Device and method for coupling a laser beam into a double-clad fibre
Patent Information
- Application Number
- EP2023754203
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2022-08-04
- Filing Date
- 2023-08-03
- Publication Date
- 2025-06-11
Smart Images
Figure 1.1
Abstract
Description
[0001] Device and method for coupling a laser beam into a double-clad fiber
[0002] Technical area
[0003] The present invention relates to a device and a method for coupling a laser beam of a laser into a double-clad fiber.
[0004] State of the art
[0005] In recent years, the advancement of laser systems has led to a new type of material processing based on the application of a laser beam to a workpiece. To transport the laser beam from the typically stationary laser to a target location, the laser beam is often coupled into a multi-clad fiber, particularly a double-clad fiber. The target location can be, for example, a processing optics system that shapes the laser beam and then applies the shaped laser beam to a workpiece to be processed.
[0006] An important machining process is the cutting and machining of workpieces, where the laser beam can create a perforation in the workpiece along a cutting line, along which the workpiece can be separated. Another important machining process is the joining of two joining components. The interface between adjacent components is exposed to the laser beam to create a melt, which, after solidification, forms a weld seam between the two components.
[0007] The different processing processes typically have different requirements for characteristic laser beam parameters, such as the focus diameter, intensity distribution, or beam profile characteristics at the processing point. Changing the processing process therefore involves significant retooling effort on the optical system.
[0008] EP2556397 discloses that the laser beam components coupled into the inner or outer core of the multi-clad fiber provide different beam profile characteristics and beam qualities in the outcoupled laser beam. To achieve this, the laser beam is split into two partial laser beams by a mechanically retractable wedge splitter, which are coupled into different cores of the double-clad fiber. However, using a wedge splitter to adjust the beam quality is complex, both mechanically and in terms of adjustment effort.
[0009] US10914902 shows further different variants of coupling a laser beam into a double-clad fiber, in which a polarization splitting of an incident laser beam is achieved by means of birefringent elements, and the two resulting partial laser beams are imaged into different cores of the double-clad fiber.
[0010] Description of the invention
[0011] Based on the known prior art, it is an object of the present invention to provide an improved device for coupling a laser beam, as well as a corresponding method.
[0012] The object is achieved by a device for coupling a laser beam having the features of claim 1. Advantageous further developments emerge from the subclaims, the description, and the figures.
[0013] Accordingly, a device for coupling a laser beam from a laser into a double-clad fiber is proposed, comprising a first birefringent optical element, in particular an optical wedge or an optical plane-parallel plate, which is configured to split the laser beam incident on its beam entrance surface into two partial laser beams, wherein the two partial laser beams have first exit angles and / or first beam offsets to the beam exit surface normal, wherein the two partial laser beams are polarized along the base polarization components of the first birefringent optical element, comprising a polarization rotator configured to adjust the polarization of the incident partial laser beams and thus provide polarization-adjusted partial laser beams, comprising a second birefringent optical element,wherein the beam exit surface of the second birefringent optical element is first traversed by the polarization-adjusted partial laser beams, and wherein the first exit angles and / or the first beam offsets of the partial laser beams from the first birefringent optical element are the second incidence angles and / or the second beam offsets of the polarization-adjusted partial laser beams relative to the beam exit surface normal of the second birefringent optical element, wherein the second birefringent optical element is configured to split the polarization-adjusted partial laser beams into two partial laser beams each, wherein the two partial laser beams each have second exit angles and / or second beam offsets to the beam entry surface normal of the second birefringent optical element,wherein the two partial laser beams are each polarized along the base polarization components of the second birefringent optical element, and wherein the second exit angles and / or the second beam offsets of the partial laser beams, whose polarization corresponds to that of the original partial laser beams, compensate for the respective first exit angles and / or the first beam offsets, and with coupling optics configured to couple the partial laser beams with compensated first exit angles and / or first beam offsets into the inner core of the double-clad fiber and to couple the other partial laser beams into the annular core of the double-clad fiber.
[0014] The laser can be a continuous wave laser or a pulsed laser, in particular an ultrashort pulse laser.
[0015] A continuous wave laser provides a continuous laser beam so that laser energy is continuously transported along the laser beam.
[0016] In contrast, a pulsed laser only provides laser energy during specific time intervals, the length of which is known as the pulse length. The energy transport by the laser pulses also occurs along the laser beam. In particular, a pulsed laser can also be an ultrashort pulse laser, where the pulse duration of the laser pulses can be less than 10 ps, preferably less than 1 ps.
[0017] Instead of individual laser pulses, the laser can also provide bursts, for example, with each burst comprising the emission of multiple laser pulses. The laser pulses can be emitted very closely over a specific time interval, with intervals ranging from a few picoseconds to hundreds of nanoseconds. These bursts can, in particular, be so-called GHz bursts, in which the sequence of consecutive laser pulses of the respective burst occurs in the GHz range.
[0018] The wavelength of the laser beam can be, for example, between 200nm and 2000nm, preferably 257nm or 343nm or 515nm or 1030nm.
[0019] A birefringent optical element comprises a beam entrance surface and a beam exit surface, whereby the birefringent optical element itself may comprise or consist of a birefringent material. In particular, a birefringent optical element can be designed as a wedge or as a plane-parallel plate. The optical wedge, by parallelizing the beam entrance surface and the beam exit surface, transitions into a plane-parallel plate, so that an optical wedge can also be understood as a generic term for optical wedges and plane-parallel plates.
[0020] Birefringence is the ability of an optical material to split the incident laser beam into two perpendicularly polarized laser beams. This occurs due to the anisotropy of the refractive indices of the optical material, which depends on the polarization and the angle of incidence of the light relative to the optical axis of the optical material.
[0021] If a laser beam falls on a beam entrance or exit surface of a birefringent optical element, the polarization of the incident laser beam is projected onto the optical axis of the optical material of the birefringent optical element, whereby the laser beam is split into partial laser beams according to its polarization components, whereby the partial laser beams are polarized along the base polarization states of the birefringent optical element.
[0022] The partial laser beams can exit the beam entrance or exit surface of the birefringent optical element at an exit angle relative to the beam entrance surface normal or the beam exit surface normal, so that the partial laser beams are spatially separated by propagation through the birefringent optical element. In particular, the respective exit angles of the partial laser beams can be different, so that the partial laser beams exhibit an angular offset from one another.
