Abstract:
A method of minimizing contamination of optical components of a laser resonator is disclosed. The resonator components are located in an enclosure, which may contain contaminants including water vapor and organic favor released by the optical components, mounts of the optical components, or the enclosure itself. The enclosure may also contain suspended particulate matter. In order to reduce the level of these contaminants, a purging system extracts gas from the enclosure and passes the gas through a desiccant, an organic vapor trapping material, and a particulate matter filter then returns the extracted gas to the enclosure. The purging system is particularly useful for ultrafast lasers and ultraviolet lasers where the power of the laser radiation increases the probability of destabilizing reactions between laser radiation and contaminants.

Description:
CLAIM OF PRIORITY 
     This application is a continuation of U.S. patent application Ser. No. 09/901,857, filed Jul. 9, 2001 (now U.S. Pat. No. 6,798,813), entitled “CLOSED-LOOP PURGING SYSTEM FOR LASER,” which is hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates generally hermetic sealing of lasers. The invention relates in particular to a closed-loop purging system for water vapor, organic vapor and particulate content in an enclosure surrounding an ultrafast laser resonator or an ultraviolet (UV) laser resonator. 
     BACKGROUND 
     Ultrafast lasers are generally regarded as being lasers that deliver output radiation in pulses having a duration of a few hundred femtoseconds or less. One common ultrafast laser is a Ti:sapphire laser, which can be arranged to deliver output radiation at wavelengths between about 700 nanometers (nm) and about 1000 nm. The pulses delivered often have a relatively low energy, for example, tens of millijoules (mJ) to as little as tens of nanojoules (nJ). The short pulse-duration can cause the pulses to have a very high peak power, for example, on the order of gigawatts per square centimeter (GW/cm 2 ) in certain locations in a resonator. 
     The very high peak powers delivered by such lasers can rapidly cause damage to optical components of the lasers, absent measures to inhibit such damage. Laser damage to optical components may be exacerbated by defects on or in optical surfaces of the components. Accordingly, it is not unusual that at least some portion of the optical components of an ultrafast laser are generated by so-called super-polishing techniques which yield surfaces having a surface smoothness of atomic dimensions, for example, about 4 Ångstrom Units (Å) root-mean-square (RMS) or less. Optical coatings for such super-polished components, reflective coatings in particular, are often deposited by ion-beam sputtering (IBS). IBS is a coating deposition method that can provide coatings having a high degree of chemical perfection and very low defect content. This minimizes absorption and scattering of radiation by the coatings. However, a super-polished, IBS-coated optical component can be as much as about five or more times more expensive than a similar component polished and coated by more conventional methods. Such additional expense can be wasted if the components are later contaminated by particulate matter, condensates, vapors, or the like. 
     It is not unusual in commercial laser manufacture to assemble lasers in clean-room conditions to minimize particulate deposition on optical components of the lasers. In such a case, it would be usual to place at least the optical resonator of the laser in an enclosure sufficiently sealed to minimize at least ingress of particulate contaminants, and preferably also, ingress of contaminants in gaseous or vapor form. Such an enclosure may be purged, before sealing, with filtered dry nitrogen, dry air or the like. 
     By implementing one or more above-discussed measures during manufacturing and assembly, an ultrafast laser may be operated for a total as long as several thousand before the performance of the laser becomes significantly diminished by laser damage to one or more optical components thereof. It is believed, however, even if an enclosure could be perfectly hermetically-sealed, damage to optical components may result from contamination of optical components by outgassing products of the optical components, adhesives and the enclosure itself. Outgassing products can be generated while the laser is operating and also while the laser is not operating. 
     It is believed that the most problematical of the outgassing products are organic vapors, which can be released from material such as adhesives, elastomer seals, and any plastic materials used in the construction of the enclosure. Water vapor may also be released from components of the enclosure or optics therein. The water vapor and the organic vapors can condense directly on surfaces of the optical components. The water vapor and organic vapors together or in combination can react with laser radiation while the laser is operating. Products of the reactions can also condense or be deposited on the optical surfaces. These reaction products may include particulate matter such as carbon particles or soot. Most of these reaction products, if condensed or deposited on the optical surfaces can increase the vulnerability of the optical surfaces to damage by the laser radiation. Even if reaction products were only present within the atmosphere of the enclosure this could still result in unstable operation of the laser. 
     BRIEF SUMMARY 
     The present invention is directed to a method of minimizing contamination of optical components of a laser, the components being located in a gaseous atmosphere within an enclosure. The gaseous atmosphere can contain contaminants including water vapor, organic vapor, and suspended particulate matter. These contaminants may be present at some low level, for example, hundreds of parts per billion or less, immediately after the components are placed in the enclosure. The contaminant level can increase with both operational and non-operational time of the laser. 
