Abstract:
A thin disk laser includes a thin disk of a host material incorporating a laser gain medium. The disk has opposite first and second surfaces, at least one of which is non-planar. The first surface is coated with a high reflectivity coating. The second surface has an anti-reflection coating thereon. The shape and mounting of the laser is such that mismatch of the coefficients of thermal expansion between the disk laser and the mount does not affect scaling of the laser to larger size disks for higher power lasers

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates generally to disk lasers and, more particularly, to high power doped thin-disk lasers and methods for making them. 
       BACKGROUND 
       [0002]    A thin disk laser, sometimes referred to as an “active mirror,” is an optical amplifier gain medium, typically but not necessarily disk-shaped, in which stimulated emission of light, i.e., “lasing,” is produced when a pump laser illuminates the disk, resulting in gain in emitted light. Conventional thin-disk lasers may be made from ytterbium (Yb) doped yttrium aluminum garnet (YAG) disks bonded to heat sinks or heat spreaders, such as diamond or copper. In conventional thin disk laser systems, the crystal of the gain medium, which may have an anti-reflection (AR) coating on the front side thereof that is effective at both the incident pump and emitted laser radiation wave-lengths, is fixed to the heat sink/spreader with a layer of indium or equivalent bonding solder or adhesive. The heat sink may be liquid cooled (e.g., with water), or by use of a thermoelectric (TE) cooler, from the back side. 
         [0003]    The significant difference in the respective coefficients of thermal expansion (CTE) values of the heat sink and the disk laser prevents device scaling to larger diameters for operation at increased power output. In particular, if the bonding operation occurs at room temperature or above, larger devices are limited to operation at close to the assembly temperature to prevent catastrophic failure. 
         [0004]    More particularly, conventional thin disk lasers typically operate at room temperature, which is usually the same temperature at which the thin disk laser system (comprising, for example, a diamond heat sink, adhesive and Yb:YAG gain medium:host material) is assembled, and at reduced power levels, so that CTE issues are neither confronted nor resolved. Conventional designs may also incorporate an outer region of the disk that is not pumped to aid in disk integrity, and support heat dissipation requirements. However, as the disk is operated at increasing power output levels, sufficient temperature differences can occur across the disk area, as well as the interfacial bonding region between the disk and the heat spreader, to cause catastrophic failure due to the differences in the respective material CTE&#39;s. 
         [0005]    Additionally, it is known that the efficiency and performance of such devices improve as their operating temperature is lowered, for example, to cryogenic temperatures. However, the difference between CTE&#39;s of the disk and heat spreader may again lead to failure upon cooling below the assembly temperature. The foregoing CTE issues thus severely limit scaling of the device size to produce higher output powers. In addition, the resulting thermally induced stresses may introduce undesirable aberration in the output beam by distorting the laser crystal optically, and otherwise degrade the quality or brightness of the laser light produced. 
         [0006]    In light of the foregoing, there is a need for thin disk laser design and assembly methods that eliminate or reduce the CTE mismatch problem to enable device scaling for higher output. 
       BRIEF SUMMARY 
       [0007]    In accordance with the exemplary embodiments described herein, thin-disk lasers and methods for manufacturing them are disclosed that overcome the above and other problems of prior art thin-disk lasers, thereby enabling device scaling for higher power and brightness laser light output. 
         [0008]    In one exemplary embodiment, a laser includes a thin disk comprising a host material doped with a laser gain medium, having opposite first and second surfaces, wherein at least one of the two surfaces is non-planar. The first surface may have a planar, a concave or a convex shape and is coated with a high reflectivity (HR) coating. The second surface has an anti-reflection (AR) coating thereon, and may have a concave, planar, or convex shape. 
         [0009]    In another exemplary embodiment, a laser assembly includes a thin disk laser, the disk comprised of host material and laser gain medium, having opposite first and second surfaces, wherein at least one of the two surfaces has a non-planar shape. The first surface may have a concave, a planar, or a convex shape and a high reflectivity (HR) coating thereon, and the second surface may have a planar, a concave or a convex shape with an antireflection (AR) coating thereon. A housing supports the thin disk laser and has a hollow interior for sealingly retaining and circulating coolant fluid in direct contact with the first surface. The assembly further includes an apparatus to illuminate the second surface with a pump beam of a selected wavelength of laser light to excite the thin disk laser. The assembly further includes a second mirror which, in combination with the thin disk laser, forms a resonant laser cavity, and which is operable to transmit a portion of the laser light as an output beam. 
