Regular Papers

Asynchronous All-optical Signal Processing
Achieved Using Raman Soliton EffectMasao Kato†
Abstract
Error-free asynchronous all-optical bit-by-bit self-signal recognition and simultaneous demultiplexing
have been successfully demonstrated using a signal processor based on the intensity-dependent self-frequency shift induced by the Raman soliton effect. The processor demultiplexed overlapping signals in“contended optical packets” with a recognition sensitivity of better than 3 dB. Clear open eye diagrams
were successfully obtained. The ability to achieve three-stage cascade error-free operation was verified.
Furthermore, photonic conversion of the multiplexing format from WDM (wavelength division multiplexing) to OTDM (optical time division multiplexing) was experimentally demonstrated. Four WDM
channels with a 200-GHz channel spacing were converted into one OTDM signal with various repetition
rates of up to 100 GHz.

1. Introduction
Photonic packet-switched networks will be able tosupport future intelligent systems in which telecommunications, computer communications, and multicasting coexist. These networks will provide a highspeed data rate, format transparency, and configurability, which are needed for converting different data
formats. In general, photonic packet-switched networks can be divided into two categories: slotted(synchronous) and unslotted (asynchronous) [1].
Unslotted networks are easier and cheaper to build,
more robust, and more flexible than slotted networks.
However, in unslotted networks the probability ofcontention is higher because packet behavior isunpredictable and less noticeable. The term “contended packets” refers to optical packets that overlap
in the time domain, which are difficult to recognize
and demultiplex without an optical buffer.
Efforts to achieve buffer-free packet contention resolution are focusing on all-optical devices using highspeed optical time-division multiplexing (OTDM).
† NTT Photonics Laboratories
Atsugi-shi, 243-0198 Japan
E-mail: katom@aecl.ntt.co.jp

26

They are faster than electronic circuits and let us use
the optical frequency domain for signal processing.
However, in conventional all-optical devices, the synchronization of bit frequency and phase requires anextracted electrical clock to drive the optical source
and thus generate control pulses for device operation.
This will prevent conventional all-optical devices
from becoming useful.
One of the most promising ways of obtaining useful all-optical devices is to develop an asynchronous
device that operates without synchronization of thebit frequency and phase. My colleagues and I havedeveloped an asynchronous all-optical signal processor based on the Raman soliton effect. This technique
has been used for generating only necessary multiwavelength pulses [2] and for a novel asynchronous
all-optical demultiplexer for ultra-fast OTDM transmission, called the AAA-DEMUX [3]. The term
Raman soliton refers to a soliton pulse with an intensity-dependent self-frequency shift [4]-[7]. This originates from intra-pulse stimulated Raman scattering.
The downshift of a carrier frequency is proportional
to the input pulse intensity when the pulse is propagating in a fiber with an anomalous group-velocity
dispersion. Therefore, contended optical packets withdifferent amplitude levels are automatically self-sigNTT Technical Review

Regular Papers

nal recognized and demultiplexed bit-by-bit, depending on the signal intensity.
This paper describes how we demonstrated errorfree asynchronous all-optical bit-by-bit self-signal
recognition and demultiplexing from contended optical packets without the use of an optical buffer, control pulse or bit-phase synchronization. The measured processing sensitivity was better than 3 dB andthe measured BER (bit error rate) shows that we successfully achieved three-stage cascade operation.
Photonic conversion of the multiplexing format
achieved by intensity-dependent self-frequency shiftof Raman soliton is also described. Experimental
results show that the novel asynchronous all-opticalsignal processor is a low-noise device that is promising for use in contention resolution in unslotted photonic packet-switched networks.

intra-pulse stimulated Raman scattering and thedownshift of the carrier frequency is proportional tothe intensity of the signals propagating in a fiber withan anomalous group-velocity dispersion.
The signal processor is very simple and consists ofthree components: an erbium-doped fiber amplifier(EDFA), a highly nonlinear fiber for Raman soliton
generation (RS fiber), and an optical bandpass filter(OBF) (Fig. 2). The amplitude level of each opticalsignal is used as a label. Thus, all the signals can be
automatically converted into WDM (wavelength
division multiplexing) signals, according to theiramplitude levels. Therefore, this processor can use
the optical frequency domain for signal processing.
Low-speed WDM signals with wavelength of λ 1,
λ 2, λ 3, and λ 4 are coupled in a photonic converter
composed of variable attenuators (VATTs), variable
time-shifters (VTSs), a wavelength combiner (WC),an EDFA, and an RS fiber. All the WDM channels areattenuated by VATTs to equalize their amplitude levels. After time shifting, the amplitude-controlled
WDM channels are combined by the wavelength
combiner and amplified to the Raman soliton power
regime of the RS fiber. Since the downshift of a carrier frequency is proportional to the input pulse intensity when the pulse is propagating in a fiber with an

