Abstract:
A light scanning device, including a light source including plural light emitting elements arranged linearly, a deflection section that deflects plural light beams emitted from the light source to scan a surface to be scanned, a photosensor that receives at least one of the plural light beams that are deflected by the deflection section, a signal generation section that generates a signal when a light energy amount received at the photosensor reaches a predetermined amount, and a control section that starts scanning of the surface to be scanned by each light beam after a predetermined amount of time passes from a point in time when the signal is generated by the signal generation section, the light receiving surface of the photosensor being inclined to receive light beams emitted from at least two light emitting elements among the plural light emitting elements, is provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims priority under 35 USC 119 from Japanese Patent Application No. 2005-216541, the disclosure of which is incorporated by reference herein. 
   BACKGROUND 
   1. Technical Field 
   The present invention relates to a light scanning device for scanning a surface to be scanned by deflecting plural light beams emitted from a light source by a deflection section. 
   2. Related Art 
   In an image forming device in an electronographic system, for high resolution and high speed, a light scanning device has been widely used, which simultaneously emits plural light beams from a light source in which plural light emitting elements such as a semiconductor laser is arranged two-dimensionally and deflects them on the same deflection surface to scan a photoreceptor at the same time by plural light beams simultaneously. In addition, as a light source of this light scanning device, a Vertical Cavity Surface Emitting Laser (a so-called VCSEL) has been widely used because of its high degree of freedom in arrangement of the semiconductor laser and low manufacturing cost. 
   In the light scanning device, generally, a photodetector is arranged so that light beam at starting of a main scanning can enter upon. This photodetector generates a scanning onset signal (hereinafter, called as a SOS signal) in accordance with timing of detection of the scanned beam and a driving circuit of a light source controls a start position of the main scanning based on a SOS signal generated by the photodetector. 
   As shown in  FIG. 9 , as a photodetector, one configured by a photodiode applying a current depending on a light incident amount, an amplifier OP for perform I (current)/V (voltage) conversion by amplifying the inputted current, a threshold power source SP for generating a voltage indicating a threshold, and a comparator CP for comparing each output voltage of the amplifier OP and the threshold power source SP has been widely known. In this photodetector, the SOS signal is made a high level when the output voltage of an amplifier OP is not less than a threshold. 
   It is general that a single mode oscillation (oscillation in a single wave length) is required for the light scanning device in order to obtain a minute beam spot, however, if the single mode oscillation is made in the VCSEL, there is a tendency that the light emission output is small. Therefore, in the case of scanning a photodiode PD by lighting only one VCSEL, the light energy amount received by the photodiode PD is small. So it may be required that the amplification gain of the photodiode PD is increased or the threshold voltage is decreased. However, in this case, this makes the scanning easily affected by the noise. Therefore, a method to increase a light energy amount received by the photodiode PD by lighting plural VCSELs of which positions in the main scanning directions are close to each other and scanning the photodiode PD is devised. 
   According to this method, when there is no displacement in the positions in the main scanning directions of plural VCSELs (Δ=0 μm), plural light beams emitted from the plural VCSELs at the same time enters the photodiode PD at the same time, so that, as shown in a graph of  FIG. 10 , a received light energy profile is formed, which has one rising and one falling and has the maximum value larger than the light emission energy amount of each VCSEL. Therefore, there are only two cross points between the received light energy profile and an energy level corresponding to a threshold vale of generation of a SOS signal without raising the amplification gain of the photodiode PD or lowering the threshold voltage, and this makes it possible to generate a SOS signal stably. Further, the graph of  FIG. 10  shows a received light energy profile in a main scanning direction in the photodiode PD when three VCSELs having Gaussian distribution with a beam diameter of 60 μmare lighted at the same time to scan the photodiode PD. 
   However, if there is a displacement in the positions of the main scanning directions of plural VCSELs (Δ=100 μm), there is a difference in times that plural light beams emitted from the plural VCSELs at the same time enter the photodiode PD. Therefore, as shown in the graph of  FIG. 10 , rising and falling are repeated for each light beam and the received light energy profile of which the maximum value is substantially equivalent to the light emission energy amount of each VCSEL is formed. Therefore, in order to make only two cross points between the received light energy profile and the energy level corresponding to the threshold vale of generation of the SOS signal, the amplification gain of the photodiode PD should be increased or the threshold voltage should be increased, and this makes the affect of the noise easy to receive. 
