Improvements in or relating to laser beam shapers

Abstract

A laser beam shaping optical component 10 comprising an array of beam shaping optical elements, each element having an off-centre apex on a surface thereof. The element surfaces may be profiled upon t

Description

The present invention relates to laser beam shapers and in particular, though not exclusively, to refractive optical components formed of arrays of beam shaping optical elements. Laser beam shaping is increasingly used in laser material processing, additive manufacture, and biomedical fields. Output beam profiles can be tailored to the requirements of the application, and do not need to be limited to typical Gaussian or super Gaussian appearances. Beam shaping can be achieved through refractive or diffractive optical elements (ROEs / DOEs), or more complex approaches such as coherent beam combination and multiplane light conversion. ROEs can provide beam shaping solutions with high transmission and shaping efficiency in a single optical component, without the need for bulky or complex systems. Laser beam shaping using an array of lenses (lens array homogeniser / diffracting homogeniser) in a single optical component is a known method for producing a homogenised laser spot at the focus of a lens from a single input laser beam. The lenses are identical and the lens aperture shape and F-number dictates the shape and size of the output laser spot. Such laser beam shaping optical components with a conventional basic lens array have disadvantages in that they only produce a flat top output of a fixed size and the output beam profile is limited to typically symmetrically uniform square, rectangular or hexagonal profiles based on the sub-aperture shape in the array. It is an object of the present invention to provide a beam shaping optical component which obviates or mitigates at least one of the disadvantages of the prior art. According to a first aspect of the present invention there is provided a laser beam shaping optical component comprising an array of beam shaping optical elements, each beam shaping optical element having an off-centre apex on a surface thereof. In the prior art lens arrays, the lenses in the array are centred within their sub-apertures, i.e. the lens apex is in the centre of the aperture. In contrast, the elements in the sub-apertures of the present invention are located off-axis or off-centre within the sub-aperture. This beam shaping method comprising of arrays of identical optical elements with off-centre apexes, produces an asymmetrical laser spot profile for a beam passing though the laser beam shaping optical component. This redistribution of intensity can be used for pre and / or post heating of a workpiece in laser welding. Preferably, the off-centre apex provides an off-centre surface origin from which the element surface is profiled on an X-Y plane orthogonal to a Z-axis being an intended direction of a beam passing though the laser beam shaping optical component. The off-centre surface origin may be displaced in the X and / or Y directions in the array. In this way, the apex position is chosen to control the degree of asymmetry in the output of the beam shaper. An angular profile of the elements may be radially symmetric or X / Y separable functions about the off-centred surface origin. In this way, the angular profile of the elements can be customised to generate tailored output intensity distributions and so control the shape of the output of the beam shaper. The angular profile of the elements may comprise a conventional spherical lens curvature. Alternatively, the angular profile of the elements may comprise a conventional conic or higher order aspheric lens curvature. The angular profile may follow an arbitrary polynomial with odd, even or both terms, either radially or in an X / Y separable manner. In this way the output intensity distributions can be tailored to particular material and process condition requirements for laser material processing such as welding. Leading and / or trailing intensity along the weld direction, as well as lateral "wings" either side can all be tuned to adjust the heat profile across a workpiece. Preferably the elements are arranged in an array with a pitch smaller than a size of the input laser beam. In this way a majority of elements contributes to the intensity distribution of the output laser beam. Preferably the element has a tessellating shape. The element shape may be one of a group comprising: square, rectangular and hexagonal. Preferably, the laser beam shaping optical component is part of an assembly including a processing head to support the beam shaping optical component. More preferably the assembly includes an input laser source to provide the laser beam. In this way, there is provided laser beam shaping optical apparatus comprising: a single laser source providing an input beam; a single beam shaping optical component on which at least one surface is contoured; and a focusing lens to focus a shaped output beam from the single beam shaping element onto a focal plane; characterised in that: the at least one surface of the beam shaping optical component comprises an array of beam shaping optical elements, each beam shaping optical element having an off-centre apex on a surface thereof . Preferably, a lens is also mounted in the processing head. More preferably the assembly