Abstract

We report a theoretical study and numerical analysis of the coupling efficiency of a grating coupler in a slab waveguide having a nonlinear guiding medium characterized by a saturable index of refraction. A phenomenological relation is assumed for the saturation of the nonlinearity. For a regular (constant period) grating a minimum appears in the plot of coupling efficiency vs incident power. This minimum shifts toward lower powers with decreasing saturation value (by saturation value we mean the maximum allowed change in the waveguide propagation constant). The origin of this minimum is due to the fact that a grating coupler is a distributed coupler where coupling of the incident beam to a guided mode is distributed over a distance along the propagation direction. Similar results are obtained for a linearly chirped grating coupler. We show that, for such a grating and a given saturation value, the proper choice of grating chirp rate can result in high coupling efficiencies at high power levels without any need for a detuning of the incident beam to optimize the coupling efficiency.

© 1989 Optical Society of America