Abstract:
A method of reversing a laser beam in a machine includes orienting a pair of reflective surfaces at an angle of approximately 90° relative to each other. The laser beam is directed in a first direction such that the laser beam sequentially impinges upon a first of the reflective surfaces reflects off of the first reflective surface, impinges upon a second of the reflective surfaces, and reflects off of the second reflective surface in a second direction substantially parallel to and opposite the first direction. A pivot axis is substantially parallel to each of the reflective surfaces. Each of the reflective surfaces is pivoted about the pivot axis by a substantially equal angle to thereby change a position of the laser beam after reflecting off of the second reflective surface.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to mirror assemblies in a laser scanning machine, and, more particularly, to mirror assemblies for optical path reversal in an electrophotographic machine. 
     2. Description of the Related Art 
     In order to provide a color electrophotographic printer with a same printing speed as a monochrome electrophotographic printer, it is known to substantially simultaneously produce the images on each respective electrophotographic drum for each of the colors of the printer. This is known as “tandem color laser electrophotographic architecture” and requires four laser print image lines to be respectively generated by cyan, magenta, yellow and black printheads, all of which must be packaged in a reasonably sized machine housing. 
     In order to minimize the size of a tandem color laser printer system, a compact laser printhead is required. Meeting this compactness objective requires folding the optical path back over top (or under) the mirror motor. To accomplish this, a 180° reversal of the optical path is achieved by a set of mirrors mounted as a pair, which are oriented at an angle of approximately 90° relative to each other. This 90° mirror set is located between the scanning polygon mirror and the first f-theta lens. In order to avoid undesirable bow and spot distortions occurring at the image plane, it is critical that the scanned laser beam pass through the desired optical axis of each lens. Thus, extreme care must be taken to physically align the source of the laser beam and/or the lens such that the scanned laser beam passes through the desired optical axis of the lens. Further, the laser beam and/or the lens must be provided with a shiftable mounting in order to perform such alignment. 
     What is needed in the art is a method of reversing a laser beam between a scanning mirror and a lens in an electrophotographic machine Such that a physical alignment procedure does not need to be performed on the source of the laser beam and/or the lens. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of reversing a laser beam and aligning the reversed laser beam with the optical axis of a lens without having to physically align either the source of the laser beam or the lens. 
     The invention comprises, in one form thereof, a method of reversing a laser beam in a machine. A pair of reflective surfaces are oriented at an angle of approximately 90° relative to each other. The laser beam is directed in a first direction such that the laser beam sequentially impinges upon a first of the reflective surfaces, reflects off of the first reflective surface, impinges upon a second of the reflective surfaces, and reflects off of the second reflective surface in a second direction substantially parallel to and opposite the first direction. A pivot axis is substantially parallel to each of the reflective surfaces. Each of the reflective surfaces is pivoted about the pivot axis by a substantially equal angle to thereby change a position of the laser beam after reflecting off of the second reflective surface. 
     An advantage of the present invention is that the laser beam can be aligned with the optical axis of a lens without having to physically align either the source of the laser beam or the lens. 
     Another advantage is that the alignment procedure can be performed quickly, resulting in minimal assembly time. 
     Yet another advantage is that only minimal changes occur in the length of the optical path, and hence, no translation of the reversing mirrors is required. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a schematic, side view of one embodiment of a pivotable mirror assembly of the present invention in an unrotated position; 
     FIG. 2 is a schematic, side view of the pivotable mirror assembly of FIG. 1 in a rotated position; 
     FIG. 3 is a schematic, side view of the superposition of the two different positions of the pivotable mirror assembly of FIGS. 1 and 2; 
     FIG. 4 is a plot of beam separation versus the angle of rotation of the mirror assembly of the present invention for various positions of the pivot axis; and 
     FIG. 5 is a plot of the change in optical path length versus the angle of rotation of the mirror assembly of the present invention for various positions of the pivot axis. 
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate one preferred embodiment of the invention, in one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, and, more particularly, to FIG. 1, there is shown one embodiment of a rotatable mirror assembly  10  of the present invention. Mirror assembly  10  includes a mirror set  12  in the form of a pair of planar mirrors  14  and  16  which are fixedly mounted relative to one another at a substantially right (90°) angle at respective projected edges  18  and  20  which intersect at line  32 . An incoming laser beam  22  reflects off of mirror  14  as indicated at  24 , and then reflects off of mirror  16  as an outgoing laser beam  26 . Outgoing laser beam  26  is directed toward a stationary lens  28  having an optical axis  30 . Incoming laser beam  22  forms a 45° angle θ 1  with mirror  14 , and outgoing laser beam  26  forms a 45° angle θ 2  with mirror  16 . This will be referred to herein as the “unrotated position” of mirror set  12 . 
     To insure that outgoing laser beam  26  is coincident with optical axis  30  of the lens  28  in the presence of real parts tolerances, the 90° mirror set  12  is rotated about a pivot axis P that is parallel to a line of intersection  32  of projected edges  18 ,  20  of the two mirrors  14 ,  16 . That is, mirror set  12  is pivotable in directions parallel to the plane of the page about pivot axis P. The rotation of mirror set  12  about pivot axis P causes a distance X3 between incoming beam  22  and outgoing beam  26  to increase or decrease (depending upon the direction of rotation) to achieve the desired adjustment. X3 o  is defined as the value of X3 in the unrotated position of FIG. 1 wherein incoming beam  22  and outgoing beam  26  are oriented at 45° angles relative to mirror  14  and  16 , respectively. In the particular preferred embodiment presented in FIG. 4, X3 o  is 13.9 mm, but the invention is general for any value of X3. The 90° angle between mirrors  14 ,  16  insures that the incoming beam  22  and the outgoing beam  26  remain parallel to each other throughout the rotation of mirror set  12 . 
