GB2083687A - Circulating gas laser - Google Patents

Abstract

In a circulating-gas laser including a circulation system having a gas propelling means 34 for propelling lasing gas into and lasing gas products out of the lasing system, and a cooling system for cooling the lasing gas products, the gas propelling means includes at least one propelling surface having thereon a catalyst system containing a catalyst which promotes formation of lasing gas from lasing gas products, eg. the recombination of CO and O2 into CO2. The propelling means may comprise a fan having fan blades 35 coated with a catalyst system 42 which is catalytically active at low temperatures, for example an SnO2-supported catalyst. Providing the catalyst in this form rather than, for example, in a conventional honeycomb structure lying across the gas flow minimises the impedance to gas flow whilst maximising contact between catalyst and lasing gas products. <IMAGE>

Description

SPECIFICATION Improvements in or relating to gas lasers The invention concerns improvements in or relating to gas lasers and is particularly, but not exciusively, concerned with the use of a catalyst in a circulatinggas laser, the catalyst having the effect of improving the performance of the laser.

Atypical gas laser includes a resonant cavity within an envelope containing a gaseous lasing medium which includes an active lasing-gas. An energy source is provided for pumping the active lasing gas to an excited state in order to induce lasing. Typically the energy source consists of a uniform electric discharge across the lasing medium between a principal anode discharge electrode and a principal cathode discharge electrode.

In a "TE" ("Transversely Excited") laser the discharge occurs transverse of the laser axis, this mode of operation often being used in high pressure lasers.

A gas laser may be pulsed or "CW" ("Continuous Wave") and may be unsealed with a flowing-gas system in which the lasing medium is continuously renewed from an external supply or sealed. A sealed laser may employ a circulating-gas system in which a gas-propelling means circulates the lasing medium around the laser inside the envelope, this system often being employed in a pulsed laser having a high pulse repetition frequency and an electric discharge energy source. In this type of laser, the gas lying between the principal discharge electrodes (i.e. in the discharge volume) during one pulse of discharge is incapable of being replaced by more gas within the envelope by natural convection before the next pulse of discharge occurs. A gas-propelling means is therefore employed to aid natural convection in renewing the discharge volume.

Examples of gas lasers having radiation at micrometre wavelengths are lasers which have an active lasing-gas with generalised molecuiarformula MX where M is a hydrogen isotope and Xis a halogen for example, deuterium fluoride.

A CO2 laser, employing, typically a lasing medium consisting of a mixture of CO2, N2 and He is a gas laser which is exploited for its radiation at 10.6 um.

A disadvantageous phenomenon in the aforementioned gas lasers is the breakdown of the active lasing-gas under the influence of the electric discharge. Unlike an unsealed, flowing-gas system in which the gaseous products are continuously replaced, in a sealed laser the breakdown products remain in the lasing medium and result in degradation of laser performance. In a CO2 laser, for example, the CO2 dissociates to CO and 02, resulting in the breakdown of the uniform electric discharge into localised arcs, thus resulting in a lowering of both output power and laser life. The problem becomes more severe as the CO2 pressur in the laser increases.

A known method of alleviating the problem of arcing in a CO2 laser which is often used in high pressure CO2 lasers involves irradiation of the lasing medium in the discharge volume by a UV source prior to firing the discharge. The UV causes release of electrons by photoionisation of the medium (ie "pre-ionisation") thus promoting a uniform discharge between the principal discharge electrodes.

A second known method of alleviating the problem of arcing in a sealed CO2 laser is the inclusion of a heated platinum wire in contact with the lasing medium, the platinum catalysing the recombination of the CO2 dissociation products, CO and 02. A disadvantage of this method of catalysis is that the platinum wire must be heated to an operating temperature of approximately 1 000 C to be effective.

This therefore entails a high power consumption with a consequent high power supply weight and also causes a deterioration in the quality of the output beam due to heating and expansion of the resonant cavity. Additionally, the life of the platinum wire is limited due to its fragility.

In a system having a catalyst which acts upon a flowing fluid it is known to direct the fluid through a matrix in which the catalyst is included. It is known, for example; in a chemical reaction between flowing gaseous reactants which produce gaseous products, to pass the reactants through a wire mesh in which a catalyst is incorporated. If the reaction requires minimum attenuation of gas pressure as the reaction proceeds, the mesh is a disadvantage as it impedes the gas flow.

