DE102023100646A1 - Device and method for generating a laser beam with a defined beam shape - Google Patents

Abstract

The present invention relates to a device for generating a high-power laser beam with a predetermined beam shape, comprising a beam shaping module (120, 220, 320) for generating a shaped laser beam (160, 260, 360) with a first defined beam shape from an input laser beam (150, 250, 350); and an optical amplifier module (130, 230, 330) for generating an amplified laser beam (170, 270, 370) with a second defined spatial beam shape from the shaped laser beam, wherein the optical amplifier module comprises an optically active element which increases the intensity of a free-space laser beam as it passes through.

Description

Technical area

The present invention relates to a device and a method for generating a laser beam with a defined beam shape, in particular for a laser beam of an ultrashort pulse laser (USP).

State of the art

In the field of laser technology, a spatial shaping of the beam profile generated is in many cases advantageous in order to achieve certain properties. This is particularly true for the use of laser beams for material processing. Processing methods include, for example, laser cutting as a contactless material separation process, the processing of surfaces or superficial material layers, the removal of materials, the processing of transparent materials or various variants of laser welding and similar joining processes. All of these processes benefit from a targeted influence on the beam shape in order to adapt the beam properties as precisely as possible to the materials and conditions used. At the same time, very high pulse energy peaks are often required in these applications, so short and ultra-short pulses (USP) with high power are used.

Beam shaping includes simple determination of the beam diameter and focus positions with defined power as well as the generation of complex beam profiles, such as ring-shaped beam cross-sections, tophat profiles with uniform energy density over a flat area, or the determination of defined energy distributions within the beam cross-section.

In order to generate a desired beam shape, beam shaping elements such as diffractive optical elements are used in the area of the output beam, with which the final beam profile can be determined. Variable beam shaping is of particular interest in order to be able to use different beam shapes within a process and to maintain a high level of productivity in the overall process. The switching speed of the beam shaping can certainly be a limiting factor. Controllable beam shaping elements or changers for diffractive elements are available, which enable rapid switching - but these elements are often not suitable for high performance and therefore cannot be used in all applications. If the power is too high, problems such as phase drift initially arise, which must be constantly corrected and worsens the beam shaping. The damage threshold of conventional beam shaping elements is also far below the performance values desired in the high-performance range, which can be in the kW range.

From the patent application WO 2019063662 A device for generating a laser beam with a top hat profile is known in which a Gaussian beam is homogenized using a diffractive optical element and this homogenized beam is radiated into a multimode fiber in which higher order modes are excited. After the multimode fiber, the beam can be additionally amplified. Due to the use of multimode radiation, this method is not suitable for ultrashort pulse applications. Non-homogeneous beam shapes cannot be achieved in a targeted manner either.

In the publication “ Radially polarized emission with 635 W of average power and 2.1 mJ of pulse energy generated by an ultrafast thin-disk multipass amplifier” (Optics Letters 40.24 (2015), pp. 5758-5761 ) described the generation of a radially polarized laser beam, whereby a beam is first radially polarized using polarization elements and thereby converted into a donut shape. This beam is then amplified in a multipass amplifier, whereby depolarization effects occur due to the reflections in the amplifier. To compensate for these depolarization effects, a lambda/4 plate was attached to the amplifier output.

Description of the invention

Based on the known prior art, it is an object of the present invention to provide an improved device for generating a laser beam with a defined beam shape and a corresponding method.

The object is achieved by a device for generating a laser beam with the features of claim 1. Advantageous further developments emerge from the subclaims, the description and the figures.

Accordingly, a device for generating a laser beam with a predetermined beam shape is proposed, which comprises a beam shaping module for generating a shaped laser beam with a first defined beam shape from an input laser beam, and an optical amplifier module for generating an amplified laser beam with a second defined spatial beam shape from the shaped laser beam, wherein the optical amplifier module comprises at least one optically active element which determines the intensity of a free-space laser beam during passage.

With such a device, it is also possible to use beam shaping modules that are only designed for comparatively low laser powers and that could not previously be used with higher target powers of the output beam. The selection of possible beam shaping modules for targeted applications in material processing, manufacturing or medicine is expanded considerably, so that flexible and quickly switchable beam shaping modules can also be used for high-performance applications such as for UKP (ultrashort pulse) lasers. This allows optimized beam profiles such as vortex, flattop or freeform profiles to be generated.

It is preferred that the first defined beam shape is determined at least partially as a function of properties of the optical amplifier module. This dependency can be explicitly defined, but can also be determined or taken into account implicitly by evaluating the achieved second beam shape of the amplified laser beam. This can improve the quality of the second defined beam shape of the amplified laser beam, since various effects that influence the beam shape in the amplifier can be at least partially compensated.

In one possible embodiment, the amplifier module comprises an optical disk amplifier, wherein the optically active element comprises a material disk for at least one passage of the laser beam. To increase the amplification, however, several passes of the laser beam can also be provided, so that it is a so-called multipass amplifier (or multipass cell, MPC). The multiple passes are achieved by reflection on mirror elements, e.g. mirror arrays in the amplifier; the back of the material disk can also be used to reflect the beam, e.g. by means of a suitable coating. A disk amplifier is well suited to amplifying a freely propagating beam, especially at high peak powers, since larger beam diameters are also possible.

