Apparatus and method for processing a workpiece made of a birefringent material

Abstract

The present invention relates to an apparatus and to a method for processing a workpiece (1) made of a birefringent material having at least one optical axis (OA) by means of ultrashort laser pulses of a laser beam (20) from an ultrashort pulse laser (2), wherein the laser beam (20) is given a non-diffractive beam shape by a beam shaping device (3), wherein the laser beam (20) is given a polarisation distribution by a polarisation shaping device (4), wherein the laser beam (20) is focussed by processing optics (5) into a focal zone in a focal plane in the workpiece (1), wherein the workpiece (1) is irradiated with the laser beam and thereby processed, wherein the polarisation distribution compensates for the birefringence of the material of the workpiece (1).

Description

[0001] Device and method for machining a workpiece made of a birefringent material

[0002] Technical area

[0003] The present invention relates to a device and a method for machining a workpiece made of a birefringent material.

[0004] State of the art

[0005] The processing of transparent materials using ultrashort laser pulses is well known. The short pulse length and high peak power can lead to nonlinear absorption of the pulse energy in the material, allowing the processing of materials that are essentially transparent even for the laser light wavelength used.

[0006] However, when processing birefringent materials, it is problematic that the laser beam is refracted into an ordinary and an extraordinary beam depending on the angle of incidence and the orientation of the material's optical axis. Due to the diverging beam shape, homogeneous material modification cannot be achieved, and the shape of the introduced material modifications on the upper surface differs from the shape on the lower surface of the material. This makes cutting the material particularly difficult.

[0007] Segmented beam-shaping elements and laser processing systems are known from DE102019128362B3. A laser device with controlled orbital angular momentum is known from EP 3305456B1. Spiral phase plates are known from US 10218145 B1.

[0008] Description of the invention Starting from the known prior art, it is an object of the present invention to provide an improved device for machining a workpiece of a birefringent material, as well as a corresponding method.

[0009] The object is achieved by a method for machining a workpiece of a birefringent material having the features of claim 1. Advantageous developments of the method emerge from the subclaims as well as the present description and the figures.

[0010] Accordingly, a method for machining a workpiece made of a birefringent material with at least one optical axis is proposed using ultrashort laser pulses of a laser beam from an ultrashort pulse laser. A beam-shaping device imparts a non-diffracting beam shape with an intensity distribution to the laser beam. A polarization-shaping device imparts a local polarization distribution to the laser beam. The laser beam is focused by processing optics into a focal zone in a focal plane within the workpiece. The workpiece is exposed to the laser beam and thus machined. According to the invention, the polarization distribution compensates for the birefringence of the workpiece material.

[0011] Birefringence is the property of an optical material to impart different propagation properties to light of different polarizations. This effect can be used to split an incident laser beam into two perpendicularly polarized laser beams. This occurs due to the different refractive indices of the optical material, which depend on the polarization and the angle of incidence of the light relative to the optical axis of the optical material.

[0012] For the purposes of this disclosure, partial laser beams polarized perpendicular to one another are understood to mean linearly polarized partial laser beams whose polarization directions are at an angle of 90° to one another. However, partial laser beams polarized perpendicular to one another are also understood to mean circularly polarized partial laser beams with opposite directions of rotation, ie, left- or right-circularly polarized partial laser beams.

[0013] For example, a material can have one optical axis or several optical axes.

[0014] In a material with only one optical axis, the optical axis defines a so-called index ellipsoid, which indicates the size of the respective refractive index depending on the angle of the laser beam to the optical axis and the polarization of the laser beam. The material can be sapphire.

[0015] Sapphire has an ordinary refractive index of n0=1 ,766 and an extraordinary refractive index of n a =1.758 at a wavelength of 633nm. Sapphire is also extremely scratch-resistant, making it ideal for use as a cover or protective glass for camera lenses.

[0016] The thickness of the workpiece can be between 300pm and 3000pm, typically between 500pm and 2000pm.

[0017] This allows the workpiece to be used in a particularly versatile manner and to provide the protective effect mentioned above.

[0018] The laser provides the laser pulses of the laser beam, with the individual laser pulses forming the laser beam in the beam propagation direction. The pulse duration of the laser pulses can be between 300 fs and 12 ps, for example, between 300 fs and 100 ps. The wavelength of the laser pulses can be between 300 nm and 3000 nm, preferably between 900 nm and 2200 nm.

[0019] Instead of individual laser pulses, the laser can also provide laser bursts, with each burst comprising the emission of several laser pulses. For a specific time interval, the laser pulses can be emitted very closely one after the other, at intervals of a few picoseconds to nanoseconds. The laser bursts can in particular be GHz bursts, in which the sequence of consecutive laser pulses of the respective burst takes place in the GHz range. A burst can, for example, comprise between 2 and 20 laser pulses, preferably between 2 and 10 laser pulses, with the time interval between the laser pulses being between 10 ns and 50 ns. However, a burst can also comprise between 30 and 300 laser pulses, with the time interval between the laser pulses being between 100 ps and 1000 ps.

[0020] The repetition rate of the laser pulses and / or laser bursts can be greater than 1 kHz, preferably greater than 10 kHz. For example, the repetition rate can also be 100 kHz or more. Accordingly, the laser can emit, for example, more than 1,000 pulses per second or more than 10,000 pulses per second.

[0021] The pulse energy of the laser pulses can be greater than 1 pJ, for example 2 pJ.

