IMAGING OPTICS FOR MATERIAL PROCESSING USING LASER RADIATION AND LASER PROCESSING HEAD WITH SUCH

Description

[0001] The invention relates to an imaging optics for material processing with laser radiation and a laser processing head equipped with such beam shaping optics.

[0002] When processing workpieces using laser beams, it is often necessary to adjust the focus position, i.e., the position of the laser beam focal point relative to the laser processing head and / or the workpiece, as well as the focus diameter. This is necessary, for example, when switching from thin to thick sheet metal during laser cutting. If the focus diameter cannot be varied over a sufficiently wide range, the processing optics must be replaced, which reduces the productivity of the laser processing system.

[0003] An optical system with which the image size can be changed while the object and image position are constant, i.e. the focus diameter can be changed during material processing with laser radiation, is known in technical optics as a zoom system. Zoom systems enable the adjustment of the image scale, i.e. the ratio of focus diameter to the diameter of the laser beam source (e.g. the laser exit surface of an optical fiber), for example by using an afocal telescope between the collimation and focusing optics. Such afocal telescopes usually consist of three or four lens groups. Afocal telescopes are described, for example, in US 4353617 A and EP 0 723 834 A. Typically, two movements are required to adjust the focus diameter without changing the focus position.One movement of a lens or lens group of the zoom system serves to vary the focal diameter, while a second movement of another lens or lens group of the zoom system compensates for the change in focal position, i.e., shifts the focus back to its original position. For industrial material processing with laser radiation in the multi-kilowatt range, however, it is important to keep the number of optical elements small, as undesirable effects generally increase with the number of optical elements and reinforce each other. One example of this is thermal focus shift, i.e., the change in the refractive power or focal length of a lens or optical system due to the temperature-dependent refractive index and / or the thermal expansion of the lens material.

[0004] DE 198 25 092 C2 discloses a laser system for generating a focused laser beam with a variable focal diameter. A negative (concave) optical element (diverging lens), which is also movable in the direction of the optical axis, is provided between a fixed collimator lens and a focusing lens movable in the direction of the optical axis. By varying the distance between the diverging lens and the focusing lens, the diameter of the laser beam focus can be varied. To ensure that the focal plane remains in the workpiece plane, the system consisting of the diverging lens and focusing lens must be shifted accordingly in the direction of the optical axis of the system.

[0005] The disadvantage here is that a high level of mechanical effort must be put into the focusing area, since the proximity to the processing point and the associated thermal stress and contamination risks due to metal splashes, smoke and the like place particular demands on robustness and accessibility.

[0006] DE 20 2010 006 047 U1 discloses a beam-shaping unit for focusing a laser beam. It comprises a collimator optics consisting of two converging optical elements movable along the optical axis in the laser processing head, and a focusing optics. The collimator optics are constructed in the manner of a Keplerian telescope. The first movable optical element serves to adjust the focus diameter, and the second movable optical element to compensate or adjust the focus position.

[0007] Since there is a real intermediate focus in the collimation area, the known optical system has a large overall length.

[0008] From DE 10 201 1 1 17 607 A1, which forms the basis of the preamble of claim 1, another optical system for laser radiation with variable imaging scale is known, in which a collimating lens and a focusing lens are provided. The collimating lens consists of a first movable converging lens group, a second movable diverging lens group, and a third fixed or axially movable converging lens group. By shifting the first two lens groups relative to each other, the focus diameter can be adjusted. If the third lens group is also axially adjustable, it is also possible to adjust the axial focus position.

[0009] DE 10 2008 048 502 A1 describes an invention which relates to an optical device for focusing a laser beam into a working focus, in particular for focusing a laser beam in a laser processing head for material processing, with an objective arrangement which comprises a collimator and a focusing group, in order to make it possible to shift the position of the working focus along the optical axis without changing the power density in the working focus, assuming constant laser power, the invention provides that the focusing group has a first and a second lens group which is arranged so as to be displaceable in the axial direction and a front lens group which is arranged fixedly relative to the collimator, wherein the movably arranged lens groups are designed such that the axial position of the working focus relative to the front lens group can be adjusted by shifting them, without the magnification changing.

[0010] Here, too, the movable optical elements are in the collimation range, but the number of optical elements used here makes the usual thermal problems in laser material processing, such as thermal focus shift, particularly critical.

