Study On The Diffraction Characteristics Of Novel Coupling Grating Applied On Holographic Waveguide

The new helmet-mounted display technology represented by holographic waveguide has received extensive attention of researchers for the advantages of small size,light weight,and high diffraction efficiency.It utilizes the diffraction effect of the holographic optical elements and the total internal reflection characteristics of the waveguide optical elements simultaneously.The reflection volume holographic grating(RVHG),advanced in high diffraction efficiency and less diffraction orders,has become an ideal coupling optical element in the helmet mounted display system of the holographic waveguide(HW-HMDs).In this paper,a graphical user interface of the grating simulation software based on the rigorous coupled wave analysis(RCWA)theory is developed by MATLAB,which can easily analyze and design various gratings.And a finite element method(FEM)simulation model of HW-HMDs and gratings based on COMSOL software is developed.The effectiveness of the simulation mothed is proved by the simulation analysis of the RVHG with the mutual verification from RCWA and FEM methods.A novel coupling grating structure is proposed through the analysis of the diffraction characteristics of the VHG and the subwavelength binary grating.We demonstrate that the high diffraction efficiency of the novel coupling grating must satisfy the wave vector matching both from the theoretical deduction and simulation experiment,that is to say: the wave vector component of the RVHG in the direction of the binary grating is equal to the wave vector of the binary grating.Simultaneously,the redistribution of the diffraction order energy of the novel coupling grating is found through the simulation experiment of the diffraction efficiency influenced by the relative position of the two gratings in the novel coupling grating,and finally the analytical expression of the coupling effect analysis is obtained.By optimizing the geometric structure,material properties and other parameters,the novel green coupling grating is greatly improved with the optimized general RVHG.When TM and TE wave is incident vertically,the diffraction efficiency of the main diffraction order is increased by 18.5% and 4.1% respectively with less the stray light.When the incident wave is offset the Bragg condition,the diffraction efficiency can be higher,and the field view can be better.In addition,the simulation model of the HW-HMDs with the new coupling grating is established to analyze the diffraction characteristics of the monochromatic HW-HMDs.Results indicate that the diffraction efficiency of TM and TE wave can be increased by 33% and 8.1% respectively.The novel coupling grating proposed in this paper provides a new method to improve the coupling efficiency of the HW-HMDs,which is of great significance and value for the improvement of the overall imaging quality of the HW-HMDs.