WO1996037021A1

WO1996037021A1 - Diode laser-pumped solid state laser

Info

Publication number: WO1996037021A1
Application number: PCT/EP1996/001251
Authority: WO
Inventors: Keming Du
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 1995-05-19
Filing date: 1996-03-22
Publication date: 1996-11-21
Also published as: DE19518177A1

Abstract

A diode laser-pumped solid state laser with at least one rod or disc-shaped solid body (9) as the active medium and with diode lasers (10) from which the emitted radiation (11) is beamed into the solid body, wherein there are reflecting components (8) to guide and/or shape the beam, in which the solid body (9) is surrounded radially to its axis (2) by a glass or crystal surround (1) transparent to the emitted radiation, where the outer surface of the surround (1) is divided into at least two regions, one of which forms an irradiation region (7) for the emitted radiation and the other acts as a reflective surface (8) which reflects on the solid body (9) the radiation (11) passing through the surround (1).

Description

P a t e n t a n e l d ün

"Diode laser pumped solid state laser"

The invention relates to a diode laser-pumped solid-state laser with at least one rod-shaped or disk-shaped solid serving as an active medium and with diode lasers whose emitted radiation is irradiated into the solid, reflecting elements being used for beam guidance and / or shaping.

Due to the rapid development of high-power laser technology in recent years, lamps which are used for pumping solid-state lasers are increasingly being replaced by diode lasers. Due to the geometry of the active medium, diode laser beams are characterized, among other things, by the elliptical beam cross section. The large axis of this elliptical cross-section runs perpendicular to the pn junction of the diode structure (also referred to as the fast direction), while the small axis runs parallel to the plane of the pn junction (also referred to as the flow direction). In addition to this asymmetrical beam cross section, the emerging radiation in the direction of the major axis, i.e. perpendicular to the plane of the pn junction or to the active layer shows a high divergence, with a beam opening angle of up to 90 °, while in the direction of the small axis this divergence is only about 10 °.

Due to the special elliptical beam cross-section and the large divergence perpendicular to the active plane (also known as the junction plane net) and the relatively low divergence in the direction of the small axis, an efficient pumping of solid-state lasers by means of such diode lasers crucially depends on the suitable guidance and shaping of the diode laser radiation. Furthermore, the achievable laser properties, such as the beam quality and the output power, are strongly influenced by the effective dissipation of the heat which does not contribute to the laser process.

There are various concepts for coupling diode laser radiation into solid state media. An embodiment according to the preamble of claim 1 is described in "Efficient continuous-wafer TEM _Q0 Operation of a transversely diode-pumped Nd.YAG laser" by DR Walker, CJ Flood and HM van Driel, July 15, 1994, Vol 19, No. 14 / OPTICS LETTERS, page 1055 ff. Here, the diode laser radiation is radiated into the solid-state medium via elliptical mirrors. The individual laser diodes with their associated elliptical mirrors are evenly spaced around the solid.

Another structure is described in "Solid-state laser engineering" by Walter Köchner, Springer-Verlag, under the chapter "Laser-diode-pumped systems" and outlined on page 134. In this arrangement, diode arrays are distributed around a solid-state laser rod in the radial and axial directions in order to couple the radiation from the individual laser diodes into the solid.

With the above arrangements, the gain or gain distribution can be optimized depending on the application by the different focusing, but this requires focusing components, such as the elliptical or cylindrical mirrors, which increases the construction costs.

A further possibility of coupling the diode laser radiation into the solid body can be done using beam guiding elements. Starting from the prior art mentioned above, the present invention is based on the object of specifying an arrangement of a diode-laser-pumped solid-state laser in which, on the one hand, the radiation emitted by diode lasers can be coupled into the solid-state medium with a high degree of efficiency, in which others, the heat loss can be dissipated in a simple manner and which offers a great variation in structure, but nevertheless leads to a compact structure.

