METHOD FOR PRODUCING A CONTINUOUS DIFFRACTIVE OPTICAL ELEMENT AND CONTINUOUS DIFFRACTIVE OPTICAL ELEMENT

Description

[0001] The invention relates to a method for producing a continuous diffractive optical element for beam shaping a laser beam with a first wavelength.

[0002] Furthermore, the invention relates to the continuous diffractive optical element.

[0003] Diffractive optical elements, abbreviated DOEs, enable the beam shaping of a laser beam, i.e., the targeted modification of the intensity distribution of the laser beam profile perpendicular to the propagation direction. For beam shaping, the DOE is inserted into the laser beam. Different optical path lengths at the DOE cause phase modulation of the laser beam, resulting in interference patterns. The intensity of the laser beam is spatially modulated through constructive and destructive superposition. This allows the typically Gaussian intensity profile of the laser beam to be specifically modified, for example, into a doughnut-shaped intensity profile.

[0004] DOEs known in the prior art are glass or plastic substrates onto which microstructures are applied using laser lithography and / or photolithography, as well as various wet and dry chemical etching processes. For example, a blazed grating can be applied as a microstructure, i.e., an optical grating in which the individual grating lines have a triangular, sawtooth cross-section, thus maximizing the diffraction efficiency for a specific diffraction order.

[0005] However, the production of DOEs with high diffraction efficiency is very complex. To achieve high diffraction efficiency, the microstructures must have a height profile that is as continuous as possible, which in turn entails very high technical manufacturing effort. Due to the manufacturing process using a lithographic etching process, the height profile is usually approximated by a stair-step profile with a discrete number of steps. Repeating the lithographic etching process increases the number of steps and thus allows the height profile to be better approximated. Such DOEs with multiple steps are called multilevel DOEs. However, due to the process, the number of steps is limited to around 16, since after about four repetitions of the etching process, the manufacturing-related inaccuracies increase and no higher diffraction efficiency can be achieved.Thus, multilevel DOEs are physically limited in their diffraction efficiency to about 98%.

[0006] Furthermore, quasi-continuous DOEs are known from the state of the art. These are produced using a maskless lithography process employing two-photon polymerization. For this purpose, a polymer applied to a substrate is typically directly patterned using laser radiation. These quasi-continuous DOEs exhibit a diffraction efficiency equivalent to more than 50 steps. However, the use of such quasi-continuous DOEs is limited because they are unsuitable for use with high-power lasers due to the low thermal damage threshold of the polymers.

[0007] The publication Smjuk, AY, Lawandy, NM: Direct laser writing of diffractive optics in glass. Opt. Lett., 1997, Vol. 22, No. 13, pp. 1030-1032 describes a method for producing diffractive optical elements from semiconductor-doped glass using a low-power laser and a thermal expansion process.

[0008] The publication Shore, BW et al.: Design of high-efficiency dielectric reflection gratings, J. Opt. Soc. Am. A, 1997, Vol. 14, No. 5, pp. 1124-1136, describes examples of designs for purely dielectric reflection gratings, commenting on the relationship between transmission gratings and reflection gratings. Furthermore, examples of high-efficiency (95%) gratings fabricated using hafnium and silica multilayers are described.

[0009] The document US 2009 / 0 273 772 A1 describes a light-reflecting mask comprising a reflective layer provided on a substrate and reflecting light, an absorbing layer provided on the reflective layer and absorbing light, a device pattern formed in a first region of the absorbing layer, and a reflectance measurement pattern formed in a second region of the absorbing layer.

[0010] The publication DE 10 2018 200 029 A1 describes a method for creating surface structures on a laser-transparent workpiece, in particular made of glass or plastic. One or more ultrashort pulse laser pulses are focused through the workpiece surface into the laser-transparent workpiece to melt a modification inside the workpiece by heating the focal volume. The pulse parameters of the one or more ultrashort pulse laser pulses and the depth of the laser focus in the workpiece are selected such that the uppermost part of the molten modification just touches the workpiece surface, and the thermal material expansion of the molten modification causes the workpiece surface to curve outward into a convex surface structure.

