We propose and demonstrate by numerical simulation a new phase modulation format, the staggered differential phase-shift keying (SDPSK), for 100 Gbit/s applications. Non-return-to-zero (NRZ) SDPSK signals was generated by using two phase modulators, and return-to-zero (RZ) SDPSK signals with 50% duty cycle was generated by cascading a dual-arm Mach-Zehnder modulator. The demodulation of 2 bit/symbol can be simply achieved on 1 bit rate through only one Mach-Zehnder delay interferometer and a balanced receiver. By comparing the transmission characteristics of the two staggered phase modulation formats with those of NRZ-DPSK, RZ-DPSK, NRZ-DQPSK, and RZ-DQPSK, respectively, we show that, the SDPSK signal has similar chromatic dispersion and polarization-mode-dispersion tolerance to the DPSK signal with same NRZ or RZ shape, while the SDPSK signal has stronger nonlinear tolerance than the DPSK or DQPSK signal. In addition, the SDPSK signal has the best transmission performance when each signal was transmitted over 106km optical SMF+DCF, and then launched into a third-order Gaussian optical bandpass filter placed with beyond 125GHz bandwidth.
© 2008 Optical Society of America
Recently, multi-state per symbol optical modulation technique is a very hot research topic. One example is the combination of amplitude-shift-keying (ASK) and differential phase-shift keying (DPSK) signals . Another modulation scheme like differential quadrature phase-shift keying (DQPSK) has been recently proposed to achieve double spectral efficiency, relax dispersion management and improve PMD tolerance [2–5]. On the other hand, for a future 100Gbit/s Ethernet (100GbE) standard , many research groups showed interest in 100GbE and different modulation formats have been proposed, such as NRZ , duobinary  and DQPSK . The main challenge to realize 100GbE is the development of cost-effective systems. The most straightforward implementation for 100GbE that requires the least optical and electrical components is the NRZ modulation format. Many 100GbE papers have been reported using the NRZ modulation format [7, 8]. However so far, 100Gbit/s using RZ modulation format has not been reported usually.
In this paper, we propose a novel scheme to generate staggered generate differential phase-shift keying (SDPSK) signals with NRZ or RZ shape for 100Gbit/s transmission. In contrast to NRZ-DQPSK or RZ-DQPSK, SDPSK with NRZ or RZ shape requires demodulation and detection can be achieved at twice symbol rate by only one set of Mach-Zehnder delay interferometer (MZDI) and balanced receiver. This proposed transmitter and receiver are simple and potentially inexpensive. Based on numerical simulation, we will discuss the performance of SDPSK format with NRZ or RZ shape, and demonstrate RZ-SDPSK signal has a few advantages than that of the traditional phase modulation format.
2. Principle for NRZ-SDPSK signal generation and detection
Optical NRZ-SDPSK format can be generated by cascading two phase modulators. Two driven electrical binary NRZ signals (data1 and data2) are operated at half of the bit rate, and are pre-coded. After pre-coder, one signal directly drives the first PM (PM1). The other is delayed half of the bit duration (τ=0.5T=10ps, T is one symbol period) to drive the second PM (PM2), as shown in Fig. 1. In series NRZ-DQPSK modulation, the phase shifts in two phase modulators are π and π/2, respectively; while in NRZ-SDPSK modulation, each phase shift in the two phase modulators is π. As a result, an optical NRZ-SDPSK signal is generated at the output of PM2.
