Continuous Laser Generation of Ultrasound
USPTO Patent Number 8210045
Generating and detecting ultrasound is a standard method of nondestructive evaluation of materials. The amplitude or speed of the ultrasound is used to determine the strength of the material, or to sense defects. Pulsed lasers are used to generate ultrasound remotely in situations where contact transducers cannot be used or is not beneficial. In these laser-based systems, the scanning rate is limited by the repetition rates of the pulsed lasers, ranging between 10 and 400 Hz for lasers with sufficient pulse widths and energies. A hundred shots per second may at first seem like a good pace, but if one needs to scan a square meter with millimeter resolution, the scan would take between three hours and two days. In place of the pulsed laser, we believe that a high-powered continuous wave laser (think about a laser pointer with two million times the power) can be used in replace of the pulsed laser to create the ultrasound in the material. As shown in the figure, a scanning mirror sweeps the beam across the surface, generating an ultrasonic wavefront in the material. Our calculations reveal that with sufficient power, detectable ultrasound waves can be produced at a scanning rate that is more than 20,000 times faster. All components of the system are commercially available to produce such a system.
Fig1: A typical CLGU setup used for sensing generating and detecting ultrasound in materials. Changes in the wavefront as it passes through the material indicate material defects. The deflection shown is greatly exaggerated. Here a GCLAD system is used for detection, but a scanning laser interferomteric system may also be used.
The primary technology challenge is developing a fast rotating mirror that can operate at the desired angular velocity and handle the high incident power. The primary advantage is the time savings. As of 2008, the Lockheed-Martin LaserUT system had a rate of 400 inspection points per second, limited by the pulse rates of the lasers. With this scan rate and 2 mm steps, the system can scan at a rate of 5.8 m^2/hour. If steps are reduced to 1 mm, it would take 42 minutes to scan a squared meter. In contrast, CLGU could scan a single meter-long line in less than a millisecond, and a squared meter with millimeter resolution in less than a second. If realized, CLGU can be used for the rapid inspection of airplane fuselages, full inspection of composite panels and during manufacturer, and remote inspection of hot metals during production.
As a research tool, CLGU allows the study of acoustic wavefronts in materials, and can provide ultrasound ’videos’ of materials under stress and impact.
Increased ultrasound frequency enables the detection of smaller flaws. The frequency distribution created by pulsed laser generation is inversely proportional to the pulse width and cannot easily be modified. For CLGU, the center frequency distribution is determined by the scanning velocity and the beam radius. Thus, ultrasound frequency can be varied by changing either the scanning rate or the beam spot size.
We are currently exploring different approaches to funding the R & D of CLGU. As you can imagine, this requires the use of a facility with the appropriate lasers. If you have any ideas on how to get this started, feel free to share them with us.