[0011] DE 101 58 859 A1 describes a method and device for analyzing and monitoring the light intensity distribution across the cross-section of a laser beam. A laser beam is deflected using a laser mirror, and local thermal expansion of the laser mirror is interferometrically recorded based on the different light intensities incident on the laser mirror.

[0012] Accordingly, it is the object of the invention to provide a DOE and a method for producing such a DOE, wherein the DOE has a high diffraction efficiency and is suitable for use in beam shaping of high-power lasers.

[0013] This problem is solved by the features of the independent patent claims. Preferred developments can be found in the subclaims.

[0014] The invention relates to a method for producing a diffractive optical element for beam shaping a laser beam with a first wavelength of at least 100 nm, comprising the steps: Providing a laser mirror, wherein the laser mirror has a layered structure comprising a substrate, a dielectric layer and optionally an absorption layer, wherein the dielectric layer is adjacent to the substrate or wherein the absorption layer is located between the substrate and the dielectric layer, generating a plurality of bulges of the dielectric layer by treating the laser mirror with a series of focused heating laser beams having a second wavelength, wherein the plurality of bulges have a height perpendicular to the dielectric layer, and wherein at least one bulge has a height of at least half the first wavelength.

[0015] The core of the invention lies in the fact that a continuous DOE is produced by generating bulges in a laser mirror by means of targeted heating. The method makes it possible to produce a DOE with a very high diffraction efficiency, which, when converted to the manufacturing accuracy of quasi-continuous DOEs, can correspond to a number of steps of more than 2500. The provided laser mirror and the DOE produced by the method comprise the substrate, the optional absorption layer, and the dielectric layer, all of which are preferably parallel to one another, wherein the dielectric layer lies against the substrate or, if the absorption layer is present, the absorption layer is arranged between the substrate and the dielectric layer. In particular, it is provided that a bottom side of the dielectric layer or a bottom side of the absorption layer borders an upper side of the substrate.It is further preferred that, when the absorption layer is present, an upper side of the absorption layer borders a lower side of the dielectric layer. In other words, the upper side of the dielectric layer corresponds to the upper side of the DOE or the upper side of the laser mirror.

[0016] In contrast to the methods known in the prior art, in which microstructuring is created by adding and / or removing material, thus producing the DOE, the present method provides that the microstructuring takes place by creating the plurality of bulges in the dielectric layer of the laser mirror. Thus, after treating the laser mirror with the series of focused heating laser beams at the second wavelength, it is not necessary to apply additional material layers and / or remove material layers in order to produce the DOE with very high diffraction efficiency. In other words, by providing the laser mirror, which itself comprises the substrate and the dielectric layer, and subsequently treating the laser mirror with the focused heating laser beams, the DOE is produced directly and thus without adding and / or removing material.In contrast to the prior art methods, the method according to the invention eliminates a time-consuming lithographic and / or coating process.

[0017] During the process, bulges are created in the dielectric layer, which lead to a microstructuring of the surface of the dielectric layer. This means, in particular, that the surface of the dielectric layer is not flat after the process, but has local regions that have a different height than before the process. The height of the bulges refers to an extension of the bulge perpendicular to the dielectric layer relative to an area around the local region. Preferably, the bulges are rotationally symmetrical, with the axis of rotation of the bulges lying perpendicular to the absorption layer. For example, the bulges are circular when projected onto a plane parallel to the absorption layer. Further preferably, in the case of rotationally symmetrical bulges, the half-width of the rotationally symmetrical bulges is no greater than 6 µm.

[0018] The height of the protrusions influences the use of the DOE for beam shaping the laser beam with the first wavelength, since beam shaping with the DOE is only possible if at least one protrusion has a height of at least half the first wavelength. The process allows the production of DOEs suitable for beam shaping a laser beam with a first wavelength of at least 100 nm. In other words, this means that at least one protrusion has a height greater than 50 nm.

[0019] The method has the advantage that the bulges can be created in a targeted manner through treatment with the heating laser beam. Both the location of the bulge relative to the plane parallel to the dielectric layer and the height of the bulge can be controlled. Using the method, bulges of varying heights can be created, with a height resolution of at least 0.5 nm. In particular, the height resolution is at least 0.1 nm. In other words, the method allows the creation of very finely graded height profiles as structuring. The method thus enables the production of continuous DOEs with a very high diffraction efficiency, which, when converted to the manufacturing accuracy of quasi-continuous DOEs, corresponds to a number of steps of more than 2500.

