Abstract:
An improved architecture for optical waveguides as used in a diode-pumped alkali laser system is provided by using micro-channel-etched silicon or other metal in place of the more usual sapphire.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 62/353,759 titled “Waveguide for Diode-Pumped Alkali Lasers,” filed Jun. 23, 2016, incorporated herein by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    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 
     Field of the Invention 
       [0003]    The present invention relates to techniques for cooling waveguides, and more specifically, it relates to improved means for cooling diode pumped alkali lasers. 
       Description of Related Art 
       [0004]    The current architecture for the majority of diode-pumped alkali laser (DPAL) designs is the end-pumped configuration, where the pump light enters from one end of the gain region. In such a geometry, the pump light must be ducted down the gain region in order to efficiently pump the alkali-vapor gain medium. 
         [0005]    Current waveguide designs call for a multi-layer dielectric stack deposited on a sapphire substrate. The dielectric stack provides high reflectance for the pump radiation at 780 nm (over a specified angular range). It also provides sufficiently low reflectance for the laser radiation at 795 nm so as to prevent parasitic oscillations from forming. The lower reflectance at 795 nm also allows the transmission of spontaneous emission (fluorescence) through the waveguide. 
         [0006]    Thermal management for the waveguide is provided by means of metallic heat exchangers placed in close proximity to the waveguides. The fluorescence is incident on the face facing the waveguide and is absorbed. The heat so generated is removed by means of cooling fluid that circulates via channels machined into the heat exchanger. 
         [0007]    A schematic drawing showing the relationship between the right sapphire mounted waveguide  10  and the heat exchanger  12  is shown in  FIG. 1 . In the drawing, the helium filled gap  14  between the waveguide  10  and the heat exchanger  12  is greatly exaggerated. Although not shown in the figure, a similar arrangement exists for the left waveguide  16 . The pressure vessel  18  intercepts any fluorescence escaping out the top and bottom waveguides. 
         [0008]    In general, the thermal resistance between where the energy is deposited and the location of the cooling fluid is quite large—in the range of 2.6 to 7.6° C./W/cm 2 . Typical operation of the laser can result in fluorescence loads approaching 50 W/cm 2 , with the result that the heat exchanger can experience a temperature increase of 130-380° C. Because the heat exchanger is separated from the waveguide by a thin gap of stagnant gas, the waveguide also increases in temperature by this same amount. 
         [0009]    The large increase in waveguide temperature is deleterious for several reasons. At operating temperatures approaching 500° C., the dielectric stack can dramatically change its reflection characteristics which can lead to catastrophic failure. The temperature difference between the flowing gas in the gain cell and the waveguide leads to very poor beam quality due to large temperature gradients near the waveguide walls. 
         [0010]    The use of micro-channels to cool laser diodes and laser diode arrays is beneficial in efficiently removing heat from a laser diode bar, where the thermal flux can be on the order of 1000 W/cm 2 . Typical thermal impedances for the micro-channel design can approach 0.0125° C./W/cm 2 , which is a 200 to 600× improvement over the waveguide cooler design of  FIG. 1 . 
         [0011]    A scanning-electron-microscope image of micro-channels as etched into silicon is shown in  FIG. 2 . As can be seen, the typical channel width is on the order of 40 μm. This is extremely large by modern photo-lithographic standards and can easily be manufactured. The cooling fluid is delivered to the micro-channels by means of a plenum bonded to the silicon. Using, for example, borosilicate glass, the Si may be anodically bonded to the plenum. 
       SUMMARY OF THE INVENTION 
       [0012]    We present an improved architecture for optical waveguides as used in a diode-pumped alkali laser system. The improvement comes from using micro-channel-etched silicon or other metal in place of the more usual sapphire. This geometry allows for much more efficient heat removal, leading to more robust, lighter laser designs. 
         [0013]    Past and current uses of micro-channel technology are primarily in the area of laser diode cooling. The architecture described herein can potentially be used anywhere large amounts of heat must be removed from a surface. The invention has great utility in the area of end-pumped diode-pumped alkali lasers where the pump light must be ducted down the gain region. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The accompanying drawings, which are incorporated into and form a part of the disclosure, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
           [0015]      FIG. 1  is a schematic drawing showing the relationship between the sapphire mounted waveguide and the heat exchanger. 
           [0016]      FIG. 2  is a scanning electron micrograph of micro-channels etched in silicon. 
           [0017]      FIG. 3  is a side view drawing of an embodiment of the present improved waveguide structure. 
           [0018]      FIG. 4  shows a perspective view of an embodiment of the invention. 
           [0019]      FIG. 5  shows a plenum as it would be etched into the glass mounting block. 
           [0020]      FIG. 6  is a perspective end view drawing of a DPAL cell incorporating an embodiment of the present invention. 
           [0021]      FIG. 7  shows one T structure design for use with the embodiment of  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    To overcome the limitations of the current heat exchanger design, a new design for the waveguides, as exemplified in  FIG. 3 . In this design, heat removal is done right at the waveguide, thus eliminating the need for a separate, bulky, heat exchanger. The dielectric stack serves the same purpose as before, to provide high reflectance at the pump wavelength and lower reflectance at the laser wavelength. Because the heat is removed right at the source, the temperature of the waveguide can be much better controlled. As a result, the waveguide temperature can closely match the temperature of the bulk gas flowing down the channel. This will result in greatly improved beam quality, as the steep temperature gradients near the waveguide have been eliminated. 
