Arrangement of optical elements for the formation of structural patterns

Abstract

Arrangement of optical elements for the formation of structural patterns using direct laser interference, in which a laser beam source emits a laser beam (1) and in which a triangular or quadrilateral prism (2) formed with two triangular subprisms (2.1 and 2.2) connected to each other at a planar surface by means of a layer (2.3) having properties that divide the laser beam (1) into sub-beams (1.1 and 1.2), and the two sub-prisms (2.1 and 2.2) have two equal acute angles α, each arranged between the planar surface at which the two sub-prisms (2.1 and 2.2) are connected to each other and an outwardly facing surface, wherein the laser beam (1) is directed towards one of the outwardly facing surfaces of one of the two partial prisms (2.1 and 2.2), so that the laser beam (1) hits the layer (2.3) inside the prism (2) and there by reflection of part of the radiation at the layer (2.3) and transmission of another part of the radiation through the layer (2.3) a division of the laser beam (1) into two partial beams (1.1 and 1.2) takes place, wherein the resulting partial rays (1.1 and 1.2) strike one of the outward-facing surfaces of one of the two partial prisms (2.1 and 2.2), are reflected at these surfaces, and the partial rays (1.1 and 1.2) emerge from the surfaces of the two partial prisms (2.1 and 2.2) which are arranged opposite the acute angle α. from there onto at least one optical element (5) which is designed such that the partial beams (1.1 and 1.2) are aligned in such a way that they interfere with each other in an area of ​​a component in which a structural pattern is to be formed, characterized in that the distance (D E) between the point of impact of the laser beam (1) on the prism (2) and the tip of the two sub-prisms (2.1 and 2.2) is variable, wherein Between the prism (2) formed with the two partial prisms (2.1 and 2.2) and the at least one optical element (5) a further prism (7) is arranged, which is connected to two further partial prisms (7.1 and 7.2) by means of a layer (7.3) and is aligned at an angle greater than 0° with respect to the prism (2) on which the at least one laser beam (1) first strikes, or the partial beams (1.1 and 1.2) emerging from the prism (2) are rotatable about a common axis and are directed directly or by means of a reflecting element (3) onto an outwardly facing surface of the further prism (2), so that the two partial beams (1.1 and 1.2) can each be divided into further partial beams (8.1 to 8.4) and used to form a structural pattern.

Description

[0001] The invention relates to an arrangement of optical elements for forming structural patterns, which can be configured with a compact optical setup for generating periodically pronounced intensities of laser radiation for structuring surfaces. The method used to define the intensity distribution is laser interference structuring.

[0002] Interference patterns have previously been obtained by using one or more beam splitters. The resulting partial beams are deflected at different angles by reflective or refractive optical elements onto the same area and superimposed there to form a structured pattern. This approach requires precise alignment of the reflective elements, ensuring that the partial beams have identical path lengths, and sufficient space for the setup.

[0003] Since the structure period A of the interference pattern depends on the enclosed angle θ of the partial beams, a change in this always requires a new, complex adjustment.

[0004] Another well-known method is to achieve beam splitting using diffractive optical elements (DOEs). The partial beams are deflected after passing through a diffractive optical element at an angle that depends on the DOE and the laser wavelength. The subsequent beam paths of partial beams from a split laser beam can be influenced by two focusing optics. For this purpose, the beam splitting point at the DOE is imaged onto the surface. A first lens with its focus on the DOE parallelizes the partial beams, which are then focused and superimposed by a second focusing optic towards a component. Due to the fixed focal lengths of optical lenses, at least one of the two focusing optics must be replaced with a different focal length to change the structure period A.

[0005] On the other hand, a variable change in the structure period A can be achieved by parallelizing the partial beams obtained through a DOE using a prism or an optical configuration of equivalent effect, as described in EP 2 976 176 B1. The axial distance between the prism and the DOE determines the local incidence of the partial beams on the prism and thus the effective distance between them. A downstream optical lens focuses and superimposes the parallel partial beams, whereby the angle and the resulting period can be adjusted by the distance between the partial beams due to the fixed focal length. For precise parallelization of the partial beams, the combination of DOE and prism with respect to the emission angle (DOE) and base angle (prism) for the corresponding wavelength is crucial. Furthermore, two precisely manufactured optical elements are required.

[0006] A super-resolution microscope using optical prisms is known from WO 2017 / 085 618 A1.

