Patent Publication Number: US-2010124434-A1

Title: Light source device, optical scanning device, and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to and incorporates by reference the entire contents of Japanese Patent Application No. 2008-296196 filed in Japan on Nov. 20, 2008. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a light source device, an optical scanning device, and an image forming apparatus, and more particularly, to a light source device that can monitor light intensity of beams emitted, an optical scanning device including the light source device, and an image forming apparatus including the optical scanning device. 
     2. Description of the Related Art 
     In electrophotographic image recording, an image forming apparatus using a laser as a light source has been widely used. In this case, the image forming apparatus includes an optical scanning device to scan a surface of a photosensitive drum with beams emitted from the light source and deflected by a deflector (scanning beams), thereby forming a latent image on the surface of the photosensitive drum. 
     In image forming apparatuses, a quantity of light of scanning beams changes with a change in temperature and a change with a passage of time, and density unevenness can occur in a finally output image (an output image). To suppress this problem, in an optical scanning device, auto power control (APC) is generally performed. In the APC, a part of beams emitted from a light source is received as monitoring beams by a detector such as a photodiode, and a drive signal of the light source is controlled based on a result thereof (for example, see Japanese Patent Application Laid-open No. H10-100476, Japanese Patent Application Laid-open No. 2002-26445, Japanese Patent Application Laid-open No. 2005-274678, Japanese Patent Application Laid-open No. H6-164070, and Japanese Patent Application Laid-open No. 2007-298563). 
     Recently, image forming apparatuses are also used for simple printing as an on-demand printing system, and downsizing and high-density image quality have been desired. However, as for the conventional apparatuses disclosed in the patent documents mentioned above, it has been difficult to realize high light use efficiency and while achieving downsizing. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to at least partially solve the problems in the conventional technology. 
     According to one aspect of the present invention, there is provided a light source device that provides a light beam. The light source device includes: a light source that emits a light beam; a coupling optical system that couples the light beam; a separation optical element on which the light beam is incident through the coupling optical system, the separation optical element including an opening through which a part of the light beam having a highest light intensity passes, and reflecting other part of the light beam incident on a surrounding area of the opening as a monitoring light beam; and a light shielding member including at least one of a light shielding portion that is arranged on an optical path between the light source and the coupling optical system to shield a part of the light beam traveling toward an outside of an effective area of the coupling optical system and a light shielding portion that is arranged on an optical path between the coupling optical system and the separation optical system to shield a part of the light beam passed through the outside of the effective area of the coupling optical system. 
     Furthermore, according to another aspect of the present invention, there is provided an optical scanning device that scans a scanning surface with a light beam. The optical scanning device includes: a light source device including a light source that emits a light beam, a coupling optical system that couples the light beam, a separation optical element on which the light beam is incident through the coupling optical system, the separation optical element including an opening through which a part of the light beam having a highest light intensity passes, and reflecting other part of the light beam incident on a surrounding area of the opening as a monitoring light beam, and a light shielding member including at least one of a light shielding portion that is arranged on an optical path between the light source and the coupling optical system to shield a part of the light beam traveling toward an outside of an effective area of the coupling optical system and a light shielding portion that is arranged on an optical path between the coupling optical system and the separation optical system to shield a part of the light beam passed through the outside of the effective area of the coupling optical system; a deflector that deflects the light beam from the light source device; and a scanning optical system that condenses the light beam deflected by the deflector onto a scanning surface to be scanned. 
