Depolarisation compensator and optical system

Abstract

The invention relates to a depolarisation compensator (1) for at least partially compensating for a location-dependent variation in the polarisation (6) of a laser beam (4), comprising: a first compensation component (2) made of a birefringent material, which has a preferably planar beam incident face (2a) for entrance of the laser beam (4) and a beam exit field (2b) for exiting of the laser beam (4), a thickness (D(X, Y)) of the first compensation component (2) between the beam incident face (2a) and the beam exit field (2b) varying in a location-dependent manner in order to at least partially compensate for the location-dependent variation in the polarisation (6) of the laser beam (4); and a second compensation component (3), which compensates for a light-refracting effect of the first compensation component (2) on the laser beam (4) and does not have a birefringent effect on the laser beam (4). The invention also relates to an optical system which comprises at least one such depolarisation compensator (1).

Description

[0001]16.07.2024 DS16778P3934WO0 Depolarization compensator and optical system The present invention relates to a depolarization compensator for at least partially compensating for a location-dependent variation in the polarization of a laser beam. The depolarization compensator is used to compensate or equalize a spatial depolarization of laser radiation, more precisely of a laser beam. Depolarization is understood here as a state in which the laser beam, in an (arbitrary) plane perpendicular to the propagation direction of the laser beam, has a well-defined but not homogeneous polarization (e.g., linear, circular, or elliptical). Instead, a location-dependent variation or change in the polarization or polarization state occurs across the beam cross-section of the laser beam (spatially inhomogeneous polarization).With such spatially inhomogeneous polarization, for example, circular polarization can occur in the center of the laser beam's cross-section and elliptical polarization at the edge of the beam's cross-section, or vice versa. Depolarization or the spatially dependent variation of polarization can occur, for example, when the laser beam experiences a spatially dependent phase shift between the S-polarized component and the P-polarized component during its propagation across an optical element, for example, when a convergent or divergent laser beam is incident at an oblique angle on a (coated) mirror, when stress birefringence occurs in the strained material of transmissive optics, etc.The depolarization compensator is inserted into the beam path of the laser beam and serves to introduce a location-dependent phase shift into the optical system, which is ideally exactly opposite to the location-dependent variation in the phase shift due to depolarization, i.e. has the opposite sign and the same magnitude as the phase shift due to depolarization. Ideally, the depolarization in a viewing plane, e.g., at the "output" of the optical system, can be exactly compensated in this way. To compensate for a spatially homogeneous phase shift, it is known to use a Babinet-Soleil compensator or a Berek compensator, for example. To compensate for a location-dependent variation in polarization, adapted phase plates can be used. For this purpose, for example, stress birefringence can be locally introduced into a glass plate using an ultrashort pulse laser.EP 3712664 A1 describes a depolarization compensator for compensating the depolarization of light in laser systems. The depolarization compensator serves for spatially variable polarization control and, for this purpose, comprises a microstructured optical element with spatially varying birefringence, which was manufactured from a transparent and optically isotropic material by direct writing with an ultrashort pulse laser. The spatially varying birefringence is caused by a different orientation of the fast axes of birefringent nanogratings at different locations in the cross-section of the microstructured optical element. The nanogratings are created in the volume of the optical element during direct writing with the ultrashort pulse laser.Object of the invention: The object of the invention is to provide a depolarization compensator that is simple to manufacture and has the least possible influence on the propagation of the laser beam. The invention also aims to provide an optical system with such a depolarization compensator.Subject of the invention This object is achieved according to a first aspect by a depolarization compensator of the type mentioned at the outset, comprising: a first compensation component made of a birefringent material, which has a preferably planar beam entry surface for the entry of the laser beam and a beam exit surface for the exit of the laser beam, wherein a thickness of the first compensation component between the beam entry surface and the beam exit surface varies depending on the location in order to at least partially, in particular completely, compensate for the location-dependent variation in the polarization of the laser beam, and a second compensation component which at least partially, in particular completely, compensates for a refractive effect of the first compensation component on the laser beam and has no birefringent effect on the laser beam.In the depolarization compensator according to the invention, the beam entrance surface of the first compensation component is typically flat and oriented perpendicular to the beam direction of the laser beam, but this is not mandatory (see below). The thickness distribution of the first compensation component in the beam direction of the laser beam and the orientation of the optical axis of the birefringent material of the first compensation component are selected such that, when the laser beam passes through the first compensation component, the entire phase shift required to compensate for the location-dependently varying depolarization is impressed or generated. The beam exit surface of the first compensation component is therefore not aligned parallel to the beam entrance surface and generally does not have a flat geometry.To generate a phase-shifting effect on the laser beam, the