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) comprising a plurality of individual light guides extending parallel to one another, and comprising a superposition optical unit (2), the superimposition optical unit (2) is designed to superimpose the light emissions of the individual light guides in a target plane (3) at the exit end of the multichannel light guide (1). According to the invention, the superposition of the light emission of the individual light guides is incoherent in the target plane (3). The discovery of the invention is that the incoherent superposition of the individual emissions results in better, more uniform and more stable beam quality compared to individual large core fibers (e.g., as amplification fibers of laser systems) using surface equivalent lateral multimode.

Description

Technical Field

[0001] The invention relates to an optical system for generating high-power light, the optical system having a multi-channel light guide comprising a plurality of individual light guides extending parallel to one another, and a superposition optical unit designed to superimpose the light emissions of the individual light guides in a target plane at the outlet end of the multi-channel light guide. Background Art

[0002] High-power laser systems have many applications in industry and science. The spatial coherence of the laser emission allows the radiation to be focused into the smallest spatial region. The ideal situation is a diffraction-limited beam that produces the smallest focused spot for a given imaging optics. Poor beam quality generally results in a larger focused spot and therefore lower intensity, or requires the use of focusing optics with a larger numerical aperture (i.e. a higher divergence angle of the radiation to the focus), thus reducing the Rayleigh length (i.e. the distance over which high intensity can be maintained). The better the beam quality, the higher the achievable power density, even at greater distances.

[0003] The achievable power density determines the applications that can be addressed. Continuously emitted high-power lasers are used, for example, for cutting and welding various materials (e.g., metals), whereas pulsed lasers are used for specialized ablation or modification of materials. The excessively high peak power density of pulsed laser radiation also allows the driving of nonlinear effects, such as the frequency conversion of the primary laser radiation into a spectral range relevant for other applications, i.e., the generation of secondary radiation. This frequency conversion can be coherent (e.g., crystal frequency conversion in the form of high-order harmonic generation, spectral broadening via Kerr nonlinearity, or generation of short-wavelength coherent radiation via gas harmonics in inert gases) or incoherent (e.g., via laser-induced plasmas in gases or metals).

[0004] A prominent example of incoherent frequency conversion with high economic relevance is the generation of incoherent EUV radiation with a wavelength of 13.5 nm (92 eV photon energy) for use in the semiconductor industry by laser-induced tin plasma (see OO Versolato, “Physics of laser-driven tin plasma sources of EUV radiation for nanolithography”, Plasma Sources Sci. Technol. 28, 083001, 2019). In a high-power version, radiation from a pulsed CO2 laser is focused onto tin droplets (approximately 30 μm in diameter). The resulting plasma emits incoherently at a wavelength of 13.5 nm in all spatial directions, and the conversion efficiency of the process can be 3%-6% (targeted pre-preparation by pre-pulses is also possible).

[0005] Another industrial process chosen as an example is laser shock peening, which is used to extend the service life of components (see C. Zhang, Y. Dong and C. Ye, "Recent Developments and Novel Applications of Laser Shock Peening: A Review", Adv. Eng. Mater. 23, 2001216, 2021). Compressive stress is introduced into the material to offset the fatigue caused by tensile stress. In laser shock peening, high-energy laser pulses are used to generate pressure waves. Therefore, pulse energies of several hundred millijoules to several joules are used, and the focused spot diameter is several millimeters. The uniformity of the beam profile is crucial for uniform pressure input. The processing speed is determined by the pulse repetition frequency, which is why higher pulse repetition frequencies are strived for.

[0006] Another example of a selected application of high-energy nanosecond pulses is the laser lift-off process (see R. Delmdahl, R. and J. Brune, "Large-Area Laser-Lift-Off Processing in Microelectronics," Phys. Procedia 41, 241-248, 2013. In this process, functional thin films (e.g., displays) are produced over large areas on a solid support (substrate). The laser lift-off process enables the separation of thin films from substrates with the necessary reproducibility and film protection. High-energy nanosecond pulses in the ultraviolet spectral range are used, which penetrate the substrate and are absorbed by the absorber layer. The resulting energy input leads to the detachment of the thin film. The spatial uniformity of the energy input is crucial for this process.

