Optics: measuring and testing – By light interference – Having light beams of different frequencies
Reexamination Certificate
1998-05-22
2001-10-02
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Having light beams of different frequencies
C356S497000, C356S511000
Reexamination Certificate
active
06297884
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an interferometric instrument for scanning the rough surfaces of a test object. The interferometric instrument of the present invention includes a radiation generating unit, which emits briefly coherent radiation having different spectral components, and a beam splitter, which produces a first and a second beam component. One beam component is aimed at the test object surface to be sensed and the other beam component is aimed at a device with a reflective element for periodically changing the light path. The interferometric instrument of the present invention also has an interference element, which causes the radiation coming from the test object surface and the radiation coming from the reflecting device to interfere with one another, and a photodetector arrangement, which absorbs the interfered radiation and supplies electrical signals to an analysis circuit.
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”, App. 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 interference maximum is not easily accomplished.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an interferometric instrument that is capable of accomplishing an increased measuring accuracy. The present invention therefore calls for an arrangement which produces a frequency shift between the two interfering beam components to be provided in the optical path of the first beam component and/or in the optical path of the second beam component. The present invention also includes a beam splitting arrangement, which splits the beams into at least two spectral components and supplies them to the photodetector arrangement either directly or via additional elements on the photodetectors assigned to the components. These additional elements are provided in front of the photodetector arrangement in the optical path of the interfered radiation.
The frequency shift in the two interfering beam components makes it possible to conduct a heterodyne interferometric analysis, and, as a result, a simplified and improved detection of the interference maximum occurs. (Details regarding the heterodyne interferometric method itself can be found in the literature.) By splitting the interfered radiation in the beam splitting arrangement, the minimum phase difference can be measured through heterodyne interferometric element in the analysis circuit for each spectral component. Since the phase differences can be very precisely measured by means of heterodyne interferometry on the basis of the zero crossings, the analysis circuit can be used to precisely determine the interference maximum, which forms the envelope of the higher-frequency signal components on which the analysis is based. As a result, it is possible to precisely assign the minimum phase differences to the relatively flat interference maximum.
The present invention calls for the beam splitting arrangement to be a spectral prism in order to obtain the heterodyne signals from the light of the spectral components or the various wavelengths.
Another advantageous embodiment for obtaining the various spectral components consists of assigning opposite polarities to the various spectral components coming from the different light sources of the radiation generating unit and designing the beam splitting arrangement as a polarization beam splitter. The use of at least two light sources provides a high light intensity for the photodetectors, therefore improving the analysis.
The fact that the minimum phase difference of the interfered radiation can be analyzed for the various spectral components in the analysis circuit, using heterodyne interferometric techniques, and assigned to the interference maximum increases the accuracy of the interference maximum measurement. The design of the measuring instrument is simplified by designing the arrangement as an acousto-optical modulator, driven by a modulator driver, which is positioned between the first beam splitter and the test object in the optical path of the first beam component.
The device for changing the light path may be provided with an acousto-optical deflector arrangement with at least two acousto-optical deflectors that are followed by the stationary reflecting element, which is positioned in the optical path of the second beam component in order to change its light path. Further, the deflectors may be frequency-modulated and arranged in relation to the incoming second beam component arriving via a compensation grating and in relation to the reflecting element so that the second beam component, which is also supplied to the interference element via the compensating grating, undergoes the change in its light path when it is deflected in the deflectors. As a result of this arrangement, a simplified and more accurate analysis is achieved, since this arrangement avoids the use of a mechanically moving reflecting element. The light path can be very precisely determined and assigned to the interference maximum.
Since the two deflectors are driven by two deflector drivers with slightly different carrier frequencies, which causes the second beam component to undergo a frequency shift, it is no longer necessary to use an additional acousto-optical modulator to generate the heterodyne frequency. Instead, the existing acousto-optical deflectors, which produce a change in the light path, are used to generate the heterodyne frequency. If the modulation frequency of the carrier frequencies is a few tens of MHZ, for example, the slight frequency difference between the carrier frequencies can amount to 0.5 MHZ. The deflector drivers can consist of two driver stages of a deflector driver unit.
To achieve a simple design, the modulation frequency of the carrier frequencies can be generated by a common control unit, to which the two deflector drivers can also be connected. With a simple design for accurate analysis, for example, the first deflector deflects the incoming beam component by an angle that is variable over time, as a function of the frequency, while the second deflector resets the angular deflection so that the second beam component continues to move in the direction of incidence parallel to the first deflector. The reflecting element is designed as a diffraction grating oriented at an angle to the beam component leaving the second deflector so that the second beam component is radiated back in the direction of incidence.
According to the present invention, signal processing and analysis can be achieved by combining the control unit and a driving unit to form an analysis circuit. Information about the modulation frequency of the carrier frequencies is sent to the analysis circuit, and the analysis circuit can be used to measure the distance to the measuring point on the test object on the basis of the information and the signals.
In a suitable embodiment, a collimator is positioned between the radiation generating unit and the first beam splitter, a focusing lens is positioned between the beam splitter and the test object, and a mirror is positioned between the first beam splitter and the compensation grating.
Kenyon & Kenyon
Robert Bosch BmGH
Turner Samuel A.
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