WO1998008279A1 - Optically pumped amplifier, in particular solid-state amplifier - Google Patents

Abstract

An arrangement is disclosed for geometrically shaping a radiation field, in particular the radiation field of a diode laser array (14) which propagates in the z direction and has a radiation cross-section which is larger in a direction defined as the x direction perpendicular to the z direction than in a direction defined as the y direction, which is perpendicular to the first, and has a lower radiation quality in said direction. X, y and z form a coordinate system. The radiation is grouped into radiation fractions in the x direction and the radiation fractions are reoriented as regards their radiation cross-sections. The arrangement has at least two reflective elements (22, 23) and is characterised in that each reflective element has a pair of reflection surfaces at approximately or exactly 90 DEG to each other. This opening angle is oriented against the direction of propagation. The line at which the reflection surfaces intersect is oriented at approximately below 45 DEG to the x direction.

Description

 P a t e n t a n m e l g

"Optically Pumped Amplifier, Especially Solid State Amplifier"

The present invention relates to an optically pumped amplifier, in particular a solid-state amplifier, with an amplification medium and with an optical pump arrangement, via which pump radiation is coupled into the amplification medium transversely to its radiation direction, the pump radiation being shaped before the coupling, and the radiation portion of the pump radiation passing through the gain medium is reflected back into the gain medium by means of a reflection unit.

Optically pumped amplifiers of the type specified above are generally known.

The rapid developments in high-performance diode lasers, particularly in terms of reliability and costs, have led in particular to the use of diode lasers for the optical pumping of amplifiers and lasers. The beam properties and spectral properties of high-power diode lasers enable a variety of system configurations compared to pumping with conventional flash lamps or arc lamps. In general, the axial pump arrangement is used for lasers with an output power up to a few 10W, while the transverse pump arrangement is preferably used for scaling the output power. With regard to the geometry of the gain or amplification medium, a distinction must be made between rod lasers and slave lasers. The rod lasers have round output beams, but have large depolarization losses due to thermo-optical effects. These losses reduce the efficiency that can be achieved, particularly when polarized output radiation is desired. There is less depolarization loss in lasers with slab-shaped medium. However, this expectation is only met if a suitable pump jet distribution and cooling arrangement is available or can be provided.

A pump arrangement according to the prior art for slab lasers is now described in FIG. The slab-shaped laser medium 1 is pumped by a diode laser stack in that the radiation emanating from each linear diode laser array 3 is coupled into a coupling optics 5 via a cylindrical lens 4. Arranged on the opposite side of the laser medium 1 is a retro mirror 6, which couples the radiation or power not absorbed during the first passage of the diode laser radiation through the laser medium 1 back into the laser medium. In this way, a double passage through the laser medium 1 is realized. The distribution of the absorbed power density in the pumping direction is shown in FIG. 5 for a coupling efficiency of the pump radiation of 86%. The front edge into which the pump radiation is radiated is indicated by zero, while the rear edge from which the pump radiation emerges is designated by 1. It can be seen from this graphic representation that it is not possible to achieve homogeneous illumination or homogeneous pumping of the slab-shaped laser medium 1 with an acceptable coupling efficiency (greater than 70%) with this pump arrangement. The non-homogeneous distribution of the pump radiation in the laser medium in turn leads to thermo-optical disturbances and thus to reduced beam quality, since there is no desired one-dimensional heat conduction in the slab-shaped laser medium.

On the basis of the prior art specified above, the present invention is based on the object of solving the problems identified with the aid of the prior art, in particular with regard to those which have been absorbed To homogenize the power density of the pump radiation coupled into a gain medium over the cross section of the solid medium.

This object is achieved, starting from an optical amplifier of the type mentioned at the outset, in that the pump radiation is at least approximately polarized in that a polarization beam splitter is arranged in the beam path of the pump radiation before being coupled into the amplification medium, to which a mirror is assigned that the radiation component which is guided through the amplification medium by the polarization beam splitter and is not absorbed is reflected back into the amplification medium by the reflection unit in such a way that the radiation component which is not absorbed during this second pass through the amplification medium is guided onto the polarization splitter, the direction of polarization of which Radiation component is rotated, and that this radiation component is guided by the polarization beam splitter to the mirror assigned to it and is reflected back by the latter via the polarization beam splitter into the amplification medium.

The principle of the invention is that a suitable pump arrangement pumps a defined volume of the medium with the pump radiation. To increase the homogeneity of the pump power distribution, the polarization property of the pump radiation, preferably diode laser radiation, is used in the form of a fourfold passage through the laser medium using a polarization beam splitter.

