WO2018099851A1

WO2018099851A1 - Laser beam shaping element

Info

Publication number: WO2018099851A1
Application number: PCT/EP2017/080498
Authority: WO
Inventors: Axel Heinrici; Alexander Müller
Original assignee: Highyag Lasertechnologie GmbH
Current assignee: Highyag Lasertechnologie GmbH
Priority date: 2016-11-29
Filing date: 2017-11-27
Publication date: 2018-06-07
Anticipated expiration: 2019-05-29
Also published as: LU93326B1; DE112017000225B4; DE112017000225T5

Abstract

The invention relates to an optical device and its use for generating a desired shape of the focus of a laser beam. By means of such a device changes in the beam spot geometry can be achieved in the focus area of the laser beam. The invention provides a focus-shaping element, wherein the focus-shaping element is refractive or reflective, comprising a segmented region with an array of polygonal lenslets for generating a polygonal main spot and at least one plane surface for generating at least one secondary spot.

Description

LASER BEAM SHAPING ELEMENT

Description

FIELD OF THE INVENTION

[0001] The invention relates to an optical device and its use for generating a desired beam shape of a laser's focus. By means of such a device, changes in the beam spot geometry can be achieved in the focal region of the laser beam.

BACKGROUND ART

[0002] In material processing fiber-guided lasers are usually used. The laser beam emerging from a light guide is first coUimated in a laser processing head mounted at the end of the light guide by means of laser optics with appropriate lenses and then focused.

[0003] The collimation serves to make from the divergent laser beam cone a nearly parallel beam. Focusing serves to form the coUimated beam into a laser spot, which is then used for material processing.

[0004] The distance between collimation and focusing has no significant effect on the so- called imaging performance in terms of the size of the laser spot. In the area of the coUimated, parallel beam, therefore, different sensors and cameras can be reflected, without affecting the system. For this reason, focus-shaping elements are also arranged in this area.

[0005] For the production of cohesive connections by means of brazing or welding with additional wire with the aid of high-power lasers, in particular of galvanic or dip-galvanizing sheets exist various technical solutions in the prior art technology. Here, opto-mechanical systems are used with laser head, which project the laser radiation on the workpiece, run the head along the seam to be created and an additional material, for example, for a joining process, perform. [0006] In such known systems, a round area on the workpiece with a diameter of a few millimeters is usually irradiated with a focused laser spot during brazing. The filler material is thereby dragged over a wire, that is to say fed counter to the feed direction of the laser head to the workpiece.

[0007] A particular embodiment of laser heads generates on the workpiece a main spot for the actual joining process of two parts and two leading small areas, which are also subjected to laser power. The leading small areas ensure a heat pretreatment of the surfaces of the parts to be joined together, whereby the flow behavior of the filler material in the subsequent joining process is positively influenced.

[0008] There are different possibilities for the production of a main spot with the two additional spots: The special three foci on the workpiece can be generated by a fiber-connector with three glass fibers that were spliced to a quartz block. This results in a spot geometry with a large round main spot and two other small round side spots. A disadvantage of this solution is that this requires three laser sources.

[0009] Another implementation for generating a plurality of spots produces a special focus shape by shaping the three focuses over a focus shaping element. It is possible to create a spot geometry with a main spot and two secondary spots, with all three spots having a substantially rectangular shape. The focus element required for this is based on an easily calculated and easy-to-display geometry. A disadvantage of this embodiment is that the energy in the main laser spot is introduced almost uniformly over the width of the whole solder seam. This leads either to overheating or too slow solidification of the solder or thermal conduction welding seam in the edge region or too low energy input in the middle part of the seam.

[0010] WO 2014/045147 Al discloses an optical device comprising a first surface with a plurality of micro sized facets, each facet having a respective orientation. Said plurality of facets having an optical axis which extends parallel to the normal vector to an average orientation of all said respective orientations. The plurality of micro- sized facets comprises a meaningful pattern forming sub-set of facets. Said subset has at least one feature chosen from: equal orientation (tilt and azimuth), similar color, similar marking (scratching, frosting, ribbing), simi- lar spacing with adjacent facets. This document does not refer to an array of lenslets used in laser beam welding, but to optical flats used in a lighting device.