Optionally, the device can include at least one polarization element for linearly polarizing the laser beam in front of the amplifier module. This makes it possible to let the beam pass through the amplifier twice (double pass) and then separate it from the input beam using filter elements. The double pass also doubles the number of individual disk passes in the amplifier and thus the amplification factor. The second pass can be achieved, for example, simply by using an end mirror at the amplifier output.

The beam-forming element can effect beam formation by modulating the phase and/or the amplitude of the laser beam; it can in particular be designed to effect beam formation at least by phase modulation of the laser beam. The phase modulation enables, among other things, the polarization of the beam already described above. In addition, very complex beam shapes can also be achieved by phase modulation.

Further advantages arise when an adjustable beam-forming element is used as the beam-forming module and the first defined beam shape, which is generated by the beam-forming module, can be adjusted by controlling the adjustable beam-forming element. Depending on their design, such adjustable beam-forming elements can enable the beam shape to be switched quickly, sometimes with switching frequencies in the kHz range and faster. By using them in front of the amplifier module, these modules, which are usually not high-performance, can be used to shape the high-performance beam and thus make a significant contribution to increasing throughput and productivity with high available pulse energy in a process. Adjusting the beam profiles enables improved process control and can be used as an "optical tool change".

When using an adjustable beam-forming element, a control unit for controlling the adjustable beam-forming element can also be present, wherein the control unit is set up to determine static or dynamically variable setting parameters at least depending on the second defined beam shape. These setting parameters can be determined in different ways; in addition to manual setting, it is possible, for example, to determine setting parameters depending on the second defined beam shape on the basis of characteristic maps, functions or similar assignments that were previously derived from measurements and/or modeling.

Another possibility for controlling an adjustable beam-forming element is to provide at least one measuring device for detecting one or more parameters of a beam profile of the amplified laser beam with the second beam shape after the amplifier module (e.g. a camera or a corresponding sensor for other wavelength ranges). The The device can then further comprise a controller which is set up to regulate one or more parameters of the recorded beam profile of the amplified laser beam, namely by determining a control deviation between the one or more parameters of the recorded beam profile and corresponding parameters of a predetermined target beam profile, wherein the controller is set up to achieve the control by adapting the control of the adjustable beam shaping element as an actuator. With such active feedback control, the beam shape of the amplified output beam can be optimized freely or on the basis of predetermined start parameters. Changes during the process, e.g. due to thermal influences (warming up of the amplifier) and fluctuations in operation, can also be easily compensated in this way in order to keep the desired beam shape as constant as possible. Of course, it is also possible to change the target beam profile during operation and thus quickly achieve the desired beam shape in the output beam again by readjusting.

A micro-mirror array, a grating light valve module or a spatial light modulator (SLM) module can be used as an adjustable beam shaping module. The choice of the appropriate beam shaping module can be made depending on the desired switching speeds, the wavelength range and the power of the seed laser. The beam shaping modules mentioned (or comparable elements) can also be designed as a micro-electronic-mechanical system (MEMS), so that a compact structure is possible.

The device can also include a laser module for generating the input laser beam (seed laser), such as a fiber or diode laser. In principle, all laser shapes and all wavelength ranges for which corresponding beam-forming modules are available can be used here. Additional frequency-converting elements can also be used optionally if the wavelength ranges specified by the laser are not compatible with a specific beam-forming element, for example. Many beam-forming elements are particularly powerful in the infrared range, i.e. at wavelengths of around 800 to 1500 nm.

The input laser beam can be a fundamental mode beam (eg with a diffraction index of about M ² < 1.3) or a multimode beam (about M ² > 1.3, but also larger values, eg M ² > 2). A fundamental mode beam is preferred, especially for USP applications. For pulses, eg in the ns range, a multimode beam can also be used.

The above object is further achieved by a method for generating a defined output beam shape for a laser beam with the features of claim 10. Advantageous further developments of the method emerge from the subclaims as well as the present description and the figures.

Accordingly, a method is proposed which comprises spatially shaping an incident laser beam by a beam shaping module to obtain a shaped laser beam with a first defined beam shape; and amplifying the shaped laser beam in an amplifier module to obtain an amplified laser beam with a second defined beam shape.

Beam shaping of the seed laser beam or input laser beam in front of the amplifier module is both faster and cheaper than previous methods. In particular, beam shaping modules can be used that are otherwise not available for high-performance applications and enable, for example, faster switching times or higher resolutions.

The first defined beam shape can be determined at least partially depending on the properties of the optical amplifier module. This makes it possible to at least partially compensate for negative effects on the beam shape when passing through the amplifier module and to optimize the beam shape of the amplified beam.

It is possible to use a beam-forming element that can be adjusted by a control. In this case, the method can further comprise specifying the second defined beam shape; and determining adjustment parameters of the beam-forming module at least as a function of the second defined beam shape. The adjustment parameters can be determined, for example, on the basis of a characteristic map, a table and/or an assignment function.

In addition, it is also possible to use active control to control the beam-shaping element. In this case, determining setting parameters can further comprise detecting one or more parameters of a beam profile of the amplified laser beam with the second beam shape, determining a control deviation between the one or more parameters of the detected beam profile and corresponding parameters of the specified second defined beam shape; and controlling the beam shape of the amplified laser beam by adjusting the control of the adjustable beam-shaping element as an actuator.

