Abstract:
Using a thin disk laser gain element with an undoped cap layer enables the scaling of lasers to extremely high average output power values. Ordinarily, the power scaling of such thin disk lasers is limited by the deleterious effects of amplified spontaneous emission. By using an undoped cap layer diffusion bonded to the thin disk, the onset of amplified spontaneous emission does not occur as readily as if no cap layer is used, and much larger transverse thin disks can be effectively used as laser gain elements. This invention can be used as a high average power laser for material processing applications as well as for weapon and air defense applications.

Description:
The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to thin disk lasers, and more specifically, it relates to a means for scaling the transverse area of the laser gain sample to scale the average power of laser systems based on the thin disk laser. 
     2. Description of Related Art 
     U.S. Pat. No. 5,553,088 is directed to a laser amplifying system having a solid body arranged in a laser radiation field and including a laser active material that is pumped with a pumping light source. The solid body has a cooling surface and transfers heat created therein to a solid cooling element via the cooling surface. In this manner, a temperature gradient results in the solid body in a direction towards the cooling surface. The solid cooling element forms a carrier for the solid body. The laser radiation field propagates approximately parallel to the temperature gradient in the solid body. By enabling heat to be transferred to the solid cooling element via the cooling surface, this structure enables the solid body to be pumped at a high pumping power. Further, since the laser radiation field propagates approximately parallel to the temperature gradient in the solid body, the radiation field sees the same temperature gradient in all cross-sectional areas. Thus, the temperature gradient does not lead to an adverse effect on the beam quality of the laser radiation field at high pumping power. 
     Although the thin-disk or active mirror laser architecture is a well known and demonstrated approach to generating laser radiation, its ability to scale to high-average power is limited by transverse amplified spontaneous emission (ASE). The thin disk is motivated as a gain element for high beam quality lasers because heat is removed from the back face of the disk. This geometry leads to a situation in which the transverse thermal gradients in the laser gain sample are substantially reduced, and even completely eliminated. This allows the possibility of energy extraction in a high quality laser beam that suffers little of no optical distortion due to transverse thermal gradients. 
     To scale the average power of laser systems based on this approach, one must scale the transverse area of the laser gain sample. Although this scaling approach works up to a point, eventually the deleterious effects of transverse ASE limit further scaling. The present invention specifically addresses this ASE limitation to scaling by substantially reducing the solid angle over which spontaneously emitted photons are trapped in the laser sample. It is this reduction in the trapped solid angle of spontaneously emitted photons that enables the thin disk laser to be substantially scaled in power output beyond what has been available. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to scale thin disk lasers to obtain high average power values. 
     Using a thin disk laser gain element with an undoped cap layer enables lasers to be scaled to extremely high average output power values. Ordinarily, the power scaling of such thin-disk is limited by the deleterious effects of amplified spontaneous emission. By using an undoped cap layer diffusion bonded to the thin disk, the onset of amplified spontaneous emission does not occur as readily as when no cap is used, and much larger transverse size thin disks can be effectively used as laser gain elements. 
     In a conventional thin disk laser system, pump radiation passes through a dichroic beamsplitter and an output coupler to optically pump a thin disk of laser material. Heat generated in the laser crystal is drawn away from the crystal, in the downward direction, into a cooling block. The laser resonator is formed by a highly reflective coating on the side of the thin disk laser sample that is in contact with the cooling block, and the output coupler laser mirror that is coated to allow the pump radiation to pass through it. The thin disk geometry insures that heat will flow substantially in the downward direction in the sample and so result in no thermal gradient in a direction transverse to the laser axis. 
     Due to the total internal reflection of spontaneously emitted photons within the thin disk at its large face that is not contacted to anything, amplified spontaneous emission limits the transverse size of the thin disk that can be efficiently utilized in a laser system. The present invention reduces the solid angle over which spontaneously emitted photons are trapped and so allows the transverse size of the thin disk to be substantially increased before the deleterious effects of ASE become apparent. The present invention is constructed with identical elements as in the conventional thin disk laser described above, with the addition of an undoped crystal affixed to the opposite side of the thin disk laser sample that is in contact with the cooling block. The undoped crystal is near index matched to the thin disk crystal. 
