Patent Publication Number: US-10777964-B2

Title: Method and system for compact efficient laser architecture

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 14/820,375, filed Aug. 6, 2015, which is a divisional of U.S. patent application Ser. No. 13/284,525, filed Oct. 28, 2011, which claims the benefit of and priority to U.S. Provisional Patent Application No. 61/408,222, filed Oct. 29, 2010, the disclosures of which are hereby incorporated by reference in their entireties for all purposes. 
    
    
     STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     The United States Government has rights in this invention pursuant to Contract No. DE-AC52-07NA27344 between the U.S. Department of Energy and Lawrence Livermore National Security, LLC, for the operation of Lawrence Livermore National Laboratory. 
    
    
     BACKGROUND OF THE INVENTION 
     Projections by the Energy Information Agency and current Intergovernmental Panel on Climate Change (IPCC) scenarios expect worldwide electric power demand to double from its current level of about 2 terawatts electrical power (TWe) to 4 TWe by 2030, and could reach 8-10 TWe by 2100. They also expect that for the next 30 to 50 years, the bulk of the demand of electricity production will be provided by fossil fuels, typically coal and natural gas. Coal supplies 41% of the world&#39;s electric energy today, and is expected to supply 45% by 2030. In addition, the most recent report from the IPCC has placed the likelihood that man-made sources of CO 2  emissions into the atmosphere are having a significant effect on the climate of planet earth at 90%. “Business as usual.” baseline scenarios show that CO 2  emissions could be almost two and a half times the current level by 2050. More than ever before, new technologies and alternative sources of energy are essential to meet the increasing energy demand in both the developed and the developing worlds, while attempting to stabilize and reduce the concentration of CO 2  in the atmosphere and mitigate the concomitant climate change. 
     Nuclear energy, a non-carbon emitting energy source, has been a key component of the world&#39;s energy production since the 1950′s, and currently accounts for about 16% of the world&#39;s electricity production, a fraction that could—in principle be increased. Several factors, however, make its long-term sustainability difficult. These concerns include the risk of proliferation of nuclear materials and technologies resulting from the nuclear fuel cycle; the generation of long-lived radioactive nuclear waste requiring burial in deep geological repositories; the current reliance on the once through, open nuclear fuel cycle; and the availability of low cost, low carbon footprint uranium ore. In the United States alone, nuclear reactors have already generated more than 55,000 metric tons (MT) of spent nuclear fuel (SNF). In the near future, we will have enough spent nuclear fuel to fill the Yucca Mountain geological waste repository to its legislated limit of 70,000 MT. 
     Fusion is an attractive energy option for future power generation, with two main approaches to fusion power plants now being developed. In a first approach, Inertial Confinement Fusion (ICF) uses lasers, heavy ion beams, or pulsed power to rapidly compress capsules containing a mixture of deuterium (D) and tritium (T). As the capsule radius decreases and the DT gas density and temperature increase, DT fusion reactions are initiated in a small spot in the center of the compressed capsule. These DT fusion reactions generate both alpha particles and 14.1 MeV neutrons. A fusion burn front propagates from the spot, generating significant energy gain. A second approach, magnetic fusion energy (MFE) uses powerful magnetic fields to confine a DT plasma and to generate the conditions required to sustain a burning plasma and generate energy gain. 
     Important technology for ICF is being developed primarily at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL), assignee of this invention, in Livermore, California. There, a laser-based ICF project designed to achieve thermonuclear fusion ignition and burn utilizes laser energies of 1 to 1.3 MJ. Fusion yields of the order of 10 to 20 MJ are expected. Fusion yields in excess of 200 MJ are expected to be required in a central hot spot fusion geometry if fusion technology, by itself, were to be used for cost effective power generation. Thus, significant technical challenges remain to achieve an economy powered by pure ICF energy. 
     In addition to ICF applications, there is broad interest in the area of high-average-power lasers for materials processing, drilling, cutting and welding, military applications, and the like. Conventional high power laser designs utilize architectures with large footprints and associated costs. Thus, there is a need in the art for laser and amplifier architectures that are compact, providing high power output at reduced system cost. 
     SUMMARY OF THE INVENTION 
     The present invention relates generally to laser systems. More specifically, the present invention relates to methods and systems for generating high power laser beams using a four-pass amplifier architecture. Merely by way of example, the invention has been applied to an amplifier assembly utilizing either transverse pumping or end pumping of amplifiers in a compact architecture. The methods and systems can be applied to a variety of other laser amplifier architectures and laser systems. 
     According to an embodiment of the present invention, a laser amplifier module including an enclosure is provided. The laser amplifier module includes an input window, a mirror optically coupled to the input window and disposed in a first plane, and a first amplifier head disposed along an optical amplification path adjacent a first end of the enclosure. The laser amplifier module also includes a second amplifier head disposed along the optical amplification path adjacent a second end of the enclosure and a cavity mirror disposed along the optical amplification path. 
