DE102017217145A1 - Laser system and method for producing a top-hat approximated beam profile - Google Patents

Abstract

The laser system (1) according to the invention has a beam homogenizer (5) for homogenizing a substantially Gaussian beam profile of a laser light to a homogenized beam profile. The homogenization takes place using an optical element (7) of the beam homogenizer (5), which is configured as a refractive, reflective and / or diffractive optical element. The laser system (1) further comprises a multi-mode fiber (5) and is configured to mode-excite in the multi-mode fiber using the homogenized beam profile.

Description

The present invention relates to a laser system for producing a laser beam having a beam profile which approximates a top-hat beam profile (box profile) as well as to a method for producing a top-hat-approximated beam profile.

Beam homogenizers for laser light are known in the prior art that are configured to generate a laser beam having a beam profile that approximates an ideal top hat beam profile.

These conventional beam homogenizers usually have a field mapper or a lens array. A field mapper generates an outgoing wavefront in which, in the course of the further free propagation, the intensity of the laser light is resorted such that the top hat-approximated beam profile is generated. In typical embodiments, the field mapper comprises a plurality of aspherical lenses or a diffractive optical element. However, it has been found that with the use of field mappers, the quality of the top hat approximated beam profile is sensitive to the beam profile of the incident laser beam impinging on the field mapper. Furthermore, the quality is sensitive to the orientation of the field mapper relative to the incoming laser beam. This can impair the homogenization, in particular if no complicated adjustment devices are provided.

These dependencies are less pronounced when the beam homogenizer is based on a lens array and, in addition, field-mappers focus much less on top-hat performance compared to lens arrays. Furthermore, when lens arrays are used, the propagation distance over which the beam profile remains homogeneous is comparatively low compared to the use of field mappers.

There is therefore a need for laser systems for generating a laser beam with a top-hat-approximated beam profile, which have a high tolerance for adjustment errors and / or with respect to the beam profile of the incident laser beam. This should be concentrated as much performance in the top hat area. At the same time, there is a need for laser systems that produce a laser beam with a top-hat-approximated beam profile that remains homogeneous over as large a propagation distance as possible.

A first aspect of the invention relates to a laser system comprising a beam homogenizer for homogenizing a Gaussian or substantially Gaussian beam profile of a laser light to a homogenized beam profile. The substantially Gaussian beam profile represents a two-dimensional energy density distribution or power density distribution of the laser light. The homogenization is carried out using an optical element of the beam homogenizer. The optical element is configured as a refractive, reflective and / or diffractive optical element. The laser system further comprises a multi-mode fiber. The laser system is configured for mode excitation in the multimode fiber using the homogenized beam profile.

The beam profiles may be transverse beam profiles relative to a propagation direction of the laser light. The energy density distribution may represent an integration of the power density distribution over one or more laser pulses of the laser light.

The Gaussian or substantially Gaussian beam profile may be a beam profile of an incident laser beam arriving at the beam homogenizer. The incoming laser beam can propagate in a gaseous medium (such as air), in a vacuum, in a liquid or in a solid. The Gaussian or substantially Gaussian beam profile may be a beam profile of a beam in an optical fiber adjacent to the beam homogenizer. The optical waveguide may be an optical fiber, in particular a monomode fiber, hollow core fiber (photonic crystal fiber) or a multimode fiber.

The incoming laser beam may be a Gaussian beam (also referred to as a TEM ₀₀ mode) or substantially a Gaussian beam. Higher order modes than the TEM ₀₀ mode of the Gaussian beam may be suppressed or absent in the incoming laser beam. An M ² factor (diffraction index) of the incoming laser beam may be less than 1.7 prefers 1.5 and especially preferred 1.2 or as 1.1 , When the incident laser beam propagates in an optical fiber, such as in an optical fiber, in particular a monomode fiber, the incident laser beam may be the transverse fundamental mode or substantially the transversal fundamental mode of the optical fiber. Higher order modes than the transverse fundamental mode may be suppressed or absent.

The laser system may be configured such that the laser light propagates within the beam homogenizer at least partially in a gaseous medium (such as air) or in vacuum. In other words, the beam homogenizer can cause the mode excitation in the multimode fiber by means of free-space fiber coupling.

The beam homogenizer may be arranged in the laser light between a light exit surface of a core of an input fiber and a light entry surface of a core of the multimode fiber. The beam homogenizer may be arranged adjacent to or spaced from the light entry surface and / or from the light exit surface.

The mode excitation may include coupling at least a portion of the laser light into the multimode fiber.

The homogenized beam profile generated by the beam homogenizer may be located within its homogenized beam portion of the laser light which extends along a propagation direction of the laser light. At each position within the homogenized beam section, the beam profile of the laser light can be homogenized in each case in comparison to the beam profile of the laser beam which arrives at the beam homogenizer. The homogenized beam section may have a focus of the laser light or represent a focus. However, it is also conceivable that the focus is arranged outside the homogenized beam section, that is to say that the homogenized beam section is free from a focus. As seen along a propagation direction of the laser light, a position of the focus may correspond to a position of the light entrance surface of the multi-mode fiber. Alternatively, the position of the focus may be spaced from the position of the light entrance surface. The position of the light entry surface may be within the homogenized beam section.

A core of the multimode fiber may have a diameter which is greater than 30 μm or greater than 100 μm, in particular greater than 200 μm.

The refractive optical element may have an aspherical optically effective refractive surface. This surface may be part of an aspherical lens. The reflective optical element may have an aspherical optically effective reflective surface. This surface can be part of an aspherical mirror. The refractive and / or reflective optical element may be an array optics, in particular a lens array.

The homogenized beam profile may have a circular or substantially circular cross-section. However, there are also other cross-sectional geometries, such as rectangular or hexagonal conceivable. A cross-sectional geometry of the homogenized beam profile may correspond to a cross-sectional geometry of the core of the multi-mode fiber.

In a further aspect, the laser system according to the invention comprises a beam homogenizer for homogenizing laser light having a substantially Gaussian beam profile, which represents a two-dimensional energy or power density distribution of the laser light, into a homogenized beam profile, the homogenization using at least one optical element of the beam homogenizer a multispot beam profile is generated from the Gaussian beam profile and configured as a refractive, reflective and / or diffractive optical element, and a multimode fiber of the beam homogenizer configured to multi-mode excitation in the multi-mode fiber using the multi-spot beam profile.

