Optical system for generating high-power light

Abstract

The invention relates to an optical system for generating or guiding light, comprising a multi-channel light guide (1) which comprises a plurality of individual light guides running parallel to one another and comprising a superposing optical unit (2) which is designed to superpose light emissions of the individual light guides in a target plane (3) at an outlet end of the multi-channel light guide (1). According to the invention, the superposition of the light emissions of the individual light guides is incoherent in the target plane (3). The finding of the invention is that the incoherent superposition of the individual emissions results in an effectively better, more homogenous, and more stable beam quality than when a surface-equivalent transversally multimode individual large-core fiber is used, e.g. as an amplification fiber of a laser system.

Description

[0001] Optical system for generating high-power light

[0002] The invention relates to an optical system for generating high-power light, comprising a multi-channel optical fiber comprising a plurality of individual optical fibers running parallel to one another, and having superimposing optics designed to superimpose light emissions from the individual optical fibers at an exit end of the multi-channel optical fiber in a target plane.

[0003] High-power laser systems find numerous applications in industry and science. The spatial coherence of a laser's emission allows the radiation to be focused into the smallest spatial regions. The ideal case is a diffraction-limited beam, which produces the smallest focus spot for a given imaging optics. Poorer beam quality typically leads to a larger focus spot, and thus lower intensities, or requires the use of focusing optics with a larger numerical aperture (i.e., a higher divergence angle of the radiation from the focus), thus reducing the Rayleigh length, i.e., the distance over which a high intensity can be maintained. The better the beam quality, the higher the achievable power densities, even at greater distances.

[0004] The achievable power densities determine the addressable applications. Continuously emitting high-power lasers are used, for example, for cutting and welding a wide variety of materials (e.g., metals), while pulsed lasers are used, among other things, to specifically ablate or modify material. The elevated peak power densities of pulsed laser radiation also allow the driving of nonlinear effects, e.g., the frequency conversion of the primary laser radiation into other, application-relevant spectral ranges, i.e., the generation of secondary radiation. This frequency conversion can occur coherently (e.g., crystal-based frequency conversion in the form of the generation of higher harmonics, spectral broadening through Kerr nonlinearity, or the generation of short-wave coherent radiation through gas harmonics in noble gases), but also incoherently (e.g., through laser-induced plasmas in gases or metals).

[0005] A prominent example of incoherent frequency conversion with high economic relevance is the generation of incoherent EUV radiation at 13.5 nm wavelength (92 eV photon energy) for applications in the semiconductor industry using laser-induced tin plasmas (see O. O. Versolato, "Physics of laser-driven tin plasma sources of EUV radiation for nanolithography," Plasma Sources Sci. Technol. 28, 083001, 2019). In a high-performance version, the radiation from a pulsed CO2 laser is focused onto tin droplets (approximately 30 pm in diameter). The resulting plasma emits incoherently in all spatial directions at 13.5 nm wavelength; the conversion efficiency of this process can be 3-6% (also through targeted pre-preparation of the target using pre-pulses).

[0006] Another example of an industrial process selected for extending the service life of components is laser shock peening (see C. Zhang, Y. Dong, and C. Ye, "Recent Developments and Novel Applications of Laser Shock Peening: A Review," Adv. Eng. Mater. 23, 2001-216, 2021). This process involves inducing compressive stress into the material to counteract fatigue caused by tensile stresses. In laser peening, a pressure wave is generated using a high-energy laser pulse. Pulse energies ranging from several hundred mJ to several joules with focus spot diameters of a few millimeters are used. The homogeneity of the beam profile is essential for homogeneous pressure application. The process speed is determined by the pulse repetition frequency, which justifies the pursuit of higher pulse repetition frequencies.

[0007] Another selected application example of high-energy nanosecond pulses is the laser lift-off process (see R. Delmdahl, R. Pätzel, and J. Brune, "Large-Area Laser-Lift-Off Processing in Microelectronics," Phys. Procedia 41 , 241-248, 2013). This process involves producing a large-area functional film (e.g., a display) on a solid support (substrate). The laser lift-off process enables the separation of film and substrate with 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 leads to the detachment of the film. The spatial homogeneity of the applied energy is essential for this process.

[0008] Another example of the application of high-energy laser radiation is lithotripsy, i.e., the fragmentation of kidney or bladder stones (see NM Fried, "Recent advances in infrared laser lithotripsy [Invited]," Biomed. Opt. Express 9, 4552, 2018). This involves the use of long, high-energy laser pulses in the Joule range, preferably at a wavelength of around 2 pm due to tissue absorption.

