WO2024078661A1

WO2024078661A1 - Interferometer system and measuring method

Info

Publication number: WO2024078661A1
Application number: PCT/DE2023/100749
Authority: WO
Inventors: Jean-Michel Asfour; Thomas Kinder
Original assignee: Dioptic Gmbh; Tem Messtechnik Gmbh
Priority date: 2022-10-14
Filing date: 2023-10-09
Publication date: 2024-04-18
Also published as: DE102022127020B3

Abstract

The invention relates to a measuring method and to an interferometer system for determining a distance of an object from a reference, preferably for measuring an object or a wavefront, the interferometer system comprising: - a tunable measurement-laser source which is designed to emit a variable measurement beam within a measuring frequency range (Δv_M), - a reference laser source which is designed to emit a reference beam having a known frequency, - a measuring interferometer arrangement comprising an interferometer which is designed to interferometrically measure an interference phase on an object by means of the measurement beam and the reference beam, wherein the interferometer system (1) is designed for simultaneous measurement using the measurement-laser source (10a) and the reference laser source (10b) for determining the distance, - a phase determination unit which is designed to determine the number (Δφ_M) of phase passes of the measurement beam in the measuring interferometer arrangement during tuning of the frequency of the measurement-laser source, - a distance determination unit which is designed to determine the distance of an object from a reference on the basis of a weighted phase difference of the measured number of phase passes (Δφ_M) of the measurement beam and a value of a phase change (Δφ_V) between two measurements carried out using reference beams.

Description

Interferometer system and measurement method

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. In particular, 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.

The precise measurement of surface shapes is an important task in science and industrial production. Non-tactile (contactless) methods are preferred for measuring sensitive surfaces, but also for other surfaces. Among these, 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. In the simplest case, the functional relationship with I = cos <p is sinusoidal. In this, the phase <p is a linear function of the optical path length difference L with <p = 2nL/Ä. If an image sensor, e.g. 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.

A prerequisite for this method is that the height difference between neighboring measuring points is so small that the phase difference of the associated image points is smaller than n. In order to be able to carry out measurements on strongly curved surfaces, the wave front of the wave to be reflected can be adapted to the shape of the test object. The phase image then shows deviations of the shape of the test object from the shape of the wave front. For example, spherical mirrors or lenses are illuminated with spherical waves. In the case of non-spherical surfaces, the adaptation can be carried out using a (computer-generated) hologram, for example. In the manufacturing processes of optical components, determining the radius of curvature of the measured surfaces is an important and (possibly multiple) recurring task. For this purpose, 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.

Common arrangements in practice are, for example, Twyman-Green interferometers, Fizeau interferometers or tilted wave interferometers. It should be noted that if such interferometers work with light of exactly one constant wavelength, they can neither a) measure the shape of steps in the surface if the step height exceeds half a wavelength, nor b) determine the position of a test object in relation to the apparatus, nor c) determine the path length difference of the interfering partial waves, in particular not the thickness of parallel-surface components in which a partial wave on the front surface and the second on the rear surface and thus the path length difference is equal to the object thickness.

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.

To overcome these ambiguities and to enable absolute measurements in ranges larger than the wavelength of light used, there are some 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.

In multi-wavelength interferometry, 2 or more wavelengths are used for illumination, and differences in the phase images generated for each individual wavelength are calculated. These difference images behave like phase images that would have been generated by a light source with a much longer wavelength (namely the beat wavelength). This increases the uniqueness range for each pixel, so that larger object movements or larger step heights on the surface can be measured. In principle, however, the periodic relationship between brightness and path length difference is retained at each pixel, so that only length-Zdistance changes can still be detected. However, attempts to make the uniqueness range very large by using very large beat wavelengths usually fail because the requirements for the numerical uncertainty of the wavelength values involved then increase enormously.

Methods with variable wavelength exploit the fact that if the path length difference of the partial waves interfering at the image point is different from zero (which is generally the case), the brightness phase changes when the light wavelength changes. The phase change (i.e. the totality of the or partially completed periods) is proportional to the frequency change of the light and the proportionality factor is equal to the path length difference.

One advantage of this method is that there is no ambiguity at all, which is why the method is also called "absolute interferometry". A significant disadvantage, however, is the high sensitivity to object movements during the measurement: a change in the path length difference of just a few micrometers during the measurement can lead to measurement errors in the millimeter range. 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.

This task is solved by an interferometer system and a measuring method according to the claims.

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. In a Fizeau interferometer, for example, 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.

Please note that only the features essential to the invention are listed here. 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.

First, we will consider laser sources whose frequencies are well known. Further down, we will describe embodiments that allow an improvement in the accuracy of the frequency of the measuring beam.

