Method and laser machining system for influencing an astigmatism in a laser beam

Abstract

The invention relates to a method and a laser machining system for influencing an astigmatism in a laser beam (21), wherein the laser beam (21) is guided through a first cylindrical lens (23) and then through a second cylindrical lens (24), and the mutual spacing (d) between the two cylindrical lenses (23, 24) in the propagation direction (z) of the laser beam (21) and / or different angles of rotation of the longitudinal axes of the two cylindrical lenses (23, 24) about an axis of rotation (26) in the propagation direction (z) of the laser beam (21) have been set in such a way that an astigmatism present in the laser beam (21) is at least partially compensated for or a desired astigmatism is generated in the laser beam (21).

Description

[0001] Method and laser processing system for influencing astigmatism in a laser beam

[0002] Laser processing systems typically feature numerous optical elements for beam shaping and beam guidance. Manufacturing tolerances of these elements can lead to aberrations such as the development of astigmatism in the laser beam. Particularly in laser processing systems that generate high pulse energy from the laser beam, such astigmatism occurs due to the amplifier stages within the system. Astigmatism can lead to deviations in a laser cutting process or deform a plasma.

[0003] On the other hand, astigmatism in the laser beam can also be desirable. When laser cutting a workpiece, it can be advantageous if the point of incidence of the laser beam on the workpiece is not round but slightly elliptical, i.e., has a larger extension in the machining direction than transverse to it.

[0004] The invention is based on the object of proposing a method and a laser processing system which allow an interfering astigmatism in a laser beam to be at least largely compensated and / or a desired astigmatism to be specifically generated in a laser beam.

[0005] This object is achieved according to the invention by a method for influencing an astigmatism in a laser beam of a laser processing system, which is characterized in that the laser beam is guided through a first cylindrical lens and then through a second cylindrical lens, wherein the mutual distance between the two cylindrical lenses in the propagation direction of the laser beam and / or differing angles of rotation of the longitudinal axes of the two cylindrical lenses about a rotation axis in the propagation direction of the laser beam were adjusted in such a way that an astigmatism present in the laser beam is at least partially compensated or a desired astigmatism is generated in the laser beam.

[0006] Two cylindrical lenses, whose mutual distance is adjustable, form a telescope whose focal length is variable. A displacement of at least one of the lenses in the direction of propagation leads to either an increase or a decrease in the focal length, which can be estimated as follows: ftotal = fl * f2 / (fl + f2 - d), where fi , f2 are the focal lengths of the two lenses and d is their mutual distance.

[0007] This effect can be used to compensate for any astigmatism present in the laser beam or to deliberately introduce astigmatism into the laser beam.

[0008] If, however, one of the cylindrical lenses is rotated by an angle around the optical axis of the laser beam, a cylindrical refractive power is created

[0009] Dzyi = 2 * DL * cos 0 and a spherical refractive power

[0010] DSPH when both lenses have the same refractive power DL. The system of the two lenses can thus be optically described as a combination of a cylindrical lens and a spherical lens.

[0011] The cylindrical part of the refractive power introduces an astigmatism into the laser beam, which depends on the size of the angle 0. This astigmatism can counteract any astigmatism already present in the laser beam or can be imposed on the laser beam in a desired manner.

[0012] Of course, it is also possible to cause astigmatism in the laser beam by a combination of both measures, ie changing the mutual distance between the cylindrical lenses and rotating at least one of the cylindrical lenses about an axis in the propagation direction of the laser beam.

[0013] The laser beam can first be passed through a first cylindrical lens with a positive focal length and then through a second cylindrical lens with a negative focal length or through two cylindrical lenses with a positive focal length.

[0014] A combination of a cylindrical lens with a negative focal length and a cylindrical lens with a positive focal length forms a Galilean telescope, and a combination of two cylindrical lenses with a positive focal length can form a Keplerian telescope if the lenses are spaced appropriately. These various combinations of cylindrical lenses allow for a wide range of focal lengths for the lens system.

[0015] For example, it is also possible to choose the focal length of the first cylindrical lens smaller than the focal length of the second cylindrical lens. When using a first lens with a negative focal length and a second lens with a positive focal length, this results in an overall positive focal length and a suitable distance between the lenses. If the signs of the focal lengths of the two lenses are reversed, this results in an overall negative focal length for the system consisting of the two cylindrical lenses.

