A viable spot-size converter for coupling between a single-mode fiber and a silicon-wire waveguide

A viable spot-size converter (SSC) consisting of a horizontal linear up-taper, a vertical down-taper, and a gap region in cascade for coupling between a single-mode fiber (SMF) and a silicon-wire waveguide was fabricated and characterized. The gap region enabled significantly improved matching between the mode-field diameters of the silicon waveguide and the SMF. The fabrication process involved was significantly simplified in comparison to that required for the SSC reported earlier. The measured coupling losses per port between the fiber and the waveguide for the TM-like and TE-like modes are 3.1 and 5.2 dB, respectively. The new SSC structure is planar, robust, and easy to fabricate, and has thus demonstrated its viability.