DE102021127314A1 - Device and method for spectral broadening of a laser pulse - Google Patents

Abstract

The present invention relates to a method and a device (2) for spectrally broadening a laser pulse (100) of a laser beam (10) of a laser (1), the laser beam (10) being coupled into a resonator (22), the laser beam (10 ) passes through the resonator (22) several times, the laser pulse (100) of the laser beam (10) is spectrally broadened when passing through the resonator (22) by nonlinear interaction with a nonlinear medium, the laser beam (10) passes through the resonator at least once (22) is focused, and the laser beam (10) is coupled out of the resonator (22), the beam cross section of the laser beam (10) being elongated in the focus (30), the focus (30) preferably being a line focus.

Description

technical field

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

State of the art

It is known to use Herriott cells 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 or DE102014007159B4 . Here, a laser beam is repeatedly guided through a gas and focused in it. The laser pulse of the laser beam can then interact non-linearly with the gas due to the high intensity. The non-linear interaction leads to a self-phase modulation, which causes a spectral broadening, ie a broadening of the laser pulse in the frequency range, in the area of the focus points, essentially along the Rayleigh length.

However, the maximum usable pulse peak power of the laser beam is limited by the ionization threshold of the gas. With conventional Herriott cells, this ionization threshold of the gas is reached very quickly due to the high intensity in the area of the focus points along the Rayleigh length.

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

In addition, the strength of the nonlinear interaction is influenced by another nonlinear effect that can also occur at high pulse peak powers in the region of the focus points along the Rayleigh length, namely the formation of a Kerr lens, which can lead to catastrophic self-focusing, which can also occur at high intensities in the area of focus points along the Rayleigh length.

One way to reduce the intensity of the laser beam in the area of the focus points 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 is bought at the expense of a longer design or a longer focal length of the optics used in the Herriott cell, so that the intensity of the laser can be kept below the damage threshold of the optics used. As a result, such Herriott cells can become very large for high pulse energies. For example, conventional Herriott cells for pulse energies greater than one joule can be more than 40m long.

Herriott cells can therefore hardly be realized technically efficiently for high pulse energies.

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 of a laser beam of a laser, and a corresponding method.

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

Accordingly, a method for the spectral broadening of a laser pulse of a laser beam of a laser is proposed, the laser beam being coupled into a resonator, the laser beam passing through the resonator several times, the laser pulse of the laser beam being spectrally broadened as it passes through the resonator by nonlinear interaction with a nonlinear medium , the laser beam is focused in at least one, preferably in several or in each passage through the resonator, and the laser beam is coupled out of the resonator. According to the invention, the beam cross section is elongated in the focus, with the focus preferably being a line focus.

The ultra-short pulse laser makes the ultra-short laser pulses of the laser beam available, with the individual laser pulses forming the laser beam in the beam propagation direction. By definition, the beam propagation direction is the z-direction.

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 resonator. The coupling can be achieved, for example, by means of coupling optics. For example, a mirror as coupling optics can deflect the laser beam. In particular, it is possible for the resonator to include the in-coupling optics, or for an optical element of the resonator to act as in-coupling optics. In particular the coupling can also already include a focusing of the laser beam. However, the coupling optics can also be provided in the form of a corresponding passage opening in a mirror of the resonator.

A resonator can be generally understood as a device in which the laser beam propagates for a certain period of time. This length of time is linked to the optical path length of the resonator via the speed of light. Such a temporal storage of the laser beam is typically achieved in that the laser beam is reflected multiple times between the resonator inlet and the resonator outlet. In this case, reflection preferably takes place on at least two resonator mirrors with a reflective layer that reflects the laser beam by more than 95%, preferably more than 99%, for example, so that the reflections in the resonator result in only low energy losses, or preferably no energy losses. A passage of the laser beam through the resonator describes the covering of a distance in the resonator between two reflections.

In the present case, a non-linear interaction of the laser pulse of the laser beam with the non-linear medium is generated on the path between two reflections. For this purpose, for example, a gas atmosphere with a gas is present in the resonator as a non-linear medium. The gas can be, for example, ambient air or cleaned or dried air, or it can be in the form of an inert gas, for example helium, argon or krypton.

A liquid medium can also be used as the non-linear medium.

In order to form a gas atmosphere forming the non-linear medium, the volume of the resonator can be sealed off from the environment in order to be able to absorb and store gas or to be able to maintain a certain gas pressure via a supply line or discharge line or some other means.

If ambient air is used as the gas forming the non-linear medium, there is no need for a gas-tight sealing of the device with respect to the environment.

Proportionally, the greatest spectral broadening takes place in the focus and in particular in the area of the focus along the Rayleigh length, since the intensities are highest here. When passing through the gas in the gas atmosphere, a spectral broadening takes place in principle along the entire path of the laser pulse through the gas, but this is only worth mentioning in the high-intensity range, as explained below.

