Apparatus and method for laser machining materials

Abstract

An apparatus and method for laser processing a material. An apparatus for laser processing a material, the apparatus comprising a laser, an optical fiber and a coupler wherein: the laser is connected to the optical fiber; the optical fiber enables laser radiation to propagate along the optical fiber in a first optical mode having a first mode order, a second optical mode having a second mode order, and a third optical mode having a third mode order; the third mode order is higher than the second mode order; the second mode order is higher than the first mode order; the device is characterized in that: the coupler is configured to switch laser radiation propagating in a first optical mode to laser radiation propagating in a second order mode; and the coupler is configured to switch the laser radiation propagating in the second optical mode to the laser radiation propagating in the third-order mode.

Description

[0001] This divisional patent application is a divisional application of the invention patent application with the international application number PCT / GB2019 / 000018, the international filing date of February 1, 2019, the Chinese national phase application number 201980011482.1, and the title "Device and Method for Laser Processing of Materials". Technical Field

[0002] The present invention relates to a device and method for laser processing of materials. Background Art

[0003] Lasers are used in many laser processing applications, including cutting, welding, drilling, engraving, and additive manufacturing. In these applications, it is generally desirable to optimize the size and cross-sectional profile of the laser beam that interacts with the material. For example, the spot size of the laser beam that interacts with the material can be changed by varying the working distance between the focusing lens and the workpiece. External optics can be used to change the cross-sectional profile from Gaussian to top hat, or to a ring or annular profile. However, providing such flexibility to external optics is expensive, and the time taken to change the working distance increases the processing cost. There is a desire to be able to change the laser beam from a single fundamental Gaussian mode to a top hat or annular beam without having to adjust the optics in the processing head. There is also a desire to be able to change the spot size of the laser beam without changing the working distance.

[0004] Metal powder bed additive manufacturing systems use a fundamental Gaussian mode in order to give the smallest possible feature size in the three-dimensional structure being built. However, using the fundamental Gaussian mode means that building larger structures is slow. Therefore, there is a need to be able to switch the laser beam from the fundamental Gaussian mode, which can create small features, to a laser beam with a larger and more uniform spot size that can process larger areas more quickly.

[0005] A problem that can occur in laser drilling is that once the hole is drilled, the laser beam can damage the surface behind the hole. This problem can be solved at least in part by providing an annular laser beam. An annular beam can be created using an axicon lens or by guiding the laser radiation into the cladding or annular core of an optical fiber or other waveguide. However, such beams can diverge rapidly and do not retain their annular cross-section when they are more than 1 mm to 2 mm from the focus. Therefore, there is a need to maintain the annular beam as it leaves and passes through the focus. There is a related requirement to reduce the size of the holes that can be drilled by providing an annular laser beam with low divergence.

[0006] Laser cutting of steel is achieved by directing a laser beam onto the workpiece via a processing head, which has optics for collimating and focusing the laser beam and a conical copper nozzle for providing a high-pressure gas jet coaxial with the laser beam. The basic cutting operation involves using the laser beam to heat and melt the desired area in the workpiece and using a gas jet, known as the assist gas jet, to blow the molten material out of the bottom of the cutting zone. The cutting head moves above the workpiece while maintaining a constant distance between the nozzle tip in the cutting head and the workpiece surface. The cutting head moves along a programmed path to create the shape.

[0007] In the case of cutting stainless steel, the use of an inert assist gas avoids the formation of metal oxides on the cut-edge face of the workpiece. Metal oxides can cause problems such as weakening the welded parts, reducing the corrosion properties of stainless steel due to depletion of chromium on the cutting edge face, and increasing the wear on sliding parts due to the increased hardness of the metal oxide 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 density will provide more efficient cutting by creating a narrower molten zone. A low divergence is required so that the molten zone is narrow through the thickness of the metal. The limitation on the minimum practical focal spot is determined by the optical depth of field in combination with the material thickness. This is because the kerf width must be wide enough to allow the assist gas to travel to the bottom of the cut with sufficient pressure to cleanly remove the molten material and avoid slag on the lower cutting edge, resulting in a clean cut. For this type of cutting, the assist gas must be applied at a high pressure typically in the range of 10 bar to 20 bar. The diameter of the nozzle outlet is typically in the range of 0.5 mm to 2.0 mm, and generally thicker materials require larger nozzles.

[0008] In the case of cutting mild steel (also known as low-carbon steel) thicker than 5 mm, oxygen is typically used as the assist gas. Oxygen undergoes an exothermic reaction with the iron in the workpiece to provide additional heat, which increases the cutting speed. Oxygen is applied at a pressure typically in the range of 0.25 bar to 1 bar. These pressures are much lower compared to the pressures used for nitrogen assist gas cutting. For thick-section cutting typically in the thickness range of 10 mm to 30 mm, the kerf must be wide enough so that the oxygen assist gas can reach the bottom of the cutting zone with sufficient airflow to eject the molten material while maintaining a slag-free cut. For thick mild steel cutting, the beam is typically defocused so that the beam waist is above the sheet metal surface, causing the incident beam diameter on the sheet metal surface to be larger than the beam waist. When the divergence of the beam increases, a better-quality cut with lower edge roughness can be obtained.

[0009] Most general flatbed laser cutters are required to cut a range of metals of various thicknesses, with good quality cuts in all cases. The choice of the focal spot size is typically a compromise to meet the requirements of a wide set of process conditions. For cutting thin stainless steel, a small focal spot with low divergence is required. For cutting thick mild steel, a larger focal spot with higher divergence is needed. Flatbed cutting machines are designed to work with lasers having a fixed beam quality. To increase the processing capabilities, the cutting head may have an enhanced optical system, which first allows a limited movement of the focusing lens along the beam path to permit defocusing of the laser beam relative to the workpiece, which can increase the incident spot size, and second allows adjustment of the focal spot diameter. This has limited benefits, as a laser with a constant beam quality will have a fixed relationship between the focal spot size and the divergence, where this fixed relationship acts in the opposite way to what is desired for the cutting process scenario.

[0010] Different cutting scenarios either require a small spot with low divergence or a large spot with high divergence, while a laser with a fixed beam quality can either provide a small spot with high divergence or a large spot with narrow divergence. Therefore, it is not possible to optimize the process parameters for all metal types and thicknesses.

[0011] Fusion cutting is typically optimized by focusing the laser beam near the bottom surface of the material. The high intensity of the laser beam is preferably used to increase the cutting speed, but this may come at the cost of melt flow dynamics that can cause undesirable striations on the top surface of the cut. It is desirable to provide a laser beam having an annular cross-section at the top surface of the material and a Gaussian or top-hat profile at the bottom surface. This will provide better heat distribution on the top surface and higher intensity towards the bottom of the material, thus enhancing the cutting quality without significant loss of cutting speed.

[0012] Other material processing equipment such as, by way of example, welding, marking and additive manufacturing have similar limitations. In all of these application areas, a laser processing device is needed in which the beam parameter product of the laser can be varied and the diameter of the focused laser beam on the material being processed can be varied.

[0013] U.S. Patent Application No. US2008 / 159692 discloses a system for filtering light propagating in a waveguide. The system utilizes an adjustable periodic grating that causes mode coupling of a predetermined frequency of the light propagating in the waveguide.

[0014] WO 2014 / 118516 discloses an optical combiner that includes a bundle of input optical fibers joined to an output optical fiber having a cladding and at least one high refractive index portion within the cladding such that the high refractive index portion has a diameter that is substantially equal to or less than the outer diameter of the input fiber bundle at the joining point. By connecting different lasers to each input fiber, the output beam profile can be adjusted during material processing by turning on and off the respective one of the input lasers.

[0015] It is an object of the present invention to provide an apparatus and method for laser processing of materials that reduces or avoids the problems mentioned above. Summary of the Invention

[0016] According to a non - limiting embodiment of the present invention, there is provided an apparatus for laser processing of materials, the apparatus including a laser, an optical fiber, and a coupler,

[0017] wherein:

[0018] · The laser is connected to the optical fiber;

[0019] · The optical fiber enables laser radiation to propagate along the optical fiber in a first optical mode having a first mode order, a second optical mode having a second mode order, and a third optical mode having a third mode order;

[0020] · The third mode order is higher than the second mode order; and

[0021] · The second mode order is higher than the first mode order;

[0022] The apparatus is characterized in that:

[0023] · The coupler is configured to switch laser radiation propagating in the first optical mode to laser radiation propagating in a second - order mode; and

[0024] · The coupler is configured to switch laser radiation propagating in the second optical mode to laser radiation propagating in a third - order mode.

[0025] The coupler can be configured to couple at least 75% of the laser radiation that can propagate in the first optical mode to the third optical mode.

[0026] The coupler can be configured to switch laser radiation propagating in the first optical mode to multiple optical modes, thereby enabling a top - hat optical power distribution of the laser radiation to be formed.

[0027] The apparatus may include an optical lens arrangement configured to focus the laser radiation onto or near the surface of the material.

[0028] The device may include a lens, where the lens is defined by a front focal plane and a rear focal plane, a first optical mode is defined by a Rayleigh length, and the lens is located within two Rayleigh lengths from the distal end of an optical fiber originating from a laser, where the Rayleigh length is defined as the distance from the distal end of the optical fiber to a plane where the radius of the first optical mode has increased by a factor of the square root of two.

