DE102019127968B4 - Laser system - Google Patents

Abstract

Laser system (11) with: - a laser beam source (13) for emitting laser radiation (13A), - an optical filter system (17) which has an optical fiber (18) for guiding the laser radiation from a first end (19A) to a second end ( 19B), wherein the laser radiation (21) emerging from the second end (19B) of the optical fiber (18) forms a far-field distribution (2) in the far field (33), which has a central beam area (35) and around the central beam area (35) the far field distribution (2) arranged secondary maxima (37), and which has a spatial filter aperture (F) for optical filtering of laser radiation (21) emerging from the optical fiber (18), the spatial filter aperture (F) being designed to be in the far field (33 ) to remove at least one of the secondary maxima (37) from the far field distribution (2), and a fiber coupling unit (15) for coupling the laser radiation (13A) into the optical fiber (18).

Description

The present invention relates to a laser system, for example for material processing.

The beam quality of optical fields, which are emitted as laser radiation from high-power ultrashort pulse lasers, often differs slightly from the ideal diffraction limit. For example, a beam quality M ² can be present in the range from 1.0 to approximately 1.3, with the beam quality M ² (beam propagation factor) describing the deviation of the measuring beam from a diffraction-limited Gaussian beam with regard to beam diameter and divergence angle. The latter has an M ² of 1. With an M ² of typically less than or equal to 1.3, one still speaks of laser radiation in the basic mode or of an almost diffraction-limited beam quality, but the beam components that deviate from the Gaussian beam can have negative effects on the Use of laser radiation. For example, they can lead to an enlarged focus zone, to one or more secondary foci or to undesirable heating of optical components. Such beam components, in combination with beam-shaping elements that are intended to produce a flattop focus, for example, can reduce the homogeneity in the intensity profile and thus cause inhomogeneous removal or drilling results. Furthermore, when forming a Bessel beam from a fundamental mode beam, such beam components can cause modulations along the optical axis and thus cause modifications of varying degrees in the elongated focus zone in the transparent material.

US 9,645,309 B2 discloses a multimode optical fiber for propagating laser radiation in a fundamental mode with low bending loss and for suppressing higher order modes. Further revealed US 8,009,948 B2 a hollow core fiber with a central section of reduced diameter, whereby core diameters of different sizes can be realized along the fiber.

US 2008/0 219 620 A1 discloses a device for converting a higher order mode into a near-fundamental Gaussian form.

The DE 10 2015 005 257 A1 discloses an optical arrangement for increasing the beam quality and improving the intensity distribution.

One aspect of this disclosure is based on the task of specifying a system for reliable filtering of unwanted beam components from laser radiation. Another task can be to generate laser radiation that propagates almost in the (free field) fundamental mode for subsequent beam guidance.

At least one of these tasks is solved by a laser system according to claim 1. Further developments are specified in the subclaims.

The laser system has an optical filter system with an optical fiber for guiding laser radiation from a first end to a second end, the laser radiation emerging from the second end of the optical fiber forming a far-field distribution in the far field, which has a central beam region and around the central beam region of the far-field distribution arranged secondary maxima. Furthermore, the optical filter system has a spatial filter aperture which is designed to remove at least one of the secondary maxima from the far field distribution in the far field.

The laser system further has a laser beam source for emitting laser radiation and an optical filter system as described above, which has an optical fiber and a spatial filter aperture for optically filtering laser radiation emerging from the optical fiber. Furthermore, the laser system comprises a fiber coupling unit for coupling the laser radiation into the optical fiber and optionally a focusing unit for focusing the laser radiation optically filtered with the spatial filter aperture, in particular for material processing. The laser system can be designed as a laser amplifier system in which the laser beam source is designed as a laser amplifier stage. Alternatively or additionally, the laser system can include a laser amplifier stage for amplifying the laser radiation optically filtered with the spatial filter aperture. Optionally, a focusing unit can then be provided for focusing laser radiation amplified by the laser amplifier stage, in particular for material processing.

The spatial filter aperture can have a central transmission area and at least one aperture area protruding from the transmission area. The transmission area and the central beam area are coordinated in size in such a way that the central beam area Spatial filter aperture happens, and the position of the at least one aperture area coincides with the position of at least one of the secondary maxima and blocks this.

The optical filter system can be designed in particular to filter unwanted beam components from laser radiation and to provide laser radiation that propagates largely in a basic mode for subsequent use, for example to focus the laser beam on a workpiece during material processing.

