Title of Invention

AN INTERFEROMETRIC MEASURING DEVICE

Abstract

The invention relates to an interferometric measuring device for measuring surface characteristics, shapes, distances, distance variations, e.g. oscillations of measuring objects (7), by means of a probe part (6). The invention aims to provide a device which is easy to use and permits error-free scanning. To achieve this, the probe part (6) is subdivided into a fixed probe part (6.1) and a rotatable probe part (6.2) that is mechanically and optically coupled thereto and a beam splitter (6.3; 6.3') for creating a reference beam and a measuring beam for the interferometric measurement is located in the rotatable probe part (6.2).

Full Text

The invention relates to an interferometric measurement device for measuring surface parameters, shapes, spacings, changes in spacings, for example vibrations, of measurement objects with the aid of a probe part. An interferometric measurement device of this type is specified in DE 198 08 273 Al. In this known measurement device, the optical system of the measurement device in the interferometric measuring system is split up by means of coherence multiplexing into two subsystems, specifically a so-called modulation interferometer and a probe part. The probe part can be effectively manipulated in this way and has a measuring head which also enables measurements in relatively long, narrow bores. The measurement device is designed for mu It wavelength interferometry such that an expansion of the measuring range is achieved. In order to carry out panoramic scanning of a surface, the measurement object itself or the measurement device is driven rotating, as a rule. The drive is not always easy to execute and can have a negative effect on the measuring accuracy. In the case of a further interferometric measurement device of this type, indicated in DE 198 19 762 Al, various space-saving measuring probes are proposed for the measuring system, it being possible, as specified above, for the generation of the scanning movement to be attended by similar difficulties. EP 0 126 475 exhibits a method and a device for contactless measurement of the actual positions and/or the profile of rough surfaces, which is based on the concept of the multiwavelength heterodyne interferometer and contains one or more lasers as light source. The heterodyne technology permits the influence of vibrations to be largely suppressed on the basis of the phase evaluation; however, the abovenamed difficulties can occur with this mode of procedure, as well. It is the object of the invention to provide an interferometric measurement device of the type mentioned at the beginning with the aid of which, in conjunction with the simplest possible manipulation of the measurement device, an increased accuracy is achieved in conjunction with rotating scanning of the surface of a measurement object, and interfering influences on the measuring accuracy that are caused by the drive device are counteracted. This object is achieved with the aid of the features, as described herein. It is provided thereby that the probe part is subdivided into a fixed probe part and a rotatable probe part coupled thereto mechanically and optically, and in that a beam splitter for generating a reference beam and a measuring beam for the interferometric measurement is arranged in the rotatable probe part. The subdivision of the probe part into a fixed and a rotatable probe part renders possible a relatively simple alignment for scanning the measurement object, and design of the probe part for precisely rotating scanning. The arrangement of the beam splitter in the rotatable probe part prevents the path differences to be detected between the reference beam and measurement beam and which are caused by the surface to be measured from having superimposed on them such differences as are produced by the rotation at the transitional region between the fixed probe part and the rotating probe part. A further simplification to the manipulation results from the measures that a modulation interferometer is present which is spatially separated from the probe part and has arranged in it a short coherence light source or a spatially separated demodulation interferometer, the short coherence light source being arranged in the rotatable or in the fixed probe part or outside the probe part. A favorable design is achieved in this case by virtue of the fact that the probe part and the modulation interferometer or the demodulation interferometer are coupled to one another by a monomodal optical fiber. An advantageous design is achieved, furthermore, by virtue of the fact that the beam splitter is part of a common path interferometer arrangement. As a result, dedicated optical arms for the reference beam become dispensable and a slim design is favored. Various alternatives, known per se, for the design of the measurement device consist in that the design of the interferometer corresponds to a classic interferometer, a white light interferometer or a heterodyne interferometer. An advantageous refinement consists, moreover, in that the interferometer is designed as a multiwavelength interferometer in order to expand the measuring range, For a measurement in very narrow channels or bores such as an injection nozzle, for example, it is rendered possible by virtue of this fact that the probe part has in a measuring head for scanning the measurement object an optical measuring fiber upstream of which a fiber length is connected, and in that a separating surface is formed as beam splitter between the fiber length and the measuring fiber. For example, measurements are possible in this case in bores of a diameter between 80 pm and one mm. The end of the measuring fiber on the side of the measurement object is designed in this case in accordance with the respective measurement task. Furthermore, a contribution to a favorable design comes from the fact that the light from the light source is guided via a further optical fiber and via a fiber beam splitter into the fiber length and is guided out of the latter into the optical fiber after illumination of the measurement