         [0010]    A more complete understanding of the novel thin disk laser embodiments disclosed herein will be afforded to those skilled in the art, as well as a realization of additional advantages thereof, by a consideration of the following detailed description of one or more exemplary embodiments, particularly if considered in conjunction with the appended drawings, wherein like reference numerals are used to identify like elements illustrated in one or more of the figures thereof. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1A  is a cross-section elevation view of an exemplary embodiment of a thin disk laser formed from lightly doped yttrium aluminum garnet (YAG) in accordance with the present disclosure. 
           [0012]      FIG. 1B  is a cross-sectional elevation view of another exemplary embodiment of a thin laser disk formed from a YAG crystal with a heavily doped region and an un-doped region in accordance with the disclosure. 
           [0013]      FIG. 2A  is an enlarged cross-sectional view of the exemplary thin disk laser of  FIG. 1B , showing the effect of its shape on transverse amplified spontaneous emission to suppress parasitic oscillations. 
           [0014]      FIG. 2B  is an enlarged cross-sectional view of an exemplary thin disk laser having a planar first surface, in accordance with the present disclosure. 
           [0015]      FIG. 3  is an enlarged cross-sectional view of an exemplary thin disk laser, illustrating an exemplary embodiment of a relative mode distribution of pump and laser radiation in accordance with the disclosure. 
           [0016]      FIG. 4  is a cross-sectional elevation view of an exemplary thin disk laser mounted in a housing of an exemplary embodiment of thin-disk laser system in accordance with the disclosure. 
           [0017]      FIG. 5  is schematic cross-sectional side view of another exemplary embodiment of a thin disk laser assembly in accordance with the disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIG. 1A  illustrates the shape of an exemplary embodiment of a thin disk laser  100 A in accordance with the present disclosure, which may be formed from a host material such as YAG crystal  101  that is lightly doped with an ionic atom such as Yt-terbium (Yb). YAG crystal  101  may be oriented with the &lt;110&gt; axis parallel to the lasing axis to minimize thermally induced birefringence. Crystal  101  may be on the order of one mm thick and may be ground and polished to form thin-disk laser  105  having a first surface  110  with a convex curvature, but other curvatures are admissible, including a planar surface. The meaning of convex and concave curvature is not limited to spherical curvature. First surface  110  may have a high reflectivity (HR) coating  112  deposited by one of several well known deposition processes, including thermal evaporation, sputtering, chemical vapor deposition, or equivalent techniques well known in the art. HR coating  112  may be a metal, or alternatively, it may be a quarter-wave multilayer coating scaled to have high reflectivity over a selected range of wavelengths that may include the pump and lasing radiation. HR coating  112  may be applied to maximize radiation of amplified stimulated emission in a direction toward a second surface  115  of laser  105 . 
         [0019]    Other host materials may be used besides YAG. Such materials preferably have one or more of the following properties: higher thermal conductivity than yttrium aluminum garnet (YAG), higher absorption cross section than ytterbium (Yb) doped YAG, broader absorption than YAG, and reduced quenching effects compared to Yb:YAG. Examples of such materials include, but are not limited to: Lu 2 O 3 , YVO 4 , LuVO 4 , Sc 2 O 3 , LaSc 3 (BO 3 ) 4 , KGd(WO 4 ) 2 , and KY(WO 4 ) 2 . 
         [0020]    Other laser gain media (dopants) may be used besides Yb with the host materials indicated above. Such laser gain medium dopants known to enable solid state lasing include, but are not limited to: neodymium (Nd), thulium (Tm), holmium (Ho), and chromium (Cr). Other combinations of dopants and host materials may be determined and may be incorporated in the structures disclosed herein. 
         [0021]    The second surface  115  of laser  105  may be flat, concave or convex, provided only one of the two surfaces is planar. Additionally, second surface  115  may be coated with an anti-reflective (AR) layer  117  to minimize reflection of light away from the second surface  115  of laser  105 . AR layer  117  may be designed to provide minimized reflection at both the pump and lasing radiation wavelengths. 
         [0022]      FIG. 1B  illustrates another exemplary embodiment of a thin disk laser in accordance with the present disclosure, which is formed from a YAG crystal  151  with a heavily doped region  170  and an un-doped region  180 . In this exemplary embodiment, un-doped region  180  may be on the order of one mm thick, and heavily doped region  170  may be on the order of one hundred microns thick, or larger. YAG crystal  151  comprising the two regions may be formed by a variety of processes, including, for example, diffusion bonding of two separate crystals, one doped (i.e.,  170 ) and the other un-doped (i.e.,  180 ). Crystal  151  may be ground and polished to form thin disk laser  155  having a first surface  160  with a convex curvature on the heavily doped side. First surface  160  may preferably have a high reflectivity (HR) coating  162  deposited by one of several well known deposition processes, as described above. HR coating  162  may be a metal, or alternatively, it may be a quarter-wave multilayer coating scaled to have high reflectivity at the pump and lasing wave-lengths. HR coating may be applied to maximize radiation of amplified stimulated emission in a direction toward a second surface  165  of laser  155 . 
         [0023]    The second surface  165  of laser  155  may be flat, concave or convex. Additionally, surface  165  may be coated with an anti-reflection layer  167  that minimizes reflection of light away from second surface  165  of laser  155 . 
         [0024]    The shape of laser  105  and laser  155  may be chosen to overcome a plurality of issues that limit scaling to larger sizes and larger optical power outputs. The convex first surface  110  and  160  of crystals  105  and  155 , respectively, add strength to the disk structure when subject to the pressure of a coolant that may be applied directly to the first surface, as opposed to an intervening heat sink, e.g., a diamond heat sink, of conventional thin disk lasers. 
         [0025]    Additionally,  FIG. 2A  shows the effect of shape on transverse amplified spontaneous emission (ASE) to suppress parasitic oscillations, in accordance with the exemplary embodiment of  FIG. 1B . ASE is produced when a laser gain medium is pumped to produce a population inversion. Feedback of the ASE by the laser&#39;s optical cavity produces laser operation if the lasing threshold is reached. 
         [0026]    Excess ASE, i.e., emission generated in directions other than along the laser cavity axis, e.g., rays  220  and  230 , is an unwanted effect in lasers, since it dissipates some of the laser&#39;s power by excitation of unwanted lasing, for example, in a lateral or other direction, where it is wasted. Furthermore, unwanted ASE lasing may cause oscillations or instability in the desired lasing direction. For example, in conventional thin disk lasers with flat surfaces and vertical side walls, parasitic oscillation may arise in the direction transverse to the intended output direction created by the radial cavity. As shown in  FIG. 2A , laser disk  155  may be meniscus shaped so as to have only two surfaces, so that no resonant cavity is formed with a shape that might otherwise permit parasitic transverse oscillations to occur. When illuminated by pump radiation  210 , ASE may result in rays of stimulated light emission in a generally transverse direction being excited in heavily doped region  170 . 
         [0027]    Thus, as illustrated in  FIG. 2A , because laser  165  is shaped with a convex curvature on first surface  160  and this surface has HR coating  162  thereon, the transverse ASE rays are deflected by internal reflection out of the volume of laser  155  so as to exit the disk from second surface  115  or  165 , thus suppressing undesirable transverse lasing oscillations. In optical amplifiers, ASE limits the achievable gain and increases laser noise level. ASE (together with overheating and background scattering loss) may limit the maximum size (and power) of conventional thin disk lasers, especially if the main laser radiation field travels through a lasing medium with a short but wide gain region, which is the case with disk lasers. It may therefore be appreciated that the combination of shaping and HR coating may significantly reduce heating and losses due to transverse ASE, thus reducing a limitation to scaling for higher power. 
         [0028]    Additionally, the detailed figure (i.e., the detailed profile thickness) as ground and polished on second side  115  and  165  of crystals  105  and  155  can be determined by known modeling techniques to substantially cancel wave-front distortions (also referred to as Optical Path Distortion—OPD) due to transverse thermal gradients affecting the index of refraction and thickness dimension of laser  105  or  155 . This may involve figure correction polishing on the order of a fraction of a wavelength change in thickness (from a nominal value that ignores OPD effects) over a radial dimension on the order of one mm (i.e., a figure correction based on thermal gradient effects that is on have only two surfaces, so that no resonant cavity is formed with a shape that might otherwise permit parasitic transverse oscillations to occur. When illuminated by pump radiation  210 , ASE may result in rays of stimulated light emission in a generally transverse direction being excited in heavily doped region  170 . 
         [0029]    Thus, as illustrated in  FIG. 2A , because laser  165  is shaped with a convex curvature on first surface  160  and this surface has HR coating  162  thereon, the transverse ASE rays are deflected by internal reflection out of the volume of laser  155  so as to exit the disk from second surface  115  or  165 , thus suppressing undesirable transverse lasing oscillations. In optical amplifiers, ASE limits the achievable gain and increases laser noise level. ASE (together with overheating and background scattering loss) may limit the maximum size (and power) of conventional thin disk lasers, especially if the main laser radiation field travels through a lasing medium with a short but wide gain region, which is the case with disk lasers. It may therefore be appreciated that the combination of shaping and HR coating may significantly reduce heating and losses due to transverse ASE, thus reducing a limitation to scaling for higher power. 
