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.

An ultra-short fiber Bragg grating (US-FBG) refractive index (RI) sensor is presented and demonstrated. The US-FBG is composed of a series of cascaded microchannels which carved by femtosecond (fs) laser in the microprobe. According to the simulation results, an US-FBG with length/period of 149.8/5.35 μm is fabricated and used to measure refractive index (RI) of liquids. The experimental results show that the sensitivity of the proposed sensor can reach 600.5622 nm/RIU with a linear response range of 1.3323 to 1.3787. Such a compact size, high sensitivity and the inherent high reliability and ease of modulation of the US-FBG is expected to be used in the field of biosensing related to RI.

A fiber ring cavity laser temperature sensor is presented and demonstrated. The sensor probe consists of a fiber Bragg grating (FBG) and a heterostructure based on polymer-coated no-core fiber (NCF), and this probe also functions as a wavelength-selective component in the fiber laser system. There are multimode interference and antiresonance effects in the NCF coated with high refractive index polymer. Embedding this sensing head into an Er-doped fiber ring cavity laser system is helpful to improve the spectral quality and the detection limit (DL). The passband peak position of the heterostructure filter and FBG filter varies with the temperature, which leads to the corresponding shift in the output laser wavelengths. Experimental investigation achieves a maximum sensitivity of −3.155 nm/°C in the temperature range from −4 °C to 47 °C, and the DL is calculated as 0.005 °C. It should be noted that there are several measurement intervals in this temperature range, which can be distinguished by the cascaded FBG to obtain the accurate temperature value. Moreover, the sensor with a smaller NCF diameter has a higher temperature sensitivity. The proposed sensor can meet the actual demands of mesoscale ocean phenomena and microregion temperature detection in seawater, and it provides references for improving the sensing capability of optical methods in ocean detection and other practical application.

An optical fiber ultrashort polymer micro-nano waveguide Bragg grating (PMWG) for contact liquid temperature sensor was fabricated on the single-mode fiber tip by the femtosecond laser induced two-photon polymerization. This structure is composed of a conical waveguide, a micro-nano waveguide Bragg gating and a pair of tripods. The overall length of the structure is about 160 μm and the cross-sectional dimension of micro-nano waveguide is only 1.6*1.5 μm. The length of polymer micro-nano waveguide Bragg grating is less than 125 μm, which is about one fiftieth of the traditional FBG. The micro-nano waveguide grating was placed in deionized water. The surrounding liquid acts as the cladding of the micro-nano waveguide grating. The direct contact of the grating structure with the external liquid will result in faster and more sensitive liquid temperature response. In the liquid temperature corresponding experiment, the sensitivity of the micro-nano waveguide grating is up to 0.3385 nm/°C with deionized water temperature changing from 27.7°C to 49.2°C. It has a good application prospect in the field of optical fiber biochemical micro-nano detection, microfluidics, etc.