Simultaneous measurement of temperature and salinity of seawater based on the semi-encapsulated tapered no-core fiber

An optical fiber sensor for simultaneously measuring temperature and salinity of seawater based on polydimethylsiloxane (PDMS) and tapered no-core fiber (TNCF) is proposed. The sensing probe consists of a section of TNCF half coated with PDMS spliced between two sections of single mode fiber (SMF). PDMS is used as not only the temperature sensitive material due to its high thermal-optic coefficient but also the encapsulant. The sensing probe is immersed in the seawater with different temperature and salinity. Because the dip wavelengths in the transmission spectra have different sensitivities to the temperature and salinity of seawater, simultaneous measurement of temperature and salinity can be achieved using the transfer matrix method. The temperature and salinity responses of the sensing probe based on the SMF - TNCF - SMF (STNS) structure with TNCF half coated with PDMS are experimentally measured. The maximum temperature sensitivity is about −2.21 nm/°C with the range from 25 °C to 31 °C and the maximum salinity sensitivity is 1.43 nm/% in the salinity range of 0–9 %. In the experiment of simultaneous measurement of temperature and salinity, the average temperature and salinity relative errors are 1.54 % and 2.1 %, respectively. The proposed sensor has the advantages of simple fabrication, low cost, robust and compact structure and good stability.