DE102021122754A1 - Device for processing a material - Google Patents

Abstract

The present invention comprises a device (1) for processing a workpiece (10) by means of ultra-short laser pulses, comprising an ultra-short pulse laser (3) which is set up to emit ultra-short laser pulses (32) in a laser beam (30), a geometric phase element (4) that is set up to perform beam deflection and/or beam shaping and/or beam splitting depending on the input polarization of the incident laser beam (30) and imaging optics (8) that are set up to project the laser beam (30) or the partial laser beams (300 ) to be introduced into the workpiece (10), as a result of which the workpiece (10) is processed, with successive laser pulses (32) having different, preferably orthogonal, polarizations or the laser pulses (32) having orthogonal polarizations in pairs.

Description

technical field

The present invention relates to a device for processing a workpiece using ultra-short laser pulses.

State of the art

In recent years, the development of lasers with very short pulse lengths, especially pulse lengths of less than a nanosecond, and high average powers, especially in the kilowatt range, has led to a new type of material processing. The short pulse length and high pulse peak power or the high pulse energy of a few 100 µJ can lead to non-linear absorption of the pulse energy in the material of a workpiece, so that materials that are actually transparent or essentially transparent for the laser light wavelength used can also be processed.

A special area of application for such laser radiation is the cutting and processing of workpieces. In this case, a non-diffracting processing laser beam is preferably introduced into the material with perpendicular incidence, as a result of which material modifications are produced in the material, which damage the material in a targeted manner. This creates a kind of perforation along which the material can be separated.

During material processing, the laser beam can in particular be divided into different partial laser beams, so that parallel processing of the workpiece with the different partial laser beams is possible. The problem here, however, is when the various partial laser beams have to be positioned close to one another, since disruptive interference between the partial laser beams can then impair the processing quality.

Presentation of the invention

Proceeding from the known prior art, it is an object of the present invention to provide an improved device for machining a workpiece.

The object is achieved by a device for machining a workpiece having the features of claim 1. Advantageous developments result from the dependent claims, the description and the figures.

Accordingly, a device for processing a workpiece using ultra-short laser pulses is proposed, comprising an ultra-short-pulse laser that is set up to emit ultra-short laser pulses in a laser beam, a geometric phase element that is set up to deflect the beam and/or depending on the input polarization of the incident laser beam or carry out beam shaping and/or beam splitting and imaging optics which are set up to introduce the laser beam or the partial laser beams into the workpiece, as a result of which the workpiece is machined. According to the invention, successive laser pulses have different, preferably orthogonal, polarizations or the laser pulses have orthogonal polarizations in pairs.

The ultra-short pulse laser makes the ultra-short laser pulses of the laser beam available, with the individual laser pulses forming the laser beam in the beam propagation direction.

Instead of individual laser pulses, the laser can also provide bursts, with each burst comprising the transmission of a number of laser pulses. For a certain time interval, the emission of the laser pulses can follow one another very closely, at intervals of a few picoseconds up to hundreds of nanoseconds. The bursts can in particular be so-called GHz bursts, in which the sequence of the successive laser pulses of the respective burst takes place in the GHz range.

In this context, a sequence of individual pulses means that the laser emits several individual pulses one after the other. A sequence of individual pulses therefore includes at least two individual pulses. A sequence of bursts means that the laser emits several bursts one after the other. A sequence of bursts therefore includes at least two bursts. In particular, the bursts or individual pulses of the sequence can each be of the same type. Similar are the bursts or one zelpulse if the laser pulses used have essentially the same properties, i.e. approximately the same pulse energy, the same pulse length and - in the case of bursts - also have the same pulse spacing within the burst.

The wavelength of the laser pulses can be between 200 nm and 2000 nm, preferably 257 nm or 343 nm or 515 nm or 1030 nm and/or the pulse interval can be less than 100 ps, preferably less than 10 ps and/or the pulse duration can be less than 10 ps, preferably be less than 1 ps.

The device is therefore particularly well suited for processing a workpiece, regardless of the wavelength. Depending on the given wavelength, however, it may be necessary to adapt the optical elements of the device to the wavelength of the laser pulses.

A geometric phase element deflects an incident laser beam depending on the input polarization of the laser beam, while maintaining the polarization of the laser beam. Geometric phase elements are discussed in particular in Sakakura, M., Lei, Y., Wang, L. et al. "Ultralow-loss geometric phase and polarization shaping by ultrafast laser writing in silica glass" Light Sci. appl. Vol. 9, 15 (2020). Geometric phase elements make use of the fact that an incident light wave can also be manipulated by a parameter other than the optical path length through the optical element—as is the case, for example, with a lens. For this purpose, a birefringent structure is introduced into a carrier material to produce a geometric phase element. The birefringent structure makes it possible to carry out a polarization-dependent refraction of the incident light wave, it being possible in particular to influence the shape of the light wave deflected by the geometric phase element via the shape of the birefringent structure.

Birefringence is the ability of an optical material to separate the incident laser beam into two partial laser beams that are polarized perpendicularly to one another. This occurs due to different refractive indices of the optical material depending on the polarization and the angle of incidence of the light relative to the optical axis of the optical material.

Within the meaning of this application, partial laser beams polarized perpendicularly to one another are understood to mean linearly polarized partial laser beams whose directions of polarization are aligned at an angle of 90° to one another. However, partial laser beams polarized perpendicularly to one another are also understood to mean circularly polarized partial laser beams with opposite directions of rotation, i.e. two partial laser beams circularly polarized to the left and right. The conversion of linearly polarized partial laser beams with polarization directions aligned perpendicularly to one another into circularly polarized partial laser beams with the opposite sense of rotation can be carried out, for example, with the aid of a suitably oriented retardation plate (λ/4 plate) (see below).

A geometric phase element can theoretically achieve higher efficiencies than classic diffractive beam shapers, so that a particularly large amount of laser power is transmitted through the geometric phase element.

For example, it is possible with a geometric phase element to deflect a right circularly polarized laser beam in the +X _GP direction, while the same geometric phase element deflects a left circularly polarized laser beam in the -X _GP direction. In this case, the polarization is not changed when passing through the geometric phase element. For example, it is also possible with the imaging optics (see below) to translate the transverse deflection into different focal depths, i.e. a lateral deflection.

It is also possible for the beam deflection to be an angular deflection, so that when the laser beam passes through the geometric phase element, an angle α _GP is imposed relative to the optical axis of the device, at which angle the laser beam continues to propagate after passing through the geometric phase element. It is also possible that the geometric phase element causes only a parallel offset of the laser beam, so that the laser beam no longer propagates on the optical axis after passing through the geometric phase element, but is offset parallel to the optical axis. In particular, offset in parallel means here that the laser beam does not enclose an angle α with the optical axis after the geometric phase element.

Here and in the following, the description of the deflections relates in particular to the deflection of the focal zone of the laser beam in the focal plane of the imaging optics (see below). The deflection ±X _GP can be achieved regardless of whether the geometric phase element produces a parallel offset of the laser beam or whether the geometric phase element imposes an angle α _GP on the laser beam. A description of the deflection via a spatial deflection in the focal plane explicitly includes both possibilities.

It is also possible to split the beam with the geometric phase element. If a laser beam has two polarization components (or the laser beam is broken down into the basic polarization components of the geometric phase element), then each basic polarization component of the laser beam experiences a corresponding deflection. For example, a circularly polarized laser beam can be split into an s-polarized partial laser beam and a p-polarized partial laser beam, the s-polarized laser beam being deflected to +X _GP and the p-polarized laser beam to -X _GP .

In addition, it is possible to shape the beam using the geometric phase element. For example, certain polarizations are filtered out or deflected from a given intensity profile of the laser beam.

Successive laser pulses can have different, preferably orthogonal, polarizations or the laser pulses can have orthogonal polarizations in pairs.

