Method for laser material processing using a processing laser beam whose power profile is adjustable

Abstract

Method for laser material processing using a processing laser beam (3), wherein a first laser beam (8a), which is coupled into at least a first fiber core (9a) of an optical multicore fiber (9), and a second laser beam (8b), which is coupled into at least a second fiber core (9b) of the multicore fiber (9), are generated from a polarized input laser beam (5) using a birefringent crystal (7), wherein by influencing the polarization of the input laser beam (5) the ratio in which the power of the input laser beam (5) is divided between the first and the second laser beam (8a, 8b) is changed with a modulation frequency (f) between 1 Hz and 100 kHz, in particular between 100 Hz and 100 kHz, and thereby the first and the second laser beam (8a, 8b) can be coupled out of the multi-core fiber (9) alone or together as a processing laser beam (3). characterized by that the polarization is influenced by switching between linearly, circularly or elliptically polarized polarization by means of an electro-optic polarization modulator (6), in particular a Pockels cell or Faraday rotator, which can be controlled with the modulation frequency (f) to change the polarization of the input laser beam (5).

Description

[0001] The present invention relates to a method for laser material processing using a processing laser beam, wherein a first laser beam, which is coupled at least into a first fiber core of an optical multicore fiber, and a second laser beam, which is coupled at least into a second fiber core of the multicore fiber, are generated from an input laser beam.

[0002] Such a method and device are known, for example, from US 2015 / 0378184A1. In the Fig. In the variants shown in Figures 11A to 11K, the perpendicularly and parallel polarized components of two input laser beams are spatially separated from each other by means of a birefringent crystal and then combined to form a perpendicularly polarized first partial laser beam and a parallel polarized second partial laser beam. These two partial laser beams are each coupled into an inner fiber core and an outer fiber core (ring core) of an optical fiber, which surrounds the inner core in a ring-like fashion, and then coupled out of the fiber as a single processing laser beam.

[0003] From WO 2011 / 124 671 A1, it is known to influence the beam profile characteristics during material processing with laser radiation. In this process, one or more input laser beams are coupled by means of a coupling optic either into an inner fiber core, into a ring core surrounding the inner core, or into both the inner fiber core and the ring core of a multicore optical fiber. A processing laser beam with different beam profile characteristics, depending on the fiber cores involved, is then coupled out at the other fiber. The coupling optic is, for example, designed as a wedge plate movable into the beam path of the input laser beam, which deflects the input laser beam either into the inner core or into the ring core.This well-known method is advantageously used in both laser cutting and laser welding, as is known, for example, from WO 2014 / 060 091 A1 or WO 2018 / 091 417 A1.

[0004] US patent 2019 / 0383998A1 discloses a laser cutting system that superimposes a central beam and a concentric ring beam via a multi-core fiber to form a combined beam profile and separately controls their power densities. A control unit adjusts the ratio of center to ring beam component, particularly to the workpiece thickness and process situations such as turning points, by reducing or modulating the power, or switching off the ring beam for thin materials, in order to improve cutting quality and process stability.

[0005] From JP2010-1 17564A a wavelength selective switch (WSS) for WDM / ROADM networks is known which separates the individual wavelength channels from a WDM signal and switches them selectively to desired output ports without using MEMS mirrors. Instead of a MEMS mirror matrix (which has reliability problems due to the necessary biaxial mirror movement with an unfavorable hinge structure), the WSS uses a PLC platform with AWG(s) for spectral separation and a liquid crystal-based polarization switch with a birefringent wedge / crystal (or polarization displacer) and mirror: The liquid crystal rotates the polarization (typically 0° / 90°), the birefringent crystal converts the polarization state into a spatial deflection (beam control), and via a suitable arrangement (including a reflection path), the channels are directed with high precision onto two or more paths / ports (e.g., 1×2, cascaded 1×4) and then coupled out again via AWG(s).remultiplexed; in addition, solutions are described that address cost / module thickness and resolution losses through optimized crystal / LC arrangement and AWG design (e.g. FSR matching, polarization diversity concept). The company publication "Improving fiber laser weld quality and yield with CleanWeld" (A Coherent White Paper, 2018 Coherent, Inc., MC1018) describes an integrated approach to fiber laser welding that improves the process stability of the keyhole and weld pool through process-optimized beam profiles (including "ARM" technology), adapted feed optics / focusing heads, and application know-how. The solution combines several control levers such as intensity distribution in the focus, beam movement, and vapor / plasma removal and is used, among other things, for gap-free welding of galvanized steels, powertrain components, and Al / Cu welds in electric vehicle batteries.

