Device for determining the focal position of a processing laser beam

Abstract

The invention relates to a device (1) for determining the focal position (FL) of a processing laser beam (2) in relation to a surface (3a) of a workpiece (3), comprising: a measurement light source (4) which is designed to emit measurement light (5a, 5b) at at least two different measurement wavelengths, a processing head (13) for focusing the measurement light (5a, 5b) onto the workpiece (3), a beam guiding apparatus (12) for guiding the measurement light (5a, 5b) to the processing head (13), at least one optical element (15, 17) with chromatic aberration, which is passed through by the measurement light (5a, 5b) and is preferably arranged in the processing head (13), a detector unit (6) for capturing the intensity (I1, I2) of the measurement light (5a', 5b') reflected back by the surface (3a) of the workpiece (3), and an evaluation apparatus (19) for determining the focal position (FL) of the processing laser beam (2) in relation to the surface (3a) of the workpiece (3) using the intensity (I1, I2) captured by the detector unit (6). In the device (1), the beam guiding apparatus (12) is designed for jointly beam-guiding the processing laser beam (2) and the measurement light (5a, 5b) to the processing head (13).

Description

[0001] Device for determining the focus position of a processing laser beam

[0002] The present invention relates to a device for determining the focus position of a processing laser beam with respect to a surface of a workpiece, comprising: a measuring light source which is designed to emit measuring light at at least two different measuring wavelengths, a processing head for focusing the measuring light onto the workpiece, a beam guiding device for guiding the measuring light to the processing head, at least one optical element with chromatic aberration through which the measuring light passes and which is preferably arranged in the processing head, a detector unit for detecting the intensity of the measuring light reflected back from the surface of the workpiece, and an evaluation device for determining the focus position of the processing laser beam with respect to the surface of the workpiece based on the intensity of the measuring light detected by the detector unit.

[0003] For the purposes of this application, a workpiece is understood to be any object with respect to whose surface the focal position is to be determined. The workpiece can be an object intended for processing with the processing laser beam. However, the workpiece can also be an object that is not intended for processing with the processing laser beam and whose surface forms a reference for determining the focal position of the processing laser beam with respect to this reference.

[0004] Laser processing processes, in particular welding processes with fixed optics, robot-guided remote welding processes with scanner optics, laser cutting and a wide variety of ultrashort pulse applications require an increasing degree of sensor monitoring in order to meet growing demands for high and consistent processing quality. A fundamental distinction can be made between online and offline sensors as well as on-axis or off-axis sensor arrangements. To evaluate the processing result, depending on the laser application, there are a wide variety of sensor-detectable characteristics (e.g. welding depth, thermal field in the processing zone, position of the joining partners, etc.) and, depending on the laser application, a wide variety of laser processing parameters that must be taken into account (e.g. beam geometry, wavelength, ...) or controlled using sensors (e.g. laser power, feed rate, positioning of the processing laser beam, ...).

[0005] Crucial to the processing quality and process stability of a wide variety of laser applications is process control with a precisely adjusted and, as far as possible, consistent focus position, which can be detected by sensors. The focus position represents the distance between the focus of the processing laser and the surface of the target or workpiece to be processed, measured along the optical axis of the processing optics integrated into a processing head. For example, to avoid spatter when laser welding steel materials, welding at a focus position of -2 mm is recommended. In this case, the laser processing focus is shifted 2 mm from the surface of the workpiece into the interior of the workpiece. This value is based on empirical experience.

[0006] In practice, it is customary to maintain an accuracy of 1 / 3 of the Rayleigh length of the processing laser beam when adjusting the focus position. Achieving this is particularly challenging in laser processing with small irradiation diameters of a few 10 pm (e.g., 11 pm), small image ratios when using, for example, short focal length focusing optics in the tens of millimeters range (e.g., 65 mm), and the associated short Rayleigh lengths of significantly less than 1 mm (e.g., 0.14 mm). Focus adjustment is further complicated if the focus position or the distance between the workpiece surface and the processing optics changes during the laser process, e.g., in ablation processes using ultrashort pulse lasers, and thus the initially set focus position loses its validity over the course of the process.Emerging applications in electromobility, in particular, require frequent and, above all, rapid focus position determination, monitoring, or tracking. For example, the stator of an electric motor contains numerous hairpin pairs, with the position of the end faces to be welded varying due to manufacturing tolerances. The welding process is sensitive to deviations in the focus position; therefore, these deviations should be detected and corrected before laser processing each hairpin pair. Another example is the welding of contacts on battery blocks or contacts on different levels of other electronic assemblies.In the production of fuel cells, it is also important to align the plane field of a laser scanner optics as precisely as possible to a large-area workpiece (bipolar plate, for example 15 cm x 30 cm) in a rotational manner, so that welding paths that extend over a large length are welded as best as possible with the intended focus position over the entire workpiece.

[0007] In practical use, the focus position is still subject to fluctuations due to shape and position deviations of the workpiece (tolerances or fluctuations of the manufacturing process, tolerance of the workpiece clamping), inaccuracies in the actuators guiding the processing optics (e.g. absolute positioning accuracy of an industrial robot which guides a scanner optics) or temporal changes in the processing optics (thermal focus shift due to heating and associated distortion or change in the refractive index of the components of the processing optics).

[0008] There are various ways to determine the focus position, several of which are described below:

[0009] Creating a focus series: Several processing laser pulses are emitted one after the other onto a black anodized aluminum sheet. Between the individual laser pulses, the distance between the processing optics and the aluminum sheet varies in a constant (known) increment and direction, and the sheet is additionally shifted laterally relative to the processing laser beam. Alternatively, the focus position can be changed by moving lenses or changing the focal length of a focusing mirror within the processing optics. The distance between the processing optics or processing head and the sheet is roughly selected at the beginning of the focus series such that the processing optics passes through focus position 0 approximately in the middle of the focus series, where the focus of the processing laser beam is on the surface of the workpiece.With each laser pulse, the anodized aluminum layer is removed at the corresponding processing location, creating a bright spot that stands out from the surrounding sheet metal. Subsequent measurement of the individual processing locations serves to determine the processing location with the smallest diameter. At this processing location, processing took place close to or at focal position 0.

[0010] WO 2020 / 143861 A1 describes a method and a device for controlled laser processing of a workpiece using confocal distance measurement. This device uses an optical confocal distance measuring device with a variable-focal-length measuring light optics, the focal length of which is varied over time to acquire distance measurement data at different focal length values. Acquiring the distance measurement data involves recording the intensity of the measuring light reflected back from the workpiece to be processed, and the distance is determined based on a temporal progression of the intensity of the measuring light reflected back from the workpiece to be processed. If the focus is on the surface of the workpiece, the intensity of the measuring light is maximum. The focal position can be determined based on the temporal progression of the intensity of the reflected measuring light.

[0011] DE 10 2019 004 337 A1 describes a beam analysis device for determining an axial focus position of a measuring beam coupled from a laser beam. The device comprises a partial beam imaging device configured to receive a first measuring beam and a first selection device for forming a first partial beam from a first partial aperture region of the first measuring beam. The device comprises a detector unit with a light-sensitive detector and an evaluation unit for processing signals from the detector unit. The first selection device is arranged off-center with respect to an optical axis provided for the irradiation of the first measuring beam, and the partial beam imaging device is configured to image the first partial beam onto the detector unit to generate a first beam spot.The evaluation unit is configured to determine a lateral position of the first beam spot. A change in the axial focus position of the measuring beam is correlated with a change in the lateral position of the first beam spot.

[0012] DE 10 2018 211 166 A1 and WO 2020 007 984 A1 describe a method and a device for checking the focus position of a pulsed laser beam relative to a workpiece. The laser beam is focused on the workpiece at a plurality of positions along a trajectory, and radiation generated at a respective position upon interaction of the pulsed laser beam is detected. Using signal values ​​corresponding to the detected radiation at a respective position, the focus position is checked at at least one of the positions. For this purpose, the signal value at the position is compared with a reference value formed from the signal values.

