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Title:
LIGHT AMPLIFICATION DEVICE
Document Type and Number:
WIPO Patent Application WO/2007/144645
Kind Code:
A1
Abstract:
A light amplification device comprising an outer electrode (30) defining an annular passage therethrough (40) and an inner annular electrode (20) located within the annular passage (40), the inner and outer concentric annular electrodes defining a gas discharge region therebetween, the inner electrode being electrically isolated from the outer electrode, wherein the outer electrode is grounded and the inner electrode is operable to receive current therethrough, such that when current is passed through the inner electrode, electrical discharge occurs in the gas discharge region.

Inventors:
MASON, Paul (14 Hendremawr Close, Tycoch, Swansea SA2 9ND, GB)
PEARSON, Guy (46 Hornyold Road, Malvern, Worcestershire WR14 1QR, GB)
Application Number:
GB2007/002239
Publication Date:
December 21, 2007
Filing Date:
June 14, 2007
Export Citation:
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Assignee:
MASON, Paul (14 Hendremawr Close, Tycoch, Swansea SA2 9ND, GB)
PEARSON, Guy (46 Hornyold Road, Malvern, Worcestershire WR14 1QR, GB)
International Classes:
H01S3/038
Foreign References:
GB2194380A
US5353299A
EP1217700A2
GB2150742A
US4805182A
DE3515679C1
Attorney, Agent or Firm:
WHITFIELD, Gillian Janette (Astrum-IP Limited, 12 Enville CloseNewport, South Wales NP20 3SD, GB)
Download PDF:
Claims:

CLAIMS

1. A light amplification device comprising and outer electrode defining an annular passage therethrough and an inner annular electrode located with the annular passage, the inner and outer concentric electrodes defining a gas discharge region therebetween, the inner electrode being electrically isolated from the outer electrode, wherein the outer electrode is electrically grounded and the inner electrode is operable to receive current therethrough, such that when current is passed through the inner electrode, electrical discharge occurs in the gas discharge region.

2. A light amplification device according to Claim 1, wherein the outer electrode has an annular outer surface.

3. A light amplification device according to Claim 1 or Claim 2, further comprising a first and second mirror located at respective opposing ends of the outer electrode, each mirror being provided with receiving means within which to receive at least a portion of the inner electrode.

4. A light amplification device according to any one of Claims 1 to 3, wherein the inner electrode comprises a metal extrusion.

5. A light amplification device according to Claim 4, wherein the metal extrusion comprises one or more metals selected from aluminium and magnesium and superalloys of nickel and iron.

6. A light amplification device according to any one of Claims 3 to 5, wherein the inner electrode is electrically isolated from each of respective first and second mirrors.

7. A light amplification device according to Claim 6, wherein the portions of the inner electrode contacting each of respective first and second mirrors

are anodised if the inner electrode is aluminium or have a dielectric surface insulating layer in the case of other metals.

8. A light amplification device according to any one of Claims 3 to 7, wherein each of respective first and second mirrors defines an aperture therethrough located so as to receive a laser beam therethrough in use.

9. A light amplification device according to Claim 8, wherein the first and second mirrors are located such that their respective apertures for receiving a laser beam therethrough in use are offset from one another.

10. A method of construction of a light amplification device, comprising the steps of:

a. Providing an extruded metal annular inner electrode; b. Providing an annular outer electrode; c. Providing two mirrors for location adjacent opposing end regions of the outer electrode, each respective mirror having receiving means for receiving an outer portion of the inner electrode therein, each respective mirror further defining an aperture therethrough for receiving a laser beam therethrough in use; d. Locating the inner electrode concentrically within the outer electrode; and e. Locating each opposing outer portion of the inner electrode within the receiving means in respective mirrors and orienting the mirrors relative to one another such that the apertures for receiving a laser beam therethrough are offset from one another.

1 1. A method according to Claim 10, wherein the outer electrode comprises a metal extrusion.

12. A method of light amplification comprising the steps of:

a. Providing a light amplification device in accordance with any one of Claims 1 to 9; and b. Earthing the outer electrode and passing live current through the inner electrode so as to create a discharge within the gas discharge region.