Overall, it is therefore possible to achieve lateral beam shaping even at high and very high power levels without having to pay particular attention to the performance capability of beam shaping elements, without having to monitor a continuous phase drift of the beam shaping element. The beam profile at the output of the amplifier can be optimized and adjusted by beam shaping in the seed laser beam, so that, for example, vortex, flattop or freeform profiles can be set as desired. The beam quality remains defined by the seed laser.

Short description of the characters

• 1 schematisch das Grundprinzip beispielhafter Ausführungsformen der Erfindung;
• 2 eine beispielhafte Ausführungsform zur Erzeugung eines Laserstrahls mit vorgegebener Strahlform mit einem Strahlformungsmodul und einem Multipass-Verstärker mit einfachem Verstärkerdurchgang;
• 3 eine weitere beispielhafte Ausführungsform mit einem Strahlformungsmodul und einem Multipass-Verstärker mit zweifachem Verstärkerdurchgang;
• 4 ein Flussdiagramm eines beispielhaften Verfahrensablaufs zur Erzeugung eines Laserstrahls mit vorgegebenem Strahlprofil,
• 5 ein Flussdiagramm eines beispielhaften Verfahrensablaufs zur Ermittlung der Abhängigkeit zwischen einer vorgegebenen Ausgangsstrahlform und verwendeten Parametern; und
• 6 ein Flussdiagramm eines beispielhaften Verfahrensablaufs zur Erzeugung eines Laserstrahls mit vorgegebener Ausgangsstrahlform unter Verwendung einer aktiven Regelung.

Exemplary embodiments of the invention are explained in more detail by the following description of the figures.

• 1 schematically the basic principle of exemplary embodiments of the invention;
• 2 an exemplary embodiment for generating a laser beam with a predetermined beam shape with a beam shaping module and a multipass amplifier with a single amplifier pass;
• 3 another exemplary embodiment with a beamforming module and a multipass amplifier with dual amplifier pass;
• 4 a flow chart of an exemplary process sequence for generating a laser beam with a predetermined beam profile,
• 5 a flow chart of an exemplary process sequence for determining the dependency between a given output beam shape and parameters used; and
• 6 a flow chart of an exemplary process sequence for generating a laser beam with a predetermined output beam shape using active control.

Detailed description of implementation examples

In the following, embodiments are described with reference to the figures. Identical, similar or equivalent elements in the different figures are provided with identical or corresponding reference symbols, and a repeated description of these elements is partly omitted in order to avoid redundancies.

In 1 The structure of exemplary embodiments for beam shaping is described schematically. A system for generating laser radiation can be provided in which a free input laser beam is amplified in an active medium in order to obtain a highly amplified output beam. A disk amplifier can be selected as the active amplifier medium, for example. While such systems are basically suitable for both pulsed and continuous laser radiation, they are particularly advantageous for generating short and ultra-short laser pulses with high repetition rates and/or high pulse peak power.

For this purpose, a first laser module 110 can be used, which is often referred to as a seed laser. This first laser module 110 can, for example, comprise a parametric oscillator, a solid-state laser, e.g. as a fiber or disk laser, a diode laser or another laser module. The exact structure of this first laser module 110 is not described further here; in principle, any laser that can deliver the desired beam properties or pulse properties can be used. A laser beam is understood here to mean any type of laser radiation, i.e. in particular both pulsed and continuous laser radiation, even if the exemplary embodiments described are essentially based on pulsed lasers.

The input laser beam 150 generated by the first laser module 110 or seed laser module can, for example, initially have an approximately Gaussian beam shape without spatial beam shaping. The intensity distributions indicated in the figure for the three laser beams 150, 160 and 170 are only intended to illustrate the principle; the actual intensity distributions and beam profiles can differ significantly from this. To generate pulses in the case of a pulsed laser, various methods of temporal beam shaping can be integrated into the laser module 110. In particular, when using the system for ultrashort pulses, fundamental mode radiation (or approximately fundamental mode radiation) can preferably be used to generate pulses.

Alternatively, the input laser beam 150 can also be designed as a multimode laser beam and, for example, have a tophat beam shape.

This input laser beam 150 can then be introduced into a beam shaping module 120, which enables defined spatial beam shaping, e.g. by phase modulation or amplitude modulation. A static element can be used which is specifically manufactured to form a predetermined beam shape, such as a diffractive optical element.

Particularly suitable here are flexible, i.e. adaptable beam shaping modules 120, which are available in the Able to provide different beam shapes depending on the setting or control. For example, 120 SLM (Spatial Light Modulator) elements, controllable micromirror arrays, or so-called (grating) light valves can be used as adaptable beam shaping modules. Such beam shaping elements can also be compact in the form of microelectronic-mechanical systems (MEMS). Spatial beam shaping can, for example, include spatial modulation, phase modulation or amplitude modulation, or a combined intensity and phase modulation. Phase modulation in particular offers the advantage that a polarization for the beam can be set and very complex beam shapes are also possible. The beam shaping modules can be addressed or controlled electronically or optically, depending on the type.

The laser beam 160 formed in this way can then be coupled into the amplifier module 130, in which the beam is amplified by passing through a laser-active medium once or several times. In particular, it can be a linear optical disk amplifier that uses a laser-active medium with a comparatively small thickness (disk) for amplification, so that the result is a laser beam with higher output power or higher pulse energies. Since large beam diameters can also be used, such amplifiers are particularly suitable for scaling high-power lasers and in the short or ultra-short pulse range.

