Patent Publication Number: US-7911667-B2

Title: Optical beam scanning device, image forming apparatus, optical beam scanning method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical beam scanning device used for a laser printer, a digital copying machine or the like, and particularly to a pre-deflection optical system for guiding a light beam from a light source to a rotary deflector which deflects an incident light flux and scans it in a specified scanning direction. 
     2. Description of the Related Art 
     Hitherto, there is known a light beam scanning device which uses a rotary deflector such as a polygon mirror and scans a light flux from a light source in a main scanning direction. In such a conventional light beam scanning device, as a pre-deflection optical system to shape a divergent light beam from a light source into a light flux having a cross-sectional shape long in the main scanning direction, there are often adopted a lens to weaken the degree of divergence of the divergent light beam from the light source or to convert it into a parallel light beam or a converging light beam, and a lens to give a positive power to the light flux the degree of divergence of which has been weakened or which has been converted into the parallel light beam or the converging light beam. 
     However, in a case where a multi-beam optical system using divergent light beams from plural light sources is adopted in the light beam scanning device, when only the structure of the conventional light beam scanning device is used, it is difficult to adjust all conditions, such as, in the rotation axis direction of the rotary deflector, (1) the pitch of the light fluxes from the plural light sources, (2) focal positions and (3) beam diameters of the light fluxes, to be appropriate. 
     SUMMARY OF THE INVENTION 
     An embodiment of the invention has an object to provide a technique which can realize scanning with a light flux having a desired optical characteristic in a light beam scanning device adopting a multi-beam optical system. 
     In order to solve the problem, a light beam scanning device of the invention includes a pre-deflection optical system that shapes divergent light beams from plural light sources into a light flux having a cross-sectional shape long in a specified direction, and a rotary deflector that deflects the light flux shaped by the pre-deflection optical system and scans it in the specified direction, and is characterized in that the pre-deflection optical system includes a first optical system that weakens a degree of divergence of the divergent light beams from the plural light sources or converts them into parallel light beams or converging light beams, a second optical system including at least one lens having a negative power in a rotation axis direction of the rotary deflector, and a third optical system having a positive power in the rotation axis direction of the rotary deflector. 
     Besides, an image forming apparatus of the invention is characterized by including a light beam scanning device having a structure as stated above, image bearing means on which an electrostatic latent image is formed by a light flux scanned by the light beam scanning device, and developing means for revealing the electrostatic latent image formed on the photosensitive body. 
     Besides, an optical beam scanning method of the invention shapes divergent light beams from plural light sources into a light flux having a cross-sectional shape long in a specified direction, and deflects the shaped light flux by a rotary defector to scan it in the specified direction, and is characterized by comprising weakening a degree of divergence of the divergent light beams from the plural light sources or converting them into substantially parallel light beams or converging light beams, giving a negative power to the processed light flux by at least one lens having a negative power in a rotation axis direction of the rotary deflector, giving a positive power to the light flux to which the negative power has been given by a lens having a positive power in the rotation axis direction of the rotary deflector, and deflecting the light flux to which the positive power has been given by the rotary deflector to scan it in the specified direction. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view showing a digital copying machine as an image forming apparatus including a light beam scanning device of an embodiment of the invention. 
         FIG. 2  is a schematic block diagram showing an example of a drive circuit of a digital copying apparatus. 
         FIG. 3  is a schematic view for explaining the structure of the light beam scanning device shown in  FIG. 1 . 
         FIG. 4  is a schematic view for explaining the structure of the light beam scanning device shown in  FIG. 1 . 
         FIG. 5  is a view showing a coordinate system for defining the shape of a lens surface. 
         FIG. 6(A)  is a view showing an example of coordinate data indicating a lens surface shape of an imaging lens (fθ 1  lens)  91  at an incident surface side. 
         FIG. 6(B)  is a view showing an example of coordinate data showing a lens surface shape of the imaging lens (fθ 1  lens)  91  at an outgoing surface side. 
         FIG. 6(C)  is a view showing an example of coordinate data indicating a lens surface shape of an imaging lens (fθ 2  lens)  92  at an incident surface side. 
         FIG. 6(D)  is a view showing an example of coordinate data indicating a lens surface shape of the imaging lens (fθ 2  lens)  92  at an outgoing surface side. 
