A novel compact ultra-high sensitivity optical fiber temperature sensor based on surface plasmon resonance (SPR) is proposed and demonstrated. The sensor is fabricated by employing a helical-core fiber (HCF), which is polished as a D-type fiber on the helical-core region and coated with a layer of Au-film and polydimethylsiloxane (PDMS). The theoretical and experimental results show that the resonant wavelength and sensitivity of the proposed sensor can be effectively adjusted by changing the twisting pitch of HCF. Due to the high refractive index sensitivity of the sensor and the high thermo-optic coefficient of PDMS, the maximum sensitivity can reach -19.56 nm/°C at room temperature when the twist pitch of HCF is 2.1 mm. It is worth noting that the sensitivity can be further improved by using a shorter pitch of HCF. The proposed SPR temperature sensor has adjustable sensitivity, is easy to realize distributed sensing, and has potential application prospects in biomedical, healthcare, and other fields.

Embedding of optical fiber sensors into materials to detect and monitor the deformation and damage of structures has attracted much attention recently. However, the embedding of optical fiber sensors into metallic materials is usually difficult due to lack of suitable technique, which does not damage the optical fiber during metal processing. In this paper, we introduced a very high power ultrasonic additive manufacturing (UAM) for producing metallic structure optical fiber sensors. Single-mode optical fibers (SMFs) were successfully embedded in aluminum (Al 1100) matrix by this technique. Tensile strain experiments were performed by a fiber optical white-light interferometric sensing system, and the effect of consolidation contact pressure of UAM on the output strain-applied strain relation was examined systematically. As the appropriate contact pressure of 1300 to 1350 N was taken, the excellent and reliable out strain-applied strain relations were obtained. These results demonstrate that the UAM produced optical sensor like a “nervous system” is capable of monitoring the health condition of metallic structural device.

A tungsten disulfide (WS₂) coated surface plasmon resonance (SPR) sensor based on gradient pitch Mach-Zehnder interferometer (GP-MZI) for measuring ethanol vapor concentration is proposed and verified by experiments. Under continuous CO₂ laser heating, a MZI based on GP helix structure is fabricated by twisting single mode fiber (SMF), which can excites multi-order cladding modes. A gold film is deposited on the surface of the GP helix structure by a magnetic sputtering coating machine. WS₂ film is coated on the gold film of the GP helix structure, which increases the evanescent field strength of the twisted structure surface and enhances the interaction between SPR wave and ethanol molecules. Since the absorption of ethanol molecules by WS₂ sheets will cause the change of effective refractive index (RI) of WS₂ film, the intensity of transmission signal can be adjusted accordingly. For multi-order cladding modes, the effective RI and the effective thermo-optic coefficient vary with the modal order, so the RI and temperature sensitivity of different modal orders are also different. So, the ethanol vapor concentration, relative humidity (RH), and temperature can be simultaneously measured by monitoring the intensity of those dips with the resolution of ± 0.030 mg/L, ±0.035%RH, and ± 0.010 ℃, respectively. This sensor structure provides a promising platform for multi-parameter sensing applications.

A novel pressure vector sensor based on a short section of PANDA fiber with an orthogonal optical path Sagnac interferometer structure is proposed and experimentally demonstrated. The sensor structure was realized using a gold film, coated on the end of the fiber tip through the magnetron sputtering method. The birefringence-dependent interference dip is sensitive to external force. Therefore, pressure can be monitored by this sensor. The relationship between the force direction and the pressure sensitivity was studied. We embedded the sensor in aluminum, using ultrasonic consolidation technology, to investigate the application of its sensing properties in metal. Based on this, the influence of the embedding direction on the polarization characteristics of the fiber was analyzed. The experimental results showed that the sensor offered a high sensitivity of 2330 pm/(N·m) in its freeform and 780 pm/(N·m) after being embedded in aluminum. Due to its simple fabrication process, low cost, and high sensitivity, the pressure sensor described in this paper could be a competitive candidate in several pressure sensing applications.

An all-fiber bend sensor based on Mach-Zehnder interferometer (MZI) is proposed and experimentally demonstrated. The fiber interferometer consists of two helical structures in single mode fiber (SMF) and is fabricated by continuous arc discharge technique. The first helical structure forms a twisted long-period fiber grating (LPFG) to excite the higher-order cladding modes and acts as a beam splitter. The LPFG arrangement is adopted to improve the bending sensitivity. The second helical structure only contains two turns and can recouple the light in the cladding back into the core. The bending characteristics of the MZI are discussed. The experimental results show that high curvature sensitivities of 24.06 nm/ $\text{m}^{-1}$ and −7.524 dB/ $\text{m}^{-1}$ are obtained in the curvature range of 0-0.422m ⁻¹ . Due to its low cost, small size, simple preparation, less time consuming, it could be a competitive candidate in many bending sensing applications.

A high sensitivity strain sensor based on micro-helix micro taper long period fiber grating (MH-MTLPFG) is proposed and experimentally investigated. The MH-MTLPFG is fabricated by periodically tapering and micro helix structure on a standard single-mode fiber with arc discharge technology. The arrangement of micro helix taper makes the sensor have high strain sensing sensitivity. The experiment results show that the sensitivity of strain sensing is up to −3.478 pm/ $\mu \varepsilon $ . Compared with the conventional MTLPFG sensor, the strain sensitivity is improved by about 10 times. The proposed high sensitivity strain sensor has a good potential for accurate strain measurement, due to its low cost, simple fabrication and good performance.