Development of a Non-Zero Dispersion-Shifted Fiberwith Ultra-low Dispersion Slopeby Naomi Kumano *, Kazunori Mukasa *, Misao Sakano *2 and Hideya Moridaira *3

As a next-generation medium for overland fiber-optic transmission links thatoffers a wider range of signal wavelengths and Raman amplification, a new typeof non-zero dispersion shifted fiber (NZ-DSF) has been developed, which has a dispersion slope
only about one half that of conventional NZ-DSFs with low dispersion slope.
Generally speaking a lowering of the dispersion slope is accompanied by a contraction in theeffective area of the core (Aeff), but what is needed, on the contrary, is an expansion of Aeff toovercome non-linear phenomena that hamper WDM transmission. In the work reported here it
has been possible, by optimizing the refractive index profile, to lower the dispersion slope to only
0.020 ps/nm2/km, while maintaining Aeff at 45 µm2--about the same as for conventional DSFs.
Lowering the dispersion slope also resulted in a shift of the zero-dispersion wavelength to theshorter side, thereby not only expanding the transmission range but also enabling suppression offour wave mixing (FWM) in the pumping wavelength region when Raman amplification is applied.
It has been confirmed by a transmission simulator of the performance of the prototype NZ-DSF
that 10 Gbps transmission can be achieved, and the application of a dispersion compensator foreven higher rates is under consideration.

ABSTRACT1.

INTRODUCTION

With the achievement of higher bit rates and larger capacities using wavelength-division multiplexing (WDM) technology, there has come a need for a reduction in fiber
non-linearity together with a flatter dispersion-wavelengthcharacteristic, and several types of optical fibers havebeen proposed. In submarine and other long-haul links
there have been proposals, for example, for a dispersionmanagement link comprising single-mode fibers (SMFs)
and reverse dispersion fibers (RDFs) 1). With such a dispersion management line, total dispersion can be suppressed over a wide range in the C-band (1530-1565 nm).
On short-haul overland links, however, the adoption of dispersion management lines is difficult for reasons of installation. Attention has accordingly been turned to the nonzero dispersion-shifted fiber (NZ-DSF), which can provide
an overland transmission link consisting of a single line,
and R & D work is actively proceeding 2)~6).
Work has also be done in recent years to widen therange of optical amplifiers and this has led to a need fordispersion designs covering a wide range of wavelengths--not only in the conventional C-band (1530-1565nm)--but also including the L-band (1565-1620 nm) and Sband (1460-1530). It is also necessary to suppress or
eliminate non-linear phenomena including four-wave mixing (FWM) that is inherent in WDM, self-phase modulation(SPM) and cross-phase modulation (XPM) that cause signal waveform distortion at high power input. Also,
progress is being made in the practical application ofRaman amplifiers, creating a need to ease the limitations
on the Raman pumping region that pose a problem whenusing conventional NZ-DSFs as the transmission link.
An objective of the work reported here was to expand
the transmission range to achieve suitability for Raman
amplifiers, emphasizing the development of an NZ-DSF
with lower dispersion slope. A lowering of the dispersion
slope is generally accompanied by a reduction in theeffective core area (Aeff), but what is needed, on the contrary, is an expansion of Aeff to suppress the non-linear
phenomena. In this work the authors endeavored, by optimizing the refractive index profile, to lower the dispersion
slope while maintaining Aeff at 45 µm2--about the same asfor conventional DSFs. This paper reports the results ofthe design of the fiber and the fabrication of prototypes aswell as of confirming performance by means of a transmission simulator, and discusses work on the applicationof a dispersion compensator.

* WF Team, FITEL-Photonics Lab.
*2 Optical Transmission Sub-System Development Dept.,
FITEL-Photonics Lab.
*3 Intellectual Properties Dept.

1FiberA
B
CCharacteristics of Selected types of NZ-DSF (at 1550
nm).
Dispersion

Dispersion slope

Aeff

Attenuation(ps/nm/km)(ps/nm2/km)( µ m2 )(dB/km)

65

<0.21

8
5
5

0.07
0.09
0.045

72
55

8.0

Dispersion (ps/nm/km)

Table 1

<0.21
<0.21

A

6.0
4.0

C
B

2.0
0.0

1350

3.

