Abstract:
The present invention provides a novel and improved technique for achieving a combined RF-DC gas discharge in the optical cavity of the gas laser. The combination of the fast pre-ionization with the transverse RF discharge and the high power excitation with the highly-efficient longitudinal DC discharge allows for a high-speed laser gain modulation of the active medium of the high-power gas laser. At the same time, the all-metal electrode system allows for a very compact, reliable and low cost design of the gas laser.

Description:
REFERENCE TO PROVISIONAL APPLICATION 
     This application claims the benefit of the U.S. Provisional Application No. 60/102,063, filed Sep. 28, 1998. 
    
    
     TECHNICAL FIELD 
     This invention relates to gas lasers, especially to diffusion-cooled CO 2  lasers with improved pulsing performance. 
     BACKGROUND OF INVENTION 
     Diffusion-cooled CO 2  gas lasers have a number of useful industrial and medical applications. Traditionally, most commercial diffusion-cooled CO 2  gas lasers utilize a radio-frequency (RF) excitation discharge that is transverse to the laser&#39;s longitudinal axis. RF excitation allows for a very compact, low cost, all-metal laser tube designs, which are well known for their long life-time and reliability. Less popular, though still used mainly in medical CO 2  lasers, direct current (DC) discharge excitation, that is parallel to the laser&#39;s longitudinal axis, allows for less expensive (than RF) DC power source. It is often desirable to have a laser capable of emitting a continuous wave (CW) low power output and also capable of emitting a pulsed high power output. Unfortunately, RF excitation becomes very expensive to implement if high peak powers are required. On the other hand, the longitudinal DC discharge cannot be easily modulated which limits the pulsing rate of laser with DC excitation. However, a combination of RF excitation with DC excitation could overcome the limitations mentioned above if such excitation utilizes the low power RF and high power DC excitation circuits. 
     A prior art excitation method utilizing transverse RF discharge and longitudinal DC discharge is described by V. I. Myshenkov, et. al. in “Stability of a composite discharge sustained by static and RF electric fields. II. Mechanism for stabilizing effect of an RF field on the positive column of a DC discharge”,  Soviet Journal of Quantum Electronics,  1982, vol. 8, No. 4, pp. 397-400. This prior art method describes the diffusion-cooled CO 2  laser, which includes a glass discharge tube filled with the laser gas. A pair of long RF electrodes is located outside of the glass tube parallel to its axis so that the transverse RF discharge inside the tube is created when RF voltage is applied to the RF electrodes. A pair of short DC electrodes is located inside the glass tube on the opposite ends of the tube so that the longitudinal DC discharge inside the tube is created simultaneously with the RF discharge when DC voltage is applied to the DC electrodes. A disadvantage of this method is in the use of the glass discharge tube, which is fragile and expensive to manufacture. 
     Another prior art laser with combined transverse RF and transverse DC excitation is described in U.S. Pat. No. 5,097,472 (the &#39;472 patent) issued to Peter P. Chenausky. This laser design utilizes two elongated metal electrodes to which both the RF and DC voltage is applied. The excitation circuitry includes the RF and DC power sources as well as the elements to electrically isolate one power source from the other. The pulsed RF voltage is applied to the electrodes and a uniform transverse RF discharge lights up in the interelectrode gap. Once the RF discharge is established, it commutes the energy from the DC power source through the electrodes and through the interelectrode gap filled with the RF discharge plasma. 
     A disadvantage of the technique described in the &#39;472 patent is a very high cost of the RF power supply required to pre-ionize the gas at elevated gas pressure. Indeed, a 28 kW RF pulse is described for commuting a high power DC discharge through a 1 cm interelectrode space between 13.5 cm long electrodes in a CO 2  laser operated at 225 Torr. An average laser power output from the described electrode configuration can not exceed approximately 5-10 W for diffusion-cooled laser gas (Vitruk et. al., “Similarity and Scaling in Diffusion-Cooled RF-Excited Carbon Dioxide Lasers”,  IEEE J. Quantum Electronics,  QE-30, 1994, pp. 1623-1634), which translates to approximately $2,000 price for commercially available CO 2  lasers. At the same time the price for the 28 kW RF power source is in excess of approximately U.S. $30,000 for a vacuum tube based design, which makes this approach prohibitively expensive. 
