Selected Papers: Wavelength Converter Devices for Creating

Recent Research and Development of AllOptical Wavelength Conversion Devices
Yasuhiro Suzuki†, and Hiromu Toba
Abstract
Wavelength conversion is a key technology for flexible wavelength control in photonic networks. At
optical cross-connect nodes in photonic networks, wavelength conversion decreases the channel blocking probability and makes it possible to reuse wavelengths. The efficient use of wavelength resources
will accelerate the establishment of flexible networks. All-optical wavelength conversion is especiallyattractive because it does not need OE or EO devices and is independent of signal formats and bit rates,making photonic networks transparent with respect to these properties. This special issue focuses on the
all-optical wavelength conversion devices being studied and developed at NTT Photonics Laboratories.
This paper summarizes recent R&D progress on them.

1. Introduction
Broadband access networks are spreading and data
rates of 100 Mbit/s to a single user in the home are
becoming common. Considering the jump from bit
rates of 64 kilobits to several megabits per second andnow to 100 Mbit/s and the progress of popularization,
Gbit/s-class transmission to a single user is not too far
off. The increasing amount of communication trafficmeans more and more capacity is required in metropolitan/backbone networks. For this purpose, the introduction of wavelength division multiplexing (WDM)
technologies has been accelerated. WDM technologies
use the wavelengths of light and its application areas
have been including point-to-point, rings with an adddrop function, and mesh networks with optical crossconnects [1],[2]. This trend points to the need for thedevelopment of device/module technologies for routing, switching and optical signal processing throughwavelength control. Especially important is the development of all-optical wavelength conversion devices,which accomplish wavelength routing without converting optical signals into electrical signals.
The five articles in this special issue describe alloptical wavelength conversion devices and their† NTT Photonics Laboratories
Atsugi-shi, 243-0198 Japan
E-mail: suzukiy@aecl.ntt.co.jp

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applications, as studied and developed in NTT Photonics Laboratories. This paper summarizes recent
progress in our research and development of thesewavelength conversion devices.
2. Wavelength conversion technologies inphotonic networks
In photonic networks, WDM and wavelength routing technologies allow optical signals to be routed asthey are. At each node in such networks, wavelength
conversion decreases the channel blocking probability and makes it possible to reuse wavelengths. The
efficient use of wavelength resources will accelerate
the establishment of flexible networks. Requirements
for wavelength conversion devices are highly efficient conversion, a high bit rate, low noise, transparency to optical signal formats, and polarization
insensitivity. Wavelength converters should also be
cascadable, have a wide input power dynamic range,and operate using the same wavelength as the input
one. When these requirements are met, wavelength
conversion devices can be freely utilized within a network and at bridges between networks (Fig. 1).
Various wavelength conversion technologies havebeen developed, and they can be categorized into two
types: i) the OE/EO type, where optical signals areconverted to electrical signals and reconverted to optical signals of different wavelengths and ii) the all-optiNTT Technical Review

Photonic Networks

cal type, where optical signals are directly converted todifferent wavelength signals [3],[4]. The advantage of
the OE/EO type is that it can use conventional optical/electrical devices, such as laser diodes, photodiodes, and integrated circuits. However, the transparency with respect to signal format and bit rate is poor. On
the other hand, all-optical wavelength converter technologies have excellent transparency to signal format
and bit rate and are indispensable for the constructionof photonic networks, where such transparency is very
important. Table 1 summarizes the features of wave-

length conversion technologies, and Fig. 2 maps thesignal bit rates and conversion efficiency.
3. Recent research and development of all-optical
wavelength converter devices
3.1 Two types of all-optical wavelength
converter devices
Some all-optical wavelength converter devices are
based on optical switching. The principle is that a signal light (wavelength λ1) drives an optical switch toFig. 1. Wavelength conversion technologies in photonic networks.
Table 1. Comparison of wavelength conversion technologies.
All-optical type
Optical switch type
OE/EO type

Typicaldevices
Bit rate
Bandwidth
Conversion
efficiency
Polarization
insensitivity

PD/IC/LD(LD+MD)

Cross gain
modulation
(XGM)
SOA
LD

Optical mixing type

Differential
phasemodulation
(DPM)
SOA+
SOA+
Mach-Zehnder Mach-Zehnderinterferometer interferometer
Cross phasemodulation(XPM)~40 Gbit/s~40 Gbit/s~40 Gbit/s~160 Gbit/s(IC dependent) (RZ→NRZ)
Dependent on Gain bandwidth Gain bandwidth Gain bandwidth
light source(~30 nm)(~30 nm)(~30 nm)
Excellent

