Abstract:
A laser material processing system and method focus a laser beam to a smaller spot size with a high power density using a movable beam expander to provide high resolution laser beam for engraving and/or cutting. A movable beam focusing assembly containing a beam expanding optics and a beam focusing optics is a part of a motion system providing a high power density focused beam within the material processing area minimizing size and weight of the laser beam positioning optics and avoiding the problems inherent in handling and positioning a larger diameter beam.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority to U.S. provisional patent application number 60/527,222, filed on Dec. 4, 2003, which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention relates to laser systems and in particular to a laser material processing system providing high resolution and power density of the laser beam.  
         [0004]     2. Description of the Related Art  
         [0005]     Laser technology has numerous applications in different scientific and commercial settings. For example, laser-based imaging and fabrication, such as engraving and cutting to form artwork or other ornamental images including lettering, has become very popular, especially in the creation of artwork on various materials, such as plastics, wood, rubber and rubber-like material, paper, etc.  
         [0006]     As the state of the art of such laser technology and applications has progressed, one trend in the laser engraving field has been to produce finer and more precise detail in the engraved and/or cut images. Typically, commercially available laser beam delivery systems easily provide resolutions at which a laser beam may be positioned on the order of about 0.01 mm. using known X-Y based motion systems. An example of a known laser beam delivery system  10  is illustrated in  FIG. 1 , having a movable beam focusing assembly  34  which travels back and forth as shown by arrows  14  on a moveable arm  15  forming an X-axis, the arm  15  moving back and forth as shown by arrows  12  on stationary arms  13  and  16 , which may be rails, forming a Y-axis and positioned over a workpiece  18  of material for engraving, etching, or cutting artwork or the like with a laser beam  20  output from a laser source  22 .  
         [0007]     The system  10  operates by directing the output laser beam  20  along paths  24 - 28  parallel to the Y-axis and the X-axis, respectively, using mirrors and/or other known optical elements. A final mirror  30  and a final focusing lens  32  are located in beam focusing assembly  34  of the system  10 , such that the final mirror  30  directs the beam  20  out of the plane of the X-Y motion system to the final focusing lens  32 . The final focusing lens  32  then focuses the resulting beam to a focal spot  36  on the subject material workpiece  18 .  
         [0008]     In an example embodiment, the laser source  22  is a typical carbon dioxide laser, providing an output beam between about 3 mm. and about 5 mm. in size, which may be focused to a focal spot typically no smaller than about 0.1 mm. at a typical focal length of about 50 mm., or about 10 times larger than the resolution at which known motion systems may position the focal spot  36 . Such configurations of known laser systems generally limit the detail which may be achieved in the cut or engraved image on the workpiece  18 .  
         [0009]     One obvious solution to such limitations in detail is to focus the output beam  20  from the laser source  22  to a smaller spot. Traditionally, such focusing is done by employing a beam expander, using known optical elements, to expand the output beam from the laser source  22  by about 2 times to about 10 times. With D being the input beam diameter, the proposed solution increases D, which then allows the beam to be focused to a spot about 2 times to about 10 times smaller. Such focusing decreases the focal spot diameter d, and so such use of a beam expander provides smaller focal spot diameters than could be achieved with an unexpanded beam, as determined by the following equation: 
 
 d   (1/e{circumflex over ( )}2) =4λ f /(π D   (1/e{circumflex over ( )}2) ) 
 
 where f is the focal length, λ is the laser wavelength, and d (1/e{circumflex over ( )}2)  and D (1/e{circumflex over ( )}2)  correspond to the focal spot diameter d and the input beam diameter D measured at the 1/e 2  points of the laser beam output from the laser source. 
