Research On Design And Imaging Features Of Diffractive Waveguide For Near-Eye Display

Near-eye display(NED)system is the most popular product in personal wearable devices in the future and one of the key research directions in the field of electronic display devices in recent years.This display technique has great application prospect in military,entertainment,education,medical and other fields.In accordance with the development trend of small optical display equipment,the NED equipment also put forward high requirement for the light weight and miniaturization.Particularly,Augmented Reality(AR)based on smart glasses have gained much attention.Diffractive waveguide based on surface relief grating(SRG)to realize AR for smart glasses is recognized as the most significant and comprehensive feasible scheme.This method has certain advantages in reducing size,providing a large eye box,and realizing mass production.However,the diffractive waveguide based on SRG still has many problems to be solved,including:(1)The diffraction efficiency is low and unstable in the field of view(FOV)in coupling process,and the utilization of the optical engine needs to be improved;(2)Dispersion caused by diffraction in colorful imaging inevitably exists,which makes RGB color ratio uneven among the eye-box.These problems will eventually lead to great unevenness in the brightness and color display after the exit-pupil expansion,which is also not conducive to the visual experience.To solve the above problems,this paper studies a colorful NED based on diffractive waveguide,which can achieve high brightness uniformity and color uniformity at one-dimensional exit-pupil expansion.In this work,two separate waveguides have been used to transport lights.The upper waveguide mainly transports both green and blue lights,while the lower one transmits red light.Particularly,metagratings optimized by neural network are placed on the waveguides and act as grating couplers in our design.The red grating coupler has high diffraction efficiency and good angular stability in the range of horizontal FOV.The green and blue grating coupler is beneficial to reduce the chromatic dispersion by providing a same diffraction angle.In the out-coupling part,the uniform light intensity is realized by adjusting the transmission of out-coupler.To further verify the effectiveness of the coupler metagrating and the uniformity in both brightness and color for one-dimensional exit-pupil expansion in the waveguides system,ray-tracing simulation and analysis in three colors were implemented.Results show that the system can achieve brightness and color uniformity across the diagonal FOV=24~°at exit-pupil expansion and maintain the continuity of light beam.By calculating the ambient contrast ratio of the system,the feasibility of presenting relatively high-quality virtual images is verified.At the same time,imaging simulation is carried out with the simplified lens.The design starts from achieving the uniformity of exit-pupil expansion and focuses on improving the performance of system.The exploration and research of diffraction waveguide provides a feasible way to colorful and miniaturized NED,and lays a foundation for sample production and test system construction.