IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 15, NO. 3, MARCH 2003
473
Abstract—We report transmission of nine 25-Gb/s return-to-zero differential quadrature phase-shift keyed
(RZ-DQPSK) dense wavelength-division-multiplexing signals with 25-GHz channel spacing over 1000 km of single-mode fiber
(SMF-28) in a recirculating loop. The loop uses all erbium-doped fiber amplifiers (EDFAs) and has an amplifier spacing of 100 km with an average loss of 25 dB between EDFAs and a maximum span loss of up to 30 dB. All channels were copolarized launched.
No precompensation or postcompensation was employed. To the best of our knowledge, this is the first transmission test of multichannel RZ-DQPSK signals operating at 25 Gb/s with a spectral efficiency of 0.8 b/s/Hz. The transmission distance is limited by amplified spontaneous emission noise due to the high span losses. Nevertheless, our result indicates that upgrading the capacity of long-haul terrestrial systems using RZ-DQPSK modulation format should be feasible.
Index Terms—Differential phase-shift keying (DPSK), optical fiber communication, phase-shift keying, quadrature phase-shift keying (QPSK), wavelength-division multiplexing (WDM).
Fig. 1.
Experimental setup. C: PM coupler. A: EDFA. PS: polarization scrambler.
CR: clock recovery.
AMZ: asymmetric Mach–Zehnder interferometer. BPF: bandpass filter. SW: AO switch. FBG: circulator and fiber Bragg grating.
I. I NTRODUCTION
T HERE HAVE been an increasing number of reported transmission tests and simulations of optical differential binary and quadrature phase-shift keying (DBPSK and
DQPSK) modulation formats for nonreturn-to-zero (NRZ) and return-to-zero (RZ) pulses with direct detection that shows significantly improved system performance [1]–[7]. RZ-DBPSK modulation format is more tolerant to fiber nonlinearities such as self-phase and cross-phase modulation (SPM and
XPM) compared with RZ ON–OFF keying (OOK) format due in part to its continuous pulse sequence with no missing pulses
[2]–[4]. Interchannel XPM and ghost pulse problem [8], [9] are virtually eliminated using PSK format. However, PSK signal suffers intrachannel four-wave mixing (FWM) especially in strong pulse-overlapped transmission [10] such as terrestrial systems. Interchannel FWM also affects the performance of
PSK format depending on system parameters such as power and channel spacing. Furthermore, the accumulation of linear and nonlinear phase noise induced by amplified spontaneous emission (ASE) noise limits the transmission distance for PSK system [11]–[13]. Nevertheless, PSK with balanced detection is superior in sensitivity performance to OOK [14], [10]. PSK is also more robust to narrow-band optical filtering than OOK
[15].
By using DQPSK format to encode two bits onto each optical symbol, the spectral efficiency (SE) of DBPSK system is doubled. As a result, the data rate of DQPSK signal is two times higher than DBPSK or OOK using the same symbol rate without suffering performance degradation due to more tolerance to chromatic dispersion and PMD in fiber links [5].
RZ-DQPSK format had also shown numerically to be more tolerant to fiber nonlinearities than RZ-OOK [6]. In this letter, we report transmission of nine RZ-DQPSK dense wavelengthdivision-multiplexing (DWDM) channels at 12.5 Gsymbol/s
(OC-192 line rate with 25% overhead bandwidth reserved for advanced forward-error correction (FEC) coding), equivalent to 25-Gb/s data rate, with 25-GHz channel spacing in an erbium-doped fiber-amplifier (EDFA)-based recirculating loop. No FEC was used in the test. The loop uses single-mode fibers (SMF-28) with 100-km amplifier spacing and an average loss of 25 dB between EDFAs with a maximum span loss of up to 30 dB. No precompensation or postcompensation was employed. To the best of our knowledge, this is the first experimental transmission test of multichannel RZ-DQPSK signals operating at 12.5 Gsymbol/s with 0.8 b/s/Hz SE without polarization multiplexing or interleaving. The achievable transmission distance was over 1000 km primarily limited by ASE noise due to high span loss. Nevertheless, our result indicates that upgrading the capacity of terrestrial systems using RZ-DQPSK modulation format should be feasible.