     According to another embodiment of the present invention, a method of amplifying a laser beam is provided. The method includes receiving an input beam, directing the input beam along a first direction, and amplifying the input beam a first time using a set of amplifiers. The amplification paths through the set of amplifiers are disposed along a second direction substantially orthogonal to the first direction. The method also includes reflecting the amplified beam using a first cavity mirror, amplifying the amplified beam a second time using the set of amplifiers, image relaying the twice amplified beam along the first direction, and reflecting the amplified beam using a second cavity mirror. The method further includes amplifying the twice amplified beam a third time using the set of amplifiers, reflecting the three times amplified beam using the first cavity mirror, amplifying the three times amplified beam using the set of amplifiers, and outputting the four times amplified beam. 
     According to a specific embodiment of the present invention, a quad-beam laser amplifier module including an enclosure is provided. The quad-beam laser amplifier module includes a set of four input ports disposed on a top surface of the enclosure and a set of four output ports disposed on the second end of the enclosure. The quad-beam laser amplifier module also includes a first amplifier head disposed at a first end of the enclosure, wherein the first amplifier head includes four amplifiers, a second amplifier head disposed at a second end of the enclosure, wherein the second amplifier head includes four amplifiers, and a cavity minor operable to reflect light into the second amplifier head. 
     Embodiments of the present invention provide an amplifier module in which the number of optics is reduced in comparison with conventional designs while increasing the efficiency with which pump light is delivered to the amplifier slabs, which can be suitable for high peak power and high average power applications (e.g., 23.3 cm×23.3 cm in the transverse dimensions). Additionally, embodiments of the present invention increase the depth of field in comparison with conventional designs, enabling the use of a number of amplifier slabs, for example, ten amplifier slabs per amplifier head. Embodiments of the present invention are not limited to ten amplifier slabs and fewer or greater numbers can be utilized as appropriate to the particular implementation. Some embodiments reduce beam distortion to provide a generally “square” beam, which is pumped using diode arrays that are imaged to the center of the amplifier head. 
     Numerous benefits are achieved by way of the present invention over conventional techniques. For example, embodiments of the present invention provide laser systems useful for Laser Inertial Fusion Engine (LIFE) applications, including pure fusion LIFE engines, other users of pulsed average power lasers, and for pumping of various laser media in order to generate ultra-short laser pulses. Moreover, embodiments of the present invention provide architectures for laser systems operating in the stored energy, high average power mode of operation with performance characteristics not available using conventional designs. These and other embodiments of the invention along with many of its advantages and features are described in more detail in conjunction with the text below and attached figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a simplified plan view of a transverse pumped amplifier system according to an embodiment of the present invention; 
         FIG. 1B  is a simplified side view of the transverse pumped amplifier system illustrated in  FIG. 1A ; 
         FIG. 1C  is a simplified cross-section view of a subdivided amplifier aperture according to an embodiment of the present invention; 
         FIG. 2A  is a simplified plan view of a transverse pumped four-bean amplifier system according to an embodiment of the present invention; 
         FIG. 2B  is a simplified side view of the transverse pumped four-beam amplifier system illustrated in  FIG. 2A ; 
         FIG. 3A  is a simplified plan view of an end pumped amplifier system according to an embodiment of the present invention; 
         FIG. 3B  is a simplified side view of the end pumped amplifier system illustrated in  FIG. 3A ; 
         FIG. 4A  is a simplified plan view of an end pumped four-beam amplifier system according to an embodiment of the present invention; 
         FIG. 4B  is a simplified side view of the end pumped four-beam amplifier system illustrated in  FIG. 4A ; 
         FIG. 5A  is a simplified plan view of a pump delivery architecture according to an embodiment of the present invention; 
         FIG. 5B  is a simplified side view of the pump delivery architecture illustrated in  FIG. 5A ; 
         FIG. 6  is a laser amplifier system according to an embodiment of the present invention; and 
         FIG. 7  is a simplified flowchart illustrating a method of providing an amplified laser beam according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
     Embodiments of the present invention relate to laser systems. More specifically, the present invention relates to methods and systems for generating high power laser beams using a four-pass amplifier architecture. Merely by way of example, the invention has been applied to an amplifier assembly utilizing either transverse pumping or end pumping of amplifiers in a compact architecture. The methods and systems can be applied to a variety of other laser amplifier architectures and laser systems. 
     As described more fully below, embodiments of the present invention provide an amplifier module operable to amplify one, two, four, or more beams in a close coupling arrangement to form, in a four-beam arrangement, a “quad” amplifier utilizing either end or transverse pumping of the amplifier slabs. Accordingly, as shown in Table 1 below, embodiments of the present invention provide a single or quad amplifier module with reduced volume per beam aperture in comparison with conventional techniques. Embodiments of the present invention provide methods and systems to reduce asymmetries in the gain profiles, providing uniform gain as a function of transverse position. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Architecture 
               