[0023] The exit angles of the partial laser beams depend, for example, on the anisotropy of the refractive indices, whereby the beam entrance surface is preferably parallel to the optical axis of the optical material.
[0024] However, the exit angles can also be influenced by the fact that the beam exit surface of the birefringent optical element is inclined at an angle to the beam entrance surface and / or to the optical axis of the optical material.
[0025] However, it is also possible for the partial laser beams to exit the birefringent optical element with a beam offset. In this case, the partial laser beams run parallel to the incident laser beam, but are spaced apart perpendicular to the beam propagation direction. In such a case, the first exit angle can also be 0°.
[0026] In particular, it is also possible for the partial laser beams to have a first exit angle and a first beam offset. In general, the splitting of the partial laser beams depends on the orientation of the optical axis of the birefringent crystal, the magnitude of the anisotropy, the angular inclination of the beam exit surface relative to the optical axis, and the angle of incidence of the laser beam on the beam entrance surface and / or beam exit surface.
[0027] A first birefringent optical element is arranged in front of the polarization rotator in the beam propagation direction.
[0028] The first birefringent optical element is configured to split the laser beam incident on the beam entrance surface into two partial laser beams, wherein the two partial laser beams have first exit angles and / or first beam offsets to the beam exit surface normal, wherein the two partial laser beams are polarized along the base polarization components of the first birefringent optical element,
[0029] For example, a first partial laser beam can be s-polarized and have an exit angle of -10° to the beam exit surface normal and a second partial laser beam can be p-polarized and have an exit angle of +20° to the beam exit surface normal.
[0030] For example, a first partial laser beam can be p-polarized and have a beam offset of 0mm, while the second partial laser beam is s-polarized and has a beam offset of 2mm or 5pm.
[0031] The polarization rotator is arranged downstream of the first birefringent optical element, so that the partial laser beams in the base polarization states pass through the polarization rotator at the first exit angles. Due to the spatial separation of the partial laser beams in the first birefringent optical element, the partial laser beams also impinge on the polarization rotator at different locations. However, the effect of the polarization rotator is independent of the angle and location of incidence.
[0032] A polarization rotator allows the polarization of the incident partial laser beams to be modified. A modification can involve generating a partial laser beam in a final polarization state from a partial laser beam in a defined initial polarization state. Overall, the polarization rotator rotates the electric field vector of the partial laser beams by an angle around the respective beam propagation direction. The polarization modification thus transforms the partial laser beams into polarization-adjusted partial laser beams.
[0033] For example, a p-polarized, polarization-adjusted partial laser beam can be generated from an s-polarized partial laser beam. For example, an s-polarized, polarization-adjusted partial laser beam can be generated from a p-polarized partial laser beam. However, it is also possible for the s- and p-polarized partial laser beams to be converted to a final polarization state in which both polarization-adjusted partial laser beams are partially s-polarized and partially p-polarized. The polarization rotator can also allow the partial laser beams to pass through without modifying the polarization. Such partial laser beams are also polarization-adjusted partial laser beams.
[0034] A polarization rotator can also be a phase element, for example a lambda / 4 plate, which generates a circularly polarized laser beam from a linearly polarized laser beam, and vice versa.
[0035] The second birefringent optical element is arranged downstream of the polarization rotator in the beam propagation direction, with the polarization-adjusted partial laser beams passing through the beam exit surface of the second birefringent optical element first. The polarization-adjusted partial laser beams pass through the second optical wedge in reverse.
[0036] The polarization-adjusted partial laser beams impinge on the beam exit surface of the second birefringent optical element at the first exit angle and / or the first beam offset, wherein the first exit angles and / or the first beam offsets of the polarization-adjusted partial laser beams determine the second incident angles and / or the second beam offsets of the polarization-adjusted partial laser beams relative to the
[0037] Beam exit surfaces are normal to the second birefringent optical element.
[0038] The beam exit surface of the second birefringent optical element is understood to be the surface of the second birefringent optical element that is arranged at an angle to the optical axis. The polarization-adjusted partial laser beams that have exited the first birefringent optical element first impinge on this surface.
[0039] In the simplest case, the second birefringent optical element is identical to the first birefringent optical element and rotated 180° in the beam path. In other words, the beam exit surfaces of the first and second birefringent optical elements, which are angled relative to the optical axis, face each other, whereas the optical axes of the birefringent optical elements are (anti)parallel to each other.
[0040] The second birefringent optical element adjusts the polarization
[0041] Partial laser beams are split into two partial laser beams each, with each of the two partial laser beams having second exit angles and / or second beam offsets relative to the beam entrance surface normal of the second birefringent optical element, with the two partial laser beams being polarized along the base polarization components of the second birefringent optical element. This effect is described analogously to the first birefringent optical element.
[0042] The second exit angles and / or the second beam offsets of the partial laser beams, whose polarization corresponds to that of the original partial laser beams, compensate for the respective first exit angles and / or the first beam offsets.
[0043] This means, for example, that a partial laser beam of a first polarization, which originates from a partial laser beam of a first polarization, has a second exit angle at the beam entrance surface of the second birefringent optical element, which is opposite to and equal to the exit angle of the partial laser beam at the beam exit surface of the first birefringent optical element. The polarization-adjusted partial laser beams thus pass through the second birefringent optical element as if they were passing through the first birefringent optical element in reverse, but only if the polarization of the polarization-adjusted partial laser beam and the angle of incidence correspond to the component emerging from the first birefringent optical element. The description for the beam offsets is analogous.
[0044] For example, in a first simple example, the polarization rotator can rotate the polarization of the partial laser beams by 0°. Then, according to the above example, the s-polarized partial laser beam has an exit angle of -10° and the p-polarized partial laser beam has an exit angle of +20°. Due to the spatial separation between the first birefringent optical element and the second birefringent optical element, the two partial laser beams impinge on the second birefringent optical element at different locations. Furthermore, the two partial laser beams impinge on the beam exit surface of the second birefringent optical element at angles of -10° and +20°, respectively. As a result, both partial laser beams are deflected so that they run parallel to each other behind the second birefringent optical element, but also parallel to the incident laser beam.The first exit angles were accordingly compensated by the second exit angles.
