Laser system with optical system for spectral broadening of pulsed laser radiation and method for spectral broadening of pulsed laser radiation

Abstract

A laser system (1) comprises a laser radiation source (7) for providing a pulsed laser radiation (9), wherein the pulsed laser radiation comprises laser pulses (11) having a pulse energy in the range of 1 mJ to 100 J or in the range of 10 mJ to 1 J and a pulse duration in the range of 10 fs to 5 ps or in the range of 500 fs to 1.5 ps. The laser system (1) further comprises an optical system (3) for spectral broadening of the pulsed laser radiation (9). The optical system (3) comprises: - a first polarization setting optics (19) setting a circular polarization state (17B) of the pulsed laser radiation (9), and - a multi-pass cell (5) having at least two mirrors (25A, 25B) through which the pulsed laser radiation (9) passes forming a plurality of intermediate focus regions (29), wherein the multi-pass cell (5) is filled with a filling gas (5A) having an optical nonlinearity, wherein the filling gas (5A) enables spectral broadening of the pulsed laser radiation (9) in the intermediate focus regions (29). Further, a pressure of the filling gas (5A) is set in a pressure range in which there is an ionization behavior of the filling gas (5A) in the regime of multiphoton ionization, and a focus diameter (d) of the intermediate focus regions (29) is set such that the pulsed laser radiation (9) passes through the multi-pass cell (5) without ionization of the filling gas (5).

Description

TECHNICAL FIELD

[0001] The present invention relates to a laser system having an optical system for spectral broadening of pulsed laser radiation, and to a laser system for emitting pulsed laser radiation having high pulse energy, in particular an ultrashort pulse (USP) laser system. Furthermore, the present invention relates to a method for spectral broadening of pulsed laser radiation, in particular of an ultrashort pulse sequence. BACKGROUND

[0002] DE 10 2014 007 159 A1 discloses a method for spectral broadening of laser pulses for nonlinear pulse compression, which uses an arrangement having a series of nonlinear interaction sections, such as can be provided in a multi-pass cell, which can be configured, for example, in the form of a so-called Herriott cell. Here, the goal is spectral broadening of laser pulses, which can be carried out even in the case of pulse powers greater than the critical power of the nonlinear medium used for spectral broadening.

[0003] It is an object of the present invention to propose a system and a method for spectral broadening of pulsed laser radiation, for example of an ultrashort pulse sequence, which can be used in a compact construction, even in the case of high pulse energy and optionally high average power. In particular, here the nonlinear effects in a filling gas are to be utilized to influence the spectral broadening of laser pulses having high pulse energy in an arrangement having a spatially as small as possible footprint. SUMMARY

[0004] At least one of these objects is achieved by the laser system according to claim 1 and the method for spectral broadening of pulsed laser radiation according to claim 10. Extended solutions are specified in the dependent claims.

[0005] In a first aspect, a laser system comprises a laser radiation source for providing pulsed laser radiation. The pulsed laser radiation comprises laser pulses with a pulse energy in the range of 1 mJ to 100 J, preferably in the range of 10 mJ to 1 J and a pulse duration in the range of 10 fs to 5 ps, preferably in the range of 500 fs to 1.5 ps. Further, the laser system comprises a (at least one) optical system for spectral broadening of the pulsed laser radiation, the optical system comprising a first polarization setup optics, which setups a circular polarization state of the pulsed laser radiation, and a multipass cell with at least two mirrors. In case of a configuration of a plurality of intermediate focus zones, the pulsed laser radiation present in the circular polarization state passes through the multipass cell. The multipass cell is filled with a filling gas having an optical nonlinearity, wherein the filling gas enables a spectral broadening of the pulsed laser radiation in the intermediate focus zones. In the multipass cell, a pressure of the filling gas is setup in a pressure range, in which there is an ionization behavior of the filling gas in the regime of multiphoton ionization. Further, a focus diameter of the intermediate focus zones is setup such that the pulsed laser radiation passes through the multipass cell without ionization of the filling gas (in the intermediate focus zones).

[0006] A further aspect comprises a method for spectral broadening of pulsed laser radiation using a nonlinearity of a filling gas of a multipass cell with at least two mirrors. The multipass cell forms a plurality of intermediate focus zones. The method comprises the following steps:

[0007] - generating pulsed laser radiation comprising laser pulses with a pulse energy in the range of 1 mJ to 100 J, in particular in the range of 10 mJ to 1 J and a pulse duration in the range of 10 fs to 5 ps, in particular in the range of 500 fs to 1.5 ps,

[0008] - setting up a circular polarization state of the pulsed laser radiation for passing through the multipass cell,

[0009] - coupling the pulsed laser radiation into the multipass cell, wherein the pulsed laser radiation passes through the plurality of intermediate focus zones and nonlinearly interacts with the filling gas in the intermediate focus zones, thereby enabling a spectral broadening of the pulsed laser radiation in the intermediate focus zones,

[0010] - setting up a pressure of the filling gas in a pressure range, in which there is an ionization behavior of the filling gas in the regime of multiphoton ionization,

[0011] - setting up a focus diameter in the intermediate focus zones such that the pulsed laser radiation passes through the multipass cell without ionization of the filling gas, and

[0012] - coupling out the spectrally broadened pulsed laser radiation out of the multipass cell.

[0013] If the pressure is set to the ionization behavior of the filling gas in the range of multiphoton ionization, there is pure multiphoton ionization, in which the avalanche ionization does not substantially contribute to the ionization of the gas. The focusing geometry is set such that the passage through the intermediate focusing region without ionization means in this case that the ionization that can occur takes place only within a range in which the performability of the desired spectral broadening is not disturbed.

[0014] As a condition for setting the parameters of the multipass cell for the ionization behavior in the range of multiphoton ionization, the pressure is set in a range in which the peak intensity of the laser pulses at which the filling gas begins to ionize (here also referred to as multiphoton ionization (threshold) intensity) is substantially independent of the pressure of the filling gas or does not decrease significantly with increasing pressure of the filling gas. (Here, not significantly relates to a range of one order of magnitude of the pressure increase, as is required for a nonlinear increase.)

[0015] In some embodiments, the multipass cell is filled with helium gas as the filling gas, which has a pressure in the range of 100 Pa to 60 000 Pa, in particular in the range of 1000 Pa to 50 000 Pa. In other embodiments, the multipass cell is filled with argon gas as the filling gas, which has a pressure in the range of 100 Pa to 50 000 Pa, in particular in the range of 1000 Pa to 40 000 Pa.

[0016] In some embodiments, the focusing diameters of the intermediate focusing regions are set such that the peak intensity resulting from the pulse duration and the pulse energy of the laser pulses in the intermediate focusing regions is in the range of 50% to 110% of the multiphoton ionization (threshold) intensity.

[0017] In some embodiments, the first polarization setting optics can comprise a first wave plate, for example a λ / 4 wave plate and / or a λ / 2 wave plate.