[0022] The laser beam can have global polarization. In particular, the laser beam can be composed of several parallel partial laser beams, each partial laser beam having the same polarization. For example, all partial laser beams can be s-polarized, so that the laser beam is globally s-polarized. The beam-shaping device can be configured to impart a non-diffracting beam profile to the laser beam.

[0023] A non-diffracting beam profile refers to the beam profiles of non-diffracting beams and / or Bessel-type beams. This particularly includes beams whose transverse intensity distribution is propagation-invariant. In particular, for non-diffracting beams and / or Bessel-type beams, the transverse intensity distribution is essentially constant along the beam propagation direction.

[0024] For the definition and properties of non-diffracting beams, please refer 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. This book is expressly incorporated by reference.

[0025] Non-diffracting laser beams therefore have the advantage of being able to have a focal zone that is elongated in the beam propagation direction and significantly larger than the transverse dimensions of the focal zone. For example, this can be used to create a material modification that is elongated in the beam propagation direction.

[0026] In particular, non-diffracting beams can also have an elliptical cross-section, so that the beam cross-section is elongated in one direction in a plane perpendicular to the beam propagation direction of the laser beam.

[0027] The laser beam is guided from the beam-shaping device to the polarization-shaping device. The polarization-shaping device can be configured to impose a local polarization distribution on the intensity distribution of the laser beam.

[0028] For example, a first portion of the laser beam may be s-polarized, while a second portion of the laser beam may be p-polarized. For example, between the first and second portions of the laser beam, the polarization may transition continuously from s-polarization to p-polarization.

[0029] For example, a local polarization distribution can consist of a radially polarized laser beam. In such a case, the polarization is invariant under rotation of the laser beam around its propagation direction.

[0030] In other words, a polarization distribution means that the polarization is a local property of the non-diffracting beam. The laser beam is guided from the polarization shaping device to the processing optics. The processing optics can be configured to focus the beam-shaped and polarization-shaped laser beam into a focal zone in a focal plane within the workpiece.

[0031] The term "focus" in general can be understood as a targeted increase in intensity, whereby the laser energy converges into a "focus area" or a "focus zone." In particular, the term "focus" is used below regardless of the actual beam shape used and the methods used to achieve an increase in intensity. By "focusing" the laser pulses, the location of the increase in intensity along the beam propagation direction can be influenced, i.e., the focal plane can be selected. For example, the increase in intensity can be linear, resulting in a Bessel-shaped focus area around the focus position, as can be provided by a non-diffracting beam. Furthermore, other more complex beam shapes are also possible, whose focus zones extend arbitrarily in three dimensions.

[0032] The focus zone refers to the portion of the laser beam's intensity distribution that exceeds the modification threshold of the material being processed. The term "focus zone" emphasizes that this portion of the intensity distribution is specifically targeted, and that focusing achieves an intensity enhancement in the form of the intensity distribution.

[0033] When an ultrashort laser pulse is focused into and / or onto the workpiece, the intensity in the focal volume can lead to nonlinear absorption, for example, through multiphoton absorption and / or electron avalanche ionization processes. This nonlinear absorption leads to the generation of an electron-ion plasma, which, upon cooling, can induce permanent structural changes in the workpiece material. Since nonlinear absorption can transport energy into the bulk of the material, these structural changes can be generated inside the sample without affecting the workpiece surface.

[0034] At high laser repetition rates, the workpiece cannot cool completely between pulses, meaning that cumulative effects of the heat introduced from pulse to pulse can influence material modification. For example, the laser repetition frequency can be higher than the inverse of the workpiece's thermal diffusion time, allowing heat to accumulate in the focal zone through the gradual absorption of laser energy until the workpiece's melting temperature is reached. Furthermore, the thermal transport of heat energy to the areas surrounding the focal zone allows a larger area to be processed than the focal zone. After the ultrashort laser pulses are applied, the heated material cools rapidly, effectively freezing the density and other structural properties of the high-temperature state in the material.

[0035] 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 or cavity created by a microexplosion. The material modification produced depends on laser parameters such as the pulse duration, wavelength, pulse energy, and repetition frequency of the laser; on the material properties, including the electronic structure and thermal expansion coefficient; and on the numerical aperture (NA) of the focusing.

[0036] The material modifications are preferably introduced into the material along a separation line. The separation line describes the line of impact of the partial laser beams on the upper surface of the workpiece. A feed device moves the partial laser beams and the workpiece relative to each other at a specific feed rate, while the ultrashort pulse laser emits laser pulses into the workpiece at its repetition frequency.

[0037] "Relatively movable" means that both the laser beam can be moved translationally relative to a stationary workpiece, and the workpiece can be moved relative to the laser beam. It is also possible that both the workpiece and the laser beam can move.

[0038] As time progresses, the feed rate results in different impact points of the laser pulses on the workpiece surface. In both directions, i.e., in the feed direction and perpendicular to it, the material modifications can at least partially overlap. Even if the focal zones are discrete, the resulting material modifications can be designed by adjusting the laser parameters so that there is partial overlap or that the material modifications are connected by cracks.

[0039] The separation along the material modifications ultimately occurs, for example, through a separation step, so that the workpiece is divided into the useful part and the so-called non-useful part or section of the workpiece. However, it is also possible that a spontaneous separation of the workpiece along the separation line occurs due to internal material stresses. Such spontaneous separations are favored, for example, by cracking along the separation line. The processing of the material with a beam-shaped and polarization-shaped laser beam is improved by the fact that the polarization distribution can compensate for the birefringent effect of the workpiece material, so that a uniform material modification can be created along the thickness direction of the workpiece.

