A Two-Port Single-Mode Fiber–Silicon Wire Waveguide Coupler Module Using Spot-Size Converters

In this paper, the performance of a two-port single-mode fiber-silicon wire waveguide coupler module which utilizes an identical spot-size converter (SSC) at the input and output ports is reported. Each of the silicon (Si)-based SSCs comprised cascaded horizontal linear and vertical nonlinear up-tapers measured 300 and 200 mum in length, respectively, in a common silicon-on-insulator (SOI) substrate. The structural parameters of the tapers were designed for compactness and relaxed tolerance to fabrication errors. The total length of the two-port coupler module was 1000 mum plus the variable length of the wire waveguide connecting the two SSCs. The mode-field diameter (MFD) of the Si-wire waveguide, 0.32 times 0.46 mum
²
, was transformed to the diameter of 2.8 times 8.0 mum
²
at the wavelength of 1.55 mum (corresponding to an area expansion of about 150 times) and vice versa by the SSCs with a net transmission loss of 4.1 dB/port. The field-mismatch loss between the SSC and the single-mode fiber with the MFD of 5.2 mum was 2.1 dB/port.