Design And Research Of Crystal 2×4 90° Spatial Optical Hybrid

Coherent optical communication has become a research hotspot for researchers because of its high sensitivity,strong anti-jamming ability and rich modulation methods.As an important part of the terminal of coherent optical communication system,the performance of optical hybrid has a direct impact on the receiving performance of coherent optical communication system and plays an important role in the communication system.In this paper,according to the basic composition principle of spatial optical hybrid and making comprehensive use of the propagation characteristics of light in uniaxial crystal,a 2×4 90° crystal spatial optical hybrid suitable for free space coherent communication is designed.A series of studies on the designed optical hybrid are carried out.The main work is as follows:1.According to the birefringence effect of crystal,the phase delay effect of wave plate and the principle of polarization,a 2×4 90° spatial optical hybrid is designed.It consists of two birefringent crystals and two half wave plates to realize the beam splitting,coupling and re-splitting of the signal and local oscillator beams,and a quarter wave plate produces the necessary phase shift,thus realizing the function of 2×4 90° optical hybrid.After comprehensive analysis,the structure of the crystal is designed and the structure parameters are given.2.The performance analysis and structure optimization of the designed optical hybrid are carried out.By establishing the error analysis model of the hybrid,the effects of various factors on the phase accuracy,energy uniformity and power ratio of the are analyzed respectively.In order to optimize the performance of the hybrid,a half wave plate is added to the signal optical path,and the wave plate and the quarter wave plate of the local oscillator optical path are selected to compensate and adjust the phase and power ratio of the hybrid respectively.3.Experimental study of the optical hybrid.The designed optical hybrid is fabricated,the function of the hybrid is verified by experiments,and the hybrid is applied to the coherent optical communication system for further verification.The results of theoretical analysis show that the quarter wave plate has the most severe relative phase difference and splitter ratio between the in-phase and quadrature channels of the hybrid,which is one of the most important factors affecting the performance of the hybrid.The phase and power ratio will be changed by rotating the optical axis of the quarter wave plate,while the half wave plate with the increase of the signal optical path only affects the power ratio of the two channels.The scheme can completely compensate the phase error caused by the crystal processing error,the wave plate error,the system error and so on,and readjust the power ratio.At the same time,the mixing efficiency is only reduced to 75.9%in the worst case.Therefore,the phase compensation and splitting ratio adjustment scheme is simple,effective and feasible.The experimental results show that the hybrid has the advantages of low insertion loss and adjustable splitting ratio,and can communicate stably for a long time in the coherent optical communication system.