Research Of Horizontal Atmospheric Layer Influence On Polarization Properties Of Circularly Polarized Laser Beam

With the advent of the information age, free-space optical(FSO) communicationshas become the focus of research not only at home but also at abroad, for its advantagessuch as high transmission code rate, strong anti-electromagnetism interference, goodperformance of secrecy, small volume and light weight. In recent years, experts andscholars have done some experiments about modulation with the polarization of laserbeam, and they expected to achieve a communication system with high data rate andlow bit error rate. Meanwhile, coherent optical communication has received wideattention due to it’s high sensitivity and large capability of transmission. However, thepolarization would change due to turbulence and scattering when laser beam propagatein atmosphere, sunch the bit error rate of the PolSK system would increase and thedetection sensitivity of coherent detection would reduce. The existing research indicatedthat the effect of atmosphere between satellite and ground on laser beam can beequivalent to horizontal atmospheric propagation about10km. So, it is important tostudy the polarization property of the laser beam propagating in horizontal atmosphere.In this thesis, circularly polarized laser beam’s polarization status changes effect byhorizontal atmosphere would be studied.First of all, Designed a polarization detection system of laser beam in real timebased on Stokes vector. System optical circuit was set up under laboratory conditions.Steady state measurement experiment and dynamic measurement experiment have beendone to verify the performance of the system. The measurement of the dynamicmeasurement experiments can reach500times per second, and can reflect thepolarization status of the incident signal light in real time.Secondly, designed a Monte Carlo simulation with the consideration ofpolarization character based on the Mie scattering theory. And reasonable simulationparameters were considered in the thesis. Before simulation, analysed the rationality ofusing H-G function approximate Mie scattering phase function. The size distributionand density of three scattering particles sunch as water molecules, atmospheric aerosoland dust were both considered in the simulation, so the model can be more perfect.Analysed the polatization property of different areas of the receiving spot andleft-handed and right-handed circularly polarized laser beam were both considered.Discussesed the effect on the polarization performance when communication distanceincrease. And discussesed the polarization performance as the distance of the receivingantenna deviate from the center of the spot meantime. Thirdly, designed a Monte Carlo simulation with the consideration of polarizationcharacter based on the atmosphere bulbbles model. The intensity of the turbulentatmosphere can be described by atmosphere refractive index structure parameterC2Analysed the effect on the polarization performance as communication distance andC_n~2increase. Analysis and comparison between left-handed and right-handed circularlypolarized laser beam were performed.In the end, the polarization characteristics of left-handed circularly polarized laserbeam were measured though horizontal atmospheric layer transmission parths under theconditions of atmospheric visibility was5km-10km andC_n~2was1.825×10-13m-2/3.The degree of polarization and the Stokes vectors at the center and edges of thereceiving spot were measured.