Overall, the multispot mode excitation according to the invention with a suitable weighting of the individual spots results in the coupling conditions for modes of different symmetry groups being fulfilled simultaneously and a homogenized beam profile results at the output of the multimode fiber. In particular, for example, radially symmetric and radially anti-symmetric modes are excited (on- and off-axis coupling). This procedure increases the number of excited modes and provides better mixing (modes of different symmetry groups), thus reducing the interference contrast at the fiber output.

According to one embodiment, the homogenized beam profile has a plateau irregularity with an index which is less than 0.3 or less than 0.2 or less than 0.1 , The plateau irregularity can be defined according to the standard ISO 13694: 2015.

According to one embodiment, the homogenized beam profile has a steepness with a characteristic number less than 0.3 or less than 0.2 or less than 0.1 , The slope can be related to threshold values of 10% and 90% of a maximum value of the homogenized beam profile. The slope can be defined according to the standard ISO 13694: 2015.

In another embodiment, a proportion of at least 80% of a total energy or power of the homogenized beam profile has a beam uniformity which is more uniform than ± 10% or more uniform than ± 5% or more uniform than ± 2%. Beam uniformity may be defined according to ISO 13694: 2015.

According to a further embodiment, the laser system is configured such that the homogenized beam profile is corresponding or substantially equivalent to a super Gaussian beam profile. Beam profile with an order greater than or equal to 5 or greater equal 7 or greater equal 10 ,

According to another embodiment, the laser system is configured such that the homogenized beam profile is generated substantially at a light entrance surface of a core of the multi-mode fiber, as viewed along a propagation direction of the laser light. In other words, a position of the homogenized beam profile along a propagation direction of the laser light may be equal to or substantially equal to a position of a light entrance surface of the multi-mode fiber.

According to a further embodiment, a transverse extent of the homogenized beam profile essentially corresponds to a light entry surface of the core of the multimode fiber. The homogenized beam profile can be congruent or essentially congruent to the light entry surface.

According to another embodiment, the beam homogenizer is configured such that the laser light converges on the light entry surface of the core of the multi-mode fiber. A convergence angle of the laser light at the light entry surface of the core of the multimode fiber may correspond to an acceptance angle of the multimode fiber or substantially correspond to the acceptance angle.

According to a further embodiment, the laser system further comprises a laser amplifier for amplifying at least a portion of the laser light. The laser amplifier may be located downstream of the multimode fiber.

According to a further embodiment, at least a portion of the multimode fiber is configured such that at least a portion of the laser light can be amplified by means of the portion. At least a portion of the multi-mode fiber may be formed as an active optical fiber. A core of the active optical fiber may form an optically active medium of a fiber amplifier of the laser system. The core may be doped, for example, with rare earth metal ions. The laser system may be configured to couple pumping light into the core of the active optical fiber or into the cladding of the fiber. The pump light may be generated by a pump laser of the laser system.

According to a further embodiment, the beam homogenizer has imaging optics, which can be configured, for example, as 4f imaging optics. The beam homogenizer can generate an intermediate focus and / or a homogenized intermediate beam profile in an object plane of the imaging optics, which is homogenized in comparison to the beam profile of the laser beam which arrives at the beam homogenizer. The homogenized intermediate beam profile may be in the intermediate focus. As seen along the propagation direction of the laser light, a position of an image plane of the imaging optics which is optically conjugate to the object plane may be equal to or substantially equal to a position of the light entrance surface of the core of the multi-mode fiber.

In accordance with a further embodiment, the beam homogenizer has collimation optics. The collimating optics can be arranged downstream of the input fiber of the laser system and / or upstream of the refractive, reflective and / or diffractive optical element.

According to a further embodiment, the beam homogenizer has a beam matching optics for beam widening and / or beam narrowing of the laser light. The beam matching optics may be located downstream of the input fiber and / or upstream of the refractive, reflective and / or diffractive optical element. The beam-matching optics may be located downstream of the collimating optics. Alternatively or additionally, the collimating optics may be configured as beam-matching optics at the same time. In other words, the collimating optics may have a variable focal length for beam matching. A magnification of the beam-matching optics may be controllably variable. For different magnification values, the beam-matching optics may be afocal. Downstream of the diffractive optical element, a focusing optics can be arranged, which focuses the laser light into the intermediate focus.

According to a further embodiment, the optical element of the beam homogenizer is configured as a diffractive optical element. Alternatively, the diffractive optical element may be configured as a refractive and / or reflective optical element. The diffractive optical element may have a diffractive structure which is arranged in the laser light. The diffractive structure can be configured as a diffractive phase structure and / or as a diffractive amplitude structure. The diffractive structure may be rotationally asymmetric or axis unsymmetrical relative to a beam axis of the laser light.

The diffractive optical element may have a statistical or substantially random diffractive structure. The statistical or substantially statistical diffractive structure may be a statistical or substantially random diffractive phase and / or diffractive amplitude structure. For example, the statistical or substantially statistical diffractive structure may be calculated based on a statistical (ie random) or substantially randomly selected amplitudes and / or phase distribution for the plane in which the homogenized beam profile or the intermediate beam profile is generated.

According to another embodiment, the laser system comprises a fiber laser configured to generate at least a portion of the laser light. The fiber laser can be arranged upstream of the beam homogenizer. A laser medium of the fiber laser may comprise a single-mode fiber core.

According to another embodiment, the beam homogenizer is configured so that a spatial frequency spectrum of the homogenized beam profile is suppressed for each frequency value above a transverse boundary spatial frequency of the multi-mode fiber. Due to the suppressed part of the spatial frequency spectrum, the homogenized beam profile may deviate from an ideal top hat beam profile. A maximum value of the spatial frequency spectrum of the homogenized beam profile at the light entry surface may be corresponding to or substantially corresponding to the transverse boundary spatial frequency of the multimode fiber.

According to a further embodiment, the laser system further comprises a focusing system, which is arranged downstream of the multi-mode fiber. The focusing system may be configured to extract and focus light from a transport fiber. The focusing system may be configured to generate a focus of the extracted light on a surface of an object to be processed. The transport fiber may be the multimode fiber in which the modes were excited by the homogenized beam profile. Alternatively, the transport fiber may be an optical fiber located downstream of the multimode fiber. The transport fiber may be configured as a multimode fiber. The focusing system may comprise a collimating optics, a scanning system and / or a focusing optics. The focusing optics can be arranged downstream of the collimating optics.

