Apparatus and method for determining a beam parameter product or a marginal power of a laser beam

Abstract

A device and a method for determining a beam parameter product and / or a marginal power of a fiber-guided laser beam are provided. The device comprises a housing with a fiber socket for receiving a laser fiber with a known fiber radius and a known laser power; a pinhole arranged coaxially in the housing at a predetermined distance from the fiber socket and configured to directly absorb at least partially the laser beam; at least one temperature sensor arranged on the pinhole for detecting a temperature of the pinhole; and a control unit configured to detect the temperature of the pinhole within a predetermined measuring interval and, based on the known fiber radius, the known laser power, the predetermined distance, and the detected temperature, to determine the beam parameter product of the laser beam using a lookup table.

Description

[0001] The present invention relates to a device for determining a beam parameter product and / or a boundary power of a fiber-guided laser beam and a corresponding method. The device can be designed compactly and intended for mobile use, e.g., in a laser processing system. Technical background

[0002] In state-of-the-art laser processing systems (e.g., laser cutting and laser welding systems) that operate with disk lasers or fiber lasers, the laser source is coupled into the end effector, the so-called laser head, via a laser fiber. The end piece of the laser fiber is located in a fiber connector, which is inserted into a fiber socket at the upper entrance of the head. From the end of the laser fiber, the laser beam propagates along a propagation axis inside the laser head and passes through a number of transmissive optics, specifically lenses (imaging optics) and protective glasses (non-imaging optics).

[0003] The laser beam used is often a so-called Gaussian beam. A Gaussian beam is characterized by the fact that the intensity distribution I along the beam diameter corresponds to a two-dimensional, more precisely circular, Gaussian normal distribution, with the beam diameter increasing along its propagation axis starting at the fiber end. In other words, the distribution of intensity (i.e., power density or power per area) in each beam cross-section perpendicular to the propagation axis corresponds to a Gaussian normal distribution, as in Fig. 1. While the total laser power remains constant in each beam cross-section, the variance of the normally distributed power density increases with the increasing width of the beam.

[0004] Since the intensity, due to the Gaussian distribution, tends only asymptotically towards zero with increasing radial distance from the beam axis, no defined radius of the beam can be represented at which the intensity is exactly zero. Therefore, the beam radius w(z) at a defined distance z from the fiber end is conventionally considered to be the radius of a beam cross-section that is 1 - e -2 or ~86.5% of the total laser power, see Fig. 1.

[0005] The laser beam therefore exhibits considerable edge intensity even at a large radial distance from the beam axis. In contrast, the design of laser heads requires a beam with a sharp (not smeared) edge to avoid absorption and reflection of laser radiation from peripheral components within the head. For this purpose, a pinhole is usually installed in laser heads a short distance from the fiber end. This pinhole serves as a so-called hard aperture for the subsequent beam path by absorbing all edge radiation outside its inner hole diameter. The combination of the inner diameter of the pinhole and its distance from the fiber end results in a clearly defined aperture angle of the laser beam passing through the pinhole, beyond which the intensity of the laser beam is reliably zero (also referred to as the "numerical aperture" or "NA").Depending on the quality of the laser source and the NA, which is defined by the pinhole, it must have a certain edge power P. a of the laser beam. The absorbed laser power P a is dissipated into heat and increases the temperature of the pinhole. The fraction of the absorbed edge power P a of the laser beam can precisely determine the -2 -portion or approximately 13.5% of the total incident laser power P0, which by convention is referred to as the marginal power beyond the laser beam (see Fig. 1), but it can also be larger or smaller than this.

[0006] Due to its function, this pinhole is subjected to high thermal stress during laser processing, meaning it must be adequately cooled and is subject to increased wear. Therefore, an accurate quantification of the edge power of a laser beam absorbed by the pinhole is of great interest for the design and operation of a laser head.

[0007] The edge power of a Gaussian laser beam for radii larger than a defined radius r at a given distance z from the fiber end is approximately related to the total incident laser power or laser power P0: P(r,z)=P0e−2r2 / w2(z)

[0008] With radius r, and w(z)=w01+(z bppw02)2

[0009] The beam waist w(z) describes the 86.5% envelope (1-e -2 )*P0 from Fig. 1, within which the laser power is approximately 86.5% of the total laser power. For a given distance z from the fiber end, the beam waist w(z) depends only on the fiber radius w0 and the beam parameter product bpp. If both quantities are known, the power of edge radiation incident on a pinhole can be quantified for a known installation situation. The beam parameter product itself is the product of the fiber radius w0 and half the aperture angle φ: bpp=w0φ

[0010] Laser source manufacturers typically specify both the diameter of the laser fiber and the beam parameter product of their laser to allow users to assess the quality of the laser source. While the fiber diameter specification is usually very accurate, the beam parameter product specification is very imprecise in practice, and the actual beam parameter product can exhibit considerable scatter across different laser models. Therefore, it is common practice to only specify an upper limit for the beam parameter product, e.g., "bpp <= 4."

[0011] WO 2007 / 025563 A1 relates to a method and a device for detecting the fringe field intensity of a laser beam. The device comprises a pinhole diaphragm that can be moved along the laser beam axis. The pinhole diaphragm is designed as a perforated mirror that reflects the edge radiation of a laser onto a separately arranged absorber. The fringe field intensity is detected by calorimetric measurement of the amount of heat absorbed by the absorber. However, no beam parameter product is determined.

[0012] DE 10 2014 012 913 A1 relates to a device in which the laser beam is absorbed by an absorber, whereby the temperature of the coolant is measured three times before heating by a heating element, after heating, and after cooling of the absorber. Temperature differences are calculated from the temperature values in an evaluation unit, and the total laser power is determined based on these values. However, a determination of the beam parameter product and the edge power is not shown.

