1740 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 14, NO. 12, DECEMBER 2002
Simultaneous Repolarization of Two 10-Gb/s
Polarization-Scrambled Wavelength Channels
Using a Mutual-Injection-Locked
Laser Diode
L. Y. Chan, W. H. Chung, P. K. A. Wai, Senior Member, IEEE, B. Moses, H. Y. Tam, Senior Member, IEEE, and
M. S. Demokan, Senior Member, IEEE
Abstract—An all-optical polarizer, which can repolarize a high-
speed data signal without converting the state-of-polarization vari-
ations into amplitude jitters, is realized using a mutual injection-
locked laser diode. The all-optical polarizer is used to simultane-
ously realign the state-of-polarizations of two 10-Gb/s polariza-
tion-scrambled nonreturntozero signals withoutstate-of-polariza-
tion characterization and feedback control.
Index Terms—Fabry–Pérot laser, injection locking, polarization
control.
I. INTRODUCTION
R
EAL-TIME automatic polarization stabilization is crucial
to the deployment of all-optical switches, add–drop
multiplexers, polarization-multiplexed systems, and coherent
detection systems in optical networks. Most of the polariza-
tion control schemes proposed to date are comprised of a
polarization rotation unit and a complex feedback control unit.
Polarization control is carried out either mechanically (e.g.,
fiber squeezers/paddles, MEMS [1]) or electrooptically (e.g.,
liquid crystal or electrooptics crystal [2], and PM-segment
fibers [3]). In this letter, we demonstrate that a mutual in-
jection-locked laser diode (MILD) functions as an all-optical
polarization controller and can be used to repolarize 10-Gb/s
polarization-scrambled nonreturn-to-zero (NRZ) signals.
The MILD controls the state-of-polarization (SOP) of the
input signal not by rotating its SOP but by functioning as an
intensity-compensating polarizer. Thus, neither complicated
SOP characterization nor a speed-limiting feedback control
process is required. We further show that a single MILD can
simultaneously realign the polarizations of two polarization
scrambled 10-Gb/s signals.
Manuscript received July 3, 2002; revised August 26, 2002. This work was
supported by the Research Grant Council of the Hong Kong Special Adminis-
trative Region, China, under Project PolyU 5132/99E.
L. Y. Chan, P. K. A. Wai, and B. Moses are with the Photonics Research
Center, Department of Electronic and Information Engineering, The Hong
Kong Polytechnic University, Kowloon, Hong Kong SAR, China (e-mail:
enwai@inet.polyu.edu.hk).
W. H. Chung and H. Y. Tam are with the Photonics Research Center, De-
partment of Electrical Engineering, The Hong Kong Polytechnic University,
Kowloon, Hong Kong SAR, China.
M. S. Demokan is with the Department of Electrical Engineering, The Hong
Kong Polytechnic University, Kowloon, Hong Kong SAR, China.
Digital Object Identifier 10.1109/LPT.2002.804652
Fig. 1. The output spectra of Fabry–Pérot laser diode (FP-LD) when injected
by CW signals at different wavelength which are detuned from the FP modes.
(a) TE polarized. (b) TM-polarized CW signal.
II. POLARIZATION STABILIZATION BY INJECTION LOCKING
The FP-LD used in the experiment supports both TE mode
and TM mode emission during lasing but the double-channel
planar-buried heterostructure of the FP-LD favors the TE mode.
The power of the TM mode is less than 0.1% [4]. Fig. 1(a) and
(b) shows the output spectra of the FP-LD when injected by a
TE and a TM polarized wavelength-tunable signal, respectively.
The injected signal power is
17 dBm and the wavelength step
is 0.01 nm. Fig. 1(a) shows a typical injection-locking charac-
teristic while Fig. 1(b) shows a typical reflection spectrum of
a FP cavity. The peak of the output spectrum in Fig. 1(a) oc-
curs when the injected TE signal is spectrally aligned with a
TE longitudinal mode of the FP-LD, and the absorption min-
imum in Fig. 1(b) occurs when the injected TM mode is aligned
with a TM longitudinal mode of the FP-LD. Therefore, for any
injected signal that is spectrally aligned with a wavelength at
which the TE and the TM modes of the FP-LD coincide, the TE
component of the injected signalwillbe amplified with its inten-
sity clamped and stabilized by injection locking [5] if the power
of the TE component is above the injection-locking threshold.
