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WO2010058165A1 - Système d'interférométrie pour le profilage de surfaces, avec application à la production d'antennes de radioastronomie, et procédé correspondant - Google Patents

Système d'interférométrie pour le profilage de surfaces, avec application à la production d'antennes de radioastronomie, et procédé correspondant Download PDF

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Publication number
WO2010058165A1
WO2010058165A1 PCT/GB2009/002699 GB2009002699W WO2010058165A1 WO 2010058165 A1 WO2010058165 A1 WO 2010058165A1 GB 2009002699 W GB2009002699 W GB 2009002699W WO 2010058165 A1 WO2010058165 A1 WO 2010058165A1
Authority
WO
WIPO (PCT)
Prior art keywords
determining
interferometer
motion
radiation
roll
Prior art date
Application number
PCT/GB2009/002699
Other languages
English (en)
Inventor
John Benjamin Mitchell
Paul Howard Morantz
Original Assignee
Cranfield University
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Cranfield University filed Critical Cranfield University
Publication of WO2010058165A1 publication Critical patent/WO2010058165A1/fr

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B11/00—Measuring arrangements characterised by the use of optical techniques
    • G01B11/002—Measuring arrangements characterised by the use of optical techniques for measuring two or more coordinates
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B21/00—Measuring arrangements or details thereof, where the measuring technique is not covered by the other groups of this subclass, unspecified or not relevant
    • G01B21/02—Measuring arrangements or details thereof, where the measuring technique is not covered by the other groups of this subclass, unspecified or not relevant for measuring length, width, or thickness
    • G01B21/04—Measuring arrangements or details thereof, where the measuring technique is not covered by the other groups of this subclass, unspecified or not relevant for measuring length, width, or thickness by measuring coordinates of points
    • G01B21/047—Accessories, e.g. for positioning, for tool-setting, for measuring probes
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B9/00—Measuring instruments characterised by the use of optical techniques
    • G01B9/02—Interferometers
    • G01B9/02015—Interferometers characterised by the beam path configuration
    • G01B9/02017—Interferometers characterised by the beam path configuration with multiple interactions between the target object and light beams, e.g. beam reflections occurring from different locations
    • G01B9/02021—Interferometers characterised by the beam path configuration with multiple interactions between the target object and light beams, e.g. beam reflections occurring from different locations contacting different faces of object, e.g. opposite faces
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B9/00—Measuring instruments characterised by the use of optical techniques
    • G01B9/02—Interferometers
    • G01B9/02015—Interferometers characterised by the beam path configuration
    • G01B9/02027—Two or more interferometric channels or interferometers
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B2290/00—Aspects of interferometers not specifically covered by any group under G01B9/02
    • G01B2290/45—Multiple detectors for detecting interferometer signals