[0040] Due to the high symmetry of the laser beam with respect to the optical axis of the material, the intensity profile does not change as it propagates through the material. As a result, the resulting material modifications are homogeneous in the sense that they are similar or have the same shape on the upper and lower surfaces of the material. This leads to high-quality processing results.

[0041] The laser beam can be incident perpendicularly on the upper surface of the material, parallel to at least one optical axis of the material.

[0042] If the material has only one optical axis, the perpendicular incidence ensures that the partial laser beams that hit the material parallel to the optical axis are not refracted.

[0043] Those partial laser beams that hit the surface of the material at an angle due to the focusing are split and refracted according to the ordinary and extraordinary refractive indices.

[0044] For example, the focusing means that the partial laser beams are directed at an angle onto the material. Since the laser beam's propagation direction is parallel to the optical axis, the partial laser beams are essentially located on a conical surface with a circular base, with the apex of the cone coinciding with the beam's propagation direction. In the image of the index ellipsoid, this means that the partial laser beams are refracted evenly, since the refraction of a first partial laser beam is at least partially compensated by the refraction of a partial laser beam opposite on the cone. This allows uniform material modifications to be created in the material.

[0045] A vortex phase distribution can be imposed on the laser beam by a phase shaping device.

[0046] A vortex phase distribution describes a modification of the phase of the laser beam that increases or decreases with the distance from the beam axis and with the angle around the beam axis. In a sense, a vortex phase distribution corresponds to an azimuthal phase distribution. With a vortex phase distribution, the laser energy of the beam-shaped and polarization-shaped laser beam can be guided along the beam axis, resulting in material modifications with a smaller diameter and thus improving material processing because smaller structures can be written into the material. A detailed description of this phenomenon is given below.

[0047] A polarization shaping device can impart a radial and an azimuthal polarization distribution to the laser beam, and an elliptical phase distribution can be imparted to it by the phase shaping device. In other words, the phase shaping is performed in such a way that the focus distribution becomes elliptical.

[0048] The elliptical phase distribution allows the beam shape to be influenced to create elongated beam profiles perpendicular to the beam propagation direction. In a sense, an elliptical phase distribution of the laser beam can result in the formation of an elliptical, non-diffracting beam profile.

[0049] The elliptical beam shape allows, in particular, elliptical material modifications to be introduced into the material, which allows the crack propagation to be controlled based on the material modifications.

[0050] However, the elliptical beam shape has the disadvantage that the angles of incidence of the individual laser beams are not evenly distributed. In a sense, the laser beams incident on the material are not distributed over the surface of a cone with a round base, but rather over the surface of a cone with an elliptical base. Due to the birefringent nature of the material, the beam profile of the laser beam is refracted to different degrees as it propagates through the material, depending on the different angles of incidence.

[0051] Radial and azimuthal polarization allows the polarization of the laser beam to be locally adjusted, so that the laser beam is not distorted in birefringent materials, for example. This enables improved material processing with elliptical, non-diffracting beams.

[0052] The object is achieved by a device for machining a workpiece of a birefringent material having at least one optical axis with the features of claim 1. Advantageous further developments emerge from the subclaims, the description, and the figures.Accordingly, a device for machining a workpiece made of a birefringent material with at least one optical axis is proposed, comprising an ultrashort pulse laser configured to provide a laser beam with laser pulses, a beam-shaping device configured to impart a non-diffracting beam shape with an intensity distribution to the laser beam, a polarization-shaping device configured to impart a polarization distribution to the laser beam, and machining optics configured to focus the laser beam into a focus zone in the focal plane of the workpiece, wherein the workpiece is exposed to the laser beam and is thereby machined. According to the invention, the polarization distribution compensates for the birefringence of the workpiece material.

[0053] The beam shaping device can be designed, for example, as a diffractive optical element (DOE), a freeform surface, an axicon, or a microaxicon, or can include a combination of several of these components or functionalities.

[0054] If the beam-shaping device forms a non-diffracting laser beam from the laser beam in front of the processing optics, the depth of penetration of the focal zone into the material can be determined by focusing the processing optics. However, the beam-shaping device can also be designed so that the non-diffracting laser beam is only generated by imaging with the processing optics.

[0055] A diffractive optical element can be configured to influence one or more properties of the incident laser beam in two spatial dimensions. A diffractive optical element is a fixed component that can be used to produce a specific non-diffracting laser beam from the incident laser beam. Typically, a diffractive optical element is a specially shaped diffraction grating, where diffraction transforms the incident laser beam into the desired beam shape.

[0056] 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 it. In particular, the axicon has a cone angle a', which is calculated from the beam entrance surface to the lateral surface of the cone. This causes the edge rays of the laser beam to be refracted to a different focal point than rays closer to the axis. This results in a focal zone that is elongated in the beam propagation direction. An axicon can be designed as a transmissive or reflective axicon.

[0057] The beam-shaping device can also be implemented using a geometric phase hologram. Geometric phase holograms are described, for example, in Kim, Jihwan, et al., "Fabrication of ideal geometric-phase holograms with arbitrary wavefronts." Optica 2.11 (2015): 958-964.

