SEGMENTED BEAM SHAPING ELEMENT AND LASER PROCESSING SYSTEM

Description

[0001] The present invention relates to a beam shaping element for forming diffraction-free intensity zones in the propagation direction. The invention further relates to a laser processing system with a beam shaping element.

[0002] Transparent laser processing involves using laser radiation to create modifications in a material that is essentially transparent to laser radiation and is referred to herein as the transparent material. The absorption (volume absorption) of laser radiation occurring within the material's volume can be used, for example, for drilling, separation by induced stress, welding, modifying the refractive properties, or selective laser etching. See, for example, the applicant's applications WO 2016 / 079062 A1, WO 2016 / 079063 A1, and WO 2016 / 079275 A1.

[0003] From the scientific publication Flamm, Daniel et al.: "Beam shaping for ultrafast materials processing", Proceedings Volume 10904, Laser Resonators, Microresonators, and Beam Control XXI; 109041G (2019) https: / / doi.org / 10.1117 / 12.2511516 , A diffractive optical beam shaping element for imprinting a phase distribution onto a transverse beam profile of a laser beam is known, wherein the beam shaping element has several angular segments, wherein neighboring angular segments have a phase offset to each other and wherein the azimuthal segment widths of all existing angular segments are the same.

[0004] In these applications, it can be crucial to be able to accurately control both the geometry and the type of modification within the material. Besides parameters such as laser wavelength, pulse shape, number of pulses, and pulse energy, the beam shape can also be relevant.

[0005] For example, ultrashort pulse laser-based glass modification processes for glass cutting are often carried out with elongated focus distributions, such as those found in Bessel-like beam profiles. These can form elongated modifications in the material that extend within the material in the propagation direction of the laser radiation.

[0006] Beam shaping elements and optical setups that can provide elongated, slender beam profiles with a high aspect ratio for laser processing in the beam propagation direction are described, for example, in the aforementioned WO 2016 / 079062 A1. For example, WO 2016 / 079062 A discloses the formation of beam profiles that change along the propagation direction and can exhibit periodicity, e.g., a periodic intensity profile along the Z-axis.

[0007] One aspect of this disclosure is the objective of providing a diffractive optical beam shaping element that enables beam shaping for tailored volume absorption. In particular, the disclosure aims to provide a beam shaping element for processing transparent materials (i.e., for laser radiation of transparent materials) that can provide elongated, slender beam profiles with a high aspect ratio in the beam propagation direction for laser processing applications.

[0008] At least one of these tasks is solved by a beam shaping element according to claim 1 and by a laser processing system according to claim 10. Further developments are specified in the dependent claims.

[0009] In one aspect, a diffractive optical beam shaping element for imprinting a phase distribution onto the transverse beam profile of a laser beam comprises adjacent surface elements that form a planar lattice structure. Within this lattice structure, each surface element is assigned a phase shift value, and these phase shift values ​​define a two-dimensional phase distribution. This two-dimensional phase distribution has a beam center position that defines a radial direction within the planar lattice structure. The surface elements are assigned to a plurality of angular segments. Each angular segment has an azimuthal segment width with respect to the beam center position. The phase shift values ​​in the angular segments each form radially symmetrical phase profiles with respect to the beam center position. These radially symmetrical phase profiles form lattice functions in the radial direction that have the same lattice period.Each of the grating functions is associated with a segment grating phase, particularly with respect to the beam center position. The azimuthal segment widths of at least two adjacent angular segments differ. Additionally, the segment grating phases of at least two adjacent angular segments exhibit a segment grating phase difference between 0 and 2π.

[0010] In another aspect, a laser processing system for processing a material with a laser beam by modifying the material in a focal zone that is elongated in one direction of propagation of the laser beam is disclosed. The laser processing system comprises a laser beam source that emits a laser beam, an optical system that includes a diffractive optical beam shaping element as disclosed herein, and a beam shaping and beam guiding module with a focusing lens. The diffractive optical beam shaping element is arranged in the beam path of the laser beam to impose a two-dimensional phase distribution on the laser beam, and the two-dimensional phase distribution causes the formation of the elongated focal zone in the material by focusing the laser beam with the focusing lens.

[0011] In some embodiments, the phase distribution can be point-symmetric with respect to the beam center position. Furthermore, at least two angular segments can form identical radially symmetrical phase profiles with respect to the beam center position, whereby the angular segments can be opposite each other, particularly with respect to the beam center position.

