Laser optics and method for laser cutting by means of annular intensity distribution, and correspondingly designed laser system

Abstract

The invention relates to a laser system (1), an optical device (7) for the laser system and a method for operating the laser system (1) for machining a workpiece (2). The optics (7) comprise a light guide (5) for simultaneously and coaxially transmitting a first laser beam (18) as a single-mode or quasi-single-mode laser beam (18) and a second laser beam (21) as a multi-mode laser beam (21). The optics (7) further comprise optical means (14) arranged in the beam path of the first laser beam (18) for applying a phase singularity and / or a polarization singularity on the first laser beam (18) for generating an annular intensity distribution (18) of the first laser beam (18) having a central intensity minimum (19) on the optical axis (13) of the optics (7).

Description

TECHNICAL FIELD

[0001] The present invention relates to the field of laser technology. The present invention relates to optics for a laser system, in particular for laser cutting. Furthermore, the present invention relates to such a laser system and a method for machining a workpiece, in particular cutting a workpiece, with such a laser system. BACKGROUND

[0002] Lasers offer a wide range of useful possibilities for non-contact material processing, such as material removal or cutting of workpieces. Material can be melted and / or evaporated by the action of a laser, which can be used, for example, to produce a cutting gap by directing a laser beam on a workpiece. The resulting cutting edge of the remaining material or the cutting behavior and / or properties of the cutting gap can be influenced by adjusting the laser system and the laser.

[0003] For example, WO 2019 / 150071 A1 describes a device for laser machining of a material. The device comprises a pressing mechanism for pressing a periodically structured surface and an optical fiber together. This enables a first optical mode and a second optical mode of laser radiation propagating in the optical fiber to be coupled together. The resulting laser is intended to enable a material processing in which the disadvantages or undesirable influences known from other laser-based material processing methods are reduced.

[0004] A similar approach is described in WO 2019 / 150070 A1. The latter describes a device for laser machining of a material, which comprises a laser, an optical fiber and a coupler. The optical fiber is embodied in such a way that laser radiation can propagate in the optical fiber in a first optical mode, a second optical mode and a third optical mode, wherein the mode order of the third optical mode is highest and the mode order of the first optical mode is lowest. The coupler is configured to convert laser radiation propagating in the first optical mode into laser radiation propagating in the second optical mode. Furthermore, the coupler is configured to convert laser radiation propagating in the second optical mode into laser radiation propagating in the third optical mode. This is also intended to enable laser-based material processing, which reduces known disadvantages and undesirable influences.

[0005] Despite various known approaches to optimize laser-based material processing, it has not been possible to date to reliably achieve optimal results, for example with regard to an exact cutting gap geometry according to a desired shaping and / or with regard to thermal effects due to a temperature gradient between a material exposed to the laser and its surroundings, etc.

[0006] It is an object of the present invention to enable improved cutting performance and cutting gap quality in laser cutting. SUMMARY

[0007] This object is achieved by the subject matter of the independent claims. Further possible embodiments of the application are specified in the dependent claims, the description and the drawings. The features, advantages and possible embodiments set forth in the description in connection with one of the subject matters of the independent claims are to be considered as features, advantages and possible embodiments of the respective subject matter of the other independent claims and of any possible combination of the subject matters of the independent claims, mutatis mutandis, in combination with one or more of the dependent claims, if necessary.

[0008] The optical device according to the application can particularly advantageously be used in a laser system for laser cutting a workpiece or a material. However, other use cases are also possible. An optical device in the sense of the present application can particularly be understood as an arrangement or combination of a plurality of components or parts for transmitting, guiding and influencing or shaping or conditioning light, in particular laser light or at least one laser beam. The optical device according to the application comprises a light guide for simultaneously coaxially transmitting, i.e. guiding or directing, a first laser beam as a single-mode laser beam or a quasi-single-mode laser beam and a second laser beam as a multi-mode laser beam. In other words, the light guide is designed to transmit a first component of the light coupled into the light guide at the input side corresponding to the first laser beam, for example, in a fundamental mode or at least close to a fundamental mode or to a large extent in a fundamental mode, and a second component of the light coupled into the light guide at the input side corresponding to the second laser beam in a plurality of different modes further away from the fundamental mode. In other words, the first laser beam can have a smaller diffraction coefficient M 2 i.e. a larger or higher beam quality, compared to the second laser beam. The light guide can particularly be or comprise an optical waveguide or an optical guide cable, i.e. an optical fiber or a glass fiber. The laser beams can be guided in the light guide, for example, in a fully or partially overlapping manner or spaced apart from one another. For example, the light guide can have a light guide region for both laser beams, into which the already different laser beams can be coupled at the input side. Likewise, the light guide can have, for example, a plurality of light guide regions. Thus, the first laser beam can then be guided, for example, only or predominantly in a first light guide region of the light guide, while the second laser beam can be guided, for example, only or predominantly in a further second light guide region or, for example, partially in the first light guide region and partially in the second light guide region. For example, the light guide regions can be directly adjacent to one another or spaced apart or separated by a "cladding". This is specified in more detail elsewhere. The thickness of the cladding provided, if necessary, in the radial direction can be selected, for example, depending on the embodiment or depending on the requirements of the individual case.

