Laserresonator

Abstract

The invention relates to a laser resonator (10) with a light inlet (12) for a laser beam (14), a first mirror (20), a second mirror (22) and a light outlet (18), wherein the first mirror (20) and the second mirror (22) are arranged such that a laser beam (14) entering through the light inlet (12) can be directed onto the first mirror (20) such that the laser beam (14) is reflected from the first mirror (20) onto the second mirror (22) and exits from the light outlet (18), wherein at least one of the mirrors (20, 22) has at least one metamirror section (24).

Description

The invention relates to a laser resonator comprising (a) a light inlet for a laser beam, (b) a first mirror, (c) a second mirror, and (d) a light outlet, (e) wherein the first mirror and the second mirror are arranged such that a laser beam can be directed through the light inlet onto the first mirror in such a way that the laser beam is reflected from the first mirror onto the second mirror and exits through the light outlet. In a second aspect, the invention relates to a method for manufacturing a laser resonator with a light inlet and a light outlet, comprising the step of providing a first and a second mirror. Laser resonators are generally used to amplify laser light, select frequencies and / or increase the frequency stability of the laser beam. Laser resonators typically have at least two mirrors. After entering the resonator through the light entrance, the laser beam is reflected at least once by the first and once by the second mirror before exiting the resonator. The laser resonator has natural frequencies that depend primarily on its geometric length. Therefore, the laser resonator also acts as a frequency filter. The mirrors typically have dielectric layers to increase their reflectivity for certain wavelengths, while reflecting other wavelengths almost not at all. This ensures that the desired wavelengths exhibit higher laser power at the resonator's light output, as other wavelengths represent only a small fraction of the total laser power. Therefore, high frequency selectivity is desirable. The longer the resonator length and the larger the beam diameter of the laser beam when it hits the mirrors, the less thermal noise affects the resonator length and thus the natural frequencies. Ideally, the resonator should exhibit as little thermal noise as possible. This results in higher frequency stability of the laser beam exiting the resonator. This is particularly important in high-precision measurements, for example, in the fields of interferometry, quantum optics, optical atomic clocks, the measurement of gas properties using lasers, or the measurement of gravitational waves. Frequency stability, in this context, refers specifically to the quotient of the frequency difference, which describes the difference between a maximum and minimum laser frequency, divided by the average laser frequency. A disadvantage of known frequency-stabilized laser resonators is that higher frequency stability usually comes with a higher volume and / or weight of the resonator, as well as usually a particularly complex manufacturing, positioning and / or adjustment of the mirrors. The object of the invention is to provide an improved, in particular more frequency-stable, laser resonator. The problem is solved by a generic laser resonator characterized by the fact that at least one of the mirrors has at least one metamirror section. Furthermore, the problem is solved by a generic method characterized by the steps (b) surface structuring of a substrate and / or a coating on the substrate of the first and / or second mirror such that at least one of the mirrors has at least one metamirror section, (c) arranging and fixing the first mirror and the second mirror such that a laser beam can be directed onto the first mirror through the light entry point in such a way that the laser beam is reflected from the first mirror onto the second mirror and exits through the light exit point. The advantage of metamirror sections is that they achieve high frequency selectivity and frequency stability. Metamirror sections do not require a multitude of dielectric coatings to achieve high reflectivity for a specific, typically narrow, wavelength range. Therefore, they exhibit significantly lower thermal noise than dielectric mirrors. Furthermore, materials can be used for the metastructures that, for example, exhibit lower mechanical losses, thus further reducing noise. Frequency stability generally increases with the optical path length of the laser beam within the laser resonator. This optical path length can be increased by adjusting the mirror spacing or the number of reflections within the resonator. Increasing the number of reflections to improve frequency stability using dielectric mirrors typically comes at the cost of increased noise, a greater number of mirrors (resulting in a larger resonator volume and weight), and / or complex alignment of the numerous mirrors. An alternative method for increasing frequency stability involves increasing the distance between the mirrors, but this results in a larger resonator volume. In contrast, metamirror sections in the described arrangement in the laser resonator make it possible to realize a frequency-stable laser resonator with a small volume and weight. In the context of this description, a light inlet is understood to be, in particular, a region of the laser resonator configured such that a laser beam can enter the resonator interior through the light inlet. The light inlet is, for example, an opening in the resonator or a region of a component of the resonator that is at least 50% transmittable. A light exit point is a region of the laser resonator configured to allow a laser beam to exit the resonator interior. The light exit point can be, for example, an opening in the resonator or a region of a component that transmits at least 50%, preferably at least 90%. The light inlet is preferably located at the first or second mirror. The light outlet is preferably located at the first or second mirror. In other words, the first or second mirror preferably has an area that is at least 0.1% transmittable for the laser beam, preferably at least 0.5%. Preferably, the light entry and exit points are not formed on the same mirror. Alternatively, the light entry and exit points are formed on the same mirror. For example, the mirrors are designed and arranged in such a way that a laser beam entering through the light entrance is reflected once, twice, three times, four times or more by the first or second mirror and then exits the resonator interior through the light exit. A mirror is understood to be, in particular, an optical element designed such that at least 90% of an incident light ray is reflected by at least one section of the mirror, especially at a reflection angle corresponding to an incidence angle of ± 30°. The mirror is understood to be the entirety of all sections, which may include, for example, mirror sections, metamirror sections, transmission sections, and so on. Preferably, the first and / or second mirror has a reflectivity of at least 99% on at least one mirror section, preferably at least 99.5%, preferably at least 99.99%, preferably at least 99.997%. The first mirror is preferably spaced apart from the second mirror, preferably by at least 2 mm, preferably by a maximum of 100 meters, preferably by a maximum of 1 meter, preferably by a maximum of 50 cm. The laser resonator preferably has a