Facet optimization for edge-coupling of fiber to foundry fabricated SOI waveguides

The reduction of optical loss for integrated photonics I/O is an important area of active research. Edge coupling (end-firing) is a key I/O technology, having advantages over grating couplers in terms of spectral bandwidth and lower insertion loss¹. Low-loss edge coupling into silicon waveguides will be critical to datacenters and telecommunications systems in order to help accommodate the aggressive growth of data analytics applications². In this work, we investigate the coupling losses from optical fiber (SMF-28) into on-chip silicon waveguides using silicon nitride edge couplers with varying chip facet angles. The expected losses were simulated using Three Dimensional Finite-Difference Time-Domain (3D-FDTD) modelling and measured experimentally to close the design-fabrication loop. The chips were produced within a state-of-the-art 300 mm CMOS foundry, using edge couplers from the foundry Process Design Kit (PDK). During optimization of the photolithography and dry etching process, the facet angle deviation from 90° was minimized. Insertion loss of the SiN edge coupler was investigated via transmission measurements utilizing both cleaved fibers and fiber V-grooves. Facet angles varied from approximately 75°–90° were tested for insertion loss and trends were consistent with the 3D-FDTD modelling. Measurements were performed over a range of 1450–1650 nm using a tunable laser source and optical power meter. In addition, facet insertion loss was isolated by using propagation loss data from an in-line testing tool that measured silicon waveguides propagation losses, on wafer and in the same wavelength band.