[0009] The laser technology used today in these applications has the following disadvantages:

[0010] - The high-power CO2 laser mentioned above has a total efficiency (wallplug efficiency) of only a few percent (see 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).

[0011] - Diode-pumped solid-state lasers offer significantly higher efficiency, but suffer from thermo-optical problems with increasing output power, which manifest themselves in a deterioration of the beam quality, and thus the focusability and beam homogeneity.

[0012] - High pulse energies require a large cross-section of the active medium. In solid-state lasers (including fiber lasers), this leads to the oscillation of higher-order transverse modes, which in turn degrades beam quality and beam homogeneity. Particularly in fiber-based lasers (but also in passive transport fibers), the various transverse modes of a multimode fiber (large cross-section) are coherent with each other, i.e., even the smallest changes in the relative phase position of the various transverse modes lead to changes in the spatial emission profile and thus ultimately to instabilities in the application process (see 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, 1996).

[0013] The object of the invention is to provide an optical system, in particular a high-power laser system, which avoids at least some of the disadvantages mentioned above.

[0014] The invention solves this problem starting from an optical system of the type specified at the beginning in that the superposition of the light emissions of the individual light guides in the target plane is incoherent.

[0015] The proposed approach is to incoherently superpose the light emissions of the individual light guides forming the individual channels of the multi-channel light guide. Preferably, the beam quality should be good, i.e., the light emissions of the individual light guides are ideally (almost) diffraction-limited. It is also preferable for the individual emissions to be positioned as close to each other as possible at the exit end of the multi-channel light guide.

[0016] A multi-channel optical fiber within the meaning of the invention is any arrangement of a plurality of optically guiding structures running parallel to one another as individual optical fibers. The individual optical fibers of the multi-channel optical fiber have exit ends in a common plane, which together form the exit end of the multi-channel optical fiber. Examples of suitable multi-channel optical fibers 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 light guides of the multi-channel light guide can be two or more, preferably the number is at least three, more preferably at least 8, even more preferably at least 20, particularly preferably at least 40. In principle, any desired number is conceivable.

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

[0019] The insight of the invention is that the incoherent superposition of the individual emissions can achieve an effectively better and significantly more stable beam quality than, for example, when using a single transverse multimode large-core fiber with equivalent area, e.g., as an amplifier fiber of a laser system. An equivalent multimode fiber with equivalent area has the same core cross-sectional area as all the individual cores of the multichannel optical fiber combined. Assuming identical doping concentrations, this results in an identical fiber length. Consequently, both geometries (multichannel optical fiber or multimode fiber) are comparable in terms of stored energy and extractable laser power, limiting nonlinear effects, and fiber destruction due to excessive power densities. However, it turns out that the equivalent multimode fiber has poorer beam quality, even at a low numerical aperture. The following should be noted:

[0020] - The beam quality of the emissions from the individual light guides of the multi-channel light guide of the invention should, as already mentioned, be as good as possible, ideally almost diffraction-limited. The diffraction index of the individual emissions should accordingly be less than 3, preferably less than 2, more preferably less than 1.5, and most preferably less than 1.25. The directions of the individual emissions are preferably parallel to one another. - The cross-section of the cores of the individual light guides should preferably be as large as possible for a given distance and taking into account the individual core beam quality. Geometries such as tapered large-core fibers or other known large-core fiber designs can be helpful here, as they are known to support the best beam quality even with large core areas.The cores of the individual light guides should preferably have a diameter greater than 5 times, preferably greater than 10 times, more preferably greater than 25 times, particularly preferably greater than 50 times the wavelength of the propagating light.

[0021] - The spacing between the individual cores should preferably be as small as possible, taking into account the avoidance of optical coupling. Optical barriers within the structure of the multi-channel optical fiber can be helpful in preventing cross-coupling. For the purposes of the invention, "optical decoupling of the individual optical fibers" means that, over the entire length of the multi-channel optical fiber, preferably less than 10%, preferably less than 5%, and preferably less than 1% of the power propagating in an individual optical fiber is lost through power transfer to other individual optical fibers.

[0022] The incoherent superposition of the individual emissions is an optical transformation of any cross-sectional plane in the beam path, which is located in a region behind the exit end of the multi-channel light guide.