[0045] For example, the polarization rotator can rotate the polarization of the partial laser beams by 45°. The first polarization-adjusted partial laser beam and the second polarization-adjusted partial laser beam then exhibit mixed polarizations, i.e., they exhibit p- and s-polarization components. For example, the first polarization-adjusted partial laser beam strikes the beam exit surface of the second birefringent optical element at an incidence angle of -10°. The component of the laser beam whose polarization corresponds to that of the original laser beam, i.e., the s-component, can pass through the second birefringent optical element in reverse, so to speak. The p-polarized component of the first polarization-adjusted partial laser beam, however, is deflected.Conversely, in the second polarization-adjusted partial laser beam, the p-polarized component is guided backward through the second birefringent optical element, while the s-component is deflected.
[0046] By adjusting the polarization rotator, the laser power can be distributed between the partial laser beams that have a compensated first exit angle and the other partial laser beams.
[0047] The coupling optics allow the partial laser beams provided by the second birefringent optical element to be directed into different focal zones, in particular into the inner core or the annular core of a double-clad fiber. In particular, the partial laser beams are directed into the inner core of the double-clad fiber with a compensated first exit angle, while the other partial laser beams are directed into the annulus of the double-clad fiber.
[0048] The coupling optics can comprise a lens and / or a lens system and / or a mirror arrangement. The coupling optics can also comprise a collimating lens and a focusing lens.
[0049] The collimating lens is designed to convert bundles of non-parallel partial beams, especially divergent partial beams, into parallel partial beams. In particular, partial laser beams with uncompensated exit angles can be parallelized by a collimating lens.
[0050] The focusing lens can direct the partial beams of a beam bundle into a focal zone. In particular, this makes it possible to direct different beam bundles, such as those of the partial laser beams provided by the second birefringent optical element, into different focal zones.
[0051] A double-clad fiber with two cores has an inner fiber core and an intermediate cladding surrounding this fiber core, which is as thin and low-refractive as possible. This is followed by a single outer ring-shaped core, which is also surrounded by a second cladding with a low refractive index. Another layer of glass can be placed on top of this, which determines the outer diameter of the fiber but has no influence on its function in terms of beam guidance. The final layer can be a coating made of a plastic material such as silicone and / or nylon, which serves to protect the fiber. By using a double-clad fiber, different beam profile characteristics can be selected at the fiber output, depending on whether the fiber is coupled into the inner fiber core, the outer ring-shaped core, or both the inner fiber core and the outer ring-shaped core.
[0052] A double-clad fiber is a special case of a multiclad fiber with N cores, where a multiclad fiber comprises at least one double-clad fiber. For example, a multiclad fiber can comprise three cores: an inner core, an outer annular core, and a central annular core, with the central annular core surrounding the inner core and being arranged between the inner core and the outer core. For example, the inner core and the outer core can then form the double-clad fiber. However, it is also possible for the central core and the outer core to form the double-clad fiber, or for the inner core and the central core to form the double-clad fiber. By combining several first and second birefringent elements and polarization rotators, the intensity of the partial laser beams in the N cores of a multiclad fiber can also be adjusted.
[0053] The splitting ratio at which the laser beam is coupled into the core, the inner core, and the annular core of the double-clad fiber can be adjusted using the polarization rotator. The splitting ratio can be determined from the ratio of the powers of those partial laser beams whose polarization matches that of the original partial laser beams to the powers of the other partial laser beams.
[0054] The use of a polarization rotation device, in particular, allows for a continuous adjustment of the splitting ratio. This drastically reduces the mechanical complexity compared to the state of the art, resulting in a device with particularly stable alignment.
[0055] For example, the entire laser power can be coupled into the core of the double-clad fiber if the polarization rotator does not rotate the polarization of the partial laser beams. For example, the entire laser power can be coupled into the outer core of the double-clad fiber if the polarization rotator rotates the polarization of the partial laser beams by 90°. In particular, all other power ratios can also be adjusted by adjusting the polarization rotator.
[0056] The splitting ratio can therefore be used to adjust the beam quality of the laser beam after the double-clad fiber.
[0057] When processing materials using laser radiation, especially when using high
[0058] Power in the kW range, switching between the coupling variants enables, for example, the choice between a comparatively good beam quality with a sharp focus, as is required for a laser cutting process, and a "reduced" beam quality with an almost uniform intensity distribution in the beam cross-section, which is particularly suitable for welding processes.
[0059] To achieve high laser beam quality, the laser beam is coupled into the inner fiber core of the double-clad fiber, which in this case behaves like a conventional standard fiber whose fiber core is surrounded by a low-refractive-index intermediate cladding. If, however, a laser beam with a broadened profile, for example with uniform intensity, is required, the laser beam is coupled into the outer ring-shaped core or into both the inner fiber core and the outer ring-shaped core. Depending on the application, a laser beam with a filled circular profile corresponding to the inner fiber core, with a ring profile corresponding to the outer ring-shaped core, with a filled circular profile corresponding to both core regions together (with a narrow missing ring due to the first intermediate cladding), or with the corresponding intermediate stages of the aforementioned profile characteristics can be obtained at the output of the double-clad fiber.
[0060] The laser beam can be polarized or unpolarized.
[0061] The magnitude of the exit angles and / or beam offsets of the laser beams from the birefringent optical elements is independent of the polarization of the incident laser beam. The polarization of the incident laser beam determines only the amplitude of the individual exiting laser beams. In a sense, the possible spatial paths of the partial laser beams and the partial laser beams are determined solely by the geometry of the device's elements, whereas the input polarization merely determines how much power is to be transported along the individual paths.
[0062] The first birefringent optical element and / or the second birefringent optical element may comprise quartz glass or be formed from quartz glass.
[0063] Quartz glass has a particularly high laser damage threshold, making it particularly well-suited for use in laser material processing. Furthermore, quartz glass is particularly easy to machine, which can reduce manufacturing costs.
[0064] The first birefringent optical element and / or the second birefringent optical element may comprise calcite or be formed from calcite.
