Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/02—Constructional details
  • H01S3/03—Constructional details of gas laser discharge tubes
  • H01S3/0315—Waveguide lasers

Definitions

• TECHNICAL FIELD This invention relates to radio-frequency (RF) excited gas lasers, especially to diffusion-cooled slab C0 2 lasers.
• 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.
• liquid cooling of the electrodes is required in order to remove the heat generated in the gas discharge.
• 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.
• 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 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)).
• An all-metal slab gas laser 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 CO 2 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.
• 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.
• 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.
• FIG. 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.
• LC-circuit can be decreased by increasing the inductance, L and/or the capacitance, C.
• an increased ridge electrode surface area 110 will cause the structure capacitance, C, to increase and, therefore, cause the resonant frequency to shift down.
• a significant modification to ridge geometry has to be implemented.
• FIG. 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.
• truncating the body of the ridges 21 forms new cavities 23.
• 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.
• FIG. 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.
• 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.
• FIG 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.
• 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.
• 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 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
• FIG. 5 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.
• 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.
• 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.

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• Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Lasers (AREA)
• Engineering & Computer Science (AREA)
• Plasma & Fusion (AREA)
• Optics & Photonics (AREA)

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• 1999
  • 1999-09-17 EP EP99948271A patent/EP1123570A1/de not_active Withdrawn
  • 1999-09-17 WO PCT/US1999/021370 patent/WO2000017969A1/en not_active Ceased

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