Fig.1 10Gbit/s waveforms after 7500km transmission (a) NRZ, (b) RZ
Multi-terabit long haul DWDM transmission with VSB-RZ format
KDDI R&D Laboratories, Inc. 2-1-15 Ohara, Kamifukuoka-shi, Saitama, 356-8502
Japan
TEL: +81-492-78-7835, FAX: +81-492-78-7516, E-mail: suzuki@kddilabs.jp
Introduction
Since terabit capacity per fiber has already become commercially viable even over transpacific distances using 10Gbit/s-based DWDM transmission technologies and C-band amplifiers, now the intense research efforts have been concentrated on multi-terabit transmission system development. For such applications, higher bit rates of either 20Gbit/s or 40Gbit/s will be expected so that the number of terminals can be much reduced. For multi-terabit long-haul transmission systems such as transoceanic systems, maintaining the high spectral efficiency over long distances is a crucial issue, since the impact of increasing nonlinear penalties due to narrower channel spacing and higher bit rate will become intense as the transmission distance increases. In this paper, technical challenges for multi-terabit long-haul DWDM transmission with high spectral efficiency employing vestigial sideband (VSB) RZ signals are discussed.
Modulation Format with High Spectral Efficiency
To accommodate multi-terabit capacity within the limited bandwidth, the required spectral efficiency should be very high. To increase the spectral efficiency, various modulation formats have been proposed[1]-[9].
Duo-binary format whose spectral width is a half of the standard NRZ format is a possible way to increase the spectral efficiency[1]. However, resistance to nonlinear effects of duo-binary signals is not so large, because the phase information, which is vulnerable to interaction between ASE noise and SPM, is needed to generate the duo-binary signals. The spectral efficiency can be also doubled with polarization division multiplexing (PDM) and 0.8bit/s/Hz has been demonstrated in 10.92Tbit/s (273x40Gbit/s) over 117km transmission[2]. Considering the effects of PMD and XPM, however, PDM technique seems difficult to apply to long haul systems. Thus, both duo-binary and PDM techniques will be suitable for short haul ultra DWDM systems.
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Fig.1 10Gbit/s waveforms after 7500km transmission (a) NRZ, (b) RZ |
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Fig.2 Spectra of 20Gbit/s WDM signals with VSB-RZ format |
For long distance transmission, RZ format is proved to be a suitable signal format to overcome fiber nonlinearity. Fig.1 shows the example of waveforms of NRZ and RZ formats after 7500km transmission, where four 10Gbit/s WDM signals with a channel spacing of 0.6nm were transmitted over multiple 40km hybrid fiber spans of large core NZ-DSF and low-dispersion slope NZ-DSF. Since all the pulses in RZ format are isolated each other and have the same waveform independently of the data pattern, the pattern dependence of SPM-induced waveform distortion observed in conventional NRZ format can be mitigated. To improve the nonlinear waveform distortion further, chirped RZ(CRZ) signal format[3] and dispersion managed soliton (DM-soliton)[4] have been proposed. These formats have large resistance to SPM-GVD effect, but they require the wider bandwidth than conventional RZ signals. To increase the spectral efficiency maintaining good transmission performance, modified RZ formats with less spectral width and larger tolerance of optical power such as carrier-suppressed RZ (CS-RZ)[5] and alternate chirp RZ (AC-RZ)[6] have been proposed. Since the alternating chirp between the adjacent pulses entails almost zero residual dispersion, the use of AC-RZ seems difficult in practical application. In terms of robustness to optical power and dispersion tolerance, CS-RZ format is promising, but, spectral efficiency is not so high as VSB-RZ format described in next section.
