Abstract:
An optical scanning device includes a plurality of optical scanning, units each including a light source emitting a light flux, a scanning input optical system directing the light flux emitted from the light source to a deflector. The deflector deflects the light flux so as to cause the light flux to scan a surface to be scanned. A scanning and imaging optical system condenses the light flux deflected by the deflector so as to form a beam spot thereof on the surface to be scanned. The optical scanning device scans the surface to be scanned continuously through coordinated movements of the plurality of optical scanning units, and the respective scanning and imaging optical systems of adjacent at least two of the plurality of optical scanning units have one lens in common.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to an optical scanning device employed in an optical writing unit of a laser printer, a laser copier or the like, and deflecting a laser light emitted from a laser light source and thus scanning a surface to be scanned while forming a beam spot on the surface to be scanned, and, in particular, an optical scanning device employing a plurality of optical scanning units each including a light source, a first optical system, a deflector and a second optical system, and continuously scanning one surface to be scanned through coordinated movements of the respective optical scanning units. 
     2. Description of the Related Art 
     Recently, high density writing, high-speed writing and miniaturization are demanded for laser printers, laser copiers and so forth. Accordingly, for optical scanning devices which occupy a large space from a laser light source to a surface to be scanned, it is demanded to deal with high density writing, high-speed writing and miniaturization. 
     However, an optical scanning device, such as for example including a single laser light source which emits a light flux and a single deflector which deflects the light flux a surface to be scanned is scanned (raster-scanned) by means of a beam spot, and the optical scanning device needs to deflect the light flux extending from a deflector to the surface to be scanned through a wide range of the surface to be scanned. Because there is a limit to the angle by which the light flux extending from the deflector to the surface to be scanned is deflected for scanning, long distance is needed between the deflect and surface to be scanned. Thus, a large is needed in the optical scanning device. Accordingly, it is difficult to miniaturize the optical scanning device. 
     Further, it is necessary to reduce the diameter of a beam spot to deal with high-density writing. For this purpose, it is necessary to design the optical system subsequent to the deflector to have a small focal length. Also from this point, it is difficult to achieve high-density writing using a single laser light source and a single deflector. 
     An optical scanning device in the related art will now be described with reference to a figure. 
     FIG. 1 shows a configuration of the optical scanning device in the related art. 
     As shown in the figure, the optical scanning device  100  includes a light source  1  such as for example a semiconductor laser which emits a divergent laser light flux, a coupling lens  2  which is for example a collimator lens to transform the divergent light flux emitted from the light source  1  into an approximately parallel light flux, a stop  3  which reduces the diameter of the light flux made to be approximately parallel by the coupling lens  2  and cuts an unnecessary flux portion, a line-image forming optical system  4  which, as a cylindrical lens for example, has a refracting power in sub-scanning direction, and a mirror  5  which bends the light path of the light flux exiting from the line-image forming optical system  4  by reflecting it and thereby directs it to a deflection reflective surface  6   a  of a deflector  6 . The deflector  6  deflects in a main scanning direction the light flux so as to become like a line, long in the main scanning direction by the line-image forming optical system  4  and incident on the deflection reflective surface  6   a  by rotating the deflector at a uniform angular velocity. An fθ lens  7  corrects the light path of the light flux deflected by the deflector  6  for linearly scanning a photosensitive body  9 . A long-dimensional lens  8  corrects the surface inclination of the light flux occurring from the deflection reflective surface  6   a . For example, when photosensitive body  9 , which is a surface to be scanned, is scanned by a beam spot formed of the deflected light flux, a synchronization detecting sensor  12  is used for establishing synchronization between the light flux emitted from the light source  1  and the rotation angle of the deflector  6  based on the incident light flux. 
     The light source  1 , coupling lens  2 , stop  3 , line-image forming optical system  4  and mirror  5  constitute a scanning input optical system  15 . Further, the fθ lens  7  and long-dimensional lens  8  constitute a scanning and imaging optical system  18 . 
