Methods and laser processing devices for reducing the impact of thermal lens effects during laser material processing

Abstract

The present invention relates to methods and laser processing devices for reducing the impact of thermal lens effects during laser material processing. In this case, an amplitude mask or phase mask (8) is inserted in the beam path of the processing laser (1) or of an additional measurement laser (13), said mask, in the focal plane in the region of the processing plane (6), generating a characteristic pattern which characteristically changes outside the focal plane. The pattern is detected by a camera (9) in the processing plane (6) during the laser processing and, on the basis of a change in the pattern, an actuating element is activated which activates optical components (5) for shifting the focus position in such a manner that a focus shift, generated by thermal lens effects, in the processing plane (6) is compensated for. Alternatively, the actuating element can also correspondingly change the distance to the processing plane (6). The methods and laser processing devices permit direct measurement of the focus shift in the processing plane and reliable compensation for said focus shift during laser material processing.

Description

[0001] Methods and laser processing equipment for reducing the effects of thermal lensing during laser material processing

[0002] Technical application area

[0003] The present invention relates to methods and laser processing devices for reducing the effects of thermal lens effects during laser material processing, which lead to a shift of the focal plane of the processing laser beam from the processing plane, wherein the shift of the focal plane from the processing plane is compensated by adjusting one or more optical components in the beam path of the processing laser beam, by adjusting which the focal plane of the processing laser beam can be shifted relative to the processing plane, or by changing the distance to the processing plane during laser material processing via an actuator.

[0004] When processing with laser radiation, laser powers of several kW are often used. Optical lenses and other optical elements that guide and / or shape the radiation are transparent to the laser light and absorb only a very small portion of the laser power. However, even this small proportion is often sufficient to heat up the lenses or optical elements. This leads to the so-called thermal lens effect. The low thermal conductivity of optical materials causes a strong temperature gradient. The resulting refractive index gradient and the thermal deformation of the lens surfaces also contribute to the refractive power of the lens. This changes the focal length of the optical system, which defocuses the radiation during processing and thus reduces the intensity of the laser radiation on the workpiece.This can reduce processing quality or even require the interruption of the processing process. Other optical elements, such as protective glass or diffractive optical elements, in the beam path of the processing laser beam can also contribute to the thermal lensing effect. Since the change in focal length occurs during and dependent on the processing process, and therefore usually also varies during the processing process, it has not been possible to measure and compensate for the thermal focus shift during the processing process directly in the processing plane.

[0005] State of the art

[0006] A. Gatej et al., “Methods for compensation of thermal lensing based on thermo-optical (TOP) analysis”, in Optical Modelling and Design III, F. Wyrowski, JT Sheridan, J. Tervo, Y. Meuret, eds. (SPIE, 2014), 91310F, describes a method for determining the focus shift before the process starts by measuring the temporal maximum intensity using a CCD camera. Blomster et al., “Optics performance at high-power levels”, in Solid State Lasers XVII: Technology and Devices, WA Clarkson, N. Hodgson, RK Shori, eds. (SPIE, 2008), 68712B measure the beam caustics using beam profile measurements. Based on the results of the characterization carried out, the focus position is then adjusted later in the machining process within the framework of a predictive control, as described, for example, in 0. Pütsch et al., "Real-time capable laser beam control for compensating thermal effects", DGaO-Proceedings 2011.With these known techniques, the optical system is characterized with respect to thermal effects prior to the process. However, this pre-process characterization cannot adequately capture time-varying aspects such as laser power fluctuations, temperature fluctuations in the test environment, or contamination of the optics, which gradually increases with the process duration and also leads to greater thermal effects.

[0007] Furthermore, it is known that thermal effects can be observed during laser material processing due to the partial coupling of radiation from the beam path to the processing plane. The thermal displacement of the coupled-out radiation can now be measured separately. However, since the beam path and thus the number of irradiated or irradiated optical elements have changed due to the coupling, a precise statement about the thermal effects for the entire beam path up to the processing plane is not possible.

[0008] As an alternative to experimental characterization of the optical system, thermal simulations are also performed prior to the process. Predictive control is also possible based on the simulated thermal shifts. However, the precision of the prediction is limited by the accuracy of the simulation.

