Simultaneous Measurement of Seawater Salinity and Temperature With Composite Fiber-Optic Interferometer

Abstract

A seawater salinity and temperature simultaneous measurement sensor is proposed and demonstrated in this article, which consists of a Fabry-Perot interferometer (FPI) and Mach-Zehnder interferometer (MZI) composite fiber-optic structure. The femtosecond laser micromachining technique is used to modify the single-mode fiber (SMF) and inscribe three consecutively connected waveguides, in which the modified areas are corroded by hydrofluoric (HF) acid to form in-fiber channels. One internal channel forms a FPI and is selectively filled with the low refractive index UV adhesive to measure temperature. The other channel is combined with the waveguide structure to constitute a MZI, which has a wavelength shift response to salinity and temperature variations. In theoretical modeling, the sensing performances of the composite interference structure are analyzed separately. The experimental results show that the salinity and temperature sensitivities are −2.323 nm/‰ and 2.176 nm/°C, respectively. Besides, the repeatability, stability, and measurement error are tested, which shows that the sensor has good sensing performance and has the potential for seawater parameter measurements.