DESIGN OF AN NZ-DSF WITH ULTRALOW DISPERSION SLOPE

In considering the refractive index profile of an NZ-DSF
with low dispersion slope, the authors first undertook areview of the dual-shaped refractive index profile of a 2layered structure (see Figure 2-a) Such a profile is easy tofabricate, and in the past has frequently been considered
for transmission link fibers. The electrical field distribution
is substantially Gaussian, making for easy connection toother fibers. As a result of simulations in which each parameter of the dual-shaped profile was minutely altered, it
was found that with this profile the dispersion slope could
not be lowered further than 0.06 ps/nm2/km making it virtually impossible to obtain the target value. Next an analogous investigation was made into the W-shaped profile(see Figure 2-b) which, like the dual-shaped profile, is inwide use, but it was found that lowering its dispersion
slope would also be difficult.

2

1500

1550

L
1600

Raman pumping region

Signal transmission
region-6.0

DEVELOPMENT OF NZ-DSFs WITHLOW DISPERSION SLOPE

Table 1 shows the characteristics of selected types of NZDSF that have been proposed in the past. Fiber A is characterized by a comparatively large dispersion value at awavelength of 1550 nm, and a zero-dispersion wavelength
shifted to the short side; fiber B by an expanded Aeff, andfiber C by a dispersion slope smaller than that of the others.
Looking at fiber C, an NZ-DSF with low dispersion
slope, it attains an Aeff of 55 µm2 while the lowering of dispersion slope is limited to 0.045 ps/nm2/km. And since thezero-dispersion wavelength is in the Raman pumping
region, we may say that fibers B and C are specificallyunsuited for transmission links using a distributed Raman
amplifier (see Figure 1).
In this work the objective was to lower the dispersion
slope to 0,020 ps/nm2/km, i.e., less than half the value forfiber C in Table 1, while maintaining Aeff at 45 µm2, about
the same as that of the conventional DSF. And in order tomake it a fiber that is also applicable in transmission links
using distributed Raman amplifiers, it was an objectivethat the zero-dispersion wavelength be shorter than 1430
nm 7)~10). This paper not only describes the design and fabrication of prototype NZ-DSFs, but also considers a system model applying a distributed Raman amplifier andreports on the results of a simulation of the characteristicsof the prototype NZ-DSF as a transmission link.

1450

C

-2.0
-4.0

2.

1400

S-bandWavelength (nm)Figure 1

Chromatic dispersion of selected NZ-DSFs.

α

∆1

α

∆1

∆2
∆2
a) Dual-shaped core

Figure 2b) W-shaped core

Refractive index profile of a 2-layered structure.

Since the foregoing investigations demonstrated the difficulty of achieving our target with the 2-layered structure
we proceeded to study a more complex 3-layered structure profile, and determined that its use would make itpossible to lower the dispersion slope to the target valueof 0.020 ps/nm2/km. It was also discovered, however that
if the dispersion slope was low there would be a strong
tendency toward degradation of other characteristics suchas a contraction of Aeff and a shift to a longer cutoff wavelength. We accordingly revised the design, reviewing indetail the relative refractive index difference ∆, the ∆'s
structural parameter α, the diameter ratio, and the like, following an optimization process that achieved, in a balanced manner, the target values for the various characteristics--dispersion, dispersion slope, Aeff, cutoff wavelength,
bending loss, and so on.

4.

TRANSMISSION CHARACTERISTICS

Prototypes were then fabricated in accordance with thedesign described above. Within a range of refractive index
profiles indicated by the results of the simulation, the synthesizing conditions and drawing conditions were optimized resulting in completion of an NZ-DSF with ultra-low
dispersion slope capable of being used in a link with low
loss and low polarization-mode dispersion (PMD).
Characteristics of the prototype NZ-DSFs are shown in
Table 2, where dispersion, dispersion slope, mode field
diameter (MFD), effective core area (Aeff) and attenuation
are the values at 1550 nm; λc is the cutoff wavelength,with a length of 2 m; λ0 is the zero-dispersion wavelength;
bending loss is the value of loss with respect to 1550 nm

Furukawa Review, No. 22

2002

Typical transmission characteristics of prototype NZDSFs (at 1550 nm).
123TargetDispersion (ps/nm/km)

4.2

4.9

5.7

4~8

Dispersion slope (ps/nm2/km)

