GB2214700A

GB2214700A - Gas discharge lasers

Info

Publication number: GB2214700A
Application number: GB8801241A
Authority: GB
Inventors: Christopher John Briscoe
Original assignee: CAMBRIDGE LASERS Ltd
Current assignee: CAMBRIDGE LASERS Ltd
Priority date: 1988-01-20
Filing date: 1988-01-20
Publication date: 1989-09-06
Also published as: GB8801241D0

Abstract

A gas discharge ion laser comprises a plasma tube 10 communicating with an external ballast volume 40 which serves to reduce the drop in pressure occuring in the plasma tube during use of the laser. The external ballast volume is within, or forms, a support structure. The support structure is provided for mounting the plasma tube 10 and the reflectors 24 and 26 of the laser optical resonator cavity. The support structure may comprise a thick walled support tube 32 extending alongside the plasma tube 10 and brackets 36 for securing the reflectors 24 and 26 to the support tube. <IMAGE>

Description

GAS DISCHARGE LASERS The present invention relates to a gas discharge laser and in particular to an ion laser.

Known gas discharge ion lasers comprise a plasma tube, within which a gas discharge is generated during use by the application of a DC voltage between the anode and the cathode and the passing of a high direct current. The plasma tube is arranged between reflectors which are accurately aligned to cause multiple reflection of the emitted light thus defining an optical resonator cavity within which light amplification occurs by stimulated emission of radiation.

A rigid structure is required to mount the plasma tube and support the reflectors at the opposite ends of the resonator cavity. The structure must prevent movement apart of the reflectors and any angular movement of the reflectors relative to one another and relative to the plasma tube. In view of the heat generated in the vicinity of the plasma tube during use, the design of the support structure requires careful, thought and many designs have previously been put forward in order to afford the necessary stiffness while compensating for effects of thermal expansion.

Within the plasma tube, the ion discharge occurs in a narrow tube and is confined by the bore to follow a path along the narrow tube. A coaxial magnet surrounds the bore. The effect of the migration of ions down the length of the tube is to create a pressure differential and this is avoided by the provision of a return path for the gas to equalise the pressures at the opposite ends of the plasma tube. A further effect noted during prolonged running periods is that the pressure of the gas gradually drops due to absorption of the gas into the walls of the plasma tube. This effect is mitigated by the provision of an external gas reservoir or ballast volume to reduce the drop in gas pressure through this loss of gas.

The high power consumed by the gas discharge generates substantial amounts of heat which must in some way be dissipated. To this end, the wall of the plasma tube may be formed of a good thermal conductor, such as beryllia, and this is in turn surrounded by a jacket through which a coolant fluid, such as water is circulated.

From the above brief description of a ion laser, it will be appreciated that the construction of the plasma tube is itself complex and this further complicates the design of the structure required to support it and the reflectors of the resonator cavity.

As a result of the overall complexity, known ion lasers have generally been expensive to manufacture and to service. Maintenance of the plasma tube, in particular, was made difficult by the design of the support structure and the need to break seals in the plasma tube before it could, in some cases, be withdrawn from the support structure for servicing or repair.

With a view to mitigating at least some of the foregoing disadvantages, the present invention provides a gas discharge ion laser comprising a plasma tube communicating with an external ballast volume, an optical resonator cavity defined between reflectors arranged adjacent opposite ends of the plasma tube, and a support structure for mounting the plasma tube and the reflectors of the optical resonator cavity, wherein the support structure comprises a thick walled support tube extending alongside the plasma tube and brackets for securing the reflectors to the support tube, the external ballast volume which serves to reduce the drop in pressure occurring in the plasma tube during use of the laser being contained in the hollow space within the support tube.

Preferably, the brackets on the support tube are clamped about the outer surface of the support tube and abutments are formed on the support tube to define the axial positions of the brackets.

Conveniently, the support tube is formed of a material, such as INVAR > having a a negligibly small coefficient of thermal expansion.