Good

Four wave
mixing(FWM)

Difference
frequency
generation
(DFG)

Fiber
SOA

QPM-LN＊~1 Tbit/s~1 Tbit/s~40 nm~60 nm

Good

Good

Fair

Fair

Excellent

Good

Good

Good

Fair

Fair

Input level

Receiverdependent(~-17 dBm
@10 Gbit/s)-10 dBm@2.5 Gbit/s~5 dBm@40 Gbit/s-10 dBm@10Gbit/s~8 dBm@40 Gbit/s

0~5 dBm@40 Gbit/s~-20 dBm(SOA+DFB)-15~5 dBm

Discrete/batch
conversion

Discrete

Discrete

Discrete

DiscreteBatchavailableBatchavailable

Issues• Dependent
on bit rate
and format• ChirpVol. 1 No. 1 Apr. 2003

Narrow input • Low conversion • Low conversion• Narrow input • dynamic
range efficiency
efficiency
dynamic• Operation• Polarization- • Polarizationrange
stability
dependent
dependent＊QPM-LN: Quasi-Phase Matching LiNbO3

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Selected Papers

Fig. 2. Characteristics of wavelength conversion
technologies.

convert the signal light to a different signal light(wavelength λ2) and the bit patterns of the former are
copied to the latter. Optical switches that use the nonlinearity of a semiconductor gain medium have been
actively studied.
Such semiconductor optical switches can use crossgain modulation (XGM), cross phase modulation(XPM), or differential phase modulation (DPM). The
XGM types are based on a gate switch that utilizes
the gain variation that occurs when a signal lightchanges the carrier density in a semiconductor opticalamplifier (SOA). The XPM and DPM types are basedon an interference switch that uses refractive index
change. The merits of these wavelength conversion
devices using SOAs include compactness, high conversion efficiency, and polarization insensitivity [4].
Other all-optical converters are based on opticalmixing, the so-called coherent type. These make useof nonlinear effects in materials. When a signal light(wavelength λ1) and a pump light (wavelength λ2)
are incident to a nonlinear material, a converted light(wavelength λ3) is generated. The most important
methods of optical mixing are four-wave mixing(FWM) and difference frequency generation (DFG).
They enable wavelength conversion at very high bit
rates and simultaneous multi-wavelength conversion.
3.2 Optical switch types: Cross Gain
Modulation (XGM)
In an XGM type, a signal light (wavelength λ1)
decreases device gain due to saturation of the gain of
an SOA that occurs when it is incident to an SOA anddecreases the amplification ratio of a converted light(wavelength l2) (Fig. 3). Based on this principle, thebit patterns of the signal light and the converted light28

are reversed.
By utilizing a polarization-insensitive SOA, polarization-insensitive wavelength conversion can beachieved. The extinction ratio of XGM is still notlarge enough (about 10 dB). The input dynamic range
is wide (7-10 dB). It was initially considered that thefrequency characteristics were limited by a mechanism related to carrier lifetime in an SOA. However,
XGM was found to involve a more complicatedmechanism (described below) and higher bit rate
operation than expected was observed. The factorsthat increase bit rate in XGM include reduction ofcarrier lifetime induced by a signal light and the filtering effect in an SOA. These factors have been clarified to be very effective for a long SOA with a highinjection current. Over-40-Gbit/s operation by XGM
has been reported [5], although this was accompanied
by a change of the signal format from return-to-zero(RZ) to non-return-to-zero (NRZ). However, XGM
has problems in that the output signal exhibits chirping and conversion efficiency is degraded when a signal light with a short wavelength is converted to one
with a long wavelength.
3.3 Optical switch types: Cross Phase
Modulation (XPM)
XPM is based on the principle that the phase modulation of the SOA in an interferometer causes achange in the interference state and switching so thatthe intensity of the converted light is modulated. The
phase modulation of the SOA is a result of the refractive index change that occurs when the signal light
induces carrier variation. Typical interferometers are
the Michelson type and Mach-Zehnder type (Fig.4).
An SOA as a phase modulator is principally set in one
arm of an interferometer. However, for fine tuning of
the interference state, in many cases, an SOA isplaced in each of the two arms.
For XPM, the phase change of converted light passing through one arm in the interferometer should be
set at π by controlling the intensity of the signal light

Fig. 3. Optical switch type: Cross gain modulation (XGM).

