The Study On The Fabrication Of Active Liquid Crystal Polarization Grating

We define an electric control optical element,in which the director distribution of the liquid crystal molecule rotates 180° along the planar substrates,called the active liquid crystal polarization grating(LCPG).The LCPG,which used in optical imaging system,laser communication system,laser radar system and so on,has the advantages of ease of processing and rapid prototyping,and the diffraction efficient of 100 percent.However,in the country there is no report on the fabrication of active LCPG.Thus,it is necessary to invest the massive man-power and resource to fill the technology gap.Responding to that demand,this dissertation develops the study on the fabrication of active LCPG.The thesis explore the internal reason for low diffraction efficiency of LCPG and attribute to the alignment of liquid crystal molecule with defect.In summary,there are three viewpoints.First,the paper is concerned with the effect of the critical thickness on the thickness of liquid crystal cells.We define the critical thickness is the maximum thickness which can ensure all the liquid crystal molecules align the orientation without defect.The established conclusion that the critical thickness exists can be concluded from the experimental results.Next,the constraint conditions of critical thickness under strong anchoring are quantitatively studied.It provides theoretical analysis and basis for the definition of the upper limit of thickness in the preparation of liquid crystal cells.This conclusion explained the reason for poor products in the fabrication of small grating period.Second,the thickness error can not be eliminated in the preparation of liquid crystal cells.Therefore,the paper quantitatively studied the error on the diffraction efficiency.To compensate the error,a series of nematic mixed liquid crystals with increasing birefringence in turn are proposed.Then,we modulate the birefringence by replacing liquid crystal materials and applying the voltage,matching between optical delay and the cell thickness.In this way,diffraction efficiency can be raised to 94.4%.But this leads to a new problem: for one LCPG,the optimal fine-tuning voltage of different positions differences.Third,the above problem will greatly reduce the application and practicability of active LCPGs.Therefore,the effect of imperfect recording lights on the diffraction efficiency is discussed in the paper.It is found that the matching relationship between diffraction efficiency and liquid crystal cell thickness changes when recording lights are orthogonal circular polarized lights with different intensity.Based on this,the solutions for Gaussian beam shaping and expanding the aperture are proposed.In addition to exploring the causes and solutions of low diffraction efficiency,the theoretical interpretation with diffraction efficiency of 100%,large steering angle and modulating polarization stage is also discussed in the paper.Besides,the fabrication process of active LCPG is given in detail.And a series of preparation criteria are put forward.Also,a set of simple and effective methods and standards for evaluating environment vibration are designed,and the visualization of weak vibration is realized.The research conclusions presented in this dissertation lay a solid foundation for the practical process of active liquid crystal polarization grating.