Highly sensitive fiber Bragg grating refractive index sensors

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 $3\mu \mathrm{m}$ , 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 $\sim 1.5\mu \mathrm{m}$ 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 $1.4\times {10}^{-5}$ . 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.