EP0263118A1 - A multiple metal reservoir discharge tube for metal vapour lasers - Google Patents

Abstract

A metal vapor laser, intended to be excited by a pulsed electric discharge whose volume is defined by a chamber (12) extending between two laser electrodes (2), comprises several reservoirs (13) of metals placed at intervals regular inside the chamber over its entire length. These reservoirs are intended to prevent the molten metal from amalgamating to form an accumulation of extended metal and to allow the operation of the laser in positions other than strictly horizontal positions. The metal reservoirs are preferably defined by a series of rings (6) made of a material coaxial with the axis of the laser and arranged along a tube (3, 8, 9) forming the chamber.

Description

"A Multiple Metal Reservoir Discharge Tube for Metal Vapour Lasers" This invention relates to lasers and has particular application to lasers of a type wherein a metal vapour is excited by a pulsed electrical discharge.

Pulsed metal vapour lasers allow the generation of ultra-violet, visible and infrared radiation. Many examples of these devices are well-known in the scientific literature and they have applications in areas as diverse as the separation of atomic isotopes, using copper as the lasant, communications, with manganese as the lasant, treatment of cancer, by photochemotherapy, with gold as the lasant, and photochemical research, with strontium as the lasant. These applications, inter alia, require the greatest possible efficiency for generating the laser radiation and some additionally require laser beams of uniform circular cross-section of small (1-lOmm) diameter, produced from devices having small departures from the strict horizontal plane.

It is the aim of the present invention to improve the efficiency and increase the operating lifetime of metal vapour lasers 'while producing output beams of uniform circular cross-section with diameters as small as 1-lOmm and to allow metal vapour lasers to be operated in positions in which the laser axis does not have to be strictly horizontal.

Metal vapour lasers generally comprise two electrodes between which there is a column of gas known as the discharge volume. Initially, the discharge volume contains a buffer gas and beads (uniform or otherwise) of a metal placed along the bottom of the innermost surface of the discharge chamber. The application of a fast and high voltage pulse to the discharge volume produces a stream of electrons which heat the gas and thereby transfer heat to the inner walls of the chamber which substantially confine the active volume. The walls of the chamber and the metal therein are both heated; with repeated discharges and appropriate insulation the temperature may rise sufficiently to melt and vapourise the metal which can then be electrically excited and lasing may ensue. This class of laser is well known in the art.

Hitherto, the technique generally employed for introducing metal to the discharge chamber has been to place small pieces of metal, distributed as uniformly as possible, along the bottom of the innermost surface.

However, when the discharge is operating, the uniform nature of the distribution is lost as vibration, shock waves and running of the discharge vessel off the horizontal axis cause the majority of the individual pieces of metal to coalesce into one large, centrally- located accumulation. This accumulation is held together by the surface tension of the molten metal. The lasing action now results from the evaporation of metal vapour from the centrally located accumulation into the discharge volume. Long term running of the laser causes migration of the metal £o the cold ends of the discharge vessel where it again coalesces to form large accumulations. This technique of metal distribution exhibits several disadvantages. Firstly, both the accumulating of the metal in the centre of the vessel, and the subsequent redeposition at the ends, aperture the laser resonator, reducing the optimum output and the system efficiency. Secondly, the non-uniform distribution of metal vapour in the discharge vessel reduces the efficiency of extraction of the laser energy because not all of the vessel is operating at the correct vapour pressure for the optimum generation of laser emission. Thirdly, the accumulation produces a beam cross-section that is non-circular and this is deleterious for many applications. Fourthly, the discharge vessel must be run in a horizontal plane or the molten metal will roll out of the vessel and not be available for further generation of laser emission. The above effects restrict the diameter of a commercially viable metal vapour laser discharge vessel to greater than approximately 20mm and limit the efficiency of operation of the laser. These restrictions are undesirable as there is significant demand for a metal vapour laser with a beam diameter in the range of 1 to 10mm, such beams being generated by discharge vessels of approximately the same diameter.

The present invention consists in a metal vapour laser having a tubular chamber defining a gas discharge volume characterized in that along the chamber are provided means defining a plurality of metal reservoirs located uniformly along the length of the chamber.

It has been found that the deleterious coalescing effects can be eliminated in small diameter discharge vessels by separating the beads of metal with spacer segments of refractory material in a manner so as to form a multiplicity of uniformly spaced, individual metal reservoirs. The spacer segments are arranged co-axially and transform the wall of the discharge vessel into a multiple reservoir structure that provides individual wells for the location of metal beads and prevents the molten metal from coalescing and accumulating. Individual reservoirs can be of any desired length but are generally configured so as to maximise their number. Enough metal can then be loaded into the discharge vessel to permit a commercially viable time (of say, at least 400 hours) between metal reloads to be achieved in a small diameter vessel.