[0007] CN 202 948 246 U relates to lithography with a suitably designed optical device.

[0008] An optical arrangement for laser interference structuring is described in DE 10 2017 205 889 A1 and DE 10 2018 200 036 B3.

[0009] The scientific publication “Fabrication of superhydrophobic and ice-repellent surfaces on pure aluminium using single- and multiscaled periodic textures” by S. Milles et al. describes an optical setup with a diffractive optical element (DOE) for splitting an incident laser beam into two partial beams and a prism for parallelizing these partial beams, whereby the partial beams are subsequently refracted towards each other by a lens in the direction of a workpiece to be structured, so that they interfere with each other there.

[0010] DE 595 211 A describes the construction of a Köster prism consisting of two partial prisms for splitting an incident light beam into two partial beams which are aligned parallel to each other after exiting the prism, in order to improve the evaluation of an interference structure within the framework of an interference measurement technique.

[0011] It is therefore an object of the invention to provide an extremely compact structure for direct laser interference structuring (DLIP), while maintaining the flexibility of a variable structure period, with a reduction of the required optical elements and the required build volume.

[0012] According to the invention, this problem is solved with an optical arrangement having the features of claim 1. Advantageous embodiments and further developments can be realized with features specified in dependent claims.

[0013] According to the invention, a triangular or quadrilateral prism is formed with two triangular sub-prisms which are connected to each other on a planar surface by means of a layer having properties that divide at least one laser beam into sub-beams.

[0014] The two partial prisms have two equal acute angles α, each located between the planar surface where the two partial prisms are connected and an outward-facing surface.

[0015] The at least one laser beam is directed onto one of the outward-facing surfaces of one of the two sub-prisms. The at least one laser beam should preferably be directed perpendicularly onto the outward-facing surface of one of the two sub-prisms in order to minimize reflection losses.

[0016] The splitting of the laser beam into partial beams occurs at the layer to which the two partial prisms are connected, through reflection of part of the radiation at the layer and transmission of another part of the radiation through the layer. For this to happen, at least one laser beam strikes the layer.

[0017] The resulting partial beams then strike an outward-facing surface of one of the two partial prisms. From there, the reflected partial beams strike at least one optical element, which is designed to align the partial beams, in particular to focus them, so that they interfere with each other in a region of a component where a structural pattern is to be formed. The function of the at least one optical element can also be the surface of the two individual prisms from which the partial beams emerge.Accordingly, the received partial beams are influenced, either alone or in addition to the at least one optical element, by selecting the angle of incidence of the at least one laser beam on the outward-facing surface of one of the two partial prisms, as well as the angles α, β and γ of the two partial prisms, taking into account the wavelength of the at least one laser beam and the optical refractive index of the two partial prisms, such that the partial beams are aligned in such a way that they interfere with each other in an area of ​​a component in which a structural pattern is to be formed.

[0018] The acute angle α is located on the side of the two partial prisms that is opposite the surface where the partial rays emerge from the prism.

[0019] The at least one laser beam should be modified before striking one of the outward-facing surfaces of one of the two partial prisms, preferably by adjusting its polarization to match the beam splitter layer, in order to ensure a uniform distribution of energy into the partial beams. The spatial intensity distribution of the laser beam directed at the respective surface of one of the two partial prisms can be freely adjusted. The at least one laser beam can, for example, be shaped into a line using a cylindrical lens system or converted into a flat-top profile using suitable optical elements.

[0020] Starting from a typically radially Gaussian distribution of laser radiation energy across the cross-sectional area of ​​a laser beam, a linear beam cross-section can be chosen as an example of a more uniform spatial intensity distribution. Linear polarization is not strictly necessary. However, the polarization should be selected to match the respective beam splitter layer or adjusted beforehand according to the physical principles of the layer used for beam splitting, so that the energy ratio of the partial beams can be as similar as possible. For example, a linear polarization of the laser beam emitted by the respective beam source can be rotated into the correct spatial orientation, or the laser beam can be switched from linear to circularly polarized radiation.

[0021] The two sub-prisms can each be identical. Advantageously, two surfaces on each of the two sub-prisms can be aligned at an angle α of 30° and two surfaces at an angle β of 60° to each other. This results in the two connected sub-prisms forming an isosceles triangle where the three surfaces have the same length between the edges with the acute angles.

