Abstract:
A mirror for laser application and a method for manufacturing the mirror is disclosed herein. The mirror comprises several layers including a top layer used as the mirror surface. A cooling structure including a plurality of cooler layers are providing between the top layer, several intermediate layers and a bottom layer. Connections are provided for allowing a coolant to flow within the mirror. The mirror is constructed by stacking the layers on top of one another and connecting adjacent layers together by direct copper bonding or active soldering.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention refers to a cooled mirror for laser applications and in particular to a cooled mirror with a mirror body having a mirror surface and comprising several layers with their surfaces connected in a stack-like arrangement and forming a cooler structure with connections for the inlet and outlet of a coolant, whereby a first layer forming the top of the mirror contains or forms the mirror surface. The present invention refers also to a method for the manufacture of the said cooled mirror for laser applications.  
           [0003]    2. Description of the Prior Art  
           [0004]    In laser devices, for example in such devices used for processing parts, mirrors are generally needed for reflecting the laser beam, and are moved by a drive for processing of a part. Although the laser beam is generally sent as a widened beam to the mirror used in order to reduce the power density, it is common practice to actively cool such mirrors, using a coolant that flows through a cooler structure of the mirror body, preferably with a liquid coolant, for example such as water. The widened laser beam is then focused in a focal point by means of a lens for processing of the respective part. The focal point must be of high quality, i.e. in particular a pre-defined diameter, for example a circular diameter, and there must be an extremely homogeneous distribution of the laser power within the focal point. The part can be processed to meet quality requirements only if these conditions are fulfilled by a respective high-quality beam.  
           [0005]    Therefore, mirrors for laser devices or the mirror surfaces of such mirrors must possess a high degree of mechanical and thermal stability, i.e. high stability against external forces and/or pressure, against temperature deviations or deformations etc., thus avoiding any resulting alterations and/or deformations of the mirror surface. Miniscule alterations and/or deformations, such as warping of the mirror surface in the sub-micrometer range already cause a serious decline in the beam quality of the laser beam reflected on the mirror, especially a decline in the shape or diameter of the beam and the quality of the laser beam focal point or focus on a surface of the part to be processed. Furthermore, minimal deformations of a mirror surface cause significant operating time differences in the laser beam, thus also adversely affecting the quality of the laser beam focal point and therefore the quality of the processing of the part with the laser.  
           [0006]    Numerous suggestions for cooled mirrors for laser devices are known in the art. All of these suggestions are unsatisfactory.  
           [0007]    A cooled mirror for a higher power laser is known, as is described in U.S. Pat. No. 4,443,059. In this mirror, the mirror surface consists of a layer made from a material with high thermal conductivity, of which, however, the thermal expansion coefficient is adapted as nearly as possible to the material of the components connected to the cooled mirror and therefore in particular to the thermal expansion coefficient of the connected cooler structure. Therefore, the materials recommended for the layer comprising the mirror surface are molybdenum, tungsten, silicon carbide or silicon nitride. The cooler structure is made from a further plate that is provided with a plurality of grooves on one side that are divided by stays and form cooling channels for the coolant. By means of the stays, this cooler structure is connected directly with the plate possessing the mirror surface. The side of the plate comprising the cooler structure facing away from the mirror surface is connected with a support body with a honeycomb profile containing channels for the inlet and outlet of the coolant.  
           [0008]    The disadvantage is the complex and also non-symmetrical design of the mirror, so that the axis direction is perpendicular to the mirror surface, causing thermal deformations (bimetal effect). Another disadvantage is that the individual stays of the plate forming the cooler structure connect directly to the layer possessing the mirror surface without an additional intermediate layer, which causes uneven cooling and therefore deformations of the mirror surface. A final disadvantage of the known mirror is that it has a high material mass, which is undesirable for laser mirrors that are moved for processing of parts.  
           [0009]    The groove-like cooling channels used also have the disadvantage that the stiffness of the mirror on the mirror surface in the longitudinal direction of the grooves is greater than in the transverse direction. In the necessary finishing of the mirror surface, for example by means of milling with a diamond tool, this causes structures to be formed in the mirror surface due to the varying stiffness, which also adversely affects the beam quality. This is a disadvantage of all known mirrors that use cooling channels that extend parallel to the mirror surface.  
