US20220399695A1 - Optical system for increasing the contrast of pulsed laser radiation, laser system and method for increasing the contrast of pulsed laser radiation - Google Patents

Optical system for increasing the contrast of pulsed laser radiation, laser system and method for increasing the contrast of pulsed laser radiation Download PDF

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US20220399695A1
US20220399695A1 US17/889,396 US202217889396A US2022399695A1 US 20220399695 A1 US20220399695 A1 US 20220399695A1 US 202217889396 A US202217889396 A US 202217889396A US 2022399695 A1 US2022399695 A1 US 2022399695A1
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laser radiation
pulsed laser
intermediate focus
multipass cell
optical system
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Thomas Metzger
Sebastian Stark
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Trumpf Scientific Lasers GmbH and Co KG
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Trumpf Scientific Lasers GmbH and Co KG
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    • 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
    • H01S3/0057—Temporal shaping, e.g. pulse compression, frequency chirping
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/3501—Constructional details or arrangements of non-linear optical devices, e.g. shape of non-linear crystals
    • G02F1/3503—Structural association of optical elements, e.g. lenses, with the non-linear optical device
    • 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
    • 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
    • H01S3/02—Constructional details
    • H01S3/03—Constructional details of gas laser discharge tubes
    • H01S3/036—Means for obtaining or maintaining the desired gas pressure within the tube, e.g. by gettering, replenishing; Means for circulating the gas, e.g. for equalising the pressure within the tube
    • 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
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/08—Construction or shape of optical resonators or components thereof
    • H01S3/08095—Zig-zag travelling beam through the active medium
    • 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
    • H01S3/09—Processes or apparatus for excitation, e.g. pumping
    • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
    • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
    • H01S3/094076—Pulsed or modulated pumping
    • 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
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/10038—Amplitude control
    • 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
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/11—Mode locking; Q-switching; Other giant-pulse techniques, e.g. cavity dumping
    • 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
    • H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
    • H01S3/22—Gases
    • H01S3/2207—Noble gas ions, e.g. Ar+>, Kr+>
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/3511—Self-focusing or self-trapping of light; Light-induced birefringence; Induced optical Kerr-effect
    • 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
    • H01S3/0071—Beam steering, e.g. whereby a mirror outside the cavity is present to change the beam direction
    • 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
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/10061—Polarization control

Definitions

  • Embodiments of the present invention relate to optical systems for increasing the contrast of pulsed laser radiation and to laser systems, in particular ultrashort pulse (USP) laser systems, for emitting pulsed laser radiation with high power and/or high pulse energy. Furthermore, embodiments of the present invention relate to a method for increasing the contrast of pulsed laser radiation, in particular of ultrashort pulse trains.
  • USP ultrashort pulse
  • the contrast between primary laser pulses having a high peak intensity and the background is a significant parameter.
  • the laser radiation that forms the background usually consists of a radiation pedestal attributable to ASE, and/or of laser pulses which arrive temporally before or after the primary laser pulses and the peak intensity of which is significantly lower than the peak intensity of the primary laser pulses.
  • the aim here is to largely remove the ASE background and the laser pre- and postpulses from the pulsed laser radiation and thus to largely restrict the intensity contribution for the interaction to the primary laser pulses.
  • the prior art discloses contrast enhancement methods in which a laser beam is guided in a gas-filled hollow core fiber (HCF) in order for example to cause a rotation of an elliptical polarization on account of nonlinear effects.
  • HCF gas-filled hollow core fiber
  • NER Nonlinear Ellipse Rotation
  • the NER is accompanied by a nonlinear spectral broadening of the intensive laser pulses.
  • a constant gas condition can be provided along the HCF.
  • the gas density can decrease along the fiber in order to prevent unwanted nonlinear effects or ionization from arising.
  • NER is used to improve the contrast of sub-4 fs laser pulses using an HCF filled with argon gas.
  • DE 10 2014 007159 A1 discloses a method for the spectral broadening of laser pulses for nonlinear pulse compression using a multipass cell constructed in the form of a so-called Herriott cell, for example.