     The optical scanning device  100  shown in FIG. 1 operates as follows: 
     A divergent light flux  20  emitted from the light source  1  is transformed into an approximately parallel light flux  20   a  by the coupling lens  2 , has an unnecessary light flux portion thereof cut by the stop  3 , is condensed in sub-scanning direction by the line-image forming optical system  4 , is reflected by the mirror  5 , and, thus, is incident on the deflection reflective surface  6   a  of the deflector  6  becoming like a line long in.main scanning direction. 
     The deflector  6  rotates at the uniform angular velocity in direction of arrow  30 , and the entrance angle and exit angle of the incident light flux  20   a  with respect to the deflection reflective surface  6   a  change with the rotation of the deflector  6 . Accordingly, the incident light flux  20   a  exits therefrom as the light flux  20   b → 20   c → 20   d  in the stated order as a result of being deflected by the deflection reflective surface  6   a  in main scanning direction according to the rotation of the deflector  6 . 
     Each light flux  20   b ,  20   c ,  20   d  exiting from the deflector  6  has the light path thereof corrected by the fθ lens  7  so as to scan the photosensitive body  9  linearly in direction indicated by arrow  50 , and has the surface inclination of the deflection reflective surface  6   a  corrected by the long-dimensional lens  8 . 
     Each light flux  20   b ,  20   c ,  20   d  corrected by the fθ lens  7  and the long-dimensional lens  8  forms a beam spot on the-photosensitive body  9 . 
     The synchronization detecting sensor  12  detects, for example, the light flux  20   e  exiting from the deflector  6 , compares the timing of the thus detected light flux with the timing of the predetermined light flux emitted from the light source  1 , and eliminates the difference therebetween. Thereby, synchronizes the rotational angle of the deflector  6  with the light flux emitted from the light source  1 . 
     In order to miniaturize an optical scanning device such as that  100  shown in FIG. 1, for example, as disclosed in Japanese Laid-Open Patent Application No. 61-11720, two deflectors are used, thereby a scanning length to be scanned by each deflector is reduced, and miniaturization is achieved. 
     However, in such a method, as a result of two deflectors being employed, it is important to secure continuity between light fluxes (scanning laser beams) exiting from the respective deflectors, precisely. However, Japanese Laid-Open Patent Application No. 61-11720 does not clearly disclose how to secure continuity between the light fluxes precisely. 
     On the other hand, Japanese Laid-Open Patent Application No. 10-68899, for example, discloses a method in that two sets of deflectors and optical components for the respective deflectors are disposed stepwise, and, also, light fluxes from the respective deflators are made to be continuos by a beam splitter, and, thereby, continuity between the light fluxes is precisely secured. 
     However, in such a method, a beam splitter is needed as an extra component, and, also, a high level of position adjustment technology and so forth are needed for securing continuity between light fluxes from respective sets of deflectors and optical components disposed stepwise through the beam splitter. 
     Accordingly, it is difficult to actually manufacture it. 
     Further, in order to further miniaturize an optical scanning device, it is necessary to secure continuity between light fluxes from respective ones of more than two sets of deflectors and optical components through a beam splitter. However, it is further difficult to actually manufacture it because a further high level of position adjustment technology is needed. 
     SUMMARY OF THE INVENTION 
     The present invention has been devised in order to solve the above-described problems, and, an object of the present invention is to provide an optical scanning device for which miniaturization is easy, and, also, adjustment and so forth needed manufacturing it are easy. 
     An optical scanning device according to the present invention comprises: 
     a plurality of optical scanning units each comprising: 
     a light source emitting a light flux; 
     a scanning input optical system directing the light flux emitted from the light flux to a deflector; 
     the deflector deflecting the light flux for causing the light flux to scan a surface to be scanned; and 
     a scanning and imaging optical system condensing the light flux deflected by the deflector so as to form a beam spot thereof on the surface to be scanned, 
     wherein: 
     the optical scanning device scans the surface to be scanned continuously through coordinated movements of the plurality of optical scanning units; and 
     the respective scanning and imaging optical systems of adjacent at least two of the plurality of optical scanning units have one lens in common. 
     In this configuration, the optical scanning device employs the plurality of optical scanning units. Accordingly, it is possible to miniaturize the device. Further, because the lens is provided which the respective optical scanning units have in common, it is possible to provide the optical scanning device for which adjustment and so forth in manufacturing is easy. 