[0029] The lens may be positioned such that the distal end of the optical fiber is located at the front focal plane.

[0030] The lens may include a graded index lens.

[0031] The optical fiber may have a plurality of cores, and a third optical mode and a first optical mode may propagate in different ones of these cores. At least one of these cores may be an annular core surrounding another one of these cores.

[0032] The coupler may include at least one squeezing mechanism, the at least one squeezing mechanism including a periodic surface defined by a pitch. The periodic surface may be positioned adjacent to the optical fiber. The squeezing mechanism may be configured to squeeze the periodic surface and the optical fiber together using a squeezing force, thereby coupling the first optical mode to a second optical mode and coupling the second optical mode to a third optical mode.

[0033] The device may be configured to apply different squeezing forces depending on a desired output mode.

[0034] The pitch may be a variable pitch that is chirped along the length of the periodic surface. The variable pitch may have a first pitch and a second pitch, where the first pitch couples the first optical mode and the second optical mode together, and the second pitch couples the second optical mode and the third optical mode together.

[0035] The squeezing mechanism may be configured to deform the optical fiber in a helical manner when applying the squeezing force.

[0036] Laser radiation may be defined by a beam parameter product, and the coupler may enable an increase in the beam parameter product by increasing the squeezing force.

[0037] The device may include a long period grating configured to couple the third optical mode to a plurality of optical modes, thereby enabling the laser radiation to have a top hat or an annular ring profile.

[0038] The long period grating may include a second squeezing mechanism, the second squeezing mechanism including a periodic surface defined by a pitch; the periodic surface is positioned adjacent to the optical fiber; and the squeezing mechanism may be configured to squeeze the periodic surface and the optical fiber together using a squeezing force.

[0039] The device may be configured to emit a single individual optical mode from the optical fiber.

[0040] The optical fiber may include a substantially homogeneous core, thereby avoiding unintended mode coupling between optical modes.

[0041] The present invention also provides a method for laser processing a material, the method comprising:

[0042] ● Providing a laser that emits laser radiation;

[0043] ● Providing an optical fiber through which the laser radiation can propagate in a first optical mode having a first mode order, a second optical mode having a second mode order, and a third optical mode having a third mode order; and

[0044] · Coupling the laser radiation into the first optical mode of the optical fiber;

[0045] wherein

[0046] · The third mode order is higher than the second mode order; and

[0047] · The second mode order is higher than the first mode order;

[0048] The method is characterized by the following steps:

[0049] · Providing a coupler configured to switch the laser radiation propagating in the first optical mode to laser radiation propagating in a second-order mode and to switch the laser radiation propagating in the second-order mode to laser radiation propagating in a third-order mode; and

[0050] · Using the laser radiation to laser process the material.

[0051] At least 75% of the laser radiation propagating in the first optical mode can be switched to the third optical mode.

[0052] The laser radiation propagating in the first optical mode can be switched to a plurality of optical modes including the third optical mode, thereby forming a top-hat optical power distribution of the laser radiation.

[0053] The first optical mode may be defined by a Rayleigh length, where the Rayleigh length is defined as the distance from the distal end of the optical fiber to the plane where the radius of the first optical mode has increased by a factor of the square root of two, and the method may include the following steps: providing a lens defined by a front focal plane and a rear focal plane; and positioning the lens within two Rayleigh lengths from the distal end of the optical fiber originating from the laser.

[0054] The method may be such that the lens may include a gradient index lens.

[0055] The method may include the step of focusing the laser radiation to form a beam waist on or near the surface of the material.

[0056] This method can enable the first optical mode to be the fundamental mode of the optical fiber.

[0057] This method can enable the third optical mode to have an azimuthal mode number of at least 3 and a radial mode number of at least 1.

[0058] This method can enable the coupler to include at least one squeezing mechanism, which includes a periodic surface defined by a pitch. The periodic surface can be positioned adjacent to the optical fiber. The squeezing mechanism can be configured to squeeze the periodic surface and the optical fiber together using a squeezing force.

[0059] This method may include the following steps: providing a controller to apply a defined control signal to the squeezing mechanism in order to select a desired third optical mode. The step of selecting the third optical mode can be achieved by adjusting the squeezing force.

[0060] This method may include the following steps: providing a controller to apply a defined control signal to the coupler for selecting different optical output modes.

[0061] This method may include the following steps: selecting the first optical mode; and using laser radiation to pierce the material.

[0062] The step of laser processing the material includes: selecting the third optical mode; and using laser radiation to cut the material.

[0063] This method may include the following steps: switching the laser radiation to a top-hat optical power distribution; and using the laser radiation to cut the material.

[0064] This method may include the step of using laser radiation to weld the material.

[0065] This method may include the step of using a laser to sinter the material, wherein the material is in the form of metal powder before sintering.

[0066] This method may include the step of using a laser to drill the material.

[0067] The present invention also provides a method for cutting a material using the device of the present invention, the method including: using a lens arrangement to focus the laser on the material; selecting a Gaussian profile to pierce the material; and selecting a top-hat optical power distribution to cut the material.

[0068] The present invention also provides a method for welding a material using the device of the present invention, the method including: using a lens arrangement away from the focus to project the laser; and using the device of the present invention to change the working spot size to optimize the welding process by changing the spot size and profile.

[0069] The method of the present invention may alternatively or additionally include one or more steps required for the above optional aspects of the apparatus of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0070] Embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which:

[0071] Figure 1 an apparatus for laser processing of materials according to the present invention is shown;

[0072] Figure 2 the intensity distribution of the guided modes of an optical fiber is shown;

[0073] Figure 3 the waist formed by a laser beam is shown;

[0074] Figure 4 and 5 the beam diameter in which the optical mode diverges from the distal end of the optical fiber and is imaged by a lens is shown;

[0075] Figure 6 a negative lens for increasing the divergence of a laser beam is shown;

[0076] Figure 7 a short-focus lens formed on the distal end of an optical fiber is shown;

[0077] Figure 8 a short-focus lens formed on an end cap joined to the distal end of an optical fiber is shown;

[0078] Figure 9 a short-focus lens made of a graded-index optical fiber is shown;

[0079] Figure 10 the beam diameter in which the optical mode is imaged by a short-focus lens is shown;

[0080] Figure 11 the top-hat power distribution at the distal end of an optical fiber is imaged as the top-hat power distribution at the focus of a laser beam by the apparatus according to the present invention;

[0081] Figure 12 the near-field top-hat power distribution at the distal end of an optical fiber is imaged as the far-field of the top-hat power distribution at the focus of a laser beam by the apparatus according to the present invention;

[0082] Figure 13 an extrusion mechanism in which an optical fiber is periodically bent along its length is shown;

[0083] Figure 14 an extrusion mechanism in which an optical fiber is periodically compressed along its length is shown;

[0084] Figure 15 Shows an extrusion mechanism including four periodic surfaces, where each periodic surface is arranged at a right angle to its adjacent surface;

[0085] Figure 16 Shows an extrusion mechanism including three periodic surfaces arranged at 60 degrees relative to each other;

[0086] Figure 17 Shows an extrusion mechanism including three components for twisting an optical fiber into a helix;

[0087] Figure 18 Shows details of one of the three components;

[0088] Figure 19 Shows the results of an experiment in which the fundamental mode is coupled to higher-order modes through a cascading process;

[0089] Figure 20 Shows the fundamental LP 0,1 mode and LP 3,1 through the evolution of the beam waist;

[0090] Figures 21 to 25 Shows the influence of the selection and position of the lens at the distal end of the optical fiber on the beam diameter of various optical modes after being focused by the focusing lens;

[0091] Figure 26 Shows the temperature profile of the workpiece after being irradiated by the annular mode;

[0092] Figure 27 Shows the temperature profile of the workpiece after being irradiated by the fundamental mode; and

[0093] Figure 28 Shows an optical fiber with an annular core. Detailed Description

[0094] Figure 1 Shows a device for laser processing material 11, which includes a laser 1, an optical fiber 2, and a coupler 125,

[0095] where:

[0096] · The laser 1 is connected to the optical fiber 2;

[0097] · The optical fiber 2 enables laser radiation 13 to propagate along the optical fiber 2 in a first optical mode 21 with a first mode order 24, a second optical mode 22 with a second mode order 25, and a third optical mode 23 with a third mode order 26;

[0098] ● The third mode order 26 is higher than the second mode order 25; and

[0099] ● The second mode order 25 is higher than the first mode order 24;

[0100] The device is characterized in that:

[0101] ● The coupler 125 is configured to switch the laser radiation propagating in the first optical mode 21 to the laser radiation propagating in the second-order mode 22; and

[0102] ● The coupler 125 is configured to switch the laser radiation propagating in the second optical mode 22 to the laser radiation propagating in the third-order mode 23.

[0103] The coupler 125 can be configured to couple at least 75% of the laser radiation that can propagate in the first optical mode 21 to the third optical mode 23. The coupler 125 can be configured to couple at least 90% of the laser radiation that can propagate in the first optical mode 21 to the third optical mode 23.

