Sensing performance of a refractometric optical sensor platform based on multimode interference couplers

This paper focuses on characterization of the sensing performance of a refractometric sensing platform based on multimode interference couplers (MMICs). Platform fabrication exploited a low-cost process using photocurable organic-inorganic hybrid sol-gel materials which were structured to form optical waveguides by direct UV laser writing. The sensing principle is based upon the high sensitivity of the optical field distribution formed in the MMIC toward changes in the refractive index of its environment. Simulations demonstrated the importance of correctly specifying the length of the MMIC section and illustrated that longer platforms are more sensitive due to a greater shift in self-image position. To characterize sensing performance, a porous sol-gel humidity sensing enrichment layer was coated on the MMIC. Relative humidity was detected by the system with a resolution of 0.097%. Refractive index resolution of the platform was determined to be ∼ 2 × 10^-6 RIU, which is an analyte-independent value and illustrates the generic nature of this platform. As such, this platform has immense potential for future applications as a label-free and real-time biosensor platform.