Efficient coupling to a waveguide by combined gratings in a holographic waveguide display system

Gratings are widely used as coupling parts in a waveguide display system for achieving a much lighter and more compact system, but their diffraction efficiency needs to be improved. Two combined gratings for integrating a subwavelength binary grating and volume holographic grating (VHG) are applied as incoupler and outcoupler of a holographic waveguide display system. Two basic design rules are put forward to guarantee the maximum diffraction energy guided into the waveguide and finally coupled out to enter into the user's eyes: one is the grating vector matching rule, the other is the refractive index matching rule on the interface of the binary grating and the VHG. The finite element method is used to simulate the couple-in parts and the whole waveguide display system. The combined grating with the metal binary grating is different from that with a dielectric binary grating for achieving higher diffraction efficiency and an additional second peak in the diffraction efficiency curve varied with the relative position between the binary grating and the VHG. The simulation results indicate that a VHG+Ag combined grating can obtain much higher diffraction efficiency compared to gold, aluminum, and other dielectric materials. In addition, several factors such as the Bragg wavelength, the index modulation of VHG, binary grating thickness, and the filling factor of the binary grating are discussed for the VHG+Ag combined grating. Moreover, a higher diffraction efficiency in the holographic waveguide system can be obtained by using VHG+Ag-VHG+Ag combined gratings as the incoupler and outcoupler.