The polarizations of the laser pulses are orthogonal in pairs if two consecutive laser pulses are polarized orthogonally to one another. For example, a first laser pulse is s-polarized and a second laser pulse is p-polarized, or vice versa.

An imaging optic is set up to introduce the partial laser beams into the workpiece, as a result of which the workpiece is machined. In particular, the partial laser beams are focused in a respective focal zone. The focal zones are in the so-called focal plane. In this case, imaging optics can include, for example, a mirror system or a lens system. The imaging optics can also include a telescope system, for example a relay telescope. The imaging optics transfer the laser beams into a respective focal zone.

In this case, the geometric phase element can be arranged in particular in the far field of the imaging optics. Thus, the machining area on the workpiece is largely linked to the beam shaping area via an angle-to-space transformation. For example, the element can be located directly in front of the imaging optics.

In this case, the focal zone is understood to mean in particular that part of the intensity distribution of the laser beam which is greater than the modification threshold of the material. The word focal zone makes it clear that this part of the intensity distribution is provided in a targeted manner and that an intensity increase in the form of the intensity distribution is achieved by focusing.

Material processing can consist, for example, in that the material is melted so that the material can be joined to another. However, it is also possible that processing consists of bringing about material modifications in the material. In particular, it is possible to separate the material along a dividing line by processing the material.

For this purpose, the device can have a feed device which is set up to move the workpiece and the laser field relative to one another with a feed along a trajectory.

The material modifications are introduced into the material of a workpiece along a trajectory. The trajectory describes the line of impact of the laser field on the surface of the workpiece. By means of a feed, for example, the laser beam and the workpiece are shifted relative to one another at a feed rate, so that the laser pulses hit the surface of the workpiece at different locations as time progresses.

For this purpose, the device can have a scanner unit, for example, which is set up to scan the laser field over the workpiece, the scanner preferably being a galvanic scanner. esp In particular, a galvanic scanner allows the laser field to be positioned precisely and quickly over the workpiece.

The material modifications introduced into transparent materials by ultrashort laser pulses are divided into three different classes, see K. Itoh et al. "Ultrafast Processes for Bulk Modification of Transparent Materials" MRS Bulletin, vol. 31 p.620 (2006): Type I is an isotropic refractive index change; Type II is a birefringent refractive index change; and Type III is a so-called void. The material modification produced depends on laser parameters such as the pulse duration, the wavelength, the pulse energy and the repetition frequency of the laser, on the material properties such as the electronic structure and the thermal expansion coefficient, as well as on the numerical aperture (NA) of the imaging optics.

The isotropic type I refractive index changes are attributed to localized melting by the laser pulses and rapid resolidification of the transparent material of the workpiece. For example, with fused silica, the density and refractive index of the material is higher when the fused silica is rapidly cooled from a higher temperature. So if the material in the focus volume melts and then cools down quickly, the quartz glass has a higher refractive index in the areas of material modification than in the unmodified areas.

The type II birefringent refractive index changes can arise, for example, as a result of interference between the ultrashort laser pulse and the electric field of the plasma generated by the laser pulses. This interference leads to periodic modulations in the electron plasma density, which leads to a birefringent property, i.e. direction-dependent refractive indices, of the transparent material when it solidifies. A type II modification is also accompanied, for example, by the formation of so-called nanogratings.

The voids (cavities) of the type III modifications can be generated with a high laser pulse energy, for example. The formation of the voids is attributed to an explosive expansion of highly excited, vaporized material from the focus volume into the surrounding material. This process is also known as a micro-explosion. Because this expansion occurs within the bulk of the material, the microblast leaves behind a less dense or hollow core (the void), or submicron or atomic-scale microscopic defect, surrounded by a densified shell of material. Due to the compression at the impact front of the microexplosion, stresses arise in the transparent material, which can lead to spontaneous cracking or can promote cracking.

In particular, the formation of voids can also be associated with type I and type II modifications. For example, Type I and Type II modifications can arise in the less stressed areas around the introduced laser pulses. Therefore, if a type III modification is introduced, then in any case a less dense or hollow core or a defect is present. For example, in a type III modification of sapphire, the microexplosion does not create a cavity, but rather an area of lower density. Due to the material stresses that occur in a type III modification, such a modification is often accompanied by cracking or at least promotes it. The formation of type I and type II modifications cannot be completely prevented or avoided when introducing type III modifications. Finding "pure" Type III modifications is therefore not likely.

However, it is also possible to use the device to process, in particular to cut, opaque materials such as metals or sheet metal. Here, material is vaporized and removed by the high-energy excitation of the material of the workpiece.

A polarization splitter can be set up to split the incident laser beam into two partial laser beams, with the two partial laser beams having an orthogonal output polarization.

A polarization splitter makes it possible to split an incident laser beam into its linear polarization components and to spatially separate the respective polarization components. Such polarization splitters are typically also based on birefringence of the laser beam.

The outgoing laser beams broken down into polarization components by the polarization splitter can also have an angular offset and/or a spatial offset in relation to one another. This also allows are due to the anisotropy of the refractive indices for different polarization directions of the optical material of the polarization element.

For example, the partial laser beams can have an angular offset after passing through the polarization splitter. This means that the partial laser beam with a first polarization does not run parallel to the partial laser beam with a second polarization behind the polarization splitter.

In order to generate the angular offset (without spatial offset), the polarization splitter can have a beam exit surface which is inclined at an angle to the beam entry surface. In this case, the optical axis of the birefringent crystal is typically aligned parallel to the beam entry surface. In this case, the two partial laser beams exit the beam exit surface at the same location and with a defined angular offset from the birefringent crystal.

For example, the partial laser beams can have a spatial offset after passing through the polarization splitter. This means that the partial laser beam with the first polarization runs parallel to the partial laser beam with the second polarization behind the birefringent polarization element. However, both partial laser beams are shifted parallel to each other, so that there is a finite distance between the two partial laser beams.

In order to generate the spatial offset (without angular offset), the polarization splitter can have, for example, beam entrance and beam exit surfaces that are aligned parallel and are generally planar. In this case, the optical axis of the birefringent crystal is typically aligned at an angle to the beam entry surface. If the laser beam hits the beam entry surface perpendicularly, a pure spatial displacement is generated on the beam exit surface.

The distances between the individual partial laser beams with different polarization can be defined via the polarization element, for example during production or by orienting the optical axis of the crystal to the incident laser beam.

For example, a polarization splitter cannot deflect a first partial laser beam and deflect a second partial laser beam. Accordingly, the first partial laser beam would continue to propagate on the optical axis, but not the second partial laser beam. It is also possible that both partial laser beams are deflected in opposite directions ±X _P . It is also possible that both partial laser beams are deflected in the same direction but to different extents. The deflection of the partial laser beams relates here - exactly as in the case of the geometric phase element - to the deflection of the focal zones in the focal plane. A deflection ±X _P can thus be generated in this case in that the partial laser beam is given an angle α _P relative to the optical axis. However, such a deflection can also be achieved in that the partial laser beam is merely subjected to a parallel offset as it passes through the polarization splitter.

For example, a left circularly polarized laser beam can be split into its polarization components, with the p-polarized partial laser beam being deflected to ±X _P , without loss of generality, and the s-polarized partial laser beam being deflected to −X _P .

For example, a laser beam linearly polarized at 45° can be split into its polarization components, with the p-polarized partial laser beam being deflected to + _YP and the s-polarized partial laser beam being deflected to _-YP .

In particular, the device can have a λ/4 plate which is set up to convert a circular polarization of the incident laser beam into a linear polarization or a linear polarization into a circular polarization, the λ/4 plate being in front of the geometric phase element in the direction of beam propagation or after the polarization splitter.

This makes it possible to generate a right-hand circularly or left-hand circularly polarized laser beam from an s- or p-polarized laser beam of an ultrashort pulse laser, which is then deflected by the geometric phase element according to the circular polarization.