[0020] With regard to the height of the bulges and the half-width of rotationally symmetric bulges, it is preferred that a topographic image of the dielectric layer be created using a Mirau interferometer to determine these parameters. The Mirau interferometer comprises an illumination source with a short coherence length, preferably a green LED with a central wavelength of λ = 530 nm, a camera for image acquisition (DCC1545M, Thorlabs), and a Mirau microscope objective (CF Plan 20x DI, Nikon). A front facet of the Mirau microscope objective preferably acts as a beam splitter, with half of the LED light being reflected onto a reference mirror inside the Mirau microscope objective. This half of the LED light preferably passes through the reference arm of the interferometer. The LED light transmitted through the front facet is preferentially reflected back by the dielectric layer of the DOE and superimposed with the LED light of the reference arm.To ensure that an interference pattern is visible on the camera despite the short coherence length of the LED light, the path lengths of both interferometer arms are preferably chosen to be identical except for a few wavetrains (z 0 ≈ z Ref .). A topography of the dielectric layer of the DOE is preferably created by slowly displacing the DOE by about one micrometer using a piezo crystal (time scale ~ seconds) while the interference images are recorded with the camera. Subsequently, a brightness sequence is created from the stored images (typically 50 interference images) for each pixel of the camera (x = 1 to 1000; y = 1 to 1000). By fitting a function ~ sin(ωt + ϕ 0 ) with angular frequency ω, a phase ϕ 0 (x, y) can be determined for each pixel.This is preferably done for all pixels (x, y) of the camera, so that a phase map can be created in which the adjusted phase can be entered at any location in the camera image. Within the phase map, phase jumps can occur between neighboring pixels, since the adjusted phase is limited to the range 0 to 2π. By expanding the range beyond 0 to 2π, the phase discontinuities can be resolved (phase unwrapping). The phases ϕ 0 (x, y) in the phase map can then be converted into heights h(x, y) using the following formula: . h x y = λ 2 π φ 0 x y 2

[0021] In this way, a topographical map of the dielectric layer of the DOE is created. To determine the height of the bulges, a region of the topographical map that has no bulges is preferably used to set a reference height to zero. The data points of the topographical map of the dielectric layer are then preferably used to fit a function corresponding to the height profile of the bulge to regions with a bulge. In the case of rotationally symmetric bulges, a two-dimensional Gaussian function is preferably fitted. Further preferably, the height of the bulge and the half-width of the bulge are determined using the fitted function.

[0022] According to a preferred development, the provided laser mirror is suitable for reflecting laser light of the first wavelength. The laser mirror, which is provided in the first step of the method and essentially represents the blank of the DOE, is preferably a highly reflective laser mirror for reflecting the laser beam with the first wavelength. Highly reflective in this context means that the laser mirror has a reflectance for the first wavelength of more than 99%. In particular, it is provided that a pre-coated dielectric laser mirror can be used. It is further preferably provided that the laser mirror is suitable for reflecting laser light with a first wavelength greater than 100 nm.With regard to the use of the DOE for beam shaping of high-power lasers, it is further preferred that the laser mirror be suitable for reflecting laser light with a power greater than 100 W / cm. This also means that the laser mirror preferably has a high damage threshold of at least 0.5 J / cm2 at a pulse duration of a few picoseconds, i.e., 1-30 ps.

[0023] In this context, according to a preferred development of the invention, the provided laser mirror and the diffractive optical element have a transmission of T ≤ 10 -2 < for the first wavelength. In particular, the method provides that the bulges are generated in such a way that the dielectric layer retains its highly reflective properties. This also means that the dielectric layer is not destroyed by treating the laser mirror with the series of focused heating laser beams at the second wavelength. Preferably, the transmission is determined by measuring the power of the laser at the first wavelength, upstream and downstream of the diffractive optical element, using a power-calibrated photodiode.

