Abstract:
A gas laser oscillator having at least three discharge tubes disposed along the optical axis, and a spacer having an opening centered on the optical axis. The spacer is disposed between a partially reflective mirror and the closest discharge tube. Further, the discharge tubes are disposed in series along the optical axis, and satisfy the following three formulas simultaneously: 
     
       
         
               
               
               
             
                   
                   
               
                   
                 Formula 1 
                 r1/r2 &gt; 1.0 
               
                   
                 Formula 2 
                 L2/(L1 + L2) &lt; 0.85 
               
                   
                 Formula 3 
                 r3/r2 &lt; 1.4 
               
                   
                   
               
           
              
             
             
              
              
              
              
             
          
         
       
     
     where the sum of lengths of a pair of discharge tubes disposed at both ends in optical axis direction is L 1 , the inside diameter of these discharge tubes is r 1 , the sum of lengths of the other discharge tubes in the optical axis direction is L 2 , the inside diameter of these discharge tubes is r 2 , and the inside diameter of the opening of the spacer is r 3.

Description:
This is a divisional application of Ser. No. 09/120,886, filed Jul. 23, 1998,noe Pat. No. 6,249,535. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a gas laser oscillator having an optical axis that is matched with the axial direction of the discharge tube, and more particularly to a gas laser oscillator capable of obtaining a laser beam of high quality. 
     FIG. 4 is a schematic block diagram of a conventional gas laser oscillator. In FIG. 4, reference numeral  1  is a discharge tube made of glass or other dielectric material, and the inside of the discharge tube  1  is filled with laser gas, or laser gas is circulated by a gas circulating apparatus not shown in the drawing. Reference numerals  2  and  3  are electrodes disposed at both ends of the discharge tube  1 , reference numeral  4  is a high voltage power source connected to the electrode  2  and electrode  3 , and reference numeral  5  is a discharge space inside the discharge tube  1  lying between the electrode  2  and electrode  3 . Reference numeral  6  is a fully reflective mirror disposed toward one opening of the discharge space  5 , and reference numeral  7  is a partially reflective mirror disposed toward the other opening of the discharge space  5 , and the fully reflective mirror  6  and partially reflective mirror  7  form an optical resonator. Reference numeral  8  is a laser beam emitted from the partially reflective mirror  7 . 
     In the conventional gas laser oscillator, the operation is described below. Discharge occurs in the discharge space  5  between the electrode  2  and electrode  3  connected to the high voltage power source  4 . By this discharge, the laser gas in the discharge space  5  is excited by the discharge energy. The excited laser gas is set in a state of resonance by the optical resonator formed by the fully reflective mirror  6  and partially reflective mirror  7 , and being optically amplified by this resonance, the laser beam  8  is issued from the partially reflective mirror  7 . This laser beam  8  is used in various applications of laser processing. 
     FIG.  5 ( a ) and FIG.  5 ( b ) are diagrams for explaining the operation of the optical resonator in the gas laser oscillator, showing more specifically the structure of the gas laser oscillator. In FIG. 4, only one discharge tube is shown, but generally, as shown in FIG.  5 ( a ) and FIG.  5 ( b ), plural discharge tubes  1  are disposed in series along the optical axis. Although mere cylindrical forms are expressed in FIG.  5 ( a ) and FIG.  5 ( b ), same as the discharge tube  1  in FIG. 4, an electrode  2  and an electrode  3  are disposed at both ends of each discharge tube  1 , and a high voltage power source  4  is connected between each pair of electrodes, that is, electrode  2  and electrode  3 , and a discharge space is formed inside of each discharge tube  1 . 
     In the gas laser oscillator shown in FIG.  5 ( a ) and FIG.  5 ( b ), when discharge occurs in the discharge space  5 , a standing wave  10  is formed. The property of this standing wave  10  is determined by the size of the resonance space  9  and the curvature of the fully reflective mirror  6  and partially reflective mirror  7 . This property of standing wave is known as TEM (transverse electromagnetic) mode order. Generally the lower the TEM mode order, the better is the laser beam converging, and it is known that higher processing performance is obtained. For example, the smaller the inside diameter of the discharge tube  1 , the narrower is the resonance space, and therefore oscillation of high-order TEM mode is suppressed, the TEM mode order becomes lower and light converging is enhanced, so that a laser beam of high processing performance is obtained. 
