WO2023066995A1

WO2023066995A1 - Device and method for spectrally broadening a laser pulse

Info

Publication number: WO2023066995A1
Application number: PCT/EP2022/079092
Authority: WO
Inventors: Yanik PFAFF; Sandro KLINGEBIEL; Peter Krötz; Michael Rampp
Original assignee: Trumpf Scientific Lasers Gmbh + Co. Kg
Priority date: 2021-10-21
Filing date: 2022-10-19
Publication date: 2023-04-27
Also published as: DE102021127328A1

Abstract

The present invention relates to a device (2) for spectrally broadening a laser pulse (100) of a laser beam (10), said device comprising an optical coupling unit (20), at least one non-linear optical unit with a lens effect (224, 224'), and an optical out-coupling unit (26), wherein the optical coupling unit (20) is designed to couple the laser beam (10) into the at least one lens element (224, 224'), and wherein the optical out-coupling unit (26) is designed to couple out the laser beam (10), wherein the laser pulse (100) of the laser beam (10) is spectrally broadened using a non-linear interaction with the at least one non-linear optical unit with a lens effect (224, 224'), and the laser beam (10) is focused by the optical coupling unit (20) and/or the at least one non-linear optical unit with a lens effect (224, 224'), wherein the beam cross-section is elongated in the focus (30), wherein the focus (30) is preferably a line focus.

Description

Device and method for spectral broadening of a laser pulse

technical field

The present invention relates to a device and a method for spectrally broadening a laser pulse of a laser beam.

State of the art

It is known to use devices for spectral pulse broadening of an ultra-short laser pulse of a laser beam by means of self-phase modulation, see T. Nagy et al. (2021) "High-energy few-cycle pulses: post-compression techniques", Advances in Physics: X, 6:1 , 1845795. Here, a laser beam is passed through the optical elements of the device, whereby the laser pulse of the laser beam is non-linear with the Material of the optical elements interacts. The non-linear interaction leads to a self-phase modulation, which leads to a spectral broadening, ie a broadening of the laser pulse in the frequency range. Between the optical elements of the device, the laser beam is typically focused to allow control over beam propagation and beam properties via optical imaging between the optical elements of the device.

However, the maximum power of the laser beam that can be used is limited by the ionization threshold of the gas, for example the air, which is located between the optical elements of the device. In conventional devices, this ionization threshold of the gas is reached very quickly due to the high intensity in the area of the focus point, essentially along the Rayleigh length, between the optical elements of the device.

The maximum power of the laser beam that can be used is also limited by the laser-induced destruction threshold of the optical elements used in the device.

In addition, the strength of the nonlinear interaction in the optical elements of the device is affected by another nonlinear effect, the so-called catastrophic Self-focusing, which also occurs at high intensities around focal points along the Rayleigh length, is limited.

One possibility of reducing the intensity of the laser beam in the area of the focus points between the optical elements of the device is to increase the diameter of the focus points. This can be achieved, for example, by a smaller aperture angle of the laser beam, which would lead to a reduction in intensity in the focus points if the laser power remained the same. However, a smaller opening angle of the laser beam comes at the price of a longer design or a longer focal length of the optical elements used in the device, so that the intensity of the laser can be kept below the damage threshold of the optical elements used.

As a result, such devices for high pulse energies can become very large.

Presentation of the invention

Proceeding from the known prior art, it is an object of the present invention to provide an improved device for spectrally broadening a laser pulse and a corresponding method.

The object is achieved by a device for spectrally broadening a laser pulse with the features of claim 1. Advantageous developments result from the dependent claims, the description and the figures.

Accordingly, a device is proposed for the spectral broadening of a laser pulse of a laser beam, comprising in-coupling optics, at least one non-linear optic with a lens effect, and a de-coupling optic, wherein the in-coupling optics are set up to couple the laser beam into the at least one non-linear optic with a lens effect, and wherein the Decoupling optics are set up to decouple the laser beam, the laser pulse of the laser beam being spectrally broadened by a nonlinear interaction with the at least one nonlinear optics with a lens effect and the laser beam being focused by the coupling optics and/or the at least one nonlinear optics with a lens effect. According to the invention, the beam cross section is elongated in the focus, with the focus preferably being a line focus.

The laser for providing the laser beam is preferably an ultra-short pulse laser and provides ultra-short laser pulses of the laser beam, with the individual laser pulses forming the laser beam in the beam propagation direction. By definition, the beam propagation direction is the z-direction. The pulse peak power of the laser pulses of the laser beam can be greater than 3 megawatts (MW) and less than 2 terawatts (TW), preferably greater than 30 MW and particularly preferably 50 MW to 500 gigawatts (GW).

The pulse peak power is determined as the quotient of the pulse energy E _P of the laser pulse of the laser beam divided by the pulse duration t _P of the laser pulse multiplied by a factor s for the pulse shape: P « s - ^E P rp ^d ^L p

For example, for a Gaussian laser pulse, the factor s=0.94.

As a result, laser pulses in particular can be generated with a particularly large bandwidth.

Coupling can mean that the laser beam is deflected from its original trajectory to a trajectory on which the laser beam can pass through the arrangement of non-linear optics with a lens effect. For example, a mirror as coupling optics can deflect the laser beam. In particular, the coupling can already include a focusing of the laser beam.

In this context, non-linear optics with a lens effect is an optical element which has an optically imaging property. In particular, the non-linear optics with lens action can focus the laser beam. It is also possible that the non-linear optics with a lens effect images the laser beam from the object plane into an image plane.

The non-linear optics with a lens effect can be designed in particular in the form of a lens element.

The non-linear optics with a lens effect can also be provided as reflective optics. This can mean that the characterizations given in this description for a lens element (transmissive optics) are also to be understood analogously for a mirror, with the mirror preferably being designed in such a way that the laser beam first passes through a transparent body made of a nonlinear material and then a rear reflection layer is reflected. Nonlinear interaction can take place through the interaction of the laser beam with the nonlinear material of the transparent body.

