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ofc.2014.w3f.5

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W3F.5.pdf
OFC 2014 © OSA 2014
Simultaneous Phase Regeneration of CoWDM BPSK Signals
by Hybrid Optical Phase Squeezer
1
Takayuki Kurosu1, Mingyi Gao2, Shu Namiki1
National Institute of Advanced Industrial Science and Technology (AIST),
1-1-1 Umezono, Central 2, Tsukuba, 305-8568, Japan.
School of Electronics and Information Engineering, Soochow University,
Suzhou, Jiangsu Province, 2150066, China
Email: { t.kurosu, shu.namiki}@aist.go.jp, mygao@suda.edu.cn
2
Abstract: We propose a new concept of hybrid optical phase squeezer and demonstrate
simultaneous phase regeneration of two CoWDM BPSK signals. A gain extinction ratio of
20dB is achieved with a pump power of only 3dBm.
OCIS codes: (060.2330) Fiber optics communications, (060.4510) Optical communications (190.4380), Nonlinear
optics, four-wave mixing, (190.4970) Nonlinear optics, fibers
1. Introduction
Phase sensitive amplification (PSA) is attracting great attention because it enables all-optical regeneration of phase
encoded signals through its functionality of quantizing optical phase. The performance of PSA is characterized by
the contrast of phase sensitive gain, which is called gain extinction ratio (GER). As large optical nonlinearity is
necessary to achieve high GER, regeneration of PSK signals have been demonstrated using specially arranged
nonlinear materials [1-3]. In the sideband-assisted PSA, on the other hand, GER can be infinitely high even with
small optical nonlinearity if all the relevant parameters are optimized [4]. In contrast to the all-optical approach, the
hybrid method that combines an optical parametric process and an electrical modulator, is free from the requirement
mentioned above and enables to achieve arbitrarily high GER in any condition. Furthermore, it provides wide
operation flexibility, e.g. processing of WDM signals. In this paper, we propose a new concept to perform
quantization of optical phase, which we call hybrid optical phase squeezer (HOPS). Using 2-level HOPS based on
SOA, simultaneous phase regeneration of two 10.75-Gb/s CoWDM BPSK signals was successfully demonstrated.
2. Principle of HOPS
For an M-level optical PSK signal, phase regeneration can be achieved by interfering it with a conjugated (M-1)th
phase harmonic [5]. Fig. 1 illustrates the concept to realize this by HOPS in the case of M = 2. The signal
(frequency: s) is first mixed with a pump (frequency: p) to generate a phase conjugate (idler) by four-wave mixing
(FWM) in a nonlinear medium. Then, the signal and idler are filtered out and equalized in amplitude. After
amplification by a standard optical amplifier such as EDFA, the signal-idler pair is launched into an amplitude
modulator (AM), which is driven sinusoidally as when generating CS-RZ pulses. The AM splits the input signal into
two components whose frequencies are up- and down-shifted by the modulation frequency, . If the modulation
frequency is equal to the frequency difference between the signal and the pump, e.g.  =p - s, the up- (or down-)
converted signal and the down (or up-) converted idler are located in the same frequency of p and interfere. The
amplitude of the interference signal has a periodicity of  with respect to the input signal phase. This characteristic
ensures that for the input of a BPSK signal, the interference signal becomes the phase regenerated BPSK signal. In
the case when the pump laser is phase locked to the signal, it is done by properly adjusting the pump phase. When
the pump laser is in the free running condition, it can be done by stabilizing the interference signal through phase
control of the microwave signal driving the AM. It is straightforward to extend the scheme of 2-level HOPS shown
in Fig.1 to the M-level PSK signals with M > 2.
In HOPS, a GER higher than 25 dB can be easily achieved irrespective of the conversion efficiency of FWM, if
Fig. 1 Principle of 2-level hybrid optical phase squeezer for phase regeneration of a BPSK signal.
FWM: Four-wave mixing, AM: Amplitude modulator.
978-1-55752-993-0/14/$31.00 ©2014 Optical Society of America
W3F.5.pdf
OFC 2014 © OSA 2014
the power levels of the signal-idler pair are equalized adequately. Furthermore, for a multi-channel input signal, the
signal-idler pair mixing can be done simultaneously using a single AM, provided that both the signal and the pump
are obtained from optical frequency combs (OFC) with the same repetition rate. Therefore, HOPS is suitable for the
phase regeneration of coherent WDM (CoWDM) signals [6].
2. Experiment and results
EDFA PC
OCG
0±m f
WSS-1
0
EDFA PC
Mod
f =43GHz
CW
Port-1
ch.1
Sig
0 -3f
Port-2
l=1550nm
F
PM
ch.2
0 +f EDFA
OC
ch.1
PC
PC
ch.2
Pump
PC
SOA
EDFA PC
43GHz
MZM
ch.1
f1 -f1
0 -2f
0 +f 0 +2f
0 +2f
VCO
SOA
0 -3f 0 -2f
F
0 -2f
PLL
VCFS
672MHz
10.75Gb/s
BPSK
WSS-2
PPG
F
OC
F
ch.2
f2
PD
-f2
0 +2f
ch.1
0 +2f
ch.2
0 -2f
OMA
Fig. 2 Experimental setup. OCG: Optical comb generator, WSS: Wavelength selective switch, Mod: BPSK modulator, PPG: Pulse pattern
generator, F: Filter, PM: Phase modulator, PC: Polarization controller, OC: 3-dB optical coupler, MZM: Amplitude modulator,
SOA: Semiconductor optical amplifier, VCFS: Voltage controlled frequency shifter, OMA: Optical modulation analyzer.
