EP4111553A1

EP4111553A1 - Laser system comprising an optical system for the spectral broadening of pulsed laser radiation and method for the spectral broadening of pulsed laser radiation

Info

Publication number: EP4111553A1
Application number: EP21708637.0A
Authority: EP
Inventors: Thomas Metzger; Sebastian Stark
Original assignee: Trumpf Scientific Lasers GmbH and Co KG
Current assignee: Trumpf Scientific Lasers GmbH and Co KG
Priority date: 2020-02-26
Filing date: 2021-02-26
Publication date: 2023-01-04
Also published as: CN115210967A; WO2021170815A1; US20220416493A1; EP4111554A1; US20220399695A1; CN115210968A; WO2021170814A1

Abstract

The invention relates to a laser system (1) comprising a laser radiation source (7) for providing pulsed laser radiation (9), wherein the pulsed laser radiation comprises laser pulses (11) with pulse energies in the region of 1 mJ to 100 J or in the region of 10 mJ to 1 J and with pulse durations in the region of 10 fs to 5 ps or in the region of 500 fs to 1.5 ps. The laser system (1) also comprises an optical system (3) for the spectral broadening of the pulsed laser radiation (9). The optical system (3) comprises: a first polarisation adjustment lens (19), which adjusts a circular polarisation state (17B) of the pulsed laser radiation (9); and a multipass cell (5) having at least two mirrors (25A, 25B), through which the pulsed laser radiation (9) passes forming a plurality of intermediate focus zones (29), wherein the multipass cell (5) is filled with a filler gas (5A), which has an optical non-linearity, wherein the filler gas (5A) brings about a spectral broadening of the pulsed laser radiation (9) in the intermediate focus zones (29). In addition, a pressure of the filler gas (5A) is adjusted within a pressure range, in which there is ionisation behaviour of the filler gas (5A) within the scope of multiphoton ionisation, and focus diameters (d) of the intermediate focus zones (29) are adjusted in such a way that the pulsed laser radiation (9) passes through the multipass cell (5) without ionisation of the filler gas (5).

Description

LASER SYSTEM WITH OPTICAL SYSTEM FOR SPECTRAL BROADENING OF PULSED LASER RADIATION AND METHOD FOR SPECTRAL BROADENING OF PULSED LASER RADIATION

The present invention relates to laser systems with an optical system for the spectral broadening of pulsed laser radiation and, in particular, ultra-short pulse (USP) laser systems for emitting pulsed laser radiation with a high pulse energy. The invention also relates to a method for the spectral broadening of pulsed laser radiation, in particular special ultrashort pulse trains.

DE 10 2014 007159 A1 discloses a method for the spectral broadening of laser pulses for non-linear pulse compression using an arrangement with a sequence of non-linearly interacting sections, as is the case in a multipass cell, which, for example, is constructed in the form of a so-called Herriott cell , can be provided. The aim is a spectral broadening of laser pulses that can also be carried out at 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 that can be used in a compact structure, especially with high pulse energies and optionally high average powers, for the spectral broadening of pulsed laser radiation, for example ultrashort pulse trains. In particular, non-linear effects in a filler gas should be used to influence the spectral broadening of laser pulses with high pulse energies in an arrangement that is as spatially small as possible.

At least one of these objects is achieved by a laser system according to claim 1 and a method for spectral broadening of pulsed laser radiation according to claim 10. Further developments are given in the subclaims.

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 pulse energies in the range from 1 mJ to 100 J, preferably 10 mJ to 1 J, and pulse durations in the range from 10 fs to 5 ps, preferably 500 fs to 1.5 ps. Furthermore, the laser system comprises (at least) one optical system for the spectral broadening of the pulsed laser radiation with a first polarization setting optics which have a circular polarization state of the pulsed laser radiation and a multi-pass cell with at least two mirrors. The multiple passage cell is from the pulsed laser radiation - which was in the circular Polarisationszu - traversed with the formation of a plurality of intermediate focus zones. The multi-pass cell is filled with a filling gas which has an optical non-linearity, the filling gas causing a spectral broadening of the pulsed laser radiation in the intermediate focus zones. In the multi-passage cell, a pressure of the filling gas is set in a pressure range in which the filling gas exhibits ionization behavior in the context of multiphoton ionization. Furthermore, the focus diameters of the intermediate focus zones are set in such a way that the pulsed laser radiation passes through the multiple passage cell without ionization of the filling gas (in the intermediate focus zones).

In a further aspect comprises a method for spectral broadening of a pulsed laser radiation using a non-linearity of a filling gas of a multiple passage cell with at least two mirrors. The multi-pass cell forms a plurality of intermediate focus zones. The procedure consists of the following steps:

- Generating a pulsed laser radiation, the laser pulses with a pulse energy in a range from 1 mJ to 100 J, in particular in a range from 10 mJ to 1 J, and pulse durations in a range from 10 fs to 5 ps, in particular in a range of 500 fs to 1.5 ps,

- Setting a circular polarization state of the pulsed laser radiation for passing through the multi-pass cell,

- Coupling of the pulsed laser radiation into the multiple passage cell, the pulsed laser radiation passing through the plurality of intermediate focus zones and interacting non-linearly with the filling gas in the intermediate focus zones so that a spectral broadening of the pulsed laser radiation is effected in the intermediate focus zones,

- Setting the pressure of the filling gas in a pressure range in which an ionization behavior of the filling gas is present in the context of multiphoton ionization,

- Setting the focus diameter in the intermediate focus zones in such a way that the pulsed laser radiation passes through the multi-pass cell without ionizing the filler gas, and

- Decoupling of the spectrally broadened pulsed laser radiation from the multiple passage cell.

If the pressure for an ionization behavior of the filling gas is set in the context of the Mehrphotonenio nization, there is in particular a pure multiphoton ionization in which Avalanche ionization essentially does not contribute to the ionization of the gas. Setting the focus geometry so that the intermediate focus zone is traversed without ionization means in this case that any ionization that may occur occurs only to an extent that does not interfere with the feasibility of a desired spectral broadening.

As a condition for setting the parameters of the multi-pass cell for an ionization behavior in the context of multiphoton ionization, the pressure is set in a range in which a peak intensity of a laser pulse at which ionization of the filling gas begins (also referred to herein as multiphoton ionizing (threshold) intensity ), is essentially independent of the pressure of the filling gas or decreases insignificantly with increasing pressure of the filling gas. (Insignificant here refers to the extent of a pressure increase in the order of magnitude that is necessary for a desired increase in non-linearity.)

In some embodiments, the multiple passage cell is filled with He gas as the filling gas at a pressure in a range from 100 Pa to 60,000 Pa, in particular in a range from 1,000 Pa to 50,000 Pa. In other embodiments, the multiple passage cell is filled with Ar gas as the filling gas at a pressure in a range from 100 Pa to 50,000 Pa, in particular in a range from 1,000 Pa to 40,000 Pa.

In some embodiments, the focus diameters of the intermediate focus zones are set such that a peak intensity, which results from the pulse duration and the pulse energy of the laser pulses in the intermediate focus zones, is in the range of 50% to 110% of a multi-photo ionizing (threshold) intensity.

In some embodiments, the first polarization adjustment optics may comprise a first wave plate, for example a 1/4 wave plate and / or a 1/2 wave plate.

In some embodiments, the optical system can further include at least one of the following optical components:

- A pulse duration setting system for setting a pulse duration of the laser pulses of the 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, - a coupling mirror for coupling the pulsed laser radiation into the multiple passage cell,

- An output mirror for forwarding the pulsed laser radiation emerging from the multiple passage cell and

- A second optical telescope arrangement which is set to collimate the pulsed laser radiation emerging from the multiple passage cell.

In some embodiments, the multi-pass cell can

- With a predetermined or adjustable number of intermediate focus zones, and / or

- With intermediate focus zones which have essentially the same diameter and the same Rayleigh length, and / or

- with intermediate focus zones, which are arranged on top of one another, next to one another and optionally partially overlapping,

- In a resonator structure with, in particular the same, radii of curvature of the at least two mirrors, optionally in a confocal or concentric arrangement, and / or

- In a resonator-like structure with, in particular the same, radii of curvature of the at least two mirrors, optionally in an almost confocal or almost concentric arrangement, and / or

- In an arrangement in which the at least two mirrors comprise a plurality of mirror segments, an intermediate focus zone being formed between each two mirror segments, and the intermediate focus zones being traversed one after the other,

as a cell filled with a noble gas such as helium or argon as filling gas, the same pressure being present in each of the intermediate focus zones, and / or

- Be designed for the stepwise non-linear spectral broadening of the pulsed laser radiation passing through in the intermediate focus zones.

In some embodiments, the laser system can further comprise a second polarization adjustment optics for returning the circular polarization state to a linear polarization state. The second polarization setting optics can be arranged downstream of the multiple passage cell and in particular comprise a second, in particular achromatic, wave plate, for example a 1/4 wave plate and / or a 1/2 wave plate. In some embodiments, the laser system can further include at least one of the following optical components:

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

- An optical pulse duration compressor system to compensate for a dispersive contribution of the optical system and / or to compress the laser pulses of the laser beam over time, which have undergone the nonlinear spectral broadening in at least one of the intermediate focus zones,

- A beam splitter for separating, output from the multi-pass cell, under different polarization states and

a control system which is designed to compensate for a reduction in the non-linearity of the filling gas due to the set circular polarization for setting a pressure of the filling gas in the multi-passage cell.

In some embodiments, the method may further include at least one of the following steps:

- Providing He gas as filling gas and setting the pressure in a range from 100 Pa to 60,000 Pa, in particular in a range from 1,000 Pa to 50,000 Pa,

- Providing Ar gas as filling gas and setting the pressure in a range from 100 Pa to 50,000 Pa, in particular in a range from 1,000 Pa to 40,000 Pa, and

- Increasing the pressure of the filling gas to increase the non-linearity in such a way that a decrease in a non-linearity of the filling gas in the case of circular polarization based on a non-linearity of the filling gas, which is present at the same pressure and with linear polarization, is compensated.

In some embodiments, the method may further include the step of:

- Setting the focus diameter of the intermediate focus zones so that a peak intensity, which results from the pulse duration and the pulse energy of the laser pulses in the intermediate focus zones, is in the range of 50% to 110% of a multiphoton ionizing (threshold) intensity.

- Setting the polarization of the spectrally broadened pulsed laser radiation for a subsequent beam path and - Carrying out a dispersion compensation of the spectrally broadened pulsed Laserstrah treatment.

In some embodiments of the method, at least one of the following parameters of the multi-pass cell can furthermore be set:

- a dispersion of the laser pulses accumulated in the multi-pass cell,

- Focus diameter of the intermediate focus zones, and

- Rayleigh lengths of the intermediate focus zones.

According to the invention it is generally proposed to use a gas-filled multiple passage cell for the spectral broadening of pulsed laser radiation with circular polarization. This has the advantage that the gas-filled multiple passage cell can be constructed with a reduced length. This is because by reducing the maximum E-F eid strength due to the irradiation of circularly polarized laser radiation (i.e. more pulse energy is required to reach the ionization threshold), the multiple passage cell can be formed with a smaller diameter in the intermediate focus zones. This requires smaller radii of curvature of the mirrors that build up the multi-pass cell, and thus leads to a shortening of the multi-pass cell, for example confocally or concentrically constructed, (compared to a multi-pass cell operated with linear polarization). The multi-pass cell is preferably operated with parameters in the range of multi-photon ionization; in other words, outside the range of (electron) avalanche ionization, in which a large number of free electrons are created during the ionization process. Multiphoton ionization is the predominant ionization process in "thin" gases, as they exist at the pressures specified here for noble gases.

Operating the multi-passage cell with parameters (including density, pulse length, pulse energy) that characterize the ionization process of multiphoton ionization allows the non-linearity in the filling gas to be increased by increasing the pressure in the multi-passage cell, whereby the pressure increase reduces the circular breakdown required for ionization. Pulse energy (with constant pulse duration and focus size as well as circular polarization) is essentially not influenced. For helium as filler gas Mehrphotonenionisation place in the loading range from 10 ^{13 to} 10 ¹⁵ Watt / cm ² at a pressure of 1,000 Pa to several hundred mbar (n * 10 ⁴ Pa), for example, 60,000 Pa instead. In some embodiments, the mirrors of the multiple passage cell are designed as convex mirrors, the radii of curvature in particular matching and / or the distance between the mirrors being in a range from 95% to 105% of the sum of the radii of curvature. Alternatively or additionally, 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 laser pulse of the pulsed laser radiation through the multiple passage cell. As an alternative or in addition, at least one of the mirrors can further comprise a plurality of mirror segments which the pulsed laser radiation strikes at least once when the pulsed laser radiation circulates through the multiple passage cell.

In some embodiments, the multiple passage cell is designed such that a laser pulse of the pulsed laser radiation, the spectrum of which is to be broadened in the optical system, undergoes essentially no change in the pulse duration and / or pulse energy in the intermediate focus zones.

The spectral broadening can optionally be combined with a subsequent pulse duration compression, for example, in order to generate pulsed laser radiation with a short pulse duration and high peak intensities.

Concepts are disclosed herein that allow aspects from the prior art to be improved at least in part. In particular, further features and their usefulness emerge from the following description of embodiments with reference to the figures. The figures show:

1 shows an exemplary schematic representation of a laser system with an optical system for spectral broadening,

2A to 2C are exemplary sketches for explaining a Herriott cell as an example of a multi-pass cell and

3 shows a schematic flow diagram to explain an exemplary procedure for spectral broadening.

Aspects described here are based in part on the knowledge that a shortening of a multiple-passage cell, which is used for the spectral broadening of high-intensity laser radiation, is achieved by reducing the maximum EF strength due to irradiation can be successfully implemented with high-intensity laser radiation in circular polarization. It was recognized that, for the advantageous use of circular polarization, the parameters of the laser radiation for the respective filling gas are preferably in the range of multi-photo ionization, so that an increase in pressure (and thus the gas density in the multi-passage cell) does not or only slightly affects the E-field strength required for ionization has an effect.

It is known that in the case of circularly polarized laser radiation, the pulse energy required to ionize a filling gas is increased significantly, e.g. by a factor of 3 to 10. For circularly polarized laser radiation, the non-linear portion of the refractive index of the filling gas present in the intermediate focus zones is also smaller, so that a The desired non-linear effect only occurs at higher intensities / pulse energies. For example, the non-linearity in the case of circular polarization is reduced to a third of the value of the non-linearity that would be present in the case of linear polarization.

In order to compensate for the reduced non-linearity, it is proposed to increase a gas pressure in the multi-passage cell by a compensation factor (for example by a factor of 3).

In this context, it was recognized that for He gas a multiple passage cell can be operated at a pressure in the range around or below one bar (100,000 Pa) for e.g. ultrashort laser pulses in the multiphoton absorption range, in which the pulse energy required for ionization is almost independent of the pressure . Correspondingly, the pressure in the multiple passage cell can be increased without any significant effect on the ionization behavior and the non-linearity reduced due to the circular polarization can be compensated for.

In other words, for a gas-filled multiple passage cell there is the possibility of configuring the optical beam path (essentially with regard to the spectral broadening given by parameters of the intermediate focus zones) and parameters of the filling gas (essentially given by a gas pressure in the multiphoton absorption range depending on the type of gas) set that when using circular polarization a desired spectral broadening is achieved in a short and compact structure of the multiple passage cell. With the estimate that the length of a multi-pass cell is scaled proportionally to the square root of the ratio of the pulse energy required for ionization with linear polarization to the pulse energy required for ionization with circular polarization, the multi-pass cell can be significantly shortened in this way. This results in a possible shortening by a factor of 3. In other words, the length of a Herriott cell, for example, can be shortened by setting a circular polarization for the laser radiation to be spectrally broadened, with the boundary condition, the Herriott cell, at the same time to operate as “close to the ionization threshold” as possible for the non-linear interaction, can be maintained.

A multi-passage cell can be constructed with a pair of mirrors, such as, for example, with the Herriott cell explained below in connection with the figures. In general, a multi-pass cell provides multiple pass through intermediate focus zones. Intermediate focal zones can be formed between optical elements, e.g. between reflections on mirrors / mirror segments. See also DE 10 2014 007159 A1 mentioned at the beginning. A plurality of intermediate focus zones can, for example, also be formed in modular structures of Herriott-like cells with several mirror segments. One or more folds of the beam path can be made in the multiple passage cell.

Fig. 1 shows a laser system 1, which has an optical system 3 for spectral broadening. The optical system 3 is based on the use of a multiple passage cell 5 filled with filling gas 4 (for example a Herriott cell), the filling gas 4 serving as a nonlinear (Kerr) medium. Noble gases, for example, are used as filling gases. At very high intensities, helium with a high ionization threshold (approx. 3 times higher ionization threshold than argon) can be used. At lower but still high intensities in the multiple passage cell 5, argon or another noble gas can be used as the non-linear medium, for example.

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 a few mJ, for example at least 20 mJ, e.g. a few hundred millijoules, and a pulse duration At in the range of a few hundred femtoseconds (FWHM pulse duration) and less, e.g. 500 fs. The laser pulses 11 form, for example, an ultra-short pulse train. Depending on the laser radiation source 7, the laser radiation 9 can further include lower-level laser radiation 13, which is indicated by way of example in FIG. 1 as (secondary) prepulse 13A or postpulse 13B.

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 indicated in FIG.

In the present example it is assumed that 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 laser pulses 11 and any low-energy laser radiation 13 that may be present are linearly polarized.

The optical system 3 has first polarization adjustment optics 19. In this the laser radiation 9 is circularly polarized. The circular polarization can generally be set with a wave plate in the beam path in front of the multiple passage cell 5 / Herriott cell (for example with a zero or few order wave plate). For setting the circular polarization state of the laser radiation 9, the first polarization setting optics 19 in FIG. 1 comprise, for example, a first 1/4 wave plate 19A. In Fig. 1, at the output of the first polarization setting optics 19, a circular E-field vector to illustrate a circular polarization state 17B is indicated.

Alternatively, the circular polarization can be set using Faraday rotators, Pockels cells or other suitable polarization-influencing elements. The setting of the polarization in the first polarization setting optics 19 can preferably be done independently of the Intensi ity if the waveplates work with an anisotropic refractive index, for example with a birefringent crystal.

To set the circular polarization state of the laser radiation 9, for example, the first 1/4 wave plate 19A is set at an angle between a fast axis of the 1/4 wave plate and the polarization plane with respect to the plane of polarization of the laser radiation 9 such that the angle is approximately 45 ° . For example, the angle is in the range from 42 ° - 48 °, so that there may be an ellipticity of the polarization (eg also due to the adjustment).

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.

The multi-passage cell 5 comprises two concavely curved mirrors 25A, 25B which, in a gas-filled environment, form a beam path 5A running back and forth between the mirrors 25A, 25B. Between the mirrors 25A, 25B, the pulsed laser radiation passes through a focus zone area in each passage, in which intermediate focus zones with a correspondingly high intensity of the pulsed laser radiation are formed. In the intermediate focus zones, there is an interaction of the laser radiation with a filling gas introduced into the multiple passage cell, which leads to non-linear effects, for example to the intended spectral broadening or - if the intensities in the intermediate focus zones are too high to be avoided - to an optical breakdown / one excessive ionization of the filling gas. The operation of the multiple passage cell thus takes place in the field of tension of a sufficiently occurring spectral broadening of the laser radiation and an avoidance of ionization effects on the laser radiation.

A step-by-step non-linear spectral broadening is caused by the high intensity present in each intermediate focus zone and by the non-linearity of the refractive index of the gaseous medium in the multiple passage cell 5.

A Herriott cell is an example of a multi-pass cell into which pulsed laser radiation can be coupled for a multi-pass. The Herriott cell is formed by two concave mirrors, for example in a concentric or confocal resonator arrangement (or almost in a concentric or confocal resonator-like arrangement with an offset of up to a few millimeters from the ideal concentric or confocal arrangement), generally also in a different resonator configuration, along a common optical axis 27 (given by the specific arrangement) are aligned with one another. In this case, the mirrors 25A, 25B are also referred to as Herriott or End mirrors. If the laser radiation 9 is offset to the optical axis 27 in the When multiple passage cell 5 is introduced, the laser radiation 9 is circulated several times back and forth on a predetermined, usually elliptical (circular) pattern.

2A illustrates schematically the beam path between the mirrors 25A, 25B (two mirror segments) with the formation of an intermediate focus zone 29, assuming a correspondingly adapted mode of the injected 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 configured 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 are indicated schematically on the mirror surfaces. In the impingement areas 33, the laser radiation 9 strikes as centrally as possible before it is reflected back from there like that in the direction of the center of the multiple passage cell 5 / the resonator arrangement. In FIGS. 2B and 2C one can also see a coupling-in opening 35A and a coupling-out opening 35B. The areas on the surface of the mirrors 25A, 25B ready for reflection 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; i.e., multiple intermediate focus zones are traversed in the multi-pass cell. Furthermore, at least one of the mirrors 25A, 25B can also be built up from individual discrete mirror elements, wherein a reflection (impact area 33) can preferably take place on a single mirror element. For example, twelve intermediate focus zones 29 are traversed.

As an alternative to beam coupling and beam decoupling through openings in the mirrors, smaller mirror elements which engage in the multiple passage cell can be used and, for example, positioned at the positions of the openings 35A, 35B.

Referring to the indicated in Fig. 1 beam path 5A, the pulsed Laserstrah treatment 9 is repeatedly ge leads through intermediate focus zones in the center of the multi-passage cell. Due to the focusing of the laser pulses during the pulse duration At of the laser pulses 11, high intensities form in the intermediate focus zones, which keep the refractive index of the gas 4 nonlinear. The nonlinear behavior of the Refractive index of the gas 4 can be used for the spectral broadening of the pulsed Laserstrah treatment 9.

After the predetermined number of passes through the multi-pass cell 5, the laser radiation 9 leaves the multi-pass cell 5 and hits an output mirror 37 reflecting the coupled-out laser radiation. The output mirror 37 directs the laser radiation 9 through a second telescope arrangement 39, which recollimates the laser radiation 9 .

A length of a multi-pass cell for nonlinear compression is given by a distance between the mirrors 25A, 25B. In the radial direction, the extent of the multiple passage cell depends on the intended number of revolutions.

The length of the multi-pass cell is essential for the integration of a multi-passage cell in an optical structure, as this can be a few meters (e.g. up to 10 m and more). The length of a multi-pass cell is determined by two factors:

- A damage threshold for the end mirror. The damage threshold requires - with a given beam intensity - a minimum size of the reflected laser beam ("minimum adjustable beam diameter") on the end mirrors. With regard to a single pass, this minimum size, together with the curvature of the end mirror, determines the focus diameter in the intermediate focus zone. (Together with the number of reflection zones required, the minimum size of the reflected laser beam on the end mirrors also defines a diameter of the end mirror.)

- An ionization threshold of the filling gas present in the multiple passage cell. The ionization threshold limits the intensity that can be introduced into the intermediate focus zone, ie the intensity that can be used for the non-linear interaction. The ionization threshold thus determines a “maximum beam intensity that can be coupled in” for specified parameters of the intermediate focus zone. If more ionization occurs in the intermediate focus zone, the laser radiation passing through the multiple passage cell can be disrupted and, for example, assume an intensity distribution deviating from the Gaussian beam profile or a reduced transmission. From these two boundary conditions - which beam diameter is allowed on the end mirrors and which focus diameter and thus which intensity should be present in the center of the multi-passage cell - the length of the multi-passage cell results.

If it is possible to increase the pulse energy required for ionization, for example through the use of circular polarization proposed herein, a shorter multiple passage cell can be constructed and used for the spectral broadening. A shorter multi-pass cell corresponds to shorter focus lengths (i.e., e.g. a smaller radius of curvature of the end mirror of a Herriott cell), which results in a smaller focus diameter in the intermediate focus zones, in which the intensity required for ionization with the existing pulse energy and pulse duration is not achieved or may not be significantly exceeded.

The non-linearity, which is reduced due to the circular polarization and which the laser radiation experiences when passing through the multi-pass cell, is compensated for by increasing the pressure of the filling gas.

With regard to the pulse energy to be used, it is selected for a given pulse duration and focus geometry in such a way that a (pulse) peak intensity is present in the intermediate focus zone that is in the range or slightly below the multiphoton ionization that is onset. The (pulse) peak intensity is the upper limit, for example, a maximum of 10% above the intensity assigned to the ionization threshold; the intensity assigned to the ionization threshold in the multiphoton ionization range is referred to herein as the multiphoton ionizing intensity. With a view to fluctuations in the laser parameters, for example, the set (pulse peak intensity can be reduced, for example to half the multiphoton ionizing intensity (lower limit of the pulse peak intensity). In other words, the geometry of the multi-pass cell is adjusted to a peak intensity of the laser pulses present (pulse energy / pulse duration / circular polarization) so that the pulse duration and the pulse energy of the circularly polarized laser pulses in the intermediate focus zones result in a pulse peak intensity that is in the range of 50% to 110% of a multi-photon ionizing intensity - the multi-photon ionizing intensity results in the intermediate focus zones at a (minima len) ionization pulse energy of circularly polarized laser pulses leading to the ionization of the filling gas Peak intensity in a range from 50% to 100% or in a range from 60% to 105% or in a range from 60% to 95% or in a range from 70% to 90% of the multiphoton ionizing intensity for circular polarization.

To adjust the polarization of the laser radiation 9 after the spectral broadening, the laser system 1 can have an arrangement of one or more wave plates (e.g. 1/4, 1/8, 1/2, 1-wave plates). The embodiment shown by way of example in FIG. 1 comprises a second (achromatic) 1/4 wave plate 43. The second 1/4 wave plate 43 converts the laser radiation emerging in the circular polarization state back into linear polarization. Optionally, additional wave plate (s) (e.g. a 1/2 wave plate) for aligning the plane of polarization can be provided before or after the second 1/4 wave plate 43.

Optionally, the optical system 3 can also have an optical beam splitting system 41. In the embodiment shown as an example in Fig. 1, the optical beam splitting system 41 comprises the second 1/4 wave plate 43 and a beam splitter 45 shown as a beam splitter cube. Further optical elements for separating different polarizations include thin-film polarizers and, for example, Wollaston prism arrangements. The beam splitter 45 can be used for beam cleaning of any beam components with other (non-circular) polarization states that may be generated in the multiple passage cell 5. These can arise, for example, in the event of low-energy laser radiation 13 and an incompletely circular polarization in the multi-pass cell 5, if, due to an intensity-dependent rotation of elliptical polarization states in the focus of the multi-pass cell 5, the orientations of the main axes of the slightly elliptically polarized laser pulses 11 and the slightly elliptically polarized low-energy laser radiation 13 at the output of the multi-pass cell differ. Further structures for splitting beam parts with different elliptical polarization states are known from the prior art.

Due to the non-linearity of the refractive index n of the gaseous Kerr medium in the multiple passage cell 5 - i.e. for an intensity-dependent refractive index n = n_0 + n_2 * I (r; t) with the gas-specific refractive index parameters n_0 and n_2 and the intensity profile I (r; t) in the intermediate focus zone - there is a spectral broadening for the laser pulses 11. In Fig. 1, a laser pulse 1 G of the useful beam portion 9A is indicated by way of example, which has been freed from pre- and post-pulses here with play.

For the subsequent use of the laser pulses 1 G, which have been spectrally broadened, the laser pulses 1 G can be added to a compressor 49, for example. The compressor 49 is shown by way of example in FIG. 1 as a chirped mirror compressor. A useful laser radiation 9A ′, which comprises a sequence of compressed laser pulses 11 ″, can thus be output at an output of the laser system 1.

In contrast to the structure known from the prior art, which uses an HCF, the configuration proposed herein, using, for example, a Herriott cell, can enable a predetermined / adjustable number of intermediate focus zones 29 to be passed through. In addition, a focus diameter d can be set 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.

In addition to the ability to adjust the size of the intermediate focus zone 29, e.g. through the radii of curvature of the mirrors 25A, 25B and through a corresponding telescopic arrangement for mode adjustment, which can be connected upstream of the multi-passage cell, the gas pressure is adjusted with regard to the non-linearity. It is noted that if there is a high spatial proximity of the various continuous intermediate focus zones in the multiple passage cell, the same gas pressure is given 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.

If the mirrors 25A, 25B form, for example, a concentric resonator (mirror from approx. 2 * Rm with identical radii of curvature Rm), the intermediate focus zones 29 essentially all have the same diameter d and accordingly have the same Rayleigh lengths Lr. In general, the distance between the mirrors 25A, 25B is in a range from 95% to 105% of the sum of the radii of curvature. An intense laser pulse 11 propagates sequentially through these intermediate focus zones 29 and interacts repeatedly with the gas 4 in the case of electrical Field strengths that can cause non-linear effects on the refractive index n and thus on the spectrum of the laser pulse 11.

The use of the Herriott structure described herein provides various pre-determinable and / or operationally adjustable parameters in the design of the intermediate focus zones and the non-linear conditions present therein. For setting the parameters, the optical system 3 can, for example, have a control system 61, which via control connections 63 with the pulse duration setting system 15, optionally the polarization setting optics 19 (in particular for setting the angular positions of the first 1/4 wave plate 19A and optionally a l / 2-wave plate), the telescope assemblies 21, 39 (in particular special for setting the distance between telescopic lenses 21 A, 21B), a pressure setting device 65 for setting the gas pressure (see Fig. 1) and / or the subsequent Wel lenplatten (for example, for Adjustment of the angular position of the second (achromatic) 1/4 wave plate 43) and optionally the optical beam splitting system 41 is connected.

With the help of the control system 3, for example, the following can be set:

the pulse duration At and / or the dispersion and / or the spectral bandwidth of the laser pulses 11 of the pulsed laser radiation 9,

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

- a circular polarization of the pulsed laser radiation 9,

- the focus diameter d in the intermediate focus zones 29,

- the Rayleigh lengths Lr of the intermediate focus zones 29 and

a gas pressure of the filling gas 4 in the intermediate focus zones 29.

As shown in FIGS. 2B and 2C, the laser radiation 9 strikes the mirrors 25A, 25B repeatedly (multiple times in each case). In addition, 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 laser pulses 11 can be acted upon directly. For example, 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. In addition, each of the mirrors 25A, 25B can be constructed from a plurality of mirror segments with predetermined dispersive properties, each of the mirror segments being dispersed over a desired pulse duration as it passes through the Multi-pass cell 5 is adapted. Correspondingly, 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. When the pulsed laser radiation circulates through the multiple passage cell, 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.

In other words, 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.

Due to the non-linear spectral broadening, 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 circular polarization is used in the multiple passage cell for this purpose. In this way, the spectral broadening can possibly take place more smoothly 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. The following are geometric parameters for the implementation of a multi-pass cell is considered within the framework of the concepts presented here.

The limitation of the pulse energies of laser pulses, which can be spectrally broadened with a multi-pass cell (and optionally increased in contrast), results from avoiding laser-induced damage to the (Herriott) mirror and from the ionization threshold value of the gas used. The highest possible ionization threshold when using helium as gas 4 in the multiple passage cell 5 is approximately 3.42 × 10 ^L 14 W / cm ²

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.

For the spectral broadening, the non-linear interaction with the filling gas is decisive, which, as already mentioned, decreases with circular polarization. The lower non-linearity is compensated for by increasing the pressure.

It is known that an increase in pressure can affect the ionization process. A distinction is made here between the area of multiphoton ionization (low pressure / low density, short pulses) and the area of avalanche ionization (higher pressure / higher density, long pulses). In the field of multiphoton ionization, the energy required for the ionization for helium as the filling gas can be viewed essentially as independent of the gas pressure in the multiple passage cell. For other noble gases there is a dependency in the form of a slight decrease, whereby the decrease becomes smaller when the gas pressure decreases. In the area of avalanche ionization, there is a greater decrease in the energy required for ionization with increasing gas pressure, since denser gas promotes the formation of the electron avalanche.

If you are in the area of multiphoton ionization, the change from linear polarization to circular polarization can lead to a considerable shortening of the Carry out a multiple pass cell (e.g. a Herriott cell). Compared to Ar gas, He gas has a lower non-linearity, but a much higher ionization threshold. Due to the high ionization threshold, He gas can be used for the spectral broadening of ultra-short pulses with pulse energies greater than 20 mJ as a filling gas in a multi-pass cell, with the ionization threshold e.g. at 500 fs and pressures below 100,000 Pa in the area of multiphoton ionization.

The inventors have recognized that in a multiple passage cell for He gas operated with pulse energies in the range of 20 mJ and more, an increase in pressure has little influence on the pulse energy required for ionization, so that He gas as filler gas together with circular polarization to shorten the Herriott -Cell can be used.

To illustrate the shortening, it is assumed that linearly polarized ultra-short laser pulses with pulse energies of a few 10 mJ, for example 200 mJ, are coupled into a He-filled multiple passage cell. In order to provide a sufficiently non-linear interaction with these high pulse energies in the multiple passage cell, a pressure in the range from 10,000 Pa to 20,000 Pa must be set, for example. The length of the multiple passage cell is then, for example, approx. 10 m.

If, on the other hand, a circular polarization is set for the ultrashort laser pulses with pulse energies of a few 10 mJ, for example 200 mJ, the energy required for ionization is increased, but the non-linearity is also reduced. The multi-pass cell can now be operated comparably close to the ionization threshold value, i.e., higher peak intensities can be set in the intermediate focus zones by increasing the length of the multi-pass cell to e.g.

5 m is reduced. With the knowledge that with He gas the pressure has almost no influence on the ionization threshold with such pulse energies, the pressure can be increased from e.g. 20,000 Pa to 40,000 Pa in order to provide a sufficiently non-linear interaction. If the multi-pass cell is operated in the parameter range of multi-photo ionization, the positive effect of raising the ionization threshold due to the circular polarization can be (almost completely) exploited.

Obviously, the concepts proposed herein for using He gas as fill gas and circular polarization in a multi-pass cell provide one possibility is to reduce the length of the multi-pass cell with a pulse energy of at least 20 mJ and pulse durations of 500 fs.

Similarly, there are corresponding parameter ranges for other filling gases such as Xe, Kr, Ar or Ne gas, in which circular polarization enables a shortening of the length of multiple passage cells operated in the area of multi-photon ionization. It should be noted here, however, that there may be a slight decrease in the pressure dependency, depending on the operating point at which a multi-pass cell is operated in the multiphoton ionization range.

Assuming that the pulse energy required for ionization increases by a factor of 3 for a multi-pass cell filled with Ar gas due to the circular polarization, the length of the multi-pass cell is approx. 1.7 times shorter (“V3”). To compensate for the reduced non-linearity due to the circular polarization, the pressure of the Ar gas, for example, is increased from 15,000 Pa (in the unabridged structure) to 45,000 Pa (in the shortened structure).

It is noted that - if the pressure increase to be made for Ar gas, for example, occurs outside of the multiphoton ionization and in the area of an avalanche-like ionization process - the pulse energy required for ionization can decrease with increasing pressure. This counteracts the desired increase in the pulse energy required for ionization due to the circular polarization.

For the more compact structure of multiple passage cells proposed here, it is important that the electrical field strength required for ionization is increased for circular (possibly slightly elliptically modified circular) polarized light 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 to be smaller. As a result, there is generally the possibility of reducing the mirror spacing for circular polarization (for example a spacing of the mirrors 25A, 25B shortened by a factor of V3, ie approx. A factor of 2) compared to a multiple pass cell operated with linear polarization e.

The shorter structure leads to cost savings in the case of non-linear compression by means of spectral broadening. For the use of high-intensity laser radiation and with a view to the damage threshold of the mirrors, 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 shorter. For ultra-short pulses, the mirrors are also broadband, for example designed for a wavelength range from, for example, 700 nm to, for example, 1100 nm 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. Furthermore, the mirrors can provide a dispersion contribution or none at all, so that dispersive coatings may also have to be taken into account.

Exemplary parameters for a multiple passage cell and the mirrors on which it is based are explained below with reference to FIGS. 2A to 2C. For coated mirrors, for example, 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. Assuming a Gaussian 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.

Based on this, for example, a beam radius of approximately 9 mm results for 200 mJ pulses or a converted 1 / e2 beam diameter of approximately 13 mm on the mirrors 25 A, 25B. This estimate can apply equally to linear as well as circular polarization.

For circularly polarized light (with a (maximum) electric field strength that is reduced compared to linearly polarized light), 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 that the The same beam diameter is available on the mirror.

With reference to the flowchart shown in FIG. 3, the steps in the procedure proposed herein for the spectral broadening of pulsed laser radiation using a multi-pass cell through which circular polarization is passed are explained. In step 71, pulsed laser radiation is generated which includes laser pulses with a pulse energy in the range from 1 mJ to 100 J, preferably 10 mJ to 1 J, and pulse durations in the range from 10 fs to 5 ps, preferably 500 fs to 1.5 ps .

In step 73, the pulsed laser radiation is circularly polarized for passing through the multi-pass cell - i.e., usually before entering the multi-pass cell.

In step 75 the spectral broadening of the pulsed laser radiation takes place. For this purpose, the pulsed laser radiation is coupled into the multiple passage cell. The multiple passage cell is formed, for example, by at least two concave mirrors which define a multiple passage of intermediate focus zones; e.g. form a (in particular concentric or confocal) resonator or a resonator-like arrangement. In step 75, the multiple passage cell is traversed a number of times with the formation of a plurality of intermediate focus zones. The multi-pass cell is filled with a filling gas that has an optical non-linearity that causes the spectral broadening of the pulsed laser radiation in the intermediate focus zones.

In step 77, the spectrally broadened pulsed laser radiation is decoupled from the multiple passage cell.

In step 79, for example, a linear polarization of the spectrally broadened pulsed laser radiation can be set and / or, for example, the spectrally broadened pulsed laser radiation can be compressed.

Furthermore, in order to be able to use a compact structure of the laser system (in particular the multi-passage cell with a short length) with a desired spectral broadening, the pressure of the filling gas of the multi-passage cell is set in a step 81 A in a pressure range in which an ionization of the filling gas is carried out Laser pulses, the pulse duration of which is comparable to the pulse duration of the laser pulses to be spectrally broadened, would take place in the context of a multi-photon ionization process. In other words, there is an ionization behavior of the filling gas in the context of multiphoton ionization. This makes it possible to set the pressure of the filler gas to increase the nonlinearity so high that a decrease in a nonlinearity of the filler gas with circular polarization starting from a Non-linearity of the filling gas, which is present at the same pressure and with linear polarization, is compensated for.

For the desired spectral broadening, the focus diameter in the intermediate focus zones is also set in a step 81B (e.g. by selecting the radii of curvature of the mirrors of the multi-passage cell and the beam diameter on the mirrors) in such a way that the pulsed laser radiation passes through the multi-passage cell without ionizing the filler gas. The focus diameter in the intermediate focus zones is selected to be as small as possible, but with a safety margin with regard to optical damage to the mirrors and avoidance of (stronger) ionization of the filling gas.

It is also possible for the laser radiation to run through a sequence of successive multiple pass cells one after the other. This allows gas conditions, mirror configurations and dispersion curves to be set in a differentiated manner for the groups of intermediate focus zones present in the individual multi-pass cells, whereby an “intermediate compression” can also be provided between individual multi-pass cells.

It is explicitly emphasized that all features disclosed in the description and / or the claims are viewed as separate and independent of one another for the purpose of the original disclosure as well as for the purpose of restricting the claimed invention, regardless of the combinations of features in the embodiments and / or the claims should be. It is explicitly stated that all range specifications or specifications of groups of units disclose every possible intermediate value or subgroup of units for the purpose of the original disclosure as well as for the purpose of restricting the claimed invention, in particular also as a limit of a range specification.

Claims

1. Laser system (1) with:

- A laser radiation source (7) for providing pulsed laser radiation (9), the ge pulsed laser radiation laser pulses (11) with pulse energies in a range from 1 mJ to 100 J or in a range from 10 mJ to 1 J and pulse durations in a range of 10 fs to 5 ps or in a range of 500 fs to 1.5 ps, and

- An optical system (3) for the spectral broadening of the pulsed laser radiation (9) comprising:

- a first polarization adjustment lens (19) which adjusts a circular polarization state (17B) of the pulsed laser radiation (9), and

- A multiple passage cell (5) with at least two mirrors (25A, 25B) through which the pulsed laser radiation (9) traverses to form a plurality of intermediate focus zones (29), the multiple passage cell (5) being filled with a filler gas (5A) , which has an optical non-linearity, the filling gas (5A) causing a spectral broadening of the pulsed laser radiation (9) in the intermediate focus zones (29), a pressure of the filling gas (5A) being set in a pressure range in which an ionization behavior of the filling gas (5A) is present in the context of multiphoton ionization, and

Focus diameter (d) of the intermediate focus zones (29) are set in such a way that the pulsed laser radiation (9) passes through the multiple passage cell (5) without ionizing the filling gas (5).

2. Laser system (1) according to claim 1, wherein the multiple passage cell (5) is filled with He gas as filling gas at a pressure in a range from 100 Pa to 60,000 Pa, in particular in a range from 1,000 Pa to 50,000 Pa.

3. The laser system (1) according to claim 1, wherein the multiple passage cell (5) is filled with Ar gas as filling gas at a pressure in a range from 100 Pa to 50,000 Pa, in particular in a range from 1,000 Pa to 40,000 Pa.

4. Laser system (1) according to one of the preceding claims, wherein the focus diameter (d) of the intermediate focus zones (29) are set such that a Spit zenintensität that is in the pulse duration and the pulse energy of the laser pulses (11) in the intermediate focus zones ( 29) results in the range of 50% to 110% of a multiphoton ionizing the intensity.

5. Laser system (1) according to one of the preceding claims, wherein the first polarization setting optics (19) have a first wave plate (19A), optionally a 1/4 wave plate (19A) and / or a 1/2 wave plate (19B) , includes.

6. Laser system (1) according to one of the preceding claims, 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 (At) of the laser pulses (11) of the pulsed laser radiation (9),

- A first optical telescope arrangement (21) which is set to image the pulsed Laserstrah treatment (9) in a predetermined mode in the multi-passage cell (5) and which is optionally arranged downstream of the first polarization setting optics (19),

- A coupling mirror (23) for coupling the pulsed laser radiation (9) into the multiple passage cell (5),

- A decoupling mirror (37) for forwarding the pulsed laser radiation (9) emerging from the multiple passage cell (5) and

- A second optical telescope arrangement (39) which is set to collimate the pulsed laser radiation (9) emerging from the multiple passage cell (5).

7. Laser system (1) according to any one of the preceding claims, wherein the multiple passage cell (5) is formed

- With a predetermined or adjustable number of intermediate focus zones (29), and / or

- With intermediate focus zones (29) which have essentially the same diameter (d) and the same Rayleigh length (Lr), and / or

- With intermediate focus zones (29) which are arranged one on top of the other, next to one another and optionally partially overlapping one another,

- In a resonator structure with, in particular the same, radii of curvature of the at least two mirrors (25A, 25B), optionally in a confocal or concentric arrangement, and / or - In a resonator-like structure with, in particular the same, radii of curvature of the at least two mirrors (25 A, 25B), optionally in an almost confocal or almost concentric arrangement, and / or

- In an arrangement in which the at least two mirrors (25 A, 25B) comprise a plurality of mirror segments, an intermediate focus zone (29) being formed between each two mirror segments, and the intermediate focus zones (29) traversed one after the other,

- As a cell filled with a noble gas such as helium or argon as filling gas, the same pressure being present in each of the intermediate focus zones (29), and / or

- For the gradual non-linear spectral broadening of the pulsed laser radiation passing through (9) in the intermediate focus zones (29).

8. The laser system (1) according to any one of the preceding claims, further comprising: a second polarization adjustment optics for returning the circular polarization state to a linear polarization state (47 A, 47B), wherein the second polarization adjustment optics is arranged downstream of the multi-pass cell (5), and wherein the second polarization setting optics, in particular a second, in particular achromatic wave plate (43), optionally a 1/4 wave plate (43) and / or a 1/2 wave plate.

9. Laser system (1) according to one of the preceding claims, wherein the laser system (1) further comprises at least one of the following optical components:

- a pulse duration setting system (15) for setting the pulse duration (At) of the laser pulses (11),

- An optical pulse duration compressor system (49) for compensating for a dispersive contribution of the optical system (9) and for the temporal compression of the laser pulses (11) of the laser radiation that have experienced the nonlinear spectral broadening in at least one of the intermediate focus zones (29) ,

- A beam splitter (45) for separating, from the multi-passage cell (5) output NEN, different polarization states and

- A control system (61) which is designed to compensate for a reduction in the non-linearity of the filling gas due to the set circular polarization for setting a pressure of the filling gas (4) in the multi-passage cell (5).

10. A method for the spectral broadening of a pulsed laser radiation (9) using a non-linearity of a filling gas of a multi-passage cell (5) with at least two mirrors (25A, 25B) which form a plurality of intermediate focus zones (29), with the steps:

- Generating a pulsed laser radiation (9), the laser pulses with a pulse energy in a range from 1 mJ to 100 J or in a range from 10 mJ to 1 J, and pulse durations in a range from 10 fs to 5 ps or in a range of Includes 500 fs to 1.5 ps,

- Setting a circular polarization state (17A) of the pulsed laser radiation (9) for passing through the multiple passage cell (5),

- Coupling of the pulsed laser radiation (9) into the multiple passage cell (5), the pulsed laser radiation (9) passing through the plurality of intermediate focus zones (29) and interacting non-linearly with the filling gas (4) in the intermediate focus zones (29), so that a spectral broadening the pulsed laser radiation (9) is effected in the intermediate focus zones (29),

- Setting the pressure of the filling gas (5A) in a pressure range in which an ionization behavior of the filling gas (5A) is present in the context of multiphoton ionization,

- Setting focus diameters (d) in the intermediate focus zones (29) in such a way that the pulsed laser radiation (9) passes through the multiple passage cell (5) without ionizing the filling gas (5), and

- Decoupling of the spectrally broadened pulsed laser radiation (9) from the multiple passage cell (5).

11. The method of claim 10, further comprising at least one of the following steps:

- Providing He gas as filling gas and setting the pressure in a range from 100 Pa to 60,000 Pa, in particular in a range from 1,000 Pa to 50,000 Pa, and

- Providing Ar gas as filling gas and setting the pressure in a range from 100 Pa to 50,000 Pa, in particular in a range from 1,000 Pa to 40,000 Pa,

12. The method of claim 10 or 11, further comprising

- Setting the focus diameter (d) of the intermediate focus zones (29) in such a way that a peak intensity, which is reflected in the pulse duration and the pulse energy of the laser pulses (11) in the Intermediate focus zones (29) results in the range from 50% to 110% of a multi-photon ionizing intensity.

13. The method according to any one of claims 10 to 12, further comprising: setting the polarization of the spectrally broadened pulsed laser radiation (9) for a subsequent beam path and / or

- Carrying out a dispersion compensation of the spectrally broadened pulsed Laserstrah treatment (9).

14. The method according to any one of claims 10 to 13, further comprising setting at least one of the following parameters of the multi-pass cell (5):

- a dispersion of the laser pulses (11) accumulated in the multiple passage cell (5),

- Focus diameter (d) of the intermediate focus zones (29), and

- Rayleigh lengths (Lr) of the intermediate focus zones (29).