In-Line Microfiber-Assisted Mach–Zehnder Interferometer for Microfluidic Highly Sensitive Measurement of Salinity

We present a microfluidic U-shaped micro-cavity sensor by splicing a segment of a microfiber of a few hundred micrometers in length tapered from a single-mode fiber (SMF) between two SMFs with a predesigned lateral offset for highly sensitive salinity measurement. The proposed sensing probe serves as an in-line microfiber-assisted Mach-Zehnder interferometer (MAMZI) with an ultra-high refractive index sensitivity of 104 nm/RIU. Three Mach-Zehnder interferometer structures with different cavity lengths of 351.82, 242.56, and 181.31 μm are fabricated, by which microfluidic sensing systems are established for in-line measurement of sodium chloride (NaCl) solution. Experimental results indicate that the detection limit of NaCl solution is as low as 4 × 10 ^-3 wt% and the response time is less than 15 s, which would make the MAMZI-based microfluidic measuring system play an important role in label-free biological and chemical detection applications.