Classifications

• • 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
  • H01S3/14—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

• the present invention relates to optical systems for increasing the contrast of pulsed laser radiation and laser systems, in particular ultra-short pulse (USP) laser systems for delivering pulsed laser radiation with high power and / or high pulse energy.
• the invention also relates to a method for increasing the contrast of pulsed laser radiation, in particular special ultra-short pulse trains.
• the contrast between primary laser pulses with a high peak intensity and the background is an essential parameter.
• the laser radiation that forms the subsurface consists of a radiation platform that goes back to ASE and / or laser pulses that arrive before or after the primary laser pulses and whose peak intensity is significantly lower than the peak intensity of the primary laser pulses.
• the aim is to largely remove the ASE background and the pre- and post-laser pulses from the pulsed laser radiation and thus largely limit the intensity contribution to the interaction to the primary laser pulses.
• HCF hollow core fiber gas-filled hollow core fiber
• NER nonlinear elliptical polarization rotation
• HCF gas-filled hollow core fiber
• NER nonlinear ellipse rotation
• NER nonlinear ellipse rotation
• a constant gas condition can be provided along the HCF.
• differential pumping can reduce the gas density along the fiber in order to avoid undesired non-linear effects or ionization.
• NER used to improve the contrast of sub-4 fs laser pulses using an HCF filled with argon gas.
• DE 102014007159 A1 discloses a method for the spectral broadening of laser pulses for non-linear pulse compression using a multipass cell, which is constructed, for example, in the form of a so-called Herriott cell.
• the aim is a spectral broadening of laser pulses, which can also be carried out with a pulse power that is greater than the critical power of the non-linear medium used for the spectral broadening.
• One object of the invention is to propose systems and methods which can also and in particular at high pulse energies and high average powers for a contrast increase of pulsed laser radiation, for example ultrashort pulse trains.
• non-linear effects are to be used to influence the polarization of laser pulses and their propagation.
• At least one of these objects is achieved by an optical system for increasing the contrast of pulsed laser radiation according to claim 1, a laser system according to claim 12 and a method for increasing the contrast of pulsed laser radiation according to claim 14. Further developments are given in the dependent claims.
• an optical system for increasing the contrast of pulsed laser radiation using a non-linear elliptical polarization rotation comprises:
• An optical beam splitting system for splitting the beam components with differently directed elliptical polarization states.
• a laser system in particular an ultra-short pulse (USP) laser system, for emitting pulsed laser radiation (9)
• USP ultra-short pulse
• a laser radiation source that emits pulsed laser radiation that includes primary laser pulses, in particular with pulse energies pulse durations in the range of a few hundred femtoseconds and 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 non-linear elliptical polarization rotation comprises the following steps:
• At least one of the following parameters of the optical system can be set or adjustable for a rotation of the alignment of one of the elliptical polarization states, in particular for setting a rotation angle of 90 °:
• the optical system being designed as a gas-filled cell and the optical system having 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 also be set or adjustable:
• the first polarization adjustment optics comprise a first wave plate, optionally a 1/4 wave plate and / or a 1/2 wave plate.
• the optical system can also include at least one of the following optical components:
• a pulse duration setting system for setting a pulse duration of primary laser pulses of the ge pulsed laser radiation
• a first optical telescope arrangement which is set to image the pulsed laser radiation in a predetermined mode in the multi-passage cell and which is optionally arranged downstream of the first polarization setting optics,
• the multiple passage cell can be designed:
• a non-linear gaseous medium in particular a noble gas such as helium or argon, the same gas pressure being present in each of the intermediate focus zones,
• At least one of the mirrors of the multiple passage cell can be designed as a convex mirror, the radii of curvature in particular matching and / or the distance between the mirrors being in a range of 95% to 105% of the sum of the radii of curvature.
• at least one of the mirrors can be designed as a dispersive mirror, the dispersion contribution of which compensates for a dispersive contribution of at least one passage of a primary laser pulse of the pulsed laser radiation through the multiple passage cell.
