Device and method for manipulating pulsed laser radiation

Abstract

The invention relates to a method for manipulating pulsed laser radiation (2), having the following steps: - providing pulsed laser radiation (2); - spatially separating the frequency components of the pulsed laser radiation (2) to a first predetermined extent in a direction perpendicular to a propagation direction of the pulsed laser radiation (2); - guiding pulsed laser radiation (2), spatially separated to the first predetermined extent, to a multiple-pass gas cell (8); - increasing the spectral bandwidth of the pulsed laser radiation (2) in the multiple-pass gas cell (8) for temporal pulse compression of the pulsed laser radiation (2). The invention also relates to a corresponding device (1).

Description

[0001] Device and method for manipulating pulsed laser radiation

[0002] The invention relates to a device and a method for manipulating pulsed laser radiation.

[0003] Over the past two decades, hollow-core fibers (HCFs) have been used for pulse compression due to their spectral broadening properties and waveguide geometry, which allows extending the length of the nonlinear optical interaction to several meters with negligible dispersion. However, multi-pass gas cells (MPCs) have emerged as a practical alternative to HCFs for achieving spectral broadening through self-phase modulation (SPM) in gases. Compared to HCFs, MPCs offer more compact setups and less demanding beam alignment stability constraints. Nevertheless, both solutions suffer from unfavorable spatial scaling of the setups when handling pulses with higher energies.

[0004] External pulse compression using MPC is an extremely powerful technique for shortening the duration of sub-ps laser pulses (cf. M. Hanna, X. Deien, L. Lavenu, F. Guichard, Y. Zaouter, F. Druon, and Patrick Georges, "Nonlinear temporal compression in multipass cells: theory" J. Opt. Soc. Am. B 34, 1340-1347 (2017)). Despite its enormous success and versatility, the MPC method faces the challenge of handling high energy due to unfavorable geometric scaling requiring a rapid increase in the distance between the concave mirrors and the corresponding size of the vacuum chamber.

[0005] Two material parameters – the ionization threshold of the gas used as the self-phase modulation (SPM) medium (typically Ar for higher pulse energies) and the destruction threshold of the dielectric coating on the cell mirrors – determine the length scaling of the MPC as a function of the pulse energy. While the strength of the SPM can be controlled by adjusting the noble gas pressure, this does not apply to the plasma losses. The latter can only be suppressed by a sufficiently large beam waist to keep the peak intensity of the pulse at a safe low level. Due to the loose focusing, a long propagation distance to the mirror is required to increase the beam size and reduce the fluence to a safe level, which for currently available coatings is ~ 0.5–0.6 J / cm 2for sub-ps pulses . In M . Kaumanns , D . Kormin, T . Nubbemeyer, V . Pervak, and S . Karsch, " Spectral broadening of 112 mJ, 1 . 3 ps pulses at 5 kHz in a LG10 multipass cell with compressibility to 37 fs" Opt . Lett . 46 , 929- 932 ( 2021 ) an attempt was made to reduce the fluence by converting the fundamental spatial mode of the laser beam into a higher-order mode , which allows a significantly larger spot size .

[0006] MPCs are therefore limited in pulse energy and corresponding pulse intensity due to the damage threshold of the optics and the ionization limit of the gas in the cell. Consequently, the energy scaling of the laser pulses also requires a spatial scaling of the MPCs, i.e., larger MPCs are needed for higher pulse energies.

[0007] The object of the invention is to mitigate or eliminate one or more of the disadvantages of the prior art. In particular, the object of the invention is to propose a device and a method for providing pulsed laser radiation, preferably for pulse compression, particularly in the amplification of chirped pulses (chirped pulse amplification, CPA) and / or for CPA, which allow higher pulse energies and / or enable a smaller size of the MPC.

[0008] This is achieved by a method for manipulating pulsed laser radiation comprising the steps:

[0009] - Providing pulsed laser radiation;

[0010] - spatially separating the frequency components of the pulsed laser radiation to a first predetermined extent in a direction perpendicular to a propagation direction of the pulsed laser radiation;

[0011] - guiding the pulsed laser radiation spatially separated to the first predetermined extent to a multi-pass gas cell;

[0012] - Increasing the spectral bandwidth of the pulsed laser radiation in the multi-pass gas cell for temporal pulse compression of the pulsed laser radiation.