optical axis of the birefringent first compensation component, or more precisely, the optical axis of the birefringent material of the first compensation component, is typically aligned transversely to the beam propagation direction of the laser beam. Since the beam exit surface is not aligned parallel to the beam entrance surface, the first compensation component generates a refractive effect on the laser beam. To compensate for the refractive effect ("lens") of the location-dependently varying thickness distribution of the first compensation component, the depolarization compensator has a second compensation component that compensates as completely as possible for the refractive effect of the first compensation component on the laser beam and that itself has no birefringent effect on the laser beam.The depolarization compensator formed in this way is comparatively easy to manufacture and particularly suitable for high-power laser beams. In one embodiment, the second compensation component has a beam entrance surface adjacent to the beam exit surface of the first compensation component, as well as a planar beam exit surface, which is preferably aligned parallel to the beam entrance surface of the first compensation component. In this case, the second compensation component forms a "counterpart" to the first compensation component. The beam entrance surface of the second compensation component has a surface shape that corresponds to the "negative" of the surface shape of the first compensation component.The beam exit surface of the second compensation component is flat and typically parallel to the (flat) beam entrance surface of the first compensation component and therefore generally perpendicular to the beam direction of the laser beam. The refractive index of the second compensation component is matched to the refractive index of the first compensation component (see below). The depolarization compensator described here thus ideally has the optical effect of a plane-parallel plate on the laser beam, generating a defined, location-dependent phase shift between the S-polarization and the P-polarization, or between two mutually perpendicular polarization directions.In a further development of this embodiment, the second compensation component is formed from a birefringent material whose optical axis is aligned perpendicular to the beam entrance surface of the first compensation component. Preferably, the birefringent material of the second compensation component coincides with a birefringent material of the first compensation component. As described above, the laser beam typically impinges perpendicularly on the beam entrance surface of the first compensation component, i.e., the optical axis of the birefringent material of the second compensation component is aligned parallel to the beam direction of the laser beam, which means that the second compensation component has no birefringent effect on the laser beam.As described above, to generate the phase-shifting effect, the optical axis of the birefringent material of the first compensation component is typically aligned transversely to the beam propagation direction of the laser beam or parallel to the beam entrance plane of the first compensation component. It is generally advantageous if the birefringent material of the second compensation component matches the birefringent material of the first compensation component, since in this case their refractive indices match. In this case, the birefringent materials differ from one another only in their orientation or in their intersection direction relative to the crystallographic axes. In an alternative embodiment, the second compensation component is formed from a non-birefringent (solid-state) material.In this embodiment, a material is selected for the second compensation component whose refractive index, or more precisely the real part of the refractive index, deviates as little as possible from the refractive index of the birefringent material of the first compensation component at the wavelength of the laser beam. For example, the refractive index of the non-birefringent material can correspond to the ordinary refractive index of the birefringent material or, for example, the mean value between the ordinary refractive index and the extraordinary refractive index of the birefringent material. The non-birefringent material can be an amorphous material or a crystalline material without birefringent effect. In a further development of this embodiment, the first compensation component is made of quartz and the second compensation component is made of quartz glass.The ordinary refractive index of single-crystal quartz is 1.544, the extraordinary refractive index is 1.553 (each at a wavelength of approximately 590 nm); the refractive index of quartz glass is in the same order of magnitude. The difference between the refractive indices of the two compensation components is therefore so small that the depolarization compensator has virtually no refractive effect on the laser beam. In an alternative embodiment, the first compensation component is made of sapphire, and the second compensation component is made of a non-birefringent material containing aluminum oxide. Sapphire is crystalline Al2O3, which has a birefringent effect. The material containing aluminum oxide can be an amorphous material, such as alon (aluminum oxynitride, AlN-Al2O3), or a crystalline material without birefringent effect, such as spinel (MgAl2O4).Sapphire has an ordinary refractive index of approximately 1.768 and an extraordinary refractive index of approximately 1.760. Alon has a refractive index of approximately 1.79, and spinel has a refractive index of approximately 1.719; these refractive indices are very close to each other. In another embodiment, the beam exit surface of the first compensation component deviates from a flat geometry, and the beam entrance surface of the second compensation component has a geometry complementary to the beam exit surface of the first compensation component. In this case, the beam exit surface is typically a freeform surface. The thickness of the first compensation component varies irregularly across the cross-section of the laser beam.In this case, the second compensation component forms a "counterpart" to the first compensation component with a beam entrance surface whose surface shape corresponds to the "negative" of the surface shape of the first compensation component (see above). For the production of the depolarization compensator described above, one can proceed, for example, as follows: - Determine the depolarization to be compensated or the spatially resolved phase shift to be compensated by measurement or simulation. - Select a suitable birefringent material for the first compensation component, for which a corresponding non-birefringent "counterpart" for the second compensation component exists, which has the smallest possible refractive index difference (see above), or use the same birefringent material in a different "section" so that the optical axis runs parallel to the beam propagation direction (see above).