         FIG. 7  is a view showing combinations of shapes of two cylindrical lenses to form a linear image of a light flux on a reflecting surface of a polygon mirror and results of an evaluation function for optimization. 
         FIG. 8  is a schematic view for explaining a beam interval adjustment in a pre-deflection optical system  70 . 
         FIG. 9(A)  is a schematic view for explaining the beam interval adjustment in the pre-deflection optical system  70 . 
         FIG. 9(B)  is a schematic view for explaining the beam interval adjustment in the pre-deflection optical system  70 . 
         FIG. 10  is a schematic view for explaining the beam interval adjustment in the pre-deflection optical system  70 . 
         FIG. 11(A)  is a view showing an example of a structure of a support part  75   h.    
         FIG. 11(B)  is a view showing the example of the structure of the support part  75   h.    
         FIG. 12  is a view showing a change in beam interval of beam interval in a case where one of a finite lens  72 , a cylindrical lens  74  and a cylindrical lens  75  is moved in an optical axis direction. 
         FIG. 13  is a view showing a wave aberration in a case where one of the finite lens  72 , the cylindrical lens  74  and the cylindrical lens  75  is moved in the optical axis direction. 
         FIG. 14  is a view showing another example of a support part to support the cylindrical lens  75  movably in the optical axis direction. 
         FIG. 15  is a view showing a structural example in which a cylindrical lens  74   a  having a negative power in a sub-scanning direction and a cylindrical lens  74   b  are disposed between an aperture  73  and the cylindrical lens  75 . 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, embodiments of the invention will be described with reference to the drawings. 
       FIG. 1  is a view showing a digital copying machine (for example, MFP: Multi Function Peripheral, etc.) as an image forming apparatus including a light beam scanning device of an embodiment of the invention. 
     As shown in  FIG. 1 , a digital copying apparatus  1  includes, for example, a scanner part  10  as image reading means and a printer part  20  as image forming means. 
     The scanner part  10  includes a first carriage  11  formed movably in a direction of an arrow, a second carriage  12  driven and moved by the first carriage  11 , an optical lens  13  to give a specified imaging characteristic to a light beam from the second carriage  12 , a photoelectric conversion element  14  that photoelectrically converts the light beam to which the specified imaging characteristic has been given by the optical lens  13  and outputs an electric signal, a document table  15  to hold a document D, a document fixing cover  16  to press the document D to the document table  15 , and the like. 
     The first carriage  11  includes a light source  17  to illuminate the document D, and a mirror  18   a  to reflect a reflected light beam, which has been reflected by the document D illuminated by a light beam emitted by the light source  17 , to the second carriage  12 . 
     The second carriage  12  includes a mirror  18   b  to bend the light beam transmitted from the mirror  18   a  of the first carriage  11  by 90°, and a mirror  18   c  to further bend the light beam, which has been bent by the mirror  18   b , by 90°. 
     The document D placed on the document table  15  is illuminated by the light source  17 , and reflects the reflected light beam in which the variations of light and shade corresponding to the presence/absence of an image are distributed. The reflected light beam of the document D is incident on the optical lens  13  as image information of the document D through the mirrors  18   a ,  18   b  and  18   c.    
     The reflected light beam guided to the optical lens  13  from the document D is condensed onto the light receiving surface of the photoelectric conversion element (CCD sensor)  14  by the optical lens  13 . 
     When the start of image formation is instructed from a not-shown operation panel or an external device, the first carriage  11  and the second carriage  12  are once moved by driving of a not-shown carriage driving motor to a home position determined so that they have a specified positional relation relative to the document table  15 , and then are moved at a specified speed along the document table  15 , so that the image information of the document D, that is, the image light beam reflected from the document D is cut out to have a specified width in the direction in which the mirror  18   a  is extended, that is, in a main scanning direction and is reflected toward the mirror  18   b , and is successively taken out in units of the width cut out by the mirror  18   a  and in a direction perpendicular to the direction in which the mirror  18   a  is extended, that is, in a sub-scanning direction, and all image information of the document D is guided to the CCD sensor  14 . Incidentally, an electric signal outputted from the CCD sensor  14  is an analog signal, and is converted into a digital signal by a not-shown A/D converter, and is temporarily stored as an image signal into a not-shown image memory. 
     In the manner as stated above, the image of the document D placed on the document table  15  is converted into, for example, an 8-bit digital image signal indicating the light and shade of the image by a not-shown image processing part for each line along a first direction in which the mirror  18   a  is extended, by the CCD sensor  14 . 