[0009] Instead of active compensation, passive compensation options are also known. In this case, the optical system is designed with optical elements whose thermal effects compensate each other through the appropriate choice of material. Such optical elements are commercially available.

[0010] WO 2012 / 041 351 A1 describes a device for laser material processing in which a radiation pattern of electromagnetic radiation is generated on a partially reflecting surface in the beam path in front of the focus of the focused laser radiation and an image of the pattern is captured and processed in order to determine the focus position and to change it if necessary.

[0011] DE 20 2005 010 715 Ul discloses an adjustment device for a flying laser welding head, whose distance from the workpiece is programmable for defined focusing of the laser beam using a visible alignment laser beam. To determine the exact focusing, the distance, shape, and brightness of alignment laser spots generated on the workpiece surface are recorded with an observation camera.

[0012] From DE 10 2011 054 941 B3 a device and a method for correcting the thermal shift of the focus position of a processing laser beam are known, in which the current focus position is determined with a focus sensor or a wavefront sensor.

[0013] JP 2007-253 200 A describes a method for determining the correct focusing position of a processing laser beam in a laser welding device. For this purpose, two pilot laser beams of visible light are guided to the workpiece such that their points of impact overlap when the processing laser beam is correctly focused on the workpiece surface. If the focus is incorrect, the points of impact are separated from each other.

[0014] The object of the present invention is to provide a method and a device for reducing the effects of thermal lens effects in laser material processing, with which a focus shift in the processing plane caused by thermal lens effects can be directly detected and compensated by controlling an actuator.

[0015] Description of the invention

[0016] The object is achieved by the methods and the associated laser processing devices of patent claims 1, 2, 9 and 10. Advantageous embodiments of the methods and the laser processing devices are the subject of the dependent patent claims or can be derived from the following description and the exemplary embodiments.

[0017] The two proposed process alternatives and associated laser processing devices differ in the number of lasers used. In the first process alternative, only the processing laser is used, while in the second process alternative, a measuring laser is also used. In both processes and associated laser processing devices, the shift in the focal plane due to thermal lens effects is compensated for in one embodiment by adjusting one or more optical components in the beam path of the processing laser beam during laser material processing via an actuator, by adjusting which the focal plane of the processing laser beam can be shifted relative to the processing plane. This can, for example, be a movable F-theta lens or a dynamic focusing unit as a component of the optical system for laser material processing.In another embodiment, compensation is achieved by changing the distance to the processing plane during laser material processing via an actuator. The change in distance can be achieved by adjusting the processing plane, i.e., the workpiece currently being processed, by adjusting the entire laser processing head, or simply by adjusting (in the sense of changing the distance to the processing plane) the optical focusing arrangement.

[0018] The first process alternative is characterized by the fact that an amplitude or phase mask is used in the beam path of the processing laser beam, which creates a pattern in the focal plane of the processing laser beam that is characteristically deformed outside the focal plane. The pattern created by the amplitude or phase mask in the processing plane is recorded during laser material processing, and any deformation of the pattern in the processing plane is counteracted by adjusting one or more optical components or the distance to the processing plane via the actuator. The pattern can be recorded continuously or quasi-continuously, i.e. as individual measurements at short time intervals.

[0019] The second method alternative of the proposed method is characterized in that a measuring laser beam is coupled into the beam path of the processing laser beam up to the processing plane, which measuring laser beam passes through the one or more optical components for compensating the shift of the focal plane (if provided in the corresponding embodiment) and all optical elements in the beam path of the processing laser beam that cause thermal lens effects during laser material processing, and thus has the same focal plane as the processing laser beam. In the beam path of the measuring laser beam, before it is coupled into the beam path of the processing laser beam, an amplitude or phase mask is used, which generates a pattern in the focal plane of the measuring laser beam that is characteristically deformed outside the focal plane.As with the first alternative method, the pattern generated by the amplitude or phase mask in the processing plane is recorded continuously or quasi-continuously during laser material processing, and deformation of the pattern in the processing plane is counteracted by adjusting one or more optical components or the distance to the processing plane via the actuator. The measuring laser beam preferably has a different wavelength than the processing laser beam, so that coupling into the beam path of the processing laser beam can occur via a dichroic mirror.

