Abstract

Experimental and theoretical investigations into the polarization properties of planar dielectric waveguide grating couplers are presented. The ellipticity and azimuth of the polarization ellipse of radiation modes coupled out by several different grating couplers were measured as functions of the angle between the incident guided-wave and the grating rulings. The polarization states were, in general, elliptical, and this finding agrees with the theoretical predictions in the literature. However, a major difference between TE and TM excitations was observed. The ellipticity of the radiation modes was vanishingly small when the guided wave was TE-polarized, but it was very large when the guided wave was TM polarized. This implies that a TE mode is much easier to work with than is a TM mode. The polarization state of the evanescent tail of the guided wave in the grating region is responsible for the polarization state of the radiation modes. We present simple induced dipole model that can qualitatively explain the observed polarization phenomena. Numerical results based on an exact theory and a perturbation theory are also given. The results of this research have significant implications for the design of focusing waveguide grating couplers in optical data storage heads, which will be described in a companion paper by Hwang et al.

© 1990 Optical Society of America