Analysis and design of optical guided-wave devices for quasi-phasematched second harmonic generation and Bragg deflection

摘要

Integrated optics-based approaches to beam steering, beam shaping, beam collimation, and quasi-phasematched (QPM) second harmonic generation (SHG) of light offer significant advantages over conventional approaches based on bulk optics. The research in this dissertation addresses the analysis and design of optical guided-wave devices for both efficient quasi-phasematched second harmonic generation in diffused channel waveguides, as well as Bragg deflection of beams in planar waveguides. It is known that the normalized SHG efficiency depends on the linear properties of the waveguide through the overlap of the modal fields at the fundamental and second harmonic wavelengths. To analyze the linear modal properties, a fast and accurate modeling tool, based on an improved, semi-vector, Fourier method of analysis, is presented. The tool incorporates the Wentzel-Kramers-Brillouin (WKB) and effective index methods to accurately determine the computational parameters required for the numerical calculation in the Fourier method so that automatic variation of the waveguide parameters is permitted. Using the modeling tool, the dependence of the SHG process on the waveguide parameters is investigated in detail, leading to waveguide designs with improved mode confinement, and consequently higher SHG efficiency. The phasematching characteristics of these improved designs are also calculated, and it is found that non-critical phasematching, or phasematching with wide tolerances to variations in the waveguide parameters, is possible in certain cases. The analysis of the waveguide-SHG process also indicates that efficiency can be improved by utilizing thin films on the waveguide surface to ensure that the peaks of the fundamental and second harmonic modes are coincident. This contributes to higher SHG efficiency through improved mode overlap as well, but it is demonstrated that this approach is clearly distinct from and independent of the mode-confinement approach. The analysis of planar overlays is based on recursion relations for the phase shift upon reflection at interfaces. The significance of this approach is demonstrated through the "matching" of the diffused waveguide to an independently-designed, multilayer overlay for the purposes of obtaining specific modal characteristics in the "integrated" structure. On a different note but incorporating similar approaches for analysis and design, beam shaping, steering, and collimation in planar waveguides, using Bragg gratings with finite area and non-uniform depth variation, are also discussed.