WO2008077384A1

WO2008077384A1 - Laterally pumped solid-state laser

Info

Publication number: WO2008077384A1
Application number: PCT/DE2007/002285
Authority: WO
Inventors: Konrad Altmann
Original assignee: Las-Cad Gmbh
Priority date: 2006-12-22
Filing date: 2007-12-19
Publication date: 2008-07-03
Also published as: DE102007004083A1; DE112007002753A5

Abstract

The invention relates to a solid-state laser comprising a block (20) of a laser material, said block (20) being composed of at least three layers (1, 2, and 3). At least one laser-active layer (2) is disposed between two inactive layers (1, 3). The solid-state laser further comprises at least one pump source (14, 18) for emitting at least one pump beam (4) in order to pump the laser-active layer. The laser also comprises at least two reflectors (5 and 6) which are inclined relative to one another and by means of which the reflected pump beam (7) is redirected into the doped layer (2) so as to intersect the incident pump beam (4) within the doped layer, resulting in a solid-state laser that has an improved beam quality and is more efficient.

Description

Laterally pumped solid state laser

In contrast to end-pumped laser systems, it is much more difficult for laterally pumped laser systems to achieve an optimal overlap between pump light and laser mode. In conventional systems, this is made up of one or more pump light sources, e.g. B. laser diode arrays coming pump light either directly from the side in a cylindrical crystal irradiated or focused with lenses in the axis region of the crystal. The so-called "slow axis" of the diode arrays in these arrangements is usually parallel to the crystal axis. Since the intensity of the pump light after entering the crystal decreases exponentially as a result of the absorption and therefore a considerable part of the pump power is absorbed in the immediate vicinity of the entry site, the laser mode is formed along most of the crystal axis and thus the distance between the entry point of the crystal Pump beam and the laser mode at the currently realizable crystal diameters at least the order of 1 mm, the overlap of the density distribution of the absorbed pump light with the laser mode is poor and thus the efficiency of the laser low.

In order to provide an improvement, an arrangement has been proposed in the patent application AK 102 42 701.1, in which the pump beam is irradiated in a laser-active plate, is reflected after passing through the plate on the opposite side and is then directed to a mirror through which the pump beam is redirected back to the original impact area. In the additional application AKZ 102 42 701.1 has also been proposed that the plate consists of three layers, with only the middle layer is laser active, whereby the density distribution of the absorbed pump light can be concentrated more. However, this arrangement has the disadvantage that there are asymmetries in the density distribution of the absorbed pumping light, which leads to a reduction in the efficiency and to a deterioration in the quality of the laser beam.

By contrast, the object of the invention is to provide a solid-state laser with improved steel quality and higher efficiency. This object is achieved by a solid state laser according to claim 1 in a surprisingly simple manner. Advantageous developments of the invention are the subject of the dependent claims. According to the invention, a collimated pump beam coming from a light source, for example a laser diode array, is irradiated into a laser material which is composed of at least 3 layers, wherein a laser-active layer is located between two inactive layers. After passing through the laser-active layer, the pump beam is deflected back into the laser-active layer by at least two mutually inclined reflectors so that it crosses the originally incident beam there.

The formation according to the invention, which is preferred in comparison to the state of the art, of two mutually inclined reflection surfaces, causes reflected and irradiated pump beams to intersect within the laser-active medium, which ensures that the latter overlap optimally. The thickness of the laser-active layer is preferably chosen so that as little pump power as possible is absorbed outside the region in which the two pump beams intersect. This leads to a solid-state laser with high efficiency and beam quality.

The development of the invention according to claim 2 provides that the pump beam is deflected by another reflector approximately in itself, so that it passes through the laser-active medium in the region of the intersection of the previous beam path twice. As a result, a high percentage of the radiated pump power is spatially concentrated in the crossing region of the pump beams, which further increases the efficiency of the laser. In contrast to previous pumping arrangements, a very good overlap between the density distribution of the absorbed pumping light and the laser beam is achieved in this way.