To generate the multispot beam profile, the beam homogenizer preferably has at least one diffractive optical element with a one-dimensional or two-dimensional phase mask or alternatively at least one refractive optical element with a one- or two-dimensional phase mask, in particular a one- or two-dimensional microlens array. Using two diffractive optical elements, the first element can generate the target beam profile and the second element can cure phase noise to produce a propagating supergauss / top hat.

Finally, the invention also relates to a method for homogenizing laser light having a substantially Gaussian beam profile, which represents a two-dimensional energy or power density distribution of the laser light, to a homogenized beam profile, in particular to a TopHat beam profile, wherein the beam profile in a multimode fiber to a mode excitation leads.

The laser system according to the invention can be used for generating an input beam for a laser amplifier or additionally or alternatively for generating a laser beam for material processing. The material processing may include laser ablation, laser cutting, laser drilling, laser welding, and / or laser marking.

• 1 eine schematische Darstellung eines erfindungsgemäßen Lasersystems gemäß einer ersten Ausführungsform;
• 2a, 2b eine transversale Leistungsdichteverteilung an einer Lichtaustrittsfläche einer Multimodenfaser bei einer Modenanregung mit Hilfe eines Gaußstrahls (2a) und eines homogenisierten Strahlprofils (2b);
• 3a, 3b eine Illustration zur Berechnung der Plateaugleichförmigkeit und der Strahlgleichförmigkeit eines homogenisierten Strahlprofils, welches durch einen Strahlhomogenisierer des in 1 gezeigten Lasersystems erzeugt wird;
• 4a eine transversale Phasenverteilung eines Laserlichts in einer Ebene eines diffraktiven optischen Elements des Strahlhomogenisierers des in 1 gezeigten Lasersystems;
• 4b eine transversale Leistungsdichteverteilung des Laserlichts in einer Ebene einer Lichteintrittsfläche einer Multimodenfaser des in 1 gezeigten Lasersystems;
• 4c eine transversale Phasenverteilung des Laserlichts in der Ebene der Lichteintrittsfläche der Multimodenfaser des in 1 gezeigten Lasersystems;
• 5a eine transversale Phasenverteilung eines Laserlichts in einer Ebene einer statistischen diffraktiven Phasenstruktur eines Strahlhomogenisierers in einem erfindungsgemäßen Lasersystem gemäß einem zweiten Ausführungsbeispiel;
• 5b eine transversale Leistungsdichteverteilung des Laserlichts in einer Ebene einer Lichteintrittsfläche einer Multimodenfaser im Lasersystem gemäß dem zweiten Ausführungsbeispiel;
• 5c eine transversale Phasenverteilung des Laserlichts in der Ebene der Lichteintrittsfläche der Multimodenfaser im Lasersystem gemäß dem zweiten Ausführungsbeispiel;
• 6 eine schematische Darstellung eines erfindungsgemäßen Lasersystems gemäß einer zweiten Ausführungsform;
• 7a-7c das am Ende einer Multimodefaser gemessene Strahlprofil bei einem zentral (7a) und mit unterschiedlichen Offsets ( 7b, 7c) eingekoppelten Grundmodestrahl;
• 8a, 8b die Phasenmaske eines eindimensionalen, diffraktiven optischen Elements und das zugehörige Intensitätsprofil; und
• 9a, 9b die Phasenmaske eines zweidimensionalen, diffraktiven optischen Elements und das zugehörige Intensitätsprofil.

Further advantages and advantageous embodiments of the subject invention will become apparent from the description, the claims and the drawings. Likewise, the features mentioned above and the features listed further can be used individually or in combination in any combination. The embodiments shown and described are not to be understood as exhaustive enumeration, but rather have exemplary character for the description of the invention. Show it:

• 1 a schematic representation of a laser system according to the invention according to a first embodiment;
• 2a . 2 B a transverse power density distribution at a light exit surface of a multimode fiber in a mode excitation with the aid of a Gaussian beam ( 2a) and a homogenized beam profile ( 2 B) ;
• 3a . 3b an illustration for calculating the plateau uniformity and the beam uniformity of a homogenized beam profile, which by a beam homogenizer of the in 1 shown laser system is generated;
• 4a a transverse phase distribution of a laser light in a plane of a diffractive optical element of the beam homogenizer of the in 1 shown laser system;
• 4b a transversal power density distribution of the laser light in a plane of a light entrance surface of a multimode fiber of the in 1 shown laser system;
• 4c a transverse phase distribution of the laser light in the plane of the light entry surface of the multimode fiber of the in 1 shown laser system;
• 5a a transverse phase distribution of a laser light in a plane of a statistical diffractive phase structure of a beam homogenizer in a laser system according to the invention according to a second embodiment;
• 5b a transverse power density distribution of the laser light in a plane of a light entrance surface of a multi-mode fiber in the laser system according to the second embodiment;
• 5c a transverse phase distribution of the laser light in the plane of the light entrance surface of the multi-mode fiber in the laser system according to the second embodiment;
• 6 a schematic representation of a laser system according to the invention according to a second embodiment;
• 7a-7c the beam profile measured at the end of a multimode fiber at a central ( 7a) and with different offsets ( 7b . 7c) coupled basic mode beam;
• 8a . 8b the phase mask of a one-dimensional, diffractive optical element and the associated intensity profile; and
• 9a . 9b the phase mask of a two-dimensional, diffractive optical element and the associated intensity profile.

The 1 schematically shows the structure of a laser system 1 according to a first embodiment. The laser system 1 is for editing an object surface 25 a workpiece configured. Such a machining of a workpiece may, for example, be a laser ablation, a laser welding or a laser marking.

The laser system 1 is configured to have a machining focus in a machining area 21 to create. At least at one position within the machining focus 21 the laser light has a beam profile which, in a good approximation, corresponds to a top hat beam profile (also referred to as a flat top). The Top Hat beam profile is an ideal beam profile in the form of a box. For example, a beam profile that closely approximates a top hat beam profile may be a higher order super Gaussian profile. Many applications in the area of material processing benefit from a multi-mode beam profile, which corresponds as closely as possible to such a top-hat beam profile.