[0013] DE 195 00 684 C1 relates to a power meter that also only determines the total power of a laser. The power meter operates without a liquid coolant, with a laser beam S being absorbed by a structure 3, causing the structure 3 to heat the air flow L. The thermometer 4 measures the temperature of the heated air flow L. This measured temperature and the room temperature result in a temperature difference that is proportional to the average laser power.

[0014] EP 4 035 820 A1, DE 32 46 290 A1, US 2015 / 0 276 473 A1 and US 4 793 715 A relate to further prior art for the present invention. Summary of the invention

[0015] The user of a laser source connected to a laser processing head is therefore interested in determining the beam parameter product of their laser source as a measure of the quality of the laser source and / or the edge power of the laser beam that strikes and is absorbed by a pinhole with a predetermined inner diameter at a predetermined distance of the pinhole from the fiber end. The laser source to be measured should not require laborious transport to a stationary test station. Rather, there is a need for a mobile measuring device that can be easily transported to the laser source to be measured and quickly set up and dismantled there.

[0016] It is therefore an object of the present invention to provide a device for determining the beam parameter product of a laser beam and / or the edge power.

[0017] It is a further object of the present invention to provide a compact and mobile device for determining the beam parameter product of a laser beam and / or the edge power.

[0018] It is a further object of the present invention to provide a method for determining the beam parameter product and / or the edge power of a laser beam.

[0019] At least one of these problems is solved by the subject-matter of the independent claims.

[0020] According to one aspect of the present disclosure, the device for determining a beam parameter product and / or the edge power of a fiber-guided laser beam comprises a housing with a fiber socket for receiving a laser fiber with a known fiber radius. The device further comprises a pinhole diaphragm arranged coaxially in the housing at a predetermined distance from the fiber socket or the fiber end and configured to at least partially absorb (in particular directly) incident edge radiation of the laser beam. The device further comprises at least one temperature sensor arranged on the pinhole diaphragm for detecting a temperature or at least one temperature value of the pinhole diaphragm.The device also comprises a control unit configured to detect the temperature of the pinhole at at least one predetermined measurement time and / or continuously within a predetermined measurement interval when the laser beam is irradiated with a known laser power during the measurement interval. The control unit is further configured to determine the beam parameter product and / or the edge power of the laser beam using a lookup table based on the known fiber radius, the known laser power, the predetermined distance, and the detected temperature. The device can be referred to as a calorimetric measuring device.

[0021] The laser fiber here is the laser fiber that guides or radiates the laser beam. The laser fiber can be connected to a laser source for generating the laser beam, i.e. the laser fiber can be a laser fiber connected to a laser source for generating the laser beam. The pinhole is arranged coaxially to the fiber socket, i.e. an optical axis of the pinhole and an optical axis of the fiber socket are coaxial. Thus, an optical axis of a laser fiber accommodated in the fiber socket or a beam axis of the laser beam radiated by a laser fiber accommodated in the fiber socket can be coaxial to the (optical axis of the) pinhole and / or run through the center point (of the opening) of the pinhole. The edge power of the laser beam here refers in particular to the absorbed power of the edge radiation of the laser beam incident on the pinhole.

[0022] An analytical (i.e., computational) relationship between the detected temperature or the temperature value at the position of the at least one temperature sensor and the beam product parameter is not possible due to the complexity of the differential equations required for this purpose. However, the device provided can be used to implement an indirect method that assigns the beam parameter product and / or the boundary power to the detected temperature values in accordance with at least one boundary condition in a lookup table, thereby making it accessible. The laser beam is preferably a Gaussian beam.

[0023] By measuring the temperature of the pinhole over a short irradiation period and comparing it with a lookup table, an unknown beam parameter product of a Gaussian laser beam can be determined. From this beam parameter product, the edge power can be directly determined for a specific installation situation of the pinhole in a laser head.

[0024] The pinhole has an opening ("hole") with a known diameter. This diameter can be referred to as the (smallest) inner diameter or the clear diameter of the pinhole. The position of the pinhole or the distance between the pinholes can be related to the (smallest) inner diameter or clear diameter. The pinhole can be arranged in the housing, for example, using a holder. The pinhole can be arranged in the housing such that the optical axis of the pinhole is aligned coaxially with the beam axis of the laser beam.

[0025] The pinhole can have an anti-reflective coating and / or a light-absorbing coating. The pinhole can have a light-absorbing surface and / or a coating made of a light-absorbing material, at least on the side facing the fiber socket. Preferably, the pinhole can comprise a light-absorbing material or be made entirely of a light-absorbing material, such as copper. The pinhole is preferably a single piece.

[0026] The pinhole can be arranged coaxially to the fiber socket in the housing. The pinhole can be arranged in the housing such that the edge radiation of the laser beam of a laser fiber arranged in the fiber socket impinges (in particular directly and / or immediately) on the pinhole, i.e., in particular without prior reflection by another component, such as a pinhole mirror. In other words, the device according to the invention does not require the use of a separate absorber, which absorbs the edge radiation to be measured, or a coolant flow. As a result, both the size of the device according to the invention and its installation, transport, and operating costs are significantly reduced compared to previously known solutions.

[0027] The pinhole comprises at least one temperature sensor, preferably three temperature sensors. The three temperature sensors can be evenly distributed in the circumferential direction and / or arranged at different distances from a central axis or optical axis of the pinhole. The pinhole can have a rotationally symmetrical structure. In the case of multiple temperature sensors, temperature values can be recorded at the respective, i.e., different, positions on the pinhole.