NTT Technical Review

Selected Papers

and converted light and the injection current to the
SOA. The refractive index change necessary for
XPM is induced by gain change of 3-4 dB in an SOA.
Because the gain change for XPM is smaller than thatfor XGM, the chirp in wavelength conversion can be
reduced. XPM also has a waveform reshaping effectbecause of the interferometer configuration. By usinga polarization-insensitive SOA, polarization sensitivity in wavelength conversion can be reduced. By optimizing the device and input signal light conditions,
we can accomplish wavelength conversion with an
extinction ratio of over 15 dB and a high bit rate of 40
Gbit/s for an RZ signal. The wavelength range of
wavelength conversion is about 30 nm, which is comparatively wide. For 10-Gbit/s-class NRZ signals,
XPM wavelength converters have been developed forpractical use by focusing on the reduction of input
signal power, simplification of module assembly, andfabrication of units [6],[7].
This special issue introduced an XPM converter
that uses planar lightwave circuit hybrid integration
technology. The converter is hybrid integrated with
an SOA and PLC.
Because XPM has an interferometer configuration,
the characteristics of the wavelength converter depend
on input signal intensity, and the dynamic range of
input signal intensity is about 2-3 dB, which is comparatively narrow. To enlarge the dynamic range, amethod in which an input level equalizer is placed at
the input port of the wavelength converter has been
proposed. XPM enables higher-bit-rate wavelength
conversion than XGM. However, at 40 Gbit/s, thewaveform of converted output signals degrades due tothe limit of carrier relaxation time in an SOA.
3.4 Optical switch types: Differential Phase
Modulation (DPM)
A DPM-type wavelength converter can overcome
the bit rate limitation in the XPM wavelength con-Fig. 4. Optical switch type: Cross phase modulation (XPM).

Vol. 1 No. 1 Apr. 2003

verter. DPM is based on the following. Original signal and delayed signal are incident to the SOAs of
XPM interferometor, so that the lights are phasemodulated at each SOA (Fig. 5).
This method cancels out the effect of slow carrier
relaxation time in an SOA and enables high-bit-ratewavelength conversion and optical signal processing.
So far, 40-Gbit/s wavelength conversion in the RZ
format has been demonstrated using a semiconductor
monolithic integrated device consisting of a symmetric Mach-Zehnder interferometer [8]. Wavelength
conversion of 168 Gbit/s has also been observed
using the delay loop method [9]. The bit rate forwavelength conversion has been remarkably
increased through the development of ultrafast optical signal generation techniques.
For practical use, besides high bit rate operation,
wavelength conversion to a light with the same wavelength as that of the signal light is important. In conventional wavelength conversion, the signal light(wavelength λ1) and the converted light (wavelengthλ2) are divided by a filter at the output port. Therefore, when the wavelength of the signal light is thesame as that of the converted light, filtering is noteffective and conversion is difficult. To solve thisproblem, the signal light and the converted light canbe filtered by changing the order of the transverse
guided modes of the two lights [10]. In this method,
it is necessary to control the guided modes of the signal and converted light exactly. Further improvementof the technique is required for stable operation.
A device including a parallel amplifier structure(PAS) has been proposed as a way to divide the signal and converted light [11]. In the PAS, SOAs areincorporated in both arms in a Mach-Zehnder interferometer. A semiconductor wavelength conversion
device consisting of a Sagnac interferometer monolithically integrated with the PAS (Sagnac interfer-

Fig. 5. Optical switch type: Differential phase modulation (DPM).

