Optics for pulse compression of a pulsed laser beam and laser system

Abstract

The invention relates to an optical device (1) for pulse compression of a pulsed laser beam (6), comprising a grating device (2) with at least one diffraction grating (3, 4). The optical device (1) comprises beam expansion means, in particular at least one beam expansion optical element (7), for forming a divergent pulsed laser beam (6) which enters the grating device (2) divergently and generally passes through the grating device (2) divergently. The invention also relates to a laser system with a laser source for generating a pulsed laser beam (6) and an optical device (1) for pulse compression designed as described above.

Description

Technical Field

[0001] This invention relates to an optical device for pulse compression of a pulsed laser beam, the optical device comprising a grating device having at least one diffraction grating. The invention also relates to a laser system having such an optical device. Background Technology

[0002] Optical devices with grating compressors containing one or more diffraction gratings are used for pulse compression, such as in chirped pulse amplification (CPA) systems. In a CPA system, the laser pulse of a pulsed laser beam is broadened in a stretcher, amplified in an amplifier, and compressed in a compressor. Grating compressors are commonly used as compressors in CPA systems. Due to the high pulse peak power in CPA systems, a large beam diameter pulsed laser beam is required in the compressor to prevent unwanted nonlinear effects (Kerr lensing) or to prevent damage to the diffraction grating of the grating compressor in the worst case. A large beam diameter requires a large diffraction grating, i.e., a diffraction grating with a large grating area, which leads to high production costs.

[0003] US 5,847,863 discloses a system for amplifying ultrashort optical pulses, particularly for chirped pulse amplification. In this system, an fiber stretcher is combined with a grating compressor. A telescope is positioned in the beam path of the collimated beam and is used to compensate for the phase mismatch between the fiber stretcher and the grating compressor. As an example, the grating compressor can be a Treacy-type grating compressor, having a first diffraction grating for beam spreading and a second diffraction grating for beam collimation. Summary of the Invention

[0004] Purpose of the invention

[0005] The present invention is based on the objective of providing an optical device for pulse compression and a laser system having such an optical device, both of which can be implemented with a compact structure even at high pulse peak power.

[0006] Invention Task

[0007] According to the invention, this objective is achieved by an optical device of the type proposed at the outset, having a beam-expanding device, particularly at least one beam-expanding optical element, for forming a diverging pulsed laser beam that, for pulse compression, diverges into and typically passes through the grating device. The laser beam typically maintains its diverging beam shape as it passes through the grating device; that is, the laser beam is typically neither collimated nor focused within the grating device.

[0008] According to the present invention, it is proposed to irradiate a pulsed laser beam not in a collimated manner but divergently (more precisely, at a divergence angle pre-given by a beam-expanding device) onto a (typically non-imaging) grating device. The beam-expanding device may have one or more beam-expanding optical elements, for example, in the form of transmissive optical elements (e.g., in the form of lenses) and / or in the form of reflective optical elements (e.g., in the form of (curved) mirrors).

[0009] When the laser beam enters the grating device, that is, when it illuminates the first diffraction grating in the beam path, the pulse duration of the pulsed laser beam is still long, and therefore the peak power or peak intensity is relatively low. As a result, with proper design of the grating device, nonlinear effects can be avoided even with a relatively small beam diameter, and the optical elements are not damaged. Therefore, the diverging laser beam typically has a small beam diameter when entering the grating device, and only a small grating area is required at the first diffraction grating.

[0010] When the laser beam propagates through the grating device, i.e., after diffraction at the first diffraction grating, the pulse duration decreases and the pulse peak power increases. However, when the diverging laser beam is sufficiently extended, the corresponding increase in peak intensity can be compensated by increasing the grating area onto which the laser beam illuminates, thus avoiding the aforementioned nonlinear effects and without damaging the optical unit. Under limited conditions, the required grating area for one or more diffraction gratings of the grating device can be reduced by 50% in this way. In this way, a cost-effective grating device with a compact structure can be realized.

[0011] In grating devices used for pulse compression, the laser beam typically diffracts four times at the diffraction grating structure (more precisely, along the diffraction plane or along multiple parallel diffraction planes). Spectral splitting and spectral merging of the spectral components of the pulsed laser beam occur in the corresponding diffraction planes. To produce quartet diffraction, the grating device can have four diffraction gratings, each passed through by the laser beam only once. In this case, from the first to the fourth diffraction grating in the beam path, the grating area increases in the direction perpendicular to the diffraction plane because the beam diameter of the diverging laser beam also increases as it propagates through the grating device. The larger beam diameter required for the larger grating area allows for the avoidance of nonlinear effects and prevents damage to the optical elements due to increased pulse peak power.

[0012] By using a laser beam that diverges into the grating device, the grating area, particularly the grating area of ​​the first three diffraction gratings, can be reduced in the direction perpendicular to the diffraction plane compared to a grating device where the laser beam enters in a collimated manner. The reduction in grating area is greatest for the first diffraction grating, and decreases further for the second and third diffraction gratings. At the fourth diffraction grating, the grating size perpendicular to the diffraction plane generally corresponds to the grating size of the grating device where the laser beam enters in a collimated manner.

[0013] Generally, the laser beam passes through at least one of the diffraction gratings in the grating device at least twice in order to reduce the number of diffraction gratings in the grating device. In this case, the laser beam illuminates the diffraction grating (more precisely, the diffraction grating structure) multiple times in different surface regions of the diffraction grating.

[0014] In one embodiment, the grating device has at least one deflection device for deflecting a laser beam after it has passed through at least one diffraction grating. This deflection device is configured to redirect the laser beam back to the at least one diffraction grating that has already been passed through. Preferably, the deflection device has at least two reflective surfaces for deflecting the laser beam. The deflection of the laser beam by means of the deflection device allows it to pass through the same diffraction grating multiple times.

[0015] As an example, the deflection device that redirects the laser beam back to (at least one) diffraction grating can be a prism, particularly a roof prism, or multiple prisms or a group of prisms. The deflection device can also be one or more mirrors, such as in the form of a roof mirror. In this case, the reflecting surfaces are generally planar, at which the pulsed laser beam is reflected by total internal reflection. Generally, at least two reflecting surfaces are required to reflect the laser beam back to the at least one diffraction grating.

