THEORY AND FABRICATION OF SUB-MICRON GRATINGS ON NONLINEAR OPTICAL WAVEGUIDES.

Abstract

Because of their compatibility with the planar concept of integrated optics, grating couplers offer the most satisfactory means of coupling light into thin film optical waveguides. The purpose of this dissertation has been to study the behaviour, both theoretically and experimentally, and fabrication of grating couplers in nonlinear waveguides. A theory of nonlinear grating couplers is presented based on a coupled-mode approach. The dependence of coupling efficiency on incident beam intensity, beam size, beam position, incident angle, chirp rate, and waveguide losses have been examined all in the presence of nonlinearities in the waveguide. It is reported that, in the presence of nonlinearities, the coupling efficiency decreases with increasing incident power. Different ways of optimizing the coupling efficiency at high incident power levels are presented. These include adjusting the beam size, the coupling angle, and chirping the grating. A new technique is reported for fabrication of regular period, chirped, and curved photoresist gratings. The experimental arrangement is essentially based on Lloyd's mirror fringes and is characterized by its stability, simplicity, and versatility. We also report on successful use of Reactive Ion-Beam Etching (RIBE) with C₂F₆ gas in producing very smooth and deep gratings with high aspect ratios in different waveguide structures. Experimental coupling efficiencies of up to 40% are reported in polystyrene waveguides using etched grating couplers. Experiments are reported in support of the theoretical findings of this dissertation using a polystyrene waveguide with thermal nonlinearity.