All-optical modulation format conversion from NRZ-OOK to RZ-QPSK using parallel SOA-MZI OOK/BPSK converters.

Abstract

A novel all-optical modulation format conversion from non-return-to-zero on-off-keying (NRZ-OOK) to return-to-zero quadrature-phase-shift-keying (RZ-QPSK) is proposed and experimentally demonstrated. The proposed format conversion scheme is based on parallel Mach-Zehnder interferometric (MZI) OOK/binary-PSK (BPSK) converters, consisting of integrated semiconductor optical amplifiers (SOAs). We experimentally demonstrate that in both decoded channels of the converted RZ-QPSK signal bit error rate (BER) curves show almost the same receiver sensitivity at a symbol-rate of 10.7 Gsymbol/s. In addition, a reasonable dispersion tolerance of the converted signal up to +295 ps/nm is observed. The numerical simulation based upon carrier rate equation verifies the experimental results.

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All-optical NRZ-OOK to RZ-QPSK conversion using
parallel SOA-MZI OOK/BPSK converters
Ken Mishina∗1, Suresh M. Nissanka∗1, Akihiro Maruta∗1,
Shunsuke Mitani∗2, Kazuyuki Ishida∗2, Katsuhiro Shimizu∗2,
Tatsuo Hatta∗1,∗2,∗3, and Ken-ichi Kitayama∗1
[*1]Graduate School of Engineering, Osaka University,
2-1 Yamada-oka, Suita, Osaka 565-0871 Japan,
TEL: +81-6-6879-7728, FAX: +81-6-6879-7688,
mishina@ac.eie.eng.osaka-u.ac.jp
[*2]Mitsubishi Electric Corporation
[*3]Optoelectronic Industry and Technology Development Association (OITDA)
Abstract: We propose a novel all-optical NRZ-OOK/RZ-QPSK modulation format converter
using parallelized SOA-MZIs and demonstrate the proof-of-the-principle experiment at 10.7
GSymbol/s by using the test parallel SOA-MZI OOK/BPSK converters.
c ?2006 Optical Society of America
OCIS codes: (060.5060) Phase modulation; (070.4340) Nonlinear optical signal processing
1. Introduction
Differential phase-shift-keying (DPSK) modulation formats are promising techniques to enhance the per-
formance for long-haul transmission systems [1], [2]. The differential quadrature PSK (DQPSK) modulation
format, which has twice as high spectral efficiency as differential binary PSK (DBPSK), has been extensively
studied for the 40 Gb/s long-haul densely wavelength division multiplexed (DWDM) systems. Therefore, it
is likely that at a gateway node between long-haul backbone network and metro area network (MAN), a
transparent modulation format conversion between the PSK signals and conventional on-off-keying (OOK)
becomes a key technique for seamless photonic networking in the near future. We have proposed an all-optical
OOK to BPSK converter using semiconductor optical amplifier-based Mach-Zehnder interferometer (SOA-
MZI) [3]. All-optical OOK/BPSK conversion based on optical fiber nonlinearity have been reported [4].
To the best of our knowledge, however, all-optical modulation format conversion from OOK to QPSK has
never been investigated. In this paper, we propose for the first time an all-optical modulation format con-
version from NRZ-OOK to RZ-QPSK. The converted waveforms and spectra at the bit-rate of 21.4 Gb/s
are experimentally demonstrated to confirm the principle of operation by using the test parallel SOA-MZI
OOK/BPSK converters. We also confirm that the fixed output pattern after balanced receiving can be
generated in agreement with the calculated pattern from the fixed input patterns.
2. Principle of Operation
Figure 1 shows the schematic diagram of the proposed modulation format converter. The basic configuration
consists of an SOA-MZI OOK/BPSK modulation format converter (SOA-MZI#1) on the upper arm and a
phase shifter and another SOA-MZI#2 in tandem on the lower arm. The SOA-MZI OOK/BPSK converters
convert NRZ-OOK data signal to RZ-BPSK data signal by using cross-phase modulation (XPM) in SOA.
NRZ-OOK data 1 (λ0)
1 1 0 0
RZ clock (λ1)
CW (λ2)
Converted RZ-QPSK
data signal (λ1)
−3π/4 π/4 −π/4 3π/4
NRZ-OOK/RZ-QPSK
modulation format convertor
NRZ-OOK data 2 (λ0)
1 0 1 0
SOA
SOA
SOA
SOA
SOA-MZI#1
SOA-MZI#2
π π 0 0
−π/2 π/2 −π/2 π/2
∆π/2
BPF
<Port1>
<Port3>
<Port2>
Fig. 1. Schematic diagram of the proposed
modulation format converter.
