Laser processing machine with frequency comb-based pitch sensor and associated method with frequency comb-based pitch measurement

Abstract

According to the invention, a laser processing machine (1) for laser processing at least one workpiece (5) by means of a processing laser beam (3), comprising a laser beam generator (2) for generating the processing laser beam (3) and comprising a processing optics (4), has a frequency comb-based distance sensor (6), the processing optics are used for directing the processing laser beam (3) to the at least one workpiece (5), and the frequency comb-based distance sensor is used for measuring a distance in order to determine a geometrical characteristic of the at least one workpiece (5). The invention relates to a frequency comb-based distance sensor (6), comprising a frequency-shifted feedback laser source (11) for generating a sensor laser beam (12) having a frequency comb that is spectrally shifted over time, a measuring section (13) and a reference section (14), a detector (17) and an evaluation device (17), the sensor laser beam (12) is divided into a measurement beam (15) and a reference beam (16) on the measurement path and the reference path, and the returned measurement beam (15) and the reference beam (16) are superimposed on the detector, the evaluation device determines a distance value on the basis of a frequency difference from the two frequency combs of the measuring section (13) and the reference section (14), said frequency difference being derived from a propagation time difference of the measuring and reference beams (15, 16).

Description

Technical Field

[0001] The invention relates to a laser processing machine for laser processing at least one workpiece by means of a processing laser beam and comprising a laser beam generator for generating the processing laser beam and processing optics for directing the processing laser beam toward the at least one workpiece. The invention also relates to a method for detecting geometrical features on at least one workpiece during laser processing. Background Art

[0002] Laser processing machines of this type having an additional interferometric distance measuring device in the form of a scanning OCT (optical coherence tomography) distance sensor are known, for example, from DE 10 2019 132 619 A2 or DE 10 2020 203 983 A1.

[0003] Measuring the distance to determine geometric workpiece features (joining position, weld penetration depth, seam geometry) is a key requirement for sensors for controlling laser welding processes. For this purpose, OCT distance sensors with a reference path (reference arm) that can be adjusted by motorized compensation and implemented in the form of an additional component have been developed in recent years. The shoebox-sized OCT distance sensor is usually arranged in a switch cabinet and connected to the processing optics via a fiber. The reference arm is either integrated in the OCT sensor or installed in the vicinity of the OCT scanner, separated from the processing optics. Only when the measuring range is reduced can path length differences of a few micrometers be resolved, i.e. the resolution and measuring range cannot be flexibly adapted, but hardware changes to the OCT distance sensor are required.

[0004] However, the interferometric distance measurement device using the OCT distance sensor in the laser processing machine has the following disadvantages:

[0005] - Due to the very small measuring range (only about 12 mm), an adjustable reference arm is required;

[0006] The z resolution is not sufficient for the z resolution of less than 12 μm, for example 1 μm or in the sub-μm range, which is required when laser processing thin sheets, for example sheets with a thickness of 50 μm or 100 μm;

[0007] Integration into the process optics is not possible, but rather requires a complex and expensive spectrometer design with free-beam optics. Summary of the invention

[0008] In contrast, the object of the present invention is to develop a laser processing machine and a method of the type mentioned at the outset in such a way that the above-mentioned disadvantages can be overcome.

[0009] According to the present invention, this task is achieved by a frequency comb-based distance sensor, which has a frequency-shifted feedback laser source, a measuring section and a reference section (also called a measuring arm or a reference arm), a detector and an analysis device, wherein the frequency-shifted feedback laser source is used to generate a sensor laser beam, which has a frequency comb that shifts spectrally over time, on which the sensor laser beam is divided into a measuring beam and a reference beam, on which the returning measuring beam and reference beam are superimposed, and the analysis device determines the distance value based on the frequency difference of the two frequency combs from the measuring section and the reference section, which is derived from the propagation time difference of the measuring beam and the reference beam.

[0010] Preferably, the frequency comb is a cw (continuous wave) frequency comb, but pulsed frequency combs are also possible.

[0011] In principle, spacing measurements based on frequency combs are known and are described, for example, in DE 10 2012 001754 A1, WO 03 / 061084 A1, WO 2018 / 005987 A1 and JP 2021-021744 A1.