[0006] Thick workpieces (> 6 mm) can be cut faster and with higher quality using a processing laser beam that primarily emerges from the ring core of the optical fiber. Thinner workpieces, on the other hand, are cut with a processing laser beam that primarily emerges from the inner fiber core of the optical fiber and therefore exhibits higher beam quality. Today, the power distribution between core and ring beams is predefined for a specific workpiece thickness. This power distribution allows for a significantly higher feed rate and improved cut edge quality, depending on the sheet thickness. When cutting laminated workpieces, the lower-quality laser beam emerging from the ring core can be used to vaporize the laminate.

[0007] In contrast, the present invention aims to make a method of the type mentioned at the outset more variable, so that a greater variety of processing options becomes possible in material processing.

[0008] This problem is solved according to the invention in the aforementioned method by coupling the first and second laser beams individually or jointly out of the multi-core fiber as a processing laser beam and by changing the ratio of the laser power between the first and second laser beams with a modulation frequency between 1 Hz and 100 kHz, preferably between 100 Hz and 100 kHz.

[0009] The change in the ratio of laser power is achieved by influencing the polarization of the input laser beam and subsequently splitting the input laser beam into partial beams of different intensity.

[0010] The ratio of laser power can also be changed by influencing the propagation of the input laser beam.

[0011] The inventive method makes it possible to switch the laser power of the input laser beam either completely (100% at a time) rapidly back and forth between the different cores of the multi-core fiber or to divide it in a specific ratio (e.g., with maximum values ​​of 70% to 30%) between at least two partial laser beams, couple the individual partial laser beams into different cores of the multi-core fiber, and couple them out of the multi-core fiber together as a processing laser beam, rapidly modulating the laser power between the maximum values. Preferably, the first (inner) fiber core is surrounded by the second (outer) fiber core (ring core) in a ring-like manner.

[0012] By rapidly changing the power distribution between the different fiber cores, the power distribution reaching the workpiece can be optimally matched to the workpiece thickness. It is assumed that, due to the slow thermal processes within the workpiece, the rapid power distribution has a time-averaged effect, thus achieving power distributions that lead to particularly advantageous cutting and welding results. Preferably, the modulation frequency is selected depending on the processing speed and / or the workpiece thickness.

[0013] To influence the propagation of the input laser beam, the input laser beam is preferably deflected by means of an acousto-optic modulator modulated with the modulation frequency and switched with the modulation frequency between a non-deflected, first laser beam and a deflected, second laser beam, wherein the non-deflected laser beam is coupled, for example, into the inner fiber core and the deflected laser beam into the outer fiber core.

[0014] To influence the polarization, the input laser beam is preferably split by a birefringent crystal into an ordinary and an extraordinary (partial) laser beam, whose polarization directions are perpendicular to each other. The laser power can be divided between the two partial beams with any value between 0% and 100%, depending on the original polarization direction of the input laser beam entering the crystal. Subsequently, for example, using a focusing lens, the ordinary laser beam and the extraordinary laser beam are coupled into different fiber cores, e.g., the ordinary laser beam into the inner core and the extraordinary laser beam into at least the ring core of the optical fiber. In this way, modulation of the laser power in the different fiber cores between predefined maximum values ​​(e.g., 70% to 30%) is possible.Since the beam-carrying optical fiber is not polarization-preserving, the polarization of the laser beams coupled into the fiber is lost again on their way to the workpiece. Material processing therefore takes place as usual with a randomly polarized beam.

[0015] The spatial separation of the ordinary and extraordinary partial beams can be determined by the length of the birefringent crystal. Biaxial crystals (LBO, BBO) are suitable, but uniaxial crystals such as quartz, sapphire, or LiNbO3 are preferred. The crystal length is in the millimeter or centimeter range.