[0013] Another option for determining the focus position of a processing laser beam is a commercially available sensor, also known as a CalibrationLine sensor. This sensor essentially consists of a pinhole with a photodiode mounted behind it. The sensor is mounted away from the workpiece to be processed. To determine the focus position, the processing optics are placed over the pinhole so that it coincides with the assumed position of the laser focus. The pinhole is then scanned laterally by deflecting the processing laser beam (e.g. using a scanner optics or a fixed optics attached to a robot). During this process, the intensity values ​​measured at the photodiode are recorded. If the scan is repeated at different distances between the laser optics and the pinhole, a three-dimensional intensity map is created, from which, among other things, the focus position of the processing laser beam can be calculated.Another option is cutting a focus comb: Using a utility program, several comb-shaped cuts are cut into a component. The focus position is varied from cut to cut, and the smallest gap is then determined by checking with a feeler gauge.

[0014] It is also possible to determine the focus position indirectly using a distance sensor that measures the distance between the workpiece and an arbitrary reference point. The reference point could, for example, be the zero point of the optical coherence tomography (OCT) measurement range. In this case, however, an additional step is required beforehand: establishing a reference between the focus position of the processing laser and the reference point of the distance sensor. This is achieved using a supplementary sensor solution, for example, the CalibrationLine sensor described above, which first measures the focus position of the processing laser beam away from the workpiece to be processed.Additional reference features of the CalibrationLine sensor provide the distance sensor, for example the OCT sensor, with the possibility of geometric referencing to the coordinate system of the CalibrationLine sensor and thus also the reference to the focus position detected by the CalibrationLine sensor.

[0015] The known methods described above have in common that they carry out a focus position determination, which individually or in combination:

[0016] - only takes place with indirect reference to the workpiece, since the measurement cannot take place on it (CalibrationLine sensor / generation of focus series);

[0017] - only takes place with indirect reference to the focus position, since the focus position is reconstructed from a measured value that is not directly related to it (e.g. OCT: measures a distance on the workpiece to a reference point set for metrological reasons, which does not correspond to the focus position 0 and which therefore has to be related to the focus position 0 using additional tools (calibrated), e.g. using the CalibrationLine sensor); - compared to fast process cycle times, is too slow for a focus measurement at the beginning of each machining process (CalibrationLine sensor / generation of focus series);

[0018] - is technically or economically difficult due to the requirement to process on different, tolerance-dependent levels (CalibrationLine sensor / generation of focus series);

[0019] - involves manual effort (creation of focus series);

[0020] - not all interference influences in the optical path or in the beam path of the processing laser beam that affect the focus position are recorded (WO 2020 / 143861 A1 and DE 10 2019 004 337 A1);

[0021] - is expensive if the corresponding sensor technology is only to be used for the purpose of focus position measurement (OCT).

[0022] DE 10 2009 059 245 A1 describes a device for detecting and adjusting the focus of a laser beam during laser processing of workpieces, comprising: an optical device for supplying and focusing a laser beam emitted by a processing laser, which has a focusing element arranged in a processing head, at least a first and a second adjustment light source that emit radiation of different wavelengths, an optical device for supplying and focusing the radiation emitted by the adjustment light sources onto the surface of the workpiece to be processed, an optical decoupling device for coupling out the radiation of the adjustment light sources that is reflected back from the surface of the workpiece to be processed and that has a chromatic aberration, a detector for detecting the intensities of the reflected radiation of the adjustment light sources,and an evaluation device for determining the position of the focus of the laser beam of the processing laser in relation to the surface of the workpiece to be processed.

[0023] The basis for the detection of the focus position described in DE 10 2009 059 245 A1 is the chromatic confocal principle, which utilizes the wavelength-dependent refractive index of electromagnetic radiation (primarily UV, VIS, NIR, IR) as it passes through optical elements and the resulting chromatic aberration. Thus, the focus of broadband light emanating from a point light source, which is collimated by lenses and then refocused, experiences a spectral spread after focusing: Since the various wavelength components of the broadband light are focused at different positions along the optical axis, its focus appears to be elongated overall. If a target, e.g.When a workpiece is placed within the focal area, primarily light of the wavelength in focus enters and passes through the optics along the same path it already traveled on its way through the optics to the target. If all light reflected from the target is directed onto an aperture acting as a spatial filter, this primarily allows light of the wavelength focused on the surface of the target to pass through. This light is captured by a detector unit (often a spectrometer), whereby the spectral information allows direct conclusions to be drawn about the position of the target relative to any other wavelength of the measuring light.

[0024] Object of the invention

[0025] The invention is based on the object of improving a device for determining, in particular for monitoring and, if necessary, correcting, the focus position of a processing laser beam, which is based on the chromatic confocal principle.

[0026] Subject of the invention

[0027] This object is achieved by a device of the type mentioned above, in which the beam guiding device is designed for the joint beam guidance of the processing laser beam and the measuring light to the processing head. The beam guiding device is typically designed for coaxial beam guidance of the processing laser beam and the measuring light. The processing laser beam and the measuring light are typically guided coaxially onto the workpiece by processing optics in the processing head. The beam guiding device also serves to guide the measuring light reflected back from the surface of the workpiece. Unlike what is described in DE 10 2009 059 245 A1, the coupling of the measuring light into the beam path of the processing laser beam in the device described here does not take place in the processing head, but in the beam path in front of the processing head, typically in a laser device (see below).), which is arranged at a distance from the processing head. This extends the shared beam path of the processing laser beam and the measuring light, or they share the same beam path. Therefore, when determining the focus position using the measuring light, all optical elements that influence the focus position of the processing laser beam are taken into account. This allows for correct determination of the focus position even in the event of a thermal focus shift, if an end cap has slipped in the connector of a fiber optic cable, or if a fiber optic cable is replaced.

[0028] The device designed in this way is also robust and has a direct relationship to the processing laser beam: In the device described here, the determination of the focus position takes place directly on the workpiece and not on a target that is used away from the workpiece for a focus series or not as with the CalibrationLine sensor described above, in which the detection of the focus position also takes place away from the workpiece to be processed.

[0029] The focus position determined using the fixture can be compared with a target focus position, meaning the fixture can be used to monitor the focus position. If the focus position determined using the fixture deviates from the target focus position, the focus position can be corrected. In particular, the distance between the processing optics and the workpiece surface can also be corrected if necessary.

[0030] In one embodiment, the measuring light source and preferably the detector unit are integrated into a laser device for providing the processing laser beam, which preferably has a processing laser source for emitting the processing laser beam, and the beam guiding device is designed for the joint beam guidance of the processing laser beam and the measuring light from the laser device to the processing head. Typically, the detector unit and the measuring light source are arranged in a common sensor unit which has an output from which the measuring light exits, wherein the output generally simultaneously forms the input for the measuring light reflected back from the workpiece. On its way from the laser device to the processing head or to the processing optics, the measuring light follows the same path as the processing laser beam, i.e. the measuring light also moves through the same transport medium (free beam or fiber optic cable, see below).In a fiber-guided sensor unit, the measurement light is preferably extracted via a (common) output fiber, which can be designed as a single-mode or multimode fiber. In the free-beam design of the sensor unit, the step of extracting the measurement light is already anticipated, e.g., by early extraction of the measurement light directly at the exit of the measurement light source.

[0031] In the embodiment described here, a chromatic confocal detection of the focus position is realized, which is integrated into the assembly of laser device, beam guidance device (usually fiber optic cable or free beam, e.g. in the case of USP laser applications) and processing head or processing optics. The processing optics of the processing head can be implemented as a rigid or scanning structure. The device designed in this way is compact, easy to integrate and has a high degree of cost-effectiveness, since the measuring light source and the detector unit are integrated into the laser device and do not have to be newly purchased for each existing processing optics or for each processing head. The device can also be assembled from standard parts, so that manufacturing costs are low. The determination of the focus position can also be carried out within a few milliseconds and can therefore, for example,be carried out before each welding.