[0016] In an extended embodiment, the deflection device is configured to generate beam deflection in at least one beam deflection direction. The laser beam deflected at the deflection device is typically parallel to the laser beam entering the deflection device and extends in the opposite direction to the laser beam entering the deflection device, and is deflected relative to the entering laser beam in at least one beam deflection direction by a predetermined beam deflection. This beam deflection enables the deflected laser beam to illuminate or pass through a different surface region of the diffraction grating compared to the first crossing of the diffraction grating.

[0017] In the case of a deflection device, or at least one of the deflection devices, the beam deflection of the laser beam generally extends in a beam deflection direction perpendicular to the diffraction plane, in which the laser beam is diffracted and recombined by the diffraction grating. However, at least one of the deflection devices can also produce a beam deflection of the laser beam extending in a plane parallel to the diffraction plane.

[0018] In a further extension, the deflection device is configured to generate beam deflection in two beam deflection directions and has at least three reflecting surfaces for deflecting the laser beam. The deflection device can have a single deflection element comprising (at least) three reflecting surfaces. This deflection element typically functions as a retroreflektor. The geometry of the reflecting surfaces of this deflection element does not necessarily need to be square, as in the case of a conventional cubic retroreflektor. However, the three reflecting surfaces can also be distributed among multiple deflection elements of the deflection device, for example, distributed among two or more prisms, which typically collectively also function as retroreflektors. With this deflection device, combined deflection of the laser beam can exist in beam deflection directions perpendicular to and additionally parallel to the diffraction plane. As an example, this deflection is advantageous if the grating device has only a single diffraction grating.

[0019] In the case of a grating compressor or a grating device, the laser beam is typically deflected in a first direction (e.g., vertically), with a first beam shift after an even-numbered diffraction, and deflected in a second direction perpendicular to the first direction (e.g., horizontally), with a second beam shift after an odd-numbered diffraction. For example, in a Treacy-type grating compressor, vertical deflection occurs after two diffractions or after two diffraction gratings. In the case of a grating compressor with only a single diffraction grating, horizontal deflection is generally implemented after the first and third diffractions, and vertical deflection is generally implemented after the second diffraction. However, in principle, other configurations are possible when the laser beam is deflected in a grating device.

[0020] In one extended embodiment, the grating device has a first diffraction grating and a second diffraction grating through which a laser beam passes sequentially. A deflection device is configured to redirect the laser beam back to the second diffraction grating (and also the first diffraction grating) by a beam deflection, preferably extending in a beam deflection direction perpendicular to the diffraction plane. In this case, the laser beam passes through the first and second diffraction gratings for the first time, and then, deflected at the deflection device, passes through the first and second diffraction gratings a second time in the opposite direction with a parallel deflection (and a larger beam cross-section). Typically, no imaging optics are arranged between the first and second diffraction gratings.

[0021] The deflection device is typically positioned at a relatively small distance from the second diffraction grating, such that the cross-sectional area of ​​the laser beam passing through the second diffraction grating for the first time and the cross-sectional area of ​​the laser beam passing through the second diffraction grating after being deflected at the deflection device have virtually the same size. Therefore, the surface areas on the second diffraction grating filled or required by the laser beam and the deflected laser beam have approximately the same size. Consequently, the beam deflection produced by the deflection device typically corresponds to approximately half the height of the second diffraction grating in the beam deflection direction. In the optical arrangement described herein, the two diffraction gratings, or more precisely, their diffraction grating structures, are generally oriented parallel to each other, but this is not necessarily required.

[0022] In a further extended embodiment, the first and second diffraction gratings are arranged to be laterally offset from each other along the beam offset direction, which extends perpendicular to the diffraction plane of the grating devices. For cases where no optical elements are arranged between the two diffraction gratings, the diverging laser beam or diverging caustics generally require that the first and second diffraction gratings be offset from each other in the beam offset direction to ensure that the center of the beam cross-section of the laser beam and the center of the beam cross-section of the deflected laser beam are respectively centered on the corresponding diffraction-determined surface regions of the two diffraction gratings. The extension of the corresponding diffraction-determined surface region on the corresponding diffraction grating in the beam offset direction increases with the increase of the extension of the beam cross-section in the beam offset direction.

[0023] In one extended embodiment, at least one additional deflection device is arranged between the first and second diffraction gratings. This additional deflection device generates a lateral deflection of the laser beam along a beam deflection direction that extends perpendicular to the diffraction plane of the grating device. Unlike the aforementioned deflection device, this additional deflection device does not redirect the laser beam back to the diffraction grating it has already passed through. The additional deflection device can generate only a lateral (parallel) deflection of the laser beam, such that the propagation direction of the laser beam is maintained after passing through it. This is, for example, when the additional deflection device is constructed as a prism or a planar parallel plate arranged at an angle relative to the propagation direction of the laser beam to generate beam deflection. However, the additional deflection device can also be constructed to not only laterally deflect the laser beam but also, for example, deflect the laser beam at a predetermined angle in the diffraction plane. In this case, the two diffraction gratings are typically not oriented parallel to each other but are also oriented at an angle to each other in the diffraction plane.

[0024] Additional deflection devices can be used to generate lateral offsets, allowing the center of the beam cross-section of the laser beam and the center of the beam cross-section of the deflected laser beam to irradiate the respective diffraction-oriented surface regions of the two diffraction gratings in the beam offset direction, without requiring the two diffraction gratings to be offset in the beam offset direction perpendicular to the diffraction plane for this purpose. In this embodiment, the two diffraction gratings can therefore be positioned at the same height in the beam offset direction. However, the lateral offset generated by at least one additional deflection device can also be combined with the lateral offset generated by the offset of the diffraction gratings in the beam offset direction.

[0025] In a further embodiment, the lateral offset between the two diffraction gratings in the beam offset direction, or the lateral offset generated by at least one additional deflection device (or optionally a combination of lateral offsets), is given by the following formula:

[0026] ΔH=1 / 4H G (H A -H E ) / (H A +H E ),

[0027] Among them, H G H represents the extension of the first diffraction grating in the beam shift direction. E This represents the extension of the laser beam cross-section in the beam offset direction when it first passes through the first diffraction grating, and H A This represents the extension of the beam cross-section in the beam deflection direction of the laser beam that has been deflected when it passes through the first diffraction grating for the second time.

