Femtosecond laser-inscribed fiber-optic sensor for seawater salinity and temperature measurements

In this study, a fiber-optic sensor based on Fabry-Perot interferometer (FPI) and Mach-Zehnder interferometer (MZI) cascaded structure is proposed and demonstrated for seawater salinity and temperature simultaneous measurements. A section of hollow-core fiber (HCF) with a U-shaped defect is spliced between single-mode fibers to form FPI. The U-shaped defect inscribed by the femtosecond laser facilitates liquid flow-through and enables the salinity measurement. The MZI composes of laser-induced optical waveguides written in the cladding of the coating-retained single-mode fiber. There is an interaction between the evanescent field of the in-fiber waveguides and the coating due to the written waveguide is close to the cladding-coating interface. Part of the incident light is reflected by HCF to form the FPI spectrum, while the transmitted light in the core is coupled to the written waveguides and guided along the cladding region, which finally returned to the core to obtain the MZI spectrum. Theoretical analysis and experimental verification are carried out, and the results show that the proposed sensor offers a salinity sensitivity of 0.244 nm/‰ and a temperature sensitivity of − 2.767 nm/℃. Such a cascaded interferometer has advantages of high sensitivity and compact size, which is expected to be widely used in seawater parameter measurements due to the flexible structure and precise control.