DE102021120509A1 - Device for processing a material - Google Patents

Abstract

The present invention relates to a device (1) for processing a material (2) using ultrashort laser pulses from an ultrashort pulse laser (3), comprising a resonator (4) with a resonator input (40) and a partially reflecting resonator output (42), the resonator (4) thereto is set up to reflect the laser beam (30) of the ultrashort pulse laser multiple times in the resonator chamber (44), with a segment (320) of the laser beam (30) being coupled out at the partially reflecting resonator output (42) with each reflection, radial manipulation optics (5), set up to manipulate the laser beam (30) radially, with the cross section through the laser beam (30) yielding a laser contour (32), imaging optics (6) set up to project the segments (320) of the laser beam (30 ) to be introduced into the material (2), as a result of which the material (2) is processed, the resonator (4) being designed in such a way that the lateral extent of the laser contour (32) increases with each passage ng of the resonator (4) increases or decreases, the lateral extent of the segments (320) of the laser beam (30) decoupled one after the other from the resonator (4) increasing or decreasing accordingly, and the imaging optics (6) the lateral extent of the decoupled Segment (320) of the laser beam (32) translated into a longitudinal and / or lateral focal zone (F), wherein the position of the focal zone (F) is correlated with the lateral extent.

Description

technical field

The present invention relates to a device for processing a material using ultrashort laser pulses of an ultrashort pulse laser.

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.

For this purpose, non-diffracting beam shapes are preferably used, which have a focal zone that is elongated in the beam propagation direction, as a result of which the material can also be processed at particularly great material depths. However, the aforementioned ultra-short-pulse processing methods using non-diffracting beam shapes are limited in certain applications, for example due to the build-up of stresses in the material or the pulse power to be coupled. In particular, in the case of more complex jet shapes, for example vortex Bessel jets, shielding effects can also occur in deeper material areas, so that only superficial material processing is possible.

Presentation of the invention

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

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

Accordingly, a device for processing a material using ultrashort laser pulses from an ultrashort pulse laser is proposed, comprising a resonator with a resonator input and a partially reflecting resonator output, the resonator being set up to reflect the laser beam of the ultrashort pulse laser multiple times in the resonator space, with a Segment of the laser beam is decoupled, radial manipulation optics, which are set up to manipulate the laser beam radially, the cross section through the laser beam yielding a laser contour, and imaging optics, which are set up to introduce the segments of the laser beam into the material, whereby the material is processed. According to the invention, the resonator is designed in such a way that the lateral extent of the laser contour increases or decreases with each passage through the resonator, with the lateral extent of the segments of the laser beam successively decoupled from the resonator increasing or decreasing accordingly, with the imaging optics measuring the lateral extent of the decoupled segment of the laser beam is translated into a longitudinal and/or lateral focal zone, the position of the focal zone being correlated with the lateral extent.

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. The bursts or individual pulses are of the same type if the laser pulses used have essentially the same properties, ie approximately the same Pulse energy, the same pulse length and - in the case of bursts - also have the same pulse spacing within the burst.

A resonator can be a Fabry-Perot interferometer, for example, or a ring resonator, but other resonators can also be used that are based on multiple reflections of the laser beam.

The laser beam of the ultrashort pulse laser is coupled through the resonator entrance into the resonator volume of the resonator chamber. In this case, the resonator input is ideally designed in such a way that the laser beam can couple into the resonator space, but can no longer couple out through the resonator input. In this case, the resonator space is the space that is delimited by the resonator input and the resonator output.

The resonator exit is the part of the resonator through which the laser beam is coupled out of the resonator space. The resonator output is designed to be partially reflective, so that part of the laser beam that falls on the resonator output is reflected back into the resonator space. The part of the laser beam that is not reflected back into the resonator chamber by the partially reflecting resonator outlet is coupled out of the resonator through the resonator outlet. The part that is reflected back into the resonator chamber can be reflected back into the resonator chamber at the resonator input.

This results in a gradual decoupling of the laser power from the resonator chamber through the resonator output. A segment of the laser beam can be understood here as the energy segment of the laser beam that is coupled out of the resonator.

• Ein Laserstrahl trifft auf den Resonatorausgang, wobei das erste Segment des Laserstrahls aus dem Resonator ausgekoppelt wird. Anschließend wird der nicht-ausgekoppelte Teil des Laserstrahls zurück zum Resonatoreingang reflektiert, von wo aus er erneut zum Resonatorausgang reflektiert wird. Nach diesem erneuten Durchgang kann ein zweites Segment des Laserstrahls aus dem Resonator ausgekoppelt werden. Das erste und das zweite Segment des Laserstrahls werden somit zu unterschiedlichen Zeiten aus dem Resonator ausgekoppelt und weisen einen Gangunterschied auf.

Due to the geometric arrangement of the resonator input and the resonator output, as well as due to the optical path covered by the laser beam between the partial reflections at the resonator output, there is a path difference between two segments of the laser beam that are coupled out one after the other. This can be understood as follows:

• A laser beam strikes the resonator exit, with the first segment of the laser beam being coupled out of the resonator. The part of the laser beam that has not been coupled out is then reflected back to the resonator input, from where it is reflected again to the resonator output. After this renewed passage, a second segment of the laser beam can be coupled out of the resonator. The first and the second segment of the laser beam are thus coupled out of the resonator at different times and have a path difference.

In other words, the path difference of the segments is determined by the length of the optical path that the laser beam travels between two consecutive partial reflections at the resonator exit.

For example, the optical path that the second segment has to cover between the reflection at the resonator output and the decoupling at the resonator output can be 3 cm long. The speed of light is c = 30 cm/ns, so this optical path is covered within 0.2 ns (the distance has to be traversed twice). The path difference between the first and the second partial laser beam is therefore 0.2 ns. Compared to this path difference, the pulse duration of the laser pulses, which can be between 10 fs and 50 ps, appears infinitesimally short. This means in particular that a second segment of the laser beam which hits the material of a workpiece can no longer interact, for example interfere, with the first segment of the laser beam. One can also say that the temporal coherence of the decoupled segments is broken.

A radial manipulation optics is set up to manipulate the laser beam radially. In particular, this can mean that the laser beam converges or diverges after the radial manipulation optics and the partial laser beams can therefore lie on the surface of a cone or multiple cones. By projecting the radially manipulated partial laser beams onto a plane after the radial manipulation optics, a so-called laser contour results as a cross section of the laser beam. Depending on the configuration of the radial manipulation optics, the shape of the laser contour can be a circle or an ellipse.

If the laser beam of the ultrashort pulse laser is manipulated radially by a radial manipulation optics and the radially manipulated partial laser beams enter the resonator or are in the resonator, then it follows that the lateral extension of the laser contour changes with each passage through the resonator. In particular, the lateral extent can increase or decrease with each passage through the resonator.

For example, the laser beam can have divergent partial laser beams after the radial manipulation optics, so that a laser contour with a diameter D and thickness d falls on the resonator output. Accordingly, the laser beam with the laser contour is partially decoupled from the resonator—in the form of a segment—and partially reflected back into the resonator chamber animals. The part of the laser beam that is reflected back into the resonator space expands as a result of further propagation along the optical path to the resonator entrance and has a diameter of 2D with a thickness of 2d there. The laser beam is reflected from the resonator entrance back to the resonator exit, where the lateral extent of the laser contour has grown to 3D with a thickness of 3d. Due to the multiple reflections, concentric rings with a diameter of D, 3D, 5D, .

