Optical: systems and elements – Lens – Plural focal length
Reexamination Certificate
2000-05-04
2001-09-04
Schwartz, Jordan M. (Department: 2873)
Optical: systems and elements
Lens
Plural focal length
C359S577000
Reexamination Certificate
active
06285514
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to ultrasonic laser-based contactless inspection systems, and more particularly to such systems with two and three-dimensional imaging, electronically controllable steering, focusing and phasing capabilities and self-calibration capabilities.
2. Description of the Related Art
Ultrasonic waves are commonly used to probe a variety of materials, particularly for thickness gauging and flaw detection. The sound waves have usually been generated with a contact piezoelectric transducer (PZT). The launched waves propagate through the material, reflecting from interfaces (in thickness gauging applications) or internal features (in flaw detection applications). The scattered sound propagates back to the surface of the workpiece, causing the surface to vibrate at the ultrasound frequency. This vibration is usually detected with a contact PZT similar to the one used to generate the sound.
Optical detection techniques, such as those described in C. B. Scruby and L. E. Drain,
Laser Ultrasonics, Techniques and Applications
, Adam Hilger, New York (1990), pages 325-350, can be used in place of the piezoelectric transducers to remotely detect the workpiece vibrations. Generally, a laser probe beam is directed onto the workpiece. When the surface vibrates it imparts a phase shift onto the reflected beam. This phase shift is detected with a photodetector after mixing the reflected probe beam with a stable reference beam and measuring the amplitude and frequency or phase of the detector output intensity fluctuations. The reference beam originates from the same laser source as the reflected probe beam, and the output signal from the photodetector or an electronic phase detector corresponds to the surface motion.
One problem with laser detection systems is that extraneous mechanical noise sources can cause additional low frequency vibrations at the surface. These additional vibrations are picked up by the reflected probe beam and reduce the signal-to-noise ratio of the detected signal.
Another problem is low sensitivity. Typically, the workpiece surface that is being probed has a diffusely reflecting or scattering quality. Consequently, the reflected beam is highly aberrated and its wavefront is mismatched with respect to the reference beam. The aberrated reflected beam produces a “speckle” field distribution on the optical detector that is used to detect the optical interference between the reflected and reference beams. The phase relationship between the reflected probe beam and the reference beam is maintained only over a single “speckle” diameter. Consequently, the phase relationship can be set optimally only for light within the speckle area; light within other speckles will have a different and generally nonoptimal phase relationship with the reference beam. The resulting detector signal can thus be thousands of times weaker, due to multiple speckle capture, than would be the case if the surface were a perfect mirror (in which all light would be in a single speckle).
One prior laser-based ultrasonic detection system, described in U.S. Pat. No. 5,131,748 entitled “Broadband Optical Detection of Transient Motion From a Scattering Surface by Two-Wave Mixing in a Photorefractive Crystal”, issued Jul. 21, 1992 to Jean-Pierre Monchalin, et al., addresses the wavefront matching problem. In this system the reflected probe beam optically interferes inside a photorefractive crystal with a “pump” beam that is derived from the same laser. The two beams form a refraction grating inside the crystal that diffracts the pump beam in the propagation direction of the probe beam so that the beams overlap and have substantially matching wavefronts when they exit the crystal. However, the grating matches the phases of the diffracted pump beam and the probe beam, which causes tee system's sensitivity to the surface vibrations to be very small in the case of a homodyne detection system. To overcome this problem, a second frequency shifted pump beam is superimposed onto the first pump beam, permitting heterodyne detection. The second pump beam is close enough in frequency to the first pump beam to be Bragg matched to the index grating and, therefore, diffracts off this grating. A second index grating is not written by the second pump beam and the probe beam because the crystal cannot respond fast enough to the moving fringe grating produced between the beams (the frequency shift between the beams results in non-stationary fringes). As a result, the second pump beam only diffracts off the first stationary grating (written by the first pump beam and the probe beam), and the relative phase between it and the probe beam is preserved.
Although this technique improves the system's sensitivity, it suffers from many limitations. Although the wavefronts of the probe beam and the diffracted pump beam are matched, they are matched to the aberrated and highly speckled wavefront of the probe beam rather than to the clean wavefront of the pump beam, resulting in a degradation of the coherently detected beam. Second, if the workpiece surface is de-polarizing, the sensitivity of the detector goes down. In addition, if the workpiece surface contains highly contrasting features (for example, pits, rust, spots, etc.), the two-wave mixing amplification may result in nonuniform “print-through” (due to pump depletion), which degrates the system performance. Finally, the Monchalin system does not automatically compensate for extraneous acoustic noise sources which cause additional vibrations at the surface. These spurious noise sources can dominate the signal limit of the desired feature to be detected and, moreover, can lie in the same ultrasonic frequency band, thereby obscuring the sending of the desired signal. Also, a single detection spot will limit the spatial resolution of the system, so the precision mapping of the desired feature's location may not be resolvable. Furthermore, the use of a single detection spot will place a limit on the sensitivity of the diagnostic, since beyond the threshold probe intensity, optical damage to the sample can result.
An alternative interferometric technique for detecting ultrasound uses a “self-referencing” interferometer that produces an output proportional to a temporal difference vibration signal, rather than to the displacement, of the moving workpiece surface. Time-delay interferametry, described in the Scruby et al. book, pages 123-127, is one such technique. In time-delay interferometry the probe beam that is reflected from the workpiece surface is split into two interferometer beams and then recombined at a standard photodetector, with one of the beams time-delayed with respect to the other such as by having it traverse a longer distance. The two beams are collinear when they are recombined at the photodetector, and the light intensity at the photodetector is proportional to the velocity of the workpiece surface. Ideally, the reflected readout beam is interfered with a time-delayed replica of itself and the wavefronts of the two interfering beams are substantially matched. Consequently, a phase shift in one leg of the interferometer is common to all speckles, and all speckles can be detected optimally. Unlike a conventional interferometer, which has a flat frequency response to phase shifts, a time-delay interferometer has a band-pass type of response. The time-delay interferometer suppresses both the low frequency (below ultrasonic frequencies), as well as certain ultrasonic frequency vibrations.
With this type of system, however, the speckles are often so plentiful and small as to not be compatible with each other after one arm of the interferometer has been time-delayed. The time delay is most easily accomplished with a multi-mode fiber, which further increases the number of speckles and scrambles their locations, making speckle registration impossible. Thus, bulky, long-path-length and is expensing precision “re-imaging” interferometer such as a Fobery-Perot unit, is employed. (Furthermore, th
O'Meara Thomas R.
Pepper David M.
Duraiswamy V. D.
Hughes Electronics Corporation
Sales M. W.
Schwartz Jordan M.
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