 
         [0010]     In the prior art, a beam expander would typically be placed at a convenient location in the beam path close to the position where the beam  20  exits the laser source  22 . However, such an implementation presents a problem in that an expanded beam with a larger diameter requires larger and often heavier optics to direct the expanded beam along the axes and arms of the motion system. In addition to increasing costs, such a need for larger and heavier optics has other undesirable effects on the design of the motion system, such as requiring larger clearances for the beam, as well as stronger and/or larger motors to move the larger optics and support structures of the motion system, which ultimately drives up the purchase and maintenance costs to implement the laser beam delivery system  10 .  
       BRIEF SUMMARY OF THE INVENTION  
       [0011]     A laser material processing system and method focuses a laser beam to a smaller spot size with a high power density using a movable beam expander to provide high resolution laser beam for engraving and/or cutting. A movable beam focusing assembly containing at least one beam expanding optical element and at least one beam focusing optical element is a part of a motion system providing a high power density focused beam within the material processing area minimizing size and weight of the laser beam positioning optics and avoiding the problems inherent in handling and positioning a larger diameter beam. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0012]     Preferred embodiments of the invention are disclosed hereinbelow with reference to the drawings, wherein:  
         [0013]      FIG. 1  is a top perspective view of a laser beam delivery machine with a motion system of the prior art;  
         [0014]      FIG. 2  is a top perspective view of a laser beam delivery system of the present invention;  
         [0015]      FIG. 3  is a top perspective and partial cut-away view of the beam expander assembly for use in the laser beam delivery system of  FIG. 2 ;  
         [0016]      FIGS. 4-6  are side cross-sectional views of alternative embodiments of the beam expander assembly of  FIG. 3 ;  
         [0017]      FIG. 7  is a side cross-sectional view of a laser delivery system of  FIG. 2  incorporating the beam expander assembly of  FIG. 3 ; and  
         [0018]      FIG. 8  is a side cross-sectional view of an alternative embodiment of the laser delivery system of  FIG. 2  incorporating a collimator. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]     As shown in  FIGS. 2-8 , a laser material processing system  40  and method include a beam focusing assembly  42 , shown in greater detail in  FIGS. 3-6 , contained in a moveable housing  44  for engraving, cutting, or etching a workpiece  18  composed of any known material capable of being modified by engraving, cutting, or etching by a laser beam. As shown in a partial cutaway view of the housing  44  in  FIGS. 2-3 , at least one beam expanding optical element is positioned within the housing  44  to receive a laser beam  20  and to generate therefrom a substantially divergent beam  48 . The at least one beam expanding optic includes any known optical elements for expanding a laser beam.  
         [0020]     In a preferred embodiment, the at least one beam expanding optical element includes a beam expanding lens  50 . As used herein in connection with the present invention, the term “lens” is defined herein to include any known device for controlling and/or modifying the characteristics and/or direction of radiation including a laser beam. The beam focusing assembly  42  moves back and forth as shown by arrows  14  on the arm  15  of the motion system forming an X-axis and the arm  15  moves back and forth as shown by arrows  12  on the stationary arms  13  and  16  forming a Y-axis with the final beam focusing region  52  located at the comparable position of the final beam focusing assembly  34  of the prior art laser beam delivery system  10  shown in  FIG. 1 . In an example embodiment, the beam expanding lens  50  expands the beam by about 3× magnification, and the focusing lens  32  focuses the substantially divergent beam  48  as a resulting focused beam  54  to a spot significantly smaller than the focal spot from an unexpanded beam using the laser beam delivery system  10  of the prior art lacking the beam expanding optical element in the final beam focusing assembly  34  shown in  FIG. 1 .  
         [0021]     The housing  44  of the beam focusing assembly  42  may include additional optical elements such as the mirror  30  to redirect the substantially divergent beam  48  to the focusing lens  32 . Other lenses or mirrors for use in expanding and/or focusing the substantially divergent beam  48  may also be included in addition to the beam expanding lens  50 .  