[0020] In both process alternatives, a pattern generated by the amplitude or phase mask in the processing plane is recorded during laser processing and, based on a change in the pattern, a focus shift is inferred. This shift is then compensated for by suitable control of the actuator, in particular by displacement of one or more optical components, for example a lens of the optical system. The adjustable optical component(s) can also be one or more liquid lenses whose focal length(s) is / are changed via the actuator, or one or more adjustable mirrors. The adjustment of the one or more optical components (or the change in the distance to the processing plane) to compensate for the focus shift is preferably carried out by control of the actuator via a control system.In the proposed methods and associated laser processing devices, the actuator itself is preferably formed by one or more actuators, for example piezoelectric or electromechanical actuators.

[0021] By using the phase or amplitude mask, which creates a characteristic pattern in the focal plane that undergoes characteristic deformation outside the focal plane, a shift in the focal plane during the machining process can be observed and measured. This provides a direct measurement method with which the influence of thermal lens effects can be compensated for using the actuator that adjusts a variable focusing unit, for example. This direct measurement in the machining plane automatically takes into account time-varying aspects such as fluctuations in laser power, temperature fluctuations in the test environment, or changes in thermal effects caused by gradually increasing contamination of the optics. This also takes into account the influence of the protective glass and the process gas on the thermal lens effects.The focus shift can therefore be detected very reliably and compensated during the entire processing process.

[0022] Preferably, the methods and the associated laser processing devices utilize an amplitude or phase mask, in which the distance to the focal plane can be determined from the deformation of the pattern. This information can then be used to select a higher adjustment speed, for example, of the optical component(s) at a greater distance than at a shorter distance from the focal plane, thereby accelerating the control process.

[0023] A so-called Bahtinov mask is particularly preferred as the amplitude mask. The pattern generated by the Bahtinov mask in the focal plane represents the Fourier transform of the Bahtinov mask. If this pattern is viewed outside the focal plane, it deforms in a characteristic way, allowing the distance to the focal plane to be determined.

[0024] During laser material processing, the pattern in the processing plane is preferably observed using a so-called coaxial observation system. Coaxial observation systems share the beam path with the processing laser and are therefore always aligned to the interaction area of ​​the laser beam with the workpiece. If the image or pattern resulting from the introduction of the amplitude or phase mask in the observation plane is observed with a coaxially aligned camera, the shift in the focal plane during the process can be optimally observed and measured or used to control the actuator. The associated laser processing devices accordingly have at least one processing laser, an optical arrangement for focusing a processing laser beam from the processing laser into a processing plane and, if necessary, for guiding it across the processing plane.In one embodiment, the optical arrangement comprises one or more optical components in the beam path of the processing laser beam, which can be adjusted via an actuator of the laser processing device such that they shift the focal plane of the processing laser beam relative to the processing plane. In the other embodiment, the distance of the optical arrangement can be adjusted via an actuator of the laser processing device. In the first alternative of the laser processing device, an amplitude or phase mask is arranged in the beam path of the processing laser beam, which generates a pattern in the focal plane of the processing laser beam that is characteristically deformed outside the focal plane.In the second alternative, the laser processing device additionally has a measuring laser and at least one coupling element which couples the measuring laser beam into the beam path of the processing laser beam in such a way that the measuring laser beam passes through the one or more optical components adjustable via the actuator (if provided in the corresponding embodiment) and all optical elements in the beam path of the processing laser beam which cause thermal lens effects during laser material processing, and thereby has the same focal plane as the processing laser beam. An amplitude or phase mask is arranged in the beam path of the measuring laser beam before it is coupled into the beam path of the processing laser beam, which pattern generates a characteristic deformation outside the focal plane.In this alternative, the processing laser beam does not pass through this amplitude or phase mask. In both alternatives, the laser processing device comprises a camera with which the pattern generated by the amplitude or phase mask in the processing plane is captured during laser material processing, as well as a control device that counteracts deformation of the pattern in the processing plane by adjusting one or more optical components or changing the distance to the processing plane via the actuator during laser material processing.

