Research Of Modulation And Demodulation Technology In Repeater-less High Speed Optical Communication System

Transmission rate and distance are two important indicators of optical communication system.For general optical fiber communication system,relay amplification is the main technical means to extends transmission distance.But in some scenarios such as satellites and oceanic optical communication systems,amplifying equipments is not proper to use,so repeater-less transmission technology in long-haul optical communication system has become one of the hot research topic in optical communication field.First we summarized the development status of non-repeater optical communication system.Based on its loss-limited characteristic,we studied various kinds of modulation formats with high energy efficiency,and digital coherent demodulation approach and damage compensation algorithm applying to these above modulation formats.On this basis,in LEO-GEO inter satellites laser communication system,we designed a digital coherent modulation and demodulation system based on BPSK,and researched the system performance in the presence of wide-range frequency offset caused by Doppler shift effects,and phase noise caused by laser linewidth.We studied and developed frequency offset and phase noise estimation algorithms based on pilot sequences,working in super low OSNR conditions.Comparing to previous ones,new estimation algorithms has larger estimation range,higher estimation precision and less calculation quantity.On the other side,in overland repeater-less fiber communication links,we designed a digital coherent modulation and demodulation system based on PQ-4PPM,and researched the effects of accumulated CD and PMD in fiber link.And then we studied and designed CD and PMD estimation and compensation algorithm based on Golay sequences and signal demodulation algorithm,which can significantly reduce system bit error rate compared to previous algorithm.