     Moreover, according to still another aspect of the present invention, there is provided an image forming apparatus that forms an image on a recording medium. The image forming apparatus includes: at least one image carrier; and at least one optical scanning device that scans the at least one image carrier by a light beam that is modulated based on image information. The optical scanning device includes: a light source device that provides the light beam; a deflector that deflects the light beam from the light source device; and a scanning optical system that condenses the light beam deflected by the deflector onto a scanning surface to be scanned. The light source device includes a light source that emits a light beam, a coupling optical system that couples the light beam from the light source, a separation optical element on which the light beam coupled by the coupling optical system is incident, the separation optical element including an opening through which a part of the light beam having a highest light intensity passes, and reflecting other part of the light beam incident on a surrounding area of the opening as a monitoring light beam, and a light shielding member including at least one of a light shielding portion that is arranged on an optical path between the light source and the coupling optical system to shield a part of the light beam traveling toward an outside of an effective area of the coupling optical system and a light shielding portion that is arranged on an optical path between the coupling optical system and the separation optical system to shield a part of the light beam passed through the outside of the effective area of the coupling optical system. 
     The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram for explaining a configuration of a laser printer according to an embodiment of the present invention; 
         FIG. 2  is a schematic diagram of an optical scanning device in  FIG. 1 ; 
         FIG. 3  is a schematic diagram for explaining a light source device in  FIG. 2 ; 
         FIG. 4  is a schematic diagram for explaining a surface-emitting laser array included in a light source in  FIG. 3 ; 
         FIG. 5  is a schematic diagram for explaining an effective area of a coupling lens; 
         FIG. 6  is a schematic diagram for explaining an operation of a light shielding member; 
         FIG. 7  is a schematic diagram for explaining a normal direction of a light shielding surface in a light shielding portion of the light shielding member; 
         FIG. 8  is a schematic diagram for explaining a first aperture plate in  FIG. 3 ; 
         FIG. 9  is a schematic diagram for explaining a second aperture plate in  FIG. 3 ; 
         FIG. 10  is a schematic diagram for explaining a holding member; 
         FIG. 11  is a block diagram for explaining a configuration of a light source controller; 
         FIG. 12  is a schematic diagram for explaining a first modification of the holding member; 
         FIG. 13  is a schematic diagram for explaining an operation of a light shielding member; 
         FIG. 14  is a schematic diagram for explaining a second modification of the holding member; 
         FIG. 15  is a schematic diagram for explaining a fixing member; and 
         FIG. 16  depicts a schematic configuration of a color printer. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Exemplary embodiments of the present invention will be explained below in detail with reference to the accompanying drawings.  FIG. 1  depicts a schematic configuration of a laser printer  1000  as an image forming apparatus according to an embodiment of the present invention. 
     The laser printer  1000  includes an optical scanning device  1010 , a photosensitive drum  1030 , an electrification charger  1031 , a developing roller  1032 , a transfer charger  1033 , a neutralizing unit  1034 , a cleaning unit  1035 , a toner cartridge  1036 , a paper feed roller  1037 , a paper feed tray  1038 , a registration roller pair  1039 , a fuser roller pair  1041 , an ejection roller pair  1042 , an ejection tray  1043 , a communication controller  1050 , and a printer controller  1060  that generally controls these units. These units are accommodated at predetermined positions of a printer housing  1044 . 
     The communication controller  1050  controls two-way communications between an upper-level device (such as a personal computer) and the laser printer  1000  via a network. 
     The photosensitive drum  1030  is a columnar member, and a photosensitive layer is formed on a surface thereof. That is, the surface of the photosensitive drum  1030  is a scanning target surface. The photosensitive drum  1030  rotates in an arrow direction in  FIG. 1 . 
     The electrification charger  1031 , the developing roller  1032 , the transfer charger  1033 , the neutralizing unit  1034 , and the cleaning unit  1035  are arranged near the surface of the photosensitive drum  1030 . These are arranged along a rotation direction of the photosensitive drum  1030  in an order of the electrification charger  1031 , the developing roller  1032 , the transfer charger  1033 , the neutralizing unit  1034 , and the cleaning unit  1035 . 
     The electrification charger  1031  uniformly charges the surface of the photosensitive drum  1030 . 
