A Comb-Like Profiled Fiber (CPF) Compressor and an Ultrashort Pulse Light Source
by Atsushi Oguri *, Koji Igarashi *, Hideaki Tobioka *, Takeshi Akutsu *, Jun-ichi Shinozaki *,Masanori Takahashi *, Jiro Hiroishi *, Takeshi Yagi *, Ryo Kawahara *, Kazurou Okamura *,Nobuyuki Kagi *, Shun-ichi Matsushita *, Shu Namiki * and Misao Sakano *Ultra-short (picosecond and femtosecond range) pulse light sources using optical fibers are attracting attention, not only in the field of telecommunications, butalso in such fields as optical metrology, materials processing, bio-photonics, andphysics. As the optical fiber compressor for generating ultra-short pulses, we have proposed a
comb-like profiled fiber (CPF) which is an alternate concatenation of highly nonlinear fiber(HNLF) and single-mode fiber (SMF). In the present work we examined whether or not uniform
output pulse characteristics could be obtained from a CPF compressor even if the wavelength or
repetition rate of the input pulse is changed. As a result we were able to confirm that output
pulse characteristics were uniform over wavelengths from 1530 to 1560 nm, and over repetition
rates from 5 to 500 MHz.

ABSTRACT1.

INTRODUCTION

Ultra-short pulse light sources using optical fibers are
attracting attention from the standpoint of compactness,
low cost and stable performance, in the fields of ultrahigh-speed optical communication and optical metrology,
as well as materials processing, bio-photonics, physics,etc.
As the optical fiber compressor for generating ultrashort pulses, we have reported on a comb-like profiled
fiber (CPF) which is an alternate concatenation of highly
nonlinear fiber (HNLF) and single-mode fiber (SMF) 1)~3). A
CPF compressor can be flexibly designed with respect toits dispersion profile, making it very advantageous interms of ease of manufacture. In the present work, in aneffort to confirm the further utility of the CPF compressor,
we examined whether or not uniform output pulse characteristics could be obtained even if the wavelength or repetition rate of the input pulse changes. And we report that
we have been able to confirm that output pulse characteristics were uniform over wavelengths from 1530 to 1560nm 4), and over repetition rates from 5 MHz to 500 MHz 5).
1.1

Importance of Wavelength-Tunable Ultra-short
Pulse Light Source
To increase the capacity of optical transmission systems,
research has been carried forward on wavelength-divisionmultiplexing (WDM) systems, in which signals are multiplexed on the wavelength axis, and on time-division multiplexing (TDM) systems, in which signals are multiplexed* FITEL-Photonics Lab., R&D Div.

on the time axis. Because TDM systems suffered fromproblems such as limits on the speed of electronic circuitry, it is WDM systems that have been more actively
brought into practical use. Attention is, however, being
paid to optical TDM (OTDM) systems, which have a smaller number of transmitter/receiver units and easier batch
control than WDM, and in this sort of ultra-high-speed
optical communications applications, it has become
important to develop all-optical signal processing, which
eliminates optical-electric conversion. The key device inrealizing all-optical signal processing is an ultra-shortpulse light source, which works as a stable clock pulse
train. And for wavelength-division multiplexing of OTDM
systems there must be clock pulse trains at various wavelengths, that is to say, an ultra-short pulse light source.
1.2

Expectations for a Repetition Rate-Tunable Ultrashort Pulse Light Source
In the fields of optical metrology, materials processing,bio-photonics, physics, etc., attention is being given fromthe standpoint of compactness, low cost and stable operation, to the development of an ultra-short pulse lightsource using optical fiber. Further improvement in its utility can be anticipated if it can be made tunable by repetition rate, wavelength, pulse width, pulse energy and soon. For the mode-locked fiber lasers that have been ingeneral use as ultra-short pulse light sources, however,cavity length is fixed, so that it was difficult to change therepetition rate to any great degree. In contrast light sources that use an optical pulse compressor can vary the repetition rate of the seed pulse, holding promise that repetition rate tunable response can be achieved. If the repeti-

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A Comb-Like Profiled Fiber (CPF) Compressor and an Ultra-short Pulse Light Sourcetion rate can be controlled, average power can be controlled while maintaining the various pulse characteristics,
thereby increasing the utility of the light source.

