Bending vector sensing is demonstrated based on periodic core-offset ultra-long period fiber grating, which is mainly composed of splicing two fiber ends with a purposed lateral offset in a single mode fiber (SMF).Through the measured resonance wavelength and the peak attenuation of the core-offset ultra-long period fiber grating(CO-ULPFG), the curvature amplitude and orientation can be simultaneously determined in vertical directions, reaching up to -1.833 nm/m-1 and -1.674 nm/m-1 at the 0° and 180° orientations range and reaching up to -0.583 nm/m-1 and -0.479 nm/m-1 at the 90° and 270° orientations ranging from 0 m-1 to 7.041 m-1, respectively. The bending sensitivity in different directions is 6-10 times that of LPFG with similar structure. Because of the excellent linear response, the effect of temperature on bending sensing can be eliminated by using a measurement matrix. The advantages of bending direction recognition, high bending sensitivities and compact structure make the sensor attractive for structural deformation monitoring.
An optical fiber Febry-Perot (FP) temperature sensor based on virtual vernier effect is proposed. The parallel vernier effect is realized by superposing virtual reference interferometer to improve the sensitivity of temperature detection. This method can effectively avoid the measurement error caused by the external interference of the physical reference interferometer. The FP temperature sensor was made by sealed with thermosensitive polymer into a capillary. By measuring the wavelength shift of the interference valley, the temperature sensitivity of the sensor is 2.205 nm/°C. The generative approach of the virtual interference spectrum is proposed and the theoretical simulation is carried out. The physical and virtual reference interferometers were respectively used to carry out the comparative experiments of the vernier sensitization. The temperature sensitivities obtained by the physical and the virtual reference interferometers were 10.053 nm/°C and 9.911 nm/°C in the temperature range of 51-55°C, respectively. The sensitivity magnifications were 4.559 and 4.495 times respectively, which proved the feasibility of the FP sensor demodulation by using virtual vernier effect. This method can obtain an optical fiber temperature sensing system with high-sensitivity, low complexity and low cost.
A fiber optic refractive index sensor based on tapered capillary is proposed. A multimode interferometer generated by annular high-order mode is fabricated by fused thin-walled capillary between single-mode fibers, which has high refractive index sensitivity. The experimental results show that the sensor has a piecewise-linear refractive index response with gradually increasing sensitivity in the range of 1.3328~1.4553. It is found that the increase of capillary length is beneficial to the increase of refractive index sensitivity. For the sample with 10 mm capillary, the RI sensitivity is up to 7474.959 nm/RIU, the temperature sensitivity is only 9.48 pm/°C and the measurement error is ±2.12×10-3. The results show that the sensor has good accuracy and repeatability in refractive index measurement. The sensor is suitable for high refractive index range in biological process monitoring, where high refractive index sensitivity and low temperature cross sensitivity are required.
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