  • the aim is a spectral broadening of laser pulses which can be carried out even in the case of a pulse power which is greater than the critical power of the nonlinear medium used for the spectral broadening.
  • Embodiments of the present invention provide an optical system for increasing contrast of pulsed laser radiation.
  • the optical system includes a first polarization setting optical unit for setting an elliptical polarization state of the pulsed laser radiation, and a multipass cell having at least two opposing mirrors.
  • the pulsed laser radiation passes the multipass cell with formation of a plurality of intermediate focus zones.
  • the multipass cell is filled with a gas having an optical nonlinearity that causes an intensity-dependent rotation of an alignment of the elliptical polarization state of the pulsed laser radiation, such that the multipass cell outputs beam portions having differently aligned elliptical polarization states on account of the intensity-dependent rotation.
  • the optical system further includes an optical beam splitting system for splitting the beam portions having differently aligned elliptical polarization states.
  • FIG. 1 shows an exemplary schematic illustration of a laser system comprising an optical system for contrast enhancement according to embodiments
  • FIGS. 2 A, 2 B, and 2 C show exemplary schematic diagrams for elucidating a Herriott cell as an example of a multipass cell according to embodiments.
  • FIG. 3 shows a schematic flow diagram for elucidating an exemplary procedure for contrast enhancement according to embodiments.
  • Embodiments of the present invention provide systems and methods which can be used for increasing the contrast of pulsed laser radiation, for example of ultrashort pulse trains, even and in particular in the case of high pulse energies and high average powers.
  • embodiments of the present intention utilize nonlinear effects for influencing the polarization of laser pulses and the propagation thereof.
  • an optical system for increasing the contrast of pulsed laser radiation using a nonlinear elliptical polarization rotation comprises:
  • a first polarization setting optical unit for setting an elliptical polarization state of the pulsed laser radiation
  • a multipass cell having two opposing mirrors forming in particular a concentric or confocal resonator, wherein the pulsed laser radiation repeatedly passes through the multipass cell, in particular the resonator, with formation of a plurality of intermediate focus zones, wherein the multipass cell is filled with a gas having an optical nonlinearity which causes an intensity-dependent rotation of an alignment of the elliptical polarization state of the pulsed laser radiation, such that the multipass cell outputs beam portions having differently aligned elliptical polarization states on account of the intensity-dependent rotation, and
  • an optical beam splitting system for splitting the beam portions having differently aligned elliptical polarization states.
  • a laser system for emitting pulsed laser radiation ( 9 ) comprises
  • a laser radiation source which outputs pulsed laser radiation comprising primary laser pulses, in particular having pulse energies and pulse durations in the range of a few hundred femtoseconds or less,
  • a pulse duration setting system for setting the pulse duration of the primary laser pulses
  • the laser system can comprise an optical pulse duration compressor system for compensating for a dispersive contribution of the at least one optical system and optionally for temporally compressing the primary laser pulses of the laser radiation if they have experienced a nonlinear spectral broadening in at least one of the intermediate focus zones.
  • a method for increasing the contrast of pulsed laser radiation using a nonlinear elliptical polarization rotation comprises the following steps:
  • a multipass cell having two mirrors forming in particular a concentric or confocal resonator, wherein the multipass cell, in particular the resonator, is repeatedly traversed with formation of a plurality of intermediate focus zones, wherein the multipass cell is filled with a gas having an optical nonlinearity which causes an intensity-dependent rotation of an alignment of the elliptical polarization state of the pulsed laser radiation in the intermediate focus zones, as a result of which beam portions having differently aligned elliptical polarization states are generated in the multipass cell,
  • At least one of the following parameters of the optical system can be set or settable:
  • the optical system is embodied as a gas-filled cell and the optical system has a pressure setting device for setting the gas pressure in the gas-filled cell, and/or
  • At least one of the following parameters of the optical system can be set or settable:
  • the first polarization setting optical unit comprises a first waveplate, optionally a ⁇ /4 waveplate and/or a ⁇ /2 waveplate.
