Fabrication and characterization of variable groove depth grating waveguide couplers.

Abstract

Variable groove depth grating waveguide couplers (VGDGWCs) can be used to couple light into or out of planar waveguides. In principle an out-coupled light beam with any arbitrary irradiance profile can be obtained. VGDGWCs that produce out-coupled beams exhibiting a Gaussian irradiance profile are examined in this report. An expression for the grating groove depth variation necessary to produce Gaussian beams from VGDGWCs is derived. The design of a VGDGWC using two different materials for the host planar waveguide is presented. The designs are optimized with a consideration of the waveguide materials, the fabrication process, and the out-coupled beam quality. Two methods, that utilize ion etching to fabricate VGDGWCs are presented. Each method operates by limiting the cross sectional area of an ion beam. A scanning slit is used for one method and a moving variable width aperture is employed for the second. The ion etching process for each of the fabrication methods is characterized by a series of ion beam etching experiments. The results from the ion beam etching experiments are used to optimize the fabrication process. The variable width aperture fabrication method in combination with a post etching process, is found to produce the optimum results. The measured irradiance profile and the wavefront quality of the out-coupled beams from the fabricated VGDGWCs are compared to theory. The variable width aperture fabrication method is found to produce out-coupled beams with a nearly perfect Gaussian irradiance profile. The measured wavefront quality of out-coupled beams shows good agreement with theory.