Ultra compact and sensitive optical fiber interferometric refractive index sensor

An ultra-compact and sensitive Fabry-Perot interferometer liquid refractive index sensor is presented. The device consists of an inner reflection mirror in single-mode fiber written by femtosecond laser and a hollow-core fiber. Two micro-holes are drilled on the side wall of the hollow-core fiber by femtosecond laser micromachining, which enables the liquid freely enters or leaves the hollow-core cavity. An enhanced and precisely controllable amplification factor can be obtained for the Vernier effect used, which provides an ultra-high refractive index sensitivity of − 73417.9 nm/RIU (refractive index unit). The device has ultra-compact size, simple structure, high sensitivity, and is featured with easy fabrication and good reproducing capability, thus exhibiting high potential in many sensing applications in biomedicine and chemical industry.