The lateral expansion of the laser contour can be adjusted by the distance between the resonator entrance and the resonator exit. In particular, with the divergence angle remaining the same, a shorter distance can cause a smaller lateral extension of the laser contour, and a larger distance can cause a larger lateral extension of the laser contour. In particular, the path difference of the segments of the laser beam also changes accordingly.

An imaging optic can focus the segments of the laser beam successively decoupled by the resonator into a focal zone in the material, as a result of which the material is processed.

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 for processing to involve causing material modifications in the material or causing material removal.

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 type I isotropic refractive index changes are attributed to localized melting caused by the laser pulses and rapid resolidification of the transparent material. 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 a small area higher 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.

In this case, the imaging optics convert the lateral extent of the laser contour of the segment of the laser beam that is coupled out into a longitudinal and/or lateral focal zone, with the position of the focal zone being correlated with the lateral extent of the laser contour.

For example, a first laser contour of a first diameter can be translated into a first focal zone, a second laser contour of a second diameter can be translated into a second focal zone, and a third laser contour of a third diameter can be translated into a third focal zone. Depending on the design of the imaging optics, all three focal zones can be offset laterally, that is to say, for example, can be shifted perpendicular to the optical axis in a plane, for example the focal plane of the imaging optics. However, it is also possible for the three focus zones to run parallel to the optical axis on one axis, or for them to run on the optical axis but for all three focus zones to lie in different focal planes along the direction of beam propagation. It is also possible that the three focal zones lie in different focal planes and are shifted to the optical axis.

Each segment of the laser beam that is coupled out can produce its own beam shape in the focal zone, with the beam shape being a lateral multispot or a ring focus or an autofocusing beam or a non-diffractive beam or a diffractive beam.

For example, the first focal zone can be that of a Gaussian laser beam, the second focal zone can be an annular focal zone, and the third focal zone can be an autofocussing beam.

The imaging optics can be a lens system or a mirror system, which has one, two or more mirrors and/or lenses. For example, the imaging optics can be a telescope and/or comprise a segmented diffractive optical element or a correspondingly shaped asphere or free-form optics (see below).

The radial manipulation optical element can be an inverse axicon or an axicon or an axicon telescope, with the axicon angle determining the aperture angle of the cone or of the radial manipulation, or it can be a diffractive optical element or it can be a free-form surface.

A diffractive optical element is set up to influence the incident laser beam in one or more properties in two spatial dimensions. A diffractive optical element is a fixed component that can be used to produce a specific non-diffractive laser beam from the incident laser beam. 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.

A free-form surface is generally a light-refracting surface with which a laser beam can be converted from a first intensity distribution into a second intensity distribution. The shape of the free-form surface allows the phase distribution and intensity distribution of the laser beam to be adjusted in a targeted manner.

An axicon is a conically ground optical element that forms a non-diffracting laser beam from an incident Gaussian laser beam as it passes through. In particular, the axicon has a cone angle that is calculated from the beam entry surface to the lateral surface of the cone. As a result, the marginal rays of the Gaussian laser beam are refracted to a different focal point than rays close to the axis. This results in a focal zone that is elongated in the beam propagation direction directly behind the axicon.

An inverse axicon is accordingly the negative imprint of an axicon. In other words, in this case a glass body has a conical indentation with the cone angle. The incident Gaussian laser beam is fanned out as it passes through, whereby a non-diffracting beam can be generated by focusing the fanned out beams.

In particular, the above optical elements can also be combined with one another in order to produce a beam quality of the processing laser beam that is as ideal as possible, or a phase curve that is as conical as possible.

For example, two axicons can be assembled into an axicon telescope and form the radial manipulation optics, with the cone tips of the axicons either pointing towards or away from each other. It is also possible to combine two inverse axicons to form an axicon telescope and form the radial manipulation optics, with the conical indentations genes either point at each other or point away from each other. It is also possible for an inverse axicon and an axicon to be assembled into an axicon telescope, with the cone of the axicon and the cone-shaped depression of the inverse axicon pointing towards each other or pointing away from each other, in either case either the axicon or the inverse Axicon can be traversed by the laser beam in the beam propagation direction.

The decoupled segments of the laser beam can be successively introduced into the material, with the path difference between two consecutive segments of the laser beam being greater than the pulse duration of the laser pulse.

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

Typically, the interference in the case of two segments 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 chronologically consecutive segments. In other words, the coherence of the segments is broken by the temporal path difference.

The imaging optics can comprise a segmented beam shaping element, with each segment being set up to generate its own beam shape, with each individual beam shape being able to be selected from the list of lateral multispots, ring foci, autofocusing beams and non-diffracting beams.

In particular, the segmented beam-shaping element can be a segmented diffractive optical element. For example, each segment of the segmented beam-shaping element can provide its own diffraction pattern, so that a different beam is formed depending on where the laser contour hits the segment. In particular, the segments of the segmented beam-shaping element can be concentric circles
For example, a lateral multispot can be generated from a first laser contour and an (abruptly) autofocusing beam from a second laser contour, while a non-diffracting beam or a Gaussian beam is generated from a third laser contour.

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. In particular, a material modification that is elongated in the beam propagation direction can be produced as a result.

In particular, non-diffracting beams can be used to generate elliptical non-diffracting beams that have a non-radially symmetrical transverse focal zone. For example, elliptical quasi-non-diffracting rays have a main maximum that coincides with the center of the ray. The center of the ray is given by the place where the main axes of the ellipse intersect. In particular, elliptical, quasi non-diffracting beams can result from the superimposition of a plurality of intensity maxima, in which case only the envelope of the intensity maxima involved is elliptical. In particular, the individual intensity maxima do not have to have an elliptical intensity profile.

Lateral multispots are understood to be laterally offset focal zones that can be formed from a partial laser beam. Ring foci have an annular cross-section. Autofocusing beams are a new class of beams whose maximum intensity increases by orders of magnitude just before the focal point. For more information on this beam class, see Efremidis et al., "Abruptly autofocusing waves," Opt. Lett. 35, 4045-4047 (2010).

The time-averaged intensity profile of the segments of the laser beam introduced into the material can correspond to a non-diffracting beam or Bessel beam.

In particular, the averaging over time can be understood to mean the averaging over the intensity profile over the time that elapses until a single pulse has completely left the resonator. Since residual energy always remains in an ideal resonator, an average can also be used over a build-up time. For example, a threshold time can be reached when the segments coupled out at the resonator output no longer transport enough energy to enable material processing, since the intensity in the potential focus zone is below the modification threshold of the material.

For example, five successively introduced laser beams into the material can be averaged to form a total beam by averaging over time. Since the laser beams can be introduced into the material one after the other, the locations of the focal zones can overlap completely or partially.

