| Free-space laser communication system is widely studied for its high transmission rate,strong confidentiality,strong directivity and many other characteristics which are obviously better than radio frequency wireless communication system.For the satellite-ground laser communication and the horizontal laser communication,the energy attenuation of the laser signal will be caused by the influence of atmosphere,and the wavefront distortion will occur at the same time,which will reduce the performance of the laser communication.Adaptive optics technology can compensate the optical wavefront distortion caused by atmospheric turbulence in real time and carry out high-precision correction,so as to restore the communication ability of space optical communication system.At present,the design of the adaptive system of laser communication is still based on the design method of large aperture telescope imaging optics.The received energy and bit error rate?BER?of laser communication system are evaluated by sterler-ratio?SR?,and only peak energy is considered.However,the BER of laser communication is calculated by calculating the energy integral of the whole target surface.Therefore,the SR method cannot accurately calculate the energy value of the received target surface,and therefore it is impossible to accurately predict the bit error rate of communication.Therefore,it is necessary to adopt accurate energy calculation method to analyze the influence of adaptive correction on laser communication,so as to provide theoretical basis for the design of adaptive system of laser communication.In addition,because of the real-time random changes in atmospheric turbulence,especially in horizontal communication links,the turbulence changes are more drastic.In this way,strong turbulence will occur in the communication process,leading to the inability of Shack-Hartmann wavefront sensor?S-H WFS?to detect wavefront aberration,resulting in a sharp decline in correction accuracy and communication failure.For this reason,this paper mainly focuses on the current application of adaptive systems in laser communication,the lack of evaluation criteria between correction effect and communication performance,as well as the problem of undetectability of strong turbulence wavefront.Aiming at the problem that the adaptive system cannot detect wavefront in strong turbulence,this paper proposes a hybrid detection method combining wavefront sensorless technology and S-H detector.At the beginning,the wave-front detection and correction are carried out by using the wavefront sensorless technology.In this way,the detection range of the system is increased without reducing the system bandwidth.SPGD algorithm is adopted for phase recovery without wavefront detection.Aiming at the problem of multiple iterations of SPGD algorithm for wavefront recovery,an optimization algorithm using Zernike aberration to replace the deformation mirror voltage as the disturbance object is proposed to reduce the number of iterations by two orders of magnitude.At the same time,the method of replacing the center light intensity with the annular light intensity as the performance index is put forward.In addition,the correlation between S-H WSF standard spot array and spot array under strong turbulence is proposed as the basis for switching between wavefront sensorless and S-H detection.The simulation and experimental results show that when the correlation is 0.81,the detection mode can be switched.Finally,on the basis of simulation analysis,the laboratory strong turbulence detection verification experiment is carried out.The results show that the wave-front recovery algorithm can improve the coupling efficiency from 0.012 to 0.35 and reduce the BER from 10-1 to 10-4.After entering the S-H detection,the coupling efficiency increased to 0.55 and the BER decreased to 10-6.The results show that the hybrid wavefront detection method can effectively solve the problem of insufficient detection ability of the system under strong turbulence,and provide a technical approach for the detection of the adaptive system of strong turbulence laser communication.In view of the lack of evaluation criteria for the influence of adaptive correction on communication performance in space laser communication,this paper deduces the quantitative relationship expression between system correction residual,coupling efficiency and communication bit error rate according to the power in the bucket?PIB?model,and conducts simulation analysis.For the ideal BER of 10-9 communication,for the application requirements with the BER less than 10-6,the RMS value of the correction residual is required to be less than 0.67rad,and the system bandwidth of3dB should be 1.6 times of Greenwood frequency.At the same time the adaptive correction verification experiment of 9 km horizontal turbulence was carried out,and the experimental results were almost exactly coincident with the theoretical curve derived in this paper.The average deviation of coupling efficiency was less than 0.02,and the average deviation of BER was less than 0.1 order of magnitude,which confirmed the effectiveness of the theoretical analysis and provided theoretical support for the theoretical design of the adaptive system for spatial laser communication.In order to meet the requirements of adaptive correction of laser communication on a 9km horizontal cross-sea link,the adaptive system for space laser communication was designed and adjusted,and an optical pupil imaging method for telescope based on optical fiber light source was proposed to realize the accurate measurement of exit pupil position and back intercept of the telescope.In order to solve the problem that the optical axis of the detection branch can not be determined,the method of reverse incidence of the micrometer collimating telescope is adopted.Realize the axis of the accurate calibration.Finally,laboratory and field validation experiments were carried out,and the results showed that for the static aberration of the indoor experiment,the corrected wavefront RMS value of 2.01rad decreased from0.29rad,and the bit error rate decreased from 10-5 to 10-7,by two orders of magnitude.At the same time,real-time correction of atmospheric turbulence at 40Hz frequency in is realized,after correction,the average BER decreased from 2x10-2 to 3.7x10-7,reduced the five orders of magnitude;For the adaptive correction experiment of 9km cross-sea laser communication,after correction,the wavefront residual RMS decreased from 4.68rad to 0.71rad,the coupling efficiency increased by 30%,and the bit error rate decreased from 6.7x10-3 to 2.84x10-6,reducing by three orders of magnitude.In this paper,a series of research work has been carried out to solve the problems of strong turbulence wavefront detection in space laser communication and the evaluation criteria in the design of adaptive correction system.The research results of this paper provide theoretical basis and technical support for the design of adaptive system of space laser communication,which will vigorously promote the application of adaptive optics technology in space laser communication,so as to promote the wide application of space laser communication in military and civilian fields. |
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