For example, the imaging properties of the at least one non-linear optics with a lens effect can be used to direct the laser beam from the in-coupling optics to the out-coupling optics to direct. In the case of several non-linear optics with a lens effect, a concatenation of images, that is to say a successive sequence of images, can be used here in order to guide the laser beam from the in-coupling optics to the out-coupling optics. This creates foci between the optical elements, ie between the coupling optics and the at least one nonlinear optics with lens effect, between the nonlinear optics with lens effect and between the at least one nonlinear optics with lens effect and the decoupling optics.

The foci lie in a gaseous or liquid medium, for example in air, arranged between the respective optical elements. In other words, the foci are not in the material of the non-linear optics with a lens effect or in the material of the coupling optics or the coupling optics. There is at least one focus that is not within the materials.

When the laser beam is transmitted through an optical element of the device, in particular when it is transmitted through nonlinear optics with a lens effect, a nonlinear interaction of the laser pulse of the laser beam with the optical material of the optical element is generated in the present case.

The reason for the non-linear interaction of the laser beam with the material of the in-coupling optics, the out-coupling optics or the non-linear optics with lens effect can be the so-called Kerr effect, according to which the optical properties, in particular the refractive index n, of the material of the respective optical element can be changed by applying a electric field can be varied. In particular, a laser pulse can provide the electrical field, with high-energy laser pulses or laser pulses with particularly high power providing particularly large electrical fields.

A variation of the refractive index is generated in the optical element due to the high laser intensity of the laser pulse:

Here, the linear refractive index is given by no and the nonlinear refractive index is given by no. In this case, the intensity I depends on the beam propagation direction z and on the time t due to the variable intensity of the laser pulse.

To characterize a non-linear interaction, the so-called B-integral is used, which is a measure of a non-linear phase shift of the laser beam as it passes through the optical element. The B integral is defined as: = — fn ₂ /(z, t) dz'

B ² where A is the wavelength of the laser beam. It is already clear from the B integral that a large nonlinear refractive index n2 and/or a large intensity I of the laser beam lead to a nonlinear phase shift.

In this case, the intensity of the laser pulse typically has a strong time dependency, since the laser increases within the pulse duration from a vanishing intensity to a maximum peak intensity and then falls again to a vanishing intensity. The temporal dependence of the laser energy, i.e. the temporal progression of the laser pulses, results in a change in the refractive index over time:

Ugr phase shift of the laser beam can now be calculated using _{c 2l0} ^ur| write dm as

A "A" k ₀ ^A o

This results in a non-linear phase shift of after a distance L or the thickness L of the optical element has been covered

(L, t) = &) _o t - Ln(I)

From this it is immediately apparent that the non-linear interaction causes a frequency change, which is caused by the non-linear refractive index n2:

Due to the positive time derivative, the rising edge of the laser pulse leads to a reduction in frequency and thus to a red shift of the laser pulse. The falling edge at the end of the pulse, on the other hand, leads to an increase in frequency due to the negative time derivative and thus to a blue shift.

The laser pulse is thus spectrally broadened by the propagation in the material of the non-linear optics with a lens effect and/or the gas surrounding the non-linear optics with a lens effect. The associated process is called self-phase modulation. It should be pointed out in particular that the self-phase modulation also depends on the length of the path covered or the thickness of the respective optical element. Accordingly, the self-phase modulation also scales with the number of optical elements of the device.

Another important non-linear effect is the so-called self-focusing. As already shown, the refractive index n depends on the intensity. In the case of a Gaussian laser beam, for example, whose cross section always has the shape of a Gaussian bell curve, the intensity is greatest in the middle of the beam. As a result, the optical element provides an additional lens effect for the laser beam above a certain power.

The self-focusing occurs here from the so-called critical pulse peak power Pent, where the power P of the laser is proportional to the laser intensity I and to the cross-sectional area A: P oc I A. The self-focusing can lead to an increase in the laser intensity in particular, so that the intensity in the optical element of the device exceeds a damage threshold. Self-focusing is therefore advantageously avoided.

However, the above non-linear interaction takes place not only in the optical elements of the device, but also in the foci lying in the gaseous or liquid medium between the respective optical elements. Here, the laser pulse interacts non-linearly with the gaseous or liquid medium. In particular, the gaseous or liquid medium can also lead to self-focusing.

In this case, focusing is a deliberately brought about increase in intensity of the laser beam, the intensity of the laser beam being defined by the laser energy per cross-sectional area. The focus of the laser beam is the point along the beam propagation direction where the cross-sectional area of the laser beam is minimized. Analogous to this, the focus of the laser beam is the point along the beam propagation direction where the intensity is maximum. The high intensity between the optical elements of the device can also lead to self-focusing and/or ionization of the gaseous or liquid medium, for example the air, which surrounds the device.

The cross-sectional area of the laser beam is measured in the x-y plane, which is orthogonal to the z-axis. The beam diameter is defined as the diameter of the cross-sectional area along the x-axis and the y-axis, respectively. In this case, the x and y axes are not necessarily perpendicular to one another. Rather, they can also enclose a different angle with one another in order to depict the symmetry of the cross section of the laser beam in a particularly simple manner. Alternatively and/or additionally, the focus can also be defined by minimizing the beam diameter with respect to a beam plane. A ray plane is any plane in which the direction of ray propagation lies. For example, the xz plane and the yz plane are beam planes. If the laser beam is focused, then the beam diameter with respect to the xz plane and/or the yz plane is also minimized.

According to the invention, the beam cross section is elongated in the focus.

This can mean that the beam diameter is larger with respect to a first beam plane than with respect to a second beam plane. For example, the laser beam is elongated when the ratio of the beam diameters with respect to the x-axis and the y-axis is greater than 1:2, preferably greater than 1:3, preferably greater than 1:5, particularly preferably greater than 1:10. For example, a focused laser beam may have a diameter of 1 cm along the x-axis and a diameter of 5 cm along the y-axis. Then the beam cross-section is elongated.

