Optical system for producing high-power light - Patent Application 20070122947

Abstract

The present invention relates to an optical system for generating or guiding light, comprising a multi-channel optical waveguide (1) with a plurality of individual optical waveguides running parallel to one another, and a superposition optical unit (2) designed to superpose the light emissions from the individual optical waveguides onto a target plane (3) at the exit end of the multi-channel optical waveguide (1). The invention proposes that the superposition of the light emissions of the individual optical waveguides is incoherent at the target plane (3). The invention has been found to provide a substantially better, more homogeneous and more stable beam quality than would be possible with surface-equivalent, transversely multimode, individual large-core fibers, for example as amplification fibers in laser systems.

Description

[Technical Field]

[0001] The present invention relates to an optical system for producing high-power light, having a multi-channel optical waveguide including a plurality of individual optical waveguides running parallel to one another, and a superposition optical unit designed to superpose the emitted light of the individual optical waveguides onto a target surface at the exit end of the multi-channel optical waveguide. [Background technology]

[0002] High-power laser systems have numerous applications in industry and science. The spatial coherence of laser emission allows the radiation to be focused into the smallest spatial area. The ideal case is a diffraction-limited beam, which produces the smallest focal spot for a given imaging optics. Poor beam quality typically results in either a larger focal spot and therefore lower intensity, or requires the use of focusing optics with a larger aperture number (i.e., a larger divergence angle of the radiation relative to the focal point), thus shortening the Rayleigh length (i.e., the distance over which high intensity can be maintained). The better the beam quality, the higher the power density that can be achieved, even over long distances.

[0003] The achievable power density determines the applications that can be addressed. Continuously emitting high-power lasers are used, for example, for cutting and welding various materials (e.g., metals), while pulsed lasers are particularly used for material ablation or modification. The excessive peak power density of laser radiation can also drive nonlinear effects, such as frequency conversion of the primary laser radiation to other spectral regions depending on the application (i.e., generating secondary radiation). This frequency conversion can occur not only in coherent conditions (e.g., crystal-based frequency conversion in the form of harmonic generation, spectral broadening due to Kerr nonlinearity, or generation of short-wave coherent radiation from gas harmonics in noble gases), but also in incoherent conditions (e.g., laser-induced plasma in gases or metals).

[0004] A typical example of economically viable incoherent frequency conversion is the generation of incoherent EUV radiation at a wavelength of 13.5 nm (photon energy 92 eV) by laser-induced tin plasma for semiconductor industry applications (see Non-Patent Document 1). In a powerful version, radiation from a pulsed CO2 laser is focused onto a tin droplet (diameter approximately 30 μm). The resulting plasma emits incoherently in all spatial directions at a wavelength of 13.5 nm, and the conversion efficiency of this process can be 3-6% (also depending on the target preparation by pre-pulsing).

[0005] Another industrial process chosen as an example is laser shock peening to extend the service life of components (see non-patent document 2). Compressive stresses are introduced into materials to counteract fatigue caused by tensile stresses. In laser peening, pressure waves are generated using high-energy laser pulses. Pulse energies of a few hundred mJ to a few joules and focal spot diameters of a few millimeters are used here. The uniformity of the beam profile is essential for uniform pressure input. The process speed is determined by the pulse repetition frequency, which is why higher pulse repetition frequencies are being sought.

[0006] Another selected application of high-energy nanosecond pulses is the laser lift-off process (see non-patent document 3). In this process, functional films (e.g., displays) are produced over large areas on a solid carrier (substrate). The laser lift-off process allows the separation of the film from the substrate while maintaining the necessary reproducibility and protection of the film. High-energy nanosecond pulses in the UV spectral range are used, which penetrate the substrate and are absorbed by an absorbing layer. The resulting energy input causes the film to peel off. Spatial uniformity of the energy input is essential in this process.

[0007] Another application of high-energy laser radiation is lithotripsy, i.e., the fragmentation of kidney or bladder stones (see Non-Patent Document 4). For this purpose, high-energy long laser pulses in the Joule range are used, preferably at wavelengths around 2 μm, due to tissue absorption.

[0008] The laser technology currently used for these applications has the following drawbacks: The high-power CO2 lasers mentioned above have a total efficiency (wall plug efficiency) of only a few percent (see Non-Patent Document 5).

[0009] - Diode-pumped solid-state lasers are significantly more efficient, but as output power increases, thermo-optical problems arise that manifest as a degradation of beam quality, and therefore a loss of focusability and beam uniformity.

[0010] - To achieve high pulse energies, the cross-sectional area of ​​the active medium needs to be large, which in solid-state lasers (including fiber lasers) leads to oscillation of higher-order transverse modes, resulting in degradation of beam quality and beam uniformity.