2.

OPTICAL PULSE COMPRESSIONTECHNOLOGIES

2.1 Adiabatic Soliton Compression
Adiabatic soliton compression is a term for techniques
whereby optical pulses are compressed, imparting to thebasic soliton a gentle perturbation inside the fiber. They
are advantageous in that there is so little degradation intime waveforms or spectral profiles.
There are four main techniques for adiabatic soliton
compression, namely: 1) a method using dispersionDDF

Dispersion

SDPF

decreasing fiber (DDF) 6); 2) a method using step-like dispersion profiled fiber (SDPF), in which a plurality of opticalfibers having different dispersions are used, changing thedispersion profile in the longitudinal direction of the fiber7); 3) a method using comb-like dispersion profiled fiber(CDPF), in which two optical fibers having different dispersions are used, changing the dispersion profile in the longitudinal direction of the fiber 8); and 4) a method using
Raman compression, in which gain is gently imparted in
the longitudinal direction of the fiber 9).
Figure 1 shows schematic profiles of DDF, SDPF and
CDPF fibers. Table 1 compares the four methods of adiabatic soliton compression.
As Figure 1 shows, in the DDF method it is possible togently decrease the dispersion constituting the perturbation, so that an ideal adiabatic soliton compression canbe achieved. It is difficult, however, to manufacture an
optical fiber in which dispersion parameters decrease in
an ideal manner along the length of the optical fiber.
Accordingly the SDPF and CDPF methods have been
proposed, using a combination of a plurality of opticalfibers of differing dispersions, to achieve a DDF profile byapproximation. Whereas SDPF requires several types of
optical fiber, CDPF can be manufactured with two types,
and is thus preferable from the practical point of view.
What is more in CDPF, fibers of greatly differing dispersion are connected in multiple steps, making it superior
from the standpoint of stimulated Brillouin scattering, as
well.

Distance

Figure 1

Schematic profiles of DDF, SDPF and CDPF fibers.

Table 1Comparison among methods of adiabatic soliton
compression.

MethodAdvantages

DDF

Ideal adiabatic compression
is possible

Fiber is difficult to make

SDPF

Fiber is relatively easy to
makeMultiple types of fiber are
required

Optical Pulse Compression Using Comb-like
Profiled Fiber
The ideal CDPF comprises a nonlinear medium and a dispersion medium in a multistep connection configuration.
Generally zero dispersion-shifted fiber (DSF) is used forthe nonlinear medium and single-mode fiber (SMF) is
used for the dispersion medium, and they are connectedby fusion splicing.
We are proposing a novel comb-like profiled fiber (CPF)
in which a pair of HNLF and SMF is multistep connected.
Since HNLF is used instead of DSF, it has, as shown in

Nonlinearity

2.2
CDPF

Disadvantages

CDPF

Can be designed with two
types of fiber

Dispersion

Number of connection
points is large
Number of connection
points is large

Suppressive effect of stimulated Brillouin scattering
Raman

Compression ratio can becontrolled using gain compression

Longitudinal distribution ofgain is difficult to control

Distance

Figure 2

Schematic of nonlinearity coefficient and dispersion
profile of CPF.

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A Comb-Like Profiled Fiber (CPF) Compressor and an Ultra-short Pulse Light Source

2.3 Development of Highly Nonlinear Fiber
We have also developed a highly nonlinear fiber (HNLF)
with a large nonlinearity coefficient and a small dispersion
value, suited for use in the CPF 10). Table 2 gives a comparison of typical characteristics for SMF and DSF, whichhave been used in conventional pulse compressors, andfor the HNLF that we have developed. The nonlinearity
coefficient of HNLF is more than ten times greater thanthat of SMF, and about ten times that of DSF.

