DEVICE, LASER SYSTEM AND METHOD FOR COMBINING COHERENT LASER BEAMS

An apparatus for combining a plurality of coherent laser beams includes a splitting device for splitting an input laser beam into the plurality of coherent laser beams, a plurality of phase setting devices for adjusting a respective phase of one of the coherent laser beams, and a beam combining device for combining the coherent laser beams, which emanate from a plurality of grid positions of a grid arrangement, to form at least one combined laser beam. The beam combining device has a microlens arrangement with exactly one microlens array for forming the at least one combined laser beam.

FIELD

The invention relates to an apparatus for combining a plurality of coherent laser beams, comprising: a splitting device for splitting an input laser beam into the plurality of coherent laser beams, a plurality of phase setting devices for adjusting a respective phase of one of the coherent laser beams, and a beam combining device for combining the coherent laser beams, which emanate from a plurality of grid positions of a grid arrangement, to form at least one combined laser beam. The invention also relates to a method for combining a plurality of coherent laser beams, in particular by means of such an apparatus.

BACKGROUND

Within the meaning of this application, “coherent laser beams” is to be understood in the context of a temporal coherence of the laser beams. In general, the laser beams may have a reduced degree of spatial coherence, that is to say the laser beams can be spatially partially coherent, that is to say this does not necessarily relate to single mode laser beams. By way of example, the laser beams can be produced by multi-mode sources and can for example form a higher-mode Gaussian mode, e.g., a Laguerre-Gaussian mode, a Hermite-Gaussian mode or superpositions thereof. However, the laser beams are preferably coherent both in time and space.

In the case of a coherent beam combination, a plurality of laser beams which emanate from a plurality of grid positions of a grid arrangement are superposed to form a combined laser beam, which has a correspondingly higher power. Such a beam combination can be implemented—virtually without loss of beam quality—diffractively, reflectively, for example by way of a segmented mirror, interferometrically or by way of polarization coupling.

US 2013 010 7343 A1 has described a laser system which comprises a laser source in the form of a seed laser and an optical gain system which produces an amplified laser output. The laser system may contain a phase control circuit having a phase modulation functionality for a plurality of optical amplifiers, which comprises a sensor for measuring the overall output intensity of the optical amplifiers. The phase control circuit can change a phase or relative phase relationship between individual optical amplifiers from the total thereof in order to maximize the overall output intensity of the optical amplifiers. The laser system can contain a coherent far-field combination means for combining the output of the optical amplifiers, which comprises a pair of microlens arrays.

US 2013 010 7343 A1 consequently has disclosed the practice of using a (micro)lens arrangement with a pair of microlens arrays as a beam combining device for the coherent combination of a plurality of coherent laser beams to form a combined laser beam. A beam combining device for forming at least one combined laser beam is also described in DE 10 2018 211 971 A1 and WO 2020/016336 A1, said beam combining device having a microlens arrangement with at least two microlens arrays. The beam combination using the at least two microlens arrays is based on the principle of an imaging (two-stage) homogenizer. How such a microlens arrangement should be optimized in view of its parameters (pitch of the microlenses, focal length of the microlenses or the microlens arrangement, spacing of the microlens arrays, . . . ) in order to produce a combined laser beam with an optimized, high beam quality (>90% combining efficiency) is also specified there. When the beam path is reversed, this principle facilitates a homogeneous distribution of the generated intensity peaks, and hence a high beam splitting efficiency.

Very different laser application processes, e.g., additive manufacturing, marking, and welding (both micro-welding and macro-welding) or laser switching processes in laser networks, require a fast deflection of a focal position of a laser beam (scanning) and/or the spilt of a laser beam for the alignment with a plurality of focal positions (beam splitting). Some laser material processing processes, e.g. the separation of transparent materials, possibly require high mean laser powers (of the order of kW) and high pulse energies (of the order of mJ).

SUMMARY

In an embodiment, the present disclosure provides an apparatus for combining a plurality of coherent laser beams includes a splitting device for splitting an input laser beam into the plurality of coherent laser beams, a plurality of phase setting devices for adjusting a respective phase of one of the coherent laser beams, and a beam combining device for combining the coherent laser beams, which emanate from a plurality of grid positions of a grid arrangement, to form at least one combined laser beam. The beam combining device has a microlens arrangement with exactly one microlens array for forming the at least one combined laser beam.

DETAILED DESCRIPTION

The invention is based on the object of providing an apparatus, a laser system, and an associated method for combining coherent laser beams, which even in the case of high laser powers allow virtually complete maintenance of the beam quality during the combination and which additionally allow a fast deflection of a combined laser beam and/or a splitting of the beam of a combined laser beam with a specified division of the input power to be carried out.

According to the invention, this object is achieved by an apparatus of the type set forth at the outset, in which the beam combining device has a microlens arrangement with exactly one microlens array.

The inventors have recognized that when the coherent laser beams are combined in an imaging homogenizer having (at least) two microlens arrays, (at least) one microlens array is situated in the focus or the focal plane of another microlens array. This may lead to burned areas in the microlens array, leading to a loss of power and making material processing at high mean laser powers and mean pulse energies more difficult. In the worst-case scenario, the microlens array situated in the focal plane of the other microlens array can be destroyed.

The present application therefore proposes the use of only a single microlens array for the purposes of combining the coherent laser beams, said microlens array being used with defined parameters (pitch of the microlenses, focal length of the microlenses, . . . ) for combining the coherent laser beams. The principle of the non-imaging, single-stage homogenizer is implemented using such a beam combining device, that is to say only a single microlens array is still used for combination purposes. This reduces the homogeneity, and so beam splitting with a sufficient homogeneity can no longer occur.

The inventors have recognized that although a sufficient beam splitting efficiency cannot be obtained during beam splitting, suitably chosen parameters facilitate a beam combination with a sufficiently high combination efficiency of, e.g., more than approximately 65% (in the case of three coherent laser beams), more than approximately 85% (in the case of five coherent laser beams) or higher even when a single-stage homogenizer is used. This is possible, inter alia, because the intensity of the coherent laser beams can be suitably chosen (e.g., homogeneously) at the grid positions. If moreover the number of coherent laser beams is increased, for example to a number greater than 10, it is even possible to achieve a combination efficiency of more than 90%.

Within the meaning of this application, a microlens arrangement with exactly one microlens array is understood to mean that only the microlenses of a single microlens array bring about the beam combination in a respective direction in which the beam combination is implemented (e.g., in the X-direction or in the Y-direction). In the case where there is a beam combination in two directions (e.g., in the X-direction and in the Y-direction), the exactly one microlens array can have two crossed cylindrical lens arrays within the meaning of this definition, with the microlenses of a respective cylindrical lens array acting only in one direction (X-direction or Y-direction). In this case, the two cylindrical lens arrays are typically arranged immediately adjacently, that is to say these are located (approximately) in a common plane. For the two-dimensional beam combination, two crossed cylindrical lens arrays can be replaced by a single microlens array which has square or rectangular microlenses, for example.

The conditions described in DE 10 2018 211 971 A1 or in WO 2020/016336 A1 for the optimal parameters of such a microlens arrangement also apply accordingly to the single-stage homogenizer described here. However, the (effective) focal length of the microlens arrangement with the at least two microlens arrays is replaced by the focal length of the exactly one microlens array. DE 10 2018 211 971 A1 and WO 2020/016336 A1 are in the totality thereof incorporated in the content of this application by reference.

The splitting device for splitting the input laser beam can for example be a conventional 1-to-N coupling device, for example in the form of one or more microlens arrays, a fiber splitter, a plurality of series-connected beam splitter cubes, polarization beam splitters, a diffraction grating for splitting the beam, etc. The input laser beam can be a seed laser beam produced by a laser source or the input laser beam can be produced from a seed laser beam of a laser source, for example by way of a split and coherent combination.

Alternatively, a plurality of laser sources, for example in the form of fiber oscillators, laser diodes, etc., may also serve to produce the plurality of coherent laser beams such that a splitting device can be dispensed with. In this case, a laser system containing the at least one laser source comprises a control device for driving the laser diodes or the laser sources in order to produce the coherent laser beams. The laser source(s) can be designed to produce ultrashort pulse laser beams, that is to say coherent laser beams which have a pulse duration of less than 10−12s, for example.

In principle, the grid positions of the grid arrangement can be formed along a straight line or curve (one-dimensional grid arrangement) or along a plane or curved surface (two-dimensional grid arrangement). Along the grid arrangement, the coherent laser beams are separated or spaced apart from one another to such an extent that the desired fill factor is obtained. The grid positions of the grid arrangement can be formed at the end faces of fibers (emission areas) or of other emitters, from where a respective coherent laser beam is emitted. In this case, the fibers, more precisely their end faces, are arranged in a grid arrangement and the grid positions correspond to the emission areas on the end faces of the fibers. However, the grid positions or the grid arrangement may also correspond to the near field or the far field of the emission areas, that is to say the grid positions can be arranged along a curve or an area in space on which the emission surfaces are imaged or focused such that the spatial distribution of the grid positions corresponds to the—optionally scaled—spatial distribution of the emission areas.

Consequently, the grid arrangement forms a curve or an area in space, along which there is a desired distance between the grid positions or between the coherent laser beams. By way of example, if a Fourier lens is used to input couple the coherent laser beams (see below), the desired distance is present in the focal plane of the Fourier lens.

In an embodiment the coherent laser beams emanate from a plurality of grid positions arranged in a first direction, with the coherent laser beams and the microlens array satisfying the following condition:

where N denotes a number of the grid positions arranged in the first direction X, pxdenotes a pitch of the microlenses of the microlens array in the first direction, λLdenotes the laser wavelength, and fMLdenotes the focal length of the microlens array.

In the case where the grid positions in the grid arrangement are additionally arranged in a second direction that is preferably perpendicular to the first, the coherent laser beams and the microlens arrangement typically additionally satisfy the following condition:

where M denotes a number of the grid positions arranged in the second direction and pYdenotes a pitch of the microlenses of a respective microlens array in the second direction.

The inventors have recognized that the beam quality of an individual coherent laser beam is virtually fully maintained, even in the case of a single-stage homogenizer, during the combination to form the combined laser beam if equations (1) and/or (2) above are satisfied.