The above object is achieved in a diode-laser-pumped solid-state laser in that the solid body is enveloped by a glass or crystal enveloping body which is transparent to the emitted radiation in the radial direction of its axis, the outer surface of the enveloping body being divided into at least two areas , one of which forms an irradiation area for the emitted radiation and the other area serves as a reflection surface which reflects radiation passing through the enveloping body onto the solid body.

According to the invention, a specially shaped glass or crystal envelope, preferably in the form of a cylinder or cylinder section, is divided in its outer configuration in the circumferential direction into individual areas or surface sections, these sections alternately providing an irradiation area for the emitted diode radiation , which is coupled into the body via these surfaces, and reflection surfaces form. This means that radiation that is coupled in via the irradiation regions is either radiated directly into the solid-state rod, which is preferably embedded tightly in the enveloping body, or at the respective reflection surfaces, for example by coating the outside of the enveloping body in the Area of these reflecting surfaces are formed, is reflected. At the same time, these reflection surfaces serve to dissipate heat loss that arises within the enveloping body through thermal conduction to the outside environment. It is evident that a simple component is obtained in this way, with which the one defines the diode laser radiation for pumping the solid body to the solid body rod, on the other hand heat dissipation to the outside environment is achieved. The individual surface areas can be dimensioned differently in terms of their extent and in the circumferential direction of the enveloping body. In other words, this means that the area over which the radiation from the diode lasers is radiated into the enveloping body can only be selected so large that it is just sufficient to absorb all of the radiation emitted by the diode lasers, while the remaining areas can then be used, designed as reflecting surfaces, to dissipate the heat loss to the outside. The above-mentioned dimensioning and profiling possibilities with regard to the enveloping body are particularly advantageous with simple means in that the geometry of the enveloping body can be adapted to the different beam cross-sections of the diode lasers, as explained at the beginning.

The enveloping body is preferably constructed as a cylindrical part with end faces delimiting in the axial direction, which are then totally reflective for the diode laser radiation propagating in the enveloping body. The diode laser radiation is radiated into the envelope body in the form of the irradiation regions of the envelope body and guided to the solid body; Radiation, which is directed towards the end faces of the enveloping body due to the beam divergence or the reflections within the enveloping body, is reflected there by total reflection, so that no losses due to radiation emerging from the enveloping body occur.

With respect to its cross section, the enveloping body is preferably constructed symmetrically to a plane that contains the axis of the enveloping body. This ensures that the solid body is pumped uniformly from the different directions by the radiation from the diode laser. When constructing the enveloping body, care should be taken to ensure that the reflective surfaces reflect the irradiation areas over which the diode beam radiation is radiated into the enveloping body, so that the irradiated radiation either impinges directly on the solid body or on the reflecting surface of the enveloping body lying behind or opposite the irradiation surface, and then, reflected on this reflecting surface, is directed at the solid body to become. It follows from this that it is advantageous that at least one reflection surface is assigned to each irradiation area.

In a preferred development, the reflection surfaces are designed such that they focus the radiation onto the solid body. In this case, the reflection surfaces are dimensioned and curved such that they preferably focus the radiation that is radiated into the enveloping body over the respective irradiation area that is opposite this reflection surface.

However, in order to radiate the radiation from the diode lasers directly into the solid body without reflection losses, on the one hand the diode lasers can be dimensioned such that their beam cross section approximately illuminates the cross section of the solid body, so that almost all of the radiation from the diode lasers impinges directly on the solid body rod. Such defined illumination can, however, also be achieved in that the surfaces of the irradiation areas focus or expand the radiation of the diode laser in one direction or the other (also depending on the position of the elliptical beam cross section of the diode laser) by means of a defined curvature .

It is known that individual diode lasers can be combined to achieve so-called diode laser arrays or diode laser field arrangements, for example to linear field arrangements in which the beam exit windows of the individual diode lasers lie on one line, or else to achieve higher powers and power densities also for multi-dimensional field arrangements, ie for those field arrangements in which several linear laser diode arrays are in turn combined. With such diode laser arrays, large surface areas can be uniformly illuminated, which in turn is exploited with regard to the enveloping body in such a way that the irradiation areas can be chosen to be relatively large, which can then be uniformly illuminated with such diode laser field arrangements or whose radiation is uniform over the each irradiation area is irradiated into the enveloping body. In one embodiment, the size of the irradiation areas is then selected to be approximately the same size as that of the reflection surfaces.