Definitions

  • the present invention relates to a method and apparatus for interferometric self-referencing of a measuring device.
  • the invention relates to a self-referencing, interferometric, long- trace, height-measuring profiler.
  • ELTs extremely large telescopes
  • a typical ELT segment would be 1.5m width and be highly aspheric with a very long radius of curvature (approximately 84m for a planned European ELT).
  • the long radius of curvature makes it impractical for a full aperture measurement to be made from the centre of curvature as this would require the construction of a tower at least 84m high.
  • the test set-up would be shortened by auxiliary folding, focussing and corrective optical elements.
  • Misalignments of these elements can introduce unwanted aberrations into the tested wavefront (ideally of the same profile as the surface being tested), leading to errors in the determination of the test element surface profile. It is for this reason that it is important to be able to verify the full aperture test by an independent measurement to characterise and/or eliminate any unwanted errors.
  • Profilers whether contact or non-contact, are commonly used to characterise high spatial frequency surface features over short length scales. For these applications, the accuracy of motion of a scanning head of. the profiler over the object under test and for low spatial frequencies is not critical. However, for long-trace profiling, to determine the overall surface figure, the errors in mechanical motion of the scanning head cannot be ignored.
  • X-ray optics typically, are very flat even when compared to the long radius optics for ELTs and slope variations are thus very small. It is known to detect the position of a beam centroid; this is a geometric approach which determines the centre of a cross-section of a beam of radiation.
  • n diffraction order
  • v perpendicular velocity
  • d grating period
  • US-A-2006/0077396 discloses interferometer systems for measuring displacement including a displacement interferometer.
  • the displacement interferometer includes a displacement converter responsive to a measuring beam of light.
  • the displacement converter is configured to transform movement thereof in a direction orthogonal to the measuring beam of light into a change in path length between a reflective surface of the displacement converter and the measuring beam of light.
  • the displacement converter may include a transmission grating and a displacement mirror or a reflecting grating.
  • apparatus for determining displacement relative to a reference beam of radiation comprising an emitting arrangement serving to emit said beam, wherein said determining is by an interferometer detecting movement perpendicular to said beam of a received end of said beam.
  • pointing instability of the beam of radiation may be determined with an interferometer. This determination enables a surface profile of an object to be measured highly accurately without a mechanical straightness reference artefact. Rather, the straightness of propagation of the reference beam of radiation is used as an absolute reference.
  • the beam of radiation is a beam of light, i.e. visible radiation, for example a laser beam, and the beam could be emitted as a pencil beam, as a scanning pencil beam moving in a plane, or as a planar beam.
  • a method comprising emitting a beam of radiation to form a positional reference, using said positional reference while using said beam for determining positional displacement of said beam and, in using said positional reference, compensating for said displacement of said beam.
  • the accuracy of determination of positional displacement by using a beam of radiation as a positional reference can be greatly improved.
  • the method is particularly useful in compensating for instabilities in the pointing direction of a reference straightness beam of a surface profiler.
  • an optical device movable by roll motion about an axis, the optical device being arranged in a plane oblique to the axis, a roll motion-detecting beam of radiation impinging upon the optical device at a radius from the axis, and a determining arrangement serving to determine the roll motion from a change in the optical path length of the roll motion-detecting beam.
  • the determining of roll motion of the optical device in particular of a means carrying the optical device, can be greatly simplified.
  • the optical device is preferably a diffraction grating associated with an interferometer.
  • roll motion of the optical device can be determined and compensated for in a high- accuracy measuring system, such as a surface profiler.
  • a high- accuracy measuring system such as a surface profiler.
  • the direction of propagation of that light beam should have a mathematical component substantially parallel to the surface of the object being scanned at all points along the profile scan section.
  • the reference beam is nominally parallel to the best-fit straight line through that section.
  • That measurement beam may be locally generated or derived from the straightness reference beam.
  • the surface height may be measured by a mechanical contact probe. The height of the surface may then be measured with reference to the position of the scanning head. A problem that occurs is then to compensate for errors in that measured height due to non-straight motion of the scanning head over the surface. This problem is alleviated by detecting motion of the scanning head perpendicular to the direction of the straightness reference beam.
  • Figure 1 shows two per se known interferometer configurations sensitive to perpendicular motion
  • Figure 2 shows a diagrammatic elevational view of a first version of a surface profiling system using perpendicular interferometers and constituting a first embodiment of the present invention
  • Figure 3 shows a similar view to Figure 2, but of a second version of the surface profiling system and constituting a second embodiment of the present invention
  • Figure 4 shows a similar view to Figures 2 and 3, but of a third version of the surface profiling system and constituting a third embodiment of the present invention
  • Figure 5 is a diagrammatic representation of components of an interferometer sensitive to roll motion and constituting a further embodiment of the present invention
  • Figure 6 shows a similar view to Figure 4, but of a fourth version of the surface profiling system incorporating similar components to that of Figure 5 and constituting a yet further embodiment of the present invention.
  • Figure 1 shows interferometer component configurations sensitive to perpendicular motion which can be interpreted as a geometrical change in the path length of the beam.
  • OPD optical Path Difference
  • the uppermost diagram in Figure 1 shows a diffraction grating 102 in the Littrow configuration where the diffraction angle equals the incident angle of a beam 104 which can be displaced by distance D owing to beam pointing instability, and the beam 104 is returned along its incident path.
  • the grating groove profile is usually manufactured or blazed so that the efficiency is greatest for a given wavelength and blaze angle.
  • the lowermost diagram of Figure 1 shows the geometry for a reflection grating 102a with the diffracted beam 104a retro-reflected by a mirror 106. The same geometry would also apply to a transmission grating.
  • Figure 2 shows a diagrammatic layout of a long-trace profiler 200 which comprises a laser source 206 for supplying a laser beam for transmission by an optical fibre 208.
  • Supports S indicate where components are mechanically coupled into a constant spatial relationship.
  • the reference of the profiler 200 is derived from a single-mode, fibre- delivered, laser beam collimated by a lens 210.
