Some Applications Of DSP Algorithm To Coherent Optical Communications System

Coherent optical communications technology is the key enabler for the next generation optical backbone networks. Through beating between the signal and local oscillator, coherent detection theoretically is able to obtain the intact information of the incoming optical signal. Moreover, coherent detection enabled by digital signal processing can perform fiber impairment compensation and channel equalization conveniently at the receivers. Thus, the applications of high-order modulation format and polarization division multiplexing (PDM) become possible to provide higher system capacity and longer transmission distance than the traditional TDM and WDM systems owing to the higher spectral efficiency. Coherent optical communications have no additional requirement on the fiber so that the fiber links implemented in the past decades will serve just fine. The signal distortion caused by fiber impairments such as chromatic dispersion, polarization mode dispersion as well as the laser phase noise can be compensated with minimal power penalty. Starts from the introduction of the principle of optical coherent detection, this thesis describes the transmitter, fiber channel, the coherent receiver and DSP of coherent optical communications system in sequence.In this thesis, a novel high-order modulation format generation method is proposed. The proposed method requires lower levels of electrical signal in the transmitter compared to the traditional ones and generates minimal modulation chirp. To understand the coherent receiver, a new perspective of equivalent decomposition coordinate is proposed and based on which the orthogonalization algorithm is presented.This thesis presents two novel adaptive chromatic dispersion estimation and compensation algorithms based on delay-tap sampling (DTS) technique and timing error detection (TED), respectively. In comparison with others, the DTS based algorithm provides larger scanning steps and comparable estimation accuracy while the TED based algorithm requires less symbols to complete the estimation and also provides better estimations. The relationship between the two proposed algorithm and the "nature express" algorithm is given. In addition, a new algorithm applicable to the Nyquist system is proposed and investigated via simulations. This thesis reviews the symbol timing phase recovery algorithms used in optical coherent receivers. The fundamental correspondence between the Gardner’s and Godard’s TED, and between Lee’s and Godard’s TED is established. In addition, the performance of TPR algorithms in the Nyquist system is investigated via simulations.This thesis reviews the polarization demultiplexing algorithms and proposed two algorithms that have the ability of joint equalization. Although they will suffer from the laser frequency offset in the system, the self-homodyne passive optical network experiment demonstrates that the modified CMA can perform the joint polarization demultiplexing and carrier phase recovery with negligible OSNR penalty.