[0012] In the frequency comb-based distance measurement (Frequency-Shifted-Feedback Ranging=FSF-Ranging) according to the invention with the aid of a frequency-shifted feedback laser source, a frequency comb with frequency comb teeth that are spectrally shifted over time is split in an interferometric measurement structure into a measurement section and a reference section, which have optical paths of different lengths. The light returning from the measurement section and the reference section is superimposed on the detector. In this case, the frequency difference of the two frequency combs, which results from the propagation time difference, leads to a clock frequency, from which the path length difference between the measurement section and the reference section, i.e. a relative distance value or - in the case of a known reference section - an absolute distance value, can then be derived.

[0013] In frequency comb-based distance measurements, mechanical tracking of the reference arm can be dispensed with due to the large unambiguity range at the maximum path differences to be expected in laser processing (approximately 150 mm to 200 mm).

[0014] For laser material processing, frequency comb-based distance measurement offers significant advantages due to a high axial resolution in the 1 μm to sub-μm range and a large axial measuring range. In addition, there are advantages in terms of signal quality, since the signal strength does not decrease quadratically with the measuring distance, but only according to a linear curve. The proposed distance sensor device can be used for various purposes in laser processing, primarily for seam orientation control, weld penetration depth measurement, seam quality assessment in the back-run (Nachlauf) and focus position control during laser welding; in micromachining for high-precision surface (structure) control, for controlling layer removal between individual removal steps and in general, for example, for component orientation and shape recognition and, in this context, for adaptive adaptation and orientation of the geometry to be processed. In addition, applications in generative production (LMF / LMD) are conceivable, such as layer thickness measurement, detection of component orientation during production on existing components.

[0015] Based on the welding depth measurement experiments, the suitability of the frequency comb-based spacing measurement according to the invention was confirmed: the frequency comb-based spacing measurement behaves robustly with respect to interfering light emitted by the process and is able to detect spacing values ​​from the vapor capillary with sub-μm resolution during laser processing.

[0016] Compared to conventional OCT distance measurement, the frequency comb-based distance measurement according to the present invention has the following advantages:

[0017] - Mechanical compensation adjustment of the reference path (reference arm) can be completely omitted, thereby eliminating the need for measuring stops and wear-prone actuators and achieving lower costs;

[0018] - No need for a reference arm detached from the frequency comb based spacing sensor;

[0019] - Higher resolution than in the case of conventional OCT spacing sensors, currently in the sub-μm range;

[0020] -Can be used for micromachining;

[0021] - Independent of resolution and measuring range, ie, high resolution can also be achieved with a large measuring range; for example, with a resolution of 100 nm, the measuring range is 300 mm. The measuring range can be set on the software side.

[0022] - Arbitrary definition of small measurement windows within a large unambiguity range; thereby increasing the measurement rate and suppressing artifacts;

[0023] - The signal drop-off over the measured distance is linear, not quadratic;

[0024] -Higher signal-to-noise ratio and larger measurement range under defocus conditions;

[0025] - No need for a spectrometer with free-beam optics;

[0026] - Cost-effective detectors with simpler data evaluation (diodes instead of image lines);

[0027] - By cascading the individual ranges, the measuring range is significantly increased, up to several meters.

[0028] Preferably, the frequency comb-based spacing sensor has an additional seed laser source, which is used to generate an injection laser, and the phase of the injection laser is modulated at a frequency that is variable in time. The injection laser is fed to the frequency-shifted feedback laser source, whereby, under the characteristic modulation frequency, a strong intensity over-high and greatly improved separation clarity of the beat signal are generated. Since the characteristic modulation frequency changes with the path length difference between the measuring arm and the reference arm, the characteristic modulation frequency can also be attributed to the spacing value. In the case of an unknown spacing, the corresponding characteristic modulation frequency is determined by recording the RMS (root mean square) signal of the detector in the known time variation curve of the applied phase modulation:

[0029] A modulation frequency that can be attributed to a distance is a modulation frequency at which the RMS signal becomes maximum, whereby the distance can be determined with sufficiently high accuracy.

