WO2024078661A1 - Système d'interféromètre et procédé de mesure - Google Patents

Système d'interféromètre et procédé de mesure Download PDF

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Publication number
WO2024078661A1
WO2024078661A1 PCT/DE2023/100749 DE2023100749W WO2024078661A1 WO 2024078661 A1 WO2024078661 A1 WO 2024078661A1 DE 2023100749 W DE2023100749 W DE 2023100749W WO 2024078661 A1 WO2024078661 A1 WO 2024078661A1
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WIPO (PCT)
Prior art keywords
measuring
measurement
laser source
interferometer
phase
Prior art date
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PCT/DE2023/100749
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German (de)
English (en)
Inventor
Jean-Michel Asfour
Thomas Kinder
Original Assignee
Dioptic Gmbh
Tem Messtechnik Gmbh
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Application filed by Dioptic Gmbh, Tem Messtechnik Gmbh filed Critical Dioptic Gmbh
Publication of WO2024078661A1 publication Critical patent/WO2024078661A1/fr

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    • 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
    • G01B9/02001—Interferometers characterised by controlling or generating intrinsic radiation properties
    • G01B9/02002—Interferometers characterised by controlling or generating intrinsic radiation properties using two or more frequencies
    • G01B9/02004—Interferometers characterised by controlling or generating intrinsic radiation properties using two or more frequencies using frequency scans
    • 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
    • G01B9/02001—Interferometers characterised by controlling or generating intrinsic radiation properties
    • G01B9/02007—Two or more frequencies or sources used for interferometric measurement
    • 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
    • G01B9/00—Measuring instruments characterised by the use of optical techniques
    • G01B9/02—Interferometers
    • G01B9/02055—Reduction or prevention of errors; Testing; Calibration
    • G01B9/0207—Error reduction by correction of the measurement signal based on independently determined error sources, e.g. using a reference interferometer
    • 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
    • G01B9/02055—Reduction or prevention of errors; Testing; Calibration
    • G01B9/02075—Reduction or prevention of errors; Testing; Calibration of particular errors
    • G01B9/02076—Caused by motion
    • 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
    • G01B2290/60—Reference interferometer, i.e. additional interferometer not interacting with object

Definitions

  • the invention relates to an interferometer system and a measuring method for measuring an object or a wavefront, in particular based on a Fizeau interferometer.
  • the invention relates to an absolute measuring interferometer based on tunable lasers (in particular DFB laser diodes), preferably with a (in particular atomic) frequency reference.
  • Non-tactile (contactless) methods are preferred for measuring sensitive surfaces, but also for other surfaces.
  • interferometry is particularly important because it can detect height differences of well under a micrometer on the surface using relatively simple image sensors (cameras) in conjunction with a suitable light source and a few optical elements. Advanced systems can even detect unevenness on the order of an atom's diameter.
  • Interferometers and interferometric measurements are well known. The measurements are based on the wave nature of light: a (sufficiently coherent) wave emitted by a light source is split into two partial waves, which are superimposed again after being reflected from a test object after they have traveled different paths.
  • the power density (intensity, brightness I) at the location of the superposition changes periodically with the difference in the lengths of the two paths, with the period being equal to the wavelength X.
  • each image point in the form of a camera, is located at the location of the superposition in such a way that (possibly after a mathematical transformation) each image point can be assigned a point on the test object surface ("object point"), the intensity of the image point carries information about the spatial position of the object point.
  • Such (simple) interferometric measuring methods have the advantage that they enable measurements with a very high accuracy, but the disadvantage that the results are not always clear. Due to the periodicity of the path-brightness relationship, no geometric measurement value (distance or similar) can be obtained from the brightness alone, because the phase at a pixel can at best be determined from the brightness value modulo 2n. By comparing the phases of neighboring pixels, however, information about their distance difference can be obtained, since, for example, in areas where the test object has a depression, the reflected light travels a slightly longer distance, and in raised areas a shorter distance. The total of all height differences together gives the shape of the test object.
  • the wave front of the wave to be reflected can be adapted to the shape of the test object.
  • the phase image shows deviations of the shape of the test object from the shape of the wave front.
  • spherical mirrors or lenses are illuminated with spherical waves.
