DE102022127020B3 - Interferometer system and measuring method - Google Patents

Abstract

The invention relates to a measuring method and an interferometer system for determining a distance of an object to a reference, preferably for measuring an object or a wavefront, the interferometer system comprising: - a tunable measuring laser source, designed to produce a variable measuring beam within a measurement frequency range ΔvM, - a comparison laser source, designed to emit a comparison beam with a known frequency, - a measurement interferometer arrangement comprising an interferometer, designed for the interferometric measurement of an interference phase on an object by means of the measurement -Beam and the comparison beam, the interferometer system (1) being designed for a simultaneous measurement with the measuring laser source (10a) and the comparison laser source (10b) for determining the distance, - a phase determination unit, designed to determine the number ΔφM of phase passes of the measuring beam in the measuring interferometer arrangement during a sweep 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 sweeps ΔφM of the measuring beam and a value of a phase change ΔφV between two measurements with comparison beams.

Description

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 manufacturing. Non-tactile (non-contact) methods are preferred for measuring sensitive surfaces, but also for other surfaces. Among these, interferometry is of particular importance because it can detect height differences of well under one micrometer on the surface using comparatively 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 atomic 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 reflection on a test object after they have traveled different paths. The power density (intensity, brightness 1) at the location of the superposition changes periodically with the difference in the lengths of both paths, where the period is equal to the wavelength λ. In the simplest case, the functional connection with I = cos φ is sinusoidal. Therein the phase φ is a linear function of the optical path length difference L with φ = 2πL/λ. If there is an image sensor at the location of the overlay, e.g. in the form of a camera, such that (possibly after a mathematical transformation) a point on the test object surface (“object point”) can be assigned to each image point, the intensity of the image point carries information about the spatial position of the object point.

Such (simple) interferometric measurement methods have the advantage that they enable measurements with a very high level of 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, etc.) can be obtained from the brightness alone, because the phase at an image point can at best be determined modulo 2π from the brightness value. However, by comparing the phases of neighboring pixels, 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 further path, and in raised areas it travels a shorter distance. The totality of all height differences taken together results in the shape of the test specimen.

A prerequisite for this method is that the height difference between neighboring measuring points is so small that the phase difference of the assigned image points is smaller than π. 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 specimen. 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. For non-spherical surfaces, the adjustment can be done, for example, using a (computer-generated) hologram. In the manufacturing processes of optical components, the determination of the radius of curvature of the measured surfaces is an important and recurring task (possibly several times). For this purpose, the test specimen is illuminated with spherical waves. It appears “smooth” when the spherical centers of the wave and the reflecting surface match. The distance of the surface from this center is the desired radius of curvature.

1. a) die Form von Stufen in der Oberfläche vermessen können, wenn die Stufenhöhe eine halbe Wellenlänge überschreitet, noch
2. b) die Position eines Prüflings in Bezug auf die Apparatur bestimmen können, noch
3. c) die Weglängendifferenz der interferierenden Teilwellen bestimmen können, insbesondere nicht die Dicke von parallelflächigen Bauteilen, bei denen eine Teilwelle an der Vorderfläche und die zweite an der rückwärtigen Fläche reflektiert wird und somit die Weglängendifferenz gleich der Objektdicke ist.

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 a constant wavelength, they neither

1. a) can still measure the shape of steps in the surface if the step height exceeds half a wavelength
2. b) be able to determine the position of a test specimen in relation to the apparatus
3. c) can determine the path length difference of the interfering partial waves, in particular not the thickness of parallel-surfaced components in which one partial wave is reflected on the front surface and the second on the rear surface and thus the path length difference is equal to the object thickness.

The determination of radii in particular therefore requires additional means for measuring distances between a test specimen and marked points or surfaces of the apparatus. Common methods even require moving the test specimen a distance equal to the radius of curvature. The measurement uncertainty and duration of the additional measuring equipment limits the accuracy the radius determination and the throughput of the production line.