[0058] Preferably, the beam-shaping device comprises an axicon and particularly preferably an axicon and an imaging element.

[0059] The non-diffracting beam can be transferred to the rear focal plane of the processing optics by an imaging element. For example, the axicon can have a spherically ground back surface, so that the axicon and the imaging element are formed as a single piece.

[0060] The polarization shaping device may be a segmented polarizing plate.

[0061] A segmented polarization plate comprises one or more segments that impose a local polarization distribution on the laser beam. For example, the laser beam components can pass through different segments of the polarization plate and thus become polarized. After passing through the polarization plate, the laser beam therefore exhibits a polarization distribution determined by the polarization of the laser beam components.

[0062] The segments of the polarization plate can comprise waveplates, in particular λ / 2 plates. This allows the polarization of the partial laser beams to be locally polarized according to the orientation of the waveplates.

[0063] The polarization shaping device can also be a locally polarizing nanograting.

[0064] Such a polarization-shaping device comprises a plurality of diffracting and / or scattering structures, with the scattering and / or diffraction depending on the polarization of the incident partial laser beams and on their relative orientation to the diffracting and / or scattering structure. Depending on the alignment and orientation of the structures, a local polarization of the laser beam can be achieved.

[0065] A nanograting has the advantage that it can also provide complex lattice shapes, so that complex polarization distributions can be generated.

[0066] A polarization shaping device can also be a geometric phase hologram, see above.

[0067] A beam expander can be arranged upstream of the polarization-shaping device in the beam propagation direction. A beam expander can increase the diameter of the laser beam from a first diameter to a second diameter. This allows the partial laser beams to be spaced further apart, so that the spatial separation at which they impinge on the polarization-shaping device is greater. Accordingly, it is easier to provide a polarization-shaping device because the polarizing structures can be distributed over a larger area.

[0068] Processing optics can be an optical imaging system that can consist of one or more components. A component can be, for example, a lens, an optically imaging freeform surface, a Fresnel zone plate, or an objective such as a microscope objective or an F-theta objective, or a reflective off-axis paraboloid. The focusing optics can direct the laser beam into a focus zone. The positioning of the focus zone in the beam propagation direction can be adjusted using the focusing optics. For example, by adjusting the focusing optics, a focus zone can be placed on the surface of the workpiece, or in the workpiece, or partially on the surface and partially in the workpiece.

[0069] The beam propagation direction is determined in particular by the optical axis of the focusing optics. The optical axis is perpendicular to the entrance surface of the lens.

[0070] The device may comprise a phase shaping device configured to impart a phase distribution to the laser beam.

[0071] A phase-shaping device can, for example, be an optically transmissive element with a thickness variation. The thickness variation can generate a time-of-flight difference between the partial laser beams through the element, so that the partial laser beams exit the element with a phase difference.

[0072] However, the time difference can also be caused by birefringent structures, where the polarization-dependent, different refractive indices lead to a phase shift.

[0073] The phase difference allows the laser beam to be shaped in such a way that the partial laser beams have a different phase and thus the laser beam as a whole has a changed phase front.

[0074] The phase-shaping device can be a spiral phase plate. A spiral phase plate is a phase-shaping device with a thickness variation, where the thickness of the spiral phase plate increases at a certain pitch, similar to that of a screw, around the axis of the spiral phase plate. Accordingly, the phase shift also varies with the azimuth angle around the axis of the spiral phase plate.

[0075] Vortex phase distributions can be generated particularly easily using a spiral phase plate.

[0076] The phase shaping device and the beam shaping device can be formed in one piece, in particular realized in a generalized axicon.

[0077] Generalized axicons are described, for example, in Chen, Keyou, et al. "Generalized axicon-based generation of nondiffracting beams." arXiv preprint arXiv: 1911.03103 (2019).

[0078] A one-piece design can, for example, consist in the otherwise flat back of the axicon having a corresponding thickness variation of a spiral phase plate.

[0079] For example, this can reduce the need for an additional optical element in the beam path and the associated power loss. Furthermore, additional adjustment effort is eliminated, since the phase-shaping device and beam-shaping device cannot be adjusted relative to each other.

[0080] A one-piece design is particularly achieved with generalized axicons. Generalized axicons are diffractive optical beam-shaping elements that enable any phase distribution to be applied to a transverse beam profile of a laser beam. Generalized axicons are optical elements composed of various surface elements, with each surface element generating its own phase shift. The planar grating structure formed by the surface elements allows any phase distribution to be imposed on the laser beam, corresponding to the superposition of the phase distributions of the phase element and the axicon.

[0081] The phase shaping device and the polarization shaping device can be arranged in front of the beam shaping device and in front of the processing optics.

[0082] For example, the laser beam can pass through the polarization shaping device to the phase shaping device and then be imaged by the beam shaping device and an imaging element into the back focal plane of the processing optics. The processing optics can then focus the laser beam into the focal zone in the workpiece material. The phase shaping device and the polarization shaping device can be arranged downstream of the beam shaping device and upstream of the processing optics.

[0083] For example, the laser beam can pass through the beam-shaping device and be imaged by an imaging element into the rear focal plane of the processing optics. The polarization-shaping device and the phase-shaping device can be arranged in the rear focal plane. After passing through the polarization-shaping device and the phase-shaping device, the processing optics can then focus the laser beam into the focal zone in the workpiece material.

