Research On Optical System Of See-through Near-Eye Display Based On Waveguide Technology

The system of see-through waveguide near-eye display is a compact dual-channel visual optical system combining a microdisplay,an imaging eyepiece,and a waveguide,and has shown great application value and potential in the military and civilian fields.Users can observe the real environment of the outside world through the waveguide while observing the virtual information provided by the microdisplay,and can obtain the virtual display information in real time and effectively under special circumstances.Optical waveguide technology can deflect the image by 180°,which greatly simplifies the optical path.The exit pupil of the eyepiece can also be expanded,so that the eyepiece with a small exit pupil diameter can meet the requirements of the system,which is beneficial to the miniaturization and lightweight design of the optical system.The viewing window formed after exit pupil expansion can relax the tolerance of the eye position to meet the needs of different people and multiple functions.However,the one-dimensional exit pupil expansion only expands the horizontal direction of the exit pupil of the eyepiece,and the vertical direction of the formed viewing window is still small,which may cause image loss.The two-dimensional expansion of the holographic waveguide has the problem of small brightness in the viewing window,which affects the human eye’s observation of virtual information.In this paper,the optical system of see-through waveguide near-eye display is designed and optimized,in order to further expand the range of the viewing window and improve the display brightness in the viewing window.In this paper,two main waveguide structures: geometric waveguide and holographic waveguide are simulated.The exit pupil expansion methods and their advantages and disadvantages are analyzed in depth.A two-dimensional exit pupil expansion scheme combining a geometric waveguide and a holographic waveguide is proposed to enlarge the viewing window while ensuring the brightness in the viewing window.Firstly,the stray light and light energy loss during the coupling of the two waveguides are analyzed.The exit pupil position of the eyepiece and the length and period of the input coupling grating are analyzed,which improves the coupling efficiency in the waveguide coupling process.First of all,for the problems of stray light and light energy loss during the coupling of two waveguides,a data model of the exit pupil position of the eyepiece and the length and interval of partially reflecting mirrors in the geometric waveguide was established.Secondly,according to the waveguide parameters and the visual characteristics of the human eye,an eyepiece optical system with a small exit pupil diameter(2.5mm)was designed using optical design software Zemax.Secondly,according to the constraints of light and the structure characteristics of the waveguide,the structural parameters of the geometric waveguide and holographic waveguide were determined.Then,according to the waveguide parameters and the visual characteristics of the human eye,an eyepiece optical system with a small exit pupil diameter(2.5mm)was designed using optical design software Zemax.Finally,the optical simulation software Lighttools is used to simulate the overall optical system integrated with the eyepiece system and the geometric waveguide and holographic waveguide system.Aiming at the problem of image loss caused by too large horizontal expansion interval in the simulation,a horizontal half mirror was added to the center of the holographic waveguide to prevent the image loss in the viewing window.The imaging quality,exit pupil irradiance distribution,optical efficiency,and brightness uniformity of various typical fields of view of the system are analyzed,and all aspects meet the visual characteristics of human eyes.The designed system has a field of view of 30° × 40°,the MTF of each field of view is greater than 0.5 at 40lp/mm,and the distortion is less than 1%.The system’s viewing window is similar in shape to a right-angle trapezoid,with an upper base of 13 mm,a lower base of 18 mm,and a height of 28 mm.The minimum optical efficiency in the viewing window is 48.1%.