In the case of non-diffracting rays, for example, the elongated focal zones lying in the optical axis can be directly connected to one another in the longitudinal direction, so that an average focal zone is generated whose length is the sum of the lengths of the individual focal zones, with the average focal zone again being the corresponds to the focal zone of a zero-order non-diffracting beam. However, it is also possible, for example, for the focal zones not to overlap spatially but to be separated in the longitudinal direction. Then, for example, a kind of perforation line (dashed line) can be generated as a focal zone, which corresponds to a non-diffracting higher-order beam.

The device can also have a feed device, which can be set up to move the laser beam and the material relative to one another with a feed, the spatial offset caused by the feed being smaller than the size of the averaged intensity profile.

The material modifications are introduced into the material of a workpiece along a trajectory. The trajectory describes the line of impact of the laser beam 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. The feed rate is selected in such a way that the displacement caused by the feed is smaller than the size of the averaged intensity profile.

This is intended in particular to ensure that the averaged intensity profile is not distorted.

This is based on the following idea: If, for example, 10 segments are coupled out by the resonator, which are to be averaged to form the entire intensity profile, it takes about 10 ns to introduce the averaged intensity profile into the material, provided the segments have a path difference of 1 ns each. If, for example, the mean intensity profile now has a lateral cross-section of about 2 μm, then the above condition is still met for a feed rate of 0.2 μm/ns=200 m/s. If the feed rate were to be faster, the averaged intensity profile would be distorted, which would reduce the quality of the material processing (this can be problematic, especially with longer path differences). With significantly shorter path differences, however, the different segments hit the material almost instantaneously, so that the averaged focal zone is not distorted.

The other adjustable elements of the embodiments described above can also be adjusted during different process phases using fast piezo actuators. For example, the distance between the resonator input and the resonator output can be adjusted and/or the length of the optical path can be adjusted by the delay optics and/or the positioning of the filling level and thus the degree of filling can be adjusted and/or the focusing can be adjusted and/or the Distance of inverse axicon and axicon of axicon telescope can be adjusted, and so on. This makes it possible to control the lateral extensions and the path differences of the segments with segment accuracy, so that the intensity curves introduced into the material can be set particularly precisely. In particular, in the case of the embodiments of the device according to the invention, the laser beam can also be coupled in and coupled out into and out of the resonator via quickly switchable polarizations and polarization switches. Since the incoupling and/or outcoupling is polarization-dependent, a particularly good incoupling into the resonator and/or outcoupling from the resonator can be made possible.

A flow plate can be set up to attenuate the spatially separated segments of the laser beam to different extents, with the flow plate being arranged in or directly after the resonator, or being designed in one piece with one of the optical elements of the device, preferably in the form of a coating.

In this way, in particular, a targeted beam shaping of the averaged intensity profile can be undertaken.

As already described above, the various segments in the form of transmission orders of the resonator are spatially separated by the fanned-out laser beam. Likewise, the various segments are focused into different focal zones by the imaging optics. The spatial separation makes it possible, in particular, to assign different intensities to the different transmission orders.

For example, if the resonator output has a partial reflection of 25%, then after the first passage through the resonator, 75% of the energy still remains in the resonator, with 25% of the energy being coupled out. After the second pass, 56.25% of the energy is still in the resonator, with 18.75% of the original energy being coupled out during the second pass. After the third pass, 42.19% of the energy is still in the resonator, with 14.06% of the original energy being coupled out on the third pass. After the fourth pass, 31.64% of the energy is still in the resonator, with 10.05% of the original energy being coupled out on the fourth pass. Finally, after the fifth pass through the resonator, 23.7% of the original energy still remains in the resonator and 7.9% of the original energy is coupled out.

Suppose the modification threshold of the material is at a value that corresponds to 8% of the original energy in the example above. Then only the first four segments would contribute to the averaged intensity profile. In particular, however, the successively decoupled segments have less and less energy. Accordingly, an averaged intensity profile would not be homogeneous along the beam propagation direction.

This can be remedied if a gradient plate scales the different segments to the same value. This can happen, for example, by using a neutral density filter or, in general, an attenuation filter at the location of the first decoupled segment to reduce the decoupled energy to 10% of the original energy. At the location of the second decoupled segment, the decoupled energy can also be attenuated to 10% of the original energy, and so on. This allows the averaged intensity profile to be homogenized. In particular, however, different levels of attenuation are required for the different segments.

It is also possible to scale the extracted energy by designing the partially reflecting resonator output. For example, the reflectivity can increase in such a way that segments coupled out one after the other have energies of the same magnitude.

For example, a radially symmetric gradient plate can be used to tailor a longitudinally elongated, averaged intensity profile. As a result, mean longitudinal intensity distributions that are similar to flat-top can be generated, which are robust to variations in the input beam profile, adjustment or beam quality.

In one embodiment of the invention, the radial manipulation optics can fan out the laser beam and the imaging optics can translate the lateral extent of the decoupled segment of the laser beam into a longitudinal focal zone, with the position of the focal zone in the beam propagation direction being correlated with the lateral extent.

A radial manipulation optics element can, optionally in combination with further optics, form a non-diffracting laser beam from a diffracting laser beam, in this first embodiment additionally, for example, from a Gaussian laser beam. However, the description of the first exemplary embodiment applies analogously to other beam shapes.

A non-diffracting beam can be generated from a plane wave field or from parallel partial laser beams of a Gaussian laser beam if the radial manipulation optical element refracts all partial laser beams of the Gaussian laser beam at the same angle β to the optical axis. As a result, partial laser beams close to the axis overlap shortly after the radial manipulation optical element in the beam propagation direction on the optical axis and thus form an increased laser intensity, while off-axis beams only overlap later in the beam propagation direction and form an increased laser beam intensity.

In this way, an essentially constant laser intensity can be generated over a longitudinal length, ie a distance on the optical axis, parallel to the direction of beam propagation. In the beam propagation direction behind the elongated focal zone, however, the partial laser beams are fanned out, for example fanned out conically. The cross section through the cone is accordingly a circle or ring.

However, it is also possible that the plane wave field breaks away the partial laser beams of the Gaussian laser beam at the same angle β′ from the optical axis due to the radial manipulation optical element. The partial laser beams can thus also be fanned out directly after passing through the radial manipulation optical element. In the present embodiment, therefore, such a distinction is not relevant.

When forming a laser contour with a radial manipulation optical element, the thickness d of the ring is constant in the direction of beam propagation, since all partial laser beams run at the same angle to the optical axis.

The imaging optics convert the lateral extent of the laser contour of the segment of the laser beam that is coupled out into a focal zone that is elongated along the optical axis, with the position of the focal zone being correlated with the lateral extent of the laser contour.

For example, the focal zone of laser contours with a small lateral extent is located behind the focal zone of laser contours with a large lateral extent in the direction of beam propagation. For example, a ring-shaped laser contour from the above example with a diameter D can be focused into a focal zone whose center is at +5Z. A 3D diameter ring can be focused into a focal zone at +3Z and a 5D diameter ring can be focused into a focal zone at +Z.

The large number of focal zones is to a certain extent stacked along the optical axis by the large number of decoupled segments, with the different lateral extensions of the laser contours resulting in a spatial offset of the various focal zones. In addition, there is a time delay with which the different partial laser beams are introduced into the material in the different focal zones, which results from the path difference of the segments in the resonator.

