Improved integration density with ultra-fast laser written fiber fan-in/fan-out for broadband coupling to silicon photonics

There is an evolving need for higher integration density for silicon photonic integrated circuits. Broadband operation and polarisation insensitivity are highly coveted. Despite their versatility, the commonly used grating couplers are limited by their high insertion loss, narrow bandwidth, polarisation dependence and their large physical footprint. Inverse taper edge couplers, by contrast, offer lower insertion losses, with relative wavelength and polarisation insensitivity. They are of the same dimension as silicon strip waveguides, hence, coupler spacing is constrained only by the physical size of the coupled fibre, typically amounting to a minimum pitch of 127 µm. To address this constraint, we demonstrate an ultra fast laser inscribed fan-in/fan-out (FIFO) interposer, in boro-aluminosilicate glass (Corning Eagle XG). The FIFO remaps a standard V-groove array of single-mode fibers from a pitch of 127 µm to 50 µm at the silicon interface, effectively increasing the number of ports per millimetre from 8 to 20. Denser arrangements are theoretically possible with the minimum pitch constrained only by the mode-field diameter of the waveguides. The FIFO employs a novel multi-pass waveguide morphology with a high index contrast of 1.12 × 10^-2, allowing for a reduced mode-field diameter of 5.4 µm at 1550 nm. For coupling to a 500 × 220 nm inverse taper with a tip width of 182±1 nm, we achieve a theoretical coupling loss 3.7 dB (4.4 dB measured). Across the telecommunication band (1520 - 1625 nm), we measure a flat wavelength response (0.35 dB variation) and a peak polarisation dependent loss of 0.8 dB.