  • the collimated beam has superior beam pointing stability because it is isolated from the thermal environment of the laser.
  • the collimated beam is split by way of a beam- splitting and -steering optical arrangement 211 into two parallel beams 212a and 212b.
  • the beam 212a goes to the first perpendicular interferometer 202 (Z- axis interferometer) connected to a scanning head 214.
  • a diffraction grating 216 in the first interferometer 202 is coupled to a height measuring probe 218 which may be a contact (stylus) or non-contact (optical) probe, a contact probe being shown.
  • the grating 216 and an interferometer beam splitter 220 are also coupled so that the output from the first interferometer 202 to a detector 222 is sensitive to movements in the z-axis direction (shown in Figure 2).
  • the first interferometer 202 can be compensated for variations in the incident position of the laser beam due to pointing instability because of the presence of a second interferometer 204 (pointing interferometer), included to monitor this type of motion through receiving the beam 212b.
  • the first and second interferometers 202 and 204 are referenced to the straightness of the laser beam 212b, with compensation for reference beam pointing instability.
  • the second interferometer 204 is coupled to a surface 224 being measured and so does not move in the z- axis direction and is sensitive only to beam pointing variations.
  • the output from the second (pointing) interferometer 204 to a second detector 230 varies with movement of the beam 212b perpendicular to itself. Errors in the probe z-axis motion due to beam pointing instability may be compensated for given a knowledge of the probe x-axis position.
  • the probe x- axis position is measured by using a conventional displacement measuring interferometer (DMI) 226 and a mirror 228 mounted on the probe 218, the output of the interferometer in this respect going to a third detector 232.
  • DMI displacement measuring interferometer
  • the DMI 226 may also be utilised to measure angular pitch errors in the motion of the scanning head 214 to compensate for cosine errors, the output of the interferometer in this respect going to a fourth detector 234.
  • the DMI 226 can be optically connected to the first or second interferometers 202 or 204 by way of a beam splitter 227 (in Figure 2, the DMI 226 is optically connected to the first, z-axis interferometer 202).
  • the DMI 226, the mirror 228 and the third and fourth detectors 232 and 234 are optional, the same data being obtainable by independent measurement.
  • the beam-splitting and -steering optics 211 following the collimator lens 210 and the interferometers 202 and 204 themselves be of monolithic construction. Residual non-common pointing instabilities due to index gradients in air refractive index should be minimised by careful environmental control of currents and temperature.
  • the beam paths to the first and second perpendicular interferometers 202 and 204 might be enclosed in a common bellows arrangement and flushed with helium. Issues relating to temperature are not relevant if non-expansive materials such as "INVAR" (Registered Trade
  • FIG. 3 An alternative embodiment of the profiler is shown in Figure 3 where like features of Figure 2 are shown with like reference numerals with the prefix '3'.
  • the beam splitter 311 (instead of the beam-splitting and - steering optics 211 of Figure 2) is mounted on the scanning head 314 of the profiler 300 and serves to direct the beam of light 312 to the first, z-axis interferometer 302 and to the second, pointing interferometer 304.
  • a possible disadvantage with mounting this beam splitter 311 on the scanning head 314 is that it may introduce beam deviations due to scan motion errors that would cause an error in the output signal from the second pointing interferometer 304 to the detector 330.
  • the positions of the grating 316 and a mirror 317 in the first perpendicular interferometer 302 may be interchanged.
  • supports S indicate where components are mechanically coupled into a constant spatial relationship.
  • the DMI 326 is optically connected to the first, z-axis interferometer 302.
  • the profiler 400 comprises a separate probe 418 and scanning head 414 for z-axis measurement and a further displacement measuring interferometer (DMI) 450, for example a Zygo. differential plane mirror interferometer.
  • DMI displacement measuring interferometer
  • the further DMI 450 receives a beam of radiation split from the beam 412a by way of a beam splitter 452, and a probe z-position detector 454 measures the position of the probe
  • the DMI 426 is optically connected to the second, pointing interferometer 404.
  • the probe 418 comprises a mirror 456 mounted at its upper end for the purpose of optically communicating with the
  • supports S indicate where components are mechanically coupled into a constant spatial relationship.
  • Cosine errors due to roll motions may also be compensated for by employing a roll-sensitive development of the perpendicular interferometer.
  • a configuration of an interferometer using a diffraction grating that is sensitive to roll motion (rotation about the optical axis) is shown in Figure 5.
  • the upper diagram in Figure 5 shows a plan view from above of components of an interferometer sensitive to roll motion and the lower diagram shows a side elevation of those components.
  • An incident beam of radiation 500 is split, by way of a. beam splitter 506, into two spaced-apart parallel beams of radiation 502 and 504 separated by a distance L.
  • the spaced-apart beams 502 and 504 impinge upon a diffraction grating 508 (shown in the Littrow configuration) and are returned along their incident paths to a detector 510.
  • a roll motion of the grating 508 occurs about an axis parallel to the x-axis, one of the spaced-apart beams 502 or 504 gets longer while the other gets shorter, thereby imparting the necessary phase shift for the interferometer to measure that roll motion as detected by the detector 510.
  • the change in path length for a small roll angle, ⁇ is given by:
  • OPD 2Z. sin a. tan ⁇
  • FIG. 6 An embodiment of the profiler, similar to that shown in Figure 4 and incorporating similar components to those shown in Figure 5, is shown in Figure 6. Again, like reference numerals are shown with a prefix '6'.
  • the profiler 600 is sensitive to roll motion about an axis parallel to the x-axis.
  • a reflecting diffraction grating 608 (in the Littrow configuration) is connected to the scanning head 614.
  • the spaced apart beams 658 and 660 are returned along their incident paths to a 3-axis interferometer 626.
  • the 3-axis interferometer 626 has three outputs to three detectors; a detector 632 for detecting linear displacement of the scanning head 614; another detector 634 for detecting angular pitch in the scanning head 614; and a further detector 662 for detecting the roll motion about the aforementioned axis.
  • the 3-axis interferometer 626 is optically connected to the pointing interferometer 604 by way of a beam splitter 627.
  • the support S on the right-hand side of the profiler system 200, 300, 400 or 600 could be omitted and be replaced by a mirror fixed relative to the surface 224, 324, 424 or 624 being measured, the mirror optically communicating with the second, pointing interferometer 202, 302, 402 or 602.
  • Such a profiling system 200, 300 or 400 can thus be utilised to perform very high accuracy measurements and be self-compensating for not only beam pointing instabilities, but also for errors in the mechanical motion of the scanning head 214, 314 or 414 across the surface 224, 324 or 424.
  • Such applications may include three- dimensional co-ordinate measuring machines (CMM), and on-machine metrology for precision machining and precision motion control.
  • CCM three- dimensional co-ordinate measuring machines

