Selected Papers

Cross-phase Modulation Wavelength Converter
using PLC Hybrid Integration Technologies
Rieko Sato†, Toshio Ito, Ikuo Ogawa, and Hiroshi Okamoto
Abstract
We assembled a cross-phase modulation (XPM) wavelength converter by using semiconductor optical amplifier (SOA) and planar lightwave circuit (PLC) hybrid integration technologies. We clarified the
optimal current conditions for low input power operation and successfully achieved -10 dBm input signal power operation at 10 Gbit/s without using a signal preamplifier. For stable control of XPM operation, we introduced a novel tuning technique to optimize the input power of the XPM wavelength converter. Using this technique, we could easily set the optimal input power for variable wavelength conversion among 30 wavelengths. We also fabricated an XPM wavelength converter unit containing a variable continuous wave source and confirmed simultaneous and automatic operation.

1. Introduction
All-optical wavelength converters are importantcomponents for future photonic networks based on
dynamic wavelength path routing [1]. Various types
using semiconductor optical amplifiers (SOAs) havebeen reported. Cross-phase modulation wavelength
converters (XPM-WCs) [2]-[5] show high total performance compared with other means of wavelength
conversion using SOAs (cross-gain modulation(XGM) and four-wave mixing) in terms of regeneration capability [6], low chirp characteristics on single-mode fiber transmission, noise reduction [7], andso on. As one method of assembling such XPM-WC
devices, we have applied hybrid integration technologies [8], [9] on a PLC (planar lightwave circuit) platform. These technologies allow an optical modulewith at least two I/O (input/output) ports, like an
XPM-WC, to be assembled with sufficient reproducibility, and the characteristics of the SOA and
PLC can be optimized independently. Moreover, it
should be possible to add other functions by insertinga filter or grating and to expand to large-scale-integration circuits by attaching PLCs directly [10].
When such XPM-WCs operate in an optical cross† NTT Photonics Laboratories
Atsugi-shi, 243-0198 Japan
E-mail: rieko@aecl.ntt.co.jp

32

connect or optical transport system [11], there will besome problems to be solved. One is that the wavelength converter should operate at low optical powerso that the system’s level diagram can be flexibly
designed. For example, a parallel arrangement of
XPM-WCs is possible if the required optical power isreduced. In conjunction with this, low power consumption (or low drive current) for the SOA is alsoneeded. In 10-Gbit/s trials reported to date, a relatively high input power of about 0 dBm has always
been used. Although an XPM-WC with a monolithically integrated optical preamplifier did achieve 10Gbit/s operation at input power of -10 dBm, the SOA
required a high total current (400–500 mA) [3].
Another issue is a simple control technique for the
XPM-WC. It is necessary to adjust two or more tuning parameters (e.g., input signal power, input continuous wave (CW) light power, SOA drive currents,
and wavelengths) and operate at the optimal operating point. In addition, these parameters must be readjusted when they are changed. From the practicalstandpoint, it is important to be able to set the optimalvalue of these tuning parameters easily. However,there has not been a full report on this before.
This paper describes approaches to optimal operation that addresses the above issues. Using the optimal tuning technique, we develop a control unit thataims to achieve simultaneous operation with a variable CW source.
NTT Technical Review

Selected Papers

2. SOA-PLC hybrid module and approach to
optimal tuning

Converted output power
Pc-out (dBm)