2. Operating principle
The principle of the developed asynchronous alloptical processor is amplitude-division demultiplexing based on the intensity-dependent self-frequency
shift of gigahertz soliton pulses. The all-optical selfsignal recognition and demultiplexing is schematically shown in Fig. 1. The frequency shift is caused byPulse trains @ λ0

P1
P0
Time

λ2

Wavelength

Intensity

P2

λ1
λ0

RS fiberTime

Fig. 1. Schematic concept of bit-by-bit self-signal recognition and simultaneous demultiplexing.
WDM signals

Photonic converter

λ1

OTDM signal

λ2

λ5

I

RS fiber
OBF

λ3

VATT

VTS

WC

λ4
EDFA

λ1 λ2 λ3 λ4

I
Time

Time

λ1 λ2 λ3 λ4Fig. 2. Schematic configuration of photonic converter.

Vol. 3 No. 4 Apr. 200527

Regular Papers

anomalous group-velocity dispersion, WDM channels with different amplitude levels can be converted
to the same wavelength (λ 5).

should be positive and, in this case, it was +2.0
ps/nm/km. To verify the capability of cascade operation, we measured the BER performance and eye-diagrams after the signal had passed through three signal
processors connected in series. Here, contended
packets were not used in the tests for the second andthird signal processing stages; instead, OBFs with a
3-nm passband were used in the second and third
processors.
The experimental setup for converting the multiplexing format from WDM to OTDM is shown in
Fig. 4. To generate low-speed WDM channels, weused a repetition rate of 20 MHz, a pulse width of
0.57 ps, and a mode-locked fiber laser (MLFL) witha wide spectrum range of over 9.0 nm FWHM (fullwidth at half maximum). The output spectrum fromthe laser is shown in Fig. 5 and the dotted lines show
the spectra of WDM channels. We used a pair of
AWGs with a 200-GHz channel spacing for spectral
slicing, and four WDMs (λ 1: 1556.0 nm, λ 2: 1557.6
nm, λ 3: 1559.2 nm, λ 4: 1560.9 nm) without polarization multiplexing. A 2.5-km-long RS fiber was used
and it had a second-order dispersion of +2.0
ps/nm/km at 1555 nm for Raman soliton generation.
The experimentally measured dependence of the
wavelength shift on input peak power for each WDM
channel is shown in Fig. 6. These values were allmeasured in the same way. The pulse widths of input
WDM channels 1 – 4 (λ 1 – λ 4) were 4.4, 4.7, 4.7, and
4.7 ps, respectively. It is clear that WDM channels
with different amplitude levels can be converted tothe same wavelength, e.g., 1575 nm (λ 5). Therefore,

3. Experiment
The experimental setup for self-signal recognitionand demultiplexing is shown in Fig. 3. A 3-ps, 9.95GHz optical pulse train at 1555 nm was generated
from a newly designed super stable pulse source [8].
The generated pulse train was obtained by synthesizing its Fourier spectrum, which is composed of higher-order sidebands produced by a Fabry-Perot resonator integrated with an electro-optic phase modulator. The train was modulated by a LiNbO3 intensity
modulator at 9.95 Gbit/s with a 231–1 PRBS (pseudorandom bit stream). After coding, the data signal was
amplified and connected to a 1.1-km-long highly
nonlinear (HNL) fiber. This enabled us to accomplish
adiabatic soliton compression to less than 0.8 ps.
The contended packets are optical packets thatoverlap in the time domain. Therefore, we emulated
the overlapping signals in the contended optical packets using a PLC (planar lightwave circuit) multiplexer with 1.5-ps relative delay lines and a 3-dB relative
intensity difference. After overlapping, the contended
packets were injected into the processor without synchronization of the bit frequency and phase. The
processor consisted of an EDFA, a 2.5-km RS fiber,
and an arrayed waveguide grating (AWG) filter witha channel spacing of 200 GHz. To generate soliton
pulses, the second-order dispersion of the RS fiber
λ0

HNL-fiber:
1.1 km

EDFA

OPG

EDFA
AWG

PLC

MOD

λ2

RS fiber
2.5 km

EDFA
DATA

λ1

PC

PPG
9.95 GHz

EDFA

λ11
OBF

EDFA

λ111
OBF

CLOCK

λ1

9.95 GHz640 GHzλ2

RS fiber
2.5 km
EDF A
BPF

λ22

BER

OR

RS fiber
2.5 km
EDF
A

DATA
BPF

λ222

OPG: optical pulse generator, MOD: modulator, PPG: pulse pattern generator, PLC: PLC multiplexer,
PC: personal computer, BPF: bandpass filter, OR: optical receiver, BER: bit error rate test set

Fig. 3. Experimental setup for signal processing of overlapping signals in contended optical packets and for cascade
operation examination.