     The invention differs from the conventional thin disk geometry due to the inclusion of the undoped cap layer that, in one embodiment, is diffusion bonded to the laser crystal. Because the surface of the thin disk that was previously not contacted to anything is now contacted to an undoped crystal that is near index matched to the thin disk crystal, spontaneously emitted photons are not trapped by total internal reflection at this face of the thin disk. Because photons that impinge on this diffusion bonded surface are not confined to the gain loaded crystal, they are not as effective as they previously were in generating ASE. In effect, the use of the undoped cap layer has transformed the thin disk from a geometry in which ASE was largely trapped within the thin disk to a geometry in which the ASE is unconfined. The unconfined geometry of the diffusion bonded sample allows scaling to much higher power level lasers than would be possible without the use of the cap layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a conventional thin disk laser system. 
     FIG. 2 shows the basic elements of the present invention, which is constructed with identical elements as described in FIG. 1 with the addition of an undoped crystal  22  affixed to the opposite side of the thin disk laser sample that is in contact with the cooling block. 
     FIG. 3 shows a laser system based on this approach. 
     FIG. 4 shows an expanded view of the thin disk with a diffusion bonded cap layer. 
     FIGS. 5A and 5B show a high average power gain module based on a thin disk, diffusion bonded to an undoped lens duct. 
     FIG. 6 shows the lens duct formed by a focusing lens and canted surfaces. 
     FIG. 7 shows a hollow lensing duct designed such that the focal length of the lens at the large end of the device has a focal length approximately equal to the axial length of the device. 
     FIGS. 8A and 8B show that the sides of the hollow lensing duct can be continuously graded shapes that take a rectangular input end down to an output end, which may take on variously shaped exit holes beyond simple squares or circles, such as rectangles, octagons and ovals. 
     FIG. 9 shows an embodiment of a 100 kWatt class laser. 
     FIG. 10 shows a scaleable multimodule laser weapon system. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A conventional thin disk laser system is shown schematically in FIG.  1 . Pump radiation  10  passes through a dichroic beamsplitter  12  and output coupler  14  to impinge on a thin disk  16  of laser material at, or near, normal incidence. Heat generated in the laser crystal  16  is drawn away from the crystal, in the downward direction in the figure, into the cooling block  18 . The laser resonator is formed by a highly reflective coating  20  on the side of the thin disk laser sample  16  that is in contact with the cooling block  18 , and the output coupler laser mirror  14  that is coated to allow passage of the pump radiation. An output beam  19  is shown in the figure to be reflected from dichroic beamsplitter  12 . The thin disk geometry insures that heat will flow substantially in the direction of the cooling block  18  in the sample and so result in no thermal gradient in a direction transverse to the laser axis. 
     Due to the total internal reflection of spontaneously emitted photons within the thin disk at its large face that is not contacted to anything, amplified spontaneous emission limits the transverse size of the thin disk that can be efficiently utilized in a laser system. The present invention reduces the solid angle over which spontaneously emitted photons are trapped and so allows the transverse size of the thin disk to be substantially increased before the deleterious effects of ASE become apparent. An embodiment of the gain medium of the present invention has a thickness that is less than about 1 mm. FIG. 2 illustrates the invention, which is constructed with identical elements as described in FIG. 1, with the addition of an undoped crystal  22  affixed to the opposite side of the thin disk laser sample  16  that is in contact with the cooling block  18 . Undoped crystal  22  is near index matched to the thin disk crystal. The requirement on index matching can be stated in terms of the gain that would be seen by a ray traveling in the vertical direction of the thin disk shown in FIG.  2 . Calling G the 1-way gain seen by such a ray, the gain seen by a ray that travels through the sample two ways, first down hitting the high reflector and then back up hitting the index matched interface, will be given by G 2 . To ensure that the fractional energy of this ray which is reflected back into the sample on hitting the index matched interface is small so as not to contribute to amplified spontaneous emission, the condition that must be satisfied is 
     
       
         G 2 R&lt;&lt;1 
       
     
     where R is the reflectivity due to any index mismatch at the diffusion bonded interface. Calling the index of the sample n 0 , and the index mismatch between the sample and the capping layer Δn, the reflectivity R that would be seen by a ray intercepting the interface at normal incidence is given by 
     
       
         R=(((n 0 +Δn)/n 0 −1)/((n 0 +Δn)/n 0 +1)) 2 . 
       
     
     Combining the previous two equations and solving for Δn, yields the condition to be satisfied by the index matching as 
     
       
         Δn&lt;&lt;2n/(G−1). 
