Abstract:
A gas laser device. The device includes a laser chamber having at least one active gas and a device for purifying gas. The purified gas is in communication with the free exchange of gas with the chamber. This device may be used with high powered gas lasers.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an active gas laser device comprising integrated means of purifying the active gas. 
     2. Discussion of the Background 
     A distinction is made between several categories of gas lasers, depending on their sensitivity to operating conditions and as a function of the rate at which their active medium (gas) degrades. 
     A first category includes sealed gas lasers for which operation and operating conditions do not change the characteristics of the active gas or gases, or only change them very little. 
     The performances of this type of laser are very stable in time and no action on the active medium is necessary. 
     For example, this first category includes HeNe lasers, C 0   2  lasers with wave guides, and low power argon or krypton lasers. 
     A second category includes sealed gas lasers for which operation and operating conditions slowly change the characteristics of the active gas or gases. These lasers require very little technical action on the active medium. 
     For example, the second category includes argon or krypton lasers with an output power of the order of one Watt. A service operation is necessary on these lasers after about 1000 or 2000 hours of operation. Usually, the sealed head of these lasers is replaced. 
     Finally, there is a third category of lasers for which operation and operating conditions quickly change the active gas mixture. Therefore, the amplifying medium of these lasers must be regenerated frequently. 
     The third category includes high power CO 2  lasers (of the order of a kilowatt) and excimer lasers. 
     Excimer lasers require servicing about once every week, and the performances of these lasers degrade continuously between two service operations. 
     For lasers in the third category, it is possible to work without recovering the gas, or to renew all or as part of the gas mixture, whichever is preferred. Some active media may also be purified and regenerated. 
     Therefore, the device according to the present invention is more particularly applicable to lasers in which the active medium may be regenerated, particularly by purification of gas contained in the laser chamber. Excimer lasers form part of the above described third category and will be explained in more detail below. 
     An excimer laser is a gas laser capable of emitting light within the ultra-violet spectral range, in a pulsed mode. 
     A cavity in an excimer laser contains a gas mixture that forms an amplifying medium and which produces the laser effect. This mixture comprises essentially an halogen compound (fluorine/chlorine) in molecular form, a rare gas such as krypton, xenon or argon, and a buffer gas, for example such as neon or helium. 
     The energy efficiency of the laser and the quality of the light beam emitted depend on a number of parameters and operating conditions, including the partial pressures of the various gases in the gas mixture and excitation of the amplifying medium (gas mixture). 
     Pre-ionization, for example by X-ray or ultra-violet radiation, or by a corona effect preliminary discharge, contributes to controlling the electrical pumping discharge, in other words controlling excitation of the amplifying medium. 
     The purity of gases in the amplifying medium is another parameter that is important for obtaining good energy efficiency and a high quality beam. 
     The purity of the gases in the amplifying medium contained in a chamber may be affected by chemical reactions. For example, these reactions take place between the excited halogen compounds and the walls of the chamber. 
     Furthermore, it is found that the laser cavity sealing elements may also form sources of pollution of the amplifying medium. 
     Finally, the quality of the gases in the amplifying medium may be degraded by parasite chemical combinations. New molecules formed by chemical combination of the various molecules of the mixture can absorb radiation and therefore reduce the performances of the laser. 
     In order to prevent a drop in the quality of the laser beam (stability in amplitude, beam uniformity, energy) and in the efficiency of the laser cavity, it is possible to either renew gases in the active medium, particularly by adding new halogen compounds, or to eliminate undesirable gases in the gas mixture. 
     In particular, a number of undesirable halogen molecules appearing in the gas mixture may be trapped by a cryogenic effect outside the laser. For example, these molecules include CC 1   4, CF   4, HF.    
     Elimination of undesirable molecules requires precise control of the temperature of a cryogenic trap. This temperature varies as a function of the compounds to be eliminated and therefore as a function of the molecules initially present in the active medium mixture. 
     Table I below contains examples of pure gases that may be added to the mixture to regenerate it and the temperature of a cryostat capable of trapping undesirable molecules, for a number of active molecules. 
     
