Interferometer system and measuring method

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 (ΔvM), - 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 (ΔφM) 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 (ΔφM) of the measurement beam and a value of a phase change (ΔφV) between two measurements carried out using reference beams.

Description

[0001] Interferometer system and measurement method

[0002] The invention relates to an interferometer system and a measurement 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.

[0003] 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, as well as other surfaces. Among these, interferometry is particularly important because, using comparatively simple image sensors (cameras) in conjunction with a suitable light source and a few optical elements, it can detect surface height differences of well under a micrometer. Advanced systems can even detect irregularities on the order of an atom's diameter.

[0004] Interferometers and interferometric measurements are well known. These 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 after being reflected from a test object, having traveled different paths. The power density (intensity, brightness I) at the point of superposition changes periodically with the difference between the lengths of the two paths, with the period equal to the wavelength X. In the simplest case, the functional relationship is I = cos <p sinusförmig. Darin ist die Phase <p eine lineare Funktion der optischen Weglängendifferenz L mit <p = 2nL / Ä. Befindet sich am Ort der Überlagerung ein Bildsensor, z.B. in Form einer Kamera, derart, dass (ggf.If (after a mathematical transformation) each pixel can be assigned a point on the test object surface (“object point”), the intensity of the pixel carries information about the spatial position of the object point.

[0005] Such (simple) interferometric measuring methods have the advantage that they enable measurements with a very high degree 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 or similar) can be obtained from the brightness alone, because the phase at an image point can at best be determined from the brightness value modulo 2n. By comparing the phases of neighboring image points, however, information about their distance difference can be obtained. For example, in areas where the test object has a depression, the reflected light travels a slightly further path, and in raised areas a shorter path. The totality of all height differences together determines the shape of the test object.

[0006] 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 n. In order to still be able to carry out measurements on strongly curved surfaces, the wavefront of the wave to be reflected can be adapted to the shape of the test object. The phase image then shows deviations of the test object shape from the shape of the wavefront. For example, spherical mirrors or lenses are illuminated with spherical waves. For 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 repeatedly) recurring task. For this purpose, the test object is illuminated with spherical waves. It appears "smooth" when the sphere centers of the wave and the reflecting surface coincide.The distance of the surface from this center is the desired radius of curvature.

[0007] Common arrangements in practice include Twyman-Green interferometers, Fizeau interferometers, or tilted-wave interferometers. It should be noted that since such interferometers operate with light of exactly one constant wavelength, they cannot 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 relative to the apparatus, nor c) determine the path length difference of the interfering partial waves, especially not the thickness of parallel-surface components, where one partial wave is reflected at the front surface and the second at the rear surface, and thus the path length difference is equal to the object thickness.

[0008] Radius determination, in particular, therefore requires additional means for measuring distances between a test piece and designated points or surfaces of the equipment. Common methods even require moving the test piece a distance equal to the radius of curvature. The measurement uncertainty and time required by the additional measuring equipment thus limit the accuracy of radius determination and the throughput of the production line.

[0009] To overcome these ambiguities and to enable absolute measurements in ranges larger than the used light wavelength, there are some interferometric methods, e.g. those that work with two beams of different wavelength and whose unambiguity lies in the range of the beat wavelength (multi-wavelength interferometry), or those that are based on a change in the wavelength.

[0010] In multi-wavelength interferometry, two or more wavelengths are used for illumination, and differences between the phase images generated at 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 unambiguousness range for each image point, so that larger object movements or larger step heights on the surface can be measured. Fundamentally, however, the periodic relationship between brightness and path length difference at each image point remains intact, so that only length-Z-distance changes can still be detected. Attempts to make the unambiguousness 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.

[0011] Variable-wavelength methods 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 total number of fully or partially traversed periods) is proportional to the change in frequency of the light, and the proportionality factor is equal to the path length difference.

