On-Chip Integration of a Fabry--Perot Fiber Interferometric Sensor With Digital Microfluidics for Droplet Refractive Index Sensing

Digital microfluidics (DMF) enables programmable manipulation of discrete droplets and has emerged as a versatile platform for miniaturized and automated chemical and biological assays. However, the integration of compact optical sensing units for label-free measurements at fixed and addressable locations within DMF systems remains challenging. Here, we propose the on-chip integration of a Fabry–Perot (F–P) fiber interferometric sensor with DMF for refractive index (RI) sensing. By vertically embedding a single-mode optical fiber into the top plate of a parallel-plate DMF device, a low-finesse F–P cavity is formed between the cleaved fiber end face and the bottom multilayer reflector. Electrowetting actuation enables reliable droplet extraction, transport to the sensing site, and replacement with fresh droplets while maintaining good volumetric consistency over repeated extraction cycles. Sucrose droplets with different RI were programmably generated and delivered to the sensing site. Variations in droplet RI induce stable and repeatable spectral shifts in the reflection signal, enabling quantitative RI sensing. Experimental results show that the proposed device exhibits a linear response for nanoliter-scale droplets over the RI range of 1.3368–1.3493, with a sensitivity of 914.91 nm/RIU. The proposed sensor combines the compactness of optical fiber interferometric sensing with the programmability of DMF, providing a compact integration strategy for label-free RI measurement in discrete droplet systems.