EP4259373A1

EP4259373A1 - Laser machining a partly transparent workpiece using a virtually non-diffracting laser beam

Info

Publication number: EP4259373A1
Application number: EP21801073.4A
Authority: EP
Inventors: Daniel FLAMM; Jonas Kleiner
Original assignee: Trumpf Laser und Systemtechnik GmbH
Current assignee: Trumpf Laser und Systemtechnik Se
Priority date: 2020-12-11
Filing date: 2021-10-25
Publication date: 2023-10-18
Also published as: DE102020133145A1; KR20230112726A; WO2022122238A1; US20230311245A1; CN116917077A

Abstract

The invention relates to a method for machining the material of a workpiece (9) using a virtually non-diffracting laser beam (5), said workpiece (3) having a material which is partly transparent to the virtually non-diffracting laser beam (5) and has a linear absorption. The method has the steps of: irradiating (step 103) a raw pulsed laser beam (5') into an optical beam shaping system (13) in order to form a virtually non-diffracting laser beam (5) with a focal zone (7) extending in a longitudinal direction (z) for machining the material of the workpiece (3), wherein a phase is imprinted on the beam cross-section of the raw laser beam (5') by means of the optical beam forming system (13) in order to form phase-imprinted laser radiation (5_PH), and focusing (step 107) the phase-imprinted laser radiation (5_PH) onto the partly transparent material of the workpiece (3) so that the virtually non-diffracting laser beam (5) is formed and the focal zone (7) has an intensity distribution which can be adjusted along the longitudinal direction (z). The phase impression is set such that when the phase-imprinted laser radiation is focused onto the partly transparent material of the workpiece (3), the resulting intensity distribution (BG_2h(+)) of the virtually non-diffracting laser beam (5) on the focal zone (7) is at least approximately constant in the longitudinal direction (z).

Description

LASER MACHINING OF A PARTIALLY TRANSPARENT WORKPIECE WITH A

QUASI-NON-DIFFRACING LASER BEAM

The present invention relates to a method for material processing of a partially transparent workpiece with a quasi-non-diffracting beam. Furthermore, the invention relates to a laser processing system.

Regardless of the linear absorption, a workpiece can be processed using the non-linear absorption of high-intensity laser radiation. For this purpose, one or more modifications can be produced in a workpiece with the high-intensity laser radiation if non-linear absorption of the high-intensity laser radiation occurs in the material of the workpiece. Modifications can affect the structure of the material and can be used, for example, for drilling, cutting by induced stresses, effecting a modification of the refractive behavior or for selective laser etching. See, for example, the applications WO 2016/079062 A1, WO 2016/079063 A1 and WO 2016/079275 A1 of the applicant in the field of processing essentially transparent workpieces. Beam-shaping elements and optical systems with which long, slim beam profiles can be provided in the beam propagation direction with a high aspect ratio for laser processing are described, for example, in WO 2016/079275 A1.

In the case of partially transparent workpieces, the material of the workpiece has a linear absorption capacity with regard to laser radiation. For example, partially transparent workpieces have an absorption (independent of the intensity of the irradiated laser radiation) with absorption coefficients in the range from approx. 0.1/mm to approx. 2.5/mm, corresponding to typical transmissions in the range from 90% to 10% per millimeter Material thickness, for example 60% per 1 mm glass thickness. Laser processing of partially transparent workpieces differs from laser processing of a material that is essentially transparent to the laser radiation, ie has negligible linear absorption, in that the laser radiation propagating in the material is also linearly absorbed by the material. Thus, the more laser radiation is absorbed, the further the laser radiation propagates through the material. One aspect of this disclosure is based on the object of enabling laser processing of a partially transparent workpiece with a focal zone that is elongated in the direction of propagation. In particular, beam shaping approaches, such as those developed for the laser processing of transparent workpieces, should also be applicable to partially transparent workpieces.

At least one of these objects is achieved by a method for material processing of a workpiece according to claim 1, by a laser processing system according to claim 14 and by a method for forming a beam shaping element according to claim 19. Further developments are specified in the dependent claims.

One aspect of this disclosure includes a method for material processing of a workpiece with a quasi-non-diffracting laser beam, the workpiece having a material that is partially transparent to the quasi-non-diffracting laser beam and has linear absorption for laser radiation in the frequency range of the quasi-non-diffracting laser beam, which is independent of a laser radiation intensity, the steps:

Radiation of a pulsed raw laser beam into an optical beam shaping system to form a quasi-non-diffracting laser beam with a focal zone extending in a longitudinal direction for the material processing of the workpiece, with the optical beam shaping system being used to impress a phase on a beam cross section of the raw laser beam to form phase-impressed laser radiation, and

Focusing of the phase-imposed laser radiation in the partially transparent material of the workpiece, so that the quasi-non-diffracting laser beam is formed and the focus zone has an intensity distribution that can be adjusted along the longitudinal direction, the phase imprint being set in such a way that when the phase-imposed laser radiation is focused in the partially transparent material of the workpiece, resulting intensity distribution of the quasi-non-diffracting laser beam in the focal zone is at least approximately constant in the longitudinal direction.

In a further aspect, this disclosure relates to a laser processing system for material processing of a workpiece with a quasi-non-diffracting laser beam, wherein the workpiece has a material that is partially transparent to the quasi-non-diffracting laser beam and has linear absorption for laser radiation in the frequency range of the quasi-non-diffracting laser beam. which is independent of an intensity of the laser radiation. the Laser processing equipment includes a laser beam source that outputs a pulsed laser beam, and a beam shaping optical system for beam shaping the laser beam to form the quasi-non-diffractive laser beam having a focal zone extending in a longitudinal direction. The optical beam shaping system includes beam adjustment optics that are set up to output the laser beam as a raw laser beam with a beam diameter, and a beam shaping element that is set up to impress a phase on a beam cross section of the raw laser beam to form phase-impressed laser radiation for a predetermined beam diameter of the raw laser beam that when the phase-impressed laser radiation (5_PH) is focused into the partially transparent material of the workpiece (3), the quasi-non-diffracting laser beam (5) is generated with a resulting intensity distribution that is at least approximately constant in the longitudinal direction in the focal zone. The laser processing system also includes a workpiece holder for storing the workpiece, with the optical beam shaping system and/or the workpiece holder being set up to bring about a relative movement between the workpiece and the quasi-non-diffracting laser beam, in which the quasi-non-diffracting laser beam moves along a scanning trajectory in the material of the workpiece is positioned.

In a further aspect of this disclosure includes a method for material processing of a workpiece with a quasi-non-diffracting laser beam, wherein the workpiece has a material that is partially transparent to the quasi-non-diffracting laser beam and has linear absorption for laser radiation in the frequency range of the quasi-non-diffracting laser beam is independent of a laser radiation intensity, the steps:

Radiation of a pulsed raw laser beam into an optical beam-shaping system to form a quasi-non-diffracting laser beam with a focal zone extending in a longitudinal direction for the material processing of the workpiece, with the optical beam-shaping system being used to impress a phase on a beam cross-section of the raw laser beam in such a way that the quasi-non-diffracting laser beam has an intensity distribution on the focal zone that can be adjusted in the longitudinal direction, in particular adjusted to vary, such as a variable intensity distribution, and

Adjusting the phase imprinting in such a way that a resulting intensity distribution of the quasi-non-diffracting laser beam is at least approximately constant in the longitudinal direction when radiating into the partially transparent material of the workpiece at the focal zone. In a further aspect of this disclosure, a method for material processing of a workpiece with a quasi-non-diffracting laser beam comprises, the workpiece having a material which is partially transparent to the quasi-non-diffracting laser beam and which has linear absorption for laser radiation in the frequency range of the quasi-non-diffracting laser beam is independent of a laser radiation intensity, the steps:

Radiation of a pulsed raw laser beam into an optical beam shaping system, which is set up to output phase-impressed laser radiation for the formation of a quasi-non-diffracting laser beam with a focal zone extending in a longitudinal direction for the material processing of the workpiece, with the optical beam shaping system being used to impress a phase on a beam cross-section of the Raw laser beam takes place in such a way that the quasi-non-diffracting laser beam in the focal zone has an intensity distribution that is variable in the longitudinal direction, without taking into account linear absorption,

Adjusting the phase imprinting such that when the phase imprinted laser radiation is irradiated into the partially transparent material of the workpiece, a resulting intensity distribution of the quasi-non-diffracting laser beam in the focal zone is at least approximately constant in the longitudinal direction, and

Radiating the phase-impressed laser radiation into the partially transparent material of the workpiece to form the quasi-non-diffracting laser beam for material processing.

Another aspect of this disclosure includes a method for forming a, in particular diffractive optical, beam-shaping element that is intended for use in material processing of a workpiece in an optical beam-shaping system for beam-shaping a quasi-non-diffracting laser beam from a raw laser beam, the workpiece being a quasi-non-diffracting laser beam has partially transparent material that has a linear absorption for laser radiation in the frequency range of the quasi-non-diffracting laser beam, which is independent of a laser radiation intensity. The procedure includes the steps:

providing a linear absorption parameter of the partially transparent material in the frequency range of the quasi-non-diffracting laser beam, in particular by measuring a linear absorption parameter;

Defining a target intensity distribution as the resultant intensity distribution to be achieved in the workpiece along an optical axis of the quasi-non-diffracting laser beam, in which for modifying the material of the workpiece at a plurality of Positions along the optical axis, at least in sections, an intensity of the target intensity distribution is above an intensity threshold for non-linear absorption, which is dependent on a laser radiation intensity that is present in each case;

Specification of a transversal beam profile, in particular a beam diameter, of the raw laser beam onto which a two-dimensional phase distribution is to be impressed;

Calculate a two-dimensional phase distribution for the transverse beam profile by:

- Subdividing the transversal beam profile into, in particular ring-shaped, beam cross-sectional areas,

- assignment of, in particular identical linear, phase increases in the radial direction over the beam cross-section areas as the initial phase distribution, and

- Iterative adjustment of the phase increases in the beam cross-section areas and calculation of the intensity distribution along the optical axis that results in the workpiece after the raw laser beam has passed through the optical beam shaping system, taking into account the linear absorption given by the linear absorption parameter, until a two-dimensional phase distribution that compensates for the linear absorption is present, with which the target intensity distribution along the optical axis in the workpiece results as the resulting intensity distribution; and

Providing the beam-shaping element with the linear absorption-compensating two-dimensional phase distribution.

In a further aspect, this disclosure relates to a method for material processing of a workpiece with a quasi-non-diffracting laser beam, the workpiece having a material which is partially transparent to the quasi-non-diffracting laser beam and which has linear absorption for laser radiation in the frequency range of the quasi-non-diffracting laser beam is independent of a laser radiation intensity. The procedure includes the steps:

Radiation of a pulsed raw laser beam into an optical beam shaping system for beam shaping of the raw laser beam, wherein the optical beam shaping system is set up to impress a phase on a beam cross section of the raw laser beam in such a way that laser radiation of the raw laser beam is directed to a plurality of positions in the workpiece arranged along an optical axis in one Entry angle range (from, for example, approx. 5° to approx. 25° in the partially transparent material - corresponding to approx. 40° in air) with respect to the optical axis and forms the quasi-non-diffracting laser beam at the majority of positions, with intensity losses due to the linear Absorption occur during the propagation of the laser radiation in the partially transparent material to the plurality of positions, and

Adjusting the phase imprinting in such a way that laser radiation is guided at several angles from the angle of incidence range to at least one position of the plurality of positions, so that in the partially transparent material at the plurality of positions, despite the intensity losses that occur, an intensity threshold for nonlinear absorption is Material present laser radiation intensity depends is exceeded.

