GB2582331A

GB2582331A - Apparatus for laser processing a material

Info

Publication number: GB2582331A
Application number: GB1903752.2A
Authority: GB
Inventors: John Keen Stephen; Andre Codemard Christophe; Botheroyd Iain
Original assignee: SPI Lasers UK Ltd
Current assignee: Trumpf Laser UK Ltd
Priority date: 2019-03-19
Filing date: 2019-03-19
Publication date: 2020-09-23
Also published as: GB201903752D0

Abstract

Apparatus for laser processing a material 10, comprising; a) a plurality of first lasers 1; b) at least one second laser 2; c) a plurality of first optical fibres 3; d) at least one second optical fibre 4; e) a controller 9; f) a first lens 17; g) a laser processing head 18, and, h) a second lens 19, wherein; the first lasers emit optical radiation 11 at a first wavelength 13, the second laser (2) emits optical radiation 12 at a second wavelength 14, the first lens collimates the first and second optical radiation through the laser processing head, and the second lens focusses the first and the second optical radiation to form a beam waist 15 onto a surface of the material, such that, a first intensity profile 21 of the first optical radiation at the beam waist is different from a second intensity profile 22 of the second optical radiation at the beam waist. The first and second optical fibres may be coupled to a third optical fibre, which may comprise a central core, a ring core surrounding the central core and a cladding surrounding the ring core. Further aspects are directed towards methods of laser processing materials.

Description

Apparatus for Laser Processing a Material

Field of Invention

This invention relates to an apparatus for laser processing a material.

Background to the Invention

Laser cutting of steel is achieved by directing a laser beam to a work-piece via a process head which has optics for collimating and focusing the laser beam and a conical copper nozzle for providing a high pressure gas jet which is co-axial with the laser beam. The basic cutting operation involves using the laser beam to heat and melt a metal sheet work-piece, and using a gas jet, known as an assist gas jet, to blow the molten material out of the bottom of the cut-zone. The cutting head is moved over the sheet metal whilst maintaining a constant distance between a nozzle tip in the cutting head and the work-piece surface. The cutting head is moved in a programmed path to create the desired sheet metal profile.

In the case of cutting stainless steel, the use of an inert assist gas avoids the creation of metal oxides on the cut-edge faces of the work-piece. The metal oxides can cause problems such as weakening welded parts, reduction of the corrosive properties of the stainless steel owing to a depletion of chromium on the cut-edge faces, and increased wear on sliding parts owing to the increased hardness of the metal oxides compared to stainless steel. Since the only heat source for this cutting process is provided by the focused laser beam, a smaller focal spot size with a higher energy power density will provide more efficient cutting by generating a narrower molten region. Low divergence is required so that the melt region is narrow through the thickness of the metal. The limit on the smallest practical focused spot is determined by the optical depth of field in conjunction with the material thickness. This is because the cut-width (kerf) must be wide enough to allow the assist gas to travel to the bottom of the cut with sufficient pressure to cleanly remove molten material and avoid dross on the lower cut edge in order to generate a clean cut. For this type of cutting, the assist gas must be applied with high pressure, typically in the range of 10 to 20 bar. The diameter of the nozzle outlet is normally in the range 0.5 mm to 2.0 mm, and in general thicker materials require larger nozzles.

In the case of cutting mild steel (also known as low-carbon steel) thicker than 5 mm, it is typical to use oxygen as the assist gas. The oxygen exothermically reacts with the iron within the work-piece to provide additional heat which increases the cutting speed. The oxygen is applied at pressures typically in the range 0.25 bar to 1 bar. These pressures are much lower compared to those used for nitrogen assist gas cutting. For thick section cutting, typically in the range 10 mm to 30 mm thickness, the kerf must be wide enough so that the oxygen assist gas can reach the bottom of the cutting zone with sufficient gas flow to eject the molten material whilst maintaining a dross-free cut. It is typical for thick mild steel cutting for the beam to be defocussed such that the beam waist is above the sheet metal surface so that the incident beam diameter on the sheet metal surface is larger than the beam waist. Better quality cuts with lower edge roughness can be obtained when the divergence of the beam is increased.

