Abstract:
A laser diode/pre-scan assembly associated with a printhead for a laser printer is presented. The laser diode/pre-scan assembly includes a pair of collimation lenses that are de-centered from the axes of a pair of laser beams to direct the pair of beams inwardly in a process direction and into a single pre-scan lens. A corresponding method of constructing a laser diode/pre-scan assembly for a laser printer is also presented.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to a laser diode/pre-scan assembly for use in a laser printer and, more particularly, to a method of converging and directing a laser beam by de-centering a collimation lens relative to the laser beam center axis in a laser diode/pre-scan assembly. 
   BACKGROUND OF THE INVENTION 
   Laser printers typically include a printhead for generating a scanning laser beam to selectively charge an image that is to be printed onto a surface of a photosensitive drum from which the image will subsequently be transferred to a medium that is to be printed. Typically, the printhead will comprise a laser/diode pre-scan assembly for generating a laser beam, a scanner assembly to sweep the beam in a scan direction and a post-scan assembly to focus the beam and direct it onto the surface of the photosensitive drum at a proper position. 
   A printhead for a modern color laser printer may include four separate laser diodes to generate four separate laser beams corresponding, for example, to the colors yellow, cyan, magenta and black. In the pre-scan optical assembly, the individual beams are collimated and directed onto facets of a single rotating polygonal mirror in the scanning assembly. The facets of the rotating mirror sweep the individual beams across the surfaces of a plurality of mirrors and through f-theta lenses within the post-scan assembly. The beams then scan across surfaces of four corresponding photosensitive drums within the printer. 
   Referring now to  FIG. 1 , a conventional laser diode/pre-scan optical arrangement for use in a laser printer is shown diagrammatically and referred to generally by reference numeral  100 . The conventional arrangement  100  includes a first laser diode  110  emitting a first laser beam  112 , having a first beam center axis  114 . 
   The first laser beam  112  diverges in both a process direction P and in a scan direction S upon leaving the first laser diode  110 . In  FIG. 1 , the scan direction S is a direction in and out of the plane of the paper and is indicated by a point S. In the illustrated conventional arrangement  100 , the first laser beam  112  diverges in the process direction P at an angle of about 8 degrees and in the scan direction S at an angle within a range of about 25 degrees to about 35 degrees. 
   A first structure  116 , defining a first aperture  118 , is positioned in the path of the first laser beam  112  such that a center portion  120  of the first laser beam  112  passes through the first aperture  118  and a peripheral portion of the first laser beam  112  represented by rays  122  and  124  is blocked by the first structure  116 . The first aperture  118  is generally oval in shape. 
   After passing through the first aperture  118 , the center portion  120  of the first laser beam  112  strikes a first surface  126  of a first collimation lens  128 . 
   The first collimation lens  128  has optical power in the process direction P and in the scan direction S. The first collimation lens  128  further has an optical axis  130  passing through a mechanical center  132  of the first collimation lens  128 . In the conventional arrangement  100  illustrated in  FIG. 1 , the first collimation lens  128  is positioned such that the optical axis  130  is substantially coaxial with the first beam center axis  114 . 
   The first collimation lens  128  has a focal length in the focus direction F defined as a distance between the mechanical center  132  of the lens  128  and a point (not shown) where light rays that are parallel with the lens  128  optical axis  130  will converge to a point after passing through lens  128 . In the conventional arrangement illustrated in  FIG. 1 , the first collimation lens  128  is positioned relative to the first laser diode  110  in the focus direction F such that a distance F 1  between the point where the first laser beam  112  is emitted from the first laser diode  110  and the mechanical center  132  of the first collimation lens  128  is substantially equal to the focal length of the first collimation lens  128 . In this fashion, the rays of the generally diverging first laser beam  112  emitted from the first laser diode  110  are collimated by the first collimation lens  128  such that a substantially collimated first laser beam  134  comprising substantially parallel rays  136 ,  138  and  140  is created as the first laser beam  114  passes through the first collimation lens  128 . 
   The substantially collimated first laser beam  134  now strikes a pre-scan lens  142 . The pre-scan lens  142  is a cylindrical lens having optical power in the process direction P only. The pre-scan lens  142  causes the rays  136 ,  138  and  140  of the substantially collimated first laser beam  134  to bend inward in the process direction P and further causes the rays  136 ,  138  and  140  to converge in the process direction P such that a converging first laser beam  144  comprising rays  146 ,  148  and  150  is created. The beam  144  is directed toward and converges to a point  152  in the process direction on a surface  154  of a scanner mirror, shown only partially in  FIG. 1 . 
   The conventional arrangement  100  also includes a second laser diode  158  emitting a second laser beam  160 , having a second beam center axis  162 . In the conventional arrangement  100  illustrated, the second laser diode  158  is separated from the first laser diode  110  in the process direction P such that the second beam center axis  162  is separated from the first beam center axis  114  by a distance P 1  in the process direction P. 
   The second laser beam  160  diverges in both the process direction P and in the scan direction S upon leaving the second laser diode  160 . The second laser beam  160  diverges in the process direction P at an angle of about 8 degrees and in the scan direction S at an angle within a range of about 25 degrees to about 35 degrees. 
   A second structure  164  defining a second aperture  166  is positioned in the path of the second laser beam  160  such that a center portion  168  of the second laser beam  160  passes through the second aperture  166  and a peripheral portion of the second laser beam  160  represented by rays  170  and  174  is blocked by the second structure  164 . The second aperture  166  is generally oval in shape. 
   After passing through the second aperture  166 , the center portion  168  of the second laser beam  160  strikes a first surface  176  of a second collimation lens  178 . In the illustrated conventional arrangement  100 , the structure  164 , defining the aperture  166 , is positioned about 1 mm in the focus direction F from the first surface  176  of the second collimation lens  178 . 