[0007] Another example of an application of high-energy laser radiation is lithotripsy, i.e., the breaking up of kidney or bladder stones (see N.M. Fried, “Recent advances in infrared laser lithotripsy [Invited],” Biomed. Opt. Express 9, 4552, 2018). High-energy, long laser pulses in the Joule range are used for this purpose, with a wavelength of about 2 μm being preferred due to tissue absorption.

[0008] Current laser technologies used for these applications have the following disadvantages:

[0009] The total efficiency (wall plug efficiency) of the above-mentioned high-power CO2 lasers is only a few percent (see K. Kellens, G. Costa Rodrigues, W. Dewulf, J. R. Duflou, GC Rodrigues, W. Dewulf, and J. R. Duflou, "Energy and resource efficiency of laser cutting processes", Phys. Procedia 56, 854-864, 2014).

[0010] Diode-pumped solid-state lasers offer significantly higher efficiency, but suffer from thermo-optical problems as output power increases, which manifests itself as a deterioration in beam quality and, consequently, focusability and beam uniformity.

[0011] High pulse energies require a large cross-section of the active medium. In solid-state lasers (including fiber lasers), this leads to oscillations of higher-order transverse modes, which in turn deteriorate the beam quality and beam uniformity.

[0012] Especially in fiber-based lasers (but also in passive transmission fibers), different transverse modes of multimode fibers (large cross-section) are coherent with each other, i.e., even the smallest changes in the relative phase of the different transverse modes lead to changes in the spatial emission profile and thus ultimately to instabilities in the application (see. BYZel'dovich, DZ Anderson and MA Bolshtyansky, "Stabilization of the speckle pattern of a multimode fiber undergoing bending", Opt. Lett. Vol. 21, Issue 11, pp. 785-78721, 785-787, 1996). Summary of the Invention

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

[0014] This object is achieved according to the invention starting from an optical system of the aforementioned type, since the superposition of the light emissions of the individual light guides in the target plane is incoherent.

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

[0016] For the purposes of the present invention, a multichannel light guide is any arrangement of a plurality of light guide structures extending parallel to one another as individual light guides. The individual light guides of the multichannel light guide thus have exit ends in a common plane, which together form the exit end of the multichannel light guide. Examples of suitable multichannel light guides are known in 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 a multi-channel light guide may be two or more, preferably a number of at least three, more preferably at least 8, even more preferably at least 20, particularly preferably at least 40. In principle, any number is conceivable.

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

[0019] The insight of the present invention is that, by incoherent superposition of the individual emissions, a better and more stable beam quality can be achieved, compared to, for example, using area-equivalent single transverse multimode large-core fibers (e.g. as amplifier fibers for laser systems). An area-equivalent multimode fiber has the same core cross-sectional area as all the individual cores of a multichannel light guide. Assuming the same doping concentration, this results in the same fiber length. As a result, the two geometries (multichannel light guide and multimode fiber) are comparable in terms of stored energy and extractable laser power, limiting nonlinear effects and fiber destruction due to excessive power density. However, it has been shown that even at low numerical apertures, area-equivalent multimode fibers have a poorer beam quality. The following factors should be taken into account:

[0020] As already mentioned, the beam quality of the emissions from the individual light guides of the multi-channel light guide of the present invention should be as good as possible, ideally nearly diffraction-limited. Therefore, the diffraction index of the individual 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 emissions are preferably parallel to one another.

[0021] At a given distance and taking into account the single-core beam quality, the core cross-section of the individual light guides should preferably be designed to be as large as possible. For this purpose, geometries such as tapered large-core fibers or other known large-core fiber designs, which are known to support optimal beam quality even with large core areas, may be helpful. The diameter of the core of the individual light guides should preferably be greater than 5 times the wavelength of the propagating light, preferably greater than 10 times, more preferably greater than 25 times, and particularly preferably greater than 50 times.

[0022] To avoid optical coupling, the distance between the individual fiber cores is preferably as small as possible. In this case, optical barriers within the multichannel lightguide structure help prevent excessive coupling. In the context of the present invention, "optical decoupling of the individual lightguides" means that, over the total length of the multichannel lightguide, the power propagated in the individual lightguides is preferably lost by less than 10%, preferably less than 5%, and preferably less than 1%, due to power transfer to other individual lightguides.

[0023] The incoherent superposition of the individual emissions is an optical transformation of an arbitrary cross section in the beam path, which is located in the region behind the exit end of the multi-channel light guide.