In some developments, the optical fiber can be designed in such a way that the laser radiation in the optical fiber is guided in a (basic) mode, with a phase jump between the central beam region and in the far field distribution after the optical fiber, which optionally forms after a collimation optics unit the secondary maxima is present. The far field distribution can have (first) intensity-free regions between the central beam region and the secondary maxima, in particular due to the phase jump. Further intensity-free areas and phase jumps can form further out (in the radial direction). The spatial filter aperture can have a transmission region and at least one aperture region, the transmission region allowing laser radiation incident on the spatial filter aperture to pass through (pass through), and the at least one aperture region blocking laser radiation incident on the spatial filter aperture (F) (preventing further propagation in the beam direction). A geometric course of the transition between the transmission region and the at least one aperture region can be adapted to the course of the intensity-free regions and the spatial filter aperture can be arranged in the beam path in such a way that the spatial filter aperture transmits the central beam region and (in the radial direction) outside the first intensity-free regions lying laser radiation blocks, in particular cuts off or deflects.

In some developments, the filter system can comprise a collimation optics unit (e.g. designed as a fiber coupling unit) which is arranged to collimate the emerging laser radiation and is in particular designed as a focusing lens and/or mirror unit or as a microscope objective. The collimation optics unit is spaced from the second end of the optical fiber in such a way that a beam path section is formed downstream of the collimation optics unit, in which (essentially) the far-field distribution is given and in which the spatial filter diaphragm is arranged.

In some developments, the optical fiber of the optical filter system can be designed such that, in particular for laser radiation that is guided with the optical fiber in a (basic) mode, there is a phase jump in the far field distribution between the central beam area and the secondary maxima. The far field distribution can have intensity-free regions between the central beam region and the secondary maxima, particularly due to the phase jump.

In some developments, the optical fiber of the optical filter system can be designed as a single-mode optical fiber, in particular as a hollow-core optical fiber or step-index optical fiber, which essentially carries laser radiation only in a basic mode. Additionally or alternatively, the optical fiber can be designed as an ultra-short pulse optical fiber, in particular as a hollow core optical fiber, photonic crystal optical fiber such as Kagome optical fiber or tubular optical fiber or as a multimodal hollow core optical fiber.

In some developments, the optical fiber can be designed as a hollow core optical fiber of the Kagome type with seven or nine secondary maxima in the far field and the transmission range of the spatial filter diaphragm can have a corresponding seven-fold or nine-fold rotational symmetry. Optionally, the spatial filter panel can have a polygonal opening forming the transmission region.

In some developments, the optical fiber can be designed as a hollow core optical fiber of the tubular type with four to ten, optionally eight, glass web rings and correspondingly four to ten, optionally eight, secondary maxima in the far field and the transmission range of the spatial filter diaphragm can have a corresponding number of rotational symmetry . The spatial filter panel can optionally have a polygonal opening forming the transmission region.

In some developments of the optical filter system, the central beam area can have a constant phase. In particular, the central beam area can be an area of the field profile with a constant phase.

In some developments, the optical fiber of the optical filter system can be designed as a hollow core optical fiber, which has a hollow core that is surrounded by a material structure, in particular glass webs. The material structure can form a hollow core inner wall, which approaches a center of the hollow core in sections and / or which has a cross-sectional profile with an integer rotational symmetry (with an integer greater than 1, ie n-fold with n >1; in general, "n" stands for a natural number) and/or whose cross-sectional shape approximates a polygonal basic geometry, in particular a hexagonal or an octagonal geometry.

In some developments, the hollow core inner wall can have a cross-sectional shape which leads to the far field distribution of the fundamental mode.

Furthermore, the spatial filter diaphragm can have a diaphragm structure which delimits a transmission region of the laser radiation which approximates the cross-sectional course of the far-field distribution of a free-field fundamental mode, excluding the secondary maxima.

In some developments, the secondary maxima of the optical fiber can be distributed azimuthally, in particular with an n-fold rotational symmetry with an integer n > 1, around the central beam area, and the spatial filter diaphragm can have a transmission area and aperture areas. The aperture areas are also arranged azimuthally, and in particular with the same n-fold rotational symmetry with n > 1, distributed around the transmission area.

In some developments of the material structure, this can form a hollow core inner wall, the cross-sectional shape of which forms a form of a fundamental mode of the optical fiber in the near field. Furthermore, the spatial filter aperture can have a transmission area and aperture areas such that a geometric progression of the transition between the transmission area and the aperture areas is adapted to the shape of the basic mode. Optionally, the geometric course of the transition between the transmission region and the aperture regions can follow the course of the phase change at least in sections. Alternatively or additionally, the geometric course can essentially surround the central beam area, which is in particular given by a constant phase of the laser radiation.

In some developments of the optical filter system, intensity components of a (free-field) fundamental mode that deviate from rotational symmetry in the narrower sense (circular symmetry) can be removed from the far-field distribution using the spatial filter diaphragm. Alternatively or additionally, laser radiation in at least one of the secondary maxima of the far field distribution can interact with the spatial filter diaphragm, in particular be absorbed or deflected by it.

