Simultaneous measurement of salinity and temperature of seawater based on U-shaped tapered no-core fiber

An all fiber seawater salinity and temperature sensor based on U-shaped tapered no-core fiber (UTNCF) is proposed for the first time. The sensing structure is composed of a piece of UTNCF spliced between two sections of single mode fiber (SMF). The seawater with different salinity and temperature is used as the cladding of UTNCF. Because the dips of transmission spectra have different sensitivities to the surrounding seawater salinity and temperature, the measurement of salinity and temperature of the seawater can be realized. The sensitivities of optical fiber salinity sensors based on no-core-fiber (NCF), tapered NCF (TNCF), and UTNCF are measured. On this basis, UTNCF is selected as the probe of salinity and temperature sensor. The temperature response of the sensor based on UTNCF is also measured. The salinity sensitivity of 0.69 nm/% in the salinity range of 0–15 % and the temperature sensitivity of −0.11 nm/°C in the temperature range of 19.2−35.3°C are achieved. The problem of cross sensitivity between temperature and salinity is solved by using matrix transfer method. The sensor is simple to operate, low in cost and has good stability.