28

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Regular Papers

VATT

VTS
RS fiber
2.5 km

AWG

VATT

VTS

OBF

AWG (WC)

OSC

MLFL
EDFA
VATT

VTS

VATT

VTS

O/E

Trigger

MLFL: mode-locked fiber laser, OSC: oscilloscope, O/E: optical-to-electrical converter

Fig. 4. Experimental setup for photonic converter.

1538

λ2
λ1

λ3

λ4
Outputspectrumfrom MLFL

Wavelength (nm)Intensity (10 dB/div)

1590

1580

1570

1560

1558

1578

Wavelength (nm)1550

λ1 λ2 λ3 λ4
0

0.25

0.5
0.75
Input peak power (W)

1.0

Fig. 5. Spectrum waveform.

Fig. 6. Experimental dependence of the wavelength shift
on input peak power.

the input WDM signals were identically attenuatedand time-shifted into various time slots.

However, after passing through the first processor,
the contended signals were simultaneously demultiplexed to 2 × 9.95 Gbit/s WDM signals at 1561 nm(channel 1) and 1563 nm (channel 2). By using a 3dB intensity difference as a label, we were able toachieve demultiplexing. Thus, error-free asynchronous all-optical demultiplexing from overlapping signals was successfully achieved without the use of
optical buffers, a control pulse, or bit-phase synchronization. The demultiplexed signals exhibited clear
eye opening, as shown in Fig. 8. They were coupled
into the second and third signal processors to examine the possibility of cascade operation. The WDM
signals output from the second processor were 1565
(channel 1) and 1576 nm (channel 2) and those fromthe third processor were 1581 (channel 1) and 1584

4. Results and discussion
Figure 7 shows an autocorrelation trace of theoverlapping signals measured after the PLC multiplexer with a relative delay time of 1.5 ps. Signals
with a width of 0.8 ps were used and the intensity difference between the overlapping signals was 3 dB.
The multiplexed signals are in the longitudinal mode
at 640 GHz, which is induced by the relative timedelay of 1.5 ps. It should be noted that they cannot be
demultiplexed by a bandpass filter. Moreover, they
are not easy to distinguish because of the overlappingof the envelope when observed in the time domain.
Vol. 3 No. 4 Apr. 200529

Regular Papers(a)Intensity (a.u.)(a)

1.5 ps50 ps/div(b)
Intensity (10 dB/div)(b)
50 ps/div

640 GHz

Fig. 7. Overlapping signals with relative intensity
difference of 3 dB in contended optical packets.
(a) Autocorrelation trace measured after PLC
multiplexer with relative delay time of 1.5 ps and(b) spectrum of the overlapping signals.

30

10–4

9.95 Gbit/s
PRBS 231–1

BER

10–5
10–6
10–7

2nd

10–8
10–9
10–10
10–11
–26

3rd

1st
CH 1
CH 2
–24

–22

–20

–18

–16

–14

Received power in photodiode (dBm)

Fig. 9. BER performance measured after 1st, 2nd, and3rd signal processors.

2nd: 1576 nm

Intensity (10 dB/div)

nm (channel 2).
Figure 9 shows the BER performance of thedemultiplexed 9.95-Gbit/s signals, Fig. 10 shows the
spectra of the demultiplexed signals measured after
the second and third processors, and Fig. 11 shows
eye-diagrams measured after the third processor.
Clearly open eyes and error-free demultiplexing by
the cascade operation were obtained. The power
penalties of the second and third demultiplexing
stages relative to the first were 2 and 6 dB, respectively, at a BER of 1 × 10 –9. The measured BER performance confirmed a processing sensitivity of better
than 3 dB and the ability to achieve three-stage-cascade operation from overlapping signals.
The characteristics of the overlapping pulses wereclarified as follows. When the autocorrelation trace ofthe overlapping pulses was measured, we could not
determine whether there was one pulse or a pair ofpulses. In general, when a pulse train is divided andmultiplexed again with a short delay time, it is difficult to distinguish between one and two pulse trains
because of the overlapping of the envelope whenobserved in the time domain. In this case, observationin the frequency domain provides the required resolution. Since the pulse train is periodic, the spectrum
is discrete and the envelope of the Fourier transformis the pulse shape. The frequency spacing is proportional to the repetition frequency of the pulse train.
With overlapping pulse trains, the fundamental fre-Fig. 8. Eye-diagrams of the demultiplexed signals
measured after 1st processor. (a) Channel 1:
1561 nm and (b) channel 2: 1563 nm.