     The printer part  20  includes a light beam scanning device  21  as an exposure device described in a later section, and an electrophotographic image forming part  22  capable of forming an image on a recording sheet P as a medium on which an image is to be formed. 
     The image forming part  22  includes a drum-shaped photosensitive body (hereinafter referred to as a photosensitive drum)  23  that is rotated by an after-mentioned main motor so that its outer peripheral surface moves at a specified speed, and is irradiated (scanned) with a laser beam L from the light beam scanning device  21  so that an electrostatic latent image corresponding to image data, that is, an image of the document D is formed, a charging device  24  to give a surface potential with a specified polarity to the surface of the photosensitive drum (image bearing means)  23 , a developing device (developing part, developing means)  25  to selectively supply a toner as a visualizing agent to the electrostatic latent image formed on the photosensitive drum  23  by the light beam scanning device and performs development (revealing an image), a transfer device  26  that gives a specified electric field to the toner image formed on the outer periphery of the photosensitive drum  23  by the developing device  25  and transfers it onto the recording sheet P, a separating device  27  that releases and separates the recording sheet P on which the toner image has been transferred by the transfer device and the toner between the recording sheet P and the photosensitive drum  23  from electrostatic attachment (from the photosensitive drum  23 ), a cleaning device  28  that removes the transfer residual toner remaining on the outer peripheral surface of the photosensitive drum  23  and returns the potential distribution of the photosensitive drum  23  to the state before the surface potential is supplied by the charging device  24 , and the like. Incidentally, the charging device  24 , the developing device  25 , the transfer device  26 , the separating device  27  and the cleaning device  28  are sequentially arranged along the arrow direction in which the photosensitive drum  23  is rotated. Besides, the laser beam L from the light beam scanning device  21  is irradiated to a specified position K on the photosensitive drum  23  between the charging device  24  and the developing device  25 . 
     The image signal read from the document D by the scanner part  10  is converted into a printing signal in a not-shown image processing part by a processing such as, for example, a contour correction or a gradation processing for halftone display, and is further converted into a laser modulation signal to change the light intensity of the laser beam emitted from an after-described semiconductor laser element of the light beam scanning device  21  into one of the intensity at which the electrostatic latent image can be recorded on the outer periphery of the photosensitive drum  23  to which a specified surface potential has been given by the charging device  24  and the intensity at which the latent image is not recorded. 
     Each of semiconductor laser elements of the light beam scanning device  21  described below is intensity-modulated in accordance with the foregoing laser modulation signal, and emits a light beam so that the electrostatic latent image is recorded at a specified position of the photosensitive drum  23  correspondingly to the specified image data. The light beam from the semiconductor laser element is deflected in a first direction as the same direction as a reading line of the scanner part  10  by a polygon mirror (rotary deflector) as after-described light scanning means in the light beam scanning device  21 , and is irradiated onto the specified position K on the outer periphery of the photosensitive drum  23 . 
     The photosensitive drum  23  is rotated in the arrow direction at a specified speed, so that, similarly to the movement of the first carriage  11  and the second carriage  12  of the scanner part  10  along the document table  7 , the laser beam from the semiconductor laser element (light source) sequentially deflected by the polygon mirror is exposed onto the outer periphery of the photosensitive drum  23  for each line at specified intervals. 
     In the manner as stated above, the electrostatic latent image corresponding to the image signal is formed on the outer periphery of the photosensitive drum  23 . 
     The electrostatic latent image formed on the outer periphery of the photosensitive drum  23  is developed with the toner from the developing device  25 , is transported to a position opposite to the transfer device  26  by the rotation of the photosensitive drum  23 , and is transferred onto the recording sheet P, which is taken out from a sheet cassette  29  by a paper feed roller  30  and a separation roller  31  and is supplied while the timing is adjusted by an aligning roller  32 , by the electric field from the transfer device  26 . 
     The recording sheet P on which the toner image has been transferred, together with the toner, is separated by the separating device  27 , and is guided to a fixing device  34  by a transport device  33 . 
     The toner (toner image) is fixed by the heat and pressure from the fixing device  34  onto the recording sheet P guided to the fixing device  34 , and the sheet is discharged to a tray  36  by a paper discharge roller  35 . 