object. Regarding the design and the mode of operation of the interferometric measurement device per se, reference may be made to the prior art mentioned at the beginning, in which still further sources in the literature are specified in relation to interferometric measurement devices, The invention is explained in more detail below with the aid of exemplary embodiments and with reference to the drawings, in which: figure 1 shows a first exemplary embodiment of an interferometric measurement device with a modulation interferometer and a probe part spatially separated therefrom, in a schematic illustration, and figure 2 shows a further exemplary embodiment of an interferometric measurement device, in the case of which a demodulation interferometer and a probe part spatially separated therefrom are provided. In the case of an interferometric measurement device 1 shown in figure 1, a component of a modulation interferometer 2 and a component with a probe part 6 are arranged spatially separated from one another and connected to one another via a preferably monomode optical fiber 5. Provided for the purpose of picking up measuring light that is guided from a scanned object surface of a measurement object via the monomode optical fiber 5 is a receiver arrangement 4 with a spectral element 4,2 and a photodetector arrangement 4.1 whose output signals are passed on to an evaluation device 8 for computational evaluation, which can also, in addition, take over control tasks of the interferometric measurement device 1. The modulation interferometer comprises a short coherence, broadband light source 3, for example a super luminescent LED, as well as two modulators 2.1, in particular acoustooptical modulators, a time-delay element 2.2 arranged in one arm, for example a plane-parallel plate, two beam splitters, one for splitting the light beam into two partial light beams fed to the two modulators 2.1, and the other for uniting the divided light beams, as well as two deflecting elements. Such a modulation interferometer is specified, for example, in DE 198 19 7 62 Al mentioned above, the mode of operation also being described in more detail. The probe part 6 has a fixed probe part 6.1 and a rotatable probe part 6.2 that is mechanically and optically coupled thereto and in which a beam splitter 6.3 is arranged. The arrangement of the beam splitter 6.3 in the rot at able probe part 6.2 has the advantage that owing to the rotation no path differences can be produced between the reference beam generated by the beam splitter 6.3 and the measurement beam, but the changes produced in the path difference are to be ascribed to the surface properties or shape, spacing, change in spacing, for example vibrations, of the scanned surface of the measurement object 7. The light from the short coherence light source 3 of the modulation interferometer 2 is collimated with the aid of a lens and split between the two partial light beams. The modulation interferometer is designed, for example, according to the principle of a Mach-Sender interferometer. The two partial light beams are mutually displaced in frequency with the aid of the modulators 2.1. The frequency difference is a few kHz, for example- In one arm of the modulation interferometer 2, the time-delay element 3 effects a difference in the optical paths of the two partial light beams that is longer than the coherence length of the light source 3. The two partial light beams are superimposed in the downstream beam splitter and coupled into the monomode optical fiber 5. The partial light beams do not interfere because of the optical path difference- The light is guided via an optical conductor to the probe part 6 and coupled out there. Apart from the beam splitter 5.3, the rotatable probe part 6.2 contains further optical elements, which focus the light beam fed onto the surface of the measurement object 7 that is to be measured. The optical path from the beam splitter 6.3 to the measurement surface compensates the optical path difference introduced in the modulation interferometer 2. The light beam is split up with the aid of the beam splitter 6.3 into the measuring beam guided to the measurement object and a reference beam. By way of example, the wall of a bore is scanned and the variation in the shape of the inner cylinder is measured owing to the rotation of the rotatable probe part 6.2. The light reflected in this case by the measurement surface is superimposed with the reference beam and launched the optical fiber 5- The light beams of the measuring beam and of the reference beam can interfere because of the compensation of path difference. The light phase difference contains information on the spacing from the measurement surface. The light guided to the modulation interferometer 2 via the optical fiber 5 is coupled out and is decomposed with the aid of the spectral element 4.2, for example a grating or a prism, into a plurality of spectral components of the wavelengths A,i, Xzr ...Xn, and focused on the photodetector arrangement 4.1. Each photodetector supplies an electric signal with the differential frequency generated by the modulators 2.1, and a phase Acp that is related to the measurement variable AL of the spacing from the measurement object 7 and the associated wavelength Xn in accordance with the relationship Acp == (2 • n/Xn) • AL. The spacing AL, which may be greater than individual light wavelengths, can be uniquely determined by measuring the phase differences between the signals from a plurality of photodetectors (multiwavelength heterodyne interferometry). The evaluation is performed by means of the evaluation device 8. The further interferometric measurement device 1 shown in figure 2 also operates in a way similar to that in figure 1. However, in this case the interferometric measurement device 1 is coupled into a demodulation interferometer 2' and a probe part 6, remote therefrom and coupled by means of the optical fiber 5, and which is likewise subdivided into the fixed probe part 6.1 and the rotatable probe part 6.2, The short coherence light source 3, for example a super luminescent diode, is located in the rotatable part 6.2 in this case. Its light is launched via a further optical fiber 6.4, likewise preferably a monomode optical fiber, by means of a fiber beam splitter 6.3' into a fiber length 6. 