[0011] The invention is based on the object of providing an imaging optics system for material processing using laser radiation that allows flexible adjustment of the focus position and focus diameter in a structurally simple and compact manner. A further object of the invention is to provide a laser processing head with such an imaging optics system.

[0012] This object is achieved by the imaging optics according to claim 1 and the laser processing head according to claim 2. Advantageous embodiments of the invention are described in the subclaims.

[0013] The invention relates to imaging optics for laser radiation with adjustable focus position and adjustable focus diameter. A zoom system comprises collimator optics consisting of a first movable lens or lens group with a positive focal length and a second movable lens or lens group with a negative focal length, as well as a focusing element. The first movable lens or lens group with a positive focal length serves to image a working laser beam source into a virtual intermediate focus, and the second movable lens or lens group (212) with a negative focal length serves to image the virtual intermediate focus toward infinity, so that the focusing optics images the working laser beam source into its focal point.

[0014] Furthermore, a laser processing head with a housing through which a working laser beam can be guided is equipped with an imaging optics according to the invention for generating a working focus.

[0015] According to the invention, a maximum adjustment range of the focus position can be achieved with a reduced number of optical elements or lens groups and a minimum overall length.

[0016] In an advantageous development of the laser processing head according to the invention, it is provided that the first movable lens or lens group and the second movable lens or lens group can each be displaced by an actuator for adjusting the focus diameter and / or focus position.

[0017] In principle, it is conceivable for the two actuators to be coupled to each other via gears, so that the displacements of the lenses or lens groups occur synchronously and / or proportionally to each other. However, a practical embodiment of the invention is characterized by the actuators being independent of each other. This not only has the advantage of allowing for a mechanically simple design, but also that, when changing the working laser radiation, the wavelength dependence of the focal lengths of the optical elements can be easily taken into account when adjusting the focus diameter and / or focus position.

[0018] The position of the movable lenses or lens groups relative to each other and to the focusing optics is adjusted depending on the type of processing and the laser wavelength by a control unit, which is preferably programmable.

[0019] The invention is explained in more detail below with reference to the drawings. They show: Figure 1 a simplified, schematic representation of a laser processing head according to the invention, and Figures 2 (a) to (c) simplified, schematic representations of the imaging optics according to the invention, wherein the lens groups assume different positions relative to one another.

[0020] In the figures of the drawing, identical components are provided with identical reference symbols.

[0021] Figure 1 shows a laser processing head 10 through which a working laser beam 12, supplied via an optical fiber 11, is guided and directed onto a workpiece 14. The divergent working laser beam 121 emerging from the optical fiber 11 is focused by an imaging optics 20 into a working focus 15 on, above, or below a surface 16 of the workpiece 14.

[0022] The imaging optics 20 comprises a collimator optics 21 and a focusing optics 22. The collimator optics 21 comprises a first movable lens or lens group 211 with a positive focal length and a second movable lens or lens group 212 with a negative focal length. The first and second movable lenses or lens groups 211, 212 can each be moved in the axial direction along the optical axis 17 by respective actuators A, B to adjust the focus diameter and / or focus position, as indicated by the arrows.

[0023] The actuators A, B, which are independent of each other, are controlled by a control unit 30, as indicated by the dashed lines 31, 32, so that the position of the movable lenses or lens groups 211, 212 relative to each other and to the focusing optics 22 can be adjusted depending on the laser wavelength and the type of processing.

[0024] The control unit 30, which is advantageously programmable via a suitable wireless or wired interface 33, can be integrated, in a manner not shown in detail, into a machine control system of a laser processing device with which the laser processing head 1 according to the invention is used. Thus, all information required for adjusting the lenses or lens groups 211, 212, such as laser wavelength, processing type (such as laser cutting or welding), material type (such as metal or plastic), sheet thickness, and the like, can be easily provided to the control unit 30.

[0025] In the Figures 2 (a) to (c)the positions of the movable lens groups 211, 212 relative to each other, to the focusing optics 22 and to a working laser beam source 18 formed by the exit surface of the optical fiber 11 are shown, as they are suitable for cutting thin sheet metal with a thickness of about 5 mm or less, medium sheet metal with a thickness of about 5 mm to about 10 mm or thick sheet metal with a thickness of more than about 10 mm.

