Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping
  • H01S3/0057—Temporal shaping, e.g. pulse compression, frequency chirping
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
  • H01S3/10053—Phase control
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping
  • H01S3/0071—Beam steering, e.g. whereby a mirror outside the cavity is present to change the beam direction
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping
  • H01S3/0085—Modulating the output, i.e. the laser beam is modulated outside the laser cavity
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/23—Arrangements of two or more lasers not provided for in groups H01S3/02 - H01S3/22, e.g. tandem arrangements of separate active media
  • H01S3/2308—Amplifier arrangements, e.g. MOPA
  • H01S3/2316—Cascaded amplifiers

Definitions

• the invention relates to an optical arrangement for pulse compression of a pulsed laser beam, comprising: a grating arrangement which has at least one diffraction grating.
• the invention also relates to a laser system with such an optical arrangement.
• Optical arrangements that have a grating compressor with one or more diffraction gratings (diffractive gratings) are used, for example, for pulse compression in chirped pulse amplification (CPA) systems.
• CPA chirped pulse amplification
• the laser pulses of the pulsed laser beam are stretched in a stretcher, amplified in an amplifier and compressed in a compressor.
• Lattice compressors are often used as compressors in a CPA system. Due to the high pulse peak power in CPA systems, a large beam diameter of the pulsed laser beam is required in the compressor to avoid unwanted non-linear effects (Kerr lens) or im to prevent the diffraction grating of the grating compressor from being destroyed in the worst case.
• the large beam diameter requires large diffraction gratings, ie diffraction gratings with a large grating area, which lead to high manufacturing costs.
• a system for amplifying ultra-short optical pulses has become known from US Pat. No. 5,847,863, which can be used in particular for chirped pulse amplification.
• a fiber stretcher is combined with a grating compressor.
• a telescope is placed in the beam path of the collimated beam to compensate for a phase mismatch between the fiber stretcher and the grating compressor.
• the grating compressor may be of the Treacy type, having a first grating for beam expansion and a second grating for beam collimation.
• the object of the invention is to provide an optical arrangement for pulse compression and a laser system with such an optical arrangement which can be implemented in a compact design even with high pulse peak powers.
• an optical arrangement of the type mentioned at the outset which has a beam-expanding device, in particular at least one beam-expanding optical element, for forming a divergent, pulsed laser beam, which divergently enters the grating arrangement for pulse compression and the grating arrangement typically passes divergently.
• the laser beam typically maintains its divergent beam shape as it traverses the grating assembly, i.e., the laser beam is typically neither collimated nor focused in the grating assembly.
• the pulsed laser beam is not collimated into the (typically non-imaging) grating arrangement, but divergent, specifically with a divergence angle predetermined by the beam-expanding device.
• the beam-expanding device can have one or more beam-expanding optical elements, for example in the form of transmitting optical elements, for example in the form of lenses, and/or in the form of reflecting optical elements, for example in the form of (curved) mirrors.
• the pulse duration of the pulsed laser beam is still long and the pulse peak power or peak intensity is comparatively low, so that if the grating arrangement is designed correctly, even with a comparatively small beam diameter, non-linear effects are avoided and no optics are damaged.
• the divergent laser beam therefore typically has a small beam diameter and requires only a small grating area on the first diffraction grating.
• the pulse duration is reduced and the pulse peak power increases.
• the corresponding increase in peak intensity can be compensated for with sufficient expansion of the divergent laser beam by increasing the grating area on which the laser beam impinges, so that the non-linear effects described above are avoided and no optics are damaged.
• the required grating area of the diffraction grating(s) of the grating arrangement can be reduced by 50% in this way. In this way, a cost-effective lattice arrangement can be implemented in a compact design.
• the laser beam is typically diffracted four times at a diffractive grating structure, namely along a diffraction plane or along several parallel diffraction planes.
• a spectral splitting and a spectral combining of the spectral components of the pulsed laser beam takes place in a respective diffraction plane.
• the grating arrangement can have four diffraction gratings, through which the laser beam passes only once.
• the grating area of the diffraction grating increases from the first diffraction grating in the beam path to the fourth diffraction grating in the beam path in a direction that runs perpendicular to the diffraction plane, since the beam diameter of the divergent laser beam also increases during propagation through the grating arrangement. Due to the larger beam diameter, which requires a larger grating area, non-linear effects can be avoided or the destruction of the optics by the increasing pulse peak power can be prevented.
• the particular grating area of the first three diffraction gratings can be reduced in a direction perpendicular to the diffraction plane compared to a grating arrangement into which the laser beam enters collimated.
• the reduction in grating area is greatest for the first diffraction grating and decreases for the second and third diffraction gratings.
