WO2021074380A1 - Système de filtre optique et système laser - Google Patents

Système de filtre optique et système laser Download PDF

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Publication number
WO2021074380A1
WO2021074380A1 PCT/EP2020/079216 EP2020079216W WO2021074380A1 WO 2021074380 A1 WO2021074380 A1 WO 2021074380A1 EP 2020079216 W EP2020079216 W EP 2020079216W WO 2021074380 A1 WO2021074380 A1 WO 2021074380A1
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WO
WIPO (PCT)
Prior art keywords
diaphragm
optical fiber
laser radiation
spatial filter
laser
Prior art date
Application number
PCT/EP2020/079216
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German (de)
English (en)
Inventor
Daniel FLAMM
Malte Kumkar
Dirk Sutter
Original Assignee
Trumpf Laser Gmbh
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Publication date
Application filed by Trumpf Laser Gmbh filed Critical Trumpf Laser Gmbh
Publication of WO2021074380A1 publication Critical patent/WO2021074380A1/fr

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Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02295—Microstructured optical fibre
    • G02B6/02314—Plurality of longitudinal structures extending along optical fibre axis, e.g. holes
    • G02B6/02319—Plurality of longitudinal structures extending along optical fibre axis, e.g. holes characterised by core or core-cladding interface features
    • G02B6/02323—Core having lower refractive index than cladding, e.g. photonic band gap guiding
    • G02B6/02328—Hollow or gas filled core
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/02—Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
    • B23K26/06—Shaping the laser beam, e.g. by masks or multi-focusing
    • B23K26/064—Shaping the laser beam, e.g. by masks or multi-focusing by means of optical elements, e.g. lenses, mirrors or prisms
    • B23K26/066—Shaping the laser beam, e.g. by masks or multi-focusing by means of optical elements, e.g. lenses, mirrors or prisms by using masks
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
    • G02B27/09—Beam shaping, e.g. changing the cross-sectional area, not otherwise provided for
    • G02B27/0927—Systems for changing the beam intensity distribution, e.g. Gaussian to top-hat
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
    • G02B27/09—Beam shaping, e.g. changing the cross-sectional area, not otherwise provided for
    • G02B27/0938—Using specific optical elements
    • G02B27/0988—Diaphragms, spatial filters, masks for removing or filtering a part of the beam
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02295—Microstructured optical fibre
    • G02B6/02314—Plurality of longitudinal structures extending along optical fibre axis, e.g. holes
    • G02B6/02342—Plurality of longitudinal structures extending along optical fibre axis, e.g. holes characterised by cladding features, i.e. light confining region
    • G02B6/02361—Longitudinal structures forming multiple layers around the core, e.g. arranged in multiple rings with each ring having longitudinal elements at substantially the same radial distance from the core, having rotational symmetry about the fibre axis
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02295—Microstructured optical fibre
    • G02B6/02314—Plurality of longitudinal structures extending along optical fibre axis, e.g. holes
    • G02B6/02342—Plurality of longitudinal structures extending along optical fibre axis, e.g. holes characterised by cladding features, i.e. light confining region
    • G02B6/02366—Single ring of structures, e.g. "air clad"
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02295—Microstructured optical fibre
    • G02B6/02314—Plurality of longitudinal structures extending along optical fibre axis, e.g. holes
    • G02B6/02342—Plurality of longitudinal structures extending along optical fibre axis, e.g. holes characterised by cladding features, i.e. light confining region
    • G02B6/02371—Cross section of longitudinal structures is non-circular
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/42—Coupling light guides with opto-electronic elements
    • G02B6/4296—Coupling light guides with opto-electronic elements coupling with sources of high radiant energy, e.g. high power lasers, high temperature light sources
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
    • G02B27/0025—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00 for optical correction, e.g. distorsion, aberration
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/005—Diaphragms
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping

Definitions

  • the present invention relates to an optical filter system.
  • the invention also relates to a laser system, for example for material processing.
  • a beam quality M 2 can be in the range from 1.0 to approx. 1.3, the beam quality M 2 (beam propagation factor) describing the deviation of the measuring beam from a diffraction-limited Gaussian beam in terms of beam diameter and divergence angle.
  • the latter has an M 2 of 1.