     On the other hand, in the gas laser oscillator having thus explained construction, of the electric energy supplied from the high voltage power source  4 , all energy excluding the portion converted into the laser beam  8  becomes heat. Therefore, to maintain the parallelism between the fully reflective mirror  6  and partially reflective mirror  7  by preventing deformation due to this generated heat, it is necessary to cool the fully reflective mirror  6  and partially reflective mirror  7  and the peripheral parts supporting them. 
     Concerning cooling of the fully reflective mirror  6  and partially reflective mirror  7  and their peripheral parts in the conventional gas laser oscillator, as disclosed in Japanese Laid-open Patent No. 56-90588, the construction being shown in FIG.  8 . As shown in FIG. 8, the fully reflective mirror  6  and partially reflective mirror  7  for resonance are respectively held by a flange  31  and a flange  32 . By coupling these flanges  31  and  32  through a support element  33 , the parallelism of the fully reflective mirror  6  and partially reflective mirror  7  necessary for laser oscillation is maintained. A passage  35  is provided inside the support element  33 , and it is intended to cool by passing oil or other cooling medium in this passage  35 . In the conventional gas laser oscillator, the passage  35  of the cooling medium inside the support element  33  was straight from the inlet to the outlet of the cooling medium. 
     The high voltage power source  4  is, as shown in FIG. 15, composed of a switching power source  44 , a step-up transformer  45 , and a rectifying and smoothing circuit  46 . Generally, the gas laser oscillator is composed of plural discharge tubes, and each discharge tube requires the step-up transformer  45  and rectifying and smoothing circuit  46 . In one switching power source  44 , the primary side of plural step-up transformers  45  can be connected, and therefore only one switching power source  44  is enough for plural discharge tubes. 
     The step-up transformer  45  is composed of a step-up transformer main body  49  and a transformer container  47  as shown in FIG. 16, and the transformer container  47  is filled with insulating oil  48 , and the step-up transformer main body  49  composed of coil and core is immersed in the insulating oil  48 . A top plate  50  is disposed in the upper part of the transformer container  47 , and an oil feed port  51  provided in the top plate  50  is sealed with an oil cap  52  except when feeding oil, so that the entire step-up transformer  44  is in a sealed structure. 
     The conventional gas laser oscillator thus constructed had several problems. 
     First, to lower the TEM mode order, in the discharge tube  1  shown in FIG.  5 ( a ), when the inside diameter of the discharge tube  1  is reduced as shown in FIG.  5 ( b ), scattered beam  8   a  is likely to occur in the resonance space  9 , and scattered beam  8   a  mixes into the laser output. FIG. 6 shows an output mode in a conventional gas laser oscillator. The axis of abscissas in FIG. 6 denotes the distance toward outside from the center of the output laser beam, and position  0  indicates the center. The axis of ordinates represents the energy density of the laser beam. FIG. 6 shows that scattered beam  8   a  is present in the peripheral region A of the laser beam  8 . Laser cutting by using such a laser beam causes an increase in the thermal effects around the cut section due to the scattered beam  8   a  included in the peripheral region, and lowers the cutting quality. As explained above, when attempting to improve the light converging and enhance the processing performance by lowering the TEM mode order, the scattered beam mixes into the output laser beam to increase the thermal effect range, which leads to a first problem of deterioration of processing quality. 
     As mentioned herein, in the gas laser oscillator, of the electric energy supplied from the high voltage power source  4 , all energy except for the portion converted into the laser beam becomes heat  36 . Such heat  36  was dissipated, conducting to the parts composing the gas laser oscillator, such as flanges  31  and  32  existing around the resonance space  9  or the support element  33  for coupling them, through the laser gas filling the resonance space  9  as shown in FIG.  9 . 
