Apparatus and method for transmitting pulsed laser radiation using hollow-core optical fibers

Abstract

A device (11) for transmitting pulsed laser radiation having a pulse energy that results in a high-intensity laser pulse (3C) with a compressed pulse duration comprises a pulse duration setting device (13), a hollow-core fiber (1), and a fiber coupling input device (17). The pulse duration setting device is configured to receive the pulsed laser radiation and to set the transmission pulse duration (Δt_t) of the laser radiation. The hollow-core fiber has a hollow core (4A) surrounded by a material (4B), wherein the hollow-core fiber (1) transmits a laser pulse (3B) coupled into the first fiber end (1A) in the hollow core (4A) to a second fiber end (1B) and outputs the laser pulse at the second fiber end. The hollow-core fiber (1) is configured to operate with a beam trajectory parameter value (X, α) present at the first fiber end (1A), the beam trajectory parameter value being within a target tolerance range (ΔX_S). The fiber coupling device is configured to receive laser radiation set to the transmission pulse duration (Δt_t) and couple the laser radiation into the hollow core fiber (1) with a beam trajectory parameter value (X, α), the beam trajectory parameter value being within the target tolerance range (ΔX_S). The transmission pulse duration (Δt_t) is set in such a way that for all beam trajectory parameter values (X, α) within the target tolerance range (ΔX_S), the laser radiation is coupled into the hollow core (4A) in such a way that the material and / or structure (37) of the hollow core fiber (1) is retained during the coupling.

Description

Technical Field

[0001] The present invention relates to a device for transmitting high-power laser pulses using a hollow-core optical fiber. In addition, the present invention relates to a method for transmitting high-power laser pulses using a hollow-core optical fiber. Background Art

[0002] In conventional optical fibers, such as step-index solid-core fibers, the guidance of laser radiation can be affected by possible interactions of the laser light with the fiber material. In particular, damage to the fiber material caused by this interaction limits the maximum transmittable pulse energy and pulse power, making it impossible to use solid-core fibers with high-intensity laser radiation, in particular with high-power laser pulses. High-power laser pulses are understood herein to be so-called ultrashort pulses having a pulse duration in the range of a few femtoseconds (fs) to a few hundred picoseconds (ps) and a pulse energy in the range of microjoules (μJ) to millijoules (mJ). High-power laser pulses can be constructed in various spectral ranges, for example in the IR spectral range, the green spectral range, the UV spectral range, and the deep UV spectral range.

[0003] Optical fibers with a hollow fiber core, in which the high-intensity laser radiation propagates, can avoid or at least reduce interactions with the fiber material that could damage the fiber. Accordingly, optical fibers with a hollow core (hollowcore fibers) and, in particular, photonic crystal fibers (HCPCFs) with a hollow core are suitable for transmitting high-power laser pulses. Such fibers have a light-guiding region in which laser pulses with pulse durations in the nanosecond to femtosecond range can be transmitted. In this light-guiding region, the peak intensity does not reside in the fiber material, but rather in a region that is essentially free of solids. Filling the hollow core with, for example, air or nitrogen, or evacuating it, can also allow the optical properties of the fiber, such as its nonlinearity, to be adapted.

[0004] The concepts described below relate particularly to the transmission of laser pulses using hollow-core optical fibers (also referred to herein as hollow-core fibers or simply hollow fibers), such as HCPCFs. In the case of HCPFs, a hollow core that guides the laser radiation is surrounded by a photonic "crystal" structure. The photonic structure is designed in such a way that coupling of light from the hollow core into the photonic structure is optically inhibited or impossible, thereby keeping the laser radiation "trapped" in the hollow core. Such light guiding is also referred to as "photonic-band-gap-guiding," "inhibited-coupling," or "anti-resonant-guiding," depending on the fiber type. Exemplary fiber types include inhibited-coupling (IC) HCPCFs (e.g., Kagome-type or hypocycloidal-core Kagome-type HCPCFs) or bandgap HCPCFs.

[0005] The transition between the hollow core and the photonic structure forms the (innermost) core wall structure. If laser radiation is coupled into a hollow-core fiber, then during transmission, typically only a low intensity is present in the region of the core wall structure; due to the design of the fiber, the main portion of the power remains in the center of the hollow core.

[0006] In general, for achieving good light guidance efficiency with optical fibers, it is important that the laser radiation to be coupled in is matched to the optical fiber parameters with regard to beam propagation parameters, such as spot size and divergence.

[0007] Additionally, the position of the beam path relative to the fiber end into which it is coupled is important. In other words, for good coupling into a hollow-core fiber, the beam path of the laser radiation should be adapted to the subsequent trajectory of the light-guiding region of the fiber in terms of its position relative to the fiber end (beam center position) and angle of incidence. Beam center position and angle of incidence are also referred to herein as beam trajectory parameters.

[0008] For coupling in, fibers have so-called beam trajectory parameter tolerances. These tolerances are related to the position of the beam center and the angle of incidence of the incident laser radiation, and describe the position range and angle of incidence range within which the focused laser beam can strike the fiber without damaging it. For example, depending on the fiber type, the permissible beam center position range and angle of incidence range lie within a range of ±5% to ±10% of the mode field diameter and within a range of ±5% to ±10% of the beam divergence.

[0009] Fiber coupling devices are typically used to ensure the correct alignment of the beam path required for coupling. This alignment involves, for example, positioning the coupling lens relative to the light-guiding region. Thus, misalignment of the coupling lens relative to the light-guiding region can impair coupling efficiency. During operation, the beam path (beam center and angle of incidence) should remain as stable as possible relative to the fiber end; in other words, the beam trajectory parameters in the coupling region should be within tolerances.

[0010] After the laser radiation is transmitted by the optical fiber, the laser radiation emitted from the optical fiber is delivered to the subsequent optical components by means of a fiber coupling output device. Summary of the Invention

[0011] One aspect of the present disclosure is based on the task of enabling the transmission of high-power laser pulses with a hollow-core optical fiber while maintaining a sufficient tolerance window with respect to the coupling of the laser radiation into the hollow-core optical fiber.

[0012] At least one of these objects is achieved by a device for transmitting pulsed laser radiation according to claim 1 or claim 15 or claim 16 and by a method for transmitting pulsed laser radiation using a hollow-core fiber according to claim 11. Developments are described in the dependent claims.

[0013] In one aspect, an apparatus for delivering pulsed laser radiation having a pulse energy resulting in high-intensity laser pulses (3C) with compressed pulse duration, the apparatus having:

[0014] a pulse duration setting device, which is designed to receive the pulsed laser radiation and to set a transmission pulse duration of the laser radiation,

[0015] A hollow-core fiber having a hollow core surrounded by a material, wherein the hollow-core fiber transmits laser pulses coupled in at a first fiber end in the hollow core to a second fiber end and outputs the laser pulses from the second fiber end, wherein it is further provided that the hollow-core fiber is operated with beam trajectory parameter values present at the first fiber end, said beam trajectory parameter values being within a target tolerance range, and

[0016] A fiber coupling input device is configured to receive laser radiation set to the transmission pulse duration and couple the laser radiation into the hollow-core optical fiber with a beam trajectory parameter value within the target tolerance range.

[0017] Here, the transmission pulse duration is set in such a way that for (all) beam trajectory parameter values within the target tolerance range, the following coupling-in of the laser radiation into the hollow core is given: in this coupling-in, the material and / or structure of the hollow-core optical fiber is retained.

[0018] In another aspect, a system for providing high-intensity laser pulses for material processing of a workpiece includes: a laser pulse source configured to output laser pulses having a spectral width and a pulse energy required for material processing as pulsed laser radiation; an apparatus for transmitting the pulsed laser radiation, as described above, comprising a hollow-core fiber and a pulse duration setting device; and a laser processing head configured to focus the laser pulses emitted from the hollow-core fiber onto the workpiece for material processing. In some embodiments, the pulse duration setting device of the apparatus for transmitting the pulsed laser radiation can be configured as a separate component assigned to the hollow-core fiber or as part of the laser pulse source (a pulse duration stretching device or (non-maximum compression) pulse duration compression device disposed therein).

[0019] In another aspect, a method for transmitting pulsed laser radiation from a laser pulse source using a hollow core optical fiber having a hollow core surrounded by a material, wherein the hollow core optical fiber is configured to transmit the laser radiation in the hollow core from a first fiber end to a second fiber end, the method comprising the steps of:

[0020] generating pulsed laser radiation comprising a laser pulse having an initial pulse duration,

[0021] setting the peak power of the pulsed laser radiation present at the first optical fiber end, and

[0022] coupling the pulsed laser radiation into the hollow-core optical fiber,

[0023] The peak power is set by extending the pulse duration from the initial pulse duration to the transmission pulse duration or by non-maximally compressing the laser pulses in such a way that, for beam trajectory parameter values at the first fiber end that are within the target tolerance range provided for operating the hollow-core fiber, coupling of laser pulses with the transmission pulse duration into the hollow core takes place, with the material and / or structure of the hollow-core fiber being maintained.