[0012] In some embodiments, the grating functions can each exhibit a portion of a sawtooth grating phase profile, wherein the slope of an ascending region in each of the sawtooth grating phase profiles can correspond to a predetermined axicon angle associated with the diffractive optical beam shaping element. The predetermined axicon angle can be in the range of 0.5° to 40° for generating a real Bessel beam intermediate focus with the laser beam downstream of the diffractive optical beam shaping element, or in the range of -0.5° to -40° for using a virtual Bessel beam intermediate focus upstream of the diffractive optical beam shaping element.

[0013] The azimuthal segment widths of the diffractive optical beam shaping element can, in some embodiments, range from 2π / 300 to π. Optionally, each of the plurality of angular segments can have the same predetermined segment width.

[0014] In some embodiments, the two-dimensional phase distribution can be characterized by one or more axes of symmetry.

[0015] In some embodiments, the lattice period can be constant in the radial direction.

[0016] In some embodiments, the grating period can have an aspherical, preferably quadratic, and particularly preferably a linear profile in the radial direction. Alternatively or additionally, a collimation phase distribution can be integrated into the two-dimensional phase distribution, which is radially symmetrical over the majority of angular segments with respect to the beam center position.

[0017] In some embodiments of the laser processing system, a beam center position of the diffractive optical beam shaping element can be aligned with a beam center of a transverse beam profile of the laser beam.

[0018] Furthermore, the laser processing system can include a workpiece holder, whereby a relative positioning capability of the beam-shaping and beam-guiding module and a workpiece provided by the workpiece holder as the material to be processed is provided.

[0019] The concepts described herein relate in particular to (three-dimensional) beam profiles that are diffracting-free (non-diffractive) in the direction of propagation. Since there is no significant change in intensity in the beam profile along the direction of propagation, modifications can be created in the material that are continuous in the direction of propagation.

[0020] This document reveals concepts that allow for at least partial improvements to aspects of the prior art. In particular, further features and their advantages become apparent from the following description of embodiments with reference to the figures. The figures show: Fig. 1A A schematic sketch of a laser system with a beam-shaping element for processing a material with, for example, an elongated focus zone. Fig. 1B A schematic sketch to illustrate the imprinting of an axicon phase and a Bessel focus zone. Fig. 2A A schematic representation of a planar phase distribution of a beam-shaping element having several segments. Fig. 2B A schematic comparative representation of two radial phase profiles of segments of the beam-shaping element. Fig. 2A Fig. 2C shows a representation of a radial phase distribution with a lattice period increasing in the radial direction. Fig. 3A and 3B illustrate a planar phase distribution of a first example of a segmented beam-shaping element. Fig. 3C and 3D show calculated intensity distributions in space, as they would be with a phase distribution of the Fig. 3A in a material, Figs. 4A and 4B illustrate a planar phase distribution of a second example of a segmented beam-shaping element, and Figs. 4C and 4D show calculated intensity distributions in space, as they would be with a phase distribution of the Fig. 4A can be produced in a material.

[0021] The aspects described here are partly based on the understanding that the intensity profile, periodically formed along the Z-axis and resulting from beam diffraction in the direction of propagation, can influence the formation of modifications during laser processing. For example, material weakenings that can occur with such intensity profiles may be too far apart in the direction of propagation, thus affecting the quality of the cutting result.

[0022] Accordingly, the concepts disclosed herein aim to generate diffraction-free beam profiles in the direction of propagation (i.e., longitudinally). The inventors have recognized that specific phase profiles can be imprinted on several (azimuthal) angular segments using a beam-shaping element. The segment-specific phase profiles are designed such that together they generate a beam profile that is essentially diffraction-free in the direction of propagation. A diffraction-free configuration can be achieved if phase profiles are imprinted on the angular segments that exhibit the same grating characteristics in the radial direction (for example, a constant / radially uniformly changing "same" grating period) and differ in their "absolute phase," here referred to as the segment grating phase Θj, by a segment grating phase difference (ΔΘj in ). Fig. 2B ), distinguishing between different types of gratings. Here, the condition of "same grating period" refers to the phase components required to form an elongated focal zone, as they are applied to the input beam in each segment. The grating period can also vary radially, for example, when a lens is superimposed, which can result in an aspherical, linear, or quadratic change in the radial direction.

[0023] The inventors have further recognized that for shaping a three-dimensional beam profile, the azimuthal widths of the segments and the azimuthal positions of the segments can be varied without losing the feature of being able to form a beam profile (a focus distribution) without diffraction in the direction of propagation.

