Study On Correction Technology Of Large Amplitude Distorted Wavefront In Free-space Coherent Optical Communication

Adaptive optics,as a real-time distorted wavefront correction technology,has been widely used in the field of wireless laser communication.The wavefront of a long-distance laser communication system will be significantly distorted owing to the infl uence of the external turbulent environment and the processing error of the optical elements.With the continuous improvement of the wavefront correction technology,there is the gradual improvement in the accuracy of wavefront correction and the actuator resolution.However,the large amplitude distorted wavefront is beyond the correction range of a single deformable mirror.To address the correction range of the deformable mirror,a series of investigations on large amplitude distorted wavefront correction technology were carried out in this study.The main contributions of this study are:(1)The distorted wavefront amplitude increases with the increase of turbulence,and the increase in distortion is possible to exceed the correction range of the deformable mirror.Uncorrected or partially corrected distorted wavefront will directly affect the communication performance of the system.Subsequently,the relationship between distorted wavefront and mixing efficiency,and the periodic influence of the distorted wavefront on the system bit error rate(BER)under different modulation modes were analyzed.(2)According to the periodic effect of distorted wavefront on BER and on the basis of PID control complete correction algorithm,the large amplitude distorted wavefront residual correction algorithm was proposed in this study.Simulation analysis of two correction algorithms were conducted to improve the performance for free space coherent optical communication system.(3)The wavefront peak to valley value,mixing efficiency,signal-to-noise ratio,coupling power,IF signal amplitude,and BER were used as evaluation indexes to correct the large amplitude distorted wavefront to verify the effectiveness of the residual correction algorithm.Compared with the complete correction algorithm,the results demonstrated that the residual correction algorithm guarantees adaptability to the external environment and improves the performance of the communication system.The results show that with the increase of Zernike coefficient,the distorted wavefront has 2? periodicity influence on BER of coherent optical communication system.For the large amplitude distorted wavefront,the residual correction method is used to replace the PID control complete correction algorithm.The residual part of the distorted wavefront located outside the 0?2? principal value interval is corrected,and the 0?2? principal value interval is reserved to ensure that the deformable mirror has a sufficient range for correction.The wavefront peak to valley value,mixing efficiency,signal-to-noise ratio,coupling power,IF signal amplitude,and BER were used as evaluation indexes to correct the large amplitude distorted wavefront in indoor(0.5m)and 1.2km,10km,and 100km experiments in outdoor fields.When the residual correction algorithm was adopted,the BER of the system was 10-11,10-6,10-5,and 10-2.When the complete correction algorithm was used,the BER was 10-10,10-3,10-3,and 10-1.Therefore,the residual algorithm is suitable for coherent optical communication systems and improves the communication performance.