     The lens which the respective scanning and imaging optical systems have in common may comprise a plastic lens. Thereby, it is easy to form aspherical surfaces and/or free curved surfaces on the lens, and it is possible to provide inexpensive optical scanning device. 
     An optical scanning device according to another aspect of the present invention comprising: 
     a plurality of optical scanning units each comprises: 
     a light source emitting a light flux; 
     a scanning input optical system directing the light flux emitted from the light flux to a deflector; 
     the deflector deflecting the light flux for causing the light flux to scan a surface to be scanned; and 
     a scanning and imaging optical system condensing the light flux deflected by the deflector so as to form a beam spot thereof on the surface to be scanned, 
     wherein: 
     the optical scanning device scans the surface to be scanned continuously through coordinated movements of the plurality of optical scanning units; and 
     in order to reduce a difference between a diameter of a beam spot at a scanning end position of an optical scanning unit of the plurality of optical scanning units scanning the surface to be scanned and a diameter of a beam spot at a scanning beginning position of another optical scanning unit of the plurality of optical scanning units scanning the surface to be scanned subsequently, the respective scanning and imaging optical systems of the plurality of optical scanning units satisfy the following expression (1): 
     
       
         ( Bn (−)−&lt; B &gt;)×( Bn +1(+)−&lt; B &gt;)≧0  (1) 
       
     
     where: 
     m: the total number of the plurality of optical scanning units; 
     n: any integer in the range of 1≦n≦m−1; 
     Bn(−): a diameter of beam spot at the scanning end position of an n-th optical scanning unit of the plurality of optical scanning units; 
     Bn+1(+): a diameter of beam spot at the scanning beginning position of an (n+1)-th optical scanning unit of the plurality of optical scanning units; and 
     &lt;B&gt;: an average of diameters of beam-spots of the plurality of optical scanning units scanning the surface to be scanned. 
     In this configuration, the optical characteristics at joined portions of the respective optical scanning units are approximated by one another. Thereby, the optical scanning device is not likely to generate lines of density difference and/or the like at the joined portions only as a result of the microprocessor or the like precisely controlling switching of the outputs of the respective optical scanning units. 
     The scanning and imaging optical system may be approximately telecentric in main scanning direction. Thereby, shifts otherwise occurring at the joins with change in position on the photosensitive body do not occur. 
     An optical scanning device according to another aspect of the present invention comprises: 
     a plurality of optical scanning units each comprising: 
     a light source emitting a light flux; 
     a scanning input optical system directing the light flux emitted from the light flux to a deflector; 
     the deflector deflecting the light flux for causing the light flux to scan a surface to be scanned; and 
     a scanning and imaging optical system condensing the light flux deflected by the deflector so as to form a beam spot thereof on the surface to be scanned, 
     wherein: 
     the optical scanning device scans the surface to be scanned continuously through coordinated movements of the plurality of optical scanning units; and 
     a synchronization detecting light path for at least one optical scanning unit of the plurality of optical scanning units is provided between light paths of the respective scanning and imaging optical systems of adjacent two optical scanning units of the plurality of optical scanning units, and, also, a light directing part directing a light flux of the synchronization detecting light path to the outside of the light paths of the respective scanning and imaging optical systems is provided there. 
     In this configuration, even when more than two deflectors are used, it is possible to detect synchronization of the deflectors precisely. 
     When the synchronization detecting light path comprises a plurality of synchronization detecting light paths, the single light directing part may direct the plurality of synchronization detecting light paths. In this configuration, the synchronization detecting part may be used in common. Thereby, it is possible to save the space, to improve the space utilization efficiency, to reduce the influence of variation in characteristics of a plurality of synchronization detecting sensors, and to reduce the costs by reducing the number of synchronization detecting sensors. 