With regard to an arrangement of the irradiation areas relative to the respective reflection surfaces, an odd number of irradiation areas is selected, in which case, from the point of view that the number of reflection areas corresponds to the number of irradiation areas and these each have the same size, that means that there is symmetry with respect to a plane which contains the axis of the enveloping body, the preferred construction already mentioned above can be achieved, in each of which an irradiation region is opposite a reflection surface. So that the outer geometry of the enveloping body is not too complicated, but on the other hand uniform radiation of the solid body is achieved, the number of irradiation areas should not exceed an odd number between three and eleven.

The enveloping body according to the invention also offers the possibility of embedding, in addition to the arrangement of a solid body along the axis of the enveloping body, several solid bodies, for example three, which are then preferably arranged symmetrically around the axis of the enveloping body in order to achieve the symmetry mentioned.

In order to support the dissipation of the heat loss via the reflection surfaces, cooling devices can be arranged on the outer surfaces of the reflection surfaces. It can be seen that, despite such cooling devices, a compact structure can be achieved. Furthermore, additional cooling measures can be taken in that the solid body is cooled directly in the enveloping body by, for example, channels or channels surrounding the solid body being provided in the enveloping body in order to pass a cooling fluid there.

The respective diode lasers are preferably arranged with the large axis of their elliptical beam cross section substantially perpendicular to the axis of the enveloping body, so that, in particular with regard to an enveloping body which is formed in the form of a short cylinder section, for example in the form of a disk-shaped one In part, the beam is directed towards these end faces of the enveloping body with a high beam divergence. As a result, a high packing density of the diode lasers used for pumping can be achieved.

Further details and features of the present invention result from the following description of exemplary embodiments with reference to the drawings. Show in the drawings

FIG. 1 schematically shows an embodiment of a cylindrical envelope body according to the invention with a single solid rod which is embedded therein,

FIG. 2 shows a disk-shaped enveloping body with a cross-sectional shape corresponding to the enveloping body of FIG. 1,

FIG. 3 shows schematically the beam path within an enveloping body, as shown in FIGS. 1 and 2, specifically with regard to a diode laser field arrangement,

FIG. 4 shows the arrangement of FIG. 3, the beam path of the three diode laser field arrangements used being shown schematically,

Figure 5 schematically shows a diode laser with the exit surface emerging typical beam cone,

FIGS. 6A to 6D schematically show different one- and two-dimensional diode laser field arrangements which can be used in connection with the enveloping body,

FIG. 7 shows an arrangement comparable to the illustration in FIG. 4, but in contrast to the arrangement in FIG. 4, the irradiation areas are dimensioned smaller compared to the reflection areas in the circumferential direction of the envelope and the slow direction of the beam cross section of the diode laser parallel to the axis of the enveloping body or of the solid rod,

8 shows an arrangement corresponding to the arrangement of FIGS. 3 and 4, but with three solid-state rods embedded in the enveloping body, and

FIG. 9 shows an arrangement corresponding to FIG. 1, with a cooling channel being provided around the solid body in relation to FIG. 1.

1 shows a perspective view schematically of a cylindrical enveloping body 1 which is delimited in the direction of the axis 2 by end faces 3 which run approximately perpendicular to the axis 2. A continuous bore 4 runs along the axis 2 and is intended to receive a solid body rod of a diameter corresponding to the cross section of the bore 4.

The cylindrical surface 5 is divided into six surface segments 6, three flat surface segments 6 each forming irradiation areas 7, while the other three curved surface segments 6 form reflection surfaces 8. While the enveloping body 1 of FIG. 1 has the shape of an elongated cylinder, an enveloping body 1 is shown in FIG. 2, which has the same cross-sectional shape as the enveloping body 1 of FIG. 1, also with six surface segments, three segments of which are irradiation areas 7 and three Segments form reflection surfaces 8, but are disc-shaped, ie have only a small extension in the direction of the axis 2.

The enveloping bodies 1, as shown in FIGS. 1 and 2, are each shown in a plan view of their end face 3 in FIGS. 3 and 4. In FIG. 3, the three irradiation areas 7 of the enveloping body 1 are each assigned a diode laser array or a diode laser field arrangement 10 (also referred to in the figures as "DL array"). The slow direction runs perpendicular to the axis 2 of the enveloping body 1

The radiation which is emitted by the respective diode laser field arrangements 10 is radiated into the enveloping body 1 via the irradiation regions 7. The radiation 11 either strikes the solid-state rod 9 directly, as indicated in FIG. 3 by the central beam emanating from the lower diode laser array 10, or passes the solid-state rod 9, strikes the opposite reflection surface 8 and from there, due to the curvature of the reflection surface 8, it is focused on the solid body rod 9. A corresponding radiation emanates from the two other diode laser field arrangements 10, as shown in FIG. 4, so that the solid-state rod 9 is irradiated uniformly from all sides of the arrangement.

The irradiation areas 7 are dimensioned such that their size corresponds in each case to the extent of the diode laser field arrangement 10, so that the entire irradiation area 7 is illuminated by the radiation from the diode laser field arrangement 10. Furthermore, as is clear from FIGS. 3 and 4, the arrangement is arranged symmetrically to a fictitious plane which contains the axis 2 of the enveloping body 1. The reflection surfaces 8, which are preferably passed from the outside to the Envelope applied reflecting layers are formed, are totally reflective for the diode laser radiation. As can be seen from FIGS. 3 and 4, the reflecting surfaces 8 form a relatively large surface area, with respect to the entire enveloping body 1, which is suitable for dissipating heat loss generated in the enveloping body 1 to the outside environment. A measure is therefore taken at the same time by these reflection surfaces to cool the enveloping body, which is very important for the laser properties of the solid-state laser that can be achieved, such as beam quality and output power.

The individual areas, i.e. The irradiation areas 7 and the reflection surfaces 8 can be dimensioned in a defined manner in their radial and axial extent. While the irradiation regions 7 are shown in the figures as flat surface parts, they can also have a curvature in order to either focus or expand the radiation emitted by the diode lasers of the field arrangement 10, the beam cross-section, the is typical for a diode laser, which is shown in FIG. 5 and will be explained below, is taken into account.

In order to keep losses at the two end faces 3 of the enveloping body 1 low, they are preferably totally reflective, in order to reflect radiation 11, which emanates from the diode laser field arrangements 10 and strikes these end faces 3, back into the enveloping body 1. If necessary, these end faces 3 can be curved in order to achieve a focusing effect; flat end faces 3 are preferred, however, in order to achieve a simple geometry of the enveloping body 1, the radiation components which strike these end faces 3 within the enveloping body 1 being kept low by a defined arrangement of the diode lasers of the field arrangement 10 .

If necessary, additional cooling measures can be taken in the area of the reflection surfaces 8 on the outside of the envelope body 1 be, in order to dissipate the heat generated in the enveloping body 1 faster to the outside. Since the heat loss is transported through the volume of the enveloping body 1, the enveloping body is preferably made of a highly heat-conducting and highly transmissive material, such as sapphire.

As already mentioned above, the elliptical beam cross section of a diode laser must be taken into account when orienting the diode laser field arrangement 10 relative to the irradiation regions 7 or the axis 2 of the enveloping body 1.

