Abstract

A focusing grating coupler consists of a chirped, curved grating on the surface of a dielectric waveguiding structure. Due to the geometry of the grating rulings, such a coupler can convert a guided waveguide mode to a radiation mode having a point focus above or below the plane of the waveguide. Such focusing couplers have been proposed as the central coupling and focusing elements in CD ROMS and magnetooptic disk drives. In spite of these applications, little detailed study of their polarization properties has been presented in the literature. In this presentation we discuss the polarization properties of focusing grating couplers. Based on the local normal mode theory of waveguide gratings, we have analyzed the coupling induced by the grating between orthogonal polarization components for the case of radiation modes. The analysis is based on an extension of the ray trace method of Heitmann and Ortiz¹ and implementing the coupling strengths for guided to radiation mode coupling obtained from local normal mode theory. The method produces a near field amplitude which is propagated to the far field using Huygens-Fresnel theory. Resultant far field spot profiles are presented and implications for appropriate use of focusing grating couplers in integrated optical systems are discussed.

© 1989 Optical Society of America