Optimized optical coupling to silica-clad photonic crystal waveguides.

Silica-clad silicon photonic crystal waveguides (PCWs) are promising components for various applications because of their simple fabrication and generation of slow light. However, an optical loss higher than 4 dB occurs when they are simply coupled to input/output silicon wire waveguides. To reduce the optical loss, we proposed a junction structure in which light in the waveguide is first coupled to a high-group-velocity radiation mode at an expanded core and subsequently converted to the slow-light mode in a tapered core of the PCW. The coupling loss at a junction is calculated to be 0.28 dB at its minimum and less than 0.5 dB for the wavelength range of 12 nm. We measured a coupling loss of 0.46 dB for the device fabricated by the silicon photonics process. This low-loss junction well supports the practical use of PCWs.