           [0010]    A further light-weight mirror for laser applications is described in U.S. Pat. No. 5,002,378. For the cooling of this known mirror, however, there is no cooler structure with connections for the inlet and outlet of a coolant, i.e. the mirror is not connected to an external coolant circuit. In this case, the cooler design is based on the heat-pipe cooling principle.  
           [0011]    A further mirror structure cooled by a coolant, is shown in U.S. Pat. No. 3,781,094, in which the cooler structure consists of a plurality of perpendicularly crossing cooling channels that are all limited by stays. The disadvantage of this known construction is again the lack of symmetry in the direction perpendicular to the mirror axis, especially also of the cooler structure, in addition to the manufacturing expense. Moreover, the stays limiting the cooling channels of this known mirror likewise extend directly to the layer possessing the mirror surface, so that no homogeneous cooling of the mirror surface is achieved, which means that thermal deformations and faults in the mirror surface are unavoidable.  
           [0012]    In a further known mirror for laser applications, a plurality of separate cooling channels are formed by using corrugated plates in the cooler structure as shown in U.S. Pat. No. 4,387,962. This known construction is likewise complex and does not possess the required symmetry.  
           [0013]    A further known cooled mirror for laser applications described in DE 33 39 076 A1 has an extremely complex cooler structure and consists essentially of a plurality of posts that extend from the bottom of a plate forming the mirror surface and that are subjected to circulation by a coolant emitted from nozzles. The disadvantage here is likewise the very complex and therefore expensive construction in addition to the lack of a symmetrical design.  
           [0014]    A further known cooled mirror for laser applications shown in U.S. Pat. No. 4,770,521 has a cooler structure formed directly beneath a layer comprising the mirror surface. Here the layer comprising the mirror surface is separated by a further layer parallel to this layer by a plurality of polygonal spacers, so that a space is formed between the two layers for circulation of a coolant. The disadvantage of this known cooler is likewise a complex and expensive design. Since the spacers are directly connected with the layer forming the mirror surface, no homogeneous cooling of the mirror surface is achieved.  
           [0015]    Finally, a known cooled mirror for laser applications described in U.S. Pat. No. 4,403,828 consists of a very massive block that is provided on top with several grooves, each of which forms a cooling channel. A plate bearing the mirror surface is attached on the top by soldering to the stays of the grooves. The disadvantages here include the high weight of the mirror, making it unsuitable for a large number of applications. Moreover, there is no symmetrical design.  
         SUMMARY OF THE INVENTION  
         [0016]    The object of the present invention is to describe a cooled mirror for laser applications with a mirror surface that possesses a high degree of thermal and mechanical stability despite the simple design of the mirror.  
           [0017]    The design according to the present invention brings significant advantages:  
           [0018]    For one thing, the cooler structure is achieved by means of a particularly simple construction in that a plurality of structured copper layers, i.e. layers provided with a plurality of openings, are connected with each other on the surface by means of direct copper bonding (DCB) technology in such a manner that these openings in the copper layers form an extensive network of channels for the coolant, and the stays surrounding the openings in the single copper layers also serve as continuous posts. The longitudinal sides of these posts lie perpendicular to the plane of the mirror surface and extend across the entire thickness of the cooler structure, i.e. starting from one of the copper layers adjacent to the first intermediate layer to the copper layer adjacent to the second intermediate layer of the cooler structure.  
           [0019]    This design not only achieves the flow of coolant through the cooler structure, continuously branching out in all three perpendicular spatial axes and continuously supplying fresh coolant to the area of the first layer comprising or possessing the mirror surface, but also, due to the posts, achieves a high degree of overall mechanical stability and stiffness for the mirror. This prevents deformations of the mirror surface from thermal effects as well as mechanical force and/or pressure. In particular, the posts prevent the swelling of the cooler structure and therefore the deformation of the mirror surface due to the pressure of the coolant or unavoidable deviations in the pressure of the coolant during operation.  
