GB2031643A - Lasers - Google Patents

Abstract

A gas discharge laser including a cathode electrode made of a noble metal. Preferably the noble metal is silver or platinum. The gas discharge may be produced in continuously flowing carbon dioxide.

Description

SPECIFICATION Improvements in or relating to gas discharge lasers The present invention relates to gas discharge excited lasers and, more specifically, to such lasers in which the lasing medium includes carbon dioxide.

Many gas discharge excited carbon dioxide lasers include cathode electrodes made of copper. A problem which arises is that corrosion of the copper leads to a deterioration of the stability of the discharge, which in turn results in the formation of filamentary arcs in the discharge. This is a serious problem, as it is necessary to dismantle and clean the cathode after every few tens of hours or so of operation of the laser. A further problem which arises in the operation of such lasers is the dissociation of the carbon dioxide in the lasing medium. This results in a loss of efficiency in operation of the laser, and is usually overcome by means of a separate catalyst to recombine the carbon monoxide and oxygen, or by replacement of the lasing medium.

According to the present invention, there is provided a gas discharge laser including a cathode electrode made of a noble metal.

Preferably the noble metal is silver or platinum.

The invention will now be described, by way of example, with reference to the accompanying drawing which is a diagrammatic representation of a gas discharge laser embodying the invention.

Referring to the drawing, a gas discharge laser consists of a closed system of ducting 1 through which a gaseous lasing mixture containing carbon dioxide 2 is circulated by means of a fan 3. The temperature of the lasing medium 2 is controlled by means of two heat exchangers 4 and 5. A region 6 of the system of ducting 1 is adapted to act as an optical cavity 7 in which lasing action takes place by means of a system of mirrors 8 and output window 9. In the region 6 there are situated two pairs of electrodes 10 and 11, by means of which electric discharges can be maintained to excite the lasing medium. The cathode electrodes 10' and 11' are made of silver, and the anodes 10" and 11" are made of nickel.

The use of silver for the cathodes 10' and 11' instead of the more usual copper has a number of advantages. Although the silver does tend to tarnish in use, silver oxide is unstable and decomposes under the action of light and at temperatures higher than 160or, which are reached in operation. Hence the cathodes 10' and 11' are to a large extent self-cleaning. In any case, silver oxide is a relatively good conductor of electricity so that the electrical properties of the cathodes 10' and 11' are relatively unaffected by the presence of any silver oxide.

Cathode lifetimes of hundreds of hours are obtained with silver cathodes.

Another advantage is that silver is a known catalyst for oxidising reactions, and also silver oxide is efficient at dissociating ozone to form oxygen. In practice it has been found that whereas, using copper cathodes decomposition of the carbon dioxide is often > 30%; using silver cathodes it is less than 10%, much reducing the need for additional catalysts.

It is known to use catalysts in carbon dioxide lasers to prevent deterioration of the lasing medium, but in existing designs of lasers internal catalysts beds or grids rapidly become poisoned by copper sputtered from the cathodes. By making the cathodes 10' and 11' from silver, they act as catalysts themselves, and even if separate silver catalytic regions are used, they are replenished rather than poisoned by material sputtered from the cathodes 10' and 11'.

Another noble metal which can be used for the cathodes 10' and 11' is platinum. Not only is it more active as a catalyst than silver, but its use tends to reduce the dissociation of the carbon dioxide in the first place. In addition it does not form oxides thereby improving the stability of the operation of the laser and reducing even further the need for dismantling of the laserforcleaning purposes. It is, of course, more expensive than silver, but in the larger sizes of laser for use in industrial situations where the loss of production caused by having to take the laser out of operation to dismantle it and clean the electrodes is important, then the extra expense is acceptable.

Athird noble metal which can be used is gold. This metal shares with platinum the advantage that it does not form an oxide layer, and has the advantage that it is less expensive. On the other hand, it does not have the same catalytic properties as silver or platinum.

1. A gas discharge laser including a cathode electrode made of a noble metal.

2. A gas discharge laser according to Claim 1, wherein the noble metal is silver, platinum or gold.

3. A gas discharge laser according to Claim 2, wherein the noble metal is platinum.

4. A gas discharge laser substantially as hereinbefore described with reference to the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Improvements in or relating to gas discharge lasers The present invention relates to gas discharge excited lasers and, more specifically, to such lasers in which the lasing medium includes carbon dioxide. Many gas discharge excited carbon dioxide lasers include cathode electrodes made of copper. A problem which arises is that corrosion of the copper leads to a deterioration of the stability of the discharge, which in turn results in the formation of filamentary arcs in the discharge. This is a serious problem, as it is necessary to dismantle and clean the cathode after every few tens of hours or so of operation of the laser. A further problem which arises in the operation of such lasers is the dissociation of the carbon dioxide in the lasing medium. This results in a loss of efficiency in operation of the laser, and is usually overcome by means of a separate catalyst to recombine the carbon monoxide and oxygen, or by replacement of the lasing medium. According to the present invention, there is provided a gas discharge laser including a cathode electrode made of a noble metal. Preferably the noble metal is silver or platinum. The invention will now be described, by way of example, with reference to the accompanying drawing which is a diagrammatic representation of a gas discharge laser embodying the invention. Referring to the drawing, a gas discharge laser consists of a closed system of ducting 1 through which a gaseous lasing mixture containing carbon dioxide 2 is circulated by means of a fan 3. The temperature of the lasing medium 2 is controlled by means of two heat exchangers 4 and 5. A region 6 of the system of ducting 1 is adapted to act as an optical cavity 7 in which lasing action takes place by means of a system of mirrors 8 and output window 9. In the region 6 there are situated two pairs of electrodes 10 and 11, by means of which electric discharges can be maintained to excite the lasing medium. The cathode electrodes 10' and 11' are made of silver, and the anodes 10" and 11" are made of nickel. The use of silver for the cathodes 10' and 11' instead of the more usual copper has a number of advantages. Although the silver does tend to tarnish in use, silver oxide is unstable and decomposes under the action of light and at temperatures higher than 160or, which are reached in operation. Hence the cathodes 10' and 11' are to a large extent self-cleaning. In any case, silver oxide is a relatively good conductor of electricity so that the electrical properties of the cathodes 10' and 11' are relatively unaffected by the presence of any silver oxide. Cathode lifetimes of hundreds of hours are obtained with silver cathodes. Another advantage is that silver is a known catalyst for oxidising reactions, and also silver oxide is efficient at dissociating ozone to form oxygen. In practice it has been found that whereas, using copper cathodes decomposition of the carbon dioxide is often > 30%; using silver cathodes it is less than 10%, much reducing the need for additional catalysts. It is known to use catalysts in carbon dioxide lasers to prevent deterioration of the lasing medium, but in existing designs of lasers internal catalysts beds or grids rapidly become poisoned by copper sputtered from the cathodes. By making the cathodes 10' and 11' from silver, they act as catalysts themselves, and even if separate silver catalytic regions are used, they are replenished rather than poisoned by material sputtered from the cathodes 10' and 11'. Another noble metal which can be used for the cathodes 10' and 11' is platinum. Not only is it more active as a catalyst than silver, but its use tends to reduce the dissociation of the carbon dioxide in the first place. In addition it does not form oxides thereby improving the stability of the operation of the laser and reducing even further the need for dismantling of the laserforcleaning purposes. It is, of course, more expensive than silver, but in the larger sizes of laser for use in industrial situations where the loss of production caused by having to take the laser out of operation to dismantle it and clean the electrodes is important, then the extra expense is acceptable. Athird noble metal which can be used is gold. This metal shares with platinum the advantage that it does not form an oxide layer, and has the advantage that it is less expensive. On the other hand, it does not have the same catalytic properties as silver or platinum. CLAIMS

1. A gas discharge laser including a cathode electrode made of a noble metal.

2. A gas discharge laser according to Claim 1, wherein the noble metal is silver, platinum or gold.

3. A gas discharge laser according to Claim 2, wherein the noble metal is platinum.

4. A gas discharge laser substantially as hereinbefore described with reference to the accompanying drawing.