Evaluation with Correction Factor:

In the above section, by assuming the variation in the

scattering coefficient ^(z) along a fiber is negligible

compared to the variation in the MFD, the distributed MFD can be obtained from (9). However, when the

scattering coefficient changes greatly along a fiber, its length dependence must be taken into account. It has been considered a measurement procedure for the transmission line composed of different types of fiber, by taking into account the difference between the

scattering coefficients of each fiber.

When the scattering coefficient and the refractive index

change along the fiber are taken into account, the MFD

W{X,z) = W{X,z,)\0

20

(10)

Using this relation, correction factor K is rewritten as l+0.62A(z)|f 50-A(z) K=mLog

l + 0.62(zj (I 50-A(zJ

(11)

where A is the relative index difference of Si02 glass

in %.

D. Improved Correction Factor K:

The backscattered power received by the OTDR also influenced by the group refractive index of fiber under

test. Therefore, it is taken into account in literature [3] and [4] and can be witnessed from (1). Group refractive index can be expressed in terms of refractive index of

optical fiber as [7] dn N^n-X . (12)

dX

where n is the refractive index of fiber and ? is the operating wavelength.

By neglecting the second term on the right hand side of

(12), it can be assumed that the group refractive index

is nearly equal to the refractive index of fiber. Therefore, (8) yields

/ {A,z) = 101og + 20 log

W{X,z^

W{X,z)

(13;

and finally, the correction factor A' has to be altered as

[8,9]

K = \QLog

I+0.62/!(z) |[ 50-A(r) |[50-A(z)|^

l+0.62^z„)J[50-A(z )J |50-/^z

(14)

Thus, we have improved the correction factor A, which

then affect on the measurements of MFD and

consequently the measurements of chromatic

dispersion.

E. Chromatic Dispersion Evaluation:

The chromatic dispersion s is the sum of the material

dispersion s^ and waveguide dispersion s„, given by

Where s,„ and s^ are expressed as [7], [10]

(15)

85
a =

c dX'

2n cn dX

d r J_

(16)

(17)

The material dispersion %, can estimated from the dopant concentration in an optical fiber by using

Sellmeier's relation [11]. On the other hand, the waveguide dispersion can be estimated by

determining the wavelength dependence of the mode

field diameter. The empirical relationship between the mode field diameter 2 W and. the normalized frequency

Fhas been reported by Marcuse as [12]. W

a

(18)

(19)

W (X Y (X

where a is the core radius and ?c is the cutoff wavelength. Here, wavelength dependence of mode

field diameter 2 W can be approximated as W{X,z) = g^ (/)+ (z)A- ' + (z)A- (20)

Equation (20) has been used to approximate the

distribution of the derivative of MFD in 07) at the wavelengths of interest. By substituting (20) into (17), the waveguide

dispersion s „ can be obtained from

X

2jc'cnW'{X,z)

X 1- 2A

pf(a, |g,(_-)A-+6g,(_-)A^

(21)

Here, the coefficients, go(z), gi(z) and g2(z) are determined by solving (20) with the least squares method, when mode field diameters are given at more

than three wavelengths [9].

III. EXPERIMENTAL RESULTS

A. Bidirectional Measurement:

Two OTDR traces have been acquired bi-directionally.

The pulse width and averaging time were LO^is and 20

minutes per wavelength, respectively, during the experiment and all four available wavelengths of the equipment were used. The OTDR took 50,000 samples

for averaging with 1 meter resolution.

Four wavelengths of 1.31, 1.45, 1.55 and 1.625^m, were available for measuring the backscattered signal

power with the Anritsu MW9076 OTDR. The

experiment has been performed around a fusion-spliced

optical transmission link. The transmission link composed of two different kinds of optical fibers, i.e. 2, 15 and 10km long single mode fiber (SMF) and 20km

long Reverse Dispersion Fiber (RDF). The

specifications of the fibers used in our experiment are

listed in Table 1. TABL&I

FIBER SPECIFICATION

Parameters

Fiber AFiber BFiber CFiber D

MFD @ 1.31 (im) 9.53 9.32 9.59 MFD @ 1.55 (im) 10.69 10.35 10.71 5.59 CD (a), \ 55\im (ps/nm/km) 19.5 19.1 19.7 -26.7

Fiber Length (km) 2 15 10 20

B. Mode Field Diameter Measurement:

Fig.l shows experimental results for the mode field diameter distribution of the dispersion managed

transmission line. Here, in order to normalize the imperfection contribution, a 2km single mode fiber had

connected in front of the link under test. The value of

MFD at the reference point i.e. after 2km long

reference fiber, on the available set of wavelengths,

should be known in advance. Therefore, the reference

fiber shall be fully characterized in terms of MFD by one of the standard methods [13], e.g., the far field pattern (FFP) technique or variable aperture far field (VAFF) technique. A comparable results are presented

in Fig.l, with previous and our proposed new

correction factor K. The difference between two results can be observed from the expanded view. Black line shows results with new correction factor and pink line

Fig.l Measured mode field diameter distribution at 1550nm.

86
shows results with old correction factor. An average MFD measurement can be observed from Table II.

C. Chromatic Dispersion Measurement:

The measurements of chromatic dispersion distribution are shown in Fig.2. These results are calculated from

the results of MFD measurements depicted in Fig.l. Here, the results are also presented with previous and

our newly proposed correction factor K. The two

different colors of line are separating the two results. Black line shows results obtained from new correction

factor and pink line shows results with old correction

factor. A clear difference between two results is evident through an expanded view shown in Fig.2. An average CD measurement is depicted in Table II.

I

-20 o L -30 '

Distance (km)

Fig.2 Measured chromatic dispersion distribution at

1550nm.

TABL&n

AVERAGE MEASUREMENT

Fibers MFD

1.55|im

(urn)

CD 1.55 ^im (ps/nm/km)

Fiber Length

(km)

A

OldK.

10.6534 19.6653 NewK

1065454 19 6451 2

B

Old K

10.25473 19 1597 New K

10.25582 19.1382 15

C

Old K

10.65177 19.7872 NewK

10.65291 19.767 10

D

OldK

5.000725 -26.4762 NewK

5.001254 -26.5721 20

Accumulated

Old K

-4.9259 CD

New K

-7.4088 47

In this table the accumulated CD is defined as a sum of

the dispersion occurred at each of the fiber sections A,

B, C and D that construct the dispersion managed

system. Her, it is noted that the combination of fibers used in our experiment made not exactly zero but almost zero accumulated dispersion.

IV. CONCLUSION

The described, OTDR based nondestructive technique

with our proposed enhanced correction factor clearly

improved the measurement of the chromatic dispersion

distribution along the optical fiber transmission system.

The technique was compared with the previously

established technique with old correction factor K. The

results are in good agreement with the fibers specifications shown in table - 1. ACKNOWLEDGEMENT

The authors would like to thank Dr. I. Morita of KDDI

R&D laboratories Inc. for providing us the fiber samples used in their experiments. They are also

grateful to Mr. Y. Hinata of A-Tic company for his assistance in splicing of different kinds of fiber.

REFERENCES

[1] M. Suzuki and N. Edagawa, J. ofLightwave Technology,

vol. 21, no. 4, April 2003.

[2] K.Nakajima, M.Ohashi and M.Tateda, J. of Lightwave

Technology, vol. 15, No.7, pp. 1095- 11 01,July 1997.

[3] M.S. O'SuUivan and R.S. Lowe, "Interpretation of SM fiber OTDR signatures," Proc. SP/E'86 vol. 661,

Optical Testing and Metrology (1986), pp 171-176.

[4] Luc B. Jeunhomme, "Single -Mode Fiber Optics

Principles and Applications, " optical engineering/vol.

23, second edition,pp. 187.

[5] P. Di. Vita and U. Rossi, Electron. Lett., vol. 15, pp. 467-469, July, 1979.

[6] M. Ohashi and M. Tateda, Electron. Lett., vol. 29, pp. 426-427, 1993.

[7] John M. Senior, "Optical fiber communications

principles andpractice, " Prentice-Hall, 1985.

[8] A. Rossaro, M. Schiano, T. Tambosso, and Davide

D'Alessandro, IEEE J. on Select. Topics in Quantum

Electronics, vol. 7, no.3, pp.475-483, May/June 2001.

[9] M. Imran Baig, T. Miyauchi and M. Imai, Proc. of WFOPC2005, IV.2, pp. 146-151, June 2005.

[10] M. Imran Baig, T. Miyauchi, M. Imai and S. Sato, Optical Engineering, 2006 (to be published).

[1 1] J.W. Fleming, Electron. Lett., vol. 14, no. 1 1, May 1978.

[12] D. Marcuse, Bell System Technical Journal, vol. 56, pp. 703-718, 1977.

[13] "ITU-T Contribution G.650," Dec. 1997. Definition and

test methods for the relevant parameters of SM fibers.

87
Assessment of the potential accuracy of a four wavelength OTDR

to measure Chromatic Dispersion.

Abstract: Modem multi-wavelength Optical Time Domain Reflectometers (OTDRs)

are capable of measuring chromatic dispersion of installed links. This paper describes

the sources of uncertainty associated with this measurement along with techniques to assess each contribution. The potential accuracy based on the assessment of one four

wavelength OTDR is calculated.

Introduction

OTDRs can be used to measure the total chromatic dispersion of a fibre link. The technique is a bi- directional version of the Spectral Group Delay in the Time Domain technique'^ and relies on measuring

the variation in the time of flight of the optical fibre with wavelength. In the case of OTDRs the distance

to a well-defined end reflection is measured at a number of wavelengths (typically four). These distances

are converted back to time of flight, i(A.i), for each of the discrete measurement wavelengths, X,. These

time of flight data points are fitted to a time delay model and the resulting model parameters are used to calculate the chromatic dispersion. In this paper only three term models have been considered, the three- term Sellmeier equation, and the three-term quadratic equation,

xCk) = a_2')C~ +a2X' and t(/V) = a^ + a,?^ -i- a^X,' , (1) & (2)

where ai are the fitted parameters. The accuracy of the chromatic dispersion depends on accurate measurements and accurate fibre time delay models. The critical measurement is the change of time delay

for the different wavelengths. This depends on the ability to repeatably locate a well-defined end

reflection while ensuring that there is no zero offset between the different laser sources. The latter is only

important if the OTDR does not perfonn a cut back measurement. It is also necessary to know the wavelength of each laser source and less critically the overall distance scale uncertainty.

Assessment of OTDR

The source wavelengths can be measured using a calibrated optical spectrum analyser (OSA). It is necessary to operate the lasers in their normal pulsed mode since a continuous wave mode can change the operating temperature and thus wavelength. This means that care must be taken not to overload the OSA

even though the average power may be quite low. As a test a variable attenuator is inserted between the OTDR and OSA and the attenuation increased in steps until the total power measured on the OSA

decreases in sympathy. The uncertainty of the wavelength measurement is combined with the deviation of

the measure centre wavelengths from their internal OTDR software values to calculate the OTDR

wavelength uncertainty, gx. For the OTDR tested ox was found to be 0.75 nm although by adjusting the OTDR software values this could be reduced.

David Ives, Joan Hall and Christian Hart, Photonics Group

National Physical Laboratory, TWl 1 OLW, United Kingdom.

OTDR

FC/PC

Termini

Variable Delay Line

Figure 1 Experimental arrangement to assess the distance resolution and repeatability of the automated

event recognition software
The distance resolution and the repeatabiHty of the automated event recognition software were assessed

using the experimental arrangement in Figure 1. The OTDR was connected through a variable optical

delay to a 35 km fibre sample. Since the OTDR sample resolution was stated to be 50 mm the variable

optical delay was scanned in small, 5 mm, steps over a 100 mm range and the distance to the end

reflection recorded for each OTDR wavelength. The OTDR was operated in its automated chromatic

dispersion measurement mode allowing it to optimise the pulse width and event recognition features. The

OTDR used a 10 ns pulse and determined the position of the end reflection from the position of the peak

of the reflected signal. The OTDR measured distance was converted into time delay using the OTDR

assumed group index for each wavelength. The variable optical delay time was subtracted from the OTDR time delay for each measurement to leave the fixed time delay of the 35 km fibre. The standard

deviation was taken for each wavelength and combined with the uncertainty of the variable optical delay

to calculate the OTDR fime resolution, Otr. For the OTDR tested Gtr was found to better than 0.05 ns. The zero offset distance of the different wavelengths is critical to avoid biasing the chromatic dispersion

results. This was determined using the same set up of Figure 1 but by replacing the 35 km fibre with a 4 m patch lead. Again the OTDR distance to the end reflection was recorded, using the automated

chromatic dispersion software, for each wavelength while the optical delay was incremented in 5 mm

steps over a 1 00 mm range. The OTDR measured distance was converted into time delay using the OTDR

assumed group index for each wavelength and the variable optical delay time subtracted as before. The

standard deviation was taken for the whole data set, all the wavelengths together; to obtain the zero offset uncertainty, gzo- For the OTDR tested Ozo was found to be 0.5 ns. This uncertainty could be removed by

carrying out a cut back measurement and this would have reduced the uncertainty contribution to 0.05 ns due only to the short length repeatability, however the OTDR tested did not have this feature.

Figure 2 Assessment of OTDR distance scale Figure 3 Typical OTDR trace, deviation using a re-circulating delay line. The distance scale deviation affects the overall distance accuracy and thus the overall chromatic

dispersion accuracy. The distance scale deviation can be easily assessed using a re-circulating delay

line^^l The re-circulating delay line consists of a lead in fibre connected to a 50/50 coupler which is joined

back on itself by a loop fibre. The fourth port of the coupler is given a fiat end face to provide a welldefined reflection event, see Figure 2. The OTDR trace. Figure 3, shows a sequence of reflection events spaced at half the loop length. The distance to these events is fitted by linear regression to obtain the spacing, which is compared to the loop length to obtain the distance scale deviation. The distance scale deviation is combined with the uncertainty in its measurement to obtain distance scale uncertainty, ax- For the OTDR tested Cx was found to be 0.00003 m.m '. This is likely to be the least significant

uncertainty contribution to the measurement of chromatic dispersion.

Fitting Equations

The uncertainty due to the fitting equation can be estimated by comparing the chromatic dispersion

calculated firom a three term fit to time delay data at the four measurement wavelengths with chromatic

89
dispersion calculated from a high order fit to a larger time delay data set measured using the phase shift technique. The latter is taken as a true estimate of the chromatic dispersion of the fibre under test. The

best three term fits for calculating the chromatic dispersion in the two wavelength ranges and for five

fibre types are shown in the Table 1 (It should be noted that this is based on very limited measurements, one fibre of each type and is for guidance only. Further work is required.).

Fibre Type Zero Dispersion Chromatic Best three term fit for Wavelength (nm) Dispersion Chromatic Dispersion results

at 1550 nm in wavelength window (nm)

(ps.nm'.km') 1250- 1450 1450 - 1650

SMF28

TM

1313 16.8 Sellmeier Sellmeier

DSF 1547 0.3 Quadratic Quadratic

LEAF

TM

1575 . -2.9 Quadratic Quadratic

TRUEWAVE 1449 4.4 Sellmeier Quadratic

DCF -106.7 Quadratic

Table 1 Fibre type vs best three term fit. This is the best three term fit for calcu ating the chromatic

dispersion in the range shown using all four wavelength points.

Q

<

0

-

-0.5 SMF 2S CTM) DSF

LEAF OM)

TRUa«(AVE(TM) DCF 1200 1300 1400 1500

Wavelength (nm)

1600 1700

Figure 4 Difference between the chromatic dispersion and the best fit 3 term equation divided by the empirical derived error of equation 3 or 4. The discontinuity in the curve for the TRUEWAVETM fibre is caused by a change in the best 3 term fit equation, see Table 1

.

An empirical relation of the maximum difference between the chromatic dispersion and the best three term fit was developed and found to be correct for the five fibres in the ranges shown in Table 1 . For the

three term Sellmeier this was found to be

ADCD-3T Sellmeier < V0.25' +0.05'D' +0.0025'(?i- < X >) (ps.nm-'.km"'), (3)

where <A,> is the mean of the four measurement wavelengths (nm), X is the wavelength (nm) where the difference is being calculated and D the chromatic dispersion (ps.nm '.km"'). For the three term Quadratic

the maximum difference was found to be given by

|^DcD-3TQuadrat,c| < ^0-25' + O.OS' D' + 0.00?' (^- < X >f (ps.nuf' .km"' ). (4)

Figure 4 shows the difference between the chromatic dispersion and the best three term fit as a fraction of

the empirically derived error of equations 3 and 4. This is less than +1 showing that the empirically

derived maximum error is correct over this wavelength range for these fibres.

Uncertainties

The uncertainty in the chromatic dispersion due to the OTDR is calculated by combining the OTDR

resolution, offset and wavelength uncertainties into a single relative time uncertainty for each wavelength.

90
o^(X) ps.km' , cy,ay--^ + ^+ D(Xra,\ (5)

This relative time uncertainty is used to calculate the chromatic dispersion uncertainty, <Jd ps.nm '.km"',

through the statistics of the fitting process. The uncertainty in the zero dispersion wavelength, a>.o nm, is calculated through the dispersion slope, S(A,) ps.nm '.km"' and

^>o=^^' C6)

The measurement uncertainty is combined with the distance scale deviation, the effect of the assumed

fitting equations, the temperature dependence of the fibre^^^ and the overall repeatability of the measurement. The measurement uncertainties are dominant for short fibres (<50 km) while the effect of

the fitting equations become dominant for longer fibres (>50 km).

Measurements

Table 2 show the zero dispersion wavelength and chromatic dispersion at 1550 nm measured by the NPL

phase shift system and by the OTDR with their 95% confidence intervals. The results are within the 95%

confidence intervals.

Fibre Zero Dispersion Wavelength (nm) Chromatic Dispersion at 1550 nm

(ps.nm'.km"')

Phase Shift OTDR Phase Shift OTDR

SMF28

TM

DSF

LEAF

TM

TRUEWAVE

TM

1313.1 ±0.1

1546.6 ±0.1

1574.7 ±0.1

1448.8 ±0.1

1331.4± 19.0 1542.0 ± 12.6 1574.7 ±21.1

1454.6 ±21.3

16.76 ±0.25

0.26 ±0.01

-2.95 ±0.05

4.36 ±0.07

18.2± 1.5 0.7 ± 1.0 -3.2 ±2.0

4.3 ±2.1

Table 2 Zero dispersion wavelength and chromatic dispersion at 1550 nm measured using both phase

shift and OTDR techniques. Uncertainties shown are 95% confidence intervals. 3 term fit as suggested in Table 1. Conclusion

The significant uncertainties in the calculation of the zero dispersion wavelength and chromatic

dispersion from OTDR data are the accurate measurement of time at known wavelengths and the quality

of the fitting equation. Techniques to assess the OTDR accuracy have been described. An empirical

relationship for the error due to the fitting equation has been developed and compared for five fibre types.

For short fibres the accurate measurement of the time of flight is the dominant component while for longer fibres the error between using a three term fit and the true curve is the dominant component. The

best uncertainty on the zero dispersion wavelength in the long fibre regime is expected to be around

± 3 nm, while the best dispersion uncertainty is expected to be around ± 0.3 ps.nm '.km"'. Acknowledgements

We gratefully acknowledge the financial support of the UK Department of Trade and Industry. © Crown

Copyright 2006. Reproduced by permission of the Controller of HMSO. Extracts from this report may be

reproduced provided the source is acknowledged and the extract is not taken out of context

References

[1] "Optical fibres. Measurement methods and test procedures. Chromatic dispersion" lEC 60793-1-42:2001

[2] "Calibration of optical time-domain reflectometers (OTDRs)" lEC 61746:2005

[3] M C Wicks, "Temperature dependence of Zero Dispersion Wavelength in Single Mode Optical Fibres", NPL

Report, June 1996.