[0024] According to a preferred development of the invention, the absorption layer is made of silicon or gold and / or the substrate is made of glass, CaF 2 , MgF 2 or sapphire and / or the dielectric layer is made of SiO 2 , Ta 2 O 5 , TiO 2 , HfO 2 , Al 2 O 3 , MgF 2 , LaF 3 , and / or ZrO 2 . The silicon in the absorption layer is particularly preferably amorphous silicon. It is further preferably provided that the substrate is made of quartz glass in order to enable the beam shaping of a high-power laser beam. It is further preferably provided that the dielectric layer comprises several layers of different materials, wherein a first material has a high refractive index with respect to the first wavelength and a second material has a low refractive index with respect to the first wavelength. It is further preferably provided that the dielectric layer consists of alternating layers of the high refractive index material and the low refractive index material.In particular, it is provided that the dielectric layer is designed such that the laser beam with the first wavelength is reflected by the dielectric layer. In other words, this means that the reflectance of the dielectric layer for the first wavelength is greater than 99%, both for the provided laser mirror and for the DOE produced by the method.

[0025] In connection with the creation of the bulges, the method provides that, when treating the laser mirror with a series of focused heating laser beams, the heat input of the heating laser beam into a volume of the dielectric layer or into a volume of the absorption layer of the laser mirror is at least 30 kJ / cm3. This makes it particularly easy to create bulges with a height of more than 50 nm.

[0026] In connection with the height of the bulges, the method provides that the height of the bulges can be influenced by a heat input of the heating laser beam per volume into the absorption layer and / or into the dielectric layer. According to a preferred development, when treating the laser mirror with a series of focused heating laser beams, the second wavelength, a power of the heating laser beam, a focusing of the heating laser beam, a heating duration, the absorption layer of the laser mirror, the dielectric layer of the laser mirror, and / or a layer thickness of the absorption layer are selected such that at least one bulge has a height of at least half the first wavelength.

[0027] The heat input of the heating laser beam can be controlled by various parameters. Preferably, the wavelength of the heating laser beam, i.e., the second wavelength, is matched to an absorption spectrum of the absorption layer and / or the dielectric layer. This ensures that a portion of the heating laser beam is absorbed by the absorption layer and / or the dielectric layer.

[0028] In this context, according to a preferred development, when treating the laser mirror with a series of focused heating laser beams, the second wavelength is between 200 and 700 nm and the absorption layer of the laser mirror is made of silicon, or that the second wavelength is between 200 and 2000 nm and the absorption layer of the laser mirror is made of gold, and / or the second wavelength is between 100 and 2000 nm. By means of these combinations of wavelength of the heating laser beam and absorption layer, the bulges with a height of more than 50 nm can be produced very reliably. If the absorption layer is omitted, a UV laser with a second wavelength in the range of 100 to 400 nm is preferably used to treat the laser mirror with the series of focused heating laser beams.

[0029] As already mentioned, the heat input of the heating laser beam per volume into the absorption layer and / or dielectric layer can be influenced by the power of the heating laser beam. In this regard, according to a preferred embodiment, when treating the laser mirror with a series of focused heating laser beams, the power of the heating laser beam is at least 10 mW. Particularly preferably, the power is between 10 and 1000 mW.

[0030] It is further preferred that, when treating the laser mirror with a series of focused heating laser beams, the power of the heating laser beam is varied. This allows the height of the protrusions to be easily varied. In particular, by varying the power of the heating laser beam, it is particularly easy to create protrusions of varying heights and to achieve height resolutions of at least 0.5 nm.

[0031] With regard to the creation of the bulges, according to a preferred development, when treating the laser mirror with a series of focused heating laser beams, the heating laser beam strikes the absorption layer and / or dielectric layer perpendicularly through the substrate of the laser mirror. In other words, this means that a rear side of the laser mirror is irradiated with the series of focused heating laser beams. It is further preferred to match the substrate of the laser mirror and the wavelength of the heating laser beam, i.e., the second wavelength, to one another so that the heating laser beam is essentially not absorbed by the substrate.

[0032] As already mentioned, the heat input of the heating laser beam per volume into the absorption layer and / or dielectric layer can be controlled by various parameters. In this context, it is provided that when treating the laser mirror with a series of focused heating laser beams, the heating laser beam is focused onto the absorption layer and / or dielectric layer with a half-width of at most 5 µm. It is particularly preferably provided that the heating laser beam has a half-width of 1.8 µm or less. This makes it possible to generate heat input locally in the absorption layer and / or dielectric layer, so that the bulges in the dielectric layer are also local. In particular, it is possible to use this focusing to create the plurality of bulges, which are preferably spaced apart by 3 µm or less than 3 µm.The distance between two bulges preferably refers to the peak-to-peak distance of two bulges in the plane perpendicular to the absorption layer.