In principle, amplification can be achieved with just one pass through the laser disk; however, due to the comparatively low amplification, a larger number of passes through the laser disk are typically used to achieve the desired output power (multipass cell or multi pass cell, MPC). The multiple passes can be achieved using reflective elements or surfaces, whereby the surfaces of the active medium itself can also be used for reflection. Various designs are known, such as the active mirror amplifier.

However, for many beam shapes, each disk passage in the amplifier causes more or less strong linear and/or non-linear distortions of the output beam shape 170 compared to the input beam shape 160. The beam profile can be disturbed, for example, by linear effects and/or by non-linear effects (e.g. Kerr lens). In general, beam propagation, e.g. in the linear multi-pass amplifier, is influenced by the beam radius of the laser beam, the beam quality, the disk curvature and the distance between the disk and the reflective array. According to the invention, the beam shaping 120 upstream of the amplifier module 130 can therefore be specifically adjusted so that the non-linear transfer function of the optical amplifier module 130 is completely or largely compensated. It is therefore possible that for a defined, desired output beam profile 170, a significantly different input beam profile 160 is initially specifically generated by the beam shaping module 120. After passing through the amplifier module 130 with the distortions taken into account, an output beam 170 with the desired beam shape can then be obtained.

Optionally, the beam forming module 120 can also be actively controlled or regulated via a control unit 140 for this purpose. For example, the achieved output beam shape 170 can be measured and used to control or also for feedback control of an active beam forming module 120 such as an SLM element, as shown by the dashed arrows in 1 is indicated.

In other variants, a predetermined control can be used, which can be implemented, for example, using data from test runs and with suitable calibrations or characteristic maps. Various examples of the control and regulation 140 of an active beam forming module 120 are described in more detail below.

2 shows a possible embodiment with a single overall passage of a shaped beam 260 through an optical amplifier module 230. The amplifier module 230 can be designed such that one or more passages of the shaped beam 260 through the amplifying medium can be provided. In the case of a multi-pass disk amplifier, a predetermined number of disk passages can be provided within the disk amplifier 230, e.g. 18 passages. However, this number is only given as an example and can be selected depending on the amplifier type, the characteristics of the laser beam and other conditions.

The seed laser module 210 can be any module for generating the laser signal with the desired spectral properties, e.g. with the desired wavelength, pulse length, power or pulse energy and spectral width. In principle, these values can be freely selected, adapted to the amplifier medium.

However, the methods and devices described are particularly suitable for cases in which a seed laser beam 250 is generated with a comparatively low power, e.g. with a power below 100 watts on the beam shaping module. With sufficient amplification, such as through a multi-pass amplifier, a seed laser output of around 5 to 10 watts can be sufficient, although values above or below these limits can of course also be selected, e.g. in the range of around 100 W. The wavelengths of the seed laser can, for example, be in the infrared range of around 800 to 1500 nm, e.g. 1030 nm for a Yb:YAG laser. Many beam shaping modules are particularly powerful in these ranges. However, if suitable beam shaping modules are available, any other wavelength can also be selected, e.g. in the green wavelength range or even 2 µm lasers such as thulium or holmium lasers. Known beam shaping elements cover a range from approx. 400 to 2200 nm. The seed laser can, for example, be an optical parametric oscillator that uses a fiber-, rod-, or disk-shaped active medium, a fiber laser, or a diode laser.

The input laser beam 250 generated by the seed laser module 210 can then be guided to the beam shaping module 220 via suitable optical elements. In this case, a beam expansion can optionally be carried out first by an optical lens system 212 (telescope system). It is also possible to guide the input laser beam 250 through a polarization element 290, e.g. a lambda/2 plate, and thus convert the linearly polarized laser light into a circular polarization. The polarization can also be carried out at a different location than shown here.

The polarized and expanded beam can now be directed to the beam-shaping element 220. As shown here in the figure, this can be a reflective beam-shaping module. The beam-shaping module 220 generates, preferably by phase modulation, a shaped laser beam 260 with a first defined beam shape, which can be statically specified by the design of the beam-shaping module 220 or adapted by controlling the beam-shaping module 220 by means of a control unit. The first defined beam shape 260 can be specified depending on the properties (i.e. the transfer function) of the subsequent amplifier module 230 in such a way that distorting effects are at least partially compensated, so that the desired second defined beam shape 270 of the output beam is present after the amplifier module 230. Since the (single or multiple) passage through the amplifier module 230 can change the beam shape more or less, the first defined beam shape of the shaped input beam 260 can also deviate more or less from the second defined beam shape of the output beam 270. In principle, possible distortions are also increased with an increasing number of passes through the amplifier medium, so that the number of passes should be taken into account for the optimization of the beam formation.

In the example shown here, a spatial filter 222 is arranged after the beam-forming module 220, which can be implemented, for example, by a pinhole diaphragm with or without additional lens elements. With such a spatial filter 222, higher modes of the first defined beam shape can optionally be trimmed and/or scattering or diffraction of the beam-forming module can be limited.

The laser beam 260 formed with the first defined beam shape can then be coupled into the amplifier module 230, which can be, for example, the disk amplifier already described as a multi-pass cell with multiple disk passes. The multiple disk passes can be achieved by a suitable arrangement of (curved) reflective elements or reflective surfaces. The active medium of the amplifier module, i.e. in this example the laser disk, can be optically pumped in a known manner by a pump unit such as a diode laser. With each pass through the active medium, the laser beam is further amplified, so that ultimately an amplified output beam is obtained.

