Abstract

Waveguide gratings, like most holographic devices, exhibit aberrations when used at other than the design wavelength. For high numerical aperture devices the usable spectral range may be very small, perhaps less than the expected shift in a laser diode light source. Unlike the conventional hologram, the waveguide devices may, in principle, be made self-compensating by matching the spectral change of optical path lengths in the guide layer and the freespace propagation path. This requires a variable propagation constant for the waveguide which may be achieved by controlling the thickness and/or the index of refraction of the guide layer. In this paper we describe the theory of this method and calculate the necessary range of refractive index. For large numerical aperture devices, variations of index of 50% or better are needed.

© 1989 Optical Society of America