EP4111554A1

EP4111554A1 - Optical system for increasing contrast of pulsed laser radiation, laser system and method for increasing contrast of pulsed laser radiation

Info

Publication number: EP4111554A1
Application number: EP21708638.8A
Authority: EP
Inventors: Thomas Metzger; Sebastian Stark
Original assignee: Trumpf Scientific Lasers GmbH and Co KG
Current assignee: Trumpf Scientific Lasers GmbH and Co KG
Priority date: 2020-02-26
Filing date: 2021-02-26
Publication date: 2023-01-04
Also published as: CN115210967A; WO2021170815A1; US20220416493A1; US20220399695A1; EP4111553A1; CN115210968A; WO2021170814A1

Abstract

The invention relates to an optical system (3) for increasing contrast of pulsed laser radiation (9) using a non-linear eliptical polarisation rotation, comprising: a first polarisation adjustment lens (19) for adjusting an elliptical polarisation state (17B) of the pulsed laser radiation (9); a multipass cell (5) comprising two opposing mirrors (25A, 25B), forming a concentric resonator, which is passed through multiple times by the pulsed laser radiation (9) forming a plurality of intermediate focus zones (29), wherein the multipass cell (5) is filled with a gas (5A), which has an optical non-linearity, which brings about an intensity-dependent rotation of an orientation of the eliptical polarisation state (17B) of the pulsed laser radiation (9), such that the multipass cell (5) outputs beam parts with differently orientated elliptical polarisation states on the basis of the intensity-dependent rotation; and an optical beam splitting system (41) for splitting the beam parts with differently orientated elliptical polarisation states.

Description

OPTICAL SYSTEM FOR INCREASING THE CONTRASTER OF PULSED LASER RADIATION, LASER SYSTEM AND PROCESS FOR INCREASING CONTRASTER

OF PULSED LASER RADIATION

The present invention relates to optical systems for increasing the contrast of pulsed laser radiation and laser systems, in particular ultra-short pulse (USP) laser systems for delivering pulsed laser radiation with high power and / or high pulse energy. The invention also relates to a method for increasing the contrast of pulsed laser radiation, in particular special ultra-short pulse trains.

When using pulsed laser radiation, the contrast between primary laser pulses with a high peak intensity and the background is an essential parameter. Usually, the laser radiation that forms the subsurface consists of a radiation platform that goes back to ASE and / or laser pulses that arrive before or after the primary laser pulses and whose peak intensity is significantly lower than the peak intensity of the primary laser pulses.

In order to reduce the interaction of the background, for example during an experiment or during material processing, an increase in contrast is usually carried out. The aim is to largely remove the ASE background and the pre- and post-laser pulses from the pulsed laser radiation and thus largely limit the intensity contribution to the interaction to the primary laser pulses.

Processes for contrast improvement are known from the prior art, in which a laser beam is guided in a gas-filled hollow core fiber (HCF hollow core fiber) in order, for example, to cause a rotation of an elliptical polarization due to non-linear effects. This is known as nonlinear elliptical polarization rotation (also nonlinear ellipse rotation, NER). In the HCF, in addition to the NER, there is a non-linear spectral broadening of the intensive laser pulses. In particular, a constant gas condition can be provided along the HCF. Alternatively, differential pumping can reduce the gas density along the fiber in order to avoid undesired non-linear effects or ionization. The use of NER in connection with an HCF is disclosed, for example, in “Generation of high-fidelity few-cycle pulses via nonlinear ellipse rotation in stretched hollow-fiber compressor” by N. Khodakovskij, CLEO 2018, OSA 2018. Furthermore, in “Contrast improvement of sub-4 fs laser pulses using nonlinear elliptical polarization rotation ”by N. Smijesh et al., Optics Letters, Vol. 44, No. 16, August 15, 2019, NER used to improve the contrast of sub-4 fs laser pulses using an HCF filled with argon gas.

Furthermore, DE 102014007159 A1 discloses a method for the spectral broadening of laser pulses for non-linear pulse compression using a multipass cell, which is constructed, for example, in the form of a so-called Herriott cell. The aim is a spectral broadening of laser pulses, which can also be carried out with a pulse power that is greater than the critical power of the non-linear medium used for the spectral broadening.

One object of the invention is to propose systems and methods which can also and in particular at high pulse energies and high average powers for a contrast increase of pulsed laser radiation, for example ultrashort pulse trains. In particular, non-linear effects are to be used to influence the polarization of laser pulses and their propagation.

At least one of these objects is achieved by an optical system for increasing the contrast of pulsed laser radiation according to claim 1, a laser system according to claim 12 and a method for increasing the contrast of pulsed laser radiation according to claim 14. Further developments are given in the dependent claims.

In one aspect, an optical system for increasing the contrast of pulsed laser radiation using a non-linear elliptical polarization rotation comprises:

- a first polarization setting optics for setting an elliptical polarization state of the pulsed laser radiation,

- A multi-pass cell with two opposing mirrors, which in particular form a concentric or confocal resonator, wherein the multi-pass cell, in particular the resonator several times, is traversed by the pulsed laser radiation with the formation of a plurality of intermediate focus zones, the multi-pass cell with a gas is filled, which has an optical non-linearity that causes an intensity-dependent rotation of an alignment of the elliptical polarization state of the pulsed laser radiation, so that the multi-pass cell due to the intensity-dependent rotation outputs beam components with differently aligned elliptical polarization states, and

- An optical beam splitting system for splitting the beam components with differently directed elliptical polarization states.

In a further aspect, a laser system, in particular an ultra-short pulse (USP) laser system, for emitting pulsed laser radiation (9)

- A laser radiation source that emits pulsed laser radiation that includes primary laser pulses, in particular with pulse energies pulse durations in the range of a few hundred femtoseconds and less,

- optionally a pulse duration setting system for setting the pulse duration of the primary laser pulses,

- At least one optical system as disclosed herein for increasing the contrast of the pulsed laser radiation using a non-linear elliptical polarization rotation in a plurality of intermediate focus zones of a multi-pass cell. Optionally, the laser system can comprise an optical pulse duration compressor system for compensating for a dispersive contribution of the at least one optical system and optionally for temporally compressing the primary laser pulses of the laser radiation if they have experienced a nonlinear spectral broadening in at least one of the intermediate focus zones.

