Feasibility study of focusing grating couplers for optical data storage.

Abstract

In this dissertation, some analysis tools for the study of integrated devices applied in optical-disk memory system are investigated. These applications include the gradient-index media used in waveguide lenses and the waveguide focusing grating coupler. Previous studies of gradient-index components have largely been either strictly geometrical in nature or based on the split-step procedure, which are not sufficient for design and analysis purposes. In this dissertation, some beam propagation methods based on paraxial optics are presented, which facilitate diffraction propagation calculation in all types of optical systems that have modest amounts of aberrations, including gradient-index elements. It is also shown that not only is it possible to apply the beam propagation method accurately to stigmatic gradient-index media, but also a propagation distance of any length may be taken without using the split-step method. The Maxwell's fisheye lens is discussed in detail to show application of these methods. Waveguide focusing grating couplers are modeled in detail. The polarization theory of these devices differs from the theory of conventional gratings because of the conversion of guided waves to radiated waves. A simple theory based on induced dipoles is presented to study the polarization conversion effect in radiated field of grating coupler under general case of oblique incidence. Calculations of this model indicate that TE guided mode will be well-behaved but a high degree of variation of elliptical polarization will be generated by TM guided mode. It is also shown that despite the simplicity of the analytical formulations, the agreement with experiment is high and adequate for practical design purpose. Also, tolerances for fabrication errors in focusing grating coupler are described and a systematic method to evaluate tolerances is presented.