Intraguide mode conversions of silica-on-silicon transfer waveguides and SOI tapered structures for highly efficient chip-fiber butt coupling.

An advanced fiber-waveguide coupling scheme is crucial to the effective developments and applications of silicon-on-insulator (SOI) waveguide photonic integrated chips. In this paper, a regime of the mode conversions within guided channels is investigated to realize a highly efficient fiber-chip coupling with a silica-on-silicon (SOS) transfer waveguide that can be realized with the local oxidation of silicon (LOCOS) technique. With this regime, the optical-field overlap process of two optical guided modes between the fiber and the SOI waveguide through a gap is simplified to the two loss contributions: (i) the ultralow coupling loss of the fiber-SOS transfer waveguide and (ii) the Fresnel loss of the SOS transfer waveguide and an SOI tapered structure. As a result, all the mode conversions in this system are operated within the guided channels, so it is referred to as an intraguide mode conversion. Further, the overall mode conversion efficiency of such a fiber-SOS-SOI system is modeled, in which the three key processes, the fiber-SOS gap mode conversion, the SOS-SOI interface mode conversion, and the SOI taper profile mode conversion, are individually investigated and optimized. The simulation values and the experimental results are agreeable with each other for both fiber-SOS waveguide coupling loss and fiber-SOI waveguide coupling loss. Consequently, a fiber-chip butt-coupling loss of 0.8-1.0 dB/facet is demonstrated. Finally, the conditions for realizing the effective LOCOS structure and process are discussed.