A schematic of an XPM wavelength converter circuit is shown in Fig. 1. It consists of a two-channel
SOA (SOAmod and SOApc for modulation and phase
control, respectively) integrated with spot-size converters (SS-SOAs) on a PLC platform. In Figure 1(a),the CW light at a wavelength of λc input at port 1 isdivided by the 3-dB coupler c1 and sent into the twoarms of the interferometer. Then, it is injected into
both SOAs, multiplexed by another 3-dB coupler c2,
and output at port 2. The signal light at a wavelengthof λs is injected into SOAmod through port 3. The
input signal light causes a depletion of carrier density in SOAmod and thereby an increase in its refractive
index. The relative phase difference of the interferometer ∆φ should change from 2nπ to (2n+1)π as aresult of the change in the refractive index in SOAmod.
The phase modulation, caused by the signal input to
SOAmod, is converted to an intensity modulation andthe converted output signal is output at port 2. The
current to SOApc Ipc was set to “inverted-mode” conversion. Figure 1(b) shows an example of the static
wavelength conversion curve in “inverted mode”, inwhich the converted output signal power changes ininverse proportion to the input signal power.
The approach to optimal tuning is drawn schematically in Fig. 2. There are four parameters (Imod, Ipc,
Ps-in, and Pc-in) to determine the tuning condition. Imodand Ipc are the currents to SOAmod and SOApc, respectively, and Ps-in and Pc-in are the input powers of thesignal and CW lights, respectively. For a constantcurrent Imod, we examine the relationships among
pairs of the other parameters. As the first approachfrom the relationship between Ipc and Ps-in, we clarify
the optimal Ipc for low optical power operation andfind the minimum input signal power at a low total
current [12]. As the second approach from the relationship between Ps-in and Pc-in, we propose a tuning
technique to optimize the two input powers.
Figure 3 shows the structure of a polarization
insensitive SS-SOA array [14]. Spot-size converters
are butt-coupled to both sides of the 1.55-µm bulk
active region of the SOA. The SS-SOA is 1200 µm
long (active region: 600 µm, spot-size converter
region: 300 µm) and both facets are treated with anantireflection coating. The waveguide is 0.4 µm thickat the butt-joint and 0.2 µm thick at the facet.
A photograph of a hybrid module and a drawing of(a)

Input signal powerPs-in (dBm)(b)

Fig. 1. (a) Schematic of XPM-WC,(b) wavelength conversion curve.

Imod

Constant

Ipc
(1)
Ps-in

Pc-in
(2)

Fig. 2. Approach to XPM optimal tuning.

Vol. 1 No. 1 Apr. 2003

33

Selected Papers

device assembly are shown in Fig. 4. The SS-SOA
array was flip-chip bonded onto the PLC platform,
which has a silica-on-silicon structure, using a passive alignment technique [8]. The average couplingloss between the SS-SOA and the PLC waveguide
was estimated to be about 4 dB. The total insertion
loss from port 1 to the SS-SOA, including the coupling loss between the SS-SOA and the PLC waveguide, was estimated to be about 11 dB. The optical
couplers (c1, c2, and c3 in Fig. 2) are multi-mode
interference couplers. The PLC waveguide has arefractive index difference of 0.75%. The PLC chipsize is 30 × 3 × 1 mm3. An array of four single-mode
fibers was directly attached to each side of the PLC
using UV-curable adhesive. A built-in Peltier device
in the package precisely controls the module temperature.
3. Low input power and low current operation
Low input power operation is difficult, especiallywhen the drive current of the SOA is low. The reason
is as follows. As described in the previous section,
there are two currents that have to be controlled inwavelength conversion: Imod and Ipc, which work forsignal modulation and phase control, respectively.
We usually adjust these currents to get the maximum
output extinction ratio. But one problem occurs if wemake them lower. When Imod is high enough, forexample, more than 200 mA, a good output eye pattern is observed. But when it is lowered, the signal
degradation becomes strong (Fig. 5(b)). This is due toa consequent increase in the carrier lifetime in the
SOA. Under these conditions, since the eye opening
will deteriorate, stable low-optical-power operation isdifficult.
In order to improve the eye pattern without increasing Imod, we focused on the other SOA current Ipc,
because the output signal amplitude is controlled byit. To confirm the effectiveness of using Ipc, we measured the dependence of the eye opening ratio and theextinction ratio on Ipc (Fig. 5(a)) keeping Imod and the
input signal power constant. In Fig. 5(a), the left axis
shows the eye opening ratio (the ratio between the
maximum output amplitude A and the minimum
value B) at 10 Gbit/s, and the right axis shows theextinction ratio measured under the static condition.
At operating point a shown Fig. 5, where the maximum extinction ratio is obtained, the eye opening
ratio is inadequate (Fig. 5(b)). When the extinction
ratio decreases as Ipc is reduced from operating pointa shown Fig. 5, the maximum output amplitude A
also decreases. Therefore, the eye opening ratio
improves and the maximum value of 0.98 is obtained
at operating point b shown Fig. 5. Figure 5(c) shows
the improved eye pattern at the optimal operating
point b shown Fig. 5, where Ipc was reduced to 85
mA.
At the optimal operating point b shown Fig. 5, we
examined 10-Gbit/s low power operation. We measured the bit error rate (BER) characteristics for boththe input signal (base line) and the converted output
signal. The input signal was modulated at 10 Gbit/s(NRZ format, PRBS 231-1), and its wavelength was
1558.7 nm. The wavelength of CW light was 1535.1

Fig. 3. Structure of SS-SOA array.
Ps-in = -10 dBm

0.9

14

0.8

13

0.7

12

0.6

11

0.5
70

34

Eye opening = B/AAB(b) 100 ps

1080 90 100 110 120 130
Current Ipc (mA)(a)

Fig. 4. Hybrid XPM module and device assembly.