[0024] In some embodiments of the device, the target tolerance range can be defined by permissible fluctuations in the beam parameters of the pulsed laser radiation during operation. These beam parameters can include, in particular, the beam center position and / or the angle of incidence. The permissible fluctuations in the beam center position can be a few percent of the mode field diameter, for example, within a range of ±1%, ±2%, ±5%, or ±10% of the mode field diameter, and / or the permissible fluctuations in the angle of incidence can be a few percent of the beam divergence, for example, within a range of ±1%, ±2%, ±5%, or ±10% of the beam divergence. Furthermore, the hollow-core fiber can be assigned a maximum pulse peak power within the target tolerance range, above which no damage-free coupling into the hollow-core fiber is achieved across the entire target tolerance range. Accordingly, the transmission pulse duration of the laser radiation at the first fiber end is set such that the peak power of the laser radiation at the first fiber end is less than or equal to the maximum pulse peak power.

[0025] In some embodiments, the hollow-core fiber can include structural elements, in particular glass structures or photonic crystal structures, which form an inner core wall surrounding the hollow core. The pulse duration setting device and the fiber coupling device can be configured such that, for beam trajectory parameter values within a target tolerance range, in particular at the extreme values of the target tolerance range, the intensity distribution of the laser radiation and the spatial overlap of the structural elements is such that the intensity in the region of these structural elements is below a threshold intensity for destruction of the core wall.

[0026] In some embodiments, the pulse duration setting device can be configured as a pulse duration stretching device to stretch the laser pulses of the pulsed laser radiation having an initial pulse duration by a stretching factor to the transmission pulse duration via at least one optical element and to output these laser pulses as temporally stretched laser pulses.

[0027] Optionally, the pulse duration setting device may comprise a diffraction grating, a volume Bragg grating, a prism, a grating prism and / or a dispersive mirror for use in transmission or in reflection.

[0028] In some embodiments, the transmission pulse duration can be set by means of the pulse duration setting device in such a way that damage-free coupling into the hollow-core fiber occurs within the target tolerance range with respect to the material of the hollow-core fiber.

[0029] Alternatively or additionally, the pulse duration setting device designed as a pulse duration stretching device can be configured to stretch an initial pulse duration in the range of 50 fs to 1 ps to the transmission pulse duration by a stretching factor of at least 1.5, in particular at least 10 or at least 100.

[0030] In some configurations, the pulse duration setting device can be provided as a pulse duration compression device to compress the laser pulses of the pulsed laser radiation (only) with respect to the pulse duration to the transmission pulse duration.

[0031] In some embodiments, the pulse duration setting device, in particular as a pulse duration compression device, can be configured to temporally compress the pulse duration of laser pulses of the pulsed laser radiation to the transmission pulse duration via at least one optical element, the laser pulses being output by the laser amplifier system with a pulse duration significantly longer than the minimum possible pulse duration associated with the existing spectral bandwidth, and thus in particular outputting the laser pulses as temporally stretched laser pulses relative to the minimum possible pulse duration. Optionally, the pulse duration setting device can also be configured to set the pulse duration of the laser pulses to the transmission pulse duration by a stretching factor of at least 1.5, in particular at least 10 or at least 100, relative to the minimum possible pulse duration in the range of 50 fs to 1 ps. Furthermore, an additional safety factor of at least 2, 10 or 100 can be taken into account in determining the transmission pulse duration.

[0032] In some embodiments, the beam parameters of the pulsed laser radiation may include the following group of parameters:

[0033] - beam propagation parameters, including mode field diameter, aperture angle and beam quality value,

[0034] - pulse parameters, including pulse energy, pulse duration, and repetition rate, and

[0035] - beam tracking parameters, which include the beam center position at the first fiber end and the direction of incidence of the pulsed laser radiation onto the first fiber end.

[0036] Here, the peak power of the pulsed laser radiation can be adapted by means of the pulse duration setting device in such a way that, under fixedly set beam propagation parameters and fixedly set pulse parameters, the laser radiation can be coupled into the hollow core in a non-destructive manner when the beam trajectory parameters are within the target tolerance range.

[0037] In some embodiments of the system, the laser pulse source can include a laser amplifier system and a pulse duration compression device.

[0038] In some embodiments of the system, the laser processing head can include a pulse duration matching device configured to shorten the transmission pulse duration to the processing pulse duration. Optionally, the laser processing head can include at least one diffraction grating, volume Bragg grating, prism, grating prism, and / or dispersion mirror for use in transmission or reflection.

[0039] In some embodiments of the system, the device for transmitting pulsed laser radiation can be configured such that, without extending the initial pulse duration, the pulse energy at the first fiber end results in a peak power that causes damage to the hollow-core fiber. In some embodiments of the system, the device for transmitting pulsed laser radiation can be configured such that, in the event of maximum possible dispersion compression of the laser pulses, the pulse energy at the first fiber end results in a peak power that causes damage to the hollow-core fiber. Damage may typically occur in the event of compression to a pulse duration shorter than the transmission pulse duration.

[0040] In some extended solutions, the method may further include the following steps:

[0041] Determine the maximum pulse peak power coupled in with the beam trajectory parameter values within the entire target tolerance range, and

[0042] The transmission pulse duration is set in such a way that the peak power of the pulsed laser radiation, in particular the laser pulses, is less than or equal to the maximum pulse peak power.

[0043] In some embodiments of the method, the extension of the pulse duration from the initial pulse duration to the transmission pulse duration can be based on a stretching factor of at least 1.5, in particular at least 10 or 100, and optionally based on an additional safety stretching factor of at least 2, 10, or 100. The coupling of the laser pulses into the hollow-core optical fiber can be performed by means of a fiber coupling device that provides a beam path for the pulsed laser radiation, the beam path providing, at the first fiber end, beam trajectory parameter values within the target tolerance range for damage-free coupling into the hollow-core optical fiber at the transmission pulse duration.

[0044] In some extended solutions, the method may further include the following steps:

[0045] The pulsed laser radiation is coupled out from the hollow core optical fiber,

[0046] shortening the pulse duration of the laser pulse of the pulsed laser radiation coupled out from the hollow-core optical fiber from the transmission pulse duration to the processing pulse duration, and

[0047] The outcoupled pulsed laser radiation is focused to provide high-intensity laser pulses for material processing of the workpiece.

[0048] In another aspect, which can be expanded in particular according to the embodiments summarized above, a device for transmitting pulsed laser radiation having a pulse energy that results in high-intensity laser pulses with a compressed pulse duration is disclosed. The device includes a pulse duration compression device configured to receive the pulsed laser radiation and to set a transmission pulse duration for the laser radiation, wherein the pulse duration compression device is configured to compress the laser pulses of the pulsed laser radiation to the transmission pulse duration. Furthermore, the device includes a hollow-core fiber having a hollow core surrounded by a material, wherein the hollow-core fiber transmits laser pulses coupled in at a first fiber end in the hollow core to a second fiber end and outputs the laser pulses at the second fiber end. The hollow-core fiber is configured to be operated with beam trajectory parameter values present at the first fiber end that are within a target tolerance range. Furthermore, the device includes a fiber coupling device configured to receive the laser radiation set to the transmission pulse duration and couple the laser radiation into the hollow-core fiber with beam trajectory parameter values within the target tolerance range. The transmission pulse duration is set in such a way that for all beam trajectory parameter values within the target tolerance range, a coupling of the laser radiation into the hollow core occurs in which the material and / or structure of the hollow-core fiber is retained.

[0049] In another aspect, which can be expanded in particular according to the embodiments summarized above, a device for transmitting pulsed laser radiation having a pulse energy that results in high-intensity laser pulses with a compressed pulse duration is disclosed. The device includes a pulse duration setting device configured to receive the pulsed laser radiation and to set a transmission pulse duration of the laser radiation. The pulse duration setting device is configured as a pulse duration compression device to compress the laser pulses of the pulsed laser radiation to the transmission pulse duration. The device also includes a hollow-core fiber having a hollow core surrounded by a material. The hollow-core fiber transmits laser pulses coupled in at a first fiber end in the hollow core to a second fiber end and outputs the laser pulses at the second fiber end. The hollow-core fiber is provided for operation with beam trajectory parameter values present at the first fiber end that are within a target tolerance range. The device also includes a fiber coupling device configured to receive laser radiation set to the transmission pulse duration and couple the laser radiation into the hollow-core fiber with beam trajectory parameter values within the target tolerance range. The transmission pulse duration is set such that, for all beam trajectory parameter values within the target tolerance range, the laser radiation is coupled into the hollow core in such a way that the material and / or structure of the hollow-core fiber is retained.