91
Critical Issues Relating to FTTH/PON Testing

by

Dr. Andre Girard

Senior Member of Technical Staff EXFO E.O. Engineering, Inc. 400 Godin Avenue, Quebec City, QC Canada G1 M 2K2

Tel.: 1-418-683-0913 #3138; Fax: 1-418-683-2170

E: andre.girard(a)exfo.com : Web: www.exfo.com

Presented at SOFM 2006, Boulder, CO, USA, September 19-20, 2006

Summary

Fiber-to-the-home (FTTH) Passive Optical Networks (PON) is currently in major deployment or demonstration around the world. The technology is currently being supported by international

standards organizations such the International Telecommunication Union (ITU-T), the Institute of

Electrical and Electronic Engineers (IEEE) and the International Electrotechnical Commission.

This paper will address the following issues:

• Market and driving application motivation compared to other access technologies together

with the related bandwidth requirements.

• Description of PON topologies and architectures

• Critical aspects and challenges during PON installation, operation and maintenance.

• Technical challenges having forced the development of new test instrumentation.

A textbook, Girard, A. "FTTx PON Technology and Testing", 191pp, © 2005 will be distributed in conjunction with the presentation.

1 Market and applications

PON is one of many solutions for supporting bandwidth and services provisioning in the access

network as shown in Figure 1. Typical modern basic triple-play applications and future ones make

the total downstream bandwidth demand at levels between 30 to 40 Mbit/s, as shown in Table 1. PON represents an attractive solution for bringing these high-bandwidth consuming applications

to home and small office, home office (SOHO) residences. PON is considered a cost-effective

and very long-term maintaining solution because it uses no active elements between the service

provider's central office and the subscriber's premises, as shown in Figure 1. Totally passive

components are used in the field; and consequently they substantially contribute to improve the

network reliability, decrease maintenance costs, and provide a much better life cycle cost and

return on the investment.

FTTH PON is based on two basic architectures: point-to-point (P2P) (Figure 2) and point-tomultipoint (P2MP) (Figure 3), with the later further divided in broadband PON (BPON), Ethernetready PON (EPON) and gigabit-capable PON (GPON).

The P2P architecture operates on the same downstream and upstream wavelength (1310 nm)

and has no bandwidth provisioning limitation. However, it has a poor premises count per cable

and is consequently considered more expensive and less cost effective than the P2MP

architecture for the FTTH market.

92
CO RT Node

(Neighborhood)

Curb

(Pole) Subscriber's

Premises

DSLAM = DSL Access Multiplexer 3IVIb (1-Play)

DSLAWl/ATM

Aggregation

FTTN

Aggregation

FTTC

Aggregation

FTTx

OLT

HFC

Headend

CWITS/QAM

DSLAM/ADSL ADSL

< 3 kmADSL

Modem

<30Mb (2-Play)

DSLAM

100s Homes

ADSL2+ <1 km

Aggregation

VDSL2 <1.5 km

ADSL2+

DSL Modem + STB

30-40Mb+ (3-Play) ONU 1<3DI)|DSL Modem +

8-12 Homes | m STB

I VDSL2

BPON+ Optional RF Overlay/EPON/GPON

30^0Mb (3-Play)

<20 km of fiber (one fiber only, P2MP)

ONT+STB+

Home Network

30IVIb (3-Play)

Cable Modem

+ STB

Fiber Node

Fiber i Copper I Coax

Figure 1 -Solutions for bandwidth and service provisioning in the access network

Bandwidth (Mbit/s)

Single

appl.

Maximum

Applications MPEG-2 MPEG-4

WM9

Voice (per channel) 0.064 0.5 (Murti-lines) Web browsing

, „.„.^_,„ 5-10(2 PC's)

File download (10s-6MpixelJPEG picture*) 2 SD

MPEG-2 4-6 10/2TV

TV Broadcasting

(per channel)

MPEG-4 2 5/2TV

MPEG-2 20 20/1 TV

HD MPEG-4 9 8- Windows Media 9 8 10/1 TV

Others (e.g. e-healtli/e-safety probing/monitoring) 5 10 TOTAL (perpremises) -30

J/MPEG: Joint/fWotion Picture Experts Group ' Real-time = 50 Mbit/s minimum

Table 1 - Typical home applications and related bandwidth demand

The P2MP architecture operates on different downstream (1490 nm + 1550 nm if RF analog

video overlay is used as shown in Figure 4) and upstream (1310 nm) wavelengths and provides

bandwidth on a shared basis. Even with the additional use of a power splitter between the CO

transmitter and the premises, it has a very large premises count per cable (a 144-fiber cable serves 4608 homes) and is consequently considered the most attractive solution for the FTTH

PON market.

93
Figure 2 - Point-to-point {P2P) PON architecture

Figure 3 - Point-to-multi-point {P2MP) PON architecture

Figure 4 - Spectral bands used in telecommunication networks

94
2 Critical PON issues

Even if PON is an attractive business solution for tiie premises access network, there are some

technical issues that installers and operators will have to take into consideration in order to ensure successful deployment and avoid market disruption if anything goes wrong.

The choice of transmitting video on analog RF signal using 1550 nm separate wavelength brings

a certain number of critical issues such as human safety, stimulated Brillouin scattering non-linear

effect and composite second-order beat noise all due to the very high power used. Furthermore, an expensive transmitter is required together with narrowband filtered cooled DFB laser with

external modulation. Localized laser heating may also happen especially in case of very high

power RF video transmission together with dirty or bad connection.

In order to keep the cost of the premises equipment down, an inexpensive transmitter and

receiver are used. However the choice of a Fabry-Perot laser also brings technical issues such as mode partitioning noise and mode hopping due to the multi-frequency nature of that laser.

Macrobending is another issue related to excessive fiber curvature such as in fiber management

system causing loss of light. It is mostly workmanship-related. It is defined in fiber specifications as in the ITU-T Recommendations G.65x series. The longer the wavelength the more sensitive it

is and could reach 0.5 dB at 1550 nm. Apart of fiber discontinuity, voids, gaps, misalignment,

mismatch, and angular cracks, one of the worst issue related to network performance is still related to connector cleanliness or lack of verification that connectors are in fact clean before

connecting.

3 Testing requirements

3. 1 Physical layer

Adequate testing during network construction, installation and commissioning will always

minimize costly and time-consuming afterwards troubleshooting efforts. The following is a list of areas in which particular attention should be provided.

easurement using OTDR

(1) Construction and installation

• Qualification of installed outside plant equipment through optical loss and optical return loss (ORL) m and automated optical loss test set

• End-to-end link characterization assessing splices and cable plant, connectors and splitters

with bidirectional loss and ORL measurement

(2) Service activation and troubleshooting

• Verification of each optical signal power to be within specification

• Problem source/location identification

PON link characterization using an OTDR has however created new OTDR requirements and

consequently a new product development. For instance, a much shorter dead zone is required in order to detect the closest drop terminal from the premises (see Figure 3). A much larger dynamic

range is also required to go through the lossy splitter (see Figure 3). The OTDR should also be

able to measure through the splitter, distinguishing between the splitter loss (approximately

17.5 dB for a 1x32 splitter) and the end of fiber, in order to characterize the link after the splitter.

Finally, high resolution is required to characterize closely-spaced events.

These have brought the development of a new handheld-type PON-optimized OTDR operating at

95
1650 nm for in-service activation and troubleshooting.

Optical power measurement at the CO is required to ensure that sufficient power is delivered to the premises. This is done only during the initial activation because it cannot be repeated without

Interrupting service for the entire network. A new wavelength-isolating PON power meter measures the power of each wavelength simultaneously without traffic interruption. Power

thresholds may be used in order to provide pass, warning or fail status for each wavelength

simultaneously.

Each time new premises are added to the PON, downstream and upstream optical powers at the

drop terminal and at the premises should be measured. Because of the bursty nature of the

upstream 1310-nm traffic, the new wavelength-isolating PON power meter must be capable of simultaneously measuring the continuous downstream power at 1550 nm and 1490 nm, and the

bursty upstream power at 1310 nm each for BPON, EPON and GPON.

3,2 Protocol layer

On of the advantages of using ATM in PONs (BPON and GPON) is that it includes management

features to guarantee quality of service (QoS). This allows network service providers to guarantee

transmission rates, error rates, and other charactehstics to their customers. Two types of testing can be performed with ATM-based PONs for ensuring QoS. Those are non-intrusive testing (in- service) used to monitor live traffic for service-affecting faults, and intrusive testing (out-ofservice) used for circuit turn-up and troubleshooting faults. An end-to-end QoS test can be run in order to determine if performance will degrade over time. ^ :

There are two levels to ATM testing. One is cell level testing which requires wire-speed ATM

traffic generation and cell analysis. The second is packet level analysis which requires

reassembly of cells into packets and analysis of upper layer protocol layers. The packet latency

and loss measurement is an example of the later.

In case of EPON, Ethernet was not designed with QoS in mind and, originally, offered no means

to differentiate between low and high phority data. This made it difficult to combine different types

of services, such as email and voice communications, over the same link while ensuring that transfer rates met pre-established criteha. Various solutions have been proposed since then to overcome this shortcoming. One improvement is to manage network traffic by grouping similar

types of traffic together (for example, email, streaming video, voice, large document file transfer)

and treating each type as a class with its own level of service priority. This technique is called class of service (CoS).

Regardless of the CoS specified for any particular type of data, various factors such as network

congestion can affect the actual rate at which the data is transferred. For this reason, specific

tests are required to verify Ethernet performance in order to ensure that the SLA requirements

are met.

The data-communication industry has put together a test methodology to address the issues of

performance verification at the layer-2 and layer-3 levels. The Internet Engineering Task Force

(IETF) has developed RFC 2544 ("Benchmarking Methodology for Network-Interconnect Devices",

IETF, Ed. S. Bradner & J. McQuaid, The Internet Society, March 1999) in order to specify the

requirements and procedures for testing the network performance characteristics. When these

measurements are performed, they provide a baseline for service providers to define SLAs with

their customers.

96
3.3 Component testing

Characterizing PON passive components in manufacturing is a critical step in ensuring the

optimum PON performance. Important parameters such as insertion loss, spectral uniformity,

bandwidth, isolation (for passive WDM components while called crosstalk for systems) central channel wavelength, polarization dependence, and optical return loss must be controlled and

tested during development, engineering and production.

Three test methods are currently available depending on the device complexity and performance

requirement: one based on broadband light sources (BBS), one based on swept delay system

and a test method based on swept-wavelength (or frequency) system. A possible compromise

between testing time and system complexity may be a system using a tunable scanning laser, a

polarization controller and multi low-polarization-dependent response (PDR) detectors, capable of performing IL, ORL and PDL measurements.

For active components, the most critical parameters to be considered in production testing are:

• Center wavelength

• Bandwidth (spectral distribution and linewidth)

• Frequency stability/temperature control (when applicable)

• Output power stability/control

• Life time (failure in time - FIT).

4 Conclusion

FTTH PON has never provided before such service applications and bandwidth provisioning to residences and SOHO. However there are still and will be technical and market issues that will need to be looked at and in some cases overcome. Proper testing, measurement and monitoring

strategy and instrumentation will have to be carefully implemented. The following five market

conditions will have to be met before FTTH PON becomes a wide spread reality:

• A growing telecommunication market o Opened, highly competitive market o No single dominant service provider

• Favorable Government FTTH policy / regulations

• Educated middle-class population knowing, accepting and desiring telecommunication

technologies

• Optical fiber used as a marketing incentive

• Favorable conditions for consuming HDTV

When these conditions will be met together with yet-to-be-found killer applications, FTTH PON

will become the only long term suitable transport solution.

97
WDM-PON for FTTx

Wayne V. Sorin , Chang-Hee Lee"^, Byoung Y. Kim ,

' Novera Optics, Inc., 2620 Augustine Dr., Santa Clara, CA 95054

" Korea Advanced Institute ofScience and Technology^, Daejeon, Korea

Introduction

WDM-PON is a general purpose and extremely efficient future-proof optical transport

technology for use in Access and Metro transport networks. It enables highly efficient use of

the outside fiber plant by providing point-to-point optical connectivity to multiple remote

locations through a single feeder fiber.

Figure 1 . WDM-PON supports multiple services

Figure 1 illustrates the general FTTx (Fiber-to-the-x) architecture using a WDM-PON

(Wavelength Division Multiplexed Passive Optical Network). As can be seen in the figure,

this general-purpose architecture can serve multiple applications for both the business and

residential customer. This functionality is possible since each end point is connected to the

central office through a dedicated bidirectional optical channel. This virtual point-to-point

PON architecture enables large guaranteed bandwidths, bit rate independency, protocol

transparency, graceful upgradeability, high QoS with excellent security and privacy.

Until recently, WDM (i.e. dense WDM) has been used only for long-haul and metro

applications and was not thought to be economical for the access market. One reason for this

98
is that each remote site would have needed a unique transceiver (i.e. a wavelength-stabilized

DFB laser) to match the WDM channel defined by the optical transport layer. These

differently "colored" transceivers raised concerns regarding the high operational costs

(installation, management and inventory) associated with managing each remote access

location.

This limitation has been solved through the development of an innovative technology that has

eliminated the need for complex wavelength-stabilized lasers [1,2]. By utilizing an "optical

injection seeding" technique, simple and identical Fabry-Perot lasers can now be used at all the remote ONU (Optical Network Unit) locations. Although all the transmitters are identical,

each one operates at a different DWDM wavelength through the use of a novel automatic

wavelength-locking technique. The term "wavelength-locked" WDM-PON will be used to describe a WDM-PON system that utilizes identical transceivers by means of automatic

wavelength locking. In this paper we will review the concepts behind a wavelength-locked

WDM-PON and will discuss a measurement instrumentation opportunity for this future-proof access architecture.

Comparison with Conventional WDM system

It is useful to first describe how a wavelength-locked WDM-PON differs from a more

conventional long-haul WDM transmission system. Conventional WDM systems, as

illustrated at the top of figure 2, typically carry unidirectional traffic over each fiber

transmission link. This allows the use of unidirectional optical amplifiers that are normally

required in long-haul applications. Therefore, bidirectional traffic requires two separate data

links, one for eastbound traffic and another for westbound traffic. In contrast a WDM-PON

provides the same fianctionality using only a single bidirectional data link. This is possible by

using a cyclic or periodic AWG that can support both a separate upstream and downstream

wavelength on each of its "n" output fibers. To achieve this fianctionality the AWG must be

designed to operate on a high diffraction order (just as in a classical bulk-optics diffraction

grating). By requiring only single fiber connections at each location, this simplified

architecture becomes much more suitable for access network applications.

Another important difference is eliminating the need for "n" different laser sources (i.e. multiple wavelength-stabilized DFB lasers) at the "n" transceiver locations. By using

automatically wavelength-locked FP-LDs (Fabry-Perot laser diodes), each remote transceiver can be identical and interchangeable with all the other remote transceivers. Identical

transceivers are critical for minimizing inventoiy and management costs in an access network

application. Wavelength locking of FP-LDs will be described in the following section.

Also of importance is the recent development of athermal AWGs that enable the remote node

to be completely passive. Previously AWGs required heaters to keep their WDM channels

locked onto the ITU wavelength grid. This active power requirement was acceptable in

convenfional long-haul applications since the AWGs (together with the temperature stabilized

DFB lasers) could be located in temperature-controlled environments (i.e. central offices).

99
Conventional WDM

Same Functionality

n

Figure 2. Comparison with a conventional WDM system.

In summary, a wavelengtii-locked WDM-PON differs from a conventional WDM long-haul

system by (i) enabling bidirectional transmission over each of its optical fibers, (ii) providing

a point-to-multipoint architecture through a passive and environmentally hardened remote

Mux/DeMux, and (iii) using identical and interchangeable automatically wavelength-locked

FP-LDs. .

Automatic Wavelength Locking in a WDM-PON

Figure 3 illustrates the operation of automatic wavelength locking in a WDM-PON system.

An un-modulated BLS (Broadband Light Source) located at the OLT (Optical Line Terminal)

in the central office is used to generate seeding signals for "locking" the wavelengths of the

remotely located identical FP-LDs. The BLS seeding signal is transmitted downstream

through the single feeder fiber into the passive remote node containing the athermal and

cyclic AWG. At this location the BLS wavelength spectrum is divided or "sliced" into "n"

narrowband DWDM (dense WDM) channels by the demultiplexing function of the AWG.

Each spectral slice is then transmitted through a single distribution fiber and injected into a remotely located FP-LD. When the FP-LD is current modulated with the electrical data

signal, the injected seed signal forces the laser to operate in a narrow wavelength range

100
defined by the optical passband of the DWDM transmission Hnk. This wavelength locking

process can be easily understood when one realizes that the FP-LD basically acts as an optical

amplifier that modulates, amplifies and reflects the injected BLS seeding signal. The FP-LD

is not capable of free-lasing due to the gain saturation caused by the amplified seeding signal.

This results in a stable narrow-band output data signal, free from any of the noise associated

with mode-hopping found in standard free-running FP-LDs.

Unmodulated BLS OLT chi ««- n n n n

I 1 1 I —» ch 1 RN

cyclic AWG

- n n n n FPLD

Z7

Spectrum before locking

1530nm 1560nm

Spectrum after locking

downstream band upstream band mmiiMim^iWBifitmm

1530nm 1560nm

1 2 n 1 2 n

Figure 3. Basic description of automatic wavelength locking

The lower right hand side of figure 3 shows the FP-LD wavelength spectrum before and after

applying the seeding or "locking" signal. Without the application of the locking signal, the

FP-LD lases in multiple wavelength modes (see top insert on the right). This spectrum is unsuitable for data transmission through the DWDM transmission link due to the generation

of mode partition noise caused by the wavelength filtering of the AWG. After injection of

the locking signal the multimode spectrum is transformed into a quasi single-mode signal (see

bottom insert) similar to that of a DFB laser. This "DFB-like" signal is automatically aligned

to the DWDM channel defined by the optical transport layer. This wavelength locking

process results in a "plug and play" functionality where all the remote FP-LDs are identical

and interchangeable but can operate at different wavelengths without the need of any complex

control or locking circuitry.

Figure 3 also illustrates the bidirectional functionality of a WDM-PON. Simultaneously

along with the downstream BLS signal, "n" independent downstream data wavelengths are transmitted in a different wavelength band (shown at bottom left of figure 3). Due to the

cyclic nature of the AWG, both a spectral slice of the BLS and one downstream data

wavelength are demultiplexed and sent to each remote ONU. Each ONU transceiver uses an

identical dichroic band-splitting filter which separates the two bands, directing the

downstream BLS seeding wavelength into the FP-LD and the downstream data wavelength

into a standard optical receiver. The modulated upstream data signal generated by the

101
wavelength-locked FP-LD returns along the same optical path as the downstream BLS

seeding signal.

System Description

Figure 4 shows a typical configuration for a wavelength-locked WDM-PON. Wavelengthlocked FP-LDs are used at both the central office and the remote ONUs. All the ONU

transceivers are identical and interchangeable. The central office OLT houses the BLS, a Mux/DeMux and the "n" downstream wavelength-locked laser sources.