3rd: 1584 nm

1st: 1561 nm

Input:
1555 nm

1500

1600

1550
Wavelength (nm)1st: channel 1, 2nd: channel 2, 3rd: channel 2

Fig. 10. Spectra of the demultiplexed signals measured
after 2nd and 3rd signal processors.

NTT Technical Review

Regular Papers(a)Input WDM
signals
Intensity (10 dB/div)

50 ps/div

Output OTDM
signal(b)
50 ps/div
1480

1530

1580

1630

Wavelength (nm)Fig. 12. Output spectrum of RS fiber for photonic conversion.
(a)Fig. 11. Eye diagrams measured after 3rd processor.
(a) Channel 1: 1581 nm and (b) channel 2: 1584 nm.

Vol. 3 No. 4 Apr. 2005

Intensity (a.u.)
1575 nm(b)

Intensity (a.u.)

50 ps/div

1575 nm(c)1575 nmIntensity (a.u.)

quency of the original pulse train and the frequency
synthesized from the overlapping pulse trains coexist
in the observed spectrum. In this experiment, the fundamental frequency of the overlapping pulse trains
was 9.95 GHz and the synthesized frequency fromthe overlapping was 640 GHz owing to the time delayof 1.5 ps as shown in Fig. 7(b). Moreover, since
uncompressed pulse energy remained around thecompressed pulse as a broad pedestal, the waveformof the overlapping pulses was distorted. Therefore, it
was more difficult to decide whether there was one
pulse or a pulse pair from observations in the time
domain.
The output spectrum of the RS fiber is shown in
Fig. 12 and the dashed line shows the spectrum of theamplified spontaneous emission (ASE) of the EDFA
measured without WDM signals for comparison. The
OTDM signal converted from the four WDM signals
was generated at 1575 nm. The output was filtered by
a 1.0-nm OBF at 1575 nm to isolate the OTDM signal. Figures 13(a) and (b) show sampling oscilloscope traces with time separation of 200 and 50 ps,
which corresponded to data rates of 5 and 20 Gbit/s,
respectively. Figure 13(c) shows the autocorrelation
trace of the OTDM signal with a time separation of 10
ps corresponding to 100 Gbit/s, which was converted
from two orthogonally polarized WDM channels(channels 1 and 2). There are three pulses in the autocorrelation trace and the intensity of the center pulse
is twice that of the other pulses; however, this is a feature of the autocorrelation traces of double pulses.
The measured pulse width of the generated OTDM

50 ps/div

10 ps

10 ps
2.0 ps(calibrated 1.3 ps)

Fig. 13. Converted OTDM signals generated from 4 WDM
signals. Time separation is (a) 200 ps (5 Gbit/s),(b) 50 ps (20 Gbit/s), and (c) 10 ps (100 Gbit/s).

signal was 2.0 ps, calibrated to 1.3 ps assuming sech2
pulses. We found that four WDM channels with a
200-GHz channel spacing were successfully converted into one OTDM signal with various repetition
rates of up to 100 GHz achieved by the intensitydependent self-frequency shift of the Raman soliton.
31Regular Papers

References
Masao Kato
[1]

[2]

[3]

[4]
[5]
[6]

[7]

[8]

32

D. J. Blumenthal, R. J. Feuerstein, and J. R. Sauer, “First demonstration of multihop all-optical packet switching,” Photonics Technology
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asynchronous all-optical amplitude-division demultiplexing based onpolarization-independent GHz Raman soliton in fiber,” in Proceedings Conference on Laser and Electro-optics (CLEO), Post deadline
paper CPDB4, 2002.
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M. Nakazawa and K. Kurokawa, “Femtosecond soliton transmissionin 18 km-long dispersion-shifted, distributed erbium-doped fibre
amplifier,” Electron. Lett., Vol. 27, pp. 1369-1371, 1991.
M. Kato, K. Fujiura, and T. Kurihara, “Generation of super-stable 40
GHz pulse trains without supermode competition from a Fabry-Perot
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Lett., Vol. 40, pp. 299-301, 2004.

Research Engineer, Photonic Processing
Devices Research Group, NTT Photonics Laboratories.
He is currently taking a Ph.D. degree at the
University of Tokyo, Tokyo. He joined NTT
Laboratories in 1995, where he was engaged inresearch on nonlinear optics in optical fiber.
Since 2001, he has been engaged in research on
all-optical signal processing. He received the1999 Young Scientist Award for the Presentationof an Excellent Paper from Japan Society ofApplied Physics (JSAP). He is a member of
JSAP.

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