Abstract:
A cooling system for use in with a transmissive optical element of a high average power laser (HAP). The system includes at least one optically transmissive element (TOC) that is held by a differential pressure in thermal contact with a heat sink assembly. In one embodiment, the heat sink assembly includes an optically transparent heat conductor (THC) attached to at least one face of the TOC. A vacuum formed between adjacent faces of the TOC and THC urges the facing planar surfaces into thermal contact with one another. Waste heat generated in the TOC is conducted to the THC. The temperature gradient inside the TOC is maintained substantially parallel to the direction of a laser beam being directed through the THC so that a given phase front of the beam exposes TOC material to the same temperature. As a result, the TOC does not perturb the phase front of the laser beam.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to cooling techniques for optical elements employed in laser systems, and more particularly, to conductive face-cooled optical elements for use in optical systems which are required to process/handle the output of High Average Power (HAP) laser systems.  
       BACKGROUND OF THE INVENTION  
       [0002]     Overview  
         [0003]     Transmissive optical components used in high-average power (HAP) lasers experience significant heat load due to absorption of optical energy and other processes. This heat must be removed (often in real time) for the transmissive optical component to operate correctly and efficiently. For example, the process of frequency conversion in a nonlinear laser material generates heat within the nonlinear material medium due to absorption. This heat must be removed if the frequency converter is to operate efficiently at a significant power level. Also, the process of storing energy in a solid state laser amplifier material also generates heat within the laser medium that must be removed, especially if the amplifier is to operate at a significant input power. Other transmissive optical components subjected to heat load and requiring cooling include crystals used in Pockels cells and glass used in Faraday rotators.  
         [0004]     Traditional Methods of Heat Removal  
         [0005]     A traditional method of heat removal from solid state crystalline materials employed in laser systems is to remove the heat from the sides of the materials, in a direction transverse to the direction of laser energy propagation. The removal of heat in a transverse direction causes thermal gradients in this direction. This creates several problems. In general, temperature gradients generate thermal-optical stress and index variations, which in turn cause thermal aberrations that distort the laser beam. More specifically, in most frequency conversion materials, the temperature variation in a direction transverse to the direction of propagation of the laser beam must be maintained to within a very small tolerance range. The presence of a thermal gradient in this direction severely limits the aperture size and the power loading allowed in a laser system design. Transverse cooling is described in a paper entitled “The Potential of High-Average-Power Solid State Lasers,” by J. L. Emmett et al., Document No. UCRL-53571, dated Sep. 25, 1984, available from the National Technical Information Service, and hereby incorporated by reference into the present application.  
         [0006]     Conventional beam shaping techniques have been used to cool crystals whereby the laser beam is optically flattened in one transverse direction. This allows the crystal to be cooled along a greater length, and reduces the path from the center of the beam to the edge of the crystal where it is cooled. However, this method is not practical in all applications and requires a relatively high degree of complexity in the associated optics.  
         [0007]     In some crystalline materials, and in particular beta-barium borate (BBO), the direction of greatest thermal conductivity in the material is also aligned closely with the direction of optical propagation. In order to efficiently remove heat from materials with this property, the heat must therefore be removed from the optical faces. One method of face cooling is a convective process, normally achieved using a flowing gas. In this method a gas is forced at high velocity across the faces of the crystal. The chief disadvantage of this method is that it requires a complex, active cooling system, and is therefore less suitable for applications requiring low cost, weight and volume, and a high degree of reliability. Also the engineering to implement this method is complex because the gas flow across the optical surfaces must be very uniform to avoid optical distortion.  
         [0008]     U.S. Pat. No. 5,363,391, entitled “Conductive Face-Cooled Laser Crystal”, and issued to Steven C. Matthews et al on Nov. 8, 1994 and hereby incorporated by reference, discloses and claims techniques for passively removing heat from an optical element in a laser system through its optically transmissive faces ( FIG. 1 ). Heat is removed by way of optically transmissive heat conducting media disposed adjacent the optically transmissive surfaces of the optical element. Heat is transferred out of the optical element in a direction parallel to the direction of propagation of optical radiation, thus minimizing problems associated with thermal gradients. Devices employing optical elements such as nonlinear frequency conversion crystals and laser crystals may utilize this heat management approach to achieve better performance. Heat is transferred to the heat conducting media by direct contact or through narrow gas-filled gaps disposed between the optical element and the heat conducting media.  