[0050] The aforementioned prerequisite for aligning the beam path within this tolerance range also applies to hollow-core fibers. In hollow-core fibers, drift in the laser radiation's beam path can result in laser radiation impinging on the photonic structure outside the hollow core region, thereby increasing its exposure to the intense regions of the laser beam. Consequently, excessively high peak powers can damage or destroy the photonic structure. In particular, regions of the core wall structure, which are primarily responsible for light guidance, can be damaged, ultimately rendering the fiber unsuitable for transmitting laser radiation.

[0051] Accordingly, for such fiber-based arrangements for transmitting pulsed laser radiation, in particular for coupling in pulsed laser radiation with predetermined parameters (e.g., pulse energy, pulse duration, repetition rate), a tolerance range is typically determined, which indicates the extent to which variations in the beam center position or the angle of incidence due to drift of the beam path are permitted without causing damage or destruction of the photonic structure. The goal here is, for example, that the aforementioned beam center position range and angle of incidence range—depending on the fiber type—be within a range of ±5% to ±10% of the mode field diameter and within a range of ±5% to ±10% of the beam divergence.

[0052] With regard to the use of high-intensity laser pulses, the concepts presented herein allow for a wider tolerance range for coupling into hollow-core fibers than would be determined without implementing these concepts, taking into account potential damage or destruction of the photonic structures, and thus provide a more robust beam guidance system for industrial applications. Due to the reduced pulse peak power during coupling into the hollow-core fiber, the threshold for damaging the hollow-core fiber, particularly photonic structures such as the core wall structure, is increased by the laser pulses with reduced pulse peak power. Since fiber damage can be avoided or at least reduced in frequency, malfunctions in systems using hollow-core fibers for transmitting high-intensity pulsed laser radiation and in components of such systems can be reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] This document discloses concepts that allow at least some improvements in aspects of the prior art. Further features and advantages thereof will be apparent from the following description of embodiments according to the accompanying drawings. The accompanying drawings show:

[0054] Figure 1A A schematic diagram is shown for illustrating the coupling of pulsed laser radiation into a hollow-core optical fiber for beam trajectory parameters within a tolerance range;

[0055] Figure 1B A schematic diagram is shown for illustrating the coupling of pulsed laser radiation into a hollow-core optical fiber for beam trajectory parameters outside a tolerance range;

[0056] Figure 2A and Figure 2B A schematic diagram is shown for illustrating the use of a device for transmitting pulsed laser radiation with a pulse duration setting device in laser-based material processing;

[0057] Figure 3A and Figure 3B A plan view of an exemplary hollow-core fiber coupling input end is shown for illustrating variations in beam trajectory parameters; and

[0058] Figure 4 A diagram is shown to illustrate the adjustability of the beam intensity present during coupling-in when using a pulse duration setting device.

[0059] The aspects described herein are based in part on insights into the tolerance ranges of beam trajectory parameters obtained in hollow-core optical fibers intended for transmitting laser radiation. Thus, it has been recognized that the higher the peak power of the laser radiation coupled in to be transmitted, the smaller the tolerance range for permissible beam drift. DETAILED DESCRIPTION

[0060] Figure 1A The use of a hollow-core optical fiber 1 (transmission optical fiber) for transmitting high-power laser pulses 3A is illustrated. The high-power laser pulses 3A are generated by a laser pulse source 3 and each have an initial pulse duration Δt. The high-power laser pulses 3A are focused onto a first fiber end 1A of the hollow-core optical fiber 1 by means of a fiber coupling-in device comprising a lens 5A. In particular, the beam profile of the high-power laser pulse 3A overlaps with the hollow core 4A of the hollow-core optical fiber 1, so that only a small intensity portion of the beam profile overlaps with the material 4B of the hollow-core optical fiber 1 surrounding the hollow core 4A. In the case of ideal coupling-in (that is, there is minimal overlap of the beam profile with the material), for fixed beam parameters, the maximum intensity I_max represents the destruction threshold of the optical fiber type (see also Figure 4 ).

[0061] If the beam trajectory parameters of the beam path of the incident laser radiation lie within tolerances that are often very narrow due to the high intensities, for intensities below the maximum intensity I_max, the laser radiation can be guided in the hollow core 4 to the second fiber end 1B. The high-power laser pulses emerge from the second fiber end as highly divergent laser radiation. Figure 1A In the embodiment, the outgoing laser radiation is collimated by means of a further lens 5B and guided, for example as a free beam, to a laser processing head 7 having a focusing lens 7A in order to process a workpiece 9. Figure 1A , it is shown that the processing pulse duration Δt' of the high-power laser pulse 3A' emitted from the hollow-core fiber 1 can (substantially) correspond to the initial pulse duration Δt of the laser pulse 3A coupled in. It is worth noting that there may be dispersion effects of the hollow-core fiber 1, so that the pulse duration may change (fiber-dependent stretching or shortening of the pulse duration) during the propagation of the laser pulse in the hollow-core fiber 1.

[0062] If the beam trajectory parameters (beam center, angle of incidence) of the high-power laser pulse 3A lie outside a (narrow) tolerance range during incorporation, or if the peak intensity of the high-power laser pulse 3A is too high, beam intensities can occur in the region of the material 4B (e.g., in the region of the photonic structure) that damage the first fiber end 1A. This obviously also applies to intensities that are equal to or above the maximum intensity I_max. If the hollow-core fiber 1 is damaged, this can ultimately result in no laser radiation being emitted from the hollow-core fiber 1 that can be used to process the workpiece 9.

[0063] Figure 1B The following situation is illustrated by way of example: in this case, the lens 5A is misaligned relative to the first fiber end 1A, so that the beam center of the beam profile of the high-power laser pulse 3A is not centered on the hollow core 4A and the core wall 4C of the hollow-core fiber 1 is therefore ablated or thermally deformed, for example, by the incident laser radiation.

[0064] The maximum destruction peak power, also called the destruction threshold, is the maximum destruction peak power from which the photonic structure is destroyed, since the maximum intensity I_max is reached, and from which the hollow-core fiber can no longer be reliably used for transmitting high-power laser pulses for any possible coupling-in case (i.e., even in the case of ideal coupling-in). In the case of coupling-in, the destruction threshold, and therefore also the tolerance range considered herein, increases with the pulse duration of the incident laser pulse, since, for a destructive interaction of the laser radiation with the fiber material, the longer the pulse duration, the more pulse energy must be applied.

[0065] The inventors have now recognized that for industrial laser systems that use hollow-core fibers to transmit high-power laser pulses from a laser pulse source, a sufficiently large tolerance range (target tolerance range) must be provided when coupling into the hollow-core fiber. For example, depending on the fiber type, the tolerance range includes variations / fluctuations of the beam center within a range of ±5% to ±10% of the mode field diameter and variations / fluctuations of the beam angle within a range of ±5% to ±10% of the beam divergence. Within the target tolerance range, fiber damage should generally be prevented from occurring even in the event of variations in beam trajectory parameters that may occur during operation (tolerable miscoupling). The tolerance range is defined by a tolerance range threshold in the form of an (ultimate) intensity I_Lim. The (ultimate) intensity I_Lim is sufficiently below the maximum intensity I_max, i.e., below the "ideal" damage threshold, that any fluctuations that may occur during operation are covered by the target tolerance range. This tolerance range threshold, which is reduced relative to the damage threshold, serves to protect the hollow-core fiber 1. In particular, this protection should be ensured even in the case of pulse energies (e.g. in the μJ range, i.e. greater than 1 μJ or greater than 10 μJ) which in principle (in the case of pulse duration compression) could lead to high-intensity laser pulses with peak intensities above the destruction threshold.

[0066] The inventors have now recognized that if, due to a "long" transmission pulse duration, the effect (intensity) of the transmitted laser pulse always remains below the tolerance range threshold for the fiber type, then destruction of the hollow-core fiber can be prevented despite high pulse energies (which in principle could lead to peak intensities above the destruction threshold). Accordingly, the inventors propose increasing the tolerance range in the coupling-in process to at least a target tolerance range by not exceeding the tolerance range threshold for peak intensity / pulse energy in the hollow-core fiber due to a pulse duration that is longer than the minimum pulse duration achievable, for example, due to the current bandwidth (e.g., due to an extension of the transmission pulse duration before coupling-in or due to an incomplete compression of the laser pulse duration before coupling-in).

