Abstract:
Systems, configurations and methods of using laser diodes in ring-shaped arrays placed a distance away from thin disk solid-state laser gain media, which provide uniformly absorbed pump power distribution with high absorption efficiency. This results in major improvements in the scalability and ruggedness of such lasers and disk laser amplifiers. Use of the diode laser pump configurations of the invention results in compact, robust and scalable lasers that produce high quality, high power outputs.

Description:
This invention claims the benefit of priority from U.S. Provisional patent application 60/461,336 filed Apr. 9, 2003. 

   FIELD OF THE INVENTION 
   This invention relates to high power output laser systems and more particularly to those high power laser systems and methods of producing high power lasers with diode arrays, which are compact, use fewer optical elements and have increased ease of adjustment of power delivery and laser output. 
   BACKGROUND AND PRIOR ART 
   The current art for disk laser pumping involves either complex arrays of mirrors to redirect pump light from a conventional array into the disk many times to achieve both efficient absorption and uniformity or an array of diodes placed around the rim of the disk pumping through the rim towards the center. Both suffer from complexity and the former is not as rugged as applications demand. Also, both suffer from scalability limitations for higher power. 
   The current art is represented by the laser technology such as Giesen&#39;s multi-pass face pumped thin disk laser (see C. Stewen, K. Contag, M. Larionov, A. Giesen, and H. Hugel, “A 1-kW CW thin disc laser”,  IEEE Journal of Selected Topics in Quantum Electronics,  2000, 6(4): 650-657). Patents related to the work of Giesen and others include, U.S. Pat. No. 6,438,152 to Contag et al. disclosing a complex system with a plurality of pumping branches and U.S. Pat. No. 6,577,666 B2 to Erhard et al. with a plurality of optical refocusing legs. Vetrovec&#39;s proposal of edge pumping a disk gain medium (see J. Vetrovec, “Compact active mirror laser (CAMIL)”,  SPIE, Photonics West Laser &#39; 2002  Conference , San Jose, Calif., Jan. 22–26, 2001) also requires complex systems with many optical components and also have pump power delivery and laser outputs which are most often highly non-uniform. U.S. Pat. Nos. 6,603,793 B2 and 6,625,193 B2 to Vetrovec provide several arrangements of gain elements, diode arrays, optical medium and optical coatings to achieve high power lasers; however, all arrangements are unlike the arrangement of elements in the present invention. 
   In addition to the above, annular or circular arrangements of laser diode bars mounted in a dielectric block are disclosed in U.S. Pat. No. 5,627,850 to Irwin et al. and U.S. Pat. No. 6,647,037 B2 to Irwin. U.S. Pat. No. 6,661,827 B2 to Lam et al. discloses a radial array of laser diodes mounted in a segmented conductive ring surrounding a laser rod. 
   None of the prior art arrangements of diode bars, gain elements, or optical elements have the configuration of an open ring as disclosed herein. Not only does the present invention have a unique configuration, the invention meets the commercial need for a laser pump that is scalable to high power laser output, uses fewer optical elements, and is easy to adjust the pump power delivery and laser output to provide improved uniformity. 
   SUMMARY OF THE INVENTION 
   It is a primary objective of the present invention to develop a laser pump that is scalable to high power laser output and uses fewer optical elements. 
   Another object of the present invention is to provide a more efficient disk laser pump source that is scalable to higher powers and of increased ruggedness. 
   A further object of the present invention is to provide a disk laser pump that is easy to manufacture and compatible with both disk laser and disk amplifier configurations. 
   Preferred embodiments of the invention include a new configuration of diode laser bars to face pump thin disk solid state lasers comprising an array of diode bars placed on a washer shaped substrate and a method of modifying the output of a high average power disk laser comprising the steps of: operating said disk laser with an array of diode bars placed on a washer shaped substrate which allows laser light to reach the disk-shaped gain medium through its central hole; and, cooling said diode array from its back surface whereby said combination provides a more efficient disk laser pump source which is scalable to higher powers and is of increased ruggedness. 