In some developments, the optical filter system can comprise a rotation unit to which the spatial filter aperture or the hollow core optical fiber is attached and which is designed to adapt the azimuthal orientation (around the beam axis) of the spatial filter aperture to the far field distribution. With the rotation unit, the angular positions of aperture areas can be adjusted in such a way that they correspond to the angular positions of the secondary maxima of the far field distribution.

The concepts described herein can enable a (high) performance mode filter, at the output of which laser radiation is essentially in the Gaussian mode.

• 1 eine schematische Skizze eines Lasersystems mit einem optischen Filtersystem basierend auf einer ersten beispielhaften Lichtleitfaser und
• 2 eine schematische Skizze eines Lasersystems mit einem optischen Filtersystem basierend auf einer zweiten beispielhaften Lichtleitfaser.

Concepts are disclosed here that make it possible to at least partially improve aspects of the prior art. In particular, further features and their usefulness emerge from the following description of embodiments based on the figures. Of the figures show:

• 1 a schematic sketch of a laser system with an optical filter system based on a first exemplary optical fiber and
• 2 a schematic sketch of a laser system with an optical filter system based on a second exemplary optical fiber.

Aspects described herein are based in part on the knowledge that a (high-performance) optical fiber, such as a (hollow-core) photonic crystal fiber (HC-PCF), can be used as part of a “mode” filter (also herein referred to as an optical filter system). An HC-PCF provides a light guidance mechanism based on photonic bandgaps or inhibited coupling. A distinction is made between fibers of the “Kagome” type (with 7 or 19 cells, where the number of cells corresponds to the number of missing star-like cells in the core corresponds) and fibers of the “tubular” type (with a number of 4, 5, 6, ... tubes or (glass web) rings in the refractive index profile).

Examples of hollow core fibers generally include single-mode or multi-mode hollow core optical fiber. Further examples of single-mode optical fibers are, in particular, step-index optical fibers, which essentially carry laser radiation only in one (fiber) basic mode.
Based on the laser radiation emerging from the fiber, it was recognized that with the combination of fiber and a spatial filter arranged in a generated far field, the beam quality can be improved by blocking/filtering power ranges that are not assigned to the (free space) fundamental mode. (Free space here is understood to mean the propagation of laser radiation in free space, for example in air or in a gas or in a vacuum, in contrast to the propagation of laser radiation guided in a fiber.) Such an optical filter system can in particular make it possible to achieve an almost “ideal “To deliver beam quality with M ² ~ 1 for use in material processing for high-intensity laser pulses. For this purpose, one can consider an overlap integral I_Overlapp_filtered of the filtered optical field (U_filtered) to the field of a Gaussian-shaped fundamental mode (U fundamental mode), which is given by: $$I\_\ddot{U}berlapp\_Gefiltert=\int \int dxdyU\_Gefiltert\left(x,y\right)xU\_GrundmOde\left(x,y\right).$$

With appropriately adjusted spatial scaling parameters, overlap values of (well) over 99% can be achieved. If there is an overlap integral of the filtered optical field to the field of a Gaussian-shaped fundamental mode that is greater than 99%, one can speak of (almost) “ideal” beam quality.

wobei U_in das Einkoppelfeld (d.h. das in die Faser eingekoppelte Feld) und U Grundmode das Feld des Grundmodes der z.B. HC-PCF ist. Bei einem reduzierten Moden-Überlappintegral können sich Leistungsverluste ergeben.It was further recognized that the use of a hollow core fiber can make it possible for a significant proportion of the power of the coupled laser radiation to be carried by the basic mode of the hollow core fiber, depending on the type and degree of aberrations when coupling in wavefront aberrated or multi-mode optical fields. However, deviations from the field distribution of the fundamental mode during coupling are usually not stably guided by the hollow core fiber. Deviations can be determined, for example, with a mode overlap integral (I_mode overlap), which is given by: $$I\_MOden-\ddot{U}berlapp=\int \int dxdyU\_in\left(x,y\right)xU\_GrundmOde\left(x,y\right),$$

where U_in is the coupling field (ie the field coupled into the fiber) and U fundamental mode is the field of the fundamental mode of, for example, HC-PCF. A reduced mode overlap integral can result in power losses.

In general, the laser radiation emerging from the hollow core fiber can already have an (increased) high beam quality M ² in the range of, for example, approximately 1.1 to 1.2. The concepts proposed here now make it possible to transform the beam quality M ² closer or almost completely to the diffraction limit through spatial frequency filtering.

