| Over the past few years, with the improvement of high-speed digital signal processing (DSP) and analog-to-digital conversion, coherent optical transmission has become a research hotspot again. The combination of coherent detection and DSP technique make it possible to perform linear impairments electrically, such as chromatic dispersion and polarization mode dispersion in electrical domain. Also this combination enables carrier phase recovery and polarization tracking digitally, which have been recognized as major obstacles for the conventional coherent optical transmission. Coherent receiver based on DSP technique has the simple configuration, with the significant hardware-transparent. Due to its ability to perform linear/nonlinear transmission impairment compensation in the electrical domain, the coherently-detected receiver makes it possible to streamline the transmission links and reduce costs. Also, multi-level modulation formats are supported by the digital coherent receiver, on which the high spectral efficiency (SE) can be achieved easily. Due to these advantages, the coherently-detected fiber-optics systems are considered to be the very promising candidate for the next generation optical network. The fiber-optics transmission capacity can be improved significantly with the combination of coherent receiver and quadrature phase shift keying (QPSK)/quadrature amplitude modulation (QAM). If the polarization-multiplexing (PM) technique is used, the capacity is doubled.The combination of polarization-multiplexing quadrature phase shift keying(PM-QPSK) and the digitally coherently-detected has been consided to be the most promising candidate for the implementation of40/100optical networks. Any linear impairment, such as roup velocity dispersion (GVD) and polarization mode dispersion (PMD), can be compensated by this transmission system in electrical domain. As a result, nonlinear transmission impairments expericenced by signals durng transmission has become a major concern for coherently-detected PM-QPSK modulated signals. These nonlinear transmission distortions include self-phase modulation(SPM)、cross phase modulation(XPM)、 four-wave-mixing(FWM), nonlinear phase noise(NLPN) and so on. For wavelength-division-multiplexing transmission, inter-channel XPM is the major nonlinearity, especially for PM signals. XPM results in not only the phase disturbance experienced by optical signal, but the other severe nonlinear impairments-the so called inter channel cross polarization modulation (XPolM). Some previous experimental and analytical foundings indicate that the XPolM-induced distortions have become the major limiting factor for40G WDM PM-QPSK signals. For100G WDM signals, XPolM leads to the same level of nonlinear penalty as the other fiber nonlinearities. We have investigated the nonlinearities in40/100G PM-QPSK transmission systems in this paper. The major studies and contributions of this paper including:(1) We have studied, for the first time, the SPM-induced polarization scattering for single-channel PM-QPSK over dispersion managed link. We found that due to the different disturbance experienced by the X and Y polarization component of arbitrary pulse pair, the phase difference of them is no longer constant but randomized around the ideal value. This random process leads to polarization scattering for arbitrary pulse pair. Moreover, we found that appropriate pre-compensation is helpfui to reduce the SPM-induced polarization scattering. These results hold true for both non return-to-zero (NRZ) and return-to-zero (RZ) pulse format.(2) Using numerical simulations, the impact of nonlinear signal-noise interactions (NSNI) between the amplified spontaneous emission noise (ASE) and the information signal on polarization-multiplexed quadrature phase-shift keying (PM-QPSK) systems at42.8(112)-Gbit/s is investigated over dispersion-managed (DM) link. Both symbol-aligned and symbol-interleaved formats are considered and compared. We find that for symbol-aligned PM-QPSK systems, the impact of NSNI on system performance seems rather weak due to the strong inter-channel cross-polarization modulation (XPolM). However, when the symbol-interleaved format is used, in which the XPolM is suppressed significantly, the system performance is seriously degraded by NSNI, especially at low bit-rate. Results of1000-km transmission employing standard single-mode fiber (SSMF) over DM link show that for42.8-Gbit/s coherent PM-QPSK systems, the nonlinear threshold (NLT) will decrease from5.8dBm to0.6dBm due to the nonlinear signal-noise interactions when symbol-interleaved RZ format is used.(3) Considering the polarization mode dispersion(PMD), the transmission penalty induced by nonlinear signal-noise interactions (NSNI) between the amplified spontaneous emission noise (ASE) and the information signal is investigated numerically for40(100)G dispersion-managed PM-QPSK systems. We show that for single-channel PM-QPSK systems, PMD is helpful to reduce the NSNI-induced penalty because differentail group delay (DGD), introduced by PMD, walk off the X and Y component of PM signals. The walking off of the X and Y component means lower spike power and thus smaller nonlinearities. For multi-channel PM-QPSK system, however, the NSNI-induced nonlinear penalty is significantly enhanced by PMD, especially at low bit-rate. The walking off between X and Y component, caused by DGD, reduce the polarizations that make contribution to XPolM and thus the XPolM-induced impairment is reduced. On the other hand, just as it for symbol-interleaved format, the suppression of XPolM means that the systems are sensitive to NSNI. Our results show that due to the NSNI, the reduction of allowed input power that gives1-dB Q penalty after1600-km nonlinear transmission will increase from1dB without PMD to3.7dB with PMD for42.8-Gbit/s coherent return-to-zero (RZ)-PM-QPSK systems.(4) The impact of filter shape and filter bandwidth on closed-spaced PM-QPSK systems is investigated. The optical multiplexer is modeled by super-Gaussian (SG) filter with1to4order. We found that, although a higher-order filter generally leads to better system performance at the optimal filter bandwidth, this benefit comes at the cost of reduced tolerance to variation in filter bandwidth and center-frequency shifts. The closer channel spacing, the more pronounced these findings. It is found that all filters exhibit the same performance if they have more than1.1Rs bandwidth for WDM PM-QPSK systems with Rs symbol-rate and50GHz spacing in b2b operation. The intra-channel symbol interference (ISI) is the major distortion if the filter’s bandwidth is less than1.1RS. Compared with lower-order filter, in this narrow bandwidth domain, a higher-order filter generally leads to more severe ISI impairment.(5) A method estimating the system performance of digital coherent PM-QPSK systems was presented in this paper. The quality parameter, mean degree of polarization (DOP) of PM-QPSK signals, obtained through the complex signal constellation after coherent receiver, was introduced to assess system performance. With the help of the simulative results of112-Gbit/s PM-QPSK transmission systems, we found that the mean DOP is proportional to the Q-factor of the systems. Moreover, it is shown that the linear relation between the mean DOP and Q-factor is uncorrelated with the other system parameters such as the channel number, the dispersion map and so on. |
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