13. A light amplification device substantially as hereinbefore described and with reference to the accompanying figures.

14. A method of construction of a light amplification device substantially as hereinbefore described and with reference to the accompanying figures

15. A method of light amplification substantially as hereinbefore described and with reference to the accompanying figures.

Description:

LIGHT AMPLIFICATION DEVICE

Background to the invention: The present invention relates to a light amplification device for the manufacture of a Carbon Dioxide laser (CO 2 laser) or a Carbon Monoxide laser (CO laser) excited by a Radio Frequency (RF) field of electro-magnetic energy. The present invention also relates to a method of assembly of such device.

CO 2 lasers are used extensively throughout industry for engraving, marking, scribing, cutting, heat treatment and many other applications (known as laser material processing) and are applied to numerous materials. CO 2 lasers are also widely used in medical, scientific and military applications.

The light energy emitted by these devices has a wavelength of typically 10.6 (10.59μm) microns but can range from 9.1μm to l lμm. CO lasers are much less common at the present time and emit light typically with a wavelength in the range of 4 to 7 microns.

Critical to many applications is the amount of laser power emitted from the source and also the shape or profile of the laser beam. In most instances a pure Gaussian energy distribution of the beam profile is required as this yields the highest degree of focussability of the beam, resulting in smaller focussed spot sizes and consequently higher energy densities at the material or target surface. This provides higher precision and faster material processing rates.

Other key parameters required from such devices are high power, air cooling, long sealed-off lifetime (if used in sealed mode), high power to length ratio, good power and pointing stability, very high frequency pulsing capability and low cost of ownership.

Conventionally, RF CO 2 lasers have inferior and variable beam quality and in some cases significant power drift and variation with time due to heating and tuning effects within the laser. Pulsing the laser to provide rapidly modulated optical pulse streams is also limited by issues relating to depth of modulation (laser output not switching off but being partly modulated in synchronicity with a control signal) and super-heating of the gas within the Laser Gas Discharge region. Laser output powers can be limited when attempting to produce high quality Laser beam profiles. Many designs are costly to manufacture and are often based upon 'waveguide' and 'slab' laser technologies that use ceramic parts. These designs can also severely restrict the Laser beam quality and create entrapped volumes of undesirable gases as well as being complex to assemble and align optically, and comprising multiple component parts.

Summary of the Invention

The present invention seeks to address the problems of the prior art.

Definitions

The use of the term 'laser' is intended to include lasers such as gas lasers, chemical lasers, molecular lasers, atomic lasers, EUV lasers and X-ray lasers and other devices relying upon the stimulated emission of photons as the fundamental physical process of light output generation.

The use of the term 'electrode' is intended to include a device or means of producing energy or power input to a laser and/or producing electrons and/or collecting electrons and/or gas ions.

The term 'gas discharge' is intended to include a device or system where the medium consists of ions or electrons.

The term 'super-alloy' is used to indicate a material that has extreme oxidation resistance even at high temperature amongst other properties such as being dielectric and/or having good mechanical properties with respect to its application to a laser.

UHV and HVAC indicate ultra-high-vacuum and high vacuum respectively.

RF is intended to include radio frequency fields and waves oscillating between IMHZ and 300MHZ.

AC means alternating current flow

A 'Heatpipe' (Hereinafter referred to as Heatpipe) is a device that has extremely high thermal conductivity. This may be over 1000 times that achievable from 'conventional' conductive or radiative or convective processes. The Heatpipe uses the latent heat of vaporisation of a liquid (often water) held in a closed atmosphere at low pressure as a means of conducting heat away rapidly from a heat source. The device comprises essentially 3 mains parts such as an evaporator, conductor and condenser. The evaporator part is placed in good thermal contact to the source of the heat that it is desirable to remove, the conductor section is the part that transfers the heat to a more convenient, less thermally sensitive place and the condenser section is then connected via a good thermal contact to a convenient source of heat- sink to remove and isolate the re-distributed heat. The condensed liquid then flows back from the condenser into the evaporator either by means of a Wicking material or hydrophile, thus the evaporator is replenished with evaporant and the cycle repeats itself ad infinitum.

