In this letter, we propose a compact hybrid fiber-optic sensor for seawater temperature and salinity simultaneous measurements. The device consists of a hollow-core fiber (HCF)-based Fabry-Perot interferometer (FPI) and no-core fiber (NCF)-based anti-resonance (AR) structure. A U-shaped groove is inscribed in the HCF for salinity sensing, and a section of the polymer coating is retained on the NCF for temperature measurement. The incident light is partially reflected at the HCF fusion end-faces and forms the FPI spectrum, while the transmission light in the NCF cladding is coupled to the polymer coating and forms the AR phenomenon. In theoretical modeling, the temperature and salinity responses of the hybrid sensor are analyzed separately, and the corresponding sensitivities are calculated theoretically. In the experiment, the results show that the temperature and salinity sensitivities are -4.948 nm/°C and 0.235 nm/‰, respectively. In addition, the calibration test, time stability, and repeatability of the sensor are also evaluated by the experiments. The construction method of this hybrid sensing structure is flexible and accurate, so it is expected to be applied in the seawater parameter measurements.

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.

A novel magnetic field measurement method based on the magneto-volume effect is proposed. The magnetic field sensing structure fabricated by the proposed measurement method is made by the magnetic fluid (MF) partially filled into the hollow core fiber (HCF). The presence of air provides conditions for the change of MF volume with the magnetic field in the proposed sensing structure. Therefore, the magneto-volume effect is introduced into the magnetic field sensing mechanism. From 109.6 to 125.8 Gs, the peak wavelength shift is −65480 pm and the sensitivity is −4219.15 pm/Gs, which is 28.16 times higher than that of the fabricated contrast sensing structure. The sensitivity of the magnetic field sensing structure is greatly improved by introducing the magneto-volume effect compared with the contrast sensing structure. The response time and recovery time of the sensing structure are equally 0.2 s. Therefore, the sensing structure has high sensitivity, good linearity, and short response time, which has potential application prospects in magnetic field sensing.