A multifunctional optical fiber sensor fabricated by asymmetric offset splicing is proposed in this Letter. The light is divided into several parts at the offset interface, among which the transmitted light forms the Mach–Zehnder interference (MZI) spectrum while the reflected light forms the Fabry–Perot interference (FPI) spectrum. The online monitoring system is built to create a better light distribution at the offset interface. Theoretical analysis and experimental verification are carried out. The results of the experiment show that the proposed sensor has good characteristics of salinity and temperature, and the salinity sensitivity is as high as − 2.4473 n m / ‰ in the range of 20–40‰; the temperature sensitivity is better than 2.17 nm/°C in the range of 28–48 °C. The two interferometers involved have different responses to temperature and salinity, contributing to the effective elimination of cross-sensitivity. The proposed optical fiber sensor has the benefits of compact size, high sensitivity, and multispectral measurement function.

In this paper, a novel platform for simultaneous measurement of relative humidity (RH) and temperature using dual Fabry–Perot interferometers (FPIs) based on C-shaped fiber was demonstrated. The sensor was composed by splicing two sections of C-shaped fiber between single mode fiber (SMF). Polydimethylsiloxane (PDMS) and polyvinyl alcohol (PVA) are filled in the two sections of C-shaped fibers to increase sensitivity to temperature and RH. This is the first demonstration that solid polymer materials can be added to the C-shaped fiber interferometers for sensing. In our experiment, RH sensitivities of -0.128 nm/%RH and 0.038 nm/%RH in the range of 20%RH to 45%RH, and temperature sensitivities of 0.022 nm/°C and -0.722 nm/°C in the temperature range of 15 °C to 45°C, were acquired for dual FPIs, respectively. We verified that it is possible to use the sensitivity matrix method to measure two parameters simultaneously. In addition, it has the benefits of simple structure, excellent stability and high sensitivity, and has a broad application prospect in agriculture, food processing and environmental measurement.

A temperature compensated magnetic field sensor using magnetic fluid (MF) filled exposed core fiber (ECF) has been proposed to detect magnetic field strength and temperature (MFT) simultaneously in this article. The designed sensor was formed by splicing an ECF between two sections of single-mode fibers (SMFs), in which the ECF’s exposed side was filled with MF. ECF is a multimode fiber that supports multiple modes of light transmitting and produces mode interference in the core. Different order modes of light excited in the ECF have individual sensitivities to MFT. Therefore, interference spectra of two different pair modes can be monitored to measure MFT simultaneously. The results show that the sensor responds well to MFT. The maximum MFT sensitivities are −0.18 nm/mT and −0.16 nm/lpzrptcircC. The limit of detections (LOD) of MFT is 1.7 mT and 1.9 lpzrptcircC. The sensor is particularly suitable for simultaneous detection of MFT in narrow spaces, such as industrial flaw detection, aerospace, and medical applications.

A reflective optical fiber sensor that is composed of two Fabry Perot cavities is proposed for temperature and salinity measurement. One of the Fabry Perot cavity which is fabricated by offset splicing is exposed to the ambient directly to sense the ambient salinity, the other is employed to compensate for the temperature coupling effect. A thin layer of the gold film is coated on the refractive index change interface, which makes the spectrum quality is no longer limited by the refractive index difference between the two sides of the interface. By applying the frequency division multiplexing (FDM) technique and cavity length demodulation technology, the measurement range is no longer restricted to the free spectrum range (FSR), which can realize salinity measurement in the full measurement range. The proposed sensor can realize the salinity sensitivity better than 50 nm/‰ and temperature sensitivity of 6.85 nm/°C in the cavity length domain. It is promising for salinity measurement application for its merits of low cost, easy fabrication, and long sensing distance.

A magnetic field measurement method based on the magneto-volume effect of magnetic fluid (MF) and Fabry-Perot (FP) interferometer (FPI) is proposed, which has higher magnetic field sensitivity and shorter response time than the traditional optical fiber methods. The MF is partially injected into the fabricated FP cavity composed of two sections of SMFs and a capillary glass tube (CGT), where the FPI is made up of optical fiber end face and MF end face. When the external magnetic field is applied to the sensor, the volume change of MF will cause the change of air cavity length. Therefore, the reflection spectrum changes with external magnetic field due to the magneto-volume effect of MF. Experimental results show that the magnetic field sensor shows a sensitivity of 268.81 pm/Gs from 15.5 to 139.7 Gs even with a small amount of MF. The response time is 0.2 s at a step magnetic field of 7.7, 31.0, 46.5, and 139.7 Gs. In this work, the magnetic field sensor boasts high sensitivity, good repeatability, short response time, and low cost, which shows a great potential in applications.

A special open-cavity Mach–Zehnder salinity sensor is presented and verified in this Letter, which has obvious advantages in salinity sensitivity and loss. The open-cavity structure is composed of a short section of etched double-side hole fiber spliced between a pair of multimode fibers and connected in series between a pair of single-mode fibers, which is the SMF-MMF-etched DSHF-MMF-SMF structure proposed in the paper. According to the experiment results, when the cavity length is about 100 µm, the salinity sensitivity of the sensing probe can reach 2 nm/‰, and its refractive index (RI) sensitivity can be more than 10,000 nm/RIU, while having a low loss of − 15 d B and a detection limit of 0.23‰. Based on its characteristics, the sensor is a prospective online monitor of ocean salinity. At the same time, it also provides a low-cost way to construct an open cavity instead of femtosecond inscribing.