  • the optical system can furthermore comprise at least one of the following optical components:
  • a pulse duration setting system for setting a pulse duration of primary laser pulses of the pulsed laser radiation
  • a first optical telescope arrangement which is set to image the pulsed laser radiation onto a predefined mode in the multipass cell and which is optionally arranged downstream of the first polarization setting optical unit,
  • an input coupling mirror for coupling the pulsed laser radiation into the multipass cell
  • a second optical telescope arrangement which is set to collimate the pulsed laser radiation emerging from the multipass cell.
  • the multipass cell can be embodied:
  • intermediate focus zones which are produced with the opposing mirrors for example in a resonator set-up with, in particular identical, radii of curvature of the mirrors, wherein the intermediate focus zones have substantially an identical diameter and an identical Rayleigh length,
  • a cell filled with a nonlinear gaseous medium in particular a noble gas such as helium or argon, wherein the same gas pressure is present in each of the intermediate focus zones,
  • At least one of the mirrors of the multipass cell can be embodied as a convex mirror, wherein the radii of curvature match, in particular, and/or a distance between the mirrors lies in a range of 95% to 105% of the sum of the radii of curvature.
  • at least one of the mirrors can be embodied as a dispersive mirror, the dispersion contribution of which compensates for a dispersive contribution of at least one pass of a primary laser pulse of the pulsed laser radiation through the multipass cell.
  • at least one of the mirrors can comprise at least one mirror segment on which the pulsed laser radiation impinges at least once during the circulation of the pulsed laser radiation through the multipass cell.
  • the multipass cell can be embodied in such a way that a primary laser pulse of the pulsed laser radiation, the contrast of which pulse is intended to be increased in the optical system, experiences substantially no change in the pulse duration and/or pulse energy in the intermediate focus zones.
  • the beam portions having differently aligned elliptical polarization states can comprise a useful beam portion with primary laser pulses and a residual beam portion with low-intensity laser radiation.
  • the residual beam portion forms a background composed of a radiation pedestal attributable to ASE, and/or of laser pulses which arrive temporally before or after the primary laser pulses and the peak intensity of which is significantly lower than the peak intensity of the primary laser pulses.
  • the radiation background can have in particular low-intensity laser pulses preceding the high-intensity laser pulses and/or low-intensity laser pulses succeeding said high-intensity laser pulses.
  • the optical beam splitting system for splitting the beam portions having differently aligned elliptical polarization states can comprise:
  • a second polarization setting optical unit for returning each of the differently aligned elliptical polarization states to a linear polarization state, wherein in particular the beam portions having differently aligned elliptical polarization states that are output by the multipass cell are converted into a useful beam portion and a residual beam portion having differently aligned linear polarization states, and
  • a beam splitter which outputs the useful beam portion and the residual beam portion on different beam paths.
  • the second polarization setting optical unit can comprise a second waveplate, optionally a ⁇ /4 waveplate and/or a ⁇ /2 waveplate.
  • the optical system can furthermore comprise a control system, which is configured for rotating the alignment of one of the elliptical polarization states, in particular for setting a rotation angle of 90°, at least for setting one of the following parameters:
  • gas conditions, in particular gas type and/or gas pressure, in a first optical system can differ from corresponding gas conditions in a second optical system.
  • the latter can furthermore comprise the following step:
  • a pulse spectrum of the pulsed laser radiation can broaden, in particular from intermediate focus zone to intermediate focus zone, on account of a nonlinear spectral broadening in the multipass cell, while the increase in contrast simultaneously takes place.
  • Embodiments of the present invention use the approach of NER using a gas-filled multipass cell, for example a Herriott cell, for enhancing the contrast of pulsed laser radiation, wherein this can optionally be effected together with a spectral pulse broadening and a subsequent pulse duration compression.
  • the concepts proposed herein have the advantages that the increase in the pulse contrast can be carried out in the case of high average powers (greater than a few 100 W into the kW range) and very high pulse energies (into the J range) and can be implemented in an easily realized and very efficient way.
  • an elliptical polarization state is understood to mean a polarization state in which a polarization ellipse is present, such as can be caused for example by setting an angle in a range of between 0° and 45° or in a range of between 45° and 90° (analogously in a range of between 45° and 135° or in a range of between 135° and 180°) of a fast axis of a ⁇ /4 waveplate and a polarization plane of an incident linearly polarized laser beam.