[0024] The azimuthal width of a segment (also referred to here as segment width) is the range of the azimuthal angle assigned to that segment. The segment width, for example, ranges from 2π / 200 to π. Each spatially defined segment is assigned a segment lattice phase Θj. Furthermore, symmetry requirements such as point symmetry about a center of the phase mask or one or more axes of symmetry can be imposed on the phase distribution, such that segments correspond in their phase distribution. For example, opposite segments can have identical phase distributions if they have point symmetry. In the example of the Fig. 4B When subdivided into 200 segments, the number of segment lattice phases Θj to be determined was 50 due to the symmetry.

[0025] The position of a segment (also referred to herein as segment position) can be specified, for example, by a central angle of the respective segment.

[0026] The use and exemplary forms of training of beam-shaping elements are discussed below in connection with the Figuren 1 bis 4D described.

[0027] Fig.1 Figure 1 shows a schematic representation of a laser processing system 1 for processing a material 3 with a laser beam 5. The processing causes a modification of the material 3 in a focal zone 7. As shown in Fig. 1A As indicated, the focus zone 7 can generally be elongated in a propagation direction 9 of the laser beam 5. For example, the focus zone 7 is a focus zone of a Bessel beam or an inverse Bessel beam, as it can form in an essentially transparent material.

[0028] The laser processing system 1 comprises a laser beam source 11, which generates and emits the laser beam 5. The laser beam 5 is, for example, pulsed laser radiation. Laser pulses have, for example, pulse energies that lead to peak pulse intensities, which cause volume absorption in the material 3 and thus the formation of the modification in a desired geometry.

[0029] For beam shaping and guidance, the laser processing system 1 further comprises an optical system 13. The optical system 13, also called processing head, comprises a diffractive optical beam shaping element 15 and a beam shaping and beam guiding module 17 with a focusing lens 17A.

[0030] In Fig. 1A Not shown are other beam-guiding components of the optical system 13 such as mirrors, lenses, telescope arrangements, filters and control modules for aligning the various components.

[0031] Finally, the laser processing system 1 includes a schematically indicated workpiece holder 19 for storing a workpiece. Fig. 1A The workpiece is the material 3 to be processed. It can be, for example, a glass disc or a disc that is largely transparent at the laser wavelength used, made of ceramic or crystalline material (e.g., sapphire or silicon). For processing the material 3, a relative movement occurs between the optical system 13 and the material 3, so that the focus zone 7 can be formed at different positions along a predetermined trajectory. For separating the material 3 into two parts, the trajectory then determines the path of a separation line.

[0032] In general, the laser beam 5 is determined by beam parameters such as wavelength, spectral width, temporal pulse shape, formation of pulse groups, beam diameter, transverse beam profile / input intensity profile, transverse input phase profile, input divergence and / or polarization.

[0033] According to Fig. 1A The laser beam 5 is fed to the optical system 13 for beam shaping, i.e., for converting one or more of the beam parameters. Typically, for laser material processing, the laser beam 5 will be approximately a collimated Gaussian beam with a transverse Gaussian intensity profile, generated by the laser beam source 11, for example, an ultrashort pulse high-power laser system. With regard to the laser radiation that can be used, reference is made by way of example to the laser systems and parameters described in the applicant's aforementioned applications.

[0034] The optical system 13 is usually assigned an optical axis 21, which is preferably defined by a symmetry point of the beam shaping element 15 (e.g. by a beam center position 23 of the diffractive optical beam shaping element 15, see Fig. 2A ). In the case of a rotationally symmetric laser beam 5, a beam center of a transverse beam profile of the laser beam 5 can coincide with the beam center position 23 along the optical axis 21 of the optical system 13.

[0035] The beam shaping element 15 is a spatial light modulator. It can, for example, be implemented as a fixed diffractive optical element. Furthermore, the beam shaping element 15 can be implemented electronically by time-dependent adjustment of a programmable diffractive optical element (liquid crystal display (SLM spatial light modulator)). Typically, such beam shaping elements are digitized beam shaping elements designed to imprint a phase profile onto the transverse beam profile of a laser beam. The digitization can involve the use of discrete values ​​for the phase shift and / or the transverse grating structure.

[0036] In general, an adjustable diffractive optical beam shaping element can allow very fine phase shifts at a coarser lateral resolution, unlike, for example, a lithographically fabricated, fixed diffractive optical element. A fixed diffractive optical element can, for example, have plane-parallel steps, where the material thickness in the region of a step determines the extent of a phase shift. The lithographic fabrication of the plane-parallel steps can enable high lateral resolution.