In some developments of the method, the phase imprinting on the beam cross-section of the raw laser beam can be set in such a way that laser radiation reaches a plurality of positions in the workpiece arranged along an optical axis in an angle of arrival, which in particular is in the range of, for example, approx. 5° to approx ° in the partially transparent material of the workpiece - corresponding to approx. 40° in air - is guided with respect to the optical axis and forms the quasi-non-diffracting laser beam with the resulting intensity distribution at the majority of positions, with intensity losses due to linear absorption during propagation of the laser radiation in the partially transparent material to enter the plurality of positions. The phase imprint can be set in such a way that at least one position of the plurality of positions laser radiation is guided at several angles out of the angle of incidence range, so that in the partially transparent material at the plurality of positions an intensity threshold for non-linear absorption is exceeded despite the intensity losses that occur, wherein the non-linear absorption in the partially transparent material depends on the respective intensity of the laser radiation.

In some developments of the method, laser radiation directed at a first angle to the at least one position of the plurality of positions can have a phase difference of less than Pi/4 with respect to laser radiation directed at a second angle to the at least one position of the plurality of positions is performed, has. Alternatively or additionally, the phase imprinting can be set in such a way that the laser radiation is guided in a rotationally symmetrical manner to the plurality of positions, so that each of the plurality of angles represents a local cone angle. In some developments of the method, adjusting the phase imprint can include adjusting phase increases in the radial direction, which are imprinted on beam cross-sectional areas of the raw laser beam, and/or adjusting geometric parameters of beam cross-sectional areas in which one or more phase increases are imprinted.

In some developments of the method, the beam cross-sectional areas can comprise at least two annular or ring-segment-shaped beam cross-sectional areas and the phase increases for the two annular or ring-segment-shaped beam cross-sectional areas can be set in such a way that laser radiation from the two annular or ring-segment-shaped beam cross-sectional areas of a common position of the plurality of positions among two different cone angles is supplied.

In some developments of the method, in addition to setting the phase imprint in beam cross-section areas, the intensity components of a raw laser beam intensity that are assigned to the beam cross-section areas can also be set in order to bring about the resulting intensity distribution of the quasi-non-diffracting laser beam in the focal zone.

In some developments of the method, the phase imprinting can be adjusted for a predefined transversal intensity distribution of the raw laser beam, in particular a predefined beam diameter of the raw laser beam, and a predefined linear absorption of the partially transparent material of the workpiece. For a material with a linear absorption that deviates from the specified linear absorption of the partially transparent material, the transversal intensity distribution, in particular a beam diameter, of the raw laser beam can be adjusted with unchanged phase imprinting, by an intensity component of a raw laser beam intensity that is supplied to one position of the plurality of positions will increase or decrease.

In some developments of the method, the phase imprint can be set in such a way that a decrease in intensity of the quasi-non-diffracting laser beam due to the linear absorption in the partially transparent material is compensated for at least in sections. In some developments of the method, the resulting intensity distribution of the quasi-non-diffracting laser beam along the optical axis can have an intensity distribution or an envelope intensity distribution that includes deviations from an average intensity of the quasi-non-diffracting laser beam in the range of up to 10%, with the average intensity refers to the part of the focal zone where there is a non-linear interaction with the material of the workpiece. Optionally, the intensity distribution or the envelope intensity distribution can in particular be essentially constant.

In some developments of the method, the partially transparent material can be modified based on the non-linear absorption at a plurality of positions in the focus zone, despite the intensity losses that occur. The modification of the partially transparent material can extend over a length of the quasi-non-diffracting laser beam or consist of a series of modification zones along the quasi-non-diffracting laser beam.

In some developments of the method, a laser beam with a Gaussian transverse intensity profile can be used as the raw laser beam, and the optical beam shaping system can be set up to shape a Bessel-Gaussian beam as a quasi-non-diffracting laser beam. Additionally or alternatively, a transverse extent of the quasi-non-diffractive laser beam in the focal zone can change along the optical axis and/or a transverse extent of the quasi-non-diffractive laser beam can depend on angles of incidence at a position of the focal zone, with which laser radiation is used to form the quasi- non-diffractive laser beam is incident on the optical axis at the position of the focal zone.

In some developments, the method can also include the following steps:

Setting beam parameters of the raw laser beam in such a way that the partially transparent material of the workpiece is modified,

Positioning at least a portion of the quasi-non-diffractive laser beam in the workpiece or

Bringing about a relative movement between the workpiece and the quasi-non-diffracting laser beam, in which the quasi-non-diffracting laser beam is moved along a scanning trajectory in the workpiece, so that a series of modifications is written into the workpiece along the scanning trajectory. In some developments of the method, the optical beam-shaping system can include a diffractive optical beam-shaping element and the diffractive optical beam-shaping element can have surface elements that adjoin one another, which build up a surface grating structure and to which a phase shift value is assigned, the phase shift values having a two-dimensional phase distribution corresponding to the set phase impression define. When the raw laser beam is radiated into the optical beam-shaping system, the phase imprinting can be effected with the diffractive optical beam-shaping element by impressing the phase distribution on the raw laser beam.

In general, the beam-shaping optical system may include a beam-shaping element formed according to the method of forming a beam-shaping element disclosed herein.

In some developments of the method for forming a beam-shaping element, the iteratively adapted phase increases in conjunction with intensity components of the raw laser beam present in the beam cross-section areas can cause a redistribution of the laser radiation contributing to the quasi-non-diffracting laser beam along the optical axis to form the target intensity distribution.

In some developments of the method for forming a beam-shaping element, a phase increase corresponds to an angle at which the laser radiation is guided to the optical axis. The two-dimensional phase distribution compensating for the linear absorption can be determined iteratively in such a way that laser radiation is guided to at least one position of a plurality of positions along the optical axis at a plurality of angles.

In some developments of the method for forming a beam-shaping element, the beam-shaping element can have surface elements which adjoin one another and are provided with phase shift values which are set in accordance with the two-dimensional phase distribution compensating for the linear absorption. In particular, the beam-shaping element can be embodied as a Fresnel-Axicon-like diffractive optical element whose phase shift values are fixed, or as a spatial light modulator whose phase shift values have been adjusted according to the phase distribution compensating for the linear absorption. In some developments, the method for forming a beam-shaping element can also include:

Deriving a height profile, in particular a thickness profile of an optical material or a mirror profile, from the two-dimensional phase distribution compensating for the linear absorption, a local height corresponding to a local phase shift value, and

forming a refractive or reflective axicon optic with the height profile as the beam shaping element.

In some developments of the laser processing system, the phase imprinting on a beam cross-section of the raw laser beam can be adjusted with the beam-shaping element in such a way that laser radiation of the raw laser beam is guided to a plurality of positions in the workpiece arranged along an optical axis in an angle of arrival range with respect to the optical axis and the quasi - forms non-diffracting laser beam at the plurality of positions. Intensity losses can occur due to the linear absorption during the propagation of the laser radiation in the partially transparent material to the plurality of positions, wherein the phase imprint can also be set such that laser radiation is guided at several angles from the angle of arrival range to at least one position of the plurality of positions, so that in the partially transparent material, an intensity threshold for non-linear absorption, which is dependent on a laser radiation intensity that is present in each case, is exceeded at the majority of positions despite the intensity losses that occur.

In some developments, the laser processing system can also include a controller that is set up to adjust the beam adjustment optics in such a way that the beam diameter on the beam shaping element is larger or smaller than the specified beam diameter by variations in the linear absorption with respect to the linear absorption for which the phase imprint was determined , to balance.

In some developments of the laser processing system, the beam-shaping element can be designed as a diffractive optical element, a spatial light modulator or a modified refractive or reflective axicon. In some developments of the laser processing system, the phase imprint can be designed so that the resulting intensity distribution has an intensity distribution or an envelope intensity distribution that includes deviations from an average intensity of the quasi-non-diffracting laser beam in the range of up to 10%, with the average intensity refers to a part of the focal zone in which a non-linear interaction with the material of the workpiece takes place. The intensity distribution or the envelope intensity distribution can in particular be essentially constant.

The concepts disclosed herein relate to the approach that a quasi-non-diffractive laser beam formed by means of an optical beam shaping system can have an intensity distribution in the focal zone in the formation of an elongated focal zone in air that varies in the longitudinal direction (usually in the direction of beam propagation) (i.e., is variably formed / was set), so that when this quasi-non-diffracting laser beam is irradiated into a workpiece to be machined, a resulting intensity distribution within the workpiece is preferably approximately constant. In particular, the course of the variable intensity distribution in air is matched to the linear absorption behavior of the material of the workpiece. The resulting intensity distribution is to be understood as meaning the intensity distribution present in the partially transparent material, whereas the variable intensity distribution mentioned is present without interaction with the linearly absorbing material of the workpiece (i.e., for example in air). For material processing, an approximately constant intensity distribution in the material includes, for example, deviations from an average intensity of the laser beam in the range of e.g. up to 10%, with the average intensity referring to the part of the focal zone in which the (nonlinear) interaction with the material of the workpiece takes place.

Those skilled in the art will appreciate that phase imprinting performed in accordance with the present invention can be accomplished using a refractive, diffractive, and/or reflective beamforming system. In addition to impressing the phase, it can also be provided that the beam shaping system is used to impress the amplitude of the raw laser beam.

The concepts disclosed herein relate in particular to beam shaping, which involves the arrival of beam components at an angle of arrival onto a beam axis of the laser beam for the Formation of an elongated focus zone caused by interference of the beam components. During material processing, the jet components are fed in partly through the material of the workpiece. Sections of the elongated focal zone lying downstream in particular are thus based on laser radiation which propagates through the material along an optical path whose length is of the order of magnitude of the length of the focal zone.

Furthermore, the beam shaping described herein relates to beam shaping that generates a quasi-non-diffracting beam for forming the elongated focal zone along the beam axis in the partially transparent workpiece. The linear absorption can affect the intensity distribution along the focal zone, particularly in the case of focal zones of this type that extend in the direction of propagation over a significant length of up to a few millimeters. A (longitudinal) intensity distribution along the elongated focal zone considered in this context is characterized by the progression of a maximum of the intensity in the direction of propagation. In some embodiments, the course of a quasi-non-diffracting beam can have several local intensity maxima along the elongated focal zone, so that in these embodiments a function enveloping the local intensity maxima can be used for the (longitudinal) intensity distribution (envelope intensity distribution). Furthermore, a transversal intensity distribution of the quasi-non-diffracting beam can be observed at every position in the propagation direction, in particular at every local intensity maximum.

With regard to laser processing, one speaks of an elongated focal zone if a three-dimensional intensity distribution with regard to a target threshold intensity is characterized by an aspect ratio (as the extension of the quasi-non-diffracting beam in the propagation direction in relation to the lateral extension across the quasi-non-diffracting beam (diameter of the intensity maximum)) of at least 10:1, for example 20:1 and more or 30:1 and more, or 1000:1 and more. In the case of a modulated intensity distribution, the aspect ratio can be related to the aforementioned enveloping function of the intensity distribution.

A quasi-non-diffracting beam can lead to a modification in the material with a similar aspect ratio or to an arrangement of several modification zones, which are delimited by an envelope with a corresponding aspect ratio, with a correspondingly sufficient intensity in the elongated focal zone. The modification/the Arrangement of several modification zones can preferably extend over a length of the quasi-non-diffracting laser beam (5).

In general, with the quasi-non-diffracting beams disclosed herein with such aspect ratios in the material, a maximum change in the transverse extent of the intensity distribution over the focal zone can be in the range of 50% and less, for example 20% and less, for example in the range of 10% and less lie on an average transverse extent, where the average transverse extent relates to that part of the focal zone in which the (non-linear) interaction with the material of the workpiece takes place. The same applies correspondingly to a maximum change in the transverse en/lateral extent of the modification.

The concepts described here are intended to generate elongated focus zones and correspondingly elongated modifications with high aspect ratios even in partially transparent materials.