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Length Measuring Devices By Optical Means (AREA)

Abstract

L'invention porte sur un appareil pour déterminer le déplacement par rapport à un faisceau de rayonnement de référence, l'appareil comprenant un dispositif d'émission servant à émettre le faisceau, la détermination se faisant par un interféromètre détectant un mouvement perpendiculaire du faisceau d'une extrémité reçue du faisceau. De cette manière, l'instabilité de pointage du faisceau de rayonnement de référence peut être déterminée avec un interféromètre. Une telle détermination permet à une surface d'un objet d'être mesurée de façon extrêmement précise sans élément de référence de rectitude mécanique. La rectitude de propagation du faisceau de rayonnement de référence est plutôt utilisée comme référence absolue.
PCT/GB2009/002699 2008-11-18 2009-11-18 Système d'interférométrie pour le profilage de surfaces, avec application à la production d'antennes de radioastronomie, et procédé correspondant WO2010058165A1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GBGB0821015.5A GB0821015D0 (en) 2008-11-18 2008-11-18 Apparatus and method
GB0821015.5 2008-11-18

Publications (1)

Publication Number Publication Date
WO2010058165A1 true WO2010058165A1 (fr) 2010-05-27

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WO (1) WO2010058165A1 (fr)

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2022021907A (ja) * 2020-07-22 2022-02-03 株式会社ミツトヨ てこ型光学式変位センサ

Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4436424A (en) * 1981-07-27 1984-03-13 Gca Corporation Interferometer using transverse deviation of test beam
US4538911A (en) * 1982-01-15 1985-09-03 Carl-Zeiss-Stiftung Three-dimensional interferometric length-measuring apparatus
US5455677A (en) * 1993-03-15 1995-10-03 Matsushita Electric Industrial Co., Ltd. Optical probe
US6344656B1 (en) * 1997-12-04 2002-02-05 Taylor Hobson Limited Surface measuring apparatus having relative displacement between a probe and its carriage
US20060077396A1 (en) * 2004-10-07 2006-04-13 Dong-Woon Park Interferometer systems for measuring displacement and exposure systems using the same
US20070146722A1 (en) * 2005-12-23 2007-06-28 Trutna William R Jr Littrow interferometer

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4436424A (en) * 1981-07-27 1984-03-13 Gca Corporation Interferometer using transverse deviation of test beam
US4538911A (en) * 1982-01-15 1985-09-03 Carl-Zeiss-Stiftung Three-dimensional interferometric length-measuring apparatus
US5455677A (en) * 1993-03-15 1995-10-03 Matsushita Electric Industrial Co., Ltd. Optical probe
US6344656B1 (en) * 1997-12-04 2002-02-05 Taylor Hobson Limited Surface measuring apparatus having relative displacement between a probe and its carriage
US20060077396A1 (en) * 2004-10-07 2006-04-13 Dong-Woon Park Interferometer systems for measuring displacement and exposure systems using the same
US20070146722A1 (en) * 2005-12-23 2007-06-28 Trutna William R Jr Littrow interferometer

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2022021907A (ja) * 2020-07-22 2022-02-03 株式会社ミツトヨ てこ型光学式変位センサ
JP7430457B2 (ja) 2020-07-22 2024-02-13 株式会社ミツトヨ てこ型光学式変位センサ

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