The laser system 1 has a laser 24 which laser light is generated and configured in the embodiment shown as a fundamental mode fiber-coupled laser, for example diode laser with single mode coupling. Ground mode fiber lasers provide high flexibility in temporal and spectral pulse shaping for the laser light produced. The laser light of the fundamental mode fiber laser is transmitted through a single mode fiber 2 a jet homogenizer 5 fed. The incoming laser beam 3 , which of the single-mode fiber 2 is emitted and on the beam homogenizer 5 is incident, is a Gaussian beam with high accuracy. Therefore, by using the fundamental mode fiber laser, it can be ensured that there are no changes in the beam profile of the incoming laser beam 3 occur. Alternatively, however, it is also conceivable that the laser system 1 a laser of a different type. Furthermore, it is also conceivable that the laser light using a multi-mode fiber and / or as a free beam to Strahlhomogenisierer 5 to be led.

As in 1 can be seen causes the Strahlhomogenisierer 5 a free space coupling between the single mode fiber 2 and a multi-mode fiber 4 , That into the multimode fiber 4 Coupled laser light is using the multimode fiber 4 to an optional amplifier 18 transported. That from the amplifier 18 Outgoing, amplified laser light is transmitted through the multimode fiber 4 or through another multimode fiber 23 a machining head 19 supplied, depending on whether in the amplifier 18 the laser light from the multimode fiber 4 is decoupled. The amplifier 18 may be formed for example as a fiber amplifier. In this case, at least a portion of the multimode fiber 4 be formed as an active optical fiber. However, there are other configurations of the amplifier 18 conceivable.

The machining head 19 is configured as a focusing system which has collimation optics 20 and a focusing optics 22 having. The machining head 19 also has an optional scanning system 30 on which in the laser light between the collimation optics 20 and the focusing optics 22 is arranged. Through the focusing optics 22 becomes the laser beam, which by the Kollimationsoptik 20 collimated, focused in the editing area to the machining focus 21 to create. A beam profile of the laser beam, at least at a position within the machining focus 21 , is a top-hat-approximated beam profile.

The jet homogenizer 5 is configured so that from the gaussian beam profile of the incoming laser beam 3 a homogenized beam profile on a light entry surface 10 the multimode fiber 4 is produced. The homogenized beam profile is also a top-hat-approximated beam profile. The mode excitation in the multimode fiber 4 is therefore done using the homogenized beam profile.

It has been shown that with the help of mode excitation in the multimode fiber 4 under Using the homogenized beam profile a machining focus 21 can be generated in a processing area, wherein a beam profile at a position within the processing focus 21 in a good approximation corresponds to a top hat beam profile. The top hat-approximated beam profile is less sensitive to the orientation of the beam homogenizer 5 to the incoming laser beam 3 from. Lasers are used, which an incoming laser beam 3 With a fluctuating beam profile, there is a reduced sensitive dependence of the top-hat-approximated beam profile in the machining focus 21 from these fluctuations over conventional approaches. Fluctuating beam profiles can arise, in particular, when the laser light can not be guided through a monomode fiber, for example, when high pulse energies are to be provided in the processing area.

It has also been shown that with the help of mode excitation in the multimode fiber 4 using the homogenized beam profile, a comparatively large propagation distance D in the processing region can be obtained, via which the beam profile corresponds to the top hat beam profile to a good approximation. It could be proven that the coupling of the laser light into the multimode fiber 4 with the aid of the homogenized beam profile, a significantly higher number of modes (mode mixture) within the multimode fiber 4 can be excited compared to a coupling using a Gaussian beam profile.

As in 2a and 2 B through an experiment on an exemplary multimode fiber 4 is shown, the power density distribution at a light exit surface of the multimode fiber 4 more homogeneous, if a higher number of modes within the multimode fiber is excited. The core of the multimode fiber 4 which was used in the experiment has a diameter of 75 μm.

2a shows the power density distribution at the light exit surface of the multimode fiber 4 in a case where a Gaussian beam is incident on the light entrance surface 10 the multimode fiber 4 is coupled. Higher order modes can be found in the multimode fiber 4 for example, by coupling (spot size, position), bends (micro, macro) and targeted inhomogeneity of the transverse fiber profile along z in the multimode fiber 4 stimulate. Therefore, the power density distribution at the light exit surface of the multi-mode fiber 4 an interference pattern due to mode interference within the multimode fiber 4 is generated and the shape of which strongly depends on the location of the multimode fiber 4 depends.

In comparison shows 2 B the power density distribution at the light exit surface of the multimode fiber 4 when at the light entry surface 10 a homogenized beam profile is generated, as in the 1 shown laser system 1 he follows. How to 2 B can be seen, the interference contrast of the interference pattern is significantly suppressed. Thus, when a higher number of modes is excited, the interference contrast decreases, and the power density distribution at the light emitting surface of the multi-mode fiber becomes more homogeneous.

For the laser system 1 it has proven to be advantageous if the homogenized beam profile at the light entry surface 10 the multimode fiber 4 has a plateau uniformity with a score which is less than 0.3 , preferably as 0.2 and especially preferred 0.1 , By definition, for the ideal top-hat beam profile, the plateau uniformity has an index of zero. The plateau uniformity may be measured on a power density distribution or on an energy density distribution of the homogenized beam profile. In pulsed lasers, for example, the energy density distribution can be used as a measured variable, wherein the energy density distribution can represent an integration of the power density distribution over one or more laser pulses.