     The support element  33  is a member for maintaining the parallelism between the fully reflective mirror  6  and partially reflective mirror  7 , and when uniformity of temperature distribution in the support element  33  is lost due to the conducting heat  36 , the support element  33  is thermally deformed, and accurate parallelism between the fully reflective mirror  6  and partially reflective mirror  7  cannot be maintained. To avoid this inconvenience, the support element was cooled by passing a cooling medium in the support element  33 . However, in the conventional gas laser oscillator, the passage  35  for the cooling medium was straight from the inlet to the outlet of the cooling medium inside the support element  33 . Accordingly, heat convection occurs in the cooling medium inside the passage  35 , and temperature distribution of the cooling medium itself is not uniform. Due to heat convection of the cooling medium itself, the temperature is higher in the upper part and the temperature is lower in the lower part of the support element  33 , and thus the temperature distribution is uneven, and thermal distortion occurs. This thermal distortion leads to a second problem of making it difficult to maintain the accurate parallelism between the fully reflective mirror  6  and partially reflective mirror  7 . 
     In the step-up transformer  45  of the conventional high voltage power source, the step-up transformer main body  49  was contained in the transformer container  47 , and the transformer container  47  was in a sealed structure. Due to the heat generated in the step-up transformer main body  49 , the temperature of the insulating oil  48 , in which the transformer main body  49  is immersed, and the air  53  in the transformer container  47  are raised. When the transformer container  47  is closed by the top plate  50  and oil cap  52 , the internal atmospheric pressure in the transformer container is raised, and a pressure difference occurs between the inside and outside of the transformer container  47 . This pressure difference causes the insulating oil  48  to leak out of the transformer container  47 . 
     To eliminate the pressure difference between the inside and outside of the transformer container  47 , as shown in FIG. 17, a penetration hole was provided in the oil cap  52 . As a result, occurrence of a pressure difference between the inside and outside of the transformer container  47  could be prevented, but the insulating oil  48  splashed up and leaked during transportation. FIG.  18  and FIG. 19 are modified examples of the penetration hole provided in the oil cap  22 , but it was a third problem that leakage of the insulating oil  48  could not be prevented completely. 
     SUMMARY OF THE INVENTION 
     The invention is devised to solve the above-described problems, and it is a first object thereof to offer a gas laser oscillator capable of obtaining a laser beam of high quality by suppressing occurrence of scattered beam while lowering the output laser TEM mode order. 
     It is a second object of the invention to offer a gas laser oscillator capable of maintaining parallelism of fully reflective mirror and partially reflective mirror for composing an optical resonator, and obtaining a stable laser beam, by preventing thermal deformation of the support element and other members due to heat generated by laser oscillation. 
     It is a third object of the invention to offer a gas laser oscillator capable of preventing the occurrence of pressure difference between inside and outside of the transformer container, and also preventing the insulating oil in the transformer container from leaking out during transportation. 
     The gas laser oscillator, constructed in accordance with a first embodiment of the invention, comprises: 
     at least three discharge tubes disposed in series along the optical axis of laser beam for forming a discharge space inside, 
     a fully reflective mirror disposed toward one opening of the discharge space for composing a terminal mirror, 
     a partially reflective mirror disposed toward the other opening of the discharge space for composing an output mirror, and 
     a spacer disposed between the partially reflective mirror and the closest discharge tube, having an opening in the center of the optical axis of laser beam. 
     The discharge tubes are disposed in series along the optical axis, the sum of lengths of a pair of discharge tubes disposed at both ends in the optical axis direction supposed to be L 1 , the inside diameter of these discharge tubes supposed to be r 1 , the sum of lengths of the other discharge tubes in the optical axis direction supposed to be L 2 , the inside diameter of these discharge tubes supposed to be r 2 , and the inside diameter of the opening of the spacer supposed to be r 3  satisfy the following three formula simultaneously. 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Formula 1 
                 r1/r2 &gt; 1.0 
               