[0013] This is further achieved by a device for manipulating pulsed laser radiation, comprising:

[0014] - a device for providing pulsed laser radiation;

[0015] - a separation element for spatially separating the frequencies of the pulsed laser radiation to a first predetermined extent in a direction perpendicular to a propagation direction of the pulsed laser radiation;

[0016] - a multi-pass gas cell for increasing the spectral bandwidth of the pulsed laser radiation for temporal pulse compression of the pulsed laser radiation, wherein the multi-pass cell is arranged in the optical path of the pulsed laser radiation behind the separation element.

[0017] As the laser pulse energy increases, the intensity at the laser focus exceeds the threshold of the optimal range. To avoid an increase in intensity with a simultaneous increase in energy, it is necessary to increase the size of the laser focus. Where I=intensity. of the laser focus, i=laser pulse duration. The size of the laser focus r [ r oc f / a, r=radius of the laser focus, f=distance of the focus, a=radius of the laser spot on the mirror] can be increased in the MPC:

[0018] - by reducing the size ( a ) of the laser on the mirrors => limited by laser damage on the mirrors

[0019] - by increasing the distance ( f ) between the mirrors and the focus => limited by the restrictions on the maximum permissible size of the MPC .

[0020] By spatially separating the beam prior to feeding it to the multi-pass gas cell, the size of the collimated beam in the multi-pass gas cell is increased (particularly at the mirrors of the multi-pass gas cell). At the same time, the size of the beam at the focus is maintained. It is noteworthy that the intensity at the mirrors decreases even faster than the fluence, as the effective pulse duration at the mirrors changes due to the monochromatizing effect of spatial chirping. This is important because the damage mechanism advantageously changes from an avalanche-dominated effect (for fs pulses) to a thermally dominated effect (>>ps), allowing the use of higher fluences. Thus, the pulse duration of the collimated beam is increased, whereas the original pulse duration is restored when the beam is focused.As a result, it is possible to increase the laser energy in the optics without changing the properties of the laser pulse (duration) and the beam (spot size) at the focus, where the desired spectral bandwidth increase normally takes place. By reducing the fluence on the optical elements of the multi-pass gas cell, the maximum pulse energy limit is increased, for example, by a factor of between 1.4 and 2 for a fixed length of the multi-pass gas cell. For a fixed pulse energy, the length of the multi-pass gas cell can be reduced, for example, by a factor of 1.4 to 2.

[0021] Temporal broadening means the temporal stretching of the frequency components, i.e. the introduction of a temporal chirp (used synonymously with spectral chirp). Temporal broadening describes the reverse process to temporal compression. Spatial separation means the introduction of a spatial chirp. Diffraction into different angles means the introduction of an angular chirp. Preferably, the spatial separation of the frequency components of the pulsed laser radiation is not reversed to the first predetermined extent before the pulsed laser radiation is guided to the MPC. Preferably, the pulsed laser radiation is broadened when injected into the MPC compared to the originally provided pulsed laser radiation by the factor by which the pulsed laser radiation was broadened to the first predetermined extent during the spatial separation.

[0022] The multi-pass gas cell is in particular filled with a filling gas, for example argon, which has an optical non-linearity. The multi-pass gas cell contains in particular a medium with non-linear optical properties, in which the pulsed laser radiation acquires a non-linear phase through self-phase modulation. The MPC preferably has at least two mirror elements, between which the pulsed laser radiation is reflected multiple times. In particular, the multi-pass gas cell has two opposing mirrors which form a concentric resonator through which the pulsed laser radiation passes multiple times. The non-linear medium, in particular argon, is preferably provided between the mirrors. It is of course also possible for a plurality of non-linear elements to be arranged as a non-linear medium between the mirrors. The pulsed laser radiation is in particular injected into the multi-pass cell.The MPC preferably comprises two spherical mirrors, particularly those with low dispersion (e.g. Optoman).

