WO2004068657A1

WO2004068657A1 - Regenerative amplifier having a resonator-internal dispersion compensation and seed impulse without positive dispersion

Info

Publication number: WO2004068657A1
Application number: PCT/EP2004/000673
Authority: WO
Inventors: Angelika Beyertt; Adolf Giesen; Detlef Nickel
Original assignee: Forschungsgesellschaft Für Strahlwerkzeuge-Fgsw -Mbh
Priority date: 2003-01-30
Filing date: 2004-01-27
Publication date: 2004-08-12
Also published as: DE10304401A1

Abstract

The invention relates to a laser system (100) for generating output laser pulses (70), comprising a seed laser (10) for generating a seed laser pulse (20) from which an injected laser pulse (48) can be generated that can be injected via an injecting element (68) into a regenerative amplifier, which comprises a resonator (50), a controllable coupling element (80), which is mounted therein, and a solid disc (76) serving as laser-active medium. This injected laser pulse can be amplified in the regenerative amplifier in the form of an amplifier laser pulse by multiple revolutions until the amplifier laser pulse can be extracted from the regenerative amplifier in the form of an extracted laser pulse (70). The aim of the invention is to improve a laser system of the aforementioned type whereby enabling the output laser pulses with short pulse durations to be attained with the least possible effort. To this end, the invention provides that, in order to generate output laser pulses in the span of fewer than five picoseconds, the injected laser pulse (48), relative to the seed laser pulse (20), is essentially free of a prolongation of pulse duration caused by positive dispersion. The invention also provides that, in the laser active medium (76), the maximum energy density per amplifier pulse is less than one joule per squared centimeter.

Description

REGENERATIVE AMPLIFIER WITH RESONATOR INTERNAL DISPERSION COMPENSATION AND SEED MPULS WITHOUT POSITIVE DISPERSION

The invention relates to a laser system for generating output laser pulses, comprising a seed laser for generating a seed laser pulse, from which a coupling laser pulse can be generated, which into a resonator and a controllable coupling element arranged therein and a solid-state disk with a laser-active medium having regenerative amplifiers can be coupled in via a coupling element and can be amplified in the regenerative amplifier as an amplifier laser pulse by means of multiple cycles until the amplifier laser pulse can be coupled out of the regenerative amplifier as a decoupling laser pulse.

In known laser systems of this kind, the coupling laser pulse is increased in terms of pulse duration by elements with positive dispersion compared to the seed laser pulse, in order to reduce the power density per amplifier laser pulse in the regenerative amplifier, so that no damage to the individual components of the regenerative amplifier occurs when the amplifier laser pulse is amplified.

To this pulse duration increase there are further pulse duration increases due to elements of positive dispersion in the regenerative amplifier, so that overall the coupling laser pulse has a larger pulse duration enlargement than the coupling laser pulse, which finally reduces the formation of a coupling laser pulse "with the desired pulse duration by compression by means of elements with a large negative dispersion must become. Such a procedure is not only complex in terms of the type of components used, but also has the disadvantage that high energy losses occur when the output laser pulse is very strongly compressed in order to form the output laser pulse.

The invention is therefore based on the object of improving a laser system of the generic type in such a way that output laser pulses with short pulse durations can be achieved with as little effort as possible.

This object is achieved according to the invention in a laser system of the type described in the introduction in that, in order to generate output laser pulses in the region of less than five picoseconds, the coupling laser pulse relative to the seed laser pulse is essentially free of a pulse duration increase caused by positive dispersion and in that in the laser-active medium the maximum energy density per amplifier laser pulse is less than one joule per square centimeter.

In particular, when considering the pulse duration enlargement, the pulse duration of the seed laser pulse after passing through components that are usually used and are not specifically designed for pulse duration enlargement are taken as a basis.

The advantage of the solution according to the invention can be seen in the fact that by avoiding a pulse duration increase caused by positive dispersion before coupling the coupling laser pulse into the regenerative amplifier and by reducing the energy density in the regenerative amplifier it is possible to obtain coupling laser pulses whose pulse duration increase is far less than that according to the prior art, so that even when such a coupling laser pulse is compressed, the expenditure on equipment and the energy losses are lower than in the known solutions.

An even more advantageous embodiment in which damage to optical elements in the regenerative amplifier can be avoided provides that the maximum energy density per amplifier laser pulse is less than one hundred millijoules.

In an advantageous embodiment of the invention, such low maximum energy densities can be achieved with a pulse duration of the coupling laser pulse of less than one hundred picoseconds, so that an optically disadvantageous broadening of the pulse duration to larger values is not necessary.

It is even more favorable for the optical boundary conditions if the pulse duration of the coupling laser pulse is less than fifty picoseconds.