         [0009]     U.S. Pat. No. 6,330,256, entitled “Method and apparatus for non-dispersive face-cooling of multi-crystal nonlinear optical devices”, and issued to Robert W. Byren et, al on Dec. 11, 2001 and incorporated by reference herein, teaches how to use the face-cooling method taught in U.S. Pat. No. 5,363,391 with multiple nonlinear crystal formats used primarily for second harmonic generation without the need for air-path rephasing between the crystals ( FIG. 2 ). One or more birefringent crystals are cut and oriented such that there is no dispersion between the fundamental and second harmonic wavelengths within each crystal. The birefringent crystals are then disposed in a heat-conducting housing, sandwiched between two or more nonlinear crystals and used as the face-cooling medium. The multiple crystal assembly may be further sandwiched between optically transmissive windows which need not be birefringent or non-dispersive, these windows being used to protect the outermost nonlinear crystals and/or provide additional face cooling. This causes the heat generated in the nonlinear crystals by absorption at the fundamental and second harmonic wavelengths to flow longitudinally (direction of beam propagation) into the face-cooling medium, thereby minimizing any transverse thermal gradient in the nonlinear crystals and the attendant dephasing loss. The crystals can be dry stacked with a very small gas-filled gap as taught in U.S. Pat. No. 5,363,391, immersed in a liquid or gel of suitable refractive index, bonded with suitable optical cement, optically contacted, or diffusion-bonded together to form a composite crystal.  
         [0010]     The above-described systems and methods rely on heat transfer from the transmissive optical component to a heat sink by means of conduction due to a mechanical contact, optical contact, bonded joint, or a narrow gas-filled gap. It is well known, however, that transmissive optical components exposed to thermal load tend to warp significantly. Unless an external force is provided, the effective contact area in mechanically and optically contacted joints is, therefore, significantly reduced, which typically leads to increased temperatures and warpage. Bonded joints typically use organic adhesive which has a low thermal conductivity and, therefore, impedes effective heat transfer. In addition, bonded joints cause increased stresses in the transmissive optical component since its transverse thermal expansion is now constrained by attachment to a heat sink. Finally, heat conduction through narrow gas-filled gaps is rather limited even when gasses with high thermal conductivity are used. Because of the above limitations, there is a need for an improved method for cooling transmissive optical components in HAP lasers.  
       SUMMARY OF THE INVENTION  
       [0011]     The present invention is directed to a heat sink assembly ideally suited for face cooling a optical transmissive component (TOC) receiving a laser beam in a high average power (HAP) laser system. The heat sink assembly enables cooling of the TOC without introducing temperature gradients along the TOC in a direction normal to the direction of propagation of the laser beam. The heat sink assembly of the present invention further accomplishes this without the need for physically bonding the TOC to the heat sink assembly.  
         [0012]     The heat sink assembly includes a transparent heat conductor (THC) that has one face thereof held closely adjacent a face of the TOC such that a seal is formed between the two faces. The seal may be formed by an independent sealing member interposed between the two faces. In one preferred embodiment, the heat sink assembly includes a THC having an O-ring groove formed in a face portion thereof, while the THC comprises a disk having parallel planar faces, with at least one, and preferably both, of the surfaces being formed as optically flat surfaces.  
         [0013]     The THC may also include an O-ring seated in the O-ring groove, and a vacuum pump-out groove formed in the same surface as the O-ring groove, and radially inwardly of the O-ring groove. The vacuum pump-out groove includes a channel that opens onto an exterior surface of the THC to enable a vacuum to be applied to the pump-out groove. The vacuum urges the TOC against the THC, thus compressing the O-ring to form a sealed cavity defined by the O-ring and the facing surfaces of the TOC and the THC. Advantageously, no bonding or adhesives are required to maintain the faces of the TOC and the THC in contact with one another; the pressure differential between the cavity and the ambient environment performs this “holding” function. The heat sink assembly also includes a heat sink member in thermal contact with a peripheral portion of the THC.  
         [0014]     In operation, heat generated in the TOC by a laser beam is conductively transmitted to the THC via the above-described contact, and then to the heat sink member. Since no physical bonding of the facing surfaces of the TOC and THC is needed, there is no tendency for the heat sink to cause stresses in the TOC to develop by impeding its transverse thermal expansion during operation. Avoiding a mechanically formed thermal coupling between the facing surfaces eliminates the possibility of excessive warpage of the face of the TOC, during operation, from degrading the transfer of thermal energy from the TOC to the THC.  
         [0015]     In one alternative preferred embodiment, the TOC is sandwiched between a pair of THCs. Each THC is otherwise constructed as described above. Face cooling of both opposing surfaces of the TOC can be accomplished.  
         [0016]     In another alternative preferred embodiment, a plurality of TOCs are disposed adjacent one another, but separated by a plurality of THCs. Thus, each TOC is sandwiched between a pair of THC. Each THC is constructed as described above and includes an associated heat sink component. Face cooling is achieved on both opposing faces of every TOC.  
         [0017]     In another alternative preferred embodiment, the TOC includes a reflective coating on the face in contact with the THC of the heat sink assembly. The THC can include a plurality of pump-out channels formed to open onto the face of the THC that faces the TOC. The THC can also incorporate the O-ring groove and O-ring as described previously. The pump-out channels are in communication with a portion that opens onto an exterior surface of the THC so that an external means can be used to form a pressure force for holding the TOC in intimate thermal contact with the THC. The pressure force is applied by the pressure differential between the ambient atmosphere and the coolant.  
         [0018]     The features, functions, and advantages can be achieved independently in various embodiments of the present inventions or may be combined in yet other embodiments. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0020]      FIG. 1  is a diagram of a prior developed technique for passively removing heat from an optical element of a laser system;  
         [0021]      FIG. 2  is a diagram of a prior developed system that uses the cooling method of  FIG. 1  with multiple, non-linear crystal formats;  
         [0022]      FIGS. 3   a  and  3   b  illustrate side cross-sectional side and end views, respectively, of a first preferred embodiment of the heat sink assembly of the present invention;  
         [0023]      FIG. 4  illustrates a side cross-sectional view of a second preferred embodiment of the heat sink assembly of  FIG. 3  that achieves face cooling on both opposing faces of an optical element;  
         [0024]      FIG. 5  illustrates a cross-sectional side view of a third alternative preferred embodiment of the heat sink assembly in which every optical element has both of its opposing faces cooled by a heat sink assembly;  
         [0025]      FIG. 6  is a side cross-sectional view of a fourth alternative preferred embodiment in which the thermal heat conductor (THC) incorporates a heat exchanger having a coolant flowed through it for cooling an optical element that is held in close proximity to the THC by an external holding element; and  
         [0026]      FIG. 7  is a side cross-sectional view of a fifth alternative preferred embodiment in which a coolant is flowed directly through microchannels formed in a thermal heat conductor (THC) for removing heat from an optical element held in close proximity to a face of the THC. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
         [0028]     The invention discloses a method for cooling optical components of a high average power, solid state laser (HAPSSL), and thus increasing the power-handling capability of the optical components. The various preferred embodiments enable many tactical and strategic laser systems which would otherwise be too costly to manufacture.  
         [0029]     Referring to  FIGS. 3   a  and  3   b , there is shown a cooled optical assembly  10  in accordance with a first preferred embodiment of the present invention comprising a transparent optical component (TOC)  22  and a heat sink assembly  48 . The cooled optical assembly  10  receives an incident laser beam  26  and transmits it therethrough to form a laser beam  26 ′. The heat sink assembly  48  further comprises an optically transparent heat conductor (THC)  24  and a heat sink  28 . The TOC  22  is a flat member made of suitable optically transparent material having two large and generally parallel surfaces. The perimeter of the TOC  22  is preferably circular, elliptical, oval or polygonal in shape. Large TOC surfaces  52  and  52 ′ are adapted to receive and transmit laser beams and for this purpose can be equipped with appropriate optical coatings. In addition, surface  52 ′ is machined to optical flatness and mated with THC surface  42 .  
