DEVICE, LASER SYSTEM AND METHOD FOR COMBINING COHERENT LASER BEAMS

Description

[0001] The invention relates to a laser system according to independent claims 1, 21 and 22. The invention further relates to a method for combining coherent laser beams according to independent claims 23, 26 and 27.

[0002] For the purposes of this application, "coherent laser beams" means that the laser beams are temporally coherent with each other. In general, the laser beams may exhibit a reduced degree of spatial coherence, i.e., they may be partially coherent in space; that is, they are not necessarily single-mode laser beams. For example, the laser beams may be generated by multimode sources and may form, for instance, a higher-mode Gaussian mode, such as a Laguerre-Gaussian mode, a Hermite-Gaussian mode, or superpositions thereof. Preferably, however, the laser beams are coherent in both time and space.

[0003] In beam combining, multiple laser beams are superimposed to form a single, combined laser beam with a correspondingly higher power output. In coherent tiled aperture beam combining, also known as side-by-side beam combining, the cross-sectional area of ​​the combined laser beam is increased, but the divergence is reduced, which is generally not possible without a loss of power or beam quality. In coherent filled aperture beam combining, multiple laser beams are superimposed to form a single, combined laser beam that typically has the same cross-sectional area and ideally the same divergence as a single laser beam. Such beam combining can be achieved—with virtually no loss of beam quality—diffractively, interferometrically, or via polarization coupling.

[0004] US patent 2017 / 0201063 A1 describes a system for phase-matching a number M of laser sources arranged in a periodic spatial configuration. The system includes means for collimating and aligning the number M of laser beams onto a combining diffractive optical element comprising a phase grating. The diffractive optical element is located in the object plane of a Fourier lens. The system also includes means for phase-matching the laser sources based on a negative feedback signal originating from the combined laser beams.

[0005] US Patent 7,924,894 B2 discloses a high-power laser system comprising a master oscillator and multiple fiber laser amplifiers for generating output beamlets. A diffractive optical element combines the output beamlets into a combined output beam. A control device minimizes errors resulting from the combination of the output beamlets, which would otherwise reduce the quality of the combined output beam. The phase of the beamlets is adjusted using phase modulators in response to an error signal.

[0006] US Patent 8,248,700 B1 describes systems and methods for coherent beam combination. The system comprises an array of coherent laser emitters that generate an array of output beams, which are collimated by an array of collimation lenses. The collimated output beams are focused by a lens into a focal plane containing a phase screen to generate one or more coherently superimposed beams from the array of output beams, corresponding to a phase combination of the array. The phase screen may be a diffractive optical element.

[0007] YU CX ET AL: "Coherent beam combining of large number of PM fibres in 2-D fibre array", ELECTRONICS LETTERS, THE INSTITUTION OF ENGINEERING AND TECHNOLOGY, GB, Vol. 42, No. 18, 31 August 2006 (2006-08-31), pages 1024-1025, ISSN: 0013-5194, DOI: 10.1049 / EL:20061938, discloses a device for combining coherent laser beams, comprising phase adjustment devices and a microlens arrangement. Aufgabe der Erfindung

[0008] The invention is based on the objective of providing a laser system and an associated method which make it possible to preserve the beam quality of each individual laser beam almost completely in the at least one combined coherent laser beam. Gegenstand der Erfindung

[0009] This problem is solved according to the invention by a laser system according to claims 1, 21 and 22.

[0010] The inventors have recognized that, with a suitable determination of the (relative) phase or phase differences between the coherent laser beams, a microlens arrangement can be used as a beam combining device for the coherent combination of a plurality of coherent laser beams into at least one combined laser beam with high beam quality. While it is generally known to use a microlens arrangement or an imaging homogenizer for homogenizing radiation, in particular for homogenizing a laser beam, according to the invention the microlens arrangement is used for the coherent combination of a plurality of laser beams.With a suitable design of the microlens arrangement, for example a suitable choice of the focal lengths of the microlenses of the microlens arrays, as well as a suitable adaptation of the phase front of the laser beams that are directed onto the microlens arrangement, in particular by a suitable choice of the (relative) phases δφ n of the coherent laser beams, a high efficiency in coupling can also be achieved.

[0011] While it is theoretically possible to achieve phase matching solely through suitable coupling optics for coupling the coherent laser beams into the microlens arrangement—that is, through one or more appropriately designed optical elements—in practice, phase adjustment devices are typically used to individually adjust the (relative) phases δφ n of the coherent laser beams. Additionally, suitable coupling optics can be used in combination to perform phase matching. This is advantageous because phase adjustment devices are generally required anyway to correct or control the phases of the coherent laser beams, for example, to compensate for temperature-related influences on the device.

[0012] Phase-adjusting devices are used to adjust the phase of coherent laser beams, typically before they exit an emission surface, i.e., before they reach the microlens array via free-space propagation. A variety of methods exist for implementing phase-adjusting devices, which are typically designed to set a variable phase delay. For example, the phase-adjusting devices can be modulators in the form of electro-optical modulators (EOMs), spatial light modulators (SLMs), optical delay lines in the form of mirror arrays, or similar devices. If the coherent laser beams are guided in a fiber before emission from an emission surface, phase adjustment can be achieved, for example, by applying tension to the fiber using piezoelectric actuators, by influencing the fiber temperature, etc.

[0013] If the coherent laser beams are guided across an emission surface in a gain medium and optically amplified before emission, the specific gain characteristic of the respective optical amplifier can be adjusted, thus establishing a phase relationship between the individual channels or laser beams. The gain characteristic can represent the overall gain and / or the absolute output power of the individual coherent beams. Different phase relationships can be caused by varying numbers of excited laser-active ions in the gain medium or by differing optical intensity profiles, which, due to optical nonlinearity, lead to a change in phase either within the gain medium itself or in another optical element.

[0014] In one aspect of the invention, the coherent laser beams and the microlens arrangement satisfy the following equation: N = p 2 / λ L f E where N is the number of coherent laser beams for the combination, p is the grid spacing of the microlenses in a given microlens array, λL is the laser wavelength of the coherent laser beams, and fE is the (effective) focal length of the microlens arrangement. In the simplest case, the microlens arrangement comprises two microlens arrays with identical focal lengths, spaced apart by their focal lengths. In this case, the focal length of the microlens arrangement coincides with the (combined) focal length of the two microlens arrays.

[0015] The inventors have recognized that when combining the laser beams into a single coherent laser beam, the beam quality of that beam is almost completely preserved if equation (1) above is satisfied. This exploits the fact that a microlens array or an imaging homogenizer, irradiated with a coherent, collimated laser beam, produces a diffraction pattern with N diffraction spots of equal intensity if equation (1) is satisfied (see the article by M. Zimmermann et al., "Refractive Micro-optics for Multi-spot and Multi-line Generation," Proceedings of LPM2008—the 9th International Symposium on Laser Precision Microfabrication). The inventors propose reversing the beam path through the imaging homogenizer and arranging the emission surfaces for the coherent laser beams at the positions where the diffraction spots are generated in the cited article.to emit the coherent laser beams in the direction of the microlens array. If the coherent laser beams have (approximately) the same intensity, reversing the beam direction will produce a combined, coherent laser beam of high beam quality.

[0016] It is understood that equation (1) cannot be followed exactly in practice. If equation (1) is deviated from, the beam quality of the superimposed laser beam deteriorates.

[0017] In one embodiment, the microlens arrangement comprises at least three microlens arrays and is designed to adjust its (effective) focal length f E. Adjusting the focal length of the microlens arrangement is necessary when the number N of coherent laser beams used for the combination is changed and equation (1) is still to be satisfied, since the other parameters in equation (1), i.e., the laser wavelength λ L and the microlens grid spacing p, cannot be readily changed.

[0018] The effective focal length fE of the microlens arrangement refers to a beam beam that passes through one microlens per microlens array in the beam path (corresponding to a sub-aperture). A microlens arrangement that can be used as a beam combining device in the device according to the invention has, with respect to a beam beam, one microlens per microlens array, in the notation of matrix optics, i.e., as an imaging matrix. A B C D The following properties typically apply: A=0; |B|=f E . Furthermore, D<1 usually also holds (for a substantially collimated exit beam); C then depends on A, B, and D. The use of at least three microlens arrays is required to satisfy the second condition |B|= f E for variable f E.

[0019] In the simplest case, each microlens array within the microlens assembly is implemented by its own multilens array component. However, it is also possible for multiple microlens arrays in the beam path to be implemented by a single microlens array component, provided the beam path passes through this microlens array component multiple times, for example, because the beam path is deflected at a reflective optical element.

[0020] There are various ways to adjust the effective focal length of the microlens array: In one advanced design, the microlens array is configured to adjust its (effective) focal length by changing the (optical) distance between at least a first microlens array and a second microlens array. For two (thin) (micro)lenses, for example, an effective focal length fE can be determined from the focal lengths of the individual lenses f1a and f2a and the distance ds between the lenses, according to the following basic relationship: 1 f E = 1 f 1 a + 1 f 2 a − ds f 1 a ∗ f 2 a

[0021] From this basic relationship, the effective focal length fE of a microlens arrangement with any number of microlens arrays (usually three or four) can be determined (iteratively, if necessary). Since changing the number of coherent laser beams used for combination is generally not time-critical, the adjustment or variation of the effective focal length of the microlens arrangement can be performed manually by moving the microlens arrays to predetermined positions. Optionally, the device can include a motorized adjustment mechanism and an electronic control unit to automatically move to the various positions. The adjustment mechanism can include a carriage on which one microlens array or one microlens array component, or at least two microlens array components, are arranged at a fixed distance.The carriage can be designed to be movable relative to at least one other microlens array component, and in particular relative to at least two microlens array components, along the beam propagation direction. For example, a first and third microlens array in the beam path can be fixed in position, and a second and fourth microlens array in the beam path can be arranged on the movable carriage, or vice versa.

[0022] To combine coherent laser beams into a single combined laser beam, it is typically necessary or advantageous to couple adjacent coherent laser beams into the microlens array with a predefined angular difference δθ, where δθ = λL / p, and λL is the laser wavelength and p is an (identical) grid spacing of the microlenses in a given microlens array. To fulfill this condition, the emission surfaces from which the coherent laser beams emerge can be aligned relative to each other at the respective angular difference δθ and, for example, arranged on a circular arc. Focusing of the coherent laser beams can then be achieved, for example, using individual lenses positioned in the beam path of each coherent laser beam; however, it may also be possible to omit such lenses.

[0023] In a further embodiment, the device has a movement mechanism for generating a preferably adjustable lateral offset between at least a first microlens array and at least a second microlens array of the microlens arrangement. The movement mechanism shifts the grids in which the microlenses of the two microlens arrays are arranged laterally, i.e., perpendicular to the direction of propagation of the combined laser beam or to the optical axis along which the microlens arrays are spaced apart from one another. The lateral shift is generally by a non-integer multiple of the grid spacing of the microlenses, so that successive microlenses or sub-apertures in the direction of propagation are laterally offset from one another. Depending on the size orDepending on the amount of lateral offset, the combined laser beam can be selectively deflected or split to form two or more combined laser beams, which generally have the same power. The laser energy and beam quality of the individual coherent laser beams can be essentially maintained. The lateral offset allows the optical parameters used to form a single combined laser beam to be modified so that the combination results in several well-defined bundles (beam splitting) of laser beams with uniformly distributed power. Alternatively or additionally, a single laser beam can be formed during the combination process, propagating parallel to the optical axis (beam deflection), provided that the angular distribution of the far field is imaged with a lens or similar device and thereby converted into a spatial distribution.This method for beam splitting and / or beam deflection can be implemented at control rates of << 1 kHz.