The aspects disclosed herein relate in particular to the laser-based material processing of a partially transparent workpiece, the linear absorption of which is given by an absorption coefficient in the range from approx. 0.1/mm to approx. 2.5/mm.

Concepts are disclosed herein that allow at least some aspects of the prior art to be improved. In particular, further features and their usefulness result from the following description of embodiments with reference to the figures. From the figures show:

Fig. 1 Figures to clarify quasi-non-diffracting beams in comparison with a Gaussian beam,

2 shows a schematic sketch of a laser processing system for material processing,

3A to 3F schematic sketch to clarify the formation of a quasi-non-diffracting beam in a partially transparent workpiece,

Fig. 4 schematic representations to clarify the effect of the linear

absorption onto a quasi-non-diffracting ray, 5 shows a flow chart to illustrate a method for material processing of a workpiece that consists of a partially transparent material,

6A to 6C exemplary representations of radial height profiles of an axicon and a modified axicon as well as a radial phase curve,

7 shows a schematic illustration to clarify the adjustment of a longitudinal intensity distribution of the quasi-non-diffracting beam in the propagation direction when there is linear absorption in a workpiece by adjusting the phase imprint,

8 schematic illustrations of a quasi-non-diffracting beam formed according to the invention in a partially transparent workpiece and

9 shows a flowchart to clarify a method for forming an, in particular diffractive, optical beam-shaping element.

The aspects described herein relate in particular to the use of non-diffractive radiation in material processing. Non-diffracting beams ("non-chff active beams") - alternatively also known as propagation-invariant beams - can be formed by wave fields that correspond to the Helmholtz equation suffice and a clear separability into a transverse (ie, in the x and y direction) dependency and a longitudinal dependency (ie, a dependency in the z direction/direction of propagation) of the shape exhibit.

Here ' is the wave vector with its longitudinal/axial and transversal

components y) _any complex-valued function that depends only on the transversal coordinates x and y. Since the z-dependence in Equation 2 exhibits pure phase modulation, is an intensity of a function solving Equation 2 is propagation-invariant and is called “non-diffractive”: (equation 3) This approach provides different solution classes of the Helmholtz equation in different coordinate systems, such as so-called Mathieu rays in elliptical-cylindrical coordinates or so-called Bessel rays in circular-cylindrical coordinates.

See also J. Turunen and AT Friberg, “Propagation-invariant optical fields”, in Progress in optics, 54, 1-88, Elsevier (2010) and M. Woerdemann, “Structured Light Fields: Applications in Optical Trapping, Manipulation, and Organization”, Springer Science & Business Media (2012).

A large number of types of non-diffracting beams can be realized to a good approximation. These realized non-diffractive beams are referred to herein as "quasi-non-diffractive beams" or "spatial non-diffractive beams" or, for convenience, "non-diffractive beams". Quasi-non-diffracting beams, i.e. optically shaped/optically implemented non-diffracting laser beams, in contrast to the theoretical construct, have finite power. A length L of a propagation invariance assigned to them is also finite.

In comparison with intensity representations of a conventional Gaussian focus (see the propagation behavior of a Gaussian focus in figure (a) of Fig. 1), Fig. 1 shows the propagation behavior of quasi-non-diffracting rays using intensity representations in figures (b) and (c) . Figures (a), (b) and (c) each show a longitudinal section (x-z plane) and a transverse section (x-y plane) through the focus of a Gaussian beam and quasi-non-diffractive beams, respectively, which appear in z -direction, with arrows 2 additionally clarifying the direction of propagation in the z-direction (e.g. also in FIGS. 4 and 7). Figure (b) also shows a far-field transverse distribution F of the quasi-non-diffracting beam. See Fig. 2 for the position of the far-field distribution. When generating a quasi-non-diffracting beam with an axicon, only one spatial frequency is generated in the far-field, which is due to the (fixed) cone angle of the axicon.

Figure (b) refers to a rotationally symmetrical, quasi-non-diffracting beam, in this case a Bessel-Gaussian beam. Figure (c) refers to an asymmetric quasi-non-diffracting beam as an example. For a Bessel-Gaussian beam, panels (d) and (e) of Figure 1 also show details of a central intensity maximum. Figure (d) of FIG. 1 shows an intensity profile in a transversal sectional plane (x- y-plane) and a transverse intensity curve in the x-direction. Image (e) of FIG. 1 shows details of the central intensity maximum in a section in the direction of propagation (z-direction).

For the comparison of the quasi-non-diffracting beam with a Gaussian beam, a focus GF diameter ^u o of the Gaussian focus is defined, with the Gaussian focus being established via the second moments. Furthermore, an associated characteristic length of the Gaussian beam

, <• , T -ZR = 7i (d2 ^F ) ² /4A , ™ , over the Rayleigh length ■ ⁷ defined as a distance starting from the

Focus position is set at which the beam cross-section has increased by a factor of 2. Furthermore, for a quasi-non-diffracting beam, a transverse focal diameter JND o is defined as the transverse dimension of a local intensity maximum, where the VD transverse focal diameter ^H is given by the shortest distance of directly adjacent, opposite intensity minima (e.g. intensity drop to 25%). See, for example, Figures (b) and (d) in FIG. 1. The longitudinal (axial, in the direction of propagation) extent of the almost propagation-invariant intensity maximum can be regarded as a characteristic length L of the quasi-non-diffracting beam. It is defined by an intensity drop to 50%, starting from the local intensity maximum, in the positive and negative z-direction, see images (c) and (e) of Fig. 1.

Here, a quasi-non-diffracting ray is assumed if, for similar transverse dimensions, eg “o ~ “o , the characteristic length L of the quasi-non-diffracting ray clearly exceeds the Rayleigh length of the associated Gaussian focus, especially if L > 10z _R

(Quasi-) Bessel rays, also known as Bessel-like rays, are examples of a class of (quasi-) non-diffractive/propagation-invariant rays. With such rays, the transverse field distribution obeys near the optical axis in good

Approximation of a Bessel function of the first kind of order n. A subset of this class of rays are the so-called Bessel-Gaussian rays, which are widely used because they are easy to generate. A Bessel-Gaussian beam can be formed, for example, by illuminating an axicon of refractive, diffractive or reflective design with a collimated Gaussian beam. An associated transverse field distribution in the vicinity of the optical axis in the area of an associated elongated focal zone obeys in good Approximation of a Bessel function of the first kind of order 0 wrapped by a Gaussian distribution, see figure (d) of Fig. 1.

Typical Bessel-Gauss beams, which can be used to process transparent materials, have a diameter of the central intensity maximum on the optical axis in the range of on. The associated length L of a quasi-non-diffractive beam can easily exceed 1 mm, see figure (b) of Fig. 1. A focus of a Gaussian JGF JND _ or

beam with ^u o ~ ^u o E ¹¹¹ is characterized by a focal length in air of only a wavelength λ of 1 pm, see figure (a) in FIG. 10 R, for example 100 times or more or even 1000 times or more the Rayleigh length.

Figure (f) of Figure 1 shows an inverse Bessel-Gaussian beam as an example of another quasi-non-diffracting beam. It can be seen how the longitudinal intensity distribution of the inverse Bessel-Gaussian beam is inverted in comparison to the Bessel-Gaussian beam with respect to the propagation direction by imaging a virtual Bessel-Gaussian beam (see the publications mentioned at the beginning).

Aspects described herein are based in part on the recognition that when a workpiece made of a partially transparent material is to be processed with a quasi-non-diffractive beam, the linear absorption affects the intensity emitted along the quasi-non-diffractive beam, i.e. in the elongated focal zone , exists. This is particularly the case when the quasi-non-diffracting beam forms, for example, in an interference-based focal zone of a Bessel-Gaussian beam. Accordingly, beam shaping, as used to process essentially transparent workpieces, is no longer expedient, since the processing along the quasi-non-diffracting beam generated in this way would be carried out with different interaction conditions (due to the decreasing intensity in the direction of propagation) or no longer spatially in the required extent would take place.

In order to obtain the formation and properties of the quasi-non-diffracting beam, for example with a Bessel beam-like beam profile, in the partially transparent workpiece, it is proposed here that the absorbing beam that occurs when passing through the workpiece To counteract the effect by "increasing the injection of intensity along the focal zone". Correspondingly, a quasi-non-diffracting beam is formed with an intensity distribution that increases along the direction of propagation--ignoring the linear absorption--as would form, for example, in the case of a comparison workpiece without linear absorption or, for example, in air. The increase in the intensity distribution (without linear absorption in the workpiece) can then at least partially compensate for the decrease in intensity due to the linear absorption.

On the one hand, an increasing intensity distribution (without linear absorption in the workpiece) can be achieved by a special adaptation of the phase imprint (caused, for example, by a special shape of the geometry of the axicon or a specially designed phase distribution of a diffractive optical element).

On the other hand, known phase impressions with modified beam parameters can be used. For example, phase imprinting/beam-shaping optics can be designed in such a way that, without linear absorption in the workpiece, they cause a homogenized intensity distribution in the propagation direction for a given beam diameter by distributing laser power evenly along the focal zone, in particular by intensifying intensity in downstream sections of the quasi-non-diffracting beam redistributed. An example is a homogenized Bessel-Gaussian beam. In one embodiment of the invention, such phase imprinting/beam shaping optics can now be used with a varied, e.g. increased, beam diameter, where the beam diameter is chosen such that more intensity is redistributed into downstream portions of the quasi-non-diffractive beam, so as to reduce the linear counteract absorption.

It was thus recognized that an influencing of the intensity distribution along the propagation direction of a quasi-non-diffracting beam can be counteracted at least in sections by linear absorption during propagation in the workpiece. In addition, it was recognized that, with appropriate measures, beam shaping concepts and beam shaping components that were developed for essentially transparent workpieces can be used to form a quasi-non-diffracting beam in an absorbent material. It was recognized that, despite linear absorption during the propagation of laser radiation in a partially transparent workpiece, a quasi-non-diffracting beam can be generated with an intensity distribution that is essentially constant in the direction of propagation in the material of the workpiece. Correspondingly, extended modifications can also be written into a workpiece with partial transparency. As with essentially transparent workpieces, structural modifications produced in this way can, for example, enable a cutting process or be used to remove material.

Fig. 2 shows a schematic representation of a laser processing system 1 for processing a workpiece 3 with a quasi-non-diffractive (laser) beam 5. The concepts disclosed herein are aimed specifically at the processing of workpieces made of a material that is partially transparent with respect to the laser beam 5 and causes a linear absorption of the laser beam 5 accordingly. The workpiece 3 can be, for example, a partially transparent (e.g. colored) glass, such as a glass pane, or an object that is partially transparent for the laser wavelength used, such as a pane, in a ceramic or crystalline design (for example made of aluminum oxide or zirconium oxide such as sapphire, e.g. natural or artificially colored sapphire ) be. For example, the material in the spectral range of the laser beam 5 absorbs 50% of the intensity of a laser beam passing through over a length of 1 mm. In general, the material of the workpiece can have absorption coefficients in the range from approx. 0.1/mm to approx. 2.5/mm, with corresponding transmissions in the range from 90% to 10% per millimeter of material thickness, for example 50% per 1 mm glass thickness.

The processing with the quasi-non-diffracting laser beam causes a modification of the material of the workpiece 3 in a focus zone 7 that is formed by the quasi-non-diffracting laser beam 5 . As indicated in FIG. 2, the focal zone 7 is generally elongated in a direction of propagation (direction of propagation; here the z-direction) of the quasi-non-diffracting laser beam 5 . For example, the focal zone 7 can be formed as a focal zone of a Bessel-Gaussian beam or an inverse Bessel-Gaussian beam.

