DEVICE FOR SPECTRAL BROADENING OF A LASER PULSE AND LASER SYSTEM

Abstract

A device for spectrally broadening a laser pulse is disclosed. The device includes a multipass arrangement having a convex mirror and a concave mirror, the convex mirror and the concave mirror being arranged relative to each other such that a laser pulse coupled into the multipass arrangement is reflected at least once from the concave mirror to the convex mirror and at least once from the convex mirror to the concave mirror. Further, the device includes a nonlinear optical medium arranged at least partially within the multipass arrangement such that the laser pulse coupled into the multipass arrangement passes through the nonlinear optical medium multiple times. The disclosure also relates to a laser system having a device according to the disclosure for spectral broadening of a laser pulse.

Claims

1. A device for spectrally broadening a laser pulse, the device comprising: a multipass arrangement having a convex mirror and a concave mirror, the convex mirror and the concave mirror being arranged relative to each other such that a laser pulse coupled into the multipass arrangement is reflected at least once from the concave mirror to the convex mirror and at least once from the convex mirror to the concave mirror; and a nonlinear optical medium arranged at least partially within the multipass arrangement such that the nonlinear optical medium is passed through multiple times by the laser pulse coupled into the multipass arrangement.

2. The device according to claim 1, wherein the nonlinear optical medium is passive.

3. The device according to claim 1, wherein the device is passive.

4. The device according to claim 1, wherein the multipass arrangement is configured such that the laser pulse coupled into the multipass arrangement is reflected multiple times, optionally more than ten times, from the concave mirror to the convex mirror and multiple times, optionally more than ten times, from the convex mirror to the concave mirror.

5. The device according to claim 1, wherein the multipass arrangement is configured such that the laser pulse coupled into the multipass arrangement is reflected from the concave mirror directly to the convex mirror and from the convex mirror directly to the concave mirror.

6. The device according to claim 1, wherein the multipass arrangement further comprises one or more deflection mirrors.

7. The device according to claim 1, wherein the nonlinear optical medium comprises a solid medium and/or a gaseous medium.

8. The device according to claim 7, wherein the solid-state nonlinear optical medium is formed at least partially of sapphire and/or SiC and/or fused silica and/or diamond.

9. The device according to claim 7, wherein the device is arranged in a pressure chamber and/or is formed as a pressure chamber and wherein the gaseous medium is provided in the pressure chamber.

10. The device according to claim 1, wherein the device and/or the multipass arrangement comprises at least one dispersive optical element configured to at least partially compensate or overcompensate for spectral dispersion caused in the nonlinear optical medium.

11. The device according to claim 10, wherein the dispersive optical element is formed as a dispersive coating of the concave mirror and/or the convex mirror, which is configured to at least partially compensate or overcompensate for spectral dispersion caused in the nonlinear optical medium.

12. The device according to claim 1, wherein the concave mirror and/or the convex mirror comprise a recess for coupling the laser pulse into the multipass arrangement and/or for coupling the laser pulse out of the multipass arrangement.

13. The device according to claim 1, wherein the multipass arrangement comprises or is configured as a Herriott cell.

14. A laser system comprising the device for spectrally broadening a laser pulse according to claim 1.

15. A method of spectrally broadening a laser pulse, the method comprising: providing a multipass arrangement having a convex mirror and a concave mirror for spectral broadening of a laser pulse, in which the convex mirror and the concave mirror are arranged relative to one another such that a laser pulse coupled into the multipass arrangement is reflected at least once from the concave mirror to the convex mirror and at least once from the convex mirror to the concave mirror and the laser pulse propagates for spectral broadening through a nonlinear optical medium arranged in the multipass arrangement.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] The disclosure will now be described with reference to the drawings wherein:

[0046] FIG. 1 shows a schematic representation of a conventional device for spectral broadening of a laser pulse with a conventional related art multipass arrangement 20;

[0047] FIG. 2 shows a schematic representation of a device for spectral broadening of a laser pulse according to an exemplary embodiment of the disclosure;

[0048] FIG. 3 shows a schematic representation of a multipass arrangement according to another exemplary embodiment;

[0049] FIG. 4A shows a schematic explanation for the stability criteria of a concave-convex multipass arrangement;

[0050] FIGS. 4B to 4E show exemplary courses of reflection point curves on a mirror surface for different values of parameters;

[0051] FIG. 5 shows in a diagram the course of the beam radius along the propagation length through the Herriott cell or multipass arrangement;

[0052] FIG. 6 shows in a graph the beam radius and the cumulative B-integral versus the propagation length through the multipass arrangement;

