Light Energy Tracing And Imaging Analysis Of Holographic Waveguide Display

After two decades of development since the first presentation of the augmented reality display technology in the 1990s,it gradually shows great application potential.Holographic waveguide display,as one of the main solutions to achieve augmented reality,has also received extensive attention and research.On the one hand,the holographic waveguide display product's light efficiency is an important indicator to measure its comprehensive performance.Due to the introduction of VHGs and slab waveguides,the calculation of light efficiency is more complicated than traditional display device.In addition,the light efficiency of current products is estimated by the overall energy utilization rate,which cannot characterize the brightness of the image seen by eye in each field of view.So it is urgent to establish a model to calculate the brightness of the image in different directions when input one luminous flux.On the other hand,a complete holographic waveguide display system includes five parts:a micro-display image source,a collimation system,volume holographic gratings(VHGs),a plate waveguide,and the human eye.The existing optical design software cannot simulate the imaging effect of the whole system.Therefore,there is an urgent need to improve the current optical design software.The content of this article mainly focuse on the above two urgent needs,including the following three parts:(1)A light energy tracing model for holographic waveguide display is established.According to the propagation process of light,the effects of collimation system,entrance and exit VHG and eye pupil on the beam energy is analyzed and the energy loss caused by each part is modeled.Then combining the brightness of image source,the brightness in different fields of view is calculated.The input parameters required by the model include the size of the waveguide plate,the luminous flux and relative spectrum of the micro-image source,the diffraction efficiency of VHGs,the size and position of the pupil,etc.The output results include the energy loss in each field of view and the image brightness when the unit luminous flux is input.Finally,the brightness of the image at different field of view was measured experimentally,which verified the accuracy of the model within a certain error range.(2)Implementation and optimization of the VHG components in ZEMAX.The article detailly introduces the method of using dynamic link library to realize VHG surface and arbitrary polygonal VHG objects in sequential mode and non-sequential mode of ZEMAX,which solves the problem that ZEMAX can not simulate the diffraction efficiency of VHG and any shape VHG.Then the paper puts forward the idea of using Monte Carlo method to optimize VHG,and elaborate on its implementation process in ZEMAX.This method avoids the simulation method based on light splitting,and solves the problems of slow speed and low accuracy when the number of input rays is large.When the number of rays emitted by the light source is 5 million and the minimum relative ray intensity of the trace is 10^-9,the simulation results show that the simulation speed after optimization is increased by more than five times compared with that before optimization,and the energy loss caused by threshold is also significantly reduced.(3)Analysis and optimization of holographic waveguide display imaging quality.The improved ZEMAX software is used to simulate the imaging effects of the one-dimensional pupil expansion structure and the two-dimensional pupil expansion structure.The dark streaks and uneven color distribution in the simulation image are analyzed in detail.In order to further expand the FOV,the article also simulates the holographic waveguide display system with a multiple composite grating and a multilayer glass substrate.Through the powerful MTF analysis function of ZEMAX,the key factors affecting the MTF are summarized.Finally,according to the defects in the simulation results,the optimization methods from three aspects:shape of VHGs,energy utilization rate and image uniformity are proposed.Aiming at the light efficiency and imaging quality of holographic waveguide display,this paper proposes a method to quickly calculate the system light effect and simulate the imaging effect.The calculation results of the model and the simulation results of the software can quickly evaluate the influence of various factors on the system and provide guidance for experimental optimization.