DE102022110078A1 - Device and method for modifying the beam profile of a laser beam - Google Patents

Abstract

Provided is a device for selectively changing the beam profile of a laser beam, the device comprising: at least one laser beam source, which comprises a fiber laser, for generating a laser input beam; A collimation unit for collimating the laser input beam; A coupling unit which has at least one optical element which is displaceably arranged in the beam path of the laser input beam; and A multiclad fiber comprising a core region and at least one ring region surrounding the core region; The coupling unit is designed to couple the laser input beam proportionally into the core region and/or the at least one ring region of the multiple clad fiber in a predeterminable ratio, so that a laser output beam with a selectively selectable beam profile can be provided at an exit end of the multiple clad fiber. Furthermore, a method for selectively changing the beam profile of a laser beam that can be carried out using the device is provided.

Description

Field of invention

The present invention relates to the field of laser processing. In particular, the invention relates to a device and a method for selectively changing the beam profile of a laser beam.

State of the art

Techniques for selectively changing the beam quality of a laser beam by selectively coupling a laser input beam into a multiple clad fiber with a central core region and at least one ring region surrounding the core region are generally known and, for example, in WO 2011/124671 A1 described.

Solid-state lasers, which are generally suitable for using this technology, can be divided into different groups, depending on the type of laser-active medium used. A particularly efficient group of solid-state lasers are fiber lasers, in which a doped optical fiber is used as the laser-active medium. Compared to rod or disk lasers, fiber lasers are characterized, among other things, by the fact that the laser radiation generated is guided in an optical fiber from the outset, which means that comparatively complicated and bulky guidance of the laser beam in the free beam can be avoided.

In order to maintain the slim design of the fiber laser even during the transition between two fibers, the fibers are connected or welded together using so-called “splicing”. In order to provide a laser beam with a variable beam profile by using a multiple clad fiber, the fibers of several fiber lasers are spliced together with the entrance end of a multiple clad fiber in such a way that at least one fiber is connected to the core region and at least one of the fibers is connected to the ring region of the multiple clad fiber, as for example in the US 9,620,925 B2 described. By selectively switching individual input lasers on and off, the beam profile of the laser output beam combined in the multiple clad fiber can be changed.

There are also areas of application in which fiber lasers are converted into a free beam and then coupled into a multiple clad fiber. For example, in the WO 2016/201278 A1 describes a technique for laser drilling in which a laser beam diverging from an input fiber (feed fiber) is focused by means of a lens into the annular cladding part of a transport fiber in order to obtain an annular beam profile for laser processing.

The present invention is based on the object of improving the prior art. In particular, the flexibility in setting the beam profile of a laser beam based on fiber lasers should be improved by utilizing the full power spectrum available.

The invention

To achieve the object underlying the invention, a device for selectively changing the beam profile of a laser beam is provided. The device includes at least one laser beam source, which includes a fiber laser, for generating a laser input beam. A fiber laser includes a doped optical fiber as a laser-active medium. The laser beam source can comprise a total of several laser modules with respective fiber lasers. If the laser beam source comprises several laser modules, it can further comprise a laser combiner in which the fibers of several laser modules are brought together into a common fiber, preferably by means of splicing, and combined to form a common laser input beam. For example, a laser module can provide a laser power of up to 500 W or up to 1 kW or more.

The device further comprises a collimation unit for collimating the laser input beam. In other words, the laser input beam is converted from a fiber (input fiber) into a free beam and collimated - i.e. parallelized - by means of the collimation unit, which can include, for example, a collimation lens or a collimation mirror.

The device further comprises a coupling unit which has at least one optical element which is displaceably arranged in the beam path of the laser input beam, as well as a multiple clad fiber comprising a core region and at least one ring region surrounding the core region. The coupling unit is designed to couple the laser input beam proportionally into the core region and/or the at least one ring region of the multiple clad fiber in a predeterminable ratio, so that a laser output beam with a selectively selectable beam profile can be provided at an exit end of the multiple clad fiber. For example, a part of the laser input beam can be coupled as a first partial beam into the core region of the multiple clad fiber and a second part of the laser input beam can be coupled as a second partial beam into a ring region of the multiple clad fiber. Alternatively, the Laser input beam can be completely coupled either into the fiber core or into the at least one fiber ring. Coupling into the multiple clad fiber can be understood as focusing the laser input beam or the respective partial beam onto an input-side fiber end of the multiple clad fiber in the corresponding area (core area or ring area).