Remote Optical Central Office (CO) Node Network Unit (RN) (ONU)

Athermal AWG

Optical Line Terminal

Figure 4. Wavelength-locked WDM-PON system configuration

A single feeder fiber is used to connect the OLT to the environmentally hardened passive

remote node. From the remote node, "n" distribution fibers are used to connect to "n" remote

ONUs. In summary, over a single feeder fiber this WDM-PON architecture provides a dedicated and bidirectional optical point-to-point connection between "n" transceivers in the

central office and "n" remotely located ONUs. There are no special requirements for

addressing or managing the multiple remote ONUs.

One important advantage of the WDM-PON architecture is the ability to completely

characterize all the optical fiber paths by using a single tunable-wavelength OTDR located at the central office [3]. This is possible since the different DWDM wavelengths provide a unique and low-loss optical path to each remote ONU which can be used to measure

individual Rayleigh backscatter signals. This level of optical characterization is not possible

in a more conventional TDM-PON (Time Domain Modulation) due to the remote node power

splitter that prevents unambiguously identifying the backscatter signatures from the individual

fibers after the power splitter. It is expected that a demand for wavelength-tunable OTDRs

will emerge as WDM-PONs grow in popularity [4].

102
Summary

A wavelength-locked WDM-PON is an efficient and future-proof WDM transport

architecture optimized for the access network. It provides a point-to-point optical connection over a shared fiber plant by allocating a pair of dedicated wavelengths for each ONU. To

reduce both capital and operating costs, a newly developed optical seeding technique enables

the use of automatic wavelength locking of identical low-cost Fabry-Perot laser diodes. The

benefits of a wavelength-locked WDM-PON access system are; identical wavelengthindependent DWDM ONT/ONUs, simple point-to-point dedicated connectivity, bit-rate and

protocol independency, high security and privacy, simple fumre data-rate upgradeability, and

complete fiber characterization through use of a wavelength-tunable OTDR.

References:

[1] H. D. Kim, S. G. Kang and C. H. Lee, "A Low-Cost WDM Source with an ASE Injected

Fabry-Perot Semiconductor Laser," IEEE Photonics Technology Letters, vol. 12. pp 1067-

1069, Aug. 2000

[2] D. J. Shin, Y. C. Keh, J. W. Kwon, E. H. Lee, J. K. Lee, M. K. Park, J. W. Park, Y. K.

Oh, S. W. Kim, I. K. Yun, H. C. Shin, D. Heo, J. S. Lee, H. S. Shin, H. S. Kim, S. B. Park, D.

K. Jung, S. T. Hwang, Y. J. Oh, D. H. Jang and C. S. Shim, "Low-Cost WDM-PON With

Colorless Bidirectional Transceivers," IEEE Journal of Lightwave Technology, vol. 24, pp.

158-165, Jan 2006

[3] J. H. Park, J. S. Baik and C. H. Lee, "Fault-localization in WDM-PONs," Optical Fiber

Communications Conference, paper JThB79, Mar. 2006

[4] S. J. Park, C. H. Lee, K. T. Jeong, H. J. Park, J. G. Ahn and K. H. Song, "Fiber-to-theHome Services Based on Wavelength-Division-Multiplexing Passive Optical Network," IEEE

Journal of Lightwave Technology, vol. 22, pp. 2582-2591, Nov. 2004

103
FTTx Deployment Issues

Jimmy Salinas

AT&T

Senior Manager-Technical Support

is59Q6(a)at&t.com

FTTx is talked about in many forms, ( FTTN, FTTP, Brown Field, Green Field, Fiber to the Curb, and Fiber to the Home) an the fonns all are just FTTx technology applied to match the suiTounding. A good example of this is the FTTH Council report of April 25,

2006 covering the 936 communities in 47 states already being served Via Fiber-to-the - Home technology today. Everything from small developments to entire communities,

ILEC to DEV/CLEC joint ventures.

In this presentation we will start with FTTP technology and then jump to FTTN

technology with an understanding that FTTP technology is only applied to the local loop

out of the serving Central Office, excluding the backbone network and local rings needed

to get the signal to the local office serving the local loop. FTTP networks are made of an

Optical Line Termination unit in the Central Office, the PON local loop including the

splitter, and a H-ONT Gateway unit. There are many issues with FTTP technology, but

the main issue is "Fiber and Connector Cleaning" with 75% of all troubles being due to dirty connector related issues. This is followed by a misunderstanding of the maximum

and minimum PON Loss allowed.

Optical link loss budgets for FTTP PON systems are in two parts, the first being the

maximum allowed Optical Link Loss of about 25.0 dB followed by a minimum Optical

Link Loss of -14.0 dB. In fact if the optical loss is to low attenuation will need to be

added to keep the signal from overdriving the receivers. The Optical Link Budget

includes items like fiber loss per Kft, Fusion Splice loss. Connector loss, and 1x32 or 1x16 Splitter loss. Within the Central Office the P-OLT is connected to the world by

items like Voice Gateways and Broadband Optical Boxes that connect all other services

to the P-OLT unit. Within the Optical OSP Link you have the fiber, a splitter (1x32 or 1x16) and the Fiber Serving Terminal that serve the Home. The FTTP local loop network

is finished by the application of a Home-Optical Network Temiinal ( H-ONT) which

serves as a gateway to the customer providing POTS, Ethernet, VoIP, and IPTV service.

Trouble issues with the application of PON technology are the following

1 . Dirty Connectors and Bad Splice Points

2. Micro Bending

3. Max Light Loss Budget exceeded

4. Minimum Link Loss not maintained

5. 120 AC needed to power the H-ONT and to charge the back up battery

104
FTTN systems come with a difference set of problems including the following

1 . A max of 12 Kft between the CO and the SAI ( Based on 26 GA NL-Loaded

Cable) and a max of 9.0 dB loss between the CO and the NID for the POTS Lines

2. A max of 4 Kft ( of Drop ) between the SAI and the NID for Data

3. Depending on the VDSL/POTS drop length, Bridge Tap cable can be an issue

As with FTTP technology, the FTTN NID (H-ONT) converts the POTS and Optical Data

signal from the network into POTS and VDSL services on the customer's side of the NID

using a VRAD/HPNA signal that is further converted into Data and Video through a deplexer / router unit.

Due to the fact that FTTX technologies are all built on local loops, short to medium in

length and newer type fibers, as well as the improvements in fiber manufacturing ( Core

placement and Core shaping) and splicing equipment doing automatic alignment, testing

and splicing simultaneously, problems like dirty connectors, optical loss, optical return

loss, and reflectance can be minimized and detected using tools like Fiber Scope, Bidirectional Power Meters, and Optical Time Domain Reflectometers to rest and correct

all trouble issues. As well as Jitter and Propagation Delay are not being major issue when

FTTx/PON technology is correctly applied.

Other problems like Chromatic Dispersion and Polarization Mode Dispersion (PMD)

should not be issues with FTTx technology. This being the case because of the fiber

manufacturing methods used today to create a more uniforme shape, size, and placement

of the core within the center of the fiber. As well as most FTTx systems being designed

today to work in a lOM and 1 .0 M range with both problems only being an issue above

the lOGb/s range of operation.

Place note that the Local and Backbone networks being used to support the local loop

FTTN or FTTP application are a different matter. Because of the bandwidth and

technologies being used to transport the signal between the SHE (Super Head End), VHO

(Video Hub Office), and the Local Central Office, extensive system and fiber testing will

be required to insure overall system performance requirements.

Fiber system testing requirements can be reduced by following a few of the design items

1. No cable manufactured before 1997

2. No Depressed Clad Fiber or Dispersion-Shifted Fiber

3. All Fusion spliced cable, no mechanical splices

4. No damage cable that has been restored

5. No cable that has temperature induced cable loss

6. No Biconic connectors, replace with SC type connectors

7. Clean all fiber connection points every time accessed

105
FTTx is the future with the abihty to dehver greater bandwidth and data rates (20 to 25M),

with lower installation and maintenance cost along with the ability to grow as the

customer's need grow.

If proper design, application, and engineering guide lines are applied when creating your

FTTx network of the future along with the supporting interoffice network. There is no

reason for your network of the future to not grow with your customer's needs at a minimal cost to you the network provider. In fact this network should be ready for all your customer's anticipated future needs ( 1 to 24M and up) for many years to come.

Thank You for Your Attention and Time

Jimmy Salinas

106
Special requirements for measurement techniques

used for POF and thick glass fibers

OlafZiemann, Polymer Optical Fiber Application Center

University of Applied Sciences Nurnberg, Wassertorstrafie 10, 90489 Nurnberg/Germany

Ph./Fax: ++49 91 1 5880-1060/ -5060

e-mail: olaf.ziemann@pofac.fh-nuernberg.de, web: www.pofac.de

Summary

Optical fibers are well established in the field of data comnnunication and local area networks. It is obvious, that a high number of different measurement technologies and devices for optical

fiber have been developed and produced over the last years. Examples are spectrometers,

reflectometers and dispersion measurement setups. The fibers typically used in these areas are made from silica glass having core diameters between 10 pm (single mode) and 62.5 pm

(graded index multi mode). Operation wavelengths are around 850 nm, 1,300 nm and 1,550 nm. Of cause, all the measurement devices are optimized for these conditions.

In a number of new applications, fibers are used newly for short-range data communication as

well. The high reach and transmission capacity of communication glass fibers is not required

here. On the other hand side, special attention must be taken for easy installation and low cost

components. The solution is the use of large core diameter optical fibers like POF (Polymer

Optical Fiber) and PCS (Plastic Clad Silica).

The measurement problems for thick fibers are different sometimes. The existing devices can

be used only in a limited manner, due to deviating parameters. This paper will give an overview

to typical problems, existing solutions an open questions of measurement techniques for thick

optical fibers (detailed overview: [1], [2], [3]).

1. Thick optical fiber overview

We like to define "thick" optical fibers as all versions with a core diameter higher than the

classical telecommunication fibers (10 pm for single mode and 50 pm for multi mode. The

following list includes some of the most important fibers. • Step index type POF: core diameter: 1 mm, NA: 0.30 to 0.65; core material: PMMA; single ore double optical cladding • Multi core type POF: core diameter: 1 mm, NA: 0.19 to 0.50; core material: PMMA; 19 to >600 connected cores; single or double coating • Multi core glass fibers: developed by Schott Glas as an example; about 400 single fibers

(each 50 pm diameter) as a loose bundle; NA about 0.50 (see [4] and [5]) • PMMA-GI-POF: core diameter: 0.5 to 1 mm, NA: 0.30 to 0.40; core material: doped or copolymerized PMMA

• SI- and semi-GI-PCS: 200 pm diameter silica glass core with a polymer optical cladding,

NA: 0.30 to 0.50; also available with a week graded index profile in the core ([6], [7]) • PF-GI-POF: the core diameter is 120 pm for all existing fibers (62.5 pm and 200 pm

included in the standard; core material is CYTOP® by AGC; graded index profile is formed

by dopants (detailed description of measurement problems in [8])

Cross-areas of the different fibers with a comparison of the size of communication fibers are shown in Fig. 1

.

107
St-POF

: 1000 |jm

MS-POF

0: 1000 |jm

SM-GOF MM-GOF MM-GOF

0:10 Mm 0: 50 pm 0: 62.5 pm

Fig. 1 : Comparison of different optical fibers

MC-GOF

0: 1000 Mm

SI-PCS

5: 200 Mm

PMMA-GI-POF

0: 900 Mm

PF-GI-POF

0: 120 Mm

On a first view, on can see the big differences in the core diameters of the fibers. This

influences the requirements for measurement technique, first of all the size of detectors. But

there are even more differences existent. The Numerical Aperture (NA) and the core diameter of

different available fibers s presented in Fig. 2.

1

1

1 r Numerical Aperture

1.0

0.8

0.6

0.4

0.2

0.0

10 20 50 100 200

Fig. 2: Comparison of different optical fibers

500 1000 2000

The perfluorinated GI-POF is more or less comparable with glass multimode fibers. Most of the

existing measurement devices, optimized for MM-GOF can be use with small changes. But for

MC-GOF, PMMA based POF and the PCS the situation is different, even more, if one take into

account, that the operation wavelength is between 450 nm and 780 nm, which is an unused

range for silica glass fibers.

108
If a standard optical device is used for thick fibers, the most frequent error is, that only center

low order modes are detected. This leads to a incorrect measurement of loss, bandwidth, bending behavior and more or less all other optical parameters ad described in the next chapter.

2. Light propagation in multimode fibers

Light passes through multi mode fibers along many different ways (about 50 for a MM-GOF, but

2,000,000 for a St.-POF). For MM-GOF, these paths are more or less identical, but in thick

fibers there a very strong mode dependent processes. First of all, for POF and PCS the

cladding has a much higher attenuation than the core. This gives a high mode dependent loss. The results of mode dependent loss for a POF and two different PCS (with a step index and a semi-graded index profile) are shown in Figs. 3 and 4.

90

80

70

60

50

40

30

20

10

excess loss

[dB/km]

POF with An = 0.48 core diameter: 980 [im

acceptance angle: ± 28.6°

launch angle [°]

200

150

100

50

0

excess loss [dB/km]

i s.emi

'OS

-Gl 50 m to 5 m

F

)l PC ;l

is Ml en

0 4 8 12 16 20 24 28 -25 -20 -15 -10

Fig. 3: Influence of the cladding absorption on the loss ([3]), left Fig. 4: Mode dependent loss in 200 mm PCS ([3]), right

-5 0 5 10 15 20 25

There is a wide range of consequences. The attenuation and much stronger the bandwidth

depends on the launching and the detection conditions. Experimental results are given in Fig. 5

(length dependent loss for 4 sources with different emission angles, see [9]) and Fig. 6 (length

and launch dependent bandwidth, [10]).

2 5 10 20 50 100

Fig. 5: Length dependent loss for 4 sources with different emission angles

109
3 dB opt. bandwidth [MHz]

An=0.05

An=0.09

O-An=0.19

^An=0.33

-®-An=0.48

-#-An=0.64

5 10 20 50 100

'^"9^^

Fig. 6: POF length and launch dependent bandwidth

It can be seen clearly, that it is very important to define and to guarantee the launching and

detection conditions for correct and reproducible measurements. The same situation is true for

other parameters, like bending loss, connector loss and OTDR measurements.

A second effect besides the mode dependent loss can be seen in both figures as well. The

curves are not parallel, but the converge for higher lengths. The reason is the internal mode

mixing (mainly on the core cladding interface). The mode mixing is very strong for SI-POF but

also important for GI-POF and PCS. In theory, it is sufficient to take a fiber in front of the test object, with is much longer than the mode coupling length to be launch independent. In the

reality it is not possible, because of the high loss of the required length. The influence of mode

mixing can be seen again in Fig. 7. A collimated light beam is coupled into a St.-POF under an

angle of 10°. The far field is measured after 10 m (left) and 100 m (right).

Fig. 7: Far field pf a St.-POF (collimated light, 1 0 m and 1 00 m fiber)

3. Examples for available solutions

A lot of special devices have been developed, which can be used for POF and other thick fibers.

We will present some of them. A major problem is the bandwidth limitation due to the large area

of the required photo diodes. For a Si-pin photo diode with about 1 mm diameter, the possible

bandwidth is about 1 GHz. This is not sufficient to measure the bandwidth of GI-POF even for short distances. Figure 8 shows a setup combined from a special red laser source (70 ps pulse

width) and a Hamamatsu optical oscilloscope (20 GHz bandwidth) for measurements in the time

domain (e.g. [1 1]).

110
Fig. 8: Fast laser pulse source and Optical Oscilloscope Hamamatsu OOS-01A/IS

The correct measurement of spectral attenuation can be made, if the launching conditions are

close to EMD (Equilibrium Mode Distribution, see [12]). A possible setup is shown in Fig. 9. The

core elements are the monochromator with a high NA and the special optics for adapting the

launching conditions to the POF. The integration sphere enables the detection of all modes.

This principle is now part of the European standardization.

Light

Fig. 9: Setup for spectral loss measurements under EMD conditions

One of the most powerful methods for the investigation of optical fibers is the OTDR (Optical

time Domain reflectometer). Two different devices for POF and/or PCS are on the market

(Fig. 10, [13], [14]).

Fig. 10: Available POF OTDR

We have shown in [14] that an ODTR can be used for bandwidth measurement with some

restrictions (see also [15]). New principles and improved devices are under development. For

details for the measurement of index profiles, lifetime and far field see [1 6], [1 7] and [1 8].

111
4. Open Problems and Summary

The principles of optical measurements on thick optical fibers are the same as for conventional

communication glass fibers. The higher diameter and NA and the different operation

wavelength, together with the influence of mode dependent effects requires new and adapted

solutions. We have developed several setups for measuring the different optical parameters like spectral attenuation and bandwidth.

The biggest task for the future is the definition of parameters in the standardization and the

production of devices based on these standards.

References

[1] W. Daum, J. Krauser, P. E. Zamzow, O. Ziemann: „POF - Polymer Optical Fibers for Data

Communication", Springer, Berlin 2002

2] H. Poisel: „POF Measurement Techniques", WorkPOF 2006, Rio de Janeiro, 27.04.2006

3] W. Daum, J. Krauser, P. E. Zamzow, O. Ziemann: „POF - Polymer Optical Fibers for Data

Communication - Handbook for thick Optical Fibers", 2'''' edition, Springer, Berlin to be

published 2007

4] J. Warrelmann, R. Schnell: „GF-Bundel fur Datenkommunikation", 15. ITG-Fachgruppentreffen 5.4.1 , Offenburg, 25./26.03.2003

5] M. Luber, A. Bachmann, J. Vinogradov, O. Ziemann, H. Poisel: „Comparison of glass fiber

bundles and step index POF", POF'2004, Nurnberg, 27.-30.09.2004, pp. 393-398

6] O. Ziemann, C.-A. Bunge, H. Poisel, K.-F. Klein: „Bandwidth of thick glass and polymer

optical fibers", POF'2004, Nurnberg, 27.-30.09.2004, pp. 140-146

7] O. Ziemann, H. Poisel, A. Bachmann, K.-F. Klein: „Bandwidth measurements on SI- and

Semi-GI-PCS", POF'2005, HongKong, 19.-22.09.2005, pp. 251-254

8] T. Ishigure: „POF Test & Measurement", Tutorial, POF'2003, Seattle, 14.-17.09.2003

9] M. Luber, O. Ziemann, A. Droge, J. Hanson, R. HengI, R. Renk: „Characterisation of EMD

Quality for Different Light Sources", POF'2002, Tokyo, 18.-20.09.2002, pp. 161-164

10] C.-A. Bunge, O. Ziemann, M. Bloos, A. Bachmann: „Theoretical and Experimental Investigation of FF and Bandwidth for Different POF", POF'2002, Tokyo, 18.-20.09.2002,

pp. 217-220

11] A. Bachmann, K.-F. Klein, H. Poisel, O. Ziemann: „Differential mode delay measurements

on graded index POF", POF'2001 , Amsterdam, 27.-30.09.2001, pp. 57-62

12] D. Peitscher, G. Schulte, H. Muhlen, O. Ziemann, J. Krauser: „Correct Definition and

Measurement of Spectral Attenuation for Step Index Polymer Optical Fibers", POF'2000,

Boston, 05.-08.09.2000, pp. 214-220

13] J. Brendel: „Applications of optical time-domain reflectometry for POF test and measurement", POF'2001, Amsterdam, 27.-30.09.2001, pp. 45-50

14] O. Ziemann, M. Bloos, K.-H. Friedrich, L. v.-d. Vegt: „Measurement of pulse broadening in SI- and GI-PMMA-POF with an OTDR", POF'2005, HongKong, 19.-22.09.2005, pp. 57-60

15] R. Gaudino, E. Capello, G. Perrone: „POF bandwidth measurements using OTDR",

POF'2004, Nurnberg, 27.-30.09.2004, pp. 153-158

16] C.-A. Bunge, G. Kramer, H. Poisel: „Measurement of refractive-index profile by lateral illumination". Poster, POF'2004, Nurnberg, 27.-30.09.2004, pp. 513-520

17] B. Gunther, W. Czepluch, K. Mader, S. Zedler: ..Multiplexer for Attenuation Measurements

During POF Durability Testing", POF'2000, Boston, 05.-08.09.2000, pp. 209-213

18] K.-F. Klein, A. Bachmann, M. Bloos, O. Ziemann: ..Comparison of Different FFMeasurement Setups", POF'2002, Tokyo, 18.-20.09.2002, pp. 259-262

112
Characterization of Hole-Assisted Fibers as Single

Polarization Fibers and Polarization Maintaining Fibers

Xin Chen, Joohyun Koh, Ming-Jun Li, and Daniel A. Nolan

Science and Technology Division, Corning Incorporated, Corning, NY14831

Chenx2@corninq.com

Abstract

Hole-assisted single polarization fibers (SPFs) and polarization maintaining fibers (PMFs)

have attracted significantly interest recently due to its wide range of applications. Holeassisted SPF and PMF also have special properties that require understanding for proper tlber deployment. In this paper, we review the novel properties of a hole-assisted fiber that we

recently implemented as either a SPF or PMF. We measured the fiber birefringence and the fiindamental mode cutoff wavelengths, which directly determine the performance of such

fiber as a SPF. We also comprehensively explored other unique characteristics of these fibers such as the length dependence of the fundamental mode cutoff, the effects of bending on the

fiber cutoff wavelengths, and the performance of such fiber over a wide range of

temperatures. To address the issue of selecting target fiber in a non-destructive and cost

effective manner, we developed a novel method to distributedly measure the cutoff wavelength drifting along the entire length of the fiber.