[0028] The at least one temperature sensor can detect the temperature (i.e., the temperature value) at at least one predetermined point in time within the predetermined measuring interval (i.e., at the at least one predetermined or predefined measuring time). The measuring interval can begin with activation or irradiation of the laser beam. The measuring time can therefore correspond to an irradiation duration of the laser beam. The control unit determines the temperature of the pinhole diaphragm using the temperature sensor, preferably at the end of the measuring interval. In other words, the at least one measuring time of the at least one temperature sensor can be at the end of the measuring interval. The laser beam can be irradiated throughout the entire measuring interval.

[0029] The lookup table can be stored as a value table in the control unit. The lookup table can comprise a link between predetermined temperature values (or reference temperature values) for each temperature sensor and / or for the at least one predetermined measurement time with a respective beam parameter product and / or with a respective edge power of the laser beam for a number of boundary conditions. In other words, the lookup table can specify different values for the beam parameter product and / or for the edge power depending on several reference temperature values for each temperature sensor and for each predetermined measurement time, as well as depending on boundary conditions. The boundary conditions can comprise different fiber radii and / or different laser powers and / or different distances between the fiber socket or the fiber end and the pinhole, in short: pinhole distance.For this purpose, the lookup table can be represented by a data structure, wherein the data structure comprises a first attribute in which the predetermined temperature values or reference temperature values (possibly for different temperature sensors and / or for different measurement times) are stored, and at least one further attribute of a second attribute in which the different fiber radii are stored, a third attribute in which the different laser powers are stored, and a fourth attribute in which the different pinhole distances are stored, as well as a fifth attribute or result attribute in which the associated beam parameter products and / or the associated edge powers are stored.The data structure can comprise a plurality of rows, wherein each row comprises at least one combination of temperature value, fiber radius, laser power, and distance, which is linked to a corresponding beam parameter product and / or a corresponding edge power. Furthermore, each row can comprise at least one piece of position information or sensor identification and / or measurement time information with regard to the temperature value. Using the lookup table, the associated beam parameter product and / or the associated edge power can thus be read out based on a query consisting of a combination of temperature value, fiber radius, laser power, and / or pinhole distance. Therefore, no complex calculation or data analysis is necessary to determine the beam parameter product or the edge power during operation; instead, a beam parameter product or the edge power can be looked up based on the recorded temperature value.The lookup table is preferably initially provided offline, i.e., at the factory. It may be possible to update the lookup table after its initial provision, particularly via remote access.

[0030] Since an analytical (i.e., computational) relationship between the local temperature profiles at the position of at least one temperature sensor and the laser power absorbed at the pinhole is not possible, an indirect method is provided that assigns the temperature values and the absorbed edge power or the beam parameter product in a lookup table. Thus, for a given pinhole, the beam parameter product or the edge power can be determined precisely and efficiently for any laser fiber or laser source using the lookup table.

[0031] By determining the beam parameter product, the intensity distribution or power distribution P(r, z) for any beam cross-section of a laser beam can be determined according to the above-mentioned equation (1). In other words, the 86.5% envelope curve can be determined based on the determined beam parameter product. The most accurate knowledge of this envelope curve enables better, particularly predictive, control of the thermal load on a pinhole during laser processing.

[0032] The predetermined measurement interval can be a short irradiation period in the range of 0.1 seconds to 3 seconds. The length of the measurement interval can be selected based on the expected temperature rise caused by the laser radiation impinging on the pinhole. Such a short irradiation time significantly reduces the operating time of the device until a result for the beam parameter product or the edge power is available, since the system does not have to be in a steady state.

[0033] During the predetermined measuring interval, temperature values can be recorded discretely at different measuring times and / or continuously over the measuring interval.

[0034] The distance between the pinhole and the fiber socket or fiber end influences which portion of the laser beam's laser power can pass through the pinhole, while the remaining portion of the laser power is absorbed by the edge of the pinhole. There is a selected pinhole distance at which 86.5% of the laser power can pass through the pinhole opening. At this pinhole distance, the remaining 13.5% of the laser power is absorbed by the pinhole and converted into heat. At this selected pinhole position, the beam parameter product, which is defined as the product of the fiber radius and half the aperture angle of the laser beam, or more precisely, half the aperture angle of the 86.5% envelope of the laser beam, can be directly "read" by the measuring device according to the invention. The half aperture angle results from the tangent between the radius of the pinhole and the pinhole distance.

[0035] In one embodiment, the device further comprises a vertical adjustment device which is configured to set a distance or different distances of the pinhole to the fiber socket (ie to the end of the laser fiber accommodated therein).

[0036] The vertical adjustment device can comprise several suspensions at different positions, whereby the pinhole can be arranged at these different positions using the suspensions to set different predetermined distances to the fiber socket, i.e., to the output of the laser fiber. This vertical adjustment device has the advantage that various predefined pinhole distances can be easily, reliably, and reproducibly adjusted, thereby further increasing the accuracy in determining the beam parameter product or the edge power.

[0037] Advantageously, the vertical adjustment device comprises a guide rail on which the pinhole can be moved to set various predetermined distances, e.g., parallel to a beam axis of the laser beam. In this embodiment, the distance between the pinhole and the fiber socket is continuously adjustable. The advantage of the vertical adjustment device according to this embodiment lies in maximum flexibility in adjusting the pinhole distance.

[0038] The guide rail can run along or parallel to a beam axis of the laser beam and / or to an optical axis of the pinhole.

[0039] In one embodiment, the distance between the pinhole and the fiber socket is adjustable automatically or motorized by the control unit and / or manually by the user of the device.

[0040] In one embodiment of the device, the at least one temperature sensor is or comprises a resistance thermometer and / or a thermocouple. A PT-100 or PT-1000 temperature sensor is particularly suitable as a resistance thermometer.