[0012] One advantage of this method is the complete elimination of ambiguity, which is why it is also called "absolute interferometry." A significant disadvantage, however, is its high sensitivity to object movement 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 planar form measurement therefore rely on the test object and the medium being irradiated (the air) remaining stationary during the measurement.

[0013] 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.

[0014] This problem is solved by an interferometer system and a measuring method according to the claims.

[0015] An inventive (absolute measuring) interferometer system allows the measurement of an object or a wavefront by determining the distance of points on 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 on the object to a point on a reference surface. The interferometer system comprises the following components:

[0016] - A tunable measuring laser source, designed to generate a variable measuring beam within a (in particular predetermined but always known) measuring frequency range AV M TO emit,

[0017] - A reference laser source designed to emit a reference beam with a known frequency,

[0018] - 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 a simultaneous measurement with the measuring laser source and the comparison laser source for the determination of the distance,

[0019] - A phase determination unit designed to determine the number of Acp M of phase sweeps of the measuring beam in the measuring interferometer arrangement during a tuning of the frequency of the measuring laser source,

[0020] - 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 Acp Mof the measuring beam and a value of a phase change Acp v between two measurements with reference beams.

[0021] Please note that only the essential features of the invention are listed here. The interferometer system includes all other components that make up an interferometer system, such as optical components, mounts, recording units, or adjustment units.

[0022] The interferometer system thus comprises two independent laser sources: a measurement laser source and a reference laser source. The measurement laser source is tunable. The reference laser source only needs to emit a reference beam at a single frequency, but can also be designed to emit a variable reference beam within a reference frequency range. The reference beam can be considered a beam for determining distance changes due to (relative or absolute) movement of the object.

[0023] 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.

[0024] The frequency of the reference beam of the reference laser source must be known. This frequency can be determined, for example, by presetting, by selecting components and parameters, or by frequency measurement.

[0025] The laser sources are preferably laser diodes, especially distributed feedback lasers (DFB). These are laser diodes in which the active material is periodically structured. The structures of varying 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 measurement laser source can be widely tuned, in particular over more than 100 MHz or more than 1 GHz, especially over more than 10 GHz or more than 100 GHz. Tuning of the reference laser source is not absolutely necessary.

[0026] We will first consider laser sources whose frequencies are well-known. Further on, we will describe embodiments that allow for improvements in the accuracy of the measurement beam's frequency.

[0027] The measuring interferometer arrangement comprises at least one interferometer, but can also comprise two or more interferometers, e.g., one interferometer for each of the two laser sources. It is important, however, that the same object is always measured with all beams. The measuring interferometer arrangement preferably comprises additional components such as lenses, prisms, beam splitters, mirrors, a reference surface, 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.

[0028] A preferred embodiment of the measuring interferometer arrangement is a Fizeau interferometer, in which an object is measured relative to a reference surface, e.g., a flat or curved mirror. The basic design 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 using interferometry.

[0029] The interferometer system must be designed for simultaneous measurement with the measuring laser source and the reference laser source. "Simultaneous" in the context of this invention means that both measurements must be performed during or for determining the distance, either simultaneously or alternately. This does not mean that the measurements with the measuring laser source are performed first and then the measurements with the reference laser source, but rather that multiple measurements with the reference laser source must be performed during the tuning of the measuring laser source, e.g., at least at the beginning and end of the tuning and also (especially multiple times) during the tuning (e.g., between measurements during the tuning). This can be achieved, for example, with a variable aperture that alternately allows only the beam of a single laser source to pass through. However, measurements can also be performed simultaneously, e.g.,with two interferometers, one scanning the object with the measurement beam and the other with the reference 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 within the meaning of the invention is a simultaneous measurement with both beams or an alternating measurement in which the beams (in particular multiple or multiple times) radiate at different times from one another.

[0030] Preferably, a single interferometer is used, at least if the two different beams from the measuring laser source and the reference laser source can be separated from each other. This can be achieved, for example, by irradiating the beams into the measuring interferometer arrangement at different times (e.g., using a so-called "chopper") or by separating the beams with filters.

