High Repetition-Rate Ultra Short-Pulse Train Generation
by Short Comb-like Dispersion Profiled Fiber
Using High-Nonlinearity DSF
by Masateru Tadakuma *, Osamu Aso *, Misao Sakano * and Shu Namiki *Not only broadband DWDM transmission systems but also high bit rate transmission of over 40 Gbps per channel is investigated for the next generation of fiber
optical networks. For ultra high-speed signal processing that is not available with electrical components, it is effective to use all-optical signal processing with optical fiber devices using nonlinear effects through fibers. For the purpose of developing a high speed optical pulse generator asa standard clock for all-optical signal processing, we have studied high repetition rate pulse generation by short length Comb-like Dispersion Profiled Fiber (CDPF) using high nonlinearity dispersion shifted fiber with about six times more nonlinearity than conventional DSF and 1.3 µm
zero dispersion single mode fiber. To eliminate pedestal noise in the short pulse, we constructed
the CDPF by incorporating a dispersion imbalanced nonlinear optical loop mirror and generated a
100 GHz 380 fs pulse train.

ABSTRACT1.

INTRODUCTION

For next-generation optical networks, it is expected thattransmission capacity will be increased by making highchannel density and wide band WDM systems, andincreasing data bitrate per wavelength channel. The highbitrate per channel system has great potential for cost
reductions in comparison with wide broaden DWDM systems of the same transmission capacity, because of afewer number of channels and simple control of wavelength dependence. To realize a high-speed transmission
system of more than 40 Gbps per channel, electrical signal processing speed at transmitters and receivers is thelimiting factor. Accordingly, all-optical signal processing isnecessary for signal switching and down rate conversion
to use electrical signal processing. For an advanced optical transmission system using all-optical signal processing, it is desirable to have a new technique whereby the
optical fiber devices use nonlinear effects through fibers.
In this report, for the purpose of developing an opticalclock pulse generator that is used for the optical signal
processing, we have researched short pulse generation
by Comb-like Dispersion Profiled Fiber (CDPF) using ahigh nonlinearity fiber.

*

Optical Transmission Sub-Systems Development Dept.,
FITEL-Photonics Lab.

2.

HIGH REPETITION RATE PULSE
SOURCE USING PULSE COMPRESSION
THROUGH OPTICAL FIBER

Short optical pulse sources with high repetition rates are
expected to play a key role in high capacity Optical Time
Division Multiplexed (OTDM) transmissions. Since ElectricTime Division Multiplexing (ETDM) methods have difficulty
generating pulses at a repetition rate higher than 100
GHz, one may want to have an alternative to generatesuch pulse trains through other means. Pulse sources
used for high bit rates of over 40 Gbps are mostly investigated using OTDM with low bit rate pulse sources 1), 2).
Stand-alone high repetition rate ultra short pulse generation methods were suggested earlier, such as modelocked DFB laser diode 3), and a method using modulation
instability through the fiber 4), but it is desirable to generatehigh repetition pulses in a more stable and simple way for
all optical signal processing. In such a case, pulse compression of beat modulation between two optical carriers
is an attractive method. This technique requires only standard tunable laser sources and optical fibers, but permits
us to tune the repetition rate and the center wavelength ofthe pulse train simply by adjusting the frequencies of the
input lasers. In the process of propagating through theoptical fiber, the pulse width of the beat pulse is compressed by nonlinear and dispersion effects 5). The techniques of beat signal conversion were previously demonstrated with Dispersion-Decreasing Fiber (DDF) 6), 7). An
alternative approach is to profile the fiber dispersion using
segments of conventional fibers with different dispersions.

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High Repetition-Rate Ultra Short-Pulse Train Generation by Short Comb-like Dispersion Profiled Fiber Using High-Nonlinearity DSF