  • a polarization ellipse of the electric field forms in these angle ranges.
  • 90% of the laser beam can be present in an s-polarization state and 10% in a p-polarization state.
  • An elliptical polarization state thus differs from a linear polarization state (having only one portion in the s- or p-polarization state) and also from a circular polarization state (angle of 45° of the ⁇ /4 waveplate or identical portions in the s-polarization state and in the p-polarization state).
  • aspects described herein are based in part on the insight that a nonlinearly intensity-dependent rotation of an alignment of an elliptical polarization can be caused on account of an intensity-dependent phase rotation (self-induced ellipse rotation).
  • the intensity dependence of the elliptical polarization rotation has the effect that the polarization of a pulse experiences a different rotation near the peak of the pulse compared with the low-intensity regions.
  • high-intensity beam portions formed by the primary laser pulses provided for an application
  • low-intensity laser radiation for example formed by the pre- and postpulses accompanying the primary laser pulses.
  • the implementation of NER in the context of a multipass cell provides diverse setting parameters and control parameters, thereby enabling contrast enhancement in an optical system for a wide variety of parameter configurations of a laser system.
  • pulsed laser radiation can be freed of low-energy laser radiation (e.g. laser pre- or postpulses) by virtue of only laser pulses having a high intensity (the primary laser pulses discussed) being influenced in terms of their polarization with regard to the downstream beam path.
  • the influencing is effected by way of the nonlinearity of the refractive index of the gas in intermediate focus zones of the Herriott cell in such a way that the primary laser pulses can pass through an optical beam splitting system without any loss if possible.
  • the alignment of the polarization ellipse of the low-energy laser radiation portion does not change within the Herriott cell, and so said low-energy laser radiation portion can be removed from the laser radiation after leaving the Herriott cell by means of the optical beam splitting system.
  • laser pre- and/or postpulses can be removed from the laser radiation if the rotation of the polarization ellipse is set only for the primary laser pulses on account of the high intensity thereof in such a way that only the primary laser pulses can pass through a preset beam splitter, for example.
  • FIG. 1 shows a laser system 1 comprising an optical system 3 for increasing the contrast.
  • the optical system 3 is based on the use of a multipass cell 5 filled with gas 4 (for example a Herriott cell), the gas 4 serving as a nonlinear Kerr medium.
  • gas 4 for example a Herriott cell
  • helium can be used in the case of very high intensities in the multipass cell 5 .
  • argon or some other noble gas can be used as a nonlinear medium.
  • the laser system 1 generally comprises a laser radiation source 7 , which outputs laser radiation 9 .
  • the laser radiation 9 comprises primary laser pulses 11 having a pulse energy in the range of e.g. a few hundred millijoules and a pulse duration ⁇ t in the range of a few hundred femtoseconds or less, which form an ultrashort pulse train, for example.
  • the laser radiation 9 furthermore comprises low-energy laser radiation 13 , indicated by way of example as prepulses 13 A or postpulses 13 B in FIG. 1 .
  • the laser radiation source 7 can optionally have a pulse duration setting system 15 for setting the pulse duration of the primary laser pulses 11 , wherein the pulse duration setting system 15 can also be assigned to the optical system 3 , as indicated in FIG. 1 .
  • the optical system 3 has a first polarization setting optical unit 19 .
  • the laser radiation 9 is elliptically polarized.
  • FIG. 1 indicates at the output of the first polarization setting optical unit 19 an electric field vector circulating on a polarization ellipse in order to clarify an elliptical polarization state 17 B by way of example, the semimajor axis of the polarization ellipse running in the plane of the drawing by way of example.
  • the first polarization setting optical unit 19 comprises a waveplate (e.g. a ⁇ /4 waveplate or a ⁇ /8 waveplate, etc.).
  • the first polarization setting optical unit 19 comprises a first ⁇ /4 waveplate 19 A. Furthermore, the first polarization setting optical unit 19 can optionally have, for example upstream of the first ⁇ /4 waveplate 19 A, a ⁇ /2 waveplate 19 B for aligning the orientation of the elliptical polarization state.