1. Introduction

Recently there has been increasing interest in using single-polarization fibers in applications such

as fiber lasers, fiber-optic gyroscopes, and fiber sensors to deal with unwanted and detrimental

polarization effects. As a result, different approaches in achieving single polarization operation in optical fibers have been explored [1-4]. Although a solid type single polarization fiber was

studied in details in early times [1], more recent studies focused on single polarization fiber with

air holes either in hole-assisted fibers or microstructured fibers [2-6].

In the past few years, we have proposed, implemented and commercialized a hole-assisted single

polarization fiber [5-6]. In studying this new type of fiber, we have identified many novel

properties that require special attention in order to use the fiber properly. In this paper, we focus on the characterization of the single polarization fibers based on the dual air hole design. We

review the novel properties of a hole-assisted fiber that we recently implemented as either a SPF

or PMF including some very recent results. In Section 2, we study the fiber birefringence and the fundamental mode cutoff wavelengths, which directly detennine the performance of such fiber as a SPF. We also comprehensively explored other unique characteristics of these fibers such as the

length dependence of the fundamental mode cutoff, the effects of bending on the fiber cutoff

wavelengths, and the performance of such fiber over a wide range of temperatures. To simplify

the task of selecting target fiber, in Section 4, we describe a novel method to distributedly measure the cutoff wavelength drifting along the entire length of fiber in a non-destructive and

cost effective manner.

2. Hole-assisted Single Polarization and Polarization Maintaining

Fibers

Figure 1 shows a schematic of the proposed dual air hole fiber structure. The fiber consists of an

ellipfical core with two air holes placed next to the core and along the minor axis of the ellipse.

The two air holes play critical roles. They induce form birefringence through a high index

113
contrast between the core and the air holes. The air holes also create fundamental mode cutoffs. The fundamental cutoff wavelengths between the two polarization modes are different because of

the high birefringence; in between the fiber is a SPF. In order to have sufficiently high

birefringence and single polarization bandwidth, which is defined by the difference of the

fundamental mode cutoff wavelengths between the two polarization modes, the core delta should

be sufficiently high and appropriate dimension of the core especially the minor core dimension

should be chosen so that the single polarization operation window is located at the desired

wavelength range. At the wavelength range below the first fundamental cutoff and higher than the

higher order cutoffs, the fiber is highly birefringent, behaving like a PMF. Therefore, depending on the wavelength that the fiber is operated, the dual air hole fiber can serve either as a PMF or a SPF.

Core Air hole

Cladding

-60

-70 —

-80

-90

1^ (a)

I tM.jl.nLIJl.J

1000 1020 1040 1060 1080

-70

-90

110

Z. (b)

I I I I I I I I I I II ill I I I I I I I I 1220 1240 1260 1280 1300 1320 1340 1360

1450 1500 1550

Wavelength (nm)

1600

Figure 1. Cross section of a dual air holes Figure 2. Measured cutoff wavelengths of the two

fiber as SPF and PMF. polarization modes.

3. Characterization of Hole-Assisted Fibers

Over the past few years, we have fabricated many fibers according to the design in Fig. 1 by

outside vapor deposition and with different parameters. With these fibers, we have experimentally

studied many aspects of the fiber characteristics.

First we study several transmission characteristics of the fibers. An experimental setup that is capable of measuring the transmission signals over a wide range of wavelengths and that uses a white light source and an optical spectrum analyzer was constructed. We used a polarizer to obtain the transmission spectrum of each polarization mode by aligning the polarizer with the

birefringent axis of the fiber. In Fig. 2 we show the transmitted signal for three fibers which have

single polarization operations at 1060nm (Fig. 2a), 1310nm (Fig. 2b), and 1550nm (Fig. 2c). The

bandwidths of the fibers in Fig. 2 judged at the 3dB down from the plateau region are 35nm,

53nm, and 59nm respectively for 1060nm, 1310nm and 1550nm wavelength windows. The

ability to control the center of a single-polarization window offers flexibility for applications

operating at different wavelengths.

In early experiments, to explore experimentally the relation between the core A and the

bandwidth of single-polarization operation, we used two core A values in the fiber fabrication.

Subsequent measurements were conducted to characterize several properties of the fibers. Results

for five fibers are summarized in Table 1 . We compare the bandwidth of each fiber to the core A

114
and group beat length. The beat length is measured by a wavelength scanning method in the PM

regime by using a broadband light source, placing the fiber between two cross polarizers and

observing the modulation pattern in the spectrum. We have found that, in general, a higher core A

is linked to higher fiber birefringence and a higher single polarization bandwidth. This

observation agrees with the results of the theoretical modeling we conducted.

Fiber Delta Cutoffl Cutoff2 Bandwidth Beatlength Loss (%) (nm) (nm) (nm) (mm) (dB/m)

1 1.1 1147.8 1177.4 29.6 3.0 0.03

2 1.1 1066.6 1098 31.4 2.8 0.03

3 1.1 911.2 935.5 24.3 * 0.08 4 2 972.0 1014.0 42.0 1.1 1.76

5 2 1477.7 1533.1 55.4 * 0.1

Table 1 . Measurement results of the single polarization fibers. Beatlength is normalized to value at 980nm.

We then studied the dependence of the cutoff wavelength on the fiber length. Figure 3 plots the measured cutoff wavelengths of the two polarization modes as a function of fiber length for fiber

2 of Table 1 . When the fiber length was cut from 5 to 1 m the cutoff wavelengths of both

polarization modes were shifted higher by 14 nm. This suggests that one can fine tune the center wavelength for the single-polarization operating window in a given wavelength range by

adjusting the fiber length, which offers additional flexibility for dedicated applications. We also

looked into the loss properties of the fibers. One can measures loss by obtaining the transmission

powers at a longer length (3 m), followed by a cutback to a shorter length (1 m). Typical losses of

the preferred polarization listed in Table 1 are less than 0.1 dB/m. More accurate attenuation

measurement by OTDR of certain fibers has shown attenuation of less than 0.02dB/m. As singlepolarization fiber is used mostly in short lengths, the current loss level is sufficient for most

applications, although lower loss is desired.

1120

c

>

153

1060

** j:

Cutoffl CutofO — i:

. . . 1 . 1 1 1 1 1 1 1 1 1 1 1 1 1

1.0 2.0 3.0 4.0 5.0 Length (m)

Figure 3. Measured fiber cutoff wavelengths for

different fiber lengths.

-60 ^ -70

i- -80

B^ -90 3 -60

-80

-90

Polarization Mode 1

OcmA

- 1 1 1 1 1 1 1

- Polarization Mode 2

=. I I 1000 1020 1040 1060 1080 1100 Wavelength (nm)

Figure 4. Fundamental cutoff wavelengths shift at different bending diameter.

We also studied the effect of the bending by measuring the transmission spectrum of each

polarization mode at different bending diameters. Although the bending even at several

millimeters in diameter has very minor effect to the fiber attenuation in wavelength region below

the fijndamental mode cutoff, it has larger effect to the cutoffs. The results are shown in Fig. 4. As the bending diameter decreases, the spectrum curve shifts toward lower wavelengths, resulting

in the down-shift of the fundamental mode cutoffs. The effects apply to both polarization modes.

The results in Fig.4 vary with the orientation of the bending, which is not controllable. Larger

distortion of the transmission spectrum has been observed for smaller bending diameter. Knowing

the bending property can be very usefiil in selecting fibers for proper deployment.

115
Another important property of single-polarization fibers is the extinction ratio for singlepolarization operation, which is defined by the relative strength in decibels of the power

transmitted through the fiber between the two polarization modes. As our concern is with the two

linear polarization modes, the standard measurement of polarization-dependent loss 10 (PDL) can be inteipreted as the extinction ratio measurement of a single-polarization fiber. We measured

fiber 5 by using a PDL meter (Profile PAT-9000B) with a tunable laser source. In Fig. 5 we show

the measured PDL for three lengths of fiber 5 across the single-polarization wavelength window.

No significant difference among the three lengths was found. Even when the fiber length was as short as 27.8 cm, the average PDL, i.e., the extinction ratio, was 60 dB. Note that the

measurements may be constrained by the dynamic range. The actual extinction ratio could be

higher. The PDL results suggest that compact fiber-polarizer applications with short pieces of

fiber are feasible. The ability to reach a high extinction ratio in a short length of fiber is a

significant advantage over other single-polarization fibers, which were reported to have lower

extinction ratios with linear length scaling.

80

70 _ 60

CO3 50 J 40 g 30

20

100

Figure

' Ail

L

-

101.3 cm ^ JftIVs

53.9cm !)/T \

M

27.8 cm '/ /

h4 1 1

5.0

3.0 —

1.0

1.0 —

-3.0

1420 1440 1460 1480 1500 1520 1540 Wavelength (nm)

5. Measured PDL for different fiber lengths.

-5.0

Relative ERl

Relative ER2

I I I I I I -40 -20 0 20 40 60 80 100 Temperature (C)

Figure 6. Relative ER as a function of temperature.

-100

1 1 50 200 1250 1300

Wavelength (nm)

Figure 7. Transmission spectaim for both polarization modes measured at three different temperatures.

Recently, we studied the temperature related properties of dual air hole fibers as either a PMF or SPF. As a PM fiber, the main concern is if the extinction ratio can be maintained

throughout the range of temperatures that the fiber can be operated at, such as from -35°C

to 85°C. For 500m long dual air hole fiber, we measure the PM extinction ratio (ER) to be around 25 dB at room temperature. We then prepared 100m portion of it in loose coil

condition and measure the extinction ratio as a function of the temperature. Two slightly

different ERs are obtained because there are two polarizer alignment conditions as related

to two birefringent axes of the fiber. The change of the ERs relative to one ER at room

temperature is shown in Fig. 6. It is found the ER is maintained rather stable for the wide

range temperature. We also measure the fiber over the range of temperature to see how

the cutoff shifts with the temperature with the results shown in Fig. 7. Over 1 17 degree of

temperature change, the cutoff wavelength shifts in general upward and by less than 3nm.

This suggests that the fiber optical properties are largely independent of the temperature,

and the birefringence is primarily contributed by form birefringence. Our further

116
numerical model, which can take into account the thermal and stress effect is in reasonably good agreement with the experimental observation.

4. Distributed Cutoff Measurement

The location of the fundamental mode cutoff wavelength of single polarization fibers is very

sensitive to the geometry of the fiber as laid out in Figure 1. A minor change in the fiber

geometry can alter the fundamental mode cutoff significantly. In practice, the air pressures within

air holes applied during the fiber drawing process has most of the effect to alter the core dimension. The shift can move somehow randomly and gradually upward or download in wavelength over the entire fiber span. However, for a given portion of the fiber, the shift typically

stays in one direction.

Because of the fluctuation of the cutoff wavelengths, the selection of the target fiber can be a

difficult and cumbersome task. Many 2 meter length of fiber samples are cut from different

regions of the fiber to determine the cutoff wavelength of the fiber at a specific location, and

further prediction is made by linearly interpolate the measured results. This method is labor

intensive and the frequent cutback incurs loss of useful fibers.

OTDR

EDFA Tunable

Filter

Tunable

Filter

^ Detector Pulse Generator AO Cell Controller

Optical

Circulator

AO Cell FUT

Figure 8. Schematic of the tunable OTDR used for measuring the cutoff wavelengths distributed.

In order to simplify the fiber selection process, we have developed a novel distributed cutoff

measurement method based on the use of a wavelength tunable optical time domain reflectometer

(OTDR), which measures the cutoff shift of the fiber along the fiber. Figure 8 illustrate the scheme of the method. A commercial OTDR working at a fixed wavelength such as 1550nm is first converted into a wavelength tunable OTDR over the wavelength range of between 1530nm

and 1560nm. The tunable hght source is obtained through a two stage amplification of EDFA

working at the C-band with wavelength selecdon done by a fiber Bragg-grating based tunable

filter. The pulsed light output from the commercial OTDR is detected by a detector and used as a triggering source for a pulse generator. The pulse generator further chops the CW light from the

EDFAs into pulsed light at a specified wavelength through the use of an acoustic-optic modulator

(AO Cell). Two optical circulators are used to loop the light into the fiber under test (FUT) and

the back-scattered light back into the OTDR.

In the actual test, we launched the light from the fiber end with higher fundamental mode cutoff

wavelength, which can be found by measuring two short fiber samples taken from both end of the

fibers. In the scheme of Fig. 8 where no polarizer is used, the light will propagate till no light can

propagate further, or when the cutoff or complete extinction of the light is reached. The reached

cutoff is for the second polarization mode, which has a longer cutoff wavelength than the other

polarization mode. Since the cutoff wavelength shift toward a lower wavelength along the fiber,

117
depending on the wavelength of the Hght launched into the fiber, the length of the fiber that light can propagate varies. The information is captured by a series of OTDR traces taken at different

wavelengths. We illustrate the results from one fiber samples in Figure 9. It is shown that when

the wavelength increases, the OTDR traces fall at a shorter distance from the light launching end.

By choosing a proper threshold level, we can obtain the wavelength of the second cutoff as a function of the position as in Fig. 9(b). Since the fiber has a single polarization bandwidth of

55nm, we can pick the right portion of the fiber based on a specified requirement. For example,

for fiber that operates as SPF between 1540 nm and 1560nm, the first 60m of the fiber meets the

requirement. In this example, we illustrate the measurement of second cutoff wavelength. Should

we insert a polarizer into the light launching end and proper align it, we can gain sole access to the first cutoff The measurement scheme above can then be used to each polarization mode

separately.

1590 r ^ 1580

Cutoff 2

1.15 1.20 1.25 Position (km)1.30 100

Length (m)

200

Figure 9. (a) OTDR traces at different wavelengths. The fiber under test starts at around 1.16kni. (b) The

wavelength of the second cutoff as a function of the fiber position.

5. Conclusion

We comprehensively studied various properties of the dual air hole fibers both as a PMF and as a SPF. We also developed a novel method to measure the fiber fundamental mode cutoff

distributed along the fiber in a non-destructive and cost effective manner. The understanding

gained from the measurement efforts not only helps to facilitate the deployment the fibers in different environment but also helps us to verify theoretical model which strengthens our further research capability in this area.

References

1. M. J. Messerly, J. R. Onstott, and R. C. Mikkelson, "A broad-band single polarization optical fiber", J. Lightwave Technol. 9, 817 (1991).

2. H. Kubota, S. Kawanishi, S. Koyanagi, M. Tanaka, and S. Yamaguchi, "Absolutely single polarizadon

photonic ctystal fiber", IEEE Photonics Technol. Lett. 16, 182 (2004)

3. J. R. Folkenberg, M. D. Nielsen, and C. Jakobsen, "Broadband single-polarization photonic crystal

fiber". Optics Lett. 30, (2005)

4. T. Schreiber, O. Schmidt, F. Roser, J. Rothhardt, J. Limpert, "High power operation of a lownonlinearity single polarization photonic crystal fiber", Photonics West 2006, WA3.

5. D. A. Nolan, G. E. Berkey, M.-J. Li, X. Chen, W. A. Wood, and L. A. Zenteno, "Single-polarization

fiber with a high extinction ratio". Optics Lett. 29, 1855 (2004)

6. D. A. Nolan, M.-J. Li, X. Chen, and J. Koh, "Single polarization fibers and applications", in Optical

Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference on CD-ROM (Optical Society of America, Washington DC, 2006) OWAl

118
Micro-Tomographic Reconstruction for Arbitrary Refractive Index Profiling of

Optical Fibers

N.M. Dragomir', X. M. Goh', G. Baxter^ and A. Roberts'

'School of Physics, The University of Melbourne, Melbourne, Vic, 3010, Australia "School of Electrical Engineering, Victoria University of Technology, Melbourne, 8001, Victoria, Australia

Tel: +61 03 8344 5985, Fax: +61 03 9347 4783

n.dragomir@physics.unimelb.edu.au

Abstract: We demonstrate a micro tomographic method to measure arbitrary refractive index

profiles of optical fibers and grating structures. The method involves the use of a traditional bright field microscope in conjunction with Quantitative Phase Microscopy. The fiber is rotated through 180 degrees and the phase data is collected, then the filtered back projection technique

is used in the computed two-dimensional profile distribution. The complete micro tomographic

method makes it possible to reconstruct the optical fiber index distribution without a priori

knowledge of the optical fiber.

1. Introduction

Optical fibers, especially those with complex internal structure require a non-destructive technique

for a fiiU determination of the three-dimensional refractive index (RI) distribution. In addition, knowledge

of the refractive index distribution in many photonic devices including long period gratings and fiber Bragg

gratings is fundamental to modeling their behavior and optimizing their performance.

Roberts et al reported the use of a non-destmctive method to measure the two-dimensional (2D)

refractive index distribution of circularly symmetric optical fibers [1] using Quantitative Phase Microscopy

(QPM) [2]. A single transverse phase image of an optical fiber is measured, from which the refractive index

profile is calculated by using the Abel inversion method [3]. In addition, QPM has been used with polarized

light to determine the retardation introduced by a specimen [4], which permits the simultaneous mapping of

the refractive index and birefringence in an optical fiber. Furthermore, it has also been demonstrated that a comprehensive measurement of the distribution of residual axial stress within an axially symmetric fiber

[5] is possible when polarized light is used. Recently, differential longitudinal evolution in the fiber refractive index profile in the vicinity of a fusion splice has been quantified using QPM [6]. The method

has also been proved for the tomographic reconstruction of asymmetric fibers [7], whereby the three- dimensional (3D) refractive index distribution of a double core fiber has been reconstructed via a Radon

transform from a series of transverse phase images obtained for different angular orientations. We illustrate here new experimental results demonstrating the reconstruction of the three- dimensional refractive index distribution of a section of an ion implanted long period grating structure incorporated in a graded-index multimode fiber. This method is very timely as a wide selection of new

optical fibers with complex structures and various enter the telecommunication and sensors market [8,9].