  • the adaptation can be carried out using a (computer-generated) hologram, for example.
  • determining the radius of curvature of the measured surfaces is an important and (possibly multiple) recurring task.
  • the test object is illuminated with spherical waves. It appears "smooth" if the sphere centers of the wave and the reflective surface coincide. The distance of the surface from this center is the radius of curvature sought.
  • Radius determination in particular therefore requires additional means for measuring distances between a test object and designated points or surfaces of the equipment. Common methods even require the test object to be moved over a distance equal to the radius of curvature. The measurement uncertainty and duration of the additional measuring equipment therefore limits the accuracy of radius determination and the throughput of the production line.
  • interferometric methods e.g. those that work with two beams of different wavelengths and whose unambiguity lies in the range of the beat wavelength (multi-wavelength interferometry), or those that are based on a change in wavelength.
  • phase change i.e. the totality of the or partially completed periods
  • proportionality factor is equal to the path length difference
  • Absolute interferometers for surface shape measurement therefore rely on the test object and the medium through which it is irradiated (the air) not moving during the measurement.
  • the object of the present invention was to overcome the disadvantages of the prior art and to provide an interferometer system and a measuring method for measuring an object or a wavefront, by means of which a very precise absolute measurement can be achieved in a very large measuring range.
  • An inventive (absolute measuring) interferometer system allows measurement of an object or a wavefront by determining a distance from points of the object to a reference.
  • This reference is a point at a known position or a set of points on a reference surface.
  • the shape of an object is determined as the distance of the respective point of the object to a point on a reference surface.
  • the interferometer system comprises the following components:
  • a tunable measuring laser source designed to emit a variable measuring beam within a (in particular predetermined but always known) measuring frequency range AV M ZU,
  • a reference laser source designed to emit a reference beam with a known frequency
  • a measuring interferometer arrangement comprising an interferometer designed for the interferometric measurement of an interference phase on an object by means of the measuring beam and the comparison beam, wherein the interferometer system is designed for simultaneous measurement with the measuring laser source and the comparison laser source for determining the distance,
  • a phase determination unit designed to determine the number Acp M of phase passes of the measuring beam in the measuring interferometer arrangement during a tuning of the frequency of the measuring laser source
  • a distance determination unit designed to determine the distance of an object to a reference based on a weighted phase difference of the measured number of phase passes Acp M of the measuring beam and a value of a phase change Acp v between two measurements with comparison beams.
  • the interferometer system includes all other components that make up an interferometer system, such as optical components, holders, recording units or adjustment units.
  • the interferometer system therefore comprises two independent laser sources, a measuring laser source and an additional comparison laser source.
  • the measuring laser source is tunable.
  • the comparison laser source only has to emit a comparison beam with a single frequency, but can also be designed to emit a variable comparison beam within a comparison frequency range.
  • the comparison beam can be viewed as a beam for determining distance changes due to (relative or absolute) movements of the object.
  • the measuring beam emitted by the measuring laser source lies in the measuring frequency range M, which must be known and is preferably predetermined, e.g. by measurement or selection of suitable components and/or operating parameters.
  • the frequency of the comparison beam of the comparison laser source must be known.
  • the frequency can be determined, for example, by presetting, by selecting the components and parameters or by measuring the frequency.
  • the laser sources are preferably laser diodes, in particular distributed feedback lasers (DFB). These are laser diodes, in which the active material is periodically structured.
  • the structures of changing refractive index form a one-dimensional interference grating or an interference filter (Bragg mirror).
  • An example would be two DFB laser diodes at wavelengths of 633 nm and 795 nm, of which at least the measuring laser source can be tuned widely, in particular over more than 100 MHz or more than 1 GHz, in particular more than 10 GHz or more than 100 GHz. Tuning of the comparison laser source is not absolutely necessary.
  • the measuring interferometer arrangement comprises at least one interferometer, but can also have two or more interferometers, e.g. one interferometer for each of the two laser sources. It is important, however, that one and the same object is always measured with all beams.
  • the measuring interferometer arrangement preferably comprises other components such as lenses, prisms, beam splitters, mirrors, reference surfaces and a holder for the object to be measured.
  • a detector e.g. a camera or an image sensor with imaging optics, is also part of the measuring interferometer arrangement.