[0031] Interferometric measurements are well known in the art and are based on a partial beam being reflected from the object and interfering with another partial beam. In a Fizeau interferometer, this other partial beam originates, for example, from the reference surface.

[0032] One point of the object can be measured at a time, e.g., using an interferometer with a point detector, whereby the object's surface is scanned to measure it. Movements of the object are compensated for by the measurements with the reference beam. However, it is particularly preferred to perform a planar measurement, e.g., using an interferometer that has a camera as a detector (or at least an image sensor matrix with imaging optics). In this case, it is preferable 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.

[0033] In the following calculations and examples, we will preferably assume a reference surface, but without excluding other embodiments.

[0034] 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 connected to this image sensor. The phase determination unit determines the number of phase sweeps during a frequency tuning of the measuring laser source.

[0035] When tuning the reference 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.

[0036] 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 or cosine function, the measured intensity will vary between maxima and minima, whereby the transition from one maximum to the next (2n) is referred to here as the "phase sweep". These changes are counted and result in a number of phase sweeps. Even if this number results in an integer value in the simplest case (counting all maxima or minima) one could already use it to perform distance calculations. However, since intermediate values ​​can also be estimated using an image sensor, this number is preferably a rational number and also indicates intermediate steps (e.g. starting from a minimum via another minimum to the next maximum, the number would correspond to 1.5).If the detuning is continuous, the measured time can also be used as a measure of the phase sweeps. For example, if a phase sweep lasts exactly 1 s and 34.567 s were measured during the tuning, then the number of phase sweeps can be specified as 34.567. In this case, the phase determination unit is preferably designed to determine the tuning time and the duration of a specified number of phase sweeps (including one).

[0037] The distance determination unit is designed to calculate values. Suitable calculation units are known and can be implemented, for example, in a computer system. The distance of an object to a reference is determined using the measured number of phase passes Acp. M and the measuring frequency range Av Mwhich should ideally be well known. To compensate for minimal movements of the object during the measurement, the distance is calculated based on a weighted phase difference of the measured number of phase sweeps Acp M of the measuring beam and the value (0 to 2n) of a phase change Acp v between two measurements using reference beams. This is explained in more detail below.

[0038] 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:

[0039] - 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,

[0040] - 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, whereby this measurement is carried out before tuning the measuring laser source,

[0041] - 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 Mthe interference phase during tuning in the measuring interferometer arrangement by means of the phase determination unit,

[0042] - multiple emission of a reference beam from the reference laser source onto the object and measurement of another 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 measuring beams and reference beams radiate simultaneously or alternately for (the mutually independent) measurements,

[0043] - Calculation of the distance of the object to the reference from the ratio of the measured phase transitions Acp M , from A <p v = | Acpvi - Acp V 2| , of the measuring frequency range Av M , and preferably also a frequency v M one of the measuring beams, in particular the first measuring beam, and a frequency v vone of the comparison beams, in particular the first comparison beam, forming a weighted phase difference Arp, particularly preferably according to the formula Arp = Acp M - Acp v • v M / v v .

[0044] First, the measuring laser source is tuned to a first frequency v M To ensure this is done very accurately, this frequency is preferably stabilized using a stabilization unit. For example, the measuring laser source is set to a frequency of a J2 transition and stabilized using lock-in technology.

[0045] Once the measuring laser source has been adjusted, a first measurement can be taken. For this purpose, a first measuring beam from the measuring laser source is emitted at this frequency, strikes 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 then interferes with another partial beam (which was reflected, for example, from a reference surface). The resulting interference pattern is measured, and from this the phase relationship between these two partial beams, which is referred to as the "interference phase," is determined. An image sensor reproduces this interference phase as an intensity value, and a pixel matrix as the image sensor reproduces it as a matrix of intensity values.