In a further aspect, a method for increasing the contrast of pulsed laser radiation using a non-linear elliptical polarization rotation comprises the following steps:

- Setting an elliptical polarization state of the pulsed laser radiation,

- Coupling of the pulsed laser radiation into a multi-passage cell with two mirrors, which in particular form a concentric or confocal resonator, the multi-passage cell, in particular the resonator being traversed several times, forming a plurality of intermediate focus zones, the multi-passage cell being filled with a gas that has an optical non-linearity that causes an intensity-dependent rotation of an alignment of the elliptical polarization state of the pulsed laser radiation in the intermediate focus zones, whereby beam components with differently directed elliptical polarization states are generated in the multi-pass cell,

- Decoupling of the beam components with differently oriented elliptical polarizations from the multiple passage cell and - Separating the decoupled beam components into a useful beam component with primary laser pulses and a remaining beam component with low-intensity laser radiation.

In some developments of the optical system, at least one of the following parameters of the optical system can be set or adjustable for a rotation of the alignment of one of the elliptical polarization states, in particular for setting a rotation angle of 90 °:

a gas pressure in the intermediate focus zones, the optical system being designed as a gas-filled cell and the optical system having a pressure setting device for setting the gas pressure in the gas-filled cell, and / or

a dispersion present in the multi-pass cell.

Optionally, at least one of the following parameters of the optical system can also be set or adjustable:

- an ellipticity of the set elliptical polarization of the pulsed laser radiation,

- a number of intermediate focus zones in the multi-pass cell,

- Focus diameter in the intermediate focus zones and

- Rayleigh lengths of the intermediate focus zones.

In some developments, the first polarization adjustment optics comprise a first wave plate, optionally a 1/4 wave plate and / or a 1/2 wave plate.

In some developments, the optical system can also include at least one of the following optical components:

- A pulse duration setting system for setting a pulse duration of primary laser pulses of the ge pulsed laser radiation,

- a first optical telescope arrangement which is set to image the pulsed laser radiation in a predetermined mode in the multi-passage cell and which is optionally arranged downstream of the first polarization setting optics,

- a coupling mirror for coupling the pulsed laser radiation into the multiple passage cell,

- An output mirror for forwarding the pulsed laser radiation emerging from the multiple passage cell and - A second optical telescope arrangement which is set to collimate the pulsed laser radiation emerging from the multiple passage cell.

In some further developments of the optical system, the multiple passage cell can be designed:

- With a predetermined or adjustable number of intermediate focus zones, which are generated with the opposing mirrors, for example in a resonator structure with, in particular the same, radii of curvature of the mirrors, the intermediate focus zones having essentially the same diameter and the same Rayleigh length,

- for the formation of intermediate focus zones, which are arranged next to each other and optionally partially overlying each other,

- as a cell filled with a non-linear gaseous medium, in particular a noble gas such as helium or argon, the same gas pressure being present in each of the intermediate focus zones,

- With an essentially constant pulse duration and essentially constant pulse energy of the pulsed laser radiation passing through and / or with a substantially constant pulse duration present in the intermediate focus zones

- For the gradual non-linear spectral broadening of the pulsed laser radiation passing through in the intermediate focus zones.

In some developments of the optical system, at least one of the mirrors of the multiple passage cell can be designed as a convex mirror, the radii of curvature in particular matching and / or the distance between the mirrors being in a range of 95% to 105% of the sum of the radii of curvature. Alternatively or additionally, at least one of the mirrors can be designed as a dispersive mirror, the dispersion contribution of which compensates for a dispersive contribution of at least one passage of a primary laser pulse of the pulsed laser radiation through the multiple passage cell. Alternatively or additionally, at least one of the mirrors can comprise at least one mirror segment which the pulsed laser radiation strikes at least once when the pulsed laser radiation circulates through the multiple passage cell.

In some developments of the optical system, the multiple passage cell can be designed in such a way that a primary laser pulse of the pulsed laser radiation, its contrast in optical system is to be increased, experiences essentially no change in the pulse duration and / or pulse energy in the intermediate focus zones.

In some developments of the optical system, the beam components with differently oriented elliptical polarization states can comprise a useful beam component with primary laser pulses and a residual beam component with low-intensity laser radiation. The remaining beam portion forms a background from a radiation platform originating from ASE and / or from laser pulses that arrive before or after the primary laser pulses and whose peak intensity is significantly lower than the peak intensity of the primary laser pulses. In particular, the radiation background can have low-intensity laser pulses preceding the high-intensity laser pulses and / or subsequent low-intensity laser pulses.

In some developments of the optical system, the optical beam splitting system for splitting the beam components with differently oriented elliptical polarization states can include:

- A second polarization setting optics for returning each of the differently aligned elliptical polarization states to a linear polarization state, in particular the beam components output by the multi-passage cell with differently oriented elliptical polarization states being converted into a useful beam component and a remaining beam component with differently oriented linear polarization states be, and

- a beam splitter that outputs the useful beam portion and the remaining beam portion on different beam paths.

In particular, the second polarization adjustment optics can comprise a second wave plate, optionally a 1/4 wave plate and / or a 1/2 wave plate.

In some developments, the optical system can also include a control system that is designed to rotate the alignment of one of the elliptical polarization states, in particular to set a rotation angle of 90 °, at least to set one of the following parameters:

- a pulse duration of primary laser pulses of the pulsed laser radiation,

- a pulse energy of primary laser pulses of the pulsed laser radiation,

- an ellipticity of the set elliptical polarization of the pulsed laser radiation, - focus diameter in the intermediate focus zones,

- Rayleigh lengths of the intermediate focus zones and

- a gas pressure of the gas in the intermediate focus zones.