[0053] FIG. 7 shows the temporal power curve of the simulated laser pulse after spectral broadening and compression and the simulated spectrum after spectral broadening and compression;

[0054] FIG. 8 shows the measured spectrum and the output spectrum determined by means of a FROG measurement after spectral broadening with the concave-convex device according to the exemplary embodiment;

[0055] FIG. 9 shows the measured spectrum and the output spectrum determined by means of a FROG measurement after spectral broadening with a conventional concave-concave Herriott cell;

[0056] FIG. 10 shows the calculated spectral overlap for both axes after broadening with the concave-convex device according to the exemplary embodiment;

[0057] FIG. 11 shows the calculated spectral overlap for both axes after broadening with the conventional concave-concave Herriott cell; and

[0058] FIG. 12 shows a schematic diagram of a laser system 200 according to an exemplary embodiment.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0059] In the drawings, the same or similar elements in the various exemplary embodiments are designated with the same reference signs for the sake of simplicity. The terms laser beam and laser pulse are used as synonyms, since pulsed laser radiation is also to be described in terms of the optical path in the form of a laser beam.

[0060] FIG. 1 shows a schematic diagram of a conventional device 10 for spectral broadening of a laser pulse with a conventional concave-concave multipass arrangement 20 according to the related art, which is designed as a Herriott cell (HC). This conventional multipass arrangement 20 has two concave mirrors 21 and 22, which are arranged relative to each other in such a way that an in-coupled laser beam is reflected between the two concave mirrors 21 and 22. Due to the concave shape of the reflecting surfaces of the mirrors 21 and 22, the laser beam is focused, whereby the focal plane is arranged centrally between the two mirrors 21 and 22. Accordingly, the laser beam 40 has the largest beam diameter at the reflection surfaces of the mirrors 21 and 22 and the smallest beam diameter in the focal plane.

[0061] Furthermore, the device 10 has a nonlinear optical medium 30, which is in solid state form. The nonlinear medium 30 is arranged in the focal plane, since this is where the smallest beam diameter and thus the greatest intensity of the laser pulses prevail, which is decisive for the nonlinear optical effects and in particular for the spectral broadening.

[0062] In addition, the device 10 has an in-coupling and out-coupling mirror 23 by means of which a laser beam 40 can be coupled into and out of the multipass arrangement 20.

[0063] The optical path of the laser beam 40 is drawn by means of an exemplary line and shows that the laser beam 40, after coupling into the multipass arrangement 20, travels around the multipass arrangement 20 several times before the laser beam 40 is coupled out again by the in-coupling and out-coupling mirror 23. In this process, the laser pulse passes through the nonlinear medium 30 in the focal plane, in which the desired nonlinear optical processes for spectral broadening take place, after each reflection at the mirrors 21 and 22, i.e., twice per complete circulation.

[0064] While high intensities are required in the nonlinear optical medium 30 in the focal plane, the intensity of the laser pulses must be significantly lower at the reflection surfaces of the mirrors 21 and 22 to ensure that the destruction threshold of mirrors 21 and 22 is not exceeded. For this purpose, the laser beam must have a sufficiently large beam diameter at the reflection surfaces of the mirrors 21 and 22, which is achieved by a sufficiently large distance of the mirrors from the focal plane and a correspondingly large focal length of the mirrors 21 and 22. This is accompanied by the fact that the diameters of the concave mirrors 21 and 22 must also be selected to be correspondingly large. In the illustration shown, the mirror distance d.sub.0 corresponds to the sum of the focal lengths of the mirrors 21 and 22, which in the example shown is d.sub.0/2 in each case.

[0065] Since a large focus is typically desired in the focal plane to spectrally broaden high-intensity laser pulses, concave mirrors with a long focal length are required. Smaller focal lengths would result in a smaller mode size in the focus, increasing undesirable effects in the nonlinear optical medium 30. The consequence of the large focal lengths to be selected accordingly is that the distance d.sub.0 must be chosen to be correspondingly large in order to ensure a sufficient beam diameter on the reflection surfaces of the mirrors 21 and 22. This brings with it the disadvantage that the multipass arrangements 20 according to the related art usually have very large spatial dimensions, in particular a large length, which is not infrequently several meters. This can be a major challenge for the use in laser systems, especially for industry, with regard to the space requirements of the laser system.