In order to overcome these ambiguities and enable absolute measurements in areas 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 uniqueness is in the range of the beat wavelength (multi-wavelength interferometry), or those that are based on 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 produced 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 remains intact at each pixel, so that only changes in length/distance can still be detected. However, the attempt to make the uniqueness range very large by using very large beat wavelengths usually fails because the requirements for the numerical uncertainty of the wavelength values involved then increase enormously.

Variable wavelength methods exploit the fact that if the path length difference of the partial waves interfering at the pixel 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 fully or partially completed periods) is proportional to the frequency change of the light and the proportionality factor is equal to the path length difference.

DE 10 2011 076 458 A1 discloses, for example, a frequency shifting interferometer which is arranged to measure an optical profile of a test object with a tunable light source. A sequence of interference images of the test object is recorded together with a measurement of the beam frequencies at which the interference images are formed. A limited number of captured interference images of the test object are selected so that the monitored beam frequencies approximately match a predetermined beam frequency spacing pattern. Further processing follows based on the selected interference images.

One advantage of this method is the complete elimination of ambiguity, which is why the method is also called “absolute interferometry”. However, a significant disadvantage is the high sensitivity to object movements during the measurement: even a change in the path length difference of 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 (the air) being irradiated not to move 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.

• - Eine durchstimmbare Mess-Laserquelle, dazu ausgelegt, einen variablen Mess-Strahl innerhalb eines (insbesondere vorbestimmten aber stets bekannten) Mess-Frequenzbereichs Δv_M zu emittieren,
• - Eine Vergleichs-Laserquelle, dazu ausgelegt, einen Vergleichs-Strahl mit einer bekannten Frequenz zu emittieren,
• - Eine Messinterferometer-Anordnung umfassend ein Interferometer, ausgelegt zur interferometrischen Messung einer Interferenzphase an einem Objekt mittels des

Mess-Strahls und des Vergleichs-Strahls, wobei das Interferometer-System für eine simultane Messung mit der Mess-Laserquelle und der Vergleichs-Laserquelle für die Bestimmung der Distanz ausgestaltet ist,

• - Eine Phasen-Bestimmungseinheit, ausgelegt zur Bestimmung der Anzahl Δφ_M von Phasendurchläufen des Mess-Strahls in der Messinterferometer-Anordnung während einer Durchstimmung der Frequenz der Mess-Laserquelle,
• - Eine Distanz-Bestimmungseinheit, ausgelegt zur Bestimmung der Distanz eines Objekts zu einer Referenz basierend auf einer gewichteten Phasendifferenz der gemessenen Anzahl von Phasendurchläufen Δφ_M des Mess-Strahls und einem Wert einer Phasenänderung Δφ_V zwischen zwei Messungen mit Vergleichs-Strahlen.

An (absolute measuring) interferometer system according to the invention allows a 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 from the respective point of the object to a point on a reference surface. The interferometer system includes 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 Δv _M ,
• - A reference laser source designed to emit a reference beam at a known frequency,
• - A measuring interferometer arrangement comprising an interferometer, designed for the interferometric measurement of an interference phase on an object using the

measuring beam and the comparison beam, the interferometer system being 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 Δφ _M of phase passes of the measuring beam in the measuring interface ferometer arrangement while tuning 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 Δφ _M of the measurement beam and a value of a phase change Δφ _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 includes two independent laser sources, a measuring laser source and an additional comparison laser source. The measuring laser source can be tuned. The comparison laser source need only output a comparison beam at a single frequency, but may also be configured to emit a variable comparison beam within a comparison frequency range. The comparison beam can be viewed as a beam for determining changes in distance due to (relative or absolute) movements of the object.

The measuring beam emitted by the measuring laser source lies in the measuring frequency range Δv _M , which must be known and is preferably predetermined, for example by measuring or selecting suitable components and/or operating parameters.

The frequency of the comparison beam from the comparison laser source must be known. The frequency can be determined, for example, by presetting, by selecting the components and parameters or by frequency measurement.