0.016

0.020

0.029

<0.020

MFD ( m)

7.5

7.6

8.0

Aeff ( µm2)

43

45

48

>45

λ c (nm)
λ 0 (nm)

1156

1273

1446

<1460

1396

1387

1393

<1430

Attenuation (dB/km)

0.216

0.218

0.207

Bending loss φ 20 mm (dB/m)

4.0

1.0

0.5

PMD (ps/km1/2)

0.026

0.027

0.048

C
#2

4.0

B

2.0
0.0

1350

1400

1450

S

C

L-band

1500

1550

1600

1650

-2.0
-4.0

<1

when bent to a 20-mm diameter; and PMD is the value forpolarization mode dispersion. In Table 2 prototype #1 has
a dispersion slope of 0.016 ps/nm2/km, meeting the target,
but Aeff is somewhat low--45 µm2. Prototype #3, on theother hand, with an Aeff of 48 µm2, meets the target but
has a dispersion slope that is somewhat too large--0.029
ps/nm2/km. Prototype #2 has a dispersion slope of 0.020
and Aeff of 45 µm2, thereby meeting both targets. In all
three the lower dispersion slope resulted in a short-side
shift in zero-dispersion wavelength λ0 to below 1430 nm.
The values for the other characteristics--cuttoff wavelength, λc, bending loss, PMD, etc.--were also satisfactory.

5.

6.0

Dispersion (ps/nm/km)

Prototype number

8.0

CHROMATIC DISPERSION CHARACTERISTICS AND RAMAN GAIN EFFICIENCY

To investigate the effect of lower dispersion slope, a comparison was made among the three types of NZ-DSFs
described in the foregoing section, which all have a dispersion at 1550 nm of 5 ps/nm/km but different dispersion
slopes. As a result of prototype fabrication, fiber #2, which
has a dispersion slope of 0.020 ps/nm2/km while maintaining an Aeff of 45 µm2, was selected. As a comparison,
Figure 3 shows the dispersion curves for fiber B withexpanded A eff in Table 1 (dispersion slope of 0.090
ps/nm2/km and Aeff of 72 µm2), and fiber C with low dispersion slope in Table 1 (dispersion slope of 0.045
ps/nm2/km and Aeff of 55 µm2). In the C-band the three
NZ-DSFs show similar dispersion characteristics, whereas
in the L-band, prototype #2, which achieved an ultra-low
dispersion slope, had the greatest suppression of accumulated dispersion.
The range shown shaded in gray was the region inwhich the absolute value of dispersion was less than ±2
and the efficiency with which FWM occurred was higher. If
the wavelength at which the dispersion curve enters this"gray zone" is shown by a broken line, it can be seen that,the broken lines for B, C and #2, which have a large dispersion slope, shift toward shorter wavelength (to the left)
in that order, and FWM interference in the S-band isavoided for prototype #2.
As described above it was possible, by lowering the dis-Wavelength (nm)Figure 3

Comparison of dispersion curves of selected NZDSFs.

8.0

Dispersion (ps/nm/km)

Table 2

Raman pump
Region6.0

Transmission
Region

4.0
2.0
0.0
1350

1400

1450

1500

1550

1600

1650

-2.0
-4.0

Wavelength (nm)Figure 4

Chromatic dispersion of prototype #2 in transmission
and Raman pumping regions.

persion slope, to achieve a flat dispersion characteristics
and avoid accumulated dispersion on the long-wavelengthside, and also to widen the range in which the occurrenceof FWM can be avoided toward the short-wavelength side,
thereby expanding the range supporting transmission to
include the full range of the S-, C- and L-bands.
Next to be discussed is the suitability of NZ-DSF for
Raman amplifiers. There are two types of Raman amplifier: the discrete type and the distributed type, and bothplay important roles in the technology of optical communications. Great attention has been paid to the efficacy of
Raman amplification, and a particularly large number ofexperiments in high-capacity transmission using distributed-type Raman amplification have been reported.
Reports are also beginning to appear, on the other hand,that some NZ-DSFs are unsuited for use in transmission
links that use distributed-type Raman amplification due tothe effects of the zero-dispersion wavelength (λ0). For thisreason most recent reports of experiments using Raman
amplification involve the use of SMFs as the transmission
link 11), 12).
As is generally known, in amplifying the signal transmission band of 1530-1620 nm (C- and L-bands), it is necessary to input a pumping light of 1430-1520 nm. In the caseof prototype #2, Figure 4 shows that, in both the signaltransmission region and the pumping region for Raman
amplification, the dispersion curve does not enter the grayzone so that interference by FWM with the pumping lightis avoided.

