Patent Publication Number: US-2006012669-A1

Title: Laser scanning optical unit used in image forming apparatus

Description:
This application is based on Japanese Patent Application(s) No(s). 2004-208723 filed in Japan on Jul. 15, 2004, the entire content of which is hereby incorporated by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a laser scanning optical unit, particularly, to a laser scanning optical unit disposed so as to correspond to each photoreceptor in a tandem electrophotographic image forming apparatus having a plurality of photoreceptors disposed in parallel.  
      2. Description of Related Art  
      Generally, as systems that form color images by electrophotography, various tandem systems have been proposed in which four photoreceptor drums are disposed in parallel on the route of movement of the transfer belt, images of the three primary colors (magenta, yellow and cyan) and black are formed on the photoreceptor drums, respectively, and the images are superimposed one on another on the sheet.  
      Moreover, these tandem systems employ a configuration in which a laser scanning optical unit is disposed so as to correspond to each of the photoreceptor drums to simplify the laser scanning optical unit.  
      When a laser scanning optical unit is provided so as to correspond to each photoreceptor drum, in tandem image forming apparatuses of this type, to avoid an increase in the size of the apparatus itself, it is required that the laser scanning optical units be compact, particularly, the disposition pitch of the photoreceptor drums be minimized.  
      Conventionally used laser scanning optical units of this type are broadly divided into three kinds which are shown in  FIGS. 5, 6  and  7 , respectively.  
      In the laser scanning optical unit shown in  FIG. 5 , a laser beam LB deflected for scanning by a deflector  3  passes through a first scanning lens  6 , is bent 90 degrees downward by a mirror  7 , and then, passes through a second scanning lens  8  to be directed to a photoreceptor drum  9 . In this case, the disposition pitch P of the photoreceptor drum  9  is restricted by the dimension A of the laser scanning optical unit. The dimension A which is subject to constraints such as the two-dimensional size of the deflector  3 , the axial thickness of the first scanning lens  6  and the width of the mirror  7  is comparatively large, so that it is difficult to reduce the pitch P.  
      In the laser scanning optical unit shown in  FIG. 6 , the laser beam LB deflected for scanning by the deflector  3  is bent by mirrors  7   a  and  7   b,  passes through the first scanning lens  6 , is bent downward by a mirror  7   c  and then, passes through the second scanning lens  8  to be directed to the photoreceptor drum  9 . Like the optical unit shown in  FIG. 5 , this optical unit is disadvantageous in reducing the size of the tandem image forming apparatus because the disposition pitch P of the photoreceptor drum  9  is restricted by the dimension A of the optical unit.  
      In the laser scanning optical unit shown in  FIG. 7 , the laser beam LB deflected for scanning by the deflector  3  is directed to the photoreceptor drum  9  while passing through the first and second scanning lenses  6  and  8  without the intervention of a mirror. This optical unit has an advantage that it can be comparatively thin because it is necessary for the dimension A only to be equal to the thickness of the deflector  3  and the scanning lenses  6  and  8 . However, since the optical path is not bent, the optical path length becomes the length B of the optical unit as it is, and this rather increases the image forming apparatus in size.  
     OBJECT AND SUMMARY  
      Accordingly, an object of the present invention is to provide a compact laser scanning optical unit in which the disposition pitch of a plurality of photoreceptors can be small and that is optimum for tandem image forming apparatuses.  
      To solve the above-mentioned object, a first aspect of the invention is a laser scanning optical unit provided so as to correspond to each of a plurality of photoreceptors disposed in parallel. The laser scanning optical unit is provided with a laser light source, a deflector, a scanning optical element, a reflecting mirror, and a housing containing these members. The laser beam incident on the deflector is deflected for scanning in a direction opposite to the direction of disposition of the photoreceptor, and the optical path along which the laser beam deflected for scanning by the deflector and having passed through the scanning optical element is directed to the photoreceptor while being bent by the reflecting mirror disposed in the last stage is set so as to be substantially orthogonal to the rotation axis of the deflector and pass through a space between two planes that define the outermost part of the housing to reach the photoreceptor.  
      Moreover, a second aspect of the invention is a laser scanning optical unit provided with the same elements as those of the first aspect of the invention. The optical path along which the laser beam deflected for scanning by the deflector and having passed through the scanning optical element is directed to the photoreceptor while being bent by the reflecting mirror disposed in the last stage is set so as to pass a plane substantially orthogonal to the rotation axis of the deflector within the outer range of the housing to reach the photoreceptor.  