  • M 2 typically less than or equal to 1.3 one speaks of laser radiation in the fundamental mode or of an almost diffraction-limited beam quality, but the beam components deviating from the Gaussian beam can have negative effects when using the laser radiation.
  • Such beam components can lead to an enlarged focus zone, to one or more secondary focusses or to undesired heating of optical components.
  • Such beam components can also reduce the homogeneity in the intensity profile in combination with beam-shaping elements that are intended to produce a flat-top focus, for example and thus cause inhomogeneous removal or drilling results.
  • beam components can cause modulations along the optical axis and thus cause modifications of different strengths in the elongated focus zone in the transparent material.
  • US 9,645,309 B2 discloses a multimode optical fiber for propagating Laserstrah treatment in a fundamental mode with low bending loss and for suppressing higher order modes. Furthermore, US Pat. No. 8,009,948 B2 discloses a hollow core fiber with a central section of reduced diameter, as a result of which core diameters of different sizes are realized along the fiber.
  • One aspect of this disclosure is based on the object of specifying a system for reliable filtering of undesired beam components from laser radiation. Another task can be to generate a laser radiation that propagates almost in the (free field) fundamental mode for a subsequent beam guidance. At least one of these objects is achieved by an optical filter system according to claim 1 and by a laser system according to claim 13. Further developments are given in the subclaims.
  • an optical filter system has an optical fiber for guiding laser radiation from a first end to a second end, wherein the laser radiation emerging from the second end of the optical fiber forms a far field distribution in the far field that has a central beam area and around the central beam area Having far field distribution arranged secondary maxima. Furthermore, the optical filter system has a spatial filter diaphragm which is designed to remove at least one of the secondary maxima from the far field distribution in the far field.
  • a laser system has a laser beam source for outputting laser radiation and an optical filter system as described above which has an optical fiber and a spatial filter diaphragm for optical filtering of laser radiation emerging from the optical fiber.
  • 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 diaphragm, 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.
  • the laser system can comprise a laser amplifier stage for amplifying the laser radiation optically filtered with the spatial filter diaphragm.
  • a focusing unit can then be provided for focusing laser radiation amplified with the laser amplifier stage, in particular for material processing.
  • the spatial filter screen can have a central transmission area and at least one screen area protruding towards the transmission area.
  • the transmission area and the central beam area are matched in size so that the central beam area passes the spatial filter screen, and the position of the at least one screen area coincides with the position of the at least one of the secondary maxima and blocks this.
  • the optical filter system can in particular be designed for filtering undesired beam components from laser radiation and for providing laser radiation that is generated largely in a basic mode for subsequent use, for example to focus the laser beam on a workpiece during material processing.
  • the optical fiber can be designed in such a way that the laser radiation is guided in the optical fiber in a (basic) mode, with a phase jump between the central one in the far field distribution after the optical fiber, which is optionally formed after a collimation optical unit Beam area and the secondary maxima is present. Since the far field distribution, in particular due to the phase jump, can have (first) intensity-free areas between the central beam area and the secondary maxima. Further intensity-free areas and phase jumps can develop further outward (in the radial direction).
  • the spatial filter diaphragm can have a transmission area and at least one diaphragm area, the transmission area allowing laser radiation incident on the spatial filter diaphragm to pass through, and the at least one diaphragm area blocking laser radiation incident on the spatial filter diaphragm (F) (preventing further propagation in the direction of the beam ).
  • a geometric course of the transition between the transmission area and the at least one diaphragm area can be adapted to the course of the intensity-free areas and the spatial filter diaphragm can be arranged in the beam path in such a way that the spatial filter diaphragm transmits the central beam area and (in the radial direction) outside the first intensity-free area Laser radiation in areas is blocked, in particular cut off or deflected.
  • the filter system can include a collimation optics unit (e.g. designed as a fiber decoupling unit) which is arranged to collimate the emerging laser radiation and is designed in particular 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.
  • the optical fiber of the optical filter system can be designed in such a way 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, in particular on due to the phase jump, have intensity-free areas between the central beam area and the secondary maxima.
  • 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 only guides laser radiation in one basic mode.
  • 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.
  • 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 area of the spatial filter diaphragm can have a corresponding seven-fold or nine-fold rotational symmetry.
  • the spatial filter screen can have a polygonal opening that forms the transmission area.