[0009] Furthermore, the optical device according to the application comprises an optical arrangement for imposing a phase singularity and / or a polarization singularity on the first laser beam, i.e. on the single-mode or quasi-single-mode component of the light guided in the light guide or leaving the light guide on the output side. Thus, the optical arrangement is configured or implemented to produce a ring-shaped intensity distribution of the first laser beam having a central intensity minimum on the optical axis of the optical device. In other words, the optical arrangement can thus, for example, convert an at least substantially Gaussian intensity distribution of the first laser beam into a ring-shaped intensity distribution. Various ways of achieving this effect exist and correspondingly various ways of implementing the optical arrangement exist, which are explained in more detail elsewhere. For example, the optical arrangement can be or comprise a spiral phase plate and / or a spatial light modulator, such as, for example, a liquid crystal display or a DMD (digital micro-mirror device) for imposing a phase singularity and / or an S-retarder for imposing a polarization singularity.

[0010] According to embodiments, the respective imposed phase or polarization singularity can also be checked in a metrological manner, for example by means of an interferometer or by means of a liquid crystal display in combination with a camera or by means of a rotating polarization filter or the like.

[0011] In the optical device according to the application, the optical arrangement is arranged in the intended light or beam path of the at least first laser beam. Along the longitudinal direction of this beam path, i.e. in or with respect to the intended beam propagation direction of the first laser beam in or through the optical device, the optical arrangement is arranged downstream of the light guide. Thus, the optical arrangement can be arranged directly at the output end of the light guide, for example by being plugged or fused thereon or the like, or can be arranged at a distance from the output end of the light guide, so that the first laser beam transmitted in the light guide can leave the light guide and then enter the optical arrangement or be irradiated through the optical arrangement. According to embodiments, the optical arrangement can be or comprise a single optically active element or component or an arrangement or combination of several components.

[0012] The optical arrangement can likewise be arranged in the beam path of the second laser beam, i.e. be irradiated by the second laser beam during operation. In practice, however, only the single-mode or quasi-single-mode component of the laser irradiating the optical arrangement is significantly influenced by the optical arrangement. Thus, the phase and / or polarization singularity is imposed on the first laser beam by the optical arrangement and a ring-shaped intensity distribution is thereby produced, while the second laser beam can be irradiated through the optical arrangement at least substantially unchanged or unaffected and thus can maintain its original intensity distribution, i.e. the intensity distribution imparted upstream of the optical arrangement, and thus can also exhibit this intensity distribution downstream of the optical arrangement.

[0013] The first laser beam can here be used for actually cutting or melting the respective material exposed to the laser beam. Thus, the first laser beam can be used to form the actual cutting gap. The first laser beam can be particularly effectively and purposefully optimized for this task, for example by suitably implementing or adapting the optical device for the specific case or use case, for example the material to be cut.

[0014] At the same time, the second laser beam can be available or used for other tasks or effects. For example, the second laser beam can also or only hit the respective workpiece or material outside the actual cutting gap or at least in the entry region of the cutting gap is substantially absorbed. This makes it possible, for example, to round or round off the otherwise sharp-edged geometry of the upper, i.e. entry-side, cutting edge of the cutting gap that occurs, for example, when using the first laser beam alone. Such rounding can be particularly advantageous for the flow and thus, for example, enable more efficient or effective removal of molten or evaporated material from the cutting gap. In other words, the second laser beam can be used to shape the upper region of the cutting gap as an entry funnel when viewed in cross section. In addition, by irradiating the region surrounding the actual cutting gap with the second laser beam, it is possible to flatten or adjust the temperature gradient in the respective workpiece or material. This can prevent or reduce the respective undesirable thermal effects or stresses, etc.

[0015] Due to the common coaxial guidance of the laser beams in the light guide, a reliable and consistent overlap or spatial relationship between the first and second laser beams is always easily achieved. However, unlike the first laser beam, since the second laser beam is a multimode laser beam, the optics can be able to achieve a different or selective influence of the two laser beams. This means that, despite the superposition or coaxial or concentric arrangement or guidance of the two laser beams, different individual intensity profiles of the two laser beams can ultimately be obtained. This enables a particularly simple and flexible adaptation to different requirements or use cases.

[0016] Furthermore, it has been shown that with the annular intensity profile of the first laser beam generated by the optical device, i.e. by applying phase and / or polarization singularities, which annular intensity profile or central intensity minimum does not only exist in the central focal point, but can be propagated, i.e. maintained, over more than one Rayleigh length in longitudinal direction, i.e. as seen in the light direction or beam propagation direction along the beam path. In particular, the annular shape can be propagated or maintained from the central focal point in positive and negative longitudinal or beam propagation direction, respectively, for at least one Rayleigh length, i.e. for a total of at least two Rayleigh lengths. Thus, depending on the use case, the annular intensity profile or central intensity minimum can be achieved or maintained over a length which can be within the thickness range of the workpiece or material to be cut. This is typically not the case for prior methods for generating or using an annular intensity profile of a cutting laser beam and can represent a significant advantage in practice. Thus, the present invention makes it possible to achieve a particularly uniform, i.e. constant, absorption intensity over the absorption front - within a narrower range than prior methods. This makes it possible to improve the burr-free properties of the resulting cutting gap or resulting cutting edge, for example when laser cutting metal sheets, in particular at high feed rates. Thus, the present invention enables a particularly high cutting gap quality and a particularly high feed rate, i.e. a particularly high cutting performance, at the same time.

[0017] In addition, the present invention provides the possibility to purposefully shape the cutting gap at depth or in the lower region, i.e. in the exit region. This lower region or exit region is the region of the cutting gap which is located on the side of the respective workpiece or material which faces away from the optics during operation. Thus, the present invention can provide a further degree of freedom for shaping the cutting gap by means of an appropriate parameterization or adaptation of the first laser beam by means of the correspondingly designed optical device. This means, for example, that the region of the cutting gap which is located below the rounding-off by the second laser beam, if necessary on the input side, can be purposefully adapted or shaped.

