Transmission of 20 × 20 Gb/s RZ-DQPSK signals over 5090 km with 0.53 b/s/Hz spectral efficiency

Abstract

We demonstrate a transmission of 20 channels at 20 Gb/s using RZ-DQPSK format over 5090 km of dispersion-managed fiber with distributed Raman amplifiers. For this condition, RZ-DQPSK signals are mainly degraded by single channel distortion.

Full text

Transmission of 20 x 20 Gb/s RZ-DQPSK signals over
5090 km with 0.53 b/s/Hz spectral efficiency
Kazuyuki Ishida, Katsuhiro Shimizu, Takashi Mizuochi, and Kuniaki Motoshima
Information Technology R&D Center, Mitsubishi Electric Corporation, 5-1-1, Ofuna, Kamakura, Japan
ishidak@isl.melco.co.jp
Dany-Sebastien Ly-Gagnon and Kazuro Kikuchi
Research Center for Advanced Science and Technology University of Tokyo 4-6-1 Komaba, Meguro-Ku, Tokyo, Japan
Abstract: We demonstrate a transmission of 20 channels at 20 Gb/s using RZ-DQPSK format
over 5090 km of dispersion-managed fiber with distributed Raman amplifiers. For this condition,
RZ-DQPSK signals are mainly degraded by single channel distortion.
©2003 Optical Society of America
OCIS codes: (060.4510) Optical communications
1. Introduction
Modulation formats have been a major concern for the next generation of submarine systems that are
expected to exhibit a larger transmission capacity through higher spectral efficiencies. Phase-shift keying (PSK)
formats were seen as incapable of transoceanic transmission because of their susceptibility to nonlinear effects.
However, the recent availability of new types of fibers and distributed Raman amplifiers made it possible to
transmit PSK signals over thousands of kilometers. In particular, recent experiments presented that 40 Gb/s RZ
differential phase-shift keying (RZ-DPSK) has a potential to transmit over transoceanic distances [1,2]. In
addition, RZ differential quadrature phase-shift keying (RZ-DQPSK) [3] is a promising format owing to its high
spectral efficiency as compared to RZ-DPSK. It is expected that RZ-DQPSK possesses a larger tolerance to
chromatic dispersion and polarization-mode-dispersion than RZ-DPSK. Recently, short distance transmission
experiments presenting the high spectral efficiency of this format were reported; e.g., 1.6 b/s/Hz (8 x 40 Gb/s)
transmission over 200 km with polarization multiplexing [4], 0.8 b/s/Hz (9 x 25 Gb/s) transmission over 1000
km [5], 0.8 b/s/Hz (8 x 20 Gb/s) transmission over 310 km [6]. Numerical simulations also suggested that RZ-
DQPSK has a higher tolerance to fiber nonlinearities in comparison with RZ on-off keying (RZ-OOK) [7].
However, its nonlinear tolerance in long-haul transmission has yet to be confirmed in order to clearly
demonstrate the full potential of this modulation format. In this paper, we report a 20 x 20 Gb/s RZ-DQPSK
wavelength division multiplexing (WDM) transmission using dispersion managed fiber and distributed Raman
amplifiers (DRA) and show its feasibility for ultra long-haul transmission systems.
2. Spectrum efficiency of RZ-DQPSK
We compared 20 Gb/s RZ-DQPSK with 20 Gb/s RZ-DPSK by numerical simulations. Figure 1 (a) shows
the back-to-back Q factor as a function of the spectral efficiency for an optical signal-to-noise ratio (OSNR) of
10 dB/nm. Figure 1 (b) shows the Q penalty due to nonlinear effects after a transmission of 363 km. All
channels were co-polarized and the demultiplexing filters were optimized for each modulation format. The
model of the transmission line and repeaters used for simulation was in the same configuration as the one seen
in Fig.2 and the fiber input power was set to –2.5 dBm/ch to estimate fiber nonlinear effects after 363 km. At the
same bit rate, back-to-back simulations show that RZ-DPSK suffers from considerable crosstalk as compared to
RZ-DQPSK for high spectral efficiencies. However, at a distance of 363 km, the Q penalty due to the nonlinear
effects of RZ-DQPSK is larger than that of RZ-DPSK; it might become non-negligible in long-haul
transmission. These results suggest that RZ-DQPSK has a potential for high spectral efficiencies in back-to-back
condition, although this advantage can only be exploited if RZ-DQPSK shows an acceptable nonlinear tolerance
in long-haul transmission. In the following experiment, the nonlinear tolerance of RZ-DQPSK will be
investigated for a spectral efficiency of 0.53 b/s/Hz.
5
7
9
11
13
0.4 0.5 0.6 0.7 0.8 0.9 11.1
Spectral Efficiency [b/s/Hz]
Q Factor [dB]
RZ-DQPSK
RZ-DPSK
0.0
0.2
0.4
0.6
0.8
1.0
0.4 0.5 0.6 0.7 0.8 0.9 11.1
Spectral Efficiency [b/s/Hz]
Q Penalty [dB]
RZ-DQPSK
RZ-DPSK
(a) Back-to-back Q factor versus spectral efficiency. (b) Q penalty after 363 km versus spectral efficiency.
Fig. 1. Comparison RZ-DQPSK and RZ-DPSK by numerical simulations.