  • a unit for recording the measurements (images) is part of the interferometer system, in particular part of the measuring interferometer arrangement or the distance determination unit.
  • a preferred embodiment of the measuring interferometer arrangement is that of a Fizeau interferometer, in which an object is measured relative to a reference surface, e.g. a flat or curved mirror.
  • a reference surface e.g. a flat or curved mirror.
  • the basic structure of a Fizeau interferometer is known in the art. It is a special interferometer that can be used to assess the optical quality of surfaces and optics.
  • the measuring principle is based on comparing a surface to be measured with a reference surface of a reference surface using interferometry.
  • the interferometer system must be designed for simultaneous measurement with the measuring laser source and the comparison laser source. “Simultaneous” means The purpose of this invention is that during or for determining the distance, both measurements must be carried out, either simultaneously or alternately. This does not mean that the measurements are carried out first with the measuring laser source and then with the comparison laser source, but rather that during the tuning of the measuring laser source, multiple measurements must be carried out with the comparison laser source, e.g. at least at the beginning and at the end of the tuning and also (in particular multiple times) during the tuning (e.g. between measurements during the tuning). This can be achieved, for example, with a variable aperture that alternately lets through only the beam of a single laser source. However, measurements can also be carried out simultaneously, e.g.
  • a simultaneous measurement in the sense of the invention is a simultaneous measurement with both beams or an alternating measurement in which the beams (in particular multiple or many times) radiate in at different times from one another.
  • a single interferometer is used, at least if the two different beams of the measuring laser source and the comparison laser source can be separated from each other.
  • This can be achieved, for example, by having the beams enter the measuring interferometer arrangement at different times (e.g. by means of a so-called "chopper") or by separating the beams from each other using filters.
  • Interferometric measurements are well known in the art and are based on the fact that a partial beam of a beam is reflected from the object and interferes with another partial beam. In a Fizeau interferometer, this other partial beam comes from the reference surface, for example.
  • One point of the object can be measured at a time, e.g. with an interferometer with a point detector, whereby the surface of the object is scanned to measure it. Movements of the object are detected by the measurements with the comparison beam is compensated.
  • a surface measurement is carried out, e.g. with an interferometer which has a camera as a detector (or at least an image sensor matrix with imaging optics). In this case, it is preferred to emit the beams in the form of radiation cones. This is also preferred for measuring a wavefront and a corresponding interferometer system could be used as or in a wavefront sensor.
  • the phase determination unit can be part of the measuring interferometer arrangement or exist independently of it. For example, when recording using an image sensor (single pixel or pixel matrix), the phase determination unit can also be located in a computing unit that is connected to this image sensor via data technology. The phase determination unit determines the number of phase passes during a tuning of the frequency of the measuring laser source.
  • this (or another) phase determination unit can be used to determine its phase transitions. The use is identical to that for the measurement beam.
  • phase pass When the frequency of the measuring beam is tuned, its wavelength changes and thus also the phase measured in the measuring interferometer arrangement. Since the light waves follow a sine function or cosine function, the measured intensity will vary between maxima and minima, whereby the transition from one to the next maximum (2n) is referred to here as a "phase pass". These changes are counted and result in a number of phase passes. Even if this number results in an integer value in the simplest case (counting all maxima or minima), distance calculations could already be carried out with it. However, since intermediate values can also be estimated with an image sensor, this number is preferably a rational number and also indicates intermediate stages (e.g. starting from a minimum via another minimum to the next maximum, the number would correspond to 1.5).
  • the measured time can also be used as a measure for the phase passes. If, for example, a phase pass lasts exactly 1 s and 34.567 s were measured during the tuning, then the number of phase passes can be specified as 34.567.
  • the phase determination unit is preferably designed to determine the time of the tuning and to determine the duration of a specified number of phase passes (even one).
  • the distance determination unit is designed to calculate values. Suitable calculation units are known and can be implemented in a computer system, for example.
  • the distance of an object to a reference is determined using the measured number of phase passes Acp M and the measurement frequency range Av M , which should ideally be well known. In order to compensate for minimal movements of the object during the measurement, the distance is determined based on a weighted phase difference of the measured number of phase passes Acp M of the measurement beam and the value (0 to 2n) of a phase change Acp v between two measurements with comparison beams. This is explained in more detail below.
