Laser-induced micro-cavity on a fiber end with a harmonic-Vernier effect for salinity sensing

In this paper, a fiber optic salinity sensor based on Fabry–Perot interference (FPI) with a harmonic-Vernier effect is suggested. An open cavity and a resin solid cavity are grown at the end of the fiber by a unique fiber-end photopolymerization and integration (FEPI) technique. When seawater fills the open cavity, it changes the optical path length (OPL) of the interferometer directly. While the OPL of the reference interferometer is hardly impacted by the seawater. The insertion of the harmonic envelope shifts greatly with salinity. Tunable sensing performances are achieved by adjusting the cavity length ratio(CLR) according to a theoretical model. The maximum sensitivity of 6.53 nm/‰is attained in the range of 25 ‰to 45 ‰when its CLR approaches 1.18. In addition, the measuring accuracy, consistency, and temperature compensation method are also experimentally investigated. The suggested sensor is a prospective competitor in the field of measuring the salinity of seawater due to the advantages of high sensitivity, compact size, large dynamic range, and adjustable capabilities.