DISPERSION ADJUSTMENT UNIT

Description

[0001] The present invention relates to a dispersion adjustment unit for electromagnetic radiation with a spectral width. Furthermore, the present invention relates to a method for dispersion adjustment for laser pulses.

[0002] Laser pulses have a spectral width that determines the achievable minimum duration of the laser pulse. The broader the underlying frequency spectrum, the shorter the pulse duration of the laser pulse can be. However, the dispersion of the material being passed through (referred to herein as material dispersion) and possibly self-phase modulation at high peak powers generally lead to a divergence of the spectral components. Therefore, especially for short and ultrashort laser pulses with pulse durations in the ps range and shorter, dispersion adjustment of the optical path is usually carried out if the divergence of the laser pulses is to be prevented or reversed. Optical structures that counteract a dispersive broadening of the pulse duration are referred to herein as pulse compressor units. Examples of such dispersion adjustment units include, for example:Grating pair or prism pair based compressor units (grating or prism compressors for short) that use a spreading of the spectral components of the laser pulse, referred to herein as angular dispersion, to generate different optical path lengths.

[0003] Furthermore, amplified pulses in particular can lead to high intensities, which can cause, among other things, non-linear effects such as self-focusing, e.g. in the amplifier laser medium. Such non-linear optical effects can have a detrimental effect on the beam and pulse quality and accordingly on the entire amplification process. Accordingly, amplifier configurations are designed such that either an actively stretched laser pulse is amplified or the pulse lengthening occurs during the amplification process. Optical structures of such dispersion adjustment units, which cause a dispersive broadening of the pulse duration of a laser pulse, are referred to herein as pulse stretcher units. Examples of such dispersion adjustment units include, for example, grating pair or prism pair-based stretcher units (in short: grating or prism stretchers), which, for example,to utilize angular dispersion opposite to that of a later-used pulse compressor unit, for example, with an integrated lens system. Furthermore, a laser pulse can be guided through materials with corresponding dispersive properties for pulse broadening, for example, optical fibers, possibly with integrated (chirped) fiber Bragg gratings.

[0004] The fundamental parameter for grating or prism compressors and grating or prism stretchers is the extent of spectral broadening. This depends, for example, on the grating constant of the grating used or the refractive index of the prism used. In general, the dispersion can be adjusted by the spacing of the gratings or prisms, which determines the path length differences caused by angular dispersion. In grating compressors or stretchers, the phase accumulated by diffraction contributes in particular to the dispersion. In general, the better the dispersion can be compensated, the closer the pulse duration can be to the spectrally achievable pulse duration. The shorter the pulse duration, the more orders of dispersion must be taken into account during compression. For fine adjustment, thin glass plates positioned in the beam path can also be used, which make the dispersion properties of the beam path adjustable through material dispersion.

[0005] In general, the above concepts for pulse stretching and pulse compression are used in so-called CPA (chirped pulse amplification) to generate ultrashort pulses with high pulse energies. For example, to generate (ultra-)short pulses with a fiber laser system, an input laser pulse from a fiber laser is typically stretched in time (e.g., in a fiber-based stretcher or in a grating stretcher), amplified in a fiber amplifier unit, and then compressed in time (e.g., in a grating compressor). Similar setups for generating (ultra-)short pulses can be based on disk laser systems, for example, in machine tools. Typically, very precise and technically sophisticated compression is required to reverse the dispersive divergence of the spectral components.

[0006] Various approaches for precise dispersion control are known in the prior art, for example, in grating compressors. For example, WO 2015 / 117128 A1 and US Pat. No. 7,822,347 B1 disclose concepts in which the change in pulse duration is controlled via the grating spacing of the stretcher or compressor and additionally fine-tuned using a chirped FBG (Fiber Bragg Grating). Further approaches are known from US Pat. Nos. 7,729,045 B2 and 8,780,440 B2. Furthermore, DE 10 2010 018967 A1 discloses an OPO system with dispersion compensation based on material dispersion, in which one (or two) glass plates are positioned in the beam path. Furthermore, US 2008 / 0304127 A1 discloses a pulse shaper with an SLM, US 6,272,156 B1 the use of a stretcher-compressor arrangement in an optical fiber-based transfer of a pulsed laser beam, US 2011 / 0255563 A1 a pulse shaper for a laser system and US 2006 / 0159137 A1 a pulsed laser system with a grating compressor.

[0007] Furthermore, WO 2010 / 028837 A1 discloses a device for amplifying light pulses with a stretcher, at least one amplifier, and a compressor. The stretcher and the compressor generate substantially compensating dispersions and are constructed, for example, with prisms or diffraction gratings. The dispersion is further adjusted with an additional optical element of variable dispersion and / or by self-phase modulation of the light pulses. The additional optical element is arranged in front of the stretcher or between the stretcher and the amplifier. For example, an additional pair of prisms is used.

[0008] WO 2015 / 140512 A1 discloses a high-power laser with chirped pulse amplification for generating extremely powerful ultrashort pulses. In one embodiment, the high-power laser comprises a transmission grating stretcher with a first diffraction grating for dispersing a seed laser pulse, and transmission optics that collect the dispersed laser pulse and guide it to a transmission diffraction grating. The transmission diffraction grating collimates the laser pulse onto a reflector, which reflects the laser pulse back through the pulse stretcher.

[0009] CN 104795718 A discloses a chirped pulse amplification laser with fourth-order dispersion compensation. In addition to a grating stretcher and a grating compressor, the dispersion compensation includes a further dispersion corrector comprising a first grating, a second grating, a rectangular prism, and a mirror. The further dispersion corrector serves to correct the fourth-order dispersion and uses a grating structure on a prism, so that the dispersive contribution of the dispersion corrector is determined by the grating and the prism, with a corresponding effect on the fourth-order dispersion.

[0010] One aspect of this disclosure is based on the object of providing an extended dispersion adjustment in, for example, grating pair and prism pair-based dispersion adjustment units. A further aspect of the invention is based on the object of enabling a dispersion adjustment in such dispersion adjustment units that has the least possible impact on the beam quality and the beam path downstream of the dispersion adjustment unit and, in particular, is based on an easily controllable adjustment of an optical component.

