Compact waveguide grating couplers operating in the strong coupling regime

Since both photonic crystal and high index contrast waveguide photonic devices allow for tighter bend radii and thus reduced die size, they are being actively investigated for use in dense planar lightwave circuits (PLCs). To fully realize highly integrated PLCs, an efficient optical connection interface between single mode fibers or fiber arrays and high-index-contrast waveguides is required. The core size of single-mode fiber is typically 4--9mum while the core size of high-index-contrast waveguides is usually less than 1 to 2mum. Due to the mode size mismatch between the fiber and the waveguide, coupling light into small waveguides is challenging. This results in prohibitive insertion loss and extreme difficulty in fiber alignment for simple pigtail coupling, in which the end of a fiber is aligned and placed next to the end face of a waveguide. In addition, with input and output coupling taking place only along the edge of a chip at the plane of the waveguide device layer, this edge coupling arrangement severely limits the number of optical I/Os in dense PLCs. Moreover, fiber alignment has to be done after the wafer is diced into separate chips, and the end facet of the waveguide needs to be cut and polished. As a result, alignment and packaging accounts for the main manufacturing cost of optical components. In order to fully realize the potential of highly integrated PLCs, a new coupling approach needs to be developed, especially one that permits input and output to a 2-D array of fibers.; The goal of this dissertation is to develop a high efficiency coupling method based on grating couplers operating in the strong coupling regime for fibers oriented normal to the waveguide plane without any intermediate optics between the fibers and waveguides. Development of this capability allows for relaxed fiber alignment requirements and wafer-scale alignment and testing.; In this dissertation, three types of grating couplers that operate in the strong coupling regime are investigated. These are (1) stratified waveguide grating couplers (SWGCs), (2) slanted grating couplers (SLGCs) and (3) embedded slanted grating couplers (ESGCs). With the use of micro genetic algorithm (muGA) in conjunction with the finite difference time domain method, specific designs based on different high index contrast waveguides are presented. A simplified k-vector diagram together with the rigorous coupled wave analysis (RCWA) mode solver method have been suggested and explored in order to analyze and understand the physical principles of strong grating couplers. On the basis of this understanding, a systematic design procedure for uniform slanted grating couplers is developed and shown to be effective. This design procedure avoids the computational requirements needed for a purely numerical design based on the muGA method.