Most general purpose flatbed laser cutting machines are required to cut a range of metals of varying thicknesses, with the cuts an being of good quality. The choice of focal spot size is typically a compromise of the requirements needed to meet the wide set of process conditions. For cutting thin stainless steel a small focal spot is needed with low divergence. For cutting thick mild steel a larger focal spot is needed with higher divergence. The flatbed cutting machines are designed to work with a laser having a fixed beam quality. In order to increase the processing capabilities, the cutting head may have an augmented optical system, firstly to enable limited movement of the focusing lens along the beam path to allow defocusing of the laser beam relative to the work-piece which can increase the incident spot size, and secondly to allow the focal spot diameter to be adjusted. This has limited benefit since a laser having constant laser beam quality will have a fixed relationship between the focal spot size and divergence, with this fixed relationship working in the opposite way to that desired by the cutting process regimes.

Different cutting regimes require either a small spot with low divergence or a large spot with high divergence whereas laser having a fixed beam quality can either provide a small spot with high divergence, or a large spot with narrow divergence. It is therefore not possible to optimize process parameters for all metal types and thicknesses.

Similar limitations arise with other material processing equipment, such for example as welding, marking, and additive manufacturing. In all these application areas, there is a need for a laser processing apparatus in which the beam parameter product of the laser is able to be varied, and the diameter of the focused laser beam on the material being processed is able to be varied.

An aim of the present invention is to provide an apparatus and method for laser processing a material which reduces or avoids the aforementioned problems.

The Invention: According to a first non-limiting aspect of the present invention, there is provided apparatus for laser processing a material, which apparatus comprises a plurality of first lasers, at least one second laser, a plurality of first optical fibres, at least one second optical fibre, a controller, a first lens, a laser processing head, and a second lens, wherein: * the first lasers emit first optical radiation at a first wavelength; * the second laser emits second optical radiation at a second wavelength; * the first optical fibres guide the first optical radiation from respective ones of the first lasers; * the second optical fibre guides the second optical radiation from the second laser; * the first lens collimates the first and the second optical radiation through the laser processing head; and * the second lens focusses the first and the second optical radiation to form a beam waist onto or near a surface of the material; the apparatus being characterized in that: * the first optical fibres and the second optical fibre are configured such that a first intensity profile of the first optical radiation at the beam waist is different from a second intensity profile of the second optical radiation at the beam waist; and * the controller is configured to control a power of the first optical radiation emitted by the first lasers and a power of the second optical radiation emitted by the second laser.

The first optical fibres and the second optical fibre may form an array in which the first and the second optical fibres are arranged side by side.

The array may be a two dimensional array.

The array may be a one-dimensional array.

The two-dimensional array may comprise the second optical fibre surrounded by at least one ring of the first optical fibres.

The first lens may be configured to image end faces of the first optical fibres to form the beam waist.

The first optical fibres and the second optical fibre may be coupled to a third optical fibre.

The first optical fibres and the second optical fibre may be spliced to the third optical fibre.

The first optical fibres and the second optical fibre may be coupled to the third optical fibre with a first coupler.

The first optical fibres and the second optical fibre may be coupled to the third optical fibre independently of each other.

The third optical fibre may comprise a central core, a ring core surrounding the central core, and a cladding surrounding the ring core.

The ring core may overlay the first cores, and the central core may overlay the second core The first optical fibres may be arranged in concentric rings, and the ring core may overlay more than one of the rings of the first cores.

The third optical fibre may have a plurality of ring cores that are concentric with each other, and each of the ring cores may overlay different ones of the first cores.

The first lens may be configured to image an end face (8) of the third optical fibre to form the beam waist.

The second optical fibre may be a multimode optical fibre, and the apparatus may include a second coupler configured to couple a first optical mode that can propagate along the second optical fibre to at least one second optical mode that can propagate along the second optical fibre. The first optical mode may be a fundamental mode. The second optical mode may be a ring mode. The ring mode may comprise a plurality of high intensity lobes around its azimuth.

The second coupler may comprise a long period grating configured to couple the first optical mode to the second optical mode. The second coupler may comprise an actuator configured to induce the mode coupling upon demand from the controller.

The central core of the third optical fibre may have the same optical guiding properties as the second core of the second optical fibre.

The central core may have a larger diameter than a diameter of the second core of the second optical fibre.

The first wavelength may be the same as the second wavelength.

The first wavelength may be different from the second wavelength. The first wavelength may be between two and three times the second wavelength.