[0104] The coupler 125 can include at least one squeezing mechanism 3. The squeezing mechanism 3 can include at least one periodic surface 6 defined by a pitch 7. The periodic surface 6 is positioned adjacent to the optical fiber 2. The pitch 7 is selected such that the periodic surface 6 couples the first optical mode 21 and the second optical mode 22 together. The squeezing mechanism 3 is configured to squeeze the periodic surface 6 and the optical fiber 2 together using a squeezing force 12, thereby coupling the first optical mode 21 to the second optical mode 22.

[0105] Figure 1 The device may include a lens 4 defined by a front focal plane 14 and a rear focal plane 15. The first optical mode 21 can be defined by the Rayleigh length 217 as shown in the reference Figure 20 The lens 4 is located within two Rayleigh lengths 217 from the distal end 16 of the optical fiber 2. Preferably, the lens 4 is located within one of the Rayleigh lengths 217 from the distal end 16 of the optical fiber 2.

[0106] The lens 4 can be positioned such that the distal end 16 of the optical fiber 2 is located at the front focal plane 14.

[0107] At least one long-period grating 127 can optionally follow the first squeezing mechanism 3. The long-period grating 127 can have a uniform period 7 along its length 8, or can have a chirped period 7 along its length 8. The first squeezing mechanism 3 can be configured to couple the first optical mode 21 to the second optical mode 22. The long-period grating 127 can be configured to couple the second optical mode 22 to the third optical mode 23. The long-period grating 127 can be configured to couple the second optical mode 22 to multiple third optical modes 23.

[0108] The first squeezing mechanism 3 can be configured to couple the first optical mode 21 to the third optical mode 23. The long-period grating 127 can be configured to couple the third optical mode 23 to a plurality of optical modes (not shown). Preferably, the long-period grating 127 can be configured such that the optical modes are excited approximately uniformly, such that the laser radiation 13 can have a top-hat or annular profile.

[0109] The long-period grating 127 can include a fiber Bragg grating.

[0110] The long-period grating 127 can include a second squeezing mechanism 129 such as Figure 1 shown.

[0111] The laser 1 is connected to the optical fiber 2. The laser 1 is shown as having an output optical fiber 9, which is connected to the optical fiber 2 at the joint 10. The laser 1 can be a laser that emits laser radiation from the optical fiber 9 in the fundamental mode. The joint 10 can include a taper configured to emit the fundamental mode of the optical fiber 2. The joint 10 can be such that at least two modes of the optical fiber 2 are emitted. Alternatively, the laser 1 can be a laser that emits laser radiation in a plurality of transverse modes. The laser 1 can be a fiber laser, a disk laser, a rod laser, a slab laser, or a solid-state laser. The device can be sold with or without the laser 1.

[0112] The optical fiber 2 can be positioned anywhere suitable within the device. Thus, for example, the optical fiber 2 can include one or more optical fibers spliced together. It can be advantageous to locate the first squeezing mechanism 3 in proximity to the laser 1 such that the control signal for controlling the laser can also be used to control the first squeezing mechanism 3. This avoids costly cables and control systems. The first squeezing mechanism 3 can be encapsulated with the laser 1 or can be located outside the encapsulation of the laser in a transmission optical fiber that conveys the laser radiation 13 from the laser 1 to the distal end 16 of the optical fiber 2.

[0113] The optical fiber 2 includes a core 31 and a cladding 32, as Figure 2 shown. The core 31 is defined by a core diameter 18 and a glass cladding diameter 19. The core diameter 18 can be between 20 μm and 150 μm, preferably between 50 μm and 105 μm, and more preferably 50 μm. The glass cladding diameter 19 can be between 150 μm and 500 μm, and preferably between 150 μm and 250 μm. Preferably, the ratio of the glass cladding diameter 19 to the core diameter 18 is at least 5, and more preferably at least 10, in order to avoid microbending and uncontrolled coupling between the modes.

[0114] The optical fiber 2 can be Figure 28The optical fiber 281 shown. The optical fiber 281 has an annular core 282 surrounding the core 31. The annular core 282 can be designed such that it supports the third optical mode 23 described in reference Figure 1 .

[0115] Referring again to Figure 2 , the optical fiber 2 is considered to guide the optical mode 20. The optical mode 20 includes several lobes 27, where the intensity of the optical mode 20 has local maxima. There are sixteen lobes 27 around the azimuth 29 of the optical fiber 2, and four lobes 27 along its radius 28. Conventionally, the optical mode 20 is an LP p,q mode, where p is the azimuthal mode number and q is the radial mode number. The number of lobes 27 around the azimuth 29 is equal to twice the azimuthal mode number, and the number of lobes 27 along the radius 28 is equal to the radial mode number q. The shown mode 20 is an LP 8,4 mode because there are sixteen lobes 27 around the azimuth 29 and four lobes 27 along the radius 28. The mode order of the optical mode 20 is given by:

[0116] Mode order = p + 2q - 1

[0117] In this example, the mode order of the optical mode 20 = 15.

[0118] Figure 3 It is shown that the laser radiation 13 having a beam diameter 39 that varies with distance is focused to the focal point 34. The laser radiation 13 has a beam waist diameter 35 equal to 2ω0 at the focal point 34. The beam waist diameter 35 is often referred to as the spot size. The laser radiation 13 diverges from the focal point 34 at a divergence angle 36 equal to α. The product of half of the beam waist diameter 35 and the divergence angle 36 is defined as the beam parameter product BPP 33:

[0119] BPP = α.ω0

[0120] The beam parameter product 33 is a measure of the beam quality of the laser radiation 13. The beam parameter product 33 is related to the beam quality M 2 value 37 and λ (the wavelength 5 of the laser radiation 13) by the following equation:

[0121] BPP = M 2 .λ / π

[0122] The diffraction-limited Gaussian mode has a beam quality M 2 value 37 equal to its mode order. If these modes have the same beam waist diameter 35, the divergence angle 36 is proportional to its mode order. The beam waist diameter 35 is often referred to as the spot size.

[0123] Optical modes guided by optical fibers are generally not perfectly diffraction-limited Gaussian modes. For example, a single-mode optical fiber has an M 2 value of about 1.1. However, for a first approximation, the optical mode has an M 2 value equal to the mode order. Similarly for a first-order approximation, the optical mode propagating along the optical fiber 2 has a waist diameter approximately equal to the core diameter 18 shown in the reference Figure 2 35. Therefore, if the laser radiation 13 propagates along the optical fiber 2 as a set of optical modes with different azimuthal mode numbers p and different radial mode numbers q, the divergence angle 36 of the laser radiation 13 in each optical mode will be given by:

[0124] α = M 2 .λ / (π.ω0)

[0125] which is approximately:

[0126] α = (p + 2q – 1).λ / (π.ω0)

[0127] where the waist diameter 35 2ω is approximately the core diameter 18 of the optical fiber 2.

[0128] Therefore, for a first order, the laser radiation 13 emitted from the distal end 14 of the optical fiber 2 will be emitted as a group of optical modes, each group of optical modes having the same waist diameter 35, and in which the divergence angle 36 changes such that the divergence angle 36 increases with the mode order of the optical mode.

[0129] Taking into account the diffraction and refraction of the lens 4, the lens 4 placed such that the distal end 16 of the optical fiber 2 is at its front focal plane 14 will produce a waist at its rear focal plane 15, at which the light field is an amplified spatial Fourier transform of the field of the laser radiation 13 propagating along the optical fiber 2. In other words, referring to Figure 1 , the lens 4 converts the incident angle into a displacement in the rear focal plane 15. Therefore, a set of modes having approximately the same waist diameter 35 at the distal end 16 of the optical fiber 2 and diverging from the front focal plane 14 at different divergence angles 36 will be transformed into a set of modes having different waist diameters 35 in the rear focal plane and diverging from the rear focal plane 15 at substantially the same divergence angle 36.

[0130] The magnification at the rear focal plane 15 is given by the ratio of the focal length of the lens 4 to the Rayleigh length of the field at the distal end 16 of the optical fiber 2. The Rayleigh length is defined as the distance from the distal end 16 of the optical fiber 2 to the plane at which the radius of the light beam has increased by a factor of the square root of two. For example, if the focal length of the lens 4 is equal to the Rayleigh length, the radius of the light beam at the rear focal plane 15 will be equal to the radius of the light beam at the distal end 16 of the optical fiber 2. If the focal length of the lens 4 is twice the Rayleigh length, the width of the beam waist at the rear focal plane 15 will be twice the width at the distal end 16 of the optical fiber 2, and the divergence 36 of the light beam will be half the divergence of the light beam emitted from the distal end 16 of the optical fiber 2.