[0028] If no deflection device or additional optical element for generating beam deflection is arranged between the two diffraction gratings, a lateral offset ΔH of the two diffraction gratings, as given above, is required to ensure that the center of the beam cross-section of the laser beam and the center of the beam cross-section of the deflected laser beam are centered on the diffraction-provided surface area of ​​the two diffraction gratings in the beam deflection direction. When the two diffraction gratings are arranged at the same height, the aforementioned lateral offset ΔH is generated by (at least one) additional deflection device to ensure that the two beam cross-sections are centered on the corresponding diffraction-provided surface area of ​​the diffraction gratings in the beam deflection direction. For this purpose, two additional deflection devices can be used: a first additional deflection device causes a lateral offset +ΔH in the laser beam propagating from the first diffraction grating to the second diffraction grating, and a second additional deflection device causes an equal but opposite lateral offset -ΔH in the deflected laser beam propagating from the second diffraction grating to the first diffraction grating. However, this can also be achieved using a single additional deflection device, for example constructed in the form of a planar parallel plate through which the laser beam passes at an angle to the plane normal. In principle, a first component of the lateral offset ΔH can be generated by means of this additional deflection device, and a second component of the lateral offset ΔH can be generated by the lateral offset of the two diffraction gratings relative to each other.

[0029] In an extended embodiment, the at least one additional deflection device is configured to generate a beam offset in the beam offset direction in addition to a lateral offset, the beam offset corresponding to the beam offset of the deflection device. In this case, for example, in addition to the lateral offset required to centrally arrange the corresponding center of the beam cross-section in the beam offset direction within the diffraction-defined area, an additional beam offset, corresponding in magnitude to the beam offset generated by the deflection device, can be generated by the additional deflection device. In this case, two additional deflection devices can be used, which deflect the laser beam and the laser beam deflected at the deflection device parallel to each other in the beam offset direction, each having the magnitude of the beam offset generated by the deflection device. However, for this purpose, a single additional deflection device, for example in the form of a prism, can be used, which, in addition to the beam offset or lateral offset in the beam offset direction, also deflects the laser beam perpendicular to the beam offset direction (i.e., within the diffraction plane).

[0030] In an alternative embodiment, the grating device has a single diffraction grating and a first deflection device for generating beam deflection in a first beam deflection direction and a second deflection device for generating beam deflection in a second beam deflection direction, preferably in the first beam deflection direction, wherein the first deflection device and the second deflection device are preferably arranged on opposite sides of the diffraction grating. In this embodiment, the laser beam passes through the single diffraction grating of the grating device four times. The two deflection devices allow the laser beam to irradiate the diffraction grating in four different surface regions. The second deflection device may have one or more deflection elements for generating beam deflection in both the first and second beam deflection directions.

[0031] In an extended embodiment, the second deflection device is preferably configured to generate a lateral deflection in the first beam deflection direction, in addition to generating a beam deflection corresponding to the magnitude of the beam deflection of the first deflection device in the first beam deflection direction. This lateral deflection is given by the following formula:

[0032] ΔH=1 / 4H G (H A -H E ) / (H A +H E ),

[0033] Among them, H G H represents the extension of the diffraction grating in the first beam offset direction. E This represents the extension of the laser beam cross-section in the first beam offset direction when it first passes through the diffraction grating, and H A This represents the extension of the beam cross-section of the laser beam deflected in the first beam deflection direction during its last passage through the diffraction grating.

[0034] Similar to the embodiment with two diffraction gratings described above, a lateral offset in the first beam offset direction is required to ensure that the beam cross-section is centered in the first beam offset direction and illuminates the diffraction-oriented surface region of the diffraction grating. To ensure this, for example, the edges (where the two reflecting surfaces of the first deflector are adjacent to each other) can be offset laterally by half (ΔH / 2) relative to the edges (where two of the three reflecting surfaces of the second deflector are adjacent to each other). The second deflector can be configured to generate beam offset in the first beam offset direction and additionally generate lateral offset in the first beam offset direction. However, the second deflector may also generate lateral offset only in the first beam offset direction without generating beam offset, as described above with respect to the additional deflector(s) with two diffraction gratings in the embodiment.

[0035] In a further embodiment, the extension of the beam cross-section of the laser beam emitted from the grating device in the direction perpendicular to the diffraction plane of the grating device is at least 1.5 times, preferably at least 1.7 times, and particularly preferably at least 2.0 times, the extension of the beam cross-section in the direction perpendicular to the diffraction plane of the laser beam entering the grating device. In the case of a circular beam cross-section, the extension of the beam cross-section in the beam offset direction corresponds to the diameter of the beam cross-section or the beam diameter of the laser beam.

[0036] At the first diffraction grating of the grating device in the beam path (more precisely, at its diffraction grating structure), the beam cross-section, or its extension in a direction perpendicular to the diffraction plane (which typically corresponds to the beam offset direction), is measured for the incoming laser beam. Correspondingly, at the last diffraction grating of the grating device in the beam path, the beam cross-section, or its extension in a direction perpendicular to the diffraction plane, is measured for the outgoing laser beam; this last diffraction grating may optionally correspond to the first diffraction grating (see above). Due to the divergence and causation of the laser beam, the beam cross-section increases continuously as it passes through the grating device. Although the divergence angle is relatively small (see below), if the diffraction gratings are relatively far apart (which may optionally be on the order of meters), the extension of the beam cross-section in the grating device generally increases significantly in a direction perpendicular to the diffraction plane, corresponding to the beam diameter in the case of a circular beam cross-section.

[0037] In a further embodiment, the beam-expanding optics are configured to generate a divergence angle for the laser beam entering the grating device, said divergence angle being between 0.5 mrad and 100 mrad. The selection of a suitable divergence angle for the laser beam entering the grating device depends on several parameters, such as the distance between the diffraction gratings. The divergence angle should not be chosen to be too large to prevent aberrations or phase errors from becoming too significant as they pass through the grating device, as this could lead to a degradation of the beam quality, especially if the optics or grating compressor operates near a nonlinear transition of diffraction conditions. Grating compressors or grating devices can typically be designed such that an acceptable reduction or degradation in beam quality is achieved without requiring additional measures to improve beam quality.