• Stufe 1:
  • Eine potentiell aberrierte Laserstrahlung (1,0 < M² < ca. 1,3) des Hochleistungs-Ultrakurzpulslasers wird mittels einer Fasereinkopplungseinheit (z.B. ein Mikroskop-Objektiv) in die Hohlkernfaser eingekoppelt. Von der Hohlkernfaser wird lediglich die Grundmode geführt. Abhängig von der Strahlqualität des eingekoppelten Feldes kann es zu Transmissionsverlusten kommen. Am Ausgang einer beispielsweise wenige Dezimeter langen Hohlkernfaser kann bereits eine verbesserte Strahlqualität vorliegen, die durch die Strahlqualität des (Faser-) Grundmodes der Hohlkernfaser vorgegeben ist.
• Stufe 2:
  • Je nach Faserdesign der Hohlkernfaser kann der (Freiraum-) Grundmode der austretenden Laserstrahlung im Nahfeld eine nicht „rotationssymmetrisch im engeren Sinne“, allgemeine eine n-zählig rotationssymmetrische Geometrien mit n > 1, beispielsweise mehreckige wie z.B. hexagonal-artige, Feldverteilung aufweisen. Diese Feldverteilung unterscheidet sich leicht von der idealen (Freiraum-) Grundmode und kann z.B. eine Strahlqualität M² von ca. 1,1 aufweisen.

In other words, achieving improved (almost “ideal”) beam quality using hollow core fiber and spatial frequency filtering using the example of a high-performance ultra-short pulse laser takes place in two stages:

• Step 1:
  • A potentially aberrated laser radiation (1.0 <M ² <approx. 1.3) from the high-power ultrashort pulse laser is coupled into the hollow core fiber by means of a fiber coupling unit (eg a microscope lens). Only the basic mode is carried by the hollow core fiber. Depending on the beam quality of the coupled field, transmission losses can occur. At the output of a hollow core fiber, for example a few decimeters long, there can already be an improved beam quality, which is determined by the beam quality of the basic (fiber) mode of the hollow core fiber.
• Level 2:
  • Depending on the fiber design of the hollow core fiber, the basic (free space) mode of the emerging laser radiation in the near field can have a field distribution that is not “rotationally symmetrical in the narrower sense”, generally an n-fold rotationally symmetrical geometries with n > 1, for example polygonal such as hexagonal-like. This field distribution differs slightly from the ideal (free space) basic mode and can, for example, have a beam quality M ² of approximately 1.1.

For the present considerations, the ideal (free space) basic mode is rotationally symmetric in the narrower sense, whereby “rotationally symmetric in the narrower sense” is understood to mean that a field distribution can be rotated about an axis of rotation by any angle without the field distribution changing. An integer rotational symmetry of a field distribution (with an integer greater than 1, i.e. n-fold with n >1; generally "n" here stands for a natural number) exists if the field distribution only extends by predetermined angles (e.g. 360°/n). can be rotated around an axis of rotation without changing the field distribution.

The laser radiation emerging from the hollow core fiber is collimated. By collimating the laser radiation, an associated far field distribution can be obtained in which the field components that impair the beam quality are spatially separated from the useful beam (i.e. offset from the beam axis/offaxis). With regard to the far field distribution, spatial frequency filtering, i.e. blocking the unwanted beam components, can be carried out with a geometrically adapted, for example hexagonal, high-performance aperture. The aperture can, for example, be designed to be refractive or diffractive. In this way, a laser beam with an (almost) ideal, well-defined beam quality can be guided, for example, onto a workpiece to be processed after renewed focusing.

It is noted that in the first step the optical filtering can also be implemented using a multi-mode hollow core fiber (also referred to as few-mode HCF), whereby the beam components of higher modes are either retained as part of the filtered laser radiation or in a similar way the secondary maxima of the fundamental mode of the hollow core fiber are removed from the laser radiation using the spatial filter.

Referring to 1 A laser system 11 includes a laser beam source 13, a fiber coupling unit 15 and an optical filter system 17.

The laser system 11 is, for example, a laser oscillator or a laser amplification system and generates laser radiation 13A, for example pulsed high-intensity laser radiation with pulse durations in the nanosecond to femtosecond range and pulse energies in the range from 1 µJ to 100 mJ. The generated laser radiation 13A may have a beam quality that deviates from an ideal Gaussian beam quality. In order to avoid disadvantages when using such laser radiation, as already mentioned, the optical filter system 17 proposed here can be used.

The optical filter system 17 comprises a hollow core optical fiber 18, a collimation optics unit 23 (also referred to as a collimation unit or fiber coupling unit) and a spatial filter diaphragm F. The collimation optics unit 23 (and also the fiber coupling unit 15) can be, for example, a lens and / or mirror unit or a microscope Include lens that is configured and arranged to collimate the emerging laser radiation 21 (focusing the laser radiation 13A). For example, the collimation optics unit 23 is spaced from the second end 19B in such a way that a beam path section is formed downstream of the beam in which the far field distribution 2 is given, which can be viewed as the far field of the coupled-out laser radiation.