[0131] Figure 4 and Figure 5 shows how the beam diameter 39 varies for the LP 0,1 mode 41, LP 2,1 mode 42, LP 4,1 mode 43, LP 6,1 mode 44, LP 8,1 mode 45 and LP 10,1 mode 46 as a function of the distance 49 from the distal end 16 of the optical fiber 2. The beam diameter 39 is shown as the difference in the radial distance 40 between the higher and lower lines for each mode, as Figure 4 indicated by the beam diameter 39 of the LP 4,1 mode 43 at a distance of 4 mm from the distal end 16 of the optical fiber 2. For clarity, some of the several optical modes 41 to 46 are omitted in Figure 5 The optical fiber 2 has a core diameter 18 of 50 μm. The beam diameters 39 of the optical modes 41 to 46 are assumed to have a beam waist diameter 35 equal to the core diameter 18, i.e., a beam waist diameter 35 of 50 μm at the distal end 16 of the optical fiber 2. The modes 41 to 46 diverge from the distal end 16 at different divergence angles 36 because the modes 41 to 46 have different mode orders and thus different beam diameter products 33. The lens 4 is positioned such that the distal end 16 of the optical fiber 2 is at the front focal plane 14 of the lens 4. The lens 4 converts the angle incident on the lens 4 into the distance from its optical axis in the rear focal plane 15. The modes 41 to 46 each form a beam waist 48 at the rear focal plane 15, at which they each have a mode field diameter 35 that is different from one another. As Figure 15 shown, as the modes 41 to 46 diffract away from the rear focal plane 15, they converge to have the same divergence angle 36.

[0132] Referring again to Figure 1, the device includes an optical lens arrangement 50 configured to image the rear focal plane 15 of the lens 4 onto or near the surface of the material 11. The optical lens arrangement 50 is shown to include a collimating lens 51, a laser scanner 52, and a focusing lens 53. Other optical lens arrangements 50 are possible. Referring to Figure 4 and 5 The described optical modes 41 to 46 have the same divergence angle 36 away from the rear focal plane 15. Modes 41 to 46 have different mode orders and thus have different beam quality M 2 values 37 and different beam parameter products 33. Modes 41 to 46 will thus have different beam waist diameters 35 at the focus 34. Ignoring the effects of optical defects such as aberrations, the beam waist diameter 35 at the focus 34 on the surface of the material 11 will be equal to the magnification of the beam waist diameter 35 of the optical lens arrangement 50 and the corresponding mode at the rear focal plane.

[0133] Therefore, the lens 4 has converted the near field of the laser radiation 13 at the front focal plane 14 into the far field of the laser radiation 13 at the rear focal plane 15. Thus, the imaging of the rear focal plane 15 onto the surface of the material 11 will also be the far field of the laser radiation 13. The ability to image the far field of the laser radiation 13 provides some important advantages compared to imaging the near field of the laser radiation 13. These advantages include faster piercing speeds, faster cutting speeds, and better edge quality when cutting certain materials. Additionally, the requirement to adjust the relative position of the material 11 and the focusing lens 53 can often be avoided, which provides a significant cost advantage.

[0134] Reference Figure 4 , the lens 4 reduces the divergence angle 36 of the laser radiation 13 emitted from the optical fiber 2. As Figure 6 shown, a negative lens 61 can be utilized to increase the divergence angle 36. The negative lens 61 is placed such that the rear focal plane 15 is between the lens 4 and the negative lens 61. In making the thin lens approximation, the distance 63 between the front focal plane 14 and the lens 4 is the focal length 65 of the lens 4. The distance 64 between the rear focal plane 15 and the lens 4 is also the focal length 65 of the lens 4.

[0135] Referring again to Figure 1, the magnification of the lens 4 is given by the ratio of the focal length of the lens 4 to the Rayleigh length of the laser radiation 13 emitted from the distal end 16 of the optical fiber 2. In order to increase the divergence angle 36, the magnification should be as small as possible. This means that the focal length 65 of the lens 4 should be as short as possible, ideally not exceeding four times the Rayleigh length of the beam from the optical fiber 2. This is because the general purpose is to image the field at the rear focal plane 15 of the lens 4 onto a small focal spot size 34 on the material 11 using a focusing lens 53 with an appropriately large focal length, in order to protect the focusing lens 53 from spatter from the workpiece. If the field to be imaged onto the target is too large and the divergence angle 36 is too low, the system of imaging optics may become inconveniently large. Therefore, there are practical limitations to the magnification of the lens 4.

[0136] Figure 10 shows how the beam diameter 39 changes with distance 49 from the distal end 16 of the optical fiber 2 when the lens 4 has a focal length of 400 μm for LP 0,1 mode 41, LP 2,1 mode 42, LP 4,1 mode 43, LP 6,1 mode 44, LP 8,1 mode 45 and LP 10,1 mode 46. This divergence ratio Figure 5 diverges faster.

[0137] A shorter focal length 65 can be achieved by forming the lens 4 on the output of the optical fiber 2, as Figure 7 shown. The lens 4 can be formed by melting the glass of the optical fiber 2 using, for example, an electric arc, a flame, or a laser. When the lens 4 is formed, the dopant 71 that defines the core 31 will diffuse. The distal end 16 of the optical fiber 2 is where the guidance provided by the core 31 ends. The lens 4 positions the distal end 16 at the front focal plane 14.

[0138] The lens 4 can be formed on an end cap 81 connected to the optical fiber 2, as Figure 8 shown. The end cap 81 can be shaped using a carbon dioxide laser or by diamond turning. End caps are often spliced onto the ends of optical fibers for delivering high - power laser beams in order to prevent optical damage at the glass - to - air surface. The lengths 82 of the lens 4 and the end cap 81 are such that the front focal plane 14 in the glass material of the end cap 81 is at the front surface 83 of the end cap 81. The end cap 81 is preferably made of silica.

[0139] The end cap is preferably assembled onto Figure 1 the optical fiber 2 as shown. The silica end cap has a refractive index of approximately 1.5, and therefore the lens 4 must be moved closer to the optical fiber 2 for compensation in order to ensure that the distal end 16 is at the front focal plane 14 of the lens 4.

[0140] Figure 9Shown is a lens 4 made from a graded-index optical fiber 91 having a core 92 and a cladding 93. The core 92 has a refractive index profile 94 that varies with radius 95. The refractive index profile 94 is preferably a parabolic profile. Such an optical fiber refocuses an image on its front face after a refocusing length L R 97, where the refocused image is inverted. The refocusing length L R 97 is half of the pitch length at which the image at the start of the pitch is reformed. The length 96 of the graded-index optical fiber 91 is preferably equal to half of the refocusing length 97 or an odd integer multiple of half of the refocusing length 97. That is, the length 96 can be 0.5L R 、1.5L R 、2.5L R 、3.5L R etc. Subsequently, the front focal plane 14 is at the front face 88 of the graded-index optical fiber 91, and the rear focal plane 15 is at the rear face 89 of the graded-index optical fiber 91. Preferably, an end cap 98 is joined to the graded-index optical fiber 91 to prevent optical damage caused by a high-power laser beam. The end cap 98 is preferably fused silica. The length 99 of the end cap 98 can be between 1 mm and 5 mm.

[0141] Figure 9 The device shown in Figure 5 and 10 is made using a graded-index optical fiber having a length 96 of approximately 0.4 mm. The diffraction ratios of modes 41 to 46 Figure 6 are faster than the diffraction shown in

[0142] Figure 11 . Referring again to Figure 11 , the greater divergence 36 of the laser beam 13 means that a negative lens 61 is no longer needed to make the beam compatible with an imaging lens system of reasonable size.

[0143] When cutting a shiny metal, the common procedure is to use a focused high-intensity laser beam to pierce the material 11, and once piercing is achieved, the focus is moved out and a wider spot size is used to cut the material 11. A laser beam profile with a high peak intensity at the center is desired to minimize the speed of piercing the material 11. For cutting, a more uniform top-hat profile is desired to achieve a clean cut. A well-homogenized laser beam containing many optical modes propagating along a multimode fiber will have a top-hat profile in the near field and a more pointed profile in the far field. Thus, the ability to focus the far-field profile of the laser radiation 13 emitted from the distal end 16 of the fiber 2 onto the material 11 and approximate the near-field profile away from the focus results in a laser beam with far more desirable characteristics for this process.

[0144] A top-hat distribution can be obtained in the Figure 1 device by applying a squeezing force 12 to squeeze the fiber 2. As will be explained below, if the squeezing force is relatively mild, the individual modes are coupled together. If the squeezing force 12 is increased, more and more modes are coupled together and it is possible to obtain a top-hat distribution.

[0145] Figure 12 The Figure 11 device is shown, but with the lens 4 in place. The lens 4 is implemented as a graded-index fiber 91. Other forms of the lens 4 are possible, including the lenses described with reference to Figure 1 , 6 , 7 and 8. The near-field profile 111 has been converted into a far-field profile 121 at the rear focal plane 15 of the lens 4. The collimating lens 51 and the focusing lens 53 image the far-field profile 121 to form a far-field profile 122 at the focus 34. There are near-field profiles 123 on either side of the focus 34. If the near-field profile 111 is a top-hat distribution, the far-field profile 122 is more Gaussian and thus more useful for piercing the material during a cutting application. Once pierced, the near-field profile 123 can be used to cut the material. Referring to Figure 1 , the material 11 can have a thickness 124. The thickness 124 can be between 1 mm and 25 mm, or greater. Arranging the focus 34 within the material 11 facilitates piercing the material 11. Once pierced, the far-field profile 123 projected onto the surface 17 of the material 11 can be used to cut the material 11.

