GB2212322A

GB2212322A - Gas discharge laser apparatus

Info

Publication number: GB2212322A
Application number: GB8726571A
Authority: GB
Inventors: David Ian Wheatley
Original assignee: Ferranti PLC
Current assignee: Ferranti International PLC
Priority date: 1987-11-13
Filing date: 1987-11-13
Publication date: 1989-07-19
Also published as: GB8726571D0

Abstract

A first discharge electrode of a laser comprises a number of separate conductive portions (12) located adjacent to one another within the laser envelope (10), whilst a second discharge electrode (14) is provided within the envelope spaced apart from the first discharge electrode. A power source (15) applies a voltage between the first and second discharge electrodes so that a current may flow between them through the gas. Switching means (16) connect the conductive portions (12) of the first electrode to the power source (15) in sequence so that the discharge current flows between the second electrode (14) and the energised portion of the first electrode (12). Each conductive portion of the first electrode may be connected to the power supply via a common ballast component. The switching means may operate so that one conductive portion of the first electrode is permanently connected to the power source, the remaining portions being connected in succession. The apparatus ensures uniform flow discharge with a single ballast component. <IMAGE>

Description

GAS DISCHARGE LASER APPARATUS Many types of laser use as the pumping mechanism an electric discharge produced in a gaseous active medium.

Several techniques as available for producing the discharge, frequently involving producing a flow of current between two electrodes in contact with the gas. The discharge may be the uniform glow-type discharge or the streamer or arc-type discharge. In the interests of efficiency and stability the glow-type discharge is preferred. However it is sometimes difficult to ensue that this form of discharge is produced, particularly in dc lasers where an arc tends to form a point on one electrode to a point on the other.

It is known to provide, in a dc laser, a number of anode electrodes and a single cathode electrode. Each anode is connected to a power supply through a separate ballast component. Ihis arrangement is commonly used in high power lasers of the transverse gas flow type, particularly to enable a larger current to be carried. However, the need to provide a number of separate ballast components is inconvenient, expensive and wasteful of power.

It is an object of the invention to provide gas discharge laser apparatus in which the possibility of an arc discharge being formed is substantially reduced.

According to the present invention there is provided gas discharge laser apparatus which includes an envelope containing a gaseous active medium, a first discharge electrode having at least two conductive portions located adjacent to one another within said envelope, a second discharge electrode located within said envelope spaced apart from the first electrode, a power source operate to apply a voltage between said first and second electrodes such that a discharge current may flow through the gas between the electrodes, and switching means operable to connect the power source to the conductive portions of the first electrode in sequence such that the discharge current flows between the second electrode and the energised portion of the first electrode.

The invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic side view of laser apparatus according to a first embodiment of the invention; Figure 2 is a sectional view along the line II-II of Figure 1; Figure 3 is a schematic block diagram of the switching means for the laser of Figures 1 and 2; Figure 4 illustrates the switching sequence of the switching means of Figure 3; Figure 5 is a schematic side view of a laser according to a second embodiment of the invention; and Figure 6 is a sectional view along the line VI to VI of Figure 5.

Referring now to Figure 1 this shows a cylindrical envelope 10 closed at one end and having a side arm 11 near to that closed end. Positioned in this side arm 11 are a number of conductive metal pins 12, arranged parallel to one another and to the axis of the side arm as shown in Figure 2. Each of the pins 12 has an electrical connection 13 passing through the wall of the side arm. Near to the open end of the envelope 10 is a cylindrical metal section 14 forming part of the wall of the envelope 10. A power source 15 supplies a current at high potential to the metal section 14, which forms the cathode of the laser and, through the switching means 16, to the pins 12 which form the anodes of the laser.

In use gaseous laser active medium enters the envelope through the side arm 11 and leaves through the open end of the envelope. The closed end will usually support a mirror, whilst a second mirror will be provided beyond the open end of the envelope 10. The entry of the gas through the side arm 11 produces turbulence ia the gas flow which helps to stabilise the gas discharge between anode and cathode.