[0024] The movement device typically serves to laterally displace one of the microlens arrays. For the purpose of misaligning or displacing the at least one microlens array, the movement device may, for example, comprise a piezoelectric element, a voice coil, a mechanical device (possibly in the form of a camshaft), etc., or be formed from such a component.

[0025] In a further training, the first and second microlens arrays for forming two combined laser beams have a lateral offset Δ, for which the following applies: Δ = ± p / 2 N , where N denotes the number of coherent laser beams and p a grid spacing of the microlenses of a respective microlens array:

[0026] In this case, the microlenses of the first microlens array are offset by the amount Δ in one direction (e.g., the X-direction) relative to the microlenses of the second microlens array. The two combined laser beams, or rather the two diffraction orders into which they are diffracted, are also offset in the X-direction.

[0027] In a further development, the first and second microlens arrays for combining an even number N of coherent laser beams into a combined laser beam exhibit a lateral offset Δ, for which the following holds: Δ = ± p / 2 N + 1 , where N denotes the number of coherent laser beams and p a grid spacing of the microlenses of a respective microlens array, wherein for a preferably adjustable distance d between the first microlens array and the second microlens array: d = p 2 / N λ L , where λL denotes the laser wavelength. The distance d between the two laterally offset microlens arrays can usually be adjusted by means of a motorized adjustment mechanism in the manner described above in connection with the setting of the effective focal length fE of the microlens arrangement; however, it is also possible that the distance d between the two microlens arrays is constant and cannot be changed.

[0028] In a further development, the first and second microlens arrays for forming a combined laser beam diffracted into a diffraction order B other than zero exhibit a lateral offset Δ, for which the following applies: Δ = p / N B , where N denotes the number of coherent laser beams and p a grid spacing of the microlenses in a given microlens array. The diffraction order B takes on non-zero integer values, i.e., B = ±1, ±2, .... The lateral offset Δ allows the combined laser beam to be deflected into a specific diffraction order, i.e., at a predetermined angle corresponding to that order. The intensity of the diffracted combined laser beam decreases depending on the diffraction order, for example, by approximately 3% for the ±1st diffraction order and by approximately 10% for the ±2nd diffraction order; thus, the efficiency of the beam combination remains high in this case as well.

[0029] In the event that a plurality of laser beams are to be combined two-dimensionally into a common laser beam and the microlens arrangement for this purpose has two pairs of one-dimensional, directly one behind the other, crossed microlens arrays, the motion device can be used to set a separately adjustable lateral offset of the microlenses of a respective one-dimensional microlens array in two different, for example perpendicular directions (X, Y).

[0030] If two-dimensionally structured microlens arrays are used for the two-dimensional combination, independent adjustment of the lateral offset in two directions is not possible. If such a two-dimensionally structured microlens array is laterally offset in one direction (e.g., the X-direction), the combined laser beam will also be deflected in the second direction (e.g., the Y-direction), or four combined laser beams will be generated instead of two combined laser beams.

[0031] In a further embodiment, the device includes a coupling optic for coupling the coherent laser beams into the microlens arrangement. The coupling optic is preferably configured to couple adjacent coherent laser beams with a predetermined angular difference δθ into the microlens arrangement, where δθ = λL / p, where λL is the laser wavelength and p is the grid spacing of the microlenses in a respective microlens array. In this case, a coupling optic is used that is arranged between the emission surfaces from which the coherent laser beams emerge and the microlens arrangement. The coupling optic is not strictly necessary but can be advantageous, for example, when setting up the device.In particular, the coupling optics can be used to satisfy the requirement of an angular difference δθ without requiring the beam exit directions of the coherent laser beams from the emitter surfaces to be aligned at an angle to each other. For example, in this case, the emitter surfaces can be arranged in a line, meaning the beam exit directions, or Poynting vectors, of the coherent laser beams are aligned parallel to each other. The use and design of the coupling optics and the arrangement of the emitter surfaces depend on the specific conditions, such as the laser source used. For instance, the use of coupling optics is advantageous if the emitter surfaces form the end faces of parallel fibers.

[0032] In a further development, the coupling optics include at least one focusing device, in particular at least one focusing lens, for focusing the majority of coherent laser beams onto the microlens arrangement. The use of a focusing lens positioned essentially at a distance of its focal length from the microlens arrangement (Fourier lens) has proven advantageous for fulfilling the requirement for the angular difference δθ, provided the emission surfaces are arranged at a suitable distance from one another. In this case, the coherent laser beams can strike the focusing lens essentially parallel to each other and are focused onto the microlens arrangement.

[0033] In a further development, the coupling optics for correcting phase front curvature in the plurality of coherent laser beams comprise at least one additional imaging optic, in particular at least one additional lens. As described above, at least a portion of the relative phase of the coherent laser beams required for coherent combination can be adjusted using optics comprising one or more optical elements. Adjusting an absolute (i.e., independent of the specific laser beam) portion of the laser beams' phase front can be considered a correction of the phase front curvature. However, the use of (an additional) optics for phase front correction is not strictly necessary if the device, in particular the coupling optics, is suitably designed (su). The additional optics are typically arranged in the beam path of the coherent laser beams upstream of the focusing device.

[0034] To generate multiple coherent laser beams, the laser source can consist of a single laser, for example, in the form of a (fiber) master oscillator. The seed laser radiation generated by the laser is split into multiple N laser beams, for example, using a conventional 1-to-N coupling device, such as a single microlens array. Alternatively, multiple laser sources, such as fiber oscillators, laser diodes, etc., can be used to generate multiple N coherent laser beams. In this case, the laser system includes a control device for driving the laser diodes to generate the coherent laser beams. The laser source(s) can be configured to generate ultrashort pulse laser beams, i.e., coherent laser beams with a pulse duration of less than, for example, 10⁻¹² s.

[0035] The coherent laser beams generated in the laser source(s) are typically guided to the emission surfaces by means of a plurality N of beam guides, for example, in the form of fibers. The individual beam guidance of the laser beams allows for individual manipulation of each beam to adjust the relative phases appropriately using the phase adjustment device. The beam guides can include a corresponding plurality N of amplifiers or amplifier chains, for example, in the form of fiber amplifiers, to amplify the laser beams before they are emitted from the emitter surfaces towards the microlens array. The phase adjustment devices can be arranged in the beam path upstream or downstream of the beam guides and / or act directly on the beam guides, e.g., in the form of fibers.The advantage of such a laser system with individual amplifiers is not only to achieve higher laser powers, but also that the errors that accumulate in the amplifier setup affect each of the coherent laser beams differently and are therefore averaged out when forming the combined laser beam.

[0036] In another embodiment, the coherent laser beams emitted from the emission surfaces exhibit a Gaussian beam profile, a donought-shaped beam profile, or a top-hat beam profile. As described above, the temporal coherence of the laser beams is important for superposition; however, the laser beams can be generated by spatially partially coherent laser sources, such as multimode laser sources. In other words, the coherent laser beams are not necessarily single-mode laser beams that are superimposed in the microlens arrangement to form a combined single-mode laser beam with a larger beam diameter. In the diffraction pattern or multi-spot profile generated by the microlens arrangement when it is traversed in reverse by a substantially collimated laser beam, the individual spots exhibit, for example,The beam profile may be essentially Gaussian, essentially donought-shaped, or essentially top-hat-shaped. However, it is generally advantageous for the coherent laser beams used for the coherent combination to be single-mode laser beams combined into a single-mode laser beam. Since compliance with equation (1) produces a homogeneous distribution of N individual spots, the intensity of the coherent laser beams emerging from the emitter surfaces should also be as homogeneous as possible, i.e., of equal magnitude.

[0037] In another embodiment, the laser system includes a control unit designed and programmed to adjust the (effective) focal length of the microlens arrangement depending on the number N of emitted coherent laser beams. The number of emitted coherent laser beams can change, for example, when additional laser sources are added to or removed from the laser system. The laser system described here is scalable, meaning that, in principle, any number of coherent laser beams can be superimposed. By adjusting the (effective) focal length of the microlens arrangement to the respective number N of laser sources, a modular design of the laser system can be achieved.

[0038] Optionally, the control device for controlling the at least one laser source can be configured to determine a number N of emitted coherent laser beams. In this case, individual laser sources can be selectively switched on or off, for example, to adjust the intensity of the combined laser beam. To change the number N of emitted coherent laser beams, the outermost laser sources of the laser system are typically switched on or off with respect to a central axis of the microlens array. The number of microlenses in the microlens array is independent of the number N of coherent laser beams used for combination.

[0039] In another embodiment, the distance between the focusing device of the coupling optics and the microlens arrangement is adjustable, and the control device is configured to adjust the distance depending on the number N of emitted coherent laser beams. In this case, a number N of parallel coherent laser beams can be directed onto the focusing device, for example, onto a focusing lens, without requiring additional optics for phase correction. If the focusing device has a focal length f₂, the microlens arrangement, more precisely the first microlens array of the microlens arrangement, is ideally arranged at a distance L₂ from the focusing device, which is given by: L₂ = f₂ - p₂ / (λ LN) or L₂ = f₂ - f₉.To adjust the distance L2, the control device can, for example, act on the adjustment mechanism of the microlens arrangement described above. By deviating the distance L2 of the microlens arrangement from the focal length f2 of the focusing device by p2 ≤ (λLN), a common component of the phase front, or a common phase front curvature of the laser beams incident on the microlens arrangement, is adjusted such that the coherent laser beams form a combined, single laser beam when passing through the microlens arrangement or through the imaging homogenizer. The condition for the distance L2 is also considered fulfilled if the right side deviates from the left side by less than 5%, preferably by less than 2%.

[0040] In another embodiment, the focusing device is arranged at a distance of its focal length f₂ from the microlens arrangement, and the focal length f₂ of the focusing device, a focal length f₁ of the further imaging optics, and a focal length f₉ of the microlens arrangement satisfy the following condition: f₁ = -f₂ / f₉. As described above, the further imaging optics, which may comprise one or more optical elements, serve to correct a phase-front curvature common to all laser beams. Assuming that the further imaging optics are arranged in front of the focusing device at a distance of its focal length f₂, it is necessary for the focal lengths to satisfy the above condition for this purpose. The condition for the focal lengths is also considered satisfied if the right-hand side deviates from the left-hand side of the above condition by less than 5%, preferably by less than 2%.

