EP1123570A1

EP1123570A1 - Truncated ridge waveguide for all-metal slab gas laser excitation

Info

Publication number: EP1123570A1
Application number: EP99948271A
Authority: EP
Inventors: Peter Vitruk
Original assignee: Individual
Current assignee: Individual
Priority date: 1998-09-21
Filing date: 1999-09-17
Publication date: 2001-08-16
Also published as: WO2000017969A1

Abstract

The present invention is directed to a high power, ridge waveguide, RF excited slab gas laser (10) with improved ridge electrode design. It is achieved by truncating the ridges (21) in order to decrease resonant frequency of the ridge waveguide structure from microwave into VHF band, (30-300 MHz), which is the most suitable for CO2 laser excitation. The present invention is characterized by a much simpler liquid cooling scheme of the slab electrodes through truncated ridges, which connect electrodes to the inner wall (420) of a vacuum-sealed laser tube (421). A high Q-factor of the truncated ridge waveguide resonator allows for greater RF power coupling efficiency into the gas discharge plasma than is achieved in slab electrode systems resonated with coil inductors. Low voltage RF excitation is applied to both electrodes in anti-phase, which prevents unwanted discharges from electrodes to the inner walls of the tube and to laser resonator mirrors.

Description

TRUNCATED RIDGE WAVEGUIDE FOR ALL-METAL

SLAB GAS LASER EXCITATION

TECHNICAL FIELD This invention relates to radio-frequency (RF) excited gas lasers, especially to diffusion-cooled slab C0₂ lasers.

BACKGROUND OF INVENTION The design of a high power RF excited slab gas laser has to meet several strict electrical, mechanical and thermal requirements. First of all, RF power should be delivered to the discharge electrodes, located within a vacuum-sealed laser tube, and then uniformly distributed over the large area gas discharge plasma. Resonant inductors have to be attached between the electrodes to ensure proper input impedance and uniform voltage distribution between the electrodes. The electrodes have to be spaced with high precision with respect to each other and within the tube with enough space around each of them. Also, liquid cooling of the electrodes is required in order to remove the heat generated in the gas discharge. Finally, electrodes and other internal components of the laser tube have to be securely attached within the tube in order to withstand mechanical vibrations and shocks as well as thermal differential expansion, etc.

Many of the prior art RF excited slab laser designs (e.g. Hobart et. al, US Patent #5,123,028) are based on grounding one of the discharge electrodes, while the other electrode is supplied with RF voltage through a single high-voltage electrical connector. A disadvantage of this design is in the complexity of the live RF electrode support structure. Additionally, the electrical connector has a finite inductive impedance, resulting in the possibility of exceedingly high voltage on the connector, which is a further disadvantage of this approach due to the risk of unwanted RF discharges around the connector. Cooling water is brought into the vacuum-sealed laser tube by metal pipes, which are then pressed into the electrodes. Cooling pipes used for RF electrode have in-line dielectric insulators to electrically de-couple this electrode from the ground. A disadvantage of this cooling scheme is in its limited efficiency due to the finite thermal resistance of the cooling pipe-to- electrode interface.

Another prior art laser design (Allcock, US Patent # 4,817,108) provides liquid cooling of the RF electrode by arranging the water flow through hollow inductors. These water carrying metal inductors de-couple the RF electrode from the ground potential. A disadvantage of this design is the significant dimensions of the coiled water pipes, which leads to unnecessary and undesirable increases in the laser tube dimensions.

Another yet prior art laser design (Kruger et. al., US Patent # 5,224,117) uses a microwave excited, diffusion cooled, ridge waveguide electrode structure, which can be directly water cooled through the ridge electrodes with no need for water-to-vacuum sealed fittings. This laser does not require additional elements such as an electrode support structure (because ridge electrodes are just a part of the tube), resonant coils, cooling pipe support structure, etc. A disadvantage of this design is its applicability to the microwave range of excitation frequencies (in excess of approximately 1 GHz), which does not prove to be the most effective and practical for CO laser excitation (Vitruk, Hall and Baker, IEEE J Quantum Electron., 30, 1623 (1994)).

It is an object of the present invention to simplify and improve the electrode support structure design used in high power, slab gas lasers with RF excitation in VHF band (30- 300 MHz). It is a further object of the current invention to improve and simplify the liquid cooling of slab laser electrode structure. It is also an object of the current invention to increase the Q-factor of the electrode resonant structure in order to improve the efficiency of RF power coupling into plasma.