The use of a hollow tube has several advantages in addition to the compactness which results from containing the ballast volume within its hollow centre.

There is a weight reduction as compared with a solid tube of the same diameter and a corresponding reduction in material costs. Furthermore, a hollow tube has less tendency to sag than a solid tube thus imparting greater structural rigidity to the resonator cavity.

The support tube may itself be a sealed container for the ballast gas but it is preferred that the ballast volume should be in a sealed container positioned within the support tube and connected to the plasma tube by a flexible steel bellows passing though a slot in the end of the support tube. In this case, after removal of one of the reflector mounts from one of the ends of the support tube, the plasma tube and the ballast volume container can be withdrawn from the support for repair or replacement. This considerably simplifies servicing and permits replacement of the plasma tube without the need to remove the laser from its site.

The invention will now be described further, by way of example, with reference to the accompanying drawing which is a sectional plan view of a laser head.

In the drawing, which only shows the laser head, the plasma tube assembly is generally designated 10 and its internal construction is not shown in detail. The plasma tube assembly is formed of a beryllia tube surrounded by a water jacket and an electromagnet.

Pipes 12 supply cooling water to the water jacket.

The beryllia tube is an argon or krypton filled discharge tube having a heated cathode at one end and an anode at the other. In operation, a voltage of around 300 volts DC is applied across the discharge tube and the current flowing through the tube is of the order of 30 amps DC.

A regulated power supply is required for the plasma tube and this is housed separately. The semiconductor elements, usually field effect transistors, of the output stage of the regulator also require water cooling on account of the significant power dissipation.

Conventionally, these elements, which are referred to as the pass bank, are mounted with the remainder of thepower supply in the separate housing and consequently water and electrical connections are required between the laser head and the power supply.

By contrast, in the illustrated embodiment, the pass bank 14 is mounted in the laser head. The pass bank is mounted on a copper plate 16 of heavy gauge and the latter has a part circular recess along its length into which there is soldered a copper pipe 18. The coolant water flows through the pipe 18 prior to entering the water jacket of the plasma tube.

Also arranged within the laser head is a flow switch 20 through which the water flows after leaving the water jacket of the plasma tube. The flow switch contains a paddle which rotates at a rate related to the flow rate of the water. A circuit monitoring the rotation of the paddle acts to cut off the power supply when insufficient cooling water is circulating.

By virtue of the fact that the pass bank, the flow switch and the water jacket are all contained in the laser head, all the connections of the coolant water circuit (which are not shown in the drawing) within the laser head can be rigid pipes, for example copper or stainless steel, whereas previously flexible connections were always used. The use of rigid pipes and reliable pipe couplings considerably reduces the risk of water leakage, which can be the cause of serious damage.

As water is no longer required for the components of the power supply housed in the separate housing, these are fully safeguarded against damage from water leakage.

The ends of the plasma tube projecting from the water jacket and the electromagnet are terminated in Brewster windows 22. These permit the passage, with minimum reflection, of light waves polarised in one plane while fully reflecting light having the transverse polarisation. The light passing through these windows along the axis of the plasma tube is reflected by reflectors at both ends of the plasma tube to form the optical resonant cavity required for light amplification by stimulated emission of radiation, or lasing as it is called more briefly.

The reflector 24 at one end is a partial reflector which transmits some of the incident radiation as the output radiation and reflects the remainder for the purpose of lasing. The reflector 26 at the other end of the plasma tube, on the other hand is a fully reflecting mirror 30 preceded by a prism 28. The prism, which may optionally be omitted, acts to refract the incident light so that only light of one wavelength within the line spectrum of the argon gas is reflected by the mirror 30 to return on the axis of the plasma tube.

Sturdy mounts fixed relative to each other and relative to the plasma tube are required on which to support the reflectors and the prism. These mounts act as reference surfaces for adjustment of the angles of the reflecting and refracting surfaces of the optical resonant cavity.