5 that is coupled by means of a fiber connector in a measuring head 6.6 to a measuring fiber 6.7 facing the measurement object 7. The surface of the measurement object 7, for example a very narrow bore of an injection nozzle, is optically scanned by means of the measuring fiber 6.2, which is designed at its free end for illuminating the measurement surface and picking up the light reflected thereby. The exit surface of the fiber length 6.5 at the transition to the measuring fiber 6.7 is coated such that it has the function of a beam splitter 6,3. The light is split up at this beam splitter 6.3 into two partial beams, the measuring beam and the reference beam. The reference beam is coupled back into the fiber length 6.5 and guided into the demodulation interferometer 2' via an optical coupler 6.8 at the transition between the rotatable probe part 5,2 and the fixed probe part 6,1. The measuring beam is coupled out of the measuring fiber, whose end is specifically treated, for example ground at an angle of 45° and aluminized^ and illuminates the inner wall, which is to be measured, of the small bore in the measurement object 7. The measuring fiber 6.7 has a diameter of 125 pm, for example. The light reflected by the wall of the bore is coupled into the demodulation interferometer 2' via the measuring fiber 6.7, the fiber beam splitter 6.3' and the optical coupler 6.8, and superimposed with the reference beam. The two beams cannot interfere, since the coherence length of the light source 3 is shorter than half the measuring fiber 6.7, The demodulation interferometer 2' is designed, for example, according to the principle of a Mach-Sender interferometer. The incoming light is split up into two partial light beams in the demodulation interferometer 2' , The time-delay element 2.2, for example likewise a plane-parallel glass plate, is used in on^ arm of the demodulation interferometer 2' and resets the difference of the optical paths between the measuring beam and the reference beam that was forced in the measuring head. The frequencies of the two partial light beams are mutually shifted with the aid of the modulators 2.1, for example likewise acoustooptical modulators, the frequency difference also being a few kHz in this case, for example. The two partial light beams capable of interference are superimposed in a further beam splitter, coupled out, decomposed into a plurality of spectral components with wavelengths of Xi, X2, - . An with the aid of the spectral element 4.2, for example a grating or prism, and focused on the photodetector arrangement 4.1. The evaluation is then performed in accordance with the exemplary embodiment according to figure 1- The transmission of information from the rotating probe part 6.2 to the fixed probe part 6.1 is performed via the optical coupler 6.8, which can be designed, for example, in the form of two Grin {= graduate [sic] index) lenses arranged at the fiber ends of the corresponding optical fibers 5. Since the optical coupler 6.8 is located downstream of the fiber beam splitter 6.3' or the beam splitter 6.3 in the light path, any possible small instances of tilting or displacements of the two probe parts 6.1, 6.2 cause no interference during rotation, and so the rotation during scanning produces no falsifications of the measurement result, WE CLAIM : 1. An interferometric measurement device for measuring surface parameters, shapes, spacings, changes in spacings, for example vibrations, of measurement objects with the aid of a probe part (6), the probe part (6) is subdivided into a fixed probe part (6.1) and a rotatable probe part (6.2) coupled thereto mechanically and optically, a beam splitter (6,3; 6.3') for generating a reference beam and a measuring beam for the interferometric measurement is arranged in the rotatable probe part (6.2), and a demodulation interferometer (2') physically separated from the probe part (6), characterized in that a short coherence light source (3) being arranged in the rotatable probe part (6). 2. The measurement device as claimed in claim 1, wherein the probe part (6) and the demodulation interferometer (2') are coupled to one another by a monomode optical fiber (5). 3. The measurement device as claimed in claim 1 or 2, wherein the beam splitter (6.3) is part of a common path interferometer arrangement. 4. The measurement device as claimed in one any of the preceding claims, wherein the construction of the interferometer corresponds to a classic interferometer, a white light interferometer or a heterodyne interferometer. 5. The measurement device as claimed in claim 4, wherein the interferometer is constructed as a multiwavelength interferometer in order to expand the measuring range. 6. The measurement device as claimed in any one of the preceding claims, wherein the probe part (6) has in a measuring head (6.6) for scanning the measurement object (7) an optical measuring fiber (6.7) upstream of which a fiber length (6,5) is connected, and a separating surface is formed as beam splitter (6.3) between the fiber length (6.5) and the measuring fiber (6.7). 7. The measurement device as claimed in any one of claims 2 to 6, wherein the light from the light source (3) is guided via an optical fiber (6.4) and via a fiber beam splitter (6.3') into the fiber length (6.5) and is guided out of the latter into the optical fiber (5) after illumination of the measurement object (7).

Documents:

764-chenp-2003-abstract.pdf

764-chenp-2003-claims filed.pdf

764-chenp-2003-claims granted.pdf

764-chenp-2003-correspondnece-others.pdf

764-chenp-2003-correspondnece-po.pdf

764-chenp-2003-description(complete)filed.pdf

764-chenp-2003-description(complete)granted.pdf

764-chenp-2003-drawings.pdf

764-chenp-2003-form 1.pdf

764-chenp-2003-form 18.pdf

764-chenp-2003-form 26.pdf

764-chenp-2003-form 3.pdf

764-chenp-2003-form 5.pdf

764-chenp-2003-other documents.pdf

764-chenp-2003-pct.pdf

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