[0084] For example, in both cases the imaging element can also be arranged at a distance from the beam-shaping device.

[0085] In all cases, the laser beam can be expanded using a beam expander before passing through the above-mentioned optical elements.

[0086] A feed device can be configured to move the laser beam and the workpiece relative to each other with a feed rate. For example, the workpiece can be mounted on the feed device, with the laser beam being stationary, and the workpiece being moved beneath the laser beam using an XYZ table. However, the workpiece can also be mounted stationary, and the laser beam can be guided over the workpiece using one or more deflection optics. It is also possible, however, for both the laser beam and the workpiece to be moved.

[0087] Short description of the characters

[0088] Preferred further embodiments of the invention are explained in more detail in the following description of the figures. In the figures:

[0089] Figure 1 is a schematic representation of a device according to the prior art

[0090] Technology;

[0091] Figures 2.1, 2.2, 2.3 are schematic representations of the intensity distribution of a non-diffracting laser beam in a material according to the prior art;

[0092] Figures 3.1, 3.2, 3.3 are further schematic representations of the intensity distribution of an elliptical non-diffracting laser beam in a material according to the state of the art; Figures 4.1 and 4.2 are schematic representations of the material processing according to the state of the art.

[0093] Technology;

[0094] Figure 5 is a schematic representation of the alignment of

[0095] State-of-the-art beam propagation direction and polarization in a birefringent material;

[0096] Figures 6.1, 6.2 are schematic representations of the device according to the invention;

[0097] Figures 6.3 schematic representation of the functioning of the inventive

[0098] Device and method;

[0099] Figures 7.1, 7.2, 7.3 are schematic representations of the intensity distribution of a non-diffracting laser beam in a birefringent material according to the method according to the invention;

[0100] Figures 8.1, 8.2, 8.3 show further schematic representations of the intensity distribution of a non-diffracting laser beam in a birefringent material according to the method according to the invention;

[0101] Figures 9.1, 9.2, 9.3 show further schematic representations of the intensity distribution of an elliptical non-diffracting laser beam in a birefringent material according to the method according to the invention;

[0102] Figure 9.4 is a schematic representation of a superposition of radial and azimuthal polarization;

[0103] Figures 10.1 and 10.2 are schematic representations of polarization shaping devices;

[0104] Figure 11 is a schematic representation of a generalized axicon;

[0105] Figure 12 is a schematic representation of a spiral phase plate; and

[0106] Figure 13 is a schematic representation of a sapphire crystal.

[0107] Detailed description of preferred embodiments

[0108] Preferred embodiments are described below with reference to the figures. Identical, similar, or equivalent elements in the different figures are provided with identical reference numerals, and a repeated description of these elements is partially omitted to avoid redundancies.

[0109] Figure 1 shows a device according to the prior art. The laser 2 generates a laser beam 20 in which ultrashort laser pulses propagate along the beam propagation direction z. The laser beam 20 can be expanded, for example, by a beam expansion device 7 from a first beam diameter to a second beam diameter. The expanded laser beam 20 is then converted by a beam shaping device 4 into a non-diffracting laser beam, i.e., the intensity profile I of the laser beam is manipulated so that it is suitable for the desired material processing. The beam shaping device 4 can, for example, comprise an axicon 40, which forms a non-diffracting laser beam elongated in the beam propagation direction z. After the axicon 40, the laser beam 20 can be transferred by an imaging element 42 into a rear focal plane of the processing optics 5.The processing optics 5 then focus the laser beam 20 into a focus zone in or on the workpiece 1, where the material of the workpiece is exposed to the laser pulses and is thereby processed.

[0110] Figures 2.1 to 2.3 show the propagation of such a non-diffracting laser beam 20 in a birefringent material for different depths, e.g.

[0111] Figure 2.1 a shows the intensity profile of laser beam 20 at the surface z = 0.00 mm of the birefringent material. In particular, it can be seen that the entire energy of the laser beam is concentrated in the zeroth maximum of the intensity distribution I. Figure 2.1 b shows the polarization of the laser beam at the surface in the form of white lines. The laser beam is globally linearly polarized along the x-axis.

[0112] Figure 2.2a shows that, at a depth of z=0.65 mm in the material, the laser beam energy is no longer concentrated solely on the zeroth maximum, but rather the first maximum carries laser energy at the expense of the zeroth maximum. This leads to the material modifications introduced in deeper layers of the material being greater, or to the fact that they can no longer be introduced if the depth falls below a processing threshold. Figure 2.2b shows that the polarization of the laser beam 20 has changed, with elliptical polarization states forming around the beam axis.

[0113] Figure 2.3a shows, at a depth of z=1.30 mm in the material, that the energy transport in the zeroth maximum is negligibly small compared to the first maximum. Thus, only material modifications with the diameter of the first maximum can be introduced. Figure 2.3b also shows that the polarization of the laser beam 20 has also changed further. Figure 3.1a shows the intensity profile of a laser beam with an elliptical beam cross-section. The intensity profile comprises dumbbell-shaped intensity maxima into which the laser energy is guided. Typically, the energy in the three intensity maxima arranged one above the other in the y-direction is so large that they induce an elliptical material modification in the material. This elliptical material modification then has a preferred direction along which cracks can propagate in the material.

[0114] Figure 3.1 b shows that the laser beam is still circularly polarized at the surface.

