Development of Highly Nonlinear Fibers for
Optical Signal Processing
by Jiro Hiroishi *, Ryuichi Sugizaki *, Osamu Aso *2,
Masateru Tadakuma *2 and Taeko Shibuta *3

Nonlinear optical phenomena occurring in optical fibers result in noise and waveform distortion that are factors in signal degradation. It is therefore desirable that
nonlinear phenomena in fibers used as the transmission path be reduced as much as possible.
On the other hand consideration is being given to methods of optical signal processing that makeuse of the nonlinear phenomena occurring in the fibers. For example, by actively making use ofsuch nonlinear phenomena as four-wave mixing (FWM) and self-phase modulation (SPM), it ispossible to combine optical signals of multiple wavelengths to achieve wavelength conversion,pulse compression and the like 1)~4). Such techniques for utilizing nonlinear phenomena are considered promising in terms of the next generation of high-speed optical signal processing andlong-haul optical transmission. A highly nonlinear optical fiber has been developed for wavelength conversion using FWM that has a dispersion slope of 0.02 ps/nm2/km or less. This fiber
relaxes the dependence of the pump wavelength in wavelength conversion using FWM, broadening the conversion bandwidth. We also report on a polarization-maintaining highly nonlinear fiberand a highly nonlinear fiber with reduced clad diameter.

ABSTRACT1.

INTRODUCTION

Nonlinear optical phenomena occurring in optical fibers
result in noise and waveform distortion that are factors insignal degradation. It is therefore desirable that nonlinear
phenomena in fibers used as the transmission path be
reduced as much as possible. By actively making use ofsuch nonlinear phenomena as four-wave mixing (FWM)
and self-phase modulation (SPM), however, it is possible
to combine optical signals of multiple wavelengths toachieve wavelength conversion, pulse compression, soliton transmission, waveform shaping, and so on. To makeuse of these nonlinear phenomena in optical signal processing requires that a suitable fiber be available. We
have embarked on the development of a highly nonlinear
dispersion-shifted fiber (HNL-DSF) for the purpose of optical signal processing using nonlinear phenomena, and as
part of that process have developed a highly nonlinear
fiber intended for wavelength conversion, which has a dispersion slope of 0.02 ps/nm2/km or less and a large nonlinear coefficient. This fiber relaxes the dependence of thepump wavelength on the zero-dispersion wavelength inwavelength conversion using FWM, broadening the conversion bandwidth. We also report on a polarization-main*

WF Team, FITEL-Photonics Lab.
Optical Transmission Sub-Systems Development Dept.,
FITEL-Photonics Lab.
*3 Furukawa Techno-Research Co., Ltd.
*2

taining highly nonlinear fiber and a highly nonlinear fiberwith reduced clad diameter.

2.

NONLINEARITY IN OPTICAL FIBERS

2.1 Nonlinear Phenomena
When a light signal of high power impinges on an opticalfiber, the refractive index changes in accordance with thepower of the signal. The refractive index n may be
expressed asn= n0 + n2 . I(1)where:
n0 is the linear refractive index,
n2 is the nonlinear refractive index, andI is the power density of the signal
As a result of this, a variety of nonlinear phenomena
occur in the optical fiber, including self-phase modulation(SPM), cross-phase modulation (XPM), four-wave mixing(FWM), Brillouin scattering, and so on. SPM is the occurrence of an independent phase shift, while XPM is aphase shift when signals of differing wavelengths propagate simultaneously in the same direction. FWM is thephenomenon whereby, when signals of two or more wavelengths impinge, a new signal is produced of a wavelength
determined by a certain rule. Pulse compression using
SPM and soliton transmission become possible.
Wavelength conversion using FWM is also possible.

Furukawa Review, No. 23

2003

21

Development of Highly Nonlinear Fibers for Optical Signal Processing

We may consider nonlinear phase shift as an index of
the effect of nonlinear optical phenomena. We may represent the nonlinear phase shift ΦNL during SPM by theequationΦNL = (2π/λ ) . (n2/Aeff ) . I . Leff(2)

where:λ is the wavelength,
Aeff is the effective area of the core, andLeff is the effective length of the fiber.
2.2 Wavelength Conversion by FWM
Following is a simplified explanation of the process of
wavelength conversion using four-wave mixing (FWM).
FWM is a nonlinear phenomenon whereby, as can beseen in Figure 1, a converted signal (idler signal) is produced by the input of pump light and signal (probe) light ofdiffering wavelengths, such as to satisfy the frequency
conditions set forth in the equation
fconv = 2 . fpump - fsignal(3)

where:
fpump is the pump frequency,
fsignal is the signal frequency, andfconv is the converted signal frequency.
Figure 1 shows a single signal but in wavelength conversion using FWM it is possible, as shown in Figure 2, toperform batch conversion of signals of several wavelengths using a single pump. This type of wavelength conversion has the further advantage that it proceeds at thesame speed as the propagation of light in the fiber.

