Silicon nitride biosensor with an optimized nonlinear fiber-to-chip coupler

Abstract

Silicon nitride is a CMOS-compatible platform ideal for label-free optical biosensors; however, achieving efficient fiber-to-chip coupling typically requires large device footprints, which limit chip integration. This work presents a Mach-Zehnder interferometric biosensor integrated with a compact nonlinear inverse taper designed to address this challenge. The proposed coupler achieves a coupling efficiency of 99.28% within a total length of only 700 μm. This design yields a footprint reduction of 68.2% compared to conventional linear tapers, which require millimeter-scale lengths to achieve comparable performance. Furthermore, the biosensor uses an optimized rib waveguide geometry to maximize sensing performance. The sensing capability was validated through simulations of blood serum refractive index changes, in which the system demonstrates a sensitivity of 1630 nm/RIU and a limit of detection of 8.59×10-5 RIU. These results highlight the potential of the proposed design as a compact, portable and sensitive lab-on-chip biosensing platform.