Abstract:
A vario-astigmatic beam expander is capable of collimating an astigmatic light beam, or inducing astigmatism in a well-collimated beam, by passing the light beam through a combination of spherical and cylindrical lenses, whereby both the degree of astigmatism and the axis of astigmatism induced are continuously adjustable. The beam expander has applications in industrial laser processing systems.

Description:
COPYRIGHT NOTICE  
       [0001]    2007 Electro Scientific Industries, Inc. A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 37 CFR §1.71(d). 
       TECHNICAL FIELD  
       [0002]    This disclosure concerns using optical elements to modify properties of a light beam. 
       BACKGROUND INFORMATION  
       [0003]    In an industrial laser processing system, it may be desirable for a laser beam to have a symmetrically round cross section and for the laser beam to be collimated, that is, with light rays propagating along and parallel to an optic axis. However, in certain applications, it may be preferable to de-focus the laser beam by forcing some of the light rays to converge or diverge away from the optic axis. Such a beam with light rays that converge or diverge asymmetrically is defined as astigmatic. As an astigmatic laser beam propagates along a path through space, the laser beam spot on a target becomes increasingly asymmetric, changing shape from circular to elliptical, or “anamorphic.” Anamorphic laser beam spots, like ellipses, are characterized by their eccentricity, a measure of elongation of the ellipse. The ability to de-focus a laser beam may be advantageous when creating an autofocus control feature or protecting a workpiece from excess energy absorption (laser burning). Conversely, a laser may produce an astigmatic beam in applications requiring a well-collimated beam with no astigmatism. In such a case it is preferable to force all the light rays in the system to align with the optic axis. 
         [0004]    Correcting astigmatism in a poorly collimated beam, or inducing astigmatism in a well-collimated beam, may be achieved by passing the laser beam through a cylindrical lens, either alone or in combination with a spherical lens. A spherical lens has one or more curved surfaces that resemble the surface of a sphere; a cylindrical lens has one or more curved surfaces that resemble the surface of a cylinder. Whereas a spherical lens, such as a typical piano-convex or plano-concave lens, causes parallel rays of light to converge or diverge in all directions, a cylindrical lens causes convergence or divergence in a single plane. Thus, while spherical lenses are used to magnify or reduce image size proportionally, cylindrical lenses are used to stretch an image along a particular axis. Although a single cylindrical lens can correct or introduce astigmatism, it cannot affect the degree of asymmetry in a beam. A system of cylindrical lenses, arranged in a telescope configuration, can affect the symmetry of the beam independent of the astigmatism. 
       SUMMARY OF THE DISCLOSURE  
       [0005]    A preferred embodiment of a vario-astigmatic beam expander is capable of either introducing a continuously variable degree of astigmatism into a well-collimated laser beam or correcting a degree of astigmatism in a poorly collimated laser beam. The vario-astigmatic beam expander is based on a traditional telescope, which is comprised of two spherical lenses. Substituting a pair of cylindrical lenses for the second spherical lens allows astigmatism to be adjusted by rotating the principal axes of the two cylindrical lenses relative to each other. The angle between the principal axes is defined as the rotation angle. When the principal axes of the two cylindrical lenses are orthogonal, i.e. the rotation angle is 90 degrees, there is no astigmatism in the emerging beam, and the spot shape is circular with zero eccentricity. Moving the rotation angle away from an orthogonal orientation causes the beam to become increasingly astigmatic, and the spot shape to become more elongated. Rotating the pair of cylindrical lenses together causes rotation of the axis of astigmatism 
         [0006]    Additional aspects and advantages will be apparent from the following detailed description of preferred embodiments, which proceeds with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0007]      FIGS. 1A ,  1 B, and  1 C are diagrams of three prior art anamorphic telescopes made from various configurations of prisms. 