[0037] Depending on its design, a beam shaping element can be used in transmission or reflection to impose a phase profile on a laser beam. In general, the beam shaping elements proposed herein can, for example, be used in the optical setups described in the aforementioned applications of the applicant.

[0038] In connection with the Figuren 2A bis 4D The basic features are explained using examples.

[0039] Structural beam shaping elements that cause phase imprinting and are formed over a surface are also referred to as phase masks, whereby the mask refers to the phase of the two-dimensional phase distribution.

[0040] The two-dimensional phase distribution of the concepts disclosed herein is designed in particular for the generation (after focusing with the focusing lens 17A) of an elongated focus zone in the workpiece.

[0041] Fig. 1B This illustrates the generation of a "real" Bessel beam focus zone 81, such as can be imaged onto a workpiece using a telescopic arrangement. To form the Bessel beam focus zone 81, a transverse axicon phase distribution is imprinted onto the laser beam 5. The axicon phase distribution corresponds to the phase that is transversely imprinted onto the incident beam 5 by a (rotationally symmetric) axicon 83 made of a material with refractive index n. The axicon 83 is characterized by an axicon angle γ, which specifies the angle of the conically tapered axicon tip. A diffractive optical element can reproduce such axicon phase distributions with corresponding phase shift values ​​provided on surface elements (see also Fig. 2A ).

[0042] Fig. 1B Figure 1 further shows an angle δ to a propagation direction 9' of the incident laser radiation, at which the (transverse) beam components are directed to the Bessel beam focus zone 81. The angle δ depends on the axicon angle γ and the refractive index n. A maximum possible length l 0 of the Bessel beam focus zone 81 results from the beam diameter of the incident laser radiation and the angle δ.

[0043] Fig. 1B Figure 85A further illustrates the resulting transverse intensity distribution 85A and a longitudinal intensity distribution 85B, each shown in cross-section and as an intensity profile through the beam axis (in the X direction) and along the beam axis (in the Z direction), respectively. An example of the diameter d0 of a principal maximum 87 in the transverse intensity distribution 85A is also shown.

[0044] As in Fig. 1B Even before the focal zone is shown in the workpiece, a focal zone corresponds to a three-dimensional intensity distribution that determines the spatial extent of the interaction and thus the extent of the modification in the material 3 to be processed. As an elongated focal zone, a fluence / intensity is thus generated in a region elongated in the direction of propagation 9 within the material 3, which is higher than the threshold fluence / intensity of this material relevant for processing / modification.

[0045] A focal zone is typically described as elongated when the three-dimensional intensity distribution with respect to a target threshold intensity is characterized by an aspect ratio (extent in the direction of propagation relative to the lateral extent) of at least 10:1, for example, 20:1 and more or 30:1 and more, e.g., 10000:1. Such an elongated focal zone can lead to a modification of the material with a similar aspect ratio. In some embodiments, for example, (partial) focal zones can also form that run parallel to each other in the direction of propagation, each of which has a corresponding aspect ratio. For the in Fig. 1B The Bessel beam focus zone 81 shown is the ratio of the longitudinal extent to the transverse extent of the principal maximum 87 on the optical axis given by r0 = 2.405 / kr, with kr=2*π / λ*(n-1)*γ, where γ is the axicon angle, λ is the wavelength and n is the refractive index of the axicon material.

[0046] In particular, with Bessel beam profiles, the energy can be directed laterally into the elongated focal zone (in) essentially over the entire length of a modification to be induced. Fig. 1B The energy is introduced at an angle δ to the propagation direction 9' into the Bessel beam focus zone 81 (not yet shown) for volume absorption. In this sense, a Gaussian beam cannot produce a comparable elongated focus, since the energy input is essentially longitudinal and not lateral.

[0047] Returning to beam shaping, it shows Fig. 2A schematically a two-dimensional phase distribution 25 of a diffractive optical beam shaping element 15, as it can be imprinted with the diffractive optical beam shaping element 15 via a transverse beam profile. Fig. 2B Figure 26 shows an associated graph to illustrate radial phase profiles as they are formed in the two-dimensional phase distribution 25 in the radial direction starting from the beam center position 23 of the diffractive optical beam shaping element 15 in corresponding segments.

