GB2343990A

GB2343990A - Solid state laser having a monolithic pumping cavity

Info

Publication number: GB2343990A
Application number: GB9927308A
Authority: GB
Inventors: Guenter Hollemann; Igor Kucma; Aleksandr Levoskin; Arthur Mak; Vjaceslav Boutcenkov; Alekseij Petrov
Original assignee: Jenoptik Jena GmbH; Jenoptik AG
Current assignee: Jenoptik AG
Priority date: 1998-11-18
Filing date: 1999-11-18
Publication date: 2000-05-24
Also published as: DE19854004A1; JP2000164959A; FR2786035A1; GB9927308D0

Abstract

A solid state laser has a rod-shaped active element 1, at least one diode laser row 7 for pumping the active element 1. There is a pumping cavity which surrounds the active element 1 and the pumping cavity is characterized by a monolithic, cylindrical block 2 with a channel in the direction of the cylinder axis in which the active element 1 is fixed by means of adhesive 4. The block 2 has a reflecting layer on the surface.

Description

1 2343990 Solid state laser having a monolithic pumping cavity The

invention relates to a solid state laser having an active element (AE) in the f orm of a laser rod, at least one diode laser row, aligned parallel to the axis of the laser rod, and a so-called pumping cavity (a reflector arrangement which surrounds the AE and retroreflects onto the AE the pumping radiation, emitted by the diode laser row, which is not directed directly onto the AE and/or is not completely absorbed by the first traversal of the AE). The person skilled in the art will understand, moreover, that moreover further means belong to the solid state laser, such as a resonator arrangement or a cooling arrangement which, however, are of no importance for explaining the essence of the solid state laser according to the invention, and are therefore left out of account.

Solid state lasers having an AE made f rom a three-level medium which emit in the eye-safe spectral region are promising light sources with a high potential of possible applications. The following may be named as examples: laser radar equipment, laser distance measuring equipment and laser speed measuring equipment, equipment for recording air pollution, and medical laser equipment.

By contrast with other optically pumped lasers, lasers having an AE made from a three-level medium are more sensitive to pumping inhomogeneities because of their normally high laser threshold. Coactivated three-level media such as Yb:Er:glass, Tm:Ho:YAG, Yb:Ho:YAG etc. are even particularly sensitive because of the missing saturation response of the absorption.

In the case of three-level media, small inhomogeneities in the pumping energy distribution, which have no influence on the absorption efficiency in the case of four- level media, can already lead to a substantial reduction in the overall laser efficiency.

The decisive parameters for an effective and homogeneous pumping radiation absorption are essentially the shape and the size of the pumping 2 cavity, the reflectivity of the surf ace of the pumping cavity, the optical properties and the thermal conductivity of the medium between- the reflecting surface and the AE, the number and position of the gaps through which the pumping radiation is directed into the pumping cavity, the thermal input into the AE and the pumping wavelength.

Various solutions with cylindrically directed reflecting surfaces into which the pumping radiation is launched into the pumping cavity through one or more gaps are known from the prior art.

Patent Specifications US 3,821,663,

US 4,969,155, US 5,033,058 and US 5,307,365 and the publications in Optics Letters 17, page 1785 (1992) by H. Ajer, S. Landro, G. Rustad and K. Stenersen would require mention in this connection.

The use of a pumping cavity having a direct reflecting surface and a cylindrical shape with circular cross section, as disclosed in US 5,033, 058 and US 5,307,365 necessarily leads to an inhomogeneous angular distribution of the pumping radiation, and thus to inhomogeneities in the spatial pumping power density in the AE. Moreover, these solutions additionally require, for example, optical elements which are arranged between the laser light source and the surface of the pumping cavity and/or directly on the surface of the pumping cavity, in order to match the angular distribution of the pumping radiation to the solid angle which is given by the AE (geometrically conditioned). This renders the overall arrangement more complicated in design, and thus more expensive. of course, losses also occur owing to the additional optical elements.

