Pointing Error Correction For Free Space Optical Communication Based On Adaptive Optics

With the rapid development of society and the arrival of information age,the traditional communication is unable to satisfy the requirement of people.The application of internet and big date technology give an explosive growth to the information,so the communication network and communication service needs to be improved to accommodate the development of society.Compared with the traditional communication,free space optical(FSO)communication has the advantages of large bandwidth potentiality,large information capacity,excellent confidentiality and unregulated spectrum.These advantages make FSO communication become one of the research hotspots of new communication technology.Due to the existing of atmospheric effect,FSO communication has not been popular in our daily life.The main problem is that the laser beam will be affected by random refractive-index fluctuation known as atmospheric turbulence when it is transmitted in the atmospheric channel,which will cause adverse impact on the performance of FSO communication system.Additionally,pointing errors caused by the atmospheric turbulence will also degrade the performance of FSO communication system in regard of bit error rate and coupling efficiency.Therefore,it's essential to solve this problem caused by atmospheric turbulence for the application of FSO communication technology.In order to compensate the wavefront aberration,adaptive optics(AO)has been applied to FSO communication system and it has been proved to be an effective approach to solve this problem.After reading a large number of literatures,an AO system is set up in our laboratory to complete the task of correcting pointing error.The AO system which is set up in our laboratory consist of three parts,they are wavefront detection unit,wavefront control unit and wavefront correction unit.High speed camera is used as detector to measure the real time error;computer is used as controller which is similar to the brain of the system to calculate output signal for corrector;fast steering mirror(FSM)is used as corrector to compensate pointing error caused by atmospheric turbulence.After introducing the typical proportional integral differential(PID)algorithm and fuzzy proportional integral algorithm,PI control algorithm which is an effective and commonly-used control algorithm in industry has been selected to control FSM in real time for doing experiments.The proportional coefficient is adjusted by experiment and the experimental data show that the standard deviations of pointing error reduce obviously in the condition of close loop.According to the experimental date,efforts should be made to improve the performance of PI control algorithm to adjust the condition of strong turbulence.Therefore,the proportional with proportional integral(P-PI)algorithm is proposed.When the atmospheric turbulence is strong,a larger proportional coefficient is applied to approach target position.When the pointing error is reduced to a certain range,the typical PI algorithmwith a smaller proportional coefficient is applied.The experimental result show that the rise time of P-PI control algorithm is only about half of PI control algorithm.Compare with PI control algorithm,the standard deviations of pointing error with P-PI control is smaller under the effect of sine function and pseudo random function.After analyzing the relationship between BER and pointing errors in focal plane of FSO communication system,the result shows that the small pointing error leads to a lower BER.