           [0020]    Another particular advantage of the present invention is the fact that the mirror has a completely symmetrical design with respect to the consecutive layers that are perpendicular to the mirror surface, which means that changes in temperature cannot cause deformations of the mirror surface. The intermediate layer made of the material with a reduced heat expansion coefficient that is connected directly to the first layer with the mirror surface, not only further improves the thermal stability of the first layer comprising or possessing the mirror surface, but this intermediate layer also especially ensures an even cooling effect on the first layer comprising or possessing the mirror surface. How crucial the mechanical stiffness and stability of a mirror for laser applications is, can be seen from the fact, for example, that even the bending of tubes for the inlet and outlet of coolant can cause changes in the mirror surface, which adversely affects the quality of the laser beam.  
           [0021]    Use of the DCB technology prevents solder layers between the individual copper layers. This is essential not only for preventing corrosion, but also creating fine structures for the structured copper layers, i.e. the layers provided with holes and openings, which produces a high degree of homogeneity in cooling of the mirror surface. Furthermore, the use of the DCB technology also provides a high degree of stability, since there are no solder layers, which can be softer than the copper of the copper layers.  
           [0022]    A very essential advantage of the design according to the present invention consists in the fact that the cooler structure, and therefore also the mirror in general, possesses the same stability in both axis directions defining the plane of the mirror surface on the top or mirror surface. This is a prerequisite for proper finishing of the mirror surface. The same stiffness of the mirror in both spatial axes defining the mirror surface is achieved by the fact that the cooler structure consists of the copper layers that are stacked one above the other and connected on the surface which each other and that are structured, i.e. provided with openings, instead of using groove-shaped cooling channels.  
           [0023]    Moreover, the DCB technology improves the ability to process the copper during manufacture of the mirror surface, i.e. texturing of the copper is achieved, which enables significantly more even treatment of the surface.  
           [0024]    The manufacture of the mirror surface in the present invention is achieved by suitable finishing of the top of the first layer facing away from the cooler structure, preferably by means of machining using a tool with a defined cutter, and thereby preferably by means of a diamond tool. The suitable process is also known as diamond turning.  
           [0025]    The copper used in the mirror according to the present invention is preferably oxygen-free copper. This significantly increases the life of the mirror surface, due to the increased resistance against corrosion.  
           [0026]    In the embodiment according to the present invention, the first intermediate layer made of the material with a reduced heat expansion coefficient that is directly connected to the first layer of copper comprising or possessing the mirror surface achieves a high degree of thermal stability for the mirror surface, so that for example a change in the laser power and/or temperature deviations from switching the laser on and off do not result in a negative deformation or alteration of the mirror surface.  
           [0027]    In order to prevent the bimetal effect, i.e. warping of the mirror and especially of the mirror surface formed by the top due to temperature deviations, the multi-layer mirror body is symmetrical to a middle plane and therefore has a second intermediate layer made of the material with a reduced heat expansion coefficient that is connected to the second layer of copper forming the bottom of the mirror.  
           [0028]    For the at least one intermediate layer made from the material with a reduced heat expansion coefficient, a material such as ceramic can also be used, which additionally provides for improved stability of the mirror.  
           [0029]    As described above, the cooler structure consists of several connected cooler layers that are then structured in such a way that they form a cooler or cooler structure area with a fine structure of passageways for the coolant that continuously branch out in two, or preferably three spatial axes extending perpendicular to each other. The continuous posts bridging the cooler structure not only improve the cooling effect but also ensure the necessary mechanical stability and stiffness, especially pressure resistance of the mirror in the area of the cooler structure. Additionally, copper or a copper alloy is used for the cooler layers due to its high heat conducting properties.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]    The present invention will be more clearly understood from the following detailed description of a representative embodiment thereof in conjunction with the following drawings:  
         [0031]    [0031]FIG. 1 is a simplified representation in side view of a mirror for laser applications according to the present invention;  
         [0032]    [0032]FIG. 2 is a top view of the mirror shown in FIG. 1;  
         [0033]    [0033]FIG. 3 is a top view of the layout of an embodiment of a structured cooler layer for use in the cooled laser mirror of FIGS. 1 and 2; and  
         [0034]    [0034]FIG. 4 a top view of the layout formed by two connected cooler layers. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]    The mirror generally designated  1  in the figures is intended for use in high-energy lasers, for reflecting the laser beam. The mirror  1  is constructed of several layers that are connected with each other on the surface in a suitable manner. In particular, the mirror  1  has the design described below, where the various areas and layers are connected with each other beginning from the top  2  of the mirror toward the bottom  3  of the mirror in the following order:  
         [0036]    top copper layer  4  that forms the mirror surface on its open top side  2 ;  
         [0037]    intermediate layer  5 ;  
         [0038]    top copper or end layer  6  for the cooler structure  7  located beneath;  
         [0039]    bottom copper or end layer  8  for the cooler structure  7 ;  
         [0040]    intermediate layer  9 ;  
         [0041]    bottom copper layer  10  that forms the bottom  3  of the mirror  1 .  