     Another disadvantage of the method described in &#39;472 patent is a very low efficiency of the transverse DC excitation if a low pressure gas discharge is used. Indeed, a voltage across a 1 cm long DC gas discharge at 30 Torr is described in &#39;472 patent as 400 Volts. It is well known in the art of electrical gas glow discharges that approximately 250-300 Volts is distributed across the cathode fall region, which does not contribute constructively to the creation of the laser gain in the DC discharge. Therefore, no more than approximately 30-40% of the DC power dissipated in the transverse DC discharge is actually deposited in the plasma column, in which the laser gain is created. 
     Another prior art laser with combined transverse RF and transverse DC excitation is described in the U.S. Pat. No. 5,596,593 (the &#39;593 patent) issued to Katherine D. Crothall et. al. This prior art laser includes two pairs of elongated electrodes, which form a four walls surrounding a square bore of a CO 2  gas laser. First electrode pair is used to create a transverse RF discharge, while the second pair of electrodes is utilized to sustain a transverse DC discharge. Similarly to &#39;472 patent, a disadvantage of the method described in &#39;593 patent is a low efficiency of the transverse DC gas discharge excitation due to significant voltage drop across the cathode fall region of the DC discharge. 
     It is an object of present invention to simplify and improve the combined RF and DC excitation method used in high peak power, diffusion-cooled gas lasers. Present invention allows for a very simple, light-weight, low-cost, long life-time, all-metal design of the high power, efficient pulsed gas laser for either industrial or medical applications. 
     SUMMARY OF INVENTION 
     The present invention overcomes the above discussed and other disadvantages of the prior art by providing a novel and improved technique for achieving a combined RF-DC gas discharge in the optical cavity of the gas laser. A method for producing a beam of laser energy with the all-metal gas laser according to present invention consists of the following steps. First, a plurality of elongated metal electrodes is provided, at least two of the electrodes having a plurality of dielectrically coated surface areas and a plurality of metal surface areas located on a discharge surfaces of the electrodes. Second, the spacing between the metal surface areas is provided to be substantially greater than the spacing between the discharge electrodes. Third, an RF voltage is applied between at least two electrodes to establish a transverse RF gas discharge between the discharge surfaces of the two electrodes to which the RF voltage is applied. And finally, a DC voltage is applied between at least two electrodes, thereby producing a longitudinal DC gas discharge through said transverse RF gas discharge with the DC gas discharge current flowing between the metal surface areas of the two electrodes to which the DC voltage is applied. 
     The combination of the fast pre-ionization with the transverse RF discharge and the high power excitation with the highly-efficient longitudinal DC discharge allows for a high-speed modulation of the active medium of the high-power gas laser. At the same time, the all-metal electrode system allows for a very compact, reliable and low cost design of the gas laser. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1A is an isometric schematic diagram of the electrode system of the all-metal gas laser with combined RF-DC excitation according to first embodiment of present invention. 
     FIG. 1B is a side-view schematic diagram of the all-metal gas laser with combined RF-DC excitation according to second embodiment of present invention. 
     FIG. 2A is a cross-sectional schematic diagram of the all-metal gas laser with combined RF-DC excitation according to third embodiment of present invention. 
     FIG. 2B is an isometric schematic diagram of the electrodes  210  and  220  from FIG. 2A according to third embodiment of present invention. 
     FIG. 2C is an isometric schematic diagram of the electrodes  220  and  240  from FIG. 2A according to forth embodiment of present invention. 
     FIG. 3A is a cross-sectional schematic diagram of the all-metal gas laser with combined RF-DC excitation according to fifth embodiment of present invention. 
     FIG. 3B is a cross-sectional schematic diagram of the all-metal gas laser with the combined RF-DC excitation according to sixth embodiment of present invention. 
     FIG. 3C is an isometric schematic diagram of the electrodes  320  and  330  from FIG.  3 B and the transverse RF gas discharge according to sixth embodiment of present invention. 