[0025] The proposed methods and associated laser processing equipment offer particular advantages for laser processing processes where higher powers (>10W) are used and where thermal lensing effects are therefore to be expected.

[0026] Short description of the drawings

[0027] The proposed methods and associated laser processing equipment are explained in more detail below using exemplary embodiments in conjunction with the drawings. Herein:

[0028] Fig. 1 shows a first example of the proposed laser processing device;

[0029] Fig. 2 shows a second example of the proposed laser processing device;

[0030] Fig . 3 a third example of the proposed

[0031] Laser processing device; and Fig. 4 shows an example of a characteristic pattern as generated with a Bahtinov mask in and out of the focal plane.

[0032] Ways to implement the invention

[0033] Figure 1 shows a first example of the proposed laser processing device in a schematic representation. In this example, a processing laser 1 with a pilot laser is used. The processing laser radiation can, for example, be at a wavelength of 1064 nm, the wavelength of the pilot laser at 632 nm. The pilot laser serves as a measuring laser for detecting the focus shift. In this example, the processing laser 1 emits both the processing laser beam 2 and the measuring laser beam 3, which is first coupled out of the beam path of the processing laser beam via a dichroic mirror 7 and then coupled back in via a further dichroic mirror 7, as shown in Figure 1.In the decoupled section, the measuring laser beam 3 passes through an amplitude or phase mask 8, by means of which a pattern is generated in its focal plane in the region of the processing plane 6 of the laser processing device, which pattern changes characteristically outside the focal plane. This pattern is recorded and evaluated by the camera 9 with a lens coaxial to the processing and measuring laser beams and used to control an actuator for adjusting the focus position of the processing laser beam 2 in the processing plane 6. In this example, the actuator is only indicated by the double arrow on the focusing lens 5, which in this example is mounted so that it can be adjusted accordingly. This is - as in the following figures - only a simplified representation of the optical system of the laser processing device, which can also comprise several lenses.During processing, the processing and measuring laser beams are guided over the processing plane 6 by means of a scanner 4. In this example, the radiation of the measuring laser beam 3 is first linearly polarized using a linear polarizing filter 10. This ensures that light is not deflected onto the camera 9 on the way to the focal plane, since the polarizing beam splitter cube 11 only allows light polarized perpendicular to it to pass through. Radiation that is instead propagated to the focal plane and back again passes twice through an λ / 4 plate 12. This allows the polarization to be rotated and the beam splitter cube 11 to be passed in the direction of the camera 9. In general, the polarization is not retained during reflection at the processing plane. This leads to a weakening of the intensity observed at the camera 9.

[0034] Figure 2 shows a further example of a laser processing device according to the present invention, in which a separately arranged measuring laser 13 is used in addition to the processing laser 1. The beam paths of the measuring laser beam 3 and the processing laser beam 2, which here again have different wavelengths, are coaxially superimposed via a dichroic mirror 7 and focused into the processing plane 6 via the scanner 4 and the focusing lens 5. Here too, an amplitude or phase mask 8 is arranged in the beam path of the measuring laser beam 3 before it is coupled into the beam path of the processing laser beam 2, as in Figure 1, which amplitude or phase mask 8 generates a pattern in the focal plane of the measuring laser beam 3 in the region of the processing plane 6, said pattern changing characteristically outside the focal plane.This pattern is again captured by camera 9 with a lens coaxial with the processing and measuring laser beams and used to control an actuator for adjusting lens 5 and thus compensating for the focus shift. In this example, the measuring laser beam 3 is again initially linearly polarized via the linear polarizing filter 10, as already described in connection with Figure 1.

[0035] 1 . This also applies to the arrangement of the X / 4 plate 12 and the polarizing beam splitter cube 11 in front of the camera 9 .

[0036] Figure 3 shows another example of the proposed laser processing device, in which only the processing laser 1 is used. An amplitude or phase mask 8 is inserted into the beam path of the processing laser 1, which creates a pattern in the focal plane of the processing laser beam 2 in the region of the processing plane 6, which pattern changes characteristically outside the focal plane. This pattern is applied coaxially to the processing laser beam.