The TM component, however, is always suppressed. As a re-
sult, an injection-locked FP-LD can act as an intensity-com-
pensating polarizer with TE polarized output. Fig. 2(a) depicts
the Poincaré sphere representation of the SOP of a CW signal
with a power of
17.9 dBm. When the polarization of the CW
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CHAN et al.: SIMULTANEOUS REPOLARIZATION OF TWO 10-Gb/s POLARIZATION 1741
(a) (b)
Fig. 2. Poincaré spheres for the polarization-scrambled signal (a) before and
(b) after injection locking.
Fig. 3. Experimental setup for dual-wavelength polarization compensation
using MILD. Notation: Distributed feedback laser (DFB); polarization
controller (PC); intensity modulator (MOD); polarization scrambler (PS);
intensity coupler (COUP); circulator (CIR); Fabry–Pérot laser diode (FP-LD);
optical spectrum analyzer (OSA); polarizer (Pol); erbium-doped fiber
amplifiers (EDFAs); bandpass filter (BPF); and photodiode (PD).
signal is varied randomly by hand using a polarization scram-
bler, the SOP wanders randomly over the Poincaré sphere. After
injection locking of the FP-LD, the SOP of the FP-LD output
is confined to a small spot on the Poincaré sphere even when
the polarization state of the CW signal is varied randomly as
shown in Fig. 2(b). The applied current of the FP-LD is
where is the threshold current. The degree of polarization
(DOP) for the output signal after polarization stabilization by
the scheme is over 95% at a FP-LD current of
[6]. We
emphasize that only those FP modes having exact overlapping
of the TE injection-locking peak and the TM absorption min-
imum will give optimal performance in the proposed scheme.
In order to realign the polarization of a high bit-rate signal, it is
necessary to simultaneously inject aCW stabilizer signal (wave-
length matched with another FP-LD mode) with the input high
bit-rate signal such that mutual injection locking of the FP-LD
occurs. The functions of the CW stabilizer signal are to suppress
the FP-LD modes during the “0” bits of the polarization varying
input signals and to increase the response speed of the proposed
scheme by shortening the fall-time of the compensated signal
under stimulated emission.
III. S
IMULTANEOUS REPOLARIZATION OF TWO 10-Gb/s
S
IGNALS
Fig. 3depicts the experimentalsetup for thepolarization com-
pensation of both one 10-Gb/s NRZ signal and two 10-Gb/s
NRZ signals using the proposed scheme. First, we study the
repolarization of only one 10-Gb/s signal, which is generated
by externally modulating the 1546.54-nm signal from a tunable
laser. The SOP of the modulated signal is varied by a polariza-
tion scrambler (PS) which operate at a sinusoidal frequency of
152 kHz. Fig. 4(a) shows the eye diagrams of a polarization-
Fig. 4. Eye diagrams of a polarization scrambled 10-Gb/s signal measured
after a polarizer (a) without injection locking, (b) with single wavelength
injection locking, and (c) mutual injection locking with a CW stabilizer signal.
The polarization scrambling rate is 152 kHz.
Fig. 5. BER performance for the 10-Gb/s input signal ( ) without polarization
scrambling and (
) polarization compensated signal after MILD. Both are
measured after a polarizer.
scrambled signal measured by a photodiode (PD) after passing
through a polarizer (Pol). The polarization-scrambled signal is
injected into the MILD, which comprises of a DFB (the CW
stabilizer signal) with an emitting wavelength of 1548.7 nm, a
FP-LD with an applied current of
and one circulator for
separating the output polarization compensated signal from the
input signals of the FP-LD. The FP-LD and the DFB are ther-
mally tuned such that the polarization-scrambled signal and the
CW stabilizer signal are within the injection-locking range of
two different FP modes. The injected powers to the FP-LD are
0.73 dBmand
4.69 dBmforthe 10-Gb/s 1546.54-nm polariza-
tion scrambled signal and the CW stabilizer signal, respectively.