The development of the invention according to claim 4 provides that the required according to claim 1 layers of laser active and inactive material using the technique of diffusion or adhesive-free bonding are interconnected. This technique makes it possible to join workpieces with finely polished surfaces without adhesive in such a way that no optical interference occurs at the boundary layer of the two materials. For this purpose, it is advantageous if the surfaces of the two workpieces are flat, as characterized in claim 3.

As an alternative to this, techniques are already being used today to pull crystals with layers of different doping out of the melt, which is achieved by changing the Concentration of the laser-active atoms in the melt during the process of crystal growth is realized, as characterized in claim 5. In these processes, there is no sharp interface between doped, ie, laser active and undoped regions, which, however, tends to have a positive effect on the technology of the present invention. A further alternative according to claim 5 is to produce the laser-active and inactive layers of ceramic laser material.

In a preferred embodiment of the present invention, the mirrors on which the pump beam is reflected are formed as surfaces of the laser crystal. This also has the advantage that their positioning and inclination during the production of the crystal can be realized very accurately. This eliminates the need for external mirror and costly adjustment effort.

According to a further improvement, the reflective surfaces of the laser crystal which form the reflectors are antireflective or highly reflective coated. Alternatively or additionally, the arrangement of the components can be selected such that the pump beam impinges at an angle on the surfaces, which is greater than the critical angle of total reflection, whereby a coating is unnecessary.

The development according to claims 10 and 11 provides, according to the current technology of pumping light sources, that the emitting surface of the pumping light source is significantly longer in one preferred direction than in the other, or that a series of small emitting surfaces are arranged along a preferential direction, such as this eg in the case of laser diode arrays.

The development according to claim 12 provides that the block of the laser material is cooled on two lateral surfaces 12 and 13, which can be realized technically with very little effort.

The development according to claim 13 provides that the coming of a diode array pump beam is narrowed in the direction of the "slow axis" by focusing. As a result, the density distribution of the absorbed pump light is even more concentrated, which is particularly important for 3-level laser systems because of the reabsorption of the laser beam occurring there. The development according to claim 14 provides that from three or more diode arrays coming pumping rays radiate into a block of laser material. These diode arrays are arranged so that the "slow axis" of the two outer diode arrays is inclined relative to the "slow axis" of the middle diode array in such a way that the pump beams in the laser-active layer at least partially overlap.

The development according to claims 15 to 20 provides that the reflector 8 is replaced by two mutually inclined reflectors 21 and 22, whereby it is achieved that the pump beam is not reflected back at this point in itself, but is deflected so that he traverses the area of the crossing point of 11 again coming from another direction. Only after the pump beam has passed through the area of the crossing point a third time, it is almost by the reflector 23 is directed back into itself.

The invention will be described below by way of example with reference to preferred exemplary embodiments in conjunction with the drawing. In this show:

Fig. 1 shows a section through an inventive arrangement transversely to the longitudinal extent of the laser rod, which is formed as a block of laser active and inactive layers, wherein a penetrating from above in the block pumping beam is reflected several times on the surfaces of the block, so that the pump beam passes through a small area of the doped layer four times.

2 shows a section through an arrangement according to the invention parallel to the "slow axis" of a laser diode array whose beam is narrowed in the direction of the "slow axis" by a lens and irradiated into a block which, analogous to FIG. 1, is composed of laser-active and inactive layers is

Fig. 3 shows a section through an inventive arrangement parallel to the "slow axis" of three laser diode arrays, wherein the "slow axis" of the outer diode arrays is inclined relative to the "slow axis" of the middle diode array to irradiation in overlapping areas of the laser active material to enable. Fig. 4 shows a section through an inventive arrangement transverse to the longitudinal extent of the laser rod according to Fig. 1 of another embodiment of the invention, in which a from above in the block penetrating pump beam is reflected several times on the surfaces of the block, so that the pump beam a small area passes through the doped layer six times.

Fig. 5 shows a section through an inventive arrangement similar to that shown in Fig. 1, but different from Fig. 1 laser rod and pumping arrangement were rotated so that the laser beam 10 extends in the plane of the intersecting pumping beams through the intersection point 11.

Fig. 6 shows a section through an inventive arrangement in which the laser rod shown in Fig. 5 is introduced together with a non-linear optical element 27 into a resonator with the end mirrors 25 un d 26.