The term 'heat-sink' is intended to indicate a method or means of removal of or a redistribution of heat away from a particular area or position

The term 'laser resonator' is intended to indicate the optical system that provides stable optical feedback to and confines and propagates and stabilizes the laser beam with the optical cavity and the space formed between 2 or more mirrors and or partial reflectors

The term 'laser superstructure' is intended to indicate any enclosure or mainframe or main mount or main support or any mechanical or other embodiment that assists in adding thermo-mechanical and optical stability to the laser

A first aspect of the present invention provides a light amplification device comprising an outer electrode defining an annular passage therethrough and an inner annular electrode located within the annular passage, the inner and outer concentric electrodes defining a gas discharge region therebetween, the inner electrode being electrically isolated from the outer electrode, wherein the outer electrode is substantially electrically grounded and the inner electrode is operable to receive (live) current therethrough, such that when current is passed through the inner electrode, electrical discharge occurs in the gas discharge region.

It is conventional to electrically ground or earth the inner electrode and pass current through the outer electrode. In contrast to conventional arrangements, the polarity of the inner and outer electrodes of the present invention is reversed. Previously this has required that the arrangement is contained within an outer casing in order to prevent access to the live outer electrode. In the present invention, by passing current through the inner electrode and placing the outer electrode at ground (earth) potential, there is no need for an additional casing in which to house the arrangement as the inner electrode is already electrically isolated from the outer electrode and so the outer electrode is sufficient also to perform the function of the outer casing.

The outer electrode may also have an annular outer surface. However, it will be appreciated that the outer surface contours of the outer electrode may vary and may include fins for facilitating heat transfer as discussed below.

The outer surface of the outer electrode may be circular, square, rectangular, finned, or any other suitable contour known to the skilled person. However, none of these contours affect the true annular nature of the discharge geometry between the two electrodes where the plasma forms although, the outer surface contour of the outer electrode may have an impact on the overall cooling and laser efficiency of the device.

In one embodiment, the light amplification device further comprises a first and second mirror located at respective opposing ends of the outer electrode, each mirror being provided with receiving means within which to receive at least a portion of the inner electrode.

In one embodiment, the light amplification device is further provided with respective first and second supporting discs located adjacent opposing ends of the outer electrode. The inner electrode is preferably positioned along the central long axis of the outer electrode by means of the two supporting discs positioned at either end of the inner electrode. Optionally, these supporting discs may also support or assist in supporting the inner electrode securely and accurately in position and can also serve to hold the first and second mirrors of the amplification device. In this way, the inner electrode is supported so that none of the discharge volume is obstructed, thereby allowing the optical beam to utilise the maximum fraction of the gain volume.

Preferably, at least a portion of the inner electrode may be formed at least in part from an extrusion such as a low cost UHV or HVAC compatible metal extrusion. Alternatively, or in addition, at least a portion of the outer electrode comprises such

a metal extrusion. It is to be appreciated that each of the inner and outer electrodes may comprise a single metal extrusion. By using a metal extrusion rather than a machined part, the manufacturing process of the inner electrode is significantly simplified and wastage minimised.

The metal extrusion may comprise one or more metals selected from aluminium and magnesium and metal alloys commonly known as superalloys of iron and/or nickel. However, aluminium is the preferred metal due to its favourable properties. The use of such materials both supports the conductive properties required of the inner electrode, and also allows the inner electrode to be isolated from each of the first and second respective mirrors, for example, by anodising (in the case of aluminium) or surface oxidation of the portion of the inner electrode contacting the receiving means of each of the respective first and second mirrors. Anodising or surface oxidation of the aforementioned portions of the inner electrode facilitates the electrical isolation of the inner electrode from the respective first and second mirrors without requiring the use of, for example, glass or ceramic seals or the like. Thus, the construction of the device is once again significantly simplified.

Alternatively, the outer electrode may be a mechanically machined part composed of a UHV or HVAC compatible material. Preferably, the UHV or HVAC compatible material is of low cost, such as, but not restricted to aluminium or Invar™.

Similarly, rather than being a metal extrusion, at least a part of the inner electrode may be composed of a low cost UHV or HVAC compatible material, such as, but not restricted to aluminium or Invar™.