includes an input laser source to provide the laser beam. The laser beam shaping optical component may be positioned between the laser source and the lens. Alternatively, the lens may be positioned between the laser source and the laser beam shaping optical component. In this way the single lens may be considered as a relay lens. Preferably, the assembly includes a collimating lens and a focusing lens mounted in the processing head. In a preferred embodiment the laser beam shaping optical component is arranged between the collimating lens and a focusing lens, with the collimating lens positioned between the laser source and the laser beam shaping optical component. Alternatively, the laser beam shaping optical component is arranged between the laser source and the collimating lens. Optionally, the focusing lens is arranged between the collimating lens and the laser beam shaping optical component with the collimating lens arranged between the laser source and the focusing lens. The invention provides a method for generating asymmetric laser beam profiles using a refractive, transmissive array of beam shaping elements with off centre surface origins and tailored angular profiles. The angular profile of the elements may be customised to generate tailored output intensity distributions. The profiles may be radially symmetric or X / Y separable functions about the off-centred origin. The output intensity distributions may be tailored to particular material and process condition requirements for laser material processing such as welding. The beam shaper may be used in a rotational mount to adjust the direction of the asymmetric profile to match the direction of welding. Embodiments of the present invention will now be described with reference to the accompanying drawings. Figure 1 illustrates a surface of a refractive optical beam shaping component comprising an array of beam shaping optical elements, according to an embodiment of the present invention; Figures 2 (a)-(d) are plots of an example surface profile of a beam shaping optical element for use in the array of an optical beam shaping component according to an embodiment of the present invention; Figure 3 is a schematic illustration of an assembly including a beam shaping optical component according to an embodiment of the present invention; Figures 4(a)-(c) are a representative output beam profile showing a laser spot shape from the assembly of Figure 2; Figures 5(a) and 5(b) are schematic illustrations of an assembly including a beam shaping optical component according to embodiments of the present invention; and Figures 6(a) and 6(b) are schematic illustrations of an assembly including a beam shaping optical component according to embodiments of the present invention. Referring initially to Figure 1 of the drawings there is illustrated a refractive optical beam shaping component, generally indicated by reference numeral 10, comprising an array of beam shaping optical elements 12. Each beam shaping optical element 12 has an off-centre apex 14 which produces an asymmetrical laser spot profile 32 for a beam 30 passing though the laser beam shaping optical component 10 as seen in Figures 3 and 4(a). This redistribution of intensity can be used for pre and / or post heating of a workpiece in laser welding. Figure 1 shows an array arranged as 11 x 11 micro-optic elements 12, the component 10 may have any number i.e. n x n and shape of elements 12 as long as they are tessellating shapes such as square, rectangular or hexagonal. The component is generally formed by micro-machining of around 10mm2 square or rectangular 1mm thick substrate plate made of fused silica, quartz glass, sapphire, ZnSe or the like. The present applicants use a freeform direct-write process in fused silica as described in US9089927, incorporated herein by reference, to create the component. The individual elements 12 are constructed with an off-centre apex 14. In this specific example, the surface follows a cubic function: z(x,y) = zx(x) + zy(y) zx = AxQx-xo03 zy = Ay(ly-yol)3 Ax = —3e — 8, Ay = —1.2e — 8, x0 = 250, y0 = 0 Other functions are possible. Tuning the dependency on x and y coordinates, or alternatively r and theta coordinates, controls the shape of the output profile. In this example, the cubic coefficients control the maximum extent of the output spot, while the offsets control the degree of asymmetry. Figures 2(a)-(d) show an individual element 12 with off-centre apex 14. Figure 2 (a) is a top view in the x-y plane with shading to indicate height in the z-axis. Figure 2(b) is the asymmetric surface 16 shown in the isometric view. Figure 2 (c) is slice in the x-z plane showing the asymmetry and off-axis apex and Figure 2 (d) is a slice in the y-z plane. An example system setup, assembly 20, is shown in Figure 3. This may be considered as a laser beam shaping optical apparatus. A laser source 22, often from a fibre, is relayed onto a workpiece 24 using a collimating 26 and focusing 28 lens. The elements 12 are arranged in the array of the component 10 with a pitch smaller than the size of the input laser beam 30. The beam shaper component 10 can be positioned anywhere in the collimated space between the two lenses 26,28 and these components 26,10,28 are mounted in a processing head (not shown). The arrangement produces a focused laser spot 32 at the focal plane 34, which is at the position