• at least one of the mirrors can comprise at least one mirror segment which the pulsed laser radiation strikes at least once when the pulsed laser radiation circulates through the multiple passage cell.
• the multiple passage cell can be designed in such a way that a primary laser pulse of the pulsed laser radiation, its contrast in optical system is to be increased, experiences essentially no change in the pulse duration and / or pulse energy in the intermediate focus zones.
• the beam components with differently oriented elliptical polarization states can comprise a useful beam component with primary laser pulses and a residual beam component with low-intensity laser radiation.
• the remaining beam portion forms a background from a radiation platform originating from ASE and / or from laser pulses that arrive before or after the primary laser pulses and whose peak intensity is significantly lower than the peak intensity of the primary laser pulses.
• the radiation background can have low-intensity laser pulses preceding the high-intensity laser pulses and / or subsequent low-intensity laser pulses.
• the optical beam splitting system for splitting the beam components with differently oriented elliptical polarization states can include:
• a second polarization setting optics for returning each of the differently aligned elliptical polarization states to a linear polarization state, in particular the beam components output by the multi-passage cell with differently oriented elliptical polarization states being converted into a useful beam component and a remaining beam component with differently oriented linear polarization states be, and
• - a beam splitter that outputs the useful beam portion and the remaining beam portion on different beam paths.
• the second polarization adjustment optics can comprise a second wave plate, optionally a 1/4 wave plate and / or a 1/2 wave plate.
• the optical system can also include a control system that is designed to rotate the alignment of one of the elliptical polarization states, in particular to set a rotation angle of 90 °, at least to set 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.
• this can also include the step:
• a pulse spectrum of the pulsed laser radiation assigned to the primary pulses in particular from intermediate focus zone to intermediate focus zone, can also widen due to a non-linear spectral broadening in the multiple passage cell, while the contrast increase takes place at the same time.
• a gas-filled multiple passage cell for example a Herriott cell
• contrast enhancement of pulsed laser radiation which can optionally be done together with a spectral pulse broadening and a subsequent pulse duration compression.
• the concepts proposed here have the advantages that the increase in pulse contrast at high average powers (greater than a few 100 W up to the kW range) and very high pulse energies (up to the J range) and can be implemented in a very efficient and easy to implement.
• an elliptical polarization state is understood to mean a polarization state in which an ellipse of polarization is present, as can be achieved, for example, by setting an angle in a range between 0 ° and 45 ° or in a range between 45 ° and 90 ° (analogously in a range between 45 ° and 135 ° or in a range between 135 ° and 180 °) a fast axis of a 1/4 wave plate and a polarization plane of an incident linearly polarized laser beam can be effected.
• a polarization ellipse of the E-field is formed in these angular ranges.
• an elliptical polarization state 90% of the laser beam can be in an s-polarization state and 10% in a p-polarization state.
• an elliptical polarization state differs from a linear polarization state (with only one component in the s or p polarization state) and from a circular polarization state (angle of 45 ° of the 1/4 wave plate or identical components in the s polarization state and in the p-polarization state).
• FIG. 1 shows an exemplary schematic representation of a laser system with an optical system for contrast enhancement
• 2A to 2C are exemplary sketches for explaining a Herriott cell as an example of a multi-pass cell and
• FIG. 3 shows a schematic flow diagram to explain an exemplary procedure for contrast enhancement.
• aspects described here are based in part on the knowledge that, due to an intensity-dependent phase rotation, a non-linear intensity-dependent rotation of an alignment of an elliptical polarization can be brought about (self-induced ellipse rotation).
• the intensity dependence of the elliptical Polarization rotation has the effect that the polarization of a pulse close to its peak experiences a different rotation than the low-intensity areas. This makes it possible to separate high-intensity beam components (formed by the primary laser pulses intended for an application) from low-intensity laser radiation (for example formed by the pre- and post-pulses accompanying the primary laser pulses).
• the implementation of the NER as part of a multi-pass cell provides a wide range of setting parameters and control parameters, so that contrast enhancement in an optical system is made possible for a wide variety of parameter constellations in a laser system.