     The optical scanning device  1010  irradiates beams modulated based on image information from the upper-level device onto the surface of the photosensitive drum  1030  charged by the electrification charger  1031 . Accordingly, a latent image corresponding to the image information is formed on the surface of the photosensitive drum  1030 . The formed latent image moves toward the developing roller  1032  with a rotation of the photosensitive drum  1030 . A configuration of the optical scanning device  1010  will be described later. 
     Toner is stored in the toner cartridge  1036  and the toner is supplied to the developing roller  1032 . 
     The developing roller  1032  causes the toner supplied from the toner cartridge  1036  to adhere on the latent image formed on the surface of the photosensitive drum  1030 , to visualize the image information. The latent image on which the toner adheres (hereinafter, “toner image” for convenience) moves toward the transfer charger  1033  with the rotation of the photosensitive drum  1030 . 
     Recording paper  1040  is stored in the paper feed tray  1038 . The paper feed roller  1037  is arranged near the paper feed tray  1038 , and the paper feed roller  1037  takes out the recording paper  1040  one by one from the paper feed tray  1038 , and carries it to the registration roller pair  1039 . The registration roller pair  1039  temporarily holds the recording paper  1040  taken out by the paper feed roller  1037 , and sends it out toward a gap between the photosensitive drum  1030  and the transfer charger  1033 , matched with the rotation of the photosensitive drum  1030 . 
     A voltage with an opposite polarity to the polarity of the toner is applied to the transfer charger  1033 , to electrically attract the toner on the surface of the photosensitive drum  1030  to the recording paper  1040 . A toner image on the surface of the photosensitive drum  1030  is transferred to the recording paper  1040 . The recording paper  1040  to which the toner image is transferred is carried to the fuser roller pair  1041 . 
     Heat and pressure are applied to the recording paper  1040  by the fuser roller pair  1041 , and thus the toner is fixed on the recording paper  1040 . The recording paper  1040  on which the toner is fixed is carried to the ejection tray  1043  via the ejection roller pair  1042 , and sequentially stacked on the ejection tray  1043 . 
     The neutralizing unit  1034  neutralizes the surface of the photosensitive drum  1030 . 
     The cleaning unit  1035  removes toner remaining on the surface of the photosensitive drum  1030  (residual toner). The surface of the photosensitive drum  1030  with the residual toner being removed returns to a position facing the electrification charger  1031 . 
     The configuration of the optical scanning device  1010  is explained next. 
     The optical scanning device  1010  includes, as shown in  FIG. 2  as an example, a light source device  10 , a cylindrical lens  31 , a scanning-beam reflecting mirror  32 , a polygon mirror  33 , a deflector-side scanning lens  35 , an image-surface-side scanning lens  36 , two photo-detection mirrors ( 37   a,    37   b ), and two photo-detection sensors ( 38   a,    38   b ). These units are incorporated at predetermined positions in a housing. 
     In the present specification, in an XYZ three-dimensional rectangular coordinate system, a direction along a longitudinal direction of the photosensitive drum  1030  is explained as a Y-axis direction, and a direction along an optical axis of the respective scanning lenses (the deflector-side scanning lens  35  and the image-surface-side scanning lens  36 ) is explained as an X-axis direction. Further, in the following explanations, a direction corresponding to a main scanning direction is abbreviated as “main-scanning corresponding direction”, and a direction corresponding to a sub-scanning direction is abbreviated as “sub-scanning corresponding direction” for convenience. 
     The light source device  10  includes, as shown in  FIG. 3  as an example, a light source  11 , a coupling lens  12 , a light shielding member  13 , a first aperture plate  14 , a monitoring-beam reflecting mirror  15 , a second aperture plate  16 , a condenser lens  17 , a light receiving element  18 , and a light source controller  22 . The light source  11 , the light receiving element  18 , and the light source controller  22  are respectively mounted on a same circuit board  19 . 