[0024] It should be noted that the advantages of multi-channel light guides over multimode large-core fibers are realized when the individual light guides of spatially coherent emission are closely packed, so that the total area of ​​the resulting incoherent superposition of the coherent individual emissions does not become unnecessarily large. These design criteria can be specifically implemented in multi-core fibers, as the known concept of multi-core fibers ideally supports dense packing of the individual cores. Ideally, in the case of multi-channel light guides, the ratio of the distance between the cores of the individual light guides to the core diameter should be less than 20, preferably less than 10, more preferably less than 5, and particularly preferably less than 3.

[0025] The advantages of the method of the present invention are summarized as follows:

[0026] The optical system has a simple and compact design.

[0027] In a fiber-based implementation, the optical system of the present invention as a laser system (see below) offers high efficiency and can directly pump a multi-channel light guide as the lasing medium with semiconductor diodes.

[0028] The geometry of a multi-channel light guide as an elongated waveguide, for example 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 area can be used to dissipate the introduced heat. Therefore, this approach offers the possibility of emitting high average powers.

[0029] The inversion stored in the doped core and therefore the extractable light output is determined by the nature of the dopant and the doping concentration, which defines and limits the extractable light power of each individual lightguide. Multichannel lightguides increase the extractable power according to the number of individual lightguides.

[0030] Various rare earth elements are possible dopants: ytterbium ions can address the wavelength range of approximately 1 μm, erbium ions around 1.5 μm, and thulium ions around 2 μm. It is also conceivable that the optical fiber cores differ from each other in terms of doping. This makes it possible to achieve (optionally also dynamically) varying the emission wavelength by selecting the pump wavelength.

[0031] The requirements regarding nonlinear effects and material degradation are distributed over several individual light guides, which in turn improves the overall performance.

[0032] According to the present invention, the superposition of the individual transmissions is incoherent, meaning the relative phases of the individual transmissions have no influence. Consequently, the elements for detecting and stabilizing the phase positions of the individual channels, as is required for coherent superposition, are omitted. Consequently, the present invention results in a very simple design.

[0033] Due to the incoherent superposition, the exact position of the individual light guides in the cross section of the multichannel waveguide is not critical, thus allowing a great deal of freedom in the design of the multichannel waveguide. There are also large tolerances in production regarding the positional accuracy and dimensions of the individual light guides.

[0034] As mentioned above, the beam quality of the incoherent superposition of the multi-core light guide is significantly better than that of a multimode fiber with equivalent area.

[0035] These advantages make the optical system according to the invention (implemented as a laser system, i.e. having a multi-channel light guide as laser medium) particularly suitable for the above-mentioned applications, i.e. the incoherent superposition laser radiation generated thereby can be used to generate ultraviolet light (in particular EUV light) from laser-induced metal plasma or gas plasma (e.g. tin plasma) for material processing by laser shock peening, for separating thin films from substrates by laser ablation, or also for generating laser pulses for fragmenting kidney stones or bladder stones (lithotripsy).

[0036] These advantages also apply without reservation to optical systems with purely passive multichannel light guides (without core doping), for example, as transmission fibers. In addition to the advantages in terms of beam quality, the following is also important: in conventionally used passive multimode transmission fibers, the inherent coherence of the different transverse modes with one another results in the resulting intensity distribution being highly sensitive to relative changes in the phases of the individual transverse modes. In passive multimode fibers, these phase changes originate from external influences (e.g., position changes or stresses caused by fiber contacts); in active fibers, the laser-induced heat input predominates. These disadvantages are eliminated by the present invention.

[0037] In one possible embodiment, the cores of the individual light guides may have different diameters. At least one of the cores may also be designed as a hollow core.

[0038] In another possible embodiment, the individual light guides are individual doped light guide fibers, preferably (rare earth ion doped) dual core fibers, or also individual active light guide fibers in a multi-channel light guide combined as a single fiber.

[0039] The superposition optics of the optical system may be variable and designed to generate (optionally dynamically generate) different beam profiles in the target plane.

