Laser system with a multipass amplifier configured for thermal load conservation

Abstract

The present invention relates to a laser system (1) comprising a multipass amplifier (2), in particular a multipass disk laser amplifier, for amplifying laser light to provide an amplified output beam (14) and a control unit (60), wherein the multipass amplifier (2) comprises a laser active medium (12), wherein the control unit (60) is configured to maintain the thermal load on the laser active medium (12), determined by at least two different power sources, substantially constant over a range of the laser output power of the output beam (14).

Description

[0001] Laser system with a multipass amplifier configured for thermal load maintenance

[0002] Technical area

[0003] The present invention relates to a laser system having a multipass amplifier for amplifying laser light, wherein the multipass amplifier comprises a laser-active medium.

[0004] State of the art

[0005] To amplify incident laser light in a multipass amplifier, a laser-active medium is required. To generate the population inversion necessary for amplification, the laser-active medium is irradiated with pump light from a pump laser of a suitable wavelength. The energy thus stored in the laser-active medium is tapped by irradiating laser light from a seed laser of a suitable wavelength (which differs from the pump light wavelength) through stimulated emission, thereby amplifying the seed laser light.

[0006] To increase the amplification efficiency in a laser system, the typical aim is to maximize the path length traveled by the pump radiation and / or the seed radiation within the laser-active medium. For this reason, so-called multipass amplifiers are used, among other things. On the one hand, pump light is guided through the laser-active medium several times by geometrically guiding the pump beam through the laser-active medium using suitable optics. On the other hand, the seed beam, and thus the laser beam to be amplified, is also guided through the laser-active medium several times using another suitable optical structure according to the multipass concept before the amplified laser beam is tapped for final use. It should be noted that in such a multipass amplifier, the seed beam, in contrast to regenerative amplifiers, for example, is guided along geometrically different paths during its passages.This has the particular advantage that the coupling and decoupling within the amplifier arrangement occurs purely geometrically, and, for example, no optical switch is required for active coupling or decoupling, as is the case with regenerative laser amplification systems. In general, the laser-active medium of a laser amplification system can have very different geometries and, above all, can have various advantages and disadvantages depending on its aspect ratio (i.e., the ratio between the depth or height of the medium and its lateral extent). The optical intensities within the active media play a particularly important role, and can vary greatly for different aspect ratios.Especially with aspect ratios significantly smaller than 1 (such as in fiber laser systems), the optical intensities reach very high values ​​even at comparatively low power levels, leading to nonlinear behavior. Accordingly, the achievable power levels are significantly influenced by the geometry of the laser-active medium.

[0007] Regardless of the geometry of the laser-active medium, however, thermal effects exist within the laser-active medium, which can have a negative impact on beam quality. In general, pump light that is not converted into laser power in the outcoupled, amplified beam generates predominantly heat, thus creating a thermal load on the laser-active medium. This thermal load is particularly relevant for multipass amplifiers, since the heat deposited in the active medium is correspondingly high due to the multiple passages of the radiation.

[0008] This thermal load can be approximately quantified as the difference between pump power and the extracted power, i.e. the laser output power of the laser amplification system.

[0009] Despite the typically applied cooling of the active media in laser amplification systems, the thermal load in the medium leads to undesirable thermally induced optical effects. This is primarily due to the fact that the heating by the pump light leads to a changed lensing effect of the medium and thus to the aforementioned undesirable optical effects.

[0010] To achieve constant beam parameters and consistent beam quality with variable laser output power, existing laser amplification systems operate at a specific, fixed laser output power characterized by high beam quality. To vary the output power, downstream optical elements are used to reduce or regulate the power, rather than changing the pump power and / or seed power. This type of operation is also referred to as biased operation. Accordingly, a laser system operating in biased operation requires additional downstream components to enable the desired adjustability of the laser output power. This increases the cost and complexity of the system.

[0011] From the document DE102019131507A1, a regenerative disk laser amplifier is known in which the pulse energy of a seed laser can be attenuated before coupling into a resonator in order to avoid a deviation in the pulse energy of the amplified output laser pulses in the event of deviations in the amplification time and / or changes in the repetition rate.

[0012] However, by appropriately attenuating the seed laser pulse energy, which allows the pulse energy to be kept constant while changing the repetition rate by altering the amplification time (i.e., the number of revolutions of the laser beam in the resonator), the extracted laser power of the system also changes, thereby changing the thermal load in the active medium, in this case, in particular a laser-active disk. To counteract the resulting temperature change of the disk, the pump power of the system can be adjusted to compensate for the thermal load change, thus suppressing unwanted thermally induced optical effects.

[0013] Accordingly, the thermal load maintenance according to DE102019131507A1 is designed to keep the thermal load constant over a range of the laser pulse repetition rate. However, the described system is not designed to guarantee thermal load maintenance over a range of the extracted laser output power of the laser system.

[0014] Description of the invention

[0015] Based on the known prior art, it is an object of the present invention to provide an improved laser system for amplifying laser light, which ensures a nearly constant beam quality as well as beam parameters over a range of laser output power.

[0016] The problem is solved by a laser system having the features of claim 1. Advantageous further developments emerge from the subclaims, the description, and the figure.

[0017] Accordingly, a laser system comprising a multipass amplifier, in particular a multipass disk laser amplifier, for amplifying laser light to provide an amplified output beam, and a control unit is proposed, wherein the multipass amplifier comprises a laser-active medium. According to the invention, the control unit is configured to keep the thermal load on the laser-active medium, which is determined by at least two different power sources, substantially constant over a range of the laser output power of the output beam.

[0018] The active medium of the multipass amplifier can be disk-shaped, with a disk-shaped active medium encompassing all shapes in which the lateral dimensions are significantly larger than their thickness. However, a disk-shaped active medium also includes one and / or more transmissive disks, which can be coated with an antireflective coating on one or both sides. The active medium of the multipass amplifier does not have to be disk-shaped, but can also have other geometries.

[0019] The thermal load of a laser-active medium of a laser system with a multipass amplifier is primarily determined by the pump power of the pump laser and the seed power of the seed laser, which are designed to irradiate the laser-active medium.

[0020] The pump radiation that is absorbed in the laser-active medium, but not extracted by the stimulated emission induced by the seed radiation, is predominantly deposited as heat in the laser-active medium. If, for an initial consideration, one restricts oneself to these power sources necessary for laser amplification (pump power and seed power), without excluding other power sources that lead to heat input and / or heat removal into / from the laser-active medium, the thermal load P th of the laser-active medium can be approximately calculated as

[0021] ^th — ^pump — ^ex — ^pump — ^(^pump) ^seed < where P pump the pump power of the pump laser and P ex is the extracted power of the amplified laser output beam. The latter is referred to as laser output power. After the second equal sign, P ex= K(P pump ) P seed used. The laser output power P ex is therefore directly proportional to the seed power P seed , where the proportionality factor the gain factor, which in turn is a function of the

[0022] Pump power P pump The concrete functional relationship between the amplification factor K and the pump power P pump depends essentially on the energy level structure of the laser-active laser medium used. In general, however, it can be stated that the amplification factor K increases with increasing pump power P pump increases, as long as the laser-active medium is not destroyed by excessive pump power and / or its properties are not changed to such an extent that conventional laser operation is no longer possible. From the above considerations and the equation for the thermal load, it is clear that an increase in the pump power, depending on the used (constant) seed power Pseed and the present amplification factor K, either an increase or decrease of the thermal load P th can result in.

[0023] For common gain factors K and typically used seed powers P seed However, it can be assumed that an increase in the pumping power P pump also leads to an increase in the thermal load P th leads.

[0024] An increase in seed performance P seed in turn, at a finite (constant) pump power Ppump, typically leads to a decrease in the thermal load P th as long as the gain is not saturated by the seed power.

[0025] Preferably, the laser output power range extends over the entire range of achievable laser output powers of the multipass amplifier or extends only over a portion of this range. In other words, the thermal load can be kept essentially constant over a given output power range, eliminating the need for downstream power regulation elements while maintaining high beam quality.

[0026] In particular, a repetition rate of laser pulses of the output beam is constant.

[0027] Preferably, the repetition rate of the laser pulses of the output beam is constant across the laser output power range of the output beam, while the thermal load on the laser-active medium is kept substantially constant by the control unit. In particular, the repetition rate of the laser pulses of the output beam is not varied across the laser output power range of the output beam.

[0028] The term “maintaining substantially constant” the thermal load on the laser-active medium is understood herein to mean that the thermal load has a maximum relative fluctuation of +10%, preferably +5%, particularly preferably +2%.

[0029] However, the present invention is not limited to the above examples regarding the change in thermal load due to changes in seed power and pump power. Rather, any other change in thermal load Pth , which is caused by a change in seed performance P seed and / or the pump power P pump induced. Furthermore, other power sources besides the seed power and the pump power are conceivable, which may result in a (positive or negative) contribution to the thermal load of the laser-active medium and may also have a direct influence on the laser output power.

[0030] The present invention makes particular use of the special relationship between thermal load P thand the seed and pump power to keep the thermal load essentially constant across a range of laser output power by appropriately adjusting the seed and pump power. This enables a laser system that achieves variable laser output power with a constant thermal load. The latter particularly improves the usability of the amplified output beam, as its beam quality and beam parameters can be kept virtually constant across the entire laser output power range through thermal load maintenance.

[0031] Accordingly, a seed laser and a pump laser are preferably provided as power sources contributing to the thermal load on the laser-active medium, which are configured to irradiate the laser-active medium, wherein the control unit is configured to jointly adapt a seed power of the seed laser and a pump power of the pump laser over the range of the laser output power such that the thermal load on the laser-active medium is substantially constant.

[0032] Preferably, the control unit is configured to keep the thermal load on the laser-active medium substantially constant over the range of the laser output power by adjusting the powers of the power sources contributing to the thermal load of the laser-active medium by manual adjustment and / or by a mathematical model and / or by an allocation table.

[0033] Preferably, the control unit is configured to keep the beam quality and / or at least one beam parameter of the output beam, preferably defined by the waist diameter and / or the waist position and / or the diffraction index, substantially constant over the range of the laser output power, wherein the beam quality and / or the beam parameter preferably has a maximum relative fluctuation of +10%, preferably +5%, particularly preferably +2% over the range of the laser output power.

[0034] The beam parameters are essentially determined by the waist diameter and the waist position, with the beam quality being primarily characterized by the diffraction index of the amplified output beam. A monitoring device can preferably be provided for monitoring the beam quality and the beam parameters, in particular the waist diameter, the waist position, and the diffraction index of the amplified output laser beam.

[0035] In a further preferred embodiment, the pump laser is configured to generate an extended, blurred pump spot on the laser-active medium. This allows for a more homogeneous behavior of the laser-active medium, which can further improve the beam quality.

[0036] Preferably, the pump laser is configured to generate a pump spot on the laser-active medium whose diameter is a factor of 1 to 1.5 larger than the diameter of the seed beam or, if multiple seed beams impinge on the laser-active medium, the seed spot resulting from the seed beams on the laser-active medium. This can prevent or reduce the influence of edge regions of the pumped volume on the beam quality, allowing the beam quality to be further improved.

[0037] Preferably, a temperature measuring unit is provided, which is configured to determine the thermal load of the laser-active medium by measuring the temperature of the active medium. In this way, the constant maintenance of the thermal load can be monitored. The temperature measuring unit can also be provided to regulate the power sources such that the thermal load is kept substantially constant.

[0038] In addition to the seed and pump power, external power sources can also be used to maintain the thermal load on the laser-active medium of the laser amplification system. Unlike the seed and pump power, these power sources have no direct influence on the laser system's output power, but they do have a direct influence on the thermal load. For the sake of accuracy, however, it should be noted that an indirect influence on the laser output power is also induced by the temperature change in the laser-active medium caused by the thermal load. This effect, however, will be neglected.

[0039] In particular, means for cooling, i.e., for heat dissipation from the laser-active medium, can be used. Accordingly, a cooling unit can be provided, which, as a heat sink, can reduce the thermal load of the laser-active medium. The corresponding cooling capacity of the cooling unit, which is dissipated from the medium, can be defined as quantified. For example, water cooling can be used, which is in thermal contact with the laser-active medium. Within the scope of the invention, any other method that achieves a cooling power P kühl for the laser-active medium. Furthermore, means for heating, i.e., for introducing heat into the laser-active medium, can be used. Accordingly, a heating element can be provided, which, as a heat source, can increase the thermal load of the laser-active medium. The corresponding heat output of the heating element, which is introduced into the medium, can be defined as P heizbe quantified.

[0040] For example, a heater can be used that is directed at the laser-active medium. Any other method that has a heating power P heiz for the laser-active medium.

[0041] Preferably, the power sources contributing to the thermal load of the laser-active medium may accordingly comprise the cooling power of a cooling unit configured to cool the laser-active medium and / or the heating power of a heating element configured to heat the laser-active medium, which have no direct influence on the laser output power of the output beam.

[0042] Preferably, the control unit is configured to adjust the heating power of the heating element and / or the cooling power of the cooling unit at constant seed power and changing pump power such that the thermal load on the laser-active medium is substantially constant over the laser output power range, or to adjust the heating power of the heating element and / or the cooling power of the cooling unit at constant pump power and changing seed power such that the thermal load on the laser-active medium is substantially constant over the laser output power range, or to adjust the effectively radiated pump power on the laser-active medium at maximum pump power and changing seed power by selectively coupling out pump power using an auxiliary resonator and / or an absorber such thatthat the thermal load on the laser-active medium is essentially constant over the range of laser output power.

[0043] To quantify the thermal load of the laser-active medium, a temperature measuring unit can also be provided that can measure the temperature of the laser-active medium. For example, the temperature measuring unit can be an infrared thermal camera directed at the active medium. Any other device suitable for measuring the temperature of the laser-active medium is also included within the scope of the invention.

[0044] The laser output power range over which the thermal load of the medium is kept constant can extend either over the entire laser output power achievable for the system or only over a portion of this range. In other words, the range can be any, while the beam quality is still maintained. To adjust the power sources contributing to the thermal load of the laser-active medium, in particular the power quantities P pump , P seed , kühl and Pheiz. ' st A control unit is provided which adjusts the power sources for thermal load maintenance according to suitable regulations.

[0045] In a particularly preferred embodiment, the control unit can be configured to control the seed power P seed and the pump power P kühlto simultaneously adjust the laser output power of the laser system over a range such that the thermal load on the laser-active medium is constant, with the heating power of the heating element and the cooling power of the cooling element being zero or constant. Particularly preferably, the cooling power can be constant while the heating power is zero, so that the thermal load on the laser-active medium is kept constant but as low as possible.

[0046] For the typical case where an increase / decrease in the pump power P pump also to an increase / decrease of the thermal load P th and an increase / decrease in seed performance P seed in turn leads to a decrease / increase in the thermal load P th , the services P pump and P seedbe increased / decreased simultaneously, so that the laser output power continuously increases / decreases while the thermal load remains constant. In this case, no additional adjustment of the heating and / or cooling power is necessary. In a preferred embodiment, however, the laser-laser-active medium is still cooled by the cooling unit with a constant cooling power in order to reduce thermally induced optical effects with a negative impact on beam quality, or even to prevent thermally induced destruction of the laser-active medium.

[0047] In an alternative embodiment, thermal load maintenance can also be achieved using the control unit at a constant seed power. Accordingly, only the pump power can be increased / decreased to continuously increase / decrease the laser output power. The resulting change in thermal load can be compensated by appropriately adjusting the heating power of the heating element and / or the cooling power of the cooling unit.

[0048] In a typical exemplary case, increasing / decreasing the pumping power at constant seed power would also lead to an increase / decrease in the thermal load, which can be compensated by a corresponding increase / decrease in the cooling power of the cooling unit and / or by a decrease / increase in the heating power of the heating element.

[0049] In a further alternative embodiment, the thermal load maintenance can be achieved by means of the

[0050] The control unit can also be used at constant pump power. Accordingly, only the seed power can be increased / decreased to continuously increase / decrease the laser output power. The resulting change in thermal load can be compensated by appropriately adjusting the heating power of the heating element and / or the cooling power of the cooling unit.

[0051] In a typical exemplary case, the increase / decrease in seed power at constant pump power would lead to a decrease / increase in thermal load, which can be compensated by a corresponding increase / decrease in the heating power of the heating element and / or by a decrease / increase in the cooling power of the cooling unit.

[0052] In a further embodiment, thermal load maintenance can be achieved by means of the control unit even at maximum (constant) pump power. Accordingly, only the seed power can be increased / decreased to continuously increase / decrease the laser output power. The resulting decrease / increase in the thermal load can be compensated by a suitable coupling of the radiated pump power using an auxiliary resonator, so that the thermal load in the laser-active medium remains constant. Instead of an auxiliary resonator, an absorber could also be used, which can compensate for the aforementioned decrease / increase in the thermal load.

[0053] The control unit can keep the power of the power sources contributing to the thermal load of the laser-active medium constant by manual adjustment and / or by a mathematical model and / or by an allocation table.

[0054] With manual adjustment of the power sources, the control unit can be implemented by a skilled operator of the laser system. This operator can adjust the powers contributing to the thermal load according to the aforementioned embodiments such that the thermal load remains constant over a range of laser output power.

[0055] Alternatively, a control unit can use a mathematical model to calculate the powers contributing to the thermal load such that the thermal load is constant over a range of laser output power. This requires a functional dependence of the thermal load P thThe power sources contributing to the thermal load must be known, in particular their dependence on seed and pump power. Such a mathematical model can be implemented, for example, via software in an electronic control unit, such as a computer, which is connected to the power sources contributing to the thermal load in the laser system and can control them accordingly. For example, given a given laser output power, the electronic control unit can specify the powers contributing to the thermal load through a real-time calculation in accordance with the aforementioned embodiments and keep them constant by controlling the power sources across the laser output power range.

[0056] Another alternative to thermal load maintenance by a control unit can be achieved through a predefined allocation table. The allocation table is characterized by the fact that, for a given laser output power, suitable values ​​are specified for the powers contributing to the thermal load, which lead to thermal load maintenance across the laser output power range. The allocation table can, for example, be precalculated or determined empirically.

[0057] For example, an electronic control unit can be used, such as a computer, which uses the pre-defined allocation table to control the power sources according to the aforementioned embodiments and thus keep the thermal load constant over the range of the laser output power.

[0058] The inventive control unit for thermal load maintenance on the laser-active medium over a range of laser output power maintains the thermally induced optical effects discussed above (in particular, the altered lensing effect of the laser-active medium) at a nearly constant level. Accordingly, the beam quality and beam parameters of the amplified output beam, in particular its waist diameter, waist position, and diffraction index, remain nearly constant, or at least more constant than in a laser system without thermal load maintenance.

[0059] In particular, the maximum relative fluctuation of said values ​​for the beam quality or beam parameters corresponds to a maximum of +10%, preferably +5%, particularly preferably +2%.

[0060] To quantitatively verify the beam quality and beam parameters, the laser amplification system can include a monitoring device for monitoring the beam quality and beam parameters of the amplified output laser beam. For example, the beam quality and beam parameters can be monitored using a CCD camera into which a portion of the output beam can be coupled. Alternative devices for measuring the beam quality and beam parameters are not excluded.

[0061] The above-described object is also achieved by a method for amplifying a seed laser beam with the features of claim 12. Accordingly, a method for amplifying a seed laser beam in a multipass amplifier comprising a laser-active medium is proposed to provide an amplified output beam. According to the invention, the thermal load on the laser-active medium, which is determined by at least two different power sources, is kept substantially constant over a range of the laser output power, wherein the thermal load on the laser-active medium is preferably kept constant within a range of +10%, preferably +5%, particularly preferably +2%.

[0062] Preferably, the power sources contributing to the thermal load of the laser-active medium comprise the seed power of a seed laser and the pump power of a pump laser, which are configured to irradiate the laser-active medium, wherein the seed power of the seed laser and the pump power of the pump laser are jointly adjusted over the range of the laser output power such that the thermal load on the laser-active medium is kept substantially constant.

[0063] Preferably, the thermal load on the laser-active medium is kept substantially constant by adjusting the powers of the power sources contributing to the thermal load of the laser-active medium by manual adjustment and / or by a mathematical model and / or by an allocation table.

[0064] Preferably, the beam quality and / or at least one beam parameter of the output beam, in particular defined by the waist diameter and / or the waist position and / or the diffraction index, is kept substantially constant, wherein the beam quality and / or the beam parameter preferably has a maximum relative fluctuation of +10%, preferably +5%, particularly preferably +2% over the range of the laser output power.

[0065] Short description of the figure

[0066] Preferred further embodiments of the invention are explained in more detail in the following description of the figure. It shows:

[0067] Figure 1 Schematic representation of a laser system with a multipass amplifier (multipass disk laser amplifier) ​​with different power sources contributing to the thermal load of the laser-active medium.

[0068] Detailed description of preferred embodiments

[0069] Preferred embodiments are described below with reference to the figure. Figure 1 schematically shows a laser system 1 comprising a multipass amplifier 2 in the form of a multipass disk laser amplifier. The multipass amplifier 2 comprises a laser-active medium 12 and serves to amplify laser light to provide an amplified output beam 14.

[0070] The laser system 1 has a seed laser 10 and a pump laser 20, whose associated seed laser beam 11 and pump laser beam 21 are coupled into the multipass amplifier 2.

[0071] The laser-active medium 12 can be provided as a disk laser. Disk lasers are solid-state lasers with a laser-active medium in the shape of a disk. Typically, these disks are coated on the back with a highly reflective layer and therefore also serve as a mirror. Thus, a laser beam striking the disk is guided twice through the laser-active medium due to the back reflection.

[0072] In general, the disk of a disk laser encompasses all geometric shapes whose lateral dimensions are significantly larger than their thickness. This enables, in particular, more efficient cooling of the laser-active medium 12, since a heat sink can be mounted on the side of the reflective layer across its relatively large lateral extent. In addition to this efficient cooling, the special shape of the laser-active medium 12 of a disk laser enables heat dissipation with a temperature gradient that is almost exclusively perpendicular to the lateral extent. This leads to a reduction in the thermal effects on the disk that occur during operation and can negatively impact beam quality.

[0073] Although a single irradiation of a laser-active disk with a rear-side reflective layer already results in a double passage of radiation through the laser-active medium, the absorption of incident laser light is typically still relatively low due to the thinness of the laser-active disk. To improve amplification, as already described above, so-called multipass amplifiers (2) are constructed, among other things. These multipass amplifiers guide a pump beam and a seed beam multiple times through the disk using suitable optical structures. However, the latter also results in greater thermal stress on the disk.

[0074] Despite cooling, this thermal load on the disk leads to undesirable thermally induced optical effects. This is primarily due to the relatively localized and inhomogeneous heating caused by the pump light in the area of ​​the disk irradiated by the pump light ("pump spot"). The associated local temperature changes and inhomogeneities lead to a changed lensing effect of the disk and thus to the aforementioned undesirable optical effects.

[0075] These effects include, among other things, a geometric curvature of the front side of the disk caused by the pump spot. This curvature causes a change in the propagation of the (seed) laser beam, thus altering the beam parameters and potentially reducing the beam quality.

[0076] Furthermore, the local temperature change on the disk leads to a change in the density of the laser-active material of the disk and thus to a change in the local refractive index of the disk material. This, in turn, is associated with a change in the optical properties of the disk, resulting in altered beam parameters and potentially a deterioration in beam quality.

[0077] Another effect concerns the generation of a thermal gas lens, which is induced by the heating of the surrounding gas in front of the laser-active disk. The changing density due to the increased temperature of the gas influences the local refractive index distribution in front of the disk, which in turn undesirably affects the propagation of the seed beam.

[0078] In the exemplary arrangement shown in Figure 1, a laser-active medium 12 in the form of a laser-active disk is mounted on the right-hand end of the multipass amplifier 2. The disk has a highly reflective layer 120 applied to the rear. Preferably, the laser-active medium 12 is in thermal contact with a cooling unit 30 on the rear side, so that heat induced by laser radiation can be dissipated. The cooling unit 30 can, for example, be a water cooling system.

[0079] In particular, the multipass amplifier 2 comprises suitable optical devices for geometrically folding the laser beams 11 and 21. In the example shown, the multipass amplifier 2 comprises a pump optics 3, which comprises, for example, a parabolic mirror 22 and retroreflectors 23. After the pump laser beam 21 is coupled into the multipass amplifier 2, the pump laser beam 21 is directed onto the parabolic mirror 22, which in turn reflects the pump laser beam 21 back onto the laser-active medium 12.

[0080] The pump laser beam 21 impinging on the highly reflective layer 120 is in turn directed onto the parabolic mirror 22, whereupon the beam 21 impinges on a retroreflector 23. The retroreflector 23 reflects the pump laser beam 21 back onto the parabolic mirror 22, so that another pass through the laser-active medium 12 takes place. A plurality of suitably arranged retroreflectors 23 can thus achieve numerous passes through the laser-active medium 12, thereby increasing the effective path traveled by the pump radiation 21 in the laser-active medium 12 and the associated effectively introduced absorbed pump power.

[0081] In a typical exemplary construction, the pump optics 3 can also have an end mirror (not shown in Figure 1) onto which the pump laser beam 21 impinges after multiple passages through the laser-active medium 12 and is arranged such that the pump beam propagates back the entire way via the laser-active medium 12, the parabolic mirror 22 and the retroreflector 23.

[0082] In a further optional embodiment, the pump optics 3 can also provide an auxiliary resonator 24 in the beam path of the pump laser beam 21 instead of an end mirror. This can be used to extract part of the pump power of the pump laser beam 21 and thus represents a possibility for adjusting the effectively introduced pump power into the laser-active medium 12.

[0083] In a further optional embodiment, the pump optics 3 can also provide an absorber (not shown in Figure 1) in the beam path of the pump laser beam 21 instead of an end mirror or an auxiliary resonator 24. This absorber can also be used to couple out part of the pump power of the pump laser beam 21 and thus represents a further adjustment option for the effectively introduced pump power into the laser-active medium 12.

[0084] Furthermore, in the exemplary embodiment presented, the multi-pass amplifier 2 has a mirror arrangement 13 comprising a plurality of individual mirrors 130 in a special arrangement. After the seed laser beam 11 is coupled into the multi-pass amplifier 2, the seed laser beam 11 is guided through the mirror arrangement 13 onto the laser-active medium 12. The highly reflective layer 120 on the back of the laser-active medium 12 reflects the coupled seed laser beam 11 back onto the mirror arrangement 13 and strikes one of the mirrors 130. The mirrors 130 are designed such that the seed laser beam 11 is reflected back onto the disk 12. By using a plurality of suitably designed mirrors 130, numerous passages of the seed laser beam 11 through the disk 12 can be achieved, whereby the laser gain is increased with each passage.Accordingly, after the last passage of the seed laser beam 11 through the laser-active medium 12, the amplified output laser beam 14 is coupled out of the mirror arrangement 13 and from the multipass amplifier 2.

[0085] The multiple passes of the seed laser beam 11 and the pump laser beam 21 through the laser-active medium 12 primarily ensure the heat input into or heat removal from the laser-active medium 12. Accordingly, the heat input or the thermal load in the laser-active medium 12 can be influenced by adjusting the pump power of the pump laser 20 and the seed power of the seed laser 10, as discussed in the section illustrating the invention.

[0086] In principle, within the scope of the invention, further laser power sources which, in addition to the seed laser 10 and the pump laser 20, apply laser power to the laser-active medium 12 and thereby introduce heat into the laser-active medium 12 and / or remove heat from the laser-active medium 12 can also be taken into account for adjusting the thermal load of the laser-active medium 12.

[0087] Further power sources that can lead to a change in the heat input of the laser-active medium 12 are, for example, the cooling power of a cooling unit 30, for example a water cooling system, and / or the heating power of a heating element 40, for example a radiant heater, which is directed onto the laser-active medium 12.

[0088] In contrast to the laser powers, i.e., the seed power and the pump power, the cooling power and the heating power have no direct influence on the laser output power of the amplified output beam 14. An indirect influence can arise from the thermally induced optical effects on the laser-active medium 12 or from the temperature change of the laser-active medium 12, which, however, have a comparatively insignificant influence on the output power. Any other power sources that can lead to heat input into or heat removal from the disk 12 and do not have a direct influence on the output power of the output beam 14 can be used within the scope of the invention to maintain the thermal load of the laser-active medium 12.

[0089] To measure the thermal load of the laser-active medium 12, a temperature measuring unit 50 can also be provided, for example in the form of an infrared thermal camera. By determining the temperature of the laser-active medium 12 via the thermal camera, conclusions can be drawn about the thermal load of the laser-active medium 12.

[0090] A control unit 60 is provided, which is configured to adjust two or more of the aforementioned thermal power sources that influence the thermal load of the laser-active medium 12 such that the thermal load on the laser-active medium 12 is substantially constant across a range of the laser output power of the amplified output laser beam 14. A substantially constant thermal load on the laser-active medium 12 is understood herein to mean, in particular, a thermal load that exhibits a relative fluctuation of the values ​​of +10%, preferably ±5%, particularly preferably +2%.

[0091] As a result, a substantially constant beam quality and / or at least one substantially constant beam parameter of the output laser beam 14 can be achieved across the laser output power range. In particular, key parameters representing the beam quality and the beam parameters, such as the waist diameter, the waist position, and the diffraction index, can be kept substantially constant across a range of laser output powers using the thermal load maintenance according to the invention.

[0092] A substantially constant development of beam quality and / or a beam parameter over a range of laser output power can be quantified by a relative fluctuation of those values ​​of +10%, preferably ±5%, particularly preferably +2%. These values ​​relate in particular to the waist diameter, the waist position, and the diffraction index of the output beam 14.

[0093] The connecting lines connected to the control unit 60, which, among other things, connect the seed laser 10, the pump laser 20, the cooling unit 30, the heating element 40, and the temperature measuring unit 50 to the control unit 60, illustrate in Figure 1 that these elements can be suitably adjusted. Accordingly, the power sources contributing to the thermal load of the laser-active medium 12 in the embodiment illustrated in Figure 1 are, for example, manually adjusted such that, for any laser output power of the amplified output beam 14, the thermal load of the laser-active medium 12 is kept substantially constant. The constant maintenance of the thermal load can be checked by an operator using the temperature measuring unit 50 for each output power of the laser output beam 14.

[0094] Alternatively, the control unit 60 can be provided in the form of a computer, a computer program product, or a programmable controller. Using such an electronic control, the thermal load can be maintained, for example, via a mathematical model implemented in the control unit 60. The latter incorporates all power sources contributing to the thermal load of the laser-active medium 12 and calculates the theoretically expected thermal load from the values. The desired laser output power and a fixed thermal load value, which remains unchanged over any laser output power range, serve as input parameters. The remaining power sources are then calculated such that the thermal load remains constant for any laser output power.

[0095] Alternatively, electronic control can also be achieved via an assignment table. For this purpose, an empirically determined or pre-calculated table is generated which, for a specified thermal load on the laser-active medium 12, provides suitable values ​​for the power sources contributing to the thermal load of the laser-active medium 12 in order to achieve the desired thermal load value. These values ​​are created for a range of laser output powers of the output beam 14 so that the electronic control unit 60 can access the values ​​when the desired output power is set and adjust the corresponding power sources in such a way that the thermal load remains constant over any range of the laser output power.For example, for a desired laser output power, a seed power and a pump power are specified and adjusted accordingly for the seed laser 10 and the pump laser 20.

[0096] Compared to the previously described alternative using a mathematical model, a predefined allocation table for thermal load maintenance can be advantageous, since in the latter case, the electronic control unit 60 does not have to perform real-time calculations, but can simply access the precalculated values ​​from the table. Accordingly, advantages can result in terms of the speed with which the control unit 60 can adjust the performance parameters appropriately.

[0097] The control unit 60 can also regulate the pump power of the pump laser 20 and the seed power of the seed laser 10 to a predetermined temperature value based on the values ​​measured by the temperature measuring unit 50 via a control, for example a PID control.

[0098] To check the beam quality and the beam parameters of the laser output beam 14, a monitoring device 70 is also provided, for example in the form of a CCD camera. The monitoring device 70 is preferably configured to measure the waist diameter, the waist position, and the diffraction index of the amplified output beam 14. For this purpose, for example, a portion of the output beam 14 can be coupled out (e.g., via a partially transparent mirror) in order to be coupled into the monitoring device 70 and measured. Further measurements of beam parameters are not excluded. Furthermore, a connection between the monitoring device 70 and the control unit 60 is provided, whereby, for example, a skilled operator or an electronic control unit such as a computer can check the beam quality.In particular, the monitoring device 70 is intended within the scope of the invention to represent a possibility of checking the constancy of the beam quality and the beam parameters over any range of the laser output power.

[0099] Particularly preferably, the beam quality and the beam parameters should be essentially constant over the entire range of the laser output power due to the thermal load maintenance of the laser-active medium 12.

[0100] Concrete and exemplary methods for thermal load maintenance over a range of laser output power based on the laser system 1 shown in Figure 1 have already been described in detail in the section illustrating the invention.

[0101] In a preferred embodiment of the laser system 1 according to Figure 1, a reduction in the thermal load can be achieved by the pump laser 20 generating a blurred, extended pump spot on the laser-active medium 12, whereby the beam quality of the amplified output beam 14 is improved by reducing undesirable thermal effects on the laser-active medium 12.

[0102] In a likewise preferred embodiment of the laser system 1 with a multipass amplifier, the amplification efficiency can be increased by ensuring that the diameter of the pump beam 21 on the laser-active medium 12 corresponds at least to the diameter of the seed beam 11 on the laser-active medium 12 or, if multiple seed beams impinge on the laser-active medium 12, to the diameter of the seed spot resulting from the seed beams. Specifically, the diameter of the pump beam 21 can have an extension that is a factor of 1 to 1.5 larger than the diameter of the seed beam 11 or the diameter of the seed spot.

[0103] Where applicable, all individual features presented in the embodiments may be combined and / or exchanged without departing from the scope of the invention.

[0104]

[0105] 1 laser system

[0106] 2 multipass amplifiers (multipass disk laser amplifiers)

[0107] 3 Pump optics

[0108] 10 Seed laser source

[0109] 11 Seed laser beam

[0110] 12 Laser-laser active disc of the multi-pass amplifier

[0111] 120 Highly reflective layer

[0112] 13 Mirror arrangement of the multipass amplifier

[0113] 130 mirrors of the mirror arrangement

[0114] 14 Output laser beam

[0115] 20 pump lasers

[0116] 21 Pump laser beam

[0117] 22 parbolic mirrors for pump optics

[0118] 23 Retroreflector

[0119] 24 Auxiliary resonator

[0120] 30 cooling units

[0121] 40 heating element

[0122] 50 temperature measuring unit

[0123] 60 control unit

[0124] 70 Beam quality monitoring device

Claims

Claims 1. Laser system (1) comprising a multipass amplifier (2), in particular a multipass disk laser amplifier, for amplifying laser light to provide an amplified output beam (14) and a control unit (60), wherein the multipass amplifier (2) comprises a laser-active medium (12), characterized in that the control unit (60) is configured to keep the thermal load on the laser-active medium (12), which is determined by at least two different power sources, substantially constant over a range of the laser output power of the output beam (14).

2. Laser system (1) according to claim 1, characterized in that the range of the laser output power extends over the entire range of the achievable laser output powers of the multi-pass amplifier (2) or extends only over a part of this range and / or the control unit (60) is configured to keep the thermal load on the laser-active medium constant in a range of +10%, preferably +5%, particularly preferably +2%.

3. Laser system (1) according to claim 1 or 2, characterized in that the control unit (60) is configured to keep the thermal load on the laser-active medium (12) substantially constant over the range of the laser output power by adjusting the powers of the power sources contributing to the thermal load of the laser-active medium (12) by manual adjustment and / or by a mathematical model and / or by an assignment table.

4. Laser system (1) according to one of the preceding claims, characterized in that a seed laser (10) and a pump laser (20) are provided as power sources contributing to the thermal load on the laser-active medium (12), which are configured to irradiate the laser-active medium (12), wherein the control unit (60) is configured to jointly adjust a seed power of the seed laser (10) and a pump power of the pump laser (20) over the range of the laser output power such that the thermal load on the laser-active medium (12) is substantially constant.

5. Laser system (1) according to one of the preceding claims, characterized in that the control unit (60) is configured to control the beam quality and / or at least one The beam parameters of the output beam (14), preferably defined by the waist diameter and / or the waist position and / or the diffraction index, are to be kept substantially constant over the range of the laser output power, wherein the beam quality and / or the beam parameter preferably exhibits a maximum relative fluctuation of +10%, more preferably +5%, and most preferably +2% over the range of the laser output power. A laser system (1) according to any one of the preceding claims, characterized in that a monitoring device (70) is provided for monitoring the beam quality and the beam parameters, in particular the waist diameter, the waist position, and the diffraction index of the amplified output laser beam (14). A laser system (1) according to any one of the preceding claims, characterized in that one or the pump laser (20) is configured to generate an extended, blurred pump spot on the laser-active medium (12).Laser system (1) according to one of the preceding claims, characterized in that one or the pump laser (20) is configured to generate a pump spot on the laser-active medium (12), the diameter of which is a factor of 1 to 1.5 larger than the diameter of the seed beam (11) or, if several seed beams (11) strike the laser-active medium (12), the diameter of the seed spot resulting from the seed beams (11) on the laser-active medium (12). Laser system (1) according to one of the preceding claims, characterized in that a temperature measuring unit (50) is provided, which is configured to determine the thermal load of the laser-active medium (12) by measuring the temperature of the laser-active medium (12).Laser system (1) according to one of the preceding claims, characterized in that the power sources contributing to the thermal load of the laser-active medium (12) comprise the cooling power of a cooling unit (30) configured for cooling the laser-active medium and / or the heating power of a heating element (40) configured for heating the laser-active medium (12), which have no direct influence on the laser output power of the output beam (14). Laser system (1) according to one of the preceding claims, characterized in that the control unit (60) is configured to… to adjust the heating power of the heating element (40) and / or the cooling power of the cooling unit (30) at constant seed power and changing pump power such that the thermal load on the laser-active medium (12) is essentially constant over the range of laser output power, or to adjust the heating power of the heating element (40) and / or the cooling power of the cooling unit (30) at constant pump power and changing seed power such that the thermal load on the laser-active medium (12) is essentially constant over the range of laser output power, or to adjust the effectively radiated pump power onto the laser-active medium (12) at maximum pump power and changing seed power by selectively coupling out pump power using an auxiliary resonator (24) and / or an absorber such that the thermal load on the laser-active medium (12) is essentially constant over the range of laser output power.Method for amplifying a seed laser beam in a multipass amplifier (2) comprising a laser-active medium (12) to provide an amplified output beam (14), characterized in that the thermal load on the laser-active medium (12), which is determined by at least two different power sources, is kept substantially constant over a range of laser output power, wherein the thermal load on the laser-active medium (12) is preferably kept constant in a range of +10%, preferably +5%, and particularly preferably +2%.A method according to claim 12, characterized in that the power sources contributing to the thermal load of the laser-active medium (12) comprise the seed power of a seed laser (10) and the pump power of a pump laser (20), which are configured to irradiate the laser-active medium (12), wherein the seed power of the seed laser (10) and the pump power of the pump laser (20) are jointly adjusted over the range of the laser output power such that the thermal load on the laser-active medium (12) is kept substantially constant. A method according to one of claims 12 and 13, characterized in that the thermal load on the laser-active medium (12) is kept substantially constant by adjusting the powers of the power sources contributing to the thermal load of the laser-active medium (12). Power sources are adjusted by manual setting and / or by a mathematical model and / or by an assignment table. A method according to any one of claims 12 to 14, characterized in that the beam quality and / or at least one beam parameter of the output beam (14), in particular defined by the waist diameter and / or the waist position and / or the diffraction coefficient, are kept substantially constant, wherein preferably the beam quality and / or the beam parameter has a maximum relative fluctuation of +10%, more preferably +5%, and most preferably +2% over the range of the laser output power.

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