[0041] In a further development, the focal length of the additional imaging optics is adjustable, and the control device is designed to adjust the (effective) focal length of the additional imaging optics as a function of the number N of emitted coherent laser beams. Since the effective focal length of the microlens arrangement changes when the number N of emitted coherent laser beams changes, it is typically necessary to adjust the focal length of the additional imaging optics to the focal lengths in order to fulfill the above condition, unless the distance between the focusing device and the microlens arrangement is adjusted. This adjustment is possible, for example, if the additional imaging optics has two (thin) lenses whose distance ds is changed, whereby the effective focal length f₁ or f₂ changes.f E is changed according to the basic equation (2) above, where in this case f 1a , f 2a denote the focal lengths of the thin lenses of the further imaging optics.

[0042] In one aspect of the invention, the phase adjustment devices are designed for the nth coherent laser beam (n = 1, ..., N ) to set a (relative) phase δφ n, which is given by: δφ n = − 2 π / λ L f E m n λ L / p 2 where p denotes a grid spacing of the microlenses of a respective microlens array, f E the focal length of the microlens arrangement and λ L the laser wavelength.

[0043] The following applies to the running index mn: m n = − N + 1 2 + n .The relative phase δφ n differs for each individual coherent laser beam and is therefore set using the phase-adjusting devices, not by one or more optical elements of the coupling optics. Individual adjustment of the (relative) phase of the coherent laser beams before they strike the microlens array is necessary to generate a suitable phase front for the coherent combination of the laser beams. A control unit can be used to regulate the phase-adjusting devices, adapting the individual (relative) phases to the properties of the combined laser beam.

[0044] In another embodiment, adjacent emitter surfaces are arranged equidistantly along the direction and have a distance δx from each other, which is given by δx = λL f² / p, where λL is the laser wavelength, f² is the focal length of the focusing device, and p is a grid spacing of the microlenses in a respective microlens array. The coherent laser beams typically emerge from emitter surfaces that are identically spaced from each other. If the laser beams are parallel, the emitter surfaces are typically arranged along a common direction or line (e.g., in the X-direction) that is perpendicular to the common beam propagation direction of the laser beams. In this case, the distance δx between the laser beams or the emitter surfaces is typically determined by the condition above.

[0045] In an alternative embodiment, adjacent emission surfaces are arranged and aligned equidistantly on a circular arc, and adjacent coherent laser beams with a predetermined angular difference δθ are coupled into the microlens arrangement, for which the following applies: δθ = λ L / p , where λL is the laser wavelength and p is the grid spacing of the microlenses in a given microlens array. As described above, arranging the emission surfaces on a common circular arc with an angular difference δθ that satisfies the condition stated above has proven advantageous.

[0046] In one embodiment, the coupling optics comprise a beam-shaping optic for shaping each coherent laser beam with a predetermined, and in particular adjustable, divergence angle upon coupling into the microlens arrangement. In the examples described above, the coherent laser beams are typically coupled into the microlens arrangement in a collimated state. The beam-shaping optics can, for example, comprise two or more lenses arranged in series, which spread all coherent laser beams, or several lenses or other optical elements, such as a number of individual lenses corresponding to the plurality of coherent laser beams or a (further) microlens array, can be provided to generate the divergence angle (which is generally the same for all coherent laser beams).If the emission surfaces are arranged on a circular arc, the distance D between the arc and the microlens array can be reduced in this way, thus achieving a more compact design. For example, if the beam-shaping optics are formed by a lens with a focal length f and the (effective) focal length of the microlens array is denoted by fE, then the distance D = fE - fE. The distance D can be smaller than the distance L2 = f2 - fE described above between the focusing device and the first microlens array, since the focal length f of the beam-shaping optics can be chosen to be smaller than the focal length f2 of the focusing device.

[0047] In a further training, the laser system has an even number N of emitter surfaces and the beam-shaping optics have a focal length f that satisfies the following condition: f = p 2 / λ L , where λL denotes the laser wavelength and p a grid spacing of the microlenses in a given microlens array. This condition generally applies to the combination of an even number N of coherent laser beams, i.e., it is independent of the number N of coherent laser beams. This condition on the focal length of the beam-shaping optics also defines the most compact design of the device; i.e., the focal length f should not be less than the number N of coherent laser beams, i.e., f ≥ p2 / λL.

[0048] The above considerations assumed a one-dimensional arrangement of the emitter surfaces, which can be arranged, for example, along a common line or on a common arc. The emitter surfaces lie in a common plane with the microlenses of the microlens arrays, in which the beam propagation directions of the laser beams also lie. In this case, the microlenses are typically cylindrical lenses.

[0049] As an alternative to a one-dimensional coherent combination of laser beams, it is also possible to combine a plurality of N x M laser beams two-dimensionally into a single laser beam. In this case, the emitter surfaces are arranged in a two-dimensional grid or lattice, whereby the distances between adjacent emitter surfaces can be the same or different in both directions. The grid with the emission surfaces can extend in a plane (e.g., an XY plane) or on a curved surface, e.g., a spherical shell. In the first case, the laser beams emerging from the emitter surfaces are typically aligned parallel to each other, while in the second case, they can be directed, for example, towards the center of the spherical shell where the microlens array is located.

[0050] The periodicity of the grid with the emitter surfaces determines the grid spacing of the microlenses in two different, for example, perpendicular directions (X, Y). In this case, 2-dimensional microlens arrays can be used whose grid spacings px, p, Y may differ in the two mutually perpendicular directions X, Y, depending on the grid's periodicity. The microlenses of the 2-dimensional microlens array will accordingly have different curvatures in the X and Y directions; that is, they are not cylindrical lenses. It is also possible to replace each 2-dimensional microlens array with two 1-dimensional microlens arrays containing cylindrical lenses, where the cylindrical lenses of the 1-dimensional microlens arrays are oriented perpendicular to each other.

[0051] The relationship between the two-dimensional grating with the emitter surfaces and the two-dimensional microlens arrays is analogous to the relationship between the Bravais grating and the reciprocal grating. Accordingly, the arrangement of the emitter surfaces can also correspond to a closest-packed arrangement, i.e., a hexagonal grating. In this case, the microlenses of the microlens arrays are also arranged in a hexagonal configuration. The combination of the coherent laser beams in the two linearly independent, not necessarily perpendicular, grating directions is fundamentally independent; that is, the conditions or equations given above apply to both grating directions independently of each other.

[0052] Only when adjusting the relative phase of the laser beams do the contributions in the two mutually perpendicular directions add up, i.e., for a number N x M of laser beams arranged in a rectangular grid (in the X-direction or Y-direction), the following applies for the respective phase δφ n,m: δφ n , m = − 2 π / λ L f E , X p n λ L / p x 2 − 2 π / λ L f E , Y q m λ L / p Y 2 , where px denotes the grid spacing of the microlenses in the X direction and py the grid spacing of the microlenses in the Y direction, and where f E,X denotes the effective focal length of the microlens arrangement in the X direction and f E,Y the effective focal length of the microlens arrangement in the Y direction. The following applies in each case: p n = − N + 1 2 + n with n = 1, ... , N and q m = − M + 1 2 + m with m = 1, ... , M .

[0053] In another embodiment, the laser system has an even number N of emitter surfaces, and the microlens array includes a phase-shifting device configured to generate a constant phase shift of π between beams of the coherent laser beams passing through adjacent microlenses of a given microlens array. A diffraction pattern typically has a 0th diffraction order and, symmetrically to it, an even number of higher orders, i.e., an odd total number of diffraction orders. Therefore, to superimpose an even number N of coherent laser beams along a direction (e.g., the X-direction), it is typically necessary to eliminate the 0th diffraction order. For this purpose, a phase-shifting device can be used, which suppresses the 0th diffraction order by destructive interference.

[0054] The phase-shifting device can, for example, be designed as a phase-shifting element in which first and second passing elements are formed alternately in a direction transverse to the beam propagation direction, the passage of a first passing element relative to the passage of a second passing element generating a phase shift of π. Such a phase-shifting element can be selectively inserted into or removed from the beam path depending on the (even or odd) number of coherent laser beams, for which purpose, for example, the adjustment mechanism described above can be used.

[0055] Alternatively, the phase-shifting device can be integrated into one of the microlens arrays. In this case, the profiling of the microlens array includes a thickness step at the interface between two adjacent microlenses, which generates the phase shift of π. With this design, a separate phase-shifting element can be omitted. For example, the thickness of the microlens array changes gradually across the width of a microlens aperture or grid spacing, thus creating a phase shift of π across the aperture width; this thickness change is superimposed on the actual lens profile. It is also possible to design every second microlens with a uniform additional thickness, which, analogous to the phase-shifting element described above, generates the phase shift.

[0056] The laser system described above also allows, in principle, the coherent combination of ultrashort pulse laser beams without requiring additional modifications. However, if a relatively large number of coherent laser beams are to be combined along a spatial direction (for example, on the order of ten or more laser beams), it is advantageous to implement a wavelength-dependent correction.

[0057] In a further embodiment, the laser system comprises a first wavelength-dispersive element, in particular a first diffraction grating, arranged upstream of the microlens arrangement for spectrally splitting the coherent laser beams into several beam bundles which pass through the microlens arrangement in spatial separation, and a second wavelength-dispersive optical element, in particular a second diffraction grating, arranged downstream of the microlens arrangement for spectrally combining the beam bundles to form the combined laser beam with one wavelength.

[0058] In this embodiment, the laser beams, which have a uniform wavelength λL or a uniform wavelength spectrum upon emission, are spectrally split into several beams by means of a first wavelength-dispersive optical element. These beams typically correspond to different diffraction orders of the wavelength-dispersive optical element (and thus different wavelengths). The respective beams pass through the microlens arrangement spatially separated and essentially parallel to each other. The coupling optics can include an optical element, for example, in the form of a collimating lens, to collimate or align the beams emerging from the first wavelength-selective element. By means of further optics, for example, a focusing lens, the beams are directed onto the second wavelength-dispersive optical element after passing through the microlens arrangement.The focus is achieved by the spectral merging of the beam bundles into a single laser beam. The equations and relationships mentioned above, which include the wavelength λL, apply accordingly to the central wavelength of the spectral distribution of the beam bundles, which does not necessarily have to coincide with the wavelength λL of the laser beams emitted from the emitter surfaces.

[0059] In this advanced version, the coupling optics feature a collimation lens designed as an aspherical lens. The curvature of the aspherical lens surface is chosen to provide wavelength-dependent correction of the phase front of the laser beams, or more precisely, of the individual beam bundles. As described above, such wavelength-dependent correction is generally only necessary when a relatively large number of ultrashort pulse (USP) laser beams are to be coherently superimposed.

[0060] Another aspect of the invention relates to a method according to claims 23, 26 and 27.

[0061] Changing the number N of combined laser beams requires changing the (effective) focal length of the microlens arrangement. For this purpose, the adjustment mechanism described above can be used, for example.

[0062] In another variant, the coherent laser beams are coupled in using coupling optics that include a focusing device. The method further comprises: setting a distance between the focusing device and the microlens array, given by: L2 = f2 - p2 / (N λL), where f2 is the focal length of the focusing device, p is the grid spacing of the microlenses in a given microlens array, and λL is the laser wavelength. As described above, in addition to adjusting the absolute phase differences, a correction of the (global) phase front curvature is typically required, which depends on the number N of coherent laser beams. In this example, no additional imaging optics are typically needed to correct the phase front curvature.

