Study On Wave Front Aberration Correction Algorithm For Wave Front Sensor-less Free Space Optical Communication Systems

The rapid development of society and technology has promoted an increasing demand for the speed and the quality of communications,and the expansion and upgrade of widely used wireless communication network technologies have been restricted due to the limited spectrum resources that can be allocated.At the same time,with the characteristic of no-application for spectrum,free-space optical communication(FSOC)technology has attracted much attention.Free-space optical communication is a communication technology with the atmosphere as the transmission medium and the light beam as the source.With the advantages of simple laying,high transmission rate,large transmission capacity and high security,FSOC has applied to interstellar communications,satellite communications and near-earth communications,which can also be applied to areas of mountains and rivers that are not suitable for laying electric cables.At present,it has been widely used in military and astronomical fields.However,the atmosphere attenuation has effect on the beam with information,particularly common atmospheric turbulence,which can make the wave front of the beam to be distorted in amplitude and phase,which will result in loss of beam energy.Further,with the interference of atmospheric turbulence,the fiber coupling rate at the receiver of the communication system is reduced,the bit error rate is increased,and the communication quality may not be guaranteed.Therefore,it is of great significance to study how to eliminate wave front aberration caused by disturbances such as atmospheric turbulence.At present,adaptive optics technology which is widely used in the fields of biomedical and astronomical imaging,works best in correcting distortion of free-space optical communication systems.The main principle of adaptive optics technology to correct distortion as follows: first,the wave front sensor or detector captures the wave front information of the beam;then,according to the correction algorithm and phase conjugation principle,the controller calculates the control voltage which corresponds to the desired compensating phase;and last,adding the control voltage to the corrector to compensate wave front distortion,which makes the compensated wave front phase plane tends to flat.By breaking the limitation of the number of sub-apertures of wave front sensors and high cost,the sensor-less adaptive optics technologies achieve a lot of attention than the traditional adaptive optics technology.As a result,our research mainly focuses on the sensor-less adaptive optics technology.In this technology,the correction algorithm plays an important role,and affects the correction result and performance of the wave front distortion.Therefore,a novel correction algorithm with good correction performance is worth proposing and studying.Through the study of two typical algorithms,Jacopo Antonello(JA)method with fast convergence but less iteration and Stochastic Parallel Gradient Descent(SPGD)algorithm with many practical applications but slow convergence speed and easy to fall into local extreme value,we propose a sensor-less hybrid correction algorithm based on the idea of combing the advantages of both and avoid the disadvantages.This algorithm compensates the low order aberration of wave front by JA method,while SPGD algorithm compensates the high order aberration and the residual low order aberration after correction by the JA method.The simulation results not only verify the convergence and validity of the sensor-less hybrid correction algorithm,but also draw the conclusion that the correction result of the algorithm is better than JA method and the convergence speed of the correction curve is faster than SPGD algorithm.The faster the convergence speed of the correction algorithm and the better the correction result,the more it meets the requirements of real-time and reliability of the free-space optical communication system.Therefore,we have improved the hybrid algorithm by replacing the SPGD algorithm for correcting high order wave front distortion with the simulated annealing algorithm,which is introduced into the adaptive optics system in recent years.The computer simulation experiments also prove that the convergence speed of the improved hybrid correction algorithm is significantly improved.