A Highly Nonlinear Fiber Module and its Application to the Generation of
Ultra-High Repetition-Rate Sub-Picosecond Optical Pulse Trains
by Koji Igarashi * , Hideaki Tobioka * , Shinji Kuroki * , Ryo Miyabe * , Jiro Hiroishi * ,Masanori Takahashi * , Takeshi Yagi * , Osamu Aso * 2 and Shu Namiki *Work is going forward on examining the efficacy of highly nonlinear fiber(HNLF) with respect to various types of highly nonlinear optical devices, andon developing modules. Studies on the adaptation of HNLF to the generation of ultra-highrepetition-rate sub-picosecond optical pulse trains, one of its applications, have demonstrated
the efficacy of HNLF with respect to generating 160-GHz 0.8 ps ultra-high purity soliton trainsusing comb-like dispersion-profiled fiber (CDPF). An HNLF module having a conveniently usable
configuration has also been developed. Studies have also been carried out on applying thismodule in subsystems, and its efficacy with respect to optical delay circuitry and all-opticalregeneration has also been demonstrated.

ABSTRACT1.

INTRODUCTION

The technology of nonlinear optical signal processing
is indispensable in optical fiber transmission systems
having single-channel bit rates in excess of 100 Gbit/s.
Various semiconductors and dielectrics have been
considered for use as the nonlinear material, and devices
using them have been fabricated. Of them all it is opticalfiber that has been found to be superior in terms of
the high speed, low noise and low loss of its nonlinear
response. In fact there have been a large number of
proposals for methods of using four-wave mixing 1) andnonlinear polarization rotation 2) in optical fibers, and fortechniques for using nonlinear fiber loop mirrors 3), andexamples exhibiting superior broad-band properties andultra high-speed characteristics have been reported.
One of the most important issues in improving theperformance of these nonlinear optical fiber devices isincreasing the nonlinearity of the fiber. Despite the factthat the third-order nonlinear coefficient of silica glass
is not inherently large, its nonlinearity can be increased
by such means as adding material with a high refractive
index to the fiber core, or reducing the core diameter.
Indeed highly nonlinear fibers (HNLFs) have beenreported that have nonlinear coefficients some ten times
greater than that of conventional fiber 4), 5), and discussions
are progressing on their application in subsystems 4), 6)~8).
Against this background the authors are investigating
the efficacy of HNLF with respect to a variety of nonlinear* Fitel-Photonics Lab., R&D Div.
* 2 Yokohama Lab., R&D Div.

optical signal processing devices and subsystems. At
the same time studies are also being carried out on thedevelopment of modules. When HNLF is to be used ina subsystem, it is indispensable that its configuration
be of outstanding stability and reliability, and be easy
to use, and development of HNLF modules is the mostimportant issue. This paper will present an HNLF modulethat advances the development of ultra-high repetitionrate sub-picosecond soliton train generation 8), as anapplication of HNLF.

2.

GENERATION OF ULTRA-HIGH
REPETITION-RATE HIGH PURITY SUBPICOSECOND SOLITON TRAINS

Optical transmission systems exceeding 100 Gbit/s perchannel require a clock pulse generator with a repetition
frequency corresponding to the bit rate. Since it isdifficult, based on methods using conventional electronic
circuit technologies, to operate in the range above 100
GHz, the normal way of achieving the desired ultra-highrepetition rate is by the dividing, delaying and combiningof a 10- or 40-GHz pulse train to achieve time-divisionmultiplexing. It is not, however, a simple matter to control
and stabilize the delay, and another problem is presented
by coherence degradation of the pulse train.
It was at this point that the authors identified a methodof converting a dual-mode optical beat signal into an
optical soliton train by a process of adiabatic soliton
compression in the optical fiber 9). Since it is possible,
by combining two continuous waves (CW) having awavelength difference corresponding to the desired

