Laser-ultrasound detection systems: a comparative study with Rayleigh waves

Using laser-generated Rayleigh waves in the thermoelastic regime as an ultrasonic source, a range of ultrasonic detector systems have been evaluated experimentally for their sensitivity. Systems examined were those based on electromagnetic acoustic transducers (EMATs), a non-steady state photo-emf detector, a confocal Fabry-Perot interferometer (CFPI) and a differential optical beam deflection (OBD) detector. These detector systems had their own range of frequency responses within the range of approximately 100 kHz to 140 MHz. However, experimental conditions in this study limited the upper frequency range to 22 MHz. For comparison, signal-to-noise ratios were normalized with respect to both the experimental bandwidth, and, in the case of laser detection, the incident laser intensity. Results showed that for remote monitoring at a distance greater than 0.2 m, optical systems based on the photo-emf detector and the CFPI offered similar performances from a machine-finished aluminium surface. OBD measurements are not possible on such a surface. However, for typical stand-off distances of 0.25 mm or less, cost-effective EMATS provided an alternative form of detector.