WO2023066995A1 - Dispositif et procédé d'élargissement spectral d'une impulsion laser - Google Patents

Dispositif et procédé d'élargissement spectral d'une impulsion laser Download PDF

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Publication number
WO2023066995A1
WO2023066995A1 PCT/EP2022/079092 EP2022079092W WO2023066995A1 WO 2023066995 A1 WO2023066995 A1 WO 2023066995A1 EP 2022079092 W EP2022079092 W EP 2022079092W WO 2023066995 A1 WO2023066995 A1 WO 2023066995A1
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Prior art keywords
optics
laser beam
lens effect
linear
laser
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PCT/EP2022/079092
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German (de)
English (en)
Inventor
Yanik PFAFF
Sandro KLINGEBIEL
Peter Krötz
Michael Rampp
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Trumpf Scientific Lasers Gmbh + Co. Kg
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Publication of WO2023066995A1 publication Critical patent/WO2023066995A1/fr

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Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/3501—Constructional details or arrangements of non-linear optical devices, e.g. shape of non-linear crystals
    • G02F1/3503—Structural association of optical elements, e.g. lenses, with the non-linear optical device
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping
    • H01S3/0057—Temporal shaping, e.g. pulse compression, frequency chirping
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping
    • H01S3/0092—Nonlinear frequency conversion, e.g. second harmonic generation [SHG] or sum- or difference-frequency generation outside the laser cavity
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S2301/00—Functional characteristics
    • H01S2301/08—Generation of pulses with special temporal shape or frequency spectrum
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping

Definitions

  • the present invention relates to a device and a method for spectrally broadening a laser pulse of a laser beam.
  • 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.
  • 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.
  • 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.
  • 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.
  • a device 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.
  • 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.
  • 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
  • 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.
  • a mirror as coupling optics can deflect the laser beam.
  • the coupling can already include a focusing of the laser beam.
  • non-linear optics with a lens effect is an optical element which has an optically imaging property.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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:
  • the linear refractive index is given by no and the nonlinear refractive index is given by no.
  • the intensity I depends on the beam propagation direction z and on the time t due to the variable intensity of the laser pulse.
  • B-integral is a measure of a non-linear phase shift of the laser beam as it passes through the optical element.
  • 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:
  • 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 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.
  • the refractive index n depends on the intensity.
  • the intensity is greatest in the middle of the beam.
  • 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.
  • 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.
  • the laser pulse interacts non-linearly with the gaseous or liquid medium.
  • the gaseous or liquid medium can also lead to self-focusing.
  • 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.
  • 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.
  • 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.
  • 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.
  • the beam cross section is elongated in the focus.
  • the beam diameter is larger with respect to a first beam plane than with respect to a second beam plane.
  • 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.
  • 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.
  • 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.
  • the laser beam is not focused on and/or in the non-linear optics with a lens effect.
  • the elongated focus is not positioned on and/or in the non-linear optics with lens action.
  • 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.
  • cylinder optics can only focus the laser beam along one beam plane.
  • a line focus is generated by an optic that only focuses the laser beam with respect to one axis.
  • 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.
  • the entire non-linear interaction process between the laser and the optical elements of the device can be easily controlled.
  • 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.
  • ionization of the medium means that the non-linear interaction of the laser pulse can no longer be controlled or suppressed, or proceeds uncontrolled.
  • 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.
  • 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.
  • 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.
  • a cylindrical mirror is a non-rotationally symmetrical mirror.
  • 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.
  • 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.
  • the in-coupling optics and the laser pulse there is a spectral broadening of the laser pulse.
  • 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.
  • the beam diameter remains the same in the x-z plane, or the laser beam remains collimated with respect to the x-z plane.
  • 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.
  • 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.
  • the cross-sectional area of the laser beam is also minimized.
  • 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.
  • the laser beam 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.
  • 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.
  • a spherical mirror is a rotationally symmetrical mirror.
  • a spherical lens is a rotationally symmetrical lens.
  • a parabolic mirror or a parabolic lens is also rotationally symmetrical.
  • 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.
  • the axes for the description can therefore be freely selected.
  • the laser beam hits the first spherical lens element after the in-coupling optics.
  • the laser beam has its original beam shape again due to the divergence after the focus after the coupling optics.
  • 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.
  • the laser beam is collimated with respect to the y-z plane, whereas the laser beam is focused in the x-z plane.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • the laser beam can be coupled out of the device by a coupling-out lens system.
  • the decoupling optics can in particular collimate the laser beam again.
  • 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.
  • the sagittal plane can coincide with the x-y plane and the y-z plane can coincide with the meridional plane.
  • 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.
  • 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.
  • non-linear optics with a lens effect 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.
  • the arrangement of lens elements can include two non-linear optics with a lens effect.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • all optical elements can have a focal length of 20 cm.
  • 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.
  • 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.
  • LIDT laser-induced damage threshold
  • 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.
  • 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.
  • the total broadening of the laser pulse can be determined with the number of optical elements.
  • 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.
  • the beam cross section is elongated in the focus, with the focus preferably being a line focus.
  • FIG. 1 shows a schematic representation of the device
  • Figure 2 is a schematic representation of the location dependency of different
  • 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
  • FIG. 6 shows a further schematic representation of a device.
  • FIG. 1 A first embodiment of a proposed device 2 is shown schematically in FIG.
  • a laser 1 provides a laser beam 10 in which the laser pulses 100 of the laser 1 propagate.
  • the laser pulses 100 can be ultra-short laser pulses.
  • 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.
  • the laser pulses 100 have a frequency bandwidth that is intended to be broadened by the device 2 .
  • 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 .
  • 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.
  • 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.
  • 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.
  • FIG. 1 Various beam characteristics in a prior art device are shown in FIG.
  • 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.
  • 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.
  • 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.
  • 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 .
  • 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 .
  • 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.
  • 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.
  • the progression of the beam diameter D is shown when the beam cross section in the focus 30 is elongated.
  • the laser beam 10 is initially focused by the in-coupling optics 20 in a beam plane, here the yz 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 .
  • 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.
  • 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.
  • the intensity here is also so low that ionization of the liquid or gaseous medium 240, for example air, is avoided.
  • 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.
  • the intensity of the laser beam 10 in the optical elements 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.
  • 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.
  • the line focus 30 for spectral broadening.
  • 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.
  • 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.
  • 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 .
  • 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 ⁇ , as an example for the device 2 from FIG. 3D.
  • 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.
  • the in-coupling optics 20 In the side view, the in-coupling optics 20 have the cylinder curvature, so that a focusing of the laser beam 10 is achieved.
  • 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.
  • 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.
  • the mirror elements 20, 220 have a coating with which the laser pulse 100 can interact non-linearly.
  • 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.
  • FIG. 5 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.
  • 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.
  • 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 .
  • 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.
  • 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.
  • 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.

