Highly-sensitive fiber-optic F-P salinity sensor based on vernier effect

A new reflective fiber-optic Fabry-Perot (F-P) salinity sensor is proposed with a section of open liquid cavity, which ensures that the seawater flows in and out freely, and the seawater can directly participate in optical transmission. Variation of seawater salinity will cause change of the effective refractive index in the optical path, which in turn affects the output spectrum. By controlling the length of the spliced fiber to form a reflection spectrum with Vernier effect, the sensitivity of the sensor to salinity changes is greatly improved. The results show that the sensor can reach a salinity sensitivity of 2.50883 nm/‰ (11511.07 nm/RIU) in the salinity range of 0 ‰–26.15 ‰, which is approximately 13.17 times higher than that of the sensor without combining Vernier effect. The sensor has good robustness, high sensitivity, and is easy to fabricate to package, which can be applied in the marine monitoring field.