Research On Diffractive Optical Elements For Volume Holographic Grating Recording

In recent years,as an important coupling element in a holographic optical waveguide system,volume holographic gratings(VHG)have attracted much attentions in the headwear display and near-eye display.The holographic optical waveguide modulates the beam direction via volume holographic grating to realize turning imaging,which makes the whole system simple and miniaturized.Therefore,the preparation of VHG with high diffraction efficiency is particularly important.The traditional recording method of VHG is holographic interference,which has the disadvantages of complex optical path,high environmental requirement and uncontrollable intensity of interference fringes.This paper analyzes and studies the above problems.Diffractive optical elements(DOE)can directly modulate the amplitude and phase of light waves and realize some functions that can not be completed by traditional devices.The paper proposes a method to design a DOE to modulate laser beam to generate intensity-controlled fringes.The recording path of this method is simple and the environmental requirement is not high.The emphasis is that the intensity of fringes can be controlled,which makes the fabrication of a VHG relatively convenienced.In this paper,the theoretical basis and optimization algorithm for the design of diffractive optical elements for VHG recording are detailed,and the problem of quantization of diffractive optical elements is studied.In this paper,the interference fringes with sinusoidal intensity distribution are used as the output light field.Aiming at this special output light field,the output plane is divided into three parts: the first signal region where the bright fringes is located,the second signal region where the dark fringes is located and the non-signal region.On the basis of this optimization algorithm,the small period equidistant fringe DOE,the small period non-equidistant fringe DOE and the large period equidistant fringe DOE are designed respectively.The simulation results show that compared with traditional GS algorithm,the uniformity of the fringes produced by the small period equidistant fringe DOE increases from 76.34% to 87.33%,the fringe contrast increases from 24.92 to 67.15,and the error function decreases from 8.33% to 5.69%;the uniformity of the fringes produced by the small period non-equidistant fringe DOE increases from 76.74% to86.09%,the fringe contrast increases from 23.47 to 66.95,and the error function decreases from 8.52% to 5.71%;the uniformity of the fringes produced by the large period equidistant fringe DOE increases from 80.15% to 88.06%,the fringe contrast increases from 28.42 to 68.95,and the error function decreases from 8.26% to 5.24%.The phases of large period equidistant fringe DOE designed by two algorithms are loaded on a liquid crystal spatial light modulator and verified experimentally.Both the simulation and experimental results show that the algorithm is effective.With the consideraiton that the DOE phase obtained by the optimization algorithm has to be quantized for practical fabrication,its influence on DOE optical performance is analyzed by software simulation.After that,the optimization algorithm of regional iterative multi-order quantization is proposed to reduce the processing difficulty.Compared with direct quantization,the evaluation indexes of each order quantization are obviously improved.Finally,the DOE with binary phase is fabricated and tested.