[0033] According to a further preferred development of the invention, when treating the laser mirror with a series of focused heating laser beams, the heating duration of the heating laser beam is between 1 µs and 1 ms. This has proven particularly suitable for generating bulges with heights greater than 50 nm. A shorter heating duration can lead to destruction of the provided laser mirror. A longer heating duration can lead to destruction of the dielectric layer, which could, in particular, lead to the loss of the highly reflective properties of the DOE.

[0034] Not only the previously mentioned parameters—namely, the second wavelength, the power of the heating laser beam, the focusing of the heating laser beam, the heating duration, the optional absorption layer, and the dielectric layer of the laser mirror—influence the height of the bulges, but also the thickness of the absorption layer. The thickness of the absorption layer refers to the extent of the absorption layer perpendicular to its length. According to a preferred embodiment, the thickness of the absorption layer is greater than 30 nm. Particularly preferably, the thickness of the absorption layer is greater than 40 nm. This makes it particularly easy to generate a high heat input per unit volume into the absorption layer, so that the bulges have a height of more than 50 nm.

[0035] According to a preferred development of the invention, when the laser mirror is treated with a series of focused heating laser beams, a plurality of bulges arranged in a grid-like manner are created on an area of ​​at least 0.25 cm². The grid-like arrangement of the bulges preferably relates to the plane parallel to the absorption layer, wherein the grid-like arrangement of the bulges can be oblique-angled, rectangular, centered-rectangular, hexagonal, or square. Deviations from the translational symmetry of the grid-like arrangement are also possible. More preferably, the plurality of bulges are present on the area of ​​at least 0.25 cm², wherein the bulges are preferably spaced apart by 3 µm or less than 3 µm. The distance between two bulges preferably relates to the peak-to-peak distance of two bulges in the plane perpendicular to the absorption layer.

[0036] In principle, it is possible for the heating laser beam to be deflected in two dimensions during treatment while the laser mirror is not displaced. In this way, the grid-like arrangement of bulges is reliably achieved. However, according to a preferred development of the method, when the laser mirror is treated with a series of focused heating laser beams, the laser mirror is displaced during treatment along a displacement direction perpendicular to the heating laser beam, and the heating laser beam is deflected perpendicular to the displacement direction during treatment. This enables the particularly rapid and precise production of a DOE whose dielectric layer has a plurality of bulges arranged in a grid-like manner over an area of ​​at least 0.25 cm².Furthermore, this provides the necessary precision to ensure that the majority of bulges are preferably spaced at a distance of 3 µm or less than 3 µm from each other. By moving the laser mirror during treatment along the direction of movement perpendicular to the heating laser beam and deflecting the heating laser beam perpendicular to the direction of movement during treatment, the majority of bulges can be created on an area of ​​0.25 cm² in less than 60 minutes.

[0037] Further advantages and technical features of the method for producing a DOE will become apparent to the person skilled in the art from the description of the DOE, the description of the device for carrying out the method and the description of the exemplary embodiment.

[0038] Furthermore, the invention relates to a diffractive optical element, DOE, for beam shaping of a laser beam with a first wavelength of at least 100 nm, wherein the diffractive optical element has a layered structure comprising a substrate, an absorption layer and a dielectric layer, wherein the absorption layer is located between the substrate and the dielectric layer and is designed to absorb a heating laser beam by generating heat input into the absorption layer, wherein the dielectric layer has a plurality of bulges, wherein the bulges have a height perpendicular to the dielectric layer, and wherein at least one bulge has a height of more than half the first wavelength.

