Optical Fiber Sensor for Temperature and Salinity Based on Femtosecond Laser-Inscribed Semiopen Cavity and Waveguide

A compact semiopen cavity (SOC) sensing structure for simultaneous measurement of temperature and salinity is proposed. This structure is formed by splicing lead-in single-mode fiber (SMF), hollow core fiber (HCF), and lead-out SMF, which is called SHS structure for short. Based on this structure, femtosecond laser micromachining technology (FT) is introduced to inscribe waveguide and cavity, realizing the integration of Fabry–Perot interferometer (FPI) and Mach–Zehnder interferometer (MZI). The writing of the cladding waveguide provides the basis for the realization of MZI, and the SOC realizes the direct contact between the sensing arm and the measured substance, which provides a basis for high-sensitivity measurement. According to the experimental results, the high resolution of 0.01 °C in temperature and 0.03‰ in salinity is realized. Moreover, it has good stability, repeatability, and reproducibility. The sensing structure shows the prospect of measuring ocean temperature and salinity.