I. Introduction
Silicon (si)-based photonic nanowire waveguide structures may enable the monolithic integration of electronic, optoelectronic, and optical devices. Such Si-based integrated optoelectronic chips possess high potential for providing various optical functionalities in silicon platforms such as high-speed optical networks as well as facilitating integrated nonlinear applications. However, the great disparity in size between the mode-field diameters (MFDs) of standard single-mode fibers (SMFs) and Si-wire waveguides will result in excessive coupling losses (higher than 25 dB) when they are directly butt-coupled. Clearly, a robust and efficient spot-size converter (SSC) is needed. Various kinds of SSCs have been reported [1]–[10]. These SSCs, in general, require either very fine lithographies for fabrication or complicated alignment. Recently, we reported a prototype of an SSC that utilizes a horizontal linear taper and a vertical nonlinear taper in cascade [11]. The length of this prototype SSC was relatively large (2 mm), and the facet of either the SSC or the SMF must be tilted in order to obtain optimum coupling.