[0039] The core of the invention thus lies in the fact that the different optical path lengths required for beam shaping of the laser beam are implemented at the DOE by bulges in the dielectric layer. This makes it possible to achieve a very high diffraction efficiency, which, when converted to the manufacturing accuracy of quasi-continuous DOEs, can correspond to a number of steps of more than 2500. It is preferably provided that the height of at least one bulge of the DOE is greater than 50 nm. Thus, the DOE is preferably suitable for beam shaping of laser beams with a first wavelength greater than 100 nm. Furthermore, it is preferably provided that the bulges are Gaussian with respect to a plane perpendicular to the dielectric layer and have a half-width of at least 2 µm. More preferably, the half-width is not greater than 6 µm.

[0040] In particular, the DOE is a reflective DOE. This means that the beam shaping of the laser beam with the first wavelength occurs by reflection of the laser beam from the diffractive optical element. In contrast to DOEs, where beam shaping occurs by transmission of the laser beam through the DOE (transmittive DOEs), this is preferably a DOE in which the laser beam is reflected. This has the advantage that, unlike transmissive DOEs, there are only minimal absorption losses, enabling high efficiency.

[0041] Furthermore, a device for carrying out the above method is described below, wherein the device comprises a heating laser for generating a heating laser beam with the second wavelength, a laser mirror positioning device for providing a laser mirror, a focusing device for focusing the heating laser beam onto the laser mirror, a deflection device and a controller, wherein the laser mirror positioning device is designed to displace the laser mirror in a displacement direction, the deflection device is designed to deflect the heating laser beam perpendicular to the displacement direction and the controller is designed to control the heating laser, the deflection device and the laser mirror positioning device. In particular, it is provided that the deflection device is designed to deflect the heating laser beam in one direction.In other words, the deflection device is preferably a 1-dimensional deflection device. These have the advantage of being able to adjust very large deflection angles with high precision and speed. In order to be able to create a grid-like arrangement of bulges using the device, the laser mirror positioning device is designed such that the laser mirror can be displaced in the displacement direction. The laser mirror positioning device is preferably a computer-controlled positioning stage with nanometer resolution.

[0042] For further advantages and technical features of the method for producing the diffractive optical element, the diffractive optical element and the device for carrying out the method, reference is made to the figures and the further description.

[0043] The invention is explained below by way of example with reference to the drawings using a preferred embodiment.

[0044] In the drawing show Fig. 1 shows a schematic representation of the method for producing a diffractive optical element and a sectional representation of the diffractive optical element according to a preferred embodiment of the invention, and Fig. 2 shows a schematic device for carrying out the method for producing the diffractive optical element.

[0045] Figure 1shows a schematic representation of the method for producing a diffractive optical element 10, abbreviated DOE 10, as well as a sectional view of the DOE 10 according to a preferred embodiment of the invention. The method provides two steps, wherein in a first step, a laser mirror 12 is provided as a blank for the DOE 10. The laser mirror 12 is suitable for reflecting a laser beam with a first wavelength λ 1 . In the present case, it is a laser mirror 12 for reflecting laser beams with a wavelength λ 1 of 532 nm, i.e. laser light in the green wavelength range. Furthermore, the laser mirror 12 is suitable for a high-power laser and has a high damage threshold. In this exemplary embodiment, the laser mirror 12 has a layered structure comprising a substrate 14, an absorption layer 16 and a dielectric layer 18.All three layers 14, 16, 18 are arranged parallel to one another, with the absorption layer 16 being arranged between the substrate 14 and the dielectric layer 18. The dielectric layer 18, in turn, comprises a plurality of material layers 20, 22 made of two different materials, which are layered alternately, wherein the first material has a high refractive index relative to the first wavelength λ 1 and the second material has a low refractive index relative to the first wavelength λ 1.

[0046] In a second step of the method for producing the DOE 10, bulges 24 of the dielectric layer 18 are created. In the preferred embodiment, these are rotationally symmetric bulges that are created at an interface between the dielectric layer 18 and the absorption layer 16. A rotation axis 26 of the rotationally symmetric bulges 24 is perpendicular to the absorption layer 16. As shown in Figure 1As can be seen, in the present case, the dielectric layer 18 does not lie on the absorption layer 16 at the bulge 24, but is detached from the absorption layer 16 at the bulge 24. At the bulge 24, there is a cavity 28 between the dielectric layer 18 and the absorption layer 16. When viewed in section through the bulge 24 along a plane perpendicular to the absorption layer 16, the bulge 24 is Gaussian with respect to this plane.

