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• A highly sensitive temperature and strain sensor based on Lyot filter with Vernier effect has been demonstrated. • Because the interference spectra of the Lyot filter and fiber Sagnac interferometer are both relatively uniform, it is easy to achieve the Vernier effect. • The FSRs of the Lyot filter and fiber Sagnac interferometer can be regulated conveniently, it is easy to achieve higher magnification based on Vernier effect. • Compared with the sensors based on Lyot structure with Vernier effect, the sensor in this work realized highest magnification and sensitivity. A highly sensitive temperature and strain sensor cascaded with a fiber Lyot filter (FLF) and a fiber Sagnac interferometer (FSI) is proposed in this paper. The temperature and strain characteristics of Lyot filter without and with Vernier effect are experimentally studied. Because the free spectral ranges (FSRs) of the two interferometers can be controlled simply by adjusting the length of polarization-maintaining fiber (PMF), the Vernier effect can be realized conveniently. High temperature and strain sensitivity measurement is obtained by this structure for the first time. The experimental results indicate that the temperature and strain sensitivity of the cascade structure can reach −61.52 nm/°C and 58.63 pm/με respectively, which is about 50 times higher than that of a single FLF whose temperature and strain sensitivity is 1.23 nm/°C and 1.21 pm/με. By carefully optimizing the structural parameter, the sensitivity can be further improved, which provides a new choice for temperature and strain detection. [ABSTRACT FROM AUTHOR]

Single-mode-no-core-single-mode (SNS) optical fiber structures have valuable potential to encapsulate as high-precision temperature sensors, due to their great sensitivity. This paper is to improve the temperature sensitivity of SNS fibers by studying the parameters of fiber length, diameter, and refractive index of the seal liquid. Transmission spectra of the fibers are given in both aspects of experiment and numerical simulation. We find that using a slim no-core fiber section sealed with liquid of high refractive index can effectively improve the sensitivity, while the choice of fiber length mainly affects the width of transmission peak. The results may contribute to the development of high-sensitivity optic temperature sensor. [ABSTRACT FROM AUTHOR]

• An unclosed fiber optic current sensor based on the helically wound spun highly birefringent fiber half waveplate on the tunnel boring machine is proposed and experimentally demonstrated. • The unclosed fiber optic current sensor is designed to coil the spun highly birefringent fiber around the measured current clockwise and counter clockwise alternately in order to be mounted on the measured object with large diameter easily. • The helically wound spun highly birefringent fiber half waveplate is proposed to accumulate the Faraday effect. • The good performance of the unclosed fiber optic current sensor is confirmed by the laboratory experiment. In the application of the fiber optic current sensor that is mounted on tunnel boring machine (TBM) for measuring the probe current in tunnel geological prediction, the field installation and the influence of the linear birefringence caused by external perturbations are problems needing to be solved. In this paper, an unclosed fiber optic current sensor (UFOCS) based on the helically wound spun highly birefringent (SHB) fiber half waveplate is proposed and demonstrated. The UFOCS is designed to coil the fiber around the measured current clockwise and counter clockwise alternately in order to be mounted on the measured object with large diameter easily. The all-fiber half waveplate made by the helically wound SHB fiber is used to accumulate the Faraday effect. The simulation is carried out based on the polarization model and a linear response is obtained. The laboratory experiment is conducted and the results confirm the good performance of the UFOCS. [ABSTRACT FROM AUTHOR]

• A novel optical fiber sensor based on MMF-FCF-MMF structure is proposed. • The maximum fringe visibility of the interference resonance dips exceeds 26.3 dB. • The MMFs are used to improve the light coupling efficiency between the input SMF and the FCF. • The sensor has advantages of simple fabrication process, good stability, repeatability and high fringe visibility. In this paper, a novel optical fiber sensor based on multimode fiber (MMF) - four-core fiber (FCF)-MMF structure is proposed and experimentally demonstrated. The device consists of a segment of FCF sandwiched between two MMFs. The MMFs are used to improve the light coupling efficiency between the input single mode fiber (SMF) and the FCF, and the FCF is used as sensing arm. Due to the multipath interference configuration embedded in one fiber, higher phase sensitivity can be obtained in our scheme. The maximum fringe visibility of the interference resonance dips exceeds 26.3 dB. The interference fringe of the sensor would shift with the variation of liquid level and temperature. So it is possible to measure dual-parameter simultaneously by simply monitoring the different wavelength shift. For 10 pm wavelength resolution, the resolution of the sensor is 0.14 cm and 2.08 °C in liquid level and temperature, respectively. Moreover, the proposed sensor has the advantages of simple fabrication process, good stability, repeatability and high fringe visibility, which mean it has a broad application prospect in liquid level measurement. [ABSTRACT FROM AUTHOR]

• A Pb2+ sensor with BP nanosheets integrated tapered optical fiber is manufactured. • The exciting cladding mode interferes with the fundamental mode in the core, realizing rapid response to external changes. • The adsorption complexes formed by the electrostatic adsorption of BP nanosheets and Pb2+ will change the effective refractive index of the fiber surface, and cause the interference spectrum to shift. • The on-line optical fiber sensor not only exhibits high sensitivity for the detection of trace Pb2+ but also has a very low detection limit. In this paper, a high-sensitivity Pb2+ on-line sensor based on black phosphorus (BP) integrated tapered fiber is proposed. The BP nanosheets which are deposited on the surface of the tapered fiber by light deposition technology are obtained by one-pot solvothermal reaction and ultrasonic peeling in the laboratory. Due to the electrostatic adsorption between BP and Pb2+, the effective refractive index (RI) of the tapered fiber surface is changed by the adsorption complexes, which eventually causes the interference wavelength to shift. The experimental results show that the sensor has a sensitivity of 0.03714 nm/ppb when the range of Pb2+ solution concentration is 0.1–105 ppb, and its limit of detection (LOD) is 0.0206 ppb. The response time of 1 ppb and 106 ppb Pb2+ solution is 44 s and 49 s, respectively. Meanwhile, the sensor has good stability after repeated measurement of the same concentration solution. In general, the sensor has the characteristics of simple preparation, high sensitivity, low detection limit, integration, and low-cost, and has potential application prospects to detecting trace Pb2+ in water quality monitoring. [ABSTRACT FROM AUTHOR]