However, it is also possible to form right-hand circularly and left-hand circularly polarized partial laser beams from the split s-polarized or p-polarized partial laser beams that arise when passing through the polarization splitter.

In particular, using a λ/4 plate allows flexibility in the design of the device. In particular, however, a polarization dependency of the material processing can also be taken into account here, so that the workpiece can be processed with particularly high quality.

The polarization splitter can be arranged behind the geometric phase element in the beam propagation direction. This has the effect that the laser beam is first deflected by the geometric phase element and, in addition to this deflection, a beam deflection and splitting occurs through the polarization splitter. However, it is also possible for the polarization splitter to be arranged in front of the geometric phase element in the beam propagation direction. The laser beam is then caused to first undergo beam deflection and splitting by the polarization splitter and then undergo deflection by the geometric phase element.

The beam deflection of the polarization splitter can take place perpendicularly or parallel to the beam deflection of the geometric phase element. However, any angle between the deflections can also be set.

For example, a right circular laser beam can be deflected by the geometric phase element to +X _GP . The deflected laser beam is then split by the polarization splitter into a p-polarized partial laser beam and an s-polarized partial laser beam, with the polarization splitter additionally deflecting the p-polarized partial laser beam to ±X _P and the s-polarized partial laser beam to -X _P in parallel. The right-hand circularly polarized laser beam is split into its polarization components and deflected by the geometric phase element and the polarization splitter in vector notation according to (+X _GP +X _P ,0) _P and (+X _GP -X _P ,0) _S . The index describes the polarization of the partial laser beam.

For example, a left-hand circular laser beam can be deflected by the geometric phase element to -X _GP . The deflected laser beam is then split by the polarization splitter into a p-polarized partial laser beam and an s-polarized partial laser beam, with the polarization splitter additionally deflecting the p-polarized partial laser beam to ±X _P and the s-polarized partial laser beam to -X _P in parallel. The left-hand circularly polarized laser beam is split and deflected into its polarization components by the geometric phase element and the polarization splitter in vector notation according to (-X _GP -X _P ,0) _S and (-X _GP +X _P ,0) _P .

For example, a right circular laser beam can be deflected by the geometric phase element to +X _GP . The deflected laser beam is then split by the polarization splitter into a p-polarized partial laser beam and an s-polarized partial laser beam, with the polarization splitter additionally deflecting the p-polarized partial laser beam to + _YP and the s-polarized partial laser beam to _-YP . The right-hand circularly polarized laser beam is split into its polarization components and deflected by the geometric phase element and the polarization splitter in total in vector notation according to (+X _GP + Y _P ) _P and (+X _GP , - Y _P ) _S .

For example, a left-hand circular laser beam can be deflected by the geometric phase element to -X _GP . The deflected laser beam is then split by the polarization splitter into a p-polarized partial laser beam and an s-polarized partial laser beam, with the polarization splitter additionally deflecting the p-polarized partial laser beam to + _YP and the s-polarized partial laser beam to _-YP . The left-hand circularly polarized laser beam is split into its polarization components and deflected by the geometric phase element and the polarization splitter in vector notation according to (-X _GP , _-YP ) _S and (-X _GP , + _YP ) _P .

By splitting the laser beam into two mutually orthogonally polarized s and p components, the focal zones of the two partial laser beams can be placed close to one another without undesired interference occurring in the adjacent focal zones. As a result, the workpiece can be processed with particularly high quality.

The device can have a beam shaping device which is set up to split the incident partial laser beams into a plurality of partial partial laser beams, the beam shaping device being arranged behind the polarization splitter in the beam propagation direction.

A beam-shaping device can be a diffractive optical element, for example. A diffractive optical element is set up to focus the incident laser beam in two spatial dimensions affect one or more properties. A diffractive optical element is a fixed component that can be used, for example, to produce a specific non-diffracting laser beam from the incident laser beam. A diffractive optical element can also be suitable for splitting an incident laser beam into several partial laser beams. Typically, a diffractive optical element is a specially shaped diffraction grating, whereby the incident laser beam is brought into the desired beam shape by the diffraction.

In this case, for example, the number of partial laser beams can be defined via the diffractive optical element. In particular, the diffractive optical element can be used to determine whether the split partial laser beams lie on a one-dimensional line or on a two-dimensional grid.

In this case, the splitting can be effected independently of the polarization of the laser beam. In particular, the polarization of the laser beam is retained as it passes through the diffractive optical element. However, possible positional deviations or angular offsets from the optical axis are taken into account when passing through the diffractive optical element, so that a splitting and deflection by the diffractive optical element is effected in addition to a previous splitting and deflection.

For example, a diffractive optical element can be set up to split a laser beam into 4 partial laser beams $$\left(\begin{array}{c}+X_{DOE}\\ +Y_{DOE}\end{array}\right),\left(\begin{array}{c}+X_{DOE}\\ -Y_{DOE}\end{array}\right),\left(\begin{array}{c}-X_{DOE}\\ +Y_{DOE}\end{array}\right),\left(\begin{array}{c}-X_{DOE}\\ -Y_{DOE}\end{array}\right).$$

Such splitting takes place in particular for each partial laser beam that originates from splitting and deflection by a geometric phase element and a polarization splitter.

sowie $${\left(\begin{array}{c}+X_{DOE}-X_{GP}\\ +Y_{DOE}-Y_P\end{array}\right)}_P,{\left(\begin{array}{c}+X_{DOE}-X_{GP}\\ -Y_{DOE}-Y_P\end{array}\right)}_P,{\left(\begin{array}{c}+X_{DOE}-X_{GP}\\ +Y_{DOE}-Y_P\end{array}\right)}_P,{\left(\begin{array}{c}-X_{DOE}-X_{GP}\\ -Y_{DOE}+Y_P\end{array}\right)}_P.$$

For example, a left-hand circular laser beam is split and deflected into (-X _GP , _-YP ) _S and (-X _GP ,+YP _{) P} by the geometric phase element and a polarization splitter. When these partial laser beams pass through the diffractive optical element, the partial laser beams are split into several partial laser beams: $${\left(\begin{array}{c}+X_{DOE}-X_{GP}\\ +Y_{DOE}-Y_P\end{array}\right)}_S,{\left(\begin{array}{c}+X_{DOE}-X_{GP}\\ -Y_{DOE}-Y_P\end{array}\right)}_S{\left(\begin{array}{c}+X_{DOE}-X_{GP}\\ +Y_{DOE}-Y_P\end{array}\right)}_S{\left(\begin{array}{c}-X_{DOE}-X_{GP}\\ -Y_{DOE}-Y_P\end{array}\right)}_S$$

as well as $${\left(\begin{array}{c}+X_{DOE}-X_{GP}\\ +Y_{DOE}-Y_P\end{array}\right)}_P,{\left(\begin{array}{c}+X_{DOE}-X_{GP}\\ -Y_{DOE}-Y_P\end{array}\right)}_P,{\left(\begin{array}{c}+X_{DOE}-X_{GP}\\ +Y_{DOE}-Y_P\end{array}\right)}_P,{\left(\begin{array}{c}-X_{DOE}-X_{GP}\\ -Y_{DOE}+Y_P\end{array}\right)}_P.$$

sowie $${\left(\begin{array}{c}+X_{DOE}+X_{GP}\\ +Y_{DOE}-Y_P\end{array}\right)}_S,{\left(\begin{array}{c}+X_{DOE}+X_{GP}\\ -Y_{DOE}-Y_P\end{array}\right)}_S,{\left(\begin{array}{c}-X_{DOE}-X_{GP}\\ +Y_{DOE}-Y_P\end{array}\right)}_S,{\left(\begin{array}{c}-X_{DOE}+X_{GP}\\ -Y_{DOE}-Y_P\end{array}\right)}_S.$$