  • the optical fiber can be designed as a hollow-core optical fiber of the tubular type with four to ten, optionally eight, glass segments and correspondingly four to ten, optionally eight, secondary maxima in the far field and the transmission area of the spatial filter screen can have a corresponding number in terms of rotational symmetry exhibit.
  • the spatial filter screen can optionally have a polygonal opening forming the transmission area.
  • the central beam area can have a constant phase.
  • the central beam region can be a region of the field profile with a constant phase.
  • 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 by glass bars.
  • the material structure can form a hollow core inner wall, which in sections approximates a center of the hollow core and / or which has a cross-section with an integral rotational symmetry (with an integer greater than 1, ie n-ary with n>1; in general, "n "For a natural number) and / or whose cross-sectional extension approximates a polygonal basic geometry, in particular a hexagonal or octagonal geometry.
  • the inner wall of the hollow core can have a cross section which leads to the far field distribution of the basic mode.
  • the spatial filter diaphragm can have a diaphragm structure which delimit a transmission range of the laser radiation which approximates the cross-sectional profile of the far field distribution of a free field basic mode, excluding the secondary maxima.
  • the secondary maxima of the optical fiber can be distributed azimuthally, in particular with 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 diaphragm areas.
  • the diaphragm areas are also arranged azimuthally and in particular with the same n-fold rotational symmetry with n> 1 distributed around the transmission area.
  • this can form a hollow core inner wall, the cross-sectional profile of which forms a form of a basic mode of the optical fiber in the near field.
  • the spatial filter screen can have a transmission area and screen areas such that a geometric course of the transition between the transmission area and the screen areas is adapted to the shape of the basic mode.
  • the geometric course of the transition between the transmission area and the diaphragm areas can follow the course of the phase change, at least in sections.
  • the geometric course can essentially surround the central beam area, which is given in particular by a constant phase of the laser radiation.
  • intensity components of a (free-field) basic mode that deviate from rotational symmetry in the narrower sense (circular symmetry) can be removed from the far-field distribution with the spatial filter diaphragm.
  • laser radiation can interact with the spatial filter diaphragm in at least one of the secondary maxima of the Fernfeldver distribution, in particular be absorbed or deflected by this.
  • the optical filter system can comprise a rotation unit to which the spatial filter diaphragm 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 diaphragm to the far field distribution. With the rotation unit, the angular positions of diaphragm 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 make a (high) performance suitable mode filter possible, at the output of which laser radiation is essentially in the Gaussian mode.
  • FIG. 1 shows a schematic sketch of a laser system with an optical filter system based on a first exemplary optical fiber
  • FIG. 2 shows a schematic sketch of a laser system with an optical filter system based on a second exemplary optical fiber.
  • a (high-performance) optical fiber such as a (hollow-core) photonic crystal fiber (HC-PCF), as part of a “mode” filter (herein also referred to as an optical filter system) can be used.
  • An HC-PCF provides a light guide mechanism based on photonic bandgaps or an inhibited coupling.
  • fibers of the "Kagome” type with 7 or 19 cells, the number of cells corresponding to the number of missing star-like cells in the nucleus
  • fibers of the "Tubular" type with a number of 4, 5, 6, .. . Tubes or (glass bar) rings in the refractive index profile).
  • hollow core fibers generally include single-mode or multimodal hollow-core optical fibers.
  • single-mode optical fibers are, in particular, step-index optical fibers which essentially only guide laser radiation in one (fiber) basic mode. Based on the laser radiation emerging from the fiber, it was recognized that the combination of fiber and a spatial filter arranged in a generated far field can improve the beam quality by blocking / filtering power ranges that are not assigned to the basic (free space) mode.
  • free space 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, enable an almost “ to deliver ideal “beam quality with M 2 ⁇ 1 for use in material processing for high-intensity laser pulses.
  • I filtered overlap ⁇ dxdy U filtered (x, y) x U basic mode (x, y).
  • I mode overlap ⁇ dxdy U in (x, y) x U basic mode (x, y), where U_in is the coupling field (ie the field coupled into the fiber) and U basic mode is the field of the basic mode of the HC-PCF, for example .
  • a reduced mode overlap integral can result in power losses.
  • 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, approx. 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 by spatial frequency filtering.