       
     
     The invention differs from the conventional thin disk geometry due to the inclusion of the undoped cap layer  22  that, in one embodiment, is diffusion bonded to the laser crystal. Because the surface of the thin disk that was previously not contacted to anything is now contacted to an undoped crystal that is near index matched to the thin disk crystal, spontaneously emitted photons are not trapped by total internal reflection at this face of the thin disk. Because photons that impinge on this diffusion bonded surface are not confined to the gain loaded crystal, they are not as effective as they previously were in generating ASE. In effect, the use of the undoped cap layer has transformed the thin disk from a geometry in which ASE was largely trapped within the thin disk to a geometry in which the ASE is unconfined. By direct calculation, it has been shown that the unconfined geometry of the diffusion bonded sample allows scaling to much higher power level lasers than would be possible without the use of the cap layer FIG. 3 shows a laser system based on this approach. In one embodiment, the active laser crystal  16  is YbAG with Yb 3+  serving as the lasing ion. As in FIGS. 1 and 2, the laser crystal has an undoped, index-matched crystal  22  that is diffusion bonded to laser crystal  16 . Pump radiation is delivered to the thin disk from a diode pump array  24 , through a hollow non-imaging beam delivery optic  26  described below. The diode array is configured with a hole to allow the 1.03 μm laser radiation to exit. 
     FIG. 4 shows an expanded view of the thin disk  16  with a diffusion bonded cap layer  22 . The pump radiation is delivered to the thin disk from the diode pump array shown in FIG. 3, and is focused by the hollow lensing duct  26  through the undoped crystal and onto the thin disk laser crystal  16 . In one variation of this embodiment, a tapered undoped crystal  28  is placed between the hollow lens duct  26  and the undoped cap layer  22  to provide a further means for reducing ASE and to prevent the laser beam from damaging the hollow lens duct. The undoped cap layer  22  is attached to a high performance cooled backplane  18 . The high performance cooled backplane  18  is described in U.S. Pat. No. 5,548,605 incorporated herein by reference. The incorporated patent is specifically directed to a monolithic microchannel heatsink. Although at the time of invention it was directed toward cooling diode arrays, the backplane cooler part of the invention is applicable to cooling any large two dimensional area as required in the present thin disk laser. 
     FIGS. 5A and 5B show a high average power gain module based on a thin disk  16  diffusion bonded to an undoped lens duct  50 . In one embodiment, the thin disk  16  comprises Yb:YAG and the lens duct  50  comprises undoped YAG. Pump light from a laser diode array  52  is delivered through the undoped lens duct  50  onto the thin disk  16 . Embodiments of lens duct designs usable in the present invention are described in U.S. Pat. No. 5,307,430, which is incorporated herein by reference. The cap layer  22  can be an integral part of the undoped lens duct  50 , or can be another section of undoped, index matched material that is bonded (e.g., diffusion bonded) to the lens duct and the thin disk. The thin laser disk  16  is affixed directly to a microchannel backplane cooler  18 , and does not include a highly reflective coating on the side of the thin disk laser sample that is in contact with the cooling block. This embodiment includes a brewster angled optical cavity comprising an output coupler  54  and a high reflector  56 . 
     The hollow non-imaging beam delivery optic  26  that is included in the embodiments of FIGS. 3 and 4 avoids obstructing access to the end of the this laser disk by extending the basic idea disclosed in U.S. Pat. No. 5,307,430 to a hollow lens duct. These hollow lens ducts have a lens at their input end with a small hole located in the lens to allow optical access to the end of the laser rod or slab at which the lens duct is located. 
     In FIG. 6, the lens duct  110  is formed by a focusing lens  112  and canted surfaces  114  that transfer pump light down to the small end of the lens duct where it is delivered to the thin disk laser (not shown). Additionally, the laser diode array  118  is configured to allow the laser cavity radiation to exit the pump delivery assembly without any interference from these components. In this laser configuration, pump light from laser diode array  118  is focused by lens duct  110  into the thin disk laser. In general the pump light that is delivered through the hollow lensing duct to the thin laser disk suffers some depolarization due to the fact that it may make multiple reflections in traversing the lensing duct that do not preserve polarization. 
     The hollow lensing duct shown in FIG. 7 is designed such that the focal length of the lens  180  at the large end of the device has a focal length approximately equal to the axial length of the device. This lens serves to focus down light, which approaches from the left-hand side in the figure, to a focal spot located near the end of the tapered region of the lensing duct. The hole  182  in the lens is located such that it is on the axis of the lensing duct and is approximately of the same size or slightly larger than the exit hole  184  at the small end of the lensing duct. The reflective sides  186 ,  188  of the hollow lens duct are shown to be planar. However, other configurations are possible. For example, the reflective sides can be continuously curved, as in a funnel, with a round input end and a round output end. Alternatively, as shown in FIGS. 8A and 8B, the sides can be continuously graded shapes that take a rectangular input end  190  down to an output end  192 , which may take on variously shaped exit holes beyond simple squares or circles, such as rectangles, octagons and ovals. 