[0011] At least one of these objects is achieved by a dispersion adjustment unit according to claim 1, by a dispersion adjustment unit according to claim 3, by a laser system according to claim 12 and by a method for dispersion adjustment according to claim 15. Further developments are specified in the subclaims.

[0012] In one aspect, a dispersion adjustment unit for electromagnetic radiation with a spectral width, in particular for laser pulses, comprises an arrangement with at least one dispersive element for generating angular dispersion in an angular dispersion range delimited by two interaction regions of the electromagnetic radiation with the at least one dispersive element, in which individual spectral components of the electromagnetic radiation are assigned optical paths extending at an angle to one another. Furthermore, the dispersion adjustment unit has an optical unit arranged in the angular dispersion range with an optical element that transmits the electromagnetic radiation and causes an incident angle-dependent parallel offset of the individual spectral components of the electromagnetic radiation with respect to the propagation of the individual spectral components upstream and downstream of the optical unit.

[0013] In a further aspect, a dispersion adjustment unit for electromagnetic radiation with a spectral width, in particular for laser pulses, comprises at least one pair of dispersive elements for generating angular dispersion. The pair of dispersive elements is arranged such that optical paths extending at an angle to one another are assigned between the dispersive elements of the pair of dispersive elements. Furthermore, the dispersion adjustment unit comprises an optical element arranged in the beam path between the dispersive elements and rotatably mounted for adjusting the angular position of the optical element with respect to the beam path.

[0014] In a further aspect, a laser system comprises a laser pulse source for generating spectrally wide laser pulses and a pulse stretcher or a pulse compression or both with at least one dispersion adjustment unit as described above.

[0015] Furthermore, the laser system can have a control unit and a pulse duration measuring device for outputting a pulse duration-dependent measurement signal to the control unit, wherein the control unit is designed, for example, to adjust the angular position of the optical element with respect to the beam path as a function of the pulse duration-dependent measurement signal.

[0016] In a further aspect, a method for dispersion adjustment for laser pulses comprises the steps of: providing an angular dispersion component by spectrally fanning out and combining a pulsed laser beam in an angular dispersion range; providing an optical element in the angular dispersion range, which provides propagation directions in the optical element that are dependent on an entrance angle of spectral components with respect to an entrance surface of the optical element; and changing the propagation direction and / or the propagation length in the optical element, thereby influencing the angular dispersion component, in particular while maintaining the propagation directions of spectral components,wherein changing the propagation direction and / or the propagation length causes an angle-dependent parallel offset of the individual spectral components of the electromagnetic radiation with respect to the propagation of the individual spectral components,For example, by inserting a rotatable glass plate (as an example of an optical element transmitting electromagnetic radiation) into the spectrally expanded region of an arrangement of at least one dispersive element, additional dispersion adjustment parameters of the associated dispersion adjustment unit are obtained. The spectrally dependent angle of incidence on the glass plate leads to different path lengths in the glass plate for the various spectral components and to a different beam path after the glass plate. This results in an incident angle-dependent parallel offset of the individual spectral components. The spectral dependence of the refractive index and thus the change in material dispersion is largely negligible compared to the dispersion change due to the changed parallel offset.

[0017] The rotatable glass plate can thus be used to modify the temporal compression of a laser pulse in a (e.g. grating / prism / grism) compressor.

[0018] To adjust the dispersion, the glass plate is mounted rotatably, for example, above a control unit. Such an arrangement can be implemented with little or no impact on beam quality. In general, the thickness of the glass plate (generally the optical element for dispersion adjustment) allows the adjustment range or sensitivity of the dispersion modification to be selected by rotation.

[0019] Similar effects on the beam path of individual spectral components during dispersion adaptation can also be achieved using an optical element designed as a double-wedge structure and / or an electro-optical modulator and / or two acousto-optical modulators.

[0020] A prerequisite for wavelength- / angle-of-incidence-dependent modification of angular dispersion is generally that a refractive index change is provided within an optical unit. For an implementation in air / nitrogen, the refractive index of the optical element must be different from 1 to provide refraction that depends on the angle of incidence.

[0021] For example, in the case of a folded grating compressor with a double pass, a plane-parallel plate has the advantage of minimizing the impact on beam quality. However, in an unfolded grating compressor, an optical unit with a single optical element can also be provided on only one side, for example, if the stretching or compression is small, thus minimizing the impact on beam quality in the asymmetric beam path.

[0022] The optical element preferably has a geometry that, in particular, parallel-shifts the direction of an incoming (monochrome) beam—assuming a non-perpendicular incidence. As a result, the propagation directions of the individual spectral components in the spectrally divergent region remain the same before and after the optical element, so that the angular relationship between the beam paths of the individual spectral components is maintained, but the degree of fanning out can be varied.

[0023] The optical element is transparent in the used spectral range and generally has plane-parallel input and output surfaces. It is, for example, a plane-parallel quartz plate or a system of optical elements that provides plane-parallel input and output surfaces. The input and output surfaces can, for example, be anti-reflective coated to prevent power losses and interference.

[0024] Furthermore, the optical unit can comprise two glass plates (as optical elements), which can be rotated relative to one another, for example. With appropriate alignment of the glass plates, a beam offset-free zero position can be achieved, in which essentially only the material dispersion of the glass plates contributes to the dispersion properties of the dispersion adjustment units. Furthermore, the optical unit can comprise a thin plate for fine adjustment and a thick plate for coarse adjustment of the dispersion properties.

[0025] The concepts and embodiments disclosed herein may have the following advantages. The pulse duration can be adjusted independently of or in addition to a movement of one of the dispersive elements (e.g., displacement of the second grating in a grating compressor for distance adjustment). For example, in the case of a grating compressor, the beam quality is very sensitive to the orientation of the grating structures. As a result, adjusting the pulse duration requires a very complex grating holder, which should be able to be moved very precisely without influencing the orientation of the grating structure. Furthermore, the sensitivity of the dispersion adjustment, and thus the fine adjustment of the pulse duration of the laser pulses, to the rotation of the plate can be selected by the material thickness of the optical element, e.g., a plane-parallel plate of a few millimeters or centimeters. The material thickness can, for example,be chosen so that a compressor structure is as stable as possible and / or the pulse duration can be actively adjusted or varied by rotating the optical element using a motor or piezo element. In general, the material thickness is selected according to the desired adjustment range, whereby small angular changes have a greater effect on a thick optical element than on a thin optical element. Furthermore, a double-wedge configuration, for example, can make the initial thickness of the optical element adjustable. In a double-wedge configuration, at least one of the wedges can be moved independently of the dispersive element, for example the grating / prism / grism (or gratings / prisms / grisms) of a compressor or stretcher.