Spectral Filtering Method
Another approach to increase spectral efficiency is the spectral filtering method with narrow band optical bandpass filter at the transmitter end[7]-[9] or at the receiver end[10],[11]. With spectral filtering at the transmitter, vestigial sideband (VSB) RZ signal has been generated and 20Gbit/s, 100WDM over 4000km transmission[7],[8] and 10Gbit/s, 200WDM over 9200km transmission[9] have been demonstrated with a high spectral efficiency of 0.6Bit/s/Hz and 0.53bit/s/Hz, respectively. With OBPF and OTDM signals, band-limited RZ signal can be generated and 2.56Tbit/s (40Gbit/s, 64WDM) unrepeatered transmission over 230km was also demonstrated with high spectral efficiency of 0.8bit/s/Hz[12]. With vestigial sideband demultiplexing (VSB-demux) for NRZ signals at the receiver, a spectral efficiency of 0.64bit/s/Hz without PDM[10] and 1.28bit/s/Hz with PDM[11] have been achieved and 10.2Tbit/s (256x42.7Gbit/s) over 100km transmission experiments have been demonstrated[11].
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Fig.3 Eye-diagrams for 20Gbit/s VSB-RZ signal and conventional RZ signal(WDM with channel spacing of 0.28nm) |
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Fig.4 Spectra of VSB-RZ signals after 4260km transmission (a) before transmission (b) after transmission |
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Fig.5 Q-factor of 2Tbit/s (100x20Gbit/s) sugnals after 2700km |
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Fig.6 Q-factor versus transmission distance |
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Fig.7 Spectral efficiency versus transmission distance |
2 Tbit/s VSB- RZ Long-Haul Transmission
The spectra of 20Gbit/s VSB-RZ WDM signals are shown in Fig.2. With VSB technique, the spectral crosstalk between adjacent WDM channels can be avoided as shown in Fig.2. Fig. 3 shows the waveforms of VSB-RZ signals and the conventional RZ signals for WDM transmission with a channel spacing of 0.28nm (35GHz). Clear eye opening can be obtained for VSB-RZ signal, though the eye-diagram of RZ signal is distorted due to large crosstalk. A major concern in VSB-RZ transmission is the growth of the subdued spectral sideband due to fiber nonlinearity during the transmission. Fig.4 shows the spectra before and after 4260km transmission. The fiber span used in the experiment consists of Aeff-enlarged positive dispersion fiber and slope-compensating DCF. The extinction ratio of over 25dB between the carrier and the filtered sideband was maintained even after 4260km transmission. The tolerance of the residual dispersion of VSB-CRZ was larger than that of the conventional RZ signals because of narrower bandwidth[8]. Feasibility of VSB-RZ long haul transmission has been demonstrated through the experiment of 35GHz-spaced-20Gbit/s 100WDM VSB-RZ transmission using dispersion managed fiber described above[7]. VSB-RZ signals were generated with narrow band OBPFs at the transmitter and spectral efficiency of 0.6bit/s/Hz has been achieved. Fig. 5 shows Q-factor after 2700km transmission. Average Q-factor was 14.2dB. Fig.6 shows the Q-factor versus transmission distance of channel 52, which represents a typical average transmission performance of this transmission line. The Q-factor changes almost linearly with distances over 2000-4000km regime. At 4260km, Q-factor was 12.6dB. The transmission distance will be further increased by optimising the channel spacing[8]. The effectiveness of VSB-RZ format for ultra long-haul DWDM systems has been also confirmed by the 10Gbit/s, 200WDM VSB-RZ transmission over 9240km with a spectral efficiency of 0.53bit/s/Hz[9].
Applicability of these modified RZ formats and VSB technique as well as conventional RZ and NRZ formats to ultra long haul DWDM systems is still under investigation[7]-[9], [12]-[15]. Fig. 7 shows the spectral efficiency versus transmission distance for various modulation formats. Note that these figures do not represent the exact comparison since the Q-factors obtained at the reported transmission distance are not the same.
Conclusion
Various modulation formats with high spectral efficiency have been proposed for multi-terabit long haul DWDM systems. VSB-RZ seems a promising modulation format providing narrow spectral width and excellent robustness against nonlinear effects.
References