For example, a right-hand circularly polarized laser beam is split and deflected by the geometric phase element and a polarization splitter to (+X _GP , + _YP ) _P and (+X _GP , -YP _{) S} . When these partial laser beams pass through the diffractive optical element, the partial laser beams are split into several partial laser beams: $${\left(\begin{array}{c}+X_{DOE}+X_{GP}\\ +Y_{DOE}+Y_P\end{array}\right)}_P,{\left(\begin{array}{c}+X_{DOE}+X_{GP}\\ -Y_{DOE}+Y_P\end{array}\right)}_P,{\left(\begin{array}{c}-X_{DOE}+X_{GP}\\ +Y_{DOE}+Y_P\end{array}\right)}_P,{\left(\begin{array}{c}-X_{DOE}+X_{GP}\\ -Y_{DOE}+Y_P\end{array}\right)}_P$$

as well as $${\left(\begin{array}{c}+X_{DOE}+X_{GP}\\ +Y_{DOE}-Y_P\end{array}\right)}_S,{\left(\begin{array}{c}+X_{DOE}+X_{GP}\\ -Y_{DOE}-Y_P\end{array}\right)}_S,{\left(\begin{array}{c}-X_{DOE}-X_{GP}\\ +Y_{DOE}-Y_P\end{array}\right)}_S,{\left(\begin{array}{c}-X_{DOE}+X_{GP}\\ -Y_{DOE}-Y_P\end{array}\right)}_S.$$

A total of eight s- or p-polarized partial laser beams are therefore generated from a circularly polarized laser beam by the above arrangement. It is therefore possible to generate a defined laser field through the imaging optics by selecting the geometric phase element, polarization splitter and beam shaping device, with the polarization of the laser field being able to be adjusted locally.

In particular, by selecting the geometric phase element, the polarization splitter and the diffractive optical element, it can be realized that adjacent partial partial laser beams have mutually orthogonal polarization. This can make it possible for the partial partial laser beams to be introduced into the material of the workpiece particularly close to one another, with interference of the partial partial laser beams being able to be avoided. As a result, a particularly high density of focal zones can be generated in the focal plane of the imaging optics.

However, a beam shaping device can also have other advantages. In particular, a beam shaping device can shape each partial laser beam into a non-diffracting beam. Non-diffracting rays and/or Bessel-like rays are to be understood in particular as rays in which a transverse intensity distribution is propagation-invariant. In particular, in the case of non-diffractive beams and/or Bessel-type beams, a transverse intensity distribution is essentially constant along the beam propagation direction.

With regard to the definition and properties of non-diffracting rays, reference is made to the book "Structured Light Fields: Applications in Optical Trapping, Manipulation and Organization", M. Wördemann, Springer Science & Business Media (2012), ISBN 978-3-642-29322- 1 referenced. This is expressly and fully referred to.

Accordingly, non-diffracting laser beams have the advantage that they can have a focal zone that is elongated in the direction of beam propagation and that is significantly larger than the transverse dimensions of the focal zone. For example, a material modification that is elongated in the beam propagation direction can be produced in this way.

However, it is also possible for the beam-shaping device to introduce the different partial partial laser beams into focal zones of different depths in the direction of beam propagation. For example, the focal plane can be tilted overall as a result. In other words, the optical axis of the device is at an angle to the surface normal of the focal plane. For example, so-called chamfer beams can be generated with which the edges of a workpiece can be chamfered.

The device can have a pulse delay device which is set up to variably set the time offset of the laser pulses.

The time offset is the time that elapses before two consecutive laser pulses are introduced into the material.

The pulse delay device can be implemented, for example, by one or more birefringent crystals. In this case, the birefringent crystal has different refractive indices for different directions of polarization, which are translated into a path difference when passing through the birefringent crystal, the path difference depending on the thickness of the birefringent crystal. In this case, the path difference can in particular be greater than the pulse duration, so that the incoming laser pulse is split into two outgoing laser pulses with orthogonal polarization. The method described is described, for example, in Fraggelakis et al. "Double- and Multi-Femtosecond Pulses Produced by Birefringent Crystals for the Generation of 2D Laser-Induced Structures on a Stainless Steel Surface," Materials, 12(8), 1257 (2019).

The pulse delay device can also be implemented, for example, by a delay optics onto which a laser pulse is directed. A deceleration optics can include a mirror system, which deflects the laser pulse onto a deceleration path and finally directs it back to the original trajectory of the laser beam.

For example, if a laser pulse is to be delayed between a first optical element and a second optical element, the laser pulse can be directed after the first optical element through the delay optics onto the delay path and, after passing through the delay path, can be directed through the second optical element. The laser pulse thus covers a longer path on the delay path and accordingly arrives later at the second optical element, as if the laser pulse had propagated directly from the first optical element to the second optical element.

For example, a pulse delay device can have four optical elements, with all four optical elements being mirrors, for example, which preferably have a highly reflective coating. The four mirrors can all be arranged on the corner points of a rectangle. For example, the first and fourth mirrors can stand between the first and second optical elements mentioned above. After the first optical element, the first mirror can therefore reflect the laser pulse at a right angle to a second mirror. The second mirror reflects the laser pulse at right angles to a third mirror. The third mirror reflects the laser pulse at right angles to the fourth mirror. The fourth mirror reflects the laser pulse at right angles to the second optical element. The reflections at a right angle thus allow the fourth mirror to direct the laser beam onto a path through the second optical element which the laser beam would have passed through if it had not been deflected by the deceleration optics. A pulse delay occurs at the second optical element simply because of this delay line.

In particular, however, two opposite sides of the rectangle formed by the four mirrors mentioned can be lengthened or shortened, so that the length of the delay line can be adjusted. Typically, the distance between the first and the second mirror and the distance between the third and the fourth mirror are adjusted in order to keep the optical adjustment of the entire device as uncritical as possible. The length of the delay line can be set, for example, using fast piezo actuators or a linear table.

The pulse delay device described above only allows the pulse intervals of successively emitted laser pulses to be set. However, it is also possible to set a finer adjustment of the pulse delay using a pulse delay device. For this purpose, in the example above, the first and fourth mirrors can be replaced by beam splitters—which are called the first and fourth beam splitters below—to clarify the analogy to the example above.

Again, the beam splitter and the fourth beam splitter can be between the first and second optical element mentioned above. The first beam splitter can split the incoming laser pulse into a transmitted part and a reflected part. The transmitted part is therefore transmitted directly to the fourth beam splitter. There, the transmitted part is broken down again into a transmitted part, which is routed to the second optical element, and a reflected part, which is routed into a beam trap, for example.

The part of the laser pulse reflected at the first beam splitter is directed at a right angle to the second mirror. The second mirror reflects the laser pulse at right angles to a third mirror. The third mirror reflects the laser pulse at right angles to the fourth beam splitter. The fourth beam splitter splits the laser pulse again into a reflected part and a transmitted part. The reflected part is directed at a right angle to the second optical element, the trajectory of the laser pulse again coinciding with that of the laser pulse transmitted at the first beam splitter. The part transmitted at the fourth beam splitter can be guided into a beam trap, for example.

Due to the different distances covered, the laser pulses have a path difference behind the fourth beam splitter (and before the second optical element). The path difference is the time difference between the laser pulses that occurs when they pass through the delay line. The path difference between two pulses can be adjusted very finely. In particular, two laser pulses can also have a path difference of zero, ie they can completely overlap.

Overall, a further degree of freedom is generated by a pulse delay device, which makes it possible to set the pulse intervals with the device. In particular, the laser pulses can also be introduced into the workpiece at close intervals.

However, the pulse delay device also makes it possible to adapt the time interval between successive pulses in accordance with the material processing process and the laser pulse duration used.

The successive laser pulses can be understood to mean, in particular, laser pulses that have passed through the different paths of the pulse delay device, regardless of the actual path difference.