            
           
           
               
               
               
            
               
                   
                 Single Beam Amplifier Module 
                 Quad Beam Amplifier Module 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 Length 
                 Width 
                 Height 
                 Volume 
                 Length 
                 Width 
                 Height 
                 Volume 
               
               
                   
                 (m) 
                 (m) 
                 (m) 
                 (m 3 ) 
                 (m) 
                 (m) 
                 (m) 
                 (m 3 ) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 End Pumped 
                 8.32 
                 2.2 
                 1.35 
                 24.7 
                 8.32 
                 3.66 
                 2.38 
                 72.5 
               
               
                 Transverse Pumped 
                 9.5 
                 1.3 
                 1.28 
                 15.8 
                 9.5 
                 2.2 
                 2.38 
                 49.7 
               
               
                   
               
            
           
         
       
     
     Embodiments of the present invention provide a passive four pass architecture suitable for high average power operation.  FIG. 1A  is a simplified plan view of a transverse pumped amplifier module according to an embodiment of the present invention. Utilizing embodiments of the present invention, an amplifier module with dimensions of 9.5 m in length, 1.3 m in width, and 1.28 m in height is provided. Embodiments of the present invention are not limited to these particular dimensions, but these dimensions provide an indication of the compact size that the architectures discussed herein provide. Referring to  FIG. 1A , a laser beam (for example, provided by a preamplifier module) is injected into the amplifier system using injection mirror  110 . The laser beam may be multiplexed as suitable for high power operation. The injected beam reflects from mirror  112  down into the amplifier module and then reflects from polarizer  114  due to the polarization of the injected laser beam, which is aligned with respect to the transmission axis of polarizer  114 . According to an embodiment of the present invention, the injected laser beam is characterized by an s-polarization and polarizer  114  is aligned to reflect the s-polarization and pass the p-polarization, although other embodiments can utilize a p-polarization state or other suitable polarization state. In some embodiments, the beam is injected utilizing angular multiplexing through a transport telescope. 
     The beam passes through the quarter waveplate  116 . The quarter waveplate converts the s-polarized injected laser light to circular polarization (e.g., left-handed circular polarization) and lowers the B-integral. As described more fully below, the passive 4-pass architecture described herein utilizes the fact that right-handed polarization becomes left-handed upon normal incidence reflection from a mirror. The injected light then passes through two amplifier heads with image relaying used between amplifiers and end mirrors as described below. Referring to  FIG. 1A , mirror  120  directs the light into amplifier  124 , which is pumped along a transverse direction by diode arrays  126  and  128 . In embodiments of the present invention, amplifier  124  can include multiple amplifier slabs, also referred to as slablets. Elements  120 ,  122 ,  124 ,  126 , and  128  can be referred to as amplifier head  129 . Additional description related to transverse pumping of laser amplifiers is provided in U.S. patent application Ser. No. 12/940,869, entitled “Transverse Pumped Laser Amplifier Architecture,” filed on Nov. 5, 2010, the disclosure of which is hereby incorporated by reference in its entirety for all purposes. 
     As illustrated in  FIG. 1 , quarter waveplate  116  is positioned in a specific location in the architecture. In addition to this implementation, the quarter waveplate  116  could also be located in front of the back mirror  140  illustrated in  FIG. 1A  in order to allow injection of light with a linear polarization, (e.g., s-polarization in the example that follows), which would then pass through the quarter waveplate becoming circularly polarized (e.g., left hand circular polarization in this example), reflect off mirror  140 , changing the handedness of the beam (e.g., becoming right circularly polarized) and exit the quarter waveplate in the p-polarization state as appropriate for the second and third amplification passes, then change to s-polarization for the fourth amplification pass. The quarter waveplate in front of cavity mirror  140  could also be replaced by a 45° Faraday rotator in order to obtain the same net effect on the polarization. One of ordinary skill in the art would recognize many variations, modifications, and alternatives. 
