Experimental Study On Non Common Path Aberration Calibration Of Coherent Optical Communication System

Free space optical communication uses laser signal as the carrier,which provides a line of sight,wireless and high bandwidth communication link between remote stations.In free space coherent optical communication,atmospheric turbulence makes the optical signal produce phase fluctuation,which seriously affects the reliability of the communication link.As one of the most promising technologies to suppress atmospheric turbulence,adaptive optics has been widely used in free space laser communication,astronomical imaging,beam shaping and other fields.Adaptive optics technology has achieved great success in many fields,but it still needs to be studied in eliminating system errors.The optimization of adaptive optics system is very important to improve the performance of coherent optical communication system.In this paper,the experimental study of coherent optical communication with non common path aberration calibration is carried out.The main work of this paper is as follows:1.According to the principle of coherent optical communication system,the analytical expression of intermediate frequency current signal was derived.The components and working principle of the adaptive optics system were described.The error classification and how to eliminate the influence of the error in the adaptive optics system were introduced.2.Aiming at the problem that non common path aberrations affect the quality of communication system,Stochastic Parallel Gradient Descent(SPGD)algorithm was used to calibrate non common path aberrations.The source of non common path aberration was explained,and the process of transforming non common path aberration into reference point information of wavefront sensor was analyzed.The indoor experimental system was designed with the optical power coupled into the single-mode fiber and the intermediate frequency voltage of the coherent receiving system as the evaluation indexes.The experimental results showed that the optical power coupled into the single-mode fiber could be increased by 12 dBm and the intermediate frequency voltage could be increased by 800 mV after non common optical path aberration calibration.3.The experimental platform of coherent optical communication system was built to complete the field experiment.The experimental results showed that the non common path aberration calibrated adaptive optics system can effectively correct the wavefront distortion of the beam.In cloudy,sunny and rainy days,when the adaptive optics system was closed-loop,the optical power coupled into the single-mode fiber increased from-41.54 dBm,-44.20 dBm and-43.72 dBm to-30.03 dBm,-33.41 dBm and-34.60 dBm,respectively;The peak to peak value of intermediate frequency voltage increased from 200 mV,170 mV and 50 mV to 640 mV,380 mV and 260 mV respectively;The coherent receiving gain was increased by 20.3 dB/MHz,13.4 dB/MHz and 10.05 dB/MHz,respectively.