In principle, it is possible for the first beam splitter to be a polarizing beam splitter, for example a thin-film polarizer. Accordingly, the first polarizing beam splitter transmits a first polarization component and reflects a polarization component orthogonal thereto. The reflected laser pulse passes through the delay line, while the transmitted laser pulse is routed directly to the fourth beam splitter. At the fourth beam splitter, the laser pulses with the orthogonal polarizations can be recombined to form a laser beam, so that the laser beam has laser pulses with different polarizations.

To adjust the polarization of the laser pulses, the pulse delay device can have at least one wave plate, in particular λ/2 plates and/or λ/4 plates.

A λ/2 plate allows the polarization of an incident laser beam to be rotated. For example, an s-polarization can be converted into a p-polarization. For example, a right-hand circular laser beam can be converted into a left-hand circular laser beam. With a λ/4 plate, a circularly polarized laser beam can be generated from a linearly polarized laser beam and vice versa.

For example, the first optical element mentioned above can be a λ/2 plate, with which the polarization of the incident laser beam is rotated in such a way that a specific splitting can be achieved at the first polarized beam splitter. In particular, it can be achieved in this way that the partial laser beam reflected at the first beam splitter has the same intensity as the transmitted partial laser beam. In principle, however, a different power distribution of the partial laser beams can also be achieved in this way.

For example, the above second optical element can be a λ/4 plate. The λ/4 plate can be used to translate the laser pulses with p or s polarization, which are combined at the fourth beam splitter, into right-hand circular and left-hand circular polarizations, for example.

The polarization properties of the laser pulses can thus be effectively adjusted by the wave plates. In particular, the pulse delay device with the polarization setting can be used to construct a complex laser beam that guides differently polarized laser pulses with different amplitudes and variable time intervals.

In particular, orthogonal polarization is of particular relevance when using a geometric phase element, since only orthogonal polarization causes a complete change in the position of the individual laser pulses through the geometric phase element. If the laser pulses are not polarized orthogonally to one another, the two cases will be superimposed.

The time interval between two consecutive laser pulses can be greater than the duration of the laser pulses.

Since the path difference between two consecutive laser pulses is greater than the duration of the laser pulses, the laser pulses do not overlap in the material. Accordingly, the laser energy of the first pulse is introduced into or applied to the material before the laser energy of the second laser pulse. As a result, the different laser pulses cannot interact with one another, so that interference of the laser pulses, for example, is effectively suppressed.

Typically, the interference in the case of two laser pulses entering the material, which overlap in time and space, ensures inhomogeneous material processing, since the laser radiation of the segments is superimposed depending on location and time. This behavior is effectively prevented by the successive laser pulses. In other words, the coherence of the segments is broken by the temporal path difference.

The time interval between the laser pulses of the adjacent partial partial laser beams of the same polarization can be greater than the pulse duration.

Adjacent partial partial laser beams of the same polarization are generated when laser pulses with alternating polarization are emitted by the pulse delay device.

For example, it was shown above that a right-hand circularly polarized laser beam can be deflected to (+ _XGP + _XP ,0) _P and (+ _XGP - _XP ,0) _S by the geometric phase element and the polarization splitter. A left circularly polarized laser beam can be classified according to (-X _GP - X _P , 0) _S and (-X _GP + X _P , 0) _P are deflected. Accordingly, two adjacent laser pulses with the same polarization are produced: (-X _GP + X _P , 0) _P , (+X _GP + X _P , 0) _P . These laser pulses can interfere with one another, for example, if the pulse spacing due to the pulse delay device is less than the pulse duration. This should be avoided here.

The laser pulses are introduced, for example, with a time interval of 2 ps while the pulse duration of the ultra-short laser pulses is, for example, only 300 fs. Even with a small or negligible spatial offset, there is no interference of the laser pulses or the partial laser beams.

The time average of the intensity distribution corresponds to that of a flat-top laser beam.

In particular, the mean over time can be understood to mean the averaging over the entire intensity over at least the time that corresponds to the path difference of the laser pulses. However, it is also possible to average the intensity over a different period of time, for example over the time required for the laser pulse to cover the delay path.

The averaging over time makes it possible, in particular, to map a more complex intensity profile of the laser beam with the partial partial laser beams. This can be advantageous in particular when the processes in the material of the workpiece take place over a longer time scale than the duration of the material processing.

If, for example, 5 laser pulses are introduced into the material of the workpiece in a total of 500ps, which decays over a period of 1µm for the material processing or the process triggered by it in the material, then the laser pulses are introduced almost instantaneously. Despite the time offset, the processing characteristics of the ultra-short laser pulses are retained.

For example, the different partial partial laser beams, which are introduced into the workpiece at different times because of the pulse delay device, can produce a flat-top beam cross-section on average. In particular, a flat-top beam profile corresponds to a homogeneous and sharply defined beam cross-section, with the homogeneous part of the beam cross-section having an equally high intensity everywhere, but the intensity falling quickly to an almost vanishing value across the sharp boundary. In this case, the partial laser beams themselves can have a different beam cross section.

This can be an advantage, for example, if the beam quality of the laser is not sufficient for classic flat-top beam shaping. In any case, complex beam shapes can be realized by averaging the partial laser beam intensities in the focal plane.

character list

• 1 eine schematische Darstellung der Fokussierung eines Laserpulses zum Bearbeiten eines Werkstücks;
• 2A, B eine schematische Darstellung der Funktionsweise eines geometrischen Phasenelements;
• 3A, B, C, D eine schematische Darstellung der Funktionsweise eines Polarisationsaufspalters,
• 4 eine schematische Darstellung der Funktionsweise einer Strahlformungsvorrichtung;
• 5A, B, C, D, E, F eine schematische Darstellung der Funktionsweise der erfindungsgemäßen Vorrichtung;
• 6A, B eine schematische Darstellung der Pulsverzögerungsvorrichtung;
• 7A, B eine schematische Darstellung der Funktionsweise eines geometrischen Phasenelements mit Laserpulsen unterschiedlicher Polarisation;
• 8 eine schematische Darstellung der Funktionsweise einer erfindungsgemäßen Vorrichtung mit Laserpulsen unterschiedlicher Polarisation; und
• 9A, B eine weitere schematische Darstellung der Funktionsweise einer erfindungsgemäßen Vorrichtung.

Preferred further embodiments of the invention are explained in more detail by the following description of the figures. show:

• 1 a schematic representation of the focusing of a laser pulse for machining a workpiece;
• 2A, B a schematic representation of the functioning of a geometric phase element;
• 3A, B , C , D a schematic representation of how a polarization splitter works,
• 4 a schematic representation of the functioning of a beam shaping device;
• 5A, B , C , D , E , f a schematic representation of the operation of the device according to the invention;
• 6A, B a schematic representation of the pulse delay device;
• 7A, B a schematic representation of the functioning of a geometric phase element with laser pulses of different polarization;
• 8th a schematic representation of the functioning of a device according to the invention with laser pulses of different polarization; and
• 9A, B a further schematic representation of the functioning of a device according to the invention.

Detailed description of preferred embodiments

Preferred exemplary embodiments are described below with reference to the figures. Elements that are the same, similar or have the same effect are provided with identical reference symbols in the different figures, and a repeated description of these elements is sometimes dispensed with in order to avoid redundancies.

In 1 A typical apparatus for machining a workpiece 10 is shown schematically. In this case, an ultra-short pulse laser 3 made a laser beam 30 available. The ultrashort laser pulses 32 generated by the ultrashort pulse laser 3 run in the laser beam 30. The laser beam 30 is focused into a focal zone 80 by imaging optics 8, the focal zone 80 being in the workpiece 10 or on the surface of the workpiece 10. The position of the focal zone 80 is described here by the focal plane 800 . In the focus zone 80 material modifications are introduced by the irradiated laser beam 30, through which the workpiece 10 is processed. The material modifications can be, for example, isotropic changes in the refractive index (type I), birefringent changes in the refractive index (type II) and/or the creation of so-called voids or cavities. During the machining process, the workpiece 10 is arranged on a feed device 12 so that the material modifications can be introduced along a machining trajectory. In this case, the feed device 12 can be an XYZ table, through which the laser beam 30 and the workpiece 10 can be moved relative to one another.