[0030] Frequency comb-based distance measurement makes it possible to scan the distance values ​​of a reduced segment of the unambiguous range according to the measurement window and to increase the measurement rate by the associated reduction of the phase modulation range. At the same time, interfering signals (unwanted reflections) can be suppressed by corresponding positioning of the measurement window. In order to define the measurement window position, the measurement method requires as input an expected value of the phase modulation frequency or of the distance, which corresponds to the difference in path length between the measurement object (workpiece) and the reference object (such as a protective glass). Depending on the characteristics of the data preprocessing in the frequency comb-based distance sensor, for a single measurement, only one distance value is recorded or also the entire distance value spectrum is recorded along the measurement axis (limited by the measurement window). Accordingly, the post-processing and evaluation of the data is carried out according to its complexity (point measurement or scanning line measurement). In the simplest case, this post-processing and evaluation can be carried out directly in the machine control device; in complex cases, an image processing PC or a high-performance graphics card is required for this.

[0031] Furthermore, the size of the unambiguity range (possibly in combination with the possibility of cascading the unambiguity ranges along the measuring axis) allows the reference section to be implemented as a component of the measuring section. Preferably, therefore, the reference section is completely contained in the measuring section, i.e. the reference arm is completely integrated into the measuring arm, so that the temperature drifts of the measuring arm and the reference arm are identical and therefore cancel each other out. Furthermore, this measure allows smaller sensor dimensions.

[0032] In the simplest case, the end of the measuring fiber is used as the reference plane. In a particularly preferred embodiment of the laser processing machine according to the invention, an optical element is arranged in the beam path of the sensor laser beam, and the reflectivity of the side of the optical element facing the frequency comb-based spacing sensor for the sensor laser beam is at least 1%, preferably at least 5%, so as to transmit the sensor laser as the measuring beam and reflect the sensor laser as the reference beam. According to the invention, instead of minimizing the reflectivity of the optical element for the sensor laser beam as conventionally in order to reduce radiation losses, the reflectivity of the optical element is increased so as to use the portion of the sensor laser beam reflected on the optical element as the reference beam. Such a structure cannot be realized with the help of conventional OCT solutions, because the path length difference between the optical element and the workpiece in the beam path cannot be compensated. In an advantageous extension of this embodiment, the optical element is formed by a protective glass, by a focusing lens of a processing optical device, or by a fiber end of a fiber common to the measuring beam and the reference beam.

[0033] Preferably, the processing optical system has a processing scanner (macro scanner), for example in the form of a galvanometer with mirrors, in order to be able to deflect the processing laser beam one-dimensionally or two-dimensionally on at least one workpiece.

[0034] In a particularly preferred embodiment of the laser processing machine according to the invention, a reflector is arranged not only in the beam path of the processing laser beam but also in the beam path of the sensor laser beam, which directs the processing laser beam and the sensor laser beam coaxially with respect to one another. For example, the reflector can be an inclined beam splitter, which is reflective for the processing laser beam and transmissive for the sensor laser beam, or vice versa, or a scraper mirror, a hole mirror, etc. In an advantageous extension of this embodiment, a sensor scanner (microscanner) is arranged between the frequency comb-based spacing sensor and the reflector, which deflects the sensor laser beam one-dimensionally or two-dimensionally in order to scan at least one workpiece. The sensor scanner is therefore located upstream of the processing scanner in order to enable a relative movement between the measuring beam and the processing laser beam.

[0035] Particularly advantageously, the frequency comb-based distance sensor is implemented at least partially, in particular exclusively, in fiber optics, which allows a compact sensor structure and a high robustness to mechanical vibrations and shocks as well as a compact design. In contrast to OCT, the frequency comb-based distance measurement does not require a space-demanding interferometer structure. In combination with the already described omission of a separate reference arm and the implementation in compact fiber optics based on the principle, this allows space-saving integration in laser processing optics. For higher measurement rates and a highly integrated design, the frequency comb-based distance sensor can alternatively also be implemented as a silicon-based photonic circuit.

[0036] Preferably, the frequency comb-based distance sensor is fastened to the processing optics or integrated into the processing optics, especially when implemented in fiber optics. Therefore, additional expensive sensor device switch cabinets are no longer required. For example, the frequency comb-based distance sensor is a highly integrated hardware module that can be screwed onto or into the processing optics in a modular manner. However, alternatively, the frequency comb-based distance sensor can also be arranged in the laser or in a separate switch cabinet. In this case, the measuring light needs to be guided to the processing optics via a fiber parallel to the light-conducting cable of the processing laser. The measuring light can also be guided to the processing optics in the fiber of the processing laser, but for this purpose, there needs to be sufficient spectral separation of the two light components.