     FIG. 3D is an isometric schematic diagram of the electrodes  320  and  330  from FIG.  3 B and the longitudinal DC gas discharge according to sixth embodiment of present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1A is an isometric schematic diagram of the electrode system of the all-metal gas laser with combined RF-DC excitation according to first embodiment of present invention. The electrode system  10  consists of the metal electrodes  101  and  102  having a discharge surfaces  103  and  104  disposed so as to form an inter-electrode slab discharge gap  105  filled with the laser gas. The metal electrodes  101  and  102  are dielectrically coated with the exception of the surface areas  106  and  107 . The metal surface areas  106  and  107  are made in the form of the strips parallel to each other and perpendicular to the longitudinal axis  130 . The length of the strips  106  and  107  equals the widths of the electrodes  101  and  102 . The area of the strips  106  and  107  is chosen so as to spread the cathode and anode parts of the discharge uniformly along the strips  106  and  107 . 
     The electrodes  101  and  102  are electrically connected to the DC power source  120  and to the RF power source  124 . Once a transverse RF discharge is ignited between the electrodes  101  and  102 , then a longitudinal DC discharge can be established through the RF discharge between the metal surface areas  106  and  107 . The laser gas pressure (in the range 30-100 Torr), the frequency (in the range 40-100 MHz) and the power of the RF power source  124  and the spacing between electrodes  101  and  102  (in the range 1-10 mm) are chosen to optimize the RF discharge characteristics such as its uniformity along the discharge surfaces  103  and  104 , the power density in the discharge gap  105 , etc. The voltage of the DC power source  120  is chosen to optimize the characteristics of the combined RF-DC discharge such as its power density and its uniformity in the direction perpendicular to the axis  130 , etc. The spacing between the metal strip  107  and either of the strips  106  is substantially greater than the spacing between the electrodes  101  and  102  in order to ensure large length of the DC discharge. At the same time the spacing between strips  107  and  106  is not exceeding a certain critical value above which the DC discharge is no longer uniform in the direction perpendicular to the axis  130 . Large length DC discharge results in greater voltage along the plasma column of the DC discharge relatively to the cathode fall voltage, so that the efficiency of the DC power deposition in the active lasing medium is increased. 
     The transverse RF slab discharge is characterized by the positive voltage-current (V-I) characteristics so it can uniformly fill the interelectrode gap  105 . However, the plasma column of the self-sustained DC gas discharge has negative V-I characteristics and additional measures are needed in order to prevent its thermal instability and to stabilize the spatially uniform plasma column. The DC slab discharge plasma uniformity is achieved in the present invention by two means. First of all, the RF discharge pre-ionizes the gas so that the DC electric field can be somewhat less than needed for the self-sustaining the DC plasma column. As a result, the thermal instability of the plasma column could be prevented. Secondly, the areas of the metal strips  106  and  107  could be chosen so that the cathode and anode parts of the DC discharge fully fill the areas of the metal strips  106  and  107 . Therefore, the cathode fall region is characterized by the positive V-I characteristics leading to increased stability of the DC discharge as a whole. 
     FIG. 1B is a side-view schematic diagram of the all-metal gas laser with combined RF-DC excitation according to second embodiment of present invention. The electrode system  10  consists of the metal electrodes  101  and  102  having a discharge surfaces  103  and  104  disposed so as to form an inter-electrode discharge gap  105  filled with the laser gas. The metal electrodes  101  and  102  are dielectrically coated with the exception of the surface areas  106  and  107 . The metal surface areas  106  and  107  are made in the form of the strips parallel to each other and perpendicular to the laser axis  130 . The electrodes  101  and  102  are electrically connected to the DC power source  120  and to the RF power source  124 . Also shown are laser resonator mirrors  126  and  127  as well as laser housing  128  for containing a laser gas. The gas discharge performance of the electrode system  10  in FIG. 1A is similar to the performance of the electrode system of the laser  10  in FIG. 1B with the exception of the greater number of metal surface areas  106  and  107 , which allows for greater length of the electrodes in the direction of the laser axis  130  and leads to increased power of the laser  10 . FIG. 1B also shows additional elements such as capacitors C, C 1  and an inductor L in the electrical circuit of the laser  10 . The capacitor C is blocking a DC current from reaching the RF power supply  124 , while the inductive choke L and capacitor C 1  filter out the RF interference on the DC power supply  120 . 
