Research Of Phase Matching In Free-space Coherent Optical Communications

Over the past a dozen of years, as the earth observation satellite and remote sensing satellite are applied extensively, the mass observation data acquired by the satellite shall be transferred to the ground timely and fast. However, the near-earth orbit satellite suffers short communication time within the border while faces various limitations in setting stations beyond the border. The real-time and speedy communication link shall be established. The traditional wireless microwave communication suffers slow communication speed and unsound disturbance-resistance ability, which could hardly meet the requirements of satellite communication. What’s more, it is difficult to guarantee the safety of wireless microwave communication for the sensitive data information communication. The real-time and safe communication of mass data has become the bottleneck for the development of satellite communication.Space optical communication is the communication type taking optical signal as the information carrier and earth atmosphere or aerospace free space as the transmission channel. Compared with traditional wireless microwave communication method, free space optical communication boasts advantages of wide communication band width, large information capacity, strong disturbance resistance and anti-interception and capturing ability, small volume and quality as well as low power rate. Space optical communication could be divided into direct detection and coherent detection in accordance with the different detection methods. The advantage of direct detection is the simple structure and disadvantage is the relatively low detection sensitivity. On the contrary, the coherent detection enjoys popularity among people with its high detection sensitivity, serving as a detection model featuring great development potential.The development of space optical communication is limited by the influences exerted by atmospheric channel on transmission beam. The influences of atmospheric link on the phase front of transmission beam mainly present in two aspects. One is that it causes the phase position between signal light and local oscillator light generates random piston motion error. The other is that it causes serious distortion of wave-front phase of signal light. The influences of these two aspects make the mixing efficiency of coherent detection be extremely low and limit severely the application of space coherent communication.In terms of the random phase piston error, modulating the phase position of local oscillator light could solve out-of-step problem between signal light and the local oscillator light. As for the wave-front phase distortion error, it could adopt the method combining the adaptive optics(AO) technology with other wave-front phase compensation technique to make compensation. The working types of AO technology would be different for the optical fiber coherent detection and space coherent detection.For the optical fiber coherent detection, the major function of AO technology lies in improving the coupling efficiency of the systematic optical fiber. In the experiment, after complementing the wave-front phase distortion through AO technology, the coupling efficiency of SMF improved from 1.3% to 46.1%; the BPSK coherent communication link with 5Gbps was established in stable; and the error rate was lower than 10-9. In terms of the single coherent light communication system in the space detection, the major function of AO system is to improve the coherent mixing efficiency. After the supplementation of AO technique, the mixing efficiency increased from 1% to 30%. Furthermore, the mixing efficiency could be improved to 70% if adopting the space filter technique to supplement higher wave-front face distortion.On the basis of the theory, simulation and experiment, this paper demonstrates: the combination of AO technique and other phase compensation methods could establish speedy, safe and stable space coherent light communication link in the medium turbulent flow condition. The research work carried out by this paper is the beneficial exploration for the pragmatic utilization of coherent light communication system in the atmospheric link, which will play certain promotion functions for the application of space coherent light communication technology.