     As shown in  FIG. 4  as an example, the light source  11  has a two-dimensional array  100  in which forty light emitting units are arranged two-dimensionally and formed on one board. An M direction in  FIG. 4  is the main-scanning corresponding direction (same as the Y-axis direction), and an S direction is the sub-scanning corresponding direction (same as the Z-axis direction). A T direction is a direction inclined from the M direction toward the S direction. 
     The two-dimensional array  100  has four light-emitting arrays in which ten light emitting units are arranged with equal intervals along the T direction. These four arrays of light emitting units are arranged in the S direction with equal intervals so that these arrays have equal intervals therebetween when these are orthogonally projected on a virtual line extending in the S direction. In the present specification, “interval between the light emitting units” refers to a distance between centers of two light emitting units. 
     Each of the light emitting unit is a vertical-cavity surface-emitting laser (VCSEL) having an emission wavelength of 780 nanometers. That is, the two-dimensional array  100  is a surface-emitting laser array having forty light emitting units. 
     It is assumed that beams are emitted from the light source  11  toward a +X direction. 
     The coupling lens  12  is arranged on a +X side of the light source  11 , to make the beams emitted from the light source  11  substantially parallel beams. As shown in  FIG. 5  as an example, the coupling lens  12  has an effective area with high forming accuracy and a non-effective area with low forming accuracy around the effective area. 
     The light shielding member  13  is arranged on a +X side of the coupling lens  12 , and as shown in  FIG. 6  as an example, transmits beams having passed through the effective area of the coupling lens  12  and shields the beams having passed through the non-effective area. 
     A normal direction of a light shielding surface in a light shielding portion of the light shielding member  13  is inclined, as shown in  FIG. 7  as an example, with respect to a direction parallel to an optical axis of the coupling lens  12  (the X-axis direction). An inclination angle θ of the light shielding surface in the normal direction satisfies a relation of L×tan(90-θ)&gt;1, using a distance L in the X-axis direction between the light shielding surface and the light source  11 . 
     As shown in  FIG. 8  as an example, the first aperture plate  14  is arranged on a +X side of the light shielding member  13  and has an opening to define a beam diameter of the beams having passed through the light shielding member  13 . A surrounding surface of the opening of the first aperture plate  14  is coated with aluminum or silver to have high reflectivity. The first aperture plate  14  is arranged so that a portion of incident beams having the largest light intensity passes substantially a center of the opening. 
     The first aperture plate  14  is also arranged to be inclined with respect to a virtual surface orthogonal to a direction parallel to the optical axis of the coupling lens  12  for using beams reflected by a surrounding area of the opening as monitoring beams. The first aperture plate  14  is arranged such that a traveling direction of the beams reflected by the surrounding area of the opening becomes a −Z direction (see  FIG. 3 ). 
     The beams having passed through the opening in the first aperture plate  14  are beams emitted from the light source device  10 . 
     The monitoring-beam reflecting mirror  15  is arranged on a −Z side of the first aperture plate  14 , to fold back an optical path of the beams (monitoring beams) reflected by the first aperture plate  14  in a −X direction. 
     As shown in  FIG. 9  as an example, the second aperture plate  16  is arranged on a −X side of the monitoring-beam reflecting mirror  15  and has an opening to define a beam diameter of the monitoring beams reflected by the monitoring-beam reflecting mirror  15 . The size and shape of the opening in the second aperture plate  16  are determined according to the size and shape of the opening in the first aperture plate  14 . 
     The condenser lens  17  is arranged on the −X side of the second aperture plate  16 , to condense the monitoring beams having passed through the opening in the second aperture plate  16 . 
     The light receiving element  18  is arranged on the circuit board  19  and on the −X side of the condenser lens  17 , to receive the monitoring beams. The light receiving element  18  outputs a signal corresponding to a quantity of received light (a photoelectric conversion signal). 
     An optical system arranged on the optical path of the monitoring beams between the first aperture plate  14  and the light receiving element  18  is also referred to as a monitoring optical system. In the present embodiment, the monitoring optical system includes the monitoring-beam reflecting mirror  15 , the second aperture plate  16 , and the condenser lens  17 . 