The apparatus of the invention provides significant advantages over the prior art in that it can be used to optimize many different material processing operations. For example, piercing of the material can be performed with a bell-shaped or gaussian second intensity profile. Once pierced, the material can be cut using a second intensity profile that is ring shaped or top hat. Alternatively or additionally, the material can be cut using a first intensity profile that is ring shaped. The choice of which intensity profiles are optimum will vary depending on the type of material and its thickness.

This can be advantageous when cutting or welding high reflectivity materials that have reflectivities greater than 75% at 1060nm. For example, copper and aluminium have a higher reflectivity at a first wavelength of 1060 nm and a lower reflectivity at a second wavelength of 532 nm. Once laser processing has commenced with the second optical radiation from the second laser, the absorption of the material greatly improves at the first wavelength. Indeed, there is little technical advantage, if any, in processing the material with each of the first and the second lasers emitting at 532nm compared with processing the material with the second laser 2 emitting at a second wavelength 14 of 532nm, and the first laser 1 emitting at a first wavelength of 1060nm.

The ability to select from different wavelengths when processing the material gives greater choice for optimizing processes. Frequency doubled lasers emitting at 532nm are also more expensive than lasers that do not use frequency conversion, and the frequency doubling crystals require servicing. The apparatus of the invention thus provides a lower cost and more reliable solution for the processing of high reflectivity materials.

The first laser and the second laser may be each be selected from a group comprising fibre lasers, disk lasers, slab lasers, semiconductor lasers and solid-state lasers. The lasers can generate single mode or multimode outputs. The lasers might produce different wavelengths for example infra-red, green, blue or ultra-violet.

In a further non-limiting aspect, there is provided a method of laser processing a material, using an apparatus as described above, comprising: * causing the first lasers to emit first optical radiation at a first wavelength; * causing the second laser to emit second optical radiation at a second wavelength; * guiding the first optical radiation from respective ones of the first lasers using the first optical fibres; * guiding the second optical radiation from the second laser using the second optical fibre; * collimating the first and the second optical radiation through the laser processing head by the first lens; * focusing, by the second lens, the first and the second optical radiation, to form a beam waist onto or near a surface of the material; and the method further comprising: * configuring the first optical fibres and the second optical fibre such that a first intensity profile of the first optical radiation at the beam waist is different from a second intensity profile of the second optical radiation at the beam waist; and * configuring the controller to control a power of the first optical radiation emitted by the first laser and a power of the second optical radiation emitted by the second laser.

In a yet further non-limiting aspect, there is provided a method for laser processing a material, comprising: * emitting first optical radiation from a plurality of first lasers, at a first wavelength; * emitting second optical radiation from at least one second laser, at a second wavelength; * guiding the first optical radiation from respective ones of the first lasers using a first optical fibre; * guiding the second optical radiation from the second laser by a second optical fibre; * collimating, with a first lens, the first and second optical radiation through a laser processing head; * focusing, with a second lens, the first and second optical radiation to form a beam waist onto or near a surface of the material; characterized by: * configuring the first optical fibres and the second fibre such that a first intensity profile of the first optical radiation at the beam waist is different from a second intensity profile of the second optical radiation at the beam waist; and * using a controller to control a power of the first optical radiation emitted by the first lasers and a power of the second optical radiation emitted by the second laser.

Brief Description of the Drawings

Embodiments of the invention will now be described solely by way of example and with reference to the accompanying drawings in which: Figure 1 shows apparatus for laser processing a material according to the present invention; Figure 2 shows a two dimensional array of the first optical fibres and the second optical fibre; Figure 3 shows a one dimensional array of the first optical fibres and the second optical fibre; Figure 4 shows a third optical fibre having a central core and a ring core; Figure 5 shows an alternative third optical fibre having a number of discrete multimode cores; and Figure 6 shows an array of first optical fibres surrounding a central capillary.