   Further objects and advantages of this invention will be apparent from the following detailed description of the presently preferred embodiments, which are illustrated schematically in the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1A  shows a top view of the ring of diode bars in an array of diode bars placed on a flat washer shaped substrate. 
       FIG. 1B  shows one group of arrayed diode bars delivering light to thin disk of a solid-state laser gain medium. 
       FIGS. 2A ,  2 B and  2 C show the results of calculating the absorbed pump power density at the entrance surface of a Yb:YAG laser disk when it is pumped by an array such as shown in  FIGS. 1A and 1B . 
       FIGS. 2D ,  2 E and  2 F show the distribution of absorbed pump power at the back surface of a Yb:YAG laser disk when it is pumped by an array such as shown in  FIGS. 1A and 1B . 
       FIG. 3  shows a laser diode array configured on the inside of a conical ring for face-pumping a thin disk laser. 
       FIG. 4  is a schematic illustration of a beam prism controlled single emitting element of a laser diode bar with cooling system. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Before explaining the disclosed embodiments of the present invention in detail, it is to be understood that the invention is not limited in its application to the details of the particular arrangements shown since the invention is capable of further embodiments. Also, the terminology used herein is for the purpose of description and not of limitation 
   According to this invention, the above objects are met by incorporation of a new configuration of diode laser bars to face pump thin disk solid-state lasers. In  FIG. 1  there is shown an array of diode bars  10 , having a length of approximately 1 centimeter (cm), placed on a flat washer shaped substrate  12 . The washer shaped substrate  12  allows laser light to reach the disk-shaped gain medium located below the central hole  14  of the substrate while arraying the diode bars  10  in such a manner as to be contiguous with the circular geometry of the substrate disk. The diode bars  10  can be fitted with beam control prisms shown as  18  in  FIG. 4  or other optical elements to allow for control of the angular spread of the emitted light and its direction of propagation away from the surface of the array. An example of a beam control prism and other optical elements to control laser emitted light are shown in U.S. Pat. No. 5,208,456, which is incorporated by reference. Optical elements useful in focusing a light beam on the desired plane may be passive or in-line optics, including, but not limited to, lenses, mirrors and prisms. The array of diode bars  10  can be cooled from the back surface of the substrate  12  with a cooling device  200  (shown in  FIG. 4 ). In  FIG. 1 , one group of the arrayed diode bars  10   a , spaced approximately 15 cm from the thin disk  16 , are shown delivering light to thin disk  16  of a solid-state laser gain medium. The diode bars are placed on the side of the ring facing the gain medium and the backside is cooled using various techniques known by those skilled in the art, such as, but not limited to, those described in U.S. Pat. Nos. 6,480,514, 5,105,430 and 5,105,429, which are incorporated by reference. An exemplary cooling technique can be a spray cooling system  200 . The pump power pattern  17  shows some spill over of light off the edge of the disk  16 . This can be controlled by adding slow axis angular spread control to the beam control prisms or by aiming the light in different manners.  FIG. 1  shows that the diode bars  10  are stacked along radii of the washer shaped substrate  12  with their lengths perpendicular to the radii. This is one embodiment. Other embodiments would include those orienting the diode bar lengths at other angles with respect to the radii. 
   The choice of which diode bar  10  directs its light to which location on the surface of the disk gain medium depends on the particular application and the need for more uniformity or for more pump absorption efficiency. Again, any set of such choices is within the scope of the present invention. 
     FIGS. 2A ,  2 B,  2 C,  2 D,  2 E, and  2 F show the results of calculating the absorbed pump power density in a Yb:YAG laser disk  16  when it is pumped by an array such as shown in  FIGS. 1A and 1B . The concentration of Yb is approximately 10 (atomic weight) at. %. The size of Yb:YAG thin disk is approximately 50 millimeters (mm) in diameter and approximately 2 mm in thickness. The distance between diode lasers and Yb:YAG is approximately 150 mm, approximately 132 diode bars are used and total power is approximately 5280 watts (W). The divergence angles of the diode lasers are approximately 20 and approximately 8 degrees. The incident angle of the diode laser is approximately 14 degrees. The diode lasers can be arranged as a ring with an inner radius of approximately 40 mm and the outer radius of approximately 56 mm. The uniformity of pump light distribution due to the array is excellent. 
