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Characterization of Laser Ultrasonic in Ablation Regime Using Filter and Hilbert Transform
D. Mangaiyarkarasi, Vinuthashri, T. Zhenkui, K. O. Y. Keng, C. Long, S. H. Wei, S. Yali, C. Lianwei and M. Hong

Laser generated ultrasound in ablation regime provides a large enhancement of longitudinal mode signal which is useful to investigate defects and monitor thickness of offshore structures in real time. However, the simultaneous generation of bulk and surface acoustic waves (SAW) create complexities in the detection of appropriate type of ultrasonic wave. This is due to strong surface waves, which propagate along the surface interface regions interfering with the reflected longitudinal mode signals from the defects as well as from the rear substrate surface. Thus, the signal is mostly overlapped or hidden in the tail of the surface waves detected causing the determination of the arrival time of the ultrasound a challenging one. This paper utilizes the property of different frequency distributions of the bulk and surface waves and applies filtering functions together with Hilbert transform (HT), which gives instantaneous frequency and amplitude in order to separate and extract the detailed features of the ultrasound signal. A single, distinct, unipolar waveform (positive peak) obtained with bandpass filter followed by HT combination enhances the signal amplitude which is essential for B and C scans in the laser ultrasonics where the threshold voltages are set by the positive peak voltages.

Keywords: Laser ultrasonics, ablation, plasma physics, pulsed laser, signal processing, hilbert transform, surface acoustics waves

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