[0040] In one possible embodiment, the individual light guides are arranged linearly or in an array or matrix in the cross section of the multichannel light guide. The "packing density" of the individual light guides can also be increased by a hexagonal arrangement. A random distribution of the individual light guides over the cross section of the multichannel light guide is also conceivable. Furthermore, in order to increase the filling factor, i.e., the proportion of the cross section through which light passes, it is conceivable (and subsequently) to enlarge the mode field diameters of the individual cores at the inlet and / or outlet end of the multichannel light guide (e.g., by targeted heat input, i.e., thermal expansion or tapering of the multicore fiber).

[0041] Another possible way to increase the filling factor of a multi-core emission is to use a lens array, in particular a microlens array external to the multi-channel light guide. This can, for example, be placed directly in front of the exit end of the multi-channel light guide (at a corresponding working distance), but can also be placed further along 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 means of the lens array, the filling factor can be increased by at least a factor of 1.2, preferably by at least a factor of 1.5, and more preferably by a factor of at least 2, with higher factors also possible.

[0042] In one possible embodiment, the individual light guides of the multi-channel light guide each form the laser medium in the optical resonator. If no coupling occurs between the individual light guides, all individual light guides can perform laser emission independently of each other. Therefore, one or more free beam resonators can be placed around the multi-channel light guide (by means of a suitable reflector arrangement), which are used by all individual light guides, but each individual light guide performs laser oscillation independently, i.e. independently of the other individual light guides. This allows the realization of superimposed incoherence in the target plane. It is also conceivable that the independent optical resonators can be realized by reflective coated end faces of the individual light guides or by Bragg gratings (FBGs) engraved on the ends of the individual light guides. Even Fresnel reflection at the free ends of the individual light guides is sufficient to form an optical resonator without further measures.

[0043] Other elements can also be arranged within or outside the resonator, such as a (temporal) light modulator for generating pulsed laser radiation (by mass switching, cavity dumping or mode coupling).

[0044] A MOPA (micro-oscillator amplifier) ​​arrangement can also be implemented. Here, for example, a laser oscillator with a multichannel lightguide designed according to the present invention generates low-power laser radiation, whereby the emission pattern consists of spatially incoherent beams. The laser oscillator can be operated in a temporal operation regime (via a suitable modulation scheme) depending on the application requirements (continuous wave emission (CW) or pulsed to ultrashort laser pulses). Alternatively, a conventional single emitter can be used as the light source, whereby its emission is appropriately split between the individual lightguides of the multichannel lightguide. The laser light generated in this way is coupled downstream into an active laser multichannel lightguide with an appropriate number and arrangement of individual lightguides, where it is amplified to a higher power (and possibly pulse energy). This procedure can also be repeated, i.e., additional multichannel lightguides can be connected in series as amplifiers. The individual emitted modes can also be coupled into a multichannel lightguide serving as a passive transmission fiber. Thus, the individual emissions can be transmitted to the application. Frequency conversion processes (e.g., four-wave mixing or Raman scattering) can also be driven in the individual lightguides of the multichannel lightguide downstream of the amplifier.

[0045] When using a single emitter as a light source in the optical system of the invention implemented as a MOPA system, it must be ensured that for the incoherent superposition at the output of the downstream multi-channel light guide, the path length of the individual emissions to superposition in the target plane is greater than the coherence length of the light originating from the single emitter. Therefore, all light sources with a sufficiently large spectral width are conceivable, for example, mode-coupled ultrashort pulse lasers or the (optionally time-stretched) emission of superluminescent diodes.

[0046] In the MOPA concept, an optical modulator (e.g., arranged between an oscillator and an amplifier) ​​can modify the temporal characteristics of the emission, for example, in order to generate pulses from a temporally continuous laser light, or in order to adapt the pulse shape to the requirements of the application (e.g., to generate a pre-pulse or an overboost at the beginning of the pulse), or to shape the pulse in order to influence saturation-related pulse shaping in the amplifier.

[0047] The method also offers the possibility of emitting the individual light guides independently of one another in time, for example, by independently temporally modulating the laser light in the individual channels of a multichannel light guide. Thus, it is conceivable, for example, to generate one or more prepulses from a certain number of individual light guides and to emit a higher-energy main pulse from another individual light guide. To this end, it is sufficient to time-shift the light pulses into the individual light guides. For example, by appropriately doping the individual light guides or by corresponding frequency modulation of the pump light, the prepulses can have an emission wavelength different from that of the main pulse. One possible embodiment provides that the cores of the individual light guides have different diameters, so that the one or more prepulses and the main pulse generate different spot sizes in an incoherent superposition. In another possible embodiment, one or more individual light guides are formed by a hollow fiber core, suitable, for example, for transmitting ultrashort laser pulses with high peak pulse powers, and are surrounded by the other individual light guides of an active or passive multichannel light guide.