[0018] The optics can also have or comprise further components, such as, for example, collimation optics, i.e. partial optics for collimating the laser light exiting from the light guide; and / or focusing optics, i.e. partial optics for focusing the (if necessary collimated) laser light into a focal point or focal region or onto the workpiece or material to be cut or machined, respectively; and / or a protective glass on the output side or periphery and / or adjustment means for setting or adapting the optics, for example by moving at least one optical component along the optical axis, i.e. in the light or beam propagation of the laser beam and / or perpendicular to the optical axis; and / or a respective holder for fixing said components and / or a housing surrounding said components, etc.

[0019] In one possible embodiment of the present invention, the light guide is configured or designed to guide the diffraction index M 2 <2, in particular M 2first laser beam having a diffraction coefficient M < 1.7 2 >2 or M 2 second laser beam having a diffraction coefficient M > 1.7. In other words, the first laser beam can be guided in the light guide in at least almost diffraction-limited manner or almost at the diffraction limit, and the second laser beam can be guided in a non-diffraction-limited manner, i.e. further away from the diffraction limit. This can be achieved, for example, by suitable materials for the light guide regions, i.e. the core light guide regions, and the cladding regions surrounding them and / or the diameters of the light guide regions, if necessary depending on the wavelength used for the laser beams. For example, the first laser beam can be considered to be a M 2 pure single-mode laser beam having a diffraction coefficient M < 1.3 and M 2 quasi-single-mode laser beam having a diffraction coefficient M < 1.5. The different diffraction coefficients M 2 of the two laser beams proposed here and the corresponding beam qualities have proven useful in practice. For example, it can be reliably achieved even when both laser beams illuminate the optical device that only the first laser beam or at least predominantly the first laser beam is affected, i.e. its intensity distribution is changed, while the second laser beam can remain at least substantially unaffected.

[0020] In a further possible embodiment of the application, the light guide has an inner core or inner light guide region for transmitting the first laser beam and an outer core or outer light guide region for transmitting the second laser beam. The outer light guide region surrounds the inner light guide region in an annular manner, resulting in an intensity distribution of the second laser beam that surrounds the first laser beam in a corresponding annular manner. This annular intensity distribution of the second laser beam is here caused by the annular or tubular shape of the outer light guide region. On the other hand, the inner light guide region can for example be completely cylindrical in shape, i.e. take the form of a circular disc in cross section. The inner light guide region can in particular form the center of the light guide. In contrast, the outer light guide region can surround the center, i.e. the central longitudinal axis of the light guide, coaxially or concentrically. Since the second laser beam can remain at least substantially unaffected by the optical device or can propagate around the optical device, an inner and an outer ring of the intensity distribution of the two laser beams is created downstream of the optical device, the inner ring being formed by the first laser beam and the outer ring of the intensity distribution concentrically surrounding the inner ring being formed by the second laser beam.

[0021] For this purpose, the light guide can be embodied as, for example, a "2-in-1" fiber or an "n-in-1" fiber, with n ≥ 2, or as a "multi-clad" fiber or can comprise such a fiber. In this case, a cladding material or a cladding region can be arranged radially between the inner light guide region and the outer light guide region in a cross-sectional plane perpendicular to the light propagation direction or to the longitudinal direction of the light beam path or to the beam propagation direction of the laser beam. Likewise, the outer light guide region can be surrounded by such a cladding material or a cladding region. This embodiment prevents an undesired coupling of the two laser beams. This can prevent, for example, the center or a deeper region of the cutting gap from being influenced by the second laser beam. This in turn can enable a particularly simple adjustment or implementation of the desired influence on the respective workpiece or material in the respective application. For example, the intensity or power or power density of the center of the cutting gap can be adjusted or adapted only by appropriate adaptation of the first laser beam and thus in particular easily.

[0022] By means of the cladding region, i.e. the cladding surrounding the inner light guide region or first light guide region provided for guiding the first laser beam, an escape of photons of the first laser beam into the surrounding outer light guide region or second light guide region provided for guiding the second laser beam can be avoided or at least reduced compared to a structure without such a cladding between the light guide regions. This enables a correspondingly higher power density of the first laser beam and a better control or adjustability of the power in the outer ring formed mainly by the second laser beam over the entire intensity profile. This enables a particularly flexible and versatile application of the optical device or the respective laser system according to the application.

[0023] However, it is also possible to use a light guide structure with an inner light guide region or first light guide region provided for guiding the first laser beam and an outer light guide region or second light guide region surrounding the inner light guide region or first light guide region and provided for guiding the second laser beam, but without a cladding arranged between these light guide regions. Thus, the light guide regions can then be, for example, directly adjacent to one another. In this case, the outer light guide region or second light guide region does not necessarily have to be exposed to the laser individually on the input side. This can enable a simpler design of the respective laser system if necessary and represents, for example, a practical approach for laser cutting of components or materials in combination with hot forming.

[0024] In a further possible embodiment of the application, the optical device has at least one focusing device for focusing the laser beam into a focal point or a focal region which is located outside the optical device on the output side. This focusing device is arranged in the beam path downstream of the light guide when viewed in the longitudinal direction of the beam path or in the beam propagation direction of the laser beam. In particular, the focusing device can be embodied such that it is illuminated by both the first laser beam and the second laser beam. The focusing device can be, for example, or comprise a focusing lens. The focusing device can thus be a single focusing element or can be embodied as a multipart partial optical device of the optical device according to the application, i.e. as necessary comprising a plurality of optical elements. In particular, the focusing device can be configured or designed to produce a focused diameter of at most 600 pm, preferably at most 300 pm, for the first laser beam. The focusing device enables, for example, particularly effective and efficient laser cutting. The focused diameter presented here can be used to achieve a hydrodynamic adaptation and optimization of the cutting gap geometry.

