Optics: measuring and testing – By light interference – Having light beams of different frequencies
Reexamination Certificate
1998-05-26
2001-06-05
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Having light beams of different frequencies
C356S511000, C356S497000
Reexamination Certificate
active
06243169
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an interferometric instrument for scanning the rough surfaces of a test object. The present invention includes a radiation generating unit, which emits briefly coherent radiation, and a first beam splitter, which produces a first and second beam component. According to the present invention, one beam component is aimed at the surface to be sensed, and the other beam component is aimed at a device with a reflecting element for periodically changing the light path. The interferometric instrument of the present invention further includes an interference element, which causes the radiation coming from the surface and the radiation coming from the reflecting device to interfere with one another, and a photodetector which receives the radiation.
BACKGROUND INFORMATION
A known interferometric instrument is described in the publication by T. Dresel, G. Häusler, H. Vanzke entitled “Three-Dimensional Sensing of Rough Surfaces by Coherence Radar,” Appl. Opt., Vol. 3, No. 7, dated Mar. 1, 1992. This publication proposes an interferometer with a briefly coherent light source and a piezoelectric mirror for sensing rough surfaces. In the measuring instrument, a first beam component in the form of a light wave radiated back from a test object has a second beam component in the form of a reference wave superimposed upon it. The two light waves have a very short coherence length (just a few &mgr;m) so that the interference contrast reaches its maximum when the optical path difference is zero. A reflecting element in the form of a piezoelectric mirror is provided for changing the light path of the reference wave. The distance to the test object can be determined by comparing the position of the piezoelectric mirror with the time at which the interference maximum occurs. However, the precise measurement of the position of this piezoelectric mirror is relatively complicated.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an interferometric instrument having a simplified layout and capable of achieving an increased measuring accuracy.
In order to achieve this object, the interferometric instrument of the present invention includes a device for changing a light path of a received beam component. This device has an arrangement for producing a parallel shift and positioned in an optical path of one of the first and second beam components. This arrangement is followed by a stationary reflecting element. This layout avoids the need to provide a mechanically driven element, thus making it possible to achieve a simple and more accurate analysis.
According to an embodiment of the present invention, the arrangement for producing the parallel shift has an acousto-optical deflector arrangement that is positioned in the optical path of one of the first and second beam components. This acousto-optical deflector arrangement is followed by the stationary reflecting element or by an element for deflecting the second beam component. The acousto-optical deflector arrangement is frequency-modulated and positioned in relation to the incoming beam component and in relation to the reflecting element so that the second beam component supplied to the interference element undergoes a change in its light path when it is deflected in the acousto-optical deflector arrangement. The acousto-optical deflector arrangement, in combination with the stationary reflecting element that follows it, is used to produce a change in the light path without any mechanical movement, simply by modulating the driving frequency of the acousto-optical deflector arrangement. At the same time, knowledge of the modulation frequency makes it possible to easily detect the light path, thus determining the distance to the test object by measuring the interference maximum.
In the interferometric instrument of the present invention, the acousto-optical deflector arrangement includes a first acousto-optical deflector and a second acousto-optical deflector that are positioned consecutively in the optical path. The first acousto-optical deflector deflects the second beam component around an angle by an angular deflection that is variable over time as a function of the frequency, while the second acousto-optical deflector resets the angular deflection so that the second beam component continues to move in the direction of incidence parallel to the first acousto-optical deflector. The reflecting element includes a diffraction grating oriented at an angle to the second beam component leaving the second acousto-optical deflector so that the second beam component is radiated back in the direction of incidence. These measures cause the first beam component, which strikes the diffraction grating, to return along its own path at any modulation frequency, i.e., at any angle of deflection, with the light path varying with the modulation frequency.
In the interferometric instrument of the present invention, the first acousto-optical deflector and the second acousto-optical deflector are driven by a common deflector driver and information about a modulation frequency is sent to an analysis circuit, which also receives an output signal of a photodetector. The analysis circuit can be used to measure the distance to the measuring point on the test object on the basis of the frequency information and the output signal. Since the light path is directly dependent on the modulation frequency without being subject to inertia, it is always precisely detected, and the position of the test object can be reliably determined.
The interferometric instrument of the present invention also includes a collimator that is positioned between the radiation generating unit and a first beam splitter, while a second beam splitter is located between the first beam splitter and the test object in order to direct the first beam component onto the test object via a focusing lens and to direct the first beam component reflected from the test object to an interference element that is in the form of an additional beam splitter. In addition, a third beam splitter is positioned between the first beam splitter and the first acousto-optical deflector and used to direct the second beam component returning from the first acousto-optical deflector to the additional beam splitter so that the second beam component will interfere with the first beam component reflected from the test object.
REFERENCES:
patent: 5321501 (1994-06-01), Swanson et al.
patent: 39 06 118 (1990-08-01), None
patent: 41 08 944 (1992-09-01), None
patent: 43 36 318 (1995-04-01), None
patent: 195 22 262 (1997-01-01), None
patent: 2 325 737 (1998-12-01), None
T. Dressel et al,“Three-Dimensional Sensing Of Rough Surfaces By Coherence Radar”, Applied Optics, vol. 31, No. 7, Mar. 1, 1992, pp. 919-925.
Drabarek Pawel
Kuehnle Goetz
Kenyon & Kenyon
Robert & Bosch GmbH
Turner Samuel A.
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