Study On The Optical System Of Space Coherent Optical Communication Terminal

Laser communication is an important technology to solve the problem of high-speed communications, dues to its advantages such as high optical gain, high communication rate, high anti-interception and anti-jamming capability. Since Germany TESAT Company achieved 5.6Gbps inter-satellite link coherent optical communication rate, coherent optical communication technology has become recognized as the best solution of compact, lightweight and high-rate of the optical communication terminal. Aiming space coherent optical communication technical difficulties, this paper conducted the following studies:Firstly, In order to improve the mixing efficiency of heterodyne detection, the method that shaping Gaussian beam as local oscillator light is proposed. Mixing efficiency formulas are given for shaping and not shaping local oscillator Gaussian beam. By the numerical simulation, obtain that mixing efficiency of the shaping local oscillator laser is about 0.1 larger than not shaping, and get the best solution of each device parameters of coherent detection. Secondly, the influence of optical system of space coherent optical communication terminal on SNR is studied. Numerical simulation of the influence of optical antenna wave front error, 90 degree optical hybrid beam offset, beam tilt and relative phase shift error was done, the simulation result show: the tolerance of the four errors are λ/20, R0/20(R0 is the width of signal beam), 0.1mrad and ±10°. Thirdly, optical antenna system index requirements of the space coherent optical communication terminal were presented, and a reasonable optical antenna system was designed according to the index. The off-axis forms and coaxial form design features were compared when designing the telescope. Fourth, the principle of 90° optical hybrid was derived using Jones matrix. Established the principle model of 90° optical hybrid by Optisystem software, analyzed the indicators error of relative phase shift. The results show that: by rotating λ/4 wave plate can compensate relative phase shift error, but will result in I, Q branch splitting ratio changes, by rotating the λ/2 wave plate splitting ratio will be compensated. On the basis of analysis of the engineering shortcomings of the original 90°space optical hybrid structure, two improved structures based on polarization lateral shearing interferometer have been proposed. Fifth, a free space 90° optical hybrid has been designed and fabricated, and the experimental platform have been built to detect the optical hybrid’s index: insert losses, power uniformity, relative phase shift and mixing efficiency. The detection experiment show: the optical hybrid’s relative phase shift between I and Q is 90.73°, the insertion Losses from signal and LO are 0.58 dB, 0.53 dB, and the loss of mixing efficiency is less than 0.82 dB, and all meet the requirements.