It can thereby be achieved that the intensity of the laser beam in the focus between the optical elements of the device is reduced, since the laser energy is distributed over a larger area than with a punctiform focus. Due to the reduced intensity in the focus, in particular laser pulses with greater pulse powers can also be spectrally broadened before catastrophic self-focusing occurs between the optical elements. At the same time, the low intensity of the laser beam in the focus avoids exceeding the ionization threshold of the air. This allows for a shorter construction of the device, since larger opening angles can be used. For example, the device can be made shorter by a factor of 4 than conventional devices.

The focus between the optical elements can be a line focus.

The elongated focus can be arranged in a liquid or gaseous medium, preferably air, between the coupling-in optics and non-linear optics with a lens effect and/or between at least two non-linear optics with a lens effect and/or between a non-linear optics with a lens effect and the decoupling optics, in particular in each case an elongated focus is preferably arranged between all of the non-linear optics with a lens effect in the liquid or gaseous medium.

In particular, the laser beam is not focused on and/or in the non-linear optics with a lens effect. In particular, the elongated focus is not positioned on and/or in the non-linear optics with lens action. In particular, the laser beam has and/or has its original beam shape in non-linear optics with a lens effect. This is to be understood in particular as the beam shape which the laser beam incident on the in-coupling optics and/or the laser beam emerging from the laser has.

The laser beam incident on the coupling optics and/or the laser beam emerging from the laser has a Gaussian beam shape, for example.

A line focus can be generated by optics that only focus the laser beam with respect to one beam plane. In particular, cylinder optics can only focus the laser beam along one beam plane.

By definition, a line focus is generated by an optic that only focuses the laser beam with respect to one axis. However, a focus is generally a line focus if the ratio of the beam diameters with respect to the x-axis and the y-axis is greater than 1:10, preferably greater than 1:100. Accordingly, astigmatic optics can also produce a line focus.

An interaction with the gaseous or liquid medium, for example the air, can be particularly well controlled or avoided by means of a line focus between the optical elements of the device.

The nonlinear phase shift of the laser pulse in the optical elements of the device can be between TT/20 and 4TT, preferably between TT/10 and 2TT, per nonlinear interaction per optical element.

As a result, the entire non-linear interaction process between the laser and the optical elements of the device can be easily controlled. For example, the spectral broadening can be adjusted via the number of optical elements of the device.

The intensity of the laser beam in the focus between the optical elements of the device can be smaller than the ionization threshold of the gaseous or liquid medium present there.

Smaller than the ionization threshold can mean that in the focus volume in which, for example, more than 50%, preferably more than 75%, particularly preferably more than 90% of the spectral broadening is collected, less than 50%, preferably less than 25%, particularly preferably less than 10% of the medium present there are ionized. Ionization of the gaseous or liquid medium changes the refractive index and the non-linear refractive index of the medium, since the distribution of the electrical charge carriers in the medium changes. Eventually, ionization of the medium means that the non-linear interaction of the laser pulse can no longer be controlled or suppressed, or proceeds uncontrolled. In addition, a large loss of laser power is associated with the formation of a plasma state of a gas.

The device, preferably the decoupling optics of the device, can have an optical compressor which is set up to increase the laser pulse duration to less than 0.9 times, preferably to less than 0.7 times, due to the spectral broadening of the laser pulse to compress the original laser pulse duration.

This is clearly based on the fact that the frequency space and the time period are linked to one another by a Fourier transformation. For example, a bandwidth Af in the frequency domain is inversely proportional to the duration of the signal At in the time domain:

<x 1/At-

For example, a laser pulse can originally have a pulse duration of 1 ps and then, after spectral broadening, have a Fourier-limited pulse duration of 0.7 ps, which can be achieved by subsequent pulse compression.

Due to the spectral broadening of the laser pulse, a shortening of the laser pulse duration can thus be brought about, in particular by a pulse compressor arranged in the decoupling optics or a pulse compressor connected downstream. A corresponding pulse compression can also be achieved by at least one chirped mirror, which in particular can also be part of the decoupling optics or can be designed in one piece with the decoupling optics.

The coupling-in optics and the coupling-out optics can be non-rotationally symmetrical optics, preferably cylindrically symmetrical optics.

An optic is non-rotationally symmetrical if it breaks the symmetry of an incident laser beam. This can be achieved, for example, by the surface of the optics being non-rotationally symmetrical. For example, a cylindrical mirror is a non-rotationally symmetrical mirror. For example, a cylindrical lens is a non-rotationally symmetrical lens. If the mirror or lens has a parabolic cross-section (rather than cylindrical) and a longitudinal axis, then the mirror or lens is also non-rotationally symmetrical. A so-called asphere can also be non-rotationally symmetrical optics. For example, a collimated laser beam with a Gaussian beam profile falls in the z-direction onto a cylindrical lens, which is the launching optic, with the cylinder axis being parallel to the x-axis. The cylindrical symmetry then results in two relevant beam planes in which the propagation and focusing of the laser beam can be described, namely the xz plane and the yz plane. In particular, due to the non-linear interaction between the in-coupling optics and the laser pulse, there is a spectral broadening of the laser pulse.

On the one hand, the laser beam is transmitted in the x-z plane without modification of the beam propagation (note: "x as without modification"), since the cylindrical lens has no curvature in this plane. In other words, the beam diameter remains the same in the x-z plane, or the laser beam remains collimated with respect to the x-z plane.

On the other hand, the laser beam is focused in the y-z plane (note: "y as in cylinder"), since the cylindrical lens has the cylinder curvature in this plane. If the beam diameter in the y-z plane is plotted as a function of the z coordinate, the beam diameter in the y-z plane steadily decreases until it reaches a minimum in the focus. The beam diameter then increases again with respect to the y-z plane. In other words, the laser beam converges in the y-z plane towards the focus and then diverges away from the focus.

