High-sensitive FBG-based adaptive fiber laser acoustic sensing system

The all-optical fiber-based intelligent sensing system is one key technology for acoustic/ultrasonic structural health monitoring. Damages such as cracking or impact loading in civil, aerospace, and mechanical structures can generate transient ultrasonic waves, which can reveal the structural health condition. Hence, there is a great need to develop a high precision adaptive sensor for large-value strain signals with large frequency range that can extent to several hundred kilohertz in ultrasonic/acoustic sensing. In this work, we explore an intelligent system based on a fiber Bragg grating (FBG) and an erbium-doped fiber amplifier (EDFA), composing as a fiber cavity that offers significant advantages and higher performance in ultrasonic/acoustic sensing applications. The ASE light emitted from the EDFA and reflected by a FBG is amplified in the fiber cavity and coupled out by a 90:10 coupler, which is demodulated by an unbalanced Mach-Zehnder interferometer (MZI) composed by a 2X2 coupler and a 3 x3 coupler. As the reflective spectrum of the FBG sensor changes due to excited acoustic waves, the shift of the laser output wavelength is subsequently converted into a corresponding phase change. We theoretically and experimentally calculate the three output signals using a differential cross-multiplication (DCM) algorithm to directly demodulate the wavelength shift of the FBG sensor. The experimental results demonstrate that the proposed FBG acoustic sensing system has high sensitivity and can respond the ultrasonic waves into the hundreds of kilohertz frequency range, which shows a potential for acoustic emission detection in practical applications.