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. 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. with two interferometers, one of which scans the object with the measuring beam and the other with the comparison beam (possibly from a different direction), or a chopper arrangement can be used that alternately allows only one of the beams to pass through to an interferometer, or a filter arrangement can be used that directs beams from an interferometer to different detectors depending on their wavelength. In short: 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.

Preferably, 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. However, it is particularly preferred that 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.

In the following calculations and examples, we will preferably assume a reference surface, but without excluding other embodiments.

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.

When tuning the comparison 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.

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). If the tuning is continuous, 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. In this case, 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 according to the invention 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:

- setting the measuring laser source to a first frequency v _M , and preferably stabilizing this frequency with a stabilization unit, emitting a first measuring beam of the measuring laser source with this frequency onto an object in the measuring interferometer arrangement, and measuring an interference phase with the measuring interferometer arrangement,

- setting a comparison laser source to a first frequency v _v , and preferably stabilizing this frequency with a stabilization unit, emitting a first comparison beam of the comparison laser source with this frequency onto the object in the measuring interferometer arrangement and measuring an interference phase cp _Vi with the measuring interferometer arrangement, this measurement being carried out before tuning the measuring laser source,

- Tuning the measuring laser source over the measuring frequency range Av _M , emitting further measuring beams from the measuring laser source and measuring the number of Phase transitions Acp _M of the interference phase during tuning in the measuring interferometer arrangement by means of the phase determination unit,

- multiple emission of a comparison beam from the comparison laser source onto the object and measurement of a further interference phase cp _V 2 with the measuring interferometer arrangement, whereby this measurement is carried out simultaneously with the tuning of the measuring laser source, whereby “simultaneous” means that the measuring beams and comparison beams are emitted simultaneously or alternately for (the mutually independent) measurements,

- Calculating the distance of the object to the reference from the ratio of the measured phase transitions Acp _M , of A<p _v = | Acpvi - Acp _V 2| , the measuring frequency range Av _M , and preferably also a frequency v _M of one of the measuring beams, in particular the first measuring beam, and a frequency v _v of one of the comparison beams, in particular the first comparison beam, to form a weighted phase difference Arp, particularly preferably according to the formula Arp = Acp _M - Acp _v • v _M /v _v .

First, the measuring laser source is set to a first frequency v _M . To ensure that this can be done very precisely, this frequency is preferably stabilized using a stabilization unit. For example, the measuring laser source is set to a frequency of a J ₂ transition and stabilized using lock-in technology.

Once the measuring laser source has been set up, a first measurement can be taken. For this, 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. In the measuring interferometer arrangement, 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.

Setting 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. For example, 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. Depending on the desired accuracy, such a measurement can often be carried out during tuning, e.g. by having a chopper alternate between the measuring beam and the comparison beam during tuning. Based on this, the distance D is then calculated from the ratio of the measured phase transitions Acp _M and Acp _v = |cpvi - <pv ₂ |, the measuring frequency range Av _M and preferably also a frequency v _M of the first measuring beam and a frequency v _v of the comparison beam, forming a weighted phase difference Arp.

The basic principles of distance determination and further preferred embodiments of the invention are described below. It is pointed out that a preferred device can also be designed analogously to the corresponding description of the method and vice versa and that in particular individual features of different embodiments can also be combined with one another.

If one considers the formulas known in the state of the art for calculating distances, one finds that for highly accurate measurements the measuring frequency range Av _M must be known very precisely. Achieving this can be problematic. To improve the accuracy in this regard, 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.

In this case, 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.

In the event that the comparison laser source is also tuned, 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. In this case, 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. In a preferred measuring method, a measurement of at least the measuring beams is carried out on the said reference interferometer arrangement with the known reference distance D _R . During the tuning of the measuring laser source over the measuring frequency range Av _M , in addition to measuring the number of phase transitions Acp _M , 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. Such 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.

In the event that the comparison laser source is also to be tunable, 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. 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.

In order to separate the interference patterns of the two laser sources that arise during the measurement, the laser beams are alternately blocked out using the selection unit (e.g. a chopper). When viewing with a camera, this is preferably done at half the camera rate when reading out with a camera.

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). It should be noted here that alternatively or additionally 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, can be measured with an interferometer. Two preferred cases can be distinguished: 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. In the first case, it is preferred to use small, compact interferometers. In the second case, it is preferred that 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.

Preferred is a staggered distance calculation in which the distance is first calculated using the weighted phase difference, then a distance calculation is carried out based on a two-wavelength method and then a further distance calculation is carried out using a one-wavelength method.

A preferred measurement procedure is described below. First, an absolute measurement is carried out with a variable synthetic wavelength (WSV) without prior knowledge of the distance, then an absolute measurement with prior knowledge of the distance (2-wavelength measurement with beat wavelength A) and finally a 1-wavelength measurement.

