Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
2002-04-24
2004-03-02
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
Reexamination Certificate
active
06700666
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of materials analysis, and in particular to a method and apparatus for probing an object in motion with the aid of small surface displacements generated, for example, by ultrasonic energy.
2. Description of Related Art
Two-beam adaptive phase demodulators are used for noncontact and non-invasive measurements of the small surface displacements produced by ultrasonic waves propagating in an object. These devices are based on the illumination of the surface of the object by a probing laser, collection of the light scattered by the surface of the object and its mixing with a beam directly derived from the laser (the pump beam). They are adaptive in the sense they can operate with light scattered by rough surfaces and having speckles and in the presence of various ambient perturbations. The ultrasonic waves to be detected are often generated by a pulsed laser. The combination of generation and detection in a technique called laser-ultrasonics is recognized to be a powerful and versatile technique for inspecting objects and industrial products. When the object is in rapid motion along the line-of-sight of the probing laser or when the laser beam is rapidly scanned, these two-beam adaptive phase demodulators become very insensitive and in practice useless. This invention provides a practical solution to this problem and allows the two-beam adaptive phase demodulators to be used even in the case of rapid motion of either the object or the probing laser beam.
Laser-ultrasonics is a powerful nondestructive evaluation technique based on the generation and the detection of ultrasound with lasers. A short pulse high power laser is used for generating the ultrasonic waves using either a thermoelastic mechanism (noninvasive) or material ablation. The detection of the ultrasonic waves is performed with a second laser coupled to an optical demodulator which measures the phase modulation of the laser light scattered or reflected by the surface. This phase modulation is induced by the small ultrasonic displacement of this surface. Several solutions have been developed to detect and demodulate efficiently the scattered light in the practical cases of industrial interest where the surface is rough and the scattered light has speckles.
A first type of solution is based on time-delay interferometers that have been field-widened. J. -P. Monchalin,
Optical Detection of Ultrasound
, IEEE Trans. Sonics, Ultrasonics, Freq. Control, UFFC-33, p. 485-499 (1986). Particularly useful and practical are the various confocal Fabry-Perot schemes described in several US patents, for example, J. -P. Monchalin, Optical Interferometric Reception of Ultrasonic Energy, U.S. Pat. No. 4,659,224; J. -P. Monchalin, Broadband Optical Detection of Transient Surface Motion From a Scattering Surface, U.S. Pat. No. 4,966,459; and R. Heon and J. -P. Monchalin, Optical detection of a surface motion of an object using a stabilized interferometric cavity, U.S. Pat. No. 5,137,361. Such a demodulator has a rather short response time to a change of phase or frequency of the collected scattered light (across the whole field or change of the distribution): values of 100 ns or shorter are typical and depend upon the cavity length and mirrors reflectivity. Therefore such systems easily tolerate strong vibrations and motions of the probed object. Such motions cause not only a change of the speckle pattern but also, if they have a component along the line-of-sight a change of frequency (Doppler effect). Finally it should be noted that, for proper operation of the Fabry-Perot based devices, the laser frequency should be locked to the cavity length, which requires a laser with an adjustable frequency or an adjustable cavity length and locking electronics.
A second solution consists in the various two-beam adaptive phase demodulator schemes, in which the beam scattered by the surface is mixed with a beam directly derived from the laser (the pump beam) in a sensing material. This sensing material has trapping sites for the charges produced by the photoelectric effect. After charge motion by diffusion or drift (if an electric field has been applied) and trapping, a charge distribution is set in the material and results in an electric field distribution (the space-charge field distribution). Three different schemes based on this approach are known. First, two-wave mixing (TWM) in a photorefractive crystal operated in the diffusion regime, i.e. without any voltage applied to the crystal, was proposed by Monchalin: J. -P. Monchalin, R. K. Ing, Broadband Optical Detection of Transient Motion From a Scattering Surface by Two-Wave Mixing in a Photorefractive Crystal, U.S. Pat. No. 5,131,748; R. K. Ing and J. -P. Monchalin,
Broadband Optical Detection of Ultrasound by Two
-
Wave Mixing in a Photorefractive Crystal
, Appl.Phys.Lett. vol. 59, pp.3233-3235, 1991; and A. Blouin, J. -P. Monchalin,
Detection of ultrasonic motion of a scattering surface by two
-
wave mixing in a photorefractive GaAs crystal
, Appl. Phys. Lett. 65, 932, (1994). For the TWM setup, the signal beam at the output of the crystal is composed of the signal beam transmitted through the crystal and of the beam from the pump beam diffracted by the grating, which is wavefront adapted to the signal beam. This signal beam at the output of the crystal is then sent to an optical detector or to a balanced receiver. This TWM-based ultrasonic demodulator was later extended to a crystal under a transient applied voltage. A. Blouin, P. Delaye, D. Drolet, J. -P. Monchalin and G. Roosen, Sensitive and Fast Response Optical Detection of Transient Motion From a Scattering Surface by Two-Wave Mixing, U.S. Pat. No. 5,680,212. P. Delaye, A. Blouin, D. Drolet, L. A. de Montmorillon, G. Roosen, J. -P. Monchalin,
Detection of ultrasonic motion of a scattering surface by photorefractive InP:Fe under an applied field
, Journal of the Optical Society of America, B 14, 1723, (1997). Instead of a photorefractive crystal, the TWM device can also be operated with a photorefractive polymer material (Klein, M. B., Bacher, G. D., Grunnet-Jepsen, A., Wright, D., Moerner, W. E.,
Homodyne detection of ultrasonic surface displacements using two
-
wave mixing in photorefractive polymers
, Optics-Communications. vol.162, no.1-3; Apr. 1, 1999; p.79-84), photorefractive quantum wells or with other non-linear materials. A sketch of the TWM optical demodulator scheme is shown in FIG.
1
.
In
FIG. 1
, laser
1
outputs a beam that is passed through a beam splitter
2
. A portion of the output beam is directed at the sample
3
, which is excited with ultrasonic energy, and a second portion is directed at the photorefractive crystal
4
, where it is combined with the beam scattered off the sample
3
after passing through a lens L and polarizer
5
.
The photorefractive crystal is energized with a pulse high voltage source
6
and outputs a beam
7
that passes through adjustable quarter wave plate
8
and polarizing beam splitter
9
. The polarized output beams are received by photodetectors D
1
and D
2
and passed to the respective inputs of differential amplifier.
A second scheme based on the photo-electromotive force (p-EMF) was proposed by Petrov. M. P. Petrov, I. A. Sokolov, S. I. Stepanov, G. S. Trofimov, ‘Non-steady-state photo-electromotive-force induced by dynamic gratings in partially compensated photoconductors’, J. Appl. Phys. 68, 2216, (1990). The p-EMF-based device does not requires the crystal to have electro-optic properties, which means that the written space-charge field does not necessarily produces a refractive index grating. A third scheme, proposed more recently is based on the self-polarization modulation effect. K. Paivasaari, A. A. Kamshilin,
Adaptive sensors of rough
-
surface ultrasonic vibrations based on the polarisation self
-
modulation effect
, Fourth International Conference on Vibration Measurements by Laser Techniques: Advances and Applications, SPIE Proceedings vol. 4072, 70, (2000).
Two-beam ada
Blouin Alain
Drolet Denis
Monchalin Jean-Pierre
Padioleau Christian
(Marks & Clerk)
Lyons Michael A.
National Research Council of Canada
Turner Samuel A.
LandOfFree
Ultrasonic vibration detection using frequency matching does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Ultrasonic vibration detection using frequency matching, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Ultrasonic vibration detection using frequency matching will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3280989