[0016] Astigmatism can occur or be desired in different directions of the laser beam. Therefore, it is advantageous if the first and second cylindrical lenses can be rotated together about an axis running in the propagation direction of the laser beam in order to compensate for or create astigmatism in a plane with a different radial orientation relative to the laser beam.

[0017] The mutual spacing of the cylindrical lenses and / or the differing rotation angles of the longitudinal axes of the two cylindrical lenses can preferably be adjusted before commissioning the laser processing system and can be changed during processing of a workpiece with the laser processing system. Any astigmatism present in the laser beam can be detected by measurements before processing a workpiece and compensated for by adjusting the spacing and / or the rotation angles of the two cylindrical lenses accordingly. If a more elliptical shape of the laser beam's impact spot on the workpiece is desired during processing of a workpiece, this can be achieved by adjusting the mutual spacing and / or the rotation angles of the lenses during workpiece processing.In addition, the distance and / or the angle of rotation can be adjusted when changing the feed speed of the laser beam in order to achieve an optimal shape of the point of impact of the laser beam on the workpiece.

[0018] The invention also relates to a laser processing system with a laser beam source and a device for influencing an astigmatism in the laser beam, which is characterized in that the device for influencing an astigmatism in the laser beam has at least two cylindrical lenses arranged one behind the other in the propagation direction of the laser beam, which are arranged so as to be adjustable in their mutual distance in the propagation direction of the laser beam and / or whose longitudinal axes can be aligned at different angles of rotation about a rotation axis running in the propagation direction of the laser beam.

[0019] This laser processing system makes it possible to carry out the method according to the invention for neutralizing astigmatism present in a laser beam and / or for deliberately introducing astigmatism into a laser beam. One of the cylindrical lenses can have a positive focal length and the other cylindrical lens a negative focal length. The lenses thus form a Galilean telescope, whose magnification factor is preferably chosen close to 1 so that the roundness of the laser beam cross-section is not negatively affected. If both lenses have the same refractive power and their longitudinal axes are aligned parallel to each other, the overall system represents a lens with an infinite focal length.If one of the lenses is now rotated by an angle around an axis in the propagation direction of the laser beam, the overall system receives a cylindrical and a spherical refractive power component, whereby the cylindrical component causes the generation of an astigmatism in the laser beam, the size of which depends on the angular difference between the longitudinal axes of the two cylindrical lenses.

[0020] In an alternative design, both cylindrical lenses can have a positive focal length, thus forming a Keplerian telescope. If lenses with a short focal length are used, a small change in the mutual separation of the lenses is sufficient to achieve a relatively large change in the refractive power of the overall arrangement.

[0021] It is also possible to provide a lens group with more than two cylindrical lenses, with at least the mutual spacing of two of the cylindrical lenses being adjustable. In a group of three cylindrical lenses, the first and third cylindrical lenses can be lenses with a positive focal length, and the middle cylindrical lens can be a lens with a negative focal length.

[0022] A design in which the first and third cylindrical lenses are lenses with a negative focal length and the middle cylindrical lens is a lens with a positive focal length is also possible.

[0023] The cross-sectional shape of the cylindrical lenses can be freely selected. However, for manufacturing reasons, a design with a flat side and a convex or concave curved side is advantageous. When designing the device for influencing astigmatism in the laser beam that allows astigmatism to be generated by varying the angles of rotation of the longitudinal axes of the cylindrical lenses, it is advisable to select the focal lengths such that the angles between the longitudinal axes of the cylindrical lenses can be adjusted between 0° and ±10°. Even small angular differences between the longitudinal axes result in a relatively large astigmatism. Preferably, only one of the lenses can be rotated about an axis running in the direction of propagation of the laser beam. The other lens can be fixed.

[0024] If the astigmatism is to be created by changing the mutual distance between the cylindrical lenses, distance changes of 0-10mm can preferably be adjustable.

[0025] Further advantages can be achieved if the device for influencing astigmatism in the laser beam or the two cylindrical lenses are arranged so that they can rotate around an axis in the direction of propagation of the laser beam by an angle of ±45°. This makes it possible to compensate for or create astigmatism in any radial direction of the laser beam, not just in the x and y spatial directions.