15
Extinction ratio (dB)

Eye opening

1(c)Fig. 5. (a) Dependence of eye opening and extinction
ratio on current Ipc, (b) eye Pattern at operation
point, (c) eye Pattern at operation point.

NTT Technical Review

Selected Papers

Vol. 1 No. 1 Apr. 2003

Bit error rate

10-6
10-7
10-8
10-9
10-10
10-12
Average received power (dBm)

Fig. 6. 10-Gbit/s bit error characteristics and output eye
pattern.
4
3

2

1

0
-11 -10

-9

-8

-7

-6

-5

-4

Input signal power, Ps-in (dBm)

Pc-out (dBm)

Fig. 7. 10-Gbit/s low input power operation.

max. ER(a)
Pc-out-XGM (dBm)

4.1 Optimization of input power using
monitored signal
In achieving optimal XPM wavelength conversion,
input power is an important tuning parameter. This isbecause ∆φ (the amount of phase change in SOAmod)should be kept at π for optimal XPM operation, and it
is determined by the input power ratio of the twolights (signal and CW). In this section, in order todetect and keep the state of optimal XPM operation,
we focus on the amplified signal (see Fig. 1) at
SOAmod. The wavelength of the amplified signal is
the same as the input signal.
Figure 8(a) shows the schematic of the converted
output signal power Pc-out as a function of the input
signal power P s-in . With increasing P s-in , P c-out
changed from the maximum to the minimum value.
The amount of Pc-out change is defined as the extinction ratio (ER). The input signal power where ER is
maximum is defined as Ps-in-conv. Fig. 8(b) shows thechange in Pc-out-XGM. This is the converted output signal power when Ipc = 0 mA. In other words, it is theoutput power converted by cross-gain modulation(XGM). The Pc-out-XGM reduction when the input signal power increases to Ps-in-conv is equal to the gainchange ∆Gc (dB) in SOAmod. Here, ∆Gc should be

10-5

Ps-in-conv∆Pc-out-XGM = ∆Gc(b)∆Ps-out = ∆Gs

Ps-out (dBm)

4. Optimal tuning technique and its application
to variable XPM-WC

10-4

Power penalty (dB)
(BER of 10-9)

nm. The input CW power was set to be optimal foreach input signal power. Fig. 6 shows the results of
BER measurement at input signal power of -10 dBm
and its output eye pattern, when Ipc was the optimalvalue (85 mA). The black and white circles show the
BER of the input signal and the converted output signal, respectively. The excess power penalty at BER of
10–9 was less than 0.5 dB. The power penalty for each
input signal power less than -4 dBm is plotted in Fig. 7.
The black circles show the value when Ipc was optimal (85 mA) and the white circles show the value
when Ipc was not optimal (110 mA). In the optimal
condition, a power penalty of less than 0.5 dB was
maintained down to -10 dBm of input signal power,
whereas in the unoptimized condition, it was morethan 1 dB when the input signal power was lower than
-6 dBm. If we take a 1-dB power penalty as a threshold, then the minimum operating input power was
improved by at least 4 dB. Moreover, the input CW
light power at input signal power of -10 dBm was also
-10 dBm, and the SOA total current was reduced to
215 mA. These results indicate that Ipc optimization
is effective for low-power and low-current operation.

Psat
Optimum

w/o CWPs-outwith CW(c)
Ps-in(dBm)

Fig. 8. Output power of (a) converted output signal Pc-out(XPM), (b) converted output signal Pc-out-XGM(XGM, Ipc =0 mA), (c) amplified signal Ps-out.