PERFORMANCE REQUIREMENTS FORHIGHLY NONLINEAR FIBERS

As can be seen from Equation (2), increasing the nonlinear phase shift can be accomplished in terms of opticalfiber characteristics by raising the value of n2/Aeff --that isincreasing n2 and/or decreasing Aeff. The value of n2 isdetermined by the material used. In optical fibers based

FIBER DESIGN

A study was made on simultaneously decreasing the coefficient of nonlinearity and lowering the dispersion slope.
Figure 4 shows representative refractive index profiles for
optical fibers. Simulations were run, and the results

Pump
Signal4.

Converted
Fiber with low dispersion slope
Fiber with conventional dispersion slopef2

Figure 1

Dispersion (ps/nm/km)

3.

on silica glass, the core is doped with germanium, which
increases the refractive index, and by increasing theamount of germanium dopant n 2 can be increased.
Increasing the difference between the refractive indexesof the core and clad improves the efficiency of light confinement, making it possible to narrow the region of lighttransmission--that is to say, to decrease effective area Aeff.
Further, in wavelength conversion using FWM, it is necessary, in satisfying the conditions for phase matching,that the pump wavelength match the zero-dispersion
wavelength of the fiber. Thus if, for example, the pumpwavelength is set at 1550 nm, the absolute value of thefiber’s wavelength dispersion at 1550 nm should be as
small as possible. Whether or not wavelength conversion
makes use of FWM, it is required, even for pulse compression using SPM or some other nonlinear phenomenon, soliton transmission, or wave shaping, that the highly
nonlinear fiber used should have a value of wavelength
dispersion that corresponds to its nonlinearity.
If the wavelength dispersion slope of the highly nonlinear fiber used is low, the bandwidth within the desired
wavelength dispersion of that fiber will be wider, rendering
it more useful. Figure 3 shows the wavelength dispersion
characteristics of a conventional fiber with a higher dispersion slope and a fiber with a low dispersion slope. It canbe seen that the same wavelength dispersion spread covers a wider bandwidth when the dispersion slope is low
than when it is high.
In terms of the characteristics of a highly nonlinear fiber,
low Aeff. and low dispersion slope must be combined witha short cut-off wavelength with respect to the wavelengths
used.
Based on the foregoing discussion, we proceeded,using nonlinear phenomena, to develop a highly nonlinear
fiber which not only has a large coefficient of nonlinearity,
but also offers the desired wavelength dispersion and a
low wavelength dispersion slope.

Conversion of a single wavelength by FWM.

Pump
Signals

Converted

1

0

-1

-2
1520

λ

1540

1560

1580

Wavelength (nm)Figure 2

Conversion of multiple wavelengths by FWM.

Figure 3

Wider bandwidth achieved by lower dispersion slope.

Furukawa Review, No. 23

2003

22

Development of Highly Nonlinear Fibers for Optical Signal Processing(a)Figure 4(b)(c)

Aeff ( µ m2)

15

(d)

Representative refractive index profiles.

14

13

12

∆1

0.2

0.4

0.6

0.8

Ra

Figure 7∆2

Ra-dependence of effective area Aeff.

a
1500

b

Ra =a/b

W-shaped refractive index profile detail.

λ c (nm)

Figure 5

1400

Dispersion slope (ps/nm2/km)

0.04

1300

1200

0.035

1100
0.2

0.4

0.6

0.8

Ra

0.03

Figure 8

Ra-dependence of cutoff wavelength λc.

0.025

0.02
0.2

Table 1
0.4

0.6

Characteristics of highly nonlinear fibers by simulation when dispersion at 1550 nm is 0 ps/nm/km.

0.8

Ra
Fiber

Figure 6

Ra-dependence of dispersion slope.

showed that the W-shaped profile in Figure 4(b), withdepressed layers of low refractive index around the center
core, gave the optimum balance between the two characteristics mentioned.
We then examined in detail the parameters of the Wshaped profile. In Figure 5 let the outer diameter of thecenter core be a, the outer diameter of the depressed layers be b, and the ratio of a to b be Ra = a/b. Changes in Ra
produce the variations in fiber optical characteristics
shown in Figures 6 through 8, in which wavelength dispersion at 1550 nm is zero.
It can be seen from Figures 6 through 8 that there is an
optimum parameter satisfying the characteristics of lower
Aeff, lower dispersion slope and shorter cut-off wavelength.
Based on the results of simulations, we selected fibers
A and B having a good balance of characteristics, as
shown in Table 1, as candidates for prototype manufacture. The characteristics shown in Table 1 are when dispersion at 1550 nm is 0 ps/nm/km.