[0026] In general, the concepts proposed herein allow for a reduction in the required installation space for a dispersion adjustment unit. Furthermore, the dispersion adjustment can be performed without a translational movement of a dispersive element (e.g., grating, prism, or grism), so that, in particular, only a minimal influence, if any, on the beam path is exerted during dispersion adjustment.

[0027] Disclosed herein are concepts that allow aspects of the prior art to be improved, at least in part. In particular, further features and their usefulness will become apparent from the following description of embodiments with reference to the figures. The figures show: Fig. 1 shows a schematic representation of a laser system with a dispersion adjustment unit based on a rotatable glass plate for pulse duration compression (compressor). Fig. 2A-2B shows exemplary calculations of rotation angle-dependent dispersion contributions. Fig. 3A-3D shows exemplary calculations of rotation angle-dependent dispersion orders for a small lattice constant. Fig. 4A-4B shows exemplary calculations of intensity curves to illustrate the dispersion adjustment. Fig. 5A-5D shows exemplary calculations of rotation angle-dependent dispersion orders for a large lattice constant. Fig. 6 shows exemplary calculations of intensity curves to illustrate the pulse shaping. Fig. 7 shows a schematic representation of a dispersion adjustment unit based on a rotatable glass plate for pulse duration stretching (stretcher). Fig. 8A-8C shows exemplary embodiments of optical units with multiple rotatable optical elements for dispersion adjustment units.9 shows an exemplary flow diagram of a method for dispersion adjustment, Fig. 10 shows a schematic representation of another optical unit based on a double-wedge structure for a dispersion adjustment unit, Fig. 11 shows a schematic representation of a laser system with a dispersion adjustment unit based on an electro-optical modulator for pulse duration compression and Fig. 12 shows a schematic representation of a laser system with a dispersion adjustment unit based on a single dispersive element for pulse duration compression.

[0028] The aspects described herein are based in part on the realization that additional possibilities for influencing the dispersion properties exist by intervening in the angle-dispersive propagation section of a grating, prism, or grism compressor (or stretcher). It was further recognized that the angular dispersion can be modified by an entrance-angle-dependent parallel offset of the individual spectral components.

[0029] In the following, with reference to Fig. 1 The concept disclosed herein for dispersion adjustment for an ultrashort pulse system - exemplary for laser systems that usually perform dispersion adjustment - is described using a dispersion adjustment unit for pulse duration compression based on a rotatable glass plate.

[0030] In Fig. 1An ultrashort pulse system 1 comprises a laser pulse source 3 for generating spectrally broad laser pulses and a dispersion adjustment unit 5 for pulse duration compression, which comprises an optical unit 4 explained below. The laser pulse source 3 can be designed, for example, as a laser oscillator or laser oscillator-amplifier combination. Furthermore, a similar dispersion adjustment unit (see, for example, Fig. 7 ) can be integrated into the laser pulse source 3 as part of a pulse stretching process.

[0031] In the ultrashort pulse system 1, laser pulses with spectral widths of, for example, 1 nm and greater and pulse energies of, for example, 0.1 µJ and greater are fed to the dispersion adjustment unit 5 as input beam 6. The spectral width of the laser pulses necessitates dispersion adjustment of the beam path in order to provide a specific pulse shape (intensity profile) of the laser pulses at a target location with a desired pulse duration, for example, the shortest possible or one adapted to a processing method.

[0032] The dispersion adjustment unit 5 is, for example, constructed in the form of a folded grating compressor. The grating compressor comprises a pair of dispersive elements in the form of a first grating 7A and a second grating 7B, which provide two interaction regions of the gratings with the laser radiation. The spectrally dependent diffraction conditions generate an angular dispersion after the first grating 7A. This means that in an angular dispersion region 8 between the dispersive elements (gratings 7A and 7B), the optical paths for the individual spectral components run at an angle to one another in a fan-out plane.

[0033] In Fig. 1Three optical paths 8A, 8B, 8C of the beam path are shown as examples, each for a central wavelength λ 0 as well as a longer (λ> λ 0 ) and shorter (λ< λ 0 ) wavelength. The optical paths 8A, 8B, 8C of the different wavelengths run at an angle to one another in a fanning area 8. The second grating 7B effects an alignment / parallelization of the optical paths 8A, 8B, 8C onto a reflector element 9 which effects folding. The reflector element 9 is, for example, a deflection prism or a roof mirror, so that the return path experiences the same optical conditions, but is, for example, offset in height to enable a separation of the compressed laser pulses of an output beam 10 from the input beam 6 at a pick-off mirror 10A. In Fig. 1 The dispersive elements 7A, 7B are designed, for example, as transmission gratings. Alternatively, reflection gratings, prisms, or grisms can be used.

[0034] According to the concepts disclosed herein, the dispersion adjustment unit 5 comprises the optical unit 4, which provides a further parameter for adjusting the dispersion. In the Fig. 1 In the embodiment shown, the optical unit 4 comprises as an optical element, for example, a transmitting plane-parallel plate 11. The optical element, ie the plate 11, is made of quartz, YAG, sapphire or SF 10, for example. It is arranged between the first grating 7A and the second grating 7B in the fan-out region 8. In Fig. 1The plate 11 is shown oversized in order to more clearly depict the optical paths. Due to the spectral dispersion, the individual wavelengths impinge on an entrance surface 11A of the plate 11 at different angles, pass through the plate 11 on optical paths of different lengths, and exit the exit surface 11B at different distances from one another. With a plane-parallel plate, the angular relationship between the wavelengths is maintained. Accordingly, the plate 11 causes an entrance angle-dependent parallel offset of the individual spectral components of the laser radiation with respect to the propagation of the individual spectral components before and after the plate 11. Fig. 1 The alignment of the optical paths 8A, 8B, 8C is carried out accordingly with gratings 7A, 7B arranged parallel to each other.