Manuscript received October 10, 2002; revised December 3, 2002.
The authors are with CeLight, Inc., Silver Spring, MD 20904 USA.
Digital Object Identifier 10.1109/LPT.2002.807934
II. E XPERIMENTAL S ETUP AND R ESULTS
Fig. 1 shows the transmission test setup for multichannel
RZ-DQPSK signals. Nine standard distributed feedback (DFB)
1041-1135/03$17.00 © 2003 IEEE

474 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 15, NO. 3, MARCH 2003 lasers with 25-GHz spacing were combined together using polarization-maintaining (PM) couplers and coupled into a lithium niobate (LN) Mach–Zehnder modulator (MZM).
The DFB lasers have a typical linewidth of 2 MHz. The quadrature-biased MZM was driven by a 12.5-GHz sine wave producing an RZ optical pulse train with approximately 60% duty cycle. The extinction ratio of the RZ pulse was only
6 dB in order to reduce its spectral spreading and to minimize adjacent channel interference due to the tight channel spacing.
The RZ optical pulse was encoded with DQPSK signal using a QPSK modulator with two push–pull LN MZMs biased at their respective dc transmission minima and driven by two complementary 12.5 Gb/s NRZ data streams with peak-to-peak voltage of 2 V . The two data streams were pseudorandom bit sequences (PRBSs) with a word length of and they were time delayed relative to each other by 3.44 ns or 43 b to simulate uncorrelated data streams at the output of the QPSK modulator. The aggregate bit rate of the DQPSK signal was, therefore, 25 Gb/s at 12.5 Gsymbol/s. No precoding of the data stream was employed. The QPSK modulator was composed of discrete components consisting of two 2 2 3-dB PM couplers connected to a parallel pair of LN MZMs, optical phase shifters, and optical delay lines for RZ pulse and data timing alignment.
In order to maintain optical phase orthogonality ( ) of the in-phase ( ) and quadrature-phase ( ) components of the
DQPSK signal, the two LN-based phase shifters were computer-controlled that minimizes the amplitude variation of the output. The computer accepts an input low-frequency feedback signal derived from one of the QPSK modulator outputs. The phase stabilization control loop was needed to compensate slow environmental perturbations on the QPSK modulator. The two fiber path lengths of the QPSK modulator were matched to less than 1 mm. Minimizing the difference of the two fiber path lengths of the QPSK modulator suppresses amplitude noise at the output as a result of the laser wavelength fluctuation. All the components of the QPSK modulator have PM fiber pigtails and were fusion spliced together.
The RZ-DQPSK signals were amplified before being launched into a recirculating loop through an acoustooptic
(AO) switch. All the channels were copolarized. The loop consists of two 102-km spans of SMF-28 fiber and dispersion compensating fibers (DCFs). The path-averaged dispersion of the loop was approximately 0.045 ps/nm/km at 1545.32 nm
(center channel). EDFAs were used to boost the launch optical power into the SMF-28 fiber and to compensate for the losses in the DCFs, AO switch, and the 3-dB coupler. A polarization scrambler operating at up to 800 kHz was inserted into the loop to minimize polarization-dependent loss (PDL). An extra
10-dB loss was added after one of the booster EDFA to reduce its output power. The average loss between EDFAs was 25 dB with one of the span loss up to 30 dB. Due to such a high span loss, the transmission distance is primarily limited by ASE noise as well as the gain nonuniformity of our EDFAs. Typical launched power into the SMF-28 fiber was 0 dBm per channel.
The output of the loop was amplified and followed by a circulator and a fiber Bragg grating (FBG) filter to select the center channel for bit-error-rate (BER) measurement. The
FBG filter has a reflection bandwidth of 18.2 GHz. A coil of 20-km SMF-28 fiber ( 340 ps/nm) was used to reduce
Fig. 2.
