| Liquid crystal polarization grating is a new kind of grating based on geometric phase principle.It operates on circularly polarized light and has Electro-optic tunability and polarization tenability.Its first-order diffraction efficiency can reach 100 % in theory.It has a wide range of applications in optical switches,laser scanning systems,filters,and polarization imaging systems.It is known as the fourth-generation optical component and has important research significance and application value in the field of photoelectricity.However,at present,there are difficulties in the preparation of liquid crystal polarization gratings based on the traditional polarization holographic method of two-beam interference optical pathways.In addition,there are also problems such as slow response speed and secondary diffraction phenomena in liquid crystal polarization gratings.This paper focuses on these problems,and finally realizes a large angle grating deflection system based on the design of liquid crystal polarization grating and liquid crystal phased array.Firstly,this paper designs the exposure path of the liquid crystal polarization grating during the Cengde interference to solve the problem that it is difficult to prepare the large periodic grating.In the study: The Mach-Zehnder interferometric exposure light path of the liquid crystal polarization grating is proposed,and the design and building of the light path is completed,which solves the problem of the difficulty of preparing large periodic polarization grating by traditional light path.After the laser beam is filtered through a small hole,it is straightened by a lens.Then the PBS(polarization beam prism)is used to divide the beam into a beam of S light and a beam of P light.The two beams of light are reflected by the mirror and then reach the other PBS to achieve the superposition of the double beams.Place a 1/2 wave before the first PBS to adjust the ratio of the two beams of light.Place a 1/4wave before the second PBS to convert two orthogonal lines of polarized light into two orthogonal circlarly polarized light.Under the second PBS,a High-precision rotating table is installed to control the angle of the polarization light of two orthogonal circles to control the period of polarization grating.The liquid crystal polarization grating with pitch as small as 3 ?m to 500 ?m has been successfully fabricated by using this Mach-Zehnder light path,which solves the difficulty of fabricating large period polarization grating by traditional light path.Secondly,in this paper,the secondary diffraction problem of liquid crystal polarization grating is studied in depth,and the reason for the appearance of secondary diffraction light is analyzed from the theoretical model and experimental reason of liquid crystal polarization grating.In the study: The theoretical model of the secondary diffraction liquid crystal polarization grating is established and the experimental reasons for the secondary diffraction of the polarization grating are analyzed.It is found that the polarization grating with secondary diffraction is not uniform in width of adjacent stripes when observed under polarizer microscope.It shows that the liquid crystal molecules are no longer Linearly arranged within a grating cycle.In this paper,a theoretical model of the nonlinear arrangement of liquid crystal molecules is presented.And through the vector Fourier transform,it is found that there is indeed a secondary diffraction light at this time.Then the curves of the optical efficiency of primary and secondary diffraction with the ratio of adjacent stripes are calculated.Subsequently,by studying the polarization holographic recording field of the liquid crystal polarization grating,it is found that when the light intensity of the two superimposed beams is unequal and the polarization state is not circular polarized light,that is,when the 1/2 wave sheet and the 1/4 wave sheet in the optical path have adjustment errors,The long axis of the polarized light field formed is no longer a linear change,and the light field-oriented liquid crystal molecule is no longer a linear arrangement.At this time,the uneven grating stripes observed under the polarizer microscope are formed.The above theoretical analysis is verified byexperiments and the secondary diffraction light of liquid crystal polarization grating is effectively suppressed by adjusting the optical path.Thirdly,in view of the slow response speed of traditional active liquid crystal polarization grating,a dual-frequency liquid crystal polarization grating based on hybrid nematic liquid crystal cell is designed in this paper.In the study: A liquid crystal polarization grating structure combining hybrid nematic liquid crystal cell with dual-frequency liquid crystal is proposed.The response speed of submillisecond can be realized at low operating voltage(10 VRMS).The hybrid nematic liquid crystal cell have the advantages of no threshold voltage and low operating voltage compared to parallel liquid crystal cell,but the phase delay of hybrid nematic liquid crystal cell is only half that of parallel cell.The use of dual-frequency liquid crystals can double the phase delay of hybrid nematic liquid crystal cell and significantly reduce the relaxation time of liquid crystal polarization gratings.For hybrid nematic dual-frequency liquid crystal polarization gratings,when a low frequency saturation voltage is applied to the liquid crystal cell,the liquid crystal molecules are aligned perpendicular to the substrate;When a high-frequency saturation voltage is applied,the liquid crystal molecules are arranged parallel to the substrate.In this way,liquid crystal molecule is also driven by the voltage during the fall of the liquid crystal molecule,which can greatly reduce the response time of the liquid crystal molecule,and the phase delay of the liquid crystal layer in the equilibrium state is the same as that of the parallel cell with a high-frequency saturation voltage applied.Therefore,the hybrid arrangement of dual-frequency liquid crystal polarization gratings can achieve submillisecond magnitude response speed at lower operating voltage.Then,based on the hybrid nematic liquid crystal polarization grating model,a two-dimensional liquid crystal polarization grating capable of diffracting a beam into a beam array is designed and realized.In the study: A hybrid arrangement model of two-dimensional liquid crystal polarization gratings is presented,and the polarization characteristics,deflection angles and diffraction efficiency formulas of the twodimensional liquid crystal polarization gratings are derived by means of Jones matrix combined with grating vector diffraction theory.The two-dimensional liquid crystal polarization grating with three different grating vector angles is realized in the laboratory.Their secondary diffraction efficiency can reach 90 %.Finally,in the research of practical system,the laser beam scanning system with high angle and high precision is realized by combining the liquid crystal polarization grating and liquid crystal phased array.Specifically: A two-dimensional non-mechanical high-precision beam deflection scanning system with large angle is proposed by combining liquid crystal polarization grating with liquid crystal phased array.Four liquid crystal polarization gratings are added together to achieve a one-dimensional beam deflection within a large angle(± 20 °).Then the same set of liquid crystal polarization gratings is placed with grating vector rotating 90°,in this way,a two-dimensional large-angle beam deflection can be achieved.In order to achieve high precision beam deflection,the polarization grating and liquid crystal phased array are connected by light path designed,and the scanning step length of the grating is filled with liquid crystal phased array.Based on this method,a beam scanning system with high scanning accuracy and ± 20 ° deflection range is realized.Through the research of this thesis,it is expected that this kind of competitive optical device can be applied in the fields of Lidar,laser communication,laser weapon and so on. |
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