FEMTOSECOND COMPRESSION OF
OPTICAL PULSES USING CPF
COMPRESSORS

We evaluated the wavelength tunability and repetition rate
tunability of femtosecond compression using CPF, andreport the results below. Specifically, we evaluated theuniformity of characteristics from the optical spectra andautocorrelation traces while varying the wavelength of the
input pulses to the CPF in order to obtain optimum operation. In the same way we evaluated the uniformity of characteristics while varying the repetition rate of the input
pulses to the CPF in order to obtain optimum operation.
Table 2

Wavelength-Tunable Femtosecond Compression
Using CPF Compressors
Figure 3 is a schematic showing the experimental set-up
for CPF compression of a 40-GHz pulse train. The 40-GHz
pulse train was generated by external modulation of acontinuous wave (CW) light beam of wavelength λ in froma wavelength-tunable laser by means of a LiNbO3 modulator (LNM). This was then amplified by an erbium-doped
fiber amplifier (EDFA) and adjusted to the optimal input
power Pin for CPF compression. The CPF comprises 12
HNLF-SMF pairs, totaling 1.4 km. Figure 4 shows the profiles of dispersion and nonlinearity coefficient for the 12pair CPF. For the HNLF dispersion is 1 ps/nm/km or lessand the dispersion slope is 0.02 ps/nm2/km. To suppress
stimulated Brillouin scattering, two isolators were inserted
along the CPF.
Figure 5 shows autocorrelation traces and optical spectra of the input pulse and the 3- 8- and 12-pair output
pulses of the CPF, where λ in is 1550 nm and Pin is 23.6
dB. It can be seen that for both the autocorrelation traces

30
20
10
0
30
20
10
0
0

Comparison of characteristics among selected fibers.
Parameter (@ 1550 nm)

0.5

SMF

DSF

NLF

Attenuation coefficient (dB/km)

0.20

0.21

1.16

Zero-dispersion wavelength (nm)

1310

1550

1500~1600

Dispersion slope (ps/nm2/km)

0.09

0.07

0.013

Effective area (µm2)

80

50

9.7

Nonlinearity (1/W/km)

1.3

2.7

25.1

Figure 4

CPF input�

LNM
P in

CW
t

λ in

Figure 3

Intensity (a.u.)

TLS

λ

λ in

λ

2.4 psCPF 8-pair output
0.85 ps
CPF 12-pair outputtλSchematic diagram of wavelength-tunable ultra-shortpulse light source.

CPF 3-pair output∆ t ∆ν
= 0.36

CPF 8-pair output

0.31

CPF 12-pair output

0.33

0.50 pst

λ in

CPF input

8.5 psCPF CPF CPF
3-pair 5-pair 4-pair

EDFA

1.5

Profiles of dispersion and nonlinearity coefficient for
12-pair CPF.

CPF 3-pair output

40 GHz1
Z (km)

Spectrum (10dB/div)

3.

3.1

Nonlinearity (w-1km-1) Dispersion (ps·nm-1·km-1)

Figure 2, a comb-like profile not only for dispersion but
also for the nonlinearity coefficient.
For the nonlinear medium we have used an optical fiberwith a small dispersion and high nonlinearity, so that it ispossible both to obtain an ideal nonlinear medium and toachieve a compressor of shorter length. Also, as in the
SDPF, since we have multistep connection of opticalfibers with greatly differing dispersion values, it is superior
in suppressing stimulated Brillouin scattering. Note however that stimulated Brillouin scattering is not suppressed
completely, so that isolators must be inserted as needed.

-10

-5

0

5

10

1540

1550

1560

Delay time (ps)Wavelength (nm)a) Autocorrelation tracesb) Optical spectra

Figure 5

Characteristics of input and various output pulses ofCPF.

