Abstract:
A laser system and method. The inventive laser includes an annular gain medium; a source of pump energy; and an arrangement for concentrating energy from the source on the gain medium. In a more specific implementation, a mechanism is included for rotating the gain medium to effect extraction of pump energy and cooling. In the illustrative embodiment, the pump source is a diode array. Energy from the array is coupled to the medium via an array of optical fibers. The outputs of the fibers are input to a concentrator that directs the pump energy onto a pump region of the medium. In the best mode, plural disks of gain media are arranged in an offset manner to provide a single resonator architecture. First and second mirrors are added to complete the resonator. In accordance with the inventive teachings, a method for pumping and cooling a laser is taught. In the illustrative embodiment, the inventive method includes the steps of providing a gain medium; pumping energy into a region of the gain medium; moving the medium; extracting energy from the region of the medium; and cooling region of the medium.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to lasers. More specifically, the present invention relates to high energy lasers. 
     2. Description of the Related Art 
     High energy lasers are currently being evaluated for a variety of military and industrial applications. The implementation of a high energy weapon-class laser system is currently-limited to large platforms only due to the relatively low power per weight ratio numbers in the present approaches. 
     Prior approaches for weapons class lasers include chemical and gas lasers, which have already demonstrated weapon level power. Other diode-pumped solid-state bulk and fiber-based approaches also have been proposed. The conventional chemical laser is a large and highly complex system. In addition, the chemical handling requirement makes this an extremely cumbersome and undesirable approach. The relatively clean diode-pumped solid-state laser approach is much more desirable. 
     However, diode-pumped solid-state lasers using bulk active media have problems such as high beam distortion as the beam propagates through the amplifier chain. Compensation for this distortion currently requires active (deformable mirror) or passive (such as non-linear phase conjugation) techniques. In addition, the gain elements for bulk diode-pumped solid-state lasers are currently comprised of complex, expensive composite slabs, which are prone to damage. In addition, the master oscillator-power amplifier (MOPA) approach, which is typically, used to improve the beam quality of bulk solid-state lasers, limits the ultimate optical (and, therefore, the overall) conversion efficiency, which results in increased power and waste heat extraction real-estate requirements. 
     Fiber lasers have inherently high efficiencies because they operate at very high laser beam intensities and allow for 100% pump power absorption and can be cooled efficiently due to their inherently high surface to volume ratio relative to traditional bulk solid-state lasers. Fiber lasers, however, are limited with respect to the maximum power that they can operate at due to intensity damage threshold limits. Fiber lasers, therefore, typically require coherent phasing of multiple fiber oscillators to achieve high power levels. This adds a number of problems and associated complexities. Principal of which is alignment, sensitivity to vibration, and lack of a reliable and robust approach for coherent combining of multiple individual laser beams. Another problem is the requirement of complex beam combining/shaping optics to form one compact output beam out of an array of multiple individual beams. Production of a high quality output beam then requires scaling, dissipation of heat and some approach for dealing with the high concentration of energy in a small volume. This leads to increased system complexity and associated high costs. 
     Hence, a need exists in the art for a relatively unsophisticated system or method for substantially increasing the output power, efficiency, and beam quality of high-energy lasers. 
     SUMMARY OF THE INVENTION 
     The need in the art is addressed by the system and method of the present invention. In the illustrative embodiment, the inventive system comprises an annular gain medium; a source of pump energy; and an arrangement for concentrating energy from the source on the gain medium. 
     In a more specific implementation, a mechanism is included for rotating the gain medium to effect extraction of pump energy and cooling. In the illustrative embodiment, the pump source is a diode array. Energy from the array is coupled to the medium via an array of optical fibers. The outputs of the fibers are input to a concentrator that directs the pump energy onto a pump region of the medium. 
     In the best mode, plural disks of gain media are arranged in an offset manner to provide a single resonator architecture. First and second outcoupler mirrors are added to complete the resonator. 
     In accordance with the inventive teachings, a method for pumping and cooling a laser is taught. In the illustrative embodiment, the inventive method includes the steps of providing a gain medium; pumping energy into a region of the gain medium; moving the medium; extracting energy from the region of the medium; and cooling region of the medium. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified representation of an illustrative offset pumping/extraction arrangement implemented in accordance with the present teachings. 
         FIG. 1   a  is a front view of the annular gain medium of the disk of  FIG. 1  in accordance with an illustrative embodiment of the present teachings. 
         FIG. 1   b  is an isolated view of a single sector of the gain medium of  FIG. 1   a.    
         FIG. 1   c  is a magnified view of the single sector of  FIG. 1   b.    
         FIG. 2  is a front view showing an illustrative embodiment of the disk assembly of the present invention with an associated jet impingement cooling manifold. 
         FIG. 3  is an edge view of the disk assembly of the illustrative embodiment. 
         FIG. 4  shows a resonator architecture utilizing multiple counter-rotating annular gain media in accordance with an illustrative embodiment of the present teachings. 
         FIG. 5  is a simplified front view of the resonator of  FIG. 4  with concentrators in accordance with the present teachings. 
         FIG. 6  is a simplified side view of the resonator of  FIG. 4 . 
         FIG. 6   a  is a diagram that shows an arrangement in which the gain media of the disks are disposed in a parallel relation tilted with respect to the incident beam at the Brewster angle, 
                   θ   Brewster     =       tan     -   1       ⁡     (       n   gainmedium       n   air       )         ,         
in accordance with the present teachings.
 
