Abstract:
A disclosed light source apparatus includes a light source including multiple light emitting devices; an optical element configured to transmit light emitted from the light source; a tube configured to hold the optical element; and a tube holder configured to fix the tube by holding an end of the tube close to the light source or by holding the end of the tube on the light emission side away from the light source. A positional displacement preventing member is provided at the other end of the tube on a light emission side away from the light source in such a manner as to be slidable on the tube in the direction of an optical axis and prevent positional displacement of the tube in all directions perpendicular to the optical axis.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed to a light source apparatus used in an optical scanning apparatus which is mounted on an image forming apparatus, such as a digital copying machine or a laser printer, and in particular to a light source apparatus having multiple light emitting devices. 
     2. Description of the Related Art 
     It is conventionally known that changes in temperature of an optical scanning apparatus cause changes in wavelength of its light source, positional displacement due to expansion and contraction of optical components, and changes in the refractive index or the like, thereby causing a change in an image location (at which an image is formed), on a surface to be scanned, in the optical axis direction of the scanning light. 
     For example, Patent Document 1 proposes a technology that adjusts linear expansion coefficients of members supporting the light source and the collimator lens in such a manner that the distance between the light source and the collimator lens does not change, thereby preventing change in the image location, on the surface to be scanned, in the optical axis direction of the scanning light. 
     Patent Document 2 devises the lens structure of the collimator lens, whereby even if the wavelength of the light source changes, it is possible to prevent change in the image location, on the surface to be scanned, in the optical axis direction of the scanning light. 
     Patent Document 3 proposes to provide a ring-shaped member on the tube of the collimator lens in order to hermetically seal the space between the collimator lens and the light source. In this case, however, the collimator lens is fixed based on the position of the ring-shaped member, and therefore, the ring-shaped member does not shift in the optical axis direction even if the ambient temperature changes. Accordingly, the technology of Patent Document 3 leaves the problem that a change in wavelength of the light source due to environmental change causes change in the image location, on the surface to be scanned, in the optical axis direction of the scanning light. 
     If the tube of the collimator lens is made long, the position of the collimator lens can be shifted in relation to that of the ring-shaped member according to expansion and contraction of the tube. However, in this case, an end face of the tube away from the ring-shaped member is not fixed in the directions perpendicular to the optical axis, and therefore, the collimator lens may shift in a perpendicular direction. This sometimes causes the problem that the emitted light beam changes in a direction perpendicular to the optical axis. 
     Patent Document 5 proposes a technology that provides a spring means and a screw means in order to adjust the position of the collimator lens in the optical axis direction and further provides another spring means in the direction perpendicular to the optical axis. Accordingly, it is possible to prevent the positional displacement of the collimator lens in the perpendicular directions during the adjustment of the collimator lens in the optical axis direction. In this case, since the collimator lens is pressed in a unidirectional direction perpendicular to the optical axis under the force of the spring means, positional displacement in the pressing direction (e.g. the perpendicular direction) is prevented. However, there are gaps in the horizontal direction, and the position of the collimator lens may therefore be changed after the adjustment.
     [Patent Document 1] Japanese Examined Patent Application Publication No. H01-28381   [Patent Document 2] Japanese Examined Patent Application Publication No. H06-85233   [Patent Document 3] Japanese Laid-open Patent Application Publication No. 2006-284653   [Patent Document 4] Japanese Laid-open Patent Application Publication No. 2004-170771   [Patent Document 5] Japanese Laid-open Patent Application Publication No. 2002-131677   

     As described above, there are conventionally proposed technologies that prevent change in the image location, on the surface to be scanned, in the optical axis direction of the scanning light caused by a change in temperature; however, not all the problems have been solved. 
     The structures of the light source apparatuses disclosed in Patent Documents 1 and 2 are directed to dealing with change in the image location, on the surface to be scanned, in the optical axis direction of the scanning light. Accordingly, neither Patent Document 1 nor Patent Document 2 takes into account a positional change in the directions perpendicular to the optical axis of the scanning light. 
     In the case of a synthesized light source apparatus that synthesizes two light sources, there are two scanning light beams, as described in Patent Document 4. Particularly in this case, change in the relative positional relationship in the directions perpendicular to the optical axis results in a change in the scan interval, thereby causing defects such as variation in the scanning pitch. 
