Coherent light generators – Particular resonant cavity – Specified cavity component
Reexamination Certificate
2001-02-16
2002-08-27
Leung, Quyen (Department: 2828)
Coherent light generators
Particular resonant cavity
Specified cavity component
C372S061000, C372S034000
Reexamination Certificate
active
06442187
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a laser device comprising a laser active medium, an optical resonator system defining an optical axis, exciting means for exciting said laser active medium and enabling a stimulated emission of radiation of said laser active medium, and cooling means having first and second cooling elements arranged in spaced opposing relationship with surfaces facing each other, wherein said laser active medium is provided between said cooling elements along said optical axis.
Laser devices as indicated above are generally known in the art. Basically, all kinds of laser devices have to deal with the problem of heat dissipation. For example, in a solid state rod laser device the surface of the rod which is subjected to excitation by a pumping light source such as a Xe lamp, is cooled and thus, a temperature gradient dependent on the cooling power and efficiency of the cooling means is generated. Thermal effects inside the rod strongly influence the optical properties of the laser active material such as the refractive index varying in dependence of the temperature distribution and birefringence, as well as of the further optical elements, namely the spherical resonator mirrors. It is difficult and costly to design a high quality laser system which is free of thermal degradation when the amount of heat generated during laser operation increases due to a desired increase of output power.
Consequently, laser devices were designed to improve both of the following aspects:
(i) increasing heat dissipation in order to achieve higher output power per unit volume of active laser material, and
(ii) minimising the influence of the temperature gradients on the optical properties of the laser device.
In the field of gas lasers using RF-excitation so called slab lasers or zig-zag lasers were developed. These lasers have opposing RF electrodes formed of rectangular plates having a reflective surface. These electrodes are arranged such that therebetween a volume is formed with the resulting gap being filled with a laser active gas. The distance between these electrodes is typically about 2 to 4 mm and may be increased up to 1 cm if an additional gas flow is provided, while the width of the gap in the direction perpendicular to the distance is in the order of several cm. The electrodes are cooled and thus, the heat is dissipated from the laser gas by conduction cooling with the surfaces of the electrodes. As a result a large cooling area and thus a large cooling power is established.
Simultaneously, this structure provides a temperature gradient which is substantially directed merely perpendicular to the surfaces of the electrodes. except for distortions at the lateral ends of the plates. This structure tends to cancel the effect of temperature gradients since the laser beam zigzags in the plane of temperature variation. However, the relatively large distance of the plates in the cm range results in a wide range of reflecting angles in this plane. Therefore, the resulting made of the radiation is no longer determined by the spherical resonator mirrors to be single mode but consequently the radiation is a multi mode one. However, many applications of laser devices require a fine focusing of the laser beam to achieve a high power density and thus, a multi mode beam is not desired.
Furthermore, so called “waveguide” laser devices are known in the art wherein the radiation field in both of the transverse directions is confined by highly polished and highly reflective side walls. The radiation mode is completely determined by the waveguide cavity, whereas the resonator is merely composed of plane mirrors. In order to obtain single mode radiation from a waveguide laser the dimensions of the waveguide cavity are restricted to merely a few mm (2-4 mm). Waveguide lasers have excellent thermal properties, but the output power is low due to the small active volume.
U.S. Pat. No. 5,123,028 disposes a CO
2
stab laser including a waveguide arrangement formed by a pair of spaced apart planar electrodes having opposed light reflecting surfaces. The confinement of the radiation in the plane parallel to the electrode surfaces is attained by a negative branch unstable resonator.
WO 95/2909 describes a CO
2
slab waveguide laser wherein the light propagation path in plane parallel to the waveguide surfaces is folded by spherical mirrors.
GB 2276031 discloses a solid state laser device comprising a slab-shaped laser medium which has a pair of optically smooth surfaces. In the width dimension of the slab-shaped laser medium an unstable free space resonator is provided, whereas in the narrow spaced thickness direction of the slab medium confinement of the radiation is achieved by the optically smooth surfaces of the medium.
In U.S. Pat. No. 4,719,639 a laser device is disclosed, wherein by means of highly polished and highly reflective electrodes having a small distance of less than 5 mm in one transverse dimension waveguide conditions are created, while the other transverse dimension remains “open”, i.e. the optical cavity in this direction is confined by spherical resonator mirrors. An arrangement as mentioned in the above disclosure succeeds in increasing the active volume in comparison to the waveguide laser, while on the other hand, a single mode radiation can be obtained. However, the distance of the electrodes is limited to typically 2-3 mm. In addition the electrodes require a high optical quality as well as high parallelism.
SUMMARY OF THE INVENTION
In view of the above mentioned problems and disadvantages of the prior art it is therefore an object of the present invention to provide a laser device of a compact structure having increased optical power and outputting finely focusable radiation.
The above mentioned object is solved by a laser device comprising a laser active medium, an optical resonator system defining an optical axis exciting means for exciting said laser active medium and enabling a stimulated emission of radiation of said laser active medium, and cooling means having first and second cooling elements arranged in spaced opposing relationship with surfaces facing each other, wherein said laser active medium is provided between said cooling elements along said optical axis and the laser device is characterised in that an optical element is provided, arranged within the optical path formed by said optical resonator system and having a refractive power in a first plane along the optical axis and perpendicular to said surfaces, differing from a refractive power in a second plane along the optical axis and perpendicular to said first plane, wherein said refractory power in said first plane of said optical element is adjusted so as to prevent interaction of the lowest order radiation mode with the surfaces of said first (
61
) and second (
62
) cooling elements.
The term “refractive power” refers to all kinds of optical elements, particularly to refractive, diffractive and reflective optical elements having the ability to collimate or disperse a light beam.
The term “along the optical axis” includes all arrangements of said planes parallel to the optical axis or the optical axis being within said planes.
“A plane along the optical axis perpendicular to said surfaces” represents a plurality of planes, which are parallel if the surfaces of the cooling elements are even and include a certain angular range if the surfaces are bent. The above definition may also include the case in which the “planes” are no longer even but comprise a certain curvature according to the spaced relationship of the cooling elements, for instance if the surfaces of the cooling elements are cylindrical with non-coinciding centres of curvature.
The definition of radiation mode used in this application is, with reference to the related art, to be understood in the following way:
In case of a free space propagation of the laser beam the TEM∞ mode constitutes the so-called fundamental or lowest order mode whereas the all other modes are indicated as higher order modes.
For a la
Dutov Alexander Ivanovich
Kuleshov Alexey Alexeevich
Novoselov Nikolay Alexeevich
Sokolov Alexander Alexandrovich
Gosudarstvennoye Predpriyatie Nauchnoissledovatelsky Institut La
Greenblum & Bernstein P.L.C.
Leung Quyen
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