maximum of three mirrors, preferably exactly two mirrors. The mirrors preferably have a maximum extension direction of a surface on which the laser beam is reflected of at least 1 mm, preferably at least 2 mm, preferably at least 5 mm, preferably a maximum of 2 meters, preferably a maximum of 1 meter, preferably a maximum of 50 cm, preferably a maximum of 20 cm. Preferably, the first mirror is arranged such that a light beam striking the first mirror at an angle of incidence of 90° is reflected from the first mirror to the second mirror. Preferably, the same applies to a light beam striking the first mirror at an angle of incidence between 89° and 91°. Preferably, the light entry point is located at the second mirror. Preferably, the first mirror and the second mirror each have at least one metamirror section, preferably at least two metamirror sections each. A metamirror section is understood in particular to be an area of ​​the surface of the mirror which has a surface structure whose smallest structures measure between 20 nanometers and 700 nm. In other words, the metamirror section has a surface structure that affects the amplitude and phase of laser beams striking it, in particular reflecting them anomalously. At least one metamirror section, and in particular at least a majority of the metamirror sections, preferably has at least one at least partially periodic surface structure that anomalously reflects an incident laser beam. Preferably, all metamirror sections have, for example, at least one at least partially periodic surface structure that anomalously reflects an incident laser beam. Anomalous reflection is understood to mean, in particular, that an incident laser beam is reflected at least 50%, preferably at least 75%, at a reflection angle that does not correspond to the angle of incidence. In contrast, normal reflection is understood to occur when the angle of incidence equals the angle of reflection. It is possible that one or both mirrors have a metamirror section, in particular exactly one, that shows normal reflection. Preferably, the mirror comprises (a) a substrate and (b) a metamirror section having a reflective structure incorporated into the substrate and / or into a coating on the substrate. The coating is particularly not a dielectric coating and / or, without the reflective structure, has a reflectivity of particularly a maximum of 49%, and more specifically a maximum of 30%. For example, the coating comprises at least one of the following materials: aluminum oxide (Al₂O₃), silicon nitride (SiN₃), tantalum oxide (Ta₂O₃), diamond, silicon, germanium. The substrate is understood to be, in particular, a solid-state region of the mirror having a depth of at least 100 µm, and in particular at least 1 mm. The substrate comprises, for example, silicon, silicon oxide, other silicon compounds, sapphire, diamond, quartz glass, titanium silicate glass and / or a ceramic, which is in particular polycrystalline and / or contains magnesium, aluminum and / or silicon, for example, such a ceramic available under the commercial name NexCera. The combination of substrate and coating is preferably selected to match the desired reflectivity and reflection angle, and optionally to the wavelength of the laser beam and / or the temperature. Different combinations result in different refractive indices, which can depend on the wavelength and temperature. Reflective structuring is understood to mean, in particular, a surface structure created by locally removing parts of the substrate and / or parts of a coating located on the substrate, and which alters the amplitude, phase, and / or reflection angle of a light beam reflected by the reflective structuring compared to the substrate without structuring. In particular, the reflective structuring is designed such that at least 70%, preferably at least 90%, preferably at least 95%, preferably at least 99%, preferably at least 99.9% of the intensity of an incident light beam is reflected by the metamirror section. The advantage of reflective structuring is that the structure depth can be less than 3 µm, particularly less than 2 µm, and especially at most 1.9 µm, which is smaller compared to commonly used dielectric layers, whose depth is generally greater than 2 µm. This advantageously reduces noise through the metamirror sections while still achieving high reflectivity. Preferably, the area fraction of a surface on which the substrate or coating for reflective structuring is at least partially removed is at least 35% of the total area of ​​the metamirror section, preferably at least 40%, preferably at least 45%. Furthermore, reflective structuring advantageously enables the production of multiple areas with different specifications for the reflection (angles) and / or transmission (angles) of the laser beam side by side on the same substrate within a single structuring process, so that a mirror is obtained which has multiple areas with different reflectivities, reflection angles, transmissivities and / or transmission angles in a single component. This, in turn, allows the laser beam to be reflected back and forth between the mirrors multiple times before exiting the resonator through the light exit, without requiring a large number of mirrors. This increases the optical path length and thus the frequency stability. Metamirror sections make this achievable with simpler manufacturing and adjustment compared to a possible alternative mirror arrangement for increasing the optical path length, which uses several independently tiltable dielectric mirrors or mirror sections that must be manufactured and / or adjusted individually and are susceptible to environmental influences such as temperature fluctuations with regard to their exact alignment, which in turn reduces frequency stability. The reflective structuring is preferably applied to or within the substrate using electron beam lithography and / or an etching process. Electron beam lithography advantageously enables the fabrication of very small structures, for example, with a feature width of less than 100 nm, with a high accuracy of, for example, at least 20 nm. Feature width is defined as the width of the structure, particularly along the direction parallel to an averaged surface of the substrate. Preferably, the reflective structuring has a cross-section with a depth of at most 5 µm, preferably at most 2 µm, preferably at most 1 µm, preferably at most 0.5 µm. Depth refers in particular to a height difference in the cross-section of the reflective structuring from the highest elevation to the deepest depression. It is possible that the reflective structuring is arranged exclusively in the coating located on the substrate. It is possible that the reflective structuring is incorporated into the coating in such a way that the substrate is exposed in certain areas, but this is not necessary. Due to the shallow depth, the noise of the laser beam, especially the frequency, amplitude and / or phase of the laser beam, is reduced. Preferably, the reflective structuring has a plurality of geometric surface structures, preferably in the form of crosses and / or ridges. For example, the crosses and / or ridges are arranged parallel to each other, and / or are oriented such that a polarization direction of the laser beam is parallel to a longitudinal orientation of the ridges. Preferably, the crosses and / or webs have an approximately rectangular cross-section or a cross-section composed of rectangular sections, for example in