Figure 3 is a schematic block diagram showing the arrangement of the switching means of the invention. Each of the anode electrodes 12 is connected by means of a separate switch 30 through a common ballast component 31 to the power source (not shown). Each switch has a control input which is connected to a common shift register 32. A clock input 33 is also applied to the shift register 32.

The shift register 32 responds to a regular train of clock pulses to activate each switch 30 in turn so that the power source is connected to the electrodes in turn. This causes the discharge between anode and cathode to be switched from one anode pin 12 to another continually. Figure 4 illustrates a suitable timing sequence provided by the shift register 32. The references 12(1), 12(2) etc. denote the different ones of the anode pins 12. It will be noted that the timing sequence allows for a slight overlap between the energisation of separate anode pins to prevent the discharge from being extinguished.

The rate of switch is, to some extent, dependent upon the mode of operation of the laser. Preferably the duration of a complete switching cycle, shown in Figure 4 as taking a time T, must be less than any other limiting time period affecting the operation of the laser. If, for example, the laser is energised in a pulsed manner at, say, SKhz, then the time T must be less than 200uS. It is possible for the time T to be as little as 20uS, so that the limitation or switching cycle time presents no real problem.

If gas flow turbulence presents a problem in any way the arrangement of the laser envelope may be changed as shown in Figures 5 and 6. These show the laser envelope 10 as being a simple cylinder with gas entering at one end and leaving at the other. In such an arrangement the anode electrodes 51 may simply project inwards through the side of the envelope. The cathode would probably still best be provided by a cylindrical conducting section 52 of the envelope as before. Such an arrangement of electrodes may well be preferred in a multiple-tube laser.

The number of separate anode electrodes may vary from two upwards, and, if there are more than two, some electrodes may be left permanently connected to the power source, sharing the discharge current with other, switched, electrodes. The timing sequence could be changed from the simple sequence shown in Figure 4.

Both of the embodiments show axial flow lasers, that is the gas flows in the same direction as the laser radiation produced by the laser. The multiple-anode arrangement could be used equally well in a transverse flow laser, in which the gas flows across the axis of the laser radiation.

The invention may also be applied to the sealed laser, though the benefits which are experienced in a flowing-gas laser would not all be obtained.

The multiple switched electrode could be the cathode electrode, though the multiple-anode arrangement described is more useful in an axial flow laser.

Claims

1. Gas discharge laser apparatus which includes an envelope containing a gaseous active medium, a first discharge electrode having at least two conductive portions located adjacent to one another within said envelope, a second discharge electrode located within said envelope spaced apart from the first electrode, a power source operable to apply a voltage between said first and second electrodes such that a discharge current may flow through the gas between the electrodes, and switching means operable to connect the power source to the conductive portions of the first electrode in sequence such that the discharge current flows between the second electrode and the energised portion of the first electrode.

2. Laser apparatus as claimed in Claim 1 in which the envelope has a side arm in which the conductive portions of the first discharge electrode are located.

3. laser apparatus as claimed in either of Claims 1 or 2 in which the switching means is operable to connect each conductive portion of the first electrode to a common ballast component connected to the power source,

4. laser apparatus as claimed in Claim 3 in which the switching means is operable such that each conductive portion of the first electrode is connected to the power source for equal periods of time, extinction of the discharge being prevented by ensuring overlap between the periods during which successive portions are connected to the power source.

5. laser apparatus as claimed in Claim 3 in which the switching means is operable such that at least one conductive portion of the first electrode is permanently connected to the power source, the remaining portions being connected to the power source for equal periods of time in succession.

6. laser apparatus as claimed in any one of the preceding claims in which the switching means includes a separate switch operable to connect each conductive portion of the first electrode to the power source and a shift register responsive to a train of clock pulses to control the operation of the switches.

7. Laser apparatus as claimed in any one of Claims 1 to 6 in which the gaseous active medium passes continuously through the envelope past the first and second electrodes.

8. Gas discharge laser apparatus substantially as herein described with reference to the accompanying drawings.