   The second collimation lens  178  has optical power in the process direction P and in the scan direction S. The second collimation lens  178  further has an optical axis  180  passing through a mechanical center  182  of the second collimation lens  178 . In the conventional optical arrangement illustrated in  FIG. 1 , the second collimation lens  178  is positioned such that the optical axis  182  is substantially coaxial with the second beam center axis  162 . 
   The second collimation lens  178  has a focal length in the focus direction F defined as a distance between the mechanical center  182  of the lens  178  and a point (not shown) where light rays that are parallel with the optical axis  180  of the lens  178  will converge to a point after passing through the lens  178 . In the conventional arrangement  100  illustrated in  FIG. 1 , the second collimation lens  178  is positioned relative to the second laser diode  158  in a focus direction F such that a distance F 1  between the point where the second laser beam  160  is emitted from the second laser diode  158  and the mechanical center  182  of the second collimation lens  178  is substantially equal to the focal length of the second collimation lens  178 . In this fashion, the rays of the generally diverging second laser beam  160  emitted from the second laser diode  158  are collimated by the second collimation lens  178  such that a substantially collimated second laser beam  184  comprising substantially parallel rays  186 ,  188  and  190  is created as the second laser beam  160  passes through the second collimation lens  178 . 
   The substantially collimated second laser beam  184  now strikes the pre-scan lens  142 . The pre-scan lens  142  is a cylindrical lens having optical power in the process direction P only. The pre-scan lens  142  causes the rays  186 ,  188  and  190  of the substantially collimated second laser beam  184  to bend inward in the process direction P and further causes the rays  186 ,  188  and  190  to converge in the process direction P such that a converging second laser beam  192  comprising rays  194 ,  196  and  198  is created. The second beam  192  is directed toward and converges to a point in the process direction that is near or at the same point  152  on the surface  154  of the scanner mirror  156  where the converging first laser beam  144  strikes the surface  154  of the scanner mirror  156 . 
   In the conventional optical arrangement illustrated, the distance P 1  in the process direction between the first beam center axis  114  and the second beam center axis  162  is substantially equal to a distance P 2  between the optical axis  130  of the first collimation lens  128  and the optical axis  180  of the second collimation lens  178 . Additionally, as previously mentioned, the first and second beam center axes  114  and  162  of the first and second laser beams  112  and  160  are substantially coaxial with the first and second optical axes  130  and  180 , respectively, of the first and second collimation lenses  128  and  178 . As a result, the first and second collimation lenses  128  and  182  serve to collimate the first and second laser beams  112  and  160 , respectively, creating the substantially collimated laser beams  134  and  184 . 
   SUMMARY OF THE INVENTION 
   In accordance with a first aspect of the present invention, a laser pre-scan assembly for use in a laser printer is provided The pre-scan assembly may comprise a first laser diode for emitting a first laser beam having a first beam center axis and a second laser diode for emitting a second laser beam having a second beam center axis. The first beam center axis and the second beam center axis may be separated from one another by a first distance in the process direction. The pre-scan assembly may further comprise a first collimation assembly comprising a first collimation lens having a first optical axis and causing the first laser beam to converge in a scan direction and in the process direction, and a second collimation assembly comprising a second collimation lens having a second optical axis and causing the second laser beam to converge in the scan direction and in the process direction. The first optical axis may be separated from the second optical axis by a second distance in the process direction. The pre-scan assembly may further comprise a pre-scan lens configured to further converge the first laser beam and the second laser beam in the process direction and direct the first and second laser beams onto or near a common point on a surface of a scanner mirror. The first collimation lens and the second collimation lens may be positioned relative to the first laser diode and the second laser diode such that the first distance is greater that the second distance so that the first collimation lens directs the first laser beam inwardly in the process direction toward the pre-scan lens and the second collimation lens directs the second laser beam inwardly in the process direction toward the pre-scan lens. 
   The first collimation lens may direct the first laser beam inwardly in the process direction toward the pre-scan lens an amount within a range of from about 0.25 degree to about 1 degree and the second collimation lens may direct the second laser beam inwardly toward the pre-scan lens an amount within a range of from about 0.25 degree to about 1 degree. 
   The first collimation assembly may further comprise a first structure defining a first aperture for receiving the first laser beam. The first structure may be configured to block a portion of the first laser beam passing therethrough. The second collimation assembly may further comprise a second structure defining a second aperture for receiving the second laser beam. The second structure may be configured to block a portion of the second laser beam passing therethrough. 
   The first structure defining the first aperture may have a first dimension in the scan direction and a second smaller dimension in the process direction, and the second structure defining the second aperture may have a first dimension in the scan direction and a second smaller dimension in the process direction. 
   A distance in a focus direction between the first laser diode and the first collimation lens may be greater than a distance defined by a focal length of the first collimation lens, and a distance in the focus direction between the second laser diode and the second collimation lens may be greater than a distance defined by a focal length of the second collimation lens. 
   The first laser beam may converge or nearly converge in the process direction to a first point on the surface of the scanner mirror and the second laser beam may converge or nearly converge in the process direction to a second point on the surface of the scanner mirror and the first point may be near or the same point as the second point. 
   The distance between where the first laser beam and the second laser beam strike the surface of the scanner mirror may be within a range of from about 0 microns to about 200 microns. 
   The first collimation lens may comprise a double convex spherical lens and the second collimation lens may comprise a double convex spherical lens. 
   The pre-scan lens may comprise a cylindrical lens having power in the process direction. 