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A Comb-Like Profiled Fiber (CPF) Compressor and an Ultra-short Pulse Light Source

and the optical spectra shown in Figure 5, there was good
agreement between the experimental values (solid lines)
and the fitting with sech2 (broken lines). It will also beseen that as the optical pulses were compressed as they
propagated in the CPF, to the extent that at the 12-pair
output they were compressed to a pulse width of 500 fs.
The time-bandwidth product ∆ t∆ v at the 12-pair output
was 0.33, and the peak-to-pedestal ratio Rpp was 15 dB.
Figure 6 shows the dependence of pulse width ∆ t, timebandwidth product ∆ t∆ v, CPF input power Pin and peakto-pedestal ratio Rpp on the input wavelength of the tunable light source λ in, when λ in was varied in the range of1530~1560 nm and Pin was fine-tuned so that the CPF
12-pair output pulse width was 500 fs. From Figure 6 it
can be seen that by optimizing Pin, it is possible to keep
∆ t to 550 fs or less and ∆ t∆ v to 0.36 or less. It was also
confirmed that Rpp was 14 dB or less.
Next we examined the wavelength tunability of the CPF.
Wavelength tunability is significantly affected by wavelength dependence of the 2nd-order dispersion β 2 of thedispersion medium. That is to say it is significantly affected by β 3. Thus the larger the ratio |β 2 /β 3 |, the moreadvantageous in terms of wavelength tunability. Figure 7
is a schematic diagram showing the 2nd-order dispersion
for CPF and DDF. Since 2nd-order dispersion β 2 is greaterfor CPF than for DDF, the influence of 3rd-order disper-

0.2

∆t

Repetition Rate Tunable Femtosecond
Compression Using CPF Compressors
Figure 8 is a schematic diagram showing a repetition rate
tunable ultra-short pulse light source. A distributed feedback laser diode (DFB-LD) is gain switched by a 400-ps
electrical pulse from a pulse pattern generator (PPG), anda seed pulse is generated by chirp compensation using adispersion compensating fiber (DCF). This seed pulse is
first amplified by the 1st-stage EDFA to the pulse energythat is optimum for CPF compression, after which it is
input to the CPF. It is then amplified to a pulse energy of
50 pJ by the normal-dispersion EDFA (ND-EDFA). The
CPF consists of 13 HNLF-SMF pairs, with a total fiber
length of 2.5 km. In the ND-EDFA we make use of twostages of EDFA comprising 20 m of EDF with a dispersionof -4 ps/nm/km. Then 2 m of SMF is connected after the
ND-EDFA. The repetition rate of the seed pulse can becontrolled by the repetition rate of the electrical pulse
used to gain switch the DFB-LD.
Figures 9 through 11 show the autocorrelation traces
and optical spectra for the seed pulse output, CPF output
and ND-EDFA output respectively, and correspond to thepoints in Figure 8 indicated by A, B and C respectively.
From the results in Figure 9 for the autocorrelation traces for the seed pulse output, it can be seen that the input
pulses to the CPF showed a certain amount of deviation,in the order of 12 to 17 ps. This is attributed to such factors as the band of the electrical amplifier and the adjustment method.

0
DCF

40

26

Pin

30

24

20

22

Rpp

10

20
1540

1550

CPF
EDFA

DFB-LD

A

1560

Figure 8

λ in (nm)

Dependence of pulse width ∆ t, time-bandwidth product ∆ t∆ v, CPF input power Pin and peak-to-pedestal
ratio Rpp on wavelength λ in.

frep= 500 MHz

Wavelength tuning range

C

frep= 500 MHz

50 MHz14.5 ps
5 MHz

50 MHz5 MHz

16.5 ps-20 -10
CPF (SMF)
Wavelength

Figure 7

Intensity (a.u.)