         FIG. 6   b  is a diagram that shows an arrangement similar to that of  FIG. 6   a  with the exception in that in  FIG. 6   b , the gain media of the disks are alternately tilted at the Brewster angle with respect to the incident beam. 
         FIG. 6   c  is a diagram that shows an arrangement in which the disks are disposed in a parallel configuration, but comprise wedge geometry gain media with gain faces tilted at the Brewster angle with respect to the laser beam/optic axis. 
         FIG. 7  shows a side view of an illustrative embodiment of the disk assembly of the present invention in an edge pumped mode of operation with a cooling manifold. 
         FIG. 7   a  is a schematic side view of a motorized disk drive in accordance with an illustrative embodiment of the present teachings. 
         FIG. 7   b  is a magnified view of an illustrative embodiment of the cooling manifold in accordance with the present teachings. 
         FIG. 8  is a simplified front view showing an arrangement for two-sided pumping of a disk implemented in accordance with an illustrative alternative embodiment of the present teachings. 
         FIG. 9  is a simplified edge view of the arrangement of  FIG. 8 . 
         FIG. 10  is a graph showing rotation frequency dependence on the disk diameter. 
         FIG. 11  is a diagram which shows the dependence of the number of required disk sub-assemblies as a function of doping density for the three different power categories. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     Illustrative embodiments and exemplary applications will now be described with reference to the accompanying drawings to disclose the advantageous teachings of the present invention. 
     While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility. 
       FIG. 1  is a simplified representation of an illustrative offset pumping/extraction arrangement implemented in accordance with the present teachings. As discussed more fully, the arrangement  10  includes a source of pump energy  13 ,  15  and  17 ; an annular gain medium  37 ; and an arrangement  11  for concentrating energy from the source on the gain medium. In the illustrative embodiment, pump energy is provided by a plurality of diode arrays of which three are shown  13 ,  15  and  17 . Each array includes plural laser diodes of which three are shown  19 ,  21  and  23  with respect to the first array  13 . Diode arrays are well-known in the art and may currently be purchased from such vendors as Quintessence of Sylmar, Calif.; NP Photonics of Seattle, Wash.; or Coherent of Santa Clara, Calif. Nonetheless, those skilled in the art will appreciate that the invention is not limited to the manner by which pump energy is provided to the medium. 
     The arrangement  11  includes plural bundles  25 ,  27  and  29  of optical fibers. As illustrated in  FIG. 1 , pump energy from each diode in each array is coupled to an optical fiber, in a bundle  25 ,  27  or  29 , at one end thereof and to a concentrator  31  at a second end thereof. In accordance with the present teachings, the concentrator  31  concentrates the energy from the optical fibers onto the gain medium  37  of a disk  33  of a disk assembly  30 . In the illustrative embodiment, the gain medium  37  is disposed about a metallic hub  39  in an annular configuration. 
       FIG. 1   a  is a front view of the annular gain medium  37  of the disk  33  of  FIG. 1  in accordance with an illustrative embodiment of the present teachings.  FIG. 1   b  is an isolated view of a single sector of the gain medium  37  of  FIG. 1   a .  FIG. 1   c  is a magnified view of the single sector of  FIG. 1   b . As shown in  FIGS. 1   a - c , in the illustrative embodiment, the gain medium  37  is comprised of a plurality of individual facets or sectors of a polygon.  FIG. 1   c  shows the detail of the gain medium annulus. HR inner disk coating  55  provides for dual pass absorption and undoped index matched edge coatings  51 ,  53  for robust damage resistance. Each sector includes a doped annular gain medium  38  sandwiched between rings of undoped material with appropriate index match (e.g. undoped YAG in the case of Er:YAG gain medium)  51  and  53 . A coating  55 , highly reflective at the pump wavelength, is disposed inside the inner ring  51  of undoped YAG and a broadband antireflective coating  57  is provided on the outer ring  53  of undoped YAG. 