[0066] FIG. 2 shows a schematic representation of a device 100 for spectral broadening of a laser pulse according to an exemplary embodiment of the disclosure. The device 100 has a multipass arrangement 120 comprising a concave mirror 121 and a convex mirror 122. The concave and convex mirrors 121, 122 are thereby arranged relative to each other such that a laser beam 140 coupled into the multipass arrangement 120 is reflected several times between the two mirrors 121, 122 before the laser beam is coupled out of the multipass arrangement 120 again. For coupling the laser beam 140 into and out of the multipass arrangement 120, the concave mirror 121 has an in-coupling and out-coupling aperture 123 through which the laser beam 140 can pass during in-coupling and out-coupling to enter or leave the multipass arrangement 120 accordingly.

[0067] In addition, the device 100 includes a nonlinear optical medium 130 arranged in the multipass arrangement 120. The nonlinear optical element is arranged apart from the center of the multipass arrangement 120 and is located close to the convex mirror 122, since the laser beam 140 has a smaller diameter there than at other positions in the multipass arrangement 120, which are closer to the concave mirror 121. The nonlinear optical medium 130 is thereby arranged and formed in such a way that the laser beam 140 passes through the nonlinear optical medium 130 after each reflection, i.e., twice per circulation in the multipass arrangement 120. For this purpose, it may be advantageous if the nonlinear medium 130 has approximately a similar lateral extent as the convex mirror 122 to ensure that the laser beam passes through the nonlinear optical medium 130 in all circulations. According to the exemplary embodiment shown, the nonlinear medium 130 is in solid state form. According to other exemplary embodiments, the device 100 may additionally or alternatively comprise a gaseous nonlinear medium. For this purpose, for example, the multipass arrangement 120 or the device 100 may be formed as a pressure chamber which can be filled with a suitable gas at the desired pressure.

[0068] As can be seen in FIG. 1, the distance d.sub.0 of the two mirrors 121 and 122 differs from the focal length f1 of at least the concave mirror 121 and optionally also from the focal length f2 (see FIG. 4A) of the convex mirror 122. Here, the focal length f1 of the concave mirror 121 is longer than the distance of the concave mirror 121 from the convex mirror, so that the focal plane F1 of the concave mirror 121 lies outside the multipass arrangement 120. Since the convex mirror 122 is a diverging mirror, its focal plane or focus (not shown) lies outside the multipass arrangement 120. As a result, the laser beam is not focused within the multipass arrangement 120 and, consequently, undesirable effects such as exceeding the destruction threshold of the mirrors 121 and 122, ionization of air or other gases in the multipass arrangement 120, and critical self-focusing can be easily avoided. Nevertheless, in order to achieve a sufficiently high B-integral and an associated desired spectral broadening of the laser pulse, the nonlinear optical medium 130 can be adapted with respect to its nonlinear refractive index and/or its thickness and/or the number of circulations of the laser beam 140 in the multipass arrangement 120 can be increased compared to conventional concave-concave multipass arrangements 20.

[0069] To achieve control of the dispersion of the laser pulse already in the multipass arrangement 120, the mirrors 121 and 122 can each be provided with an optional dispersive dielectric coating 150 on their reflection surface. This can be designed in such a way that the dispersion which the laser pulse collects during propagation through the nonlinear optical medium 130 is at least partially compensated. Optionally, the dispersion can also be overcompensated, for example to achieve self-compression of the laser pulse. For example, the dispersive coating(s) 150 may be configured to at least partially compensate for the GDD and TOD that the laser pulse collects in the nonlinear optical medium 130. In other exemplary embodiments, only one of the mirrors 121 and 122 may have such a dispersive coating 150. According to further exemplary embodiments, neither of the mirrors 121 and 122 may comprise a dispersive coating. Optionally, the device 100 or a laser system using the device 100 may include dispersive optical elements (not shown), such as dispersive dielectric mirrors, to control and/or compensate for dispersion elsewhere.

[0070] FIG. 3 shows a schematic representation of a multipass arrangement 120 according to a further exemplary embodiment. This multipass arrangement 120 differs from the multipass arrangement 120 shown in FIG. 2 in that it has a deflection mirror 124 in addition to the concave mirror 121 and the convex mirror 122. The multipass arrangement 120 is constructed in such a way that the laser beam 140 is reflected from the concave mirror 121 to the deflection mirror 124 and via the deflection mirror 124 to the convex mirror 122. On the return path of the circulation of the laser beam 140 from the convex mirror 122 to the concave mirror, a deflection also takes place by the deflection mirror 124. According to the exemplary embodiment shown, the concave mirror has a significantly larger diameter than the convex mirror and also has a recess 125 through which the laser beam 140 can pass through the concave mirror 121. In this case, the convex mirror 122 is arranged behind the concave mirror 121 so that the laser beam 140 passing through the recess 125 can strike the convex mirror and be reflected back by the convex mirror through the recess 125. According to other exemplary embodiments (not shown), the convex mirror may also be arranged in front of the concave mirror.

