Cell for frequency conversion of laser radiation and arrangement for frequency conversion of laser radiation

Abstract

The present invention relates to a cell for frequency conversion of laser radiation, comprising - two optical elements that transmit at least some of the data; - a crystal that converts a wavelength of the laser radiation passing through the crystal, wherein the crystal is arranged inside the cell and between the two optical elements; - wherein the two optical elements are arranged such that an unconverted laser beam passes through the first optical element upon entering the cell, enters the crystal at a first entry surface and exits the crystal at a first exit surface, wherein at least part of the laser beam exiting the crystal has a converted wavelength, wherein the converted and the unconverted laser beam enter the second optical element along a second entry surface, wherein at least part of the laser beam exits the second optical element along a second exit surface and - wherein at least one of the two optical elements is arranged such that the laser beam strikes the optical element at the Brewster angle.

Description

[0001] The invention relates to a cell for frequency conversion of laser radiation and an arrangement for frequency conversion of laser radiation.

[0002] Lasers are used in many areas, such as communications, materials processing, medicine, reading CDs or DVDs, measuring distances, speeds or surface properties, as well as in high-precision atomic clocks and research.

[0003] Depending on the application, a suitable choice of wavelength, power, line width and other parameters such as continuous beam / pulsed laser, pulse duration and repetition rate is important.

[0004] In addition to the selection of specific laser media, frequency conversion technology is also used to generate specific wavelengths. This involves modifying the original wavelength of the laser medium using nonlinear materials, such as special crystals. Frequency conversion typically involves frequency doubling (second harmonic generation SHG), frequency tripling (third harmonic generation THG), or sum frequency generation (SFG). Typical frequency-converting materials include lithium niobate, potassium dihydrogen phosphate, potassium titanyl phosphate, beta-barium borate, cesium lithium borate, and lithium triborate; thin films of diethylaminosulfur trifluoride; periodically poled polymers; special nanomaterials; or liquid crystals. High frequency conversion efficiency is essential to achieve high laser power at the desired converted wavelength.

[0005] However, materials that enable high frequency conversion efficiency require complex precautions to protect the surfaces from contamination. This is because any contamination absorbs large portions of the laser power, leading to heating and damage to the surfaces, thus significantly reducing the efficiency of frequency conversion. Furthermore, many of the currently preferred materials for frequency conversion are hygroscopic, meaning they attract and bind water. However, the accumulation of moisture or liquid particles on the crystals also leads to damage to the crystals. Therefore, additional protection against humidity is required.

[0006] Another problem is that the optical intensities typically encountered in frequency conversion, especially at short wavelengths in the UV range, often lead to damage and / or a short lifetime of optical coatings on or near the nonlinear medium.

[0007] Currently, the crystals are usually placed in a small, hermetically sealed cell with at least two windows. The cell is only slightly larger than the crystal itself. The cell's window surfaces are arranged (almost) perpendicular to the incident light and are anti-reflective coated.

[0008] An alternative is described in US9651849B2. Here, the crystals are arranged in an optical resonator, and the entire resonator is hermetically sealed from the external volume.

[0009] In both cases, only materials with very low outgassing rates may be used, otherwise deposits will form on the crystals, which in turn will result in absorption of the laser light and subsequent damage.

[0010] Furthermore, it is usually necessary to spatially separate the frequency-converted light from the unconverted light after frequency conversion, e.g., to maximize the power of the unconverted light in an optical resonator. Currently, this separation is often achieved using a separate dichroic mirror, especially when generating UV light. This requires space and thus enlarges the optical resonator.

[0011] This also results in the disadvantage that the mirror must reflect the fundamental wavelength and therefore transmits the converted wavelength. However, the converted wavelength is usually comparatively short, often in the blue or UV range, which can lead to strong absorption and thus undesirable power loss in the mirror's substrate material.

[0012] An alternative separation option is the use of a (Brewster) dichroic mirror between the crystal and a subsequent resonator mirror. The mirror reflects the converted wavelength and transmits the fundamental wavelength, for which it is Brewster-oriented. The Brewster angle minimizes reflection losses for the fundamental wavelength.

[0013] Furthermore, in a crystal with a Brewster output facet, the fundamental and converted wavelengths are refracted at slightly different angles, so that - as a further alternative - a large distance to the following folding mirror of the resonator ensures that the converted beam does not hit it and is thus decoupled.

[0014] In addition, it is possible to spatially separate converted and unconverted light using a prism outside a resonator.

[0015] These methods have several serious disadvantages. These include the fact that an anti-reflection coating on the input window and the crystal front facet can destroy the coating, thus significantly reducing performance and efficiency. Furthermore, the transmission of the fundamental wavelength is comparatively low compared to a Brewster window / facet, the resonator losses are comparatively high, and the power enhancement of the pump wavelength and the conversion efficiency are comparatively low.