[0022] Advantageously, the point of impact of at least one laser beam on the surface of one of the two partial prisms can be changed by a relative movement of the focal spot of the laser beam on the outward-facing surface and the position of the prism, so that the distance D EThe point of impact relative to the tips of the two partial prisms of the resulting prism, and thus the angle of incidence θ of the partial beams into the region where they interfere, can be changed. The at least one laser beam can be moved accordingly by a translational shift. However, it should always strike the surface perpendicularly to avoid radiation losses due to reflection.

[0023] The at least one laser beam can be directed onto the outward-facing surface by means of a suitable reflective optical element.

[0024] Between the prism formed by the two partial prisms and the at least one optical element, a further prism can be arranged. This further prism consists of two more partial prisms bonded together by a layer. A further laser beam is directed perpendicularly onto a surface of this further prism, and the further laser beam is transmitted and reflected by the further prism in such a way that further partial beams from this further prism are directed onto the at least one optical element. The further prism can be designed analogously to the prism already described and can therefore be used in the same way. The points of impact of the laser beams on the respective partial prisms of the second prism should be at different distances D. E exhibit.

[0025] Alternatively to this embodiment, a further prism can also be arranged offset from the prism positioned upstream in the beam path of the partial beams, such that the respective planes of the two prisms, which are spanned by the surface of the material-bonded connection of the partial prisms, are arranged parallel to each other at a lateral distance. The edges of the two prisms that are first arranged on the partial prisms in the beam path of the partial beams can be aligned parallel to each other and arranged in two different parallel planes.

[0026] This makes it possible to ensure that the partial rays emerging from the prism positioned in the beam path of the partial rays in front of the further prism have different entry point distances D. Eimpinge upon an outward-facing surface. This can also be enhanced by having the partial prisms of this prism shaped differently, and in particular by orienting the surfaces from which the partial rays emerge from the partial prisms at different angles, so that these partial rays emerge from these partial prisms at different angles due to optical refraction. Alternatively, or in addition, the partial rays emerging from the first partial prism in the beam path can also be refracted at different entry point distances D using reflective elements. E allow the impact to occur on the outward-facing surfaces of the further prism.

[0027] According to the invention, a further prism is arranged between the prism formed by the two partial prisms and the at least one optical element. This further prism comprises two further partial prisms connected to each other by a layer and is aligned at an angle greater than 0°, preferably in the range of 30° to 120°, with respect to the prism onto which the at least one laser beam first strikes. The two partial beams emerging from the prism can then be directed by means of a reflecting element onto an outwardly facing surface of the further prism, so that the two partial beams can each be divided into further partial beams and used to form a structural pattern. The relative rotation of the partial beams with respect to the plane in which the layer is arranged between the two partial prisms of the further prism can be adjusted as described above by rotating the prism or by a suitable additional optical element (e.g., a reflector).Dove prism) which rotates the partial beams around a common axis.

[0028] The at least one optical element can be at least one focusing optical lens, at least one focusing reflective element, or at this position in the beam path of the partial beams as a device designed for spatial modulation of the partial beams.

[0029] In the beam path of the partial beams, a Dove prism or another beam-splitting optical element, designed to achieve higher-order interference of the partial beams, can be positioned between the prism or a further prism. A Dove prism can rotate around an axis aligned parallel to the optical axis of the partial beams to change the orientation of structural elements that form a structural pattern. Since these are essentially periodically occurring structural elements formed by direct laser interference, different structural patterns can also be created by having the same or a changing structural period A.

[0030] A metal and / or a dielectric material of suitable thickness can be used as the material for the beam-splitting layer. Suitable layer materials include, for example, silver, aluminum, gold, chromium, and combinations of thin layers with different refractive indices, such as SiO2, TiO2, Al2O3, Ta2O5, MgF2, LaF3, and AlF3. The symmetrical, metallurgical bond between the two partial prisms can be achieved, for example, with optical adhesives based on acrylate or epoxy resins.

[0031] The layer, or a layer system consisting of several layers, can be applied to the surface of one of the two partial prisms using a vacuum coating process with a suitable layer thickness. If the layer is made of or contains a metal, it can also be used as a solder layer for the metallurgical bonding of the two partial prisms.

