DEVICE FOR SCANNING A TARGET PLANE WITH SEVERAL LASER BEAMS, IN PARTICULAR FOR LASER MATERIAL PROCESSING

Description

Technical application area

[0001] The present invention relates to a device for scanning a target plane with a plurality of laser beams, in particular processing laser beams for laser material processing, wherein the device has at least two laser beam sources, a deflection device for each of the laser beam sources which is designed to dynamically deflect a laser beam emitted by the respective laser beam source, and one or more focusing optics for focusing the laser beams into the target plane.

[0002] In the field of laser material processing, the process speed is limited by various influences. Firstly, the processing process itself can limit the speed. This is the case, for example, with selective laser melting (LPBF: Laser Powder Bed Fusion), where the remelting of a metal powder can in practice occur at a maximum speed of approximately 1 m / s. Secondly, the available laser power or the inertia of the beam deflection system can also limit the process speed. However, increasing the process speed is very important, especially when using laser material processing in an industrial environment.

[0003] Another problem is the generally large installation space of the beam deflection systems and lenses used in laser material processing relative to the scanning range or scan field and the spot diameter of the laser beam in the target plane. Typically, the beam deflection of high-power laser radiation is achieved by two mirrors with galvanometer drives. The laser beams are usually focused into the target plane using a fiber collimator and an F-theta lens. Especially in applications that require a small laser spot diameter and thus short focal lengths and short working distances, the comparatively large installation space of the lenses and deflection units prevents overlap of the working ranges of the individual laser beams. State of the art

[0004] The approach pursued so far in laser material processing has been to increase process speed through parallelization, i.e., simultaneous processing with multiple laser beams. Various techniques are known for this parallelization.

[0005] For example, it is known to split a laser beam into several sub-beams by diffraction at a diffractive optical element (DOE) and direct them onto the workpiece via a scanner. However, the sub-beams cannot be spatially and temporally modulated separately, so the application range of this technique is limited.

[0006] As part of the FutureAM integration project (https: / / www.futuream.fraunhofer.de), a system with five scan heads arranged side by side is currently being implemented. Each laser beam covers a specific processing area in the target plane. The aim is to achieve the greatest possible overlap between the processing areas of the laser beams of the individual scan heads. The system contains five fiber lasers, each of whose laser radiation is collimated and directed onto the target plane via a 2D galvanometer scanner and an F-theta lens. By appropriately modifying commercial scanner modules, a center-to-center distance of 85 mm between the scan heads was achieved. However, the design of the scanner modules and the F-theta lenses limit the overlap, so that a maximum of three scanners can operate in an area of ​​approximately 42 mm x 179 mm.

[0007] DE 19933825 A1 (which forms the basis for the preamble of claim 1) describes a laser processing device with multiple laser beam sources for generating laser beams of different wavelengths, a scanner device for shifting the relative beam positions, which moves the workpiece, and a beam mixing device for aligning the optical axes. Variable mirrors that can change their curvature allow the focus position of the individual laser beams to be changed along the beam axis.

[0008] US 2015 / 0321286 A1 discloses a device for scanning a target plane with multiple laser beams according to the preamble of patent claim 1.

[0009] In JP 2003 / 334685 A, relay optics are used in a laser processing device to superimpose two laser beams. The two laser beams are superimposed via polarization beam splitters and focused on the workpiece using a common focusing optic.

[0010] WO 2009 / 108543 A2 describes an exposure system for triggering a multiphoton reaction in a resin. After passing through a first galvanometer, a beam passes through two optical relays connected in series, between which a second galvanometer is arranged. The second galvanometer is positioned at the inner image of the first optical relay, and thus the inner image of the first optical relay is also the subject of the second optical relay.

[0011] The object of the present invention is to provide a device for scanning a target plane with a plurality of laser beams, which enables a complete overlap of the scanning areas of the individual laser beams in the target plane. Description of the invention

[0012] The object is achieved with the device according to patent claim 1. Advantageous embodiments of the device are the subject of the dependent patent claims or can be derived from the following description and the exemplary embodiments.

[0013] The proposed device comprises at least two laser beam sources, a deflection device for each of the laser beam sources for dynamic one- or two-dimensional deflection of a laser beam emitted by the respective laser beam source, and one or more focusing optics for focusing the laser beams into the target plane. The laser beam sources are selected or configured such that the laser beams emitted by the different laser beam sources have different central wavelengths. In the proposed device, these laser beams of different central wavelengths are combined in a superposition arrangement by means of one or more dichroic mirrors tuned to the different central wavelengths after passing through the deflection device.