In the following, the spatial properties of the components installed in the device 1 according to the invention will first be discussed. Then the temporal properties and the combination with the spatial properties are discussed.

2 shows a schematic representation of the functioning of a geometric phase element 4. The geometric phase element 4 allows the laser beam 30 to be deflected as a function of its polarization. For example, in 2A shown that a laser beam 30 with a right-hand circular polarization can be deflected in the +X _GP direction. The deflection of the laser beam 30 by the geometric phase element 4 therefore means that the focal zone 80 is offset by the geometric phase element 4 in the focal plane 800 .

In 2 B the case is shown in which the incident laser beam 30 is left circularly polarized. The left circular laser beam 30 is deflected to -X _GP by the geometric phase element. The mechanism described therefore makes it possible to displace a laser beam 30 due to its polarization, or to adjust the position of the focal zone 80 in the focal plane 800 . In particular, it is possible for the rotation of the geometric phase element 4 about the optical axis of the device to cause the deflection not in the x-direction but in the y-direction or a mixture thereof. This allows the focal zone 80 to be placed in the two-dimensional focal plane 800 around the optical axis of the device 1 (not shown).

In the 3A , 3B two embodiments of a polarization splitter 5 are shown schematically, with which it is possible to split a laser beam 30 into different polarization components. In this case, the polarization splitters 5 are birefringent polarization splitters 5 and can be provided, for example, in the form of a birefringent crystal. Different birefringent materials can be used as the crystal material for the polarization splitter 5, e.g. alpha-BBO (alpha-barium borate), YVO4 (yttrium vanadate), crystalline quartz, etc.

The polarization splitter 5 in 3A is wedge-shaped, ie a planar beam entry surface 52 for entry of an incoming laser beam 30 and a planar beam exit surface 54 of the polarization splitter 5 are aligned at a (wedge) angle to one another. The or an optical axis 56 of the crystal material is aligned parallel to the beam entry surface 40 .

The laser beam 30 entering the polarization splitter 5 perpendicularly to the beam entry surface 52 is split at the beam exit surface 54, which is inclined at an angle to the beam entry surface 52, into two partial laser beams 300, which are perpendicular to one another (s- and p-polarized). In 3A is, as is common practice, the s-polarized partial laser beam 300 characterized by a point, while the second, p-polari oriented partial laser beam 300 is marked by a double arrow. The first, p-polarized partial laser beam 300 is refracted less than the second, s-polarized partial laser beam 300 when exiting the polarization splitter 5, so that an angular offset α between the first and the second partial laser beam 300 occurs. The first and second partial laser beams 300 emerge from the polarization splitter 5 at the same point on the beam exit surface 54, which means that an angular offset α is generated, but no spatial offset between the two partial laser beams 300.

At the in 3B In the polarization splitter 5 shown, the beam entry surface 52 and the beam exit surface 54 are aligned parallel to one another and the optical axis 56 of the crystal material is aligned at an angle of 45° to the beam entry surface 52 . In this case, the laser beam 30 striking perpendicularly to the beam entry surface 52 is divided at the beam entry surface 52 into a first, ordinary partial laser beam 3000 and a second, extraordinary partial laser beam 300AO. The two partial laser beams 300 exit parallel at the beam exit surface 54, that is to say without an angular offset, but with a spatial offset Δx.

The two in 3A , 3B polarization splitters 5 shown differ fundamentally in that the in 3A polarization splitter 5 shown has an angular offset α (without spatial offset) and that in 3B polarization splitter 5 shown generates a spatial offset Δx (without angular offset).

In the 3C, D the effect of the described polarization splitter 5 is shown. 3C shows the case in which the incident laser beam 30 is left-circularly or right-circularly polarized. As described above, a p-polarized partial laser beam 300 and an s-polarized partial laser beam 300 are generated from a circularly polarized laser beam 30 . Depending on the rotation of the polarization splitter 5 about the optical axis 58 of the polarization splitter (which typically coincides with the optical axis of the device 1), a beam deflection can occur in a positive or negative x-direction, or in a positive or negative y-direction or a mixture thereof . In the 3A until 3D is also shown that the s-polarized beam is not deflected, so that the p-polarized partial laser beam 300 would always be rotated about the s-polarized partial laser beam 300 by a rotation described above. In fact, it is possible that the p-polarized sub-laser beam is not deflected and the s-polarized sub-laser beam 300 is deflected (as the p-polarized sub-laser beam 300 and the s-polarized sub-laser beam 300 swap roles in the above description) or that both partial laser beams are deflected. In particular, the p-part laser beam can be deflected along the positive x-axis or y-axis and the s-polarized part can be deflected along the negative x-axis or y-axis.

In 3D the generation of circularly polarized partial laser beams 300 after the polarization splitter 5 is shown. A λ/4 plate 50 is arranged behind the polarization splitter 5 for this purpose. The λ/4 plate 50 generates a left circularly polarized partial laser beam 300 from a p-polarized partial laser beam 300 and a right circularly polarized partial laser beam 300 from an s-polarized partial laser beam 300 or vice versa. It is thus possible to process the workpiece 10 with circularly polarized light, which can have a positive effect on the processing properties, for example.

In the 3C , 3D shows that the first partial laser beam 30 is not deflected and a second partial laser beam is deflected to −X _P . In general, however, it can be written that the first partial laser beam is deflected to ±X _P and the second partial laser beam 30 to −X _P is deflected.

In 4 a beam shaping device 6 is shown. A beam shaping device 6 can in particular be a diffractive optical element which rearranges an incoming laser intensity distribution of the laser beam 30 to another—generally arbitrary—laser intensity distribution. For example, the beam-shaping element 6 can shape three outgoing partial laser beams 300 from a single incident laser beam 30 . In this case, the partial laser beams 300 can lie on a line or a straight line. However, it is also possible with the beam-shaping device 6 to split the incident laser beam 30 into N times M partial laser beams 300, with N and M being natural numbers. For example, five times 7 partial laser beams 300 can be generated with the beam shaping device 6, the focal zones 80 of which lie in the focal plane 800, which form lattice points of a lattice and are therefore particularly regularly spaced from one another. In this case, the splitting takes place without dependence on the polarization. Accordingly, the beam shaping device can process all types of polarizations of the laser beam 30 equally well.

In 5A a first embodiment of the proposed device 1 is shown. Here, the laser beam 30 of the ultrashort pulse laser 3 is first guided through a geometric phase element 4, the laser beam 30 being deflected as a function of its polarization. Subsequently, the deflected laser beam 30 is guided through a polarization splitter 5, the laser beam 30 being split into two partial laser beams 300 according to its polarization and being deflected. As a result, the imaging optics 8 in the focal plane 800 achieve a distribution of the focal zones 80 that reflects the influence of the various beam deflections in the beam path. For example, the total beam deflection is the sum of the individual deflections.

For example, the incident laser beam 30 is right-hand circularly polarized. A right-hand circularly polarized laser beam 30 can be deflected by the geometric phase element 4 along the positive x-axis. Thus, without the polarization splitter, the laser beam 30 could be deflected to +X _GP . Without the geometric phase element 4, the right-hand circular laser beam 30 would be split and deflected along the negative and the positive x-axis, for example, to ±X _P and −X _P , for example. The geometric phase element 4 and the polarization splitter 5 now produce focus zones 80 at (+X _GP +X _P ) _P and (+X _GP -X _P ) _S , the partial laser beams 300 having a mutually orthogonal polarization. This makes it possible, for example, to place the adjacent partial laser beams 300 close to one another, for example by selecting the splitting angle by the polarization splitter 5, without disturbing interference effects occurring during the machining of the workpiece 10.