of the workpiece 24. The beam shaper component 10 may be used in a rotational mount to adjust the direction of the asymmetric profile to match the direction of welding. Figures 4 shows the output from the example beam shaper component 10 used in the configuration of Figure 3. It can be seen that the output profile is highly asymmetrical in x (Figure 4(b)), owing to the apex position offset of 250 pm, which is half of the pitch of the lens / element 12. In y, the output is symmetrical (Figure 4(c)), as the apex 14 of the unit cell / element 12 has not been shifted in y. The cubic nature of the surface 16 results in a decaying tail preceding / following the primary output spot. This slows the rate of change of temperature in laser welding. The beam shaping array element 12 can be adjusted to alter the output laser spot 32 to match processing requirements. The position of the element apex 14 dictates the degree and direction of asymmetry in the output laser spot 32. Offsetting in Y will adjust Y asymmetry, and likewise in X. The profile 16 of the element 12 can be adjusted to alter the profile shape. The profile 16 may be radially equal or separable in X and Y. The profile 16 may follow a radius of curvature as a lens, or can take the form of polynomial terms. These terms may be odd, even, or both. Additionally, the aperture shape of the element 12 can be adjusted. This can be square (as shown in Figure 1), rectangular, or hexagonal. Changing the aperture shape will change the shape of the output spot 32 accordingly, also depending on the profile 16 chosen, as will be recognised by those skilled in the art. The beam shaper component 10 can be mounted in different positions within a processing head. Reference is now made to Figures 5(a) and 5(b) which illustrate assemblies 20a,20b being laser beam shaping optical apparatus, according to further embodiments of the present invention. Like parts to the assembly 20 of Figure 3 have been given the same reference numerals to aid identification of the component parts. For assembly 20a, the beam shaping component 10a is positioned in the divergent path before the collimating lens 26a. The input laser beam 30a from the single laser source 22a first passes through the beam shaping component 10a, before the collimating lens 26a and focusing lens 28a to create a laser spot 32a of the desired output profile at the focal plane 34a. In Figure 5(b), the beam shaping component 10b, is now located after the focusing lens 28b. The input laser beam 30b from the single laser source 22b first passes through the collimating lens 26b and then the focusing lens 28b before passing through the beam shaping component 10b to create a laser spot 32b of the desired output profile at the focal plane 34b. Used in this configuration the shaped laser spot 34b will be reduced in size compared to mounting the beam shaper component 10 in the collimated beam path, as shown in Figure 3. The beam shaper component 10 could also be mounted in a processing head consisting of a single lens. Reference is now made to Figures 6(a) and 6(b) of the drawings, which illustrate assemblies 20c,20d. In each assembly 20a,20b the laser source 22c,22d produces a single laser beam 30a,30d which passes through a beam shaping component 10c, lOd either before, Figure 5(a), or after, Figure 5(b), the single lens 28c,28d. Here the lens 28c,28d may be considered as a relay lens. A focused laser spot 32c,32d with the desired output beam profile is formed at the focal plane 34c,34d where a workpiece 24 can be located. The principal advantage of the present invention is that it provides a beam shaping optical component using a refractive, transmissive array of beam shaping elements with off centre surface origins and tailored angular profile which can be used for generating asymmetric laser beam profiles. A further advantage of the present invention is that it provides a beam shaping optical component in which leading, trailing, and lateral intensity output beam profiles can be engineered independently and tailored to the requirements of the application. A still further advantage of the present invention is that it provides a beam shaping optical component which can provide an asymmetric output beam giving the advantage of pre and / or post heating of a workpiece in conjunction laser welding, without the need for additional complex optical systems. A yet further advantage of the present invention is that it provides a beam shaping optical component for which the beam shaping is performed with a single optical element installed within the optical train of the laser system. This does not require additional passes or additional lasers, as the pre or post heating is achieved in line with the primary laser spot. A still further advantage of the present invention is that it provides a beam shaping optical component which produces an output profile that is largely independent of the input beam profile. This is advantageous if the input beam fluctuates temporally. It will be appreciated by those skilled in the art that modifications may be made to the invention herein described without departing from the scope thereof. For example, while a refractive optical component is described, the component may be a reflective optic. Additionally, the lens can be machined on a surface of the substrate with the array on the opposite surface of the substrate.