     After a first amplification pass through amplifier  124 , light is reflected off of mirror  122  toward a relay telescope represented by lenses  130  and  132 . In addition to image relaying, the relay telescope provides spatial filtering functionality. A 90° polarization rotator  134  is positioned between amplifier head  129  and amplifier head  136 , which includes two turning mirrors and an amplifier transversely pumped by two diode arrays. The 90° polarization rotator  134  compensates for thermal birefringence in the amplifier slabs among other benefits. In embodiments in which the beam is in a circularly polarized state during amplification, thermal birefringence will tend to introduce ellipticity into the beam, which is removed by the multiple passes through polarization rotator  134 . In the illustrated embodiment, image relaying is utilized between the amplifier heads  129  and  136 . 
     Thermal birefringence is a potentially debilitating loss associated with isotropic gain media under thermal load. The adverse impacts of thermal birefringence has led some system designers to utilize Brewster&#39;s angle designs. After a first amplification pass through amplifier heads  129  and  136 , the beam is image relayed to cavity mirror  140 , where the circular polarization is modified from left-to-right (assuming the left handedness as discussed above). In some embodiments, mirror  140  is a deformable mirror operable to reduce or remove distortion from the amplified beam. As illustrated, a relay telescope is disposed along the optical path between the exit turning mirror of amplifier head  136  and the cavity mirror  140 . 
     The beam, after the first pass through amplifier heads  129  and  136 , reflects off mirror  140  and makes a second amplification pass through amplifier heads  136  and  129 . On passing through quarter waveplate  116 , the polarization is converted from right-handed circular polarization to p-polarization in this embodiment and, therefore, passes through polarizer  114 . The beam is then image relayed using a relay telescope including lenses  142  and  144  to Pockels cell  146 , through polarizer  148 , which is crosses with respect to polarizer  116 , to the second cavity mirror  150  then back through polarizer  148  and Pockels cell  146 . 
     On the third pass through amplifier heads  129  and  136 , the twice-amplified beam is in a left handed circular polarization state. After reflection off deformable mirror  140  and the fourth pass through heads  129  and  136 , the quarter waveplate  116  converts the polarization to the s-polarization, which results in reflection off of polarizer  114  and mirror  112  into the upper level of the amplifier module. A transport telescope at the level of mirror  112  and discussed in relation to  FIG. 1B  transmits the light to output window  152 . 
     Additional description related to Pockels cells suitable for use with embodiments of the present invention, particularly a Gap Coupled Electrode Pockels Cell, is provided in U.S. patent application Ser. No. 12/913,651, entitled “Electro-Optic Device with Gap-Coupled Electrode,” filed on Oct. 27, 2010 and hereby incorporated by reference in its entirety for all purposes. 
     Referring to  FIG. 1A , the amplifier heads  129  and  136  are excited using transverse pumping. As described in relation to  FIGS. 3A-4B , the amplifiers can also be excited using end pumping. In these pumping designs, it is possible to utilize state-of-the-art high power diode bar technology combined with low diode pitch to create very high array intensities. In some embodiments, high pump intensities are provided through the use of polarization combining to double the effective array intensity. This pump source is then image relayed to the center of the amplifier. This architecture maintains the source intensity and divergence of the diode bars, which means the pump radiation has a relatively large Rayleigh range, providing efficient flat pump profile intensity to relatively thick amplifiers (for example, where the amplifier depth is close to the amplifier width and height). Additional description related to polarization combining techniques and transverse pumping of laser amplifiers is provided in U.S. patent application Ser. No. 12/940,869, entitled “Transverse Pumped Laser Amplifier Architecture,” filed on Nov. 5, 2010, the disclosure of which is hereby incorporated by reference in its entirety for all purposes. 
       FIG. 1B  is a simplified side view of the transverse pumped amplifier system illustrated in  FIG. 1A . Referring to  FIG. 1B , the relay telescope between the two amplifier heads  129  and  136  is illustrated by lenses  130  and  132  and the transport telescope used to direct the beam to the output window is illustrated by lenses  160  and  162 . In some embodiments, longitudinal spatial filters (not shown) are utilized to improve the system performance. Additional description related to relay telescopes and spatial filters are provided in U.S. patent application Ser. No. 12/544,988, entitled “Spatial Filters for High Average Power Lasers,” filed on Aug. 20, 2009, the disclosure of which is hereby incorporated by reference in its entirety for all purposes. 
     Spatial filtering serves as one form of gain isolation, limiting parasitic light between the two high gain amplifiers. Additionally, spatial filtering resets the B-integral, enabling higher extraction efficiency while maintaining beam quality on the last pass through the amplifiers. Relay imaging improves extraction efficiency by reducing vignetting associated with the multiplexing angle and by enabling higher contrast beams with larger mode fill to extract the power from the amplifier. Additionally, relay imaging enables lower quality optics to be used while maintaining a high contrast beam. Although relay telescopes are illustrated in some embodiments of the present invention in order to improve the beam quality, they are not required by the present invention and are optional in some designs. Additionally, the use of spatial filters between the amplifier heads are also optional for designs less constrained by parasitic issues. 