the shape of a T or an inverted L. Other cross-sections are possible, for example, those with a column shape that tapers upwards in sections and / or one or more, in particular a maximum of ten, approximately ellipsoidal sections arranged one above the other. "Approximately" in this context refers in particular to a maximum deviation of the cross-section from an ideal cross-section of the rectangle, ellipsoid, and other aforementioned shapes of a maximum of 20%, in particular a maximum of 10%, and in particular a maximum of 5%, of the total cross-sectional area. Preferably, the spacing between the ridges and / or crosses is constant or periodic with a period of preferably a maximum of 2 µm, preferably a maximum of 0.5 µm, preferably a maximum of twice the central wavelength of the laser beam, and preferably a maximum of the central wavelength of the laser beam. The short period advantageously results in an anomalous reflection of the laser beam at the reflective structure. Along a top-view axis that is orthogonal to the cross-sectional direction and, for example, runs along the direction of the structure's width, the reflective structuring exhibits, for example, a multitude of crosses, rectangles, rounded rectangles, circles, ellipses, triangles, and / or other shapes. For example, the reflective structuring of a single metamirror section exhibits a multitude of exactly one of these shapes, the precise shape of which preferably differs from one another by a maximum of 5% of their top-view area along the top-view axis. For example, a single metamirror section may contain a multitude of crosses whose top-view shape, the shape along the top-view axis, differs only by a maximum of, say, 2%, in that some of the crosses have slightly more rounded corners than others. Alternatively, a single metamirror section may also have different shapes in top view. Alternatively, it is possible, for example, that the structure width of the individual struts is not constant, but increases from a left end to a center along the direction of the structure width and then decreases from a center to a right end, or vice versa. This causes the metamirror section to reflect as if it were curved, even though the metamirror section is not actually curved. Using numerical simulation, for a given angle of incidence and a desired angle of reflection and / or for a desired reflectivity and / or transmittance, a corresponding concrete form of reflective structuring can be determined, for example using numerical Maxwell solvers based on methods such as Rigorous coupled wave analysis (RCWA), Finite Elements (FEM) or Finite Difference Time-Domain (FDTD). This shape can then be incorporated into the substrate or coating using a structuring process, such as etching or electron beam lithography. Preferably, the first mirror has at least two metamirror sections in the form of a first first-mirror metamirror section and a second first-mirror metamirror section. Preferably, the first first-mirror metamirror section has a first first-mirror axis and the second first-mirror metamirror section has a second first-mirror axis that differs from the first first-mirror axis. Preferably, the first mirror metamirror sections are designed such that a laser beam falling through the light entrance onto the first first mirror metamirror section is reflected onto the second mirror, and from the second mirror is reflected onto the second first mirror metamirror section. The first and / or second first-mirror metamirror section is preferably located within the metamirror section. In other words, the metamirror section preferably includes the first-mirror metamirror sections. The term "mirror axis" refers to an axis, specifically a vector, that defines the reflection orientation of the metamirror section. In particular, the mirror axis is the straight line along which a light ray, when incident on the mirror along the mirror axis, is reflected back along the same path as its incident direction. Due to the specific properties of the metamirror section, a mirror axis may not necessarily be the normal to a coarse orientation of the metamirror section. A coarse orientation is a plane that represents the average total surface area of ​​the metamirror section, disregarding the fine structure formed by the reflective patterning. If the metamirror section reflects like a paraboloid mirror, the term "mirror axis" specifically refers to the axis of rotational symmetry along which a laser beam incident below the mirror axis is reflected back along the same path. Preferably, the first mirror metamirror sections are immovable relative to each other, preferably formed in one piece and / or on the same substrate. Preferably, the second mirror has at least two metamirror sections in the form of a first second-mirror metamirror section and a second second-mirror metamirror section. Preferably, the first second-mirror metamirror section has a first second-mirror axis and the second second-mirror metamirror section has a second second-mirror axis that differs from the first second-mirror axis. Preferably, the first mirror metamirror sections and the second mirror metamirror sections are designed such that a laser beam falling through the light entrance onto the first first mirror metamirror section is reflected from the first first mirror metamirror section onto the first second mirror metamirror section, is reflected from the first second mirror metamirror section onto the second first mirror metamirror section, is reflected from the second first mirror metamirror section onto the second second mirror metamirror section and, indirectly or directly, then reaches the light exit point. The same advantages and preferred embodiments as for the metamirror sections of the first mirror apply to the metamirror sections of the second mirror. The characteristic that the laser beam then reaches the light exit point directly means, in particular, that the laser beam, after being reflected by the second secondary mirror metamirror section, reaches the light exit point without further reflections. The characteristic that the laser beam then reaches the light exit point indirectly means, in particular, that the laser beam reaches the light exit point only after further reflections. Preferably, the first mirror has a light exit point, which is preferably designed such that an incoming laser beam is transmitted through it with at least 50%, preferably at least 75%, preferably at least 99% of the input power of the laser beam and is deflected by less than 2°, preferably less than 1°. Preferably, the first mirror and the second mirror each have a third first-mirror metamirror section and second-mirror metamirror section, and preferably each have a fourth metamirror section, preferably at least six metamirror sections each, preferably a maximum of 100 metamirror sections each. The higher the number of metamirror sections, the more different reflections, reflection angles, transmissions, and / or transmission angles are possible on the same mirror. This makes it possible to increase the number of revolutions of the laser beam within the resonator, and thus the optical path length and consequently the frequency stability, without increasing the overall volume of the resonator. For example, with four metamirror sections per mirror, plus an additional light entry on the second mirror and a light exit from the first mirror, an incoming laser beam can travel through the resonator a total of nine times, resulting in an