2. Quantitative Phase Tomograpliy

A significant number of previously published articles have discussed the principle of QPM [1, 7]. Here, it is sufficient to note that in the context of non-destructive fiber characterization it is based on the measurement of the phase shift introduced into an optical wavefield when it is transmitted transversely

through an optical fiber. Thus its phase is uniquely detennined by using an algorithm based on the transport

of intensity equation for a set of intensity images at known defocus. Since a transverse phase measurement of the fiber represents a projection through the refractive index

distribution within the fiber, by obtaining a series of phase measurements as the fiber is rotated around its axis through 180 degrees, standard tomographic techniques can be used to reconstruct the three- dimensional refractive index distribution. Figure 1 shows the coordinate system and the micro tomographic geometry based on a conventional transmission microscope where the fiber sample is rotated within the field of view of a lens. The z-axis of in the Cartesian coordinate system (x ,y ,z) defines the axis of rotation of the fiber sample; the rotated coordinate system is defined by (p, 9, z) and is related to the reference system thi'ough the angle of

119
rotation 0, measured from the positive x-axis. A miniature stepper motor turns the fiber sample which is placed between a glass slide and coverslip.

TrariOTis^ion

piezoetectnc

posrtioner

iuminafon

Soiirce

M«yog8ri lamp

Fig. 1. Schematic of the micro tomographic imaging setup. The fiber (8 cm length) without its plastic jacket is mounted side-on in the tomographic mount using a standard fiber chuck. The fiber chuck is mounted on a micro stepper motor that pemiits a full 360° rotation. The fiber is placed in a pool of index matching fluid (Cargille Laboratories) fonned between two other identical fibers located on either side of the specimen, acfing as spacers to minimize the introduction of spurious tilts into the phase images.

Let A n(x, y, z) denote the refractive index distribution of the optical fiber, the measured phase of

the optical fiber, (p (p. 0, z) for a distinct position, 0 , is proportional to the integration of the refractive index

distribution through the fiber specimen and can be expressed as a Radon transform [10]: ^(p,0,r)=/cj A«(pcos0 -5 sin 0,p sin 0 +5'cos0, z)(i5 (1)

where the wavenumber k = 2n/ X and A is the wavelength.

A number of mathematical algorithms to reconstruct a quantitative map of the RI distribution from

a series of projections, (pip, 6,z), as 0 is varied through 180° are available. Here filtered backprojection was used [10].

3. Experiments and Results

The fibre sample used in this work was a section of a graded index multimode fibre (Coming

62.5/125 |im) containing a long period grating (LPG) (500 \xm period). The LPG was fabricated by an ion- implantation process [11] carried out using a focused ion beam of singly charged light ions (¥C ions at a concentration of lO"" ions/cnr) accelerated through a 2.4 MeV potential). Upon entering the fibre the protons travel to a depth which is determined by their energy, mass and the nature of materials in the fibre cladding and core. This produces a compaction leading to a significant increase of the RI in both the cladding and the core. Images of the optical fiber were obtained using a conventional bright field microscope (Olympus

BX60) equipped with a commercial implementation of QPM [12]. Intensity images were recorded with a

120
cooled camera (Roper Scientific CoolSNAP), a 12-bit camera with a 1392 x 1040 pixel CCD image sensor (Sony ICX285AL) and a 6.45 )im pixel size. The subsequent intensity images of the optical fiber samples

under investigation were magnified by a lens system consisting of objectives lenses (UplanApo 20x/0.70

NA for the LPG) and a 0.5 x relay lens to couple the microscope intermediate image into the camera CCD

chip. Fig. 1 . The numerical aperture (NA) of the condenser was chosen to be less than the NA of the objective lens being used and it was set to 0.2 to improve spatial coherence. The illumination, provided by

a lOOW halogen lamp was filtered using a band pass filter centered on 52 1± 10 nm. Control of the defocus

positions at either side of the plane of interest was achieved using a piezo-driven nanopositioning system

(PIFOC, Physik Instrumente, Germany).

Fig. 2. Reconstructed refractive index distribution obtained from transverse phase images collected over

180° in 2" steps, (a) transverse 2D slice through the ion implanted region of a graded-index multi-mode

fiber; (b) rendered three-dimensional tomography plot for a 60 \xm length of the fiber.

A total of 90 transverse phase images of the fiber were acquired at an angular interval of 2° using a defocus of ±3 jim. The phase images were first aligned using an algorithm based on centering on the maximum phase. The Rl of a series of slices through the fiber was obtained using filtered backprojection.

This was achieved by applying the IDL (V6.1) RADON function to 'ramp' filtered data. These slices through the fiber were then fused together to produce a 3D RI distribution of the sample. Figure 2 (a) illustrates a transverse section through the reconstructed RI corresponding to the irradiated region of the multi-mode fiber. In Fig. 2(a) a parabolic index variation of the fiber itself is apparent as is an index

enhanced arc through the fiber corresponding to the end-of-range of the ions. As predicted prior to implantation, the end-of-range passes close to the centre of the core of the fiber.

0.04

0.03 ^

^ 0,02

0.01 E0.00

=1 ' 1

1 1 1 11

1 1 1

1

1 1

/ \ ^/"'"'^

//

//

//

1

1

Unirradiated regiori = - — Irradiated region :

1

1 ~ i~ r 1 '\ 1 1 1 1 1 1 1 L_ 1 1 1 1 \-j—1—j—i—1

-60 -40 -20 0 20 40 60

X (wm)

Fig. 3. Comparison of reconstructed RI through the central region of unirradited (solid line) and irradiated (dashed

line) for LPG data. An increase of the RI with -0.005 can be observed through the irradiated section.

121
A 3D volume rendering of the RI distribution of the LPG is illustrated in Fig. 2 (b), where changes

in the RI distribution along the fiber length can be seen. A line profile through the centre of the fiber of the unirradiated region (solid line) and through the iiradiated region (dashed line), is shown in Fig. 3. This

shows that the index increase at the end-of-range is of the order of 0.005, while a more subtle increase in index of approximately 0.002 is apparent between the iiradiated surface and the end-of-range, while, as expected there is no change in index below the end-of-range.

3. Conclusions

This paper illustrates a micro tomographic method to measure the refractive index distribution using phase infonnation of an optical fiber section with arbitrary refractive index distribution. It can be

seen that QPM is a powerful tool for the characterization of optical fibers and photonic devices. In the case of non-symmetric distribution of the RI of optical fiber micro tomographic reconstruction of the 3D RI

distribution of optical fibers was obtained. Ongoing research is aimed at a more comprehensive assessment of the accuracy and sensitivity of the technique as well as the development of an improved rotation mount.

Acknowledgement

The Financial support from the Australian Research Council and the provision of the samples by OFTC are acknowledged.

References

[I] A. Roberts, E. Ampem-Lassen, A. Barty, K. A. Nugent, G. W. Baxter, N. M. Dragomir, and S. T. Huntington, "Refractive-index profiling of optical fibers with axial symmetry by use of quantitative phase

microscopy," Opt. Lett.. 27: 2061-2063, 2002.

[2] A. Barty, K. A. Nugent, D. Paganin, and A. Roberts, "Quantitative optical phase microscopy,"

Opt. Lett., 23: 817-819, 1998.

[3] M. Kalal and K. A. Nugent, "Abel inversion using fast Fourier transforms," Appl. Opt., 27: 1956-

1959, 1988.

[4] A. Roberts, K. Thorn, M. L. Michna, N. M. Dragomir, P. M. Farrell, and G. W. Baxter, "Detennination of bending-induced strain in optical fibers by use of quantitative phase imaging," Opt.

Lett. ,21: 86-88,2002.

[5] N. M. Dragomir, G. W. Baxter, and A. Roberts, " Phase sensitive imaging techniques applied to optical fibre characterisation," 7th Optical Fibre Measurements Conference, Teddington, UK, 107-111,

2005.

[6] N. M. Dragomir, E. Ampen-Lassen, G. W. Baxter, P. Pace, S. T. Huntington, A. J. Stevenson, P. Farrell and A. Roberts " Analysis of Changes in Optical Fibres during Arc-Fusion Splicing by Use of

Quantitative Phase Imaging" Microsc. Res. Teclm. (accepted), 2006.

[7] A. Barty, K. A. Nugent, A. Roberts, and D. Paganin, "Quantitative phase tomography," Opt.

Comm., 175: 329-336, 2000.

[8] X. Feng, M. T. Monro, P. Petropoulos, V. Finazzi, D.J. Richardson, "Extruded single-mode highindex-core one-dimensional microstructured optical fiber with high index-contrast for highly nonlinear

opfical devices " Appl. Phys. Lett. 87: 08 110, 2005.

[9] A. Wang, A. George, J. Liu, J. Knight, "Highly birefringent lamellar core fiber", Opt. Exp.

13(16):5988-5993,2005.

[10] F. Natterer, The Mathematics of Computerized Tomography. Stuttgart: John Wiley and B G

Teubner, 1986.

[II] M. L. von Bibra, A. Roberts, and J. Canning, "Fabrication of long-period fiber gratings by use of

focused ion-beam iiTadiafion," Opt. Lett., 26: 765-767, 2001.

[12] QPm V2.1 latia Lid. AmtraWa, http://www.iatia.com.ait.

122
Fictive temperature distribution in highly Ge-doped multimode

optical fibers

M. Lancry\ I. Flammer^ D. Simons^ B. Poumellec\ C. Depecker"*, P. Nouchi^ and M.

Douay^

^ 'ICMMO/LPCES. UMR CNRS-UPS 8182, Universite Paris Sud (XI), 91405 Orsay, France

"Draka Comteq France. Site Alcatel de Marcoussis, Route de Nozay, 91460 Marcoussis, France

'Draka Comteq Fiber BV, Zwaanstraat 1, 5651 CA Eindhoven, Netherlands

_'^LSPES. UMR 8008. Universite des Sciences et Technologies de Lille, 59655 Villeneuve d' Ascq, France

^PhLAM, UMR 8523, Universite des Sciences et Technologies de Lille, 59655 Villeneuve d"Ascq, France

Abstract:

hi this study. FTIR FPA (Focal Plan Array) detector was used to image the 'bond-stretching' vibration

mode observed near a=l 120 cm"' of highly Ge-doped graded index multimode optical fibers (GI-MMF).

Next, as calibration curves between a and the fictive temperature Tf are not available in the literature for highly Ge-doped glasses (above 7 w %), we have determined our own calibration curves from 1 to 30 w%

in Ge. Then, we have applied these corrections to our a measurements in GI-MMF in order to estimate,

for the first time to our knowledge, the fictive temperamre distribution within MMF cross section.

1. Introduction

It is well known that the fictive temperature characterizes the glass structural state and is linked to glass properties such as density [1]. mechanical fatigue resistance [I] or Rayleigh scattering loss [2]. In general, a glass sample can exhibit a fictive temperature Tf of its surface different from that of its bulk.

This situation can occur, for example, when a glass is rapidly cooled from the liquid as it is done in fiber

production. In this case, a higher Tf is expected on the fiber surface than in the bulk due to a faster coohng

rate at the surface. One objective of the present work is to examine this occurrence in multimode optical

fibers. In this view, the 'bond-stretching' vibration observed near 1120 cm ' in the IR reflection spectra was monitored to determine the fictive temperature distribution in MMF cross sections.

However, the radial variation of Tf has been studied before with some divergent results [2-5]. One

problem is that the reflection peak position varies not only with Tf but also with the material composition

[2 and 5]. This is a problem as optical fiber has different composition in the core and in the cladding

surrounding the core. Therefore, calibration curves between Tf and the IR band peak position must be

available for each material composition. Unfortunately, the fictive temperature of highly Ge-doped glasses

(> 6.3 w %) cannot be rehably estimated due to the lack of calibration curves between Tf and OmoIn the following, after a brief description of the experimental procedure, we determine here caUbration

cur\'es for germanosilicate glasses from \ w9c to 30w% using only one sample. Next, using these curves, we are able to determine the Tf of Ge-doped optical fibers whatever the Ge content may be.

2. Experimental procedure

It has been shown that the fundamental asymmetric vibration band located near 1 120 cm"' is the most

sensitive to structural change and thus to the Tf changes. Thus, this band was monitored in the IR

reflection spectra to determine the fictive temperature for GI-MMF. Furthermore, this method is well

adapted to realize Tf cross-section profile in optical fibers [2-5] whereas IR transmission measurements

are less precise (lower slope of the calibration curve) and imply to prepare thin sample (around 100 |im).

FTIR reflectance spectra were thus recorded by means of a FTIR Spothght 300 Perkin Elmer equipped

with the new technology FPA (Focal Plane Array) MCT (Mercury Cadmium Telluride) detector. The

instrument provides 6:1 imaging on MCT detector, resulting in nominal resolution of 6.25 |am. The

123
spectra have been recorded in the 800 cm"' to 2000 cm"' spectral range. The reproducibihty of our measurements of the IR peak wavenumber is around 0.1 cm"'.

3. Determination of calibration curves between the IR peak wavenumber o and the fictive

temperature Tf in GI-MMF stretched preform rods

3.1. Samples preparation and treatments

The main objective of this study is to determine calibration lines for the Si-O-Si asymmetric stretching

band position versus fictive temperature for highly Ge-doped glasses. This study has never been reported

to our knowledge because it is too difficult to cut a highly Ge-doped preform rod into slices without

breaking it due to the high stress level. We have thus circumvented this tricky point by reducing the

preform diameter by a factor of 100. The preform was indeed stretched into smaller rods called capillaries

in the following. Next, we have cut these capillaries to the desired length to realize the calibration

standards. In order to achieve different uniform fictive temperatures in the calibration standards, the

samples were held at various temperatures for long time periods, long enough to ensure full structural relaxation (up to a few 100s hours). The typical temperatures selected were between 1223 K and 1523 K.

After heating treatment, these samples were rapidly quenched in water to fix the uniform fictive temperature at the heating temperature. Then, the capillaries were polished and the IR reflection spectra were measured as a function of the radial position (and thus the Ge content).

3.2. 2D distribution of the IR peak wavenumber o in GI-MMF stretched preform heated at various temperature

Figure 1 displays the 2D distribution of the peak wavenumber a related to the Si-O-Si asymmetric

stretching band recorded in reflection. The circle (black solid line) corresponds to the core part within the

capillary. In this figure, the stronger the blue color (or the darker in gray levels), the higher the peak

wavenumber a (in cm"'). These results indicate that the concentric distribution of o is quite uniform for a

fixed radial position r whereas it changes strongly according to the radial position r.

Fig. 1 : 2D distribution of the reflectance peak wavenumber a in the core of the MMF capillary cross section

Figure 2 shows the IR peak wavenumber profiles a = f (r) of a silica asymmetric stretching band

around -1120 cm"' as a function of the radial distance. In this Figure, one can clearly see that the IR

wavenumber follows a parabolic profile in the core region. Furthermore, the higher the heating

temperature (i.e. Tf); the lower the IR wavenumber along the whole profile. Now, as the germanium

concentration profile follows a well-known parabolic law (from 0 to 30 w % in Ge), we are able to determine calibrations curves for several Ge concentrations.

3.3, Determination of caUbration curves: influence of the Ge concentration

In this part, we will extract the data obtained above in order to determine calibration curves between

the fictive temperature and the IR reflection peak wavenumber for Ge concentrations up to 30 w %. Figure

3 displays several of these calibration curves for various Ge contents from lw% to 30w%. The least

124
square regression analysis reveals the following relationship between fictive temperature and IR peak

position: a(Ge, Tf) = a(Ge) - b(Ge) x Tf, where the coefficients a and b could depend on the Ge content. Based on those cahbration curves, the fictive temperatures of the highly Ge-doped (up to 30 w %) optical

fibers can be estimated once the composition of the core is known.

1120

1115

E 1110

1105

1100

-too

1125

1120

e 1115 u

i 1110 >

1105

1100

-200 -100 0 100 200 Radial position ( ^m)

400 1200 1250 1300 1350 1400 1450 1500

Fictive temperature (K) Fig. 2: Radial profiles of the reflectance peak wavenumber a for MMF stretched preforms heat treated at various

temperatures. Fig. 3: The relationship between IR peak wavenumber and the fictive temperature for GeO^-doped

bulk silica for various Ge concentrations.

In the following, we will investigate more precisely the effect of the Ge content on the calibration curves

parameters: i.e. the ordinate at the origin (coefficient a) and the slope b. We have thus extracted these

values from the above linear regressions for various Ge concentrations up to 30 w%. We found that the

coefficient a follows a linear relationship with the Ge content: a = 1129.8 - 0.469 x [Ge] with an

uncertainty Aa around + 0. 1 . In contrast, the slope b appears to be independent on [Ge] when compared to our measurements uncertainty: b = 0.0102 ± 0.0002.

Now, we have extracted relation (1) from these results to estimate the fictive temperature profile from

the IR peak wavenumber a. This allows us to estimate the fictive temperature for any radial position r provided that the Ge concentration [Ge] is known. Using the above reported uncertainties on a, b and o, we can estimate an uncertainty around 2 % in the fictive temperature determination.

a{Ge)-a{r) _ 1129 .8 - 0.469 .[Gg ](r) - cr(0 (i)

r (r) =

0.0102

4. Application: measurements of fictive temperature distribution in multimode optical fibers

Graded index multimode prototype fiber (GI-MMF) PI was realized for these experiments by Draka

Comteq using internal deposition process (CVD based). Fiber has silica outer cladding with a diameter of

125|im and a GeOi-doped core with a diameter of 50|im. The typical refractive index profile follows a parabolic law within the core. The maximum Ge concentration of the investigated fibers is around 15 ±

0.05 w%. Next, fiber was cleaved and then mounted in epoxy resin at 90 ± 1 degrees off the horizontal

direction using a V-groove metal support. Finally, the mounted fiber was etched for 30s in 10% HF-10%

H2SO4 solution and the IR reflection spectra were recorded by means of a FTIR spectrometer.

Figure 4 shows the estimated fictive temperature distribution on the cross section of the PI fiber. The

Tf along the entire cross-section varied with radial position. Firstly, the determined fictive temperatures of

the PI fiber at the near-surface region are ~ 200 K higher than the lowest in the interior of the fiber. Secondly, there is an increase i~ 150 K) of the fictive temperature from the tube towards the center of the core.

125
1600 Core

1200 -

- 1100

-60 -40 -20 0 20 40 60

Radial position r (\im)

Fig. 4: Estimated fictive temperature as a function of the radial position for GI-MMF fiber. The soHd Hnes are guides

for eyes.

In the GI-MMF fiber, pure silica used for the outer cladding has the highest viscosity while the Gedoped core has the lowest viscosity. Typically when cooled at a constant rate, a glass with higher viscosity

is expected to acquire a higher fictive temperature. Therefore, the core is expected to have 1/ a non

uniform T| due to the Ge profile (lower Tf in the center) and 2/ a lower fictive temperature than the outer- cladding. However, in our measurements, the fictive temperature of the core is higher in the center (higher

Ge content) and even higher than in the interior (e.g. from -50 to -25 )im) of the outer-cladding. One

explanation could be that during the fiber cooling, the outer-cladding becomes rigid before the core. Thus,

when the core changes from the supercooled liquid to the glass state, it is constrained by the outer- cladding and cannot change its volume freely. This might produce a higher fictive temperature state in the core when compared with unconstrained cooling. Another possibility will be that the stress distribution

influences our a measurements resulting in non-reliable calibration curves. Indeed, the stress distribution

is different in the calibration capillaries and in the fiber samples.