• the grating size perpendicular to the diffraction plane generally corresponds to the grating size of a grating arrangement with a laser beam entering the grating arrangement in collimated form.
• the laser beam passes through at least one of the diffraction gratings of the grating arrangement at least twice in order to reduce the number of diffraction gratings of the grating arrangement.
• the laser beam hits the diffraction grating, more precisely the diffractive grating structure of the diffraction grating, several times in different surface areas.
• the grating arrangement has at least one deflection device for deflecting the laser beam after it has passed through at least one diffraction grating, the deflection device being designed to deflect the laser beam back to the at least one diffraction grating that has already passed through.
• the deflection device preferably has at least two reflection surfaces for deflecting the laser beam. The deflection of the laser beam with the help of the deflection device makes it possible to run through one and the same diffraction grating several times.
• the deflection device which deflects the laser beam back to the (at least one) diffraction grating, can be, for example, a prism, in particular a roof prism, or several prisms or prism groups.
• the deflection device can also be one or more mirrors, for example in the form of roof mirrors.
• the reflection surfaces are generally flat surfaces on which the pulsed laser beam is reflected by total reflection. At least two reflection surfaces are generally required for the reflection of the laser beam back to the at least one diffraction grating.
• the deflection device is designed to generate a beam offset in at least one beam offset direction.
• the laser beam deflected at the deflection device typically runs parallel and in the opposite direction to the laser beam entering the deflection device and is offset in at least one beam offset direction by a predetermined beam offset to the incoming laser beam.
• the beam offset makes it possible for the deflected laser beam to hit or pass through the diffraction grating that has already passed through in a different surface area than was the case when it first passed through the diffraction grating.
• the beam offset of the laser beam usually runs in a beam offset direction that runs perpendicular to the diffraction plane in which the laser beam is expanded by the diffraction gratings and brought together again by diffraction.
• at least one of the deflection devices it is also possible for at least one of the deflection devices to produce a beam offset of the laser beam that runs in a plane parallel to the diffraction plane.
• the deflection device is designed to generate a beam offset in two beam offset directions and has at least three reflection surfaces for deflecting the laser beam.
• the deflection device can have a single deflection element which comprises the (at least) three reflection surfaces.
• Such a deflection element typically fulfills the function of a retroreflector.
• the geometry of the reflection surfaces of such a deflection element is not necessarily square, like this is the case with a conventional cube-shaped retroreflector.
• a combined deflection of the laser beam can take place in a beam offset direction perpendicular to the diffraction plane and additionally parallel to the diffraction plane.
• Such a deflection is advantageous, for example, if the grating arrangement has only a single diffraction grating.
• the laser beam is deflected in a first direction (e.g. vertically) with a first beam offset after an even number of previous diffractions and in a second direction after an odd number of previous diffractions. to the first perpendicular direction (e.g. horizontal) with a second beam offset.
• a vertical deflection takes place after two diffractions or after two diffraction gratings.
• a horizontal deflection usually takes place after the first and third diffraction and a vertical deflection after the second diffraction.
• a horizontal deflection usually takes place after the first and third diffraction and a vertical deflection after the second diffraction.
• other configurations are also possible for the deflection of the laser beam in the grating arrangement.
• the grating arrangement has a first and a second diffraction grating through which the laser beam passes in succession, and the deflection device is designed to direct the laser beam with a beam offset that preferably runs in a beam offset direction that is aligned perpendicular to a diffraction plane , to return to the second diffraction grating (and also to the first diffraction grating).
• the laser beam passes through the first and second diffraction gratings, between which there are typically no imaging optical elements, for the first time and then (with a larger beam cross section) a second time - offset in parallel - in the opposite direction from that at the deflection device pass through the deflected laser beam.
• the deflection device is typically arranged at a comparatively small distance from the second diffraction grating, so that the beam cross section of the laser beam when it first passes through the second diffraction grating and when it passes through the second diffraction grating for the second time after deflection at the deflection device are practically the same size.
• the surface areas on the second diffraction grating that are filled or required by the laser beam or by the deflected laser beam are therefore approximately the same size.
• the beam offset that is generated by the deflection device therefore typically corresponds to approximately half the height of the second diffraction grating in the beam offset direction.
• the two diffraction gratings, more precisely their diffractive grating structures, are generally aligned in parallel in the optical device described here, but this is not absolutely necessary.
• the first diffraction grating and the second diffraction grating are offset from one another by a lateral offset along a beam offset direction that runs perpendicularly to a diffraction plane of the grating arrangement.
• At least one further deflection device is arranged between the first diffraction grating and the second diffraction grating, which generates a lateral offset of the laser beam along a beam offset direction that runs perpendicular to a diffraction plane of the grating arrangement.