  • the setting of the polarization in the first polarization setting optical unit 19 is independent of the intensity since the waveplates operate with an anisotropic refractive index, for example with a birefringent crystal.
  • the ⁇ /4 waveplate is set in relation to the polarization plane of the laser radiation 9 in such a way that an angle between a fast axis of the ⁇ /4 waveplate and the polarization plane is not 0°, 45°, 90° or 135°.
  • Corresponding angle ranges can be assigned to other waveplates or combinations of waveplates for producing elliptical polarization. Setting the angle of the waveplate makes it possible to set an ellipticity of the polarization ellipse. In combination with the upstream ⁇ /2 waveplate 19 B, an alignment of the polarization ellipse can furthermore be effected.
  • these two beam portions On account of the linear polarization present identically for the primary laser pulses 11 and the low-energy laser radiation 13 at the input of the first polarization setting optical unit 19 , these two beam portions also have the same elliptical polarization state 17 B upon leaving the first polarization setting optical unit 19 .
  • FIG. 1 furthermore shows a telescope arrangement 21 for matching the mode (generally beam parameters such as beam diameter and beam divergence) of the pulsed laser radiation 9 prior to input coupling into the multipass cell 5 by means of an input coupling mirror 23 .
  • mode generally beam parameters such as beam diameter and beam divergence
  • the multipass cell 5 embodied as a Herriott cell, for example, comprises two concavely curved mirrors 25 A, 25 B, which form a beam path 5 A running back and forth repeatedly between the mirrors 25 A, 25 B in a gas-filled environment.
  • the multipass cell 5 for example in the geometry of a Herriott cell—can be formed by two concave mirrors aligned with one another for example in a concentric or confocal resonator arrangement, generally also in any other resonator configuration, along a common optical axis 27 (given by the concentric arrangement).
  • the mirrors 25 A, 25 B are also referred to as Herriott mirrors. If the laser radiation 9 is introduced into the multipass cell 5 in a manner offset with respect to the optical axis 27 , the laser radiation 9 will circulate repeatedly there and back on a predefined, usually elliptical (circular) pattern.
  • FIG. 2 A schematically illustrates the beam path between the mirrors 25 A, 25 B with the formation of an intermediate focus zone 29 , presupposing a correspondingly matched mode of the input-coupled laser radiation 9 .
  • the intermediate focus zone 29 has for example a focus diameter d and a Rayleigh length Lr and lies in the region of a plane of symmetry 31 of the resonator arrangement, embodied in a concentric fashion in the example in FIG. 1 .
  • FIGS. 2 B and 2 C show plan views of the mirrors 25 A, 25 B, in which circularly arranged impingement regions 33 on the mirror surfaces are indicated schematically.
  • the laser radiation 9 impinges as centrally as possible in the impingement regions 33 before it is reflected back from there again in the direction of the center of the multipass cell (here the resonator arrangement).
  • FIGS. 2 B and 2 C furthermore reveal an input coupling opening 35 A and also an output coupling opening 35 B.
  • the regions available for the reflection on the surface of the mirrors 25 A, 25 B are circular area sections having a diameter D.
  • the number of circulations can be of arbitrary magnitude, in principle; for example, 5 to 100 intermediate focus zones can be traversed; that is to say that a plurality of intermediate focus zones are traversed in the multipass cell.
  • at least one of the mirrors 25 A, 25 B can also be constructed from individual discrete mirror elements, wherein a reflection can preferably take place on one individual mirror element. For example, twelve intermediate focus zones 29 are traversed.
  • smaller mirror elements engaging in the Herriott cell can be used and positioned at the positions of the openings 35 A, 35 B, for example.
  • the pulsed laser radiation 9 is repeatedly guided through intermediate focus zones in the center of the multipass cell 5 .
  • High intensities form in the intermediate focus zones on account of the focusing of the primary laser pulses during the pulse duration ⁇ t of the primary laser pulses 11 , and result in a nonlinear behavior of the refractive index of the gas 4 .