5. Conclusion

New technology FTIR detector (FPA) allows us to perform rapidly fictive temperature radial profile

within optical fiber cross-section with 6.25)im spatial resolution. Firstly, we have determined the

calibration curves between fictive temperature and the ER peak wavenumber related to the Si-O-Si

asymmetric stretching band for high Ge-content (from 1 to 30 w%) silica glasses. Next, using these

curves, we have estimated for the first time to our knowledge the fictive temperature radial profile in GIMMF. We show that Tf is higher at the fiber edges presumably due to the faster cooling and there is also an increase of Tf in the center of the core.

6. Bibliography

[1] A. Agarwal and M. Tomozawa, J. of Non-Cryst. Solids 209, 1997

[2] D.L. Kim et al., J. of Non-Cryst. Solids 286, 2001

[3] D.L. Kim et al., J. of Light. Technol. 19 (8), 2001

[4] P. Helander, J. of Mat. Science 39, 2004

[5] J.W. Hong, PhD thesis, Rensselaer Institute, NY, 2003

126
Monitoring OSNR in Dynamic WDM Network

Y. C. Chung

Korea Advanced Institute ofScience and Technology

Department ofElectrical Engineering and Computer Science

373-1 Giiseong-dong, Yuseong-gu, Daejeon 305-701, Korea

ychung@ee. kaist. ac. kr

Future optical networks are envisioned to be dynamically reconfigurable - optical channels are added/dropped

or cross-connected directly in the optical layer using wavelength-division-multiplexing technology. However,

for the efficient operation and maintenance of these networks, it is essential to monitor various parameters

including the channel power, optical frequency, OSNR, Q-parameter, and optical path, etc [1]. In this paper, we

will review the current status of these monitoring techniques with an emphasis on the OSNR monitoring

techniques.

Previously, OSNR has been measured by using the linear interpolation technique, in which the ASE noise was

measured in between the channels and then interpolated into the signal's wavelength. However, in dynamically

reconfigurable networks. WDM signals are added/dropped or cross-connected directly in the optical layer. Thus,

each channel could traverse through different routes and different number of optical amplifiers. As a result, the accumulated noise level could be quite different from channel to channel. However, these noises located within

the signal's bandwidths (and consequently the "true" value of OSNR) cannot be measured by the conventional

linear interpolation technique [1].

Recently, a unique OSNR monitoring technique has been proposed by utilizing the different polarization

properties of signals and ASE noises [2]-[4]. Up till now, this polarization-nulling technique appears to be the only practical solution capable of measuring the in-band ASE noise. However, the performance of this technique could suffer from PMD, PDL, nonlinear birefringence, in-band crosstalk (caused by FWM or multi- path interference), and polarization fluctuafion [2], [5]-[6]. To overcome this problem, it has been proposed to improve the polarization-nulling technique by using an additional optical bandpass filter or measuring the noise

power at the slope of the signal's spectrum [3]-[4]. It has also been reported that this technique could endure the

fast fluctuation of the state-of-polarization (SOP) of optical signal in the aerial fiber caused by wind and

electrical current in the neighboring power lines [5], [7].

To evaluate the possibility of using the polarization-nulling technique in the real systems, we investigated the

effects of PMD, nonlinear birefringence, PDL, and fast polarization fluctuation. The results showed that this technique could monitor the OSNR with accuracy of better than ±1 dB, even when DGD and in-band crosstalk were as large as 60 ps (@ OSNR < 27 dB) and 20 dB (@ OSNR=20 dB), respectively. The effect of the signal

depolarization caused by nonlinear birefringence was measured to be negligible even in a highly nonlinear

transmission link. We also investigated the effect of the partially polarized ASE noise caused by PDL. The

results showed that, as long as the PDL/span was smaller than 0.2 dB (as in most current systems [8]), the OSNR

could be monitored accurately by using the polarization-nulling technique even in a long-distance system. For example, when the PDL/span was 0.2 dB, the probability that the error in the measured OSNR became larger than 1 dB was merely 10"^ in the transmission link made of 50 amplifier spans. To verify the practicality of the polarization-nulling technique, we measured the OSNR of the optical signals transmitted through a 120-km long

aerial fiber link for a week. In this aerial fiber link, the SOP of the optical signal was measured to be fluctuated

at -0.3 Hz and 60 Hz due to winds and electric currents in the neighboring power lines, respectively. Despite of

these slow and fast fluctuations, the polarization-nulling technique could monitor OSNR with accuracy better than ±1 dB (@ OSNR = 19-28 dB). No significant degradation in the monitoring accuracy was observed

during this long-term measurement. We also evaluated the performance of the polarization-nulling technique in an ultra long-distance transmission link by using a 640-km long recirculating loop. The results showed that this technique could measure the OSNR accurately even in the transmission link longer than 3200 km.

In summary, we reviewed the current status of various optical monitoring techniques for WDM networks with an emphasis on the OSNR monitoring technique. The results show that the polarization-nulling technique could

be used in modem dynamic WDM networks and enhance their operation and maintenance.

127
References

[1]. Y. C. Chung, "Performance monitoring in optical networks (Tutorial)", Proc. APOC 2003, 5282-46, 2003.

[2]. J. H. Lee, D. K. Jung, C. H. Kim, and Y. C. Chung, "OSNR monitoring technique using polarizationnulling method", IEEE Photon. Technol. Lett. 13, 88-90, 2001.

[3]. J. H. Lee and Y. C Chung, "Improved OSNR monitoring technique based on polarization-nulling method".

Electron. Lett. 37, 972-973, 2001.

[4]. H. Y. Choi, J. H. Lee, S. B. Jun, and Y. C. Chung, S. K. Shin and S. K. Ji, "Improved polarization-nulling

technique for monitoring OSNR in WDM network," Proc. OFC 2006, paper 0ThP2, 2006.

[5]. C. H. Kim, Y. B. Lee, S. K. Ji, S. D. Choi, S. P. Hamn, M. S. Kim, M. W. Park, S. K. Shin, I. B. Kim, J. H.

Lee, D. H. Hyun, and Y. C. Chung, "Performance of an OSNR monitor based on the polarization-nulling

technique", OSA Journal of Optical NetM'orking, 3, 388-395, 2004

[6]. J. H. Lee and Y. C. Chung, "Effect of polarization-dependent loss on optical signal-to-noise ratio monitoring technique based on polarization-nulling method", Op/Zc.? isxpre^^, 13,5045-5049,2006

[7]. J. Wuttke, P. M. Krummrich, and J. Rosch, "Polarization oscillations in aerial fiber caused by wind and

power-line currenf, IEEE Photon. Technol. Lett. 15, 882-884, 2003.

[8]. T. Lima, A. O. Lima, Y. Sun, H. Jiao, J. Zweck, C. R. Menyuk, and G. M. Carter, "A receiver model for optical fiber communication systems with arbitrarily polarized noise," J. Lightwave Technol., 23, pp. 1478-

1490,2005

128
Optical signal quality monitoring based on asynchronous amplitude histogram

for DPSK systems

H. Takara, T. Ohara, and B. Kozicki*

NTT Network Innovation Laboratories, NTT Corporation, 1-1 Hikari-no-oka, Yokosuka, Kanagawa, 239-0847 Japan

* Currently at Osaka University, 2-1 Yamadaoka, Suita, Osaka, 565-0871 Japan, e-mail : takara.hidehiko@lab.ntt. co.jp

Abstract

We propose a method of monitoring the optical signal quality of NRZ-DPSK and RZ-DPSK systems that

utilizes asynchronous amplitude histogram. It is shown that the proposed method can provide information about the level of noise and the amount of chromatic dispersion for DPSK systems.

1. Introduction

Optical signal quality monitoring is an important issue in optical networks and should satisfy a number of

general requirements [1, 2]. There are several approaches for this purpose including both digital and analog

techniques. Those receiving most attention are optical spectrum monitoring [3], pilot tone monitoring [4], polarization nulling [5], and RF spectrum analysis [6]. These methods can provide information about signal

parameters. However, they are limited either to a certain signal format or to a particular degradation.

In this paper, we propose an extension of the optical signal quality monitoring method based on

asynchronous amplitude histogram to cover a wide range of signal formats. To confirm our approach, we apply

it to DPSK signals. Amplitude histograms have been shown to provide information about the level of noise through the averaged Q-factor and about the amount of residual dispersion [7] for intensity-modulated signals

at bit rates from 2.5 to 40 Gbifs. It has been verified that the same device can support both NRZ and RZ

modulation. In order to provide even greater flexibility of the asynchronous amplitude histogram method, we

investigate its application in monitoring of dispersion and noise level of NRZ-DPSK and RZ-DPSK signals.

The following sections describe the theoretical and experimental results of application of asynchronous

amplitude histogram method to the monitoring of DPSK signals.

2. Asynchronous amplitude histogram

Asynchronous amplitude histogram optical performance monitoring consists of two parts: the sampling

device and the algorithm extracting the desired information about the signal quality. Sampling is performed

asynchronously. Therefore, the key advantages of this method are that the same device can be used to monitor a wide variety of optical signals and that the expensive clock recovery circuit can be eliminated. The sampler

modeled and used in this research consists of an internal clock, electrical pulse generator, electro-absorption

modulator (EAM), O/E converter, and a signal processing unit with analog-to-digital (A/D) converter, as shown in Fig. 1

.

EAM PD A/D Analysis

A

Pulse generator

Fig. 1 Fundamental configuration of optical signal quality monitor

129
The clock operates at a frequency of 100 MHz and is not synchronized to the incoming bit stream. It stimulates

the pulse generator, which, in turn, drives the electro-absorption modulator to provide a narrow optical gate for sampling the signal waveforms. The sampled signals are received by a photodetector and processed

electronically. This arrangement is simpler than the traditional electronic sampling, because the photodetector

bandwidth can be lower than the bandwidth of the incoming signal as the signal is sampled in the optical

domain. The electrical signal is processed to form an amplitude histogram. Signal parameters can be calculated from histogram details.

3. NRZ-DPSK monitoring

When an asynchronous amplitude histogram is constructed from an NRZ-DPSK signal, a single peak is created with the mean value equal to the average power of the signal. Considering the fact, that the ASE noise

affects the intensity of the signal in the same fashion as in the case of IM signals, we adopted a similar measure

of noise for the phase-modulated signal. With the single peak present, the modified Q-factor is: a.,-^ (1)

avg . ^ where jUgx'g and Oavg are the average mean and average standard deviation of the histogram peak, respectively.

An example of NRZ-DPSK histogram and the corresponding values are shown in the inset of Fig. 2. The performance of asynchronous amplitude histogram for measuring noise in a 10 Gbit/s NRZ-DPSK

signal was estimated by simulation and experiment. Light from a tunable laser diode, emitted at 1550.2 rmi, is modulated by a phase modulator and, subsequently, attenuated and amplified. The tunable attenuator allows to modify the optical signal-to-noise ratio (OSNR) of the signal. The optional span of fiber is used for adjusting

the level of dispersion (as explained in the following paragraphs). The power reaching the monitor is kept at a

constant level of -2 dBm. The value obtained by the monitor is compared to the OSNR measured by the optical

spectrum analyzer whose resolution is 0. 1 nm.

10 - Number ol times

I I ! I 20 30 40 50 OSNR (dB) Fig. 2 Relationship between averaged Q-factor and OSNR for NRZ-DPSK signal

The results of the averaged Q-factor measurement are plotted as a function of OSNR in Fig. 2. The

experimental results are compared to the simulation results (line). In the experiment, the OSNR was changed

from 27 to 49 dB. In that range the agreement between the simulation and the experiment results is very good.

Moreover, the simulation considered OSNR values down to 15 dB. As can be observed in the figure, the

relation between the averaged Q-factor and the signal OSNR is linear in the range 15 to 40 dB OSNR.

In addition to monitoring the noise level, the asynchronous amplitude histogram method can provide

information about the level of acquired dispersion. We propose an extension of this method. An NRZ-DPSK

signal is a constant-amplitude signal with phase transitions between the bits. The temporal transitions in phase

cause frequency chirp. When the signal is transmitted through a dispersive medium, the chromatic dispersion

produces the phase-to-amplitude modulation, which manifests itself as a variation from the constant amplitude

at the bit transitions. This can be observed in the asynchronous amplitude histogram, as shown in the inset of

Fig. 3. The effect yields a pedestal with peaks at both ends, whose width is proportional to the acquired

130
chromatic dispersion; without affecting the standard deviation of the central peak. This measurement was

demonstrated in [8]. However, for large levels of dispersion (>200 ps/nm), the standard deviation of the

pedestal peaks cannot be calculated because of the excessive spread. We propose to calculate the level of

dispersion by the following equation,

Fdis.nrz = IJ-ml Hm (2),

where Hgo is the level of 90% of cumulative histogram distribution and //„, is the mean level of the entire histogram; Fdis,nrz is obtained by normalizing jigo with

Using the factor Fdis.nr-., verification tests were done using simulations and experiments by employing a number of dispersion compensating modules. The results for a 10 Gbit/s NRZ-DPSK signal are plotted in Fig.3.

The asynchronous amplitude histogram method provides a way to measure residual dispersion within the range -1000 to +1000 ps/nm. This range is much broader than the one presented in ref [8] (-170 - +320 ps/nm).

Chromatic dispersion (ps/nm) Fig.3 Relationship between Fd,s.nrz and chromatic dispersion for NRZ-DPSK signal

4. RZ-DPSK monitoring

We also propose a simple way to measure the noise and dispersion in RZ-DPSK signals by using

asynchronous amplitude histograms. Similar to the NRZ-DPSK case, the information in the RZ-DPSK signal is encoded in the phase of the

signal. However, the phase transitions, clearly visible in the former one, in the latter format occur when the

intensity reaches the "0" level. Therefore, the dispersion does not develop a pedestal in the histogram due to the

phase-to-intensit>' conversion. The actual asynchronous amplitude histogram of an RZ-DPSK signal is shown

in the inset of Fig. 4(a). The two peaks are formed because of the retum-to-zero intensity modulation. The

dispersion influences the signal (and the histogram) through chirp created by intensit>' modulation, thereby

reducing the spacing between the histogram peaks without a change in average power. Moreover, as the signal traverses the amplifiers, the ASE broadens the histogram peaks and effectively reduces the rate between the height of histogram peak and the height of histogram valley. These observations allow us to derive the parameters needed for estimating the level of noise and dispersion in the RZ-DPSK signal. In order to verify the applicability of the above effects, the following two parameters, Fsm.rz and Fdu.rz are proposed; a confirming simulation was performed on a 10 Gbit/s RZ-DPSK signal.

Fs„r.r. = N,l N„ (3)

Fdis.rz = {Ml- IUo)l M.m (4), where A', is the number of sampling points of the higher histogram peak. Parameter is the number of

sampling points of the histogram valley at the middle level, while parameters /// and are the levels of higher

and lower histogram peak, respectively.

131
The simulation results of OSNR dependence assuming constant dispersion are presented in the graph in Fig.

4(a). Noise parameter Fs„r,rz is plotted versus OSNR. For a wide range of noise levels (OSNR 15-40 dB), the parameter provides a clear relation to the OSNR. We also estimated the relation between the dispersion

parameter Fjis.ri and the actual acquired dispersion. The results are plotted in Fig. 4(b). The parameter also shows a good relationship to the chromatic dispersion. It is found that signal quality monitoring can be performed in the range from -600 to +600 ps/nm. From these results, it is expected that the proposed

parameters, Fs„r.rz and Fjis.rz- can be applied to evaluate the signal quality of RZ-DPSK signals.

5. Summary

This contribution proposed a method of monitoring the optical signal quality of DPSK systems based on

asynchronous amplitude histograms. Parameters are derived from the histograms and shown to be in a close

relation to the actual performance of the signal in terms of dispersion and noise level. The asynchronous

amplitude histogram method has been proven to be a flexible method for optical performance monitoring of

both intensity- and phase-modulated signals.

This work was supported in part by the National Institute of Information and Communications Technology

(NICT) of Japan.

References

[1] G. Bendelli, C. Cavazzoni, R. Girardi, and R. Lano, ECOC 2000, vol. 4, pp. 1 13-116, Sep. 2000.

[2] R. Giles, SOFM2002, pp. 19-24, 2002.

[3] K. Park, S. Shin, and Y. Chung, Electronics Letters, vol. 35, pp. 415-417, 1999.

[4] H.C. Ji, K.J. Park, J.H. Lee, H.S. Chung, E.S. Son, K.H. Han, S.B. Jun, and Y.C. Chung, J. Optical

Networking, vol. 3, pp. 510-533, 2004.

[5] J. Lee, D. Jung,, C. Kim, and Y. Chung, IEEE Photonics Technology Letters, vol. 13, pp. 88-90, 2004.

[6] C. Dorrer, and X. Liu, IEEE Photonics Technology Letters, vol. 16, pp. 1781-1783, 2004.

[7] I. Shake, and H. Takara, IEEE J. Lightwave Technology, vol. 20, pp. 1367-1373, 2002.

[8] Z. Li, C. Lu, Y. Wang, and G. Li, IEEE Photonics Technology Letters, vol. 17, pp. 1998-2000, 2005.

132
New methods for investigating mode coupling in multimode fiber:

Impact on high-speed links and channel equalization

Stephen E Ralph and Amp Polley

School of Electrical and Computer Engineering

Georgia Institute of Technology, Atlanta, Georgia 30332

K. D. Pedrotti, R. P. Dahlgren and J. A. Wysocki

Department of Electrical Engineering

University of California, Santa Cruz, CA 95064-1077

Introduction

Multimode fiber has reemerged as a potentially high performance optical transport media with

under exploited capabilities. By examination of the impulse response of MMF with large temporal

resolution and large dynamic range, enabled by measurements with sub-picosecond jitter, we quantify the impact on the impulse response and determine the mode coupling. These results together with numerical

models enable us to accurately depict the transmission of ultrafast data streams, and thereby quantify

system performance increases in the context of adding optical and electronic signal processing to legacy

and modem multimode fiber. We first provide an overview of the mode coupling theoretical results, which lead to the fiber impulse response for various levels of coupling strength. This provides a motivation for the new

experimental methods which are subsequently discussed. The experimental results are then fit to the mode

coupling theory, providing a measure of mode coupling strength in modem multimode fiber. We then

discuss the impact of mode coupling on the perfonnance of electronic equalization methods. Lastly we

outline our new results which exploit Raman gain to selectively amplify only one mode group and thereby

improve the fiber impulse response.

Mode coupling theory

A set of coupled equations describing the power flow in a waveguide with N modes can be derived via a perturbation approach [1].

where, Pp is the power in the n* mode at time t at a distance z. Yn is the attenuation coefficient for nth mode

and dn.k is the coupling coefficient between two modes n and k. In deriving ( 1 ) a number of assumptions

are made: a) the imperfections in the waveguide such as changes in diameter, elliptical core deformations,

or random bends of the axis lead to coupling of modes; b) these imperfections appear in a random fashion

along the length of the waveguide and a statistical ensemble average can be taken; c) the waveguide

perturbation is uncorrelated to the field amplitude beyond a certain correladon length; d) the distance required to create any change in field amplitude is large compared to correlation length. The assumptions

c) and d) essentially make the waveguide perturbation fiinction and field amplitudes uncorrelated over the

entire waveguide. It should be noted that the perturbation interacts strongly with the phases of the field and

the effect is integrated over the length of the waveguide. The spatial frequencies of the perturbation are determined via a Fourier analysis and are used to quantify the interaction with the propagation phase

difference of the modes. The contribution is contained in the mode coupling coefficient dn.k given by

where, E„ is the radial electric field profile of the n" mode, n(r) is the refractive index profile, A^,a is the difference in propagation constant of mode n and k and A is the mode coupling strength. The electric field

133
profiles and the propagation constants as well as the group delays used later are all numerically determined

using our mode solver which allows arbitrary core index profiles. The electric field profiles in (3) are nonnalized such that:

/ (4)

There are wave vector degeneracies among the modes and complete mode coupling is assumed between the these modes which form a mode group as suggested by (2). Furthermore, since the adjacent mode groups have the smallest z1/?„.a, coupling is dominated by them. Considering this the average power Pg of the mode

group g is found by solving the equations:

-+T

dz ' dt ^+1

-1

-1

^g-i(^ir-i-^J for g even

(5)

odd

The coupling coefficient dg between two adjacent mode groups g and g+1 can also be found using an approximate analytical solution [2] and we find that it matches well with the exact value obtained using (2), (3) and (4) directly -

' 2

nk,.