[0047] In the preferred embodiment, the substrate 14 of the laser mirror 12, and thus also of the DOE 10, is made of quartz glass, and the absorption layer 16 is made of amorphous silicon. Furthermore, the layer thickness 30 of the absorption layer is 40 nm. The layer thickness 30 refers to the extent of the absorption layer 28 perpendicular to its length.

[0048] The bulges 24 of the dielectric layer 18 are produced by irradiating the laser mirror 12 with a series of focused heating laser beams 38 (only in Figure 2 shown) is treated with a second wavelength λ 2 , wherein a heat input 54 of the heating laser beam 38 into a volume of the absorption layer 16 of the laser mirror 12 is greater than 30 kJ / cm 3 . In this way, at least one bulge 24 of the bulges 24 produced has a height 32 of at least half the first wavelength λ 1 . Thus, at least one bulge 24 has a height 32 of at least 266 nm.

[0049] Figure 2shows a schematic diagram of a device 34 for carrying out the method for producing the DOE 10. The device 34 comprises a heating laser 36 for generating the heating laser beam 38 with the second wavelength λ 2 , a focusing device 40 for focusing the heating laser beam 38, a laser mirror positioning device 42 and a controller 44. In the preferred embodiment, the second wavelength λ 2 of 532 nm is generated by the heating laser 36. Furthermore, the device 34 comprises a deflection device 46 for deflecting the heating laser beam 38. In the present case, this is implemented with a 1-dimensional galvo scanner 46, which is designed to deflect the heating laser beam 38 in one direction. Figure 2The direction of the heating laser beam 38 to the laser mirror 12 corresponds to the z-direction. The galvo scanner 46 allows the heating laser beam 38 to be deflected in the x-direction. Furthermore, the device 34 includes an acousto-optical modulator 52, which can change the intensity and thus the power of the heating laser beam 38.

[0050] In order to create a grid-like arrangement of bulges 24 of the dielectric layer 18 using the device, the laser mirror positioning device 42 is designed such that the laser mirror 12 is displaceable. Here, the displacement direction of the laser mirror 12 is the y-direction, i.e., perpendicular to the direction of the heating laser beam 38 and perpendicular to the deflection direction of the galvo scanner 46.

[0051] Furthermore, the controller is designed to control the heating laser 36, the acousto-optical modulator 52, the deflection device 46 and the laser mirror positioning device 42.

[0052] Furthermore, in the preferred embodiment, the focusing device 40 for focusing the heating laser beam 38 is implemented by a confocal microscope. The confocal microscope (f objective = 2 cm; f tube = f 2 = 30 cm) focuses the heating laser beam 38 onto the absorption layer 16.

[0053] The heating laser 36, the focusing device 40, the laser mirror positioning device 42, and the deflection device 46 are arranged relative to one another with the aid of a mirror 50 and lenses 48 such that the heating laser beam 38 strikes the absorption layer 16 of the laser mirror perpendicularly. The heating laser beam 38 strikes a rear side of the laser mirror 12 and penetrates through the substrate 14 to the absorption layer 16. The absorption of the heating laser beam 38 results in a local heat input 54 of at least 30 kJ / cm³ in the absorption layer 16, which leads to the formation of the bulge 24. List of reference symbols

[0054] 10 Diffractive optical element, DOE 12 Laser mirror 14 Substrate 16 Absorption layer 18 Dielectric layer 20 Material layer made of first material 22 Material layer made of second material 24 Bulge 26 Rotation axis 28 Cavity 30 Layer thickness of the absorption layer 32 Height of the bulge 34 Device 36 Heating laser 38 Heating laser beam 40 Focusing device, confocal microscope 42 Laser mirror positioning device, positioning stage 44 Controller 46 Deflection device, galvo scanner 48 Lens 50 Mirror 52 Acousto-optical modulator 54 Heat input

Claims

1. Method for producing a diffractive optical element (10) for beam shaping a laser beam having a first wavelength (λ1) of at least 100 nm, comprising the steps of: providing a laser mirror (12), wherein the laser mirror (12) has a layered structure comprising a substrate (14) and a dielectric layer (18), wherein the dielectric layer (18) abuts against the substrate (14), or wherein the laser mirror (12) has a layered structure comprising a substrate (14), a dielectric layer (18) and an absorption layer (16), wherein the absorption layer (16) is located between the substrate (14) and the dielectric layer (18); creating a plurality of bulges (24) of the dielectric layer (18) by treating the laser mirror (12) with a series of focused heating laser beams (38) having a second wavelength (λ2), wherein said plurality of bulges (24) have a height (32) perpendicular to said dielectric layer (18), and wherein at least one bulge has a height (32) of at least one-half of said first wavelength (λ1).