Second-order dispersion β 2

Output end of DDF

BSchematic diagram of repetition rate tunable ultrashort pulse light source

11.5 ps0

ND-EDFA

PPG

0
1530

Figure 6

3.2

Spectrum (10dB/div)0

Pin (dBm)

0.4

Rpp (dB)∆ t (ps)∆ t ∆ν
1∆ t ∆ν

0.6

2sion β 3 is less apparent, and wavelength tunability is moreeasily achieved.

Schematic diagram of 2nd-order dispersion for CPF
and DDF.

0

10

20

Delay time (ps)a) Autocorrelation traces
Figure 9

1556
1557
1558
Wavelength (nm)b) Optical spectra

Seed pulse output characteristics.

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A Comb-Like Profiled Fiber (CPF) Compressor and an Ultra-short Pulse Light Source

frep= 500 MHz

frep= 500 MHz ∆ t ∆ν = 0.35100

0.48 ps
5 MHz
0.50 ps
-4

-2

0

2

50 MHz

5 MHz

A

10
1

B

0.1

C

∆ t ∆ν = 0.38
100

1540

4

∆ t∆ν = 0.36

∆ t (ps)

50 MHz

1560

1580

Delay time (ps)Wavelength (nm)a) Autocorrelation tracesb) Optical spectra

ε p (pJ)

Spectrum (10dB/div)

Intensity (a.u.)

0.48 ps

C

10
B

1

A

0.1
1

Figure 10 CPF output characteristics.

10

100

1000

fref (MHz)frep= 500 MHz

Figure 12 Dependence of pulse width ∆t and pulse energy εp on
repetition rate frep at outputs from seed pulse (A), CPF(B), and ND-EDFA (C).

frep= 500 MHz
Spectrum (10dB/div)

Intensity (a.u.)

0.30 ps
50 MHz0.30 ps
5 MHz
0.31 ps
-4

-2

0

2

4

50 MHz

4.
5 MHz

1540

1560

1580

Delay time (ps)Wavelength (nm)a) Autocorrelation tracesb) Optical spectra

Figure 11 ND-EDFA output characteristics.

The broken line plot in Figure 10 shows the fitting of
sech2 to the experimental results. It can be seen that there
is a good fit to the experimental results for the autocorrelation traces and the optical spectra. When pulse widthand peak-to-pedestal ratio at CPF output were estimatedfrom the autocorrelation traces, we obtained values of<500 fs and >10 dB respectively. From this it can beseen that the CPF is responsive to a wide range of repetition rate tuning. Further, it can be seen from Figure 9 thateven when the width of the pulse input to the CPF changes by several picoseconds, an output of constant pulse
width can be obtained.
From the results in Figure 11 for the autocorrelation
traces for the ND-EDFA output, it can be seen that theoutput pulse width is constant at approximately 300 fs,
irrespective of the repetition rate. It can also be seen thatthe 500-fs CPF output pulse is being further compressed
to 300 fs by the nonlinearity effect within the ND-EDFA,
and by dispersion compensation by the SMF.
Figure 12 summarizes the results of Figures 9 through11, showing the dependence of pulse width and pulse
energy on the repetition rate. Values at seed pulse output
are indicated by A, at CPF output by B and at ND-EDFA
output by C, corresponding to the points indicated by A,
B and C in Figure 8. It can be seen that over a range ofrepetition rates from 5 MHz to 500 MHz, a uniform train of
300-fs, 50-pJ pulses is obtained.

CONCLUSION

We have described a CPF compressor in which the nonlinearity coefficient and dispersion value have a comb-like
distribution along the length of the optical fiber due tomultistep connection of HNLF-SMF pairs. It has been confirmed that, at wavelengths ranging from 1530 to 1560
nm, that the CPF compressor can achieve uniform compression of approximately 500 fs. It has further been confirmed that in the 5-MHz to 500-MHz range uniform output
pulses can be obtained. Since the CPF compressor isoutstandingly easy to manufacture and responds flexibly
to both wavelength tuning and repetition rate tuning, it isexpected to find applications in the fields of optical communication and optical metrology, as well as bio-photonics, materials processing, physics, etc.

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