If the pumping cavity is limited to a reflecting layer directly to the surface of the AE, as proposedin US 3,821,663, this leads.to the formation of so-called "hot zones" of excessive pumping power density.

In the case of expanded dimensions of the pumping cavity, convective cooling of the AE becomes difficult.

3 Consequently, in the case of high pulse repetition rates with a high average power, the heat generated in the AE is dissipated via liquid cooling. The result here, again, is a comparatively high outlay on design, and the efficiency is worsened because of the shading of surface regions by the holding elements required.

It is the object of the invention to create a solid state laser of simple design in which, by contrast with known similar solid state lasers, a high homogeneity is achieved in the pumping light distribution around the AE, and a substantially higher absorption efficiency is achieved.

This object is achieved according to the invention for a solid state laser by means of the characterizing features of claim 1. Advantageous designs are set forth in the subclaims.

It is essential to the invention that the pumping cavity comprises a monolithic block. The material of the block must be a material which is transparent to the pumping radiation, for example optical, molten or crystalline glass, sapphire and undoped yttrium. aluminum garnet (YAG). The block material must have a high thermal conductivity in order to be able to dissipate effectively the heat produced in the AE.

The concrete selection of the material is determined by the crosssectional shape of the pumping cavity and the required average output power of the laser.

The block has a cylindrical shape, it being possible to have different cross-sectional shapes which are explained in more detail with the aid of the exemplary embodiments. A tubular channel running in the direction of the cylinder axis is common to all designs. In this case, the channel axis can coincide with the cylinder axis or run parallel to it. The AE is fitted in this channel by means of - an adhesive.

The outer surface is provided with a reflecting layer, it being possible to consider both, coatings which reflect in a scattering fashion and ones which reflect - 4 in a directed fashion, depending on the cross-sectional shape. The surface layer has at least one gap running parallel to the cylinder axis, via which the pumping radiation from the diode laser row arranged upstream 5 thereof is launched into the block.

Just like other radiation components radiating past the AE, the radiation components which are not absorbed after directly striking the AE strike the AE again after one or more reflections at the reflecting surface of the block.

Since a plurality of traversals of the radiation through the AE are possible, the requirements placed on the stability of the pumping wavelength are reduced. This circumstance is important because the wavelength - of the diode lasers shifts when they are heated up. Moreover, laser diodes exhibit a tolerance in the wavelength from row to row, and likewise from diode to diode inside a row.

A particularly high absorbtivity is achieved for a 20 solid state laser having an AE made from a three-level medium (k: absorption coefficient of the three-level medium at the pumping wavelength, and d: diameter of the AE).

In the case of kd < 0.1, the losses at the reflecting layer increase very sharply on the assumption of realistic reflectivity values.

In the case of kd > 5, the inhomogeneity of the absorbed energy distribution becomes significant giving the assumption of ideal homogeneous illumination. A high homogeneity in the pumping light distribution, and a high absorption efficiency are achieved under the following conditions:

A. Block having a diffusely reflecting coating:

In this case, a large component of the inner surface is diffusely reflecting.

A-1. The coating of the surface of the block is coated in a diffusely reflecting fashion. The coating can consist, for example, of metal oxide powder such as MgO, Zr02, ZnO or others. The reflectivity can assume values of up to 98%.

A-2. Before the vapor deposition of the reflecting coating, the block has a smooth surface without special requirements placed on the exact geometry. In a special case, the cross- sectional shape of the block can be circular. The absorption efficiency of a block having a diffusely reflecting surface depends only insignificantly on its cross-sectional shape. The optimum ratio of the maximum cross section of the block (for a circle: the diameter D) and of the diameter of the AE depends essentially on the value kd. For kd = 0.1 - 5.0, the optimum ratio D/d is within the limits 1-4.