           [0008]      FIGS. 2A and 2B  are diagrams of, respectively, prior art Keplerian and Galilean beam expanders, which represent two examples of traditional telescopes made from spherical lenses. 
           [0009]      FIG. 3A  is a schematic of an embodiment of a vario-astigmatic beam expander, which includes a single spherical element and a pair of cylindrical elements of the same magnifying power. 
           [0010]      FIG. 3B  is an isometric view of an optical module embodying the vario-astigmatic beam expander of  FIG. 3A . 
           [0011]      FIG. 4A  is a ray diagram of a prior art fixed beam expander (telescope), which has no effect on astigmatism. 
           [0012]      FIG. 4B  is a ray diagram of a vario-astigmatic fixed-ratio beam expander in a zero-astigmatism configuration, which produces an optical output equivalent to that produced by the configuration in  FIG. 4A . 
           [0013]      FIG. 4C  is a ray diagram of a vario-astigmatic fixed-ratio beam expander in an astigmatic configuration. 
           [0014]      FIGS. 5A and 5B  are drawings showing differences between beam spots formed by anamorphic and astigmatic beams, respectively. 
           [0015]      FIG. 6A  represents a schematic combination of  FIG. 3A  and  FIG. 5B  depicting a vario-astigmatic beam expander deployed in a system implemented with scan mirrors and a scan lens. 
           [0016]      FIG. 6B  is a contour plot of light intensities for an image produced by the system of  FIG. 6A  as predicted by a computer model. 
           [0017]      FIG. 6C  is a pair of irradiance plots obtained by sectioning the contour plot shown in  FIG. 6B  along its x- and y-axes. 
           [0018]      FIG. 7  is a ray diagram of an alternative embodiment of a vario-astigmatic beam expander, in which crossed cylindrical lenses are positioned at the system input. 
           [0019]      FIGS. 8A ,  8 B, and  8 C are ray diagrams of three configurations of a conventional zoom beam expander with no provision for astigmatism. The expansion ratio in each configuration is adjusted by varying the distances between successive pairs of the three lens elements. 
           [0020]      FIG. 9  is a ray diagram of a zoom beam expander using a pair of cylindrical lenses adjusted for zero astigmatism. 
           [0021]      FIG. 10  is a ray diagram of a zoom beam expander using a pair of cylindrical lenses adjusted for a selected amount of variable astigmatism. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0022]    A “beam expander” expands a beam of parallel light rays about an optic axis (represented in the accompanying drawing as a line formed of alternating dots and dashes), to form a larger diameter beam. Beam expanders can be constructed with lenses or prisms. Both prisms and lenses magnify by decelerating light rays, causing them to bend. Prisms have straight surfaces; lenses have curved surfaces. The difference in index of refraction between glass and air determines how much deceleration occurs, and the angle of the glass surface presented to the incident light beam controls which rays within the light beam are bent first. 
         [0023]      FIGS. 1A ,  1 B, and  1 C are diagrams showing telescopic properties of three exemplary prior art prism configurations. In each example, the output beam is wider in one plane than the input beam. Hence, these are magnifying prisms, and each system is classified as a telescope. Prisms can correct asymmetry, but not astigmatism. Likewise, they can introduce asymmetry to a symmetric beam, but they cannot introduce astigmatism. Because each of the resulting light beams in  FIGS. 1A-1C  is collimated, they are all non-astigmatic. However, the change in image shape makes the resulting images asymmetric, or “anamorphic.” 
         [0024]    With reference to  FIG. 1A , a two-prism system  100  includes prisms  102  and  104  that are separated by an air gap  106 . Prisms  102  and  104  have substantially the same index of refraction and are of substantially the same shape. An input beam  108  of size do defined by parallel light rays  110  enters prism  102 , and after propagation through prism  102 , air gap  106 , and prism  104 , exits prism  104  as an output beam  112  of size d 1  defined by parallel light rays  114 . Prisms  102  and  104  are positioned and oriented relative to each other so that prism  102  angularly displaces principal light ray  108   p  of input beam  108  from its original direction of propagation to form an intermediate principal light ray  108   i  in air gap  106 . Prism  104  angularly displaces intermediate principal light ray  108   i  from its direction of propagation to form principal light ray  112   p  of output light beam  112  propagating in a direction parallel to, but laterally offset by a distance Δy from, the original direction of propagation of principal light ray  108   p.    