     The reflective sides of the lens duct can be fabricated by machining an appropriately shaped cavity in a solid piece of metal, followed by polishing, and, if appropriate, applying a highly reflective coating such as gold or silver. Alternatively, these sides can be made out of individual pieces of a material such as glass or metal with an appropriate metallic or dielectric coating applied to their surface so as to render them highly reflective. 
     The lens at the input surface will generally be an ordinary, commercially available lens that is modified by cutting it to the shape of the input aperture and by fabricating a hole in its center to allow optical access to the end of the laser rod or slab. Although the optical access hole  194  is shown to be round in FIG. 8A, it can have other shapes if appropriate. For example, if a hollow lens duct were to be used to pump a laser slab with a rectangular input aperture, then it might be advantageous to have the optical access hole fabricated with a rectangular cross-section to match this aperture. 
     The invention is usable in an aggressive laser architecture particularly suited to the needs of point defense for aircraft and ships: the High-Brightness Tactical Engagement Laser (High-BriTE). The laser employs a thin Yb 3 Al 5 O 12  gain element oriented such that the laser beam propagates along the same direction as the heat flow, resulting in no thermal distortions to first order. A more advanced embodiment of the laser system employs multiple thin Yb 3 Al 5 O 12  gain elements oriented such that the laser beam propagates along the same direction as the heat flow, resulting in no first order thermal distortions. By using an extremely thin gain element, operation is possible at very high thermal power densities before stresses rupture the material, resulting in unprecedented compactness and performance at high average powers. Furthermore, the output power of this design scales with the area of the gain region, which simultaneously increases the area over which heat is removed. Edge effects, which may result in some beam distortions, are also minimized at high power and large aperture. Modeling performed to date indicates that amplified spontaneous emission only becomes a limiting factor for output powers &gt;100 kW. This system incorporates three innovations not presently used in solid-state laser systems: the use of high-purity Yb 3 Al 5 O 12  as the thin gain element in order to achieve the shortest possible pump absorption length while maintaining high thermal conductivity, a high-performance microchannel-cooled backplane capable of dissipating heat fluxes of 1 kW/cm 2 , and an index-matched undoped “cap” on the thin gain element to prevent trapping of amplified spontaneous emission, enabling high power scaling. Beam quality and wavefront aberrations can be improved through the use of aspheric and/or adaptive optics, along with understanding the impact of edge effects and deviations from ideal cooling. 
     The application of high power lasers to point defense systems involving aircraft and ships is an issue of great current interest. It is estimated that at least 20 kWatts of continuous-wave power is needed to defeat an incoming missile, with stringent requirements on laser beam quality. Systems like this will also be able to defend transport vehicles in enemy territory. In addition, increasing concerns about the safety of commercial aircraft also suggest that speed-of-light defenses may one day be advisable against possible Stinger (shoulder-held) missile attacks. It is believed that solid-state laser technology will ultimately be able to out-perform chemical laser systems, although the full potential of the solid-state approach has not yet been realized. 
     The main system considerations for laser weapon systems are: power, beam quality, efficiency, compactness and magazine depth. The power requirement of &gt;20 kWatts, at a beam quality of a few times diffraction limited, is set by the need to “kill” an incoming missile. High efficiency is required to reduce prime power demands, while a compact system is desirable in order to deploy practical systems on aircraft or ships. The recovery time of the system between “shots” and the amount of expendable inventory (coolant, electrical energy, etc.) will determine whether the system can respond to repeated threats. Diode-pumped solid state lasers are poised to meet the requirements of a laser based point defense system, with the High-BriTE architecture incorporating cutting-edge approaches to the requirements described above. 
