Study On Compensation For Distorted Wavefront In Free Space Optical Communication

The growth of global communication service demands increase of communication capacity,thus valuable spectrum resource has become more and more scarce.The expansion of the spectrum to higher frequencies can meet this demand.With high communication frequency,wide frequency band,high transmission rate,and excellent confidentiality,optical communication has been one of the significant and effective communication technologies.Recently free space optical communication(FSOC)with abundant frequency resource and flexible network plays an important role in satellite communication,emergency communication and special use access networks,etc.However,optical signals in FSOC system will propagate in atmosphere and be disturbed by the atmospheric turbulence which perturbs the phase of the optical signal and causes aberrations.The atmospheric turbulence results in wavefront distortion which increases bit error rate and degrades communication quality.Therefore,the problem about wavefront distortion caused by atmospheric turbulence is always an important research topic.Adaptive optics(AO)is an effective way to compensate wavefront distortion caused by atmospheric turbulence.When AO system works,data of the wavefront are detected by the wavefront sensor then transmitted to the wavefront controller which uses certain algorithm to calculate the voltages applied on the wavefront corrector.At last the wavefront corrector produces the conjugate wavefront of the previous distorted wavefront to counteract the previous distortions.The key point of this paper is to study an algorithm that controls the wavefront corrector.Having summarized the existing algorithms that compensate the distorted wavefront in AO system,we propose a Hybrid Compensation(HC)algorithm which combines the Direct-gradient(DG)algorithm and Stochastic Parallel Gradient Descent(SPGD)algorithm to better compensate the distortions.The DG algorithm needs no wave-front reconstruction and has little computation pressure,but it is relatively incapable of correcting high-order aberrations,while SPGD algorithm needs no detections of wavefront and has simple procedures,but its speed of correcting seriously distorted wavefront is low.Therefore,we put them together by using the DG module to compensate the lower orders of aberrations and the SPGD module to compensate the higher ones in order to improve overall effectiveness,reduce iterations and increase convergence rate.In order to investigate the performance of HC using Zernike polynomials,this paper present the simulation of wavefront in different orders and levels of turbulence for the HC scheme with Strehl Ratio(SR)as the performance metric.Our simulation results show that the HC scheme can better take the advantages from both the DG and the SPGD algorithm to reach higher SR and require much less iterations to attain the same SR by the two algorithms separately,especially for the seriously distorted wavefront.Thus the proposed HCscheme can obviously let more light power be coupled into the fiber,which will definitely reduce the bit error rate and improve communication quality of FSOC system.Meanwhile,this paper conducts the experiments of the AO system based on our established platform.This work includes the establishment and calibration of the system,and real compensation of the distorted wavefront.The experiment results show that our constructed AO system can make the Peak-Valley value of the wavefront less than 1?m for the distorted wavefront.The AO system shows its effectiveness well and has made a firm foundation for the realization of a real FSOC system.