[0025] In a possible further development of the application, the optical device, i.e. the element or component for producing the annular intensity distribution of the first laser beam, is arranged between at least a portion of the focusing device and the focal point or the focal region when viewed in the beam propagation direction of the first laser beam, i.e. along the beam path. The arrangement of the optical device presented here can enable a particularly simple structure of the optical device. For example, the focusing device provided or, as necessary, the collimation device or collimation lens arranged in particular close to the output end or end of the light guide. This makes it easy to avoid unnecessary large expansion of the laser beam. The optical device does not necessarily have to be arranged in the 2f plane of the focusing device, i.e. at a distance from this plane corresponding to twice the focal length. This also enables a particularly flexible arrangement of the optical device or an arrangement adapted to the respective installation space conditions in the respective application and thus a corresponding flexible structure of the optical device as a whole.

[0026] In a possible further development of the application, the optical device has a protective glass arranged in the beam path on the output side. This protective glass can thus be arranged, in particular when viewed in the beam propagation direction of the laser beam, i.e. in the longitudinal direction of the beam path, downstream of the focusing device, but upstream of the focus region which is located outside the optical device. Such a protective glass can for example prevent or minimize the ingress of dust or dirt into the optical device. Furthermore, it is provided here that the optical means for generating the ring-shaped intensity distribution of the first laser beam are arranged on or formed in the protective glass. In the former case, the protective glass can for example serve as a holder or support for the optical means. In the latter case, the optical means can for example be formed by suitably shaping or machining the protective glass, in particular its surface. In both cases, an additional holder for the optical means can for example be eliminated and a particularly robust and durable positioning of the optical means can be achieved.

[0027] In a further possible embodiment of the application, the optical means for generating the ring-shaped intensity distribution of the first laser beam are arranged at a longitudinal position along the beam path at which at least the first laser beam has its maximum diameter. This refers at least to the region of the optical device according to the application, i.e. for example from the light guide to the focus or focus region which is located outside the optical device on the output side. Since the power of the first laser beam is predetermined, the surface energy density of the first laser beam is lowest at the point with the maximum diameter. By arranging the optical means at this point along the beam path, each surface element of the optical means is thus exposed to as small an intensity or power of the first laser beam as possible. This enables the respective load on the optical means to be minimized, for example to avoid damage or to increase its service life. For example, the optical means can be arranged on the side of the focusing device facing the light guide, mentioned elsewhere, in particular between the focusing device and the collimation device mentioned elsewhere, when viewed along the beam path.

[0028] In another possible embodiment of the application, the optical device has an end element which is arranged directly at the output-side end of the light guide. This end element can thus be plugged or fused, etc. onto the output-side end of the light guide, for example. If the light guide is an optical fiber, for example, the end element can be an optical fiber end connector. Furthermore, it is provided here that the optical arrangement for generating the ring-shaped intensity distribution of the first laser beam is arranged or formed in this end element. Likewise, the end element can be the output-side end region of the light guide. In this case, the optical device can be formed directly on or in the output-side end, i.e. on the respective end face of the light guide, for example. By means of the embodiments of the application presented here, it can be easily achieved that the first laser beam and the second laser beam are already precisely and consistently arranged relative to one another or superimposed on one another at the point of entry into the optical arrangement. This can make it possible to dispense with a corresponding additional beam shaping or beam guidance, if necessary. In addition, it can be achieved that at least the first laser beam always, i.e. even in the event of vibrations, etc., illuminates the optical arrangement in the same way, for example, in the same region and in the same direction. This makes it possible to achieve a persistent consistency, i.e. constancy, of the intensity distribution, thereby achieving a particularly high consistency and reliability of the properties of the optical device or of the laser system equipped with the optical device. The end element can have a range of up to several centimeters in the longitudinal direction or in the beam propagation direction, for example.

[0029] In a possible further development of the application, the end element comprises a machined end region of the light guide. The optical arrangement is then formed by a height profile shaped on the output-side end face, through which the laser thus travels in the longitudinal direction or in the beam propagation direction during operation. Such a height profile can be etched onto or into the end face of the light guide, for example, or produced by selective material removal, for example by means of a laser lithography process, etc. By means of the further development of the application presented here, the optical device can be composed of particularly few components and can be particularly robust. For example, any relative movement between the light guide and the optical arrangement can be eliminated in this way. This makes it possible to achieve particularly stable and persistent consistent output-side beam properties. Depending on the size and embodiment of the light guide and the available manufacturing processes, the height profile forming the optical arrangement can be formed on the entire end face of the light guide or only in the region of the light guide region which transmits the first laser beam. The former variant can be particularly easy to manufacture, while the latter variant can completely avoid any influence of the optical arrangement on the second laser beam.

