DE102022101043B3 - Methods and laser processing equipment for reducing the effects of thermal lens effects in laser material processing - Google Patents

Abstract

The present invention relates to methods and laser processing devices for reducing the effects of thermal lens effects in laser material processing. An amplitude or phase mask is used in the beam path of the processing laser or an additional measuring laser, which generates a characteristic pattern in the focal plane in the region of the processing plane, which characteristically changes outside of the focal plane. The pattern is captured in the processing plane with a camera during laser processing and based on a change in the pattern, an actuator is controlled, which controls optical components for shifting the focus position in such a way that a focus shift in the processing plane caused by thermal lens effects is compensated. Alternatively, the actuator can also change the distance to the processing plane accordingly. The methods and laser processing devices enable direct measurement of the focus shift in the processing plane and reliable compensation for this focus shift during laser material processing.

Description

Technical field of application

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 displacement of the focal plane of the processing laser beam from the processing plane, the displacement of the focal plane from the processing plane being caused by adjusting one or more optical components in the beam path of the processing laser beam, the focal plane of the processing laser beam can be displaced relative to the processing plane by adjusting it, or by changing the distance from the processing plane during laser material processing via an actuator.

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 only absorb a very small part of the laser power. But even this small amount is sufficient in many cases 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 leads to a defocusing of the radiation during processing and thus to a reduction in the intensity of the laser radiation on the workpiece. This can reduce the editing quality or even require the editing process to be aborted. 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 lens effect. Since the focal length changes during and as a function of the machining process, i.e. it usually also varies during the machining process, it was previously not possible to measure and compensate for the thermal focus shift directly in the machining plane during the machining process.

State of the art

From A. Gatej et al., "Methods for compensation of thermal lensing based on thermo-optical (TOP) analysis", in Optical Modeling and Design III, F. Wyrowski, J.T. Sheridan, J. Tervo, Y. Meuret, eds. (SPIE, 2014), 91310F, a method for determining the focus shift before the process is carried out by measuring the temporal maximum intensity using a CCD camera is known. O. Blomster et al., "Optics performance at high-power levels", in Solid State Lasers XVII: Technology and Devices, W.A. Clarkson, N. Hodgson, R.K. Shori, eds. (SPIE, 2008), 68712B measure the beam caustic by means of beam profile measurements. Based on the results of the characterization carried out, the focus position is then adjusted later in the processing process as part of a predictive control, as described, for example, in O. Pütsch et al., "Real-time laser beam control for the compensation of thermal effects", DGaO Proceedings 2011. With these known techniques, the optical system is characterized with regard to the thermal effects before the process is carried out. However, this characterization before the process does not adequately depict aspects that change over time, such as power fluctuations of the laser, temperature fluctuations in the test environment, or soiling of the optics that gradually increases with the duration of the process, which also leads to greater thermal effects.

Furthermore, it is known that thermal effects can be observed during laser material processing as a result of partial decoupling of radiation from the beam path up to the processing plane. The thermal displacement can now be measured separately for the emitted radiation. However, since the beam path and thus also the number of irradiated or irradiated optical elements has changed due to the decoupling, it is not possible to make a precise statement about the thermal effects for the entire beam path up to the processing level.

As an alternative to an experimental characterization of the optical system, thermal simulations are also carried out before the process. Predicative control is also possible based on the simulated thermal displacements. However, the precision of the prediction is limited by the accuracy of the simulation.

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 selection of suitable materials. Corresponding optical elements are commercially available.

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

The DE 20 2005 010 715 U1 discloses an adjustment device for a cantilevered laser welding head, whose distance from the workpiece for defined focusing of the laser beam can be programmed with the aid of a visible adjustment laser beam. To determine the exact focus, the distance, shape and brightness of adjustment laser spots generated on the workpiece surface are recorded with an observation camera.

From the DE 10 2011 054 941 B3 a device and a method for correcting the thermal displacement 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.

The JP 2007-253 200A 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 in such a way that their points of impingement are superimposed on the workpiece surface when the processing laser beam is correctly focused. If the focus is not correct, the points of impact are separated from each other.

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

Presentation of the invention

The object is achieved with the methods and the associated laser processing devices of patent claims 1, 2, 9 and 10. Advantageous configurations of the methods and the laser processing devices are the subject matter of the dependent patent claims or can be found in the following description and the exemplary embodiments.