It goes without saying that the output power depends on the input power and the amplifier design, among other things, and that the pulse energies in pulsed laser radiation also depend on the pulse length. With the principles presented here, it is possible to achieve output powers of more than 500 watts, even with fast beam shaping, with pulse energies of around 1 mJ or more being the goal. However, the effects described also occur with other, even lower, output powers or pulse energies.

The amplified output beam 270 has experienced various distortions when passing through the active amplifier medium, but possibly also due to the reflection processes. Without compensating beam shaping, the beam shape of the amplified output beam 270 would therefore only correspond approximately to the beam shape before the amplifier in exceptional cases, e.g. in the case of a simple ring shape. The distorting effects are also dependent on many different conditions; for example, the thermal influences due to the heating of the amplifier ker elements play a major role, so that different distortions occur at the beginning than in continuous operation which has achieved a certain equilibrium. In order to obtain a second defined beam shape of the amplified output beam at this point, the properties of the amplifier module must therefore be taken into account in order to at least partially compensate for the distortions by suitable selection of the first defined beam shape, which is set by the beam shaping module. The second defined beam shape is selected such that the beam is adapted for the respective application (e.g. laser drilling, welding, etc.). If an adjustable beam shaping module 220 is used, temporal changes in the second defined beam shape and, depending on this, corresponding changes in the first defined beam shape can also be provided within an application, e.g. different beam shapes during material removal and subsequent surface processing. In other cases, the second defined beam shape may be constant, while the first defined beam shape may still change over time, e.g. depending on the heating and the associated thermal effects in the amplifier.

In the structure shown here, beam parts were coupled out at various points 292, 294 in order to measure the beam shapes or beam profiles as well as the power of the laser beam. Between the (optional) spatial filter 222 and the amplifier module 230, the laser beam 260, which has not yet been amplified but has already been shaped with the first defined beam shape, can be coupled out, for example via several deflection elements and a coupling-out element 292 (e.g. a partially transparent mirror), and guided to a measuring device 280 which can record the beam profile. This can be, for example, a camera with a suitable sensor element such as a CCD or CMOS sensor in the visible range, or other corresponding sensors for lasers in the non-visible range. Special measuring devices such as a Shack-Hartmann sensor or a beam quality measuring device can also be used here.

The recorded beam profiles can then be evaluated, for example, by software, saved for later use and/or shown on a display for a user to check. In a corresponding manner, the amplified beam 270 with the second beam shape can also be decoupled 294 from the beam and recorded by a measuring device 280 to measure the beam profile. In the case shown here, the same measuring device is used for both beam profiles and one beam is blocked in each case by a pivotable absorber 296, which is used in one of the two positions shown. In this way, it is possible to determine a dependency between the first defined beam shape, which is generated by the beam shaping module 220, and the second defined beam shape, which is present at the output of the amplifier 230. For the evaluation, individual parameters of the beam profiles recorded by the measuring device 280 can be taken into account, e.g. using image processing methods. For example, brightness, contrast and/or color values of different image areas can be evaluated in order to obtain information about the existing beam shape. In addition, the power 282 of the residual beam can be measured after the coupling out 294 of the measuring beam. The power value obtained can be taken into account for the evaluation either directly or with a corresponding correction for the coupled out portion.

The dependency between the first and the second defined beam shape is essentially determined by various properties of the amplifier module and the conditions during beam generation; nevertheless, it is not absolutely necessary to explicitly take these amplifier properties into account. By evaluating a comparison between the first and the second defined beam shape in a given system as described above, it is possible to specify the required parameters for the beam shaping module without knowing the amplifier properties exactly. For this purpose, for example, test runs can be carried out under different conditions, e.g. with different laser powers, different amplifier settings and different first beam shapes, each of which is evaluated over time. A characteristic map or a function can then be determined, for example, from the relationship between the first defined beam shape set at a specific point in time, which was achieved by controlling the beam shaping module with known parameters, and the second defined beam shape obtained as a result at the output of the amplifier. A desired second defined beam shape can then be specified for an application, and using the map or function, the first defined beam shape and/or the parameters for the beam forming module can be derived. Since the dependency will vary depending on the power in most cases, the measured power of the output beam can be taken into account as a parameter of the map or function.

However, it is also possible, for example, to define different performance ranges within specified limits and a separate characteristic map or a separate function can be determined for each of these areas. In a similar way, it is conceivable not to evaluate the temporal progression of the various parameters and the beam shapes obtained separately, but to define time ranges directly after switching on, after reaching an equilibrium state and after certain running times, for example, and then to derive characteristic maps or functions for each of these time ranges. It goes without saying that as a simplified variant, an evaluation of the parameters can be carried out in an approximate equilibrium state, e.g. after a specified running time, without taking temporal changes into account.

Alternatively, the dependency can also be determined or evaluated without explicitly measuring the first defined beam shape before the amplifier. Even without knowledge of the first beam shape, at least the parameters with which the beam shaping module was designed (in the case of a non-controllable module) or with which the beam shaping module is controlled at a certain point in time are known. The second beam shape, i.e. the shape of the laser beam after the output of the amplifier module, can then be recorded in a suitable manner and a dependency between the second beam shape and the set parameters can be evaluated over time.