This type of beam generation can be used to optimize the laser beam for the absorption behavior of the material. For example, the material can be processed from "inside" to "outside" or vice versa. It is also possible that interference between individual segments is largely avoided since the temporal coherence between the segments is broken.

In a further embodiment of the invention, the radial manipulation optical element can be arranged in front of the resonator entrance; the resonator entrance can be the hole of a reflecting perforated mirror, with the reflecting perforated mirror being arranged reflecting into the resonator space.

A pinhole mirror is a mirror with a hole in it. The laser beam, which has been radially manipulated by the radial manipulation optical element, can be coupled into the resonator through the hole in the perforated mirror. Since the laser beam can be coupled into the resonator without the laser beam having to propagate through a (partially) transparent surface, the effectiveness or the power output of the laser beam increases. The mirrored side of the perforated mirror points in the direction of the resonator chamber or the resonator exit. This ensures that the laser beams can be reflected from the resonator input to the resonator output. In addition, the perforated mirror preferably has a highly reflective coating of more than 80%, preferably more than 99%, so that the smallest possible energy loss occurs during reflection at the perforated mirror.

The resonator input and/or the partially reflecting resonator output can be tilted radially symmetrically, so that the path difference of the segments of the laser beam can be adjusted independently of the distance.

Tilted radially symmetrically can mean that the surface of the resonator inlet and/or the resonator outlet has surface curvature. Such a tilting makes it possible to adjust the reflection angle, so that the lateral expansion of the laser contour can be adjusted in this way. With a large distance between the resonator input and the resonator output, it can thus be avoided that the laser contours become too large. At the same time, however, the large path difference of the segments can be used. In particular, small path differences of the segments with large lateral dimensions of the laser contours can also be set.

A filling mirror can be arranged behind the hole of the hole mirror in the resonator, which reflects the laser beam onto the reflecting side of the hole mirror, the longitudinal degree of filling of the averaged intensity distribution being increased.

The filling mirror can be set up to transmit a first part of the incident laser radiation to the resonator outlet and to reflect a second part back to the resonator inlet. As a result, the first part and the second part of the laser radiation cover different optical paths until they are coupled out of the resonator output. In particular, the the parts therefore have different diameters at the resonator exit. Accordingly, the filling level can be used to ensure that the diameter of the laser contour can be set not only by the distance between the resonator input and the resonator output, but also by the distance between the resonator input and the filling level.

For example, the segments according to the above example with the diameters of the laser contours D, 3D, 5D and so on are coupled out at the resonator output. It can now be particularly advantageous to arrange the filling level in such a way that the laser beam has a laser contour with a diameter of 2D at the resonator output after being reflected back to the resonator input through the filling level and the subsequent forward reflection to the resonator output. The segment of the laser beam with a laser contour with a diameter of 2D is also partially decoupled at the resonator output and partially reflected back to the resonator input. During a subsequent passage through the resonator, the segment of the laser beam originally separated by the filling mirror has a laser contour with a diameter of 4D, and so on. Accordingly, a dense ring pattern can be generated at the resonator exit by the filling level, for example with the diameters D, 2D, 3D, 4D, 5D, and so on.

It is also possible that rings which are adjacent due to the filling level have different distances, for example D, 1.5D, 3D, 3.5D, and so on. As a result, the filling level can also be used to generate special averaged intensity profiles.

According to a further embodiment of the invention, the resonator can be a ring resonator, the resonator having a first radial manipulation optical element and a second radial manipulation optics element, the second radial manipulation optics element being arranged downstream of the first radial manipulation optics element, with the second radial manipulation optics element preferably collimating the partial laser beams and with particularly preferably, the first radial manipulation optical element and the second radial manipulation optical element are axicons, which together form an axicon telescope.

A ring resonator consists of at least three optical elements, namely the resonator input, the resonator output and the delay optics.

The resonator input is set up to couple the laser beam into the resonator and has, for example, a highly reflective reflective coating on the resonator chamber side. The partially reflecting resonator output is set up to partly decouple the segments of the laser beam and partly direct them onto a delay line. The deceleration optics directs the laser beam, which is partially reflected from the resonator output, via the deceleration path to the resonator input.

The retarding optics comprise at least one mirror, but typically two mirrors. The typically two mirrors preferably form the corner points of a rectangle together with the resonator input and the resonator output. As a result, the laser beam can be reflected from the resonator output onto a first mirror of the deceleration optics. The first mirror reflects the laser beam to the second mirror of the retardation optics. The second mirror of the deceleration optics reflects the laser beam to the resonator entrance. From the resonator entrance, the laser beam traverses the path just described again, a segment of the laser beam being coupled out of the resonator with each passage through the resonator.

The delay line is the optical path that the laser beam takes during a passage through the resonator, counted from the reflection at the resonator output to the renewed interaction with the resonator output. The path difference results from the length of the optical path via the speed of light.

This path difference can advantageously be variably adjusted by adjusting the delay line. For example, the retarding optics can be spatially further removed from the resonator exit, so that the distance that the light travels during one resonator passage increases. If, as described above, the mirrors of the retarding optics, the resonator input and the resonator output form the corner points of a rectangle, then the optical path can easily be lengthened by simultaneously lengthening the opposite sides of the rectangle. Because the opposite sides of the rectangle are lengthened, the optical alignment is preserved.

The resonator can have a first radial manipulation optics element and a second radial manipulation optics element, with the second radial manipulation optics element preferably collimating the partial laser beams.

Analogously to the description above, the laser beam is manipulated radially by the first radial manipulation optical element, so that a Laser contour is generated. In this case, the laser contour is divergent, for example, which means that the partial laser beams of the laser beam do not run parallel to one another, but instead all partial laser beams run at an angle β′ to the optical axis.

The partial laser beams of the laser beam can again be aligned parallel to the optical axis, ie collimated, by the second radial manipulation optical element downstream of the first radial manipulation optics element. After this parallelization, the laser contour has the diameter D, with a segment of the laser beam being coupled out with the laser contour during the subsequent interaction with the resonator output. The part of the laser beam remaining in the resonator is deflected back to the first radial manipulation optical element via the delay line. There, the laser beam with the laser contour with the diameter D is again manipulated radially by the first radial manipulation optics element and then parallelized by the radial manipulation optics element, so that the laser contour finally has a diameter 2D, and so on.

Accordingly, the lateral extent of the laser contour can be adjusted by the spatial distance between the first radial manipulation optical element and the second radial manipulation optical element. The path difference between two consecutive segments of the laser beam can be adjusted by the length of the delay path, ie in particular by the positioning of the delay optics. The use of a ring resonator therefore has the advantage that the path difference of the segments and the lateral extent of the laser contours can be adjusted independently of one another.

In this case, the first radial manipulation optical element and the second radial manipulation optical element can be configured as an inverse axicon or axicon in a particularly simple manner.

An inverted axicon and an axicon together form what is known as an axicon telescope. If the cone angle of the inverse axicon and the axicon are the same, the optical effects of the two elements cancel each other out, so that the lateral expansion of the laser contour has increased to a certain diameter after passing through the axicon telescope, but is not divergent or fanned out .

According to a further embodiment of the invention, the resonator can be a ring resonator, with a first radial manipulation optical element being arranged in front of the resonator entrance and a second radial manipulation optical element being arranged in the ring resonator and imaging optics being arranged partially in the ring resonator.

