The Research In The Modulation And Launch Technology Of High-speed Coherent Optical Communication

Free space optical communication becomes a new direction of optical communications because of its advantages as broad bandwidth, high bit rate, high security and good performance on anti-jamming. With the high sensitivity, long distance relay, good selectivity, communication capacity ,a variety of modulation and many other advantages, the coherent optical communication becomes a hot research from various countries' scholars and a very potential choice of the future development of space laser communication technologies.In this paper, we carried out a detailed study of coherent optical communications Launch System.We selected the signal light wavelength of 1550nm and DFB semiconductor laser in Launch System.We chose external modulation and external LiNbO₃ MZ modulator. In order to meet the high-speed, we used differential phase-shift keying (DPSK) modulation, and we carried out a detailed study of RZ-DPSK and CSRZ-DPSK modulation signal. We simulated their output time-domain diagram and spectrum diagram.Through the analysis of simulation results, we reached that the CSRZ-DPSK signal had better performance than RZ-DPSK signal, which meet the high-speed coherent optical communication launch system. In addition, we studied the drive circuit of high-speed LiNbO₃ modulator .In order to carry out coherent demodulation,coherent optical communication requires the signal light with a defined frequency and phase.This requires that laser had a very narrow spectral width and frequency stability. We used the weak CFP cavity optical feedback method to press laser narrow linewidth and stabled the laser frequency by Saturated absorption, which meet the requirements of the system specifications.