   SUMMARY 
   An aspect of the invention is a light scanning device including: a light source including plural light emitting elements that are arranged linearly; a deflection section that deflects plural light beams emitted from the light source to scan a surface to be scanned; a photosensor that receives at least one of the plural light beams that are deflected by the deflection section; a signal generation section that generates a signal when an amount of light energy received at the photosensor reaches a predetermined amount; and a control section that starts scanning of the surface to be scanned by each light beam after a predetermined time passes from a point in time when the signal is generated by the signal generation section; wherein a light receiving surface of the photosensor is inclined so as to receive light beams emitted from at least two light emitting elements among the plural light emitting elements. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the invention will be described in detail with reference to the following figures, wherein: 
       FIG. 1  is a perspective view showing a light scanning device according to a first exemplary embodiment; 
       FIG. 2  is a schematic diagram explaining the operation of the light scanning device according to the first exemplary embodiment; 
       FIG. 3  is a graph showing a received light energy amount profile on a light receiving surface of a photosensor of the light scanning device according to the first exemplary embodiment; 
       FIGS. 4A ,  4 B, and  4 C show a sensor adjusting mechanism of the photosensor of the light scanning device according to the first exemplary embodiment;  FIG. 4A  is a front view;  FIG. 4B  is a side view; and  FIG. 4C  is a plan view; 
       FIG. 5  is a block diagram showing the configuration of an exposure control section of the light scanning device according to the first exemplary embodiment; 
       FIG. 6  is a timing chart showing the state of a substantial part signal of the exposure control section according to a second exemplary embodiment; 
       FIG. 7  is a perspective view showing a light scanning device according to the second exemplary embodiment; 
       FIGS. 8A ,  8 B, and  8 C show a sensor adjusting mechanism of a photosensor of the light scanning device according to the second exemplary embodiment;  FIG. 8A  is a front view;  FIG. 8B  is a side view; and  FIG. 8C  is a plan view; 
       FIG. 9  is a circuit diagram showing a circuit configuration example of a photodetector; and 
       FIG. 10  is a graph showing a received light energy amount profile on a light receiving face of a conventional light scanning device. 
   

   DETAILED DESCRIPTION 
   With reference to the drawings, the exemplary embodiments of the invention will be described below. 
   First Exemplary Embodiment 
   As shown in  FIG. 1 , a light scanning device  10  according to the first exemplary embodiment is provided with a VCSEL array  16  having plural VCSELs  16 A arranged two-dimensionally. At the side of light emission of the VCSEL array  16 , a collimator lens  36 , a cylindrical lens  38 , and a polygon mirror  40  are disposed in that order, and further, at the side of light deflection of the polygon mirror  40 , an fθ lens  44  and a photoreceptor  46  are disposed in that order. 
   The laser beams emitted from the VCSEL array  16  are made to be approximately parallel beam by the collimator lens  36 . This laser beam is converged in the sub scanning direction by the cylindrical lens  38  to be focused on a reflection surface of the polygon mirror  40 . Then, it is deflected by the rotation of the polygon mirror  40  to be imaged on the photoreceptor  46  via the fθ lens  44 . Further, the main scanning is carried out by rotation in an arrow A direction of the polygon mirror  40  and the sub scanning is carried out by rotation in an arrow B direction of the photoreceptor  46 . 
   On the other hand, a reflection mirror  48  is provided at a position where the main scanning is started by the laser beam. Further, in the reflection direction of this reflection mirror  48 , a lens  49  having a positive power in the sub scanning direction and a photosensor  50  configured by a photodiode are provided so that the laser beam when the main scanning is started enters the photosensor  50  passing through the lens  49 . Then, in accordance with incidence of the laser beam in the photosensor  50 , a SOS signal is generated by a photodetector for a SOS signal detection  52  (refer to  FIG. 5 ) which will be described later. 
   The photodetector for SOS signal detection  52  according to this exemplary embodiment is configured in the same way as the photodetector shown in  FIG. 9 . Here, the photodiode PD shown in  FIG. 9  corresponds to the photosensor  50  of this exemplary embodiment. 
   In addition, the reflection surface of the polygon mirror  40  and the light receiving surface  50 A of the photosensor  50  are in a conjugate relation. Even if optical surface tangle of the reflection surface of the polygon mirror  40  occurs, the incidence position of the light beam in the light receiving surface  50 A of the photosensor  50  is not shifted in the sub scanning direction. 