For example, the resonator input can be a perforated mirror. The laser beam is fanned out in front of the perforated mirror by the first radial manipulation optical element and coupled into the resonator through the hole in the perforated mirror.

Because the imaging optics are partially arranged in the resonator, the entire imaging optics is now divided into a resonator-side lens system and focusing optics, with the focusing optics being arranged after the resonator exit.

Within the resonator, the laser beam is imaged into one another by at least two lenses of a resonator-side lens system of the imaging optics, the first lens being at the focal point of the second lens and vice versa. In particular, a first lens can be arranged in front of the second radial manipulation optical element and a second lens on the delay path. With each passage, the second radial manipulation optical element fans out the laser beam further, with the lens system on the resonator side preventing strong divergence on the delay path through the imaging. Through the resonator output, a segment of the laser beam is coupled out in each pass and focused on the workpiece. In this case, the lens in front of the second radial manipulation optical element and the focusing optical system together produce the one focal zone.

The partially reflecting resonator output can be designed as a polarization splitter with a wave plate, in which case the segment of the laser beam to be coupled out can be set via the local polarization, which can be different for each segment of the laser beam.

The segment to be coupled out is set via the local polarization, which can be set differently for each segment. Appropriately locally adapted wave plates can be produced e.g. by means of nanogratings in glass.

A λ/4 plate or a λ/2 plate can be arranged in the resonator, which changes the polarization of the laser beam between each multiple reflection.

In cavity configurations that rely on bouncing the laser beam back and forth, such as a Fabry-Perot cavity, the laser beam can have a λ/4 between each reflection at the Pass through the resonator output twice. This effectively causes the λ/4 plate to act as a λ/2 plate, such that the polarization of the laser beam is rotated about the optical axis of the effective λ/2 plate on each pass. The segments to be coupled out can thus be selected by choosing a polarization splitter.

With a resonator configuration that relies on round trip through the resonator, such as a ring resonator, the laser beam can only traverse a λ/2 plate once per pass. As a result, the polarization of the laser beam can be further rotated with each pass. Here, too, the segments to be decoupled can be selected by selecting a polarization splitter.

character list

• 1A, B schematische Darstellungen der Erzeugung von nicht-beugenden Strahlen gemäß dem Stand der Technik;
• 2A, B, C, D, E, F, G, H schematische Darstellungen einer Ausführungsform der Vorrichtung;
• 3A, B, C, D schematische Darstellungen von Verlaufsplatten;
• 4A, B, C, D, E schematische Darstellungen einer weiteren Ausführungsform der Vorrichtung; und
• 5A, B schematische Darstellungen einer weiteren Ausführungsform der Vorrichtung.

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

• 1A, B schematic representations of the generation of non-diffracting beams according to the prior art;
• 2A, B , C , D , E, F, G, H schematic representations of an embodiment of the device;
• 3A, B , C , D schematic representations of gradient plates;
• 4A, B , C , D , E schematic representations of a further embodiment of the device; and
• 5A, B schematic representations of a further embodiment of the device.

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 the 1A 1 is a schematic representation of the generation of a non-diffracting beam according to the prior art. In this case, a laser beam 30 of an ultra-short pulse laser 3 is guided through a radial manipulation optical element 5 . In this case, the laser beam 30 has a Gaussian intensity distribution I. The radial manipulation optical element 5 is designed here, for example, as an axicon 550, which is a conically ground optical element. The conical shape of the axicon 550 directs the laser beam 30 at an angle β to the optical axis A of the device 1 . In particular, all partial laser beams of the laser beam 30 are tilted by the angle β to the optical axis A. In the first focal zone F, the partial laser beams overlap so that the laser intensity is excessive. In the focal zone F, the laser beam already has the properties of a non-diffracting beam. In particular, the focal zone F is already elongated here in the beam propagation direction.

After the first focus zone F, the laser beam is introduced into the material 2 of a workpiece through imaging optics 6, which can include two lenses 600, 602, for example. In this case, a reduced imaging of the non-diffracting beam into the focal zone F in the material 2 essentially takes place.

An alternative device for generating a non-diffracting beam is in 1B shown. In this case, the laser beam 30 with the Gaussian intensity distribution is directed onto another manipulation optical element 5 . The radial manipulation optics element 5 can be designed here in particular as an inverse axicon 50 . The inverse axicon 50 is, so to speak, the negative impression of the axicon 550. This means that the partial laser beams are not refracted at an angle β to the optical axis, but are refracted away from the optical axis by an angle β'. In particular, this does not result in a first focus zone F. Nevertheless, the laser beam 30 fanned out by the radial manipulation optical element 5 can be imaged in a material 2 by an imaging optical system 6 .

In any case, the material 2 is processed by the ultra-short laser pulses of the laser beam 30 in that material modifications are introduced into the material 2 or material is removed.

In 2A a first proposed embodiment of the device 1 is shown. The device 1 comprises a resonator 4, a radial manipulation optics element 5 and imaging optics 6. The resonator 4 is formed here by a resonator inlet 40 and a resonator outlet 42, which together enclose the resonator chamber 44. In this case, the resonator input 40 is designed in particular as a perforated mirror 400 , the perforated mirror 400 having a hole 402 and a mirror coating 4000 . In this case, the mirroring 4000 can be a highly reflective mirroring, which is arranged on the side of the perforated mirror 400 that faces the resonator space 44 .

A radial manipulation optical element 5 is arranged in front of the resonator input 40, which can be, for example, an axicon, an inverse axicon or a diffractive optical element or a free-form surface. The laser beam 30 , which is made available by an ultrashort pulse laser 3 , is radially manipulated by the radial manipulation optical element 5 . The beam diameter of the Gaussian beam is typically chosen to be comparatively small here, with this determining the longitudinal length of the focal zone F of the segment 320 in conjunction with the diameter of the radial manipulation.

The partial laser beams run at an angle β′ to the optical axis of the device 1 due to the fanning out. After passing through the resonator space 44, the partial laser beams strike the resonator outlet 42. The projection of the partial laser beams onto the resonator outlet 42 is a ring here (in particular in the case of a conically ground axicon). This ring is in particular the laser contour 32 of the laser beam 30 on the resonator output 42, the laser contour 32 having a diameter D.

It should be noted that for the sake of clarity in the 2A until 2H only the central beam of the partial laser beams split by the radial manipulation element 5 is shown. The partial laser beams have a finite extent as shown in the enlarged view in 2A is shown. In fact, this finite extension (the thickness d of the ring) leads to a finite length of the focal zone F, also shown in the enlargement.

The resonator output 42 is partially reflective, so that part of the laser beam 30 is reflected back into the resonator space 44 . The part of the laser beam that is not reflected is coupled out through the resonator output 42 . The part of the laser beam 30 that is coupled out is called in particular the segment 320 of the laser beam 30 .

In other words, the radial manipulation optical element 5 generates a laser contour 32 on the resonator output 42, which is partially decoupled from the resonator 4 in the form of an energy segment 320.