2. Method according to claim 1, wherein the provided laser mirror and the diffractive optical element have a transmission of T ≤ 10-2 for the first wavelength (λ1).

3. Method according to any one of the preceding claims, wherein the absorption layer (16) consists of silicon or gold, the substrate (14) consists of glass, CaF2, MgF2 or sapphire and / or the dielectric layer (18) consists of SiO2, Ta2O5, TiO2, HfO2, Al2O3, MgF2, LaF3, and / or ZrO2.

4. Method according to any one of the preceding claims, wherein when treating the laser mirror (12) with a series of focused heating laser beams (38), a heat input of the heating laser beam (38) into a volume of the dielectric layer (18) or into a volume of the absorption layer (16) of the laser mirror (12) is at least 30 kJ / cm3.

5. Method according to any one of the preceding claims, wherein when treating the laser mirror (12) with a series of focused heating laser beams (38), the second wavelength (λ2), a power of the heating laser beam (38), a focusing of the heating laser beam (38), a heating duration, the absorption layer (16) of the laser mirror (12), the dielectric layer (18) of the laser mirror (12) and / or a layer thickness (30) of the absorption layer (16) are selected in such a way that at least one bulge (26) has a height (32) of at least half of the first wavelength (λ1).

6. Method according to any one of the preceding claims, wherein the layer thickness (16) of the absorption layer (16) is greater than 30 nm.

7. Method according to any one of the preceding claims, wherein when treating the laser mirror (12) with a series of focused heating laser beams (38), the second wavelength (λ2) is between 200 and 700 nm and the absorption layer (16) of the laser mirror (12) is made of silicon, or the second wavelength (λ2) is between 200 and 2000 nm and the absorption layer (16) of the laser mirror (12) is made of gold and / or the second wavelength (λ2) is between 100 and 2000 nm.

8. Method according to any one of the preceding claims, wherein when treating the laser mirror (12) with a series of focused heating laser beams (38), a power of the heating laser beam (38) is at least 10 mW.

9. Method according to any one of the preceding claims, wherein when treating the laser mirror (12) with a series of focused heating laser beams (38), the heating laser beam (38) is focused with a full width at half maximum of at most 5 µm onto the absorption layer (16) and / or the dielectric layer (18).

10. Method according to any one of the preceding claims, wherein when treating the laser mirror (12) with a series of focused heating laser beams (38), the heating duration of the heating laser beam is between 1 µs and 1 ms.

11. Method according to any one of the preceding claims, wherein when treating the laser mirror (12) with a series of focused heating laser beams (38), the laser mirror (12) is displaced during the treatment along a displacement direction perpendicular to the heating laser beam (38) and the heating laser beam (38) is deflected during the treatment perpendicular to the displacement direction, or the laser mirror (12) is displaced during the treatment along two displacement directions orthogonal to one another, both perpendicular to the heating laser beam (38).

12. Diffractive optical element (10) for beam shaping of a laser beam having a first wavelength (λ1) of at least 100 nm, wherein the beam shaping of the laser beam with the first wavelength (λ1) is effected by reflection of the laser beam at the diffractive optical element (10), wherein the diffractive optical element (10) has a layer-like structure comprising a substrate (14), a dielectric layer (18) and an absorption layer (16), wherein the absorption layer (16) is located between the substrate (14) and the dielectric layer (18) and is designed to absorb a heating laser beam (38) while generating a heat input into the absorption layer (16), wherein the dielectric layer (18) has a plurality of bulges (24), wherein the bulges (24) have a height (32) perpendicular to the dielectric layer (18), and wherein at least one bulge (24) has a height (32) of at least half of the first wavelength (λ1) wherein the diffractive optical element (10) has been produced by a method according to any of the preceding claims.