wobei ΔE_FWHM die volle Halbwertsbreite in einem Energiedichtehistogramm N(E) (gezeigt in 3b) ist. Das Energiedichtehistogramm N(E) gibt die Anzahl N der Positionen in der Querschnittsebene des homogenisierten Strahlprofils an, an welchen die Energiedichte E aufgezeichnet wurde. Das zugehörige zweidimensionale homogenisierte Strahlprofil ist in er 3a durch einen eindimensionalen Schnitt illustriert. Das Maximum 28 des Energiedichtehistogramms, an dem die volle Halbwertsbreite ΔE_FWHM gemessen wird, bezieht sich auf das Plateau 27 der Energiedichteverteilung. Daher endet die rechte Flanke 29 des Maximums 28 häufig beim Maximalwert der Energiedichteverteilung E_max .The plateau irregularity is defined according to the standard ISO 13694: 2015 and is based on the 3a and 3b illustrated. The content of this standard is incorporated by reference in its entirety. In accordance with this standard, the plateau uniformity U _PE for an energy density distribution of the homogenized beam profile is defined by $$U_{Pe}=\frac{\mathrm{\Delta }{e}_{FWHM}}{e_{max}}.$$

where ΔE _{FWHM is} the full half _width in an energy density histogram N (E) (shown in FIG 3b) is. The energy density histogram N (E) indicates the number N of positions in the cross-sectional plane of the homogenized beam profile at which the energy density E was recorded. The associated two-dimensional homogenized beam profile is in he 3a illustrated by a one-dimensional cut. The maximum 28 of the energy density histogram at which the full half _width ΔE _{FWHM is} measured relates to the plateau 27 the energy density distribution. Therefore, the right flank ends 29 of the maximum 28 often at the maximum value of the energy density distribution E _max ,

wobei ΔH_FWHM die volle Halbwertsbreite in einem Leistungsdichtehistogramm N(H) (nicht gezeigt in 3a und 3b) ist, welches die Anzahl N der Positionen in der Querschnittsebene des homogenisierten Strahlprofils angibt, an welchen die Leistungsdichte H aufgezeichnet wurde. Die volle Halbwertsbreite ΔH_FWHM bezieht sich auf das Plateau der Leistungsdichteverteilung.Accordingly, the plateau uniformity U _PH for the power density distribution of the homogenized beam profile is defined by $$U_{PH}=\frac{\mathrm{\Delta }{H}_{FWHM}}{H_{max}}.$$

where ΔH _{FWHM is} the full half _width in a power density histogram N (H) (not shown in FIG 3a and 3b) which indicates the number N of positions in the cross-sectional plane of the homogenized beam profile at which the power density H has been recorded. The full half _width ΔH _FWHM refers to the plateau of the power density distribution.

Furthermore, it has proved to be advantageous if the homogenized beam profile has an edge steepness with an index which is less than 0.3 , prefers 0.2 and especially preferred 0.1 , As explained more fully below, the slope is based on threshold values of 10% and 90% of a maximum value of the homogenized beam profile. The slew rate can be calculated based on the energy or power density distribution. For the energy density distribution is the maximum value E _max in 3a shown.

wobei A_η die Bestrahlungsfläche bezeichnet, welche diejenigen Werte des Strahlprofils repräsentiert, welche den Anteil η des Maximalwertes (der Energie- bzw. Leistungsdichteverteilung) übersteigen. Entsprechend bezeichnet A_ε die Bestrahlungsfläche, welche diejenigen Werte des Strahlprofils repräsentiert, welche den Anteil ε des Maximalwertes übersteigen. η und ε repräsentieren daher Schwellenwerte. Für die angegebenen Werte in der vorliegenden Offenbarung soll η auf 10% des Maximalwertes und ε auf 90% des Maximalwertes festgelegt sein. Für das ideale Top-Hat-Strahlprofil weist die Flankensteilheit eine Kennzahl von 0 auf.The slope is defined according to the Standard EN ISO 13694: 2015 , In accordance with this standard, the edge steepness S _{η, ε is} defined according to the following expression: $$S_{\eta \text{.}\epsilon }=\frac{A_{\eta }-A_{\epsilon }}{A_{\eta }}.$$

where A _η denotes the irradiation area which represents those values of the beam profile which exceed the proportion η of the maximum value (the energy or power density distribution). Correspondingly, A _ε denotes the irradiation area, which represents those values of the beam profile which exceed the proportion ε of the maximum value. η and ε therefore represent threshold values. For the given values in the present disclosure, η should be set to 10% of the maximum value and ε to 90% of the maximum value. For the ideal top-hat beam profile, the slope has a ratio of 0.

wobei der Mittelwert E_ϱave (illustriert in der 3a) aus den Werten der Energiedichteverteilung innerhalb der Bestrahlungsfläche A_ϱ berechnet wird. Die Bestrahlungsfläche A_ϱ (illustriert in der 3a) ist eine Fläche, welche diejenigen Werte der Energiedichteverteilung repräsentiert, welche einen Anteil ρ des Maximalwertes der Energiedichteverteilung E_max übersteigen.Furthermore, it has proven to be advantageous if a proportion of at least 80% of a total energy or power of the harmonized beam profile has a beam uniformity which is more uniform than ± 10%, preferably ± 5% and particularly preferred ± 2%. The beam uniformity is defined in accordance with the standard ISO 13694: 2015 and is calculated using the 3a illustrated. The beam uniformity may be determined based on the energy density distribution or the power density distribution. In accordance with this standard, the calculation of the beam uniformity U _ρ results from the energy density distribution: $$U_{\rho }=\frac{1}{e_{\rho \text{ave}}}\sqrt{\frac{1}{A_{\rho }}{\displaystyle \iint {\left[e\left(x.y\right)-e_{\rho \text{ave}}\right]}^2}}\text{d}x\text{d}y.$$

the mean E _ρave (illustrated in the 3a) is calculated from the values of the energy density distribution within the irradiation area A _ρ . The irradiation area A _ρ (illustrated in _FIG 3a) is an area representing those values of the energy density distribution which is a fraction ρ of the maximum value of the energy density distribution E _max exceed.

wobei der Mittelwert H_κave aus den Werten der Leistungsdichteverteilung innerhalb der Bestrahlungsfläche A_κ berechnet wird. Die Bestrahlungsfläche A_κ ist eine Fläche, welche diejenigen Werte der Energiedichteverteilung repräsentiert, welche einen Anteil κ des Maximalwertes der Leistungsdichteverteilung H_max übersteigen.Accordingly, the beam uniformity U _{ρ is} based on the power density distribution: $$U_{\kappa }=\frac{1}{H_{k\text{ave}}}\sqrt{\frac{1}{A_{\kappa }}{\displaystyle \iint {\left[H\left(x.y\right)-H_{\kappa \text{ave}}\right]}^2}}\text{d}x\text{d}y.$$

wherein the mean value H _{kave is calculated} from the values of the power density distribution within the irradiation area A _κ . The irradiation area A _κ is an area which represents those values of the energy density distribution which comprise a portion κ of the maximum value of the power density distribution H _max exceed.

