Joint-Carrier Phase Tracking Techniques For Coherent Optical Transmission Systems With Phase-Locked Carriers And Fiber Nonlinearity

In order to circumvent the electronic bottleneck and satisfy the exponentially ever-increasing capacity demands of optical communication systems in a cost-effective manner,superchannels via optical parallelism are introduced to increase the spectral efficiency of coherent wavelength division multiplexing(WDM)systems.The term superchannel refers to a set of channels that are co-generated,co-transmitted and co-detected as a single entity.In this context,the superchannel can be realized by using either frequency division multiplexing(FDM i.e.,spectral superchannel),space division multiplexing(SDM i.e.,spatial superchannel)or a combination of FDM and SDM(i.e.,hybrid superchannel).Also,polarization division multiplexing(PDM)can be applied to double the capacity of each channel.The superchannel can be created using either discrete or comb laser sources.When spectral WDM superchannel is enabled by the use of optical frequency combs,the WDM channels can be closely packed requiring a smaller or no guard-bands between the channels,since the frequencies of the channels are stable and drift in a correlated manner.This greatly increases the spectral efficiency,besides reducing the hardware complexity and power consumption as the large number of independent laser sources are replaced by a single laser comb source.But these benefits come at the cost of phase noise originating from the linewidths of the lasers used for the transceiver combs as well as fiber nonlinearities,which must be estimated and compensated using carrier phase estimators.In this case,an essential parameter of phase noise estimation algorithms is the average length,which can be optimized adaptively for improved performance.On the other hand,the increasing popularity of DSP-enabled coherent-intradyne receivers in optical communication systems have underscored the need for DSP algorithms with improved phase noise tolerance and/or reduced complexity(and power consumption).In standard PDM and/or comb-based WDM superchannel transmission systems,the same transmitter and local oscillator(LO)lasers are used for transmitting and detecting the subchannels.As a result,the carrier phase noise is correlated among the subchannels such that the broadband phase coherence provided by phase-locked carriers can be exploited via joint-carrier phase tracking to improve phase-noise tolerance or simplify and reduce the DSP complexity of the carrier phase recovery.The main principle behind joint-carrier phase tracking techniques is independent of the algorithm used for the phase estimation as long as the phase can be readily estimated and applied to the subchannels.However,as joint-carrier phase tracking techniques rely on the relative phases of the subchannels being same,any phase-difference among the channels arising from the comb generation process and/or fiber transmission effects may impair and degrade their performance.In view of this,this thesis includes joint-polarization and joint-channel carrier phase tracking techniques designed for single-wavelength PDM and single-polarization WDM superchannel systems respectively with phase-locked carriers.Two adaptive joint-polarization carrier phase tracking techniques based on the Viterbi-Viterbi carrier phase estimator are proposed and their performances are evaluated for PDM-QPSK/16QAM signal formats with the combined effect of laser and nonlinear phase noises.Extensive simulation results showed that the proposed adaptive phase estimation schemes outperform their conventional implementation without adaptation.On the other hand,the two complementary methods that may be used to achieve ultra-high spectrally efficient superchannel in coherent optical communication systems are seamless multiplexing of spectrally shaped single-carrier-and OFDM-modulated signals.In this context,we designed two master-slave joint-channel carrier phase-tracking methods based on the popular blind phase search(BPS)and pilot-subcarrier phase estimation algorithms for Nyquist-and OFDM-WDM superchannel systems respectively with phase-locked carriers.As the phase noise is correlated among the OFDM bands,fewer dedicated pilot-subcarriers for carrier phase estimation can be multiplexed(or co-transmitted)with the data subcarriers at unprecedented low pilot-rate,which significantly reduces the pilot-overhead.We evaluate the feasibility of these joint-channel carrier phase tracking schemes via comprehensive simulation of long-haul superchannel signals based on different modulation signal formats with laser linewidth and fiber nonlinearities.For both WDM superchannel systems enabled by optical frequency combs based on Nyquist and OFDM signals,the simulation results showed that the performance of these joint-channel carrier phase tracking schemes closely followed that of the conventional independent carrier phase tracking technique.In addition,for the first time,to the best of our knowledge,we identify the interplay between fiber chromatic dispersion(CD)and LO laser linewidth as the origin of channel-dependent differential phase,which severely degrade the performance of joint-channel carrier phase tracking techniques,besides the channel phase-differences from the comb generation process.Also,we found out that the CD-induced differential phase drifts in orders of magnitude much slower than the correlated phase noise originating directly from the transceiver laser linewidths.Besides fiber length(i.e.,accumulated CD)and modulation formats,the impact of the CD-induced differential phase on the performance of joint-channel carrier phase tracking techniques depends on the spectral distance between the side channel(s)and the central channel,with severe performance degradation observed for outer channels than the inner channels.The dependence of the differential phase on the channel count may be attributed to the fact that frequency combs are mainly characterized by only two frequencies:the central frequency and the frequency separation between the lines.In contrast,fiber nonlinearity does not contribute to the differential phase among the subchannels.Finally,we conducted an experiment to demonstrate the feasibility and performance of the BPS-based master-slave carrier phase tracking technique using a 40 GHz-spaced 2-channel comb-based WDM system with 12.5 Gbaud 16QAM signal format up to 100km fiber transmission distance.