[0035] The optical unit 4 may further comprise an angle adjustment device 17 for adjusting the angular position of the plate 11 with respect to a rotation axis 17A. To control the angle adjustment device 17, the laser system 1 may further comprise a control unit 13 and a pulse duration measuring device 15.

[0036] It can be seen that a rotation of the plate 11 changes the optical paths in the plate 11 and the distances between the wavelengths on the exit surface 11B. Accordingly, the rotation influences the dispersion properties of the dispersion adjustment unit 5, so that the pulse duration of the laser pulses in the output beam 10 can be adjusted by rotating the plate 11.

[0037] In contrast to the concept described herein, the arrangement disclosed in the aforementioned DE 10 2010 018967 A1 is based on a glass plate that is not arranged between the gratings. Thus, the structure there exhibits a comparatively significantly lower material dispersion contribution and does not have the geometric effect of an optical element arranged between the gratings, which additionally changes the pulse duration, as is the case with the arrangement disclosed herein.

[0038] Using the information provided in connection with Fig. 1With the dispersion adjustment unit 5 described above, the pulse duration could be varied between 200 fs and 1 ps using a plane-parallel, approximately 1.5 mm thick glass plate without significantly impairing the beam or pulse quality. The adjustment was more reproducible and more convenient than conventional adjustment by adjusting the distance between the compressor grids 7A, 7B. Obviously, the variation of the pulse duration is not limited to the range 200 fs to 1 ps, but can be selected, among other things, by the thickness and material of the glass plate (for a given adjustment range of the rotation angle δ).

[0039] In the following, the functional principle is explained in general terms, but with reference to the Fig. 1The embodiment shown is summarized. Those skilled in the art can apply the functional principle analogously to the alternative embodiments described below within the scope of this disclosure. Due to the diffraction at the first grating 7A, the various spectral components impinge on the plate 11 at different angles. As a result, the phase difference between the spectral components changes upon rotation of the plate 11 due to several effects: 1. The path length in the optical unit (e.g. in plate 11) differs for the various spectral components. This leads to different optical path lengths (the dependence of the refractive index on the wavelength is negligible in most configurations) and thus to a phase difference. In the case of plate 11, the phase difference depends, for example, on its orientation in the beam path, i.e. generally on the angle of incidence, e.g. of a central wavelength, on plate 11. 2. Due to the different angles of incidence, the spectral components experience different beam offsets when exiting plate 11. As a result, the rotation of plate 11 can change: a) the path length between plate 11 and the 2nd interaction region (impact region on grating 7B) for the various spectral components, b) the size (i.e. the fanning out) of the beam at the 2nd interaction region.Interaction area (impact area at grating 7B) and thus the phase contribution of the 2nd dispersive element (grating 7B) and c) the path distance between the 2nd dispersive element (grating 7B) and the reflector element 9 for the various spectral components.

[0040] The contributions of these effects to the phase difference between the different spectral components or the dispersion are shown in the Figures 2A and 2B for a glass plate. The Figures 2A and 2B illustrate the β 2 coefficient of the phase (ϕ=β 0 +β 1 *(ω-ω 0 )+β 2 / 2*(ω-ω 0 ) 2< +...) taking into account the previously explained amounts of the functional principle. The β 2 coefficient is given as a function of the angle of rotation of the glass plate, where Fig. 2Bshows the contributions in the area around the 0° position of the glass plate enlarged. In the 0° position, the central wavelength hits the glass plate perpendicularly (angle of incidence 0°). The given values ​​correspond to a setup with a vertical grating spacing I grating of 50 mm, a plate thickness of 10 mm, and a grating constant of 588 nm. Figures 2A and 2B The graphs are assigned to the contributions as follows: 21 (dotted line): Compressor without plate, where the distance between the compressor grids is adjusted so that β 2 is the same as with the plate at a rotation angle δ of the 0° position. 23 (dashed line): The plate's contribution to the phase for the path between the grids is taken into account according to points 1) and 2a) of the operating principle. 25 (dash-dotted line): The plate's contribution to the phase for the path between the grids and the contribution of the 2nd grid are taken into account according to points 1), 2a) and 2b) of the operating principle. 27 (solid line): All contributions of the plate are taken into account according to points 1), 2a), 2b) and 2c) of the operating principle.

[0041] It can be seen that in the angular range shown, all contributions are relevant for the dispersion adjustment, since they are essentially of the same order of magnitude (see in particular Fig. 2B ).

[0042] If one compares the contributions to β 2 of the plate rotation with the β 2 contributions of the length change of the grid compressor, they are also of a comparable order of magnitude.

[0043] As in the Figures 3A to 3D As shown, especially for small lattice constants, the ratio of the dispersion orders changes significantly less than β 2 itself when the glass plate is rotated. Figures 3C and 3D As an example, a relative delta - rel. Δ(β3 / β2) or rel. Δ(β4 / β2) - is plotted, which characterizes the ratio of the dispersion orders. For example, the relative delta rel. Δ(β3 / β2) is given by: 1 − β 3 β 2 β 3 , 0 β 2 , 0 ∗ 100 %

[0044] Since rotating the glass plate has approximately the same effect as changing the grating spacing, a change in the spacing can be replaced by a change in the angle of rotation during dispersion adjustment. The glass plate can thus be used to adjust or control the pulse duration / pulse shape, especially for small grating constants.

[0045] The Figures 4A and 4B illustrate an example calculation for pulse compression for a pulse whose spectral width allows a minimum possible duration of 300 fs. Fig. 4A shows an intensity curve during an adjustment of the dispersion adjustment unit, where the compressor has a suboptimal position of the glass plate or a suboptimal distance between the compressor grids. By optimizing the angular position of the glass plate, the pulse duration can be significantly compressed, as shown in the intensity curve of the Fig. 4B is shown.

[0046] For very short pulses with high spectral widths, the pulse quality (especially the pulse duration and pulse shape) reacts sensitively to the higher orders β 3 , β 4 ... of β. Thus, the pulse duration can be varied by rotating the glass plate in only a small angular range, which results in the optimization of the pulse quality as a further application of the concepts disclosed herein, especially for large lattice constants and (ultra-)short pulses.