Step-like dispersion profiled fiber consists of several segments of dispersion shifted fiber with different dispersion 8).
With Come-like Dispersion Profiled Fiber (CDPF) that consists of alternating segments of Dispersion Shifted Fiber(DSF) and Single Mode Fiber with 1.3 µm zero dispersion(SMF) respectively, picosecond pulse generations areobserved 9), 10). A soliton pulse train was also generatedusing a CDPF comprising only of two pairs of DSF and
SMF 11), 12). A transmission test using a pulse train generated by such a method was also carried out 13).
A schematic of CDPF configuration is illustrated in
Figure 1. A beat signal synthesized from two CW sources,
which have a slightly different wavelengths, is launched
into a DSF. In the DSF, Self Phase Modulation (SPM)
induces a chirp in a pulse. In the following SMF, theanomalous dispersion compensates the chirp and thepulse width is compressed. After repeating this process
several times, the beat signal is converted into a pulse
train. Previous work on a CDPF used conventional standard DSF and SMF. In this type of fiber, the nonlinear
chirp effect through the DSF plays an important role inpulse compression, and it is effective to use highly nonlinear (HNL-) DSF to shorten DSF segments 14), 15).
The main feature of the CDPF in this report is the use ofa high nonlinearity fiber for DSF.
The transmission characteristics of the HNL-DSF are
shown in Table 1. The nonlinear coefficient γ (γ = n2 /Aeff .
2π/λp, n2 is the nonlinear refractive index, Aeff is the effective area, and λp is the wavelength) of this fiber is about
six times larger than the value of the conventional DSF;
therefore, we can shorten the fiber to obtain an equivalent

DSF

SMF

DSF

nonlinear effect compared to a standard DSF.
It is desirable to shorten the fiber using HNL-DSF for thefollowing reasons: (1) as fiber length decrease, the stateof polarization fluctuation between two tone signals
becomes smaller to produce larger Four Wave Mixing(FWM), and (2) a shorter fiber may have less dispersion
fluctuations along the fiber length to better maintain thezero-dispersion condition for obtaining a large and broader bandwidth SPM.

3.

PULSE GENERATION WITH 5 PAIRS OF
CDPF USING HNL-DSF

We have constructed a CDPF using HNL-DSF and commercially available SMF 16). The fiber consisted of 5 pairsof DSF and SMF, and had a total length of 693.6 m. The
composition of fibers in CDPF is shown in Table 2. The
experimental setup is shown in Figure 2. A dual frequency
beat signal was generated by two tunable-wavelength
lasers (HP8368F). The wavelength space between two
waves was set at 0.831 nm to generate a pulse train
approaching a 100 GHz repetition rate. The center wavelength was tuned to coincide with the zero dispersion
wavelength of the HNL-DSF to generate a nonlinear effectefficiently. Two tunable laser sources were phase modulated using the built-in modulation function to suppress
Stimulated Brillouin Scattering (SBS). Two signal powers
were set equally and the polarization states were aligned
in parallel by adjusting polarization controllers to minimize
insertion loss of the polarizer. Both waves are combinedby a fiber-fused coupler. The beat signal was amplified by

SMF

Dispersion (ps/nm/km)

Table 1

Characteristics of high nonlinearity fiber.
Parameter

Longitudinal
direction

Value

Attenuation coefficient

0.61 dB/km

Zero dispersion wavelength1565.5 nm

Dispersion slope (@ λ = λ0 )0.029 ps/nm2/km
17.5 W-1 km-1

Nonlinear coefficient
Pulse waveform

Table 2
Optical spectrum

Figure 1

Configuration of five pairs of CDPF.

Pair of fibers

Dispersion @1565 nm

3

4

5

DSF

-0.015 ps/nm/km

160 200 100

50

30

SMF

+17 ps/nm/km

144

0.3

0.3

Pulse compression process using comb-like dispersion profiled fiber. (Example of two pairs of DSF
and SMF)

1

2

7

2

Polarization controller
DSF SMF

Polarizer

Wavelength tunable
light source

50:50
coupler

EDFA

Figure 2

DSF SMF

Short comb-like dispersion
profiled fiber
Attenuator

Optical spectrum
analyser
Autocorrelator

Experimental setup.

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SHG Intensity (a.u.)High Repetition-Rate Ultra Short-Pulse Train Generation by Short Comb-like Dispersion Profiled Fiber Using High-Nonlinearity DSF

-15

-10

-5

0

5

10

15

Time (ps)Figure 3

Autocorrelation trace of fiber input beat pulse.

0.305. The duty cycle of the pulse output from the 5th pairof fiber segments in CDPF was 0.034 and the period-toduration ratio T/∆τ (T is the period and ∆τ is the FWHM of
the pulse) reached 29.3.
To achieve high compression ratio in CDPF, many pairsof comb-like segments are needed and the scheme
becomes complex. The nonlinear phase shift around thepeak of the beat modulation is approximately parabolicand the beat modulation is compressed due to linear chirp
compensation through SMF. Therefore, this implies thatthe foot of the pulse is not compressed and remains aspedestal noise. As shown in Figure 5, it should be notedthat the pulses had pedestal noise.

