Performance Analysis Of Coherent Free Space Optical Communication System Based On Adaptive Optics

Free space optical communication?FSOC?is a wireless communication mode with light as carrier,and atmosphere as channel.For the advantages of fast transmission speed,no frequency permission,high security and flexibility,FSOC has become one of research focuses in wireless communication,and it is widely used in satellite-ground communication,satellite-satellite communication and border and coast defense.With the rising demands of high bandwidth and low cost access for“last kilometer”problem in the wireless communication,FSOC has gained more and more attentions in mobile communication.However,limited by laser power,the no-relay transmission distance of traditional FSOC system based on intensity modulation/direct detection is short.In the meanwhile,limited by the modulation mode,intensity modulation/direct detection FSOC system is difficult to satisfy the demands of high communication speed.Coherent FSOC is an effective way to improve receiver sensitivity and no relay communication distance.Distinguishing from the intensity modulation/direct detection system,coherent FSOC has stronger sensitivity,more flexible modulation,higher transmission rate and longer no relay communication distance.Thus,the coherent FSOC has become one of the most important development tendencies.As the coherent FSOC system uses the atmosphere as the transmission medium,the turbulence effect is inevitable.Due to the coherent reception mode,the influence of wavefront phase mismatch caused by atmospheric turbulence on the communication performance is much serious.Therefore,efficient channel compensation technology is a strong guarantee for the long distance coherent FSOC.Adaptive optics?AO?technology is considered as an effective way for FSOC channel compensation.It uses real-time measurement and closed-loop correction to suppress the effect of atmospheric turbulence on the performance of coherent FSOC system.AO technology has been widely used in medical imaging and space surveillance.This paper mainly pays attention to the atmospheric channel compensation techniques based on AO,particulally focusing on the performance analysis of coherent FSOC system based on AO.After introduction of AO system and basic principle of coherent FSOC,our research is performed in the following three aspects,which include wavefront sensor-less?WFSless?technology,wavefront sensing technology based on phase diversity?PD?and the experiment analysis of the Shark-Hartmman?SH?wavefront sensor.We discuss two optimization algorithms based on stochastic parallelism gradient descent?SPGD?and modified artificial fish school?MAFS?in WFSless technology,and evaluate their effects on channel compensation of the coherent FSOC system respectively.The main research works of this paper focus on the following:1.According to the problems existing in the intensity modulation/direct detection system,we expound the background and significance of the channel compensation technology in the coherent FSOC system.And then we introduce the development of the coherent FSOC system and AO technology at home and abroad.2.The basic principle of AO and its basic structure are introduced.And then the basic principles of tilt and high-order aberrations correction are given respectively.The high-order aberrations correction principle based on SH wavefront sensor and the WFSless technique is provided respectively.In addition,the principle of coherent reception of the coherent FSOC system is explained.The basic flow of heterodyne detection and homodyne detection is described respectively.The demodulation mode of heterodyne detection and homodyne detection are discussed from two aspects:synchronous mode and asynchronous mode.Combined with Kolmogorov atmospheric turbulence theory,the mathematical model of atmospheric turbulence and its effect on laser beam are given.The expression of the atmospheric space-domain characteristics is presented,which lays the necessary theoretical foundation for the following works.3.We focus on the analysis of the influence of WFSless-AO unit based on SPGD algorithm on the mixing efficiency of coherent FSOC system.Accoring to the approximate relationship between mixing efficiency and Strehl ratio,the influence of WFSless technique of the SPGD algorithm on the Strehl ratio is evaluated.Moerover,the evaluation of WFSless-AO unit based on SPGD algorithm for the performance of the coherent FSOC system is given through numerical simulations and experimental analysis.The results show that the AO unit based on SPGD can effectively improve the mixing efficiency of the coherent FSOC system under different atmospheric turbulence conditions,thus the communication performance of coherent FSOC system is upgraded.Different from SH wavefront sensor,WFSless technology based on SPGD algorithm is no wavefront sensor,simple and easy to implement.4.Aiming at low-efficiency problem of the SPGD algorithm,we analyze the effect of WFSless AO based on MAFS algorithm on the atmospheric turbulence compensation of the coherent FSOC system.The static and dynamic aberrations correction methods are discussed respectively,and the dynamic aberrations correction model is constructed.The results show that MAFS can effectively correct the wavefront distortion caused by atmospheric turbulence under different sampling time and proper visual range.In addition,the performance of the MAFS algorithm,the traditional artificial fish school algorithm,the SPGD algorithm and the SA algorithm are compared.The comparison results show that compared with the SPGD algorithm and the SA algorithm,MAFS algorithm has higher convergence speed.Furthermore,MAFS has better convergence value than traditional artificial fish school algorithm.5.A wavefront sensing technology based on PD is introduced to solve the problem of low energy efficiency of SH wavefront sensor.And we analyze the influence of AO technology based on PD wavefront sensing on bit error rate?BER?of coherent FSOC system.Under the condition of binary phase shift keying?BPSK?modulation and homodyne detection,the relationship between BER and mixing efficiency of the coherent FSOC system is given,and BER performance of the system both before and after AO correction based on PD wavefront sensing is evaluated by numerical simulations.The results show that the system BER decreases significantly after calibration.In addition,the effect of received photon number of single bit on the system BER is analyzed.The results show that when received photon number of single bit is more than 15,BER can reduce to 10^-9.6.An AO experimental system with a SH wavefront sensor,double-stage tilt correction unit and 97 element continuous surface deformation mirror is built.The turbulence suppression ability under different atmospheric temporal characteristics and space-domain characteristics is discussed.The experimental results show that AO is an effective way to improve the performance of coherent FSOC system.For same Greenwood frequency,BER reduces 1² orders of magnitude with the increase of atmospheric coherence length for the BPSK modulation and homodyne detection coherent FSOC system.Under the same atmospheric coherence length condition,the BER increases 2³ orders of magnitude with the increase of atmospheric Greenwood frequency.That means that atmospheric temporal characteristics have more effect on the performance of the coherent FSOC system than space-domain characteristics when the parameters of receiving antenna aperture and sub-aperture are determined.