optical path length nine times the geometric length of the resonator. This advantageously increases frequency stability while simultaneously reducing the volume and weight of the resonator and minimizing thermal noise. However, with a constant overall mirror size, the individual primary mirror metamirror sections have a smaller reflective surface as their number increases, so that too high a number is detrimental to the stability and tolerance of the resonator to minimal deviations of the laser beam entering the light entrance. Preferably, the reflective surface area of ​​each of the metamirror sections is at least 500 µm², preferably at least 100 µm², preferably at least 50 µm², and preferably at least 5 µm². In particular, each of the metamirror sections is preferably at least 5%, preferably at least 10%, and preferably at least 15% larger than the impact area of ​​a laser beam striking the metamirror section. The impact area is understood to be the area illuminated by a laser beam striking the metamirror section. The advantage is that, with appropriate arrangement and, if necessary, adjustment, the entire laser beam impacts the metamirror section, and that minor deviations, for example, in the angle of incidence of the laser beam into the resonator, do not directly result in the corresponding metamirror section no longer being completely illuminated by the laser beam. This avoids losses within the resonator. Preferably, the light entry and / or light exit point has a substrate with a surface structure introduced into the substrate or into the coating, which is introduced into and / or onto the substrate in particular by means of an etching process and / or electron beam lithography. Preferably, the light entry point is an area on the first or second mirror, preferably on the second mirror. Preferably, the light exit point is an area on the first or second mirror, preferably on the first mirror. Preferably, the light entry and / or light exit points of the metamirror sections are different, i.e., non-overlapping. Preferably, the first and / or second mirror has at least one antireflective region on a back side facing away from the resonator interior and / or opposite the metamirror sections. The antireflective region is understood to be, in particular, a surface area of ​​the mirror configured such that the reflectivity of a laser beam striking the antireflective region is less than 1%, preferably less than 0.2%, and preferably less than 0.1%, at least within a predetermined wavelength range. The antireflective region is preferably designed as an antireflective coating, for example with one or more dielectric layers, or as a surface structure that is preferably introduced into the substrate and / or a coating on the substrate of the first and / or second mirror by means of an etching process or electron beam lithography. Preferably, the antireflective region is designed such that it comprises at least one impact surface through which the laser beam enters and / or exits the laser resonator. It is possible for the antireflective region to comprise a continuous surface or several spaced-apart sub-surfaces. The advantage of the antireflective region is that losses at the entry of the laser beam into and / or exit of the laser resonator are reduced. Preferably, the antireflective region is configured to include at least one primary surface located exactly opposite the metamirror sections of the corresponding mirror. For example, the primary surface is arranged on a surface facing away from the interior, opposite the front of the mirror facing the interior. The advantage of such a primary surface is that any portions of the laser beam not reflected by the metamirror sections are prevented from being reflected back into the laser resonator at the rear of the mirror. In particular, etalon reflection effects between surfaces of the mirror are reduced or eliminated.In particular, the first surface is preferably arranged such that no laser beam, which can strike the substrate of the mirror at an angle between, in particular, 60° and 120° and subsequently one of the metamirror sections, can strike the first surface without having previously struck it. Preferably, the antireflective region is designed such that, in addition to the primary surface, it comprises a secondary surface, which is preferably arranged adjacent to the primary surface and / or whose total area is preferably at least 10%, and preferably at least 20%, of the total area of ​​the primary surface. The advantage of such a secondary surface is that losses are reduced even with slight deviations from a perpendicular laser beam entering the mirror. Preferably, the first and / or second mirror has a curvature region with a non-zero effective curvature, which is preferably a region of the metamirror section. The curvature region is preferably designed such that it acts on an incident laser beam like a curved mirror surface, in particular like a concave mirror surface. This advantageously increases the resonator stability. The curved area of ​​the first mirror preferably has a reflective pattern on a substrate, which is applied to the substrate by means of an etching process and / or electron beam lithography. It is possible, but not necessary, for the rough surface of the curved area to actually have a curvature, particularly a concave one. The reflective pattern achieves a comparable effect through anomalous reflection even without a corresponding actual curvature. Preferably, the curvature region is partially transmittable, preferably with a transmittance of at least 0.05%, preferably at least 0.1%, preferably at most 80%, and preferably at most 50%, where these values ​​refer to the fraction of a laser beam located inside the resonator and incident on the curvature region that is transmitted such that it subsequently ends up outside the resonator. Preferably, the partially transmittable curvature region has a reflectivity that, when added to the transmittance, is at least 80%, preferably at least 90%, preferably at least 99%, and preferably at least 99.9%. Preferably, the curvature area is at least partially overlapping with the light emission point; preferably, the curvature area and the light emission point overlap completely. Preferably, the first mirror has a transmission area configured such that a laser beam is transmitted through it with at least 90%, preferably at least 98%, preferably at least 99%, of the laser beam's incident power, and thus exits the laser resonator through the light outlet. The transmission area preferably deflects the laser beam by less than 2°, preferably less than 1°, during transmission. A plurality of the metamirror sections, preferably all metamirror sections, and if present on the first and / or second mirror, preferably also the curvature area and / or the light entry and / or light exit, are preferably arranged in a checkerboard pattern and / or have a rectangular, in particular square, overall surface. Preferably, the laser resonator has a spacer on which the first and second mirrors are arranged such that a distance between the first and second mirrors is defined. Preferably, the spacer is designed such that the first and / or second mirrors are not movable or tiltable relative to each other. The first and / or second mirror is preferably mechanically and / or materially connected to the spacer or formed in one piece. For example, the first and / or second mirror is each connected to the spacer by means of molecular