[0041] In one embodiment of the device, the control unit is configured to detect a plurality of temperature values or a temporal temperature profile within the predetermined measuring interval using the at least one temperature sensor. In particular, the at least one temperature sensor can continuously detect the temperature of the aperture or the temporal temperature profile. The control unit can further be configured to determine a temporal temperature gradient from the temporal profile of the detected temperature and, based thereon, to determine the beam parameter product and / or the edge power of the laser beam.

[0042] The temporal profile of the temperature of the pinhole at the position of the at least one temperature sensor can be recorded by a time series measurement with a predetermined measurement frequency. By measuring a temporal profile, the measurement error in recording the temperature of the pinhole can be minimized. In one embodiment, the temperature measurements of the time series can be smoothed before determining the beam parameter product and / or the edge power, for example, by calculating a moving average.

[0043] A temporal temperature gradient can be created between the lowest temperature value or one of the first temperature values in the time series and the highest temperature value or one of the last temperature values in the time series, for example by calculating the difference, preferably with respect to the measurement interval. The temporal temperature gradient can also be determined by calculating a numerical derivative, for example using three-point or five-point models. It is advisable to determine the temporal temperature gradient from the recorded temperature profile using the same method as the corresponding temperature values stored in the lookup table.

[0044] Depending on the laser power, the irradiation time during which the laser beam is irradiated into the device or onto the aperture and / or the measurement interval in which at least one temperature value or the temporal progression of the temperature is recorded can be between 0.1 seconds and 3 seconds, or 0.5 seconds, 1 second, 1.5 seconds, 2 seconds, 2.5 seconds, or 3 seconds. In other words, the measurement interval can correspond to the irradiation time or be the irradiation time.

[0045] Experiments have shown that a copper pinhole in the device according to the invention heats up to 40 °C during a measurement interval of one second when irradiated with a laser fiber at average laser power. After 3 seconds, the copper pinhole can heat up to a temperature of 60-70 °C. Temperature increases of this magnitude are sufficiently reproducible and do not require active cooling of the pinhole, for example, by means of a liquid or gaseous coolant stream, which significantly reduces installation, transport, and operating costs. Even without cooling, the pinhole can cool down sufficiently quickly after a measurement interval to subsequently determine another beam parameter product or edge power after a short interruption.

[0046] In one embodiment of the device, the temperature sensor is arranged on a side of the pinhole facing away from the fiber socket and / or is accommodated in a recess, preferably a radial recess, in the pinhole.

[0047] This arrangement of the temperature sensor prevents the temperature sensor from being directly exposed to the laser beam during the measuring interval, so that sufficient measurement accuracy of the temperature of the pinhole can be guaranteed.

[0048] In one embodiment of the device, several temperature sensors are arranged at the same angular distance from one another or evenly distributed in the circumferential direction of the pinhole and / or arranged at different distances from the center of the pinhole.

[0049] In an advantageous embodiment, three temperature sensors are each arranged at an equal angular distance of 120° from one another, i.e. evenly distributed in the circumferential direction of the pinhole. In other words, the positions of the three temperature sensors can each have an angle of substantially 120° to one another. Furthermore, the positions of the three temperature sensors can each have different radial distances from the optical axis of the pinhole. This arrangement of the temperature sensors enables the parallel recording of three temperature values per measurement time, whereby the accuracy in determining the beam parameter product and / or the edge power can be further increased. For this purpose, it can be advantageous, if the pinhole is of one-piece and homogeneous design, to compare the three recorded temperature values or temperature profiles with one another, taking into account the thermal conductivity of the material from which the pinhole is made.For the arrangement of the three temperature sensors, for example, radial recesses can be provided on the perforated plate, in each of which a temperature sensor is arranged.

[0050] The recess can be formed as a blind bore. The recess can be milled. If multiple recesses are provided for multiple temperature sensors, it is advantageous to produce the recesses using the same method or procedure.

[0051] The advantage of using a pinhole aperture is that only the edge radiation of the laser beam is absorbed during the measurement interval, while the central radiation of the laser beam, which accounts for the majority of the intensity, can pass through the aperture. This prevents the pinhole aperture from heating up excessively during the determination of the beam parameter product or the edge power, which would otherwise require cooling.

[0052] In addition, a separate absorber that absorbs the edge radiation to be measured can be dispensed with, which considerably simplifies the construction and operation of the device.

[0053] The pinhole can be designed to be replaceable within the device. This allows for easy replacement if the pinhole becomes damaged. Furthermore, pinholes with different inner diameters can be used, e.g., to determine the absorbed edge power for a given pinhole spacing.

[0054] In one embodiment, the device further comprises a laser beam absorbing element for absorbing laser radiation passing through the pinhole, wherein the laser beam absorbing element is arranged in the housing downstream of the pinhole in the beam propagation direction.

[0055] In other words, in this embodiment, the pinhole can be arranged in the housing between the fiber socket and the laser beam absorbing element.

[0056] The laser beam-absorbing element can be a beam trap and / or a dummy sheet. The laser beam-absorbing element can be configured to prevent damage to the device housing or the floor and / or the escape of beam energy from the housing when determining the beam parameter product or the edge power.

[0057] In one embodiment, the pinhole has a known inner diameter. The control unit can further be configured to determine a marginal power of the laser beam using the lookup table based on the known fiber radius, the known laser power, the predetermined distance, the inner diameter of the pinhole, and the detected temperature.

[0058] For this purpose, values for the edge power for various (predetermined) inner diameters of the pinhole can be stored in another attribute of the lookup table. Alternatively or additionally, the edge power can be calculated using the beam parameter product, for example by calculating or approximating the integral of the function P(r, z) (see equation (1)) using the parameter radius r, where the parameter r is greater than or equal to the inner radius of the pinhole. In this way, the edge power of the laser fiber can be directly determined for a specific installation situation of a pinhole (i.e., for a specific pinhole spacing and a specific inner diameter) in a laser head.