- Computational adjustment of the thickness distribution and axial alignment of the birefringent material of the first compensation component so that the location-dependent variation in the phase shift is compensated. The surface shape of the beam exit surface of the first compensation component resulting from the location-dependent variation in the thickness of the first compensation component determines the – complementary – surface shape of the second compensation component. The depolarization compensator designed in the manner described above compensates precisely “in advance” for the phase shift of the two polarization components induced in one or more optical components with a depolarizing effect through the location-dependent birefringence by generating a location-dependent phase shift with an opposite sign. For this, the location-dependent phase shift must be known in advance (e.g., based on simulations or measurements).The thickness distribution of the first compensation component can now be adjusted to precisely achieve the required compensation effect. However, depending on the "shape" and magnitude of the phase shift, the ideal surface shape of the beam exit surface of the first compensation component can be difficult to manufacture, especially considering the correct axial position of the optical axis of the birefringent material. It may therefore be advantageous to reduce the complexity of manufacturing the depolarization compensator. In a further development, the beam exit surface of the first compensation component is flat and aligned at a wedge angle to the beam entry surface of the first compensation component, and the beam entry surface of the second compensation component is flat and aligned at an (identical) wedge angle to the beam exit surface of the second compensation component.This embodiment assumes that the phase shift caused by the disturbance in the beam path is sufficiently linear in the vicinity of a central offset, so that a purely linear phase correction is sufficient to effectively compensate for the depolarization of the laser beam. Ideally, sufficiently good compensation can be achieved by combining two uniaxial depolarization compensators (e.g., the first depolarization compensator acts only along the horizontal axis, the second depolarization compensator acts only along the vertical axis). A uniaxial linear correction of the phase shift can be easily achieved by a first compensation component in the form of a birefringent wedge and a corresponding non-birefringent counterpart, which also has a wedge shape.In a further development, the first compensation component and the second compensation component are designed to vary the respective wedge angle. In this way, the linear compensation of the phase shift can be flexibly adapted to the respective compensation task. In this case, the depolarization compensator typically has a fixing device to fix the two sub-elements of the first and second compensation components relative to one another once the desired wedge angle of the respective compensation component has been set. In a further development of this embodiment, the first compensation component has two sub-elements that can be rotated relative to one another to vary the wedge angle of the first compensation component, and the second compensation component has two sub-elements that can be rotated relative to one another to vary the wedge angle of the second compensation component.In this case, the beam exit surface of the second sub-element of the first compensation component, located in the beam path of the laser beam, borders the beam entrance surface of the first sub-element of the second compensation component. Rotating the two sub-elements relative to each other provides a simple way to adjust the wedge angle. The two sub-elements of the first compensation component are made of a birefringent material. In this case, at least one of the two sub-elements of the second compensation component is typically made of a non-birefringent material.In a further development, the first sub-element of the first compensation component has a cylindrical beam exit surface, which is adjacent to a cylindrical beam entry surface of the second sub-element of the first compensation component, and the first sub-element of the second compensation component has a cylindrical beam exit surface, which is adjacent to a cylindrical beam entry surface of the second sub-element of the second compensation component. In this case, the first sub-element and the second sub-element of the first compensation component are designed as cylindrical lenses whose cylinder axes are aligned parallel to the optical axis of the birefringence of the birefringent material of the first and second sub-elements. The beam exit surface of the first sub-element is concave, the beam entry surface of the second sub-element is convex (or vice versa), and the radius of curvature of the two lenses or surfaces is identical.The two sub-elements of the first compensation component can therefore be rotated relative to each other along the adjacent cylindrical beam entry and exit surfaces. The second compensation component is designed in the same way. In the depolarization compensator described above, which compensates for a linear depolarization, a spatially homogeneous phase shift component can additionally remain, which is determined by the thickness of the