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, in the second process alternative, a measuring laser is also used. In both methods and associated laser processing devices, the shift in the focal plane due to thermal lens effects is compensated 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, via the adjustment of which the focal plane of the processing laser beam can be displaced relative to the processing plane. This can be, for example, a displaceable f-theta lens or a dynamic focusing unit as part of the optical system for laser material processing. In another embodiment, the compensation takes place by changing the distance from the processing plane during the laser material processing via an actuator. The distance can be changed by tracking the processing plane, i.e. the workpiece currently being processed, by tracking the entire laser processing head or just by tracking (in the sense of changing the distance to the processing plane) the optical arrangement for focusing.

The first method alternative is characterized in 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 of the focal plane. The pattern generated by the amplitude or phase mask in the processing plane is detected during the laser material processing and a deformation of the pattern in the processing plane is counteracted by adjusting the one or more optical components or the distance to the processing plane via the actuator. The pattern can be detected continuously or also quasi-continuously, ie as individual measurements at short time intervals.

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 contains the one or more optical components to compensate for the shift in the focal plane (if provided in the corresponding configuration) 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 outside of the focal plane characteristically deformed. The pattern generated by the amplitude or phase mask in the processing plane is recorded continuously or quasi-continuously during the laser material processing, just as in the first method alternative, and a deformation of the pattern in the processing plane by adjusting the one or more optical components or the distance to the Working level counteracted via the actuator. The measuring laser beam preferably has a different wavelength than the processing laser beam, so that it can be coupled into the beam path of the processing laser beam via a dichroic mirror.

In both process alternatives, a pattern generated by the amplitude or phase mask in the processing plane is thus detected during laser processing and a focus shift is inferred from a change in the pattern, which is then detected by suitable control of the actuator, in particular by shifting the one or more optical components (n), for example a lens of the optical system, is compensated. The adjustable optical component(s) can also be one or more liquid lens(es) 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 from the processing plane) to compensate for the shift in focus is preferably carried out by controlling 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.

By using the phase or amplitude mask, which generates a characteristic pattern in the focal plane that deforms outside of the focal plane, a shift in the focal plane can be observed and measured during the processing process. In this way, there is a direct measurement method with which the influence of the thermal lens effects can be compensated with the help of the actuator, which adjusts a variable focusing unit, for example. This direct measurement in the processing plane automatically takes into account aspects that change over time, such as power fluctuations of the laser, temperature fluctuations in the test environment or a change in the thermal effects caused by gradually increasing soiling 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 thus be recorded very reliably and compensated for during the entire machining process.

An amplitude or phase mask is preferably used in the method and the associated laser processing devices, with which the distance from the focal plane can be determined from the deformation of the pattern. This information can then be used in order to select a higher adjustment speed, for example of the optical component(s) at a greater distance than at a smaller distance from the focal plane, and thereby to speed up the regulation.

A so-called Bahtinov mask is particularly preferably used as the amplitude mask. The pattern generated by the Bahtinov mask in the focal plane represents the Fourier transformation of the Bahtinov mask. If this pattern is viewed outside of the focal plane, it deforms in a characteristic way, so that the distance to the focal plane can be inferred.

In the case of laser material processing, the observation of the pattern in the processing plane is preferably carried out with a so-called coaxial observation system. Coaxial observation systems share the beam path with the processing laser and are thus always aligned with 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 can be optimally observed and measured during the process or used to control the actuator .

The associated laser processing devices correspondingly have at least one processing laser, an optical arrangement for focusing a processing laser beam of the processing laser in a processing plane and, if necessary, for guiding over the processing plane. In one configuration, 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 between 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 arranged phase mask, which generates a pattern in the focal plane of the processing laser beam, which is deformed outside of the focal plane characteristic. In the second alternative, the laser processing device also 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 affects the one or more optical components that can be adjusted via the actuator (if provided in the corresponding configuration) and passes through 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. 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. The processing laser beam does not therefore pass through this amplitude or phase mask in this alternative. In both alternatives, the laser processing device has a camera with which the pattern generated by the amplitude or phase mask in the processing plane is recorded during the laser material processing, as well as a control device that allows a deformation of the pattern in the processing plane by adjusting the one or more optical Components or change in the distance to the processing plane counteracts via the actuator during laser material processing.

The proposed methods and associated laser processing devices offer particular advantages in laser processing methods in which higher powers (>10W) are used and in which thermal lens effects are therefore to be expected.

character list

• 1 ein erstes Beispiel der vorgeschlagenen Laserbearbeitungseinrichtung;
• 2 ein zweites Beispiel der vorgeschlagenen Laserbearbeitungseinrichtung;
• 3 ein drittes Beispiel der vorgeschlagenen Laserbearbeitungseinrichtung; und
• 4 ein Beispiel für ein charakteristisches Muster, wie es mit einer Bahtinovmaske in- und außerhalb der Fokusebene erzeugt wird.

