Highly sensitive fiber Bragg grating refractive index sensors

TLDR

The study combines fiber Bragg grating technology with wet chemical etch‑erosion to demonstrate two types of refractive index sensors on single‑mode optical fibers. The authors fabricate two end‑of‑fiber sensors by wet‑etching single‑mode fibers: a 3 µm‑radius FBG for four liquids and a 1.5 µm‑radius fiber Fabry‑Pérot interferometer for isopropyl alcohol solutions, both measured via reflected signals. The FFPI sensor outperforms the FBG sensor, achieving a sensitivity that detects refractive‑index changes as small as 1.4 × 10⁻⁵.

Abstract

We combine fiber Bragg grating (FBG) technology with a wet chemical etch-erosion procedure and demonstrate two types of refractive index sensors using single-mode optical fibers. The first index sensor device is an etch-eroded single FBG with a radius of 3μm, which is used to measure the indices of four different liquids. The second index sensor device is an etch-eroded fiber Fabry-Pérot interferometer (FFPI) with a radius of ∼1.5μm and is used to measure the refractive indices of isopropyl alcohol solutions of different concentrations. Due to its narrower resonance spectral feature, the FFPI sensor has a higher sensitivity than the FBG sensor and can detect an index variation of 1.4×10−5. Since we can measure the reflection signal, these two types of sensors can be fabricated at the end of a fiber and used as point sensors.