Small-area bends and beamsplitters for low-index-contrast waveguides

We explore the use of air trenches to achieve compact high efficiency 90degrees waveguide bends and beamsplitters for waveguide material systems that have low refractive index and low refractive index contrast between the core and clad materials. For a single air interface, simulation results show that the optical efficiency of a waveguide bend can be increased from 78.4% to 99.2% by simply decreasing the bend angle from 90degrees to 60degrees. This can be explained by the angular spectrum of the waveguide mode optical field. For 90 bends we use a micro-genetic algorithm (muGA) with a 2-D finite difference time domain (FDTD) method to rigorously design high efficiency waveguide bends composed of multiple air trenches. Simulation results show an optical efficiency of 97.2% for an optimized bend composed of three air trenches. Similarly, a single air trench can be designed to function as a 90degrees beamsplitter with 98.5% total efficiency. (C) 2003 Optical Society of America.