  • the nonlinear behavior of the refractive index of the gas 4 results in beam portions of the laser radiation 9 having different polarization states and can thus be used for increasing the contrast of the pulsed laser radiation 9 .
  • the laser radiation 9 leaves the multipass cell 5 and impinges on an output mirror 37 , which reflects the output-coupled laser radiation.
  • the output mirror 37 directs the laser radiation 9 through a second telescope arrangement 39 , which recollimates the laser radiation 9 .
  • the optical system 3 furthermore has an optical beam splitting system 41 .
  • the optical beam splitting system 41 comprises a second waveplate 43 and a beam splitter 45 .
  • the second waveplate 43 is generally set in such a way that the elliptical polarization state is converted into a linear polarization again. Further set-ups for splitting beam portions having different elliptical polarization states are known from the prior art.
  • a polarization ellipse rotated by ⁇ is indicated by way of example as an elliptical polarization state 17 C in FIG. 1 .
  • the orientation of the polarization ellipse of the low-energy laser radiation 13 results from the ordinary refractive index of the gas 4 and remains substantially unchanged.
  • the elliptical polarization states of the primary laser pulses 11 and of the low-energy laser radiation 13 thus change their relative alignment with respect to one another upon every pass through an intermediate focus zone 29 .
  • the alignments of the two polarization ellipses downstream of the multipass cell 5 differ by 90°, for example, after traversal of the waveplate 43 this results in two linear polarization states 47 A, 47 B aligned orthogonally to one another for the primary laser pulses 11 and the low-intensity laser radiation 13 .
  • the beam splitter 45 is correspondingly aligned, the beam splitter 45 distributes the primary laser pulses 11 and the low-intensity laser radiation 13 between two different beam paths for a useful beam portion 9 A and a residual beam portion 9 B, which correspond to the orthogonal linear polarization states 47 A, 47 B.
  • FIG. 1 indicates by way of example a primary laser pulse 11 ′ of the useful beam portion 9 A, from which pre- and postpulses have been removed.
  • the pre- and postpulses form the residual beam portion. Even if the resulting beam portions are not polarized fully orthogonally to one another, significant portions of the pre- or postpulses can be removed from the useful beam path.
  • the primary laser pulses 11 ′ can be added to a compressor 49 , for example.
  • the compressor 49 is illustrated by way of example as a chirped mirror compressor in FIG. 1 .
  • a useful laser radiation 9 A′ comprising a train of compressed primary laser pulses 11 ′′ can thus be output at an output of the laser system 1 , the compressed primary laser pulses 11 ′′ having an increased contrast.
  • the configuration proposed herein e.g. using a Herriott cell can enable a predetermined/settable number of intermediate focus zones 29 to be traversed.
  • a beam diameter d in the intermediate focus zones is settable and can for example also be coordinated with the laser power, pulse duration, etc., and the gas 4 by way of the radius Rm of curvature of the mirrors 25 A, 25 B.
  • the radius Rm of curvature is identical for both mirrors, for example, or is at least of the same order of magnitude.
  • the gas pressure can be set in regard to the nonlinearity. It is noted that on account of the high spatial proximity of the various intermediate focus zones being traversed in the multipass cell, the same gas pressure is present in each of the intermediate focus zones.
  • the optical beam parameters and beam properties in the various intermediate focus zones are very similar, with the result that similar nonlinear effects are present as well.
  • the intermediate focus zones 29 substantially all have the same diameter d and have correspondingly identical Rayleigh lengths Lr.
  • the distance between the mirrors 25 A, 25 B lies in a range of 95% to 105% of the sum of the radii of curvature.
  • the optical system 3 can comprise a control system 61 , for example, which is connected via control connections 63 to the pulse duration setting system 15 , the polarization setting optical unit 19 (in particular for setting the angular positions of the first ⁇ /4 waveplate 19 A and optionally the ⁇ /2 waveplate 19 B), the telescope arrangements 21 , 39 (in particular for setting the distance between telescope lenses 21 A, 21 B), a pressure setting device 65 for setting the gas pressure (see FIG. 1 ) and/or the optical beam splitting system 41 (in particular for setting the angular position of the second ⁇ /4 waveplate 43 ).