• the further deflection device does not direct the laser beam to the one that has already passed through it diffraction grating back.
• the additional deflection device can only generate a lateral (parallel) offset of the laser beam, which retains its propagation direction after passing through the additional deflection device.
• the further deflection device is designed as a prism or as a plane-parallel plate which is arranged inclined at an angle in relation to the direction of propagation of the laser beam in order to generate the beam offset.
• the further deflection device it is also possible for the further deflection device to be designed not only to displace the laser beam laterally, but also to deflect it at a predetermined angle, for example in the diffraction plane.
• the two diffraction gratings are typically not aligned in parallel, but are also aligned at an angle to one another in the diffraction plane.
• the additional deflection device can be used to generate a lateral offset that enables 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 hit the surface area of the two diffraction gratings intended for the respective diffraction in the beam offset direction in the middle, without
• the two diffraction gratings must be offset in the beam offset direction perpendicular to the diffraction plane.
• the two diffraction gratings can thus be positioned at the same height in the beam offset direction.
• a lateral offset between the two diffraction gratings in the beam offset direction or a lateral offset generated by the at least one further deflection device is given by:
• AH 1/4 HG (HA - HE) / (HA + HE), where HG is the extent of the first diffraction grating in the beam offset direction, HE is the extent of the beam cross section of the laser beam in the beam offset direction when passing through the first diffraction grating for the first time and HA denote the extension of the beam cross section of the deflected laser beam in the beam displacement direction when passing through the first diffraction grating for the second time.
• the lateral offset AH of the two diffraction gratings specified 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 meet the two diffraction gratings in the beam offset direction in the middle of a surface area intended for diffraction, if there is no deflection device or no other optical elements are arranged between the two diffraction gratings that produce a beam offset.
• the above-mentioned lateral offset AH is generated by the (at least one) additional deflection device in order to achieve that the two beam cross-sections in the beam offset direction are centered on a respective surface area provided for the diffraction of the diffraction grating meet.
• a first additional deflection device generates a lateral offset +AH of the laser beam, which propagates from the first diffraction grating to the second diffraction grating
• a second additional deflection device generates an opposite lateral offset -AH of the deflected beam, which is the same amount Laser beam propagating from the second diffraction grating to the first diffraction grating.
• a single further deflection device for this purpose, which is designed, for example, in the form of a plane-parallel plate through which the laser beam passes at an angle to the surface normal.
• the at least one further deflection device is designed to generate, in addition to the lateral offset, a beam offset in the direction of the beam offset, which beam offset corresponds to the beam offset of the deflection device.
• a beam offset in the direction of the beam offset which beam offset corresponds to the beam offset of the deflection device.
• an additional beam displacement can be generated by the further deflection device, the amount of which corresponds to the beam displacement generated by the deflection device.
• two further deflection devices can be used, which offset the laser beam and the laser beam deflected at the deflection device in parallel with the amount of beam displacement generated by the deflection device in the beam displacement direction.
• a single further deflection device for this purpose, for example in the form of a prism or the like, which deflects the laser beam in addition to the beam offset or the lateral offset in the beam offset direction also perpendicular to the beam offset direction, ie within the diffraction plane.
• the grating arrangement has a single diffraction grating and a first deflection device for generating a beam offset in a first beam offset direction and a second deflection device for generating a beam offset in the second beam offset direction and preferably in the first beam offset direction, with the first and second deflection device are preferably arranged on opposite sides of the diffraction grating.
• the laser beam passes through the single diffraction grating of the grating arrangement four times.
• the two deflection devices allow the laser beam to hit the diffraction grating at four different surface areas.
• the second deflection device can have one or more deflection elements in order to generate the beam offset in the first beam offset direction and in the second beam offset direction.
• the second deflection device is designed to generate a lateral offset in the first beam offset direction, preferably in addition to a beam offset that corresponds to the amount of the beam offset of the first deflection device in the first beam offset direction, which is given by
• AH 1/ 4 HG (HA - HE) / (HA + HE), where HG is the extent of the diffraction grating in the first beam displacement direction, HE the extension of the beam cross section of the laser beam in the first beam offset direction when first passing through the diffraction grating and HA the extension of the beam cross section of the deflected laser beam in the first beam offset direction when last passing through the diffraction grating.
• the lateral offset in the first beam offset direction is required analogously to the embodiment described above with the two diffraction gratings in order to achieve that the beam cross section in the first beam offset direction impinges centrally on a respective surface area of the diffraction grating provided for the diffraction.
• an edge at which the two reflection surfaces of the first deflection device adjoin one another can be offset by half the lateral offset (AH/2) to an edge at which two of the three reflection surfaces of the second deflection device adjoin one another.