Back-to-back BER versus received power of 12.5 Gb/s RZ-DQPSK signal for a single channel with (triangle) and without (square) the FBG filter and for nine channels with the FBG filter (circle). The solid line depicts simulation result. Insets show the received eye diagrams.
the pulse distortion due to the negative dispersion of more than 340 ps/nm added by the FBG filter [4]. A portion of the signal was tapped off for clock recovery. The selected center channel was amplified to compensate for the FBG filter loss and followed by a 1.3-nm-wide bandpass filter to reduce spontaneous-spontaneous beat noise. The filtered RZ-DQPSK signal was demodulated using an asymmetric Mach–Zehnder
(AMZ) interferometer. The fiber-based AMZ demodulator has a differential delay of 80 ps and a PDL of approximately
0.3 dB at differential phase shift ( ). A thermoelectric cooler and a resistive heater provide control of . The polarization-dependent frequency shift was 340 MHz. The two outputs of the AMZ demodulator were directed to a balanced photoreceiver with a bandwidth of 15 GHz.
was adjusted to approximately or to obtain an open eye pattern at the balanced photoreceiver output which corresponds to and components of the demodulated DQPSK signal. The output bit patterns for the and signals are different. To measure the BER, the error detector was programmed with the expected demodulated DQPSK data sequence of bits calculated according to and the two input applied to the QPSK modulator.
long PRBS data
Back-to-back BER measurement was performed for the case of single-channel input, single channel with the FBG filter and nine channels input for . As shown in Fig. 2, for the single-channel case 3 dB of dispersion penalty was caused by the FBG filter alone. The linear interchannel crosstalk penalty was approximately 2 dB at 10 BER. Similar result was obtained for . The sensitivity for the case of a single channel without the FBG filter was approximately
21.5 dBm due to the limitation of our receiver. With improved receiver, a sensitivity of 36 dBm (OSNR: 16.5 dB) can be achieved. The solid line shows simulation result based on calculation of the probability distribution function (pdf) of the RZ-DQPSK signal. Detail of the model will be published elsewhere. Good agreement of the simulation with the measured data for the single-channel case down to a BER of
10 was obtained. For lower BER, measurement error and limitations of the simulation model account for the discrepancy.
Fig. 3 shows the optical spectrum of the nine RZ-DQPSK

CHO et al.: TRANSMISSION OF 25-Gb/s RZ-DQPSK SIGNALS WITH 25-GHz CHANNEL SPACING 475
III. S UMMARY
We have demonstrated transmission of nine 25-GHz-spaced
RZ-DQPSK channels at 25 Gb/s over 1000 km of SMF-28 fibers with 100-km EDFA spacing. To the best of our knowledge, this is the first experiment reported on multichannel transmission of 25-Gb/s RZ-DQPSK signals with 0.8 b/s/Hz SE. Based on our result, RZ-DQPSK format could be a feasible choice for next-generation high capacity long-haul terrestrial systems.
Fig. 3.
Optical spectrum of transmitted RZ-DQPSK channels after 1200 km.
Noise bandwidth: 0.01 nm.
A CKNOWLEDGMENT
The authors wish to thank M. Roberts and C. Kerr for technical assistance.
Fig. 4.
BER performance of the received 12.5 Gb/s RZ-DQPSK center channel versus distance. The square and circle denote
1 = +=4 and 0=4. The input PRBS length was
2 0 1. Insets show the received eye diagrams for back-to-back and after 1000 km. The vertical streaks near the outer edge of the eye are caused by gating of the sampling oscilloscope.
channels after 1200 km of transmission. Note that the signal power of the center channel was equal to or below the other eight channels. Therefore, the center channel suffers the most
ASE-noise impairment and the BER performance of the other eight transmitted channels should be similar or better than the center channel. The BER of the received center channel versus distance is shown in Fig. 4 for
10 of corresponds to the and outputs. Insets show eye diagrams of the 12.5-Gb/s received center channel for back-to-back and after 1000 km of transmission. The uncorrected BER for the detected channel at 1200 km was approximately 2 10 . This transmission result is consistent with our simulation result which predicts a transmission distance of around 1000 km at a BER of 10
(uncorrected). With advanced high net coding gain FEC codes such as Turbo Product Code [16], [17] a corrected BER below should be feasible with 25% overhead. Although no
FEC code was employed in our test, the performance of standard FEC code on DBPSK has shown to be similar for OOK format even though their noise statistics are different [3]. It is expected that the FEC code performance on DQPSK should be comparable with DBPSK despite their different pdfs.
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