[0038] In a further embodiment, the optical device has at least one correction device, particularly a phase correction device, for at least partially compensating for the degradation of the beam quality of the laser beam, which can be attributed to the divergence of the laser beam as it enters the grating device.

[0039] As described above, in the case of a conventional grating compressor, the laser beam enters the grating device in a collimated manner. Compared to the beam quality of a laser beam entering the grating device in a collimated manner, a laser beam entering the grating device divergently typically leads to a deterioration in the beam quality of the laser beam after passing through the grating device. For the purposes of this application, the beam quality of the laser beam is understood as a K-factor, for which the following applies: K = 1 / M 2 , where M 2 This represents the beam quality factor. Correction devices are typically phase correction devices because phase correction of the laser beam allows for improvement of beam quality without light loss. However, in principle, other types of correction devices that correct phase errors in the spatial domain can also be used, such as those in the form of neutral density filters or apertures.

[0040] In one extended embodiment, the correction device is arranged in the beam path in front of or behind the grating device. Correction for at least partial compensation of beam quality degradation can be performed before or after passing through the grating device. Specifically, a first correction device can be arranged in the beam path of the laser beam in front of the grating device, and a second correction device can be arranged in the beam path of the laser beam behind the grating device. The correction device can also be arranged within the grating device. When the compensation device is arranged within the collimated beam path, it can, in principle, be positioned anywhere therein. When the correction device is arranged in the emitted beam path and configured for phase correction, it is advantageous, in principle, to arrange the correction device at the location where the phase error to be compensated is greatest. When the correction device corrects phase errors in the spatial domain and is configured for, for example, a neutral density filter, or in the form of an aperture, the correction device should be arranged at the location where the phase error is least.

[0041] In one extended embodiment, the phase correction device is constructed as a diffractive optical element. In principle, the phase correction device can also be implemented differently, for example, in the form of a delay plate with spatially dependent phase shifts or delays. However, the phase correction device in the form of a diffractive optical element can be particularly easily integrated into a grating device.

[0042] In one extended embodiment, the phase correction device is integrated into the diffraction grating of the grating device, that is, integrated into the diffraction structure (grating structure). In this case, the diffraction structure (grating structure) of the diffraction grating is designed such that the diffraction grating additionally generates phase correction to counteract the degradation of the laser beam's beam quality. In principle, the degradation of the laser beam's beam quality, which can be attributed to divergence when the laser beam enters the grating device and can be attributed to the corresponding diffraction grating, can be almost completely corrected by the phase correction device integrated into the diffraction grating. Using such a phase correction device, phase errors that can be attributed to previous or subsequent diffraction or diffraction gratings can be partially compensated. Therefore, two or more phase correction devices can also be integrated into two or more diffraction gratings. When the laser beam passes through the same diffraction grating at least twice in different surface regions, phase correction is appropriately matched in the corresponding surface regions. In particular, the diffractive optical element can be integrated into the first diffraction grating of the grating device in the beam path.

[0043] A compensation device is needed if the degradation of beam quality should be offset at a given divergence angle, if the beam quality should be improved when there is no optimal beam quality in front of the grating device, or if the beam cross-section of the laser beam emitted from the grating device must be increased at higher pulse peak power and the beam cross-section of the laser beam entering the grating device needs to be reduced accordingly so as not to increase the required grating area.

[0044] In particular, the compensation device can be configured to partially compensate for the degradation of the beam quality K in the diffraction direction or diffraction plane, so that the beam quality K does not decrease by more than 0.1 when passing through the grating device.

[0045] In a further embodiment, the optical device has a collimation device, particularly at least one collimating optical element, for collimating the laser beam after it has passed through the grating device. It has proven advantageous to integrate the grating device into a beam telescope formed by a beam expander and a collimation device. The beam telescope increases the beam cross-section of the collimated laser beam illuminating the beam expander, that is, the laser beam is expanded as it passes through the grating device. As an example, the collimation device may have one or more transmissive optical elements (e.g., in the form of lenses) and / or one or more reflective optical elements (e.g., in the form of (curved) mirrors).

[0046] In principle, one or more diffraction gratings of a grating device can be constructed in either a transmissive or reflective manner. In both cases, the required grating area can be significantly reduced overall by allowing the laser beam to diverge into the grating device.

[0047] A further aspect of the invention relates to a laser system having a laser source for generating a pulsed laser beam and optics configured as described above for pulse compression of the pulsed laser beam. As an example, the laser system may be an ultrashort pulse system comprising a laser source for generating spectrally broad laser pulses. As an example, the laser source may be a laser oscillator, but it may also be a combination of a laser oscillator and an amplifier. Such a laser source has an oscillator (e.g., a fiber optic oscillator) for generating laser pulses and an amplifier device for amplifying the laser pulses or pulsed laser beams, the amplifier device having one or more optical amplifiers. The laser source may have a pulse stretcher for broadening the pulse duration of the laser pulse. The pulse stretcher may be located in front of or within the amplifier device. As an example, the laser source may be configured to generate laser pulses having a spectral width of, for example, greater than 1 nm and a pulse energy of, for example, greater than 1 mJ. In the case of such a laser system, the aforementioned optics (more precisely, grating devices) can be used as a dispersion matching unit (also called a pulse compressor) for pulse duration compression.

[0048] Further advantages of the invention will become apparent from the description and drawings. Similarly, the features described above and those to be presented can be used independently or in any desired combination as a plurality. The embodiments shown and described are not to be construed as exhaustive, but rather have exemplary characteristics for summarizing the invention. Attached Figure Description

[0049] The attached diagram shows:

[0050] Figures 1a and 1b are schematic diagrams of optical devices for pulse compression of pulsed laser beams, respectively. These optical devices feature Treacy-type grating devices with two transmission or reflection diffraction gratings and deflection devices in the form of prisms.

[0051] Figure 2a , Figure 2b The schematic side view of the optical device shown in Figures 1a and 1b illustrates a pulse shape of a diverging pulsed laser beam generated by a beam spreading element and passing through a grating device with a transmission or reflection diffraction grating.

[0052] Figures 3a to 3c A schematic diagram showing the cross-section of a diverging laser beam on two diffraction gratings as it passes through a grating device is shown. A schematic diagram also shows the lateral offset of the two diffraction gratings or the parallel offset of the laser beam, which is laterally offset by two additional deflection devices.