[0037] The frequency comb-based distance sensor can be mounted directly on the processing optics or alternatively can also be integrated in a laser separate from the processing optics, wherein in this case the sensor laser beam and the processing laser beam are guided from the laser to the processing optics via a common fiber. Since the distance sensor can already have a fiber for guiding the measuring / reference radiation, this fiber can coincide with the fiber for guiding the processing laser radiation from the laser to the optics. In this case, for example, the common fiber can be designed at its end to be slightly reflective for the sensor laser beam of the distance sensor, so that the common fiber also serves as a reference path.

[0038] The invention also relates to a method for detecting a geometric (depth) feature on at least one workpiece during laser processing by means of the above-described laser processing machine, wherein a measuring beam of a frequency comb-based distance sensor is directed toward a geometric feature of the workpiece and a distance value or depth of the geometric feature is determined from a frequency difference of two frequency combs from a measuring section and a reference section, which is derived from a propagation time difference of the measuring beam and a reference beam. By scanning the geometric feature one-dimensionally or two-dimensionally with the measuring beam, a corresponding depth profile of the geometric feature can be obtained. For example, the geometric feature can be a joint position, a weld penetration depth or a seam geometry in the case of laser welding. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Further advantages of the present invention are apparent from the description and the drawings. Likewise, the features mentioned above and still to be further explained can be applied individually or in multiples in any combination. The embodiments shown and described should not be understood as an exhaustive enumeration, but rather have exemplary characteristics for the description of the present invention. The drawings show:

[0040] Figure 1 A first specific embodiment of a laser processing machine according to the invention is schematically shown, which has fixed optics for processing the laser beam and has a frequency comb-based spacing sensor;

[0041] Figure 2 A second specific embodiment of a laser processing machine according to the invention is schematically shown, which has programmable focusing optics for the processing laser beam and has a frequency comb-based distance sensor. DETAILED DESCRIPTION

[0042] In the following description of the figures, the same reference numerals are used for identical or functionally identical components.

[0043] exist Figure 1 The laser processing machine 1 schematically shown in the figure has a laser beam generator 2 for generating a processing laser beam 3, a processing optical device 4 (here in the form of a fixed optical device) for directing the processing laser beam 3 onto a workpiece 5, and a frequency comb-based distance sensor 6, which is used to measure the distance in an interferometric manner in order to determine geometrical features (joining position, welding depth, seam geometry in the case of laser welding) on ​​a workpiece surface 7 of the workpiece 5.

[0044] The processing optical system 4 comprises a reflective, tilted mirror 8 for the processing laser beam 3, at which the processing laser beam 3 is deflected by 90°, and a focusing lens 9, which serves to focus the deflected processing laser beam 3 onto the workpiece 5 and a protective glass 10, through which the focused processing laser beam 3 emerges from the fixed optical system 4.

[0045] The frequency comb-based distance sensor 6 has in a known manner a frequency-shifted feedback laser source (frequency comb generator) 11 for generating a sensor laser beam 12 having a cw frequency comb spectrally shifted over time, a measuring section 13 and a reference section 14 on which the sensor laser beam 12 is divided into a measuring beam 15 or a reference beam 16 as described below, a detector 17 on which the returning measuring beam 15 and the reference beam 16 are superimposed, and an evaluation device 18 for determining a distance value from a frequency difference of the frequency comb of the returning measuring beam 15 and the reference beam 16 derived from the propagation time difference of the measuring beam 15 and the reference beam 16. The sensor laser beam 12 is thus a laser beam generated in the distance sensor 6. Optionally, the distance sensor 6 can also have an additional seed laser source 19 for generating injection laser light, which is fed to the frequency-shifted feedback laser source 11. The phase of the injected laser light is modulated with a temporally variable frequency. The optical circulator 20 serves to guide the light emitted by the laser source 11 onto the workpiece 5 and to guide the back-reflected measuring beam 15 and reference beam 16 onto the detector 17. The optical circulator 20 is, for example, a single-mode fiber circulator.

[0046] The sensor scanner 21 is adjacent to the distance sensor 6 in order to deflect the sensor laser beam 12 one-dimensionally or two-dimensionally, i.e. in the x-direction and / or the y-direction, on the workpiece surface 7 and thus scan a region of the workpiece surface 7, for example, in a line scan. For example, the sensor scanner 21 can have a scanning mirror that can be deflected about two axes or - for example - Figure 1 It is shown that two scanning mirrors 22 are provided, each of which can be pivoted about an axis.