The part of the laser beam 30 that is not coupled out is reflected back to the resonator input 40 . The lateral extension of the laser contour 32 of the laser beam 30 is already so large that the laser beam 30 falls on the mirror coating 4000 of the resonator input 40 and is not coupled out of the resonator 4 through the hole 402 . The laser beam 30 is then reflected again from the resonator inlet 40 in the direction of the resonator outlet 42 . At the resonator exit 42 the diameter of the laser contour 32 has increased due to the longer optical path L covered in the resonator 4 . In particular, the diameter of the laser contour 32 on the resonator output 42 can now be 3D. With each passage through the resonator 4, the diameter or the lateral extent of the laser contour 32 increases in the present example, so that the diameter of the outcoupled segments 320 also becomes larger and larger.

In particular, however, the thickness d of the generated segments 320 or of the rings is the same for each passage through the resonator 4 . This is due to the fact that all partial laser beams are fanned out by the same angle β′ by the radial manipulation optical element 5 .

In particular, the thickness d of the laser contour 32, or of the rings, depends directly on the angle β', as simple trigonometric considerations show.

The longer running time of the laser beam due to the multiple reflections is not only reflected in an increased diameter D of the laser contour 32 . Rather, the different segments 320 are decoupled at different times due to the increased propagation time in the resonator 4 . The time difference between two consecutively decoupled segments 320 is called the path difference Δt. The path difference Δt is directly related to the optical path L that the laser beam 30 travels between successive interactions with the resonator output 42: 1 / Δt = c / L.

An imaging optics 6 is arranged after the resonator 4, which focuses the different segments 320 in a respective focal zone F. FIG. In this embodiment, in each of the focal zones F, a non-diffracting beam is formed with a focal zone F that is elongated in the beam propagation direction (however, any other beam shapes can also be generated by the imaging optics, see below). In particular, the segments 320, which have a small lateral extent on the resonator output 42, are focused in a focal zone F that is offset in the direction of beam propagation. Due to the time offset of the segments 320, the material 2 is accordingly processed from the inside to the outside. In this way, in particular, shielding effects that make processing of the material 2 more difficult can be avoided. In addition, the focus zones F can differ chen lateral locations are introduced, so that the focal zones F are not on one axis, but can also be shifted perpendicular to the optical axis.

By varying the distance between the resonator inlet 40 and the resonator outlet 42, the longitudinal distance of the focal zones F of the different segments 320 can be adjusted, as a comparison of FIG 2A until 2C shows. 2 B has a large distance between the resonator input 40 and the resonator output 42, so that the fanned-out laser beam 30 has to cover a long optical path in the resonator 4. As a result, the lateral extent of the laser contours 32 which are projected onto the resonator output 42 increases. Due to the greater difference in the lateral dimensions, the longitudinal distance between the focal zones F also increases.

Due to the larger optical path, successive segments 320 are also introduced later into the material 2, as shown above.

In 2C the resonator input 40 and the resonator output 42 have a smaller distance than in 2 B on. As a result, the focal zones F after the imaging optics 6 move closer together. However, it should be pointed out that the length of the individual focal zones F remains the same since this is only determined by the thickness d of the segments 320 on the resonator outlet 42 .

Typically, the distance between the resonator inlet 40 and the resonator outlet 42 is selected in such a way that the segments 320 are successively absorbed in the material 2 . This can be used to determine that the path difference Δt of the segments must be at least as large as the pulse duration. For example, if an ultra-short laser pulse has a pulse duration of 50 ps, then the path difference Δt of two consecutive segments 320 should also be at least 50 ps. If this is the case, the various segments 320 are introduced into the material 2 one after the other—successively.

In particular, it is also possible to set the path difference Δt and the spatial offset of the focal zones F independently of one another, as in 2D and 2E is shown. For this purpose, for example, the resonator inlet 40 and the resonator outlet 42 can have a radially symmetrically tilted surface. This makes it possible, for example, as in 2D shown to remove the resonator input 40 and the resonator output 42 from one another, ie to increase the path difference Δt of the segments 320, but to select the longitudinal spacing of the focal zones F small. Conversely, a surface curvature can also make it possible for the resonator inlet 40 and the resonator outlet 42 to be arranged closer to one another, as a result of which the path difference Δt is reduced. At the same time it can be achieved that the longitudinal distance between the focal zones F is increased.

In the case of a hole mirror 400, it can be the case in particular that the geometric distances for coupling the laser beam 30 into the resonator 4 are predetermined by the size of the hole 402 and by the radial manipulation optical element 5 used. Accordingly, the shape of the focus zones F can only be influenced to a limited extent. This can be remedied by a so-called axicon telescope 56, which as part of the imaging optics 6 in 2F and 2G is trained.

An axicon telescope comprises an inverse axicon 50 and an axicon 550 through which the segments 320 after the resonator exit 42 pass in sequence. If the segments 320 first pass through the inverse axicon 50 and then the axicon 550, the refraction at the aforementioned elements 50, 550 causes the segments 320 to be introduced into the material 2 at an obtuse angle. If the segments 320 first run through the axicon 550 and then the inverse axicon, the segments 320 can be introduced into the material at a more acute angle. Overall, the so-called effective processing angle (or angle of incidence on the material 2) can thus be adjusted. The effective machining angle can be further adjusted by selecting the cone angle.

In 2H a possibility for increasing the longitudinal degree of filling of the focal zones F in the material 2 is shown. The degree of filling describes the density of the focal zones F along the optical axis.

In this case, a so-called filling level 404 is arranged in the resonator 4 after the resonator input 40 . The laser beam 30 fanned out by the radial manipulation optical element 5 is partially reflected back by the filling mirror 404 to the resonator input 40 (dashed lines) and partially transmitted to the resonator output 42 (continuous lines). To a certain extent, the laser beam 30 is already divided into two segments 320 at the filling level 404 , the segments 320 running in opposite directions after the separation in the resonator 4 . In particular, for the segment 320 reflected back to the resonator input 40, the optical path through the filling mirror 404 for the first pass from the resonator input 40 to the resonator output 42 is extended, whereby the Fanning out during propagation in the resonator 4 is increased. Accordingly, segment 320 reflected back by filling level 404 has a larger diameter D during the first interaction with resonator output 42 than the part transmitted through filling level 404 .

It should be noted at this point that the resonator 4 of 2 can in principle also be manufactured as a monolithic element with clearly defined optical properties.

In the 3A until 3D different transmission courses of a course plate 46 are shown schematically. In this case, the profile plate 46 can be set up to attenuate or transmit the spatially separated laser contours 32 to different extents. As a result, the intensity of the individual focus zones F can be influenced.

In this case, the profile plate 46 can in principle be arranged at any point in the beam path where the laser beam 30 is broken down into spatially separated segments 320 . The location with the greatest separation can be selected particularly advantageously, for example the resonator output 42 or the resonator chamber side of the resonator input 40. It is particularly possible that a gradient plate 46 as a coating on one of the optical elements, for example resonator input 40, resonator output 42 or one or several lenses of the imaging optics 6, is applied. However, it is also possible for a gradient plate 46 to be formed separately, that is to say to be formed as an independent optical element, and to be arranged accordingly in the beam path.

