Double-Stage Taper for Coupling Between SOI Waveguides and Single-Mode Fiber

A low-loss polarization-independent and wavelength-insensitive mode converter is demonstrated for coupling between standard single-mode fiber (SMF) and 1.5-$\mu$m-thick silicon waveguides. This mode converter consists of a double-stage taper fabricated using planar processing. Optical testing results show facet loss of approximately $-$1.5 dB/facet, for TE and TM polarizations across a wide wavelength range.