In some developments of the laser system, gas conditions, in particular gas type and / or gas pressure, in a first optical system can differ from corresponding gas conditions in a second optical system.

In some developments of the method, this can also include the step:

- Setting at least one of the following parameters for rotating the alignment of one of the elliptical polarization states in the multi-pass cell, in particular for setting a rotation angle of 90 ° for primary laser pulses of the pulsed laser radiation:

- a gas pressure in the intermediate focus zones and / or

a dispersion of the primary laser pulses accumulated in the multi-pass cell.

Optionally, the following can also be set:

- a pulse duration of the primary laser pulses of the pulsed laser radiation,

- a pulse energy of the primary laser pulses of the pulsed laser radiation,

- an ellipticity of the set elliptical polarization state of the pulsed laser radiation,

- Focus diameter in the intermediate focus zones and / or

- Rayleigh lengths of the intermediate focus zones.

In some developments of the method, a pulse spectrum of the pulsed laser radiation assigned to the primary pulses, in particular from intermediate focus zone to intermediate focus zone, can also widen due to a non-linear spectral broadening in the multiple passage cell, while the contrast increase takes place at the same time.

According to the invention, it is generally proposed to use the NER approach using a gas-filled multiple passage cell, for example a Herriott cell, for contrast enhancement of pulsed laser radiation, which can optionally be done together with a spectral pulse broadening and a subsequent pulse duration compression. The concepts proposed here have the advantages that the increase in pulse contrast at high average powers (greater than a few 100 W up to the kW range) and very high pulse energies (up to the J range) and can be implemented in a very efficient and easy to implement.

In the context of the NER approach considered here using a gas-filled multi-passage cell, an elliptical polarization state is understood to mean a polarization state in which an ellipse of polarization is present, as can be achieved, for example, by setting an angle in a range between 0 ° and 45 ° or in a range between 45 ° and 90 ° (analogously in a range between 45 ° and 135 ° or in a range between 135 ° and 180 °) a fast axis of a 1/4 wave plate and a polarization plane of an incident linearly polarized laser beam can be effected. A polarization ellipse of the E-field is formed in these angular ranges. In an exemplary elliptical polarization state, 90% of the laser beam can be in an s-polarization state and 10% in a p-polarization state. Thus, an elliptical polarization state differs from a linear polarization state (with only one component in the s or p polarization state) and from a circular polarization state (angle of 45 ° of the 1/4 wave plate or identical components in the s polarization state and in the p-polarization state).

Concepts are disclosed herein that allow aspects from the prior art to be improved at least in part. In particular, further features and their usefulness emerge from the following description of embodiments with reference to the figures. The figures show:

1 shows an exemplary schematic representation of a laser system with an optical system for contrast enhancement,

2A to 2C are exemplary sketches for explaining a Herriott cell as an example of a multi-pass cell and

3 shows a schematic flow diagram to explain an exemplary procedure for contrast enhancement.

Aspects described here are based in part on the knowledge that, due to an intensity-dependent phase rotation, a non-linear intensity-dependent rotation of an alignment of an elliptical polarization can be brought about (self-induced ellipse rotation). The intensity dependence of the elliptical Polarization rotation has the effect that the polarization of a pulse close to its peak experiences a different rotation than the low-intensity areas. This makes it possible to separate high-intensity beam components (formed by the primary laser pulses intended for an application) from low-intensity laser radiation (for example formed by the pre- and post-pulses accompanying the primary laser pulses). The implementation of the NER as part of a multi-pass cell provides a wide range of setting parameters and control parameters, so that contrast enhancement in an optical system is made possible for a wide variety of parameter constellations in a laser system.

Furthermore, the inventors have recognized that with the help of the NER concepts disclosed herein, when used in a Herriott cell, pulsed laser radiation can be freed from low-energy laser radiation (e.g. pre- or post-laser pulses) by only using laser pulses with a high intensity (the primary laser pulses mentioned ) can be influenced in their polarization with a view to the subsequent beam path. The influence takes place through the non-linearity of the refractive index of the gas in the intermediate focus zones of the Herriott cell in such a way that the primary laser pulses can pass through an optical beam splitting system with as little loss as possible. In contrast to the primary laser pulses, the alignment of the polarization ellipse of the low-energy laser radiation component within the Herriott cell does not change, so that this low-energy laser radiation component can be removed from the laser radiation with the optical beam splitting system after leaving the Herriott cell. In other words, pre- and / or post-laser pulses can be removed from the laser beam if the rotation of the polarization ellipse is only set for the primary laser pulses due to their high intensity so that only the primary laser pulses can pass through a preset beam splitter, for example.

1 shows a laser system 1 which has an optical system 3 to increase the contrast. The optical system 3 is based on the use of a multiple passage cell 5 filled with gas 4 (for example a Herriott cell), the gas 4 serving as a non-linear Kerr medium. For example, helium can be used in the multi-pass cell 5 at very high intensities. At lower but still high intensities in the multiple passage cell 5, for example argon or another noble gas can be used as the non-linear medium. The laser system 1 generally comprises a laser radiation source 7 which emits laser radiation 9. The laser radiation 9 comprises primary laser pulses 11 with a pulse energy in the range of, for example, a few hundred millijoules and a pulse duration Δt in the range of a few hundred femoseconds and less, which, for example, form an ultra-short pulse train. Depending on the laser radiation source 7, the laser radiation 9 also includes low-energy laser radiation 13, which is indicated by way of example in FIG. 1 as prepulse 13A or postpulse 13B. Furthermore, the laser radiation source 7 can optionally have a pulse duration setting system 15 for setting the pulse duration of the primary laser pulses 11, wherein the pulse duration setting system 15 can also be assigned to the optical system 3, as indicated in FIG. 1.

In the present example it is assumed that the laser radiation 9 with a linear polarization 17A is present at the output of the pulse duration setting system 15 or at the output of the laser radiation source 7, the polarization vector of which is indicated in FIG. 1 as an example orthogonal to the plane of the characters. That is, both the primary laser pulses 11 and the low-energy laser radiation 13 are linearly polarized, generally in the same direction.

