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Bent-fiber intermodal interference based dual-channel fiber optic refractometer.

We present a novel dual-channel fiber optic interferometer based on intermodal interference from single-mode fiber (SMF) bending. This dual-channel interferometer has simple structure, consisting of two bare fiber semicircular bending regions with different bending radiuses connected by a section of straight fiber. A dual-channel interferometer with bending radiuses of 4 mm and 4.3 mm is fabricated and refractive index (RI) sensing is realized by measuring the wavelength shift of the resonance dips in the transmission spectrum of the dual-channel interferometer. In the RI range of 1.3403 to 1.3726, the corresponding RI sensitivities for these two channels are 207 and 245 nm/RIU (refractive index unit) and the RI resolutions are about 6.57 × 10⁻⁵ RIU and 5.55 × 10⁻⁵ RIU, respectively.