• pulsed laser radiation when used in a Herriott cell, pulsed laser radiation can be freed from low-energy laser radiation (e.g. pre- or post-laser pulses) by only using laser pulses with a high intensity (the primary laser pulses mentioned ) can be influenced in their polarization with a view to the subsequent beam path.
• the influence takes place through the non-linearity of the refractive index of the gas in the intermediate focus zones of the Herriott cell in such a way that the primary laser pulses can pass through an optical beam splitting system with as little loss as possible.
• the alignment of the polarization ellipse of the low-energy laser radiation component within the Herriott cell does not change, so that this low-energy laser radiation component can be removed from the laser radiation with the optical beam splitting system after leaving the Herriott cell.
• pre- and / or post-laser pulses can be removed from the laser beam if the rotation of the polarization ellipse is only set for the primary laser pulses due to their high intensity so that only the primary laser pulses can pass through a preset beam splitter, for example.
• the laser system 1 shows a laser system 1 which has an optical system 3 to increase the contrast.
• the optical system 3 is based on the use of a multiple passage cell 5 filled with gas 4 (for example a Herriott cell), the gas 4 serving as a non-linear Kerr medium.
• gas 4 for example a Herriott cell
• helium can be used in the multi-pass cell 5 at very high intensities.
• argon or another noble gas can be used as the non-linear medium.
• the laser system 1 generally comprises a laser radiation source 7 which emits laser radiation 9.
• the laser radiation 9 comprises primary laser pulses 11 with a pulse energy in the range of, for example, a few hundred millijoules and a pulse duration ⁇ t in the range of a few hundred femoseconds and less, which, for example, form an ultra-short pulse train.
• the laser radiation 9 also includes low-energy laser radiation 13, which is indicated by way of example in FIG. 1 as prepulse 13A or postpulse 13B.
• 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 laser radiation 9 with a linear polarization 17A is present at the output of the pulse duration setting system 15 or at the output of the laser radiation source 7, the polarization vector of which is indicated in FIG. 1 as an example orthogonal to the plane of the characters. That is, both the primary laser pulses 11 and the low-energy laser radiation 13 are linearly polarized, generally in the same direction.
• the optical system 3 has first polarization adjustment optics 19.
• the laser radiation 9 is elliptically polarized.
• FIG. 1 an E-field vector revolving around a polarization ellipse to illustrate an elliptical polarization state 17B is indicated by way of example at the output of the first polarization adjustment lens 19, the long semiaxis of the polarization ellipse running in the plane of the drawing as an example.
• the first polarization setting optics 19 comprise a wave plate (for example a 1/4 wave plate or a 1/8 wave plate, etc.).
• the first polarization adjustment optics 19 comprise a first 1/4 wave plate 19A.
• the first polarization adjustment optics 19 can optionally have a 1 ⁇ 2 wave plate 19B, for example arranged in front of the first 1 ⁇ 4 wave plate 19A, for aligning the orientation of the elliptical polarization state.
• the setting of the polarization in the first polarization setting optics 19 is independent of the intensity, because the wave plates work with an anisotropic refractive index, for example with a birefringent crystal.
• the 1/4 wave plate with respect to the polarization plane of the laser radiation 9 is set in such a way that an angle between a fast axis of the 1/4 wave plate and the plane of polarization is not 0 °, 45 °, 90 ° or 135 °.
• Corresponding angular ranges can be other wave plates or combinations of Waveplates can be assigned to generate elliptical polarization.
• an ellipticity of the polarization ellipse can be set.
• the polarization ellipse can also be aligned.
• 1 also shows a telescope arrangement 21 for adapting the mode (generally of beam parameters such as beam diameter and beam divergence) of the pulsed laser radiation 9 prior to coupling into the multiple passage cell 5 with a coupling mirror 23.
• mode generally of beam parameters such as beam diameter and beam divergence
• the multiple passage cell 5 for example designed as a Herriott cell, comprises two concavely curved mirrors 25A, 25B which, in a gas-filled environment, form a multiple beam path 5A running back and forth between the mirrors 25A, 25B.