The laser sources are preferably laser diodes, in particular distributed feedback lasers (abbreviation: DFB, German: laser with distributed feedback). 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 the wavelengths 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 . It is not absolutely necessary to tune the comparison laser source.

Laser sources whose frequencies are well known are initially considered here. Embodiments are described below that allow an improvement with regard to the accuracy of the frequency of the measuring beam.

The measuring interferometer arrangement includes at least one interferometer, but can also have two or more interferometers, e.g. one interferometer for each of the two laser sources. However, it is important that all beams always measure one and the same object. The measuring interferometer arrangement preferably includes further 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 prior 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. For the purposes of this invention, “simultaneous” means that both measurements must be carried out during or for determining the distance, at the same time or alternately. This does not mean that the measurements are carried out first with the measuring laser source and then the measurements with the comparison laser source, but rather that measurements must be carried out several times with the comparison laser source during the tuning of the measuring laser source, for example at least at the beginning and at the end of the tuning and also (especially often) during the tuning (e.g. between measurements during the tuning). This can be achieved, for example, with a variable aperture that alternately only allows the beam from a single laser source to pass through. However, measurements can also be carried out simultaneously, for example with two interferometers, one of which probes 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 alternates only one of the beams passes to an interferometer, or it can with A filter arrangement is 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 irradiate (in particular several times or multiple times) at a time offset from one another.

A single interferometer is preferably used, at least if the two different beams of the measuring laser source and the comparison laser source can be separated from one another. This can be achieved, for example, by radiating the beams into the measuring interferometer arrangement at different times (e.g. using a so-called “chopper”) or by separating the beams from each other using filters.

Interferometric measurements are well known in the prior art and are based on the fact that a partial beam of a beam is reflected on the object and interferes with another partial beam. In a Fizeau interferometer, for example, this other partial beam comes from the reference surface.

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 the object. Movements of the object are compensated for by the measurements with the comparison beam. However, it is particularly preferred that an area measurement is carried out, for example 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, a reference surface is preferably assumed, but without excluding other embodiments.

The phase determination unit can be part of the measuring interferometer arrangement or can 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 in terms of data technology. The phase determination unit determines the number of phase cycles during a sweep 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 of the measuring beam.

When the frequency of the measuring beam is tuned, its wavelength changes and with it 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, with the transition from one to the next maximum (2π), here referred to as “phase transition”. These changes are counted and result in a number of phase runs. Even if this number results in an integer value in the simplest case (counting all maxima or minima), you could already use it to calculate distances. However, since intermediate values can also be estimated with an image sensor, this number is preferably a rational number and also indicates intermediate levels (e.g. starting from a minimum through another minimum to the next maximum, the number would correspond to 1.5). If the tuning is continuously detuned, the measured time can also be used as a measure for the phase cycles. For example, if a phase cycle lasts exactly 1 s and 34.567 s were measured during the tuning, then the number of phase cycles can be given 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 fixed number of phase cycles (even one).

The distance determination unit is designed to calculate values. Suitable computing units are known and can be implemented, for example, in a computing system. The distance of an object to a reference is determined by means of the measured number of phase cycles Δφ _M and the measurement frequency range Δv _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 Δφ _M of the measuring beam and the value (0 to 2π) of a phase change Δφ _V between two measurements with comparison beams . This will be explained in more detail below.