[0065] The first birefringent optical element and / or the second birefringent optical element can comprise BBQ (barium borate) or be formed from BBQ. The base thickness of at least one birefringent optical element can be between 1 mm and 50 mm, preferably between 1 mm and 10 mm.
[0066] The base thickness is the length of the longest side in the cross-section of an optical fiber, which is neither the beam entrance surface nor the beam exit surface and where the input laser beam and the generated partial laser beams also lie in the cross-sectional plane.
[0067] Such a base thickness enables a particularly robust design of the optical wedges.
[0068] For a plane-parallel plate, the base thickness corresponds to the thickness of the plate.
[0069] The polarization rotating device can be electronically controlled.
[0070] This allows the polarization rotator to receive and implement electronic control signals. For example, the polarization rotator can be motorized so that it can be rotated.
[0071] Electronic control can, for example, consist of the polarizations of the partial laser beams being continuously rotated from the initial polarization state to the final polarization state by the polarization rotator. In particular, all intermediate polarization states for both partial laser beams can thus be set by the polarization rotator.
[0072] However, the polarization of the partial laser beams can also be switched between two states using the polarization rotation device. This can be particularly advantageous when switching between two processing steps.
[0073] The polarization rotator can be a rotatably mounted lambda / 2 plate or a Pockels cell.
[0074] A Pockels cell is an optoelectronic device that can modify the polarization of a laser beam passing through it by applying a control voltage. In particular, it is possible to rotate the polarization of the laser beam. Accordingly, switching, rotating, or modifying the polarization can be performed particularly easily by controlling the voltage.
[0075] In particular, a Pockels cell eliminates the need for moving parts in the device, thus achieving exceptional mechanical stability. The above-mentioned object is further achieved by a method for coupling a laser beam with the features of claim 10. Advantageous developments of the method are evident from the subclaims as well as the present description and the figures.
[0076] Accordingly, a method for coupling a laser beam from a laser into a double-clad fiber is proposed, wherein the laser beam incident on the beam entrance surface of a first birefringent optical element is split into two partial laser beams, wherein the two partial laser beams have first exit angles and / or first beam offsets to the beam exit surface normal, wherein the two partial laser beams are polarized along the base polarization components of the first birefringent optical element, wherein the polarization of the incident partial laser beams is adjusted using a polarization rotator and thus polarization-adjusted partial laser beams are provided, wherein the polarization-adjusted partial laser beams first pass through the beam exit surface of a second birefringent optical element,wherein the first exit angles and / or the first beam offsets of the partial laser beams from the first birefringent optical element form the second incidence angles and / or the second beam offsets of the polarization-adjusted partial laser beams relative to the beam exit surface normal on the second birefringent optical element, wherein the polarization-adjusted partial laser beams are split by the second birefringent optical element into two partial laser beams each, wherein the two partial laser beams each have second exit angles and / or second beam offsets to the beam entry surface normal of the second birefringent optical element, wherein the two partial laser beams each are polarized along the base polarization components of the second birefringent optical element and wherein the second exit angles and / or the second beam offsets of the partial laser beams, the polarization of which,which corresponds to the original partial laser beams, each compensating for the first exit angle and / or first beam offsets, and the partial laser beams with compensated first exit angle and / or first beam offsets are coupled into the inner core of the double-clad fiber with a coupling optics, and the other partial laser beams are coupled into the annular core of the double-clad fiber with the coupling optics.
[0077] The splitting ratio with which the laser beam is coupled into the inner core and the ring-shaped core of the double-clad fiber can be adjusted using the polarization rotator.
[0078] The splitting ratio can be determined from the ratio of the powers of those partial laser beams whose polarization corresponds to that of the original partial laser beams and the powers of the other partial laser beams. The above-stated object is further achieved by a system for processing a material having the features of claim 13. Advantageous developments of the method emerge from the subclaims as well as the present description and the figures.
[0079] Accordingly, a system for processing a workpiece with the laser beam of a laser is proposed, comprising a laser, a double-clad fiber, a device for coupling the laser beam of the laser into the double-clad fiber, processing optics and a workpiece, wherein the coupling device is configured to couple the laser beam of the laser with a splitting ratio into the inner core of the double-clad fiber and into the annular core of the double-clad fiber, wherein the double-clad fiber is configured to guide the laser beam from the input of the double-clad fiber to the output of the double-clad fiber, wherein the processing optics are configured to form a processing laser beam from the laser beam after the output of the double-clad fiber, to focus the processing laser beam and to apply the processing laser beam to the workpiece, and thereby to process the workpiece.
[0080] The processing optics enable the laser beam provided by the double-clad fiber to be introduced into the workpiece with the appropriate quality. Specifically, the laser beam is introduced into the workpiece in a focal zone for processing. Only through focusing with processing optics and the resulting convergence of the laser beam into the focal zone is an intensity boost achieved in the focal zone, allowing the workpiece to be processed.
[0081] For example, a machining operation can involve separating a workpiece along a parting line, chamfering an edge, creating a predetermined breaking point, or applying a specifically directed material stress, etc. However, the system can also be used to process, particularly cut, opaque materials such as metals or sheet metal. In this process, material is vaporized and removed through high-energy excitation of the workpiece material.
[0082] However, it is also possible for the workpiece to comprise two joining partners that are to be joined together. The joining partners can be arranged on top of one another such that the interfaces of the joining partners, across which the joining partners are to be joined, face one another. Applying pressure to the interface can thus lead to local melting of the material of the joining partners, with the resulting melt bridging the common interface between the joining partners and permanently joining the joining partners upon cooling. The system can also have a feed device that is designed to move the workpiece and the laser beam relative to one another with a feed along a trajectory, with the feed preferably taking place perpendicular to or parallel to the splitting of the laser beam.
[0083] For example, a feed moves the laser beam and the workpiece relative to each other along the trajectory at a feed rate, so that as time progresses, different points of impact of the laser beam on the surface of the workpiece result.
[0084] The beam quality of the processing laser beam after the output of the double-clad fiber can be adjusted by the splitting ratio with which the partial laser beams are coupled into the double-clad fiber.
