Selected Papers
Signal Processing Using All-optical Wavelength Conversion–Application using Hybrid-integrated XPM Device and SIPAS
Toshio Ito†, Rieko Sato, Yasuo Shibata, and Yoshihiro
Kawaguchi
Abstract
The explosive demand for data traffic requires high-speed all-optical signal processing by wavelength
conversion using a device such as a planar lightwave circuit (PLC) hybrid-integrated cross-phase modulation (XPM) device or monolithically integrated SIPAS. An XPM device operates with a low input
power of –10 dBm and can be used to adjust the wavelength or monitor the polarization mode dispersion(PMD) of a WDM channel, while SIPAS provides high-speed operation. This paper reports bit-rate andformat conversion between conventional high-speed (10 Gbit/s) NRZ signals and extremely high-speed(40 Gbit/s) RZ signals by multi-wavelength conversion. It also discusses PMD compensation, which isan important issue, especially for extremely high-speed signals.

1. Introduction
In the near future, the huge amount of data traffic
will require a high bit rate of 10 Gbit/s, even in metropolitan networks, as shown in Fig. 1. The long-haul
backbone network, which serves a number of metropolitan networks, will need an extremely high bit rateof more than 40 Gbit/s. Therefore, bit-rate conversion
between 10- and 40-Gbit/s networks is indispensable,
because most metropolitan users will not use the full
40-Gbit/s capacity. All-optical bit-rate conversion
has the advantage of bit-rate independent operationand has the potential to overcome the speed limit ofelectrical devices. Prior to bit-rate conversion, a simple technique that adjusts the phase of the 10-Gbit/schannels is needed. Furthermore, the conversion
should operate for any wavelength division multiplexing (WDM) channel wavelength.
For all-optical bit-rate conversion, extremely highspeed wavelength conversion using differential-phasemodulation (DPM) is one of the most promising
approaches [1], [2]. However, DPM devices usuallyneed the return-to-zero (RZ) format, while 10-Gbit/s† NTT Photonics Laboratories
Atsugi-shi, 243-0198 Japan
E-mail: toshio@aecl.ntt.co.jp

52

WDM systems use the non-return-to-zero (NRZ) format. This RZ format is also convenient for highspeed optical transmission, because the short-pulse
RZ signal reduces the signal degradation from fiber
nonlinearities, such as self-phase modulation [3], [4].
Bit-rate conversion is being developed at laboratories around the world. TDM-to-WDM conversion
and WDM-to-TDM reconversion (TDM: time division multiplexing) have been demonstrated using a
supercontinuum source [5]. TDM-to-WDM signal
format translation has been achieved using bi-directional four-wave mixing (FWM) in a semiconductor
optical amplifier (SOA) [6] or monolithic MachZehnder interferometer (MZI) module [7]. Mikkelsen
et al. reported RZ-to-NRZ conversion using a monolithically integrated active Michelson interferometer[8]. However, a total experiment comprising bit-rate
and format conversion/reconversion, as well as phaseadjustment for any WDM channel, had not been done
until the present work.
This paper reports a successful demonstration offull bit-rate conversion from 10-Gbit/s WDM channels of any wavelength to a 40-Gbit/s channel, including NRZ-to-RZ format conversion and reconversion.
The key technique is wavelength conversion using amonolithic Sagnac Interferometer integrated with a
Parallel-Amplifier Structure (SIPAS) [9]. The key
NTT Technical Review

Selected Papers

Long haul

LAN: 10 Gbit/s
NRZ

PMD
compensation

Inter-LANbridgeLANLAN

40 Gbit/s
RZ

LAN: 10 Gbit/s
NRZ

Fig. 1. Network configuration in the near future.

procedure, which is the phase adjustment of 10Gbit/s signals, used slight wavelength conversion
with hybrid-integrated cross-phase modulation(XPM) device [10].
Another important issue for extremely high-speed
networks is the compensation of polarization mode
dispersion (PMD). Monitoring the state of the differential group delay (DGD) is indispensable for PMD
compensation, as shown in Fig. 2, and several
approaches have been implemented using electronic
circuits. A simple method for monitoring DGD is tomeasure the intensity of half the frequency of the
input signal. This is an elegant method because it
requires only a photodiode (PD), a narrow band-passfilter (BPF), and a power meter [11], [12]. However,it requires a high-speed PD, which limits network
speed upgrades to 100 Gbit/s. The BPF should bestrictly adjusted to the input frequency, which disturbs the bit-rate transparency of photonic networks.
For PMD compensation of extremely high-speed
signals (> 80 Gbit/s), optical monitoring techniques
are expected. Rosenfeldt et al. performed the world’s
first demonstration of PMD compensation at 80
Gbit/s, using a polarization-resolved evaluation of thedegree of polarization (DOP) combined with a polarization scrambler at the fiber input [13]. Anothertechnique for 160-Gbit/s compensation monitors theeye opening of the data signals using an optical sampling eye monitoring method [14]. This techniqueprovides the most reliable information because thePMDequalizer