[0048] In the Figuren 2A und 2B The underlying phase shift values ​​(phase in rad) from -π to +π are shown in grayscale or specified as phase values. As explained below, phase distribution 25 and the radial phase profiles for reproducing an axicon-like phase imprint exhibit an exemplary sawtooth structure.

[0049] The two-dimensional phase distribution 25 is segmented (subdivided) into several angular segments (hereinafter also referred to as segments). The beam shaping element 15 can be arranged – like an axicon modified according to the invention – in the beam path of the laser beam 5 to imprint a phase according to the two-dimensional phase distribution 25 onto the transverse beam profile of the laser beam 5.

[0050] In Fig. 2A are parameters of the two-dimensional phase distribution 25 and parameters of an associated planar grid structure illustrated, where the planar grid structure implements the two-dimensional phase distribution 25.

[0051] The planar lattice structure is constructed using adjacent surface elements 15A and their associated phase shift values. Here, the surface elements 15A refer to spatial structural units of the planar lattice structure, which can be used to effect a preset phase shift for the incident laser radiation according to an assigned phase shift value. A surface element 15A thus acts on a two-dimensional section of the transverse beam profile of the laser beam 15 and modifies the phase by the phase shift value. Surface elements correspond to the previously mentioned aspect of digitization. Examples of surface elements 15A are shown in Fig. 2A indicated in the upper right corner of the phase distribution 25, whereby the size ratio between the exemplary rectangular surface elements and the phase dependence depends on the manufacture of the beam shaping element.

[0052] The surface elements 15A form a plurality of angular segments, each exhibiting a uniform radial development of the phase (phase distribution) over the associated angular segment surfaces. The angular segments each have an azimuthal segment width with respect to the beam center position 23. In Fig. 2A The azimuthal segment width is illustrated by an azimuthal angle range Δβj for an angular segment. The in Fig. 2A The angle segment areas shown correspond to circular sector areas, which may be truncated by the basic shape of the beam shaping element 15 in the outer edge region. Thus, in Fig. 2A An example of a square basic shape of the beam shaping element 15 is shown.

[0053] For example, the azimuthal segment widths can range from 2π / 300 to π. Optionally, each of the plurality of angular segments can have the same predetermined segment width, e.g., an azimuthal angle range of 2π / 200.

[0054] In the phase distribution 25 of the Fig. 2A Furthermore, the previously mentioned beam center position 23 is shown, to which the center of the incident laser beam 5 is adjusted. The beam center position 23 defines radial directions in the planar lattice structure (in Fig 2A starting in the plane of the drawing at the beam center position 23), along which the radial phase profiles are present.

[0055] For the phase distribution 25 of the Fig. 2A shows Fig. 2B radial phase profiles 27A and 27B (as phase profile curves) are formed by the course of the phase shift values ​​through two opposing segments 31, 31' and 33, 33', respectively. In particular, this shows Fig. 2B the course of the phase shift values ​​along the X-axis at Y=0 (line 29A in Fig. 2A Phase 27A includes the phase curve through segments 31 and 31', as well as the phase shift values ​​through segments 33 and 33', which are adjacent to segments 31 and 31' (phase curve 27B). Both phase curves 27A and 27B pass through the beam center position 23 (X = 0 and Y = 0).

[0056] The segments exhibit phase shift values ​​that change depending on the radial distance r from the beam center position 23 according to an exemplary periodic function (e.g., a sawtooth grating). It can be seen that the phase profiles show a linear increase in the radial direction. In an angular segment, the phase shift values ​​with respect to the beam center position 23 form a radially symmetric phase profile, which can be assigned to a grating function in the radial direction. In other words, the phase profiles -27A, 27B are piecewise rotationally symmetric. In the segments in Fig. 2B The lattice functions have the same lattice period Tr in the radial direction and regardless of the radial position.

[0057] A phase shift can occur between two adjacent angular segments. This is in Fig. 2B The segment lattice phase difference ΔΘj is given as an example. The segment lattice phase Θj can be specified at the beam center position 23, for example, with respect to a rotationally symmetric lattice whose phase is "zero" at one origin of the lattice. The segment lattice phase Θj is shown as an example in Fig. 2B "0" for angular segments 31, 31' and "(-π)" for angular segments 33, 33'. This corresponds to a segment lattice phase difference ΔΘj of (-π) between the segment lattice phases Θj of adjacent angular segments 31 and 33, as well as between adjacent angular segments 31' and 33'.