[0023] It should be noted that the advantages of a multi-channel optical fiber over a multimode large-core fiber are particularly evident when the spatially coherently emitting individual optical fibers are arranged closely together, so that the total area of ​​the ultimately incoherent superposition of the coherent individual emissions does not become unnecessarily large. These design guidelines can be specifically implemented in a multi-core fiber, since the well-known multi-core fiber concept ideally supports dense packing of the individual cores. Ideally, in a multi-channel optical fiber, the ratio of the spacing of the cores of the individual optical fibers to the core diameter should be less than 20, preferably less than 10, more preferably less than 5, and most preferably less than 3.

[0024] The advantages of the invention’s approach are summarized:

[0025] - The optical system has a simple and compact design.

[0026] - In a fiber-based implementation, the optical system of the invention offers high efficiency as a laser system (see below) and it is possible to pump the multi-channel optical fiber as a laser medium directly with semiconductor diodes.

[0027] - The geometry of the multi-channel optical fiber as an elongated waveguide, e.g., in the form of an active multi-core fiber in a laser system (see below), distributes the laser-induced heat input over a large length, and the large fiber cladding surface can be used to dissipate the introduced heat. Consequently, this approach offers the possibility of emitting high average powers.

[0028] - The inversion stored in the doped cores, and thus the extractable light output, is determined by the properties of the dopants and the dopant concentration, which determines and limits the extractable light output for each individual light guide. The multi-channel light guide increases the extractable power according to the number of individual light guides.

[0029] - Various rare earth elements can be used as dopants; ytterbium ions can address the wavelength range around 1 pm, erbium ions around 1.5 pm, and thulium ions around 2 pm. It is also conceivable for the light-guiding cores to differ in their doping. This makes it possible to achieve a (optionally dynamic) change in the emission wavelength by selecting the pump wavelength. - Requirements regarding nonlinear effects and material destruction are distributed across several individual light guides, which in turn enables an increase in overall performance.

[0030] - According to the invention, the superposition of the individual emissions is an incoherent superposition, meaning the relative phase position of the individual emissions is irrelevant. This eliminates the need for elements for detecting and stabilizing the phase position in the individual channels, as is required for coherent superposition. The invention thus makes the design extremely simple.

[0031] - Due to the incoherent superposition, the exact position of the individual optical fibers across the cross-section of the multi-channel waveguide is irrelevant, allowing considerable flexibility in the design of the multi-channel waveguide. Likewise, large manufacturing tolerances exist regarding the positioning accuracy and size of the individual optical fibers.

[0032] - The beam quality of the incoherently superimposed total emission of the multi-core optical fiber can, as already mentioned above, be significantly better compared to an area-equivalent multimode fiber.

[0033] These advantages make the optical system according to the invention, realized as a laser system, ie with the multi-channel light guide as the laser medium, particularly suitable for the applications mentioned above, namely the incoherently superimposed laser radiation generated thereby can be used to generate UV light (in particular EUV light) from a laser-induced metal or gas plasma (e.g. tin plasma), for material processing by laser shock peening, for separating a film from a substrate by laser lift-off, or also for generating laser pulses for fragmenting kidney or bladder stones (lithotripsy).

[0034] The aforementioned advantages also apply without restriction to an optical system with a purely passive multi-channel optical fiber (without core doping), e.g., as a transport fiber. In addition to the advantage in terms of beam quality, the following is also relevant: In conventionally used passive multimode transport fibers, the intrinsic coherence of different transverse modes results in a high sensitivity of the resulting intensity profile to relative changes in the phase position of the individual transverse modes. In passive multimode fibers, these phase changes originate from external influences (e.g., stress due to a change in position or contact with the fiber); in active fibers, the laser-induced heat input dominates. These disadvantages are eliminated by the invention.

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

[0036] In a further possible embodiment, the individual light guides are individual doped light-conducting fibers, preferably double-core fibers (doped with rare earth ions), or also individual passive light-conducting fibers, which are combined in the multi-channel light guide, as it were as a bundle of individual fibers.

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

[0038] In one possible embodiment, the individual light guides have a linear, array, or matrix-like arrangement in the cross-section of the multi-channel light guide. Likewise, the packing density of the individual light guides can be increased by a hexagonal arrangement. A random distribution of the individual light guides across the cross-section of the multi-channel light guide is also conceivable. Furthermore, to increase the fill factor, i.e., the portion of the cross-section through which light passes, a (subsequent) widening of the mode field diameter of the individual cores (e.g., by targeted heat input, i.e., thermal widening or taper of the multi-core fiber) at the inlet and / or outlet end of the multi-channel light guide is conceivable.