SUMMARY OF INVENTION An all-metal slab gas laser according to present invention consists of a metal tube and a pair of endplates forming a vacuum envelope for containing a laser gas, a laser resonator mirrors placed at the opposite ends of the tube and a pair of elongated metal ridges located on the inner walls of the metal tube to define a ridge waveguide. The ridges have the discharge surfaces disposed so as to define a gap in which a slab gas discharge lasing medium is sustained by applying the alternating electrical current between the ridges. At least one of the ridges is truncated in order to increase the ridge waveguide structure inductance and to decrease the resonant frequency from the microwave band into the VHF band (30-300 MHz), which is the most suitable for CO₂ laser excitation.

The present invention is characterized by much simpler and more efficient electrode cooling. It is achieved by supplying water directly to the electrodes through the ridge structure.

A high Q-factor is an intrinsic feature of the truncated ridge waveguide resonator in the present invention, since the RF current is conducted along the low resistance, large area surfaces of the inside walls of the waveguide. The high Q-factor of the truncated ridge resonator allows for greater RF power coupling efficiency into the gas discharge plasma than is achieved in prior art slab electrode systems resonated with coil inductors.

Additionally, electrodes are driven in anti-phase with low RF voltage, which prevents unwanted RF discharges from electrodes to the walls of the tube and to laser resonator mirrors.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an isometric schematic diagram of prior art ridge waveguide. FIG. 1A is an equivalent electric diagram of the ridge waveguide for cut-off conditions.

FIG. 2 is an isometric schematic diagram of the first embodiment of the truncated ridge waveguide according to present invention.

FIG. 3 is an isometric schematic diagram of the second embodiment of the truncated ridge waveguide according to present invention. FIG. 4 is an isometric schematic diagram of the third embodiment of the truncated ridge waveguide according to present invention.

FIG. 5 is an isometric schematic diagram of the all-metal slab gas laser with truncated ridge waveguide according to present invention.

DETAILED DESCRIPTION OF THE INVENTION Figure 1 is an isometric schematic diagram of prior art ridge waveguide. A ridge waveguide 10 consists of ridges 11 and cavities 12 formed by the inner walls 120 of the waveguide 10. An inter-ridge gap 100 between the ridges 11 in Figure 1 defines a capacitance C, which is distributed along the surfaces 110 of the ridges 11. Figure 1 A is an equivalent electric diagram of the ridge waveguide 10 for cut-off conditions in which inter- ridge gap 100 is represented by a capacitor C. Each cavity 12 on either side of the ridge 11 in Figure 1 represents a one-loop inductor 21, which is distributed along the waveguide 10. The surfaces 110 of the ridges 11 serve as discharge electrodes to which RF or microwave voltage can be applied in order to sustain a gas discharge in the inter-ridge gap 100.

An equivalent electric diagram of the ridge waveguide for the cut-off conditions (S. Ramo, J. Whinnery and Th. Van Duzer, "Fields and waves in communication electronics",

John Wiley & Sons, Second Edition, p. 470), which is relevant to the slab gas laser excitation, is presented in Figure 1A. The cut-off condition corresponds to the inter-ridge voltage uniformity along the waveguide 10 and it is achieved at the parallel resonance in the LC-circuit shown in Figure 1A. The resonant frequency, f_res=I/2 π' (L C)^' of an

LC-circuit can be decreased by increasing the inductance, L and/or the capacitance, C. For example, an increased ridge electrode surface area 110 will cause the structure capacitance, C, to increase and, therefore, cause the resonant frequency to shift down. In order to increase the structure inductance, L, a significant modification to ridge geometry has to be implemented.

Figure 2 is an isometric schematic diagram of the first embodiment of the truncated ridge waveguide according to present invention. A ridge waveguide 20 consists of the metal ridges 21 and cavities 22 formed by the inner walls 220 of the metal tube 221. An inter- ridge gap 200 between the ridges 21 defines an inter-ridge capacitance, which is distributed along the surfaces 210 of the ridges 21. The inter-ridge gap 200 and waveguide cavities 22 are left unchanged as in the prior art ridge waveguide. At the same time, truncating the body of the ridges 21 forms new cavities 23. Greater current density on the surface of the truncated ridges 21 increases the magnetic field intensity in the volume of the waveguide, which together with greater volume of the waveguide 20 increases both the magnetic flux and the inter-ridge inductance. The resonant frequency of such truncated ridge waveguide 20 is substantially lower than in the prior art ridge waveguide. The inter-ridge gap 200 can be filled with a gas discharge active lasing medium if an RF voltage at the appropriate frequency is applied between the ridges 21.