To increase the contrast, the optical system 3 has first polarization adjustment optics 19. In this the laser radiation 9 is elliptically polarized. In FIG. 1, an E-field vector revolving around a polarization ellipse to illustrate an elliptical polarization state 17B is indicated by way of example at the output of the first polarization adjustment lens 19, the long semiaxis of the polarization ellipse running in the plane of the drawing as an example. To set this elliptical polarization state of the laser radiation 9, the first polarization setting optics 19 comprise a wave plate (for example a 1/4 wave plate or a 1/8 wave plate, etc.). In the example of FIG. 1, the first polarization adjustment optics 19 comprise a first 1/4 wave plate 19A. Furthermore, the first polarization adjustment optics 19 can optionally have a ½ wave plate 19B, for example arranged in front of the first ¼ wave plate 19A, for aligning the orientation of the elliptical polarization state. For this purpose, the setting of the polarization in the first polarization setting optics 19 is independent of the intensity, because the wave plates work with an anisotropic refractive index, for example with a birefringent crystal. To set the elliptical polarization state of the laser radiation 9, for example, the 1/4 wave plate with respect to the polarization plane of the laser radiation 9 is set in such a way that an angle between a fast axis of the 1/4 wave plate and the plane of polarization is not 0 °, 45 °, 90 ° or 135 °. Corresponding angular ranges can be other wave plates or combinations of Waveplates can be assigned to generate elliptical polarization. By setting the angle of the wave plate, an ellipticity of the polarization ellipse can be set. In combination with the upstream ½ wave plate 19B, the polarization ellipse can also be aligned.

Due to the identical linear polarization for the primary laser pulses 11 and the low-energy laser radiation 13 present at the input of the first polarization setting optics 19, these two beam portions also have the same elliptical polarization state 17B when they leave the first polarization setting optics 19.

1 also shows a telescope arrangement 21 for adapting the mode (generally of beam parameters such as beam diameter and beam divergence) of the pulsed laser radiation 9 prior to coupling into the multiple passage cell 5 with a coupling mirror 23.

The multiple passage cell 5, for example designed as a Herriott cell, comprises two concavely curved mirrors 25A, 25B which, in a gas-filled environment, form a multiple beam path 5A running back and forth between the mirrors 25A, 25B.

The multi-passage cell 5 can - for example in the geometry of a Herriott cell - be formed by two concave mirrors which, for example, in a concentric or confocal resonator arrangement, generally also in a different resonator configuration along a common optical axis 27 (given by the concentric arrangement ) are aligned. In this case, the mirrors 25A, 25B are also referred to as Herriott mirrors. If the laser radiation 9 is introduced into the multiple passage cell 5 offset from the optical axis 27, the laser radiation 9 is circulated several times back and forth on a predetermined, usually elliptical (circular) pattern.

2A illustrates schematically the beam path between the mirrors 25A, 25B with the formation of an intermediate focus zone 29, assuming a correspondingly adapted mode of the coupled laser radiation 9. The intermediate focus zone 29 has, for example, a focus diameter d and a Rayleigh length Lr and lies in the region of a plane of symmetry 31 of the resonator arrangement, which is embodied concentrically in the example in FIG. 1. Figures 2B and 2C show plan views of the mirrors 25A, 25B, in which circularly arranged impact areas 33 on the mirror surfaces are indicated schematically. In the impingement areas 33, the laser radiation 9 strikes as centrally as possible before it is reflected back from there, as in the direction of the center of the multiple passage cell (here the resonator arrangement). In FIGS. 2B and 2C one can also see a coupling-in opening 35A and a coupling-out opening 35B. The areas ready for reflection on the surface of the mirrors 25A, 25B are circular surface sections with a diameter D. The number of revolutions (intermediate focus zones 29) can in principle be as large as desired; for example, 5 to 100 intermediate focus zones can be traversed; that is, several intermediate focal zones are traversed in the multi-pass cell. Furthermore, at least one of the mirrors 25A, 25B can also be built up from individual discrete mirror elements, wherein a reflection can preferably take place on a single mirror element. For example, twelve intermediate focus zones 29 are passed through.

As an alternative to beam coupling and beam decoupling through openings in the mirrors, smaller mirror elements that engage in the Herriott cell can be used and positioned, for example, at the positions of the openings 35A, 35B.

Referring to the indicated in Fig. 1 beam path 5A, the pulsed Laserstrah treatment 9 is repeatedly ge leads through intermediate focus zones in the center of the multiple passage cell 5. Due to the focusing of the primary laser pulses during the pulse duration At of the primary laser pulses 11, high intensities develop in the intermediate focus zones, which lead to a non-linear behavior of the refractive index of the gas 4. The non-linear behavior of the refractive index of the gas 4 leads, as explained below, to beam components of the laser radiation 9 with different polarization states and can thus be used to increase the contrast of the pulsed laser radiation 9.

After the predetermined number of passes through the multi-pass cell 5, the laser radiation 9 leaves the multi-pass cell 5 and hits an output mirror 37 reflecting the coupled-out laser radiation. The output mirror 37 directs the laser radiation 9 through a second telescope arrangement 39, which recollimates the laser radiation 9 . The optical system 3 also has an optical beam splitting system 41 for splitting the beam components of the laser radiation 9 with different polarization states. In the embodiment shown by way of example in FIG. 1, the optical beam splitting system 41 comprises a second wave plate 43 and a beam splitter 45. The second wave plate 43 is generally set such that the elliptical polarization state is converted back into a linear polarization. Further structures for dividing beam components with different elliptical polarization states are known from the prior art.

Due to the non-linearity of the refractive index n of the gaseous Kerr medium in the multiple passage cell 5 - i.e. for an intensity-dependent refractive index n = n_0 + n_2 * I (r; t) with the gas-specific refractive index parameters n_0 and n_2 and the intensity profile I (r; t) in the intermediate focus zone - there is only a rotation of the polarization ellipse by an angle of rotation DQ for the primary laser pulses 11, the angle of rotation DQ being proportional to the intensity I (r; t) and n_2. A polarization ellipse rotated by DQ is indicated by way of example in FIG. 1 as an elliptical polarization state 17C.