[0039] Another possible approach to increasing the fill factor of the multi-nuclear emission is the use of a lens array, in particular a microlens array outside the multi-channel light guide. This can be placed, for example, directly in front of the exit end of the multi-channel light guide (at the corresponding working distance), but also further along the beam path. Each individual lens of the lens array corresponds to one or more individual light guides of the multi-channel light guide. The lens array can increase the fill factor by at least a factor of 1.2, preferably by at least a factor of 1.5, more preferably by at least a factor of 2, although higher factors are also possible.

[0040] In one possible design, the individual light guides of the multi-channel light guide each form a laser medium in an optical resonator. If there is no coupling between the individual light guides, all individual light guides can be made to emit lasers independently of one another. For example, one or more free-beam resonators (using a suitable reflector arrangement) can be placed around the multi-channel light guide. While these resonators are used by all individual light guides, each individual light guide performs the laser oscillation independently, i.e., independently of the other individual light guides. This makes it possible to achieve incoherence of the superposition in the target plane. It is also conceivable that independent optical resonators could be realized by reflectively coated end surfaces of the individual light guides or by Bragg gratings (FBGs) inscribed into the ends of the individual light guides.Even the Fresnel reflection at the free ends of the individual light guides can be sufficient to form an optical resonator without any further measures.

[0041] Additional elements can also be arranged in or outside the resonator, e.g. (temporal) light modulators for generating pulsed laser radiation (by Q-switching, cavity dumping or mode locking).

[0042] An oscillator-amplifier arrangement (MOPA) is also feasible. For example, a laser oscillator configured according to the invention with a multi-channel optical fiber generates low-power laser radiation, with the emission pattern consisting of spatially incoherent beams. The laser oscillator can operate (using a suitable modulation scheme) in a temporal operating regime according to the requirements of the application (continuous emission (cw) or pulsed up to ultrashort laser pulses). Alternatively, a conventional single emitter can be used as the light source, with its emission being distributed accordingly among the individual optical fibers of the multi-channel optical fiber. The laser light generated in this way is coupled downstream into a laser-active multi-channel optical fiber with a suitable number and arrangement of individual optical fibers, where it is amplified to higher power levels (and possibly pulse energies). This step can also be repeated, i.e.Additional multi-channel optical fibers can be passed through in series as amplifiers. The pattern of individual emissions can also be coupled into a multi-channel optical fiber serving as a passive transport fiber. This allows the individual emissions to be transported to the application. Frequency conversion processes can also be driven in the individual optical fibers of a multi-channel optical fiber connected downstream of the amplifier (e.g., four-wave mixing or Raman scattering).

[0043] When using a single emitter as the light source in an optical system of the invention implemented as a MOPA system, it is important to ensure that the path lengths of the individual emissions up to the point of superposition in the target plane are greater than the coherence length of the light originating from the single emitter for the incoherent superposition at the output of the downstream multi-channel optical fiber. Thus, all light sources with a sufficiently large spectral width are conceivable, e.g., the (optionally time-stretched) emission of a mode-locked ultrashort pulse laser or a superluminescence diode.

[0044] In the MOPA concept, a light modulator (e.g. arranged between oscillator and amplifier) ​​can modify the temporal characteristics of the emission, e.g. to generate pulses from a temporally continuous laser light or to adapt pulse shapes to the requirements of the application (e.g. to generate pre-pulses or an increase at the beginning of a pulse) or to shape the pulses to influence saturation-induced pulse shaping in the amplifier.

[0045] The approach described also offers the possibility of the emissions from the individual light guides occurring independently of one another in time, e.g. through independent temporal modulation of the laser light in the individual channels of the multi-channel light guide. This makes it conceivable, for example, to generate one or more pre-pulses from a certain number of individual light guides and to emit a higher-energy main pulse from further individual light guides. This only requires a temporally staggered coupling of light pulses into the various individual light guides. In this case, it is also possible, e.g. through suitable different doping of the individual light guides or through corresponding frequency modulation of the pump light, for the pre-pulses to have an emission wavelength different from that of the main pulse.One possible embodiment provides for the cores of the individual optical fibers to have different diameters, resulting in different spot sizes for the pre-pulse(s) and the main pulse during incoherent superposition. In another possible embodiment, one or more of the individual optical fibers are formed with a hollow core, which is suitable, for example, for transporting ultrashort laser pulses with high peak power, surrounded by additional individual optical fibers of the active or passive multi-channel optical fiber.