[0014] The combination is preferably carried out in such a way that, in one position of the respective deflection devices, hereinafter also referred to as the zero position, all laser beams are completely superimposed by the superposition arrangement, i.e., they run on the same optical axis up to the target plane. It is also possible for the laser beams to be combined by the superposition arrangement in such a zero position of the deflection devices such that they pass through the focusing optics at a close distance from one another, parallel to one another. Because the deflection devices can be controlled independently of one another and thus the laser beams can be deflected independently of one another, complete superposition or parallelism only occurs in a specific position of the respective deflection devices.The dichroic mirrors of the superposition arrangement represent edge filters whose slope must be selected depending on the wavelength differences between the respective central wavelengths of the laser beams. With a given edge slope, the distances between the central wavelengths can also be selected appropriately. In one embodiment, the different central wavelengths of the laser beam sources lie within a wavelength range Δλ of 200 nm, preferably within a wavelength range Δλ of 100 nm, and particularly preferably within a wavelength range Δλ of 50 nm.

[0015] The laser beams are thus dynamically deflected one- or two-dimensionally via deflection devices and preferably focused into the target plane via a common focusing optics. Laser beams of different central wavelengths are used and spectrally superimposed or combined so that they pass through the common focusing optics. Preferably, at least two of the laser beams are guided simultaneously across the target plane at a distance from each other.

[0016] The proposed device allows the scanning area or scan field of the individual laser beams to be completely superimposed in the target plane, while each laser beam can still be deflected independently of the others. The number of laser beams that can be used with the device depends solely on the available laser beam sources or central wavelengths and the possibilities for separation using appropriate edge filters. Up to ten or more laser channels can be combined with a 100% overlap of the working areas. Preferably, at least three laser beams, in particular processing laser beams for laser material processing, are used or combined. This allows laser material processing systems to be scaled and parallelized accordingly.This immediately results in a significant increase in the productivity of laser material processing, especially in micro SLM, SLE and stereolithography.

[0017] The superposition arrangement is also designed as several relay optics arranged one behind the other. As a result, a deflection angle generated by the deflection device produces a smaller offset perpendicular to the beam direction on optical elements located downstream of the superposition arrangement. This allows for larger deflection angles to be generated without being limited by the downstream optical elements.

[0018] The laser beam sources used in the proposed device must be available for different central wavelengths or be adjustable accordingly. Examples of such laser beam sources are primarily diode lasers, which are commercially available, for example, as fiber-coupled modules with stabilized central wavelengths between 800 and 1000 nm. For applications requiring small spot diameters and thus high beam quality, ytterbium fiber lasers, for example, are a suitable beam source. Ytterbium fiber lasers can be tuned and stabilized in a range of 1000–1100 nm with a bandwidth of approximately 5 nm or less. Furthermore, ultrashort-pulse lasers can also be combined, for example, pulsed ytterbium fiber lasers. With short-pulse lasers, the spectral broadening resulting from Fourier transformation of the short-time signal must be taken into account.For example, a laser pulse with a central wavelength of 1070 nm and a pulse length of 100 fs has a spectral width of approximately 17 nm. This must be taken into account when selecting the separation of the central wavelengths and the dichroic filters in the superposition arrangement. Beam sources with comparatively large separation of the central wavelengths can also be combined in the proposed device and method, for example, beam sources with central wavelengths of 1064 nm and 532 nm.

[0019] Dichroic mirrors can be technically implemented as dichroic edge filters with an edge steepness of less than 1 nm. 2D galvanometer scanners are preferably used as deflection devices. Of course, the deflection devices can also be implemented in other known ways, for example, using individual mirrors that can be deflected in two directions.