1 shows the laser material block 20 of a solid-state laser transverse to the beam direction. The block 20 consists of a thin plate 2 of laser active material, which is arranged between two plates 1 and 3 of inactive material. The lateral boundary surfaces of the laser rod need not necessarily be flat or parallel to each other, as shown in the drawing. The plates are z. B. connected by means of diffusion bonding so that occurring at the boundary layers optical interference z. B. absorption or scattering of the laser beam are negligible. The four edges of the block are chamfered, creating there flat boundary surfaces 5, 6, 8 and 9, which are inclined to each other. In this case, a surface 9 as an irradiation surface and three further surfaces 8, 5 and 6 are formed as reflection surfaces. The first two reflecting surfaces 6, 5 are inclined relative to each other and arranged with respect to the laser-active layer 2 so that they face the incident pumping beam. A pumping beam 4, the z. B. comes from a diode array and preferably by a cylindrical lens in the direction of the so-called. "Fast axis", which is arranged parallel to the plane collimated occurs from obliquely above through the surface 9, which is antireflectively coated for the pumping light in the Block 20 a. The pump beam then traverses the laser-active layer 2, is reflected back on the surface 6 in the direction of the surface 5 and from there into the laser-active layer in such a way that it intersects the originally incident pump beam at the point of intersection 11. Then, the pump beam is formed by the reflector 8 formed surface, which for the pump radiation highly reflective coated, approximately inwardly directed back so that it passes through the laser-active layer in the region of the intersection point 11 twice more. As a result, a substantial absorption of the pump beam in a very small area in the vicinity of the crossing point 11 is achieved, unlike conventional transverse pumping arrangements. By means of the intersecting pumping beams, it is achieved that the center of gravity and the maximum of the resulting density distribution of the absorbed pumping light are largely brought into coincidence with the maximum of the beam profile of the laser beam 10 which is perpendicular to the pumping beams. As a result, a laser beam of very high beam quality is generated. With high doping of the laser-active layer, typically 90% and more of the radiated pump power can be absorbed. The transverse extent of the region thus pumped can typically be less than 1 mm. The angle of incidence α of the pumping beam on the surfaces 5 and 6 is expediently chosen so that it is greater than the critical angle of total reflection, in which case a coating of these surfaces is unnecessary, which would otherwise have to be applied if the angle is below the critical angle. The heat developed by absorption of the pumping light is dissipated through the side surfaces 12 and 13 of the block by solid state contact with a cooling element or a cooling medium of high thermal conductivity. The reflecting surface 8 can be replaced by an external mirror.

In order to further increase the absorbed pumping power, it is proposed in the embodiment of FIG. 4 to replace the reflector 8 by two mutually inclined surfaces 21 and 22, which ensures that the pumping beam is not reflected back into itself but is deflected in such a way he traverses the area of the crossing point of the 11 coming from another direction again. Only after the pumping beam has passed through the area of the crossing point a third time, it is almost by the surface formed as a reflector 23 is directed back into itself. It is thereby achieved that the pumping beam traverses the region of the crossing point 11 six times, whereby an almost complete absorption of the pumping power is achieved. The angle of incidence of the pumping beam on the reflectors 21 and 22 are preferably chosen so that they are greater than the critical angle of total reflection. If the geometry of the assembly does not allow this, an anti-reflective coating of the surface 21 and / or the surface 22 may be applied. To simplify the technical production of the crystal rod, it is proposed that the surfaces 9 and 22 lie in one plane. In order to be able to realize this, it may be necessary for the pump beam 4 to occur inclined to the surface normal to the surface 9. In order to further increase the density of the absorbed pump light distribution, it is proposed to focus the pump beam in the direction of the so-called "slow axis". Such a modification is of particular interest for 3-level lasers in order to avoid reabsorption losses of the laser beam in the laser-active medium. A corresponding inventive arrangement will be described below with reference to FIG. 2. Coming from a diode array 14, already collimated by a cylindrical lens in the direction of the "fast axis" pump beam is focused by a further cylindrical lens 15 so that the beam profile in the direction of the "slow axis" becomes narrower and in the block 20 of FIG irradiated. The inclined surfaces are not shown for reasons of clarity. In contrast to Fig. 1, however, this block is now shown rotated by 90 °, so that the deflection of the pump beam within the block is now perpendicular to the plane. In order to avoid thermal deformation of the upper and lower boundary surfaces of the block, it is additionally proposed to add blocks 17 of undoped material at the upper and lower boundary surfaces.