Feedback

Coupler

DGD
monitorFig. 2. Configuration of PMD compensator.

Vol. 1 No. 1 Apr. 2003

eye opening has a high correlation with the bit error
rate (BER), which is the final target for improvements. However, both techniques may need expensive equipment for each WDM channel, which would
make them difficult to apply to large WDM systems
with numerous channels.
We have proposed a novel technique for monitoring
the DGD state of an optical fiber using a hybrid integrated wavelength converter. Thanks to the XOR
operation of the wavelength conversion, this technique is bit-rate-independent and format-independent. Hybrid integration is very attractive because aplanar lightwave circuit (PLC) can easily implementa polarization beam splitter (PBS), one of the key
components for PMD compensation. By monitoring
the output power from the wavelength converter, we
successfully compensated for the PMDs of 40- and80-Gbit/s RZ, 40-Gbit/s NRZ, and 20-Gbit/s Manchester format signals. The measured power penalty
was less than 0.8 dB.
2. Bit-rate conversion
2.1 Configuration for full bit-rate conversion
The configuration of the demonstration system isshown in Fig. 3. The key features are as follows.
I) NRZ/RZ and MUXFour 10-Gbit/s NRZ WDM channels with a 100GHz channel spacing (A) were launched into a polarization-independent electro absorption (EA) modulator [15] with 10-GHz sinusoidal modulation. The
modulator simultaneously converted the four NRZ
signals to RZ ones with a pulse width of 25 ps (B). A
fiber loop was used to arrange the piled-up RZ pulses into a serial bit-stream (MUX), using with a single
modulator and a single fiber, which is a simple andextensible technique for bit-stream formation. This
technique can also be used in larger-scale WDM systems, e.g., ones with 8 or 16 channels.
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II) Wavelength fitting and phase adjustment
In confirming NRZ/RZ and MUX operation, there
are two important issues: the channel spacing, whichmust be nearly equal, and the phases of the four
WDM channels, which must be adjusted. Using a
multi-channel XPM module, we arranged the input
WDM channels to have an equally spaced fixedwavelength. Then a small wavelength adjustment(e.g., 0.1 nm) provided a phase shift at the fiber loop,
which assured the phase adjustment at the EA modulator. Thermal tuning of the CW source achieved thisphase adjustment.
III) Bit-rate conversion using SIPAS
For 40-Gbit/s bit-rate conversion, four different
wavelengths of the multiplexed bit-stream were converted to a single wavelength by using a SIPAS witha gating window of less than 25 ps (C in Fig. 4). DPM
can compensate for the slow carrier recovery time of
SOAs to achieve high-speed wavelength conversion.
The SIPAS has low wavelength dependency andoffers the possibility of filter-less operation.
IV) DEMUX and RZ/NRZ
An EA modulator was also used to demultiplex theconverted 40-Gbit/s stream to the 10-Gbit/s RZ format(D in Fig. 4). Another DPM device with a gating window of 100 ps enlarged the pulse width, thus completingthe 10-Gbit/s RZ-to-NRZ reconversion (E in Fig. 4).
100 GHz spacing
Randomwavelength