[0032] The at least one laser beam is split into two parallel partial beams. The optical system is an equilateral prism, consisting of two individual prisms that may be cemented together via a beam splitter layer. Such prisms are also known as "Koester prisms." The laser beam is directed onto the prism by a laser beam source itself or by deflection so that it strikes the prism face orthogonally. This allows for purely transmitting behavior into the prism, and the individual laser beam is split into two partial beams at the beam splitter layer. Both partial beams reflected from the prism faces can be reflected by total internal reflection at the interface between the two media and thus parallelized. Depending on the respective local point of impact of the laser beam on the prism face, the distance D can be SThe relationship between the parallel partial beams can be influenced. A downstream optical imaging system, such as a fixed-focus lens, a lens system (spherical, aspherical, cylindrical, etc.), or a galvanometer scanner, can be used to influence the superposition and focusing of the partial beams to create interference. To adjust the beam splitting of the partial beams in a 50:50 energy ratio, and thus contribute to precise interference from both partial beams, the polarization of the laser radiation can be aligned beforehand by at least one polarizing optical element or system.

[0033] The variable entry point distance D E the point of impact of the laser beam relative to the tip of the prism and the associated changes in the beam distance D SThe relationship of the partial beams to each other and the beam angle θ enclosed between the partial beams at constant focal length f and wavelength λ lead to an influence on the structure period A of the structure pattern to be formed according to:

[0034] Beam spacing: DS=4DE3

[0035] Included angle: Θ=2 tan−1(DS2f)

[0036] Period: Λ=λ2 sin(Θ2)

[0037] The invention will be explained in more detail below by way of example.

[0038] This shows: Fig. 1 a schematic representation of an example of an arrangement in which a laser beam hits a prism at different positions, which is split into two partial beams and Fig. 2 a schematic representation of another example of an arrangement and Fig. 3 a schematic representation of an example according to the invention with two prisms.

[0039] At the in Fig. In the arrangement shown in Figure 1, a laser beam 1 strikes a surface perpendicularly onto one of the two partial prisms 2.1 and 2.2, which is enclosed by the two acute angles of prism 2.1. The two partial prisms 2.1 and 2.2 form a prism 2, in which they are bonded together by means of layer 2.3, in this example a dielectric coating and acrylate resin adhesive.

[0040] In prism 2 shown, all surfaces have the same length and are aligned at equal angles of 60° to each other. Accordingly, the two partial prisms 2.1 and 2.2 each have an angle γ of 90° at the surface where they are joined. The other two angles of the two identical triangular prisms 2.1 and 2.2 are 30° for angle α and 60° for angle β.

[0041] In the form not shown, the angles can also have other values, so that angles β and γ can also be smaller or larger. The angles of the two partial prisms 2.1 and 2.2 should be chosen taking into account the wavelength of the laser beam 1 and the refractive index of the two partial prisms 2.1 and 2.2 in order to form a structural pattern by utilizing the interference of the partial beams.

[0042] As in Fig. As can be seen, the laser beam 1, which is directed perpendicularly into the prism 2, strikes layer 2.3 in the prism 2. At layer 2.3, part of the laser radiation is reflected, and the other part of the laser radiation passes through layer 2.3 due to its transparency, so that the laser beam 1 is split into two partial beams 1.1 and 1.2.

[0043] Both partial beams 1.1 and 1.2 strike a surface of prism 2 enclosed by an angle of 30° and an angle of 60° of one of the two partial prisms 2.1 and 2.2, respectively. There, they are reflected so that they are aligned parallel to each other and exit prism 2 at a distance from one another. In this example, partial beams 1.1 and 1.2 encounter a focusing optical lens, as an example of an optical element 5. By means of the focusing lens, they are deflected towards an area where partial beams 1.1 and 1.2 interfere with each other, thus forming a corresponding structural pattern.

[0044] Large-scale structural patterns can be formed through the relative movement of a component and its arrangement. These structural patterns do not necessarily have to be formed on the surface of a component. They can also be created beneath the surface of a component by modifying the component material.

[0045] Out of Fig. Figure 1 also shows how the structural period A of a structural pattern can be influenced in a very simple way. The laser beam 1 strikes the surface of one of the two subprisms 2.1 and 2.2 at different positions, such that the impact positions have different distances D. E between the point of impact and the tip of prism 2 or prism 2.1.

[0046] It is evident that with a larger distance D E the distance D Sthe partial rays 1.1 and 1.2, and therefore the angle of incidence θ, is larger than with a shorter distance D. E , as he was chosen in the right-hand illustration.

[0047] On the left in the Fig. In the representation shown in Figure 1, the structural period A of the resulting structural pattern is smaller than in a structural pattern as can be formed according to the representation on the right.