         [0030]     The THC  24  is a flat member having two large and generally parallel surfaces. It is made of optical material having good thermal conductivity and is substantially transparent at the optical wavelengths of the laser beams  26  and  26 ′. Suitable materials for THC  24  include sapphire, single crystal diamond, polycrystalline diamond, yttrium aluminum garnet (YAG) in a single crystal or polycrystalline form, and gallium gadolinium garnet (GGG) in a single crystal or polycrystalline form. The perimeter of the THC  24  is preferably circular, elliptical, oval or polygonal in shape. At least some portions of the THC  24  perimeter are attached to and in thermal communication with the heat sink  28 . Heat sink  28  provides both mechanical support and heat removal for THC  24 . For the latter purpose, the heat sink  28  is cooled either by conduction or by convection. For example, a conductively cooled heat sink  28  can be thermally attached to a solid-state cooler operating in accordance with the Peltier effect. A convectively cooled heat sink  28  may, for example, include a liquid or gas-cooled heat exchanger.  
         [0031]     The large THC surfaces  42  and  42 ′ receive and transmit laser beams and, for this purpose, can be equipped with appropriate optical coatings. THC surface  42  includes an O-ring groove  46  and pump-out grove or channel  34 . The pump-out groove  34  is connected to a vacuum pump using a pump-out hole  36 . In addition, THC surface  42  is machined to optical flatness. The cooled optical assembly  10  is formed when the O-ring grove  46  in the heat sink assembly  48  is equipped with an appropriate vacuum-sealing O-ring  32 , the THC surface  42  is mated with the TOC surface  52 ′, and vacuum suction is applied to the pump-out hole  36  in the presence of atmosphere  38 . The pressure differential between the pressure of atmosphere  38  and the reduced pressure in the pump-out groove  34  generated by the vacuum suction forces the TOC surface  52 ′ and THC surface  42  to contact. Over time, gas molecules trapped between the surfaces  52 ′ and  42  migrate to the pump-out groove  34  and are removed. As a result, good thermal communication is established between the TOC  22  and THC  24 .  
         [0032]     When the cooled optical assembly  10  is operated with laser beam  26  and waste heat is generated inside the TOC  22 , this heat is conducted to the THC  24 . During this process the temperature gradient inside the TOC  22  is very nearly parallel to the direction of the laser beam  26 . This means that a given phase front of the laser beam exposes TOC  22  material of the same temperature. As a result, the TOC  22  does not perturb phase fronts of the laser beam  26 . The pressure differential that clamps the TOC  22  on to THC  24  can be further increased if the cooled optical assembly  10  is operated in a pressure chamber and atmosphere  38  is at or above ambient atmospheric pressure. In particular, if the pressure of atmosphere  38  is substantially above ambient atmospheric pressure, the pump-out hole  36  can be connected to ambient atmosphere rather than a vacuum pump.  
         [0033]     Referring now to  FIG. 4 , there is shown a cooled optical assembly  11  in accordance with a second preferred embodiment of the present invention comprising the transparent optical component (TOC)  22  and heat sink assemblies  48   a  and  48   b . The transparent optical component (TOC)  22  is the same as practiced with the first preferred embodiment  10 . The heat sink assembly  48   a  is the same as the heat sink assembly  48 , and the heat sink assembly  48   b  is a mirror image of the heat sink assembly  48   a . In contrast to the first embodiment  10  where only the surface  52 ′ was cooled, in this preferred embodiment both TOC surfaces  52  and  52 ′ are respectively cooled by the heat sink assemblies  48   a  and  48   b . As in the first embodiment, thermal communication between the TOC  22  and the heat sink assemblies  48   a  and  48   b  is assured by hydrostatic pressure generated by the pressure differential between the atmosphere  38  and the reduced pressure in the pump-out groves  34 .  