Another possibility is to use active control to set the parameters of the beam shaping module. The results of the beam profile measurement of at least the second beam shape can be evaluated, the deviation between the actually measured second beam shape and the specified second beam shape can be determined and fed to a controller as a control deviation. Such a controller can therefore use the measurement parameters of the second beam shape as the actual value and regulate to the specified beam shape parameters as the target value by using the setting parameters of the beam shaping module as the manipulated variable of the controlled system. Active control of this type can be implemented by a software or hardware controller, whereby the controlled system can be implemented in any way and additional factors such as the pulse duration of the pulsed beam, characteristics of the laser application (variable movement, continuous repetition of a single process, etc.) or switching times of the beam shaping module can be included in the controller parameterization.

It is also possible to use machine learning methods to obtain suitable parameters for setting the beam forming module from the recorded values. For example, measurement data of at least the second beam profile together with the parameters used, e.g. the recorded power values and the setting parameters of the beam forming module, can be used as input values for a learning model. Various methods of supervised or unsupervised learning as well as reinforcement learning can be used. Depending on the model used, it is also possible not to record beam profile parameters from the measurement data, but to use the recorded images of the beam profile directly as input data for the learning model, for example for a convolutional neural network.

After a training phase, the learning model can be used for continuous control in a similar way to an active control system. If further data from the second beam shape is recorded after amplification, this (optionally together with the desired beam shapes) can also be fed to the learning module to enable retraining. For this purpose, data can also be recorded and collected at certain intervals and then used at a suitable time to retrain the model.

Alternatively or in addition to the practical methods described above for determining the optimal parameters for setting the beam forming module, simulations and models can also be used that make it possible to calculate at least approximately the distortions to be expected from the amplifier pass. Such models can also be used in particular to initially delimit the parameter space that is to be used for the measurements, evaluations or controls described. For example, a theoretical model or a numerical simulation can be used to roughly preselect the first defined beam shape and determine the required parameters for the beam forming module, and with these initial parameters, a refined optimization of the parameters can then be achieved by the described determination of characteristic maps, by machine learning, or by active control.

3 shows a further embodiment of a device for generating a shaped laser beam, which in essential parts corresponds to the embodiment of 2 All details described there and not repeated here are therefore also applicable to this example. However, here an end mirror 334 is attached to the output of the amplifier module 330, which leads to a second passage of the laser beam through the amplifier module 330. Accordingly, the number of of the individual disk transitions within the amplifier 330 and thus also the amplification of the intensity of the output laser beam 370. This variant requires a linear polarization of the laser beam; therefore, further polarization elements 390a, 390b, 390c are provided in the optical path. A beam with a defined polarization direction can be generated by an optical isolator 391 after the first polarization element 390a (for example a wave plate such as a lambda/2 plate); the light is circularly polarized before the beam shaping module 320 by a further polarization element or lambda/2 plate 390b and linearly polarized again before entering the amplifier 330 by a third polarization element 390c. Alternatively, an isolator 391 could also be used after the beam shaping module 320, for example.

The output laser beam 370 is guided out of the amplifier 330 on the same path as the input laser beam. A polarization filter 398, which transmits light of one polarization and redirects light of another polarization, can then be used to redirect only the amplified beam from the common path. For this purpose, the polarization of the output laser beam 370 can be rotated again in front of the polarization filter 398 by lambda/2 plates or other polarization elements (not shown here). A thin film polarizer (TFP), for example, can be used as a polarization filter, but other elements such as polarizers based on birefringent crystals are also conceivable. Accordingly, the beam guidance also changes compared to the embodiment from 2 as shown, but essential parts such as beam coupling 392, 394, beam profile measurement 380, power measurement 382, the use of spatial filter 322 or telescope 312 and others remain essentially the same.

It goes without saying that the arrangement and position of the individual elements are shown here only as examples, and that in particular the use and number of e.g. beam splitters, coupling elements and deflection elements can be freely selected depending on the structure used in each case. If a measurement of the beam profile for one or both beams is not required in the subsequent application and the setting is made purely on the basis of the previously determined parameters, the corresponding components can be omitted. If, on the other hand, for example, the active control described is used, at least a beam profile measurement of the amplified beam is retained. The amplified output beam 270, 370 after the amplifier module 230, 330 can optionally be guided into the application area via further components, e.g. focusing modules and deflections, for example onto the material surface to be processed.

4 , 5 and 6 show exemplary process sequences for generating a laser beam with a defined beam shape as a flow chart.

This shows 4 a flow chart of an exemplary method sequence in which the parameters for beam shaping are obtained from a function or a characteristic map or determined using similar methods without resorting to active control. To do this, in step 400, the desired output beam profile, i.e. the second defined beam shape, is first specified. Certain beam shape parameters, such as the diameter and intensity distribution of the beam, can be freely selected using suitable means. In other embodiments, e.g. in control software, the user can additionally or alternatively be given various conventional beam shapes, for example a flat-top profile, a top-hat profile, a Gaussian profile, a ring or donut profile or a freely selectable free-form profile. In addition, other parameters such as the desired output power can also be specified in this step.

If the beam shaping is controlled by an automated process (e.g. a complex manufacturing process with several actuators), the desired properties of the output beam can be taken into account when developing the automation control, while in the automation only the setting parameters for the beam shaping module are optionally defined statically or dynamically. Different beam shapes and thus different parameters can also be provided for different steps. In this case, step 400 is therefore carried out when developing the automation control.