[0059] Based on the determined edge power, the thermal load of the pinhole for any laser source in the laser material processing operation can be predicted, so that efficient and effective cooling of the pinhole can be set up.

[0060] In one embodiment of the device, the control unit comprises a memory unit on which the lookup table is stored. Alternatively or additionally, the device comprises a communication interface for wireless and / or wired communication with an external database.

[0061] The device can also have a user interface (UI) by means of which the user can configure a measurement interval and / or activate or terminate a measurement. The determined value for the beam parameter product and / or for the marginal power of the laser beam can be displayed to the user by means of the user interface. The user interface can also be configured to receive commands from the user for operating the device, in particular for setting at least one measurement time, for defining a measurement interval and / or for entering boundary conditions, and / or to forward them to the control unit. Alternatively or additionally, the user can configure a measurement interval and / or measurement times, activate or terminate a measurement by means of an application on a terminal device that is in communication with the device via the communication interface.end and / or read out the determined values for the beam parameter product and / or the edge power of the laser beam.

[0062] In one embodiment, the user interface comprises a query module that allows a user, after determining the beam parameter product of the laser beam, to query the expected edge power for a number of predetermined pinhole spacings or pinhole installation situations.

[0063] Storing the lookup table on an internal memory unit of the device has the advantage that the device can be offered as a stand-alone module or product that can determine or calculate the beam parameter product and / or the marginal power of the laser beam offline, i.e. without a network connection.

[0064] Advantageously, the device is designed as a mobile device.

[0065] A mobile device is specifically defined as a portable device and / or a device that can be moved to the respective location of the laser fiber to be measured with minimal installation and transport effort. This distinguishes the device from a stationary or immobile aperture test bench.

[0066] According to a further aspect of the invention, a method is provided for determining a beam parameter product and / or the edge power of a fiber-guided laser beam. The method comprises the following steps: inserting or receiving a laser fiber (i.e. a laser fiber connected to a laser source for generating the laser beam) with a known fiber radius into a fiber socket; irradiating the laser beam with a known laser power through the laser fiber during a predetermined measuring interval onto a pinhole arranged at a predetermined distance from the fiber socket; detecting a temperature of the pinhole at at least one predetermined measuring time within the predetermined measuring interval, e.g. by means of at least one temperature sensor arranged on the pinhole; and determining the beam parameter product orthe edge power of the laser beam based on the known fiber radius, the known laser power, the predetermined distance and the detected temperature using a lookup table.

[0067] It is advantageous to carry out the steps in the order given.

[0068] In other words, in a first step, the laser fiber can be connected to the device described above or inserted into the fiber socket in order to determine an unknown beam parameter product or edge power for a laser fiber with a known fiber diameter. The pinhole can preferably be adjusted to any distance from the fiber socket in the housing using a vertical adjustment device. The laser beam can be switched on with a known total or laser power, and the pinhole can be irradiated for a period appropriate to the total power of the laser beam, preferably between 0.1 seconds and 3 seconds, until the laser beam is switched off again.

[0069] At least one temperature sensor arranged on the pinhole can detect or measure at least one temperature or at least one temperature value and / or a predetermined number of discrete temperature values and / or a temperature profile that is at least partially continuous during the irradiation of the pinhole by the laser fiber. Based on this, the combination of the total power, the distance of the pinhole from the fiber end, the fiber diameter or fiber radius, and the measured temperature and / or temperature profile can be searched in the pre-created lookup table, and the beam parameter product correlated or linked to the combination and / or the edge power correlated or linked to the combination can be determined.

[0070] From the temporal course of the temperature, a temporal temperature gradient, ie a temperature gradient over the irradiation time or the measurement interval, can be determined.

[0071] In one embodiment of the invention, the method for increasing the accuracy can be carried out repeatedly for several distances between the pinhole and the fiber end, and the beam parameter products or edge powers determined from the lookup table can be arithmetically averaged.

[0072] In one embodiment, the method comprises, in the step of detecting a temperature of the pinhole, detecting a temporal profile of the temperature in the predetermined measurement interval and determining a temporal temperature gradient from the temporal profile of the detected temperature. In this embodiment, the step of determining the beam parameter product or the edge power of the laser beam can be performed using the lookup table based on the temporal temperature gradient.

[0073] By recording a temperature profile over the irradiation time compared to a (single) temperature value, the accuracy of the determined beam parameter product or edge power is increased, as the heat transfer to the pinhole during irradiation is better captured. A temperature gradient can be determined from the recorded temperature profile (particularly independent of the temperature of the pinhole at the start of the laser beam). The temperature gradient can result exclusively from the irradiation. In this way, measurements can also follow one another in close succession, whereby the pinhole does not have to have completely cooled down by the start of the subsequent measurement.

[0074] In an embodiment according to which the pinhole has a known inner diameter, the method further comprises determining an edge power of the laser beam based on the known fiber radius, the known laser power, the predetermined distance, the inner diameter of the pinhole and the detected temperature or the temporal temperature gradient by means of the lookup table, wherein the lookup table is preferably determined based on a three-dimensional and time-resolved simulation model.

[0075] In this embodiment, the absorbed edge power at the aperture can be directly determined for a specific arrangement consisting of a laser fiber with an unknown beam parameter product and a pinhole aperture (or aperture for short) with a specific geometry and a defined distance between the aperture and the fiber end (e.g., in a laser processing head that is already in operation). For this purpose, the laser fiber in question is connected to the disclosed device, and its pinhole aperture is adjusted to a predetermined distance.

[0076] The method also works if the inner diameter of the pinhole in the device according to the invention differs from that of a pinhole in the system under test. To achieve this, the distance of the pinhole to the fiber end in the device simply needs to be adjusted accordingly so that the resulting aperture angle of the remaining (unshielded) laser beam in the propagation direction behind the pinhole is equal to the aperture angle of the pinhole in the system under test.