• the multi-passage cell 5 can - for example in the geometry of a Herriott cell - be formed by two concave mirrors which, for example, in a concentric or confocal resonator arrangement, generally also in a different resonator configuration along a common optical axis 27 (given by the concentric arrangement ) are aligned.
• the mirrors 25A, 25B are also referred to as Herriott mirrors. If the laser radiation 9 is introduced into the multiple passage cell 5 offset from the optical axis 27, the laser radiation 9 is circulated several times back and forth on a predetermined, usually elliptical (circular) pattern.
• FIG. 2A illustrates schematically the beam path between the mirrors 25A, 25B with the formation of an intermediate focus zone 29, assuming a correspondingly adapted mode of the 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, which is embodied concentrically in the example in FIG. 1.
• Figures 2B and 2C show plan views of the mirrors 25A, 25B, in which circularly arranged impact areas 33 on the mirror surfaces are indicated schematically. In the impingement areas 33, the laser radiation 9 strikes as centrally as possible before it is reflected back from there, as in the direction of the center of the multiple passage cell (here the resonator arrangement).
• FIGS. 2B and 2C one can also see a coupling-in opening 35A and a coupling-out opening 35B.
• the areas ready for reflection on the surface of the mirrors 25A, 25B are circular surface sections with a diameter D.
• the number of revolutions (intermediate focus zones 29) can in principle be as large as desired; for example, 5 to 100 intermediate focus zones can be traversed; that is, several intermediate focal zones are traversed in the multi-pass cell.
• at least one of the mirrors 25A, 25B can also be built up from individual discrete mirror elements, wherein a reflection can preferably take place on a single mirror element. For example, twelve intermediate focus zones 29 are passed through.
• smaller mirror elements that engage in the Herriott cell can be used and positioned, for example, at the positions of the openings 35A, 35B.
• the pulsed Laserstrah treatment 9 is repeatedly ge leads through intermediate focus zones in the center of the multiple passage cell 5. Due to the focusing of the primary laser pulses during the pulse duration At of the primary laser pulses 11, high intensities develop in the intermediate focus zones, which lead to a non-linear behavior of the refractive index of the gas 4.
• the non-linear behavior of the refractive index of the gas 4 leads, as explained below, to beam components of the laser radiation 9 with different polarization states and can thus be used to increase the contrast of the pulsed laser radiation 9.
• the optical system 3 also has an optical beam splitting system 41 for splitting the beam components of the laser radiation 9 with different polarization states.
• the optical beam splitting system 41 comprises a second wave plate 43 and a beam splitter 45.
• the second wave plate 43 is generally set such that the elliptical polarization state is converted back into a linear polarization. Further structures for dividing beam components with different elliptical polarization states are known from the prior art.
• 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 essentially unchanged.
• the elliptical polarization states of the primary laser pulses 11 and the low-energy laser radiation 13 thus change their relative alignment to one another with each passage through an intermediate focus zone 29.
• the beam splitter 45 distributes the primary laser pulses 11 and the low-intensity laser radiation 13 to two different beam paths for a useful beam portion 9A and a residual beam portion 9B, which correspond to the orthogonal linear polarization states 47A, 47B.
• a primary laser pulse 1G of the useful beam portion 9A is indicated by way of example, which has been freed from pre- and post-pulses.
• the pre- and post-pulses form the remaining beam portion. Even if the resulting beam components are not completely orthogonal to one another are polarized, substantial portions of the pre- and post-pulses can be removed from the useful beam entrance.
• the primary laser pulses 1 G can be added to a compressor 49, for example.
• the compressor 49 is shown in Figure 1 for example as a chirped mirror compressor.
• a useful laser radiation 9A ′ can thus be output at an output of the laser system 1 which comprises a sequence of compressed primary laser pulses 11 ′′, the compressed primary laser pulses 11 ′′ having an increased contrast.
• the configuration proposed herein can enable a predetermined / adjustable number of intermediate focus zones 29 to be traversed, for example using a Herriott cell.
• a beam diameter d can be adjusted in the intermediate focus zones and can also be adjusted to the laser power, pulse duration, etc. and the gas 4, for example, via the radius of curvature Rm of the mirrors 25A, 25B.