[0026] Figure 2 (a) shows, for example, the position of the first and second movable lenses or lens groups 211, 212 of the collimator optics 21 for cutting thin sheet metal, in which the working laser beam 12 in the area of ​​the focus 15 can have a relatively small diameter with a relatively short Rayleigh length. For thin sheet metal, small focus diameters of approximately 100 µm to 150 µm are common.

[0027] If the imaging properties of a lens system are determined according to the following equation m = NA / NA ' where m is the magnification, NA is the numerical aperture of the system on the object side and NA' is the numerical aperture of the system on the image side, the embodiment in Fig. 2a , where the numerical aperture of the image side NA' is approximately equal to the numerical aperture of the object side NA, a magnification of m ≅ 1. With a fiber diameter of about 100 µm, the optimal focus diameter for cutting thin sheet metal is obtained.

[0028] If the beam diameter is to be reduced, the first movable lens group 211 with a positive focal length is moved closer to the working laser beam source 18, causing the image of the working laser beam source 18 generated by the first movable lens group to move away from the first movable lens group 211. Consequently, for focus position compensation, the second movable lens group 212 with a negative focal length must be shifted until the image of the working laser beam source 18 is located at the focal point of the second movable lens group 212 with a negative focal length, so that it is imaged towards infinity by this second movable lens group 212. Consequently, the laser focus 15 is again located at the focal point of the focusing optics 22.

[0029] In order to obtain a particularly long Rayleigh length with a large focus diameter, as is desired when cutting thick sheets, the first movable lens assembly 211, as shown in Figure 2 (c)shown, is moved even closer to the working laser beam source, while the second movable lens assembly 212 must be moved even closer to the focusing optics 22. In particular, Figure 2 (c) shows that the numerical aperture of the image side NA' is smaller than the numerical aperture of the object side NA, resulting in a magnification m > 1.

[0030] The major advantage of the imaging optics 20 according to the invention is that, in addition to the simple and flexible adjustment of the focus position and focus diameter, it also enables an extended adjustment range for the focus position. In particular, the adjustment range for the focus positions can be maximized without adjusting the focusing optics 22, since the focus diameter and focus position are adjusted by means of the movable lens groups 211, 212 in the collimation range and not with optical elements of the focusing range. This particularly exploits the fact that changes in the focus position are linear with the change in position of the focusing optics 22, while changes in the focus position are quadratic in relation to the change in an optical component in the collimation range. This means that even small displacements of the movable lens groups 211, 212 in the collimation range result in large displacements of the focus position.

[0031] A further advantage of the imaging optics according to the invention is that the use of a collimator optics 21, which according to the invention is constructed in accordance with a Galilean telescope, enables a short overall length of the imaging optics 20 without a real intermediate focus.

[0032] All this is achieved with a total of only three lens groups, two of which are for collimation and one for focusing, with only the first two being movable according to the invention. This makes the optical zoom system of the imaging optics according to the invention very simple in construction and at the same time resistant to thermal focus shift.

Claims

1. Imaging optics (20) for material machining by means of laser radiation, consisting of: a collimator optics (21) for collimating a divergent working laser beam (121) and a focusing optics (22) for focusing a working laser beam (12) on a workpiece (14) to be machined, characterised in that the collimating optics (21) comprises a first movable lens or lens group (211) with positive focal length for imaging a working laser beam source (18) in a virtual intermediate focal point and a second movable lens or lens group (212) with negative focal length for imaging the virtual intermediate focal point to infinity, and wherein only the first lens or lens group (211) and the second lens or lens group (212) are movable.

2. The laser machining head (1) with a housing (10) through which a working laser beam (12) can be passed, and with imaging optics (20) according to claim 1 for producing a working focus (15).

3. The laser machining head (1) according to claim 2, characterised in that the first movable lens or lens group (211) and the second movable lens or lens group (212) are displaceable by respective actuating drives (A, B) in order to adjust at least one of a focal diameter and a focal position.

4. The laser machining head (1) according to claim 3, characterised in that the actuating drives (A, B) operate independently of one another.

5. The laser machining head (1) according to claim 3 or 4, characterised in that the position of the movable lenses or lens groups (211, 212) relative to one another and relative to the focusing optics (22) is adjusted in dependence on the type of machining and the laser wavelength by means of a control unit (30).

6. The laser machining head (1) according to claim 5, characterised in that the control unit (30) is programmable.