[0048] In another possible embodiment, the optical system may include two or more multi-channel light guides, whereby the corresponding superposition optical unit is designed to superimpose the total emissions of the two or more multi-channel light guides in a spatial region. For example, the individual emissions may enter the spatial region from different spatial directions and be incoherently superimposed there.

[0049] The total emission of the optical system can also be the superposition of the emissions of two multi-channel light guides with orthogonal polarizations at a polarizer. In addition, the total emission can be the superposition of the emissions of two or more multi-channel light guides with different wavelengths at one or more spectrally selective elements (e.g., volume Bragg gratings, dichroic mirrors, prisms, gratings, prism gratings, or combinations of these elements).

[0050] In another possible embodiment, a nonlinear optical element can be provided for frequency conversion of the superimposed light emissions of the individual light guides. Frequency conversion can be performed in a conventional crystal-based manner (generating second or third harmonics, etc.) or in a laser-induced plasma (e.g., in a gas target or a metal target, such as a tin target for generating EUV radiation with a wavelength of 13.5 nm). This approach is particularly advantageous because the emission from the plasma is spatially incoherent, and therefore the nonlinear process does not require spatially coherent radiation. As a prerequisite for efficient frequency conversion, it is sufficient to achieve the necessary light intensity. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] In the following, embodiments of the present invention are explained in more detail with reference to the accompanying drawings. They show:

[0052] Figure 1 : Experimental demonstration of the incoherent superposition of a total emission consisting of several individual emissions in a common focused spot;

[0053] Figure 2 : According to the present invention, different beam profiles are generated by incoherent superposition;

[0054] Figure 3 : Schematic diagram of the first embodiment;

[0055] Figure 4 : A schematic diagram of a second embodiment;

[0056] Figure 5 : A schematic diagram of a third embodiment;

[0057] Figure 6 : A schematic diagram of a fourth embodiment;

[0058] Figure 7 : A schematic diagram of a fifth embodiment;

[0059] Figure 8 : A schematic diagram of a sixth embodiment;

[0060] Figure 9 : A schematic diagram of a seventh embodiment;

[0061] Figure 10 : A schematic diagram of an eighth embodiment;

[0062] Figure 11 : A schematic diagram of a ninth embodiment;

[0063] Figure 12 : A schematic diagram of a tenth embodiment;

[0064] Figure 13 : Schematic diagram of different configurations of multi-channel light guides;

[0065] Figure 14 : Other possible configurations of multi-channel light guides;

[0066] Figure 15 : Other possible configurations of multi-channel light guides;

[0067] Figure 16 : Other possible configurations of multi-channel light guides;

[0068] Figure 17 : Other possible configurations of multi-channel light guides;

[0069] Figure 18 : Schematic diagram of incoherent frequency conversion using the optical system of the present invention. DETAILED DESCRIPTION

[0070] Figure 3The basic structure of the optical system according to the present invention is shown. It comprises a multi-channel light guide 1 comprising a plurality of individual light guides extending parallel to one another, in this case in an array of 4×4 individual light guides (visible in the cross-sectional view on the left). The individual light guides are each formed by a light guide core (black circle) surrounded by a common (circular in cross section in this case) cladding of the multi-channel light guide 1. A superposition optical unit 2 is provided to provide a superposition optical unit 2 at the outlet end (at the Figure 3 In FIG, the right end of the multi-channel light guide 1 in FIG, the divergent light emissions of the individual light guides are incoherently superimposed in the target plane 3. It can be seen that the individual emitted light beams meet at different angles in the target plane 3.