     Even in the case where two light sources are not synthesized, if the scanning light is shifted in a direction perpendicular to the optical axis, it goes off from a position at which the optical sensor detects the printing start timing. This results in negative effects, such as inability to print due to misdetection or a decrease in light intensity due to deflection of the scanning light from the lens. 
     As in the case of Patent Document 5, even if positional displacement in one direction (e.g. the vertical direction) is prevented, the position of the collimator lens may be changed after adjustment since there are gaps in another direction (the horizontal direction), thereby causing the same negative effects as mentioned above. In particular, in the case of using expansion and contraction of the tube of the collimator lens in a proactive manner, as in embodiments of the present invention, the presence of gaps is undesirable since positional displacement is likely to occur. 
     One example of a light source having multiple light emitting devices is a surface emitting laser. In general, the collimator lens which renders parallel light emitted from the light source has larger aberration on the peripheral part compared to the central part. Large aberration tends to lead to a large beam spot. Therefore, in the case of using a surface emitting laser, the light emitting devices are disposed within the image circle (a region having an aberration of a predetermined value or less) of the collimator lens.  FIG. 10  shows an example where nine light emitting devices are provided. A light source array region  70  and an image circle region  71  approximately coincide with each other and not much room is left between them. In this condition, if the position of the collimator lens is displaced in a direction perpendicular to the optical axis as described above, the image circle is displaced, whereby some light emitting devices on the periphery of the light source array region  70  undesirably go outside of the image circle. 
     It is expected that, in the future, a larger number of light emitting devices than in the case of  FIG. 10  will be required and the light source array region therefore increases in size. In this case, a collimator lens having a large image circle is necessary. However, there is a limit to the size of the image circle, and the light source array region  70  and the image circle region  71  approximately coincide with each other and not much room is left between them, as illustrated in the example of  FIG. 10 . Therefore, in this case also, positional displacement of the collimator lens in a direction perpendicular to the optical axis causes positional displacement of the image circle. Accordingly some light source devices on the periphery of the light source array region  70  undesirably go outside of the image circle. 
     SUMMARY OF THE INVENTION 
     The present invention aims at providing a light source apparatus having less positional displacement in all directions perpendicular to the optical axis of the scanning light in the case where the light source apparatus has a structure in which the image location, on the surface to be scanned, in the optical axis direction of the scanning light is corrected using expansion and contraction of the supporting members of the light source and the collimator lens. The present invention also aims at providing a synthesized light source apparatus, an optical scanning apparatus and an image forming apparatus each having such a light source apparatus. 
     In order to solve the above problems, one aspect of the present invention is a light source apparatus including a light source including multiple light emitting devices; an optical element configured to transmit light emitted from the light source; a tube configured to hold the optical element; and a tube holder configured to fix the tube by holding an end of the tube close to the light source or by holding the end of the tube on the light emission side away from the light source. A positional displacement preventing member is provided at the other end of the tube on a light emission side away from the light source in such a manner as to be slidable on the tube in the direction of an optical axis and prevent positional displacement of the tube in all directions perpendicular to the optical axis. 
     Another aspect of the present invention is an optical scanning apparatus including a light source apparatus and a rotating polygon mirror. The light source apparatus includes a light source having multiple light emitting devices; an optical element configured to transmit light emitted from the light source; a tube configured to hold the optical element; a tube holder configured to fix the tube by holding an end of the tube close to the light source or by holding an end of the tube on a light emission side away from the light source; and a positional displacement preventing member provided at the other end of the tube on a light emission side away from the light source in such a manner as to be slidable on the tube in the direction of an optical axis. The light source apparatus is configured to prevent positional displacement of the tube in all directions perpendicular to the optical axis. The rotating polygon mirror is configured to deflect, for scanning, light beams emitted from the light source apparatus. 