depolarization compensator or the first compensation component. This spatially homogeneous phase shift component can be achieved either by existing methods for compensating homogeneous phase shifts (e.g.Berek compensator), or, analogous to the "adjustable wedge" method described above, an "adjustable thickness" element made of the birefringent material is realized by two oppositely oriented, movable wedges (similar to a Babinet compensator). In an alternative embodiment, a liquid refractive index adjustment medium is introduced between the beam entrance surface and the beam exit surface of the second compensation component. In this case, the "counterpart" of the first compensation component is not a solid, but rather a liquid made of a refractive index adjustment medium whose refractive index matches the refractive index of the birefringent material of the first compensation component as precisely as possible. In particular, the entire first compensation component can be embedded or introduced into a container filled with the refractive index adjustment medium.In this way, the first compensation element does not create a lens effect, since no refractive index differences occur transversely to the beam propagation direction of the laser beam. A mixture of water and glycerin, for example, can be used as the refractive index adjustment medium, whereby the refractive index can be adjusted by the proportion of glycerin in the mixture. Alternatively, a refractive index adjustment medium tailored to the respective birefringent material can be used. In this embodiment, the beam entry surface of the first compensation element can fundamentally deviate from a flat geometry. In this case, to avoid a light-refracting effect, a beam entry surface into the container and a beam exit surface from the container, which corresponds to the beam exit surface of the second compensation component, are typically aligned parallel to one another.If the first compensation component has a flat beam exit surface, a "wedge angle" between the beam entry surface of the container and the beam exit surface of the first compensation component can be varied by rotating the first compensation component in the liquid of the container. In a further embodiment, the second compensation component has a medium with a refractive index that deviates by no more than 0.2, preferably by no more than 0.1, from the mean value of the ordinary refractive index and the extraordinary refractive index of the first compensation component. As described above, the medium of the second compensation component can be a solid or a liquid, the refractive index of which should be as close as possible to the refractive index of the medium of the first compensation component.A further aspect of the invention relates to an optical system comprising: a beam source for generating a laser beam, at least one optical component that generates a location-dependent variation in the polarization of the laser beam, and at least one depolarization compensator, which is designed as described above, for at least partially compensating for the location-dependent variation in the polarization of the laser beam generated by the at least one optical component. As described above, the laser beam generated by the beam source typically strikes the (planar) beam entrance surface of the first compensation component of the depolarization compensator perpendicularly and exits the depolarization compensator perpendicularly to the (planar) beam exit surface. The alignment of the laser beam is based on the beam direction of the central beam in the center of the beam cross-section of the laser beam.The depolarization compensator can generally be arranged in the beam path before or after the optical component which generates the location-dependent variation in the polarization of the laser beam. In order to simplify the calculation of the required location-dependent compensation effect, it is advantageous if the depolarization compensator is arranged in the collimated beam path, but this is not absolutely necessary. In one embodiment, the optical component which generates the depolarization forms a (usually coated) mirror onto which the laser beam impinges convergently or divergently. The mirror can be a deflecting mirror with a flat mirror surface, but this is not absolutely necessary. In this embodiment, the laser beam usually impinges on the mirror surface at an angle, i.e. at an angle of incidence other than 90°. Due to the convergent or divergent angle of incidence, the laser beam is incident on the mirror surface.With divergent incidence, the angles of incidence of the laser beam differ in the center and at the edge of the beam cross-section. The phase shift between the portion of the laser beam polarized perpendicular to the plane of incidence (S-polarization) and the portion of the laser beam polarized parallel to the plane of incidence (P-polarization) depends on the angle of incidence of the laser beam on the mirror surface. Due to the angular dependence of the phase shift between the S- and P-polarized portions of the laser beam, a phase shift results between the two polarization components, which is also location-dependent due to the location dependence of the angle of incidence distribution. For example, a laser beam that is circularly polarized before hitting the mirror can, after reflection from the mirror, have an elliptical polarization state towards the edge of the beam cross-section, i.e. the polarization state becomes spatially inhomogeneous.Perpendicular to the plane of incidence, the angular deviation between the central ray and the marginal ray of the laser beam's cross-section is determined only by the divergence angle of the laser beam and is therefore usually small. In the plane of incidence, the angular difference is also determined only by the beam divergence, but a comparatively large central angle is achieved by the angle of incidence on the mirror surface. Since the phase shift between S and P polarization often increases significantly with increasing angle of incidence, the phase shift in the plane of incidence is therefore comparatively large compared to the phase shift perpendicular to the plane of incidence. Such a mirror is therefore an example of an