IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 15, NO. 4, APRIL 2003
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Comparison of Return-to-Zero Differential Phase-Shift Keying and ON–OFF Keying in Long-Haul Dispersion Managed Transmission Chris Xu, Xiang Liu, Linn F. Mollenauer, Fellow, IEEE, and Xing Wei
Abstract—Performance of return-to-zero (RZ) differential phase-shift keying (DPSK) in ultralong-haul dense wavelength-division-multiplexing (WDM) dispersion managed transmission is studied experimentally and compared with conventional ON–OFF keying (OOK) in a 10-Gb/s system. We show that, while OOK out-performs phase-shift keying in a low spectral efficiency WDM system, the performance of DPSK is comparable to OOK at 10-Gb/s transmission with a spectral efficiency of 0.2. Furthermore, RZ DPSK is advantageous in a high spectral efficiency (e.g., 0.4) system and our numerical simulation results show superior performance of DPSK at 10 Gb/s with 25-GHz channel separation. Index Terms—Amplitude shift keying, differential phase-shift keying (DPSK), optical fiber communication, optical solitons, phase-shift keying (PSK).
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IFFERENTIAL phase-shift keying (DPSK) has been studied for fiber optic transmissions in the past [1]–[7]. Nonetheless, applications of DPSK in dense wavelength-division-multiplexing (WDM) ultralong-haul (ULH) optical communications have only recently attracted a lot of interests [8]–[11]. It was also realized lately that return-to-zero (RZ) DPSK has several added advantages [3], [8], [9]. The ability of DPSK to eliminate cross-phase modulation (XPM) penalties and its significant improvement in receiver sensitivity when employing balanced receivers [3] have generated many new excitements and impressive transmission results. Furthermore, implementation of DPSK with direct detection is straightforward at 10 and 40 Gb/s [4], [6], [9]. It is, however, also realized that nonlinear phase noise caused by amplitude fluctuations and self-phase modulation (SPM) poses new limitations on any PSK system. Since SPM and XPM depend on the intensity, amplified spontaneous emission (ASE) noise or nonlinear interactions caused amplitude fluctuations will translate into phase noise through both SPM and XPM. It is previously known that a PSK system is fundamentally limited by ASE and SPM-induced nonlinear phase noise, the Gordon–Mollenauer effect [12]. In this letter, we show that the relative performances of ON–OFF keying (OOK) and PSK depend on the spectral efficiency of the system. While OOK out-performs PSK at low spectral efficiencies, the performance of DPSK is comparable to OOK at 10-Gb/s transmission with a spectral efficiency of Manuscript received November 15, 2002; revised December 23, 2002. C. Xu, X. Liu, and L. F. Mollenauer are with Bell Laboratories, Lucent Technologies, Holmdel, NJ 07733 USA (e-mail:
[email protected]). X. Wei is with Bell Laboratories, Lucent Technologies, Murray Hill, NJ 07974 USA. Digital Object Identifier 10.1109/LPT.2003.809317
Fig. 1. Experimental setup. For simplicity, only half of the transmitter is shown. AWG: array waveguide grating. Inset: schematic of RZ DPSK encoding. “ ” and “ ” signs indicate the phases of the pulses.