In order to increase the pump power absorbed in the laser active layer, it is further proposed to irradiate into the block with three or more diode arrays. As shown in FIG. 3, this can be achieved by arranging further diode arrays 18 next to a central diode array 14 whose beam direction is inclined with respect to the beam direction of the middle diode array with respect to the "slow axis". This ensures that the beams of the middle and the two outer diode arrays penetrate into overlapping regions of the laser-active material 2 and are absorbed there.

Alternatively, it is proposed to increase the absorbed pump power by arranging several diodes in the direction of the "slow axis" and irradiating them into a correspondingly long block or into a plurality of blocks arranged next to one another.

Of course, instead of two reflection surfaces, a plurality of reflection surfaces can also be used. In addition, the reflective surfaces do not necessarily have to be formed on the block 20 of the laser material, although this is advantageous.

In FIG. 5, unlike FIG. 1, the laser rod and the pump arrangement are rotated by 90 ° so that the laser beam 10 runs in the plane of the intersecting pump beams through its intersection point 11. To effectively use this arrangement, it is advantageous to use a diode array instead of a diode array. Diode arrays to use an all-round collimated pump beam, which comes for example from a fiber. In addition, preferably the angle at which the pump beams intersect, as small as possible and the thickness of the doped layer slightly larger than in Fig. 1 z. B. typically 2 mm selected to achieve the highest possible power density of the absorbed in the laser beam pump radiation. The achievable in this way power density of the absorbed pump radiation in the crossing region of the pump beams is very high and is far above that which is achieved with currently known pumping arrangements.

Preferably, therefore, this arrangement is used in particular in 3-level lasers, optical parametric oscillators and frequency multiplication, z. To generate a blue laser beam using an Nd: YAG crystal. It is proposed to introduce for this purpose inside or outside the resonator optically non-linear material in the laser beam.

Fig. 6 shows a typical arrangement in which the pumped laser rod 20 is incorporated together with a block 27 of non-linear optical material in a resonator with the end mirrors 25 and 26. In an analogous manner, the block 27 can also be introduced outside the resonator into the laser beam. To make the frequency multiplication as effective as possible, it is proposed to use this optically non-linear material, which is periodically poled, whereby quasi-phase matching conditions (QCM) are generated. This use of periodically poled materials in accordance with the present invention achieves a much more efficient frequency conversion than with the use of ordinary nonlinear crystals. Eligible materials are z. Periodically poled LiNbO ₃ (PPLN), Li-TaO ₃ (PPLT) or KTP (PPKTP).

Claims

claims:

A solid-state laser comprising a block (20) of a laser material consisting of at least three layers (1, 2 and 3), wherein at least one laser-active layer (2) is located between two inactive layers (1,3), and at least one Pump source (14,18) for delivering at least one pumping beam (4) for pumping the laser-active layer, characterized in that at least two mutually inclined reflectors (5 and 6) are provided, through which the reflected pumping beam (7) into the laser-active layer ( 2) is deflected back so that it crosses the incident pumping beam (4) within the doped layer.

2. Solid state laser according to claim 1, characterized in that a third reflection surface (8) is provided, through which the at the first two reflectors (5, 6) from below reflected pump beam (7) is reflected back almost in itself.

3. Solid-state laser according to claim 1 or 2, characterized in that the interfaces between the laser-active and the inactive layers are flat.

4. Solid-state laser according to one of the preceding claims, characterized in that the laser-active and inactive layers z. B. are connected to each other by means of diffusion or adhesive bonding.

5. Solid state laser according to one of the preceding claims, characterized in that the laser-active and inactive layers continuously merge into one another z. B. be realized by changing the concentration of the laser-active atoms in the melt during the process of crystal growth, or that the laser-active and inactive layers of ceramic laser material.

6. Solid-state laser according to one of the preceding claims, characterized in that the deflectors provided for deflecting the pump beam reflectors (5, 6, 8) by a suitable arrangement and / or coating of surface regions of the laser material (20) can be realized.

7. Solid state laser according to one of the preceding claims, characterized in that the laser material consists of a block (20) composed of three layers (1, 2 and 3), wherein only the middle layer (2) is laser active, and that the edges (5, 6, 8, 9) of this block are ground flat to form radiating and reflecting surfaces, the pumping beam (4) penetrating the radiating surface (9) in the block passing through the laser active layer, from the first reflecting surface (6) is reflected to the second reflection surface (5) and is directed from here towards a third reflection surface (8), crossing the original incident beam within the laser active layer, and then at the third reflection surface (8) approximately in itself is deflected back.

8. Solid-state laser according to one of the preceding claims, characterized in that the irradiation surface (9) for the pump beam antireflective, the third reflection surface (8), however, is coated highly reflective.

9. Solid-state laser according to one of the preceding claims, characterized in that the pumping beam at the first and second reflection surface (5 and 6) is reflected at an angle α to the solder on these surfaces, which is greater than the critical angle of total reflection of the laser beam at the Inside of the reflective surface.

10. Solid state laser according to one of the preceding claims, characterized in that the emitting surfaces of the pump light sources are significantly longer in one direction than in the other, or that a series of small emitting surfaces are arranged along a preferred direction.

11. Solid state laser according to one of the preceding claims, characterized in that the beam emanating from the pump light sources beam is collimated or focused such that its beam profile in a preferred direction is substantially wider than in the other, and that the reflectors (5,6,8 ) are arranged for the deflection of the beam so that it crosses within the laser-active layer along the beam axis of the laser beam generated.

12. Solid-state laser according to one of claims 7 to 10, characterized in that the crystal is cooled on at least one of the outer surfaces, through which neither a pump nor a laser beam passes, is cooled ..

13. Solid-state laser according to one of the preceding claims, characterized in that a lens (15) is arranged in the pump beam (4) coming from a diode array (14) through which the beam is additionally focused in the direction of the "slow axis".

14. Solid state laser according to one of the preceding claims, characterized in that three or more diode arrays (14, 18) are provided, which radiate into the block (20), and that the "slow axis" of the two outer diode arrays (18) relative to the "Slow axis" of the middle diode array (14) is inclined.

15. Solid state laser according to one of the preceding claims, characterized in that the reflector 8 is replaced by two further mutually inclined reflectors (21 and 22), by which the reflected pump beam (7) is deflected back into the doped layer (2) in that it traverses the latter again from another direction in the region of the crossing point of the pumping beams (4 and 7).

16. Solid state laser according to claim 15, characterized in that the deflected by the reflectors (21 and 22) pump beam (24) after passing through the laser active layer (2) by a reflector (23) is reflected back almost in itself.

17. Solid-state laser according to claim 15 or 16, characterized in that the block (20) is suitably ground, so that two surfaces (21 and 22) are formed, at which the pump beam is reflected from the inside.

18. Solid state laser according to one of claims 15 to 17, characterized in that the pump beam impinges on the surface 21 and / or the surface 22 at an angle which is smaller than the critical angle of total reflection.

19. Solid state laser according to one of claims 15 to 18, characterized in that the surface 21 and / or the surface 22 is coated antireflective for the pumping beam and that the surface 23 is coated for the pumping beam highly reflective.

20. Solid-state laser according to one of claims 15 to 19, characterized in that the inlet surface (9) for the pumping beam and the reflection surface (22) lie in one plane.

21. Solid state laser according to one of claims 1 to 9 or 12, characterized in that the pump beam is collimated on all sides and that the laser beam is in a plane with the intersecting pump beams and passes through the intersection point.

22. Solid state laser according to claim 21, characterized in that inside or outside of the laser resonator non-linear optical elements (27) are mounted.

23. Solid state laser according to claim 22, characterized in that the nonlinear optical elements are periodically poled, whereby quasi-phase matching conditions (QCM) are generated.