It is understood that equation (1) cannot be exactly observed in practice. The beam quality of the superposed laser beam deteriorates in the case where there is a deviation from equation (1). Within the context of this application, equation (1) above is considered satisfied if the right-hand side of equation (1) deviates by no more than 20%, preferably by no more than 10%, in particular by no more than 5% from the (integer) value N on the left-hand side of equation (1), that is to say if the following applies: |N−px2/(λLfML)|≤0.2, preferably <0.1, in particular <0.05. A corresponding statement also applies to the equation (2), i.e., |M−pY2/(λLfE)|<0.2, preferably <0.1, in particular <0.05.

In a further embodiment, the apparatus is designed to input couple coherent laser beams that are adjacent in the first direction into the microlens arrangement with a specified angle difference δθx, for which the following applies:

where λLdenotes the laser wavelength and pxdenotes a pitch of the microlenses of the microlens array in the first direction.

To combine the coherent laser beams to form a combined laser beam, it is typically necessary or advantageous for adjacent coherent laser beams to be input coupled into the microlens arrangement with the angle difference δθxspecified further above. In order to satisfy this condition, the grid positions from where the coherent laser beams emanate can be aligned at the respective angle difference δθxwith respect to one another and, for example, can be arranged equidistantly on a circular arc. In this case, focusing of the coherent laser beams can be implemented for example with the aid of single lenses or using a further microlens array, which are/is arranged in the respective beam path of one of the coherent laser beams, but the provision of such lenses can optionally also be dispensed with. A corresponding condition applies to the angle difference between adjacent coherent laser beams in the second direction Y, that is to say the following applies: δθy=λL/py. The aforementioned condition is considered satisfied if the following applies: |δθx−λL/px|<0.2, preferably <0.1, in particular <0.05, or if the following applies: |δθy−λL/py|<0.2, preferably <0.1, in particular <0.05.

In a further embodiment the apparatus comprises an input coupling optical unit for input coupling the coherent laser beams into the microlens arrangement, the input coupling optical unit comprising at least one focusing device, in particular at least one focusing lens, for focusing the plurality of coherent laser beams onto the microlens arrangement. In this case, use is made of an input coupling optical unit which is arranged between the grid positions from which the coherent laser beams emanate and the microlens arrangement. In the case where the beam paths of the coherent laser beams are too long to satisfy the conditions specified above, the input coupling optical unit may comprise a telescopic optical unit, for example in the form of at least two lenses.

The input coupling optical unit is not mandatory but may be advantageous, for example when setting up the laser system or the apparatus. In particular, the input coupling optical unit can be used to satisfy the aforementioned condition in relation to the angle difference δθxor δθywithout for this purpose the beam emergence directions of the coherent laser beams at the grid positions having to be aligned at an angle with respect to one another. The use of a focusing lens which is arranged substantially at a distance of its focal length from the microlens arrangement (Fourier lens) was found to be advantageous to this end. In this case, the coherent laser beams can strike the focusing lens with substantially parallel alignment to one another and are focused on the microlens arrangement, more precisely on the microlens array. The focus or the beam diameter of the coherent laser beams incident on the one microlens array is substantially larger than the partial foci that would be incident on the first microlens array when two microlens arrays are used. Moreover, the beam diameter of the combined laser beam formed at the one microlens array is adjustable by way of the fill factor of the coherent laser beams at the grid positions and by way of the pitch of the microlenses of the microlens array.

By way of example, the grid positions can be arranged on a line in this case, that is to say the beam emergence directions or the Poynting vectors of the coherent laser beams are aligned parallel to one another. The use or the design of the input coupling optical unit and the arrangement of the grid positions depend on the boundary conditions, for example on the utilized laser source. By way of example, the use of an input coupling optical unit lends itself to the case where the grid positions form the end faces of fibers running in parallel.

In a development the coherent laser beams emanate from a plurality of grid positions which are arranged in a first direction and which have a distance δxfrom one another which is given by:

where λLdenotes the laser wavelength, fFLindenotes the focal length of the focusing device, and pxdenotes a pitch of the microlenses of the microlens array in the first direction. In the case where the grid positions are additionally arranged in a second direction (e.g., Y direction), the following applies accordingly to the distances δy in the second direction Y: δy=fFLin/py, where pydenotes the pitch of the microlens array in the second direction Y.

In the case where the laser beams run in parallel, the grid positions are typically arranged in a common direction or line (e.g., in the X-direction) and optionally additionally in a common line in the Y-direction, which run(s) perpendicular to the common direction of the beam propagation of the laser beams. In this case, the distance δx between the laser beams or grid positions is typically defined by the aforementioned condition. The aforementioned condition is considered satisfied if the following applies: |δx−λLfFLin/px|<0.2, preferably <0.1, in particular <0.05 and/or |δy−λLfFLin/py|<0.2, preferably <0.1, in particular <0.05.

In an alternative embodiment the coherent laser beams emanate from a plurality of grid positions arranged in a first direction, the grid positions being arranged at a distance from the focal length fMLof the microlens array in front of the microlens array and the grid positions having a distance δx from one another, which is given by

where pxdenotes a pitch of the microlenses of the microlens array in the first direction. In the case where the coherent laser beams are additionally also arranged in a second direction, which is preferably perpendicular to the first direction, the following applies accordingly to the distances δy in the second direction: δy=py, where pydenotes the pitch of the microlenses of the microlens array in the second direction. The aforementioned condition is considered satisfied if the following applies: |δx−px|<0.2, preferably <0.1, in particular <0.05 and |δy−py|<0.2, preferably <0.1, in particular <0.05.

In the embodiment described here, the grid positions are arranged in the focal plane of the microlens array in the beam path of the coherent laser beams upstream of the microlens array. The inventors have recognized that the microlens array acts as a diffraction grating and that, in the case of near field diffraction, the brightness distribution of the microlens array repeats at certain Talbot distances, where the brightness distribution exactly corresponds to the structure of the diffraction grating itself. This is the case for the microlens array in the object-side focal plane. Therefore, the distance between the grid positions in the focal plane should correspond to the pitch of the microlenses of the microlens array.

In principle, the condition specified further above in relation to the angle δθx, δθybetween adjacent coherent laser beams should also be observed in this embodiment. However, the resultant angles δθx, δθyare negligibly small in the present embodiment since the focal length is comparatively short. Although the focal length of the microlens array depends on the pitch of the microlenses and increases with increasing pitch, the pitch itself depends on the angle δθx, δθyby way of the relationship δθx=λL/pxor δθy=λL/pyspecified above. The angle δθx, δθytherefore decreases with increasing pitch and also remains negligible in the case of a large pitch or relatively long focal lengths. Therefore, the coherent laser beams can typically be radiated onto the microlens array with parallel alignment without the use of an input coupling optical unit in this embodiment. The typical order of magnitude of the focal length fMLof the microlens array is less than approximately 70-80 mm and is shorter than the Rayleigh length of the coherent laser beams at the typically utilized wavelengths.

In a development the coherent laser beams have at the grid positions a beam diameter 2 ωfMLxin the first direction, which is given by:

where denotes the laser wavelength. What applies as a matter of principle is that the plurality of coherent laser beams at the grid positions should reproduce the diffraction pattern as accurately as possible in the focal plane, said diffraction pattern arising in the case of a reversal of the beam direction, that is to say for the case where the microlens array is passed in the reverse direction. This can be achieved, inter alia, by virtue of the fact that the coherent laser beams satisfy the condition in relation to the beam diameter 2 ωfmLxspecified above. In this case, the beam diameter 2 ωfmLxdenotes the distance between two points in the intensity or power distribution (generally: a Gaussian distribution) in the first direction, where the maximum intensity or the peak power has dropped to 50%, that is to say the beam diameter 2 ωfMLxdenotes the full width at half maximum.

In the case where the grid positions are additionally also arranged in the second direction, the following applies accordingly to the beam diameter in the second direction: 2 ωfMLy=λLfML/py. As a rule, a respective coherent laser beam has a rotationally symmetric beam profile. In this case, the following applies: 2 ωfMLy=2 ωfMLxand hence py=px. The aforementioned condition is considered satisfied if the following applies: |2 ωfMLx−λLfML/px|<0.2, preferably <0.1, in particular <0.05 and/or |2 ωfMLy−λLfML/ py|<0.2, preferably <0.1, in particular <0.05.

In the case where the grid positions correspond to the end faces of optical fibers, the beam diameter at the respective grid position is substantially defined by the diameter of the optical fiber, more precisely the diameter of the beam emergence area on the end face of the optical fiber. It is possible with the aid of a suitable beam shaping device to change the beam diameter of the coherent laser beams following the emergence from the respective optical fiber such that a desired diameter is set at the respective grid position of the grid arrangement. By way of example, the beam shaping device may for this purpose comprise a plurality of (spherical) collimation or focusing lenses, in the focal plane of which the grid arrangement is formed.

It was found that the intensity of the coherent laser beams at the respective grid position has a comparatively small influence on the combination efficiency. The maximum intensities of the coherent laser beams may be equal in size in the present embodiment, as is also the case in the embodiment described further above. However, the coherent laser beams preferably have a respective maximum intensity at the grid positions, the envelope of which corresponds to an intensity distribution of the combined laser beam at the microlens array. The intensity distribution of the combined laser beam at the microlens array typically is a Gaussian distribution, which forms the envelope of the maximum intensities.

In a development, the coherent laser beams have a fill factor FFxin the first direction, for which the following applies: FFx<0.4, preferably FFx<0.3. Accordingly, it is advantageous if the following applies to the fill factor FFyin the second direction Y: FFy<0.4, preferably FFy<0.3.

The fill factor FFxin the first direction X is defined as FFx=2 ωfMLx/δx. Accordingly, the fill factor FFyin the second direction Y is defined as FFy=2 ωfMLy/δy. The distance δx and δy between the grid positions in the X-direction and Y-direction, respectively, denotes the distance between the centers of the beam profiles of adjacent coherent laser beams. It was found that the fill factor FFxor FFyshould not be chosen to be too large in the present embodiment since the fill factor FFx, FFyinfluences the beam diameter 2 ωMLAxand 2 ωMLAy, respectively of the combined laser beam, as will be described below.