[0047] The sensor laser beam 12 is coupled into the processing optical system 4 coaxially with the processing laser beam 3 via the transmissive, tilted mirror 8 for the sensor laser beam 12 and directed toward the workpiece 5 through the focusing lens 9 and the protective glass 10. The reflectivity of the side of the protective glass 10 facing the distance sensor 6 is at least 1%, preferably about 5%, for the sensor laser beam 12, so that the sensor laser beam 12 is split at the protective glass 10 into a transmitted measuring beam 15 and a reflected reference beam 16. The measuring beam 15 is reflected at the workpiece surface 7 and returns to the distance sensor 6 via the processing optical system 4 and the sensor scanner 21. The measuring section 13 is defined by the sensor laser beam 12 and the optical path taken by the measuring beam 15 in total after the sensor laser beam is split, and the reference section 14 is defined by the sensor laser beam 12 and the optical path taken by the reference beam 16 in total after the sensor laser beam is split. That is, the reference section 14 is completely contained in the measuring section 13.

[0048] In a frequency comb-based distance measurement, a cw frequency comb with frequency comb teeth that are spectrally shifted over time is split in an interferometric measurement configuration by means of a frequency-shifted feedback laser source (Frequency-Shifted-Feedback Ranging=FSF-Ranging, frequency-shifted feedback ranging) 11 into a measurement section 13 and a reference section 14, which have optical paths of different lengths. The measurement beam 15 and the reference beam 16 returning from the measurement section 13 and the reference section 14 are superimposed on a detector 17. In this case, the frequency difference of the cw frequency comb of the measurement beam 15 and the reference beam 16, which results from the propagation time difference, leads to a clock frequency, from which the path length difference between the measurement section 13 and the reference section 14, i.e. a relative distance value or—in the case of a known reference section 14—an absolute distance value, can be derived by an evaluation unit 18. However, the spacing information is only present in the beat spectrum in a scattered manner, and accordingly, the spacing determination is inaccurate. If the frequency-shifted feedback laser source 11 is fed with additional injection light from a seed laser source 19, the phase of which is modulated at a frequency that is variable in time, then at a characteristic modulation frequency, a strong intensity super-high and greatly improved separation clarity of the beat signal are generated. Since the characteristic modulation frequency changes with the distance length difference between the measurement section 13 and the reference section 14, the characteristic modulation frequency can also be attributed to the spacing value. In the case of an unknown spacing, the corresponding characteristic modulation frequency is determined by recording the RMS signal of the detector 17 in the known time variation curve of the applied phase modulation: the following modulation frequency is the frequency that can be attributed to the spacing: at this modulation frequency, the RMS signal becomes maximum.

[0049] Figure 2 The laser processing machine 1 shown in FIG. Figure 1 The difference between the laser processing machine 1 and the processing optical device 4 is that the processing optical device 4 is not designed as a fixed optical device, but as a programmable focusing optical device. The programmable focusing optical device is a two-axis laser scanner (xy), which can optionally also have a collimating lens 23 that can be moved along the beam axis (z). The collimating lens 23 is arranged in front of the reflector 8 in the beam path of the processing laser beam 3, whereby the focus of the processing laser beam 3 can be moved along the beam direction z. In addition, the processing optical device 4 has a processing scanner 24 between the reflector 8 and the focusing lens 9 in order to deflect the processing laser beam - and therefore also the sensor laser beam 12 - on the workpiece surface 7 one-dimensionally or two-dimensionally, that is, in the x-direction and / or y-direction. The processing scanner 24 can have, for example, a scanner mirror that can be deflected around two axes or - as Figure 2As shown in FIG. 1 , there are two scanner mirrors 25 that can be deflected around an axis. Figure 2 As further shown in , the sensor laser beam 12 or the measuring beam 15 can be deflected relative to the processing laser beam 3 and independently of the processing laser beam 3 by means of a sensor scanner 21 in order to measure the workpiece surface 7, for example, during forward movement and backward movement of the processing laser beam 3 at the welding position.

[0050] Based on the weld penetration depth measurement test, the suitability of the frequency comb-based spacing measurement was confirmed: the frequency comb-based spacing measurement behaves robustly with respect to interfering light emitted by the process and is able to detect spacing values ​​from the steam capillary (keyhole) with sub-μm resolution during the laser welding process.