[0063] In an alternative variant, coupling is achieved using a coupling optic comprising a focusing device and a further imaging optic. The method further includes: setting a focal length f1 of the further imaging optic, which is given by: f1 = -f2 / fE, where f2 is a focal length of the focusing device and fE is a focal length of the microlens arrangement. The focal length of the further imaging optic can be set, for example, by changing the distance between two or more optical elements of the imaging optic. The further imaging optic can be used to correct the (global) phase curvature of the laser beams. Further advantages of the invention will become apparent from the description and the drawing.

[0064] They show: Fig. 1 a schematic representation of a laser system with a device for combining a plurality of coherent laser beams into a combined laser beam, Fig. 2 a representation of the device of Fig. 1 with a beam combining device comprising a coupling optic and a microlens arrangement with two microlens arrays, Fig. 3a, a representation of a microlens arrangement with three microlens arrays and with an adjustment mechanism as well as the beam path through a sub-aperture of the microlens arrangement, Fig. 4, a representation of a device analogous to Fig. 2 , which are part of the laser system of Fig. 1 is, with a coupling optic that has an imaging optic with adjustable focal length for correcting a phase front curvature, Fig. 5 a representation of a device analogous to Fig. 4 , which are part of the laser system of Fig. 1 is, in which emission surfaces for the emission of the coherent laser beams for correcting the phase curvature are each arranged at an angle to each other, Fig. 6 a representation of a device analogous to Fig.4 , which are part of the laser system of Fig. 1 is, in which the coupling optics are formed by a focusing lens, Fig. 7 a representation of a beam combining device of a device that is part of the laser system of Fig. 1 is, in which the emission surfaces are arranged on a circular arc, Fig. 8a,b Representations of two beam combination devices of devices that are part of the laser system of Fig. 1 are, in which the emission surfaces are each arranged in a grid, Fig. 9a-c Representations of three microlens arrangements of a beam combining device of a device that is part of the laser system of Fig. 1 is, wherein the microlens arrangements each have a phase-shifting device, Fig. 10 Representations of a beam combining device of a device that is part of the laser system of Fig. 1 is, wherein the beam combining device comprises two diffraction gratings and an aspherical lens for wavelength-dependent correction in the coherent combination of ultrashort pulse (USP) laser beams, Fig. 11 a representation of a beam combining device analogous to Fig. 7 , which is part of a device that is part of the laser system of Fig. 1 is, wherein the beam combining device has a beam-shaping optic for shaping coherent laser beams which enter the microlens arrangement with a respective predetermined divergence angle, Fig. 12 a representation of a beam combining device analogous to Fig. 6 , which is part of a device that is part of the laser system of Fig. 1 is, wherein the beam combining device has a motion device for generating a lateral offset between the second microlens array and the first microlens array, Fig. 13a-c representations of the far field of the beam combining device of Fig. 12 without or with a lateral offset of two microlens arrays for deflecting the combined laser beam, as well as Fig. 14a-c representations of the far field of the beam combination device of Fig. 12 without or with a lateral offset of two microlens arrays to form two combined laser beams.

[0065] In the following description of the drawings, identical reference symbols are used for identical or functionally equivalent components.

[0066] Fig. 1 Figure 1 shows an exemplary setup of a laser system 1, which includes a laser source 2 for generating a number N of coherent laser beams 3.1, ..., 3.N. For this purpose, the laser source 2 has a mode-locked fiber master oscillator 4 that generates seed laser radiation with a laser wavelength λL. This seed laser radiation is split into the number N of coherent laser beams 3.1, ..., 3.N in a conventional 1-to-N coupling device 5, for example, in the form of a microlens array. The laser beams 3.1, ..., 3.N pass through a corresponding number N of phase-adjusting devices 6.1, ..., 6.N, which allow the adjustment of a respective individual phase δφn of the laser beams 3.1, ..., 3.5 (n = 1, ..., N) by effecting a suitable phase delay. The phase adjustment devices 6.1, ..., 6.N can, for example, be designed as electro-optic modulators, as acousto-optic modulators, etc.

[0067] Following the phase-adjusting devices 6.1, ..., 6.N, the coherent laser beams 3.1, ..., 3.N pass through a corresponding number N of amplifier fibers 7.1, ..., 7.N to amplify the coherent laser beams 3.1, ..., 3.N. The end faces of the amplifier fibers 7.1, ..., 7.N serve as emission surfaces 8.1, ..., 8.N from which the coherent laser beams 3.1, ..., 3.N are emitted. The phase-adjusting devices 6.1, ..., 6.N can also be arranged downstream of the amplifier fibers 7.1, ..., 7.N or act directly on the amplifier fibers 7.1, ..., 7.N, for example by generating an adjustable mechanical stress on them.

[0068] The coherent laser beams 3.1, ..., 3.N are deflected by a deflection device 9, which has a plurality of deflection mirrors, in order to increase the fill factor, i.e., to reduce the distance between adjacent laser beams 3.1, ..., 3.N. It is understood that the deflection device 9 is not strictly necessary. In the example shown, the coherent laser beams 3.1, ..., 3.N enter a beam combining device 10, which has a microlens arrangement 11 or an imaging homogenizer for the coherent combination of the laser beams 3.1, ..., 3.N to form a combined laser beam 12.

[0069] As in Fig. 1 As can be seen, a portion 12a of the combined laser beam 12 is coupled out via an output coupling device in the form of a partially transmissive mirror 13 and strikes a detector 14. The detector 14 is in signal communication with a control unit 15a of the laser system 1, which controls the phase adjustment devices 6.1, ..., 6.N in order to adjust the individual phases δφ n of the laser beams 3.1, ..., 3.N depending on the properties of the detected portion 12a of the combined laser beam 12. The control unit 15a can, in particular, enable the control of the phase adjustment devices 6.1, ..., 6.N to generate desired (target) phases δφ n of the laser beams 3.1, ..., 3.N. Although in the example shown the number N of phase adjustment devices 6.1, ..., 6.N corresponds to the plurality N of laser beams 3.1, ..., 3.N, a number of N - 1 phase adjustment devices 6.1, ..., 6.N-1 is usually sufficient.At the one in . Fig. 1 In the laser system 1 shown, a high beam quality of, for example, M = 1.3 of the combined laser beam 12 can be achieved, while errors due to nonlinear effects in the individual amplification of the laser beams 3.1, ..., 3.N in the amplifier fibers 7.1, ..., 7.N average out each other, so that a significant increase in the power of the laser beams 3.1, ..., 3.N can be achieved.

[0070] Fig. 2 shows a device 16 of the Fig. 1 for combining an (exemplary) number N = 3 coherent laser beams 3.1, 3.2, 3.3 using a beam combining device 10, which comprises a microlens arrangement 11 with two microlens arrays 17a,b and a coupling optic 18. The device 16 also includes three phase-adjusting devices 6.1, 6.2, 6.3 for adjusting the phases δφ 1 , δφ 2 , δφ 3 of the three laser beams 3.1, 3.2, 3.3 such that, in combination with the coupling optic 18, a phase front is formed at the microlens arrangement 11, which enables the coherent combination of the laser beams 3.1, 3.2, 3.3 to form the combined laser beam 12 while maintaining beam quality. The emitter surfaces 8.1, 8.2, 8.3 are arranged along a line in the X direction and the laser beams 3.1, 3.2, 3.3 enter the coupling optics 18 in parallel alignment along a uniform propagation direction (Z direction).

[0071] The emitter surfaces 8.1, 8.2, 8.3 and the coherent laser beams 3.1, 3.2, 3.3 are arranged equidistantly, i.e., at equal intervals δx, along the X-direction. The coupling optics 18 are configured to couple adjacent coherent laser beams 3.1, 3.2, 3.3 with a predetermined angular difference δθ into the microlens arrangement 11, for which the following applies: δθ = λ L / p, where λ L is the (uniform) wavelength of the laser beams 3.1, 3.2, 3.3 and p is a grid spacing (pitch) of the microlenses 20a,b of a respective microlens array 17a,b.

[0072] To generate the angular difference δθ, the coupling optics 18 has a focusing device in the form of a focusing lens 19, more precisely a cylindrical lens, which focuses the laser beams 3.1, 3.2, 3.3 onto the microlens arrangement 11, more precisely onto the first microlens array 17a of the microlens arrangement 11. To fulfill the condition for the angular difference δθ, the following applies to the Fig. 2 In the example shown, the emitter surfaces 8.1, 8.2, 8.3 are arranged at a distance δx, which is given by δx = λ L f 2 / p, where f 2 denotes the focal length of the focusing lens 19, which is in Fig. 2 is arranged at a distance L2 from the microlens arrangement 11, which corresponds to its focal length f 2, i.e., L2 = f 2 .

[0073] Provided that the intensities of the laser beams 3.1, 3.2, 3.3 emerging from the emission surfaces 8.1, 8.2, 8.3 are equal, the microlens arrangement 11 can be used to achieve the following: Fig. 2 The coherently superimposed laser beam 12 shown can be generated if the microlens arrangement 11 and the combined laser beams 3.1, 3.2, 3.3 satisfy the following equation (1): N = p 2 / λ L f E where N denotes the number of coherent laser beams (here: N = 3) and fE denotes the focal length of the microlens arrangement 11. Equation (1) should be adhered to as precisely as possible, since deviations lead to a deterioration of the beam quality of the combined laser beam 12.

[0074] In the example shown, the microlenses 20a of the first microlens array 17a have a first focal length fa and the microlenses 20b of the second microlens array 17b have a second focal length fb, where fa = fb. In the example shown, the two microlens arrays 17a,b are arranged at a distance d from each other, which corresponds to the focal lengths fa and fb, respectively, and the resulting focal length fE of the microlens arrangement 11.

[0075] In the example shown, the laser beams 3.1, 3.2, 3.3 emerging from the emission surfaces 8.1, 8.2, 8.3 are single-mode beams, meaning they each have a Gaussian profile with a half-value wR,0. Alternatively, the laser beams 3.1, 3.2, 3.3 could have a different beam profile with a potentially reduced degree of spatial coherence, for example, a donut-shaped beam profile or a top-hat beam profile. In order to form a combined laser beam 12 with a corresponding Gaussian profile with a larger half-value width w R > w R,0 from the laser beams 3.1, 3.2, 3.3 in the microlens arrangement 11, it is necessary that the laser beams 3.1, 3.2, 3.3 are directed onto the microlens arrangement 11 with a phase front or with individual phases δφ n (θ) (dependent on the angle of incidence θ), for which the following applies: δφ n θ = = 2 π / λ L f E θ 2 − 2 π / λ L f E λ L / p m n θ , where in the present example: mn = -1, 0, +1.

[0076] For the phase front or the individual phases δφ n (x) in spatial space, the following approximation holds: δφ n x = = 2 π / λ L 1 2 f 1 x 2 − 2 π / λ L f E m n λ L / p 2 , where x = 0 denotes the central axis of the beam combining device 10, along which the central laser beam 3.2 propagates, and f1 denotes the focal length of a further imaging optic 21 in the form of a cylindrical lens, which is used to correct the phase curvature and which, in the simplest case (in a one-dimensional combination), can be designed as a cylindrical lens. The further imaging optic 21 is arranged at a distance L1 in front of the focusing lens 19, which corresponds to the focal length f2 of the focusing lens 19.