[0071] As mentioned above, the superimposed light from the individual emissions of the multi-channel light guide 1 is characterized by a high beam quality compared to a multimode optical fiber of equivalent area. Figure 1 A further advantage is shown. The uniformity and stability of the focus obtained in the target plane 3 or at the point of use are also advantageous. For example, the size of the individual emissions can be adjusted by means of the 4f imaging system, for example magnification, whereby the relationship between the diameter of the individual emissions and the distance of the individual emissions remains unchanged. A further lens focuses these individual emissions extending in parallel into the target plane 3 of application. The light beams of each individual emission are thus focused. The different light beams therefore hit the same focused spot at different angles and overlap incoherently there. This results in an intensity distribution in the focused spot with a correspondingly increased power density. The increase in the power density at the point of use is therefore "traded" for an increased angular spectrum; as described above, this angular spectrum is generated by the lateral spread of the individual emissions forming the total emission. In Figure 1 In the figure, a cross section of the beam path is shown in each case, the emission of the ytterbium-doped multi-channel light guide according to the invention (mode field diameter 28 μm, core-to-core distance 82 μm) is magnified 33 times using a lens system and focused on the target plane 3 using another lens (f=40 mm). Figure 1 The top row of images in the figure shows the emission from a single light guide / core (left), an array of nine light guides / cores (center), and an array of 21 light guides / cores (right). The bottom row of images shows that incoherent superposition generates a uniform Gaussian-shaped intensity distribution, and the size of the intensity distribution does not change when more individual emissions are added (the spot diameter is approximately 70 μm). Assuming uniform distribution of optical power in the individual light guides, the power density increases by a factor of 21 (equal to the number of cores).

[0072] In contrast, imaging in multimode fibers cannot generate such a uniform intensity distribution in the focus due to the coherence of the individual transverse modes with each other and the different phases imposed on the radiation during its propagation in the multimode fiber. A speckle pattern is always generated, which changes over time due to variations in the relative phase positions of the different transverse modes (e.g., due to small interferences). This makes the generated light unusable for many applications. The present invention herein provides a remedy.

[0073] Figure 3 The basic method of the invention shown also offers the possibility to generate a variety of beam profiles and to switch dynamically between them. By superposition of corresponding incoherent beams behind a multi-channel light guide, for example Figure 2 Beam profile shown. The beam profile is determined by suitable 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 different distances behind a lens, which is located at the focal length (here 60 mm) from the exit end of the multichannel waveguide 1. Each of these temporally stable and spatially uniform patterns (left: flat top, middle: super Gaussian profile, right: Gaussian profile) can be imaged with corresponding imaging optics onto a target plane 3 with a corresponding target size and the associated target power density can be obtained based on the emission power characteristics of the individual emissions. In addition to small spot sizes and high power densities, spatially flat (flat top) profiles can also be provided for the corresponding applications (e.g., for the highest pulse energies for laser shock peening). By adjusting the imaging optics, it is possible to switch between these profiles, even dynamically.

[0074] The proposed approach also offers the possibility to dope individual light guides with different dopants, thereby changing the emission wavelength by switching the pump wavelength (e.g., 793 nm for thulium and between 910-980 nm for ytterbium), whereby all emissions still represent an incoherent superposition and thus generate a flat-top beam or a focus that can be modulated in its wavelength.

[0075] Figure 4 Another embodiment of a multi-channel light guide as a laser oscillator is shown, which in its simplest version emits continuous laser radiation. The laser resonator is surrounded by a single light guide that is actively doped (e.g. with ytterbium, erbium or thulium ions), the core of which ( Figure 4The pump cladding (shown in shaded areas) is integrated into the common pump cladding and can be formed, for example, by coating or Fresnel reflection on the end faces of the individual light guides. The laser resonators of the individual light guides can also be formed by introducing fiber Bragg gratings (FBGs). The FBGs can be inscribed independently in each core, but it is also possible to do so across the entirety of the individual cores. The FBGs form independent resonators for each core, ensuring incoherent emission between the cores.

[0076] Figure 5 Another embodiment is shown in which mutually incoherent pulsed radiation is generated from the individual emissions of an actively doped multi-channel light guide (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 temporally modulated individual emissions are then propagated via a superposition optical system 2 to a target plane 3 (not shown here).

[0077] exist Figure 6 In another embodiment, the continuous or pulsed individual emissions generated by the multicore oscillator 5 are amplified in one or more further actively doped multichannel optical waveguides (multicore amplifiers 7 ) and subsequently incoherently superimposed using a superposition optical unit 2 .

