Research On GS Phase Retrieval Algorithm For Laser Beam Two-dimensional Deflection

Spatial light modulators play an important role in the field of modern optics that can achieve phase modulation and amplitude modulation of light waves.These modulators can also obtain high light energy utilization and high diffraction efficiency,have the advantages of phase flexibility and real-time performance,and are widely used in the fields of spatial filtering,image processing,digital holography.In order to improve the scanning range and the scanning accuracy of the optical communication system,making the space optical communication terminal has the ability of multi-user instant communication;in order to reduce the structural complexity of the communication system and make the operation simple and the calculation accurate,the thesis will use a liquid crystal two-dimensional spatial light modulator to achieve two-dimensional deflection of multiple beams.Based on the two-dimensional scalar diffraction method,an improved GS algorithm is used to achieve independent control of multiple beams and analyze the diffraction efficiency,root mean square error and the deflection accuracy of the beam,which provided a guarantee for the multi-user instant communication of space optical communication.The specific research contents are as follows:(1)The physical properties of liquid crystal are introduced.According to the theory of liquid crystal elastic body,the distribution of the direction vector under different applied electric field is analyzed.Based on the electro-optic birefringence effect of liquid crystals,the variation of the optical path difference of the incident beam under different voltages and liquid crystal box thicknesses is analyzed.Based on the basic diffraction theory of light,the structural characteristics of liquid crystal spatial light modulation are analyzed.(2)Based on the liquid crystal phased array model,the theory of one-dimensional deflection of single beam using periodic grating method and non-periodic grating method is summarized.A general solution based on the liquid crystal spatial light modulator using an appropriate two-dimensional phase to directly realize the two-dimensional deflection of a single beam is proposed.Based on the theory of microwave phased array,the subaperture method and cross subarray method are proposed to realize the twodimensional deflection of multiple beams,and the two-dimensional phase structure model is established.The multi-beam deflection is achieved through the sub-aperture method,and the two-beam,three-beam and four-beam defletion is achieved through computer optimization processing.It is obtained that when the sub-aperture weight vectors are different,the light intensity of the generated beam is also different.The uniform cross subarray method and the non-uniform cross subarray method are used to analyze the generation of multiple beams.Finally,the phase distribution and beam distribution obtained by the sub-aperture method and the cross subarray method are compared and it is concluded that the beam obtained by the cross sub-array method has higher resolution.(3)According to the laser beam shaping method of the liquid crystal spatial light modulator,an improved GS algorithm is proposed to achieve the beam deflection,so that the energy center of the outgoing beam is offset from the incident center direction and concentrated in one or several points in the far field.The characteristic of this algorithm is that the multi-beam generated by interference on the image surface is used as the target beam.The two-dimensional phase distribution on the input surface is calculated by the algorithm.The Gaussian parallel beam passes through the diffraction plane to reconstruct the wavefront and generate multiple beam spots with controlled intensity and controlled angle.The results show that the far-field double-beam using the improved GS algorithm has a two-dimensional radial deflection angle of 0° to 1.21°,and the diffraction efficiency and root mean square error fluctuate in the range of 0.5 ~ 0.92 and 0.0046 ~ 0.019,respectively.This result is significantly improved compared to the original GS algorithm.(4)Based on the liquid crystal spatial light modulator,an improved GS algorithm is used to verify the deflection of multiple beams.By analyzing the deflection errors of the two-beam and four-beam at different deflection angles,it is concluded that as the radial deflection angle increases,the deflection error of the two-beam gradually increases.When the double beam is deflected within the range of 0 ° ~ 1.1 °,the deflection error fluctuates within 0 ~ 0.003 mrad.Finally,the controllability and effectiveness of the improved GS algorithm are verified through experiments.