[0046] In another possible embodiment, the optical system can comprise two or more multi-channel optical fibers, with the respective superposition optics being designed to superimpose the total emissions of the two or more multi-channel optical fibers in a spatial region. For example, the individual emissions can enter the spatial region from different spatial directions and be superimposed there incoherently.

[0047] The total emission of the optical system can also be a superposition of the emissions from two multi-channel optical fibers of orthogonal polarization on a polarizer. Furthermore, the total emission can be a superposition of the emissions from two or more multi-channel optical fibers of different wavelengths on one or more spectrally selective elements (e.g., volume Bragg grating, dichroic mirror, prism, grating, grism, or a combination of these elements).

[0048] In yet another possible embodiment, a nonlinear optical element can be provided for frequency conversion of the superimposed light emissions of the individual light guides. The frequency conversion can be conventionally crystal-based (generation of the second or third harmonic, etc.) or in a laser-induced plasma (e.g., in gaseous targets or in metallic targets, such as tin targets for generating EUV radiation at a wavelength of 13.5 nm). This approach is particularly advantageous because the plasma emission is spatially incoherent and thus no spatially coherent radiation is required for the nonlinear process. Achieving the required light intensity is sufficient for efficient frequency conversion.

[0049] Exemplary embodiments of the invention are explained in more detail below with reference to the drawings. They show:

[0050] Figure 1 : Experimental evidence of the incoherent

[0051] Overlay of a single emission consisting of several individual emissions

[0052] Total emission in a common focus spot;

[0053] Figure 2: Generation of different beam profiles by incoherent superposition according to the invention;

[0054] Figure 3: schematic representation of a first

[0055] embodiment;

[0056] Figure 4: schematic representation of a second

[0057] embodiment;

[0058] Figure 5: schematic representation of a third

[0059] embodiment;

[0060] Figure 6: schematic representation of a fourth

[0061] embodiment;

[0062] Figure 7: schematic representation of a fifth

[0063] embodiment;

[0064] Figure 8: schematic representation of a sixth

[0065] embodiment; Figure 9: schematic representation of a seventh

[0066] embodiment;

[0067] Figure 10: schematic representation of an eighth

[0068] embodiment;

[0069] Figure 11 : schematic representation of a ninth

[0070] embodiment;

[0071] Figure 12: schematic representation of a tenth

[0072] embodiment;

[0073] Figure 13: Illustrations of different designs of the multi-channel light guide;

[0074] Figure 14: further possible designs of the multi-channel light guide;

[0075] Figure 15: further possible designs of the multi-channel light guide;

[0076] Figure 16: further possible designs of the multi-channel light guide;

[0077] Figure 17: further possible designs of the multi-channel light guide;

[0078] Figure 18: Illustration of the incoherent frequency conversion using the optical system of the invention.

[0079] Fig. 3 shows the basic structure of an optical system according to the invention. It comprises a multi-channel light guide 1, which has a plurality of individual light guides running parallel to one another, here in an array-like arrangement with 4x4 individual light guides (visible on the left in the cross-sectional view). The individual light guides are each formed by a light-guiding core (dark circle) which is surrounded by a common cladding of the multi-channel light guide 1 (here circular in cross-section). A superposition optics 2 is provided for incoherently superimposing the diverging light emissions from the individual light guides at the exit end of the multi-channel light guide 1 (the right-hand end of the multi-channel light guide 1 in Fig. 3) in a target plane 3. As can be seen, the light rays of the individual emissions converge at different angles in the target plane 3.

