Research On Large Angle Beam Deflection Technology Based On Liquid Crystal Polarization Grating

The current ladar system usually adopts mechanical beam deflection technology,which can realize large field of view and high power beam scanning.However,it has the disadvantages of large volume,complex system,slow scanning speed,and non-random pointing.In recent years,with the development of basic research of micro-optical devices and the improvement of the fabrication process,non-mechanical beam deflection technology represented by liquid crystal phased arrays has become a research focus in the field of beam control.However,since the liquid crystal phased array device has an optical backhaul region,its deflection range and efficiency are limited.The liquid crystal polarization grating is an ultra-thin diffractive optical element,which can ideally reach nearly 100% diffraction efficiency and the maximum deflection angle has reached a practical level.Therefore,the large angle beam deflection technology based on liquid crystal polarization grating is studied in this paper.The main research contents is as follows:(1)Based on the light transmission matrix,systematically study the mechanism of the influence of the structure of the liquid crystal polarization grating on the diffraction characteristics.The finite difference time domain tool is used to simulate the beam deflection principle of the liquid crystal polarization grating.The results show that the finite difference time domain simulation and the analytical expression of the equation have good agreement.The ideal single-order diffraction efficiency can reach 99.6%.Secondly,the monolithic liquid crystal polarization grating is tested and the beam deflection performance is verified.The results show that 0-order light cannot be eliminated.By applying a 2V voltage to the liquid crystal polarization grating,0-order light is completely eliminated,which further improves the diffraction efficiency of ± 1 order,and the diffraction efficiency is basically stable at about 95%.(2)In order to solve the problem that it is difficult to achieve beam deflection of multiangles and a large field of view with a single liquid crystal polarization grating,multiple liquid crystal polarization gratings are designed to cascade,and two types of cascaded liquid crystal polarization gratings are introduced.Theoretical comparison analysis shows that the improved rough polarization module design has 3.71% higher diffraction efficiency than the binary rough polarization module design.Aiming at the problem that the cascade of liquid crystal deflection gratings requires multiple driving voltages,a multi-channel liquid crystal programmable controller based on MCU is designed in this paper to achieve flexible control of the beam deflection angle.The function of the controller is verified.(3)Establish a large-angle beam deflection experiment platform for liquid crystal polarization gratings.Aiming at the problem that the diffraction efficiency of light passing through the effective area of the cascaded liquid crystal polarization grating is reduced at the same position,an improved coarse polarization module design is used to construct a beam deflection experiment device.Different driving voltages are applied to measure the spot energy,and the relationship between diffraction efficiency and voltage is obtained.The experimental results show that when the voltage is in the range of 2-2.5V,the diffraction efficiency reaches the highest.Finally,using the precise voltage measured in the experiment,a one-dimensional beam scan with the angular resolution of 1°,the field of view of 12° and the system deflection efficiency of more than 70% was realized at the wavelength of 1064 nm.The research in this paper lays a foundation for the practical progress of domestic large-angle beam deflection systems based on liquid crystal polarization gratings.