Salinity sensing based on microfiber knot resonator

Salinity measurement has become one of the key technologies in marine detection since salinity is an important parameter in oceanography. This paper demonstrates salinity sensing in NaCl solution based on the microfiber knot resonator by estimating its sensitivity from the relationship between the resonance wavelength red-shifts and the increment of salinity. The dependence of sensing sensitivity on microfiber diameter and probing wavelength are also investigated. Results show that sensing sensitivity increase with the decreasing microfiber diameter (3.5 μm-2.5 μm) and the increasing probing wavelength (1500 nm-1650 nm), which are in good agreement with theoretical results. By optimizing our sensing system, the maximum sensitivity is 21.18 pm/‰, which is 16 times higher than that of fiber Bragg grating. Such type of salinity sensor demonstrated here develops a new optical method to measure salinity of liquid and offers useful references for measuring salinity in actual seawater so that ultra-compact optical devices for salinity sensing of ocean can be further developed.