     An optical scanning device according to another aspect of the present invention comprises: 
     a plurality of optical scanning units each comprising: 
     a light source emitting a light flux; 
     a scanning input optical system directing the light flux emitted from the light flux to a deflector; 
     the deflector deflecting the light flux for causing the light flux to scan a surface to be scanned; and 
     a scanning and imaging optical system condensing the light flux deflected by the deflector so as to form a beam spot thereof on the surface to be scanned, 
     wherein: 
     the optical scanning device scans the surface to be scanned continuously through coordinated movements of the plurality of optical scanning units; and 
     a synchronization detecting light path for at least one optical scanning unit of the plurality of optical scanning units is provided between light paths of the respective scanning and imaging optical systems of adjacent two optical scanning units of the plurality of optical scanning units, and, also, a synchronization detecting part detecting a light flux of the synchronization detecting light path is provided there. 
     Also in this configuration, even when more than two deflectors are used, it is possible to detect synchronization of the deflectors precisely. 
     When the synchronization detecting light path comprises a plurality of synchronization detecting light paths, the single synchronization detecting part may detect light fluxes of the plurality of synchronization detecting light paths. Also in this configuration, the synchronization detecting part is used in common. Accordingly, it is possible to save the space, to improve the space utilization efficiency, to reduce the influence of variation in characteristics of a plurality of synchronization detecting sensors, and to reduce the costs by reducing the number of synchronization detecting sensors. 
     Further, as a result of the above-described features of the present invention being combined, it is possible to miniaturize the optical scanning device, to prevent density difference from occurring in the joints of the respective optical scanning units, and to establish synchronization of the deflectors precisely, in the optical scanning device. 
     Other objects and further features of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a configuration of an optical scanning device in the related art; 
     FIG. 2 shows a configuration of an optical scanning device in a first embodiment of the present invention; 
     FIGS. 3A and 3B show a diameter of beam spot in the optical scanning device in the first embodiment of the present invention; 
     FIG. 4 shows a configuration of an optical scanning device in a second embodiment of the present invention; 
     FIG. 5 shows a configuration of an optical scanning device in a third embodiment of the present invention; 
     FIG. 6 shows a configuration of an optical scanning device in a fourth embodiment of the present invention; 
     FIG. 7 shows a configuration of an optical scanning device in a first variant embodiment of the second embodiment of the present invention; 
     FIG. 8 shows a configuration of an optical scanning device in a first variant embodiment of the third embodiment of the present invention; 
     FIG. 9 shows a configuration of an optical scanning device in a second variant embodiment of the second embodiment of the present invention; 
     FIG. 10 shows a configuration of an optical scanning device in a second variant embodiment of the third embodiment of the present invention; 
     FIG. 11 shows a configuration of an optical scanning device in a first variant embodiment of the fourth embodiment of the present invention; 
     FIG. 12 shows a configuration of an optical scanning device in a second variant embodiment of the fourth embodiment of the present invention; and 
     FIG. 13 shows a configuration of an optical scanning device in a third variant embodiment of the fourth embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will now be described with reference to figures. 
     In FIGS. 2 through 13, the same reference numerals are given to components having the same functions as those of the components shown in FIG. 1, and the duplicated description is omitted. 
     FIG. 2 shows an optical scanning device in a first embodiment of the present invention. 
     Further, each of the optical scanning units  201 A through  201 D has the same components as those of the configuration of the optical scanning device  100  in the related art shown in FIG.  1 . For these same components in FIG. 2, letters A through D corresponding to the respective optical scanning units  201 A through  201 D are added immediately subsequent to the same reference numerals as those used in FIG. 1, and the duplicated description is omitted. 
     Further, although the scanning and imaging optical system  18  of the optical scanning device  100  shown in FIG. 1 is a non-telecentric optical system, scanning and imaging optical systems  18 A through  18 D of the respective optical scanning units  201 A through  201 D are approximately telecentric optical system. 
     It is noted that the telecentric optical system is an optical system which causes a scanning light flux  20  to be incident on a surface to be scanned  9  at a right angle (90°) and form an image thereon. However, in the first embodiment, the scanning.and imaging optical systems  18 A thorough  18 D may be optical systems each of which causes a scanning light flux to be incident on the surface to be scanned  9  at an approximately right angle (not precisely 90° but may be one in a range of 80° through 90°) and form an image thereon. Accordingly, the scanning and imaging optical systems  18 A through  18 D are expressed as approximately telecentric optical systems. 