               
                   
                 Formula 2 
                 L2/(L1 + L2) &lt; 0.85 
               
               
                   
                 Formula 3 
                 r3/r2 &lt; 1.4 
               
               
                   
                   
               
             
          
         
       
     
     The gas laser oscillator of another embodiment of the invention comprises: 
     discharge tubes disposed along the optical axis of laser beam for forming a discharge space inside, 
     a fully reflective mirror disposed toward one opening of the discharge space for composing a terminal mirror, 
     a partially reflective mirror disposed toward other opening of the discharge space for composing an output mirror, 
     a first flange for holding the fully reflective mirror, 
     a second flange for holding the partially reflective mirror, and 
     a support element, being a member for keeping parallelism between the fully reflective mirror and the partially reflective mirror by coupling the first flange and the second flange, and having a spiral medium passage for passing cooling medium disposed inside thereof. 
     In the support element of the gas laser oscillator, plural spiral cooling medium passages are provided for passing cooling medium. 
     The gas laser oscillator also comprises: 
     discharge tubes disposed along the optical axis of laser beam for forming a discharge space inside, 
     a fully reflective mirror disposed toward one opening of the discharge space for composing a terminal mirror, 
     a partially reflective mirror disposed toward other opening of the discharge space for composing an output mirror, and 
     a high voltage power source including a switching power source for generating discharge inside the discharge tubes, a step-up transformer, and a rectifying and smoothing circuit, 
     in which the step-up transformer includes: 
     a step-up transformer main body, 
     a transformer container for storing insulating oil inside for immersing the step-up transformer main body in the inside insulating oil, and 
     an oil cap having a penetration hole and also including a filter having resistance to passing of insulating oil in the penetration hole, being fitted to the transformer container. 
     In the gas laser oscillator of the invention, the penetration hole provided in the oil cap of the transformer container penetrates the oil cap in the vertical direction, and is provided with a filter having resistance to passing of insulating oil at a lower portion in the penetration hole. 
     In the gas laser oscillator of the invention, the penetration hole provided in the oil cap of the transformer container has one end opened to the lower end of the oil cap, and other end opened to the outer circumference of the upper part of the oil cap, and is provided with a filter having resistance to passing of insulating oil at a lower portion in the penetration hole. 
     In the gas laser oscillator of the invention, the penetration hole provided in the oil cap of the transformer container has one end opened to the upper end of the oil cap, and other end opened to the outer circumference of the lower part of the oil cap, and is provided with a filter having resistance to passing of insulating oil at a lower portion in the penetration hole. 
     In the gas laser oscillator of the invention, the pore size of the filter of the oil cap disposed in the transformer container is 0.55 mm or less. 
     According to the first embodiment of the gas laser oscillator, of the series of discharge tubes arranged in series, the resonance space formed in the other discharge tubes other than a pair of discharge tubes disposed at both ends is relatively narrowed, the TEM mode order of the laser beam is lowered. Besides, since scattered beam caused in the discharge tubes disposed at ends is intercepted by the spacer and is not delivered outside, so that mixing of scattered beam into the laser beam is prevented. The TEM mode order of the laser beam is lowered, light converging is improved, and mixing of scattered beams into the laser beam is prevented, so that an excellent laser beam high in processing performance and small in thermal effects in the processing peripheral area is obtained. 
     According to the gas laser oscillator of the second embodiment, by forming the passage for cooling medium inside the supporting element spirally, the temperature distribution of the support element is uniform, and thermal distortion of the support element can be eliminated. Since the support element is formed by coupling the flange for holding the fully reflective mirror and the flange for holding the partially reflective mirror, as the thermal distortion of the support element is eliminated, it is easier to maintain the parallelism between the fully reflective mirror and partially reflective mirror, so that a stable laser beam can be obtained. 