[0011] U.S. Patent Application No. 2013/265755A1 refers to a lens system including one or more insertable lenses that are secured from movement relative to a protective transparent media that itself is securable to a lighting enclosure with a light source in the lighting enclosure. The insertable lenses can be removable or alternately oriented to alter the distribution of light or the projection direction of light, respectively. This document does not refer to laser beam shaping, but to the distribution of light or the projection direction of light.

[0012] In document EP 0 706 072 A2 a unit for splitting a laser beam into a plurality of beams is provided separately from a converging unit so that both units can be manufactured easily, replaced at low cost and are less likely to be polluted or damaged during laser machining. A laser beam produced by a laser oscillator and guided into a machining head through a transmitter is split into a plurality of beams by a plane reflecting mirror. The laser beams thus split are reflected by another reflecting mirror having a single paraboloidal surface so as to be converged on focal points. The first plane reflecting mirror comprises two semicircular mirrors which can be inclined independently of each other. This document refers to a laser beam splitter by using an array of angled mirrors, but not to a monolithically element for splitting a single beam into separate beams generating multiple foci and for focus shaping.

[0013] Beam shaping by means of a lens array can be found in the prior art in the documents WO 2006066706 A2, US020060209310A1, DE 112005003207 B4. [0014] With none of the known devices, it is possible to form a plurality of foci from a single laser source, without the energy being introduced evenly across the surface of the main spot transversely to the feed direction of a main spot. There is further a need for a device allowing to generate a plurality of foci and to shape the foci.

BRIEF DESCRIPTION OF THE INVENTION [0015] The invention is therefore based on the objective to provide a device for focus shaping, in which a main spot and secondary spots are formed. The power density in the main spot is essentially homogeneous and corresponds in a cross-section to a top-hat distribution. The spot geometry ensures that the spot is sharply delimited at the edge transversely to the feed direction and at the same time a longer exposure time of the laser is achieved in the middle of the soldering seam. As a result, the energy input in the middle of the seam is higher than at the edge.

[0016] Within the context of the present disclosure main and secondary spots refer to spots of a split laser beam. The main spot is in terms of area and power larger than a secondary spot.

[0017] The term lenslets within the meaning of the present disclosure refers to small lenses and not to facets within the meaning of optical flats. The lenslets are periodic and thus congruent with the exception of a parallel shift.

[0018] The invention provides a device comprising a focus-shaping element which is refractive or reflective, comprising a first and a second part, wherein said first part comprises a segmented region with an array of polygonal lenslets for generating a polygonal main spot and wherein said second part comprises at least one plane surface for generating at least one secondary spot.

[0019] It is envisaged that the focus- shaping element comprises similar shaped polygonal lenslets, wherein the polygonal lenslets may have three, four or six corners. [0020] Furthermore, it is provided that the at least one plane surface can be angled to the segmented region and the second part may also comprise at least two plane surfaces, wherein the plane surfaces can be angled to each other.

[0021] The focus-shaping element may be monolithic.

[0022] It is also intended that the surface of a lenslet is continuously differentiable, wherein the surface of the segmented area may be continuous, but not continuously differentiable. [0023] The focus-shaping element can be arranged in a laser beam optic in a collimated laser beam, wherein the focus- shaping element can also be arranged movably transversely to the beam direction (longitudinal axis) of the collimated laser beam.

[0024] For the focus-shaping element is provided that it may consist of zinc sulfide, quartz glass, aluminum or copper or a combination of aluminum and copper.

[0025] The focus-shaping element may have a diameter of up to 45 mm, a depth of the opti- cally effective parts of up to 6 mmm and refractive elements of up to 8 mm.

[0026] Furthermore, the angle of the plane surfaces of the focus- shaping element to the segmented area can be up to 2 °, wherein the angle of at least two plane surfaces can be up to 1.5 o

[0027] It is further envisaged that up to 60% of the total area of the focus shaping element comprises polygonal lenslets 5.

[0028] Another object of the invention relates to a laser beam optic, comprising a focus- shaping element, wherein the focus-shaping element is refractive or reflective, comprising a first and a second part, wherein said first part comprises a segmented region with an array of polygonal lenslets for generating a polygonal main spot and wherein said second part comprises at least one plane surface for generating at least one secondary spot. [0029] For the laser optics is further provided that the polygonal lenslets may have a similar shape, wherein the polygonal lenslets may have three, four or six corners.

[0030] The laser optics can furthermore comprise focus-shaping element that is arranged between a coUimation unit and a focusing unit, wherein the focusing unit and the coUimation unit can be arranged so as to be movable relative to one another and transversely to the longitudinal axis of the laser beam. [0031] The laser optics may have a focal length of 123 mm to 200 mm.