     Since the 90° mirror set  12  is located between a scanning polygon mirror and the first f-theta lens  28 , the optical path length between a surface  34  of the polygon mirror and the surface of first f-theta lens  28  is critical to the performance of the optical system. Unfortunately, the optical path length from the scanning polygon surface  34  to the surface of first lens  28  can also increase or decrease due to this rotation of the 90° mirror set  12 . This change in optical path length can cause undesirable changes in the optical performance of the system. 
     In the unrotated embodiment of FIG. 1, the pivot axis P of the 90° mirror set  12  is placed at a general location, wherein D2 is a normalized distance from reflected beam  24  to pivot axis P in a direction parallel to both incoming beam  22  and outgoing beam  26 , and D 3  is a distance from incoming beam  22  in a direction perpendicular to both beams  22  and  26 . 
     When mirror set  12  is in a rotated position (FIG.  2 ). pivot axis P is at the same location as in FIG. 1, but mirror set  12  has been rotated in a counterclockwise direction about pivot axis P such that incoming beam  22  is oriented at an angle of less than 45° relative to mirror  14 , and outgoing beam  26  is oriented at an angle of greater than 45° relative to mirror  16 . 
     FIG. 3 shows a comparison of the optical paths of the laser beam before and after rotation of mirror set  12  through an angle θ r . That is, FIG. 3 shows the optical paths of the laser beam both when mirror set  12  is in the rotated position of FIG.  2  and when mirror set  12  is in the unrotated position of FIG. 1 wherein incoming beam  22  forms a 45° angle with mirror  14 . In general, the optical path length before rotation (X1 o +X3 o +X2 o ) is not equal to the optical path length after rotation (X1b+X3b+X2b). Which optical path is longer depends upon the location of pivot axis P as well as the degree and direction of rotation of mirror set  12 . 
     A beam separation distance S is defined in FIG. 3 as a distance between a current position of outgoing beam  26  and the position of outgoing beam  26  before rotation, i.e., in the unrotated position. FIG. 4 is a plot of beam separation distance S versus the angle of rotation of mirror set  12  for various positions of pivot axis P. As can be determined from FIG. 4, the change in beam separation S becomes greater for a given change in angle of rotation θ r  of mirror set  12  as D2 decreases. In other words, beam separation S per degree of rotation of θ r  becomes greater as pivot axis P is moved further from the junction  32  of mirror set  12 , i.e., to the right in FIGS. 1-3. A counterclockwise (CCW) rotation of angle θ r , as shown in FIG. 3, causes a decrease in separation distance S as defined in FIG.  4 . Likewise, a clockwise (CW) rotation of angle θ r  causes an increase in separation distance S. 
     In assembly of the printhead in which rotatable mirror assembly  10  is placed, the adjustment of beam separation distance S may require a less sensitive gain between angle of rotation θ r  of mirror set  12  and the change in distance of separation S between the two outgoing beams  26 . In this case, moving pivot axis P toward the junction  32  of the two mirrors  14 ,  16 , i.e., to the left in FIGS. 1-3, results in a positive increase in D2 and a lower gain. That is, movement of pivot axis P toward junction  32  results in a larger angle of rotation θ r  being required to achieve the same beam separation distance S. This is can also be determined from FIG.  4 . 
     FIG. 5 shows the relationship between the change in optical path length of the laser beam and angle of rotation θ r  of mirror set  12  for various locations, D3, of pivot axis P. It has been found that changes in the “horizontal” position, D2, of pivot axis P parallel to incoming beam  22  and outgoing beam  26  causes less than 0.1 micron change in path length over the same +/−2° rotation of mirror set  12 , but, as shown in FIG. 4, changing D2 does linearly affect beam separation S. In contrast, changing the location of the pivot “vertically” (perpendicular to incoming beam  22  and outgoing beam  26 ), D3, does impact the change in path length as shown in FIG. 5, but causes a change of less than one micron in beam separation distance S. In conclusion, changing the pivot location by changing D2 affects beam separation S, but does not substantially affect the path length. Changing D3 affects the path length, but does not substantially affect beam separation S. From FIG. 5 it can be seen that locating the pivot axis in the center of the unrotated beam  24  location (D2=0, D3=X3 o /2) results in less than 10 microns of path length change over 2° of rotation and less than 30 microns of path length change if a worst case error in the location of pivot axis P occurs. Thus, there is no need to translate the 90° mirror set  12  in a direction parallel to incoming beam  22  and outgoing beam  26  in order to correct for path length change after mirror set  12  has been rotated to achieve the desired beam separation S. Such rotation of mirror set  12  is needed to adjust the location of outgoing beam  26  to compensate for the tolerances of the particular set of components that make up a particular printhead. 
     If a lower gain position of D2 is chosen, i.e., pivot axis P is located closer to junction  32 , a trade-off must be made because, as is shown in FIG. 4, a larger angle of rotation θ r  is needed to achieve the same required change in beam separation S. As is shown in FIG. 5, a larger range of change of angle θ r  to achieve a desired change in beam separation S increases the undesired change in optical path length. The model presented here (as shown in FIGS. 4 and 5) can be used to determine the appropriate location of pivot axis P to achieve an acceptable trade-off. For any value of D2 that is chosen, FIG. 5 shows that it is best to locate pivot axis P at X3 o /2 to maintain a minimal and symmetric path length change as a function of angular rotation θ r . 
     While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.