Furthermore, the optical filter system 17 can comprise a rotation unit R, R', which is provided for the relative alignment of the hollow core optical fiber 18 with respect to the spatial filter diaphragm F. The rotation unit R, R' can, for example, rotate the hollow core fiber or the spatial filter diaphragm about the beam axis and thus align it to one another at the rotation angle. The rotation unit is designed to adapt the orientation of the spatial filter diaphragm F to the far field distribution 2. In general, the angular positions of the aperture areas 43 are adjustable with respect to the beam axis and preferably correspond to angular positions of the secondary maxima 37 of the far field distribution 2.

Furthermore, you can see in 1 a focusing unit 27, which is arranged downstream of the optical filter system 17 and which focuses the filtered beam onto a material surface for material processing (as an example of an application).

With regard to the beam path of the laser radiation 13A generated by the laser beam source 13, the laser radiation in 1 divided into a laser radiation 21 emerging from the hollow core optical fiber 18, an optically filtered laser radiation 25 and a resulting (diffraction-limited focused) laser beam 29.

As in 1 shown is the laser radiation to be filtered 13A coupled with a fiber coupling unit 15 (for example an (aspherical) microscope objective) into the hollow core optical fiber 18 at a first end 19A. If the hollow core optical fiber 18 is designed as a basic mode optical fiber, the coupled laser radiation is only guided in a (fiber) basic mode of the hollow core optical fiber 18.

In 1 1, an exemplary crystal structure is shown in a fiber cross section 51 for a Kagome-type hollow core optical fiber 18 with seven cells. The crystal structure forms a hollow core 53 around a center Z. This allows high-intensity laser radiation to spread in an area that is filled with gas (or even gas-free under vacuum). A material structure 55 on which the crystal structure is based comprises a plurality of glass webs 57, which in particular form a hollow core inner wall 59.

It can be seen that the geometry of the hollow core 53 is not rotationally symmetrical in the narrower sense, but rather has an integer number (here, for example, 6) of crystal cells that protrude furthest in the direction of the center Z. The cross section of the hollow core inner wall 59 is rotationally symmetrical with a count of 6 (“6-fold rotationally symmetric”). Accordingly, the basic (fiber) mode of the hollow core optical fiber 18 is not rotationally symmetrical, and the beam profile of the laser radiation 21 emerging at a second end 19A of the hollow core optical fiber 18 also deviates from rotational symmetry in the narrower sense.

1 shows a near-field amplitude distribution 1 with an exemplary field distribution 31 in the near field of the second end 19B of the hollow core optical fiber 18. The near-field amplitude distribution 1 is shown as a grayscale image. A phase representation is unnecessary because there is a uniform phase at this point. A near-field amplitude distribution 1' represents a correspondingly schematic line drawing of the near-field amplitude distribution 1. It can be seen that the field distribution 31 corresponds in its geometric shape and in particular in its number to the course of the hollow core inner wall 59 in the fiber cross section 21.

The field distribution 31 in the near field is converted into a far field distribution in a far field 33 by the focusing unit 27, for example also a micro lens.

1 shows a far-field amplitude distribution 2 in the far field 33. The far-field amplitude distribution 2 includes a central beam region 35 and six schematically highlighted secondary maxima 37. The underlying symmetry is therefore six-fold. Between the central beam area 35 and each of the secondary maxima 37 there is an intensity-free area 39, which is due to a phase jump 39A in the field distribution (schematically in 1 indicated by dashed lines). As shown for the near field 1 a far-field amplitude distribution 2' as a schematic line drawing of the far-field amplitude distribution 2.

As an example, a beam quality M2eff of approximately 1.12 was determined for the field distributions shown in the near field and in the far field 33.

The spatial filter diaphragm F is arranged in the far field 33. The spatial filter aperture F is shaped and aligned with respect to the geometry of the far-field amplitude distribution 2 so that the desired optical filtering occurs. In other words, the far field amplitude distribution 2 (or the laser radiation 21 emerging from the fiber) is spatial frequency filtered after collimation with an adapted aperture. The entire field distribution in the far field is called the spatial frequency distribution. In the far field, the spatial frequencies whose phase differs from the central distribution with constant phase by just “Pi” (π) are now filtered. In areas further out there are further spatial frequencies that again have the phase value “0”, i.e. are again in phase with the main maximum. However, these areas have little performance and can therefore be neglected and thus also filtered.

The spatial filter diaphragm F, which is matched to the fiber cross section 51, comprises a hexagonal transmission region 41 and diaphragm regions 43 which are assigned to the sides of the hexagon. The aperture areas 43 block the secondary maxima 37 of the far field amplitude distribution 2, with at least one aperture area 43 generally coinciding in position with the position of at least one of the secondary maxima 37. However, the transmission area 41 and the central beam area 35 are matched in size to one another in such a way that the central beam area 35 passes the spatial filter diaphragm F. An example is in 1 a field component that strikes one of the aperture areas 43 and is thus filtered out of the laser radiation is indicated with dots. You recognize in 1 thus, that of a rotation sym In the narrower sense, deviating intensity components of a basic mode can be removed from the far field distribution 2 using the spatial filter diaphragm F.