[0046] Setting a comparison laser source to the first frequency v v, the emission of a first comparison beam and the measurement of an interference phase cp Vi This corresponds to the first measurement with the measurement beam described above and is carried out analogously, except that the reference laser source now radiates a beam, which is called the "reference beam" for easier differentiation. The reference beam preferably has a different wavelength than the measurement beam, but this is not absolutely necessary. For example, the reference laser source is tuned to a frequency of the Rb-DI transition at 795 nm and stabilized using lock-in technology.

[0047] Now the measuring laser source is moved over the measuring frequency range Av M tuned, e.g., over 100 GHz. This means that the frequency of the measuring beam is from v Mto a different frequency (continuously). Meanwhile, the measuring laser source continues to emit measuring beams, and measurements of the interference phase continue to take place. However, the interference phases will continuously change due to the change in the wavelength of the measuring beam, resulting in phase transitions as described above, which manifest themselves as intensity fluctuations on the image sensor. The number of phase transitions Acp M The interference phase during tuning in the measuring interferometer arrangement is now counted during tuning. In addition to integer changes, initial changes are also preferably recorded quantitatively, e.g., based on the tuning speed (see above), which improves the accuracy of the result.

[0048] The measurements with the reference laser source, which are carried out simultaneously with the tuning, may be preceded by a re-tuning of the reference laser source to a frequency, especially if measurements are to be taken at a different frequency. If the first frequency v is used again, vmeasured, it is merely preferable to stabilize the reference laser source so that the deviation of the frequency of the reference beam is small compared to the previous measurement. Basically, this step is otherwise the same as the previous measurement with the reference laser source, except that it is carried out simultaneously or alternately (i.e., simultaneously) with the tuning. If the position of the object (absolute or relative, e.g., to a reference surface) has changed in any way, this will be reflected in the measured interference phase. Depending on the desired accuracy, such a measurement can be carried out multiple times during tuning, e.g., by a chopper alternating between the measuring beam and the reference 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- <pv2|, des Mess-Frequenz- bereichs Av M and preferably also a frequency v M of the first measuring beam and a frequency v v of the reference beam, forming a weighted phase difference Arp.

[0049] The following describes the principles of distance determination and further preferred embodiments of the invention. It should be noted that a preferred device can also be configured 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.

[0050] If one considers the formulas for distance calculation known in the state of the art, one finds that for high-precision measurements the measuring frequency range Av Mmust be known very precisely. Achieving this can be problematic. To improve accuracy in this regard, a preferred interferometer system comprises a reference interferometer arrangement including 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.

[0051] The distance determination unit is in this case particularly preferred for determining the distance of an object to a reference based on the measured number of phase sweeps 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.

[0052] 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 sweeps 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 at 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 also a measurement of the number of phase transitions Acp Rin the reference interferometer arrangement using a phase determination unit.

[0053] The distance D is then calculated 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 benchmark in a sense.

[0054] Preferably, the interferometer system 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 tunings over a measuring frequency range Av Mgreater than 10 GHz or even greater than 100 GHz are preferred. Such a tuning unit is generally known in the state of the art and can be implemented, for example, by variable voltage or current control of the measuring laser source.

[0055] If the reference laser source is also to be tunable, the interferometer system preferably includes a corresponding tuning unit designed to tune the frequency of a reference beam from the reference laser source. The specifications for the measurement frequency range preferably apply to the reference frequency range.

[0056] The interferometer system preferably comprises a stabilization unit for stabilizing one of the laser sources to a frequency. The general principle of such 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. For this purpose, the beam guiding unit preferably guides the beams from the laser sources to a single light guide by means of 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.

[0057] The interferometer system preferably comprises a selection unit, e.g., a so-called "chopper," which is known in the art. Such a selection unit is designed to alternately block the beam of one of the two laser sources, so that at one measurement time, only the measurement beam of the measurement laser source enters the measurement interferometer arrangement, and at another measurement time, only the comparison beam of the comparison laser source enters the measurement interferometer arrangement.

[0058] To separate the interference patterns created by the two laser sources during the measurement, the laser beams are alternately filtered out using a selection unit (e.g., a chopper). When viewing with a single camera, this is preferably done at half the camera rate.