• the second deflection device can be designed to generate a beam offset in the first beam offset direction and additionally a lateral offset in the first beam offset direction.
• the second deflection device it is also possible for the second deflection device to generate only a lateral offset, but no beam offset in the first beam offset direction, as described above in relation to the further deflection device(s) in the embodiment with the two diffraction gratings.
• the steel cross-section of the laser beam emerging from the grating arrangement extends in a direction perpendicular to a diffraction plane of the grating arrangement by at least a factor of 1.5, preferably at least by a factor of 1.7, particularly preferably by at least one A factor of 2.0 larger than an extension of the beam cross section of the laser beam entering the grating arrangement in the direction perpendicular to the diffraction plane.
• the extension of the beam cross section in the beam displacement direction corresponds to the diameter of the beam cross section or the beam diameter of the laser beam.
• the beam cross-section or its extent in the direction perpendicular to the diffraction plane which typically corresponds to the or a beam offset direction, is the first in the beam path for the incoming laser beam Diffraction grating of the grating arrangement, more precisely measured at its diffractive grating structure. Accordingly, the beam cross-section or its extension in the direction perpendicular to the diffraction plane for the emerging laser beam is measured at the last diffraction grating of the grating arrangement in the beam path, which can possibly match the first diffraction grating (see above). Due to the divergent caustics of the laser beam, the beam cross section of the laser beam increases continuously as it passes through the grating arrangement.
• the extension of the beam cross section in the direction perpendicular to the diffraction plane which corresponds to the beam diameter in the case of a circular beam cross section, generally increases significantly in the grating arrangement if the diffraction gratings are at a comparatively large distance from one another , which may be in the order of meters.
• the beam-expanding optical element is designed to generate a divergence angle of the laser beam entering the grating arrangement that is between 0.5 mrad and 100 mrad.
• the choice of a suitable divergence angle, with which the laser beam enters the grating arrangement depends on several parameters, for example the distance between the diffraction gratings.
• the divergence angle should not be too large in order to avoid that the aberrations or phase errors when passing through the grating arrangement become too large, as this can lead to a deterioration in the beam quality of the laser beam, especially if the optical arrangement or the Grating compressor is operated near the transition to non-linearity of the diffraction condition.
• the grating compressor or the grating arrangement can often be designed in such a way that an acceptable decrease or deterioration in the beam quality takes place without additional measures having to be taken to improve the beam quality.
• the optical arrangement has at least one correction device, in particular a phase correction device, for at least partially compensating for a deterioration in the beam quality of the laser beam due to the divergence of the laser beam when it enters the grating arrangement.
• a phase correction device for at least partially compensating for a deterioration in the beam quality of the laser beam due to the divergence of the laser beam when it enters the grating arrangement.
• the correction device is typically a phase correction device, since the correction of the phase of the laser beam enables the beam quality of the laser beam to be improved without light losses. In principle, however, it is also possible to use other types of correction devices that correct the phase error in the spatial domain, for example in the form of gray filters, for example in the form of diaphragms, or the like.
• the correction device is arranged in the beam path before the grating arrangement or in the beam path after the grating arrangement.
• the correction for at least partially compensating for the deterioration in the beam quality can take place before or after passing through the grating arrangement.
• a first correction device to be arranged in the beam path of the laser beam before the grating arrangement and for a second correction device to be arranged in the beam path of the laser beam after the grating arrangement.
• the correction device can also be arranged within the grid arrangement. In the event that the compensation device is arranged in the collimated beam path, its position is fundamentally arbitrary.
• the correction device is arranged in the divergent beam path and is designed as a phase correction device, it is basically favorable if it is arranged at a position at which the phase error to be compensated is at a maximum.
• the correction device corrects the phase error in the spatial domain and is designed, for example, as a gray filter, for example in the form of an aperture, the correction device should be arranged at a position at which the phase error is minimal.
• the phase correction device is designed as a diffractive optical device element formed.
• the phase correction device can also be designed in a different way, for example in the form of a delay plate with a phase shift or delay that varies depending on the location.
• a phase correction device in the form of a diffractive optical element can be integrated particularly easily into the grating arrangement.
• the phase correction device is integrated into a diffraction grating of the grating arrangement, i.e. into the diffracting structure (grating structure).
• the diffractive structure (grating structure) of the diffraction grating is designed in such a way that it also generates a phase correction in order to counteract a deterioration in the beam quality of the laser beam.
• the deterioration of the beam quality of the laser beam which is due to the divergence when the laser beam enters the grating arrangement and which is due to a respective diffraction grating, can be almost completely corrected by a phase correction device that is integrated in this diffraction grating.
• phase errors that can be attributed to previous or subsequent diffractions or diffraction gratings can be partially compensated for with such a phase correction device. It is therefore also possible for two or more phase correction devices to be integrated into two or more diffraction gratings. If the laser beam passes through one and the same diffraction grating at least twice in different surface areas, the phase correction in the respective surface area is suitably adjusted.
• the diffractive optical element can be integrated into the first diffraction grating of the grating arrangement in the beam path.
• the compensation device is required if a deterioration in beam quality is to be counteracted at a given divergence angle, the beam quality is to be increased if it was not optimal before the grating arrangement, or if the beam cross-section of the exiting from the grating arrangement is higher at higher pulse peak powers Laser beam has to be enlarged and the beam cross section of the laser beam entering the grating arrangement has to be reduced accordingly in order not to increase the required grating area.
• the compensation device can in particular be designed to partially compensate for the deterioration of the beam quality K in the diffraction direction or in the diffraction plane, so that the beam quality K does not decrease by more than 0.1 when passing through the grating arrangement.
• the optical arrangement has a collimating device, in particular at least one collimating optical element, for collimating the laser beam after it has passed through the grating arrangement.
• a collimating device in particular at least one collimating optical element, for collimating the laser beam after it has passed through the grating arrangement.
• the beam telescope increases the beam cross-section of the collimated laser beam that hits the beam-expanding device, i.e. the laser beam is expanded as it passes through the grating arrangement.
• the collimating device may, for example, have one or more transmitting optical elements, e.g. in the form of lenses, and/or one or more reflecting optical elements, e.g. in the form of (curved) mirrors.
• the diffraction grating(s) of the grating arrangement can in principle be designed to be transmissive or reflective. In both cases, the required grating area can usually be significantly reduced by the laser beam entering the grating arrangement divergently.
• a further aspect of the invention relates to a laser system which has a laser source for generating a pulsed laser beam and an optical arrangement for pulse compression of the pulsed laser beam, which is designed as described above.
• the laser system can be, for example, an ultra-short pulse system that includes a laser source for generating spectrally wide laser pulses.
• the laser source can be a laser oscillator, for example, but the laser source can also be designed as a laser oscillator-amplifier combination.
• Such a laser source has an oscillator, for example a fiber oscillator, for generating laser pulses and an amplifier arrangement for amplifying the laser pulses or the pulsed laser beam, which has one or more optical amplifiers.
• the laser source can have a pulse stretcher for stretching the pulse durations of the laser pulses.
• the pulse stretcher can be positioned in front of or inside the amplifier arrangement.
• the laser source can be designed, for example, to generate laser pulses with spectral widths of, for example, 1 nm or more and pulse energies of, for example, 1 mJ or more.
• the optical arrangement described above, more precisely the grating arrangement can serve as a dispersion adjustment unit for pulse duration compression (also called a pulse compressor) in such a laser system.
• FIG. 1 a,b schematic representations of an optical arrangement for pulse compression of a pulsed laser beam, which has a Treacy-type grating arrangement with two transmitting or reflecting diffraction gratings and a deflection device in the form of a prism, each in a plan view,
• FIGS. 1a, b schematic side views of the optical arrangements of FIGS. 1a, b with a pulse shape of a divergent, pulsed laser beam, which is generated by a beam-expanding element and passes through the grating arrangement with the transmitting or with the reflective diffraction gratings,
• FIG. 5 shows a schematic representation of an optical arrangement for pulse compression, which has a single diffraction grating and two deflection devices, and
• FIG. 6 shows a schematic representation of a laser system which has a laser source for generating a pulsed laser beam and an optical arrangement for pulse compression of the pulsed laser beam.
• 1a and 2a show an optical arrangement 1, which has a Treacy-type grating arrangement 2 with a first diffraction grating 3 operated in transmission and a second diffraction grating 4 operated in transmission, as well as a deflection device 5 in the form of a roof prism.
• the two diffraction gratings 3, 4 are aligned parallel to one another and diffract a pulsed laser beam 6, which passes through the grating arrangement 2, along a YZ plane of an XYZ coordinate system, which is also referred to below as the diffraction plane.
• the laser beam 6 is spectrally expanded and spectrally combined, as indicated by dashed lines in FIG. 1a.
• the laser beam 6 After passing through the first and second diffraction gratings 3, 4, the laser beam 6 passes through the deflection device 5 and is deflected by it, more precisely retroreflected, with the deflection device 5 generating a beam offset AX in a beam offset direction X of the XYZ coordinate system, which is perpendicular to the diffraction plane YZ is aligned.