   Referring now to  FIG. 2A , a three-dimensional plot  20  shows the distribution of absorbed pump power at the entrance surface of a disk pumped by diode laser light from an array of diode lasers, as shown in  FIGS. 1A and 1B .  FIG. 2B  shows a plot  21  of the cross section of the distribution of absorbed pump power in  20  taken parallel to the x axis in  20  and through the center of the disk.  FIG. 2C  is a plot  22  of the cross section of the distribution of absorbed pump power in  20  is taken parallel to the y axis in  20  and through the center of the disk. 
     FIG. 2D  is another three-dimensional plot  25  showing the distribution of absorbed pump power at the back surface of a disk pumped by diode laser light from an array of diode lasers, as shown in  FIGS. 1A and 1B .  FIG. 2E  is a plot  26  of the cross section of the distribution of absorbed pump power in  25  taken parallel to the x axis in  25  and through the center of the disk.  FIG. 2F  is a plot  27  of the cross section of the distribution of absorbed pump power in  25  taken parallel to the y axis of  25  and through the center of the disk. 
   The efficiency of absorption is approximately 78%. By simply accepting a lower efficiency, the roll off in absorbed pump power at the rim of the disk  16  can be eliminated. As indicated above and shown in  FIG. 3 , there are other means, to achieve an improvement in uniformity without giving up absorption efficiency. 
     FIG. 3  shows an alternative to the flat washer array, and is another embodiment of the present invention. By arraying the diode bars  30  on the inside of a conical ring  32 , some of the diode bars  30  are closer to the thin disk  34  than others. Line D is the distance between the diode bars  30  and the thin disk  34 , a distance that can be variable. The slow axis spread problem alluded to above can be compensated for, by this approach, as the near diode bars  30   a  produce light  36  that spreads less and spills over the rim of the disk less when it reaches the surface of the disk  34 . This provides a wider range of choices as to which area of the disk  34  is illuminated by which diode bar  30 . 
   In  FIG. 4 , a single laser diode bar  40  measuring approximately one centimeter (cm) in width contains nineteen emitting elements  42 . A quantum-well active layer  50  with emitting elements  42  is sandwiched between a top insulating plate  52 , a second top cathode layer  54  and a bottom anode layer  56 . A cooling system  200 , preferably, a spray cooling system, is connected to the substrate of the laser diode emitting elements  42 . The cooling system  200  (not shown in the drawings) is also attached to the back of the laser diode bars in  FIGS. 1 and 3 . The beam controlled prism  18  is nearly one-quarter of the optical fiber placed on the front of the emitting area  44  of the emitting elements  42  and can be adjusted by the micro-mechanical system which can adjust the direction and the fast-axis divergence angle of the laser diode output beam  58 . The beam-controlled prism  18  is connected to the laser diode bar  40  by placing the prism on the groove  45  of the substrate of the laser diode bar  40 . 
   Thus, the present invention provides an efficient disk laser pump source in a novel configuration with an array of diode bars placed on a washer shaped substrate, cooled from the back surface while laser light emitted from the diode bars is focused to a disk-shaped gain medium located below the central hole of the substrate. The high power laser array system is compact, robust and easy to scale to high power laser output, uses fewer optical elements, and it is easy to adjust the pump power delivery and laser output. 
   The advantages of the invention are less cost, more efficient disk laser pump sources, scalability to higher powers, greatly increased ruggedness, ease of manufacture and compatibility with both disk laser and disk amplifier configurations. It is useful to pump high average power disk lasers for manufacturing, medical and military applications. Manufacturing applications include, but are not limited to, materials processing. Military uses include directed energy weapons that demand the very high beam quality that disk lasers provide. 
   While the invention has been described, disclosed, illustrated and shown in various terms of certain embodiments or modifications which it has presumed in practice, the scope of the invention is not intended to be, nor should it be deemed to be, limited thereby and such other modifications or embodiments as may be suggested by the teachings herein are particularly reserved especially as they fall within the breadth and scope of the claims here appended.