Suspended optical fiber-to-waveguide mode size converter for silicon photonics

In this paper, an efficient and novel optical fiber-to-waveguide mode size converter for Si Photonics devices with sub-micron waveguides is developed on the SOI platform. This optical converter is composed of a suspended SiO(2) waveguide and overlapped Si nano-tapers located in the center of suspended SiO(2) waveguide. Laterally connected SiO(2) beams provide structural support for the suspended SiO(2) waveguide. The optical input signal from the optical fiber is launched into the suspended SiO(2) waveguide, and then coupled into the Si nano-taper. The measured coupling loss using a lensed fiber with 5 microm spot diameter is 1.7 ~2.0 dB/facet for TE mode and 2.0 ~2.4 dB/facet for TM mode in the wavelength range of 1520 ~1600 nm. When a cleaved fiber with 9.2microm spot diameter is used, the coupling losses for both TE and TM modes are less than 4.0 dB/facet at 1550 nm. For the case of lensed fiber, the alignment tolerances for both TE and TM modes are about +/- 1.7 microm for 1 dB excess loss in both X and Y axes. The alignment tolerances for both modes of TE and TM are relaxed, exceeding +/- 2.5 microm for 1 dB excess loss in both X and Y axes when a cleaved fiber is used.