Functionality of optical field transformation in irregular waveguide structures

Summary form only given. Periodic optical elements such as photonic bandgap structures have been widely explored. While these structures perform effectively in some applications, the limited degrees of freedom can dictate their size. We adopt the concept of irregular structures, which have been shown to provide better performance and functionality than periodic structures, with significantly reduced size. We demonstrate both optical mode converter and mode-selective optical cavity structures that can be fabricated using standard semiconductor processing techniques. Frequency-dependent mode conversion illustrates the prospect for wavelength division multiplexing elements and laser cavity mode control. The mode converter function can lead to optimal power splitters. The general concept of our irregular waveguide structure is depicted where the variable width sections provide for field transformation. We treated the discrete waveguide widths as optimization variables in a cost function involving the input field and the desired output field. A multi-resolution strategy was used to arrive at particular designs, where solutions with coarse steps in waveguide width with a small number of sections were refined to smaller steps in width and a larger number of sections during the iterative process. We have demonstrated the functionality achievable with optimized, irregular optical field transformation components. The compact design geometries we have obtained are within the resolution limits of electron beam lithography, and could be fabricated on a semiconductor wafer with current technology.