Claims

1. A laser beam shaping optical component to produce an asymmetric laser spot profile for a laser beam passing through the optical beam shaping optical component, comprising an n x n array of identical beam shaping optical elements, the beam shaping optical elements having an off-centre apex on a surface thereof.

2. A laser beam shaping optical component according to claim 1 wherein the off-centre apex provides an off-centre surface origin from which the element surface is profiled on an X-Y plane orthogonal to a Z-axis being an intended direction of a beam passing though the laser beam shaping optical component.

3. A laser beam shaping optical component according to claim2 wherein the off-centre surface origin is displaced in theX direction in the array.

4. A laser beam shaping optical component according to claim2 wherein the off-centre surface origin is displaced in theY direction in the array.

5. A laser beam shaping optical component according to claim2 wherein the off-centre surface origin is displaced in theX and Y directions in the array.

6. A laser beam shaping optical component according to claim2 wherein the off-centre surface origin is displaced in theX and / or Y directions in the array.7 . A laser beam shaping optical component according to any one of claims 2 to 6 wherein an angular profile of the elements is radially symmetric about the off-centred surface origin.8 . A laser beam shaping optical component according to any one of claims 2 to 6 wherein an angular profile of the elements is X / Y separable functions about the off-centred surface origin.

9. A laser beam shaping optical component according to claim 7 wherein the angular profile of the elements comprises a spherical lens curvature.

10. A laser beam shaping optical component according to claim 8 wherein the angular profile of the elements is a conic lens curvature.

11. A laser beam shaping optical component according to claim 8 wherein the angular profile of the elements is a higher order aspheric lens curvature.

12. A laser beam shaping optical component according to claim 7 or claim 8 wherein the angular profile of the elements is selected from a group consisting of; an arbitrary polynomial with odd terms; an arbitrary polynomial with even terms: and an arbitrary polynomial with odd and even terms .

13. A laser beam shaping optical component according to any preceding claim wherein the elements are arranged in an array with a pitch smaller than a size of an intended input laser beam.

14. A laser beam shaping optical component according to any preceding claim the elements have a tessellating shape.

15. A laser beam shaping optical component according to claim 14 the shape is one selected from a group consisting of: square; rectangular; hexagonal; octagons with squares; and alternating triangles.

16. A laser beam shaping optical component according to any preceding claim wherein the laser beam shaping optical component is part of an assembly and the assembly includes an input laser source to provide a laser beam.

17. A laser beam shaping optical component according to claim 16 wherein the assembly includes a focusing lens.

18. A laser beam shaping optical component according to claim 17 wherein the laser beam shaping optical component is positioned between the laser source and the focusing lens.

19. A laser beam shaping optical component according to claim 17 wherein the focusing lens is positioned between the laser source and the laser beam shaping optical component.

20. A laser beam shaping optical component according to claim 17 wherein the assembly includes a collimating lens.

21. A laser beam shaping optical component according to claim 20 wherein the laser beam shaping optical component is arranged between the collimating lens and the focusing lens, with the collimating lens positioned between the laser source and the laser beam shaping optical component.

22. A laser beam shaping optical component according to claim 20 wherein the laser beam shaping optical component is arranged between the laser source and the collimating lens, with the collimating lens between the laser beam shaping optical component and the focusing lens.

23. A laser beam shaping optical component according to claim 20 wherein the focusing lens is arranged between the collimating lens and the laser beam shaping optical component with the collimating lens arranged between the laser source and the focusing lens.

24. A laser beam shaping optical component according to any one of claims 16 to 23 wherein the assembly includes a rotational mount and the laser beam shaping optical component is located in the rotational mount.

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