[0023] The device preferably has a laser radiation source for providing the pulsed laser radiation. The pulsed laser radiation is in particular an ultrashort pulse. The pulsed laser radiation (originally provided (i.e. before carrying out the remaining steps of the method), in particular by the laser radiation source) comprises laser pulses with pulse energies in the range from 0.5 mJ to 25 mJ, preferably 1 mJ to 14 mJ, in particular 2.5 mJ, and pulse durations in the range from 10 fs to 5 ps, preferably 100 fs to 500 fs, in particular 250 fs.

[0024] The extent of the broadening refers in particular to the factor between the beam waist before the broadening and after the broadening (in the direction perpendicular to the propagation direction). It is advantageous if, during the spatial separation of the frequency components of the pulsed laser radiation to the first predetermined extent, a broadening of the pulsed laser radiation in the direction perpendicular to the propagation direction of the pulsed laser radiation occurs by at least 1.5 times, preferably at least twice, particularly preferably at least three times.

[0025] Preferably, the method comprises the following step, which is carried out before increasing the spectral bandwidth of the pulsed laser radiation in the multi-pass gas cell (and before guiding the pulsed laser radiation spatially separated to the first predetermined extent to a multi-pass gas cell):

[0026] - temporal pulse compression (of the frequency components) of the pulsed laser radiation in a dispersion arrangement which has at least two dispersive elements, in particular optical gratings, which are designed to spatially separate the frequency components of the pulsed laser radiation by a second predetermined amount and, after propagation over an optical path length which differs depending on the frequency, to combine the frequency components of the pulsed laser radiation according to the second amount, so that temporal pulse compression of the pulsed laser radiation is effected. This can be a temporal pulse compression as carried out in CPA.Preferably, a collimated, laterally expanded (broadened) beam is obtained by breaking the symmetry between an incoming and an outgoing pass through the dispersion arrangement, in particular the at least two dispersive elements. It is advantageous if, during spatial separation of the frequency components of the pulsed laser radiation to the second predetermined extent, the pulsed laser radiation is broadened in the direction perpendicular to the direction of propagation of the pulsed laser radiation to at least 1.5 times, preferably at least twice, particularly preferably at least 3 times. Preferably, during spatial merging, at least a corresponding narrowing takes place.It is advantageous if, during the spatial merging of the frequency components of the pulsed laser radiation to the second predetermined extent, the pulsed laser radiation is narrowed in the direction perpendicular to the propagation direction of the pulsed laser radiation to equal to or less than 2 / 3 times, preferably equal to or less than half, particularly preferably equal to or less than 1 / 3 times. In particular, during the spatial merging of the frequency components of the pulsed laser radiation to the second predetermined extent, the original spot size and the original overlap of the various frequency components are restored.

[0027] It is advantageous if, in order to spatially separate the frequency components of the pulsed laser radiation by the second predetermined amount, the frequency components of the pulsed laser radiation are diffracted into different angles using a first of the at least two dispersive elements, and the frequency components of the pulsed laser radiation diffracted into different angles are collimated using a second of the at least two dispersive elements; and in order to spatially combine the frequency components of the pulsed laser radiation according to the second amount, the frequency components of the pulsed laser radiation are diffracted into different angles using the second of the at least two dispersive elements, and the frequency components of the pulsed laser radiation diffracted into different angles are collimated using the first of the at least two dispersive elements. The dispersion arrangement can comprise a Treacy pair compressor (optionally additionally with a separation element, see below).) .

[0028] It is advantageous if the spatial separation of the frequency components of the pulsed laser radiation takes place to the first predetermined extent in the direction perpendicular to the propagation direction of the pulsed laser radiation in that a first optical path length of the pulsed laser radiation between the first of the at least two dispersive elements and the second of the at least two dispersive elements during the spatial separation of the frequency components of the pulsed laser radiation differs by the second predetermined extent from a second optical path length of the pulsed laser radiation between the second of the at least two dispersive elements and the first of the at least two dispersive elements during the spatial merging of the frequency components of the pulsed laser radiation in accordance with the second extent. In this way, a controlled extent of spatial chirp can be introduced into the resulting beam.