[0146] Referring again to Figure 1 , the squeezing mechanism 3 can be the Figure 13 squeezing mechanism 130 shown in

[0147] The extrusion mechanism 3 can be Figure 13 the extrusion mechanism 140 shown in. The first periodic surface 131 and the second periodic surface 132 are arranged in-phase with each other such that they periodically extrude the optical fiber 2 with a pitch 7 without significantly bending the optical fiber 2. The optical fiber 2 has an extrusion pressure that periodically changes with a pitch 7 along its length. The pitch 7 can be uniform or chirped as shown. The chirp can be monotonic or non-monotonic.

[0148] Figure 15 An extrusion mechanism 150 is shown that includes four members 151 arranged at an angle 154 to each other. The first periodic surface 131 and the second periodic surface 132 can be out-of-phase with each other, in which case the optical fiber 2 is periodically bent along its length. If the relative phases of the first periodic surfaces 131 of the orthogonal members 151 are out-of-phase with each other, the optical fiber 2 can be deformed into a helical shape. Alternatively, the first periodic surface 131 and the second periodic surface 132 of each member 151 can be in-phase with each other, in which case the optical fiber 2 is periodically pressurized along its length.

[0149] Figure 16 An extrusion mechanism 160 is shown that has three members 151 arranged at 120 degrees to each other. Each mechanism 150 has a first periodic surface 131. The first periodic surfaces 131 can be arranged to be spatially out-of-phase with each other by 120 degrees along their lengths, in which case the optical fiber 2 is twisted into a helical shape. Alternatively, the first periodic surfaces 131 can be arranged to be in-phase with each other along their lengths, in which case the optical fiber 2 is periodically pressurized along its length.

[0150] Figure 17 An extrusion mechanism 170 is shown that includes three members 175, each member having two periodic surfaces 171 and 172 as shown in reference Figure 18 . The members 175 are arranged at 120 degrees to each other. The periodic surfaces 171 and 172 are spatially out-of-phase with each other by 120 degrees along their lengths, and thus the optical fiber 2 is deformed in a substantially helical manner.

[0151] Referring to Figure 1 , the first optical mode 21 has an effective refractive index of β1 / k, while the second optical mode 22 has an effective refractive index of β2 / k, where β1 and β2 are the propagation constants of the first optical mode 21 and the second optical mode 22, respectively, and k is the wave number related to the wavelength λ5 of the laser radiation 13 according to k = 2π / λ. It is useful to consider the difference in propagation constants Δβ = β1 – β2. For making reference Figure 1The extrusion mechanism 3 shown couples the first optical mode 21 to the second optical mode 22, and there needs to be a spatial frequency component in the twist of the optical fiber 2 along its length equal to Δβ / 2π. This occurs if the periodicity (defined as the reciprocal of the pitch 7) is equal to Δβ / 2π, or a harmonic of the periodicity is equal to Δβ / 2π. However, it is also important to consider the symmetry of the perturbation of the optical fiber 2 compared to the optical mode.

[0152] If p is non-zero, the azimuthal dependence of the electric field of each LP p,q mode guided by the core of the optical fiber 2 can be expressed as follows:

[0153] E(r,θ) = E(r).cos(pθ)

[0154] E(r,θ) = E(r).sin(pθ)

[0155] where E(r) is the radial dependence of the electric field.

[0156] As described in the reference Figure 13 , when the optical fiber 2 has a linear sinusoidal deflection along its length, then for symmetry considerations, only one of the cos(pθ) and sin(pθ) orientations will be coupled when the pitch 7 is equal to 2π / Δβ. More generally, if p is an odd integer and the pitch 7 is equal to 2π / (β A –β B ), then the LP 01 mode guided by the core 31 can be coupled to the LP p,q mode guided by the same core, where β A and β B are the propagation constants of the optical modes coupled together. However, unless there are significant harmonics in the sinusoidal deflection, the coupling to the LP 11 mode will be the strongest. If p is an even integer, the symmetry of the perturbation is incorrect. By a similar symmetry argument, if the optical fiber has a sinusoidal deflection along its length, the linear extrusion mechanism will not couple the LP 01 mode to the LP 0q mode either.

[0157] As described in the reference Figure 14 , if the periodic surface 6 is periodically compressed along its length, then mode coupling will be caused by the photoelastic effect. By symmetry considerations, the LP 01 mode will not be coupled to the LP11 mode because the symmetry is incorrect. However, if the pitch 7 is equal to 2π / (β A –β B ), where β A and β B are the propagation constants of the optical modes coupled together, then the LP 01Capable of coupling to the LP 21 mode, or more generally coupling to the LP p,q mode, where p = 2, 4, 8, etc.

[0158] As referenced Figures 15 to 18 above, when the optical fiber 2 has a helical twist, then by symmetry arguments, the LP 01 mode can couple to the LP p,q mode in both the cos(pθ) and sin(pθ) orientations when the pitch 7 is equal to 2π / Δβ and p is an odd integer. However, if p is an even integer, then the LP 01 mode will not couple, or the LP 01 mode will couple to the LP 0q mode. Thus, the amount of mode coupling provided by the squeezing mechanism shown in Figures 15 to 18 is at least twice the amount of mode coupling provided by the linear squeezing mechanism shown in Figure 13 .

[0159] Therefore, the helical squeezing mechanisms referenced Figures 15 to 18 where the optical fiber 2 is perturbed in a helical manner are advantageous because they couple more orientations of the modes together compared to the linear squeezing mechanism shown in Figure 13 . Additionally, the squeezing force 12 required to provide the coupling and thus the maximum deflection of the optical fiber 2 is smaller, which results in less stress being applied to the optical fiber 2 and thus higher reliability. Experimentally, it has been observed that the optical fiber 2 can be pulled from a helical squeezing mechanism such as Figure 17 with a pulling force of less than 1 Newton (N). This is significantly less than the pulling force required to pull the optical fiber 2 from a linear squeezing mechanism such as Figure 13 in cases where the helical and linear squeezing mechanisms induce a similar level of mode coupling in the optical fiber 2. Thus, a smaller squeezing force 12 is applied to the optical fiber 2 in the helical squeezing mechanism, which means higher mechanical reliability.

[0160] The core 31 preferably has a uniform refractive index profile without refractive index fluctuations across its radius. The optical fiber 2 is preferably a step-index optical fiber or an optical fiber with a graded-index core. This helps to selectively excite individual modes or groups of modes in the optical fiber 2 using the squeezing mechanism 3. This also helps to maintain the mode shape during narrowing in the joint 10, thus enabling robust single-mode excitation of the optical fiber 2.

[0161] Referencing Figure 2 , the optical fiber 2 can have a core diameter 18 of 50 μm, a numerical aperture of 0.22, and a glass cladding diameter 19 between 250 μm and 500 μm. Preferably, the ratio of the glass cladding diameter 19 to the core diameter 18 is at least 5, and more preferably at least 10, in order to avoid microbending and uncontrolled coupling between the optical modes.

[0162] Table 1 shows the calculated period Λ for efficient coupling between different LP p,q optical modes in a 50 μm, 0.22 numerical aperture step-index fiber, where the period Λ does not change along the length of the fiber. It can be seen that the pitch Λ needs to be 7.9 mm to efficiently couple between the LP 0,1 mode and the LP 1,1 mode. These modes will be coupled together using a squeezing mechanism 3 that bends the fiber 2 with a pitch Λ. The LP 1,1 mode can be coupled to the LP 2,1 mode using a second mechanism 129 located between the first mechanism 3 and the distal end 16 of the fiber 2. The required pitch Λ is 6.0 mm. In both cases, the squeezing force 12 of the respective squeezing mechanisms 3, 129 can be adjusted in order to obtain the desired amount of mode coupling between the optical modes. Specifically, it is possible to couple to a single higher-order mode with a coupling efficiency of more than 95%. In order to couple to even higher-order modes, a third squeezing mechanism 3 and possibly a fourth squeezing mechanism 3 are required, or the squeezing force 12 must be increased in order to generate spatial harmonics in the approximate sinusoidal perturbation of the fiber 2. Such a squeezing force can result in a large amount of mode coupling, which can result in a top-hat output profile of the laser radiation 13 at the distal end 16 of the fiber 2.

[0163] <![CDATA[LP 1,1 > <![CDATA[LP 2,1 > <![CDATA[LP 3,1 > <![CDATA[LP 4,1 > <![CDATA[LP 5,1 > <![CDATA[LP 6,1 > <![CDATA[LP 7,1 > <![CDATA[LP 0,1 > <![CDATA 7.9 > 3.4 2.0 1.4 1.0 0.8 0.6 <![CDATA[LP 1,1 > <![CDATA 6.0 > 2.7 1.6 1.1 0.8 0.6 <![CDATA[LP 2,1 > <![CDATA 4.9 > 2.2 1.4 1.0 0.7 <![CDATA[LP 3,1 > <![CDATA 4.1 > 1.9 1.2 0.9 <![CDATA[LP 4,1 > <![CDATA 3.6 > 1.7 1.1 <![CDATA[LP 5,1 > <![CDATA 3.2 > 1.5 <![CDATA[LP 6,1 > <![CDATA 2.9 >

[0164] Table 1: Periods (mm) for coupling between optical LP p,q modes

[0165] Alternatively or additionally, at least one of the squeezing mechanisms 3 may have a variable period Λ that is chirped along the length 8 of the squeezing mechanism 3. In order to transfer power between the modes using the underlined coupling lengths in Table 1, the period Λ of the squeezing mechanism 3 should change from at least 7.9 mm at its input end (the end closest to the laser 1) to a period Λ of no more than 2.9 mm at its output end (the end closest to the distal end 16).