[0080] As explained above, the light of the multi-channel optical fiber 1 superimposed from the individual emissions is characterized by a high beam quality compared to that of a multimode fiber with equivalent area. Fig. 1 illustrates a further advantage. The homogeneity and stability of the resulting focus in the target plane 3 or at the application site is also advantageous. The dimensions of the individual emissions can be adjusted, e.g. enlarged, using a 4f image, for example, whereby the relationships between the diameters of the individual emissions and the distances between the individual emissions remain unchanged. A further lens focuses these parallel individual emissions into the target plane 3 of the application. The beam of each individual emission is focused. The different beams strike the same focus spot at different angles and superimpose there incoherently.This leads to an intensity distribution in the focus spot with a correspondingly added power density. The increase in power density at the point of application is therefore "paid for" with an enlarged angular spectrum; this angular spectrum results from the lateral extension of the individual emissions forming the overall emission, as explained above. In Fig. 1, which shows a cross-section of the beam path, the emission of an ytterbium-doped multi-channel optical fiber according to the invention (28 pm mode field diameter, core-to-core spacing 82 pm) was magnified by a factor of 33 using a lens system and focused onto the target plane 3 using a further lens (f = 40 mm). The top row of images in Fig. 1 shows the emission of a single optical fiber / core (left), an array of nine individual optical fibers / cores (middle), and an array of 21 individual optical fibers / cores (right).The lower row of images shows that the incoherent superposition produces a homogeneous Gaussian intensity profile whose size does not change upon addition of further individual emissions (spot diameter approx. 70 pm). Assuming an evenly distributed light power in the individual light guides, the power density is increased by a factor of 21 (equal to the number of cores). In comparison, imaging a multimode fiber cannot produce such a homogeneous intensity distribution in the focus due to the coherence of the individual transverse modes to one another and the differently imposed phases during propagation of the radiation in the multimode fiber. A speckle pattern is always generated, which changes over time due to changes in the relative phase position of various transverse modes (e.g., due to minute disturbances). This makes the generated light unusable for many applications. The invention provides a remedy here.

[0081] The basic approach of the invention illustrated in Fig. 3 also offers the possibility of generating a wide variety of beam profiles and dynamically switching between them. By appropriate incoherent beam superposition behind the multi-channel optical fiber, the beam profiles shown in Fig. 2 are obtained. The beam profile is specified by suitable optics. Shown in each case is the incoherent superposition of an array of 10x10 individual emissions (individual Gaussian beams with a diameter of 60 pm and a spacing of 150 pm) at various distances behind a lens located at the distance of its focal length (here 60 mm) from the exit end of the multi-channel waveguide 1.Each of these temporally stable and spatially homogeneous patterns (left: flat-top, center: super-Gaussian profile, right: Gaussian profile) can be imaged using appropriate imaging optics onto a target plane 3 with corresponding target dimensions and—depending on the emitted power characteristics of the individual emissions—an associated target power density. This allows for small spot sizes and thus high power densities, as well as spatially flat (flat-top) profiles (e.g., highest pulse energy for laser shock peening) for the respective application. By adjusting the imaging optics, it is possible to switch between these profiles, even dynamically.

[0082] The proposed approach also offers the possibility of doping the individual light guides with different dopants and thus changing the emission wavelength by switching the pump wavelength (e.g., between 793 nm for thulium and 910-980 nm for ytterbium), whereby all emissions nevertheless represent the incoherent superposition and thus generate a flat-top beam or focus whose wavelength can be modulated. Fig. 4 illustrates, as a further embodiment, a multi-channel light guide as a laser oscillator, which in its simplest form emits continuous laser radiation. The laser resonator around the actively doped (e.g., with ytterbium, erbium, or thulium ions) individual light guides, whose cores (shown hatched in Fig. 4) are integrated in a common pump cladding, can be formed, for example, by coated end faces or Fresnel reflections at the end faces of the individual light guides.Likewise, the laser resonators of the individual optical fibers can be formed by integrated fiber Bragg gratings (FBGs). The FBGs can be written independently in each core, but are also possible across the entire set of individual cores. The FBGs form an independent resonator for each core, ensuring incoherent emission from core to core.

[0083] Fig. 5 shows a further embodiment with the generation of mutually incoherent pulsed radiation from the individual emissions of the actively doped multi-channel optical fiber (multi-core oscillator 5). A modulator 6, e.g., an active (e.g., acousto-optical or electro-optical) modulator or a passive amplitude modulator (e.g., a saturable absorber), can optionally be incorporated. The temporally modulated individual emissions subsequently propagate through a superposition optics 2 to the target plane 3 (not shown here).

[0084] In the further embodiment shown in Fig. 6, the continuous or pulsed individual emissions generated by the multi-core oscillator 5 are amplified in one or more additional actively doped multi-channel optical fibers (multi-core amplifiers 7). Subsequently, the incoherent superposition is performed using superposition optics 2.

[0085] In the further embodiment shown in Fig. 7, the emissions of a laser system 9 are transported in the individual optical fibers of a passive multi-channel optical fiber (multicore transport fiber 8). For this purpose, the emission of the laser system 9, optionally of a multicore laser system, e.g., consisting of a multicore oscillator 5 and a multicore amplifier 7, can be coupled into the multicore transport fiber 8 (free-beam coupling or fiber connection via a splice) and guided therein, e.g., to the application. Subsequently, the incoherent superposition takes place again using superposition optics 2.