[0077] The first term of the above equation (4) for the position-dependent phases δφ n (x) corresponds to an imaging optic in the form of a (cylindrical) lens 21, if its focal length f 1 at the in Fig. 2 The device 16 shown fulfills the following condition: f 1 = - f 2 2< / f E . The further imaging optics 21 thus serve to adapt the curvature of the phase front φ n (x) of the laser beams 3.1, 3.2, 3.3 incident on the microlens arrangement 11, in order to satisfy the above equation (4) with respect to the first summand.

[0078] To adjust the (individual) phases δφ n of the laser beams 3.1, 3.2, 3.3, which correspond to the second summand in the equation above, the phase adjustment devices 6.1, 6.2, 6.3 are controlled by the control device 15a so that they generate the following phase δφ n for the nth coherent laser beam 3.1, ..., 3.N: δφ n = − 2 π / λ L f E m n λ L / p 2 .

[0079] In the present example, i.e., with a number of N = 3 laser beams 3.1, 3.2, 3.3, the following results for the three (relative) phases to be set δφ 1 , δφ 2 , δφ 3: δφ 1 = + 2 π / λ L f E λ L / p 2 δφ 2 = 0 δφ 3 = − 2 π / λ L f E λ L / p 2

[0080] The number N of coherent laser beams 3.1, ..., 3.N can be changed by modifying the laser source 2 or the laser system 1 as a whole, for example by adding or removing emitter surfaces 8.1, ..., 8.N, phase-adjusting devices 6.1, ..., 6.N, etc. The number N of coherent laser beams 3.1, ..., 3.N used for the combination can also be changed by the control device 15a controlling the laser source 2 to change the intensity of certain laser beams 3.1, ..., 3.N, for example, to selectively switch certain laser beams 3.1, ..., 3.N on or off.

[0081] When the number N of coherent laser beams 3.1, ..., 3.N is changed, it is necessary to adjust the beam combining device 11 appropriately to satisfy the conditions described above, including equation (1). For this purpose, the microlens arrangement 11 can be configured to adjust its (effective) focal length f E and has at least three microlens arrays 17a-c with focal lengths fa , fb , fc , as is exemplified for the microlens arrangement 11 of Fig. 3a is shown.

[0082] The effective focal length f E refers to a beam of light which passes through one microlens 20a-c per microlens array 17a-c (corresponding to one sub-aperture), cf. Fig. 3b . A microlens arrangement 11, which is used in the beam combining device 10, has, with respect to a beam bundle to each microlens 20a-c per microlens array 17a-c in the notation of matrix optics, i.e. as an imaging matrix A B C D The following properties typically apply to the input vector (r,θ) and the output vector (r',θ'): A=0; |B|=f E . Furthermore, D<1 usually also holds (for a substantially collimated beam); C then depends on A, B and D. The use of at least three microlens arrays 17a-c is required to satisfy the second condition |B|= f E for variable f E.

[0083] A motorized adjustment mechanism 22 (for example, with a driven gear engaging a rack) allows the positions of slides 23, 24, which carry the first and third microlens arrays 17a, 17c of the microlens arrangement 11, to be adjusted motorically by means of an electronic (programmable) control device 15, and thus a distance d1 between the first and second microlens arrays 17a, 17b and a distance d2 between the second and third microlens arrays 17b, 17c can be set independently of each other. In the example shown, the second microlens array 17b is fixed in position.

[0084] For a desired effective focal length f E according to equation (1), the distances to be set d 1 , d 2 can be determined as follows for given focal lengths fa , fb , fc and the slides 23, 24 can be moved along guides to corresponding adjustment positions: d 2 = f b 1 + f E f a and d 1 = f b + f a + f b f a f E

[0085] If the effective focal length f E of the microlens arrangement 11 is changed, it is typically also necessary to adjust the coupling optics 18 accordingly. In the case of the Fig. 2 In the device 16 shown, this can be achieved by making the focal length f1 of the further imaging optics 21 adjustable in order to satisfy the condition f1 = -f2 / fE. For this purpose, the further imaging optics 21 can be formed by two (thin) lenses 21a, 21b, the distance d3 between which is adjustable, as shown in Fig. 4 is shown. By choosing the distance d3 between the lenses 21a, 21b, the (effective) focal length f 1 of the further imaging optics 21 changes so that the above condition can be fulfilled, for example if the number N of laser beams 3.1, ..., 3.N is increased from N = 3 to N = 5, as is done in Fig. 4 as indicated, in which two additional, outer laser beams 3.4, 3.5 or emitter surfaces 8.4, 8.5 were added.

[0086] Fig. 5 shows a device 16 of the Fig. 1 , in which the coupling optics 18 do not have any additional imaging optics 21 for correcting the phase front δφ n (x). For the adaptation of the phase front δφ n (x), the emitter surfaces 8.1, 8.2, 8.3 are each aligned at an angle δγ to each other, which corresponds to the angle at which the laser beams 3.1, 3.2, 3.3 emerge from the in Fig. 2 or in Fig. 4 The imaging optics 21 shown exit. The adaptation of the phase front δφ n (x), more precisely of the first term in the above equation (4), can in this case be achieved without providing an additional imaging optics, i.e. the coupling optics 18 only has the focusing lens 19.

[0087] Imaging optics 21 for adjusting the phase front curvature can also be dispensed with if the emitter surfaces 8.1, 8.2, 8.3 are arranged along a line in the X direction and aligned parallel to each other, as is the case in Fig. 6 is shown. In this case, the distance L2 between the focusing lens 19 and the microlens arrangement 11, more precisely the first microlens array 17a of the microlens arrangement 11, is suitably chosen as follows: L2 = f 2 - p 2< / (N λ L ). The distance L2 depends on the device 16 of Fig. 6 This also depends on the number N of laser beams 3.1, ..., 3.N, so that it must also be adjusted if the number N of laser beams 3.1, ..., 3.N changes. For this purpose, the control device 15 can act on the focusing lens 19 and / or on the adjustment mechanism 22 of the microlens arrangement 11, which is already present in this case, in order to move the focusing lens 19 and the microlens arrangement 11 relative to each other.

[0088] At the in Fig. 7 device 16 shown Fig. 1 The beam combination device 10 has only the microlens arrangement 11 and no coupling optics. In this example, the emitter surfaces 8.1, 8.2, 8.3 are aligned on a circular arc 29 at a respective difference angle : δθ = λ L / p relative to each other. The following applies to the fill factor FF of the laser beams 3.1, 3.2, 3.3 in this case: FF = 2 p / π w , where w denotes the beam diameter of the combined laser beam 12. The distance D is given by: D = w 2 π / λ L 4 p 2 / π FF 2 λ L .

[0089] To deal with the in Fig. 7 To achieve a fill factor FF of, for example, approximately 35% with the device shown in 16 at a grid spacing p of approximately 500 µm, a comparatively large distance D of approximately 2.5 m is required.

[0090] In connection with Fig. 2 bis Fig. 7 In the devices described in 16, the laser beams 3.1, ..., 3.N were combined one-dimensionally. Fig. 8a,b each shows a device 16 of the Fig. 1 , in which a number N (here: N = 3) x M (here: M = 3) of emission surfaces 8.1.1, ..., 8.NM are arranged in a two-dimensional grid. In which in Fig. 8a In the example shown, the emission surfaces 8.1.1, ... 8.NM are arranged in a rectangular grid in a common plane (XY plane), and the beam propagation directions of all laser beams 3.1.1, ..., 3.NM run parallel (in the Z direction). Analogous to Fig. 6 The coupling optics 18 in the device 16 of Fig. 8a merely a focusing device in the form of a focusing lens 19, which is in Fig. 8a The microlenses 20a,b of the microlens arrays 17a,b of the microlens arrangement 11 are each arranged in a corresponding rectangular grid and aligned parallel to the XY plane.

[0091] Depending on the spacing of the emitter surfaces 8.1.1, ... 8.NM and on the periodicity of the grating in the X and Y directions, the grid spacings px, pY of the microlenses 20a,b can also differ in the two mutually perpendicular directions X, Y. The microlenses 20a,b accordingly exhibit potentially different curvatures in the X and Y directions; that is, they are not cylindrical lenses. The combination of the coherent laser beams 3.1.1, ... 3.NM in the two linearly independent directions X, Y, which are perpendicular in the example shown, is fundamentally independent; that is, the conditions and equations given above apply to both directions X, Y independently of each other.

[0092] Only when adjusting the relative phase of the laser beams 3.1.1, ... 3.NM do the contributions in the two mutually perpendicular directions add up, i.e., for a number of N x M laser beams arranged in a rectangular grid (in the X-direction or Y-direction), the following applies for the respective phase δφ n,m: δφ n , m = − 2 π / λ L f E , X p n λ L / p x 2 − 2 π / λ L f E , Y q m λ L / p Y 2 where px denotes the grid spacing of the microlenses in the X direction and py denotes the grid spacing of the microlenses in the Y direction, and where f E,X denotes the effective focal length of the microlens arrangement 11 in the X direction and f E,Y denotes the effective focal length of the microlens arrangement 11 in the Y direction, where: p n = − N + 1 2 + n with n = 1, ... , N and q m = − M + 1 2 + m with m = 1, ... , M .

[0093] Fig. 8b shows a device 16 analogous to Fig. 8a , in which the two two-dimensional microlens arrays 17a,b of the microlens arrangement 11 are replaced by four one-dimensional microlens arrays 17a-d. The microlens arrays 17a-d each have a plurality of microlenses 20a-d in the form of cylindrical lenses, wherein the microlenses 20a,c of the first and third microlens arrays 17a,c and the microlenses 20b,d of the second and fourth microlens arrays 17b,d are oriented perpendicular to each other in the X-direction and the Y-direction, respectively.

[0094] In the examples described above, an odd number N of coherent laser beams 3.1, ..., 3.3 were superimposed. If an even number N of laser beams 3.1, ..., 3.N is to be superimposed, a phase-shifting device 25 can be used, as shown below. Fig. 9a -cThe phase-shifting device 25 is integrated into a respective first or second microlens array 17a,b of the microlens arrangement 11 and generates at a transition 27 between beam bundles 26 the adjacent microlenses 20b of the second microlens array 17b (see figure). Fig. 9a ) or adjacent microlenses 20a of the first microlens array 17a (see Fig. 9b,c ) pass through, each with a phase shift of π. This allows the 0th diffraction order to be eliminated by destructive interference, so that an even number N of coherent laser beams 3.1, ..., 3.N can be combined to form the combined laser beam 12.

[0095] At the in Fig. 9a In the microlens arrangement 11 shown, a thickness step 28 is provided at the transition between adjacent microlenses 20b of the second microlens array 17b. The additional thickness of each microlens 20b along the X-direction increases linearly from an initial value z0 of additional thickness to an additional thickness corresponding to z0 plus the thickness corresponding to the phase difference π across the width or grid spacing p of the respective microlens 20b. The initial value z0 serves to approximate the location of the phase step with respect to the X-direction to the focal plane behind the third microlens array 17c. The sawtooth profile is provided for all microlenses 20b of the second microlens array 17b.