[0020] The proposed device is suitable for all applications in which a target plane is to be scanned simultaneously with multiple laser beams whose scanning ranges should have the largest possible overlap. The proposed device can be used very advantageously in the field of laser material processing, for example, for laser welding, cutting, drilling, deposition welding, SLM / LPBF, micro-LPBF, 3D printing, marking, laser ablation, stereolithography, SLE, or ISLE. The device can also be used in metrology such as LIDAR, OCT, or pyrometry. Short description of the drawings

[0021] The proposed device is explained in more detail below using exemplary embodiments in conjunction with the drawings. Fig. 1 a schematic representation of an arrangement of two scan heads for laser material processing according to the prior art; Fig. 2 a schematic representation of an exemplary embodiment of a device not according to the invention (deflection units in zero position); Fig. 3 a schematic representation of the conditions in the device of the Figure 2 with different beam deflection; Fig. 4 a schematic representation of an exemplary realization of a superposition arrangement of the device as a relay optics; Fig. 5 a schematic representation of a further example of a superposition arrangement of the device not according to the invention as a relay optics; Fig. 6 a representation of the wavelength-dependent transmission and reflection of the two in the example of Figure 5 inserted dichroic mirrors; Fig. 7 a schematic representation of an exemplary embodiment of the proposed device with several relay optics in the zero position of the deflection units; Fig. 8a schematic representation of the conditions in the device of the Figure 7 with different beam deflection; Fig. 9 a schematic representation of a further exemplary embodiment of a device not according to the invention with pre-focusing of the laser beams; and Fig. 10 a schematic representation of the intermediate focal points in the image planes and the process plane in the design of the Figures 7 and 8 . Ways to implement the invention

[0022] In known devices for laser material processing with multiple laser beams, several laser scanning heads with corresponding deflection units AE i for dynamic beam deflection and lenses O i for focusing the laser beams into the process plane PE are arranged next to each other, as shown in Fig. 1is indicated schematically. Especially in applications that require a small laser spot diameter FP i and short working distances AA, as is the case with Micro-SLM / LPBF, for example, an overlap of the scanning or working ranges AB i of the individual laser beams in the process plane cannot be achieved.

[0023] This problem is solved with the proposed device, also known as a multi-spectral scanner, by first deflecting the laser radiation from several beam sources with different central wavelengths, then spectrally combining or superimposing them, and finally guiding them into the process plane via a common focusing optics. This allows a 100% overlap of the scanning or working ranges AB i for a large number of laser beams or laser channels.

[0024] The following in connection with the Figures 2 to 6 and with Figure 9The embodiments described are not in accordance with the invention with regard to the overlay arrangement and are for illustrative purposes only.

[0025] An exemplary embodiment of a device for laser material processing is shown in the Figures 2 and 3 shown schematically. In this example, the laser beams from three laser sources are directed into the process plane PE. The device is, of course, not limited to this number of laser beams or laser beam sources. Fig. 2shows the situation without beam deflection by the deflection devices AE i (zero position). The laser radiation exits the fibers (or fiber lasers) F 1 , F 2 and F 3 and is collimated by the collimators K i and guided into the deflection units AE i. The radiation does not necessarily have to be guided via fibers. Free-beam coupling or direct beam exit from a laser, e.g. a diode laser, is also possible. The deflection units AE i are shown only schematically as blocks here and in the other figures. The laser radiation is completely superimposed using a superposition arrangement of, in this example, three dichroic mirrors DS i and imaged into the process plane PE via the mirror S and the objective O. The dichroic mirrors DS i act as spectral edge filters with the steepest possible band edge, which transmit radiation up to a cutoff wavelength and reflect radiation above this wavelength.Without beam deflection, the laser radiation from the three different beam sources is imaged into the same focal point FP = FP 1 = FP 2 = FP 3 in the process plane PE.

[0026] Fig. 3 shows the beam path of this device with different beam deflections of the laser beams by the respective deflection units AE i schematically, here only based on the optical axes. The radiation from the various beam sources is then imaged into the different focus points FP 1 , FP 2 and FP 3 in the process plane PE, as shown in Fig. 3 In the design of the Figures 2 and 3 The first mirror DS 1 does not necessarily have to be a dichroic mirror. In this example, DS 1 can also be a conventional mirror. Mirror S can be omitted or replaced with a 2D galvanometer scanner to further increase the scanning or working range of the device.

[0027] When designing the Figures 2 and 3 The beam height on the dichroic mirrors DS i , the mirror S and the objective O, which increases with the deflection angle, can be disadvantageous, as this severely limits the deflection angle of the deflection units AE i . To solve this problem, the superposition arrangement can be designed as a multi-spectral relay optics (MSRO), as shown in the Figures 4 to 8 In such a relay optics, the laser radiation from the two or more object planes OE i is imaged 1:1 into an image plane BE, as in the superposition arrangements of the Fig. 4 for two laser beams and in Fig. 5 for three laser beams. The MSRO can also have tilted, anti-reflection coated, plane-parallel substrates PP to compensate for the beam offset when passing through the dichroic mirrors DS. Such a plane-parallel substrate PP is Fig. 4 shown as an example.