[0085] Short description of the characters
[0086] Preferred further embodiments of the invention are explained in more detail in the following description of the figures. In the figures:
[0087] Figure 1A, B, C, D shows a schematic representation of a birefringent optical element;
[0088] Figure 2A, B shows a schematic representation of the beam path between two birefringent optical elements;
[0089] Figure 3A, B, C, D, E, F shows a schematic representation of the beam path between two birefringent optical elements when the polarization of the partial laser beams is adjusted with a polarization rotator;
[0090] Figure 4A, B, C a schematic representation of the coupling of the partial laser beams into a double-clad fiber with a coupling optics; and
[0091] Figure 5A, B is a schematic representation of a system for processing a
[0092] Materials.
[0093] Detailed description of preferred embodiments
[0094] Preferred embodiments are described below with reference to the figures. Identical, similar, or equivalent elements in the different figures are provided with identical reference numerals, and a repeated description of these elements is partially omitted to avoid redundancies.
[0095] Figures 1 to 4 show various aspects of the device 1 according to the invention. In particular, the birefringent optical element is depicted as a wedge, without loss of generality. However, it is clear that an optical wedge, as a general embodiment of a plane-parallel plate, always incorporates the properties of the plane-parallel plate.
[0096] Figure 1A schematically shows a first optical wedge 10, wherein the first wedge 12 has a beam entrance surface 100 and a beam exit surface 102. The first wedge 10 is configured to split the laser beam 20 incident on the beam entrance surface 100 into two partial laser beams 200, 202. These partial laser beams 200, 202 have first exit angles N102 to the beam exit surface normal.
[0097] The partial laser beams 200, 202 are generated by the birefringent properties of the first optical wedge 10. The material of the first wedge 10 has different refractive indices for different polarization directions of the incident laser beam 10, so that the incident laser beam 10 is split according to the base polarization component of the material of the first wedge 10. The partial laser beams 200, 202 are accordingly polarized along the base polarization components of the first optical wedge 10. Without loss of generality, it is assumed in the following that the partial laser beam 200 is s-polarized, while the partial laser beam 202 is p-polarized.
[0098] The first exit angles can be determined, on the one hand, by the entrance angle of the incident laser beam 20 and the shape of the beam exit surface 102, in particular the inclination compared to the beam entrance surface 100. On the other hand, the first exit angles are also determined by the inclination of the optical axis O of the birefringent medium and the relative inclination of the optical axis to the beam exit surface 102. In the present case, the optical axis of the birefringent medium is oriented parallel to the beam entrance surface 100. The beam exit surface 102 is therefore at an angle to the optical axis O.
[0099] Here, the splitting into the partial laser beams 200, 202 is independent of the polarization of the incident laser beam 20. The laser beam 20 can be polarized or unpolarized accordingly. In any case, in the birefringent medium, only the electric field vector of the incident laser beam 20 is projected onto the optical axis of the optical wedge 10, so that the polarization of the incident laser beam 20 only determines the laser power transported along the partial laser beams 200, 202. The magnitude of the spatial splitting, however, always remains the same. Figure 1B shows that a partial laser beam 200, which impinges on the beam exit surface 102 at the exit angle shown with a correspondingly selected polarization, here an s-polarization, exits the optical wedge 10 at the entrance angle of the incident laser beam 20. Such a partial laser beam 200 can accordingly pass through the optical wedge 10 backwards.
[0100] For comparison, Figure 1C shows a partial laser beam 200 that, although incident on the beam exit surface 102 at the same exit angle, has a different base polarization for this exit angle, namely a p-polarization. Accordingly, the partial laser beam 200 is not directed onto the path of the incident laser beam 20, but rather onto the path of another laser beam 20'. The p-polarized laser beam 200 cannot simply pass backward through the optical wedge, but is deflected.
[0101] If, as shown in Figure 1D, the partial laser beam 200 has both s- and p-polarization components and impinges on the beam exit surface 102 of the first optical wedge 10 at the exit angle of the s-polarized laser beam, then the s-polarized component can pass backward through the optical wedge 10. The p-polarized component, however, is deflected onto a different path, as shown in Figure 1C. The optical wedge 10 splits such partial laser beams 200 accordingly.
[0102] Figure 2A schematically illustrates a first optical wedge 10 and a second optical wedge 12. Without limiting the generality, it can be assumed at this point that the first wedge 10 and the second wedge 12 are identical in design. The second wedge 12 is merely rotated by 180° in the beam path. This ensures that the first exit angles of the partial laser beams 200, 202 from the beam exit surface 102 of the first optical wedge 10 correspond to the second angles of incidence of the partial laser beams 200, 202 on the beam exit surface 122 of the second optical wedge 12. In a sense, the s- and p-polarized partial laser beams 200, 202 pass through the second wedge 12 backward, as shown in Figure 1B.
[0103] However, due to the spatial distance between the first optical wedge 10 and the second optical wedge 12, the partial laser beams 200, 202 have different impact locations on the beam exit surface 122 of the second optical wedge 12. However, the two generated partial laser beams 2000, 2020 run parallel to each other after the second optical wedge 12. In a sense, both partial laser beams 2000, 2020 run parallel to the originally incident laser beam 20. In other words, the second exit angles of the partial laser beams 2000, 2020, whose polarization corresponds to that of the original partial laser beams, compensate for the respective first exit angles. Figure 2B shows a first optical wedge 10 and a second optical wedge 12, although the two wedges are not identical in design. In particular, the base thickness B of the two optical wedges 10, 12 is different.The base thickness B of at least one optical wedge 10, 121 can be between 1 mm and 50 mm, preferably between 1 mm and 10 mm. The base thickness B of the first optical wedge 10 can be, for example, 5 mm, and the base thickness B of the second optical wedge 12 can be 10 mm. In particular, the first optical wedge 10 and / or the second optical wedge 12 can comprise quartz glass or be made of quartz glass. This enables particularly simple and cost-effective production, while the optical wedges 10, 12 are simultaneously particularly insensitive to high laser powers.
[0104] In Figures 2A, B, the partial laser beams 2000, 2020 have the same polarization as the base polarization components of the optical material of the optical wedges 10, 12 associated with the respective paths. In Figure 3A, however, a polarization rotator 14 is arranged between the first and second optical wedges 10, 12. The polarization rotator 14 is configured to rotate the polarization of the partial laser beams 200, 202 and to provide polarization-adjusted partial laser beams 200', 202'.