[0032] In a further embodiment, the beam guiding device comprises a fiber optic cable for the joint beam guidance of the processing laser beam and the measuring light. In this case, one fiber end of the fiber optic cable on the processing head typically serves as an exit aperture and as an entrance aperture for the chromatic confocal focus position determination or for the sensor unit provided for this purpose. The fiber optic cable acts as a spatial filter and primarily allows the reflection of that measuring wavelength of the measuring light whose focus is on the surface of the workpiece to pass through. For the case described here in which the beam guiding device comprises a fiber optic cable, it is advantageous if the coupling into the fiber optic cable has the smallest possible beam waist (taking into account the critical angles of anyexisting output fiber of the measuring light and the fiber optic cable), which enables optimal coupling efficiency for fiber optic cables with different core diameters (e.g. 50 pm to 400 pm).

[0033] In the simplest case, the chromatic confocal response of the entire system is present as an unfiltered intensity signal that is recorded by the detector unit. Therefore, the sensor structure or the sensor unit is sensitive to any measurement light reflections that do not originate from the workpiece. Since the transport medium in the form of the fiber optic cable between the laser device and the processing optics in the processing head guides the measurement light both to the workpiece and from the workpiece back to the laser device or the sensor unit contained therein, the useful signal (i.e. the portion of measurement light reflected by the workpiece and usable for analysis) would be disruptively superimposed by reflections arising in the transport medium. In a fiber-guided processing laser beam or measurement light, for example, the reflections occur at the fiber inlet and fiber outlet (the glass-to-air refractive index transition generates approximately 4% backreflection at the vertical end face of the fiber optic cable without an anti-reflective coating).Therefore, especially when using standard fiber optic cables, double-clad fibers, or BrightLine Weid fibers, precautions must be taken to reduce back reflections at the fiber entry and exit. For this purpose, anti-reflective end caps can be attached to the fiber optic cable, for example.

[0034] In a further development of this embodiment, the fiber optic cable is designed as a hollow-core fiber or as a multiple-clad fiber, in particular as a double-clad fiber. In a hollow-core fiber, the light is guided in a hollow core. With hollow-core fiber optic cables, there is therefore generally no need for an end cap or an anti-reflective coating, since the light guided in the hollow core does not experience any change in refractive index when entering or leaving the fiber. A double-clad fiber has a light-guiding ring between the light-guiding core and the enclosing cladding. The ring can be used, for example, to guide the reflected measuring light, while the core is used to guide the measuring light to the workpiece. Instead of a double-clad fiber, a multiple-clad fiber designed as a triple-clad fiber or a quad-clad fiber can also be used.

[0035] In a further development, the detector unit is designed to separately detect the intensity of the measuring light reflected back from the surface of the workpiece in a core, a cladding, and / or a ring of the fiber optic cable. The measuring light reflected from the workpiece and evaluated is also referred to below as useful light. Separate detection of the intensity also refers to the sole detection of the intensity of the measuring light reflected back from the surface of the workpiece in the core, the cladding, or the ring of the fiber optic cable.

[0036] When fiber optic cables are used as beam guidance devices, their core forms the entrance aperture of the chromatic confocal measurement principle. Since the cladding of all fiber optic cable types described above, as well as the ring of a double-clad fiber or a BrightLine Weid fiber optic cable, represents an additional entrance surface for the measurement light, is suitable for guiding the measurement light, and has a larger cross-section than the fiber core, a joint evaluation of the useful light component from the cladding of the fiber optic cable or the ring of the fiber optic cable with the useful light component from the core of the fiber optic cable increases the uncertainty of the measurement principle.

[0037] It is therefore possible and advantageous to remove the useful light component contained in the sheath and the ring of the fiber optic cable before it enters the sensor device. Appropriate precautions can be taken, e.g., in the form of etched sheath surfaces, to isolate the useful light component contained in the sheath of the fiber optic cable. To remove the measurement light component contained in the ring of the fiber optic cable, an additional entrance aperture can be provided in the sensor device. In this case, the detector unit separately records the intensity of the useful light component reflected back in the core of the fiber optic cable.

[0038] The detector unit can also be designed to selectively detect the intensity of the useful light component reflected back in the core, the cladding, or the ring of the fiber optic cable. The detector unit can optionally have a separate detector for detecting the useful light component reflected back in the core, the cladding, and, if applicable, the ring of the fiber optic cable. It is also possible to detect only the useful light originating from the cladding and / or the ring of the fiber optic cable, while completely omitting the useful light originating from the core of the fiber optic cable.

[0039] Alternatively, the detector unit can be designed to separately detect the useful light originating from the sheath or ring of the fiber optic cable, separate from the detection of the useful light component in the core of the fiber optic cable. This allows for a two-stage measurement process, in which the evaluation of the useful light originating from the sheath and / or ring serves to determine the focus position in an inaccurate manner with an enlarged measurement range, and the sole evaluation of the useful light component originating from the core of the fiber optic cable serves to determine the focus position accurately with a narrow measurement range.

[0040] Two or more measuring ranges of different sizes, in which the focus position is determined with different measurement accuracy, can be advantageous for determining the focus position for the following reasons, among others: The processing optics in the processing head (fixed optics or scanner optics) focus both the processing laser beam and the measuring light via a corresponding focusing optics, for example in the form of a focusing lens. The processing optics generates the focus spread of the measuring light required for the sensor principle through chromatic aberration. Assuming otherwise constant

[0041] Under certain conditions, the focal length of the processing head's processing optics determines the degree of focal spread of the measuring light along the optical axis, thus ensuring the adaptability of the sensor technology: As the focal length and depth of field of the processing laser beam decrease, the accuracy required for determining the focus position increases. However, as the focal length decreases, the Rayleigh length of the measuring light also decreases, thus enabling higher-resolution focus position determination (with a smaller measuring range).

[0042] In an alternative embodiment, the beam guidance device is designed for the joint guidance of the processing laser beam and the measuring light in free-beam propagation. In this case, the processing laser beam and the measuring light are guided to the processing head in air as the transport medium.

[0043] An entrance aperture is provided in the laser device or in the sensor unit to perform the chromatic confocal measurement. An exit fiber or an exit-side fiber end of the sensor unit's exit fiber can serve as the exit aperture for the measurement light, which simultaneously forms the entrance aperture for the reflected measurement light. If the beam guidance in the sensor unit occurs entirely or partially in free-beam propagation, an exit aperture can be provided as a separate component.

[0044] The choice of transport medium (fiber optic cable or free beam) and the type of fiber optic cable depend on the application. If the sensor unit is adapted to a disk, fiber, or diode laser, it is usually a conventional fiber optic cable with a light-guiding core and cladding, or a double-clad fiber or BrightLine Weid fiber, which can guide light in different proportions in the core and the ring. If adapted to an ultrashort pulse laser, it is typically a hollow-core fiber optic cable or a free beam.

[0045] In a further embodiment, the device comprises a coupling device for coupling the measuring light from the measuring light source into the beam path of the processing laser beam and for coupling the measuring light reflected back from the surface of the workpiece out of the beam path of the processing laser beam, wherein the coupling device is preferably arranged in the laser device. The coupling and decoupling of the measuring light typically takes place in the laser device before the joint coupling of the measuring light and the processing laser beam into the beam guiding device. The coupling and decoupling of the measuring light generally takes place in the free beam. The coupling device for coupling and decoupling the measuring light can be designed in various ways, for example as a (possibly tightly tolerated) dichroic beam splitter, as a perforated mirror, or as a scraper mirror.In this embodiment, the coupling device is typically arranged in a stationary manner and enables a time-parallel processing of the workpiece with the processing laser beam and a determination of the focus position of the processing laser beam with the aid of the measuring light.