      In the laser scanning optical units according to the first and second aspects of the invention, since the laser beam bent by the reflecting mirror disposed in the last stage is set so as to be substantially orthogonal to the rotation axis of the deflector and pass through the space between the two planes that define the outermost part of the housing to reach the photoreceptor, or since the optical path is set so as to pass a plane substantially orthogonal to the rotation axis of the deflector within the outer range of the housing to reach the photoreceptor, the width of the optical unit is small, so that the tandem image forming apparatus can be reduced in size by reducing the disposition pitch of the photoreceptor drums. Further, since the laser beam incident on the deflector is deflected for scanning in a direction opposite to the direction of disposition of the photoreceptor and the optical path is appropriately bent, the distance between the photoreceptor and the optical unit can be reduced.  
      Moreover, an image forming apparatus according to a third aspect of the invention is an image forming apparatus provided with: a plurality of photoreceptors disposed along a rotatable transfer belt; and a plurality of laser scanning optical units provided for the photoreceptors, respectively. Each of the laser scanning optical units is provided with: a laser light source; a deflector disposed so as to be rotatable about an axis parallel to the conveyance direction of the transfer belt; a first mirror that reflects a laser beam deflected for scanning by the deflector in a direction opposite to the direction of disposition of the photoreceptor, in a direction parallel to the conveyance direction of the transfer belt; and a second mirror that reflects the laser beam reflected by the first mirror, toward the photoreceptor.  
      The deflector may include a polygon mirror and a motor that rotates the polygon mirror so that the laser beam reflected by the second mirror passes through a space on the side opposite to the polygon mirror with respect to the motor.  
      Moreover, an optical system may be disposed between the first mirror and the second mirror.  
      Further, as the plurality of laser scanning optical units, laser scanning optical units of the same structure may be disposed in the same positional relationship with respect to the photoreceptors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      These and other objects and features of the invention will become clear from the following description taken in conjunction with the preferred embodiments thereof with reference to the accompanying drawings, in which:  
       FIG. 1  is a schematic block diagram showing a laser scanning optical unit according to a first embodiment of the present invention;  
       FIG. 2  is a perspective view showing a relevant part of the laser scanning optical unit;  
       FIG. 3  is a schematic block diagram showing a relevant part of a tandem electrophotographic image forming apparatus provided with the laser scanning optical unit;  
       FIG. 4  is a schematic block diagram showing a laser scanning optical unit according to a second embodiment of the present invention;  
       FIG. 5  is a schematic block diagram showing the first example of the conventional laser scanning optical unit;  
       FIG. 6  is a schematic block diagram showing the second example of the conventional laser scanning optical unit; and  
       FIG. 7  is a schematic block diagram showing the third example of the conventional laser scanning optical unit. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Hereinafter, embodiments of the laser scanning optical unit according to the present invention will be described with reference to the attached drawings.  
      (First embodiment, see FIGS.  1  to  3 )  
      In  FIGS. 1 and 2 , a laser scanning optical unit  10 A comprises a housing  11  in which a light source  12 , a deflector  13 , a first scanning lens  16 , a reflecting mirror  17   a,  a second scanning lens  18  and a reflecting mirror  17   b  are provided, and a laser beam is emitted to a photoreceptor drum  9  to form an image (electrostatic latent image).  
      The deflector  13  is structured so that a polygon mirror  14  provided with four reflecting and deflecting surfaces is rotated about a rotation axis  13   b  thereof at constant speed by a motor  15 . The laser beam emitted from the light source  12  is incident on the reflecting and deflecting surface of the polygon mirror  14  from a direction perpendicular to the plane of  FIG. 1 , and the laser beam LB is deflected for scanning in a direction opposite to the direction of disposition of the photoreceptor drums  9 .  
      The laser beam LB deflected for scanning by the deflector  13  passes through the first scanning lens  16 , is bent 90 degrees by the reflecting mirror  17   a,  passes through the second scanning lens  18 , and is then bent upward by the reflecting mirror  17   b  to be directed to the photoreceptor drum  9 .  
      The optical path OP bent by the reflecting mirror  17   b  disposed in the last stage to be directed to the photoreceptor drum  9  is set so as to be substantially orthogonal to the rotation axis  13   a  of the deflector  13  and pass through a space between planes S 1  and S 2  that define the outermost part of the housing  11  to reach the photoreceptor  9 .  