[0059] The interferometer system preferably comprises an auxiliary interferometer designed to determine a tuning speed (change in frequency and / or phase over 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 serves 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 be this reference interferometer arrangement. In principle, the reference interferometer arrangement can also be assigned to the laser sources or comprise interferometers assigned to the laser sources (but this is not mandatory).The auxiliary interferometer is specifically designed to additionally monitor the mode purity of one of the laser sources. If the tuning speed is known, a phase sweep can be quantified very precisely by measuring the time taken to determine the phase sweeps. This makes it possible to specify the number of phase sweeps as a rational number, e.g., 100,437 sweeps. This increases the accuracy of distance determination.

[0060] Each laser source, i.e., the comparison laser source and / or the measurement laser source, can be measured with an interferometer. Two preferred cases can be distinguished: Each laser source includes its own auxiliary interferometer for determining 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 preferable to use small, compact interferometers. In the second case, it is preferable that the interferometer used corresponds in design and measurement principle to the measurement 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 in this interferometer are eliminated just as in the measurement interferometer arrangement.

[0061] The interferometer system preferably comprises additional components that are generally known in the prior art and serve to improve handling, eliminate interfering effects, or improve measurement accuracy. Preferred additional components include, for example, optical Faraday isolators, elements for coupling into a fiber optic cable for a measurement interferometer or for a reference interferometer, or elements for dichroic beam superposition.

[0062] Using the values ​​measured within the scope of the method according to the invention, further distance calculations can preferably be performed to improve the result. It should be noted that with many interferometric distance calculations, unambiguousness no longer exists 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 performed.

[0063] This calculation is preferably based on a single-wavelength method or a two-wavelength method, which is basically known in the prior art. What is special is that the distance already determined within the scope of the method according to the invention is used to establish unambiguity. Both measurement beams and comparison beams can be used as beams, wherein the images in question have preferably been generated in particular consecutively 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.

[0064] 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.

[0065] A preferred measurement procedure is described below. It begins with an absolute measurement using a variable synthetic wavelength (WSV) without prior knowledge of the distance, followed by an absolute measurement with prior knowledge of the distance (2-wavelength measurement with beat wavelength A), and finally, a 1-wavelength measurement.

[0066] If the distance is already known down to A / 2 (e.g. immediately after the absolute measurement), the measuring laser source is set to a frequency of the iodine transition and stabilized. The phases are then recorded at the detectors for the measuring and reference beams in the measuring interferometer and the distance is determined from the phase difference, whereby the integer components of cp / 2n can be reconstructed from the absolute measurement. The measurement uncertainty should be, if possible, less than half the light wavelength of the reference laser source (i.e., safely within the unambiguous range of an incremental single-wavelength measurement). Finally, the single-wavelength measurement known in the state of the art is carried out, whereby the integer components of cp / 2n can be reconstructed from the absolute measurement and the two-wavelength measurement.

[0067] During single- and dual-wavelength measurements, the respective wavelengths of the laser sources involved serve as the scale. This is determined from the frequencies, which are traceable via the 12- or Rb-frequency standard, and the refractive index of the air, which is determined separately. For absolute measurements, the reference interferometer represents the scale. This should be calibrated beforehand.

[0068] For precise phase measurement in the interferometer, it is advantageous to determine the amplitudes, offsets, and phase relationships between the components of the quadrature signals (i.e., the overall position of the signal ellipse in the xy plane). To do this, both laser sources are preferably tuned slightly one after the other, pairs of values ​​are recorded, and a correction is performed by fitting a second-order Heydemann curve. This correction should be repeated automatically before each absolute measurement (regardless of the method).

[0069] The advantage of the interferometer system according to the invention is that it can measure distances of up to 2 m with an absolute uncertainty of 0.2 pm. This accuracy can be further increased by using the additional measurements mentioned above.