[0067] In other words, if the beam trajectory parameters are to be variable to a certain extent during coupling, a corresponding tolerance range for the beam trajectory parameters during coupling can be set by setting / extending the pulse duration of the laser pulse to be correspondingly long using a pulse duration setting device (e.g., a pulse duration stretching device or a non-maximum-compression pulse duration compression device). In other words, the inventors propose that the pulse duration be stretched prior to coupling into the hollow-core transmission fiber, either by separate pulse duration extension or by non-maximum-performed pulse duration compression (setting), in order to reduce the (pulse) peak power. This is because if the laser pulses are set to a sufficiently long pulse duration, damage to, for example, the core wall network can be avoided or at least reduced within a predetermined tolerance range for coupling. If the limits of the tolerance range are reached (e.g., the maximum permissible deviation of the beam center from the central axis of the hollow-core fiber within the target tolerance range is exceeded—more than, for example, ±5% or ±10% of the mode field diameter, or the maximum permissible deviation of the incident angle from the axial incidence within the target tolerance range is exceeded—more than, for example, ±5% or ±10% of the beam divergence), a realignment of the beam path can be performed in order to re-establish the desired transmission characteristics without previously damaging the transmission fiber and possibly without even requiring fiber exchange.

[0068] The concept proposed herein of extending the pulse duration before coupling in can also be referred to as "chirped pulse transmission." This pulse duration extension allows the transmission of laser pulses that already have the full pulse energy required for laser processing, but whose (pulse) peak power is reduced for this transmission. The transmission pulse duration is extended (either solely by extending the pulse duration or by compression that is not performed to the maximum extent) within a range that ensures that, in the event of misalignment, the hollow-core fiber is not damaged within the tolerance range required for beam guidance (typically within the target tolerance range specified for fiber applications).

[0069] Figures 2A to 4 The method according to the invention is illustrated by way of example.

[0070] Figure 2A Shown is a system 10 for laser-based processing of a material 9 . The system 10 has a laser pulse source 3 , a device 11 for transmitting pulsed laser radiation, and a laser processing head 21 .

[0071] The laser pulse source 3 outputs pulsed laser radiation in the form of laser pulses 3A with an initial pulse duration Δt. The pulsed laser radiation is fed to an apparatus 11 which enables flexible transmission of the pulsed laser radiation by means of the hollow core optical fiber 1 to the location where material processing is intended.

[0072] The device 11 for transmitting pulsed laser radiation comprises a hollow-core optical fiber 1 , a pulse duration stretching device 13 and a fiber-coupled input device 17 and optionally a fiber-coupled output device 19 .

[0073] If the hollow-core fiber 1 is designed as a pure transmission fiber, it accordingly does not affect the transmitted laser radiation. For example, self-phase modulation, spectral broadening, or spectral shifts during transmission in the hollow-core fiber 1 used as a pure transmission fiber can be avoided. In some embodiments, the transmission can be accompanied by spectral broadening in the fiber in a targeted manner by filling the hollow core with a gas; such a transmission fiber can be part of the device described herein for transmitting pulsed laser radiation, provided that the gas filling does not lead to optical conditions that damage the fiber during coupling in, and the spectral broadening does not lead to damage to the fiber during transmission. Furthermore, there may be dispersion effects of the fiber on the pulsed laser radiation.

[0074] For example, for hollow core fiber, Figure 3A A plan view of a fiber end of a kagome fiber 31 (as an example of a microstructured fiber, also known as a photonic crystal fiber) is schematically shown. The fiber end shown provides a fiber end face through which laser radiation is to be coupled into the kagome fiber 31 .

[0075] In the center, the light-guiding region of the kagome fiber 31 can be seen, which is provided by the kagome fiber 31 as a central hollow core 33. The hollow core 33 has a substantially circular aperture opening surrounded by a perforated cladding structure 35. The perforated cladding structure 35 is composed of channels 37 that surround the hollow core 33 and enable optical guidance of light in the hollow core 33. In these "photonic" structures, the channels 37 are formed by a specific arrangement of a wall network 37A.

[0076] The photonic structure defines a hollow core 33 by means of an innermost wall section 39. Figure 3A The wall segment 39 shown in FIG. 1 forms the core wall.

[0077] The goal of transmitting laser radiation with a Kagome fiber 31 is to guide the laser radiation with as little loss as possible and with as little influence as possible on disturbances in the transmission conditions. Figure 3A An exemplary rotationally symmetrical intensity distribution 41 (beam profile) of an incident laser beam is schematically illustrated in FIG. In the illustrated case, ideally, the beam center 41A of the laser beam (given by the maximum intensity at the center of the intensity distribution) coincides with the center axis 31A of the kagome fiber 31, so that in the kagome fiber 31, the beam profile of the laser radiation can also propagate in the center of the hollow core 33.

[0078] Reference again Figure 2A The pulse duration stretching device 13 is shown as a grating stretcher with two gratings 15. The dispersion splitting and combining of the wavelengths forming the laser pulses is schematically illustrated.

[0079] Typically, the pulse duration stretching device 13 comprises at least one optical element, such as a diffraction grating, a volume Bragg grating, a prism and / or a grating prism (grism). Figure 2A As shown in the example, two optical elements can be arranged in the beam path in order to extend the pulse duration of the laser pulse 3A. Further optical elements, such as folding mirrors, focusing mirrors or lenses, can be provided to form a pulse duration matching device that can be set with respect to the dispersion contribution (e.g. Figure 2A The pulse duration stretching device 13 or Figure 2B Another exemplary pulse duration matching device, in particular a compressor for finely setting the pulse duration, is disclosed in DE 10 2016 110 947 A1 of the present applicant. For example, such a compressor can be used for incomplete compression of laser pulses (see Figure 2B in the structure).

[0080] exist Figure 2A , laser pulses 3B are shown, which have a frequency chirp and are correspondingly temporally extended with a transmission pulse duration Δt_t, emerging from the pulse duration stretching device 13. The transmission pulse duration Δt_t is therefore significantly longer than the theoretical lower limit for compression of the laser pulses of the laser pulse source 3, which is determined by the time-bandwidth product. The extended laser pulses 3B are fed to a fiber-coupled input device 17.

[0081] For example, the fiber coupling-in device 17 comprises a (coupling-in) lens 5. The lens 5 is arranged in such a way that it focuses the laser pulse onto the fiber end face of the hollow-core fiber 1 at an angle of incidence α, wherein the beam center position X is located in the region of the aperture opening of the hollow core 4A.

[0082] Figure 2A The range Δα for the angle of incidence α and the range ΔX for the position X of the beam center are schematically illustrated. The ranges Δα and ΔX define the tolerance ranges for the values of the beam trajectory parameters "angle of incidence α" and "beam center position X." For example, they lie within a few percent of the mode field diameter and beam divergence (e.g., within a range of ±1%, ±2%, ±5%, or ±10%), for example, within a range of ±5% to ±10% of the mode field diameter and within a range of ±5% to ±10% of the beam divergence.

[0083] to this end, Figure 3BThe position of beam center 41A is illustrated, and this beam center is irradiated onto the fiber end while being displaced by an offset ΔXd relative to the central axis 31A of the Kagome fiber 31. The interaction region 43 is indicated by dashed lines. This interaction region does not yet significantly overlap with the photonic structure 35 (minimum overlap region 45 with the core wall), thereby preventing damage to the hollow-core fiber at the offset ΔXd. It can be seen that if the offset exceeds the offset ΔXd, the interaction region 43 will overlap with the photonic structure 35. In other words, the offset ΔXd defines the tolerance range for the beam trajectory parameter "beam center position." Similarly, variations in the incident angle α that exceed the tolerance range for the beam trajectory parameter "incident angle" can cause the laser pulse to interact with the material of the hollow-core fiber 1.

[0084] According to the present invention, Figure 2A In the embodiment of the invention, the pulse duration is extended by the pulse duration extension device 13 (or, according to Figure 2B , by means of the incomplete compression in the compressor 13 ″, the incomplete compression) is set in such a way that within the tolerance range of the beam trajectory parameters, as is required for the corresponding use of the hollow-core fiber, no damage to the hollow-core fiber occurs.

[0085] Reference again Figure 2A At the output end of hollow-core fiber 1, a fiber coupling device 19 is shown. This fiber coupling device collimates the laser radiation that diverges from the hollow-core fiber, for example, by means of lens 5B. Since hollow-core fiber 1 is designed as a transmission fiber, the laser pulses 3B' emitted at the fiber end have a pulse duration Δt_t' that substantially corresponds to the transmission pulse duration Δt_t. This is true as long as no laser radiation-fiber interaction (e.g., nonlinear effects such as pulse duration shortening / spectral broadening) occurs in the transmission fiber.