[0105] The polarization of the polarization-adjusted partial laser beams 200', 202' here only partially corresponds to the base polarization component of the respective path on the beam exit surface 122 of the second optical wedge 12. The second optical wedge 12 is therefore configured, analogously to the first optical wedge 10, to split the polarization-adjusted partial laser beams 200', 202' into two partial laser beams 2000, 2002 and 2020, 2022, each of which is polarized along the base polarization component of the second wedge 12. Here, the partial laser beams 2000, 2002, 2020, 2022 have second exit angles to the beam entry surface normal N120 of the beam entry surface 120 of the second wedge 12.
[0106] Here, only the second exit angles of those partial laser beams 2000, 2020 compensate for the first exit angles whose polarization corresponds to that of the original partial laser beams 200, 202. The other partial laser beams 2002, 2022 diverge accordingly.
[0107] If the polarization rotation device 14 rotates the polarization of the partial laser beams 200, 202 such that each polarization-adjusted partial laser beam 200', 202' does not have components polarized according to the required base polarization components, the laser power is converted overall into partial laser beams 2002, 2022 that diverge, i.e., do not run parallel to the incident laser beam 20, as shown in Figure 3B. If the polarization rotation device 14 rotates the polarization of the partial laser beams 200, 202 such that each polarization-adjusted partial laser beam 200', 202' has components polarized exactly according to the required base polarization components, the laser power is converted overall into partial laser beams 2000, 2020 whose first exit angles are compensated, as shown in Figure 2A.
[0108] The polarization rotator 14 can be electronically controlled for this purpose. For example, the polarization rotator 14 can be a rotatably mounted lambda / 2 plate, wherein the electronic control of the rotatable mounting allows the adjustment of a rotation angle, with the rotation of the lambda / 2 plate in turn enabling the rotation of the polarization of the partial laser beams 200, 202. However, it is also possible for the polarization rotator 14 to be a Pockels cell, wherein a polarization rotation can be adjusted via electronic control of the Pockels cell.
[0109] Figure 3C shows the situation in which the incident laser beam 20 strikes the beam entrance surface of the first optical wedge 10 at an angle of incidence. This makes it possible to ensure that both partial laser beams exit the optical wedge at an exit angle and also strike the polarization rotator 14 at the exit angle. There, the polarization direction of the partial laser beams can be rotated, with the polarization-rotated partial laser beams subsequently striking the second optical wedge 12 at the respective exit angle. By configuring the optical wedges 10, 12, the spacing of the parallel partial laser beams can be adjusted, particularly compared to Figure 3A, in order to achieve a better adaptation of the shape of the partial laser beams to the double-clad fiber.
[0110] In Figure 3D, the optical wedges are designed analogously to Figure 3A, except that the optical crystal axis is not antiparallel. This also allows the spacing of the parallel partial laser beams to be adjusted.
[0111] In Figure 3E, the birefringent optical elements 10, 12 are designed as plane-parallel plates. Here, the optical crystal axes are, for example, at an angle of 45° to the beam entrance surface of the plane-parallel plates 10, 12. The incident laser beam 20 is split into two orthogonally polarized partial laser beams, which exit the first plane-parallel plate with only a beam offset but no exit angle. The two partial laser beams impinge directly onto the second plane-parallel plate, whose optical crystal axis is rotated relative to the first crystal axis such that the two partial laser beams recombine behind the second plane-parallel plate 12. In a sense, the beam offsets and the exit angles of 0° are compensated for.Due to the perpendicular incidence of the laser beam, strictly speaking, one can no longer speak of s- and p-polarized components of the incident laser beam, but rather of ordinary and extraordinary beams. The extraordinary beam contains a beam offset after passing through the plane-parallel plate, while the ordinary beam passes through the plane-parallel plate without any beam offset. The magnitude of the beam offset between the two partial laser beams depends on the difference in the refractive indices, the base thickness of the plane-parallel plate, and the alignment of the optical axis.
[0112] Figure 3F shows an embodiment according to the invention in which a polarization rotator 14 is arranged between the first and second plane-parallel plates 10, 12. Due to the adjusted polarization of the partial laser beams, analogous to the previous embodiments, the second beam offsets now compensate for those first beam offsets whose associated partial laser beams have a polarization that corresponds to the original polarization of the partial laser beams. In other words, for example, in the lower s-polarized partial laser beam, a p-polarized and an s-polarized component is generated by the polarization rotator 14. Only the first beam offset of the s-polarized component of the partial laser beam is compensated by the second plane-parallel plate and the second beam offset.
[0113] Figures 4A, B, and C show that a coupling optics 16 is arranged behind the second optical wedge 12. The coupling optics 16 is configured to couple the partial laser beams 2000, 2020 with a compensated first exit angle into the inner core 30 of a double-clad fiber 3 and to couple the other partial laser beams 2002, 2022 into the annular core 34 of the double-clad fiber 3.
[0114] The coupling optics 16 can comprise a lens and / or a lens system and / or a mirror arrangement, wherein in Figure 4 the coupling optics comprises only one lens 16.
[0115] The double-clad fiber shown also has a so-called intermediate cladding between the inner core 30 and the ring-shaped core 34, with which the partial beams coupled into the various cores of the double-clad fiber are held and guided in the various cores.
[0116] The polarization rotator 14, which is arranged in front of the second optical wedge 12, allows the total intensity of the partial laser beams 2020, 2000 with the compensated exit angles to be adjusted relative to the total intensities of the partial laser beams 2022, 2002 without compensated exit angles. In particular, the various partial laser beams 2002 and 2022 propagate as a unit in the annular core 34. This means that the partial laser beams 2002, 2022 form a common radiation field and are indistinguishable within the double-clad fiber and beyond the double-clad fiber. The same applies to the partial laser beams 2000, 2020 that are coupled into the inner core of the double-clad fiber. Thus, the polarization rotator 14 can be used to adjust the power components of the laser beam 20 that are to be coupled into the inner core 30 of the double-clad fiber 3 or the ring-shaped core 34 of the double-clad fiber 3.The polarization rotator 14 can therefore, in particular, determine the splitting ratio. In particular, the polarization rotator 14 can be used to set whether the entire laser power is coupled into the core 30 of the double-clad fiber or whether the entire laser power is coupled into the ring 34 of the double-clad fiber.