A typical structure of an individual diode laser 12 is shown schematically in FIG. 5, but without a heat sink and further supply devices. The structure has a heavily doped p ⁺ phase 13, a p phase 14 and an n phase 15. The reference numeral 20 denotes the exit surface of an emitting zone of the active medium, which has a certain extent in the direction of the plane of the transition the p-phase 14 and the n-phase 15, while it runs relatively narrow perpendicular to these planes. Because of the geometry of the active medium, the indicated typical radiation cone emerges from the exit surface 20 and has an elliptical cross section, the major axis 16 running perpendicular to the plane of the pn junction, while the minor axis 17 of the elliptical cross section runs parallel to the plane of the pn junction. Typically, the large extent of the jet cone perpendicular to the plane of the pn junction, which is also referred to as the "fast direction", has an angle of divergence 18 of approximately 90 °, while the jet cone in the direction of the minor axis 17 or perpendicular to the fast direction, which is also referred to as the "slow direction", has a divergence angle 19 of approximately 10 °.

In the embodiments which have been described so far, as shown in FIGS. 1 to 4, the diode laser array arrangement 10 is preferably arranged such that the slow direction of the individual diode lasers 12 Field arrangement runs perpendicular to the axis 2 of the solid body rod 9 or the enveloping body 1. In this way, an extremely high pump power density can be achieved and an extremely compact pulse laser can thus be constructed.

Examples of the structure of diode laser field arrangements 10 which can be used in the embodiments of the enveloping body 1 are shown in FIGS. 6A to 6D. Since individual laser beam sources in the form of diode lasers can only be scaled to higher powers to a limited extent, a greater number of individual diode lasers are combined to form different arrays or field arrangements in order to achieve higher laser powers and power densities, as is shown in the figures.

A distinction can be made here between linear field arrangements, as shown in FIGS. 6A and 6C, double-linear field arrangements, as shown for example in FIG. 6B, and two-dimensional arrays in accordance with the schematic representation of FIG. 6D.

As FIGS. 6A to 6D show, the dimensions and power densities of diode laser field arrangements 10 in the irradiation areas can be used accordingly in order to irradiate the radiation from the individual laser diodes into the enveloping body 1.

FIG. 7 shows an exemplary embodiment of a diode-laser-pumped solid-state laser with an enveloping body 1, in which the irradiation regions 7, in their radial extent around the axis 2 of the enveloping body 1, are dimensioned much smaller than those between the individual irradiation regions 7 lying reflection surfaces 8, viewed in the radial direction the reflection surfaces 8 being approximately five times larger than the irradiation regions 7. Again, three irradiation regions 7 are provided, each with an intermediate reflection surface 8. The arrangement of Figure 7 is suitable for such structures of a diode laser-pumped solid-state laser, in which the Slow direction of the individual diode lasers of the diode laser field arrangement 10 runs parallel to the cylinder axis 2 of the solid-state rod 9. The diode laser radiation is in turn radiated into the envelope body 1 via the three radiation regions 7, which are flat and are formed on the glass body by a highly transiting coating. Part of the diode radiation is radiated directly onto the solid-state rod 9, as the radiation pattern shows, while the other part strikes the reflection surface 8 lying behind the field-body rod 9 in the beam direction, from where it emerges due to the concave curvature the reflection flat 8, in which the solid-state rod 9 is focused. The effectiveness of the laser can be increased by the multiple reflection within the glass body.

The embodiment of FIG. 7 has the advantage over the embodiment of FIG. 3 that with a certain size of the enveloping body 1 more diode arrays 10 can be arranged around the circumference of the enveloping body 1. In addition, the pump power density distribution in the solid can be set more easily. This concept is advantageous for continuously operated (cw) lasers.

Furthermore, the embodiment of FIG. 7 has the advantage that the large-scale dimensioning of the reflection surfaces 8, in comparison to the embodiment of FIGS. 3 and 4, enables a higher heat dissipation of heat loss within the envelope body 1 to be achieved.

FIG. 8 schematically shows a further embodiment, the shape of the enveloping body 1 and the arrangement of the diode laser field arrangements 10 corresponding to the embodiment of FIGS. 3 and 4, in which, however, three solid rods 9 are embedded in the enveloping body 1 compared to the earlier embodiment. The individual solid bars are symmetrically evenly distributed around the axis 2 of the enveloping body 1, two solid bars 9 being aligned with their axes 22 such that the axes of the respective two solid bars 9 lie in one plane. gene, which runs parallel to the plane of the irradiation regions 7. This embodiment has the advantage that the thermal load on the solid-state medium is reduced, and in addition the area which is opposed to the radiation of the diode laser field arrangements 10 is increased.