     Further, each of the optical scanning units  201 A through  201 D has the same components as those of the configuration of the optical scanning device  100  in the related art shown in FIG.  1 . For these same components in FIG. 2, alphabets A through D corresponding to the respective optical scanning units  201 A through  201 D are added immediately subsequent to the same reference numerals as those used in FIG. 1, and the duplicated description is omitted. 
     Further, in the first embodiment, the respective optical scanning units  201 A through  201 D are controlled by a micro processor or the like not shown in the figure, thereby, coordinated movements thereof are achieved, and, as a result, scanning is performed continuously on the single surface to be scanned  9  by them. 
     Although each of the rotation directions  30 A through  30 D of the respective deflectors  6 A through  6 D of the optical scanning units  201 A through  201 D may be individually determined to be either clockwise or counterclockwise direction, it is assumed that the respective rotation directions  30 A through  30 D are all the clockwise direction, for the sake of simplification of description. Thereby, the respective scanning directions  50 A through  50 D of the optical scanning units  201 A through  201 D are all the downward direction as shown in FIG. 2, and, the scanning direction  50  of the entirety of the optical scanning device  200  is the downward direction as shown in the figure. 
     The reason why each of the scanning and imaging optical systems  18 A through  18 D of the first embodiment is different from the scanning and imaging optical system  18  shown in FIG.  1  and is of an approximately telecentric optical system is to minimize shift amounts at the joined portion occurring with a change in position on the photosensitive body. 
     In the case of approximately telecentric optical systems as shown in FIG. 2, there is no space between adjacent light fluxes  20   b  and  20   d  of the respective optical scanning units  201 A through  201 D scanning the surface  9  to be scanned, and it is necessary to form the long-dimensional lenses  70 A through  70 D in such a manner that there is no space between adjacent long-dimensional lenses. 
     For example, if the long-dimensional lenses  70 A through  70 D are provided separately, it is necessary to make a design so that each long-dimensional lens  70 A through  70 D has a length longer than the effective length by which the light flux  20  scans the surface  9  to be scanned, in consideration of manufacturing error, assembling error and so forth of each lens. However, when the long-dimensional lenses  70 A through  70 D are formed integrally as in the embodiment, the length of each long-dimensional lens  70 A through  70 D naturally coincides with the effective length by which the light flux  20  scans the surface  9  to be scanned, and no assembling error between the respective lenses is generated. Accordingly, no extra lens portion is needed. 
     Accordingly, when the optical systems are approximately telecentric, it is preferable that the long-dimensional lenses  70 A through  70 D are formed integrally according to the first embodiment. 
     In FIG. 2, it is expressed that there is a space between the scanning end position (position at which an image is formed of the light flux  20   d ) of the optical scanning unit  201 A and the scanning beginning position (position at which an image is formed of the light flux  20   b ) of the optical scanning unit  201 B so that they are not continuous, for example. However, this expression is made for clearly showing the scanning end position of the optical scanning unit  201 A and scanning beginning position of the optical scanning unit  201 D. Actually, the surface  9  to be scanned is scanned by the optical scanning units  201 A through  201 D continuously without a gap. 
     Further, when each optical scanning unit  201 A through  201 D is made to be a nontelecentric optical system, there is a space in the long-dimensional lens  70  at position between adjacent light flux  20   d  of which an image is formed on the surface  9  to be scanned at the scanning end position and light flux  20   b  of which an image is formed on the surface  9  to be scanned at the scanning beginning position. Accordingly, it is not necessary to dispose the long-dimensional lenses  70 A through  70 D without a space therebetween as in the case of approximately telecentric optical systems. In this case, a lens obtained from only connecting the two long dimensional lenses  70 A and  70 B of the optical scanning units  201 A and  201 B, or a lens obtained from only connecting the two long-dimensional lenses  70 C and  70 D of the optical scanning units  201 C and  201 D may be configured, for example. 
     As a method of configuring the lens  70  obtained from connecting the long-dimensional lenses  70 A through  70 D, there is a method in that the lens  70  is formed of a plastic lens molded from such a plastic material that it is easy to form curved surfaces. Since the long-dimensional lenses  70 A through  70 D are aspherical lenses, the surfaces of the lens  70  obtained from connecting them are very complicated curved surfaces, and, therefore, it is very difficult to form the aspherical lens  70  of a glass material in consideration of process of grinding and so forth. 