  • a measuring method for determining a distance of an object to a reference with an interferometer system according to the invention, preferably for measuring an object or a wavefront, comprises the following steps:
  • the measuring laser source is set to a first frequency v M .
  • this frequency is preferably stabilized using a stabilization unit.
  • the measuring laser source is set to a frequency of a J 2 transition and stabilized using lock-in technology.
  • a first measurement can be taken.
  • a first measuring beam from the measuring laser source is emitted at this frequency, hits an object in the measuring interferometer arrangement, and a partial beam of the measuring beam is reflected by this object.
  • this reflected partial beam now interferes with another partial beam (which was reflected on a reference surface, for example) and interferes with it.
  • the resulting interference pattern is measured and the phase relationship between these two partial beams, which is referred to as the "interference phase", is determined from this.
  • This interference phase is reproduced by an image sensor as an intensity value, and by a pixel matrix as an image sensor as a matrix of intensity values.
  • a comparison laser source to the first frequency v v , emitting a first comparison beam and measuring an interference phase cp Vi corresponds to the first measurement with the measuring beam described above and is carried out analogously. log, except that the comparison laser source now emits a beam that is called the "comparison beam" for easier differentiation.
  • the comparison beam preferably has a different wavelength than the measurement beam, but this is not absolutely necessary.
  • the comparison laser source is set to a frequency of the Rb-Dl transition at 795 nm and stabilized using lock-in technology.
  • the measuring laser source is now tuned over the measuring frequency range Av M , e.g. over 100 GHz. This means that the frequency of the measuring beam is changed (continuously) from v M to another frequency. During this time, measuring beams from the measuring laser source continue to be emitted and measurements of the interference phase continue to take place. However, the interference phases will change continuously due to the change in the wavelength of the measuring beam and, as described above, phase transitions will occur which manifest themselves as intensity fluctuations on the image sensor. The number of phase transitions Acp M of the interference phase during tuning in the measuring interferometer arrangement is now counted during tuning. In addition to integer changes, changes that have already begun are also preferably recorded quantitatively, e.g. based on the tuning speed (see above), which improves the accuracy of the result.
  • Measurements with the comparison laser source that are carried out simultaneously with the tuning can be preceded by a readjustment of the comparison laser source to a frequency, particularly if measurements are to be taken at a different frequency. If measurements are to be taken again at the first frequency v v , it is simply preferable to stabilize the comparison laser source so that the deviation of the frequency of the comparison beam is small compared to the previous measurement. Basically, this step is otherwise the same as the previous measurement with the comparison laser source, except that it is carried out at the same time or alternately (i.e. simultaneously) with the tuning. If anything has changed in the position of the object (absolute or relative, e.g. to a reference surface), this will be reflected in the measured interference phase.
  • a preferred interferometer system comprises a reference interferometer arrangement containing an interferometer with a known reference distance D R .
  • This reference interferometer arrangement is designed to determine a frequency change of the measuring laser source.
  • the distance determination unit is particularly preferably designed to determine the distance of an object from a reference based on the measured number of phase passes of the measuring beam and the known reference distance D R. This is explained in more detail below in the context of the corresponding measuring method.
  • the reference interferometer arrangement preferably comprises a further interferometer with a (possibly further) known reference distance, designed to determine a frequency change of the comparison laser source.
  • the (or a further) phase determination unit is preferably additionally designed to determine the number of phase passes during a detuning of the frequency of the comparison laser source in the reference interferometer arrangement.
  • a measurement of at least the measuring beams is carried out on the said reference interferometer arrangement with the known reference distance D R .
  • a measurement of the number of phase transitions Acp R in the reference interferometer arrangement is also carried out by means of a phase determination unit.
  • the distance D is then determined from the reference distance D R and a ratio based on the number of measured phase transitions, in particular by means of weighted phase differences (see above).
  • the reference distance D R serves as a kind of yardstick.
  • the interferometer system preferably comprises a tuning unit which is designed to tune the frequency of a measuring beam of the measuring laser source, wherein the tuning unit is preferably designed to tune the measuring laser source such that the amount of change in the frequency of the measuring beam is greater than 1 GHz, wherein tuning over a measuring frequency range Av M greater than 10 GHz or even greater than 100 GHz is preferred.