[0011] In particular, in fiber-based lasers (as well as passive transmission fibers), the different transverse modes of a multimode fiber (large cross section) are coherent with each other, i.e., even the smallest change in the relative phase position of the different transverse modes leads to a change in the spatial emission profile, which ultimately results in instability of the application process (see non-patent document 6). [Prior art documents] [Non-patent literature]

[0012] [Non-Patent Document 1] OO Versolato, “Physics of laser-driven tin plasma sources of EUV radiation for nanolithography,” Plasma Sources Sci. Technol. 28, 083001, 2019 [Non-patent document 2] C. Zhang, Y. Dong, and C. Ye, “Recent Developments and Novel Applications of Laser Shock Peening: A Review,” Adv. Eng. Mater. 23, 2001216, 2021 [Non-patent document 3] R. Delmdahl, R. Patzel, and J. Brune, “Large-Area Laser-Lift-Off Processing in Microelectronics,” Phys. Procedia 41, 241-248, 2013 [Non-patent document 4] NM Fried, “Recent advances in infrared laser lithotripsy [Invited],” Biomed. Opt. Express 9, 4552, 2018 [Non-Patent Document 5] K. Kellens, G. Costa Rodrigues, W. Dewulf, JR Duflou, GC Rodrigues, W. Dewulf, and JR Duflou, “Energy and resource efficiency of laser cutting processes,” Phys. Procedia 56, 854-864, 2014 [Non-patent document 6] BY Zel'dovich, DZ Anderson, and MA Bolshtyansky, “Stabilization of the speckle pattern of a multimode fiber undergoing bending,” Opt. Lett. Vol. 21, Issue 11, pp. 785-787 21, 785-787, 1996 Summary of the Invention [Problem to be solved by the invention]

[0013] It is an object of the present invention to provide an optical system, in particular a high-power laser system, which avoids at least some of the above-mentioned drawbacks. [Means for solving the problem]

[0014] This problem is solved by the invention by starting from an optical system of the type described above, in which the superposition of the emitted light from the individual light guides in the target plane is incoherent.

[0015] It is proposed to provide a non-coherent superposition of the emitted light of the individual optical waveguides forming the individual channels of the multi-channel optical waveguide. Preferably, the beam quality should be good, i.e., the emitted light of the individual optical waveguides should ideally be (near) diffraction limited. Preferably, the individual emissions at the exit end of the multi-channel optical waveguide are positioned as close as possible to each other.

[0016] A multi-channel optical waveguide in the sense of the present invention is any arrangement of a plurality of optical waveguide structures that run parallel to each other as individual optical waveguides, whereby the individual optical waveguides of the multi-channel optical waveguide have their exit ends in a common plane and together form the exit end of the multi-channel optical waveguide. Examples of suitable multi-channel optical waveguides are known from the prior art (see A. Klenke, C. Jauregui, A. Steinkopff, C. Aleshire, and J. Limpert, "High-power multicore fiber laser systems," Prog. Quantum Electron. 84, 100412, 2022).

[0017] The number of individual optical waveguides of the multi-channel optical waveguide can be 2 or more, preferably at least 3, more preferably at least 8, even more preferably at least 20, and particularly preferably at least 40. In principle, any number is envisaged.

[0018] In one possible embodiment, each individual optical waveguide is formed by an optical waveguide core or another optical waveguide structure, which are surrounded by a common cladding of the multi-channel optical waveguide. At least one of the optical waveguide cores, preferably only a portion of the optical waveguide cores, and particularly preferably all of the optical waveguide cores, can be doped with ions of a rare earth element, preferably erbium, ytterbium, or thulium, to enable optical amplification. The common cladding can be advantageously designed to guide pump light for optically pumping at least one doped core.

[0019] The present invention has discovered that by incoherently superimposing the individual emissions, it is possible to achieve a substantially better and significantly more stable beam quality than, for example, when using an area-equivalent single transversely multimode large-core fiber as, for example, the amplifier fiber of a laser system. The area-equivalent multimode fiber has a core cross-sectional area equal to the total area of ​​the individual cores of the multi-channel optical waveguide. The same doping concentration results in the same fiber length. As a result, both geometries (multi-channel optical waveguide and multimode fiber) are comparable in terms of stored energy and extractable laser power, and suppress fiber destruction due to nonlinear effects and excessive power densities. However, area-equivalent multimode fibers have been shown to have inferior beam quality even with a low aperture count. The following points should be noted: As already mentioned, the beam quality of the light emitted by the individual optical waveguides of the multi-channel optical waveguide of the present invention should be as good as possible, ideally close to diffraction limited. Therefore, the diffraction coefficient of the individual light emissions should be less than 3, preferably less than 2, more preferably less than 1.5, and particularly preferably less than 1.25. The directions of the individual light emissions are preferably parallel to one another.