  • a control system 61 for example, which is connected via control connections 63 to the pulse duration setting system 15 , the polarization setting optical unit 19 (in particular for setting the angular positions of the first ⁇ /4 waveplate 19 A and optionally the ⁇ /2 waveplate 19 B), the telescope arrangements 21 , 39 (in particular for setting the distance between telescope lenses 21 A, 21 B), a pressure setting device 65 for setting the gas pressure (see FIG. 1 )
  • control system 3 The following can be set, for example, with the aid of the control system 3 :
  • the laser radiation 9 repeatedly impinges on the mirrors 25 A, 25 B (a number of times in each case).
  • the mirrors can supplementarily be used for dispersion matching by virtue of their being embodied as dispersive mirrors. If the mirrors 25 A, 25 B have a dispersive effect at least in the case of one of the reflections, it is possible to directly affect the dispersion and thus the pulse duration of the primary laser pulses 11 .
  • one or more of the impingement regions 33 can be provided with a dispersive layer.
  • a dispersive coating 51 is indicated in a dashed manner for the mirror 25 B by way of example in FIG. 2 A .
  • each of the mirrors 25 A, 25 B can be constructed from a plurality of mirror segments having predetermined dispersive properties, the dispersion of each of the mirror segments being matched to a desired pulse duration in the pass through the multipass cell 5 .
  • the dispersion present in the multipass cell 5 is composed of a dispersion contribution of the dispersive mirrors and a dispersion contribution in the gas-filled volume along the beam path 5 A.
  • One exemplary mirror segment 53 is indicated in FIG. 1 .
  • the laser radiation impinges (depending on the size of the mirror segment) at least once on the mirror segment, which usually has at least an extent of the magnitude of the beam diameter D on the mirror surface.
  • the concepts proposed herein allow a dispersion accumulated during passage through the gas-filled volume to be at least partly compensated for by suitable dispersive mirror coatings (chirped mirrors) in order for example to maintain comparable pulse durations in the intermediate focus zones or to vary the pulse durations in a targeted manner.
  • the multipass cell 5 enables the high-intensity primary laser pulses to be spectrally broadened stepwise in the intermediate focus zones.
  • the nonlinear spectral broadening is brought about by the high intensity present in each case in an intermediate focus zone and by the nonlinearity of the refractive index of the gaseous medium in the multipass cell 5 .
  • the pulse spectrum can change from intermediate focus zone to intermediate focus zone, specifically substantially with a constant pulse duration and constant pulse energy.
  • the pulse duration can additionally be set.
  • the pulse duration can change (shorten or lengthen) from pass to pass. Accordingly, the peak intensities in the intermediate focus zones remain substantially constant even in the case of a nonlinear spectral broadening.
  • a further advantage can arise in connection with the nonlinear spectral broadening if laser radiation having elliptical polarization is used for this purpose in the multipass cell.
  • the spectral broadening can possibly be effected inherently more smoothly across the frequency spectrum, such that a less structured spectrum can arise. This can have a positive effect on the subsequent pulse shaping and/or pulse compression.
  • FIG. 2 A shows the formation of an intermediate focus zone in a Herriott cell, assuming curved Herriott mirrors. Geometric parameters for the implementation of a multipass cell in the context of the concepts presented herein are considered below.
  • the limiting of the pulse energies of primary laser pulses which can be increased in terms of contrast (and optionally be spectrally broadened) by means of a multipass cell results from an avoidance of laser-induced damage to the (Herriott) mirrors and from the ionization threshold value of the gas used.
  • a highest possible ionization threshold value is approximately 3.42 10 ⁇ circumflex over ( ) ⁇ 14 W/cm2.
  • the laser-induced damage to the mirrors 25 A, 25 B dictates a minimum diameter of the laser radiation 9 on the curved mirrors 25 A, 25 B.
  • the ionization threshold value determines the smallest possible focus diameter d in the intermediate focus zones 29 with regard to avoiding an ionization of the gas 4 .