     According to the gas laser oscillator of the second embodiment, by forming a penetration hole in the oil cap of the transformer container, and disposing a filter for resisting passing of insulating oil in this penetration hole, if the insulating oil splashes due to vibration during transportation, the insulating oil will never leak out of the transformer container. Besides, since the oil film formed in the filter provided in the penetration hole is easily broken by the pressure difference between inside and outside of the transformer container, the internal pressure of the transformer container is nearly kept constant, and insulating oil will not leak out due to the pressure difference between inside and outside. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of a gas laser oscillator constructed in accordance with a first embodiment of the invention. 
     FIGS.  2 ( a )-( c ) are diagrams showing the effects of a discharge tube and spacer shape on convergence of a laser beam in the gas laser oscillator in the first embodiment of the invention. 
     FIGS.  3 ( a )-( b ) are diagrams showing effects by the gas laser oscillator in the first embodiment of the invention. 
     FIG. 4 is a sectional view showing a schematic representation of a conventional gas laser oscillator. 
     FIGS.  5 ( a )-( b ) are structural diagrams explaining the operation of an optical resonator in the conventional gas laser oscillator. 
     FIG. 6 is a diagram showing the output mode of laser beam in the conventional gas laser oscillator. 
     FIGS.  7 ( a )-( b ) are sectional views showing a construction of a gas laser oscillator constructed in accordance with a second embodiment of the invention. 
     FIG. 8 is a structural diagram explaining the cooling medium passage of the support element in the conventional gas laser oscillator. 
     FIG. 9 is a partial diagram showing conduction and radiation of heat in a gas laser oscillator. 
     FIG. 10 is a sectional view showing a structure of a step-up transformer of a gas laser oscillator in a third embodiment of the invention. 
     FIG. 11 is a sectional view showing a first embodiment of an oil cap used in a step-up transformer of a gas laser oscillator of the invention. 
     FIG. 12 is a sectional view showing a second embodiment of an oil cap used in a step-up transformer of a gas laser oscillator of the invention. 
     FIG. 13 is a sectional view showing a third embodiment of an oil cap used in a step-up transformer of a gas laser oscillator of the invention. 
     FIG. 14 is a diagram showing the relation of pore size of the filter of the invention and the leak amount of insulating oil during transportation. 
     FIG. 15 is a block diagram showing a high voltage power source used in a gas laser oscillator. 
     FIG. 16 is a sectional view showing a structure of a step-up transformer of a conventional gas laser oscillator. 
     FIG. 17 is a sectional view of a conventional oil cap having a straight penetration hole. 
     FIG. 18 is a sectional view of a conventional oil cap having an inverted L-shaped penetration hole. 
     FIG. 19 is a sectional view of a conventional oil cap having a lateral U-shaped penetration hole. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a construction of a first embodiment of a gas laser oscillator of the present invention. Same members as the members in FIG.  4  and FIG. 5 are identified with same reference numerals also in FIG.  1  and their explanations are omitted. FIG. 1 is a sectional view of a gas laser oscillator, and in FIG. 1, reference numeral  21  is a spacer having an opening having a specified inside diameter with the center on the optical axis of a laser beam  8 , and reference numerals  22  and  23  are a plurality of (six in FIG. 1) discharge tubes disposed in series along the optical axis of the laser beam, and a pair of discharge tubes  22  out of these discharge tubes are disposed at both ends of the optical axis, that is, at both ends closest to a fully reflective mirror  6  and a partially reflective mirror  7 , while the other discharge tubes  23  are disposed between the pair of discharge tubes  22 . 
     In FIG. 1, the discharge tubes  22  and discharge tubes  23  are shown as mere tubular bodies, but in the individual discharge tubes  22  and discharge tubes  23 , same as the discharge tubes  1  shown in FIG. 4, a pair of electrodes, equivalent to electrode  2  and electrode  3 , are disposed at both ends of each tubular body. A high voltage power source is connected between each pair of electrodes, and a discharge space is formed individually in the discharge tubes  22  and discharge tubes  23 . However, such arrangement is not directly related to the nature of the invention, in FIG. 1, the discharge tubes  22  and discharge tubes  23  are expressed as mere tubular bodies. 
     Herein, supposing the inside diameter of the pair of discharge tubes  22  disposed at both ends of the optical axis to be r 1 , 
     the sum of the lengths of the pair of discharge tubes to be L 1  (=11+12), 
     the inside diameter of the other discharge tubes  23  except for the pair of discharge tubes  22  to be r 2 , 
     the sum of the length of the other discharge tubes  23  except for the pair of discharge tubes  22  to be L 2  (=13+14+15+16), and 
     the inside diameter of the opening of the spacer to be r 3 , 
     the gas laser oscillator of the first embodiment is composed so as to satisfy the following three formulas simultaneously. 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Formula 1 
                 r1/r2 &gt; 1.0 
               