     On the other hand, the photosensitive drum  23  after the toner image (toner) has been transferred on the recording sheet P by the transfer device  26  is made to face the cleaning device  28  as a result of the subsequent rotation, the transfer remaining toner (residual toner) remaining on the outer periphery is removed, return is made to a state (initial state) before the surface potential is supplied by the charging device  24 , and next image formation becomes possible. 
     The above process is repeated, so that a continuous image forming operation becomes possible. 
     As stated above, with respect to the document D set on the document table  15 , the image information is read by the scanner part  10 , the read image information is converted into the toner image by the printer part  20  and is outputted to the recording sheet P, so that copying is performed. 
     Incidentally, although the description of the image forming apparatus has been made while the digital copying machine is used as an example, for example, a printer apparatus in which an image reading part does not exist may be used. 
       FIG. 2  is a schematic block diagram showing an example of a drive circuit of a digital copying apparatus including a light beam scanning device having the structure as described above. 
     A CPU  110  as a main control device is connected with a ROM (Read Only Memory)  102  in which a specified operation rule and initial data are stored, a RAM  103  to temporarily store inputted control data, a shared (image) RAM  104  which holds image data from the CCD sensor  14  or image data supplied from an external device and outputs the image data to an image processing circuit described below, an NVM (nonvolatile memory)  105  which holds, even when power application to the copying apparatus  1  is stopped, data stored up to that time by battery backup, an image processing device  106  which adds a specified image processing to the image data stored in the image RAM  104  and outputs it to a laser driver described below, and the like. 
     The CPU  110  is also connected with a laser driver  121  to cause a semiconductor laser element (light source)  71  of the light beam scanning device  21  to emit light, a polygon motor driver  122  which drives a polygon motor  50 A to rotate a polygon mirror  80 , a main motor driver  123  which drives a main motor  23 A to drive the photosensitive drum  23  and a transport mechanism of an accompanying sheet (member to be transferred), and the like. 
       FIG. 3  and  FIG. 4  are schematic views for explaining the structure of the light beam scanning device shown in  FIG. 1 . Incidentally,  FIG. 3  is a schematic plan view in which optical elements arranged between the light source (semiconductor laser element)  71  included in the light beam scanning device and the photosensitive drum (object to be scanned)  23  are seen from a direction perpendicular to the main scanning direction (first direction) as the direction parallel to the direction in which the light flux directed to the photosensitive drum from the polygon mirror is-scanned by the polygon mirror and the turnover by the mirror is expanded and explained, and  FIG. 4  is a schematic sectional view shown such that a sub-scanning direction (second direction) perpendicular to the direction shown in  FIG. 3 , that is, the main scanning direction is on a plane. 
     As shown in  FIG. 3  and  FIG. 4 , the light beam scanning device  21  includes the laser array element (plural light sources)  71  to emit the laser beam (light flux) L of, for example, 785 nm, a lens  72  (first optical system, first light flux processing means) to convert the cross-sectional shape of the laser beam L emitted from the laser array element  71  into a converging light beam, a parallel light beam or a divergent light beam, an aperture  73  to limit the light amount (light flux width) of the laser beam L having passed through the lens  72  to a specified magnitude, a lens  74  (second optical system, second light flux processing means) and a lens  75  (first optical system, first light flux processing means), in which a power is given only in the sub-scanning direction in order to shape the cross-sectional shape of the laser beam L the light amount of which has been limited by the aperture  73  into a specified cross-sectional beam shape. As stated above, the light beam scanning device of this embodiment adopts the so-called multi-beam optical system in which divergent light beams from plural light sources arranged at positions different from each other in the sub-scanning direction are used as the light sources. 
     Hereinafter, the effect obtained by adopting the multi-beam optical system will be described in brief. In the image forming apparatus, the sub-scanning direction in the optical system corresponds to the transport direction of the transfer member, and the main scanning direction indicates a direction perpendicular to the transport direction in the transfer member surface. Besides, it is assumed that an image surface indicates a transfer member surface, and an imaging surface indicates a surface on which an image of a beam is actually formed. 
     In general, an image process speed (paper transport speed), an image resolution, a motor rotation speed, the number of polygon mirror surfaces, the number of beams, a motor rotation speed, an image frequency and the like have the following relation. 
     