[0037] 2 is captured by the camera 9 with objective via a partially transparent mirror 14 and used to control the actuator for adjusting the focusing lens 5 to compensate for the focus shift. In order to avoid excessive attenuation of the processing laser beam 2 by the amplitude or phase mask 8, the latter is designed such that it only uses a small proportion of the intensity of the processing laser beam 2 to generate the pattern in the focal plane. In the lower left part of Figure 3, an example of a top view of the amplitude or phase mask is shown. Only the marked area in the upper right corner of this cross section generates the characteristic pattern. The remaining area does not influence the processing laser beam 2. Therefore, hardly any energy is withdrawn from the process.In order to capture the resulting very faint pattern with camera 9, in this example, an aperture 15 is used on camera 9, as can be seen in the cross-section in the lower right-hand part of Figure 3. The center of aperture 15 blocks the portions of processing laser beam 2 that were not influenced by amplitude or phase mask 8 and thus do not contribute to the characteristic pattern. This allows the pattern to be clearly captured with camera 9.

[0038] Finally, Figure 4 shows, as an example, in sub-image A), a pattern created with a Bahtinov mask in the focal plane. The lines of this pattern intersect at a point in the center. Outside the focal plane, these lines shift relative to each other, so that the camera captures, for example, a pattern as can be seen in sub-image B) of Figure 4.

[0039] Reference symbol list

[0040] 1 processing laser

[0041] 2 processing laser beam

[0042] 3 measuring laser beam

[0043] 4 scanners

[0044] 5 Focusing lens

[0045] 6 processing level

[0046] 7 dichroic mirrors

[0047] 8 Amplitude or phase mask

[0048] 9 Camera with lens

[0049] 10 linear polarizing filters

[0050] 11 polarizing beam splitter cube

[0051] 12 x 4-plate

[0052] 13 measuring lasers

[0053] 14 partially transparent mirror

[0054] 15 aperture

Claims

Patent claims 1. Method for reducing the effects of thermal lens effects in laser material processing, which lead to a shift of a focal plane of a processing laser beam (2) from a processing plane (6), in which - the displacement of the focal plane from the processing plane (6) is compensated by adjusting one or more optical components (5) in the beam path of the processing laser beam (2), by adjusting which the focal plane of the processing laser beam (2) can be displaced relative to the processing plane (6), or by changing a distance to the processing plane (6) during laser material processing via an actuator, characterized in that - an amplitude or phase mask (8) is inserted in the beam path of the processing laser beam (2), which creates a pattern in the focal plane of the processing laser beam (2) which is characteristically deformed outside the focal plane, - the pattern generated by the amplitude or phase mask (8) in the processing plane (6) is recorded during laser material processing, and - a deformation of the pattern in the processing plane (6) is counteracted by adjusting the one or more optical components (5) or changing the distance to the processing plane (6) via the actuator.

2. Method for reducing the effects of thermal lens effects in laser material processing, which lead to a shift of a focal plane of a processing laser beam (2) from a processing plane (6), in which - the displacement of the focal plane from the processing plane (6) in a first alternative by adjusting one or more optical components (5) in the beam path of the processing laser beam (2), by adjusting which the focal plane of the processing laser beam (2) can be displaced relative to the processing plane (6), or in a second alternative by changing a distance to the processing plane (6) during laser material processing via an actuator, characterized in that - a measuring laser beam (3) is coupled into the beam path of the processing laser beam (2), which measuring laser beam (3) passes through all optical elements in the beam path of the processing laser beam (2) which cause thermal lens effects during laser material processing, and in the first alternative additionally passes through the one or more optical components (5) for compensating the displacement of the focal plane and thereby has the same focal plane as the processing laser beam (2), - an amplitude or phase mask (8) is inserted in the beam path of the measuring laser beam (3) before coupling into the beam path of the processing laser beam (2), which is in the focal plane of the measuring laser beam (3) creates a pattern that is characteristically deformed outside the focal plane, - the pattern generated by the amplitude or phase mask (8) in the processing plane (6) is recorded during laser material processing, and - a deformation of the pattern in the processing plane (6) is counteracted by adjusting the one or more optical components (5) or changing the distance to the processing plane (6) via the actuator.

