All-Fiber Photoacoustic Excitation and Detection for Lamb Wave in Structural Health Monitoring

All-fiber photoacoustic (PA) excitation and detection system offers a promising candidate for Lamb wave-based structural health monitoring. However, relatively low sound pressure generated by optical fiber PA transducer poses a significant challenge in the excitation and detection of Lamb waves within structures. This study investigates the limitations imposed by the laser-induced damage threshold (LIDT) of carbon nanoparticles-polydimethylsiloxane (PDMS) film, which restricts the maximum sound pressure of optical fiber PA transducers. It was found that the localized high heat contributes significantly to material damage, which can be mitigated by improving the material thermal conductivity. Through optimizing the weight ratio of carbon nanoparticles, the LIDT of PA transducer was increased to 23.71 μJ/pulse, achieving a sound pressure of 2.71 MPa. Then, a two-segment chirped fiber grating (TSCFG) structure was simulated and fabricated with optimized structural parameter, enabling a high ultrasound sensitivity of 1.843 V/Pa at 80 kHz through grating chirp rate demodulation, representing a 30-fold improvement over conventional uniform fiber Bragg grating (FBG) sensors. The integration of the high-pressure optical fiber PA transducers with the high-sensitivity TSCFG array successfully facilitated all-fiber excitation and detection of Lamb waves in composite structures, demonstrating the effectiveness of this configuration in locating delamination defects in composite laminate.