Optimized optical coupling to silica-clad photonic crystal waveguides.

Abstract

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 4dB 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 0.28dB at its minimum and less than 0.5dB for the wavelength range of 12nm. We measured a coupling loss of 0.46dB for the device fabricated by the silicon photonics process. This low-loss junction well supports the practical use of PCWs.