A compact, highly sensitive optical fiber displacement and curvature radius sensor is\npresented. The device consists of an adiabatic bi-conical fused fiber taper spliced to a single-mode\nfiber (SMF) segment with a flat face end. The bi-conical taper structure acts as a modal coupling\ndevice between core and cladding modes for the SMF segment. When the bi-conical taper is bent by\nan axial displacement, the symmetrical bi-conical shape of the tapered structure is stressed, causing a\nchange in the refractive index profile which becomes asymmetric. As a result, the taper adiabaticity is\nlost, and interference between modes appears. As the bending increases, a small change in the fringe\nvisibility and a wavelength shift on the periodical reflection spectrum of the in-fiber interferometer is\nproduced. The displacement sensitivity and the spectral periodicity of the device can be adjusted by\nthe proper selection of the SMF length. Sensitivities from around 1.93 to 3.4 nm/mm were obtained\nfor SMF length between 7.5 and 12.5 cm. Both sensor interrogations, wavelength shift and visibility\ncontrast, can be used to measure displacement and curvature radius magnitudes.
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