     The facets of the polygon can all be polished with exclusively straight edge geometries for ease of fabrication. An index matching ceramic glaze (e.g. a glaze with an index matched to YAG) can be applied along the edges, before the final slab cutting, and polished to the objective design thickness. Subsequently, the facets are bonded together via an optical contact technique or a very thin ceramic glaze interface in order to provide for nearly seamless gain geometry. The outer edge should then undergo a “touch-up” smoothing polish in order to provide for an efficient robust optical pump coupling interface. The gain medium may be coated with an antireflection coating. Brewster angled geometry or facets may be used as well. 
     In accordance with the present teachings, energy from the concentrator  31  excites a pumped region  41  on the gain medium  37 . The disk  33  is spun such that robust thermal management may be effected via: 1) the thin disk geometry and 2) a movement of the area exposed to deposited waste heat. As the disk  33  is spun, the pumped region is moved to position  43  at which the energy is extracted. 
     The inventive remote pumping implementation provides for a removed brightness requirement at the diode arrays. The diode arrays can be passively cooled fiber-optic pigtail packages that can then be joined in a branch type arrangement as shown in  FIG. 1  in order to increase the brightness provided at the pump region  41 . 
       FIG. 2  is a front view showing the disk assembly  30  with an associated jet impingement cooling manifold  45 . In the best mode, the gain medium  37  is an annular erbium yttrium-aluminum-garnet (Er:YAG) structure. However, the present teachings are not limited to the shape or type of gain medium employed. Those of ordinary skill in the art will appreciate that other gain media or other shapes may be used without departing from the scope of the present teachings. 
       FIG. 3  is an edge view of the disk assembly  30  of the illustrative embodiment. As shown in  FIG. 3 , the jet impingement manifold  40  has two sets of ducts  42  and  44  disposed on opposite sides of the gain medium  37 . The air ducts are pneumatically coupled to a forced air-cooling system (not shown) of conventional design and construction. 
     In accordance with the present teachings, the pumped region is spatially separated from the resonator/extraction region enabling a low distortion resonator extraction mode of operation. The multiplexing of the individual disk gain modules can be implemented such that the extraction gain profile dynamics is uniform. This accomplished by an offset axis disk multiplexing shown in  FIG. 4 . 
       FIGS. 4-6  show a multiplexing of disk modules in a single laser cavity for most efficient power scaling.  FIG. 4  shows a resonator architecture utilizing multiple counter-rotating annular gain media in accordance with an illustrative embodiment of the present teachings. As shown in  FIG. 4 , the resonator  20  includes plural disk assemblies implemented in accordance with the present teachings of which only three are shown  30 ,  32 , and  34 . 
       FIG. 5  is a simplified front view of the resonator of  FIG. 4  with concentrators  31  in accordance with the present teachings. 
       FIG. 6  is a simplified side view of the resonator of  FIG. 4 . As shown in  FIGS. 4-6   c , in the illustrative embodiment, the disks are mounted in an offset (off-axis) orientation to enable the extraction zones thereof to be in optical alignment within a resonator region formed between a high reflection mirror  49  and an outcoupling mirror  48 . When lasing occurs, a beam is created by mirrors  48  and  49  which passes through the extraction zones of the spinning disks. 
     The disks  33  are pumped continuously through the edge via free-space coupled light guide/concentrators  31 . The excited volume of each disk  33  is then transported within the resonator cavity  22 , where it spatially overlaps with the resonator mirrors  48  and  49  as well as with the other discs&#39; excited annular regions. While the excited region of each disk is outside of this overlap resonator/cavity region, it will have insufficient gain for laser oscillation. However, once it enters the resonator cavity/overlap region, the resonator mirror feedback and the cumulative (additive) gain from the other, discs&#39; excited annular volumes will result in laser oscillation with high extraction efficiency. 
     The offset axis disk module multiplexing with counter-rotating dynamics of the present invention provides for a uniform extraction mode profile. The adjustment of both angular velocity and offset disk axes positions can provide for dynamic tuning of the extraction mode profile to compensate for small phase distortions. This is illustrated in  FIGS. 6   a - 6   c .  FIGS. 6   a - c  show various arrangements by which Brewster angles are used to mitigate the need for a high reflection coating on each sector of the gain media of the disks. Using Er:YAG as the gain medium as an example, the Brewster angle is: 
     