[0058] In some embodiments, the lattice functions exhibit (only) a portion of a (radial) sawtooth lattice phase profile with repeating rise regions 41, in which the phase e.g. runs from (-π) to π (corresponding to the linear phase change of an axicon see Fig 1B That is, the proportion of the sawtooth lattice phase profile is a first aspect discussed herein in the choice of phase profile. Further proportions in the phase profile are possible, which can be superimposed on the concepts disclosed herein.

[0059] As an example of integrating an additional phase component, a phase component of a far-field optic, arranged downstream of the beam-shaping element 15 in the optical system 13, can be incorporated into the phase distribution. In this way, a collimation phase distribution can be integrated into the two-dimensional phase distribution, which, for example, is radially symmetrical over the majority of angular segments with respect to the beam center position. (See also the applicant's aforementioned applications.)

[0060] In graph 26' of the Fig. 2C is an overlay of one in Fig. 2B The sawtooth grating phase profile (axicon phase component) with a (rotationally symmetric) lens phase component is illustrated. It can be seen that for a phase profile 27' through two opposing segments, the grating period increases with increasing radius (distance from the beam center position 23), i.e., Tr" < Tr'. The grating period in the radial direction can, for example, have an aspherical, preferably a quadratic, and most preferably a linear profile, as would be the case, for instance, if one were to integrate a lens function.

[0061] If the radial dependence of the lattice period is the same for all segments, the radially symmetric phase profiles in the radial direction in the different segments also form lattice functions that have the same lattice period.

[0062] According to the invention, in some embodiments, the slope of a rise region 41 in each of the sawtooth lattice phase profiles can be assigned a predetermined axicon angle (axicon angle γ in Fig. 1B ). This corresponds to the diffractive optical beam shaping element 15 and determines the formation of the Bessel beam. The predetermined axicon angle can, for example, be in the range of 0.5° to 40° (typically in the range of 1° to 5°) for generating a real Bessel beam intermediate focus with the laser beam downstream of the diffractive optical beam shaping element (31). For a virtual Bessel beam intermediate focus upstream of the diffractive optical beam shaping element 15, the predetermined axicon angle can, for example, be in the range of -0.5° to -40° (typically in the range of -1° to -5°).

[0063] To implement the concepts disclosed herein, the azimuthal segment widths of at least two adjacent angular segments differ. Alternatively or additionally, the segment lattice phases Θj of at least two adjacent angular segments exhibit a segment lattice phase difference ΔΘj between 0 and 2π, particularly when all segments have the same segment widths between 0 and π or between π and 2π. In some embodiments, at least two adjacent angular segments may additionally exhibit a segment lattice phase difference ΔΘj of π.

[0064] One can recognize in Fig. 2A Furthermore, the phase distribution 25 is, for example, point-symmetric with respect to the beam center position 23. Furthermore, at least two angular segments, in particular those opposite each other with respect to the beam center position 23, can form identical radially symmetric phase profiles with respect to the beam center position 23. In general, the two-dimensional phase distribution can be characterized by one or more axes of symmetry; in Fig. 2A Axes of symmetry run along the X-axis, the Y-axis, and through the diagonals of the square basic shape of the beam shaping element.

[0065] As already mentioned, beam shaping systems can have different segment sizes, as determined, for example, by the azimuthal segment width (the azimuthal angle range Δβj). Furthermore, the azimuthal position of an angular segment (also referred to here as segment position) can vary, thus offering diverse optimization possibilities. Fig. 2A A segment position for the segment with the azimuthal angle range Δβj is clarified by an angle αj with respect to the X-axis, i.e., the angle between line 29A and the diagonal, which is designated as line 29B in Fig. 2A for example, through the middle (in the azimuthal direction) of the segment.

[0066] The following is based on the Figuren 3A bis 3D and 4A bis 4D The results described for generating elongated diffraction-free focus zones are to be understood as exemplary. It will be understood that, according to the invention, further focus distributions can be realized by changing the initial parameters of segment grating phase difference and optionally one or more of the parameters of segment position, segment width, and number of segments.

