Low-loss spot size converter based on the integration of graded-index waveguide and 3D tapered waveguide

A low-loss spot size converter (SSC) on the SOI platform has been proposed consisting of a graded-index(GRIN) waveguide build by the ion implantation process and a 3D tapered waveguide for optical coupling between the single-mode fiber and the silicon waveguide. When the refractive index of SiO2 is set as 1.44 ~ 1.458, the coupling losses can be confined within 0.3 ~ 0.6 dB in the wavelength range of 1.5~1.6 μm. As the refractive index of SiO2 varies with wavelength, device exhibits different performances as the cladding size changes. When the upper cladding size is 6μm × 8μm, the polarization-dependent loss of the SSC is less than 0.21 dB in the wavelength range of 1500 ~ 1600nm. The TE mode coupling loss is the lowest when the upper cladding size is 5.5μm × 11μm, and the coupling loss of TE mode is between 0.25dB ~ 0.34 dB in the wavelength range of 1.5-1.6 μm. The TM mode coupling loss is the lowest when the upper cladding size is 6μm×9μm, and the coupling loss of TE mode is between 0.17 ~ 0.48 dB in the wavelength range of 1.5-1.6 μm. In addition, the refractive indexes of both top and bottom oxide are at the same value and the starting taper tip width can be maintained as 200 nm suitable for the lithography resolution of the current commercial silicon photonics fabs.