         [0034]     Referring now to  FIG. 5 , there is shown a cooled optical assembly  12  in accordance with a third preferred embodiment comprising multiple transparent optical components (TOC)  22  and multiple heat sink assemblies  48  and  148 . The heat sink assembly  148  is similar to the heat sink assembly  48 , except that a THC  124  has two O-ring groves  46  with O-rings  32 , and two pump-out grooves  34  formed on its opposing surfaces and formed as a T-shaped channel (when viewed end-wise). In addition, a pump-out hole  136  is connected to both opposing pump-out grooves  34 . In this fashion the THC  124  can receive an TOC on each of its facial surfaces  142  and  142 ′.  
         [0035]     Referring now to  FIG. 6 , there is shown a cooled optical assembly  13  in accordance with a fourth preferred embodiment comprising a TOC  222  attached to a heat exchanger assembly  272 . The TOC  222  is similar to the TOC  22  except that the surface  252 ′ facing the heat exchanger assembly  272  has a reflective coating  272   a  for high-reflectivity at the operating wavelengths of cooled optical assembly  13 . The heat exchanger assembly  272  comprises a rigid body  288  having a surface  280  that includes interconnected microchannels  274 , and a heat exchanger  276  machined or otherwise formed on one or more interior surfaces of the rigid body  288 . Except for the microchannels  274  and an O-ring grove  346 , the surface  280  is machined to substantially optical flatness. The rigid body  288  can be provided in two pieces to more easily permit the heat exchanger  276  to be formed, and then secured together such as by diffusion, bonding, brazing or any other suitable attachment means. The rigid body  288  is made of thermally conductive material preferably having a high Young&#39;s modulus and made sufficiently thick to have a high degree of mechanical rigidity. Preferred materials for the rigid body  288  include silicon, copper, copper alloys, tungsten, and tungsten carbide.  
         [0036]     The cooled optical assembly  13  also includes a clamp  292  that gently presses the TOC  222  towards the surface  280 , thereby squeezing the O-ring  232 . When the pressure in the microchannels  274  is reduced below the pressure of atmosphere  38 , the TOC  222  experiences a hydrostatic pressure forcing its reflectively coated surface  252 ′ into contact with the surface  280 , thereby establishing good thermal communication between the TOC  222  and the rigid body  288 . Waste heat produced in the TOC  222  by the laser beam  26  is then conducted through the high-reflectivity optical coating  272   a  on the surface  252 ′ into surface  280  of the rigid body  288 . Preferably, the heat exchanger  276  is placed in close proximity to the microchannels  274 , thereby reducing the thermal gradient inside the rigid body  288 . The heat exchanger  276  is cooled by a coolant  282  entering the rigid body through a header  284  and drained through a header  286 .  
         [0037]     Referring now to  FIG. 7 , there is shown a cooled optical assembly  14  in accordance with a fifth preferred embodiment comprising a TOC  322  attached to a heat exchanger assembly  372 . The TOC  322  is similar to the TOC  222  practiced with the fourth embodiment of the invention and common components in  FIG. 7  are denoted by reference numerals increased by  100  over those used in  FIG. 6 . However, with optical assembly  14 , microchannels  374  are formed to provide the heat exchanger, and a coolant  382  is now flowed directly through the microchannels  374 . To assure positive attachment of the TOC  322  to the surface  380 , the coolant pressure in the microchannels  274  is kept substantially lower than the pressure of atmosphere  38 . The TOC  22  can be made of an optically active medium suitable for frequency doubling, amplification, or rotation of polarization. In testing, optical assembly  14 , with a rigid body  388  made of silicon and a heat load of 18 watts/cm 3 , maintained the TOC  322  flat to 1/10 th  of an optical input waveform having a wavelength of 1.03 μm  
         [0038]     While various preferred embodiments have been described, those skilled in the art will recognize modifications or variations which might be made without departing from the inventive concept. The examples illustrate the invention and are not intended to limit it. Therefore, the description and claims should be interpreted liberally with only such limitation as is necessary in view of the pertinent prior art.