Preferred Embodiment Figure 1 shows apparatus for laser processing a material 10, which apparatus comprises a plurality of first lasers 1, at least one second laser 2, a plurality of first optical fibres 3, at least one second optical fibre 4, a controller 9, a first lens 17, a laser processing head 18, and a second lens 19, wherein: * the first lasers 1 emit first optical radiation 11 at a first wavelength X1 13; * the second laser 2 emits second optical radiation 12 at a second wavelength X2 14; * the first optical fibres 3 guide the first optical radiation 11 from the first lasers 1; * the second optical fibre 4 guides the second optical radiation 12 from the second laser 2; * the first lens 17 collimates the first and the second optical radiation 11, 12 through the laser processing head 18; and * the second lens 19 focusses the first and the second optical radiation 11, 12 to form a beam waist 15 onto or near a surface 16 of the material 10; the apparatus being characterized in that: * the first optical fibres 3 and the second optical fibre 4 are configured such that a first intensity profile 21 of the first optical radiation 11 at the beam waist 15 is different from a second intensity profile 22 of the second optical radiation 12 at the beam waist 15; and * the controller 9 is configured to control a power of the first optical radiation 11 emitted by the first lasers 1 and a power of the second optical radiation 12 emitted by the second laser 2.

The first intensity profile 21 and the second intensity profile 22, shown in Figure 1, are ring shaped profiles. Top-hat, gaussian, and bell-shaped profiles are also possible.

The first optical fibres 3 and the second optical fibre 4 are shown forming an array 5 in which the first and the second optical fibres 3, 4 are arranged side by side. The array 5 can be a two dimensional array 25, such as shown in Figure 2, or a one-dimensional array 31, such as shown in Figure 3. Referring to Figure 2, the two dimensional array 25 can comprise the second optical fibre 4 surrounded by at least one ring of the first optical fibres 3. The first optical fibres 3 may comprise a first core 26 and a first cladding 27. The second optical fibre 4 may comprise a second core 28 and a second cladding 29. A diameter 24 of at least the first cladding 27 or the second cladding 29 is preferably as small as possible in order that the first cores 26 and the second core 28 are as close as possible. This is advantageous in order to achieve the maximum permissible brightness of the first intensity profile 21 shown with reference to Figure 1. The first cladding 27 may be etched with hydrofluoric acid in order to reduce the diameter 24.

Referring to Figure 1, the first lens 17 may image end faces 6 of the first optical fibres 3 to form the beam waist 15.

The first optical fibres 3 and the second optical fibre 4 may be coupled to a third optical fibre 7. The coupling may be performed by splicing the first optical fibres 3 and the second optical fibre 4 to the third optical fibre 7. The first optical fibres 3 and the second optical fibre 4 may be coupled to the third optical fibre 4 with a first coupler 23.

The third optical fibre 7 may be the optical fibre 40 shown with reference to Figure 4. The optical fibre 40 comprises a central core 41, a ring core 42 surrounding the central core 41, and a cladding 43 surrounding the ring core 42.

When used with the two dimensional array 25 shown in Figure 2, the optical fibre 40 and the coupling of the first optical fibres 3 and the second optical fibre 4 may be such that the ring core 42 overlays the first cores 26, and the central core 41 overlays the second core 28.

The optical fibre 40 is such that one ring of the first cores 26 overlays the ring core 42. If higher optical powers are needed, then the ring core 42 can be made larger such that more than one ring of the first cores 26 can overlay the ring core 42. Alternatively or additionally, the optical fibre 40 can have additional ring cores 42 that are arranged to be concentric with each other, and different ones of the first cores 26 can be configured to overlay the additional ring cores 42.

The third optical fibre 7 may alternatively be the optical fibre 100 shown with reference to Figure 5. The optical fibre 100 comprises a central core 101, a number of discrete multimode cores 102 surrounding the central core 101, and a cladding 103.

Another method of coupling to optical fibre 40 is to use a two dimensional array 55 as shown in Figure 6. This consists of a number of first optical fibres 3 assembled around a central capillary 60. When used with optical fibre 40, array 55 is used such that the ring core 42 overlays the first fibre cores 26. The core 41 of the optical fibre 40 then overlays the core 28 of second optical fibre 4 independently of the array 55. This allows for array 55 and optical fibre 4 to be independently manipulated or tapered prior to coupling to optical fibre 40.

The first lens 17 may image an end face 8 of the third optical fibre 7 to form the beam waist 15.

Referring to Figure 1, the second optical fibre 4 may be a multimode optical fibre, and the apparatus may include a second coupler 50 configured to couple a first optical mode 51 that can propagate along the second optical fibre 4 to at least one second optical mode 52 that can propagate along the second optical fibre 4. The first optical mode 51 is preferably a fundamental mode. The second optical mode 52 is preferably a ring mode. The ring mode may comprise a plurality of high intensity lobes around its azimuth. The second coupler 50 can comprise a long period grating configured to couple the first optical mode 51 to the second optical mode 52. The second coupler 50 can comprise an actuator 53 to induce the mode coupling upon demand from the controller 9. Other forms of second coupler 50 are also possible.