       
         
           
             
               
                 
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                       P 
                       * 
                       D 
                     
                     
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                       * 
                       m 
                       * 
                       N 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
             
               
                 
                   
                     f 
                     m 
                   
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                       2 
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                       * 
                       P 
                       * 
                       
                         D 
                         2 
                       
                       * 
                       W 
                     
                     
                       
                         25.4 
                         2 
                       
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                       m 
                       * 
                       N 
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                       A 
                     
                   
                 
               
               
                 
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         P (mm/s): process speed (paper transport speed) 
         D (dpi): image resolution (number of dots per inch) 
         Vr (rpm): polygon motor rotation speed 
         fm (Hz): image frequency 
         N: number of polygon mirror surfaces 
         m: number of beams 
         W: scanning width 
         A: half value of scanning angle 
       
    
     Thus, for realization of high speed and high resolution, it becomes necessary that the polygon mirror rotation speed is raised, the image frequency is raised, or the number of polygon mirror surfaces is increased. 
     However, when the polygon mirror is rotated at high speed, a load to the motor is large, and the motor cost is raised. Besides, the occurrence of noise, vibration, and heat is large, and countermeasures against these become necessary. Besides, since the image frequency has a limit, it is effective to increase the number of beams by adopting the multi-beam scanning optical system. 
     The lens  72  is a single finite focal lens, a collimate lens, or a lens to weaken the degree of divergence of a divergent light beam, the lens  74  is a cylindrical lens having a negative power in the sub-scanning direction, and the lens  75  is a cylindrical lens having a positive power in the sub-scanning direction. A pre-deflection optical system  70  includes the lens  72 , the aperture  73 , the lens  74 , and the lens  75 . The pre-deflection optical system  70  has a function to shape divergent light beams from plural light sources into a light flux having a cross-sectional shape long in the main scanning direction. 
     In the direction in which the laser beam L to which a specified sectional beam shape is given by the pre-deflection optical system  70  travels, the polygon mirror  80  is provided which is formed integrally with a not-shown polygon mirror motor rotating at a specified speed and scans the laser beam L, the cross-sectional beam shape of which has been shaped into the specified shape by the cylindrical lenses  74  and  75 , in the main scanning direction to the photosensitive drum (surface to be scanned)  23  positioned at a later stage. 
     Between the polygon mirror  80  and the photosensitive drum  23 , an imaging optical system  90  is provided which forms an image of the laser beam L, which is continuously reflected by each reflecting surface of the polygon mirror  80 , substantially linearly along the axial direction of the photosensitive drum  23 . 
     The imaging optical system  90  includes imaging lenses (normally called as fθ lenses)  91  and  92  that irradiate the laser beam L, which is continuously reflected by the respective reflecting surfaces of the polygon mirror  80 , from one end to the other end of the photosensitive drum  23  in the longitudinal (axial line) direction at the exposure position K shown in  FIG. 1  while the position on the photosensitive drum  23  at the time of irradiation to the photosensitive drum  23  is made to be proportional to the rotation angle of each reflecting surface of the polygon mirror  80 , and that can provide the convergence in which a specified relation is given based on the angle at which the polygon mirror  80  is rotated, so that a specified cross-sectional beam diameter is obtained at any position on the photosensitive drum  23  in the longitudinal direction, dustproof glasses  93  and  94  to prevent toner, dust, paper powder or the like floating in the image forming part  22  from going around and entering a not-shown housing of the light beam scanning device  21 . Here, at least one of the imaging lenses  91  and  92  is formed of a plastic lens. 
