Adaptive multi-channel fiber Bragg grating interrogation system for rapid detection of acoustic emission and impact

Conventional approaches to simultaneous multi-point detection of acoustic emission (AE) rely on the use of an array of piezoelectric sensors and these have certain limitations: electromagnetic interference, short distance of signal transmission, and heavy cabling overheads. Sensing systems based on fiber-optic sensors are therefore attractive for practical applications in structural health monitoring (SHM) owing to a number of advantages over their electronic counterparts. Among fiber-optic sensors, fiber Bragg grating gratings (FBGs) have their own unique features to be used for detection of AE to predict early failure in structures. We propose and develop a FBG dynamic strain sensing system for rapid AE detection by using a tunable single longitudinal mode Erbium-doped fiber (EDF) ring laser frequency tracking scheme as a demodulation technique. A proportional-integral-derivative (PID) laser controller is integrated with the signals from the photodetector in a feedback loop to continuously lock the laser wavelength to the middle-reflective wavelength of the FBG, thus stabilizing the operating point against low frequency perturbations, while the high frequency dynamic strain shifts the FBG reflection spectral profile. The reflected power varies in proportion to the applied strain which can be directly derived from the AC photocurrent of the reflected signal. We demonstrate both onechannel and four-channel adaptive AE demodulating systems that have the sensing capability to monitor the high frequency dynamic strain using four cascaded FBGs on a loaded aluminum plate.