(6)

where, gror is the total number of mode groups present and a is the radius of the fiber. A split step method

[3] is used to evaluate (5) for a range of mode coupling strengths /i. Figure 1 depicts the impulse response for a 62.5|im multimode fiber (MMF) subject to a center launch Gaussian spot excitation consistent with the mode size of single mode fiber. The intrinsic bandwidth is that of a lOGbps channel. The nature of the channel response is strongly dependent on mode

coupling. For A< lO"* the coupling has negligible effect and we observe the arrival of the distinct mode

groups. On the other hand, for A>10'' the response becomes Gaussian due to strong mode coupling. For

intermediate coupling, energy is seen to "fill in" the valleys between the primary mode groups. Essentially, some energy propagates with a group velocity correspond to some average of other distinct groups. This observation suggests the new experimental method; observe the impulse response with

sufficient temporal resolution and dynamic range to observe this fill-in effect. Then the coupling

coefficient can be estimated by fitting the results to the numerical model.

0)CQ

Pio

ftm

0

>

-H

0)u

OJ

10

10

10

10

10

10

Fig. 1. Numerical estimate of

the effects of mode coupling on received pulse shape for an 8km MMF. For weak

coupling (A < lO'*), the arrival of distinct mode groups is observed. For strong coupling

(A > lO'^), the received pulse

is nearly Gaussian. For

intermediate coupling, energy exchange between the modes

results in energy arrival at times other than that of the

distinct mode groups and the "valleys" are gradually filled

in. [Ref 3] Time [ns]

134
Measurement methods

Previous methods to determine the mode couphng strength in (MMF) are essentially based on comparing the evolution of mode power distribution (MPD) observed experimentally with that predicted by

the mode coupling theory [1], The far field analysis method is used to determine the mode power

distribution at the end of the fiber. Our method relies more on comparing the evolution of temporal

impulse response of the fiber with that predicted by a MMF model including the mode coupling. Apart

firom the change in MPD, the other obvious effect of mode coupling is the energy filling between the

distinct mode groups and the corresponding change in the temporal response. The MPD at launch is dependent on the excitation and the specific fiber and is not directly measurable. Therefore, instead of

comparing the change in MPD, we focus on the impulse response, which is sensitive to the mode coupling

coefficient and, more importantly, is directly related to the channel performance.

Impulse response measurements of the MMF are done with sufficient temporal resolution and

dynamic range to separate the mode groups distinctly and quantify the energy arriving between them. To

minimize the chromatic dispersion, 16 ps FWHM pulses at 1550 nm are launched. A mode locked laser followed by bandpass filters is used to generate the nearly transform-limited pulses. A detector-sampling

module with net bandwidth of 20 GHz is used as the receiver. A GRIN lens couples all modes of 50 |Lim

MMF in the detector with nearly 100% efficiency. The signal is launched into the MMF core with a single

mode fiber (SMF) at different offsets which allows control of the power distribution in the excited modes.

To reduce the noise, the response is effectively averaged for 32 minutes and 3 orders dynamic range is obtained. Care is taken to minimize the jitter and drift in the acquired wavefonns.

The use of 1550 nm helps to insure that the primary modes are temporally separated due to large differendal modal delay (DMD) at 1550 nm for fibers optimized at 1310 nm. Coupling strength A, which

is a function of micro-bending perturbation and mechanical properties of fiber, is expected to be only

slightly dependent on the wavelength of operation. By comparing the measured impulse responses of 4.4 km 50 |im MMF with numerically generated impulse response we estimate a coupling strength A of 1x10^. The observed uniformity of arrival of the mode groups suggest that the fibers examined can be described by

an alpha profile [4] without any major index profile defects. Hence, in the numerical model, we assume the AP's to be that of a fiber with a pure alpha index profile. The modal delays and the mode power

distribution at the launch are adjusted for different correlation length to match the received waveform. We

also estimate the coupling strength A of typical 62.5 |im FDDI grade fiber to be 5x10^.

I I

Time (ns)

Fig. 2. Comparison of measured MMF response and the numerical estimate using a mode

coupling strength of A=lxlOl Energy arriving between the primary mode groups is due to mode

coupling. For this fiber, at 1550nm, the lower order modes arrive later. The lower order modes

are seen to exhibit lower coupling compared to the higher order mode groups as predicted by the theory.

135
The method described above is specifically suitable for MMFs with relatively low mode coupling

strength and therefore, requires relatively long fiber lengths in order to observe the effects of coupling. In

that regime, modes can be resolved temporally and the estimation of mode coupling strength is accurate and the impulse response is useful in estimated the efficacy of mitigation methods. We note that for smaller lengths of fiber, the temporal resolution may be made sufficient by using shorter launch pulses and

a receiver with larger bandwidth. However, the dynamic range of the measurement is often limited by the

tail of the detector response and an appropriate length of fiber should be chosen to clearly observe the mode

coupling effects. Nonlinear methods of measuring optical pulses such as autocorrelation and cross- correlation may also be used. However, the nonlinear interaction between different modes needs to be

quantified and hence extracting the infonnation quantitatively from the obsei-ved response is challenging.

In plastic optical fibers (POFs) strong mode coupling is observed and complete mode coupling is observed within 10-30 m of fiber length [6]. A method suitable in this regime illuminates the fiber end at an angle to preferentially excite higher order modes (HOMs) [7,8], mode coupling changes the MPD to couple power into the lower order modes (LOMs). The far field patterns of LOM and HOM are disk-like and annular-shaped respectively. For an appropriate length of fiber, where mode coupling effect is intermediate, transition of far field image from disk to annular shape occur at a particular angle. From the measured angle and fiber length, the mode coupling strength can be found to a first order approximation.

The extracted mode coupling strength can be used in the numerical model for verification and prediction.

Figure 3 illustrates the mipulse response of POF over 200m, no structure is observed, indicative of the strong coupling regime. However, the response is suitable for >10Gbps hnks.

1

^0.8

d

-0.6

rac

0.2

1D 100 200 300 400

Delay (ps)

Fig. 3. Response of 200 m of 50 nm core perfiuorinated polymer based POF (GigaPOF-50SR

from Chromis Fiberoptics). Absence of any modal stmcture and nearly Gaussian response

indicate a strong mode coupling regime. _ ,

Electronic Equalization Analysis

Different electronic equalization methods to mitigate channel impairments are extensively used for copper and wireless links. For 10 Gbps data transmission via opfical channels less complex structures such

as feedforward equalizer (FFE) and decision feedback equalizer (DFE) architectures are implemented. FFE,

which is essentially a linear filter, attempts to invert the channel and in the process necessarily increases the noise-to-signal ratio of channel. DFE partially avoids the problem by removing the post-curser inter symbol interference (ISI) estimated from the previously detected bits, using a feedback filter. However, for both cases the equalization process incurs a signal-to-noise (SNR) penalty which can be translated to an

optical power penalty. Though the equalization penalty depends on the specific filter architecture, number

of filter taps etc., the ideal equalization penalty for infinite length DFE (PIE-D) can be computed which

provides a lower limit for the penalty of specific implementations [5].

136
We compute the ideal equalization penalty for the 10 Gbps, 62.5^m MMF links with and without

mode coupling for different fiber lengths and two launch conditions: center launch (0-3 jam offset) and

offset launch (17-23 |im offset). The range of each launch condition accounts for the fiber coupler

misalignment tolerance and the offset launch is under consideration as a part of an IEEE standard [9]. Based upon the measured results a coupling strength ^ = 5x10^ is assumed. We examine two fiber index

profiles, a pure alpha profile and a profile characterized by a dip in the index on center. This irregularity is common in some legacy MMF.

Figure 4 depicts the equalizer penalty for these cases and shows that coupling systematically

increases the residual equalizer penalty. Additionally, the increase depends strongly on fiber index profile.

This can be understood by first considering the equalizer penalty in the absence of coupling. Initially, the penalty linearly increases with fiber length. This results from the temporal spreading of the mode groups. As the energy slowly spreads outside of a single bit slot the penalty increases. After some fiber length only

the dominant mode group is left in one bit slot and all others mode groups contribute to ISl. In this regime

the infinite equalizer penalty no longer increases. The transition point depends on the relative DMD of the

fiber and since the center dip has larger DMD it shows that it has already reached this saturated penalty

regime by 300m for center launch, Fig. 4a. Now consider the action of mode coupling. First we note that the DMD dominates the equalizer

penalty. However, once the saturated penalty regime is reached coupling increases the penalty, in some

cases by substantial amounts. This results from the slow coupling of energy out of the dominant mode

group into the other groups which contribute to ISI. The center dip profile is more susceptible to this due to the larger mode coupling found with center dip profiles. Similarly, the effect of mode coupling is larger for offset launch. Fig. 4 b since the higher order modes, which are preferentially exited with offset launch,

exhibit larger coupling than the lower order mode groups. We note that though the general observations remain unchanged, larger coupling strength as measured for 50 }im MMF will have larger impact on the equalization penalty.

0 2 4 6 8

Fiber Length (km)

0 2 4 6 8

Fiber Length (km)

Fig. 4. Impact of mode coupling on equalizer penalty as the fiber length is increased: (a) Center launch (b) Offset launch. For each launch condition, both the pure alpha and center dip fibers are shown [4].

Raman amplification in MMF

We use Raman amplification in MMF as an optical signal processing technique to boost the signal

strength as well as improve upon the signal integrity. Numerical calculations show that Raman gain of the lowest order mode LPoi in 62.5 |im MMF is comparable to that of SMF. This results from the larger Ge02

concentration in the core and hence the larger Raman gain coefficient which offsets the somewhat larger

effecdve area of the lowest mode group compared to SMF [10]. Furthermore, the larger effective area of

the higher order modes reduces the Raman gain of the higher order modes compared to that of LPoi. This

enables Raman gain to selectively amplify the LPoi mode and thereby reduce the inter symbol interference

137
from higher order modes. We experimentally show for co-propagating pump set up and for near-center launch with a SMF, the response of MMF is improved via mode selective gain.

Fig. 5. Improvement in the impulse response via mode

selective Raman amplification.

200 mW of 1450 nm pump and

16 ps pulses at 1500 nm are launched in 9 km of 62.5 |j,m

MMF via SMF at 3 |am offset from the center of the core.

Time (ns)

Conclusions

We have shown that mode coupling effects in multimode fiber can be readily observed and quantified using

a new high temporal resolution, high dynamic range impulse response characterization method. The effects of coupling are shown to increase the power penalties for subsequent electronic filtering. We also demonstrate an optical signal processing method using mode selective Raman gain that effectively reduces

intersymbol interference and enables high speed long reach MMF links.

Acknowledgements

We thank Kasyapa Balemarthy for some of the numerical calculations, and we thank Chromis Fiberoptics

Inc for POF fiber samples.

References:

[1] D. Marcuse, Theoiy ofDielectric Optical Waveguides, Academic Press, 1991.

[2] R. Olshansky, "Mode Coupling Effects in Graded-Index Optical Fibers," Applied Optics, vol. 14, no. 4, pp. 935-945, April 1975.

[3] K. Balemarthy, A. Policy, and S. E. Ralph, "Electronic Equalization of Multi-km lOGb/s Mulfi-Mode

Fiber Links: Mode Coupling Effects," submitted to JLT.

[4] i. A. Bwck, Fundamentals of Optical Fibers, iohn^Wty &?>om, 1995.

[5] J. G. Proakis, Digital Communications, McGraw-Hill, 1995.

[6] S. Zheng, X. Jin, and X. Zhang, "Analysis of the effects of mode coupling on the bandwidth

characteristics of step-index plastic optical fiber," Microwave and Optical Technology Letters, vol. 48,

no. 3, March 2006.

[7] W. A. Gambling, D. N. Payne, and H. Matsumura, "Mode conversion coefficients in opdcal fibers,"

Applied Optics, vol. 14, no. 7, pp. 1538-1542, July 1975.

[8] K.D. Pedrotfi, R. P. Dahlgren, J. A. Wysocki, S. E. Ralph, R. Gandhi and A. Policy, "Multi-gigabit

Transmission over POF", POF World West, Santa Clara CA, June 21-22, 2006.

[9] IEEE 802.3aq lOG-LRM Task Force http://www.ieee802.org/3/aq

[10] A. PoUey and S. E. Ralph, "Raman amplification in multi-mode fiber: Reducfion of inter-symbol

interference via mode selective gain," OAA 2006.

138
A differential mode delay measurement for a multimode

optical fiber with Fourier-domain low-coherence

interferometry

J.Y. Lee, T-J. Ahn, and D.Y. Kim

Department ofInformation and Communications. Gwangjii Institute of Science and Technology

1 Oryong-dong. Buk-gu, Gwangju 500-712. Republic of Korea

We present a novel differential mode delay (DMD) measurement technique for a conventional multimode

fiber (MMF) using Fourier-domain low-coherence interferometry (fLCI) with a modified Mach-Zenhder

interferomter. An optical spectrum analyzer and a broadband source were used to obtain time resolved

optical interference signal. The measured interference signal is Fourier transformed to obtain time delay

information. A scanning offset launching method is used to excite every available mode in an MMF. A

conventional MMF with its length of 9 m is tested to demonstrate the validity of our proposed method. The

experimental results of our proposed method agree well with results obtained using a conventional timedomain measurement method. Our proposed measurement method can measure a short length multimode

optical fiber with temporal DMD resolution better than 0. 1 1 ps / m.

1. Introduction

An MMF is an essential component for 10 Gb/s next generation local area network (LAN) transmission system.

Differential mode delay (DMD) of an MMF is a decisive factor in detennining the bandwidth of an optical

transmission system [1]. The concepts of DMD and the study of its measurement method have still been under

development. The conventional time-domain DMD measurement system uses complicated and expensive

instruments such as a pulsed laser and a fast detection system with a sampling oscilloscope in order to measure

broadening in a transmitted optical pulse [2]. Even though this method is an industry standard, it is not easy for the

time-domain measurement technique to obtain the DMD of an optical multimode fiber with large chromatic

dispersion. It is also hard to measure a short length MMF or a multimode PCF with the time-domain technique.

Therefore, there is a need for an alternative DMD measurement method that can be used for a sample with a short

length or high chromatic dispersion. In this study, we present an alternative novel differential mode delay (DMD)

measurement technique for an MMF using Fourier-domain low-coherence interferometry (fLCI) [3]. A MachZehnder interferometer is used with a broadband source, and an optical spectrum analyzer is employed to detect

spectral interference signal. A conventional MMF with its length of 9 m was tested to demonstrate the validity of

this proposed method.

139
2. Experiment and results

[Fusion splicer] Offset launching

Fig. 1 Our proposed experimental setup based on fLCI

The experimental apparatus of our proposed technique is illustrated schematically in Fig. 1. The basic configuration

is a modified Mach-Zehnder interferometer with an optical broadband source and an optical spectrums analyzer

(OSA) for a spectral detection. The main body consists of a fiber type 3 dB coupler and a beam splitter (BS). An

amplified spontaneous emission (ASE) light source made of an Erbium doped fiber was used as an optical source. ASE light is launching into the interferometer and is split by a fiber coupler (50:50) into both a reference arm and a sample arm. The output through a sample MMF and another output through a reference fiber are coUimated with

two lenses and combined with a bulk beam splitter. The combined beam is coupled into an OSA with another lens. A

9 m long commercially available MMF (InfiniCor. SX +50/125, Coring Inc.) is inserted into one ami of an

interferometer as a test fiber. A single mode fiber whose length is also about 9 m is inserted into another arm of an

interferometer as a reference delay line. The output end of the reference single mode fiber is aligned with a 3-axis

linear translation stage to obtain maximum visibility in a spectral interferogram. The length of a single mode fiber is matched to those of MMF to cancel out the chromatic dispersion effect of the sample fiber, which is the major

obstacle in a time-domain DMD measurement system. This allows us to extend the capability of the measurement

range of our proposed measurement method dramatically.

A scanning offset launching method with a butt-coupled single mode fiber is used for an excitation of all possible

modes in an MMF [2]. All scanning launching processes are operated with a commercially available fusion suplicer. After a precision horizontal alignment between a center position of a single mode fiber and an MMF, the single

mode fiber whose core diameter is about 9 micron is scanned vertically with a step size of lum. The gap between

the end faces of a SMF and a MMF have been sustained within 10 jam during a complete measurement. An OSA

detects a spectral interference signal for each offset position. The spectral sweeping range of the OSA is 20 nm, and

its sweeping time is less than a second. A fiber polarization controller (PC) is put in the reference arm of

interferometer to acquire the maximum visibility of a spectral interferogram. A captured interference signal is transferred to a computer via a GPIB interface. All processing is done by a specially made LabView program

automatically. Detailed theoretical expressions about a differential mode delay measurement method with fLCI can be found in Ref [4]

140
1545 1550 1555

Wavelength[nm]

(a)

1560

iE. 0

T3

:d

E

<

Time [ps/m]

3.3 3.9 4.4 5.0 5.6 6.1 6.7

30 35 40 45 50 55 60 Time [ps]

(b)

Fig. 2. (a) A measured spectral interferogram by using our proposed method when the offset between the launching SMF and an MMF is zero, (b) Calculated relative time delay graphs obtained from Fourier transformation of the spectral interferogram shown in Fig. 2(a)

A measured spectral interferogram at zero offset position between the launching SMF and the sample MMF for a conventional MMF is shown in Fig. 2(a). This spectrum contains the inforination of a relative modal delay between

the fundamental mode of the reference SMF and a mode group of the test fiber excited by a given offset launching

position. In order to obtain modal delay information from this interferogram, we need to do a numerical Fourier

transformation. We have first converted the wavelength axis of Fig. 2(a) into a frequency axis and used a cubic

spline fitting algorism to obtain a regularly spaced data in frequency domain before doing a numerical Fourier

transformation. Then, the rescaled spectrum is Fourier transformed to obtain the inode delay distribution of the test sample [5]. Fig. 2(b) is a calculated modal delay from the interferogram shown in Fig. 2(a). The top axis of the graph shows the modal delay normalized by the length of the fiber.

