Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer
Patent
1999-07-14
2000-05-16
Kim, Robert H.
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrometer
356349, 356354, G01B 902
Patent
active
060644826
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to an interferometric measuring device for measuring rough surface profiles on a test object using a radiation-generating unit, which supplies input radiation that is coherent over a short distance, having a beam-splitter device for producing a reference beam, which is directed at a device having a reflecting element for periodically changing the path of light, and for producing a measuring beam which is directed at the test object, including a hyterodyne element, where the reference beam coming from the test object and the reference beam coming from the device are made to interfere, and including a photodetector which picks up the interfered radiation and feeds it to an evaluation device.
BACKGROUND INFORMATION
A conventional interferometric measuring device is described in the publication, T. Dresel, G. Hausler, H. Venzke "Three-Dimensional Sensing of Rough Surfaces by Coherence Radar", App. Opt., vol. 31, no. 7 of Mar. 1, 1992. This publication proposes using an interferometer having a light source that is coherent over a short distance and a piezoelectrically driven reflector to measure the profile of rough surfaces. In the measuring device, a first beam component, in the form of reference wave, is superimposed on another beam component, in the form of a measuring beam, which is reflected off of a test object. The two light waves have a very short coherence length (a few .mu.m), so that the interference contrast reaches a maximum when the optical path difference is zero. To change the path of light of the reference wave, the reflecting element is provided in the form of the piezoelectrically driven reflector. The distance to the test object is able to be determined by comparing the position of the piezoelectrically driven reflector to the time of occurrence of the interference maximum. In this context, difficulties can arise when precisely determining the interference maximum and its association to the path of light, since uniquely defining the position of the piezoelectrically driven reflector entails substantial outlay.
SUMMARY OF THE INVENTION
The object of the present invention is to provide an interferometric measuring device which will enable the design to be simplified and a high measuring accuracy to be achieved.
A modulation-interferometer arrangement is provided, whose design is such that, using a first beam splitter of the beam-splitter device, in addition to the reference beam, a second beam component is also formed. The device for altering the path of light is a parallel-shifting arrangement disposed at least in the path of rays of the reference beam, and the reflecting element is a retro grating. Disposed in the path of rays of the reference beam upstream from the parallel-shifting arrangement, is a compensation grating, where the reference beam is diffracted both before as well as after passing through the parallel-shifting arrangement. Arranged in the path of rays of the second beam component is a pair identical to that in the path of rays of the reference beam, including an additional compensation grating and an additional retro grating arranged downstream therefrom, the optical path lengths of the thus formed two arms for the reference beam and for the second beam component of the modulation-interferometer arrangement exhibiting a difference greater than the coherence length. The reference beam fed back via the compensation grating, and the second beam component fed back via the additional compensation grating, are joined to form an intermediate beam. A demodulation-interferometer arrangement is provided, whose design is such that the intermediate beam is split into two further arms of the demodulation-interferometer arrangement using an additional beam splitter, the one arm terminating in a reflector, and the other arm terminating in the test object surface, and the two other arms exhibiting the same path difference as the arms of the modulation-interferometer arrangement the beams redirected by (reflected off) the reflector
REFERENCES:
patent: 4995726 (1991-02-01), Fujita et al.
patent: 5064257 (1991-11-01), Shinoda et al.
patent: 5933237 (1999-08-01), Drabarek
T. Dresel et al., "Three-Dimensional Sensing Of Rough Surfaces By Coherence Radar," Applied Optics, vol. 31, No. 7, Mar. 1, 1992, pp. 919-925.
Kim Robert H.
Robert & Bosch GmbH
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