[0115] Figures 3.2a and 3.2b show that at a depth of z=0.65 mm, the intensity profile and polarization distribution have already changed drastically. In particular, the transported laser energy has been shifted into the "dumbbell plates" of the dumbbell-shaped intensity distribution.

[0116] Figure 3.3a shows that at a depth of z=1.30 mm, the original beam profile is significantly altered. In particular, the elliptical structure of three superimposed intensity maxima no longer exists, but has been split in the x-direction. The polarization distribution of laser beam 20 has also changed further.

[0117] Figures 4.1a and 4.2a show another intensity profile of a laser beam 20 at a depth of z=0.6mm and z=1.3mm. In particular, a symmetry reversal of the intensity profile can be observed. For example, the crescent-shaped intensity maximum at z=0.65mm is open upwards, while at z=1.3mm it is open downwards.

[0118] The consequences of such variable intensity profiles in the material thickness during material processing are shown in Figure 4.2. Figures 4.2a and b show two material modifications on the upper surface of a birefringent material 1. The material modifications have an elliptical cross-section, resulting in targeted crack propagation along the long axis of the ellipse. The defined shape of the material modification thus allows crack propagation to be controlled and, for example, guided from one material modification to a neighboring material modification. By appropriately arranging the material modifications along a desired parting line on the workpiece, the workpiece can be separated, for example, without any further separation step.

[0119] However, Figures 4.2c and d show the corresponding material modifications on the underside of material 1. Due to the propagation of the laser beam 20 in the birefringent material, the intensity distribution has changed, so that the material modifications on the underside exhibit a different symmetry and a different orientation. For example, the material modification in Figure 4.2a still allows crack propagation on the upper surface, while the corresponding material modification in Figure 4.2c consists of three individual modifications that no longer allow spontaneous crack propagation due to the lack of a preferred direction.

[0120] The material modification in Figure 4.2b also allows crack propagation along a specific direction on the upper surface. However, Figure 4.2d shows that two different material modifications are formed on the lower surface. The direction of crack propagation of these material modifications differs from the modifications on the upper surface. As a result, a smooth and high-quality parting line cannot be achieved overall.

[0121] The above-mentioned disadvantages of the prior art are based on the fact that the birefringent material has a refractive index that depends on the direction and angle of incidence of the partial laser beams on the material 1. Typically, the laser beam is globally polarized, for example, linearly polarized, as shown in Figure 5. For example, the electric field always points along the y-axis. If such a laser beam 20 is focused onto the material 1 at an aperture angle of the processing optics 5, all partial laser beams lie on a cone, in the center of which lies the beam propagation direction z. With linear polarization, the angle between the electromagnetic field and the propagation direction of the partial laser beams changes locally on the cone, as shown by the projection of the propagation directions onto the xy plane.There are configurations in which the electric field component E coincides with the propagation direction, but also configurations in which the electric field component E is perpendicular to the propagation direction.

[0122] Since the refraction in the birefringent material depends on the direction of the partial laser beams and the polarization relative to the optical axis, birefringence and thus divergence of the laser beam energy can be avoided if the polarization is locally adapted to the direction of the partial laser beams relative to the optical axis of the birefringent material.

[0123] Figure 6.1a schematically shows a first embodiment of the device according to the invention, with which such an adjustment can be made. The laser beam 20 of the laser is guided through a beam expander 7 in order to increase the diameter of the laser beam from a first diameter to a second diameter. The laser beam is then guided to a polarization shaping device 3, which imparts a polarization distribution to the laser beam 20 so that the partial laser beams have locally different polarization directions. The laser beam 20 can then be guided through an optional phase shaping device 6 in order to impart a phase distribution to the laser beam 20 so that the partial laser beams have locally different phases.

[0124] The laser beam 20 is then guided through a beam-shaping device 4, wherein a beam-shaping element 40 imparts, in particular, a non-diffracting beam shape to the laser beam. The imaging element 42 of the beam-shaping device 4 can then guide the laser beam 20 into the rear focal plane F of the processing optics 5, from where the processing optics 5 focuses the laser beam 20 into a focal zone located in or on the material 1 of the workpiece, wherein the exposure of the material 1 to the laser beam 20 results in material processing.

[0125] Optionally, in all devices according to the invention, both the polarization shaping device and the phase shaping device can be rotated about their axes (dashed arrow). This allows, for example, the orientation of the polarization distribution or the orientation of the phase front to be determined by the phase shaping device.

[0126] Figure 6.1 b shows a further schematic embodiment of the device according to the invention according to Figure 6.1 a, in which the imaging element 42 is arranged spatially spaced from the beam-shaping element 40.

[0127] Figure 6.2a shows a further schematic embodiment of the device according to the invention, in which the polarization shaping device 3 and the phase shaping device 6 are arranged in the rear focal plane F of the processing optics 5.

[0128] Figure 6.2b shows a further schematic embodiment of the device according to the invention according to Figure 6.2a, in which the imaging element 42 is arranged spatially spaced from the beam-shaping element 40.