       
         
               
               
               
               
               
             
           
               
                                                TABLE I 
               
               
                   
               
               
                      Active molecules 
                 XeCl 
                 XeF 
                 ArF 
                 KrF 
               
               
                   
               
             
             
               
                       Pure gases added to 
                   
                   
                   
                   
               
               
                 the active mixture 
               
               
                 Ar 
                   
                   
                 * 
               
               
                 Kr 
                   
                   
                   
                 * 
               
               
                 Xe 
                 * 
                 * 
               
               
                 F 2   
                   
                 * 
                 * 
                 * 
               
               
                 HCl 
                 * 
               
               
                 Ne 
                 * 
                 * 
                 * 
                 * 
               
               
                 Cryostat temperature 
                 130° K 
                 130° K 
                 90° K 
                 100° K 
               
               
                   
               
             
          
         
       
     
     FIG. 1 illustrates a particular example embodiment of a known type of gas purifier associated with a gas laser. 
     Reference  10  in the figure denotes a laser chamber containing an active amplifying medium in the form of a gas mixture. Reference  12  denotes a cryogenic trap purifier connected to chamber  10  in order to regenerate the gas mixture. 
     An outlet  14  from the chamber  10  is connected to the purifier  12  through a pipe  16 . Gas taken from chamber  10  passes through the purifier  12  and, after being purified, in other words separated from its undesirable constituents, is reinjected into chamber  10  through a pipe  18 . Pipe  18  connects the purifier  12  to a gas inlet  20  to chamber  10 . 
     The purifier  12  includes a dust filter  22 , a circulation pump  24  and a cryostat  26 , in order starting from its inlet. These elements are connected to a tube  27 . Furthermore, isolating valves  28 ,  30 , are provided at the purifier inlet and outlet irrespectively. 
     The circulation pump  24  circulates the gas mixture from the chamber  10  through a heat exchanger  32  in the cryostat in which the gas is cooled. The gas is cooled by means of a heat conducting core  34 , the first end of which is immersed in a bowl of liquid nitrogen  36 . A second end of the core  34  is equipped with an electrical heating resistance  35  that precisely adjusts a heat balance in the core and therefore the temperature of gases passing through the heat exchanger  32 . 
     The cryostat traps impurities in the gas mixture by condensation and partial crystallization of these impurities, in the bottom part of tube  27 . 
     A reverse flow heat exchanger  38  is also provided between the gas outlet from purifier  12  and the gas inlet. This exchanger pre-cools the gas to be purified by retrieving at least part of the enthalpy from the purified cold gas. 
     Finally, an extraction valve  40  and a vacuum pump  42  are provided to empty the purifier  12 . 
     Periodically, isolating valves  28 ,  30  of the purifier  12  are closed to isolate the purifier  12  from the laser chamber  10 . The part of the tube between the valves is then heated to enable evaporation and extraction of accumulated residues of impurities, using the vacuum pump. 
     A second inlet  21  into the laser chamber  10  is connected to gas cylinders  44 ,  46  and  48  containing the rare gas, the gas containing halogen compounds and the buffer gas respectively. These gas cylinders are used to inject the new gas mixture into the chamber  10 . 
     A device of this type is known and is commercially available. See, an article entitled “The GP-2000X Series Excimer Laser Gas Purifiers” by Oxford Lasers (1990), an article entitle “Cryogenic Processing of Excimer Laser Gas Mixtures” by Oxford Lasers (1991), and European Patent Publication No. EP-A-430 411). 
     Note that installations of the type described above are not included in lasers, but are available as auxiliary equipment. 
     A number of obstacles make it impossible to include cryogenic purification equipment directly in the laser device. 
     The first obstacle is related to vibrations emitted by vacuum and circulation pumps. These vibrations could disturb the stability of the laser beam. 
     Furthermore, a complete system including a laser and purification equipment would be particularly cumbersome and difficult to transport. 
     Finally, the global cost of a complete system of this type would be very high, particularly due to the complexity of the purification device. 
     Furthermore, purification devices conforming with FIG. 1 include a number of difficulties related to circulation of gases from the laser chamber. Pipes from the purifier must be equipped with mechanical and electromechanical means for controlling gas pressures and flows. 