  • an improved (almost "ideal") beam quality is achieved using hollow core fibers and spatial frequency filtering using the example of a high-power ultrashort pulse laser in two stages:
  • 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 objective). Only the basic mode is carried by the hollow core fiber. Transmission losses can occur depending on the beam quality of the coupled-in field. At the output of a hollow core fiber, for example a few decimeters long, there may already be an improved beam quality, which is predetermined by the beam quality of the (fiber) basic mode of the hollow core fiber.
  • the (free space) basic 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 n-numbered rotationally symmetrical geometries with n> 1, for example polygonal such as hexagonal-like, field distribution Field distribution differs slightly from the ideal (free space) basic mode and can, for example, have a beam quality M 2 of approx.
  • the ideal (free space) basic mode is rotationally symmetrical in the narrower sense, whereby the term “rotationally symmetrical in the narrower sense” means that a field distribution can be rotated around an axis of rotation by any angle without the field distribution changing changes.
  • An integer rotational symmetry of a field distribution (with an integer greater than 1, ie n-ary with n>1; in general, "n” stands for a natural number) exists if the field distribution is only around a given angle (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.
  • an associated far-field distribution can be obtained in which the Feldan parts that degrade the beam quality are spatially separated from the useful beam (ie offset to the beam axis / offaxis).
  • spatial frequency filtering i.e. the blocking of the undesired beam components
  • the diaphragm can, for example, be refractive or diffractive.
  • the optical filtering can also be implemented by means of a multi-mode (multi-modal) hollow core fiber (also referred to as few-mode HCF), with the beam components of higher modes either being retained as part of the filtered laser radiation or similar to how the secondary maxima of the fundamental mode of the hollow core fiber are removed from the laser radiation with the spatial filter.
  • a multi-mode (multi-modal) hollow core fiber also referred to as few-mode HCF
  • a laser system 11 comprises 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 pj to 100 mJ.
  • the generated laser radiation 13 A can have a beam quality that deviates from an ideal Gaussian beam quality.
  • the optical filter system 17 proposed herein 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 decoupling unit) and a spatial filter aperture F.
  • the collimation optics unit 23 (and also the fiber coupling unit 15) can, for example, be a lens and / or mirror unit or comprise a microscope objective which is configured and arranged for collimating the exiting laser radiation 21 (focusing the laser radiation 13A).
  • the collimation op- tikica 23 spaced from the second end 19B such that a beam path section is formed downstream in which the far field distribution 2 is given, which can be viewed as the far field of the coupled laser radiation.
  • 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 screen F.
  • the rotation unit R, R ‘ can, for example, rotate the hollow core fiber or the spatial filter screen around the beam axis and thus align it at the angle of rotation.
  • the Rotationsein unit is designed to adapt the orientation of the spatial filter diaphragm F to the far field distribution 2.
  • the angular positions of the diaphragm regions 43 can be adjusted with respect to the beam axis and preferably correspond to angular positions of the secondary maxima 37 of the far field distribution 2.
  • a focusing unit 27 which is arranged downstream of the optical filter system 17 and which focuses the filtered beam for material processing (as an application example) on a material surface.
  • the laser radiation in Fig. 1 is subdivided into a laser radiation 21 emerging from the hollow-core optical fiber 18, an optically filtered laser radiation 25 and a resulting (diffraction-limitedly focused) laser beam 29 .
  • the laser radiation 13A to be filtered is coupled into the hollow-core optical fiber 18 at a first end 19A with a fiber coupling unit 15 (for example an (aspherical) microscope objective).
  • a fiber coupling unit 15 for example an (aspherical) microscope objective.
  • the hollow-core optical fiber 18 is designed as a basic-mode optical fiber, the coupled-in laser radiation is only guided in one (fiber) basic mode of the hollow-core optical fiber 18.
  • 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 designed to be rotationally symmetrical in the narrower sense, but rather has an integral number (here, for example, 6) of crystal cells protruding furthest in the direction of the center Z.
  • the cross section of the hollow core inner wall 59 is rotationally symmetrical with the number 6 (“6-fold rotational symmetry”).
  • the basic (fiber) mode of the hollow-core optical fiber 18 is not rotationally symmetrical, and the beam profile of the laser radiation 21 exiting at a second end 19A of the hollow-core optical fiber 18 also deviates from rotational symmetry in the narrower sense.
  • a near-field amplitude distribution 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 gray-scale image. A phase representation is not necessary, since a uniform phase is present at this point.