For the calculation of the values for the homogenized beam profile given in this disclosure, ρ and κ are respectively selected such that 80% of the total energy or the total power, integrated over the beam cross-section, are within the irradiation area A _ρ or A _κ .

Furthermore, it has proved to be advantageous if the homogenized beam profile is corresponding to or substantially corresponding to a super Gaussian beam profile with an order greater than / equal to 5, preferably greater than or equal to 7 and more preferably greater than or equal to 10 ,

und entsprechend ist für die Leistungsdichteverteilung das Super-Gauß-Strahlprofil definiert durch: $$H\left(r\right)=H_0\text{exp}\left[-2{\left(\frac{r}{w}\right)}^n\right],$$

wobei r bei einem runden Strahlquerschnitt der Betrag des Radiusvektors in der Querschnittsebene ist, dessen Ursprung mit der Strahlachse des Laserlichts zusammenfällt, oder bei einem z.B. rechteckigen oder elliptischen Strahlquerschnitt auch nur x oder y sein kann. Der Parameter n ist die Ordnung des Super-Gauß-Strahlprofils und w ist ein Maß für die transversale Ausdehnung des Super-Gauß-Strahlprofils. Ein Super-Gauß-Strahlprofil der Ordnung 2 entspricht einem Gauß-Strahlprofil. Mit zunehmender Ordnung nähert sich das Super-Gauß-Strahlprofil dem Top-Hat-Strahlprofil an.For the energy density distribution, the super Gaussian beam profile is defined by: $$e\left(r\right)={\mathrm{<figure-callout\; id="0"\; label="e"\; filenames="DE102017217145A1\_0008.png,DE102017217145A1\_0011.png"\; state="\{\{state\}\}">e</figure-callout>}}_0\text{exp}\left[-2{\left(\frac{r}{w}\right)}^n\right].$$

and, correspondingly, for the power density distribution, the super Gaussian beam profile is defined by: $$H\left(r\right)={\mathrm{<figure-callout\; id="0"\; label="H"\; filenames="DE102017217145A1\_0008.png,DE102017217145A1\_0011.png"\; state="\{\{state\}\}">H</figure-callout>}}_0\text{exp}\left[-2{\left(\frac{r}{w}\right)}^n\right].$$

where r in the case of a round beam cross section is the magnitude of the radius vector in the cross-sectional plane whose origin coincides with the beam axis of the laser light or, in the case of, for example, a rectangular or elliptical beam cross-section, can only be x or y. The parameter n is the order of the super Gaussian beam profile and w is a measure of the transverse extent of the super Gaussian beam profile. A super Gaussian beam profile of order 2 corresponds to a Gaussian beam profile. As the order increases, the super Gaussian beam profile approaches the top hat beam profile.

As based on the 1 can be seen, the laser light converges on the light entry surface 10 the multimode fiber 4 , A convergence angle of the laser light at the light entry surface 10 essentially corresponds to an acceptance angle of the multimode fiber 4 , The convergence angle is defined as the angle between an edge line of the laser light and the beam axis, ie, half the opening angle. The acceptance angle is the maximum angle of incidence relative to the axis of the multimode fiber 4 , under which a ray of light on the light entry surface 10 may come to mind in the multimode fiber 4 to be forwarded by total reflections. This will be the numerical aperture of the multimode fiber 4 illuminated, so that a high number of modes is excited. At the same time a power loss is avoided by a too large convergence angle. An excitation of many modes is also achieved in that the homogenized beam profile substantially congruent with the light entry surface 10 the multimode fiber 4 is. For example, the parameter w of the super Gaussian distribution may be the radius of the core of the multimode fiber 4 correspond or substantially correspond. This ensures on the one hand that a high proportion of the laser light intensity in the multimode fiber 4 is coupled. In addition, this ensures the excitation of a high number of modes within the multimode fiber and thus a comparatively good homogenization of the top-hat-approximated beam profile, which is within the processing focus 21 located.

The desired extension of the homogenized beam profile at the light entry surface 10 and the desired convergence angle are achieved by imaging optics 6 of the Strahlhomogenisierers, which allows an intermediate focus 9 on the light entry surface 10 the core of the multimode fiber 4 maps. In the embodiment shown, the imaging optics 6 For example, formed as a 4f imaging optics, which two refractive optical elements 15 and 16 each having positive refractive power. However, there are other configurations of the imaging optics 6 conceivable.

In the intermediate focus 9 a homogenized beam profile is generated, which compared to the beam profile of the incoming laser beam 3 is homogenized. For this purpose, the Strahlhomogenisierer 5 a diffractive optical element 7 on which collimated laser light is incident. Through a focusing optics 8th , the laser light is emitted by the diffractive optical element 7 goes out to the intermediate focus 9 in a focal plane of the focusing optics 8th focused. It is conceivable that the laser system 1 is configured so that the light entry surface 10 the multimode fiber 4 at the place of the intermediate focus 9 is arranged. In this case, the beam homogenizer points 5 no imaging optics 6 on. Optionally, it may be useful to aperture 14 . 26 to use parasitic diffraction order of the diffractive optical element 7 hide.

Alternatively or additionally, it is also conceivable that the homogenized beam profile at the light entry surface 10 the multimode fiber 4 is generated using reflective and / or refractive optical elements. For example, the beam homogenizer 5 have two optical elements. Each of the optical elements may be configured as an aspheric lens or aspheric mirror.

The collimated laser light for illuminating the diffractive optical element 7 is by a collimation optics 11 and an optional beam expander 13 generated respectively upstream of the diffractive optical element 7 are arranged. Between the collimation optics 11 and the beam expander 13 is optional an isolator 12 When placed in laser light, it can also be placed elsewhere to allow back reflection of parts of the laser light into the laser 24 to prevent.

The diffractive optical element 7 has a diffractive phase structure. Additionally or alternatively, it is conceivable that the diffractive optical element 7 has a diffractive amplitude structure. By avoiding a diffractive amplitude structure, however, a higher proportion of the power of the incident laser beam can as a rule be used 3 be concentrated in the top hat of the top hat approximated beam profile, which is in the processing focus 21 located. The diffractive amplitude structure and / or the diffractive phase structure can be calculated, for example, on the basis of an iterative Fourier transform algorithm (IFTA).