The purpose of the invention is to provide a circulating-gas laser having a catalyst system which gives minimum impedance to the circulation of gas, the catalyst system incorporating a catalyst which does not require a high operation temperature.

According to the present invention there is provided a circulating-gas laser having a lasing system for initiating, amplifying and emitting laser radiation, a circulation system having a gas propelling means for propelling lasing gas into and lasing gas products out of the lasing system, a cooling system for cooling the lasing gas products, the gas propelling means having a propelling surface or surfaces which include(s) a catalyst system containing a catalyst which promotes formation of lasing gas from the cooled lasing gas products.

Preferably the gas propelling means is a fan having fan blades, preferably on which the catalyst system is deposited.

In a circulating-gas CO2 laser having an electric discharge for inducing lasing, wherein the discharge causes dissociation of CO2 to CO and 21 a preferred catalyst system is tin iV oxide-supported-palladium which is known to be effective at temperatures of -26 C and above, or tin IV oxide-supportedplatinum.

The invention will now be described by way of example only with reference to the accompanying diagrams of which: Figure 1 shows diagramatically a sealed, circulating-gas, pulsed UV-pre-ionisation, "TEA" ("Transversely Excited Atmospheric Pressure") CO2 laser having a high pulse repetition frequency and consisting of a recirculation system and a lasing system.

The lasing system is now shown in detail and the recirculation system is shown diagrammatically in partial cross section in Figure 1.

Figure 2 is a section along line AA' of Figure 1 showing the lasing system in detail.

Figure 3 is a partial section along line BB' in Figure 1 looking in the direction indicated by the arrows, showing diagrammatically the lasing system in detail.

Figure 4 is a circuit diagram for the lasing system of Figure 2 and Figure 3.

In Figure 1 a sealed, circulating-gas, pulsed, UV-pre-ionisation TEA CO2 laser having a high pulse repetition frequency consists of a recirculation system 30, and a lasing system.

A lasing medium 2, consisting of a mixture of 40% CO2, 20% N2 and 40% He (percentages by volume) at atmospheric pressure is contained within and is free to flow between the recirculation system and the lasing system.

The recirculation system includes a recirculation channel 31, enclosed within a channel wall 32, and a fan having a driving means (not shown in the diagram), a fan rotor 34, and fan rotor blades 35.

The channel wall 32, consists of an outer wall 36, and an inner wall 37, the inner and outer walls being spaced apart so as to provide a liquid cooling duct 38, therebetween, the liquid cooling duct containing a liquid coolant which is circulated within the duct by a liquid coolant circulation system (not shown in the diagram).

The recirculation channel 31 connects with the lasing system via gas-tight joints 39 and 40, which permit flow of the lasing medium between the recirculation and the lasing systems, but prevent escape of gas to outside the laser so that the lasing and recirculation systems together form a sealed laser.

When in use, the fan driving means drives the fan so as to cause circulation of the lasing medium 2, within the laser in the direction shown by arrow 41.

In Figure 2, the lasing system, having an axis 5, includes an envelope 1, of glass and expansionmatched Ni-Fe-Co alloy which confines the lasing medium 2.

Afully reflecting plane mirror of gold-plated copper 3, and an 85% reflecting plane mirror of multilayer dielectric-coated germanium 4, both per pendicularto the axis 5, define two ends of a resonant cavity within the lasing system.

Pressed nickel Rogowski-profiled cathode 6, and anode 7, mounted in alumina insulation 8 and 9, and separated by alumina spacers 10 and 11, constitute principal discharge electrodes which provide means of inducing a pulsed electric discharge transverse to the lasing system's axis 5. Connections to the electrodes are made via cylinder copper electrical connection tubes 12 and 13.

UV pre-ionisation radiation results from sequential sparking across two series of six spark gaps, one series down a first side of the inside of the envelope 1, and the other series inside the envelope on the side opposite the first side. The spark gaps, one of which is labelled 14 in Figure 2 and Figure 3, are 2 mm wide and lie between pointed ends of tungsten 'T' pins, two of which are labelled 15a and 15b in Figure 2 and Figure 3. The 'T' pins are connected to earth via 4pF capacitors, two of which are labelled 16 and 17 in Figure 3 and Figure 4.

Energy for the spark discharge is provided by a 30 kV HT supply labelled 18 in Figure 4. This charges 900 pF capacitors 19 and 20 which then dischargeto generate sparks in sequence between the tungsten 'T' pins when spark gap 21 is triggered. The sparks thus generated between the 'T' pins, result in UV radiation which pre-ionises the lasing medium.

Following the triggering of spark gap 21 and the pre-ionisation of the lasing medium, discharge between the principal discharge electrodes 6 and 7, is induced after a delay time by triggering spark gap 22 which allows a 10 nF capacitor 23 which is charged by 30 kV HT supply 24, to discharge to earth between electrodes 6 and 7. The pre-ionisation of the medium before the triggering of spark gap 22 promotes uniform discharge between the principal discharge electrodes.

Avariable delay 25, connected between the two spark gaps 21 and 22 governs the delay time before the triggering of the two.

On inducing the uniform discharge between the principal discharge electrodes, the CO2 is pumped to an excited molecular state, thus resulting in lasing and causing a pulse of laser radiation to be emitted from mirror 4.

The principal discharge also has the effect of causing dissociation of some of the CO2 to CO and 02. In order to counteract this, the fan rotor blades 35 are coated with a catalyst system 42 of tin IV oxide-supported-palladium. As the lasing medium is recirculated by the fan it comes into contact with the fan blades and the catalyst system which thus promotes recombination of CO and 02 to CO2 at the temperature of the gas contiguous to the fan blades.

An advantage of fan blades coated with a catalyst system of tin IV oxide-supported-palladium as a means of catalysis is that the catalyst system is effective at ambient temperatures around the fan blades and does not require means for maintaining it at temperatures other than ambient temperatures.

The use of a catalyst system deposited on the blades of a fan in a CO2 laser employing the two functions of catalysing oxidation of CO by 02 to CO2 and circulation of gas has the advantage that the catalyst system gives the minimum of impedance to the flow of recirculating gas. A minimum of power is therefore expended in causing gas circulation.

The invention is not restricted to the use of tin IV oxide-supported-palladium as the catalyst system.

Other catalysts which promote combination of 02 and CO to CO2 may be used, for example tin IV oxide-supported-platinum.

The quantity and disposition of catalyst system depends upon for example, the pressure and volume of CO2 within the laser, the pulse repetition frequency and the acceptable degradation in performance.

In order to maintain an equilibrium composition of lasing medium, sufficient catalyst system is used to cause recombination of CO and 02 to CO2 at the same rate as dissociation of CO2 to CO and 02.

In the past CO2 lasers have had limited useful lives due to the breakdown of CO2 and therefore an unsealed flowing gas laser having a system for renewing the lasing medium has been necessary where prolonged life has been required. The requirement for a renewal system has involved bulky peripheral apparatus and thus the laser has not been compact and easily portable. However, by incorporating into a sealed laser a catalyst system which promotes oxidation of CO by 02 to CO2, a sealed, compact, portable laser with a long useful life is obtained.

The invention is not confined to CO2 circulatinggas lasers, but includes other circulating-gas lasers in which a chemical change is induced in the lasing medium by the lasing system.

Claims (7)

1. Acirculating-gas laser including a lasing system for initiating, amplifying and emitting laser radiation, a circulation system having a gas propelling means for propelling lasing gas into and lasing gas products out of the lasing system, a cooling system for cooling the lasing gas products, the gas propelling means including at least one propelling surface having thereon a catalyst system containing a catalyst which promotes formation of lasing gas from the cooled lasing gas products.

2. A circulating-gas laser as claimed in claim 1 wherein the gas propelling means comprises a fan having at least one fan blade with at least one fan blade surface overlayed by the catalyst system.

3. A circulating-gas laser as claimed in either claim 1 or claim 2 wherein the lasing gas products include gases which are the dissociation products of at least one component gas of the lasing gas and wherein the catalyst promotes recombination of the dissociation products.

4. A circulating-gas laser as claimed in claim 3 wherein the at least one component gas of the lasing gas comprises CO2 which partially dissociates to CO and 02 during lasing.

5. A circulating-gas laser as claimed in claim 4 wherein the catalyst system consists of SnO2- supported catalyst.

6. A circulating-gas laser as claimed in claim 5 wherein the catalyst is platinum or palladium or both.

7. A circulating-gas laser substantially as herein described with reference to the accompanying drawings.