[0053] Figure 4 A schematic diagram illustrating the correlation between the degradation of laser beam quality and the minimum beam diameter.

[0054] Figure 5 A schematic diagram of an optical device for pulse compression is shown, which has a single diffraction grating and two deflection devices, and

[0055] Figure 6 A schematic diagram of a laser system is shown, which has a laser source for generating a pulsed laser beam and optics for pulse compression of the pulsed laser beam.

[0056] In the following description of the accompanying drawings, the same reference numerals are used for the same or functionally identical parts. Detailed Implementation

[0057] Figure 1a and Figure 2a An optical device 1 is shown, which has a Treacy-type grating device 2, which has a first diffraction grating 3 operating in a transmission manner and a second diffraction grating 4 operating in a transmission manner, and also has a deflection device 5 in the form of a roof prism. The two diffraction gratings 3 and 4 are oriented parallel to each other and diffract a pulsed laser beam 6 passing through the grating device 2 along the YZ plane of the XYZ coordinate system, which is also referred to below as the diffraction plane. The laser beam 6 is spectrally expanded and spectrally combined in the diffraction plane YZ (or in a plane parallel to the diffraction plane YZ), as indicated by the dashed line in FIG1a. After passing through the first diffraction grating 3 and the second diffraction grating 4, the laser beam 6 passes through the deflection device 5 and is deflected (more precisely, reflected back by the deflection device), which produces a beam offset ΔX in the beam offset direction X of the XYZ coordinate system, perpendicular to the orientation of the diffraction plane YZ.

[0058] Figure 1a and Figure 2a Optical device 1 is used for pulse compression of laser beam 6, such as based on the pulse shape P when laser beam 6 enters grating device 2. E and the pulse shape P emitted from grating device 2 A It can be seen that... Figure 2a As can be seen, the laser beam 6 enters the grating device 2 divergently and maintains its divergent beam shape as it passes through the grating device 2 (i.e., as it passes through the first diffraction grating 3 and the second diffraction grating 4, and as it passes through the deflection device 5).

[0059] To generate a diverging pulsed laser beam 6, the optical device 1 has a beam expander, which, in the example shown, is configured as a first lens 7 arranged in the beam path in front of the grating device 2. A collimating device, in the form of a second lens 8, is arranged in the beam path behind the grating device 2. The first lens 7 and the second lens 8 form a beam telescope for the laser beam 6, which is generated by a laser source (not depicted here) and collimated onto the first lens 7. In the example shown, the first lens 7 and the second lens 8 are spherical lenses, but cylindrical lenses could also be used.

[0060] Figure 1b and Figure 2b The following optical device 1 is shown: In this optical device, the grating device 2 has two reflective diffraction gratings 3, 4 instead of two transmissive diffraction gratings 3, 4. (See Figure 1a and...) Figure 2a The optical device 1 shown in Figure 1b and Figure 2b The laser beam 6 in the optical device 1 shown illuminates the first diffraction grating 3 at an angle relative to the grating normal in the diffraction plane YZ. Figure 2b In the shown side view, the angle at which the laser beam 6 illuminates the first diffraction grating 3 and the angle at which the emitted laser beam 6 is reflected at the first diffraction grating 3 are not shown in the diffraction plane YZ, but are shown at an angle to the diffraction plane ZY, in order to improve the clarity of the representation of the laser beam 6 propagating between the two diffraction gratings 3 and 4. In other respects, Figure 1b and Figure 2b The structure of the optical device 1 shown corresponds to that in Figure 1a and Figure 2a The optical device 1 shown has transmission diffraction gratings 3 and 4.

[0061] For example, based on the planar diagram showing two diffraction gratings 3 and 4 Figures 3a to 3c It can be seen that the size of the beam cross-section of the laser beam 6 increases as it passes through the grating device 2, specifically, from the minimum extension H of the first beam cross-section S1a in the beam offset direction X when it first passes through the first diffraction grating 3. E By increasing the second beam cross-section S2a and the third beam cross-section S2b (of almost the same size) when passing through the second diffraction grating 4, the beam has the maximum extension H in the beam offset direction X when passing through the first diffraction grating 3 for the second time. A The fourth beam cross-section S1b. In Figure 3a and Figure 3b The corresponding beam cross sections S1a, S1b, S2a, and S2b are represented in a circular manner because, for clarity, the spectral sectorization and combination representation of the spectral components of the laser beam 6 in the diffraction plane YZ are omitted.

[0062] The following applies to the extension H of the beam cross section S1b of the laser beam 6 emitted from the grating device 2 in the beam offset direction X. A The extension H of the beam cross section S1a of the laser beam 6 entering the grating device 2 in the beam offset direction X E The ratio between them: H A / H E ≥1.5, preferably ≥1.7, especially ≥2.0. The increase in the extension of the beam cross-section of the laser beam 6 in the beam offset direction X when passing through the grating device 2 is advantageous for the laser drag of the optical device 1, because the pulse duration of the laser beam 6 pulses is shortened and the pulse peak power increases during propagation in the grating device 2.

[0063] As from Figures 3a to 3c As can be seen in the example shown, the first diffraction grating 3 and the second diffraction grating 4 have the same extension H in the beam offset direction X. G In the example shown, the beam offset ΔX generated by the deflection device 5 corresponds to the extension H of the second diffraction grating 4 in the beam offset direction X. G Half of it.

[0064] As from Figures 3a to 3c Similarly, it can be seen that, since the scattered beam of the laser beam 6 needs to be laterally offset by ΔH in the beam offset direction X, the center of the first beam cross-section S1a and the second beam cross-section S2a when passing through the first diffraction grating 3 and the second diffraction grating 4 for the first time, and the center of the third beam cross-section S2b and the fourth beam cross-section S1b when passing through the second diffraction grating 4 and the first diffraction grating 3 in the opposite direction, are centered in the surface region set for the corresponding diffraction relative to the beam offset direction X.