The energy transported through the segments 320 typically decreases with each pass through the resonator 4 . This is already due to the fact that the resonator output 42 is partially reflecting and always decouples a certain part of the energy still present in the resonator 4 . In other words, each segment 320 has only about (1-R)^N the original energy, where N is the passage through the resonator 4 and R is the reflectivity of the resonator output 42 . The intensity I of the different segments 320 in the focal zones F can be adjusted by means of a corresponding gradient plate 46 .

For example, in 3A a possible radial transmission function T for the travel plate 46 is shown. In this case, the transmission increases linearly with the radius R of the profile plate 46 . If the gradient plate 46 is arranged in the beam path in such a way that R=0 coincides with the optical axis of the device 1, then each segment 320 (numbered here with N=1, . . . , 5) is uniformly transmitted or attenuated. In particular, the transmission of the gradient plate 46 increases with the radius R, so that segments 320 from a higher resonator passage N are transmitted more strongly. In this way, the continuously weakening transmission orders N of the resonator 4 can be counteracted to a certain extent.

It is also possible to arrange the gradient plate 46 in the beam path in such a way that R=0 does not coincide with the optical axis A. As a result, an inhomogeneous intensity distribution can be brought about in each segment 320, which can lead to particular intensity distributions in the focal zone F.

In 3B a corresponding non-linear dependence of the transmission function is shown.

In 3C a stepwise transmission function is shown.

In 3D another non-linear transmission function is shown.

In 4A a further embodiment of the device 1 according to the invention is shown. Here, the resonator 4 is a ring resonator 4'. A ring resonator 4' also has a resonator input 40 and a resonator output 42. In the present example, the resonator input 40 is designed as a perforated mirror 400 so that a Gaussian laser beam 30 can couple into the ring resonator 4 through the hole 404 in the resonator input 40 . The laser beam 30 is then shaped by an axicon telescope 56 (see below) and is then transmitted to the partially reflecting cavity exit 42 . A segment 320 of the laser beam 30 is coupled out at the resonator exit 42 and is introduced into a material 2 through imaging optics 6, so that the material 2 is processed. The part of the laser beam 30 reflected at the resonator output 42 is guided onto at least one deflection mirror, but in the present example two deflection mirrors, which together form the deceleration optics 440 . The mirrors of the deceleration optics 440, the resonator input 40 and the resonator output 42 are in this case on the corners of a rectangle. The path difference can be adapted by simultaneously varying two opposite sides of the rectangle.

The reflected laser beam 30 is guided again to the resonator input 40 by the deceleration optics 440, with the resonator input 40 now again guiding the laser beam 30 through the axicon telescope 56 and further fanning it out, and so on. The ring resonator 4 is therefore called ring resonator, because the laser beam 30 can be guided through the axicon telescope 56 again and again on an (almost) ring-shaped path.

In 4B two consecutive passes of the laser beam 30 through the resonator 4 are shown. First, the Gaussian laser beam 30 is coupled into the resonator 4 through the hole 402 or hole mirror 400 and passed through the inverse axicon 50 . By passing through the inverse axicon 50, the Gaussian laser beam 30 is fanned out. After propagation to the axicon 550, a lateral expansion of the laser contour 32 of the laser beam 30 has formed due to the fanning out. When passing through the axicon 550, the angular offset is removed, so that the laser beam 30 is collimated to a certain extent and the lateral extension of the laser contour 32 remains constant for the further passage in the resonator 4. After the axicon 550, part of the laser beam 30 is coupled out of the resonator 4 in the form of a segment (not shown). The part remaining in the resonator is reflected via the deceleration optics at the hole mirror 400 and passed through the axicon telescope 56 again. As a result of the laser beam 30 passing through the inverse axicon 50 again, the laser beam 30 is fanned out again, with the lateral extension of the laser contour 32 of the laser beam 30 having increased again after propagation to the axicon 550 . The laser beam 30 is parallelized again by the axicon 550 and then partially decoupled through the resonator output.

It can thus be clearly seen that in the embodiment, the lateral extension of the laser contour 32 increases with each passage through the resonator. The increase in the lateral extension of the laser contours 32 can thus be adjusted by the distance between the inverse axicon 50 and axicon 550. Furthermore, the time offset of the decoupled segments 320 can be adjusted by the delay distance, which is given by the positioning of the delay optics. Since the lateral extent of the laser contours 32 is constant after passing through the axicon 550, the longitudinal offset of the focal zones F and the time offset of the introduction of the segments 320 into the material 2 can be set independently of one another.

A possible design of the imaging optics 6 is shown in 4C sketched. The inverse axicon 50 and the two lenses of the imaging optics 6 result in a reduced focusing of the segments 320 onto or into the material 2 . The beam paths outline three successive revolutions of the laser beam 30 in the resonator 4, with the near-axis rays shown on the axicon being coupled out in time before the off-axis rays. The processing of the material 2 thus starts in the volume of the material 2 and is guided in the direction of the imaging optics 6 with each pass. The path difference Δt can be set completely independently of one another by varying the resonator length (or the resulting optical path length L), the spatial modulation along the optical axis by the variable axicon telescope 56 and the focusing conditions by the imaging optics 6 . This is possible up to large path differences Δt of several nanoseconds.

If particularly large path differences are to be generated, it may be that any remaining divergence of the laser beam 30 must be avoided. With a structure according to the 2 this divergence always occurs, for example, since the laser beam 30 is fanned out at every point of the resonator 4 . The divergence of the laser beam 30 can be avoided, for example, by using a relay telescope, i.e. a 4f structure.

Another possible imaging optics 6 is in 4D shown. There, the imaging optics are composed of a segmented beam-shaping element 64 and a lens 602. In particular, the segmented beam-shaping element 64 is a segmented diffractive optical element which is described more precisely in 4E is shown. The segmented diffractive optical element 64 has different radial zones or. Each segment allows the laser contour 320 impinging there to be imaged in a different focal zone or also in a different focal plane.

In 5A a further embodiment of the device 1 according to the invention is shown. Here, as a resonator 4 as in the 4 a ring resonator is also shown, but in which elements 600 , 602 of the imaging optics 6 are arranged within the resonator space 44 . The elements 600, 602 are thus part of the imaging optics 60 on the resonator side, while the actual imaging takes place through the focusing optics 62 outside of the resonator 4.

In 5A a laser beam 30 is already fanned out in front of the resonator input 40 . The resonator input 40 can be designed here in particular as a perforated mirror 400 , the laser beam 30 being coupled in through the hole 402 of the perforated mirror 400 .

Within the resonator 4, the laser beam 30 is imaged into one another by the lenses 600, 602 of the imaging optics 6 during each passage through the resonator 4. In other words, it stands Lens 600 in focus of lens 602 and vice versa.

The axicon 550 further fans out the laser beam 30 with each pass. A segment 320 is coupled out of the resonator 4 and focused into or onto the material 2 with each passage via the partially reflecting resonator output 42 already described above. Here, the lens 600 and the focusing optics 62 produce a reduced image. Adjusting the length of the optical path L within the resonator 4 requires adjusting the focal lengths of the lenses 600, 602.