[0029] It is preferred if the spatial separation of the frequency components of the pulsed laser radiation to the first predetermined extent in the direction perpendicular to the propagation direction of the pulsed laser radiation during the temporal pulse compression of the pulsed laser radiation is brought about by a separation element in the dispersion arrangement. Advantageously, the dispersive elements already available can thus be used to achieve the spatial chirp to the first extent. Preferably, the spatial separation to the first extent (in contrast to the spatial separation to the second extent, which serves for the temporal pulse compression) is not reversed. The separation element brings about in particular the difference in the optical path length described above.

[0030] It is preferred if the pulsed laser radiation is reflected in the dispersion arrangement by a mirror, wherein the pulsed laser radiation is propagated in the dispersion arrangement (from an input of the dispersion order) along a first optical path to the mirror and, after reflection at the mirror, is propagated along a second optical path (to an output of the dispersion order), wherein the at least two dispersive elements are located in the first optical path and in the second optical path, wherein the separation element is located only in the first optical path or only in the second optical path.

[0031] It is advantageous if the separation element has four mirrors. This allows a controlled degree of spatial chirp to be achieved in a simple manner.

[0032] Preferably, the method is a method for CPA. Preferably, the method further comprises the following steps, which are carried out before spatially separating the frequency components of the pulsed laser radiation to the first predetermined extent:

[0033] - temporal broadening / stretching of the pulsed laser radiation (preferably to the inverse extent of the temporal pulse compression with the dispersion arrangement);

[0034] - Amplifying the time-broadened pulsed laser radiation.

[0035] With reference to the device, it is advantageous if it has a dispersion arrangement, preferably a grating pair pulse compressor, in particular a Treacy pair pulse compressor, for the temporal pulse compression of the pulsed laser radiation with at least two dispersive elements, in particular optical gratings. The dispersive elements are in particular designed to spatially separate the frequency components of the pulsed laser radiation by a second predetermined amount and, after propagation over an optical path length which differs depending on the frequency, to combine the frequency components of the pulsed laser radiation according to the second amount, so that temporal pulse compression of the pulsed laser radiation is effected.The dispersive elements are in particular designed so that, in order to spatially separate the frequency components of the pulsed laser radiation by the second predetermined amount, the frequency components of the pulsed laser radiation are diffracted into different angles using a first of the at least two dispersive elements and the frequency components of the pulsed laser radiation diffracted into different angles are collimated using a second of the at least two dispersive elements; and in order to spatially combine the frequency components of the pulsed laser radiation according to the second amount, the frequency components of the pulsed laser radiation are diffracted into different angles using the second of the at least two dispersive elements and the frequency components of the pulsed laser radiation diffracted into different angles are collimated using the first of the at least two dispersive elements.

[0036] It is advantageous if the dispersion arrangement has a mirror which is arranged such that the pulsed laser radiation is propagated in the dispersion arrangement along a first optical path to the mirror and, after reflection at the mirror, is propagated along a second optical path, wherein the at least two dispersive elements are located in the first optical path and in the second optical path, wherein the separation element is provided only in the first optical path or only in the second optical path.

[0037] It is preferred if the separation element has four mirrors which in particular achieve an extension of the optical path.

[0038] It is advantageous if the device comprises:

[0039] - a broadening device for temporally broadening the pulsed laser radiation;

[0040] - an amplification device for amplifying the temporally broadened pulsed laser radiation; wherein the separation element and the multi-pass cell are provided in the optical path of the pulsed laser radiation after the broadening device and the amplification device. The amplification device can have one or more amplification stages.

[0041] The device for providing pulsed laser radiation may comprise a laser beam source. Preferably, the device for providing pulsed laser radiation comprises: the laser beam source, the broadening device, the amplification device, and optionally the dispersion arrangement. The MPC preferably serves as a post-compressor. Advantageously, the post-compression effect (i.e., spectral broadening) occurs primarily at the laser focus in the center of the post-compressor.

[0042] Fig. 1 shows schematically a preferred embodiment of a device for manipulating pulsed laser radiation.

[0043] Fig. 2 shows schematically a preferred embodiment of a known dispersion arrangement.

[0044] Fig. 3 shows schematically a preferred embodiment of a dispersion arrangement of the device for manipulating pulsed laser radiation, with a separation element.

[0045] Fig. 4 shows schematically a preferred embodiment of a multi-pass gas cell of the device for manipulating pulsed laser radiation.

[0046] Fig . 5 shows the fluence and the beam waist as a function of the extent of spatial separation .

[0047] Fig. 6a shows the peak power and Fig. 6b the spot size on the way to the focus.

[0048] Fig. 7a shows normalized intensity as a function of wavelength and Fig. 7b the normalized intensity as a function of pulse duration. ll

[0049] Fig. 1 schematically shows a preferred embodiment of a device 1 for manipulating pulsed laser radiation 2. In this embodiment, the device 1 is set up for CPA. The device 1 has a device 3 for providing pulsed laser radiation 2, in particular a laser beam source. The pulsed laser radiation 2 is guided to a broadening device 4 for temporally stretching the pulsed laser radiation 2. The temporally broadened laser radiation 2 is amplified in an amplification device 5. The temporal pulse compression of the amplified pulsed laser radiation 2 takes place in a dispersion arrangement 6. The dispersion arrangement 6 also has a separation element 7 for spatially separating the frequencies of the pulsed laser radiation 2 to a first predetermined extent in a direction perpendicular to a propagation direction of the pulsed laser radiation 2.Alternatively, the separation element 7 can be provided independently (outside) of the dispersion arrangement 6. The pulsed laser radiation 2 which has been (again) temporally compressed and spatially broadened to the first extent is guided to a multi-pass gas cell 8 which is designed to increase the spectral bandwidth of the pulsed laser radiation 2 for the (further) temporal pulse compression of the pulsed laser radiation 2. As a result of the spatial broadening to the first extent, the size of the collimated beam in the multi-pass gas cell 8 is increased. At the same time, the size of the beam in the focus of the MPC 8 is maintained. The pulse duration of the collimated beam is increased, whereas the original pulse duration is restored when the beam is focused. As a result, it is thus possible to increase the laser energy in the optics without changing the properties of the laser pulse (duration) and the beam (spot size) in the focus.

[0050] The device 1 can also comprise only the device 3 for providing the pulsed laser radiation 2, the separation element 7 and the multi-pass gas cell 8.

[0051] Fig. 2 shows schematically a preferred embodiment of a known dispersion arrangement 6, which can also be used in the device 1 for manipulating pulsed laser radiation 2 (this is particularly preferred if the dispersion arrangement 6 and the separation element 7 are designed separately). In this embodiment, the dispersion arrangement 6 is in particular a Treacy pair pulse compressor. The dispersion arrangement 6 has an input mirror IM, a center mirror RM and an output mirror OM. An upper plane of the dispersion arrangement 6, which represents a first optical path 9 of the pulsed laser radiation 2 in the dispersion arrangement 6 from the input mirror IM to the center mirror RM, is shown in partial figure (a).A lower plane of the dispersion arrangement 6 , which represents a second optical path 10 of the pulsed laser radiation 2 in the dispersion arrangement 6 from the center mirror RM to the output mirror OM, is shown in part figure (b).

[0052] The dispersion arrangement 6 has a first optical grating G1 and a second optical grating G2, both of which lie in the first optical path 9 and the second optical path 10. The pulsed laser radiation 2 is coupled into the dispersion arrangement via the input mirror IM. The first optical grating G1 causes the frequency components of the pulsed laser radiation 2 to be fanned out at different angles (angular chirp). By way of example, two frequency components of the pulsed laser radiation 2 are shown in Fig. 2, namely an exemplary low-frequency (e.g. red) frequency component as a dashed line 2a and an exemplary high-frequency (e.g. blue) frequency component 2b as a dotted line. The second optical grating G2 causes an exactly opposite fanning out of the frequency components of the pulsed laser radiation 2 ( angular chirp ) , so that the pulsed laser radiation 2 is collimated again .The pulsed laser radiation 2 is then reflected back at the central mirror RM and then runs along the second optical path 10 in the lower plane of the dispersion arrangement 6. This means that, in total, the two optical gratings G1, G2 in the first optical path 9 spatially separate the frequency components of the pulsed laser radiation 2 by a second predetermined amount (spatial chirp).

[0053] In the second optical path 10, the second optical grating G2 and the first optical grating G1 act in exactly the opposite way to the first optical path 9. That is, the second optical grating G2 causes the frequency components of the pulsed laser radiation 2 to be fanned out at different angles (angular chirp). The first optical grating G1 causes the frequency components of the pulsed laser radiation 2 to be fanned out in exactly the opposite way (angular chirp), so that the pulsed laser radiation 2 is collimated again. At the same time, the second optical grating G2 and the first optical grating G1 bring about a merging of the frequency components of the pulsed laser radiation 2 according to the second extent, so that the spatial chirp is canceled out again. Since the frequency components 2 of the pulsed laser radiation travel different optical path lengths, a temporal chirp is thus caused or canceled out, in particular a temporal pulse compression is caused.At the output mirror OM, the pulsed laser radiation 2 leaves the dispersion arrangement 6 .

[0054] Fig. 3 shows schematically a preferred embodiment of a dispersion arrangement 6 of the device 1 for manipulating pulsed laser radiation 2. In this case, the spatial separation of the frequency components of the pulsed laser radiation 2 to the first predetermined extent in the direction perpendicular to the propagation effect of the pulsed laser radiation 2 during the temporal pulse compression of the pulsed laser radiation 2 is brought about by a separation element 7 which is provided in the dispersion arrangement 6. Otherwise (i.e. apart from the separation element), the dispersion arrangement is designed as described in connection with Fig. 2, so that the same reference numerals are used as in Fig. 2 and reference is made to the above description. In this case, the spatial separation to the first extent is brought about by breaking the symmetry between the incoming beam (first optical path 9) and the outgoing beam (second optical path 10).For this purpose, the separation element 7 is only provided in the first optical path, but not in the second optical path. While the first optical grating G1 and the second optical grating G2 in the first optical path 9 bring about a spatial separation to the second extent, and the second optical grating G2 and the first optical grating G1 in the second optical path 10 bring about a spatial merging of the frequency components to the second extent, the separation element 7 brings about a spatial separation to the first extent. The fact that the separation element 7 is only provided in the first optical path 9 means that the spatial separation to the first extent is not reversed, so that the pulsed laser radiation 2 leaving the dispersion arrangement 6 is also spatially separated to the second extent (spatial chirp).

[0055] In particular, the separation element 7 is provided between the first optical grating G1 and the second optical grating G2 in the first optical path 9, and there causes the path length between the first optical grating G1 and the second optical grating G2 to be extended compared to the path length between the second optical grating G2 and the first optical grating G1 in the second optical path 10. As a result, the spatial chirp caused in the first optical path 9 is not completely reversed in the second optical path 10 (but only to the second extent), so that the resulting pulsed laser radiation 2 has a spatial chirp to the first extent when leaving the dispersion arrangement 6 at the output mirror OM, as can be seen from the distance between the two exemplary frequency components 2a, 2b (dotted and dashed lines). To extend the path length, the separation element 7 has four mirrors M1, M2, M3, M4.

[0056] Fig. 4 shows schematically a preferred embodiment of a multi-pass gas cell 8 of the device 1 for manipulating pulsed laser radiation 2. The pulsed laser radiation 2, after it has been spatially broadened to the first extent, is focused with a mirror 11 and injected into the multi-pass gas cell 8. The multi-pass gas cell 8 has two opposing mirrors CM1, CM2, which form a concentric resonator through which the pulsed laser radiation 2 passes several times. As can be seen, the size of the beam is retained in the focus. This means that a round beam is present in the focus, whereas the beam spots are elongated at the mirrors CM1, CM2.

[0057] In conjunction with Figures 4 to 7b, the values ​​achieved for an exemplary embodiment of the method and device according to the invention are shown. Specifically, Figure 5 shows the fluence and beam waist as a function of the extent of spatial separation; Figure 6a shows the peak power and Figure 6b the spot size on the path to the focus; Figure 7a shows normalized intensity as a function of wavelength and Figure 7b the normalized intensity as a function of pulse duration.

[0058] A Yb CPA grating compressor (as shown in Fig. 3) with a 4-mirror bypass was used to introduce the spatial chirp, and the spatially broadened beam was injected into a standard MPC containing two r=-300 low-dispersion spherical mirrors (Optoman) spaced ~590 mm apart. As shown in Fig. 4, the 1-D beam spot extension is periodically repeated through the 2f-2f imaging condition, so that the output plane of the spatial chirp is not tilted. This flexible adjustment of the spot size allows easy control of the fluence (cf. Fig. 5) on the mirror while maintaining the peak intensity in the beam waist (Figs. 6a, 6b).

[0059] In particular, Fig. 5 shows the reduction in peak fluence on mirrors CM1 and CM2. Fig. 7a shows the experimental SPM broadening of 2.5 mJ pulses in 1 bar of Ar, and Fig. 7b shows the Fourier limit of the pulse. The spectral and temporal input intensities are shown.

[0060] Notably, the intensity on mirrors CM1 and CM2 decreases even faster than the fluence, as the effective pulse duration for the coating of mirrors CM1 and CM2 changes due to the monochromatizing effect of spatial chirping. This is important because the damage mechanism changes from an avalanche-dominated effect (for fs pulses) to a thermally dominated effect (for ps pulses), allowing the use of higher fluences. Figs. 7a and 7b show our experimentally determined spectral broadening of 2.5-mJ 250-fs pulses that were spatially chirped to stretch the beam spot by ~x2, which was the maximum expansion technically limited by the aperture of the optics installed in the setup.

[0061] Note that the spatial chirp remaining in the beam after spectral broadening and pulse recompression is not critical for many applications. Focusing converts the spatial chirp into an angular chirp, which allows for the development of a phase-matched bandwidth in frequency conversion (cf. B.A. Richman, S.E. Bisson, R. Trebino, M.G. Mitchell, E. Sidick, and A. Jacobson. "Achromatic phase matching for tunable second harmonic generation using a grating." Opt. Lett. 22, 1223-1225 (1997)) and wavefront manipulation (H. Vincenti and F. Quere. "At-tosecond lighthouse: how to use spatiotemporally coupled light fields to generate isolated as pulses." Phys. Rev. Lett.108, 113904 (2012) ), while in processes such as higher-order harmonic generation in a gas cell, which is confined to a tiny fraction of the Rayleigh length of the driving beam, both the angular and spatial chirp are cancelled within the short interaction distance.

Claims

Patent claims:

1. A method for manipulating pulsed laser radiation (2) comprising the steps: - Providing pulsed laser radiation (2); - spatially separating the frequency components of the pulsed laser radiation (2) to a first predetermined extent in a direction perpendicular to a propagation direction of the pulsed laser radiation (2); - guiding the pulsed laser radiation (2) spatially separated to the first predetermined extent to a multi-pass gas cell (8); - Increasing the spectral bandwidth of the pulsed laser radiation (2) in the multi-pass gas cell (8) for temporal pulse compression of the pulsed laser radiation (2).

2. Method according to claim 1, wherein during the spatial separation of the frequency components of the pulsed laser radiation (2) in the first predetermined extent, a broadening of the pulsed laser radiation (2) in the direction perpendicular to the propagation direction of the pulsed laser radiation (2) to at least 1.5 times, preferably at least twice, particularly preferably at least 3 times, takes place.

3. Method according to one of the preceding claims, comprising the following step, which is carried out before increasing the spectral bandwidth of the pulsed laser radiation (2) in the multi-pass gas cell (8): - temporal pulse compression of the pulsed laser radiation (2) in a dispersion arrangement (6) which has at least two dispersive elements (Gl, G2), in particular optical gratings, which are designed to spatially separate the frequency components of the pulsed laser radiation (2) by a second predetermined amount and, after propagation over an optical path length which differs depending on the frequency, to combine the frequency components of the pulsed laser radiation (2) according to the second amount, so that temporal pulse compression of the pulsed laser radiation (2) is effected.

4. The method according to claim 3, wherein, for spatially separating the frequency components of the pulsed laser radiation (2) by the second predetermined amount, the frequency components of the pulsed laser radiation (2) are diffracted into different angles using a first (Gl) of the at least two dispersive elements (Gl, G2), and the frequency components of the pulsed laser radiation (2) diffracted into different angles using a second (G2) of the at least two dispersive elements (Gl, G2); and for spatially combining the frequency components of the pulsed laser radiation (2) according to the second amount, the frequency components of the pulsed laser radiation (2) are diffracted into different angles using the second (G2) of the at least two dispersive elements (Gl, G2), and the frequency components of the pulsed laser radiation (2) diffracted into different angles using the first (Gl) of the at least two dispersive elements (Gl, G2).

5. The method according to claim 4, wherein the spatial separation of the frequency components of the pulsed laser radiation (2) takes place to the first predetermined extent in the direction perpendicular to the propagation direction of the pulsed laser radiation (2) in that a first optical path length of the pulsed laser radiation (2) between the first (Gl) of the at least two dispersive elements (Gl, G2 ) and the second (G2) of the at least two dispersive elements (Gl, G2 ) during the spatial separation of the frequency components of the pulsed laser radiation (2) differs by the second predetermined extent from a second optical path length of the pulsed laser radiation (2) between the second (G2) of the at least two dispersive elements (Gl, G2 ) and the first (Gl) of the at least two dispersive elements (Gl, G2 ) during the spatial merging of the frequency components of the pulsed laser radiation (2) in accordance with the second extent.

6. Method according to one of claims 3 to 5, wherein the spatial separation of the frequency components of the pulsed laser radiation (2) to the first predetermined extent in the direction perpendicular to the propagation direction of the pulsed laser radiation (2) during the temporal pulse compression of the pulsed laser radiation (2) with a separation element (7) in the dispersion arrangement (6).

7. The method according to claim 6, wherein the pulsed laser radiation (2) is reflected in the dispersion arrangement (6) with a mirror (RM), wherein the pulsed laser radiation (2) in the dispersion arrangement (6) along a first optical path (9) to the mirror and, after reflection at the mirror, is propagated along a second optical path (10), wherein the at least two dispersive elements (Gl, G2) are located in the first optical path (9) and in the second optical path (10), wherein the separation element (7) is only located in the first optical path (9) or only in the second optical path (10).

8. Method according to one of claims 6 or 7, wherein the separation element (7) has four mirrors (M1, M2, M3, M4).

9. Method according to one of the preceding claims, further comprising the following steps, which are carried out before spatially separating the frequency components of the pulsed laser radiation (2) to the first predetermined extent: - temporal broadening of the pulsed laser radiation (2); - Amplifying the time-broadened pulsed laser radiation ( 2 ) .

10. Device for manipulating pulsed laser radiation (2), comprising: - a device (3) for providing pulsed laser radiation (2); - a separation element (7) for spatially separating the frequencies of the pulsed laser radiation (2) to a first predetermined extent in a direction perpendicular to a propagation direction of the pulsed laser radiation (2); - a multiple-pass gas cell (8) for increasing the spectral bandwidth of the pulsed laser radiation (2) spatially separated to the first predetermined extent for temporal pulse compression of the pulsed laser radiation (2), wherein the multiple-pass gas cell (8) is arranged in the optical path of the pulsed Laser radiation (2) is arranged after the separation element (7) and the multi-pass gas cell (8) is arranged to receive the pulsed laser radiation spatially separated to the first predetermined extent.

11. Device according to claim 10, comprising a dispersion arrangement (6), preferably a grating pair pulse compressor, in particular a Treacy pair pulse compressor, for temporal pulse compression of the pulsed laser radiation (2) with at least two dispersive elements (Gl, G2), in particular optical gratings.

12. Device according to claim 11, wherein the dispersion arrangement (6) has a mirror (RM) which is arranged such that the pulsed laser radiation (2) in the dispersion arrangement (6) is propagated along a first optical path (9) to the mirror (RM) and, after reflection at the mirror (RM), is propagated along a second optical path (10), wherein the at least two dispersive elements (G1, G2) are located in the first optical path (9) and in the second optical path (10), wherein the separation element is provided only in the first optical path (9) or only in the second optical path (10).

13. Device according to one of claims 10 to 12, wherein the separation element (7) has four mirrors (M1, M2, M3, M4).

14. Device according to one of claims 10 to 13, comprising: - a broadening device (4) for temporally broadening the pulsed laser radiation (2); - an amplification device (5) for amplifying the temporally broadened pulsed laser radiation (2); wherein the separation element (7) and the multi-pass gas cell (8) are provided in the optical path of the pulsed laser radiation (2) after the broadening device (4) and the amplification device (5).

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