Figure 3 is an isometric schematic diagram of the second embodiment of the truncated ridge waveguide according to present invention. A ridge waveguide 30 consists of metal ridges 31 and cavities 32 formed by the inner walls 320 of the metal tube 321. An inter-ridge gap 300 between the ridges 31 defines an inter-ridge capacitance, which is distributed along the surfaces 310 of the ridges 31. The inter-ridge gap 300 is left unchanged, while the waveguide cavity 32 is expanded into the body of the ridges 31 by truncating the ridges 31. The greater volume 32 of the waveguide 30 results in increased inter-ridge structure inductance. As a result, the resonant frequency of this truncated ridge waveguide 30 is substantially lower than in the prior art ridge waveguide. The inter-ridge gap 300 can be filled with a gas discharge active lasing medium if an RF voltage at the appropriate frequency is applied between the ridges 31.

Figure 4 is an isometric schematic diagram of the third embodiment of the truncated ridge waveguide according to present invention. A ridge waveguide 40 consists of metal ridges 41 and cavities 42 formed by the inner walls 420 of the metal tube 421. An inter- ridge gap 400 between the ridges 41 defines an inter-ridge capacitance, which is distributed along the surfaces 410 of the ridges 41. The waveguide volume 42 is substantially increased by the combination of truncations of the ridges 41 according to first and second embodiments, shown in Figures 2 and 3. The structure of the truncated ridge 41 includes two major components: posts 43 and electrodes 44. Posts 43 connect the electrodes 44 to the inner walls 420 of the waveguide 40. Significantly greater current density on the post surfaces 43 of the truncated ridges 41 together with much greater volume 42 of the waveguide 40 increases the magnetic flux and the inter-ridge inductance. Additionally, the electrodes 44 of the ridges 41 are made wider in order to increase the inter-ridge capacitance as well as the capacitances from each ridge electrode 44 to the inner walls 420 of the waveguide 40. Consequently, the resonant frequency of such truncated ridge waveguide 40 is substantially lower than in the prior art ridge waveguide. The inter-ridge gap 400 can be filled with a gas discharge active lasing medium if an RF voltage at the appropriate frequency (5-20% lower than the resonant frequency of the ridge waveguide 40) is applied between the ridges 41.

Also shown in Figure 4 are the electrical feed-through connectors 45 through which the electrical energy is supplied from RF power source 450 to electrodes 44 in order to sustain a gas discharge active lasing medium in inter-electrode gap 400 between the ridge electrodes 44. The electrical connectors 45 are brought into waveguide 40 through the openings 46 made in the walls 420 of the metal tube 421. Due to a symmetrical ridge structure design, the electrodes 44 can be driven in anti-phase, which reduces the voltage on the RF feed-through connectors 45 by half (if compared to voltage necessary to sustain RF discharge between electrodes). This eliminates the unwanted discharges from the electrodes 44 and from connectors 45 to the inner walls 420 of the metal tube 421. The electrodes 44 and some of the posts 43 could be made hollow, so that liquid coolant (such as water) could be pumped through them in order to remove the heat generated by the plasma in the gap 400 between electrodes 44. The resonant frequency of such truncated ridge waveguide structure is f_res=l/2 π¹ (L (C_E-_E + 1/2 Cε-τ))^~ , where CE-E is inter-electrode capacitance, CE-T is electrode-to-tube capacitance and L is inter-electrode inductance defined by the posts 43, electrodes 44 and the inner walls 420 of the metal tube 421.

According to the present invention, the posts 43 in the truncated ridge waveguide 40 do not just de-couple electrodes 44 from the ground potential like in the prior art laser (Patent '108). The posts 43 act as a part of resonant inductors, which are necessary for proper impedance matching and to achieve the uniform RF voltage distribution along the electrodes 44. An approximate mathematical expression for the inter-ridge structure inductance contains two major terms. The first term is proportional to the inside diameter of the waveguide 40, D, while the second term is proportional to a product of D and

Log(D/d), where d is a diameter of the post 43. Thus, in order to maximize this inductance, one needs to maximize the diameter of the waveguide 40 and to minimize the diameter and number of the posts 43 connecting the electrodes 44 to the waveguide 40.

