| Optical coherent communication systems, based on high-order modulation format, coherent detection and digital signal processing, has been considered as a promising technology for next generation high-speed optical communication systems due to its ability for achieving high spectral efficiency and compensation for impairments in electric field. In this thesis, we conduct theoretical and experimental investigation on the high-speed optical coherent communication systems based on dual polarization16ary quadrature amplitude modulation (DP-16QAM) modulation format, coherent detection and digital signal processing. This work is jointly supported by National High Technologies Development Program (863)"Key technologies in160Gb/s optical transmission systems using multi-fiber pump" and Canada Natural Sciences and Engineering Research Council strategic project grants "Digital signal processing for fiber-optic communications". The main contributions of this thesis are listed as follows:1. An improved4-level signal generation method is proposed. High quality4-level signal at baud-rates of14Gbaud/s,28Gbaud/s,32Gbaud/s and40Gbaud/s are experimentally demonstrated. With the improved4-level signal generation, Only2.7dB and3.5dB OSNR penalty for the112Gb/s and the224Gb/s DP-16QAM transmitter are investigated, respectively. The performance of a112Gb/s DP-16QAM transmission system is experimentally investigated. The optimal launch power is-3.5dBm. With FEC threshold of3.8×10-3, a maximum transmission distance of1470km is demonstrated for112Gb/s DP-16QAM signal only with linear impairments compensation.2. A complete set of digital signal processing algorithms for DP-16QAM modulation format is developed. Including IQ imbalanced compensation, resampling, fix frequency domain equalizer for dispersion compensation, digital square and filtering clock recovery, polarization recovery and adaptive equalization, carrier frequency offset estimation and compensation, carrier phase noise estimation and compensation, symbol decision and bit error rate calculation. The performance of the digital signal processing algorithms is verified both by simulation and experimental data. The performance of Gram-Schmidt orthogonalization procedure based IQ imbalance compensation algorithm is first experimentally demonstrated in DP-16QAM system.3. Digital back-propagation (DBP) based fiber nonlinearity compensation is numerically and experimentally studied in long-haul DP-16QAM transmission systems. The scaling factor and step-size of DBP are fully studied both numerically and experimentally. The results show that the scaling factor is insensitivity to step-size, launch power and transmission distance. Considering the balance between computation complexity and performance, the optimal step-size of DBP is1span/step. Using DBP with a step size of1step/span, a2dB improvement of optimal launch power and a maximum transmission distance of2400km is experimentally demonstrated for a112Gb/s DP-16QAM transmission system, which improves the maximum transmission distance by63%compared to the case only with CD compensation.4. A modified QPSK partitioning carrier phase estimation (CPE) algorithm is proposed. Combined with the QPSK partitioning algorithm, simplified QPSK partitioning algorithm or blind phase search (BPS) algorithm, three novel linewidth tolerant, low-complexity two-stage carrier phase estimation algorithms for the16-QAM modulation format are proposed. The linewidth tolerance, estimation performance, linear and nonlinear transmission performance is comparable to the BPS algorithm. Reductions in the hardware complexity by factors of about1.5-10are achieved in comparison to the BPS algorithm.5. In order to further reduce the complexity of CPE algorithms, a quasi-multiplier free CPE algorithm for16QAM modulation format is proposed. Combined with simplified QPSK partitioning algorithm, a linewidth-tolerant, low-complexity two-stage feed-forward CPE algorithm is introduced for DP16-QAM. Numeral and experimental results show that the proposed algorithm achieves comparable performance to the BPS algorithm but with reduction in the hardware complexity by a factor of17. To our best knowledge, this is the lowest hardware complexity feed-forward two-stage CPE algorithm for DP-16QAM reported so far.6. Linear interpolation is introduced to improving the performance of carrier phase compensation. Linear phase interpolation (LPI) based carrier phase compensation algorithm is proposed. Numeral and experimental results show that the performance of LPI algorithms are better than the block averaging based algorithms, and are comparable to the sliding window based algorithms. However, significant reduction in the hardware complexity is demonstrated for LPI based carrier phase compensation algorithms in comparison to sliding window based algorithms. |
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