Refractive index sensing measurement based on periodically tapered small core singlemode fibre

An all-fibre refractive index sensor with a simple periodical tapers configuration is proposed and investigated experimentally. The proposed fibre refractive index sensor consists of a small core singlemode fibre with tapers periodically fabricated along the fibre using a focused CO2 laser beam, and sandwiched between two standard singlemode fibres. Such a structure can be used for sensing of refractive index by measuring the dip wavelength shift of the multimode interference within the small core fibre cladding. A minimum sensitivity of 125 nm/RIU is measured for a refractive index of 1.33 and a maximum sensitivity of 383 nm/RIU for a refractive index of 1.38. The proposed refractive index sensor benefits from simplicity and low-cost and achieves a competitive sensitivity compared with other fibre-optic sensors.