[0096] In the microlens arrangement 11 of Fig. 9b The phase-shifting device 25 is integrated into the first microlens array 17a, and a thickness step 28 also occurs at the transition 27 between adjacent microlenses 20a. The additional thickness of each microlens 20a along the X-direction increases linearly from 0 (zero) additional thickness to an additional thickness corresponding to the phase difference π across the width of the microlens 20a. This sawtooth profile is provided for all microlenses 20a of the first microlens array 17a.

[0097] At the in Fig. 9c In the variant shown, the first microlens array 17a is also equipped with an integrated phase-shifting device 25. At the transition 27 between adjacent microlenses 20a, a thickness change 28 occurs, which generates the phase shift of π. The additional thickness provided by the integrated phase-shifting device 25 is maintained uniformly across the entire width of each microlens 20a, and this additional thickness is absent in alternating microlenses 20a' of the first microlens array 17a.

[0098] Alternatively to the one in Fig. 9a-c The phase-shifting device 25 shown, which is integrated into a respective microlens array 17a,b, can be designed as a separate component, e.g., in the form of a phase-shifting plate. Such a phase-shifting plate (not shown) has alternating first and second pass-through elements that are aligned with the microlenses 20a-c or the associated beams 26. The pass-through elements thus have a width in the X-direction corresponding to the aperture or grid spacing p. The first pass-through elements generate a phase shift of 0 (zero) over their entire width in the X-direction, and the second pass-through elements generate a phase shift of π over their entire width in the X-direction.

[0099] In this case, the phase-shifting device 25 can be removed from the beam path as needed (e.g., pulled out in the X direction) or retracted into the beam path, depending on whether an odd or even number N of laser beams 3.1, ..., 3.N is to be coherently combined. A motorized adjustment mechanism, controlled by the electronic control unit 15, can also be provided for this purpose. Generally, the phase-shifting device 25 is preferably placed approximately in the focal plane of a respective microlens array 17a-c (or a respective sub-aperture), or directly behind the first microlens array 17c of the beam path emanating from the emission surfaces 8.1, ..., 8.N.

[0100] Fig. 10 shows a device 16 of the Fig. 1 , which is designed for wavelength-dependent correction during the superposition of ultrashort pulse laser beams 3.1, .., 3.N. The device 16 has a number of emission surfaces 8.1, ..., 8.N for the emission of the coherent laser beams 3.1, ..., 3.N, of which in Fig. 10 For the sake of simplicity, only three laser beams 3.1, 3.2, 3.3 are shown, which strike a first wavelength-dispersive element in the form of a first diffraction grating 30. At the first diffraction grating 30, the laser beams 3.1, 3.2, 3.3 are spectrally split and, in the example shown, form three beam bundles 26a-c, each with different wavelengths λ₁, λ₂, λ₃. The three beam bundles 26a-c pass through a collimating lens 31 and subsequently emerge parallel through the microlens array 11. The three beam bundles 26a-c then pass through the two microlens arrays 17a,b of the microlens array 11, each through different microlenses 20a, 20b of the respective microlens array 17a,b, and thus remain separated.A focusing lens 32 positioned in the beam path behind the microlens arrangement 11 focuses the beam bundles 26a-c onto a second wavelength-dispersive optical element in the form of a second diffraction grating 33, which spectrally combines the beam bundles 26a-c and generates the combined laser beam 12, whose wavelength spectrum matches that of the coherent laser beams 3.1, 3.2, 3.3 before superposition.

[0101] In the example shown, the coupling optics 18 are formed by the collimation lens 31, which is designed as an aspherical lens and has a locally variable curvature in the X-direction, i.e., perpendicular to the propagation direction of the beams 26a-c. The curvature of the aspherical collimation lens 31 is selected to be location-dependent such that a wavelength-dependent correction of the phase front can be performed when combining the ultrashort pulse (USP) laser beams 3.1, 3.2, 3.3. However, such a wavelength-dependent correction is generally only necessary if a comparatively large number of USP laser beams 3.1, ..., 3.N (e.g., with N > 10) are to be coherently superimposed.

[0102] Fig. 11 shows a device 16 of the Fig. 1 analogous to Fig. 7 , in which the emission surfaces 8.1, 8.2, 8.3 are arranged on a circular arc 29. As described above in connection with Fig. 7 As described, the emission surfaces 8.1, 8.2, 8.3 are aligned relative to each other at a respective difference angle δθ = λ L / p. The coupling optics 18 comprise three beam-shaping optics in the form of three lenses 18a-c, which are attached to the output coupling unit of the emitter surfaces 8.1, 8.2, 8.3. The lenses 18a-c serve to generate a predetermined divergence angle α D at which the coherent laser beams 3.1, 3.2, 3.3 enter the microlens arrangement 11. If lenses 18a-c each have the same focal length f, the distance D between the circular arc 29 and the microlens arrangement 11 is given by the condition D = f - f E, where f E denotes the effective focal length of the microlens arrangement 11. By widening the beam with the aid of lenses 18a-c or with the aid of other beam-shaping optics, for example with the aid of two or more lenses arranged one behind the other, the distance D can be reduced compared to the case where a Fourier lens orFocusing device 19 (see . Fig. 6 ) with (larger) focal length f 2 is used (L2 = f 2 - f E = f 2 - p 2< / (λ LN)) can be reduced and the overall construction of the device 16 can be made more compact, which reduces the susceptibility to interference.

[0103] By selectively specifying the divergence angles α D of the coherent laser beams 3.1, 3.2, 3.3, the interference condition can not only be modified to achieve shorter installation distances, but also an even or arbitrary number of coherent laser beams 3.1, 3.2, 3.3, ... can be combined without having to use the methods described above in connection with Fig. 9a-c The described phase-shifting device 25 is required.

[0104] For the combination of an even number N of coherent laser beams 3.1, 3.2, 3.3, ... it is required that the beam-shaping optics 18a-c, more precisely the lenses 18a-c, have a focal length f that satisfies the following condition: f = p 2 / λ L , and this is independent of the (even) number N of coherent laser beams 3.1, 3.2, 3.3,... The above condition for the focal length f also represents a lower limit, which in the Fig. 11 The setup shown must not be undercut, i.e., the following should generally apply to the focal length f (i.e., regardless of the number N): f = p 2< / λ L .

[0105] In the event that the number of laser beams 3.1, 3.2, ... to be combined is to be varied, the coupling optics 18 can be designed in the form of beam-shaping optics for setting a variable divergence angle α D and for this purpose may, for example, have an adjustable focal length f.

[0106] The combination of an even number of coherent laser beams 3.1, 3.2, ... into a combined laser beam 12 is not only possible in the case of the Fig. 11 The device shown in 16 is possible, but also in the case of the one shown in Fig. 12 device 16 shown Fig. 1 , which are essentially the in Fig. 6 The device shown in 16 corresponds to the one described. Fig. 12 The device 16 shown has a movement device 35 for displacing the second microlens array 17b in the X-direction, i.e., laterally or transversely to the propagation direction Z of the combined laser beam 12, thereby generating an adjustable lateral offset Δ between the microlenses of the first microlens array 17a and the microlenses of the second microlens array 17b, as shown in Fig. 12 can be seen.

[0107] For this purpose, the motion device 35 can, for example, comprise a piezoelectric element, a voice coil, a mechanical drive including a camshaft, etc., which can be controlled by the control device 15. The control device 15 can be in the form of a control computer or other suitable hardware and / or software and controls the motion device 25 to laterally displace the second microlens array 17b.

[0108] The in Fig. 12 The device 16 shown can also be used to move the combined laser beam 12 in its position or to change its angle, or to generate two or possibly more than two combined laser beams 12a, 12b, as described below in connection with Fig. 13a-c und Fig. 14a-c is described, in which the angular distribution of the far field in a plane X, Y perpendicular to the propagation direction Z of the combined laser beam 12 is shown.

[0109] Fig. 13a Figure 1 shows the case where no lateral offset Δ is generated between the two microlens arrays 17a,b, i.e., where Δ = 0 mm. In this case, the combined laser beam 12 is diffracted into the 0th diffraction order, so that it is not deflected and propagates along the optical axis in the Z direction. Fig. 13b shows the case where, with the help of the movement device 35, a positive lateral offset Δ (displacement in the positive X direction, cf. Fig. 12 ) is set, for the case of three laser beams 3.1, 3.2, 3.3 described here, the following applies: Δ = p / N x B = p / 3 (here: N = 3, B = +1). The combined laser beam 12 is deflected into the +1st diffraction order and thus does not propagate along the optical axis in the Z direction, but is deflected at an angle θ X of approximately +2 mrad in the positive X direction. Fig. 13c shows the analogous case when a lateral offset Δ with a negative sign (Δ = - p / N x B = - p / 3 (here: N = 3, B = - 1)) is generated to deflect the combined laser beam 12 into the -1st diffraction order, so that it is deflected at an angle θ X of approximately -2 mrad in the negative X direction.

[0110] Fig. 14a shows the Fig. 13a analogous case that no lateral offset Δ is generated between the two microlens arrays 17a,b, i.e. that Δ = 0 mm, so that the combined laser beam 12 is not deflected and propagates along the optical axis in the Z direction. Fig. 14b shows the case where, with the help of the movement device 35, a positive lateral offset Δ (displacement in the positive X direction, cf. Fig. 12 ) is set, for the case of three laser beams 3.1, 3.2, 3.3 described here, the following applies: Δ = + p / (2 N) = p / 6 (here: N = 3). With such a choice of lateral offset Δ, the combined laser beam 12 is split; more precisely, two combined laser beams 12a, 12b with the same power are generated, of which the first laser beam 12a is set as in Fig. 14a The first laser beam is propagated in the Z-direction, while the second laser beam 12b is diffracted into the -1st diffraction order. Accordingly, in the Fig. 14c In the example shown, a negative lateral offset Δ (displacement in the negative X-direction) is set, for the case of three laser beams 3.1, 3.2, 3.3 described here, the following applies: Δ = - p / (2 N) = - p / 6 (here: N = 3). In this case as well, two combined laser beams 12a, 12b with the same power are generated, the first of which, laser beam 12a, is generated as in Fig. 14a propagated in the Z direction, while the second laser beam 12b is diffracted into the +1st diffraction order.

[0111] By appropriately choosing the amount of the lateral offset Δ, the in Fig. 13b ,c the deflection shown also with the in Fig. 14b,c The splitting or formation of two or possibly more than two combined laser beams 12a, 12b,... shown can be combined. In this case, too, the laser energy and beam quality of the coherent laser beams 8.1, 8.2, 8.3 can be largely preserved; that is, even with diffraction to the second diffraction order, the losses are only about 10%. It is understood that more than three laser beams 3.1, 3.2, 3.3, originating from one laser source 2 or possibly from several coherent laser sources 2, can also be deflected or split in the manner described above.