The central core 41 can have the same optical guiding properties as the second core 28. This can be achieved by ensuring the central core 41 has the same diameter and numerical aperture as the second core 28. This can be advantageous to ensure that the optical modes propagating along the second optical fibre 4 can couple adiabatically to the same optical modes propagating along the third optical fibre 4. Thus if the controller 9 were to control the coupler 50 such that it switched from outputting the first optical mode 51 to the second optical mode 52, then that second intensity profile 22 would be made to change from the profile of the first optical mode 51 to the profile of the second optical mode 52.

The central core 41 can have a larger diameter than a diameter of the second core 28. This can be advantageous if it is desired to couple from a first optical mode 51 of the second core 28 to a group of first optical modes of the central core 41, and from a second optical mode 52 of the second core 28 to a group of second optical modes of the central core 41. Such coupling can be configured to preserve a bell shaped second intensity profile 22 when coupling a fundamental mode, and a ring shaped second intensity profile 22 when coupling a ring mode.

The apparatus of Figure 1 provides significant advantages over the prior art in that it can be used to optimize many different material processing operations.

Piercing of the material 10 can be performed with a bell-shaped or gaussian second intensity profile 22. Once pierced, the material 10 can be cut using a second intensity profile 22 that is ring shaped or top hat. Alternatively or additionally, the material 10 can be cut using a first intensity profile 21 that is ring shaped. The choice of which intensity profiles are optimum will vary depending on the type of material and its thickness.

The first wavelength 13 may be the same as the second wavelength 14.

The first wavelength 13 may be different from the second wavelength 14.

The first wavelength 13 can be two times or three times the second wavelength 14. This can be advantageous when cutting or welding high reflectivity materials that have reflectivities greater than 75% at 1060nm. For example, copper and aluminium have a higher reflectivity at a first wavelength 13 of 1060 nm and a lower reflectivity at a second wavelength 14 of 532 nm. Once laser processing has commenced with the second optical radiation 12 from the second laser 2, the absorption of the material greatly improves at the first wavelength 13. Indeed, there is little technical advantage, if any, in processing the material 10 with each of the first and the second lasers 1, 2 emitting at 532nm compared with processing the material with the second laser 2 emitting at a second wavelength 14 of 532nm, and the first laser 1 emitting at a first wavelength of 1060nm.

The ability to select from different wavelengths when processing the material 10 gives greater choice for optimizing processes. Frequency doubled lasers emitting at 532nm are also more expensive than lasers that do not use frequency conversion, and the frequency doubling crystals require servicing. The apparatus of the invention thus provides a lower cost and more reliable solution for the processing of high reflectivity materials.

The first laser 1 and the second laser 2 can be selected from a group comprising fibre lasers, disk lasers, slab lasers, and rod lasers. The first laser 1 and the second laser 2 can be frequency doubled lasers, or frequency tripled lasers. Preferably when processing high reflectivity materials, the first laser 1 has a first wavelength 13 that is double or triple the second wavelength 14 of the second laser 2.

Second Preferred Embodiment With reference to Figures 1 & 4, an N:1+1 combiner characterized by the delivery fibre 40 having a core 41 and an inner cladding 42, N fibre lasers 1 are delivered by fibres 3 and coupled to the inner cladding 42 of the delivery fibre 7, independently the laser 2 feeding the core 41 of the delivery fibre 40 can be adjusted from a near single-mode to a number of distinct higher order single modes, these modes are retained through propagation through the combiner and then the delivery fibre. In one embodiment fibre laser 2 is delivered in a near single mode fibre 54 and connected via splice 56 to fibre 4 that supports a number of modes. The coupling between these two fibres should be such that the output of fibre 4 is few-moded with a beam quality factor M2 <4. The LPG apparatus of 50, 51 and 53 can be used to excite a number of high order modes 52.