For example, the first resonator mirror and the second resonator mirror can be round and face each other. For example, the first and second resonator mirrors can form two end faces of an enclosure that seals off a gas atmosphere. For example, a cover that seals off the gas atmosphere can be cylindrical, so that only a cylinder jacket needs to be placed around the two sides of the resonator. However, it can also be the case that the cladding is cubic or has a different shape and is formed between the mirrored sides of the resonator. However, other geometries are also conceivable.

In order to bring about a significant nonlinear interaction between the laser pulse and the nonlinear medium, high intensities are necessary. The laser beam is therefore focused on each passage through the resonator, with the focus being in the non-linear medium at least once, preferably always, during passages through the resonator.

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 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 x-z plane and the y-z plane are beam planes. When the laser beam is focused, then the beam diameter is also minimized with respect to the x-z plane and/or the y-z plane.

The reason for the non-linear interaction of the laser beam with the non-linear medium can be the Kerr effect, according to which the optical properties, or the refractive index n, of the non-linear medium can be varied by applying an electric field. In particular, a laser pulse can represent the electrical field, with high-energy laser pulses or laser pulses with particularly high power providing particularly large electrical fields. This effect is intensified by focusing the laser pulse.

In the non-linear medium, for example the gas, a variation of the refractive index is generated by the high laser intensity in the focus: $$n\left(I\right)={\mathrm{<figure-callout\; id="0"\; label="n"\; filenames="DE102021127314A1\_0013.png,DE102021127314A1\_0014.png"\; state="\{\{state\}\}">n</figure-callout>}}_0+n_{2}I\left(\mathrm{e.g},t\right).$$

The linear refractive index is given here by n ₀ and the non-linear refractive index is given by n ₂ . 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. For example, in the case of argon, the non-linear refractive index is n _2,Ar = 1.1×10 ^-24 m ² /W at a gas pressure of 100 mbar.

wobei λ die Wellenlänge des Laserstrahls ist. Aus dem B-Integral wird bereits deutlich, dass ein großer nichtlinearer Brechungsindex n₂ und/oder eine große Intensität I des Laserstrahls zu einem nichtlinearen Phasenschub führen.To characterize a non-linear interaction, the so-called B-integral can be used, which is a measure of a non-linear phase shift of the laser beam as it passes through the gas. The B integral is defined as: $$B=\frac{2\pi }{\lambda \int n_{2}I\left(\mathrm{e.g},t\right)\text{double}},$$

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

Assuming the nonlinear refractive index n ₂ is constant and the laser beam is focused, then significant contributions to the B-integral arise only in the regions of the laser beam where it has a high intensity, i.e. in and around the focus, especially in the area of the Rayleigh length. This can be the case, for example, in the interval in which the intensity of the laser beam is greater than 1/e of the intensity at the focus.

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: $$\frac{\mathrm{i.e}n\left(I\right)}{\mathrm{i.e}t}=\frac{{\mathrm{<figure-callout\; id="2"\; label="n"\; filenames="DE102021127314A1\_0013.png,DE102021127314A1\_0014.png"\; state="\{\{state\}\}">n</figure-callout>}}_2\mathrm{i.e}I\left(\mathrm{e.g},t\right)}{\mathrm{i.e}t}.$$

und $k_0=\frac{2\pi }{{\lambda }_0}$

schreiben als $$\varphi \left(z,t\right)={\omega }_{0}t-kz={\omega }_{0}t-\frac{2\pi }{{\lambda }_0}zn\left(I\right).$$

The phase shift of the laser beam can now be $\lambda =\frac{\mathrm{<figure-callout\; id="0"\; label="c\; n\; ƒ"\; filenames="DE102021127314A1\_0013.png,DE102021127314A1\_0014.png"\; state="\{\{state\}\}">c</figure-callout>}}{n{ƒ}_{}}=\frac{{\lambda }_0}{n}$

and ${\mathrm{<figure-callout\; id="0"\; label="k"\; filenames="DE102021127314A1\_0013.png,DE102021127314A1\_0014.png"\; state="\{\{state\}\}">k</figure-callout>}}_0=\frac{2\mathrm{<figure-callout\; id="0"\; label="\pi \; \lambda "\; filenames="DE102021127314A1\_0013.png,DE102021127314A1\_0014.png"\; state="\{\{state\}\}">\pi </figure-callout>}}{{\lambda }_{}}$

write as $$\varphi \left(\mathrm{e.g},t\right)={\omega }_{0}t-k\mathrm{e.g}={\omega }_{0}t-\frac{2\mathrm{<figure-callout\; id="0"\; label="\pi \; \lambda "\; filenames="DE102021127314A1\_0013.png,DE102021127314A1\_0014.png"\; state="\{\{state\}\}">\pi </figure-callout>}}{{\lambda }_{}}\mathrm{e.g}n\left(I\right).$$