Figure 2 shows the principle for the NRZ-SDPSK generation and detection in time. After pre-coding, two streams are then staggered by 0.5 symbol period and used to modulate the same optical carrier phase. Two streams are staggered, and then changed the optical carrier phase. Only one set of MZDI with one bit duration delay can realize the detection of the NRZ-SDPSK signal. From the detected staggering data streams, odd bits were directly chosen to restore data1, and even bits were chosen to restore data2. For example, data1 is “10100111”, and data2 is “01101001”, after pre-coding, they were transmitted. In the receiver, the detected data is “1001110001101011”, while the NRZ-SDPSK signal was demodulated by one set of MZDI with one bit duration delay. In the detected “1001110001101011”, odd bits stream is “10100111”, and even bits stream is “01101001”. Hence, the input data1 and data2 can be restored from the odd bits and even bits of the detected data at the receiver, respectively. Since the coding and decoding rule is very simple, the method is effectively. Furthermore, as we known, series NRZ-DQPSK modulation can be described as two independent NRZ-DPSK modulations, and it’s demodulation and detection needs two sets of MZDIs. However, NRZ-SDPSK signal can be detected by only one MZDI. Hence, this method is cost-effective. In practice, the phase modulation format with RZ shape is widely used in long-haul optical communication systems due to its superior performance . The RZ pulse carver can be implemented cascading a dual-arm Mach-Zehnder modulator (MZM) after two phase modulators.
3. Numerical simulations
The tolerance of SDPSK to some transmission impairments is evaluated through numerical simulations using commercial software. A comparison with currently used phase modulation formats, such as DPSK and DQPSK, is shown. As shown in Fig.3, at the transmitter, a CW light beam at 193.1THz generated is fed into PM1. Two data streams from 50Gbit/s pattern generator, with 215-1 NRZ binary sequence, are pre-coded and staggered to drive two phase modulators, respectively. One is directly driven PM1, and the other is delayed 10ps to drive PM2, and each phase shift in two phase modulator is π. Then, an optical NRZ-SDPSK signal with 100Gbit/s rate is generated by two phase modulators. On the other hand, for RZ carving, one dual-arm MZM driven by a sinusoidal voltage at half of the bit rate is employed. By biasing at the quadrature point with a Vπ voltage swing, duty cycle of 50% can be achieved. The electrical clock sinusoidal signal and the measured optical intensity signal are shown in Fig. 3(i) and (ii).
At the receiver, SDPSK signal demodulation is provided by an MZDI with 10ps delay between its two arms. According the above mentioned rule, to restore the input data1 and data2, the detected data sequence must be divided into two data steams, one is extracted from the odd bits, and the other is extracted from the even bits. For the balanced receiver, the detected very clear and opening eye diagrams of NRZ-SDPSK and RZ-SDPSK for the back-to-back case are show in Fig. 4(a) and (b), respectively. Fig. 5(a) and (b) show optical spectra diagrams of the SDPSK signal with NRZ and RZ shape, respectively. The optical spectrum is measured with a resolution bandwidth of 1.5625GHz. It is obviously to see, comparing RZ-SDPSK signal, the measured optical spectra of the NRZ-SDPSK signal is compacter.
As we know, for long-distance transmission, the dispersion compensation is always compensated in each amplifier span. In our proposed configuration, the generated 100Gbit/s NRZ- or RZ-SDPSK signal is transmitted over 90km standard single-mode fiber (SMF) and is completely dispersion-compensated by a piece of 16km dispersion-compensating fiber (DCF). The transmission optical fiber parameters are shown in Table.1. After the NRZ-SDPSK or RZ-SDPSK signal passing through the 90km SMF, it is amplified by an erbium-doped fiber amplifiers (EDFA1), and after the 16km DCF, it is amplified by EDFA2, to compensate signal attenuation in the SMF and DCF, respectively. At the receiver, the optical signal is filtered by an optical band pass filter (OBPF) with a bandwidth of 100GHz to suppress the excessive amplified spontaneous emission of the EDFAs. NRZ-SDPSK or RZ-SDPSK signal is detected by an erbium-doped fiber amplifier-preamplified receiver with two ideal PIN photodiodes, in which noise figure is 5 dB. An electrical low pass filter (LPF) with 70GHz bandwidth (not optimized) is applied at the SDPSK receiver to flatten the optical power distribution within one bit and to remove the amplitude fluctuation induced by the noise.
