High-Sensitivity Salinity and Temperature Sensing in Seawater Based on a Microfiber Directional Coupler

A simple, high-sensitivity, and low-cost optical sensor for the measurement of temperature and salinity in seawater is demonstrated based on a microfiber directional coupler. The shifts of sensing dips are highly sensitive to the change of salinity and temperature in seawater with highest sensitivities at 930 pm/% and -160 pm/°C for coupler assembled by 3.79and 2.20-μm-diameter fibers. The temperature sensitivity can be further improved to -1130 pm/°C for smaller fibers with diameters of 3.50 and 1.35 μm. To keep the stable structure in liquid, a methodology of coating using ethyl cellulose ethoce film is proposed. By ethyl cellulose ethoce solvent coating on the coupling area, the coupler can be operated stably in seawater. The sensor demonstrated here is easy to construct, is low cost, is high sensitivity, and is compatible with the optical fiber system, which may find applications in developing miniature and multifunction optical sensors used in seawater and other liquid surroundings.