Ultra-sensitive refractive index sensor based on an extremely simple femtosecond-laser-induced structure.

We demonstrate here an extremely simple, compact, and robust refractive index (RI) probe sensor based on a femtosecond-laser induced refractive index-modified dot (RIMD) fabricated near the end face of a single-mode fiber. The RIMD and the fiber end face form a Fabry-Perot interferometer, which is highly sensitive to surrounding RI. The fabrication process of the RIMD involves only one step and takes ∼0.1  s, which is extremely short, compared with other techniques. The proposed sensor exhibits an ultra-high sensitivity of ∼2523.2  dB/RIU at an RI of 1.435, which is one to two orders of magnitude higher than that of the existing intensity-modulated RI sensors. Moreover, the proposed sensor has the distinct advantages of compact size (∼50  μm), easy fabrication, and no temperature cross-sensitivity. The advantages of the device make it a promising candidate for applications in designing highly sensitive sensors in a biochemical and environmental measurement field.