A-3. If the block is too large for the heat deposited in the AE to be dissipated on its surfaces, or for it to be integrated into a special arrangement, it is possible to reduce one dimension of the cross section of the block virtually to the diameter of the AE. The other dimension of the cross section is to be enlarged, and the diode radiation is to be launched into the block on this axis.

A-4. In order not noticeably to reduce the efficiency by losses in the pumping radiation at the gaps, it is desirable for the gap width ts, to fulfil the following condi t i on: ts, < 2 - P - (1 - K,f 1.) IN,j, (P: perimeter of the block cross section, Kref I reflectivity coefficient of the reflecting layer, N.,j: number of the gaps). The factor 2 on the right-hand side of the inequality takes account of the reflection by a gap inside the block in the case of diffuse illumination.

As an example, an an [sic] optimum diameter D = 4 5 mm is yielded for d = 2 mm, Krefi = 0.98 and kd 0.5 - 2.0. When two gaps are present, their width may not exceed 0.25 mm. In the case of applications with kd > 2 0, even the application of gaps with a larger width is possible without appreciable impairment of the efficiency. Gaps of this size can be produced simply, and it is likewise simple to position the aperture of the diode laser rows relative to the gaps.

B. Block having specularly reflecting coating.

It is assumed that substantial components of 5 the inner surface of the block are specularly reflecting.

B-1. The reflectivity in the interior of the block can be achieved by a metal layer which is deposited on the outside of the block (gold, silver, aluminum and others), or by a dielectric multiple layer system of high reflectivity for the pumping wavelength. A portion of the surface can also remain uncoated, employing total internal reflection. The selection of the reflection coefficient depends on the number of traversals of the radiation through the block, which is essentially given by the optical density of the AE. If the optical density is high enough (kd > = 0.2 0.3), the coating can be performed by aluminum deposited chemically or in vacuum, Krefi = 0.92 - 0.95 in the near infrared (IR) spectral region.

B-2. The pumping radiation enters the block on one or two opposite sides, specifically through one or two opposite gaps or one or two opposite groups of gaps if a plurality of diode laser rows are used.

B-3. The cross section of the block has two flat or approximately flat surfaces which are arranged parallel or approximately parallel to one another and to the plane of symmetry of the divergence angle of the diode radiation which enters the block through a gap or a plurality of gaps. The surfaces can be flat or deviate from a flat geometry. The extent of the surfaces in the direction "AE - gap" is 6W, in which case 8W > 0.3-D needs to be satisfied, where D: maximum dimension of the cross section. Permissible deviations Sh from the flat geometry (relative to a mean plane) must be 8h < 0.2-D. Some. possible variants of such an aiiangement are represented in Figures 4, 5 and 6. The dimensions depend on the optical density of the AE. If the optical density of the AE varies within the limits 7 kd = 0.2 - 5.0, the quotient of maximum cross section to AE diameter D/d = 1. 5 - 6. 0, and the quotient of minimum cross section to AE diameter is 1.0 - 2.5.

B-4. In order not perceptively to reduce the efficiency 5 by radiation losses owing to leakage of radiation through the gaps, the gap width ts, or the overall gap width of a group of closely neighboring gaps must fulfil the following condition:

tr,j << d-exp(2-kd) for pumping at one end t.,, << d-exp(kd) for pumping at two ends.

For example, for an AE diameter of 2 mm and absorption coefficient of 5 cm-1, the permissible overall gap width without substantial reduction in efficiency is < 0.5 1.5 mm.

B-5. In the case of pumping intake at two ends, there is an axis of symmetry, and the AE is arranged coaxiall.y therewith. In the case of pumping intake at one end, the AE is displaced away from the gap or from the gaps onto the opposite wall, the distance of the AE from the wall not being permitted to exceed the diameter of the AE.