If the distance is already known to within A/2 (e.g. immediately after the absolute measurement), 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.

During single and dual wavelength measurements, 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. In absolute measurements, the reference interferometer represents the scale. This should be calibrated beforehand.

For a precise phase measurement in the interferometer, it is advantageous to determine the amplitudes, offsets and the phase relationship between the components of the quadrature signals (i.e. the overall position of the signal ellipse in the x-y plane). To do this, 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.

Examples of preferred embodiments of the device according to the invention are shown schematically in the figures.

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.

Figure 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. In a special embodiment, for example, the measuring laser source 10a can be stabilized to an iodine transition and the comparison laser source 10b to a rubidium transition. Basically, all possible laser media can be used instead of laser diodes 11.

It should be noted that not all other components except the laser diode need to be included. For example, the comparison laser source (10b) does not necessarily have to have a tuning unit 14 or a reference interferometer arrangement 13. On the other hand, stabilization units 12 are highly recommended. As for a stabilization unit 12, the one shown here is equipped with a coupling medium K, e.g. iodine or rubidium.

In the special interferometer system 1 according to Figure 1, 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.

Optionally, 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. 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.

So that in such a combination M, V can be measured simply with just a single beam, 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.

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. In the example considered here, only the measuring beam M of the measuring laser source 10a is tuned, which is why the phase determination unit 16 here 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.

A distance determination unit 17, which is designed to determine the distance of the object O to the reference surface 8, calculates the distance from the known distance of the D _R in the reference interferometer arrangement 13 and the determined numbers Acp _M and Acp _R of phase passes to D = A(p/Acp _R • D _R . 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.

In 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. In addition, the comparison laser source 10b is set to a first frequency v _v , and this frequency is stabilized with a stabilization unit 12. This produces two beams, a measuring beam M and a comparison beam V, each with stabilized frequencies. These beams M, V are alternately radiated into the measuring interferometer arrangement 9 by means of the selection unit 2. For example, 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.

In step II, the interference phase of the measuring beam M and the comparison beam V is measured using the measuring interferometer arrangement 9. In 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.

In 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. During this tuning, 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. In this example, 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.

After the recordings, in 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 .

In 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.

Finally, it should be noted that the use of indefinite articles such as "a" or "an" does not exclude the possibility that the features in question may be present in multiple instances. For example, "a" may also be read as "at least one". The expression "number" is also to be read as "at least one". Terms such as "unit" or "device" do not exclude the possibility that the elements in question may consist of several interacting components which are not necessarily housed in a common housing, even if the case of a comprehensive housing is preferred. List of reference symbols

1 I nterferometer system

2 Selection unit

3 Beam guidance element

4 beam splitters

5 Imaging optics

6 Image sensor

7 Collimator

8 Reference surface

9 Measuring interferometer arrangement

10 Laser source

10a Measuring laser source

10b Comparison laser source

11 Laser diode

12 Stabilization unit

13 Reference interferometer arrangement

14 Tuning unit

15 beam splitters

16 Phase determination unit

17 Distance determination unit

K Coupling medium

M measuring beam

O Object

V Comparison beam

Claims

Expectations

1. Interferometer system (1) for determining a distance of an object (O) to a reference (8), comprising:

- a tunable measuring laser source (10a) designed to emit a variable measuring beam (M) within a measuring frequency range Av _M ,

- a comparison laser source (10b) designed to emit a comparison beam (V) with a known frequency,

- a measuring interferometer arrangement (9) comprising an interferometer, designed for the interferometric measurement of an interference phase on an object (O) by means of the measuring beam (M) and the comparison beam (V), wherein the interferometer system (1) is designed for a simultaneous measurement with the measuring laser source (10a) and the comparison laser source (10b) for the determination of the distance,

- a phase determination unit (16) designed to determine the number Acp _M of phase passes of the measuring beam (M) in the measuring interferometer arrangement (9) during a tuning of the frequency of the measuring laser source (10a),

- a distance determination unit (17) designed to determine the distance of an object (O) to a reference (8) based on a weighted phase difference of the measured number of phase passes Acp _M of the measuring beam (M) and a value of a phase change Acp _v between two measurements with comparison beams (V).

2. Interferometer system according to claim 1, comprising a reference interferometer arrangement (13) containing an interferometer with a known reference distance D _R , designed to determine a frequency change of the measuring laser source (10a), wherein the distance determination unit is preferably used to determine the distance of an object (O) to a reference (8) based on the measured number Acp _M of the phase passes of the measuring beam (M) and the known reference distance D _R.

3. Interferometer system according to one of the preceding claims, comprising a tuning unit (14) which is designed to tune the frequency of a measuring beam (M) of the measuring laser source (10a), wherein the tuning unit (14) is preferably designed to tune the measuring laser source (10a) so that the amount of change in the frequency of the measuring beam (M) is greater than 100 MHz, in particular greater than 1 GHz.