[0071] The multipass arrangement 120 according to this exemplary embodiment is similar in design to a Cassegrain telescope. The structure of the multipass arrangement 120 shown offers the advantage that the spatial extent of the multipass arrangement 120, in particular its length, can be reduced by deflecting the laser beam 140, and the multipass arrangement 120 can therefore be designed to save space. This can be advantageous, in particular, for use in laser systems that have a severely limited amount of space. Furthermore, this offers the advantage that despite the reduced spatial dimensions, the path length of the optical path of the laser beam in the multipass arrangement 120 can be maintained or even increased. Furthermore, the exemplary embodiment offers the advantage, in particular compared to the concave-concave Herriott cells known from the prior art, that the beam diameter of the laser beam 140 at the deflection mirror has a sufficient size and, in particular, is larger than on the convex mirror 122 and, therefore, there is no need to fear exceeding the destruction threshold of the deflection mirror.

[0072] FIG. 4A shows a schematic explanation for the stability criteria of a concave-convex multipass arrangement 120 as known from geometrical optics. The multipass arrangement 120 shown in FIG. 4A has a concave mirror 121 whose reflecting surface has a radius of curvature R.sub.1, which is shown by means of an arrow and a corresponding circumference 1001. Furthermore, the multipass arrangement 120 has a convex mirror 121 whose reflection surface has a radius of curvature R.sub.2. Since the reflection surface of the convex mirror 122 is curved outward, the center of the circumference 1002 with radius of curvature R2 is located behind the convex mirror 122. The concave-convex multipass arrangement 120 is then considered stable, in the sense of a stable resonator which allows a plurality of circulations of a laser beam between the mirrors before the laser beam is decoupled from the resonator or from the multipass arrangement 120, when the concave mirror 121 and the convex mirror 122 are spaced apart from each other such that the circumferential circles 1001 and 1002 spanned by their radii of curvature R.sub.1 and R.sub.2 intersect and form an overlap. This is the case in the configuration shown, as the two circumferential circles 1001 and 1002 intersect at points 1003. In full three-dimensional view, these are not merely points of intersection, but rather a corresponding circle of intersection, which, however, can be seen in the two-dimensional projection shown as merely two points of intersection. The dashed line 1004 marks the mode volume of the resonator or of the multipass arrangement 120, in which the resonant beam trajectories in the multipass arrangement 120 propagate. Beams outside the mode volume leave the multipass arrangement 120 and therefore do not propagate resonantly in the multipass arrangement 120.

[0073] In the following, specific examples of devices for spectral broadening of a laser pulse according to exemplary embodiments of the disclosure and in particular concave-convex multipass arrangements are explained, without, however, limiting the disclosure to these examples. The exemplary embodiments are also partially characterized and compared to a conventional related art concave-convex Herriott cell.

Example 1

[0074] In the following, a specific example of a device 100 for spectral broadening of a laser pulse with a multipass arrangement 120 according to an exemplary embodiment is explained in detail and compared with a conventional related art Herriott cell.

[0075] For comparison with the prior art, we use a conventional Herriott cell, which is known and described in the prior related (M. Kaumanns et al., “Multipass spectral broadening of 18 mJ pulses compressible from 1.3 ps to 41 fs,” Optics letters, vol. 43, no. 23, pp. 5877-5880, 2018, doi: 10.1364/OL.43.005877). This comprises a multipass arrangement with two spherical concave mirrors, each with a radius of curvature of −1,5 m and spaced d.sub.0=2,98 m apart. This multipass arrangement is designed in such a way that an in-coupled laser pulse passes through N=23 full circulations in the multipass arrangement before the laser beam is coupled out from the multipass arrangement again and completes 45 revolutions through the nonlinear optical medium. According to the example shown, the parameter M has the value M=22.

[0076] The multipass arrangement is constructed in the manner of a Herriott cell, i.e., the reflection points at which the laser beam is reflected on the cell mirrors lie on a circle or an ellipse. For a given number N of reflections on the cell mirror, several different cell configurations are possible, which differ in the order of the reflections. The parameter M−1 indicates how many neighboring reflection points lie between two temporally successive reflection points, i.e., how many reflection points are “jumped over.” An alternative consideration is how many full circles/ellipses are described by the reflection point pattern on the cell mirrors. The ratio of M and N indicates the stability and thus do as well as the mode size in the Herriott cell.

