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A novel parallel Fabry-Perot interferometer (FPI) assisted by a concave cavity structure and an external reflector, which can be employed for large-range displacement and temperature sensing, is proposed and experimentally demonstrated. Therein, the concave cavity consists of a single mode fiber (SMF), a graded index fiber (GIF) and a hollow core fiber (HCF), which are fusion spliced to realize flat and concave surfaces of SMF-GIF and GIF-HCF, respectively. The certain length of GIF is utilized to serve as an collimator, facilitating the reduction of the divergence angle and the expansion of the mode field radius of the propagating beam. Thus the propagating beam can split into the concave cavity structure’s sidewall and inner air area, when the concave cavity size is smaller than the mode field diameter of the beam from GIF. The external Fabry- Perot interferometer (EFPI), formed by GIF endface and the external reflector, is utilized to detect displacement. Meanwhile, the intrinsic Fabry-Perot interferometer (IFPI), formed by the endfaces of GIF and HCF, serves as a temperature sensor. The length of the silica cavity changes with temperature due to the thermal expansion and thermo-optical effect of silica. In consequence, the corresponding wavelength shift can be accurately observed in response to temperature variations. The experimental results indicate the displacement sensing range of EFPI is up to 30 mm, which is greatly increased compared to other FPI displacement sensors. Moreover, the proposed sensor has a temperature sensitivity of 11.52 pm/◦C in the range of 30◦C 100 ◦C.
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