   As shown in  FIG. 2 , in the light scanning device  10  according to this exemplary embodiment, the photosensor  50  is scanned in a state of a VCSEL group (hereinafter, referred to as “a SOS detection group”) being lighted, the group emitting light beams which beam spots formed on the photoreceptor  46  are arranged linearly, and then, the SOS signal is generated in accordance with the output from the photosensor  50  depending on the scanning. 
   As shown in  FIG. 1  and  FIG. 2 , the plural VCSELs  16 A (for example, three as shown in  FIG. 2 ) configuring the SOS detection group are arranged on a line inclined toward a side of the downstream of the main scanning direction (a direction parallel to a surface(s) at which each members are disposed shown in  FIG. 1 ) with respect to the sub scanning direction (a direction perpendicular to the surface(s) at which each members are disposed shown in  FIG. 1 ). The photoreceptor  46  is scanned by row of the light beams inclined to the downstream of the main scanning direction with respect to the sub scanning direction. On the other hand, the light receiving surface  50 A of the photosensor  50  is scanned by row of the light beams inclined toward the upstream of the scanning direction in the light receiving surface  50 A with respect to the sub scanning direction. Further, since the light beams are reflected by the reflection mirror  48  to enter the light receiving surface  50 A of the photosensor  50 , in the photoreceptor  46  and the light receiving surface  50 A of the photosensor  50 , the scanning directions thereof are inverted and the inclined directions thereof with respect to the sub scanning direction of the row of the light beams to be scanned are inverted. 
   Here, as shown in  FIG. 2 , the light receiving surface  50 A of the photosensor  50  is formed in the shape of a rectangle in which a direction inclined toward the side of the upstream in the scanning direction on the light receiving surface  50 A with respect to the sub scanning direction is a longitudinal direction. In addition, the longitudinal direction of the light receiving surface  50 A and the rows of the VCSELs  16 A are made to be parallel to each other. In other words, an edge at the side of the upstream of the scanning direction of the light receiving surface  50 A is made to be parallel to the row of the VCSELs  16 A. Therefore, as compared to a case in which the longitudinal direction of the light receiving surface  50 A of the photosensor  50  is made to be parallel to the sub scanning direction, there is a smaller difference in timings that the plural light beams emitted from the plural VCSELs  16 A configuring the SOS detection group enter the light receiving surface  50 A of the photosensor  50 . 
   Thereby, in the case of lighting the all of the plural VCSELs  16 A configuring the SOS detection group, as shown in the graph of  FIG. 3 , there is a small difference in timings of rising and falling for respective light beams, and the received light energy profile of which maximum value is larger than the light emission energy amount of each VCSEL  16 A is formed on the light receiving surface  50 A of the photosensor  50 . As a result, it is possible to have two cross points between the received light energy profile and the energy level corresponding to the threshold for generating the SOS signal without raising the amplification gain of the photodiode PD or lowering the threshold voltage. 
   Particularly, since the edge at the side of the upstream in the scanning direction of the light receiving surface  50 A is made to be parallel to the row of the VCSELs  16 A, plural light beams emitted from the plural VCSELs  16 A enter the light receiving surface  50 A of the photosensor  50  at the same time. Thereby, a received light energy profile having rising and falling once, of which maximum value is plural times (for example, three times as shown in the drawing) as the light emission energy amount of each VCSEL is formed. 
   Accordingly, the stable SOS signal that is not easily affected by the noise can be generated by the photodetector for SOS signal detection  52 , so that the accuracy for the control of the scanning start position in the main scanning direction at the photoreceptor  46  can be improved. 
   In addition, as shown in  FIG. 1 , the reflection surface of the polygon mirror  40  and the light receiving surface  50 A of the photosensor  50  are in a conjugate relation by the lens  49  disposed between the polygon mirror  40  and the photosensor  50  and having the positive power in the sub scanning direction. As a result, even if the surface tangle of the reflection surface of the polygon mirror  40  occurs, the incidence position of the light beams to the light receiving surface  50 A of the photosensor  50  is not shifted to the sub scanning direction. 