[0077] In this way, based on the determined beam parameter product, the marginal power with which the aperture is subjected in the respective installation situation during operation can be determined from the previously created lookup table.

[0078] The lookup table can be created using a simulation model. The simulation model can be, for example, a 3D CFD simulation based on the finite volume method or a finite element simulation. The spatially and temporally resolved simulation uses numerical approximation methods to solve the nonlinear higher-order differential equations that describe the three-dimensional, time-dependent heat conduction through the pinhole and the convective heat transfer to the ambient air around the pinhole.

[0079] An estimated laser edge power can be specified as a boundary condition on the inner surface of the pinhole, i.e., the surface defining the opening of the pinhole facing the fiber socket. This edge power on the pinhole can be determined using formulas (1) and (2).

[0080] In a special embodiment of the invention, results of experiments in more complex test devices, in particular in a stationary aperture test bench according to the state of the art, which determines the edge power on the pinhole using a calorimetric method, can be used to specify formula (1) by applying correction factors.

[0081] The simulation can model the irradiation time of the pinhole, which, depending on the laser power, can range from 0.1 seconds to 3 seconds. The simulations provide the temperature values and / or temperature profiles, in particular the temporal temperature gradients, at the position of at least one temperature sensor over the simulation time or irradiation time.

[0082] To validate the simulation model, experiments can be conducted with the device described above. For this purpose, a laser fiber can be connected to the arrangement, the fiber diameter and beam parameter product of which are already known with sufficient accuracy in advance. The pinhole can then be irradiated with a known laser power at various distances from the fiber socket, and the measured values of at least one temperature sensor can be recorded. In the simulation model, the boundary conditions of beam parameter product and fiber diameter can be adjusted to match the laser fiber from the experiments. To do this, the parameters of the simulation model can be varied until the simulation results agree with the measured values.

[0083] In a second step, the experimentally validated simulation model can be solved with different beam parameter products, fiber diameters, laser powers and distances of the pinhole from the fiber socket as boundary conditions and the temperature gradients correlated to the respective boundary conditions at the positions of at least one temperature sensor can be determined.

[0084] From the obtained results, the lookup table can be created, with the help of which temperature gradients measured during operation can be quickly assigned to a correlated beam parameter product or a marginal power on the pinhole. Short description of the characters

[0085] Embodiments of the disclosure are illustrated in the figures and are described in more detail below. They show: Fig. 1 a schematic representation of a Gaussian laser beam; Fig. 2 shows a first embodiment of an apparatus for determining a beam parameter product of a laser beam according to one aspect of the invention; Fig. 3 shows a second embodiment of an apparatus for determining a beam parameter product of a laser beam according to one aspect of the invention, in which the edge power of the laser beam can also be determined; Fig. 4a and Fig. 4b shows two embodiments of a pinhole of the device for determining a beam parameter product and / or a marginal power of the laser beam according to one aspect of the invention; and Fig. 5 is a flowchart illustrating an embodiment of the method for determining a beam parameter product and / or a marginal power of the laser beam according to one aspect of the invention. Detailed description of the characters

[0086] In the following, unless otherwise stated, the same reference symbols are used for identical and equivalent elements.

[0087] Fig. Figure 1 shows a schematic representation of a Gaussian laser beam emanating from the end of a laser fiber and propagating in the z-direction. In every beam cross-section along the z-direction, the intensity distribution of the laser fiber follows a Gaussian normal distribution. Conventionally, the beam waist w(z) is defined as a central region of the Gaussian curve, encompassing 86.5% of the laser power, i.e., the area below the Gaussian curve.

[0088] This allows an envelope curve to be specified along the propagation direction, as explained above. However, to determine the envelope or beam waist, precise knowledge of the beam parameter product bpp is required.

[0089] Fig. Figure 2 shows a first embodiment of the device 1 according to one aspect of the invention. The device 1 comprises a housing 20 with a fiber socket 21, in which a laser fiber 10 can be received, e.g., by means of a fiber connector (not shown). When the laser fiber 10 is received in the fiber socket 21, the fiber end can be flush with the fiber socket 21. The laser fiber 10 has a known fiber radius w0.

[0090] The device 1 can be oriented vertically, i.e., the fiber socket 21 can be arranged at the top of the housing 20. In this case, the laser fiber 10 is inserted into the fiber socket 21 facing downwards, i.e., toward the floor.

[0091] The device 1 further comprises a pinhole 30, which is arranged in the housing 20 such that the laser beam 11 of the laser fiber 10 can impinge (directly and immediately) on the pinhole 30. In other words, the optical axis of the pinhole 30 is aligned coaxially with the beam axis 12. The pinhole 30 is configured to at least partially absorb the incoming laser radiation, i.e., to absorb marginal radiation of the incoming laser radiation. The pinhole 30 can be made of copper. The pinhole 30 can furthermore have an anti-reflective coating. The pinhole 30 can be formed in one piece. The pinhole 30 can be adjusted to a predetermined distance z from the fiber socket 21 or from the fiber end by means of a vertical adjustment device 22.

[0092] The vertical adjustment device 22 can be a suspension that specifies predetermined installation positions of the pinhole 30. Alternatively or additionally, the vertical adjustment device can comprise a clamping device. Furthermore, the vertical adjustment device can comprise a guide rail (not shown) arranged parallel to the propagation direction of the laser radiation or the beam axis 12. The distance between the pinhole 30 and the fiber socket 21 can be continuously adjustable on the guide rail. In addition, the guide rail can have a scale suitable for reading the distance of the pinhole from the outside.