[0086] In order to generate the desired (pulse) peak power with the high-intensity laser pulses after beam transmission, the exiting laser pulses 3B' can be temporally compressed downstream of the hollow-core fiber by means of appropriate dispersion compensation in a pulse duration matching device, for example, using a grating or prism compressor. For example, the grating compressor at the fiber output end can be set in such a way that, after beam transmission, the desired processing pulse duration, and therefore the peak power required for laser processing, is available at the target location, for example, in the material to be processed of the workpiece 9.

[0087] As in Figure 2AAs shown in , for this purpose, the laser processing head 21 can be configured as a compression laser processing head. That is, in addition to the focusing lens 7A, the laser processing head 21 also includes a pulse duration matching device 23, shown as a grating compressor, for example. The pulse duration matching device 23 is configured so that the laser pulse 3B' supplied thereto is output as a compressed laser pulse 3C having a processing pulse duration Δt'. The processing pulse duration Δt' corresponds, for example, approximately to the initial pulse duration Δt of the laser pulse 3A. Typically, the pulse duration matching device 23 is configured so that the processing pulse duration of the laser pulse 3C used for laser processing can be set for a set transmission pulse duration range.

[0088] Furthermore, the compressed laser pulse 3C has a pulse energy which corresponds to the pulse energy of the laser pulse 3A of the laser pulse source 3 , excluding losses in the pulse duration stretching device 13 , the pulse duration matching device 23 , the hollow-core fiber 1 , and the coupling-in and coupling-out devices 17 , 19 .

[0089] The workpiece 9 can now be laser processed using the compressed laser pulses 3C.

[0090] Due to the defined transmission pulse duration, beam trajectory parameters (beam center position and direction of incidence) can be varied during operation within a target tolerance range without endangering the transmission fiber. Recalibration (and / or, in some cases, a further pulse duration increase) is only necessary if the limits of the target tolerance range are reached or exceeded. Recalibration can be performed manually or automatically.

[0091] In damage threshold experiments, it was determined that, given the fiber geometry examined by way of example, laser pulses with peak powers exceeding 2 GW are no longer reliably transmitted. However, if the laser pulses are temporally stretched for coupling into the fiber (and correspondingly compressed again afterward), robust transmission of the same laser pulses can be ensured. For example, high-power laser pulses with pulse energies in the range of 100 μJ or greater and pulse durations of several hundred femtoseconds result in pulse peak powers that, given sufficient overlap with the fiber material, damage the material structure. Pulse duration extension (generally using longer pulse durations) prevents damage to the fiber structure and thus enables transmission. The concepts disclosed herein can be particularly advantageous if, for non-stretched pulses (pulse durations within the maximum temporal compressibility of the laser pulses), pulse peak powers of 25 MW or greater (pulse parameters of, for example, approximately 10 μJ or more, in the range of several hundred femtoseconds, e.g., 300 fs) are to be transmitted via hollow-core fibers.

[0092] In order to be able to determine the required length of the pulse duration (extension / incomplete compression of the pulse duration), i.e. the transmission pulse duration, the determination of the destruction threshold for the corresponding hollow-core fiber design (e.g. photonic bandgap, suppressed coupling, antiresonance, negative curvature hollow-core fiber) can be performed as follows:

[0093] 1. The laser pulses of the laser pulse source are directed to the ideal and stable beam parameters (especially with respect to mode field diameter, opening angle, M 2 value, stability with respect to position and angle) is coupled into the optical fiber.

[0094] 2. By gradually increasing the peak power, find the upper limit of power—the maximum pulse peak power—at which the hollow-core fiber design can still guide the laser pulse nondestructively.

[0095] 3. When using a beam with fluctuating parameters (e.g. M 2 For laser pulse sources with high sensitivity (e.g., fluctuations in value, fluctuations in position and angular stability, and fluctuations in pulse stability), the maximum pulse peak power can be additionally reduced in value to ensure non-destructive transmission. This reduced pulse peak power corresponds to a (limiting) intensity I_Lim determined for the target tolerance range, which is lower than the maximum possible pulse peak intensity I_max.

[0096] 4. Reduction of the peak power by stretching the laser pulse relative to the minimum possible pulse duration (either by temporally extending the pulse duration with a pulse duration stretching device to reduce the peak power or by performing non-maximum compression with a pulse duration compression device in the laser pulse source).

[0097] 5. In addition, a safety factor can also be taken into account in the elongation factor in order to achieve robust and reliable fiber guidance. The additional reduction in peak power due to the safety factor is performed, for example, by at least a factor of 2, preferably a factor of 10, and particularly preferably a factor of 100, so that the maximum pulse peak power present at the fiber end is lower by this factor (safety factor) than the pulse peak power determined for the target tolerance range.

[0098] Figure 4 A diagram is shown to illustrate the adjustability of the beam intensity for two initial situations (e.g., laser pulses from two different laser beam sources). In this diagram, the intensity (I_s axis) in a structured region (such as that introduced in a damage-critical portion of a hollow-core fiber) is plotted as a function of the beam center position (x axis). Figure 3A In the case of the Kagome fiber 31 shown in FIG, said crucial part will be given, for example, by a thin-walled mesh 37A made of glass, usually by a realized photonic structure.

[0099] In addition, Figure 4 The destruction threshold intensity I_max is plotted in the diagram. The destruction threshold intensity is specific to the hollow-core fiber considered in each case. If the intensity I_s is below the threshold intensity I_max in the case of an optimal beam center position, the pulsed laser radiation can be transmitted without damaging the hollow-core fiber. If the intensity I_s is greater than or equal to the destruction threshold intensity I_max, the hollow-core fiber is damaged even in the case of ideal alignment. It is worth noting that the intensity of the laser radiation in the hollow-core fiber results from the pulse peak power and the mode field area of the hollow-core fiber as power per unit area, since the mode field area is predetermined for the hollow-core fiber considered and does not change. It is also worth noting that the mode field area generally varies from hollow-core fiber to hollow-core fiber, in particular for different fiber types.

[0100] In addition, Figure 4 The diagram in FIG shows the tolerance range threshold intensity I_Lim (threshold intensity for short). This tolerance range threshold intensity is determined in a manner specific to the respective hollow-core fiber under consideration within the scope of the desired operation. If the intensity I_s at the beam center is below the tolerance range threshold intensity I_Lim, the pulsed laser radiation can be transmitted without damaging the hollow-core fiber. If the intensity I_s is above the tolerance range threshold intensity I_Lim, there is a risk of damaging the hollow-core fiber, and coupling into the hollow-core fiber should be avoided.

[0101] exist Figure 4In FIG, the tolerance range resulting from the plotted intensity curve and the threshold intensity I_Lim is plotted for the beam trajectory parameter “beam center”. If the tolerance range meets the required range for operation of the associated laser system, the transmission pulse duration has been adequately adapted.

[0102] In general, the x-axis describes the deviation from the optimal input coupling condition, which may be due, for example, to a change in the mode field diameter at the fiber input or to a shift in the beam center position away from the fiber central axis ( Figure 3B The optical fiber is subjected to a shift in the optical fiber intensity (ΔXd) and / or due to a decrease in the angle of incidence of the transmitted laser radiation. Such a drift can be caused, for example, by temperature fluctuations in the optical components. Plotted on the y-axis is the intensity / peak power that interacts with the critical parts of the optical fiber and can damage the optical fiber if it exceeds a threshold intensity I_Lim and can damage the optical fiber if it exceeds a damage threshold intensity I_max.

[0103] The first example (line L1) shows the intensity curve of an input laser pulse whose pulse duration is not specified for the transmission fiber. The associated tolerance range ΔX_L1 may not enable robust operation in every application of a system using the transmission device. Peak power can be reduced by adapting the pulse duration (extending it to the transmission pulse duration). Line L1_t extends below line L1 in the diagram; that is, the peak intensity decreases and the tolerance range ΔX_L1_t increases. This tolerance range is specified for laser pulses that result in an intensity in the critical region that is less than the tolerance range threshold intensity I_Lim.

[0104] If, in the first example, the target tolerance range ΔX_S required for operation lies between the tolerance range ΔX_L1 and the tolerance range ΔX_L1_t, the use of a hollow-core fiber for transmitting the pulsed laser radiation can be achieved by extending the pulse duration.