-10

4.

Power (dBm)-20

PEDESTAL NOISE SUPPRESSEDPULSE SOURCE USING DI-NOLM

-30
-40
-50
-60
-70
1530

1540

1550

1560

1570

1580

1590

1600

Wavelength (nm)Optical spectrum of output pulse.

SHG Intensity (a.u.)

Figure 4

-15

-10

-5

0

5

10

15

DI-NOLM (Dispersion-Imbalanced Nonlinear Optical Loop
Mirror) is effective to eliminate pedestal noise in the shortpulse 17)~19). DI-NOLM is a Sagnac loop comprising fibers
with different dispersions, and by propagating the input
pulse split along the two directions in DI-NOLM, the split
pulses experience different nonlinear phase shifts inclockwise and counterclockwise directions because they
stretch differently, and the peak powers evolve differently
due to the asymmetric dispersion of the loop. This feature
results in the loop discriminating the transmission,
depending on the derivative of electric field amplitude withrespect to time. Thus, the center of the pulse can be
adjusted to transmit only through the loop, while thepedestal noise is rejected.
Our next goal is to achieve more efficient pulse compression than with the above-mentioned 5 pairs-CDPF
and simultaneous pedestal suppression using a simple
configuration. We have constructed a short 3 pair CDPF
with HNL-DSF, and SMF and incorporated them into thisCDPF DI-NOLM, which does both beat pulse compression
and pedestal noise suppression at the same time 20).

Time (ps)Figure 5

Autocorrelation trace of output pulse.

an Erbium Doped Fiber Amplifier (EDFA). The launchedbeat signal power to the first DSF of CDPF was set at+27.5 dBm, which is below the limit of SBS. Figure 3
shows the SHG autocorrelation trace of the input beat signal. The optical spectrum of the output pulse and the SHG
autocorrelation trace after transmission of the 5th pair offiber segments in CDPF are shown in Figures 4 and 5. For
the pulse transmitted through the 5th pair of fiber segments, the pulse repetition rates were 104 GHz and themeasured autocorrelation full-width at half-maximum(FWHM) was 505 fs, which gives a pulsewidth of 328 fs,
assuming a sech2 pulse-intensity profile. Also, by fitting asech2 envelope to the peaks in Figure 4, the FWHM of thespectrum was measured to be 7.59 nm. Therefore, theestimated time-bandwidth product was approximately

5.

EXPERIMENTAL SETUP AND RESULTS

The experimental setup is shown in Figure 6. We constructed the CDPF and fiber loop mirror using the HNLDSF and commercially available SMF. The CDPF consistsof 2 segments of DSF and one segment SMF and the loop
mirror consists of 2 segments of SMF and one segment of
DSF. The characteristics of fibers in CDPF and DI-NOLM
are shown in Table 3. The zero dispersion wavelength of
HNL-DSF approaches 1557 nm and the other characteristics of the HNL-DSF are the same as those shown in
Table 1. A dual frequency beat signal was generated by
two tunable-wavelength lasers. Two wavelengths wererespectively set at 1557.04 nm and 1557.84 nm, because
the wavelength space between two waves was set at 0.8
nm to generate a pulse train at a near 100 GHz repetition
rate. Two signal powers were set to be equal and thepolarization states were aligned to be parallel by adjusting

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High Repetition-Rate Ultra Short-Pulse Train Generation by Short Comb-like Dispersion Profiled Fiber Using High-Nonlinearity DSF