3. Method according to claim 2, characterized in that the measuring laser beam (3) has a different wavelength than the processing laser beam (2).

4. Method according to one of claims 1 to 3, characterized in that an amplitude or phase mask (8) is used, in which a distance to the focal plane can be determined from the deformation of the pattern.

5. Method according to one of claims 1 to 4, characterized in that a Bahtinov mask is used as the amplitude mask (8).

6. Method according to one of claims 1 to 5, characterized in that the adjustment of the one or more optical components (5) or change in the distance to the processing plane (6) is carried out by controlling the actuator via a control system.

7. Method according to one of claims 1 to 6, characterized in that the pattern produced by the amplitude or phase mask (8) in the processing plane (6) is recorded coaxially to the processing laser beam (2) with a camera (9).

8. Method according to one of claims 1 to 7, characterized in that an amplitude or phase mask (8) is used which attenuates the processing laser beam (2) by less than 0.5% as it passes through.

9. Laser processing device with at least - a processing laser (1) , - an optical arrangement for focusing a processing laser beam (2) of the processing laser (1) into a processing plane (6), optionally also for guiding it over the processing plane (6), wherein either the optical arrangement has one or more optical components (5) in the beam path of the processing laser beam (2), which are adjustable via an actuator of the laser processing device such that they shift a focal plane of the processing laser beam (2) relative to the processing plane (6), or a distance of the optical arrangement from the processing plane (6) is adjustable via an actuator of the laser processing device, - an amplitude or phase mask (8) in the beam path of the processing laser beam (2) which generates a pattern in the focal plane of the processing laser beam (2) which is characteristically deformed outside the focal plane, - a camera (9) with which the pattern generated by the amplitude or phase mask (8) in the processing plane (6) during laser material processing, and - a control device which counteracts a deformation of the pattern in the processing plane (6) by adjusting the one or more optical components (5) or by changing the distance of the optical arrangement from the processing plane (6) via the actuator during laser material processing.

10. Laser processing device with at least - a processing laser (1) , - an optical arrangement for focusing a processing laser beam (2) of the processing laser (1) into a processing plane (6), optionally also for guiding it over the processing plane (6), wherein either the optical arrangement in a first alternative has one or more optical components (5) in the beam path of the processing laser beam (2), which are adjustable via an actuator of the laser processing device such that they shift a focal plane of the processing laser beam (2) relative to the processing plane (6), or in a second alternative a distance of the optical arrangement to the processing plane (6) is adjustable via an actuator of the laser processing device, - a measuring laser (13), - a coupling element (7) which couples a measuring laser beam (3) of the measuring laser (13) into the beam path of the processing laser beam (2) in such a way that the measuring laser beam (3) contacts all optical elements in the beam path of the processing laser beam (2) which, during the laser material processing, produce thermal lens effects cause, and in the first alternative additionally passes through the one or more optical components (5) to compensate for the shift of the focal plane and thereby has the same focal plane as the processing laser beam (2), - an amplitude or phase mask (8) in the beam path of the measuring laser beam (3) before coupling into the beam path of the processing laser beam (2), which creates a pattern in the focal plane of the measuring laser beam (3) which is characteristically deformed outside the focal plane, - a camera (9) with which the pattern generated by the amplitude or phase mask (8) in the processing plane (6) can be captured during laser material processing, and - a control device which counteracts a deformation of the pattern in the processing plane (6) by adjusting the one or more optical components (5) or by changing the distance of the optical arrangement from the processing plane (6) via the actuator during laser material processing.

11. Laser processing device according to claim 10, characterized in that the measuring laser (13) has a different wavelength than the processing laser (1).

12. Laser processing device according to one of claims 9 to 11, characterized in that the amplitude or phase mask (8) is selected so that from the deformation of the pattern a distance to the focal plane can be determined.

13. Laser processing device according to one of claims 9 to 12, characterized in that the amplitude mask (8) is a Bahtinov mask.

14. Laser processing device according to one of claims 9 to 13, characterized in that the pattern generated by the amplitude or phase mask (8) in the processing plane (6) is captured by the camera (9) coaxially to the processing laser beam (2).

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