         [0042]    The intermediate layers  5  and  9  are made of a material that has a much lower heat expansion coefficient than the copper of the copper layers, namely equal to or less than 10×10 −6  [1/K].The intermediate layers  5  and  9  are preferably made from the same material.  
         [0043]    The cooler structure  7  consists of a plurality of copper layers  11  that are connected with each other on the surface, the surfaces of which are also parallel to the top  2  and bottom  3  of the mirror b.  
         [0044]    As depicted in FIGS. 3 and 4, these cooler layers  11  are structured, i.e. provided with a plurality of openings  11 ′ and  11 ″ and material stays surrounding these openings, in such a way they form two chambers  12  and  13  for the inlet and outlet of a preferably liquid coolant, for example water, and between these chambers a cooling area  14  with a finely structured network of passageways formed by the openings  11  for the coolant, which paths continuously branch out in all three perpendicular spatial axes, so that the coolant flows through this cooling area  14  being constantly diverted in all three spatial axes, thus producing an intensive cooling effect.  
         [0045]    Furthermore, the cooler layers  11  are structured in such a way that continuous posts  15  made of copper are formed from the structuring or sections of the material stays surrounding the openings  11 ′ in the connecting cooler layers, the longitudinal sides of which posts are perpendicular to the levels of the layers of the mirror  1  and, as the remaining wing-like sections of the structured cooler layers  11  protruding from these posts  15 , are also subjected to the intensive flow of coolant. The posts  15  are needed for the effective infusion of the heat to be discharged into the cooling area  14  or the coolant. The posts  15  are also needed for the mechanical stability of the mirror, especially to prevent the expansion of the cooler structure  7  and therefore of the mirror  1  in general or warping of the mirror surface formed by the top  2  by the pressurized coolant. The mutual distance of the posts  15  is, for example, 1.0-8 mm, whereby the diameter of these posts is approximately half the distance between the posts.  
         [0046]    In the depicted embodiment, the layers  4 ,  6 ,  8  and  10  and the cooler layers  11  are made of a copper foil. The intermediate layers  5  and  9  are made of ceramic, for example aluminum oxide or aluminum nitride ceramic. The intermediate layers  5  and  9  can also be made of a metal matrix compound material with a heat expansion coefficient equal to or less than 10×10 −6  [1/K], e.g. of copper silicon carbide or aluminum silicon carbide.  
         [0047]    If the intermediate layers  5  and  9  are made of ceramic, especially aluminum oxide ceramic, then the individual layers are to be connected with each other on the surface in an especially simple manner using the common DCB technology, as described for example in U.S. Pat. No. 3,744,120 or in DE-PS 23 19 854. Another suitable method of connecting the layers is active soldering technology.  
         [0048]    As shown in the figures, the layers  4 - 6  and the cooler layers  11  in the depicted embodiment have a circular shape with the same diameter, so that the part of the mirror  1  formed by these layers has the shape of a flat cylinder. The copper layers  8  and  10  and the intermediate layer  9  in the depicted embodiment both are square with the same edge dimensions, which are larger than the diameter of the circular layers  4 - 6  and  11 . The layers  8 - 10  therefore form a flange  16  with a square cross-section extending evenly beyond the remaining circumference of the mirror  1  and on the same axis as the cylindrical part of the mirror  1 . The corner areas the flange  16  are provided with fastening holes  17  for fastening the mirror to a holder not depicted.  
