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 (SMF) 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$50~\mu \text {m}$ </tex-math></inline-formula> and low reflectivity of ~0.21% is inscribed by a femtosecond laser line-by-line (LbL) technology and used to fabricate the FPI sensor. The fabricated FPI sensor with the short cavity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$129~\mu \text {m}$ </tex-math></inline-formula> 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.