The proposed methods and associated laser processing devices are explained in more detail below using exemplary embodiments in conjunction with the drawings. Here show:

• 1 a first example of the proposed laser processing device;
• 2 a second example of the proposed laser processing device;
• 3 a third example of the proposed laser processing device; and
• 4 an example of a characteristic pattern generated with a Bahtinov mask inside and outside the focal plane.

Ways to carry out the invention

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 have a wavelength of 1064 nm, for example, and the wavelength of the pilot laser can be 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 decoupled from the beam path of the processing laser beam via a dichroic mirror 7 and then coupled in again via a further dichroic mirror 7, as is shown in 1 is shown. The measuring laser beam 3 passes through an amplitude or phase mask 8 in the decoupled section, through 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 outside of the focal plane. This pattern is recorded and evaluated with the camera 9 with a lens coaxial to the processing and measuring laser beam and used to control an actuator for adjusting the focal 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. As in the following figures, this is only a simplified representation of the optical system of the laser processing device, which can also include several lenses. The processing and measuring laser beams are guided over the processing plane 6 by means of a scanner 4 during processing. In this example, the radiation of the measuring laser beam 3 is first linearly polarized via a linear polarizing filter 10 . This ensures that light is not already deflected onto the camera 9 on the way to the focal plane, since the polarizing beam splitter cube 11 only lets through light that is polarized perpendicularly to this end. Radiation that is instead propagated to the focal plane and back again passes through a λ/4 plate 12 twice. As a result, the polarization can be rotated and the beam splitting cube 11 can be passed in the direction of the camera 9 . In general, polarization is not preserved upon reflection from the processing plane. This leads to an attenuation of the intensity observed at the camera 9.

2 FIG. 12 shows another example of a laser processing device according to the present invention the 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 in turn have different wavelengths here, are superimposed coaxially via a dichroic mirror 7 and focused into the processing plane 6 via the scanner 4 and the focusing lens 5 . Here too, 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 already in FIG 1 an amplitude or phase mask 8 is arranged, which generates a pattern in the focal plane of the measuring laser beam 3 in the region of the processing plane 6, which characteristically changes outside of the focal plane. This pattern is in turn recorded with the camera 9 with a lens coaxial to the processing and measuring laser beam and used to control an actuator for adjusting the lens 5 and thus compensating for the shift in focus. In this example, too, the measuring laser beam 3 is once again linearly polarized via the linear polarizing filter 10, as has already been done in connection with FIG 1 was explained in more detail. This also applies to the arrangement of the λ/4 plate 12 and the polarizing beam splitter cube 11 in front of the camera 9.

3 FIG. 12 shows another example of the proposed laser processing device, in which only the processing laser 1 is used. In this case, an amplitude or phase mask 8 is introduced into the beam path of the processing laser 1, which generates 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 captured coaxially to the processing laser beam 2 with the camera 9 with a lens 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 weakening of the processing laser beam 2 by the amplitude or phase mask 8, this is designed in such a way that it only uses a small intensity component of the processing laser beam 2 to generate the pattern in the focal plane. In the lower left part of the 3 For this purpose, a top view of the amplitude or phase mask is shown as an example. Only the marked area at the upper right corner of this cross-section creates the characteristic pattern. The remaining area does not affect the processing laser beam 2. Hardly any energy is therefore withdrawn from the process. In order to be able to capture the very faint pattern thus generated with the camera 9, a diaphragm 15 is used on the camera 9 in this example, as is shown by way of example in cross section in the lower right part of FIG 3 can be seen. The center of the aperture 15 blocks the portions of the processing laser beam 2 that were not influenced by the amplitude or phase mask 8 and therefore do not contribute to the characteristic pattern. The pattern can thus be clearly captured by the camera 9 .

4 finally shows an example in sub-figure A) of a pattern generated with a Bahtinov mask in the focal plane. The lines of this pattern cross at a point in the middle. Outside the focal plane, these lines shift against each other, so that the camera captures a pattern, for example, as shown in sub-figure B) of 4 can be seen.