The following applies to the beam diameter 2 ωMLAxof the intensity distribution of the combined laser beam at the microlens array in the first direction X:

Unlike the beam diameter 2 ωfMLxof the coherent laser beams, the beam diameter 2 ωMLAxdenotes the 1/e2width, that is to say the distance between two points where the peak power has dropped to 1/e2of the maximum, that is to say approximately 13.5% of the peak power. Accordingly, the following applies to the beam diameter 2 ωMLAyof the combined laser beam at the microlens array in the second direction Y: 2 ωMLAy=4 py/(πFFy). The above condition is considered satisfied if the following applies: |2 ωMLAx−4 px/(πFFx)|<0.2, preferably <0.1, in particular <0.05 and |2 ωMLAx−4 px/(πFFx)|<0.2, preferably <0.1, in particular <0.05.

As emerges from the relationship above, the diameter 2 ωMLAxof the combined laser beam12and hence the illumination of the microlens array 17 reduces with increasing fill factor FFxin the first direction X. The higher the fill factor FFx, the lower the combination efficiency. In principle, what applies is that the fill factor FFx, FFyshould be smaller, the greater the number of coherent laser beams in the respective direction.

In a further embodiment, the apparatus comprises a control device designed or programmed to adjust a respective phase of one of the coherent laser beams on the basis of an arrangement of the respective grid position within the grid arrangement in order to combine the coherent laser beams to form at least one laser beam that is diffracted into at least one order of diffraction. The order of diffraction can be the zeroth order of diffraction or an order of diffraction that differs from the zeroth order of diffraction.

The phases can be chosen in such a way that there is a combination into the zeroth order of diffraction that is optimized in view of the beam quality. The phases of, or phase differences between, the coherent laser beams may also be chosen such that the combined laser beam is diffracted into at least one higher order of diffraction in order to carry out a controlled beam deflection or a controlled beam split. In the case where an even number of coherent laser beams are combined there is no zeroth order of diffraction, that is to say the combined laser beam is always diffracted into at least one (half integer) order of diffraction in this case.

The phase of a respective coherent laser beam can be adjusted individually with the aid of the control device on the basis of the arrangement of the grid position of the grid arrangement assigned to the respective coherent laser beam so that the coherent laser beams are no longer combined to form a single or individual laser beam but are combined into two or more well-defined bundles or into two or more combined laser beams, which are diffracted with a defined power distribution or power division into different orders of diffraction (beam splitting) or into a single laser beam which is diffracted into an order of diffraction that differs from the zeroth order of diffraction (beam deflection).

The proposed approach is based on the concept of the optical phase array (OPA), in the case of which a set of absolute phases of the one-dimensional or two-dimensional grid arrangement of the coherent laser beams is chosen such that there is constructive interference at well-defined orders of diffraction. In the case of a one-dimensional or two-dimensional grid arrangement (array), the phases of the coherent laser beams to be combined can be chosen such that it is possible to add or remove individual combined laser beams, groups of combined laser beams or an entire array of combined laser beams, which corresponds to a set of orders of diffraction, in a targeted manner. For a respectively desired group of combined laser beams intended to be produced by the apparatus, it is possible for example to choose a suitable set of (absolute) phases by means of an iterative optimization algorithm in order to activate or deactivate the diffraction to certain orders of diffraction in a targeted manner. In this way it is possible to realize a variable beam split or deflection and power division. The iterative optimization algorithm can be a stochastic or randomized algorithm, for which a homogeneous power division or intensity distribution is specified as start values for example.

The phase setting devices serve to adjust the respective phase of the coherent laser beams and may be arranged at any desired location in front of the microlens arrangement where the coherent laser beams are separated from one another and no longer overlap. These phase setting devices are required, inter alia, because, e.g., thermal effects, vibrations or else air turbulence lead to optical path length differences in the individual channels. There are a number of options for realizing the phase setting devices, which are typically designed to set a variable phase lag: By way of example, the phase setting devices can be modulators in the form of EOMs (electro-optic modulators, for example in the form of liquid crystals), SLMs (spatial light modulators), optical retardation paths in the form of mirror arrangements, electro-mechanical modulators, for example in the form of piezo-mirrors, or the like. In the case where the coherent laser beams are guided in a fiber on the beam path upstream of the grid arrangement, it is possible to apply a tensile stress to the fiber, for example by means of piezo actuators, for the purposes of adjusting the phase; it is also possible to influence the temperature of the fiber, etc. The control device can be realized in the form of hardware and/or software, for example in the form of a microcontroller, an FPGA, an ASIC, etc. The control device is designed to suitably act on the phase setting devices, for example by way of suitable electronic (control) signals. Since the addition of a phase factor that is identical for all coherent laser beams does not change the result of the coherent beam combination, a total of N−1 phase setting devices are sufficient in the case of a total of N coherent laser beams to be combined in one direction.

The coherent laser beams produced in the laser source or sources can be guided to the grid arrangement with the aid of a plurality of beam guiding devices for example in the form of fibers. The individual beam guidance of the laser beams renders it possible to act thereon on an individual basis, in order to suitably set the relative phases with the aid of the phase setting device. The beam guiding devices may comprise an appropriate number of amplifiers or amplifier chains, for example in the form of fiber amplifiers, in order to amplify the laser beams before these are emitted in the direction of the microlens arrangement from the grid positions. The phase setting devices can be arranged upstream of the beam guiding devices or downstream of the beam guiding devices in the beam path, and/or can act on the beam guiding devices for example in the form of fibers. Alternatively, following the split in the splitting device, the coherent laser beams can reach the grid arrangement by way of free beam propagation, said grid arrangement for example being able to be located in a focal plane of a Fourier lens or at any other location where the coherent laser beams are spaced apart from one another to a sufficient degree. In the focal plane of such a Fourier lens or at the other location, the coherent laser beams—optionally after a suitable deflection—have the desired fill factor, that is to say a desired ratio between the extent or beam diameter of the respective laser beams in a respective spatial direction and the distance between the centers of adjacent laser beams, as was described further above.

In a development the control device is designed to adjust a respective fundamental phase of one of the coherent laser beams, in the case of which fundamental phase the beam combining device combines the coherent laser beams to form one laser beam that is diffracted into exactly one order of diffraction. Consequently, exactly one combined laser beam is produced in the case of the fundamental phase, said combined laser beam being diffracted into the zeroth order of diffraction (if present) or into an order of diffraction that differs from the zeroth order of diffraction in order to deflect the combined laser beam.

In a development the grid positions are arranged in a first direction and the control device is designed, for the purposes of combining the coherent laser beams to form the exactly one combined laser beam that is diffracted into the exactly one order of diffraction Bk,xin the first direction, to set the respective fundamental phase δφaof a coherent laser beam at an a-th grid position in the first direction which is given by:

where the following applies:

with a=1, . . . , N, where N is a number of the grid positions arranged in the first direction and where Bk,xis an integer or half integer, for which the following applies:

In the case where the number N of coherent laser beams is odd, the order of diffraction Bk,xassumes integer values. In the case where an even number N of coherent laser beams are combined, the order of diffraction Bk,xassumes half integer values.

In a development the grid positions in the grid arrangement are additionally arranged in a second direction that is preferably perpendicular to the first direction and the control device is designed, for the purposes of combining the coherent laser beams to form the exactly one combined laser beam that is diffracted into the exactly one order of diffraction Bk,xin the first direction and into exactly one order of diffraction Bj,yin the second direction, to set the respective fundamental phase δφa,bof a coherent laser beam at an a-th grid position in the first direction and at a b-th grid position in the second direction which is given by:

where the following applies:

with b=1, . . . , M, where M is a number of the grid positions arranged in the second direction and where Bj,yis an integer or half integer, for which the following applies:

In a further development the splitting device for splitting an input laser beam into the plurality of coherent laser beams is designed as a further microlens arrangement with at least two further microlens arrays, and the control device is designed, for the purposes of combining the coherent laser beams to form the exactly one combined laser beam that is diffracted into the exactly one order of diffraction Bk,xin the first direction and preferably diffracted into the exactly one order of diffraction Bj,yin the second direction, to set twice as much of the fundamental phases.

It was found that the values for the fundamental phases δφa, δφa,bspecified in the equations above need to be doubled for the special case that a respective microlens arrangement is used for dividing an input laser beam into the plurality of coherent laser beams and for combining the coherent laser beams. What applies in principle is that a doubling of the fundamental phases is required for the special case of two microlens arrangements, which optionally may have identical designs, in relation to the case where a fiber splitter or any other optical device is used for combination purposes. Consequently, doubling the fundamental phases is not restricted to the equations specified further above but applies in general.

In a further development the control device is designed to set the respective phase of one of the coherent laser beams that is composed of the respective fundamental phase and an additional phase. The additional phase facilitates a split of the combined laser beam into two or more orders of diffraction or a fast change in the order of diffraction into which the combined laser beam is diffracted. Preferably, the fundamental phases are chosen in such a way in the case described here that the beam combining device combines the coherent laser beams—without the additional phase—into the zeroth order of diffraction. An assumption is made below that the fundamental phases are chosen such that there is a combination of the laser beam into the zeroth order of diffraction as a result of the fundamental phases.

It was found that analytic relationships can be found for the choice or definition of the phases of the coherent laser beams in the case of discrete scanning in special cases, said analytic relationships being reproduced below.

In a development the grid positions are arranged spaced apart (equidistantly) in a first direction and the control device is designed, for the purposes of combining the coherent laser beams to form a single combined laser beam that is diffracted into an order of diffraction Bk,xthat differs from the zeroth order of diffraction, to set the respective additional phase Δφaof a coherent laser beam at an a-th grid position in the first direction which is given by:

where N denotes a number of the grid positions arranged in the first direction and Bk,xdenotes an integer or half integer, for which the following applies:

In the first direction, the grid positions are arranged at the same distance from one another (equidistantly). In this case, the grid positions can be arranged on a line that extends in the first direction, that is to say the beam emergence directions or the Poynting vectors of the coherent laser beams are aligned parallel to one another. Alternatively, the grid positions can also be arranged equidistantly from one another on a circular arc, for example, which extends in or along the first direction.