[0047] For large lattice constants, the ratio of β 2 to the higher orders β 3 , β 4 ... changes significantly more than for small lattice constants. This is illustrated by the Figures 5A to 5D , in which analogous to the Figures 3A to 3DExemplary angular dependences of β 2 and the relative ratios to β 2 for a grating constant of 588 nm are shown. The contributions to the higher orders can now be further used to optimize the pulse quality by adjusting the angle of incidence, ie, the rotation of the glass plate, and the spacing of the compressor gratings.

[0048] Fig. 6shows an example calculation for a suboptimal tuning of a stretcher-compressor system when compressing a pulse with a spectral width that would allow a minimum achievable pulse duration of 70 fs. Without the use of a glass plate, the dashed line 31 shows an intensity curve that is the result of pulse compression based purely on the compressor distance adjustment. This results, for example, in a pulse duration of a good 120 fs after the compressor without a glass plate. If the system is supplemented according to the invention with a glass plate in the compressor and both the distance between the compressor grids and the rotation angle δ of the glass plate are optimized, the pulse duration can be shortened further, e.g. to below 80 fs, as shown by the solid line 33 of an example calculated intensity curve.

[0049] Returning to Fig. 1the laser system 1 comprises the control unit 13 and the pulse duration measuring device 15 and the optical unit 4 comprises the angle adjustment device 17 for adjusting the angular position of the optical element 11 depending on a pulse duration measurement.

[0050] For example, the pulse duration measuring device 15 is designed to output a pulse duration-dependent measurement signal (e.g., an autocorrelation signal). For example, a (non-reflected) portion of the output beam 10 tapped at a mirror 19 is fed to the pulse duration measuring device 15. The pulse duration measuring device 15 transmits the measurement signal to the control unit 13, which outputs a control signal to the angle adjustment device 17. The control unit 13 uses, for example, an optimization algorithm to vary the angular position (and possibly the spacing of the dispersive elements) to shorten the pulse duration and to adjust it with respect to a shortest pulse duration or a pulse duration (or pulse shape) required for a specific material processing application.

[0051] The angle adjustment unit 17 can, for example, be a motorized and / or piezoelectric actuator-rotatable mount for the optical element, which allows a particularly continuous adjustment of the angle of rotation δ. This allows, for example, an adjustment, particularly a control, of the pulse duration by means of an actively controlled, particularly controlled, angle of rotation adjustment.

[0052] Fig. 7 shows an exemplary implementation of the dispersion adjustment concepts disclosed herein in a dispersion adjustment unit 50 designed as a grating stretcher. In order to achieve inverted angular dispersion contributions, a lens system (e.g. a telescope arrangement) can be provided in a stretcher between the interaction regions on the at least one dispersive element, wherein Fig. 7Two transmission gratings 7A', 7B' are shown as examples. As with compressors, the at least one dispersive element can alternatively be designed as a reflection grating, prism, or grism.

[0053] The arrangement that focuses the laser beam causes an image of the first grating, so that the fanned-out spectral components converge. An example is shown in Fig. 7 a telescope arrangement 51 with two lenses 51A, 51B is shown. An optical element 11' designed as a plane-parallel plate is inserted after the telescope arrangement 51 in front of the second grating 7B'. It is - like the plate 11 in Fig. 1 - rotatably mounted and can be adjusted with regard to the angle of incidence to adapt the dispersion behavior of the stretcher arrangement.

[0054] As in Fig. 1 the dispersion adjustment unit 50 is folded with a reflector element 9'. In contrast to the compressor arrangement of the Fig. 1The optical element 11' is arranged in a converging fan-out region. Alternatively or additionally, an optical element may be further arranged in front of the first lens 51A. Furthermore, the focusing arrangement may comprise one or more lenses and / or mirrors.

[0055] The dispersion aspects previously described with regard to the compressor arrangement can be transferred to the stretching arrangement, in particular the functional principle and the application possibilities for pulse duration and pulse shape adjustment.

[0056] The Figures 8A to 8C show further designs of rotatably mounted optical elements for optical units that can be used in stretcher and compressor arrangements.

[0057] Fig. 8Adepicts an optical wedge pair 35 (double-wedge structure) known, for example, from ultrashort optics - as an example of a multi-wedge arrangement - with, for example, identical wedges 35A, 35B, which can be used as a rotatable optical element of an optical unit 4. The wedges 35A, 35B each have two flat side surfaces that converge at an acute angle. The wedges 35A, 35B are arranged in the fanned-out beam path in such a way that one of the side surfaces of the wedges acts as an entrance surface 11A or exit surface 11B of the optical element, running parallel to one another. The other sides form a thin air gap 37 lying between the wedges, for example with a substantially constant thickness.

[0058] At least one of the wedges 35A, 35B is mounted for displacement (e.g., on a linear displacement stage) (arrow 39), so that the thickness of the wedge pair 35 can be adjusted depending on the insertion position of the wedges 35A, 35B, and the insertion can be used as an additional adjustment variable for dispersion adjustment. The angle adjustment is achieved by rotating the entire double wedge 35 (arrow 41A).

[0059] In some embodiments, the double wedge can be used as a special embodiment of a glass plate in which the two wedges can be rotated together by the same angle.

[0060] Multi-wedge arrangements are also a possibility for creating an embodiment in which no mechanical rotation of the optical element is necessary. This is exemplified below in connection with Fig. 10 Further embodiments that do not require mechanical movement are described in connection with Fig. 11 described as examples and are based, for example, on a change in the refractive index, for example with an EOM or a birefringent crystal.

[0061] Returning to optical units with rotatable optical elements, Fig. 8B a further embodiment which allows the angular dispersion to remain essentially unchanged in an initial position. For this purpose, the optical unit comprises a pair of identical, e.g. plane-parallel, transmitting plates 43A, 43B, which can be rotated in opposite directions (arrows 41B, 41B'). This allows, with a corresponding alignment (dashed in Fig. 8B indicated) the parallel offset of the first plate 43A is compensated by the second plate 43B, so that in this position the dispersion is determined solely by the distance of the gratings and the material dispersion of the plates 42A, 43B.