[0129] All of the devices according to the invention mentioned have a feed device (not shown) with which the laser beam 20 and the workpiece can be displaced relative to one another. This allows the material modification to be introduced at different locations on the workpiece. In particular, the material modifications can be introduced along a desired separating line, enabling particularly high-quality separation of the workpiece material along the separating line. Figure 6.3 describes one possible mode of operation of the invention. For example, in Figure 6.3a, the laser beam 20 enters the material parallel to the optical axis at z=0. The laser beam 20 is focused, with the partial laser beams lying on a conical surface in the material 1. The individual partial laser beams k1, k2, shown in Figure 6.3b, are refracted into ordinary and extraordinary beams according to the refractive indices of the index ellipsoid.The essence of the invention can be seen in the fact that the polarization of the laser beam 20 is locally adjusted such that each partial laser beam has only one electric field component in the direction that the energy is transported exclusively in the ordinary beam or the extraordinary beam. As a result, the beam propagation of the polarization-adjusted laser beam in the birefringent material can be compared, to a good approximation, with that of a non-polarization-adjusted laser beam in a non-birefringent material.

[0130] Figure 7 shows a method according to the invention in which the laser beam 20 is guided through one of the aforementioned devices according to the invention. In particular, the laser beam 20 is guided through a beam expansion device 7, a polarization shaping device 3, and a beam shaping device 4.

[0131] In Figure 7.1, for example, an intensity distribution I of the laser beam 20 with an annular zeroth maximum can be seen on the surface of material 1 at z=0 mm. Figure 7.1b shows that the laser beam 20 is radially polarized. The electric field component points radially away from the center of the intensity distribution. Figures 7.2a and 7.3a show that the energy is still transported in the zeroth intensity maximum of the intensity distribution I, even at depths of 0.65 mm and 1.3 mm. Furthermore, the intensity distribution I is not modified by the propagation in the birefringent material, just as the polarization distribution shown in Figures 7.2b and 7.3 is not modified.

[0132] Such a polarization distribution makes it possible to guide the laser beam 20, elongated in the beam propagation direction, through a birefringent material 1 without disruptive beam divergence. Accordingly, the material modifications introduced on the top and bottom sides of the material 1 have the same shapes and symmetries.

[0133] Figure 8 shows a method according to the invention in which the laser beam 20 is guided through one of the aforementioned devices according to the invention. In particular, the laser beam 20 is guided through a beam expansion device 7, a polarization shaping device 3, a phase shaping device 6, and a beam shaping device 4. The phase shaping device 6 imparts a vortex phase distribution to the laser beam 20. In contrast to the intensity profile of Figure 7, the intensity profile of Figure 8.1a now has a closed zeroth maximum in which the energy is transported. In a sense, the vortex phase distribution allows the laser beam 20 from Figure 7 to be concentrated to a zeroth maximum in Figure 8. The laser beam 20 is also radially polarized, as shown in Figure 8.1b.

[0134] The beam shape and polarization are preserved when passing through the birefringent material, as a comparison of the intensity distributions in Figures 8.1, 8.2 and 8.3 for the depths z=0mm, z=0.65 and z=1.30mm shows.

[0135] Figure 9 shows a further method according to the invention, in which the laser beam 20 is guided through one of the aforementioned devices according to the invention. In particular, the laser beam 20 is guided through a beam expansion device 7, a polarization shaping device 3, a phase shaping device 6, and a beam shaping device 4. The polarization shaping device 3 imparts an azimuthal and radial polarization distribution to the laser beam 20, while the phase shaping device 6 imparts an elliptical beam shape to the laser beam.

[0136] Mixed states of radial and azimuthal polarization are also called "higher-order Poincare sphere beams," see Naidoo, Darryl, et al. "Controlled generation of higher-order Poincare sphere beams from a laser." Nature Photonics 10.5 (2016): 327-332.

[0137] The beam shape remains largely unchanged when passing through the birefringent material, as shown by a comparison of the intensity distributions in Figures 9.1, 9.2, and 9.3 for depths z=0 mm, z=0.65 mm, and z=1.30 mm. However, the polarization changes for the different depths.

[0138] Figure 9.4 shows a possible azimuthal and radial polarization distribution of the laser beam.

[0139] Figure 10.1 shows a possible embodiment of a polarization-shaping device. The polarization-shaping device 3 has different segments 30, each segment 30 corresponding to a λ / 2 plate, each of which rotates the polarization of the incident laser beam 20 by a specific amount. By superimposing the various partial laser beams that pass through the polarization-shaping device 30, a radial polarization of the laser beam 20 can be generated, for example.

[0140] Another possible embodiment of a polarization-shaping device 3 is shown in Figure 10.2. The polarization-shaping device 3 is a locally polarizing nanograting. The locally polarizing nanograting has nanoscopic diffraction structures 32 that cause polarization-dependent scattering of the laser beam 20. For example, only those partial laser beams whose polarization is parallel to the diffraction structure are scattered. In this case, the partial laser beams that impinge on region A can be strongly scattered, and the partial laser beams that impinge on region B are not scattered. This can, for example, result in an overall radial polarization. However, by aligning the nanoscopic diffraction structures, any desired polarization distribution can be generated.

[0141] Figure 11 shows a so-called generalized axicon 400 with which the functional properties of a phase shaping device 6 and an axicon 40 can be provided.

[0142] A generalized axicon 400 enables the imposition of a phase distribution on a transverse beam profile of a laser beam 20. It comprises adjacent surface elements 402 that form a planar grating structure in which each surface element 402 is assigned a phase shift value, and the phase shift values ​​define a two-dimensional phase distribution. The surface elements 402 are assigned to a plurality of angular segments 404, 404', each angular segment 404, 404' having an azimuthal segment width relative to the beam center position. Using the angular segments 404, 404' and surface elements 402, it is possible to generate any desired beam shape.