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0.2. Furthermore, RZ DPSK is advantageous in a high spectral efficiency (e.g., 0.4) system and our numerical simulation results show superior performance of DPSK at 10 Gb/s with 25-GHz channel separation. Transmission experiments were performed in an all-Raman amplified, dispersion managed system (Fig. 1). The WDM sources consist of 64 DFB lasers in a single 25 nm band (1555–1580 nm), spaced at 50 GHz. Odd and even channels are modulated independently by two transmitters consisting of two LiNbO Mach–Zehnder modulators (MZM) in series. The first modulator of each transmitter driven by a 5-GHz clock is a pulse carver (PC) generating 33 duty-cycle RZ pulses. The second MZM is the data modulator (DM), imposing data either as phase modulation (biased at the null point) or as intensity modulation (biased at the mid-point). Thus, switching between DPSK and OOK format can be accomplished by simply changing the bias point of the MZM. An optical 3-dB combiner combines the even and odd channels. The combined channels are sent to a precompensation module (precomp) with a dispersion of approximately 300 ps/nm. The recirculating loop comprises six spans of TrueWave Reduced Slope [(TWRS) ps/nm/km] fiber. Each span consists of 100 km of TWRS followed by nearly slope matching dispersion compensation fiber (DCF). The resulting residual dispersion per span ranges from 12 to 22 ps/nm. A dynamic gain equalizing filter (DGEF) is placed within the loop after the six spans to equalize channel powers. Two discrete Raman amplifiers are also used in the loop to compensate for losses from the DGEF itself, the acoustooptic switches (AOS) as well as a 3-dB coupler. Our measurement showed that the loop has an average loss of 32.4 dB 100 km. A tunable bandpass filter (BPF) is used to
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Fig. 2. RZ DPSK at low and high channel power. The path-averaged powers are indicated.
select the wavelength channel at the receiver end. The signal is then sent through a post dispersion compensation coil (postcomp). A delay interferometer (DI) with a 1-bit delay converts the incoming RZ DPSK signal into intensity-modulated signals at its two output ports. These signals are differentially detected by balanced photodiodes. The bit error rate (BER) is then measured on the regenerated data. Forward error correction was not employed in our measurements. One key limitation in any ULH PSK system is the amplitude noise to phase noise conversion via nonlinear effects such as SPM and XPM [12]. Such nonlinear transmission penalty increases strongly with the increase of the signal power. Fig. 2 shows RZ DPSK transmission at two different signal power levels. Clearly, the system performance is significantly better at the lower transmission power, with an error-free (defined as ) transmission distance of 5400 km. The total nonlinear phase shift due to SPM is estimated to be 1 radian at 5000 km with 12.9 dBm/channel, which is close to the optimum value for ULH PSK transmission [12]. The performance degradation at higher channel powers originates from the Gordon–Mollenauer effect. It is well known that a balanced receiver for DPSK provides close to 3-dB improvements in receiver sensitivity for a system limited by ASE beat noise [3]. This improvement, however, is reduced when the Gordon–Mollenauer effect becomes dominant [13]. This is confirmed by our experiments in which we compared a balanced receiver with a single ended receiver at different signal power levels at a transmission distance of 4200 km. A 3-dB advantage for the balanced receiver is obtained at 12.9 dBm/channel, while the advantage reduces to 1.5 dB at 10.5 dBm/channel. Thus, it is essential to be in the quasi-linear transmission region in order to obtain the 3-dB receiver sensitivity advantage for DPSK. In principle, the SPM mediated Gordon–Mollenauer effect is a single-channel transmission penalty and is independent of WDM transmission. The generalized, XPM mediated, Gordon–Mollenauer effect is a concern in dense WDM. Nonetheless, its effect is relatively small at a spectral efficiency of 0.2 [14]. Thus, we expect the relative transmission performance of DPSK when compared with OOK to improve as the spectral efficiency of the system increases. Furthermore, the improvement in receiver sensitivity for DPSK with balanced detection allows error-free transmission at significantly lower signal powers (even lower pulse energies), leading to additional reduction in nonlinear transmission penalty, which becomes
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 15, NO. 4, APRIL 2003
Fig. 3. Single-channel RZ-OOK and RZ DPSK transmission. The path-averaged powers are 12.9 dB and 10.5 dBm for DPSK and OOK, respectively.