A particularly favorable solution provides that at least one dispersion element with negative dispersion is provided to form the coupling laser pulse, which widens the coupling laser pulse compared to the seed laser pulse due to its negative dispersion with regard to its pulse duration.

The advantage of this solution can be seen in the fact that the pulse duration enlargement caused by the dispersion element with negative dispersion in again the regenerative amplifier is compensated for by the fact that the amplifier laser pulse continuously runs through elements of the regenerative amplifier with positive dispersion, so that a total pulse duration increase results due to positive dispersion in the decoupling laser pulse, which is less by the effects of the dispersion element arranged in front of the regenerative amplifier with negative dispersion ,

Furthermore, the advantage of this solution is to be seen in the fact that all energy losses caused by the dispersion element with negative dispersion for forming the coupling laser pulse are unproblematic, since these power losses occur before the amplification in the regenerative amplifier and are therefore easily compensated for by an increase in the amplification can be.

It is particularly favorable in this solution if the at least one dispersion element has such a large negative dispersion that the outcoupling laser pulse is increased by a maximum of a factor of five as compared to the seed laser pulse, that is to say that all the effects of positive dispersion are regenerative Due to the preceding negative dispersion, amplifiers only have the effect of increasing the pulse duration by a factor of five, which in some embodiments as such can be tolerated without additional measures and in other embodiments in which such a pulse duration increase cannot be tolerated, measures for further compensation which are not very expensive require this increase in pulse duration.

A particularly advantageous solution provides that the at least one dispersion element for forming the coupling laser pulse has such a large negative Dispersion has that during the number of revolutions of the amplifier laser pulse in the regenerative amplifier essentially compensates for pulse duration increases caused by the positive dispersion of the coupling element, so that the effects of a component of the regenerative amplifier leading to pulse duration increases in the output laser pulse can be largely compensated.

It is even more advantageous if the at least one dispersion element for forming the coupling laser pulse has such a large negative dispersion that it essentially compensates for pulse duration increases caused by positive dispersion during the number of revolutions of the amplifier laser pulse in the regenerative amplifier.

With regard to the arrangement of the dispersion element contributing to the formation of the coupling laser pulse and having a negative dispersion, no further details have so far been given.

An advantageous solution provides that the at least one dispersion element is arranged between an optical isolator and the regenerative amplifier.

It is even more advantageous if the at least one dispersion element is arranged between the regenerative amplifier and a mode adaptation unit. A further advantageous solution provides that the at least one dispersion element is arranged following a pulse selector, so that the dispersion element is only penetrated by the laser pulses selected by the pulse selector.

In principle, it would be conceivable to arrange the at least one dispersion element with negative dispersion for shaping the coupling laser pulse so that this is also passed through by the coupling laser pulse and thus twice, namely once by shaping the coupling laser pulse from the seed laser pulse and another time by shaping the output laser pulse from the Coupling laser pulse takes effect.

However, this solution has the disadvantage that the dispersion element must be designed for the outputs of the output laser pulse.

For this reason, a particularly favorable solution provides that the dispersion element is arranged in front of a pulse separator which separates the coupling laser pulse from the coupling laser pulse, so that the coupling laser pulse no longer penetrates the dispersion element to form the coupling laser pulse.

Another advantageous exemplary embodiment of the solution according to the invention provides that the outcoupling laser pulse has a pulse duration increase of a maximum of one hundred compared to the seed laser pulse, so that the measures required for compressing the outcoupling laser pulse are far easier to implement than in the prior art, in which pulse duration enlargements are usually used by a factor of a thousand compared to the seed laser pulse. For this reason it is provided in this solution, for example, that the outcoupling laser pulse is shortened by at least one dispersion element with negative dispersion arranged downstream of the regenerative amplifier.

Such a small pulse duration reduction is simple and can be implemented with little effort.

It is preferably provided that the pulse duration is shortened by a maximum of one hundred.

This solution is even more advantageous if the pulse duration is shortened by a maximum of fifty, more advantageously by a maximum of twenty.

With regard to the formation of the at least one dispersion element used to form the coupling laser pulse and / or to form the output laser pulse, no further details have so far been given. An advantageous solution provides that the at least one dispersion element comprises at least one pair of gratings.

Another advantageous solution provides that the at least one dispersion element comprises a pair of prisms.

The pair of prisms preferably comprises Brewster prisms. Particularly favorable optical and amplification ratios can be achieved in the solution according to the invention if the coupling laser pulse and / or the coupling laser pulse do not have excessively long pulse durations.

It is preferably provided that the coupling laser pulse has a pulse duration of less than twenty picoseconds.

It is even better if the pulse duration of the coupling laser pulse is less than ten picoseconds.

It is further preferably provided that the outcoupling laser pulse has a pulse duration of less than twenty picoseconds.

It is even more favorable if the outcoupling laser pulse has a pulse duration of less than ten picoseconds.