In 5B a further embodiment of the device 1 according to the invention is shown. In this case, a polarized laser beam 30, for example a left-hand circularly polarized laser beam 30, is first deflected by a geometric phase element 4, the laser beam 30 being deflected along the negative x-direction. The following polarization splitter 5 splits the laser beam 30 into two partial laser beams 300 polarized orthogonally to one another. Each of these partial laser beams 300 can then be split into a plurality of partial partial laser beams 3000 by a beam shaping device 6, for example a 2×1 diffractive optical element, which generates two partial partial laser beams 3000 from an incident partial laser beam 300. In the focal plane 800, for example, there is a distribution of focal zones 80 which corresponds to the sum of the individual deflections by an optical element.

For example, a left circularly polarized laser beam 30 results in a deflection to −X _G . In addition, the left-polarized laser beam 30 (according to 3C ) is split into a p-polarized and an s-polarized partial laser beam 300 along the x-axis.

In is the same device 1 according to the invention as in 5B shown. However, the incident laser beam is now right-hand circularly polarized. As a result, the incident laser beam 30 is now deflected by the geometric phase element 4 in the positive x-direction. Overall, in the focal plane 800, a distribution of the focal zones 80 as in 5B achieved, with the difference that the focal zones 80 are now offset along the positive x-axis in accordance with the different deflection by the geometric phase element 4 . In particular, it can be achieved with the device 1 that differently polarized laser beams 30 are imaged in partial partial laser beams 3000 which are positioned precisely “with a gap” in the focal plane 800 .

In 5D until 5F further options for beam deflection are shown. As already discussed, a rotation of the polarization splitter 5 about the optical axis of the device 1 or a rotation of the geometric phase element 4 about the optical axis of the device 1 can cause a deflection in the x-direction or y-direction or a mixture thereof. In particular, the geometric phase element 4 can cause a deflection in a first direction and the polarization splitter 6 can cause a deflection in another direction, in particular a direction orthogonal to the first direction. For example, the geometric phase element 4 can cause a deflection in the x-direction and the polarization splitter can cause it in the y-direction.

In 5D The successive deflection and splitting of a left circularly polarized laser beam 30 is shown schematically. The left-hand circular laser beam 30 is deflected by the geometric phase element 4, for example, in the positive y-direction and then deflected by the polarization splitter 5 in the x-direction. Finally, the beam-shaping device 6 generates 2×2 partial partial laser beams 3000 from the split partial laser beams 300 .

In 5E the same breakdown is shown in a tabular view. In particular, in 5F in comparison, the splitting of a right circularly polarized laser beam 30 is shown. Here, the deflection by the geometric phase element takes place in the negative y-direction. Here, too, the beam shaping device 6 finally causes a laser field 34 to be generated in the focal plane 800.

The previous description of the figures referred only to the spatial aspects of the beam deflection. The time aspects will now be discussed further.

In particular, the ultra-short pulse laser 3 can be used to generate ultra-short laser pulses 32 which propagate along the laser beam 30 . In this case, the laser pulses 32 have a pulse duration of less than 10 ps, preferably less than 1 ps, for example 10 fs. The distance between the individual laser pulses is less than 1 ms, preferably less than 1 μm, for example 1 ns. However, it is also possible, for example, to generate double pulses in the form of laser bursts, one burst comprising two laser pulses. The pulse spacing between two laser pulses of a laser burst is less than 100 ps, preferably less than 10 ps, for example 5 ps. This corresponds to repetition frequencies of the laser pulses 32 of more than 10 GHz, preferably more than 100 GHz, for example 200 GHz. What was described with regard to the individual laser pulses applies to the time intervals between the laser bursts.

Furthermore, the wavelength of the laser pulses 32 is between 200 nm and 2000 nm, preferably 257 nm or 343 nm or 515 nm or 1030 nm.

The problem here is that the laser pulses 32 typically have a fixed time interval due to the laser 3 . In addition, the polarization of the laser pulses 32 is predetermined by the laser 3 . In order to influence the polarization and the time interval between the different laser pulses 32, the pulse delay device 7 of 6A be used.

The pulse delay device 7 shown comprises four optical elements: a first polarizing beam splitter 711, a second mirror 702, a third mirror 703 and a fourth polarizing beam splitter 714.

When a circularly polarized laser pulse 32 is emitted by the laser 3, it can be directed to the first polarizing beam splitter 711. Should the laser 3 provide s-polarized or p-polarized laser pulses 32, these can be rotated with an optional λ/2 plate 70 and directed onto the polarizing beam splitter 711 in order to adjust the intensity distribution behind the first beam splitter 711.

The polarizing beam splitter 711 splits the circularly polarized laser pulse 32 into an s-polarized component and a p-polarized component. For example, the p-polarized component is transmitted through the polarizing beam splitter 711, while the s-polarized component is reflected at the polarizing beam splitter. However, this transmission or reflection can also take place exactly the other way round.

In the given example, the s-polarized component is reflected to a second mirror 702 , reflected from there to a third mirror 703 and finally directed to the fourth beam splitter 714 . The fourth beam splitter 714 transmits part of the laser pulse 32, for example, into a beam trap 74, while the other part is reflected and guided, for example, to a geometric phase element 5 (not shown).

The p-polarized component is transmitted at the first polarizing beam splitter 711 and directed to the fourth beam splitter 714 . At the fourth beam splitter 714, part of the laser pulse 32 is reflected to form a beam trap 74, whereas the part transmitted through the fourth beam splitter 714 is now recombined with the s-polarized part. Combined here means that the two s-polarized and p-polarized laser pulses 32 again form a laser beam 30 or propagate on the same trajectory.

The s-polarized component takes more time to traverse the pulse delay device 7 than the p-polarized component because the s-polarized component has to travel an additional path. Accordingly, the two laser pulses 32 have a path difference that is correlated with the length of the optical path through the pulse delay device 7 . To set the pulse delay, it is possible to change the transit time of the s-polarized laser pulse 32 by varying the length L to improve the optical path. For an even finer tuning, such a delay line can also be installed analogously in the path of the p-polarized component, as in 6B shown.

The path difference depends on the transit time of the laser pulses 32 along the different optical paths covered. The speed of light is 0.3 mm/ps, so that it is sufficient for a path difference of the laser pulses of 1 ps if the laser pulses 32 cover an optical path that differs by 0.3 mm.

If the laser pulses 32 of the laser 3 have a time interval of 6 ps and a circular polarization, a pulse delay device 7 can be used to generate a p-polarized and an s-polarized laser pulse 32 alternately every 3 ps. For this, the path difference between the laser pulses must correspond to 3ps, which corresponds to a difference in the optical path lengths of the s-polarized and p-polarized components of 0.9mm.

However, it is also possible to arrange the p-polarized laser pulse 32 not in the middle between two s-polarized laser pulses 32 (or vice versa) by setting a different path length difference.

7A shows the case of a geometric phase element 4 through which two mutually orthogonally polarized laser pulses 32 run one after the other. In this case, the first laser pulse 32 is left-circularly polarized and the second laser pulse 32 is right-circularly polarized. In the focal plane 800, the left-hand circularly polarized laser beam 30 is introduced at -X _GP by the geometric phase element 4 and the imaging optics 8 and, after a delay time that corresponds to the pulse spacing by the pulse delay device 7, the right-hand circularly polarized laser beam 30 at +X _GP .

In the event that the pulse delay device 7 generates linearly polarized laser pulses 32, in 7B a λ/4 plate 40 is arranged in front of the geometric phase element 4 . The λ/4 plate 40 can, for example, generate a left circularly polarized laser pulse 32 from the s-polarized laser pulse 32 . Furthermore, the λ/4 plate 40 can generate a right circularly polarized laser pulse 32 from a p-polarized laser pulse 32 . Since the p-polarized laser pulse 32 passes through the λ/4 plate 40 after the s-polarized laser pulse 32, the left-hand circularly polarized laser pulse 32 is first introduced at -X _GP in the focal plane and, after the delay time by the pulse delay device, the right-hand circularly polarized laser pulse 32 at +X _gp .