35

Selected Papers

Convertedsignal (λ1-λ30)

Input signal (λ16)
XPM
λ1-λ30
SSG

OR
AWG

ATT

CW

ED

O/E

(a)
0
uncontrolled Pc-in

36-3-5.2 dBm-6-6
optimized Pc-in

-9
0

λ16
input signal

(b)
-2

-9
10
8

< 1 dB

-4

6

-6

4

-8

2

< 1 dB-10

0
1Fig. 10.
Fig. 9. Experimental setup for variable XPM
wavelength conversion.

0-3

Amplifed
signal (λ16)

Control of Pc-in for constant Ps-out (λ16)

33Ps-out (λ16) (dBm)

4.2 Variable XPM-WC
In this section, we discuss the variable wavelength
conversion of an XPM-WC using the input power
tuning technique described above. When we change
the output wavelength from λ1 to λ2 smoothly, it isnecessary to optimize the input power of the CW lightwhile changing its wavelength. At this time, we only
need to measure and keep the monitor signal power
instead of observing the eye pattern of the converted
output signal. The CW light power is optimized sothat the monitor signal power may keep the optimalvalue. Figure 9 shows the experimental setup for thePower penalty (dB)This equation shows that the total output power isalways constant in the saturation region. Therefore,
when the optimal gain change ∆Gc (= ∆Pc-out-XGM)
occurs in SOAmod, ∆Ps-out (with CW) also becomes
constant. In an experiment, we only need to measure
the average output power of Ps-out (with CW) andkeep it constant. Then the optimal condition will bemaintained. As an example of the optimal value of Ps-out(with CW), if ∆Gc = 3dB, it will be 3 dB below Psat.
Hereafter, we call the average output power of Ps-out(with CW) the monitor signal power.

Pc-in (dBm)

Pc-out-XGM + Ps-out (with CW) = Psat (mW: constant)variable wavelength conversion of the XPM-WC.
The CW source is a super-structure grating (SSG)
DBR laser. We employed 30 wavelengths (λ1-λ30)
from 1539.8 nm with a channel spacing of 100 GHz.
The wavelength of the input signal was set to λ16. The
input signal was modulated at 10 Gbit/s (NRZ format,
PRBS 231-1), and the average input power was set to-8 dBm. The monitor signal power was measured at
the output of the arrayed waveguide grating (AWG)
filter. The input CW light power was optimized sothat the monitor signal power would hold. We measured the BER characteristics and the converted output signal power for every wavelength change.
Figure 10 shows the results of wavelength change
operation. The horizontal axis shows the opticalchannel number. The left axis of Fig. 10(a) shows
input CW light power Pc-in and the right axis showsmonitor signal power Ps-out. The white circles show
the uncontrolled Pc-in (outputs of the SSG-DBR-LD).
The gray circles show the optimized Pc-in. They were
adjusted so that the monitor signal power (black diamonds) might be fixed at -5.2 dBm. In Fig. 10(b), theleft axis shows the converted output signal power Pc-out
and the right axis shows the excess power penalty inthe back-to-back condition. Among the 30 channels(wavelength range of 24 nm), the Pc-out change wasless than 1 dB (-3.6 to -4.6 dBm) and the power

Pc-out (dBm)

constant in order to keep optimal phase change ∆φ = π.
Figure 8(c) shows the change in amplified signal
power Ps-out. The dotted line shows the curve before
input of CW light (w/o CW) and the solid line shows
the curve after input of CW light (with CW). Whenthe input signal power reaches Ps-in-conv, Ps-out (w/o
CW) reaches the saturated output power Psat. When
the CW light is input, the total gain is distributed
between the two lights (signal and CW). Therefore Ps-out(with CW) no longer reaches Psat when the input signal power reaches P s-in-conv . P s-out (with CW) is
reduced by a definite value from Psat (= ∆Ps-out).
These relationships are summarized as:

5

10
15
20
25
Output wavelength numder

30

Results of variable wavelength conversion.
(a) Left axis: uncontrolled and optimized input
CW power and Right axis: monitor signal
power. (b) Left axis: converted output signal
power Right axis: back-to-back power penalty.