     Yet another aspect of the present invention is an image forming apparatus including a photoconductor; a charging device configured to charge the photoconductor; an optical scanning apparatus configured to scan the surface of the photoconductor with light beams to form, on the photoconductor, an electrostatic latent image corresponding to image information to be recorded; a developing device configured to supply toner to the electrostatic latent image to form a toner image; a transfer device configured to transfer the toner image onto a recording medium; and a fixing device configured to fix the transferred toner image to the recording medium. The optical scanning apparatus includes a light source apparatus and a rotating polygon mirror. The light source apparatus includes a light source having multiple light emitting devices; an optical element configured to transmit light emitted from the light source; a tube configured to hold the optical element; a tube holder configured to fix the tube by holding an end of the tube close to the light source or by holding an end of the tube on a light emission side away from the light source; and a positional displacement preventing member provided at the other end of the tube on a light emission side away from the light source in such a manner as to be slidable on the tube in the direction of an optical axis. The light source apparatus is configured to prevent positional displacement of the tube in all directions perpendicular to the optical axis. The rotating polygon mirror is configured to deflect, for scanning, light beams emitted from the light source apparatus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a light source apparatus according to the first embodiment of the present invention; 
         FIG. 2  is a schematic structural view of an optical scanning apparatus using the light source apparatus of  FIG. 1 ; 
         FIG. 3  is a schematic structural view of an image forming apparatus using the optical scanning apparatus of  FIG. 2 ; 
         FIG. 4  is a cross-sectional view of a light source apparatus according to the second embodiment of the present invention; 
         FIG. 5  is a structural diagram of a synthesized light source apparatus according to the third embodiment of the present invention; 
         FIG. 6  is a structural diagram of a synthesized light source apparatus according to the fourth embodiment of the present invention; 
         FIG. 7  shows spots formed on a photoconductor according to an embodiment of the present invention; 
         FIG. 8  shows spots formed on a photoconductor according to another embodiment of the present invention; 
         FIG. 9  shows the relationship between a tube and an O-ring according to the first embodiment of the present invention; 
         FIG. 10  shows the relationship between a device arrangement of a surface emitting laser and an image circle; 
         FIG. 11  shows a spot alignment obtained by synthesizing light of surface emitting lasers; and 
         FIG. 12  shows another spot alignment obtained by synthesizing light of surface emitting lasers. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments that describe the best mode for carrying out the present invention are explained next with reference to the drawings. The following descriptions are merely examples of the favorable embodiments of the present invention, and do not limit the scope of the present invention claimed in the appended claims. 
     a. First Embodiment 
     The first embodiment of the present invention is explained with reference to  FIGS. 1 ,  2 ,  3 ,  7  and  9 .  FIG. 1  is a cross-sectional view of a light source apparatus according to the first embodiment of the present invention.  FIG. 2  shows a schematic structure of an optical scanning apparatus using the light source apparatus.  FIG. 3  shows a schematic structure of an image forming apparatus using the optical scanning apparatus.  FIG. 7  shows spots formed on a photoconductor.  FIG. 9  shows an O-ring  41  mounted on a tube  8 . The O-ring  41  is mounted on the circumference of the tube  8 , that is, surrounds the tube  8  360 degrees around the optical axis so as to prevent displacement of the tube  8  in all directions perpendicular to the optical axis. 
     First, the schematic structure of the image forming apparatus according to the present embodiment is described with reference to  FIG. 3 . A drum-shaped photoconductor  18  used for forming a toner image is rotated by a motor (not shown) at a constant circumferential velocity. The photoconductor  18  is uniformly charged with a particular polarity by a charging device  10 , and then exposed to light emitted from an optical scanning apparatus  11 , whereby an electrostatic latent image corresponding to image information to be recorded is formed on the photoconductor  18 . A developing device  12  is provided downstream of the exposing location in the rotation direction of the photoconductor  18 , and forms a toner image on the photoconductor  18 . 
     A print sheet  13  which is a medium on which recording is performed is conveyed by a conveying device  14 , such as paired conveying rollers. Subsequently, a transfer device  15  charges the backside of the print sheet  13  with a polarity opposite to that of the toner, thereby transferring to the print sheet  13  the toner image formed on the photoconductor  18 . Toner remaining on the photoconductor  18  after the transfer is removed by a cleaning device  16 . The print sheet  13  having the toner image transferred from the photoconductor  18  is conveyed to a fixing device  17 . The fixing device  17  includes a heating roller  17   a  heated at a constant temperature and a pressing roller  17   b  abutting the heating roller  17   a . Pressure is applied to the print sheet  13  carrying the toner image while passing through the fixing device  17 , whereby the toner image is fused and fixed onto the print sheet  13 . After the fixing process, the print sheet  13  is ejected and laid outside the image forming apparatus with other print sheets  13  in a stack. 