[0030] According to a further possible embodiment of the application, the optical arrangement is or comprises a spiral phase plate. Such a spiral phase plate can impose a phase singularity on the first laser beam. The ring-shaped intensity distribution generated by such a spiral phase plate can propagate, i.e. remain, over particularly long lengths or paths in the beam propagation direction or in the longitudinal direction of the beam path, in particular for a diffraction order M 2a laser beam of < 1.5. Thus, even if the workpiece material to be cut is particularly thick, an exposure to a corresponding uniform intensity can be achieved within the cutting gap. Likewise, a particularly large distance between the optical device and the workpiece or material to be cut or processed can be selected without negatively affecting the cutting or processing result. This can enable a corresponding simple and safe relative positioning between the optical device or the laser system and the workpiece or material, as well as a particularly efficient and effective removal of the molten or evaporated material towards the side. The use of a spiral phase plate for the optical arrangement or as the optical arrangement can enable a particularly easy adaptation to different requirements or use cases. For example, the number of 2π phase jumps, i.e. the "topological charge", and / or the number of height steps and / or the modulation depth of the spiral phase plate can be changed for this purpose. For example, for practical applications, the topological charge number m can be selected in the range of 1 to 50, in particular in the range of 1 to 5. In particular, the topological charge number can be selected or set to be smaller than the maximum angular mode order of the second laser beam. This ensures that the second laser beam is not affected or not significantly affected by the optical arrangement, i.e. in this case the spiral phase plate.

[0031] In a possible further development of the application, the spiral phase plate has at least 16 steps, i.e. height steps or different height levels, in particular at least 32 or at least 64 steps. This is based on the recognition that although fewer steps, for example 4 or 8 steps, etc., can be used for the spiral phase plate in principle, a higher number of steps enables a higher efficiency. For example, with at least 32 steps, an efficiency of at least 97% can be achieved.

[0032] In a further possible embodiment of the application, the optical arrangement is or comprises an S retarder (S-Verzögerungsplatte). It can in particular be or comprise a component or material having a position-dependent birefringence. This enables a polarization singularity to be imposed on the first laser beam, for example by shaping of a radial or angular polarization or a mixture thereof. For example, the S retarder can convert a linear polarization into a radial or angular polarization, or a circular polarization into an optical vortex, i.e. a ring-shaped intensity distribution. With such an S retarder, particularly advantageous propagation properties of the intensity distribution of the first laser beam can be achieved. In addition, the S retarder has no or only marginal or negligible influence on the second laser beam. Likewise, a segmented polarization converter or a geometric phase hologram (GPH) can be used as the optical arrangement or as part thereof.

[0033] The application also relates to a laser system, which can be configured in particular for laser cutting. The laser system according to the application has at least one laser source, an optical device according to the application and a coupling device for coupling the various laser components or laser beams into the light guide of the optical device. The laser system according to the application can in particular be a laser system mentioned in connection with the optical device according to the application or a laser system corresponding thereto. The laser system according to the application can also have further components. For example, the laser system can comprise a power distribution device for adjusting the power distribution or power ratio between the first laser beam and the second laser beam. This can be configured in particular so that the power of the first laser beam and / or the second laser beam or the power proportion thereof can be adjusted steplessly in the range from 0% to 100% of the respective available power. Likewise, the laser system can have, for example, a control device, in particular a closed control loop, with which the power of the first laser beam and / or the second laser beam can be controlled automatically, for example, depending on the feed rate of the optical device or the focal region relative to the workpiece or material to be machined. Likewise, the laser system can have, for example, a cutting or process gas system for generating a cutting or process gas flow and directing the cutting or process gas flow coaxially with the laser beam, the flow direction of the cutting or process gas flow corresponding to the propagation direction of the laser beam, i.e. the light propagation direction. Such a process gas system can comprise, for example, a control unit for controlling and stabilizing the gas pressure of the cutting or process gas and / or a gas nozzle or the like. For example, the gas nozzle can surround the laser beam downstream of the optical device when viewed in the propagation direction. Likewise, the laser system can have a distance sensor for monitoring the distance between the respective workpiece or material to be machined and a predetermined reference point fixed in position relative to the laser system, for example a specific component or component point of the laser system such as a gas nozzle. This can be coupled, for example, to a control device or a controlled drive in order to guide or maintain the laser system or the respective reference point or the respective component, i.e. for example the gas nozzle or the optical device, at a specified distance from the respective workpiece or material. Likewise, the laser system can have a drive device or adjustment device for producing a relative movement between the laser system, in particular the optical device, and the respective workpiece or material to be cut or machined.

[0034] In one possible embodiment of the application, the laser system is configured to generate different wavelengths and / or different polarization states of the first and second laser beams. In other words, the first and second laser beams can thus have different wavelengths and / or polarization states during operation of the laser system. The use of different wavelengths can enable or adapt to maximum absorption conditions in view of the different primary beam incidence conditions of the two laser beams on the respective workpiece. For example, the first laser beam can primarily be incident in a grazing manner, for example at an angle between 70° and about 87°. On the other hand, the second laser beam can primarily be incident orthogonally, i.e. for example at an angle between 0° and 45°. In order to achieve maximum absorption at the primarily grazing beam incidence conditions of the first laser beam, for example wavelengths in the near infrared can be particularly advantageous. On the other hand, in order to achieve maximum absorption at the primarily orthogonal beam incidence conditions of the second laser beam, wavelengths in the visible range can be particularly advantageous. For example, wavelengths in the range of 0.8 pm to 2.1 pm can be used for the first laser beam. For the second laser beam, for example wavelengths in the range of 0.2 pm to 0.8 pm can be used.

[0035] Depending on the embodiment and use case, different polarization states, for example circular polarization states or angular polarization states or radial polarization states or randomly distributed polarization states or combinations thereof, can be used for the two laser beams. By selecting or using suitable polarization states, i.e. polarization states adapted to the respective conditions or to the respective use case, the absorption efficiency or the energy efficiency can be improved. Likewise, by adapting the polarization states, it is possible to purposefully exert an influence on the resulting temperature distribution of the absorption front. Thus, the appropriate adaptation or adjustability of the laser system provides for a corresponding flexibility or adaptation to different situations and use cases.