[0020] The cross section of the core of the individual optical waveguides should preferably be designed to be as large as possible, taking into account the beam quality of a single core at a given distance. In this case, geometries such as tapered large-core fibers or other known large-core fiber designs, which are known to support the best beam quality even with large core areas, may be beneficial. The core of the individual optical waveguides should preferably have a diameter greater than 5 times, preferably greater than 10 times, even more preferably greater than 25 times, and particularly preferably greater than 50 times the wavelength of the propagating light.

[0021] - the distance between the individual cores should preferably be as small as possible, taking into account the avoidance of optical coupling. In this case, optical barriers within the structure of the multi-channel optical waveguide may be useful to avoid overcoupling. In the sense of the present invention, "optical isolation of the individual optical waveguides" preferably means that less than 10%, preferably less than 5%, preferably less than 1% of the power propagating in an individual optical waveguide is lost over the entire length of the multi-channel optical waveguide due to power transfer to other individual optical waveguides.

[0022] The incoherent superposition of the individual emissions is an optical transformation at any cross section within the beam path located in a region behind the exit end of the multi-channel optical waveguide.

[0023] It should be noted that the advantages of multi-channel optical waveguides over multimode, large-core fibers are realized when the individual optical waveguides, emitting spatially coherently, are closely spaced so that the total area of ​​ultimately incoherent superposition of the coherent individual emissions is not unnecessarily large. These design guidelines can be specifically implemented in multi-core fibers, since the known concept of multi-core fibers ideally supports dense packing of the individual cores. Ideally, for multi-channel optical waveguides, the ratio of the distance between the cores of the individual optical waveguides to the core diameter should be less than 20, preferably less than 10, even more preferably less than 5, and especially preferably less than 3.

[0024] The advantages of the present approach are summarized below: - The optical system has a simple and compact design.

[0025] In a fiber-based realization, the optical system of the present invention as a laser system (see below) is highly efficient and allows for direct pumping of a multi-channel optical waveguide as a laser medium with a semiconductor diode.

[0026] - The geometry of multi-channel optical waveguides as elongated waveguides (e.g., as active multicore fibers in laser systems (see below)) allows the laser-induced heat input to be distributed over long distances, and the large cladding area of ​​the fiber can be used to dissipate the introduced heat. As a result, this approach offers the possibility of radiating high average powers.

[0027] - The inversion stored in the doped core, and therefore the extractable optical power, is determined by the dopant properties and doping concentration, which define and limit the optical power that can be extracted from each individual optical waveguide. Multi-channel optical waveguides increase the extractable power with the number of individual optical waveguides.

[0028] - Various rare earth elements are possible dopants, with ytterbium ions covering wavelength ranges around 1 μm, erbium ions around 1.5 μm, and thulium ions around 2 μm. It is also envisaged that the photoconductive cores may differ from one another in terms of doping, which allows the emission wavelength to be varied (optionally dynamically) by choosing the pump wavelength.

[0029] - Nonlinear effects and material degradation requirements are distributed over multiple individual optical waveguides, allowing for improved overall performance.

[0030] According to the present invention, the superposition of the individual emissions is a non-coherent superposition, i.e. the relative phase positions of the individual emissions have no effect. Therefore, the elements for detecting and stabilizing the phase positions of the individual channels, which are required in coherent superposition, are omitted. Therefore, the present invention significantly simplifies the design.

[0031] Due to the incoherent superposition, the exact location of the individual optical waveguides in the cross section of the multi-channel waveguide is not critical, and therefore there is a great deal of freedom in the design of the multi-channel waveguide. Large manufacturing tolerances are allowed regarding the positional accuracy and size of the individual optical waveguides.

[0032] The beam quality of the incoherently superimposed total emission of a multicore optical waveguide can be significantly improved compared to an area-equivalent multimode fiber, as already mentioned.

[0033] These advantages make the optical system according to the invention, when implemented as a laser system, i.e. with a multi-channel optical waveguide as a laser medium, particularly suitable for the above-mentioned applications: the incoherently superimposed laser radiation generated thereby can also be used for generating UV light (especially EUV light) from laser-induced metal or gas plasmas (e.g. tin plasma), for material processing by laser shock peening, for separating films from substrates by laser lift-off, or for generating laser pulses for the fragmentation of kidney or bladder stones (lithotripsy).

[0034] The above-mentioned advantages also unconditionally apply to optical systems with purely passive multi-channel optical waveguides (without core doping), for example as transmission fibers. In addition to the advantages with regard to beam quality, the following point is also important: in conventionally used passive multimode transmission fibers, the inherent coherence between different transverse modes with respect to each other makes the resulting intensity profile very sensitive to relative changes in the phase position of the individual transverse modes. In passive multimode fibers, these phase changes are due to external influences (e.g., changes in the fiber position or stress due to contact), while in active fibers, laser-induced heat input dominates. These drawbacks are overcome by the present invention.

[0035] In one possible embodiment, the cores of the individual optical waveguides may have different diameters. At least one of the cores may also be designed as a hollow core.