         [0049]    The bottom  3  is provided with connections  18 , one connection  18  for each of the chambers  12  and  13 . These connections, which are also made of copper, are fastened in a suitable manner, e.g. by means of DCB technology, to the layer  10 . For each connection  18  there is a continuous bore hole  19  in the layer  8 - 10 , through which the connection  18  is connected with its chamber  12  or  13 . For use of the mirror  1 , the connections  18  are connected to an advance or return element for the coolant.  
         [0050]    The intermediate layer  5  made of the material with a reduced heat expansion coefficient, for example of ceramic, is needed essentially to reduce the actual heat expansion of the layer  4 , caused for example by switching the laser beam reflected by the mirror  1  on and off or by changes in the power of the laser beam and/or by deviations in the coolant flow to the extent that the quality and function of the mirror  1  is not adversely affected by temperature changes in the layer  4 . In order to ensure this, the thickness of the intermediate layer  5  is greater than the thickness of layer  4 . The thickness of the layer  5  is preferably between approximately 0.2 and 5 mm and the thickness of the layer  4  between approximately 0.1 and 0.6 mm.  
         [0051]    Furthermore, the mirror  1  has a design that, with respect to the order of layers and the materials used for these layers, is symmetrical to an imaginary middle plane M that lies parallel to the top  2  and bottom  3  and also parallel to the layers  4 - 6 ,  8 - 10  and  11  and extends approximately in the middle of the cooler structure  7 . For this reason, the intermediate layer  9  is provided for between the copper layer  10  forming the bottom  3  and the copper layer  8  closing the cooler structure  7  on the bottom, which (layer  9 ) is of the same material as the layer  5  and also has the same thickness as the layer  5 . Furthermore, the layer  10  has the same thickness as the layer  4  and the layers between layers  5  and  9  on both sides of the middle plane M have the same thickness.  
         [0052]    For reasons of simplifying the manufacturing process, all copper layers  4 ,  6 ,  8 ,  10  and  11  are made of a copper sheet or copper foil of the same thickness. The symmetrical design to the middle plane M also prevents the “bimetal effect”.  
         [0053]    The flange  16  in the depicted embodiment has an inherent symmetrical design to the middle plane M′ of the intermediate layer  9 . Therefore, the copper layers  8  and  10  located on each side of this intermediate layer have the same thickness. This symmetrical construction of the flange to the middle plane M′ prevents warping of the flange  16  due to temperature deviations during use, therefore making adjustment of the mirror  1  unnecessary. Due to the use of ceramic for the intermediate layer  9 , the flange  16  retains sufficient mechanical stability despite the use of copper for the layers  8  and  10 .  
         [0054]    The top  2  is processed to form a mirror surface, i.e. for example polished and/or diamond milled, in such a way that the surface roughness is less than 10 nm and deviations in the flatness are in any case less than 1 μm. In order to refine the mirror surface and/or improve the properties, especially the optical properties (reflection properties) of this mirror surface, a further thin layer of metal (precious metal), such as Ni, Au, Ag, Pt, Pd or an alloy can be applied to the finished top  2 , for example with a thickness of 0.1 to 20 μm.  
         [0055]    In the embodiment described above the layers  4 , and for reasons of symmetry also the layers  6 ,  8 ,  10  and  11  are made of copper, since this material is especially easy to process, possesses excellent heat conducting properties and therefore ensures optimum cooling of the mirror  1  on the mirror surface, and since copper also has good reflection properties for the standard laser optical wave length.  
         [0056]    The copper used for the layers  4 ,  6 ,  8 ,  10  and  11  is preferably low-oxygen or oxygen-free copper, which significantly increases the life of the mirror surface. To produce the mirror surface the layer  4  is machined or milled on its top surface facing away from the other layers, using a machining tool with at least one defined blade, for example a diamond tool, so that the mirror surface can be said to be diamond milled. The known “diamond turning process” is suitable for this processing step.  
         [0057]    The present invention was described above based on a representative embodiment. Of course, numerous modifications and variations are possible without abandoning the inventive idea on which the invention is based. For example, it is possible to apply a hardened or tempered layer, for example of diamond, to the mirror surface for protection and/or optical refinement, e.g. by means of physical vacuum deposition (PVD). It is also possible, for example, that individual layers of the mirror consist of two single layers connected with each other on the surface.