[0067] The concepts described herein can generally be represented as follows: In general, an ideally thin axicon modulates the input field with a radially symmetric phase distribution with a linear rise: Φ axi< ( r ) = k r r (kr = 2*pi / Tr with Tr the grating period of the sawtooth grating). Here, kr is given by the axicon angle γ, the refractive index n, and the wavelength λ (small-angle approximation assumed): k r = 2 π ( n - 1)-γ / λ (See e.g.: J. Leach, GM Gibson, MJ Padgett, E. Esposito, G. McConnell, AJ Wright, and JM Girkin, "Generation of achromatic bessel beams using a compensated spatial light modulator", Opt. Express 14, 5581-5587(2006)) The concepts disclosed herein retain the radial dependence to ensure a diffraction-free focal zone in the direction of propagation, while an azimuthal dependence is added by forming angular segments: Φ( r , ϕ ) = k r r + Θ( ϕ )

[0068] Regarding the influence on the formation of the intensity ranges, the azimuthal dependence Θ( ϕ ) selectable and can, for example, exhibit jumps between segments. (For infinitesimally small angular segments, ΔΘj becomes Θ( ϕ The azimuthal dependence in the angular segments can be determined using an optimization algorithm for given target parameters. For example, the optimization algorithm can start with input parameters such as the segment grid phase, number and / or position of angular segments, or it can incorporate one or more of these input parameters into the optimization.

[0069] The following will be discussed in the Figuren 3A bis 3D and 4A bis 4D To illustrate the possibilities of the concepts proposed herein, two exemplary phase distributions on a beam-shaping element and resulting intensity distributions are shown in a lateral section through the generated focus zone and a section along the axial direction.

[0070] The starting point of the two exemplary phase distributions is that the angular segments have the same (lattice) period of a radially symmetric sawtooth lattice.

[0071] The Figuren 3A und 3B Figure 47 shows an azimuthal distribution 47 of the segment grating phases Θj and a two-dimensional phase distribution 49 of an exemplary beam-shaping element, in which the azimuthal segment widths of the adjacent angular segments differ. In accordance with the concepts disclosed herein, a "modified diffractive axicon" is subdivided into j = 12 angular segments. In each segment region, a radially symmetric phase profile around a defined optical axis, here the beam center position 23 of the beam-shaping element, with a segment grating phase Θj, is present. Thus, the optical axes of the respective angular segments coincide. The angular segments alternately exhibit segment grating phases Θj of 0 and (-π), and corresponding segment grating phase differences ΔΘj of π exist between adjacent angular segments.

[0072] For example, a segment has a segment position at angle αj and a segment width (angle range Δβj) in Fig. 3A marked.

[0073] The Figuren 3C und 3D Figure 51 shows a transverse intensity distribution (transverse beam profile 51) and a cross-section along the propagation direction (longitudinal beam profile 53) after focusing the phase-imprinted beam. The transverse beam profile 51 reveals the formation of a specially shaped main maximum 55 and several secondary maxima 57. The secondary maxima 57 lie, for example, below a relevant threshold fluence / intensity of a material being processed. The longitudinal beam profile 53 shows the formation of an elongated focal zone 59, which exhibits no diffraction effects along the propagation direction.

[0074] The Figuren 4A und 4B Figure 1 shows an azimuthal distribution 61 of the segment grating phases Θj and a two-dimensional phase distribution 63 of another exemplary beam-shaping element, in which the segment grating phases Θj differ substantially between all adjacent angular segments. Segment grating phase differences ΔΘj between 0 and 2π exist between adjacent angular segments, with the azimuthal segment widths of all angular segments being equal. The two-dimensional phase distribution 63 is subdivided into j = 200 angular segments. In each segment region, a radially symmetrical phase profile around a defined optical axis, here the beam center position 23 of the beam-shaping element, is present.

[0075] One can also recognize in Fig. 4A The sawtooth lattices in the angular segments are characterized by identical lattice periods. In other words, the phase imprints in the angular segments each correspond to an axicon with the same axicon angle, but with different phase contributions at the axicon tip. Accordingly, the lattice functions (phase profiles) are phase-shifted relative to each other.

[0076] The Figuren 4C und 4DFigure 65 shows a transverse intensity distribution and a cross-section along the propagation direction (longitudinal beam profile 67) after focusing the phase-imposed beam. The transverse beam profile 65 reveals the formation of four aligned principal maxima 69 surrounded by several secondary maxima 71. The secondary maxima 71 lie, for example, below a relevant threshold fluence / intensity of the material being processed, so that four linearly aligned modifications can form when processed with a laser pulse. In the longitudinal beam profile 67, which intersects the four principal maxima 69, four elongated focus zones 73 are correspondingly visible, which show no diffraction effects along the propagation direction.

[0077] It is explicitly emphasized that all features disclosed in the description and / or the claims are to be considered separate and independent of one another for the purpose of the original disclosure as well as for the purpose of limiting the claimed invention, irrespective of the combinations of features in the embodiments and / or the claims. The subject matter of the invention is defined in the accompanying claims.