In step 402, the specified output beam profile and a function, a characteristic map, a table or a similar assignment can be used to determine which setting parameters are to be used for the beam shaping module in order to obtain the specified second beam shape after amplification. These can be static setting parameters or setting parameters that change over time, for example to compensate for thermal effects or to obtain a constant beam shape with changing laser power or with fluctuating pointing.

In step 404, an input laser beam can be generated by the seed laser module that meets the desired criteria, e.g. with a predetermined pulse length, repetition rate and power Optionally, the values used here for generating the input laser beam can also be retrieved in step 400 by the user or from a corresponding control file.

The generated input laser beam may be shaped by the adjustable beam shaping module in step 406 using the beam shaping module adjustment parameters selected in step 402.

In step 408, the shaped beam can be amplified by the optical amplifier module to obtain the desired output laser beam. This can finally be directed to the work area, e.g. the workpiece, in step 410 and thus applied.

5 shows a flow chart of an example process sequence for determining the dependency between a given output beam shape and the parameters used. In step 500, starting parameters for the beam forming module can optionally be set first, which can be obtained, for example, from simulations or models for beam forming and beam amplification. In other cases, the parameters that generate a first beam shape that corresponds to the desired output beam shape can also be selected as the starting parameters. The desired beam shape can therefore be selected as the starting point without any compensation for the amplifier effects, which is then adjusted in the following steps.

In step 502, a seed laser beam with known properties is generated, shaped using the starting parameters from step 500 and amplified in the amplifier module. At least the output beam obtained in this way is fed to a measuring device in step 504 in order to record the beam profile. Optionally, other values such as the beam profile of the shaped input beam and the power of the input and/or output beam can also be measured in step 504. The measured values are saved in raw form or in processed form together with the known setting parameters. In step 506, parameters of the test setup are optionally changed, e.g. the setting parameters of the beam shaping module or the laser power of the seed laser module. The beam shape obtained is then measured again and the recorded profile parameters are saved with the current setting parameters. The change of parameters and new measurements can be repeated as often as desired. Likewise, any number of measurements can be taken over time without actively changing parameters, e.g. to record thermal changes.

Finally, in step 508, the data acquired from the measurements are used to determine a dependency between the known setting parameters and the second beam shapes of the output beam obtained with them. The determined dependency can then be saved in step 510 as a function, as a multi-dimensional characteristic map, as a table or in another form and can now be used to set the beam shaping module (e.g. in step 402 from 4 ) be applied.

6 shows a flow chart of another exemplary method for generating a laser beam with a predetermined output beam shape using an active control. In step 600, analogous to step 400, 4 the desired beam shape of the output beam is determined. In step 602, starting parameters for setting the beam shaping module are determined, which can be obtained from a characteristic map or function, for example, as in step 402, or can also be determined from a theoretical simulation, from a model or directly from the beam shape of the output beam without compensation, as in step 502. In step 604, the generation of the seed laser beam, the beam shaping with the current setting parameters (ie during the first run with the starting parameters from step 602) and the amplification of the shaped input laser beam are summarized.

The beam profile of the amplified output laser beam obtained in this way is recorded and evaluated in step 606. In step 608, a deviation of the recorded parameters from step 606 from the desired beam shape parameters from step 600 can be determined and defined as a control deviation. Finally, in step 610, the required adjustment of the setting parameters for the beam shaping module can be determined from the control deviation from step 608 via a suitable control system. Beam shaping can then be carried out again with these adjusted setting parameters, so that steps 604 to 610 are repeated as often as desired.

The described processes can of course be supplemented by intermediate steps. In some cases, steps can also be carried out in a different order or in parallel to one another, e.g. the determination of the new setting parameters for the control or the evaluation of a recorded measurement can take place at the same time as the beam formation with the previous setting parameters.

In all embodiments, further modifications are possible, which can also be combined in any way with each other and with the variants already described.

For example, a blaze grating can be integrated into the beam forming module. Such a grating can help to compensate for errors in the beam forming by diffracting the incorrectly formed beams and then removing them with an optional spatial filter (pinhole). It goes without saying that the angle dependence of the grating and the spatial filter can be coordinated accordingly.

It is also possible for the laser beam to be frequency converted after the beam-forming module (before or after the amplifier module), e.g. using a frequency doubling element. This can, for example, enable the use of beam-forming elements that are not available for all wavelengths.

A laser beam with a defined polarization, such as that used in connection with 3 can of course also be used independently of the double pass through the amplifier described there.

The use of shaped laser beams is useful in many areas of technology, especially in material processing, e.g. in material removal, in joining or separating materials using laser beams, in additive processes, in modifying surfaces, but also when used for medical purposes and in research. It is also understood that the methods and devices described here are not limited to the applications mentioned as examples in these areas; rather, they can be used for all areas in which the generation of shaped laser beams, in particular with short and ultra-short laser pulses, is desired. Of course, depending on the application, elements and systems that have not been expressly described here can also be added or modified. For example, the principle described can be applied to a variety of different wavelengths and operated with different types of seed lasers. The use of additional or modified optical elements in the beam path at any point in the system, such as lenses, beam splitters, measuring devices, deflecting mirrors, can also be adapted to the respective application.