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Selected Papers

ometer with parallel amplifier structure (SIPAS)) has
been proposed and demonstrated.
In a SIPAS, the signal light induces a refractive index
change in an SOA of the PAS and phase modulation.
After that, the signal light passes through the PAS anddoes not enter the Sagnac interferometer. The converted light is differentially phase modulated at the SOA of
the PAS while circulating in the Sagnac interferometerso that the bit pattern of the signal light is copied to theconverted light. The SIPAS device/module and its
application are described in this special issue.
3.5 Optical mixing types: Four-wave mixing (FWM)
FWM uses the third-order nonlinearity of materials
for wavelength conversion. When the signal light (frequency ωi) and the pump light (frequency ωp) are incident to a nonlinear material, the converted light (frequency 2ωp-ωi) is output (Fig. 6). An optical fiber [12]and an SOA [13] are typically used as nonlinear materials. FWM has good linearity and also enables wavelength conversion for 1-Tbit/s-class ultrafast signals.
It also enables simultaneous conversion of multiple
wavelengths. However, the conversion efficiency is
low and decreases as the difference between the wavelength of the signal and pump lights increases. For
higher conversion efficiency, a converter structure inwhich the SOA and a distributed feedback laser diode(DFB-LD) are monolithically integrated has been proposed. The structure does not need strong external
pump lights. For practical use, the key issues are
increasing efficiency, broadening the bandwidth, andreducing the polarization sensitivity.
3.6 Optical mixing types: Difference Frequency
Generation (DFG)
DFG uses the second-order nonlinearity of materials for wavelength conversion. When the signal light(frequency ωi) and pump light (frequency ωp) are
incident to a second-order nonlinear material, theconverted light (frequency ωp-ωi) is output (Fig. 7).
However, phase matching among the signal, pump,and converted lights is required for high conversion
efficiency. Therefore, most of the devices for DFG
have a periodic domain inversion structure for quasiphase matching (QPM).
For highly efficient wavelength conversion, it isimportant to increase the length of the uniform QPM
optical waveguide, improve the optical damage tolerance, and reduce polarization sensitivity [14]. Wavelength conversion by DFG can be done for Tbit/sclass signals. In addition, the wavelength bandwidth
is wide, the wavelength conversion range is large, and
30

Fig. 6. Optical mixing type: Four wave mixing (FWM).

Fig. 7. Optical mixing type: Difference frequency
generation (DFG).

multiple wavelengths can be converted simultaneously. DFG devices that use QPM LiNbO3 are describedin this special issue.
4. Conclusion
All-optical wavelength conversion devices were
overviewed. All-optical wavelength conversion technologies provide transparency to signal format andbit rate and are promising for constructing photonic
networks. Hereafter, besides the development of fabrication technology for modules and units, increased
efficiency, stability, and operability are necessary forpractical use. All-optical wavelength converter
devices perform not only wavelength conversion but
also high-grade optical signal processing. The wavelength conversion technologies described here should
lead to the creation of photonic networks.
References[1] H. Yoshimura, K. Sato, N. Takachio, “Future
photonic transport networks based on WDM
technologies,” IEEE Communications Magazine, Vol. 37, No. 2, pp. 74-81, 1999.
[2] K. Sato, N. Yamanaka, Y. Takigawa, M. Koga,
S. Okamoto, K. Shiomoto, E. Oki, W. Imajuku,“GMPLS-based photonic multilayer router(Hikari router) architecture: an overview of traffic engineering and signaling technology,” IEEE
Communications Magazine, Vol. 40, No. 3, pp.
96-101, 2002.
NTT Technical Review