[0077] FIGS. 4B to 4E show exemplary reflection patterns for different values of the parameters N and M on a mirror surface and designate some angles of the reflection points. The curves of the reflection points are shown for N=6 and N=7. The reflection patterns in FIGS. 4B to 4E differ in the values of the parameter M, which is M=1 (FIG. 4B), M=2 (FIG. 4C), M=3 (FIG. 4D) and M=4 (FIG. 4E), respectively. It can be seen that, despite the same number of circulations (parameter N), the arrangement of the reflection points on the mirrors can differ significantly for different parameters M. The dashed line represents the circulation pattern for N=6, while the solid line represents the circulation pattern for N=7. The angles refer to a 0° position at the rightmost point at the 3 o'clock position.

[0078] This conventional device has as a limiting factor the destruction threshold of the optical coating of the concave mirrors, which was determined to be 0.25 J/cm.sup.2. To ensure reliable operation of this conventional device, the fluence to laser radiation was set to 66% of the damage threshold, i.e., 0.17 J/cm.sup.2. The eigenmode of the conventional Herriott cell has a diameter of 5.4 mm, which enables a maximum pulse energy of

[00002]$E_{\max }=\frac{0.17J}{{\mathrm{cm}}^2}.Math.{\left(0.27\mathrm{cm}\right)}^2.Math.\pi /2=18.6\mathrm{mJ}.$

[0079] Another limiting factor is gas ionization, which occurs with argon as the nonlinear optical medium used at a gas pressure of 600 mbar at a pulse energy of about 18.3 mJ. At a pulse duration of 1.3 ps, this corresponds to an intensity at the focus of approx

[00003]$2.Math.10^{13}\frac{W}{{\mathrm{cm}}^2}.$

The proportionality factor of the ionization threshold to the gas pressure was determined by measurements to be approx.

[00004]$\sim p^{-\frac{1}{2.5}}$

where p is the gas pressure of argon.

[0080] According to a first exemplary embodiment of the disclosure, which is compared with the prior art, the multipass arrangement of the device has a spherical concave mirror with a radius of curvature of R.sub.1=−11.0 m and a spherical convex mirror with a radius of curvature of R.sub.2=8.5 m, which are arranged at a distance of d.sub.0=3.07 m from each other. The multipass arrangement is designed in such a way that a coupled laser beam propagates N=23 full revolutions in the multipass arrangement before the laser beam is coupled out again.

[0081] FIG. 5 shows in a diagram the course of the beam radius in μm (vertical axis) along the propagation length through the conventional Herriott cell or multipass arrangement in mm (horizontal axis). The solid line represents the course for the conventional concave-concave Herriott cell, while the dashed line represents the course of the beam radius for the concave-convex multipass arrangement according to the exemplary embodiment. It can be seen that only the conventional HC has a highly focused eigenmode, which leads to a beam radius of less than 500 μm at about 1,500 mm propagation length. This can lead to undesirable ionization of the existing gas atmosphere. In the concave-convex multipass arrangement according to the exemplary embodiment, on the other hand, there is no strong focusing within the multipass arrangement, so that the course of the beam radius shown with the dashed line is always greater than 2,000 μm and therefore no undesirable ionization is to be feared.

[0082] Although the concave-convex multipass arrangement has approximately the same length as the conventional concave-concave HC (the difference in length is only 3%) and has a beam diameter on the convex mirror that is approximately 15% smaller than on the concave mirror, resulting in an approximately 33% increase in fluence, ionization of gas is nevertheless completely prevented in the multipass arrangement because there is no focus of the laser beam in the multipass arrangement. The lower intensity of the laser beam in the concave-convex multipass arrangement compared to the focus in the conventional concave-concave HC offers the advantage that the concave-convex device can be used for spectral broadening and compression of laser pulses with significantly larger pulse energies, especially for laser pulses of such intensities which cannot be broadened and compressed in a conventional concave-concave HC due to the limitations described above. In order to achieve an appropriate B-integral with a concave-convex multipass arrangement, which depends on the intensity of the laser pulse as explained above, for laser pulses with more moderate energies the nonlinear medium can be adapted to exhibit a correspondingly higher nonlinear refractive index. For this purpose, for example, when using a gaseous nonlinear optical medium, such as argon, the gas pressure can be increased and additionally or alternatively a solid nonlinear optical medium with a significantly higher nonlinear refractive index can be used.

Example 2

[0083] In the following, a device according to another exemplary embodiment of the disclosure is described, which is designed for spectral broadening and compression of laser pulses with a pulse energy of 0.5 J and a pulse duration of 1.3 ps (FWHM).

[0084] Conventional concave-concave HCs are per se unsuitable for such an application, since this would require length scaling, which would make such an HC inaccessible for practical use due to its considerable spatial length.

[0085] The device according to the further exemplary embodiment has a concave-convex multipass arrangement with a spherical concave mirror with a radius of curvature R.sub.1=−50.0 m, a convex mirror with a radius of curvature of R.sub.2=32 m and a mirror spacing of d.sub.0=18.35 m. The multipass arrangement is designed in such a way that a coupled laser beam remains in the multipass arrangement for N=49 circulations and M=1. Since a concave-convex multipass arrangement is used, the multipass arrangement or the optical path in the multipass arrangement can be folded by means of a deflection mirror, as shown for example in FIG. 3. The length of the multipass arrangement can thus be reduced to well below 8 m. The pulse energy of the laser pulses to be broadened is sufficiently high to use argon gas at a pressure of 1 bar as the nonlinear optical medium for the broadening, so that a cumulative B integral per passage through the nonlinear optical medium of about 2.8 can be achieved.

[0086] FIG. 6 shows in a graph with the solid line the beam radius in μm (left vertical axis) and with the dashed line the cumulative B integral in arbitrary units (right vertical line) versus the propagation length through the multipass arrangement. Since the multipass arrangement is completely filled with argon, the propagation length of the laser pulse through the multipass arrangement is equal to the propagation length through the nonlinear optical medium.

[0087] The calculated output pulses after spectral broadening and compression in the device according to the second exemplary embodiment is shown in FIG. 7. With the device according to this exemplary embodiment, the spectrum of the pulse according to the calculations is broadenable to a bandwidth of about 60 nm (FWHM), which is compressible to a pulse duration of less than 50 fs by means of a compensation of the GDD in the amount of −35,000 fs.sup.2. Non-dispersive optics in the multipass arrangement were assumed. The performance of the device can be further improved by compensating the GDD of 300 fs.sup.2 of argon per cycle. The peak intensity and peak fluence in the device thereby occur at the convex mirror with about 4-10.sup.11 W/cm.sup.2 and 0.5 J/cm.sup.2, respectively. Both values are well below the destruction threshold of the optical elements and the ionization threshold of argon.

[0088] FIG. 7 shows in the upper graph the temporal power curve of the simulated laser pulse after spectral broadening and compression and in the lower graph the simulated spectrum after spectral broadening and compression.

Example 3

[0089] In the following, an example of a device for spectral broadening of a laser pulse is explained according to another exemplary embodiment and compared with another conventional concave-concave system.

[0090] The device according to the exemplary embodiment has a concave-convex multipass arrangement with N=19, M=1, R.sub.1=−0.5 m, R.sub.2=0.25 m, and d.sub.0=0.26 m.

[0091] For comparison, a conventional Herriott cell with N=19, M=18, R.sub.1=−0.3 m, R.sub.2=−0.3 m, d.sub.0˜.sub.0˜0.596 m was used. When used for spectral broadening of laser pulses of the commercially available laser system of the type PHAROS from the manufacturer LIGHT CONVERSION with an output pulse energy of 200 μJ and a pulse duration of 270 fs, a fused silica plate with a thickness of 6.35 mm can typically be placed as a nonlinear optical medium about 50 mm away from one of the mirrors to cause a B integral of about 0.6 when propagating through the fused silica plate. In this case, the laser pulse has a high enough peak power to cause significant nonlinear effects in the ambient air. The B integral due to propagation of the laser pulse through the air is therefore about 0.7.

[0092] In the proposed device according to this exemplary embodiment, the 6.35 mm thick fused silica plate can be placed at a distance of 56 mm from the concave mirror, resulting in a B-integral of 0.6. In contrast, the B-integral due to free propagation through air is much smaller than that of the conventional Herriott cell due to the shorter optical paths and larger beam diameters, and is as low as 0.04.

[0093] Therefore, by means of a device according to the disclosure based on a concave-convex multipass arrangement, self-phase modulation in air can be dramatically reduced and almost completely avoided.

Example 4

[0094] In another experimental comparison, the spectral broadening of laser pulses was presented with another device based on a concave-convex multipass arrangement according to another exemplary embodiment and a conventional concave-concave Herriott cell.