               
                   
                 Formula 2 
                 L2/(L1 + L2) &lt; 0.85 
               
               
                   
                 Formula 3 
                 r3/r2 &lt; 1.4 
               
               
                   
                   
               
             
          
         
       
     
     Thus, as long as the shapes of the discharge tubes  22 , discharge tube  23  and spacer  21  simultaneously satisfy the three formulas (formula 1, formula 2, and formula 3), the inside diameter r 2  of the discharge tubes  23  is smaller than the inside diameter r 1  of the discharge tubes  22 . As a result, the internal portion of the discharge tubes  23  in the resonance space  9  is relatively narrower than the internal portion of the discharge tubes  22 , and the TEM mode order of the laser beam  8  is lowered. On the other hand, the inside diameter r 1  of the discharge tubes  22  disposed at both ends in the optical axis direction is larger than the inside diameter of the discharge tubes  23  disposed in the middle, and therefore when the scattered beam  8   a  generated in the resonance space  9  inside the discharge tubes  23  passes through the inside of the discharge tubes  22 , it diverges toward the outside of the resonance space due to diffraction. This scattered beam  8   a  diverging toward the outside is intercepted by the spacer  21 , and is not delivered outside, thereby preventing the scattered beam  8   a  from mixing into the output laser beam  8 . 
     FIGS. 2 ( a ), ( b ) and ( c ) are diagrams showing the effects of the inside diameter and length of the discharge tubes  22  and discharge tubes  23  and inside diameter of the spacer  21  on the converging performance of laser beam. In these diagrams, as the parameters for evaluating the converging performance of laser beam, the width of the heat affected zone by cutting a mild steel plate by laser beam was used. The width of the heat affected zone was measured on the basis of the cut-off line. 
     FIG.  2 ( a ) shows the relation between r 1 /r 2  change and heat affected width, supposing L 2 /(L 1 +L 2 ) to be 0.5 and r 3 /r 2  to be 1. As clear from this diagram, when the value of r 1 /r 2  is larger than 1, that is, when the inside diameter r 1  of the discharge tubes  22  disposed at both ends in the optical axis direction is larger than the inside diameter r 2  of the discharge tubes  23  disposed in the middle, the occurrence of scattered beams  8   a  is suppressed, and the heat affected width is smaller. 
     FIG.  2 ( b ) shows the relation between L 2 /(L 1 +L 2 ) change and heat affected width, supposing r 1 /r 2  to be 1.11 and r 3 /r 2  to be 1. As clear from this diagram, when the value of L 2 /(L 1 +L 2 ) exceeds 0.85, the heat affected width increases suddenly, and hence the value of L 2 /(L 1 +L 2 ) should be less than 0.85. 
     FIG.  2 ( c ) shows the relation between r 3 /r 2  change and heat affected width, supposing r 1 /r 2  to be 1.11 and L 2 /(L 1 +L 2 ) to be 0.5. As clear from this diagram, when the value of r 3 /r 2  exceeds 1.4, the heat affected width increases suddenly. Hence, the value of r 3 /r 2  must be set at less than 1.4. It suggests that the intercepting effect of scattered beam  8   a  by the spacer  21  is reduced when the inside diameter r 3  of the opening of the spacer  21  is too large compared with the inside diameter r 2  of the discharge tubes  23 . In other words, the spacer  21  having an opening of an appropriate inside diameter is effective for intercepting the scattered beam  8   a.    
     FIG. 3 is a diagram that explains the effect of the first embodiment of the gas laser oscillator of the invention. FIG. 3 ( a ) shows the output mode of the output laser beam  8  of the gas laser oscillator according to the first embodiment of the invention. As is clear from a comparison between FIG.  3 ( a ) and FIG. 6, the laser beam issued from the gas laser oscillator of the invention is free from scattered beam in the peripheral region A, and a laser beam of high quality is obtained. 
     FIG.  3 ( b ) compares with heat affected width by cutting of mild metal plate, between the conventional gas laser oscillator and gas laser oscillator of the first embodiment of the invention. The embodiment of the invention and the prior art is compared by representing the heat affected width on the axis of ordinates. As is clear from FIG.  3 ( b ), in the gas laser oscillator of the invention, the heat affected width can be notably decreased as compared with the conventional gas laser oscillator. 
     As explained herein, according to the gas laser oscillator of the first embodiment of the invention, the TEM mode order of output laser beam can be lowered, and mixing of scattered beam into the output laser beam can be prevented, so that an output laser beam of high converging performance and high quality can be obtained. Therefore, by using this gas laser oscillator, a high processing performance is obtained, and laser processing of high quality is realized. 
     FIG.  7 ( a ) shows a structural example of a gas laser oscillator constructed in accordance with a second embodiment of the invention. The basic components are the same as in the conventional gas laser oscillator explained in FIG. 4, but the construction is newly described below including the basic components. In FIG.  7 ( a ), reference numeral  1  is a discharge tube,  2  and  3  are electrodes,  4  is a high voltage power source for supplying electric power for discharging between the electrodes  2  and  3 ,  6  is a fully reflective mirror, and  7  is a partially reflective mirror, and the fully reflective mirror  6  and partially reflective mirror  7  are combined to form an optical resonator. Reference numeral  8  is a laser beam issued through the partially reflective mirror  7 , reference numeral  9  is a resonance space, reference numeral  31  is a flange for holding the fully reflective mirror  6 , reference numeral  32  is a flange for holding the partially reflective mirror  7 , reference numeral  33  is a support element coupling the flange  31  and flange  32 , and reference numeral  34  is a passage for passing cooling medium disposed inside the support element  33 . 
     In this embodiment, the passage  34  of cooling medium is formed spirally inside the support element  33 . In such passage  34 , the coo ling medium flows spirally inside the support element  33 , and thereby the support element  33  is cooled uniformly without causing a temperature difference between upper part and lower part of the support element  33  due to convection of cooling medium. Therefore, thermal distortion, conventionally induced by temperature difference in the parts of the support element  33 , does not occur in the present invention. As a result, it is easy to maintain the parallelism between the fully reflective mirror  6  and partially reflective mirror  7  held by the flange  32  and flange  33  coupled by the support element  33 . 
     FIG.  7 ( b ) shows a modified example of the gas laser oscillator in the second embodiment of the invention. In the structural example shown in FIG.  7 ( a ), the passage  34  for cooling medium provided in the support element  33  was one system of spiral passage, but plural systems may be formed as in a passage  35  shown in FIG.  7 ( b ). For example, a passage  35   a  is provided inside the support element  33 , a second passage  35   b  and a third passage  35   c  are disposed on the parts close to the surface of the support element  33 . By forming plural systems of the passage  35  for the cooling medium, the cooling effect is enhanced and more uniform cooling is realized. In FIG.  7 ( b ), the inlets and outlets of the plural systems of passages  35   a ,  35   b ,  35   c  are gathered at one position each, but the inlets and outlets of plural systems of passages for cooling medium may be also located independently in each passage. Therefore, the inlets and outlets of plural systems of passages for cooling medium are also located independently in each passage, and the cooling medium flowing direction may be reverse in each passage, and the temperature distribution of the support element  33  may be more uniform. 
     Alternatively, the support element  33  may be divided into a plurality of sections, and at least one system of spirally formed passage for cooling medium may be provided for each divided section of the support element. 
     As explained herein, according to the gas laser oscillator of the second embodiment of the invention, since the support element can be cooled uniformly, the temperature distribution is uniform, the support element is free from thermal distortion, and it is easy to maintain the parallelism between the fully reflective mirror and partially reflective mirror for composing the optical resonator, so that a stable laser beam may be obtained. 
     FIG. 10 shows a step-up transformer  45  used in a gas laser oscillator in a third embodiment of the invention. In FIG. 10, a transformer container  47  contains an insulating oil  48 , and a step-up transformer main body  49  is fixed in the transformer container  47  so as to be completely immersed in the insulating oil  48 . The transformer container  47  has a top plate  50  so as to keep airtight at the junction, and an oil cap  52  having a vertical penetration hole  54  is fitted to the top plate  50  so as to keep airtight at the junction. A filter  55  having resistance to passing of the insulating oil  48  is provided in the vertical penetration hole  54  provided in the oil cap  52 . The filter  55  is made of foamed urethane or a similar material that has resistance to oil. 
     Thus, in this step-up transformer  45 , the insulating oil  48  splashing up due to vibration during transportation sticks to the top plate  50 , or partly invades into the penetration hole  54  in the oil cap  52  to be absorbed on the filter  55 . On the pores of the filter  55  absorbing the insulating oil  48 , an oil film is formed due to surface tension of the insulating oil  48  itself. By this oil film, passing of insulating oil  48  is blocked, and leakage of insulating oil from the transformer container  47  is prevented. 