[0026] The likelihood of astigmatism occurring in a laser beam from a laser processing system increases with the number of optical elements through which the laser beam is passed before hitting a workpiece. Each of these elements can contain dimensional deviations or other defects that can cause astigmatism. Therefore, laser systems that generate high pulse energies are particularly affected. Such laser processing systems typically have devices for amplifying the laser energy, which can cause astigmatism in the laser beam. In this case, it is advantageous if the device for influencing astigmatism in the laser beam is located behind these devices in the beam path of the laser beam. Any astigmatism caused by these amplification devices can thus be compensated for.

[0027] However, it is also possible to arrange a device for influencing astigmatism in the laser beam in the laser beam source itself. Astigmatism can also occur during the generation of the laser beam, which can then be immediately corrected. If the intentional generation of astigmatism in the laser beam is desired, arranging the device for influencing astigmatism in the laser source is also advantageous. Of course, the device for influencing astigmatism can also be arranged at a different location in the beam path of the laser beam. Furthermore, the provision of several such devices in the beam path is possible.

[0028] Further features and advantages of the invention will become apparent from the description, the claims, and the drawings. According to the invention, the above-mentioned and further-described features can be used individually or in combination in any convenient way. The embodiments shown and described are not intended to be exhaustive, but rather are exemplary in nature for describing the invention.

[0029] Detailed description of the invention and drawing

[0030] Fig. 1 shows a beam pattern of a Gaussian laser beam;

[0031] Fig. 2 shows a schematic representation of a first device for

[0032] Influencing astigmatism in a laser beam;

[0033] Fig. 3 shows a schematic representation of a first device for influencing an astigmatism in a laser beam;

[0034] Fig. 4 shows a diagram of the caustics of a laser beam without astigmatism; Fig. 5 shows a diagram of the caustics of a laser beam in which astigmatism was generated using the device of Fig. 2;

[0035] Fig. 6 shows a diagram of the caustics of a laser beam in which astigmatism was generated using the device of Fig. 3.

[0036] The astigmatism of a laser beam is usually normalized to the Rayleigh length of the beam. Laser beams are Gaussian beams. Therefore, Fig. 1 illustrates how the Rayleigh length ZR is defined using the beam pattern of a Gaussian laser beam 10, where a longitudinal section through the beam 10 is shown in the x-direction and the laser beam 10 propagates in the z-direction. The curves 11, referred to as caustics, are the boundary curves of the beam 10, whose course in the z-direction is shown. The z-position z xThe point at which beam 10 has its smallest radius wo is called the beam waist. The radius w(z) increases on both sides of the beam waist. The Rayleigh length ZR is the distance that beam 10 needs to travel in the z direction to double its cross-sectional area compared to the beam waist. The beam radius w there is w = 2wo.

[0037] Fig. 1 shows the beam profile in a longitudinal section through the beam 10 in the x-direction. If the longitudinal section through the beam 10 in the y-direction (not shown here) has the same shape, ie the beam waist has the same z-position z x = z y , there is no astigmatism in the laser beam 10. However, if there is a shift in the beam waist, the astigmatism A of the laser beam 10 is defined as follows:

[0038] Fig. 2 shows the structure of a device 20 for influencing astigmatism in a laser beam 21 of a laser processing system (not shown in detail here). The device 20 is arranged in the beam path of a laser beam 21 propagating in the z-direction, which has a divergent beam shape due to a telescopic lens 22 arranged upstream of the device 20. However, this is not mandatory. Astigmatism can also be generated in a convergent or collimated laser beam 21 by the device 20.

[0039] Two cylindrical lenses 23, 24 are arranged one behind the other within the device 20, with the second cylindrical lens 24 being arranged so as to be displaceable in the propagation direction z of the laser beam 21 by a distance Ad of, for example, 5 mm, as indicated by the double arrow 25. This allows the mutual distance d between the cylindrical lenses 23, 24 to be varied. The first cylindrical lens 23 can be a lens with a negative focal length fi of, for example, -2000 mm, and the second cylindrical lens 24 a lens with a larger positive focal length f2 of, for example, +2400 mm, whereby the overall arrangement of the two cylindrical lenses 23, 24 has a positive focal length ftotal. However, this configuration is not mandatory. Both cylindrical lenses 23, 24 could also have a positive focal length, or the first cylindrical lens 23 could have a positive focal length and the second cylindrical lens 24 a negative focal length.