Fig. 3 shows measured modal delays for all available mode groups of the sample MMF excited by the single mode

coupled offset launching method. There is 1 |um difference in offset launching position between adjacent waveforms

in this figure. The horizontal axis is normalized by the length of the fiber. The top graph is when the offset position

is +25 |im while the bottom one is when the offset position of the launching SMF is -25 |im. The intensity of each

waveform was normalized by its maximum peak. From those measured modal delay profiles in Fig. 3 we can

calculate the DMD of the sample MMF, which is defined by the 25% threshold level between the first leading edge

and the last trailing edge of all waveforms. The DMD of this MMF is obtained to be 1.65 ps/in. From the Full width

at half maximum of peaks in Fig. 3, the temporal resolution of our proposed measurement method is estimated to be

about 0.98 ps for 9 m sample length. Therefore, the minimuin measurable DMD in our ineasurement system is about

0.11 ps/m (= 0.98 ps / 9m). The experiinental results of our proposed inethod agree well with the results obtained

from a conventional time-domain measurement method [6]. Comparing with the results obtained with a time-domain

method, our measured DMD profiles shows asyminetric shape with respect to offset launching positions around the center of the sample MMF. We think that this asymmetric shape comes from mismatches in polarization state and

overlap integral between the transmitted reference signal from an SMF and an excited high order optical signal from

a sample MMF.

141
Time delay [ps/m]

3.3 3.9 4.4 5.0 5.6 6.1 6.7

30 35 40 45 50 55 60

Time delay [ps]

Fig. 3. Differential mode delay profile by our proposed method

3. Conclusion ^

We have demonstrated a new, powerful DMD measurement method using fLCI for a conventional multimode fiber. We have shown that offset launched DMD profiles can be obtained with subpicosecond accuracy for a conventional

MMF with less than 10m length. Measurement results have good agreement with a conventional time-domain DMD

measurement method. Our proposed DMD measurement technique as several advantages compared with

conventional time-domain techniques; it has a high sensitivity and a good temporal resolution (< 0.11 ps/m), and it can be used for a short length MMF. We expect our technique to be an alternative practical method for determining

the modal dispersion of a conventional multimode optical fiber.

References

[1] Mechels, S.E.; Schlager, J.B.; Franzen, D.L, "High-resolution differential-mode delay measurements in optical fibers using a frequency-domain phase-shift technique" PTL, Vol 9, pp.794 -796, (1997)

[2] TIA-455-220-A, Differential Mode Delay Measurement of Multimode Fiber in the Time Domain, Telecommunication

Industry Association (2003)

[3] Adam Wax, Changhuei Yang, Joseph A. Izatt, "Fourier-domain low-coherence interferometry for light-scattering spectroscopy", Opt. Lett. 28, 1230-1232, (2003)

[4] J.Y. Lee, T-J Ahn, S Moon, Y. C Youk, Y. M Jung, K. Oh, and D. Y Kim, "Fourier-domain low coherence interferometry for

differential mode delay analysis of an optical fiber". Opt. Lett, (accepted)

[5] C. Dorrer, D. Belabas, J. P. Likforman, and M. Joffre, "Spectral resolution and smapling issues in Fourier-transform

spectral interferometry", J. Opt. Soc. Am. B, 17, 1795-1802, (2000)

[6] T. -J. Ahn, and D. Y. Kim, "High resolution differential mode delay measurement for a multimode optical fiber using a modified optical frequency domain reflectometer", Opt. Express, vol.13, No. 20, 8256-8262, (2005)

142
143
144
AUTHOR INDEX

iocs

Abrardi. L. 51, 59 Ito, F. 33

Ahn, T-J. 1 39 X V J, -L/

.

88

Andersson, A. 30 XAiLiliii Ld^ XX. 33

Arii, M. 33

Arvidsson R 6 Tj^qoti T f.

Baig, M.I. 84 Kim, B.Y. 98

Rarwond Ct P 63 Kim, D.Y. 79 139

Raxter O 1 19 Knabe K

Koh I 1 1 3 Carrasco-Sanz A 51, 59 129

Chamnavere A 75 Krpf^pr S 18

Chen, H. 26 Krohn. D. 1 Chen X 113

Chung, Y.C. 127 Lancry, M. 123

Coddington, I. 45 Lao, D. 37

Corredera P 51, 59 T arsson A 6

Corvvin. K.L. 55 Lear, K.L. 14

T ecfEiiche V 75

Dahlgren. R.P. 133 Lee, C.-H. 98

J—' W L/ ^ ^ IV W 1 , . 123 Lee, J.Y. 139

Douay. M. 123 Li, M.-J. 113

Dragomir, N.M. 119 Liu, X. 41

T'dward's C S 63 Marpolis J- V XCXX t-,^ 1 1 l3 XX. H Si_J . 63

Vfartm-I one/ S 51, 59

Flammer T 123 McFerran, J.J. 45

X Iw^^Cllt^ iVX. 18 TVfivauchi J. T X X y u d w XXX ^ TX . 84

Moon S 79

vjciiitL/y, J-/. 26

Giffnrd D 1X 8\J Npwbiirv N R 45

Gill P 63 Nilsson X ^ XXL7 i3 WXX^ XHX < -FJ—< > 6

Oirarrl A 92 Nolan X ^ \_/XUXX^ DJ—^ m AJ. X. • 113

Gnh X M 119 Nouchi X ^ W L-t-V'XXX^ PX • 123

75

51, 59 Ogushi, L 33

Ohara, T. 129

Hall, J. 88

Hanson, T.A. 67 Pedrotti, K.D. 133

Hart. C. 88 Polley, A. 133

He, G. 26 Poumellec, B. 123

Hedekvist, P.O. 30

Hemanz, M.L. 51, 59 Ralph, S.E. 133

Rangaswamy, S. 37

Imai, M. 84 Roberts, A. 119

145
Rowley, W.R.C. 63

Salinas, J. 104^

Sauron, F. ~ . 75

Schinn, G.W. 26

Simons, D. 123

Seller, B. 18

Sorin, W.V. 98- Sun, X. 41

Swafford, R. 22

Swann, W.C. 45

Takara, H. 129

Tanaka, K. 33' Thapa, R. 55

Theberge, J. 26

Turukhin, A. 75

van Doom, E. 37

Vayshenker, I. 22

Wang, Y. 10

Washburn, B.R. 55

Weaver, O.L. 55

Wolfe, M. 18. Wysocki, J.A. 133

Xiao, J. 41

Xu, C.-Q. 10

Yang, S. 22

Yuan, G. 14

Ziemann, O. 107

G'JCy U.S. GOVERNMENT PRINTING OFFICE: 2006 — 561-987 / 45005 Region No. 8

146
SOFM 2006 PROGRAM

A NIST symposium for photonic andfiber measurements

September 19-20, 2006 - Boulder, Colorado

Tuesday, September 19, 2006

8:30 Opening Remarks: Paul Williams, NIST, Symposium Co-Chair

Session I. Fiber Sensors, Chair: Gordon Day, NIST

8:45 INVITED: Photonic Sensor Market Evolution, Opportunities and Metrology Needs, David Krohn, Light Wave Venture LLC

9: 15 Modulation Function Study of Coupling Based Intensity Modulated Fiber-Optic Sensors, Johan Jason' Hans-Erik Nilsson', Bertil Arvidsson''^, Anders Larsson^, ^ Department ofInformation Technology and Media, Mid-Sweden University, ^Fiberson AB, ^Ericsson Network Technologies AB

9:30 Characteristics of Spun Fiber Grating Sensors, Yong Wang and Chang-Qing Xu, Department ofEngineering Physics, McMaster

University, Canada

9:45 Metrology of Integrated Waveguide Devices and Systems Using Near-Field Scanning Optical Microscopy, Guangwei Yuan and Kevin L. Lear, Electrical and Computer Engineering Department, Colorado State Universit)' 10:00 BREAK

Session II. Power-Related Metrology, Chair: Alan McCurdy, OFS

10:30 Return Loss Measurement in the Presence of Variable Insertion Loss Using Optical Frequency Domain Reflectometry, Stephen Kreger, Mark Froggatt, Dawn Gifford, Matthew Wolfe, and Brian Soller, Luna Technologies

10:45 High-Power Nonlinearity of Optical Fiber Power Meters for Pump Lasers at 980 and 1480 nm, I. Vayshenker and S. Yang, NIST, R. Swafford, OZ Optics Ltd.

1 1 :00 Novel Approach for Non-Intrusive "Clip-On" Fiber Monitoring, Gang He, Daniel Gariepy, Hongxin Chen, Jean Theberge, and Gregory W.

Schinn, EXFO Electro-Optical Engineering, Inc. 11:15 Measurement Technique for Stimulated Brillouin Scattering Threshold, Per Olof Hedekvist and Anne Andersson, SP, Swedish National

Testing and Research Institute

1 1 :30 Butt-Joint Splice with Refractive Index Matching Material for High Power Light in Optical Fiber Communications, Ikutaro Ogushi,

Hisashi Izumita, Kuniaki Tanaka, Fumihiko Ito, and Masahito Arii, NTTAccess Net^vork Sendee Systems Laboratories. NTT Corp., Japan

1 1 :45 High Speed PRBS for Detection of Defects in Fibers and Fiber Connectors, S. Rangaswamy, D. Lao, and E. van Doom, Intelligent

Automation Inc. 12:00 Experimental Investigation on Special Shaped Fibers Coupling with InP-Based Planar Lightwave Circuit Chips, Xu Liu, Jinbiao Xiao,

Xiaohan Sun, Department ofElectronic Engineering, Southeast University, P.R. China

12:15 LUNCH

Session III. Spectroscopic and Frequency Metrology, Chair: Janet Jackel, Telcordia

1:45 INVITED: Fiber Frequency Combs: Development and Applications, N. R. Newbury, W. C. Swann, I. Coddington, and J. J. McFerran,

NIST

2:15 Generation of a Frequency Comb by FWM in SOAs Enhanced by Raman, Ana Carrasco-Sanz^, Sonia Martin-Lopez^, Miguel GonzalezHerraez^ '', Pedro Corredera", Maria Luisa Hemanz^'and Laura Abrardi'', ^Departamento de Metrologia, Instituto de Fisica Aplicada, (CSIC)

Madrid, ^Departcmiento de Electronica, Universidad de Alcald, Spain

2: 30 Comparison of Saturated Absorption Spectra of Acetylene Gas Inside Photonic Bandgap Fibers, K. Knabe, R. Thapa, O. L. Weaver, B. R. Washburn, and K. L. Corwin, Dept. ofPhysics. Kansas State University 2:45 CW Supercontinuum Generation in Standard Optical Fiber over the O, E, S and C Communication Bands, Laura Abrardi'', Sonia MartinLopez^, Ana Carrasco-Sanz'', Miguel Gonzalez-Herraez Pedro Corredera" and Maria Luisa Hemanz^, ^Departamento de Metrologia.

Instituto de Fisica Aplicada, (CSIC), Madrid, ^ Departamento de Electrdnica, Universidad de Alcald, Spain

3:00 A C-Band Optical Frequency Synthesizer Using 1.5 Micron NICE OHMS Frequency Standards and an Optical Frequency Comb

Generator, C. S. Edwards' H.S. Margolis', G.P. Barwood', P. Gill' and W.R.C. Rowley', ^National Physical Laborato)y, 'Blackett Laboratory, Imperial College, UK

3:15 BREAK

Session IV. Dispersion Metrology, Chair: Greg Schinn, EXFO

3:45 INVITED: PMD: Measurement Method Linkages, T.A. Hanson, Corning Inc. 4:15 PMD Measurements Using Fixed-Analyzer Technique on a 1000km Amplified Link that Includes ROADMs, Vincent Lecoeuche', Fabian

Sauron' and Andre Champavere', Alexey Turukhin"^ and Enrico Gonzales", ^JDSU Commtest Division, 'JDSU Optical Network Research

4:30 Chromatic Dispersion Measurement Technique Utilizing an Unstable Supercontinuum Pulse Source, Sucbei Moon and Dug Y. Kim.

Gwangju Institute ofScience Technology (GIST), Republic ofKorea

4:45 Accurate Measurement of Chromatic Dispersion Distribution along Dispersion Managed Transmission System, Mirza Imran Baig, Tohru

Miyauchi and Masaaki Imai, Dept. ofElectrical & Electronic Engineering, Muroran Institute of Technology', Japan

5:00 Assessment of the Potential Accuracy of a Four Wavelength OTDR to Measure Chromatic Dispersion, David Ives, Joan Hall and Christian

Hart, National Physical Laboratory, UK

5:15 SESSION CLOSE

6:30 CONFERENCE RECEPTION / DINNER - RED LION INN

147
Wednesday, September 20, 2006

Session V. Tutorial - FTTx Measurement Issues, Cliair: Andre Cliampavere, JDSU

8:30 INVITED; Critical Issues Relating to FTTH/PON Testing, Andre Girard, EXFO E.O. Engineering. Inc. 9:00 INVITED: WDM-PON for FTTx, Wayne V. Sorin', Chang-Hee Lee", Byoung Y. Kim', ^Novera Optics. Inc.. ^^Korea Advanced Institute ofScience and Technology

9:30 INVITED: FTTx Deployment Issues, Jimmy Salinas, .4 Mr

10:00 BREAK

Session VI. Specialty Fibers, Chair: Tom Hanson, Corning Inc. 10:30 INVITED: Special Requirements for Measurement Techniques Used for POF and Thick Glass Fibers, Olaf Ziemann, Polymer Optical

Fiber Application Center, University ofApplied Sciences Niirnberg, Germany

1 1 :00 INVITED: Characterization of Hole-Assisted Fibers as Single Polarization Fibers and Polarization Maintaining Fibers, Xin Chen,

Joohyun Koh, Ming-Jun Li, and Daniel A. Nolan, Corning Inc.

1 1 :30 Micro-Tomographic Reconstmction for Arbitrary Refractive Index Profiling of Optical Fibers, N.M. Dragomir , X. M. Goh', G. Baxter^ and A. Roberts', ^School ofPhysics. The Universit}' ofMelboitrne. 'School ofElectrical Engineering. Victoria University of Technology,

Australia

1 1:45 Fictive Temperature Distribution in Highly Ge-Doped Multimode Optical Fibers, M. Lancry', I. Flammer", D. Simons', B. Poumellec', C.

Depecker'', P. Nouchi^ and M. Douay\ ^ICMMO/LPCES, Universite Paris Sud (XI), ^Draka Comteq France. ^Draka Comteq Fiber BV,

Netherlands. ^LSPES. Universite des Sciences et Technologies de Lille. France. ^PhLAM, Universite des Sciences et Technologies de Lille. France

12:00 LUNCH

Session VII. Systems Metrology, Chair: Dug Young Kim, GIST

1 :30 INVITED: Monitoring OSNR in Dynamic WDM Network, Y. C. Chung, Korea Advanced Institute ofScience and Technology,

Department ofElectrical Engineering and Computer Science

2:00 Optical Signal Quality Monitoring Based on Asynchronous Amplitude Histogram for DPSK Systems, H. Takara', T. Ohara', and B. Kozicki", ^ NTT Netyvork Innovation Laboratories, NTT Corp.. 'Osaka University, Japan

2:15 INVITED: New Methods for Investigating Mode Coupling in Multimode Fiber: Impact on High-Speed Links and Channel Equalization,

Stephen E. Ralph, Arup Policy, Georgia Institute of Technology, K. D. Pedrotti, R. P. Dahlgren, J. A. Wysocki, Dept. ofElectrical Engineering. University of California. Santa Cruz

2:45 A Differential Mode Delay Measurement for a Multimode Optical Fiber with Fourier-Domain Low-Coherence Interferometry, J.Y. Lee, TJ. Ahn, and D.Y. Kim, Department ofInformation and Communications, Gwangju Institute ofScience and Technology, Republic ofKorea

3:00 BREAK

Session VIII. Workshop

3:30 - 5:00 Fiber and Photonic Metrology Roadmapping

5:00 SYMPOSIUM CLOSE

SYMPOSIUM COMMITTEE

P.A. Williams, NIST, Co-Chau', T.J. Drapela, NIST, Co-Chair

A. Barlow, PerkinElmer

A. Champavere, JDSU

G.W. Day, NIST

S.C. Fleming, Univ. ofSydney

D. Franzen, NIST

N. Gisin, Univ. of Geneva

M. Hackert, U.S. Navy

T.A. Hanson, Corning

D. Humphreys, NPL

J. Jackel, Telcordia D.Y. Kim, GIST

P. Kolesar, SYSTIMAX Solutions

D. Krohn, Light Wave Venture G. Kuyt, Draka Comteq

A. McCurdy, OFS

G. W. Schinn, EXFO

C. Shaar, Photon Kinetics H. Takara, NTT

Sponsored by the: National Institute of Standards and Technology

in coopertation with the IEEE Lasers and Electro-Optics Society

And the Optical Society of America

OSA Optical Society of America Nisr ^I^AuSj LASERS & ELECTRO-OPTICS SOCIETY National Institute of Standards and Technology

Technology Administration, U.S. Department of Commerce

148
Technical Publications

Periodical

Journal of Research of the National Institute of Standards and Technology—Reports NIST research and

development in metrology and related fields of physical science, engineering, applied mathematics, statistics, biotechnology, and information technology. Papers cover a broad range of subjects, with major emphasis on measurement methodology and the basic technology underlying standardization. Also included from time to time are survey articles on topics closely related to the Institute's technical and scientific programs. Issued six times a year.

Nonperiodicals

Monographs—Major contributions to the technical literature on various subjects related to the Institute's scientific and technical activities. Handbooks—Recommended codes of engineering and industrial practice (including safety codes) devel- oped in cooperation with interested industries, professional organizations, and regulatory bodies. Special Publications—Include proceedings of conferences sponsored by NIST, NIST annual reports, and other

special publications appropriate to this grouping such as wall charts, pocket cards, and bibliographies.

National Standard Reference Data Series—Provides quantitative data on the physical and chemical properties of

materials, compiled from the world's literature and critically evaluated. Developed under a worldwide program

coordinated by NIST under the authority of the National Standard Data Act (Public Law 90-396). NOTE: The

Journal of Physical and Chemical Reference Data (JPCRD) is published bimonthly for NIST by the American

Institute of Physics (AlP). Subscription orders and renewals are available from AIP, P.O. Box 503284, St. Louis,

MO 63150-3284.

Building Science Series—Disseminates technical information developed at the Institute on building materials,

components, systems, and whole strucmres. The series presents research results, test methods, and performance

criteria related to the structural and environmental functions and the durability and safety characteristics of building

elements and systems.

Technical Notes—Studies or reports which are complete in themselves but restrictive in their treatment of a subject.

Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often

serve as a vehicle for final reports of work performed at NIST under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under procedures published by the Department of Commerce in Part 10, Title 15, of the Code of Federal Regulations. The standards establish nationally recognized requirements for products, and provide all concerned interests with a basis for common understanding of

the characteristics of the products. NIST administers this program in support of the efforts of private-sector

standardizing organizations.

Order the following NISTpublications—FIPS and NISTIRs—from the National Technical Information Service,

Springfield VA 22161.

Federal Information Processing Standards Publications (FIPS PUB)—Publications in this series collectively constitute the Federal Information Processing Standards Register. The Register serves as the official source of

information in the Federal Government regarding standards issued by NIST pursuant to the Federal Property and

Administrative Services Act of 1949 as amended. Public Law 89-306 (79 Stat. 1 127), and as implemented by

Executive Order 1 1717 (38 FR 12315, dated May 1 1,1973) and Part 6 of Title 15 CFR (Code of Federal

Regulations).

NIST Interagency or Internal Reports (NISTIR)—The series includes interim or final reports on work performed

by NIST for outside sponsors (both government and nongovernment). In general, initial distribution is handled by

the sponsor; public distribution is handled by sales through the National Technical Information Service, Springfield,

VA 22161, in hard copy, electronic media, or microfiche form. NISTlR's may also report resuUs of NIST projects of

transitory or limited interest, including those that will be published subsequently in more comprehensive form.
U.S. Department of Commerce

National Bureau of Standards and Technology

325 Broadway

Boulder, CO 80305-3328

Official Business

Penalty for Private Use $300