         [0030]    Additionally, the detailed figure (i.e., the detailed profile thickness) as ground and polished on second side  115  and  165  of crystals  105  and  155  can be determined by known modeling techniques to substantially cancel wave-front distortions (also referred to as Optical Path Distortion—OPD) due to transverse thermal gradients affecting the index of refraction and thickness dimension of laser  105  or  155 . This may involve figure correction polishing on the order of a fraction of a wavelength change in thickness (from a nominal value that ignores OPD effects) over a radial dimension on the order of one mm (i.e., a figure correction based on thermal gradient effects that is on second surfaces. Thus, fractional wavelength correction of the optical path can be effected as a function of the radial position to take into account thermal distortions that may occur in radial symmetry during lasing operation. 
         [0031]    As illustrated in  FIG. 3 , detailed figure correction polishing may also be made more beneficial by shaping the incident pump radiation  210  to have a pump beam profile  310  so that the intensity gradient is gradual. For convenience, the following discussion is with respect to thin disk laser embodiment  155  of  FIG. 1B , but applies equally to thin disk laser embodiment  105  of  FIG. 1A . An exemplary shape of pump beam profile  310  may be, for example, approximately Gaussian, or a similar intensity profile, such that the intensity transition from maximum to zero in a radial direction is not abrupt, as in a conventional “top-hat” intensity profile of the type discussed below. 
         [0032]    Thus, in the absence of strong gradients in pump beam profile  310  energy intensity, there may also be a smooth gradient in the laser mode profile  320  that is produced, further reducing strong thermal gradients in the radial direction that typically lead to thermal stress failure of disk  105  or  155 . Thus, pump beam profile  310  and laser mode profile  320  may both be taken into account during predictive modeling in determining the figure correction required. 
         [0033]    Shaping the pump beam profile  310  energy relative to the laser mode profile may also enhance the efficiency of lasing. By matching the profile overlap of the pump beam profile  310  of energy intensity to the laser mode profile  320  that is being excited, the input pump energy may be applied to substantially excite emission only in the area of the disk that supports laser mode  320 . 
         [0034]    In a typical resonator laser cavity, both the mirror formed by HR coating  112  or  162  formed on crystal  105  or  155  and an external mirror, such as, for example, a “scraper mirror” or a gradient reflectivity mirror (GRM), that completes the resonator cavity determine the stability and beam mode shape. For a stable laser mode, one or both mirrors forming a laser cavity must have a degree of concave surface of reflection. The focal lengths of the one or more mirrors will determine, to a large extent, the shape and diameter of the cavity mode, and is determined by the characteristics of the mirror(s). Thus, laser beam profile  320  and diameter are determined by a number of geometric parameters and material characteristics, and not only by the shape and extent of lasing material (i.e., dopant and host material, such as Yb:YAG). 
         [0035]    Conventional disk lasers, which are typically uniformly illuminated with a “top-hat” profile pump beam may have significant thermally induced aberrations at the edge of the pumped area, rendering only a limited central portion of the doped crystal area usable for high beam quality lasing. Thus, profile shaping and control of the pump beam and the laser beam may render the laser beam aberration-free over a larger portion of laser disk  105  or  106 . 
         [0036]      FIG. 4  illustrates an exemplary thin disk laser mounted in a housing  420  of an exemplary embodiment of a thin-disk laser system. In accordance with the particular exemplary embodiment illustrated, thin disk laser  105  or  155  may be seated over an opening  410  extending into a hollow housing  420  containing circulating coolant  430 . Laser  105  or  155  may be sealingly supported in housing  420  via one or more O-rings  440  or an equivalent compressible sealant, which may provide both stress relief for laser  105  or  155 , and seal housing against leakage of coolant  430 . Where the coolant is selected to maintain a cryogenic temperature to further boost the efficiency of disk laser  105  or  155 , O-rings  440  or an equivalent compressible sealant may be chosen to preferably have flexibility at the operating temperature of coolant  430 , which in one exemplary embodiment, may be maintained at a temperature of about 100 degrees K, or lower. 
         [0037]    Referring to  FIG. 5 , another exemplary embodiment of a thin disk laser assembly  500  includes thin disk laser  105  or  155  mounted in housing  420  which provides coolant  430 , which may be recirculated by connection to an external chiller (not shown) for heat rejection. Pump radiation  210  is configured to form an intensity profile as described above in connection with  FIG. 3 . The first surface  110  or  160  of laser  105  or  155 , respectively, with respective HR coatings  112  or  162 , form one mirror of the resonator cavity of assembly  500 . Mirror  510  forms the second mirror, completing the cavity. Other embodiments may be contemplated, including complex pump beam configurations, use of specialized scraper mirror, gradient reflectivity mirrors, and folded path cavities, which are in the scope of the disclosure. 
         [0038]    Embodiments described above illustrate but do not limit the invention. It should also be understood that numerous modifications and variations are possible in accordance with the principles of the present invention. Accordingly, the scope of the invention is defined only by the following claims and their functional equivalents.