This results in a nonlinear phase shift of after a distance L has been traversed through the nonlinear medium $$\varphi \left(L,t\right)={\omega }_{0}t-\frac{2\pi }{{\lambda }_0}Ln\left(I\right),$$

From this it is immediately apparent that this causes a change in frequency caused by the nonlinear refractive index n ₂ : $$\omega \left(t\right)=\frac{\mathrm{i.e}\varphi }{\mathrm{i.e}t}=-\frac{2\pi }{{\lambda }_0}\mathrm{<figure-callout\; id="2"\; label="L\; n"\; filenames="DE102021127314A1\_0013.png,DE102021127314A1\_0014.png"\; state="\{\{state\}\}">L</figure-callout>}{n}_{}{}_2\frac{\mathrm{i.e}I\left(t\right)}{\mathrm{i.e}t}.$$

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 propagation in the non-linear medium. The associated process is called self-phase modulation. It should be noted in particular that the self-phase modulation also depends on the length of the route traversed. Accordingly, the self-phase modulation also scales with the number of passes through the resonator.

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 center of the beam. As a result, the non-linear medium provides a lens effect for the laser beam above a certain power.

A catastrophic self-focusing occurs here from a critical pulse peak power P _crit where the power P of the laser is proportional to the laser intensity I and the cross-sectional area A: $$P\propto IA.$$

The critical pulse peak power is a material-dependent parameter and depends on the wavelength and the non-linear refractive index n ₂ . The catastrophic self-focusing destroys the beam shape of the laser beam entering the resonator to a certain extent and is to be avoided at best. In other words, catastrophic self-focusing results in beam collapse.

In the case of pulse peak powers lying below the critical pulse peak power P _crit , slight self-focusing can already occur, but this does not yet lead to a beam collapse.

The laser beam entering the resonator can have a Gaussian profile. However, other beam profiles are also conceivable, such as top-hat beam profiles,

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.

In this way it can be achieved that the intensity of the laser beam is reduced in the focus, 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 larger pulse peak powers can also be spectrally broadened before catastrophic self-focusing occurs. At the same time, the low intensity of the laser beam in the focus avoids exceeding the ionization threshold of the gas. This enables a shorter construction of the resonator, since larger opening angles can be used. In short, the elongated beam focus allows the spectral broadening of high-energy laser pulses.

The focus can be a line focus.

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.

The method can be controlled particularly well by means of a line focus, since the interaction zones between the laser beam and the gas are particularly uniform and only have to be controlled and monitored with respect to one beam plane.

The intensity of the laser beam can be smaller than the ionization threshold of a gas used as non-linear medium.

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% , more preferably less than 10% of the gas present there are ionized.

Ionization of the gas changes the refractive index and the non-linear refractive index, since the distribution of the electrical charge carriers in the gas changes. Eventually, ionization of the gas means that the spectral broadening of the laser pulse can no longer be controlled or is uncontrolled.

The pulse power of the laser pulse of the laser beam can be less than the critical pulse peak power.

This avoids catastrophic self-focusing and hence beam collapse.

The non-linear phase shift, given by the B integral, can be between π/20 and 4π, preferably between π/10 and 2π, per pass.

As a result, the entire interaction process between the laser and the gas can be easily controlled.

The laser beam can be passed through the resonator in two consecutive passes are focused so that the elongated beam cross-sections of the foci are in different beam planes or in the same beam plane.

For example, the laser beam can be focused in a first pass through the resonator so that the elongated beam cross-section is in focus in the x-z plane and in a second pass so that it is in the y-z plane. However, it can also be the case that the laser beam is focused in two consecutive passes, for example in the x-z plane.

This has the advantage that the resonator and the in-coupling optics can be constructed and configured in different ways in order to enable adjustment that is as simple and inexpensive as possible.

Corresponding embodiments are presented further below.

The laser beam can pass through the resonator and thus in particular the non-linear medium more than 4 times, preferably more than 8 times, particularly preferably more than 10 times.

As a result, on the one hand, the spectral broadening to be achieved can be divided over several passages through the resonator, so that the intensity of the laser beam can be kept safely below the critical pulse peak power. Accordingly, the laser beam does not have to be focused too much in order to achieve the desired spectral broadening.

On the other hand, as shown above, the spectral broadening depends on the length L in which the laser pulse interacts with the nonlinear medium. For example, if the laser beam traverses the nonlinear medium more than 4 times, then the laser beam can also collect a B integral that is 4 times larger. In this way, particularly large spectral broadening of the laser pulses can also be achieved.