[0036] In another possible embodiment, the individual optical waveguides are individual doped optically conductive fibers, preferably double-core fibers (doped with rare earth element ions), or individual passive optically conductive fibers combined, so to speak, as a single fiber bundle within the multi-channel optical waveguide.

[0037] The overlay optics of the optical system are variable and can be designed to generate different beam profiles at the target plane, optionally dynamically.

[0038] In one possible embodiment, the individual optical waveguides have a linear arrangement or an array or matrix arrangement in the cross section of the multi-channel optical waveguide. The "packing density" of the individual optical waveguides can also be increased by a hexagonal arrangement. A random distribution of the individual optical waveguides over the entire cross section of the multi-channel optical waveguide is also envisioned. Furthermore, it is envisioned that the mode field diameter of the individual cores is (subsequently) expanded at the entrance and / or exit ends of the multi-channel optical waveguide (e.g., by targeted heat input, i.e., thermal expansion or tapering of the multi-core fiber) to increase the fill factor, i.e., to increase the proportion of the cross-sectional area through which light passes.

[0039] Another possible approach to increasing the fill factor of the multi-core light emission is to use a lens array, in particular a microlens array, outside the multi-channel light guide. This can be placed, for example, just before the exit end of the multi-channel light guide (at the corresponding working distance), but can also be placed further back in the beam path. Each individual lens of the lens array corresponds in each case to one or more individual light guides of the multi-channel light guide. By utilizing a lens array, the fill factor can be increased by at least 1.2 times, preferably at least 1.5 times, and even more preferably at least 2 times, with even higher magnifications being possible.

[0040] In one possible embodiment, each individual optical waveguide of the multi-channel optical waveguide forms a laser medium within an optical resonator. If no coupling occurs between the individual optical waveguides, all individual optical waveguides can lase independently of one another. Therefore, one or more free-beam resonators used by all individual optical waveguides can be arranged around the multi-channel optical waveguide (by appropriate reflector arrangements), but each individual optical waveguide lases independently, i.e., independently of other individual optical guides. This allows for superposition incoherence at the target plane. It is also envisioned that independent optical resonators can be realized by applying reflective coatings to the end faces of the individual optical waveguides or by Bragg gratings (FBGs) inscribed at the ends of the individual optical waveguides. Fresnel reflections at the free ends of the individual optical waveguides alone may be sufficient to form an optical resonator without additional measures.

[0041] Further elements may also be arranged intra- or extra-resonantly to generate pulsed laser radiation (by quality switching, cavity dumping or mode coupling), such as for example (temporal) optical modulators.

[0042] Oscillator-amplifier arrangements (MOPAs) are also feasible. For example, a laser oscillator designed according to the present invention and equipped with a multi-channel optical waveguide generates low-power laser radiation, whose emission pattern consists of mutually spatially incoherent beams. The laser oscillator can operate (with appropriate modulation) in various temporal operating regimes, from continuous wave (cw) or pulsed emission to ultrashort laser pulses, depending on the application requirements. Alternatively, a conventional single emitter can be used as the light source, and its emission can be appropriately divided among the individual optical waveguides of the multi-channel optical waveguide. The laser light thus generated is downstream coupled into a laser-active multi-channel optical waveguide equipped with an appropriate number and arrangement of individual optical waveguides, where it is amplified to higher power (and possibly pulse energy). This step can also be repeated, i.e., additional multi-channel optical waveguides can be passed in series as amplifiers. The individual emission patterns can also be coupled into a multi-channel optical waveguide that functions as a passive transmission fiber. Thus, the individual emissions can then be transmitted to the application. It is also possible to drive frequency conversion processes (eg, four-wave mixing or Raman scattering) in individual optical waveguides of a multi-channel optical waveguide located downstream of the amplifier.

[0043] When using a single emitter as the light source in the optical system of the present invention realized as a MOPA system, it is necessary to ensure that the path length of the individual emissions to the superposition in the target plane is longer than the coherence length of the light emitted from the single emitter, in order to achieve incoherent superposition at the output of the downstream multi-channel optical waveguide. Thus, any light source with a sufficiently wide spectral width is contemplated, such as, for example, the (optionally time-stretched) emission of a mode-coupled ultrashort pulse laser or a superluminescent diode.

[0044] In the MOPA concept, the temporal characteristics of the light emission can be changed by an optical modulator (e.g., placed between the oscillator and the amplifier) ​​to, for example, generate pulses from a temporally continuous laser beam, or to tailor the pulse shape to the application requirements (e.g., to generate a pre-pulse or over-elevation at the beginning of the pulse), or to shape the pulse to influence saturation-related pulse shapes in the amplifier.