In 8th the device 1 according to the invention is shown. In this case, a left-hand circularly polarized laser pulse 32 and then a right-hand circularly polarized laser pulse 32 are successively passed through the pulse delay device 7 to the geometric phase element 4, with the geometric phase element 4 in the focal plane 800 causing a deflection of the laser beam 30 as in 7A, B is effected. However, the deflected laser beams 30 are split by the polarization splitter 5 according to their polarization components.

In this way, laser fields 34 can be generated which have a high focal zone density. For example, the focal zones 80 can spatially overlap in the focal plane 800 without disturbing interference of the partial laser beams 300 if the time interval between the successively introduced laser pulses 32 is greater than the pulse duration of the laser pulses 32. This breaks the coherence between the laser pulses 32. Overall, a higher quality can thus be achieved when machining a workpiece 12 .

In 9A another embodiment of the proposed device 1 is shown. In this case, a left-hand circularly polarized laser pulse 32 and then a right-hand circularly polarized laser pulse 32 are guided through the geometric phase element 4 one after the other, where a deflection is effected in accordance with the polarization. The deflected laser beam is then passed through a polarization splitter 5, the laser beam 00 being split into two partial laser beams 300 according to its polarization components. The partial laser beams 300 are passed through a beam shaping device 6, which splits the partial laser beams 300 into partial partial laser beams 3000, which are then focused into the respective focal zones 80 in the focal plane 800 by the imaging optics. A laser field 34 can also be generated in this way, with the partial partial laser beams 3000 lying either on a straight line or on a two-dimensional grid (as in 9B shown). In this case, adjacent partial partial laser beams 3000 of different polarization can be introduced into the workpiece 10 at the same times, while adjacent partial partial laser beams 3000 can be introduced into the workpiece 10 at different times.

In 9B a situation is shown in which the beam deflection by the geometric phase element 4 occurs perpendicularly to beam splitting by the polarization splitter 5. As a result, the generated laser fields 34 change in particular from the 5E and 5F off with each other. In the configuration mentioned, it is possible in particular for neighboring focal zones 80 to always have a different polarization or a path difference, so that the time-averaged laser field 34 can have a particularly high focal zone density without disruptive interference effects occurring between the partial partial laser beams 3000.

In 9C another situation is shown which can be brought about with the proposed device 1. In this case, the beam shaping device 6 is designed in such a way that the partial partial laser beams 3000 have a different penetration depth into the workpiece 10 . In particular, the focal plane 800 formed by the individual focal zones 80 is tilted relative to the z-axis or the optical axis of the device 1 . Such a distribution of the focal zones 80 of the partial partial laser beams 3000 makes it possible, for example, to provide what is known as a chamfer beam, which can be used, for example, for chamfering the workpiece. In particular, the pulse sequence and polarization are therefore determined by the pulse delay device 7, the geometric phase element 4 and the polarization splitter 5, while the shape of the laser field 34 can be set particularly easily by the beam shaping device 6.

In 9D a further situation is shown which can be brought about with the device 1 according to the invention. The beam shaping device 6 is designed in such a way that the focal zones 80 of the partial laser beams 3000 lie within a limited area of the focal plane 800. In particular, all focal zones 80 are illuminated with the same intensity, so that on average over time, i.e. in particular on average over two consecutive laser pulses 32 with orthogonal Polarization, a homogeneous intensity distribution in the focal plane 800 arises. The mean intensity distribution is in 9E shown, with black representing low intensity and white representing high intensity. The mean intensity distribution has a sharp limit here. Such an intensity distribution corresponds to that of a so-called flat-top beam. With the device 1 according to the invention, it is therefore also easily possible to generate complex beam shapes and beam geometries.

As far as applicable, all individual features that are presented in the exemplary embodiments can be combined with one another and/or exchanged without departing from the scope of the invention.

Reference List

1

contraption

10

workpiece

12

feed device

14

trajectory

3

Ultrafast Laser

30

laser beam

300

partial laser beam

3000

partial laser beam

32

laser pulse

34

laser field

4

geometric phase element

40

A/4 plate

5

polarization splitter

50

A/4 plate

52

beam entry surface

54

jet exit surface

56

optical axis of the birefringent crystal

6

beam shaping device

7

pulse delay device

70

λ/2 plate

72

λ/4 plate

701

first mirror

702

second mirror

703

third mirror

704

fourth mirror

711

first beam splitter

714

fourth beam splitter

74

beam trap

8th

imaging optics

80

focus zone

800

focal plane

a

Angular offset between the partial laser beams

V

feed

Claims (14)

Device (1) for processing a workpiece (10) by means of ultra-short laser pulses (32), comprising an ultra-short pulse laser (3) which is set up to emit ultra-short laser pulses (32) in a laser beam (30), a geometric phase element (4), which is set up to carry out a beam deflection and/or beam shaping and/or beam splitting depending on the input polarization of the incident laser beam (30), imaging optics (8) which are set up to project the laser beam (30) or the partial laser beams (300) into the workpiece (10), whereby the workpiece (10) is machined, characterized in that successive laser pulses (32) have different polarizations, preferably mutually orthogonal, or the laser pulses (32) have orthogonal polarizations in pairs. device after claim 1 , characterized by a polarization splitter (5), which is set up to split the incident laser beam (30) into two partial laser beams (300), the two partial laser beams (300) having orthogonal output polarizations, further wherein the polarization splitter (5) in the beam propagation direction behind or is arranged in front of the geometric phase element (4). Device (1) after claim 1 or 2 , characterized in that the beam deflection of the polarization splitter (5) takes place perpendicularly or parallel to the beam deflection of the geometric phase element (4). Device (1) according to one of the preceding claims, characterized by a beam shaping device (6) which is set up to split the incident laser beam (30) into a plurality of partial laser beams (300), the beam shaping device (6) being located behind the polarization splitter ( 5) is arranged. Device (1) after claim 4 , characterized in that the partial laser beams (300) lie on a one-dimensional line or on a two-dimensional grid. Device (1) according to one of the preceding claims, characterized by a λ/4 plate which is set up to convert a circular polarization into a linear polarization or a linear polarization into a circular polarization, the λ/4 plate being in the beam propagation direction before the geometric phase element (4) or after the polarization splitter (5). Device (1) according to one of the preceding claims, characterized by a pulse delay device (7) which is set up to set the time offset of the laser pulses (32) variably. Device (1) according to one of the preceding claims, characterized in that the time interval between two successive laser pulses (32) is greater than the duration of the laser pulses (32). Device (1) according to one of the preceding claims, characterized in that the time average of the intensity distribution (I) corresponds to that of a flat-top laser beam. Device (1) according to one of the preceding claims, characterized in that adjacent partial partial laser beams (3000) of the laser field (34) have a mutually orthogonal polarization and/or that the time interval between the laser pulses (32) of the adjacent partial partial laser beams (3000) of the same polarization is greater is than the pulse duration. Device (1) according to one of the preceding claims, characterized in that the geometric phase element (4) is arranged in the far field of the imaging optics (8). Device (1) according to one of the preceding claims, characterized by a feed device (12) which is set up to move the workpiece (10) and the laser field (34) relative to one another with a feed (V) along a trajectory (14). Device (1) according to one of the preceding claims, characterized by a scanner unit (120) which is set up to scan the laser field (34) over the workpiece (10), the scanner unit (120) preferably having a galvanic scanner is. Device (1) according to one of the preceding claims, characterized in that - the wavelength of the laser pulses (32) is between 200 nm and 2000 nm, preferably 257 nm or 343 nm or 515 nm or 1030 nm and/or - the pulse spacing is less than 100 ps, preferably is less than 10ps and/or the pulse duration is less than 10ps, preferably less than 1ps.

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