[0117] A particularly advantageous feature is that by splitting the polarization-adjusted partial laser beams 200', 202' into partial laser beams 2000, 2002, 2020, and 2022, a particularly homogeneous power distribution can be achieved on the entrance surface of the double-clad fiber. This reduces the thermal load on the entrance surface.
[0118] By adjusting the splitting ratio using the polarization rotator, the beam quality behind the double-clad fiber 3 can also be adjusted. If the portion of the laser radiation in the annular core 34 of the double-clad fiber 3 behind the double-clad fiber provides a flat laser beam, while the portion of the laser radiation in the inner core 30 of the double-clad fiber provides a collimated laser beam, the overall beam quality can be adjusted by adjusting the polarization rotator 14.
[0119] Figure 4A also shows that 25% of the laser power is coupled into the annular core 34 by the partial laser beams 2002, 2022, while 25% of the laser power is also coupled into the inner core 30 of the double-clad fiber by the partial laser beams 2020, 2000.
[0120] Figure 4B shows that 50% of the laser power is coupled into the ring-shaped core 34 by the partial laser beams 2002, 2022, while no laser power is coupled into the inner core 30 of the double-clad fiber.
[0121] Figure 4C shows that none of the laser beams is coupled into the annular core 34, while the laser power is coupled into the inner core 30 of the double-clad fiber at 50% each by the partial laser beams 2020, 2000. Figure 5A schematically shows a system according to the invention for processing a workpiece 5. In particular, the processing of a workpiece can consist of joining two joining partners. The joining partners 50, 52 are arranged one on top of the other at a common interface.
[0122] Laser 2, for example, provides ultrashort laser pulses. These can be introduced into the interface between the joining partners 50, 52 in the form of a sequence of single pulses or in the form of a sequence of bursts.
[0123] The laser beam 20 of the laser 2 is guided through the device 1 for coupling the laser beam 20 into the double-clad fiber, wherein the device 1 couples the laser beam 20 into the annular core 34 and the inner core 30 of the double-clad fiber 3 according to the splitting ratio predetermined by the polarization rotator 14.
[0124] The double-clad fiber guides the laser beam 20 to the processing optics 4. The processing optics can comprise an outcoupling optic, with which the laser beam 20 is decoupled from the ring 34 and the core 30 of the double-clad fiber 3. However, the processing optics 4 can also be arranged downstream of an outcoupling optic. For example, the processing optics 4 focuses the laser beam 20 with the beam quality determined by the splitting ratio onto the common interface of the two joining partners 50, 52.
[0125] In order to focus the laser beam 20 into the common interface of the joining partners 50, 52, the first joining partner 50 in the beam propagation direction must be transparent to the wavelength of the laser 2. For example, the first joining partner 50 can be a glass or a crystal or a ceramic or a plastic. For example, the second joining partner 52 can be opaque or transparent. For example, the second joining partner 52 can be a metal or a semiconductor or a plastic or a ceramic.
[0126] At the interface, the laser pulses are absorbed in such a way that the material of the joining partners 50, 52 melts and bonds with the other joining partner 52, 50 across the interface. As soon as the melt cools, a permanent connection is formed between the two joining partners 50, 52. In other words, the two joining partners 50, 52 are joined together in this area by welding.
[0127] The laser beam and the joining partners can be moved and / or positioned relative to each other by means of a feed device 6 with a feed rate V between 0.01 mm / s and 1000 mm / s, preferably between 0.1 mm / s and 300 mm / s. For this purpose, the joining partners can be positioned, for example, on a feed device 6. This allows the laser beam 20 to be moved along a joining seam over the joining partners 50, 52, so that the joining partners 50, 52 can be joined along the joining seam.
[0128] Figure 5B schematically shows the system according to the invention for processing a workpiece 5, wherein the processing here consists in separating a workpiece 5. Analogous to Figure 5A, the laser beam 20 is introduced into the workpiece 5 along a trajectory 54 along which the material is to be separated. By introducing the laser beam 20, the workpiece 5 is damaged in a targeted manner along the trajectory, so that the workpiece 5 can be separated along the trajectory 54.
[0129] In order to switch between the processing processes of Figure 5A and Figure 5B, only one adjustment of the polarization rotation device 14 has to be made, so that there is no changeover effort.
[0130] Where applicable, all individual features presented in the embodiments may be combined and / or exchanged without departing from the scope of the invention.