[0016] The polarization of the input laser beam is preferably controlled by means of a steerable polarization modulator. The polarization modulator can be, for example, an electro-optic modulator (EOM), in particular a Pockels cell, a Faraday rotator, or a rotatable delay plate. The input laser beam striking the polarization modulator can be linearly or elliptically polarized. For example, laser beams from disk lasers can be linearly polarized easily and efficiently using a Brewster window integrated within the resonator, and linearly polarized laser beams from fiber lasers or non-polarization-coupled diode lasers are also known.

[0017] Polarization can be influenced, for example, by rotating the polarization direction or principal axis of a linearly or elliptically polarized input laser beam and / or by switching between linearly, circularly or elliptically polarized polarization.

[0018] Preferably, the limit values ​​for the ratio in which the power of the input laser beam is divided between the first and the second laser beam can be determined by the amplitude of the voltage applied to an electro-optic modulator serving as a polarization modulator.

[0019] Rapid switching (> 100 Hz) of the power distribution between the inner fiber core and the toroidal core enables a new approach to laser cutting: The laser power guided in the toroidal core precedes the cutting process (low intensity at perpendicular incidence), and the power guided in the inner core acts deep into the cutting gap (high intensity at steep cutting fronts, i.e., large angles). Modulating the power distribution influences the absorption of the laser power in the cutting gap, allowing the absorption to be specifically controlled and adapted to the process conditions (e.g., workpiece thickness, cutting speed, or contour shape [corners]) by changing the modulation conditions (e.g., modulation frequency).

[0020] The variable power distribution between the inner fiber core and the ring core is specifically designed to improve the cutting quality of nitrogen melt cutting. By switching or modulating the power on the workpiece, a minimal surface roughness can be achieved on the cut edge while simultaneously increasing the feed rate. The modulation frequency is advantageously increased with increasing feed rate. In this way, clean, rework-free cut edges can be obtained with significantly increased productivity.

[0021] The inventive method can also be used advantageously in laser welding: The modulation of the laser power between the fiber cores and the adjustable power distribution lead to a reduced occurrence of spatter and inclusions (pores) in the weld seam.

[0022] According to another aspect, a device for laser material processing using a processing laser beam, comprising - an optical multi-core fiber with at least two, in particular different, fiber cores, and - a birefringent crystal which generates from a polarized input laser beam a first laser beam, which is coupled at least into the first fiber core of an optical multicore fiber, and a second laser beam, which is coupled at least into the second fiber core of the multicore fiber, wherein the first and the second laser beam are coupled out of the multicore fiber alone or together as a processing laser beam, wherein a polarization modulator, which can be controlled with a modulation frequency between 1 Hz and 100 kHz, preferably between 100 Hz and 100 kHz, is arranged in the beam path of the input laser beam in front of the birefringent crystal for changing or adjusting the polarization of the input laser beam.

[0023] The polarization modulator can be an electro-optic modulator (EOM), in particular a Pockels cell, a Faraday rotator, or a rotating retardation plate.

[0024] Alternatively, a device for laser material processing using a processing laser beam relates to - an optical multi-core fiber with at least two, in particular different, fiber cores and - an acousto-optic modulator which generates from an input laser beam a first, non-deflected laser beam, which is coupled at least into the first fiber core of an optical multicore fiber, or a second, deflected laser beam, which is coupled at least into the second fiber core of the multicore fiber, wherein the first or the second laser beam is coupled out of the multicore fiber alone or together as a processing laser beam and wherein the refractive index in the acousto-optic modulator or the diffraction grating of the acousto-optic modulator can be switched between two states with a modulation frequency between 1 Hz and 100 kHz, preferably between 100 Hz and 100 kHz.

[0025] Preferably, the first (inner) fiber core is surrounded in a ring shape by the second (outer) fiber core (ring core).

[0026] Further advantages and advantageous embodiments of the subject matter of the invention can be found in the description, the drawings, and the claims. Likewise, the features mentioned above and those listed further below can be used individually or in any combination. The embodiments shown and described are not to be understood as an exhaustive list, but rather serve as examples for illustrating the invention.