       
         
           
             
               
                 
                   
                     θ 
                     Brewster 
                   
                   = 
                   
                     
                       
                         tan 
                         
                           - 
                           1 
                         
                       
                       ⁡ 
                       
                         ( 
                         
                           
                             n 
                             YAG 
                           
                           
                             n 
                             air 
                           
                         
                         ) 
                       
                     
                     = 
                     
                       
                         61 
                         ° 
                       
                       ⁢ 
                       
                           
                       
                       . 
                     
                   
                 
               
               
                 
                   [ 
                   1 
                   ] 
                 
               
             
           
         
       
     
       FIG. 6   a  is a diagram that shows an arrangement in which the gain media of the disks are disposed in a parallel relation tilted at Brewster angle with respect to the incident beam  46 . 
       FIG. 6   b  is a diagram that shows an arrangement similar to that of  FIG. 6   a  with the exception in that in  FIG. 6   b , the gain media of the disks are alternately tilted at Brewster angle with respect to the incident beam  46 . 
       FIG. 6   c  is a diagram that shows an arrangement in which the disks are disposed in a parallel configuration, but comprise wedge geometry gain media with gain faces tilted at Brewster angle with respect to the beam  46 . In  FIG. 6   c , the gain media are constructed as shown in  FIG. 1   c.    
     The modular disk subassembly illustrated in  FIGS. 4-6   c  allows for ready scaling to very high (e.g., megawatt-class) power levels. 
     Edge Pumping 
     Edge pumping can be implemented with a reflective inner radius  55  in order to reflect the pump energy in a double-pass arrangement. In this way, pump uniformity across the gain rim can be achieved. This is illustrated in  FIGS. 7 and 8 . 
       FIG. 7  is a simplified side view of an illustrative embodiment of the disk assembly of the present invention in an edge pumped mode of operation with a cooling manifold. In  FIG. 7 , rims  50  and  52  are used to stabilize the edge of the medium  37 . As shown in  FIG. 7 , optical pump energy is supplied to the medium  37  of the disk  33  at the edge of the gain medium thereof via its broadband antireflection coating  57 . While the disk is spun and edge pumped, the medium  37  is cooled by a gas via jet impingement structures  42  and  44  of the cooling manifold  40 . Each disk is spun by a motor (not shown) via a shaft  64 . An illustrative arrangement for spinning the disks is provided in  FIG. 7   a.    
       FIG. 7   a  is a schematic side view of a motorized disk drive in accordance with an illustrative embodiment of the present teachings. As shown in  FIG. 7   a , the inventive disks are spun by a conventional motorized drive with either conventional (steel ball) or air bearings. A bearing mounting is shown in  FIG. 7   a . The bearing mounting consists of a base and a precision bearing support  60  with a bearing  58  mounted inside a precision bore (not shown). A threaded bearing retainer  58  is used to secure an outer bearing  56  to the support. A bearing shaft  64  mounts to the inner bearing retainer and is clamped with a similar threaded bearing retainer  58 . The rotating disk  33  is attached to the top of the bearing shaft  64 . A DC motor  70  with an attached optical encoder (not shown) is mounted to a lower motor support plate  72 . The motor  70  is attached to the plate  72  and a motor shaft  64  connects to the bearing shaft  64  via a coupler  76 . 
     A mechanical bearing spindle  56  is included having a duplex pair ball bearing  58  mounted inside the bearing support  60 . Such duplex pair bearings  58  have been used on airborne gimbals and precision laser pointing devices. The duplex pair bearing  58  reduces the axial and radial play in the shaft  64 . This design places a controlled amount of compression from one bearing into the second bearing to remove the end play. This precision is needed to maintain, in the best mode, an air jet gap of 0.025 inches on both sides of the disk  33 . The geometry of the bearing support and bearing fits are precision machined for operation over an extended temperature and vibration range. 