For example, delivery fibre 40 has core diameter 41 equal to 50pm, core NA 0.22, inner cladding 42 equal to 180 am, inner cladding NA equal to 0.22 and an outer cladding diameter equal to 327 tim, outer cladding NA equal to 0.45. Laser 2 has a single-mode output fibre 54 with core diameter equal to 13.5 tun / cladding diameter equal to 125 pm that is spliced to a multimode fiber with core diameter equal to 50 am / core NA = 0.22/ cladding diameter equal to 125 pm. Depending on the application laser 1 can be delivered via a single mode fibre or could be a multimode source optionally of a different wavelength.

This apparatus enables the output of delivery fibre 40 to deliver a number of distinct energy distributions that are advantageous for material processing applications. These applications include laser metal cutting, welding, selective laser melting and wire additive manufacturing.

The apparatus can be switched between a high brightness source ideal for cutting of highly reflective materials, piercing and drilling to a large area annular beam ideal for cutting of thick section mild steel.

A further example of the processing advantage of this source is for spatter-free welding of reflective materials and coated materials. The outer annular beam acts to preheat the material the high order ring beam generated in the core precisely controls the melt pool formed.

A further example is powder bed additive manufacturing. Fusion of the metallic powder requires precise energy control that prevents defects and pores that might otherwise compromise the structural integrity of the pail. Single lasers can operate in only one regime with either the processing rate or feature size compromised. This source can be switched between single mode for high precision, high order annular core mode for medium precision and also a large area annular beam ideal for fusing the material at a high rate with low defects. Uniquely the source can be switched between these three modes to optimize productivity and quality.

It is to be appreciated that the embodiments of the invention described above with reference to the accompanying drawings have been given by way of example only and that modifications and additional components may be provided to enhance performance. Individual components shown in the drawings are not limited to use in their drawings and they may be used in other drawings and in all aspects of the invention. The invention also extends to the individual components mentioned and/or shown above, taken singly or in any combination.