         [0025]    With reference to  FIG. 1B , a four-prism system  120  eliminates the lateral offset of the principal axes of an input beam  122  and an output beam  124 . System  120  constitutes two two-prism systems  100  arranged in optical series and includes prisms  126 ,  128 ,  130 , and  132 , adjacent ones of which are mutually spaced apart by air gaps. Prisms  126 ,  128 ,  130 , and  132  have substantially the same index of refraction and are of substantially the same shape. Prisms  126 ,  128 ,  130 , and  132  are positioned and oriented relative to one another to produce from input beam  122  an output beam  124  having its principal light ray  124   p  that is coaxial with principal light ray  122   p  of input beam  122 . 
         [0026]    With reference to  FIG. 1C , a single prism  140  also does not produce a lateral offset of the principal light rays of an input beam  142  and an output beam  144 . Light rays  146  of input beam  142  enter prism  140  at its input surface  148  and undergo internal reflection at a glass/air interface  149  to form parallel light rays  150  that propagate through prism  140  and exit its output surface  152  as output beam  144  of parallel light rays  154 . Principal light ray  144   p  of output beam  144  is coaxial with principal light ray  142   p  of input beam  142 . The advantage of this single prism configuration is that it produces a magnified output beam  144  using only one optical element, ie., prism  140 . The  FIG. 1C  example also illustrates, however, the inherent inefficiency of prismatic systems in that each time a light beam encounters a glass/air interface, a portion of the incident light beam energy is transmitted and the remaining energy is reflected. The amount of energy in the transmitted or reflected component of light propagation that is not recaptured in the system is therefore lost. 
         [0027]      FIGS. 2A and 2B  show examples of, respectively, Keplerian and Galilean telescopes built with lenses rather than prisms. Lenses bend the propagation directions of incident light according to the indices of refraction, curvatures of glass surfaces, and distances between successive elements of the lenses. While manufacturing curved lenses is more difficult than manufacturing flat prisms, an advantage of lenses over prisms is that they are optically axial, i.e., the output beam is coaxial with the input beam. This means that no lateral offset occurs. 
         [0028]    A Keplerian telescope  160  shown in  FIG. 2A  includes a convex-plano lens  162  that receives an input light beam  164  formed of parallel light rays  166  and converges them to a principal focus  168  at a focal length f 1 . The image focused at f 1  becomes a source image for a second, larger piano-convex cylindrical lens  170  with focal length f 2 . Lens  170  collimates the light rays incident to it and produces an output light beam  172 . Input light beam  164  and output light beam  172  are coaxial. A Galilean telescope  180  shown in  FIG. 2B  includes a concave-plano lens  182  that diverges light rays  166  of input light beam  164 , which a plano-convex lens  184  collimates to produce output light beam  172 . The greater width of output light beam  172  as compared with the width of input light beam  164  indicates that telescopes  160  and  180  magnify images carried by input light beam  164 . Lenses  170  and  184  ensure production of collimated output light beams  172 . 
         [0029]      FIG. 3A  shows a preferred embodiment of a vario-astigmatic beam expander  200 , which is based on Galilean telescope  180  of  FIG. 2B . Beam expander  200  comprises a spherical lens  202  for isotropic beam expansion greater than one and first and second cylindrical lenses  206  and  208  of the same magnifying power for symmetric beam expansion greater than one. (Cylindrical lenses  206  and  208  take the place of spherical lens  184  in Galilean telescope  180 .) Spherical lens  202  and cylindrical lenses  206  and  208  are arranged in optical series along a system optic axis  210 . 