Due to the spectral broadening of the laser pulse, the laser pulse duration can then be compressed in a subsequent compressor (for example a lattice compressor) to less than 0.9 times, preferably to less than 0.7 times, the original 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 Δf in the frequency domain is inversely proportional to the duration of the signal Δt in the period: $$\mathrm{\Delta }\text{f}\propto \frac{1}{\mathrm{\Delta }\text{t}}.$$

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 downstream pulse compressor. A corresponding pulse compression can also be achieved by at least one chirped mirror, which in particular can also be part of a decoupling optics.

The pulse peak power P _p of the laser pulses of the laser beam can be from 80 megawatts (MW) to 1 petawatt (PW). 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: $$P_p\approx s\frac{E_p}{t_p}$$

For example, for a Gaussian pulse, the factor s is ≈ 0.94.

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

The object set above is also achieved by a device for spectrally broadening a laser pulse of a laser beam of a laser having the features of claim 8. Advantageous developments of the method result from the dependent claims and the present description and the figures.

Accordingly, a device for spectrally spreading a laser pulse of a laser beam is proposed, comprising a resonator in which a nonlinear medium is present, and coupling-out optics, the coupling-in optics being set up to couple the laser beam into the resonator, the resonator being set up to Leading the laser beam multiple times through the non-linear medium, the laser pulse of the laser beam being spectrally broadened by a non-linear interaction with the non-linear medium, the resonator and/or the coupling optics being set up to focus the laser beam, the laser beam being at least one preferably at several or at each, passage through the resonator is focused, the off coupling optics is set up to decouple the laser beam from the resonator. According to the invention, the beam cross section is elongated in the focus. In this case, the focus can in particular be a line focus.

In a first exemplary embodiment, the coupling-in optics can be a non-rotationally symmetrical mirror, preferably a cylinder mirror, and the resonator can have at least two opposing, rotationally symmetric resonator mirrors, preferably spherical resonator mirrors, with the decoupling optics preferably being a non-rotationally symmetrical mirror, preferably a cylinder mirror.

A mirror is non-rotationally symmetrical if the surface has no rotational symmetry. In particular, a non-rotationally symmetrical mirror is not a spherical mirror. For example, a cylindrical mirror is a non-rotationally symmetrical mirror. If the mirror has a parabolic cross-section (rather than cylindrical) and a longitudinal axis, then it is also non-rotationally symmetrical. A so-called asphere can also be a non-rotationally symmetrical mirror. In addition to a spherical mirror, a parabolic mirror is also rotationally symmetrical.

In the following example, a cylindrical mirror is used as a representative of a non-rotationally symmetrical mirror and a spherical mirror is used for a rotationally symmetrical resonator mirror.

For example, a collimated laser beam falls in the z-direction onto a convex cylinder mirror, which is the launching optics, with the cylinder axis 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 x-z plane and the y-z plane.

On the one hand, the laser beam is reflected in the x-z plane without modification of the beam propagation (note: "x as without modification"), since the cylindrical mirror 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 cylinder mirror 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 at the focal point. 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 a focus between the in-coupling optics and the first resonator mirror, which can also already be in the non-linear medium.

After passing through the focus, the laser beam diverges in the y-z plane. Subsequently, the laser beam impinges on the first resonator mirror where the laser beam is collimated with respect to the y-z plane, whereas the laser beam is now focused in the x-z plane by interaction with the first spherical resonator mirror. To a certain extent, the focal plane is changed by the combination of non-rotationally symmetrical (cylindrical) coupling optics and a rotationally symmetrical (spherical) resonator mirror.

Accordingly, the laser beam has a focus in the x-z plane between the first and the second spherical resonator mirror, or during the first passage through the resonator. The focus here extends into the y-z plane and is therefore elongated, or a line focus.

After the first pass, the laser beam hits the second spherical resonator mirror where the laser beam is re-collimated with respect to the x-z plane and is focused with respect to the y-z plane.

Accordingly, focusing of the laser beam alternates between the x-z plane and the y-z plane on successive passes through the resonator. In particular, in this embodiment the laser beam is focused only once in each pass. In other words, the same alignment of the beam cross section is only achieved with every second pass, which corresponds to a so-called 4f image.

The coupling optics can also be designed in the form of a passage opening in one of the mirrors of the resonator, so that in this way a Coupling of the laser beam can be achieved in the resonator.

• In der Meridionalebene wird der Laserstrahl dank der kleineren Brennweite in Strahlausbreitungsrichtung zuerst fokussiert, sprich der Strahldurchmesser bezüglich der Meridionalebene des Laserstrahls wird in dem ersten Fokus minimiert. Entsprechend ist der Strahldurchmesser bezüglich der Meridionalebene im Fokus kleiner als in der Sagittalebene. Es ergibt sich somit bei einem astigmatischen Spiegel ein Linienfokus, der sich in der Sagittalebene erstreckt.