In a development of this embodiment the grid positions of the grid arrangement are additionally arranged in a second direction that is perpendicular to the first direction and the control device is designed, for the purposes of combining the coherent laser beams to form a single combined laser beam that is diffracted into an order of diffraction Bk,xin the first direction that differs from the zeroth order of diffraction and into an order of diffraction Bk,yin the second direction that differs from the zeroth order of diffraction, to set an additional phase Δφa,bof a coherent laser beam at an a-th grid position in the first direction and at a b-th grid position in the second direction which is given by:

where M denotes a number of the grid positions in the second direction and Bj,ydenotes an integer or half integer, for which the following applies:

Observing the aforementioned conditions for the additional phases Δφaor Δφa,band for the fundamental phases δφaor δφa,bfacilitates a deflection without loss of efficiency. However, it is understood that the aforementioned conditions cannot be exactly observed in practice. The beam quality of the deflected laser beam deteriorates in the case where there is a deviation from the aforementioned conditions. Within the context of this application, the aforementioned conditions are considered satisfied if the right-hand side deviates by no more than 20%, preferably by no more than 10%, in particular by no more than 5% from the value Δφaor Δφa,bon the left-hand side, that is to say if the following applies: |Δφa+2(π/N)(a−(N+1)/2)Bk,x|<0.2, preferably <0.1, in particular <0.05. A corresponding statement also applies to Δφa,b, i.e., |Δφa,b+((2π/N)(a−(N+1)/2)Bk,x(2π/M)(b−(M+1)/2))Bj,y|<0.2, preferably <0.1, in particular <0.05. A corresponding statement also applies to the fundamental phases δφaor δφa,b, i.e., |δφa+π/N(ma+Bk,x)2|<0.2, preferably <0.1, in particular <0.05 or |δφa,b+π/N(ma+Bk,x)2+π/M(mb+Bj,y)2|<0.2, preferably <0.1, in particular <0.05.

The additional phase Δφa,bis set at an a-th grid position in the first direction, which simultaneously forms a b-th grid position in the second direction. In the case where the grid positions in the grid arrangement are arranged only in the first direction the coherent laser beams are combined to form a single laser beam which is diffracted into the zeroth order of diffraction in the second direction (i.e., Bk,y=0). Consequently, the formula for the additional phase Δφaspecified further above arises for the one-dimensional case.

In this development, a plurality of N×M laser beams, rather than a one-dimensional coherent combination of a laser beams, are combined in two dimensions to form one or more laser beams. In this case, the grid positions are arranged in a two-dimensional grid arrangement, with the distances between adjacent grid positions typically being the same in both directions if the number of grid positions is the same in both directions (i.e., N=M) or—should N not equal M—being chosen to be different. In this case, the grid or the grid arrangement with the grid positions can extend in a plane (e.g. XY-plane) or on a curved surface, for example on a spherical shell. The laser beams emanating from the grid positions are typically aligned in parallel in the first case and can in the second case be aligned for example in the direction of the center of the spherical shell, where the microlens arrangement is arranged.

In this case, the periodicity of the grid with the grid positions specifies the pitches of the microlenses in two different, for example perpendicular directions (X, Y). In this case, it is possible to use a 2-dimensional microlens array whose pitches px, pYoptionally differ in the two perpendicular directions X, Y on the basis of the periodicity of the grid. Accordingly, the microlenses of the 2-dimensional microlens array have an optionally different curvature in the X-direction and in the Y-direction, that is to say these are not cylindrical lenses. It is also possible to form a 2-dimensional microlens array by combining two 1-dimensional microlens partial arrays with cylindrical lenses, with the cylindrical lenses of the 1-dimensional microlens partial arrays being aligned perpendicular to one another and being arranged in the same plane, that is to say even in this case the microlens arrangement has only a single microlens array and does not act as an imaging homogenizer.

The relationship between the 2-dimensional grid with the grid positions and the 2-dimensional microlens array is analogous to the relationship between the Bravais lattice and the reciprocal lattice. Accordingly, the arrangement of the grid positions can also correspond to the highest density packing, that is to say a hexagonal lattice. The microlenses of the microlens array are likewise arranged in a hexagonal arrangement in this case.

In an embodiment, the control device is designed to vary the respective phase of one of the coherent laser beams on the basis of an arrangement of the respective grid position within the grid arrangement in order to change an order of diffraction into which the at least one combined laser beam is diffracted. In this way it is possible to realize an extremely quick, discrete scanning process, within the scope of which the at least one diffracted laser beam jumps or is moved back and forth between different orders of diffraction. In this case, the apparatus can serve as a scanner device or as a beam shaping unit.

The scanning process can be carried out using a laser beam that is diffracted into a single order of diffraction, but it is also possible to realize a discrete scanning process using a laser beam that is split among two or more orders of diffraction (at most ±(N−1)/2 orders of diffraction), that is to say using two or more combined laser beams. In this case, the phase relationship or the phase of a respective coherent laser beam required to diffract or split the combined laser beam into at least two different orders of diffraction can be set with the aid of the control device. By varying the phase of the coherent laser beams, it is possible to change the power distribution among the various orders of diffraction into which the at least two combined laser beams are diffracted. In this way it is possible to implement a discrete scanning process with a number of combined laser beams, with the scan field being between the −((N−1)/2)-th order of diffraction and the (N−1)/2-th order of diffraction and N denoting the number of coherent laser beams (in the respective scanning direction).

The control device can set or vary the respective phase of the coherent laser beams on the basis of a parameter table stored in a memory device in order to move the at least one combined laser beam along a specified (discrete) trajectory. The respective phases to be set can also be specified to the control device from the outside, for example by a user, or the phases to be set can be specified or varied on the basis of at least one measured variable which is measured for example with the aid of a sensor arrangement, that is to say there can be closed-loop control of the phases to a respective target value. In the case of the at least one combined laser beam or at least one combined laser beam not being diffracted into the zeroth order of diffraction during the beam combination, it is generally necessary to use a sensor array or optionally a spatially resolving sensor for the phase detection.

In the case where the combined laser beam is imaged by means of a lens or an imaging optical unit, the (at least one) combined laser beam no longer propagates along the optical axis but with a parallel offset from the optical axis. The magnitude of the parallel offset of the combined laser beam depends on the higher order of diffraction (±1, ±2; ±0.5, ±1.5, etc.), into which said combined laser beam is diffracted. In the case where the grid positions are arranged in a two-dimensional grid arrangement, the (at least one) combined laser beam can thus be offset in two typically perpendicular directions parallel to the optical axis, to be precise within a further grid arrangement that corresponds to the grid arrangement of the coherent laser beams.

In a development the control device is designed to vary the respective additional phase of the coherent laser beams for the purposes of changing a first order of diffraction, into which a first combined laser beam is diffracted, and/or for the purposes of changing a second order of diffraction, into which a second combined laser beam is diffracted. In this embodiment the coherent laser beams are combined by the beam combining device to form at least two diffracted laser beams. To achieve this, the respective (additional) phases of the combined laser beams are suitably chosen, for the purposes of which an iterative, for example stochastic, optimization algorithm can be used in order to vary or set the (±(N−1)/2-th or zeroth) order of diffraction of the first combined laser beam and the (±(N−1)/2-th or zeroth) order of diffraction of the second combined laser beam in a targeted manner. It is understood that variable beam splitting is not restricted to two combined laser beams but can also be performed with more than two combined laser beams.

In a further embodiment the control device is designed to adjust a respective additional phase of the coherent laser beams for the purposes of producing a specified, in particular different power of the at least two combined laser beams that are diffracted into different orders of diffraction. In particular, the control device can be designed to vary the respective additional phase of one of the coherent laser beams on the basis of an arrangement of the respective grid position of the coherent laser beam within the grid arrangement, in order to change the specified, in particular different power or the power distribution over time.

The input power can be distributed equally among the respective combined laser beams but it is also possible to implement a specified, differing distribution of the input power among the at least two laser beams combined in different orders of diffraction, and optionally to vary this distribution over time.

In the case where the coherent laser beams are combined to form a first combined laser beam that is diffracted into the zeroth order of diffraction and a second combined laser beam that is diffracted into the ±1st order of diffraction in the first direction, the input power p can for example be split among the 0th and the ±1st order of diffraction as follows: p0=C p; p±1=(1−C)p, where 0<C<1. For the two cases of C=1 and C=0, respectively, only one combined laser beam is produced, which is diffracted into the 0th or into the ±1st order of diffraction. In the case of C=0.5, half of the input power p is diffracted into the 0th order of diffraction and the other half is diffracted into the ±1st order of diffraction.

For the additional phase of a respective coherent laser beam at an a-th grid position in the first direction, which produces the aforementioned power distribution with the factor C, the following applies:

where a component of the input power p is diffracted into the −1st order of diffraction for a positive sign in the equation above and a component of the input power is diffracted into the +1st order of diffraction for a negative sign in the equation above. The equation above can be generalized to the two-dimensional case in a manner analogous to the aforementioned equations for the additional phase Δφa, with the following formula arising for the additional phase Δφa,b:

The factor C can be chosen to be constant or can be varied in a time-dependent manner. In the latter case, the apparatus can be operated in the style of an acousto-optic or electromechanical component in the form of deflectors or modulators. The formulae above for the additional phase apply generally for the case where the input power is intended to be split between two immediately adjacent orders of diffraction. For the case where the fundamental phase is set such that there is a diffraction of the coherent laser beams into the +1st order of diffraction, the input power is split between the +1st order of diffraction and the +2nd order of diffraction.

In the case of a number M of more than two combined laser beams, the split can for example be realized in the form of a (linear) power ramp, in the case of which a first combined laser beam is diffracted with a maximum power pk.maxinto the k-th order of diffraction and the remaining M−1 combined laser beams are diffracted into the remaining M−1 orders of diffraction with a power that has been reduced in relation to the maximum power pk.max. By way of example, for the power distribution in the form of a power wedge, the following may apply: a/M pk,max, where a =1, . . . , M. For the example of a total of five diffracted combined laser beams, proportions of the maximum power pk,maxof 100%, 80%, 60%, 40% and 20% arise.