4. Interferometer system according to one of the preceding claims, comprising a stabilization unit (12) for stabilizing the beam (M, V) of one of the laser sources (10) to a predetermined frequency, preferably

- a first stabilization unit (12), in particular with a first coupling medium (K), designed for optical stabilization of the measuring laser source (10a) to a predetermined frequency and/or

- a second stabilization unit (12), in particular with a second coupling medium (K), designed for optical stabilization of the comparison laser source (10b) to a predetermined frequency, wherein a preferred coupling medium (K) comprises in particular iodine or rubidium.

5. Interferometer system according to one of the preceding claims, comprising a beam guiding element (3), preferably a light guide, e.g. a glass fiber, designed to guide the beam (M, V) of at least one of the laser sources (10), in particular both laser sources (10), into the measuring interferometer arrangement, wherein the beam guiding element (3) is preferably designed such that during a measurement the beams (M, V) of both laser sources (10) are guided by means of light guides onto a single light guide, and is particularly preferably V- or Y-shaped.

6. Interferometer system according to one of the preceding claims, comprising a selection unit (2), in particular a chopper, designed for alternately blocking the beam (M, V) of one of the two laser sources (10) in each case, so that at one measurement time only the measurement beam (M) of the measurement laser source (10a) falls into the measurement interferometer arrangement (9) and at another measurement time only the comparison beam (V) of the comparison laser source (10b) falls into the measurement interferometer arrangement (9), wherein the interferometer system (1) is preferably designed such that during a measurement by means of the measurement interferometer arrangement (9) an alternating distance measurement is carried out with the measurement beam (M) and the comparison beam (V).

7. Interferometer system according to one of the preceding claims, comprising an auxiliary interferometer designed to determine a tuning speed of one of the laser sources (10), in particular of the measuring beam (M), and in particular additionally designed to monitor a mode purity of one of the laser sources (10).

8. Measuring method for determining a distance of an object (O) to a reference (8) with an interferometer system (1) according to one of the preceding claims, comprising the steps:

- setting the measuring laser source (10a) to a first frequency v _M , and preferably stabilizing this frequency with a stabilization unit (12), emitting a first measuring beam (M) of the measuring laser source (10a) with this frequency onto an object (O) in the measuring interferometer arrangement (9), and measuring an interference phase with the measuring interferometer arrangement (9),

- setting a comparison laser source (10b) to a first frequency v _v , and preferably stabilizing this frequency with a stabilization unit (12), emitting a first comparison beam (V) of the comparison laser source (10b) with this frequency onto the object (O) in the measuring interferometer arrangement (9) and measuring an interference phase cp _Vi with the measuring interferometer arrangement (9), this measurement being carried out before tuning the measuring laser source (10a),

- tuning the measuring laser source (10a) over the measuring frequency range Av _M , emitting further measuring beams (M) of the measuring laser source (10a) and measuring the number of phase transitions Acp _M of the interference phase during tuning in the measuring interferometer arrangement (9) by means of the phase determination unit (16),

- multiple emission of a further comparison beam (V) of the comparison laser source (10b) onto the object (O) and measurement of a further interference phase cpv2 with the measuring interferometer arrangement (9), this measurement being carried out simultaneously with the tuning of the measuring laser source (10a),

- Calculating the distance of the object (O) to the reference (8) from the ratio of the measured phase transitions Acp _M , of Acp _v = | Acpvi - Acp _V2 |, of the measuring frequency range AV _M , and preferably also of a frequency v _M of one of the measuring beams (M) and of a frequency v _v of one of the comparison beams (V), forming a weighted phase difference Arp, particularly preferably according to the formula Acp ⁼ A<PM - Acpv ■ VM/VV.

9. Measuring method according to claim 8, wherein a measurement of at least the measuring beams (M) is carried out on a reference interferometer arrangement (13) with a known reference distance D _R , and during the tuning of the measuring laser source (10a) over the measuring frequency range Av _M , in addition to measuring the number of phase transitions Acp _M , a measurement of the number of phase transitions Acp _R of the interference phase in the reference interferometer arrangement (13) is also carried out by means of the phase determination unit (16), wherein the distance is then determined from the reference distance D _R and a ratio based on the numbers of measured phase transitions, in particular by means of weighted phase differences.

10. Measuring method according to claim 8 or 9, wherein a further calculation of the distance is additionally carried out based on a two-wavelength method, in particular using the measured values for a measurement with a measuring beam (M) and a measurement with a comparison beam (V), and/or wherein a further calculation of the distance is additionally carried out based on a one-wavelength method based on the measured values, in particular using the measured values for a measurement with a measuring beam (M) or a measurement with a comparison beam (V).