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An in-fiber refractometer based on asymmetrical twin-core fiber

We demonstrate a compact fiber Michelson interferometer for refractive index (RI) sensing which is composed of an asymmetrical twin-core fiber (TCF) with a central core and a side core. By chemical etching a segment of the TCF to make the side core is nearly exposed, the effective RI of the side core mode is sensitive to the RI change of the external medium. Therefore, the optical path length difference between the central and side cores will vary with the ambient refractive index, which leads to a shift of the transmission spectrum of the Michelson interferometer. The experimental results show that the device has a refractive index resolution of ~10-5 in the range of 1.34-1.38.