A further aspect of the invention relates to a laser system, comprising: a seed laser source for producing a seed laser beam, and an apparatus as described further above for combining the plurality of coherent laser beams, with the seed laser beam preferably forming the input laser beam of the apparatus. The seed laser source is preferably designed to produce the seed laser beam with a spectral bandwidth of less than 100 nm, particularly preferably less than 50 nm, in particular less than 10 nm, and preferably with a spatial fundamental mode (single mode laser beam). The seed laser beam can be guided to the apparatus either directly or by way of suitable beam-guiding optical elements. Prior to entry into the above-described apparatus, the seed laser beam may be amplified in at least one optical amplifier. Particularly in this case it is possible to optionally completely dispense with the provision of amplifiers, for example in the form of gain fibers, for amplifying the individual coherent laser beams in the apparatus. As a result of amplifying the seed laser beam prior to entry into the apparatus it is optionally possible to dispense with active closed-loop control of the phases of the individual coherent laser beams. In this case, a static phase—or varying phase for the targeted modification of the respective order of diffraction—can be set at the respective phase setting devices, and need not be corrected. Alternatively, it is possible for the input laser beam itself to be a combined laser beam, as will be described in more detail below.

In an embodiment the laser system additionally comprises a further apparatus for combining a plurality of further coherent laser beams, comprising: a further splitting device for splitting the seed laser beam or the (further) input laser beam into the plurality of further coherent laser beams, a plurality of further phase setting devices for adjusting a respective phase of one of the further coherent laser beams, and a further beam combining device for combining the further coherent laser beams emanating from a plurality of further grid positions of a further grid arrangement, with the further beam combining device comprising a further microlens arrangement having at least one further microlens array, and a further control device which is designed to adjust the respective phase of one of the further coherent laser beams on the basis of an arrangement of the respective further grid position within the further grid arrangement in order to combine the coherent further laser beams to form exactly one laser beam that is diffracted into exactly one order of diffraction, said diffracted laser beam forming the input laser beam of the splitting device of the apparatus. To avoid the above-described problems in the case of high mean laser powers, it was found to be advantageous if the further microlens arrangement also comprises exactly one microlens array; however, this is not mandatory. In particular, the power of the coherent laser beams in further apparatus can be so low that the use of two (or more) microlens arrays is also possible.

In this case, the further control device of the further apparatus is designed or programmed to combine the further coherent laser beams to form a laser beam that is diffracted into the zeroth order of diffraction or into an order of diffraction that differs from the zeroth order of diffraction, by virtue of the fundamental phases described further above in conjunction with the apparatus being set.

In this embodiment, a further apparatus for combining a plurality of further coherent laser beams is used to produce the input laser beam for the apparatus described further above. In this case, the further apparatus forms an amplified combined further laser beam, which forms the input laser beam of the apparatus, from the seed laser beam. In this case it is also possible to optionally completely dispense with the provision of amplifiers within the apparatus, in particular in the beam path downstream of the splitting device. Since an amplified input laser beam is input coupled into the apparatus it is optionally possible to dispense with an active phase adjustment or closed-loop phase control in the apparatus such that the deflection of the at least one combined laser beam in the apparatus is not slowed down by closed-loop phase control. An active stabilization of the phase settings by means of a control loop, which is provided in the further apparatus for combining the further coherent laser beams, is simplified in this case since only the zeroth order of diffraction needs stabilization.

A further aspect of the invention relates to a method for combining a plurality of coherent laser beams, in particular by means of the above-described apparatus, the method comprising: input coupling the plurality of coherent laser beams emanating from a plurality of grid positions arranged in a grid arrangement into a microlens arrangement having exactly one microlens array, and combining the coherent laser beams in the microlens arrangement to form at least one combined laser beam. As was described further above, the coherent combination of the laser beams with a sufficient combination efficiency can also be obtained with the aid of a single microlens array should the parameters be suitably chosen, as described further above in the context of the apparatus.

In a variant, the method comprises: adjusting a respective phase of one of the coherent laser beams on the basis of an arrangement of the respective grid position in the grid arrangement in order to combine the coherent laser beams to form at least one laser beam that is diffracted into at least one order of diffraction, with the method preferably comprising: varying the respective phase of the coherent laser beams on the basis of an arrangement of the respective grid position within the grid arrangement in order to change an order of diffraction into which the at least one combined laser beam is diffracted. As a result of varying the phases there can be a highly dynamic, discrete scanning process in one or two directions.

As was described further above in conjunction with the apparatus, the method for combining the plurality of laser beams can also deviate in a targeted fashion from the fundamental phases or from phase differences between the coherent laser beams for a combination in the zeroth or in a higher order of diffraction which is optimized in view of beam quality in order to carry out a controlled, fast beam deflection or controlled beam split. In the case of a beam deflection or the beam split with suitably chosen additional phases of the individual coherent laser beams there is a negligible loss of efficiency for the respective order of diffraction. The additional phases of the individual coherent laser beams may in particular satisfy the equations for Δφaor for Δφa,b, which are specified further above in the context of the laser system or the apparatus. The fundamental phases δφaand δφa,btypically also satisfy the equations described further above in the context of the apparatus.

In a further variant the method comprises: varying the respective additional phases of the coherent laser beams for the purposes of changing a first order of diffraction, into which a first combined laser beam is diffracted, and/or for the purposes of changing a second order of diffraction, into which a second combined laser beam is diffracted, proceeding from a respective fundamental phase in the case of which the beam combining device combines the coherent laser beams to form a single laser beam that is diffracted into exactly one order of diffraction.

As was described further above, varying the phases can realize a highly dynamic beam split, in the case of which two, three or optionally more (at most N or N×M) combined laser beams can be produced and/or in the case of which the position or the alignment of at most N−1 or at most (N−1)×(M−1) combined laser beams can be changed. It is understood that the scanning process described further above in the context of a single combined laser beam can also be combined with the split among two or more combined laser beams.

In a further variant the method comprises: adjusting a respective additional phase of the coherent laser beams for the purposes of producing a specified, in particular different power of the at least two combined laser beams that are diffracted into different orders of diffraction proceeding from a respective fundamental phase, in the case of which the beam combining device combines the coherent laser beams to form a single laser beam that is diffracted into exactly one order of diffraction. As was described further above in the context of the apparatus, the input power can be distributed equally among the two or more combined laser beams but it is also possible to deviate from an equal distribution among the plurality of combined laser beams in a targeted fashion.

As was described further above, it is advantageous if the coherent laser beams and the microlens arrangement satisfy the conditions N=px2/(λLfML) and M=pY2/(λLfML) specified above (with an identical focal length fMLbeing assumed). It is also advantageous if adjacent coherent laser beams are input coupled into the microlens array with a specified angle difference δθxor δθy, for which the following applies: δθx=λL/pxor δθy=λL/py.

Further advantages of the invention will become apparent from the description and the drawing. Likewise, the features mentioned above and those that will be explained further can be used in each case by themselves or as a plurality in any desired combinations. The embodiments shown and described should not be understood as an exhaustive enumeration, but rather are of illustrative character for outlining the invention.

In the following description of the drawings, identical reference signs are used for identical or functionally identical components.

FIG. 1ashows an exemplary structure of a laser system1which comprises a laser source2for producing a seed laser beam2a. To this end, the laser source2comprises a mode-coupled fiber-master oscillator, which produces the seed laser beam2aat a laser wavelength4. The seed laser beam2aof the laser source2is fed as an input laser beam9to an apparatus5for combining a number N of coherent laser beams3.1,3.2, . . . ,3.N. The apparatus5comprises a conventional 1-to-N splitting device4, for example in the form of a fiber splitter, in order to split the input laser beam9, which corresponds to the seed laser beam2a, into the number N of coherent laser beams3.1, . . . ,3.N. The coherent laser beams3.1, . . . ,3.N run through a corresponding number N of phase setting devices6.1, . . . ,6.N, which allow a respective individual phase δφa+Δφaof the coherent laser beams3.1, . . . ,3.N (a=1, . . . , N) to be set by virtue of bringing about a suitable phase lag. By way of example, the phase setting devices6.1, . . . ,6.N can be designed as electro-optic modulators or deflectors, for example using liquid crystals, as acousto-optic modulators or deflectors, as electromechanical modulators or deflectors, for example in the form of actuatable piezo-mirrors, etc.

After the phase setting devices6.1, . . . ,6.N, the coherent laser beams3.1, . . . ,3.N run through a corresponding number N of gain fibers7.1, . . . ,7.N in order to amplify the coherent laser beams3.1, . . . ,3.N. The end faces of the gain fibers7.1, . . . ,7.N serve as emission surfaces or form grid positions8.1, . . . ,8.N at which the coherent laser beams3.1, . . . ,3.N are emitted. The phase setting devices6.1, . . . ,6.N may also be arranged downstream of the gain fibers7.1, . . . ,7.N or may act directly on the gain fibers7.1, . . . ,7.N, for example by virtue of producing an adjustable mechanical stress on the gain fibers7.1, . . . ,7.N.

The coherent laser beams3.1, . . . ,3.N can be deflected to a deflection device with a plurality of deflection mirrors, not depicted here, in order to increase the fill factor, that is to say to reduce the distance between adjacent laser beams3.1, . . . ,3.N or grid positions8.1, . . . ,8.N. It is understood that the deflection device is not mandatory. In the example shown, the coherent laser beams3.1, . . . ,3.N enter a beam combining device10when aligned parallel to one another, said beam combining device comprising a microlens arrangement11in the form of a non-imaging homogenizer with one microlens array17for coherently combining the laser beams3.1, . . . ,3.N in order to form a combined laser beam12or a plurality of combined laser beams12a,b(the latter not being shown inFIG. 1a).

As is evident fromFIG. 1a, a component12cof the combined laser beam12is output coupled via an output coupling device in the form of a partly transmissive mirror13and is incident on a spatially resolving detector14, for example in the form of a sensor array or a camera. The detector14is signal-connected to a control device15of the laser system1, the control device controlling the phase setting devices6.1, . . . ,6.N in order to adjust the individual phases δφa+Δφaof the laser beams3.1, . . . ,3.N on the basis of the properties of the detected component12aof the combined laser beam12. The control device15can in particular facilitate closed-loop control of the phase setting devices6.1, . . . ,6.N in order to produce desired (target) phases δφa+Δφaof the laser beams3.1, . . . ,3.N on the basis of the properties of the detected component12aof the combined laser beam12.