     Further, although the fθ lenses  7 A through  7 D are individual lenses in the first embodiment as shown in FIG. 2, it is also possible to form a single lens by connecting the fθ lenses  7 A through  7 D similarly to the lens  70 , for example. 
     As described above, in the first embodiment, the plurality of (four) optical scanning units are used, and, also, the respective scanning and imaging optical systems  18 A through  18 D of adjacent at least two optical scanning units have the single lens ( 70 , and/or a lens obtained as a result of the lenses  7 A through  7 D being connected) in common. Thereby, it is possible to miniaturize the optical scanning device, and, also, to make easier the adjustment work in manufacturing it. 
     Further, in the first embodiment, as described above, the long-dimensional lenses  70 A through  70 D of the respective optical scanning units  201 A through  201 D are formed to be the single lens  70 . Thereby, even when the optical systems are approximately telecentric optical systems, it is possible for the optical scanning device to scan the surface  9  to be scanned continuously by the optical scanning units  201 A through  201 D. However, when the diameter of beam spot is different for each of the respective optical scanning units  201 A through  201 D, the diameter of beam spot suddenly changes at the join of the light fluxes  20   d  and  20   b  of adjacent optical scanning units. Thereby, density difference occurs, and, as a result, the density may become discontinuous. 
     In order to solve this problem, it is preferably to make the diameters of beam spots at adjacent scanning end position and scanning beginning position to be approximately the same. For this purpose, it is necessary to cause the optical characteristics of the scanning and imaging optical systems  18 A through  18 D at the adjacent portions directing the respective light fluxes  20   d  and  20   b  to the adjacent scanning end position and scanning beginning position to approximate one another. 
     In order to reduce the difference between the diameter of beam spot of the optical scanning unit of the adjacent optical scanning units  201 A through  201 D scanning the surface  9  to be scanned at the scanning end position and the diameter of beam spot of the optical scanning unit of the adjacent optical scanning units  201 A through  201 D scanning the surface  9  to be scanned subsequently at the scanning beginning position, the respective scanning and imaging optical systems  18 D through  18 D are configured to satisfy the following expression (1): 
     
       
         ( Bn (−)−&lt; B &gt;)×( Bn +1(+)−&lt;B&gt;)≧0  (1) 
       
     
     where: 
     m: the total number of the optical scanning units; 
     n: any integer in the range of 1≦n≦m−1; 
     Bn(−): the diameter of beam spot of the n-th optical scanning unit at the scanning end position; 
     Bn+1(+): the diameter of beam spot of the (n+1)-th optical scanning unit at the scanning beginning position; and 
     &lt;B&gt;: the average of the diameters of beam spots of the optical scanning units scanning the surface to be scanned. 
     When the respective scanning and imaging optical systems  18 A through  18 D do not satisfy the above expression (1), that is, the respective scanning and imaging optical systems  18 A through  18 D satisfy the following expression (2), discontinuity of density occurs at the joined portions of the light fluxes  20  on the surface  9  to be scanned directed by the respective scanning and imaging units  18 A through  18 D, and, as a result, lines of density difference are generated at the joins: 
     
       
         ( Bn (−)−&lt; B &gt;)×( Bn +1(+)−&lt;B&gt;)&lt;0  (2) 
       
     
     Further, because change in diameter of beam spot at the joins is preferably small to the utmost, it is preferable that the respective scanning and imaging optical systems  18 A through  18 D also satisfy the following expression (3) expressing the inclination of change of the expression (1): 
     
       
         { d ( Bn (−)−&lt; B &gt;)/ dy}×{d ( Bn +1(+)−&lt; B &gt;)/ dy }≧0  (3) 
       
     
     where y: length in main scanning direction. 
     As a result of the above-mentioned expressions (1) and (3) being satisfied, the first embodiment is not likely to generate lines of density difference and/or the like at the joins only as a result of the microprocessor or the like not shown in the figure precisely controlling switching of the outputs of the respective optical scanning units. 