     Therefore, relay telescopes, spatial filters, and the like are not required by the present invention and may be optional in some implementations. One of ordinary skill in the art would recognize many variations, modifications, and alternatives. 
     As illustrated in  FIG. 1B , polarization combining is used in the diode array pumps  126  and  128  to increase the pump intensity coupled to the amplifier  124 . As described more fully throughout the present specification, a variety of polarization combination techniques can be implemented in conjunction with embodiments of the present invention. Light is reflected by mirror  112  and the polarizer  114  into the lower plane including the amplifier heads and the relay telescope  130  and  132 . In the side view illustrated in  FIG. 1B , the gas cooling system is illustrated as providing helium gas cooling to the amplifier slabs in each of the amplifiers. Embodiments of the present invention provide line replaceable units (LRUs) with gas cooling and electrical and other connections that are removable for LRU replacement. One of ordinary skill in the art would recognize many variations, modifications, and alternatives. 
     The folding of the beam line between the amplifier heads to the first cavity mirror and the second cavity mirror after the first two amplification passes enables a compact design not available using conventional designs. The compact design discussed herein enables high power operation that makes these designs suitable for a wide variety of applications, particularly applications in which the laser amplifier module is transportable. 
     Although  FIGS. 1A and 1B  illustrate a single aperture for each of the amplifiers in the amplifier heads, this is not required by embodiments of the present invention. In alternative embodiments, the aperture is subdivided into smaller contiguous apertures to decrease the beam aperture dimension.  FIG. 1C  is a simplified cross-section view of a subdivided amplifier aperture according to an embodiment of the present invention. In the amplifier illustrated in  FIG. 1C , the aperture is subdivided into four subapertures  170 A,  170 B,  170 C, and  170 D across the transverse direction. Accordingly, during amplification, light passes through the amplifier into the plane of the image. In these embodiments, the input beam, rather than being a single beam, can be provided as a set of generally parallel and coherent input beams. The subdivision of the aperture facilitates the manufacturing process since optical elements with smaller transverse dimensions can be utilized. Additionally, regions  172  between the optical elements have increased propagation losses, reducing the impact of transverse amplified spontaneous emission. Although four subapertures  170 A,  170 B,  170 C, and  170 D are illustrated in  FIG. 1C , the present invention is not limited to this particular configuration and other subaperture configurations can be utilized according to embodiments of the present invention. 
     In addition to or rather than the amplifiers, other system components can be formed using subaperture techniques, including the polarization rotators, the frequency converters, the Pockels cell, and the like. In embodiments in which the amplifier is provided as a subaperture system, the diode array pumps can be divided as well, providing for gaps that can include cooling elements or other suitable elements. One of ordinary skill in the art would recognize many variations, modifications, and alternatives. In some embodiments, the preamplifier module (e.g., a fiber oscillator) can be integrated into the amplifier module rather than being provided from an external source. In embodiments in which spatial filter/relay imaging is included in the amplifier module, there is associated space that can be used for the preamplifier module location (i.e., resulting from the amplifier column height relative to the beam height). Locating the preamplifier module in the amplifier module will reduce the impact of issues related to an external location of the preamplifier module, which may require a rigid connection via image relay telescope to the amplifier module. Additionally, vibrations or displacements of the preamplifier module relative to the amplifier module can result in pointing errors, improper injection, and/or efficiency loss, issues which are ameliorated by the integration of the preamplifier module in the amplifier module. 
       FIG. 2A  is a simplified plan view of a transverse pumped four-bean amplifier system according to an embodiment of the present invention. In the embodiment illustrated in  FIGS. 2A and 2B , an amplifier module with dimensions of 9.5 m in length, 2.2 m in width, and 2.38 min height is provided, although embodiments of the present invention are not required to have these exact dimensions. The architecture illustrated in  FIGS. 2A and 2B  shares some similarities with the architecture illustrated in  FIGS. 1A and 1B , but utilizes four amplifiers per amplifier head in a two-by-two configuration as illustrated in  FIG. 2B . Thus, these embodiments can be referred to as a “quad” configuration. 