Even though the number N of phase setting devices6.1, . . . ,6.N corresponds to the plurality N of laser beams3.1, . . . ,3.N in the example shown, a number of N−1 phase setting devices6.1, . . . ,6.N−1 is generally sufficient. In the laser system1shown inFIG. 1a, it is possible firstly to attain a high beam quality of, e.g., M=1.3 of the combined laser beam12and secondly to attain a significant increase in the power of the laser beams3.1, . . . ,3.N as a result of the gain in the gain fibers7.1, . . . ,7.N.

FIG. 1bshows a laser system1which substantially differs from the laser system1shown inFIG. 1ain that the coherent laser beams3.1, . . . ,3.N in the apparatus5are not amplified with the aid of a plurality of gain fibers7.1, . . . ,7.N or with the aid of other optical amplifiers. Rather, the seed laser beam2ais amplified in a gain fiber7in the laser system1shown inFIG. 1b. The amplified seed laser beam2ais supplied to the apparatus5as input laser beam9. The apparatus5ofFIG. 1bis designed analogously to the apparatus5shown inFIG. 1a. The splitting device4can be designed in various ways, for example as a beam splitter, e.g., in the form of a plurality of series-connected beam splitter cubes, as a polarization beam splitter, as a segmented mirror or as a microlens arrangement with (at least) two microlens arrays. The use of two microlens arrays for beam splitting is possible even at relatively high powers if the system parameters are suitably chosen (small form factor, large pitch) so that the second microlens array is not arranged in the focal plane of the first microlens array.

In the case of a splitting device in the form of a microlens array, the grid positions8.1, . . . ,8.N of the coherent laser beams3.1, . . . ,3.N are not formed at the end faces of the gain fibers7.1, . . . ,7.N but are situated in a focal plane of a microlens array, second in the beam path, of the splitting device4, that is to say in the far field or the focal plane of the second microlens array of the splitting device4. The grid positions8.1, . . . ,8.N of the coherent laser beams3.1, . . . ,3.N in the focal plane form a grid arrangement16, in which adjacent grid positions8.1, . . . ,8.N have the same distance from one another, that is to say are arranged equidistantly.

In the apparatus1shown inFIG. 1b, the phase setting devices6.1, . . . ,6.N are designed to set the phases δφa+Δφaof the laser beams3.1, . . . ,3.N in free-beam propagation. By way of example, the phase setting devices6.1, . . . ,6.N can be electro-optic or acousto-optic modulators or deflectors. In the case of the apparatus5shown inFIG. 1b, the control device15also serves to control the phase setting devices6.1, . . . ,6.N. The active control of the phases δφa+Δφaof the laser beams3.1, . . . ,3.N described in the context ofFIG. 1acan be dispensed with in the apparatus5shown inFIG. 1b, at least in the case where the radiant fluxes of the laser beams3.1, . . . ,3.N are not too high, that is to say the control device15can set the (static) target phases δφa+Δφaat the phase setting devices6.1, . . . ,6.N without a correction being required. On account of the non-required active phase adjustment or control, deflection of the laser beam or the combined laser beams12,12a,bcan be quicker in the case of the apparatus5than in the case for the apparatus5shown inFIG. 1a.

FIG. 1cshows a laser system1which has the same form as that ofFIG. 1b, with the laser system1ofFIG. 1ccomprising a further apparatus5′ for combining a plurality N of further laser beams3.1′, . . . ,3.N′, instead of the amplifier7shown inFIG. 1b, for the purposes of amplifying the seed laser beam2a, the further apparatus being designed analogously to the apparatus5shown inFIG. 1a. The seed laser beam2ais fed to the further apparatus5′ as an input laser beam9′, and is split into a number N of further coherent laser beams3.1′, . . . ,3.N′ by means of a further 1-to-N splitting device4′. The number N of further coherent laser beams3.1, . . . ,3.N run through a corresponding number N of further phase setting devices6.1′, . . . ,6.N′, which allow a respective individual (fundamental) phase δφaof the further coherent laser beams3.1′, . . . ,3.N (a=1, . . . , N) to be set by virtue of bringing about a suitable phase lag.

After the further phase setting devices6.1′, . . . ,6.N′, the further coherent laser beams3.1′, . . . ,3.N′ run through a corresponding number N of further gain fibers7.1′, . . . ,7.N′ in order to amplify the further coherent laser beams3.1′, . . . ,3.N′. The end faces of the further gain fibers7.1′, . . . ,7.N′ serve as emission surfaces or form further grid positions8.1′, . . . ,8.N′ at which the further coherent laser beams3.1′, . . . ,3.N are emitted. The individual phases δφaof the further coherent laser beams3.1′, . . . ,3.N are controlled with the aid of a further control device 15′ or are controlled on the basis of a detector signal of a further detector14′, the latter detecting a component12a′ of the further laser beam12′ combined with the aid of the further apparatus5′, said component being output coupled at a further output coupling device13′.

The control device15′ of the further apparatus5′ shown inFIG. 1cis designed or programmed to set the individual (fundamental) phases δφaof the further coherent laser beams3.1′, . . . ,3.N′ on the basis of an arrangement of the further grid position8.1′, . . . ,8.N′ assigned to the respective further laser beam3.1′, . . . ,3.N so that the coherent further laser beams3.1′, . . . ,3.N are combined to form a laser beam12′ that is diffracted into the zeroth order of diffraction. The combined laser beam12′ forms the input laser beam9for the apparatus5for combining the coherent laser beams3.1, . . . ,3.N, which is designed as depicted inFIG. 1b. By amplifying the seed laser beam5in the further apparatus5′, it is possible like inFIG. 1bto dispense with the amplification of the input laser beam9in the apparatus5.

The laser systems1shown inFIG. 1a-care suitable for high mean laser powers of the order of kW and high pulse energies of the order of mJ since the beam combining device10and the microlens arrangement11each have only one microlens array17,17′.

FIG. 2ashows a beam combining device10analogous to the apparatus5offigures 1a-cfor combining an (exemplary) number of N=5 coherent laser beams3.1, . . . ,3.5. The beam combining device10comprises a microlens arrangement11having exactly one microlens array17, and comprises an input coupling optical unit18. Five phase setting devices not depicted inFIG. 2aserve to adjust the phases δφ1+Δφ1, . . . , δφ5+Δφ5of the five laser beams3.1, . . . ,3.5, in such a way that, in combination with the input coupling optical unit18, a phase front is formed at the microlens arrangement11, which facilitates a coherent combination of the laser beams3.1, . . . ,3.5to form the combined laser beam12, where possible while completely maintaining the beam quality. In this case, the grid positions8.1, . . . ,8.5are arranged in a line in the X-direction and the laser beams3.1, . . . ,3.5enter the input coupling optical unit18with a parallel alignment along a uniform propagation direction (Z-direction).

In this case, the grid positions8.1, . . . ,8.5and the coherent laser beams3.1, . . . ,3.5are arranged equidistantly, that is to say with same distances δx, along the X-direction. The input coupling optical unit18is designed to input couple adjacent coherent laser beams3.1, . . . ,3.5into the microlens arrangement11or the microlens array17with a specified angle difference δθx, for which the following applies: δθx=λL/px, where λLdenotes the (uniform) wavelength of the laser beams3.1, . . . ,3.5and pxdenotes a pitch of the microlenses20of the microlens array17in the X-direction.

To produce the angle difference δθx, the input coupling optical unit18comprises a focusing device in the form of a focusing lens19, more precisely a cylindrical lens, which focuses the laser beams3.1, . . . ,3.5on the microlens arrangement11, more precisely on the microlens array17of the microlens arrangement11. To satisfy the condition in relation to the angle difference δθx, the grid positions8.1, . . . ,8.5in the example shown inFIG. 2aare arranged in a one-dimensional grid arrangement16with a distance δx given by δx=λLfFLin/px, where fFLindenotes the focal length of the focusing lens19, which inFIG. 2is arranged at a distance of its focal length fFLinfrom the microlens array17.

As an alternative to the arrangement on a common line, the grid positions8.1, . . . ,8.5may also be arranged in a one-dimensional grid arrangement16on a circular arc extending in the X-direction. In this case, coherent laser beams3.1, . . . ,3.5at the respective grid positions8.1, . . . ,8.5are aligned with respect to one another at a respective difference angle δθx=λL/px.

Under the assumption that the intensities of the laser beams3.1, . . . ,3.5emanating from the grid positions8.1, . . . ,8.5are the same size, the coherently superposed laser beam12shown inFIG. 2acan be produced by means of the microlens arrangement11if the microlens array17and the combined laser beams3.1, . . . ,3.5satisfy the following equation (1):

where N denotes the number of coherent laser beams (in this case: N=5) and fives, denotes the focal length of the microlens array17. Equation (1) should be observed as exactly as possible since deviations lead to a deterioration of the beam quality of the combined laser beam12.

The laser beams3.1, . . . ,3.5that emanate from the grid positions8.1, . . . ,8.5are single mode beams in the example shown, that is to say these each have a Gaussian profile. Alternatively, the laser beams3.1, . . . ,3.5can have a different beam profile with an optionally reduced degree of spatial coherence, for example a donut-shaped beam profile or a top hat beam profile. The beam diameter, more precisely the full width at half maximum of the beam profile of the coherent laser beams3.1, . . . ,3.5, which is Gaussian in the example shown, is denoted by 2 ωFLMLxinFIG. 2a. The fill factor FFxof the coherent laser beams3.1, . . . ,3.N in the X-direction is defined as the ratio of the beam diameter 2 ωFLMLxto distance δx between adjacent coherent laser beams3.1, . . . ,3.N: FFx=2 ωFLMLx/δx. The beam diameter 2 ωFLAx, more precisely the 1/e2width, of the combined laser beam12on the microlens array17depends on the fill factor FFxaccording to the formula below: 2 ωFLAx=4 px/(πFFx).

The beam combining device10represented inFIG. 2bdiffers from the beam combining device10represented inFIG. 2ain that there is no input coupling optical unit18, in particular no Fourier lens19, present. The grid arrangement16with the grid positions8.1, . . . ,8.5arranged at equidistant distances δx is arranged in the focal plane of the microlens array17upstream of the microlens array17in the case of the beam combining device10represented inFIG. 2b, that is to say at the distance of the focal length fMLupstream of the microlens array17. What is exploited here is that on account of the Talbot effect the brightness distribution of the microlens array17is repeated in the focal plane in which the grid arrangement16is arranged. Accordingly, the beam combining device10shown inFIG. 2brequires that the grid positions8.1, . . . ,8.N are arranged at a distance δx from one another which corresponds to the pitch pxof the microlenses20of the microlens array17in the X-direction, that is to say the following applies: δx =px.