  • Both parameters together define a required length of the multipass cell 5 , i.e. the distance between the mirrors from which the concentric resonator is constructed, for example, and also the radius of curvature thereof.
  • the electric field strength required for an ionization is increased for circularly/elliptically polarized light in comparison with linearly polarized light, such that for a comparable beam diameter D on the mirrors 25 A, 25 B, the possible focus diameter d in the intermediate focus zones 29 can be chosen to be smaller.
  • the mirrors For the application of high-intensity laser radiation, it may be necessary for the mirrors to withstand pulse energies of a few 100 mJ in conjunction with pulse durations of a few 100 fs, for example 500 fs or shorter.
  • the mirrors should furthermore be of wideband design, e.g. designed for a wavelength range of e.g. 700 nm to 1100 nm, for example, for ultrashort pulses from a titanium-sapphire laser or 900 nm to 1100 nm for ultrashort pulses from lasers that emit around 1000 nm, such as Nd:YAG or Yb:YAG.
  • the mirrors may or may not provide a dispersion contribution, and so dispersive coatings should possibly also be taken into account.
  • a laser-induced damage threshold value of approximately 0.5 J/cm2 in the case of a pulse duration of approximately 500 fs.
  • This threshold value is usually assigned to the beam center. Assuming a Gaussian beam, e.g. a threshold value of approximately 0.1 J/cm2 thus results for the approximately 500 fs laser pulse, and so given a safety factor of 3, for example, the maximum permissible fluence would be approximately 0.03 J/cm2.
  • Step 71 involves setting an elliptical polarization state 17 A of the pulsed laser radiation 9 .
  • Step 73 involves input coupling the pulsed laser radiation 9 into a multipass cell 5 formed from two concave mirrors 25 A, 25 B, which form e.g. a concentric or confocal resonator.
  • the multipass cell 5 e.g. a concentric or confocal resonator
  • the multipass cell 5 is repeatedly traversed with formation of a plurality of intermediate focus zones 29 , wherein the multipass cell 5 is filled with a gas 4 having an optical nonlinearity which causes an intensity-dependent rotation of an alignment of the elliptical polarization state 17 B of the pulsed laser radiation 9 in the intermediate focus zones 29 . Accordingly, beam portions having differently aligned elliptical polarization states are generated in the multipass cell 5 .
  • Step 75 involves output coupling the beam portions having differently aligned elliptical polarizations out of the multipass cell 5
  • step 77 involves separating the output-coupled beam portions into a useful beam portion 9 A with primary laser pulses 11 ′ and a residual beam portion 9 B with low-intensity laser radiation.
  • At least one of the following parameters for rotating the alignment of one of the elliptical polarization states in the multipass cell 5 can be set in a preceding or operation-accompanying step 79 :
  • the optical systems proposed herein allow for the possibility of providing substantially very similar (comparable) conditions of the nonlinearity in the intermediate focus zones of the multipass cell, for example on account of the comparable gas pressure and the comparable pulse durations and pulse energies (and thus pulse peak intensities) in the intermediate focus zones.
  • the pulse durations and pulse peak intensities can furthermore be set by means of the abovementioned dispersive mirrors/mirror segments for the compensation of (often only small) pulse duration lengthening effects during the individual passes through the multipass cell.
  • dispersive mirror segments for the compensation of (often only small) pulse duration lengthening effects during the individual passes through the multipass cell.
  • the laser radiation can pass through a sequence of successive multipass cells one after another. This allows gas conditions to be set in a differentiated manner with respect to the groups of intermediate focus zones respectively present in the individual multipass cells.
  • the recitation of “at least one of A, B and C” should be interpreted as one or more of a group of elements consisting of A, B and C, and should not be interpreted as requiring at least one of each of the listed elements A, B and C, regardless of whether A, B and C are related as categories or otherwise.
  • the recitation of “A, B and/or C” or “at least one of A, B or C” should be interpreted as including any singular entity from the listed elements, e.g., A, any subset from the listed elements, e.g., A and B, or the entire list of elements A, B and C.
  • Telescope lenses 21 A, 21 B are Telescope lenses 21 A, 21 B

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