[0078] exist Figure 7 In another embodiment, the emission of laser system 9 is transmitted in a separate light guide of a passive multi-channel light guide (multi-core transmission fiber 8). To this end, the emission of laser system 9 (optionally a multi-core laser system, e.g., consisting of a multi-core oscillator 5 and a multi-core amplifier 7) can be coupled into multi-core transmission fiber 8 (free beam coupling or via a spliced ​​fiber connection) and guided therein, for example, to an application. Subsequently, incoherent superposition is performed again using superposition optical unit 2.

[0079] Incoherent superposition can be achieved using different superposition optical units 2. The simplest example is a single lens 10 which superimposes the different emissions in the target plane 3. Figure 8 An example of such a superposition is shown using a 4x4 arrangement of multi-channel light guides (multi-core optical fiber) having 16 signal cores.

[0080] In order to adjust the size or spatial extent of the superposition, and thus the resulting intensity, another optical system 11 (e.g. a telescope consisting of two lenses) can be used, such as Figure 9 As shown, it images the intermediate plane 12 onto the target plane 3 .

[0081] exist Figure 10In the embodiment, the superposition is performed by a cylindrical lens 13 which is suitable for generating an elliptical beam in the intermediate plane 12 or in the object plane 3; by correspondingly selecting the focal length of the cylindrical lens 13 and the spacing of the elements, a uniform line focus is obtained. Figure 10 A top view (top) and a side view (bottom) of the structure are shown.

[0082] exist Figure 11 In the embodiment in FIG, a lens array 14, optionally a microlens array, is used behind the multi-channel light guide 1, whereby ideally a microlens is placed in the beam path of each individual emission. In this way, the beam diameters of the individual emissions can be varied depending on their spacing. This makes it possible, for example, to increase the spatial packing density, i.e. the filling factor of the total emission, i.e. the ensemble of the individual emissions. Subsequently, an incoherent superposition is performed on the target plane 3 or the intermediate plane 12 by means of the superposition optical unit 2. 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 filling factor. At greater distances, a conventional lens array 14 can also increase the filling factor.

[0083] Figure 12 It is shown that the pulses emitted by the separate light guides arrive at the target plane 3 simultaneously or at arbitrarily offset times (eg two sets of pulses with a time difference Δt).

[0084] In addition to superimposing the optical unit 2, the multi-channel light guide 1 itself can also be adapted to subsequent applications. Thus, the refractive index profiles of the individual cores of the multi-channel light guide 1 designed as an active or passive multi-core fiber can be adapted to achieve a specific output beam profile. This illustrates Figure 13 Therefore, in conventional step-index fibers, a Gaussian beam profile with an adapted refractive index profile ( Figure 13 a) Forming a flat so-called "flat top" beam profile ( Figure 13 b) There are many design degrees of freedom. The refractive index profile of the individual light guides can be adapted to find a compromise between the distance of the individual transmissions, the beam quality of the individual transmissions, and the coupling of the individual transmissions. Special fiber designs for the individual light guides are also possible, such as photonic crystal fibers or large-pitch fibers.

[0085] Furthermore, different dopants 15 (e.g. ytterbium, erbium or thulium) may be introduced into the cores of different individual light guides, e.g. Figure 14a. This allows emission at different wavelengths by simply switching the pump wavelength, whereby the individual emissions are incoherently superimposed. It should be noted that different core diameters of the individual light guides can be chosen for the different wavelengths in order to achieve the same spot diameter during superposition. For example, the emission wavelength of thulium doping is approximately 2 μm, which means that the beam parameter product is, by definition, twice worse than the emission at 1 μm (even assuming a diffraction-limited beam quality). However, longer wavelengths also allow the core diameter to be increased by a factor of two, while the V parameter remains unchanged, and therefore the number of modes in the individual light guides remains unchanged.

[0086] In general, the core geometries of the individual light guides can be designed differently. Figure 14 In Figure 2b, fiber cores 16 of different sizes are introduced into the multi-channel light guide to generate beams of different diameters at the target plane 3. This can be combined with the use of different dopants in the individual light guides so that different emission wavelengths can generate different spot sizes and intensities at the target plane 3. In addition to using conventional active or passive cores in separate light guides, one or more passive (optionally gas-filled) hollow core waveguides 17 can also be integrated, such as Figure 14 As shown in c.

