A silicon-based spot-size converter between single-mode fibers and Si-wire waveguides using cascaded tapers

Theoretical and experimental performances of a robust Si-based spot-size converter (SSC) for efficient coupling between a single-mode fiber and a photonic Si-wire waveguide are reported. The SSC is comprised of cascaded horizontal linear and vertical nonlinear up tapers measured $2.0\mathrm{mm}$ and $86\mu \mathrm{m}$ in length, respectively, in a silicon-on-insulator substrate. The fine modal field of a Si-wire waveguide, $0.54\times 0.38\mu {\mathrm{m}}^2$ in diameters, was expanded to diameters of $5.1\times 9.2\mu {\mathrm{m}}^2$ with measured net transmission loss of $0.5\mathrm{dB}$ at the wavelength of $1.55\mu \mathrm{m}$ by the miniature SSC.