      In other words, the optical path OP directed to the photoreceptor drum  9  while being bent by the reflecting mirror  17   b  disposed in the last stage is set so as to pass planes substantially orthogonal to the rotation axis  13   a  of the deflector  13  within the outer range of the housing  11  to reach the photoreceptor drum  9 . Here, the outer range of the housing  11  means a range including the substantially occupied space when it is assumed that the housing  11  is a rectangular parallelepiped (the range including the space shown by the dotted line in  FIG. 1 ).  
      In the first embodiment, the optical path OP runs on the rear surface side (the side where the motor  15  is placed) of the deflector  13 .  
       FIG. 3  shows a condition where the above-described laser scanning optical unit  10 A is incorporated in a tandem electrophotographic image forming apparatus. The laser scanning optical unit  10 A is disposed below the photoreceptor drums  9  so as to correspond to each of the four photoreceptor drums  9  disposed in parallel. These laser scanning optical units  10 A are all the same, and are set in the same positional relationship with respect to the photoreceptors.  
      Immediately above the photoreceptor drums  9 , a transfer belt  20  is set so as to be rotatable in the direction of the arrow X. In this image forming apparatus, the axes of the polygon mirrors  14  are parallel to the conveyance direction of the transfer belt, and the laser beam reflected by the reflecting mirror  17   a  is parallel to the conveyance direction of the transfer belt. In this embodiment, the axes of the polygon mirrors  12  are horizontally disposed.  
      While non-illustrated units such as a charging unit, a developer unit and a transferring unit are disposed around each photoreceptor drum  9 , since the structures and workings of these members are known, descriptions thereof are omitted.  
      Images (electrostatic latent images) of magenta, yellow, cyan and black are formed on the photoreceptor drums  9  by the laser beams emitted from the optical units  10 A, and after toner is attached thereto, the images are successively superimposed on the transfer belt  20  to be primarily transferred, thereby forming a color image. The formed color image is secondarily transferred onto the sheet conveyed along the path shown by the arrow Y.  
      In the laser scanning optical unit  10 A having the above-described structure, since the optical path OP is set so that the laser beam bent by the reflecting mirror  17   b  disposed in the last stage passes the planes substantially orthogonal to the rotation axis  13   a  of the deflector  13  within the outer range of the housing  11  to reach the photoreceptor drum  9 , the width A of the optical unit  10 A is small, so that the tandem image forming apparatus can be reduced in size by reducing the disposition pitch P of the photoreceptor drums  9 .  
      Further, since the laser beam incident on the deflector  13  is deflected for scanning in a direction opposite to the direction of disposition of the photoreceptor drums  9  and the optical path is appropriately bent, the distance between the photoreceptors  9  and the optical units  10 A, that is, the distance B from the photoreceptor drums  9  to an end of the housings  11  can be reduced.  
      &lt;Second embodiment, see  FIG. 4 )  
      A laser scanning optical unit  10 B according to this second embodiment basically has a similar structure to that of the first embodiment. The laser scanning optical unit  10 B is different in that the optical path OP of the laser beam bent by the reflecting mirror  17   b  is set so as to run on the side, opposite to the side of the motor  15 , of the deflector  13 . The condition where the laser scanning optical unit  10 B is incorporated in a tandem image forming apparatus can be understood by referring to  FIG. 3 . Therefore, the workings and effects of the second embodiment are similar to those of the first embodiment.  
      Except this, the structure is similar to that of the first embodiment. Therefore, in  FIG. 4 , the same members as those of  FIG. 1  are denoted by the same reference numerals and overlapping descriptions are omitted.  
      Although the present invention has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications are apparent to those skilled in the art. Such changes and modifications are to be understood as included with in the scope of the present invention as defined by the appended claims unless they depart therefrom.  
      The laser scanning optical unit according to the present invention is not limited to the above-described embodiments and may be modified in various manners without departing from the gist thereof.  
      In particular, details of the structure of the housing  11  and the structure and positional relationship of the scanning lenses  16  and  18  are arbitrary. Moreover, in the tandem structure shown in  FIG. 3 , the positional relationship between the photoreceptor drums  9  and the optical units  10 A may be such that the optical units  10 A are disposed above the photoreceptor drums  9 .