Temperature-insensitive refractometer based on an RI-modulated singlemode-multimode-singlemode fibre structure.

A temperature-insensitive refractometer based on a refractive index (RI)-modulated singlemode-multimode-singlemode (RMSMS) fibre structure is proposed and experimentally demonstrated. In this investigation, a combination of no-core fibre (NCF) and multimode fibre (MMF) regions provides an RI modulation region due to the difference in RI between the NCF and the MMF. In effect, by periodically embedding the NCF within the MMF section of a singlemode-multimode-singlemode (SMS) fibre structure, a long-period grating (LPG) can be effectively introduced in the MMF section, and the excited cladding modes are therefore able to sense surrounding RI variation. The modulation parameters are determined from the numerical simulations, and the experimental results show the maximum RI sensitivity of the fabricated sample is as high as 206.96 nm/RIU. In addition, the proposed RMSMS fibre structure is proven to be unaffected by external temperature variation (in the wavelength domain), which is a very attractive feature in practical sensing applications.