The laser processing system 1 comprises a laser beam source 11 (for example an ultra-short pulse high-power laser system) which generates and outputs a laser beam 5″. The laser beam 5″ is, for example, pulsed laser radiation. For material processing, laser pulses of the pulsed laser radiation have, for example, pulse energies that lead to pulse peak intensities in the quasi-non-diffracting beam that cause non-linear absorption in the material of the Workpiece 3 and thus bring about a formation of a modification in a geometry predetermined by the intensity profile of the quasi-non-diffracting beam.

For guidance and beam shaping, the laser processing system 1 also includes an optical beam shaping system 13. The optical beam shaping system 13 can be provided at least partially in a processing head of the laser processing system 1, which can be spatially aligned relative to the workpiece 3.

The optical beam shaping system 13 comprises beam shaping optics 15 for impressing the phase on a raw laser beam 5'. In FIG. 2, the laser radiation emerging from the beam-shaping optics 15 represents phase-impressed laser radiation 5 PH that is used to shape the quasi-non-diffracting beam 5 . Beam portions 5A, 5B and 5C of the phase-impressed laser radiation 5 PH are indicated by way of example. Diffractive optical beam-shaping elements and refractive or reflective optics implementations can be used as beam-shaping optics, it being possible for these to be embodied here as essentially equivalent optical means with regard to a transverse phase imprinting to be carried out.

The beam-shaping optics 15 is, for example, an axicon, a hollow-cone axicon, a (hollow-cone) axicon lens/mirror system, a reflective axicon lens/mirror system, these components being linear in their phase-impressing property Absorption in a workpiece were modified to produce a formation of increasing intensity distributions in comparison materials without linear absorption (see Figure 6B). Modified geometries of an axicon or inverse axicon deviate from the linear dependence of the thickness of the conventional cone-shaped axicon on a radial distance from the beam axis.

The beam shaping optics 15 can also be a programmable or permanently written diffractive optical beam shaping element, in particular a spatial light modulator (SLM spatial light modulator). For example, a diffractive optical beam-shaping element has surface elements that adjoin one another (see also FIG. 8, images (d1) and (d2)), which build up a surface grating structure in which each surface element is assigned a phase shift value. With the help of specially selected phase shift values, a geometry of a (hollow cone) axicon can be simulated, for example, where the Phase imprint can also be modified with respect to the implementation of a conventional axicon. For exemplary configurations of the optical beam shaping system 13 and in particular the beam shaping optics 15, reference is made to the publications mentioned at the outset. These also reveal that in the axial direction quasi-homogenized intensity distributions can be generated in elongated focal zones of Bessel-Gauss beams as an example of a quasi-non-diffracting beam in a transparent material. The homogeneity in intensity can be continuous along the elongated focal zone or there can be a sequence of intensity maxima with, for example, comparable intensity values along the focal zone.

The beam-shaping optics 15 can be set up to prevent beam portions of a raw laser beam 5' returning to the laser beam 5" from entering at an angle of incidence 6' onto a beam axis 9 for formation of the quasi-non-diffracting laser beam 5 along the beam axis 9 in the workpiece 3 by interference of the to cause beam components. For workpieces made of a partially transparent material, the entry angle 6' is in an entry angle range of, for example, approximately 5° to approximately 25° with respect to the beam axis 9 in the partially transparent material (correspondingly up to approximately 40° in air). For an elongated material processing, there are preferably comparable intensities in at least several sections of the quasi-non-diffracting laser beam 5 that cause non-linear absorption in the partially transparent material. For this purpose, for example, specially adapted angles of incidence 6' can be provided (see also Fig. 6B), which cause intensity components to be rearranged in the direction of propagation in order to adapt the intensity along the focal zone/the quasi-non-diffracting beam.

Optionally, the optical beam shaping system 13 includes a beam adjustment optics 17A, for example in the form of a first telescope (shown schematically in FIG. 2 using lenses LI A and L2_A). The beam adjustment optics 17A are set up to adjust a beam diameter of the laser beam 5″ and to feed the laser beam 5″ as a raw laser beam 5′ with a raw laser beam diameter D to the beam shaping optics 15 .

In FIG. 2, a Gaussian intensity distribution G with beam diameter D is indicated schematically in an intensity diagram I(y) for the raw laser beam 5′. By varying the spacing of the lenses LI A and L2_A, the beam adjustment optics 17A can be used to adjust the beam size at the beam shaping optics 15 . In FIG. 2, beam shaping with an axicon-like phase imprint is shown as an example with beam paths for different beam cross-sectional areas of the raw laser beam 5' (eg corresponding to intensity rings in the intensity diagram I(y)). An axicon cross-section 15A is indicated schematically in FIG. 2 by way of example. In the case of an axicon-like phase imprint, the laser radiation is guided rotationally symmetrically at positions along the optical axis 9, with each of the angles of arrival representing a local cone angle which acts on an intensity ring in the intensity diagram I(y).

The formation of a quasi-non-diffracting beam 5 is shown enlarged in FIG. 3A (for a fixed angle of incidence) and in FIG. 3B (for angles of incidence that are variably set in a range of angles of incidence).

In a z-y section plane along the beam axis 9 shown in Fig. 3A, an exemplary beam path for a Bessel-Gauss beam - as it can be used, for example, in the absence of linear absorption, i.e., for processing a transparent workpiece 3_o - based on Schematic beam components for the formation of a quasi-non-diffracting beam in . (Radial) beam components 5A, 5B, 5C are again indicated, which enter the beam axis 9 of the laser beam 5 at an angle of incidence (predetermined by the cone angle of the axicon) 6 in air or 6′ in the material.

In this case, laser radiation of the beam portion 5A, which is associated with a (radially inner) beam cross-sectional area R A of the raw laser beam 5' around the beam center, forms an initial section 6A of the quasi-non-diffracting laser beam. Laser radiation of the beam portion 5B, which is assigned to a central annular beam cross-sectional area R_B of the raw laser beam 5', forms a central section 6B of the quasi-non-diffracting laser beam. Laser radiation of the beam portion 5C, which is assigned to an outer ring-shaped beam cross-sectional area R_C of the raw laser beam 5', forms an end section 6C of the quasi-non-diffracting laser beam.

The quasi-non-diffracting beam is formed along the beam axis 9 in the transparent workpiece 3_o by interference of the beam components 5A, 5B, 5C (over a length /., see also FIG. 1). It can be seen that the beam portions 5B, 5C lying further to the outside cover a longer path in the material and thus—in the case of a partially transparent material—would be exposed to stronger linear absorption than the beam portion 5A lying further to the inside. Correspondingly, when using a conventional axicon (with a fixed cone angle) for beam shaping, the intensities present in the focus zone at sections 6A, 6B, 6C of the quasi-non-diffracting beam are affected to different extents by linear absorption.

Returning to FIG. 2, optical paths of the laser radiation of the beam portions 5A, 5B, 5C are indicated schematically, starting from the beam-shaping element 15 to the focal zone 7. The portion of the optical paths in the partially transparent material of the workpiece 3 is essential for the linear absorption. These portions of the optical paths are provided with the reference symbols 5A′, 5B′ and 5C′ for the laser radiation of the beam portions 5A, 5B, 5C in FIG. 3A .

As can also be seen in FIG. 2, each of the beam cross-sectional areas R A , R_B, R_C is assigned an intensity component I_A, I_B, I C of the intensity of the raw laser beam 5'. As will be appreciated by those skilled in the art, Fig. 2 and Fig. 3A simplify the mapping of beam cross-sectional area, intensity fraction, section of the quasi-non-diffracting laser beam.

Variations in the angle of arrival 6' can now be adjusted by adjusting the phase imprint for the material processing of a workpiece made of a partially transparent material. This is shown schematically for the partially transparent workpiece 3 in FIG. 3B.

For example, the phase imprint is set such that laser radiation varies in its angle of incidence onto the beam axis 9 along the quasi-non-diffracting laser beam or the quasi-non-diffracting laser beam is formed at a position/on a section by laser radiation from a number of angles of incidence. For example, in FIG. 3B, laser radiation 5B_T is more flat than laser radiation 5A_T; Laser radiation 5C_T is more steep than laser radiation 5B_T; Laser radiation 5D_T falls even more steeply than laser radiation 5C_T. With a corresponding choice of the angle of incidence for the different beam components, the intensity of the laser radiation, which is guided to the different sections 6A_T, 6B_T, 6C_T along the optical axis 9, in order to constructively interfere there and form the quasi-non-diffracting laser beam, can be adjusted to the different intensities Influences of linear absorption can be adjusted. If one considers FIGS. 3A and 3B as a ray-optical comparison, in FIG. usually in the transparent material). In Fig. 3B, the generation of a (quasi) non-diffractive laser beam is effected with a plurality of specifically set local cone angles with resulting varying arrival angles 5'_1, 6'_2. It is noted that in FIG. 3B , for example, laser radiation 5C_T and laser radiation 5D_T impinge on the optical axis 9 next to one another for the sake of clarity. Depending on the position of the contributing beam cross-section areas and the associated phase increases (angle of arrival), laser radiation is guided to a position on the optical axis 9 at a number of angles (from an angle of arrival area assigned to the beam-shaping element 15 ). The respective phase difference, which is present in the focal zone 7 due to the different phases accumulated along the various optical paths, is included in a (constructive/destructive) superimposition of the laser radiation at a plurality of angles.

3C also shows a transverse far-field distribution F T , as can be present when generating a quasi-non-diffracting laser beam that is homogenized in a partially transparent material. For the position of the far-field distribution F T see Fig. 2. The far-field distribution F T shows a spatial frequency spectrum that has several frequencies (corresponding to the angle δ'_l, 6'_2) based on the spatial interferences. Compared to the generation of a quasi-non-diffracting laser beam homogenized in a transparent material, the weighting of intensities of the spatial frequencies for the generation of the quasi-non-diffracting laser beam homogenized in the partially transparent material is adapted to the linear absorption behavior.

Referring to Fig. 2, the optical beam shaping system 13 also includes an imaging system 17B, which is designed, for example, in the form of a second telescope (shown schematically in Fig. 2 using lenses LI B, L2_B) for imaging a real or virtual beam path in the partially transparent workpiece 3 is. The imaging system 17B can also be used to adjust the length of the quasi-non-diffracting beam in the workpiece 3, for example by changing the focal length of the imaging system 17B. The person skilled in the art will recognize that the lens L1_B can also be combined with the beam-shaping element 15, as in the publications mentioned at the outset. A far-field distribution of the quasi-non-diffracting laser beam is also formed in the imaging system 17B (for example the far-field distribution F of FIG. 1 image (b) or the far-field distribution FT of FIG. 3C). The position PF of the far field is indicated schematically in FIG. 2 by an intermediate focus between the lenses LI B , L2_B.

The optical beam shaping system 13 can have further beam-guiding components such as, for example, deflection mirrors, filters and control modules for aligning and adjusting the various components.

The laser processing system 1 also includes a workpiece holder 19, indicated schematically in Fig. 2, for storing and optionally moving the workpiece 3.

For the processing of the workpiece 3, a relative movement takes place between the optical beam shaping system 13 (the quasi-non-diffracting laser beam) and the workpiece 3, so that the quasi-non-diffracting beam 5/the focal zone 7 is at different positions along a predetermined (processing) trajectory T im Workpiece 3 can be formed. The quasi-non-diffracting laser beam 5 can preferably be moved along the scanning trajectory, so that a series of modifications is written into the workpiece along the scanning trajectory T. For e.g. separating the workpiece 3 into two parts, the trajectory T then determines the course of a subsequent separating line.