[0051] Alternative Figure 1 and 2 Alternatively to the cw frequency comb described in , a pulsed frequency comb may also be used.

Claims

1. A laser processing machine (1) for laser processing of at least one workpiece (5) by means of a processing laser beam (3), It has a laser beam generator (2) for generating the machining laser beam (3). It has a processing optical system (4) for directing the processing laser beam (3) toward the at least one workpiece (5). Features A frequency comb based distance sensor (6) for measuring distances in order to determine geometrical features of at least one workpiece (5), wherein the frequency comb based distance sensor (6) comprises a frequency shifted feedback laser source (11) for generating a sensor laser beam (12) having a frequency comb spectrally shifted over time, a measuring section (13) and a reference section (14), a detector (17) and an evaluation device (18), wherein the frequency shifted feedback laser source is used to generate a sensor laser beam (12) having a frequency comb spectrally shifted over time, wherein on the measuring section and the reference section the sensor laser beam (12) is divided into a measuring beam and a reference beam (15, 16), at the detector the returning measuring beam (15) and the reference beam (16) are superimposed, and the evaluation device ascertains a distance value from a frequency difference of two frequency combs from the measuring section (13) and the reference section (14) derived from a propagation time difference of the measuring beam (15) and the reference beam (16).

2. The laser processing machine (1) according to claim 1, characterized in that: The frequency comb based spacing sensor (6) has an additional seed laser source (19) for generating injection laser light, the phase of which is modulated at a temporally variable frequency, wherein the injection laser light is fed to the frequency shift feedback laser source (11).

3. The laser processing machine (1) according to claim 1 or 2, characterized in that: The reference section (14) is completely contained in the measuring section (14).

4. The laser processing machine (1) according to any one of the preceding claims, characterized in that An optical element (9, 10) is arranged in the beam path of the sensor laser beam (12), the reflectivity of which on the side of the sensor laser beam (12) facing the frequency comb-based distance sensor (6) is at least 1%, in order to divide the sensor laser beam (12) into a transmitted measuring beam (15) and a reflected reference beam (16).

5. The laser processing machine (1) according to claim 4, characterized in that: The optical element is formed by a protective glass (9), by a focusing lens (9) of the processing optics (4) or by a fiber end of a fiber common to the measuring beam (15) and the reference beam (16).

6. The laser processing machine (1) according to any one of the preceding claims, characterized in that The processing optical system (4) has a processing scanner (24) for deflecting the processing laser beam (3) one-dimensionally or two-dimensionally on the at least one workpiece (5).

7. The laser processing machine (1) according to any one of the preceding claims, characterized in that A mirror (8) is arranged in the beam path of the machining laser beam (3) and in the beam path of the sensor laser beam (12), which aligns the machining laser beam (3) and the sensor laser beam (12) coaxially with respect to one another.

8. The laser processing machine (1) according to claim 7, characterized in that A sensor scanner (21) is arranged between the frequency comb-based distance sensor (6) and the reflector (8), which deflects the sensor laser beam (12) one-dimensionally or two-dimensionally in order to scan the at least one workpiece (5).

9. The laser processing machine (1) according to any one of the preceding claims, characterized in that The frequency comb-based spacing sensor (6) is implemented at least partially in fiber optics or as a silicon-based photonic circuit.

10. The laser processing machine (1) according to any one of the preceding claims, characterized in that The frequency comb-based distance sensor (6) is fastened to the processing optical system (4) or is integrated into the processing optical system (4).

11. The laser processing machine (1) according to any one of the preceding claims, characterized in that The frequency comb-based distance sensor (6) is integrated in a laser which is separated from the processing optical system (4), wherein the sensor laser beam (12) and the processing laser beam (3) are guided from the laser to the processing optical system (4) via a common fiber.

12. Method for detecting geometric features on at least one workpiece (5) during laser processing by means of a laser processing machine (1) according to any one of the preceding claims, wherein: The measuring beam (15) of the frequency comb-based distance sensor (6) is directed toward a geometric feature of the workpiece (5), and a distance value of the geometric feature is determined based on a frequency difference of the frequency comb of the measuring beam (15) and the reference beam (16) obtained from a propagation time difference between the measuring beam (15) and the reference beam (16).

13. The method according to claim 12, characterized in that The geometric feature is scanned one-dimensionally or two-dimensionally.

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