NRZ/RZ

2.2 Experimental results for full bit-rate conversion
The experimental setup is shown in Fig. 4. Four 10Gbit/s WDM channels were converted to equal spacing using a multi-channel PLC-SOA hybrid integrated MZI module. The continuous wave (CW) andaverage signal powers of the XPMs were -2 to 2 dBm.
A 10-MHz clock with a wavelength of 1310 nm was
supplied, utilizing the unoccupied port of the XPM
module. This clock is indispensable for synchronization of bit rate conversion. The four converted signals
were coupled to an optical fiber with dispersion of
150 ps/nm. Therefore, a small wavelength adjustmentof less than 0.5 nm provided a phase shift of 75 ps.
Four phase-adjusted WDM channels with 100-GHz
spacing from 1552 to 1555 nm were launched into an
EA modulator with 10-GHz 4V sinusoidal modulation (A and B in Fig. 4). A second 2-km fiber loopwith a total wavelength dispersion of –25 ps per 100
GHz was used to arrange the piled-up RZ pulses into
a serial bit-stream with a pulse width of 25 ps. Part ofthe bit-stream was monitored and the wavelengths
converted by the XPM modules were controlled tokeep the most suitable values.
10- to 40-Gbit/s bit-rate conversion was done using
DPM in a SIPAS. Four different wavelengths of themultiplexed bit-stream were converted to a single
wavelength (1551 nm) by using the SIPAS with a gat-

MUX

Fiber
loop

DEMUX
Bit rate conversion

Fiber
loop

XPMs

EA

SIPAS

DPM

(E)

(C)

EA
(A)

(D)

RZ/NRZ(B)

Splitter
CouplerFig. 3. Configuration of full bit-rate conversion.

XPMs

Clock
Fiber
loop

10 Gbit/s
NRZ

EA
(A)

Monitor

Fiber
loop
(B)

SIPAS
MZI

(C)
Delay

EA

(D)

SOA

(E)

ReceiverFig. 4. Experimental setup of full bit-rate conversion.

54

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Selected Papers

ing window of less than 25 ps (C in Fig. 4). The CW
and averaged signal powers were about 10 dBm. The
current of the SOAs was about 200 mA.
A second EA modulator was used to demultiplex
the 40-Gbit/s stream into 10-Gbit/s RZ format (D inFig. 4). Another DPM device, consisting of a spotsize converter SOA (SS-SOA) and a PLC asymmetric MZI with a gating window of 100 ps, enlarged thepulse width, thus completing 10-Gbit/s RZ-to-NRZ
conversion (E in Fig. 4). The input current of the SSSOA was 80 mA. The CW and signal powers wereabout 0 dBm.
Figure 5 shows the four 10-Gbit/s bit stream signals
before the SIPAS. By adjusting the wavelength ofchannel #3 by 0.1 nm (phase shift of 15 ps), we could
avoid the cross point of the NRZ signal. Figure 6
shows the eye patterns measured at points A to E,
shown in Fig.4. Clear eye openings were observed for
NRZ-to-RZ conversion (Fig. 6(b)), 10-to-40-Gbit/s
conversion (Fig. 6(c)), 40-to-10-Gbit/s reconversion(Fig. 6(d)), and RZ-to-NRZ reconversion (Fig. 6(e)).
Figure 7 shows the BERs at points (A) to (E). The
receiver sensitivity at a BER of 10–9 was less than -32
dBm. Only a small power penalty of less than 0.8 dB

ch. 1

2

3

4

without phase adjustment

was observed. These results indicate that the above
techniques will be useful for future bit-rate conversion.
3. PMD compensation
3.1 DGD monitoring technique using a hybrid
integrated wavelength converter
Figure 8 shows a schematic view of the hybrid integrated XPM device. We use PLC hybrid integration,
which is attractive for constructing optical modules
with fiber array pigtails.
The DGD monitoring technique is as follows. The
input optical powers of the polarization states (TEand TM) are each fed to both arms of the XPM
device. The device is normally set to the cross state.
This minimizes the output power of the converted
light at the bar port, which is the monitor port of the
DGD. If a large DGD exists, the phase difference
between the two arms switches the converted lightfrom the cross port to the monitor bar port, whose
output power increases as shown in Fig. 9. A minimum output power means the DGD value is also at
the comfortable minimum state, which should be kept
by controlling the PMD equalizer. As a result, theXPM device operates as an XOR circuit. The monitor
port shows “0” when the input powers of the twoarms are equal, while it shows “1” when they are
unequal, as shown in Fig. 10. This circuit has the

10-4

Bit error ratewith phase adjustment

25 ps

Fig. 5. Eye patterns before the SIPAS.

(a)

10-5

back-to-backfull bit rate conversion

10-6
10-7
10-8

100 ps
10-10
10-12
-36

(b)

-34 -32 -30 -28 -26

Received optical power (dBm)

Fig. 7. BER of full bit-rate conversion.

(c)

(d)

CW input
25 ps

(e)

Fig. 6. Eye patterns of full bit-rate conversion.

Vol. 1 No. 1 Apr. 2003

TE input
DGD monitor
TM input

Fig. 8. DGD monitor using hybrid integrated XPM device.