[0027] They show: Fig. 1A, Fig. 1B schematically two devices for laser material processing with an acousto-optic modulator ( Fig. 1A) or a birefringent crystal ( Fig. 1B) for deflecting or splitting an input laser beam into two (partial) laser beams; Fig. 2 schematically another device for laser material processing with a birefringent crystal for splitting a linearly polarized input laser beam into two partial beams. Fig. 3A, Fig. 3B schematically shows the beam path of a non-phase-shifted input laser beam ( Fig. 2A) and an input laser beam phase-shifted by π ( Fig. 2B); Fig. Figure 3C schematically shows the beam path of a linearly polarized input laser beam from which an elliptically polarized laser beam is generated; and Fig. 3D schematic of the beam path of a linearly polarized input laser beam, from which a circularly polarized laser beam is generated.

[0028] The in Fig. 1A The schematically shown device 1 serves for laser processing of a workpiece 2 by means of a processing laser beam 3 and comprises a laser beam generator 4 for generating an input laser beam 5, an acousto-optic modulator 16 which can influence the propagation of the input laser beam 5 by means of a modulated controllable piezo actuator 21, and an optical fiber 9 with an inner fiber core 9a and a ring core 9b surrounding the inner fiber core 9a in a ring-like manner.

[0029] In an acousto-optic modulator, a sound wave generates a standing wave in a crystal. This wave creates a diffraction grating in the crystal through refractive index modulation, which, through diffraction, deflects the laser beam propagating in the crystal. Depending on whether the piezo actuator 21 is switched on or off, the input laser beam 5 either passes through the acousto-optic modulator 16 without changing direction and emerges as an undeflected laser beam 8a, or the input laser beam 5 is deflected by diffraction in the acousto-optic modulator 16 and emerges as a deflected laser beam 8b. The undeflected laser beam 8a is coupled, for example, only into the first fiber core 9a, and the deflected laser beam 8b, for example, only into the second fiber core 9b of fiber 9.Either the undeflected beam 8a or the deflected beam 8b is then coupled out of the optical fiber 9 as a processing laser beam 3 and focused onto the workpiece 2 by means of a focusing optic 10. By means of a modulated control of the piezo actuator 21 with a modulation frequency that can be in a value range of 1 Hz to 100 kHz, the device 1 enables rapid switching of the laser power of the input laser beam 5 between the first and the second fiber cores 9a, 9b of the optical fiber 9.

[0030] In an alternative version, the device 1 comprises, as in Fig. Figure 1B shows a laser beam generator 4 for generating a polarized input laser beam 5 with polarization direction or electric field vector. E→, a polarization modulator 6 controllable for influencing the polarization of the input laser beam 5, a birefringent crystal 7 which divides the polarized input laser beam 5 into an ordinary, first (partial) laser beam 8a and into an extraordinary, second (partial) laser beam 8b, whose polarization directions or electric field vectors E→P E→S The optical fiber 9 consists of two optical elements positioned perpendicular to each other, and an optical fiber 9 with an inner fiber core 9a and a ring core 9b surrounding the inner fiber core 7a. The first laser beam 8a is coupled, for example, only into the first fiber core 9a, and the second laser beam 8b, for example, only into the second fiber core 9b. The two partial beams 8a, 8b are then coupled out of the optical fiber 9 together as a single processing laser beam 3 and focused onto the workpiece 2 by means of a focusing optic 10. Since the beam-carrying optical fiber 9 is not polarization-preserving, the polarization of the partial beams 8a, 8b coupled into the optical fiber 9 is lost again on the way to the workpiece 2. The material processing therefore takes place as usual with a statistically polarized processing laser beam 3.The polarization modulator 6 is driven by a pulse generator 11 with a modulation frequency that can be in a range of 1 Hz to 100 kHz.

[0031] The spatial separation of the two partial beams 8a, 8b can be determined by the length of the birefringent crystal 7. Biaxial crystals (LBO, BBO) are suitable, but uniaxial crystals such as quartz, sapphire, or LiNbO3 are preferred. The length of the birefringent crystal 7 is in the millimeter or centimeter range.

[0032] The laser beam generator 4 can, for example, be a linearly polarized fiber laser, diode laser or a linearly polarized disk laser, in which a linearly polarized input beam 5 is coupled out using a resonator-internal Brewster window.

[0033] The polarization modulator 6 is, for example, a λ / 2 plate rotating at the modulation frequency, a Faraday rotator or an electro-optic modulator (EOM), in particular a Pockels cell.