4a illustrates the transverse phase distribution caused by the diffractive Phase structure of the diffractive optical element 7 is produced. The gray values represent the phase angle in radians in the plane of the diffractive optical element 7 , 4b illustrates the transverse power density distribution of the homogenized beam profile in a plane of the light entry surface 10 the multimode fiber 4 , where the gray values represent arbitrary units. 4c illustrates the transverse phase distribution of the homogenized beam profile in the plane of the light entry surface 10 the multimode fiber 4 , where the gray values represent the phase angle in radians. In the 4a shown rotationally symmetric configuration of the transverse phase distribution generated on the light entry surface 10 a substantially planar phase front, as in 4c you can see. This planar phase front causes only rotationally symmetric modes in the multimode fiber 4 be stimulated.

However, it has been shown that an even larger mode mixture and thus an even better homogenization of the top-hat-approximated beam profile in the machining focus 21 and a longer propagation distance D in the processing area can be achieved when beamforming techniques using statistical phase structures are used. The mode of action of statistical phase structures is similar to that of diffractive diffusers.

Accordingly, in a second embodiment, the diffractive optical element 7 configured as a statistical diffractive phase structure. Such a statistical diffractive phase structure can be calculated, for example, using an iterative Fourier transform algorithm. In this case, for example, a statistical (random) or essentially statistical phase distribution in a plane of the homogenized beam profile is assumed and the transverse phase distribution in the plane of the diffractive optical element is calculated by a plurality of iterations.

5a illustrates the transverse phase distribution in the plane of the diffractive optical element 7 in the laser system 1 according to the second embodiment. The gray values represent the phase angle in radians. The diffractive structure of the diffractive optical element 7 is configured as a statistical diffractive phase structure. 5b illustrates the power density distribution in the plane of the light entry surface 10 the multimode fiber 4 , where the gray values represent arbitrary units. In the second embodiment, the statistical diffractive phase structure generates point-like fluctuations in the plateau region of the power density distribution. These variations are due to the mode mixing in the multimode fiber 4 attenuated and are in the top hat-approximated beam profile, which is in the processing focus 21 is no longer recognizable.

5c shows the transverse phase distribution in the plane of the light entry surface 10 the multimode fiber 4 in the laser system 1 according to the second embodiment. The gray values represent the phase values in radians. A comparison between the 4c and 5c shows that due to the statistical diffractive phase structure, the light on the light entry surface 10 the multimode fiber 4 no longer forms a plane wavefront. On the other hand is on the light entry surface 10 the phase distribution is more statistical in nature. It could be shown that this results in a larger mix of modes in the multimode fiber 4 can be achieved. Compared to the first embodiment, this additionally results in an excitation of modes in the multimode fiber 4 , which have no rotationally symmetric field distributions, which increases the number of propagating modes. Thus, the interference contrast in the top hat-approximated beam profile, which is in the processing focus 21 is located, lowered and a longer propagation distance D are obtained in the processing area.

In a further alternative embodiment, the beam homogenizer 5 two diffractive optical elements. A first of the two diffractive optical elements generates the homogenized beam profile and the second diffractive optical element heals phase disturbances. It has been shown that thereby the homogeneity of the top hat-approximated beam profile him processing focus 21 further increased and further an even greater propagation distance D can be generated in the processing area.

It is also conceivable that the beam homogenizer 5 an optical delay device which delays a first portion of the laser light relative to at least a second portion of the laser light. The first part goes through a delay line, while the second part bypasses the delay line. The delay device may be located upstream of a diffractive optical element (such as that in FIG 1 shown diffractive optical element 7 ), by means of which the homogenization of the laser beam incident on the beam homogenizer takes place. An optical path length of the delay line may be longer than a longitudinal coherence length of the laser light incident on the delay device. The delay device may be configured such that the first part of the laser light is separated from the second part of the laser light with the aid of a partially transmissive mirror.

Examples of such delay devices are, for example, in US 2008/0225904 A1 and US Pat. No. 7,486,707 B2 disclosed, which are fully incorporated by reference. The delay device can cause fluctuations in the power or energy density distribution of the homogenized beam profile on the light entry surface 10 the multimode fiber 4 be reduced. It has been shown that this leads to an increase in the homogeneity of the top-hat-approximated beam profile in the machining focus 21 and leads to a larger propagation distance D in the processing area.

From the in 1 The laser system shown differs in the 6 shown laser system 1 only in that here the Strahlhomogenisierer 5 ' the diffractive optical element (DOE) 7 for generating a multispot beam profile from the gaussian beam profile and the multimode fiber 4 resulting in multi-mode excitation in the multimode fiber 4 is formed using the multi-spot beam profile.

A fundamental mode beam is transmitted through the telescope 13 expanded and propagated collimated by a 7, which generates a multi-spot profile after focusing. This becomes on the coupling-side Faserendfläche 10 the multimode fiber 4 Imaged and stimulates a variety of modes in this multi-mode fiber 4 so that mode interference is reduced. Here, the high number of modes reduces the interference contrast, so that the beam profile at the fiber output is more homogeneous. Here, the height of the registered spatial frequency must be adapted to the respective fiber NA (acceptance angle). The imaging optics 6 consisting of the lenses 15 . 16 , but can also be omitted if the lens 8th can be suitably selected. This shortens the structure significantly ( 2f Construction takes place 6f ).

The mode of action of the multispot excitation of the multimode fiber 4 can be described as follows. A ground-level Gaussian beam is applied to the fiber end face of the multimode fiber 4 with a core diameter of 100μm and a numerical aperture of NA = 0.2 shown, wherein the spot size of the exciting beam 15μm (NA = 0.1 ) is. In 7 Now the measured beam profile is at the end of a 20m long multimode fiber 4 for a central stimulus ( 7a ), an offset of about half a core diameter ( 7b) and represented for an offset of slightly less than a core diameter ( 7c) , The three launch situations are mainly differentiated by the group of modes of the multi-mode fiber 4 that's being stimulated. For central excitation, it can be seen that predominantly modes with rotational symmetry were excited. In the other cases, although modes are excited without radial symmetry, but now modes with radial symmetry can no longer be excited. All three cases indicate inhomogeneities due to mode interference. Furthermore, it can be seen that, although the group of excited modes has changed in the three situations illustrated, the numerical aperture of the injected radiation at the fiber input usually corresponds essentially to the numerical aperture of the radiation at the fiber output.

