Research On Dispersion Characteristics In Coherent Optical Communication System

With the rapidly increasing requirement of information, IM-DD systems will belimited by the limited transmission window, chromatic dispersion (CD) andpolarization mode dispersion (PMD) of the fiber. Coherent optical communicationsystem is a promising way to handle these problems. The multilevel modulation, asone character of coherent system, enables one symbol to carry several bits’information. This modulation format will increase the spectral efficiency and thedispersion tolerance. Therefore, coherent optical communication system can transmita large volume of information at relatively low speed. However, transmissionimpairments including CD, PMD and nonlinear phase noise will degenerate theperformance of coherent optical communication system too. In this thesis, we analyzethe impact of the transmitters, channels and receivers of coherent systems onchromatic dispersion, and the work is summarized as follows:Spectral efficiency and dispersion tolerance of different modulation formats aresimulated via commercial software OptiSystem. We find that duty circle of the pulsescan affect the spectral efficiency too.A novel approach of generating duty-circle tunable optical pulse is proposed. Thescheme exploits an SOA-based fiber ring to form an actively mode-locked fiber laser.The laser can produce high-repetition-rate pulses with tunable duty circle, spectralwidth and center wavelength.The possibilities of using the normal dispersion compensation methods incoherent system are analyzed. The performance of the DCF and the CFBG approachesin DPSK system is compared via simulation. The reason of lower compensationperformance of CFBG is analyzed by MATLAB.We design a digital filter based on LMS to equalize the impairment such as CDand PMD and discuss its performances. We use this filter to compensate CD and PMDof40Gbit/s NRZ-BPSK transmission over120-km SMF, and BER is0.0036.