WIENER FILTERING OF LASER-GENERATED MULTIPLE-PULSE NARROW-BAND ULTRASOUND FOR ENHANCED DETECTABILITY BY A LASER INTERFEROMETER

A digital filtering procedure is presented for enhancing the detectabi lity of multiple-pulse laser-generated narrowband ultrasound. Regardin g their spectral characteristics, multiple-pulse laser-generated acous tic signals offer several advantages as compared with conventional sin gle-pulse laser-generated ultrasound. In particular, multiple-pulse si gnals exhibit considerably narrower spectral content and offer the pos sibility for tuning the ultrasonic frequency, and yet still retain the high degree of temporal resolution possible with a single-pulse signa l. For applications in non-destructive evaluation, adapting the freque ncy response of an interferometer to match the comb-like spectral sign ature of the multiple-pulse ultrasound would facilitate remote ultraso nic inspection with enhanced sensitivity. Towards this goal, the steps are outlined for applying a digital Wiener filtering method to multip le-pulse ultrasonic signals generated with a long cavity length mode-l ocked Nd:YAG laser and detected with a path-stabilized Michelson inter ferometer. Experimental results are presented for multiple-pulse wavef orms generated at a pulse repetition frequency of 5 MHz, which demonst rate that Wiener filtering yields a significant improvement in the sig nal-to-noise ratio that is not possible with single-pulse laser ultras onic methods.