In order to verify the concept of HOPS, we demonstrated simultaneous phase regeneration of two CoWDM BPSK
signals. Fig. 2 shows
experimental
OFC with
a repetition
rate of &f =real-time
43 GHz
was generated from a CW
PD:the
Photo
detector, OMA:setup.
OpticalAn
modulation
analyzer
(coherent receiver
oscilloscope).
laser (frequency: 0 = 193.4 THz). From the OFC, two pairs of modes at {0-3f, 0+f} and {0-2f, 0+2f} were
separated by a wavelength selective switch (WSS-1). The first pair was used to generate two 10.75GHz BPSK
signals (PRBS: 215-1) and the second pair was used as pumps. A phase noise of 23-degree in amplitude was loaded
to the BPSK signals using a phase modulator driven at 672MHz. The signals and pumps were mixed by a (22) 3dB optical coupler (OC). In each OC output (labeled as ch.1 and ch.2), the signal at s was combined with a pump at
(s + f) to generate an idler at (s + 2f) by FWM in a semiconductor optical amplifier (SOA: I = 57mA). The powers
of the signal and the pump launched into the SOA were -6 dBm and 3 dBm, respectively. Using WSS-2, the signal
and the idler were filtered out from each of the SOA outputs and the power levels of the signal-idler pair were
equalized. Fig. 3(a) shows the spectrum of the WSS-2 output signal. The output of WSS-2 was amplified by an
EDFA and launched into a voltage controlled frequency shifter (VCFS), which consisted of a Mach-Zehnder
amplitude modulator (MZM) driven by the output of a VCO. From the output of VCFS, the interference signals at
(s + f) were separated using an OC and band-pass filters.
In order to generate a stable phase regenerated signal from the signal-idler pair, the modulation frequency of
-10
(a)
ch.1
ch.2
Power [dBm]
-20
-30
Sig
idler
Sig
idler
c-1)
-40
c-2)
20dB
-50
20dB
-60
-200
(b)
-100
0
200
ch.2
ch.1
-10
100
Frequency offset: -0 [GHz]
Power [dBm]
-20
-30
-40
-50
image
-60
-200
-100
0
100
Frequency offset: -0 [GHz]
200
Fig. 3 (a) Spectrum of WSS-2 output signal. (b) Spectra of VCFS output signal at high level lock (blue bold) and low level lock (brawn
dash). (c) Spectra of the regenerated signal for ch. 1 and ch. 2 with maximum output (blue bold) and minimum output (red dash).
W3F.5.pdf
OFC 2014 © OSA 2014
MZM needed to satisfy the condition  =p - s. To this end, a part of the interference signal of ch. 1 was detected
by a photo detector and used to control the frequency of VCO. If the VCO was free running, the power of the
interference signal changed rapidly. Instead, in our experiment, the VCO was controlled by a PLL so that the power
of the interference signals was stabilized and the required frequency condition was satisfied. As a result, phase
regeneration could be achieved for the two channels simultaneously. Fig. 3(b) shows the spectra of the VCFS output
signal which were recorded at two different locking points in the signal power stabilization. It is observed that the
power of the interference signal of ch. 2 varies synchronously with ch. 1 when the locking point was changed. Fig.
3(c) shows the spectra of the regenerated signal for the two channels. It is observed that a GER of 20 dB was
achieved.
The characteristics of the phase regenerated signals were evaluated using an optical modulation analyzer. Fig. 4
shows the constellation diagrams of the input and output of the HOPS. Fig. 5 shows the BER curves measured as a
function of OSNR. The phase degraded signal (input curve) showed a ~1.2 dB OSNR penalty, which was
effectively mitigated by the 2-level HOPS (output curve).
(a) ch.1
(b) ch.2
Input
Input
Output
Output
Fig.4 Constellation diagrams measured for (a) ch.1 and (b) ch.2.
-2
-2
(a) ch.1
Input
Output
Back to Back
-3
Input
Output
Back to Back
-LOG(BER)
-LOG(BER)
-3
-4
-4
-5
-5
-6
-6
-7
6.5
(b) ch.2
7.5
8.5
9.5
10.5 11.5 12.5 13.5
OSNR [dB]
-7
6.5
7.5
8.5
9.5
10.5 11.5 12.5 13.5
OSNR [dB]
Fig. 5 BER curves measured for (a) ch.1 and (b) ch.2. Back-to-back refers the measurements on the signals without phase noise.
3. Conclusion
A new concept to quantize optical phase by a hybrid method (HOPS) was presented. The 2-level HOPS employing
SOA as a nonlinear material showed successful simultaneous phase regeneration of two 10.75-Gb/s CoWDM BPSK
signals and exhibited a 20 dB GER with a pump power of only 3 dBm.
Acknowledgements - This work was partly supported by Project for Developing Innovation Systems of the Ministry
of Education, Culture, Sports, Science and Technology (MEXT), Japan. The authors thank Dr. Solis-Trapala for
proofreading the manuscript.
4 References
[1]
[2]
[3]
[4]
R. Slavik, et al., Nature Photon. 4, 690-695 (2010).
T. Umeki, et al., Opt. Express 21, 12077-12084 (2013).
S. Sygletos, et al., Opt. Express 19, B938-B945 (2011).
M. Gao, et al., Opt. Lett. 37, 1439-1441 (2012).
[5] J. Kakande, et al., Nature Photon. 5, 748-752 (2011).
[6] A.D.Ellis et al., IEEE Photon. Tech. Lett. 17, 504-506 (2005).
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