EP3117494A1

EP3117494A1 - Solid-state laser

Info

Publication number: EP3117494A1
Application number: EP15716389.0A
Authority: EP
Inventors: Daniel Kopf
Original assignee: Individual
Current assignee: Individual
Priority date: 2014-03-14
Filing date: 2015-03-10
Publication date: 2017-01-18
Also published as: US20170117681A1; AT515674A4; AT515674B1; WO2015135011A1

Abstract

A solid-state laser has an amplifying laser medium (1) for producing a laser beam (4) and a pump device (13, 14) that has at least one laser diode (16) and that produces a pump radiation (5) that impinges on a first lateral face (6) of the laser medium (1), which lateral face is parallel to a z axis and parallel to a y axis that is at right angles to the z axis. On a second lateral face (23), which is opposite the first lateral face (6), the laser medium (1) is cooled by a heat sink (22). The length of a y-1 /e² region (33) of the pump radiation (5) is shorter than the length of the first lateral face (6) of the laser medium (1) in the direction of the y axis, wherein the y-1/e² region (33) of the pump radiation (5) denotes a section of the y axis over which the intensity of the pump radiation (5) on the first lateral face (6) of the laser medium (1) has a value that is more than the maximum intensity of the pump radiation (5) on the first lateral face (6) of the laser medium (1) divided by e2. The length of a y cooling region (26), said length denoting a section of the y axis, over which a cooling strip (24) extends is less than 70% and greater than 50% of the length of a y pump region (28) of the laser medium (1), wherein the y pump region (28) denotes a section of the y axis over which 80% of the total power of the pump radiation (5) that is absorbed by the laser medium (1) is absorbed and at the two ends of which the intensity of the pump radiation (5) is of equal magnitude.

Description

Solid-state lasers

The invention relates to a solid-state laser with a reinforcing laser medium for generating a laser beam and at least one

Laser diode having pumping means, from which a pump radiation is generated, which impinges on a first side surface of the laser medium, which is parallel to a z-axis and parallel to a perpendicular to the z-axis y-axis, wherein in a direction perpendicular to the z -Axis and x axis perpendicular to the y-axis, the laser beam is parallel to the z-axis through the laser medium, the laser medium on a parallel to the z-axis and parallel to the y-axis and the first side surface opposite the second side surface is cooled by a heat sink with which it is in thermal communication, wherein the length of a y-cooling region is smaller than the length of the second side surface of the laser medium in the direction of the y-axis, wherein the y-cooling region of the laser medium covers a portion of the y-axis. Axis, over which a cooling strip extends, over which the laser medium on the second side surface with the Heat sink is thermally connected, wherein the length of a y- 1 / e ² region of the pump radiation is smaller than the length of the first side surface of the laser medium in the direction of the y-axis and the first side surface of the

Laser medium extends beyond the y-1 / e ² region of the pump radiation in both y-axis directions, the y-1 / e ² region of the pump radiation designating a y-axis portion over which the intensity of the pump radiation at the first side surface of the laser medium has a value greater than the maximum intensity of the pump radiation at the first side surface of the laser beam

Laser medium divided by e ² is. Side-pumped solid-state lasers, especially those with a zig-zag

Geometry (= laser with "Zig-Zag slab gain medium") is widespread and NdrYAG is the best-known laser medium for nanosecond lasers due to the relatively high Gain, a storage time of 250 s and the availability in large slabs (> 10cm) at comparatively low cost. In addition to Nd: YAG, reinforcing laser media (= laser-active materials) include Nd: glass, Nd: VANADAT or Yb: YAG.

For pumping solid state lasers laser diodes are used more recently instead of flash lamps. For example, a solid-state laser pumped in this manner is described in Errico Armandillo and Callum Norrie: "Diode-pumped high-efficiency high-brightness Q-switched ND: YAG slab laser", OPTICS LETTERS, Vol. 22, No. 15, August 1, 1997 , Pages 1 168 to 1 170. Laser diodes have particular advantages in terms of efficiency, pump efficiency and service life

Laser diodes combined in a common component. Barren (English "bar") are on a strip-shaped chip several laser diodes

(= Single emitter) arranged and electrically operated in parallel and mounted on a common heat sink. The individual emitters of such a bar each emit a laser beam having a much larger emission angle, for example +/- 33 °, in the direction of a so-called "fast-axis" than in one

perpendicular to this standing direction of a so-called "slow-axis", in which the radiation angle is, for example +/- 5 ° In laser diode stacks, which are also referred to as laser diode stack, several such bars with their broad sides and / or narrow sides next to each other In order to supply the laser radiation emitted by such a laser diode stack, which strongly diverges, correspondingly bundled, to the amplifying laser medium, different types of optical systems have been used, for example a microlens in the form of a cylindrical lens is known in front of the laser diodes of a respective ingot The cylinder axes are aligned in the direction of the "slow-axis", so that the strong divergence in the direction of the "fast-axis" is reduced, for example, to less than 1 °, thereby the subsequent optics for imaging the laser radiation in the amplifying laser medium considerably simplified. From WO 2014/019003 A1 it is known to use a common cylindrical mirror whose cylinder axis is aligned in the direction of the "fast axis" and which focuses the light of all laser diodes in the direction of the "slow axis", or to use such a cylindrical lens. In this case, a high pumping efficiency can be achieved with a compact design.

Various other optical systems for bundling the light emitted from laser diode bars, e.g. for pumping solid-state lasers are known, for example, from US 201 1/00641 12 A1, US 2007/0064754 A1 or JP P2004-96092 A.

One problem with solid-state lasers is the thermal effects that result from the formation of thermal lenses and / or the generation of stress-induced

Birefringence (stress-induced birefringence) leads. For example, Nd: YAG shows relatively strong such thermal effects. Stress-induced birefringence, especially in radially symmetric pump geometries, e.g. in the formation of the laser medium in the form of a cylindrical rod, a polarization rotation of parts of the beam profile and, subsequently, to a loss of a polarizing element in the resonator, resulting in active Q-switching with electro-optical

Elements (Pockels cells) can lead to a significant loss. This effect of stress-induced birefringence is minimized or almost absent in the zig-zag plate laser. The formation of a thermal lens can also be minimized in the plane of the zigzag-shaped course of the laser beam (x-z plane) in the zig-zag disk laser, but in the perpendicular thereto

Direction (direction of the y-axis) results in a non-vanishing positive thermal lens. This makes it necessary to install compensating optics, which then have a compensating effect only for a specific power range. With an Nd: YAG laser, this is already required from about 1 watt average output power, depending on how high the requirements are

Beam symmetry and astigmatism are. A solid-state laser of the type mentioned at the beginning is Donald B. Coyle et al .: "Efficient, reliable, long-lifetime, diode-pumped Nd: YAG laser for space-based vegetation topographical altimetry", APPLIED OPTICS, Vol. 27, September 20, 2004, pages 5236-5242. The laser medium, which is in the form of a "slab", that is prismatic, has a longitudinal axis extending in the direction of a z-axis.

Axial extends, and has side surfaces which are parallel to perpendicular to each other and perpendicular to the z-axis x and y axes. The laser beam passes through the laser medium zig-zag in the x-z plane. The pumping by means of laser diodes is effected by a parallel to the y- and parallel to the z-axis first side surface and the laser medium is cooled at the opposite and lying parallel to the first side surface second side surface. To one

To achieve more uniform temperature distribution in the laser medium, whereby the thermal lens can be reduced relative to the y-direction, the cooling is not over the entire extent of the second side surface in the direction of the y-axis but only over a strip with a reduced width in contrast Direction of the y-axis. This is done by one level of the second

Side surface adjacent heatsink reached. But it still comes to

Forming a, albeit reduced, thermal lens, which is compensated by the use of a negative cylindrical lens in the resonator.

The incident on the first side surface pump radiation has a substantially gauss-shaped profile in this laser. Considering the two points at which the intensity of the pump radiation with respect to the y-axis has fallen to a value of 1 / e2 of the maximum intensity, the length of this y-1 / e2 region of the pump radiation is smaller than the length the first side surface of the laser medium in the direction of the y-axis. The laser medium is thus pumped with respect to the y-direction is not substantially uniform to its edge but only in a more or less central area. This has the consequence that the limited extent of the laser medium in the y-direction does not act as an aperture for the laser radiation. If, on the other hand, the laser medium were pumped over its entire extent in the y-direction, the formation of a thermal lens in the y-direction could be substantially avoided, however The aperture then caused by the laser medium in the y-direction would have negative effects on the quality of the laser beam emitted by the solid-state laser.

The object of the invention is to provide an improved solid state laser of the type mentioned. This is achieved by a

Solid state laser with the features of claim 1.

In the solid-state laser according to the invention, the length of a y-cooling region of the laser medium is less than 70% and greater than 50% of the length of a y-pumping region of the laser medium, with the y-pumping region in both y-axis directions over the y-cooling region extends beyond. As already mentioned, the y-cooling region of the laser medium denotes a section of the y-axis, over which a cooling strip extends, via which the laser medium is connected to the heat sink. The y-pumping region of the laser medium denotes a section of the y-axis, over which 80% of the total absorbed by the laser medium power of the pump radiation is absorbed and at both ends of the intensity of the pump radiation is equal. Thus, the majority of the heat is introduced into the laser medium via the y-pumping region. It has been found that with such a design of a solid-state laser, a thermal lens in the y-direction can be at least largely or completely avoided. This can be explained by a central recess in the

Temperature distribution, as will be explained in more detail below.

In particular, this succeeds in conjunction with a beam profile of the pump radiation, which is more likely than in the direction of a Gaussian profile in the direction of a rectangular beam profile.

The fact that the profile of the pump radiation is more rectangular than Gaussian means that advantageously in the solid state laser according to the invention the difference between the length of the y-1 / e ² region of the pump radiation and the length of a y half-value region of the pump radiation is less than half is the difference between the length of a y-1 / e ² region and the length of a y-half-value region an imaginary beam of the same wavelength having a Gaussian profile and of which the length of a y-half-value region is equal to the length of the y-half-value region of the pump radiation and its radiant energy is equal to

Radiation energy of the pump radiation is. As already mentioned above, the y-1 / e ² region of the pump radiation designates a section of the y-axis, via which the intensity of the pump radiation at the first side surface of the laser medium has a value which is more than the maximum intensity of the pump radiation at the first side surface of the laser medium divided by e ² , that is more than about 13.5%. The y half-value region of the pump radiation designates a section of the y axis over which the intensity of the pump radiation at the first side surface of the laser medium has a value that is more than half the maximum intensity of the pump radiation at the first side surface of the laser medium. Analogously, the y-1 / e ² region and the y half-value region of the imaginary Gaussian beam are defined.

This means that the pump radiation has a much stronger edge drop than is the case with a Gaussian profile. The pump radiation is thus more approximate to a Gaussian profile to a rectangular profile. In particular, an advantageous solid-state laser can be formed in accordance with the invention, in which the laser beam (= the laser mode) runs zig-zag through the laser medium, in a plane perpendicular to the y-axis. It can thus be provided with a laser, for example, in one

Power range from 0 to more than 5 W average power one in

Essentially emitted symmetrical beam. In particular, a beam with a beam divergence of <250 rad (half-angle divergence) in both transverse directions and a quality factor Μ ^Λ 2 of <5, preferably <3, can be achieved over the entire power range. With the invention, for example, a pulsed solid-state laser, in particular Nd: YAG Zig-Zag laser, with> 50mJ energy and a beam with a low beam divergence (eg <250 rad Halbwinkeldivergenz) and good

Beam quality (eg Μ ^Λ 2 <5 or <3) are provided, regardless of the repetition rate, so for example, both single-pulse operation (singleshot) as well as at 50Hz or 100Hz (corresponding to 5W average power), the closely bounded

Beam parameters met.

Further advantages and details of the invention are explained below with reference to the accompanying drawings. In this show:

Fig. 1 is a highly schematic representation of an embodiment of a laser according to the invention;

2 shows an oblique view of the pumping device and the laser medium and the heat sink in greater detail;

3 is an oblique view of the radiation source of the pumping device;

4 shows a section through the laser medium and a part of the pumping device and the heat sink in the y-x-plane (section line AA of Fig. 5).

5 shows a side view of the laser medium and a part of the heat sink and of the pumping device in the direction of the y-axis (viewing direction B in FIG. 4);

6 shows a side view of the laser medium on the pumped first side surface in the direction of the x-axis (viewing direction C in FIG. 4), wherein a "pumped area" is illustrated by hatching and the cooling strip applied to the opposing second side area is shown in dotted lines is;

7 is a diagram for comparing the intensity distribution of

Pump radiation on the first side surface of the laser medium with respect to the y direction in comparison to the intensity distribution of an imaginary beam with a Gaussian profile;

Fig. 8 is a diagram in which the refractive index D of the formed thermal lens with respect to the y-direction in dependence on the length of the y-cooling region is shown. A possible embodiment of a solid-state laser according to the invention is shown schematically in FIG. It is a solid-state laser whose amplifying (active) laser medium 1 consists of a crystalline or glassy (amorphous) solid. For example, the amplifying laser medium may be Nd: YAG, Nd: glass, Nd: vanadate, Yb: YAG, Er: YAG or Ho: YAG.

The amplifying laser medium 1, which can also be referred to as a laser-active material, is arranged in a resonator, the components of which are explained in more detail below.

The amplifying laser medium 1 is formed prismatic, so it is a disk laser (= "slab laser"). Although by the emission of the

1 laser beam 4 formed in Fig. 1 zig-zag through the reinforcing laser medium 1 is shown running, he could also run straight through this. The inlet and outlet surfaces 2, 3 for the emitted laser medium 1, the resonator passing through the laser beam 4 are advantageously arranged in the Brewster angle, but this is not absolutely necessary. The reinforcing laser medium 1 is side-pumped, as is known. The pumping radiation 5 pumping the amplifying laser medium 1 thus does not enter the laser medium through the entrance and exit surfaces 2, 3, but rather through a first side surface 6. It is at an angle to the entry and exit surfaces 2, 3.

In particular, the pump radiation 5 impinges on the first side surface 6 at least substantially centrally.

The resonator comprises an end mirror 7 and a coupling-out mirror 8 in order to couple out the laser beam 4a emitted by the laser. The resonator shown is folded once, for which purpose a reversing prism 9 is arranged in the beam path. The folding could also be omitted or the resonator could be folded several times. Other folding mirrors could be provided. In order to form a Q-switch in the beam path of the resonator in the illustrated embodiment, a polarizer 10, a Pockels cell 1 1 and a quarter-wave plate 12 are arranged. The laser beam 4a emitted by the laser is thus pulsed. To form pulses, for example, other than electro-optical Q-switches, in particular acousto-optic Q-switches

be provided.

One of the arranged in the beam path mirror, in particular the

Auskoppelspiegel 8 or the end mirror 7 is preferably as known as

Gradientenspiegel formed whose reflectivity changes over the mirror surface and this is greater in a central region than in an edge region. As a result, the beam profile of the laser beam can be influenced, for example, to achieve a more rapid edge drop of the intensity, and / or the

Beam quality of the laser beam can be improved.

The pumping of the amplifying laser medium by means of a

Pumping device having a radiation source 13, which comprises a plurality of laser diodes. The optics 14 of the pumping device to those of the

Radiation source 13 delivered laser radiation advantageously the reinforcing laser medium 1, is indicated only schematically in Fig. 1.

The radiation source 13 is preferably in the form of a laser diode stack and an example of this is shown in FIG. The laser diode stack comprises a plurality of adjacent bars 15, each having a plurality of laser diodes 16 spaced apart in one direction.

The rays 17 emitted by the laser diodes 16 have a divergence more than three times as large as in the direction of an axis perpendicular to the beam axis of the respective beam 17, which is referred to as "fast-axis"

Direction of a perpendicular to the beam axis and perpendicular to the "fast-axis" standing axis, which is referred to as "slow-axis". For example, the beam angle (= the divergence) relative to the "fast-axis" +/- 33 ° (ie 66 ° Opening angle of the radiation cone) and the radiation angle relative to the "slow-axis" +/- 5 °.

The bars 15 are held on a carrier 20, cf. Fig. 2, which is mounted on a, for example, water-cooled, heat sink 21.

The optics 14 of the pumping means is formed in the embodiment of an optical component having a reflective cylindrical surface 14b, at which the emitted from the laser diode 17 of the radiation source and entering through the entrance surface 14a in the optical component beams are reflected. These

In this case, the cylinder surface 14b has a collecting effect with respect to the "slow-axis." With respect to the "fast-axis", the divergence of the beams 17 remains, only eflection or multiple reflections occur on side surfaces 14c of the optical component to cover the area of the optical component Limit radiation in this direction.

The optic 14 of the pumping device forming optical component can

For example, as shown in Fig. 2, be formed of several interconnected by bonding, made of transparent material parts. The optical component is attached to a carrier 18.

Through the exit surface 14d, which is separated here to ensure the total reflection of the laser beam 4 in the laser medium in the zig-zag-shaped course by a small gap from the laser medium 1, passes from the

Pumping radiation emitted as pump radiation to the first

Side surface 6 of the laser medium 1.

Such a pumping device is known from WO 2014/019003 A1 mentioned in the introduction to the description. An insert of a pumping device with optics, which has a cylindrical mirror or a cylindrical lens, wherein the

Cylinder axis is aligned in the direction of the "fast axis", is advantageous

Pumping the laser medium 1 could also be designed in other ways

Pumping devices are used. The first side surface 6 of the laser medium 1, through which the pump radiation 5 enters, is parallel to the z axis and parallel to the y axis, ie parallel to the yz plane. In particular, the z-axis forms the longitudinal axis of the laser medium 1.

The zigzag-shaped course of the laser beam 4 through the laser medium 1 lies in a plane that is parallel to the x-axis and parallel to the z-axis, ie parallel to the x-z plane.

Viewed in the direction of the x-axis, ie in relation to the projection in the y-z plane, the laser beam 4 (= the laser mode) runs in the direction of the z-axis through the laser medium 1 (= parallel to the z-axis). The x, y, and z axes form a Cartesian coordinate system.

The laser medium 1 is cooled by a heat sink 22. The cooling takes place on a second side surface 23 of the laser medium 1, which is parallel to the first

Side surface 6 is located, so also is parallel to the y-z plane. For this purpose, the second side surface 23 is connected to the heat sink 22. The connection with the

Heatsink 22 via a cooling strip 24. In addition, preferably on the second side surface 23 of the laser medium 1 is an (not shown in the figures) optical coating. This will be on the second

Side surface 23 applied to ensure that on the one hand, the total reflection of the zigzag-guided laser beam is maintained and the other

remaining pump radiation is reflected back into the crystal and does not impinge on the cooling strip 24. Such coatings are quite common in page-pumped zig-zag lasers. In addition, for attaching the cooling strip 24 to the second side surface 23 of the laser medium 1 or the optical coating applied thereon, it is favorable to provide a bonding material (in particular, an adhesive or a solder). The cooling strip 24 consists here of a material differing from the laser medium 1 and heat sink 22 material, which over the Connecting material on the second side surface 23 of the laser medium 1 or the optical coating applied thereto is applied. The cooling strip 24 could also be formed by a strip-shaped elevation of the heat sink 22 and would thus consist of the same material from which the rest

Heat sink 22 is made. Also in this case, the cooling strip could be applied directly or via a connecting material (in particular an adhesive or a solder) to the laser medium 1 or the optical coating applied thereto.

The thermal conductivity of the material of the cooling strip 24 is preferably greater than 5 W / mK. Away from the region over which the cooling strip 24 extends, the second side surface 23 is separated from the heat sink 22 by an air gap 25. In this area, instead of the air gap, at least in regions, a solid material could be provided, which has a heat conductivity which is less than half the thermal conductivity of the material of the

Cooling strip 24.

Preferably, the thermal conductivity of the material present outside the cooling strip 24 between the laser medium 1 and the heat sink 22 (which may in particular be gaseous or solid) is less than 2 W / mK, more preferably less than 1 W / mK.

The second side surface 23 faces the first side surface 6, i. in

As viewed in the x-axis direction, the side surfaces 6, 23 overlap at least partially, preferably at least substantially (i.e., over more than 50% of their areas). It is preferable that the first and second side surfaces 6, 23 extend over the same range with respect to the direction of the y-axis.

The laser medium 1 preferably has a prismatic shape. The base and top surfaces 37, 38 are conveniently parallel to the xz plane, which is a straight prism, in particular a parallelepiped. By way of example, the extent of the laser medium 1 with respect to the y direction is 5 mm to 15 mm, in the exemplary embodiment 8 mm. The extension of the

Laser medium in x-direction is for example 2mm to 8mm, in

Embodiment 4mm. The extension of the laser medium in the z-direction is for example 20mm to 80mm, in the embodiment about 40mm.

The cooling strip 24 is for example replaced by a graphite foil, e.g. 125 μιτι or 250 μηη formed thick. The heat conduction of such a graphite foil can be for example 16 W / mK. The connection of the graphite foil to the second side face 23 of the laser medium 1 and the heat sink 22 can be effected for example by gluing and / or jamming. In another possible embodiment, the cooling strip 24 is formed by an indium strip. Such an indium strip, for example, with the second side surface 23 of the laser medium 1 and the

Heat sink 22 are soldered. The solder is indium or AgSn (e.g., 96.5% Sn and 3.5% Ag) or the harder AuSn.

For example, the heat sink 22 may be made of copper tungsten having a coefficient of thermal expansion similar to Nd: YAG, e.g.

Copper tungsten with 85% W and 15% Cu.

Other materials for the cooling strip 24 and / or the heat sink 22 are conceivable and possible. For example, the cooling strip 24 could also be formed by a strip-shaped elevation of the heat sink 22, which is in thermal contact with the second side surface 23 of the laser medium 1, for example by being pressed or soldered to the second side surface 23.

The cooling strip 24 extends in the y-direction over a portion 26 of the y-axis, which is referred to in this document as the y-cooling region. Furthermore, the cooling strip 24 extends with respect to the z-direction over a portion 27 of the z-axis, which is referred to in this document as the z-cooling region. In this document, a y-pumping section 28 is referred to as the y-pumping region, via which 80% of the total power of the pumping radiation absorbed by the laser medium 1 is absorbed. The y-pumping region 28 is selected so that the intensity of the pumping radiation 5 at the two ends of the y-pumping region is the same. Furthermore, in this document, the z-pumping area is the section 29 of the z-axis, over which 80% of the total of the laser medium. 1

absorbed power of the pump radiation 5 is absorbed. The z-pumping area 29 is chosen so that at its two ends the intensity of the

Pump radiation is the same size. The y and z pumping areas are illustrated in FIG. 6 by a hatched area. The part of the volume of the

Lasermediums 1, which forms a cube, from the opposite

Side surfaces are formed by those parts of the first and second side surfaces 6, 23 of the laser medium 1, which are covered by the area hatched in Fig. 6, in this document referred to as "pumped volume" of the laser medium 1. In the pumped volume of the laser medium 1, therefore, the main part, namely 80%, of the absorption of the power of the pump radiation, so that according to the main part of the heat introduced by the pump radiation heat is introduced into the pumped volume of the laser medium 1.

Accordingly, the suggestions of the laser medium 1 on the

Inversion level mainly, namely 80%, in the pumped volume. The pumped volume can thus be determined from the inversion density.

The inversion density can be measured in particular by fluorescence images. For example, for this purpose, the heat sink 22 can be removed and take the recording of fluorescence images through the second side surface 23, wherein the

Pumping radiation 5 is shielded by a filter.

The pumped volume thus extends in the direction of the x-axis over the extent of the laser medium. 1 In the z-axis direction, the expansion of the pumped volume is preferably more than 50% of the expansion of the

Laser medium 1 in the z-axis direction and less than 90% of the expansion of the laser medium 1 in the z-axis direction. The expansion of the pumped volume lies in the direction of the y-axis

preferably in the range of one third to two thirds of the extent of the

Laser medium 1 in the direction of the y-axis.

The pumped volume is preferably in a central region of the laser medium 1 relative to the direction of the z-axis and to the direction of the y-axis.

The incident on the first side surface 6 pump radiation 5 has a relation to a beam with a Gaussian distribution substantially closer to a rectangular profile intensity distribution. FIG. 7 shows the distribution 35 of the intensity I of the pump radiation with respect to the y-axis. The maximum value of the intensity is 11. The zero point of the y-axis is placed at the position of the maximum value of the intensity. For comparison, the distribution 36 of the intensity of an imaginary beam of the same wavelength is plotted with a Gaussian profile having the same half-width, the maximum value of the intensity being at the zero point of the y-axis. The maximum value of the intensity is 12 here. The radiation energy of the imaginary ray, that is to say the area enclosed by the distribution 36, is equal to the radiation energy of the pump radiation.

The section 31 of the y axis, via which the intensity of the pump radiation 5 is more than half the maximum intensity of the pump radiation at the first side surface of the laser medium 1, is referred to in this document as the y half-value region of the pump radiation. The length of this section 31 thus corresponds to the half-width of the intensity profile of the pump radiation 5. Analogously, the y-

Half-range of the imaginary beam defined and the corresponding portion of the y-axis, which coincides with the section 31, in Fig. 7 with the

Reference numeral 32 denotes. In Fig. 7, the locations on the y-axis are further indicated, at which the intensity of the pump radiation 5 and the imaginary beam has dropped to a value which is 1 / e ² (ie about 3.5%) of the maximum value , The y-1 / e ² region of the Pump radiation 5 correspondingly designates the section 33 of the y-axis, via which the intensity of the pump radiation at the first side surface 6 of the

Laser medium 1 has a value which is more than the maximum intensity of the pump radiation at the first side surface 6 of the laser medium 1 divided by e ² . Analogously, the y-1 / e ² region of the imaginary ray is defined and the corresponding section of the y-axis is denoted by the reference symbol 34 in FIG.

The difference between the length of the y-1 / e ² region 33 of the pump radiation 5 and the length of the y half-value region 31 of the pump radiation 5 can be read off from FIG. 7 for the present exemplary embodiment by approximately 0.85 mm. The difference between the length of the y-1 / e ² region 34 and the length of the y half-value region 32 of the imaginary beam with the Gaussian profile can be read off from FIG. 7 at approximately 2.6 mm. This difference is thus less than half as large for the pump radiation 5 as for the imaginary beam with the Gaussian profile.

Furthermore, the length of the y-1 / e ² region 33 of the pump radiation 5 is smaller than the length of the first side surface 6 of the laser medium 1 in the direction of the y-axis, with the first side surface 6 of the laser medium 1 in both directions of the y Extends beyond the y-1 / e ² region of the pump radiation, preferably the same distance. For example, the length of the y-1 / e ² region 33 of the pump radiation 5 is 4 mm, while the length of the first side surface 6 of the laser medium 1 in the direction of the y-axis is 8 mm. As can be seen from FIG. 6, the length of the y-cooling region is smaller than the length of the laser medium 1 in the direction of the y-axis. However, the length of the y-cooling region is also smaller than the length of the y-pumping region, as will be explained below.

In FIG. 8, measured values are entered as black squares which represent the

Dependence of the refractive power D of the formed thermal lens in

Reflect the dependence on the length I of the y-cooling zone. If the y-cooling area extends over the entire y-dimension of the laser medium 1, then lies the refractive index of the thermal lens with respect to the y-direction with the operating parameters used in the experimental arrangement at more than 1 m-1. With

Reducing the expansion of the y-kiev range, the refractive index initially decreases slowly, whereby it is at a length I of the y-cooling range of 3mm, which is thus significantly smaller than the length of the y-pumping range of 4mm to a value of just 0.5m- 1 has dropped. In a further reduction of the length I of the y-cooling region, the refractive power of the thermal lens further decreases and is already negative at a length I of the y-cooling range of 2mm. At another

Reducing the length I of the y-cooling region, the thermal lens becomes highly negative, e.g. with a refractive power of -3m-1 at a length of the y-cooling region of 1 mm.

In the diagram of FIG. 8 are also values of a calculation as stars

registered, which reflect the obtained measurements well. The formation of a negative thermal lens with small expansions of the y-cooling range can be achieved by the formation of a central recess in the

Temperature profile can be explained about the y-pumping range.

By a suitable choice of the size of the y-cooling region can thus be achieved with respect to the y-direction disappearing or almost disappearing thermal lens. The length of the y-cooling region is chosen to be less than 70% and greater than 50% of the length L of the y-pumping region.

The y-pumping region extends in both directions of the y-axis beyond the y-cooling region, preferably equidistant, i. the y-cooling area is located centrally in the y-pumping area with respect to the y-direction.

Conversely, the length of the z-cooling region 27 of the laser medium 1 is advantageously greater than the length of the z-pumping region 29 of the laser medium 1. The extent of the z-cooling region in both directions of the z-axis beyond the z-pumping region will be so great chosen that inhomogeneities of Temperature distribution in the pumped volume of the laser medium 1 to the ends of its extension in the direction of the z-axis are kept as small as possible.

The beam profile of the forming laser mode is advantageously at least largely adapted to the profile of the excitation by means of the pump radiation 5 with respect to the y-direction, in particular by using a suitable

Gradientenspiegels. The beam profile of the laser beam 4 relative to the y-direction should thus be clearly opposite to a Gaussian profile in the direction of a

Rectangular profiles have shifted intensity distribution.

Analogous to the pump radiation 5, a y-half-value range and a y-1 / e ² range of the laser beam 4 in the laser medium 1 and when leaving the laser medium 1 can be defined. The y-1 / e ² region of the laser beam thus represents a section of the y-axis, over which the intensity of the laser beam 4 has a value which is more than the maximum intensity of the laser beam 4 divided by e ² . The y-half-value region of the laser beam 4 designates a section of the y-axis, over which the intensity of the laser beam 4 has a value which is more than half the maximum intensity of the laser beam 4. Specifically, the laser beam 4 is formed so that the difference between the length of the y-1 / e ² region of the laser beam and the length of the y half-value region of the laser beam is less than half the difference between the length of the y -1 / e ² range and the length of the y-half-value range of an imaginary beam of the same wavelength having a Gaussian profile and its length of the y-half-value range equal to the length of the y-half-value range of the laser beam 4 and its radiant energy is equal to the radiant energy of the laser beam 4.

For example, one, in particular pulsed, Nd: YAG laser with an average power of> 2 W and a beam divergence of <250 rad (half-angle divergence) in both transverse directions and an M ^A 2 of <5, or even <3, can be provided by the invention be without in the external laser beam 4a or also in the resonator symmetry and astigmatism compensating optics must be installed.

Legend

to the reference numbers:

1 amplifying laser medium 25 17 Laser beam

2 entrance surface 18 carriers

3 exit surface 20 carrier

4, 4a laser beam 21 heat sink

5 pump radiation 22 heat sink

6 first side surface 30 23 second side surface

7 end mirror 24 cooling strips

8 Auskoppelspiegel 25 air gap

9 Reversing prism 26 y cooling area

10 polarizer 27 z cooling area

1 1 Pockels cell 35 28 y pumping area

12 Lamda-Vietel plates 29 z pumping area

13 radiation source 31 y half-power range

14 optics 32 y half-power range

14a entrance surface 33 y-1 / e ² area

14b cylindrical surface 40 34 y-1 / e ² region

14c side surface 35 distribution

14d exit surface 36 distribution

15 bars 37 base area

16 laser diode 38 top surface

Claims

claims

1. Solid-state laser with a reinforcing laser medium (1) for generating a laser beam (4) and at least one laser diode (16) having pumping means (13, 14) from which a pump radiation (5) is generated, which on a first side surface (6 ) of the laser medium (1) which is parallel to a z axis and parallel to a y axis perpendicular to the z axis,

the laser beam (4) running parallel to the z-axis through the laser medium (1) in a direction of an x-axis perpendicular to the z-axis and perpendicular to the y-axis,

wherein the laser medium (1) is cooled by a heat sink (22) with which it is in thermal communication, on a second side face (23) lying parallel to the z-axis and parallel to the y-axis and opposite the first side face (6),

wherein the length of a y-cooling region (26) is smaller than the length of the second side surface (23) of the laser medium (1) in the y-axis direction,

wherein the y-cooling region (26) of the laser medium (1) denotes a section of the y-axis, over which extends a cooling strip (24), via which the laser medium (1) on the second side surface (23) with the heat sink (22 ) is thermally connected,

wherein the length of a y-1 / e ² region (33) of the pump radiation (5) is smaller than the length of the first side surface (6) of the laser medium (1) in the y-axis direction and the first side surface (6) of the Laser medium (1) extends beyond the y-1 / e ² region (33) of the pump radiation (5) in both directions of the y-axis,

wherein the y-1 / e ² region (33) of the pump radiation (5) designates a section of the y axis over which the intensity of the pump radiation (5) at the first side surface (6) of the laser medium (1) has a value who is more than the maximum intensity of the pump radiation (5) at the first side surface (6) of the laser medium (1) divided by e ² is

characterized in that the length of the y-cooling region (26) of the

Laser medium (1) is less than 70% and greater than 50% of the length of a y-pumping region (28) of the laser medium (1) and the y-pumping region (28) in both directions of the y-axis over the y-cooling region ( 26), wherein the y-pumping region (28) denotes a portion of the y-axis, over which 80% of the total absorbed by the laser medium (1) power of the pump radiation (5) is absorbed and at both ends of the intensity of the pump radiation (5) is the same size.

2. Solid-state laser according to claim 1, characterized in that the difference between the length of the y-1 / e ² region (33) of the pump radiation (5) and the length of a y-half-value region (31) of the pump radiation (5) is less than half the difference between the length of a y-1 / e ² region (34) and the length of a y-half-wave region (32) of an imaginary equal wavelength beam having a Gaussian profile and of which Length of a y-half-value region (32) is equal to the length of the y-half-value region (31) of the pump radiation (5) and its radiation energy is equal to the radiation energy of the pump radiation (5),

wherein the y-half-value region (31) of the pump radiation (5) designates a section of the y axis over which the intensity of the pump radiation (5) at the first side surface (6) of the laser medium (1) has a value which is more is the half of the maximum intensity of the pump radiation (5) at the first side surface of the laser medium (1), the y-1 / e ² region (34) of the imaginary beam denotes a portion of the y-axis, above which the intensity of the imaginary beam on the first side surface (6) of the

Laser medium (1) has a value that is more than the maximum intensity of the imaginary beam at the first side surface (6) of the laser medium (1) divided by e ² and the y half-value range (32) of the imaginary beam a portion of Y axis denotes, over which the intensity of the imaginary beam on the first side surface (6) of the laser medium (1) a Value which is more than half the maximum intensity of the imaginary ray at the first side surface (6) of the laser medium (1).

3. Solid-state laser according to claim 1 or 2, characterized in that a z-cooling region (27) of the laser medium (1) is greater than a z-pumping region (29) of the laser medium (1),

wherein the z-cooling region (27) of the laser medium (1) denotes a portion of the z-axis, over which extends the cooling strip (24), via which the laser medium (1) on the second side surface (23) with the heat sink (22 ) is thermally connected,

wherein the z-pumping region (29) denotes a portion of the z-axis, over which 80% of the total of the laser medium (1) absorbed power of the pump radiation (5) is absorbed and at both ends of the intensity of the pump radiation (5) of equal size is.

4. Solid state laser according to one of claims 1 to 3, characterized in that the cooling strip (24) has a thermal conductivity of more than 5 W / mK.

Solid-state laser according to one of claims 1 to 4, characterized in that apart from the region over which the cooling strip (24) extends, an air gap (25) between the second side surface (23) of the laser medium (1) and the heat sink (22 ) is located.

Solid-state laser according to one of claims 1 to 5, characterized in that the laser medium (1) has a prismatic shape, wherein the edges, the extension of the first side surface (6) of the laser medium (1) in

Limit the direction of the y-axis on both sides and the edges that the

Limit extension of the second side surface (23) of the laser medium (1) in the direction of the y-axis on both sides, parallel to the z-axis.

7. Solid state laser according to one of claims 1 to 6, characterized in that the laser medium (1) is arranged in a resonator.

8. solid-state laser according to one of claims 1 to 7, characterized in that the laser beam (4) passes through the laser medium (1) zig-zag running and in this case lies in a plane which is perpendicular to the y-axis.

9. Solid-state laser according to one of claims 1 to 8, characterized in that the laser beam (4) in the laser medium (1) and / or at its exit from the laser medium (1) has a StrahSprofil in which the difference between the length of a y -1 / e ² region of the laser beam (4) and the length of a y half-value region of the laser beam (4) is less than half the difference between the length of a y-1 / e ² region and the length of a y-half-value range of an imaginary beam of equal wavelength having a Gaussian profile and of which the length of a y-half-value range is equal to the length of the y-half-value range of the laser beam (4) and its

Radiant energy is equal to the radiant energy of the laser beam (4), the y-1 / e ² region of the laser beam (4) designating a portion of the y-axis over which the intensity of the laser beam (4) has a value greater than the maximum intensity of the laser beam (4) divided by e ² is

wherein the y-half-value region of the laser beam (4) denotes a section of the y-axis over which the intensity of the laser beam (4) has a value which is more than half the maximum intensity of the laser beam (4),

wherein the y-1 / e ² region of the imaginary ray denotes a portion of the y-axis over which the intensity of the imaginary ray has a value greater than the maximum intensity of the imaginary ray divided by e ² ,

wherein the y-half-value range of the imaginary ray designates a portion of the y-axis over which the intensity of the imaginary ray Value which is more than half the maximum intensity of the imaginary beam.

Solid-state laser according to one of claims 1 to 9, characterized in that the amount of the refractive power of a laser medium (1) formed in the operation of the solid-state laser thermal lens with respect to the y-axis is less than 0.5m-1.