[0046] In an alternative embodiment, the coupling device forms a beam switch, which can be arranged in the beam path of the processing laser beam for selectively coupling the processing laser beam or the measuring light into the beam guiding device and can be removed from the beam path of the processing laser beam. In this case, the determination of the focus position of the processing laser beam is only possible before or after, but not simultaneously with, the processing of the workpiece by the processing laser beam. The beam switch can optionally additionally supply a pilot laser beam to the beam guiding device. In this case, the laser device or the sensor unit is designed to provide a pilot laser beam in addition to providing the measuring light in order to couple the visible pilot laser beam into the beam path of the processing laser beam instead of the measuring light.

[0047] In a further embodiment, the device comprises a separation device for separating the measuring light emitted by the measuring light source from the measuring light reflected back by the workpiece, wherein the separation device is preferably arranged in a sensor unit of the laser device. Since the measuring light emitted by the measuring light source and the measuring light reflected back from the surface of the workpiece and the processing laser beam are guided coaxially between the laser device and the workpiece, the individual radiation components must be separated in front of or in the sensor unit for evaluation of the useful signal. In order to make the useful signal of the entire fiber optic cable cross-section available for evaluation even when using fiber optic cables with a larger core diameter (e.g. 400 pm) in the beam guiding device, it is advantageous to have the entrance aperture of the sensor unit orThe detection area of ​​the detector unit should ideally be larger than the smallest possible exit area for the measuring light.

[0048] As described above, the sensor unit typically comprises the detector unit and the measuring light source, as well as possibly other components such as the evaluation device and / or a control device for controlling the measuring light source. The sensor unit typically has an output for coupling out the measuring light, which simultaneously forms an input for the measuring light reflected back from the workpiece. The separation of all measuring light from the beam path of the processing laser radiation is achieved by the coupling device described above, which also affects the useful radiation originating from the workpiece.

[0049] In the simplest case, the measuring light is guided in free-beam propagation within the sensor unit, and the separation device is designed as a non-polarizing 50:50 beam splitter. However, such a beam splitter reduces both the intensity of the measuring light and the useful light reflected back from the workpiece, thus reducing the efficiency of the sensor unit and the measurement.

[0050] In a further development of this embodiment, the separation device is designed as a circulator in the form of a fiber circulator or a free-jet circulator.

[0051] In the case of a sensor unit that is fully or partially fiber-guided, the separation of emitted measuring light and wanted signal can be achieved using a fiber circulator. The fiber circulator can be connected to the measuring light source at a first port via a first fiber. A second fiber at a second port forms the exit surface for the measuring light and the entrance surface for the wanted signal, and a third fiber at the third port serves to exit the wanted signal. Since in this case the exit and entrance surfaces of the measuring light or wanted signal directly coincide, the sensor structure or sensor unit is sensitive to retroreflected measuring light, which primarily occurs when the measuring light exits the second port (glass to air transition). This can be counteracted by providing the exit or entrance fiber connected to the second port with an anti-reflective coated end cap.

[0052] In the case of a sensor unit partially or completely implemented in a free beam, the separation of the measuring light and the wanted signal is preferably implemented using a free beam circulator. A free beam circulator can, for example, comprise a Faraday rotator and other optical elements that separate the measuring light emitted to the workpiece from the measuring light reflected back from the workpiece based on different polarization states. In this case, the exit surface of the measuring signal and the detection surface of the wanted signal do not directly coincide. Therefore, the detection surface of the detector unit can easily be designed larger than the exit surface of the measuring light. The sensor unit can therefore be easily adapted to fiber optic cables with different diameters.

[0053] In an alternative development, the separation device is designed as a double-clad fiber coupler, which has a double-clad fiber with a core for guiding the measuring light emitted by the measuring light source, which is preferably designed as a single-mode fiber, and a ring for guiding the measuring light reflected by the workpiece, which is preferably designed as a multi-mode fiber.

[0054] In this embodiment, the beam guidance in the sensor unit is fully or partially fiber-guided. The output for the measuring light and the input for the measuring light reflected back from the workpiece are formed by a doubleclad fiber, or more precisely, by one end of the doubleclad fiber. The measuring light is guided to the workpiece in the core of the doubleclad fiber, and the measuring light reflected back from the workpiece is guided in the light-guiding ring. The doubleclad fiber forms part of a doubleclad fiber coupler, or this is connected to the doubleclad fiber. The doubleclad fiber coupler serves to separate the single-mode measuring light guided in the core of the doubleclad fiber from the multimode useful light guided in the ring of the doubleclad fiber.For this purpose, the doubleclad fiber can be routed in a coupling section of the doubleclad fiber coupler adjacent to another fiber, into whose core a portion of the useful radiation from the ring of the doubleclad fiber is coupled. The other fiber is typically not a doubleclad fiber.

[0055] The advantage of using a double-clad fiber in the sensor unit is that the diameter of the wanted signal entrance surface (e.g., 105 pm) can be selected larger than the diameter of the measuring light exit surface (e.g., 9 pm). This allows for optimal coupling efficiency in both directions (sensor-to-fiber optic cable for the measuring light and fiber optic cable-to-sensor optics for the wanted light) with different core diameters of fiber optic cables (taking into account the critical angles of the core and ring of the double-clad fiber and the fiber optic cable). If the beam guidance device for guiding the measuring light includes a fiber optic cable, this can also be designed as a double-clad fiber.

[0056] In one embodiment, the measuring light source comprises a first light source, in particular a first laser source, for emitting measuring light at a first measuring wavelength and a second light source, in particular a second laser source, for emitting measuring light at a second measuring wavelength.

[0057] In the simplest case, the measuring light source is designed to generate measuring light with two narrowband spectra concentrated around the first and second measuring wavelengths using two laser sources, for example two laser diodes. The foci of the measuring light at the two measuring wavelengths are shifted relative to one another by chromatic aberration along the optical axis of the processing optics of the processing head. The focus position of the processing laser beam can be determined from the ratio of the intensities of the measuring light detected by the detector unit at the first measuring wavelength and at the second measuring wavelength. The individual measuring wavelengths must be selected such that the chromatic confocal response (the measuring light reflected from the workpiece and returned to the laser device and detectable) of at least two light sources overlaps.In a fiber-guided setup, the discrete spectra of the measurement light from the individual light sources can be combined using WDM (Wavelength Division Multiplexer), circulators, or fiber couplers. In a free-space setup, the measurement light from the two measurement wavelengths can be combined using, for example, dichroic mirrors or polarization-dependent beam splitters.

[0058] Alternatively, the first light source and the second light source can be configured not as laser sources with narrowband spectra, but as light sources with comparatively broadband spectra around the respective measurement wavelength, in order to increase the measurement range at the expense of resolution. In this case, the light sources can be configured as superluminescent diodes, for example.

[0059] It is possible for the measuring light source to have at least one pair of light sources in the form of laser sources with a narrow-band spectrum and at least one pair of light sources with a comparatively broadband spectrum in order to open up at least two measuring ranges (fine measuring range and coarse measuring range) with one and the same processing optics.

[0060] In a further development, the first and / or the second measuring wavelength deviates from a processing laser wavelength of the processing laser beam. In the simplest case, the first measuring wavelength of the first light source is below and the second measuring wavelength of the second light source is above the processing laser wavelength of the processing laser beam whose focus position is to be detected. However, this is not necessarily the case; rather, the first measuring wavelength and the second measuring wavelength can both be below or above the processing wavelength of the processing laser beam, for example, if a focus position determination is to be carried out during permanently defocused processing. In order to expand the measuring range along the optical axis of the processing optics, the two light sources of the measuring light source can be supplemented by further light sources, in particular by further pairs of light sources, e.g.in the form of laser sources or light sources with a comparatively broadband spectrum, whose measuring wavelengths move further away from the processing wavelength of the processing laser beam with each additional light source. With the aid of a control device of the sensor device, either all light sources of the measuring light source, a selection of specific light sources, or just one light source at a time are activated sequentially in a time-controlled pattern, whereby the intensity of the measuring light emitted by the respective light source and reflected back from the workpiece is recorded.