[0016] For certain wavelengths, there are no sufficiently highly reflective coatings available, e.g., at wavelengths below 200 nm. Therefore, in these wavelength ranges, it is not possible to separate the converted from the unconverted light component.

[0017] Therefore, it is the object of the present invention to provide an improved cell for frequency conversion of laser radiation and an improved arrangement for frequency conversion of laser radiation.

[0018] The object is achieved with the cell for frequency conversion of laser radiation according to claim 1 and with the arrangement for frequency conversion of laser radiation according to claim 15.

[0019] The dependent claims specify advantageous embodiments.

[0020] The invention relates to a cell for frequency conversion of laser radiation, comprising two at least partially transmitting optical elements, a crystal which converts the wavelength of laser radiation passing through the crystal, wherein the crystal is arranged within the cell and between the two optical elements and wherein the two optical elements are arranged such that an unconverted laser beam passes the first optical element upon entering the cell, then enters the crystal at a first entrance surface and exits the crystal at a first exit surface, wherein the wavelength of at least a part of the laser beam exiting the crystal is converted,wherein the converted and the unconverted laser beam subsequently enters the second optical element along a second entrance surface and wherein at least a portion of the laser beam exits the second optical element along a second exit surface and wherein at least one of the at least two optical elements is arranged such that the laser beam impinges on the optical element at the Brewster angle.

[0021] The two at least partially transmitting optical elements are optical elements, which can be, for example, optical windows. The optical elements can be made of a different material than the material enclosing the cell.

[0022] The first optical element, which the laser beam passes through upon entering the cell, can be arranged at the Brewster angle to the laser beam entering the cell. Before entering the cell, the laser radiation can preferably be linearly polarized in a direction parallel to the plane of incidence into the first optical element. This minimizes losses due to reflection upon entering the cell, thereby increasing the efficiency of the laser system.

[0023] The crystal converts the wavelength of the laser beam. For example, the crystal causes frequency doubling (SHG), frequency tripling (THG), or generates a sum frequency (SFG). The crystal has an entrance surface at which the laser beam enters the crystal after passing through the first optical element. The entrance surface can be arranged at the Brewster angle to the incoming unconverted laser beam. The crystal has an exit surface at which the laser beam exits the crystal after passing through the crystal. The second optical element can be made of the same material as the first optical element, but this is not mandatory. The second optical element has an entrance surface that the laser beam hits after leaving the crystal.

[0024] With this frequency conversion cell, no coating is required, which could otherwise be quickly destroyed, resulting in a sudden drop in performance. Furthermore, the device offers the advantage of higher transmission than with all conventional coatings. This results in lower resonator losses, greater power enhancement at the pump wavelength, and higher conversion efficiency.

[0025] The cell can preferably be gas-filled. The cell can comprise additional optical elements, in particular optical windows, prisms, beam splitters, lenses, semi-transparent mirrors, and / or other elements.

[0026] The cell can preferably be a maximum of three times the length of the crystal and preferably a maximum of five times the diameter of the crystal. The length of the crystal represents the longest propagation direction of the crystal, and the diameter represents the propagation direction of the crystal orthogonal to the length.

[0027] The laser beam, both the converted and unconverted portions, can be provided to exit the cell at the exit surface of the second element. In this case, it can be advantageous if the entrance surface of the second element is oriented at the Brewster angle to the unconverted laser beam. This minimizes reflection losses.

[0028] Optionally, the converted wavelength portion is separated from the unconverted wavelength portion by an additional optical element located outside the cell. This separation can be achieved, for example, by a prism, a beam splitter, an optical element with a wavelength-dependent reflective coating, or similar optical elements. Separation using a prism, particularly a Brewster prism, is advantageous. This minimizes reflection losses of the unconverted laser beam, thereby increasing the efficiency of the laser system.

[0029] Optionally, the additional optical element can be a prism. The different refraction of the converted and unconverted wavelengths at the prism separates the laser beams. This also reduces the number of optical elements in the beam path and thus transmission losses. This increases the potential for power enhancement and thus the efficiency of the frequency conversion.

[0030] It is possible for the cell to have only one optical element, and for the laser beam to exit the cell through the first optical element instead of the second element. In this case, the cell may comprise an optical element through which the laser beam enters the cell and then into the frequency-converting crystal. Optionally, the cell may further comprise a deflecting mirror arranged such that the laser beam, after exiting the crystal, strikes the deflecting mirror and is reflected thereon. Optionally, the deflecting mirror may have a coating so that the converted portion of the laser beam is reflected by at least 95%, preferably by at least 98%. This increases the efficiency of the laser system.