[0062] Fig. 8Cshows another arrangement of two plane-parallel plates 45A, 45B, but of different thicknesses and / or different materials. For example, a thin plate 45A can be used for fine adjustment of the dispersion properties and a thick plate 45B for coarse adjustment. The dispersion is again adjusted by rotating one or both plates 45A, 45B (arrows 47A, 47B).

[0063] The Figures 8A to 8C The embodiments shown are examples of optical units consisting of several optical elements which together provide plane-parallel input surfaces and output surfaces and accordingly maintain the spectral propagation directions before and after the optical unit.

[0064] Further configurations of dispersion adjustment units include, for example, unfolded arrangements with two pairs of dispersive elements, with an optical unit arranged in at least one of the fanned-out regions. Furthermore, stretcher and compressor arrangements can be adapted to one another, particularly with regard to the optical elements.

[0065] Exemplary laser systems in which the dispersion adjustment units proposed herein can be used include laser systems with pulse energies of, for example, 0.1 µJ and greater and pulse durations in the range of, for example, 50 ps and shorter. Application areas for such laser systems include glass cutting (e.g., cutting displays and medical devices), marking, medical applications such as eye surgery, drilling of, for example, injection nozzles, and conducting scientific experiments.

[0066] Based on the concepts disclosed herein, a method for dispersion adjustment may include the following, in Fig. 9 shown, include steps.

[0067] By spectrally spreading and combining a pulsed laser beam, an angular dispersion component is provided (step 61). By introducing an optical element in the fanned-out region of the pulsed laser beam (step 63), the angular dispersion component is influenced, for example via a plane-parallel plate, a multi-wedge structure, EOM, etc. In particular, the material dispersion can also be taken into account when considering dispersion by the introduced optical element. An optical element in the form of a plane-parallel plate, for example, is rotatably mounted and thus adjustable in its angular position to the laser beam. If, in addition, a pulse duration of the laser pulse is measured after dispersion adjustment (step 65), the angular position of the optical element can be adjusted, for example, depending on the measured pulse duration to adjust the dispersion (step 67).This allows, in particular, shortening or stretching the pulse duration, as well as pulse shaping, thanks to the versatile combination of dispersion components in the dispersion adjustment unit. Furthermore, dispersion adjustment can be performed together with or in addition to angular dispersion adjustment.

[0068] Furthermore, in systems where dispersive conditions vary reproducibly depending on the requested pulse energy, e.g. due to self-phase modulation, control concepts can be implemented that adjust the optical unit, for example the angular position of the optical element or the voltage applied to the EOM, depending on the requested pulse energy. Accordingly, the previously mentioned pulse duration measurement can be replaced or supplemented by providing a pulse energy parameter. For example, for pulses with B-integral values ​​less than 1 rad (essentially no self-phase modulation), an almost orthogonal incidence on the optical element can be set. As the pulse energy increases, the optical element is then rotated to compensate for the dispersive effects on the pulse length.

[0069] Furthermore, changing the propagation direction and / or the propagation length can cause an incident angle-dependent parallel offset of the individual spectral components of the electromagnetic radiation with respect to the propagation of the individual spectral components. Changing the propagation direction and / or the propagation length can be achieved by rotating the optical element and / or by changing a thickness and / or the refractive index and / or the diffraction parameter of the optical element, in particular to change the dispersion generated in the angular dispersion range.

[0070] The method may comprise the following steps: measuring a pulse duration of the laser pulse after dispersion adjustment and adjusting the propagation direction and / or the propagation length in the optical element, in particular the angular position, as a function of the measured pulse duration to adjust the dispersion, in particular to shorten or stretch the laser pulses. Furthermore, the method may comprise the following steps: providing a self-phase modulation dispersion contribution parameter, in particular a pulse energy parameter, and changing the propagation direction and / or the propagation length as a function of the self-phase modulation dispersion contribution parameter, in particular to compensate for the stretching of laser pulses caused by self-phase modulation.

[0071] In general, an adjustment device of the optical element can be designed to adjust a dispersion contribution of the dispersion adjustment unit, in particular for assigning a self-phase modulation dispersion contribution of the optical beam path before and / or after the dispersion adjustment unit, as a function of a pulse duration-dependent measurement signal, an average pulse power parameter, a peak pulse power parameter and / or a pulse energy parameter.

[0072] In some embodiments, the thickness of the optical element can be selected such that pulse length minimization is possible without changing the distance, e.g., of the grating pair. In general, the required thickness depends on parameters of the various optical components of the dispersion adjustment unit, e.g., on the implemented dispersive angular dispersion, for example, due to the perpendicular line spacing of the gratings, and on the angle of incidence (almost orthogonal incidence or oblique incidence, e.g., at the Brewster angle with respect to the central wavelength, influences the effective thickness). Furthermore, the required thickness depends on laser beam parameters such as the duration of the laser pulses, the spectral width of the laser pulses, the laser pulse energy, the self-phase modulation tendency of the optical system, etc.

[0073] Fig. 10 further shows that a change in the thickness of a multi-wedge arrangement, shown as an example for a double-wedge structure 70 according to Fig. 8B, leads to an entrance angle-dependent parallel offset of the individual spectral components of the electromagnetic radiation with respect to the propagation of the individual spectral components before and after the optical unit.

[0074] In particular, it is shown for two thicknesses that for an incident spectral component with an optical path 71 that is not perpendicular to the entrance surface 11A of the wedge 35A, a beam offset results due to the oblique passage through the wedges 35A, 35B at an angle ε (≠ 0) with respect to the normal n to the entrance surface 11A, which beam offset depends on the thickness of the double-wedge structure 70. The thicker the double-wedge structure 70 is set, the further an exiting optical path 73, 73' is shifted parallel with respect to the incident optical path 71. Since different spectral components in the angular dispersion range are assigned optical paths that run at an angle to one another, the angle ε and accordingly the parallel beam offset vary, so that only a change in the thickness of the optical element (by a shift along the arrow 39 in Fig. 10) leads to a change in the angular dispersion and thus enables dispersion adjustment. It should be noted that the offset, as previously explained, also results in further phase effects, such as different paths between the second interaction region and the back reflector, etc.

[0075] For the sake of completeness, reference should be made to further embodiments of multi-wedge structures with three or more wedges, which are known to the person skilled in the art and can be used analogously, as long as the required optical conditions are met.