The orientation of the polarization ellipse of the low-energy laser radiation 13 results from the ordinary refractive index of the gas 4 and remains essentially unchanged. The elliptical polarization states of the primary laser pulses 11 and the low-energy laser radiation 13 thus change their relative alignment to one another with each passage through an intermediate focus zone 29.

If the alignments of the two polarization ellipses differ by 90 ° after the multiple passage cell 5, for example, after passing through the wave plate 43, two orthogonally aligned linear polarization states 47A, 47B for the primary laser pulses 11 and the low-intensity laser radiation 13 result Aligned accordingly, the beam splitter 45 distributes the primary laser pulses 11 and the low-intensity laser radiation 13 to two different beam paths for a useful beam portion 9A and a residual beam portion 9B, which correspond to the orthogonal linear polarization states 47A, 47B.

In FIG. 1, a primary laser pulse 1G of the useful beam portion 9A is indicated by way of example, which has been freed from pre- and post-pulses. The pre- and post-pulses form the remaining beam portion. Even if the resulting beam components are not completely orthogonal to one another are polarized, substantial portions of the pre- and post-pulses can be removed from the useful beam entrance.

For the subsequent use of the primary laser pulses 1 G, the contrast of which has been increased with respect to the low-intensity laser radiation, the primary laser pulses 1 G can be added to a compressor 49, for example. The compressor 49 is shown in Figure 1 for example as a chirped mirror compressor. A useful laser radiation 9A ′ can thus be output at an output of the laser system 1 which comprises a sequence of compressed primary laser pulses 11 ″, the compressed primary laser pulses 11 ″ having an increased contrast.

In contrast to the structure known from the prior art which uses an HCF, the configuration proposed herein can enable a predetermined / adjustable number of intermediate focus zones 29 to be traversed, for example using a Herriott cell. In addition, a beam diameter d can be adjusted in the intermediate focus zones and can also be adjusted to the laser power, pulse duration, etc. and the gas 4, for example, via the radius of curvature Rm of the mirrors 25A, 25B. The radius of curvature Rm is, for example, identical for both mirrors or is at least of the same order of magnitude.

In addition to the ability to adjust the size of the intermediate focus zone 29, e.g. through the radii of curvature of the mirrors 25A, 25B and through a corresponding telescope arrangement for mode adjustment, which can be connected upstream of the multi-passage cell, the gas pressure can be adjusted with regard to the non-linearity. It is noted that due to the high spatial proximity of the various continuous intermediate focus zones in the multiple passage cell, the same gas pressure is present in each of the intermediate focus zones. The optical beam parameters and beam properties are preferably very similar in the various intermediate focus zones, so that similar non-linear effects are also present.

If the mirrors 25A, 25B form, for example, a concentric resonator (mirror from approx. 2 * Rm with identical radii of curvature Rm), the intermediate focus zones 29 essentially all have the same diameter d and accordingly have the same Rayleigh lengths Lr. In general, the distance between the mirrors 25A, 25B is in a range from 95% to 105% of the sum of the radii of curvature. An intense primary laser pulse 11 propagates through this Intermediate focus zones 29 sequentially and thereby interacts repeatedly with the gas 4 at electrical field strengths which can cause non-linear effects on the refractive index n and thus on the polarization state of the primary laser pulse 11.

The use of the Herriott structure described herein provides various pre-determinable and / or operationally adjustable parameters in the design of the intermediate focus zones and the non-linear conditions present therein. For setting the parameters, the optical system 3 can, for example, have a control system 61, which via control connections 63 with the pulse duration setting system 15, the polarization setting optics 19 (in particular for setting the angular positions of the first 1/4 wave plate 19A and optionally the 1/2 -Wave plate 19B), the telescope assemblies 21, 39 (especially for adjusting the distance between telescopic lenses 21 A, 21B), a Druckeinstellvorrich device 65 for adjusting the gas pressure (see Fig. 1) and / or the optical beam splitting system 41 (especially for setting the angular position of the second l / 4 wave plate 43) is a related party.

With the help of the control system 3, for example, the following can be set:

- the pulse duration At of the primary laser pulses 11 of the pulsed laser radiation 9,

- the pulse energy of the primary laser pulses 11 of the pulsed laser radiation 9,

- an ellipticity of the set elliptical polarization of the pulsed laser radiation 9,

- the focus diameter d in the intermediate focus zones 29,

- the Rayleigh lengths Lr of the intermediate focus zones 29 and

a gas pressure of the gas 4 in the intermediate focus zones 29.

As shown in FIGS. 2B and 2C, the laser radiation 9 strikes the mirrors 25A, 25B repeatedly (multiple times in each case). In addition, the mirrors can be used to adjust the dispersion by designing them as dispersive mirrors. If the mirrors 25A, 25B have a dispersive effect in at least one of the reflections, the dispersion and thus the pulse duration of the primary laser pulses 11 can be acted upon directly. For example, one or more of the impingement areas 33 can be provided with a dispersive layer.

A dispersive coating 51 is indicated by dashed lines in FIG. 2A for the mirror 25B. In addition, each of the mirrors 25A, 25B can be constructed from a plurality of mirror segments with predetermined dispersive properties, with each of the mirror segments in its dispersion for a desired pulse duration as it passes through the Multi-pass cell 5 is adapted. Correspondingly, the dispersion present in the multiple passage cell 5 is composed of a dispersion contribution from the dispersive mirror and a dispersion contribution in the gas-filled volume along the beam path 5A.

An exemplary mirror segment 53 is indicated in FIG. 1. When the pulsed laser radiation circulates through the multiple passage cell, the laser radiation (depending on the size of the mirror segment) hits the mirror segment at least once, which usually has at least an extent the size of the beam diameter D on the mirror surface.