[0065] exist Figure 3a In the example shown, for the purpose of generating a lateral offset ΔH, the first diffraction grating 3 and the second diffraction grating 4 are offset relative to each other in the beam offset direction X. The following applies to lateral offset ΔH that enables the centering of the beam cross-sections S1a, S1b, S2a, and S2b:

[0066] ΔH=1 / 4H G (H A -H E ) / (H A +H E ),

[0067] Among them, H E This represents the extension of the (first) beam cross-section S1a of the laser beam 6 in the beam offset direction X when it first passes through the first diffraction grating 3, and H AThis indicates the extension of the (fourth) beam cross section S1b of the laser beam 6, which has been deflected when it passes through the first diffraction grating 3 for the second time, in the beam deflection direction X.

[0068] exist Figure 3b In the example shown, the two diffraction gratings 3 and 4 are arranged at the same height in the beam offset direction X. In this case, the lateral offset ΔH is generated by a deflection device 10 in the form of a planar parallel plate, which is arranged between the first diffraction grating 3 and the second diffraction grating 4 and is angularly tilted relative to the diffraction plane YZ or relative to the propagation direction of the laser beam 6, so as to generate the lateral offset ΔH. As the beam propagates from the first diffraction grating 3 to the second diffraction grating 4, the additional deflection device 10 generates a lateral offset +ΔH of the laser beam 6 with a positive sign in the example shown. Correspondingly, the additional deflection device 10 generates a lateral offset -ΔH of the laser beam 6 with the same magnitude but a negative sign.

[0069] exist Figure 3c In the example shown, a first additional deflection device 10a and a second additional deflection device 10b are arranged between the first diffraction grating 3 and the second diffraction grating 4. The two additional deflection devices 10a and 10b are in the form of prisms and are... Figure 3b The difference of the other deflection device 10 shown is that, in addition to the lateral offsets +ΔH and -ΔH, these other deflection devices also produce a beam offset in the beam offset direction X, which corresponds in magnitude to the beam offset ΔX of the deflection device 5, but has the opposite sign.

[0070] After passing through the first additional deflection device 10a, the laser beam 6 thus illuminates the second diffraction grating 4. The laser beam is not laterally deflected by a magnitude of ΔH from the center of the second beam cross-section S2a, but rather by a lateral deflection of ΔH+ΔX from the center of the second beam cross-section S2a. The deflected laser beam 6 is laterally deflected by a lateral deflection of -ΔH+ΔX at the second additional deflection device 10b. The laser beam 6 is deflected at the deflection device 5 (not in...) Figure 3c As shown in the diagram, the beam is offset parallel to the negative beam offset direction X by a beam offset of -ΔX (see the third beam cross-section S2b). It should be understood that... Figure 3c The additional deflection devices 10a and 10b shown do not necessarily need to produce lateral offsets +ΔH and -ΔH, but rather if the two diffraction gratings 3 and 4 are laterally offset by ΔH (e.g., Figure 3a If the beams of the deflection device 5 are offset from each other (as shown in the diagram), then the beam offset ΔX is sufficient.

[0071] In the example shown, the beam-expanding optics 7 are configured to generate a divergence angle α of the laser beam 6 as it enters the grating device 2, the divergence angle being between 0.5 mrad and 100 mrad. The divergence angle α should not be chosen to be too large, because a large divergence of the laser beam 6 would reduce the beam quality K (or its reciprocal 1 / M). 2 ) decrease, such as in Figure 4 (The beam quality factor M is shown) 2 The degradation can be seen from the correlation between the minimum beam diameter and the degradation. In the example shown, the beam-expanding optical element 7 is a spherical lens, but it could also be a cylindrical lens acting in a direction perpendicular to the diffraction plane YZ.

[0072] The following relationships apply to the minimum beam radius w0 (or minimum beam diameter 2w0), the half-divergence angle α / 2, and the beam quality factor M. 2 :

[0073] α / 2w0=M 2 λ / π

[0074] Where λ represents the wavelength of the laser beam 6.

[0075] In order to at least partially compensate for the beam mass K or 1 / M of the diverging laser beam 6 as it passes through the grating device 2 2 In the example shown, the optical device 1 has a first phase correction device 9a and a second phase correction device 9b. The first phase correction device 9a is arranged in the beam path in front of the grating device 2, more precisely, in the beam path in front of the beam-expanding optical element in the form of the first lens 7. The first phase correction device 9a is a diffractive optical element, but it can also be constructed, for example, as a delay plate or in any other way. The second phase correction device 9b is also formed as a diffractive optical element, which is integrated into the first diffraction grating 3. That is, the grating structure of the first diffraction grating 3 is modified in such a way that an additional phase correction exists when the laser beam 6 diffracts at the first diffraction grating 3, which counteracts the degradation of the beam quality K of the laser beam 6.

[0076] By using two phase correction devices 9a and 9b, the degradation of the beam quality K of the laser beam 6 in the diffraction plane YZ or in the diffraction direction Y can be partially compensated, so that the beam quality K will not decrease by more than 0.1 when passing through the grating device.

[0077] In principle, a single phase correction device is sufficient to perform phase correction in order to compensate for the degradation of the beam quality K of the laser beam 6 by the amount given above. As an example, it can be integrated into the first diffraction grating 3, such as... Figure 2a , Figure 2bAs shown in the diagram. The second diffraction grating 4 may also have a phase correction device or an additional phase correction device. Alternatively or additionally, the phase correction device or an additional phase correction device may be arranged in the beam path behind the grating device 2.

[0078] It should be understood that the phase correction device does not necessarily need to be arranged in the collimated beam path; instead, the phase correction device can also be arranged in the scattering beam path between the beam expander or optical unit 7 and the collimator 8 or optical unit, for example, in the beam path outside the grating device 2 (e.g., between the beam expander 7 and the first diffraction grating 3) or in the beam path between the first diffraction grating 3 and the collimator 8. The phase correction device or any other type of correction device used to compensate for the degradation of the beam quality K can also be arranged in the beam path between the two diffraction gratings 3, 4, or in the beam path between the second diffraction grating 4 and the deflection device 5.

[0079] It should be understood that optical device 1 does not necessarily need to have two diffraction gratings 3 and 4; instead, optical device may have more or fewer diffraction gratings that are passed through by laser beam 6 once or multiple times.

