Fiber Bragg Gratings (FBGs) are promising ultrasound transducers, especially for Structural Health Monitoring (SHM), since they can be seamlessly integrated into structures and multiplexed, and can also sustain harsh environments (extreme temperatures, radiations, electromagnetic environments…). However, their widespread use for ultrasound measurements has been limited until now because the sensitivity of the edge filtering technique, as implemented so far, is strongly impacted by environmental conditions such as temperature or deformation of the host structure, leading to a loss in sensitivity. Indeed, edge filtering is based on a narrow band light source locked on the side of the reflection spectrum of the FBG. Under varying environmental conditions, this reflection spectrum will shift and, without proper action, the setting will be lost. We present here a solution enabling measurements under varying environmental conditions based on the low frequency tracking of the setting point in order to keep the sensitivity of this method at an acceptable level. The tracking is based on a retroaction loop on the DC output of the photodiode, giving an image of the relative position of the narrow band light source and the reflection spectrum of the FBG. The setup was successfully tested during the 4-points bending test of a composite panel sollicited at increasing strain rates.

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