In order to reduce nonlinear effect, the input power into the SMF would be maintained 0dBm by adjusting the launched power of the continuous-wave laser. But, in the realistic transmission system, the chromatic dispersion may not always be fully compensated, because the influence of some unpredictable factors. Dispersion tolerance of SDPSK is evaluated for transmission over 90km SMF, and then post-compensation with 16km DCF. Fig. 6(a) shows eye-opening penalties (EOP) versus residual dispersion for SDPSK compared to DPSK and DQPSK with 0dBm fiber input power. In this simulation, a sequence length of 215-1 is to take into account the interactions between neighbouring bits caused by chromatic dispersion. NRZ and RZ impulse shaping with duty cycle 0.5 are considered. For NRZ, EOP of SDPSK is greater than for DPSK and DQPSK with 100Gbit/s. The mentioned NRZ-DQPSK is also generated by cascading two phase modulator. However, for RZ, SDPSK has the slightly lower EOP than DPSK, and the far greater EOP than DQPSK. Since the detection of DPSK and SDPSK need a smaller quantity of instruments, and considering the clock extraction and restoration, RZ-SDPSK is more suitable to be adopted in transmission system, if only referring chromatic dispersion tolerance.
Polarization-mode-dispersion (PMD) is another limiting factor for transmission of a high bit rate signals. In this paper, we only consider first-order-PMD-induced EOP. The curves of the EOP as function of the normalized differential group delay (DGD) are shown in Fig. 6(b). The results are measured under transmission over 90km SMF and 16kmDCF, and DGD is normalized to bit period (Tb=0.5T). As for the RZ phase modulation, the value of EOP is lower, whatever phase modulation format is chosen. Furthermore, DQPSK and SDPSK formats transmits data at the symbol rate equal to the half-bit rate, they should exhibit higher tolerance to PMD. Especially, RZ-DQPSK has the largest first-order-PMD tolerance.
The nonlinear tolerance of 100Gbit/s SDPSK is investigated with NRZ or RZ shape and compared to 100Gbit/s DPSK and DQPSK in Fig. 6(c). Here, EOP versus the fiber input power for 90km of SMF and 16km of DCF transmission were measured. Each fiber input power of six phase modulation formats is same at the input port in the SMF. In the numerical simulation, the values of EOP were calculated for each power by solving nonlinear Schrödinger equation. It is clear to see the nonlinear tolerance of three phase modulation formats with NRZ shape are lower three phase modulation formats with RZ shape. Note that, RZ-SDPSK has the largest nonlinear tolerance from 0~10dBm of fiber input power. In this range, RZ-SDPSK format is very robust against nonlinearities, and suited for transmission with fully dispersion compensation. As we know, in practice, the value of fiber input power is low for avoiding nonlinear effect, and we usually allocate 1dB EOP in an optical transmission system design . As shown in Fig. 6(c), when the value of the fiber input power is beyond 10dBm, each EOP value of six phase modulation signals is bigger than 1 dB. Hence, we did not consider powers beyond 10dBm when studying nonlinear effects.
In the above numerical simulations, an OBPF with a certain bandwidth (100GHz) is considered to suppress the excessive amplified spontaneous emission of two EDFAs. In this simulation, the fiber input power of each phase modulation format was 0dBm, and each phase modulation signal was launched into a third-order Gaussian OBPF with different bandwidth placed after 106km transmission with SMF+DCF. Fig. 6(d) compares EOP of the different modulation formats subjected to different bandwidth optical filtering. The results show SDPSK signal has slightly higher transmission robust than DPSK signal with the same impulse curve, while each one was launched into a third-order Gaussian OBPF. As long as the value of the bandwidth of OBPF is larger than 125GHz, RZ-SDPSK has the lowest value of EOP. Hence, in practice, we should choose OBPF with larger bandwidth.
We have presented an optical phase modulation and demodulation scheme for 100Gbit/s transmission applications. At the transmitter, the electrical components operate at the symbol speed, which is half the bit rate (50Gbit/s). On the other hand, the receivers can be achieved easily and cheaply. The generated RZ-SDPSK signal seems especially suited for single carrier transmission over wide-area network (WAN, ~100 km), for which the 100Gbit/s rate can be achieved at reasonable hardware overhead. Considering commercial application, RZ-SDPSK may be a good modulation format in the future 100Gbit/s optical transmission system.
This work was partially supported by the National High Technology Research and Development Program (863) of China (Grant No. 2007AA01Z263), and the program of the Ministry of Education of China for New Century Excellent Talents in University.
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