   In addition, as shown in  FIGS. 4A to 4C , the photosensor  50  is supported by a sensor adjusting mechanism  58  on a bottom surface  10 A of a light scanning device housing so as to be adjusted capable of rotating around an optical axis. The sensor adjusting mechanism  58  is configured by a positioning projection  72  that projects from the bottom surface  10 A and faces the lower left side of the photosensor  50 , an adjustment screw  73  that projects from the bottom surface  10 A and faces the lower right side of the photosensor  50 , a plate spring  74  that abuts against the upper surface of the photosensor such that the photosensor  50  and the positioning projection  72  and the adjustment screw  73  are press-contacted, and a supporting mechanism  75  that supports the photosensor  50  so as to be unable to incline to the optical axial direction. 
   The supporting mechanism  75  is configured by a support chip  76  on which a positioning projection  76 A is formed so as to face a center part at the lower side of the surface of the photosensor  50 , a plate spring  77  that abuts against the center part at the lower side of the rear surface of the photosensor  50  such that the photosensor  50  and the positioning projection  76 A are press-contacted, a supporting chip  78  on which a positioning projection  78 A facing the right upper side of the rear surface of the photosensor  50  is formed, and a supporting chip  79  on which a positioning projection  79 A facing the left upper side of the rear surface of the photosensor  50  is formed. 
   The photosensor  50  is supported in a sandwich manner by the positioning projection  76 A and the plate spring  77  at the lower center parts. In addition, the lower center part of the photosensor  50  is biased from the rear side to the front side. When a moment from the front side to the rear side acts on the upper side of the photosensor  50 , the right upper side of the rear surface and the left upper side of the rear surface of the photosensor  50  abut against the positioning projections  78 A and  79 A, respectively. Thereby, the photosensor  50  cannot be inclined to the light axial direction. 
   In addition, the photosensor  50  is supported in a sandwich manner by the positioning projection  72 , the adjustment screw  73 , and the platy spring  74  at the upper and lower surfaces thereof Here, the adjustment screw  73  can adjust the projection amount thereof from the bottom surface  10 A and by adjusting the projection amount from the bottom surface  10 A of the adjustment screw  73 , the light receiving surface  50 A of the photosensor  50  can be rotatably adjusted around the optical axis. Thereby, it is possible to suppress shifting of the incidence position of the light beam to the light receiving surface  50 A of the photosensor  50  that is generated by the optical surface tangle of the reflection surface of the polygon mirror  40  or the like. 
   Here, as a cause of the positional displacements of the beam spot on the photoreceptor  46  and the light receiving surface  50 A of the photosensor  50 , an error of interval of the light emission points on the VCSEL array  16 , an error of properties and an error of attaching positions in the light scanning device  10  may be conceived. 
   However, since the VCSEL array  16  is made by a semiconductor process, the error of intervals of the light emission points on the VCSEL array  16  is not serious. Further, with respect to the error of properties and the error of the attaching positions in the light scanning device  10 , providing a mechanism for adjusting the attaching position in a condition in which each optical member is appropriately designed and manufactured, it is possible to obtain a beam spot position approximately as calculated although there is a positional displacement to some extent. 
   Therefore, in a case of generating a SOS signal using only one group (a G 1  group) as shown in  FIG. 2  as a SOS detection group, an image with a sufficient quality can be obtained when the light scanning device  10  is applied to the image forming device by adding or subtracting a timing correction time HT obtained by the following formula (1) to or from lighting timing of the light beam of the other gropes (here, a G 2  group and a G 3  group) that are offset from the SOS detection group in the main scanning direction so that the beam position offset in design is corrected with respect to timing derived on the basis of the generated SOS signal.
 
 HT=OD/SS   (1)
 
   Where OD is a beam spot offset distance on the photoreceptor  46  and SS is a beam spot scanning speed on the photoreceptor  46 . 
     FIG. 5  shows the configuration of an exposure control section  90  of the light scanning device  10  according to the first exemplary embodiment. Here, the VCSELs in the VCSEL array  16  are configured so as to correspond to the beam spots shown in  FIG. 2 , namely, the VCSELs in the VCSEL array  16  are configured in such a manner that three VCSEL groups each having three VCSELs arranged at approximately even intervals on a line along the sub scanning direction that is perpendicular to the main scanning direction are arranged, so that the sub scanning directional position of each VCSEL is displaced with each other, along the main scanning direction. Further, assuming that respective VCSEL groups are a GI group, a G 2  group, and a G 3  group, the SOS signal is generated by the light beams from the VCSEL group only, which is the G 1  group. 