     In an illustrative implementation, the disks are spun at rates ranging from 1 Hz to 35 Hz—depending on the design performance power of the laser. In the best mode, the motor  70  rotates the disk  33  nominally at ˜1200 RPM (20 Hz). The velocity is adjusted by changing the DC voltage applied to the motor. An optical encoder (not shown) may be used to provide the exact velocity. 
       FIG. 7   b  is a magnified view of an illustrative embodiment of the cooling manifold in accordance with the present teachings. In the illustrative embodiment, the cooling manifold  40  includes thin fused quartz plates  82 ,  84  with holes laser drilled in a very dense pattern. In the illustrative embodiment, the hole diameter is 0.010 inches with center to center spacing of about 0.050 inches. 
     In the illustrative embodiment, there are four separate jet panels  90  for the top of the disk  33  and four on the lower side of the disk  92 . Each jet plate  82 ,  84  has a tube  42 ,  96  to bring the compressed air into a high pressure side of the jet plate  82 ,  84 . The manifold  90 ,  92  will distribute the pressure evenly so each jet will have the same pressure. In the best mode, the manifolds  90 ,  92  are adjustable in both directions to establish the desired spacing of 0.025 inches from the disk  33 . Preferably, both the upper and lower manifolds are adjustable. Each manifold  90 ,  92  has a supply line from a regulated air bottle (not shown) and are able to make pressure changes from the upper and lower manifolds  90 ,  92 . The manifolds  90 ,  92  are bolted together with standard face seal O-rings. 
     The inventive arrangement allows for use of a conductively cooled pump source implementation due to a greatly reduced brightness requirement at the diode array. The offset pumping/extraction geometry of the present invention can be implemented in an optimized fashion in order to achieve: 
     i) optimized transit time between the pumping and extraction regions (t≦⅕ th τ F , where τ F  is the fluorescence lifetime of the upper laser level); 
     ii) resonator cross-area transit time &lt;&lt; thermal diffusion time constant enabling thermal distortion free extraction beam mode profile; and 
     iii) residual round trip transit time back to the pump area &gt;(˜2×) thermal diffusion time constant. 
     Continuous cooling with laminar gas flow allows for phase distortion free cooling in a continuous wave operation for efficient laser operation. The pump uniformity is enhanced with either a dual pass reflection geometry or the implementation of two sided pumping as discussed more fully below. 
     Two-Sided Pumping 
       FIG. 8  is a simplified front view showing an arrangement for two-sided pumping of a disk implemented in accordance with an illustrative alternative embodiment of the present teachings. 
       FIG. 9  is a simplified edge view of the arrangement of  FIG. 8 . As shown in  FIGS. 8 and 9 , for each disk, a second bundle of optical fibers conveys pump energy via a second concentrator  31 ′ to the pump region  41  of the gain medium  37  while pump energy is applied to the pump region  41  via the first bundle of optical fibers and the concentrator  31 . As shown more clearly in the edge view of  FIG. 9 , dual sided pumping into a single region is effected via an anti-reflection coated wedge concentrator  36 . 
     For most efficient operation, in the illustrative embodiment, the laser cross-sectional gain area diameter is designed for a high intra-cavity to saturation intensity ratio (&gt;&gt;10) for efficient energy extraction. In addition, the pump region  41  to resonator extraction region  43  transit time, τ P−R , satisfies both the condition τ P−R &lt;⅕ th τ F  (where τ F  is the fluorescence lifetime of the upper laser level), and the revolution time, τ PRF ˜2×τ thermal  (where τ thermal  is the thermal diffusion time constant for the YAG disk). Finally, the transit time across the gain area, or the extraction time, τ extraction  is &lt;&lt;τ thermal  to increase the likelihood that no thermal phase distortions will occur during the extraction time. 
     The first requirement of the illustrative high energy laser (HEL) implementation is that the pump to extraction region excursion time should be ˜⅕th fluorescence time, τ F . Secondly, the excursion of the pumped (excited) region across the extraction/lasing mode diameter should be much shorter than the thermal diffusion time constant for the disk gain medium (τ resonator &lt;&lt;τ THERMAL ). This is to minimize thermal gradients and, therefore, phase distortions during the extraction time. A third requirement is that the revolution time of the pumped and extracted region back to the original starting point should be longer than the thermal diffusivity time constant of the gain medium. 
     The first condition can be written as: 
                   f   =       10   ⁢     L   R         π   ⁢           ⁢     Dτ   F                 [   2   ]               
where f is the revolution frequency, L R  is the resonator dimension, D is the diameter of the rotating disk structure and τ F  is the fluorescence lifetime of the upper laser state. The thermal diffusion time constant for a slab architecture (across the thin dimension) is:
 
                     τ   THERMAL     =         t   disc   2     ⁢   ρ   ⁢           ⁢   C       10   ⁢   k               [   3   ]               
where t disc  is the thickness of the gain slab, ρ is the density, C is the specific heat, and k is the thermal conductivity of YAG. We can also express the optimum resonator dimension/diameter from the condition:
 
                           P   out     ⁢     τ   resonator         L   R   2       =           P   out     ⁢     τ   F         10   ⁢     L   R   2         =     χ   ⁢     hυ     σ   0             ,           [   4   ]               
where P out  is the output power, hν is the photon energy, σ 0  is the stimulated emission cross-section and χ is the multiplication factor (the goal is to have the intra-cavity equivalent fluence be ˜&gt;5× saturation fluence). From this condition, we can express the resonator cross-section dimension as:
 
                     L   R     =             P   out     ⁢     σ   0     ⁢     τ   F         10   ⁢   χ   ⁢           ⁢   hυ         .             [   5   ]               
Combining the expression for L R  with the expression for f and requiring that:
 
                   f   =       1     2   ⁢     τ   THERMAL         =       5   ⁢   k         t   disc   2     ⁢   ρ   ⁢           ⁢   C                 [   6   ]               
leads to an expression for the rotating disk structure diameter:
 
     
       
         
           
             
               
                 
                   D 
                   = 
                   
                     
                       
                         
                           t 
                           disc 
                           2 
                         
                         ⁢ 
                         ρ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         C 
                       
                       
                         5 
                         ⁢ 
                         π 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         k 
                       
                     
                     ⁢ 
                     
                       
                         
                           
                             
                               P 
                               out 
                             
                             ⁢ 
                             
                               σ 
                               0 
                             
                             ⁢ 
                             10 
                           
                           
                             
                               χ 
                               F 
                               τ 
                             
                             ⁢ 
                             hυ 
                           
                         
                       
                       . 
                     
                   
                 
               
               
                 
                   [ 
                   7 
                   ] 
                 
               
             
           
         
       
     
       FIG. 10  is a graph showing rotation frequency dependence on the disk diameter. 
     The resonator dwell time is: 
     
       
         
           
             
               
                 
                   
                     
                       τ 
                       resonator 
                     
                     = 
                     
                       
                         
                           L 
                           R 
                         
                         
                           π 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           Df 
                         
                       
                       = 
                       
                         
                           τ 
                           F 
                         
                         10 
                       
                     
                   
                   , 
                 
               
               
                 
                   [ 
                   8 
                   ] 
                 
               
             
           
         
       
         
         
           
             and 
           
         
       
    
     
       
         
           
             
               
                 
                   f 
                   = 
                   
                     
                       1 
                       
                         2 
                         ⁢ 
                         
                           τ 
                           THERMAL 
                         
                       
                     
                     . 
                   
                 
               
               
                 
                   [ 
                   9 
                   ] 
                 
               
             
           
         
       
     
       FIG. 10  shows illustrative revolution rates (frequencies) for nominal disk geometries as a function of power. 
     The number of disc subassemblies and the individual disk annular gain thickness will be determined by the cooling requirement and by the maximum practical active ion (Er) doping concentration in the YAG crystal host. In the illustrative embodiment, Er concentrations on the order of ˜1.4% are used. For this regime, the number of discs employed in a megawatt-class laser is ˜30 with resonator length of ˜3 meters. This is illustrated in  FIG. 11 . 
       FIG. 11  is a diagram which shows the dependence of the number of required disk sub-assemblies as a function of doping density for the three different power categories (calculated from the maximum extractable power equation): 
                       N   D     =     χ       η   extraction     ⁢     t   disc     ⁢     n   disc     ⁢     σ   0           ,           [   10   ]               
where n disc  is the number of disks and η extraction  is the extraction efficiency (assumed to be 50%). The further assumption is that the intra cavity-to-saturation intensity ratio is ˜6× (given a 65% out coupler reflectivity).
 
     The OPEG concept/architecture provides for a traceable disc subassembly design with minimized modifications for implementing a wide range of laser power performance which makes for an attractive modular design-implementation approach. 
     Thus, the present invention has been described herein with reference to a particular embodiment for a particular application. Those having ordinary skill in the art and access to the present teachings will recognize additional modifications applications and embodiments within the scope thereof. 
     It is therefore intended by the appended claims to cover any and all such applications, modifications and embodiments within the scope of the present invention. 
     Accordingly,