[0143] In the axicon shown in Figure 11, the radial symmetry is broken by phase shifts in the angular segments. This results in an elliptical distribution of the non-diffracting laser beam along the beam axis.

[0144] Figure 12 schematically shows a phase-shaping device 6 in the form of a spiral phase plate, with which a vortex phase distribution can be generated. The phase-shaping device has a pitch, like a screw. With one rotation of the phase-shaping device about its axis, the thickness of the phase-shaping device increases by the amount Ah. This amount can be selected so that it corresponds to a multiple of the laser wavelength used, so that there is no jump at the edge of the phase-shaping device, but rather, for example, a phase difference from 6TT to OTT or from 2TT to OTT. The phase difference divided by 2TT indicates the so-called charge of the spiral phase plate. The diameter of the vortex phase distributions can be adjusted particularly easily using the charge. Integer charges are preferably used, for example, a charge of 1. Figure 13 shows the crystal structure of sapphire.The optical axis of sapphire coincides with the crystal's C-axis. If the laser beam 20 enters the sapphire parallel to the C-axis, the above-mentioned advantageous effects can be achieved particularly easily.

[0145] Where applicable, all individual features presented in the embodiments may be combined and / or exchanged without departing from the scope of the invention.

[0146] List of reference symbols

[0147] 1 workpiece

[0148] 2 lasers

[0149] 20 Laser beam 3 Polarization shaping device

[0150] 4 Beam shaping device

[0151] 40 Axicon

[0152] 400 generalized axicon

[0153] 402 Surface element 404 Angle segment

[0154] 42 imaging element

[0155] 5 Processing optics

[0156] 6 Phase shaping device

[0157] 7 beam expanders

Claims

Claims 1 . Method for machining a workpiece (1) made of a birefringent material with at least one optical axis (OA) by means of ultrashort laser pulses of a laser beam (20) of an ultrashort pulse laser (2), wherein a non-diffracting beam shape with an intensity distribution is impressed on the laser beam (20) by a beam shaping device (4), wherein a polarization distribution is impressed on the laser beam (20) by a polarization shaping device (3), wherein the laser beam (20) is focused by a machining optics (5) into a focus zone in a focus plane in the workpiece (1), wherein the workpiece (1) is exposed to the laser beam and is thereby machined, wherein the polarization distribution compensates for the birefringence of the material of the workpiece (1).

2. Method according to claim 1, characterized in that the material is sapphire and / or the thickness of the material is between 300|jm and 3000|jm, preferably between 500|jm and 2000|jm.

3. Method according to one of claims 1 to 2, characterized in that the laser beam (20) impinges perpendicularly on the upper surface of the material parallel to the at least one optical axis (OA) of the material.

4. Method according to one of claims 1 to 3, characterized in that the laser beam (20) is an elliptical non-diffracting beam.

5. Method according to one of claims 1 to 4, characterized in that a radial polarization distribution is impressed on the laser beam (20) by the polarization shaping device (3).

6. Method according to one of claims 1 to 5, characterized in that a vortex phase distribution is impressed on the laser beam (20) by a phase shaping device (6).

7. Method according to one of claims 1 to 6, characterized in that the laser beam (20) is given a radial and a azimuthal polarization distribution is impressed and by the Phase forming device (6) imparts an elliptical phase distribution.

8. Method according to one of claims 1 to 7, characterized in that the pulse duration of the ultrashort laser pulses is between 300fs and 12ps.

9. Device for machining a workpiece (1) made of a birefringent material (10) with at least one optical axis (OA), comprising an ultrashort pulse laser (2) which is configured to provide a laser beam (20) with laser pulses, a beam shaping device (4) which is configured to impart a non-diffracting beam shape with an intensity distribution to the laser beam (20), a polarization shaping device (3) which is configured to impart a polarization distribution to the laser beam (20), and machining optics (5) which is configured to focus the laser beam (20) into a focus zone in the focus plane in the workpiece (1), wherein the workpiece (1) is exposed to the laser beam (20) and is thereby machined, wherein the polarization distribution compensates for the birefringence of the material of the workpiece (1).

10. Device according to claim 9, characterized in that the beam-shaping device (4) comprises an axicon (40), preferably an axicon (40) and an imaging element (42).

11. Device according to one of claims 9 to 10, characterized in that the polarization shaping device (3) is a segmented polarization plate and / or a locally polarizing nanograting.

12. Device according to one of claims 9 to 11, characterized by a beam expansion device which is designed to increase the beam diameter of the laser beam from a first diameter to a second diameter, wherein the beam expansion device is preferably arranged in front of the polarization shaping device.

13. Device according to one of claims 9 to 12, characterized by a phase-shaping device (6) which is designed to impart a phase distribution to the laser beam (20).

14. Device according to claim 13, characterized in that the phase forming device (6) is a spiral phase plate.

15. Device according to one of claims 13 to 14, characterized in that the phase shaping device (6) and the beam shaping device (4) are formed in one piece, in particular are realized in a generalized axicon.

16. Device according to one of claims 9 to 14, characterized in that the phase shaping device (6) and the polarization shaping device (3) are arranged in front of the Beam shaping device (4) or after the beam shaping device (4) and before the processing optics (5).

17. Device according to one of claims 9 to 16, characterized by a feed device which is designed to move the laser beam (20) and the workpiece (1) relative to one another with a feed.

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