Selected Papers[3] S. J. B. Yoo, “Wavelength Conversion Technologies for WDM Network Applications,” J.
Lightwave Technol., Vol. 14, No. 6, pp. 955966, 1996.
[4] C. Joergensen, S. L. Danielsen, K. E. Stubkjaer,
M. Schilling, K. Daub, P. Doussiere, F. Pommerau, P. B. Hansen, H. N. Poulsen, A. Kloch,
M. Vaa, B. Mikkelsen, E. Lach, G. Laube, W.
Idler, and K. Wunstel, “All-Optical Wavelength
Conversion at Bit Rates Above 10 Gb/s Using
Semiconductor Optical Amplifiers,” J. Selected
Topics in Quantum Electron., Vol. 3, No. 5, pp.
1168-1180, 1997.
[5] S. L. Danielsen, C. Joergensen, M. Vaa, B.
Mikkelsen, K. E. Stubkjaer, P. Doussiere, F.
Pommerau, L. Goldstein, R. Ngo, and M. Goix,“Bit error rate assessment of 40Gbit/s all-opticalpolarization independent wavelength converter,” Electron. Lett., Vol. 32, No. 18, pp. 16881690, 1996.
[6] C. Janz, B. Dagens, A. Bisson, F. Poingt, F.
Pommerau, F. Gaborit, I. Guillemot, and M.
Renaud, “Integrated all-active Mach-Zehnder
wavelength converter with record signal sensitivity and large dynamic range at 10 Gbit/s,”
OFC’99, San Diego, U.S.A., Paper FB3, Feb.
1999.
[7] R. Sato, T. Ito, K. Magari, Y. Inoue, I. Ogawa, R.
Kasahara, M. Okamoto, Y. Tohmori, Y. Suzuki,
and N. Ishihara, “Low input power (-10 dBm)
SOA-PLC hybrid integrated wavelength converter and its 8-slot equipment,” ECOC2001,
Amsterdam, The Netherlands, Paper Th.F.2.3,
pp. 572-573, Sep. 2001.
[8] D. Wolfson, P. B. Hansen, T. Fjelde, A. Kloch,
C. Janz, A. Coquelin, I. Guillemot, F. Gaborit, F.
Poingt, and M. Renaud, “40Gbit/s all-optical
wavelength conversion in an SOA-based alla c t i v e M a c h - Z e h n d e r i n t e r f e r o m e t e r, ”
ECOC’99, Nice, France, pp. II-170-171, Sep.
1999.
[9] Y. Ueno, S. Nakamura, H. Hatakeyama, T.
Tamanuki, T. Sasaki, and K. Tajima, “168-Gb/s
OTDM wavelength conversion using an SMZtype all-optical switch,” ECOC2000, Munich,Germany, 1.1.1, pp. 13-14, 2000.
[10] D. Wolfson, T. Fjelde, A. Kloch, C. Janz, F.
Poingt, I. Guillemot, F. Gaborit, and M. Renaud,“Detailed experimental investigation of an allactive Dual-Order Mode Mach-Zehnder wavelength converter,” OFC2000, Baltimore, U. S.
A., Paper TuF3, pp. 72-74, Mar. 2000.
Vol. 1 No. 1 Apr. 2003[11] Y. Shibata, N. Kikuchi, S. Oku, T. Ito, H.
Okamoto, Y. Kawaguchi, Y. Suzuki, and Y.
Kondo, “Monolithically integrated ParallelAmplifier Structure for filter-free wavelength
conversion,” IPRM’01, Nara, Japan, Paper
FB1-5, pp. 587-590, May 2001.
[12] K. Inoue, T. Hasegawa, K. Oda, and H. Toba,“Multichannel frequency conversion experiment using fibre four-wave mixing,” Electron.
Lett., Vol. 29, No. 19 , pp. 1708-1710, 1993.
[13] T. Simoyama, H. Kuwatsuka, B. E. Little, M.
Matsuda, Y. Kotaki, and H. Ishikawa, “Highefficiency wavelength conversion using FWM
in an SOA Integrated DFB Laser,” IEEE Photon. Technol., Vol. 12, No. 1, pp. 31-33, 2000.
[14] M. Asobe, O. Tadanaga, T. Yanagawa, H. Itoh,and H. Suzuki, “Reducing photorefractive effectin periodically poled ZnO- and MgO-doped
LiNbO3 wavelength converters,” Appl. Phys.
Lett., Vol. 78, No. 21, pp. 3163-3165, 2001.

Yasuhiro Suzuki
Senior Research Engineer, Supervisor, Photonics Integration Project, NTT Photonics Laboratories.
He received the B.S. and M.S. degrees inapplied physics from the University of Tokyo,
Tokyo, Japan, in 1980 and 1982, respectively. In
1982, he joined the Musashino Electrical Communication Laboratories, Nippon Telegraph andTelephone public Corporation (now NTT),
Tokyo, Japan. After that, he was engaged inresearch and development on semiconductor
optical devices. In 1987, he moved to concentrateon superlattice disordering, polarization control
devices, optical amplifiers, and optical integrated
circuits. He is currently engaged in the development of optical signal processing devices including wavelength conversion devices and opticalmodule assembly technologies for optical metropolitan/access networks. Mr. Suzuki is a memberof the Institute of Electronics, Information andCommunication Engineers, the Japan Society ofApplied Physics and the Institute of Electricaland Electronics Engineers. He has received the
IEICE Inose Award.

Hiromu Toba
Senior Manager, Photonics Integration Project,
NTT Photonics Laboratories.
He received B.E., M.E., and Ph.D degrees from
Osaka University, Osaka, Japan, in 1978, 1980,
and 1992, respectively. In 1980, he joined NTT
Yokosuka Electrical Communications Laboratories, where he researched optical transmission
systems. His main research interests include
DWDM systems, photonic transport networks,
optical modules for optical access systems, highspeed optical transport systems and its related
devices, and photonic integrated device technologies. He has served OSA as a program cochair of OAA’98 in Colorado, USA, and as a
general co-chair of OAA’99 in Nara, Japan. He
also served IEEE as an associate editor of Photonics Technologies Letters from 1997 to 2000
and served IEICE as a Chairman of Optical Communication Systems technical group from 1999
to 2001. He received the Achievement Award
from IEICE in 1991. Dr. Toba is a member of
IEICE and IEEE.

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