To this end, the plasma tube assembly in the illustrated embodiment is mounted on a hollow thick walled tube 32 of INVAR1which has machined inner and outer surfaces. The inner surface is bored out by means of a gun drill while the outer surface is accurately finished on a lathe. Four brackets are clamped around cnrM) the INVARltube 32, the inner two brackets 34 are clamped (RTm ) about the INVARltube 32 and the plasma tube assembly 10.

The outer two brackets 36 serve as the mounts for the reflecting and refracting surfaces of the optical resonant cavity.

Trim) Because the tube 32 is of INVARA ,it does not expand significantly as a result of the temperature variations which occur during operation and the separation between the brackets 36 does not vary significantly. Also because it is hollow, the tube 32 has less tendency to sag than a solid bar of the same diameter, thereby ensuring that the brackets 36 remain parallel to one another.

The optical surfaces in the resonant cavity are not directly mounted on the brackets 36 but on mounting plates which are adjustable relative to the brackets 36 for fine positioning of the reflector surfaces. The angle of the prism 28, when present, is also adjustable and this is to permit alteration of the wavelength of the laser, it being possible in this way to select from the various lines in the gas line spectrum.

The tube 32 has a step on its outer surface to provide a fixed abutment for the brackets 36. If these are ever removed, they can be returned to precisely the same axial position.

During operation a pressure differential occurs between the ends of the plasma tube and a return path must be provided for gas to flow from the anode end back to the cathode end of the plasma tube. This return path in the illustrated embodiment is also contained within the plasma tube assembly 10.

In addition to the pressure drop at the cathode caused by the effect of ion migration and referred to above, there is a tendency for the pressure throughout the plasma tube to drop during use through gas being absorbed into the walls of the tube. This effect is mitigated if the plasma tube is connected to a large reservoir or ballast volume since the loss of a fixed mass of gas will cause a smaller variation in pressure in a large volume than it would in a smaller volume.

In the illustrated embodiment, such a ballast volume 40 is provided in a tank contained within the CIM) hollow space of the INVARXtube 32 and connected to the plasma tube by a flexible steel bellows 42 which passes through a slot in one end of the tube 32. In this way, the hollow space of the tube 32 is put to good use this permitting the construction of a more compact and lighter laser head. There is also no need for a separate set of mounting brackets for the reservoir tank.

The construction described above offers particular ease of servicing by affording easy access to the plasma tube assembly. Indeed, the whole of the assembly can be replaced on site without the need to return the laser head for recalibration. After disconnecting the water connections to the water jacket and the electrical connections to the anode and cathode of the plasma tube, the whole assembly complete with the ballast tank can be withdrawn after removal of clamps from the brackets 34 and one of the brackets 36. Only one mirror surface is disturbed in the process and as earlier stated, thanks to the shoulder on the INVARltube 32, the bracket 36 carrying this mirror surface can be returned to precisely the same axial position after replacement of repair of the plasma tube assembly.

Claims

1. A gas discharge ion laser comprising a plasma tube communicating with an external ballast volume, an optical resonator cavity defined between reflectors arranged adjacent opposite ends of the plasma tube, and a support structure for mounting the plasma tube and the reflectors of the optical resonator cavity, wherein the support structure comprises a thick walled support tube extending alongside the plasma tube and brackets for securing the reflectors to the support tube, the external ballast volume which serves to reduce the drop in pressure occurring in the plasma tube during use of the laser being contained in the hollow space within the support tube.

2. A laser as claimed in claim 1, wherein the brackets on the support tube are clamped about the outer surface of the support tube and abutments are formed on the support tube to define the axial positions of the brackets.

3. A laser as claimed in claim 1 or 2, wherein the support tube is formed of a material, such as INVAR(RTm) having a negligibly small coefficient of thermal expansion.

4. A laser as claimed in any preceding claim, wherein the ballast volume is in a sealed container positioned within the support tube and connected to the plasma tube by a flexible steel bellows passing though a slot in the end of the support tube.

5. A gas discharge ion laser constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.