[0032] Another object of the present invention is the use of a focus-shaping element or a laser optics as stated above to produce a main spot with a diameter of up to 4 mm and at least one secondary spot with a diameter of up to 2.5 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The invention will be described on the basis of figures. It will be understood that the embodiments and aspects of the invention described are only examples and do not limit the protective scope of the claims in any way. The invention is defined by the claims and their equivalents. It will be understood that features of one aspect or embodiment of the invention can be combined with a feature of a different aspect or aspects and/or embodiments of the invention.. It shows: FIG. 1: Top view onto the focus- shaping element

FIG. 2: Section through the focus-shaping element, sectional plane A-A '.

FIG. 3: Spot image generated by focus shaping element with two plane surfaces

DETAILLED DESCRIPTION OF THE INVENTION AND DRAWINGS

[0034] The present invention provides a focus-shaping element which makes it possible to produce a hexagonal main spot, in which the main energy is introduced centrally transversely to the moving direction of the spot. In addition, at least one secondary spot is produced by the invented focus- shaping element. For joining two workpieces, it is envisaged that two round secondary spots are produced.

[0035] The hexagonal main spot with the two secondary spots running in feed direction is generated via a segmented focus-shaping element. Essential in the context of the invention is the use of a monolithic element, which allows the production of one piece and thus a simple and inexpensive production. A joining of several elements is therefore not required. [0036] FIG. 1 shows a top view onto a schematic representation of a focus shaping element 1 according to the invention. In the upper area of the focus-shaping element 1, a region or segment can be seen, which is formed from hexagonal lenslets 5. Each lenslet has a continuously differentiable surface, i. e. the surface of a lenslet 5 has no edges, kinks, height jumps or misa- lignment.

[0037] The hexagonal lenslets 5 form a closed surface in the upper part of the focusing element 1 according to the invention. Between the hexagonal lenslets 5, edges are formed with the hexagonal or honeycomb pattern shown in FIG 1.

[0038] In the lower part of the focus-shaping element 1 in FIG. 1, two plane surfaces 10 can be seen. The number of plane surfaces 10 is variable.

[0039] In FIG. 2, a section through the plane A-A ' as indicated in FIG. 1 can be seen in a highly schematically simplified manner, the section going through a plane surface 10 and the segmented region with the hexagonal lenslets 5. The strong concave shape of the lenslets 5 is shown only by way of example. It will be apparent to one skilled in the art that the angle and curvature of the lenslets 5 are greatly exaggerated. In a true-to-scale cut, the element would be completely flat and at best have a slight roughness in the area of the lenslets 5. The expert also realizes that the section, as shown in Figure 1, cuts the lenslets 5 irregularly. These are shown in FIG. 2 evenly.

[0040] The continuously differentiable shape of the surface of the individual hexagonal lenslets 5 can be seen in FIG. 2. The cut plane surface 10 is angled with respect to the seg- mented region of the focus-shaping element 1. It cannot be seen that the two plane surfaces 10 shown in FIG. 1 are angled relative to each other. By the angle of the plane surfaces to each other and to the segmented area, the distance of main spot and secondary spots can be adjusted to each other. [0041] FIG. 3 shows main spot 15, generated by the hexagonal lenslets 5 of the segmented area of the focus-shaping element 1. In addition, the two secondary spots 110 can be seen produced by the plane surfaces 10 and correspond in shape and size to a spot without a focus shaping element.

[0042] The hexagon of the main spot is located centrally on the optical axis and thus corre- sponds to the position of the laser beam focus without focus-shaping element. The proportion of the laser power in the individual laser spots corresponds to the proportion of the laser power in the raw beam.

[0043] In addition to hexagons, other shapes for filling the segmented region of the focus- shaping element are conceivable. All polygons, such as triangles or squares are also conceivable. It has been shown that hexagons are particularly advantageous since the energy distribution in such a spot formed in this way is approximately greatest in the middle transverse to the direction of movement. In principle, the energy input at the edge is sharply limited and the energy deposited in the middle of a hexagon is twice as high, which in the present case can be regarded as an advantage.

[0044] The plane surfaces generate the secondary spots. It is basically conceivable to adapt the number of secondary spots to the requirements of the respective process in which the invention shall be used. Two secondary spots, for example, are particularly advantageous when joining roof seams of a car body, if both workpieces are made of galvanized steel.