  • a tuning unit is basically known in the prior art and can be implemented, for example, by a variable voltage or current control of the measuring laser source.
  • the interferometer system preferably comprises a corresponding tuning unit which is designed to tune the frequency of a comparison beam of the comparison laser source.
  • the information on the measurement frequency range preferably applies to the comparison frequency range.
  • the interferometer system preferably comprises a stabilization unit for stabilizing one of the laser sources to a frequency.
  • the general principle of such a stabilization e.g. to an atomic or molecular absorption line or to the interference maximum of a grating, is known in the prior art.
  • the interferometer system preferably comprises a beam guiding element, preferably a light guide, e.g. a glass fiber, designed to guide the light from both laser sources into the measuring interferometer arrangement.
  • the beam guiding unit preferably guides the beams from the laser sources to a single light guide using light guides, and is particularly preferably V- or Y-shaped for this purpose.
  • the term "light guide” refers to a single light-guiding element or a bundle of light-guiding elements by means of which light is guided in one direction.
  • the interferometer system preferably comprises a selection unit, e.g. a so-called "chopper", which is known in the prior art.
  • a selection unit e.g. a so-called "chopper", which is known in the prior art.
  • Such a selection unit is designed to alternately block out the beam of one of the two laser sources, so that at one measurement time only the measurement beam of the measurement laser source falls into the measurement interferometer arrangement and at another measurement time only the comparison beam of the comparison laser source falls into the measurement interferometer arrangement.
  • the laser beams are alternately blocked out using the selection unit (e.g. a chopper).
  • the selection unit e.g. a chopper.
  • the interferometer system preferably comprises an auxiliary interferometer designed to determine a tuning speed (change in frequency and/or phase as a function of time) of one of the laser sources, in particular of the measuring beam.
  • This auxiliary interferometer is assigned to one of the laser sources or both laser sources and is used to measure a property of the light of this laser source(s).
  • the reference interferometer arrangement can be designed to determine this tuning speed.
  • the auxiliary interferometer can also be assigned to the reference interferometer arrangement or be this reference interferometer arrangement. In principle, the reference interferometer arrangement can also be assigned to the laser sources or comprise interferometers that are assigned to the laser sources (but this does not necessarily have to be the case).
  • the auxiliary interferometer is designed in particular additionally to monitor a mode purity of one of the laser sources. If the tuning speed speed is known, a phase run that has already begun can be quantified very precisely by measuring the time when determining the phase runs. This makes it possible to specify the number of phase runs as a rational number, e.g. 100.437 runs. This increases the accuracy of a distance determination.
  • Each laser source i.e. the comparison laser source and/or the measuring laser source
  • Each laser source includes its own auxiliary interferometer to determine the frequency change or an auxiliary interferometer (or a reference interferometer arrangement) is used to measure both laser sources.
  • the interferometer used corresponds in terms of structure or measuring principle to the measuring interferometer arrangement and, in particular, is arranged in the same atmosphere. This has the advantage that refractive index compensation is achieved automatically and reflector movements in this interferometer are eliminated in the same way as in the measuring interferometer arrangement.
  • the interferometer system preferably includes further components that are generally known in the prior art and serve to improve handling, eliminate disruptive effects or improve measurement accuracy.
  • Preferred further components are, for example, optical Faraday isolators, elements for coupling into a glass fiber for a measuring interferometer or for a reference interferometer or elements for dichroic beam superposition.
  • the values measured in the context of the method according to the invention can preferably be used to carry out further calculations of the distance, which improve the result. It should be noted that with many interferometric distance calculations, there is no longer any unambiguousness beyond certain distance differences. However, if the distance can be determined within the unambiguousness of another determination method and the method allows a more precise determination of the distance within its unambiguousness, the "coarser" distance measurement can be used to establish unambiguousness. Preferably, after the aforementioned determination of the distance based on the measured values, a further calculation of the distance is additionally carried out.
  • This calculation is preferably based on a single-wavelength method or a two-wavelength method, which is basically known in the art. What is special is that the distance already determined as part of the method according to the invention is used to ensure uniqueness. Both measurement beams and comparison beams can be used as beams, with the images in question preferably being generated one after the other or at least within a period of less than 1 s.