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Figure 1�Schematic showing principle of CDPF,
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Recognizing this, the authors fabricated a CDPF using
HNLF, and while clarifying the compression performancefor a 160-GHz beat signal, investigated the effect of the
HNLF.
Figure 2 shows the experimental setup. Using a CDPF
made up of six HNLF-SMF pairs, the apparatus achieves
the adiabatic soliton compression of a 160-GHz beatsignal obtained by combining the output CWs fromtwo laser diodes having an optical frequency differenceof 160 GHz. The HNLF used here has a dispersionof - 0.8 ps/nm/km and a nonlinear coefficient of 24
1/W/km. The profiles for dispersion D of the CDPF andaverage dispersion for each pair D ave are depicted in
the upper panel of Figure 3, the point to note being that
Dave increases in the three pairs of the first stage anddecreases in the three pairs of the last stage.
The design is such that the beat signal is efficiently
converted into a soliton train by the increased-dispersion
CDPF of the first stage, and the soliton compression isthen performed by the decreased-dispersion CDPF. The
profile of nonlinear coefficient γ is shown in the middle
panel of Figure 3. It can be seen that through the useof HNLF in the CDPF, both the fiber dispersion and thenonlinear coefficient assume a comb-like profile, realizing
an ideal CDPF transmission path. It is also to be notedthat noise amplification through modulation instability
gain has been suppressed by using a normal dispersion
HNLF.
The autocorrelation traces and optical spectra of the
CDPF compression system are shown for the input andoutput pulse trains in Figure 4 (a) and 4 (b) respectively.
The broken line shows the results of fitting the pulse
waveform to the sech 2 waveform. Since the CDPF
output pulse waveform and the broken line are in good
agreement, not only on the autocorrelation trace but
also in the optical spectra, this shows that a high-purity
soliton train is generated. In addition, with respect to the������

repetition frequency, to produce a beat signal optically,
this method does not require a high-speed electricalcircuit. It is also possible that, since the conversion frombeat signal to a soliton train uses the process of adiabatic
soliton compression, it would be possible to generate asoliton train of extremely high purity. We may also saythat by stabilizing and controlling the continuous wave
wavelength using an electronic circuit of comparatively
low frequency (MHz order) it is possible to achieve
synchronism with an external reference signal.
The crucial point in this technique for generating an
ultra-high repetition-rate soliton train lies in adiabatic
soliton compression. As means of achieving this there
are reports of methods using Raman amplification fiber 9),
dispersion-decreasing fiber 10), and fibers with step-like 11)
and comb-like 12) dispersion profiles. Of these the authors
focused on comb-like dispersion profiled fiber (CDPF)
consisting of two types of fiber having significantly
different dispersion values. From the standpoint of easeof manufacture there is nothing remotely approaching a
CDPF in which adiabatic soliton compression is realized
by two types of fiber. The CDPF is also advantageous interms of suppression of the phenomenon of stimulated
Brillouin scattering, which limits compression. This isbecause, generally speaking, fibers with significantly
differing dispersion values have different Brillouin gain
bands 12).
Figure 1 is a schematic showing the principle of the
CDPF. Ideally the CDPF provides a transmission pathin which a nonlinear medium and a dispersion medium
repeat alternately. Generally a zero dispersion-shifted
fiber (DSF) is used for the nonlinear medium and a singlemode fiber (SMF) is used for the dispersion medium,
spliced together to form the CDPF. Here, the use of HNLF
for the nonlinear medium not only enables the realizationof an ideal nonlinear medium, but also enables thepossibility of a high degree of shortening of the fibers.

Experimental setup for 160-GHz soliton train
generation using CDPF.

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Profiles of dispersion (D) and nonlinearity
γ , and the ratio of dispersion length (LD) tononlinear length (LNL). Broken line shows
the averaged dispersion for each pair (Dave).

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Figure 4

Input and output pulse trains of CDPF.

peak ratio of the optical signal component and the noise
component on the optical spectra, not only were valuesof 40 dB or above obtained, but since there was no majorchange in the input/output spectral linewidth of the CDPF,
it would seem that noise amplification in the compression
process was adequately suppressed. From the sech2
fitting with respect to the autocorrelation trace, the time
width is calculated as 830 fs. Furthermore ∆ t∆ v was 0.34,
indicating that a substantially Fourier transform limited
pulse was obtained.
For the purposes of a quantitative discussion of theeffectiveness of using HNLF for the CDPF, the ratio
between nonlinear length and dispersion length (LD/LNL)
has been plotted in the lower panel of Figure 3 using
black circles for HNLF input and white circles for SMF
output. The point to note here is for the HNLF the values
are greater than 1, and for the SMF less than 1. Thisshows that in the HNLF the nonlinear effect is dominant,whereas it is the dispersion effect that is dominant for
the SMF. From this discussion it may be concluded thatusing HNLF in the CDPF has realized an ideal CDPF
transmission path, and as a result solitons of ultra-highquality were generated.