[0093] A temperature sensor 40 can be arranged on the pinhole 30. The temperature sensor 40 is preferably arranged on the side facing away from the fiber socket 21 in order to be protected from direct laser radiation. The temperature sensor 40 can be arranged in a recess 31 of the pinhole. Exemplary embodiments of this are shown in Fig. 4a and Fig. 4b. The temperature sensor 40 is configured to detect a temperature of the pinhole 30.

[0094] The device 1 further comprises a control unit 50 configured to determine the beam parameter product bpp and / or the edge power based on the known fiber radius w0, the distance z of the pinhole 30 to the fiber socket 21 or to the end of the laser fiber 10, and the detected temperature. To this end, the control unit 50 can control irradiation of the pinhole 30 by means of the laser beam 11 in a measuring interval and, using the temperature sensor, can detect at least one temperature, preferably a temperature profile of the pinhole, during the measuring interval. The control unit 50 can further be configured to form a temporal temperature gradient from a detected temperature profile. The control unit 50 determines the beam parameter product bpp or the edge power P aby means of a lookup table, i.e. by searching for a specific combination of fiber radius w0, distance z and temperature of the pinhole 30, which is assigned to a beam parameter product bpp correlated therewith.

[0095] Fig. 3 shows a further embodiment of the device 1. In this embodiment, an absorbing element 60, e.g. a beam trap, is advantageously arranged in the housing 20 after the pinhole 30. Otherwise, this applies to the embodiment of Fig. 2 Explained also for the design of the Fig. 3.

[0096] Fig. Figure 4a shows an exemplary embodiment of a pinhole 30. It shows a cross-section through the pinhole 30 (i.e., a cross-section perpendicular through an optical axis of the pinhole or to the beam axis of the laser beam), which can be arranged in a diaphragm holder. The pinhole 30 itself and / or its opening 32 can be round, in particular circular.

[0097] The pinhole 30 has the temperature sensor 40, which is arranged at a specific radial distance from a center point of the opening 32. The temperature sensor 40 can be arranged on the side of the pinhole 30 facing away from the fiber socket 21 in order to be protected from direct laser radiation. The temperature sensor 40 can be accommodated in a recess 31 in the pinhole. The recess 31 can extend radially and can preferably be formed on the side of the pinhole 30 facing away from the fiber socket 21. The recess 31 can be a blind bore.

[0098] Fig. 4b shows a further embodiment of a pinhole 30, which, in comparison to the example of Fig. 4a has three temperature sensors 40. The temperature sensors 40 can each be arranged in a radial recess 31. The radial recesses 31 are preferably arranged at rotationally symmetrical positions of the pinhole 30, ie, evenly distributed in the circumferential direction of the pinhole. In this embodiment, the temperature sensors 40 can each have an angle of substantially 120° to one another. Furthermore, the temperature sensors can each have different radial distances to the optical axis of the pinhole 30. This enables the detection of a radial temperature profile in the pinhole, from which a local temperature gradient can be formed. Otherwise, the Fig. 4a also apply to the embodiment of the Fig. 4b.

[0099] Fig. 5 summarizes an embodiment of the method for determining the beam parameter product bpp or the edge power of the laser beam using the device described above in a flow chart.

[0100] In a first step S1, a laser fiber 10 is provided, ie inserted into a fiber socket 21 of the device 1, preferably by means of a fiber plug.

[0101] In a second step S2, which is merely optional, a predetermined distance z between a pinhole 30 and the fiber socket 21 can be set. The distance z defines the aperture angle of the laser beam 11 after the pinhole 30. Furthermore, the smaller the distance z between the pinhole 30 and the fiber socket 21, the greater the edge power of the laser fiber 10 with which the pinhole 30 is exposed.

[0102] In a third step S3, the laser beam is irradiated through the laser fiber 10 onto the pinhole 30 during a predetermined measurement interval. In other words, a laser source connected to the laser fiber 10 is activated to generate the laser beam 11.

[0103] The laser source can be activated by the control unit 50 and / or manually by a user.

[0104] In a fourth step S4, the temperature of the pinhole 30 is detected, e.g., at least one predetermined measuring time during the measuring interval. For this purpose, at least one temperature sensor 40 is provided, which is arranged on the pinhole, as described with reference to the Fig. 2, Fig. 3, Fig. 4a and Fig. 4b. The temperature detection can be monitored or controlled by the control unit 50.

[0105] In a fifth step S5, the beam parameter product bpp of the laser beam 11 is determined. For this purpose, the control unit 50 is configured to detect the temperature of the pinhole 30 within a predetermined measuring interval and, based on the known fiber radius w0, the known laser power P0, the predetermined distance z, and the detected temperature, to determine the beam parameter product bpp and / or the edge power P a of the laser beam 11.

[0106] In one embodiment, steps S2 to S5 can be repeated for different predetermined distances z. The values of the beam parameter product bpp and / or the edge power P determined for the different predetermined distances z a can then be arithmetically averaged. The arithmetic mean can be used as the result or final value for the beam parameter product bpp or for the edge power P aThis allows the accuracy of the determination of the beam parameter product bpp and / or the edge power P a be increased.

[0107] The control unit 50 may be configured to calculate the edge power P based on the known fiber radius w0, the known laser power P0, the predetermined distance z, the inner diameter of the pinhole and the detected temperature using the lookup table. a of the laser beam 11.