[0036] The application also relates to a method for operating a laser system according to the application for machining, in particular cutting, a workpiece or a material. In the method, laser light is generated by means of at least one laser source. Furthermore, a first component of the laser light is transmitted to the optical device as a single-mode laser beam or a quasi-single-mode laser beam, and a second component of the laser light is transmitted to the optical device as a multi-mode laser beam having a ring-shaped intensity distribution in the light guide. At least the single-mode laser beam or the quasi-single-mode laser beam illuminates the optical arrangement, so that a coaxial or concentric ring-in-ring intensity distribution is generated, wherein the ring-shaped intensity distribution of the single-mode laser beam or the quasi-single-mode laser beam generated by the optical arrangement is surrounded in ring-shaped fashion by the larger ring-shaped intensity distribution of the multi-mode laser beam which is coaxial to the ring-shaped intensity distribution. In the method, the two laser beams are focused onto the workpiece to be machined, in particular cut, and the workpiece is machined, in particular cut, by means of the ring-in-ring intensity distribution. In this case, the optical arrangement between the laser source and the workpiece-side focal point of the laser beams is thus at least illuminated by the first laser beam. For machining or cutting the workpiece, a relative movement can be generated between the focal point of the laser beams and the respective workpiece, i.e. the focal point of the laser beams can be actively guided on the workpiece, for example along a path specified in the respective use case.

[0037] Further features of the application are disclosed in the following description and in relation to the accompanying drawings. The features mentioned in the description above and in the claims and those disclosed in the accompanying drawings can both separately and in any combination thereof be material for realizing the application in various forms thereof. BRIEF DESCRIPTION OF DRAWINGS

[0038] Figure 1 a partial schematic illustration of a laser system for cutting a workpiece using a ring-in-ring laser intensity distribution is shown; and Figure 2 a cross-sectional view of a corresponding simulated ring-in-ring laser intensity distribution is shown. DETAILED DESCRIPTION

[0039] Figure 1 a partial schematic illustration showing laser cutting using a laser system 1 is shown. This enables a workpiece 2 to be cut or machined, which is indicated schematically by way of example. The laser system 1 comprises at least one beam or laser source 3. Downstream of this laser source 3 is a coupling device 4 for coupling a laser beam generated by the laser source 3 into a light guide 5 of the laser system 1, which is likewise indicated schematically here. The light guide 5 can be an optical fiber, for example. This can in particular be a 2-in-1 optical fiber having an inner light guide region and an outer light guide region which surrounds the inner light guide region.

[0040] The laser 6, which is indicated here schematically, can exit from the light guide 5 at the end of the output side of the light guide, i.e. at the end of the light guide opposite the coupling device 4, as viewed in the direction of light propagation. In order to be able to process the workpiece 2 efficiently, the laser system 1 here also comprises further optical elements or devices arranged downstream of the light guide 5. These can be referred to here together with the light guide 5 as optical means 7. In particular, the optical means 7 comprise a collimation device, which is indicated here in the form of a collimation lens 8. Furthermore, the optical means 7 comprise a focusing device, which is arranged downstream of the collimation lens 8 and is indicated here as a focusing lens 9. The laser 6 can first be collimated by means of the collimation lens 8. The collimated laser 6 can then be focused into a focus region 10 located outside the laser system 1 by means of the focusing lens 9. In this focus region 10, a maximum intensity or energy density of the laser 6 can thus be achieved, and a cutting gap 11 can be produced in the workpiece 2 thereby. The optical means 7 can be embodied as transmissive optical means, as indicated here, but can also be embodied completely or partially as reflective optical means.

[0041] For safety reasons and to protect components, the laser system 1 may, for example, have a protective glass 12 on the output side, which can be irradiated by the laser 6. Depending on the embodiment, the protective glass 12 may, for example, be arranged downstream of the optical means 7 or be part of the optical means 7.

[0042] Depending on the requirements or use case, the laser system 1 or its laser source 3 can be embodied differently, for example as a CO2 laser, or as a solid-state laser, such as a disc laser, a fiber laser, a diode laser, etc. For typical laser cutting applications, the laser system 1 or the laser source 3 may, for example, have a laser power, in particular a continuous-wave power, in the range of several kilowatts. This enables, for example, the cutting of 3D sheets having a thickness of several millimeters. However, other applications or powers are also possible.

[0043] In order to be able to produce not only point-like holes in the workpiece 2, but also longitudinally extending cutting gaps 11, an adjustment or displacement device, which is not explicitly shown here for the sake of clarity, can be used to produce a relative movement between the focus region 10 and the workpiece 2. This can be used, for example, to move the workpiece 2. In addition or alternatively, the laser system 1 or its optical means 7 can thereby be moved, tilted or adjusted.

[0044] In addition, the laser system 1 can comprise a cutting or process gas system (not explicitly shown here either for the sake of clarity). This enables a cutting or process gas flow to be directed onto the cutting gap 11. In particular, this cutting or process gas flow can be directed coaxially with respect to an optical axis 13 (schematically indicated here) of the optical device 7. This optical axis 13 here corresponds to the longitudinal direction of the beam path, i.e. the main light direction or beam propagation direction of the laser light 6, at least after leaving the light guide 5. To this end, for example, a gas nozzle can be arranged on the outside of the laser system 1, which gas nozzle can surround the laser light 6 exiting through the protective glass 12. The flow direction of the cutting or process gas flow can thus correspond to the main propagation direction of the laser light 6.