[0095] Pulses of a commercially available laser system of the type PHAROS from the manufacturer LIGHT CONVERSION were spectrally broadened and compressed. The output pulses of the mentioned laser system before spectral broadening and compression have an average pulse energy of 15 μJ and a pulse duration (FWHM) of 266 fs with a resulting peak pulse power of 56.4 MW.

[0096] The device according to the exemplary embodiment of the disclosure has a multipass arrangement 120 in the form of a Herriott cell with a concave mirror 121 and a convex mirror 122, as shown in FIG. 2. The concave mirror has a radius of curvature R.sub.1=−250 mm and the convex mirror has a radius of curvature of R.sub.2=200 mm. The mirrors were coated in such a way that almost the entire GDD of the multipass arrangement is compensated. Here, the convex mirror has a dispersive coating with an effect of −140 fs.sup.2 and the concave mirror has only a highly reflective coating. The distance between the mirrors of the multipass arrangement has d.sub.0=114 mm and allows 19 reflections per mirror and correspondingly 38 propagations through the nonlinear medium, which is formed by a 3 mm thick fused silica plate with a diameter of 25.4 mm and anti-reflective coating on both sides. Coupling into and out of the multipass arrangement is accomplished by means of a Scarper mirror. Mode matching is performed by a Galilean beam expander. The eigenmode of the multipass arrangement is characterized by a Gaussian beam with a diameter of w.sub.1=336 μm on the concave mirror or w.sub.2=182 μm on the convex mirror. The nonlinear optical medium is located at a distance of d=110 mm from the concave mirror, where the beam has a diameter of w=193 μm.

[0097] FIG. 8 shows the measured spectrum (gray) and the output spectrum (black line) determined by means of a FROG measurement after spectral broadening with the concave-convex device according to the exemplary embodiment, where a pulse energy of 15 μJ was used for the FROG measurement. The error of the FROG measurement is 7×10.sup.−3 on a 256×256 grid. In the bottom graph, FIG. 8 shows the temporal profile obtained from the FROG measurement (black line), the temporal phase profile (dashed) and, for reference, the Fourier limit (FTL) (gray), and the integrated intensity in the main pulse (dotted).

[0098] Accordingly, the output spectrum has a bandwidth of more than 50 nm at 1/e.sup.2 of the spectral beam power. The Fourier limit of the spectrum is about 49 fs. Pulse compression is performed using six dispersive mirrors, each with −400 fs.sup.2 GDD. Transmission through the device and compression level was determined to be 91%. A pulse shortening by a factor of 5 was determined, resulting in a pulse duration of 53 fs (FWHM), as shown in FIG. 8. The FROG measurements showed that 80% of the energy was contained in the main pulse.

[0099] A nonlinear phase of about 0.5 rad was also obtained with this device, resulting in a high quality beam profile after passing through the device. To confirm this, the spectral homogeneity of the compressed beam was measured using scans along two axes of the beam. The sagittal and tangential spectra were measured in 0.2 mm increments. For each recorded spectrum I.sub.λ(λ) the overlapping portion with the central intensity spectrum was I.sub.λ(λ) was calculated according to the following formula:

[00005]$V=\frac{{\left[\int \sqrt{I_{\lambda }\left(\lambda \right)*I_{\lambda }^{\mathrm{ref}}\left(\lambda \right)}d\lambda \right]}^2}{\int I_{\lambda }\left(\lambda \right)d\lambda *\int I_{\lambda }^{\mathrm{ref}}\left(\lambda \right)d\lambda }$

[0100] The calculated spectral overlap is shown for both axes in FIG. 10. To quantify the overall spectral homogeneity, a weighted overlap with intensity was calculated with the formula V.sub.avg=ΣI*V/ΣI resulting in the values of V.sub.x=98.9% and V.sub.y=98.2%.

[0101] For comparison, the results of the equivalent measurements shown for the concave-convex device in FIGS. 8 and 10 are also shown for a conventional Herriott cell in FIGS. 9 and 11.

[0102] Here, the measured conventional concave-concave HC has a first concave mirror with a radius of curvature of −250 mm and a highly reflective coating. The second concave mirror has a radius of curvature of −200 mm and a dispersive coating with a GDD value of −140 fs.sup.2. The two concave mirrors are spaced 378 mm apart, allowing 19 reflections per mirror and 38 revolutions through the nonlinear optical medium, which is a fused silica plate with an antireflection coating on both sides and a thickness of 3 mm. The nonlinear optical medium is located at a distance of 110 mm from the second concave mirror. The eigenmode of the HC is characterized by a Gaussian beam with a diameter of w.sub.1=358 μm and w.sub.2=471 μm on the concave mirrors, respectively, and w=195 μm in the nonlinear medium.