[0086] The incoherent superposition can be achieved using different superposition optics 2. The simplest example is a single lens 10, which superposes the different emissions in the target plane 3. This superposition is illustrated in Fig. 8 using a multi-channel optical fiber 1 (multi-core fiber) with 16 signal cores in a 4x4 arrangement.

[0087] In order to adjust the size or spatial extent of the superposition and thus the generated intensities, a further optics 11 (e.g. a telescope consisting of two lenses) can be used, as shown in Fig. 9, which images an intermediate plane 12 onto the target plane 3.

[0088] In Fig. 10, the superposition is achieved by a cylindrical lens 13, adapted to generate an elliptical beam in the intermediate plane 12 or the target plane 3. With an appropriately selected focal length of the cylindrical lens 13 and the spacing of the elements, a homogeneous line focus is achieved. Fig. 10 shows the setup in plan view (top) and side view (bottom).

[0089] In the embodiment of Fig. 11, a lens array 14, optionally a microlens array, is used behind the multi-channel light guide 1, with ideally a microlens being placed in the beam path of each individual emission. With this approach, the beam diameter of the individual emissions can be varied in relation to their distances. For example, it is possible to increase the spatial packing density, i.e. the fill factor of the total emission, i.e. of the totality of the individual emissions. The incoherent superposition then takes place using the superposition optics 2 on the target plane 3 or the intermediate plane 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 a greater distance, a conventional lens array 14 can also increase the fill factor. Fig.12 illustrates that pulses emitted by the individual optical fibers arrive at the target plane 3 simultaneously or at arbitrarily shifted times (e.g. two groups of pulses with a time difference Δt).

[0090] In addition to the superposition optics 2, the multi-channel light guide 1 itself can also be adapted to the subsequent application. For example, the index profiles of the individual cores of the multi-channel light guide 1, which is designed as an active or passive multi-core fiber, can be adjusted to achieve a specific output beam profile. This is illustrated in Fig. 13. For example, Gaussian-like beam profiles are obtained in conventional step-index fibers (Fig. 13a), while an adjusted index profile results in a flat, so-called "flat-top" beam profile (Fig. 13b). There are numerous design degrees of freedom here. The index profile of the individual light guides can be adjusted to find a compromise between the spacing of the individual emissions, the beam quality of the individual emissions, and the coupling of the individual emissions. Special fiber designs of the individual light guides are also possible, e.g., as photonic crystal fibers or large-pitch fibers.

[0091] Furthermore, different dopants 15 (e.g., ytterbium, erbium, or thulium) can be introduced into the cores of the different individual optical fibers, as shown in Fig. 14a. This allows emission at different wavelengths by simply switching the pump wavelength, with the individual emissions being superimposed incoherently. It should be noted that, for different wavelengths, correspondingly different core diameters of the individual optical fibers can be selected to achieve the same spot diameter during superposition. For example, the emission wavelength when doped with thulium is approximately 2 pm; therefore, the beam parameter product is, by definition, a factor of two worse than the emission at 1 pm (even assuming diffraction-limited beam quality).However, the longer wavelength also allows a core diameter that is a factor of two larger with the V parameter and thus the number of modes in the single optical fiber unchanged.

[0092] In general, the geometry of the cores of the individual optical fibers can be designed differently. In Fig. 14b, cores of different sizes 16 are introduced into the multi-channel optical fiber, resulting in beams of different diameters at the target plane 3. This can be combined with the use of different dopants in the individual optical fibers, so that different emission wavelengths can generate different spot sizes and thus intensities in the target plane 3. In addition to the use of conventional active or passive cores of the individual optical fibers, the integration of one or more passive (optionally gas-filled) hollow-core waveguides 17 is also possible, as shown in Fig. 14c.

[0093] 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 illustrated in Fig. 15a. A linear pattern (Fig. 15b) or a polygonal pattern (Fig. 15c) is also possible. Hexagonal positioning of the individual light guides is advantageous due to the increase in the fill factor of the overall emission (Fig. 15d). Likewise, random positioning of the individual light guides, even with different core spacings, as shown in Fig. 15e, is in principle possible.

[0094] With a small core spacing of the individual optical fibers in the active or passive multi-channel optical fiber 1 of the invention, optical coupling between the individual optical fibers can be avoided or reduced by introducing optical barriers. This can be achieved, for example, as illustrated in Fig. 16, by materials 18, 19 with different refractive indices (Fig. 16a and Fig. 16b) or by air holes 20 (Fig. 16c) between or around the individual cores.