[0031] According to one embodiment, it can be provided that the second optical element has a coating which is reflective for the wavelength of the converted laser radiation, so that the converted laser beam is reflected thereon, and wherein the second optical element transmits the unconverted laser beam, so that at least a portion of the unconverted laser beam exits the cell, and wherein the cell further comprises a third, at least partially transmitting, optical element which is arranged such that the converted laser beam reflected by the second optical element strikes the third optical element and then at least a portion of the laser beam exits the cell at the third optical element.

[0032] This separates the portion of the laser beam with the converted wavelength from the portion with the unconverted wavelength. Compared to the state of the art, no additional dichroic mirror is required between the cell and the next fold mirror of the resonator. This allows for more compact resonators. Additionally, there is one less object in the beam path, resulting in lower absorption losses for both the unconverted and converted wavelengths.

[0033] The third optical element is arranged within the cell and such that the portion of the laser beam reflected by the second optical element impinges on the third optical element such that the majority of the laser beam is transmitted by the third optical element and exits the cell. Preferably, the portion of the laser beam reflected by the second optical element impinges on the third optical element at a 90° angle or at the Brewster angle.

[0034] In particular, the portion of the laser beam that is reflected by the second optical element does not hit the frequency-converting crystal again.

[0035] Preferably, the second optical element is arranged such that the laser beam emerging from the crystal impinges on the second optical element at the Brewster angle. The Brewster angle is wavelength-dependent, so the Brewster angle arrangement applies to either the converted or the unconverted laser beam.

[0036] According to one embodiment, it can be provided that at least two optical elements are optical windows.

[0037] The first window may be made of a material that transmits at least 90% of the laser beam at the unconverted wavelength, preferably at least 99%, preferably at least 99.8%. The second window may be made of the same material as the first window.

[0038] According to one embodiment, the crystal can be arranged such that the first entrance surface of the crystal is positioned relative to the first optical element such that the laser beam impinges on the entrance surface of the crystal at the Brewster angle. This minimizes reflection losses and thus increases the efficiency of the laser system.

[0039] According to one embodiment, it can be provided that at least one optical element is a prism, wherein the prism is arranged such that the converted laser beam exits the prism at a different angle than the unconverted laser beam.

[0040] This separates the portion of the laser beam with the converted wavelength from the portion with the unconverted wavelength.

[0041] Preferably, at least the second optical element can be a prism. This splits the laser beam emerging from the crystal into the portion with the converted wavelength and the portion with the unconverted wavelength. The prism can preferably be arranged such that the unconverted and / or converted portion of the laser beam exits the cell at the second optical element. In the case where a portion of the laser beam continues to travel within the cell after exiting the prism, this portion preferably strikes a third optical element, where it then exits the cell.

[0042] Two optical elements can be prisms. Preferably, the two prisms can be arranged so that the laser beam, after passing through the frequency-converting crystal, first strikes the first prism and then the second prism, with the converted laser beam exiting the first prism at a different angle than the unconverted laser beam. This separates the converted laser beam from the unconverted laser beam. In particular, the first prism can be arranged so that the unconverted laser beam and / or the converted laser beam exit the cell after exiting the prism.

[0043] The second prism can preferably be arranged so that it is located exclusively in the beam path of the unconverted laser beam and deflects it so that, after exiting the prism, it propagates parallel to its propagation direction after leaving the crystal. This prevents large deflection angles in the optical resonator and thus astigmatism.

[0044] For example, the cell may comprise a frequency conversion crystal and one or two prisms. The unconverted laser beam enters the cell at the first optical element and then strikes the crystal. The outgoing converted laser beam and the outgoing unconverted laser beam exit the crystal at the exit surface opposite the entrance surface and strike the first prism. The angle at which the converted laser beam strikes the first prism 26 may preferably be the Brewster angle. The converted laser beam and the unconverted laser beam exit the first prism at different exit angles, so that the two laser beams are separated. The unconverted laser beam may then strike a second prism, so that its beam path is deflected.The beam path of the unconverted laser beam after exiting the second prism can preferably run parallel to the incoming unconverted laser beam before entering the cell. The converted laser beam does not impinge on the second prism. The first prism can be arranged inside, on, or outside the cell. If the first prism is arranged inside the cell, a second prism can be arranged inside, on, or outside the cell. The first prism can be arranged inside the cell in such a way that it replaces the function of the second optical element and additionally serves to separate the converted laser beam from the unconverted laser beam.