5.

CHARACTERISTICS OF PROTOTYPES

Table 2 shows the characteristics of highly nonlinear fiber
prototypes manufactured according to the new design (AA

B

Dispersion slope (ps/nm2/km)@1550 nm

0.024

0.021

λ c (nm)

1217

1342

Aeff ( µ m2) @1550 nm

13.5

9.0

Table 2

Characteristics of Prototype Highly Nonlinear Fibers.

Fiber

A

B

C

Dispersion slope (ps/nm2/km)@1550 nm

0.016

0.013

0.031

Dispersion (ps/nm/km)@1550 nm

0.11

-0.08

0.12

λ c (nm)

1222

1354

1427

Aeff ( µ m2)@1550 nm

14.7

9.7

12.6

n 2/Aeff (×10-10/W)@1550 nm

31.0

61.9

43.2

Loss (dB/km)@1550 nm

0.48

1.16

0.83

γ

(W-1km-1)@1550 nm

12.6

25.1

17.5

PMD (ps/km1/2)@1550 nm

0.04

0.12

0.04

Splicing loss* (dB)@1550 nm

< 0.1

< 0.1

< 0.1

* to single-mode fiberand B), together with an older prototype manufactured
previously (C) for purposes of comparison. Figure 9shows the dispersion characteristic of the three fibers.
The dispersion slopes of the prototypes manufactured
to the new design were 0.016 ps/nm2/km for A and 0.013
ps/nm2/km for B, successfully achieving values less that

Furukawa Review, No. 23

2003

23

Development of Highly Nonlinear Fibers for Optical Signal Processing

Dispersion (ps/nm/km)

1

SignalHNL-DSFPC EDFA

0.5

TLS

0

OSA
A

-0.5

PC EDFA

B

Coupler

Polarizer

TLS

C
-1
1520

1540

1560

1580

Wavelength (nm)Figure 9

Pump

Figure 10 Set-up for wavelength conversion experiment.

Dispersion characteristic of prototype fibers.
1

6.

WAVELENGTH CONVERSION EXPERIMENT

To verify the superiority of the highly nonlinear fibers withlow dispersion slope, a wavelength conversion experimentusing FWM was carried out. Figure 10 shows the experimental set-up. The pump and signal light beams wereintroduced together into the highly nonlinear fiber, and thepower at the converted wavelength was measured by optical spectrum analyser (OSA). The length of the fiber used
was 200 m. Keeping the difference between the wavelengths of the pump light and signal (probe) light at 20 nm,
the wavelengths of both the pump and signal light werevaried in the vicinity of the zero-dispersion wavelength of
the highly nonlinear fiber (HNL-DSF), and the power of theconverted wavelength was measured.
Figure 11 shows the results obtained. The conversion
efficiency was highest when the pump wavelength was set
to the zero-dispersion wavelength of the fiber, anddecreased progressively as the pump wavelength departed from the zero-dispersion wavelength, but it was foundthat when fiber A was used, the drop in conversion efficiency when departing from the zero-dispersion wavelength was less than in the case of fiber C. In this it can beseen that by using a highly nonlinear fiber of low dispersion slope, the dependence of the pump wavelength during conversion was relaxed.

7.

POLARIZATION MAINTAINING HIGHLYNONLINEAR FIBER

Optical signal processing by means of nonlinear phenomena is also influenced, in addition to the wavelength dispersion of the fiber, by its polarization state. For this reason there are numerous subsystems in which polarization
maintenance is required for nonlinear fibers. Here we
report on a polarization maintaining nonlinear fiber madeof a stressed material.

#A

Normalized idler power

half that shown by the previous design (C). Prototype B in
particular combined a low dispersion slope with a large
n2/Aeff value of 61.9×10-10W-1. Both A and B prototype
fibers had a cut-off wavelength well under 1400 nm.

0.8

#C

0.6

0.4

0.2

0
-6

-4

-2

0

2

4

6

λ pump-λ 0 (nm)Figure 11 Pump wavelength tolerance.

B2O3-SiO2
SiO2
GeO2-SiO2
F-SiO2

Figure 12 Structure of PANDA highly nonlinear fiber.

Figure 12 shows the structure of a polarization maintaining highly nonlinear fiber. In this fiber the core is sandwiched between stressed material made of B2O3-SiO2,
which has a large coefficient of linear expansion than thepure silica of which the clad is made. For this reason, during the period of cooling during the fiber drawing process,a drawing strain is imparted to the stressed portion bymeans of which polarization maintenance is achieved.
Table 3 shows the characteristics obtained with thepolarization maintaining highly nonlinear fiber which isshown in Figure 12 and has the same profile as fiber A in
Table 1. As can be seen the fiber in Table 3 has outstanding performance, with a low dispersion slope. It can also
be seen that satisfactory values were obtained forcrosstalk and beat length.