     As shown in  FIG. 10 , as an example, the coupling lens  12 , the first aperture plate  14 , the monitoring-beam reflecting mirror  15 , the second aperture plate  16 , and the condenser lens  17  are held by a holding member  25  in a predetermined positional relation. The light shielding member  13  is integrally formed with the holding member  25 . 
     While the holding member  25  is an integrated member by using a mold, it can be produced by cutting. 
     Referring back to  FIG. 2 , the cylindrical lens  31  is arranged on the +X side of the light source device  10 , and images beams having passed through the opening in the first aperture plate  14  of the light source device  10 , that is, beams emitted from the light source device  10  near a deflection reflecting surface of the polygon mirror  33  via the scanning-beam reflecting mirror  32  in the Z-axis direction. 
     The optical system arranged on the optical axis between the light source  11  and the polygon mirror  33  is also referred to as a pre-deflector optical system. In the present embodiment, the pre-deflector optical system includes the coupling lens  12 , the first aperture plate  14 , the cylindrical lens  31 , and the scanning-beam reflecting mirror  32 . 
     The polygon mirror  33  has a tetrahedral mirror with a radius of an inscribed circle being  7  millimeters as an example, and each mirror becomes a deflection reflecting surface. The polygon mirror  33  deflects beams from the scanning-beam reflecting mirror  32 , while rotating around a shaft parallel with the Z-axis direction at constant velocity. 
     The deflector-side scanning lens  35  is arranged on the optical path of the beams deflected by the polygon mirror  33 . 
     The image-surface-side scanning lens  36  is arranged on the optical path of the beams via the deflector-side scanning lens  35 . The beams via the image-surface-side scanning lens  36  are irradiated onto the surface of the photosensitive drum  1030  to form an optical spot. The optical spot moves in a longitudinal direction of the photosensitive drum  1030  with the rotation of the polygon mirror  33 . That is, the optical spot scans on the photosensitive drum  1030 . The moving direction of the optical spot is “the main scanning direction”, and the rotation direction of the photosensitive drum  1030  is “the sub-scanning direction”. 
     The optical system arranged on the optical path between the polygon mirror  33  and the photosensitive drum  1030  is also referred to as a scanning optical system. In the present embodiment, the scanning optical system includes the deflector-side scanning lens  35  and the image-surface-side scanning lens  36 . At least one folding mirror can be arranged on at least one of the optical paths of the optical path between the deflector-side scanning lens  35  and the image-surface-side scanning lens  36  and the optical path between the image-surface-side scanning lens  36  and the photosensitive drum  1030 . 
     A part of beams before start of writing, deflected by the polygon mirror  33  and via the scanning optical system, enters into the photo-detection sensor  38   a  via the photo-detection mirror  37   a.  Further, a part of beams after end of writing, deflected by the polygon mirror  33  and via the scanning optical system, enters into the photo-detection sensor  38   b  via the photo-detection mirror  37   b.    
     Each of the photo-detection sensors outputs a signal corresponding to the quantity of received light (a photoelectric conversion signal). 
     The light source controller  22  includes, as shown in  FIG. 11  as an example, a pixel-clock generation circuit  215 , an image processing circuit  216 , a write control circuit  219 , and a light-source driving circuit  221 . Arrows in  FIG. 11  indicate a flow of representative signals and information, and does not indicate overall connection of respective blocks. 
     The pixel-clock generation circuit  215  obtains a time required for the beams to scan between the respective photo-detection sensors based on an output signal of the photo-detection sensor  38   a  and an output signal of the photo-detection sensor  38   b.  The pixel-clock generation circuit  215  then sets a frequency so that a preset number of pulses are accommodated within the obtained time, to generate a pixel clock signal PCLK at the frequency. The pixel clock signal PCLK generated here is supplied to the image processing circuit  216  and the write control circuit  219 . Further, the output signal of the photo-detection sensor  38   a  is output to the write control circuit  219  as a synchronization signal. 