broadening, and discussion is on-going with regard to
subsystem applications for them. Generally speaking,however, fiber devices, being subject to external
disturbance, can hardly be described as easy to use.
In addition specialized fibers like HNLF, having core
diameters that differ significantly from those of ordinary
fibers, cannot readily be joined to conventional fibers bymeans of connectors.
From this it can be seen that the development of
HNLF modules is a problem that must be addressed.
Accordingly the authors are working to develop a
compact HNLF module to enable connection withordinary fibers. Generally the dimensions of fiber devices
are determined by the diameter of the coils into which
they can be rolled. Since HNLF provides strong opticalconfinement it can be rolled into smaller coils, allowing
modules to be more compact. And enclosing the coiled
HNLF in a casing makes it possible to realize a device
that offers good resistance to external disturbance. In
addition, joining short lengths of SMF with connectors
attached by low-loss splicing to the two ends of the HNLF
facilitates connections to ordinary fibers. In the following
the techniques adopted in developing HNLF modules will
be discussed in detail.
3.1 Coil Rolling
The first step toward realizing a module in this work
was the use of a medium-sized coil 16 mm in diameter.
Characteristics before and after rolling are shown in Table
1. Not only has bend-induced loss been suppressed, but
it is also to be noted that changes in the wavelength atzero dispersion (λ 0) and the dispersion slope, which are
crucial to the performance of the device, have also been
suppressed. And although there was a certain amountof increase in polarization mode dispersion, it would notseem to be of a degree that would seriously affect device
performance.
3.2 Fiber Ends
Although it is difficult to join HNLF and SMF using
connectors, it is easy to achieve low-loss splicing
between the two. In fact splicing loss can be reduced
to 0.1 dB or better by optimizing splicing conditions.
Thus by splicing SMF with connectors attached to theends of the HNLF, as shown in Figure 5, it is possible forthese modules to be joined to ordinary fibers by low-loss
connectors, as in the case of conventional devices.

Table 1

3.

DEVELOPMENT OF HNLF MODULES

Investigations have been carried out not only into thegeneration of ultra-high repetition-rate pulse trains
referred to in the foregoing, but also into the effectivenessof HNLF in wavelength conversion and spectrum

Characteristics of HNLF before and after rolling.
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A Highly Nonlinear Fiber Module and its Application to the Generation of Ultra-High Repetition-Rate Sub-Picosecond Optical Pulse Trains

REFERENCES�� ��f
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Figure 6�� �� ��Splicing of SMF to the ends of the HNLF,
and the input and output connectors.

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HNLF module measuring 180(L)×150(W)×40(D) mm.

3.3 Module Configuration and its Performance
Figure 6 shows the HNLF module, in which coils of
HNLF are enclosed in a casing. Lengths of SMF withconnectors attached are spliced to the input and output
ends, and splicing loss is 0.2 dB or better. We may saythat such modules, resistant to external disturbance andeasy to use, are of a configuration that is advantageous
in applications to subsystems. In fact studies have been
conducted on the application of these modules to opticaldelay circuits 13) and optical regeneration in 40-Gbit/s
transmission 14), demonstrating their efficacy.

4.

CONCLUSIONAs the result of investigation of ultra-high repetitionrate pulse train generation as an application of HNLF,
generation of a 160-GHz 0.8-ps ultra-high purity soliton
train has been demonstrated by means of CDPF using
HNLF. HNLF is indispensable to the generation of suchhigh-purity solitons. Investigations are also being madeinto the development of HNLF modules. Since modules
constitute a configuration that is easy to use and highly
stable, it is important to subsystem applications. The
efficacy of these modules in optical delay circuits and alloptical regeneration has been demonstrated.

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