[0040] By changing the distance d between the two cylindrical lenses 23, 24 in the z-direction, an astigmatism can be generated in the laser beam 21, which can either counteract an astigmatism already present in the laser beam 21 or influence the shape of the laser beam 21 in the desired manner. The magnitude of this astigmatism depends on the mutual distance d between the cylindrical lenses 23, 24, as shown in Figures 4 and 5.

[0041] In order to generate astigmatism not only in the xz and yz planes of the laser beam 21, but in all planes in the radial direction of the laser beam 21, the entire device 20 is arranged to be rotatable about a rotation axis 26 extending in the direction of the optical axis of the laser beam 21, as indicated by the arrow 27.

[0042] Fig. 3 shows an alternative embodiment of a device 30 for influencing astigmatism in the laser beam 21, which in this exemplary embodiment is also first guided through a telescopic lens 22 with a negative focal length before entering the device 30. Two cylindrical lenses 31, 32 are arranged within the device 30. The first cylindrical lens 31 can, for example, have a positive focal length and the second lens 32 a larger negative focal length, so that the overall arrangement comprising the two lenses 31, 32 has a negative overall focal length. Here, too, such a configuration is not mandatory, and identical lenses or lenses with reversed focal length signs could also be used.

[0043] In the example shown, the second cylindrical lens 32 is rotatable about a rotation axis 34 in the propagation direction z of the laser beam 21 by an angle 0, which is indicated by an arrow 33. However, the first lens 31 or both lenses 31, 32 could also be arranged to rotate. The astigmatism in the laser beam 21 is caused by a difference in the angles of rotation of the longitudinal axes of the two cylindrical lenses 31, 32. This difference in the angle of rotation generates a symmetrical astigmatism in the x- and y-directions, which depends on the size of the angle 0.

[0044] In addition, the entire device 30 can be rotated about the rotation axis 34 running in the direction of the optical axis of the laser beam 21 - indicated by the arrow 35 - if an astigmatism is to be generated or compensated in planes other than the xz and yz planes of the laser beam 21.

[0045] It is also possible to combine a change in the distance between the cylindrical lenses with a relative rotation of the longitudinal axes of the angles in order to influence an astigmatism in the laser beam 21. For example, in the device 20, not only could the lens 24 be arranged so as to be displaceable in the z-direction, but one of the cylindrical lenses 23, 24 could also be arranged so as to be rotatable about the axis of rotation 26 running in the z-direction. An analogous modification can also be carried out in the device 30. In addition to the rotatability of the lens 32, one of the cylindrical lenses 31, 32 could also be arranged so as to be displaceable in the z-direction. Figures 4 to 6 illustrate the effect of different distances d between the lenses 23, 24 and different angles of rotation θ of the lens 32 on the caustic 11 (see Fig. 1) of the laser beam 21 in the x- and y-directions. Fig. 4 shows the result of a simulation with a cylindrical lens configuration that does not produce astigmatism.This is the case when lens 24 of device 20 is in its home position and the angle of rotation of cylindrical lens 32 in device 30 is 0 = 0°. The two caustics 11, 11' in the x- and y-directions coincide in this case. The beam waists in the x- and y-directions are both at z = 0 mm.

[0046] Fig. 5 shows the result of a simulation with a displacement of the cylindrical lens 24 by Ad = 5 mm. The difference Az between the beam waists of the two caustics 11, 11' is ~2 * ZR, i.e., an astigmatism of approximately 200% was generated, which can counteract any astigmatism already present in the laser beam 21 or be imposed on the laser beam 21.

[0047] An equally large astigmatism of approximately 200% can also be generated by rotating the cylindrical lens 32 in the device 30 in Fig. 3 by 0 = 5.9° around the axis of rotation 34, as shown in Fig. 6. This results in a symmetrical shift of the z-positions of the beam waists of the caustics 11, 11' in the x- and y-directions, so that their mutual distance Az is ~2 * ZR.

[0048] Overall, a relatively strong astigmatism in the laser beam can therefore be compensated or deliberately created by a relatively slight displacement of one of the cylindrical lenses 23, 24 in the z-direction and / or a small rotation of one of the cylindrical lenses 31, 32.