  • a calculation based on a two-wavelength method is carried out in particular using the measured values for a measurement with a measurement beam and a measurement with a comparison beam.
  • a calculation based on a single-wavelength method is carried out in particular using the measured values for a measurement with a measurement beam or a measurement with a comparison beam.
  • the measuring laser source is set to a frequency of the iodine transition and stabilized. Then the phases are recorded at the detectors for the measuring and comparison beams in the measuring interferometer and the distance is determined from the phase difference, whereby the integer parts of cp/2n can be reconstructed from the absolute measurement.
  • the measurement uncertainty should be as low as possible as less than half the light wavelength of the comparison laser source (ie definitely within the unambiguous range of an incremental single-wavelength measurement). Finally, the state-of-the-art single-wavelength measurement is carried out, where the integer parts of cp/2n can be reconstructed from the absolute measurement and the two-wavelength measurement.
  • the respective wavelengths of the laser sources involved serve as the scale embodiment. This results from the frequencies, which can be traced back via the 12 or Rb frequency standard, and the refractive index of the air, which is determined separately.
  • the reference interferometer represents the scale. This should be calibrated beforehand.
  • both laser sources are preferably tuned slightly one after the other, pairs of values are recorded and a correction is carried out by fitting a second-order curve according to Heydemann. This correction should be repeated automatically before each absolute measurement (regardless of the method).
  • the advantage of the interferometer system according to the invention is that distances of up to 2 m can be measured with an absolute uncertainty of 0.2 pm using such an interferometer. This accuracy can be further increased by using the additional measurements mentioned above.
  • Figure 1 shows a preferred embodiment of an interferometer system.
  • Figure 2 shows a preferred embodiment of a laser source.
  • Figure 3 shows a block diagram of a preferred embodiment of the measuring method.
  • Figure 1 shows a preferred embodiment of an interferometer system 1 for measuring an object O by determining a distance of an object O to a reference, which is formed here by a reference surface 8.
  • the interferometer system 1 comprises the following components:
  • a tunable measuring laser source 10a designed to emit a variable measuring beam M within a measuring frequency range Av M and a comparison laser source 10b, designed to emit a comparison beam V with a known frequency.
  • Each of these laser sources (10) can have a structure as shown in Figure 2.
  • FIG. 2 shows the advantageous structure of a laser source 10 for such an interferometer system 1.
  • a laser diode 11 is used to emit a beam which passes through two beam splitters 15 before exiting and is split there.
  • the beam of the laser source 10 can be tuned within a frequency range using a tuning unit 14 (optional for the comparison laser source), e.g. by changing the voltage or current.
  • One split beam runs into an (optional) reference interferometer arrangement 13 (here in the form of an auxiliary interferometer), in which the number of phase transitions is counted during tuning.
  • the other of the split beams runs into an (optional) stabilization unit 12 and the laser source 10 can thereby be stabilized, e.g. using a lock-in method.
  • the measuring laser source 10a can be stabilized to an iodine transition and the comparison laser source 10b to a rubidium transition.
  • all possible laser media can be used instead of laser diodes 11.
  • the comparison laser source (10b) does not necessarily have to have a tuning unit 14 or a reference interferometer arrangement 13.
  • stabilization units 12 are highly recommended.
  • a stabilization unit 12 the one shown here is equipped with a coupling medium K, e.g. iodine or rubidium.
  • the measuring beam M of the measuring laser source 10a and the comparison beam V of the comparison laser source 10b are brought together by means of a beam guiding element 3, which here comprises a glass fiber into which the two beams of the laser sources 10a, 10b are coupled, e.g. by means of special coupling elements.
  • the glass fibers are brought together in a Y-shaped arrangement onto one fiber, so that both the measuring beam M and the comparison beam V emerge from the same glass fiber.
  • the beams can also be coupled out, as indicated by dashed lines, and their frequency change can be measured in a reference interferometer arrangement 13.
  • a reference interferometer arrangement 13 This could represent an alternative to Figure 2, in which case the auxiliary interferometer 13 can be dispensed with and a single interferometer can be used to measure both laser sources 10a, 10b.
  • a selection unit 2 in the form of a so-called "chopper” is arranged between the laser sources 10 and the beam guide element 3.
