Application of Laser-Ultrasonics to the Non-Contact, Pulse Echo Measurement of the Thickness of Micron Thin Metallic Coatings

The general scope of this work was to use recently available ultra-short pulsed lasers to perform laser-ultrasonic pulse-echo thicknesses measurements on thin electroplated metallic coatings. Experiments were first performed using a femtosecond laser coupled to a confocal Fabry-Perot interferometer (CFPI). The results showed that the CFPI is often inappropriate for the detection of ultrasound pulses having spectral widths much larger than the interferometer's free spectral range. A second experimental session was conducted using a picosecond laser and an InPFe photorefractive interferometer. Multiple echoes of broad band compression waves (up to 500 MHz) were successfully detected on electrodeposited metallic coatings of zinc on steel substrates for thicknesses from 30 to 100 microns. The results show very little dispersion and attenuation in the zinc coatings