Passive photonic integration of lattice filters for waveguide-enhanced Raman spectroscopy

To perform waveguide-enhanced Raman spectroscopy (WERS) or fluorescence spectroscopy in a compact device, the optical fibers to couple the passive photonic circuit to the laser source and detector require attachment directly to the die. This necessitates the integration of edge couplers and waveguide-based filters to isolate the fiber background emission from the on-chip signal, while efficiently coupling the pump laser and detector to the input and output fibers, respectively. In this work, we experimentally demonstrate the successful integration of four-port lattice filters with sensing spirals and inverse-taper edge couplers in a passive photonic circuit. We further show that the four-port lattice filter enables the collection of backscattered on-chip Stokes signal, improving and simplifying overall system performance.