  • a near-field amplitude distribution V 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, e.g.
  • FIG. 1 shows a far-field amplitude distribution 2 in the far-field 33.
  • the far-field amplitude distribution 2 comprises a central beam region 35 and six schematically emphasized secondary maxima 37. The underlying symmetry is thus six-fold. Between the central beam region 35 and each of the secondary maxima 37 there is an intensity-free region 39, which results from a phase jump 39A in the field distribution (indicated schematically in FIG. 1 by dashed lines).
  • FIG. 1 shows a far field amplitude distribution 2 ‘as a schematic line drawing of the far field amplitude distribution 2.
  • 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 diaphragm F is shaped in such a way and aligned with respect to the geometry of the far field amplitude distribution 2 that the desired optical filtering occurs.
  • the far field amplitude distribution 2 (or the laser radiation 21 emerging from the fiber) is spatial frequency filtered after the collimation with an adapted diaphragm.
  • the entire field distribution in the far field is called spatial frequency distribution.
  • the spatial frequencies are filtered whose phase differs from the central distribution with constant phase by just "Pi" (p). In areas further out there are further spatial frequencies which again have the phase value “0”, ie 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 matched to the fiber cross-section 51 comprises a hexagonal transmission area 41 and diaphragm areas 43 which are assigned to the sides of the hexagon.
  • the diaphragm areas 43 block the secondary maxima 37 of the far field amplitude distribution 2, with at least one diaphragm area 43 generally coinciding in its position with the position of the at least one of the secondary maxima 37.
  • the transmission area 41 and the central beam area 35 are, however, matched in size to one another in such a way that the central beam area 35 passes the spatial filter diaphragm F.
  • a field portion which strikes around one of the diaphragm areas 43 and is thus filtered out of the laser radiation indicated with dots. It can thus be seen in FIG. 1 that intensity components of a basic mode deviating from a rotational symmetry in the narrower sense are removed from the far field distribution 2 with the spatial filter diaphragm F.
  • the spatial filter diaphragm F has a diaphragm structure that delimits a transmission area for the laser radiation, the geometry of the transmission area approximating the cross-sectional profile of a far-field distribution of a free-field basic mode, excluding the secondary maxima.
  • Fig. 1 shows a filtered far-field amplitude distribution 3 with correspondingly reduced deviations from the rotational symmetry.
  • the mapped filtered Fem field amplitude distribution 3 can be assigned a beam quality M2eff of approximately 1.02.
  • M2eff beam quality of approximately 1.02.
  • the optically filtered laser radiation 25 is now available downstream of the spatial filter diaphragm F essentially in a diffraction-limited manner.
  • 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.
  • FIG. 1 shows schematically a phase distribution 61 of the laser radiation impinging on the spatial filter diaphragm.
  • a phase of “0” can be assigned to the laser radiation in the area of the transmission area 41.
  • a phase of “Pi” can be assigned to the laser radiation in the area of the secondary maxima 37, for example.
  • the optical fiber is designed in such a way that a phase jump in the far field distribution for laser radiation that is guided in a (basic) mode with the optical fiber is present between the central beam region 35 and the secondary maxima 37. Because of the phase jump, the far field distribution has intensity-free areas between the central beam area 35 and the secondary maxima 37.
  • the central beam region 35 of the fundamental mode usually has a constant phase. Such phase differences can also cause disadvantageous effects, e.g. when focusing.
  • the laser radiation has an essentially rotationally symmetrical near-field amplitude distribution 4 in the focus.
  • a beam quality M2eff of approximately 1.02 can accordingly also be assigned to the focused laser radiation.
  • 1 shows, as a line drawing, a near-field amplitude distribution 4 'of the filtered and focused laser beam.
  • Fig. 1 shows an example of mode filtering with the aid of 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, in which case the contributions of the higher modes can also be influenced with the aid of the spatial filter screen F here.
  • FIG. 2 shows a laser system 1 V with an optical filter system 17 'based on a hollow-core optical fiber 18' of the tubular type with eight rings and additionally shows the development of the field distributions due to the optical filtering, ie in particular the field distributions in the near field, in the far field as well as in the filtered far field and in the filtered near field.
  • Fig. 2 illustrates the correspondingly assigned Grundge geometry of the spatial filter diaphragm F ‘of the optical filter system 17‘, as it was adapted to the hollow core optical fiber 18 ‘, and a resulting phase distribution 61‘.
