Laser ultrasonics hit the next level

InTech, Oct 2001 by Klein, Marvin, Pouet, Bruno, Breugnot, Sebastien, Peithmann, Konrad

Semiconductor-based receivers aid industrial inspection and process control.

LASER-BASED ULTRASOUND is a promising technique for remote inspection of demanding in-process applications involving high workpiece translation velocities and high temperatures. Thanks to their simple design and their ability to compensate for wavefront distortions due to dynamic speckle changes, atmospheric turbulence, and vibrations in the factory environment, the new adaptive photodetectors are particularly promising for these applications. We expect to see in the next few years more in-process laser ultrasonic systems reach the stage of factory demonstration and installation.

WIDE APPLICABILITY

Laser ultrasonics is a noncontact, nondestructive inspection and diagnostic tool with the potential for in situ sensing and process control across a variety of industrial and aerospace manufacturing needs. Manufacturers that use laser ultrasonics produce parts with closer tolerances, labor and material savings, and higher production yields for materials as diverse as composites, steel, aluminum, semiconductors, and paper. Laser ultrasonics can determine internal properties (thickness, temperature, defects) and monitor surface processes (thin-film deposition, case hardening, shock peening).

The basic laser ultrasonic technique involves two lasers that, in essence, replace ultrasonic contact transducers. One laser, typically a high-peak-power pulsed source, generates a pulse of ultrasound in a material upon absorption of the light, while a second continuous wave laser interferometer remotely senses the ultrasonically induced surface displacements on the workpiece. This receiver approach circumvents the need for direct contact, close proximity, or squirter systems.

Laser ultrasonics has a number of features that make it very attractive for process control applications:

* Its noncontact nature avoids mechanical loading of the workpiece and allows inspection of parts moving at high speeds (up to 20 meters per second).

* Its remote standoff capability allows inspection of parts in adverse manufacturing conditions, including high temperatures, vacuum, or plasmas.

* Scanning mirrors and fiber optics allow reconfigurable probing of complex-shaped parts without conformal surface tracking. The key enabling technology for many of the new in-process applications of laser ultrasonics is the class of new, adaptive interferometers developed to detect the small surface displacements encountered in typical measurement conditions. Unlike conventional homodyne or heterodyne interferometers, these adaptive laser ultrasonic receivers efficiently process the speckled beams received from rough surfaces and/or multi-- mode fibers. In addition, they compensate for dynamic wave-front changes resulting from beam scanning, workpiece motion, or atmospheric turbulence.

ULTRASONIC RECEIVERS FOR INSPECTION

While laser generation of ultrasound can be more efficient than generation with contact transducers, optical receivers for ultrasound are generally less sensitive than contact transducers for detection of ultrasonic signals. In recent years, there has been considerable interest in improving the performance of laser ultrasonic receivers. The specific requirements of such receivers are as follows:

* Operation in the shot noise limit, with a surface displacement sensitivity in the angstrom range and a processing band-- width of at least 10 megahertz (MHz) at a received power level of ~ 1 milliwatt

* The ability to process speckled beams from machined surfaces with high field of view

* The ability to compensate for low-- frequency wave-- front disturbances resulting from turbulence, workpiece translation, and mechanical noise

* Low cost

* Compact, rugged construction

Engineers have used laser interferometers for years to detect the small-amplitude surface displacements produced when an ultrasonic wave reaches the detected surface. Originally, passive homodyne or heterodyne interferometers with coherent detection could not operate effectively with the speckled input beams that result from interrogating a rough surface with a laser probe beam. Further, accurate path length stabilization or postprocessing electronics were required for effective operation.

The later development of time-delay interferometers, such as the confocal Fabry-Perot, has allowed the processing of light scattered from a rough surface with a large field of view. The confocal Fabry-Perot responds rapidly to changing input wave fronts and has high sensitivity, with measured values approaching the shot noise limit. But the confocal Fabry-Perot still requires stabilization of the interferometer length to a fraction of an optical wavelength, adding complexity and cost to the receiver.

More recently, a number of laser ultrasonic receivers based on adaptive reference-beam interferometers have been developed to process speckled beams with time-varying wave fronts resulting from mechanical disturbances or workpiece motion. These adaptive reference-beam interferometers have several advantages over passive reference-beam interferometers: