Abstract:
A method for manufacturing a laser-active solid having a bonded passive Q-switch is provided, which is particularly suitable for large quantities.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a solid state laser for use in a laser device of a so-called laser ignition for internal combustion engines. 
         [0003]    2. Description of Related Art 
         [0004]    In these laser ignitions, the spark plug is replaced by an ignition laser, which is able to emit a focused laser pulse into the combustion chamber of the internal combustion engine. In the focus of this laser beam, the so-called ignition point, the energy density is so high that the combustion gas-air mixture present there is ignited. 
         [0005]    Since internal combustion engines for motor vehicles are manufactured in very great quantities, the economical manufacture of these laser ignitions is of great importance. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention is based on the objective of providing a method for manufacturing a solid state laser having a passive Q-switch, which allows for the economical manufacture of large quantities of solid state lasers. 
         [0007]    According to the present invention, this objective is achieved by a method, in which a plane-parallel first wafer plate is manufactured from a laser-active material, subsequently a second plane-parallel wafer plate is manufactured from a material that is suitable as a passive Q-switch, and the first wafer plate and the second wafer plate are bonded to form a wafer block. This wafer block may then be coated on both end faces with a resonator mirror. Subsequently, this wafer block is separated into multiple passively Q-switched solid state lasers using a wire saw. 
         [0008]    The method of the present invention has the advantage that the solid state lasers and the passive Q-switches are not bonded or connected individually, but rather an entire first wafer plate and an entire second wafer plate are bonded respectively. From these wafer block thus formed, it is now possible to saw out a multitude of laser-active solids having passive Q-switches such that the economy is already considerably increased due to the marked reduction of the number of bonding processes. 
         [0009]    The present invention furthermore provides for the wafer block to be separated into a multitude of laser-active solids having passive Q-witches by using a wire saw. Such a wire saw comprises a continuous wire or a continuous wire cable that is run over and driven by pulleys. The required separating process is performed with the aid of the wire or the wire cable. 
         [0010]    In order to increase the separating effect of the wire or wire cable, the wire cable may be studded with diamond powder or small shards of industrial diamonds. Alternatively, it is also possible to guide the wire through a slurry before it reaches the cutting location, the slurry containing grinding or cutting means such a silicon carbide and/or bort. This slurry adheres to the wire or wire cable and thereby enters the cutting slot. There the cutting means contained in the slurry contribute toward an effective and quick separation of the wafer block into a multitude of laser-active solids having passive Q-switches. 
         [0011]    In order to achieve the highest possible quality of connection between the first wafer plate and the second wafer plate, the contact surfaces of the first wafer plate and the second wafer plate are cleaned of all impurities prior to bonding. 
         [0000]    Preferably, this may be done using a mixture of sulfuric acid and hydrogen peroxide. 
         [0012]    Subsequently, the contact surfaces of the first wafer plate and the second wafer plate may be etched prior to bonding and then rendered hydrophilic using diluted sulfuric acid having a pH value of 1, for example. The contact surfaces may be cleaned using a plasma-assisted dry etching method (plasma etching) for example. 
         [0013]    If required, at least the contact surfaces of the first wafer plate and the second wafer plate are rinsed at least once between the different cleaning steps using ultra-pure water. 
         [0014]    If required, all processes may be carried out at increased temperature in order to intensify the cleaning or the etching effect. 
         [0015]    The bonding process itself occurs in that the first wafer plate and the second wafer plate are pressed against each other by light pressure and are then heated for a period at temperatures of 1100° C. for example. This brings about a diffusion of the molecules of the first wafer plate and the second wafer plate and results in an intimate connection of the two wafer plates without the formation of visible boundary layer between the first wafer plate and the second wafer plate. The bonding process may take up to 48 hours. 
         [0016]    It is possible to achieve a further increase in the economy of the method of the present invention if multiple wire saws are simultaneously arranged side-by-side such that by two cuts, that is, one cut in the direction of an X axis and a second cut in the direction of a Y axis, a wafer block may be separated or taken apart into a multitude of laser-active solids having a passive Q-switch. 
         [0017]    The economy of the separation process may be increased further if multiple wafer blocks are stacked on top of each other and this stack is then divided using one or multiple wire saws. In order to protect the previously vapor-deposited mirror layers, it is further recommendable to protect the end faces of the wafer blocks by a protective layer, such as an adhesive protective film for example, particularly against mechanical damage. In addition, the protective film holds together the wafer blocks that are partially sawed apart, which substantially facilitates handling. 
         [0018]    Additional advantages and advantageous developments of the present invention may be found in the subsequent drawing, its description and the claims. All of the features described and disclosed in the drawing, its description and the claims may be essential to the present invention both individually and in any combination. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0019]      FIG. 1   a  shows a schematic representation of an internal combustion engine having a laser-based ignition device. 
           [0020]      FIG. 1   b  shows a schematic representation of the ignition device from  FIG. 1 . 
           [0021]      FIG. 2  shows a laser-active solid  44  manufactured according to the method of the present invention having a passive Q-switch  46 . 
