LASER ULTRASONIC CHIRP SOURCES FOR LOW DAMAGE AND HIGH DETECTABILITY WITHOUT LOSS OF TEMPORAL RESOLUTION

Linear frequency-modulated (chirped) acoustic signals have been genera ted using a pulsed laser spatially modulated by an absorption mask at the surface of a test material. By distributing the laser energy over an area, instead of focusing it to a point or line source, the peak po wer density of the laser source can be kept below the damage threshold of the material. The corresponding chirped ultrasonic surface wave pa cket produced by the source, although extended in time, is detected an d processed using a matched filtering technique which compresses the p acket into a pulse, thus preserving temporal resolution for accurate t ime-of-flight measurements. Matched filter processing of the chirped w ave packet has been compared with the same processing applied to a nar row-band tone burst wave packet. Processing of the chirped signal perm its easy separation of overlapped return echoes which could not be res olved when narrow-band signals were used. Finally, by compressing the energy within a chirped signal to a single detection spike, an apparen t 15-fold enhancement in signal-to-noise ratio is observed. (C) 1997 A coustical Society of America.