Towards Passive Fiber Bragg Grating-Based Measurement of Ambient Elastic Noise for SHM Under Varying Environment

Abstract

We explore here the use of fiber Bragg Gratings (FBG) transducers in optical fibers to create a minimally intrusive structural health monitoring (SHM) system. Piezoelectric transducers, commonly used for SHM, are often intrusive, so FBG transducers offer an interesting alternative thanks to their multiplexing capabilities. However, FBG can only be used as acoustic receivers and cannot emit elastic waves like piezoelectric transducers. It has ever been demonstrated in the past years that it is possible to use ambient structure-borne elastic noise, present in the structure, to retrieve the impulse response between two sensors, known as the "ambient noise crosscorrelation" method. However, passive signals are of very low amplitude, which requires a specific care in the FBG interrogation scheme, especially in realistic environment where temperature or strain variations can affect the sensing sensitivity. We present here an optimization of an optoelectronic FBG interrogation setup to measure elastic waves at ultrasonic frequencies. The system demonstrates excellent sensitivity in active signal measurements in a varying environment thanks to the realtime tracking of the setting point. Later work will present passive measurements of impulse responses under different sources of elastic noise.