Reference List

1

processing laser

2

processing laser beam

3

measuring laser beam

4

scanner

5

focusing lens

6

editing level

7

dichroic mirror

8th

amplitude or phase mask

9

camera with lens

10

linear polarizing filter

11

polarizing beam splitter cube

12

λ/4 plate

13

measuring laser

14

semi-transparent mirror

15

cover

Claims (14)

Method for reducing the effects of thermal lens effects in laser material processing, which lead to a shift in a focus plane of a processing laser beam (2) from a processing plane (6), in which - shifting the focus plane out of the processing plane (6) by adjusting one or more optical components (5) in the beam path of the processing laser beam (2), via the adjustment of which the focal plane of the processing laser beam (2) can be displaced relative to the processing plane (6), or by changing a distance from the processing plane (6) during the laser material processing via an actuator, thereby compensating characterized in that - an amplitude or phase mask (8) is used in the beam path of the processing laser beam (2), which is in the focal plane of the processing la laser beam (2) generates 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 detected during the laser material processing, and - a deformation of the pattern in the processing plane (6) counteracted by adjusting the one or more optical components (5) or changing the distance to the processing plane (6) via the actuator. Method for reducing the effects of thermal lens effects in laser material processing, which lead to a shift in a focus plane of a processing laser beam (2) from a processing plane (6), in which - the shift of the focus plane from the processing plane (6) in a first alternative by adjusting a or several optical components (5) in the beam path of the processing laser beam (2), via the adjustment of 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 from the processing plane (6) during the Laser material processing is compensated via an actuator, characterized in that - a measuring laser beam (3) is coupled into the beam path of the processing laser beam (2), which includes all optical elements in the beam path of the processing laser beam (2) which cause thermal lens effects during the laser material processing, and in the first alternative also passes through one or more optical components (5) to compensate for the shift in the focal plane, and thus has the same focal plane as the processing laser beam (2), - in the beam path of the measuring laser beam (3) before it is coupled into the beam path of the Processing laser beam (2) an amplitude or phase mask (8) is used, which generates a pattern in the focal plane of the measuring laser beam (3) that deforms outside of the focal plane characteristic - through the amplitude or phase mask (8) in the processing plane (6) the pattern generated is detected during the 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. procedure after claim 2 , characterized in that the measuring laser beam (3) has a different wavelength than the processing laser beam (2). Procedure according to one of Claims 1 until 3 , characterized in that an amplitude or phase mask (8) is used, in which a distance from the focal plane can be determined from the deformation of the pattern. Procedure according to one of Claims 1 until 4 , characterized in that a Bahtinov mask is used as the amplitude mask (8). Procedure according to one of Claims 1 until 5 , characterized in that the one or more optical components (5) are adjusted or the distance from the processing plane (6) is changed by controlling the actuator via a control system. Procedure according to one of Claims 1 until 6 , characterized in that the pattern generated by the amplitude or phase mask (8) in the processing plane (6) is detected coaxially to the processing laser beam (2) with a camera (9). Procedure according to one of Claims 1 until 7 , characterized in that an amplitude or phase mask (8) is used, which weakens the processing laser beam (2) by less than 0.5% during passage. 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) in a processing plane (6), with either the optical arrangement having one or more optical components (5) in the beam path of the processing laser beam (2), which are controlled via an actuator of the Laser processing device can be adjusted in such a way 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) can be adjusted 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) that is characteristically deformed outside of 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 detected during the laser material processing, and - A control device that 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 between the optical arrangement and the processing plane (6) via the actuator during the laser material processing. Laser processing device with at least - one processing laser (1), - an optical arrangement for focusing a processing laser beam (2) of the processing laser (1) in a processing plane (6), with either the optical arrangement in a first alternative one or more optical components (5) in the beam path of the processing laser beam (2), which can be adjusted via an actuator of the laser processing device in such a way 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 between the optical arrangement and the processing plane (6 ) can be adjusted 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 ) all optical elements in the beam path of the processing laser beam (2), which cause thermal lens effects during the laser material processing, and in the first alternative also passes through the one or more optical components (5) to compensate for the shift in the focal plane, and thereby the same focal plane as the processing laser beam (2) has, - an amplitude or phase mask (8) in the beam path of the measuring laser beam (3) before it is coupled into the beam path of the processing laser beam (2), which creates a pattern in the focal plane of the measuring laser beam (3) that characteristically deformed outside of 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 recorded during the laser material processing, and - a control device which compensates for a deformation of the pattern in counteracts the processing plane (6) by adjusting the one or more optical components (5) or by changing the distance between the optical arrangement and the processing plane (6) via the actuator during the laser material processing. Laser processing device claim 10 , characterized in that the measuring laser (13) has a different wavelength than the processing laser (1). Laser processing device according to one of claims 9 until 11 , characterized in that the amplitude or phase mask (8) is selected such that a distance from the focal plane can be determined from the deformation of the pattern. Laser processing device according to one of claims 9 until 12 , characterized in that the amplitude mask (8) is a Bahtinov mask. Laser processing device according to one of claims 9 until 13 , characterized in that the pattern generated by the amplitude or phase mask (8) in the processing plane (6) is detected with the camera (9) coaxially to the processing laser beam (2).

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