[0070] Examples of preferred embodiments of the device according to the invention are shown schematically in the figures.

[0071] Figure 1 shows a preferred embodiment of an interferometer system.

[0072] Figure 2 shows a preferred embodiment of a laser source. Figure 3 shows a block diagram of a preferred embodiment of the measurement method.

[0073] 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 here is formed by a reference surface 8. The interferometer system 1 comprises the following components:

[0074] A tunable measuring laser source 10a, designed to generate a variable measuring beam M within a measuring frequency range Av M and a reference laser source 10b configured to emit a reference beam V at a known frequency. Each of these laser sources (10) may have a structure as shown in Figure 2.

[0075] Figure 2 shows the advantageous design 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, 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., by means of a lock-in method. In a special embodiment, for example,The measurement laser source 10a is stabilized to an iodine transition, and the comparison laser source 10b to a rubidium transition. Instead of laser diodes 11, essentially any laser media can be used.

[0076] It should be noted that, apart from the laser diode, not all other components need to be included. For example, the reference laser source (10b) does not necessarily need to include a tuning unit 14 or a reference interferometer arrangement 13. However, stabilization units 12 are highly recommended. Regarding a stabilization unit 12, the one shown here is equipped with a coupling medium K, e.g., iodine or rubidium.

[0077] In the special interferometer system 1 shown in Figure 1, the measurement beam M from the measurement laser source 10a and the reference beam V from the reference laser source 10b are combined by means of a beam guiding element 3, which here comprises a glass fiber into which the two beams from the laser sources 10a, 10b are coupled, e.g., by means of special coupling elements. The glass fibers are combined in a Y-shaped arrangement onto a single fiber, so that both the measurement beam M and the reference beam V emerge from the same glass fiber.

[0078] Optionally, the beams can also be coupled out, as indicated by the dashed lines, and their frequency change measured in a reference interferometer arrangement 13. This could represent an alternative to Figure 2, in which case the auxiliary interferometer 13 can be omitted and a single interferometer can be used to measure both laser sources 10a, 10b.

[0079] To ensure that measurements of M and V can be carried out simply with just a single beam during such a combination, 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 is shaped like a wheel (see the illustration above, indicated by the arrow) with translucent and opaque regions. These regions are arranged such that, in each position of the wheel, one of the beams is obscured by an opaque region, while the other beam can shine through a translucent region.

[0080] Using the selection unit 2, a simultaneous measurement with the measuring beam M and the reference beam V is possible, with rapid alternation between the beams M, V. This results in a large number of alternating measurements. Theoretically, a simultaneous measurement with the measuring beam M and the reference 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 design 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 comprises an interferometer designed for the interferometric measurement of interference phases on an object O using the measuring beam M and the reference beam V.The beams M, V pass through a beam splitter 4, which serves here to direct the beam into a camera (with an imaging optics 5 and an image sensor 6).

[0081] The beams M, V pass through a collimator 7, which optimizes the illumination, onto the (here transparent) object O, with a portion of each beam being reflected by the object. Another portion passes through the object and is reflected by the surface of a reference surface 8. The reflected portions interfere with each other on the return path 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.

[0082] The camera image 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 frequency tuning. 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 only measures the number Acp M of phase sweeps of the measuring beam M and the number A <p v of phase sweeps of the reference beam V, while the measuring laser source is scanned 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 sweeps of the measuring beam M in the reference interferometer arrangement 13.

[0083] 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 reference interferometer arrangement 13 and the determined numbers Acp M and Acp R of phase sweeps 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 reference beam V.

[0084] Figure 3 shows a block diagram of a preferred embodiment of the measuring method for determining a distance of an object O from a reference 8 using 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.

[0085] In step I, the measuring laser source 10a is adjusted to a first frequency v M , and a stabilization of this frequency with a stabilization unit 12. In addition, the comparison laser source 10b is adjusted to a first frequency v v , and stabilizing this frequency with a stabilization unit 12. This produces two beams, a measurement beam M and a comparison beam V, each with stabilized frequencies. These beams M, V are alternately radiated into the measurement 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-DI transition at 795 nm.

