In this paper, a method is proposed to increase the field uniformity of holographic near-eye display system based on liquid crystal spatial light modulator (LC-SLM). The holographic near-eye display system consists of a projector, a display component and a receiver. The projector is used as an image source that based on LCOS, LCD, Micro-LED, DLP or other projection technique. The display component transmits images from the projector to the receiver, which contains at least one layer optical waveguide, in-coupling holographic grating and holographic out-coupling grating. The grating is a periodic structure formed by interference exposure on a photopolymer material. The exposure dosage determines its diffraction efficiency controlling the energy ratio of 0th-order transmitted light and 1st-order diffracted light. The average refractive index of the holographic grating is determined by the photopolymer material, and the refractive index modulation is determined by the interference exposure dosage. The higher refractive index modulation contributes to higher diffraction efficiency. The projection light diffracts into the optical waveguide through in-coupling holographic grating, and propagates to out-coupling holographic grating by total internal reflection. The out-coupling grating diffracts the light out of optical waveguide to the receiver that can be an eye or a CCD detector. The display component plays a key role in holographic near-eye display system, which directly determines the display performance (FOV, field uniformity, brightness, etc.). In the previous study, the energy distribution of object light and reference light in the interference exposure system is a standard Gaussian beam. If the center part of object beam and reference beam with near 70% uniformity of energy distribution is used to exposure and generate holographic grating, the diffraction efficiency of the grating can be considered to be uniform. However, for near-eye display system with uniform in-coupling and out-coupling grating, the uniformity of field of view is poor. When image source is a pure white pattern, the gray ratio between the darkest region and the brightest region is only about 10%. This is because different field of view angles correspond to different wavelengths related to different total reflection steps. For a receiver at a fixed position, the wavelength with longer total reflection step has less times of pupil expansion. Due to energy loss for each pupil expansion, the wavelength with longer total reflection step equips with higher output energy. Based on the above theoretical analysis, the diffraction efficiency of out-coupling grating should be as low as possible, which is expected to be less than 10% to obtain a better field uniformity. Actually, if the diffraction efficiency of out-coupling grating is too low (< 5%), it will cause dark brightness of display image and low energy efficiency of display system. In this paper, wide spectrum from 500 nm to 550 nm is used to achieve wide field of view and pure green display. According to Bragg formula and total reflection condition, the parameters (like grating period and slant angle) of in-coupling and out-coupling grating are optimized To improve the energy efficiency of near-eye display system, the diffraction efficiency of in-coupling grating should be as high as possible, which is expected to be above 90%. To improve the field uniformity, the light energy of each pupil expansion emitted from the out-coupling grating needs to be basically the same. Therefore, the diffraction efficiency of out-coupling grating should be gradually changing. The part of out-coupling grating closer to in-coupling grating should have smaller diffraction efficiency. To prepare the gradient out-coupling grating, we adopt the algorithm to control LC-SLM, making the liquid crystal molecules of each pixels corresponding to different phase modulation values. The LC-SLM can make the energy distribution of the object beam to be monotone varying. The energy distribution of reference beam can be uniform. After interference exposure, the out-coupling grating with diffraction efficiency changing from 5% to 10% is prepared. As a result, for improve near-eye display system, the field uniformity is increased to about 20% (2 times than previous one) with FOV of 19 degrees H*6 degrees V.