   In accordance with a second aspect of the present invention, a laser pre-scan assembly for use in a laser printer is provided. The laser pre-scan assembly may comprise: a laser diode for emitting a laser beam and a collimation assembly comprising a collimation lens having a centered optical axis. The collimation lens may be positioned relative to the laser diode such that the laser beam enters the collimation lens a spaced distance away from the optical axis so that the collimation lens directs the laser beam inwardly in a process direction. 
   The spaced distance may be from about 100 microns to about 400 microns. 
   The collimation lens may be positioned relative to the laser diode such that the collimation lens converges the laser beam in a scan direction and in the process direction. 
   The pre-scan assembly may further comprise a pre-scan lens configured to further converge the laser beam in the process direction and direct the laser beam onto a point on a surface of a scanner mirror. 
   The collimation assembly may further comprise a structure defining an aperture for receiving the laser beam. The structure may be configured to block a portion of the laser beam passing therethrough. 
   The structure defining the aperture may have a first dimension in the scan direction and a second smaller dimension in the process direction. 
   A distance in a focus direction between the laser diode and the collimation lens may be greater than a distance defined by a focal length of the collimation lens. 
   The laser beam may converge or nearly converge in the process direction to a point on the surface of the scanner mirror. 
   In accordance with a third aspect of the present invention a method of constructing a laser pre-scan assembly is presented. The method may comprise providing a first laser diode for emitting a first laser beam having a first beam center axis and as second laser diode for emitting a second laser beam having a second beam center axis. The first beam center axis the second beam center axis may be separated from one another by a first distance in a process direction. The method may yet comprise providing a first collimation assembly comprising a first collimation lens having a first optical axis and causing the first laser beam to converge in a scan direction and in the process direction. The method may yet further comprise providing a second collimation assembly comprising a second collimation lens having a second optical axis and causing said second laser beam to converge in the scan direction and in the process direction. The first optical axis may be separated from the second optical axis by a second distance in the process direction. The method further comprise providing a pre-scan lens configured to further converge the first laser beam and the second laser beam in the process direction and direct the first laser beam and the second laser beam onto or near a common point on a surface of a scanner mirror. The first collimation lens and the second collimation lens may be positioned relative to the first laser diode and the second laser diode such that the first distance is greater than the second distance so that the first collimation lens directs the first laser beam inwardly in the process direction toward the pre-scan lens and the second collimation lens directs the second laser beam inwardly in the process direction toward the pre-scan lens. 
   The first collimation lens may direct the first laser beam inwardly in the process direction toward the pre-scan lens an amount within a range of from about 0.25 degree to about 1 degree and the second collimation lens may direct the second laser beam inwardly in the process direction toward the pre-scan lens an amount within a range of from about 0.25 degree to about 1 degree. 
   Providing a first collimation assembly may further comprise providing a first structure defining an aperture for receiving the first laser beam. The first structure may be configured to block a portion of the first laser beam passing therethrough. 
   Providing a second collimation assembly may further comprise providing a second structure defining an aperture for receiving the second laser beam. The second structure may be configured to block a portion of the second laser beam passing therethrough. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following detailed description of the preferred embodiments of the present invention can best be understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals, and in which: 
       FIG. 1  is a diagrammatic representation of a conventional laser diode/pre-scan assembly illustrating collimation lenses on center with the laser beam center axes and showing substantially collimated laser beams entering the pre-scan lens; 
       FIG. 2  is a diagrammatic representation of a laser diode/pre-scan assembly of a first embodiment of the present invention illustrating collimation lenses de-centered from the laser beam center axes by repositioning the laser diodes and showing converging laser beams entering the pre-scan lens; and 
       FIG. 3  is a diagrammatic representation of a laser diode/pre-scan assembly of a second embodiment of the present invention illustrating collimation lenses de-centered from the laser beam center axes by repositioning the collimation lenses and showing converging laser beams entering the pre-scan lens. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration, and not by way of limitation, specific preferred embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and that changes may be made without departing from the spirit and scope of the present invention. 
   Optical systems used in laser printers may be characterized as having three sub-systems or assemblies, namely, a laser diode/pre-scan optical assembly, a scanning assembly, and a post-scan assembly. The scanning assembly may comprise a single rotating polygonal mirror to sweep four separate laser beams generated by four separate laser diodes in a scan direction across the surfaces of a plurality of mirrors, through lenses defining an F-theta lens system and onto the surfaces of four separate photosensitive drums. As noted above, a pair of beams striking a scanner mirror in a conventional device are focused or converged in a process direction only. However, it is believed that advantages such as improved and/or less costly post-scan assembly F-theta lens systems can be employed in color laser printers if each of one or more pairs of beams is focused at least some amount in the scan direction as well as to a point in the process direction when the beam strikes the rotating polygonal mirror. It is also desirable that each beam be directed or diverted to strike the scanner mirror at or near a point where its corresponding beam forming part of the pair of beams strikes the mirror. 
   Referring now to  FIG. 2 , a laser diode/pre-scan assembly for use in a laser printer in accordance with a first embodiment of the present invention is shown diagrammatically, referred to generally by reference numeral  200 . The illustrated laser diode/pre-scan assembly  200  includes a first laser diode  210  emitting a first laser beam  212 , having a first beam center axis  214 . The first laser beam  212  diverges in both the process direction P and in the scan direction S upon leaving the first laser diode  210 . In  FIG. 2 , the scan direction S is a direction in and out of the plane of the paper and is indicated by a point S. 
