Low reflection double stage coupling to slow light waveguides

Photonic crystal line-defect waveguides can guide optical modes with substantially reduced group velocity. Efficient coupling to slow light waveguides from silicon strip waveguides can be achieved through double stage mode conversion. First, light is coupled to a normal group velocity line-defect waveguide with anti-reflection adjustment of the photonic crystal interface. The reflected intensity at the first stage was reduced to -35 dB in this paper. Second stage consists of the abrupt change to slow light waveguide with intensity reflection about -20 dB for the mode with group velocity 0.02 c. The reflection at the second stage can be further reduced with adiabatic coupling. A gradual tapering 20 lattice constants long with a cubic taper function leads to less than -40 dB intensity reflection for the investigated mode.