     Incidentally, the light path of the laser beam L from the semiconductor laser element  71  to the photosensitive drum  23  in the light beam scanning device  21  is bent in the not-shown housing of the light beam scanning device  21  by plural not-shown mirrors and the like. Besides, curvatures of the imaging lenses  91  and  92  in the main scanning direction and the sub-scanning direction, and the light path between the polygon mirror  80  and the photosensitive drum  23  are optimized, so that the imaging lenses  91  and  92  and at least one of not-shown mirrors can also be integrally formed. 
     Subsequently, the structure of the light beam scanning device of the embodiment and its operation will be described in detail. 
     In the light beam scanning device  21  shown in  FIG. 3  and  FIG. 4 , the divergent laser beam L emitted from the laser array element  71  is converted into a converging light beam, a parallel light beam or a divergent light beam in cross-sectional beam shape by the lens  72 . 
     The laser beam L whose cross-sectional beam shape has been converted into a specified shape passes through the aperture  73  so that it is adjusted to have an optimum light flux width and light amount, and specified convergence is given only in the sub-scanning direction by the cylindrical lenses  74  and  75 . By this, the laser beam L has a linear shape extending in the main scanning direction on each reflecting surface of the polygon mirror  80 . 
     The polygon mirror  80  is, for example, a regular octahedron, and is formed so that the diameter Dp of its inscribed circle becomes 34.64 mm. 
     The laser beam L guided by each of the plural reflecting surfaces arranged on the outer peripheral surface of the polygon mirror  80  and continuously reflected and linearly scanned (deflected) by the rotation of the polygon mirror  80  is given a specified imaging characteristic by the imaging lenses  91  and  92  of the imaging optical system  90  so that the cross-sectional beam diameter becomes roughly uniform at least in the main scanning direction on the photosensitive drum  23  (image surface), and an image is formed substantially linearly on the surface of the photosensitive drum  23 . Besides, a correction is performed by the imaging lens  91  so that the rotation angle of each reflecting surface of the polygon mirror  80  and the imaging position of the light flux imaged on the photosensitive drum  23 , that is, the scanning position have a proportional relation. Accordingly, the scanning speed of the light flux linearly scanned in the main scanning direction on the photosensitive drum  23  becomes constant in the whole scanning range by the imaging lens  91 . 
     Incidentally, the imaging lenses  91  and  92  are given curvatures (sub-scanning direction curvatures) capable of also correcting a shift of a scanning position in the sub-scanning direction caused by a fact that each reflecting surface of the polygon mirror  80  is unparallel to the sub-scanning direction (that is, a slant occurs in each reflecting surface). Further, the curvature of field in the sub-scanning direction is also corrected. In order to correct these optical characteristics, the curvatures of the imaging lenses  91  and  92  in the sub-scanning direction are set so that they are changed according to the scanning position. 
     In the case where the shape of a lens surface is expressed in a coordinate system as shown in  FIG. 5 , the shape of the lens surface of the imaging lenses  91  and  92  is defined by, for example, 
                   X   =           CUY   *     y   2       +     CUZ   *     z   2           1   +       1   -     AY   *     CUY   2     *     y   2       -     AZ   *     CUZ   2     *     z   2               +       ∑     n   =   0       ⁢       ∑     m   =   0       ⁢       A   mn     ⁢     y   m     ⁢     z     2   ⁢   n                       (   3   )               
Incidentally,  FIG. 6(A)  is a view showing an example of coordinate data indicating the lens surface shape of the imaging lens (fθ 1  lens)  91  at the incident surface side,  FIG. 6(B)  is a view showing an example of coordinate data indicating the lens surface shape of the imaging lens (fθ 1  lens)  91  at the outgoing surface side,  FIG. 6(C)  is a view showing an example of coordinate data indicating the lens surface shape of the imaging lens (fθ 2  lens)  92  at the incident surface side, and  FIG. 6(D)  is a view showing an example of coordinate data indicating the lens surface shape of the imaging lens (fθ 2  lens)  92  at the outgoing surface side.
 