[0166] In the experiment, the fiber 2 had a 50 μm core diameter and a 0.22 numerical aperture. The period Λ of the squeezing mechanism 3 at the input end (i.e., the end of the squeezing mechanism 3 that receives the laser radiation 13 from the laser 1) had a period of 8 mm. The period Λ at its output end (the end that outputs the laser radiation 13 received from the laser 1) was 2.5 mm. As Figure 19 shown, by adjusting the squeezing force 12, it is possible to tune the mode coupling between the LP 01 mode and the LP 7,1 mode. The cross-section of the laser radiation 13 emitted from the fiber 2 clearly has fourteen high-intensity lobes around its azimuth, which indicates LP7,1 The strong presence of the mode. By increasing the extrusion pressure 12 starting from zero extrusion pressure, the extrusion mechanism 3 can sequentially output LP 11 (not shown), LP 2,1 、LP 3,1 、LP 4,1 、LP 5,1 (not shown), LP 6,1 and LP 7,1 optical modes. The conversion efficiency of each mode can be tuned to about 90% to 100%. Without limiting the scope of the present invention, it is believed that LP 01 modes are sequentially coupled to LP 11 、LP 2,1 、LP 3,1 、LP 4,1 、LP 5,1 、LP 6,1 and then subsequent LP 7,1 optical modes.

[0167] By adjusting the extrusion pressure 12 of the extrusion mechanism 3, any of the foregoing modes can be tuned to appear at the distal end 16 of the optical fiber 2. Different M 2 values, mode profiles, and divergence are associated with each individual mode. The required extrusion pressure 12 is repeatable and approximately linear. When the extrusion mechanism 3 is reversed, i.e., the 2.9 mm pitch is oriented at the input end of the extrusion mechanism 3, this cascading nature of mode coupling is not visible.

[0168] As shown in reference Figure 1 , a long period grating 127 can optionally follow the first extrusion mechanism 3. The long period grating 127 can have a uniform period 7 along its length 8, or can have a chirped period 7 along its length 8. The long period grating 127 can include a fiber Bragg grating. Alternatively or additionally, the long period grating 127 can include a second extrusion mechanism 129 as shown in Figure 1 .

[0169] In the experiment, the pitch 7 of the second extrusion mechanism 129 has a variable pitch that is chirped along its length 8. The period 7 of the extrusion mechanism 129 at its input end (i.e., the end that receives the laser radiation 13 from the first extrusion mechanism 3) has a period 7 of 4.4 mm. The period 7 at its output end (the end that transmits the laser radiation 13 to the distal end 16 of the optical fiber 2) is 4.0 mm. It is possible to use the first extrusion mechanism 3 to couple the LP 0,1 mode to the LP 4,1 mode in the above cascaded manner, and then use the second extrusion mechanism 129 to couple the LP 4,1 mode to the LP 3,2 mode. The LP 3,2 mode has a cos 2(6θ) or sin 2 Two rings of high optical intensity with azimuthal dependence of (6θ). LP 3,2 mode has higher optical intensity near its center than LP 3,1 mode. Similarly, it is possible to use the first squeezing mechanism 3 to couple the LP 0,1 mode to the LP 7,1 mode in the above cascaded manner, and then couple the LP 7,1 mode to the LP 5,3 mode. LP 5,3 mode has three rings of high optical intensity with azimuthal dependence of cos 2 (10θ) or sin 2 (10θ). LP 5,3 mode has higher optical intensity near its center than LP 5,1 mode. Experiments show that the device realizes the output of Gaussian fundamental LP 0,1 mode and a series of annular mode profiles from the optical fiber 2. These modes can occur individually or in combination. The device can be used to output individual modes and combinations of optical modes that can be combined to form annular profiles with different annular thicknesses. A top-hat (also known as flat-top) profile can also be generated by increasing the squeezing force 12 on the first squeezing mechanism 3 and / or the second squeezing mechanism 129 to increase mode coupling. The technique is robust and predictable.

[0170] Figure 1 The second squeezing mechanism 129 can have a period 7 that is chirped along its length 8. In the experiment, the period 7 of the squeezing mechanism 3 at its input end where it receives the laser radiation from the first squeezing mechanism 3 has a period of 3.5 mm. The period 7 at its output end where it outputs the laser radiation 13 received from the first squeezing mechanism 3 is 2.0 mm. By adjusting the squeezing forces 12 of the first squeezing mechanism 3 and the second squeezing mechanism 129, it is possible to couple the LP 0,1 fundamental mode to a large set of modes. If these modes are excited approximately uniformly, they will produce an approximately uniform output profile across the core 31 from a 50-μm core fiber. By adjusting the squeezing force 12, predictable different beam quality M 2 values can be obtained. Specifically, it is possible to achieve a top-hat profile of the laser radiation 13 at the distal end 16 of the optical fiber 2. The obtained M 2 value is approximately linear with respect to the squeezing force 12.

[0171] Figure 20 Shows how the mode profiles of the fundamental LP 0,1 mode 201 and the LP 3,1 mode 204 evolve when focused to the beam waist 34 along the axis 209 with the lens 4 in Figure 1 not in position. The fundamental mode 201 and LP3,1 The mode 204 diffracts away from the beam waist 34 at different rates. The beam diameter 207 of the fundamental mode 201 and the 3,1 beam diameter 208 of the LP mode are approximately equal at the beam waist 34 (i.e., in the near field). The beam diameter 207 is smaller than the beam diameter 208 away from the beam waist 34 in the far field. The fundamental LP 0,1 mode 201 has a far field 203 and a near field 202 that are both approximately Gaussian. Similarly, the LP 3,1 mode 204 has a far field 206 that has a profile similar to its near field 205. The near fields 202 and 205 have approximately the same beam waist diameter 35 as shown in the reference Figure 3 . The far field 206 of the LP 3,1 mode 204 is in an annular ring around the far field 203 of the fundamental mode 201. Higher-order modes with the same mode order as the LP 3,1 mode will also diffract from the beam waist in substantially the same annular ring. Higher-order modes with a larger mode order (such as the LP 7,1 mode) will diffract at a higher divergence angle 36, and their far fields will be substantially within an annular ring of an annular ring around the far fields of modes with a lower mode order. If the guided modes all have substantially the same optical power, the set of modes will form an approximately top-hat distribution of optical power at the location where their beam diameters 39 are approximately equal, and an approximately Gaussian distribution at a distance greater than the Rayleigh length 217 from the beam waist 34.

[0172] Reference Figure 3 shows that annular modes (such as the LP 3,1 mode 204) have an annular profile in both the near field and the far field. The annular mode maintains its annular profile through the focal point. Thus, if the device is operated such that it selects an individual annular mode, the annular mode will be maintained through the beam waist 34. Similarly, if the device is operated such that it selects a set of individual annular modes, these annular modes will be maintained through the beam waist 34.

[0173] The equation describing the Gaussian beam radius ω(z) as a function of the distance z along the axis 209 can be written as follows:

[0174]

[0175] where ω0 is the beam radius at the beam waist 34, and z R is the Rayleigh length. The beam waist diameter 35 at the beam waist 34 as described in the reference Figure 3 is equal to 2ω0. The Rayleigh length z R corresponds to the distance along the axis 209 at which the Gaussian beam radius ω(z) increases by a factor of the square root of 2 from the Gaussian beam radius ω0 at the beam waist 34. For a beam with a beam quality M 2The Rayleigh length z of a laser beam with a waist diameter 35 of 37, a wavelength 5 of λ, and 2ω0 R is given by:

[0176]

[0177] In Figure 20 the Rayleigh length 217 of the fundamental LP 0,1 mode is shown. Higher-order modes have a higher beam quality M 2 value of 37, and thus a shorter Rayleigh length. Therefore, compared to the fundamental LP 0,1 mode, higher-order modes diffract at a faster rate.

[0178] Figures 21 to 24 Shows how the beam diameter 39 changes for the LP 0,1 mode 41, the LP 2,1 mode 42, the LP 4,1 mode 43, the LP 6,1 mode 44, the LP 8,1 mode 45, and the LP 10,1 mode 46 as a function of the distance from the focusing lens 53 shown in Figure 1 . Figure 1 The collimating lens 51 shown in has a focal length of 100 mm, while the focusing lens 53 has a focal length of 200 mm. It is assumed that modes 41–46 each have a waist diameter 35 of 2ω0 = 66 μm when they emerge from the distal end 16 of the optical fiber 2. The wavelength is assumed to be 1.06 microns. The Rayleigh length 217 of the fundamental LP 0,1 mode is z 2 3.2 mm for an assumed beam quality M R＝ value = 1.

[0179] Figure 21 Shows the situation in the absence of the lens 4 shown in Figure 1 . As discussed with reference to Figure 3 , modes 41-46 each have substantially the same waist diameter 35, and a divergence angle 36 that increases with the mode order of the optical modes 41-46. Therefore, the desired beam divergence 36 can be selected by adjusting the squeezing force 12 of the first squeezing mechanism 3.