With the methods and devices described here, it is possible to obtain a desired beam shape of the amplified output beam and to use beam-forming modules that are not designed for high power. This means that all the advantages of such beam-forming modules, such as rapid adaptability and flexibility, can be exploited without sacrificing the desired high pulse power. While beam-forming modules of the grating light valve type, for example, can be switched relatively quickly (in the range of several hundred kHz), they are often not suitable for high power. Likewise, micromirror arrays cannot currently be used for the desired power in the kW range. The low input beam power before the amplifier, which depending on the amplifier type can be in the range of just a few watts, for example, makes it possible to use such beam-forming modules without limiting the desired output beam power.

Where applicable, all individual features and elements shown in the embodiments may be combined and/or exchanged without departing from the scope of the invention.

List of reference symbols

110, 210, 310

Seed laser module for generating the input laser beam

120, 220, 320

Beamforming module

130

optical amplifier module

140

Control unit

150, 250, 350

Input laser beam/seed laser beam

160, 260, 360

shaped laser beam with first beam shape

170, 270, 370

amplified laser beam with second beam shape

212, 312

expanding lens system

222, 322

optical spatial filter

230, 330

Disc amplifier

232, 332

Pump unit for disc amplifier

280, 380

Measuring device for beam profile measurement

282, 382

Measuring device for beam power measurement

290, 390a/b/c

Polarization element

292, 392

Output coupling element input beam

294, 394

Output coupling element output beam

296, 396

swivel absorber

334

End mirror for window amplifier

391

optical isolator

398

Polarizing filter

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 accepts no liability for any errors or omissions.

Cited patent literature

• WO 2019063662 [0005]

Cited non-patent literature

• Radially polarized emission with 635 W of average power and 2.1 mJ of pulse energy generated by an ultrafast thin-disk multipass amplifier” (Optics Letters 40.24 (2015), pp. 5758-5761 [0006]

Claims (15)

Device for generating a laser beam with a predetermined beam shape, comprising: a beam shaping module (120, 220, 320) for generating a shaped laser beam (160, 260, 360) with a first defined beam shape from an input laser beam (150, 250, 350); and an optical amplifier module (130, 230, 330) for generating an amplified laser beam (170, 270, 370) with a second defined spatial beam shape from the shaped laser beam, wherein the optical amplifier module comprises an optically active element which increases the intensity of a laser beam as it passes through. Device according to Claim 1 , wherein the first defined beam shape is determined at least partially depending on properties of the optical amplifier module (130, 230, 330). Device according to Claim 1 or 2 , wherein the amplifier module (130, 230, 330) comprises an optical disk amplifier and/or a multipass amplifier, wherein the optically active element comprises an amplifier disk for at least one passage of the laser beam. Device according to one of the preceding claims, further comprising at least one polarization element (290, 390a, 390b, 390c) for linearly polarizing the laser beam in front of the amplifier module (130, 230, 330). Device according to one of the preceding claims, wherein the beam shaping module (120, 220, 320) is configured to effect the beam shaping at least by a phase modulation of the laser beam. Device according to one of the preceding claims, wherein the beam-forming module (120, 220, 320) comprises an adjustable beam-forming element, and wherein the first defined beam shape generated by the beam-forming module is adaptable by controlling the adjustable beam-forming element. Device according to Claim 6 , further comprising a control unit (140) for controlling the adjustable beam-shaping element, wherein the control unit is configured to determine static or dynamically variable setting parameters at least as a function of the second defined beam shape (170, 270, 370). Device according to Claim 6 or 7 , further comprising at least one measuring device (280, 380) for detecting one or more parameters of a beam profile of the amplified laser beam with the second beam shape after the amplifier module, and a controller which is configured to regulate one or more parameters of the detected beam profile of the amplified laser beam by determining a control deviation between the one or more parameters of the detected beam profile and corresponding parameters of a predetermined target beam profile, wherein the controller is configured to achieve the control by adapting the control of the adjustable beam shaping element as an actuator. Device according to one of the preceding claims, wherein the beam forming module (120, 220, 320) comprises at least one of the following: a micro-mirror array, a grating light valve module, a spatial light modulator (SLM) module. Apparatus according to any preceding claim, wherein the input laser beam is a fundamental mode beam or a multimode beam. A method for generating a defined output beam shape for a laser beam, comprising: spatially shaping (406) an incident laser beam by a beam shaping module to obtain a shaped laser beam with a first defined beam shape; and amplifying (408) the shaped laser beam in an amplifier module to obtain an amplified laser beam with a second defined beam shape. Procedure according to Claim 11 , wherein the first defined beam shape is determined at least partially depending on properties of the optical amplifier module. Procedure according to Claim 11 or 12 , wherein the beam-forming module is adjustable by a control, the method further comprising: setting (400, 600) the second defined beam shape; and determining (402, 602) setting parameters of the beam-forming module at least as a function of the second defined beam shape. Procedure according to Claim 13 , wherein determining (402, 502, 602) setting parameters comprises determining the setting parameters from one of the following: a characteristic map, a table, an assignment function. Procedure according to Claim 13 or 14 , wherein determining (402, 502, 602) setting parameters further comprises: Detecting (606) one or more parameters of a beam profile of the amplified laser beam with the second beam shape, determining (608) a control deviation between the one or more parameters of the detected beam profile and corresponding parameters of the specified second defined beam shape; and regulating (610) the beam shape of the amplified laser beam by adapting the control of the adjustable beam shaping element as an actuator.

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