[0108] By developing an indirect method, in which a look-up table or value table created in advance (e.g. by simulations) is used, a complex direct calorimetric measurement of the marginal power P aand / or the beam parameter product bpp of a Gaussian laser beam. This eliminates the need for a separate absorber and, if necessary, a liquid or gaseous coolant. The irradiation time and thus the operating time until a result for the beam parameter product bpp or the edge power P a The laser beam's operating time can also be significantly reduced, as the system does not need to be in a steady state. By determining temporal and / or spatial temperature gradients of a pinhole over a short irradiation period and comparing them with the lookup table, an unknown beam parameter product bpp of a laser fiber with a Gaussian beam profile can be determined. Furthermore, the beam parameter product bpp can be used to directly determine the edge power P a in a specific installation situation of a pinhole in a laser head of a laser processing system. List of reference symbols 1 Device for determining a beam parameter product and / or a marginal power of a laser beam 10 laser fibers 11 Laser beam 12 Beam axis of the laser beam 11 20 housings 21 Fiber socket 22 Vertical adjustment device 30 pinhole 31 recess 32 aperture of the pinhole 40 Temperature sensor 50 control unit 60 Absorbing element, especially beam trap

Claims

[1] Device (1) for determining a beam parameter product (bpp) and / or a marginal power of a fiber-guided laser beam (11), the device (1) comprising: - a housing (20) with a fiber socket (21) for receiving a laser fiber (10) with a known fiber radius (w0); - a pinhole diaphragm (30) arranged coaxially at a predetermined distance (z) from the fiber socket (21) in the housing (20) in order to at least partially absorb edge radiation of the laser beam (11) impinging thereon; - at least one temperature sensor (40) arranged on the pinhole (30) for detecting a temperature (T) of the pinhole (30); and - a control unit (50) which is arranged to detect the temperature (T) of the pinhole (30) by means of the temperature sensor (40) within a predetermined measuring interval, if, during the measuring interval, the laser beam (11) is irradiated with a known laser power (P0) through the laser fiber (10) accommodated in the fiber socket (21); wherein the control unit (50) is further configured to determine the beam parameter product (bpp) and / or the edge power of the laser beam (11) by means of a lookup table based on the known fiber radius (w0), the known laser power (P0), the predetermined distance (z) and the detected temperature (T). [2] Device (1) according to claim 1, further comprising a vertical adjustment device (22) arranged to set the predetermined distance (z). [3] Device (1) according to claim 2, wherein the vertical adjustment device (22) comprises a guide rail on which the pinhole diaphragm (30) is displaceable along a beam axis (12) of the laser beam (10) for setting various predetermined distances (z). [4] Device (1) according to one of the preceding claims, wherein the temperature sensor (40) is a resistance thermometer or a thermocouple. [5] Device (1) according to one of the preceding claims, wherein the control unit (50) is configured to detect a temporal profile of the temperature (T) within the predetermined measuring interval, to determine a temporal temperature gradient from the temporal profile of the detected temperature (T) and, based thereon, to determine the beam parameter product (bpp) and / or the edge power of the laser beam. [6] Device (1) according to one of the preceding claims, wherein the temperature sensor (40) is arranged on a side of the pinhole (30) facing away from the fiber socket (21) and / or is accommodated in a recess (31) in the pinhole (30). [7] Device (1) according to one of the preceding claims, wherein a plurality of temperature sensors (40) are evenly distributed in the circumferential direction of the pinhole (30) and / or arranged at different radial distances on the pinhole (30). [8] Device according to one of the preceding claims, further comprising a laser beam absorbing element (60) for absorbing laser radiation passed through the pinhole (30), wherein the laser beam absorbing element (60) is arranged in the housing (20) downstream of the pinhole (30) in the beam propagation direction. [9] Device (1) according to one of the preceding claims, wherein the pinhole (30) has a known inner diameter, and wherein the control unit (50) is further configured to determine the edge power of the laser beam (11) based on the known fiber radius (w0), the known laser power (P0), the predetermined distance (z), the inner diameter of the pinhole (30) and the detected temperature (T) by means of the lookup table. [10] Device (1) according to one of the preceding claims, wherein the control unit (50) comprises: - a storage unit on which the lookup table is stored; and / or - a communication interface to communicate wirelessly and / or wired with an external database. [11] Device (1) according to one of the preceding claims, wherein the device (1) is designed as a mobile device. [12] Method for determining a beam parameter product (bpp) of a fiber-guided laser beam (11), the method comprising the following steps: - inserting (S1) a laser fiber (10) with a known fiber radius (w0) into a fiber socket (21); - irradiating (S3) the laser beam (11) onto a pinhole (30) arranged at a predetermined distance (z) from the fiber socket (21) through the laser fiber (10) during a predetermined measuring interval with a known laser power (P0); - detecting (S4) a temperature (T) of the pinhole (30) within the predetermined measuring interval; and - Determining (S5) the beam parameter product (bpp) and / or the edge power of the laser beam (11) based on the known fiber radius (w0), the known laser power (P0), the predetermined distance (z) and the detected temperature (T) by means of a lookup table. [13] The method of claim 12, wherein detecting the temperature (T) of the pinhole (30) comprises: - recording a temporal course of the temperature (T) in the predetermined measuring interval; and - Determining a temporal temperature gradient from the temporal course of the detected temperature (T); wherein the beam parameter product (bpp) and / or the edge power of the laser beam is determined based on the temporal temperature gradient using the lookup table. [14] A method according to any one of claims 12 or 13, wherein the apertured diaphragm (30) has a known inner diameter, and the method further comprises: - Determining the edge power of the laser beam (11) based on the known fiber radius (w0), the known laser power (P0), the predetermined distance (z), the inner diameter of the pinhole (30) and the detected temperature (T) or the temporal temperature gradient by means of the lookup table. [15] Method according to one of claims 12, 13 or 14, wherein the steps of irradiating (S3) the laser beam (11), detecting (S4) the temperature (T) of the pinhole diaphragm (30) and determining (S5) the beam parameter product (bpp) and / or the edge power of the laser beam (11) are repeated for various predetermined distances (z), and an arithmetic mean value for the beam parameter product (bpp) and / or the edge power of the laser beam (11) is formed from the values of the beam parameter product (bpp) and / or the edge power of the laser beam (11) determined for the respective distances (z).

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