  • the spatial filter diaphragm F has an octagonal opening, the orientation with respect to the secondary maxima 37‘ being oriented in such a way that the spatial filter diaphragm F Kunststoff acts as a spatial filter for the secondary maxima 37 ‘.
  • the intensity ranges of the collimated laser radiation with a different phase here marked with“ Pi ”.
  • the opening thus prevents the propagation of the laser radiation, which is present (in the radial direction to the beam axis) outside a zentreal Strahlbe rich with 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 '. You can see this also because of the improved field curves in terms of rotational symmetry (almost rotational symmetry in the narrower sense).
  • spatial filtering can improve, for example, 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.
  • FIG. 2 the reference characters not addressed are briefly summarized below. They relate to the aspects already described in connection with FIG. 1: 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, 4L transmission area, 43' aperture area, 51 'fiber cross-section, 53' hollow core, 55 'material structure, 57 glass segment, 59' hollow core inner wall, sample phase values "0", "Pi”.
  • the azimuthal distribution of the secondary maxima relates to a beam axis of the laser radiation, which is given by a beam center of the central beam area
  • the azimuthal arrangement of the diaphragm areas relates to a diaphragm center of the transmission area, the diaphragm center being adjusted so that it is with coincides with the beam axis. It can also be seen that the geometric course of the transition between the transmission area and the diaphragm areas surrounds a central area of the (free-field) basic mode.
  • Figures 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 (flat-top 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.) refractive index distributions and hollow core geometries, so that the filter geometries and the beam qualities to be achieved differ accordingly.
  • Kagome type, etc. refractive index distributions and hollow core geometries
  • spatial frequency filtering power losses can be less than 2%.
  • an optical filter system described herein at the output of each laser system in particular also at the output of amplifier stages or before and between amplifier stages, can be used to create a well-defined, reproducible interface with regard to the parameters location, angle and beam quality for to deliver a subsequent application.
  • a (post) amplifier stage 71 is indicated by dashed lines in FIG. 2, to which the filtered laser beam is fed for amplification.
  • the fiber coupling unit can be implemented as part of the optical filter system.

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  • Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • General Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Plasma & Fusion (AREA)
  • Mechanical Engineering (AREA)
  • Optical Couplings Of Light Guides (AREA)
  • Lasers (AREA)

Abstract

L'invention concerne un système de filtre optique (17) qui comprend une fibre optique (18) permettant de guider un rayonnement laser d'une première extrémité (19A) à une seconde extrémité (19B). Le rayonnement laser (21) sortant de la seconde extrémité (19B) de la fibre optique (18) forme une distribution de champ lointain (2) dans le champ lointain (33), cette distribution de champ lointain présentant une zone de faisceau central (35) et des maximums secondaires (37) disposés autour de la zone de faisceau central (35) de la distribution de champ lointain (2). Le système de filtre optique (17) comprend également un diaphragme de filtre spatial (F) conçu pour éliminer au moins un des maximums secondaires (37) de la distribution de champ lointain (2) dans le champ lointain (33). En conséquence, la qualité du faisceau du rayonnement laser peut être améliorée.
PCT/EP2020/079216 2019-10-16 2020-10-16 Système de filtre optique et système laser WO2021074380A1 (fr)

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US8009948B2 (en) 2006-07-25 2011-08-30 The Board Of Trustees Of The Leland Stanford Junior University Apparatus and methods using hollow-core fiber tapers
US20130022060A1 (en) * 2011-04-26 2013-01-24 Meggitt (France) Device for transmitting light energy and associated transmission method
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US8009948B2 (en) 2006-07-25 2011-08-30 The Board Of Trustees Of The Leland Stanford Junior University Apparatus and methods using hollow-core fiber tapers
US20080219620A1 (en) * 2007-03-07 2008-09-11 Furukawa Electric North America, Inc. Achieving Gaussian Outputs from Large-Mode-Area-Higher-Order Mode Fibers
US20130022060A1 (en) * 2011-04-26 2013-01-24 Meggitt (France) Device for transmitting light energy and associated transmission method
DE102015005257A1 (de) * 2015-04-26 2016-10-27 Keming Du Optische Anordnung zur Erhöhung der Strahlqualität und zur Verbesserung des lntensitätsprofils

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