The focal length fMLof the microlens array17is typically less than approximately 70-80 mm and is less than the Rayleigh length of the laser beams3.1, . . . ,3.5such that the condition in relation to the angle difference δθxbetween adjacent coherent laser beams3.1, . . . ,3.5need not be observed. Accordingly, the coherent laser beams3.1, . . . ,3.5can be radiated with a mutually parallel alignment onto the microlens array17in the case of the beam combining device10represented inFIG. 2b.

The divergence of the laser beams3.1, . . . ,3.5at the grid positions8.1, . . . ,8.5which for example may correspond to the end faces of gain fibers as emission surfaces and from which a respective laser beam3.1, . . . ,3.5emanates also has a negligible influence on the combination efficiency on account of the small distance fMLfrom the microlens array17. Optionally, the beam profile of the laser beams3.1, . . . ,3.N can be adapted with the aid of a suitable beam shaping device, for example by virtue of said laser beams being collimated or focused, that is to say the grid positions8.1, . . . ,8.5need not necessarily correspond to the emission surfaces on the end faces of the fibers but may for example be located in the focal plane of a plurality of (e.g., spherical) focusing lenses.

The provision of such a beam shaping device may be expedient for adapting the beam diameter 2 ωFLMLxof a respective coherent laser beam3.1, . . . ,3.5, for which the following should apply: 2 ωFLMLx=λLfML/px, provided this condition is not satisfied by the emission surfaces at the fiber ends.

The intensity of the coherent laser beams3.1, . . . ,3.5can be chosen to be identical, as is typically the case in the beam shaping device10described inFIG. 2a. However, the maximum intensities of the coherent laser beams3.1, . . . ,3.5in the case of the beam shaping device10shown inFIG. 2bhaving a respective maximum intensity at the grid positions8.1, . . . ,8.5, the envelope of said maximum intensity corresponding to an intensity distribution I of the combined laser beam12at or immediately downstream of the microlens array17, as depicted inFIG. 3, is also possible or advantageous. Like in the case of the intensity distributions of the coherent laser beams3.1, . . . ,3.5in the example shown, the intensity distribution I of the combined laser beam12at the microlens array17is a Gaussian distribution.

As was described further above, the following applies to the beam diameter 2 ωFLAxof the combined laser beam12at the microlens array17: 2 ωFLAx=4 px/(πFFx). The beam diameter 2 ωFLAxof the combined laser beam12, and hence the combination efficiency, therefore reduces with increasing fill factor FFx. In the case of the beam shaping device10shown inFIG. 2b, the following should apply to the fill factor FFxin the first direction X: FFx<0.4, preferably FFx<0.3. In principle, what applies is that the fill factor FFxshould be smaller, the greater the number N of combined coherent laser beams3.1, . . . ,3.5. In the case where the coherent laser beams are additionally combined in a second direction Y (see below), the following should apply to the fill factor FFyin the second direction Y: FFy<0.4, preferably FFy<0.3.

In order to form a combined laser beam12with a corresponding Gaussian profile with a larger beam diameter 2 ωFLAx, which is diffracted into the zeroth order of diffraction B0,xor into a higher order of diffraction Bk,x, from the laser beams3.1, . . . ,3.5with the beam diameter 2 ωFLMLxin the microlens arrangement11, it is necessary in the case of the two beam shaping devices10ofFIGS. 2a,bfor the laser beams3.1, . . . ,3.5to be radiated on the microlens array17with a phase front or with individual fundamental phases δφa(that depend on the angle of incidence θ) as specified below:

where the following applies:

with a=1, . . . , N, where N is the number of the grid positions arranged in the first direction (in this case: N=5) and where Bk,xis an integer or half integer, for which the following applies:

In the case where the number N of coherent laser beams is odd, the order of diffraction Bk,xassumes integer values. In the case where the number N of coherent laser beams is even, the order of diffraction Bk,xassumes half integer values.

The fundamental phase δφadiffers for each individual coherent laser beam3.1, . . . ,3.5and is therefore set with the aid of the phase setting devices6.1, . . . ,6.N and not with the aid of one or more optical elements of the input coupling optical unit18, even if this would also be possible as a matter of principle.

With the aid of the condition, specified above, for the fundamental phases δφa, the combined laser beam12can be diffracted into the zeroth order of diffraction B0,xin a targeted manner, in the case of which the laser beam12propagates in the Z-direction as represented inFIGS. 2a,b. By defining Bk,xas an odd or even non-zero number, the combined laser beam12can be diffracted into the corresponding order of diffraction Bk,x(in the X-direction) that differs from the zeroth order of diffraction, in the case of which corresponding order of diffraction the laser beam12propagates at an angle to the Z-direction.

In the example shown inFIGS. 2a,b, a further Fourier lens21arranged in the beam path downstream of the microlens array17at a distance of its object-side focal length fFLoutassists in imaging the combined laser beam12diffracted into the zeroth or a higher order of diffraction Bk,xinto a further grid arrangement16′ in an image-side focal plane of the further Fourier lens21. The following applies to the distances δx′ between the further grid positions8.1′, . . . ,8.5′ of the further grid arrangement 16′: δx′=δx fFLout/fFLin. Accordingly, the following applies, in the focal plane of the further Fourier lens21, to the beam diameter 2 ωFLx′ of the combined laser beam12that has been diffracted into the respective order of diffraction Bk,x: 2 ωFLMLx′=2 ωFLMLxfFLout/fFLin. The combined laser beam12that has been diffracted into the Bk,x-th order of diffraction is imaged on the Ba-th further grid position8.1′, . . . ,8.N′, with the following applying:

For the case of four coherent laser beams3.1, . . . ,3.4(N=4), represented in exemplary fashion inFIG. 4a, the following applies to the four orders of diffraction Bk,xinto which the combined laser beam12can be diffracted: B−1.5,x=−1.5, B−0.5,x=−0.5, B+0.5,x=+0.5 and B+1.5,x=+1.5. In the case where the combined laser beam12is diffracted into the +0.5-th order of diffraction B+0.5,x, the following applies to the four fundamental phases to be set δφ1, . . . , δφ4:

Below, an assumption is made that, in the case of the apparatuses5shown inFIGS. 1a-cor in the case of the further apparatus5′, the fundamental phases δφaof the laser beams3.1, . . . ,3.N or of the further laser beams3.1′, . . . ,3.N′ are set in accordance with the condition specified above (with k=0) in order to diffract the combined laser beam12or the further combined laser beam12′ into the zeroth order of diffraction B0,x.

To diffract the laser beam12into an order of diffraction Bk,xin the X-direction that differs from the zeroth order of diffraction, it is advantageous proceeding from the fundamental phase δφaset in this way to set a respective additional phase Δφaof a coherent laser beam3.1, . . . ,3.N at an a-th grid position8.1, . . . ,8.N (a=1, . . . , N), which additional phase is given by:

In this case N denotes, like further above, the number of the grid positions8.1, . . . ,8.N that is arranged on a common line in the X-direction in a one-dimensional grid arrangement16, and Bk,xdenotes an integer or half integer, for which the following applies:

For the coherent superposition into the zeroth order of diffraction B0,x, the respective additional phase Δφais added to the fundamental phase δφaspecified above. In particular for the scanning process described further below, within the scope of which the order of diffraction Bk,xis changed, it was found to be advantageous proceeding from the fundamental phase δφafor the diffraction into the zeroth order of diffraction B0,xto use the additional phase Δφafor the diffraction into (at least) one higher order of diffraction Bk,xand not to set the fundamental phase δφaaccordingly, i.e., for the diffraction into a higher order of diffraction Bk,x.

For the case of five coherent laser beams3.1, . . . ,3.5, described in exemplary fashion inFIGS. 2a,b, the following applies to the orders of diffraction Bk,xthat differ from the zeroth order of diffraction B0,xand into which the laser beam12can be diffracted: B−2,x=−2, B−1,X=−1, B+1,x=+1 and B+2,x=+2. InFIG. 4b, a respective individual additional phase Δφ1, . . . , Δφ5is specified for the five laser beams3.1, . . . ,3.5, which brings about the diffraction of the combined laser beam12into the −1st order of diffraction B−1,x. The associated far field (angle distribution) produced by means of the beam combining device10is represented inFIG. 5a.

To set the (individual) additional phases Δφaof the laser beams3.1, . . . ,3.5, the phase setting devices8.1, . . . ,8.5are controlled with the aid of the control device15such that these produce the respective correct additional phase Δφafor the a-th coherent laser beam3.1, . . . ,3.N.

In the example shown inFIG. 4b, that is to say in the case of a number of N=5 laser beams3.1, . . . ,3.5and a laser beam12that is diffracted into the −1st order of diffraction B−1,xin the X-direction, the following applies to the five additional phases Δφ1, . . . , Δφ5to be set:

To realize a discrete scanning process, in which the combined laser beam12is switched back and forth between different orders of diffraction Bk,x, the control device15can vary the respective additional phase Δφaof the coherent laser beams3.1, . . . ,3.N by virtue of acting on the (quickly switchable) phase setting devices6.1, . . . ,6.N. By way of example, the laser beam12can be moved from the −1st order of diffraction B−1,xin the X-direction to the +2nd order of diffraction B+2,xin the X-direction by virtue of the additional phases Δφ1, . . . , Δφ5shown inFIG. 3bbeing set in place of the additional phases Δφ1, . . . , Δφ5shown inFIG. 3a.

If the far field shown inFIG. 5ais imaged by means of an imaging optical unit, for example the further Fourier lens21shown inFIGS. 2a,b, the angle distribution is converted into a spatial distribution. In this way, it is possible to produce an adjustable beam offset of the combined laser beam12, that is to say the laser beam12can be offset by a desired distance in the X-direction, which depends on the order of diffraction Bk,x, from the optical axis that runs in the Z-direction in the center of the beam combining device10. In this case, the combined laser beam12can be focused in particular at a (varying) focal position in a focal plane, as shown in exemplary fashion inFIGS. 2a,bfor the focal plane of the further Fourier lens21.