The degree of polarization of the pump radiation should preferably be greater than 80%; this is the case when the gain medium is pumped with diode laser radiation.

In a simple arrangement, the direction of polarization can be rotated by means of a λ / 4 plate.

In order to achieve a simple structure, the polarization direction of the pump radiation in front of the polarization beam splitter, specifically before the first pass through it, should be arranged or rotated such that it is suitable for the polarization Beam splitter represents a p-polarization, ie that the direction of polarization is parallel to the plane of incidence.

Further details and features of the invention result from the following description of exemplary embodiments with reference to the drawing, in particular also in comparison to the prior art. In the drawing shows

FIG. 1 shows a first embodiment of an optically pumped solid-state amplifier,

FIG. 2 shows a further arrangement, which is based in principle on the arrangement of FIG. 1,

FIG. 3 shows a third arrangement in which diode laser radiation is used as pump radiation,

4 shows a solid-state laser, which is pumped with diode laser radiation, according to the prior art, and

FIG. 5 is a diagram illustrating the power absorbed in the amplifier medium with respect to the relative position in the amplifier medium along the direction of the pump radiation with a double passage of the pump radiation through the medium and a four times passage of the pump radiation through the medium.

The principle according to the invention is to pump only a defined partial volume of the medium by means of a suitable pump arrangement. Here, a polarized pump source 7, preferably a diode laser field arrangement, as is also shown in FIGS. 1 and 2, is used. Their pump radiation is coupled into the laser medium 10 to be pumped via coupling optics 8 and a polarization beam splitter 9. The radiation not absorbed in the laser medium 10 is applied to the retro-reflection device 11 on the side of the laser medium 10 opposite the polarization beam splitter 9. The direction of polarization is rotated using a λ / 4 plate. The pump radiation which in turn is not absorbed during this second passage through the laser medium 10 leads with a rotated direction of polarization to the polarization beam splitter 9 and is laterally coupled out of the beam path, which is designated by the reference symbol 12, and onto a retro mirror 13.

This radiation then leads back to the polarization beam splitter 9 and is again coupled into the laser medium 10 along the beam path 12. The use of the polarization beam splitter 9, the retro reflection unit 11 with polarization rotation and the retro mirror 13 thus enables the pump radiation to pass through the laser medium four times. In such a case, the normalized absorbed laser power density (p) in the pumping direction is given by the following equation:

p = α [e ^"αx + e-" ²¹ "+ e ^ ² ' ^* ^ + e ^ ⁴¹ ^]

in which

α: absorption coefficient

x: relative position along the direction of the pump radiation

I: Width of the slab-shaped medium in the pump beam direction

means.

In contrast to this, the normalized absorbed power density (p) in a double pass, as shown in FIG. 4, is given by:

p = α [e- + e ^ ²¹ " ⁰ ]

As now shown in FIG. 5, the quadruple passage of the pump radiation, shown in the graphic of FIG. 5 with a solid line, in comparison with a double passage, shown in the graphic in FIG. 5 with a broken line, corresponds to a pump arrangement according to the prior art, over the cross section in the direction of the beam path 12 (see Figure 1) can be made more homogeneous. There is an additional improvement in the homogeneity if the laser medium is contact-cooled from above and from below, ie perpendicular to the beam path 12 in FIG. 1, and its pump sides are thermally insulated. This means that the homogeneously absorbed power density in the pumping direction leads to one-dimensional heat conduction perpendicular to the pumping direction and thus depolarization losses practically do not occur. Furthermore, the height of the pumped cross section of the laser medium, ie perpendicular to the direction of the pump radiation and the laser radiation, can be dimensioned such that it is comparable to the mode radius. This means that a laser with high efficiency and high beam quality can be realized.

For scaling the laser power, a pump arrangement, as explained with reference to FIG. 1, can be arranged in a double arrangement, namely in the direction along the axial extent of the laser medium. Such an embodiment is shown in FIG. 2. The respective pump arrangements are arranged on both sides of the laser medium 10, the corresponding reference numerals, which are also shown and described in FIG. 1, being used for the further components, so that the corresponding explanations for the individual components in FIG. 1 correspond accordingly to the components of FIG Figure 2 transferred. Such an arrangement additionally increases the homogeneity of the absorbed power density in the sense of the integral along the axis of the laser medium.