     The image processing circuit  216  performs raster expansion on image information sent from the upper-level device via the printer controller  1060 , and after performing a predetermined halftone process, generates image data expressing gradation of each pixel for each light emitting unit based on the pixel clock signal PCLK. Upon detection of start of scan based on the output signal of the photo-detection sensor  38   a,  the image processing circuit  216  outputs the image data to the write control circuit  219 , in synchronization with the pixel clock signal PCLK. 
     The write control circuit  219  generates a pulse modulation signal based on the image data from the image processing circuit  216 , the pixel clock signal PCLK from the pixel-clock generation circuit  215 , and the synchronization signal. Further, the write control circuit  219  corrects a drive current of each light emitting unit so that the quantity of light of the beams passing through the opening in the first aperture plate  14  of the light source device  10  becomes constant. That is, the write control circuit  219  performs APC. 
     The light-source driving circuit  221  drives light emitting units in the two-dimensional array  100  based on the pulse modulation signal from the write control circuit  219 . 
     As can be understood from the above explanations, in the light source device  10  according to the present embodiment, the coupling lens  12  forms a coupling optical system, and the first aperture plate  14  forms a separation optical element. 
     As described above, the light source device  10  includes the light source  11  having the surface-emitting laser array in which a plurality of light emitting units are arranged two-dimensionally, the coupling lens  12  that couples the beams from the light source  11 , the light shielding member  13  having the light shielding surface that shields the beams having passed through the non-effective area of the coupling lens  12 , and the first aperture plate  14  that has the opening and reflects the beams entering into the surrounding area of the opening as the monitoring beams, with a part of the beams having the largest light intensity, which have passed through the light shielding member  13 , passing substantially the center of the opening. The light source device  10  further includes the light receiving element  18  that receives the monitoring beams and the monitoring optical system that leads the monitoring beams reflected by the first aperture plate  14  to the light receiving element  18 . 
     In this case, it is possible to prevent that beams, which have passed through the coupling lens  12  and are not used for any of optical scanning and monitoring of the quantity of light, from entering into the first aperture plate  14  and the monitoring optical system. Therefore, a beam diameter of the beams entering into the first aperture plate  14  and the monitoring optical system decreases than in a conventional case, thereby enabling to downsize the first aperture plate  14  and the monitoring optical system than in the conventional case. Further, because the light shielded by the light shielding member  13  has not been used at all conventionally, light use efficiency is not degraded. 
     Accordingly, downsizing is possible without degrading the light use efficiency. 
     Further, because the normal direction of the light shielding surface of the light shielding member  13  is inclined with respect to the direction parallel with the optical axis of the coupling lens  12 , it is suppressed that the beams reflected by the light shielding surface returns to the light source  11 . Therefore, it can be prevented that the optical output of the light source  11  becomes unstable due to return light. 
     Because the first aperture plate  14  and the monitoring optical system are held by the holding member  25 , it can be suppressed that a positional relation between the first aperture plate  14  and the second aperture plate  16  is deviated from a designed positional relation due to vibrations, for example. Therefore, high monitoring accuracy can be maintained. Further, an assembly operation at the time of production can be facilitated, and an inspection process and a tuning process before shipment can be simplified. 
     Because the light shielding member  13  is integrally formed with the holding member  25 , an increase in the number of parts can be suppressed. 
     Further, the first aperture plate  14  is used as the separation optical element, to separate the beams having passed through the light shielding member  13  into the scanning beams and the monitoring beams. Accordingly, the light use efficiency can be improved as compared with a case of using a half mirror as the separation optical element. When a half mirror is used as the separation optical element, the quantity of light of scanning beams decreases because the monitoring beams are separated from the scanning beams. 