FIG. 5bshows the far field of the beam combining device10, in which the five coherent laser beams3.1, . . . ,3.5are combined to form a first laser beam12athat is diffracted into a first order of diffraction B−1,x,1and a second laser beam12bthat is diffracted into a second order of diffraction B0,x,2. To this end, the additional phases Δφ1, . . . , Δφ5of the coherent laser beams3.1, . . . ,3.5are likewise suitably set. To set the additional (absolute) phases Δφ1, . . . , Δφ5, it is possible to apply an iterative optimization algorithm which runs in the control device15or which was already implemented in advance. As a rule, the phases that are suitable for a certain processing process, for example a laser cutting process, a laser welding process, a laser marking process, additive manufacturing, etc., are stored in the form of data sets or tables in the control device15itself or in an electronic memory connected to the latter, or such phases are specified by an operator.

In the example shown inFIG. 5b, the additional phases Δφ1, . . . , Δφ5are chosen such that a first laser beam12ais diffracted into the −1st order of diffraction B−1,x,1, like inFIG. 5a, and, additionally, a second laser beam12bis diffracted into the zeroth order of diffraction B0.

In the examples shown, the intensity or the power of the first and second laser beam12a,12bcan be of equal magnitude, that is to say the power produced by the seed laser source is divided equally among the two laser beams12a,b. If the condition specified above for the additional phase Δφais observed, the input power p, which is input coupled into the beam combining device10, is divided equally (50:50) inFIG. 5bamong the laser beam12bdiffracted into the 0th order of diffraction and the laser beam12adiffracted into the −1st order of diffraction, that is to say the following applies: p−1=p0=p/2.

However, it is also possible to set the proportion of the input power p that is diffracted into the respective order of diffraction Bk,x,1, Bk,x,2to deviate from an equal distribution in a targeted manner. In the example shown inFIG. 5b, a proportion of 80% of the input power p can be diffracted for example into the −1st order of diffraction and a proportion of 20% of the input power p can be diffracted into the 0th order of diffraction, that is to say, the following applies: p−1=0.8 p, p0=0.2 p. In general, the split of the input power p among the 0th and the ±1st order of diffraction can for example be implemented as follows: p0=C p; p±1=(1−C) p, where 0<C<1.

For the additional phase Δφaof a respective coherent laser beam3.1, . . . ,3.N at an a-th grid position8.1, . . . ,8.N in the X-direction, which produces the aforementioned power distribution with the factor C, the following applies:

The distribution factor C can be chosen to be constant or can be varied in a time dependent manner by the control device15. In the latter case, the apparatus5can be operated in the style of an (acousto-optic or electro-optic) modulator or deflector.

In the case of a number M of more than two combined laser beams12a,12b, . . . , the split can for example be realized in the form of a (e.g. linear) power ramp, in the case of which a first combined laser beam is diffracted with a maximum power pk,maxinto the k-th order of diffraction and the remaining M−1 combined laser beams are diffracted into the remaining M−1 orders of diffraction with a power that has been reduced in relation to the maximum power pk,max. By way of example, for the power distribution in the form of a power wedge, the following may apply: a/M pk,max, where a=1, . . . , M. For the example of a total of 5 diffracted combined laser beams, proportions of the maximum power pk,maxof 100%, 80%, 60%, 40% and 20% arise.

What applies in principle is that, by way of a suitable choice of the additional phases Δφ1, . . . , Δφ5, the coherent laser beams3.1, . . . ,3.5can be combined into two or more than two laser beams12a,12b, . . . which—with an equally distributed power or different power—are diffracted into corresponding orders of diffraction Bk,x,1, Bk,x,2, . . .

In the laser system1described in the context ofFIGS. 1a-ctoFIGS. 5a,b, the laser beams3.1, . . . ,3.N were combined one-dimensionally.FIGS. 6a-ceach show an optical arrangement in which a number N (in this case: N=3)×M (in this case: M=3) of grid positions8.1.1, . . . ,8.N.M are arranged in a two-dimensional grid arrangement16. In the example shown inFIG. 6a, the grid positions8.1.1, . . .8.N.M are arranged in a rectangular grid arrangement16in a common plane (XY-plane) and the beam propagation directions of all laser beams3.1.1, . . . ,3.N.M run parallel (in the Z-direction). In a manner analogous toFIG. 2a, the input coupling optical unit18in the optical arrangement ofFIG. 6ahas only one focusing device in the form of a focusing lens19, which is represented by a square inFIG. 6a. The microlenses20of the microlens array17of the microlens arrangement11are arranged in a corresponding, rectangular grid arrangement and are aligned parallel to the XY-plane. The microlenses20are square lenses, which act like cylindrical lenses in both directions X, Y.

In the optical arrangement shown inFIG. 6b, the grid positions8.1.1, . . . ,8.N.M are likewise arranged in a grid arrangement16or in an array, the latter however extending along a curved surface, more precisely along a spherical shell, with the beam propagation directions of the laser beams3.1.1, . . . ,3.N.M being aligned perpendicular to the spherical shell and the microlens arrangement11being arranged in the vicinity of the center of the spherical shell. An arrangement of the grid positions8.1.1, . . . ,8.N.M in a grid arrangement16extending along a different curved surface, for example along an ellipsoid, is also possible. It is possible to dispense with an input coupling optical unit18in this case.

FIG. 6cshows an optical arrangement analogous toFIG. 6a, in which the two-dimensional microlens array17of the microlens arrangement11is replaced by two one-dimensional partial microlens arrays22a,22b. The partial microlens arrays22a,22beach have a plurality of microlenses20a,bin the form of cylindrical lenses, with the microlenses20aof the first partial microlens array22aand the microlenses20bof the second partial microlens array22bbeing aligned perpendicular to one another, specifically in the X-direction and Y-direction, respectively. Unlike what is represented inFIG. 6c, the two one-dimensional partial microlens arrays22a,bcan be arranged immediately adjoining one another in a common plane, and correspond to the case represented inFIG. 6a.

It is understood that the two-dimensional combination of the coherent laser beams8.1.1, . . . ,8.N.M is also possible analogously in the beam combining device10represented inFIG. 2b, with the Fourier lens19being able to be dispensed with in this case and the rectangular or square grid arrangement16being arranged at the distance of the focal length fMLupstream of the microlens array17and the coherent laser beams3.1.1, . . . ,3.N.M being radiated with parallel alignment to one another on the microlens array17.

Depending on the spacings of the grid positions8.1.1, . . .8.N.M or the periodicity of the grid arrangement16in the X-direction and/or Y-direction, it is also possible for the pitches px, pYof the microlenses20a,bto differ from one another in the two mutually perpendicular directions X, Y. Accordingly, the microlenses20ofFIG. 6ahave an optionally different curvature in the X-direction and in the Y-direction, that is to say these are not cylindrical lenses. The combination of the coherent laser beams3.1.1, . . .3.N.M in the two linearly independent directions X, Y, which are perpendicular in the example shown, is independent as a matter of principle, that is to say the conditions or equations specified further above apply to both directions X, Y independently of one another.

Only when setting the phase of the laser beams3.1.1, . . .3.N.M do the contributions in the two mutually perpendicular directions add, that is to say the following applies in relation to the respective additional phase for a number of N×M laser beams3.1.1, . . .3.N.M arranged in a rectangular grid arrangement16(in X-direction and Y-direction):

where M denotes a number of the grid positions in the second direction Y and Bj,ydenotes an integer or half integer, for which the following applies:

Accordingly, the contributions of the fundamental phases δφa,balso add in the two mutually perpendicular directions X, Y, that is to say the following applies:

where the following applies:

with b=1, . . . , M, where M is a number of the grid positions arranged in the second direction and where Bj,yis an integer or half integer, for which the following applies:

FIG. 7shows, in a manner analogous toFIG. 4b, a two-dimensional arrangement of N=5×M=5 coherent laser beams3.1.1, . . . ,3.5.5with a respectively assigned additional phase Δφa,b(a=1, . . . , N; b=1, . . . , M) for producing a single laser beam12that is diffracted into an order of diffraction B−2,xin the X-direction and into an order of diffraction B+1,Yin the Y-direction (cf.FIG. 8) or for producing a first laser beam12adiffracted into a first order of diffraction B−2,x,1(in the X-direction), B+1,y,1(in the Y-direction) and a second laser beam12bdiffracted into a second order of diffraction B+1,x,2(in the X-direction), B−1,y,2(in the Y-direction) (FIG. 9).

To produce a single laser beam12that is diffracted into a (two-dimensional) order of diffraction Bk,x, Bk,y, an additional phase Δφa,bgiven by equation (3) above is set for an (a,b)-th grid position8.a.bin the two-dimensional grid arrangement16(cf.FIG. 6a), that is to say for an a-th grid position in the X-direction that simultaneously forms a b-th grid position in the Y-direction, or for an (a,b)-th coherent laser beam3.a.b(cf.FIG. 7).

Accordingly, the respective additional phases Δφa,bare also set with the aid of an iterative, stochastic optimization algorithm in the far field represented inFIG. 9in order to produce the first laser beam12athat is diffracted into a first order of diffraction B−2,x,1(X-direction), B+1,y,1(Y-direction) and the second laser beam12bthat is diffracted into the second order of diffraction B+1,x,2(X-direction), B−1,y,2(Y-direction).

As was described further above, the number and the arrangement of the laser beams12a,12b, . . . that are diffracted into the orders of diffraction Bk,x,1, Bk,y,1; Bk,x,2, Bk,y,2, . . . are as desired as a matter of principle and are only restricted by the number N and/or M of the coherent laser beams3.a.bused for the combination. By way of a suitable choice or variation of the additional phases Δφain a one-dimensional grid arrangement16or of the additional phases Δφa,bin a two-dimensional grid arrangement16, it is possible, in a targeted manner, to activate or deactivate individual combined laser beams, groups of combined laser beams or an entire array of combined laser beams, which corresponds to a set of orders of diffraction.

Using the above-described laser system1, it is therefore possible to obtain a (discrete) one-dimensional or two-dimensional scanning process or a targeted beam deflection and/or a targeted split of the combined laser beam12among two or more laser beams12a,12b. The combined laser beam(s)12,12a,bcan be imaged or focused on (a) (varying) focal position(s) in a focal plane with the aid of an additional optical unit, for example the Fourier lens19represented inFIGS. 2a,b.