In other words, the concepts proposed here allow a dispersion that is accumulated when passing through the gas-filled volume to be at least partially compensated for by suitable dispersive mirror coatings (chirped mirrors) in order, for example, to maintain comparable pulse durations in the intermediate focus zones or to selectively adjust the pulse durations vary.

In addition to the use of the NER discussed above, the multi-pass cell 5 enables the high-intensity primary laser pulses in the intermediate focus zones to be spectrally broadened step-by-step. The non-linear spectral broadening is caused by the high intensity present in an intermediate focus zone and by the non-linearity of the refractive index of the gaseous medium in the multiple passage cell 5.

Due to the non-linear spectral broadening, the pulse spectrum can change from intermediate focus zone to intermediate focus zone, namely essentially while the pulse duration and pulse energy remain the same. If the multiple passage cell 5 is built up by means of chirped mirrors, the pulse duration can moreover be adjusted. For example, the pulse duration can change (shorten or lengthen) from run to run. Accordingly, even with a non-linear spectral broadening, the peak intensities in the intermediate focus zones remain essentially the same.

Another advantage can arise in connection with the non-linear spectral broadening if laser radiation with elliptical polarization is used in the multiple passage cell for this purpose. So the spectral broadening can possibly be smoother about in itself across the frequency spectrum, so that a less structured spectrum can result. This can have a positive effect on the subsequent pulse shaping and / or pulse compression.

Fig. 2A shows the formation of an intermediate focus zone in a Herriott cell under the assumption of curved Herriott mirrors. In the following, geometric parameters for the implementation of a multi-pass cell are considered within the framework of the concepts presented here.

The limitation of the pulse energies of primary laser pulses, which can be increased in contrast (and optionally spectrally broadened) with a multi-pass cell, results from avoiding laser-induced damage to the (Herriott) mirror and from the ionization threshold value of the gas used. The highest possible ionization threshold when using helium as the gas 4 in the multiple ^{passage cell 5 is approximately 3.42 × 10 L} 14 W / cm 2.

The laser-induced damage to the mirrors 25A, 25B requires a minimum diameter of the laser radiation 9 on the curved mirror 25A, 25B. The ionization threshold determines the smallest possible focus diameter d in the intermediate focus zones 29 with a view to avoiding ionization of the gas 4. Both parameters together define a necessary length of the multi-passage cell 5, that is, the distance between the mirrors that build up the concentric resonator, for example their radius of curvature.

With regard to the limitation by the multiphoton ionization, it should be mentioned that this is essentially independent of the gas pressure in the multiple passage cell.

In this context, it is also important that the electrical field strength required for ionization for circular / elliptically polarized light is increased compared to linearly polarized light, so that with a comparable beam diameter D on the mirrors 25A, 25B, the possible focus diameter d in the intermediate focus zones 29 can be selected smaller ge. This generally results in a reduction in the mirror spacing (for example a spacing of the mirrors 25A, 25B shortened by at least a factor of 2) compared to a multiple-pass cell operated with linear polarization. Consequently Using a multi-pass cell for the NER can have the positive side effect that the multi-pass cell can be constructed more compactly.

For the use of high-intensity laser radiation it may be necessary for the mirrors to withstand pulse energies of a few 100 mJ with pulse durations of a few 100 fs, for example 500 fs or less. For ultra-short pulses, the mirrors are also broadband, for example, to a wavelength range from 700 nm to 1100 nm, for example, for ultra-short pulses from a titanium-sapphire laser or 900 nm to 1100 nm for ultra-short pulses from lasers that emit around 1000 nm, such as Nd: YAG or Yb: YAG. Furthermore, the mirrors can make a contribution or no contribution to dispersion, so that dispersive coatings may also have to be taken into account.

Exemplary parameters for a multiple passage cell and the mirrors on which it is based are explained below with reference to FIGS. 2A to 2C. For coated mirrors, a laser-induced damage threshold of approx. 0.5 J / cm2 with a pulse duration of approx. 500 fs can be measured. This threshold value is usually assigned to the center of the beam. Assuming a Gaussian beam, the result for the approx. 500 fs laser pulse is, for example, a threshold value of approx. 0.1 J / cm2, so that with a safety factor of, for example 3, the maximum permissible fluence is approx. 0.03 J / cm2 would.

On this basis, for example, a beam radius of approximately 9 mm for 200 mJ pulses or a converted 1 / e2 beam diameter of approximately 13 mm on the mirrors 25 A, 25B for the multi-passage cell 5 with a required focal length f of the mirrors 25A, 25B (radius of curvature Rm) and corresponding to a length of the multiple passage cell 5 (mirror spacing).

For circularly polarized light (with a reduced (maximum) electric field strength compared to linearly polarized light), a reduced mirror spacing / a shortened multiple passage cell length / resonator length L can be implemented with a correspondingly smaller radius of curvature of the mirrors 25A, 25B, provided the same Beam diameter is on the mirror. With reference to the flowchart shown in FIG. 3, the steps in the procedure proposed herein for increasing the contrast of pulsed laser radiation using a non-linear elliptical polarization rotation are explained.

In step 71, an elliptical polarization state 17A of the pulsed laser radiation 9 is set.

In step 73, the pulsed laser radiation 9 is coupled into a multiple passage cell 5, which is formed from two concave mirrors 25A, 25B which, for example, form a concentric or confocal resonator. The multi-pass cell 5 (e.g. a concentric or confocal resonator) is passed through several times with the formation of a plurality of intermediate focus zones 29, the multi-pass cell 5 being filled with a gas 4 which has an optical nonlinearity which is an intensity-dependent rotation of an alignment of the elliptical polarization state 17B of the pulsed laser radiation 9 in the intermediate focus zones 29. Accordingly, beam portions with differently aligned elliptical polarization states in the multi-passage cell 5 are generated.

In step 75, the beam portions with differently aligned elliptical polarizations are decoupled from the multiple passage cell 5 and in step 77 the decoupled beam portions are separated into a useful beam portion 9A with primary laser pulses 1G and a remaining beam portion 9B with low-intensity laser radiation.