     Because the monitoring optical system has the second aperture plate  16  that defines the beam diameter of the monitoring beams, a ratio between the quantity of light of the beams passing through the opening in the first aperture plate  14  and the quantity of light of the monitoring beams received by the light receiving element  18  can be made constant. Therefore, the light source device  10  can emit stable beams by controlling the drive of the light source  11  so that the output of the light receiving element  18  maintains a predetermined value. Accordingly, APC can be performed with a simple algorithm. 
     Because the optical scanning device  1010  according to the present embodiment has the light source device  10 , the surface of the photosensitive drum  1030  can be optically scanned accurately and stably, without increasing the size of the apparatus. 
     Because the light source  11  has a plurality of light emitting units, multiple scanning can be made simultaneously, thereby enabling to realize high-speed operations and high-density image formation. 
     Further, because the laser printer  1000  has the optical scanning device  1010 , a high quality image can be formed without increasing the size of the apparatus. 
     In the present embodiment, there has been explained a case that the normal direction of the light shielding surface of the light shielding portion of the light shielding member  13  is inclined with respect to the direction parallel to the optical axis of the coupling lens  12 . However, for example, when the light shielding surface is pearskin finished (finished as a non-gloss surface) or a paint that decreases reflectivity is applied to the light shielding surface, so that the light reflected by the light shielding surface does not return to the light source  11 , the normal direction of the light shielding surface does not have to be inclined with respect to the direction parallel to the optical axis of the coupling lens  12 . 
     In the present embodiment, a case that the light shielding member  13  is integrally formed with the holding member  25  has been explained. However, the present invention is not limited thereto, and the light shielding member  13  can be provided separately. 
     In the present embodiment, a case that the first aperture plate  14  and the monitoring optical system are held by the holding member  25  has been explained; however, the present invention is not limited thereto. For example, a part of the monitoring optical system can be held not by the holding member  25  but by the housing of the optical scanning device  1010 . 
     In the present embodiment, a case that the first aperture plate  14  is arranged so that the traveling direction of the beams reflected by the surrounding area of the opening becomes a −Z direction has been explained; however, the present invention is not limited thereto. 
     In the present embodiment, a case that the two-dimensional array  100  has forty light emitting units has been explained; however, the present invention is not limited thereto. 
     In the present embodiment, a case that the light source  11  has the two-dimensional array  100  has been explained; however, the present invention is not limited thereto. For example, the light source  11  can have a one-dimensional array in which a plurality of light emitting units are arranged in line, instead of the two-dimensional array  100 . Further, the light source  11  can have one light emitting unit instead of the two-dimensional array  100 . 
     In the present embodiment, when a size of the opening in the light shielding member  13  is set to match a size of the opening in the second aperture plate  16 , the second aperture plate  16  can be omitted. 
     In the present embodiment, a light shielding member  13 ′ that shields the beams traveling toward the non-effective area of the coupling lens  12  to limit the beam diameter of the beams entering into the coupling lens  12  can be further arranged between the light source  11  and the coupling lens  12  (see  FIGS. 12 and 13 ). The light shielding member  13 ′ can be integrated with the holding member  25  or provided separately. 
     In the present embodiment, the light shielding member  13 ′ that shields the beams traveling toward the non-effective area of the coupling lens  12  to limit the beam diameter of the beams entering into the coupling lens  12  can be arranged instead of the light shielding member  13  between the light source  11  and the coupling lens  12  (see 
       FIG. 14 ). The light shielding member  13 ′ can be integrated with the holding member  25  or provided separately. 
     In the present embodiment, the holding member  25  and the circuit board  19  can be fixed by a fixing member  26  in a predetermined positional relation (see  FIG. 15 ). Accordingly, an assembly operation at the time of production can be facilitated, and an inspection process and a tuning process before shipment can be simplified. Further, because it can be suppressed that a positional relation between the light receiving element  18  and the monitoring optical system is deviated from a designed positional relation, a large margin is not required for the size of the light receiving element  18 . That is, the size of the light receiving element  18  can be decreased. 
