Simultaneous multipoint acousto-ultrasonic sensing based on fiber wave Bragg grating sensors

We introduce a simultaneous multipoint acousto-ultrasonic (AU) sensing system using a tunable laser and fiber wave Bragg grating (FWBG) sensors. Although the demodulation technique is same as the existing method for a fiber Bragg grating (FBG), the sensor head is changed to the FWBG sensor for which the FBG is installed in a strain-free configuration and detects the AU wave not directly but in the form of a fiber-guided wave. Therefore since the strain cannot make the FBG spectrum move, multiple FBGs with an identical spectrum can be connected with multiple optical paths realized by equal laser intensity dividers. Temperature difference among the multiple FWBG sensors is passively resolved by using a short grating, which provides a wider temperature-operating region. Consequently, we can solve the problem that the FBG spectrum is easily deviated from the lasing wavelength owing to the strain. Also, the simultaneous multipoint sensing capability based on the single laser improves cost-performance ratio, reduces inspection time, and enables in-situ monitoring of a real structure exposed to large and dynamic strain. The system feasibility is demonstrated in the health monitoring examples like acoustic source localization and ultrasonic waves detection burst by a piezoelectric transducer and a pulsed laser.