[0095] In addition to the transverse structure of the multi-channel optical fiber 1, the longitudinal structure can also be adapted. For example, the diameter can be changed in a section, as shown in Fig. 17a, or over the entire length (“taper” 21), which can have a positive influence on the beam quality of the individual emissions. This taper can be advantageous in the multi-channel optical fiber 1 as a multi-core oscillator 5, but particularly also in the multi-core amplifier 7 or in the multi-core transport fiber 8. Likewise, changing the emission size of the individual cores with (optionally) unchanged outer diameter of the multi-channel waveguide 1 can be advantageous, since this can increase the fill factor of the overall emission. This is shown schematically in Fig. 17b at 22.Both an increase in the number of individual cores at the exit end (associated with a reduction in the distance between the cores) and a reduction in the size of the individual cores (associated with an increase in the mode area while maintaining the same core distance) can have a positive effect on the fill factor.

[0096] Finally, Figure 18 illustrates, as a selected application of the optical system according to the invention, the generation of a laser-induced plasma in gases or solids for the purpose of frequency conversion at 24. The incoherent superposition of the individual emissions with good overall beam quality and a homogeneous intensity profile in the target plane 3 is ideally suited for the incoherent frequency conversion process. Power densities in the range of 10 11 W / cm 2 on various metals generate plasmas that emit in the extreme ultraviolet (EUV) spectral range, but also in the soft X-ray range, at intensities of 10 17 W / cm2 Emissions in the hard X-ray range are possible.

[0097] - Patent claims -

Claims

Patent claims 1. An optical system for generating or guiding light, comprising a multi-channel optical fiber (1) comprising a plurality of individual optical fibers running parallel to one another, and having superimposing optics (2) designed to superimpose light emissions from the individual optical fibers at an exit end of the multi-channel optical fiber (1) in a target plane (3), characterized in that the superimposition of the light emissions from the individual optical fibers in the target plane (3) is incoherent.

2. Optical system according to claim 1, wherein the individual light guides have a line-shaped or array-shaped arrangement when viewed in the cross-section of the multi-channel light guide (1).

3. Optical system according to claim 1 or 2, wherein the individual light guides are each formed by a light-guiding core or another light-guiding structure.

4. Optical system according to claim 3, wherein the cores or light-guiding structures of the individual light guides are surrounded by a common cladding of the multi-channel light guide (1).

5. Optical system according to claim 3 or 4, wherein at least one of the light-guiding cores, preferably a part of the light-guiding cores, particularly preferably all light-guiding cores, has an optical amplification enabling doping with rare earth ions, preferably erbium, ytterbium or thulium.

6. Optical system according to claim 5, wherein the light-guiding cores differ from one another with regard to doping.

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

8. Optical system according to one of claims 1 to 7, wherein the cores have different diameters.

9. Optical system according to one of claims 1 to 8, wherein at least one of the cores is formed as a hollow core.

10. Optical system according to claim 1 or 2, wherein the individual light guides are doped light-conducting fibers, preferably double-core fibers.

11. Optical system according to one of claims 1 to 10, wherein the individual light guides are optically decoupled from one another.

12. Optical system according to one of claims 1 to 11, wherein the light emissions of the individual light guides are almost diffraction-limited.

13. Optical system according to one of claims 1 to 12, wherein the superposition optics (2) are variable and designed to generate different beam profiles in the target plane (3).

14. Optical system according to one of claims 1 to 13, wherein the superposition optics (2) comprises a lens array (14), wherein each lens of the lens array (14) is assigned to one or more individual light guides.

15. An optical system according to any one of claims 1 to 14, wherein the individual optical fibers each form a laser medium in an optical resonator.

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

17. Optical system according to one of claims 1 to 16, with two or more multi-channel optical fibers (1 ), wherein the superposition optics (2) respectively assigned to the multi-channel optical fibers (1 ) are designed to superpose the emissions of the two or more multi-channel optical fibers (1 ) in a spatial region.

18. Optical system according to one of claims 1 to 17, comprising a non-linear optical element provided for frequency conversion of the superimposed light emissions of the individual optical fibers.

19. Use of the optical system designed as a laser system according to one of claims 1 to 18 for Generation of UV light from a laser-induced metal or gas plasma, Material processing by laser shock peening, Separation of a film from a substrate by laser lift-off, or Breaking up kidney or bladder stones by applying laser pulses (lithotripsy). - Summary -

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