optical component that produces an essentially uniaxial depolarization state when the laser beam impinges on the mirror surface convergently or divergently.Therefore, in this case, uniaxial compensation of the depolarization is typically sufficient. Assuming that the S-to-P phase shift around the central angle can be closely approximated using a linear function, a first compensation component in the form of a simple wedge plate made of birefringent material and a corresponding counterpart can be used to compensate for the resulting uniaxial linear phase shift (see above). The exact parameters of the wedge plate(s) (wedge angle and thickness) can either be calculated analytically or determined using an optimization algorithm. Further advantages of the invention will become apparent from the description and the drawings. Likewise, the features mentioned above and those listed below can be used individually or in groups in any combination.The embodiments shown and described are not to be understood as an exhaustive list, but rather have an exemplary character for the description of the invention. They show: Fig. 1a a schematic representation of a depolarization compensator which has a first and second compensation element made of a birefringent material, Fig. 1b a schematic representation analogous to Fig. 1a, in which the second compensation element is formed from a non-birefringent material, Fig. 1c a schematic representation analogous to Fig. 1a, in which the second compensation element has a liquid refractive index adaptation medium, Fig. 2 a schematic representation of an optical system which has a depolarizing optical component in the form of a mirror and a depolarization compensator, Fig. 3 a schematic representation of the phase of the S- and P-polarized portions of the light incident on the mirror of Fig.2 incident laser beam as a function of the angle of incidence, Fig. 4a,b schematic representations of the location-dependent polarization of the laser beam reflected at the mirror of Fig. 2 without depolarization compensation (Fig. 4a) and with depolarization compensation (Fig. 4b), Fig. 5 a schematic representation of a depolarization compensator having a first and second compensation element in the form of two wedge plates, and Fig. 6a,b schematic representations of a depolarization compensator designed to adjust a wedge angle of the first compensation element and the second compensation element. In the following description of the drawings, identical reference numerals are used for identical or functionally identical components. Fig. 1a-c show three examples of a depolarization compensator 1 having a first compensation component 2 and a second compensation component 3. The first compensation component 2 is shown in Fig.1a-c are formed from a birefringent solid material, e.g., quartz or sapphire. The first compensation component 2 has a flat beam entry surface 2a for the entry of a laser beam 4 and a beam exit surface 2b designed as a freeform surface for the exit of the laser beam 4. In the example shown, the laser beam 4 strikes the beam entry surface 2a of the first compensation component 2 perpendicularly and has a beam cross-section that, in the example shown, extends over almost the entire beam entry surface 2a of the first compensation component 2; however, this is not absolutely necessary. As a rule, the beam cross-section of the laser beam 4 is somewhat smaller than the free aperture of the compensation component 2 (e.g., half the size) in order to avoid diffraction effects and power loss at the edge of the optics.A thickness D(X, Y) of the first compensation component 2 in a direction perpendicular to the beam entrance surface 2a, which corresponds to the Z direction of an XYZ coordinate system and the beam direction of the laser beam 4, varies in the example shown depending on the location X, Y both in the X direction and in the Y direction. The laser beam 4 shown in Fig. 1a-c was originally circularly polarized over its entire beam cross-section, but experienced depolarization upon reflection from an optical component, e.g. in the form of a mirror 5 shown in Fig. 2, and therefore no longer has a uniform, homogeneous polarization 6 across its beam cross-section. The location-dependent thickness D(X, Y) of the first compensation component 2 is selected such that it depolarizes the laser beam 4 at a respective location X, Y of the beam cross-section of the laser beam 4, i.e.the location-dependent variation of the polarization 6 of the laser beam 4 is compensated, so that the laser beam 4, after passing through the depolarization compensator 1, has a uniform polarization state across the entire beam cross-section. In the example shown, the uniform polarization state is a circular polarization state. To generate this, the thickness D(X, Y) of the first compensation component 2 at a respective location X, Y is selected such that the birefringent medium of the first compensation component 2 generates a phase shift that has the same magnitude but the opposite sign of the phase shift attributable to the depolarization of the laser beam 4. In this way, for example,an elliptical polarization state 6 at a respective location X, Y of the beam cross-section of the laser beam 4 before the depolarization compensator 1 can be converted into a circular polarization state 6 after passing through the depolarization compensator 1, as is indicated in Fig. 1a-c by way of example for a location X, Y. In order to generate a phase shift when the laser beam 4 passes through the first compensation component 2, an optical axis 7 of the birefringent medium of the first compensation component 2 is aligned transversely to the beam direction Z of the laser beam 4, in the example shown in the X direction. Due to the inhomogeneous thickness D(X, Y) of the first compensation component 2, the laser beam 4 is refracted in different directions when exiting the beam exit surface 2b of the first compensation component 2, depending on the location X, Y.In order to avoid the refractive effect of the depolarization compensator 1, it has the second compensation component 3, whose beam entrance surface 3a directly adjoins the beam exit surface 2b of the first compensation component 2 and has a surface shape