[0180] The beam diameters 39 are all equal to each other at the waist 34 and are not equal to each other away from the waist 34. The squeezing force 12 of the first squeezing mechanism 3 and the squeezing force 12 of the second squeezing mechanism 129 can be adjusted to excite many more fiber-guided modes, such that the near-field intensity profile 112 shown in reference Figure 11 is approximately top-hat shaped at the waist 34. As described with reference to Figure 11 , the far-field distribution 113 will then be approximately Gaussian.

[0181] Figure 22 and 23 shows the situation where the lens 4 shown in Figure 1 has a focal length of 400 microns and the distal end 16 of the optical fiber 2 is at the front focal plane 14 of the lens 4. Each of the modes 41–46 has substantially the same divergence angle 36, but has a different beam waist diameter 35. Thus, the desired beam waist diameter 35 can be selected by adjusting the squeezing force 12 of the first squeezing mechanism 3.

[0182] The beam diameters 39 are equal to each other far from the beam waist 34 and are equal to each other at the beam waist 34. Thus, as described in reference Figure 12 , the squeezing force 12 of the first squeezing mechanism 3 and the squeezing force 12 of the second squeezing mechanism 129 can be adjusted such that the intensity profile 123 far from Figure 22 and Figure 23 the beam waist 34 in Figure 22 and Figure 23 is approximately a top-hat distribution, and the intensity profile 122 at the beam waist 34 in

[0183] Figure 24 shows the situation where the lens 4 shown in Figure 1 has a focal length of 800 microns and the distal end 16 of the optical fiber 2 and the lens 4 are separated by an optical distance of 1.6 mm. Both the beam waist diameter 35 and the beam divergence 36 for each of the modes 41–46 are different from each other. Thus, the desired beam waist diameter 35 and beam divergence 36 can be selected by adjusting the squeezing force 12 of the first squeezing mechanism 3. Different combinations of different beam waist diameters 35 and beam divergences 36 can be achieved by selecting the lens 4 and its arrangement relative to the distal end 16 of the optical fiber 2.

[0184] Unexpectedly, the beam waist diameters 35 of the individual optical modes 41–46 do not occur at the same distance from the focusing lens 52. The beam diameters 39 are all equal to the beam diameter 242 at the distance 241 from the focusing lens 52. Thus, as described in reference Figure 12 , the squeezing force 12 of the first squeezing mechanism 3 and the squeezing force 12 of the second squeezing mechanism 129 can be adjusted such that the intensity profile 123 at the distance 241 is approximately a top-hat distribution. The intensity profile 122 far from the distance 241 is similar to a Gaussian with a reduced intensity at its center.

[0185] Figure 25 shows the situation where in Figure 1The lens 4 shown in [Figure] has a focal length of 800 microns and the situation where the distal end 16 of the optical fiber 2 and the lens 4 are separated by an optical distance of 400 microns. Therefore, the desired beam waist diameter 35 and beam divergence 36 can be selected by adjusting the extrusion force 12 of the first extrusion mechanism 3. Different combinations of different beam waist diameters 35 and beam divergences 36 can be achieved by selecting the lens 4 and its arrangement relative to the distal end 16 of the optical fiber 2.

[0186] Unexpectedly, the beam waist diameters 35 of the individual optical modes 41–46 do not occur at the same distance from the focusing lens 52. Regarding Figure 3 the defined beam waist diameters 35 for each of the modes 41–46 are different from each other and do not occur at the same distance from the focusing lens 52. The beam diameter 39 is all equal to the beam diameter 252 at the distance 251 from the focusing lens 52. Therefore, as referenced Figure 12 as described, the extrusion force 12 of the first extrusion mechanism 3 and the extrusion force 12 of the second extrusion mechanism 129 can be adjusted such that the intensity profile 123 at the distance 251 approximates a top-hat distribution. The intensity profile 122 away from the distance 251 is Gaussian-like with a reduced intensity at its center.

[0187] Comparing Figures 21 to 25 the beam waist diameters 35 of the various optical modes in [Figure] reveals that including the lens 4 enables different beam waist diameters 35 to be selected by adjusting the extrusion force 12 of the first extrusion mechanism 3. The lens 4 is preferably located within two Rayleigh lengths 217 from the distal end 16 of the optical fiber 2. More preferably, the lens 4 is located within one Rayleigh length 217. Even more preferably, within half of the Rayleigh length 217.

[0188] Referring Figures 21 to 25 to the extrusion force 12 described, it can be adjusted to select the individual optical modes 41 to 46, or a combination of the optical modes 41 to 46. The extrusion force 12 can be adjusted to switch between the individual optical modes 41 to 46, between combinations of the optical modes 41 to 46, or between an individual optical mode 41 to 46 and a combination of the optical modes 41 to 46.

[0189] Referring Figures 1 to 25 to the device described, it can be configured such that at least one of the desired beam waist diameter 35 and the desired divergence angle 36 can be selected by adjusting the extrusion force 12 of the first extrusion mechanism 3. As Figure 21 shown, when there is no lens 4, Figure 1 the device of [Figure] can be configured to select the desired divergence angle 36 by adjusting the extrusion force 12 of the first extrusion mechanism 3. As Figure 22 and Figure 23 shown, when the lens 4 is included in Figure 1In the device and positioned such that the distal end 16 of the optical fiber 2 is at the front focal plane 14 of the lens 4, the device can be configured to select a desired beam waist diameter 35 by adjusting the squeezing force 12 of the first squeezing mechanism 3. The inclusion of a second squeezing mechanism 129 allows for an even greater degree of selection. Higher mode coupling can be achieved by controlling the squeezing force 12 of the second squeezing mechanism 129, thereby achieving an intensity distribution that approximates a top hat profile, or an annular or ring profile, at any distance from the focusing lens 53 of the beam waist 34 of the fundamental mode 41 and at distances beyond the beam waist 34.

[0190] Referring again to Figure 1 , the squeezing mechanism 3 can include at least one actuator 55. The actuator 55 can include an electric motor and / or an electromagnet. The actuator can include a ratchet. The application of an electrical signal can be used to provide the squeezing force 12 via the actuator 55. The actuator 55 can also be used to vibrate the squeezing mechanism 3 in order to cause azimuthal averaging of the intensity of one or more optical modes.

[0191] The device can include a controller 75 for controlling the actuator 55 and thereby the squeezing force 12. The controller 75 can include a memory 76 that includes information about material parameters. Preferably, the memory 76 contains information that enables the signal driving the actuator 31 to be selected depending on the parameters of the material 11. The parameters can include the type of the material 11 and its thickness 124. This is a particularly useful aspect of the present invention because it allows the divergence 36 of the laser radiation 13 and the beam waist diameter 35 of the focused laser radiation 13 to be controlled by controlling the signal to the actuator 55. Thus, it allows for automatic tuning of relatively expensive industrial lasers 1 over a wide range of laser processing parameters depending on the material being processed.

[0192] Accordingly, the device of the present invention can be used to switch the laser radiation 13 between a substantially Gaussian mode, individual higher order modes alone or in combination, and a top hat distribution including multiple optical modes. In addition, by using the lens 4 to image the far field of the laser radiation 13 at the distal end 16 of the optical fiber, the laser radiation 13 can be switched between optical modes having different beam waist diameters 35. Reliably switching between annular modes having azimuthal variations in intensity (such as LP 5,1 or LP 9,1The ability to reliably image the mode) onto the material 11 provides important advantages in terms of cutting and welding the material 11. Such modes have novel and interesting interactions with certain materials. For example, it is possible to optimize the combination of piercing speed, cutting speed, and edge quality by selecting individual modes or a set of individual modes. The optimization process may include optimizing the relative position of the material 11 with respect to the focusing lens 53. The beam waist 34 can be positioned between the material 11 and the focusing lens 53, on the surface 17 of the material 11, within the material 11, or on the opposite side of the material 11 from the focusing lens 53. Interestingly, different optical modes in each optical mode provide the best results for piercing, cutting speed, and edge quality. The optimal choice can be different for different materials and material thicknesses.

[0193] One example application is an application where the material 11 is pre-processed before the main processing. Figure 26 The annular mode 225 for pre-heating a material (such as silicon) is shown, where the material absorption increases with temperature. Figure 26 The temperature profile 221 of the workpiece 220 under irradiation from the annular mode 225 is shown. The temperature profile 221 is uniform inside the ring, and thus the induced stress at the center of the irradiated area is reduced. The device of the present invention is then used to switch the laser radiation 13 to a substantially LP 0,1 mode 235, as Figure 27 shown. The spot size 232 of the fundamental mode 234 is arranged to be smaller than the spot size 222 of the annular mode 225. As described with reference to Figure 4 and 5 This can be achieved by using a lens 4 on the distal end 16 of the optical fiber 2. As a result, the workpiece 220 can be machined more precisely and with less damage compared to without the pre-heating step. Example processes include cutting, drilling, or cutting or forming links in a semiconductor circuit.

[0194] The device of the present invention can be used to provide the spot size and divergence of the laser radiation 13 to pierce, cut, weld, drill, sinter, mark, or engrave materials. The device may have monitoring equipment for monitoring the properties of the laser radiation 13 and thus controlling the actuator 31 to give the desired processing parameters.