[0045] The described approach also offers the possibility of emitting light from individual optical waveguides independently of one another in time, for example by independently modulating laser light in individual channels of a multi-channel optical waveguide. Thus, for example, it is conceivable to generate one or more pre-pulses from a certain number of individual optical waveguides and emit a high-energy main pulse from a further individual optical waveguide. For this purpose, it is only necessary to couple the optical pulses into the various individual optical waveguides in a time-shifted manner. For example, by providing the individual optical waveguides with different dopings or by appropriately frequency-modulating the pump light, it is also possible to make the pre-pulse have a different emission wavelength from the main pulse. One possible embodiment provides that the cores of the individual optical waveguides have different diameters, so that one or more pre-pulses and the main pulse produce different spot sizes in their incoherent superposition. In a further possible embodiment, one or more of the individual optical waveguides are formed with a hollow core, e.g., a shape suitable for the transmission of ultrashort laser pulses of high peak pulse power, and are surrounded by further individual optical waveguides of an active or passive multi-channel optical waveguide.

[0046] In a further possible embodiment, the optical system may comprise two or more multi-channel optical waveguides, whereby each superposition optical unit is designed to superpose the entire emissions of the two or more multi-channel optical waveguides in a spatial region, e.g., the individual emissions may enter the spatial region from different spatial directions and be superposed therein incoherently.

[0047] The total emission of the optical system may be the superposition of emissions from two multi-channel optical waveguides with orthogonal polarizations onto a polarizer. Additionally, the total emission may be the superposition of emissions from two or more multi-channel optical waveguides with different wavelengths onto one or more spectrally selective elements (e.g., volume Bragg gratings, dichroic mirrors, prisms, gratings, grisms, or combinations of these elements).

[0048] In yet another possible embodiment, a nonlinear optical element can be provided for frequency conversion of the superimposed emitted light of the individual optical waveguides. Frequency conversion can be performed on a conventional crystal basis (e.g., second- or third-harmonic generation) or on a laser-induced plasma (e.g., gaseous targets or metal targets such as tin targets for generating EUV radiation with a wavelength of 13.5 nm). This approach is particularly advantageous in that the nonlinear process does not require spatially coherent radiation, since the plasma emission is spatially incoherent. As a prerequisite for efficient frequency conversion, it is sufficient that the required light intensity is achieved.

[0049] Hereinafter, embodiments of the present invention will be described in more detail with reference to the drawings. [Brief explanation of the drawings]

[0050] [Figure 1] Experimental evidence of incoherent superposition of an overall emission consisting of several individual emissions at a common focus. [Figure 2] 1 illustrates the generation of different beam profiles by incoherent superposition according to the present invention; [Figure 3] Schematic diagram of a first embodiment [Figure 4] Schematic diagram of a second embodiment [Figure 5] Schematic diagram of a third embodiment [Figure 6] Schematic diagram of a fourth embodiment [Figure 7]Schematic diagram of a fifth embodiment [Figure 8] Schematic diagram of a sixth embodiment [Figure 9] Schematic diagram of a seventh embodiment [Figure 10] Schematic diagram of an eighth embodiment [Figure 11] Schematic diagram of a ninth embodiment [Figure 12] Schematic diagram of a tenth embodiment [Figure 13] Diagram showing different configurations of multi-channel optical waveguides [Figure 14] Yet another possible configuration of a multi-channel optical waveguide [Figure 15] Yet another possible configuration of a multi-channel optical waveguide [Figure 16] Yet another possible configuration of a multi-channel optical waveguide [Figure 17] Yet another possible configuration of a multi-channel optical waveguide [Figure 18] FIG. 1 illustrates noncoherent frequency conversion using the optical system of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0051] Figure 3 shows the basic structure of an optical system according to the present invention. It comprises a multi-channel optical waveguide 1, which comprises a plurality of individual optical waveguides running parallel to one another, arranged here in an array with 4 x 4 individual optical waveguides (see the cross-sectional view on the left). Each individual optical waveguide is formed by a light-conducting core (black circle), which is surrounded by a common (here, circular cross-section) cladding of the multi-channel optical waveguide 1. At the exit end of the multi-channel optical waveguide 1 (the right end of the multi-channel optical waveguide 1 in Figure 3), a superposition optical unit 2 is provided to non-coherently superpose the diverging emissions from the individual optical waveguides at the target plane 3. As can be seen, the light beams of the individual emissions converge at different angles at the target plane 3.