[0112] With the help of the in Fig. 12 The device 16 shown can also combine an even number N of coherent laser beams 3.1, 3.2, ... into a combined laser beam 12 without requiring the use of the phase-shifting device 25 described above. For this purpose, a lateral offset Δ is set, for which the following applies: Δ = ± p / 2 N + 1 , where N + 1 denotes the (odd) number of coherent laser beams 3.1, 3.2, ... that can be combined into a combined laser beam 12 by means of the device 16. For the in Fig. 12 The distance d shown between the first microlens array 17a and the second microlens array 17b is as follows in this case: d = p 2 / N λ L .

[0113] The distance d between the first and second microlens arrays 17a, 17b can be constant. Alternatively, the distance d can be adjustable. For this purpose, for example, the second microlens array 17b can be carried by a carriage whose position in the Z-direction is adjusted via a motorized adjustment mechanism using the programmable control unit 15, as described above in connection with Fig. 3a,b is shown.

[0114] In the event that a plurality of laser beams 3.1.1 to 3 nm are combined two-dimensionally, as described above in connection with Fig. 8a,b as described, can be used in the Fig. 8b In the illustrated microlens arrangement 11, which comprises two pairs of crossed, one-dimensional microlens arrays 17a-d, the movement device 35 is configured to set a lateral offset ΔX in the X direction and, independently, a lateral offset ΔY in the Y direction. For this purpose, the movement device 35 can, for example, act on the first microlens array 17a to offset it laterally in the X direction and similarly on the second microlens array 17b to offset it laterally in the Y direction. In this way, the deflection or combination into two laser beams 12a,b can occur independently in both directions (X, Y).

[0115] In the event that two-dimensionally structured microlens arrays 17a, 17b are used for the two-dimensional combination of the coherent laser beams 3.1.1 to 3.NM, as is done in Fig. 8a,b However, as shown, independent adjustment in two directions is not possible. If such a two-dimensionally structured microlens array, e.g., the first microlens array 17a, is laterally displaced in one direction (e.g., the X-direction), the combined laser beam 12 is also deflected in the second direction (e.g., the Y-direction), or four combined laser beams are formed instead of two combined laser beams 12a,b.

[0116] In summary, a coherent combination of a plurality N of laser beams 3.1, ..., 3.N can be achieved in the manner described above using standard optics, i.e., with the exception of the one described above. Fig. 10In the example shown, this can be achieved without the use of diffractive optical elements or the like. By appropriately selecting the focal lengths fa, fb, fC of the microlenses 20a-c of the microlens arrays 17a-c, possibly in combination with appropriately selecting the focal length(s) f2, f1 of optical elements 19, 21 of the coupling optics 18, as well as appropriately selecting the relative phases δφn(x) of the laser beams 3.1, ..., 3.N, high efficiency can be achieved in the coherent combination to form the combined laser beam 12 while simultaneously maintaining beam quality.

Claims

1. A laser system (1), comprising: at least one laser source (2) for generating a plurality N of coherent laser beams (3.1, ..., 3.N), a plurality of emission surfaces (8.1, ..., 8.N) for emitting the plurality of coherent laser beams (3.1, ..., 3.N), wherein the plurality of emission surfaces (8.1, ..., 8.N), as well as a device (16) for combining the plurality N of coherent laser beams (3.1, ..., 3.N), which comprises at least N-1 phase adjustment devices (6.1, ..., 6.N) for adjusting a respective phase (δφn) of one of the coherent laser beams (3.1, ..., 3.N), as well as a beam combination device (10) for combining the coherent laser beams (3.1, ..., 3.N) to form at least one combined laser beam (12; 12a,b), wherein the beam combination device (10) has a micro-lens arrangement (11) with at least two micro-lens arrays (17a,b; 17a-c) arranged one behind the other to form the at least one combined laser beam (12, 12a,b), wherein the micro-lenses (20a,b; 20a-c) of a respective micro-lens array (17a,b; 17a-c) are arranged in a direction (X), wherein the plurality of emission surfaces (8.1, ..., 8.N) are arranged equidistantly along the direction (X) or equidistantly on a circular arc (25) and in a common plane with the micro-lenses of a respective micro-lens array, characterized in that the coherent laser beams (3.1, ..., 3.N) and the micro-lens arrangement (11) satisfy the following condition: N = p 2 / λ L f E , wherein N is the number of coherent laser beams (3.1, ..., 3.N), p is an identical grid spacing of the micro-lenses (20a,b; 20a-c) of a respective micro-lens array (17a,b; 17a-c), λL denotes the laser wavelength, and fE denotes the focal length of the micro-lens arrangement (11), wherein the phase adjustment devices (6.1, ..., 6.N) are designed to adjust a phase δφn for the nth coherent laser beam (3.1, ..., 3.N), which is given by: δφn = − 2 π / λ L f E m n λ L / p 2 , wherein the following applies: m n = − N + 1 2 + n.

2. The laser system (1) according to claim 1, in which the micro-lens arrangement (11) has at least three micro-lens arrays (17a-c) and is designed to adjust its focal length (fE), wherein, preferably, the micro-lens arrangement (11) is designed to adjust its focal length (fE) by varying a distance (d1, d2) between at least a first micro-lens array (17a, 17b) and a second micro-lens array (17b, 17c).

3. The laser system (1) according to claim 1 or 2, further comprising: a movement device (35) for generating a preferably adjustable lateral offset (Δ) between at least a first micro-lens array (17a) and at least a second micro-lens array (17b) of the micro-lens arrangement (11).

4. The laser system (1) according to any one of the preceding claims, in which a first and a second micro-lens array (17a,b) have a lateral offset Δ for forming two combined laser beams (12a,b), for which offset the following applies: Δ = ± p / 2 N .

5. The laser system (1) according to any one of claims 1 to 3, in which a first and a second micro-lens array (17a,b) of the at least two micro-lens arrays have a lateral offset Δ for combining an even number N of coherent laser beams (3.1, ..., 3.N) to form a combined laser beam (12), for which offset the following applies: Δ = ± p / 2 N + 1 , wherein, for a preferably adjustable distance d between the first micro-lens array (17a) and the second micro-lens array (17b), the following applies: d = p 2 / N λ L .

6. The laser system (1) according to any one of claims 1 to 3, in which the first and second micro-lens arrays (17a,b) have a lateral offset Δ for forming a combined laser beam (12) diffracted into a diffraction order B other than zero, for which offset the following applies: Δ = p / N B .

7. The laser system (1) according to any one of the preceding claims, further comprising: a coupling optical system (18) for coupling the coherent laser beams (3.1, ..., 3.N) into the micro-lens arrangement (11), wherein the coupling optical system (18) is preferably designed to couple adjacent coherent laser beams (3.1, 3.2; 3.2, 3.3) into the micro-lens arrangement (11) with a predetermined angle difference δθ, for which the following applies: δθ = λ L / p .

8. The laser system (1) according to claim 7, in which the coupling optical system (18) has at least one focusing device, in particular at least one focusing lens (19), for focusing the plurality of coherent laser beams (3.1, ..., 3.N) onto the micro-lens arrangement (11), wherein, preferably, the coupling optical system (18) comprises, for correcting a phase front (δφn(x)) of the plurality of coherent laser beams (3.1, ..., 3.N), at least one further imaging optical system, in particular at least one further lens (21, 21 a,b).

9. The laser system (1) according to any one of the preceding claims, wherein the coherent laser beams (3.1, ..., 3.N) have a Gaussian beam profile, a doughnut-shaped beam profile, or a top-hat beam profile.

10. The laser system according to any one of the preceding claims, further comprising: a control device (15) designed to adjust the focal length (fE) of the micro-lens arrangement (11) depending on the number N of coherent laser beams (3.1, ... 3.N).

11. The laser system according to claim 10 and claim 8, in which a distance (L2) between the focusing device (19) and the micro-lens arrangement (11) is adjustable, and in which the control device (15) is designed to adjust the distance (L2) depending on the number N of coherent laser beams (3.1, ..., 3.N).

12. The laser system according to claim 8, in which the focusing device (19) is arranged at a distance (L2) equal to its focal length f2 from a first micro-lens array (17a) of the micro-lens arrangement (11), and the focal length f2 of the focusing device (19), a focal length f1 of a further imaging optical system (21), and the focal length fE of the micro-lens arrangement (11) satisfy the following condition: f 1 = − f 2 2 / f E .

13. The laser system according to claim 12 and claim 10, in which the focal length (f1) of the further imaging optical system (21) is adjustable, and the control device (15) is designed to adjust the focal length (f1) of the further imaging optical system (21) depending on the number N of coherent laser beams (3.1, ..., 3.N).

14. The laser system according to claim 8, in which adjacent emission surfaces (8.1, ..., 8.N) are arranged equidistantly along the direction (X) and are spaced apart from one another by a distance δx, which is given by δx = λ L f 2 / p .

15. The laser system according to any one of claims 1 to 13, in which adjacent emission surfaces (8.1, ..., 8.N) are arranged and aligned equidistantly on the circular arc (25), and adjacent coherent laser beams (3.1, 3.2; 3.2, 3.3) are coupled into the micro-lens arrangement (11) with a predetermined angle difference δθ, for which the following applies: δθ = λ L / p .

16. The laser system according to claim 7, in which the coupling optical system has or forms a beam-forming optical system (18) for forming a respective coherent laser beam (3.1, ..., 3.N) with a predetermined, in particular adjustable, divergence angle (αD) upon coupling into the micro-lens arrangement (11).

17. The laser system according to claim 16, having an even number N of emission surfaces (8.1, ..., 8.N), wherein the beam-forming optical system (18) has a focal length f that satisfies the following condition: f = p 2 / λ L .

18. The laser system according to any one of the preceding claims, having an even number N of emission surfaces (8.1, ..., 8.N), wherein the micro-lens arrangement (11) has a phase-shifting device (25) designed for generating a constant phase shift of π between beam bundles (26) of the coherent laser beams (3.1, ..., 3.N) that pass through adjacent micro-lenses (20a,b; 20a-c) of a respective micro-lens array (17a,b; 17a-c).

19. The laser system according to any one of the preceding claims, further comprising: a first wavelength-dispersive element, in particular a first diffraction grating (30), arranged in front of the micro-lens arrangement (11), for the spectral separation of the coherent laser beams (3.1, ..., 3.N) into multiple beam bundles (26a-c), which pass through the micro-lens arrangement (11) in a separated manner, as well as a second wavelength-dispersive optical element, in particular a second diffraction grating (33), arranged behind the micro-lens arrangement (11), for the spectral combination of the beam bundles (26a-c) for forming the combined laser beam (12) with a wavelength (λL).

20. The laser system according to claim 19 and claim 7, in which the coupling optical system (18) has a collimation lens (31) that is designed as an aspheric lens.