     Furthermore, pumps used to circulate the gases must be specially designed to resist corrosive gases in the active mixture and must be capable of operating in a pressure range varying from a vacuum up to about 10 bars. 
     These constraints also contribute to the increasing cost of purifiers. Germen Patent Publication No. DE-A-3 130 588 describes another type of laser gas purifier in which the impurities are liquefied by reducing the pressure of the gas mixture. The pressure is reduced in several steps. The gas passage in the purifier during the pressure reduction steps is caused by an alternating set of pumps and valves. 
     Thus, difficulties similar to the difficulties mentioned above also occur with the device described in German Patent Publication No. DE-A-3 130 588. 
     SUMMARY OF THE INVENTION 
     One purpose of this invention is to propose a gas laser device including means of purifying the gas(es) in the laser active mixture, without the difficulties mentioned above. 
     One particular purpose is to propose a device that is complete, independent and compact. 
     Another purpose is to propose a device in which the parasite vibrations and noise due to gas pumps are eliminated. 
     Another purpose is to avoid cumbersome tubes and pipes to connect the laser gas chamber to the purification means. 
     Another purpose is to propose a device that significantly increases the time interval between maintenance operations. 
     Finally, another purpose is to propose a simple, reliable and inexpensive device. 
     To achieve these purposes, the specific purpose of the present invention is a gas laser device including: 
     a laser chamber containing at least one active gas, and 
     gas purification means, in communication with free exchange of gas with the chamber. 
     Communication with free exchange of gas means communication in which no gas circulation is imposed. 
     More precisely, in the case of the invention, the gas exchange takes place naturally due to physical phenomena such as gravity, diffusion and Brownian displacement of particles without exerting external forces on the gases. 
     According to particular aspects of the present invention, the gas purification means may comprise a cryostat and/or a catalytic trap capable of retaining impurities and undesirable components in the gas or the laser gas mixture. 
     Furthermore, according to one particular embodiment of the device according to the present invention, the laser chamber may have one or several appendices (removable or not) connected on the chamber through isolating valves. These appendices each contains one or several gas purification means. 
     The device according to the present invention may also includes one or several built-in active gas sources connected to the laser chamber. Preferably, the source is connected in free exchange with the chamber. 
     The present invention also relates to a process for purification of a gas laser chamber in which communication with free exchange of gas takes place between the laser chamber and at least one gas purification means. 
     Other characteristics and advantages of the present invention will become clearer from the following description with reference to the figures in the attached drawings. 
     This description is given for illustrative purposes only and is in no way restrictive. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1, already described above, is a simplified diagrammatic view of a gas laser equipped with a known type of gas purifier. 
     FIG. 2 diagrammatically shows a laser device corresponding to a particular embodiment of the invention. 
     FIGS. 3 and 4 are partial views at larger scale showing appendices to the laser chamber for the device in FIG. 2, these appendices containing cryogenic gas purification means. 
     FIG. 5 is a partial view at larger scale showing an appendix to the laser chamber of the device in FIG. 2, the appendix comprising a catalytic trap. 
     FIGS. 6 and 7 are partial views at a larger scale showing the manufacture of a gas inlet appendix for the device in FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2 is a an overall view of a laser device according to the present invention. 
     The laser device comprises a laser chamber  100  containing a gas mixture that forms the active amplifying medium. The nature of the mixture depends on the type of laser; further information about this subject can be obtained from the Discussion of Background section above. 
     The laser chamber  100  is formed from a main body  102  to which a first, second and third appendix  104 ,  106  and  108  are connected. Each appendix is connected, removably or not, to the main body by means of a fitting  104   a ,  106   a ,  108   a  and an isolating valve  104   b ,  106   b ,  108   b.    
     On the example embodiment shown in FIG. 2, the first and second appendices  104  and  106  contain a cryogenic trap and a thermal catalytic trap respectively. 
     These traps, for which a more detailed description is given in the rest of the text, form gas purification means. 
     The third appendix  108  forms a gas source used to add new gas to laser chamber  100 . 
     An emptying system  110  comprises a vacuum pump  112  and is connected to the first and second appendices and to the main body  102  of the chamber. This system is designed to purge the laser device during a maintenance operation. 
     It can be seen that the emptying system  110  comprises pipes connected to the first and second appendices through extraction valves  104   c  and  106   c  and is connected to the body  102  of the chamber through an extraction valve  111 . 