forces, wherein a mounting surface of the first and / or second mirror and a mounting surface of the spacer preferably have a surface roughness of on average less than λ / 6, preferably less than λ / 8, and preferably less than λ / 10. The mounting surface is understood to be, in particular, the surface on which the first and / or second mirror and the spacer are connected or attached to each other. Preferably, the mounting surface is not located on the same surface of the mirror on which the metamirror section(s) are located. Preferably, the gas pressure inside the laser resonator is less than 0.5 bar, preferably less than 10⁻² mbar, preferably less than 10⁻⁴ mbar, preferably less than 10⁻⁶ mbar, preferably less than 10⁻⁸ mbar, and preferably less than 10⁻¹⁰ mbar. The advantage is that a laser beam interacts less with gas molecules inside the resonator, thus reducing noise. Furthermore, attaching the mirrors to the spacer is easier, for example, simply due to the low internal pressure and without additional fasteners such as adhesives or retaining elements. Preferably, the frequency stability of the laser resonator is at least 10-10, preferably at least 10-12, preferably at least 10-14, preferably at least 10-16, preferably at least 10-18. This is preferably achieved by means of the optical path length described above, which is at least three times the geometric resonator length. Additional stabilization elements, such as vibration isolation and / or temperature isolation, are also possible. Another aspect of the invention relates to a laser system with a laser resonator according to the invention and a laser source. Preferably, the laser source is configured to emit a laser beam, preferably a continuous wave beam, such that the laser beam enters the laser resonator via the light entrance. For example, the laser source has optical elements configured to collimate, deflect, filter, and / or polarize the laser beam. Preferably, the laser source is configured to adapt the beam diameter of the laser beam to the size of the metamirror sections of the mirrors, preferably such that the beam diameter is at most half, preferably at most 25%, of the largest extent of a smallest metamirror section when it hits the light entrance. Preferably, the laser source is configured for linear polarization of the laser beam. Preferably, the reflection patterns of the metamirror sections are configured such that they are optimized for the polarization of the laser beam, achieving a predetermined reflectivity and / or transmittance. Preferably, an incoming laser beam has a linewidth of at most 10 MHz, preferably at most 10 kHz, preferably at most 1 kHz, preferably at most 100 Hz, and preferably at most 1 Hz. The laser resonator preferably has such reflectivities of the metamirror sections that the linewidth of an outgoing laser beam from the laser resonator is at least 10% smaller than the linewidth of the incoming laser beam, preferably at least 15% smaller. The invention further relates to a method for manufacturing a laser resonator. (a) Providing a first and second mirror, (b) Surface structuring of a substrate of the first and / or second mirror such that at least one of the mirrors has at least one metamirror section, (c) Arranging and fixing the first mirror and the second mirror such that a laser beam can be directed onto the first mirror through the light entry point in such a way that the laser beam is reflected from the first mirror onto the second mirror and exits through the light exit point. Preferably, the surface structuring step comprises the step of introducing a reflective structure, in particular by means of an etching process and / or electron beam lithography, into the substrate and / or into a coating on the substrate. It is possible that the coating is already present on the substrate or that the coating is applied to the substrate before the surface structuring. The coating is in particular at least 0.1 µm thick, in particular at least 0.5 µm thick, in particular at least 1 µm thick, and / or at most 50 µm thick, in particular at most 20 µm thick, in particular at most 10 µm thick. Preferably, the reflective structuring is introduced in such a way that the first mirror has a first first-mirror metamirror section and a second first-mirror metamirror section, wherein the first first-mirror metamirror section has a first first-mirror axis, and wherein the second first-mirror metamirror section has a second first-mirror axis that differs from the first first-mirror axis. Preferably, the reflective structuring is introduced in such a way that the second mirror has a first second mirror metamirror section and a second second mirror metamirror section, wherein the first second mirror metamirror section has a first second mirror axis and wherein the second second mirror metamirror section has a second second mirror axis that differs from the first second mirror axis. Preferably, the method further comprises the step of surface structuring a section of the second mirror such that the section has the light entry point. Preferably, the surface structuring of the section is carried out such that a laser beam entering the resonator interior is transmitted through the light entry point with at least 95%, preferably at least 98%, preferably at least 99%, of the input power of the laser beam and / or is deflected by less than 5°, preferably less than 2°, preferably less than 1°. Preferably, the method further comprises the step of surface structuring a section of the first mirror such that the section has a light exit point. Preferably, the surface structuring of the section is carried out such that a laser beam exiting from the resonator interior is transmitted through the light exit point with at least 0.5%, preferably at least 5%, preferably at least 50%, of the input power of the laser beam. Preferably, the method further comprises the steps of: (a) providing a spacer, which preferably has an inlet opening and an outlet opening, and (b) arranging the first mirror and second mirror on the spacer such that preferably the first mirror is arranged at the inlet opening and the second mirror at the outlet opening, such that a laser beam is transmitted through the light inlet through the transmission area of ​​the second mirror, falls onto the first first-mirror metamirror section and is reflected onto the first second-mirror metamirror section, is reflected from the first second-mirror metamirror section onto the second first-mirror metamirror section, is reflected from the second first-mirror metamirror section onto the second second-mirror metamirror section and, directly or indirectly, then reaches the light outlet. For example, the mirrors are fixed to the spacer by pressing them against it. Preferably, the surface roughness of the mounting surfaces of the mirrors and the spacer is on average less than λ / 6 µm, preferably less than λ / 8 µm, and preferably less than λ / 10 µm. Here, λ is the central wavelength of the laser beam. The advantage is that molecular forces are sufficient to hold the mirrors to the spacer, so that adhesives or other fastening elements, which are usually susceptible to temperature and / or pressure fluctuations, are not required. Preferably, the distance A between adjacent left ends of individual structural elements of the reflective structuring, which are for example bridges, crosses or columns, differs by a maximum of 10% along a direction along the structure width from a central wavelength λ of the laser