One advantage of the structure of the device of the present invention is that it does not require the presence of glass or ceramic seals as the inner electrode is isolated

from the rest of the device by the anodised portion of the inner electrode in contact with the first and second mirrors. Thus, the device may be baked to facilitate a long working life. This is not possible where glass or ceramic seals are used as the seals have a tendency to crack when exposed to such extremes of temperature.

The inner electrode is preferably located such that at least a portion of the inner electrode lies within the outer electrode and is concentric with and parallel to the inner wall of the outer electrode. In the case where the inner and outer electrodes are concentric and parallel to one another, only one uniform gas discharge region persists uniformly around the circumference of the inner electrode and in between the inner and outer electrodes and along the entire length of the inner and outer electrodes. This uniform region of the light amplification device is then used as the laser gain section for the resonant optical path(s) that may be single or multiple in number that precess around the inner electrode and within the laser gain region between the inner and outer electrodes.

The inner electrode may be solid or hollow internally.

The outer electrode may have holes or voids drilled or otherwise provided in its body to facilitate a greater volume of gas reservoir in the gas chamber or, for example, to carry Invar™ laser resonator rods for additional thermo-mechanical stability of the system.

In one embodiment, each of the respective first and second mirrors defines an aperture therethrough located so as to receive a laser beam therethrough in use.

It is preferred that the respective first and second mirrors are located relative to one another such that the apertures which receive a laser beam therethrough in use are offset from one another. By off-setting the apertures accurately and controlling the

properties of the beam, the apertures can be accurately located to line up with the beam entering and leaving the gas discharge region.

The outer electrode not only creates the resonator structure and an outer casing for the gas discharge region of the device, but also acts as an electrode for the whole laser resonator as well as an effective region for, and method of, heat removal.

The inner electrode may be cooled using any one or more of the following methods:

1. The system may be a slow moving or fast-flowing gas system and non 'sealed' thus utilising, at least in part, gas flow to keep the whole discharge at a relatively low temperature;

2. The inner electrode may be hollow and fluid may be flowed through the central hollow to control the temperature of the inner electrode;

3. Where a hollow inner electrode is used, very high thermal conductivity, relatively low cost metals such as, but not limited to, copper, oxygen-free high conductivity (OFHC) copper, aluminium and copper-silver alloys and other high thermal conductivity materials can be inserted in the hollow of the inner electrode which is in contact with the outer electrode via a thin layer of electrical insulation, thus helping to keep the whole discharge at a relatively low temperature;

4. Where a hollow inner electrode is used, an 'e-VAP™' Tube (a device that has extremely high thermal conductivity) can be inserted into the hollow of the inner electrode along its length. The 'e-VAP™' Tube uses the latent heat of vapourisation of a liquid held in a closed atmosphere of low pressure as a means of conducting heat rapidly away from a heat source.

The device comprises essentially three main parts, such as evaporator, conductor and condenser. There is no wicking agent or hydrophile in the system. The condenser section is then connected to a convenient source of heat-sink and the condensed (cooled) liquid flows back into the evaporator section via gravity of by capillary action or surface tension effects on the inner surface of the 'e-VAP™' Tube. The 'e-VAP™' Tube is electrically isolated from the inner electrode via an insulating disc; and

5. Where a hollow inner electrode is used, pressurised flow of air or other gas into the hollow of the inner electrode may be used to control the temperature of the inner electrode.

The outer electrode is preferably maintained at zero volts potential (i.e. is electrically grounded). In this way, the heat generated in the discharge can be rapidly, safely and effectively removed from the thin-walled large-surface-area outer surface of the outer electrode using conventional heat exchange methods such as finned heat sinks and fans.

The outer electrode may be formed with integral high surface area fins for heat exchange. Alternatively, the fins may be additional 'bolt-on' fins which are located adjacent the fins for the purposes of efficient heat transfer. For example, the outer electrode may have an annular inner surface and an outer surface with square cross-sectional shape to which additional fins may be attached. However, this option is likely to carry more costs that an extruded outer electrode with integral fins.

It will be appreciated that the fins may be of any preferred contour and shape and the number of fins may vary depending on the heat transfer requirements.