     The extraction valves  104   c ,  106   c  and  111  are closed and the vacuum pump  112  is stopped in normal operation. 
     FIG. 3 diagrammatically shows a particular embodiment of the first appendix  104  with a cryogenic trap. 
     In FIG. 3, references  104   a ,  104   b ,  104   c  correspond to the fitting connecting appendix  104  to body  102  of the laser chamber, the isolating valve and appendix extraction valve, respectively. 
     The cryogenic appendix  104  includes comprises a chamber  120  delimited by a wall  122  made of a thermally insulating material in which a first end of a pin  124  is immersed, made of a heat conducting material such as copper or aluminum. 
     The cold chamber  120  and pin  124  form a cryogenic trap. 
     Another end of pin  124  is inserted into a bath of liquid nitrogen  126  which forms a cold source. The liquid nitrogen is contained in a reservoir  128 , also made of a thermally insulating material. The reservoir  128  is preferably closed to prevent condensation of moisture. However, it is fitted with a valve S to limit the pressure in the chamber and an orifice P through which the reservoir is filled with cryogenic fluid. 
     An electric heating resistance  130  in thermal contact with the core  124  carries a current controlled by an external electronic device  132 . The electric heating resistance  130  precisely adjusts the temperature of the pin  124  as a function of the components of the gas mixture to be trapped. 
     When the isolating valve  104   b  is open, the gas contained in the main body  102  of the laser chamber can diffuse into chamber  120  of the cryogenic appendix by free displacement of gas molecules. The entire body  102  of the chamber and the appendix are then at the same pressure. 
     In chamber  120 , when they come into contact with the cold pin  124 , halides and other impurities in the gas mixture are liquefied and/or crystallized. 
     Considering the thermal gradient between the body of the laser chamber and the cold chamber  120 , in other words the bottom of the cryogenic trap, the halides remain close to the cold point formed by the pin  124  at the bottom of the trap. 
     The impurities are kept at the bottom of the trap (chamber  120 ) by the effect of gravity and/or density, depending on their physical state. The trap is preferably turned downwards to prevent any convection that could produce an excessive thermal load. 
     Furthermore, the cold pin  124  in the cryogenic trap generates a thermal stratification effect which also results in a significant energy saving, because the only heat losses to be considered are due to conduction since there is no convection. 
     The cryogenic appendix  104  is periodically cleaned, in order to extract halides and undesirable impurities that have accumulated in it. 
     For cleaning, isolating valve  104 b is closed to isolate the chamber  120  in appendix  104  from the body  102  of the laser chamber, and extraction valve  104   c  is open. The halides and other impurities are evaporated, under the effect of natural heating by eliminating the cold source (nitrogen), and/or provoked heating for example by means of the heating resistance  130 . 
     The use of vacuum pump  112  shown in FIG. 2 then evacuates these halides and other undesirable impurities through the extraction valve  104   c.    
     FIG. 4 shows a variant embodiment of the first cryogenic trap appendix  104 . 
     Parts in FIG. 4 identical or similar to parts in FIG. 3 have the same references. Further details about these parts can be obtained by reference to the previous description. 
     In the appendix  104  in FIG. 4, one end of a heat conducting pin  124  extends into chamber  120  which may be put into communication with the laser chamber by opening the isolating valve  104   a.    
     Another end of pin  124  is in contact with a cryorefrigerator  150  and with an electrical resistance  130 . 
     For example, the cryorefrigerator  150  may be of the Gifford Mac-Mahon type, Stirling type, Joule Thomson type with pressure reduction, or tube type with pulsed gas. 
     The cold pin  124  in contact with the cryorefrigerator  150  and the electrical heating resistance  130  is kept in thermal equilibrium at a temperature that can be adjusted by varying the electrical current passing through the heating resistance. As for the appendix in FIG. 3, the temperature of the cold pin is adjusted as a function of the components of the gas mixture to be trapped. 
     FIG. 4 shows that the chamber  120  is delimited by first wall  122  made of a thermal insulating material. 
     Similarly, the entire appendix  104  is delimited by second wall  105 , also made of a thermal insulating material. 
     Furthermore, thermal insulating foam or a vacuum or any other insulating agent may be added between the first and second walls  122 ,  105 . 
     As in the device in FIG. 3, halides and impurities are trapped in chamber  120 . They are eliminated as in the previous example by natural and/or provoked heating (using the heating resistance  130 ) and using valve  104   c.    
     The device in FIG. 4 with a cryorefrigerator has an advantage over the device in FIG. 3, in that it can be sealed and can operate independently without external intervention. There is no need to supply it regularly with cryogenic liquid (nitrogen). 
     FIG. 5, described below, diagrammatically shows an embodiment of the second appendix  106  of the laser device. The second appendix  106  forms a catalytic thermal trap. 