3

Gain coefficient (1/W/km)

1.2

NZ-DSF
Conventional DSF

prototype NZ-DSF #2(ultra-low slope)

0.6

80km

80km

10-Gbps,
NRZ, 8-ch,
50-GHz spacing

1.0
0.8

80km

BER

Center wavelength:(1) 1550 nm(2) 1600 nm

NZ-DSF C

0.4

Figure 6

0.2

Pumping at

Pumping at

Pumping at(1) 1450 nm(2) 1500 nm(1) 1450 nm(2) 1500 nm(1) 1450 nm(2) 1500 nm

Configuration of system model for transmission simulation.

SMF
0.0
0.0

5.0

10.0

15.0

20.0

25.0

30.0

Table 3

Minimum bit error rate on worst channel.

Frequency shift (THz)

Figure 5

Raman gain coefficients at a pumping wavelength of1420 nm for selected fibers.

C-band (1550 nm)

L-band (1600 nm)

10 -1810 -5

10 -1710 -9NZ-DSF C(low slope, see Table 1)
Prototype NZ-DSF #2(ultra-low slope, see Table 2)

Figure 5 shows the results of measured Raman gain
coefficients at a pumping wavelength of 1420 nm forselected fibers. The Raman gain coefficient is normallyinversely proportional to Aeff, and is therefore smaller for
an SMF with a large Aeff (80 µm2) than for a conventional
DSF with a small Aeff (45 µm2). For NZ-DSFs, prototype #2(A eff = 45 µ m 2 ) has a considerably larger Raman gain
coefficient than NZ-DSF C in Table 1 (Aeff = 55 µm2), giving promise of Raman amplification characteristics thatare equivalent to those of conventional DSFs.

1.0E-03

BER on worst channel

1.0E-04

Fiber C (low slope)

1.0E-05
1.0E-06

Difference fromaccumulated dispersion

1.0E-07
1.0E-08
1.0E-09

Prototype NZ-DSF #2(ultra-low slope)

1.0E-10

6.

SIMULATING TRANSMISSION PERFORMANCE BY DISTRIBUTED-TYPE
RAMAN AMPLIFICATION

The present work consisted not only of comparing a prototype NZ-DSF fabricated to achieve an ultra-low dispersion
slope (#2 in Table 2) with an NZ-DSF with low dispersion
slope (C in Table 1) in terms of fiber characteristics, butalso of investigating the way in which the transmission
performance differed for systems in which these NZ-DSFs
were actually used.
The transmission simulations were carried in out in systems using a distributed-type Raman amplifier with thegain characteristics shown in Figure 6. The system modelfeatured 10-Gbps 8-channel NRZ signals at 50-GHz spacing, three 80-km spans of fiber, no dispersion compensation, and all transmission loss compensated by distributed-type Raman amplification. The worst-channel BER(minimum value) was compared for C-band transmission
and L-band transmission when using the NZ-DSF withultra-low dispersion slope (prototype #2 in Table 2) andthe NZ-DSF with low dispersion slope (C in Table 1).
In the simulations, first of all, with an NRZ signal of constant power input to the fibers, the conditions of Raman
pumping light sufficient to compensate completely for NZDSF loss were compared for prototype #2 (Table 2) andfiber C (Table 1). For a signal wavelength of 1550 nm (Cband) it was found that in a system using prototype #2, 64% of the power required by a system using fiber C was
sufficient. Similarly at a wavelength of 1600 nm (L-band)
the power required by prototype #2 was 65 % of that for

4

-13.0

-11.0

-9.0

-7.0

-5.0

Received power (dBm)

Figure 7

Relationship between bit error rate on the worst
channel in L-band and received power.

fiber C. It was thus confirmed that the pumping power
needed to compensate completely for transmission loss in
distributed-type Raman amplification was lower in the prototype NZ-DSF with large Raman gain.
Table 3 shows the minimum BER on the worst channel
for C-band (1550 nm) and L-band (1600 nm) transmission. In C-band transmission the BER was satisfactory forboth prototype #2 and fiber C, but it was found that in the
L-band, the BER was better for prototype #2.
Figure 7 shows the relationship between the BER on
the worst channel in the L-band and received power. For
prototype NZ-DSF #2, accumulated dispersion in the Lband was suppressed, giving better L-band transmission
characteristics than low-slope NZ-DSF C, and achieving a
BER of 10-9.
It is therefore considered that the prototype NZ-DSF #2
is capable of 10-Gbps C-band and L-band transmission
with distributed-type Raman amplification even without
dispersion compensation.