   In the illustrated embodiment, the first laser beam  212  diverges in the process direction P at an angle of about 8 degrees and in the scan direction S at an angle within a range of about 25 degrees to about 35 degrees. A first structure  216 , defining a first aperture  218 , is positioned in the path of the first laser beam  212  such that an inner portion  220  of the first laser beam  212  passes through the first aperture  218  and a peripheral portion of the first laser beam  212 , represented by rays  222  and  224 , is blocked by the first structure  216 . In the illustrated embodiment, the first aperture  218  is generally oval in shape and has a maximum dimension in the process direction P within a range of about 2 mm to about 3 mm and a larger maximum dimension in the scan direction S within a range of about 4 mm to about 5 mm. 
   After passing through the first aperture  218 , the inner portion  220  of the first laser beam  212  strikes a first surface  226  of a first collimation lens  228 . In the illustrated first embodiment assembly  200 , the first structure  216 , defining the first aperture  218 , is positioned about 1 mm in a focus direction F from the first surface  226  of the first collimation lens  228 . 
   The first collimation lens  228  is a double convex spherical glass collimation lens having optical power in the process direction P and in the scan direction S. The first collimation lens  228  further has an optical axis  230  passing through a mechanical center  232  of the first collimation lens  228 . In the first embodiment assembly  200  illustrated, the first beam center axis  214  is not coaxial with the optical axis  230  of the first collimation lens  228  as will be discussed more thoroughly herein. 
   The first collimation lens  228  has a focal length in the focus direction F defined as a distance between the mechanical center  232  of the lens  228  and a point (not shown) where light rays that are parallel with the lens optical axis  230  will converge to a point after passing through the lens  228 . In the first embodiment assembly  200  illustrated in  FIG. 2 , the first collimation lens  228  is positioned relative to the first laser diode  210  in the focus direction F such that a distance F 2  between the point where the first laser beam  212  is emitted from the first laser diode  210  and the mechanical center  232  of the first collimation lens  228  is greater than the focal length of the first collimation lens  228 . As a result, the rays of the generally diverging first laser beam  214  passing through the first collimation lens  228  are caused to converge a first amount in the process direction P and a first amount in the scan direction S, wherein the first amount in the process direction P may be slightly different from the first amount in the scan direction S. Thus, the beam  234  emerging from the first collimation lens  238  is converging in the process direction P and in the scan direction S. 
   The distance F 2  is greater than the distance F 1 , see  FIG. 1 , between the point where the first laser beam  112  is emitted from the first laser diode  110  and the mechanical center  130  of the first collimation lens  128  in the conventional arrangement  100 , see  FIG. 1 . 
   Referring again to  FIG. 2 , in addition to being focused or converged in the process and scan directions by the lens  228 , the first laser beam  212  upon passing through the first collimation lens  228  is also caused to deflect inward a first amount in the process direction P toward a pre-scan lens  242  as will be discussed more thoroughly herein. 
   The pre-scan lens  242  is a cylindrical lens having optical power in the process direction P only. The pre-scan lens  242  causes the rays  236 ,  238  and  240  of the converging first laser beam  234  to bend inward in the process direction P a second amount and further causes the rays  236 ,  238  and  240  to converge in the process direction P a second amount such that a converging first laser beam  244  comprising rays  246 ,  248  and  250  is created. 
   The pre scan lens  242  has a radius R 2  defining a focal length in the focus direction F such that the converging first laser beam  244  is directed to a point  252  on a surface  254  of a scanner mirror  256 , shown only partially in  FIG. 2 . The pre-scan lens  242  also causes the rays  246 ,  248  and  250  to converge in the process direction P a second amount such that the rays  246 ,  248  and  250  will converge or nearly converge in the process direction to the same point  252  on the surface  254  of the scanner mirror  256 . 
   The angle of convergence in the scan direction downstream from the first collimation lens  228  may be from about 0.2 degree to about 0.5 degree. It is also noted that for an aperture  218  having a size of about 4.3 mm in the scan direction, the beam size at the surface  254  of the scanner mirror  256  in the scan direction may be about 3.4 mm. 
   Referring now to  FIGS. 1 and 2 , the radius R 2  of the pre-scan lens  242  is greater than a radius R 1  of the pre-scan lens  142  in the conventional arrangement  100 . As a result, the pre-scan lens  242  of the first embodiment assembly  200  has less power in the process direction P than does the pre-scan lens  142  of the conventional arrangement  100 . Thus, the pre-scan lens  242  causes the first laser beam  234  to bend inward in the process direction P a second amount that is smaller than the amount that the pre-scan lens  142  of the conventional arrangement  100  causes the first laser beam  134  to bend inward in the process direction P. 
   The pre-scan lens  242  of the first embodiment assembly  200  also causes the first laser beam  234  to converge in the process direction P a second amount that is smaller than the amount that the pre-scan lens  142  of the conventional arrangement  100  causes the first laser beam  134  to converge in the process direction P. Because the first laser beam  234  is converging in the process direction a first amount after passing through the first collimation lens  228 , the pre-scan lens  242  need further converge the first laser beam  234  in the process direction a second amount that is smaller than the amount that the pre-scan lens  142  of the conventional arrangement  100  converges the first laser beam  134  in order for the beam to converge or nearly converge in the process direction to the point  252  on the surface  254  of the scanner mirror  256  as desired. 
   The first embodiment assembly  200  also includes a second laser diode  258 , emitting a second laser beam  260 , having a second beam center axis  262 . In the first embodiment illustrated, the second laser diode  258  is separated from the first laser diode  210  in the process direction P such that the second beam center axis  262  is separated from the first beam center axis  214  by a distance P 3  in the process direction P. The distance P 3  is greater than the distance P 1  that separates the first beam center axis  114  from the second beam center axis  162  in the conventional arrangement  100 , see  FIG. 1 . 
   The second laser beam  260  diverges in both the process direction P and in the scan direction S upon leaving the second laser diode  258 . The second laser beam  262  diverges in the process direction P at an angle of about 8 degrees and in the scan direction S at an angle within a range of about 25 degrees to about 35 degrees. 
   A second structure  264 , defining a second aperture  266 , is positioned in the path of the second laser beam  260  such that an inner portion  268  of the second laser beam  260  passes through the second aperture  266  and a peripheral portion of the second laser beam  260 , represented by rays  270  and  274 , is blocked by the second structure  264 . The second aperture  266  in the first embodiment assembly  200  is generally oval in shape and has a maximum dimension in the process direction P within a range of about 2 mm to about 3 mm and a larger maximum dimension in the scan direction S within a range of about 4 mm to about 5 mm. A center point of the second aperture  266  is spaced a distance P 2  from a center point of the first aperture  218 . 
   After passing through the second aperture  266 , the inner portion  268  of the second laser beam  260  strikes a first surface  276  of a second collimation lens  278 . In the illustrated first embodiment assembly  200 , the second structure  264 , defining the second aperture  268 , is positioned about 1 mm in the focus direction F from the first surface  276  of the second collimation lens  278 . 
   The second collimation lens  278  is a double convex spherical glass collimation lens having optical power in the process direction P and in the scan direction S. The second collimation lens  278  further has an optical axis  280  passing through a mechanical center  282  of the second collimation lens  278 . In the first embodiment assembly  200  illustrated, the second beam center axis  262  is not coaxial with the optical axis  280  of the second collimation lens  278  as will be discussed more thoroughly herein. 
   The second collimation lens  278  has a focal length in the focus direction F defined as a distance between the mechanical center  282  of the lens  278  and a point (not shown) where light rays that are parallel with the lens  278  optical axis  280  will converge to a point after passing through the lens  278 . In the first embodiment assembly  200  illustrated in  FIG. 2 , the second collimation lens  278  is positioned relative to the second laser diode  258  in the focus direction F such that a distance F 2  between the point where the second laser beam  260  is emitted from the second laser diode  258  and the mechanical center  282  of the second collimation lens  278  is greater than the focal length of the second collimation lens  278 . As a result, the rays of the generally diverging second laser beam  260  passing through the second collimation lens  278  are caused to converge a first amount in the process direction P and a first amount in the scan direction S, wherein the first amount in the process direction P may be slightly different from the first amount in the scan direction S. Thus, the beam  284  emerging from the second collimation lens  278  is converging in the process direction P and in the scan direction S. The distance F 2  is greater than the distance F 1 , see  FIG. 1 , between the point where the second laser beam  160  is emitted from the second laser diode  158  and the mechanical center  182  of the second collimation lens  178  in the conventional arrangement  100 , see  FIG. 1 . 
   Referring again to  FIG. 2 , in addition to being focused or converged in the process and scan directions by the lens  278 , the second laser beam  260  upon passing through the second collimation lens  278  is also caused to deflect inward a first amount in the process direction P toward the pre-scan lens  242  as will be discussed more thoroughly herein. 
   As previously mentioned, the pre-scan lens  242  is a cylindrical lens having optical power in the process direction P only. The pre-scan lens  242  causes the rays  286 ,  288  and  290  of the converging second laser beam  284  to bend inward in the process direction P a second amount and further causes the rays  286 ,  288  and  290  to converge in the process direction P a second amount such that a converging second laser beam  292  comprising rays  294 ,  296  and  298  is created. 
   As noted above, the pre scan lens  242  has a radius R 2  defining a focal length in the focus direction F such that the converging second laser beam  292  is directed to a point in the process direction that is near or the same point  252  on the surface  254  of the scanner mirror  256  where the converging first laser beam  244  strikes the surface  254  of the scanner mirror  256 . In the first embodiment assembly  200  illustrated in  FIG. 2 , the converging second laser beam  292  strikes the surface  254  of the scanner mirror  256  at a point that is within a range of from about 0 microns to about 200 microns of the point  252  where the converging first laser beam  244  strikes the surface  254  of the scanner mirror  256 . The pre-scan lens  242  also causes the rays  294   296  and  298  to converge in the process direction P a second amount such that the rays  294 ,  296  and  298  will converge or nearly converge in the process direction to the same point  252  on the surface  254  of the scanner mirror  256 . 
   Referring now to  FIGS. 1 and 2 , as noted above, the radius R 2  of the pre-scan lens  242  is greater than the radius R 1  of the pre-scan lens  142  in the conventional arrangement  100 . As a result, the pre-scan lens  242  of the first embodiment assembly  200  has less power in the process direction P than does the pre-scan lens  142  of the conventional arrangement  100 . Thus, the pre-scan lens  242  causes the second laser beam  284  to bend inward in the process direction P a second amount that is smaller than the amount that the pre-scan lens  142  of the conventional arrangement  100  causes the second laser beam  184  to bend inward in the process direction P. 
   The pre-scan lens  242  of the first embodiment assembly  200  also causes the second laser beam  284  to converge in the process direction P a second amount that is smaller than the amount that the pre-scan lens  142  of the conventional arrangement  100  causes the second laser beam  184  to converge in the process direction P. Because the second laser beam  284  is converging a first amount in the process direction after passing through the second collimation lens  278 , the pre-scan lens  242  need further converge the second laser beam  284  a second amount in the process direction that is smaller than the amount that the pre-scan lens  142  of the conventional arrangement  100  converges the second laser beam  184  in order for the beam to converge or nearly converge in the process direction to the point  252  on the surface  254  of the scanner mirror  256  as desired. 