[0028] Fig. 6 shows the characteristics of the Fig. 5 The dichroic mirrors DS 1 and DS 2 used. The figure shows the wavelength-dependent transmission and the wavelength-dependent reflection of these mirrors. Based on the edge profile of the spectral transmittance and spectral reflection of these mirrors, it is clearly evident that laser beams with the central wavelengths λ 1 , λ 2 , and λ 3 can be superimposed without significant losses using a combination of these mirrors.

[0029] If, according to the invention, several MSROs are arranged such that the image plane BE of one MSRO lies in one of the object planes OE of the following MSRO, the intermediate focal points generated by focus lenses FL i are spectrally superimposed in the image plane of the second MSRO. Beam superposition of three beam sources using MSROs is possible in the exemplary embodiment of the proposed device in Fig. 7 The deflection units AE i are in the zero position in this illustration. Without beam deflection, the radiation from each beam source is again imaged into a common focal point FP = FP 1 = FP 2 = FP 3 in the process plane PE. The intermediate focal points also lie on top of each other in the image planes BE i.

[0030] If the laser beam is now deflected via the deflection units AE i, the intermediate focal points ZFP i in the image planes BE i and the focal points FP i in the process plane PE are deflected accordingly. This situation is shown in Fig. 8 shown schematically. Fig. 10 shows the position of the intermediate focus points ZFP i in the individual image planes BE i ( Fig. 10a - 10c ))and the position of the focus points FP i in the process plane PE ( Fig. 10d ) schematically in plan view of the respective plane. The beam path is shown in Fig. 8 only shown using the respective optical axes of the laser beams.

[0031] In a further embodiment of a device for laser material processing, the laser beams are pre-focused via focus lenses FL i before passing through the deflection units AE i, superimposed via the dichroic mirrors DS i after passing through the deflection units and focused into the process plane PE, as shown in Fig. 9 is shown schematically. List of reference symbols

[0032] AAWorking distance AB i Scanning range AE i Deflection unit BE / BE i Image plane DS / DS i Dichroic mirror F i Fiber or fiber laser FL i Focus lens FP i Focus point KiCollimator L i Lens MSRO i Multispectral relay optics O / O i Object lens OE i Object plane PEProcess plane PPPlane-parallel plate SMirror ZFP i Intermediate focus point

Claims

1. Device for scanning a target surface (PE) with multiple laser beams, in particular for laser material processing, including - at least two laser beam sources (Fi), - for each of the laser beam sources (Fi), a deflection device (AEi) which is designed for dynamic one- or two-dimensional deflection of a laser beam emitted by the respective laser beam source (Fi), - one or more focusing optical units (FLi, O) for focusing the laser beams into the target surface (PE), and - a superposition arrangement for combining the laser beams that have propagated through the deflection devices (AEi), wherein the laser beam sources (Fi) are designed such that the laser beams emitted by the laser beam sources (Fi) have different central wavelengths, and the superposition arrangement includes one or more dichroic mirrors (DSi) tuned to the different central wavelengths, by which the laser beams are combined, characterized in that the superposition arrangement includes several multispectral relay optical units (MSROi) arranged one behind the other, which are arranged in such manner that for each pair of consecutive multispectral relay optical units (MSROi) an image plane (BE) of a multispectral relay optical unit (MSROi) lies in an object plane (OE) of the following multispectral relay optical unit (MSROi).

2. Device according to Claim 1, characterized in that the focusing optical unit (FSi, O) is formed by a F-theta lens or microscope lens shared by all laser beams.

3. Device according to Claim 1 or 2, characterized in that an optical arrangement (Ki) for collimation is arranged between the laser beam source (Fi) and the deflection device (AEi) for each laser beam.

4. Device according to Claim 1, characterized in that the focusing optical units (FLi, O) are arranged between the laser beam source (Fi) and the deflection device (AEi).

5. Device according to any one of Claims 1 to 4, characterized in that the one or more dichroic mirrors (DSi) is / are embodied as (an) edge filter(s).

6. Device according to any one of Claims 1 to 5, characterized in that the different central wavelengths are separated from each other by a distance of at least 1 nm.

7. Device according to any one of Claims 1 to 6, characterized in that the superposition arrangement is designed such that it completely superposes the laser beams in one position of the respective deflection devices (AEi).