• - Einstellen der Mess-Laserquelle auf eine erste Frequenz v_M, und bevorzugt Stabilisieren dieser Frequenz mit einer Stabilisierungseinheit, Aussenden eines ersten Mess-Strahls der Mess-Laserquelle mit dieser Frequenz auf ein Objekt in der Messinterferometer-Anordnung, und Messung einer Interferenzphase mit der Messinterferometer-Anordnung,
• - Einstellen einer Vergleichs-Laserquelle auf eine erste Frequenz v_V, und bevorzugt Stabilisieren dieser Frequenz mit einer Stabilisierungseinheit, Aussenden eines ersten Vergleichs-Strahls der Vergleichs-Laserquelle mit dieser Frequenz auf das Objekt in der Messinterferometer-Anordnung und Messung einer Interferenzphase φ_V1 mit der Messinterferometer-Anordnung, wobei diese Messung vor einem Durchstimmen des Mess-Laserquelle erfolgt,
• - Durchstimmen der Mess-Laserquelle über den Mess-Frequenzbereich Δv_M, Aussenden weiterer Mess-Strahlen der Mess-Laserquelle und Messung der Anzahl der Phasenübergänge Δφ_M der Interferenzphase während des Durchstimmens in der Messinterferometer-Anordnung mittels der Phasen-Bestimmungseinheit,
• - mehrfaches Aussenden eines Vergleichs-Strahls der Vergleichs-Laserquelle auf das Objekt und Messung einer weiteren Interferenzphase φ_V2 mit der Messinterferometer-Anordnung, wobei diese Messung simultan zu dem Durchstimmen des Mess-Laserquelle erfolgt, wobei „simultan“ bedeutet, dass Mess-Strahlen und Vergleichs-Strahlen gleichzeitig oder alternierend für (die voneinander unabhängigen) Messungen einstrahlen,
• - Berechnung der Distanz des Objekts zur Referenz aus dem Verhältnis der gemessenen Phasenübergänge Δφ_M, von Δφ_V = |Δφ_V1 - Δφ_V2|, des Mess-Frequenzbereichs Δv_M, und bevorzugt auch einer Frequenz v_M eines der Mess-Strahlen, insbesondere des ersten Mess-Strahls, sowie einer Frequenz v_V eines der Vergleichs-Strahlen, insbesondere des ersten Vergleichs-Strahls, unter Bildung einer gewichteten Phasendifferenz Δφ, besonders bevorzugt nach der Formel Δφ = Δφ_M - Δφ_V · v_M/v_V.

A measuring method according to the invention for determining a distance of an object to a reference using 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, sending out a first measuring beam from 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, sending out a first comparison beam from the comparison laser source with this frequency onto the object in the measuring interferometer arrangement and measuring an interference phase φ _V1 with the measuring interferometer arrangement, this measurement taking place before the measuring laser source is tuned,
• - tuning the measuring laser source over the measuring frequency range Δv _M , sending out further measuring beams from the measuring laser source and measuring the number of phase transitions Δφ _M of the interference phase during tuning in the measuring interferometer arrangement using 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 φ _V2 with the measurement interferometer arrangement, this measurement being carried out simultaneously with the tuning of the measurement laser source, where “simultaneous” means that measurement beams and comparison beams irradiate simultaneously or alternately for (the mutually independent) measurements,
• - Calculation of the distance of the object to the reference from the ratio of the measured phase transitions Δφ _M , from Δφ _V = |Δφ _V1 - Δφ _V2 |, the measurement frequency range Δv _M , and preferably also a frequency v _M of one of the measurement beams, in particular of the first measurement beam, as well as a frequency v _V of one of the comparison beams, in particular the first comparison beam, forming a weighted phase difference Δφ, particularly preferably according to the formula Δφ = Δφ _M - Δφ _V · v _M /v _V.

First, the measuring laser source is set to a first frequency v _M. So that this can be done very precisely, this frequency is preferably stabilized with 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, an initial measurement can be carried out. For this purpose, 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, for example, on a reference surface) 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. This interference phase is reproduced as an intensity value by an image sensor, and as a matrix of intensity values by a pixel matrix as an image sensor.

Setting a comparison laser source to the first frequency v _V , emitting a first comparison beam and measuring an interference phase φ _V1 corresponds to the previously described first measurement with the measuring beam and is carried out analogously, except for the difference that the Comparison laser source emits a beam, which is called “comparison beam” for better 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 at 795 nm is set to a frequency of the Rb-D1 transition and stabilized using lock-in technology.