In one embodiment, selected regions of the outer surface of the inner electrode and the inner surface of the outer electrode are coated with an oxide film or a metallic

super-alloy to create sufficient electrical resistance between the inner and outer electrodes so as to ensure that the gas discharge is produced only in the annular region and to protect the materials of the inner and outer electrodes from undergoing chemical and physical changes that may be caused by their interaction with the gas discharge.

Optionally, a catalytic coating such as, but not limited to, finely divided gold, may be applied to the electrode surfaces that are in contact with the gas discharge to enhance laser output power and increase operating lifetime of the laser.

A further aspect of the present invention provides a method of construction of a light amplification device, comprising the steps of: a) Providing an extruded metal annular inner electrode; b) Providing an annular outer electrode; c) Providing two mirrors for location adjacent opposing end regions of the outer electrode, each respective mirror having receiving means for receiving an outer portion of the inner electrode therein, each respective mirror further defining an aperture therethrough for receiving a laser beam therethrough in use; d) Locating the inner electrode concentrically within the outer electrode; and e) Locating each opposing outer portion of the inner electrode within the receiving means in respective mirrors and orienting the mirrors relative to one another such that the apertures for receiving a laser beam therethrough are offset from one another.

The outer electrode may also comprise a metal extrusion.

A light amplification device comprising an all-metal annular construction such as discussed above offers significant advantages over the prior art devices including, but not restricted to, one or more of the following:

1. The materials and structural design of the device are inexpensive to manufacture;

2. The device is easily assembled by semi-skilled personnel;

3. The device is durable and compact;

4. The device design is suitable for UHV vacuum processing and baking at a high temperature and there are no glass or ceramic seals included in the design;

5. The annual discharge fabrication is combined with a multi-pass Herriott cell resonator configuration. Such a free-space diffraction limited laser resonator design provides a very high quality and stable beam mode;

6. The device has an extremely efficient method of heat removal from the gas discharge and laser 'gain' region and inner electrode and can be used to give excellent mode and power stability;

7. The laser device of the present invention can be fundamentally extendable so that higher laser powers can be produced by either increasing the number of optical paths within a given cross-sectional profile and or by length scaling;

8. The laser device of the present invention may be capable of higher frequency pulsing (>100KHz) this being achievable partly due to shorter discharge sections being used and efficient cooling;

9. High peak power pulses at repetition rates >100KHz are achievable by using novel gas combinations suitable for use in the laser device according to the present invention;

10. UHV and HVAC compatible materials may be used for the laser device of the present invention to ensure long product life when the laser is in a sealed-offmode;

11. The laser device of the present invention may be used in sealed-off, slow- flow or fast-flow mode and need not be limited to CO 2 lasers or CO lasers;

12. The laser device of the present invention need not comprise any moving parts within the laser head;

13. A gas ballast (reservoir of gas) may be incorporated when in the sealed-off system to assist with enhancing the laser operational and shelf life;

14. The laser device of the present invention utilises proven RF power supply technology to operate and control the gas discharge section;

15. The laser device of the present invention has high efficiency by virtue of optimisation of the laser resonator, improved thermo-mechanical stability and extremely efficient cooling of the gas discharge, electrodes and laser superstructure; and

16. The laser device of the present invention is able to offer high laser amplification and gain from a fixed size device by increasing the diameter or cross-sectional profile of the laser resonator (not just by increasing

length) and therefore increasing the number of resonant optical paths between only two electrodes that form the annular section.

A further aspect of the present invention provides a method of light amplification comprising the steps of: a) Providing a light amplification device in accordance with any one of Claims 1 to 8; and b) Electrically grounding (earthing) the outer electrode and passing live current through the inner electrode so as to create a discharge within the gas discharge region.

in one embodiment, the complete optical path is defined by an optical system that is commonly known as a Herriot Cell. This is a multi-pass optical system formed between two mirrors that can optimally create a multi-pass arrangement that is compatible with annular discharge geometry. The optical beams precess around the inner electrode and in the space defined by the annular gas discharge section between the inner and outer electrodes and form a multi-pass resonant cavity via reflection from the two internal mirrors, each having a single hole at a point on their diameter.