[0048] At the in Fig. In the embodiment shown in Figure 2, the laser beam 1 is directed by means of a beam-deflectoring element 3, which can be an element reflecting the laser beam 1, onto an outward-facing surface of one of the two partial prisms 2.1 and linearly polarized by means of a polarization optic 6 such that the subsequently obtained partial beams 1.1 and 1.2 each have the same energies before striking the surface of the prism 2.1. The polarization optic 6 can be at least a wave plate, a polarization filter, a reflecting element, or at least one polarizer, such as a wire grid, a polymer, a Glan-Thomson element, a Glan-Taylor element, or a Brewster window.

[0049] Otherwise, the two subprisms 2.1 and 2.2, and thus prism 2, are analogous to the one in Fig. 1 shown trained.

[0050] The partial beams 1.1 and 1.2 exiting prism 2 strike at least one further optical element 4, which is arranged in the beam path of partial beams 1.1 and 1.2, for modification. This can cause a rotation of partial beams 1.1 and 1.2 about a common axis of rotation, which is possible with a Dove prism as element 4. As a result of the rotation, the spatial orientation of the angle θ enclosed by the partial beams can be changed during imaging by element 5, so that structural patterns with linear, parallel structural elements with different axis orientations can be obtained.

[0051] An optical element 4 can also be another prism 2, which can be designed like the prism 2 already described. Different distances D should be maintained between the subsequent prism in the beam path. Efor the respective partial rays exiting the first prism.

[0052] Instead of another prism 2, at least one other optical element 4 can be used, with which interference of the partial beams 1.1 and 1.2 of a higher order can be achieved.

[0053] Furthermore, in the Fig. In the example shown in Figure 2, at least one optical element 5 is arranged behind element 4 in the beam path of partial beams 1.1 and 1.2. This optical element 5 can deflect partial beams 1.1 and 1.2 so that they interfere with each other in a region where a structural pattern is to be formed. This can be achieved with a focusing optical lens (aspherical, spherical, cylindrical), at least one electroplating scanner, focusing and reflecting optical elements, or a device designed for the spatial modulation of partial beams 1.2 and 2.2.

[0054] Fig. Figure 3 shows an embodiment with two prisms 2 arranged successively between the laser radiation source and the structural pattern to be formed, each of which is also designed as a “Köster prism”.

[0055] Here too, a laser beam 1 is directed, analogous to the examples described above, onto an outward-facing surface of one of the two partial prisms 2.1 and then split into two partial beams 1.1 and 1.2 by means of the layer 2.3. The emerging partial beams 1.1 and 1.2 are directed by means of a reflective optical element 3 onto an outward-facing surface of the further prism 2, preferably perpendicular to this surface.

[0056] The second prism 2 also has a layer 2.3 designed to split the partial beams. After analogous reflection, four partial beams 1.1 to 1.4 are now directed by an optical element 5 to form a structural pattern using the interference of the four partial beams 1.1 to 1.4.

[0057] In this example, the two prisms 2 are aligned at an angle of 90° to each other.