Claims

1. A diffractive optical beam-shaping element (15) for imprinting a phase distribution on a transverse beam profile of a laser beam (5) with surface elements (15A) adjacent to one another, which build up a flat lattice structure, where each surface element (15A) is assigned a phase shift value and the phase shift values define a two-dimensional phase distribution (25), wherein - the two-dimensional phase distribution (25) has a beam center position (23) that defines a radial direction in the flat lattice structure, - the surface elements (15A) are assigned to a plurality of angular segments (31, 31'; 33, 33'), - each angular segment (31, 31'; 33, 33') has an azimuth segment width (Δβj) with respect to the beam center position (23), - the phase shift values in the angular segments (31, 31'; 33, 33') form radially symmetrical phase curves with respect to the beam center position (23), - the radially symmetrical phase curves in the radial direction form lattice functions that have the same lattice period (Tr), and - a segment lattice phase (Θ) is assigned to each of the lattice functions, and wherein the azimuthal segment widths (Δβj) of at least two adjoining angular segments (31, 31'; 33, 33') differ and the segment lattice phases (Θj) of at least two adjoining angular segments (31, 31'; 33, 33') have a segment lattice phase difference (ΔΘj) between 0 and 2π.

2. The diffractive optical beam-shaping element (15) according to claim 1, wherein the phase distribution (25) is point-symmetric with respect to the beam center position (23).

3. The diffractive optical beam-shaping element (25) according to claim 1 or 2, wherein at least two angular segments (31, 31'; 33, 33'), in particular those lying opposite one another with respect to the beam center position (23), form identical radially symmetrical phase curves with respect to the beam center position (23).

4. The diffractive optical beam-shaping element (15) according to one of the preceding claims, wherein the lattice functions each comprise a portion of a sawtooth lattice phase profile, wherein a slope of a rising portion (41) in each of the sawtooth lattice phase profiles corresponds to a predetermined Axicon angle (γ) associated with the diffractive optical beam shaping element (15).

5. The diffractive optical beam-shaping element (15) according to claim 4, wherein the predetermined Axicon angle is (γ) within the range of 0.5° to 40° for generating a real Bessel beam intermediate focus with the laser beam (5) downstream of the diffractive optical beam-shaping element (15), or within the range of -0.5° to -40° for assuming a virtual Bessel beam intermediate focus upstream of the diffractive optical beam-shaping element (15).

6. The diffractive optical beam-shaping element (15) according to one of the preceding claims, wherein the azimuthal segment widths are in the range from 2π / 300 to π and wherein optionally each of the plurality of angular segments (31, 31', 33, 33') has an equal predetermined segment width.

7. The diffractive optical beam-shaping element (3151) according to one of the preceding claims, wherein the two-dimensional phase distribution (25) is characterized by one or a plurality of axes of symmetry.

8. The diffractive optical beam-shaping element (15) according to one of the preceding claims, wherein the lattice period is designed to be constant in the radial direction.

9. The diffractive optical beam-forming element (15) according to any one of claims 1 to 7, wherein the lattice period in the radial direction has an aspherical, preferably square and particularly preferably, a linear profile.

10. A laser-machining apparatus (1) for machining a material (3) using a laser beam (5) by modifying the material (3) in a focus zone (7), the focus zone being elongated in a propagation direction (9) of the laser beam (5), with: a laser beam source (11) that emits a laser beam (5), an optical system (13) that - has a diffractive optical beam-shaping element (15) according to one of claims 1 to 9, and - has a beam-forming and beam-guiding module (17) with a focusing lens (17A), wherein the diffractive optical beam-shaping element (15) is arranged in the beam path of the laser beam (5) in order to imprint a two-dimensional phase distribution on the laser beam (5), and the two-dimensional phase distribution causes a formation of the elongated focus zone (7) in the material (3) by focusing the laser beam (5) with the focusing lens (17A).

11. The laser-machining apparatus (1) according to claim 10, wherein a beam center position (23) of the diffractive optical beam-shaping element (15) is orientated to a beam center of a transverse beam profile of the laser beam (5).

12. The laser-machining apparatus (1) according to claim 10 or 11, further with a workpiece holder (19), wherein the beam-shaping and beam-guiding module (17) and a workpiece provided by the workpiece holder (19) as the material (3) to be processed can be positioned relative to one another.