Study On Sensor-less Wave Front Correction Algorithm In Free Space Optical Communication

Free-space optical communication(FSOC)is an optical communication technology using laser as information carrier to transmit data wirelessly.As FSOC system has more flexible network and better secrecy performance,this technology is also appropriated for military use.The main problem for FSOC system is the influence from atmospheric turbulence.When laser propagates in atmosphere,random atmosphere refractive index leads to beam wandering,scattering,scintillation and power fluctuations,and affects the stability and reliability of the communication system.Therefore,how to restrain the influence of atmospheric turbulence and correct the wave front distortion more accurately has become an important topic in the research of FSOC system.Adaptive optical(AO)technology is a newly method in wave front aberrations compensating.It can monitor and correct wave front aberrations in real time and plays an increasingly important role in wave front correction systems.The emphasis of this paper is to study the correction algorithm of sensor-less AO system.According to particular optimization algorithms,the optimization methods can be classified into two kinds: model-free method and model-based method.The model-free method is most widely considered one.It aims to directly optimize performance indicators of received signals without considering wave front reconstructions.But all these algorithms require a large number of iterations or convergent time.Model-based method firstly establishes a basis function as the system model based on some principle,and then determines the corresponding system control algorithm.However,when the difference between the actual condition and the model is too large,the wave front correction error also increases.Therefore,this paper proposes a hybrid method for the wave front sensor-less adaptive optical system based on the existing algorithms.We divide the wave front aberrations into two parts,and use Martin algorithm to compensate the low-order aberrations and SPGD algorithm to compensate the high-order ones in order to improve both the compensation performance and the convergent speed of the method.In order to verify the correction effect of the hybrid method,this paper first simulates the change of phase plane during the correction and verifies with the experimental platform.In order to illustrate the advantages of the hybrid method,this paper simulates multi-group wave fronts under different distortion conditions and different turbulence conditions.The experiment simulation results indicate that the hybrid method can combine the advantages of the two methods and has faster compensating rate and better performance.Therefore,we can conclude that this hybrid method has the characteristics of fast convergence speed and good convergence performance.With this hybrid method,the wave front distortion can be correctedmore quickly and the corrected wave front can has a better Strehl ratio,which means the coupling efficiency of the correction system can be improved with faster speed and lower computational complexity.At the same time,this paper analyzes FSOC system performance when hybrid method is used in the system.We simulate bit error rate of FSOC systems with different turbulence intensities and modulation modes.The results show that the hybrid method can effectively improve the bit error rate of system,especially for the strong turbulent environment,the hybrid method can greatly reduce the bit error rate and improve the communication system performance.