NTT Technical Review

Selected Papers

References

Fig.11. Variable XPM-WC unit.

penalty was less than 1 dB. These results show theeffectiveness of the proposed optimal tuning technique using a monitor signal.
We fabricated a variable XPM-WC unit (Fig. 11). It
consists of XPM-WCs, a variable CW source (SSGDBR-LD), and a CW level controller. They operate
simultaneously using a common controller. By
employing the above tuning technique, we can easily
set the optimal input CW power to each selected
wavelength. The CW level controller is an auto gain
loop control circuit using an SOA [15]. Other tuning
parameters, such as Imod and Ipc, and module temperature are set in advance in the common controller.
According to the chosen output wavelength, all optimal tuning parameters are set automatically, and stable wavelength conversion is carried out. The wavelength switching time of this unit was estimated to beless than 5 ms, which is sufficiently fast for wavelength path routing [16].
5. Conclusion
We examined practical issues in XPM-WCs, especially low-power operation and optimal tuning. From
the relationship between Ipc and the eye opening ratio,
we clarified the optimal current condition for lowpower operation. As a result, we achieved -10 dBm
input power operation at a low total current. To maintain the optimal XPM operation, we found a condition where the monitor signal power should be constant. Using this principle, we successfully achieved
variable wavelength conversion among 30 wavelengths. To improve the functionality of the wavelength converter and make it easier to use, we developed an XPM-WC unit that operates with a variable
CW light source simultaneously and automatically.
Vol. 1 No. 1 Apr. 2003

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Kato, Y. Tohmori, M. Wada, and T. Sugie, “Passively aligned hybrid WDM module integrated
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Kasahara, M. Okamoto, Y. Tohmori, Y. Suzuki
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38

R. Kolodner, F. Pardo, R. Ruel, and R. E. Scotti,“Demonstration of distributed mesh restoration
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Rieko Sato
Research Engineer, Photonics Integration Project, NTT Photonics Laboratories.
She received the B.E. and M.E. degrees from
Tsukuba University, Ibaraki, Japan in 1992 and1994, respectively. In 1994, She joined NTT
Opto-electronics Laboratories (now Photonics
Laboratories). She is engaged in research anddevelopment on hybrid integrated optical modules based on semiconductor optical amplifiers.
Ms. Sato is a member of the Institute of Electronics, Information, and Communication Engineers (IEICE) of Japan and IEEE Lasers andElectro-optics Society.

Toshio Ito
Senior Research Engineer, Photonics Integration Project, NTT Photonics Laboratories.
He received the B.S. and M.E. degrees from
Keio University, Kanagawa, Japan, in 1986 and1988, respectively. In 1988, he joined theMusashino Electrical Communications Laboratories, where he engaged in research on parallel
processing systems and communication switching systems. Since 1993, he has been engaged inresearch on the InGaAlAs/InAlAs multi-quantum well optical switches, InGaAsP semiconductor optical amplifiers, and wavelength converters
at NTT Photonics Laboratories, Kanagawa,Japan. Mr. Ito is a member of the IEICE, JSAP,
and OSA.

Ikuo Ogawa
Senior Research Engineer, Photonics Integration Project, NTT Photonics Laboratories.
He received the B.S. and M.S. degrees inapplied physics from Waseda University, Tokyo,
Japan, in 1990 and 1992, respectively. In 1992,
he joined the NTT Opto-electronics Laboratories. Since then he has been engaged in researchand development on silica-based planar lightwave circuits and hybrid integrated opticaldevices. He now works at the NTT Photonics
Laboratories. Mr. Ogawa is a member of theInstitute of Electronics, Information and Communication Engineers (IEICE) of Japan, and theIEEE Lasers and Electro-optics Society.

Hiroshi Okamoto
Senior Research Engineer, Photonic Device
Laboratory, NTT Photonics Laboratories.
He received the B.E., M.E. and Dr. E. degrees
from Chiba University, in 1981, 1983, and 1998,
respectively. In 1983, he joined NTT ElectricalCommunications Laboratories. Since then, he
has been engaged in the MOVPE growth, semiconductor-crystal characterization, and researchand development on semiconductor devices. He
is now a Senior Research Engineer in the Photonic Device Laboratory in NTT Photonics Laboratories. Dr. Okamoto is a member of the IEEE
Laser and Electro-Optics Society (LEOS), theIEEE Components, Packaging, and Manufacturing Technology Society (CPMT), the IEEE Electron Devices Society (ED), the Institute of Electronics, Information and Communication Engineers (IEICE) of Japan, and the Japan Society ofApplied Physics (JSAP).

NTT Technical Review