[0045] The secondary spots generated by the plane surfaces have the size and shape of the laser spot without focus-shaping element. Due to the inclination of the plane surfaces, the secondary spots are slightly offset from the optical axis.

[0046] The focus-shaping element can be moved transversely to the beam direction in the collimated laser beam or the collimation unit of the laser processing head and thus the parallel beam can be moved. By shifting the focus element in the collimated laser beam, the power distribution in the individual spots can be adjusted. This advantage of the invention is only possible by using a segmented focus-shaping element. NUMERALS focus-shaping element polygonal lenslet plane surface main spot

secondary spot

Claims

1. A focus-shaping element (1), wherein the focus-shaping element (1) is refractive or reflective, comprising a first and a second part, wherein said first part comprises a segmented region having an array of polygonal lenslets (5) for generating a polygonal main spot (15) and wherein said second part comprises at least one plane surface (10) for generating at least one secondary spot (110).

2. The focus- shaping element (1) according to claim 1, wherein the first part comprises only similar shaped polygonal lenslets arranged periodically (5).

3. The focus- shaping element (1) according to claim 1 or 2, wherein the polygonal lenslets 5 have three, four or six corners.

4. The focus-shaping element (1) according to any one of claims 1 to 3, wherein the second part with the at least one plane surface (10) is angled to the first part with the segmented region.

5. The focus-shaping element (1) according to any one of claims 1 to 4, wherein the second part comprises at least two plane surfaces (10).

6. The focus-shaping element (1) according to claim 5, wherein said two plane surfaces (10) are angled to each other.

7. The focus-shaping element (1) according to any one of claims 1 to 6, wherein the focus-shaping element (1) is monolithic.

8. The focus- shaping element (1) according to any one of claims 1 to 7, wherein the surface of a lenslet (5) is continuously differentiable.

9. The focus-shaping element (1) according to any one of claims 1 to 8, wherein the surface of the segmented region is continuous but not continuously differentiable.

10. The focus-shaping element (1) according to any one of claims 1 to 9, wherein the focus-shaping element (1) is arranged in a laser beam optic in a collimated laser beam.

11. The focus-shaping element (1) according to any one of claims 1 to 10, wherein the focus-shaping element (1) is arranged movable transversely to the beam direction of the collimated laser beam.

12. The focus-shaping element according to any one of claims 1 to 11, consisting of zinc sulfide, quartz glass, aluminum or copper or a combination of aluminum and copper.

13. The focus- shaping element according to any one of claims 1 to 12, wherein the diameter of the focus- shaping element is up to 45 mm.

14. The focus-shaping element according to any one of claims 1 to 13, with a depth of the optically effective parts of up to 6 mmm, with refractive elements of up to 8 mm.

15. The focus- shaping element according to any one of claims 1 to 14, wherein the angle of the plane surfaces (10) to the segmented region is up to 2 °.

16. The focus-shaping element according to one of claims 1 to 15, wherein the angle of at least two plane surfaces (10) to each other is up to 1.5 °.

17. The focus-shaping element according to any one of claims 1 to 16, wherein up to 60% of the total area of the focus- shaping element has polygonal lenslets 5.

18. A laser optical system comprising a focus-shaping element (1) wherein the focus- shaping element (1) is refractive or reflective, comprising a first and a second part, wherein said first part comprises a segmented region with an array of polygonal lenslets (5) for generating a polygonal main spot (15) and wherein said second part comprises at least one plane surface (10) for generating at least one secondary spot (HO).

19. A laser optical system according to claim 18, wherein the polygonal lenslets (5) are similar.

20. A laser optical system according to any one of claims 18 or 19, wherein the polygonal lenslets (5) have three, four or six corners.

21. A laser optics according to any one of claims 1 to 20, wherein the focus-shaping element (1) is arranged between a collimation unit and a focusing unit.

22. A laser optics according to one of claims 18 to 21, wherein the focus-shaping element (1) and collimation unit are arranged displaceable relative to one another.

23. A laser optics according to any one of claims 18 to 22, further comprising a focal length of 123 mm to 200 mm.

24. The use of a focus-shaping element according to any one of claims 1 to 17 or a laser optics according to one of claims 18 to 23 for producing a main spot with a diameter of up to 4 mm and at least one secondary spot with a diameter of up to 2.5 mm.