     In the same way as halogens and/or undesirable impurities can be retained in a cryogenic trap as described above, they may also be dissociated and filtered by a thermal and catalytic effect. 
     The second appendix  106  comprises a chamber  160  containing a catalyst material such as platinum or titanium. For example, the chamber is delimited by a wall  162  made of one of these materials, or lined on the inside with one of these materials. 
     An electrical heating resistance  164 , shown very diagrammatically, is provided to heat the catalyst. 
     The second appendix  106  is connected to the body  102  of the laser chamber through a fitting  106   a  and through an isolating valve  106   b.    
     When the isolating valve  106   b  is opened, the chamber  160  of the appendix  106  is in communication with free exchange of gas with the chamber body  102 . 
     Molecules in the gas mixture can then diffuse freely in the chamber  160  of the second appendix. 
     In this chamber, as described above, the hot catalyst can dissociate gas molecules polluting the active medium. 
     Periodically, the second appendix  106  may also be purged in order to extract undesirable gases from it. The isolating valve  106   b  is then closed and the extraction valve  106   c  is opened for pumping. 
     Note that the catalytic thermal trap is particularly suitable for high power CO 2  type lasers. However, it can be used for other types of lasers. 
     FIG. 6 shows an example embodiment of an active gas source for the device according to the present invention in the form of a third appendix  108 . 
     Unlike the cryostat or the catalytic trap described above, the active gas source does not form a means of gas purification according to the meaning of this the present invention. However, the gas source does provide new gas, exempt of any undesirable compounds, to be added into chamber  102 . 
     The third appendix  108  includes three pressurized gas cylinders  171 ,  172  and  173  which contain halogen gases, rare gas and buffer gas respectively. 
     Appendix  108  is connected to the main body  102  of the laser chamber through a fitting  108   a  and through an isolating valve  108   b.    
     It can also be seen that each gas cylinder is fitted with a delivery valve  174 ,  175 ,  176  specific to it. 
     According to one variant, the three pressurized gas cylinders may be replaced by a single cylinder containing a gas mixture, corresponding to the gas mixture contained in the chamber. 
     According to another variant, the appendix may simply include a single cylinder containing a makeup halogen gas. 
     In particular, the addition of new gas compensates for some or all of the losses of halogen compounds due to the formation of impurities in the form of carbon or metal halides. 
     Furthermore, control means not shown in the figure and connected to delivery valves, may be provided to control the quantity of gas inlet into the chamber. For example, these means may comprise an optoelectronic laser beam control loop designed to measure the emitted energy and to control the addition of new gases as a function of this measurement. 
     FIG. 7 diagrammatically shows another example embodiment of a source of makeup active gas. 
     According to this variant, the appendix  108  corresponding to the gas source comprises a degassing chamber  180  containing pellets  182  capable of releasing a gas when they are heated. 
     The pellets  182  are made of a porous material such as ceramics containing gas trapped in the pores. These gases are one or several makeup gases as described above. 
     A heating resistance  184  is provided to heat the pellets in a controlled manner and thus extract a controlled quantity of makeup gas from it. The heating resistance may be controlled by servocontrol means for this purpose, not shown in the figures. 
     An illustration embodiment of the gas source shown in FIG. 7 is described in documents Great Britain Patent Publication No. GB-A-9 124 948 (WO 93/11065) Great Britain Patent Publication No; GBA-9 011 535, and U.S. Pat. No. 5 363 396, mentioned in the references at the end of this description. 
     Finally, note that appendix  108  is connected to the chamber body through a fitting  108   a  and through an isolating valve  108   b.    
     Finally, the present invention eliminates all constraints related to circulating pumps and tubes existing on known devices (in which the laser chamber and the purification means are in the form of independent units), in particular due to the use of a single device comprising a laser chamber and appendices including gas purification and/or makeup means. 
     A laser device according to the present invention is inexpensive to manufacture, not only due to the lack of any connecting circuits between the laser chamber and the purification means, particularly including a circulating pump and connecting tubes, but also because the use of built-in purification means simplifies manufacture of the chamber itself. 
     The built-in cryogenic trap traps moisture contained in the laser chamber the first time that the laser is used. Moisture is transformed into ice in the cryogenic trap. 
     Thus, oven drying and desiccation operations of the laser chamber may be accelerated when the laser chamber is made. 
     Furthermore, the lack of any connecting circuits increases the reliability of the laser. 
     Finally, note that although the present invention was described with reference to a particular example in which the laser device comprises several gas purification or makeup means, a simpler embodiment of the device would only include some of the described equipment.