Furukawa Review, No. 22

2002Dispersion compensation for selected transmission
fibers.
Rate of compensation

Compensation

with DCF at 1550 nm(design of DSCF)

100 %

easy

SMF
Prototype NZ-DSF #2

0.02
0.01

Dispersion (ps/nm/km)

Table 482 %(ultra-low slope, see Table 2)
NZ-DSF C

1530

1540

1550

1560

-0.01
-0.02
-0.03
Total dispersion:-0.04-0.05-0 ps/nm/km

-0.05

35 %

(low slope, see Table 1)

0

-0.06

NZ-DSF B

20 %(large Aeff, see Table 1)

Wavelength (nm)difficult

Figure 9

Total dispersion after compensation in C-band.

10

Dispersion (ps/nm/km)

0
1500

1520

1540

1560

1580

1600

1620

1640

-10
-20
-30

NZ-DSF
C-band

Total

-40

DSCF

-50
-60
-70

Wavelength (nm)Figure 87.

that 40-Gbps in the C-band can be achieved using atransmission link consisting of prototype NZ-DSF #2 andthe optimally designed DSCF.

Compensating for NZ-DSF dispersion by means of
DSCF.

DISPERSION COMPENSATOR TO
ACHIEVE EVEN HIGHER BIT-RATESIn the foregoing section we have talked about transmission using the NZ-DSF by itself, on the assumption that
there is no need for a dispersion compensator. When wecome to higher bit rates--say 40 Gbps--the dispersion tolerance in the transmission link has to be controlled to theorder of 60 ps/nm, and it becomes impossible, from theviewpoint of FWM suppression, to have a long-haul link
made of a single fiber. It thus becomes necessary to havea dispersion compensator that compensates for NZ-DSF
dispersion 13), 14).
Table 4 shows the dispersion compensation rates forselected NZ-DSFs with a DCF, used for compensation of
SMFs. Of all the NZ-DSFs examined, the DCF compensation rate was largest for prototype #2 (see Table 2) andcompensator design is easy.
By fabricating a dispersion slope compensating fiber(DSCF), designed to compensate optimally for the dispersion slope of prototype #2, the compensating rate was
100 % at 1550 nm.
Figure 8 shows the average dispersion for prototype
NZ-DSF #2 fabricated in this work, for the DSCF that optimally compensated for that NZ-DSF, and for the transmission link as a whole following compensation (marked"total"). It can be seen that over the whole of the C-band it
was virtually zero. Figure 9 shows this portion of the vertical axis on an expanded scale, and it can be seen that the
average dispersion in the C-band after compensation was
contained between -0.05 and 0 ps/nm/km. This indicates

8.

CONCLUSION

In the work the authors addressed the development of an
NZ-DSF having an ultra-low dispersion slope for the next
generation of overland fiber-optic transmission links.
Because of the priority attaching to lowering the dispersion slope, we adopted the objective of achieving a valueof 0.020 ps/nm2/km, even better than the 0.045 ps/nm2/km
of currently available NZ-DSFs. A number of profiles wereconsidered and simulation was used for parameter optimization, and a prototype was fabricated which maintained an Aeff of 45 µm2 and reached the dispersion slope
target of 0.020 ps/nm2/km. By lowering the dispersion
slope, accumulated dispersion in the long-wavelength
region was suppressed and the short-side shift in thezero-dispersion wavelength made it possible to widen therange of wavelengths in which the occurrence of FWM
could be avoided, making transmission possible over a
wider range of wavelengths.
Next the suitability for Raman amplifiers was considered. The short-side shift of the zero-dispersion wavelength made it possible to avoid the interference from
FWM due to the Raman pumping light, which has constituted a problem in the past. It was also confirmed that an
Aeff of 45 µm2 holds the promise of high-efficiency Raman
amplification.
A comparative study was also undertaken by means ofsimulation using a system model applying a distributedtype Raman amplifier and using as the transmission link
the new prototype NZ-DSF with ultra-low dispersion slope
and the existing NZ-DSF having a low dispersion slope. It
was discovered that while the existing NZ-DSF with low
dispersion slope suffered a deterioration of the bit error
rate (BER), particularly in the L-band, the prototype, withultra-low dispersion slope, was capable of 10 Gbps transmission in the C-band and L-band, even without dispersion compensation.
The application of dispersion compensators geared toeven higher transmission bit rates was also considered.
Even a DCF designed as a compensator for single-mode

