Research On Two-dimensional Exit Pupil Expansion Of Polarization Volume Grating Holographic Waveguide System

Head-mounted display devices are currently a hot spot in the field of augmented reality.Field of view and exit pupil size are two important parameters of head-mounted display devices.Due to the limitation of the Lagrangian invariant,the traditional visual system cannot achieve the maximum value of both at the same time.The optical waveguide technology can design the two independently due to its unique replication and propagation process.It is necessary to consider which coupling element to choose to realize an optical waveguide with excellent performance.Polarization volume grating has become an optical device that has attracted much attention in recent years due to its superior wavelength and angular bandwidth and unique polarization characteristics.Using it as a coupling element of an optical waveguide can realize a helmet display device with a large field of view and excellent color expression.However,the current research on the polarization volumec grating mainly focuses on applying it to the waveguide structure of the one-dimensional exit pupil,and the exit pupil size is still relatively small.Therefore,it is necessary to conduct in-depth research on the design points of the two-dimensional pupil expanded waveguide structure based on the polarization volume grating.This article starts from the design of the two-dimensional exit pupil expansion waveguide structure based on the polarization volume grating and the experimental verification,and the two-dimensional exit pupil expansion waveguide system is actually built to observe the imaging quality.The details are as follows:(1)A waveguide structure with a two-dimensional pupil expansion function was analyzed and designed,and a simulation model was established to verify the correctness of the designed structure.First,the specific parameters of each grating in the two-dimensional pupil expansion waveguide structure are studied and designed,and then the deflection angle of the intermediate grating when the total light efficiency is the highest is explored,and the problem of exit pupil continuity is analyzed to satisfy the exit pupil.The relationship between the continuous grating size and the distance between the gratings should be satisfied,and the key factors affecting the imaging quality of the two-dimensional pupil expansion system are summarized according to the simulated MTF curve.And use the established tracing model to perform imaging simulation to verify the correctness of the designed two-dimensional pupil expansion structure.Then,based on the defects of the red,green and blue monochrome imaging simulation results,a method for optimizing imaging was proposed from the shape of the polarization volume grating and the uniformity of the image,and the imaging simulation was verified.In addition,the color imaging effect of the two-dimensional pupil expansion waveguide display structure was simulated.(2)A waveguide structure with a two-dimensional pupil expansion function is prepared.First of all,experimental research is carried out from the selection of orientation layer materials,the selection of solvents,the solid-liquid ratio of the solution and the selection of environmental humidity.Then the optical path and the function of the optical instrument used in the experiment were introduced.Afterwards,the preparation process of the liquid crystal solution and the preparation process of the liquid crystal layer were experimentally explored.Finally,the actual measured diffraction efficiency is compared with the simulation result,which shows the correctness of the preparation method.(3)From the application point of view,a holographic optical waveguide display prototype with two-dimensional pupil expansion function was built.The monochromatic waveguide system with three colors of red,green and blue has been researched and prepared successively,and the imaging effect has been tested with a prototype.The imaging results show that the prototype can achieve an exit pupil size of 18mm×18mm,a field of view of 30°,and the optical efficiency of the three monochromatic images are 65.5 nit/lm,67.57 nit/lm and 57.90 nit/lm,respectively.The transmittance of ambient light to the outside world is 74%.From this result,it can also be seen that the two-dimensional pupil enlargement application of the holographic waveguide based on the polarization volume grating has excellent optical coupling function in the imaging system,and has great application prospects.Furthermore,the waveguide structure capable of color imaging with two-dimensional pupil expansion is further analyzed,and a double-layer two-dimensional pupil expansion waveguide structure that takes into account the volume and imaging effect is designed.In view of the small size of the exit pupil of the traditional visual system and combined with the current application status of the polarization volume holographic grating,this thesis extends the application of the polarization volume holographic grating to the two-dimensional exit pupil expansion waveguide structure.And through a series of simulations and experiments,the pupil expansion effect and excellent imaging quality of the two-dimensional pupil expansion waveguide structure based on the polarization volume grating are verified.