Spot size converting fiber with a 4-pm mode field diameter for low-loss coupling to silicon photonic circuits

This paper reports optical fibers for low-loss coupling with silicon photonic circuits. We fabricate both singlemode fibers and polarization-maintaining fibers with a mode field diameter of 4 pm for matching with an edge coupler. We demonstrate fusion splicing with standard transmission fibers and optical coupling with silicon photonic devices. Low-loss splicing is achieved by utilizing the thermally expanded core (TEC) technique. Additionally, a low-loss coupling of 2.5 dB/2 facets is obtained using these TEC fibers.