5

fibers delivers a compensation of 82 %, and it has been
confirmed that it is a simple matter to design specializedcompensators to deliver 100 % compensation.

REFERENCES
1) K. Mukasa, R. Sugizaki, T. Yagi, Y. Suzuki and K. Kokura; Wideband Dispersion Management Transmission Line with Medial
Dispersion Fiber (MDF), Proceeding of ECOC'00, 2-4-2, 2000.
2) Y. Liu, W. B. Mattingly, D. K. Smith, C. E. Lacy, J. A. Cline, and E.
M. De Liso; Design and fabrication of locally dispersion-flattenedlarge effective area fibers, Proceeding of ECOC'98, pp37-38, 1998.
3) D. W. Peckham, A. F. Judy, R. B. Kummer; Reduced dispersion
slope, non-zero dispersion fiber, Proceeding of ECOC'98, pp. 139140, 1998.
4) S. Bigo, S. Gauchard, S. Borne, P. Bousselet, P. Poignant, L.
Lorcy, A. Bertaina et al; 1.5Terabit/s WDM transmission of 150
channels at 10Gbit/s over 4 ✕ 100km of TeraLightTM fiber,
Proceeding of ECOC'99, PD2-9, 1999.
5) T. Ooishi et al.: Dispersion shifted fiber for ultra-broadband transmission, Proceedings of IEICE 2000, C-3-44, 2000. (in Japanese)
6) H. Kuchiami et al.: Evaluation results of terrestrial transmission
characteristics of Ultra Wave-SS fiber, Proceedings of IEICE
2001, C-3-28, 2001. (in Japanese)
7) N. Kumano, K. Mukasa, T. Yagi: Study on dispersion slope reduction of non-zero dispersion shift fiber, Proceedings of IEICE 2001,
C-3-27, 2001. (in Japanese)
8) N. Kumano et al.: Study on ultra-broadband transmission line
using ultra-low dispersion slope non-zero dispersion shift fiberand dispersion compensating fiber, Optical Communications
System Workshop 2001, OCS 2001-32, 2001. (in Japanese)
9) N. Kumano, K. Mukasa, T. Yagi: Study on non-linearity reductionof ultra-low dispersion slope non-zero dispersion shift fiber,Society Conference of IEICE 2001, C-3-107, 2001. (in Japanese)10) N. Kumano, K. Mukasa, M. Sakano, H. Moridaira, T. Yagi and K.
Kokura; Novel NZ-DSF with Ultra-low Dispersion Slope Lower
than 0.020 ps/nm2/km, ECOC'01 PD. A. 1-5, 2001.
11) W. S. Lee, Y. Zhu, B. Shaw, D. Watley, C. Scahill, J. Homan, C.
Fluder, M. Jones and A. Hadjifotiou; 2.56Tb/s capacity, 0.8b/Hz.s
DWDM transmission over 120km NDSF using polarization-bitinterleaved 80Gb/s OTDM signal, OFC'01 Technical Digest,
TuU1, 2001.
12) M. Gunkel, F. Kuppers, J. Berger, U. Feiste, R. Ludwig, C.
Schubert, C. Schmidt, H. G. Wber; 40Gb/s RZ unrepeatered
Transmission over 252km SMF using Raman Amplification,
OFC'01 Technical Digest, TuU3, 2001.
13) Yann Frignac and Sebastien Bigo; Numerical optimization ofresidual dispersion in dispersion-managed systems at 40 Gbit/s,
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6

Furukawa Review, No. 22

2002