In other words, the spatial filter diaphragm F has a diaphragm structure which delimits a transmission region for the laser radiation, the geometry of the transmission region approximating the cross-sectional course of a far-field distribution of a free-field fundamental mode, excluding the secondary maxima.

1 shows a filtered far-field amplitude distribution 3 with correspondingly reduced deviations from rotational symmetry. By way of example, the filtered far-field amplitude distribution 3 shown can be assigned a beam quality M2eff of approximately 1.02. The optically filtered laser radiation 25 is now available downstream of the spatial filter diaphragm F, essentially with limited diffraction.

1 also shows a far-field amplitude distribution 3' for the filtered far-field amplitude distribution 3 in the form of a schematic line drawing.

For further addition shows 1 schematically a phase distribution 61 of the laser radiation striking the spatial filter diaphragm. The laser radiation can, for example, be assigned a phase of “0” in the area of the transmission region 41. The laser radiation in the area of the secondary maxima 37 can, for example, be assigned a phase of “Pi”. (The previously mentioned further outer areas with a phase of “Pi” are not shown.) For example, the optical fiber is designed in such a way that for laser radiation that is guided with the optical fiber in a (basic) mode, a phase jump between the central beam area 35 and the secondary maxima 37 are present. Due to the phase jump, the far field distribution has intensity-free regions between the central beam region 35 and the secondary maxima 37. Typically, the central beam region 35 of the fundamental mode has a constant phase. Such phase differences can also cause adverse effects, for example when focusing.

When comparing the filter geometry and phase progression you can see: 1 (as in 2 ), that the geometric course of the transition between the transmission region 41 and the aperture regions 43 follows the course of the phase change in sections and essentially surrounds the central beam region 35. The (first) phase jump between the main maximum and the first secondary maximum thus defines the geometric shape of the aperture. Areas outside of this first phase jump are cut away with the selected filter geometry. Accordingly, further secondary maxima, which again have a phase of the main maximum, are also filtered. It is essential that laser radiation in at least one of the secondary maxima 37 of the far field distribution 2 can interact with the spatial filter diaphragm F and in particular is absorbed or deflected by it.

However, if the filtered laser radiation 25 is focused with the focusing unit 27, the laser radiation in the focus has a substantially rotationally symmetrical near-field amplitude distribution 4. The focused laser radiation can also be assigned a beam quality M2eff of approximately 1.02. 1 shows a near-field amplitude distribution 4' of the filtered and focused laser beam as a line drawing.

With the help of optical filtering, an almost diffraction-limited laser beam is generated, which allows, for example, high-precision material processing.

1 shows an example of mode filtering using a hollow core optical fiber 18 designed as a single-mode fiber. This may represent a preferred embodiment of the optical filter system. However, as already mentioned, multi-mode fibers can also be used, whereby the contributions of the higher modes can also be influenced using the spatial filter diaphragm F.

2 shows, as a further example, a laser system 11' with an optical filter system 17' based on a hollow core optical fiber 18' of the tubular type with eight rings (tubes) and additionally represents the development of the field distributions due to the optical filtering, ie in particular the field distributions in Near field, in the far field as well as in the filtered far field and in the filtered near field. Further clarified 2 the correspondingly assigned basic geometry of the spatial filter diaphragm F' of the optical filter system 17', as adapted to the hollow core optical fiber 18', and a resulting phase distribution 61'.

In the fiber cross section 51' you can see eight glass web rings 57' (tubes) arranged in a circle next to each other. This results in an 8-fold rotational symmetry for the near field. This can be seen in the near-field amplitude distribution 101 and 101' and was highlighted schematically in the far-field amplitude distribution 102'. The near-field amplitude distribution 101 and the far-field amplitude distribution 102 can be assigned a beam quality M2eff of approximately 1.14.

The spatial filter diaphragm F' has an octagonal opening, with the orientation relative to the secondary maxima 37' being aligned in such a way that the spatial filter diaphragm F' acts as a spatial filter for the secondary maxima 37'. Accordingly, as can be seen from the phase distribution 61', the intensity ranges of the collimated laser radiation with a different phase (marked here with “Pi”) are blocked. The opening thus prevents the propagation of the laser radiation, which is present (in the radial direction to the beam axis) outside a central beam area with a constant phase.

Beam qualities of approximately 1.03 can be assigned to the filtered far-field amplitude distributions 103, 103' and near-field amplitude distributions 104, 104'. This can also be seen in the improved field profiles in terms of rotational symmetry (almost rotational symmetry in the narrower sense). In general, spatial filtering can, for example, improve beam qualities greater than 1.1 for unfiltered laser radiation to beam qualities less than 1.1 for filtered laser radiation, which is then available for further use.