[0131] List of reference symbols
[0132] 1 device
[0133] 10 first birefringent optical element
[0134] 100 beam entrance area
[0135] 102 beam exit area
[0136] 12 second birefringent optical element
[0137] 120 beam entrance area
[0138] 122 beam exit area
[0139] 14 Polarization rotating device
[0140] 16 coupling optics
[0141] 2 lasers
[0142] 20 incident laser beam
[0143] 200 partial laser beam
[0144] 200' polarization-adjusted partial laser beam
[0145] 2000 partial laser beam
[0146] 2002 partial laser beam
[0147] 202 partial laser beam
[0148] 202' polarization-adjusted partial laser beam
[0149] 2020 partial laser beam
[0150] 2022 partial laser beam
[0151] 22 processing laser beam
[0152] 3 double-clad fiber
[0153] 30 inner core
[0154] 32 Intermediate cladding
[0155] 34 ring-shaped core
[0156] 4 Processing optics
[0157] 5 Workpiece
[0158] 50 first joining partners
[0159] 52 second joining partner
[0160] 54 Trajectory
[0161] 6 Feed device
[0162] B Base thickness
[0163] N1 Beam exit surfaces normal
[0164] O Optical axis of the birefringent medium
Claims
Claims 1 . Device (1) for coupling a laser beam (20) of a laser (2) into a double-clad fiber (3), comprising a first birefringent optical element (10), in particular an optical wedge or a plane-parallel plate, which is configured to split the laser beam (20) incident on the beam entrance surface (100) into two partial laser beams (200, 202), wherein the two partial laser beams (200, 202) have first exit angles and / or first beam offsets to the beam exit surface normal (N102), wherein the two partial laser beams (200, 202) are polarized along the base polarization components of the first birefringent optical element (10), comprising a polarization rotation device (14) configured to adjust the polarization of the incident partial laser beams (200, 202) and thus to generate polarization-adjusted partial laser beams (200', 202'), with a second birefringent optical element (12),in particular an optical wedge or a plane-parallel plate, wherein the beam exit surface (122) of the second birefringent optical element (12) is first traversed by the polarization-adjusted partial laser beams (200', 202'), and wherein the first exit angles and / or the first beam offsets of the partial laser beams (200, 202) from the first birefringent optical element (10) are the second incidence angles and / or the second beam offsets of the polarization-adjusted partial laser beams (200', 202') relative to the beam exit surface normal (N122) of the second birefringent optical element (12), wherein the second birefringent optical element (12) is configured to split the polarization-adjusted partial laser beams (200, 202') into two partial laser beams (2000, 2002, 2020, 2022) split, whereby the two partial laser beams (2000, 2002, 2020,2022) have second exit angles and / or second beam offsets to the beam entrance surface normal (N120) of the second optical wedge (12), wherein the two partial laser beams (2000, 2002, 2020, 2022) are polarized along the base polarization components of the second birefringent optical element (12), and wherein the second exit angles and / or the second beam offsets of the partial laser beams (2000, 2002, 2020, 2022), the polarization of which corresponds to that of the original, partial laser beams (200, 202), the respective first exit angles and / or the first Compensate beam offsets, and with coupling optics (16) which are designed to couple the partial laser beams (2000, 2020) with compensated first exit angle and / or first beam offsets into the inner core (30) of the double-clad fiber (3) and to couple the other partial laser beams (2002, 2022) into the annular core (34) of the double-clad fiber (3).
2. Device (1) according to claim 1, characterized in that the laser beam (20) is polarized or unpolarized.
3. Device (1) according to claim 1 or 2, characterized in that the first birefringent optical element (10) and / or the second birefringent optical element (12) comprises quartz glass or is formed from quartz glass.
4. Device (1) according to one of the preceding claims, characterized in that the base thickness (B) of at least one birefringent optical element (10, 12) is between 1 mm and 50 mm, preferably between 1 mm and 10 mm.
5. Device (1) according to one of the preceding claims, characterized in that the first birefringent optical element (10) and the second birefringent optical element (12) are identical.
6. Device (1) according to one of the preceding claims, characterized in that the polarization rotating device (14) is electronically controllable.
7. Device (1) according to one of the preceding claims, characterized in that the coupling optics (14) comprises a lens and / or a lens system and / or a mirror arrangement.
8. Device (1) according to one of the preceding claims, characterized in that the double clad fiber (3) comprises an intermediate cladding (32).
9. Method for coupling a laser beam (20) of a laser (2) into a double-clad fiber (3), wherein the laser beam (20) incident on the beam entry surface (100) of a first birefringent optical element (10) is split into two partial laser beams (200, 202), wherein the two partial laser beams (200, 202) have first exit angles and / or first beam offsets to the beam exit surface normal (N102), wherein the two Partial laser beams (200, 202) are polarized along the base polarization components of the first birefringent optical element (10), wherein the polarization of the incident partial laser beams (200, 202) is adjusted with a polarization rotation device (14), and thus polarization-adjusted partial laser beams (200', 202') are provided, wherein the polarization-adjusted partial laser beams (200', 202') first pass through the beam exit surface (122) of a second birefringent optical element (12), wherein the first exit angles and / or the first beam offsets of the partial laser beams (200', 202') from the first birefringent optical element (10) determine the second incident angles and / or second beam offsets of the polarization-adjusted partial laser beams (200', 202') relative to the beam exit surface normal (N122) on the second birefringent optical element (12), wherein the polarization-adjusted partial laser beams (200,202') are split by the second birefringent optical element (12) into two partial laser beams (2000, 2002, 2020, 2022) each, wherein the two partial laser beams (2000, 2002, 2020, 2022) each have second exit angles and / or second beam offsets to the beam entrance surface normal (N120) of the second optical wedge (12), wherein the two partial laser beams (2000, 2002, 2020, 2022) each are polarized along the base polarization components of the second optical wedge (12), and wherein the second exit angles and / or the second beam offsets of the partial laser beams (2000, 2002, 2020, 2022), the polarization of which corresponds to that of the original partial laser beams (200, 202), which compensate the first exit angles, and the partial laser beams (2000,2020) are coupled into the inner core (30) of the double-clad fiber (3) with a compensated first exit angle and / or first beam offsets using a coupling optics (16), and the other partial laser beams (2002, 2022) are coupled into the annular core (34) of the double-clad fiber (3) using the coupling optics (16). Method according to claim 9, characterized in that the splitting ratio with which the laser beam (20) is coupled into the inner core (30) and into the annular core (34) of the double-clad fiber (3) is adjusted using the polarization rotator (14).
11. Method according to claim 10, characterized in that the splitting ratio is determined from the ratio of the powers of those partial laser beams (2000, 2020) whose polarization corresponds to that of the original partial laser beams (200, 202) and the powers of the other partial laser beams (2002, 2022).
12. System for processing a workpiece (5) with the laser beam (20) of a laser (2), comprising a laser (2), a double-clad fiber (3), a device (1) for coupling the laser beam (20) of the laser (2) into the double-clad fiber (3) according to one of claims 1 to 8, a processing optics (4) and a workpiece (5), wherein the device (1) for coupling is configured to couple the laser beam (20) of the laser (2) with a splitting ratio into the inner core (30) of the double-clad fiber (3) and into the annular core (34) of the double-clad fiber (3), wherein the double-clad fiber (3) is configured to guide the laser beam (20) from the input of the double-clad fiber (3) to the output of the double-clad fiber, wherein the processing optics (4) is configured to (20) to form a processing laser beam (22) after the exit of the double-clad fiber (3),to focus the processing laser beam (22) and to apply the processing laser beam (22) to the workpiece (5), and thereby to process the workpiece (5).
13. System according to claim 12, characterized in that the beam quality of the processing laser beam (22) after the output of the double-clad fiber (3) is adjusted with the splitting ratio.