Wiener filtering of laser-generated multiple-pulse narrow-band ultrasound for enhanced detectability by a laser interferometer

Abstract

A digital filtering procedure is presented for enhancing the detectability of multiple-pulse laser-generated narrowband ultrasound. Regarding their spectral characteristics, multiple-pulse laser-generated acoustic signals offer several advantages as compared with conventional single-pulse laser-generated ultrasound. In particular, multiple-pulse signals exhibit considerably narrower spectral content and offer the possibility for tuning the ultrasonic frequency, and yet still retain the high degree of temporal resolution possible with a single-pulse signal. For applications in non-destructive evaluation, adapting the frequency response of an interferometer to match the comb-like spectral signature of the multiple-pulse ultrasound would facilitate remote ultrasonic inspection with enhanced sensitivity. Towards this goal, the steps are outlined for applying a digital Wiener filtering method to multiple-pulse ultrasonic signals generated with a long cavity length mode-locked Nd: YAG laser and detected with a path-stabilized Michelson interferometer. Experimental results are presented for multiple-pulse waveforms generated at a pulse repetition frequency of 5 MHz, which demonstrate that Wiener filtering yields a significant improvement in the signal-to-noise ratio that is not possible with single-pulse laser ultrasonic methods.