Ultracompact tapers for light coupling into two-dimensional slab photonic-crystal waveguides in the slow light regime

Optical coupling from a strip waveguide to a two-dimensional photonic crystal (PC) slab waveguide in the slow light regime is investigated. Group-velocity mismatch between optical modes is responsible for large coupling loss. Typical devices based on a W1 PC waveguide, with strip input and output waveguides on each side, are studied using finite-difference time-domain and plane-wave expansion calculations. Compact tapers based on PC lattice parameter tuning are proposed to reduce the coupling loss. At 1.6 mu m, for an optical mode with group velocity v(g)/c=0.054, the transmission can be enhanced from 18% without tapers to 95% with a 3.4-mu m-long taper on each side of the PC. (C) 2008 Society of Photo-Optical Instrumentation Engineers.