     In the present embodiment, a case that the monitoring optical system is included in the light source device has been explained. However, the present invention is not limited thereto, and at least a part of the monitoring optical system can be provided separately. 
     In the present embodiment, a case that the image forming apparatus is the laser printer  1000  has been explained; however, the present invention is not limited thereto. In short, a high quality image can be formed in a stable manner as far as the image forming apparatus includes the optical scanning device  1010 . 
     For example, the image forming apparatus can be an apparatus that irradiates laser beams directly to a medium (such as paper) that develops color by laser beams. 
     Further, the image forming apparatus can be an apparatus that uses a silver salt film as an image carrier. In this case, a latent image can be formed on the silver salt film by optical scanning, and the latent image can be visualized by a similar process to a development process in a general silver-salt photographic process. The latent image can be transferred onto printing paper by a similar process to a printing process in a general silver-salt photographic process. Such an image forming apparatus can be implemented as an optical plate-making apparatus or an optical drawing apparatus that draws CT scan images or the like. 
     For example, as shown in  FIG. 16 , the image forming apparatus can be a color printer  2000  including a plurality of photosensitive drums. 
     The color printer  2000  is a tandem-type multicolor printer that forms full color images by superposing four colors (black, cyan, magenta, and yellow). The color printer  2000  includes a photosensitive drum K 1 , a charger K 2 , a developing apparatus K 4 , a cleaning unit K 5 , and a transfer apparatus K 6  for black, a photosensitive drum C 1 , a charger C 2 , a developing apparatus C 4 , a cleaning unit C 5 , and a transfer apparatus C 6  for cyan, a photosensitive drum M 1 , a charger M 2 , a developing apparatus M 4 , a cleaning unit M 5 , and a transfer apparatus M 6  for magenta, and a photosensitive drum Y 1 , a charger Y 2 , a developing apparatus Y 4 , a cleaning unit Y 5 , and a transfer apparatus Y 6  for yellow. The color printer  2000  further includes an optical scanning device  2010 , a transfer belt  2080 , and a fixing unit  2030 . 
     The photosensitive drums rotate in a direction of arrows in  FIG. 16 , and chargers, developing apparatuses, transfer apparatuses, and cleaning units are respectively arranged in this order in a rotation direction around each photosensitive drum. 
     Each of the chargers uniformly charges the surface of a corresponding photosensitive drum. Optical scanning is performed by the optical scanning device  2010  with respect to the surface of each of the photosensitive drums charged by the charger, thereby forming a latent image on each of the photosensitive drums. 
     A toner image is then formed on the surface of each of the photosensitive drums by a corresponding developing apparatus. Further, a toner image of each color is sequentially transferred onto recording paper on the transfer belt  2080  by a corresponding transfer apparatus, and an image is finally fixed on the recording paper by the fixing unit  2030 . 
     The optical scanning device  2010  includes a light source device similar to the light source device  10  for each color. Therefore, Effects identical to those of the optical scanning device  1010  can be achieved. 
     The color printer  2000  can achieve effects identical to those of the laser printer  1000 . 
     In the tandem-type multicolor printer, color misregistration can occur between respective colors due to factors such as machine accuracy of the printer. However, correction accuracy of color misregistration between respective colors can be increased by selecting a light emitting unit to be lighted. 
     In the color printer  2000 , the optical scanning device can be provided for each color, or provided in a unit of two colors. 
     According to one aspect of the present invention, downsizing of a device is possible without degrading the light use efficiency. 
     Furthermore, according to another aspect of the present invention, a scanning surface can be optically scanned in an accurate and stable manner without causing an increase in size of a device. 
     Moreover, according to still another aspect of the present invention, a high quality image can be formed without causing an increase in size of an apparatus. 
     Although the invention has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.