Now the measuring laser source is tuned over the measuring frequency range Δv _M , for example over 100 GHz. This means that the frequency of the measuring beam is changed from v _M to another frequency (continuously). Meanwhile, 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 constantly due to the change in the wavelength of the measuring beam and, as described above, phase transitions will result, which will manifest themselves in intensity fluctuations on the image sensor. The number of phase transitions Δφ _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 preferably recorded quantitatively, for example based on the tuning speed (see above), which improves the accuracy of the result.

The measurements with the comparison laser source that take place simultaneously with the tuning can be preceded by setting the comparison laser source again to a frequency, especially if measurements are to be carried out at a different frequency. If measurements are taken again with the first frequency v _V , it is only preferred to stabilize the comparison laser source so that the deviation the frequency of the comparison beam is low compared to the previous measurement. Basically, this step is otherwise the same as the previous measurement with the comparison laser source, except that it takes place simultaneously or at a different time, alternating (ie simultaneously) with the tuning. If anything has changed in the position of the object (absolute or relative to a reference surface, for example), this will be reflected in the measured interference phase. Depending on the desired accuracy, such a measurement can often be carried out during tuning, for example by a chopper alternating between the measuring beam and the comparison beam during the tuning.

Based on this, the distance D is then calculated from the ratio of the measured phase transitions Δφ _M and Δφ _V = |φ _V1 - φ _V2 |, the measurement frequency range Δv _M and preferably also a frequency v _M of the first measurement beam and a frequency v _V of the comparison beam, forming a weighted phase difference Δφ.

The basics of distance determination and further preferred embodiments of the invention are described below. It should be noted 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 exemplary embodiments can be combined with one another.

If one looks at the formulas for distance calculation known from the prior art, one can see that for highly precise measurements, the measurement frequency range Δv _M must be known very precisely. Accomplishing this can be problematic. In order to improve the accuracy in this regard, a preferred interferometer system includes a reference interferometer arrangement containing an interferometer with a known reference distance D _R . This reference interferometer arrangement is designed to determine a change in frequency of the measuring laser source.

In this case, the distance determination unit is particularly preferably designed to determine the distance of an object to a reference based on the measured numbers of phase passes of the measuring beam and the known reference distance D _R. This will be explained in more detail below in the context of the corresponding measurement procedure.

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 designed in addition to determining the number of phase cycles during a detuning of the frequency of the comparison laser source in the reference interferometer arrangement.

In a preferred measuring method, at least the measuring beams are measured at the reference interferometer arrangement with the known reference distance D _R . During the tuning of the measuring laser source over the measuring frequency range Δv _M , in addition to measuring the number of phase transitions Δφ _M , the number of phase transitions Δφ _R is also measured in the reference interferometer arrangement using a phase determination unit.

The distance D is then determined from the reference distance D _R and a ratio based on the numbers of measured phase transitions, in particular using weighted phase differences (see above). The reference distance D _R serves, so to speak, as a benchmark.

The interferometer system preferably comprises a tuning unit which is designed to tune the frequency of a measuring beam of the measuring laser source, the tuning unit preferably being designed to tune the measuring laser source in such a way that the amount of change in the frequency of the measuring laser source is Beam is greater than 1 GHz, with sweeps over a measurement frequency range Δv _M greater than 10 GHz or even greater than 100 GHz being preferred. Such a tuning unit is basically known in the prior art and can be implemented, for example, by variable voltage or current control of the measuring laser source.

In the event that the comparison laser source should also 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. For the comparison frequency range, the information on the measurement frequency range preferably applies.

The interferometer system preferably includes a stabilization unit for stabilizing one of the laser sources to a frequency. The general principle of such stabilization, for example 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, for example a glass fiber, designed to guide the light from both laser sources into the measuring interferometer arrangement. For this purpose, the beam guidance unit preferably guides the beams from the laser sources onto a single light guide using light guides, and is particularly preferably V- or Y-shaped for this purpose. The term “light guide” means a single light-conducting element or a bundle of light-conducting elements by means of which light is guided in one direction.

