Gas discharge laser, method of operating a gas discharge...

Coherent light generators – Particular active media – Gas

Reexamination Certificate

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C372S058000, C372S061000, C372S065000

Reexamination Certificate

active

06798814

ABSTRACT:

TECHNICAL BACKGROUND
The present invention relates to a gas discharge laser having a discharge tube, in which a gas is present. The discharge tube has at least one aperture or window through which a laser beam emerges, or at which a laser beam is reflected. It may also be possible that the at least one aperture or window reflects a part of the laser beam and let through a part of the laser beam. At least one gas withdrawal point is provided for taking out a partial amount of the gas present within the gas discharge tube. The withdrawn amount of gas is guided through a filtering means, and is introduced in at least one gas inlet point in the zone of the aperture.
The invention further relates to a method of operating a gas discharge laser having a discharge tube, in which a gas is present, and which has at least one aperture through which a laser beam emerges, or at which a laser beam is reflected. At least a partial amount of the gas contained within the discharge tube is taken out, is cleaned by means of a filtering means and is fed in again in the zone of the at least one aperture. The cleaned gas flow may be fed in again in the zone of the at least one aperture in such a manner that a gas flow is formed directed away from the aperture. This can, for example, be achieved in that the cleaned gas flow will be directed towards the aperture, thereby rinsing the aperture free from possible dust particles and other adhering matter, whereupon the gas flow then is deflected into a direction directed away from the aperture. By this deflection of the gas flow, the penetration of dust particles from the interior of the discharge tube is avoided to a large extent, at least, however, impeded.
Finally, the invention relates to a novel use of a filtering means in the aforementioned technical field.
In gas discharge lasers—in particular excimer lasers—a gas is excited to emit light by means of an excitation and/or discharge reaction. The molecular or atomic compounds particularly suited for excimer lasers, which are also designated as gas or gases in the following for convenience, present energy transmissions by means of which electromagnetic radiation is preponderantly emitted in the ultraviolet spectral range. Therefore, excimer lasers nowadays represent the by far most intensive UV radiation sources. However, a high energy density is necessary for the excitation of the gases so as to generate stimulated emissions, which energy density is fed in in the form of an intensive pulsed electron beam or within the scope of a high-voltage discharge, since the effective cross-section for stimulated emission for the individual initial compounds within the gas is relatively small.
In the technical configuration of gas discharge lasers of this type, and in particular in excimer lasers, a particularly high effort in the constructional and set-up configuration has to be implemented for the components necessary for the targeted energy supply. Due to the very high energies which have to be fed in the excimer gas, impurities moreover occur in a system-contingent manner resulting, for example from local electrode fusing by compound reactions between the gas and the individual laser components. Said impurities, however, do not only affect the laser process as such, but are able to considerably reduce the beam outlet intensity of the laser due to deposits on the inner wall of the gas chamber enclosing the gas, and in particular in the zone of the initially mentioned apertures of the laser. Such deposits or dust particles hence can significantly affect the permeability of an outlet aperture or the reflection capability of a mirror. This problem increasingly occurs in lasers having a short wavelength, such as UV lasers.
From U.S. Pat. No. 4,534,034, a pumped gas discharge laser is believed to be known, wherein the gas present in the discharge tube is conveyed by means of a thereto connected but outside situated circulation pump through an electrostatic filter, so as to filter out dust particles etc. present in the gas. The cleaned gas flow is in each case fed into a zone of a aperture of the discharge tube after passing through a particular relaxation zone, so that the apertures possibly are kept clean by said gas flow, hence free from dust particles or other kinds of deposits. This solution may be subject to the problem that electrostatic dust collectors have a relatively large volume and require a separate voltage supply. Moreover, intricate relaxation zones here appear to be necessary in the gas conveyance system.
In U.S. Pat. No. 5,029,177, an excimer laser is believed to be disclosed, wherein likewise an electrostatic filter is provided for cleaning the gas in the discharge tube. For the transport of the gas flow through the electrostatic filter, the ventilator drum present in the discharge tube is used. The gas cleaned in the electrostatic filter flows in through an annular gap immediately in front of a aperture of the discharge tube. A pre-chamber in front of the apertures has a structure breaking the shock waves from the discharge, and impeding the gas flow and the movement of gas particles from the discharge towards the aperture.
From EP 0 669 047 B1, means are believed to be known for maintaining a clean laser aperture. Here, in particular, the structure of a pre-chamber in front of the aperture is described in more detail. The particular structure of the pre-chamber—as in the above-mentioned publication—serves the purpose of creating a gas flow directed away from the aperture in the direction of the discharge, so as to impede the movement of dust into the pre-chamber in front of the aperture. Here, in addition, it is disclosed that apart from the gas present in the discharge tube, a second gas is used. To avoid an electric charge of the gases, an electrically grounded screen in front of the aperture is proposed.
SUMMARY OF THE INVENTION
It is an object of the invention to specify a technically simple solution for keeping apertures or windows in gas discharge lasers clean.
It is an other object of the invention to specify a cost efficient solution for keeping apertures or windows in gas discharge lasers clean.
It is an other object of the invention to specify a technically simple method to keep apertures or windows in gas discharge lasers clean.
According to one aspect of the present invention, this is achieved by means of a gas discharge laser comprising a discharge tube, in which a gas is present. This discharge tube has at least one aperture through which a laser beam emerges, and/or from which the laser beam is reflected. In the discharge tube, at least one gas withdrawal point is present for withdrawing a partial amount of the gas present in the discharge tube, and to feed it to a sintered filter for cleaning the withdrawn amount of gas. The sintered filter cleans the infed gas as far as possible from dust particles and such like. The cleaned gas flow is then fed in via at least one gas inlet point in the area of the at least one aperture, so that a dirtying of the aperture by dust particles present in the discharge tube may be prevented. Moreover, the respective aperture will probably be rinsed free from dust particles possibly adhering thereto.
In one exemplary embodiment of the present invention, the cleaned gas flow counteracts the pressure waves generated by the discharge, which transport dust particles away from the discharge—and inter alia towards the apertures.
The present invention may be based on the idea of using for the first time a sintered filter for cleaning a gas flow withdrawn from the discharge tube. As may be derived from the preceding explanations as to the documents mentioned above, electrostatic filters have always been used up to date for cleaning gases in technical devices of the kind of interest here. Surprisingly, however, it has turned out that sintered filters, despite their high flow resistance which has to be overcome by the gas flow, can also be used in gas discharge lasers of the initially mentioned kind. With sintered filters, a cost-efficient and maintenance-fr

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