High-sensitivity salinity sensor based on optical microfiber coil resonator

A simple, compact, and high-sensitivity optical sensor for salinity measurement is reported based on an optical microfiber coil resonator (MCR). The MCR is manufactured by initially wrapping microfiber on a polymethylmethacrylate (PMMA) rod, which is dissolved to leave a hollow cylindrical fluidic channel within the coil for measurement. Based on the light propagation through the MCR, the device’s spectrum moves to long wavelengths with increased salinity in the fluid. The MCR device’s sensitivity can reach up to 15.587 nm/% with a resolution of 1.28 × 10-3%. It is also confirmed that the temperature dependence is 79.87 pm/°C, which results from the strong thermal-expansion coefficient of the low refractive index epoxy. The experimental results indicate that the device can be widely used as a high sensitivity salinity sensor in water and other liquids due to its stability, compactness, electromagnetic immunity, and high sensitivity.