[0018] In some embodiments, the optical system can further comprise at least one of the following optical components:

[0019] - a pulse duration setting system for setting the pulse duration of the laser pulses of the pulsed laser radiation,

[0020] - a first optical telescope arrangement which is provided for mapping the pulsed laser radiation into the multipass cell in a predefined mode, and which is optionally arranged beam downstream of the first polarization setting optics, arrangement,

[0021] - a coupling-in mirror for coupling the pulsed laser radiation into the multi-pass cell,

[0022] - a coupling-out mirror for further guiding pulsed laser radiation emitted from the multi-pass cell, and

[0023] - a second optical telescope arrangement arranged for collimating pulsed laser radiation emitted from the multi-pass cell.

[0024] In some embodiments, the multi-pass cell can be configured

[0025] - with a predetermined or adjustable number of intermediate focusing regions, and / or

[0026] - with intermediate focusing regions which substantially have the same diameter and the same Rayleigh length, and / or

[0027] - with intermediate focusing regions which are arranged one above the other (auf einander), next to each other (nebeneinander), optionally partially overlapping each other,

[0028] - in a resonator structure with identical radii of curvature of at least two mirrors, optionally in a confocal or concentric arrangement, and / or

[0029] - in a quasi-resonator structure with identical radii of curvature of at least two mirrors, optionally in a quasi-confocal or quasi-concentric arrangement, and / or

[0030] - in an arrangement in which at least two mirrors comprise a plurality of mirror segments, wherein an intermediate focusing region is configured between each two mirror segments and is passed through consecutively,

[0031] - as a cell filled with a noble gas such as helium or argon as a fill gas, wherein the same pressure prevails in each of the intermediate focusing regions, and / or

[0032] - for stepwise nonlinear spectral broadening of the pulsed laser radiation passing through the intermediate focusing regions.

[0033] In some embodiments, the laser system can further comprise a second polarization tuning optics for returning a circular polarization state to a linear polarization state. The second polarization tuning optics can be arranged downstream in the beam with respect to the multi-pass cell and comprise in particular a second wave plate, in particular an achromatic wave plate, such as a λ / 4 wave plate and / or a λ / 2 wave plate.

[0034] In some embodiments, the laser system can further comprise at least one of the following optical components:

[0035] - a pulse duration setting system for setting the pulse duration of the laser pulses,

[0036] - an optical pulse duration compressor system for compensating for a dispersion contribution of the optical system and / or for temporally compressing laser pulses of the laser radiation which have undergone nonlinear spectral broadening in at least one of the intermediate focus regions,

[0037] - a beam splitter for separating different polarization states output by the multipass cell, and

[0038] - a control system configured for setting the pressure of the filling gas in the multipass cell for compensating for a reduction of the nonlinearity of the filling gas due to the set circular polarization.

[0039] In some embodiments, the method can further comprise at least one of the following steps:

[0040] - providing helium as the filling gas and setting the pressure in the range of 100 Pa to 60 000 Pa, in particular in the range of 1000 Pa to 50 000 Pa,

[0041] - providing argon as the filling gas and setting the pressure in the range of 100 Pa to 50 000 Pa, in particular in the range of 1000 Pa to 40 000 Pa, and

[0042] - for increasing the nonlinearity, the pressure of the filling gas is increased such that, based on the nonlinearity of the filling gas present in the case of the same pressure and linear polarization, a reduction of the nonlinearity of the filling gas in the case of circular polarization is compensated.

[0043] In some embodiments, the method can further comprise the following steps:

[0044] - the focus diameters of the intermediate focus regions are set such that the peak intensity resulting from the pulse duration and the pulse energy of the laser pulses in the intermediate focus regions lies in the range of 50% to 110% of the multiphoton ionization (threshold) intensity.

[0045] In some embodiments, the method can further comprise at least one of the following steps:

[0046] - setting the polarization of the spectrally broadened pulsed laser radiation for a subsequent beam path, and

[0047] - performing a dispersion compensation on the spectrally broadened pulsed laser radiation.

[0048] Furthermore, in some embodiments of the method, at least one of the following parameters of the multi-pass cell can be adjusted:

[0049] - the dispersion of the laser pulses accumulated in the multi-pass cell,

[0050] - the focal diameter in these intermediate focus regions, and

[0051] - the Rayleigh length of these intermediate focus regions.

[0052] According to the present application, it is generally recommended to use a gas-filled multi-pass cell for spectrally broadening pulsed laser radiation having circular polarization. This has the advantage that the gas-filled multi-pass cell can be constructed with a reduced length. This is because by reducing the maximum electric field strength due to the incidence of the circularly polarized laser radiation (i.e. more pulse energy is required to reach the ionization threshold), a multi-pass cell with a smaller diameter in the intermediate focus region can be constructed. This makes the radii of curvature of the mirrors of the multi-pass cell smaller and thus leads to a shortening of the multi-pass cell, for example of a confocal or concentric construction, compared to a multi-pass cell operated with linear polarization. Preferably, the multi-pass cell is operated with parameters in the range of multiphoton ionization; i.e. outside the range of (electronic) avalanche ionization in which very many free electrons are generated during the ionization process. Multiphoton ionization is the dominant ionization process in "dilute" gases, such as the gases present at the pressures described herein for rare gases.

[0053] Operating the multi-pass cell with parameters characteristic of the ionization process of multiphoton ionization, in particular density, pulse length, pulse energy, allows to increase the nonlinearity in the filling gas by increasing the pressure in the multi-pass cell, where the pressure rise (in the case of the same pulse duration and focus size and circular polarization) does not substantially affect the circular breakdown pulse energy required for ionization. For helium gas as filling gas, multiphoton ionization occurs in the range of 10 13 - 10 15 Watt / cm 2 , the pressure being 1000 Pa to a few hundred millibar (n*10 4 Pa), for example 60000 Pa.

[0054] In some embodiments, the mirrors of the multi-pass cell are constructed as convex mirrors, wherein the radii of curvature are in particular uniform, and / or the spacing between the mirrors lies in the range of 95% to 105% of the sum of the radii of curvature. Alternatively or additionally, at least one of the mirrors can be constructed as a dispersion mirror, the dispersion contribution of which compensates the dispersion contribution of the laser pulses of the pulsed laser radiation at least one pass through the multi-pass cell. Alternatively or additionally, furthermore, at least one of the mirrors can comprise a plurality of mirror segments, on which the pulsed laser radiation impinges at least once when the pulsed laser radiation circulates through the multi-pass cell.

[0055] In some embodiments, the multi-pass cell is configured such that the laser pulses of the pulsed laser radiation, the spectrum of which is to be spectrally broadened in the optical system, experience substantially no variation in pulse duration and / or pulse energy in the intermediate focus region.