[0105] The second example is based on an intensity distribution on a transmission fiber that does not allow coupling in. Line L2, due to, for example, very high pulse energy, continuously extends above the destruction threshold intensity I_max and therefore also continuously above the tolerance range threshold intensity I_Lim; that is, even with ideal alignment (X=0), the fiber would be damaged. Only by reducing the peak power by lengthening the pulse duration, as described in the present invention, does at least a portion of line L2_t extend below the tolerance range threshold intensity I_Lim. Therefore, the inventive reduction in the interaction intensity / power makes it possible to transmit laser pulses with high energy within the tolerance range ΔX_L2_t. In this way, a target tolerance range can be defined that matches this example and, for example, corresponds to the tolerance range ΔX_L2_t or is slightly smaller in order to account for additional uncertainties in the beam parameters.

[0106] The pulse duration matching device (pulse duration stretching device 13 and compressor system 23) can generally use optical elements used in transmission or reflection, such as diffraction gratings, volume Bragg gratings, prisms and / or grisms and / or dispersive mirrors, such as Gires-Tournois interferometer mirrors (GTI mirrors). The mutually coordinated grating stretcher and grating compressor combination allows the introduction and compensation of large dispersion values. In the case of laser pulses with high power / high pulse energy (such as can be generated, for example, using a generator amplifier system as the laser pulse source), large dispersion values with correspondingly large stretching may become necessary.

[0107] In addition, regarding Figure 2A It is mentioned that the pulse duration setting device of the device 11 can also be arranged in the laser pulse source 3. Figure 2A The device 13' is schematically shown in FIG. It can be designed as a pulse duration stretching device, with which the pulse duration can be actively extended, or as a pulse duration compression device, which does not maximally compress the laser pulses. The pulse duration compression device is accordingly designed so that the dispersion of the laser pulses of the pulsed laser radiation remains unchanged, so that the laser pulses are compressed "only" in terms of pulse duration to the transmission pulse duration.

[0108] Figure 2B The following configuration is explained in which a pulse duration compression device 13 (also referred to herein as compressor for short) of a laser pulse source 3 is used for pulse compression of amplified laser pulses. Figure 2BAs shown, a laser pulse source 3' generates laser pulses in an amplifier system L for transmission using a device 11. The amplified laser pulses are output by the laser amplifier system L with a pulse duration that is significantly longer than the minimum possible pulse duration associated with the spectral bandwidth present after amplification. The amplified laser pulses are temporally compressed to a transmission pulse duration Δt_t by at least one optical element 15'. Consequently, the amplified laser pulses are output as temporally stretched laser pulses 3B relative to the minimum possible pulse duration and are delivered directly (i.e., without passing through a stretcher 13) to the transmission fiber 1.

[0109] With regard to an exemplary further beam guidance of the device 11 and the laser pulses 3B′ output by the device 11, reference is made to Figure 2A The above description.

[0110] according to Figure 2B In order to use the device 11, a pulse 3B is output from the laser pulse source 3' in such a way that the pulse duration of the pulse is not compressed as much as possible with respect to dispersion based on a given time-bandwidth product (English: time band width product) by means of the pulse duration compression device 13". The time-bandwidth product theoretically determines the lower limit of laser pulse compression, i.e., the pulse duration of the laser pulse under a given spectral bandwidth, wherein the pulse duration is usually given by the full width at half maximum (FWHM) of the intensity profile of the laser pulse. The time-bandwidth product of the ultrashort laser pulses actually generated usually does not reach the theoretical limit value due to the pulse shape, irremovable residual dispersion (residual linear frequency modulation), etc.

[0111] Typically, for a given pulse energy (here the pulse energy is in the μJ range, for example greater than 10 μJ, such as 100 μJ or more), the time-bandwidth product also determines the theoretically achievable pulse peak power, wherein, again due to the pulse shape, irremovable residual dispersion, etc., the theoretically achievable pulse peak power cannot be achieved with real laser pulses.

[0112] according to Figure 2B With the configuration, after compression in the compressor 13', residual dispersion remains in the beam, ie the pulse duration of the output laser pulse 3B is higher than the lower limit of the pulse duration and the pulse peak power is lower than the theoretically achievable pulse peak power.

[0113] With regard to the time-bandwidth product, the target pulse duration can now be set in such a way that the pulse peak power present at the hollow-core fiber corresponds to the (limit) intensity I_Lim or is still lower than the intensity according to the safety factor, so that the operation of the hollow-core fiber as a transmission tool can be guaranteed within the target tolerance range.

[0114] and Figure 2A The active pulse duration of the stretcher 13 is prolonged compared to Figure 2B The controlled setting of the compressor 13″ in the embodiment of the present invention has the following advantages over a setting for non-optimal dispersion compensation (also referred to herein as “misaligned” or “misaligned”): a simpler construction with fewer optical components and correspondingly reduced costs. In addition, fewer optical components can avoid intensity losses in the beam guidance, as can occur in particular in the stretcher 13. Fewer optical components can also improve the accuracy in the beam pointing / beam path.

[0115] The compressor 13' is set such that residual dispersion remains in the laser beam, i.e., the laser pulses are not completely compressed to the shortest pulse duration achievable with a given spectral bandwidth. In other words, the compressor 13' is misaligned so that a target pulse duration is obtained dispersively (due to residual dispersion) above the lower limit of the pulse duration given by the time-bandwidth product, so that the pulse peak power of the coupled-in pulses—taking into account a safety factor—is, for example, at most 50%, 10%, or even at most 1% of the permissible pulse peak power determined for the tolerance range. For example, 200 fs laser pulses with a pulse duration (FWHM pulse duration) greater than 1 ps (i.e., a FWHM pulse duration within the range of 200 fs, limited by the time-bandwidth product) can be output by the laser pulse source 3' and coupled into the hollow-core fiber 1.

[0116] In summary, the concepts disclosed herein can be implemented in accordance with Figure 2B In the embodiment of DE 10 2016 110 947 A1 mentioned above, this is done as follows with the aid of incomplete compression. First, the destruction threshold of the hollow-core fiber is determined or already exists, wherein this determination is performed within the entire tolerance range. Then, the compressor 13" is typically set in the factory for the transmission fiber used. In the embodiment according to DE 10 2016 110 947 A1 already mentioned, this setting is done, for example, by arranging the dispersion-compensating grating of the compressor 13" (or in Figure 2A In the case of the dispersion-adding grating arrangement of the elongator 13, the pitch of the grating 15' is adjusted or the orientation of the glass layer in the compressor 13" (or elongator 13) is adjusted. However, for laser processing to be carried out with the aid of a laser beam, laser pulses that are compressed as completely as possible (with a pulse duration as close as possible to the pulse duration given by the time-bandwidth product) are required. This compression can be carried out in a downstream compressor 23 and / or at least partially in a hollow-core fiber.

[0117] It is worth noting that the laser pulse source can be constructed very stably, especially with regard to the pulse energy and spectral width, so that with regard to determining the tolerance range, it is mainly the changes in the geometric parameters of the beam path, that is, the beam trajectory parameters "beam center position" and "incident angle" mentioned at the beginning that are important.

[0118] In addition to these beam pointing parameters, the tolerance range generally depends on the mode field diameter and the single-mode nature of the corresponding hollow-core fiber. The greater the single-mode nature of the corresponding fiber type, the smaller the tolerance range, since the incident transverse beam profile may overlap more with the material of the corresponding fiber type. The larger the mode field diameter of the fiber, the greater the tolerance range, since there is less material overlap.

[0119] According to the tolerance ranges typically used for transmission fibers, the beam trajectory parameters should lie within a predetermined range of the beam center's position and within a predetermined range of the angle of incidence. As previously mentioned, depending on the fiber type, the permissible variation in the beam center position can lie within a range of ±5% to ±10% of the mode field diameter (e.g., within a range of ±5 μm from the central axis of the hollow-core fiber). Alternatively or additionally—again depending on the fiber type—the permissible deviation in the angle of incidence can lie within a range of ±5% to ±10% of the beam divergence (e.g., a deviation within a range of 2 to 5 mrad).

[0120] In order to accommodate the tolerance ranges that are currently predefined in each transmission fiber application, the starting point for the present invention is to determine what intensity the innermost (glass) web can "withstand" without damage. The pulse peak power to be achieved within the tolerance range is then set to a value below the resulting damage threshold by providing a pulse duration that corresponds to the residual / additive dispersion setting.

[0121] The process in implementing the present invention may include, for example, the following steps:

[0122] 1. Determining a damage threshold of the optical fiber, in particular determining the residual dispersion / additive dispersion, starting from which a pulse peak power is generated that, at a given pulse energy and a given wavelength, within the tolerance range of the beam guidance parameters, no longer causes damage to the optical fiber.