[0052] As explained above, the superimposed light from the individual emissions of the multi-channel optical waveguide 1 is characterized by a high beam quality compared to an area-equivalent multimode fiber. Further advantages are illustrated in Figure 1, namely, the resulting uniformity and stability of the focus at the target surface 3 or point of use. For example, the individual emissions can be adapted to a 4f image in their dimensions, e.g., expanded, while the relationship between the diameter of the individual emissions and the distance of the individual emissions remains unchanged. Additional lenses focus these parallel-running individual emissions at the target surface 3 of the application. This causes the beams of each of the individual emissions to converge. This causes the different beams to strike the same focal spot at different angles, where they overlap incoherently. This results in an intensity distribution at the focal spot, correspondingly increasing the power density. Therefore, the increased power density at the point of use is "compensated" by an increased angular spectrum. This angular spectrum arises from the lateral expansion of the individual emissions that make up the overall emission, as explained above. In Figure 1, each image shows a cross section of a beam path. The light emitted from an ytterbium-doped multichannel optical waveguide (28 μm mode field diameter, 82 μm inter-core distance) according to the present invention was magnified 33 times using a lens system and focused onto a target plane 3 using an additional lens (f = 40 mm). The top images in Figure 1 show the light emitted from a single individual optical waveguide / core (left), an array of nine single optical waveguides / cores (center), and an array of 21 individual optical waveguides / cores (right). The bottom images show that the incoherent superposition produces a uniform Gaussian-shaped intensity profile whose size does not change as additional individual emissions are added (spot diameter approximately 70 μm). Assuming uniformly distributed optical power in the individual optical waveguides, the power density increases by a factor of 21 (equal to the number of cores).

[0053] In comparison, imaging with multimode fibers cannot produce such a uniform intensity distribution in the focal spot because the individual transverse modes are coherent with each other, resulting in different phases as the radiation propagates through the multimode fiber. A speckle pattern is always produced, which changes over time due to changes in the relative phase positions of the different transverse modes (e.g., with minimal disturbance). This makes the generated light unusable for many applications. The present invention provides a solution to this problem.

[0054] The basic approach of the present invention, shown in Figure 3, also offers the possibility of generating and dynamically switching between various beam profiles. The beam profile shown in Figure 2 is obtained, for example, by superimposing corresponding incoherent beams behind a multi-channel optical waveguide. The beam profile is determined by appropriate optics. In each case, the incoherent superposition of an array of 10 × 10 individual emissions (individual Gaussian beams with a diameter of 60 μm and a spacing of 150 μm) at various distances behind a lens, placed at a focal length (here, 60 mm) from the exit end of the multi-channel waveguide 1, is shown. Each of these temporally stable and spatially uniform patterns (left: flat-top, center: super-Gaussian profile, right: Gaussian profile) can be imaged onto a target plane 3 with corresponding target dimensions and associated target power densities (depending on the radiative power characteristics of the individual emissions) using corresponding imaging optics. In addition to the small spot size and thus power density, a spatially flat (flat-top) profile (e.g., maximum pulse energy in laser shock peening) can also be provided depending on the application. By adjusting the imaging optics, it is possible to switch between these profiles, even dynamically.

[0055] The proposed approach also offers the possibility to change the emission wavelength by doping the individual optical waveguides with different dopants and switching the pump wavelength (e.g., between 793 nm for thulium and 910-980 nm for ytterbium), so that all emissions exhibit incoherent superposition, generating a flat-top beam or a focal spot that can be modulated by its wavelength.

[0056] As an example of a further embodiment, Figure 4 shows a multi-channel optical waveguide as a laser oscillator, which in its simplest form emits continuous laser light. Laser cavities around individual actively doped (e.g., with ytterbium, erbium, or thulium ions) optical waveguides whose cores (shown as hatched lines in Figure 4) are integrated into a common pump cladding can be formed by coating the end faces of the individual optical waveguides or by Fresnel reflection at the end faces. Laser cavities for individual optical waveguides can also be formed by introducing fiber Bragg gratings (FBGs), which can be inscribed independently within each core, but also across each individual core. The FBGs form an independent cavity for each core, ensuring non-coherent emission between the cores.

[0057] 5 shows a further example embodiment for generating mutually incoherent pulsed radiation from individual emissions of an actively doped multi-channel optical waveguide (multi-core oscillator 5). For this purpose, a modulator 6 can optionally be introduced, such as an active (e.g., acousto-optic or electro-optic) modulator or a passive amplitude modulator (e.g., a saturable absorber). The time-modulated individual emissions then propagate through a superimposed optical system 2 to a target plane 3 (not shown here).

[0058] 6, the continuous or pulsed individual emissions generated by the multi-core oscillator 5 are amplified in one or more further actively doped multi-channel optical waveguides (multi-core amplifiers 7), followed by incoherent superposition using the superposition optical unit 2.

[0059] In the further embodiment example of Fig. 7, the emission of the laser system 9 is transmitted in the individual optical waveguides of a passive multi-channel optical waveguide (multi-core transmission fiber 8). For this purpose, the emission of the laser system 9 (optionally a multi-core laser system consisting for example of a multi-core oscillator 5 and a multi-core amplifier 7) can be coupled (free beam combining or fiber splicing) into the multi-core transmission fiber 8 and guided in this fiber, for example to the application. Afterwards, incoherent superposition is again performed using the superposition optical unit 2.