     Referring to  FIG. 2A , light is injected as illustrated by arrows  210  toward mirrors  212  and  214  and polarizers  216  and  218 . Thus, two beam paths are provided in comparison with the single beam path illustrated in  FIGS. 1A and 1B . Light in each of the beam paths then propagates toward a first quad amplifier head  220 . Amplifier head  220  includes four amplifiers, each of which can include a set of amplifier slabs. The amplifiers are pumped by four diode arrays in a transverse pumping arrangement, with one diode array pumping each of the amplifiers. Although a two-by-two configuration is illustrated in  FIG. 2B , this particular configuration is not required and a one-by-two configuration or other suitable configurations can be utilized within the scope of the invention. Because the amplifiers are only pumped from one side, an asymmetry in the transverse gain profile may result. As illustrated in  FIG. 2A , light amplified by the inner amplifiers of amplifier head  220  are amplified by the outer amplifiers of amplifier head  230 , reducing the gain asymmetry. 
       FIG. 2B  is a simplified side view of the transverse pumped four-beam amplifier system illustrated in  FIG. 2A . In this side view, the two-by-two configuration of the amplifiers in the amplifier heads is clearly illustrated as well as the amplifier slab cooling system. In this configuration, light is injected into the amplifier module at top and bottom levels and then amplified in the plane of the two sets of amplifiers disposed between the top and bottom levels. Thus, the quad configuration provides a variation on the configuration illustrated in  FIGS. 1A and 1B  in a mirrored orientation. One of ordinary skill in the art would recognize many variations, modifications, and alternatives. 
     Although some elements are omitted for the purposes of clarity, the relay telescopes between the two amplifier heads are illustrated in the lower portion of  FIG. 2A  and the transport telescopes are illustrated at the top and bottom portions of  FIG. 2B . 
       FIG. 3A  is a simplified plan view of an end pumped amplifier system according to an embodiment of the present invention. In the embodiment illustrated in  FIGS. 3A and 3B , an amplifier module with dimensions of 8.32 m in length, 2.2 m in width, and 1.35 m in height is provided, although embodiments of the present invention are not required to have these exact dimensions. As described more fully below, the end pumping of the amplifiers results in a shorter amplifier module than in other designs utilizing transverse pumping. 
     Light is injected into the amplifier module through mirror  310  and reflected off of mirror  312  into the lower portion of the amplifier module where it is reflected off of polarizer  314  toward amplifier head  329 . In the embodiment illustrated in  FIGS. 3A and 3B , the amplifier head  329  includes diode pump arrays  326  and  328 , which are optically coupled to the amplifier  324  using mirrors  330  and  332  as well as other suitable optics. Mirrors  320  and  322  are dichroic, passing light at the pump wavelength and reflecting light at the laser wavelength. Thus, the pump light passes through mirrors  320  and  322  to pump amplifier  324  using a face pumping configuration, which can include a number of amplifier slabs. 
     Because of the folded configuration for the diode array pumps with respect to the amplifier utilizing mirrors  330  and  332 , the length of the amplifier module can be decreased in comparison with other configurations. Other components in common with  FIGS. 1A and 1B  are illustrated as will be evident to one of ordinary skill in the art. As examples, a relay telescope including lenses  330  and  332 , 90° polarization rotator  334  and cavity mirror  340  are illustrated. Optics along the optical path after the first two amplification passes include Pockels cell  346 , polarizer  348  and cavity mirror  350 . 
       FIG. 3B  is a simplified side view of the end pumped amplifier system illustrated in  FIG. 3A . As illustrated in  FIG. 3B , polarization combining of the diode arrays is used to increase the pump intensity. In this embodiment, light from two arrays at the top and bottom of the set of arrays and with a first polarization is reflected twice to propagate collinearly with light from two arrays in the middle of the set of arrays. Other configurations are possible, including two arrays oriented at right angles to each other with a polarization sensitive reflector disposed between the arrays at an angle of 45° to each array. Light from a first array passes through the polarization sensitive reflector while the light from the second array is reflected to become collinear with the light from the first array. Thus, embodiments of the present invention can utilize various polarization combination designs for the diode pumps. One of ordinary skill in the art would recognize many variations, modifications, and alternatives. Transport telescope including lenses  360  and  362  used to transport light out of the cavity through output window  352  is illustrated. Utilizing the embodiment illustrated in  FIGS. 3A and 3B , a fusion class laser system is provided that produces a 6.3 kJ beam at 15 Hz in an amplifier module with dimensions of 1.3 m×2.2 m×8.3 m. 
