Abstract:
A laser emitter including: a first lens barrel portion for holding a first laser element; a second lens barrel portion for holding a second laser element whose optical axis is slanted with respect to an optical axis of the first lens barrel portion, the second lens barrel portion being provided as one unit with the first lens barrel portion; a first lens supporting portion provided at the tip of the first lens barrel portion to support a first collimator lens; and a second lens supporting portion provided at the tip of the second lens barrel portion to support a second collimator lens, in which the first and second lens supporting portions are capable of supporting the collimator lenses with an adjustment range stretching in an optical axis direction, and the first and second collimator lenses are fixed and supported at positions adjusted within the adjustment range.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a laser emitter and a laser scanning device suitable for an image exposure device that is provided in an image forming apparatus such as an electrophotographic copying machine or printer to expose a photosensitive body to light.  
           [0003]    2. Related Background Art  
           [0004]    A description is given on background art of the present invention with reference to FIGS.  9  to  14 . FIG. 9 shows an image forming apparatus for printing a color image. The image forming apparatus has an independent image bearing member (hereinafter referred to as photosensitive drum)  120  for each of four colors, that is, yellow, magenta, cyan, and black. The photosensitive drum  120  is an electric conductor with a photosensitive layer formed thereon by application, and forms an electrostatic latent image upon receiving laser light that is emitted from a scanning optical device. The image forming apparatus also has: a scanning optical device  121  for radiating laser light in accordance with image information that is sent from an image reading device (not shown), personal computer or the like; a developing unit  122  for forming a toner image on a photosensitive drum using toner which is charged by friction; an intermediate transfer belt  123  for carrying the toner image on the photosensitive drum to a sheet of transfer paper; a sheet feeding cassette  124  for storing sheets of paper to which toner images are to be transferred; a fixing unit  125  for fixing the transferred toner images to the paper by heat; a sheet delivery tray  126  on which sheets of transfer paper with images fixed thereon are stacked; and a cleaner  127  for cleaning toner that remains on the photosensitive drum.  
           [0005]    To form an image, the photosensitive drum  120  is irradiated with laser light which is emitted from the scanning optical device  121  in accordance with image information. This causes a charger to charge the photosensitive drum  120 , which then forms an electrostatic latent image. After that, toner charged by friction in the developing unit  122  adheres to the electrostatic latent image to form a toner image on the photosensitive drum  120 . The toner image is transferred from the photosensitive drum  120  to the intermediate transfer belt  123 , and then re-transferred to a sheet of paper that has been fed from the sheet feeding cassette  124  which is placed in a lower part of the main body of the apparatus. An image is thus formed on the paper. The sheet of paper to which the image is transferred is sent to the fixing unit  125  to fix toner and is discharged onto the sheet delivery tray.  
           [0006]    [0006]FIG. 10 is a diagram of an image forming portion of FIG. 9 and, since the image forming portion has a symmetrical shape, the symbols used in FIG. 9 are shown on only one side of the unit. The scanning optical device  121  of FIG. 9 emits laser light in accordance with image information, so that an electrostatic latent image is formed on the photosensitive drum  120  by the laser light after the laser light passes through a rotary multi-facet mirror  128 , fθ lenses  129  and  130 , turn-back mirrors  131   a  to  131   d , and a dust-proof glass enclosure  132 . The rotary multi-facet mirror (hereinafter referred to as polygon mirror)  128  is provided for deflection scanning of the laser light. The fθ lenses  129  and  130  are provided to let the laser light run at a constant speed and form a spot on the drum. The turn-back mirrors  131   a  to  131   d  are for reflecting the laser beam in a given direction. The dust-proof glass enclosure  132  is provided to protect the scanning optical device  121  from dust. The scanning optical device  121  is placed near the photosensitive drum  120  with the advent of the fashion of scanning optical devices of nowadays which have stopped irradiating photosensitive drums from afar as image forming apparatuses themselves have been reduced in size. As shown in FIG. 10, the scanning optical device  121  employs a method of using one polygon motor unit to irradiate four photosensitive drums, and forms two scanning groups for irradiating opposing faces of the polygon mirror  128  with plural laser beams each. The turn-back mirrors  131   a  to  131   d  are used to make the unit compact, and are each a mold lens in which two lenses are pasted together or two light paths are unitarily formed in order to let laser beams on two different light paths image on a photosensitive drum. In this collimating optics, a deflection plane for deflection scanning of laser light is necessary for each light path. Therefore, a thick polygon mirror or a two-stage structure polygon mirror is employed in this system.  
           [0007]    In contrast to an optical system that uses such a polygon mirror as the one described above, there is an optical system as the one shown in FIG. 11 which uses a thin polygon mirror  133  to obtain a thinner shape. In this optical system, laser beams enter and exit the polygon mirror  133  at different angles in a sub-scanning direction so that beams for irradiating photosensitive drums are split at a point where the beams are spaced at regular intervals from one another. Laser light is run for deflection scanning with the use of the polygon mirror  133  and then is transmitted through common fθ lenses  135  and  136 . After that, each laser beam passes through two turn-back mirrors and one concave mirror  134   b  or  134   e  and is applied to its associated photosensitive drum. To split laser light, a laser beam that is run on the lower side in FIG. 11 by deflection scanning is reflected upward in FIG. 11 toward an inner photosensitive drum by a turn-back mirror  134   d , which is placed at a point in the light path of the laser beam, so that the laser beam intersects a laser beam that is run on the upper side by deflection scanning, and the reflected light is then radiated onto the photosensitive drum by turn-back mirrors  134   e  and  134   f , which are placed in an upper part of the scanning optical device. In the optical system of FIG. 11 where light is incident obliquely, the fθ lenses, having a refractive power in the main scanning direction, work like the collimating optics described above and therefore can be arranged as in the collimating optics. On the other hand, in principle, it is difficult for the fθ lenses of FIG. 11 to assuredly condense light in the sub-scanning direction on a photosensitive drum since light enters the lenses obliquely with respect to the optical axes of the lenses.  
           [0008]    It is for this reason that the concave mirrors  134   b  and  134   e  are necessarily provided to condense laser beams in the sub-scanning direction after each laser light is split (the same effect is obtained if these concave mirrors are replaced by third imaging lenses each having a refractive power in the sub-scanning direction).  
           [0009]    There has been proposed also a modification of the above oblique incident optical system that needs to place four imaging optical elements in total in one group downstream of the polygon mirror. The modified oblique incident optical system is reduced in number of optical elements, and has an fθ lens as a second imaging lens placed downstream of the point where each laser light is split. This structure needs the second imaging lens for each laser light but no concave mirror or third imaging lens, and is therefore capable of condensing laser light on a photosensitive drum with three imaging optical elements. As a result, a scanning optical device having this structure is smaller in size than the above-described oblique incident optical system that uses concave mirrors or third imaging lenses.  
           [0010]    Such an optical system where light in a sub-scanning direction is incident obliquely has four laser light sources corresponding to four photosensitive drums. The symmetrical optical system shown in FIG. 11 are composed of two lens barrel portions  153  and  154 , which hold two light sources  151  and  152 , respectively, two collimator lenses  155  and  156 , and two electric circuit substrates  157  and  158  for causing emission of laser light. Those laser light sources are placed side by side in the sub-scanning direction at a distance as shown in a sectional view in the sub-scanning direction of FIG. 13, or placed off the sub-scanning direction with the use of a turn-back mirror  159  as shown in a top view of FIG. 14.  
           [0011]    However, the structures of FIGS. 13 and 14 both have problems. To place the laser light sources side by side in the sub-scanning direction at a distance as shown in FIG. 13, the length of the incident optical system from the laser light sources to the polygon mirror has to be set long since a short distance between the laser light sources causes the lens barrel portions to interfere with each other. This increases the size of the scanning optical device.  
           [0012]    The structure of FIG. 14 which uses the turn-back mirror to place the laser light sources off the sub-scanning direction is free from the above problem of increasing the scanning optical device in size, but the use of the turn-back mirror increases the number of parts and accordingly cost.  
         SUMMARY OF THE INVENTION  
         [0013]    The present invention has been made in view of the above, and an object of the present invention is to provide a laser emitter that is reduced in size.  
           [0014]    Another object of the present invention is to provide a laser emitter that has fewer parts and can achieve high laser position precision.  
           [0015]    Still another object of the present invention is to provide a laser emitter including:  
           [0016]    a first lens barrel portion for holding a first laser element;  
           [0017]    a second lens barrel portion for holding a second laser element whose optical axis is slanted with respect to an optical axis of the first lens barrel portion, the second lens barrel portion being provided as one unit with the first lens barrel portion;  
           [0018]    a first lens supporting portion provided at the tip of the first lens barrel portion to support a first collimator lens; and  
           [0019]    a second lens supporting portion provided at the tip of the second lens barrel portion to support a second collimator lens, in which  
           [0020]    the first and second lens supporting portions are capable of supporting the collimator lenses with an adjustment range stretching in an optical axis direction, and the first and second collimator lenses are fixed and supported at positions adjusted within the adjustment range.  
           [0021]    Still another object of the present invention is to provide a laser scanning device including:  
           [0022]    a laser unit for emitting first and second laser beams, the laser unit having: a first lens barrel portion for holding a first laser element that emits the first laser beam; a second lens barrel portion for holding a second laser element that emits the second laser beam whose optical axis is slanted with respect to the optical axis of the first laser beam; a first lens supporting portion provided at the tip of the first lens barrel portion to support a first collimator lens; and a second lens supporting portion provided at the tip of the second lens barrel portion to support a second collimator lens, the first and second lens supporting portions being capable of supporting the collimator lenses with an adjustment range stretching in an optical axis direction, the first and second collimator lenses being fixed and supported at positions adjusted within the adjustment range; and  
           [0023]    a rotary mirror for running the first and second laser beams, which are brought close to each other by exiting the laser unit, together.  
           [0024]    Further objects of the present invention will be apparent from the description below.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    [0025]FIG. 1 is a perspective view showing the entire structure of a scanning optical device according to Embodiment 1 of the present invention;  
         [0026]    [0026]FIG. 2 is a sectional view of a laser holder portion according to Embodiment 1 of the present invention;  
         [0027]    [0027]FIG. 3 is a schematic sectional view of an image forming portion which is composed of a scanning optical device according to an embodiment of the present invention, photosensitive drums, and others;  
         [0028]    [0028]FIGS. 4A and 4B are partial sectional views showing light paths of a polygon mirror portion according to an embodiment of the present invention;  
         [0029]    [0029]FIG. 5 is a partial perspective view showing how a laser holder portion according to an embodiment of the present invention is attached;  
         [0030]    [0030]FIGS. 6A and 6B are explanatory diagrams illustrating adjustment of collimator lenses according to an embodiment of the present invention;  
         [0031]    [0031]FIG. 7 is a perspective view showing the entire structure of a scanning optical device according to Embodiment 2 of the present invention;  
         [0032]    [0032]FIG. 8 is a sectional view of a laser holder portion according to Embodiment 2 of the present invention;  
         [0033]    [0033]FIG. 9 is a schematic sectional view of an image forming apparatus;  
         [0034]    [0034]FIG. 10 is a schematic sectional view of an image forming portion of the image forming apparatus shown in FIG. 9, the image forming portion being composed of a scanning optical device, photosensitive drums, and others;  
         [0035]    [0035]FIG. 11 is a schematic sectional view of an image forming portion composed of a different scanning optical device, photosensitive drums, and others;  
         [0036]    [0036]FIG. 12 is an explanatory diagram of a laser light flux directed at a photosensitive drum;  
         [0037]    [0037]FIG. 13 is a sectional view in a sub-scanning direction of the image forming apparatus of FIG. 9 showing arrangement of a laser holder portion; and  
         [0038]    [0038]FIG. 14 is a top view showing another example of arranging the laser holder portion. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0039]    Hereinafter, the present invention is described with reference to embodiments shown in the accompanying drawings.  
         [0040]    (Embodiment 1)  
         [0041]    A description is given below with reference to FIG. 1 through FIGS. 6A and 6B on a scanning optical device according to Embodiment 1 of the present invention. A scanning optical device of the present invention which is described in this embodiment and the subsequent embodiment is mounted to an image forming apparatus as shown in FIG. 9 and irradiates a photosensitive drum with laser light in accordance with image information, so that an electrostatic latent image is formed. The rest of the image forming apparatus structure is as described in Related Background Art, and accordingly any description thereof is omitted here. A scanning optical device of the present invention can be applied to an electrophotographic copying machine and other types of image forming apparatuses.  
         [0042]    [0042]FIG. 1 is a perspective view showing the entire structure of a scanning optical device  100  according to Embodiment 1. FIG. 2 is a sectional view of a laser holder portion. FIG. 3 is a schematic sectional view of an image forming portion which is composed of the scanning optical device, photosensitive drums, and others. FIGS. 4A and 4B are partial sectional views showing light paths of a polygon mirror portion. FIG. 5 is a partial perspective view showing how the laser holder portion is attached. FIGS. 6A and 6B are explanatory diagrams illustrating adjustment of collimator lenses.  
         [0043]    In Embodiment 1, the scanning optical device  100  is placed under photosensitive drums  911  to  914  and others. The scanning optical device  100  used in this embodiment has one polygon mirror  10 , and two fluxes of laser light enter the polygon mirror  10  on each side to expose the photosensitive drums  911  to  914  to irradiation light beams E 1  to E 4 . The thin polygon mirror  10  is employed to make the scanning optical device  100  an oblique incident optical system that is small in size. After exiting the polygon mirror  10 , fluxes of laser light have to be split and take upper and lower light paths. Therefore, their incident angles are equal to each other in magnitude (θ) but opposite in direction with respect to a fundamental plane, which is a plane defined by the normal line of the polygon mirror face and the polygon rotation direction as shown in FIG. 4A (the fundamental plane corresponds to the X-Y plane in FIG. 12). In general, the relative angle between the fundamental plane and deflection scanning light is preferably 3° or less from the viewpoint of image performance and this embodiment follows this notion. In this embodiment, oblique incident angles of laser light fluxes are equal in magnitude to each other and opposite in direction with respect to the fundamental plane in order to match optical characteristics. According to the optical arrangement of this embodiment, the polygon mirror  10  is placed at the center and optical paths leading to the photosensitive drums  911  to  914  are symmetrical with respect to the rotation center of the polygon mirror  10 . Therefore, the scanning group of the irradiation light beams E 1  and E 2  alone is described here.  
         [0044]    In FIGS. 1 and 2, Reference Symbol  1  denotes a laser holder (lens barrel member). The laser holder  1  has semiconductor lasers (single beam lasers)  2  and  3 , each of which is a light source having one light emission point in one housing (package), held by press fit in lens barrel holding portions (lens barrel portions)  1   a  and  1   b . Reference Symbol  4  denotes an electric circuit substrate, which is electrically connected to the semiconductor lasers  2  and  3  and has a laser driver circuit. Reference Symbol  5  denotes a beam detecting sensor (BD sensor) as synchronization detecting means provided on the electric circuit substrate  4 . The BD sensor  5  detects a light flux reflected by the polygon mirror  10  which will be described later and outputs a synchronization signal in a main scanning direction to thereby adjust scanning start timing at an end of an image. The lens barrel holding portions  1   a  and  1   b  are provided so as to slant the optical axes such that the light paths of the semiconductor lasers  2  and  3  intersect each other in a sub-scanning direction at a given angle θ. The contours of the lens barrels are partially integrated with each other. This makes it possible to hold the semiconductor lasers  2  and  3  across a short distance from each other. Diaphragm portions  1   c  and  1   d  are provided at the tips of the lens barrel holding portions  1   a  and  1   b  for the semiconductor lasers  2  and  3 , respectively, and shape light fluxes emitted from the semiconductor lasers  2  and  3  into desired, optimum, beam shapes. Provided at the tips of the lens barrel holding portions  1   a  and  1   b  beyond the diaphragm portions  1   c  and  1   d  in the main scanning direction are bonding portions  1   e  and  1   f  of collimator lenses  6  and  7 . The collimator lenses  6  and  7  convert light fluxes that have passed through the diaphragm portions  1   c  and  1   d  into approximately parallel pencils of light. As shown in FIG. 6A, the collimator lens  6  is securely held at three points by adjustment chucking portions  51   a ,  51   b , and  51   c . In this state, the irradiation position and pint of the collimator lens  6  are adjusted in the directions of three axes, X, Y, and Z, while optical characteristics of laser light are being detected. Once the position of the collimator lens  6  is determined, the collimator lens  6  is bonded and fixed to the bonding portion le by irradiating a UV-curable adhesive with an ultraviolet ray. Similarly, the collimator lens  7  is adjusted as shown in FIG. 6B by rotating the laser holder  1  by 180° and, once the position is set, is bonded and fixed to the bonding portion  1   f.    
         [0045]    The bonding portions  1   e  and  1   f  of the collimator lenses  6  and  7  are thus provided in the main scanning direction, and this makes it possible to adjust and bond the collimator lenses  6  and  7  in three axial directions relative to the laser holder  1 , which has lens barrels unified while being held close to each other.  
         [0046]    Reference Symbol  40  denotes an optical case for housing optical parts of the scanning optical device  100 . As shown in FIG. 5, a side wall of the optical case  40  has in the sub-scanning direction a fitting hole  40   a  and an elongated hole  40   b  for positioning the laser holder  1 . The laser holder  1  is attached to the optical case  40  by fitting portions  1   m  and  1   n , which are provided in the outer peripheries of the lens barrel holding portions  1   a  and  1   b , into the holes  40   a  and  40   b , respectively. The semiconductor lasers  2  and  3  can be positioned with high precision relative to the optical parts housed in the optical case  40  since the laser holder  1  is attached to the optical case  40  by fitting in the case&#39;s wall the fitting portions lm and in provided in the outer peripheries of the lens barrel holding portions  1   a  and  1   b , which hold the semiconductor lasers  2  and  3  and form light paths.  
         [0047]    Reference Symbol  8  denotes a cylindrical lens having a given refractive power only in the sub-scanning direction. The cylindrical lens  8  has unitarily formed lens portions  8   a  and  8   b  to handle light fluxes emitted from the semiconductor lasers  2  and  3 , respectively. Reference Symbol  9  represents a BD lens for imaging, on a light receiving face of the BD sensor  5 , light fluxes that have been reflected by the polygon mirror  10 . Placed right in front of the BD sensor  5  is a BD slit portion  1   g , which is opened in the laser holder  1 . The BD slit portion  1   g  is an opening that is narrow in the main scanning direction and long in the sub-scanning direction, so that light fluxes in the main scanning direction can be detected with high precision by restricting a light flux received by the BD sensor  5  to the main scanning direction. In this embodiment, the BD sensor  5  and the BD slit portion  1   g  are positioned to correspond to the semiconductor laser  2  and there is no BD sensor that is associated with the semiconductor laser  3 . This is because the semiconductor lasers  2  and  3  are provided in the single laser holder  1  in the sub-scanning direction and accordingly are identical to each other in terms of scanning start timing at an end of an image.  
         [0048]    A motor (not shown) of the polygon mirror  10  is rotated at a constant speed to thereby run light fluxes which have been emitted from the semiconductor lasers for deflection scanning. Reference Symbol  21  denotes a first imaging lens. The first imaging lens  21  is an fθ lens and, similar to second imaging lenses  22  and  23 , runs laser light at a constant speed and causes laser beams to form spots on the photosensitive drums  911  and  912 . A cylinder lens is used for the first imaging lens  21  since light fluxes emitted from the semiconductor lasers  2  and  3  enter the first imaging lens  21  at angles different from each other. The second imaging lens  22  placed to handle a light flux from the semiconductor laser  2  and the second imaging lens  23  placed to handle a light flux from the semiconductor laser  3  are used to image laser light in the sub-scanning direction. Denoted by  24  to  26  are turn-back mirrors for reflecting light fluxes in given directions. The mirror  24  is the last turn-back mirror placed to handle a light flux from the semiconductor laser  2 . The mirror  25  is a turn-back mirror for splitting and is placed to handle a light flux from the semiconductor laser  3 . The mirror  26  is the last turn-back mirror placed to handle a light flux from the semiconductor laser  3 . As shown in FIGS. 4A and 4B, light fluxes may be reflected at the same point on the surface of the polygon mirror  10  or may be at different points on the mirror surface which are offset from each other along the height of the polygon mirror  10 . When reflection points on the polygon mirror  10  are offset from each other as shown in FIG. 4B, the position of the splitting turn-back mirror  25  on the near side can be moved nearer. Here, the polygon mirror  10  corresponds to scanning means, and scanning optical means is composed of the polygon mirror  10 , the first imaging lens  21 , the second imaging lenses  22  and  23 , and the turn-back mirrors  24  to  26 .  
         [0049]    The photosensitive drum  911  is an electric conductor with a photosensitive layer formed thereon by application. Reference Symbol  921  denotes a charger for charging the photosensitive drum to a given electric potential. Reference Symbol  931  denotes a developing unit for forming a toner image on an electrostatic latent image. Denoted by  941  is a cleaner for cleaning toner that remains on the photosensitive drum. Other photosensitive drums  912  to  914  are similarly structured.  
         [0050]    Described is the part where light fluxes emitted from the semiconductor lasers  2  and  3  are run as the irradiation light beams E 1  and E 2  on the photosensitive drums  911  and  912 .  
         [0051]    Light fluxes emitted from the semiconductor lasers  2  and  3  are limited in size in flux section by the diaphragm portions  1   c  and  1   d  of the laser holder  1 , and then converted into approximately parallel pencils of light by the collimator lenses  6  and  7  before entering the lens portions  8   a  and  8   b  of the cylindrical lens  8 . Of the light fluxes incident on the cylindrical lens  8 , a component within the main scanning section is transmitted as it is whereas a component within the sub-scanning section is converged to form an approximately linear image on the same face of the polygon mirror  10 . At this point, the light enters obliquely at the angle θ in the sub-scanning direction. Then, the polygon mirror  10  is rotated for deflection scanning, causing the laser beams to exit in the sub-scanning direction at the angle θ. Of the two light fluxes that have exited the polygon mirror  10 , the light flux emitted from the semiconductor laser  2  is received by the BD sensor  5  through the BD slit portion  1   g  provided in the laser holder  1 . The BD slit portion  1   g  restricts the light flux to the main scanning direction, thereby making it possible to detect a light flux in the main scanning direction with high precision. The BD sensor  5  detects the light flux emitted from the semiconductor laser  2  and outputs a synchronization signal to adjust the scanning start timings by the semiconductor lasers  2  and  3  at an end of an image. The scanning start timing by the semiconductor laser  3  at an end of an image can be identical to the scanning start timing by the semiconductor laser  2  at an end of an image since the semiconductor lasers  2  and  3  are provided in the single laser holder  1  in the sub-scanning direction. In this way, light emission by the semiconductor laser  3  is timed with light emission by the semiconductor laser  2 . Light fluxes emitted from the semiconductor lasers  2  and  3  in sync with each other are transmitted through the first imaging lens  21 . After that, the light flux emitted from the semiconductor laser  2  is transmitted through the second imaging lens  22  and is reflected by the last turn-back mirror  24  to be run and imaged on the photosensitive drum  911  as the irradiation light beam E 1 . On the other hand, the light flux emitted from the semiconductor laser  3  is reflected downward by the splitting turn-back mirror  25 , transmitted through the second imaging lens  23 , and reflected by the last turn-back mirror  26  to be run and imaged on the photosensitive drum  912  as the irradiation light beam E 2 .  
         [0052]    As described above, the semiconductor lasers  2  and  3  are held by press fit in the lens barrel holding portions  1   a  and  1   b  of the laser holder  1  which are integrated with each other with the optical axes slanted, thereby making it possible to hold the semiconductor lasers  2  and  3  across a short distance from each other. At the same time, the collimator lenses  6  and  7  are each adjusted in three axial directions before bonded and fixed to thereby make it possible to shorten the light path length of an incident optical system that stretches to the polygon mirror  10  while optical characteristics of the semiconductor lasers  2  and  3  meet requirements.  
         [0053]    In addition, the BD sensor  5 , which is placed at a position corresponding to the semiconductor laser- 2 , and the BD slit portion  1   g , which is opened in the laser holder  1  holding the semiconductor lasers  2  and  3  by press fit in the lens barrel holding portions  1   a  and  1   b , make it possible to keep the positional relation without causing dimensional tolerance of the semiconductor lasers  2  and  3  and the BD slit portion  1   g  to influence much. This enables the BD sensor  5  to detect a light flux with high precision. Furthermore, the scanning optical device is capable of matching the scanning start timing by the semiconductor laser  3  at an end of an image to the scanning start timing by the semiconductor laser  2  at an end of an image without providing a BD sensor for the semiconductor laser  3 .  
         [0054]    Moreover, the semiconductor lasers  2  and  3  can be positioned with high precision relative to the optical parts housed in the optical case  40  since the laser holder  1  is attached to the optical case  40  by fitting the fitting portions lm and in provided in the outer peripheries of the lens barrel holding portions  1   a  and  1   b , which hold the semiconductor lasers  2  and  3  and form light paths, in the fitting hole  40   a  and the elongated hole  40   b , which are opened in the side wall of the optical case  40  in the sub-scanning direction.  
         [0055]    Moreover, with the bonding portions  1   e  and  1   f  of the collimator lenses  6  and  7  placed in the main scanning direction, the adjustment chucking portions  51   a ,  51   b , and  51   c  can solidly hold the collimator lenses at three points without interfering with the bonding portions  1   e  and  1   f  and with the collimator lenses that have previously been bonded and fixed. This makes it possible to adjust and bond the collimator lenses  6  and  7  in three axial directions relative to the laser holder  1 , which has lens barrels unified while being held close to each other. Moreover, since the collimator lenses  6  and  7  are adjusted and bonded by rotating the laser holder  1  by 180°, the same adjusting and bonding device can be used even when the light paths are slanted in the sub-scanning direction at the angle θ. Unnecessary investment in equipment thus can be avoided.  
         [0056]    (Embodiment 2)  
         [0057]    A description is given below with reference to FIGS. 7 and 8 on a scanning optical device according to Embodiment 2 of the present invention. FIG. 7 is a perspective view showing the entire structure of a scanning optical device  200  according to Embodiment 2. FIG. 8 is a sectional view of a laser holder portion.  
         [0058]    In Embodiment 2, the same components as those of Embodiment-1 are denoted by the same reference symbols and any description thereof will be omitted.  
         [0059]    In FIGS. 7 and 8, Reference Symbol  11  denotes a laser holder (lens barrel member), which is identical to the laser holder  1 . The laser holder  11  has semiconductor lasers  12  and  13 , each of which is a light source having one light emission point in one housing (package), held by press fit in lens barrel holding portions (lens barrel portions)  11   a  and  11   b . Reference Symbol  14  denotes an electric circuit substrate, which is electrically connected to the semiconductor lasers  2 ,  3 ,  12 , and  13  and has a laser driver circuit. Reference Symbols  5  and  15  denote BD sensors as synchronization detecting means provided on the electric circuit substrate  4 . The BD sensors  5  and  15  each detect a light flux reflected by the polygon mirror  10  and outputs a synchronization signal in a main scanning direction to thereby adjust scanning start timing at an end of an image. The lens barrel holding portions  11   a  and  11   b  are provided so as to slant the optical axes such that the light paths of the semiconductor lasers  12  and  13  intersect each other in a sub-scanning direction at a given angle θ. The contours of the lens barrels are partially integrated with each other. This makes it possible to hold the semiconductor lasers  12  and  13  across a short distance from each other. Diaphragm portions  11   c  and  11   d  are provided at the tips of the lens barrel holding portions  11   a  and  11   b  for the semiconductor lasers  12  and  13 , respectively, and shape light fluxes emitted from the semiconductor lasers  12  and  13  into desired, optimum, beam shapes. Provided at the tips of the lens barrel holding portions  11   a  and  11   b  beyond the diaphragm portions  11   c  and  11   d  in the main scanning direction are bonding portions  11   e  and  11   f  of collimator lenses  16  and  17 . The collimator lenses  16  and  17  convert light fluxes that have passed through the diaphragm portions  11   c  and  11   d  into approximately parallel pencils of light. The collimator lenses  16  and  17  are adjusted in irradiation position and in pint in the same manner as in the case of the collimator lenses  6  and  7 , and are bonded and fixed to the bonding portions  11   e  and  11   f.    
         [0060]    To place a BD slit portion  11   g  of the laser holder  11  opposite to the laser holder  1  and to place the BD slit portion  1   g  of the laser holder  1  opposite to the laser holder  11 , the laser holder  11  is placed on the same face of the optical case  40  such that, with the laser holder  1  rotated by 180°, the semiconductor laser  12  sits next to the semiconductor laser  3  and the semiconductor laser  13  adjoins to the semiconductor laser  2 . As a result, the distance in the main scanning direction between the semiconductor lasers  2  and  3  and the semiconductor lasers  12  and  13  can be set short to suit a reflection face of the polygon mirror  10  which reflects light toward a scanning optical system that is opposed to the polygon mirror  10 . In this way, the polygon mirror  10  makes it possible to scan a wide angle also when the semiconductor lasers  2 ,  3 ,  12 , and  13  are placed in the same direction, and accordingly can shorten the light path length of the scanning optical system. In addition, since there are no optical parts for light incidence on the wall opposite to the laser holders  1  and  11  across the polygon mirror  10 , the scanning optical device can be reduced in size by bringing this wall close to the polygon mirror  10 .  
         [0061]    The laser holder  11  is positioned with respect to the optical case  40  in a manner similar to positioning of the laser holder  1 . Accordingly, the semiconductor lasers  12  and  13  can be positioned with high precision relative to the optical parts housed in the optical case  40 .  
         [0062]    The semiconductor lasers  2  and  3  and the semiconductor lasers  12  and  13  are held by press fit in the lens barrel holding portions  1   a  and  1   b  of the laser holder  1  and in the lens barrel holding portions  11   a  and  11   b  of the laser holder  11  such that a common signal is shared within them. This enables the electric circuit substrate  14  to form a signal pattern with the common signal set at the center, and thus makes it easy to design a signal pattern. In addition, the fact that the laser holder  11  is completely identical to the laser holder  1  in every aspect including holding identical parts, facilitates assembly and management in manufacture of the laser holders and has an effect of lowering cost. Furthermore, the number of parts and cost can be reduced since the semiconductor lasers  2 ,  3 ,  12 , and  13  and the BD sensors  5  and  15  are placed on the same electric circuit substrate  14 . Moreover, the electric circuit substrate  14  placed outside of the optical case  40  can be reduced in size and the scanning optical device can accordingly be made smaller in size.  
         [0063]    Reference Symbol  18  denotes a cylindrical lens having a given refractive power only in the sub-scanning direction. The cylindrical lens  18  has unitarily formed lens portions  18   a  and  18   b  to handle light fluxes emitted from the semiconductor lasers  12  and  13 , respectively. Reference Symbol  19  represents a BD lens for imaging, on a light receiving face of the BD sensor  15 , light fluxes that have been reflected again by a reflector mirror  20  after being reflected by the polygon mirror  10 . Placed right in front of the BD sensor  15  is a BD slit portion  11   g , which is opened in the laser holder  11 . The BD slit portion  11   g  is an opening that is narrow in the main scanning direction and long in the sub-scanning direction, so that light fluxes in the main scanning direction can be detected with high precision by restricting a light flux received by the BD sensor  15  to the main scanning direction. In this embodiment, the BD sensor  15  and the BD slit portion  11   g  are positioned to correspond to the semiconductor laser  12  and there is no BD sensor that is associated with the semiconductor laser  13 . This is because the semiconductor lasers  12  and  13  are provided in the single laser holder  11  in the sub-scanning direction and accordingly are identical to each other in terms of scanning start timing at an end of an image.  
         [0064]    Light fluxes emitted from the semiconductor lasers  12  and  13  are run as the irradiation light beams E 3  and E 4  on the photosensitive drums  913  and  914 . The beams E 3  and E 4  after run for deflection scanning by the polygon mirror  10  behave as the irradiation light beams E 1  and E 2  do, until imaged on the photosensitive drums  91 . Therefore, instead of repeating the description on this part, the part where fluxes of laser light are emitted from the semiconductor lasers and then received by the BD sensor  15  is described.  
         [0065]    Light fluxes emitted from the semiconductor lasers  12  and  13  are limited in size in cross sectional area of the flux by the diaphragm portions  11   c  and  11   d  of the laser holder  11 , and then converted into approximately parallel pencils of light by the collimator lenses  16  and  17  before entering the lens portions  18   a  and  18   b  of the cylindrical lens  18 . Of the light fluxes incident on the cylindrical lens  18 , a component within the main scanning cross-section is transmitted as it is whereas a component within the sub-scanning cross-section is converged to form an approximately linear image on the same face of the polygon mirror  10 . At this point, the light enters obliquely at the angle θ in the sub-scanning direction. Then, the polygon mirror  10  is rotated for deflection scanning, causing the laser beams to exit in the sub-scanning-direction at the angle θ. Of the two light fluxes that have exited the polygon mirror  10 , the light flux emitted from the semiconductor laser  12  and reflected by the polygon mirror  10  is reflected again by the reflector mirror  20  and is then received by the BD sensor  15  through the BD slit portion  11   g  provided in the laser holder  11 . The BD slit portion  11   g  restricts the light flux to the main scanning direction, thereby making it possible to detect a light flux in the main scanning direction with high precision. The BD sensor  15  detects the light flux emitted from the semiconductor laser  12  and outputs a synchronization signal to adjust the scanning start timings by the semiconductor lasers  12  and  13  at an end of an image. The scanning start timing by the semiconductor laser  13  at an end of an image can be identical to the scanning start timing by the semiconductor laser  12  at an end of an image since the semiconductor lasers  12  and  13  are provided in the single laser holder  11  in the sub-scanning direction.  
         [0066]    As described above, the BD slit portion  11   g  of the laser holder  11  is placed opposite to the laser holder  1  and the BD slit portion  1   g  of the laser holder  1  is placed opposite to the laser holder  11 , and the laser holder  11  and the laser holder  1  are placed on the same face of the optical case  40 . As a result, the distance in the main scanning direction between the semiconductor lasers  2  and  3  and the semiconductor lasers  12  and  13  can be set short to suit the reflection face of the polygon mirror  10  which reflects light toward the scanning optical system that is opposed to the polygon mirror  10 . Therefore, the polygon mirror  10  makes it possible to scan a wide angle also when the semiconductor lasers  2 ,  3 ,  12 , and  13  are placed in the same direction, and accordingly can shorten the light path length of the scanning optical system. In addition, since there are no optical parts for light incidence on the wall opposite to the laser holders  1  and  11  across the polygon mirror  10 , the scanning optical device  200  can be reduced in size by bringing this wall close to the polygon mirror  10 .  
         [0067]    Furthermore, the number of parts and cost can be reduced since the semiconductor lasers  2 ,  3 ,  12 , and  13  and the BD sensors  5  and  15  are placed on the same electric circuit substrate  14 . Moreover, the electric circuit substrate  14  placed outside of the optical case  40  can be reduced in size and the scanning optical device can accordingly be made smaller in size.  
         [0068]    The scanning optical device  100  of Embodiment 1 employs a method in which two fluxes of laser light enter one polygon mirror on each side to expose four photosensitive drums to light. However, this is not to limit the present invention and the scanning optical device  100  can also employ other methods including: a method in which four fluxes of laser light enter one side of a polygon mirror to expose four photosensitive drums to light; and a method in which two scanning optical devices are used, each of which has two fluxes of laser light incident on one side of a polygon mirror, to expose four photosensitive drums to light. Every semiconductor laser employed in the scanning optical devices  100  and  200  of Embodiments 1 and 2 is the type that has one light emission point in one housing. Instead, the scanning optical devices  100  and  200  may employ a semiconductor laser that has plural light emission points in one housing and, in this case, the number of scanning lines for operating photosensitive drums increases proportionately, which makes the scanning devices suitable for even faster writing. In the laser holder  1 , the collimator lenses  6  and  7  are adjusted in irradiation point and in pint direction and bonded after the semiconductor lasers  2  and  3  are held by press fit. In an alternative case, the collimator lenses  6  and  7  are attached first by fitting them in fitting portions that are provided in the laser holder  1 , and then the semiconductor lasers  2  and  3  are adjusted in irradiation point and in pint direction and bonded while optical characteristics of laser light are being detected. The BD sensor  15  is placed on the electric circuit substrate  14  in the scanning optical device  200  of Embodiment 2. However, this is not to limit the present invention, and the BD sensor  15  may be placed somewhere else in the optical case  40  where the BD sensor  15  can receive a light flux that has been reflected by the polygon mirror  10  instead of the reflector mirror  20 .  
         [0069]    Embodiments of the present invention have been described above, but note that the present invention is not limited thereto and that any modification is possible as long as it is within the technical concept.