2×4 90° Optical Hybrid Based On Birefringent Electro-optic Crystal

The free space coherent laser communication has the advantages of high receiving sensitivity,is the main way to achieve long distance,large capacity and high code rate laser communication and the dominant technique of future satellite communication.As the core device of the coherent receiving system,optical hybrid plays a major role in the reception performance of coherent communication system.According to the propagation characteristics of light wave in crystal and the effect of electric field on the polarization direction and phase of wave,a 2×4 90° optical hybrid is presented.Specific work is as follows:1.According to the electro-optic effect of crystal with double reflection and birefrigent effect when the light wave is on the crystal surface,a free space 2×4 90° optical hybrid based on birefringent electro-optic crystal is designed.It consists of a phase modulation board,a birefringent electro-optic crystal and a polarization birefringent crystal,the phase modulation board is a electro-optic crystal which obtains 90° phase shift based on electro-optic effect;the birefringent electro-optic crystal controls the polarization direction of wave based on electro-optic effect,and realizes the splitting or combining of signal light and local light based on birefringent effect;based on birefringent effect,four output from the polarization birefringent crystal give the mixed beams which are phase shifted by 90° relatively to each other.Based on the bridge,the phase modulation board and birefringent electro-optic crystal are integrated together.Based on the optical hybrid,the phase modulation board and birefringent electro-optic crystal are integrated together,the quantity of hybrid components is down to two,the integration is improved.The hybrid which is for free space coherent laser communication systems has the advantages of few components,simpler structure and low losses.2.The design of free space 2×4 90° optical hybrid based on birefringent electro-optic crystal is optimized,the conclusion is that birefringence electro-optic crystal optical axis and interface normal Angle ? scope for(35°,45°)is more appropriate.The output phase of hybrid is discussed,the result shows that the phase is compensated and the 90° phase shift can be realized by the voltage on the electro-optic crystal.The scheme of phase modulation by QWP is analyzed,the phase is compensated and the 90° phase shift can be obtained by rotating the wave plate.