Post-Integrated Dual-Core Large-End Spot-Size Converter With Si Vertical Taper for Fiber Butt-Coupling to Si-Photonics Chip

We experimentally demonstrated a post-integrated dual-core spot-size converter (SSC) that had a coupling end with a 10-pm diameter at the edge of an Si-photonics chip. Although there was an extra loss increase as a result of post-integration, it can be reduced and, thus, the SSC is a promising candidate for practical use. The large coupling end enabled the SSC to be butt-coupled to a single mode fiber (SMF) theoretically at an optical loss of less than 0.7 dB, and post-integration of the SSC allowed us to apply complementary metal-oxide-semiconductor technologies to optoelectronic circuits on wafers before the SSC forming process. We attached the SSC to a 1.5-pm-thick Si rib waveguide. The observed near field patterns of light from the SSC were close to that of an SMF independently of light polarizations. The working wavelength range spanned more than 100 nm. The excess loss caused by the post-integration process was estimated to be 1 dB.