Temperature-Insensitive Refractometer Based on a Wave-Shaped Fiber Modal Interferometer Using No-Core Fiber

Abstract

We propose a novel, temperature-insensitive fiber optic sensor for refractive index (RI) measurement. Our refractometer is based on multimodal interference in a wave-shaped configuration with localized reverse curvature, which is composed of a concatenating standard single-mode fiber (SMF)-no-core fiber (NCF)-SMF structure. The wave-shaped fiber optic refractometer is composed of an SMF-NCF-SMF combination, in which the length of the NCF located at the top of the structure, is only 4 mm. The main wave-shaped fiber has a curvature of 20 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , while the wave-shaped NCF section has a curvature of -0.0055 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . The NCF section with no polymer coating, which acts as a sensing head, is modified to have a curvature opposite to the curvature of the whole wave-shaped optical fiber, by applying localized external pressure. The RI measurement sensitivity of our proposed refractometer was estimated at approximately -53.24 nm/RIU, while its temperature sensitivity was 0 pm/°C.