Overall, it can be deduced from this observation that a multispot mode excitation with a suitable weighting of the individual spots leads to the coupling conditions for modes of different symmetry groups being fulfilled simultaneously and at the output of the multimode fiber 4 gives a top hat approximated beam profile. In particular, for example, radially symmetric and radially anti-symmetric modes are excited (on- and off-axis coupling). This procedure increases the number of excited modes and provides better mixing (modes of different symmetry groups), thus reducing the interference contrast at the fiber output.

8a shows the phase mask of a one-dimensional DOE 7 and 8b the associated intensity profile in the form of a multi-spot distribution. 9a shows the phase mask of a two-dimensional DOE 7 and 9b the associated intensity profile form of a multi-spot distribution. The circle in the intensity profile corresponds to the fiber core of the multimode fiber 4 to be coupled into. Using two diffractive optical elements, the first element can generate the target beam profile and the second element can cure phase noise to produce a propagating supergauss / top hat

Alternatively to the diffractive approach (DOE), a refractive beam splitting, e.g. in the form of microlens arrays.

QUOTES INCLUDE IN THE DESCRIPTION

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

Cited patent literature

• US 2008/0225904 A1 [0074]
• US 7486707 B2 [0074]

Cited non-patent literature

• Standard EN ISO 13694: 2015 [0055]

Claims (25)

Laser system (1) comprising a beam homogenizer (5) for homogenizing a substantially Gaussian beam profile of a laser light into a homogenized beam profile; wherein the substantially Gaussian beam profile represents a two-dimensional energy density distribution or power density distribution of the laser light; wherein the homogenization is performed using an optical element of the beam homogenizer (5) configured as a refractive, reflective and / or diffractive optical element; and wherein the laser system (1) further comprises a multi-mode fiber (4) and is configured to mode-excite in the multi-mode fiber (4) using the homogenized beam profile. Laser system (1) comprising a beam homogenizer (5 ') for homogenizing laser light having a substantially Gaussian beam profile, which represents a two-dimensional energy or power density distribution of the laser light, to a homogenized beam profile, wherein the homogenization is performed using at least one optical element (7) of the beam homogenizer (5 ') which generates a multispot beam profile from the Gaussian beam profile and is configured as a refractive, reflective and / or diffractive optical element, and a multimode fiber (4) of the Beam homogenizer (5 '), which is configured for multi-mode excitation in the multi-mode fiber (4) using the multi-spot beam profile. Laser system after Claim 1 or 2 , characterized in that the homogenized beam profile has a plateau irregularity with an index which is less than 0.3 or less than 0.1. Laser system according to one of the preceding claims, characterized in that the homogenized beam profile has a slope with a characteristic number less than 0.3 or less than 0.1, based on threshold values of 10% and 90% of a maximum value of the homogenized beam profile. Laser system according to one of the preceding claims, characterized in that a proportion of at least 80% of a total energy or power of the homogenized beam profile has a beam uniformity which is more uniform than ± 10% or more uniform than ± 2%. Laser system according to one of the preceding claims, characterized in that the laser system (1) is configured such that the homogenized beam profile is substantially a super Gaussian beam profile with an order greater than or equal to 5 or greater than 10. Laser system according to one of the preceding claims, characterized in that the laser system (1) is configured to generate the homogenized beam profile substantially at a light entry surface (10) of a core of the multi-mode fiber (4), viewed along a propagation direction of the laser light. Laser system according to one of the preceding claims, characterized in that a transverse extension of the homogenized beam profile essentially corresponds to a light entry surface (10) of a core of the multimode fiber (4). Laser system according to one of the preceding claims, characterized in that the beam homogenizer (5) is configured so that the laser light converges on a light entry surface (10) of a core of the multimode fiber (4) arrives. A laser system as claimed in any one of the preceding claims, further characterized by a laser amplifier (18) for amplifying at least a portion of the laser light, the laser amplifier (18) being located downstream of the multimode fiber (4). Laser system according to one of the preceding claims, characterized in that at least a portion of the multi-mode fiber (4) is configured so that at least a portion of the laser light can be amplified by means of the portion. Laser system according to one of the preceding claims, characterized in that the beam homogenizer (5) has imaging optics (6). Laser system according to one of the preceding claims, characterized in that the beam homogenizer (5) has a collimating optics (11). Laser system according to one of the preceding claims, characterized in that the beam homogenizer (5) has beam matching optics (13) for beam expansion and / or beam narrowing of the laser light. Laser system according to one of the preceding claims, characterized in that the optical element of the beam homogenizer (5) is configured as a diffractive optical element (7). Laser system according to one of Claims 1 to 14 , characterized in that the optical element of the Strahlhomogenisierers (5) is configured as a refractive optical element (15, 16) having one or more, in particular aspherical, refractive optical surface. Laser system according to one of Claims 1 to 14 , characterized in that the optical element of the Strahlhomogenisierers (5) is configured as a reflective optical element (15, 16) having one or more, in particular aspherical, reflective optical surface. Laser system according to one of Claims 15 to 17 , characterized in that at least a part of the optical element acts as a stochastic phase mask. Laser system according to one of the preceding claims, characterized in that the laser system (1) further comprises a single-mode fiber (2) which is arranged upstream of the beam homogenizer (5). The laser system of any one of the preceding claims, further characterized by a fiber laser configured to generate at least a portion of the laser light. Laser system after Claim 20 , characterized in that a laser medium of the fiber laser comprises a single-mode fiber core. Laser system according to one of the preceding claims, characterized in that the beam homogenizer (5) is configured so that a spatial frequency spectrum of the homogenized beam profile is suppressed for each frequency value above a transverse limit spatial frequency of the multimode fiber (4). Laser system according to one of Claims 2 to 21 , characterized in that the beam homogenizer (5 ') for generating the multi-spot beam profile has at least one diffractive optical element (7) with a one- or two-dimensional phase mask. Laser system according to one of Claims 2 to 21 , characterized in that the beam homogenizer (5 ') for generating the multi-spot beam profile has at least one refractive optical element with a one- or two-dimensional phase mask, in particular a one- or two-dimensional microlens array. A method for homogenizing laser light having a substantially Gaussian beam profile, which represents a two-dimensional energy or power density distribution of the laser light, to a homogenized beam profile, in particular to a TopHat beam profile, wherein the beam profile in a multi-mode fiber (4) leads to a mode excitation.

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