Fig. 1. Refraction type salinity sensor based on optical prism method^[10]. (a) Schematic of salinity measurement system; (b) schematic of light refraction through the sample unit

Fig. 2. FBG salinity sensor. (a) Structure coated with hydrogel directly^[12]; (b) structure coated with hydrogel after etching^[13]

Fig. 3. FBG sensor that simultaneously measures salinity and temperature^[14]

Fig. 4. Salinity sensor based on F-P cavity interferometer^[21]

Fig. 5. The structure of temperature and pressure sensor based on fiber tip F-P interferometer^[18]

Fig. 6. Schematic of salinity measurement device based on Sagnac interferometer^[22]

Fig. 7. Temperature sensor based on nested annular cavity of M-Z interferometer^[28]

Fig. 8. Salinity sensor based on two-core optical fiber^[29]

Fig. 9. A cross-sectional view of PCF-based sensor^[33]

Fig. 10. Sensor system diagram based on multi-mode-single-mode-multi-mode structure^[39]

Fig. 11. The design of temperature-salinity-depth multi-parameter sensing probe based on fiber SPR^[41]. (a) Structure diagram; (b) principle diagram

Fig. 12. Schematic of salinity sensing system based on microfiber knot resonator^[50]

Fig. 13. Schematic of salinity measurement device based on Sagnac interferometer^[23]

Fig. 14. Schematic of the salinity sensor based on the microfiber-M-Z interferometer^[51]

Fig. 15. Schematic and physical photo of temperature-, salinity-, and depth-sensing experiment system based on OMC^[65]. (a) Schematic of the experiment setup for the measurement of salinity, temperature, and depth in seawater; (b) salinity measuring experimental setup and the optical microscopic image of an OMC sample (inset); (c) temperature measuring experimental setup; (d) depth measuring experimental setup

Fig. 16. Temperature-salinity-depth sensor based on OMCI^[68]. (a) Structural schematic; (b) optical microscopic image of waist region

Fig. 17. Temperature and pressure sensor structure based on PDMS encapsulated micro-nano fiber M-Z interferometer^[69].^{(a) Schematic; (b) physical image}

Fig. 18. Structure and physical photos of UFBG coated temperature and salinity sensor^[70]