LDLN
10.7 Gb/s
PRBS 27-1
NRZ-OOK
data signal 1,2
(1545.32nm)
RZ pulse
(1535.04nm)
CW light
(1542.0nm)
NRZ-OOK
to RZ-QPSK
format converter
Data 1
6.4 dBm
PC
Pulse -0.6 dBm
CW 6.2 dBm
LDLN
LD
<Port1>
<Port2>
<Port3>
1bit delayBalanced
Receiver
TDL
PC
Clock Recovery
PC
PC
Data 2
4.6 dBm
Output
port
∆φ
Fig. 2. Experimental setup.
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1534.61534.81535
Wavelength [nm]
(a)
1535.21535.4
Intensity [10 dB/div.]
50 ps/div
(b)
50 ps/div
1 0 0 0 0 0 1 0
(c)
Fig. 3. Results of measurements. (a) Optical spectra of converted signal, (b) eye diagram of electric signal
after receiver, and (c) fixed output pattern.
NRZ-OOK signal pulses 1 and 2 with the wavelength of λ0are launched into the upper arm of the MZI (port
1) and the lower arm of the MZI (port 3), as control pulses 1 and 2, respectively. RZ-clock pulse sequence
with λ1 and CW light with λ2 are launched into the MZI (port 2) as a probe pulse and an assist light,
respectively. According to the NRZ-OOK data 1 ”1” or ”0”, the phase of the probe pulse after passing
through SOA-MZI#1 shifts either by ”0” or ”π”, respectively. On the other hand, when the NRZ-OOK data
2 is ”0” or ”1”, the probe pulse after passing through SOA-MZI#2 has the phase of ”π/2” or ”−π/2” due
to the π/2 phase shifter. The probe pulses after passing through SOA-MZI#1 and #2 have the equal peak
power and cause orthogonal interference in each combination of the control pulses 1 and 2. The probe pulse
after interference has the same peak amplitude as the incoming signals due to the orthogonal interference and
four different phase values depending on the combination of the control pulses 1 and 2. Therefore, NRZ-OOK
data signal can be converted to RZ-DQPSK data signal.
3. Experiments
Figure 2 shows the experimental setup for the proposed format conversion. The NRZ-OOK data signals
1 and 2 were generated by modulating a CW of 1545.32 nm in a lithium niobate (LN) modulator with 10.7
Gb/s pseudo random binary sequence of length 27-1. The RZ clock pulse was generated by modulating a CW
of 1535.04 nm in a LN modulator by using the regenerated clock from NRZ-OOK data 2. The RZ clock pulse
was coupled with CW assist light of 1542.0 nm. The converted signal was received by a balanced receiver
after passing through a 1-bit delay interferometer, in which the phase adjuster has a phase shift of “π/4”.
While figure 3 (a) shows the optical spectrum of the converted signal as the carrier suppressed QPSK
signal, (b) shows a clear eye opening after the balanced receiver. Figure 3 (c) shows the fixed output pattern
after the balanced receiver, which corresponds to the calculated pattern “10000010” from fixed input data 1
“00110101” and data 2 “01010011”. The above mentioned results reveal that the NRZ-OOK signal can be
converted into an RZ-QPSK signal using the proposed method.
4. Conclusions
We have proposed a novel modulation format conversion scheme from NRZ-OOK to RZ-QPSK by using
parallelized SOA-MZIs. We have experimentally demonstrated the operation of the proposed converter at
10.7 Gs/s using the test parallel SOA-MZI OOK/BPSK converters. This work was partially performed under
management of the OITDA supported by New Energy and Industrial Technology Development Organiza-
tion (NEDO).
References
1. A. H. Gnauck and P. J. Winzer, “Optical phase-shift-keyed transmission,” IEEE J. Lightwave Technol., vol. 23, pp. 115–
130, 2005.
2. G. Charlet, “Progress in optical modulation formats for high-bit rate WDM transmissions,” IEEE J. Selected Topics in
Quantum Electronics, vol. 12, pp. 469–483, 2006.
3. K. Mishina, A. Maruta, S. Mitani, T. Miyahara, K. Ishida, K. Shimizu, T. Hatta, K. Motoshima, and K. Kitayama, “NRZ-
OOK-to-RZ-BPSK modulation-format conversion using SOA-MZI wavelength converter,” IEEE J. Lightwave Technol.,
vol. 24, pp. 3751–3758, 2006.
4. C. S. Langhorst, R. Ludwig, M. Galili, B. Huettl, F. Futami, S. Watanabe, and C. Schubert, “160 Gbit/s all-optical OOK
to DPSK in-line format conversion,” in Conf. Proc. of ECOC 2006, Paper PD Th4.3.5, Cannes, France, 2006.
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