           [0022]      FIGS. 3-5  show various processing stages of the method of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    In  FIG. 1   a , reference numeral  10  designates an internal combustion engine as a whole. It may be used to drive a motor vehicle (not shown). Alternatively, internal combustion engine  10  may of course also be used in a stationary application. Internal combustion engine  10  comprises multiple cylinders, only one of which is indicated in  FIG. 1  by reference numeral  12 . A combustion chamber  14  of cylinder  12  is bounded by a piston  16 . Fuel reaches combustion chamber  14  directly through an injector  18 , which is connected to a fuel pressure reservoir  20  that is also referred to as a rail. 
         [0024]    Fuel  22  injected into combustion chamber  14  is ignited by a laser pulse  24 , which is eradiated into combustion chamber  14  by an ignition device  27  that includes an ignition laser  26 . For this purpose, an light guide device  28  feeds ignition laser  26  with a pumping light provided by a pumping light source  30 . Pumping light source  30  is controlled by a control unit  32 , which also controls injector  18 . 
         [0025]    As may be gathered from  FIG. 1   b , pumping light source  30  feeds multiple light guide devices  28  for different ignition lasers  26 , which are respectively associated with one cylinder  12  of internal combustion engine  10 . Toward this end, pumping light source  30  has multiple individual laser light sources  34 , which are connected to a pulsed current supply  36 . Because of the presence of the plurality of individual laser light sources  340 , a quasi “latent” distribution of pumping light to the various ignition lasers  26  is achieved such that no optical distributors or the like are required between pumping light source  30  and ignition lasers  26 . 
         [0026]    Ignition laser  26  has, for example, a laser-active solid  44  having a passive Q-switch  46 , which in conjunction with a coupling mirror  42  and decoupling mirror  48  forms an optical resonator. When supplied with pumping light generated by pumping light source  30 , ignition laser  26  generates a laser pulse  24  in a manner known per se, which is focused by focusing optics  52  on an ignition point ZP situated in combustion chamber  14  ( FIG. 1   a ). The components located in housing  38  of ignition laser  26  are separated from combustion chamber  14  by a combustion chamber window  58 . 
         [0027]    In an isometric view,  FIG. 2  shows a laser-active solid  44  having a passive Q-switch  46 , which was manufactured according to the method of the present invention. Coupling mirror  42  (see  FIG. 1   b ) is not visible in  FIG. 2 . The boundary between laser-active solid  44  and passive Q-switch  46  is indicated by a line  60 . With respect to its optical properties, the interface indicated by line  60  is to impair the emission of the laser pulse through decoupling mirror  48  of passive Q-switch  46  as little as possible. 
         [0028]      FIG. 3  shows a first wafer plate  62  and a second wafer plate  64  in a side view. First wafer plate  62  is made of a laser-active material, while second wafer plate  64  forms passive Q-switch  46  (see  FIG. 2 ). The contact surfaces of first wafer plate  62  and second wafer plate  64  are indicated by reference numerals  66  and  68 . 
         [0029]    First wafer plate  62  and second wafer plate  64  are bonded to each other on contact surfaces  66  and  68 . For this purpose, it is necessary that these contact surfaces  66  and  68  are very even and free of any impurities. This is achieved by various cleaning steps, etching and rendering the surfaces hydrophilic, as was already explained in detail in connection with Claims 2 ff. in the introduction of the description. 
         [0030]    Once contact surfaces  66  and  68  have been prepared accordingly, first wafer plate  62  and second wafer plate  64  are placed one upon the other and pressed against each other. Through a subsequent heat treatment, at 1100° C. for example, which may last up to fifty hours, first wafer plate  62  and second wafer plate  64  bond on their contact surfaces  66  and  68  by diffusion. In  FIG. 4 , the resulting wafer block is provided with the reference numeral  70 . 
         [0031]    Subsequently, the end faces of wafer block  70  are coated with a coupling mirror  42  and a decoupling mirror  48 . The fact that wafer block  70  is coated prior to being divided into different laser-active solids  44  results in a substantial rise in productivity. 
         [0032]      FIG. 5  shows a top view of a wafer block  70  manufactured according to the method of the present invention. At the same time, sectional lines  72  and  74  are drawn in as well. The sectional lines running in parallel to the X axis are in part provided with reference numerals  72 , while the sectional lines running in the direction of the Y axis are provided in part with reference numerals  74 . 
         [0033]    In order to separate, as economically as possible, wafer block  70  into a multitude of laser-active solids  44  having a bonded passive Q-switch  46 , wafer block  70  is divided into multiple strips by multiple wire saws (not shown) disposed in parallel in one first cut in the direction of the X axis. 
         [0000]    Subsequently, these strips are divided by a second cut using multiple wire saws disposed in parallel in the direction of the Y axis into the laser-active solids  44 , shown in  FIG. 2 , having bonded passive Q-switch  46 . 
         [0034]    The wire sawing process has proved to be extraordinarily economical, both when using one wire saw as well as when using multiple wire saws disposed in parallel. As a result, the method according to the present invention achieves a significant increase in productivity and thus a significant reduction in manufacturing costs.