     By using the imaging lens  91  as stated above, since the rotation angle θ of each reflecting surface of the polygon mirror  80  is substantially proportional to the position of the laser beam L imaged on the photosensitive drum  23 , the position where the laser beam L is imaged on the photosensitive drum  23  can be corrected. 
     Besides, the imaging lenses  91  and  92  can correct the position shift in the sub-scanning direction which is caused by the deviation of inclination in the sub-scanning direction among the respective reflecting surfaces of the polygon mirror  80 , that is, the fluctuation in the amount of surface slant. In detail, the polygon mirror surface and the photosensitive drum  23  are made to have a substantially optically conjugate relation by the imaging lenses  91  and  92 , and accordingly, even in the case where the inclination defined between an arbitrary reflecting surface of the polygon mirror  80  and the rotation axis of the polygon mirror  80  is different (between the respective reflecting surfaces), the shift of the scanning position, in the sub-scanning direction, of the laser beam L guided on the photosensitive drum  23  can be corrected. 
     Incidentally, since the cross-sectional beam diameter of the laser beam L depends on the wavelength of the light flux L emitted by the semiconductor laser element  71 , it is made 655 nm, or 630 nm, or a shorter wavelength so that the cross-sectional beam diameter of the laser beam L can be made further small. 
     The return mirror after the deflection is constructed of a plane surface. That is, the surface slant correction is performed by only the fθ lens. The fθ lens may be, for example, a toric lens which has a rotation symmetric axis with respect to the main scanning axis and in which the curvature in the sub-scanning direction varies according to the scanning position. By doing so, the reflective power in the sub-scanning direction varies according to the scanning position, and scanning line bending can be corrected. Further, in the case where the curved surface in the sub-scanning direction has the rotation symmetric axis, the degree of freedom of curvature in the sub-scanning direction is extended and the correction can be performed with higher accuracy. 
     Next, the details of the structure of the pre-deflection optical system in the light beam scanning device of the embodiment and its effect will be described. 
     It is necessary that at least one of the lenses  74  and  75  in the pre-deflection optical system  70  is a cylindrical lens having a power only in the sub-scanning direction in order to correct the surface slant of the polygon mirror  80 . 
     As described above, the pre-deflection optical system  70  in this embodiment includes the lens  72  (first optical system) that weakens the degree of divergence of the divergent light beams from the plural light sources or converts them into the parallel light beams or converging light beams, the lens  74  (second optical system) made of at least one lens having a negative power in the rotation axis direction of the rotary deflector, and the lens  75  (third optical system) having a positive power in the rotation axis direction of the rotary deflector. 
       FIG. 7  is a view showing combinations of shapes of two cylindrical lenses to form an image of a light flux linearly on the reflecting surface of the polygon mirror  80  and results of an evaluation function for optimization. Here, the damped least squares method is used for the optimization. From  FIG. 7 , it is understood that the result of the evaluation function is best when the first (upstream side in the light axis OR direction of the pre-deflection optical system) cylindrical lens  74  has a negative power and the second (downstream side in the light axis OR direction of the pre-deflection optical system) cylindrical lens  75  has a positive power. 
     From the result shown in  FIG. 7 , this embodiment adopts the structure in which the result of the evaluation function is best and the cylindrical lens  74  (second optical system) is a single lens having a negative power at the incident surface side of the light flux and a flat surface at the outgoing surface side, and the cylindrical lens  75  (third optical system) is a single lens having a positive power at the incident surface side and a flat surface at the outgoing surface side. Here, the radius of curvature of the incident surface of the cylindrical lens  74  has the negative power and is, for example, −7.79 mm, and the radius of curvature of the incident surface of the cylindrical lens  75  has the positive power and is, for example, 11.49 mm. That is, the structure is such that the cylindrical lens  74  (second optical system) has the stronger power than the cylindrical lens  75  (third optical system). 
     As stated above, in the so-called multi-beam optical system in which the light source includes plural light sources, the structure is made such that the second optical system having the negative power is arranged in the pre-deflection optical system, so that a desired optical magnification can be obtained by the cooperation of the second optical system and the third optical system. That is, by this structure, in the multi-beam optical system, it becomes possible to realize that (1) the beam diameter is narrowed to a desired width in the sub-scanning direction in order to perform the surface slant correction, (2) the beam pitch on the image surface is made a desired interval, and (3) the focal position of the beam in the sub-scanning direction is made the image surface position, and a contribution can be made to the improvement of the optical characteristic as the light beam scanning device. 
     Subsequently, an adjustment method of a beam interval in the pre-deflection optical system of the light beam scanning device of the embodiment will be described.  FIG. 8  to  FIG. 10  are schematic views for explaining the beam interval adjustment in the pre-deflection optical system  70 . 
     In the case of an over illumination type, since the width of a light flux is changed by a scanning angle, the fluctuation in the light amount on the image surface becomes large.  FIG. 10  shows the width of the light flux reflected by the polygon mirror in the case where the beam incident on the polygon mirror and the light axis of the post-deflection optical system form an angle on the main scanning plane surface (case of not 0°). Here, when the reflection widths at the incident side (a) to the polygon mirror, at the center position (b) in the scanning area, and at the opposite side (c) to the incident side are made D 1 , D 2  and D 3 , D 1 &gt;D 2 &gt;D 3  is established, and the fluctuation occurs in the light amount on the image surface. Besides, the figure shows also the intensity distribution of the light flux before the incidence on the polygon mirror. Since the beam widths at (a), (b) and (c) are different, and the laser beam is such that the intensity distribution of the light flux to be used has the Gaussian distribution, Sa&gt;Sc and Sb&gt;Sc are established. Since the light amount becomes a volume of a portion used in the beam intensity distribution, when the light amount on the image surface is made Pa, Pb and Pc, Pa&gt;Pc and Pb&gt;Pc are established. Thus, the fluctuation in the light amount in the over illumination optical system is larger than that in the under illumination optical system. 