[0056] The spectral broadening can optionally be combined with, for example, subsequent pulse duration compression in order to produce pulsed laser radiation having a short pulse duration and a high peak intensity. BRIEF DESCRIPTION OF DRAWINGS

[0057] The disclosure herein allows the idea to improve aspects of the prior art at least in part. Further features and the conformity to the objects are derived from the following description of embodiments based on the drawings. In the drawings:

[0058] Figure 1 An exemplary schematic diagram of a laser system with an optical system for spectral broadening is shown,

[0059] Figures 2A to 2C An exemplary schematic diagram for illustrating an example of a Heriot cell as a multi-pass cell is shown, and

[0060] Figure 3 An exemplary flowchart for illustrating an exemplary method of spectral broadening is shown. DETAILED DESCRIPTION

[0061] The aspects described herein are based, in part, on the recognition that a shortening of a multi-pass cell for spectral broadening of high-intensity laser radiation can be successfully achieved by reducing the maximum electric field strength due to the Durchstrahlung with high-intensity laser radiation having circular polarization. It has been recognized here that, in order to advantageously use the circular polarization, the parameters of the laser radiation for the respective filling gas are preferably in the range of multiphoton ionization, so that an increase in pressure (and thus an increase in the gas density in the multi-pass cell) does not affect or only slightly affects the electric field strength required for ionization.

[0062] It is known that, in the case of circularly polarized laser radiation, the pulse energy required for ionization of the filling gas increases significantly, for example, by a factor of 3 to 10. Furthermore, for circularly polarized laser radiation, the nonlinear part of the refractive index of the filling gas present in the intermediate focus region is smaller, so that the desired nonlinear effects only occur at higher intensities / pulse energies. As an example, the nonlinearities in the case of circular polarization are reduced to one third of the nonlinearities present in the case of linear polarization.

[0063] In order to compensate for the reduced nonlinearities, it is proposed to increase the gas pressure in the multi-pass cell by a factor of the compensation factor (for example, a factor of 3).

[0064] It has been recognized in the present context that for helium, for ultrashort laser pulses in the range of multiphoton absorption, a multipass cell can be operated at pressures in the range of about or below one bar (100000 Pa), where the pulse energy required for ionization is almost independent of the pressure. Accordingly, the pressure in the multipass cell can be increased without any significant impact on the ionization performance and thus the reduced nonlinearity due to circular polarization can be compensated.

[0065] In other words, for a gas-filled multipass cell, it is possible to so adjust the configuration of the beam path (essentially with respect to the spectral broadening given by the parameters of the intermediate focusing region) and the parameters of the filling gas (essentially given by the gas pressure in the range of multiphoton absorption depending on the gas species) that the sought spectral broadening is achieved in a multipass cell structure that is as short and compact as possible, even in the case of the use of circular polarization.

[0066] The length of the multipass cell is estimated to be proportional to the square root of the ratio of the pulse energy required for ionization in the case of linear polarization to the pulse energy required for ionization in the case of circular polarization, by means of which the multipass cell can be significantly shortened in this way. This leads to an approximately possible shortening to 1 / √3. In other words, the length of a Herriott cell, for example, can be shortened by adjusting the circular polarization for the laser radiation to be spectrally broadened, wherein the boundary condition can be maintained at the same time: the Herriott cell operates "as close to the ionization threshold" for the nonlinear interaction.

[0067] The multipass cell can consist of a mirror pair, for example a Herriott cell explained below in connection with the figures. In general, the multipass cell provides for multiple passes through the intermediate focusing region. The intermediate focusing region can be formed between optical elements, for example between reflections on mirror / mirror segments. In this regard, see also DE 10 2014 007 159 A1 cited in the introduction. A plurality of intermediate focusing regions can also be constructed, for example, in a modular structure of a similar Herriott cell with a plurality of mirror segments. In the multipass cell, the beam path can be folded once or multiple times.

[0068] Figure 1 A laser system 1 with an optical system 3 for spectral broadening is shown. The optical system 3 is based on the use of a multipass cell 5, for example a Herriott cell, filled with a filling gas 4, wherein the filling gas 4 serves as a nonlinear (Kerr) medium. As an example, a noble gas is used as the filling gas. At very high intensities, helium with a high ionization threshold (the ionization threshold is about 3 times higher than that of argon) can be used. At lower but still high intensities in the multipass cell 5, for example argon or some other noble gas can be used as a nonlinear medium.

[0069] 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, which have a pulse energy in the range of several mJ (e.g. at least 20 mJ, e.g. several hundred mJ) and a pulse duration At in the range of several hundred femtoseconds (FWHM pulse duration) or less (e.g. 500 fs). For example, the laser pulses 11 constitute a sequence of ultrashort pulses.

[0070] Depending on the laser radiation source 7, the laser radiation 9 can also comprise low-energy laser radiation 13, which has a pulse energy in the range of several nJ (e.g. at least 1 nJ, e.g. several hundred nJ) and a pulse duration At in the range of several picoseconds (FWHM pulse duration) or less (e.g. 10 ps). Figure 1 For example, the low-energy laser radiation 13 is shown in

[0071] Furthermore, the laser radiation source 7 can optionally have a pulse duration setting system 15 for setting the pulse duration of the laser pulses 11, wherein the pulse duration setting system 15 can also be assigned to the optical system 3, as is Figure 1 shown.

[0072] It is assumed in the present example that, at the output of the pulse duration setting system 15 or at the output of the laser radiation source 7, there is laser radiation 9 having a linear polarization 17A, the polarization vector of which is given, for example, in Figure 1 the drawing plane. That is to say, not only the laser pulses 11, but possibly also the low-energy laser radiation 13, are linearly polarized.

[0073] The optical system 3 has a first polarization setting optics 19. In the first polarization setting optics, the laser radiation 9 is circularly polarized. The circular polarization can generally be set by means of a wave plate in the beam path in front of the multi-pass cell 5 / Herriott cell, for example by means of a zeroth- or lowth-order wave plate. In order to set the circular polarization state 17B of the laser radiation 9, Figure 1 the first polarization setting optics 19 in Figure 1 comprises, for example, a first λ / 4 wave plate 19A. At the output of the first polarization setting optics 19, a circularly circulating electric field vector is shown in order to illustrate the circular polarization state 17B.

[0074] Alternatively, the circular polarization can be set by means of a Faraday rotator, a Pockels cell or other suitable polarization-influencing elements. If the wave plate works with an anisotropic refractive index, for example with a birefringent crystal, the polarization setting in the first polarization setting optics 19 can preferably be realized independently of the intensity.

[0075] For example, in order to set the circular polarization state of the laser radiation 9, the first λ / 4 wave plate 19A is set with respect to the polarization plane of the laser radiation 9 such that there is an angle between the fast axis of the λ / 4 wave plate and the polarization plane of approximately 45°. For example, the angle is in the range of 42°-48°, so that there can be an ellipticity of the polarization (for example, also remaining as a result of the adjustment).

[0076] Furthermore, Figure 1 A telescope arrangement 21 is shown for matching the mode (generally the beam parameters, such as the beam diameter and the beam divergence) of the pulsed laser radiation 9 before the coupling-in into the multi-pass cell 5 by means of a coupling-in mirror 23.

[0077] The multi-pass cell 5 comprises two concave mirrors 25A, 25B, which form a beam path 5A that repeatedly travels back and forth between the mirrors 25A, 25B in a gaseous environment. Between the mirrors 25A, 25B, in each pass, the pulsed laser radiation passes through a focus region in which an intermediate focus region of the pulsed laser radiation with a corresponding high intensity is formed. In the intermediate focus region, the interaction of the laser radiation with the filling gas introduced into the multi-pass cell takes place and leads to nonlinear effects, for example, the desired spectral broadening, or, in the case of an avoidance of excessively high intensities in the intermediate focus region, to an optical breakdown / over-ionization of the filling gas. The operation of the multi-pass cell thus takes place in the conflict (Spannungsfeld) between the sufficient occurring spectral broadening of the laser radiation and the avoidance of ionization effects on the laser radiation.

[0078] The stepwise nonlinear spectral broadening is achieved by the high intensity present in the intermediate focus region, respectively, and by the nonlinearity of the refractive index of the gaseous medium in the multi-pass cell 5.

[0079] A Herriott cell is one example of a multi-pass cell into which pulsed laser radiation can be coupled for multiple passes. The Herriott cell is formed by two concave mirrors, which are oriented to each other along a common optical axis 27 (given by the particular arrangement) in a, for example, concentric or confocal resonator arrangement (or almost in a similar resonator arrangement that is concentric or confocal up to a few millimeters from the ideal concentric or confocal arrangement), often also in some other resonator configuration. In this case, the mirrors 25A, 25B are also referred to as Herriott mirrors or end mirrors. If the laser radiation 9 is introduced into the multi-pass cell 5 in a manner offset with respect to the optical axis 27, the laser radiation 9 will there circulate multiple times back and forth in a predefined, generally elliptical (circular) pattern.

[0080] Figure 1The beam path between the mirrors 25A, 25B (two mirror segments) in the case of the construction of an intermediate focus region 29 is schematically shown, assuming a corresponding matching mode of the coupled-in laser radiation 9. The intermediate focus region 29 has, for example, a focus diameter d and a Rayleigh length Lrand is located in the region of the symmetry plane 31 of the resonator arrangement constructed in a concentric manner in the example. Figure 2A

[0081] Figure 1 and Figure 2B A plan view of the mirrors 25A, 25B is shown, in which the irradiation regions 33 arranged in a circular manner on the mirror surfaces are schematically shown. The laser radiation 9 is as centrally as possible irradiated in the irradiation regions 33 before the laser radiation is reflected again from the irradiation regions in the direction of the center of the multi-pass cell 5 / resonator arrangement. Furthermore, the mirrors 25A, 25B are arranged in a concentric manner in the example. Figure 2C and Figure 2B The coupling-in openings 35A and the coupling-out openings 35B can be seen in the example. The area available for reflection on the surface of the mirrors 25A, 25B is a circular area segment with a diameter D. In principle, the number of cycles (intermediate focus regions 29) can be arbitrary; for example, it is possible to pass through 5 to 100 intermediate focus regions; that is to say, a plurality of intermediate focus regions in the multi-pass cell. Furthermore, at least one of the mirrors 25A, 25B can also be composed of a single discrete mirror element, wherein the reflection can (irradiation region 33) preferably take place on one single mirror element. For example, twelve intermediate focus regions 29 are passed through.

[0082] As an alternative to the beam coupling-in and beam coupling-out through openings in the mirrors, smaller mirror elements can be used which are embedded in the multi-pass cell and which are positioned, for example, at the location of the openings 35A, 35B.

[0083] Reference is made to the beam path 5A explained in Figure 2C The pulsed laser radiation 9 is repeatedly guided through the intermediate focus region in the center of the multi-pass cell 5 as a result of the focusing of the laser pulses during the pulse duration At of the laser pulses 11. A high intensity is formed in the intermediate focus region as a result of the focusing of the laser pulses during the pulse duration At of the laser pulses 11, and the high intensity leads to a nonlinear behavior of the refractive index of the gas 4. The nonlinear behavior of the refractive index of the gas 4 can be used for spectral broadening of the pulsed laser radiation 9.

[0084] After a predetermined number of passes through the multi-pass cell 5, the laser radiation 9 exits the multi-pass cell 5 and impinges on an output mirror 37 which reflects the coupled-out laser radiation. The output mirror 37 guides the laser radiation 9 through a second telescope arrangement 39 which recollimates the laser radiation 9.

[0085] The length of the multi-pass cell for the nonlinear compression is given by the spacing between the mirrors 25A, 25B. In the radial direction, the size of the multi-pass cell depends on the number of cycles provided.​

[0086] The length of the multipass cell is important for integrating the multipass cell into an optical structure, as the multipass cell can be several meters (e.g. up to 10 m or more). The length of the multipass cell is determined by two factors:

[0087] - the damage threshold of the end mirror. For a given beam intensity, the damage threshold specifies the minimum size of the laser beam reflected on the end mirror ("minimum adjustable beam diameter"). Together with the curvature of the end mirror, this minimum size determines the focus diameter of the intermediate focus for a single pass. (Together with the number of required reflection areas, the minimum size of the laser beam reflected on the end mirror also defines the diameter of the end mirror.)

[0088] - the ionization threshold of the filling gas present in the multipass cell. The ionization threshold limits the intensity that can be introduced into the intermediate focus, i.e. that can be used for nonlinear interaction. The ionization threshold thus determines for a given parameter of the intermediate focus "the maximum beam intensity that can be coupled in". If an increase in ionization occurs in the intermediate focus, the laser radiation traveling through the multipass cell can be disturbed and e.g. an intensity distribution deviating from the Gaussian beam profile, or a reduced transmission, occurs.

[0089] From these two boundary conditions (which allow the beam diameter on the end mirror and the focus diameter and thus the intensity that should be present in the center of the multipass cell) the length of the multipass cell is derived.

[0090] If the pulse energy required for ionization is successfully increased, e.g. by using the circular polarization presented here, a shorter multipass cell can be constructed and used for spectral broadening. A shorter multipass cell corresponds to a shorter focal length (i.e. e.g. to a smaller radius of curvature of the end mirror of a Herriott cell), and thus to a smaller focus diameter in the intermediate focus, in which the intensity that can be achieved with the pulse energy and pulse duration present cannot be significantly exceeded or is not allowed to be significantly exceeded by the intensity required for ionization.

[0091] The reduction of the nonlinearities experienced by the laser radiation when passing through the multipass cell due to the circular polarization is compensated by an increase in the filling gas pressure.

[0092] With regard to the pulse energy to be used, the pre-given pulse duration and focusing geometry are chosen such that in the intermediate focus region a (pulse) peak intensity occurs which is in the range of initial multiphoton ionization or slightly below the initial multiphoton ionization. The (pulse) peak intensity is located at an upper limit, for example at most 10% above the intensity assigned to the ionization threshold; the intensity assigned to the ionization threshold in the range of multiphoton ionization is referred to herein as the multiphoton ionization intensity. For example, taking into account fluctuations in the laser parameters, the adjusted (pulse) peak intensity can be reduced, for example to half the multiphoton ionization intensity (lower limit of the pulse peak intensity). In other words, the geometry of the multipass cell is coordinated with the peak intensity of the laser pulse present (pulse energy / pulse duration / circular polarization) such that in the case of the pulse duration and the pulse energy of the circularly polarized laser pulse a pulse peak intensity in the range of 50% to 110% of the multiphoton ionization intensity is obtained in the intermediate focus region - in the case of the (minimum) ionizing pulse energy which achieves ionization of the filling gas, the multiphoton ionization intensity is obtained in the intermediate focus region for the circularly polarized laser pulse. In particular, for circular polarization, the peak intensity can be in the range of 50% to 100% of the multiphoton ionization intensity, or in the range of 60% to 105%, or in the range of 60% to 95%, or in the range of 70% to 90%.

[0093] For polarization matching of the laser radiation 9 after spectral broadening, the laser system 1 can have an arrangement of one or more wave plates (e.g. λ / 4 wave plate, λ / 8 wave plate, λ / 2 wave plate, λ wave plate). For example Figure 1 The embodiment shown in Fig. 3 comprises a second (achromatic) λ / 4 wave plate 43. The second λ / 4 wave plate 43 converts the laser radiation which occurs in a circularly polarized state again into linear polarization. Optionally, a wave plate (e.g. λ / 2 wave plate) can additionally be provided for the orientation of the polarization plane in front of or downstream of the second λ / 4 wave plate 43.

[0094] Optionally, the optical system 3 can also have an optical beam splitting system 41. For example Figure 1In the embodiment shown in Fig. 1, the optical beam separation system 41 comprises a second λ / 4 wave plate 43 and a beam splitter 45, which is shown as a beam splitter cube. Other optical elements for separating different polarizations include thin film polarizers and e.g. Wollaston prism arrangements. The beam splitter 45 can be used for beam cleaning of beam portions with other (non-circular) polarization states that can occur in the multi-pass cell 5. This can occur e.g. in case of low energy laser radiation 13 and incomplete circular polarization in the multi-pass cell 5, if the orientation of the principal axes of the slightly elliptically polarized laser pulses 11 and the slightly elliptically polarized low energy laser radiation 13 differ due to an intensity-dependent rotation of the elliptical polarization state at the focal point of the multi-pass cell 5. Other structures for separating beam portions with different elliptical polarization states are known from the prior art.

[0095] Based on the nonlinearity of the refractive index n of the gaseous Kerr medium in the multi-pass cell 5, a spectral broadening occurs for the laser pulses 11, i.e. for an intensity-dependent refractive index n = n_0 + n_2 * I(r; t), where the parameters n_0 and n_2 are gas-specific refractive indices and I(r; t) is the intensity distribution in the intermediate focus region.

[0096] Figure 1 The laser pulses 11' are shown, e.g. with the useful beam portions 9A, from which the pre-pulses and post-pulses have been exemplary removed here.

[0097] For a subsequent use of the laser pulses 11' that have been spectrally broadened, the laser pulses 11' can be provided to e.g. a compressor 49. The compressor 49 compresses the laser pulses 11' in the time domain, i.e. in the temporal pulse shape, and outputs a sequence of compressed laser pulses 11". Figure 1 The compressor 49 is shown e.g. as a chirped mirror compressor. At the output of the laser system 1, the useful laser radiation 9A' comprising the sequence of compressed laser pulses 11" can thus be output.

[0098] Compared to the setup structures known from the prior art using HCFs, the use of e.g. a Herriott cell as proposed here can enable a passage through a pre-determined / adjustable number of intermediate focus regions 29. Furthermore, the focus diameter d in the intermediate focus region is adjustable and can e.g. also be coordinated with the laser power, the pulse duration, etc. and the gas 4 by the radii of curvature Rm of the mirrors 25A, 25B. For example, the radii of curvature Rm of the two mirrors are identical or at least of the same order of magnitude.

[0099] In addition to the adjustable setting of the size of the intermediate focus regions 29 (for example by the radii of curvature of the mirrors 25A, 25B and also by corresponding telescope arrangements which can be connected in front of the multipass cell for mode matching), the gas pressure is also set in relation to the nonlinearities. It is noted that in the case of a high spatial proximity of the various intermediate focus regions passing through the multipass cell, then the same gas pressure is given in each of the intermediate focus regions. Preferably, the beam parameters and the beam properties in the individual intermediate focus regions are very similar, so that there are also similar nonlinear effects.

[0100] If the mirrors 25A, 25B form a concentric resonator (in the case of the same radius of curvature Rm, the spacing between the mirrors is approximately 2*Rm), then the intermediate focus regions 29 all essentially have the same diameter d and have a corresponding identical Rayleigh length Lr. Typically, the spacing between the mirrors 25A, 25B lies in the range of 95% to 105% of the sum of the radii of curvature. The strong laser pulses 11 propagate sequentially through these intermediate focus regions 29 and in the process repeatedly interact with the gas 4 at an electric field strength which can achieve nonlinear effects of the refractive index n and thus of the spectrum of the laser pulses 11.

[0101] In the design of the intermediate focus regions and the nonlinear conditions present therein, the use of the Herriott structure described herein provides various parameters which can be predefined and / or settable during operation. In order to set the parameters, the optical system 3 can have a control system 61 which is connected, for example, via control connections 63, with the pulse duration setting system 15, optionally with the polarization setting optics 19 (in particular for setting the angular position of the first λ / 4 wave plate 19A and optionally of a λ / 2 wave plate), the telescope arrangements 21, 39 (in particular for setting the spacing between the telescope lenses 21A, 21B), a pressure setting device 65 for setting the gas pressure (see Figure 1 ) and / or a subsequent wave plate (for example for setting the angular position of the second (achromatic) λ / 4 wave plate 43) and optionally with the optical beam separation system 41.

[0102] For example, the following can be set by means of the control system 3:

[0103] - the pulse duration Δt and / or the dispersion and / or the spectral bandwidth of the laser pulses 11 of the pulsed laser radiation 9,

[0104] - the pulse energy of the laser pulses 11 of the pulsed laser radiation 9,

[0105] - the circular polarization of the pulsed laser radiation 9,

[0106] - the focus diameter d in the intermediate focus regions 29,

[0107] - the Rayleigh length Lrof the intermediate focus region 29, and

[0108] - the gas pressure of the filling gas 4 in the intermediate focus region 29.

[0109] As shown in Figure 1 and Figure 2B , the laser radiation 9 repeatedly impinges on the mirrors 25A, 25B (in each case multiple times). These mirrors can additionally be used for dispersion matching, in that they are configured as dispersion mirrors. If the mirrors 25A, 25B have a dispersive effect at least in one of the reflections, it is possible to directly influence the dispersion and thus the pulse duration of the laser pulses 11. As an example, one or more of the impingement regions 33 can be provided with a dispersion layer. This is shown in Figure 2C , for mirror 25B in dashed lines. Furthermore, each of the mirrors 25A, 25B can be composed of a plurality of mirror segments having a predetermined dispersion characteristic, the dispersion of each of the mirror segments being matched to the desired pulse duration through the multipass cell 5. Accordingly, the dispersion present in the multipass cell 5 is composed of a contribution from the dispersion of the dispersion mirrors and a contribution from the dispersion in the gas-filled volume along the beam path 5A.

[0110] Figure 2A An exemplary mirror segment 53 is shown in

[0111] In other words, the concept presented herein allows the dispersion accumulated during the passage through the gas-filled volume to be at least partially compensated for by a suitable dispersion mirror coating (chirped mirror) in order to maintain a comparable pulse duration, for example, in the intermediate focus region, or to change the pulse duration at will.

[0112] Due to the nonlinear spectral broadening, the pulse spectrum can vary from intermediate focus region to intermediate focus region, in particular with a substantially constant pulse duration and constant pulse energy. If the multipass cell 5 is configured by means of a chirped mirror, the pulse duration can additionally be set. As an example, the pulse duration can be varied (shortened or lengthened) from one pass to another. Accordingly, the peak intensity in the intermediate focus region remains substantially constant even in the case of nonlinear spectral broadening.

[0113] If laser radiation having circular polarization is used in the multipass cell, a further advantage in connection with the nonlinear spectral broadening can be produced. In this way, the spectral broadening itself can be achieved more smoothly across the spectrum, so that fewer structured spectra can occur. This can have a positive effect on the subsequent pulse shaping and / or pulse compression.

[0114] Figure 1 The construction of the intermediate focus region in a Herriot cell with curved Herriot mirrors is shown. In the following the geometrical parameters for realizing a multi-pass cell in the context of the concepts presented herein are considered.

[0115] Due to the avoidance of laser-induced (Herriot) mirror damage and due to the ionization threshold of the used gas, a limitation of the pulse energy of the laser pulses which can be spectrally broadened by means of a multi-pass cell (and optionally increased in terms of contrast) results. In the case of the use of helium as gas 4 in a multi-pass cell 5, the highest possible ionization threshold is approximately 3.42 10^14 W / cm 2 .

[0116] The laser-induced mirror damage 25A, 25B determines the minimum diameter of the laser radiation 9 on the curved mirror 25A, 25B. The ionization threshold determines the minimum possible focus diameter d in the intermediate focus region 29 with respect to the avoidance of gas 4 ionization. Both parameters jointly define the required length of the multi-pass cell 5, i.e. the spacing between the mirrors, for example, which construct a concentric resonator, and its radius of curvature.

[0117] The nonlinear interaction with the filling gas is essential for the spectral broadening, as already discussed, in the case of circular polarization, this interaction is reduced. The lower nonlinearity is compensated by the pressure increase.

[0118] It is known that an increase in pressure can influence the ionization process. Here a distinction is made between the range of multiphoton ionization (low pressure / low density, short pulses) and the range of avalanche ionization (higher pressure / higher density, long pulses). In the range of multiphoton ionization, for helium as filling gas, the energy required for ionization can essentially be considered independent of the gas pressure in the multi-pass cell. For other noble gases, a correlation in the form of a slight decrease (Abnahme) exists, wherein the decrease becomes smaller if the gas pressure is reduced. In the range of avalanche ionization, with increasing gas pressure, a strong decrease in the energy required for ionization occurs, since a denser gas promotes the formation of an electron avalanche.

[0119] If in the range of multiphoton ionization, a change from linear to circular polarization can lead to a significant shortening of the multi-pass cell, for example a Herriot cell. Helium has a lower nonlinearity than argon, but a much higher ionization threshold. Due to the high ionization threshold, helium can be used as a filling gas in a multi-pass cell for the spectral broadening of ultrashort pulses with pulse energies greater than 20 mJ, wherein the ionization threshold is, for example, in the range of multiphoton ionization at 500 fs and a pressure of less than 100000 Pa.

[0120] The inventors have recognized that in the case of a multi-pass cell operated at pulse energies in the range of 20 mJ or more, the pressure increase has almost no influence on the pulse energy required for ionization for helium, so that it is possible to use helium as a fill gas in combination with circular polarization to shorten a Herriott cell.

[0121] For the sake of brevity, it is assumed that linearly polarized ultrashort laser pulses with a pulse energy of several 10 mJ, for example 200 mJ, are coupled into a multi-pass cell filled with He. In order to provide sufficient nonlinear interaction in the multi-pass cell at these high pulse energies, it is necessary, for example, to set the pressure in the range of 10 000 Pa to 20 000 Pa. The length of the multi-pass cell is then, for example, about 10 m.

[0122] In contrast, if circular polarization is set for ultrashort laser pulses with a pulse energy of several 10 mJ, for example 200 mJ, the energy required for ionization increases, but the nonlinearity also decreases. By reducing the length of the multi-pass cell to, for example, 5 m, the multi-pass cell can then be operated relatively close to the ionization threshold, i.e. a higher peak intensity can be set in the intermediate focus region. It is known that in the case of helium, the pressure at this pulse energy has almost no influence on the ionization threshold, and in order to provide sufficient nonlinear interaction, the pressure can be increased from, for example, 20 000 Pa to 40 000 Pa. If the multi-pass cell is thus operated in the parameter range of multiphoton ionization, the positive effect of the rise in the ionization threshold due to circular polarization can be (almost completely) exploited.

[0123] The concept presented here of using helium as a fill gas and circular polarization in a multi-pass cell clearly shows the possibility of reducing the length of the multi-pass cell at a pulse energy of at least 20 mJ and a pulse duration of 500 fs.

[0124] Similarly, for other fill gases such as Xe, Kr, Ar or Ne gas, there are corresponding parameter ranges in which circular polarization makes it possible to shorten the length of a multi-pass cell operated in the multiphoton ionization range. However, it should be considered here that depending on which operating point of the multi-pass cell in the multiphoton ionization range is operated, there can be a small drop in the pressure dependence.

[0125] If it is assumed that for a multi-pass cell filled with argon, the pulse energy required for ionization increases to 3 times due to circular polarization, the length of the multi-pass cell is shortened to about 1 / 1.7 ("1 / √3"). In order to compensate for the reduced nonlinearity due to circular polarization, the pressure of the argon is increased, for example, from 15 000 Pa (in the unshortened configuration) to 45 000 Pa (in the shortened configuration).

[0126] It should be noted that if the pressure increase to be made (e.g. for argon) occurs outside the range of multiphoton ionization and in the range of an avalanche-like ionization process, the pulse energy required for ionization can drop with increasing pressure. Due to the circular polarization, this counteracts the expected increase in the pulse energy required for ionization.

[0127] For the more compact multipass cell structure proposed here, it is important that the electric field strength required for ionization is increased for circular (possibly slightly elliptically corrected circular) polarized light compared to linearly polarized light, so that for comparable beam diameters D on the mirrors 25A, 25B, the possible focusing diameter d in the intermediate focusing region 29 can be chosen to be smaller. Thereby, the possibility of a reduced mirror spacing (e.g. the spacing between the mirrors 25A, 25B, which is shortened to 1 / √3, i.e. to approximately 1 / 2) is generally obtained for circular polarization compared to a multipass cell operated with linear polarization.

[0128] In the case of nonlinear compression by means of spectral broadening, the shorter structure leads to cost savings.

[0129] For applications of high-intensity laser radiation, and taking into account the damage threshold of the mirrors, the mirrors can need to withstand several 100 mJ of pulse energy at several 100 fs pulse duration (e.g. 500 fs or less). For ultrashort pulses, the mirrors should also be broadband in design, e.g. designed for a wavelength range of e.g. 700 nm to 1100 nm for ultrashort pulses from a titanium-sapphire laser, or for a wavelength range of 900 nm to 1100 nm for ultrashort pulses from a laser emitting around 1000 nm, such as a Nd:YAG or Yb:YAG. Furthermore, the mirrors can or can not provide a dispersion contribution, and thus a dispersion coating can also need to be considered.

[0130] Reference is made below to Figure 2A Exemplary parameters of the multipass cell and the mirrors on which it is based are explained. For a coated mirror, a laser-induced damage threshold of e.g. approximately 0.5 J / cm2can be measured at a pulse duration of approximately 500 fs. This threshold is typically assigned to the beam center. Assuming a Gaussian beam, e.g. the threshold is thus approximately 0.1 J / cm2for laser pulses of approximately 500 fs, and thus, with a safety factor of 3, e.g. the maximum permissible fluence is approximately 0.03 J / cm2.

[0131] Based thereon, this leads to e.g. a beam radius of approximately 9 mm for a 200 mJ pulse, or a converted 1 / e2beam diameter of approximately 13 mm on the mirrors 25A, 25B. This estimate can be applied approximately equally for linear polarization as well as also for circular polarization.

[0132] For circularly polarized light (with reduced (maximal) electric field strength compared to linearly polarized light), as explained, assuming the same beam diameter on the mirrors, a reduced mirror spacing / shortened multi-pass cell length / resonator length L can be achieved in case of a respective smaller radius of curvature of the mirrors 25A, 25B.

[0133] The method of spectrally broadening pulsed laser radiation using a multi-pass cell with circular polarization of the passage will be explained with reference to the flow chart shown in Figures 2A to 2C Figure 3 The steps in the method of spectrally broadening pulsed laser radiation using a multi-pass cell with circular polarization of the passage proposed herein will be explained with reference to the flow chart shown in

[0134] In step 71, pulsed laser radiation is generated, which comprises laser pulses with a pulse energy in the range of 1 mJ to 100 J, preferably in the range of 10 mJ to 1 J, and a pulse duration in the range of 10 fs to 5 ps, preferably in the range of 500 fs to 1.5 ps.

[0135] Step 73 comprises circularly polarizing the pulsed laser radiation, i.e. typically before entering the multi-pass cell, for the passage through the multi-pass cell.

[0136] In step 75, spectral broadening of the pulsed laser radiation is performed. For this purpose, the pulsed laser radiation is coupled into a multi-pass cell. The multi-pass cell is formed by e.g. at least 2 concave mirrors defining multiple passes of an intermediate focus region; e.g. forming a (especially concentric or confocal) resonator or similar resonator arrangement. In step 75, the multi-pass cell is repeatedly passed through in case of a plurality of intermediate focus regions being constructed. The multi-pass cell is filled with a filling gas having an optical nonlinearity, which nonlinearity enables spectral broadening of the pulsed laser radiation in the intermediate focus region.

[0137] Step 77 comprises coupling out the spectrally broadened pulsed laser radiation from the multi-pass cell.

[0138] In step 79, linear polarization of the spectrally broadened pulsed laser radiation can be adjusted, for example, and / or compression of the spectrally broadened pulsed laser radiation can be performed, for example.

[0139] Furthermore, in order to be able to utilize the compact construction of the laser system, in particular with a multi-pass cell of short length, in the case of the sought spectral broadening, in step 81 A, the pressure of the filling gas of the multi-pass cell is set in a pressure range in which, in the context of the multiphoton ionization process, the filling gas is ionized by means of laser pulses whose pulse duration corresponds to the pulse duration of the laser pulses to be spectrally broadened. That is to say, there is an ionization behavior of the filling gas in the context of the multiphoton ionization. This allows, in order to increase the nonlinearity, the pressure of the filling gas to be set so high that, on the basis of the nonlinearity of the filling gas which exists in the case of the same pressure and in the case of linear polarization, a drop in the nonlinearity of the filling gas in the case of circular polarization is compensated.

[0140] Furthermore, in order to the sought spectral broadening, in step 81 B, the focus diameter in the intermediate focus region is set (for example by selecting the radius of curvature of the mirrors of the multi-pass cell and the beam diameter on the mirrors) such that the pulsed laser radiation passes through the multi-pass cell without ionization of the filling gas. Here, the focus diameter in the intermediate focus region is selected to be as small as possible, but with a safety margin in terms of optical damage to the mirrors and avoidance of (relatively strong) ionization of the filling gas.

[0141] Furthermore, the laser radiation can pass through a series of successive multi-pass cells in succession. This allows the gas conditions, the mirror configuration and the distribution of the dispersion on the group of intermediate focus regions which respectively exist in the individual multi-pass cells to be set in different ways, wherein also an "intermediate compression" can be provided between the individual multi-pass cells.

[0142] It is expressly emphasized that all features disclosed in the description and / or the claims are to be interpreted as individually independent inventions or combinations if so explicitly disclosed. By way of example, any feature of the description or the claims can be claimed in combination with any other feature(s) of the description or the claims, regardless of whether the combination of features is explicitly disclosed or not. It is expressly noted that the features of the claims are to be interpreted as independent inventions, independently of the features of the description.

Claims

1. A laser system (1) comprising: - a source of laser radiation (7) for providing pulsed laser radiation (9), wherein the pulsed laser radiation comprises laser pulses (11) having a pulse energy in the range of 1 mJ to 100 J or in the range of 10 mJ to 1 J and a pulse duration in the range of 10 fs to 5 ps or in the range of 500 fs to 1.5 ps, and - an optical system (3) for spectral broadening of the pulsed laser radiation (9), the optical system comprising: - a first polarization setting optics (19) setting a circular polarization state (17B) of pulsed laser radiation (9), the first polarization setting optics (19) being arranged along the beam before a multi-pass cell (5), and - a multi-pass cell (5) having at least two mirrors (25A, 25B) through which the pulsed laser radiation (9) passes in the case of a configuration of a plurality of intermediate focus regions (29), wherein the multi-pass cell (5) is filled with a filling gas (5A) having an optical nonlinearity, wherein the filling gas (5A) effects a spectral broadening of the pulsed laser radiation (9) in the intermediate focus regions (29), wherein a pressure of the filling gas (5A) is set in a pressure range in which there is an ionization behavior of the filling gas (5A) in the range of multiphoton ionization, and a focus diameter (d) of the intermediate focus regions (29) is set such that the pulsed laser radiation (9) passes through the multi-pass cell (5) without ionization of the filling gas (5A).

2. The laser system (1) according to claim 1, wherein the multi-pass cell (5) is filled with helium as a filling gas, the pressure of the filling gas being in the range of 100 Pa to 60 000 Pa.

3. The laser system (1) according to claim 2, wherein the pressure of the filling gas is in the range of 1000 Pa to 50 000 Pa.

4. The laser system (1) according to claim 1, wherein the multi-pass cell (5) is filled with argon as a filling gas, the pressure of the filling gas being in the range of 100 Pa to 50 000 Pa.

5. The laser system (1) according to claim 4, wherein the pressure of the filling gas is in the range of 1000 Pa to 40 000 Pa.

6. The laser system (1) according to any one of claims 1 to 5, wherein a focus diameter (d) of the intermediate focus regions (29) is set such that a peak intensity resulting from the pulse duration and the pulse energy of the laser pulses (11) in the intermediate focus regions (29) is in the range of 50% to 110% of a multiphoton ionization intensity.

7. The laser system (1) according to any one of claims 1 to 5, wherein the first polarization setting optics (19) comprises a first wave plate (19A).

8. The laser system (1) according to claim 7, wherein the first wave plate is a λ / 4 wave plate (19A) and / or a λ / 2 wave plate (19B).

9. The laser system (1) according to any one of claims 1 to 5, wherein The optical system (3) further comprises at least one of the following optical components: - a pulse duration setting system (15) for setting a pulse duration (Δt) of laser pulses (11) of the pulsed laser radiation (9), - a first optical telescope arrangement (21) which is provided for mapping the pulsed laser radiation (9) into the multi-pass cell (5) in a predefined pattern, - a coupling-in mirror (23) for coupling the pulsed laser radiation (9) into the multi-pass cell (5), - a coupling-out mirror (37) for further guiding pulsed laser radiation (9) emanating from the multi-pass cell (5), and - a second optical telescope arrangement (39) which is provided for collimating the pulsed laser radiation (9) emanating from the multi-pass cell (5).

10. The laser system (1) of claim 9, wherein The first optical telescope arrangement is arranged beam downstream with respect to the first polarization setting optics (19).

11. The laser system (1) according to any one of claims 1 to 5, wherein, The multi-pass cell (5) is configured - with a predetermined number or an adjustable number of intermediate focusing regions (29), and / or - with intermediate focusing regions (29) which substantially have the same diameter (d) and the same Rayleigh length (Lr), and / or - with intermediate focusing regions (29) which are arranged one above the other, adjacent to each other, - in a resonator structure with the same radius of curvature of the at least two mirrors (25A, 25B), the at least two mirrors are arranged confocally or concentrically, and / or - in a quasi-resonator structure with the same radius of curvature of the at least two mirrors (25A, 25B), the at least two mirrors are arranged almost confocally or almost concentrically, and / or - in an arrangement in which the at least two mirrors (25A, 25B) comprise a plurality of mirror segments, wherein an intermediate focusing region (29) is configured between each two mirror segments and is passed through successively, - as a cell filled with a noble gas such as helium or argon as a filling gas, wherein the same pressure prevails in each of the intermediate focusing regions (29), and / or - for stepwise nonlinear spectral broadening of the pulsed laser radiation (9) passing through the intermediate focusing regions (29).

12. The laser system (1) according to claim 11, the intermediate focusing regions being arranged partially overlapping each other.

13. The laser system (1) according to any one of claims 1 to 5, further having: a second polarization setting optic for returning the circular polarization state to a linear polarization state (47A, 47B), wherein, The second polarization setting optics are arranged beam downstream with respect to the multi-pass cell (5).

14. The laser system (1) according to claim 13, wherein The second polarization setting optics comprise a second wave plate (43).

15. The laser system (1) according to claim 14, wherein The second wave plate is an achromatic wave plate.

16. The laser system (1) according to claim 14, wherein The second wave plate is a λ / 4 wave plate (43) and / or a λ / 2 wave plate.

17. The laser system (1) of claim 11, wherein The laser system (1) further comprises at least one of the following optical components: - a pulse duration setting system (15) for setting a pulse duration (Δt) of the laser pulses (11), - an optical pulse duration compressor system (49) for compensating a dispersion contribution of the optical system (3) and for temporally compressing laser pulses (11) of laser radiation which have undergone the nonlinear spectral broadening in at least one of the intermediate focus regions (29), - a beam splitter (45) for separating different polarization states output by the multipass cell (5), and - a control system (61) configured for setting a pressure of the filling gas (5A) in the multipass cell (5) for compensating a reduction of the nonlinearity of the filling gas due to the set circular polarization.

18. A method for spectrally broadening pulsed laser radiation (9) by using a nonlinearity of a filling gas of a multipass cell (5) having at least two mirrors (25A, 25B) configured a plurality of intermediate focus regions (29), the method having the steps of: - generating pulsed laser radiation (9) comprising laser pulses having a pulse energy in a range of 1 mJ to 100 J or in a range of 10 mJ to 1 J and a pulse duration in a range of 10 fs to 5 ps or in a range of 500 fs to 1.5 ps, - setting a circular polarization state (17A) of the pulsed laser radiation (9) for passing through the multipass cell (5), - coupling the pulsed laser radiation (9) into the multi-cell (5), wherein the pulsed laser radiation (9) passing through the plurality of intermediate focus regions (29) and nonlinearly interacting with the filling gas (5A) in the intermediate focus regions (29) thereby achieving a spectral broadening of the pulsed laser radiation (9) in the intermediate focus regions (29), - setting a pressure of the filling gas (5A) in a pressure range in which there is an ionization behavior of the filling gas (5A) in a regime of multiphoton ionization, - setting a focus diameter (d) in the intermediate focus regions (29) such that the pulsed laser radiation (9) passes through the multipass cell (5) without ionization of the filling gas (5A), and - coupling out spectrally broadened pulsed laser radiation (9) from the multipass cell (5).

19. The method according to claim 18, further having at least one of the following steps: - providing helium as filling gas and setting a pressure in a range of 100 Pa to 60 000 Pa, and - providing argon as filling gas and setting a pressure in a range of 100 Pa to 50 000 Pa, - increasing the pressure of the filling gas for increasing the nonlinearity compensating a drop of the nonlinearity of the filling gas in case of circular polarization based on the nonlinearity of the filling gas which is present in case of the same pressure and linear polarization.

20. The method of claim 19, wherein, The pressure of the helium is set in the range of 1000 Pa to 50000 Pa.

21. The method of claim 19, wherein, The pressure of the argon is set in the range of 1000 Pa to 40000 Pa.

22. The method according to any one of claims 18 to 21, further having: - the focus diameter (d) of the intermediate focus region (29) is set such that the peak intensity resulting from the pulse duration and the pulse energy of the laser pulse (11) in the intermediate focus region (29) is in the range of 50% to 110% of the multiphoton ionization intensity.

23. The method according to any one of claims 18 to 21, further having: - the polarization of the spectrally broadened pulsed laser radiation (9) is set for the subsequent beam path, and / or - dispersion compensation is performed on the spectrally broadened pulsed laser radiation (9).

24. The method according to any one of claims 18 to 21, further having setting at least one of the following parameters of the multi-pass cell (5): - the dispersion of the laser pulse (11) accumulated in the multi-pass cell (5), - the focus diameter (d) of the intermediate focus region (29), and - the Rayleigh length (Lr) of the intermediate focus region (29).

Images