A compact vector magnetometer based on balloon-like fiber structure functionalized by magnetic fluid (MF) was proposed. To fabricate the sensor, the single mode fiber (SMF) was bent into a balloon-like structure, and then the shape was fixed by using the alcohol lamp flame. Because of the bending, the fiber core mode leaks into the cladding, thereby forming higher order cladding modes that are sensitive to the variations of external refractive index. When the structure is encapsulated in MF, due to the anisotropic distribution of ferromagnetic nanoparticles under magnetic field, the refractive index around the fiber structure also presents anisotropy. By measuring the refractive index of the MF under different magnetic-field directions and intensities, both the measurement of the magnetic field intensity and direction can be achieved. The experimental results show that a high magnetic field intensity sensitivity of 0.78 nm/mT and a high magnetic-field direction sensitivity of 0.094°/mT were obtained. Due to the simple fabricating method, low cost, and highly sensitivity, the proposed fiber vector magnetometer has potential applications in the navigation, aviation, and electric power industry.

In this paper, a novel optical fiber surface plasmon resonance (SPR) magnetic field sensor is proposed and experimentally demonstrated. The structure is fabricated by splicing a section of photonic crystal fiber between two multimode fibers. After the structure is coated with 10 nm Cr and 50 nm Au, the high refractive index (RI) sensitivity, from 1973.72 nm RIU ⁻¹ to 3223.32 nm RIU ⁻¹ in the range of 1.3326–1.3680, verifies the SPR sensor, which is higher than the structure based on single mode fiber with the same coating. In addition, the microscopic mechanism of the tunable characteristics of magnetic fluid RI with the ambient magnetic field is simulated by the molecular dynamics method. To measure the external magnetic field, the sensing region of the SPR sensor is fully inserted in a capillary tube, which is filled with magnetic fluid and sealed with UV glue. A maximum sensitivity of 4.42 nm mT ⁻¹ is achieved in the range of 0–24 mT, experimentally. Due to high sensitivity, simple manufacturing and compact size, the proposed sensor possesses attractive application prospects in environmental monitoring, power transmission and biomedical applications.

An all-fiber temperature and refractive dual-parameter-sensing Michelson interferometer is designed based on the waist-enlarged bitaper. At 5 mm from the fiber end, the waist-enlarged bitaper is manually spliced and the probe is formed. Since the input light encounters the waist-enlarged bitaper, it will excite high-order modes to transmit in the fiber cladding, and there will be an optical path difference between the basic mode and the higher-order mode. The light transmitted in the core and cladding is reflected upon encountering the fiber end face and the interference occurs due to the optical path difference between basic mode and higher-order mode. Changes in temperature and refractive index at the fiber probe can be detected by monitoring the interference fringes. The refractive response sensitivity is −191.06 dBm/RIU from 1.351 RIU to 1.4027 RIU, and the temperature response sensitivity is 0.12 nm/°C from 11 °C to 98 °C. Through the sensitivity matrix equation, the superimposed refractive index and temperature signals can be effectively demodulated. The sensor has the advantages of multi-parameter measurement, compact structure, low cost, easy fabrication and high reliability.

A sapphire optical fiber blackbody radiation sensor based on the high-temperature multilayer structure of wolfram and aluminum oxide has been developed. Through the analysis of Planck’s law, the linear relationship between the temperature and the square root of the voltage is theoretically deduced. Radio frequency magnetron sputtering method is used to sputter and deposit high-temperature wolfram metal film and aluminum oxide film on sapphire fiber. In order to meet the needs of real-time temperature measurement, an online acquisition system of optical signals is designed to complete the demodulation and storage of signals. The high-temperature experiment results show that the temperature response sensitivity is 0.00113 V ^1/2 /°C between 300 °C and 1750 °C, and it can work stably for more than 10 h. The sensor is calibrated in the metrology institute, where a maximum quoted error of 0.84% within a range between 900 °C and 1600 °C is proven. Compared with the traditional high-temperature measurement method, the sensor has the advantages of compact structure, antielectromagnetic interference, large measuring range, and good linearity, which can be widely used in the sensing and measurement of extreme high-temperature environments, such as aeroengine tail injection temperature monitoring and combustion chamber temperature monitoring of aeroengine simulation test benches.

In electrohydrostatic drive actuators, there is a demand for temperature and pressure monitoring in complex environments. Fiber Bragg grating (FBG) has become a promising sensor for measuring temperature and pressure. However, there is a cross-sensitivity between temperature and pressure. A gold-plated FBG is proposed and manufactured, and an FBG is used as a reference grating to form a parallel all-fiber sensing system, which can realize the simultaneous measurement of pressure and temperature. Based on the simulation software, the mechanical distribution of the pressure diaphragm is analyzed, and the fixation scheme of the sensor is determined. Using the demodulator to monitor the changes in the reflectance spectrum in real-time, the pressure and ambient temperature applied to the sensor are measured. The experimental results show that the temperature sensitivity of gold-plated FBG is 3 times that of quartz FBG, which can effectively distinguish the temperature changes. The pressure response sensitivity of gold-plated FBG is 0.3 nm/MPa, which is same as the quartz FBG. Through the sensitivity matrix equation, the temperature and pressure dual-parameter sensing measurement is realized. The accuracy of the temperature and pressure measurement is 97.7% and 99.0%, and the corresponding response rates are 2.7 ms/°C and 2 ms/MPa, respectively. The sensor has a simple structure and high sensitivity, and it is promising to be applied in health monitoring in complex environments with a high temperature and high pressure.

In this paper, a novel micro-displacement sensor with double optical fiber probes is proposed and designed, which can realize the highly sensitive sensing of longitudinal or lateral micro-displacements. The optical fiber probes are made through photopolymer formulation, and the effects of reaction time and optical power on the growth length of the probe are illustrated. The relationship between light intensity and longitudinal micro-displacement is a power function in the range of 0–100 μm at room temperature with a correlation coefficient of 98.92%. For lateral micro-displacement, the sensitivity is −2.9697 dBm/μm in the range of 0–6 μm with a linear fit of 99.61%. In addition, the linear correlation coefficient decreases as the initial longitudinal distance increases, and the function of these correlation coefficients is also linear with a linearity of 96.14%. This sensor has a simple manufacturing process, low cost, high sensitivity, and fast response speed. It is suitable for harsh environments such as strong electromagnetic interference and corrosivity, and has a broad application prospect in the field of micro-displacement sensing.