Claims

Patent claims 1. Method for influencing an astigmatism in a laser beam (21) of a laser processing system, characterized in that the laser beam (21) is guided through a first cylindrical lens (23, 31) and then through a second cylindrical lens (24, 32), wherein the mutual distance (d) of the two cylindrical lenses (23, 24) in the propagation direction (z) of the laser beam (21) and / or differing angles of rotation (θ) of the longitudinal axes of the two cylindrical lenses (31, 32) about a rotation axis (34) in the propagation direction (z) of the laser beam (21) were set in such a way that an astigmatism present in the laser beam (21) is at least partially compensated or a desired astigmatism is generated in the laser beam (21).

2. Method according to claim 1, characterized in that the laser beam (21) is guided through a first cylindrical lens (23) with a positive focal length (fi) and then through a second cylindrical lens (24) with a negative focal length (f2) or through two cylindrical lenses (23, 24; 31, 32) with a positive focal length.

3. Method according to claim 2, characterized in that the amount of the focal length (fi ) of the first cylindrical lens (23) is smaller than the amount of the focal length (f2 ) of the second cylindrical lens (24).

4. Method according to one of the preceding claims, characterized in that the first and second cylindrical lenses (23, 24; 31, 32) are rotated jointly or separately by the same angle about an axis (26, 34) running in the propagation direction (z) of the laser beam (21) in order to compensate for or generate an astigmatism in a plane with a different radial orientation with respect to the laser beam (21).

5. Method according to one of the preceding claims, characterized in that the mutual distance (d) of the cylindrical lenses (23, 24) and / or the differing angles of rotation (θ) of the longitudinal axes of the two cylindrical lenses (31, 32) are set before commissioning of the laser processing system and can be changed during the processing of a workpiece with the laser processing system.

6. Laser processing system with a laser beam source and a device (20, 30) for influencing an astigmatism in the laser beam (21), characterized in that the device (20, 30) for influencing an astigmatism in the laser beam (21) has at least two cylindrical lenses (23, 24; 31, 32) arranged one behind the other in the propagation direction (z) of the laser beam (21), which are arranged so as to be adjustable in their mutual distance (d) in the propagation direction (z) of the laser beam (21) and / or whose longitudinal axes can be aligned at different angles of rotation (θ) about a rotation axis (34) running in the propagation direction (z) of the laser beam (21).

7. Laser processing system according to claim 6, characterized in that one of the cylindrical lenses (24, 31) has a positive focal length and the other cylindrical lens (23, 32) has a negative focal length.

8. Laser processing system according to claim 6, characterized in that both cylindrical lenses (23, 24; 31, 32) have a positive focal length.

9. Laser processing system according to claim 6, characterized in that more than two cylindrical lenses are provided, wherein at least the mutual distance (d) of two of the cylindrical lenses is adjustable.

10. Laser processing system according to claim 9, characterized in that the first and third cylindrical lenses are lenses with a positive focal length and the middle cylindrical lens is a lens with a negative focal length.

11. Laser processing system according to claim 9, characterized in that the first and third cylindrical lenses are lenses with a negative focal length and the middle cylindrical lens is a lens with a positive focal length.

12. Laser processing system according to one of claims 6 to 11, characterized in that the angle (0) between the longitudinal axes of the cylindrical lenses (31, 32) is adjustable between 0° and ±10°.

13. Laser processing system according to one of claims 6 to 12, characterized in that the mutual distance (d) of the cylindrical lenses (23, 24) is adjustable between 0 - 10 mm.

14. Laser processing system according to one of claims 6 to 13, characterized in that the device (20, 30) for influencing an astigmatism in the laser beam (21) or the two cylindrical lenses (23, 24; 31, 32) are arranged so as to be rotatable together about an axis (26, 34) in the propagation direction (z) of the laser beam (21) by an angle of ±45°.

15. Laser processing system according to one of claims 6 to 14, characterized in that it has devices for amplifying the laser energy and the device (20, 30) for influencing an astigmatism in the laser beam (21) is arranged behind these devices.

16. Laser processing system according to one of claims 6 to 15, characterized in that the device (20, 30) for influencing an astigmatism in the laser beam (21) is arranged in the laser beam source.

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