[0080] Figure 5 An example of an optical device 1 with a grating device 2 is shown, which has only a single diffraction grating 3. (This is in contrast to Figures 1a, 1b, or...) Figure 2a , Figure 2b In addition to the first deflection device 5, which is similar in design to the deflection device 5 shown, the grating device 2 has a second deflection device 11 configured to generate a first beam offset ΔX in a first beam offset direction X perpendicular to the diffraction plane YZ. The first beam offset corresponds to the magnitude of the beam offset ΔX of the first deflection device 5 in the first beam offset direction X, but with the opposite sign. The second deflection device 11 is also configured to generate additional lateral offsets +ΔH and -ΔH in the first beam offset direction X, as described in more detail below.

[0081] The second deflection device 11 is further configured to deflect the laser beam 6 in the second beam deflection direction Y by a second beam deflection ΔY, wherein the second beam deflection direction Y extends in or parallel to the diffraction plane YZ. To achieve beam deflections ΔX and ΔY in two mutually perpendicular beam deflection directions X and Y, the second deflection device 11, configured as a prism assembly, has three reflecting surfaces 11a-c.

[0082] The second deflection device 11 further generates lateral offsets +ΔH and -ΔH in the first beam offset direction X, so as to ensure that the surface area of ​​the diffraction grating 3 set for the corresponding diffraction is irradiated by the laser beam 6 in a centered manner.

[0083] Here, the second deflection device 11 takes over. Figure 3c The functions of the two additional deflection devices 10a and 10b are as follows:

[0084] After the first diffraction at the diffraction grating 3, the second deflection device 11 generates a beam offset of +ΔX and a lateral offset of +ΔH. The laser beam 6, aided by this beam offset and lateral offset, illuminates the diffraction grating 3 during the second diffraction. The first deflection device 5 generates a beam offset of –ΔX in the first beam offset direction X, causing the laser beam 6 to illuminate the diffraction grating 3 again during the third diffraction with a lateral offset of +ΔH relative to the illuminating laser beam 6. Upon passing through the second deflection device 11 for the second time, the second deflection device generates a beam offset of +ΔX and a negative lateral offset of –ΔH, causing the laser beam 6 to exit the grating device 2 after the fourth diffraction at the diffraction grating 3 with a beam offset of +ΔX relative to the illuminating laser beam 6.

[0085] To achieve the deflection described above, the edge between the first reflecting surface 11a and the second reflecting surface 11b of the second deflecting device 11 is positioned with an offset of -ΔH / 2 relative to the edge between the two reflecting surfaces 5a and 5b of the first deflecting device 5 in the first beam deflection direction X. However, it should be understood that this positioning of the edge is not necessarily required to allow the deflection described above.

[0086] The first deflection device 5 is positioned at a relatively short distance from the diffraction grating 3 to ensure that the beam cross-section of the laser beam 6 remains approximately the same between the second and third diffractions. In contrast, the second deflection device 11 is positioned at a relatively long distance from the diffraction grating 3.

[0087] and Figure 5 The difference shown is that the second deflection device 11 can be configured to generate lateral offsets +ΔH and -ΔH with the aforementioned values, but does not generate a beam offset ΔX in the first beam offset direction X. In this case, the second deflection device 11 generates a beam offset ΔY in the second beam offset direction Y, which corresponds to... Figure 5 The beam offset ΔY is shown. In this case, the laser beam 6 can also pass through the diffraction grating 3 four times in different surface regions of the diffraction grating.

[0088] Without omitting the third reflecting surface 11c Figure 5 The second deflection device 11 shown can replace Figure 3c The two additional deflection devices 10a and b shown, because viewed from the first beam deflection direction X, the second deflection device 11 performs the same function as the two additional deflection devices 10a and b. If Figure 5The two reflecting surfaces 11a and 11b shown serve as an additional deflection device 10, so that the two diffraction gratings 3 and 4 are not oriented parallel to each other, but are oriented at an angle relative to each other, the angle extending in the diffraction plane YZ. For example, the two reflecting surfaces 11a and 11b that can be formed on the additional deflection device in the form of a prism allow for beam folding of the laser beam 6 in the diffraction plane YZ, and thus allow for a particularly compact structure of the optics 1.

[0089] exist Figure 5 Similar to Figures 1a and 1b, dashed lines depict the fan-shaped distribution of the laser beam 6 in the diffraction plane YZ (more precisely, the edge of the fan-shaped beam). An additional deflection device 11 ensures that the laser beam 6 passes through the diffraction grating 3 in four different mutually offset planar regions, and diffracts four times in the process. Figure 5 Similar to Figures 1a and 1b, the representation of the beam expanding element 7 and collimating element 8 of the optical device 1 is omitted.

[0090] The aforementioned optical device 1 has a compact structure and can be used, for example, as a chirped pulse amplification laser system 20 (which will be combined below). Figure 6 (For a more detailed description) the compressor. However, it should be understood that the use of optical device 1 is not limited to chirped pulse amplification systems.

[0091] Figure 6 The laser system 20 shown is an ultrashort pulse system, comprising a laser pulse source 21 for generating a laser beam 6 with a wide spectral width, and the aforementioned optical device 1 (also called a pulse compressor) for dispersion matching (more precisely, for pulse duration compression). As an example, the laser pulse source 21 can be configured as a laser oscillator, or as... Figure 6 As shown, the configuration is a laser oscillator / amplifier combination.

[0092] exist Figure 6 In the example shown, the fiber optic oscillator 22 is integrated into the laser source 21, and a dispersion matching unit 23 (also called a pulse broadener) for generating pulse broadening is integrated into the fiber optic oscillator. The structure of this dispersion matching unit is similar to that of the pulse compressor described above. Alternatively, the dispersion matching unit 23 for pulse broadening can also be configured as a fiber Bragg grating (FBG). The laser source 21 also has an amplifier chain with n amplifiers 25a-n. Figure 6 In the example shown, there is also a module 24 for amplitude matching and / or phase matching arranged in front of amplifier chains 25a, ..., 25n, which can also be integrated into amplifier chains 25a, ..., 25n.

[0093] An optical modulator 26 for selecting or matching the amplitude of a laser pulse is arranged behind the amplifier chains 25a, ..., 25n and in front of the optics 1 that performs pulse compression. A free-beam optical unit 27 has at least one beam-expanding optical element 7, as described above, for generating a diverging input beam for the grating device 2. Figure 6 The laser system 20 shown has a spectral width P of, for example, greater than 1 nm. E A laser pulse with a pulse energy of, for example, 1 mJ or higher is provided as an input laser beam 6 to an optical device 1 that serves as a dispersion adjustment unit.