Furukawa Review, No. 23

2003

24

Development of Highly Nonlinear Fibers for Optical Signal Processing

Table 3Characteristics of PANDA highly nonlinear fiber.
Item

Characteristic

Dispersion slope (ps/nm2/km)@1550 nm

0.018

Dispersion (ps/nm/km)@1550 nm

0.04

λ c (nm)1205

MFD ( µ m)@1550 nm

4.3

Loss (dB/km)@1550 nm

0.89

Crosstalk (dB/100 m)@1550 nm-32.9

Beat length (mm)

8.

4.5

HIGHLY NONLINEAR FIBER WITH
SMALL CLAD DIAMETER

Generally speaking the highly nonlinear fibers used in
optical signal processing are accommodated in subsystems in a modular configuration. This means that minimizing the space occupied by the fibers affects the compactness of the system. Single-mode fibers (SMFs) and dispersion-shifted fibers (DSFs) normally used have a clad
diameter of 125 µm, but for this work we have examined
the possibility of a highly nonlinear fiber with a clad andresin coating of smaller diameter.
Table 4 shows the characteristics obtained with a highly
nonlinear fiber with a clad diameter of 90 µm, which has
the same profile as fiber A in Table 1, and, for purposes ofcomparison, the characteristics of a highly nonlinear fiberof the same refractive index profile and a clad diameter of
125 µm.
As Table 4 shows it was possible, with a 90-µm clad
diameter, to achieve a coating diameter of only 145 µm.
This is only 58 % of the coating diameter and 34 % of thecross-sectional area of a conventional fiber with a 125-µm
clad and 250- µ m coating, enabling module size to be
reduced by 35 % and more compact subsystems to berealized.
It was also confirmed that the fiber with the 90-µm clad
was in no way inferior to the 125-µm clad fiber in terms of
performance characteristics.

9.

CONCLUSION

We have developed a highly nonlinear optical fiber with alow dispersion slope. In wavelength conversion experiments by FWM using this fiber it was confirmed that pumpwavelength tolerance was increased. Prototypes werealso built of a panda-type highly nonlinear fiber, and ahighly nonlinear fiber with reduced clad diameter to realize
more compact systems.
Optical signal processing using highly nonlinear fibers
can be used for high-speed optical-to-optical processing,as well as multiple wavelength batch conversion, pulse
compression, soliton transmission and wave shaping, andaccordingly offers great promise of playing an importantrole in the high-speed signal processing and long-haultransmission applications of the future.

Table 4Characteristics of highly nonlinear fibers with small
and conventional clad diameters.
Item

Small diameter

Conventional

fiber

diameter fiber

90

125

Clad diameter ( µ m)
Coating diameter ( µ m)

145

250

Dispersion slope (ps/nm2/km)@1550 nm

0.017

0.016

Dispersion (ps/nm/km)@1550 nm

-0.64

0.11

λ c (nm)

1213

1222

MFD ( µ m)@1550 nm

4.3

4.3

n 2/Aeff (×10-10/W)@1550 nm

31.5

31.0

Loss (dB/km)@1550 nm

0.41

0.48

Bending loss (dB/m)@1550 nm

< 0.5

< 0.5

PMD (ps/km1/2)@1550 nm

0.049

0.042

Splicing loss* (dB)@1550 nm

< 0.1

< 0.1

* to single-mode fiber

ACKNOWLEDGMENT
We would like to take this opportunity of expressing ourthanks to the following persons for their participation in thedevelopment work here described: Messrs. Kamiya,
Tamura, Onuma, Oyama and Shimotakahara of the Chiba
Fiber Fabrication Dept.; Messrs. Koaizawa, Nakamura,
Uchikoshi and Inoue of the Production Technology
Development Center; Messrs. Sakano and Namiki of the
Optical Subsystems Development Dept., Fitel Photonics
Laboratory; and Messrs. Kokura, Yagi and Kumano of the
WF Team.

REFERENCES
1) O.Aso, S.Arai, T.Yagi, M.Tadakuma and S.Namiki: Broadband
four-wave mixing short optical fibres, Electronics Letter, Vol.36
No.8 (2000)
2) O.Aso, M.Tadakuma and S.Namiki: Four-wave mixing in opticalfibers and its applications, Furukawa Review, 19 (2000), 63.
3) O.AsoÅCS.Arai, T.Yagi, M.Tadakuma, Y.Suzuki and S.Namiki:
Efficient FWM Based broadband Wavelength Conversion Using a
Short High-Nonlinearity FiberÅCIEICE Trans.Electron Vol.E83-C(2000) 816.
4) M.Onishi: Silica-based Fibers For Nonlinear Applications, OECC
(2002) 490.

Furukawa Review, No. 23

200325