• the radius of curvature Rm is, for example, identical for both mirrors or is at least of the same order of magnitude.
• the gas pressure can be adjusted with regard to the non-linearity. It is noted that due to the high spatial proximity of the various continuous intermediate focus zones in the multiple passage cell, the same gas pressure is present in each of the intermediate focus zones.
• the optical beam parameters and beam properties are preferably very similar in the various intermediate focus zones, so that similar non-linear effects are also present.
• the intermediate focus zones 29 essentially all have the same diameter d and accordingly have the same Rayleigh lengths Lr.
• the distance between the mirrors 25A, 25B is in a range from 95% to 105% of the sum of the radii of curvature.
• the optical system 3 can, for example, have a control system 61, which via control connections 63 with the pulse duration setting system 15, the polarization setting optics 19 (in particular for setting the angular positions of the first 1/4 wave plate 19A and optionally the 1/2 -Wave plate 19B), the telescope assemblies 21, 39 (especially for adjusting the distance between telescopic lenses 21 A, 21B), a Druckeinstellvorrich device 65 for adjusting the gas pressure (see Fig. 1) and / or the optical beam splitting system 41 (especially for setting the angular position of the second l / 4 wave plate 43) is a related party.
• a control system 61 which via control connections 63 with the pulse duration setting system 15, the polarization setting optics 19 (in particular for setting the angular positions of the first 1/4 wave plate 19A and optionally the 1/2 -Wave plate 19B), the telescope assemblies 21, 39 (especially for adjusting the distance between telescopic lenses 21 A, 21B), a Druckeinstellvorrich device 65 for adjusting the gas pressure
• the laser radiation 9 strikes the mirrors 25A, 25B repeatedly (multiple times in each case).
• the mirrors can be used to adjust the dispersion by designing them as dispersive mirrors. If the mirrors 25A, 25B have a dispersive effect in at least one of the reflections, the dispersion and thus the pulse duration of the primary laser pulses 11 can be acted upon directly.
• one or more of the impingement areas 33 can be provided with a dispersive layer.
• a dispersive coating 51 is indicated by dashed lines in FIG. 2A for the mirror 25B.
• each of the mirrors 25A, 25B can be constructed from a plurality of mirror segments with predetermined dispersive properties, with each of the mirror segments in its dispersion for a desired pulse duration as it passes through the Multi-pass cell 5 is adapted.
• the dispersion present in the multiple passage cell 5 is composed of a dispersion contribution from the dispersive mirror and a dispersion contribution in the gas-filled volume along the beam path 5A.
• An exemplary mirror segment 53 is indicated in FIG. 1.
• the laser radiation (depending on the size of the mirror segment) hits the mirror segment at least once, which usually has at least an extent the size of the beam diameter D on the mirror surface.
• the concepts proposed here allow a dispersion that is accumulated when passing through the gas-filled volume to be at least partially 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 selectively adjust the pulse durations vary.
• suitable dispersive mirror coatings chirped mirrors
• the multi-pass cell 5 enables the high-intensity primary laser pulses in the intermediate focus zones to be spectrally broadened step-by-step.
• the non-linear spectral broadening is caused by the high intensity present in an intermediate focus zone and by the non-linearity of the refractive index of the gaseous medium in the multiple passage cell 5.
• the pulse spectrum can change from intermediate focus zone to intermediate focus zone, namely essentially while the pulse duration and pulse energy remain the same. If the multiple passage cell 5 is built up by means of chirped mirrors, the pulse duration can moreover be adjusted. For example, the pulse duration can change (shorten or lengthen) from run to run. Accordingly, even with a non-linear spectral broadening, the peak intensities in the intermediate focus zones remain essentially the same.
• Another advantage can arise in connection with the non-linear spectral broadening if laser radiation with elliptical polarization is used in the multiple passage cell for this purpose. So the spectral broadening can possibly be smoother about in itself across the frequency spectrum, so that a less structured spectrum can result. This can have a positive effect on the subsequent pulse shaping and / or pulse compression.
