Modelling of a composite waveguide holographic display

Abstract

Optical designs of waveguide-type augmented reality displays are investigated. Displays based on volume phase holograms are notable for their small size, large exit pupil and high transmittance both in the projected image channel and in the direct vision channel. However, with an increase of the aperture, field of view and working spectral range, the spread of the values of the beam angle of incidence and the wavelength increases when solving the diffraction problem at different points on the hologram surface which imposes restrictions on spatial resolution and diffraction efficiency. To overcome this phenomenon, it is proposed to use a composite hologram which represents a volume phase grating divided into zones with independently varying parameters of the fringes tilt, their shape, the holographic layer thickness and the refraction index modulation depth. We propose an algorithm that allows ray tracing through a hologram recorded by two coherent point sources using an auxiliary aspherical mirror. The initial ray tracing in the hologram recording scheme is performed using the error function minimization by the orthogonal descent and golden section methods. Based on the results obtained, the directional vectors of the diffracted beam are calculated using the Welford equation. Using the tracing results, the hologram diffraction efficiency is computed with the Kogelnik’s coupled wave theory. The proposed algorithms are implemented in the Zemax Optics Studio software. The application of the proposed composite hologram element and the tools for operation modeling are shown on an example of display operating in the range of 510–530 nm with the field of view of 7°36ʹ × 5°48ʹ and the exit pupil diameter of 8 mm. It is shown that the proposed solutions make it possible to increase the diffraction efficiency by 3.45 times. At the same time, the spatial resolution increases by 12.7 % varying across the field of view in the range of 0ʹ44ʺ–1ʹ6ʺ. The use of composite holograms allows one to create displays with higher spatial resolution and brightness of the projected image as well as uniformity of the characteristics across the field of view.