[0123] 2. The pulse duration is set in such a way that the pulse peak power is lower than the pulse peak power determined under 1. by at least a safety factor of 2, in particular by at least a safety factor of 10 or 100. This safety factor ensures safe operation even in the event of fluctuations in the pulse energy or spectrum.

[0124] The concepts disclosed herein are particularly applicable with non-stretched (i.e., maximum achievable) pulse peak powers of at least 25 MW or more. Such pulse peak powers can be achieved, for example, with ultrashort pulses having a pulse energy of approximately 10 μJ at a pulse duration of 300 fs.

[0125] In summary, the inventive concept of using hollow-core optical fiber to transmit high-power laser pulses is based on the following points:

[0126] 1. With regard to the intensity / power present, there is a (theoretical) destruction threshold below which the fiber is damaged even with perfect coupling. The maximum pulse peak power that can be transmitted in a hollow-core fiber is determined by the existing overlap of the beam profile with the fiber structure. This applies to perfectly aligned coupling into the fiber and becomes even more important in the case of misalignment. Consequently, the maximally transmittable / coupled peak power is limited.

[0127] 2. There is a tolerance range for the coupling of laser radiation into a hollow-core fiber. This tolerance range depends on the peak pulse power, but must be of a certain size for reliable operation (target tolerance range).

[0128] 3. During operation, the coupling-in must always remain below the maximum peak power assigned to the fiber (given by the tolerance range threshold intensity). When transmitting laser pulses that can result in higher pulse peak powers, a defined stretching factor for the pulse duration is achieved during coupling-in (due to misalignment of the stretcher 13 or compressor 13″).

[0129] In summary, the system disclosed herein differs from known transmission systems in that the tolerance range for coupling the laser radiation into the hollow-core fiber is increased in that, before coupling in, the laser pulse undergoes a certain stretching factor due to the delivered additional dispersion (stretcher 13) or missing / incomplete dispersion compensation (misalignment of the compressor 13″). The stretching factor, i.e. the additionally provided dispersion or the missing dispersion compensation, depends on the transmission fiber and the pulse peak power. For example, an initial pulse duration Δt in the range of 50 fs to 1 ps can be extended by a factor of at least 1.5 (stretching factor) to the transmission pulse duration Δt_t; exemplary factors are in the range of 1.5 to several hundred; for example, the factor is greater than or equal to a factor of 2, 10 or 100 and a safety factor can be taken into account. Preferably, the factor (i.e. the residual dispersion or delivered dispersion) is kept as small as possible (e.g. a factor of less than 100) so that unnecessary dispersion correction does not have to be performed.

[0130] It is expressly emphasized that, for the purpose of original disclosure and for the purpose of limiting the claimed invention independently of the combination of features in the embodiments and / or claims, all features disclosed in the specification and / or claims should be considered separate and independent from each other. It is expressly pointed out that, for the purpose of original disclosure and for the purpose of limiting the claimed invention, all range descriptions or descriptions of unit groups disclose any possible intermediate values or unit subgroups, especially also as limitations of the range descriptions.

Claims

1. A device (11) for transmitting pulsed laser radiation having a pulse energy which, with a compressed pulse duration, results in high-intensity laser pulses (3C), comprising: a pulse duration setting device (13) which is designed to receive the pulsed laser radiation and to set a transmission pulse duration (Δt_t) of the laser radiation, A hollow core optical fiber (1) having a hollow core (4A) surrounded by a material (4B), wherein: - the hollow core optical fiber (1) transmits the laser pulse (3B) coupled into the first optical fiber end (1A) in the hollow core (4A) to the second optical fiber end (1B) and outputs the laser pulse at the second optical fiber end, and - setting to operate the hollow-core optical fiber (1) with a beam tracking parameter value (X, α) present at the first optical fiber end (1A), said beam tracking parameter value being within a target tolerance range (ΔX_S), and A fiber coupling input device (17) is configured to receive laser radiation set to the transmission pulse duration (Δt_t) and couple the laser radiation into the hollow-core optical fiber (1) with a beam trajectory parameter value (X, α), the beam trajectory parameter value being within the target tolerance range (ΔX_S). The transmission pulse duration (Δt_t) is set in such a way that for all beam trajectory parameter values (X, α) within the target tolerance range (ΔX_S), the following coupling of the laser radiation into the hollow core (4A) occurs: in this coupling, the material and / or structure (37) of the hollow-core optical fiber (1) is retained, The pulse duration setting device (13) is configured as a pulse duration compression device (13") to compress laser pulses of pulsed laser radiation output by a laser amplifier system (L) with a pulse duration longer than a minimum possible pulse duration related to the existing spectral bandwidth in time to the transmission pulse duration (Δt_t) via at least one optical element (15) and thus output them as laser pulses (3B) that are stretched in time relative to the minimum possible pulse duration.

2. The device (11) according to claim 1, wherein The target tolerance range (ΔX_S) is given by permissible fluctuations of beam parameters of the pulsed laser radiation during operation, wherein the beam parameters include the beam center position (X) and / or the angle of incidence (α), and the permissible fluctuations in the beam center position (X) are within a range of ±10% of the mode field diameter and / or the permissible fluctuations in the angle of incidence (α) are within a range of ±10% of the beam divergence, Within the target tolerance range (ΔX_S), the hollow core optical fiber (1) can be allocated with a maximum pulse peak power, and over the entire target tolerance range (ΔX_S), above the maximum pulse peak power, no coupling input is provided that does not damage the hollow core optical fiber (1), and The transmission pulse duration (Δt_t) of the laser radiation is set at the first fiber end (1A) in such a way that the peak power of the laser radiation at the first fiber end (1A) is less than or equal to the maximum pulse peak power.

3. The device (11) according to claim 1 or 2, wherein The hollow core optical fiber (1) has structural elements that construct an inner core wall surrounding the hollow core (4A), and The pulse duration setting device (13) and the fiber coupling input device (17) are arranged in such a way that, for a beam trajectory parameter value (X, α) within the target tolerance range (ΔX_S), the spatial overlap of the intensity distribution (41) of the laser radiation with the structural element is such that the intensity (I_s) present in the region of the structural element is below the destruction threshold intensity (I_max) of the core wall.

4. The device (11) according to claim 1, wherein the pulse duration setting device is configured to set the laser pulse (3B) in terms of its pulse duration to the transmission pulse duration (Δt_t) with a stretching factor of at least 1.5 relative to the minimum possible pulse duration in the range of 50 fs to 1 ps.

5. The device (11) according to claim 4, wherein The transmission pulse duration (Δt_t) is set by means of the pulse duration setting device (13) in such a way that, with respect to the material (4B) of the hollow-core optical fiber (1), damage-free coupling into the hollow-core optical fiber (1) occurs within the target tolerance range (ΔX_S).

6. The device (11) according to claim 5, wherein The pulse duration setting device (13) comprises a diffraction grating, a volume Bragg grating, a prism and / or a dispersive mirror for use in transmission or in reflection.

7. The device (11) according to claim 6, wherein The pulse duration setting device (13) comprises a grating prism.

8. The device (11) according to claim 2, wherein The beam parameters of the pulsed laser radiation include parameters from the following group: - beam propagation parameters, including mode field diameter, opening angle and beam quality value, - pulse parameters, including pulse energy, pulse duration and repetition rate, and - beam tracking parameters, said beam tracking parameters including the beam center position (X) at the first optical fiber end (1A) and the incident direction (α) of said pulsed laser radiation to said first optical fiber end (1A), and The peak power of the pulsed laser radiation is adapted by means of the pulse duration setting device (13) in such a way that, with fixed beam propagation parameters and fixed pulse parameters, the laser radiation can be coupled into the hollow core (4A) in a non-destructive manner with beam trajectory parameters that are within the target tolerance range (ΔX_S).

9. The device (11) according to claim 2, wherein The allowable fluctuation in the beam center position (X) is within the range of ±5% of the mode field diameter.

10. The device (11) according to claim 9, wherein The allowable fluctuation in the beam center position (X) is within the range of ±2% of the mode field diameter.

11. The device (11) according to claim 10, wherein The allowable fluctuation in the beam center position (X) is within the range of ±1% of the mode field diameter.

12. The device (11) according to claim 2, wherein The permissible fluctuation in the incident angle (α) is within the range of ±5% of the beam divergence.

13. The device (11) according to claim 12, wherein The permissible fluctuation in the incident angle (α) is within the range of ±2% of the beam divergence.

14. The device (11) according to claim 13, wherein The permissible fluctuation in the incident angle (α) is within the range of ±1% of the beam divergence.

