Improved 2D continuously variable output couplers for an exit pupil expander fabricated through ion beam etching.

An important goal in designing a head-mounted display device is to improve its exit pupils' output light areal uniformity. The goal cannot be reached by using an output grating coupler of uniform surface profile depth. To solve this problem, we propose a method to fabricate 2D and continuously variable depth gratings by using a specially modified reactive ion-beam etching system, which features variable speed scanning etching and ion beam cross-sectional shaping in two orthogonal dimensions. We experimentally verified that the uniformity of the light outcoupled from the coupler fabricated by using the proposed method was 52% better than that of a uniform-depth output grating coupler.