BACKGROUND

This relates generally to devices with displays, and, more particularly, to head-mounted displays.

Head-mounted displays may be used to display virtual reality and augmented reality content. A head-mounted display that is displaying augmented reality content may overlay computer-generated images on real-world objects. Displays and optical systems may be used to create images and to present those images to a user.

If care is not taken, however, the components used in displaying content for a user in a head-mounted display may not exhibit desired levels of optical performance.

SUMMARY

An electronic device may have a display system that produces images. An optical system with one or more waveguides and input and output coupler systems may be used to distribute the images to a user.

The display system may have one or more pixel arrays such as liquid-crystal-on-silicon pixel arrays. Images from the display system may be coupled into one or more waveguides by an input coupler system and may be coupled out of the waveguide in multiple image planes using an output coupler system. The input and output coupler systems may include single couplers, stacks of couplers, and tiled arrays of couplers. The couplers may be volume phase holographic gratings or other optical couplers for coupling light into and out of the upper and lower surfaces of elongated strip-shaped waveguides.

Holographic gratings in the waveguide may have fringes with constant pitch and variable period. The period at a given portion of the grating may be Bragg-matched to maximize diffraction efficiency for light of a given wavelength and incident angle. For example, a first set of fringes may have a first period that maximizes diffraction efficiency for incident light associated with a left field of view, a second set of fringes may have a second period that maximizes diffraction efficiency for incident light associated with a center field of view, and a third set of fringes may have a third period that maximizes diffraction efficiency for a right field of view.