• the laser beam 6 enters the grating arrangement 2 divergently and, when passing through the grating arrangement 2, i.e. when passing through the first and the second diffraction grating 3, 4 and when passing through the deflection device 5 its divergent jet shape.
• the optical arrangement 1 has a beam-expanding device, which in the example shown is in the form of a first lens 7, which is arranged in the beam path in front of the grating arrangement 2.
• a collimating device in the form of a second lens 8 is arranged in the beam path after the grating arrangement 2 .
• the first and the second lens 7, 8 form a beam telescope for the laser beam 6, which is generated by a laser source, not shown, and collimated onto the first lens 7 impinges.
• the first and second lenses 7, 8 are spherical lenses, but it is also possible to use cylindrical lenses.
• FIG. 1b and FIG. 2b show an optical arrangement 1 in which the grating arrangement 2 has two reflecting diffraction gratings 3, 4 instead of two transmitting diffraction gratings 3, 4.
• FIG. 1b and in Fig. 2b the laser beam 6 hits the first diffraction grating 3 at an angle to the grating normal in the diffraction plane YZ, as in the optical arrangement 1 shown in Fig. 1a and in Fig. 2a on.
• the angle at which the laser beam 6 strikes the first diffraction grating 3 and the angle at which the exiting laser beam 6 is reflected at the first diffraction grating 3 are not in the diffraction plane YZ, but under one Shown aligned angle to the diffraction plane ZY in order to increase the clarity of the representation of the between the two diffraction gratings 3, 4 propagating laser beam 6.
• the construction of the optical arrangement 1 shown in Fig. 1b and in Fig. 2b corresponds to the optical arrangement 1 shown in Fig. 1a and in Fig. 2a with the transmitting diffraction gratings 3, 4.
• the size of the beam cross section of the laser beam 6 increases as it passes through the grating arrangement 2, specifically from a minimum extension HE of one first beam cross section S1a in the beam offset direction X during the first pass through the first diffraction grating 3 via a - practically equally large - second and third beam cross section S2a, S2b when passing through the second diffraction grating 4 to a fourth beam cross section S1b with a maximum extent HA in the beam offset direction X on the second Passing through the first diffraction grating s.
• the respective beam cross sections S1a, S1b, S2a, S2b are shown in a circle in Fig. 3a, b, because the representation of the spectral fanning out and merging of the spectral components of the laser beam 6 in the diffraction plane YZ for reasons of clarity was waived.
• the ratio between the extension HA of the beam cross section S1b of the laser beam 6 emerging from the grating arrangement 2 in the beam offset direction X to the extension HE of the beam cross section S1a of the laser beam 6 entering the grating arrangement 2 in the beam offset direction X applies: HA / HE > 1, 5, preferably >1.7, in particular >2.0.
• the enlargement of the extent of the beam cross section of the laser beam 6 in the beam offset direction X when passing through the grating arrangement 2 is favorable for the laser resistance of the optical arrangement 1, since the pulse durations of the pulses of the laser beam 6 are shortened during propagation in the grating arrangement 2 and this the pulse peak power is increased.
• the first diffraction grating 3 and the second diffraction grating 4 have the same extent HG in the beam offset direction X in the example shown.
• the beam offset AX which is generated by the deflection device 5, corresponds to half the extension HG of the second diffraction grating 4 in the beam offset direction X.
• a lateral offset AH in the beam offset direction X is required to move the center of the first and second beam cross section S1a, S2a when passing through the first and second diffraction grating 3 , 4 and the center of the third and fourth beam cross section S1b, S2b when passing through the first and second diffraction grating 3, 4 in the opposite direction with respect to the beam offset direction X to be positioned centrally in a surface area provided for the respective diffraction.
• the first diffraction grating 3 and the second diffraction grating 4 are offset relative to one another in the beam offset direction X in order to generate the lateral offset AH.
• AH 1/4 HG (HA - HE) / (HA + HE), where HE is the extension of the (first) beam cross section S1a of the laser beam 6 in the beam offset direction X when passing through the first diffraction grating 3 for the first time and HA is the extension of the (fourth) beam cross section S1 b of the deflected laser beam 6 in the beam displacement direction X when passing through the first diffraction grating 3 for the second time.
• the two diffraction gratings 3, 4 are arranged at the same height in the beam offset direction X.
• the lateral offset AH is generated by a deflection device 10 in the form of a plane-parallel plate, which is arranged between the first diffraction grating 3 and the second diffraction grating 4 and is inclined at an angle to the diffraction plane YZ or to the propagation direction of the laser beam 6 to generate the lateral offset AH.
• the further deflection device 10 generates a lateral offset +AH of the laser beam 6 with a positive sign in the example shown.