     FIGS. 3A and 3B show relationship between the diameter of beam spot and image height. In the figures, (+) denotes the scanning beginning position and (−) denotes the scanning end position. FIG. 3A relates to the n-th optical scanning unit and FIG. 3B relates the (n+1)-th optical scanning unit. 
     Second and third embodiments of the present invention will now be described. 
     FIG. 4 shows a configuration of an optical scanning device in the second embodiment of the present invention, and FIG. 5 shows a configuration of an optical scanning device in the third embodiment of the present invention. 
     The difference between the configuration of the optical scanning device  300  in the second embodiment shown in FIG.  4  and the configuration of the optical scanning device  400  in the third embodiment shown in FIG. 5 is only as follows: The optical scanning device  300  uses non-telecentric optical systems while the optical scanning device  400  uses approximately telecentric optical systems. As a result, the light paths of the respective scanning and imaging optical systems  18 A through  18 D are different between the second and third embodiments. 
     The difference between the optical scanning device  300  in the second embodiment or the optical scanning device  400  in the third embodiment and the optical scanning device  200  in the first embodiment is as follows: Synchronization detecting light paths for at least one (two in these embodiments) of the optical scanning unit  301 A through  301 D or  401 A through  401 D is provided between the light path  20   d  of the scanning and imaging optical system  18  of one of the optical scanning units  301 A through  301 D or  401 A through  401 D and the light path  20   b  of the scanning and imaging optical system  18  of the other adjacent optical scanning device, and, also, mirrors  13 B,  13 C directing the light fluxes  20   e B,  20   e C of the synchronization detecting light paths to the outside of the light paths of the respective scanning and imaging optical systems  18 A through  18 D or synchronization detecting sensors  12 B,  12 C detecting the light fluxes  20   e B,  20   e C of the synchronization detecting light paths are provided there. 
     For example, as in the optical scanning units  301 B,  301 C disposed between the other optical scanning units  301 A and  301 D, no component can be disposed between the light paths  20   b B and  20   d B and between the light paths  20   b C and  20   d C because these spaces are used for scanning by the respective light fluxes. Accordingly, the places where the synchronization detecting units  12 B,  12 C for detecting synchronization of the deflectors  6 B,  6 C are disposed are limited to the narrow space between the light flux  20   d B and light flux  20   b C, the narrow space between the light flux  20   d A and light flux  20   b B or the narrow space between the light flux  20   d C and light flux  20   b D. 
     In a recent optical scanning device, due to demand for miniaturization, there is a case where other components are already closely disposed in the above-mentioned narrow spaces. In such a case, it is not possible to dispose the relatively large synchronization sensors  12 B,  12 C or the like there. 
     In order to solve this problem, in the second and third embodiments, as shown in FIGS. 4 and 5, the mirrors  13 B and  13 D which do not require a large space are disposed between the light flux  20   d B and light flux  20   b C, and the synchronization detecting sensors  13 B,  13 D are disposed in the outside of the spaces for scanning by the respective light fluxes, as shown in the figures. 
     However, when there is a sufficient space between the light fluxes  20   d B and  20   b C, or the synchronization detecting sensors  12 B,  12 C are relatively small-sized ones, the synchronization detecting sensors  12 B,  12 C may be disposed there instead of the mirrors  13 B,  13 C in FIGS. 4 and 5 (as in the embodiments shown in FIGS.  7  and  8 ). 
     It is noted that the position of the light path for the synchronization detection for each one of the optical scanning units  301 A through  301 D or  401 A through  401 D may be slightly outside of the scanning beginning end of the effective scanning range of the deflector  6  or may be slightly outside of the scanning completion end of the effective scanning range of the deflector  6 . 
     Thus, in each of the second and third embodiment, it is possible to minmiaturize the opitcal scanning device, to make easier adjustment in manufacturing it, and, also, to precisely detect synchronization of deflectors even in the optical scanning device employing more than two optical scanning units. 
     A fourth embodiment of the present invention will now be described. 
     FIG. 6 shows a configuration of an optical scanning device in the fourth embodiment employing non-telecentric optical systems. 
     The difference between the optical scanning device  500  in the fourth embodiment and each of the optical scanning devices  300  and  400  in the second and third embodiments shown in FIGS. 4 and 5 is that a single synchronization detecting sensor  12 BC is used for the two light fluxes  20   e B,  20   e C for the synchronization detection instead of the mirrors  13 B,  13 C and synchronization detecting sensors  12 B,  12 C. 