     The key to the present laser weapon invention is to pump the laser gain element with a high-radiance laser diode array, that allows the pump light from an extended array to be optically concentrated and efficiently delivered to a thin disk, in such a manner that the induced temperature gradients are aligned with the optical field in the resonator. By employing a thin gain element that is backplane-cooled, temperature gradients are achieved which do not distort the laser beam to first order. By taking this approach to the limit of an extremely thin gain medium and applying very aggressive cooling, high power densities can be achieved before reaching the ultimate limit of thermal fracture of the laser medium. In particular, the use of Yb 3 Al 5 O 12  (YbAG) is proposed as the laser material, where the laser active ion, Yb 3+ , is present at the stoichiometric concentration (1.4×10 22  ions/cm 3 ) resulting in the shortest absorption length possible while maintaining high thermal conductivity. The purity of the Yb 3 Al 5 O 12  must be sufficient that nonradiative processes leading to lifetime quenching and additional heating of the gain element are negligible in order to maximize optical efficiency and minimize heat generation: i.e. η quench &lt;&lt;η quantum defect  where η quench  is the heat generated as a result of nonradiative processes and η quantum defect  is the heat generated due to the quantum defect. A significant advantage of Yb-lasers is that η quantum defect , the heat load per absorbed pump photon, is less than ⅓ that of Nd, the most common solid state laser ion. Parasitic oscillations and amplified spontaneous emission are controlled by bonding the YBAG crystal to an appropriately configured block of index matched material so as to avoid trapping of light rays in the pumped region of the laser material. This block of index matched material may use appropriately angled faces, absorbing regions, or diffusely scattering surfaces to avoid or minimize the trapping of amplified spontaneous emission (ASE). 
     An embodiment of a 100 kWatt class laser is shown in FIG.  9 . The main components are the laser diode pump arrays  200  (50×20 cm 2 ), the YbAG laser crystal  202 , the hollow curved reflector  204  (hollow lens duct) used to concentrate the laser light, and an adaptive optic  206  to assure that the laser beam will be 1-2× diffraction-limited. Other variations of the laser resonator layout are also possible, for instance incorporating injection seeding or utilizing a tightly folded resonator. For this point design, 250 kW of diode pump power at 0.94 μm with a delivery efficiency of 85% is assumed, resulting in a predicted output power of 100 kW at 1.03 μM. Combined with an electrical-optical efficiency of 50% for the laser diode array, this results in an overall electrical efficiency of 20%. Even with the total system efficiency reduced somewhat due to power required for cooling, this represents an extremely efficient and compact system. 
     FIG. 10 shows a scaleable multimodule laser weapon system. Each module contains a laser diode pump array  300 , delivery optics  302  (e.g. hollow lens duct) used to concentrate the pump light, a tapered undoped section  304  for parasitic suppression, the YbAG thin disk gain element and backplane cooler  306 , and cavity optics (not shown) including a multilayer high reflector located between the thin disk and the backplane cooler. Angular multiplexing makes it possible to pass the extraction beam through each successive gain element. 
     FIG. 4 shows a cross-sectional view of the thin disk gain element and surrounding hardware used in the embodiments shown in FIGS. 3,  9  and  10 . Diffusion bonded to the YbAG gain element  16 , the index-matched, undoped Y 3−x Lu x Al 5 O 12  “cap”  22  is intended to prevent light ray paths from being totally internally reflected (TIR) back into the gain element, and a tapered undoped section  28  is used to direct the diode array output to the gain region. The YbAG layer has a dielectric multilayer stack  17  deposited on the free surface, on the side of the gain element  16  opposite from the undoped section, which serves as a high reflectivity mirror for the pump and laser beams. In this embodiment, the dielectric multilayer stack  17  is a broadband high reflector for the wavelength range of 0.94 μm to 1.03 μm. From a computer model, the optimum thickness of the YbAG layer is 150 μm, resulting in a temperature rise of 60 degrees C for the pump irradiance of 13.3 kW/cm 2 . Although the dielectric multilayer stack is only a few microns thick, its low thermal conductivity is estimated to contribute an additional 20 degrees C in temperature rise. Thermal stresses are maximum at the free surface of the YbAG, with a maximum stress calculated to be 48% of the fracture limit. In order to dissipate the 1 kW/cm 2  heat flux, high-performance thermal management techniques are required. Aggressive cooling is accomplished by adapting the microchannel cooled packaging for laser diodes disclosed in U.S. Pat. No. 5,548,605, titled Monolithic Microchannel Heatsink, which is incorporated herein by reference. 
     The role of the undoped, index-matched Y 3−x Lu x Al 5 O 12  overlayer is crucial for high power operation. Amplification of spontaneous emission (ASE) limits the stored power density in large systems, by effectively reducing the excitation lifetime. The index-matched over-layer prevents trapping of ASE rays which otherwise would be totally-internally-reflected. This results in a small fraction of emitted rays intercepting sufficient gain length as to reduce the stored power density. Analytic modeling of this process shows that 100 kW operation is possible using a high reflectivity (96%) output coupler in order to reduce the saturated gain, with smaller scale systems being even more straightforward. 
     Changes and modifications in the specifically described embodiments can be carried out without departing from the scope of the invention, which is intended to be limited by the scope of the appended claims.