The interferometer system preferably comprises a selection unit, for example a so-called “chopper”, which is known in the prior art. Such a selection unit is designed to alternately mask 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 falls into the measuring 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 viewed with a camera, this preferably happens at half the camera rate.

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 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 from 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 can be this reference interferometer arrangement. Basically, the reference interferometer arrangement can also be assigned to the laser sources or can include interferometers that are assigned to the laser sources (but does not necessarily have to). The auxiliary interferometer is designed in particular to monitor the mode purity of one of the laser sources. If the tuning speed is known, a phase cycle that has begun can be quantified very precisely by measuring the time when determining the phase cycles. 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 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 automatically achieved and reflector movements are eliminated in this interferometer in the same way as in the measuring interferometer arrangement.

The interferometer system preferably includes further components that are fundamentally 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 measurement interferometer or for a reference interferometer or elements for dichroic beam superimposition.

With the values measured as part of the method according to the invention, further calculations of the distance can preferably be carried out, which improve the result. It should be noted that in many interferometric distance calculations there is no longer any clarity beyond certain distance differences. However, if the distance can be determined within the unambiguity of another determination method and the method allows a more precise determination of the distance within its unambiguity, then the “rougher” distance measurement can be used to establish the unambiguity.

After an aforementioned determination of the distance based on the measured values, a further calculation of the distance is preferably carried out.

This calculation is preferably carried out based on a one-wavelength method or a two-wavelength method, which is basically known in the prior art. What is special is that the distance already determined in the context of the method according to the invention is used to create clarity. As Both measurement beams and comparison beams can be used, with the recordings in question preferably having been generated in particular successively 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 ones Values for a measurement with a measuring 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 measuring beam or a measurement with a comparison beam.

A staggered distance calculation is preferred, 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 sequence is described below. First there is an absolute measurement 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 down to Λ/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 on the detectors for the measuring and comparison beam recorded in the measuring interferometer and the distance is determined from the phase difference, whereby the integer components of φ/2π can be reconstructed from the absolute measurement.

The measurement uncertainty should, if possible, be below half the light wavelength of the comparison laser source (i.e. certainly within the clarity range of an incremental single-wavelength measurement). Finally, the one-wavelength measurement known in the prior art is carried out, where the integer components of φ/2π can be reconstructed from the absolute measurement and the two-wavelength measurement.

During the one- and two-wavelength measurements, the respective wavelengths of the laser sources involved serve as a scale embodiment. This results from the frequencies, which can be traced via the I2 or Rb frequency standard, and the refractive index of the air, which is determined separately. In absolute measurement, the reference interferometer represents the scale. This should be measured beforehand.

For 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 (overall, i.e. the position of the signal ellipse in the x-y plane. For this purpose, both laser sources are preferably slightly tuned one after the other, pairs of values are recorded and transferred The adaptation of a second-order curve according to Heydemann carries out a correction. 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 using such an interferometer, distances of up to 2 m can be measured absolutely with an uncertainty of 0.2 µm. This accuracy can be further increased by using the additional measurements mentioned above.

• 1 zeigt eine bevorzugte Ausführungsform eines Interferometer-Systems.
• 2 zeigt eine bevorzugte Ausführungsform einer Laserquelle.
• 3 zeigt ein Blockschaltbild einer bevorzugten Ausführungsform des Messverfahrens.

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

• 1 shows a preferred embodiment of an interferometer system.
• 2 shows a preferred embodiment of a laser source.
• 3 shows a block diagram of a preferred embodiment of the measuring method.

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 includes the following components:

A tunable measurement laser source 10a, designed to emit a variable measurement beam M within a measurement frequency range Δv _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 in 2 shown.

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. By means of a tuning unit 14 (optional for the comparison laser source), the beam of the laser source 10 can be tuned within a frequency range by changing the voltage or current, for example. 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, for example using a lock-in method. In a special exemplary embodiment, for example, the measuring laser source 10a can be stabilized on an iodine transition and the comparison laser source 10b on 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 need to be included except for the laser diode. 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 far as a stabilization unit 12 is concerned, the one shown here is equipped with a coupling medium K, for example iodine or rubidium.