Claims

Claims 1. Apparatus for laser processing a material (10), which apparatus comprises a plurality of first lasers (1), at least one second laser (2), a plurality of first optical fibres (3), at least one second optical fibre (4), a controller (9), a first lens (17), a laser processing head (18), and a second lens (19), wherein: * the first lasers (1) emit first optical radiation (11) at a first wavelength (13); * the second laser (2) emits second optical radiation (12) at a second wavelength (14); * the first optical fibres (3) guide the first optical radiation (11) from respective ones of the first lasers (1); * the second optical fibre (4) guides the second optical radiation (12) from the second laser (2); * the first lens (17) collimates the first and the second optical radiation (11), (12) through the laser processing head (18); and * the second lens (19) focusses the first and the second optical radiation (11), (12) to form a beam waist (15) onto or near a surface (16) of the material (10); the apparatus being characterized in that: * the first optical fibres (3) and the second optical fibre (4) are configured such that a first intensity profile (21) of the first optical radiation (11) at the beam waist (15) is different from a second intensity profile (22) of the second optical radiation (12) at the beam waist (15); and * the controller (9) is configured to control a power of the first optical radiation (11) emitted by the first lasers (1) and a power of the second optical radiation (12) emitted by the second laser (2).
2. Apparatus according to claim 1 wherein the first optical fibres (3) and the second optical fibre (4) form an array (5) in which the first and the second optical fibres (3), (4) are arranged side by side.
3. Apparatus according to claim 2 wherein the array (5) is a two dimensional array (25).
4 Apparatus according to claim 2 wherein the array is a one-dimensional array.
5. Apparatus according to claim 3 wherein the two-dimensional array comprises the second optical fibre (4) surrounded by at least one ring of the first optical fibres (3).
6. Apparatus according to any one of the preceding claims wherein the first lens (17) is configured to image end faces (6) of the first optical fibres (3) to form the beam waist (15).
7. Apparatus according to any one of claims 1 -5 wherein the first optical fibres (3) and the second optical fibre (4) are coupled to a third optical fibre (7).
8. Apparatus according to claim 7 wherein the first optical fibres (3) and the second optical fibre (4) are spiced to a third optical fibre (7).
9. Apparatus according to claim 7 wherein the first optical fibres (3) and the second optical fibre (4) are coupled to the third optical fibre (4) with a first coupler (23).
10. Apparatus according to any one of claims 7 -9 wherein the third optical fibre (7) comprises a central core (41), a ring core (42) surrounding the central core (41), and a cladding (43) surrounding the ring core (42).
11. Apparatus according to claim 10 wherein the ring core (42) overlays the first cores (26), and the central core (41) overlays the second core (28).
12. Apparatus according to claim 11 wherein the first optical fibres are arranged in concentric rings, and the ring core (42) overlays more than one of the rings of the first cores (26).
13. Apparatus according to any one of claims 7 -12 wherein the third optical fibre has a plurality of ring cores that are concentric with each other, and each of the ring cores overlays different ones of the first cores.
14. Apparatus according to any one of claims 7 -12 wherein the first lens (17) is configured to image an end face (8) of the third optical fibre (7) to form the beam waist (15).
15. Apparatus according to any one of the preceding claims, wherein the second optical fibre (4) is a multimode optical fibre, and the apparatus includes a second coupler (50) configured to couple a first optical mode (51) that can propagate along the second optical fibre (4) to at least one second optical mode (52) that can propagate along the second optical fibre (4).
16. Apparatus according to claim 15 where the first optical mode (51) is a fundamental mode.
17. Apparatus according to claim 15 or claim 16 wherein the second optical mode (52) is a ring mode.
18. Apparatus according to claim 17 wherein the ring mode comprises a plurality of high intensity lobes around its azimuth.
19. Apparatus according to any one of claims 15 -18 wherein the second coupler (50) comprises a long period grating configured to couple the first optical mode (51) to the second optical mode (52).
20. Apparatus according to any one of claims 15 -19 wherein the second coupler (50) comprises an actuator (53) configured to induce the mode coupling upon demand from the controller (9).
21. Apparatus according to any one of claims 10 -20 wherein the central core (41) of the third optical fibre has the same optical guiding properties as the second core (28) of the second optical fibre.
22. Apparatus according to any one of claims 10 -20 wherein the central core (41) has a larger diameter than a diameter of the second core (28) of the second optical fibre.
23. Apparatus according to any one of claims 1 -22 wherein the first wavelength (13) is the same as the second wavelength (14).
24. Apparatus according to any one of claims 1 -22 where the first wavelength (13) is different from the second wavelength (14).
25. Apparatus according to claim 24 wherein the first wavelength (13) is between two and three times the second wavelength (14)
26. Apparatus according to any one of claims 1 -25 wherein the first laser (1) and the second laser (2) are each selected from a group comprising fibre lasers, disk lasers, slab lasers, and rod lasers.
27 Apparatus according to any one of the preceding claims where the second laser (2) is a frequency doubled or frequency tripled laser.
28 A method of laser processing a material (10), using an apparatus as claimed in any of Claims 1 to 27, comprising: * causing the first lasers (1) to emit first optical radiation (11) at a first wavelength (13); * causing the second laser (2) to emit second optical radiation (12) at a second wavelength (14); * guiding the first optical radiation (11) from respective ones of the first lasers (1) using the first optical fibres (3); * guiding the second optical radiation (12) from the second laser (2) using the second optical fibre (4); * collimating the first and the second optical radiation (11), (12) through the laser processing head (18) by the first lens (17); * focusing, by the second lens (19), the first and the second optical radiation (11), (12) to form a beam waist (15) onto or near a surface (16) of the material (10); and the method further comprising: * configuring the first optical fibres (3) and the second optical fibre (4) such that a first intensity profile (21) of the first optical radiation (11) at the beam waist (15) is different from a second intensity profile (22) of the second optical radiation (12) at the beam waist (15); and * configuring the controller (9) to control a power of the first optical radiation (11) emitted by the first lasers (1) and a power of the second optical radiation (12) emitted by the second laser (2).
29. A method for laser processing a material (10), comprising: * emitting first optical radiation (11) from a plurality of first lasers (1), at a first wavelength (13); * emitting second optical radiation (12) from at least one second laser (2), at a second wavelength (14); * guiding the first optical radiation from respective ones of the first lasers using a first optical fibre; * guiding the second optical radiation (12) from the second laser by a second optical fibre (4); * collimating, with a first lens, the first and second optical radiation through a laser processing head (18); * focusing, with a second lens, the first and second optical radiation to form a beam waist onto or near a surface of the material (10); characterized by: * configuring the first optical fibres (3) and the second fibre (4) such that a first intensity profile (21) of the first optical radiation at the beam waist is different from a second intensity profile (22) of the second optical radiation (12) at the beam waist (15); and * using a controller (9) to control a power of the first optical radiation (11) emitted by the first lasers (1) and a power of the second optical radiation (12) emitted by the second laser (2).