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  • Electromagnetism (AREA)
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  • Engineering & Computer Science (AREA)
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Abstract

La présente invention concerne un dispositif (2) pour l'élargissement spectral d'une impulsion laser (100) d'un faisceau laser (10), ledit dispositif comprenant une unité de couplage optique (20), au moins une unité optique non linéaire ayant un effet de lentille (224, 224'), et une unité de couplage de sortie optique (26), l'unité de couplage optique (20) étant conçue pour coupler le faisceau laser (10) dans le ou les éléments de lentille (224, 224'), et l'unité de couplage de sortie optique (26) étant conçue pour sortir le faisceau laser (10), l'impulsion laser (100) du faisceau laser (10) étant élargie spectralement à l'aide d'une interaction non linéaire avec la ou les unités optiques non linéaires avec un effet de lentille (224, 224'), et le faisceau laser (10) étant focalisé par l'unité de couplage optique (20) et/ou la ou les unités optiques non linéaires avec un effet de lentille (224, 224'), la section transversale du faisceau étant allongée dans le foyer (30), le foyer (30) étant de préférence un foyer linéaire.
PCT/EP2022/079092 2021-10-21 2022-10-19 Dispositif et procédé d'élargissement spectral d'une impulsion laser WO2023066995A1 (fr)

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EP4111553A1 (fr) 2020-02-26 2023-01-04 Trumpf Scientific Lasers GmbH & Co. KG Système laser comprenant un système optique pour l'élargissement spectral d'un rayonnement laser pulsé et procédé d'élargissement spectral d'un rayonnement laser pulsé
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