     Accordingly, in the fourth embodiment, it is possible to minmiaturize the opitcal scanning device, to make easier adjustment in manufacturing it, to precisely detect synchronization of deflectors even in the optical scanning device employing more than two optical scanning units, and, also, to reduce the space, thus improving the space utilization efficiency. Further, in comparison to the case where the synchronization detecting sensors are provided for the light fluxes for synchronization detection, respectively, it is possible to reduce the influence of the variation in characteristics of the respective synchronization detecting sensors. Further, because it is possible to reduce the number of synchronization detecting sensors, it is possible to reduce the cost. 
     FIG. 7 shows a configuration of an optical scanning device in a first variant embodiment of the above-described second embodiment of the present invention shown in FIG.  4 . 
     The difference between the first variant embodiment of the second embodiment and the second embodiment is that the mirrors  13 B,  13 C in the second embodiment are replaced by the synchronization detecting sensors  12 B,  12 C in the first variant embodiment of the second embodiment. 
     FIG. 8 shows a configuration of an optical scanning device in a first variant embodiment of the above-described third embodiment of the present invention shown in FIG.  5 . 
     The difference between the first variant embodiment of the third embodiment and the third embodiment is that the mirrors  13 B,  13 C in the third embodiment are replaced by the synchronization detecting sensors  12 B,  12 C in the first variant embodiment of the third embodiment. 
     FIG. 9 shows a configuration of an optical scanning device in a second variant embodiment of the above-described second embodiment of the present invention shown in FIG.  4 . 
     The difference between the second variant embodiment of the second embodiment and the second embodiment is that the mirrors  13 B,  13 C in the second embodiment are replaced by a single mirror  13 BC in the second variant embodiment of the second embodiment. 
     FIG. 10 shows a configuration of an optical scanning device in a second variant embodiment of the above-described third embodiment of the present invention shown in FIG.  5 . 
     The difference between the second variant embodiment of the third embodiment and the third embodiment is that the mirrors  13 B,  13 C in the third embodiment are replaced by a single mirror  13 BC in the second variant embodiment of the third embodiment. 
     FIG. 11 shows a configuration of an optical scanning device in a first variant embodiment of the above-described fourth embodiment of the present invention shown in FIG.  6 . 
     The difference between the first variant embodiment of the fourth embodiment and the fourth embodiment is that the optical scanning device in the fourth embodiment uses non-telecentric optical systems while the optical scanning device in the first variant embodiment of the fourth embodiment instead uses approximately telecentric optical systems. As a result, the light paths of the respective scanning and imaging optical systems  18 A through  18 D are different between the fourth embodiment and the first variant embodiment of the fourth embodiment. 
     FIG. 12 shows a configuration of an optical scanning device in a second variant embodiment of the above-described fourth embodiment of the present invention shown in FIG.  6 . 
     The difference between the second variant embodiment of the fourth embodiment and the fourth embodiment is that the synchronization detecting sensor  12 BC in the fourth embodiment is replaced by a mirror  13 BC, and the synchronization detecting sensor  12 BC is disposed in another place to which the light fluxes reflected by the mirror  13 BC is directed in the second variant embodiment of the fourth embodiment. 
     FIG. 13 shows a configuration of an optical scanning device in a third variant embodiment of the above-described fourth embodiment of the present invention shown in FIG.  6 . 
     The difference between the third variant embodiment of the fourth embodiment and the first variant embodiment of the fourth embodiment shown in FIG. 11 is that the synchronization detecting sensor  12 BC in the first variant embodiment of the fourth embodiment is replaced by the mirror  13 BC, and the synchronization detecting sensor  12 BC is disposed in another place to which the light fluxes reflected by the mirror  13 BC is directed in the third variant embodiment of the fourth embodiment. 
     The present invention is not limited to the above-described embodiments, and variations and modifications may be made without departing from the scope of the present invention. 
     The present application is based on Japanese priority application No. 11-269230, filed on Sep. 22, 1999, the entire contents of which are hereby incorporated by reference.