Regarding 2 The reference numbers not mentioned are briefly summarized below. They refer to those already related to the 1 described aspects: 11' laser system, 13' laser beam source, 13A' laser radiation, 15' fiber coupling unit, 17' optical filter system, 19A', 19B' first end/second end of the hollow core fiber, 21' emerging laser radiation, 23' collimation optics unit, R, R' Rotation units, 25' optically filtered laser radiation, 27' focusing unit, 29' diffraction-limited laser beam, 31' field distribution, 33' far field, 35' central beam area, 37' secondary maximum, 39' intensity-free area, 41' transmission range, 43' aperture range, 51' fiber cross section, 53' hollow core, 55' material structure, 57 glass web ring, 59' hollow core inner wall, example phase values “0”, “Pi”.

In the 1 and 2 It can be seen that the secondary maxima of the optical fiber are distributed azimuthally, in particular with an n-fold rotational symmetry with n > 1 (for example n = 6 or n = 8), around the central beam area. Furthermore, the aperture areas are also arranged azimuthally, and in particular with the same n-fold rotational symmetry with n > 1 (for example n = 6 or n = 8) distributed around the transmission area. The azimuthal distribution of the secondary maxima refers to a beam axis of the laser radiation, which is given by a beam center of the central beam area, and the azimuthal arrangement of the aperture areas relates to an aperture center of the transmission area, the aperture center being adjusted in such a way that it is aligned with the beam axis matches. It can also be seen that the geometric shape of the transition between the transmission region and the aperture regions surrounds a central region of the (free field) fundamental mode.

The 1 and 2 are related to corresponding simulations that were carried out with regard to coupling efficiency, filter efficiency and application of the inventive concept to various beam shaping techniques (flattop beam profiles, etc.). It was recognized that the results depend on the respective HCF design. For example, the different types of hollow core optical fibers (Kagome type etc.) have different refractive index distributions and hollow core geometries, so that the filter geometries and the beam qualities to be achieved differ accordingly.

However, it has generally been found that power losses in spatial frequency filtering can be less than 2%.

It is further noted that the concepts of an optical filter system described herein can be used at the output of any laser system, in particular at the output of amplifier stages or before and between amplifier stages, in order to create a well-defined, reproducible interface with regard to the parameters of location, angle and beam quality for a to deliver subsequent application. An example is in 2 a (post) amplifier stage 71 indicated by dashed lines, to which the filtered laser beam is supplied for amplification.

It is noted that in some embodiments the fiber coupling unit can be designed as part of the optical filter system.

It is expressly emphasized that all features disclosed in the description and/or the claims are considered to be separate and independent from each other for the purpose of the original disclosure as well as for the purpose of limiting the claimed invention, regardless of the combinations of features in the embodiments and/or the claims should. It is expressly stated that all range statements or statements of groups of units disclose every possible intermediate value or subgroup of units for the purpose of the original disclosure as well as for the purpose of limiting the claimed invention, in particular also as a limit of a range statement.

Claims (14)