   In the illustrated first embodiment assembly  200 , the distance P 3  in the process direction P between the first beam center axis  214  and the second beam center axis  262  is greater than the distance P 1  in the process direction P between the first beam center axis  114  and the second beam center axis  162  of the conventional arrangement  100 , see  FIG. 1 . This may be accomplished by spacing the first laser diode  210  and the second laser diode  258  apart an equal amount in the process direction P relative to the positions of the first laser diode  110  and the second laser diode  158  in the conventional arrangement  100 , see  FIG. 1 . 
   Referring again to  FIGS. 1 and 2 , the distance P 2  in the process direction P between the optical axis  230  of the first collimation lens  228  and the optical axis  280  of the second collimation lens  278  is equal to the distance P 2  in the process direction P between the optical axis  130  of the first collimation lens  128  and the optical axis  180  of the second collimation lens  178  of the conventional arrangement  100 , see  FIG. 1 . The distance P 2  in the process direction P between the optical axis  230  of the first collimation lens  228  and the optical axis  280  of the second collimation lens  278  is also equal to the distance P 2  between the center point of the second aperture  266  and the center point of the first aperture  218 . The distance P 2  in the process direction P between the optical axis  230  of the first lens  228  and the optical axis  280  of the second lens  278  is less than the distance P 3  between the first beam center axis  214  and the second beam center axis  262 . As a result, the beam center axis  214  of the first laser beam  212  does not pass through the optical axis  230  of the first collimation lens  228  but rather strikes the first surface  226  of the first collimation lens  228  a spaced distance of about 100 microns to about 400 microns in the process direction P from the optical axis  230  of the first collimation lens  228  and in a direction toward a first end surface  228 A of the first lens  228 . This causes the first laser beam  212  to bend inward a first amount as previously mentioned. In the illustrated first embodiment assembly  200 , the first laser beam  212  is bent inward, i.e., in a direction toward a center point  242 A of the pre-scan lens  242 , an amount within a range of about 0.25 degree to about 1 degree by the first collimation lens  228 . The bending of the first laser beam  212  a first amount by the first collimation lens  228  and the subsequent bending of the first laser beam  212  a second amount by the pre-scan lens  242  results in the first beam  212  striking the scanner mirror  256  in the process direction at the point  252 . 
   The beam center axis  262  of the second laser beam  260  does not pass through the optical axis  280  of the second collimation lens  278  but rather strikes the first surface  276  of the second collimation lens  278  a spaced distance of about 100 microns to about 400 microns in the process direction P from the optical axis  280  of the second collimation lens  278  and in a direction toward a first end surface  278 A of the second lens  278 . This causes the second laser beam  260  to bend inward a first amount as previously mentioned. In the illustrated first embodiment assembly  200 , the second laser beam  260  is bent inward, i.e., in a direction toward the center point  242 A of the pre-scan lens  242 , an amount within a range of about 0.25 degree to about 1 degree by the second collimation lens  228 . The bending of the second laser beam  260  a first amount by the second collimation lens  278  and the subsequent bending of the second laser beam  260  a second amount by the pre-scan lens  242  results in the second beam  260  striking the scanner mirror  256  in the process direction at the point  252 . 
   Referring now to  FIG. 3 , a laser diode/pre-scan assembly for use in a laser printer in accordance with a second embodiment of the present invention is shown diagrammatically, referred to generally by reference numeral  300 . The illustrated laser diode/pre-scan assembly  300 , hereinafter, second embodiment assembly, includes a first laser diode  310  emitting a first laser beam  312 , having a first beam center axis  314 . The first laser beam  312  diverges in both the process direction P and in the scan direction S upon leaving the first laser diode  310 . In  FIG. 3 , the scan direction S is a direction in and out of the plane of the paper and is indicated by a point S. 
   In the second embodiment assembly  300 , the first laser beam  312  diverges in the process direction P at an angle of about 8 degrees and in the scan direction S at an angle within a range of about 25 degrees to about 35 degrees. A first structure  316 , defining a first aperture  318 , is positioned in the path of the first laser beam  312  such that an inner portion  320  of the first laser beam  312  passes through the first aperture  318  and a peripheral portion of the first laser beam  312 , represented by rays  322  and  324 , is blocked by the first structure  316 . The first aperture  318  in the second embodiment assembly is generally oval in shape and has a maximum dimension in the process direction P within a range of about 2 mm to about 3 mm and a larger maximum dimension in the scan direction S within a range of about 4 mm to about 5 mm. 
   After passing through the first aperture  318 , the inner portion  320  of the first laser beam  312  strikes a first surface  326  of a first collimation lens  328 . In the illustrated assembly  300 , the first structure  316 , defining the first aperture  318 , is positioned about 1 mm in a focus direction F from the first surface  326  of the first collimation lens  328 . 
   The first collimation lens  328  is a double convex spherical glass collimation lens having optical power in the process direction P and in the scan direction S. The first collimation lens  328  further has an optical axis  330  passing through a mechanical center  332  of the first collimation lens  328 . In the second embodiment assembly  300  illustrated, the first beam center axis  314  is not coaxial with the optical axis  330  of the first collimation lens  328  as will be discussed more thoroughly herein. 
   The first collimation lens  328  has a focal length in the focus direction F defined as a distance between the mechanical center  332  of the lens  328  and a point (not shown) where light rays that are parallel with the lens  328  optical axis  330  will converge to a point after passing through the lens  328 . In the second embodiment assembly  300  illustrated in  FIG. 3 , the first collimation lens  328  is positioned relative to the first laser diode  310  in the focus direction F such that a distance F 2  between the point where the first laser beam  312  is emitted from the first laser diode  310  and the mechanical center  332  of the first collimation lens  328  is greater than the focal length of the first collimation lens  328 . As a result, the rays of the generally diverging first laser beam  314  passing through the first collimation lens  328  are caused to converge a first amount in the process direction P and a first amount in the scan direction S, wherein the first amount in the process direction P may be slightly different from the first amount in the scan direction S. Thus, the beam  334  emerging from the first collimation lens  338  is converging in the process direction P and in the scan direction S. 
