Composite waveguide holographic display

Abstract

Waveguide-type displays with volume phase holograms are notable for their small size, large eyebox and high transmission in both the projected image and see-through channels. However, as the aperture, field of view and working spectral range grow, the variation of the hologram replay conditions across its’ surface increases and sets a performance limitation in terms of resolution and diffraction efficiency. In order to overcome it, we propose to use a composite hologram, i.e. a volume phase grating split into sub-apertures with independently varying parameters as the fringes tilt, their pattern, the holographic layer thickness and modulation depth. This approach allows to increase the number of free variables in the design drastically, without introducing any additional optical components. We present the optical design and modelling algorithm for a display with a composite outcoupling hologram. The algorithm relies on simultaneous raytracing through auxiliary optical systems and the diffraction efficiency computations with the coupled waves theory equations. We demonstrate its’ application on an example of polychromatic display with extended field of view. It operates in the spectral range of 480-620 nm and covers the field of 6° × 8° with 8 mm exit pupil diameter. The output hologram has a uniformized diffraction efficiency over the field and the spectral range varying from 26.0% to 65.3%, thus providing throughput around 50% for the projected image and its’ optimal overlapping with the see-through scene. At the same time, the image quality is high and uniform with the PTV angular size of the projected spot better than 1.88′ for the entire image. We compare the results for the initial design using a single classical grating with that for a composite hologram comprising of 4 sub-apertures and show the achieved gain in performance – the gain in DE is up to 13.8% and that in aberration is 0.4 ′ . Also we demonstrate that the computed parameters are feasible and provide a brief sensitivity analysis for them.