         [0022]     By utilizing the beam expanding lens  50  located in the beam focusing assembly  42  and therefore substantially adjacent to the final focusing region positionable anywhere on the workpiece  18 , the laser material processing system  40  and method of the present invention maintains the output laser beam to be relatively small until just before the final focus is to occur, and then the beam is expanded to substantially fill the focusing lens  32 , thus achieving a smaller focal spot.  
         [0023]     While  FIG. 3  shows the preferred embodiment of the beam focusing assembly in which the beam expanding lens  50  is a refractive optical element and the beam focusing lens is a refractive optical element, other embodiments may be considered and included in the present invention.  FIG. 4  shows the beam focusing assembly in cross section where the beam expanding lens  50  is a reflective optical element and the beam focusing lens  32  is a refractive optical element.  FIG. 5  shows the beam focusing assembly in cross section where the beam expanding lens  50  is a reflective optical element and the beam focusing lens  32  is at least one reflective optical element.  FIG. 6  shows the beam focusing assembly in cross section where the beam expanding lens  50  is a refractive optical element and the beam focusing lens  32  is a reflective optical element.  
         [0024]     As shown in  FIGS. 7-8 , the laser material processing system  40  and method of the present invention is shown in a side cross-sectional view in a simplified form omitting the various optical elements in the paths  24 - 28  of the laser beam  20 . In the example embodiment shown in  FIG. 7 , the beam focusing assembly  42  is positioned substantially close to the workpiece  18  and relatively far downstream from the laser source  22 . Accordingly, the beam expanding lens  50  moves with the beam focusing assembly  42  of the motion system so the distance D VARIABLE  between the laser source  22  and the beam expanding lens  50  changes, while the various optical elements in the paths  24 - 28 , shown in  FIG. 2 , continue to direct the laser beam  20  to the beam focusing assembly  42 . Accordingly, the resulting focused beam  54  and its focal spot is movable in the X-Y plane to be directed to any desired location on the surface of the workpiece  18 .  
         [0025]     In an alternative embodiment shown in  FIG. 8 , the laser material processing system  40  and method of the present invention optionally includes a collimator  56  to compensate for divergence of the output laser beam  20 . The performance of the beam expanding lens  50  of the present invention is dependent on the diameter of the output laser beam entering the beam expanding lens  50 . Since the beam from a laser is typically divergent to some degree, as the beam focusing assembly  42  moves; on the motion system, the beam focusing assembly  42  may encounter a varying size or diameter of the output laser beam incident on the beam expanding lens  50 .  
         [0026]     In some implementations of the present invention, such divergence of the output laser beam incident on the beam expanding lens  50  may be minimal or within tolerance levels to perform nominally to generate the resulting focused beam  54  with its focal spot on the workpiece  18  with little or no degradation of resolution. For example, in laser material processing systems  40  with relatively small X-Y fields or dimensions, the effect of divergence may not produce any noticeable effect in the size of the focal spot as the beam focusing assembly in the X-Y motion system travels.  
         [0027]     Alternatively, to compensate for such divergence of the laser beam, especially in X-Y motion systems with relatively large X-Y fields, the collimator  56  reduces the divergence of the output laser beam  20 , and so prevents the spot size of the output laser beam  20  from varying too much from one end of the X and Y travel to the other, for example, as D VARIABLE  varies.  
         [0028]     In a preferred embodiment, the collimator  56  is positioned substantially adjacent to the laser source  22 , for example, to be fixedly positioned in a region  58  of the laser material processing system  40  shown in  FIG. 2 . In the present invention, shown in  FIG. 8 , the optional collimator  56  generates a collimated beam  60  which is incident on the beam expanding lens  50 . Accordingly, in the alternative embodiment shown in  FIG. 8 , D VARIABLE  is measured between the final optic  62  of the collimator and the beam expanding lens  50  of the beam focusing assembly  42 , as shown in  FIG. 8 .  
         [0029]     While the preferred embodiment of the present invention has been shown and described herein, it will be obvious that such embodiment is provided by way of example only. Numerous variations, changes and substitutions will occur to those skilled in the art without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.