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A novel and in-line fiber low-frequency vibration sensor based on Butterfly-Shape Mach-Zehnder Interferometer (BSMZI) is proposed and demonstrated experimentally. The sensing element is a tapered hollow-core fiber (HCF) sandwiched between two single-mode fibers (SMFs). The frequency measured by the Butterfly-Shape vibration sensor can well match with the applied frequency, and the presence of the higher harmonics can verify that the sensor does not distort the measured vibration signal. The maximum relative error of vibration frequency is 0.27%. The intensity of vibration signal (of the fiber sensor) is linearly proportional to the driving power of vibration platform. The vibration measurement sensitivity is 13.575 dB/w in the range of 0.1 w to 0.5 w under a constant vibration frequency of 50 Hz, and the linear fitting coefficient is 0.9556. The proposed sensor has a negligible temperature sensitivity in the range of 30.3 °C–69.5 °C. Because of its compact structure, easy preparation, low production cost, high frequency measurement accuracy of low-frequency vibration, and good linearity of vibration sensing, the proposed BSMZI sensor has potential application prospect in low-frequency vibration sensing field. [ABSTRACT FROM AUTHOR]

A miniature and highly sensitive optic fiber temperature sensor using a PDMS-filled air-microbubble Fabry-Perot (FP) interferometer (PAFPI) in a hollow-core fiber (HCF) is proposed. The sensor is fabricated by fusion splicing a single mode fiber (SMF) with a HCF, which is filled with polydimethylsiloxane (PDMS) by capillary effect, to form an air microbubble cavity. As far as we know, this is the first time that PDMS is applied to fill the hollow core fiber. There are three main reflected surfaces in the PAFPI sensor, and the superimposed interference spectrum of double FPI (a thin FPI and a thick FPI) is formed. Hence, the proposed sensor combines two advantages that are the large free spectral range (FSR) of the thin FPI and the high resolution of the thick FPI, which improve the resolution of spectral demodulation while ensuring a large dynamic range. Experimental results show that the sensor has the high temperature sensitivity of 2.7035 nm/℃ with a high linear response (R-square: 0.9962) in the range of 51.2–70.5 ℃, which is almost three orders larger than that of the conventional all-fiber air-cavity FP temperature sensors (about 10 pm/℃). The stability experiment is also carried out, and the maximal variations of the wavelength shift and the extinction ratio of the interference fringes are 20.3 pm and 0.1262 dB, respectively, for a long-term testing. Therefore, it may be applied to the surrounding temperature sensing. [ABSTRACT FROM AUTHOR]