[0034] In Fig. Figure 2 shows a device 1 in which the polarization modulator is formed by a λ / 2 plate 6' rotating at frequency f. The λ / 2 plate 6' delays the electric field component of the laser radiation, which is polarized parallel to its optical axis, by half a wavelength relative to light polarized perpendicular to it. The λ / 2 plate rotates the polarization direction of the incident, linearly polarized input laser beam 5 by a fixed angle. By rotating the λ / 2 plate 6', the polarization direction of the input laser beam 5 is continuously rotated, and the beam is then split at the birefringent crystal 7 into two partial beams 8a, 8b modulated at frequency f. The power distribution is continuously variable between 0% and 100%.

[0035] Alternatively, a controllable Faraday rotator can be used as a polarization modulator 6 to change the polarization direction of the input laser beam 5. The Faraday rotator is based on the Faraday effect, in which an external magnetic field acts on a transparent medium. If this medium has a non-zero Verdet constant, the electric field vector of the passing polarized laser beam will rotate. E→. Typical transparent materials with a high Verdet constant are terbium-doped glasses or terbium-gallium garnet crystals.

[0036] In a preferred variant, which is in Fig. As shown in 3A-3D, the polarization modulator 6 is an electro-optic modulator. In an electro-optic modulator 6, the electric field vector can be changed by applying an electric voltage U. E→ of the input laser beam 5, more precisely the phase between the S and P components of the electric field vector E→ (between the two orthogonal polarization directions of the laser beam). This results in a rotation of the (resulting) field vector and / or a change in the polarization type. The phase change Δφ depends on the amplitude of the applied electrical voltage U. A Pockels cell becomes birefringent when a high electrical voltage is applied, and the refractive index of the Pockels cell, and thus the phase, can be selectively changed via the voltage. By changing the polarization of the input laser beam 5, the ratio in which the power of the input laser beam 5 is divided between the two partial laser beams 8a, 8b can be changed using the Pockels cell 6.

[0037] If no voltage is applied to the Pockels cell 6 by the pulse generator 11 (U=0V), no phase shift occurs between the electric field components (Δφ=0) due to the Pockels cell 6, i.e., the polarization direction is not rotated or the polarization type is not changed. The input laser beam 5 is transmitted by the birefringent crystal 7 as an ordinary, first laser beam 8a ( Fig. 3A) and thus 100% coupled into the inner fiber core 9a.

[0038] If a certain voltage is applied to the Pockels cell 6 by the pulse generator 11, a phase shift of π (Δφ=π) occurs through the Pockels cell 6, i.e., the resulting vector of the polarization direction of the input laser beam 5 is rotated by 90°, and the input laser beam 5 is now polarized in the P direction (electric field vector). E→P ). The input laser beam 5 is deflected by the birefringent crystal 7 as an extraordinary, second laser beam 8b ( Fig. 3B) and thus 100% coupled into the outer fiber core 9b.

[0039] The resulting vector of the polarization direction of the linearly polarized input laser beam 5 can be rotated arbitrarily by means of the voltage applied to the Pockels cell 6. This vector is then split into the two partial beams 8a and 8b at the birefringent crystal 7 according to its polarization direction. The desired ratio in which the power of the input laser beam 5 is divided between the two partial laser beams 8a and 8b can thus be set via the voltage amplitude of the pulse generator 11.

[0040] The polarization modulator 6, designed as an electro-optic modulator, therefore not only allows for fast (modulated) switching of the laser power between the different cores of an optical fiber 9, but also for a variation of the power distribution between the fiber cores.

[0041] By periodically switching the voltage U applied to the Pockels cell 6 between a lower and an upper voltage value (limit values) or by modulating the voltage U applied to the Pockels cell 6, the linearly polarized input laser beam 5 can be periodically switched between the inner fiber core 9a and the toroidal core 9b. This enables rapid, stepless switching of the beam power either from 0% to 100% or – depending on the amplitude of the applied voltage U – between flexibly selected maximum values ​​of the power distributions (e.g., between 10% and 90%, 20% and 80%, 30% and 70%, 60% and 40%, 45% and 55%, or intermediate values) with modulation frequencies between approximately 1 Hz and approximately 100 kHz between the fiber cores 9a and 9b, depending on the amplitude of the applied voltage. The temporal relationship between the two partial beams 8a, 8b can be adjusted via the duty cycle of the voltage switching pulses of the pulse generator 11 applied to the Pockels cell 6.

