Spot-size converters for rib-type silicon photonic wire waveguides

We report the fabrication of spot-size converters to efficiently connect a rib-type silicon wire waveguide with an optical fiber. The spot-size converter consists of a channel-type Si inverse taper core without a slab section and a SiO_x waveguide that covers the taper. To make the Si taper core, we eliminate the Si slab at the end of the rib waveguide using a two-step etching process. The coupling loss per connection with optical fiber is reduced from 7.5 dB to about 0.7 dB by the spot-size converter. The apparent light reflection at the step structure at boundary between rib and channel core is not observed.