   As shown in  FIG. 5 , the exposure control section  90  according to the first exemplary embodiment includes a photodetector for SOS signal detection  52 , a video signal output circuit  60 , and a VCSEL driving circuit  70 . 
   As described above, the photodetector for SOS signal detection  52  is configured in the same way as the photodetector shown in  FIG. 9 . 
   The video signal output circuit  60  is configured by an oscillator  62 , a clock phase synchronous circuit  64 , a counter circuit  66 , a timing circuit  68 , plural video memories MG  1 - 1  to MG  3 - 3 , and a SOS detection group lighting control circuit  69 . Further, the video memories MG  1 - 1  to MG  1 - 3  correspond to respective VCSELs belonging to the G 1  group, the video memories MG  2 - 1  to MG  2 - 3  correspond to respective VCSELs belonging to the G 2  group, and the video memories MG  3 - 1  to MG  3 - 3  correspond to respective VCSELs belonging to the G 3  group. 
   The SOS detection group lighting control circuit  69  outputs the signals, that can light all VCSELs belonging to SOS detection group (the G 1  group according to this exemplary embodiment), to the VCSEL driving circuit  70  for a time period that the light beams can be incident to the reflection mirror  48 . Thereby, plural light beams emitted from the VCSELs belonging to the SOS detection group are incident to the photosensor  50  and a SOS signal (refer to  FIG. 6 ) depending on a light amount level of the plural light beams is generated by the photodetector for SOS signal detection  52 . 
   In addition, in the video signal output circuit  60 , the SOS signal generated by the photodetector for SOS signal detection  52  and a clock signal generated by the oscillator  62  are inputted in the clock phase synchronous circuit  64 , and a video clock signal in synchronization with rising timing of the SOS signal is outputted. 
   To the counter circuit  66 , the SOS signal and the video clock signal are inputted, in the counter circuit  66 , a number of video clock is counted as the elapsed time from rising of the SOS signal, and a count signal indicating a count value is outputted to the timing circuit  68 . 
   The timing circuit  68  generates an LS 1  signal that becomes a high level when a time TO shown in  FIG. 6  has passed and becomes a low level after a predetermined time for allowing reading of video signal shown in  FIG. 6  passed, on the basis of the count signal inputted from the counter circuit  66 , and then, the timing circuit  68  output the signal. 
   Each of the video memories MG  1 - 1  to MF  3 - 3  is structured by a FIFO (First-In First-Out) memory, and, on the basis of the image data, a video signal for lighting each VCSEL beam that is transformed by a video signal processor (not illustrated) is stored. 
   When the LS 1  signal becomes the high level, it is inputted in the video memories MG  1 - 1  to MG  1 - 3  corresponding to the respective VCSELs of the G 1  group shown in  FIG. 2  as a reading allowing signal, video signals SG  1 - 1  to SG  1 - 3  for the VCSELs belonging to the G 1  group are outputted from the video memories MG  1 - 1  to MG  1 - 3  in synchronization with the video clock signal, and when each video signal is ON, the VCSEL driving circuit  70  lights the corresponding VCSELs. 
   Here, it is necessary to delay the lighting timings of the VCSELs belonging to the G 2  group and the lighting timings of the VCSELs belonging to the G 3  group in  FIG. 2 , with respect to the lighting timing of the VCSELs belonging to the G 1  group, by amounts corresponding to an offset OF 1  and an offset OF 2  shown in  FIG. 2  respectively. 
   As shown in  FIG. 6 , each delay time is obtained from the following formulas (2) and (3).
 
Delay time  T 1 of  G 2 group= OF 1/scanning speed  (2)
 
Delay time  T 2 of  G 3 group= OF 2/scanning speed  (3)
 
   Therefore, in order to expose by the beams of the VCSELs belonging to each of the G 2  group and the G 2  group at a predetermined position, outputting a LS 2  signal in which the LS 1  signal is delayed by the delay time T 1  and a LS 3  signal in which the LS 1  signal is delayed by the delay time T 2  from the timing circuit  68 , each signal is applied as “a video memory reading allowing signal of a G 2  group” and “a video memory reading allowing signal of a G 3  group”. 
   Then, in the same way as the lighting order of the VCSELs belonging to the G 1  group, the VCSEL corresponding to each video signal is lighted. 