In FIG. 9, in addition to the embodiment of FIG. 1, an annular space 22 is left around the solid body rod 9, through which a fluid circulates, which dissipates the heat loss within the enveloping body 1.

As the above description shows, a compact but nevertheless effective arrangement of a diode laser-pumped solid-state laser is created using the described enveloping body, in which one or more solid-state rods 9 are used as the active medium.

Claims

P a t e n t a n s r u c h e

Diode laser-pumped solid-state laser with at least one rod-shaped or disk-shaped solid serving as an active medium and with diode lasers whose emitted radiation is radiated into the solid, reflective elements being used for beam guidance and / or shaping, characterized in that that the solid body (9) is encased in the radial direction to its axis by a glass or crystal 1 envelope body (1) which is transparent to the emitted radiation, the jacket surface (5) of the envelope body (1) being divided into at least two areas (6 ), one of which forms an irradiation area (7) for the emitted radiation and the other area serves as a reflection surface (8), the radiation (11) passing through the enveloping body (1) onto the solid (9) reflected.

Diode laser-pumped solid-state laser according to Claim 1, characterized in that the end faces (3) of the enveloping body (1) which axially delimit the enveloping body (1) are designed to be totally reflective of the diode laser radiation (11) propagating in the enveloping body (1) are. 3. Diode laser-pumped solid-state laser according to claim 1 or 2, characterized in that the reflection surface (8) of the irradiation area (7) is arranged opposite.

4. Diode laser-pumped solid-state laser according to claim 3, characterized gekenn¬ characterized in that each irradiation area (7) is assigned at least one reflection surface (8).

5. Diode laser-pumped solid-state laser according to one of claims 1 to 4, characterized in that the reflection surface (8) focuses the radiation (11) on the solid body (9).

6. Diode laser-pumped solid-state laser according to claim 5, characterized gekenn¬ characterized in that the focused radiation illuminates the cross section of the solid body (9) approximately.

7. diode laser-pumped solid-state laser according to one of claims 1 to

6, characterized in that the irradiation areas (7) and / or the reflection surfaces (8) have the same size.

8. diode laser-pumped solid-state laser according to one of claims 1 to

7, characterized in that the number of irradiation areas (7) corresponds to the number of reflection surfaces (8).

9. diode laser-pumped solid-state laser according to one of claims 1 to

8, characterized in that the number of irradiation areas (7) is odd and is at least three.

10. diode laser-pumped solid-state laser according to one of claims 1 to

9, characterized in that the irradiation areas (7) and the reflecting surfaces (8) are arranged symmetrically to a plane which contains the central axis (2) of the enveloping body (1). 11. diode laser-pumped solid-state laser according to one of claims 1 to

10, characterized in that several solid bodies (9) are embedded in the hollow body (1).

12. Diode laser-pumped solid-state laser according to one of claims 1 to

11, characterized in that the enveloping body (1) is provided with cooling devices.

13. Diode laser-pumped solid-state laser according to claim 12, characterized ge indicates that the heat loss generated in the solid (9) is dissipated by heat conduction through the enveloping body (1) to the outer surface (5), the cooling devices on the outer surfaces of the reflection surfaces (8th ) are arranged.

14. Diode laser-pumped solid-state laser according to claim 12 or 13, characterized in that the cooling devices surround the solid (9) directly.

15. Diode laser-pumped solid-state laser according to claim 1, characterized gekenn¬ characterized in that the diode lasers, which are assigned to an irradiation area, are summarized to at least one array or a field arrangement (10).

16. Diode laser-pumped solid-state laser according to one of claims 1 to 15, characterized in that the respective diode lasers are arranged with the major axis of their elliptical beam cross section substantially perpendicular to the axis (2) of the envelope body (1).