     In one aspect of the present invention, the method comprises extracting gas from the atmosphere within the enclosure. The extracted gas is passed through a first medium selected to reduce the water vapor content of the extracted gas; through a second medium selected to reduce the organic vapor content of the extracted gas; and through a filter selected to reduce the particulate matter content of the extracted gas. After the extracted gas is passed through the first and second media and the filter, it is returned to the enclosure. 
     The extraction and replacement cycle preferably takes place continuously during operation of the laser such that the water vapor, organic vapor, and particulate matter content of the atmosphere in the enclosure is maintained at a minimum consistent with the selection of the media and the filter. 
     In another aspect of the invention, apparatus for carrying out the method includes a gas conditioning arrangement including the first (a desiccant) medium, the second (a medium for trapping organic vapors) medium, and the filter for trapping particulate matter. The apparatus includes a pump, which is arranged to extract gas from the enclosure and deliver the extracted gas to the gas-conditioning arrangement. The gas conditioning arrangement is configured such that the extracted air delivered thereto by the pump passes through the desiccant medium, the organic vapor trapping medium, and the filter, and is then returned to the enclosure. 
     In one preferred embodiment, the apparatus further includes first and second valves. The first and second valves are arranged such that a drying gas may be circulated through the desiccant medium for regenerating the desiccant medium while preventing the drying gas from reaching the enclosure. 
     Maintaining a low organic vapor content in a laser resonator is particularly important if the laser resonator is an ultrafast laser resonator or a laser resonator arranged to generate ultraviolet laser radiation. The relatively high-energy of ultraviolet laser radiation, multiphoton processes in the case of ultrafast lasers, generating longer wavelength radiation can increase the probability of reactions between the laser radiation and the organic vapors or their condensates. As noted above, products of these reactions, including particulate matter, can lead to unstable operation of the laser, or accelerated damage to optical components of the laser resonator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates an ultrafast laser including an ultrafast laser resonator, a source of optical pump light, a controller, and a purging system in accordance with the present invention. 
         FIG. 2  schematically illustrates details of the laser resonator of  FIG. 1 , the laser resonator being located in an enclosure cooperative with the purging system of  FIG. 1 . 
         FIG. 3  schematically illustrates one preferred embodiment of the purging system of  FIG. 1 . 
         FIG. 4  schematically illustrates another preferred embodiment of the purging system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIG. 1 , laser  20  includes a laser resonator  22 , a source  24  of optical pump light, and a purging system  26  in accordance with the present invention. In this example, laser resonator  22  is an ultrafast laser resonator delivering laser radiation in the form of ultrafast output pulses  28 . Laser  20  also includes a controller  30  arranged to control operations and parameters the laser resonator, the pump light source, and the purging system. Controller  30  controls operations of purging system  26  via electrical connections  32 ,  34 , and  36 . The controller controls operations and parameters of laser resonator  22  via electrical connections  38 ,  40 , and  42 , and controls pump light source  24  via electrical connection  43 . Purging system  26  is cooperative with laser resonator  22  via conduits  44  and  46 . Purging system  26  also includes conduits  48  and  50 , which connect with a desiccant module (not shown in  FIG. 1 ) in the purging system. The function of conduits  48  and  50  is described in detail further to hereinbelow. 
     Referring now to  FIG. 2 , laser resonator  22  includes a resonant cavity  52  terminated by mirrors  54  and  56 . Mirror  54  is a maximum reflecting mirror. The inclination of mirror  54  can be adjusted by controller  30  via electrical connection  42  and a mirror mount  57  including actuators  58 . Mirror  56  is a partially transmitting mirror, which allows output pulses  28  to be delivered from the resonant cavity. A portion of the output pulses is sampled by a beamsplitter  62  and detected by a detector  64 . Output of detector  64  is connected to controller  30  by connection  38  for use by the controller in controlling parameters of the laser resonator. The optical path of resonant cavity  54  is folded by fold mirrors  66 ,  68 ,  70 , and  72 . Folding of the optical path reduces the physical length of the resonant cavity. 