55

Selected Papers

potential to work at >100 Gbit/s, because the pushpull switching mechanism of DPM cancels the slow
relaxation of the induced refractive index change[16].
3.2 Experimental setup and results
Figure 11 shows the experimental setup for PMD
compensation. We used four types of optical signals:
40-Gbit/s RZ and NRZ signals, a 20-Gbit/s Manchester signal, and an 80-Gbit/s RZ signal.
To emulate the waveform distortion caused by
PMD, we prepared a polarization-maintaining fiber(PMF) whose DGD between the fast and slow axes
was about 30 ps. The power ratio between the two
principal polarization states was changed from 0 to
10 dB by adjusting the polarization controller (PC)before the PMF.
To equalize the PMD, a second PC and a variable
DGD emulator were used. Part of the signal was
divided for monitoring, and its polarization states
were divided by a bulk PBS module. The opticalpowers of the divided two polarization states wereequalized to within 1 dB using the SOAs and each

TE = 0DGD = 0

CW input

was fed to both arms of the hybrid integrated wavelength converter. The optical lengths between the
PBS and the wavelength converter arms were equalized using optical delay lines. Of course, this opticallength adjustment can be achieved easily if the integrated PLC-PBS is used. The wavelength converter
has input power monitor ports with power meters,
which make it possible to adjust the optical power ofboth arms.
A DFB laser with a wavelength of 1552 nm wasused as the optical source of the XPM device. With
SS-SOA current biases of 80 and 82.5 mA, the XPM
device was normally set to the cross state. It operates
as an XOR circuit. The output power from the XOR
circuit (bar port) was minimized by controlling the
PMD equalizer. The optical signal, after PMD compensation, was optically demultiplexed to 10-Gbit/s
RZ format using an EA modulator, and the BER was
evaluated.
Figure 12 shows the eye patterns for the 40-Gbit/s
RZ, 40-Gbit/s NRZ, and 20-Gbit/s Manchester signals. Though large waveform distortions wereobserved before PMD compensation, a clear eye pattern was observed afterwards. Figure 13 shows the
BER for RZ and NRZ formats. The power penalty
was less than 0.8 dB. These results confirm that format-independent PMD compensation was achieved

TM = 0TE = 1 or 0
DGD = 1

CW input

TM = 0 or 1
TE = 1
DGD = 0

CW input

80 Gbit/s RZ
40 Gbit/s NRZ, RZ
80 Gbit/s Manchester
PD

PC

PMF

EDFA PC Equalizer Coupler
FA
v-DGD
Feedback

PD

Delay

10 GHzXOR Feedback

PBS
PD

DFB-LD
PD

Feedback SOA

Delay
PMD monitor

TM = 1

Fig. 11. Expermental setup of PMD compensator.
Fig. 9. Principle of DGD monitor using hybrid
integrated XPM device.
DGD = 0TE
TM

before compensation after compensation
40 Gbit/s RZ

40 Gbit/s NRZ
XOR (detect the phase difference)
DGD ≠ 0

TE
TM

25 ps
XOR (always 0)

Fig. 10. Operation of a DGD monitor as an XOR circuit.

56

20 Gbit/s
Manchester

Fig. 12. Eye patterns for 40-Gbit/s RZ, 40-Gbit/s NRZ,
and 20-Gbit/s Manchester format signals.

NTT Technical Review

Selected Papers

by the hybrid integrated wavelength converter.
Figure 14 shows the eye patterns for 80 Gbit/s, with
DGDs of 12.5 and 25.0 ps. Again, clear eye patterns
were observed after PMD compensation. The compensated optical signal was optically demultiplexed
to 10 Gbit/s by the SIPAS, and the BER was evaluated. The minimum received optical power at BER of
10–9 was -21 dBm, and the power penalties were lessthan 0.9 dB. These results confirm the extremely highbit-rate operation of the all-optical XOR circuit.
4. Conclusion
The extremely high-speed network expected in thenear future will require bit-rate conversion and polarization mode dispersion (PMD) compensation. This
paper described novel techniques using all-opticalsignal processing. Full bit-rate conversion was successfully demonstrated using a monolithically integrated SIPAS. Bit-rate-independent and format-independent PMD compensation has been achieved usinga hybrid integrated cross phase modulation (XPM)
device.