[0060] Incoherent superposition can be achieved using different superposition optical units 2. The simplest example is a single lens 10 that superposes the different emissions at the target plane 3. Figure 8 shows an example of this superposition using a multi-channel optical waveguide (multi-core fiber) with 16 signal cores in a 4x4 array.

[0061] To adjust the size or spatial extent of the superposition and thereby the intensity generated, a further optical system 11 (e.g., a two-lens telescope) can be used to image the intermediate plane 12 onto the target plane 3.

[0062] In Figure 10, the superposition is performed by a cylindrical lens 13 adapted to produce an elliptical beam at the intermediate plane 12 or target plane 3, and a uniform line focus is obtained by appropriate selection of the focal length and element spacing of the cylindrical lens 13. Figure 10 shows a top view (top) and a side view (bottom) of the structure.

[0063] In the example embodiment of FIG. 11, a lens array 14 (optionally a microlens array) is used behind the multi-channel light guide 1, so that ideally one microlens is placed in the beam path of each individual emission. This approach allows the beam diameter of the individual emissions to be varied depending on their spacing. This makes it possible, for example, to increase the space-filling density, i.e., the fill factor of the total emission (i.e., the total of the individual emissions). The superposition optical unit 2 then performs incoherent superposition on the target surface 3 or intermediate surface 12. The microlens array 14 does not necessarily have to be placed directly behind the multi-channel light guide 1; other positions in the beam path are also suitable for adjusting the fill factor. At longer distances, a conventional lens array 14 can also be used to increase the fill factor.

[0064] FIG. 12 shows that the pulses emitted from the individual optical waveguides arrive at the target surface 3 simultaneously or at arbitrarily staggered times (for example, two groups of pulses with a time difference Δt).

[0065] In addition to the superposition optical unit 2, the multi-channel optical waveguide 1 itself can be adapted for subsequent applications. Therefore, the refractive index profile of the individual cores of the multi-channel optical waveguide 1, designed as an active or passive multicore fiber, can be adapted to achieve a specific output beam profile. This is illustrated in Figure 13. Thus, in a conventional step-index fiber, a Gaussian-like beam profile with an adapted refractive index profile (Figure 13a) produces a flat, so-called "flat-top" beam profile (Figure 13b). Many design degrees of freedom exist here. The refractive index profiles of the individual optical waveguides can be tailored to achieve a compromise between the distance between the individual emissions, the beam quality of the individual emissions, and the coupling of the individual emissions. Special fiber designs for the individual optical waveguides are also possible, such as photonic crystal fibers or large-pitch fibers.

[0066] Furthermore, as shown in Figure 14a, different dopants 15 (e.g., ytterbium, erbium, or thulium) can be introduced into the cores of different individual optical waveguides. This allows emission at different wavelengths by simply switching the pump wavelength, resulting in incoherent superposition of the individual emissions. Note that different core diameters of the individual optical waveguides can be selected for different wavelengths to achieve the same spot diameter upon superposition. For example, the emission wavelength of thulium doping is approximately 2 μm, which means that the beam parameter product is, by definition, twice as bad as the emission at 1 μm (even assuming diffraction-limited beam quality). However, as the wavelength increases, it is also possible to double the core diameter while keeping the V parameter unchanged and, therefore, the number of modes in the individual optical waveguides.

[0067] In general, the core shapes of the individual optical waveguides can be designed to be different from each other. In Figure 14b, cores 16 of different sizes have been introduced into a multi-channel optical waveguide, resulting in beams of different diameters on the target surface 3. This can be combined with the use of different dopants in the individual optical waveguides, which can produce different spot sizes at different emission wavelengths and therefore different intensities on the target surface 3. In addition to using conventional active or passive cores in the individual optical waveguides, it is also possible to incorporate one or more passive (optionally gas-filled) hollow core waveguides 17, as shown in Figure 14c.

[0068] The arrangement of the individual light guides across the cross section of the multi-channel light guide is not limited to a rectangular pattern as shown in Figure 15a. Linear patterns (Figure 15b) or polygonal patterns (Figure 15c) are also possible. A hexagonal arrangement of the individual light guides (Figure 15d) is advantageous because it improves the fill factor of the overall light emission. As shown in Figure 15e, it is also possible in principle to randomly position the individual light guides, even if the distance between the cores is different.

[0069] When the core spacing of the individual optical waveguides in the active or passive multi-channel optical waveguide 1 of the present invention is close, optical coupling between the individual optical waveguides can be avoided or reduced by introducing optical barriers. For example, as shown in Figure 16, this can be achieved by using materials 18, 19 with different refractive indices (Figures 16a and 16b), or by using air holes 20 between or around the individual cores (Figure 16c).