By keeping the beam diameter constant in the x-z plane and minimizing the beam diameter with respect to the y-z plane, the cross-sectional area of the laser beam is also minimized. In the minimum of the beam diameter with respect to the y-z plane, the laser beam thus reaches an intensity maximum. The laser beam can thus have an elongated focus between the in-coupling optics and the arrangement of lens elements.

After passing through the focus, the laser beam diverges in the y-z plane. Such a divergence can be remedied by a cylindrical decoupling optics. In other words, the laser beam can be collimated again by a decoupling optics, which is also a cylindrical lens.

The arrangement of non-linear optics with a lens effect can include at least one rotationally symmetrical lens element.

An optic is rotationally symmetrical if the symmetry of an incident laser beam is not broken. This can be achieved, for example, by the surface of the optics being rotationally symmetrical. For example, a spherical mirror is a rotationally symmetrical mirror. For example, a spherical lens is a rotationally symmetrical lens. In addition to a spherical mirror or a spherical lens, a parabolic mirror or a parabolic lens is also rotationally symmetrical.

For example, a collimated laser beam with a Gaussian beam profile falls on a spherical lens in the z-direction. Then the laser beam is focused uniformly in all beam planes. In particular, there are no specific axes that would be particularly helpful for the description. The axes for the description can therefore be freely selected.

In the above example, the laser beam hits the first spherical lens element after the in-coupling optics. On the lens element, the laser beam has its original beam shape again due to the divergence after the focus after the coupling optics. For example, a Gaussian beam shape is again present on the surface of the first lens element. Due to the non-linear interaction between the lens element and the laser pulse, a spectral broadening of the laser pulse occurs in the lens element.

At the same time, the laser beam is collimated with respect to the y-z plane, whereas the laser beam is focused in the x-z plane. To a certain extent, the combination of non-rotationally symmetrical (cylindrical) coupling optics and a rotationally symmetrical (spherical) lens element results in a change in the focal plane in which the elongated beam cross section extends.

Accordingly, after the first spherical lens element, the laser beam has a focus in the x-z plane. The focus here extends into the y-z plane and is therefore elongated, or a line focus.

In the above example, the cylindrical decoupling optics could collimate the laser beam. Accordingly, after passing through the laser beam, the laser beam could further propagate with a Gaussian beam profile. However, it must be taken into account here that the decoupling optics, or the cylinder of the cylindrical lens, is rotated by 90° in relation to the coupling optics, since the collimation plane was rotated by the interposed spherical lens element.

However, it is also possible for further lens elements to be arranged after the first lens element, so that the laser beam passes through a large number of lens elements and is thus successively broadened spectrally.

For example, the laser beam has a focus in the x-z plane between the first and second spherical lens elements. The focus here extends into the yz plane and is therefore elongated, or a line focus. The laser beam is then re-collimated with respect to the xz plane and focused with respect to the yz plane by the second spherical lens element. Depending on the number of lens elements, this process can be continued and continued.

Thus, focusing of the laser beam alternates between the x-z plane and the y-z plane on successive passes through the spherical lens elements. In other words, the same alignment of the beam cross section is only achieved with every second pass through a lens element, which corresponds to a so-called 4f image.

After the laser beam has been broadened spectrally to a sufficient degree, the laser beam can be coupled out of the device by a coupling-out lens system. In this case, the decoupling optics can in particular collimate the laser beam again. For this purpose, the decoupling optics can be non-rotationally symmetrical, in particular cylindrical, in order to eliminate the corresponding convergence of the laser beam.

The decoupling optics can have, for example, at least one chirped mirror or be a chirped mirror in order to achieve temporal pulse compression of the spectrally broadened laser pulse.

The coupling optics and/or the decoupling optics and/or at least one non-linear optics with a lens effect can be designed as a mirror element, preferably as a mirror element designed by a quartz glass element that is mirrored on the back.

The coupling optics can be an astigmatic mirror, which has two different focal lengths in two different orthogonal mirror planes, the sagittal plane and the meridional plane. For example, the sagittal plane can coincide with the x-y plane and the y-z plane can coincide with the meridional plane. For example, the mirror can have a greater focal length in the sagittal plane than in the meridional plane. The two mirror planes then result in two relevant beam planes in which the propagation and focusing of the laser beam can be described.

Thanks to the smaller focal length in the direction of beam propagation, the laser beam is first focused in the meridional plane, i.e. the beam diameter in relation to the meridional plane of the laser beam is minimized in the first focus. Accordingly, the beam diameter in relation to the meridional plane is smaller in the focus than in the sagittal plane. With an astigmatic mirror, this results in a line focus that extends in the sagittal plane. The beam diameter then increases again with respect to the meridional plane, while the beam diameter in the sagittal plane is further reduced until it is minimal in the focus of the sagittal plane. In the case of an astigmatic mirror, this results in a further line focus that extends into the meridional plane.

Astigmatic optics can therefore result in two elongated foci, or line foci, between the optical elements of the device.

Preferably less than 20 non-linear optics with a lens effect, preferably less than 10 non-linear optics with a lens effect can interact non-linearly with the laser beam.

For example, the arrangement of lens elements can include two non-linear optics with a lens effect.

In addition, the optical structure can include a telescope. The telescope can be an arrangement of two lenses that enlarge or reduce the raw laser beam to a desired beam diameter before entering the coupling optics and collimate the laser beam.

In the above example, the device could only have four lens optics, namely the coupling optics, the coupling optics and two lens elements. The optics of the telescope are not counted here. Nevertheless, the laser pulse can also interact non-linearly with the optics of the telescope. The entrance immediately before the in-coupling optics and the exit immediately after the out-coupling optics accordingly form two reference points of the device, in which the spectral broadening is also determined.

In one embodiment, each optical element of the device can be a lens, in which case the laser beam can interact non-linearly with the material of the lenses when passing through the lenses.