Furthermore, according to the procedure disclosed herein, at least one of the following parameters for rotating the alignment of one of the elliptical polarization states in the multi-pass cell 5 can optionally be set in a step 79 preceding or accompanying the operation:

- a pulse duration At of the primary laser pulses 11 of the pulsed laser radiation 9,

- a pulse energy of the primary laser pulses 11 of the pulsed laser radiation 9,

- An ellipticity of the set elliptical polarization state of the pulsed Laserstrah ment 9,

- focus diameter d in the intermediate focus zones 29,

- Rayleigh lengths Lr of the intermediate focus zones 29,

- a gas pressure in the intermediate focus zones 29 and - A dispersion of the primary laser pulses (11) accumulated in the multi-pass cell 5.

For example, an angle of rotation DQ of 90 ° for primary laser pulses 11 of the pulsed laser radiation 9 can be set

In summary, the necessary non-linear conditions for a NER for cleaning the laser radiation from low-energy contributions can be set in a multi-pass cell designed as a Herriott cell. As explained, the optical systems proposed here allow essentially very similar (comparable) conditions of the non-linearity to be provided in the intermediate focus zones of the multi-pass cell, for example due to the comparable gas pressure and the comparable pulse durations and pulse energies (and thus pulse peak intensities) in the intermediate focus zones.

In addition, the pulse durations and pulse peak intensities can also be set with the mentioned dispersive mirrors / mirror segments to compensate for (often only small) pulse duration lengthening effects in the individual passes through the multi-pass cell. In particular, when using different dispersive mirror segments, additional freedom can be gained for a dispersion compensation and thus for an adjustment of the intensity in the intermediate focus zones.

It is also possible for the laser radiation to run through a sequence of successive multiple pass cells one after the other. This allows gas conditions to be differentiated according to the groups of intermediate focus zones present in the individual multiple passage cells.

It is explicitly emphasized that all features disclosed in the description and / or the claims are viewed as separate and independent of one another for the purpose of the original disclosure as well as for the purpose of restricting the claimed invention, regardless of the combinations of features in the embodiments and / or the claims should be. It is explicitly stated that all range specifications or specifications of groups of units disclose every possible intermediate value or subgroup of units for the purpose of the original disclosure as well as for the purpose of restricting the claimed invention, in particular also as a limit of a range specification.

Claims

1. Optical system (3) for increasing the contrast of pulsed laser radiation (9) using a non-linear elliptical polarization rotation comprising:

- A first polarization setting optics (19) for setting an elliptical polarization state (17B) of the pulsed laser radiation (9),

- A multi-pass cell (5) with at least two opposing mirrors (25A, 25B) through which the pulsed laser radiation (9) traverses while forming a plurality of intermediate focus zones (29), the multi-pass cell (5) being filled with a gas (5A) is, which has an optical non-linearity that causes an intensity-dependent rotation of an alignment of the elliptical polarization state (17B) of the pulsed Laserstrah treatment (9), so that the multi-pass cell (5) outputs beam components with differently aligned elliptical polarization states due to the intensity-dependent rotation, and

- An optical beam splitting system (41) for splitting the beam components with differently oriented elliptical polarization states.

2. Optical system (3) according to claim 1, wherein for a rotation of the alignment of one of the elliptical polarization states, in particular for setting a rotation angle (DQ) of 90 °, at least one of the following parameters of the optical system (3) is set or can be set :

- A gas pressure in the intermediate focus zones (29), wherein the optical system (3) is designed as a gas filled cell and the optical system (3) has a pressure adjustment device (65) for adjusting the gas pressure in the cell filled with the gas, and /or

- A dispersion present in the multiple passage cell (5), with at least one of the following parameters of the optical system (3) optionally being set or adjustable:

- an ellipticity of the set elliptical polarization of the pulsed laser radiation (9),

- a number of intermediate focus zones (29) in the multi-pass cell (5),

- Focus diameter (d) in the intermediate focus zones (29) and

- Rayleigh lengths (Lr) of the intermediate focus zones (29).

3. Optical system (3) according to claim 1 or 2, wherein the first polarization adjustment lens (19) has a first wave plate (19A), optionally a 1/4 wave plate (19A) and / or a 1/2 wave plate (19B), includes.

4. Optical system (3) according to one of the preceding claims, wherein the optical system (3) further comprises at least one of the following optical components:

- A pulse duration setting system (15) for setting a pulse duration (At) of primary laser pulses (11) of the pulsed laser radiation (9),

- A first optical telescope arrangement (21) which is set to image the pulsed Laserstrah treatment (9) in a predetermined mode in the multi-passage cell (5) and which is optionally arranged downstream of the first polarization setting optics (19),

- A coupling mirror (23) for coupling the pulsed laser radiation (9) into the multiple passage cell (5),

- A decoupling mirror (37) for forwarding the pulsed laser radiation (9) emerging from the multiple passage cell (5) and

- A second optical telescope arrangement (39) which is set to collimate the pulsed laser radiation (9) emerging from the multiple passage cell (5).

5. Optical system (3) according to one of the preceding claims, wherein the multiple passage cell (5) is formed

- With a predetermined or adjustable number of intermediate focus zones (29) which are generated with the opposing mirrors (25A, 25B) in a resonator structure with, in particular, the same radii of curvature of the mirrors (25A, 25B), the intermediate focus zones (29 ) have essentially the same diameter (d) and the same Rayleigh length (Lr),

- For forming intermediate focus zones (29) which are arranged next to one another and optionally partially overlapping one another,

- As a cell filled with a non-linear gaseous medium, in particular a noble gas such as helium or argon, the same gas pressure being present in each of the intermediate focus zones (29),

- With an essentially constant pulse duration (At) present in the intermediate focus zones (29) and essentially constant pulse energy of the pulsed laser radiation (9) passing through and / or - For the gradual nonlinear spectral broadening of the pulsed laser radiation passing through (9) in the intermediate focus zones (29).