The pump arrangements described above with reference to FIGS. 1 and 2 can be used for all types of optical pumps. An example in which a solid-state amplifier or a laser is pumped with diode laser radiation is shown in FIG. 3. It is known that high-power diode lasers preferably emit polarized radiation with a polarization ratio of 20: 1 parallel to the PN junction. Therefore, in the arrangement of FIG. 3, the radiation emitted by a diode laser stack or a diode laser array 14, in which a plurality of linear diode laser emitter arrangements 15 are stacked one on top of the other, is collimated in each case via a cylindrical lens 16 in the almost direction, ie perpendicular to the PN junction - lubricated and used as a pump radiation source. A λ / 2 plate 17 is then inserted into the beam path of the diode laser radiation, which is used to rotate the polarization the diode laser radiation by 90 °, ie the direction of polarization behind the λ / 2 plate has a p-polarization with respect to the downstream polarizer. With a coupling optics 18, the p-polarized diode laser radiation is coupled into the laser medium 20 via a polarizer 19. Behind the laser medium 20 there is a λ / 4 plate 21 and a first retro mirror 22. With the retro mirror, the diode laser radiation not absorbed during the first passage through the laser medium 20 is coupled back into the medium 20. At the same time, the polarization is rotated through 90 ° with two passes through the λ / 4 plate. The diode laser radiation with s-polarization which is not absorbed during the second passage through the laser medium 20, ie after reflection from the retro mirror 22, is reflected by the polarizer or the polarization beam splitter 19 to a second retro mirror 23 and from there again coupled back into the laser medium 20 via the polarization splitter for a third pass. The diode laser radiation not absorbed after the third pass through the laser medium 20 is coupled back into the medium by the first retro mirror 22 and at the same time the polarization of the diode laser radiation is p-polarized again. The diode power which has not yet been absorbed after the fourth pass through the laser medium 20 then runs through the polarizer 19 and is considered to be power loss.

As can also be seen in particular from FIG. 3, the advantages of such a pump arrangement, in particular a pump arrangement with which a laser medium is pumped by means of diode laser radiation, lie in the achievable one-dimensional heat conduction, the low depolarization loss, a low thermo-optical disturbance and the possibility of the pumped Adapt volume to the laser volume by using appropriate coupling optics and corresponding retro mirrors 22 and 23 (reference is made to FIG. 3). Furthermore, there is a high degree of efficiency with a high beam quality.

The beam quality and intensity distribution from the amplifiers or lasers with a rectangular cross section can be homogenized with mirrors arranged in steps, for example, or adapted to applications by reshaping the beam cross section or dividing it into groups and then regrouping.

Claims

Patent application "Optically pumped amplifier, in particular solid-state amplifier" Patent claims 1. Optically pumped amplifier, in particular solid-state amplifier, with an amplification medium and with an optical pump arrangement, via which pump radiation is coupled into the amplification medium transversely to its radiation direction, the pump radiation being shaped before the coupling, and the beam component passing through the amplification medium Pump radiation is reflected back into the gain medium by means of a reflection unit, characterized in that the pump radiation is at least approximately polarized, that a polarization beam splitter (9; 19) is arranged in the beam path (12) of the pump radiation before it is coupled into the gain medium (10; 20) , to which a mirror (13; 23) is assigned, so that the portion of the beam which is guided through the amplification medium (10; 20) by the polarization beam splitter (9; 19) and is not absorbed by the reflection unit (11; 22) is reflected back into the gain medium (10, 20), d ate during the second pass through the gain medium (10; 20) non-absorbed beam component is guided onto the polarization splitter (9; 19), the direction of polarization of this beam component being rotated, and that this beam component is guided from the polarization beam splitter (9; 10) to the mirror (13; 23) assigned to it and is reflected by this back via the polarization beam splitter (9, 10) into the gain medium (10; 20). 2. Optically pumped amplifier according to claim 1, characterized in that the degree of polarization of the pump radiation is greater than 80%. 3. Optically pumped amplifier according to claim 1 or claim 2, characterized in that the rotation of the polarization direction is carried out in two passes through a λ / 4 plate. 4. Optically pumped amplifier according to one of claims 1 to 3, characterized in that the polarization direction of the pump radiation in front of the polarization beam splitter (9; 19) is rotated through there before the first pass so that it for the polarization beam splitter (9 ; 19) represents p-polarization. 5. Optically pumped amplifier according to one of claims 1 to 4, characterized in that the gain medium is arranged between two resonance mirrors. 6. An optically pumped amplifier according to claim 5, characterized in that the resonator is unstable in the direction of the pump radiation and is stable in the direction perpendicular to it.