15. The apparatus (11) according to claim 3, wherein The structural element is a glass structure or a photonic crystal structure (37).

16. The apparatus (11) according to claim 3, wherein The pulse duration setting device (13) and the fiber coupling input device (17) are arranged in such a way that, in the case of beam trajectory parameter values (X, α) at the extreme values of the target tolerance range (ΔX_S), the spatial overlap of the intensity distribution (41) of the laser radiation with the structural element is such that the intensity (I_s) present in the region of the structural element is below the destruction threshold intensity (I_max) of the core wall.

17. The apparatus (11) according to claim 4, wherein The elongation factor is at least 10.

18. The device (11) according to claim 17, wherein The elongation factor is at least 100.

19. A system (10) for providing high intensity laser pulses for material processing of a workpiece (9), the system comprising: A laser pulse source (3) configured to output laser pulses (3A) having a spectral width and a pulse energy required for material processing as pulsed laser radiation, The device (11) for transmitting pulsed laser radiation according to any one of claims 1 to 18, comprising a hollow-core optical fiber (1) and a pulse duration setting device (13), and A laser processing head (21) focuses the laser pulse (3B') emitted from the hollow core optical fiber (1) onto the workpiece (9) to perform material processing.

20. The system (10) of claim 19, wherein: The laser processing head (21) has a pulse duration adaptation device (23) which is designed to shorten the transmission pulse duration (Δt_t) to a processing pulse duration (Δt').

21. The system (10) of claim 20, wherein: The pulse duration matching device (23) comprises at least one diffraction grating, volume Bragg grating, prism and / or dispersive mirror for use in transmission or in reflection.

22. The system (10) of claim 21, wherein: The pulse duration matching device (23) comprises at least one grating prism.

23. The system (10) according to claim 19 or 20, wherein The device (11) for transmitting pulsed laser radiation is set in such a way that, without prolonging the initial pulse duration (Δt) or with the maximum possible dispersion compression of the laser pulses, the pulse energy at the first fiber end (1A) leads to a peak power that causes damage to the hollow-core fiber (1).

24. The system (10) of claim 19, wherein: The laser pulse source comprises a laser amplifier system (L) and a pulse duration compression device (13").

25. A method for transmitting pulsed laser radiation from a laser pulse source (3) using a hollow core optical fiber (1), the hollow core optical fiber having a hollow core (4A) surrounded by a material (4B), wherein: The hollow core optical fiber (1) is configured to transmit the laser radiation in the hollow core (4A) from a first optical fiber end (1A) to a second optical fiber end (1B), the method comprising the following steps: generating pulsed laser radiation comprising laser pulses (3A) having an initial pulse duration (Δt), setting the peak power of the pulsed laser radiation present at the first optical fiber end (1A), and coupling the pulsed laser radiation into the hollow core optical fiber (1), The peak power is set by non-maximum compression in a pulse duration setting device (13) embodied as a pulse duration compression device (13") in such a way that for a beam trajectory parameter value (X, α) at the first fiber end (1A) that is within a target tolerance range (ΔX_S) provided for operating the hollow-core fiber (1), a laser pulse (3B) with the transmission pulse duration (Δt_t) is coupled into the hollow core (4A), wherein the material and / or structure of the hollow-core fiber (1) is maintained. The pulse duration setting device (13) is configured as a pulse duration compression device (13") to temporally compress laser pulses of pulsed laser radiation output by a laser amplifier system (L) with a pulse duration longer than a minimum possible pulse duration related to the existing spectral bandwidth to the transmission pulse duration (Δt_t) via at least one optical element (15) and thus output them as temporally extended laser pulses (3B) relative to the minimum possible pulse duration.

26. The method according to claim 25, further comprising: Determine the maximum pulse peak power coupled in with the beam trajectory parameter values (X, α) within the entire target tolerance range (ΔX_S), and The transmission pulse duration (Δt_t) is set in such a way that the peak power of the pulsed laser radiation is less than or equal to the maximum pulse peak power.

27. The method according to claim 25 or 26, The laser pulse (3B) is coupled into the hollow-core optical fiber by means of an optical fiber coupling-in device (17) that provides a beam path for the pulsed laser radiation, wherein the beam path provides a beam trajectory parameter value (X, α) within the target tolerance range (ΔX_S) at the first optical fiber end (1A) for damage-free coupling into the hollow-core optical fiber (1) at the transmission pulse duration (Δt_t).

28. The method according to claim 25 or 26, further comprising: coupling the pulsed laser radiation out from the hollow core optical fiber (1), shortening the pulse duration of the laser pulse (3B') of the pulsed laser radiation coupled out from the hollow core optical fiber (1) from the transmission pulse duration (Δt_t) to the processing pulse duration (Δt'), and The outcoupled pulsed laser radiation is focused to provide high-intensity laser pulses (3C) for material processing of a workpiece (9).

29. A device (11) for transmitting pulsed laser radiation having a pulse energy which, with a compressed pulse duration, results in high-intensity laser pulses (3C), comprising: A pulse duration compression device (13) is designed to receive the pulsed laser radiation and to set a transmission pulse duration (Δt_t) of the laser radiation, wherein: The pulse duration compression device is configured to compress the laser pulses (3A) of the pulsed laser radiation with respect to the pulse duration to the transmission pulse duration (Δt_t), A hollow core optical fiber (1) having a hollow core (4A) surrounded by a material (4B), wherein: The hollow core optical fiber (1) transmits the laser pulse (3B) coupled into the first optical fiber end (1A) to the second optical fiber end (1B) in the hollow core (4A) and outputs the laser pulse at the second optical fiber end, and - setting to operate the hollow-core optical fiber (1) with a beam tracking parameter value (X, α) present at the first optical fiber end (1A), said beam tracking parameter value being within a target tolerance range (ΔX_S), and a fiber coupling input device (17) configured to receive laser radiation set to the transmission pulse duration (Δt_t) and couple the laser radiation into the hollow-core optical fiber (1) with a beam trajectory parameter value (X, α) within the target tolerance range (ΔX_S), The transmission pulse duration (Δt_t) is set in such a way that for all beam trajectory parameter values (X, α) within the target tolerance range (ΔX_S), a coupling input of the laser radiation into the hollow core (4A) is given, in which the material and / or structure (37) of the hollow-core optical fiber (1) is retained, and the pulse duration compression device is configured to temporally compress laser pulses of pulsed laser radiation output by a laser amplifier system (L) with a pulse duration longer than a minimum possible pulse duration related to the existing spectral bandwidth to the transmission pulse duration (Δt_t) via at least one optical element (15) in terms of pulse duration and thus output them as temporally extended laser pulses (3B) relative to the minimum possible pulse duration.

30. A device (11) for transmitting pulsed laser radiation having a pulse energy which, with a compressed pulse duration, results in high-intensity laser pulses (3C), comprising: A pulse duration setting device (13) is designed to receive the pulsed laser radiation and to set a transmission pulse duration (Δt_t) of the laser radiation, wherein: The pulse duration setting device (13) is provided as a pulse duration compression device for compressing the laser pulses (3A) of the pulsed laser radiation with respect to the pulse duration to the transmission pulse duration (Δt_t), A hollow core optical fiber (1) having a hollow core (4A) surrounded by a material (4B), wherein: The hollow core optical fiber (1) transmits the laser pulse (3B) coupled into the first optical fiber end (1A) to the second optical fiber end (1B) in the hollow core (4A) and outputs the laser pulse at the second optical fiber end, and - setting to operate the hollow-core optical fiber (1) with a beam tracking parameter value (X, α) present at the first optical fiber end (1A), said beam tracking parameter value being within a target tolerance range (ΔX_S), and a fiber coupling input device (17) configured to receive laser radiation set to the transmission pulse duration (Δt_t) and couple the laser radiation into the hollow-core optical fiber (1) with a beam trajectory parameter value (X, α) within the target tolerance range (ΔX_S), wherein the transmission pulse duration (Δt_t) is set in such a way that for all beam trajectory parameter values (X, α) within the target tolerance range (ΔX_S), a coupling-in of the laser radiation into the hollow core (4A) is obtained in which the material and / or structure (37) of the hollow-core optical fiber (1) is retained, The pulse duration setting device (13) is configured as a pulse duration compression device (13") to temporally compress laser pulses of pulsed laser radiation output by a laser amplifier system (L) with a pulse duration longer than a minimum possible pulse duration related to the existing spectral bandwidth to the transmission pulse duration (Δt_t) via at least one optical element (15) and thus output them as temporally extended laser pulses (3B) relative to the minimum possible pulse duration.

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