For example, the coupling optics can be a non-rotationally symmetrical mirror, preferably a cylinder mirror, and the device can have at least one coated mirror, preferably a cylinder mirror, with the coupling optics preferably also being a cylinder mirror.

Analogously to the example above, a collimated laser beam is focused by the non-rotationally symmetrical in-coupling optics in the yz plane in front of the first non-rotationally symmetrical lens element. If the cylinder axes of the coupling optics and the lens elements run parallel, then the laser beam is always focused in the yz plane. In a way, therefore achieves the same alignment of the beam cross-section with each pass, which corresponds to a so-called 2f image.

However, it can also be the case that the cylinder axes of the lens elements are orthogonal to one another. Alternating focusing in the plane of the cylinder curvature and the plane of the cylinder axis can then be brought about by a suitable choice of the focal length of the resonator mirror. However, it should be noted here that the divergence of the laser beam after a focus is kept so small that the widening of the laser beam is smaller than the dimensions of the cylinder mirror of the resonator. To a certain extent, the same alignment of the beam cross section is thus achieved with every second pass, which corresponds to a so-called 4f image.

In any case, the laser pulse of the laser beam can interact non-linearly with the non-linear material of the body of the mirror, so that a spectral broadening is achieved independently of the mirror symmetry.

In each of the examples mentioned, it is possible to replace a lens with a mirror with the appropriate symmetry. In particular, it is thus possible for the device to include lenses and mirrors.

For example, the laser beam can be reflected and elongate focused by a cylindrical mirror as in-coupling optics and passed through an array of non-linear optics with lens action, wherein the non-linear optics with lens action are lenses. The laser beam can then be coupled out of the device through a cylindrical lens as coupling-out optics.

The non-linear optics with lens action can have the same focal length or all optical elements of the device can have the same focal length.

In this way, a particularly simple construction of the device can be achieved.

Two optical elements of the device, with which the laser beam interacts non-linearly one after the other, can have a distance of between 0.8 times and 1.2 times the sum of the focal lengths of the two optical elements of the device.

For example, the cylindrical in-coupling optics can have a focal length of 30 cm and the first lens element of the device can have a focal length of 70 cm. Then the mirrors can be placed at a distance of 80cm to 120cm from each other. For example, the first lens element and the second lens element can have a focal length of 100 cm and can be set up from 160 cm to 240 cm. In this case, the same orientation of the cross-section of the focus can be achieved in every second pass.

For example, all optical elements can have a focal length of 20 cm.

The various parameters listed above can be taken into account for the design of a corresponding device.

For this purpose, the cross section of the laser beam at which the intensity in the focus reaches the ionization threshold can first be calculated from the pulse energy. The cross-section (or diameter) is related to the beam angle of the laser beam via the beam parameter product, as shown above.

In addition, the maximum intensity or fluence of the laser beam on the optical elements should be less than the laser-induced damage threshold (LIDT) so that the optical elements are not destroyed by the laser beam. The laser-induced destruction threshold is not reached when the cross-sectional area of the laser beam on the optical element exceeds a certain critical cross-sectional area. The focal length of the optical elements results from the cross-sectional area on the optical elements, the cross-sectional area in the focus and the aperture angle.

Finally, the material parameters of the optical elements can be used to calculate the non-linear phase shift using the B integral per transmission/reflection. Safety factors in particular can be taken into account here in order to reliably avoid ionization or destruction of the mirrors or lenses.

Finally, the total broadening of the laser pulse can be determined with the number of optical elements.

The object set above is also achieved by a method for spectral broadening with the features of claim 12. Advantageous developments of the method result from the dependent claims and the present description and the figures.

Accordingly, a method for the spectral broadening of a laser pulse of a laser beam is proposed, the laser beam being coupled into at least one nonlinear optics with a lens effect by coupling optics, the laser beam passing through the nonlinear optics with a lens effect, the laser pulse of the laser beam being caused by a nonlinear interaction with the nonlinear Optics with a lens effect is spectrally broadened, the laser beam being coupled out after passing through the at least one nonlinear optics with a lens effect by a decoupling optics, the laser beam being focused by the coupling optics and/or at least one nonlinear optics with a lens effect. According to the invention, the beam cross section is elongated in the focus, with the focus preferably being a line focus.

Brief description of the figures

Preferred further embodiments of the invention are explained in more detail by the following description of the figures. show:

FIG. 1 shows a schematic representation of the device;

Figure 2 is a schematic representation of the location dependency of different

State-of-the-art beam characteristics;

FIG. 3A, B, C, D, E, F shows a schematic representation of the location dependence of different beam properties in a device;

Figure 4A, B, C, D is a schematic representation of the optical elements;

FIG. 5 shows a further schematic representation of the location dependency of various

Beam properties in a device according to the invention; and

FIG. 6 shows a further schematic representation of a device.

Detailed description of preferred embodiments

Preferred exemplary embodiments are described below with reference to the figures. Elements that are the same, similar or have the same effect are provided with identical reference symbols in the different figures, and a repeated description of these elements is sometimes dispensed with in order to avoid redundancies.

A first embodiment of a proposed device 2 is shown schematically in FIG.

In this case, a laser 1 provides a laser beam 10 in which the laser pulses 100 of the laser 1 propagate. In particular, the laser pulses 100 can be ultra-short laser pulses. For example the ultra-short laser pulses can have a pulse duration of less than 100 ps, preferably less than 1 ps, for example 600 fs or 300 fs.

In this case, the laser pulses 100 have a frequency bandwidth that is intended to be broadened by the device 2 . For example, the pulse peak power of the laser pulses 100 of the laser beam 10 can be greater than 3 megawatts (MW) and less than 2 terawatts (TW), preferably greater than 30 MW and more preferably 50 MW to 500 gigawatts (GW).