     Resonant cavity  54  includes a gain medium  74  located between mirrors  66  and  70 . In this example the gain medium is Ti:sapphire, which provides optical gain in a wavelength region between about 700 and 1000 nanometers (nm). Pump light source  24 , in this example, is a frequency-doubled Nd:YVO 4  laser, delivering pump light by fold mirrors a wavelength of 532 nm. Pump light from source  24  is delivered to gain medium  74  through mirror  70 . Also located in resonant cavity  52  are two prisms  76  and  78 . The prisms are arranged to compensate for group delay dispersion of laser radiation circulating in resonant cavity  52 , and are also used to tune the output wavelength of the laser resonator. 
     Prism  78  is mounted on a movable carrier  80 , the movement of which is controlled by controller  30  via electrical connection  40 . A slit  81  defines a portion of prism  78  through which optical radiation can pass. The output wavelength of pulses  28  is changed or tuned by operating carrier  80  such that prism  78  is moved to a new location, indicated in  FIG. 2  by dotted triangle  78 A. Slit  81  is moved synchronously with the prism as indicated by line  81 A. Dotted lines  82  indicate a change in optical path in the resonator resulting from the movement of prism  78 . Laser resonator  22 , in this example, is a mode locked laser resonator. Mode locking of the laser resonator is effected by an aperture  84  located in resonant cavity  52  and cooperative with a Kerr-lens effect induced in gain medium  74  by pump light delivered from pump-light source (laser)  24 . 
     A detailed explanation of operating principles of resonant cavity  52  is not required for understanding principles of the present invention. Accordingly, such an explanation is not presented herein. A detailed explanation of an ultrafast laser including a resonant cavity similar to resonant cavity  52  is provided in co-pending application Ser. No. 09/813,507 the complete disclosure of which is hereby incorporated by reference. 
     Continuing now with reference to  FIG. 2 , optical components of laser resonator  22  are located in an enclosure indicated in  FIG. 2  by dotted line  90 . Pump light from laser  24  enters enclosure  90  via a window  92 . Laser output pulses  28  leave the enclosure via a window  94 . Other general construction principles of an enclosure such as enclosure  90  are well known to those skilled in the art to which the present invention pertains. Accordingly, such principles are not described or depicted herein. A feature of enclosure  90  specific to the present invention, however, is the connection of the enclosure to conduits  44  and  46 , which provide fluid communication between the enclosure and components of purging system  26 . 
     The interior (atmosphere)  90 A of enclosure  90  is maintained at about ambient atmospheric pressure. The atmosphere of enclosure  90  will usually be an air atmosphere. If enclosure  90  is sufficiently well sealed, however, an atmosphere of nitrogen or some other inert gas may be included. Whatever the gaseous atmosphere of enclosure  90 , it can be expected to include some finite level of contaminants, however small that level. As discussed above, these contaminants may include water vapor, organic vapors, and particulate matter. As noted above, particulate matter may include that which was present at the time that the enclosure was closed, and particulate matter generated as a result of interaction between laser radiation circulating in resonant cavity  52  and the organic vapors. 
     Referring now to  FIG. 3 , one preferred embodiment  26 A of a purging system  26  in accordance with the present invention includes a pump  102  and a gas conditioning arrangement  104 . Gas conditioning arrangement  104  includes a container  106  containing a desiccant material  108 . Desiccant material  108  is preferably silica gel, but maybe any desiccant material. Gas conditioning arrangement  104  also includes a container  110  including an organic vapor trapping material  112 . A preferred organic vapor trapping material is a high surface-area coconut-shell based activated carbon. Organic vapor traps including this material are available in various sizes from Agilent Technologies, Inc. of Palo Alto, Calif. Other suitable organic vapor trapping materials include a 5 Å molecular sieve. 
     A filter unit  114  is provided for filtering particulate matter. Filter unit  114  is preferably capable of trapping particles having a size of about 0.5 micrometers (μm) and greater, for example, a HEPA filter. One suitable HEPA filter is available from the Pall Gellman Sciences Inc. of Ann Arbor, Mich. as HEPA Capsule Part No. 12144. This filter has a pore size of 0.3 μm and has a filtering efficiency of 99.97% for 0.3 μm DOP aerosol. 
     Pump  102  extracts gas from the atmosphere of enclosure  90  via conduit  44 . The pump delivers the extracted gas via a conduit  120  and a two-way valve  122  to gas conditioning arrangement  104 . The circulation direction of gas through the purging system is indicated in  FIG. 3  by arrows A. The gas delivered by pump  102  is urged by the pump through the desiccant material (medium)  108 ; through a conduit  124 ; through another two-way valve  126 ; and then through organic vapor trapping material  112 . After passing through the organic vapor trapping material, the gas passes through HEPA filter  114  into conduit  46 , which returns the gas to enclosure  90 . As noted above, desiccant material  118  reduces the water vapor content of the gas, and organic vapor trapping material  112  reduces the organic vapor content of the gas. HEPA filter  114  reduces the particulate matter content of the gas. Valves  122  and  126  in this mode of operation prevent any of the extracted gas from escaping the purging system via conduits  48  and  50 . 