By suitable alignment techniques, the beam is allowed to pass through each hole and to fall upon the external resonant cavity mirrors in a conventional manner and the laser beam is output via the laser output coupler. The internal and external mirrors are mounted on the inner and outer faces respectively of metal flanges positioned at either end of the laser structure that are attached to the outer casing that forms the laser superstructure and that may be provided by the outer electrode.

Brief Description of the Drawings

An embodiment of the invention will now be described, by way of example only, and with reference to the accompanying drawings, in which:

Figure 1 is an isometric view of a coaxial gas discharge arrangement of a first embodiment of a light amplification device in accordance with the present invention showing the inner and outer electrodes; Figure 2 is a cross-sectional view of the embodiment of figure 1 showing the inner and outer electrodes and the mirrors and supporting discs;

Figure 3 is an isometric view of the coaxial gas discharge arrangement of figure 1 showing the inner and outer electrodes and the internal mirrors and end cap; and Figure 4 is a perspective view of a coaxial gas discharge arrangement of a further embodiment of a light amplification device in accordance with the present invention showing the outer electrode finned heat sink arrangement.

Detailed Description of the Invention A device in accordance with the present invention will now be described with reference to figures 1 to 4 in which the same figure reference numerals have been used to indicate the same features throughout.

Figure 1 shows an annular discharge arrangement 10 comprising an annular inner electrode 20 received within an annular outer electrode 30. Inner electrode 20 is received within outer electrode 30 such that the outer surface of inner electrode 20 is essentially parallel to and coaxial with the inner surface of outer electrode 30 and the gas discharge volume defined between the inner and outer electrodes 20, 30 is uniform along the whole length of the outer electrode 30.

Figure 2 is a cross-sectional view of the discharge arrangement 10 of figure 1 also showing the two inner mirrors M2 and M3, the two outer mirrors Ml and M4 and supporting discs Dl and D2. The annular RF gas discharge region 40 is shown as the shaded area in figure 2. The two inner mirrors M2, M3 form the multi-pass system and supporting discs Dl and D2 serve to hold all of the inner mirrors M2,

M3, and outer mirrors Ml, M4, as well as the inner electrode 20 in optical alignment.

In the embodiment shown in figures 1 to 4, the supporting discs Dl and D2 are supported by stand-off pillars 50. Suitable portions of the reduced diameter end sections of the inner electrode 20 and/or the stand-off pillars 50 and/or the supporting discs Dl, D2 are coated with an oxide layer to enable the discharge to run stably and only in the annular gap between the inner and outer electrodes 20, 30.

Figure 3 shows the embodiment of figure 2, with end cap 60 in place around one end of the outer electrode 30. It will be appreciated that a further end cap 60 is located at the opposing end of outer electrode 30, in use (not shown in figure 3).

Finally, figure 4 shows a further embodiment of a device in accordance with the present invention. In figure 4, the extrusion design of outer electrode 30 differs from the design shown in figures 1 to 3. The device 10 is sealed at the ends using end caps 60 (similar to those shown in figure 3). Inner electrode 20 is hollow and defines a central passage 22. Extruded outer electrode 30 defines passages 32 therethrough. In figure 4, four passages 32 are defined by the extruded outer electrode 30. However, it will be appreciated that the number, shape and positions of the passages 32 may vary in dependence upon the requirements of the device.

The extruded outer electrode 30 also serves as the vacuum envelope, the grounded RF enclosure and the finned heat sink. In this air-cooled embodiment, the coolant water is flowed through the central passage 22 in inner electrode 20 and then circulated in a closed loop through the passages 32 in extruded outer electrode 30. Heat is extracted from the whole device 10 by blowing air over fins 34 of outer electrode 30. The design of the fins 34 of outer electrode 30 serves to increase the

outer surface area available to the circulating air to facilitate efficient heat transfer from the outer electrode 30.

In the water-cooled version of this embodiment, the coolant is, in addition, circulated through a separate heat exchanger or chiller (not shown) or is constantly replenished and drained using a continuous coolant source.

Although aspects of the present invention have been described with reference to the embodiment shown in the accompanying drawings, it is to be understood that the invention is not limited to the precise embodiment shown and that various changes and modifications may be effected without further inventive skill and effort.