[0086] In step II, the interference phase of the measuring beam M and the reference beam V is measured using the measuring interferometer arrangement 9. In the measuring interferometer arrangement 9 of Figure 1, an interference pattern is automatically generated in the camera when a beam enters the arrangement and an object O is present. This pattern only needs to be recorded.

[0087] In step III, the measuring laser source 10a is detuned and further measurements are performed, which is indicated by the arrow to step II. This is repeated until the measuring laser source 10a is tuned over the desired frequency range Av M During this voting, the number of Acp M of phase sweeps of the measuring beam M for each pixel of the camera's image sensor 6. In this example, the number Acp Rof phase sweeps of the measuring beam M in the reference interferometer arrangement 13. Because the measuring beam M and the comparison beam V are always radiated simultaneously (e.g., alternately as in Figure 1) into the measuring interferometer arrangement 9, it is easy to wait during the measurement until, after tuning, a measurement of a final interference phase of the comparison beam V has been recorded.

[0088] 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 measured during tuning M (in the measuring interferometer arrangement 9 and possibly also in the reference interferometer arrangement 13), the phase difference Acp v by movement of the object O and the known (or determined) quantities over the measuring frequency range Av M , and also a frequency v Mone of the measuring beams M and a frequency v v one of the reference beams V, forming weighted phase differences Arp and Acp R .

[0089] In step V, additional calculations of the distance are then carried out based on a two-wavelength method and a one-wavelength method, in particular using the data already recorded.

[0090] Finally, it should be noted that the use of indefinite articles, such as "a" or "an," does not preclude the possibility that the features in question may be present in multiple instances. Thus, "a" can also be read as "at least one." The expression "number" should also be read as "at least one." Terms such as "unit" or "device" do not preclude the elements in question from consisting of several interacting components that are not necessarily housed in a common housing, even if a comprehensive housing is preferred. List of Reference Symbols

[0091] 1 Interferometer system

[0092] 2 selection unit

[0093] 3 Beam guidance element

[0094] 4 beam splitters

[0095] 5 Imaging optics

[0096] 6 image sensor

[0097] 7 Collimator

[0098] 8 Reference surface

[0099] 9 Measuring interferometer arrangement

[0100] 10 Laser source

[0101] 10a Measuring laser source

[0102] 10b Comparison laser source

[0103] 11 laser diode

[0104] 12 Stabilization unit

[0105] 13 Reference interferometer arrangement

[0106] 14 Tuning unit

[0107] 15 beam splitters

[0108] 16 phase determination unit

[0109] 17 Distance determination unit

[0110] K coupling medium

[0111] M measuring beam

[0112] O Object

[0113] V Comparison beam

Claims

Claims 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 generate a variable measuring beam (M) within a measuring frequency range Av M to emit, - a reference laser source (10b) designed to emit a reference 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 of Acp Mof 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 Acp M of the measuring beam (M) and a value of a phase change Acp v between two measurements with reference 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 Mthe phase sweeps of the measuring beam (M) and the known reference distance D R occurs.

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) 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.

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 the 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 measuring time only the measuring 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), wherein the interferometer system (1) is preferably designed such that during a measurement by means of the measuring interferometer arrangement (9) an alternating distance measurement is carried out with the measuring 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. A 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 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), - Calculation of the distance of the object (O) to the reference (8) from the ratio of the measured phase transitions Acp M , by Acp v = | Acpvi - Acp V2 |, of the measuring frequency range AV M , and preferably also a frequency v M one of the measuring beams (M) and a frequency v v 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 Rand 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 also a measurement of the number of phase transitions Acp R the interference phase in the reference interferometer arrangement (13) by means of the phase determination unit (16), wherein the distance 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.

10. Measuring method according to 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 wherein based on the measured values ​​additionally a further calculation of the distance is carried out based on a one-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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