The device 2 has coupling-in optics 20, an arrangement 22 of non-linear optics with a lens effect 224, 224', which are designed as lens elements in the exemplary embodiment shown, and coupling-out optics 26. The arrangement 22 of non-linear optics with a lens effect 224, 224' comprises at least one non-linear optic with a lens effect 224, 224', preferably at least one lens element.

The laser beam 100 strikes the in-coupling optics 20 in a collimated state and is guided by the latter into the arrangement 22 of non-linear optics with a lens effect 224 . In array 22 of nonlinear lensing optics 224, laser beam 10 is directed through nonlinear lensing optics 224, with each transmission through nonlinear lensing optics 224, laser beam 10 interacting nonlinearly with the optical material of nonlinear lensing optics 224 and is thus spectrally broadened. After the transmission of the laser beam 10 through the last non-linear optics with lens effect 224', the laser beam 10 is coupled out of the device 2 by the decoupling optics 26. The decoupled laser pulse 100' is spectrally broadened compared to the original laser pulse 100.

The decoupling optics 26 can collimate the laser beam 10 again and/or carry out a pulse compression and/or guide the laser beam 10 to a downstream pulse compressor.

In particular, the laser beam 10 also interacts non-linearly with the in-coupling optics 20 and the out-coupling optics 26, in which case the extent of this interaction can be less than the non-linear interaction of the laser beam 10 with the at least one non-linear optics with lens effect 224, 224' of the arrangement 22.

Various beam characteristics in a prior art device are shown in FIG. According to the prior art, the coupling optics 20 shown, the non-linear optics with a lens effect 224, and the coupling-out optics 26 are each designed as spherical lenses. This has far-reaching consequences for the beam properties. In Figure 2 is the Beam diameter D shown as a function of the beam propagation direction z for the xz plane and the yz plane. In this case, the z-direction always coincides with the beam propagation direction of the laser beam 10 .

The laser beam 10 is first colli mated before it falls on the coupling optics 20 . This can be seen from the fact that the beam diameter D in front of the in-coupling optics 20 is constant both in the x-z plane and in the y-z plane.

The laser beam 10 is transmitted and focused by the spherical in-coupling optics 20 of the prior art. During transmission, the laser pulse already interacts with the optical material of the in-coupling optics 20, as a result of which the laser pulse is spectrally broadened. In particular, the laser beam 10 is focused in the x-z plane and the y-z plane. The entire energy of the laser beam 10 is accordingly concentrated on a quasi-point-shaped focal spot, so that the intensity I increases sharply locally at the focal point 3 . This is also shown in FIG. The focal point 3 is accordingly arranged where the beam diameter D has a minimum in both beam diameters. The laser beam 10 then diverges to the second lens element and is then focused again. This continues until the laser beam 10 is coupled out of the device through the coupling-out optics 26 .

As described above, due to the high intensity and the non-linear interaction with the liquid or gaseous medium 240, here in the form of air, between the optical elements 20, 224, 26 in the quasi punctiform focus 3, the laser beam 10 or an ionization of the liquid or gaseous medium 240 come. The former is particularly the case when the power of the laser exceeds the peak pulse power Pent. Since the pulse peak power Pont is linked to the intensity I via the beam diameter D, focusing the laser beam 10 can accordingly lead to so-called catastrophic self-focusing. By ionizing the air, some of the laser energy is lost to creating a plasma state in the air.

In order to solve this problem, according to the proposed solution, the in-coupling optics 20 and/or non-linear optics with a lens effect 224 are set up to focus the laser beam 10, with the beam cross-section being elongated in the focus 30.

FIG. 3A shows a corresponding course of different beam properties of the device 2 according to the proposed solution. In particular, the progression of the beam diameter D is shown when the beam cross section in the focus 30 is elongated. In this case, the laser beam 10 is initially focused by the in-coupling optics 20 in a beam plane, here the yz plane. through the Focusing in the yz beam plane, the laser beam 10 has an unchanged beam diameter D in the x-z plane even after transmission through the in-coupling optics 20 . In the minimum of the beam diameter D, an intensity increase in the form of a line focus 30 is consequently generated. The line focus 30 extends in the x-z plane when focused in the yz plane. Due to the line focus 30, the intensity in the line focus 30 remains comparatively low, so that the pulse peak power Pent is not reached or even exceeded. As a result, catastrophic self-focusing does not occur either. Typically, the intensity here is also so low that ionization of the liquid or gaseous medium 240, for example air, is avoided. For example, the ionization threshold of air is around 20 TW/cm ² . The intensity between the optical elements can thus be 7TW/cm ² in the focus 30, so that the intensity in the focus 30 is still a safety factor 3 below the ionization threshold.

At the same time, the intensity of the laser beam 10 in the optical elements, in particular the non-linear optics with a lens effect 224, can be selected to be large enough to cause a spectral broadening of the laser pulse 100 as it passes through the optical elements. This behavior can be seen from the plotted B integral. The B integral, i.e. the non-linear phase shift, always increases in the area of propagation through the optical elements 20, 224, 26. However, the B integral only makes relevant contributions in the optical elements 20, 224, 26. During the propagation and the contribution is negligible when focusing in the air.

However, it is also possible to also use the line focus 30 for spectral broadening. In particular, however, the intensity in the air is below the ionization threshold and below the pulse peak power, so that there is neither ionization of the air nor self-focusing.

Such a line focus 30 between the optical elements can be generated in different ways. One possibility is to combine a non-rotationally symmetrical in-coupling optics 20 with at least one rotationally symmetrical lens element as non-linear optics with a lens effect 224 . Non-rotationally symmetrical optics also include astigmatic optics.

FIG. 3B shows the elongated focus 30 of a laser beam 10, which was generated by an astigmatic coupling optics 20, for example with an astigmatic lens. The beam diameter has a minimum in the focus 30 in both the x-axis and the y-axis. However, the pulse energy is distributed over a larger area compared to a point focus, so that there is neither self-focusing nor the ionization threshold of the liquid or gaseous medium being exceeded.