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Fig. 4. Dense WDM (10 Gb/s at 50-GHz channel separation) RZ-OOK and RZ DPSK transmission. The path-averaged powers are indicated.
more important in high spectral efficiency systems. Fig. 3 compares low spectral efficiency DPSK and OOK transmission (by turning OFF two neighboring channels on each side of the channel being measured). It was previously known that only the effects caused by adjacent channels need to be considered in a high channel count WDM system [15]. OOK clearly out-performed DPSK in such an extreme case. We note that the signal power ( 18 fJ/pulse or 10.5 dBm/channel) used for the RZ-OOK transmission approximately satisfies the condition for ps/nm/km). In fact, dispersion-managed soliton (with even better “single” channel OOK performance can be obtained by increasing the signal power, such as demonstrated in many soliton experiments. The results for WDM transmission are markedly different. Fig. 4 shows the transmission of DPSK and OOK at 10 Gb/s with a channel separation of 50 GHz. Comparing with the data shown in Fig. 3, WDM DPSK performance is essentially the same as that of the single channel, while the performance of WDM OOK is significantly reduced. We have measured many channels within the wavelength band and the results showed that DPSK performances are comparable or slightly better than OOK at a spectral efficiency of 0.2. The variations in channel performance 1 dB can be explained by the OSNR variations across the band. Our experiments clearly showed that transmission penalty resulting from WDM is not significant when using DPSK format. Post dispersion compensation is important for optimum performance of the transmission. High tolerance for variations of dispersion compensation is essential for a robust and low-cost system. We have measured RZ DPSK performance at variable
XU et al.: COMPARISON OF RZ DPSK AND OOK IN LONG-HAUL DISPERSION MANAGED TRANSMISSION
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performances are comparable. We further argue that RZ DPSK is advantageous in a high spectral efficiency system and our numerical simulation results showed superior performance of RZ DPSK at 10 Gb/s with 25-GHz channel separation. ACKNOWLEDGMENT The authors would like to thank A. Grant, A. Chraplyvy, C. Mckenstrie, S. Hunsche, R. Giles, R. Slusher, A. Gnauck, P. Winzer, H. Kim, and D. Fishman for valuable discussions, and C. Doerr for providing the DI and DGEF. REFERENCES
Fig. 5. Numerical simulation results of dense WDM (10 Gb/s at 25-GHz channel separation) RZ DPSK transmission with a 25-dB back-to-back factor. The path-averaged power is 14.5 dBm/channel.
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amounts of post dispersion compensation at 4200 km. We found a dispersion range of greater than 315 ps/nm for a 1-dB penalty, which is much larger than that of a dispersion managed soliton system. Intuitively, because DPSK transmission is essentially in the “linear” region, the function of post dispersion compensation in DPSK is to restore the pulse width. While in addition to restoring pulse width, post dispersion compensation also compensates some nonlinear transmission penalties in an OOK system. For example, soliton timing jitter can be reduced in part by using appropriate amount of post dispersion compensation. We have extended the comparison of DPSK and OOK at 10 Gb/s and 25-GHz channel separations through numerical simulations in the absence of experimental data. Our numerical simulations have been providing valuable insights in DPSK performance, with predictions mostly confirmed by our experiments and other recent results [9]. Fig. 5 shows the numerical simulation results of RZ DPSK at 10 Gb/s with 25-GHz channel separation. The modeling parameters are similar to our experimental setup, except that the path-averaged power is further reduced. An error-free transmission distance of 4500 km is predicted. The reach is longer than an OOK system. (For example, timing jitter caused by soliton collisions alone will limit the system reach to 3500 km for a dispersion managed soliton system at 10 Gb/s and 25-GHz channel spacing [15].) We note that there are advantages by trading XPM penalty (dominant in OOK) with SPM penalty (dominant in DPSK). SPM in a PSK system is bit-pattern independent, while XPM in an OOK system is bit-pattern dependent. Ways of compensating the Gordon–Mollenauer effect have already been proposed to enhance the performance of a PSK system [16]. The absence of significant XPM penalty also allows polarization division multiplexing in a DPSK system, further increasing the spectral efficiency and capacity of a system [17]. In summary, we have shown that OOK out-performs PSK in a low spectral efficiency system. However, at 10-Gb/s transmission with 50-GHz channel separation, RZ-OOK and RZ DPSK
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