[0042] With a phase shift other than 0, π and π / 2 (Δφ≠0, Δφ≠π, Δφ≠π / 2) an elliptically polarized input laser beam 5 is produced ( Fig. 3C). Depending on the field rotation in the Pockels cell 6, the power of the partial beams 8a, 8b generated in the subsequent birefringent crystal 7 can thus be continuously and variably adjusted. By rotating the ellipse spanned by the electric field vectors, the power ratio between the inner core 9a and the ring core 9b in the optical fiber 9 can be continuously adjusted.

[0043] With a phase shift of π / 2 (Δφ=π / 2) a circularly polarized input laser beam 5 is created ( Fig. 3D). The power distribution to the two partial beams 8a, 8b or to the inner core 9a and the ring core 9b of the optical fiber 9 is in a ratio of 50% to 50%.

[0044] Instead of coupling the first partial beam 8a only into the inner fiber core 9a and the second partial beam 9b only into the ring core 9b, the two partial beams 8a, 8b can also be distributed differently between the two fiber cores 9a, 9b. For example, the first partial beam 8a can be coupled only into the inner fiber core 9a and the second partial beam 9b into both fiber cores 9a, 9b, or vice versa. Or the two partial beams 8a, 8b can be coupled differently into each of the two fiber cores 9a, 9b.

[0045] Instead of an inner fiber core 9a and a ring core 9b, the optical fiber 9 can also have two or more parallel (identical or different) fiber cores, into which the two partial beams 8a, 8b are coupled differently.

[0046] Instead of being linearly polarized, the input laser beam 3 can also be elliptically polarized, in which case its principal polarization axis is rotated by the polarization modulator 6.

[0047] Rapid switching (> 100 Hz) of the power distribution between the inner fiber core 9a and the ring core 9b enables a new approach to laser cutting or welding: The laser power directed in the ring core 9b precedes the cutting or welding process (low intensity at perpendicular incidence), while the power directed in the inner core 9a acts deep into the kerf or keyhole (high intensity at steep kerf fronts, i.e., large angles). Modulating the power distribution influences the absorption of the laser power in the kerf or keyhole, allowing the absorption of the laser power and the dynamics of the resulting melt to be specifically controlled and adapted to the process conditions (e.g., workpiece thickness, processing speed, or contour shape [corners]) by changing the modulation conditions (e.g., modulation frequency).

[0048] In laser welding, the modulation of the power distribution leads to a reduced occurrence of spatter and inclusions (pores) in the weld seam.

Claims

[1] Method for laser material processing using a processing laser beam (3), wherein a first laser beam (8a), which is coupled into at least a first fiber core (9a) of an optical multicore fiber (9), and a second laser beam (8b), which is coupled into at least a second fiber core (9b) of the multicore fiber (9), are generated from a polarized input laser beam (5) using a birefringent crystal (7), wherein by influencing the polarization of the input laser beam (5) the ratio in which the power of the input laser beam (5) is divided between the first and the second laser beam (8a, 8b) is changed with a modulation frequency (f) between 1 Hz and 100 kHz, in particular between 100 Hz and 100 kHz, and thereby the first and the second laser beam (8a, 8b) can be coupled out of the multi-core fiber (9) alone or together as a processing laser beam (3). characterized by, that the polarization is influenced by switching between linearly, circularly or elliptically polarized polarization by means of an electro-optic polarization modulator (6), in particular a Pockels cell or Faraday rotator, which can be controlled with the modulation frequency (f) to change the polarization of the input laser beam (5). [2] Method according to claim 1, characterized by , that the limit values ​​for the ratio in which the power of the input laser beam (5) is divided between the first and the second laser beam (8a, 8b) are determined by the amplitude of the voltage (U) applied to the electro-optic polarization modulator (6). [3] Method according to any one of the preceding claims, characterized by, that during the laser material processing of a workpiece (2) the modulation frequency (f) with which the ratio of the laser power between the first and the second laser beam (8a, 8b) is set is changed, in particular depending on the processing speed and / or the workpiece thickness. [4] Method according to any one of the preceding claims, characterized by , that the first fiber core (9a) is surrounded by the second fiber core (9b) in a ring shape.

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