Patents with Abstracts
“When a laser pulse impacts an opaque material, part of the pulse is reflected and part is absorbed. Absorbed light energy is converted to thermal energy on the time scale of picoseconds (Spicer 1991), creating a thermoelastic expansion. This rapid expansion produces an acoustic wave that travels through the material. Several wave types are created simultaneously: longitudinal or compression waves, transverse or shear waves, surface acoustic waves (Rayleigh waves) in thicker plates and Lamb waves in thinner plates.
The efficiency of conversion from laser light to an acoustic wave depends on many factors, including surface reflectivity for the specified laser wavelength (Dubois 1993), temporal shape and duration of the laser pulse, (Enguehard 1997) energy of the laser pulse, and the surface area illuminated by the laser spot. Given that the choice of laser is usually governed by economical constraints, the flexibility in laser generation of acoustic waves mainly lies in the laser energy and pulse width. Purely thermoelastic generation is limited at higher intensities by the onset of surface vaporization, called ablation. (Caron 1996) Either increasing the laser energy, or decreasing the surface area illuminated by the pulse by focusing the beam, increases the power density. The proportionality of laser power density and acoustic amplitude has been derived in theory (White 1963) and demonstrated empirically (Caron 1996, Dewhurst 1982).
Many methods have been proposed to increase the efficiency of laser ultrasonic generation by spreading out the laser energy over a larger area. The use of a larger laser spot size allows the material to absorb more of the laser energy without surpassing the ablation threshold. The influence of laser energy distribution on efficiency of laser-generated ultrasound has been studied by Gonthier et al. (Gonthier 1994). Laser array sources have been implemented and described by Wagner et al. (Wagner 1990), Yang et al. (Yang 1993), Noroy et al. (Noroy 1993), and Steckenrider et al. (Steckenrider 1995). A holographic fringe generating spot was presented in von Gutfeld et al. (von Gutfeld 1983). Splitting a beam temporally by rapidly Q-switching a pulsed laser has also been shown to be an effective means of spreading the pulse power (Wagner 1990). The energy of the laser beam can be reduced by inserting beam attenuators (partially reflecting mirrors which divert a portion of the energy to beam dumps) or by reducing the amount of voltage supplied to the flash pump of the laser (Caron 1996).
Ultrasound measurements are usually classified as A, B, and C-scans. An A-scan is a single point measurement, and a B-scan is a measurement along a single line. A C-scan is an area scan where the laser generation and detection points are rastered across the surface of the sample. Alternatively, the lasers can be fixed while the material is moved by robotic arms. An important limiting factor when performing C-scans is the repetition rate of the laser. Pulsed lasers with suitable power and pulse rates operate in the range of 10 to 100 shots per second. To scan a 1 meter by 1 meter area, with a resolution of 1 mm at 100 Hz would take close to three hours. The invention described here can perform the same scan much faster.”
[Caron, US Patent 8,210,045 (7/3/2012)]
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Roger D. Corneliussen
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Copyright 2012 by Roger D. Corneliussen.
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* Date of latest addition; date of first entry is 7/3/2012.