[0045] One prism and / or two prisms can be arranged as a Brewster prism. With a Brewster prism, the light strikes one or preferably several surfaces of the prism at the Brewster angle. Preferably, the prism can be arranged so that the laser beam strikes the entrance surface of the prism at the Brewster angle. This allows the portion of the laser beam polarized parallel to the plane of incidence to be transmitted into the prism with virtually no reflection losses. Preferably, the laser beam is polarized before entering the cell so that it is polarized parallel to the plane of incidence of the prism when it strikes the prism. This minimizes reflection losses. For this reason, no anti-reflective coating is required.Particularly advantageously, after entering the prism, the laser beam can also hit the exit surface of the prism at the Brewster angle, so that almost the entire laser beam is transmitted and leaves the prism without reflection losses.

[0046] The Brewster angle is wavelength-dependent, so the Brewster angle arrangement applies to either the converted or the unconverted laser beam. Preferably, the Brewster angle is designed for both the entrance to the prism and the exit from the prism for the unconverted wavelength. This reduces the round-trip losses in an optical resonator, increasing the power gain and thus also the efficiency.

[0047] According to one embodiment, it can be provided that at least one optical element is a beam splitter, wherein the beam splitter is arranged such that the converted laser beam exits the beam splitter at a different angle than the unconverted laser beam.

[0048] Preferably, the beam splitter is arranged so that the laser beam impinges on the beam splitter after exiting the frequency-converting crystal, and the converted and / or unconverted laser beam exits the cell after exiting the beam splitter. The beam splitter can be, for example, a polarizing beam splitter (PBS), a Glan Laser, a Glan Taylor, or another type of beam splitter.

[0049] According to one embodiment, the exit surface of the crystal can be arranged such that the laser beam emerging from the crystal hits the exit surface at the Brewster angle. This minimizes reflection losses and increases the efficiency of the laser system.

[0050] The Brewster angle is wavelength-dependent, so the Brewster angle arrangement applies to either the converted or the unconverted laser beam. In one example, the outgoing converted and unconverted laser beams can impinge on the second optical element after exiting the crystal. In this case, the second optical element can be arranged so that the outgoing unconverted laser beam impinges on the second optical element at the Brewster angle. This maximizes the enhancement and thus the converted power.

[0051] In another example, three optical elements can be arranged in the cell so that the outgoing laser beams exit the crystal and impinge on two different optical elements. In this case, the second optical element can be arranged so that the unconverted laser beam impinges on the second optical element at the Brewster angle, and the third optical element can be arranged so that the converted laser beam impinges on the third optical element at the Brewster angle. This achieves maximum enhancement and maximum outcoupling.

[0052] According to one embodiment, the exit surface of the crystal can have a coating that is highly reflective for the wavelength of the converted laser beam. The exit surface of the crystal is the surface opposite the entrance surface of the crystal. The unconverted laser beam exits the crystal at the exit surface. The coating reflects the converted laser beam back at the surface opposite the entrance surface of the crystal, so that it exits the crystal at the entrance surface. This separates the unconverted laser beam from the converted laser beam. The converted laser beam can exit the cell, for example, through the first optical element.

[0053] According to one embodiment, it can be provided that at least one of the optical elements of the cell is mechanically connected to a body of the cell by an indium seal, hard soldering, gluing and / or bonding.

[0054] This enables a permanent, reliable, and precise seal for the cell. Preferably, the seal can prevent moisture from entering the cell.

[0055] According to one embodiment, it can be provided that a drying agent is arranged in the cell.

[0056] This prevents moisture from forming in the cell, which could deposit on the optical surfaces of crystals, optical elements, prisms, etc. and then absorb laser light, thereby causing damage to the optical surfaces.

[0057] The drying agent can be, for example, silica gel or another drying agent in solid, liquid or gaseous form.

[0058] According to one embodiment, at least one heating and / or at least one cooling element can be arranged in and / or on the cell. This enables a temperature change within the cell. This makes it possible, for example, to prevent condensation on the optical surfaces, which could cause damage due to absorption of the laser radiation, by maintaining a suitable temperature.

[0059] According to one embodiment, at least one temperature sensor can be arranged in and / or on the cell. A combination with a heating and / or cooling element is particularly advantageous. This allows temperature control of the cell. A suitable temperature can prevent condensation on the optical surfaces, which could cause damage due to absorption of the laser radiation.

[0060] According to one embodiment, a connection unit for a purge gas can be arranged in the cell. This allows any deposits on the optical surfaces to be removed, thus preventing damage due to absorption of the laser radiation.

[0061] According to one embodiment, channels integrated into a wall of the cell for flowing a fluid for controlled temperature variation can be arranged. This allows for controlled, precise, and uniform temperature regulation of the cell. A suitable temperature can prevent condensation on the optical surfaces, which could cause damage due to absorption of the laser radiation.

[0062] A further aspect of the invention relates to an arrangement for frequency conversion of laser radiation, comprising a crystal which converts the wavelength of laser radiation passing through the crystal, a prism, wherein the prism is arranged such that a laser beam exits the crystal at an exit surface of the crystal and strikes the prism at an entrance surface, and that the converted laser beam exits the first prism at a different angle than the unconverted laser beam, so that the converted laser beam is separated from the unconverted laser beam.

