Researches On Techniques Of Modulation And Demodulation In High-Speed Coherent Optical Communications Systems

Multi-dimensional modulation formats have been realized in coherent optical communications systems by taking full advantage of amplitudes,phases,polarizations,time and frequencies of optical carriers.The widespread application of digital coherent optical communications technologies based on digital signal processing in long-haul optic fiber transmissions,optical communication network,ultra long span repeaterless transmission,and long-haul free space optical communications,witnessed in last decade,is due to flexible and controllable modulation formats,efficient algorithms at receivers countering different channel impairments,and capability of conveying signals modulated in different rates,spectral efficiencies and power efficiencies.Theoretical and experimental researches in this paper concentrate on modulations,demodulations and DSP algorithms in high speed coherent optical communications systems.The main content and breakthroughs are exhibited as follows:1.We have theoretically investigated different modulation formats in modulation space,and derived the analytical bit error rate formula and compared the power efficiencies and spectral efficiencies of different modulation formats,including OOK?BPSK?DPSK?QPSK?PPM?FSK?QAM?PS-QPSK?PM-QPSK,mPPM-QPSK,et cetera.2.We have investigated high capacity long-haul 16QAM coherent optical communications systems,launched numerical simulations and analyzed the parameters impacting on the performance of 16QAM signals.And a 28GBaud double-carrier PM-16QAM modulation transmitting and digital coherent receiving system has been set up,by which we realize error free transmissions at a bit rate of 448 Gbit/s.3.We have achieved digital processing algorithms dealing with coherent QPSK and 16QAM signals,including orthogonal imbalance compensation,clock recovery,polarization division demultiplex,frequency offset compensation,and carrier phase recovery et cetera.And we have proposed a least mean square equalization algorithm based on k-means clustering(KM-LMS),aiming at eliminating the influence of extinction ratio due to imperfect modulators.The experimental results indicate that comparing to the traditional algorithm based on direct decision least mean square(DD-LMS),the KM-LMS algorithm has reduced the minimum required OSNR by 2.2 dB at BER of IE-3 in transmitting signals modulated by a commercial modulator with the extinction ratio of 20dB.4.We have launched highly sensitive long-haul optic free space communication experiments with 40Gbit/s QPSK signals and obtained the sensitivity of-40.3 dBm at the BER of 1E-7,only 1.3dB deteriorated than the theoretical limit based on the presented DSP algorithm.5.We have proposed a novel modulation format named Nyquist-mPPM-LQAM modulation format which combines PPM and QAM based on Nyquist shaping.For instance,comparing to QPSK,Nyquist-4PPM-QPSK has theoretically improved the receiving sensitivity by 2.3 dB without increasing the bandwidth and experimentally improved the receiving sensitivity by 1.3dB and 1.6dB,demonstrated in coherent communication experiments at 2Gbit/s and 10Gbit/s respectively.