[0195] It should be appreciated that the embodiments of the present invention described above with reference to the drawings have been given only by way of example, and modifications and additional components can be provided to enhance the performance. The individual components shown in the drawings are not limited to their uses in the drawings and can be used in other drawings and all aspects of the present invention. The present invention also extends to the individual components mentioned and / or shown above, either alone or in any combination.

Claims

1. A device for laser processing of materials, the device comprising a laser (1), an optical fiber (2) and a coupler (125), wherein: · the laser (1) is connected to the optical fiber (2); · the optical fiber (2) enables laser radiation (13) to propagate along the optical fiber (2) in a first optical mode (21) having a first mode order (24), a second optical mode (22) having a second mode order (25), and a third optical mode (23) having a third mode order (26); · the third mode order (26) is higher than the second mode order (25); and · the second mode order (25) is higher than the first mode order (24); the device is characterized in that: · the coupler (125) is configured to switch the laser radiation propagating in the first optical mode (21) to laser radiation propagating in the second optical mode (22); and · the coupler (125) is configured to switch the laser radiation propagating in the second optical mode (22) to laser radiation propagating in the third optical mode (23); the coupler (125) includes at least one squeezing mechanism (3), the at least one squeezing mechanism including a periodic surface (6) defined by a pitch (7); the periodic surface (6) is positioned adjacent to the optical fiber (2); and the squeezing mechanism (3) is configured to use a squeezing force (12) to squeeze the periodic surface (6) and the optical fiber (2) together, thereby coupling the first optical mode (21) to the second optical mode (22) and coupling the second optical mode (22) to the third optical mode (23); wherein the pitch (7) is a variable pitch that is chirped along the length of the periodic surface (6), the variable pitch having a first pitch and a second pitch, wherein the first pitch couples the first optical mode (21) and the second optical mode (22) together, and the second pitch couples the second optical mode (22) and the third optical mode (23) together.

2. The device according to claim 1, wherein, The coupler (125) is configured to couple at least 75% of the laser radiation capable of propagating in the first optical mode (21) to the third optical mode (23).

3. The device according to claim 1 or claim 2, characterized in that, The coupler (125) is configured to switch the laser radiation propagating in the first optical mode (21) to a plurality of optical modes, so as to enable the formation of a top-hat optical power distribution of the laser radiation.

4. The device according to claim 1 or claim 2, characterized in that, Including an optical lens arrangement (50), the optical lens arrangement being configured to focus the laser radiation onto or near the surface of the material.

5. The device according to claim 1 or claim 2, characterized in that, Comprising a lens (4), wherein the lens (4) is defined by a front focal plane (14) and a rear focal plane (15), the first optical mode (21) is defined by a Rayleigh length, and the lens (4) is located within two Rayleigh lengths from the distal end (16) of the optical fiber (2) originating from the laser (1), wherein the Rayleigh length is defined as the distance from the distal end (16) of the optical fiber (2) to a plane where the radius of the first optical mode (21) has increased by a factor of the square root of two.

6. The device according to claim 5, wherein The lens (4) is positioned such that the distal end (16) of the optical fiber (2) is located at the front focal plane (14).

7. The device according to claim 5, characterized in that The lens (4) comprises a graded-index lens.

8. The device according to claim 1 or claim 2, characterized in that, The optical fiber (2) has a plurality of cores (31), and the third optical mode (23) and the first optical mode (21) propagate in different cores among the cores (31).

9. The device according to claim 8, characterized in that, At least one of the cores is an annular core (282) surrounding another one of the cores (31).

10. The device according to claim 1, characterized in that, The device is configured to apply different squeezing forces (12) depending on the desired output mode.

11. The device according to claim 1, characterized in that, The squeezing mechanism (3) is configured to deform the optical fiber (2) in a helical manner when applying the squeezing force (12).

12. The device according to claim 1, characterized in that, The laser radiation is defined by a beam parameter product (33), and the squeezing mechanism (3) enables the beam parameter product (33) to be increased by increasing the squeezing force (12).

13. The device according to claim 1, wherein Comprising a long-period grating (127), the long-period grating being configured to couple the third optical mode (23) to a plurality of optical modes, whereby the laser radiation can have a top-hat or annular-ring profile.

14. The device according to claim 13, characterized in that, The long-period grating (127) comprises a second squeezing mechanism (129), the second squeezing mechanism including a periodic surface (6) defined by a pitch (7); the periodic surface (6) is positioned adjacent to the optical fiber (2); and the squeezing mechanism (129) is configured to squeeze the periodic surface (6) and the optical fiber (2) together using a squeezing force (12).

15. The device according to claim 1 or claim 2, characterized in that, The device is configured to emit a single individual optical mode from the optical fiber (2).

16. The device according to claim 1 or claim 2, characterized in that, The optical fiber (2) comprises a homogeneous core (31), thereby avoiding unintended mode coupling between the first, second, and third optical modes.

17. A method for laser processing materials, the method comprising: · Providing a laser (1) that emits laser radiation (13); · Providing an optical fiber (2) through which the laser radiation (13) can propagate in a first optical mode (21) having a first mode order (24), a second optical mode (22) having a second mode order (25), and a third optical mode (23) having a third mode order (26); and · Coupling the laser radiation (13) to the first optical mode (21) of the optical fiber (2); wherein · The third mode order (26) is higher than the second mode order (25); and · The second mode order (25) is higher than the first mode order (24); The method is characterized by the following steps: · Provide a coupler (125) that: is configured to switch the laser radiation propagating in the first optical mode (21) to laser radiation propagating in the second optical mode (22), and is configured to switch the laser radiation propagating in the second optical mode (22) to laser radiation propagating in the third optical mode (23); includes at least one squeezing mechanism (3), the at least one squeezing mechanism including a periodic surface (6) defined by a pitch (7); the periodic surface (6) is positioned adjacent to the optical fiber (2); the squeezing mechanism (3) is configured to squeeze the periodic surface (6) and the optical fiber (2) together using a squeezing force (12); wherein the pitch (7) is a variable pitch that is chirped along the length of the periodic surface (6), the variable pitch having a first pitch and a second pitch, wherein the first pitch couples the first optical mode (21) and the second optical mode (22), and the second pitch couples the second optical mode (22) and the third optical mode (23); and · Laser process the material using the laser radiation (13).

18. The method according to claim 17, wherein At least 75% of the laser radiation propagating in the first optical mode (21) is switched by the coupler to the third optical mode (23).

19. The method according to claim 17 or claim 18, characterized in that, The laser radiation propagating in the first optical mode (21) is switched to a plurality of optical modes including the third optical mode (23) to form a top-hat optical power distribution of the laser radiation (13).

20. The method according to claim 17 or claim 18, characterized in that The first optical mode (21) is defined by a Rayleigh length, wherein the Rayleigh length is defined as the distance from the distal end (16) of the optical fiber (2) to a plane where the radius of the first optical mode (21) has increased by a factor of the square root of two, and the method includes the following steps: · Provide a lens (4) defined by a front focal plane (14) and a rear focal plane (15); and · Position the lens (4) within two Rayleigh lengths of the distal end (16) of the optical fiber (2) originating from the laser (1).

21. The method according to claim 20, characterized in that, The lens (4) includes a gradient-index lens.

22. The method according to claim 17 or claim 18, characterized in that, Includes the step of focusing the laser radiation (13) to form a beam waist on or near the surface of the material.

23. The method according to claim 17 or claim 18, characterized in that, The first optical mode (21) is the fundamental mode of the optical fiber (2).

24. The method according to claim 17 or claim 18, characterized in that, The third optical mode (23) has an azimuthal mode number of at least 3 and a radial mode number of at least 1.

25. The method according to claim 17, wherein Includes the following steps: Provide a controller (75) to apply a defined control signal to the squeezing mechanism (3) to select a desired third optical mode (23), and the step of thus selecting the third optical mode (23) is achieved by adjusting the squeezing force (12).

26. The method according to claim 17 or claim 18, characterized in that Includes the following steps: Provide a controller (75) to apply a defined control signal to the coupler (125) for selecting different optical output modes.

27. The method according to claim 17 or claim 18, characterized in that Includes the following steps: Select the first optical mode (21); and use the laser radiation (13) to pierce the material.

28. The method according to claim 27, wherein The step of laser processing the material includes: selecting the third optical mode (23); and using the laser radiation (13) to cut the material.

29. The method according to claim 27, wherein It includes the steps of: switching the laser radiation (13) to a top-hat optical power distribution; and using the laser radiation (13) to cut the material.

30. The method according to claim 17 or claim 18, characterized in that It includes the step of using the laser radiation (13) to weld the material.

31. The method according to claim 17 or claim 18, characterized in that, It includes the step of using the laser (1) to sinter the material, wherein the material is in the form of metal powder before sintering.

32. The method according to claim 17 or claim 18, characterized in that, It includes the step of using the laser (1) to drill the material.

33. A method of cutting a material using the apparatus according to claim 4, comprising: Use the optical lens arrangement (50) to focus the laser (1) on the material; Select a Gaussian profile to pierce the material; and select a top-hat optical power distribution to cut the material.

34. A method of welding a material, comprising: Use the device according to claim 4, project the laser outside the focus using the optical lens arrangement (50), and use the device to change the working spot size to optimize the welding process by changing the spot size and profile.

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