Ultrashort Weak Fiber Bragg Grating-based High Sensitivity Optical Fiber Sensor for Low Refractive Index Range

We demonstrate an optical fiber refractive index (RI) sensor based on an ultrashort weak fiber Bragg grating (FBG), which exhibits a high sensitivity for the low RI range. The FBG and the single mode fiber end face form an in-fiber Fabry-Perot interferometer (FPI). Theoretical and experimental results have demonstrated that the proposed FPI sensor is highly sensitive in the low RI region owing to a proper choice of the reflectivity and the length of the FBG. The FBG with the ultrashort length of 50 μm and low reflectivity of ~0.21% is inscribed by a femtosecond laser line-by-line technology and used to fabricate the FPI sensor. The fabricated FPI sensor with the short cavity of 129 μm achieves a high sensitivity of 489.5 dB/RIU at the RI of 1.333, which is almost twice as high as that of the existing intensity-modulated in-fiber RI sensors. The FPI RI sensor prepared is used to detect the concentration of NaCl solution and shows a small measurement error. Furthermore, the proposed RI sensor has the low temperature-cross sensitivity and compact length, which is expected to be used for the RI detection in most chemical and biological applications.