Analysis of HF-Etched Uniformly Thinned Linearly Chirp Types of Refractive Index Profile Fiber Bragg Grating Sensor

This article presents a numerical and experimental analysis of the linear chirp types of refractive index profile etched fiber Bragg grating (e-FBG) sensor for the first time. Prior study via an analytical method on e-FBG-based sensing system predominantly deals with homogenous core step refractive index profile only, where the sensitivity is enhanced by etching the cladding diameter through a hydrofluoric (HF) solution. We have developed the finite-difference method (FDM) to simulate the Bragg wavelength shift of complicated arbitrary chirp types of refractive index profile e-FBG sensors and validate the results with analytical means for some trivial cases. The Bessel function theory of double-clad optical fiber also validated the experimental results for the known case. In the experiment, fiber-clad diameter has been continuously etched by using 45% concentrated HF acid while monitoring the Bragg wavelength shift on a multichannel optical interrogator system. The chirp types of the refractive index profile FBG could not be available at the time of the experiment, hence step refractive index homogenous core FBG has been taken in the proposed experiment for the proof-of-concept purpose only. Numerical results of sensitivity and tunability of chirp types of e-FBG have been well compared with the existing e-FBG-based sensor. The maximum resolution of the proposed chirped e-FBG sensor is found to be ${\approx } {10}^{-{6}}$ , which can measure the minimum change of refractive index ${\approx } {10}^{-{6}}$ .