complementary thereto. A beam exit surface 3b of the second compensation component 3 is flat and aligned parallel to the flat beam entrance surface 3a of the first compensation component 2. In the event that the second compensation component 3 is formed from a material whose refractive index deviates only slightly from or corresponds to the refractive index of the birefringent material of the first compensation component 2, the refractive effect of the first compensation component 2 on the laser beam 4 can be practically completely compensated with the aid of the second compensation component, i.e.The depolarization compensator 1 has the optical effect of a plane-parallel plate. In contrast to the first compensation component 2, the second compensation component 3 has no birefringent effect on the laser beam 4. The second compensation component 3 can, in principle, be designed in different ways. In the example shown in Fig. 1a, the second compensation component 3 is made of a birefringent material whose optical axis 8 is aligned perpendicular to the beam entrance surface 2a of the first compensation component 2 and thus parallel to the beam direction Z of the laser beam 4. Due to this alignment of the optical axis 8, the second compensation component 3 has no birefringent effect on the laser beam 4.In the example shown, the first compensation component 2 and the second compensation component 3 are made of the same birefringent material, so that the refractive indices of the two compensation components 2, 3 essentially match. In the depolarization compensator 1 shown in Fig. 1b, the second compensation component 3 is formed from a non-birefringent material that has the smallest possible difference in refractive index from the birefringent material of the first compensation component 2. In this case, the material of the first compensation component 2 can be, for example, quartz, and the material of the second compensation component 3 can be quartz glass. Alternatively, the material of the first compensation component 2 can be sapphire, and the material of the second compensation component 3 can be a non-birefringent material containing aluminum oxide, for example, alon or spinel.It is understood that other material combinations are also possible. Fig. 1c shows an example of a depolarization compensator 1 in which a liquid refractive index adaptation medium 9 is introduced between the beam entrance surface 3a and the beam exit surface 3b of the second compensation component 3. In the example shown in Fig. 1c, the entire first compensation component 3 is embedded in the liquid refractive index adaptation medium 9, whose refractive index is adapted to the refractive index of the birefringent medium of the first compensation component 2. In general, it is advantageous if the second compensation component 3 has a medium with a refractive index n2 that differs by no more than 0.2, preferably by no more than 0.1, from the mean value (arithmetic mean) of the ordinary refractive index n. 1oand the extraordinary refractive index n1ao of the first compensation component 2. The following describes in more detail how the depolarization of the laser beam 4 generated by a beam source 11 at the mirror 5, which is designed as a planar deflecting mirror, occurs with reference to an optical system 10 shown in Fig. 2. The laser beam 4 generated by the beam source 11 is focused by a focusing device 12 in the form of an objective lens to a focus position F and impinges convergently on the mirror 5. In the example shown, an angle of incidence β of the laser beam 4 onto the mirror 5 is 60° in the center of the beam cross-section ("central beam"). However, the angle of incidence varies across the beam cross-section from the center to the edge of the beam cross-section ("marginal beam") in the deflection or incidence plane Z, Y. Fig. 3 shows the phase Φ sof the portion of the laser beam 4 polarized perpendicular to the deflection plane Z, Y (S-polarization) as well as the phase Φp of the portion of the laser beam 4 polarized parallel to the deflection plane Z, Y (P-polarization) as a function of the angle of incidence β. As can be seen in Fig.3, at an angle of incidence of β = 60°, a significant phase shift Φ s - Φ p between the S-polarized and the P-polarized portion of the laser beam 4. However, perpendicular to the deflection plane Z, Y, the angle of incidence is 0° and the phase shift Φ s - Φ pis very small. As can also be seen in Fig.3, the phase shift Φs - Φp in the vicinity of 60° is approximately linearly dependent on the angle of incidence β. The dependence of the phase shift Φs - Φp on the angle of incidence β has the consequence that the polarization 6 of the initially circularly polarized laser beam 4, after reflection at the deflection mirror 5, varies depending on the location across the beam cross-section 13 of the laser beam 4 shown in Fig.4a, with the variation in the Y direction, i.e. in the deflection plane Y, Z, being significantly greater than in the X direction perpendicular to the deflection plane Y, Z. In the Y direction, the polarization state varies from circular in the center of the beam cross-section via elliptical or linear to circular at the edge of the beam cross-section, while only a slight depolarization occurs in the X direction. To compensate for the depolarization shown in Fig.4a, a depolarization compensator 1 can now be designed, which is configured as described above in connection with Fig. 1a-c. Depending on the type of depolarization, the depolarization compensator 1 shown in Fig. 1a-c may, however, be complex to manufacture. In the event that the phase shift Φs - Φp is sufficiently linear in the vicinity of a central offset, as is the case in Fig. 3 with an angle of incidence β of 60° to a good approximation, a purely linear phase correction or depolarization compensation is sufficient to compensate for the depolarization of the laser beam 4. For the depolarization described in connection with Fig. 2, such a linear phase compensation can therefore be applied. The phase correction can also be uniaxial, i.e.with a single linear depolarization compensator 1, since, as described above, no significant depolarization occurs in the X-direction. A depolarization compensator 1 for