     FIG. 2A is a cross-sectional schematic diagram of the all-metal gas laser with combined RF-DC excitation according to third embodiment of present invention. Laser  20  includes a discharge bore  200  enclosed by the discharge surfaces  211 ,  221 ,  231  and  241  of the respective electrodes  210 ,  220 ,  230  and  240 . The discharge bore  200  is connected by the longitudinal gaps  201  to the rest of the volume  202  of the laser gas. The electrodes  210 ,  220 ,  230  and  240  are separated by the gas gaps  203  and spacers  204  and  205  from each other and the housing  206 . Spacers  204  are dielectric, while the spacers  205  are metal. Resonant inductor  250  is connected between the electrodes  210  and  230  which serve as an RF electrodes between which transverse RF discharge is sustained. The electrical feed-through  280  on the electrode  230  is connected to the DC power source  260  through the RF blocking inductor L and to the RF power source  270  through the DC blocking capacitor C. 
     The inner wall  207  of the housing  206  is dielectrically coated in order to prevent the DC discharges between electrodes and the housing. Spacing  203  is chosen small enough in order to eliminate a RF discharges between the electrodes and the housing. Additionally, if the RF electrodes  210  and  230  can be driven in anti-phase with the help of inductor  250 , then electrode-to-housing voltage would be only the half of the discharge voltage, which further helps to eliminate the RF discharges between the electrodes  210 ,  230  and the housing  206 . The design and performance of the laser  20  in FIG. 2A is further explained by the FIGS. 2B and 2C. 
     FIG. 2B is an isometric schematic diagram of the electrodes  210  and  220  from FIG. 2A according to third embodiment of present invention. The metal electrodes  210  and  220  are dielectrically coated all over their discharge surfaces  211  and  221  with the exception of the areas  212  and  222 . The metal surface areas  212  and  222  serve as a DC discharge electrodes between which a longitudinal DC discharge is sustained. The structure of the electrode  230  (shown in FIG. 2A) is similar to electrode  210  shown in FIG. 2B, the structure of the electrode  240  (shown in FIG. 2A) is similar to electrode  220  shown in FIG.  2 B. The laser discharge electrodes, shown in FIGS. 2A and 2B, function as follows. The low power, high-voltage RF pulses from the RF source  270  from a transverse RF discharge in the discharge bore  200  between the discharge surfaces  211  and  231 . This triggers a high-power longitudinal DC discharge between the metal surface areas  212  of the high-DC-voltage electrodes  210  and  230  and the metal surface areas  222  of the grounded electrodes  220  and  240 . 
     FIG. 2C is an isometric schematic diagram of the DC electrodes according to forth embodiment of present invention. The forth embodiment of present invention is similar to laser shown in FIG. 2A with the exception that the DC power source is connected to the electrode  220 , the spacers  205  between the electrode  220  and the housing  206  are dielectric, the electrodes  210  and  230  are dielectrically covered all over their surfaces and electrodes  220  and  240  are dielectrically coated according to FIG.  2 C. The discharge surfaces  221  and  241  are facing the discharge bore  200  and are dielectrically coated all over except the areas  222  and  242 . The metal surface areas  222  and  242  serve as a DC discharge electrodes when high-DC-voltage is supplied to the electrode  220  while electrode  240  is grounded by the metal spacers  205  between electrode  240  and housing  206 . 
     Unlike the large discharge volume slab laser presented in FIGS. 1A and 1B, the small bore laser described in FIGS. 2A,  2 B and  2 C can function even with the negative V-I characteristics of the longitudinal DC discharge similarly to other small-bore lasers with longitudinal DC excitation. The DC discharge stabilization is achieved through the electrical ballast elements in the electrical circuit of the DC power source  260 . This feature allows for a very large separation between the DC electrodes (i.e. metal surface areas  222  and  212  or  242 ). The role of the RF pre-ionization is to ensure fast and uniform switching of the plasma in the whole volume of the discharge bore. 
     FIG. 3A is a cross-sectional schematic diagram of the all-metal gas laser with combined RF-DC excitation according to fifth embodiment of present invention. FIG. 3B is a cross-sectional schematic diagram of the all-metal gas laser with the combined RF-DC excitation according to sixth embodiment of present invention. Laser  30  in FIGS. 3A and 3B includes the discharge bore  300 , the ballast volume  310  of the laser gas, the electrodes  320  and  330  and the housing  340 . The discharge bore  300  having the longitudinal slots  301  and  302  connecting it with the ballast volume  310  and the channels  321  and  331  where the electrodes  320  and  330  are located. The width of the of the slots  302  is chosen accordingly to minimize the plasma losses and yet to provide low enough optical losses for the laser radiation propagating inside the discharge bore  300 . The inner surface  304  of the discharge bore  300  is a part of the inner wall  341  of the housing  340  which can have a members  342  extruded so as to form all the necessary features of the laser bore  300 . The inner walls  304  and  341  of the housing  340  and members  342  are dielectrically coated in order to prevent the DC discharges between electrodes and the housing. The DC power source  350  is connected between the electrodes  320  and  330  through the RF-blocking inductor L, while the RF power source  360  is connected between the electrodes  320  and  330  through the DC-blocking capacitor C. The design and performance of the laser  30  is further explained by the FIGS. 3C and 3D. 