6. Optical system (3) according to one of the preceding claims, wherein at least one of the mirrors (25 A, 25B) of the multi-pass cell (5)

- Is designed as a convex mirror, the radii of curvature in particular being the same and / or a distance between the mirrors in a range of 95% to 105% of the sum of the radii of curvature, and / or

- Is designed as a dispersive mirror, the dispersion contribution of which compensates for a dispersive contribution of at least one passage of a primary laser pulse (11) of the pulsed laser radiation (9) through the multiple passage cell, and / or

- comprises at least one mirror segment (53) on which the pulsed laser radiation (9) strikes at least once when the pulsed laser radiation circulates through the multiple passage cell.

7. Optical system (3) according to one of the preceding claims, wherein the multiple passage cell (5) is designed such that a primary laser pulse (11) of the pulsed laser radiation (9), the contrast of which is to be increased in the optical system (3), in the intermediate focus zones (29) there is essentially no change in the pulse duration (At) and / or pulse energy, and / or

, wherein the multiple passage cell (5) is designed as a, in particular concentric or confocal, resonator.

8. Optical system (3) according to one of the preceding claims, wherein the Strahlan parts with differently aligned elliptical polarization states a useful beam portion with primary laser pulses (11) and a residual beam portion with low-intensity Laserstrah treatment, in particular a radiation platform and / or comprises the high-intensity laser pulses (11) preceding low-intensity laser pulses (13A) and / or subsequent low-intensity laser pulses (13B).

9. Optical system (3) according to one of the preceding claims, wherein the optical beam splitting system (41) for splitting the beam components with differently oriented elliptical polarization states comprises: - A second polarization setting optics for returning each of the differently aligned elliptical polarization states to a linear polarization state (47 A, 47B), in particular the beam components output by the multiple passage cell (5) with differently oriented elliptical polarization states in a useful beam component (9A) and a residual beam portion (9B) with differently aligned linear polarization states (47A, 47B) are transferred, and

- A beam splitter (45), which outputs the useful beam portion (9A) and the remaining beam portion (9B) on un ferent beam paths.

10. The optical system (3) according to claim 9, wherein the second polarization setting optics comprises a second wave plate (43), optionally a 1/4 wave plate (43) and / or a 1/2 wave plate.

11. Optical system (3) according to any one of the preceding claims, further comprising a control system (61), which for a rotation of the alignment of one of the elliptical polarization states, in particular for setting an angle of rotation (DQ) of 90 °, at least for setting one of the the following parameters are formed:

- a pulse duration (At) of primary laser pulses (11) of the pulsed laser radiation (9),

- a pulse energy of primary laser pulses (11) of the pulsed laser radiation (9),

- focus diameter (d) in the intermediate focus zones (29),

- Rayleigh lengths (Lr) of the intermediate focus zones (29) and

- A gas pressure of the gas (4) in the intermediate focus zones (29).

12. Laser system (1), in particular ultra-short pulse (USP) laser system, for emitting pulsed laser radiation (9) comprising

- A laser radiation source (7) which emits pulsed laser radiation (9) which comprises primary laser pulses (11), in particular with pulse energies pulse durations (At) in the range of a few hundred femtoseconds and less,

- optionally a pulse duration setting system (15) for setting the pulse duration (At) of the primary laser pulse (11),

- At least one optical system (3) according to one of the preceding claims for increasing the contrast of the pulsed laser radiation (9) using a non-linear one elliptical polarization rotation in a plurality of intermediate focus zones (29) of a multiple passage cell (5), and

- Optionally an optical pulse duration compressor system (49) to compensate for a dispersive contribution of the at least one optical system (9) and optionally to compress the primary laser pulses (11) of the laser radiation over time, if this is a nonlinear spectral broadening in at least one of the intermediate focus zones (29) have experienced.

13. Laser system (1) according to claim 12, wherein gas conditions, in particular gas type and / or gas pressure, differ in a first optical system according to one of the preceding claims from corresponding gas conditions in a second optical system according to one of the preceding claims.

14. Method for increasing the contrast of pulsed laser radiation (9) using a non-linear elliptical polarization rotation, with the steps:

- Setting an elliptical polarization state (17A) of the pulsed laser radiation (9),

- Coupling of the pulsed laser radiation (9) into a multiple passage cell (5) with at least two mirrors (25A, 25B), which is traversed to form a plurality of intermediate focus zones (29), the multiple passage cell (5) with a gas (4 ) ge is filled, which has an optical non-linearity that causes an intensity-dependent rotation of an alignment of the elliptical polarization state (17B) of the pulsed laser radiation (9) in the intermediate focus zones (29), whereby beam components with differently aligned elliptical polarization states in the multiple passage cell (5 ) be generated,

- Decoupling of the beam components with differently oriented elliptical polarizations from the multiple passage cell (5) and

- Separation of the decoupled beam components into a useful beam component (9A) with primary laser pulses (11 ') and a residual beam component (9B) with low-intensity laser radiation.

15. The method of claim 14, further comprising

- Setting at least one of the following parameters for rotating the alignment of one of the elliptical polarization states in the multiple passage cell (5), in particular for setting an angle of rotation (DQ) of 90 ° for primary laser pulses (11) of the pulsed laser radiation (9):

- A gas pressure in the intermediate focus zones (29) and / or - A dispersion of the primary laser pulses (11) and optionally accumulated in the multiple passage cell (5)

- a pulse duration (At) of the primary laser pulses (11) of the pulsed laser radiation (9),

- a pulse energy of the primary laser pulses (11) of the pulsed laser radiation (9), - an ellipticity of the set elliptical polarization state of the pulsed laser radiation (9),

- Focus diameter (d) in the intermediate focus zones (29) and / or

- Rayleigh lengths (Lr) of the intermediate focus zones (29).

16. The method according to claim 14 or 15, wherein a pulse spectrum of the pulsed laser radiation (9) assigned to the primary pulses (11), in particular from intermediate focus zone to intermediate focus zone, is broadened due to a non-linear spectral broadening in the multiple passage cell (5), while at the same time the contrast increase takes place.