       FIG. 2  is a schematic diagram showing the internal structure of an optical scanning apparatus  11 . A light beam  21  emitted from a light source apparatus  20  to be described below passes through a cylindrical lens  23  having a predetermined curvature only in the sub-scanning direction, and is deflected by a rotating polygon mirror  24  for scanning. Subsequently, the light beam  21  passes through an fθ lens  25 , is then reflected by a folding mirror  28  and projected onto the photoconductor  18  to form an electrostatic latent image. The direction of arrow X in  FIG. 2  represents the light scanning direction (main-scanning direction). A part of the beam deflected by the rotating polygon mirror  24  is directed to an optical sensor  27  by a mirror  26 , and in response to this, the optical sensor  27  starts modulating the light beam  21  emitted from the light source apparatus  20 . 
     Note that light emitting devices of a light source  1  are arranged in a line, and therefore, optical spots are formed on the photoconductor  18  in a line. 
       FIG. 7  shows five optical spots  50  aligned on the photoconductor  18 . When the light source  1  is rotated around the optical axis, an alignment angle θ of the optical spots  50  on the photoconductor  18  is changed. In accordance with the change in the alignment angle θ, a scan interval d is also changed. Therefore, the scan interval d can be adjusted by changing the alignment angle θ. 
       FIG. 1  shows a cross-sectional view of the light source apparatus  20  according to the first embodiment of the present invention. The light source apparatus  20  mainly includes the light source  1 , a light source holder  2 , a collimator lens  9  which is an optical element mounted on the tube  8 , and a tube holder  4 . Note that  FIG. 9  shows the O-ring  41  mounted on the tube  8 . 
     The light source  1  is fixed to the light source holder  2  by welding or a screw (not shown). After the distance between the tube  8  and the light source  1  is adequately adjusted, a screw  3   c  is screwed in until the end of the screw  3   c  abuts the tube  8  and thereby fixes the tube  8  in place. Then, the positions of the light source holder  2  and the tube holder  4  in the directions perpendicular to the optical axis are determined, and the light source holder  2  and the tube holder  4  are subsequently joined into an integrated form with screws  3   a . Then, the integrated light source holder  2  and tube holder  4  are fixed to a base  5  with screws  3   b . At this point, the alignment angle θ of the spots on the photoconductor  18  is changed by positioning the light source  1  in the rotation direction around the optical axis so that the scan interval d can be appropriately adjusted. 
     According to the present embodiment, an end of the tube  8  close to the light source  1  is fixed with the screw  3   c . A groove  40  is provided at the other end of the tube on the light emission side, and the O-ring  41  which is a ring-shaped elastic body is placed in the groove  40 . The O-ring  41  is designed in such a manner as to be movable, inside the tube holder  4 , in the optical axis direction but not in the direction perpendicular to the optical axis. Accordingly, the O-ring  41  does not apply constraints on positional displacement of the end of the tube not fixed with the screw  3   c  (i.e. the end on the light emission side) in the optical axis direction, which positional displacement is caused by expansion and contraction of the tube  8 ; however, it is able to apply constraints on positional displacement in all directions perpendicular to the optical axis. Providing the O-ring  41  close to the collimator lens  9  further improves the effect of the O-ring  41  for preventing the positional displacement. 
     In addition, since being reasonable in price, the O-ring  41  creates only a modest increase in price of the light source apparatus. Note that in the present embodiment, the groove  40  and the O-ring  41  are provided on the tube  8 ; however, the same effect may be achieved by providing a similar structure on the tube holder  4 . 
