High-sensitivity laser-based ultrasonic C-scan system for materials inspection

A beam-steering C-scan system for laser generation and detection of ultraso und in opaque solids has been developed. For ultrasound detection, the syst em uses a confocal Fabry-Perot interferometer that is stabilized at its hig h-sensitivity point by implementing a novel procedure that does not require a reference beam. This stabilization procedure effectively compensates for slowly varying (up to 2 Hz) disturbances and. more important, for any chan ge in the interferometer illumination conditions while probing different sc anning points. For the latter reason, the proposed stabilization appears to be more effective than previous approaches for high-sensitivity ultrasound detection in beamsteering scans. Noise performance and sensitivity limit o f the system are evaluated. Ultrasonic echoes are detected in aluminum samp les having optically rough surfaces with single-pulse generation signal-to- noise ratios on the order of 20 dB. A C-scan inspection is performed where a known defect in the test piece is successfully detected by time-of-flight gating of the ultrasonic signals.