However, it is also possible for the coupling optics to be an astigmatic mirror which has two different focal lengths in two different, typically orthogonal, mirror planes, the sagittal plane and the meridional plane. For example, the sagittal plane can coincide with the xy plane and the yz 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.

In both of the coupling optics designed, the line focus is suitable for bringing about a non-linear interaction of the laser beam with the non-linear medium by increasing the intensity. However, due to the elongated focus, the intensity is low enough to avoid critical self-focusing. Due to the non-linear interaction with the non-linear medium, the laser pulse experiences a spectral broadening, as described above.

After the laser beam has been broadened spectrally to a sufficient extent, the laser beam can be coupled out of the resonator by means of a coupling optic. The decoupling optics can 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 decoupling optics can also be designed in the form of a passage opening in one of the resonator mirrors, so that the laser beam can be decoupled from the resonator in this way.

In this case, the resonator can have at least two resonator mirrors. This can mean that the resonator has, for example, exactly two monolithic mirrors. In particular, in the present embodiment, each mirror segment can be a spherical mirror. Accordingly, the resonator can be a modified Herriott cell. This achieves a particularly high level of mechanical stability.

However, it can also be the case that each resonator mirror is composed of several mirror segments. As a result, a higher degree of freedom of adjustment can be achieved.

In a second exemplary embodiment, the coupling-in optics can be a non-rotationally symmetrical mirror, preferably a cylinder mirror, and the resonator can have at least two opposing non-rotationally symmetrical resonator mirrors, preferably cylinder mirrors, with the decoupling optics preferably being a cylinder mirror.

Analogously to the first embodiment, the collimated laser beam is given a convergence, for example in the y-z plane, by the non-rotationally symmetrical in-coupling optics.

If the focus through the in-coupling optics is, for example, in front of the first cylinder mirror of the resonator, then a further focus zone can be generated during the first pass by appropriately selecting the focal length of the cylinder mirror of the resonator. The focus zone can also be in the y-z plane. The second cylinder mirror of the resonator can in turn produce a focus during the second pass. If the cylinder axes of the in-coupling optics and resonator optics run parallel, then the laser beam is always focused in the y-z plane. To a certain extent, the same alignment of the beam cross section is thus achieved with each pass, which corresponds to a so-called 2f image.

However, it can also be the case that the cylinder axes of the resonator are orthogonal to one another. Then, by a suitable choice of the focal length of the resonator mirror, an alternating focusing in the plane of the cylinder curvature tion and the plane of the cylinder axis. 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.

It is also possible that the resonator is a multi-pass cell, with the multi-pass cell having at least three mirrors, with the first mirror of the multi-pass cell on which the laser beam hits being coupling optics and the last mirror of the multi-pass cell on which the laser beam hits being the coupling-out optics.

In this case, a multipass cell is in particular a two-dimensional arrangement of mirrors. However, the mode of operation is analogous to that of the Herriott cell or to the resonators described above.

An advantage of the multi-pass cell is the simpler adjustment and the simpler mechanical design. As a result, a multipass cell can be produced inexpensively.

The distance between two resonator mirrors hit by the laser beam one after the other can be between 0.8 times and 1.2 times the sum or twice the sum of the focal lengths of the two mirrors.

For example, the first spherical mirror of the resonator can have a focal length of 30 cm and the second spherical mirror of the resonator 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 cylindrical mirror of the resonator can have a focal length of 100 cm and the second cylindrical mirror of the resonator can have a focal length of 100 cm. Then the mirrors can also be set up at a distance of 80 to 120. In this case, the same orientation of the cross-section of the focus can be achieved in every second pass.

In particular, the mirrors of the resonator can have the same focal length.

This allows for a simpler geometric arrangement of the mirrors and simpler adjustment.

Overall, the overall transmission of the device can be greater than 50%. As a result, power losses in particular can be reduced. In addition, a high laser power can be achieved.

In particular, the total transmission can relate to the spectral range that is to be achieved with the spectral broadening. Therefore, the transmission at any frequency of the spectral range of the broadened pulse of the overall system can be more than 50%.

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 peak power. 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 mirrors should be less than the laser-induced damage threshold (LIDT) so that the mirrors are not destroyed by the laser beam. The laser-induced damage threshold is not reached when the cross-sectional area of the laser beam on the mirror exceeds a certain critical cross-sectional area. The focal length of the mirror can thus be derived from the cross-sectional area on the mirrors, the cross-sectional area in the focus and the opening angle.

Finally, the nonlinear phase shift can be adjusted by the B integral per pass through the nonlinear medium via the selection of the nonlinear medium and in particular via the gas pressure and the type of gas.

In particular, safety factors can be taken into account in the calculations mentioned above in order to reliably avoid ionization or destruction of the mirror.

character list

• 1 eine schematische Darstellung der Vorrichtung;
• 2 eine schematische Darstellung der Ortsabhängigkeit verschiedener Strahleigenschaften gemäß Stand der Technik;
• 3A, B, C eine schematische Darstellung der erfindungsgemäßen Ortsabhängigkeit verschiedener Strahleigenschaften;
• 4A, B, C, D schematische Darstellungen unterschiedlicher Einkoppeloptiken;
• 5 eine weitere schematische Darstellung der erfindungsgemäßen Ortsabhängigkeit verschiedener Strahleigenschaften;
• 6 eine schematische Darstellung einer erfindungsgemäßen Vorrichtung;
• 7 eine weitere schematische Darstellung einer erfindungsgemäßen Vorrichtung;
• 8 eine weitere schematische Darstellung einer erfindungsgemäßen Vorrichtung;
• 9 eine weitere schematische Darstellung einer erfindungsgemäßen Vorrichtung; und
• 10 eine schematische Darstellung einer erfindungsgemäßen Multipasszelle.

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

• 1 a schematic representation of the device;
• 2 a schematic representation of the spatial dependence of different beam properties according to the prior art;
• 3A, B , C a schematic representation of the spatial dependence of different beam properties according to the invention;
• 4A, B , C , D schematic representations of different coupling optics;
• 5 a further schematic representation of the spatial dependence of different beam properties according to the invention;
• 6 a schematic representation of a device according to the invention;
• 7 a further schematic representation of a device according to the invention;
• 8th a further schematic representation of a device according to the invention;
• 9 a further schematic representation of a device according to the invention; and
• 10 a schematic representation of a multipass cell according to the invention.

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.

In 1 a first embodiment of the device 2 proposed here is shown schematically. 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.

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 P _p of the laser pulses of the laser beam can be from 80 megawatts (MW) to 1 petawatt (PW), preferably between 500 megawatts (MW) and 100 terawatts (TW).

In this case, the device 2 has coupling optics 20 , a resonator 22 , a nonlinear medium 240 present at least in the resonator 22 , and coupling optics 26 . The laser beam 100 strikes the in-coupling optics 20 in a collimated state and is guided by the latter into the resonator 22 . In the resonator 22, the laser beam 10 is reflected multiple times by the resonator sides 220, 222 and is guided through the non-linear medium 240 in the process.

With each passage through the resonator 22, the laser beam 10 is focused, so that the laser pulse 100 can interact with the non-linear medium 240 after each reflection on one of the resonator sides 220, 222 and thereby be spectrally broadened. After the last reflection of the laser beam 10 on a resonator side 222, the laser beam 10 is coupled out by the coupling-out optics 26. The decoupled laser pulse 100 ′ is spectrally broadened compared to the original laser pulse 100 . The decoupling optics 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.

The nonlinear medium 240 may be, for example, ambient air, filtered air, or a specifically selected gas, such as an inert gas. If the non-linear medium 240 is only present as ambient air, a gas-tight seal of the device 2 with respect to the environment can be dispensed with. However, if something other than ambient air is to be used as the non-linear medium 240, the device 2 must provide a gas-tight seal with respect to the environment or allow the desired gas to flow through at least the resonator 22 continuously. The gas provided as the non-linear medium can, for example, be an inert gas, for example helium, argon or krypton.

Instead of in the 1 shown separate coupling optics 20 and the separate decoupling optics 26, these can also each be formed by an opening in one of the resonator mirrors 220, 222, through which the laser beam 10 can enter the resonator 22 and exit again.

Different beam properties in a prior art resonator are in 2 shown. According to the prior art, the coupling optics 20 shown, the first resonator mirror 220 and the second resonator mirror 220 and the coupling-out optics 26 are spherical mirrors. This has far-reaching consequences for beam shaping in the resonator 22. In 2 the beam diameter D is shown as a function of the beam propagation direction z for the xz plane and the yz plane. The z-direction always coincides with the beam propagation direction of the laser beam 10, even after reflections on one of the built-in mirrors.

The laser beam 10 is first collimated before it falls on the in-coupling optics 20 . This can be seen from the fact that the beam diameter 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 reflected and focused by the spherical in-coupling optics 20 . In particular, this results in the laser beam 10 being focused in the xz plane and the yz plane. The entire energy of the laser beam 10 is accordingly concentrated on a quasi-point-like area, so that the intensity I increases locally in the focal point 3 . This is also in 2 shown. The focal point 3 is accordingly arranged where the beam diameter D has a minimum in both beam diameters. The laser beam 10 moves through the non-linear medium 240 formed by the gas, with a non-linear interaction between the gas and the laser pulse 100 leading to a spectral broadening.

After the laser beam 10 has traversed a minimum of the beam diameter D, the laser beam 10 diverges to the second mirror 222 of the resonator 22 from where it is again reflected and focused, and so on.

As described above, due to the high intensity and the non-linear interaction with the gas in the quasi-punctiform focus 3, the laser beam 10 can self-focus or the gas can ionize. The former is particularly the case when the power of the laser exceeds the critical pulse peak power P _crit . Since the critical pulse peak power P _crit 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.

In order to solve this problem, the resonator 22 and/or the in-coupling optics 20 according to the device 2 proposed here are set up to focus the laser beam 10, with the beam cross-section being elongated in the focus.

In 3A a corresponding course of different beam properties of the resonator 22 according to the proposed device 2 is shown. 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. As a result of the focusing in the yz beam plane, the laser beam 10 has an unchanged beam diameter D in the xz plane even after reflection at the in-coupling optics 20 . In the minimum of the beam diameter D, ie the focus 30 between the first and the second resonator side 220, 200, an intensity increase in the form of a line focus 30 is consequently generated. The line focus 30 extends in the xz plane when focusing in the yz plane. Due to the line focus, the intensity in the line focus 30 remains comparatively low, so that the critical pulse peak power P _crit is not reached. As a result, catastrophic self-focusing does not occur either. Typically, the intensity in this case is also so low that ionization of the gas used as non-linear medium 240 is avoided.

However, due to the sufficiently high intensity in the line focus 30, a spectral broadening of the laser pulse 100 as it passes through the non-linear medium 240 can be brought about. This behavior can be seen from the plotted B integral. The B integral, ie the non-linear phase shift, always increases in the area of the line focus 30. However, the B integral only experiences relevant contributions around the focus 30, so that the B integral follows a step function, so to speak.

Such a line focus 30 can be generated in different ways. One possibility is to combine a non-rotationally symmetrical in-coupling optics 20 with rotationally symmetrical resonator mirrors 220, 222.

In 3B the focus 30 of a laser beam 10 is shown, which was generated by an astigmatic coupling optics 20, for example an astigmatic mirror. The beam diameter has a minimum in the focus 30 in both the x-axis and the y-axis. However, the pulse peak conduction is distributed over a larger area compared to a point focus, so that neither self-focusing nor the ionization threshold of the gas used as the nonlinear medium is exceeded.

In 3C analogously, the focus of a laser beam 10 is shown, which was generated by a cylindrical in-coupling optics 20 . Here, the beam diameter is constant in the x-axis (as in 3A shown) 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.

For example, in 4A a coupling optics 20 in the form of a cylindrical mirror 20 in the side view and in 4B shown in 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 in does not focus at this level. Rather, the laser beam 10 is only reflected here.

Next, the laser beam 100 focused in the side view is guided onto the first resonator mirror 220, as in FIG 4C is shown. Accordingly, due to the spherical curvature of the mirror surface of the first resonator mirror 220, 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 resonator mirrors 220, 222, a line focus 30 can accordingly be formed alternately in the xz plane and in the yz plane.

In particular, it should be noted that with the proposed device 2, the beam diameter and intensity curves 3A can be generated.

In 5 Various beam properties of an alternative embodiment of the device 2 are shown 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 resonator mirrors 220, 222 are non-rotationally symmetrical. Accordingly, the laser beam 10 can always be focused in the yz plane. A corresponding device 2 is in 6 shown.

In 6 a device 2 is shown in which the in-coupling optics 20 and the resonator mirrors 220, 222 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 mirrors 20, 220, 222 and 26 lies in the plane of the page.

The collimated laser beam 10 falls on the in-coupling optics 20 and is reflected from there to the first resonator mirror 220, the laser beam passing through a first focus. After the first reflection at the first resonator side 220, the laser beam 10 passes through a line focus 30, which lies in the gas present as the nonlinear medium 240, so that the laser pulse 100 can interact with the nonlinear medium, so that the laser pulse 100 is spectrally broadened. Since the second resonator side 222 is also non-rotationally symmetrical and is aligned like the first resonator side 220, the laser beam 10 is always focused in the same beam plane. In this case, the laser beam 10 passes through the resonator 22, for example, 5 times.

Further embodiments of the device 2 are in 7 until 10 shown.

In 7 For example, resonator 22 consists of two opposite sides 220 and 222 each comprising a plurality of mirrors 224. The mirrors are arranged on the resonator sides 220, 222 in such a way that the laser beam 10 can pass through the non-linear medium several times. In particular, since the resonator sides 220, 222 have individual mirrors, simpler adjustment can be made possible.

In 8th are the monolithic resonator mirrors from the 1 and 6 replaced by so-called segmented mirrors 224 provided as resonator mirrors. This makes use of the fact that the laser beam 10 does not completely illuminate the resonator sides 220, 222, but only partially. In a sense, only those portions of the resonator sides 220, 222 that the laser beam 10 actually strikes are retained. Analogous to this is in 9 a case is shown in which the resonator 22 is actually made up of individual mirrors 224.

In 10 a so-called multipass cell is shown as resonator 22 . The multi-pass cell 22 has a multiplicity of mirrors 224 in one plane, by means of which the laser beam 10 is guided multiple times through the non-linear medium 240 . In particular, the first mirror is the in-coupling optics 20 and the last mirror is the out-coupling optics 26.

In principle, in the 7 until 10 each mirror taken by itself can be rotationally symmetrical or non-rotationally symmetrical. However, it is also possible for the mirror surfaces to coincide with the surface of a single rotationally symmetrical or non-rotationally symmetrical mirror.

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 List

1

Laser

10

laser beam

100

laser pulse

2

contraption

20

coupling optics

22

resonator

220

first resonator side

222

second resonator side

224

Mirror

240

nonlinear medium

26

decoupling optics

3

focus

30

line focus

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• DE 102014007159 B4 [0002]

Claims (13)

Method for spectral broadening of a laser pulse (100) of a laser beam (10) of a laser (1), the laser beam (10) being coupled into a resonator (22), the laser beam (10) passing through the resonator (22) several times, the laser pulse (100) of the laser beam (10) is spectrally broadened when passing through the resonator (22) by nonlinear interaction with a nonlinear medium, the laser beam (10) is focused during at least one passage through the resonator (22), and the laser beam (10 ) is decoupled from the resonator (22), characterized in that the beam cross section of the laser beam (10) is elongated in the focus (30), the focus (30) preferably being a line focus. procedure after claim 1 , characterized in that the intensity of the laser beam (10) in the focus (30) is less than the ionization threshold of the non-linear medium, in particular a gas (240), and/or the pulse power of the laser pulse (100) of the laser beam (10) is less than is the critical pulse peak power (P _crit ) and/or a non-linear phase shift per pass through the resonator (22) is between π/20 and 4π, preferably between π/10 and 2π. procedure after claim 1 or 2 , characterized in that the laser beam (10) is focused in two different passes, preferably in two consecutive passes, through the resonator (22) in different beam planes or in the same beam plane. Method according to one of the preceding claims, characterized in that the laser beam (10) is passed through the non-linear medium more than 4 times, preferably more than 8 times, particularly preferably 10 times. Method according to one of the preceding claims, 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 the preceding claims, characterized in that the pulse peak power of the laser pulses (100) of the laser beam (10) is between 80 MW and 1 PW, preferably between 500 ME and 100 TW. Method according to one of the preceding claims, characterized in that a gaseous or liquid medium is provided as the non-linear medium, preferably ambient air, purified air or an inert gas. Device (2) for spectral broadening of a laser pulse (100) of a laser beam (10) comprising a coupling optics (20), a resonator (22) in which a nonlinear medium is present, and a coupling optics (26), wherein the coupling optics (20) is set up to couple the laser beam (10) into the resonator (22), the resonator (22) being set up to guide the laser beam (10) multiple times through the non-linear medium, the laser pulse (100) of the laser beam (10 ) is spectrally broadened by a nonlinear interaction with the nonlinear medium, the resonator (22) and/or the coupling optics (20) being set up to focus the laser beam (10), the laser beam (10) passing through at least one time is focused on the resonator (22), the decoupling optics (26) being set up to decouple the laser beam (10) from the resonator (22), characterized in that the beam cross section is elongated in the focus (30), the focus (30) preferably a line focus. Device (2) after claim 8 , characterized in that the in-coupling optics (20) is a non-rotationally symmetrical mirror, preferably a cylinder mirror and/or the resonator (22) has at least two opposing rotationally symmetrical mirrors (220, 222), preferably spherical mirrors, the resonator ( 22) is preferably a Herriott cell and/or the decoupling optics (26) is a non-rotationally symmetrical mirror, preferably a cylindrical mirror. Device (2) after claim 8 , characterized in that the coupling-in optics (20) is a non-rotationally symmetrical mirror, preferably a cylinder mirror, and/or the resonator (22) has at least two opposing non-rotationally symmetrical mirrors, preferably cylinder mirrors, and/or the decoupling optics (26 ) is a non-rotationally symmetrical mirror, particularly preferably a cylindrical mirror. Device (2) after claim 8 , characterized in that the resonator (22) is a multi-pass cell, the multi-pass cell having at least three mirrors (224), the first mirror of the multi-pass cell on which the laser beam (10) impinges being a coupling lens (20) and the last Mirror of the multi-pass cell, on which the laser beam (10) strikes, is a decoupling optics (26). Device (2) according to one of Claims 8 until 11 , characterized in that the distance between two mirrors of the resonator (10) approached one after the other by the laser beam (10) is between 0.8 times and 1.2 times the sum of the focal lengths or twice the sum of the focal lengths of the two mirrors. Device (2) according to one of Claims 8 until 12 , characterized in that the mirrors (220, 222) of the resonator (22) have the same focal length.

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