[0094] exist Figure 6 In the laser system 20 shown, beam path dispersion matching can be performed to allow for fine adjustment of pulse duration, as described in the applicant's DE 10 2016 110 947 A1. In this way, an intensity distribution of laser pulses with a desired pulse duration (e.g., with the shortest possible pulse duration or a pulse duration suitable for the processing method) can be provided.

Claims

1. An optical device (1) for pulse compression of a pulsed laser beam (6), the optical device comprising: A grating device (2) having a first diffraction grating (3) and a second diffraction grating (4), through which the pulsed laser beam (6) passes successively, and A beam spreading device (7) is provided for forming a diverging pulsed laser beam (6) that diverges into the grating device (2) and at least one deflection device (5) for deflecting the pulsed laser beam (6) after it has passed through the first diffraction grating (3) and the second diffraction grating (4), wherein the deflection device (5) is configured to redirect the pulsed laser beam (6) back to the first diffraction grating (3) and the second diffraction grating (4) that have already been passed through, wherein the deflection device (5) is configured to redirect the pulsed laser beam (6) back to the second diffraction grating by a beam offset (ΔX). The grating (4), wherein the beam offset extends in a beam offset direction (X) perpendicular to the diffraction plane (YZ) of the grating device (2), is characterized in that the first diffraction grating (3) and the second diffraction grating (4) are arranged to be offset relative to each other by a lateral offset (ΔH) along the beam offset direction (X), and / or at least one additional deflection device (10; 10a, 10b) is arranged between the first diffraction grating (3) and the second diffraction grating (4), the at least one additional deflection device generating a lateral offset (+ΔH, -ΔH) of the pulsed laser beam (6) along the beam offset direction (X).

2. The optical device (1) according to claim 1, wherein, The beam extender (7) is at least one beam extender optical element.

3. The optical device according to claim 1, wherein, The deflection device has at least two reflective surfaces (5a, b) for deflecting the pulsed laser beam (6).

4. The optical device according to any one of claims 1 to 3, wherein, The deflection device (5) is configured to generate beam deflection (ΔX; ΔX, ΔY) in at least one beam deflection direction (X, Y).

5. The optical device according to any one of claims 1 to 3, wherein, The lateral offset (ΔH) in the beam deflection direction (X) between the first diffraction grating (3) and the second diffraction grating (4), or the lateral offset (+ΔH, -ΔH) generated by the at least one other deflection device (10a, 10b), is given by the following: ΔH = 1 / 4 H G (H A - H E ) / (H A + H E ), Among them, H G H represents the extension of the first diffraction grating (3) in the beam shift direction (X). E H represents the extension of the beam cross section (S1a) of the pulsed laser beam (6) entering the first diffraction grating (3) in the beam offset direction (X). A This indicates the extension of the beam cross section (S1b) of the deflected pulsed laser beam (6) emitted from the first diffraction grating (3) in the beam deflection direction (X).

6. The optical device according to any one of claims 1 to 3, wherein, The at least one additional deflection device (10a, 10b) is configured to generate, in addition to the lateral deflection (+ΔH, -ΔH), the following beam offset (+ΔX) in the beam offset direction (X): the beam offset corresponds to the magnitude of the beam offset (ΔX) of the deflection device (5) in the beam offset direction (X).

7. The optical device according to any one of claims 1 to 3, wherein, The beam cross section (S1b) of the pulsed laser beam (6) emitted from the grating device (2) extends (H) in the direction (X) perpendicular to the diffraction plane (Y, Z). A ) is the extension (H) of the beam cross section (S1a) of the pulsed laser beam (6) entering the grating device (2) in the direction (X) perpendicular to the diffraction plane (Y, Z). E At least 1.5 times that of ).

8. The optical device according to claim 7, wherein, The beam cross section (S1b) of the pulsed laser beam (6) emitted from the grating device (2) extends (H) in the direction (X) perpendicular to the diffraction plane (Y, Z). A ) is the extension (H) of the beam cross section (S1a) of the pulsed laser beam (6) entering the grating device (2) in the direction (X) perpendicular to the diffraction plane (Y, Z). E At least 1.7 times that of ).

9. The optical device according to claim 8, wherein, The beam cross section (S1b) of the pulsed laser beam (6) emitted from the grating device (2) extends (H) in the direction (X) perpendicular to the diffraction plane (Y, Z). A ) is the extension (H) of the beam cross section (S1a) of the pulsed laser beam (6) entering the grating device (2) in the direction (X) perpendicular to the diffraction plane (Y, Z). E At least twice that of ).

10. The optical device according to any one of claims 1 to 3, wherein, The beam expansion device (7) is configured to generate a divergence angle (α) of the pulsed laser beam (6) when it enters the grating device (2), the divergence angle being between 0.5 mrad and 100 mrad.

11. The optical device according to any one of claims 1 to 3, further comprising: At least one correction device (9a, 9b) is provided for at least partially compensating for the degradation of beam quality (K) of the pulsed laser beam (6) as it passes through the grating device (2), the degradation of beam quality being attributed to the divergence of the pulsed laser beam (6) as it enters the grating device (2).

12. The optical device according to claim 11, wherein the at least one correction device (9a, 9b) is at least one phase correction device.

13. The optical device according to claim 11, wherein, The correction device (9a) is arranged in the beam path in front of the grating device (2) or in the beam path behind the grating device (2).

14. The optical device according to claim 12, wherein, The phase correction device is a diffractive optical element.

15. The optical device according to claim 14, wherein, The phase correction device is integrated into the diffraction grating of the grating device (2).

16. The optical device according to any one of claims 1 to 3, further comprising: A collimation device for collimating the pulsed laser beam (6) after passing through the grating device (2).

17. The optical device according to claim 16, wherein, The collimation device is at least one collimating optical element.

18. A laser system (20), comprising: Laser source (21), said laser source being used to generate pulsed laser beam (6), and Optical device (1) for pulse compression of the pulsed laser beam (6) according to any one of claims 1 to 17.

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