• the additional deflection device 10 produces a lateral offset ⁇ AH of the deflected laser beam 6 of the same magnitude with a negative sign.
• a first and second further deflection device 10a, 10b are arranged between the first diffraction grating s and the second diffraction grating 4.
• the two other deflection devices 10a, 10b are designed as prisms and differ from those in FIG. 3b further deflection devices 10 shown in that in addition to the lateral offset +AH, -AH they produce a beam offset in the beam offset direction X, which corresponds to the amount of the beam offset AX of the deflection device 5, but has the opposite sign.
• the laser beam 6 After passing through the first further deflection device 10a, the laser beam 6 therefore does not strike the second diffraction grating 4 with a lateral offset of the amount AH, but with a lateral offset of AH + AX with the center of the second beam cross section S2a of the deflection device 5, not shown in Fig. 3c, in the negative beam offset direction X by a beam offset -AX parallel offset (cf. the third beam cross section S2b), before the deflected laser beam 6 at the second further deflection device 10b with a lateral offset of -AH + AX laterally is transferred.
• the further deflection devices 10a, 10b shown in FIG lateral offset AH are offset from each other, as shown in Fig. 3a.
• the beam-widening optical element 7 is designed to generate a divergence angle a of the laser beam 6 when it enters the grating arrangement 2, which angle is between 0.5 mrad and 100 mrad.
• the divergence angle a should not be chosen too large, since too large a divergence of the laser beam 6 leads to a reduction in the beam quality K (or its return 1/M 2 ), as can be seen from FIG Deterioration of the diffraction index M 2 shows as a function of the minimum beam diameter.
• the beam-expanding optical element 7 is a spherical lens, but it can also be a cylindrical lens that acts in a direction perpendicular to the diffraction plane YZ.
• a / 2 wo M 2 X / TT, where X denotes the wavelength of the laser beam 6.
• the optical arrangement 1 in the example shown has a first and second phase correction device 9a, 9b.
• the first phase correction device 9a is arranged in the beam path in front of the grating arrangement 2, more precisely in the beam path in front of the beam-widening 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 designed, for example, as a delay plate or in some other way.
• the second phase correction device 9b also forms a diffractive optical element that is integrated into the first diffraction grating 3, ie the grating structure of the first diffraction grating 3 is modified in such a way that when the laser beam 6 is diffracted at the first diffraction grating 3, a phase correction is also carried out , which counteracts a deterioration in the beam quality K of the laser beam 6 .
• the deterioration 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 for by the two phase correction devices 9a, 9b, so that the beam quality K when passing through the grating arrangement is no more than 0.1 decreases.
• a single phase correction device can be sufficient for the phase correction to compensate for the deterioration in the beam quality K of the laser beam 6 by the amount specified above.
• This can be integrated into the first diffraction grating 3, for example, as shown in FIGS. 2a, b.
• the second diffraction grating 4 it is also possible for the second diffraction grating 4 to have the or a further phase correction device.
• the or a further phase correction device can be arranged in the beam path after the grating arrangement 2 .
• the phase correction device does not necessarily have to be arranged in the collimated beam path, but can also be arranged in the divergent beam path between the beam-expanding device or optics 7 and the collimating device 8 or optics, for example in the beam path outside the grating Arrangement 2, e.g. between the beam-expanding device 7 and the first diffraction grating 3 or in the beam path between the first diffraction grating s and the collimating device 8.
• the phase correction device or another type of correction device to compensate for the deterioration in beam quality K can also be 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.
• the optical arrangement 1 does not necessarily have to have two diffraction gratings 3, 4, but can also have a larger or smaller number of diffraction gratings through which the laser beam 6 passes one or more times.
• FIG. 5 shows an example of such an optical arrangement 1 with a grating arrangement 2 which has only a single diffraction grating 3 .
• a first deflection device 5 which is designed like the deflection device 5 shown in FIGS. 1a, b or like the deflection device 5 shown in FIGS AX is formed in a first beam offset direction X perpendicular to the diffraction plane YZ, which corresponds to the amount of the beam offset AX in the first beam offset direction X of the first deflection device 5 with the opposite sign.
• the second deflection device 11 is also designed to generate an additional lateral offset +AH, -AH in the first beam offset direction X, as described in more detail below.
• the second deflection device 11 is additionally designed to displace the laser beam 6 in a second beam displacement direction Y by a second beam displacement AY, the second beam displacement direction Y running in the diffraction plane YZ or parallel to the diffraction plane YZ.
• the second Deflection device 11 which is designed as a prism group, three
• Reflective surfaces 11 a-c Reflective surfaces 11 a-c.
