Abstract

In substrate-mode devices, the propagation angle of the substrate-mode beam must exceed the critical angle at all substrate-air interfaces.¹ High spatial frequency asymmetrical profile (blazed) incoupling gratings are required to obtain this large propagation angle when the external beam is normally incident on the incoupling grating. However, the efficiency of blazed gratings is rapidly degraded as the wavelength/period ratio increases.² The maximum achievable efficiency for normal incidence polarization unselective incoupling gratings is investigated by using rigorous coupled-wave theory. The ellipticity introduced during incoupling is also evaluated. High efficiency polarization selective and unselective incouplers for obliquely incident external beams are presented. Maximum achievable selective and unselective efficiencies for these elements are summarized. Additionally, the properties of surface-relief and volume grating incouplers are compared, and fabrication limitations for normal incidence surface-relief incouplers are discussed.

© 1992 Optical Society of America