         [0023]    Another feature of the proposed design lies in its simplicity, leading to greatly reduced weight. The extremely low thermal resistance of the waveguide system allows one to handle significantly greater amounts of fluorescence than is now possible. A consequence of this is the ability to increase the concentration of the laser-active species, and thus shorten the overall length of the gain medium. As a result, one achieves a more compact system. In terms of manufacturability, Si wafers with diameters of 300 mm are routinely available, and there is the possibility of going to 450 mm diameter in the near future. Such large sizes can easily accommodate several waveguides. 
         [0024]      FIG. 3  is side view of an embodiment of an improved waveguide structure according to the present invention. The silicon micro-channel structure  30  will be described in more detail below. The structure is anodically bonded to glass manifold  32 . Multi-layer dielectric stack  34  adheres to the silicon micro-channel structure  30 . The multi-layer dielectric stack provides high reflectivity at the pump wavelength (780 nm) and lower reflectivity at the laser wavelength (795 nm). The thickness of the Si in one embodiment is within a range from approximately 1 mm. 
         [0025]      FIG. 4  shows a perspective view of an embodiment of the invention. It includes a glass mounting block  40 , a silicon micro-channel structure  42  and a multilayer dielectric stack  44 . The micro-channel is anodically bonded to the glass mounting block. The plenum of  FIG. 5 , as discussed below, is etched into the top of the glass mounting block. The silicon structure has micro-channels that are facing toward the glass mounting block. The range of thicknesses of the silicon capping layer between the micro-channels and the multilayer dielectric stack is about 20 μm to 500 μm. Each micro-channel has a width that ranges from 20 microns to 1 mm and a channel depth that ranges from 10 microns to 1 mm. The total thickness of the silicon micro-channel structure can be up to 1.2 mm. 
         [0026]      FIG. 5  shows a top view of the glass mounting block with etched plenums. This embodiment is suitable far the embodiment of  FIG. 4 . There are two cooling fluid inlet plenums ( 50 , 52 ) and one cooling fluid outlet plenum  54 . Representative micro-channels  56  are shown above the plenums. Cooling fluid enters plenums  50  and  52  at inlet ports  58  and  60  respectively. When the fluid reaches one of the micro-channels  61 , a portion of the fluid flows through the micro-channel and into plenum  54  to exit the output port  70 . Some of the cooling fluid continues to flow in the plenums  50  and  52  so that the fluid flows through other micro-channels and out of the system. As the fluid flows through the micro-channels, it comes into contact with the ˜1 mm portion of the silicon structure that is between the micro-channels and the multilayer dielectric stack. Note that other plenum structures are possible. For example, there may be only one inlet and one outlet. When there is more than one inlet plenum, each one may have the cooling fluid flowing in a direction that is opposite to the other plenum. Based on this disclosure, those skilled in the art will understand that other configurations are possible. 
         [0027]      FIG. 6  is a perspective end view drawing of a portion of a DPAL cell incorporating an embodiment of the present invention. The cell includes a window  70  through which pump light enters the cell. Four multilayer dielectric stacks  71 - 74  form the inner walls of the cell. Each stack of the four multilayer dielectric stacks  71 -  74  are in contact with a respective silicon micro-channel structure  81 - 84 . As in the embodiments of  FIGS. 3 and 4 , the micro-channels are faced away from the multilayer dielectric stacks and do face their respective glass mounting block  91 - 94 , each of which has a plenum structure etched on the surface of the glass mounting block that faces the micro-channel structure. The silicon structure is anodically bonded to the glass mounting block. Notice in the figure that stacks  74  and  72  and their associated silicon micro-structures extend the full length of their respective glass mounting blocks such that one of their ends is in contact with the upper block  91  and the other end is in contact with lower block  93 . In this configuration, stacks  71  and  73  and their respective silicon micro-structures fit between stack  74  and stack  72 . Other configurations are within the scope of this invention. 
         [0028]      FIG. 7  is an end view drawing of a DPAL cell  100  incorporating an embodiment of the present invention. The figure shows a window  102 , and four multilayer stacks  104 ,  104 ′,  106  and  106 ′ where each stack is in contact with its own silicon micro-structure  108 ,  108 ′,  110  and  110 ′ respectively which are anodically bonded to a respective glass mounting block  114 ,  114 ′,  116  and  116 ′. The silicon micro-structures include a thin layer of material that is in contact with the multilayer dielectric stack and also includes etched micro-channels as described in the previous embodiments. See, e.g., micro-channels  112 . As discussed above, each glass mounting block includes plenums to provide a flow of cooling liquid through the micro-channels and to remove the cooling liquid after it has passed through the micro-channels. In this figure, the micro-channels run in a direction that is perpendicular to the plane of the page. In this configuration, the plenums are formed so that the cooling liquid flows through the plenums in a direction that is parallel to the plane of the page. This is but one example. The direction of the micro-channels and the plenums can be reversed as well. Other configurations will be apparent to those skilled in the art based on this disclosure. 
         [0029]    The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. The embodiments disclosed were meant only to explain the principles of the invention and its practical application to thereby enable others skilled in the art to best use the invention in various embodiments and with various modifications suited to the particular use contemplated. The scope of the invention is to be defined by the following claims.