It is understood that the components described above are preferably arranged in the order in which they are mentioned in the beam propagation direction of the laser input beam.

The multiple clad fiber can in particular be designed as a double clad fiber, also referred to as a 2-in-1 fiber, with a fiber core and a fiber ring surrounding the fiber core. However, the multiple clad fiber can also comprise more than one ring area, with all ring areas preferably being arranged concentrically around the core area. The multiple clad fiber can also preferably be designed as a transport fiber that transports the laser beam from the laser beam source to a laser processing station, for example a laser cutting system or a laser welding system. The multiple clad fiber can have a length of at least 5 m to, for example, 100 m.

The collimation of the laser input beam makes the beam easier to handle, particularly with regard to the selective change in the beam profile. The proposed component arrangement in particular allows the laser input beam to be freely distributed to the core area and/or the ring area of the multiple clad fiber using the full available laser power of all fiber lasers used. This increases flexibility when using the device. The device can be used, for example, in a system for laser processing of metal, in particular plate-shaped or tubular workpieces. Typical applications are laser cutting or laser welding. However, other processing techniques that rely on the use of a laser beam are just as conceivable.

By using fiber lasers as a beam source, the device has a more compact and simpler design compared to disk lasers with the same power and can be produced more cost-effectively overall.

The coupling unit can comprise a beam splitter which is displaceably arranged in the beam path of the laser input beam in a direction transverse to the beam propagation direction, the laser input beam being at least partially deflectable relative to the laser input beam depending on the position of the beam splitter in the beam path, and wherein the coupling unit can further comprise a focusing lens , which is designed to couple a non-deflected part of the laser input beam into the core region and a deflected part of the laser input beam into the at least one ring region of the multiple clad fiber. Preferably, the beam splitter and the focusing lens can be arranged at a fixed, predetermined distance from one another in the beam path of the laser input beam. This simplifies the structure of the device and the ease of use. The proposed combination of beam splitter and focusing lens can ensure that the partial beams (deflected and non-deflected part) of the laser output beam are fed completely into the core area and/or the ring area of the multiple clad fiber, even during a change in the position of the beam splitter in the beam path. The position of the coupling of the partial beams in the core area or in the ring area at the input end of the multiple clad fiber can be predetermined by a fixed arrangement of the beam splitter and the focusing lens relative to one another. Even during the change in the beam profile, no laser radiation reaches an intermediate cladding, which is preferably arranged between the core area and the ring area of the multiple clad fiber, so to speak as a partition.

The beam splitter can preferably have a substantially disk-shaped optical element with an entrance surface and an exit surface opposite the entrance surface. The optical element can have a parallel-walled, first section and a wedge-shaped, second section. The beam splitter can also be referred to as a wedge switch. The entrance surface can preferably be flat and arranged perpendicular to the laser propagation direction. In a first position of the beam splitter in the beam path of the laser input beam, the laser input beam only passes through the first, parallel-walled section of the optical element, so that the laser input beam is not deflected. In a second position of the beam splitter, the laser input beam only passes through the second, wedge-shaped or beveled section of the optical element, so that the entire laser input beam is deflected. In an intermediate position of the beam splitter, the laser input beam partially passes through the first section and partially through the second section of the optical element, so that the direction of a first part of the laser input beam remains unchanged and a second part of the laser input beam is deflected. Depending on the positioning of the optical element in the beam path of the laser input beam, the laser input beam can be divided into a non-deflected and a deflected part can be freely selected and adjusted. Analogously, the power in the core area and/or the at least one ring area of the multiple clad fiber can be freely adjusted. Compared to an optical, in particular wedge-shaped, element that is only partially inserted into the laser input beam, the proposed optical element with the additional parallel-walled section has greater robustness, especially with large laser powers in the kW range, and enables a largely loss-free division of the laser input beam.

The beam splitter can preferably be pivotable about a pivot axis, the pivot axis being arranged parallel to the beam path of the laser input beam. A lateral or other movement of the beam splitter in the beam path of the laser input beam is also possible. Compared to other types of movement, rotating the beam splitter to change the position of the optical element in the beam path is particularly efficient and quick. A comparatively small rotational movement causes a particularly rapid movement of the optical element on a circular path section. The switching times between two positions of the optical element can be optimized in this way. Furthermore, rotational movement can occur directly from the rotational movement of a drive motor. A translation of the direction of movement is not necessary, which simplifies the drive structure.

The transition between the first section and the second section of the optical element can be designed in the form of a linear edge on the exit surface of the optical element. The line along which the edge extends can preferably intersect with the pivot axis of the beam splitter. This can simplify calibration of the device.

The optical element can be attached to a pivot arm, which is designed, in cooperation with a pivot drive, to pivot the optical element around the pivot axis.

As already indicated above, the core region and the at least one ring region of the multiple clad fiber surrounding the core region can be separated from one another by an intermediate cladding. Preferably, the intermediate cladding can have a thickness of at most 15 μm, preferably at most 12 μm, even more preferably at most 10 μm. The intermediate cladding acts as a partition between the light-conducting core and ring areas of the multiple clad fiber. The intermediate cladding should have the smallest possible thickness in order to minimize an intensity gap between the core and ring beam(s) of the laser output beam resulting at the output end of the multiple clad fiber. The core area of the multiple clad fiber can, for example, have a diameter between 50 µm and 200 µm. The outer diameter of the ring area immediately surrounding the core area can be, for example, between 200 μm and 1000 μm. Double-clad fibers (i.e. fibers with a ring area surrounding the core area) with a ratio of core diameter to ring diameter of, for example, 50 µm/200 µm, 100 µm/400 µm, 100 µm/600 µm, 200 µm/700 µm or of 300 µm/1000 µm. By increasing the difference between the diameter of the core area and the outer diameter of the ring area, the difference in beam quality between the core beam and the ring beam can be increased, and thus the spectrum can be increased in terms of adjusting the beam quality of the laser output beam.

The device according to the invention can further comprise at least one further coupling device and at least one further multiple clad fiber, as well as at least one deflection unit which is arranged in the beam propagation direction between the collimation unit and each of the coupling devices in the beam path of the laser input beam and which is designed to selectively direct the collimated laser input beam to one of the Coupling devices for coupling into one of the multiple clad fibers. The deflection unit can be designed as a mirror, which can either be pivoted into the beam path of the laser input beam or moved into it in some other way. The mirror can also be partially transparent in order to direct a first part, for example half, of the laser input radiation in the direction of a first multiple clad fiber with an upstream first coupling device and at the same time to allow a second part of the laser input radiation to pass through for coupling into a second multiple clad fiber. According to this scheme, the device can basically be expanded to include any number of laser outputs, whereby not every laser output necessarily has to be connected to a multiple clad fiber, but at least one laser output can also be connected to a single core fiber or single core fiber, with the coupling device in such a conventional laser output for example, can be replaced by a fixed focusing lens. A device with two laser outputs or, for example, with four laser outputs can be preferred, which can either be controlled individually with full laser power or together with at least one further output and divided laser power. For example, Merfachclad fibers can be connected to two laser outputs with different diameters in the core or ring area. In this way, different multiple clad fibers can be used with the same laser, which can further increase the range of available and variably selectable beam parameters. Such variability in beam splitting is currently not possible with a laser guided exclusively in fibers. Compared to a “fiber-only design,” the present device also has the advantage that optical elements in the beam path of the laser input beam can be flexibly exchanged. If the device is damaged or improved, the technical and financial effort can be limited. For example, sensors for monitoring the laser input beam, for example for detecting scattered light, back reflections or Raman beams, can be integrated comparatively easily into the device. Furthermore, the coupling divergence of the laser input beam into the multiple clad fiber is variable and adjustable.

The collimation unit and the at least one coupling unit of the device can, together with the at least one optional deflection unit, preferably be arranged on a common platform. The light guide cable from the laser beam source can be connected to the collimation unit on the input side of this platform. On the output side, a multiple clad fiber can be connected to each coupling unit.

In the beam path immediately after the collimation unit, and preferably also on the platform, a modification unit, for example in the form of a partially transparent deflection mirror, can be arranged, which is designed to filter out portions of the laser input beam with a Raman wavelength that is harmful to the further guidance of the laser input beam. In this way, among other things, subsequent elements for guiding the laser beam can be protected from damage, in particular from overheating.

The laser input beam may preferably have a wavelength of 1075 nm. The optical elements for guiding the laser input beam can preferably have a coating that is functionally adapted to the wavelength of the laser input beam. In particular, the optical element of the coupling unit can have a coating on its entrance surface and/or on its exit surface, which ensures the most complete and loss-free transmission of the laser input beam.

To achieve the object underlying the invention, a method for selectively changing the beam profile of a laser beam is also provided. In a first step, the method includes providing a laser input beam in a laser input fiber. In a second step, the method includes converting the laser input beam into a free beam at an exit end of the laser input fiber. In a third step, the method includes collimating the laser input beam and, in a fourth step, selectively deflecting at least a portion of the laser input beam. In a fifth step, the method includes coupling an undeflected part of the laser input beam into a core region of a multiple clad fiber and/or coupling a deflected part of the laser input beam into a ring region of the multiple clad fiber surrounding the core region. The coupling preferably includes focusing the corresponding partial beam onto the entrance end of the multi-clad fiber in the corresponding area (core area and/or ring area).

By using the specified method, a laser output beam with a selectively selectable beam profile can be provided at an exit end of the multiple clad fiber.

The method can be carried out using a device according to one of the variants described above.

Optional features of the method result from the variants of the device according to the invention described above and vice versa.

According to a preferred variant, the input laser radiation is provided with a laser power of at least 2 kW, preferably at least 4 kW, even more preferably at least 6 kW.

The described method for selectively changing the beam quality of a laser beam can be used, for example, in a process for laser cutting or laser welding of preferably metallic, in particular tubular or plate-shaped workpieces. For this purpose, the output end of the multiple clad fiber can be connected to a laser processing head of a laser processing system, so that the laser output beam with the desired beam profile is directed onto the workpiece surface through focusing optics in the laser processing head, preferably with the supply of a process gas jet.

By using the method according to the invention, it is possible to change the beam profile of the machining process during a machining process (for example laser cutting or laser welding). of the laser beam (laser output beam) without interrupting the process. In addition, the full laser power (nominal power) is available for any power distribution that can be selected over the cross section of the laser beam (in particular the power distribution between the core area and the ring area).

Examples of embodiments

The following description of preferred exemplary embodiments serves to explain the invention in more detail in conjunction with the drawings.

• 1 Eine Variante einer erfindungsgemäßen Vorrichtung zur selektiven Veränderung des Strahlprofils eines Laserstrahls.
• 2a-2c Verschiedene Stellungen einer Kopplungseinrichtung zur selektiven Einkopplung eines Lasereingangsstrahls in einen Kernbereich und/oder einen Ringbereich einer Doppelclad-Faser; und
• 3a-3c Verschiedene mögliche Strahlprofile eines Laserausgangsstrahls am ausgangsseitigen Ende der Doppelclad-Faser gemäß 1.

Show it:

• 1 A variant of a device according to the invention for selectively changing the beam profile of a laser beam.
• 2a-2c Different positions of a coupling device for selectively coupling a laser input beam into a core region and/or a ring region of a double-clad fiber; and
• 3a-3c Various possible beam profiles of a laser output beam at the output end of the double-clad fiber according to 1 .

For the sake of simplicity, features with the same or similar structure or function are given the same reference numerals in the figures.

1 shows a device 10 according to the invention for selectively changing the beam profile of a laser beam b. First, the laser beam b is provided as a laser input beam via an input fiber 11. The laser input beam is connected to the device 10 by a laser beam source (not shown here), which comprises at least one fiber laser, and by means of the input fiber 11. The device 10 comprises a collimation element - here in the form of a collimation lens 12. When passing through the collimation element 12, the laser beam b is collimated, i.e. (approximately) parallelized, so that it can be guided as a free beam. The laser beam b can be deflected via a modification unit - here in the form of a partially transparent deflection mirror 13 -, the modification unit 13 advantageously being designed to pass through and decouple parts of the laser beam b with a Raman wavelength that is harmful to the further guidance of the laser beam b.

The exemplary device 10 points further in the beam propagation direction (marked by corresponding arrows). 1 a deflection unit 14a. The deflection unit 14a is designed here in the form of a partially transparent mirror and deflects part of the laser beam b in the direction of a first laser output with a first double-clad fiber 16a and an upstream coupling unit 15a. A remaining part of the laser beam b transmits through the deflection unit 14a and is guided via an (optional) second deflection unit 14b - here in the form of a mirror - in the direction of a second laser output with a second double-clad fiber 16b and an upstream second coupling unit 15b. The deflection unit can optionally be moved into the beam path and removed from it. In this way, according to the configuration shown, either both laser outputs shown can be used with divided laser power (see illustration in accordance with. 1 ) or - when the deflection unit 14a is removed from the beam path - only the lower laser output can be used with full laser power. According to an alternative variant, the deflection element 14a can also be designed as a non-transparent mirror. In this case, either the upper or lower laser output can be used, each with full laser power available. You can switch back and forth between the respective modes by moving the deflection unit 14a.

The double-clad fiber 16b typically has a core region 162 or inner fiber core (e.g. made of undoped quartz glass) with a refractive index n1 and a thin first cladding surrounding the core region (e.g. made of doped quartz glass) whose refractive index n2 is lower than n1. This is followed by a ring region 164 or an outer ring core surrounding the inner fiber core (e.g. made of undoped quartz glass) with the refractive index n3, which is also surrounded by a low-refractive second cladding (e.g. made of doped quartz glass) with refractive index n4. The refractive indices n1 and n3 can be the same or different; the same applies to the refractive indices n2 and n4. This can be followed by another layer of glass (not shown), which determines the outer diameter of the fiber, but has no influence on its function in terms of beam guidance. The final finish is typically a coating made of a plastic material, such as silicone and/or nylon, to protect the fiber.

Via the coupling unit 15a, 15b, the laser beam b is selectively deflected by moving an optical element - here a beam splitter 152 - and into the core area 162 and / or the ring area 164 of the double-clad fiber 16a, 16b at an input end 161 of the double-clad fiber. Fiber 16a, 16b coupled in, so that at an output end 165 of the double-clad fiber 16a, 16b the laser beam b as a laser output beam with a selectively selectable beam profile (ie with a wide freely selectable intensity distribution in the core and/or ring area of the laser beam) can be provided. The elements of the device 10, starting with the collimation unit 12 up to and including the coupling unit 15a, 15b, can preferably be provided on a common platform, shown here by a dashed frame. The platform has at least one input-side connection (or laser input) for at least one input fiber 11, and at least one output-side connection (or laser output) for at least one multi-clad fiber 16.

In the 2a until 2c The functionality of a coupling unit 15 is shown schematically, which is suitable for use in a device 10 according to the invention. The coupling unit 15 according to the example shown comprises a beam splitter 152 and a focusing lens 154 which are arranged successively in the beam propagation direction in the beam path of the laser beam b. On the left side of the 2a until 2c a beam splitter 152 is shown in a view perpendicular to the direction of beam propagation. On the right side of the respective illustration, a coupling device 15 is shown in a plane parallel to the direction of beam propagation.

The beam splitter 152 is here in the form of a disk-shaped, (transparent) optical element 1520, which has an entrance surface and an exit surface opposite the entrance surface. The optical element 1520 has a parallel-walled, first section 1521 and a wedge-shaped, second section 1522, in which the exit surface has a slight inclination compared to the exit surface in the first section 1521. The beam splitter 152 also has a pivot arm 1523, to which the optical element 1520 is attached, and which is rotatably mounted at one end about a pivot axis 152x of the beam splitter 152. The sections 1521 and 1522 of the optical element 1520 can form a line-shaped transition on the surface of the optical element 1520 (here at the exit surface). The line along which the transition extends can preferably intersect the pivot axis 152x. The pivot axis 152x runs parallel to the beam propagation direction of the laser beam b. The beam splitter 152 also has a drive (not shown here), by means of which the optical element 1520 can be pivoted around the pivot axis 152x into various predeterminable positions within the beam path of the laser beam b. 2a shows a first pivoting position of the beam splitter 152 or the optical element 1520, in which the laser beam b is transmitted through the first, parallel-walled section 1521 of the optical element 1520 and is coupled into the core region 162 of the double-clad fiber by means of a focusing lens 154 of the coupling device 15 . In 2 B a rotational position of the beam splitter 152 is shown, in which part of the laser beam b traverses the optical element 1520 in the first section 1521 and part of the laser beam traverses the optical element 1520 in the second section 1522. Due to the wedge shape in the second section 1522 of the optical element 1520, the laser beam b transmitting in this section is deflected and coupled into the ring region 164 of the double-clad fiber 16 by means of the focusing lens 154. In a third rotational position of the beam splitter 152, the incident laser beam b transmits completely in the second section 1522 of the optical element 1520, so that the entire laser beam b is coupled into the ring region 164 of the double-clad fiber 16.

