Holographic waveguide display with a combined-grating in-coupler.

Volume holographic gratings are widely used as couplers in eyewear waveguide display systems, but they show a relative lower TM polarized energy compared to transverse-electric (TE) incidence. In this paper, we propose a novel holographic waveguide display system with a combined-grating as the in-coupler. When used as an in-coupler for a holographic waveguide display system, a subwavelength metal grating is designed onto the volume holographic grating to increase the total diffraction efficiency of the coupling gratings. Theoretical calculations show that this design increases the diffraction efficiency by 16.4% for TM polarization, 4.3% for TE mode, and 10.0% for unpolarized light, compared to a single volume holographic grating. Calculations also show that the use of this design as an in-coupler for a holographic waveguide system increases the luminance efficiency for these three modes by 26.8%, 9.0%, and 15.6%, respectively.