[0070] In addition to the transverse structure of the multi-channel optical waveguide 1, the longitudinal structure can also be adjusted. For example, the diameter can be varied ("taper" 21) in a section or over the entire length, as shown in FIG. 17a, which can have a positive effect on the beam quality of the individual emissions. This taper can be advantageous in the multi-channel optical waveguide 1 as a multi-core oscillator 5, but also in particular in a multi-core amplifier 7 or a multi-core transmission fiber 8. Varying the emission size of the individual cores, without (optionally) changing the outer diameter of the multi-channel waveguide 1, can also be advantageous, as this can increase the fill factor of the overall emission. This is shown diagrammatically in FIG. 17b at 22. Both increasing the size of the individual cores at the output end (with a concomitant decrease in core spacing) and decreasing the size of the individual cores (with a concomitant increase in mode area for the same core spacing) can have a positive effect on the fill factor.

[0071] Finally, Figure 18 shows the generation of laser-induced plasma in gases or solids for the purpose of frequency conversion as a selected application of the optical system according to the invention. The incoherent superposition of individual emissions with good overall beam quality and a uniform intensity profile at the target plane 3 is ideally suited for incoherent frequency conversion processes. 11 W / cm 2 Power densities in the range of 10 produce plasmas that emit light not only in the extreme ultraviolet (EUV) spectral region but also in the soft x-ray range. 17 W / cm 2At this intensity, emission in the hard X-ray range is also possible. [Explanation of symbols]

[0072] 1 Multi-channel optical waveguide 2. Superposition optical unit 3 Target Surface 4 Fiber Bragg Grating 5 Multicore Oscillator 6 Modulator 7 Multicore Amplifier 8 Multicore transmission fiber 9 Laser System 10 Single Lens 11 Optical System 12 Intermediate Surface 13 Cylindrical Lens 14 Lens Array

Claims

1. 1. An optical system for generating or guiding light, comprising a multi-channel light guide (1) with a plurality of individual light guides running parallel to one another, and a superposition optical unit (2) designed to superpose emitted light from the individual light guides onto a target surface (3) at the exit end of the multi-channel light guide (1), The superposition of the emitted light of the individual optical waveguides is incoherent at the target plane (3). An optical system comprising:

2. 2. The optical system of claim 1, wherein the individual optical waveguides have a linear or array-like arrangement seen in a cross section of the multi-channel optical waveguide (1).

3. 3. The optical system of claim 1, wherein each of the individual optical waveguides is formed by an optical waveguide core or another optical waveguide structure.

4. 4. The optical system of claim 3, wherein the cores or optical waveguide structures of the individual optical waveguides are surrounded by a common cladding of the multi-channel optical waveguide (1).

5. 5. The optical system according to claim 3, wherein at least one of the optical waveguide cores, preferably some of the optical waveguide cores, particularly preferably all of the optical waveguide cores, are doped with ions of a rare earth element, preferably erbium, ytterbium or thulium, to enable optical amplification.

6. The optical system of claim 5 , wherein the optical waveguide cores differ from one another in terms of doping.

7. 7. The optical system of claim 5 or 6, wherein the common cladding is adapted to guide pump light for optically pumping the at least one doped core.

8. The optical system of claim 1 , wherein the cores have different diameters.

9. The optical system of claim 1 , wherein at least one of the cores is formed as a hollow core.

10. 3. An optical system according to claim 1 or 2, wherein the individual optical waveguides are doped light-conducting fibres, preferably double-core fibres.

11. 11. The optical system of claim 1, wherein the individual optical waveguides are optically isolated from one another.

12. 12. The optical system of claim 1, wherein the emitted light in the individual optical waveguides is nearly diffraction limited.

13. 13. The optical system according to any one of claims 1 to 12, wherein the superposition optical unit (2) is variable and designed to generate different beam profiles at the target surface (3).

14. 14. The optical system of claim 1, wherein the superposition optical unit (2) comprises a lens array (14), each lens of the lens array (14) being associated with one or more individual optical waveguides.

15. 15. The optical system of claim 1, wherein each of the individual optical waveguides forms a laser medium within an optical cavity.

16. 16. Optical system according to claim 15, comprising an optical modulator (6) arranged inside or outside the resonator and designed to generate a temporal modulation of the laser emission.

17. 17. The optical system according to claim 1, comprising two or more multi-channel optical waveguides (1), wherein the superposition optical units (2) associated with each of the multi-channel optical waveguides (1) are designed to superpose the light emissions of the two or more multi-channel optical waveguides (1) in the spatial domain.

18. 18. The optical system according to any one of claims 1 to 17, comprising a nonlinear optical element provided for frequency conversion of the superimposed emitted light of the individual optical waveguides.

19. Use of an optical system designed as a laser system according to any one of claims 1 to 18, - to generate UV light from laser-induced metal or gas plasmas, - for material processing by laser shock peening, for separation of the film from the substrate by laser lift-off, or - for the fragmentation of kidney or bladder stones by exposure to laser pulses (lithotripsy) Use of optical systems.

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