The invention is to be explained below with the aid of drawings. Here, the individual figures show the parts of a solid state laser which are essential to the 25 invention, together with the respectively differing cross-sectional shape of the pumping cavity, in particular Fig. 1 with an elliptical cross section, Fig. 2 with a cross section resembling a rectangle, 30 the broad sides being convex circular arcs, and the AE being arranged eccentrically, Fig. 3 with a cross section resembling a rectangle, the broad sides being convex circular arcs, and the AE being arranged centrally, Fig. 4 with a cross section resembling a rectangle, the broad sides being convex circular arcs and the long sides being concave circular arcs, Fig. 5 with a cross section resembling a rectangle, the broad sides having convex circular arcs and 8 the long sides having a notch at the center, and Fig. 6 with a cross section resembling a rectangle, the broad sides being convex circular arcs, and the long sides being of V-shaped configuration.

The parts of a solid state laser which are essential to the invention are represented in Fig. 1.

The pumping cavity, which according to the invention is a monolithic block 2 having a cylindrical shape, here has the cross-sectional shape of an ellipse. The surface of the block 2 is coated with a layer 5 which reflects in a scattering fashion. This layer 5 has a gap 6 which runs parallel to the cylinder axis through an apex of the ellipse. This gap 6 is arranged downstream of a diode laser row 7 whose radiation is directed through the gap 6 into the block 2. Located in the block 2 is a tubular channel 3, parallel to the cylinder axis, in which the rod-shaped AE 1 is fixed by means of an adhesive 4 so that the axis X of the active element runs parallel to the cylinder axis.

In Fig. 2 and Fig. 3, the cross section of the block 2 has a shape resembling a rectangle, the broad sides being convex circular arcs. In these two examples, the surface is coated with a surface which reflects in a directed fashion. Whereas in the case of Fig. 2, just as in the case of Fig. 1, only one gap 6 is present, in the case of Fig. 3 the pumping radiation enters the block 2 through two gaps 6.

Figs 4-6 ' show further cross-sectional shapes for the 30 block 2 coated with a layer 5 which reflects in a directional fashion.

The plane-parallel long sides, represented in Figs 2 and 3, of the cross section of the block are of different configuration here. These long sides have in common that over their length 6W, which is not shorter than 0. 3 D (D = maximum dimension of the block cross section), do not exceed I a maximum deviation of 8h which is not greater than 0.2 - D.

9 It is to be seen in the case of Figs 1-6 that the AE 1 is arranged centrally in the case of a block 2 having two gaps 6, while it is located -further removed therefrom in the case of only one gap 6.

Instead of a gap 6 or two gaps 6 arranged on two sides, it is also possible for groups of gaps to be present. As a result, the pumping power can be multiplied with the aid of a plurality'of diode laser rows 7.

Claims

- 10 Patent claims

1. Solid state laser having at least one diode laser row (7) for generating the pumping radiation, a 5 rod-shaped active element (1) and a cylindrical pumping cavity which is arranged around the active element (1) such that its cylinder axis runs parallel to the axis of the active element (1) and to the diode laser row (7), characterized in that the pumping cavity is a 10 monolithic block (2) with a continuous tubular channel (3) in which the active element (1) is f ixed by means of adhesive (4), and the surface of the block (2) has a reflecting layer in which at least one gap (6) is present via which the pumping radiation emitted by the diode laser row (7) is launched into the block (2).

2. Solid state laser according to claim 1, characterized in that the block (2) has a crosssectional shape which has its largest extent (D) in the direction of the connecting line between a gap (6) and the cylinder axis.

3. Solid state laser according to claim 2, characterized in that the cross-sectional shape is elliptical.

4. Solid state laser according to claim 2, 25 characterized in that the cross-sectional shape resembles a rectangle, the broad sides having the shape of a convex circular arc, and the long sides not being shorter than 0.3 - D.

5. Solid state laser according to claim 4, 30 characterized in that the long sides are plane-parallel surfaces.

6. Solid state laser according to claim 4, characterized in that,in the middle the long sides have a deviation from the plane-parallel surface which is not greater than 0.2 - D.