     Next is described focal point displacement of the spots formed on the photoconductor  18 , associated with expansion and contraction of the tube  8 . Assume here the scanning optical apparatus as a whole having the following characteristic: if the ambient temperature changes by +30 K, the position of the focal point of each optical spot  50  on the photoconductor  18  is displaced by 10 mm to the rear side of the photoconductor in the optical axis direction. If the longitudinal magnification of the optical scanning apparatus is 100, the displacement is equivalent to 0.1 mm when converted into positional displacement in the optical axis direction of the light source  1 . In the case where the holders have a linear expansion coefficient of 23×10 −6  (the linear expansion coefficient of aluminum), the tube has a linear expansion coefficient of 90×10 −6  (that of Delrin), the distance between the light source  1  and the fixing point of the screw  3   c  is 30 mm, and the distance between the fixing point of the screw  3   c  and the collimator lens  9  is 30 mm, the distance between the light source  1  and the collimator lens  9  is changed (increased) by 0.1 mm with a temperature change of 30 K. Accordingly, by using the light source unit having the above-described structure, it is possible to offset, with the optical scanning apparatus as a whole, the optical-axis-direction displacement of the focal point of each optical spot formed on the photoconductor  18  caused due to a change in the ambient temperature. 
     b. Second Embodiment 
       FIG. 4  is a cross-sectional view of a light source apparatus according to the second embodiment. An optical scanning apparatus and an image forming apparatus of the present embodiment are the same as those according to the first embodiment. In addition, their assembly and adjustment method are also the same as in the first embodiment. 
     According to the present embodiment, the end of the tube  8  close to the light source  1  is fixed with the screw  3   c , and a direct drive bearing  30 , which is a ring bearing, is provided on the other end. Accordingly, constraint is not applied to the movement of the tube  8  in the optical axis direction; however, constraint is applied to the movement of the tube  8  in all directions perpendicular to the optical axis. Therefore, in the case where the collimator lens  9  is desired to be shifted in the optical axis direction by causing the tube  8  to expand/contract with reference to the position of the screw  3   c , using a change in the ambient temperature, positional displacement of the other end (the end on the light emission side) can be prevented in all directions perpendicular to the optical axis. 
     c. Third Embodiment 
       FIG. 5  illustrates the third embodiment and shows an example of a synthesized light source having two light source apparatuses. In  FIG. 5 , two sets of the light source apparatuses  20  (first and second light sources) according to the above embodiments are mounted on the base  5 . 
     The two light source apparatuses  20  are mounted in such a manner that light beams emitted from the two light source apparatuses  20  at an angle with each other go through the cylindrical lens  23  and substantially intersect with each other at the rotating polygon mirror  24  (see  FIG. 2 ). According to the structure, a synthesized light source is achieved with a small number of components, and therefore, it is possible to provide a light source having twice the number of beams of a single light source at a moderate price. 
     d. Fourth Embodiment 
       FIG. 6  illustrates the fourth embodiment in which two sets of the light source apparatuses  20  (first and second light sources) are fixed to the base  5 , and a reflected beam and a transmitted beam are synthesized by a prism  60 . In this case, the optical axis of the light beam emitted from each light source apparatus  20  can be made to coincide with the optical axis of the synthesized light beam. Accordingly, it is possible to achieve an optical system having small aberration, thereby obtaining a stable and small spot diameter. 
       FIG. 8  shows an example of spot alignment formed on the photoconductor  18  in the case of synthesizing two light sources as in the case of the third and fourth embodiments. As shown in  FIG. 8 , a spot  50  of the first light source and a spot  52  of the second light source are alternately aligned in a single line. For example, in the case where scanning is performed at 1200 dpi, the scan interval d is set to 0.021 mm by changing the spot alignment angle θ. With a synthesized light source, variation in the scanning pitch or the like is generally unnoticeable if the positional displacement of the optical spots of the two light sources is kept within ½ of the scan interval d. Accordingly, in the case of 1200 dpi, the positional displacement should be 0.011 mm or less. 
     In the case where the lateral magnification of the scanning optical apparatus as a whole is 10, the positional displacement of the light source in the direction perpendicular to the optical axis is amplified tenfold on the photoconductor  18 . Therefore, it is necessary to keep the positional displacement of the light source unit within 0.0011 mm. 