         [0030]    First cylindrical lens  206  has a convex surface  212  and a piano surface  214 , and second cylindrical lens  208  has a piano surface  216  and a convex surface  218 . In a preferred embodiment, cylindrical lenses  206  and  208  are positioned in proximity to each other with their respective piano surfaces  214  and  216  set in confronting relationship. Cylindrical lenses  206  and  208  are mounted for rotation about system optic axis  210  so that their respective principal axes  220  and  222  can be angularly displaced relative to each other or rotated together at a fixed angular displacement. Rotation of cylindrical lenses  206  and  208  can be accomplished by manual adjustment ( FIG. 3B ) or motive force applied by powered mechanism (not shown). 
         [0031]      FIG. 3A  shows cylindrical lenses  206  and  208  with their respective optic axes displaced by 90 degrees. An isotropically expanding input beam propagating from spherical lens  202  is of a size that is encompassed by the region of overlap of piano surfaces  214  and  216 . Cylindrical lens  206  collimates the input beam in a first plane, and cylindrical lens  208  collimates the input beam in a second, orthogonal plane. 
         [0032]    When they are rotated about system optic axis  210  such that their principal axes  220  and  222  are set at a displacement angle  230  of 90 degrees, cylindrical lenses  206  and  208  cooperate to function as a symmetric lens that imparts to the output beam no amount of astigmatism relative to that of the input beam. When they are rotated about system optic axis  210  such that their principal axes  220  and  222  assume various displacement angles  230  that differ from 90 degrees, cylindrical lenses  206  and  208  cooperate to impart to the output beam different amounts of astigmatism corresponding to the measure of displacement angle  230 . When they are rotated together about system optic axis  210  such that their principal axes  220  and  222  remain at a fixed displacement angle  230 , cylindrical lenses  206  and  208  cooperate to impart to the output beam a fixed amount of astigmatism at a variable axis of astigmatism corresponding to the extent of the rotation. Each cylindrical lens in vario-astigmatic beam expander  200  can be replaced with a multi-lens system performing the same function as a single lens. 
         [0033]      FIG. 3B  shows an optical module  240  embodying beam expander  200  of  FIG. 3A , complete with mounting and adjustment hardware. Optical module  240  includes a mounting plate  242  to which are releasably coupled a lens mount  244  for spherical lens  202  and a lens mount  246  for a tubular cell  248  in which cylindrical lenses  206  and  208  are housed. In a preferred embodiment, spherical lens  202  has a focal length of −6.21 mm, and cylindrical lenses  206  and  208  each have focal lengths of 200 mm. 
         [0034]    Lens mount  244  is attached to a translational stage  250  that is slidably mounted for movement along a surface  252  of mounting plate  242  in the direction of optic axis  210  (z-axis). Slots  254  in translational stage  250  allow for axial position adjustment of spherical lens  202  relative to cylindrical lenses  206  and  208 . The lengths of slots  254  restrict the axial position of spherical lens  202 , which a user fixes in place by tightening set screws  256  (one shown). Thumbscrews  258  provide user controllable x-axis and y-axis position adjustment of spherical lens  202 . 
         [0035]    Lens mount  246  is slidably attached to a translational stage  262  that is fixed to mounting plate  242 . An adjustment knob  264  provides x-axis position adjustment of translational stage  262  and thereby cell  248  that houses cylindrical lenses  206  and  208 . Cell  248  has mounted to its surface rotational adjustment mechanisms  268 ,  270 , and  272  for varying the orientation of cylindrical lenses  206  and  208  about optic axis  210 . Rotational adjustment mechanism  268  rotates cylindrical lens  206  about optic axis  210 ; rotational adjustment mechanism  270  rotates cylindrical lens  208  about optic axis  210 ; and rotational adjustment mechanism  272  rotates lenses  206  and  208  together about optic axis  210 , thus preserving displacement angle  230  between their principal axes  220  and  222  while rotating the axis of net cylindrical power. When lenses  206  and  208  are set with their respective principal axes  220  and  222  orthogonal to each other, the resultant focal length is approximately equivalent to a 200 mm spherical lens. The axial spacing between lenses  206  and  208  in a preferred embodiment is 0.5-1 mm. 
         [0036]      FIGS. 4A ,  4 B, and  4 C are ray diagrams corresponding to, respectively, the lens system of Galilean telescope  180  shown in  FIG. 2A  and two configurations of the vario-astigmatic beam expander  200  shown in  FIG. 3A . Comparison of  FIGS. 4A and 4B  demonstrates the equivalence of the output beams of vario-astigmatic beam expander  200  and Galilean telescope  180  when vario-astigmatic beam expander  200  is in its zero-astigmatism configuration, i.e., when principal axes  220  and  222 , corresponding to the respective cylindrical lenses  206  and  208  are orthogonally aligned. In both cases, parallel rays  166  of input light beam  164  are expanded, in similar fashion, into an intermediate divergent beam and then re-collimated into (non-astigmatic) output beam  172 . Whereas, as shown in  FIG. 4C , vario-astigmatic beam expander  200  in its astigmatic configuration, with non-orthogonally aligned principal axes  220  and  222 , ultimately produces output beam  173  with non-parallel, asymmetrically converging rays. 
         [0037]      FIGS. 5A and 5B  illustrate spot shape differences between an astigmatic beam and a collimated anamorphic beam, respectively. With reference to  FIG. 5A , a collimated light beam  280 , although composed of parallel light rays, forms an anamorphic image with an elliptical cross section  282  at the entrance surface of a focusing lens  284 . Collimated beam  280  propagates through focusing lens  284 , which converges the light rays of beam  280  to a point  286  lying in a single focal plane  288 . With reference to  FIG. 5B , an astigmatic light beam  290  forms an image with a circular cross section  292  at the entrance surface of focusing lens  284 . Astigmatic beam  290  propagates through focusing lens  284 , which converges the light rays of beam  290  to form elliptical spots  294  and  296  in separate focal planes located on either side of a plane in which there is an unfocused circular spot  298 . Thus, the light rays of astigmatic beam  290  do not converge to a point at circular spot  298 , whereas some of the light rays of astigmatic beam  290  converge at elliptical spots  294  and  296 . 
         [0038]      FIGS. 6B and 6C  present energy distribution data at one focal point of an image created by a computer model of vario-astigmatic beam expander  200 . The computer-generated data in  FIGS. 6B and 6C  correspond to the incidence of astigmatic light beam  290 , as diagrammed in  FIG. 5B . With reference to the lens diagram shown in  FIG. 6A , an initially collimated beam  300  is made astigmatic by a beam expander  302 . The configuration of lenses inside the dashed box, similar to the configuration in  FIG. 3A , includes a single spherical lens  202  that spreads a collimated beam  300  isotropically, and cylindrical lenses  206  and  208  that have been rotated to produce a slightly astigmatic output beam  304 . Two scan mirrors  306  deflect slightly astigmatic beam  304  downward through a series of optical elements  308  comprising a focusing scan lens  310  that focuses beam  304  onto a focal plane  312 , which resides, for example, on a surface of a workpiece undergoing laser processing. 
         [0039]    The graph in  FIG. 6B  is an iso-irradiance contour plot  314  of an elliptical focused laser spot  316  formed on the work surface at focal plane  312 . Elliptical focused laser spot  316  corresponds either to elliptical spot  274  or to elliptical spot  276  in  FIG. 5B , depending on which focal length distance is chosen as the position of focal plane  312 . The major axis of elliptical image  316  is rotated clockwise a few degrees relative to the vertical axis because cylindrical lenses  206  and  208  were slightly rotated as a unit. Each elliptical contour  318 - 334  represents a 10% decrease in irradiance, starting from the center out, as detailed in Table 1 below: 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Contour 
                 Low Intensity 
                 High Intensity 
               
