A new fiber Bragg grating (FBG) sensor that can measure temperature, vibration, and strain is proposed in this article. The structure of this sensor is designed and optimized via theoretical analysis and the finite-element method (FEM). With an innovatively designed L-shaped cantilever (LSC), this designed sensor can better balance the vibration measurement sensitivity and the resonant frequency. The strain measurement structure is designed as a hollow square so that the reaction force of the sensor for the measured object is only 2.76×10 ^-3 N/ με, which ensures that the strain distribution of the measured object is not affected. The designed sensor can significantly amplify the measured strain with a measurement sensitivity (5.44 pm/ με) that is four times greater than that of the bare FBGs. This article uses single-mode FBGs inscribed by a femtosecond laser (FS-FBG) to make the sensors, and the sensing performance is analyzed. The results show that this FS-FBG-based sensor can withstand temperatures up to 1100 °C.

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.

A combined stress-vibration sensor was developed to measure stress and vibration simultaneously based on fiber Bragg grating (FBG) technology. The sensor is composed of two FBGs and a stainless steel plate with a special design. The two FBGs sense vibration and stress and the sensor can realize temperature compensation by itself. The stainless steel plate can significantly increase sensitivity of vibration measurement. Theoretical analysis and Finite Element Method (FEM) were used to analyze the sensor’s working mechanism. As demonstrated with analysis, the obtained sensor has working range of 0–6000 Hz for vibration sensing and 0–100 MPa for stress sensing, respectively. The corresponding sensitivity for vibration is 0.46 pm/g and the resulted stress sensitivity is 5.94 pm/MPa, while the nonlinearity error for vibration and stress measurement is 0.77% and 1.02%, respectively. Compared to general FBGs, the vibration sensitivity of this sensor is 26.2 times higher. Therefore, the developed sensor can be used to concurrently detect vibration and stress. As this sensor has height of 1 mm and weight of 1.15 g, it is beneficial for minimization and integration.

A temperature multipoint sensing system based on three cascade Mach–Zehnder interferometers (MZIs) is introduced. The MZIs with different lengths are fabricated based on waist-enlarged fiber bitapers. The fast Fourier transformation is applied to the overlapping transmission spectrum and the corresponding interference spectra can be obtained via the cascaded frequency spectrum based on the inverse Fourier transformation. By analyzing the drift of interference spectra, the temperature response sensitivities of 0.063 nm/°C, 0.071 nm/°C, and 0.059 nm/°C in different furnaces can be detected from room temperature up to 1000 °C, and the temperature response at different regions can be measured through the sensitivity matrix equation. These results demonstrate feasibility of multipoint measurement, which also support that the temperature sensing system provides new solution to the MZI cascade problem.

A Mach–Zehnder fiber optic sensor with high refractive index response sensitivity was developed. By fabricating a waist-enlarged bitaper structure on the interference arm of a single mode–multimode–single mode (SMS) Mach–Zehnder interferometer (MZI), the spectral contrast and response sensitivity were improved. Subsequently, the response sensitivity was further improved by etching the interference arm. When a beam of light was introduced into the sensor, due to the structural mismatch between the multimode fiber and the normal transmission light, the difference between the low-order mode and the high-order mode was generated in the fiber core and the fiber cladding. In the process of transmission in the sensing arm, due to the different refractive indices of the core and cladding, the optical path difference of the high-order mode and the low-order mode was different, which eventually generated interference fringes. The experimentally measured response sensitivity of SMS MZI in the range of 1.351 RIU to 1.402 RIU is 57.623 nm/RIU; the response sensitivity of a single mode–multimode–bitaper–multimode–single mode (SMBMS) MZI is 61.607 nm/RIU; and the response sensitivity of the etched SMBMS (ESMBMS) MZI is 287.65 nm/RIU. The response sensitivity of the new ESMBMS MZI is three times higher than that of the original SMS MZI. The sensor has the characteristics of compact structure, high sensitivity, easy manufacture, and a wide range of refractive index measurements, and can be used in food processing, pharmaceutical manufacturing and other fields.

At present, flexible pressure-sensitive materials generally have problems with weak adhesion and poor wear resistance, which easily result in friction failure when used for plantar pressure detection. In this study, a flexible pressure sensor with the advantages of a wide detection range, fast recovery, and good abrasive resistance was designed and prepared based on the screen printing process. The pressure-sensitive unit with a structural size of 5 mm× 8 mm was a functional material system due to the use of graphene and epoxy resin. The influence of the different mass ratios of the graphene and epoxy resin on the sensing properties was also studied. The test results showed that when the mass ratio of graphene to epoxy resin was 1:4, the response time and recovery time of the sensing unit were 40.8 ms and 3.7 ms, respectively, and the pressure detection range was 2.5–500 kPa. The sensor can detect dynamic pressure at 0.5 Hz, 1 Hz, 2 Hz, 10 Hz, and 20 Hz and can withstand 11,000 cycles of bending. In addition, adhesion tests showed that the high viscosity of the epoxy helped to improve the interlayer bond between the pressure-sensitive materials and the flexible substrate, which makes it more suitable for plantar pressure detection environments, where friction is common.