In the 3a until 3c Examples of beam profiles of a laser output beam are shown, which can be generated with a device 10 according to the invention. In the present case, the beam profile can be understood as a cross-sectional profile of the laser output beam. The beam profile 51 according to 3a This results from a constellation as shown in 2a , when the laser input beam is coupled only into the core area of a multi-clad fiber. A beam profile 52 according to 3b This results when the laser input beam is as shown in 2c completely deflected and coupled exclusively into a ring area of the multiple clad fiber surrounding the core area. 3c shows a beam profile 53, in which the laser output beam in question has beam components both in the core area of the beam and in the surrounding ring area. Such a beam profile 53 results when parts of a laser input beam are coupled into both the core area and the surrounding ring area of a multi-clad fiber. By means of a device according to the invention and/or by means of a method according to the invention, the power and intensity of a laser output beam can be flexibly adjusted in a core region and at least one annular region of the beam profile surrounding the core region.

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• WO 2011/124671 A1 [0002]
• US 9620925 B2 [0004]
• WO 2016/201278 A1 [0005]

Claims (10)

Device for selectively changing the beam profile of a laser beam, the device comprising: At least one laser beam source comprising a fiber laser for generating a laser input beam; A collimation unit for collimating the laser input beam; A coupling unit which has at least one optical element which is displaceably arranged in the beam path of the laser input beam; and A multi-clad fiber comprising a core region and at least one ring region surrounding the core region; The coupling unit is designed to couple the laser input beam proportionally into the core region and/or the at least one ring region of the multiple clad fiber in a predeterminable ratio, so that a laser output beam with a selectively selectable beam profile can be provided at an exit end of the multiple clad fiber. Device according to Claim 1 , wherein the coupling unit comprises a beam splitter which is displaceably arranged in the beam path of the laser input beam in a direction transverse to the beam propagation direction; wherein the laser input beam can be at least partially deflected depending on the position of the beam splitter in the beam path; and wherein the coupling unit further comprises a focusing lens which is designed to couple a non-deflected part of the laser input beam into the core region and a deflected part of the laser input beam into the at least one ring region of the multiple clad fiber. Device according to Claim 2 , wherein the beam splitter has a substantially disk-shaped optical element with an entrance surface and an exit surface opposite the entrance surface; wherein the optical element has a parallel-walled first section and a wedge-shaped second section. Device according to Claim 2 or 3 , wherein the beam splitter can be pivoted about a pivot axis, the pivot axis being arranged parallel to the beam path of the laser input beam. Device according to Claim 4 , wherein a transition between the first section and the second section of the optical element forms a dividing line that intersects with the pivot axis of the beam splitter. Device according to one of the preceding claims; The core region and the at least one ring region of the multiple clad fiber surrounding the core region are separated by an intermediate cladding, the intermediate cladding having a thickness of at most 15 μm, preferably at most 12 μm, more preferably at most 10 μm. Device according to one of the preceding claims, further comprising: At least one further coupling device and at least one further multiple clad fiber; as well as At least one deflection unit, which is arranged in the beam propagation direction between the collimation unit and each of the coupling devices in the beam path of the laser input beam and which is designed to selectively direct the collimated laser input beam to one of the coupling devices for coupling into one of the multiple clad fibers. Device according to one of the preceding claims, wherein the laser input beam has a wavelength of 1075 nm. Method for selectively changing the beam profile of a laser beam, the method comprising the following steps: providing a laser input beam in a laser input fiber; converting the laser input beam into a free beam at an exit end of the laser input fiber; collimating the laser input beam; Selectively deflecting at least a portion of the laser input beam; coupling an undeflected portion of the laser input beam into a core region of a multi-clad fiber; and or Coupling a deflected part of the laser input beam into a ring region of the multiple clad fiber surrounding the core region. Procedure according to Claim 9 , wherein the input laser radiation is provided with a laser power of at least 2 kW, preferably at least 4 kW, more preferably at least 6 kW.

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