FIG. 3C shows the elongated focus 30 of a laser beam 10, which was generated by a cylindrical in-coupling optics 20. Here, the beam diameter in the x-axis is constant (as shown in Figure 3A) and only the beam diameter along the y-axis goes through a minimum. Accordingly, a so-called line focus 30 is formed by a cylinder optics.

FIG. 3D shows a device 2 according to the proposed solution, in which the diameter of the laser beam 10 of the laser 1 is first brought to a desired size by a telescope 12 . In addition, the laser beam 10 can also be collimated by the telescope 12 . The coupling optics 20 and the decoupling optics 26 are cylindrical lenses and the arrangement of the non-linear optics with lens effect 22 includes four spherical lenses 224. The beam diameter drawn for the x-z plane and the y-z plane makes it clear that the alignment of the elongated beam cross-section in Focus 30 between the optical elements 20, 224, 26 changes after each transmission.

FIG. 3E shows the generated spectra, or the normalized intensities Inorm as a function of the wavelength λ, as an example for the device 2 from FIG. 3D. It can be clearly seen that the bandwidth after transmission through the telescope 12, ie before coupling into the coupling optics (referred to as “ZL1”) has a comparatively small bandwidth. After transmission through four spherical lenses (“SL2” to “SL5”) and decoupling through the decoupling optics (called “ZL6”), the bandwidth has increased visibly.

This is also reflected in the associated pulse duration, as shown in FIG. 3F. In FIG. 3F the total intensity Itot is plotted as a function of time around the center of the pulse. While the pulse duration after the telescope was still 264 fs, the laser pulse duration could be shortened to 106 fs by pulse compression after the decoupling optics 26.

FIG. 4A shows a further possible configuration of the in-coupling optics 20 in the form of a cylindrical mirror 20 in a side view and in FIG. 4B a bird's-eye view. The collimated laser beam 10 initially falls from the right onto the mirrored surface of the coupling optics 20 and is reflected there. In the side view, the in-coupling optics 20 have the cylinder curvature, so that a focusing of the laser beam 10 is achieved. At the same time, the beam diameter D of the laser beam remains unchanged in the bird's-eye view, since the in-coupling optics 20 do not focus in this plane. Rather, the laser beam 10 is only reflected here. Next, the laser beam 10 focused in the side view is guided onto the first mirror element 220 as shown in Figure 4C, the mirror element 220 being a spherical mirror. Accordingly, due to the spherical curvature of the mirror surface of the first mirror element 240, the laser beam 10 can be collimated in the side view, while the laser beam 10 is focused in the bird's-eye view. Due to the arrangement of non-rotationally symmetrical in-coupling optics 20 and rotationally symmetrical mirror elements 220, a line focus 30 can accordingly be formed alternately in the x-z plane and in the yz plane.

In this case, the mirror elements 20, 220 have a coating with which the laser pulse 100 can interact non-linearly. In particular, it should be pointed out that the beam diameter and intensity curves of FIG. 3A can be generated with the device 2 shown here, but this was realized by means of lenses. An analogous use of lenses and mirrors with a coating can therefore always be assumed.

Various beam properties of an alternative embodiment of the device 2 are shown in FIG. 5, in which all line foci 30 lie in the same plane. This can be achieved in particular in that both the in-coupling optics 20 and the mirror elements 220 are not rotationally symmetrical. Accordingly, the laser beam 10 can always be focused in the y-z plane. A corresponding device 2 is shown in FIG.

FIG. 6 shows a device 2 in which the in-coupling optics 20 and the mirror elements 220 are non-rotationally symmetrical, or are cylindrical mirrors. In the present view, the cylinder axis is perpendicular to the plane of the page, so that the cylinder curvature of all optical elements 20, 220 and 26 lies in the plane of the page.

The mirror elements 220 are designed here as non-linear optics with a lens effect and include a correspondingly designed and shaped body 221 made of a transparent, non-linear material, for example quartz glass, which is provided with a mirror coating 222 on the back. The laser beam 10 first propagates through the non-linear material of the body 221 and is then reflected at the mirror coating 222 on the back before it again passes through the material of the body 221 and then leaves the mirror element 220 again.

When passing through the non-linear material of the body 221, the laser beam collects the B integral again due to the non-linear interaction.

The collimated laser beam 10 falls on the in-coupling optics 20 and is reflected from there to the first mirror element 220 . During the passage through the material of the body 221 is the laser pulse 100 is spectrally broadened non-linearly. The laser beam 10 then passes through a first focus 30. After the first reflection at the in-coupling optics 20, the laser beam 10 passes through a line focus 30, but the ionization threshold of the surrounding gas or the air 240 is not exceeded. Since the second mirror element 220 is also non-rotationally symmetrical and is aligned like the first mirror element 220, the laser beam 10 is always focused in the same beam plane. The laser beam 10 is reflected 10 times, for example, with each time the laser beam passes through the respective bodies 221 of the mirror elements 220, the laser pulse 100 interacts non-linearly with the material of the bodies 221 of the mirror elements 220 and is thus spectrally broadened. The last reflection at the decoupling optics 26 collimates the laser beam again.

As already indicated in FIG. 6 and 3D, the optical elements 20, 224, 26 can have a distance which corresponds to the sum of the focal lengths of two optical elements 20, 224, 26 hit by the laser beam 10 in succession. For example, the in-coupling optics 20 can have a focal length of 10 mm and the first lens element 224 or the first mirror element 220 can have a focal length of 10 mm. The distance between the two elements can therefore be 20 mm, for example.

The configurations described above for the embodiments with lens elements 224 can also be implemented analogously for coated mirror elements 220 .

As far as applicable, all individual features that are presented in the exemplary embodiments can be combined with one another and/or exchanged without departing from the scope of the invention.