[0103] FIG. 9 shows the measured spectrum (gray) and the output spectrum (black line) determined by means of a FROG measurement after spectral broadening with the conventional concave-concave device according to the exemplary embodiment, where a pulse energy of 15 μJ was used for the FROG measurement. The error of the FROG measurement is 6×10.sup.−3 on a 256×256 grid. In the right graph, FIG. 9 shows the temporal profile obtained from the FROG measurement (black line), the temporal phase profile (dashed) and, for reference, the Fourier limit (FTL) (gray), and the integrated intensity in the main pulse (dotted). A spectral broadening of more than 50 nm at the 1/e.sup.2 value of the spectral power was obtained. The corresponding Fourier transformed time limit (FTL) of this spectrum is about 53 fs (FWHM). The pulse was compressed to a pulse duration of 57 fs (FWHM) using a compressor arrangement with an overall compensation of −2,400 fs.sup.2. The transmittance of the HC was determined to be 90%. A pulse shortening by a factor of 5 was achieved and confirmed with the FROG measurements shown in FIG. 9.

[0104] The calculated spectral overlap is shown for both axes in FIG. 11 and shows that for the conventional concave-concave HC the values are V.sub.x=99.1% and V.sub.y=98.9% were determined.

[0105] Thus, it can be stated that the spectral broadening, as well as the compressibility and the spectral homogeneity of the spectrum broadened with a concave-convex device are in no way inferior to a conventional concave-concave HC. Contrary to the prevailing assumption in the related art that concave-convex multipass arrangements are disadvantageous in these respects, the inventors are thus able to disprove it.

[0106] FIG. 12 shows in a schematic representation a laser system 200 according to an exemplary embodiment, which comprises a device 100 according to an exemplary embodiment of the disclosure for spectral broadening of a laser pulse. The device 100 may be integrated into the laser system 200 or formed separately therefrom. The laser pulses provided by the laser system 200 can thereby be supplied to the device 100 before further use, in which they pass through the concave-convex multipass arrangement and are spectrally broadened therein. Further, in the device 100 or elsewhere in the laser system, the laser pulse broadened by the device 100 may be compressed using one or more dispersive optics.

[0107] The foregoing description of the exemplary embodiments of the disclosure illustrates and describes the present invention. Additionally, the disclosure shows and describes only the exemplary embodiments but, as mentioned above, it is to be understood that the disclosure is capable of use in various other combinations, modifications, and environments and is capable of changes or modifications within the scope of the concept as expressed herein, commensurate with the above teachings and/or the skill or knowledge of the relevant art.

[0108] The term “comprising” (and its grammatical variations) as used herein is used in the inclusive sense of “having” or “including” and not in the exclusive sense of “consisting only of” The terms “a” and “the” as used herein are understood to encompass the plural as well as the singular.

[0109] All publications, patents and patent applications cited in this specification are herein incorporated by reference, and for any and all purposes, as if each individual publication, patent or patent application were specifically and individually indicated to be incorporated by reference. In the case of inconsistencies, the present disclosure will prevail.

LIST OF REFERENCE SIGNS

[0110] 10 Related art spectral broadening device [0111] 20 Multipass arrangement according to the related art [0112] 21 concave mirror [0113] 22 concave mirror [0114] 23 Coupling and decoupling mirrors [0115] 30 nonlinear optical medium [0116] 40 Laser beam [0117] 100 Device for spectral broadening [0118] 120 Multipass arrangement [0119] 121 concave mirror [0120] 122 convex mirror [0121] 123 In-coupling and out-coupling opening [0122] 124 Deflecting mirror [0123] 125 Recess [0124] 130 nonlinear optical medium [0125] 140 Laser beam [0126] 150 dispersive coating [0127] 1001 Circumference with radius of curvature R1 [0128] 1002 Circumference with radius of curvature R2 [0129] 1003 Intersection points of the radii of curvature [0130] 1004 Mode volume of the multipass arrangement [0131] d.sub.0 Mirror spacing of the multipass arrangement [0132] f.sub.1 Focal length of the concave mirror [0133] F.sub.1 Focal plane of the concave mirror [0134] f.sub.2 Focal length of the convex mirror [0135] R.sub.1 Radius of curvature of the first mirror [0136] R.sub.2 Radius of curvature of the second mirror