     However, if the pore size is too large, the formation of an oil film of insulating oil  48  on the filter  55  is impaired, and the effect of the arresting passing of insulating oil  48  by the oil film is lost. FIG. 14 is a diagram showing the relation between the pore size of filter material and the leakage of insulating oil during transportation. According to FIG. 14, when the pore size of the filter material exceeds 0.55 mm, the insulating oil leaks during transportation, and it is known that, as the material for the filter  55 , an oil resistant and foaming material with pore size of 0.55 mm or less must be selected. 
     On the other hand, due to heat generation of the step-up transformer main body  49 , the atmospheric pressure in the transformer container  47  is raised, and an atmospheric pressure difference occurs between the inside and outside of the transformer container  47 . Due to this atmospheric pressure difference between the inside and outside of the transformer container  47 , the oil film formed in the pores of the filter  55  is broken, and only the air inside the transformer container  47  is discharged outside, so that the atmospheric pressure in the transformer container is kept almost constant. 
     The filter  55  is provided only in a lower portion of the penetration hole  54  of the oil cap  52 , and a space is left in the upper part of the penetration hole  54 . This space is provided so that oil drops do not pop out of the penetration hole if the oil film is broken and oil drops splash upward. Part of the insulating oil  48 , which splashes out as the oil film is torn by the atmospheric pressure difference, sticks to the inner wall of the penetration hole  54 , but moves downward due to gravity, and is absorbed on the filter  55 , and again forms an oil film. 
     It is effective whether the shape of the penetration hole  54  is an inverted L-form as shown in FIG. 18 relating to the prior art or a lateral U-form as shown in FIG. 19, and still more FIG.  12  and FIG. 13 show the shapes of the penetration hole in consideration of the effect of recovering the insulating oil  48  sticking to the inner wall of the penetration hole  54  to the filter  55 . 
     The penetration hole shown in FIG. 12 is composed of a vertical portion  54   a  and a sloped portion  54   b . The vertical portion  54   a  is opened only beneath the oil cap  52 , and does not penetrate upward. The sloped portion  54   b  penetrates obliquely upward starting from the upper end of the vertical portion  54   a , and opens to the outside of the transformer container. The filter  55  is inserted only in the vertical portion  54   a.    
     When the penetration hole is thus formed, since the filter  55  is inserted only in the vertical portion  54   a , it is advantageous that the space from the filter  55  to the outside is kept wide. If the oil film of the filter  55  is broken, oil drops only splash around the filter  55 , not reaching the outside of the transformer container  47 . Part of the oil drops sticking to the inner wall of the sloped portion  54   b  moves downward by gravity, and is absorbed in the filter  55  in the vertical portion  54   a.    
     The penetration hole shown in FIG. 13 is formed of a vertical portion  54   a  and a sloped portion  54   b . The vertical portion  54   a  is opened only above the oil cap  52 , and does not penetrate downward. The sloped portion  54   b  penetrates obliquely downward starting from the lower end of the vertical portion  54   a , and opens to the inside of the transformer container. The filter  55  is inserted only into the sloped portion  54   b.    
     When the penetration hole is thus formed, since the filter  55  is inserted only in the sloped portion  54   b , it is advantageous that the space from the filter  55  to the outside is kept wide. If the oil film of the filter  55  is broken, oil drops only splash around the filter  55 , not reaching to outside of the transformer container  47 . Part of oil drops sticking to the inner wall of the vertical portion  54   a  moves downward by gravity, and is absorbed in the filter  55  in the sloped portion  54   b.    
     In the gas laser oscillator in the third embodiment of the invention, as described herein, by installing an oil resistant filter of an appropriate pore size, having resistance to passing of insulating oil, in the penetration hole of the oil cap, it is effective to prevent insulating oil from leaking out of the oil cap due to vibration during transportation. Besides, since the oil film formed in the filter is broken by a slight atmospheric pressure difference between the inside and outside of the transformer container, the atmospheric pressure in the transformer container is almost kept constant, and therefore leakage of insulating oil due to the pressure difference between the inside and outside the transformer container does not occur.