In lasers constructed according to a preferred form of the invention surface tension of the molten metal holds the individual spacers in place. Upon cooling of the tube, the metal provides a high temperature braze that welds the segments to the walls of the discharge vessel. Typically, the structural member and the segments comprising the inner wall are made of a refractory ceramic, such as alumina or beryllia. However, other suitable materials can also be used.

There are numerous advantages to be derived from the above system, with the most notable being the elimination of the accumulation of molten metal in the discharge vessel and the subsequent apertu ing of the laser output. This enables the construction of efficient, commercially viable metal vapour lasers with vessel diameters significantly less than 20mm. These vessel diameters permit efficient transfer of electrical energy to the vessel from the power supply circuit and the resultant laser beam diameter more closely resembles that generated by competing laser technologies.

Another advantage is the far greater homogenity of metal vapour that results from the multiplicity of metal reservoirs located uniformly along the length of the vessel. This enables more uniform seeding of the discharge volume with metal vapour and results in an enhanced energy extraction efficiency. Experiments have achieved an extraction efficiency of 318mW per cubic centimetre from a 4mm diameter vessel of volume 3.45 cubic centimetres. This compares very favourably with the 30.5mW per cubic centimetre extracted from conventional 25mm diameter vessels. Another advantage is the relaxation of the requirement that the vessel be operated in a horizontal or near horizontal manner so as to prevent the molten metal from rolling out of the end of the vessel. The segmented sections act as effective barriers to the rolling of the molten metal and provide individual wells that further contain the metal.

The advantages of the relaxation of the horizontal positioning requirement and the more uniform seeding of the discharge vessel with metal vapour are both applicable to larger diameter vessels common in the art.

The invention can be better understood by reference to preferred embodiments which will now be described with reference to the accompanying drawings in which:-

Fig. 1, Fig. 2 and Fig. 3 show cross-sectional views of three different laser assemblies according to the invention.

Terminating flanges 1 and tube 3 in Figure 1 define the gas chamber 11. The terminating flanges 1 are in electrical contact with electrodes 2 between which the discharge is propagated in the discharge chamber 12. Thermal insulation 4 is provided in order to raise the temperature of the metal segments 7 to the optimum value for lasing action. The tube 5 is supported co-axially with the thermal insulation 4 and individual rings 6 are disposed so as to form the appropriate structure with wells 13 in which to locate the metal. This creates the multiplicity of metal reservoirs disclosed in this invention. Such an insulating assembly is appropriate to a copper laser. The tube 3 defining the gas chamber is sealed against the terminating flanges 1 and 2 via O-rings (not shown) or some other suitable sealing technique. The whole assembly is connected to suitable power supplies, vacuum systems and cooling systems. These are all common in the art and do not form part of the invention.

Fig. 2 shows a variation of the assembly wherein tube 8 supports the segmented sections 6 directly and no thermal insulation is provided. Such an assembly would be appropriate to a strontium or lead laser, or a copper laser whose insulation was derived externally to tube 8. Fig. 3 shows a further variation wherein tube 9 is composed of an electrically conductive metallic material that is electrically isolated from the terminating flanges 1 by electrically insulating tubes 10. Tubes 9 and 10 are jointed together by a suitable, vacuum compatible technique and tube 10 is then sealed against the terminating flanges 1. The annular sections 6 can be composed of any suitable material that may or may not be of the same composition as the supporting tube. It will be appreciated that a number of alterations can be made with respect to the preferred embodiment within the scope of the succeeding claims.

The preferred embodiments described above are given by way of illustration and not limitation.

Claims

1. A metal vapour laser having a tubular chamber defining a gas discharge volume characterized in that along the chamber are provided means defining a plurality of metal reservoirs located uniformly along the length of the chamber.

2. A metal vapour laser as claimed in Claim 1, wherein the reservoirs are annular in form and are co-axial with the laser axis.

3. A metal vapour laser as claimed in Claim 2, wherein the chamber is constructed from a tube of insulating refractory material and the reservoirs are formed by a plurality of rings of similar material arranged co-axially in the tube and spaced longitudinally along the tube, spaces between the rings constituting reservoirs for metal.

4. A metal vapour laser as claimed in Claim 2, wherein the chamber is constructed from a tube of electrically conductive metallic material and the reservoirs are formed by a plurality of rings spaced longitudinally along the tube, spaces between the rings constituting reservoirs for metal.

5. A metal vapour laser as claimed in Claim 1, arranged to produce a laser beam of between 1 and 10 millimetres in diameter.