In connection with the previous explanation of the individual exemplary embodiments, it was only considered how the effects of the positive dispersion of components of the regenerative amplifier can be compensated for by dispersion elements before or after the regenerative amplifier.

In addition to or as an alternative to the possibilities described above, in a further advantageous exemplary embodiment it is provided that at least one of the multiple revolving laser pulses in the resonator with each circulation penetrated dispersion compensation element is provided with negative dispersion, which counteracts a pulse-width-widening positive dispersion of components of the regenerative amplifier. With this solution there is the possibility of additionally counteracting the effects of the positive dispersion on the amplifier laser pulses, this solution having the advantage that the at least one dispersion compensation element in the regenerative amplifier is passed through with each revolution of the amplifier laser pulse and thus with each revolution for compensation counteracts the positive dispersion of the components of the regenerative amplifier, which are also passed through with each revolution.

It is advantageous if this dispersion compensation element at least partially compensates for a positive dispersion of optical resonator components with each revolution of the amplifier laser pulse in the resonator.

It is particularly favorable if the at least one dispersion compensation element at least partially compensates for a positive dispersion of the controllable coupling element.

With regard to the formation of the dispersion compensation element itself, no further details have so far been given.

For example, it would be conceivable to use grating pairs as the dispersion compensation elements.

However, grid pairs in particular have high optical losses. For this reason, it is particularly advantageous if the at least one dispersion compensation element is designed as an interferometer.

Such interferometers are preferably Gires Tournois interferometers, which are known from the literature.

It is particularly favorable, in particular in order to achieve the necessary resonator length with an inexpensive compact construction of the resonator, if the at least one dispersion compensation element works in reflection.

As an alternative to providing an interferometer as a dispersion compensation element, it is also possible to use a pair of prisms as a dispersion compensation element.

The pair of prisms is preferably formed from Brewster prisms which have particularly low optical losses.

In the context of the invention, it is also possible, in particular in the case of a plurality of dispersion compensation elements, to use both interferometers and prism pairs as dispersion compensation elements in the same regenerative amplifier.

In order to be able to manufacture and use the dispersion compensation elements as simply as possible, it is preferably provided that the dispersion compensation elements are arranged at several points in the radiation course in the resonator. This has the advantage that in the production of the dispersion compensation elements, their negative dispersion does not have to be exactly matched to the positive dispersion occurring in the resonator, but rather a coordination by using several dispersion compensation elements with either identical or different negative dispersion in coordination with the positive one Dispersion of the other resonator components is achievable.

Thus, the tuning can be set, for example, by selecting the number and the corresponding negative dispersion of the dispersion compensation elements, for example instead of conventional reflectors, the reflectors already present in the resonator, for example those without dispersion, possibly using reflectors with a suitable negative dispersion, which in this case are then act as dispersion compensation elements, can be replaced.

In laser systems for generating laser pulses in the picosecond and sub-picosecond range, in particular when generating laser pulses with high energy in the sub-picosecond range, it is usually necessary to provide elements with positive dispersion that increase the duration of the pulse before the resonator, in order to avoid high intensity maxima which damage the could lead optical components.

Due to the use of a cooled solid-state disk, diameters of the radiation field can be realized in the resonator, which allow the laser system to be essentially free of positive elements arranged in front of the resonator for the purpose of increasing the pulse duration Dispersion is so that the laser amplifier pulses circulating in the resonator also have pulse durations which are in the order of magnitude of the coupled-in seed laser pulse, in particular are essentially identical to it.

In particular, a polarization-rotating coupling element, preferably a Pockels cell, is provided as the controllable coupling element.

In order to prevent the outcoupled laser pulse from acting back on the seed laser, an optical isolator is preferably provided following the seed laser, in particular between the seed laser and the regenerative amplifier.

Furthermore, it is preferably provided that a mode adaptation unit is arranged between the seed laser and the regenerative amplifier, which allows the mode of the seed laser to be adapted to the mode of the regenerative amplifier, in particular the resonator thereof.

A pulse separator is preferably provided between the seed laser and the regenerative amplifier in order to be able to advantageously couple the outcoupled laser pulse and, in particular, to largely avoid a reaction on the seed laser.

Such a pulse separator can be arranged particularly advantageously between the mode adaptation device and the regenerative amplifier, so that the decoupled laser pulse emerges directly with the mode of the amplifier laser pulse in the resonator and no longer runs over the mode matching device and experiences a change in it.

In this solution, the optical isolator is preferably provided between the mode matching device and the seed laser, which provides additional protection for the seed laser against any retroactive laser pulses.

In particular, the pulse separator is constructed so that it has a polarizer and an optical rotator.

The polarizer is preferably designed as a thin-film polarizer.

With regard to the repetition rate of the outcoupled laser pulses, no further details have been given in connection with the previous exemplary embodiments. An advantageous exemplary embodiment provides that the laser system generates outcoupled laser pulses with a repetition rate of several kilohertz.

This laser system is particularly advantageous when it generates outcoupled laser pulses with a repetition rate of more than five kilohertz.

With regard to the number of revolutions of the amplifier laser pulses in the resonator, no further details have so far been given. The laser system according to the invention is particularly suitable for all those applications in which a high number of revolutions in the resonator is required for amplification.

It is preferably provided in the laser system according to the invention that the decoupling element can be controlled by a control such that the amplifier laser pulses are only decoupled from the resonator after at least twenty revolutions, better still after at least fifty revolutions, and even better after more than one hundred revolutions ,

In connection with the previous explanation of the individual embodiments of the laser system according to the invention, no further specification of the laser-active medium has been discussed.

It has proven to be particularly advantageous if materials are provided as the laser-active medium for the solid-state disk, which have a reinforcement of at least 5% per double passage through the solid-state disk with a maximum thickness of the solid disk and their optical bandwidth Generation of amplifier laser pulses shorter than ten picoseconds allowed.

Further features and advantages of the invention are the subject of the following description and the drawing of an exemplary embodiment. Show in the drawing

Figure 1 is a schematic representation of a first embodiment of the laser system according to the invention in plan view.

Fig. 2 is a schematic representation of a second embodiment of the laser system according to the invention similar to Fig. 1 and

Fig. 3 is a schematic representation of a third embodiment of a laser system according to the invention.

An embodiment of a laser system according to the invention, shown in FIG. 1, comprises a seed laser 10, which is preferably designed as a diode-pumped ytterbium glass laser oscillator or ytterbium tungsten laser oscillator and is passively mode-locked.

The mode coupling is preferably carried out by a saturable semiconductor absorber mirror.

The seed laser 10 operates, for example, with a repetition rate of more than 20 MHz, and generates seed laser pulses 20 limited by the time bandwidth with a pulse duration of approximately 300 femtoseconds. The wavelengths of the seed laser 10 are in the range of, for example, 1000 to 1100 nanometers with pulse energies in the order of 1 nanojoule.

Following the seed laser 10, an optical isolator, designated as a whole by 12, is provided, which is designed as a Faraday isolator 14, which prevents reflected laser pulses from acting back on the seed laser 10 and disturbing it.

In addition, a λ / 2 plate 16 is provided for polarization rotation.

The seed laser pulse 20 leaving the seed laser 10 is preferably coupled into the optical isolator 12 by deflecting mirrors 22 and 24 and passes through it.

In addition, a photodiode 26 and a spectrometer 28 are provided for monitoring the function of the seed laser 10, with which a laser pulse 30 coupled out parasitically to the seed laser pulse 20 can be analyzed.

The laser system is preferably triggered by means of the photodiode 26.

In order to select the seed laser pulses 20 actually used for amplification and to suppress the other seed laser pulses 20 from the multiplicity of seed laser pulses 20 generated by the seed laser 10, a pulse selector 32 is preferably provided, which in particular includes a Pockels cell 34 comprises a polarizer 36, the Pockels cell being driven accordingly for pulse selection. After passing through the optical isolator 12, the seed laser pulse 20 decoupled from the seed laser 10 passes through a mode adaptation unit 40, preferably embodied as a telescope with, for example, telescope mirrors 42a, 42b, with which an adaptation to a mode of a resonator 50 described in detail below takes place ,

Following the mode matching unit 40, the seed laser pulse 20 passes through a first dispersion element 44, which is designed as a pair of gratings with two gratings 46a, b which have a negative dispersion, twice by back reflection on a reflector 47.

Due to the negative dispersion from the seed laser pulse 20 by increasing the pulse duration, which can be a multiple of the pulse duration of the seed laser pulse 20, the dispersion element 44 forms a coupling laser pulse 48, which enters the pulse separator 52 after the dispersion element 44.

The coupling laser pulse 48 generated by the dispersion element 44 preferably has a pulse duration of less than twenty picoseconds, more preferably less than ten picoseconds.

The pulse separator 52 comprises a thin-film polarizer 54, a Faraday rotator 56, which serve to later separate the coupling-in laser pulse 48 in the coupling-in laser pulse 48 from a coupling-out laser pulse 70 that is coupled out of the resonator 50, as described in detail below.

In addition, a λ / 2 plate 58 is provided for polarization rotation. After passing through the pulse separator 52, the coupling laser pulse 48 is coupled into the resonator 50 via an adjusting unit 66, comprising, for example, two mirrors 62 and 64, namely through the passage of a thin-film polarizer 68 belonging to the resonator 50, via which the coupling laser pulse 48 is coupled into the resonator 50 takes place, the coupling laser pulse 48 as an amplifier laser pulse 60 revolves several times in the resonator 50 and is thereby amplified until the amplifier laser pulse 60 is coupled out as a coupling laser pulse 70 from the resonator 50.

The resonator 50 comprises a first end mirror 72, a second end mirror 74 and a laser-active medium 76 in the form of a thin disk which can be cooled by a cooling device 78, as described, for example, in European Patent 0 632 551, to which reference is hereby made.

Furthermore, a Pockels cell 80 is arranged in the resonator 50 between the thin-film polarizer 68 and the first end mirror 72 as a coupling element and can be controlled by a control 82, for example a so-called push / pull circuit, the control 82 generating a trigger signal from one of the end mirrors 74 assigned photodiode 75 receives.

The Pockels cell 80 is further combined with a so-called λ / 4 plate 84. A coupling laser pulse 48 coupled in via the coupling / decoupling element 68 of the resonator 50 propagates in the resonator 50 as an amplifier laser pulse 60 and first penetrates the λ / 4 plate 84 and the Pockels cell 80 until it hits the first end mirror 72, which is designed as a reflecting end mirror of the resonator 50.

Thus, the amplifier laser pulse 60 is reflected at the first end mirror 72, passes through the Pockels cell 80 and the λ / 4 plate 84 again and is thereby, when this amplifier laser pulse 60 is to be amplified in the resonator 50, by the coupling / decoupling element 68 does not pass again as a decoupled laser pulse 70 in the direction of the pulse separator 52, but reflects to a deflecting mirror 86, reflects from this to a deflecting mirror 88 and then passes through, for example, a λ / 2 plate 90 provided for polarization rotation.

After passing through the λ / 2 plate 90, the amplifier laser pulse 60 hits the laser-active medium 76, which. is in turn provided on the back with a reflector 92, which redirects the amplifier laser pulse 60 again to a deflection mirror 94 to a deflection mirror 96 and this in turn to a deflection mirror 98, from which the amplifier laser pulse 60 then strikes the second end mirror 74 and is reflected back by it.

The amplifier laser pulse 60 was amplified twice by the laser-active medium 76 due to the action of the reflector 92 before it reached the second end mirror 74. In the case of a renewed back reflection, there is a renewed reflection at the deflection mirrors 98, 96 and 94 until the amplifier laser pulse 60 passes through the laser-active medium 76 twice again, then hits the deflection mirrors 88 and 86 again via the polarization-rotating element 90 and then hits the Coupling / decoupling element 68, which reflects the amplifying laser pulse 60, which has now been amplified four times by the laser-active medium 76, back to the λ / 4 plate 84 and the Pockels cell 80 until this amplifying laser pulse 60 hits the first end mirror 72 again ,

Thus, the resonator 50, together with the laser-active medium 76, acts overall as a regenerative amplifier 100 for amplifying the incoming coupling laser pulse 48.

The Pockels cell 80 is now controlled by the control unit 82 such that the coupling laser pulse 48 originally coupled in as the amplifier laser pulse 60 passes through the resonator 50 more than approximately 100 times, even better more than approximately 150 times and more, and is thereby amplified.

Since the individual elements of the resonator 50, in particular the Pockels cell 80, have a positive dispersion with regard to the group speed, each time the amplifier laser pulse passes through the resonator 50, the negative dispersion of the dispersion element 44 is partially compensated for and thus also partially compressed with the Pulse duration increase of the dispersion element 44 afflicted amplifier laser pulse 60, which lasts as a whole until the amplifier laser pulse 60 is finally decoupled as decoupling laser pulse 70. It is thus possible to choose the negative dispersion of the dispersion element 44 so large that the increase in the pulse duration caused by this cancels out exactly with the increase in the pulse duration caused by the positive dispersion of the elements of the resonator 50.

If the negative dispersion of the dispersion element 44 is less than the positive dispersion acting on the amplifier laser pulse 60 in the total number of revolutions in the resonator 50, there is the possibility of additionally providing dispersion compensation elements which are penetrated by the rotating amplifier laser pulse 60 directly in the resonator 50 have a positive dispersion of the individual elements of the resonator 50, in particular a dispersion of the Pockels cell 80, compensating negative dispersion. Such dispersion compensation elements are, for example, the first end mirror 72, the deflection element 86 and the deflection element 98, which are designed as so-called Gires Tournois interferometer mirrors, which together with the dispersion element 44 allow compensation of the positive dispersion essentially produced by the Pockels cell 80.

Such Gires Tournois interferometer mirrors are for example from the article by F. Gires and P. Tournois, Comt. Rend. Acad. Be. (Paris) 258, 6112 (1964).

Depending on the dispersion of the other components of the resonator 50, it is now possible to provide dispersion compensation elements in a corresponding number and with a corresponding negative dispersion, which allow the Dispersion of the amplifier laser pulse 60 circulating in the resonator 50 must also be compensated for each revolution, so that essentially the pulse duration of the seed laser pulse 20 can be achieved again in the decoupling laser pulse.

After the resonator 50 has been run through several times, the Pockels cell 80 is actuated accordingly via the coupling-in / coupling-out element 68, and the amplifier laser pulse 60 is coupled out in the form of the coupling-out laser pulse 70, which then passes through the adjusting device 66 and the pulse separator 52 and is reflected by the thin-film polarizer 54 thereof and then strikes a substrate 110 as an output laser pulse 108, for example for material processing.

The Pockels cell 80 is preferably operated with cycles whose frequency is several kilohertz, preferably between 1 and 10 kHz or even possibly even more, in order to obtain a high repetition rate of the coupled-out laser pulse 70.

In principle, all materials are suitable as laser-active medium for the solid-state disk 76 which, with a maximum thickness of the solid-state disk of 0.5 mm, have a reinforcement of at least 5% per double passage through the reinforcing material, that is to say through the solid-state disk, and the bandwidth of the generation of laser pulses shorter than 10 picoseconds allowed. The laser-active medium 76 in the form of a disc less than 0.5 mm thick is preferably Yb: KYW, but similar materials, such as, for example, Yb.KGW or Yb.YAG or Yb-doped sesquioxide, for example lutetium oxide, or else semiconductor materials, can also be used.

The thickness of the disk of the laser-active medium is, for example, approximately in the order of magnitude of less than 300 μm and in particular is in the order of approximately 100 μm, and the doping of the laser-active medium is, for example, less than 20%, preferably in the order of approximately 10% ,

In order to prevent destruction of the laser-active medium, the energy density per amplifier laser pulse 60 in the regenerative amplifier 100 is advantageously less than one hundred millijoules per square centimeter, more preferably less than fifty millijoules per square centimeter.

With regard to the mode in which the resonator 50 is operated, nothing has been explained in detail so far. The resonator 50 preferably operates in TEMoo mode and the mode adaptation device 40 is set in such a way that it converts a radiation field of the seed laser pulse 20 to a radiation field corresponding to the TEM ₀₀ mode of the resonator 50.

In a second exemplary embodiment of a laser system according to the invention, shown in FIG. 2, all those elements which are identical to the first laser system are provided with the same reference numerals, so that with regard to the description thereof, reference is made in full to the explanations for the first exemplary embodiment. In the second exemplary embodiment, no dispersion element 44 is provided, but the coupling laser pulse 48 has essentially the same pulse duration as the seed laser pulse 20 after the pulse selector 32.

The positive dispersion of the individual elements of the resonator 50 is either not compensated or only partially compensated for by the additional dispersion compensation elements 72, 86 and 98, so that the output laser pulse 70 has a pulse duration increase caused by the positive dispersion, which however does not exceed a factor of 100 of the pulse duration of the Seed laser pulse 20 corresponds.

In a preferred solution, the pulse duration of the coupling laser pulse 70 which is increased by the elements of the resonator is less than twenty picoseconds, it is even better if the pulse duration is less than ten picoseconds.

This increase in the pulse duration of the outcoupling laser pulse 70 is compensated for by a dispersion element 120 following the pulse separator 75, which is designed, for example, as a pair of gratings and has two gratings 122a, 122b, which the outcoupling laser pulse 70 guided to a reflector 124 passes through twice before it is applied to the substrate 110 Output laser pulse 108 hits for material processing.

With the dispersion element 120, due to the negative dispersion, the increase in the pulse duration of the output laser pulse 70 can be compensated for to such an extent that the desired short pulse duration, preferably one pulse duration of less than five picoseconds, better still has a pulse duration in the sub-picosecond range, which in the optimal case is in the order of magnitude of the pulse duration of the seed laser pulse 20.

However, the pulse duration shortening caused by the dispersion element 120 is at most a factor of one hundred, preferably less, more preferably less than a factor of fifty.

Otherwise, the second exemplary embodiment of the laser system according to the invention operates in the same way as the first exemplary embodiment, so that the description of the contents of the first exemplary embodiment can be referred to in full in terms of its description.

In a third exemplary embodiment, shown in FIG. 3, both the dispersion element 44 and the dispersion element 120 are present, both of which, because of their negative dispersion, help to compensate for the positive dispersion, with the dispersion compensation elements 72 also possibly being present in the resonator 50 , 86 and 98 can be effective.

In the third exemplary embodiment, the dispersion element 120 is designed as a pair of gratings, but the dispersion element 44 'is in the form of a pair of prisms, in particular a pair of Brewster prisms 46'a, 46'b, which do not have a very large negative dispersion, the negative dispersion of the interacting dispersion elements 44 'and 120 may not be sufficient. In contrast to the first and second exemplary embodiments, a pair of prisms 130 comprising two prisms 132 and 134 is therefore provided as the dispersion compensation element in the regenerative amplifier 100, the pair of prisms 130 preferably being arranged between the deflecting mirror 94 and the end mirror 74.

The pair of prisms 130 can be designed in such a way that it itself has such a large negative dispersion that the positive dispersion of the Pockels cell 80 is at least partially compensated for every revolution, so that it is not necessary to include the first end mirror 72 and the deflection element 86 to train as dispersion compensation elements. Rather, they can be designed as conventional optical components.

However, it is also conceivable to provide additional dispersion compensation elements in addition to the pair of prisms 120, provided that the negative dispersion of the pair of prisms 130 is not sufficient to compensate for the positive dispersion of the Pockels cell 80. In this case, a further pair of prisms can be provided or the end mirror 72 or the deflection element 86 can also be used as a Gires Tournois interferometer mirror with negative dispersion.

In addition, it is possible with the pair of prisms 130 to cause the negative dispersion acting on the amplifier laser pulse 60. Adjust the variation of the optical path length by the pair of prisms 130 and thus also set the pulse duration of the coupling-out laser pulse 70 within limits.

Otherwise, the third embodiment is designed in the same way as the first and the second embodiment, so that with respect to the Identical reference numerals provided full reference to the description of the first and second embodiments.

In addition, with regard to the mode of operation of the third exemplary embodiment, reference is made in full to the respective statements relating to the first and second exemplary embodiments.

Claims

GODFATHER CLAIMS

1. A laser system for generating output laser pulses (108), comprising a seed laser (10) for generating a seed laser pulse (20), from which an incoupling laser pulse (48) can be generated, which in a resonator (50) and in this arranged controllable coupling element (80) and a solid-state disk (76) as a laser-active medium having regenerative amplifier (100) can be coupled in via a coupling element (68) and can be amplified in the regenerative amplifier (100) as an amplifier laser pulse (60) by multiple cycles until the amplifier laser pulse (60) can be coupled out as a decoupling laser pulse (70) from the regenerative amplifier (50), characterized in that in order to generate output laser pulses (108) in the range of less than five picoseconds, the coupling laser pulse (48) relative to the seed laser pulse ( 20) is essentially free of a pulse duration increase caused by positive dispersion and that in the laser ac tive medium (76) the maximum energy density per amplifier laser pulse (60) is less than one joule per square centimeter.

2. Laser system according to claim 1, characterized in that the maximum energy density per amplifier laser pulse is less than one hundred millijoules.

3. Laser system according to claim 1 or 2, characterized in that the coupling laser pulse has a pulse duration of less than one hundred picoseconds.

4. Laser system according to claim 3, characterized in that the coupling laser pulse has a pulse duration of less than fifty picoseconds.

5. Laser system according to one of the preceding claims, characterized in that for forming the coupling laser pulse (48) at least one dispersion element (44) with negative dispersion is provided, which the coupling laser pulse (48) relative to the seed laser pulse (20) by its negative dispersion broadened in terms of its pulse duration.

6. Laser system according to claim 5, characterized in that the at least one dispersion element (44) has such a large negative dispersion that the outcoupling laser pulse (70) compared to the seed laser pulse (20) is increased by a maximum of a factor of five pulse duration.

7. Laser system according to claim 5 or 6, characterized in that the at least one dispersion element (44) for forming the coupling laser pulse (48) has such a large negative dispersion that this during the number of revolutions of the amplifier laser pulse (60) in the regenerative amplifier ( 100) essentially compensated for pulse duration increases caused by the positive dispersion of the coupling element (80).

8. Laser system according to claim 7, characterized in that the at least one dispersion element (44) for forming the coupling laser pulse (48) has such a large negative dispersion that this during the number of revolutions of the amplifier laser pulse (60) in the regenerative amplifier (100 ) occurring pulse duration increases caused by positive dispersion essentially compensated.

9. Laser system according to one of claims 5 to 8, characterized in that the at least one dispersion element (44) between an optical isolator (12) and the regenerative amplifier (100) is arranged.

10. Laser system according to one of claims 5 to 9, characterized in that the at least one dispersion element between the regenerative amplifier (100) and a mode adjustment unit (40) is arranged.

11. Laser system according to one of claims 5 to 10, characterized in that the at least one dispersion element (44) is arranged following a pulse selector (32).

12. Laser system according to one of claims 5 to 11, characterized in that the at least one dispersion element (44) is arranged in front of a pulse separator (52).

13. Laser system according to one of the preceding claims, characterized in that the outcoupling laser pulse (70) compared to the seed laser pulse (20) has a pulse duration increase of a maximum of one hundred.

14. Laser system according to claim 13, characterized in that the outcoupling laser pulse (70) by at least one on the regenerative amplifier (100) arranged dispersion element (120) with negative dispersion experiences a pulse duration reduction.

15. Laser system according to claim 14, characterized in that the pulse duration is shortened by a factor of one hundred at most.

16. Laser system according to claim 15, characterized in that the pulse duration is shortened by a maximum of fifty.

17. Laser system according to one of claims 5 to 16, characterized in that the at least one dispersion element (44, 120) comprises at least one pair of gratings (46a, b, 122a, b).

18. Laser system according to one of claims 5 to 17, characterized in that the at least one dispersion element (44 ') comprises a pair of prisms (46'a, 46'b).

19. Laser system according to claim 18, characterized in that the pair of prisms (46'a, 46'b) comprises Brewster prisms.

20. Laser system according to one of the preceding claims, characterized in that the coupling laser pulse (48) has a pulse duration of a maximum of twenty picoseconds.

21. Laser system according to claim 20, characterized in that the coupling laser pulse (48) has a pulse duration of a maximum of ten picoseconds.

22. Laser system according to one of the preceding claims, characterized in that the coupling laser pulse (70) has a pulse duration of less than twenty picoseconds.

23. Laser system according to claim 22, characterized in that the coupling laser pulse (70) has a pulse duration of less than ten picoseconds.

24. Laser system according to one of the preceding claims, characterized in that in the resonator (50) at least one dispersion compensation element (72, 86, 98, 130) with negative dispersion penetrated by the multiply circulating amplifier laser pulse (60) is provided with each circulation, which counteracts a positive dispersion of components (80) of the regenerative amplifier (100) which increases the pulse duration.

25. Laser system according to claim 24, characterized in that the at least one dispersion compensation element (72, 86, 98, 130) with each revolution of the amplifier laser pulse (60) in the resonator (50) a positive dispersion of optical resonator components (80) at least partially compensated.

26. Laser system according to claim 25, characterized in that the at least one dispersion compensation element (72, 86, 98, 130) at least partially compensates for a positive dispersion of the controllable coupling element (80).

27. Laser system according to one of claims 24 to 26, characterized in that the at least one dispersion compensation element (72, 86, 98) comprises an interferometer.

28. Laser system according to claim 27, characterized in that the at least one dispersion compensation element (72, 86, 98) is a Gires Tournois interferometer.

29. Laser system according to claim 27 or 28, characterized in that the at least one dispersion compensation element (72, 86, 98) works in reflection.

30. Laser system according to claim 25 or 26, characterized in that the at least one dispersion compensation element (130) comprises a pair of prisms.

31. Laser system according to claim 30, characterized in that the pair of prisms (130) from Brewster prisms (132, 134) is formed.

32. Laser system according to one of the preceding claims, characterized in that the dispersion compensation elements (72, 86, 98, 130) are arranged at several points in the beam path in the resonator.

33. Laser system according to one of the preceding claims, characterized in that this decoupled laser pulses (70) generates in the sub-picosecond range.

34. Laser system according to one of the preceding claims, characterized in that the controllable coupling element is a Pockels cell (80).

35. Laser system according to one of the preceding claims, characterized in that an optical isolator (12) is provided following the seed laser (10).

36. Laser system according to one of the preceding claims, characterized in that a mode adjustment unit (40) is arranged between the seed laser (10) and the regenerative amplifier (100).

37. Laser system according to claim 36, characterized in that the mode adaptation unit (40) is designed as a telescope.

38. Laser system according to one of the preceding claims, characterized in that a pulse separator (52) is provided between the seed laser (10) and the regenerative amplifier (100).

39. Laser system according to claim 38, characterized in that the pulse separator (52) is arranged between the mode matching device (40) and the regenerative amplifier (100).

40. Laser system according to claim 37 or 38, characterized in that the pulse separator (52) has a polarizer (54) and an optical rotator (56).

41. Laser system according to claim 40, characterized in that the polarizer is a thin film polarizer (54).

42. Laser amplifier system according to one of the preceding claims, characterized in that an in / out coupling element (68) of the resonator (50) is designed as a thin film polarizer.

43. Laser system according to one of the preceding claims, characterized in that this coupled-out laser pulse (70) generates with a repetition rate of several kilohertz.

44. Laser system according to claim 43, characterized in that this outcoupled laser pulses (70) with a repetition rate of more than five kilohertz.

45. Laser system according to one of the preceding claims, characterized in that the decoupling element (80) can be controlled by a control (82) in such a way that the amplifier laser pulses (60) are only coupled out of the resonator (50) after at least twenty revolutions.

46. Laser system according to one of the preceding claims, characterized in that materials are provided as the laser-active medium for the solid-state disk (76), which has a gain of at least 5% per double pass at a maximum thickness of the solid-state disk (76) of 0.5 mm have through the solid-state disk (76) and whose optical bandwidth allows the generation of amplifier laser pulses (60) shorter than ten picoseconds.