• the second deflection device 11 additionally produces a lateral offset +AH, ⁇ AH in the first beam offset direction X, in order to achieve that a surface area of the diffraction grating 3 intended for a respective diffraction is hit centrally by the laser beam 6 .
• the function of the two other deflection devices 10a, b of FIG. 3c is taken over by the second deflection device 11:
• the second deflection device 11 After the first diffraction at the diffraction grating 3, the second deflection device 11 generates a beam offset +AX and a lateral offset +AH, with which the laser beam 6 impinges on the diffraction grating 3 in the second diffraction.
• the first deflection device 5 generates a beam offset of ⁇ AX in the first beam offset direction X, so that the laser beam 6 strikes the diffraction grating 3 again in the third diffraction with the lateral offset +AH relative to the incident laser beam 6 .
• an edge between the first and second reflection surfaces 11a, 11b of the second deflection device 11 in relation to an edge between the two reflection surfaces 5a, 5b of the first deflection device 5 in the first beam displacement direction X is - AH/2 positioned offset.
• a positioning of the edges is not absolutely necessary in order to enable the type of deflection described above.
• the first deflection device 5 is arranged at a comparatively small distance from the diffraction grating 3 in order to ensure that the beam cross section of the laser beam 6 remains approximately the same between the second and third diffraction.
• the second deflection device 11 is arranged at a comparatively large distance from the diffraction grating 3 .
• the second deflection device 11 can be designed to generate a lateral offset +AH, ⁇ AH with the amount indicated above, but no beam offset AX in the first beam offset direction X.
• the second deflection device 11 produces a beam offset AY in the second beam offset direction Y, which corresponds to the beam offset AY shown in FIG.
• the laser beam 6 can pass through the diffraction grating 3 four times in different surface areas.
• the second deflection device 11 shown in FIG. 5 can replace the two further deflection devices 10a, b shown in FIG X the same function as the two other deflection devices 10a, b. If the two reflecting surfaces 11a, b shown in FIG. 5 are used as a further deflection device 10, the two diffraction gratings 3, 4 are not parallel but aligned at an angle to one another which runs in the diffraction plane YZ.
• the two reflection surfaces 11a, 11b which can be formed in the form of a prism, e.g.
• FIG. 5 as in FIGS. 1a,b, the fanning out of the laser beam 6 in the diffraction plane YZ, more precisely the marginal rays of the fanning out, are shown in dashed lines.
• the additional deflection device 11 makes it possible for the laser beam 6 to pass through the diffraction grating 3 at four different surface areas that are offset relative to one another and is thereby diffracted four times.
• the beam-expanding element 7 and the collimating element 8 of the optical arrangement 1 are also omitted in FIG.
• the optical arrangement 1 described above has a compact design and can be used, for example, as a compressor in a chirped pulse amplification laser system 20, which is described below in connection with FIG. 6 is described in more detail.
• the use of the optical arrangement 1 is not limited to a chirped pulse amplification system.
• the laser system 20 shown in Fig. 6 is an ultra-short pulse system that has a laser pulse source 21 for generating a laser beam 6 with spectrally wide laser pulses and the optical arrangement 1 described above for dispersion adjustment, more precisely for pulse duration compression (also called a pulse compressor ) having.
• the laser pulse source 21 can be designed, for example, as a laser oscillator or, as shown in FIG. 6, as a laser oscillator-amplifier combination.
• a fiber oscillator 22 is integrated in the laser source 21, to which a dispersion adjustment unit 23 constructed similarly to the pulse compressor described above is integrated for generating a pulse stretching (also called a pulse stretcher).
• the dispersion adjustment unit 23 for pulse stretching can also be in the form of a fiber Bragg grating (FBG).
• the laser source 21 also has an amplifier chain with a number n of amplifiers 25a-n.
• another module for amplitude and/or phase adjustment 24 is arranged in front of the amplifier chain 25a, . . . , 25n, which can also be integrated into the amplifier chain 25a, . . . , 25n.
• An optical modulator 26 for selecting pulses or for adjusting the amplitude of the laser pulses is arranged after the amplifier chain 25a, ..., 25n and before the optical arrangement 1 which effects the pulse compression.
• Free-beam optics 27 have at least one beam-widening optical element 7, which serves to generate a divergent input beam for grating arrangement 2, as explained above.
• a dispersion adjustment of the beam path can be undertaken in order to finely adjust the pulse duration to do.
• an intensity profile of the laser pulses can be provided with a desired pulse duration, for example the shortest possible pulse duration or one that is adapted to a processing method.

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• 2020
  • 2020-10-19 DE DE102020213122.8A patent/DE102020213122A1/de active Pending
• 2021
  • 2021-09-17 CN CN202180071424.5A patent/CN116368699A/zh active Pending
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• 2023
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