       FIG. 4A  is a simplified plan view of an end pumped four-beam amplifier system according to an embodiment of the present invention. In the embodiment illustrated in  FIGS. 4A and 4B , an amplifier module with dimensions of 8.32 m in length, 3.66 m in width, and 2.3.8 m in height is provided, although embodiments of the present invention are not required to have these exact dimensions. The architecture illustrated in  FIGS. 4A and 4B  shares some similarities with the architecture illustrated in  FIGS. 3A and 3B , but utilizes four amplifiers per amplifier head in a two-by-two configuration as illustrated in  FIG. 4B . Thus, these embodiments can be referred to as a “quad” configuration. For purposes of clarity, some of the optical elements that are common between the single amplifier and quad amplifier configurations are omitted. One of ordinary skill in the art would recognize many variations, modifications, and alternatives. 
     Referring to  FIG. 4A , light is injected as illustrated by arrows  410  toward mirrors  412  and  414  and polarizers  416  and  418 . Thus, two beam paths are provided in comparison with the single beam path illustrated in  FIGS. 3A and 3B . Light in each of the beam paths then propagates toward a first quad amplifier head  420 . Amplifier head  420  includes four amplifiers, each of which can include a set of amplifier slabs. The amplifiers are pumped by four diode arrays in an end pumping arrangement, with one diode array pumping each of the amplifiers. Although a two-by-two configuration is illustrated in  FIG. 4B , this particular configuration is not required and a one-by-two configuration or other suitable configurations can be utilized within the scope of the invention. 
       FIG. 4B  is a simplified side view of the end pumped four-beam amplifier system illustrated in  FIG. 4A . In this side view, the two-by-two configuration of the amplifiers in the amplifier heads is clearly illustrated as well as the amplifier slab cooling system. In this configuration, light is injected into the amplifier module at top and bottom levels and then amplified in the plane of the two sets of amplifiers disposed between the top and bottom levels. Thus, the quad configuration provides a variation on the configuration illustrated in  FIGS. 3A and 3B  in a mirrored orientation. One of ordinary skill in the art would recognize many variations, modifications, and alternatives. 
     As discussed in relation to  FIG. 3A , mirrors  430  and  432  reflect pump light from diode pump arrays  426  and  428 , respectively, toward amplifier  424 . Mirrors  420  and  422  are dichroic, passing light at the pump wavelength and reflecting light at the laser wavelength. Thus, the pump light passes through mirrors  420  and  422  to pump amplifier  424 , which can include a number of amplifier slabs. Pumping of the other amplifiers is accomplished using similar arrangements of diode pump arrays, optics, and the like. A second amplifier head  436  includes another quad of end pumped amplifiers. Light exits the amplifier module through output windows  452  and  454 . 
       FIG. 5A  is a simplified plan view of a pump delivery architecture according to an embodiment of the present invention. As illustrated in  FIG. 5A , a diode pump array  510  is coupled through a homogenizer  512  and duct  514  to pump amplifier  520 . The optical elements illustrated in  FIG. 5A  can be utilized in conjunction with the architectures illustrated in  FIGS. 1A-4B  as elements of the diode pumping systems.  FIG. 5B  is a simplified side view of the pump delivery architecture illustrated in  FIG. 5A . In this embodiment, a Fresnel prism  530  is utilized to collect and focus light from the diode pump array  510 . In an embodiment, the distance between the diode pump array  510  and the amplifier  520  is 5 m, although other distances can be utilized. One of ordinary skill in the art would recognize many variations, modifications, and alternatives. 
       FIG. 6  is a laser amplifier system according to an embodiment of the present invention. The laser amplifier system  600  illustrated in  FIG. 6  utilizes a dual amplifier architecture in a cavity utilizing image relaying to improve system efficiency. An input beam  605  is injected into the cavity using injection mirror  607 . In an embodiment, the input beam is a laser pulse having an energy of 0.9 J. The input beam reflects off mirror  662 . The polarization of the input beam is a predetermined polarization (e.g., s-polarization) so that the input beam reflects off of polarizer  630 , which is aligned to reflect the polarization state of the input beam  605 . The input beam passes through quarter-waveplate  650  and makes a first pass through amplifier  616  and amplifier  614 . 
     Disposed between amplifier  616  and amplifier  614  is a spatial filter  642  in the form of a telescope and a pinhole (not shown). Other spatial filters can be utilized according to embodiments of the present invention and the pinhole filter illustrated is merely provided by way of example. After the first amplification pass through amplifier  616 , the beam passes through a polarization rotator  630  (e.g., a quartz rotator) before the first amplification pass through amplifier  614 . A relay telescope  624  is provided to relay the image formed at the center of amplifier  614  to a reflective surface of mirror  620 . Image relaying is illustrated by the group of aligned squares illustrated at the center of amplifier  614  and the surface of mirror  620  as well as other locations in the system. 