Claims

[1] Arrangement of optical elements for the formation of structural patterns by means of direct laser interference, in which a laser beam source emits a laser beam (1) and in which a triangular or quadrilateral prism (2) formed with two triangular subprisms (2.1 and 2.2) connected to each other at a planar surface by means of a layer (2.3) having properties that divide the laser beam (1) into sub-beams (1.1 and 1.2), and the two sub-prisms (2.1 and 2.2) have two equal acute angles α, each arranged between the planar surface at which the two sub-prisms (2.1 and 2.2) are connected to each other and an outwardly facing surface, wherein the laser beam (1) is directed towards one of the outwardly facing surfaces of one of the two partial prisms (2.1 and 2.2), so that the laser beam (1) hits the layer (2.3) inside the prism (2) and there by reflection of part of the radiation at the layer (2.3) and transmission of another part of the radiation through the layer (2.3) a division of the laser beam (1) into two partial beams (1.1 and 1.2) takes place, wherein the resulting partial rays (1.1 and 1.2) strike one of the outward-facing surfaces of one of the two partial prisms (2.1 and 2.2), are reflected at these surfaces, and the partial rays (1.1 and 1.2) emerge from the surfaces of the two partial prisms (2.1 and 2.2) which are arranged opposite the acute angle α. from there onto at least one optical element (5) which is designed such that the partial beams (1.1 and 1.2) are aligned so that they interfere with each other in an area of ​​a component in which a structural pattern is to be formed, characterized by , that the distance (D E) between the point of impact of the laser beam (1) on the prism (2) and the tip of the two sub-prisms (2.1 and 2.2) is variable, wherein Between the prism (2) formed with the two partial prisms (2.1 and 2.2) and the at least one optical element (5) a further prism (7) is arranged, which is connected to two further partial prisms (7.1 and 7.2) by means of a layer (7.3) and is aligned at an angle greater than 0° with respect to the prism (2) on which the at least one laser beam (1) first strikes, or the partial beams (1.1 and 1.2) emerging from the prism (2) are rotatable about a common axis and are directed directly or by means of a reflecting element (3) onto an outwardly facing surface of the further prism (2), so that the two partial beams (1.1 and 1.2) can each be divided into further partial beams (8.1 to 8.4) and used to form a structural pattern. [2] Arrangement according to any one of the preceding claims, characterized by , that the at least one optical element (5) is at least one focusing optical lens, at least one focusing reflecting element or a device designed for spatial modulation of the partial beams (1.1 and 1.2) at this position in the beam path of the partial beams (1.1 and 1.2). [3] Arrangement according to one of claims 1 to 2, characterized by , that in the beam path of the partial beams (1.1 and 1.2) between the prism (2) and the further prism (7) a Dove prism (4) or a further beam-splitting optical element, which is designed to achieve higher-order interference of the partial beams (1.1 and 1.2), is arranged. [4] An arrangement of optical elements for the formation of structural patterns by means of direct laser interference, wherein a laser beam source emits a laser beam (1) and wherein a triangular or quadrilateral prism (2) is formed with two triangular subprisms (2.1 and 2.2) connected to each other at a planar surface by means of a layer (2.3) having properties that divide the laser beam (1) into sub-beams (1.1 and 1.2), and wherein the two sub-prisms (2.1 and 2.2) have two equal acute angles α, each arranged between the planar surface at which the two sub-prisms (2.1 and 2.2) are connected to each other and an outwardly facing surface, wherein the laser beam (1) is directed towards one of the outwardly facing surfaces of one of the two sub-prisms (2.1 and 2.2) such that the laser beam (1) strikes the layer (2.3) inside the prism (2) and is directed there by reflection of part of the radiation at the layer (2.3) and transmission of another part of the radiation through the layer (2.3) a division of the laser beam (1) into two partial beams (1.1 and 1.2) takes place, wherein the resulting partial rays (1.1 and 1.2) strike one of the outward-facing surfaces of one of the two partial prisms (2.1 and 2.2), are reflected at these surfaces, and the partial rays (1.1 and 1.2) emerge from the surfaces of the two partial prisms (2.1 and 2.2) which are arranged opposite the acute angle α. wherein the angle of incidence of the laser beam (1) on the outward-facing surface of one of the two partial prisms (2.1 or 2.2) and the angles α, β and γ of the two partial prisms (2.1 and 2.2) are selected taking into account the wavelength of the laser beam (1) and the refractive index of the two partial prisms (2.1 and 2.2) such that the partial beams (1.1 and 1.2) are aligned so that they interfere with each other in an area of ​​a component in which a structural pattern is to be formed, characterized by , that the distance (D E ) between the point of impact of the laser beam (1) on the prism (2) and the tip of the two partial prisms (2.1 and 2.2) is variable. [5] Arrangement according to any one of the preceding claims, characterized by, that the laser beam (1) is directed perpendicularly to the outward-facing surface of one of the two partial prisms (2.1 and 2.2), which has been modified in its polarization or its spatial intensity distribution before striking one of the outward-facing surfaces of one of the two partial prisms (2.1 and 2.2). [6] Arrangement according to the preceding claims, characterized by , that the two subprisms (2.1 and 2.2) are each identically formed. [7] Arrangement according to the preceding claims, characterized by , that on the two partial prisms (2.1 and 2.2) two surfaces are aligned at an angle of 30° and two surfaces are aligned at an angle of 60° to each other. [8] Arrangement according to any one of the preceding claims, characterized by, that the layer (2.3) is formed from silver, aluminium, gold, chromium or as a combination of several superimposed thin layers with different optical refractive indices, such as in particular SiO2, TiO2, Al2O3, Ta2O5, MgF2, LaF3 and AlF3.

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