   The distance F 2  is greater than the distance F 1 , see  FIG. 1 , between the point where the first laser beam  112  is emitted from the first laser diode  110  and the mechanical center  130  of the first collimation lens  128  in the conventional arrangement  100 , see  FIG. 1 . 
   Referring again to  FIG. 3 , in addition to being focused or converged in the process and scan directions by the lens  328 , the first laser beam  312  upon passing through the first collimation lens  328  is also caused to deflect inward a first amount in the process direction P toward a pre-scan lens  342  as will be discussed more thoroughly herein. 
   The pre-scan lens  342  is a cylindrical lens having optical power in the process direction P only. The pre-scan lens  342  causes the rays  336 ,  338  and  340  of the converging first laser beam  334  to bend inward in the process direction P a second amount and further causes the rays  336 ,  338  and  340  to converge in the process direction P a second amount such that a converging first laser beam  344  comprising rays  346 ,  348  and  350  is created. 
   The pre scan lens  342  has a radius R 2  defining a focal length in the focus direction F such that the converging first laser beam  344  is directed to a point  352  in the process direction on a surface  354  of a scanner mirror  356 , shown only partially in  FIG. 3 . The pre-scan lens  342  also causes the rays  346 ,  348  and  350  to converge in the process direction P a second amount such that the rays  346 ,  348  and  350  will converge or nearly converge in the process direction to the same point  352  on the surface  354  of the scanner mirror  356 . 
   Referring now to  FIGS. 1 and 3 , the radius R 2  of the pre-scan lens  342  is greater than the radius R 1  of the pre-scan lens  142  in the conventional arrangement  100 . As a result, the pre-scan lens  242  of the second embodiment assembly  300  has less power in the process direction P than does the pre-scan lens  142  of the conventional arrangement  100 . Thus, the pre-scan lens  342  causes the first laser beam  334  to bend inward in the process direction P a second amount that is smaller than the amount that the pre-scan lens  142  of the conventional arrangement  100  causes the first laser beam  134  to bend inward in the process direction P. 
   The pre-scan lens  342  of the second embodiment assembly  300  also causes the first laser beam  334  to converge in the process direction P a second amount that is smaller than the amount that the pre-scan lens  142  of the conventional arrangement  100  causes the first laser beam  134  to converge in the process direction P. Because the first laser beam  334  is converging a first amount in the process direction after passing through the first collimation lens  328 , the pre-scan lens  342  need further converge the first laser beam  334  a second amount in the process direction that is smaller than the amount that the pre-scan lens  142  of the conventional arrangement  100  converges the first laser beam  134  in the process direction in order for the beam to converge or nearly converge to the point  252  on the surface  354  of the scanner mirror  356  as desired. 
   The second embodiment assembly  300  also includes a second laser diode  358 , emitting a second laser beam  360 , having a second beam center axis  362 . In the second embodiment assembly  300  illustrated, the second laser diode  358  is separated from the first laser diode  310  in the process direction P such that the second beam center axis  362  is separated from the first beam center axis  314  by a distance P 1  in the process direction P. The distance P 1  is equal to the distance P 1  that separates the first beam center axis  114  from the second beam center axis  162  in the conventional arrangement  100 , see  FIG. 1 . 
   The second laser beam  360  diverges in both the process direction P and in the scan direction S upon leaving the second laser diode  358 . The second laser beam  362  diverges in the process direction P at an angle of about 8 degrees and in the scan direction S at an angle within a range of about 25 degrees to about 35 degrees. 
   A second structure  364 , defining a second aperture  366 , is positioned in the path of the second laser beam  360  such that an inner portion  368  of the second laser beam  360  passes through the second aperture  366  and a peripheral portion of the second laser beam  360 , represented by rays  370  and  374 , is blocked by the second structure  364 . The second aperture  368  is generally oval in shape and has a maximum dimension in the process direction P within a range of about 2 mm to about 3 mm and a larger maximum dimension in the scan direction S within a range of about 4 mm to about 5 mm. A center point of the second aperture  364  is spaced a distance P 4  from a center point of the first aperture. 
   After passing through the second aperture  366 , the inner portion  368  of the second laser beam  360  strikes a first surface  376  of a second collimation lens  378 . In the illustrated second embodiment assembly  300 , the second structure  364 , defining the second aperture  368 , is positioned about 1 mm in the focus direction F from the first surface  376  of the second collimation lens  378 . 
   The second collimation lens  378  is a double convex spherical glass collimation lens having optical power in the process direction P and in the scan direction S. The second collimation lens  378  further has an optical axis  380  passing through a mechanical center  382  of the second collimation lens  378 . In the second embodiment assembly  300  illustrated, the second beam center axis  362  is not coaxial with the optical axis  380  of the second collimation lens  378  as will be discussed more thoroughly herein. 
   The second collimation lens  378  has a focal length in the focus direction F defined as a distance between the mechanical center  382  of the lens  378  and a point (not shown) where light rays that are parallel with the lens  378  optical axis  380  will converge to a point after passing through the lens  378 . In the second embodiment assembly  300  illustrated in  FIG. 3 , the second collimation lens  378  is positioned relative to the second laser diode  358  in the focus direction F such that a distance F 2  between the point where the second laser beam  360  is emitted from the second laser diode  358  and the mechanical center  382  of the second collimation lens  378  is greater than the focal length of the second collimation lens  378 . As a result, the rays of the generally diverging second laser beam  360  passing through the second collimation lens  378  are caused to converge a first amount in the process direction P and a first amount in the scan direction S, wherein the first amount in the process direction P may be slightly different from the first amount in the scan direction S. Thus, the beam  384  emerging from the second collimation lens  378  is converging in the process direction P and in the scan direction S. 