Another embodiment of the present invention is shown in Figure 5, which is an isometric schematic diagram of the all-metal slab gas laser with truncated ridge waveguide. A laser 50 includes the metal electrodes 51, the metal posts 52 and the cavities 53 formed by the inner walls 530 of the metal tube 531. The slab of lasing medium 500 is disposed in between the discharge surfaces 510 of the electrodes 51 , which also provide predominant cooling for lasing medium 500. Also, the surfaces 510 could be light reflective in order to light-guide the intracavity laser radiation inside of the laser 50. The laser 50 also includes the end-caps 54 and the laser mirrors 55 at both ends of the tube 531. The tube 531 is vacuum-sealed on both of its ends by the end-plates 54. Laser mirrors 55 could be mounted onto the end-plates 54. The source of an RF power 56 is connected to the electrodes 51 by the electrical feed-through connectors 560 through an openings 561 in the tube 531. Dielectric spacers 57 are used to maintain a precise separation between electrodes 51. Dielectric spacers 570, backed-up by set-screws 571, are used to eliminate mechanical stress from the posts 52. The posts 52 could be welded both to the tube 531 and to the respective electrodes 51. A liquid coolant (such as water) could be delivered to the electrodes 51 through the hollow posts 52. The posts 52 connect the electrodes 51 to the inner walls 530 of the tube 531.

Significantly greater current density on the post surfaces 52 together with much greater volume 53 of the tube 531 increases the magnetic flux and the inter-ridge inductance. Additionally, the electrodes 51 are made wider in order to increase the inter-ridge capacitance as well as the capacitances from each ridge electrode 51 to the inner walls 530 of the tube 531. Consequently, the resonant frequency of such truncated ridge waveguide slab laser 50 is substantially lower than in the prior art ridge waveguide. The excitation frequency is typically 5 to 20 % lower than the resonant frequency of the ridge waveguide structure in order to ensure the efficient gas discharge ignition as well as the uniformity of the discharge along the electrodes 51.

PREFERRED EMBODIMENT The preferred embodiment of the present invention is a truncated ridge waveguide structure typical for a high power (400-500 Watts) waveguide CO gas slab laser and is similar to the structure schematically shown in Figure 5. It contains two electrodes 51 (each 1cm tall and having discharge area 7.6cm x 50cm each) spaced 0.3cm apart from each other. Each electrode 51 is attached to the inner walls 530 of a square (10cm x 10cm) aluminum tube 531 by two aluminum posts 52 (diameter 0.6cm). The location of the posts 52 is chosen to maximize the inter-electrode inductance. The calculated resonant frequency for this electrode structure is approximately 90 MHz with a Q-factor in excess of approximately 400-500. The electrodes 51 are driven with approximately 4 kilo Watts of RF power at 80 MHz with a voltage phase shift of 180 degrees between the electrodes. An anti-phase, low- voltage (approximately 100 Volts RMS) RF drive of electrodes 51 allows for a simple and low-cost design of vacuum-sealed electrical feed-throughs 560, which are free from unwanted RF discharges around them. The discharge surfaces 510 of the electrodes 51 are light-reflective in order to light-guide the intra-cavity laser readiation inside of the laser 50. The laser gas mixture is He:N₂:CO =3:l:l+5%Xe, the gas mixture pressure is approximately 100 Torr and the cooling water temperature is in the range 10- 30°C.

It should be understood that even though numerous features and advantages of the present invention have been set forth in the foregoing description, the above disclosure is illustrative only. For example, the dimensions given for the various elements are exemplary only and could be modified by those skilled in the art in light of the foregoing discussions. Changes may be made in detail and yet remain within the broad principles of the present invention. Accordingly, the invention is not limited except as by the appended claims.

Claims

I claim:

1. An all-metal slab gas laser with radio-frequency excitation, comprising: a metal tube and a pair of endplates forming a vacuum envelope for containing a laser gas, said metal tube having an inner wall; laser resonator mirrors placed at the opposite ends of the tube; first and second elongated metal ridges located on the inner wall of said metal tube to define a ridge waveguide, said ridges having discharge surfaces disposed so as to define a gap therebetween; and means for applying alternating electrical current between said first and second ridges to establish a slab gas discharge lasing medium in said gap, wherein at least one of the said ridges is truncated so as to define a ridge waveguide resonance frequency in 30-300 MHz band.