[0045] The laser system 1 here is configured to achieve or enable a particularly high quality of the cutting gap 11 while at the same time achieving particularly good cutting performance. To this end, the laser light 6 is generated or used in the form of a ring-in-ring at least in the focus region 10. Figure 2 A corresponding simulated ring-in-ring intensity distribution 17 in a cross-sectional plane perpendicular to the optical axis 13 is shown. For better understanding, the intensity levels are also shown here. The intensity distribution 17 here comprises an inner ring 18 with a central intensity minimum 19, which can in particular lie on the optical axis 13 or in the center of the cutting gap 11. The inner ring 18 is surrounded by an annular intensity minimum 20. On its outside, there is an outer ring 21. The inner ring 18 and the outer ring 21 are thus regions in which the laser light 6 propagates.

[0046] The component of the laser light 6 which ultimately forms the inner ring 18 can be guided in an inner light guide region of the light guide 5, while the component of the laser light 6 which ultimately forms the outer ring 21 can be guided in an outer light guide region of the light guide 5. The component of the laser light which forms the inner ring 18 (also referred to as first laser beam here) can be guided in a diffraction-limited manner or at least relatively close to or closer to the diffraction limit, for example as a single-mode laser beam or quasi-single-mode laser beam with a diffraction index M 2 <1.5. The first laser beam can thus be guided in the light guide 5 at least predominantly in a Gaussian or Gaussian-like fundamental mode. On the other hand, the component of the laser light 6 which forms the outer ring 21 (also referred to as second laser beam here) can be guided in the light guide 5 as a multi-mode laser beam. In this case, the outer light guide region of the light guide 5 can already have an annular shape, so that the outer ring 21 is easily directly generated. The second laser beam can be guided in the light guide 5 in a non-diffraction-limited manner, i.e. further away from the diffraction limit than the first laser beam, in particular with a diffraction index M 2 >1.5.

[0047] By means of the coupling device 4, the respective mode can be selectively excited or coupled. In particular, different power fractions can be variably provided for the inner ring 18 and the outer ring 21, i.e. for example different power fractions can be coupled into the inner light guide region and the outer light guide region of the light guide 5, respectively. Power fractions of 0% to 100% are possible. However, during regular operation of the laser system 1, a power fraction greater than 0% can be provided for both the inner ring 18 and the outer ring 21, i.e. for example power can be supplied to both the inner light guide region and the outer light guide region simultaneously (i.e. the laser light 6 generated by the laser source 3).

[0048] In order to shape the first laser beam into the inner ring 18, the laser system 1 has a singularity element 14. This enables a phase singularity or a polarization singularity to be imposed on the first laser beam. To this end, the singularity element 14 can be embodied for example as a spiral phase plate or an S- retarder. As Figure 1 As indicated, the singularity element 14 can be arranged or formed for example on the inner side of the protective glass 12. However, other arrangements are also possible, which are indicated here as a first alternative position 15 between the collimation lens 8 and the focusing lens 9 and a second alternative position 16 at or between the output-side end of the light guide 5 and the collimation lens 8.

[0049] The singularity element 14 thus enables the first laser beam to be shaped into the inner ring 18, while the second laser beam can remain at least substantially unaffected. This results in a ring-in-ring intensity distribution 17, which can be maintained for example over at least two Rayleigh lengths, i.e. for example over a few millimeters, in the focal region 10 in the direction along the optical axis 13. This intensity distribution 17 can thus be present in particular at least over a large part of the thickness or material thickness of the workpiece 2.

[0050] Depending on the use case, the properties or intensity distribution 17 of the laser system, in particular of the laser light 6, can be adapted or adjusted as required. For example, a fiber laser can be used at a wavelength of approximately 1070 nm with an average power of approximately 3 kW. For the optics 7, for example a collimation lens 8 with a focal length f = 200 mm and a focusing lens 9 with a focal length f = 300 mm can be used. By selecting a focus diameter that is suitable for the respective use case, a hydrodynamic adaptation and optimization of the final cutting gap geometry can be achieved. For example, a diameter ratio between the first laser beam and the second laser beam, i.e. a ratio of the diameter of the inner ring 18 to the diameter of the outer ring 21, can be set to at most 1:8, preferably to at most 1:4, in the focal region 10 for the first laser beam, i.e. for the inner ring 18. For example, a diameter of at most 600 pm, in particular of at most 150 pm, can be set in the focal region 10 for the first laser beam, i.e. for the inner ring 18.

[0051] As regards the power, for example, the power distribution or power ratio between the inner ring 18 and the outer ring 21 can be set to about 2: 1. However, likewise, by way of example, up to 95% of the total available laser power can be guided in the inner ring 18. The larger proportion of laser power in the inner ring 18 can enable the material of the workpiece 2 to be melted or evaporated particularly effectively and quickly. On the other hand, the smaller proportion of laser power in the outer ring 21 can be sufficient to produce a radius, i.e. a rounding, at the end of the cutting gap 11 facing the laser system 1. This enables the cutting or process gas to reach the cutting gap 11 particularly effectively and efficiently. This ultimately enables a particularly smooth, i.e. burr-free, cutting edge to be achieved, thus enabling a particularly high-quality cutting gap 11 to be achieved. This applies both to relatively low feed powers or feed rates and to relatively high feed powers or feed rates.

