Bragg degenerate model for fabrication of holographic waveguide-based near-eye displays.

The coupling efficiency of light beams is a crucial factor for waveguide displays. Generally, the light beam is not coupled with maximum efficiency in the holographic waveguide without employing a prism in the recording geometry. Use of prisms in recording geometry leads to restricting the propagation angle of the waveguide to a specific value only. The issue of efficient coupling of a light beam without using prisms could be overcome via Bragg degenerate configuration. In this work, the simplified expressions of the Bragg degenerate case are obtained for the realization of normally illuminated waveguide-based displays. Using this model, by tuning the parameters of recording geometry, a range of propagation angles can be produced for a fixed normal incidence of a playback beam. Numerical simulations and experimental investigations of the Bragg degenerate waveguides of different geometries are performed to validate the model. A Bragg degenerate playback beam is successfully coupled in four waveguides recorded with different geometries and yields good diffraction efficiency at normal incidence. The quality of transmitted images is characterized using the structural similarity index measure. The augmentation of a transmitted image in the real world is experimentally demonstrated through the fabricated holographic waveguide for near-eye display applications. Bragg degenerate configuration can provide flexibility in the angle of propagation while maintaining the same coupling efficiency achievable with a prism for holographic waveguide displays.