the linear compensation of the depolarization shown in Fig. 4a is shown in Fig. 5. The depolarization compensator 1 of Fig. 5 is constructed like the depolarization compensator 1 shown in Fig. 1a, i.e. the first compensation component 2 and the second compensation component 3 are made of the same birefringent material. The depolarization compensator 1 of Fig. 5 differs from the depolarization compensator 1 of Fig. 1a in that the beam exit surface 2b of the first compensation component 2 and the beam entry surface 3a of the second compensation component 3 adjacent to it are flat.The beam exit surface 2b of the first compensation component 2 is aligned at a wedge angle α to the beam entry surface 2a of the first compensation component 2, and the beam entry surface 3a of the second compensation component 3 is aligned at an identical wedge angle α to the beam exit surface 3b of the second compensation component 3. For the phase Φ. s of the S-polarized portion of the laser beam 4 when passing through the depolarization compensator 1 of Fig.5 as a function of the lateral position Y is: ^ ^ = ^ ^^^ ^ ^^^ ( ^ ) + ^ ^ ^ ^^^^^ (^), where npol denotes the extraordinary refractive index of the material of the first compensation component 2 and n0 denotes the ordinary refractive index of the material of the second compensation component 3. L pol(y) denotes the distance traveled by the laser beam 4 in the first compensation component 2, Lunpol(y) denotes the distance traveled by the laser beam 4 in the second compensation component 3. The distance L pol (y) or L unpol (y) varies in the Y-direction, as can be seen from the dashed and dash-dotted arrows in Fig.5. If L C denotes half the thickness of the depolarization compensator 1, then: ^ ^^^ ( ^ ) = ^ ^ − ^ tan(^) For the phase Φ p of the P-polarized portion of the laser beam 4 results in: ^ ^ = 2^ ^ ^ ^ , since the P-polarized portion of the laser beam 4 "sees" the ordinary refractive index n0 during propagation through the first compensation component 2 and the second compensation component 3. For the phase difference Φ s-p = Φ s – Φ p surrendered: ^ ^^^ = ^ ^ (^^^^ − ^ ^ ) + ^ tan(^) (^ ^ − ^ ^^^ ). The phase shift Φs-p generated by the depolarization compensator 1 shown in Fig. 5 therefore depends linearly on the Y coordinate. Accordingly, the depolarization compensator 1 of Fig. 5 can substantially compensate for the linear depolarization of the laser beam 4 in the Y direction shown in Fig. 4a, as shown in Fig. 4b, which shows the beam cross-section of the laser beam 4 after passing through the depolarization compensator 1 of Fig. 5. For the example shown, numerical optimization resulted in an optimal wedge angle α of 0.358° and an optimal center thickness LC of the depolarization compensator 1 of 4.8 mm, with sapphire used as the birefringent material for the two compensation components 2, 3. Fig.6a,b show a depolarization compensator 1 in which the first compensation component 2 and the second compensation component 3 are designed to vary their respective wedge angle α1, α2, wherein Fig.Fig. 6a shows the two compensation components 2, 3 with a first wedge angle α1, and Fig. 6b shows the two compensation components 2, 3 with a second wedge angle α2. As can be seen in Fig. 6a,b, the first compensation component 2 has two sub-elements 14, 15 in the form of two cylindrical lenses, which can be rotated relative to one another to vary the wedge angle α1, α2 of the first compensation component 2. The second compensation component 3 has two sub-elements 16, 17 in the form of two cylindrical lenses, which can be rotated relative to one another to vary the wedge angle α1, α2 of the second compensation component 3.The first sub-element 14 of the first compensation component 2 has a flat beam entry surface 14a and a cylindrical, convexly curved beam exit surface 14b, which is adjacent to a cylindrical, concavely curved beam entry surface 15a of the second sub-element 15 of the first compensation component 2, which has the same radius of curvature as the beam exit surface 14b of the first sub-element 14. A flat beam exit surface 15b of the second sub-element 15 of the first compensation component 2 is adjacent to a flat beam entry surface 16a of the first sub-element 16 of the second compensation component 3. Accordingly, a concavely curved beam exit surface 16b of the first sub-element 16 of the second compensation component 3 adjoins a convexly curved beam entry surface 17a of the second sub-element 17 of the second compensation component 3, which has the same radius of curvature as the beam exit surface 16b of the first sub-element 16.A beam exit surface 17b of the second sub-element 17 of the second compensation component 3 is flat and aligned parallel to the beam entry surface 14a of the first sub-element 14 of the first compensation component 3. The first sub-element 14 of the first compensation component 2 and the second sub-element 17 of the second compensation component 3 are linearly displaceable relative to one another in the Y direction; the second sub-element 15 of the first compensation component 2 and the first sub-element 16 of the second compensation component 3 can be rotated relative to one another along the adjacent cylindrical beam exit and beam entry surfaces 14b, 15a and 16b, 17a, respectively, in order to adjust the respective wedge angle α1, α2. As shown in Fig.6a,b, the two sub-elements 14, 15 of the first compensation component 2 are formed from the same birefringent material, while the two sub-elements 16, 17 of the second compensation component 3 are formed from the same non-birefringent material. In the event that a homogeneous, non-location-dependent phase shift component remains in the linear compensation of the depolarization described in connection with Fig.5 and Fig.6a,b, which is determined by the thickness of the depolarization compensator 1 or the first compensation component 2, this can either be compensated by existing methods for compensating homogeneous phase shifts (e.g. Berek compensator), or an "adjustable thickness" element made of the birefringent material is realized by two oppositely oriented displaceable wedges (similar to a Babinet compensator), analogous to the "adjustable wedge" method described above.

