Design of an ultra-thin near-eye display with geometrical waveguide and freeform optics

TLDR

Stray light in geometrical waveguides severely degrades near‑eye display quality. The study aims to create a thin, lightweight see‑through near‑eye display by combining geometrical waveguides with freeform optics. A mathematical model and non‑sequential ray‑tracing algorithm were used to optimize stray light, while freeform surfaces on a wedge prism were designed to project the image, resulting in a display with 28° field of view, 9.6 mm exit pupil, and 20 mm pupil distance. The optimized design reduced stray light to 2% of image energy and achieved MTF > 0.5 at 30 lp/mm across the field, as confirmed by a proof‑of‑concept prototype.

Abstract

Small thickness and light weight are two important requirements for a see-through near-eye display which are achieved in this paper by using two advanced technologies: geometrical waveguide and freeform optics. A major problem associated with the geometrical waveguide is the stray light which can severely degrade the display quality. The causes and solutions to this problem are thoroughly studied. A mathematical model of the waveguide is established and a non-sequential ray tracing algorithm is developed, which enable us to carefully examine the stray light of the planar waveguide and explore a global searching method to find an optimum design with the least amount of stray light. A projection optics using freeform surfaces on a wedge shaped prism is also designed. The near-eye display integrating the projection optics and the waveguide has a field of view of 28°, an exit pupil diameter of 9.6mm and an exit pupil distance of 20mm. In our final design, the proportion of the stray light energy over the image output energy of the waveguide is reduced to 2%, the modulation transfer function values across the entire field of the eyepiece are above 0.5 at 30 line pairs/mm (lps/mm). A proof-of-concept prototype of the proposed geometrical waveguide near-eye display is developed and demonstrated.