• Fig. 2A shows the formation of an intermediate focus zone in a Herriott cell under the assumption of curved Herriott mirrors.
• geometric parameters for the implementation of a multi-pass cell are considered within the framework of the concepts presented here.
• the laser-induced damage to the mirrors 25A, 25B requires a minimum diameter of the laser radiation 9 on the curved mirror 25A, 25B.
• the ionization threshold determines the smallest possible focus diameter d in the intermediate focus zones 29 with a view to avoiding ionization of the gas 4. Both parameters together define a necessary length of the multi-passage cell 5, that is, the distance between the mirrors that build up the concentric resonator, for example their radius of curvature.
• the electrical field strength required for ionization for circular / elliptically polarized light is increased compared to linearly polarized light, so that with a comparable beam diameter D on the mirrors 25A, 25B, the possible focus diameter d in the intermediate focus zones 29 can be selected smaller ge.
• the mirrors For the use of high-intensity laser radiation it may be necessary for the mirrors to withstand pulse energies of a few 100 mJ with pulse durations of a few 100 fs, for example 500 fs or less.
• the mirrors are also broadband, for example, to a wavelength range from 700 nm to 1100 nm, for example, for ultra-short pulses from a titanium-sapphire laser or 900 nm to 1100 nm for ultra-short pulses from lasers that emit around 1000 nm, such as Nd: YAG or Yb: YAG.
• the mirrors can make a contribution or no contribution to dispersion, so that dispersive coatings may also have to be taken into account.
• a laser-induced damage threshold of approx. 0.5 J / cm2 with a pulse duration of approx. 500 fs can be measured.
• This threshold value is usually assigned to the center of the beam.
• the result for the approx. 500 fs laser pulse is, for example, a threshold value of approx. 0.1 J / cm2, so that with a safety factor of, for example 3, the maximum permissible fluence is approx. 0.03 J / cm2 would.
• a reduced mirror spacing / a shortened multiple passage cell length / resonator length L can be implemented with a correspondingly smaller radius of curvature of the mirrors 25A, 25B, provided the same Beam diameter is on the mirror.
• step 71 an elliptical polarization state 17A of the pulsed laser radiation 9 is set.
• the pulsed laser radiation 9 is coupled into a multiple passage cell 5, which is formed from two concave mirrors 25A, 25B which, for example, form a concentric or confocal resonator.
• the multi-pass cell 5 e.g. a concentric or confocal resonator
• the multi-pass cell 5 is passed through several times with the formation of a plurality of intermediate focus zones 29, the multi-pass cell 5 being filled with a gas 4 which has an optical nonlinearity which is an intensity-dependent rotation of an alignment of the elliptical polarization state 17B of the pulsed laser radiation 9 in the intermediate focus zones 29. Accordingly, beam portions with differently aligned elliptical polarization states in the multi-passage cell 5 are generated.
• step 75 the beam portions with differently aligned elliptical polarizations are decoupled from the multiple passage cell 5 and in step 77 the decoupled beam portions are separated into a useful beam portion 9A with primary laser pulses 1G and a remaining beam portion 9B 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 multi-pass cell 5 can optionally be set in a step 79 preceding or accompanying the operation:
• an angle of rotation DQ of 90 ° for primary laser pulses 11 of the pulsed laser radiation 9 can be set
• the necessary non-linear conditions for a NER for cleaning the laser radiation from low-energy contributions can be set in a multi-pass cell designed as a Herriott cell.
• the optical systems proposed here allow essentially very similar (comparable) conditions of the non-linearity to be provided in the intermediate focus zones of the multi-pass cell, for example due to 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 also be set with the mentioned dispersive mirrors / mirror segments to compensate for (often only small) pulse duration lengthening effects in the individual passes through the multi-pass cell.
• additional freedom can be gained for a dispersion compensation and thus for an adjustment of the intensity in the intermediate focus zones.
• the laser radiation can run through a sequence of successive multiple pass cells one after the other. This allows gas conditions to be differentiated according to the groups of intermediate focus zones present in the individual multiple passage cells.

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• 2021
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