     The amplified light is reflected from mirror  620 , passes back through relay telescope  624 , and makes a second pass through the set of amplifiers  614  and  616 . After passing through the quartz rotator  632  and the quarter-waveplate  650  two times, the polarization of the amplified beam is rotated to enable the beam to pass through polarizer  630 . The beam passes through relay telescope  626 , Pockels cell  652  and polarizer  660 , which is crossed with respect to polarizer  630 . Relay telescope  626  relays an image at the center of amplifier  616  to the reflective surface of mirror  622 . The intensity of the amplified beam at Pockels cell  652  is produced by two amplification passes through the set of amplifiers. Although the input beam may have passed through multiple amplifier slabs in each amplifier  614  and  616 , the beam at Pockels cell  652  is referred to as a twice amplified beam. The Pockels cell is activated to rotate the polarization of the twice amplified beam by half a wave so that it passes through polarizer  630  as the beam propagates toward the amplifiers. In an alternative embodiment, the Pockels cell could be a quarter-wave Pockels cell and polarizer  660  would be replaced with a quarter waveplate, for example, positioned adjacent relay telescope  626  to provide for polarization rotation. 
     After two more passes through the amplifiers, the beam is reflected from polarizer  630  and mirror  662  towards the final optic  672 . The beam, after four amplification passes, is transmitted through spatial filter  640  and frequency converter  670 . Relay telescope  646  relays an image of the beam at the frequency converter  670  to the final optic  672 . In some embodiments, a neutron pinhole is utilized to protect the amplifier system from neutrons emitted by fusion events. 
       FIG. 7  is a simplified flowchart illustrating a method of providing an amplified laser beam according to an embodiment of the present invention. The method  700  includes receiving an input beam and directing the input beam along a first direction ( 710 ) and amplifying the input beam a first time using a set of amplifiers ( 712 ), also referred to as amplifier heads. The amplification paths through the set of amplifiers are disposed along a second direction substantially orthogonal to the first direction. As illustrated in  FIG. 1A , the amplification path through the amplifier heads is generally aligned with the width of the amplifier module, which is orthogonal to the length of the amplifier module. Thus, in the illustrated embodiment, the first direction is along a longitudinal direction of the amplifier module, which directs the beam from a central portion of the amplifier module toward a first end of the amplifier module at which a first amplifier head is positioned. 
     The method also includes reflecting the amplified beam using a first cavity mirror ( 714 ) and amplifying the amplified beam a second time using the set of amplifiers ( 716 ). In some embodiments, the first cavity mirror is a deformable mirror that can be used to compensate for distortions in the beam. The method further includes image relaying the twice amplified beam along the first direction ( 718 ) and reflecting the amplified beam using a second cavity mirror ( 720 ). After the first two amplification passes, the method can include rotating a polarization state of the twice amplified beam using a Pockels cell in order to enable the twice amplified beam to be amplified two additional times before being coupled out of the amplifier module. 
     Additionally, the method includes amplifying the twice amplified beam a third time using the set of amplifiers ( 722 ), reflecting the three times amplified beam using the first cavity mirror ( 724 ), amplifying the three times amplified beam using the set of amplifiers ( 726 ), and outputting the four times amplified beam. In some embodiments, the input beam and the four times amplified beam are characterized by a linear polarization, for example, an s-polarization or a p-polarization. Moreover, as discussed in relations to  FIGS. 1A and 1B , the amplified beam, the twice amplified beam, and the three times amplified beam can be characterized by a circular polarization during the amplification passes through the amplifier heads. 
     According to embodiments of the present invention, image relaying is performed relay imaging between the set of amplifiers, for example, performing image relaying between amplifying the three times amplified beam using the set of amplifiers and outputting the four times amplified beam. 
     It should be appreciated that the specific steps illustrated in  FIG. 7  provide a particular method of providing an amplified laser beam according to an embodiment of the present invention. Other sequences of steps may also be performed according to alternative embodiments. For example, alternative embodiments of the present invention may perform the steps outlined above in a different order. Moreover, the individual steps illustrated in  FIG. 7  may include multiple sub-steps that may be performed in various sequences as appropriate to the individual step. Furthermore, additional steps may be added or removed depending on the particular applications. One of ordinary skill in the art would recognize many variations, modifications, and alternatives. 
     It is also understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims.