VARIABLE-CAVITY-LENGTH MODE-LOCKED NDYAG LASER FOR NONCONTACT GENERATION AND SPECTRAL CONTROL OF NARROW-BAND ULTRASOUND

A passively mode-locked, flash-lamp-pumped long-cavity Nd:YAG laser wa s developed with a cavity length that was variable in discrete increme nts from approximately 11 to 60 m, permitting adjustment of the mode-l ocked pulse repetition frequency over a range from 13 to 2.5 MHz. Mult iple-pulse acoustic signals were generated with this laser in an alumi num sample and remotely detected by a path-stabilized Michelson interf erometer. The energy in the multiple-pulse acoustic signal was confine d to a considerably reduced spectral range compared with that in a sin gle pulse. Successful laser generation of spectrally selective narrow- band ultrasound presents new opportunities to integrate advanced signa l-processing strategies with interferometric detection to enhance the sensitivity of laser ultrasonics for industrial applications.