[0061] In a further development, the measuring light source comprises a further light source, for example a laser source, which is designed to emit measuring light with a measuring wavelength that corresponds to the processing laser wavelength of the processing laser beam. Such a light source can be used for testing or calibrating the sensor unit.

[0062] In an alternative embodiment, the measurement light source is designed as a broadband light source or as a tunable light source. In the first case, the measurement light is generated by a broadband light source with a continuous spectrum to enlarge the measurement range, e.g., a superluminescence diode or a supercontinuum laser. In the second case, a tunable source (e.g., a tunable VCSEL laser diode, a broadband light source with a downstream tunable bandpass filter, etc.) is used as the measurement light source.

[0063] It is possible for the device or sensor unit to include a mode mixer for the measuring light emitted by the measuring light source. In this case, the measuring light is fiber-guided within the sensor unit. The measuring light is initially guided in a single-mode fiber and then homogenized with the help of the mode mixer and a connected multimode output fiber with the smallest possible core diameter (e.g., 50 pm). In this way, after coupling out of the sensor unit, a uniform intensity distribution can be generated in a subsequent fiber optic cable of the beam guidance device, which is often designed as a multimode fiber.

[0064] In a further embodiment, the device comprises at least two optical elements exhibiting chromatic aberration with different Abbe numbers. The Abbe number of a respective optical element, which represents a measure of the dispersion of the respective optical element, can be determined, for example, by selecting the (glass) material of the optical element. The two or more optical elements with different Abbe numbers can be arranged in the processing optics of the processing head or elsewhere in the beam path of the measuring light.

[0065] The optical elements with different Abbe numbers can be used for different purposes. For example, the two or more optical elements or their Abbe numbers can be designed such that the focal plane of a light source with a narrowband spectrum, typically in the form of a laser source whose measurement wavelength differs from the processing laser wavelength, is placed on the focal plane of the processing laser beam or coincides with the focal plane of the processing laser beam. In this case, the chromatic aberration affects the measuring light of the light source as if the measurement wavelength coincided with the processing wavelength of the processing laser beam. Alternatively, for given measuring light sources, the measuring range and resolution can be specifically influenced by the different dispersion or the different Abbe numbers.For example, two measuring light sources whose measuring wavelengths are spectrally far apart and therefore have a large difference in the axial focus position can be placed closer together or further apart, which also influences the measuring range and the measurement resolution. In scanning applications, so-called lateral chromatic aberrations can occur. In the case of a classic F-theta lens with optical elements made of quartz glass, for example, this can lead to the processing laser beam and the measuring light not being exactly aligned, and thus the measuring position on the surface deviates from the processing position of the processing laser beam on the surface. The signal levels are also influenced by this. This effect can be reduced or avoided by using an achromatic lens that has at least two optical elements with different Abbe numbers.In a classic non-achromatic lens, the lateral chromatic aberration can also be corrected by a scan deflection adapted to the wavelength, but this requires a temporal separation of the determination of the focus position and the laser processing.

[0066] The type of detector device used in the device is adapted to the type of measuring light source used.

[0067] In one embodiment, the detector unit is designed as a spectrometer. A detector unit in the form of a spectrometer is typically used when the measurement light source is a broadband light source.

[0068] If the overall structure of the sensor unit is capable of providing a chromatic confocal response which also includes the processing laser wavelength (coupling and decoupling of the measuring light into and out of the beam path of the processing laser beam via the coupling device in the form of the beam switch, but not via a beam splitter, e.g. in the form of a dichroic mirror), the focus position can be determined directly from the maximum value of the spectrum of the confocal response when using broadband measuring light or a finely tunable measuring light source (e.g. 2 nm step size).

[0069] If the confocal response spectrum does not include the processing laser wavelength, the focal position can be additionally derived from the ratio of at least two measurement wavelengths, with the first measurement wavelength being above and the second measurement wavelength below the processing laser wavelength. It is advisable to use two maxima from the confocal response spectrum for this purpose. In this case, the focal position can also be derived from part or all of the spectral response detected by the detector unit.

[0070] In case the measuring light source has individually switchable light sources with different, discrete measuring wavelengths (e.g. laser diodes) or is designed as a tunable measuring light source, a simple photodiode or a highly sensitive single-photon avalanche diode (SPAD) in Si or InGaAs design can be used as the detector unit to selectively record the chromatic confocal response of the sensor unit (for each set measuring wavelength).

[0071] In a further embodiment, the measuring light source is designed for pulsed emission of the measuring light, and the detector unit and the evaluation unit are preferably designed to distinguish measuring light reflected back from the surface of the workpiece from measuring light reflected back from other locations. The measuring light source, in particular light sources of the measuring light source that have a substantially discrete spectrum, can be designed as pulsed light sources (pulse lengths, for example, in the range from ps to ns). For the differentiation described above, it is advantageous if the detector unit is designed as a single-photon avalanche diode. In this case, the evaluation device can be designed for high-resolution dTOF (direct time of flight) evaluation using a multi-hit TDC (time to digital converter) or a high-speed ADC (analog-to-digital converter) (e.g. with 10 GS / sec).This allows, for example, the separation of reflection points occurring at different locations in the beam path of the processing laser beam, as described above, and enables the separation of the useful signal originating from the workpiece and interfering reflections to be increased, comparable to the measuring principle in an OTDR (“optical time-domain reflectometer”).

[0072] Further advantages of the invention will become apparent from the description and the drawings. Likewise, the above-mentioned and further listed features can be used individually or in combination. The embodiments shown and described are not intended to be exhaustive, but rather serve as examples for describing the invention.

[0073] They show:

[0074] Fig. 1 is a schematic representation of a device for determining the focus position of a processing laser beam, wherein the device comprises a beam guiding device with a fiber optic cable for guiding the processing laser beam and measuring light from a laser device to a processing head,

[0075] Fig. 2a is a schematic representation analogous to Fig. 1 , in which the laser device has a measuring light source with two laser diodes,

[0076] Fig. 2b is a schematic representation analogous to Fig. 2a, in which the beam guiding device is designed to guide the processing laser beam and the measuring light in free beam propagation,

[0077] Fig. 3 is a schematic representation analogous to Fig. 2a, in which the laser device has a separation device in the form of a double-clad fiber coupler to separate the measuring light supplied to the workpiece from the measuring light that is reflected back from the workpiece,

[0078] Fig. 4 is a schematic representation analogous to Fig. 1 , in which the laser device has a measuring light source in the form of a broadband light source and a detector unit in the form of a spectrometer, and

[0079] Fig. 5 is a schematic representation similar to Fig. 1, in which the laser device has a tunable measuring light source and a separation device in the form of a free-jet circulator. In the following description of the drawings, identical reference numerals are used for identical or functionally equivalent components.

[0080] Fig. 1 shows the basic structure of a device 1 for determining the focus position FL of a processing laser beam 2 with respect to a surface 3a of a workpiece 3 to be processed based on a chromatic confocal measurement. The device 1 comprises a measuring light source 4, which is designed to emit measuring light 5a, 5b at two different measuring wavelengths Ai, A2. The measuring light source 4 is arranged together with a detector unit 6 in a housing of a laser device 7, which also serves to provide the processing laser beam 2. The measuring light 5a, 5b emitted by the measuring light source 4 is collimated by a collimation lens 8, strikes a separation device 9 in the form of a 50:50 beam splitter, and is deflected by this in the direction of a focusing lens 10.

[0081] By means of a coupling device (not shown), the measuring light 5a, 5b is coupled into the beam path of the processing laser beam 2. The measuring light 5a, 5b is coupled together with the processing laser beam 2 into a fiber optic cable 11, which serves as a beam guiding device 12 for guiding the measuring light 5a, 5b to a processing head 13. The processing head 13 has processing optics 14, which in the example shown comprises collimation optics 15, scanner optics 16 for two-dimensional beam deflection, and focusing optics 17. In the example shown, the collimation optics 15 are designed as a collimation lens, and the focusing optics 17 are designed as a focusing lens, each of which exhibits a chromatic aberration.

[0082] The processing laser beam 2 and the measuring light 5a, 5b are focused by the processing optics 14, more precisely by the focusing optics 17, onto the surface 3a of the workpiece 3. In the example shown, the focus of the processing laser beam 2 is located exactly on the surface 3a of the workpiece 3, i.e. the focus position FL of the processing laser beam 2, which designates the distance of the focus of the processing laser beam 2 from the surface 3a of the workpiece 3 along the optical axis 18 of the processing optics 14, is at FL = 0. Due to the chromatic aberration of the optics 15, 17 of the processing optics 14, the foci of the measuring light 5a, 5b focused on the surface 3a of the workpiece 2 are located above or below the surface 3a of the workpiece 3. The measuring light 5a', 5b' reflected from the surface 3a of the workpiece 3, which is shown in Fig. 1 with a dot-dash or dashed line, passes through the processing optics 14 in the opposite direction and is reflected behind or behind the surface 3a of the workpiece 3.focused in front of an entrance surface of the fiber optic cable 11. The fiber optic cable 11 or its entrance-side end therefore acts as an entrance aperture or spatial filter and allows mainly measuring light 5a, 5b or reflected light of the processing laser beam 2 to pass, the focus of which lies on the surface 3a of the workpiece 3.

[0083] The reflected measuring light 5a', 5b', or more precisely, a portion of the reflected measuring light 5a', 5b', passes through the fiber optic cable 11 of the beam guiding device 12 toward the laser device 7, enters the laser device 7, and is transmitted by the 50:50 beam splitter 9 to the detector unit 6. The detector unit 6 is designed to detect the intensity h, h of the reflected measuring light 5a', 5b' at the first measuring wavelength A1 and at the second measuring wavelength A2. An evaluation device 19 integrated into the laser device 7 serves to determine the focus position FL of the processing laser beam 2 based on the intensities h, h detected by the detector unit 6.

[0084] Fig. 2a shows an example of a device 1 which differs from the device 1 shown in Fig. 1, among other things, in that the measuring light 5a, 5b is fiber-guided in a sensor unit 20 which is arranged in the laser device 7. The laser device 7 also has a processing laser source 21 which is designed to emit the processing laser beam 2. In the example shown, the processing laser source 21 is a solid-state laser source which is designed to emit a processing laser beam 2 with a processing wavelength AB of 1030 nm. It is understood that the processing laser beam 2 can also have a different processing wavelength AB, for example 515 nm. The measuring wavelengths Ai, A2 are adapted to the processing wavelength AB.The processing laser beam 2 strikes a coupling device 22', which in the example shown is designed as a beam switch, which can be arranged in the beam path 2a of the processing laser beam 2 for processing the workpiece 3 and can be removed from the beam path 2a of the processing laser beam 2 for determining the focus position FL of the processing laser beam 2. For this purpose, the coupling device 22', which in the example shown is designed as a deflecting mirror, can be automatically pivoted from the first position shown in Fig. 2a, in which it is arranged in the beam path 2a of the processing laser beam 2, into a second position located next to the beam path 2a of the processing laser beam 2 and not illustrated in Fig. 2a, as indicated by an arrow.It is understood that the movement of the coupling device 22' in the form of the beam switch for the selective coupling of the processing laser beam 2 or the measuring light 5a, 5b into the beam guiding device 12 can also be effected in other ways. To prevent damage to the laser device 7 caused by the processing laser beam 2 possibly inadvertently escaping from the processing laser source 21, a beam trap 23 is arranged in the laser device 7 in the example shown in Fig. 2a.

[0085] In the example shown in Fig. 2a, the sensor unit 20 is configured to optionally emit a pilot laser beam instead of the measuring light 5a, 5b, which, when the beam switch 22' is in the second position, is coupled into the beam guiding device 12 instead of the measuring light 5a, 5b. The pilot laser beam has a wavelength in the visible wavelength range.

[0086] In the example shown in Fig. 2a, the measuring light source 4 has a first laser source 4a in the form of a first laser diode, which is designed to emit measuring light 5a at a first measuring wavelength Ai of 980 nm. The measuring light source 4 also has a second laser source 4b in the form of a second laser diode, which is designed to emit measuring light 5b at a second measuring wavelength A2 of 1064 nm. The first measuring wavelength Ai is shorter and the second measuring wavelength A2 is longer than the processing laser wavelength AB of the processing laser beam 2. This is advantageous for determining the focus position FL, as will be described in more detail below. The measuring light source 4 can have additional light sources in order to improve the accuracy of determining the focus position FL or to enlarge the measuring range.It is also possible for the measuring light source to comprise a light source in the form of another laser source whose measuring wavelength corresponds to the processing laser wavelength AB of the processing laser beam 2. The additional laser source can be used, for example, to calibrate the sensor unit 20.

[0087] The additional or alternative use of (pairs of) light sources with a somewhat broader spectral range than the light sources in the form of laser sources 4a, 4b is also possible to increase the measuring range at the expense of resolution. In particular, the pair of laser sources 4a, 4b shown in Fig. 2a can be supplemented by another pair of light sources with a somewhat broader spectral range to create two measuring ranges (fine measuring range and coarse measuring range).

[0088] It is also possible that at least two optical elements, for example the collimation optics 15 and the focusing optics 17 of the processing head 13, have a chromatic aberration with different Abbe numbers and are designed such that the focal plane of a light source 4a, 4b, ... with a narrowband spectrum, typically in the form of a laser source whose measuring wavelength Ai, A2, ... deviates from the processing laser wavelength AB, is placed on the focal plane of the processing laser beam 2 and coincides with the focal position FL of the processing laser beam 2.

[0089] The measurement light 5a, 5b with the two measurement wavelengths Ai, A2 is combined in Fig. 2a using a wavelength combiner 24 in the form of a wavelength multiplexer. The combined measurement light 5a, 5b passes through a first fiber 27a, which couples the combined measurement light 5a, 5b at a first port into a separation device in the form of a fiber circulator 9'. The measurement light 5a, 5b is coupled out of the sensor unit 20 via a second fiber 27b at a second port of the fiber circulator 9'. The second fiber 27b has an exit-side end with an end cap 28, the end face 28a of which forms the exit surface for the measuring light 5a, 5b and the entry surface for the useful signal in the form of the measuring light 5a, 5b reflected back from the surface 3a of the workpiece 3.A third port of the fiber circulator 9' is connected to a third fiber 27c, which feeds the useful signal in the form of the reflected measurement light 5a', 5b' to the detector unit 6, which in the example shown is designed as a silicon photodiode. The fiber circulator 9' serves as a separation device for separating the measurement light 5a, 5b emitted by the measurement light source 4 from the measurement light 5a', 5b' reflected back from the surface 3a of the workpiece 3.

[0090] Since, in the example shown in Fig. 2a, the exit and entry surfaces of the measuring light or the useful signal directly coincide at the end face 28a of the end cap 28, the sensor unit 20 is sensitive to back-reflected measuring light, which primarily occurs when the measuring light exits the end cap 28 of the second fiber 27b (glass-to-air transition). In the example shown, this is counteracted by providing the end cap 28 of the second fiber 27b, which serves as the exit or entry fiber, with an anti-reflective coating applied to the end face 28a of the end cap 28 to suppress the reflection of the exiting measuring light 5a, 5b at the transition from glass to air. The fiber optic cable 11, which serves as a beam guiding device 12, also has a first end cap 29a for the entry of the measuring light 5a, 5b into the laser device 7 and a second end cap 29b for the exit of the measuring light 5a, 5b into the processing head 13.The two end caps 29a, 29b can also have an anti-reflective coating in order to suppress unwanted back reflections of the processing laser beam 2, the measuring light 5a, 5b or the back-reflected measuring light 5a', 5b'.

[0091] The measuring light 5a, 5b is guided in a core 30 of the fiber optic cable 11, which is surrounded by a sheath 32 made of glass, in which no light is to be guided (cf. the cross-section of the fiber optic cable 11 shown in Fig. 2a). In the event that the fiber optic cable 11 is designed in the form of a hollow-core fiber, the measuring light 5a, 5b is guided in the hollow core 30 of the fiber optic cable 11. In this case, the provision of anti-reflective coatings or end caps on the fiber optic cable 11 can generally be dispensed with. It is possible for the measuring light 5a, 5b to be initially guided in a single-mode fiber and mixed in a mode mixer, which can be arranged, for example, downstream of the wavelength combiner 24, before or after the first fiber 27a. The measuring light 5a, 5b is homogenized after the mode mixer in a subsequent multimode fiber with the smallest possible core diameter (e.g. 50 pm), which can be the third fiber 27c, for example.In this way, after the measuring light 5a, 5b is coupled out of the sensor unit 20, a uniform intensity distribution can be generated in the optical fiber cable 11 of the beam guiding device 12, which in this case is designed as a multimode fiber.

[0092] As can also be seen in Fig. 2a, the laser device 7 also has a control device 25, which serves to control the measuring light source 4, or more precisely the two laser sources 4a, 4b. The control of the two laser sources 4a, 4b by means of the control device 25 takes place sequentially or in push-pull. In this way, the intensity h, h of the reflected measuring light 5a, 5b of one of the two laser sources 4a, 4b can be detected by the evaluation device 19 in a respective time interval of the clocked control. Fig. 2a shows the curve of the intensities h, h as a function of the focus position FL. As can be seen in Fig. 2a, the ratio h / h between the two intensities h, h depends on the focus position FL, which is why the focus position FL can be determined by the evaluation device 19 based on this ratio. The evaluation device 19 can be designed in the form of suitable hardware and / or software.

[0093] In the example shown, the detector unit 6 detects exclusively the reflected measuring light 5a', 5b' guided in the core 30 of the fiber optic cable 11, while the reflected measuring light guided in the cladding 32 or the useful light component guided in the cladding 32 is suppressed by means of etched cladding surfaces of the fiber optic cable 11 or by other measures. In the event that the fiber optic cable 11 has a light-guiding ring (see below), an additional entrance aperture can be provided in the sensor device 20 to suppress the back-reflected measuring light 5a', 5b' guided in the ring.

[0094] Fig. 2b shows a device 1 that differs from the device 1 shown in Fig. 2a in that the beam guiding device 12 is designed to guide the processing laser beam 2 and the measuring light 5a, 5b in free-beam propagation. In this case, an end of the second fiber 27b facing away from the fiber circulator 9' forms the entrance and exit aperture of the sensor unit 20. The processing head 13 has a deflection mirror 33 for deflecting the processing laser beam 2 and the measuring light 5a, 5b.

[0095] Fig. 3 shows a device 1 which differs from the device 1 shown in Fig. 2a essentially in the design of the separation device, which is not designed as a fiber circulator 9', but as a double-clad fiber coupler 9". The double-clad fiber coupler 9" is shown in a detailed illustration at the bottom edge of Fig. 3. The double-clad fiber coupler 9" comprises a double-clad fiber 27a, which has a core 30 for guiding the measuring light 5a, 5b emitted by the measuring light source 4 and combined in the wavelength combiner 24. The core 30 of the double-clad fiber 27a is designed as a single-mode fiber. The double-clad fiber 27a also has a ring 31 for guiding the measuring light 5a', 5b' reflected by the workpiece 3, which ring 31 is designed as a multi-mode fiber and is surrounded by a cladding 32. In the embodiment shown in Fig.In the device 1 shown in Fig. 3, the beam guiding device 12 has a light guide cable 11 in the form of a double-clad fiber, in whose core 30 the measuring light 5a, 5b is guided and in whose ring 31 the measuring light 5a', 5b' reflected back from the surface 3a of the workpiece 3 is guided to the laser device 7.

[0096] The double-clad fiber coupler 9" serves to separate the single-mode measurement light guided in the core 30 of the double-clad fiber 27a from the multi-mode useful light guided in the ring of the double-clad fiber 27a. For this purpose, the double-clad fiber 27a is guided in a coupling section in the double-clad fiber coupler 9" parallel to and adjacent to another fiber 27b, which has a core 30' surrounded by a cladding 32' (without a light-guiding ring). In the coupling section, a portion of the useful radiation from the ring 31 of the double-clad fiber 27a is coupled into the core 30' of the additional fiber 27b and fed to the detector unit 6 at a first end of the additional fiber 27b. At a second end of the further fiber 27b, a beam trap 34 is attached to absorb measuring light 5a, 5b coupled from the double-clad fiber 27a into the further fiber 27b.In this case, the detector unit 6 serves to detect the reflected measuring light 5a', 5b', which is guided in the ring 31 of the optical fiber cable 11 of the beam guiding device 12, which is designed as a double-clad fiber.

[0097] In principle, it is possible for the detector unit 6 to separately detect the measuring light 5a', 5b' guided in the core 30, the ring 31, and / or the sheath 32 of the fiber optic cable 11 and reflected back from the surface 3a of the workpiece 3. This allows a two-stage measuring process in which the evaluation of the useful light originating from the sheath 32 and / or the ring 31 is used for the inaccurate determination of the focus position FL with an enlarged measuring range, and the sole evaluation of the useful light component originating from the core 30 of the fiber optic cable 11 is used for the precise determination of the focus position FL with a small measuring range. It is also possible to detect only the useful light originating from the sheath 32 and / or the ring 31 of the fiber optic cable 11 while completely omitting the detection of the useful light component originating from the core 30 of the fiber optic cable 11.

[0098] The device 1 shown in Fig. 3 has a coupling device 22 in the form of a dichroic beam splitter mirror, which serves to couple the measuring light 5a, 5b of the measuring light source 4 into the beam path 2a of the processing laser beam 2 and to couple the measuring light 5a', 5b' reflected back from the surface 3a of the workpiece 3 out of the beam path 2a of the processing laser beam 2. Instead of the dichroic beam splitter mirror, the coupling device 22 can also be designed in the form of a scraper mirror, a perforated mirror, or the like.

[0099] Fig. 4 shows an example of the device 1, which differs from the device 1 shown in Fig. 3 essentially in the design of the sensor unit 20. The sensor unit 20 has a measuring light source 4 in the form of a broadband light source, which in the example shown is designed as a superluminescence diode. The sensor unit 20 is designed to guide measuring light 5, which is emitted by the measuring light source 4, in free beam propagation. As a separation device 9 for separating the measuring light 5, which propagates in the direction of the workpiece 3, from the measuring light 5' reflected back at the surface 3a of the workpiece 3, a non-polarizing 50:50 beam splitter serves, as in Fig. 1. The sensor unit 20 also has a detector unit 6 in the form of a spectrometer.

[0100] In the sensor unit 20 shown in Fig. 4, the measuring light 5' reflected back from the surface 3a of the workpiece 3 has a larger beam diameter than the measuring light 5 emitted by the measuring light source 4. The detector unit 6 is designed to detect the reflected measuring light 5' with the larger beam diameter. By using such a detector unit 6, it is possible to detect the reflected measuring light 5' even with fiber optic cables 11 used in the beam guiding device 12 that differ in the diameter of the core 30. In particular, even with a beam guiding device 12 with a fiber optic cable 11 that has a core 30 with a large diameter of e.g. 400 pm, the useful signal of the entire cross-section of the fiber optic cable 11 can be made available to the detector unit 6 for evaluation.

[0101] The diagrams shown in Fig. 4 below the illustration of the laser device 7 or the sensor unit 20 show the qualitative course of the spectrum l(A) of the chromatic confocal response of the measuring light reflected back from the surface 3a of the workpiece 3 at different focus positions FL (from left to right: The workpiece 3 is removed from the processing head 13, starting in front of a focus position FL, which is located inside the workpiece 3, via the focus position FL = 0, until the focus position FL of the processing laser beam 2 is located in front of the surface 3a of the workpiece 3. Close to the focus position FL = 0 (cf. the middle three diagrams), the exact focus position FL can be determined from the partial or entire course of the spectral response or from a ratio of the intensities h, h of at least two measuring wavelengths Ai, A2.In the example shown, the two measuring wavelengths Ai, A2 have the same difference wavelength AA to the processing laser wavelength AB of the processing laser beam 2, and these are the measuring wavelengths Ai, A2 at which the spectral response exhibits a maximum. For a ratio h / 11 = 1, assuming wavelength-independent transmission or reflection, the focal position FL = 0 applies (see the middle of the five diagrams). Far outside the focal position FL = 0 (see the first and fifth diagrams), the maximum l ma x of the spectrum can be used to determine the focus position FL.

[0102] The device 1 shown in Fig. 5 differs from the device 1 shown in Fig. 4, among other things, in that the measuring light source 4 is tunable and, in the example shown, is designed as a tunable laser source. Furthermore, the sensor device 20 has a separation device 9'" designed as a free-jet circulator. The free-jet circulator 9'" comprises a Faraday rotator 36, an λ / 2 plate 37, two birefringent crystals 38a, b, a deflection prism 39, and a polarization beam splitter 40.

[0103] To determine the focus position FL, in the device 1 shown in Fig. 5, the measurement wavelength A of the measurement light source 4 is tuned step by step, e.g., in steps of 2 nm. With the aid of a detector unit 6, which is designed as a silicon photodiode, a time-dependent intensity profile l(t) of the reflected measurement light 5' is recorded, to which a wavelength-dependent intensity distribution l(A) corresponds. In the evaluation device 19, the maximum wavelength Amax is determined, at which the wavelength-dependent intensity distribution l(A) has its maximum. The focus position FL of the processing laser beam 2 can be determined based on the maximum wavelength Amax or the distance of the maximum wavelength Amax from the processing laser wavelength AB.

[0104] In the device 1 described above, the measuring light source 4 can be designed to emit pulsed measuring light 5a, 5b, 5 with pulse lengths, for example, in the range from ps to ns. The detector unit 6 can in this case be designed, for example, as a single-photon avalanche diode, for example in Si or InGaAs. The evaluation device 19 can be designed for high-resolution dTOF (direct Time Of Flight) evaluation, for example by means of a multi-hit TDC (Time to Digital Converter) or a high-speed ADC (e.g. with 10 GS / sec). This allows, for example, the separation of reflection points occurring at different locations in the beam path 2a of the processing laser beam 2 and enables the selectivity between the useful signal originating from the workpiece 3 and interfering reflections to be increased.

Claims

Patent claims 1 . Device (1 ) for determining the focus position (FL) of a processing laser beam (2) with respect to a surface (3a) of a workpiece (3), comprising: a measuring light source (4) which is designed to emit measuring light (5a, 5b, 5) at at least two different measuring wavelengths (Ai , A2;A), a processing head (13) for focusing the measuring light (5a, 5b, 5) onto the workpiece (3), a beam guiding device (12) for guiding the measuring light (5a, 5b, 5) to the processing head (13), at least one optical element (15, 17) with chromatic aberration, through which the measuring light (5a, 5b, 5) passes and which is preferably arranged in the processing head (13), a detector unit (6) for detecting the intensity (h, h, I) of the measuring light (5a', 5b', 5') reflected back from the surface (3a) of the workpiece (3), and an evaluation device (19) for determining the focus position (FL) of the processing laser beam (2) with respect to the surface (3a) of the workpiece (3) on the basis of the intensity (h, h, I) detected by the detector unit (6), thereby characterized in that the beam guiding device (12) is designed for the joint beam guidance of the processing laser beam (2) and the measuring light (5a, 5b, 5) to the processing head (13); 2. Device according to claim 1, in which the measuring light source (4) and preferably the detector unit (6) are integrated into a laser device (7) for providing the processing laser beam (2), which preferably has a processing laser source (21) for emitting the processing laser beam (2), and in which the beam guiding device (12) is designed for the joint beam guidance of the processing laser beam (2) and the measuring light (5a, 5b) from the laser device (7) to the processing head (13).

3. Device according to claim 1 or 2, wherein the beam guiding device (12) has a fiber optic cable (11) for the joint beam guidance of the processing laser beam (2) and the measuring light (5a, 5b, 5) to the processing head (13).

4. Device according to claim 3, wherein the optical fiber cable (11) is designed as a hollow-core fiber or as a multiple-clad fiber, in particular as a double-clad fiber.

5. Device according to claim 3 or 4, wherein the detector unit (6) is designed to separately detect the intensity (h, h, I) of the measuring light (5a', 5b', 5') reflected back from the surface (3a) of the workpiece (3) in a core (30), in a jacket (32) and / or in a ring (31) of the optical fiber cable (11).

6. Device according to claim 1 or 2, wherein the beam guiding device (12) is designed for the joint beam guidance of the processing laser beam (2) and the measuring light (5a, 5b, 5) in free beam propagation.

7. Device according to one of the preceding claims, further comprising: a coupling device (22) for coupling the measuring light (5a, 5b, 5) of the measuring light source (4) into the beam path (2a) of the processing laser beam (2) and for coupling out the measuring light (5a', 5b', 5') reflected back from the surface (3a) of the workpiece (3) from the beam path (2a) of the processing laser beam (2), wherein the coupling device (22) is preferably arranged in the laser device (7).

8. Device according to one of claims 1 to 6, further comprising: a coupling device (22') in the form of a beam switch, which can be arranged in the beam path (2a) of the processing laser beam (2) for selectively coupling the processing laser beam (2) or the measuring light (5a, 5b, 5) into the beam guiding device (12) and can be removed from the beam path (2a) of the processing laser beam (2), wherein the coupling device (22') is preferably arranged in the laser device (7).

9. Device according to one of the preceding claims, further comprising: a separation device (9, 9', 9", 9'") for separating the measuring light (5a, 5b, 5) emitted by the measuring light source (4) from the measuring light (5a', 5b', 5') reflected back from the surface (3a) of the workpiece (3), which is preferably arranged in a sensor unit (20) of the laser device (7).

10. Device according to claim 9, wherein the separation device is designed as a circulator in the form of a fiber circulator (9') or a free jet circulator (9'').

11. Device according to claim 9, wherein the separation device is designed as a double-clad fiber coupler (9"), which has a double-clad fiber (27) with a core (30) for guiding the measuring light (5a, 5b, 5') emitted by the measuring light source (4), which is preferably designed as a single-mode fiber, and a ring (31) for guiding the measuring light (5a', 5b', 5') reflected back from the surface (3a) of the workpiece (3), which ring is preferably designed as a multi-mode fiber.

12. Device according to one of the preceding claims, in which the measuring light source (4) has a first light source, in particular a first laser source (4a), for emitting measuring light (5a) at a first measuring wavelength (Ai) and a second light source, in particular a second laser source (5b), for emitting measuring light (5b) at a second measuring wavelength (A2).

13. Device according to claim 12, wherein the first and / or the second measuring wavelength (Ai, A2) deviate from a processing laser wavelength (AB) of the processing laser beam (2).

14. Device according to claim 12 or 13, wherein the measuring light source (4) has a further light source whose measuring wavelength corresponds to the processing laser wavelength (AB).

15. Device according to one of claims 1 to 11, wherein the measuring light source (4) is designed as a broadband light source or as a tunable light source.

16. Device according to one of the preceding claims, which comprises at least two optical elements (15, 17) having a chromatic aberration with different Abbe numbers.

17. Device according to one of the preceding claims, wherein the detector unit (6) is designed as a spectrometer.

18. Device according to one of the preceding claims, in which the measuring light source (4) is designed for pulsed emission of the measuring light (5a, 5b, 5), wherein preferably the detector unit (6) and the evaluation device (19) are designed to distinguish measuring light (5a', 5b', 5') reflected back from the surface (3a) of the workpiece (3) from measuring light that is reflected back from other locations.

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