[0052] Likewise, suitable, in particular different, polarization states can be selected or set for the inner ring 18 and the outer ring 21. This can enable the absorption efficiency or energy efficiency to be optimized during laser cutting by means of the laser system 1, as well as influencing the absorption front or the temperature distribution of the workpiece 2 in the region of the cutting gap 11. In particular, when a spiral phase plate is used as the singularity element 14, the first laser beam, i.e. the inner ring 18, can be linearly polarized, for example, while the second laser beam, i.e. the outer ring 21, can be unpolarized. In particular, when an S-retarder is used, the first laser beam, i.e. the inner ring 18, can be radially or angularly polarized, for example, while the second laser beam, i.e. the outer ring 21, can be unpolarized.

[0053] Overall, the described examples show how a ring-in-ring beam profile for laser applications can be obtained and used to achieve an improved cutting or machining result.

[0054] List of reference signs 1 laser system 2 workpiece 3 laser source 4 coupling device 5 light guide 6 laser 7 optics 8 collimator lens 9 focusing lens 10 focal region 11 cutting gap 12 protective glass 13 optical axis 14 singularity element 15 first alternative position 16 second alternative position 17 intensity distribution 18 inner ring 19 central intensity minimum 20 annular intensity minimum 21 outer ring

Claims

1. An optical device (7) for a laser system (1) for laser cutting, wherein The optical device (7) has a light guide (5) for simultaneous coaxial transmission of a first laser beam (18) as a single-mode or quasi-single-mode laser beam (18) and a second laser beam (21) as a multimode laser beam (21), and an optical arrangement (14) arranged in the beam path of the first laser beam (18) and downstream of the light guide (5) viewed in the beam propagation direction of the first laser beam for imposing a phase singularity and / or a polarization singularity on the first laser beam (18) for producing a ring-shaped intensity distribution (18) of the first laser beam (18) having a central intensity minimum (19) on the optical axis (13) of the optical device (7).

2. The optical device (7) according to claim 1, characterized in that The light guide (5) is designed to guide the first laser beam (18) with a diffraction index M 2 < 2, in particular M 2 < 1.7, and to guide the second laser beam (21) with a diffraction index M 2 > 2.

3. The optical device (7) according to one of the preceding claims, characterized in that The light guide (5) has an inner light guide region for transmission of the first laser beam (18) and an outer light guide region which surrounds the inner light guide region ring- shaped for transmission of the second laser beam (21) so that an intensity distribution (21) of the second laser beam (21) is obtained which surrounds the first laser beam (18) ring- shaped.

4. The optical device (7) according to one of the preceding claims, characterized in that The optical device (7) has a focusing device (9) arranged in the beam path downstream of the light guide (5) viewed in the beam propagation direction, for focusing the laser beams into a focus region (10) which is located outside the optical device (7) on the output side, in particular the focusing device is designed to produce a focus diameter of at most 600 pm, preferably at most 300 pm, for the first laser beam (18).

5. The optical device (7) according to claim 4, characterized in that The optical arrangement (14) is arranged between the focusing device (9) and the focus region (10) viewed in the beam propagation direction.

6. The optical device (7) according to claim 5, characterized in that The optical device (7) has a protection glass (12) arranged in the beam path on the output side, and the optical arrangement (14) is arranged on the protection glass (12) or is formed in the protection glass (12).

7. The optical device (7) according to one of claims 1 to 4, characterized in that The optical arrangement (14) is arranged at a longitudinal position (15) along the beam path at which at least the first laser beam (18) has its maximum diameter.

8. The optical device (7) according to one of claims 1 to 4, characterized in that The optical device (7) has an end element arranged directly at an output-side end of the light guide (5), and the optical arrangement (14) is arranged in the end element.

9. Optical device (7) according to claim 8, characterized in that the end element comprises a machined end region of the light guide (5) and the optical arrangement (14) is formed by a height profile formed on the output-side end face of the light guide (5).

10. Optical device (7) according to one of the preceding claims, characterized in that the optical arrangement (14) comprises a spiral phase plate.

11. Optical device (7) according to claim 10, characterized in that the spiral phase plate has at least 16 steps, in particular at least 32 steps.

12. Optical device (7) according to one of the preceding claims, characterized in that the optical arrangement (14) comprises an S-retarder.

13. Laser system (1) having at least one laser source (3), an optical device (7) according to one of the preceding claims and coupling means (4) for coupling the various laser components (18, 21) into the light guide (5) of the optical device (7).

14. Laser system (1) according to claim 13, characterized in that the laser system is configured to generate different wavelengths and / or different polarization states of the first laser beam (18) and the second laser beam (21).

15. Method for operating a laser system (1) according to claim 13 or 14 for machining, in particular cutting, a workpiece (2), wherein - a laser light (6) is generated by means of a laser source (3), - in the light guide (5), a first component (18) of the laser light (6) is transmitted to the optical device (7) as a single-mode or quasi-single-mode laser beam (18) and a second component (21) of the laser light (6) is transmitted to the optical device (7) as a multi-mode laser beam (21) having a ring-shaped intensity distribution (21), - the optical arrangement (14) is at least irradiated with the single-mode or quasi-single-mode laser beam (18), so that a concentric ring-in-ring intensity distribution (17) is generated, wherein the ring-shaped intensity distribution (18) of the single-mode or quasi-single-mode laser beam (18) generated by the optical arrangement (14) is surrounded in ring-shaped manner by the larger ring-shaped intensity distribution (21) of the multi-mode laser beam (21), - the two laser beams (18, 21) are focused onto the workpiece (2) to be machined, in particular cut, and the workpiece (2) is machined, in particular cut, by means of the ring-in-ring intensity distribution (17).

Images