     According to the embodiments of the present invention, the focal point displacement can be corrected by making a smooth shift of the collimator lens  9  in the optical axis direction. In addition, the positional displacement in all directions perpendicular to the optical axis is rarely seen and can be kept within 0.0011 mm. Therefore, it is possible to perform printing with unnoticeable variation in the scanning pitch even if the ambient temperature changes. 
     In the first through fourth embodiments, multiple light sources aligned in a single row are described as an example; however, the same effect can be obtained using a surface emitting laser in which light source devices are arranged as shown in  FIG. 10 . The spot arrangement on the photoconductor  18  in the synthesis process is different, and is therefore described below in the fifth and six embodiments. 
     e. Fifth Embodiment 
     The structure of the synthesized light source is the same as that in the third or fourth embodiment.  FIG. 11  shows spots formed on the photoconductor. The optical spots  52  of the second light source are located among the optical spots  51  of the first light source. In this case, if the positional displacement of the optical spots of the two light sources is kept within ½ of the scan interval d, variation in the scanning pitch or the like is unnoticeable. Accordingly, in the case of 1200 dpi, the positional displacement should be 0.011 mm or less. In the case where the lateral magnification of the scanning optical apparatus as a whole is 10, the positional displacement of the light source in the direction perpendicular to the optical axis is amplified tenfold on the photoconductor  18 . Therefore, it is necessary to keep the positional displacement of the light source unit within 0.0011 mm. According to the embodiments of the present invention, the focal point displacement can be corrected by making a smooth shift of the collimator lens  9  in the optical axis direction. In addition, the positional displacement in all directions perpendicular to the optical axis is rarely seen and can be kept within 0.0011 mm. Therefore, it is possible to perform printing with unnoticeable variation in the scanning pitch even if the ambient temperature changes. Furthermore, since the displacement of the image circle is small, light emitting devices on the periphery do not have increased spot diameters. 
     f. Sixth Embodiment 
     The structure of the synthesized light source is the same as that in the third or fourth embodiment.  FIG. 12  shows spots formed on the photoconductor. A set of the optical spots  51  of the first light source and a set of the optical spots  52  of the second light source are arranged side by side. In this case also, if the positional displacement of the optical spots of the two light sources is kept within ½ of the scan interval d, variation in the scanning pitch or the like is unnoticeable. Accordingly, in the case of 1200 dpi, the positional displacement should be 0.011 mm or less. In the case where the lateral magnification of the scanning optical apparatus as a whole is 10, the positional displacement of the light source in the direction perpendicular to the optical axis is amplified tenfold on the photoconductor  18 . Therefore, it is necessary to keep the positional displacement of the light source unit within 0.0011 mm. According to the embodiment of the present invention, the focal point displacement can be corrected by making a smooth shift of the collimator lens  9  in the optical axis direction. In addition, the positional displacement in all directions perpendicular to the optical axis is rarely seen and can be kept within 0.0011 mm. Therefore, it is possible to perform printing with unnoticeable variation in the scanning pitch even if the ambient temperature changes. Furthermore, since the displacement of the image circle is small, light emitting devices on the periphery do not have increased spot diameters. 
     In the above embodiment, the screw  3   c  for fixing the tube  8  is provided at the end of the tube  8  close to the light source  1 . However, in the case of a scanning light apparatus exhibiting a characteristic opposite to the above in terms of the focal point displacement in the optical axis direction caused by a change in the ambient temperature (i.e. the focal point is displaced toward to photoconductor when the temperature increases), the screw  3   c  may be provided at the other end of the tube  9  farther away from the light source  1 . 
     In summary, the present invention is capable of providing a light source apparatus having less positional displacement in all directions perpendicular to the optical axis of the scanning light in the case where the light source apparatus has a structure in which the image location, on the surface to be scanned, in the optical axis direction of the scanning light is designed not to change, using the combination of the linear expansion coefficients of the supporting members of the light source and the collimator lens. Also, the present invention is capable of providing a synthesized light source apparatus, an optical scanning apparatus and an image forming apparatus each having such a light source apparatus. 
     This application is based on Japanese Patent Applications No. 2008-154759 filed on Jun. 13, 2008 and No. 2009-001182 filed on Jan. 6, 2009, the contents of which are hereby incorporated herein by reference.