An open-cavity fiber Fabry-Perot interferometer fabricated by femtosecond laser micromachining for refractive index sensing

We propose and demonstrate an open-cavity fiber extrinsic Fabry-Perot (FP) interferometer (EFPI), fabricated by using 800 nm femtosecond (fs) laser micromachining, for refractive index (RI) measurement. The sensor probe consists of two single-mode fiber (SMF) and a notched hollow-core fiber (HCF) fused together to form an all-silica structure of SMFHCF-SMF. The open micro-cavity is formed by removing part of the HCF in the center of its surface by fs laser micromachining technology, allowing liquid or gaseous sample to flow in. Such a sensor can provide an extremely highprecision RI measurement of 10-6 RIU (refractive index unit) and the sensitivity can reach up to 4×105 nm/RIU within the range between 1.355 and 1.374.