Laser system (11) with: - a laser beam source (13) for emitting laser radiation (13A), - an optical filter system (17) which has an optical fiber (18) for guiding the laser radiation from a first end (19A) to a second end (19B), the laser radiation emerging from the second end (19B) of the optical fiber (18). (21) forms a far-field distribution (2) in the far field (33), which has a central beam area (35) and secondary maxima (37) arranged around the central beam area (35) of the far-field distribution (2), and which has a spatial filter diaphragm (F). optical filtering of laser radiation (21) emerging from the optical fiber (18), the spatial filter diaphragm (F) being designed to remove at least one of the secondary maxima (37) from the far field distribution (2) in the far field (33), and - a fiber coupling unit (15) for coupling the laser radiation (13A) into the optical fiber (18). Laser system (11). Claim 1 , wherein the optical fiber (18) is designed such that the laser radiation in the optical fiber (18) is guided in a mode, wherein in the far field distribution (2) after the optical fiber (18), which optionally forms after a collimation optics unit (23). , there is a phase jump (39A) between the central beam area (35) and the secondary maxima (37), and the far field distribution (2), in particular due to the phase jump (39A), has intensity-free areas (39) between the central beam area (35) and the Has secondary maxima (37). Laser system (11). Claim 2 , wherein the spatial filter aperture (F) has a transmission region (41) and at least one aperture region (43), the transmission region (41) allowing laser radiation incident on the spatial filter aperture (F) to pass through, and the at least one aperture region (43) on the spatial filter aperture ( F) blocks incident laser radiation, a geometric course of the transition between the transmission region (41) and the at least one aperture region (43) is adapted to the course of the intensity-free regions (39), and the spatial filter aperture (F) is arranged in the beam path in such a way that the spatial filter diaphragm (F) transmits the central beam area (35) and blocks laser radiation lying outside the first intensity-free areas. Laser system (11). Claim 3 , wherein the geometric course of the transition between the transmission region (41) and the aperture regions (43) - at least in sections follows the course of the phase change and / or - the central beam region (35), which is given in particular by a constant phase of the laser radiation, in Essentially surrounds. Laser system (11) according to one of the Claims 1 until 4 , further with: a collimation optics unit (23), in particular a lens and/or mirror unit or a microscope objective, which is arranged to collimate the emerging laser radiation (21), the collimation optics unit (23) being extended from the second end (19B) of the Optical fiber (18) is spaced so that a beam path section is formed downstream of the collimation optics unit (23), in which the far field distribution (2) is given and in which the spatial filter diaphragm (F) is arranged, the spatial filter diaphragm (F) has a central transmission region (41) and has at least one diaphragm region (43) protruding from the transmission region (41), the transmission region (41) and the central beam region (35) are coordinated in size in such a way that the central beam region (35) passes through the spatial filter aperture (F), and the position of the at least one aperture region (43) coincides with the position of the at least one of the secondary maxima (37) and blocks this. Laser system (11) according to one of the preceding claims, wherein the optical fiber (18) is designed as a single-mode optical fiber, in particular as a hollow core optical fiber or step index optical fiber, which essentially guides laser radiation only in a basic mode (31), and / or the optical fiber (18) is designed as an ultra-short pulse optical fiber, in particular as a hollow core optical fiber, photonic crystal optical fiber such as Kagome optical fiber or tubular optical fiber or as a multimodal hollow core optical fiber. Laser system (11) according to one of the preceding claims, wherein the central beam region (35) has a constant phase, in particular is a region of the field profile with a constant phase. Laser system (11) according to one of the preceding claims, wherein the optical fiber (18) is designed as a hollow-core optical fiber of the Kagome type with seven or nine secondary maxima in the far field and the transmission region (41) of the spatial filter diaphragm (F) has a corresponding seven-fold or nine-fold rotational symmetry and the spatial filter diaphragm (F) optionally has a polygonal opening forming the transmission region (41), or the optical fiber (18) is designed as a hollow core optical fiber of the tubular type with four to ten, optionally eight, glass web rings and correspondingly four to ten, optionally eight, secondary maxima in the far field and the transmission region (41) of the spatial filter diaphragm (F) has a corresponding number in rotational symmetry and the spatial filter diaphragm (F) optionally has a polygonal opening forming the transmission region (41). Laser system (11) according to one of the preceding claims, wherein the spatial filter diaphragm (F) has a diaphragm structure which delimits a transmission region for the laser radiation, the geometry of the transmission region corresponding to the cross-sectional course of a far-field distribution (2) of a free-field fundamental mode (35). the secondary maxima is approximated. Laser system (11) according to one of the preceding claims, wherein the secondary maxima (37) of the optical fiber (18) are distributed azimuthally, in particular with an n-fold rotational symmetry, where n is an integer greater than 1, around the central beam area (35) and the spatial filter aperture (F) has a transmission region (41) and aperture regions (43) which are also distributed azimuthally, and in particular with the same n-fold rotational symmetry with n > 1, around the transmission region (41). Laser system (11) according to one of the preceding claims, wherein Intensity components of a (free-field) fundamental mode (31) that deviate from rotational symmetry in the narrower sense are removed from the far-field distribution (2) with the spatial filter diaphragm (F) and/or Laser radiation in at least one of the secondary maxima (37) of the far field distribution (2) interacts with the spatial filter diaphragm (F), in particular is absorbed or deflected by it. Laser system (11) according to one of the preceding claims, further comprising: a rotation unit (R, R'), to which the spatial filter diaphragm (F) or the optical fiber (18) is attached and which is designed to adapt the azimuthal orientation of the spatial filter diaphragm (F) to the far field distribution (2), so that angular positions of diaphragm areas (43) are adjustable, and the angular positions of aperture areas correspond in particular to the angular positions of the secondary maxima (37) in the far field distribution (2). Laser system (11) according to one of the preceding claims, wherein the laser system (11) further comprises a focusing unit (27) for focusing the laser radiation (25) optically filtered with the spatial filter aperture (F), in particular for material processing. Laser system (11) according to one of the Claims 1 until 12 , wherein the laser system (11) is designed as a laser amplifier system, in which the laser beam source (13) is designed as a laser amplifier stage, and / or wherein the laser system (11) further has a laser amplifier stage (71) for amplifying the optically filtered with the spatial filter aperture (F). Laser radiation (25) and optionally a focusing unit for focusing laser radiation amplified with the laser amplifier stage (71), in particular for material processing.