Ultrasonic Lamb Wave Detection Based on a Tunable, Narrow-Linewidth, and Frequency-Locked Fiber Laser

Developments of advanced ultrasonic sensors have made significant impact in structural health monitoring as well as in other various fields including electronics, biomedicine, and geophysics. We describe a cost-effective, highly stabilized, and sensitive ultrasonic sensing system based on a tunable, narrow-linewidth fiber laser locked to resonance of a phase-shifted fiber Bragg grating (PS-FBG). Stable single-longitudinal-mode (SLM) operation of the fiber laser is achieved with a narrow-band self-written FBG that dynamically tracks the lasing wavelength. The SLM fiber laser with 1.48-kHz linewidth and 40-nm tuning range is frequency locked to the PS-FBG resonance in Pound-Drever-Hall configuration for robust and sensitive ultrasonic detection. The sensing system exhibits a high signal-to-noise ratio for ultrasonic detection and further application of the system for lamb wave detection makes it especially attractive for structural health monitoring.