  • the selection unit 2 has the shape of a wheel (see the illustration above, indicated by the arrow) with translucent and opaque areas. These areas are arranged in such a way that in each position of the wheel one of the beams is covered by an opaque area and the other beam can shine through a translucent area.
  • the selection unit 2 Using the selection unit 2, a simultaneous measurement with the measuring beam M and the comparison beam V is possible with a rapid alternation between the beams M, V. This means that a large number of alternating measurements are carried out. Theoretically, a simultaneous measurement with the measuring beam M and the comparison beam V would also be possible without the selection unit 2 if filters were used. However, the selection unit 2 shown here enables a simple and inexpensive structure that is robust against errors.
  • the beams M, V guided and controlled in this way then enter a measuring interferometer arrangement 9, as indicated by the dashed beam cones.
  • This measuring interferometer arrangement 9 here comprises an interferometer, designed for the interferometric measurement of interference phases on an object O by means of the measuring beam M and the comparison beam V.
  • the beams M, V pass through a beam splitter 4, which here serves to direct the beam into a camera (with an imaging optics 5 and an image sensor 6).
  • the beams M, V fall through a collimator 7, which optimizes the lighting, onto the (here transparent) object O, with a partial beam of each beam being reflected by the object. Another part passes through the object and is reflected by the surface of a reference surface 8. The reflected parts interfere with each other on the way back and are directed via the beam splitter into the camera, where they form an interference pattern.
  • the shape of the interference pattern depends on the shape of the object O (and the reference surface 8).
  • the distance between the object O and the reference surface 8 (or better: between their reflective surfaces) is the distance D to be measured.
  • the image of the camera is evaluated by a phase determination unit 16, which is designed to measure the number of phase passes of the beams M, V in the measuring interferometer arrangement 9 during a tuning of the frequency.
  • a phase determination unit 16 which is designed to measure the number of phase passes of the beams M, V in the measuring interferometer arrangement 9 during a tuning of the frequency.
  • the phase determination unit 16 only determines the number Acp M of phase passes of the measuring beam M and the number A ⁇ p v of phase passes of the comparison beam V, while the measuring laser source is tuned over the frequency range Av M. What is not shown is that at the same time the same or another phase determination unit 16 determines the number Acp R of phase passes of the measuring beam M in the reference interferometer arrangement 13.
  • the quantities Ac and Acp R were calculated as weighted phase differences from measurements with measuring beam M and comparison beam V.
  • Figure 3 shows a block diagram of a preferred embodiment of the measuring method for determining a distance of an object O to a reference 8 with an interferometer system 1 as shown, for example, in Figure 1.
  • This method can also be used to measure an object or a wavefront if several measuring points are taken at different locations using the method.
  • step I the measuring laser source 10a is set to a first frequency v M , and this frequency is stabilized with a stabilization unit 12.
  • the comparison laser source 10b is set to a first frequency v v , and this frequency is stabilized with a stabilization unit 12.
  • These beams M, V are alternately radiated into the measuring interferometer arrangement 9 by means of the selection unit 2.
  • the measuring laser source 10a is set to an iodine transition and the comparison laser source 10b to a rubidium-DI transition at 795 nm.
  • step II the interference phase of the measuring beam M and the comparison beam V is measured using the measuring interferometer arrangement 9.
  • the measuring interferometer arrangement 9 from Figure 1 an interference pattern is automatically generated in the camera when a beam enters the arrangement and an object O is present. This only needs to be recorded.
  • step III the measuring laser source 10a is detuned and further measurements are carried out, which is indicated by the arrow to step II. This is repeated until the measuring laser source 10a has been tuned over the desired frequency range Av M.
  • the number Acp M of phase passes of the measuring beam M is determined for each pixel of the image sensor 6 of the camera.
  • the number Acp R of phase passes of the measuring beam M in the reference interferometer arrangement 13 is also determined. Because the measuring beam M and the comparison beam V are always irradiated simultaneously (e.g. alternately as in Figure 1) into the measuring interferometer arrangement 9, it is easy to wait during the measurement until a measurement of a last interference phase of the comparison beam V has been recorded after tuning.
