October 2001

Laser ultrasonics hit the next level

bySebastien Breugnot , Marvin Klein , Konrad Peithmann , Bruno Pouet

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 wave-front 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 multimode 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 bandwidth 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:

No path-length stabilization is required.
The intrinsic device field of view is higher.
Mechanical stability tolerances are greatly reduced.
There is compensation of low-frequency wave-front disturbances resulting from turbulence and mechanical activity.

Two possible solutions

Research and development efforts for in-process inspection applications are focusing on approaches based on two-wave mixing or on photoelectromotive force (photo-EMF) detection. In the two-wave mixing approach, the photorefractive medium acts as an adaptive beam splitter, combining a distorted signal beam with a plane-wave reference beam and matching their wave fronts for homodyne detection. To provide linear detection of a temporally phase-modulated signal beam, the phases of the beams have a bias in quadrature for optimum sensitivity. In contrast to the confocal Fabry-Perot interferometer, no path-length stabilization is required to maintain this condition.

One feature of the two-wave mixing approach is there is no material-related upper limit on the ultrasonic signal bandwidth; the bandwidth of the photodetector controls the upper limit. In contrast, the response time of the photorefractive grating controls the wave-front distortion compensation bandwidth. The grating response time also determines the maximum scan rate for scanning applications.

Many industrial applications require a compensation bandwidth of at least 1 kilohertz. The requirement for a short response time clearly favors the photorefractive semiconductors. Experimenters have observed such bandwidth in recent experiments using bulk indium phosphide and cadmium telluride, as well as photorefractive multiple quantum wells.

The other approach under development uses a reference-beam interferometer with photo-EMF detection. In this case, the photo-EMF element performs the dual function of laser-based ultrasonic detection as well as optical distortion compensation in a single semiconductor crystal. As before, the speckled signal beam interferes with the reference beam in a photorefractive material, producing a spatially modulated conductivity pattern, which leads to the production of a spatially periodic space charge field via the normal carrier migration and trapping process. The small phase modulation on the signal beam imparted by the surface motion causes a lateral vibration of the periodic free carrier grating, which induces an ac current that is proportional to the modulation amplitude and the total power.

This current develops only when the frequency of the ultrasonic phase modulation is faster than the grating relaxation rate. When the modulation frequency is lower than the grating relaxation rate, the space charge field grating can follow the motion of the fringes, producing no current. Thus, the grating relaxation rate is equivalent to the compensation bandwidth defined above, and the photo-EMF receiver has the desirable property of reduced sensitivity for noise-related frequencies below this bandwidth.

The major advantage of the photo-EMF approach is that it combines the optical compensation and detection stages of the two-wave mixing approach into a single semiconductor element, without the need for an optical readout beam or an electro-optical response. Because no transmitted beams are required, a laser wavelength with a photon energy larger than the band gap can be used. At these wavelengths, the large value of absorption coefficient provides a fast grating relaxation rate for modest levels of probe laser power.

In the case of gallium arsenide (GaAs), this rapid relaxation rate allows compensation of wave-front distortions at bandwidths exceeding 1 MHz. The upper limit on the ultrasonic signal processing bandwidth is determined by the recombination rate, which is ~ 80 MHz in conventional semi-insulating GaAs.

A feature of the photo-EMF sensor is that fabrication of the semiconducting detector element relies on conventional semiconductor lithographic techniques. Using this technology, the sensors can be formed into compact monolithic arrays on a single wafer, with the potential for phased-array, high-resolution imaging of buried features in materials, reduced background clutter, and full-frame imaging of material features in a single shot, without beam scanning.

Control at high speed, temperature

The two critical features of the adaptive photodetectors described above are the ability to do the following:

Interrogate parts at high temperatures, up to the melting temperature of steel.
Inspect samples moving at high translation velocities, up to 30 meters per second.

For these two applications, there are no effective competing nondestructive testing approaches.

In one early demonstration of the dynamic compensation capability of a photo-EMF receiver, a Q-switched Nd:YAG laser with a 7-nanosecond pulse width at 1.06 micrometers generated the ultrasound. The sample was an aluminum plate rotated at a constant rate. The receiver used a continuous wave, diode-pumped, frequency-doubled Nd:YAG laser at 532 nanometers. The photo-EMF detector used a single crystal of semi-insulating chromium-doped GaAs.

The experimenters’ purpose was to determine the capability of the receiver to compensate for the moving speckle pattern produced by the continuous surface translation across the beam. They first obtained temporal surface displacement signals when the plate was still and then when the plate was rotating. In both plots, it is easy to identify the first sound arrival and the first echo. They observed no significant differences in the signals, confirming the ability of the photo-EMF receiver to inspect moving parts during manufacture.

Researchers first demonstrated in 1993 that they could interrogate parts with high translation velocities and high temperatures in a factory environment. In this case, the application was the real-time, online measurement of wall thickness of hot (1,000°C) seamless steel tubing immediately after hot piercing and stretching. A full inspection system was set up at Algoma Steel in Canada, and researchers got very good agreement between thickness determinations from laser ultrasonics and measurements on the cold tubing.

The most demanding sample translation velocity application to date is the inspection of paper during manufacture. Modern paper manufacturing machines operate at paper velocities of up to 30 meters per second. We have studied the use of laser ultrasonics to measure the time of flight of Lamb waves in paper. The time of flight is an index to the velocity of sound and, from this, the stiffness of the paper.

We used generation of Lamb waves by a pulsed Nd:YAG laser and detection with a photo-EMF sensor. Laboratory measurements employed a paper web simulator that operates at speeds up to 30 meters per second. We have obtained usable signals at speeds of up to 20 meters per second, and we are implementing a number of improvements that will improve the signal-to-noise ratio and thus the maximum velocity for this application. IT

Figures and Graphics

Behind the Byline

Marvin Klein, Bruno Pouet, Sebastien Breugnot, and Konrad Peithmann are associated with Lasson Technologies, Inc. in Culver City, Calif.

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