100MHzSignal generator
SMF2

1557.04nm

DSF1

DSF3

DSF2

Polarizer

Wavelength tunable
light source

SMF1

Wavelength tunable
light sourcePC

1557.84nm

Phase
modulator

50:50
coupler

EDFA

PC
SMF3
Auto-correlation meter

Optical spectrum
analyser

Figure 6

Comb-like dispersion profiled fiber length.
1

2

3

DSF

200

100

62

SMF

91

7.5

0.3

polarization controllers to minimize insertion loss of thepolarizer placed at the input port of the phase modulator.
Both waves were combined by a fiber-fused coupler. Two
tunable laser sources were jointly phase-modulated usinga phase modulator driven by a 100 MHz frequency signal
to suppress SBS. The beat signal was amplified by an
EDFA. The launched beat signal power to the first DSF of
CDPF was set at +27.7 dBm. In the DI-NOLM, the compression ratio of the counterclockwise propagating pulse
is designed to be smaller than that of the clockwise propagating pulse. Therefore, there is a difference in the nonlinear phase shift between the two pulses propagating in the
opposite directions. The length of SMF3 was optimized tobest suppress pedestal noise. The polarization states ofclockwise and counterclockwise propagating pulses were
adjusted by a lossless polarization controller set in theloop. Because the DI-NOLM is composed by a part of
CDPF, the output pulse from the fiber loop mirror is compressed and its pedestal noise is suppressed simultaneously.
The SHG autocorrelation traces of input and output
pulses of the DI-NOLM are shown in Figure 7 and Figure
8 shows the optical spectrum after DI-NOLM transmission.
The repetition rate of the output pulse from DI-NOLM was
98.9 GHz and the measured Full-Width at Half-Maximum(FWHM) of autocorrelation was 587 fs, which gives apulsewidth of 380 fs, assuming a sech2 pulse-intensity
profile. Also, by fitting a sech2 envelope to the peaks in
Figure 8, the FWHM of the spectrum was measured to be1229 GHz. Therefore, the estimated time-bandwidth product was approximately 0.467. In Figure 7, comparing the
input and output pulses, the input 1.369 ps pulse is compressed by a factor of 3.6 down to 380 fs. In Figure 9, theautocorrelation traces of the pulses after clockwise transmission from SMF3 and from loop mirror output are com-

SHG Intensity (a.u.)

Segments

-15

-10

-5

0

5

10

15

Time (ps)Figure 7

Autocorrelation traces of DI-NOLM input (dashedline) and output (solid line) pulses.

-10
-20

Power (dBm)

Table 3

Experimental setup. P.C.: polarization controller

-30
-40
-50
-60
-70
-80
1500

1520

1540

1560

1580

1600

Wavelength (nm)Figure 8

Optical spectrum of DI-NOLM output pulse.

pared. The pulse after transmission from SMF3 is equivalent to the output pulse from a straight-line CDPF configuration. By constructing the loop mirror incorporated in
CDPF, the pedestal component in the output pulse fromthe loop mirror is suppressed more than that from the'straight-line' CDPF, and the pulse profile is reshaped tonear the soliton function. The period-to-duration ratio T/∆τ
reached 26.6.

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SHG Intensity (log scale; 5dB/div.)

High Repetition-Rate Ultra Short-Pulse Train Generation by Short Comb-like Dispersion Profiled Fiber Using High-Nonlinearity DSF

-4

-2

0

2

4

Time (ps)Figure 96.

Autocorrelation trace of DI-NOLM output pulse,
showing loop mirror output (solid line) and SMF3
output clockwise propagation in the DI-NOLM(dashed line).

CONCLUSION

We aimed to develop a high repetition rate ultra shortpulse source using fiber nonlinearity for all-optical signal
processing in high bit rate transmission over 40 Gbps. We
constructed a 693.6 m long Comb-like Dispersion Profiled
Fiber using a HNL-DSF with a nonlinear coefficient that isabout six times larger than that of a conventional DSF.
Using this short CDPF, we generated a 104 GHz soliton
pulse train with a 328 fs pulsewidth and achieved a period-to-duration ratio T/∆τ of 29.3. Furthermore, to simplify
the fiber configuration and eliminate pedestal noise while
keeping a high pulse compression ratio, we constructed ashort 3-pair CDPF with HNL-DSF and SMF, and incorporated into this CDPF DI-NOLM. We generated a 100 GHz,
380 fs pulse train through this CDPF-DI-NOLM. In the output pulse, pedestal noise is suppressed more than in a 5pair straight line CDPF output. This type of high repetition
beat pulse source has potential for optical clocks using alloptical signal processing, and future problems are to
reduce timing jitter and noise, to shorten pulsewidth, andto make it more compact.

ACKNOWLEDGEMENT
We would like to thank Mr. Yagi, Mr. Hiroishi, Ms.
Kumano, and Mr. Sugisaki in the WF team for making the
HNL-DSF.

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