  • step IV the distance D of the object O to a reference surface 8 is calculated from the ratio of the phase transitions Acp M measured during tuning (in the measuring interferometer arrangement 9 and if necessary also in the reference interferometer arrangement 13), the phase difference Acp v due to movement of the object O and the known (or determined) quantities over the measuring frequency range Av M , and also a frequency v M of one of the measuring beams M and a frequency v v of one of the comparison beams V, forming weighted phase differences Arp and Acp R .
  • step V additional distance calculations are then carried out based on a two-wavelength method and a one-wavelength method, in particular using the data already recorded.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Instruments For Measurement Of Length By Optical Means (AREA)

Abstract

L'invention concerne un procédé de mesure et un système d'interféromètre en vue de déterminer une distance d'un objet à partir d'une référence, de préférence en vue de mesurer un objet ou un front d'onde, le système d'interféromètre comprenant : - une source laser de mesure accordable qui est conçue pour émettre un faisceau de mesure variable dans une plage de fréquences de mesure (ΔvM), - une source laser de référence qui est conçue pour émettre un faisceau de référence possédant une fréquence connue, - un agencement d'interféromètre de mesure comprenant un interféromètre qui est conçu pour mesurer par interférométrie une phase d'interférence sur un objet au moyen du faisceau de mesure et du faisceau de référence, le système d'interféromètre (1) étant conçu à des fins de mesure simultanée à l'aide de la source laser de mesure (10a) et de la source laser de référence (10b) pour déterminer la distance, - une unité de détermination de phase qui est conçue pour déterminer le nombre (ΔφM) de passes de phase du faisceau de mesure dans l'agencement d'interféromètre de mesure lors de l'accord de la fréquence de la source laser de mesure, - une unité de détermination de distance qui est conçue pour déterminer la distance d'un objet à partir d'une référence sur la base d'une différence de phase pondérée du nombre mesuré de passes de phase (ΔφM) du faisceau de mesure et d'une valeur d'un changement de phase (ΔφV) entre deux mesures effectuées à l'aide de faisceaux de référence.
PCT/DE2023/100749 2022-10-14 2023-10-09 Système d'interféromètre et procédé de mesure WO2024078661A1 (fr)

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Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20070024860A1 (en) * 2005-08-01 2007-02-01 Mitutoyo Corporation Dual laser high precision interferometer
US20110292405A1 (en) * 2010-05-27 2011-12-01 Thomas James Dunn Frequency-shifting interferometer with selective data processing
US20130148129A1 (en) * 2010-08-19 2013-06-13 Isis Innovation Limited Apparatus and method for measuring distance
US20130222785A1 (en) * 2012-02-29 2013-08-29 Canon Kabushiki Kaisha Measurement apparatus and method of manufacturing article

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Publication number Priority date Publication date Assignee Title
JP4869895B2 (ja) 2006-12-07 2012-02-08 富士フイルム株式会社 光断層画像化装置
DE102010006749B4 (de) 2010-02-02 2011-12-15 Bundesrepublik Deutschland, vertreten durch das Bundesministerium für Wirtschaft und Technologie, dieses vertreten durch den Präsidenten der Physikalisch-Technischen Bundesanstalt Messgerät zum Messen zumindest einer Positionsänderung und/oder zumindest einer Winkeländerung und ein Verfahren zum dynamischen Messen zumindest einer Positionsänderung und/oder einer Winkeländerung
JP5939866B2 (ja) 2012-04-05 2016-06-22 キヤノン株式会社 光干渉断層撮像装置及び撮像方法
DE102020215073A1 (de) 2020-11-30 2022-06-02 Carl Zeiss Industrielle Messtechnik Gmbh Selbstkalibrierendes optronisches Messsystem

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20070024860A1 (en) * 2005-08-01 2007-02-01 Mitutoyo Corporation Dual laser high precision interferometer
US20110292405A1 (en) * 2010-05-27 2011-12-01 Thomas James Dunn Frequency-shifting interferometer with selective data processing
US20130148129A1 (en) * 2010-08-19 2013-06-13 Isis Innovation Limited Apparatus and method for measuring distance
US20130222785A1 (en) * 2012-02-29 2013-08-29 Canon Kabushiki Kaisha Measurement apparatus and method of manufacturing article

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