Fig. 4(b) shows the polarization scrambled 10-Gb/s signal after
it is injection locked to one of the FP modes without the sta-
bilizer signal. Although repolarization occurs as shown by the
partial opening of the eyes, the intensity levels of the “1” and
“0” are still rather noisy. In order to achieve better repolariza-
tion of the high-speed signal, the CW stabilizer signal which is
wavelength matched to another FP mode is injected simultane-
ously with the input high bit rate signal such that mutual injec-
tion locking occurs. Fig. 4(c) gives the eye diagram of the po-
larization scrambled 10-Gb/s signals after it is polarization sta-
bilized using the MILD. Much better eye-opening is observed.
Fig. 5 shows the BER performance (measured after a polarizer)
of the repolarized signal. There is a
0.8-dB power penalty im-
provement compared to the original signal without polarization
scrambling due to noise suppression under injection locking [7].
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1742 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 14, NO. 12, DECEMBER 2002
Fig. 6. The spectrum of an FP-LD after it is mutual injection locked by two
polarization scrambled 10-Gb/s signals at 1548.32 and 1549.54 nm.
Fig. 7. Eye diagrams of the 1548.32 nm and 1549.54 nm signals before [(a)
and (c)] and after [(b) and (d)] they are simultaneously polarization stabilized
using MILD.
The side-mode suppression ratio of the polarization compen-
sated signal is over 30 dB. The output of the MILD is TE po-
larized. Specific SOP can be obtained using a segment of polar-
ization maintaining fiber or a slow polarization controller at the
output of the MILD.
Next, we demonstrate that a single MILD can be used to
simultaneously repolarize two polarization-scrambled 10-Gb/s
signals. The wavelengths of the two DFB lasers in Fig. 3 are
1548.32 nm and 1549.54 nm. The DFB outputs are exter-
nally modulated with the same pseudo-random bit sequences
(PRBSs) to produce two 10-Gb/s NRZ signals. The results will
be the same for independently modulated signals provided that
the CW stabilizer signal is chosen such that the CW signal
locks the FP-LD only when the inputs of both 10-Gb/s signals
are “0.” The two 10-Gb/s signals are then polarization-scram-
bled at 152 kHz and injected into the MILD for polarization
stabilization. The CW stabilizer signal is generated from a
tunable laser operated at 1552.9 nm with a power of
4.14
dBm. The FP-LD current required to stabilize the polarization
of two wavelength channels is
, which is higher than
the
required for a single channel. Fig. 6 shows the
spectrum of the FP-LD output when it is simultaneously mutual
injection locked by two polarization-scrambled 10-Gb/s signals
at wavelengths 1548.32 nm and 1549.54 nm with incident
powers
10.8 dBm and 9.5 dBm, respectively. Fig. 7(a)
and (c) depict the eye diagrams for the polarization scrambled
signals and Fig. 7(b) and (d) show the repolarized signals using
a single MILD measured after a polarizer and a bandpass filter.
Significant eye openings are observed in both signals.
IV. D
ISCUSSION AND CONCLUSION
An all-optical polarizer constructed from a mutually injec-
tion-locked laser diode is used to simultaneously repolarize two
10-Gb/s polarization-scrambled signals without amplitude jitter
penalty. No SOP characterization and feedback control process
is used. For optimal performance, the maximum number of sig-
nals that can be repolarized with a single MILD depends on the
number of modes in which the TE and TM modes overlap in the
spectrum (mode spacing difference between TE and TM modes
is about0.038nm in our case). IftheSOPs of the injected signals
are aligned with the TM polarization of the MILD such that the
power of the TE components is lower than the injection-locking
threshold, the MILD will attenuate the injected signals without
repolarization. We note that since the random birefringence of
buried optical networks typically causes only 2
to 10 fluctu-
ations in the polarization angles of the propagating signals [8],
one can use a slow polarization controller to avoid the alignment
of the SOP of the injected signal with the TM polarization of the
MILD.
A
CKNOWLEDGMENT
The authors acknowledge the comments and suggestions
from the reviewers.
R
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![Fig. 7. Eye diagrams of the 1548.32 nm and 1549.54 nm signals before [(a) and (c)] and after [(b) and (d)] they are simultaneously polarization stabilized using MILD.](/figures/fig-7-eye-diagrams-of-the-1548-32-nm-and-1549-54-nm-signals-1xj16693.webp)