In the special interferometer system 1 after 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 each comprises a glass fiber into which the two beams of the laser sources 10a, 10b are coupled, for example by means of special coupling elements. The glass fibers are brought together in a Y-shaped arrangement onto one fiber, so that both the measurement 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 be an alternative too 2 represent, whereby the auxiliary interferometer 13 can then be dispensed with and a single interferometer can be used to measure both laser sources 10a, 10b.

So that during such a combination it is easy to measure with only a single beam M, V, a selection unit 2 in the form of a so-called “chopper” is arranged between the laser sources 10 and the beam guiding element 3. The selection unit 2 has the shape of a wheel (see illustration above, which is 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 rays is covered by an opaque area and the other ray can shine through a translucent area.

By means of the selection unit 2, a simultaneous measurement with the measuring beam M and the comparison beam V with a rapid alternation between the beams M, V is possible. A large number of alternating measurements are therefore 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 using the measuring beam M and the comparison beam V. The beams M, V pass through a beam splitter 4, which is used here To direct the beam into a camera (with imaging optics 5 and an image sensor 6).

The rays M, V fall through a collimator 7, which optimizes the illumination, onto the (here transparent) object O, with a partial beam of each ray being reflected by the object. Another part penetrates through the object and is reflected by the surface of a reference surface 8. The reflected components 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 object O and reference surface 8 (better: between their reflecting surfaces) is the distance D to be measured.

The image from 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 sweep 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 Δφ _M of phase sweeps of the measuring beam M and the number Δφ _V of phase sweeps of the comparison beam V determined while the measuring laser source is tuned over the frequency range Δv _M. What is not shown is that at the same time the same or a different phase determination unit 16 determines the number Δφ _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 Δφ _M and Δφ _R of phase cycles to D = Δφ/Δφ _R · D _R .

The quantities Δφ and Δφ _R were calculated as weighted phase differences from measurements with measuring beam M and comparison beam V.

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 in, for example 1 is shown. This method can also be used to measure an object or a wavefront if the method is used to take several measuring points at different locations.

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 a stabilization unit 12. This results in two beams, a measurement beam M and a comparison beam V, each with stabilized frequencies. These beams M, V are alternately irradiated into the measuring interferometer arrangement 9 by means of the selection unit 2. For example, the measurement laser source 10a is set to an iodine transition and the comparison laser source 10b to a rubidium D1 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 1 An interference pattern is automatically generated in the camera when a beam enters the arrangement and the presence of an object O. This just 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 Δv _M. During this tuning, the number Δφ _M of phase sweeps of the measuring beam M is determined for each pixel of the image sensor 6 of the camera. In this example, the number Δφ _R of phase passes of the measuring beam M in the reference interferometer arrangement 13 is also determined.

Because measuring beam M and comparison beam V are always simultaneously (e.g. alternating as in 1 ) are irradiated into the measuring interferometer arrangement 9, you can simply 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 Δφ _M measured during tuning (in the measuring interferometer arrangement 9 and possibly also in the reference interferometer arrangement 13), the phase difference Δφ _V due to movement of the object O and the known (or determined) quantities over the measurement frequency range Δv _M , and also a frequency v _M of one of the measurement beams M and a frequency v _V of one of the comparison beams V Formation of weighted phase difference Δφ and Δφ _R.

In step V, further calculations of the distance are then carried out based on a two-wavelength method and a one-wavelength method, in particular using the data that has already been recorded.

Finally, it should be noted that the use of indefinite articles, such as “a” or “an”, does not exclude the fact that the characteristics in question can be present multiple times. So “a” can also be read as “at least one”. The expression “number” can also be read as “at least one”. Terms such as “unit” or “device” do not preclude the fact that the elements in question may consist of several interacting components which are not necessarily housed in a common housing, although the case of a comprehensive housing is preferred.