3. Transmission Setup
We constructed the RZ-DQPSK transmission setup shown in Fig. 2. The 20 CW laser-diodes were arranged
from 1550.32 to 1556.05 nm with a channel spacing of 37.5 GHz. The DQPSK signals were generated at the
two parallel Mach-Zehnder modulator (SSB modulator [8]: Sumitomo Osaka Cement Co., LTD) biased at null
points and driven by two 10.7 Gb/s data streams with a peak-to-peak voltage of 2 Vπ. The optical phase shift
between the in-phase and quadrature-phase components was set to π/2. The two data streams used were 210-1
bits long and were programmed such that both decoded streams become a PRBS 10 data stream where one was
the normal sequence (Tributary 1) and the other was the inverted sequence (Tributary 2). Their phase difference
was 64 bits. A Mach-Zehnder modulator (AM MOD) followed the SSB modulator for bit-synchronous
intensity-modulation driving it with a peak-to-peak voltage of 2 Vπ at 5.3 GHz. To de-correlate the patterns of
the adjacent channels, we measured the BER of 20 channels as the following procedure. Firstly, one of 20 CW
laser-diodes was turn off. Secondly, a tunable laser source fed to another pair of modulators was set to the
replacing wavelength and this SSB modulator generated the DQPSK signal with a phase difference of 500 bits.
All channels were co-polarized, including the measured channel and the difference of fiber input signal power
was less than 0.5 dB. All channels were low-speed polarization-scrambled at around 800 kHz to emulate every
input state of polarization to the fiber. The 363 km fiber loop consisted of 8 distributed Raman amplifier (DRA)
repeaters, two EDFA repeaters and 8 spans of fiber. Two types of Raman pumps, with wavelengths of
1430/1460 nm and 1435/1465 nm, were placed in series in order to maintain the gain flatness. The combined
Raman pumping power for the two types of DRA was 400 mW. The pump light was de-polarized by a
following polarization-maintaining fiber. Each of the fiber spans was comprised of three segments of
UltraWaveTM Ocean Fibers IDF/SLA: 14.4 km of Aeff-enlarged positive dispersion fiber (SLA), 14.4 km of
slope-compensating dispersion compensation fiber (IDF) and 14.4 km of SLA. The averaged span length at
1547 nm was 43.3 km. The residual dispersion was adjusted by using 17 km of single mode fiber. The average
dispersion for the measured path was -0.018 ps/nm/km at 1550 nm. The gain and output power of each DRA
were set to 10.5 dB and –15 dBm/ch, respectively. A polarization scrambler was intentionally inserted into the
loop to emulate every state of polarization in the loop and was driven by a combination of two frequencies 321
and 231 Hz, which preferably would be loop-synchronized frequencies [9]. At the receiver, the transmitted
signal was extracted using a filter with a 3 dB bandwidth of 0.18 nm and then decoded by a fiber based 1-bit
delay interferometer. The output signals of the constructive and destructive ports were sent to the twin pin-
photodetector of a balanced receiver. We used a single receiver to decode each 10.7 Gb/s tributary (Tributary 1,
Tributary 2) by adjusting the differential optical phase between two arms at π/4 or -π/4.
10.7 Gb/s
210-1 Data x 2 363 km per loop
SW
GEQ
SW
SSB
MOD
0.18nm
DCF DCF
DRA
EDFA
5.3 GHz Clock
Measured Signal
PSCR
+D+DCDR
ERD
DQPSK
Receiver
λ1
λ20
AM
MOD
SSB
MOD AM
MOD
x 8
-D-D
+D+D
PMF Coupler
Tunable Laser
Source
OBPF
500-bit delay
Tributary 1
or
Tributary 2
PSCR
SMF
Fig. 2. Experimental Setup.
4. Results
Figure 3 shows the Q factor of one of two tributaries as a function of the fiber launch power after a
transmission of 3630 km. The open squares indicate that all channels were modulated, including the
measurement channel. The filled circles show the case where only the measured channel was modulated and all
the others were left non-modulated. Q difference between single channel and 20 channels transmission was less
than 1 dB even if the fiber launch power was set to –11 dBm/ch. In this experiment, although the measured
channel is slightly degraded by the adjacent channels, it remains mainly degraded by single channel distortion.
Figure 4 shows the Q factor of channel 10 as a function of the transmission distance. The line in the figure
shows a calculated Q factor derived from combining the measured OSNR with the back-to-back OSNR
performance of the system. The Q factor changes linearly with the logarithm of the distance. The average Q
factor of two tributaries and their Q penalty due to fiber nonlinearity after 5800 km were 10.6 dB and 2.2 dB,
respectively. Figure 5 shows the received optical carrier spectrum and the Q factor for the 20 channels after a