[0076] In further embodiments, optical elements can be used in the optical unit 4, the optical properties of which, such as refractive index and birefringence, can be specifically controlled.

[0077] Fig. 11 shows a laser system 1' with a structure similar to that shown in Fig. 1shown. However, instead of the plate 11, an electro-optical modulator (EOM) 81 with electrodes 83 is provided. The voltage applied to the electrodes 83 can be adjusted using the control device 13. Depending on the voltage, a specific refractive index is established in the EOM 81, which determines the optical paths 8A, 8B, 8C in the angular dispersion range 8. If the voltage and thus the refractive index are changed, the optical paths also change. This is exemplified in the EOM 81 by optical paths 8A', 8B', 8C'. The voltage adjustment of the EOM 81 can, for example, allow for faster adjustment of the dispersion compared to mechanical rotation or insertion adjustment. Furthermore, due to the fixed spatial integration, an anti-reflective coating optimized for the angle of incidence can be used.Regarding the remaining components and the effect of changing the optical paths, reference is made to the previous description.

[0078] In analog configurations, arrangements of two acousto-optical modulators can be used as optical elements to adjust the optical paths.

[0079] For completeness, Fig. 12 a dispersion adjustment unit 5', which can be used, for example, instead of the dispersion adjustment units 5 in the laser systems 1 or 1'. The dispersion adjustment unit 5' is designed, for example, as a compressor, but can also be designed analogously as a stretcher. It has an optical unit 4' with an optical element for dispersion adjustment as disclosed herein. As in Fig. 1The dispersion adjustment unit 5' includes the back reflector 9, which provides a vertically offset forward and return path. A second folding occurs via a deflection prism 9', so that only one dispersive element 7 is required. This second folding allows spatially separated interaction areas between laser radiation and the dispersive element 7 to be provided on the (one) dispersive element 7. Accordingly, a compact optical structure results.

[0080] The exemplary embodiments have been described with reference to laser light, particularly in the extended spectral range of ultrashort pulse lasers (typically in the wavelength range from 200 nm to 10 µm, depending on the application). However, an application is generally transferable to electromagnetic radiation with a spectral width, provided that the concept of angular dispersion variation can be implemented.

[0081] The dispersion adjustment unit embodiments described herein may also be used in single or multiple pass applications and / or in oscillator systems with free jet compressors and / or stretchers.

[0082] As can be seen from the embodiments described herein, the optical element can be adjusted, for example, independently of the at least one dispersive element in its position and / or orientation with respect to the at least one dispersive element.

[0083] As is further apparent from the embodiments described herein, the optical element, in particular the one or more plane-parallel (glass) plates or the optical wedges, preferably consists of a material with a refractive index that is homogeneous across the optical element - in particular in the region of the beam passage. The homogeneity of the refractive index is such that the deflection of the light is not influenced by changes in the refractive index in the material, for example when the optical element is rotated. In particular, the parallel offset when the rays pass through the plane-parallel (glass) plates with a correspondingly homogeneous refractive index is not influenced / disturbed by a change in the refractive index in the material itself.

Claims

1. A dispersion adjustment unit (5) for electromagnetic radiation with a spectral width, in particular for laser pulses, with an arrangement with at least one dispersive element (7, 7A, 7B) for producing angular dispersion in an angular dispersion range (8) bounded by two interaction areas of the electromagnetic radiation with the at least one dispersive element (7, 7A, 7B), in which individual spectral components of the electromagnetic radiation are assigned optical paths (8A, 8B, 8C) running at an angle to one another, and an optical unit (4) arranged in the angular dispersion range (8) between the two interaction areas of the electromagnetic radiation with the at least one dispersive element (7, 7A, 7B) with a plane-parallel optical disc (11) that transmits electromagnetic radiation, with a plane entrance surface (11A) and a plane exit surface (11B) that are arranged parallel to one another, wherein the optical disc (11) causes a parallel offset, dependent on the angle of incidence, of the individual spectral components of the electromagnetic radiation with respect to the propagation of the individual spectral components in front of and after the optical disc (11).

2. The dispersion adjustment unit (5) according to claim 1, wherein the entrance surface (11A) and the exit surface (11B) are arranged perpendicular to a fanning-out plane spanned by the optical paths (8A, 8B, 8C) running at an angle to one another, and / or wherein the entrance surface (11A) and / or the exit surface (11B) have an antireflection coating, and / or wherein the material of the plane-parallel disc (11, 43A, 43B, 45A, 45B) is quartz, YAG, sapphire or SF 10 and / or has a thickness of at least 0.1 mm or in the range of approximately 0.5 mm to approximately 10 mm.

3. A dispersion adjustment unit (5) for electromagnetic radiation with a spectral width, in particular for laser pulses, with an arrangement with at least one dispersive element (7, 7A, 7B) for producing angular dispersion in an angular dispersion range (8) bounded by two interaction areas of the electromagnetic radiation with the at least one dispersive element (7, 7A, 7B), in which individual spectral components of the electromagnetic radiation are assigned optical paths (8A, 8B, 8C) running at an angle to one another, and an optical unit (4) arranged in the angular dispersion range (8) between the two interaction areas of the electromagnetic radiation with the at least one dispersive element (7, 7A, 7B) with an optical element (11, 70, 81) that transmits the electromagnetic radiation and causes a parallel offset, dependent on the angle of incidence, of the individual spectral components of the electromagnetic radiation with respect to the propagation of the individual spectral components before and after the optical unit (11, 70, 81).

4. The dispersion adjustment unit (5) according to claim 3, wherein the optical unit (11, 70, 81) further has an adjusting device (17, 83) designed to change the parallel offset of the individual spectral components.

5. The dispersion adjustment unit (5) according to claim 3, wherein - the optical element (11, 70, 81) has a plane entrance surface (11A) and a plane exit surface (11B) which are arranged parallel to one another, and / or the adjusting device for changing the spatial orientation of the optical element (11) in the angular dispersion range (8) is designed to change the parallel offset, or the optical element (70) is designed as a multi-wedge arrangement, in particular as a double-wedge structure, and the adjusting device is designed to change a thickness and / or an angular position of the optical element (70) designed as a multi-wedge arrangement, for the purpose of changing the parallel offset, or the optical element (81) is designed as an electro-optical modulator and the adjusting device is designed to change a refractive index of the optical element (81) designed as an electro-optical modulator for changing the parallel offset, or the optical element is designed as a pair of acousto-optical modulators and the adjusting device is designed to change a diffraction parameter of the optical element designed as a pair of acousto-optical modulators in order to change the parallel offset.