               
                   
                 Reference Number 
                 value 
                 value 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 318 
                 2015 
                 2266 
               
               
                   
                 320 
                 1763 
                 2015 
               
               
                   
                 322 
                 1511 
                 1763 
               
               
                   
                 324 
                 1259 
                 1511 
               
               
                   
                 326 
                 1007 
                 1259 
               
               
                   
                 328 
                 755 
                 1007 
               
               
                   
                 330 
                 504 
                 755 
               
               
                   
                 332 
                 252 
                 504 
               
               
                   
                 334 
                 0 
                 252 
               
               
                   
                   
               
             
          
         
       
     
         [0040]    Corresponding light intensities along the x- and y-axes are shown  FIG. 6C , each of which represents the intensity along a cut line through the contour plot  314  of elliptical image  316 . A narrower peak  336  along the x-axis results because beam  304  is well-collimated in the x-direction, whereas a wider peak  338  along the y-axis results from the expanded image in the y-direction. If the other focal length were chosen, the orientation of the focused spot would rotate 90 degrees, causing wider peak  338  to extend along the x-axis and narrower peak  336  to extend along the y-axis. 
         [0041]    An alternative embodiment  350  of vario-astigmatic beam expander  200  is shown in  FIG. 7 , with crossed cylindrical lenses  206  and  208  placed in the light path of the input beam before, instead of after, spherical lens  202 . This system is better suited to accepting an astigmatic beam, correcting the astigmatism, and then expanding the corrected beam into a collimated beam. 
         [0042]    Another application of the cylindrical lens pair  206  and  208  featured in vario-astigmatic beam expander  200  is a zoom beam expander. With reference to  FIGS. 8A ,  8 B, and  8 C, a conventional zoom beam expander  352  can be constructed with a series of three lenses,  354 ,  356 , and  358 , in which magnification is determined by varying the distances between successive pairs of the lenses. Various configurations of such an embodiment, yielding expansion ratios of between 1 and 2.5 times the initial image size can be constructed according to Table 2 below: 
         [0000]                                                      TABLE 2                       Configuration/   Distance from lens   Distance from lens 366           expansion ratio   364 to lens 366, mm   to lens 368, mm                                        1/1:1   46   78.5           2/1:1.5   57   45           3/1:2.5   74.5   12.8                        
In general, the expansion ratio of system  352  increases with increasing distance between the first two lenses, and decreasing distance between the last two lenses. Lens elements comprising  354 ,  356 , and  358  in this embodiment can be obtained from CVI of Albuquerque, N.Mex. (Part Nos. PLCC-15.0-25.8-UV, BICX-25.4-61.0-UV, and PLCC-15.0-51.5-UV, for lenses 1, 2, and 3, respectively).
 
         [0043]      FIG. 9  shows a system  360 , the light output of which is equivalent to that of system  352 , in which a first lens element  354 , a plano-concave zoom beam expander spherical lens, has been replaced by a pair of piano-concave cylindrical lenses  206  and  208  of similar and equal power, (both CVI Part No. RCCB40.0-25.4-UV), such as those used in beam expander  200  of  FIG. 3A . Values in Table 2 characterizing system  352  are equivalent for system  360 , in which principal axes  220  and  222  of cylindrical lenses  206  and  208  are orthogonally aligned in this case. 
         [0044]    A similar zoom beam expander  362  is presented in  FIG. 10 , in which cylindrical lenses  206  and  208  have been rotated with respect to each other. System  362  is, therefore, capable of collimating an astigmatic input beam, or introducing variable astigmatism to a collimated input beam, as well as providing for variable expansion by adjusting distances to second lens  356  and third lens  358 . An alternative embodiment to the configuration in  FIG. 10  can be made by replacing lens  358 , instead of lens  354 , with the cylindrical pair of lenses  206  and  208 . 
         [0045]    It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments without departing from the underlying principles of the invention. The scope of the present invention should, therefore, be determined only by the following claims.