     FIG. 3C is an isometric schematic diagram of the electrodes  320  and  330  from FIG.  3 B and the transverse RF gas discharge according to sixth embodiment of present invention. FIG. 3D is an isometric schematic diagram of the electrodes  320  and  330  from FIG.  3 B and the longitudinal DC gas discharge according to sixth embodiment of present invention. The metal electrodes  320  and  330  are dielectrically coated all over their surfaces  322  and  332  with the exception of the areas  323  and  333 . The RF transverse discharge consists of the part  300 A inside the bore  300  and parts  302 A inside the longitudinal slots  302  (see FIG.  3 B). The RF discharge  300 A- 302 A creates a fast and uniform pre-ionization needed for fast and uniform high-power DC excitation. The longitudinal DC discharge consists of the part  300 B inside the discharge bore  300  and parts  302 B inside the longitudinal slots  302  (see FIG.  3 B). 
     Similarly to laser in FIG. 2A, the small bore laser described in FIGS. 3A-3C can function with the negative V-I characteristics of the longitudinal DC discharge similarly to other small-bore lasers with longitudinal DC excitation. The DC discharge stabilization is achieved through the electrical ballast elements in the electrical circuit of the DC power source  350 . This feature allows for a very large separation between the DC electrodes (i.e. metal surface areas  323  and  333 ). The role of the RF pre-ionization is to ensure fast and uniform switching of the plasma in the whole volume of the discharge bore. 
     PREFERRED EMBODIMENT 
     The preferred embodiment of the present invention is schematically shown in FIGS. 2A and 2B. All the laser tube parts, including the 40 cm electrodes  210 ,  220 ,  230  and  240  and the housing  206  are made of anodized aluminum. Discharge surfaces  211 ,  221 ,  231  and  241  have a non-anodized surface areas (0.5×0.2 cm 2  each) forming a pattern shown in FIG. 2B with the separation between the non-anodized stripes  212  and  222  being approximately 6.7 cm. 
     The 200 W pulsed excitation of the 0.5×0.5 cm 2  square bore filled to 60 Torr of laser gas mix (He:CO 2 :N 2 :Xe=57:19:19:5) with transverse RF (at ISM frequency of 40.68 MHz) would produce approximately 20-24 Watts of peak laser power at the wavelength of 10.6 μm (assuming 10-12% electro-optical efficiency). The reduced electric field in the self-sustained plasma for the described above conditions is approximately estimated as 5 V.cm −1 .Torr −1  (Vitruk et. al., “Similarity and Scaling in Diffusion-Cooled RF-Excited Carbon Dioxide Lasers”,  IEEE J. Quantum Electronics,  QE-30, 1994, pp. 1623-1634). 
     The longitudinal DC discharge inside the 0.5×0.5 cm 2  bore (with the DC current flowing between the metal non-anodized surface areas of electrodes shown in FIG.  2 ), would require approximately 2 kV of inter-electrode DC voltage. The efficiency of the DC power deposition in plasma column could be approximately estimated as (2000-300)/2000=85% assuming the cathode fall voltage of 300 Volts. The high peak laser power performance could be achieved with the short enough RF-DC pulses (50 μsec to 0.5 msec) and low duty cycle (below 10-20%) so that laser gas could not heat up substantially neither during the single pulse nor during many consecutive short pulses. Approximately 1 A DC current pulse at 2 kV DC voltage needs to be applied during the RF pulse to the electrode  230  in FIG.  2 A. This will be sufficient to sustain a longitudinal DC discharges through the RF plasma with a total electrical power of 2200 Watts dissipated in the plasma, resulting in approximately 190-240 Watts of peak laser power radiated by the laser (assuming 10-12% electro-optical efficiency). 
     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.