6. The dispersion adjustment unit (5) according to any one of claims 3 or 5, wherein the optical element (11, 70, 81) has an entrance surface (11A) and an exit surface (11B) that are substantially parallel to one another and are arranged at a distance from each other, and / or wherein the optical element (11, 70, 81) can be adjusted independently of the at least one dispersive element (7, 7A, 7B) in its position and / or orientation in the beam path with respect to the at least one dispersive element (7, 7A, 7B).

7. The dispersion adjustment unit (5) according to any one of claims 3, 5 or 6, wherein the optical element (11, 70, 81) is mounted rotatably for adjusting the angular position of the optical element (11, 70, 81) with respect to the beam path, so that the angle of incidence and the angle of emergence of the optical paths (8A, 8B, 8C) with respect to an entrance surface (11A) and an exit surface (11B) of the optical element (11, 70, 81) can be adjusted.

8. The dispersion adjustment unit (5) according to any one of claims 3, 5, 6 or 7, wherein the optical element (11, 70, 81) has a plane entrance surface (11A) and a plane exit surface (11B), wherein the entrance surface (11A) and the exit surface (11B) are arranged parallel to one another and perpendicular to a fanning-out plane spanned by the optical paths (8A, 8B, 8C) running at an angle to one another, and / or wherein the entrance surface (11A) and / or the exit surface (11B) have an antireflection coating, and / or wherein the optical element (11, 70, 81) comprises a pair of wedges (35A, 35B) arranged opposite to each other and slidable with respect to each other, and wherein the material of the optical element (11, 70, 81) of at least one of the wedges (35A, 35B) is quartz, YAG, sapphire or SF 10.

9. The dispersion adjustment unit (5) according to any one of claims 3, 5, 6, 7 or 8, wherein rotation of the optical element (11, 70) and / or changing of a thickness and / or of the refractive index and / or of the diffraction parameter of the optical element (81) causes a change in the dispersion produced by the dispersion adjustment unit (5) while the position of the at least one dispersive element (7, 7A, 7B) remains the same.

10. The dispersion adjustment unit (5) according to any one of claims 3, 5, 6, 7, 8 or 9, further with at least one focusing element between the interaction areas of the at least one dispersive element (7, 7A, 7B), wherein the optical element (11, 70, 81) is arranged in the beam path section between one of the focusing elements and the adjoining interaction area of the at least one dispersive element (7, 7A, 7B), and / or a reflective element (9, 9') for the parallel-displaced back reflection of the optical paths (8A, 8B, 8C) in one direction, which is arranged between the interaction areas of the at least one dispersive element (7, 7A, 7B), and / or an optical convolution element (9) for returning the beam path through the interaction areas of the at least one dispersive element (7, 7A, 7B) and the optical unit, and / or a second pair of interaction areas with at least one further dispersive element and / or wherein the at least one dispersive element (7, 7A, 7B) comprises one or a plurality of optical gratings, one or a plurality of prisms and / or one or a plurality of grisms.

11. The dispersion adjustment unit (5) according to any one of claims 1, 2, 3, 5, 6, 7, 8, 9 or 10, wherein the optical unit (11, 70, 81) comprises a plurality of optical elements arranged in the beam path between the interaction areas of the at least one dispersive element (7, 7A, 7B) and wherein the optical elements are arranged in the beam path twisted in the same or opposite directions.

12. A laser system (1, 1') with a laser pulse source (3) for generating laser pulses in a broad spectrum and at least one dispersion adjustment unit (5) according to one of the preceding claims for pulse compression, pulse stretching and / or pulse optimization.

13. The laser system (1, 1') according to claim 12, further with a control unit (13) for adjusting an adjusting device (17, 83) of the at least one dispersion adjustment unit (5).

14. The laser system (1, 1') according to claim 13, further with a pulse duration measuring device (15) for outputting a pulse duration-dependent measurement signal to the control unit (13) for adjusting the adjusting device (17, 83) and / or a pulse energy parameter output device (15) for providing a self-phase modulation dispersion value to the control unit (13) for controlling the adjusting device (17, 83), and / or an amplifier unit, wherein the dispersion adjustment unit (5) for pulse stretching is arranged upstream of the amplifier unit or for pulse compression downstream of the amplifier unit.

15. A method for adjusting the dispersion of laser pulses, with the steps of providing (step 61) an angular dispersion component by spectral fanning out and combining of a pulsed laser beam in an angular dispersion range (8), which is delimited by two interaction areas of the pulsed laser beam with at least one dispersive element (7, 7A, 7B), providing (step 63) an optical element (11, 70, 81) in the angular dispersion range (8) between the two interaction areas of the electromagnetic radiation with the at least one dispersive element (7, 7A, 7B), wherein the optical element (11, 70, 81) provides directions of propagation in the optical element (11, 70, 81) that are dependent on the angle of incidence of spectral components with respect to an entrance surface (11A) of the optical element (11, 70, 81), and changing (step 67) the direction and / or the length of propagation in the optical element (11, 70, 81), whereby influence is exerted on the angular dispersion component, in particular while maintaining the directions of propagation of spectral components, wherein the changing (step 67) of the direction of propagation and / or the length of propagation causes a parallel offset, dependent on angle of incidence, of the individual spectral components of the electromagnetic radiation with respect to the propagation of the individual spectral components.

16. The method according to claim 15, wherein the changing (step 67) of the direction of propagation and / or the length of propagation is caused by rotating the optical element (11) and / or by changing a thickness and / or the refractive index and / or the diffraction parameter of the optical element (70, 81).

17. The method according to claim 15 or 16, further with the steps of measuring (step 65) the pulse duration of laser pulses after dispersion adjustment and adjusting the direction of propagation and / or length of propagation in the optical element (11), and / or further with the steps of providing a self-phase modulation dispersion value parameter, and changing the direction of propagation and / or length of propagation as a function of the self-phase modulation dispersion value parameter.