2. The laser of claim 1 wherein said laser further includes spacers between said ridges.

3. The laser of claim 1 wherein said laser further includes spacers between said ridges and said tube.

4. The laser of claim 1 wherein said means for applying alternating electrical current includes electrical power source and an electrical feed-throughs to deliver said electrical current through openings in the inner wall of the tube, said feed-throughs being vacuum sealed to the inner wall of the tube.

5. The laser of claim 1 wherein the electrical potential on the discharge surface of said first ridge is in anti-phase with respect to the electrical potential on the discharge surface of said second ridge.

6. The laser of claim 1 wherein said discharge surfaces are light reflecting surfaces.

7. The laser of claim 1 wherein said discharge surfaces are planar light reflecting surfaces.

8. The laser of claim 1 wherein said laser gas includes CO₂, N₂ and He.

9. The laser of claim 1 wherein said ridges contain a hollow channel to allow for a liquid coolant flow through the ridges in order to remove the heat generated in said gas discharge lasing medium.

10. An all-metal slab gas laser with radio-frequency excitation, comprising: a metal tube and a pair of end plates forming a vacuum envelope for containing a laser gas, said metal tube having an inner wall; laser resonator mirrors and optical elements placed at the ends of the tube; and first and second elongated metal electrodes located within said metal tube so as to define an inter-electrode capacitance as well as capacitances between the tube and each of said electrodes, said electrodes having discharge surfaces disposed so as to define a gap therebetween; and means for applying alternating electrical current between said first and second electrodes to establish a slab gas discharge lasing medium in said gap, wherein at least one of the said electrodes is connected to the tube by at least one metal post, said post together with the electrodes and the inner wall of the tube defining an interelectrode inductance that resonates with said capacitances at a frequency in 30-300 MHz band.

11. The laser of claim 10 wherein said laser further includes a spacers between said electrodes.

12. The laser of claim 10 wherein said laser further includes a spacers between said electrodes and said tube.

13. The laser of claim 10 wherein said means for applying alternating electrical current includes an electrical power source and electrical feed-throughs to deliver said electrical current through openings in the inner wall of the tube, said feed-throughs being vacuum sealed to the inner walls of the tube.

14. The laser of claim 10 wherein the electrical potential on the said first electrode is in anti- phase with respect to the electrical potential on said second electrode.

15. The laser of claim 10 wherein said discharge surfaces are light reflecting surfaces.

16. The laser of claim 10 wherein said discharge surfaces are planar light reflecting surfaces.

17. The laser of claim 10 wherein said laser gas includes CO₂, N₂ and He.

18. The laser of claim 10 wherein said electrodes and said posts are hollow, thereby allowing for liquid coolant flow through said electrodes and said posts in order to remove the heat generated in said gas discharge lasing medium.

19. An all-metal slab gas laser with radio-frequency excitation, comprising: a metal tube and a pair of endplates forming a vacuum envelope for containing a laser gas, said metal tube having an inner wall;

laser resonator mirrors placed at the opposite ends of the tube; first and second metal ridges located within said tube, each ridge including an elongated metal electrode and at least one metal post connecting said electrode to the inner wall of the tube, said electrodes located within said metal tube so as to define an inter-electrode capacitance as well as capacitances between the tube and each of said electrodes, said electrodes having discharge surfaces disposed so as to define a gap therebetween; and means for applying alternating electrical current between said first and second electrodes to establish a slab gas discharge lasing medium in said gap, wherein said posts together with the electrodes and the inner walls of said metal tube define an interelectrode inductance resonating with said capacitances at a frequency in 30-300 MHz band.

20. The laser of claim 19 wherein said laser further includes a spacers between said ridges.

21. The laser of claim 19 wherein said laser further includes a spacers between said ridges and said metal tube.

22. The laser of claim 19 wherein said means for applying alternating electrical current includes an electrical power source and electrical feed-throughs to deliver said electrical current through the openings in the inner wall of said metal tube, said feed-throughs are vacuum sealed to said inner wall of said metal tube.

23. The laser of claim 19 wherein the electrical potential on said first electrode is in antiphase with respect to the electrical potential on said second electrode.

24. The laser of claim 19 wherein said discharge surfaces are light reflecting surfaces.

25. The laser of claim 19 wherein said discharge surfaces are planar light reflecting surfaces.

26. The laser of claim 19 wherein said laser gas includes CO₂, N₂ and He.

27. The laser of claim 19 wherein said electrodes and said posts are hollow, thereby allowing for liquid coolant flow through said electrodes and said posts in order to remove the heat generated in said gas discharge lasing medium.