10-6

Bit error rate

10-7
10-8
10-9
10-10
10-11
-40

back to back40 Gbit/s NRZ
40 Gbit/s RZ
-38

-36

-34 -32-30

Received optical power (dBm)

Fig. 13. BER for 40-Gbit/s RZ and 40-Gbit/s NRZ signals.
before compensation

after compensation

DGD = 12.5 ps

DGD = 25.0 ps
50 ps

Fig. 14. Eye patterns for 80-Gbit/s RZ signals.

Vol. 1 No. 1 Apr. 2003

References[1] E. Jahn, N. Agrawal, W. Pieper, H.-J. Ehrke, D.
Franke, W. Furst, and C. M. Weinert, “Monolithically integrated nonlinear Sagnac interferometer and its application as a 20-Gbit/s alloptical demultiplexer,” Electron. Lett., Vol. 32,
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Laube, W. Idler, K. Wunstel, S. Bouchoule, C.
Kazmierski, and D. Mathoorasing, “A bit rate
flexible and power efficient all-optical demultiplexer realized by monolithically integrated
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D. Breuer, M. Martin and F. Kuppers, “Experimental comparison of 40-Gbit/s RZ and NRZ
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[5] T. Hashimoto, H. Sotobayashi, K. Kitayama,
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Jeppesen, “All-optical OTDM-to-WDM signalformat translation and OTDM add-drop functionality using bi-directional four wave mixing
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[7] St. Fischer, M. Duelk, M. Puelo, R. Girardi, E.
Gamper, W. Vogt, W. Hunziker, E. Gini, and H.
Melchor, “40-Gbit/s OTDM to 4 × 10-Gbit/s
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E. Lach, G. Laube, W. Idler, M. Schilling, and S.
Bouchoule, “40 Gbit/s all-optical wavelength
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pp. 133-134, 1997.
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Okamoto, Y. Kawaguchi, Y. Kondo, and Suzuki,“Filter-free wavelength conversion using a
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Gottwald, C. Scheerer, G. Fisher, T. Weyrauch,and W. Haase, “Polarization mode dispersion
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Toshio Ito
Senior Research Engineer, Photonics Integration Project, NTT Photonics Laboratories.
He received the B.E., M.E., and Ph. D degrees
from Keio University, in 1986, 1988, and 2003,
respectively. In 1988, he joined the Musashino
Electrical Communications Laboratories, where
he engaged in research on parallel processing
systems and communication switching systems.
Since 1993, he has been engaged in research on
the InGaAlAs/InAlAs multi-quantum well optical switches, InGaAsP semiconductor opticalamplifiers, and wavelength converters at NTT
Photonics Laboratories, Kanagawa, Japan. Dr.
Ito is a member of the IEICE, JSAP, and OSA.

Rieko Sato
Research Engineer, Photonics Integration Project, NTT Photonics Laboratories.
She received the B.E. and M.E. degrees from
Tsukuba University, in 1992 and 1994, respectively. In 1994, She joined NTT Opto-electronics
Laboratories (presently Photonics Laboratories).
She is engaged in research and development on
hybrid integrated optical module based on semiconductor optical amplifiers. Ms. Sato is a member of the Institute of Electronics, Information,
and Communication Engineers (IEICE) of Japanand IEEE Lasers and Electro-optics Society.

Yasuo Shibata
Senior Research Engineer, Photonic Device
Laboratory, NTT Photonics Laboratories.
He received the B.E. and M.E. degrees in electrical engineering from Keio University, in 1985
and 1987, respectively. In 1987, he joined NTT
Optoelectronics Laboratories, where he has been
engaged in research on optical switches and optical filters. He is currently with NTT Photonics
Laboratories, engaged in research on SOA andwavelength-conversion devices. Mr. Shibata is amember of the Japan Society of Applied Physics,
and the Institute of Electronics, Information andCommunication Engineers (IEICE).

Yoshihiro Kawaguchi
Senior Research Engineer, Photonic Device
Laboratory, NTT Photonics Laboratories.
He received the B.S. and M.S. degrees inapplied chemistry from Osaka University, in
1981 and 1983, respectively. In 1983, he joined
the Electrical Communications Laboratories,
Nippon Telegraph and Telephone Corporation(presently NTT), where he had been engaged inresearch on MOMBE growth of III-V semiconductors. He is currently working on MOVPE
growth of III-V materials for opto-electronics
devices. Mr. Kawaguchi is a member of theJapan Society of Applied Physics.

NTT Technical Review