21. A laser system (1), comprising: at least one laser source (2) for generating a plurality N × M of coherent laser beams (3.1, ..., 3.N), a plurality of emission surfaces (8.1, ..., 8.N) for emitting the plurality of coherent laser beams (3.1, ..., 3.N), wherein the plurality of emission surfaces (8.1, ..., 8.N) are arranged in a two-dimensional periodic grating along a first direction (X) and a second direction (Y) that is perpendicular to the first direction (X), as well as a device (16) for combining the plurality N × M of coherent laser beams (3.1, ..., 3.N), which comprises at least (N × M)-1 phase adjustment devices (6.1, ..., 6.N) for adjusting a respective phase (δφn,m) of one of the coherent laser beams (3.1, ..., 3.N), as well as a beam combination device (10) for combining the coherent laser beams (3.1, ..., 3.N) to form at least one combined laser beam (12; 12a,b), wherein the beam combination device (10) has a micro-lens arrangement (11) with at least two micro-lens arrays (17a, b; 17a-c) arranged one behind the other to form the at least one combined laser beam (12, 12a,b), wherein the micro-lenses (20a, 20b) of a respective micro-lens array (17a,b; 17a-c) are each arranged in a two-dimensional periodic grating, characterized in that the coherent laser beams (3.1, ..., 3.N) and the micro-lens arrangement (11) satisfy the following condition: N = p x 2 / λ L f E , X , M = p y 2 / λ L f E , Y , wherein N is the number of coherent laser beams (3.1, ..., 3.N) along the first direction (X), M is the number of coherent laser beams (3.1, ..., 3.N) along the second direction (Y), px is an identical grid spacing of the micro-lenses (20a,b; 20a-c) of a respective micro-lens array (17a,b; 17a-c) in the first direction (X), py is an identical grid spacing of the micro-lenses (20a,b; 20a-c) of a respective micro-lens array (17a,b; 17a-c) in the second direction (Y), where λL denotes the laser wavelength, fE,X denotes the focal length of the micro-lens arrangement (11) in the first direction (X), and fE,Y denotes the focal length of the micro-lens arrangement (11) in the second direction (Y), wherein the phase adjustment devices (6.1, ..., 6.N) are designed to adjust a phase δφn,m for the (n × m)th coherent laser beam (3.1, ..., 3.N), which is given by: δφ n = − 2 π / λ L f E , X p n λ L / p x 2 − 2 π / λ L f E , Y q m λ L / p Y 2 , wherein the following applies: p n = − N + 1 2 + n q m = − M + 1 2 + m .

22. A laser system (1), comprising: at least one laser source (2) for generating a plurality N × M of coherent laser beams (3.1, ..., 3.N), a plurality of emission surfaces (8.1, ..., 8.N) for emitting the plurality of coherent laser beams (3.1, ..., 3.N), wherein the plurality of emission surfaces (8.1, ..., 8.N) are arranged in a two-dimensional periodic grating along a first direction (X) and a second direction (Y) that is perpendicular to the first direction (X), as well as a device (16) for combining the plurality N × M of coherent laser beams (3.1, ..., 3.N), which comprises at least (N × M)-1 phase adjustment devices (6.1, ..., 6.N) for adjusting a respective phase (δφn,m) of one of the coherent laser beams (3.1, ..., 3.N), as well as a beam combination device (10) for combining the coherent laser beams (3.1, ..., 3.N) to form at least one combined laser beam (12; 12a,b), wherein the beam combination device (10) has a micro-lens arrangement (11) with four micro-lens arrays (17a-d) arranged one behind the other to form the at least one combined laser beam (12, 12a, b), wherein the micro-lenses (20a, 20c) of the first and third micro-lens arrays (17a, 17c) are arranged along the first direction (X) and the micro-lenses (20b, 20d) of the second and fourth micro-lens arrays (17b, 17d) are arranged along the second direction (Y), characterized in that the coherent laser beams (3.1, ..., 3.N) and the micro-lens arrangement (11) satisfy the following condition: N = p x 2 / λ L f E , x , M = p y 2 / λ L f E , y wherein N is the number of coherent laser beams (3.1, ..., 3.N) along the first direction (X), M is the number of coherent laser beams (3.1, ..., 3.N) along the second direction (Y), px is an identical grid spacing of the micro-lenses (20a, 20c) of the first and third micro-lens arrays (17a, 17c) in the first direction (X), py is an identical grid spacing of the micro-lenses (20b; 20d) of the second and fourth micro-lens arrays (17b, 17d) in the second direction (Y), where λL denotes the laser wavelength, fE,X denotes the focal length of the micro-lens arrangement (11) in the first direction (X), and fE,Y denotes the focal length of the micro-lens arrangement (11) in the second direction (Y), wherein the phase adjustment devices (6.1, ..., 6.N) are designed to adjust a phase δφn,m for the (n × m)th coherent laser beam (3.1, ..., 3.N), which is given by: δφ n , m = − 2 π / λ L f E , X p n λ L / p X 2 − 2 π / λ L f E , Y q m λ L / p Y 2 , wherein the following applies: p n = − N + 1 2 + n , q m = − M + 1 2 + m .

23. A method for combining an in particular adjustable number N of coherent laser beams (3.1, ..., 3.N) emitted from a plurality of emission surfaces (8.1, ..., 8.N) to form at least one combined laser beam (12, 12a,b), comprising: adjusting a respective phase (δφn) of one of the coherent laser beams (3.1, ..., 3.N) in at least N-1 phase adjustment devices (6.1, ..., 6.N), characterized by coupling the coherent laser beams (3.1, ..., 3.N) into a micro-lens arrangement (11), which has at least two micro-lens arrays (17a,b; 17a-c) arranged one behind the other, wherein the micro-lenses of a respective micro-lens array (17a,b; 17a-c) are arranged in a direction (X), wherein the emission surfaces (8.1, ..., 8.N) are arranged equidistantly along the direction (X) or equidistantly on a circular arc (25) and in a common plane with the micro-lenses of a respective micro-lens array, wherein a focal length fE of the micro-lens arrangement (11) is adjusted, which satisfies the following condition: f E = p 2 / λ L N , wherein p represents an identical grid spacing of the micro-lenses (20a,b; 20a-c) of a respective micro-lens array (17a,b; 17a-c) and λL denotes the laser wavelength adjusting a phase δφn of the nth coherent laser beam (3.1, ..., 3.N), which is given by: δφ n = − 2 π / λ L f E m n λ L / p 2 , wherein the following applies: m n = − N + 1 2 + n, and combining the coherent laser beams (3.1, ..., 3.N) to form the at least one combined laser beam (12, 12a,b) in the micro-lens arrangement (11).

24. The method according to claim 23, in which the coupling is performed by means of a coupling optical system (18) having a focusing device (19), wherein the method further comprises: adjusting a distance L2 between the focusing device (19) and the micro-lens arrangement (11), which is given by: L 2 = f 2 − p 2 / N λ L , wherein f2 denotes a focal length of the focusing device (19).

25. The method according to claim 23 or 24, in which the coupling is performed by means of a coupling optical system (18) having a focusing device (19) and a further imaging optical system (21), wherein the method further comprises: adjusting a focal length f1 of the further imaging optical system (21), which is given by: f 1 = − f 2 2 / f E , wherein f2 denotes a focal length of the focusing device (19).

26. A method for combining an in particular adjustable number N x M of coherent laser beams (3.1, ..., 3.N) emitted from a plurality of emission surfaces (8.1, ..., 8.N), wherein the plurality of emission surfaces (8.1, ..., 8. N) are arranged in a two-dimensional periodic grating along a first direction (X) and along a second direction (Y) that is perpendicular to the first direction (Y), to form at least one combined laser beam (12, 12a,b), comprising: adjusting a respective phase (δφn,m) of one of the coherent laser beams (3.1, ..., 3.N) in at least (N x M)-1 phase adjustment devices (6.1, ..., 6.N), characterized by coupling the coherent laser beams (3.1, ..., 3. N) into a micro-lens arrangement (11), which has at least two micro-lens arrays (17a,b; 17a-c) arranged one behind the other, wherein the micro-lenses (20a, 20b) of a respective micro-lens array (17a,b; 17a-c) are each arranged in a two-dimensional periodic grating, wherein a focal length fE,X of the micro-lens arrangement (11) in the first direction (X) and a focal length fE,Y of the micro-lens arrangement (11) in the second direction (Y) are adjusted, which satisfy the following conditions: f E , X = p X 2 / λ L N , f E , Y = p Y 2 / λ L M , wherein pX is an identical grid spacing of the micro-lenses (20a,b; 20a-c) of a respective micro-lens array (17a,b; 17a-c) in the first direction (X), pY is an identical grid spacing of the micro-lenses (20a,b; 20a-c) of a respective micro-lens array (17a,b; 17a-c) in the second direction (Y), N denotes the number of coherent laser beams (3.1, ..., 3.N) along the first direction (X), M denotes the number of coherent laser beams (3.1, ..., 3.N) along the second direction (Y), and λL denotes the laser wavelength, adjusting a phase δφn,m of the (n × m)th coherent laser beam (3.1, ..., 3.N), which is given by: δφ n , m = − 2 π / λ L f E , X p n λ L / p Y 2 − 2 π / λ L f E , Y q m λ L / p Y 2 , wherein the following applies: p n = − N + 1 2 + n , q m = − M + 1 2 + m . and combining the coherent laser beams (3.1, ..., 3.N) to form the at least one combined laser beam (12, 12a,b) in the micro-lens arrangement (11).

27. A method for combining an in particular adjustable number N × M of coherent laser beams (3.1, ..., 3.N) emitted from a plurality of emission surfaces (8.1, ..., 8.N), wherein the plurality of emission surfaces (8.1, ..., 8. N) are arranged in a two-dimensional periodic grating along a first direction (X) and along a second direction (Y) that is perpendicular to the first direction (Y), to form at least one combined laser beam (12, 12a,b), comprising: adjusting a respective phase (δφn,m) of one of the coherent laser beams (3.1, ..., 3.N) in at least (N x M)-1 phase adjustment devices (6.1, ..., 6.N), characterized by coupling the coherent laser beams (3.1, ..., 3.N) into a micro-lens arrangement (11) having four micro-lens arrays (17a-d) arranged one behind the other, wherein the micro-lenses (20a, 20c) of the first and third micro-lens arrays (17a, 17c) are arranged along the first direction (X) and the micro-lenses (20b, 20d) of the second and fourth micro-lens arrays (17b, 17d) are arranged along the second direction (Y), wherein a focal length fE,X of the micro-lens arrangement (11) in the first direction (X) and a focal length fE,Y of the micro-lens arrangement (11) in the second direction (Y) are adjusted, which satisfy the following conditions: f E , X = p X 2 / λ L N , f E , Y = p Y 2 / λ L M , wherein pX is an identical grid spacing of the micro-lenses (20a, 20c) of the first and third micro-lens arrays (17a, 17c) in the first direction (X), pY is an identical grid spacing of the micro-lenses (20b; 20d) of the second and fourth micro-lens arrays (17b, 17d) in the second direction (Y), N denotes the number of coherent laser beams (3.1, ..., 3.N) along the first direction (X), M denotes the number of coherent laser beams (3.1, ..., 3.N) along the second direction (Y), and λL denotes the laser wavelength, adjusting a phase δφn,m of the (n × m)th coherent laser beam (3.1, ..., 3.N), which is given by: δφ n , m = − 2 π / λ L f E , X p n λ L / p X 2 − 2 π / λ L f E , Y q m λ L / p Y 2 , wherein the following applies: p n = − N + 1 2 + n , q m = − M + 1 2 + m , and combining the coherent laser beams (3.1, ..., 3.N) to form the at least one combined laser beam (12, 12a,b) in the micro-lens arrangement (11).