beam 14 or half, quarter, sixth or eighth thereof, preferably by a maximum of 5%, preferably by a maximum of 2%. The invention is explained in more detail below with reference to the accompanying figures. Figure 1a shows a first embodiment of a laser resonator according to the invention, Figure 1b shows a first embodiment of a mirror of a laser resonator according to the invention, Figure 1c shows a second embodiment of a mirror of a laser resonator according to the invention, Figure 2a shows an embodiment of a reflective structuring without a coating, Figure 2b shows a first embodiment of a reflective structuring of a metamirror section with a coating, Figure 2c shows a second embodiment of a reflective structuring of a metamirror section with a coating, Figure 2d shows a third embodiment of a reflective structuring of a metamirror section with a coating, Figure 3a shows an embodiment of a reflective structuring of a metamirror section with a coating and without effective curvature.3b an embodiment of a reflective structuring of a metamirror section with coating and effective curvature, Fig. 4a and b a second and third embodiment of a laser resonator according to the invention. Figure 1a shows a schematic representation of a first embodiment of a laser resonator 10 according to the invention, which has a light inlet 12 for a laser beam 14 through which the laser beam 14 can enter a resonator interior 16. The incoming laser beam is labelled "IN". The laser resonator 10 has a light outlet 18 from which the laser beam 14 can exit the resonator interior 20. The laser resonator 10 has a first mirror 20 and a second mirror 22, wherein the first mirror 20 and the second mirror 22 are arranged such that the laser beam 14 entering through the light inlet 12 can be directed onto the first mirror 20 such that the laser beam 14 is reflected from the first mirror 20 onto the second mirror 22 and exits the light outlet 18. In this example, the first mirror 20 and the second mirror each have at least one metamirror section 24, which has a reflective structure introduced into a substrate 26 of the mirror 20, 22. This structure is introduced into the substrate 26 and / or into a coating 27 on the substrate 26, for example, by means of an etching process and / or electron beam lithography. The reflective structure in the coating 27 is shown here as an example (see Fig. 2a-d for further examples). The first mirror 20, for example, has a first first-mirror metamirror section 28.1 and a second first-mirror metamirror section 28.2. The first first-mirror metamirror section 28.1 has a first first-mirror axis A28.1, which differs from a second first-mirror axis A28.2 of the second first-mirror metamirror section 28.2. The second mirror 22 has a first secondary mirror metamirror section 30.1 and a second secondary mirror metamirror section 30.2, wherein the first secondary mirror metamirror section 30.1 has a first secondary mirror axis A30.1 which differs from a second secondary mirror axis A30.2 of the second secondary mirror metamirror section 30.2. The first-mirror metamirror sections 28.j and the second-mirror metamirror sections 30.k are designed such that a laser beam 14, which falls through the light inlet 12 onto the first first-mirror metamirror section 28.1 - marked as light path 1, is reflected from the first first-mirror metamirror section 28.1 onto the first second-mirror metamirror section 30.1 - marked as light path 2 with a dashed line, is reflected from the first second-mirror metamirror section 30.1 onto the second first-mirror metamirror section 28.2 - marked as light path 3, is reflected from the second first-mirror metamirror section 28.2 onto the second second-mirror metamirror section 30.2 - marked as light path 4 with a dashed line and, indirectly or directly, then reaches the light outlet 18. In this example, after reflection at the second secondary mirror metamirror section 30.2, the light ray 14 goes directly to the light exit 18 - marked as light path 5. In this example, the light inlet 12 is not identical to the second time-mirror metamirror section 30.2, but is optically located behind it. The same applies to the light outlet 18 and the first first-mirror metamirror section 28.1. Light paths 1 and 5 are not identical in this example, but are indistinguishable from each other in the side view shown. For example, the angle α1 between light paths 1 and 4 and the angle α2 between light paths 2 and 3 are approximately equal, with a deviation of a maximum of 2°, preferably a maximum of 1°. In this example, α1 = α2. The required reflection angles at the corresponding metamirror sections 28.j are achieved by a suitable shape of the coating 27 with reflective structuring, for example in the form of ribs and / or crosses with a corresponding cross-sectional profile for the respective reflection angles (see Figs. 2a-2d, 3a, 3b for cross-sectional examples). For example, the first mirror 20 has an antireflection area 32.1 on a back side 38.1 which is turned away from the resonator interior 16 and / or arranged opposite the metamirror sections 28.j. In this example, the light inlet 12 in the second mirror 22 is configured, preferably such that the light inlet 12 does not overlap with any of the second mirror metamirror sections 30.k. For example, the second mirror 22 has an antireflection area 32.2 on a rear side 38.2, which faces away from the resonator interior 16 and / or is arranged opposite the metamirror sections 30.k. In this case, a spacer 34 surrounds the resonator interior 16 such that light can only enter or exit the resonator interior 16 through the light inlet 12 and light outlet 18. The first and second mirrors 20, 22 are attached to the spacer 34 in this example, for instance, mechanically or by bonding. Here, the mirrors 20, 22 are pressed onto the spacer 34, and the attachment surfaces have an average surface roughness of less than 2 µm, so no adhesive or fasteners are required. The dimensions shown do not correspond to the actual dimensions. In particular, for example, the depth T of the antireflective region 32 along the z-axis is preferably less than 1 µm, while the total length L of the resonator interior 16 is, for example, at least 10 cm. Furthermore, for example, the light inlet 12, light outlet 18, and the metamirror sections 28.j, 30.k are preferably not formed in reality as structures protruding from the substrate 26.i, as shown here for the sake of simplicity, but rather as surface structures 31.m introduced into the substrate 26.i as depressions with typical depths T of less than 1 µm, in particular less than 700 nanometers. Preferably at least four metamirror sections 28.j, 30.k are arranged on the first and second mirrors 20, 22, preferably at least eight. Figure 1b shows a schematic representation of a first embodiment of a mirror 20, 22 of a laser resonator 10 according to the invention. The left part of Figure 1b shows a front face 36, for example, of the second mirror 22 with a substrate 26, a light entrance 12 in the form of a surface-structured area on the substrate 26, which is introduced into the coating 27 on the substrate 26 by means of an etching process and / or electron beam lithography, and a first second-mirror metamirror section 30.1. The first mirror 20 is configured accordingly, for example, wherein the light exit 18 is arranged instead of the light entrance 12, and the first first-mirror metamirror section 28.1 is arranged instead of the first second-mirror metamirror section 30.1. The middle and right-hand parts of Fig. 1b each show an exemplary back side 38.j of a first or second mirror 20, 22 with a substrate 26.1 or 26.2 respectively and an antireflective coating 32. The antireflective coating 32 is, for example, significantly larger than the total area of ​​all metamirror sections 30.k or 28.j, for example square and about nine times the size of the individual metamirror section 30.1, see the middle illustration of Fig. 1b, or circular with a diameter approximately four times the side length of the individual metamirror section 30.1, see the right-hand illustration of Fig. 1b. The antireflective coating 32 is, for example, formed as a multiple coating of dielectric layers on the substrate 26 or introduced into the substrate 26 by means of a surface structure 31, for example by means of an etching process and / or electron beam lithography. A central axis, which designates a center M along the y-axis through the substrate 26, is shown as a dashed line. Figure 1c shows a schematic representation of a second embodiment of a first and second mirror 20, 22 of a laser resonator 10 according to the invention. Eight metamirror sections 28.j, 30.k are incorporated into the coating 27 on the substrate 26.i, for example by electron beam lithography. Furthermore, the first mirror 20 has a light exit 18 and the second mirror 22 has a light entrance 12, which are incorporated into the substrate 26, for example by electron beam lithography, as surface structures 31. Preferably, the metamirror sections 28.j, 30.k are configured such that a light ray 14, which enters the resonator interior 16 through the light inlet 12, falls on the first first-mirror metamirror section 28.1, is reflected from there onto the first second-mirror metamirror section 30.1, from there onto the second first-mirror metamirror section 28.2, from there onto the second second-mirror metamirror section 30.2 and so on until the eighth first-mirror metamirror section 28.8, from which the light ray 14 is reflected onto the eighth second-mirror metamirror section 30.8, from where the light ray 14 is reflected onto the light outlet 18. With the exemplary mirrors 20, 22 as shown in Fig. 1c, the optical path length of the light beam 14 through the resonator interior 16 is 17 times the geometric resonator length. The light inlet 12 and the light outlet 18 are, for example, positioned centrally on the substrate 26 with respect to the x-axis and the y-axis. The metamirror sections 28.j, 30.k are arranged around them in a checkerboard pattern, here, for example, with a spacing that corresponds to approximately 40% of the side length of a single metamirror section 28.j, 30.k. A different arrangement of the light inlet 12, the light outlet 18, and the metamirror sections 30.k, 28.j is possible, as is a smaller or larger spacing between the metamirror sections 28.j, 30.k and / or the light inlet 12 and / or the light outlet 18. Figures 2a to 2d show exemplary embodiments of reflective structuring. Figure 2a shows an exemplary embodiment of a reflective structuring that is incorporated into the substrate 26.i. In this example, there is no coating on the front surface 36.i of the substrate 26.i. Figures 2b to 2d show a coating 27 on the front surface 36.i of the substrate 26.i, into which the reflective structuring is incorporated. In Fig. 2b, the reflective structuring is incorporated into the coating 27 in such a way that the substrate 26.i is not exposed by the reflective structuring, i.e., the depth T of the reflective structuring is less than the thickness D of the coating 27. In Fig. 2c, the reflective structuring has a depth T equal to the thickness D of the coating 27, so that the substrate is exposed at those points where the reflective structuring has the greatest depth T. Figure 2c schematically illustrates that the width of the remaining webs 33.i (see also Figures 3a-b) of the coating 27 can be non-uniform. In the embodiment shown in Figure 2b, however, all webs are of the same width and spaced equally far apart from adjacent webs. Figure 2d schematically shows that various combinations of cross-sectional shapes are possible, for example, T-shaped or shaped like an inverted L. It is also possible for the webs to have different depths T of reflective structuring in the coating 27. Figure 3a shows an embodiment of a reflective structuring with a coating 27 on the substrate 26.i and without effective curvature. In this example, the distance A between adjacent left-hand rib ends is the same for all ribs 33.i, as are the width B and depth T of the ribs 33.i. Preferably, the distance A differs by a maximum of 10% from a central wavelength λ of the laser beam 14 or half, a quarter, a sixth, or an eighth thereof, preferably by a maximum of 5%, and preferably by a maximum of 2%. Fig. 3b shows an embodiment of a reflective structuring with a coating 27 on the substrate 26.i and with effective curvature K. Effective curvature K means, in particular, that the metamirror section 24.i acts like a curved mirror, although in this example it does not actually have a curved surface. In this example, the curvature K is achieved by the width and spacing of the webs 33.i decreasing from left to right and then increasing again from approximately the center of the metamirror section 24.i. In this example, the distance A between adjacent left web ends is constant over the entire metamirror section 24.i. Figure 4a shows a second embodiment of a laser resonator 10 according to the invention, in which the laser beam 14 is reflected exactly twice after entering the resonator interior 16 through the light inlet 12 before exiting the resonator interior 16 through the light outlet 18. This differs from the first embodiment in Figure 1a, where the laser beam 14 is reflected four times within the resonator 10. In Figure 4a, the first mirror 26.1 has the light outlet 18 and the first first-mirror metamirror section 28.1, and, for example, no metamirror sections. The second mirror 26.2 has the light inlet 12 and a first second-mirror metamirror section 30.1, and, for example, no metamirror sections. Figure 4b shows a third embodiment of a laser resonator 10 according to the invention, in which the laser beam 14, after entering the resonator interior 16 through the light inlet 12, is reflected exactly once by the first primary mirror metamirror section 28.1 before exiting the resonator interior 16 through the light outlet 18. The first mirror 26.1 has the first primary mirror metamirror section 28.1 and, for example, no further metamirror sections. The second mirror 26.2 has the light inlet 12 and the light outlet 18 and, for example, no metamirror sections. Reference symbol list 10 Laser resonator 12 Light entrance 14 Laser beam 16 Resonator interior 18 Light exit 20 First mirror 22 Second mirror 24 Metamirror section 26.i Substrate of the i-th mirror 27 Coating (with reflective structuring) 28.j First mirror metamirror sections 30.k Second mirror metamirror sections 31.m Surface structure 32 Antireflective area 33.i Bridge of the reflective structuring 34 Spacer 36.i Front of the i-th mirror 38.i Back of the i-th mirror A28.j First mirror axis A30.k Second mirror axis i, j, k, m Trajectory indices K Effective curvature L Distance between mirrors, resonator length λ Central wavelength of the laser beam A Bridge spacing of the reflective structuring M Center Q Cross-sectional axis T Depth IN Incoming light beam OUT Out Outgoing light beam

Claims

Laser resonator (10) with (a) a light inlet (12) for a laser beam (14), (b) a first mirror (20), (c) a second mirror (22) and (d) a light outlet (18), (e) wherein the first mirror (20) and the second mirror (22) are arranged such that a laser beam (14) entering through the light inlet (12) can be directed onto the first mirror (20) such that the laser beam (14) is reflected from the first mirror (20) onto the second mirror (22) and exits from the light outlet (18), characterized in that (f) at least one of the mirrors (20, 22) has at least one metamirror section (24). Laser resonator (10) according to claim 1, characterized in that (a) the first mirror (20) and second mirror (22) each have a substrate (26.1, 26.2) and that (b) the at least one metamirror section (24) has a reflective structuring (27) introduced into the substrate (26), which is introduced into the substrate (26) in particular by means of an etching process and / or electron beam lithography. Laser resonator (10) according to claim 2, characterized in that the reflective structuring (27)(a) has a cross-section (Q) having a depth (T) of at most 5 µm, preferably at most 2 µm, preferably at most 1 µm, preferably at most 0.5 µm, and / or(b) a plurality of geometric surface structures (31.j) which are preferably periodic and / or have a form of crosses and / or ridges which are in particular arranged parallel to each other and / or have an approximately rectangular cross-section. Laser resonator (10) according to one of the preceding claims, characterized in that (a) the first mirror (20) has at least two metamirror sections (24) in the form (i) a first first-mirror metamirror section (28.1) and (ii) a second first-mirror metamirror section (28.2), (b) wherein the first first-mirror metamirror section (28.1) has a first first-mirror axis (A28.1), (c) the second first-mirror metamirror section (28.2) has a second first-mirror axis (A28.2) which differs from the first first-mirror axis (A28.1), and (d) the first-mirror metamirror sections (28.i) are configured such that a laser beam (14) which falls on the first first-mirror metamirror section (28.1) through the light entrance (12) is reflected onto the second mirror (22), and from the second mirror (22) onto the second first mirror metamirror section (28.2) is reflected. Laser resonator (10) according to claim 4, characterized in that (e) the second mirror (22) has at least two metamirror sections (24) in the form (i) a first second-mirror metamirror section (30.1) and (ii) a second second-mirror metamirror section (30.2), (a) wherein the first second-mirror metamirror section (30.1) has a first second-mirror axis (A30.1), (b) the second second-mirror metamirror section (30.2) has a second second-mirror axis (A30.2) which differs from the first second-mirror axis (A30.1), and (c) the first-mirror metamirror sections (28.i) and the second-mirror metamirror sections (30.i) are configured such that (i) a laser beam (14) entering the first first-mirror metamirror section through the light entrance (12) (28.1) falls, is reflected from the first first mirror metamirror section (28.1) onto the first second mirror metamirror section (30.1),(ii) from the first second mirror metamirror section (30.1) is reflected onto the second first mirror metamirror section (28.2),(iii) is reflected from the second first mirror metamirror section (28.2) onto the second second mirror metamirror section (30.2) and,(iv) indirectly or directly, then reaches the light exit (18). Laser resonator (10) according to one of the preceding claims, characterized in that (a) the light inlet (12) is arranged on the first or second mirror (20, 22) and / or (b) the light outlet (18) is arranged on the first or second mirror (20, 22) and (c) the light inlet (12) and / or the light outlet (18) has a surface structure (31.j) which is introduced into the substrate (26.i) of the first or second mirror (20, 22), in particular by means of an etching process and / or electron beam lithography. Laser resonator (10) according to one of the preceding claims, characterized by (a) a spacer (34) to which the first mirror (20) and the second mirror (22) are attached such that a distance (L) between the first and second mirror (20, 22) is defined, (b) wherein in particular the first and / or second mirror is mechanically and / or materially connected to the spacer (34) or is formed in one piece. Method for manufacturing a laser resonator (10) with a light inlet (12) and a light outlet (18), comprising the steps: (a) providing a first and second mirror (20, 22), characterized by the steps (b) surface structuring of a substrate (26.1, 26.2) of the first mirror (20) and / or second mirror (22) such that at least one of the mirrors (20, 22) has at least one metamirror section (24), and (c) arranging and fixing the first mirror (20) and the second mirror (22) such that a laser beam (14) can be directed through the light inlet (12) onto the first mirror (20) in such a way that the laser beam (14) is reflected from the first mirror (20) onto the second mirror (22) and exits from the light outlet (18). The method of claim 8, characterized in that the surface structuring step comprises the following steps: (a) introducing a reflective structuring (27), in particular by means of an etching process and / or electron beam lithography, into the substrate (26), such that (i) the first mirror (20) has a first first-mirror metamirror section (28.1) and a second first-mirror metamirror section (28.2), (ii) the first first-mirror metamirror section (28.1) has a first first-mirror axis (A28.1), (iii) the second first-mirror metamirror section (28.2) has a second first-mirror axis (A28.2) that differs from the first first-mirror axis (A28.1), and / or such that (b) the second mirror (22) (i) has a first second-mirror metamirror section (30.1) and a second second-mirror metamirror section (30.2) (ii) wherein the first secondary mirror metamirror section (30.1) has a first secondary mirror axis (A30.1) has,(iii) wherein the second secondary mirror metamirror section (30.2) has a second secondary mirror axis (A30.2) which is different from the first secondary mirror axis (A30.1). The method of claim 9, characterized by the steps: (a) surface structuring of a section of the second mirror (22) such that the section is the light inlet (12) through which an incoming laser beam (14) with at least 95% of an input power of the laser beam (14) is transmitted and deflected by less than 5°, preferably less than 2°, (b) providing a spacer (34) which in particular has an inlet opening and an outlet opening, (c) arranging the first mirror (20) and second mirror (22) on the spacer such that (i) the second mirror (22) is arranged at the inlet opening and the first mirror (20) is arranged at the outlet opening, (ii) a laser beam (14) is transmitted through the light inlet (14) of the second mirror (22), (iii) the laser beam (14) falls on the first first mirror metamirror section (28.1) and is directed onto the first secondary mirror metamirror section (30.1) is reflected, (iv) the laser beam (14) is reflected from the first secondary mirror metamirror section (30.1) to the second primary mirror metamirror section (28.2), (v) the laser beam (14) is reflected from the second primary mirror metamirror section (28.2) to the second secondary mirror metamirror section (30.2) and, (vi) the laser beam (14), directly or indirectly, then reaches the light exit (18).

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