   The distance F 2  is greater than the distance F 1 , see  FIG. 1 , between the point where the second laser beam  160  is emitted from the second laser diode  158  and the mechanical center  182  of the second collimation lens  178  in the conventional arrangement  100 , see  FIG. 1 . 
   Referring again to  FIG. 3 , in addition to being focused or converged in the process and scan directions by the lens  378 , the second laser beam  360  upon passing through the second collimation lens  378  is also caused to deflect inward a first amount in the process direction P toward the pre-scan lens  342  as will be discussed more thoroughly herein. 
   As previously mentioned, the pre-scan lens  342  is a cylindrical lens having optical power in the process direction P only. The pre-scan lens  342  causes the rays  386 ,  388  and  390  of the converging second laser beam  384  to bend inward in the process direction P a second amount and further causes the rays  386 ,  388  and  390  to converge in the process direction P a second amount such that a converging second laser beam  392  comprising rays  394 ,  396  and  398  is created. 
   As noted above, the pre scan lens  342  has a radius R 2  defining a focal length in the focus direction F such that the converging second laser beam  392  is directed to a point in the process direction that is near or the same point  352  on the surface  354  of the scanner mirror  356  where the converging first laser beam  344  strikes the surface  354  of the scanner mirror  356 . In the second embodiment assembly  300  illustrated in  FIG. 3 , the converging second laser beam  392  strikes the surface  354  of the scanner mirror  356  at a point that is within a range of from about 0 microns to about 200 microns of the point  352  where the converging first laser beam  344  strikes the surface  354  of the scanner mirror  356 . The pre-scan lens  342  also causes the rays  394   396  and  398  to converge in the process direction P a second amount such that the rays  394 ,  396  and  398  will converge or nearly converge in the process direction to the same point  352  on the surface  354  of the scanner mirror  356 . 
   Referring now to  FIGS. 1 and 3 , as noted above, the radius R 2  of the pre-scan lens  342  is greater than the radius R 1  of the pre-scan lens  142  in the conventional arrangement  100 . As a result, the pre-scan lens  342  of the second embodiment assembly  300  has less power in the process direction P than does the pre-scan lens  142  of the conventional arrangement  100 . Thus, the pre-scan lens  342  causes the second laser beam  384  to bend inward in the process direction P a second amount that is smaller than the amount that the pre-scan lens  142  of the conventional arrangement  100  causes the second laser beam  184  to bend inward in the process direction P. 
   The pre-scan lens  342  of the second embodiment assembly  300  also causes the second laser beam  384  to converge in the process direction P a second amount that is smaller than the amount that the pre-scan lens  142  of the conventional arrangement  100  causes the second laser beam  184  to converge in the process direction P. Because the second laser beam  384  is converging a first amount in the process direction after passing through the second collimation lens  378 , the pre-scan lens  342  need further converge the second laser beam  384  a second amount in the process direction that is smaller than the amount that the pre-scan lens  142  of the conventional arrangement  100  converges the second laser beam  184  in order for the beam to converge or nearly converge in the process direction to the point  252  on the surface  354  of the scanner mirror  356  as desired. 
   In the illustrated second embodiment assembly  300 , the distance P 4  in the process direction P between the first collimation lens mechanical center  332  and the second collimation lens mechanical center  382  is less than the distance P 2  in the process direction P between the first collimation lens mechanical center  130  and the second collimation lens mechanical center  182  of the conventional arrangement  100 , see  FIG. 1 . The distance P 4  in the process direction P is also less than the distance P 1  between the first beam center axis  314  and the second beam center axis  362 . This may be accomplished by spacing the first collimation lens  328  and the second collimation lens  378  closer to one another an equal amount in the process direction P relative to the positions of the first collimation lens  128  and the second collimation lens  178  in the conventional arrangement  100 , see  FIG. 1 . 
   Referring again to  FIGS. 1 and 3 , the distance P 1  in the process direction P between the first beam center axis  314  and the second beam center axis  362  is equal to the distance P 1  in the process direction P between the first beam center axis  114  and the second beam center axis  162  of the conventional arrangement  100 , see  FIG. 1 . As a result, the beam center axis  314  of the first laser beam  312  does not pass through the optical axis  330  of the first collimation lens  328  but rather strikes the first surface  326  of the first collimation lens  328  a spaced distance of about 100 microns to about 400 microns in the process direction P from the optical axis  330  of the first collimation lens  328  and in a direction toward a first end surface  328 A of the first lens  328 . This causes the first laser beam  312  to bend inward in a direction toward the pre-scan lens  342  a first amount as previously mentioned. In the illustrated second embodiment assembly  300 , the first laser beam  312  is bent inward toward the pre-scan lens  342  an amount within a range of about 0.25 degree to about 1 degree by the first collimation lens  328 . 
   The beam center axis  362  of the second laser beam  360  does not pass through the optical axis  380  of the second collimation lens  378  but rather strikes the first surface  376  of the second collimation lens  378  a spaced distance of about 100 microns to about 400 microns in the process direction P from the optical axis  380  of the second collimation lens  378  and in a direction toward a first end surface  378 A of the second lens  378 . This causes the second laser beam  360  to bend inward toward the pre-scan lens  342  a first amount as previously mentioned. In the illustrated second embodiment assembly  300 , the second laser beam  360  is bent inward toward the pre-scan lens  342  an amount within a range of about 0.25 degree to about 1 degree by the second collimation lens  228 . 
   While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.