The laser processing system 1 also has a controller 21 which, in particular, has an interface for a user to input operating parameters. In general, the controller 21 includes electronic control components such as a processor for controlling electrical, mechanical and optical components of the laser processing system 1. For example, operating parameters of the laser beam source 11 such as pump laser power, pulse duration, pulse energy, parameters for setting an optical element (e.g. an SLM) and Parameters for the spatial alignment of an optical element of the optical beam shaping system 13 and/or parameters of the workpiece holder 19 (for traversing the scanning trajectory T) can be set. In FIG. 2, the functional connection of the controller 21 to the various controllable components is indicated by dashed connections 21A. In general, the controller 21 can be set up to set the phase imprinting in such a way that when it is irradiated into the partially transparent material of the workpiece, i.e. when the phase imprinted laser radiation is focused into the partially transparent material of the workpiece, a resulting intensity distribution of the quasi-non-diffracting laser beam 5 in the focal zone is at least approximately constant in the longitudinal direction z. For example, the controller 21 can be set up to adjust the phase distribution of an adjustable diffractive optical element (SLM).

Alternatively or additionally, the controller 21 can be set up, for example, to set a size of at least one of the beam cross-sectional areas R A , R_B, R_C and/or at least one of the intensity components I_A, I_B, I C . The adjustment can be made in particular such that several of the intensity components of the radiation take into account an intensity loss that occurs due to the linear absorption along an optical path from the respective beam cross-sectional area to the associated section 6A_T, 6B_T, 6C_T of the quasi-non-diffracting laser beam. With beam parameters set in this way, the material in the associated sections 6A_T, 6B_T, 6C_T of the quasi-non-diffracting laser beam can be modified based on a non-linear absorption that depends on the intensity of the quasi-non-diffracting laser beam in the respective section. For example, the controller 21 for setting the sizes of the intensity components I_A, I_B, I C (and/or the beam cross-sectional areas R A, R_B, R_C) can control the telescope arrangement 13 A to increase or decrease the beam diameter D of the raw laser beam 5 'on the beam shaping optics 15 .

Alternatively or additionally, the controller 21 can be set up, for example, that for a material with a linear absorption that deviates from a linear absorption of the partially transparent material for which a phase imprint was designed, an adjustment of the transversal intensity distribution of the raw laser beam with an unchanged phase imprint ment is made to increase or decrease an intensity component of a raw laser beam intensity applied to one position of the plurality of positions, thereby compensating for the deviation in linear absorption.

In general, the laser radiation used for material processing, ie the laser beam 5", the raw laser beam 5' and the laser beam 5, is defined by beam parameters such as wavelength, spectral width, temporal pulse shape, formation of pulse groups, beam diameter, transverse intensity profile, transverse input phase profile, input divergence and/or polarization.

Exemplary parameters of laser radiation that can be used within the scope of this disclosure are:

Laser pulse energies/energy of a laser pulse group (burst): e.g. in the mJ range and more, for example in the range between 20 pj and 5 mJ (e.g. 1200 pj), typically between 100 pj and 1 mJ

Wavelength ranges: IR, VIS, UV (e.g. 2 pm > k > 200 nm; e.g. 1550 nm, 1064 nm, 1030 nm, 515 nm, 343 nm)

Pulse duration (FWHM): a few picoseconds (e.g. 3 ps) and shorter, e.g. a few hundred or a few (tens) of femtoseconds

Number of laser pulses in a burst: e.g. 2 to 4 pulses (or more) per burst with a time interval in the burst of a few nanoseconds

Number of laser pulses per modification: one laser pulse or one burst for one modification Repetition rate: usually greater than 0.1 kHz, e.g. 10 kHz

Length of the focal zone in the material: greater than 20 pm, up to a few millimeters Diameter of the focal zone in the material: greater than 1 pm, up to 20 pm and more (resulting lateral extension of the modification in the material: greater than 100 nm, e.g. 300 nm or 1 pm , up to 20 pm and more)

Feed d between two adjacent modifications: at least the lateral extent of the modification in the direction of advance (usually at least twice the extent, for example four times the extent)

The pulse duration refers to a single laser pulse. Similarly, an exposure time refers to a group/burst of laser pulses that result in the formation of a single modification at a location in the material of the workpiece. If the exposure time, like the pulse duration, is short with respect to a given feed rate, one laser pulse and all laser pulses of a group of laser pulses contribute to a single modification at one location. At lower feed rates, continuous modification zones, which include modifications that border on one another and merge into one another, can also arise. The aforementioned parameter ranges can allow material processing with quasi-non-diffracting beams that protrude up to, for example, 20 mm and more (typically 100 μm to 10 mm) into a partially transparent workpiece.

According to Fig. 2, the laser beam 5" is fed to the optical beam shaping system 13 for beam shaping, i.e. for converting one or more of the beam parameters. Usually, the laser beam 5″ and correspondingly the raw laser beam 5′ will be approximately a collimated Gaussian beam with a transverse Gaussian intensity profile.

The propagation of the laser radiation and in particular the optical beam shaping system 13 can be assigned an optical axis 9 which preferably runs through a point of symmetry of the beam shaping optics 15 (e.g. through a beam center position of an axicon (axicon tip) or a diffractive optical beam shaping element). The laser radiation is propagated along the optical axis 9. In the case of a rotationally symmetrical laser beam 5", an intensity maximum of a transversal beam profile of the laser beam 5" (Gaussian intensity distribution G in Fig. 2) can occur along the optical axis 9 of the optical beam shaping system 13. Depending on the diameter D of the intensity distribution G, a correspondingly large area of the beam shaping optics 15 is illuminated.

The optical beam shaping system 13 forms the quasi non-diffracting laser beam 5 from the raw laser beam 5 ′, which forms the focal zone 7 . For example, a Bessel-Gaussian beam with an ordinary or inverse Bessel-beam-like beam profile can be generated by means of the beam shaping optics 15.

In general, when processing partially transparent materials using non-linear absorption, it applies that as soon as non-linear absorption occurs, this absorption itself or the resulting change in the material properties can influence the propagation of laser radiation. In the case of quasi-non-diffracting beams, the beam portions used for modification further downstream can be fed to the interaction zone at an adapted angle of incidence to the focal zone axis, so that the areas of the quasi-non-diffracting beam lying upstream are not irradiated. An example of such an energy input is the Bessel-Gaussian beam, which has an annular far-field distribution whose annular width is typically small compared to the radius (see panel (b) of Figure 1). With a rotationally symmetrical Bessel-Gaussian beam, the Interaction zone / focus zone axis radial beam components supplied essentially rotationally symmetrically with this predetermined angle. The same applies to the inverse Bessel-Gauss beam and to modifications such as homogenized, asymmetric or modulated (inverse) Bessel beams.

Even if areas of non-linear absorption can be avoided when laser radiation is supplied to downstream sections, the linear absorption of the partially transparent workpiece affects the laser radiation that forms downstream sections of the quasi-non-diffracting beam.

A consideration of the effect of the absorbent material property of a partially transparent workpiece 3 is summarized with reference to FIGS. 3D to 3F. One can see that the (radial) beam components arrive at an angle of incidence β in air or an angle of incidence (cone angle) β' in the material onto the optical axis 9 of the laser beam. With a refractive index n of the workpiece, the angle of incidence β' is given by . The quasi-nonlinear beam can form along the beam axis 9 in the workpiece 3 through interference of the incoming beam components over an entire thickness d of the partially transparent workpiece 3 .

The linear absorption can be described by the "optical depth" according to . The absorption coefficient a results from it: ^!

The linear absorption takes place along the optical paths up to positions x (in connection with FIGS. 3D to 3F the propagation of the laser radiation takes place in the x-direction) on the optical axis 9 . The associated path lengths are given by .

With a modified absorption coefficient the decrease in power along the optical paths results in . The decrease in performance

(damping behavior in the material) along the optical axis results in

For example, FIG. 3E shows the attenuation behavior for a partially transparent material with a thickness d=1 mm and a refractive index of n=1.45 at a cone angle of phase imposed radiation of ß = 20°. Assuming 50% of the power is linearly absorbed in the material (PO = 1 on the entry side, Pd = 0.5 on the exit side), this results in a modified absorption coefficient a' of 0.71.

3E shows the exponential power decay P(x). To compensate for the damping behavior in the material, the inversion of P(x) gives the required compensation Figure 3F shows the compensation function Pk(x) for the values discussed above by way of example. The profile of the compensation function in the partially transparent material corresponds to the required intensity profile on the optical axis 9 of the non-diffracting beam in the event that there is no linear absorption.

In other words, the formation of a comparable intensity in the sections 6A_T, 6B_T, 6C_T of FIG. 3B requires that the contributing portions of the laser radiation 5A_T, 5B_T, 5C_T, 5D_T introduce a comparable intensity entry into the corresponding sections of the quasi-non-diffracting beam. That is, the intensity components I_A, I_B, I C of the intensity of the raw laser beam 5' for the different sections 6A_T, 6B_T, 6C_T should be comparable if a comparable nonlinear absorption (for a comparable interaction with the material) is to take place in each of the sections.

FIG. 4 illustrates the effect of the linear absorption when a homogenized Bessel beam is used for processing a partially transparent workpiece, which is generated with beam shaping optics designed for a transparent workpiece.

An intensity longitudinal section 31A through a focal zone and an associated intensity curve 31B along the beam axis 9 of the homogenized Bessel beam can be seen, as would be present in the transparent workpiece. The maximum intensity along the beam axis 9 - as used with a transparent workpiece - is essentially constant over a significant length (indicated by lines 32A, 32B in Figure 4) of the quasi-non-diffracting beam.

If such a homogenized Bessel beam is radiated into a partially transparent material, a dashed intensity profile 3 IC results, in which the intensity along the beam axis 9 decreases continuously with the penetration depth into the material due to the linear absorption. A dashed intensity curve 3 ID shows a corresponding one Reduction of the intensity for a modulated quasi-non-diffracting beam, which forms several comparable intensity maxima in the direction of propagation instead of a homogeneous intensity curve in the transparent material.

In a flow chart, FIG. 5 illustrates the method proposed here for material processing of a workpiece with a quasi-non-diffracting laser beam, the workpiece having a material that is partially transparent to the quasi-non-diffracting laser beam. Partial transparency means that the material has a linear absorption for laser radiation in the frequency range of the quasi-non-diffracting laser beam, which is independent of an intensity of the laser radiation.

The method includes step 101 of generating a raw laser beam for beam shaping. In a step 101A, the generation of the raw laser beam can generate a laser beam with a laser system (in FIG. 2: laser source 11) with beam parameters that are designed for the material processing to be carried out (sufficient power, desired pulse duration, etc.). Furthermore, in a step 101B, a geometric beam parameter such as a beam diameter of the raw laser beam can be adjusted to a beam shaping element provided for the phase imprinting, in particular the implemented two-dimensional phase distribution (for example in FIG. 2 with the beam adjustment optics 17A).

The method also includes step 103, in which the raw laser beam (in Figure 2: raw laser beam 5') with a raw laser beam intensity (here the intensity of the entire raw laser beam 5') is fed into an optical beam shaping system for beam shaping (in Figure 2: the optical beam shaping system 13, which optionally includes the beam adjustment optics) is irradiated. The optical system is set up in such a way that the raw laser beam (after beam shaping has taken place) can form the quasi-non-diffracting laser beam with a focal zone extending in a longitudinal direction for the material processing of the workpiece in the workpiece. The optical beam shaping system is used to impress a phase on a beam cross section of the raw laser beam in such a way that the quasi-non-diffracting laser beam has an intensity distribution that is variable in the longitudinal direction at the focal zone. Due to the beam shaping, sections arranged in the direction of propagation (in Fig. 3: sections 6A, 6B, 6C) of the quasi-non-diffracting laser beam are separated from beam cross-sectional areas of the raw laser beam (in Fig. 2: for example the annular cross-sectional areas RA assigned to the beam portions 5A, 5B, 5C, R_B, R_C) shaped. The representation in Fig. 2 simplified to the effect that the phase imprinting can generally be carried out freely/flexibly in such a way that laser radiation can be guided to a position of the focal zone (in the longitudinal direction) from different beam cross-sectional areas of the raw laser beam. In this case, intensity components (in FIG. 2: intensity components I_A, I_B, IC) of the raw laser beam intensity are assigned to the beam cross-sectional areas of the raw laser beam.

By irradiating (step 103) the raw laser beam into the beam-shaping optical system, beam-shaping of the raw laser beam (step 101A) is performed. In this way, a two-dimensional phase distribution is impressed (step 103A) (in particular with a diffractive optical beam-shaping element or with an axicon optics modified, e.g. (in the cone angle)) onto the beam cross-section of the raw laser beam 5' (formation of phase-impressed laser radiation). The imposed two-dimensional phase distribution causes the phase-impressed laser radiation from the beam cross-sectional areas of the raw laser beam to be fed to the sections of the quasi-non-diffracting laser beam arranged in the direction of propagation.

The aim of phase imprinting is now to achieve an at least approximately constant intensity curve over a significant length of the focal zone in the workpiece, despite the partial transparency of the workpiece.

This is implemented in step 103 by adjusting the phase imprint based on at least one of the intensity components and/or a size of at least one of the beam cross-sectional areas. The phase imprint is set in such a way that a resulting intensity distribution of the quasi-non-diffracting laser beam is at least approximately constant in the longitudinal direction when radiating into the partially transparent material of the workpiece at the focal zone. In other words, the setting is made in such a way that when assigning the intensity components for the different positions of the focal zone (in the longitudinal direction), an intensity loss is taken into account in each case, which due to the linear absorption along an optical path from the respective beam cross-section area to the associated section of the quasi-non-diffractive laser beam enters. The consideration is implemented with regard to the material processing in such a way that the material in the sections of the quasi-non-diffracting laser beam is modified based on a non-linear absorption that depends on the intensity of the quasi-non-diffracting laser beam in the respective section. In step 103, for example, for the generation of a quasi-non-diffracting laser beam with an aspect ratio of at least 1:10, in particular at least 1:100, a decrease in intensity along the quasi-non-diffracting laser beam due to the linear absorption can be compensated at least in sections. Additionally or alternatively, step 103 can include, for example, that when the quasi-non-diffracting laser beam is formed in a comparison material that has essentially no linear absorption, an intensity along the quasi-non-diffracting laser beam in the comparison material is variable, eg increasing.

In step 103, a phase imprint specifically taking into account the linear absorption can be set in the beam shaping system. For example, phase increases to be impressed in the radial direction can be set in a plurality of beam cross-sectional areas (step 103A). Furthermore, geometric parameters (such as size and position) of the beam cross-section areas can be adjusted/adjusted in the phase imprint (step 103B). In this way, sizes of beam cross-sectional areas and/or positions of beam cross-sectional areas with respect to the raw laser beam, which are exposed to a uniform phase imprint, can be adapted to predetermined intensity components of the raw laser beam. In addition to discrete, e.g. for example, multiple phase increases in the radial direction can be implemented simultaneously in a beam cross-sectional area in order to deliver laser radiation to multiple positions along the optical axis from this beam cross-sectional area.

Additionally or alternatively, in step 103, a beam diameter of the raw laser beam can also be set at the beam shaping optics in order to set the intensity components of the raw laser beam assigned to the beam cross-sectional areas (RA, R_B, R_C) (step 103C). In this way, the beam diameter can be enlarged or reduced in order to use a phase imprint, which was designed for an absorption other than linear absorption of a material to be processed, for the other linear absorption as well.

In a step 105, beam parameters of the laser beam such as pulse duration and pulse energy can be readjusted, so that the material of the workpiece is (structurally) modified in the quasi-non-diffracting beam. In a step 107, the phase-impressed laser radiation is focused into the partially transparent material of the workpiece; ie at least part of the quasi-non-diffracting laser beam is positioned in the workpiece in such a way that the linear absorption that occurs is at least partially compensated for by the phase imprint.

Furthermore, in a step 109, a relative movement between the workpiece and the quasi-non-diffracting laser beam can be carried out, in which the quasi-non-diffracting laser beam is repeatedly positioned in the material of the workpiece along a scanning trajectory, so that an arrangement/arrangement of modifications in the material of the workpiece is written along the scanning trajectory.

FIGS. 6A and 6B illustrate a modified geometry of an axicon for a homogenized Bessel-Gaussian beam for processing a partially transparent material. FIG. 6A shows a linear decrease in the thickness d of a conventional axicon with the distance from the optical axis 9. In contrast to this, FIG. 6B shows a decrease in the thickness d for a correspondingly modified axicon. An initially (radially inward) greater decrease in the thickness d can be seen, followed by a slower decrease in the thickness d and again followed by a greater decrease in the thickness d. The variation in thickness d causes intensity components to be shifted/refracted backwards in the direction of propagation into the quasi-non-diffracting laser beam. The resulting homogenized intensity distribution in the partially transparent workpiece then preferably corresponds to the intensity distribution already shown in FIG. 4 for the processing of a substantially transparent material.

As already mentioned, a corresponding phase impression can alternatively or additionally be carried out reflectively or with a diffractive optical beam-shaping element.

6C shows a phase curve (calculated in a thin element approximation) oscillating between +7t and -7t, as can be simulated with phase shift values of a diffractive optical beam-shaping element. In a rotationally symmetrical case, adjusting the phase imprinting with a diffractive optical beam-shaping element includes adjusting (sawtooth-shaped) phase increases in the radial direction that are imprinted on beam cross-sectional areas of the raw laser beam. In particular, FIG. 6C shows the phase curve corresponding to a phase imprint in a central region of the modified axicon of FIG. 6B; ie the phase profile reproduces the height profile of the modified axicon. In Fig. 6B it is difficult to see how the oscillation of the phase shift values between +K and -7t varies in their oscillation frequency in the radial direction in order to understand the deviation from the fixed cone angle.

7 illustrates the formation of intensity distributions for the material processing of partially transparent workpieces with a rotationally symmetrical optical beam shaping system and correspondingly rotationally symmetrical laser beams and intensity distributions.

7 shows the raw laser beam 5' just before it hits a conventional axicon 15B or a modified axicon 15C. Furthermore, FIG. 7 shows intensity distributions in a schematized form as they result from the beam shaping, specifically applied upwards in a substantially transparent material, ie. H. without linear absorption (intensity I(-)), or plotted downwards in a partially transparent material, i. H. with linear absorption (intensity I (+)).

With an incident Gaussian beam (example intensity distribution G l), the conventional axicon 15B forms a Bessel-Gaussian beam with a longitudinal intensity distribution BG l(-) in the transparent material and a deformed Bessel-Gaussian beam with a longitudinal intensity distribution BG_1(+ ) in the partially transparent material, where the intensity distribution BG_1(+) decreases faster than the intensity distribution BG_1(-) due to the linear absorption in the propagation direction.

For the processing of a transparent material, the modified axicon 15B can be modified, for example, in such a way that with an incident Gaussian beam with the intensity distribution G l and the corresponding beam diameter D l in the transparent material, a Bessel-Gaussian beam homogenized in the propagation direction with a homogenized intensity distribution BG h(-) (corresponding to 31B in Fig. 4). Also as indicated in FIG. 4, this homogenized intensity distribution is deformed due to the linear absorption during irradiation into a partially transparent material (intensity distribution BG_h(+); corresponding to 3 IC in FIG. 4). Provided that the corresponding beam parameters of the raw laser beam 5', such as pulse duration and pulse energy, have been set, the homogenized intensity distribution BG h(-) can generate intensities over a length L(-) in Direction of propagation lead to a non-linear absorption / interaction with the transparent material. It can be seen from the intensity distribution BG_h(+) that this length is significantly reduced when irradiating a partially transparent material.

To compensate for the linear absorption, the phase imprint, i.e. in the example of the modified axicon the decrease in the thickness d of the axicon with the distance from the beam axis 9 and in the case of a diffractive optical element the setting of the phase shift values, can be adjusted in order to “redistribute the intensity components “ to cause an at least approximately constant intensity distribution in the longitudinal direction z.

If the intensity components are redistributed in such a way that the increase in the direction of propagation is adapted to the linear absorption and the decrease in intensity is essentially compensated, a harmonized intensity distribution BG_2h(+) can be formed in the partially transparent material. In this way, the homogenized intensity distribution BG_2h (+) can generate intensities which - provided that the corresponding beam parameters of the raw laser beam 5' have been irradiated - lead to a non-linear absorption/interaction with the partially transparent material over a length L(+) in the direction of propagation. Provided a corresponding laser power is available, the length L(+) can be dimensioned comparable to the length L(-). If such a phase-impressed laser beam were radiated into a transparent material, an intensity distribution BG_2(-) results along the quasi-non-diffracting laser beam, which increases with the penetration depth.

To compensate for the linear absorption, the beam diameter of the incident raw laser beam 5' can be increased alternatively or additionally, for example with the telescope 17A (beam diameter D_2 in FIG. 7). This increases the intensity component in the cross-sectional areas R_B, R_C. For example, since the outer beam components contribute to the rear sections 6B_T, 6C_T of the quasi-non-diffracting laser beam in the phase imprint for the homogenized intensity distribution BG h(-), by increasing the beam radius in a Bessel-Gaussian beam (starting from, for example, a phase imprint for an intensity distribution BG_h(-)) homogenized in the transparent material, the absorption can be compensated for at least in sections in the partially transparent material. In other words, the intensity along the quasi-non-diffracting laser beam can be at least approximately constant (similar to the homogenized intensity distribution BG_2h (+)). In the transparent material, the intensity would be along the quasi-non-diffractive Laser beam increase (intensity distribution BG_2(-)). It is noted that the intensity profiles are shown schematically in FIG. 7 in order to indicate decreases or increases in intensity, with the exponential influences of the linear absorption also being indicated schematically.

As indicated in connection with FIG. 7, when using beam shaping optics that have been optimized for a transparent material, with an adapted beam diameter, there are differences in the intensity distributions for the transparent material or the partially transparent material. Those skilled in the art will recognize that this is due to the coarse adjustment of the intensity components in pure beam expansion. Furthermore, in mixed configurations of the beam shaping optics, intensity distributions can be generated which are suitable both for transparent materials with a given beam diameter and for partially transparent materials with a different beam diameter.

FIG. 8 illustrates details of a quasi-non-diffracting laser beam with a central intensity maximum generated in a partially transparent material. Figure (a) shows a section in the direction of propagation (z-direction), in which the pronounced central intensity maximum can be seen accompanied by radially outer (ring-shaped) secondary maxima. Figure (b) shows an intensity curve in the z-direction, which forms a plateau over essentially the entire length (homogenized intensity distribution). Figures (c1), (c2) and (c3) each show an intensity curve (beam profile) in a transversal section plane (x-y plane) at the beginning, in the middle and at the end of the plateau.

The mean beam profile at z = 75 a.u. (mid-plateau) scales in the transverse dimensions by a factor of about 2 compared to the profiles at z = 10 a.u. (beginning of the plateau) or z = 110 a.u. (end of plateau). This can be recognized, for example, by the diameter of the central maximum. The variations in the diameter of the central maximum are due to the fact that several angles of incidence contribute and a transverse extent of the quasi-non-diffracting laser beam depends on angles of incidence contributing to the optical axis at a longitudinal position of the focal zone.

In general, when using a contribution from multiple angles of arrival for the intensity at a longitudinal position, it should be noted that the angles of arrival (for a intensity that is as constant as possible throughout) if possible do not lead to a phase shift that causes destructive interference. Thus, laser radiation directed at a first angle to the at least one position of the plurality of positions preferably has a phase difference of less than ±7t/4 with respect to laser radiation directed at a second angle to the (same) at least one position of the plurality is guided by positions.

For the sake of completeness it is noted that for an inverse Bessel-Gaussian beam homogenized for a partially transparent material, ray contributions from the center of the incident raw laser beam contribute to the intensity at the end of the quasi-non-diffractive laser beam. If an intensity increase (without linear absorption) along the quasi-non-diffracting laser beam is to be achieved for such a homogenized inverse Bessel-Gaussian beam, a corresponding reduction in the size of the beam cross-section is necessary in order to increase the intensity components corresponding to the downstream sections of the beam are assigned quasi-non-diffracting laser beam.

Figures (d) and (e) in FIG. 8 show examples of central sections of diffractive optical elements/applied phase profiles for the formation of inverse Bessel-like beams. Schematically, planar elements 15a adjoining each other are indicated, which build up a planar lattice structure. Each of the surface elements 15a is assigned a phase shift value which is applied to the laser radiation passing through. The phase shift values in the grating structure together form a phase mask through which the raw laser beam passes in order to experience a corresponding phase impression.

Figure (d) belongs to a phase mask for implementing an ideal (inverse) axicon (the period in the sweep of the phase shift values does not change). A phase impression with such a diffractive optical element can be used to form an intensity distribution according to FIG. 1 image (f).

Figure (e) belongs to a phase mask for implementing an (inverse) modified axicon (the periods in the sweep of the phase shift values are radius dependent). The phase distribution is designed in such a way that, with a specific beam diameter, longitudinal homogenization in the partially transparent workpiece can be expected, taking into account the associated absorption coefficient. Will be a bigger one If the beam diameter is chosen, an intensity profile of an inverse homogenized Bessel beam can be generated in a transparent material in a good approximation, which comes close to that shown in FIG.

It is noted that with respect to Figures (d) and (e) of Figure 8, complex conjugate phase distributions (inverted sign of the phase shift values) allow implementations of corresponding real axicon optics.

Fig. 9 shows a flowchart to explain a method for forming a beam shaping element, which is intended for use in material processing of a partially transparent workpiece in an optical system for shaping a quasi-non-diffracting laser beam (with an intensity distribution resulting from the phase imprint) from a raw laser beam . The aim is to set a phase imprint for a specified transversal intensity distribution of the raw laser beam, in particular a specified beam diameter of the raw laser beam, and a specified linear absorption of the partially transparent material of the workpiece. In particular, the phase curve of a phase mask, which is produced with a diffractive optical element, can be determined with the method.

The absorption behavior of the material to be processed is given. For example, by measuring the intensity Pd in FIG. 3D, a linear absorption parameter (the “optical depth T”) of the partially transparent material in the frequency range of the quasi-non-diffracting laser beam can be provided (step 201). Based on this, one calculates (or sets) the target intensity distribution on the optical axis in the workpiece, which is required to modify the material eg over the entire thickness d or over a desired length (step 203). A target intensity distribution in the workpiece along an optical axis of the quasi-non-diffractive laser beam can be defined in such a way that the target intensity distribution has an intensity above an intensity threshold, at least in sections, which is dependent on a nonlinear absorption that is dependent on a laser radiation intensity that is present in each case , is necessary for modifying the material of the workpiece at a plurality of positions along the optical axis. For the determination of the phase distribution, a transversal beam profile of the raw laser beam (intensity profile) is also to be specified, onto which the phase distribution is to be impressed (step 205).

An optics design of an axicon-like element (e.g. modified refractive or reflective axicon or diffractive optic element) is then calculated for the target intensity distribution (step 207):

—Starting from a phase imprint with an axicon (rise angle/phase rise is constant), it is divided into radial elements where the rise angle can be changed. A phase increase corresponds to an angle of arrival at which laser radiation is guided to the optical axis. (Subdivision of the transversal beam profile into, in particular annular, beam cross-section areas (corresponding to zones of the DOE or radial areas of the axicon) - step 207A - and assignment of, in particular identical linear, phase increases in the radial direction over the beam cross-section areas as initial phase distribution - step 207B)

—Changing the rise angles in the radial elements leads to a new height profile of the now modified axicon with a correspondingly modified phase imprint. In conjunction with the known power components of the raw beam, this leads to a redistribution of the power input into the focal zone, which can be calculated.

—An, e.g. iterative, adjustment of the slope angles can be carried out until the desired target intensity distribution is presented. (Iterative adjustment of the phase increases in the beam cross-section areas and calculation of the intensity distribution along the optical axis that results in the workpiece after the raw laser beam has passed through the optical system, taking into account the linear absorption parameter, until there is a phase distribution that compensates for the linear absorption and with which the target Intensity distribution along the optical axis in the workpiece results - step 207C)

The iteratively adapted phase increases of the phase distribution compensating for the linear absorption in conjunction with intensity components of the raw laser beam present in the beam cross-sectional areas can cause a redistribution of the laser radiation contributing to the quasi-non-diffracting laser beam along the optical axis to form the target intensity distribution. For the formation of the beam-shaping element, the beam-shaping element is provided with the phase distribution compensating for the linear absorption (step 209). For this purpose, a special height profile for an optical material/mirror can be derived from the compensating phase distribution in order to form a refractive or reflective optical axicon element with the height profile from the optical material as a thickness profile of an optical material or mirror profile. Furthermore, a diffractive conversion of the compensating phase distribution can be done with a diffractive optical element (e.g. a Fresnel-Axicon-like diffractive optical element whose phase shift values are fixed, or a spatial light modulator whose phase shift values have been adjusted according to the linear absorption compensating phase distribution).

The compensating phase distribution with the plurality of contributing cone angles means that the laser beam can be viewed as a plurality of sub-beams, wherein each of the sub-beams can have a different angle of arrival at which it enters the workpiece and approaches the optical axis. The angles of incidence determined according to the method depend on the position and the intensities in the respective cross-sectional areas of the raw laser beam.

Due to the phase distribution compensating for the linear absorption, laser radiation is guided to at least one of a plurality of positions along the optical axis at a plurality of angles. For example, the beam cross-section areas of the raw laser beam include at least two annular beam cross-section areas. The phase slopes for the two annular shaped beam cross-sectional areas can be adjusted such that laser radiation from the two annular shaped beam cross-sectional areas is delivered to a common position of the plurality of positions at two different cone angles.

The terms "beam cross-sectional area" and associated "section of the quasi-non-diffracting beam" introduced herein to describe the concepts and their identification in the figures do not force a fixed assignment of a surface area to a section. Rather, a beam cross-sectional area of a diffractive optical beam-shaping element can also supply several sections of the quasi-non-diffracting beam with laser radiation if, for example, diffraction structures are superimposed. Those skilled in the art will further understand that there is no limitation to discrete sections must be made, but that continuous sections are also included as a borderline case, see the example shown in FIG. 7 of the modified axicon with a homogenized intensity distribution.

With regard to the material processing of a partially transparent material, including the non-linear absorption of the material, a quasi-non-diffracting laser beam can bring about a modification in the material that extends over the entire length of the quasi-non-diffracting laser beam. Those skilled in the art will appreciate that based on the concepts disclosed herein, a linear array/array or, for example, planar array of modification zones can also be created with the quasi-non-diffractive laser beam. For this purpose, beam shaping can be used which, for example, generates a row of local intensity maxima in the direction of propagation (see FIG. 4). The intensity maxima can be limited by an envelope profile. The envelope profile can also be shaped and, for example, correspond in its course to the intensity courses shown in FIG.

As a result of the laser-based material processing, a partially transparent workpiece can be present, into which a plurality of spaced or merging modifications have been introduced. The modifications may additionally form cracks in the material extending between adjacent modifications or generally randomly from one of the modifications into the material of the workpiece.

For the sake of completeness, it is pointed out that, in addition to an intensity distribution in a focal zone that causes a single symmetrical modification, a phase imprint can be carried out, e.g. with a diffractive optical element, which leads to an intensity distribution in the focal zone that has an asymmetric (e.g. in one direction flattened) modification or several modifications running parallel to each other (see image (c) of Fig. 1). In general, the modification or the arrangement of modifications can be generated with a laser pulse or a group of laser pulses. Exemplary phase impressions and intensity distributions are, for example, in the German patent application 10 2019 128 362.0, "Segmented beam shaping element and laser processing system", with the application date October 21, 2019 by the applicant and in Chen et al., "Generalized axicon-based generation of nondiffracting beams", arXiv: 1911.03103vl [physics. optics] 8 Nov 2019 revealed. Such asymmetric modifications or arrays of modifications can also be combined with the concepts disclosed herein for processing partially transparent materials. In other words, beam shaping, which must be carried out for such asymmetric modifications, can be combined with phase imprinting, which can compensate for the influence on the intensity along the quasi-non-diffracting beam during propagation through the material.

It is explicitly emphasized that all features disclosed in the description and/or the claims are to be regarded as separate and independent from each other for the purpose of original disclosure as well as for the purpose of limiting the claimed invention independently of the combinations of features in the embodiments and/or the claims must. It is explicitly stated that all indications of ranges or groups of units disclose every possible intermediate value or subgroup of units for the purpose of original disclosure as well as for the purpose of limiting the claimed invention, in particular also as a limit of a range indication.

Claims

patent claims

1. A method for material processing of a workpiece (3) with a quasi-non-diffracting laser beam (5), the workpiece (3) having a material that is partially transparent to the quasi-non-diffracting laser beam (5) and which is suitable for laser radiation in the frequency range of the quasi-non-diffracting laser beam (5) has a linear absorption that is independent of a laser radiation intensity, with the steps:

Radiating (step 103) a pulsed raw laser beam (5') into an optical beam shaping system (13) to form a quasi-non-diffracting laser beam (5) with a focal zone (7) extending in a longitudinal direction (z) for the material processing of the workpiece (3 ), wherein the optical beam shaping system (13) is used to impress a phase on a beam cross section of the raw laser beam (5') to form phase-imposed laser radiation (5_PH), and

Focusing (step 107) the phase-impressed laser radiation (5_PH) in the partially transparent material of the workpiece (3), so that the quasi-non-diffracting laser beam (5) is formed and the focus zone (7) along the longitudinal direction (z) adjustable intensity distribution, wherein the phase imprint is adjusted in such a way that when the phase imprinted laser radiation is focused into the partially transparent material of the workpiece (3), a resulting intensity distribution (BG_2h(+)) of the quasi-non-diffracting laser beam (5) in the focal zone (7) in the longitudinal direction (z) is at least approximately constant.

2. The method according to claim 1, wherein the phase imprint on the beam cross section of the raw laser beam (5 ') is set such that laser radiation (5A_T, 5B_T, 5C_T, 5D_T) to a plurality of along an optical axis (9) arranged positions in the workpiece (3) in a run-in angle range, which in particular includes run-in angles (5'_1, 5'_2) in the range from 5° to 25° in the partially transparent material of the workpiece (3), is guided with respect to the optical axis (9) and the quasi- forms a non-diffracting laser beam (5) with the resulting intensity distribution (BG_2h(+)) at the plurality of positions, with intensity losses due to the linear absorption during propagation of the laser radiation (5A_T, 5B_T, 5C_T, 5D_T) in the partially transparent material to the plurality of positions occur, and the phase imprint is set such that at least one position of the plurality of positions laser radiation at multiple angles from the angle of arrival range is performed, so that in the partially transparent material an intensity threshold for non-linear absorption is exceeded at the majority of positions despite the intensity losses occurring, the non-linear absorption in the partially transparent material being dependent on a respective intensity of the laser radiation.

3. The method of claim 2, wherein laser radiation directed at a first angle to the at least one position of the plurality of positions has a phase difference of less than Pi/4 with respect to laser radiation directed at a second angle to the at least one position of the plurality is guided by positions, and/or wherein the phase imprint is adjusted in such a way that the laser radiation (5A_T, 5B_T, 5C_T, 5D_T) is guided rotationally symmetrically to the plurality of positions, so that each of the plurality of angles represents a local cone angle.

4. The method according to any one of the preceding claims, wherein adjusting the phase imprint includes adjusting (step 103A) phase increases in the radial direction that are impressed on beam cross-sectional areas (RA, R_B, R_C) of the raw laser beam (5'), and/or adjusting (Step 103B) of geometric parameters of beam cross-sectional areas in which one or more phase slopes are imposed.

5. The method according to claim 4, wherein the beam cross-sectional areas (RA, R_B, R_C) comprise at least two annular or ring-segment-shaped beam cross-sectional areas (RA, R_B, R_C) and the phase increases for the two annular or ring-segment-shaped beam cross-sectional areas (RA, R_B, R_C) be set in such a way that laser radiation from the two beam cross-section areas (R A, R_B, R_C) designed in the shape of a ring or a ring segment is fed to a common position of the plurality of positions at two different cone angles.

6. The method according to any one of the preceding claims, wherein in addition to setting the phase imprint in beam cross-section areas (RA, R_B, R_C), setting (step 103C) the intensity components (I_A, I_B, IC) of a raw laser beam intensity that corresponds to the beam cross-section areas (RA, R_B , R_C) are assigned in order to bring about the resulting intensity distribution (BG_2h(+)) of the quasi-non-diffracting laser beam (5) in the focal zone (7).

7. The method according to any one of the preceding claims, wherein the phase imprinting is set for a specified transverse intensity distribution of the raw laser beam, in particular a specified beam diameter of the raw laser beam, and a specified linear absorption of the partially transparent material of the workpiece, and wherein for a material with a linear absorption , which deviates from the predetermined linear absorption of the partially transparent material, the transverse intensity distribution, in particular a beam diameter, of the raw laser beam is adjusted with unchanged phase imprinting in order to increase or decrease an intensity component of a raw laser beam intensity that is fed to one position of the plurality of positions.

8. The method according to any one of the preceding claims, wherein the phase imprint is set such that a decrease in intensity of the quasi-non-diffracting laser beam (5) due to the linear absorption in the partially transparent material is at least partially compensated.

9. The method according to any one of the preceding claims, wherein the resulting intensity distribution (BG_2h(+)) of the quasi-non-diffracting laser beam (5) along the optical axis (9) has an intensity distribution or an envelope intensity distribution that deviates from an average intensity of the quasi-non-diffracting laser beam (5) in the range of up to 10%, the average intensity relating to that part of the focal zone (7) in which a non-linear interaction with the material of the workpiece (3) takes place, and optionally the Intensity distribution or the envelope intensity distribution is in particular substantially constant.

10. The method according to any one of the preceding claims, wherein based on the non-linear absorption at a plurality of positions in the focal zone (7) despite the occurring intensity losses, the partially transparent material is modified, and the modification of the partially transparent material

- extends over a length of the quasi-non-diffracting laser beam (5) or

- Consists of a line-up of modification zones along the quasi-non-diffracting laser beam (5).

11. The method according to any one of the preceding claims, wherein a laser beam with a Gaussian transversal intensity profile is used as the raw laser beam (5') and the optical beam shaping system (13) is set up to generate a Bessel-Gaussian beam as a quasi-non-diffracting laser beam (5). to shape, and/or wherein a transverse dimension of the quasi-non-diffractive laser beam (5) in the focal zone (7) changes along the optical axis (9) and/or wherein a transverse dimension of the quasi-non-diffractive laser beam (5) at a The position of the focal zone (7) depends on angles of incidence (5'_1, 5'_2) with which the laser radiation to form the quasi non-diffracting laser beam (5) is incident on the optical axis (9) at the position of the focal zone (7).

12. The method according to any one of the preceding claims, further comprising at least one of the following steps:

Setting (step 105) beam parameters of the raw laser beam (5') in such a way that the partially transparent material of the workpiece (3) is modified,

Positioning at least a portion of the quasi - non-diffractive laser beam (5) in the workpiece (3) or

Effecting (step 109) a relative movement between the workpiece (3) and the quasi-non-diffracting laser beam (5), in which the quasi-non-diffracting laser beam (5) is moved along a scanning trajectory (T) in the workpiece (3), so that a line-up of modifications is written into the workpiece (3) along the scanning trajectory (T).

13. The method according to any one of the preceding claims, wherein the optical beam-shaping system (13) comprises a diffractive optical beam-shaping element and the diffractive optical beam-shaping element has surface elements (15a) adjoining one another, which build up a surface grating structure and to which a phase shift value is assigned in each case, wherein the phase shift values define a two-dimensional phase distribution corresponding to the set phase impression, and when the raw laser beam (5') is radiated into the optical beam-shaping system (13) (step 103), the phase impression is effected with the diffractive optical beam-shaping element by the phase distribution being applied to the raw laser beam (5' ) is impressed, and optionally wherein the optical beam-shaping system (13) comprises a beam-shaping element that was created by a method according to any one of claims 19 to 23.

14. Laser processing system (1) for material processing of a workpiece (3) with a quasi-non-diffracting laser beam (5), the workpiece (3) having a material which is partially transparent for the quasi-non-diffracting laser beam (5) and which is suitable for laser radiation in the Frequency range of the quasi-non-diffracting laser beam (5) has a linear absorption that is independent of an intensity of the laser radiation, with: a laser beam source (11) that emits a pulsed laser beam (5"), an optical beam shaping system (13) for beam shaping of the Laser beam (5") for forming the quasi-non-diffracting laser beam (5) with a focal zone (7) extending in a longitudinal direction (z).

- beam adjustment optics (17A) which are set up to output the laser beam (5") as a raw laser beam (5') with a beam diameter, and

- a beam-shaping element (15), which is set up to impress a phase on a beam cross section of the raw laser beam (5') to form phase-impressed laser radiation (5_PH) for a predetermined beam diameter (D) of the raw laser beam (5') in such a way that when focusing of the phase-impressed laser radiation (5_PH) in the partially transparent material of the workpiece (3), the quasi-non-diffracting laser beam (5) is generated with a resulting intensity distribution (BG_2h(+)), which in the focal zone (7) in the longitudinal direction (z) at least is approximately constant, and a workpiece holder (19) for supporting the workpiece (3), the optical beam shaping system (13) and/or the workpiece holder (19) being set up to allow a relative movement between the workpiece (3) and the quasi-non-diffractive To effect laser beam (5), in which the quasi-non-diffracting laser beam (5) along a scanning trajectory (T) in the material of the workpiece (3) is positioned.

15. Laser processing system (1) according to claim 14, wherein the phase imprinting on a beam cross-section of the raw laser beam (5') is set with the beam-shaping element (15) in such a way that laser radiation of the raw laser beam (5') is directed to a plurality of along an optical axis (9) arranged positions in the workpiece (3) is guided in an angle of arrival range with respect to the optical axis (9) and forms the quasi-non-diffracting laser beam (5) at the plurality of positions, and wherein intensity losses due to linear absorption during the propagation of the laser radiation occur in the partially transparent material to the plurality of positions, and the phase imprint is further set such that laser radiation is guided at a plurality of angles from the angle of incidence range to at least one position of the plurality of positions is, so that in the partially transparent material at the majority of positions, despite the occurring intensity losses, an intensity threshold for non-linear absorption, which is dependent on a laser radiation intensity present in each case, is exceeded.

16. Laser processing system (1) according to claim 14 or 15, further having a controller (21) which is set up to adjust the beam adjustment optics (17A) such that the beam diameter at the beam shaping element (15) is larger or smaller than the specified beam diameter (D) to compensate for variations in the linear absorbance relative to the linear absorbance for which the phase imprint was determined.

17. Laser processing system (1) according to one of claims 14 to 16, wherein the beam-shaping element (15) is designed as a diffractive optical element, a spatial light modulator or a modified refractive or reflective axicon (15C) and optionally wherein the beam-shaping element (15) in particular was created according to a method according to any one of claims 19 to 23.

18. Laser processing system (1) according to any one of claims 14 to 17, wherein the phase imprint is designed such that the resulting intensity distribution (BG_2h(+)) has an intensity distribution or an envelope intensity distribution that deviates from an average intensity of the quasi -non-diffractive Laser beam (5) in the range of up to 10%, the average intensity relating to a part of the focal zone in which a non-linear interaction with the material of the workpiece (3) takes place, and the intensity distribution or the envelope intensity distribution in particular is essentially constant.

19. A method for forming an optical beam-shaping element (15), in particular a diffractive one, which is intended for use in material processing of a workpiece (3) in an optical beam-shaping system (13) for beam-shaping a quasi-non-diffracting laser beam (5) from a raw laser beam (5 ') is provided, wherein the workpiece (3) has a material that is partially transparent to the quasi-non-diffracting laser beam (5) and has linear absorption for laser radiation in the frequency range of the quasi-non-diffracting laser beam (5), which is independent of a laser radiation intensity, with the steps: Providing (step 201) a linear absorption parameter of the partially transparent material in the frequency range of the quasi-non-diffracting laser beam (5), in particular by measuring a linear absorption parameter;

Defining (step 203) a target intensity distribution as the resulting intensity distribution (BG_2h(+)) to be achieved in the workpiece (3) along an optical axis (9) of the quasi-non-diffracting laser beam (5), in which to modify the material of the workpiece ( 3) at a plurality of positions along the optical axis (9), at least in sections, an intensity of the target intensity distribution is above an intensity threshold for non-linear absorption, which is dependent on a laser radiation intensity that is present in each case;

specifying (step 205) a transversal beam profile, in particular a beam diameter (D), of the raw laser beam (5') onto which a two-dimensional phase distribution is to be impressed;

Compute (step 207) the two-dimensional phase distribution for the transverse beam profile by:

- subdividing (step 207A) the transversal beam profile into beam cross-sectional areas (RA, R_B, R_C), in particular ring-shaped,

- allocating (step 207B) of, in particular identical linear, phase increases in the radial direction over the beam cross-sectional areas (RA, R_B, R_C) as an initial phase distribution, and

- iterative adjustment (step 207C) of the phase increases in the beam cross-section areas (R A, R_B, R_C) and calculation of the intensity distribution along the optical axis ( 9) taking into account the linear absorption given by the linear absorption parameter, until a two-dimensional phase distribution that compensates for the linear absorption is present, with which the target intensity distribution along the optical axis (9) in the workpiece (3) is the resultant intensity distribution (BG_2h(+ )) yields; and

Providing (step 209) the beam-shaping element (15) with the linear absorption-compensating two-dimensional phase distribution.

The method of claim 19, wherein the iteratively adjusted phase slopes are associated with those present in the beam cross-sectional areas (RA, R_B, R_C). Intensity components (I_A, I_B, IC) of the raw laser beam (5') cause a redistribution of the laser radiation contributing to the quasi-non-diffracting laser beam (5) along the optical axis (9) to form the target intensity distribution.

21. The method according to any one of claims 19 or 20, wherein a phase increase corresponds to an angle at which the laser radiation is guided to the optical axis (9), and wherein the linear absorption-compensating two-dimensional phase distribution is determined iteratively such that laser radiation to at least one position a plurality of positions along the optical axis (9) at a plurality of angles.

22. The method according to any one of claims 19 to 21, wherein the beam-shaping element (15) has surface elements (15a) adjoining one another, which are provided with phase shift values that are set in accordance with the two-dimensional phase distribution compensating for linear absorption, and wherein in particular the beam-shaping element (15) as

- a Fresnel Axicon-like diffractive optical element whose phase shift values are fixed, or

- a spatial light modulator whose phase shift values correspond to the linear one

Absorption compensating phase distribution were adjusted, is executed.

23. The method according to any one of claims 19 to 22, further comprising: