Method and arrangement for the nonlinear spectral broadening of temporally incoherent or partially coherent optical pulses

Abstract

The invention relates to a method and an arrangement for the nonlinear spectral broadening of temporally incoherent or partially coherent optical pulses of a pulsed laser beam, which have a pulse duration not limited by bandwidth. In one embodiment of the method, the pulsed laser beam is split into two partial beams, which are spectrally shifted relative to each other by a frequency difference Δf. The spectrally shifted partial beams are then at least partially superimposed such that they interfere in the time domain during or after superposition and form at least one modulated pulsed laser beam, the pulses of which exhibit a temporal modulation of intensity with a period Δt = 1 / Δf.The pulses of the modulated pulsed laser beam are then spectrally broadened by self- or cross-phase modulation in an optically nonlinear medium, achieving a large spectral broadening through this modulation. This method and setup are particularly advantageous for IFE driver lasers.

Description

Technical application area The present invention relates to a method and an arrangement for the nonlinear spectral broadening of temporally incoherent or partially coherent optical pulses having a non-bandwidth-limited pulse duration. Wide-bandwidth optical pulses are required, for example, in inertial fusion energy (IFE). Laser-driven inertial fusion offers the prospect of an inexhaustible, baseload-capable, clean, and safe energy source. One way to generate the extreme conditions under which the fusion reaction takes place is to bombard a fuel capsule with high-energy laser pulses. According to current technology, this requires a pulse energy of at least 2 MJ (in the ultraviolet spectral range), distributed across many (hundreds of) laser beamlines. The ignition of a fusion plasma and a target gain (ratio of the energy released in the fusion reaction to the incident laser energy) greater than one were demonstrated at the National Ignition Facility (NIF) of the Lawrence Livermore National Laboratory in 2022. To enable a laser-driven inertial confinement fusion power plant, in addition to high pulse energy, a high repetition rate (10-20 Hz), high wall-plug efficiency, and acceptable laser costs must be achieved. Furthermore, the laser pulses must have a wide bandwidth to avoid laser-plasma instabilities during fuel capsule compression (Δλ / λ of approximately 0.3% for indirect-drive / 1.5% for direct-drive fusion schemes). The pulses are temporally incoherent and, with pulse durations of several nanoseconds, significantly longer than bandwidth-limited. Effective in preventing plasma instabilities are both the short coherence time resulting from the wide bandwidth and the ability to average out spatial intensity peaks (speckle) on the target through spectral dispersion smoothing (SSD). The cost of laser amplifiers is determined, among other things, by the cost of the laser diodes as the pump source.Therefore, a long lifetime of the upper laser level in the laser medium is helpful because the pump pulses can then be longer and a lower pump power needs to be installed. The requirements for an IFE driver laser are not yet being met simultaneously. The resulting requirements for the laser medium are in conflict because a large gain bandwidth, a long lifetime, and a large emission cross-section (for efficient extraction) cannot be achieved at the same time. The product of these three parameters is physically limited, as described by the Füchtbauer-Ladenburg and Judd-Ofelt equations. For example, the laser / gaining medium Yb:YAG at cryogenic temperatures (approx. 160 K) has a sufficient emission cross-section for efficient extraction and a long lifetime of 1 ms, but a limited gain bandwidth of approximately 2 nm. Nd:glass has a wider bandwidth but only a lifetime of 0.3 ms. State of the art To achieve a wide bandwidth for an IFE driver laser, broadband laser media can be used, but these are associated with a small emission cross-section (lower extraction efficiency) and / or shorter lifetime. Nd:glass (e.g., Schott APG-1) can be considered broadband compared to Yb:YAG and supports a sufficient bandwidth for an indirect-drive fusion scheme. However, its lifetime is approximately three times shorter than that of Yb:YAG. Other broadband media include Yb:YLF and Nd:CaF2, but for these, either the emission cross-section is too small for efficient extraction (Yb:YLF) or the lifetime is also three times shorter than that of Yb:YAG (Nd:CaF2). It is also possible to combine several laser media with slightly different wavelengths in one amplifier, e.g., two different Nd-doped glasses (phosphate and silicate glass), as described in E. Gaul et al., “Demonstration of a 1.1 petawatt hybrid OPCPA-Nd:glass laser,” ASSP 2009, WD1(2009). However, the resulting increase in bandwidth effectively reduces extraction efficiency because only a spectral portion of the laser pulse saturates the gain for each of the laser media involved, while the fluence of the entire pulse is limited by the damage threshold. In D. Eimerl et al., “StarDriver: a flexible laser driver for inertial confinement fusion and high energy density physics,” Journal of Fusion Energy 33, 476-488 (2014), laser pulses with different wavelengths from different laser media are superimposed side by side in an aperture to increase the bandwidth. However, this method is limited by the availability of laser media with different wavelengths and suitable properties. In C. Dorrer, “Optical parametric amplification of spectrally incoherent pulses,” JOSA B 38, 792-804 (2021), a technique is described in which a nonlinear crystal is pumped with a laser pulse (with a small bandwidth) in an optical parametric amplifier (OPA). This laser pulse overlaps temporally with the seed pulse within the crystal. While this allows for a large bandwidth, the overall efficiency of such a system is lower than that of a laser amplifier. In another known technique, the pulse phase is modulated by an electro-optic modulator (EOM), thereby imprinting sidebands that increase the bandwidth. This technique is used, for example, in the driver lasers at the NIF, as described in ML Spaeth et al., “Description of the NIF laser,” Fusion Science and Technology 69, 25–145 (2016). The bandwidth is limited to approximately 200 GHz. Modulation is only possible at low power in the front end of the amplifier chain. Therefore, it does not allow a bandwidth greater than the gain bandwidth of the main amplifier. Stimulated Raman scattering (SRS) allows the nonlinear generation of new spectral components at frequency intervals determined by the Raman-active medium. For IFE driver lasers, SRS was demonstrated in a gas for frequency-tripled pulses in X. Wang et al., “Ultraviolet spectral broadening by SRRS on nitrogen pumped with signal laser injection,” (2025), achieving a bandwidth of 1%. In this process, power is only converted to long-wavelength spectral components; it is therefore lossy. Limiting factors are the conversion efficiency and a degradation of beam quality. Nonlinear spectral broadening via self-phase modulation (SPM), for example through the Kerr effect, is an established method for increasing the bandwidth of bandwidth-limited ultrashort pulses beyond the gain bandwidth of the laser medium, thereby achieving even shorter pulses. However, this method is currently only practically applicable to (nearly) bandwidth-limited pulses, as will be explained in more detail below. The nonlinear phase accumulated in the nonlinear medium by the Kerr effect (quantified as the B-integral) is limited by the associated self-focusing. The frequency shift due to SPM, and thus the spectral broadening, is determined by the temporal slope of the pulse intensity.For a bandwidth-limited pulse, the temporal slope is large, and the frequency shift at a B-integral of a few π is greater than the starting bandwidth, allowing for a large factor for spectrum broadening (>10). However, for a temporally incoherent (or chirped) pulse, which is significantly longer than the bandwidth limit and has a correspondingly smaller temporal slope, the broadening is considerably smaller and negligible for many applications. Therefore, SPM technology is seemingly unsuitable for spectral broadening of a temporally incoherent IFE driver laser to achieve a large bandwidth. Even if a temporally incoherent pulse (e.g., a flat-top profile) exhibits steep temporal edges that would result in a large frequency shift, this shift only occurs in the region of the edges. A broadening of the spectrum across the entire pulse is not possible. The object of the present invention is therefore to provide a method and an arrangement for the spectral broadening of temporally incoherent or partially coherent optical pulses having a pulse duration significantly exceeding the bandwidth limit. The method and the arrangement are intended to be particularly suitable for the significant spectral broadening of the temporally incoherent or partially coherent optical pulses of driver lasers for laser-driven inertial fusion. Description of the invention The problem is solved by the method and arrangement according to claims 1, 2 and 6. Advantageous embodiments of the method and arrangement are the subject of the dependent claims or can be found in the following description and the exemplary embodiments. The proposed method spectrally broadens temporally incoherent or partially coherent optical pulses of a pulsed laser beam that have a pulse duration not limited by bandwidth, i.e., a pulse duration above the bandwidth limit of the pulses. Partial coherence means that the pulses are not coherent over their entire duration. In a first, preferred alternative of the method, the pulsed laser beam is initially split into at least two partial beams, which are then spectrally shifted relative to each other by a frequency difference Δf. The spectrally shifted partial beams are then at least partially recombined such that they interfere in the time domain during or after the superposition and form at least one modulated pulsed laser beam whose pulses exhibit a temporal modulation of intensity with a period Δt = 1 / Δf.Temporal interference during superposition requires that the pulses of the two partial beams exhibit no mutual delay during superposition. If a mutual delay does occur during superposition, this delay is compensated for by an optical arrangement or element in the beam path of the superimposed partial beams, so that temporal interference only occurs further along the beam path of the superimposed partial beams, i.e., after this compensation of the mutual delay. The partial beams are amplified in one or more optical amplifiers before or after superposition and before the formation of the modulated pulsed laser beam. Finally, the pulses of the modulated pulsed laser beam are spectrally broadened by self-phase or cross-phase modulation in an optically nonlinear medium. In the second alternative of the method, the pulsed laser beam is superimposed with at least one other pulsed laser beam with temporally coherent optical pulses that are spectrally shifted by a frequency difference Δf relative to the temporally incoherent or partially coherent optical pulses. The superposition is again carried out such that the pulsed laser beams interfere in the time domain during or after the superposition and form at least one modulated pulsed laser beam whose pulses exhibit a temporal modulation of intensity with a period Δt = 1 / Δf. In this alternative as well, the pulsed laser beams are amplified in one or more optical amplifiers before or after the superposition and before the formation of the modulated pulsed laser beam, and the pulses of the modulated pulsed laser beam are spectrally broadened by self-phase or cross-phase modulation in an optically nonlinear medium. The proposed arrangement comprises at least one laser source, a beam splitting device, a spectral shifting device, a superposition device, an at least partially dispersive optical arrangement, one or more amplifiers, and a self-phase or cross-phase modulation device with an optically nonlinear medium. The laser source generates the pulsed laser beam with temporally incoherent or partially coherent optical pulses. The beam splitting device is arranged and configured to split the pulsed laser beam generated by the laser source into at least two partial beams. The spectral shifting device is arranged and configured to generate a mutual spectral shift of the two partial beams by a frequency difference Δf. The superposition device then at least partially recombines the spectrally shifted partial beams.The at least partially dispersive optical arrangement serves to adjust the spectral phase of the two partial beams and introduce a time delay. It is arranged and configured such that the two partial beams interfere either during or after superposition, forming at least one modulated pulsed laser beam whose pulses exhibit a temporal modulation of intensity with a period Δt = 1 / Δf. The at least partially dispersive optical arrangement can also consist of several components, one or more of which may be located in one of the partial beams and one or more of which may also be located in the beam path of the superimposed partial beams. The one or more amplifiers are arranged to amplify the two partial beams before or after superposition and before the formation of the modulated pulsed laser beam.In the self-phase or cross-phase modulation device, the pulses of the modulated pulsed laser beam are then spectrally broadened by self-phase or cross-phase modulation. In the proposed method, the pulsed laser beam, or each pulse of the pulsed laser beam, is split into two parts, for example, each with half the energy. One part is then spectrally shifted by a frequency difference Δf and subsequently superimposed with the unshifted part of the pulse. If the two parts have the same or nearly the same spectral phase, i.e., do not experience different dispersion or are delayed relative to each other, they interfere in the time domain, resulting in a modulation with a period Δt = 1 / Δf, which is given by the inverse of the frequency shift. This modulation occurs independently of the coherence properties of the input pulse. The combined pulse remains incoherent in the sense that, beyond the coherence time, there is no fixed phase relationship between different times within the pulse, and the entire spectrum is present at all times within the pulse.The coherence time is given by the inverse bandwidth of the pulse. There is also no fixed phase relationship between different frequencies of the original pulse spectrum. However, the two spectrally shifted parts of the input pulse are coherent with each other because they are derived from one another by a frequency shift, which enables temporal interference. The frequency shift can be larger than the bandwidth of the incoherent input pulse, resulting in two separate spectra, or it can be only a fraction of the bandwidth, causing the spectra to overlap. The frequency shift is preferably chosen such that, on the one hand, a sufficiently large B-integral (>π) is obtained and, on the other hand, no self-focusing occurs in the nonlinear medium. Instead of spectrally shifting one part of the input pulse relative to the other, it is also possible to spectrally shift both parts of the input pulse relative to each other to obtain the desired frequency difference Δf between the two parts. The pulses can also be split into more than two parts, each of which is then spectrally shifted by different frequency differences Δf. As with the splitting into two parts, all parts are then superimposed with the same or nearly the same spectral phase. As mentioned above, it is also possible to superimpose a coherent pulse from another laser with a suitable spectral phase onto the temporally incoherent input pulse in order to obtain the desired modulation with the period Δt = 1 / Δf. Temporal modulation is only required for the nonlinear spectral broadening achieved by SPM (or XPM) in the nonlinear medium downstream of an amplifier. Interference should not occur within the amplifier itself because the doubled intensity would more strongly limit the fluence within the amplifier due to the distortion threshold, and because SPM would generate unamplified frequencies within the amplifier due to a finite B-integral. The proposed method and associated setup achieve a large spectral broadening through self- or cross-phase modulation for optical pulses that are significantly longer than the bandwidth limit and therefore exhibit a lower temporal intensity slope, thanks to the modulation and the resulting steep edges in the pulse time profile. Without the proposed method, such optical pulses would only achieve a small spectral broadening through self- or cross-phase modulation at a limited B-integral. The pulses can be longer than the bandwidth limit for various reasons, such as a large spectral phase (chirp), i.e., an instantaneous frequency that varies over the pulse, or temporal incoherence (partial coherence), where the entire spectrum can be present in the pulse at all times. The proposed method and the associated arrangement enable the use of laser media with lower gain bandwidth for an IFE driver laser with temporally incoherent pulses, which have other advantageous properties, and thus simultaneously provide a large bandwidth, high efficiency and acceptable costs. Spectral shifting can be achieved in various ways. For smaller shifts up to a few tens of GHz, acousto-optical frequency shifters (AOFS) with potentially multiple passes can be used. For larger shifts, nonlinear processes such as sum frequency generation (SFG), difference frequency generation (DFG), or four-wave mixing (FWM) with single-frequency lasers of suitable wavelengths can be employed. Raman scattering in a medium with a sufficiently narrowband Raman peak is also possible. When generating and amplifying the shifted spectra, a difference in spectral phase for both components must be avoided to achieve time-domain interference and complete modulation. If this is not possible, a difference in spectral phase can be subsequently compensated for or even pre-compensated using suitable dispersive elements (dispersion management). For dispersion management, arrangements of one or more dispersive mirrors (dielectric mirrors), gratings, or prisms can be used, for example. Interference can also be eliminated by a delay between the two components. A delay greater than the coherence time, i.e., the inverse bandwidth (e.g., a few picoseconds), is sufficient for this purpose. A delay equal to the pulse duration, which can be orders of magnitude larger (e.g., a few nanoseconds), is not required. Switching temporal interference on and off can be achieved, for example, using suitable dielectric mirrors. In the case of separated spectra, this can impose a relative delay of a few picoseconds. While a delay between the two components is not easily achievable in the case of overlapping spectra, eliminating interference is possible through a sufficiently large variation in the spectral phase across the spectrum. While the delay must be controlled to a fraction of the coherence time, interferometric stabilization to a fraction of the wavelength is not necessary.Although the phase of the temporal modulation shifts due to corresponding path differences, the spectrum generated by SPM or XPM does not. The proposed method and the associated arrangement can be used particularly advantageously for driver lasers for laser-driven inertial fusion, but also for secondary sources in which a laser pulse interacts with a plasma. Brief description of the drawings The proposed method and the associated arrangement are briefly explained below with reference to exemplary embodiments and the drawings. Figure 1 shows an example of the temporal interference of two spectrally shifted parts of a temporally incoherent pulse with the same spectral phase; Figure 2 shows an example of the nonlinear spectral broadening of temporally incoherent pulses according to the proposed method, on the left with a spectral shift Δf greater than the pulse bandwidth δf, and on the right with a spectral shift Δf by a fraction of the pulse bandwidth; Figure 3 shows a first example of an embodiment of the proposed arrangement for the nonlinear spectral broadening of temporally incoherent optical pulses; and Figure 4 shows a second example of an embodiment of the proposed arrangement for the nonlinear spectral broadening of temporally incoherent optical pulses. Ways to implement the invention In the proposed method, the pulsed laser beam with its temporally incoherent, non-bandwidth-limited optical pulses is split into two partial beams, each with the same energy. One partial beam is spectrally shifted by a frequency difference Δf and then superimposed with the spectrally unshifted partial beam such that the two partial beams have the same spectral phase, either directly at superposition or subsequently after passing through a dispersive array. This causes them to interfere in the time domain, resulting in a modulation with a period Δt = 1 / Δf, which is given by the inverse of the frequency shift Δf. Figure 1 shows, in the left-hand subfigure, two pulses of bandwidth Nf (spectral intensity If versus frequency f) that are spectrally shifted relative to each other by a frequency difference Δf. Since the two pulses were obtained by splitting them from the same temporally incoherent input pulse, they are coherent with each other and interfere in the time domain during the superposition described above, i.e., when the delay between the two pulses is zero. This results in a modulation of the intensity of the pulse formed by the superposition, as shown in the right-hand subfigure of Figure 1 (intensity I versus time t). For a delay greater than 1 / Δf, the interference disappears (dashed line). The period of the modulation is given by the spectral shift (Δt = 1 / Δf). For comparison, the bandwidth-limited pulse for this spectrum is also shown in this subfigure (filled area).In contrast, the pulse duration τ of the non-bandwidth-limited incoherent pulse can be significantly larger (e.g., by a factor of 1000) and exhibit a correspondingly large number of intensity maxima (only 31 in the partial figure of Fig. 1). Switching off the interference by a delay >1 / δf between the two pulses causes a deviation of the pulse shape from that of a bandwidth-limited pulse in this example, as can be seen in the figure (visible at the edges). This effect is considerably weaker if the pulse duration τ of the non-bandwidth-limited pulse is larger by a greater factor than the pulse duration of the bandwidth-limited pulse. The temporal modulation can then be switched on and off by a suitable delay without significantly altering the pulse shape.The temporally modulated pulse generated by the proposed method exhibits a large temporal increase in intensity due to the modulation and therefore (compared to the input pulse without modulation) experiences a stronger nonlinear spectral broadening due to SPM when passing through a nonlinear medium, for example, quartz glass. Nonlinear spectral broadening is achieved by superimposing a time-modulated phase according to temporal interference. This creates sidebands spaced at the frequency shift, as illustrated in Fig. 2. The number (and relative intensity) of the sidebands depends on the B-integral. In the case of overlapping spectra, the generated sidebands also overlap, resulting in a continuously broadened spectrum. With a B-integral of approximately 2π, which is achievable for high-energy pulses using state-of-the-art technology, considerable spectral broadening can be achieved with the modulated pulses. In Fig.Figure 2 shows the nonlinear spectral broadening of a time-modulated pulse (spectral intensity If versus frequency f) according to the present method, exemplified by a case where the spectral shift Δf is larger than the bandwidth of the pulse δf (separated spectra, here Δf / δf = 4). The right-hand figure shows the nonlinear spectral broadening exemplified by a case where the spectral shift Δf is only a fraction of the bandwidth (overlapping spectra, here Δf / δf = 0.6). The dashed lines show the spectra before SPM, and the solid lines show the spectra after SPM with B-integral B = 2π and 3π, respectively. The broadened spectra are calculated under the assumption that the B integral, and thus the phase shift of the temporal modulation, is constant over the pulse (of arbitrary duration). Spectral broadening through SPM allows for a wider pulse bandwidth than supported by the amplifier. This enables the selective use of either a larger bandwidth compared to a system without SPM, or the same bandwidth with a laser medium that has a smaller gain bandwidth but a longer lifetime, a larger emission cross-section, and potentially other advantageous properties. For example, the proposed method and arrangement compensate for the disadvantage of the cryogenic Yb:YAG gain medium, which has an approximately 5x smaller gain bandwidth than Nd:glass (APG-1). This allows it to leverage its advantages of a 3x longer lifetime, higher Stokes efficiency, lower heating power, and higher thermal conductivity. Consequently, a bandwidth sufficient for an indirect-drive fusion scheme is achievable.To amplify two spectra with shifted frequencies, Yb:YAG can be combined with Yb:LuAG, which has very similar properties and a slightly shorter central wavelength. If the bandwidth (after amplification) of the two pulses or partial beams is 0.5 nm and their spectral shift is 0.3 nm, a bandwidth of 4 nm (B = 2π) or 6 nm (B = 3π) can be achieved according to SPM. The spectrally shifted pulses or partial beams can be amplified in separate amplifiers and only then directed to a beam splitter, where the halves of the pulses at each output are superimposed and temporally interfere, as illustrated in Fig. 4. The fact that two beamlines are coupled here is not a limitation for a fusion power plant, which requires hundreds of beamlines and is operated from the same front end.It is also possible to superimpose the spectrally shifted pulses from two amplifiers into a single beam by superimposing them at a polarization beam splitter with different polarizations. In the nonlinear medium, cross-phase modulation (XPM) then occurs instead of self-phase modulation. A direct-drive fusion scheme requires an even greater bandwidth. This could involve superimposing multiple amplifiers with different wavelengths. In contrast, generating the sidebands via SPM allows for the creation of a large number of frequencies with a single array requiring only two amplifiers with different frequencies. Suitable laser media with a suitable frequency separation include, for example, Yb:YAG and Yb:YGG (cryogenic) with a 5.8 nm separation. It is also possible to amplify both frequencies in a single medium with two emission peaks, thus requiring only one amplifier. Examples of suitable amplification media for this application are Nd:Lu₂O₃ or Nd,Lu:CaF₂. Figures 3 and 4 show exemplary embodiments of the proposed arrangement for spectral broadening of temporally incoherent pulses in IFE driver lasers. For application in laser-based inertial fusion, the laser pulses are further processed by a frequency tripler 12 for transformation into the ultraviolet spectral range. This frequency tripler can be achieved, for example, by summing the frequencies in two LBO crystals (LiB3O5). The front end 1, for example starting from a Yb-doped fiber oscillator, provides temporally incoherent pulses in both configurations. In the arrangement 2, these pulses are first split into two components, each with half the pulse energy, by the beam splitter 3 to generate spectrally shifted pulses. The two components are then spectrally shifted relative to each other by a frequency shifting device 4. This results in two pulses with central wavelengths λ1 and λ2, which are coherent with each other and, with a suitable delay, produce a temporal modulation of the intensity. After amplification in the amplifier(s) 7, this modulation is used for spectral broadening by self-phase modulation in the nonlinear medium 10. Subsequently, the modulation is switched off again by a suitable delay before the pulses are frequency tripled. This is done in the configurations shown in Fig. 3 and Fig. 4.4. Temporal interference is eliminated by a suitable device or element 11, for example, by one or more suitably designed dielectric mirrors. Since the acceptance bandwidth of frequency tripling (THG) is limited, SPM may also be performed after THG. In this case, too, the modulation is switched off again by a suitable delay, as interference is generally not desired in the respective application (e.g., driving the fusion process). Fig. 3 shows an embodiment in which the spectra of the two pulses are completely separated by the arrangement 2 to generate spectrally shifted pulses, resulting in a single beamline at the output. Due to the separated spectra, the two pulses or partial beams can then be recombined after frequency shifting using a dichroic mirror 6. In this case, only one amplifier 7 can be used, which amplifies both wavelengths or spectra. Alternatively, instead of this embodiment shown in Fig. 3, the amplification can also take place before superposition, in which case two different amplifiers 7 are used for the two pulses or wavelengths, and the partial beams are only recombined after amplification with a dichroic mirror. The pulses are then spectrally broadened in the nonlinear medium 10 by self-phase modulation. Fig. 4 shows an embodiment in which the spectra of the two pulses still overlap after frequency shifting by the arrangement 2 for generating spectrally shifted pulses, resulting in two beamlines at the output. In this case, two amplifiers 7 must be used, which can either utilize the same laser medium or two laser media with slightly shifted spectra (e.g., Yb:YAG and Yb:LuAG). After amplification, the two pulses or partial beams are then partially superimposed again by a 50:50 beam splitter 8 to form two separate beamlines, as shown in Fig. 4. The pulses of both beamlines are then spectrally broadened in the nonlinear media 10 by self-phase modulation. In the examples shown in Figs. 3 and 4, the upper partial beam is spectrally shifted relative to the lower partial beam. The lower partial beam passes through a delay and dispersion management device 5. This device 5 adjusts the spectral phase of the unshifted pulse to the spectral phase of the shifted pulse to achieve the same spectral phase. In the example shown in Fig. 3, this device 5 also ensures a suitable delay between the two pulses so that they do not interfere temporally before superposition by the dichroic mirror 6 upstream of the amplifier 7. To generate the desired modulation, a suitable device or element 9 for switching on temporal interference must then be arranged downstream of the amplifier 7 to compensate for the delay. This could, for example, be a suitably designed dielectric mirror. In the example shown in Fig.In contrast, in device 5, the two pulses or partial beams are set up so that they already interfere temporally when superimposed by the beam splitter 8. Nonlinear spectral broadening via SPM (or XPM), even for temporally incoherent pulses, allows for a wider bandwidth than supported by the laser medium. This enables the use of media with smaller gain bandwidths, longer lifetimes, and larger emission cross-sections. This makes it advantageously possible to develop an IFE driver laser that is simultaneously broadband, highly efficient, and cost-effective. Nonlinear spectral broadening via SPM (or XPM) is a lossless process. It is highly controllable because temporal interference can also be controlled by the spectral phase. For example, it is possible to control the modulation depth of the temporal interference and to maintain a constant bandwidth across the pulse, even for pulses with a defined pulse shape and increasing intensity, as required for laser-driven inertial fusion. Reference symbol list 1 Front-end 2 Arrangement for generating spectrally shifted pulses 3 Beam splitter 4 Frequency shifting device 5 Delay and dispersion management device 6 Dichroic mirror for superposition 7 Amplifier 8 Beam splitter 9 Temporal interference switching element 10 Nonlinear medium for SPM 11 Temporal interference switching element 12 Frequency tripler (THG)

Claims

Method for the nonlinear spectral broadening of temporally incoherent or partially coherent optical pulses of a pulsed laser beam, which have a non-bandwidth-limited pulse duration, in which: - the pulsed laser beam is split into at least two partial beams, - the partial beams are spectrally shifted relative to each other by a frequency difference Δf, - the spectrally shifted partial beams are superimposed again at least partially such that they interfere in the time domain during or after the superposition and form at least one modulated pulsed laser beam, whose pulses exhibit a temporal modulation of intensity with a period Δt = 1 / Δf.- the partial beams are amplified in one or more optical amplifiers (7) before or after superposition and before the formation of the modulated pulsed laser beam, and - the pulses of the modulated pulsed laser beam are spectrally broadened by self- or cross-phase modulation in an optically nonlinear medium (10). Method for the nonlinear spectral broadening of temporally incoherent or partially coherent optical pulses of a pulsed laser beam, which have a non-bandwidth-limited pulse duration, in which the pulsed laser beam is superimposed with at least one further pulsed laser beam with temporally coherent optical pulses which are spectrally shifted by a frequency difference Δf relative to the temporally incoherent or partially coherent optical pulses, wherein the superposition is carried out such that the pulsed laser beams interfere in the time domain during or after the superposition and form at least one modulated pulsed laser beam, the pulses of which exhibit a temporal modulation of intensity with a period Δt = 1 / Δf.- the pulsed laser beams are amplified in one or more optical amplifiers (7) before or after superposition and before the formation of the modulated pulsed laser beam, and - the pulses of the modulated pulsed laser beam are spectrally broadened by self- or cross-phase modulation in an optically nonlinear medium (10). Method according to claim 1, characterized in that the shift by the frequency difference Δf is carried out by one or more acousto-optic frequency shifters, by nonlinear processes, in particular sum frequency generation, difference frequency generation or four-wave mixing with single-frequency lasers of suitable wavelength, or by Raman scattering. Method according to claim 1 or 3, characterized in that an equal or at least approximately equal spectral phase of the spectrally shifted partial beams required to form the at least one modulated pulsed laser beam is generated via a dispersive optical arrangement (5). Method according to one of claims 1, 3 or 4, characterized in that the pulsed laser beam is split into a first and a second partial beam, in particular of the same energy, of which the first partial beam does not undergo any frequency shift and the second partial beam is spectrally shifted relative to the first partial beam by the frequency difference Δf. Arrangement for the nonlinear spectral broadening of temporally incoherent or partially coherent optical pulses having a non-bandwidth-limited pulse duration, comprising: - a laser source (1) configured to generate a pulsed laser beam with the temporally incoherent or partially coherent optical pulses; - a beam splitting device (3) that splits the pulsed laser beam generated by the laser source (1) into at least two partial beams; - a spectral shifting device (2) that generates a mutual spectral shift of the two partial beams by a frequency difference Δf; - a superposition device (6, 8) with which the spectrally shifted partial beams are at least partially superimposed again; - an optical arrangement (5, 9) for adjusting a spectral phase of the two partial beams and a temporal delay, wherein the optical arrangement (5, 9) is arranged and configured such thatthat the two partial beams interfere temporally either during or after superposition and form at least one modulated pulsed laser beam whose pulses exhibit a temporal modulation of intensity with a period Δt = 1 / Δf, - one or more amplifiers (7) that amplify the two partial beams before or after superposition and before the formation of the modulated pulsed laser beam, and - a device for self- or cross-phase modulation with an optically nonlinear medium (10) in which the pulses of the modulated pulsed laser beam are spectrally broadened by self- or cross-phase modulation. Arrangement according to claim 6, characterized in that the device (2) for spectral shift is designed such that the spectra of the two partial beams do not overlap after generation of the mutual spectral shift, wherein the amplifier(s) (7) are arranged such that the partial beams are amplified either in separate amplifiers (7) before the superposition device (6, 8) or in one or more common amplifiers (7) after the superposition device (6, 8). Arrangement according to claim 6, characterized in that the device (2) for spectral shift is designed such that the spectra of the two partial beams still overlap after generating the mutual spectral shift, wherein the amplifiers (7) are arranged such that the partial beams are amplified in separate amplifiers (7) before the superposition device (6, 8). Arrangement according to one of claims 6 to 8, characterized in that an optical arrangement (11) for canceling temporal interference in the beam direction is arranged behind the self-phase or cross-phase modulation device, by which the temporal interference in the time domain in the modulated pulsed laser beam is canceled. Arrangement according to claim 9, characterized in that a device (12) for frequency tripling is arranged in the direction of the beam behind the optical arrangement (11) to eliminate temporal interference. Arrangement according to one of claims 6 to 10, characterized in that the device (2) for spectral shift is designed such that it generates the spectral shift by one or more acousto-optic frequency shifters, by nonlinear processes, in particular sum frequency generation, difference frequency generation or four-wave mixing with single-frequency lasers of suitable wavelength, or by Raman scattering.

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