Reference symbol list

1

Interferometer system

2

Selection unit

3

Beam guiding element

4

Beam splitter

5

Imaging optics

6

Image sensor

7

Collimator

8th

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

Voting unit

15

Beam splitter

16

Phase determination unit

17

Distance determination unit

K

Coupling medium

M

Measuring beam

O

object

v

Comparison beam

Claims (10)

Interferometer system (1) for determining a distance of an object (O) to a reference (8), comprising: - a tunable measuring laser source (10a), designed to produce a variable measuring beam (M) within a measuring frequency range Δv _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 a Object (O) by means of the measuring beam (M) and the comparison beam (V), the interferometer system (1) for a simultaneous measurement with the measuring laser source (10a) and the comparison laser source (10b) for the determination of the distance is designed, - a phase determination unit (16), designed to determine the number Δφ _M of phase sweeps 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 Δφ _M of the measuring beam (M) and a value a phase change Δφ _V between two measurements with comparison beams (V). Interferometer system 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), the distance determination unit preferably for determining the distance of an object (O) to a reference (8) is based on the measured number Δφ _M of the phase cycles of the measuring beam (M) and the known reference distance D _R. 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), the tuning unit (14) preferably being designed to: Measuring laser source (10a) to be tuned 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. 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, preferred - 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. 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), in the measuring interferometer arrangement, wherein the beam guiding element (3) is preferably designed in such a way that during a measurement the beams (M, V) of both laser sources (10) are guided onto a single light guide by means of light guides, and particularly preferably in a V or Y shape is. Interferometer system according to one of the preceding claims, comprising a selection unit (2), in particular a chopper, designed for alternately masking out the beam (M, V) of one of the two laser sources (10), so that at a measurement time only the measurement beam (M) of the measuring laser source (10a) falls into the measuring interferometer arrangement (9) and at another measuring time only the comparison beam (V) of the comparison Laser source (10b) falls into the measuring interferometer arrangement (9), the interferometer system (1) preferably being designed in such a way that, during a measurement using the measuring interferometer arrangement (9), an alternating distance measurement with the measuring beam (M) and the comparison beam (V). 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). 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) from 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 on the object (O) in the measuring interferometer arrangement (9) and measuring an interference phase φ _V1 with the measuring interferometer arrangement (9), this measurement taking place before tuning the measuring laser source (10a), - Tuning the measuring laser source (10a) over the measuring frequency range Δv _M , sending out further measuring beams (M) from the measuring laser source (10a) and measuring the number of phase transitions Δφ _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) from the comparison laser source (10b) onto the object (O) and measurement of a further interference phase φ _V2 with the measuring interferometer arrangement (9) , whereby this measurement takes place simultaneously with the tuning of the measuring laser source (10a), - calculation of the distance of the object (O) to the reference (8) from the ratio of the measured phase transitions Δφ _M , from Δφ _V = |Δφ _V1 - Δφ _V2 |, the measurement frequency range Δv _M , and preferably also a frequency v _M of one of the measurement beams (M) and a frequency v _V of one of the comparison beams (V), forming a weighted phase difference Δφ, particularly preferably according to the formula Δφ = Δφ _M - Δφ _V · v _M /v _V . measurement method Claim 8 , wherein a measurement of at least the measuring beams (M) takes place 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 Δv _M , in addition to the measurement the number of phase transitions Δφ _M is also measured by the number of phase transitions Δφ _R of the interference phase in the reference interferometer arrangement (13) by means of the phase determination unit (16), the distance then being determined from the reference distance D _R and a ratio based on the Numbers of measured phase transitions are determined, in particular by means of weighted phase differences. measurement method Claim 8 or 9 , wherein additionally a further calculation of the distance is 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 where based based on the measured values, a further calculation of the distance is also carried out based on a single-wavelength method, in particular using the measured values for a measurement with a measuring beam (M) or a measurement with a comparison beam (V).

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