[0087] The arrangement of the individual light guides in the cross section of the multi-channel light guide is not limited to a rectangular pattern, e.g. Figure 15 a. Linear pattern ( Figure 15 b) or polygonal pattern ( Figure 15 c) is also possible. Since the fill factor of the total emission increases, the hexagonal positioning of the individual light guides ( Figure 15 d) is beneficial. Figure 15 As shown in Figure e, random positioning of the individual light guides is in principle possible even if the distances between the cores are different.

[0088] If the core spacing of the individual light guides in the active or passive multi-channel light guide 1 of the present invention is small, the optical coupling between the individual light guides can be avoided or reduced by introducing an optical barrier. Figure 16 As shown, this can be achieved by using materials 18, 19 with different refractive indices ( Figure 16 a and Figure 16 b) or by using air holes 20 between or around the individual cores ( Figure 16 c) to achieve.

[0089] In addition to the transverse structure of the multi-channel light guide 1, a longitudinal structure is also applicable. Figure 17As shown in Figure a, the diameter can be varied in one section or over the entire length ("taper" 21), which can positively influence the beam quality of the individual emissions. Such a tapered structure can be advantageous in a multi-channel waveguide 1 as a multi-core oscillator 5, but is also particularly advantageous in a multi-core amplifier 7 or a multi-core transmission fiber 8. It is also advantageous to vary the emission size of the individual cores while (optionally) keeping the outer diameter of the multi-channel waveguide 1 constant, since this can increase the filling factor of the total emission. This is particularly advantageous in Figure 17 b is schematically shown at 22. Both an increase in the size of the individual cores at the outlet (associated with a decrease in the core spacing) and a decrease in the size of the individual cores (associated with an increase in the mode area at the same core spacing) have a positive effect on the filling factor.

[0090] at last, Figure 18 As a selected application of the optical system according to the invention, the generation of a laser-induced plasma in a gas or solid for frequency conversion at 24 is shown. The incoherent superposition of the individual emissions with good overall beam quality and uniform intensity distribution in the target plane 3 is well suited for the incoherent frequency conversion process. 11 W / cm 2 Power densities in the range of 10 17 W / cm 2 At intensities of 1.5 Å, emission in the hard X-ray range is possible.

Claims

1. An optical system for generating or guiding light, the optical system having a multi-channel light guide (1) comprising a plurality of individual light guides extending parallel to one another, and having a superposition optical unit (2) designed to superimpose the light emissions from the individual light guides in a target plane (3) at the outlet end of the multi-channel light guide (1), wherein: The superposition of the light emissions of the individual light guides is incoherent in the target plane (3).

2. The optical system according to claim 1, wherein Viewed from the cross section of the multi-channel light guide (1), the individual light guides have a linear or array arrangement.

3. The optical system according to claim 1 or 2, wherein: The individual light guides are each formed by a light guide core or another light guide structure.

4. The 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. The optical system according to claim 3 or 4, wherein: At least one of the optical fiber cores, preferably a portion of the optical fiber cores, particularly preferably all of the optical fiber cores, is doped with rare earth ions, preferably erbium, ytterbium or thulium, in order to achieve optical amplification.

6. The optical system according to claim 5, wherein: The light guide cores differ from one another in terms of doping.

7. The optical system according to claim 5 or 6, wherein: The common cladding is used to guide pump light to optically pump at least one doped core.

8. The optical system according to any one of claims 1 to 7, wherein: The cores have different diameters.

9. The optical system according to any one of claims 1 to 8, wherein: At least one core is formed as a hollow core.

10. The optical system according to claim 1 or 2, wherein: The separate light guides are doped light guiding fibers, preferably dual core fibers.

11. The optical system according to any one of claims 1 to 10, wherein: The individual light guides are optically decoupled from each other.

12. The optical system according to any one of claims 1 to 11, wherein: The light emission of the individual light guides is almost diffraction limited.

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

14. The optical system according to any one of claims 1 to 13, wherein: The superposition optical unit (2) comprises a lens array (14), wherein each lens of the lens array (14) is associated with one or more individual light guides.

15. The optical system according to any one of claims 1 to 14, wherein: The individual light guides each form a laser medium in an optical resonator.

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

17. An optical system according to any one of claims 1 to 16, comprising two or more multi-channel light guides (1), wherein: The superposition optical units (2) respectively associated with the multi-channel light guides (1) are designed to superpose the emissions of the two or more multi-channel light guides (1) in a spatial region.

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

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

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