Abstract

The maximum efficiency for coupling a Gaussian beam into a planar optical waveguide primarily depends on two factors: the branching ratio and the leakage rate of the waveguide-grating structure. The preceding paper (MY2) describes a means of improving the branching ratio; this paper addresses the leakage rate. A beam coupled out by a grating coupler of constant groove depth has a characteristic exponential decay profile. Such a beam profile is not desirable for many applications. In the case of input coupling, beam profile mismatch between the exponentially decaying out-coupled beam and the incident Gaussian beam limits the input coupling efficiency. Since the leakage rate is influenced most strongly by the grating groove depth, a desirable beam profile can be obtained by varying the grating groove depth along the propagation direction of the guided wave. Based on simple, numerically and experimentally verifiable assumptions, a formula for the variation of grating groove depth for any required output beam profile is derived. The variable depth gratings are fabricated by using an ion beam etching technique and a computer controlled scanning slit placed in front of a photoresist grating mask under ion bombardment. The design, fabrication parameters, and the results of beam apodization are presented.

© 1991 Optical Society of America