     In order to adjust the interval of light fluxes from plural light sources in the sub-scanning direction on the image surface, when the LD array (semiconductor laser element  71 ) is rotation adjusted, an optical loss at the aperture  73  becomes large in the under illumination optical system. Further, in the over illumination optical system, in order to reduce the fluctuation of the light amount due to the difference of position used in the Gaussian distribution according to the deflection position, it is necessary to increase the main scanning beam diameter on the reflecting surface of the polygon mirror  80 . In this case, when the LD array is rotated, the use area is very small and the optical efficiency becomes poor. Here, when the output of the LD array is increased, the influence of crosstalk becomes high, and therefore, an increase in output is difficult. Thus, there is a problem that when the rotation adjustment of the LD array is performed, the image surface energy amount becomes insufficient. 
     Then, in this embodiment, the structure is made such that in the optical parts constituting the pre-deflection optical system, the cylindrical lens  75  (third optical system) having the positive power is supported by a support part (support means)  75   h  movably in the optical axis direction, and the cylindrical lens  75  is moved in the optical axis direction, so that the beam pitch is adjusted. 
       FIG. 11(A)  and  FIG. 11(B)  are views showing an example of the support part  75   h .  FIG. 11(A)  is a plan view in which the support part to support the cylindrical lens  75  is seen from above, and  FIG. 11(B)  is a view in which a section in a vertical surface including the optical axis is seen from a direction parallel to the main scanning direction. As shown in the figures, the support part  75   h  in this embodiment presses the cylindrical lens  75  to a holder  75   a  and supports it by a plate spring  75   s  fixed to the holder  75   a  of, for example, aluminum by a screw  75   n . Two long holes  75   m  extending in the optical axis OR direction are formed in a base portion extending in the optical axis direction from the lower part of the holder  75   a , and the support part  75   h  is fixed to a not-shown housing of the light beam scanning device  21  by screws  75   w  inserted in these long holes. 
       FIG. 12  is a view showing a change in beam interval in the case where one of the finite lens  72 , the cylindrical lens  74  and the cylindrical lens  75  is moved in the optical axis direction by 0.5 mm, and  FIG. 13  is a view showing a wave aberration in the case where one of the finite lens  72 , the cylindrical lens  74  and the cylindrical lens  75  is moved in the optical axis direction by 0.5 mm. 
     From  FIG. 12  and  FIG. 13 , it is understood that when the adjustment of the beam pitch is performed, it is most effective for the adjustment of the beam pitch to move the cylindrical lens  75  having the positive power, and further, a bad influence on an optical characteristic such as a wave aberration becomes smallest. Incidentally, it is conceivable that since the power of the cylindrical lens  75  is weaker than the power of the cylindrical lens  74 , the wave aberration is small in the case where the cylindrical lens  75  having the positive power is moved. From the foregoing reason, in this embodiment, the cylindrical lens  75  having the positive power is moved in the light axis OR direction, and the beam pitch is adjusted. 
       FIG. 14  is a view showing another example of the support part to support the cylindrical lens  75  movably in the optical axis OR direction. In the drawing, the same portions as the portions shown in  FIG. 11  are denoted by the same signs. The figure is a view in which the support part to support the cylindrical lens  75  is seen from the side. Similarly to the foregoing  75   h , a support part  75   h ′ shown in the figure presses the cylindrical lens  75  to a holder  75   a ′ and supports it by a plate spring  75   s  fixed to the holder  75   a ′ of aluminum by a screw  75   n . With respect to the holder  75   a ′, both side surfaces and a bottom surface of a base portion extending from its lower part in the optical axis OR direction are guided by a guide surface gs and a guide surface gb, and it is movable in the optical axis OR direction. The holder  75   a ′ is fixed to the guide surface gs and the guide surface gb at a desired position by a well-known fixing method such as screwing, bonding or pressing. 
     Incidentally, in the foregoing embodiment, although the structure has been described in which the second optical system having the negative power in the pre-deflection optical system is the single cylindrical lens having the negative power, limitation is not made to this, and for example, the structure can also be made such that the second optical system having the negative power is made of plural cylindrical lenses having negative power. 
       FIG. 15  is a view showing a structural example in which a cylindrical lens  74   a  having a negative power in the sub-scanning direction and a cylindrical lens  74   b  are arranged between the aperture  73  and the cylindrical lens  75  in the structure of the pre-deflection optical system shown in  FIG. 4 . As stated above, when the second optical system is constructed of plural lenses having the negative power, as compared with the case where the second optical system is constructed of a single lens, the power of each lens can be made small. By this, the lens constituting the second optical system can be made to be easily processed, the improvement in processing accuracy of the lens surface can also be expected, and the second optical system can be processed at low cost and high accuracy. 
     As stated above, according to this embodiment, the beam interval on the image surface can be narrowed without increasing the rotation angle around the light beam traveling direction of the laser array, and it is possible to provide the optical apparatus which can prevent the optical efficiency from decreasing and the beam diameter from increasing. 
     Although the invention has been described in detail while using the specific mode, it would be obvious for one skilled in the art that various modifications and improvements can be made without departing from the spirit and scope of the invention. 
     As described above in detail, according to the invention, in the light beam scanning device adopting the multi-beam optical system, it is possible to provide a technique which can realize scanning by a light flux having a desired optical characteristic.