     In one preferred cycle of operation of purging system  26 A, the extraction and return of gas from and to the enclosure takes place continually during any period in which laser  20  is operating. Operation of the purging system is started and stopped by correspondingly starting or stopping pump  102  by commands delivered thereto from controller  30  via electrical connection  32 . Continuous operation of purging system  26 A can provide that in the atmosphere of enclosure  90 , the water vapor, organic vapor, and particulate matter content of the atmosphere are maintained at minimum consistent with the materials and configuration of gas conditioning arrangement  104 . It is possible, of course, that the inventive purging system could be activated and deactivated by controller  30  based on measurements of particle count or concentrations of particular vapor species. This, however, would require providing corresponding sensors, which could increase the cost of a laser or the purging system. 
     After a period of operation, depending on the ambient atmosphere in which laser  20  is located, or the conditions of operation of the laser, desiccant material  108  may become saturated with water vapor. Should this occur, desiccant material  108  may be revived or regenerated by passing a drying gas, such as dry air or dry nitrogen, through the material, as follows. 
     Operation of pump  102  is stopped. Valve  122  is switched to prevent air from being delivered from pump  102  to the desiccant material, and to allow the drying gas to be delivered to the desiccant material via conduit  48 . Valve  126  is switched to prevent any drying gas from reaching enclosure  90  via the organic vapor trapping material, the HEPA filter, and conduit  46 . This switching allows drying gas delivered to the desiccant material via conduit  48  to pass through the desiccant material and exit the purging system via conduit  50  as indicated in  FIG. 3  by arrows D. After the desiccant material has been regenerated, valves  122  and  126  are switched back to a position that allows gas extracted from enclosure  90  to pass to the gas conditioning system and return to the enclosure via conduit  46 . Valves  122  and  126  may be operated by commands delivered thereto along connections  34  and  36  from controller  30 . 
     Another preferred embodiment  26 B of a purging system in accordance with the present invention is depicted in  FIG. 4 . Purging system  26 B is similar to purging system  26 A of  FIG. 3  with an exception that desiccant material  108  and organic vapor trapping material  112  are contained in a single container  130 . A permeable diaphragm or separator  132  separates the desiccant material from the organic vapor trapping material. One such combined desiccant and organic vapor trapping unit is available from the W.A. Hammond Drierite Company Ltd of Xenia, Ohio as Part No. 27068. 
     It is emphasized here that the sequence of vapor reduction and filtering is particularly important in the method and apparatus of the present invention. If water vapor reduction does not precede organic vapor reduction there could be a significant degradation in the efficiency of organic vapor reduction. As there is a possibility that water vapor removal materials and organic vapor trapping materials can generate particulate matter it is important that particulate matter filtering takes place following water vapor reduction and organic vapor reduction. 
     In the description of laser  20  given above, laser resonator  22 , controller  30 , and purging system  26  are described as separate units. This arrangement should not be construed as limiting the present invention. By way of example, as the size of the purging system can be relatively small compared with the laser resonator, the purging system and the laser resonator may be combined in a single unit or housing. Alternatively, the purging system may be combined in a single housing with the controller. In another arrangement, purging system  26  may be configured as a stand-alone module including a dedicated controller separate from controller  30 . One skilled in the art to which the present invention pertains may devise other configurations of the purging system, the laser resonator and one or more controllers without departing from the spirit and scope of the present invention. 
     The purging system of present invention is described above with reference to its use with a Ti:sapphire ultrafast laser. This should not be construed as limiting the present invention. The inventive purging system is applicable to other ultrafast lasers such as those including dyes or semiconductor materials as gain media. As noted above, the very high-power and short duration of ultrafast laser pulses can increase the possibility of the ultrafast laser radiation reacting with any organic contaminants that may be found in the atmosphere in the resonant cavity of the laser. Also as noted above, the inventive purging system is particularly useful in ultraviolet lasers where the high-energy of the ultraviolet radiation also increases the possibility of reactions with any organic contaminants in the laser resonator. Costs permitting, however, it may be found useful to use the inventive purging system with any other laser with the goal of extending the operating lifetime of optical components or reliability of operation of the laser. 
     The present invention is described above in terms of a preferred embodiment and other embodiments. The invention is not limited, however, to the embodiments described and depicted herein. Rather, the invention is limited only by the claims appended hereto.