Claims

1. Depolarization compensator (1) for at least partially compensating a location-dependent variation in the polarization (6) of a laser beam (4), comprising: a first compensation component (2) made of a birefringent material, which has a preferably planar beam entry surface (2a) for the entry of the laser beam (4) and a beam exit surface (2b) for the exit of the laser beam (4), wherein a thickness (D(X, Y)) of the first compensation component (2) between the beam entry surface (2a) and the beam exit surface (2b) varies depending on the location in order to at least partially compensate for the location-dependent variation in the polarization (6) of the laser beam (4), and a second compensation component (3) which at least partially, in particular completely, compensates for a light-refracting effect of the first compensation component (2) on the laser beam (4). compensated and has no birefringent effect on the laser beam (4). 2.Depolarization compensator according to claim 1, wherein the second compensation component (3) has a beam entrance surface (3a) adjacent to the beam exit surface (2b) of the first compensation component (2), as well as a planar beam exit surface (3b) that is preferably aligned parallel to the beam entrance surface (2a) of the first compensation component (2).

3. Depolarization compensator according to claim 2, wherein the second compensation component (3) is formed from a birefringent material whose optical axis (8) is aligned perpendicular to the beam entrance surface (2a) of the first compensation component (2), wherein the birefringent material of the second compensation component (3) preferably matches a birefringent material of the first compensation component (2).

4. Depolarization compensator according to claim 2, wherein the second compensation component (3) is formed from a non-birefringent material.

5. The depolarization compensator according to claim 4, wherein the first compensation component (2) is made of quartz and the second compensation component (3) is made of quartz glass.

6. The depolarization compensator according to claim 4, wherein the first compensation component (2) is made of sapphire and the second compensation component (3) is made of a non-birefringent material containing aluminum oxide.

7. The depolarization compensator according to any one of claims 2 to 6, wherein the beam exit surface (2b) of the first compensation component (2) deviates from a planar geometry and the beam entry surface (3a) of the second compensation component (3) has a geometry complementary to the beam exit surface (2b) of the first compensation component (3). 8.Depolarization compensator according to one of claims 2 to 6, wherein the beam exit surface (2b) of the first compensation component (2) is flat and oriented at a wedge angle (α; α1, α2) to the beam entry surface (2a) of the first compensation component (2), and wherein the beam entry surface (3a) of the second compensation component (3) is flat and oriented at a wedge angle (α; α1, α2) to the beam exit surface (3b) of the second compensation component (3).

9. Depolarization compensator according to claim 8, wherein the first compensation component (2) and the second compensation component (3) are configured to vary the respective wedge angle (α1, α2). 10.Depolarization compensator according to claim 9, wherein the first compensation component (2) has two partial elements (14, 15) which can be rotated relative to one another to vary the wedge angle (α1, α2) of the first compensation component (2), and wherein the second compensation component (3) has two partial elements (16, 17) which can be rotated relative to one another to vary the wedge angle (α1, α2) of the second compensation component (3). Depolarization compensator according to claim 10, wherein the first sub-element (14) of the first compensation component (2) has a cylindrical beam exit surface (14b) adjacent to a cylindrical beam entry surface (15a) of the second sub-element (15) of the first compensation component (2), and wherein the first sub-element (16) of the second compensation component (3) has a cylindrical beam exit surface (16b) adjacent to a cylindrical beam entry surface (17a) of the second sub-element (17) of the second compensation component (3). Depolarization compensator according to claim 2, wherein a liquid refractive index adaptation medium (9) is introduced between the beam entry surface (3a) and the beam exit surface (3b) of the second compensation component (3).Depolarization compensator according to one of the preceding claims, in which the second compensation component (3) has a medium with a refractive index (n2) that deviates by no more than 0.2, preferably by no more than 0.1, from the mean value of the ordinary refractive index (n1o) and the extraordinary refractive index (n1ao) of the first compensation component (2). An optical system (10) comprising: a beam source (11) for generating a laser beam (4), at least one optical component (5) that generates a location-dependent variation in the polarization (6) of the laser beam (4), and at least one depolarization compensator (1) according to one of the preceding claims for at least partially compensating for the location-dependent variation in the polarization (6) of the laser beam (4) generated by the at least one optical component (5).Optical system according to claim 14, wherein the optical component forms a mirror (5) onto which the laser beam (4) impinges convergently or divergently.

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