[0063] By using the prism, the wavelengths of the converted and unconverted laser beam are separated. This also offers the advantage of allowing the separation of very short converted wavelengths from the unconverted laser beam, for which no reflective coatings exist according to the current state of the art, e.g., wavelengths below 200 nm.

[0064] The small number of components required also minimizes the space required.

[0065] The assembly can be enclosed in a cell. This allows the assembly to be arranged in a defined environment. The cell can be filled with gas. Preferably, the humidity in the cell can be low, in particular below 1%.

[0066] According to one embodiment, it can be provided that the arrangement further comprises a second prism, wherein the second prism is arranged such that the unconverted laser beam strikes the second prism after exiting the first prism.

[0067] The second prism allows the laser beam transmitted through both prisms to propagate parallel to the laser beam emerging from the crystal, thus allowing a resonator to be constructed with a smaller folding angle and thus smaller astigmatism than in the variant with one prism.

[0068] The assembly can be arranged in a cell. The cell can be filled with gas. The first prism and the crystal can be arranged in a cell, while the second prism can be arranged outside the cell.

[0069] According to one embodiment, the first prism can be arranged such that the unconverted laser beam enters the first prism at the Brewster angle. This minimizes reflection losses and increases the efficiency of the laser system.

[0070] According to one embodiment, the first prism can be designed such that the unconverted laser beam impinges on the exit surface of the prism at the Brewster angle, where the laser beam exits the prism. This further minimizes reflection losses and further increases the efficiency of the laser system.

[0071] The exemplary embodiments explained below represent a preferred embodiment of the invention. In the exemplary embodiments, the described components of the embodiments each represent individual, independently considered features of the invention, which also further develop the invention independently of one another and are thus also to be considered as components of the invention, either individually or in a combination other than that shown.

[0072] Furthermore, the described embodiments can also be supplemented by further features of the invention already described.

[0073] Exemplary embodiments of the invention are described below. Shown are: Fig. 1 a schematic representation of a prior art cell for frequency conversion of laser radiation; Fig. 2 a schematic representation of a prior art arrangement of an optical resonator with frequency conversion of laser radiation and separation of the laser radiation of the converted wavelength from the laser radiation of the unconverted wavelength; Fig. 3 a schematic representation of a cell according to the invention for frequency conversion of laser radiation; Fig. 4 a schematic representation of a cell according to the invention for frequency conversion of laser radiation and for separating the laser radiation of the converted wavelength from the laser radiation of the unconverted wavelength; Fig. 5 a schematic representation of a cell according to the invention for frequency conversion of laser radiation with a back reflection of the laser beam after the frequency conversion; Fig. 6 a schematic representation of a cell according to the invention for frequency conversion of laser radiation and for separating the laser radiation of the converted wavelength from the laser radiation of the unconverted wavelength; Fig. 7 is a schematic representation of an arrangement according to the invention for frequency conversion of laser radiation and for separating the laser radiation of the converted wavelength from the laser radiation of the unconverted wavelength; and Fig. 8 a schematic representation of a cell according to the invention for frequency conversion of laser radiation and prisms arranged behind it for separating the laser radiation of the converted wavelength from the laser radiation of the unconverted wavelength.

[0074] Fig. 1 shows a schematic representation of a prior art cell for frequency conversion of laser radiation. The cell has a window with a transmissive coating 44 on one surface for the entrance of the incoming unconverted laser beam 50. The cell further comprises a crystal 10 that converts the frequency of the incoming unconverted laser beam 50. The crystal 10 has an anti-reflective coating 42. The cell further has windows with a transmissive coating 44 on the surface opposite the entrance surface for the exit of the outgoing unconverted laser beam 52 and the outgoing converted laser beam 60.

[0075] Fig. Figure 2 shows a schematic representation of a prior art arrangement for frequency conversion of laser radiation and for separating the laser radiation of the converted wavelength from the laser radiation of the unconverted wavelength. The arrangement comprises a crystal 10 that converts the frequency of the incoming unconverted laser beam 50. The arrangement further comprises two mirrors 71, 72, a dichroic beam splitter 28, a mirror 78 movably mounted on fast timescales, and a partially reflecting mirror 76 for the unconverted laser beam. Fast timescales here mean that the position change of the mirror 78 occurs in less than 10 ms.The aforementioned elements are arranged such that an incoming unconverted laser beam 50 is transmitted through the partially reflective mirror for the unconverted laser beam, is reflected by the movably mounted mirror 78, and the laser beam strikes the first mirror 71, from where it is reflected onto the crystal 10, which converts the frequency of the laser beam. The outgoing unconverted laser beam 52 then strikes the second mirror 72, is reflected there, and thus in turn reaches the partially reflective mirror 76 for the unconverted laser beam, thus creating an optical resonator for the unconverted laser beam.The outgoing converted laser beam 60 is either reflected by an optional beam splitter 28, which may be a dichroic mirror and onto which the outgoing unconverted and the outgoing converted laser beam 52, 60 impinge after passing through the crystal 10, and is thereby separated from the unconverted laser beam 52, or the second mirror 72 is transmissive for the wavelength of the converted laser beam 60, so that the converted laser beam is thereby separated from the unconverted 52 laser beam.

[0076] Fig. 3 shows a schematic representation of a cell according to the invention for frequency conversion of laser radiation. The cell 1 comprises a crystal 10, which converts the frequency of an incoming unconverted laser beam 50, and two optical elements 20, 22, which are at least partially transmissive. The optical elements 20, 22 can be optical windows. The first optical element 20 is arranged such that the incoming unconverted laser beam 50 passes through the first optical element and then impinges on the crystal 10. The second optical element 22 is arranged such that the laser beam emerging from the crystal impinges on the second optical element 22 with an unconverted portion 52 and a converted portion 60.The second optical element 22 is configured such that at least one of the two components, the converted laser beam 60 and / or the unconverted laser beam 52, is transmitted at the second optical element 22 and then leaves the cell 1.

[0077] Advantageously, the cell may be filled with a gas 5. The gas may be air or another gas, advantageously with a humidity of less than 1%.

[0078] Advantageously, the first optical element 20 can be arranged such that the incoming unconverted laser beam 50 impinges on the first optical element 20 at the Brewster angle. The incoming unconverted laser beam 50 can have a p-polarization 30. This, in combination with the arrangement of the first optical element 20 at the Brewster angle, makes it possible to avoid reflection losses when the laser beam enters the cell 1 through the first optical element 20. Advantageously, the crystal 10 is arranged relative to the first optical element 20 such that the unconverted laser beam 50 impinges on the crystal 10 at the Brewster angle.

[0079] Advantageously, the second optical element 22 can be arranged relative to the crystal 10 such that the outgoing unconverted laser beam 52 strikes the second optical element 22 at the Brewster angle after exiting the crystal 10.

[0080] Fig. Figure 4 shows a schematic representation of a cell 1 according to the invention for frequency conversion of laser radiation and for separating the laser radiation of the converted wavelength from the laser radiation of the unconverted wavelength. The cell 1 comprises a crystal 10 and three optical elements 20, 22, 24, which are at least partially transmissive. The optical elements can be made of a different material than the outer wall of the cell 1. The statements regarding Fig. 3 with respect to the gas 5, the incoming unconverted laser beam 50, the p-polarization 30 of the incoming unconverted laser beam 50, the first optical element 20, and the crystal 10 apply analogously. The second optical element 22 is transmissive for the outgoing unconverted laser beam 52, so that the outgoing unconverted laser beam 52 then leaves the cell 1. The second optical element 22 is reflective for the outgoing converted laser beam 60, so that the outgoing converted laser beam 60 is reflected and then impinges on the third optical element 24, at which the outgoing converted laser beam 60 is transmitted and leaves the cell 1. These properties of the second optical element 22 can be realized by means of a reflective coating 40 for the wavelength of the outgoing converted laser beam 60.Alternatively, the second optical element 22 can be a beam splitter or a prism that allows the transmission of the outgoing converted laser beam 52 and simultaneously the reflection of the outgoing converted laser beam 60. If the incoming unconverted laser beam 50 has a p-polarization 30, the outgoing converted laser beam 35 has an s-polarization, particularly when using crystal types whose frequency doubling corresponds to the frequency doubling type (ooe) referred to in the technical literature as Type 1, e.g., for the crystal materials beta-barium borate, cesium lithium borate, and lithium triborate. If the third optical element 24 is a window at a Brewster angle of 35, the latter Brewster angle lies in a plane perpendicular to the Brewster angles 20, 22.

[0081] Fig. 5 shows a schematic representation of a cell 1 according to the invention for frequency conversion of laser radiation and for separating the laser radiation of the converted wavelength from the laser radiation of the unconverted wavelength. The cell 1 comprises a crystal 10 and three optical elements 20, 22, 24, which are at least partially transmissive. The crystal 10 has a coating 61 on the exit surface, which is reflective for the outgoing converted laser beam 60, but not reflective for the outgoing unconverted laser beam 52. This separates the converted laser beam 60 from the unconverted laser beam 52. The outgoing unconverted laser beam 52 leaves the crystal 10 at the exit surface and then strikes the second optical element 22, where the unconverted laser beam 52 leaves the cell 1.After being reflected by the coating 61 at the exit surface of the crystal 10, the outgoing converted laser beam 60 strikes the third optical element 24, which is transmissive to the outgoing converted laser beam 60, so that it exits the cell 1 there. The optical elements 20, 22, 24 can be optical windows. While the incoming unconverted laser beam 50 has a p-polarization 30, the outgoing converted laser beam 35 has an s-polarization for many practically relevant crystal materials.

[0082] Fig. 6 shows a cell 1 according to the invention for frequency conversion of laser radiation with a back reflection of the laser radiation after frequency conversion. The cell 1 comprises a crystal 10 and an optical element 20, which is at least partially transmissive, as well as a mirror 79 within the cell 1. The incoming unconverted laser beam 50 enters the cell 1 through the first optical element 20 and strikes the crystal 10, which at least partially converts the frequency of the laser beam. The converted and unconverted laser beams exiting the crystal then strike the mirror 79 within the cell 1, so that both laser beams are reflected back and propagate through the crystal and subsequently leave the cell 1 through the first optical element 20.If the incoming unconverted laser beam 50 has a p-polarization 30, the outgoing converted laser beam 60 has an s-polarization 35 when it exits the cell 1, particularly when using crystal types whose frequency doubling corresponds to the frequency doubling type (ooe) referred to in the technical literature as type 1, e.g. for the crystal materials beta-barium borate, cesium lithium borate and lithium triborate.

[0083] Fig. Figure 7 shows a schematic representation of an arrangement 2 according to the invention for frequency conversion of laser radiation and for separating the laser radiation of the converted wavelength from the laser radiation of the unconverted wavelength. The arrangement 2 comprises a crystal 10, which can be like the crystal 10 of the previous figures. The arrangement 2 further comprises two optical prisms 26, 28. The first optical prism 26 is arranged such that the converted laser beam 60 exiting the crystal 10 and the exiting unconverted laser beam 52 enter the first prism 26, and that the exiting converted laser beam 60 and the exiting unconverted laser beam 52 exit the first prism 26 at different angles. As a result, the exiting converted laser beam 60 is separated from the exiting unconverted laser beam 52.The second prism is arranged such that the outgoing unconverted laser beam 52 enters the second prism 28 after exiting the first prism 26. Optionally, an arrangement 2 without the second prism 28 is also conceivable. The arrangement 2 can be arranged within or partially within a cell 1 filled with gas 5. Advantageously, the crystal 10 is arranged such that the incoming unconverted laser beam 50 impinges on the crystal 50 at the Brewster angle. The incoming unconverted laser beam 50 can have a p-polarization 30.

[0084] Fig. Figure 8 shows a schematic representation of a cell 1 according to the invention for frequency conversion of laser radiation, as well as prisms 26, 28 arranged in and / or on the cell for separating the laser radiation of the converted wavelength from the laser radiation of the unconverted wavelength. The cell 1 comprises a crystal 10 for frequency conversion and an optical element 20. The unconverted laser beam 50 enters the cell 1 at the first optical element 20 and then strikes the crystal 10. The outgoing converted laser beam 60 and the outgoing unconverted laser beam 52 exit the crystal 10 at the exit surface opposite the entrance surface and strike the first prism 26. The angle at which the converted laser beam 60 strikes the first prism 26 can preferably be the Brewster angle.The converted laser beam 60 and the unconverted laser beam 52 exit the first prism 26 at different exit angles, so that the two laser beams are separated. The unconverted laser beam 52 then strikes the second prism 28, so that its beam path is deflected. The beam path of the unconverted laser beam 52 after exiting the second prism can preferably run parallel to the incoming unconverted laser beam 50 before entering the cell. The converted laser beam 60 does not strike the second prism 28. The first prism 26 can be arranged in, on, or outside the cell 1. If the first prism 26 is arranged in the cell 1, the second prism 28 can be arranged in, on, or outside the cell 1. The first prism 26 can be arranged within the cell 1 such that it replaces the function of the second optical element 22 (cf. ). Fig.3) and additionally serves to separate the converted laser beam 60 from the unconverted laser beam 52. Reference symbol 1 cell 2 Arrangement 5 Gas 10 crystal for frequency conversion 20 first optical element (window) 22 second optical element (window) 24 third optical element (window) 26 first prism 27 second prism 28 beam splitters 30 p-polarization 35 s polarization 40 reflective coating for the converted laser beam 42 transmissive coating 44 windows with transmissive coating 50 incoming unconverted laser beam 52 outgoing unconverted laser beam 60 outgoing converted laser beam 61 Coating, reflective for the outgoing converted laser beam 71 first mirror 72 second mirror 76 first optical element with partially reflective coating for the unconverted laser beam 78 movable mirror 79 mirrors within the cell QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] US 9651849B2

[0008]

Claims

[1] Cell (1) for frequency conversion of laser radiation, comprising - two at least partially transmitting optical elements (20, 22); - a crystal (10) which converts a wavelength of the laser radiation passing through the crystal (10), the crystal (10) being arranged within the cell (1) and between the two optical elements (20, 22); - wherein the two optical elements (20, 22) are arranged such that an unconverted laser beam (50) passes through the first optical element (20) upon entering the cell (1), enters the crystal (10) at a first entrance surface and exits the crystal (10) at a first exit surface, wherein at least a portion of the laser beam (60) exiting the crystal (10) has a converted wavelength, wherein the converted (60) and the unconverted laser beam (52) enter the second optical element (22) along a second entrance surface, wherein at least a portion of the laser beam exits the second optical element (22) along a second exit surface and - wherein at least one of the two optical elements (20, 22) is arranged such that the laser beam strikes the optical element at the Brewster angle. [2] Cell (1) according to claim 1, wherein the second optical element (22) has a coating (40) reflective of the wavelength of the converted laser radiation, so that the converted laser beam is reflected and the second optical element (22) transmits the unconverted laser beam (52) so that at least a portion of the unconverted laser beam (52) exits the cell (1) through the second optical element (22), and wherein the cell (1) further comprises a third at least partially transmitting optical element (24) arranged such that the converted laser beam reflected by the second optical element (22) strikes the third optical element (24) and then at least a portion of the laser beam exits the cell (22) at the third optical element (24). [3] Cell (1) according to one of the preceding claims, wherein at least two optical elements (20, 22, 24) are optical windows. [4] Cell (1) according to one of the preceding claims, wherein the crystal (10) is arranged such that the first entrance surface of the crystal (10) is arranged relative to the first optical element (20) such that the laser beam (50) strikes the entrance surface of the crystal (10) at the Brewster angle. [5] Cell (1) according to one of the preceding claims, wherein the second optical element (22) is a prism, the prism being arranged such that the converted laser beam exits the prism at a different angle than the unconverted laser beam. [6] Cell (1) according to one of claims 1 to 5, wherein the second optical element (22) is a beam splitter (28), wherein the beam splitter (28) is arranged such that the converted laser beam (60) exits the beam splitter (28) at a different angle than the unconverted laser beam (52). [7] Cell (1) according to one of the preceding claims, wherein the exit surface of the crystal (10) is arranged such that the unconverted laser beam (52) strikes the exit surface at the Brewster angle. [8] Cell (1) according to one of the preceding claims, wherein the exit surface of the crystal (10) has a coating (40) which is highly reflective for the wavelength of the converted laser beam. [9] Cell (1) according to one of the preceding claims, wherein at least one of the optical elements is mechanically connected to the cell (1) by an indium seal, brazing, gluing and / or bonding. [10] Cell (1) according to one of the preceding claims, wherein a drying agent is arranged within the cell (1). [11] Cell (1) according to one of the preceding claims, wherein at least one heating and / or at least one cooling element is arranged in and / or on the cell (1). [12] Cell (1) according to one of the preceding claims, wherein at least one temperature sensor is arranged in and / or on the cell (1). [13] Cell (1) according to one of the preceding claims, wherein a connection unit for a purge gas (5) is arranged in the cell (1). [14] Cell (1) according to one of the preceding claims, wherein integrated channels for flowing through a fluid for controlled temperature change are arranged in a wall of the cell (1). [15] Arrangement (2) for frequency conversion of laser radiation, comprising - a crystal (10) which converts the wavelength of laser radiation passing through the crystal (10), - a first prism (26), wherein the first prism (26) is arranged such that a laser beam exits the crystal (10) at an exit surface of the crystal (10) and strikes the first prism (26) at an entrance surface, and that the converted laser beam (60) exits the first prism (26) at a different angle than the unconverted laser beam (52), so that the converted laser beam (60) is separated from the unconverted laser beam (52). [16] Arrangement (2) according to claim 15, further comprising a second prism (27), - wherein the second prism (27) is arranged such that the unconverted laser beam (52) strikes the second prism (27) after emerging from the first prism (26) and is deflected by the second prism (27) such that its path after the second prism (27) is parallel to the path between the crystal (10) and the first prism (26). [17] Arrangement (2) according to claim 16, wherein the first prism (26) is arranged such that the converted laser beam (60) enters the first prism (26) at a Brewster angle. [18] Arrangement (2) according to claim 16 or 17, wherein the first prism (26) is designed such that the converted laser beam (60) impinges at the Brewster angle on the exit surface of the first prism (26) at which the laser beam exits the first prism (26).

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