Reference character list

1 laser

10 laser beam

12 telescope 100 laser pulse

2 device

20 coupling optics

22 arrangement of lens elements

220 mirror element 224, 224' non-linear optics with lens effect

26 decoupling optics

3 focus

30 line focus

Claims

24 claims

1 . Device (2) for spectral broadening of a laser pulse (100) of a laser beam (10), comprising in-coupling optics (20), at least one non-linear optic with a lens effect (224, 224'), and out-coupling optics (26), wherein the in-coupling optics (20 ) is set up to couple the laser beam (10) into the at least one non-linear optics with a lens effect (224, 224'), and wherein the decoupling optics (26) are set up to decouple the laser beam (10), the laser pulse (100) of the laser beam (10) is spectrally broadened by a non-linear interaction with the at least one non-linear optics with a lens effect (224, 224') and the laser beam (10) passes through the coupling optics (20) and/or the at least one non-linear optics with a lens effect (224 , 224') is focused, characterized in that the beam cross-section is elongated in the focus (30), the focus (30) preferably being a line focus.

2. Device (2) according to claim 1, characterized in that the pulse peak power of the laser pulses (100) of the laser beam (10) is greater than 3 MW and less than 2 TW, preferably greater than 30 MW and particularly preferably at 50 MW to 500 GW lies.

3. Device (2) according to claim 1 or 2, characterized in that the nonlinear phase shift of the laser pulse (100) by the nonlinear interaction per lens element (224, 224 ') between TT / 20 and 4TT, preferably between TT / 10 and 2TT amounts to.

4. Device (2) according to one of the preceding claims, characterized in that the elongated focus (30) between the coupling optics (20) and a nonlinear optics with a lens effect (224, 224') and/or between at least two nonlinear optics with a lens effect (224, 224') and/or between non-linear optics with a lens effect (224, 224') and the decoupling optics (26) in a liquid or gaseous medium, preferably air, with particularly preferably an elongated focus (30) between all non-linear optics with lens action (224, 224') is arranged in the liquid or gaseous medium.

5. Device (2) according to one of the preceding claims, characterized by an optical compressor which is set up to reduce the laser pulse duration to less than 0.9 times due to the spectral broadening of the laser pulse (100). to less than 0.7 times the original laser pulse duration.

6. Device (2) according to one of the preceding claims, characterized in that the coupling optics (20) and/or the decoupling optics (26) are non-rotationally symmetrical optics, preferably cylindrically symmetrical optics and/or cylindrically symmetrical lenses.

7. Device (2) according to one of the preceding claims, characterized in that at least one of the non-linear optics with a lens effect (224, 224') is a rotationally symmetrical lens element (224, 224').

8. Device (2) according to one of the preceding claims, characterized in that the coupling-in optics (20) and the coupling-out optics (26) are each designed as lenses and wherein the laser beam (10) when passing through the lenses (20, 26) of the Coupling optics and the decoupling optics and through the non-linear optics with a lens effect (224, 224 ') interacts non-linearly with the material of the lenses and the material of the non-linear optics with a lens effect (224, 224').

9. Device (2) according to one of the preceding claims, characterized in that less than 20 non-linear optics with a lens effect (224, 224'), preferably less than 10 non-linear optics with a lens effect (224, 224') with the laser beam (10) interact nonlinearly.

10. Device (2) according to one of the preceding claims, characterized in that at least two, preferably all, non-linear optics with a lens effect (224, 224') have the same focal length or that two non-linear optics with a lens effect (224, 224') and/or or the in-coupling optics (20) and non-linear optics with a lens effect (224, 224') and/or non-linear optics with a lens effect (224, 224') and the out-coupling optics (26), with which the laser beam (10) interacts non-linearly one after the other , have a distance from one another which is between 0.8 times and 1.2 times the sum of the focal lengths of the respective optical elements.

11 . Device (2) according to one of the preceding claims, characterized in that the in-coupling optics (20) and/or the out-coupling optics (26) and/or at least one non-linear optics with lens effect is designed as a mirror element (220), preferably as a quartz glass mirrored on the back Element formed mirror element (220). Method for spectral broadening of a laser pulse (100) of a laser beam (10), the laser beam (10) being coupled into at least one nonlinear optics with a lens effect (224) by a coupling optics (20), the laser beam (10) being the nonlinear optics with a lens effect (22), the laser pulse (100) of the laser beam (10) being spectrally broadened by a nonlinear interaction with the nonlinear optics with a lens effect (224, 224'), and the laser beam (10) after passing through the at least one nonlinear optics is coupled out with a lens effect (224, 224') by a coupling-out optics (26), the laser beam (10) being focused by the coupling-in optics (20) and/or at least one non-linear optics with a lens effect (224, 224'), characterized in that that the beam cross-section is elongated in the focus (30), the focus (30) preferably being a line focus. Method according to Claim 12, characterized in that the elongated focus (30) is between the coupling optics (20) and the non-linear optics with a lens effect (224, 224') and/or between at least two non-linear optics with a lens effect (224, 224') and /or is formed between non-linear optics with a lens effect (224, 224') and the decoupling optics (26) in a liquid or gaseous medium, preferably air, and the intensity of the laser beam (10) in the focus (30) is less than the ionization threshold of the is a liquid or gaseous medium, in particular a gas (240), and/or the pulse power of the laser pulse (100) of the laser beam (10) is less than the critical pulse peak power (Pent) and/or a non-linear phase shift per pass through the resonator ( 22) is between TT/20 and 4TT, preferably between TT/10 and 2TT. Method according to Claim 12 or 13, characterized in that the spectral broadening of the laser pulse (100) makes it possible to compress the laser pulse duration to less than 0.9 times, preferably to less than 0.7 times, the original laser pulse duration. Method according to one of Claims 12 to 14, characterized in that the laser pulses (100) of the laser beam (10) are provided with a pulse peak power greater than 3 MW and less than 2 TW, preferably greater than 30 MW and particularly preferably at 50 MW to 500GW.