A Study Of Mode Degeneration And Polarization Characteristics In Coherent Optical Communication

The coherent optical communication system has high heterodyne detection sensitivity,large communication capacity,and flexible modulation method.Researching the beam transmission characteristics in a coherent optical communication system is of great significance to improving the mixing efficiency.In practice,due to the effects of thermal effects,resonator dimensions,and optical components,the beam output by the laser cannot be perfectly coherent,and in general it is partially coherent,so this article uses partially coherent Gaussian-Schell model(GSM).The GSM model is the object of study,studying the mode variation of the beam through atmospheric turbulence in a coherent optical communication system,and analyzing the polarization characteristics of the beam through the astigmatic optical system at the receiving end.The main work is as follows:1.Based on the generalized Huygens-Fresnel principle and the incoherent mode decomposition theory,the expression of the weight factor of the partially coherent GSM beam after atmospheric turbulence is derived.The influence of the optical parameters,the turbulence parameters and the transmission distance on the weight factor and the M2-factor are numerically simulated.Using the phase-screen theory,numerical simulations of the distribution of the weight factors of beam mode after transmission of different distances is performed.The coherent optical communication system of 1km is built in the outfield,and the Gauss coincidence of two-dimensional light intensity is measured under different weather conditions.2.Based on the Collins formula,the expressions of the degree of polarization,azimuth and ellipticity of the GSM beam through the astigmatic system are derived.The efects of fractional order and astigmatism coefficients on the polarization degree,elliptical azimuth and ellipticity of the on-axis and output surfaces are numerically simulated.The effect of different astigmatism coefficients on the polarization characteristics of the beam is verified experimentally.The results show that the fundamental-mode of the partially coherent GSM beam propagating in the turbulent flow is the main component,and the maximum value of the fundamental-mode varies with the radius of the beam waist.In the downlink transmission of the slant path,the beam mode is minimally affected by turbulence.The larger the astigmatism coefficient,the faster the polarization characteristic of the on-axis point changes.Astigmatism makes the polarization characteristics in the non-focal plane change from circular symmetric distribution to elliptical symmetric distribution without changing the distribution shape of the focal plane.