transmission of 5090 km. The channel power variation was well suppressed to less than 3 dB and the average
received OSNR was 14.6 dB/0.1 nm. The two tributaries for each channel were measured. The average and the
worst Q factor were 11.5 dB and 11.1 dB, respectively. Figure 6 shows the received optical eye diagram before
the 1-bit delay interferometer and the electrical eye diagram after the balanced receiver. No significant
waveform distortion was observed even after a transmission of 5090 km.
10.0
10.5
11.0
11.5
12.0
12.5
13.0
13.5
14.0
-18 -17 -16 -15 -14 -13 -12 -11 -10
Signal Input Power [dBm/ch]
Q Factor [dB]
DQPSK single channel - Tributary 1
DQPSK WDM - Tributary 1 10
11
12
13
14
15
16
17
18
19
20
100 1000 10000
Distance [km]
Q Factor [dB]
Tributary 1
Tributary 2
Q determind by OSNR 2.2dB
Fig. 3. Measured Q factor versus averaged fiber input power. Fig. 4. Measured Q factor versus transmission distance.
6
7
8
9
10
11
12
13
14
1549 1551 1553 1555 1557
Wavelength [nm]
Q Factor [dB]
-40
-35
-30
-25
-20
-15
-10
Optical Power [dBm]
Tributary 1
Tributary 2
Fig. 5. Measured Q factor and received optical spectrum Fig. 6. Received eye diagrams of CH 10.
after 5090 km transmission.
5. Conclusion
We have successfully demonstrated the transmission of 20 x 20 Gb/s RZ-DQPSK signals over 5090 km. The
Q factor for all the 20 channels after 5090 km was more than 11.1 dB. Using dispersion managed fiber and
distributed Raman amplifiers, it was experimentally shown that RZ-DQPSK signal was mainly degraded by
single channel distortion at a spectral efficiency of 0.53 b/s/Hz. This experimental result suggests the feasibility
of using RZ-DQPSK for long distances and further investigations using higher spectral efficiencies is desirable.
6. References
[1] C. Rasmussen, et al., “DWDM 40G transmission over trans-Pacific distance (10,000 km) using CSRZ-DPSK, enhanced FEC and all-
Raman amplified 100 km UltraWave™ fiber spans,” Technical Digest of OFC 2003, post-deadline paper, PD18, 2003.
[2] T. Tsuritani, et al., “70GHz-spaced 40 x 42.7Gbit/s transmission over 8700km using CS-RZ DPSK signal, all-Raman repeaters and
symmetrically dispersion-managed fiber span,” Technical Digest of OFC 2003, post-deadline paper, PD23, 2003.
[3] R. A. Griffin et al., “10 Gb/s optical differential quadrature phase shift key (DQPSK) transmission using GaAs/AlGaAs integration,”
Technical Digest of OFC 2002, post-deadline paper, FD6, 2002.
[4] C. Wree, et al., “High Spectral Efficiency 1.6-b/s/Hz Transmission (8 x 40 Gb/s With a 25-GHz Grid) Over 200-km SSMF Using RZ-
DQPSK and Polarization Multiplexing,” Photon. Technol. Lett., 15, pp. 1303 –1305, 2003.
[5] P. S. Cho, et al., “Transmission of 25-Gb/s RZ-DQPSK Signals With 25-GHz Channel Spacing Over 1000 km of SMF-28 Fiber,”
Photon. Technol. Lett., 15, pp. 473 –475, 2003.
[6] H. Kim, et al., “Transmission of 8 x 20 Gb/s DQPSK Signals Over 310-km SMF With 0.8-b/s/Hz Spectral Efficiency,” Photon. Technol.
Lett., 15, pp. 769 –771, 2003.
[7] C. Wree, et al., “RZ-DQPSK format with high spectral efficiency and high robustness towards fiber nonlinearities,” Technical Digest of
OECC 2002, Paper 9.6.6, 2002.
[8] K. Higuma, et al., “X-cut lithium niobate single side-band modulator,” Electron.Lett., 37, pp.515-516, 2001.
[9] Y. Sun, et al., “Statistics of the System Performance in a Scrambled Recirculating Loop With PDL and PDG,” Photon. Technol. Lett., 15,
pp. 1067 –1069, 2003.
(a) Before the 1-bit delay interferometer.
(b) After the balanced receiver.