The beam path for the first three passes is in 5B sketched. In this case, the laser beam 30 fanned out by the inverse axicon 50 is coupled into the resonator 4 through the hole 404 of the perforated mirror 400 . However, after a single pass, the lateral expansion of the laser beam 30 is already so large that the laser beam 30 is reflected back into the resonator space 44 by the reflecting side of the perforated mirror 400 . In other words, the lateral extent of the laser contour 32 on the resonator input 40 is larger than the hole 402 of the hole mirror 400. The fanning out is ultimately specified here in combination with the radial manipulation optical element 55. An Alvarez element design or an alternative axicon telescope 56 between the lenses 600, 602 allows further adjustment of the distance.

The partially reflecting resonator output 42 can also be designed as a polarization splitter with an adapted wave plate. The segment 320 to be coupled out is set via the local polarization, which can be set differently for each passage through the resonator 4 . Appropriately locally adapted wave plates can be produced e.g. by means of nanogratings in glass.

Another variant in which the partially reflecting resonator output transmits a linear polarization direction includes in the case of 2 a λ/4 plate between the multiple reflections. In the case of the devices of 4 and 5 a λ/2 plate can be arranged in the resonator space 44 . The direction of polarization is rotated bit by bit during each passage, so that in connection with a polarizing resonator output 42 a continuous transition between 0 and 100% decoupling is achieved.

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

feed device

2

material

3

Ultrafast Laser

30

laser beam

32

laser contour

320

segment

4

resonator

40

resonator input

400

perforated mirror

4000

reflective side

402

Hole

404

filling level

42

resonator output

44

resonator room

440

delay optics

46

gradient plate

480

polarization splitter

482

wave plate

484

λ/4 plate

5

first radial manipulation optical element

50

inverse axicon

55

second radial manipulation optical element

550

Axicon

56

Axicon telescope

6

imaging optics

60

Lens system on the resonator side

62

focusing optics

64

segmented beam shaping element

f

focus zone

I

intensity profile

L

Length of the optical path

V

feed

Claims (16)

Device (1) for processing a material (2) by means of ultra-short laser pulses of an ultra-short-pulse laser (3), comprising - a resonator (4) with a resonator input (40) and a partially reflecting resonator output (42), the resonator (4) being set up to to reflect the laser beam (30) of the ultra-short pulse laser several times in the resonator chamber (44), a segment (320) of the laser beam (30) being coupled out at each reflection at the partially reflecting resonator output (42), - radial manipulation optics (5) set up for this purpose is to manipulate the laser beam (30) radially, with the cross section through the laser beam (30) yielding a laser contour (32), - imaging optics (6) which are set up to convert the segments (320) of the laser beam (30) into to introduce the material (2), whereby the material (2) is processed, characterized in that - the resonator (4) is designed such that the lateral expansion of the laser contour (32) with each passage of the Re sonator (4) increases or decreases, with the lateral extent of the segments (320) of the laser beam (30) coupled out one after the other from the resonator (4) increasing or decreasing accordingly, and - the imaging optics (6) expanding the lateral extent of the segment coupled out (320) of the laser beam (32) is translated into a longitudinal and/or lateral focal zone (F), the position of the focal zone (F) being correlated with the lateral extent. Device (1) after claim 1 , characterized in that the path difference of the segments (320) of the laser beam (30) is determined by the length (L) of the optical path that the laser beam (30) travels between two successive partial reflections at the resonator output (42). Device (1) after claim 1 or 2 , characterized in that the radial manipulation element (5) is an inverse axicon (50) or an axicon (550) or an axicon telescope, wherein the axicon angle or angles of the axicone or the axicone determines the opening angle of the cone, or a diffractive optical element or is a free-radiating surface. Device (1) according to one of the preceding claims, characterized in that the decoupled segments (320) of the laser beam (30) are successively introduced into the material (2), the path difference between two successive segments (320) of the laser beam (30) being greater is the pulse duration of the laser pulse. Device (1) according to one of the preceding claims, characterized in that each decoupled segment (320) of the laser beam (30) generates a beam shape in the focal zone (F), the beam shape being a lateral multispot or a ring focus or an autofocusing beam or is a non-diffractive ray or a diffractive ray. Device (1) according to one of the preceding claims, characterized in that the imaging optics (6) comprises a segmented beam-shaping element, with each link of the divided radial manipulation optics element being set up to generate its own beam shape, with the beam shape being a lateral multispot or a ring focus or is an autofocusing beam or a non-diffractive beam or a diffractive beam. Device (1) according to one of the preceding claims, characterized in that the time-averaged intensity profile of the segments (320) of the laser beam (30) introduced into the material (2) corresponds to a non-diffracting beam. Device (1) according to one of the preceding claims, characterized in that a feed device (10) is set up to move the laser beam (30) and the material (2) relative to one another with a feed (V), the spatial displacement being caused by the feed (V) is smaller than the size of the averaged intensity profile. Device (1) according to one of the preceding claims, characterized in that a course plate (46) is set up to attenuate the spatially separated segments (320) of the laser beam (30) to different extents, the course plate (46) in or directly after the Resonator (4) can be arranged, or can be designed in one piece with one of the optical elements of the device (1), preferably in the form of a coating. Device according to one of the preceding claims, characterized in that the radial manipulation optics element (5) fans out the laser beam (30) and the imaging optics (6) the lateral expansion of the decoupled segment (320) of the laser beam (32) into a longitudinal focal zone ( F) translated, the position of the focal zone (F) being correlated in the beam propagation direction with the lateral extent. Device (1) according to one of the preceding claims, characterized in that - the radial manipulation optical element (5) is arranged in front of the resonator input (40), - the resonator input (40) the hole (402) of a reflecting hole mirror (400), the reflecting hole mirror (400) being arranged reflecting into the resonator space (44). Device (1) after claim 9 , characterized in that the resonator input (40) and/or the partially reflecting resonator output (42) is tilted radially symmetrically, so that the path difference of the segments (320) of the laser beam (30) can be adjusted independently of the distance from the resonator input. Device (1) according to one of claims 9 or 10 , characterized in that behind the hole (402) of the hole mirror (400) in the resonator (4) is a filling mirror (404) which reflects the laser beam (30) onto the reflecting side (4000) of the hole mirror (400), wherein the longitudinal degree of filling of the averaged intensity distribution is increased. Device (1) according to one of claims 2 until 8th , characterized in that - the resonator (4) is a ring resonator, - the resonator (4) has a first radial manipulation optical element (5) and a second radial manipulation optical element (55), the second radial manipulation optical element (5) in the beam path behind the first radial manipulation optics element (5) is arranged, - wherein preferably the second radial manipulation optics element (55) collimates the partial laser beams, - wherein particularly preferably the first radial manipulation optics element (5) and the second radial manipulation optics element (55) are axicones, which together form an axicon form telescope. Device (1) according to one of claims 2 until 8th , characterized in that - the resonator (4) is a ring resonator, - a first radial manipulation optical element (5) is arranged in front of the resonator input (40), - a second radial manipulation optical element (55) is arranged in the ring resonator (4). and the imaging optics (6) are partially arranged in the ring resonator (4), - the second radial manipulation optics element (55) preferably collimating the partial laser beams. Device (1) according to one of the preceding claims, characterized in that the partially reflecting resonator output (42) is designed as a polarization splitter (480) with a wave plate (482), the segment (320) of the laser beam (30) to be coupled out being set via the local polarization which can be different for each segment (320) of the laser beam (30).

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