Inverse-Designed Silicon Nitride Bimodal Mode Converter for Interferometric Sensors

High fringe visibility is essential for achieving high sensitivity and low detection limits in bimodal interferometric sensors, as it depends critically on balanced optical power between the interfering modes. Compact bimodal mode converters (BiMCs) that enable precise modal power control are therefore key components for integrated interferometric biosensors. However, conventional approaches based on shifted junctions or periodic waveguide structures often lack flexibility and exhibit limited efficiency, particularly for higher-order mode excitation. Here, we experimentally demonstrate compact inverse-designed BiMCs on silicon nitride (SiN) capable of exciting multiple supported mode pairs within footprints as small as 224 μm² while maintaining near-balanced modal power distributions. Devices were implemented on two SiN platforms with core thicknesses of 400 and 300 nm, confirming the versatility and fabrication robustness of the inverse-design approach. Experimental characterization shows average coupling efficiencies of 41.1% and 46.0% across the C band, with a near-ideal modal power balance of 45.1%/45.0% for the ${\text{TE}}_{00}$ - ${\text{TE}}_{02}$ pair. Furthermore, a complete ${\text{TE}}_{00}$ - ${\text{TE}}_{03}$ bimodal interferometer incorporating a 15-mm multimode waveguide achieved an extinction ratio of 20.6 dB, corresponding to a fringe visibility of $\sim$ 0.98, among the highest reported for SiN bimodal interferometers.