   The VCSEL array  16  corresponds to the light source in an aspect of the invention; the VCSEL  16 A corresponds to the light emitting element in the aspect of the invention; the photosensor  50  corresponds to the photosensor in the aspect of the invention, the photodetector for SOS signal detection  52  corresponds to the generation section in the aspect of the invention; and the SOS detection group lighting control circuit  69  corresponds to the control section in the aspect of the invention. 
   Further, the explanation of this exemplary embodiment is given assuming that the SOS detection group is a group which scans the photodetector at the earliest timing, however, the invention is not limited to this and even if the SOS detection group is defined as the other group, there is no problem if the timing circuit  68  is appropriately set. 
   In addition, the explanation of this exemplary embodiment is given assuming that the video memory reading allowing signal is delayed by the timing circuit  68  in unit of a video clock signal, however, the present invention is not limited to this and the beam spot position of the VCSELs belonging to the G 2  group and the G 3  group can be controlled with a higher degree of accuracy by providing a fine adjusting mechanism using delaying by an analog element or a logic gate. 
   In addition, the explanation of this exemplary embodiment is given assuming that the correction of the beam position offset with respect to the G 2  group and the G 3  group from the G 1  group is made in accordance with a designed value, however, the invention is not limited to this and it is obvious that the invention is made so as to make the correction in accordance with a measured value. 
   Second Exemplary Embodiment 
   According to the first exemplary embodiment, the photosensor  50  is inclined to the downstream side in the main scanning direction with respect to the sub scanning direction so as to make the light receiving surface  50 A of the photosensor  50  in parallel with the rows of the VCSELs  16 A configuring the SOS detection groups. However, according to this exemplary embodiment, the light beams are incident to the light receiving surface  50 A of the photosensor  50  after rotating the row of the light beams emitted from the VCSELs  16 A configuring each SOS detection group around the optical axis to decrease an inclined angle with respect to the sub scanning direction. 
   As shown in  FIG. 7 , in the light scanning device  100  of this exemplary embodiment, an anamorphic lens  80  is disposed in place of the above-described lens  49 . As shown in  FIGS. 8A to 8C , the anamorphic lens  80  is supported on the bottom surface  10 A of the light scanning device housing so as to be rotatably adjusted around the optical axis by a lens adjusting mechanism  81 . The lens adjusting mechanism  81  is configured by a positioning projection  82  that projects from the bottom surface  10 A and faces the lower left side of the anamorphic lens  80 , an adjustment screw  83  that projects from the bottom surface  10 A and faces the lower right side of the anamorphic lens  80 , a plate spring  84  that abuts against the upper surface of the anamorphic lens  80  such that the anamorphic lens  80  and the positioning projection  82  and the adjustment screw  83  are press-contacted, and a supporting mechanism  85  for supporting the anamorphic lens  80  so as not to be unable to incline in the optical axial direction. 
   The supporting mechanism  85  is configured by a support chip  86  on which a positioning projection  86 A is formed so as to face a center part at the lower side of the surface of the anamorphic lens  80 , a plate spring  87  that abuts against the center part at the lower side of the rear surface of the anamorphic lens  80  such that the anamorphic lens  80  and the positioning projection  86 A are press-contacted, a supporting chip  88  on which a positioning projection  88 A which faces the right upper side of the rear surface of the anamorphic lens  80  is formed, and a supporting chip  89  on which a positioning projection  89 A which faces the left upper side of the rear surface of the anamorphic lens  80  is formed. 
   The anamorphic lens  80  is supported in a sandwich manner by the positioning projection  86 A and the plate spring  87  at the lower center parts. In addition, the lower center part of the anamorphic lens  80  is biased from the rear side to the front side. When a moment from the front side to the rear side acts on the upper side of the anamorphic lens  80 , the right upper side of the rear surface and the left upper side of the rear surface of the anamorphic lens  80  abut against the positioning projections  88 A and  89 A, respectively. Thereby, the anamorphic lens  80  cannot be inclined to the light axial direction. 
   In addition, the anamorphic lens  80  is supported in a sandwich manner by the positioning projection  82 , the adjustment screw  83 , and the platy spring  84  at the upper and lower surfaces thereof Here, the adjustment screw  83  can adjust the projection amount thereof from the bottom surface  10 A and by adjusting the projection amount from the bottom surface  10 A of the adjustment screw  83 , the anamorphic lens  80  can be rotatably adjusted around the optical axis. 
   Here, by adjusting the anamorphic lens  80  in a rotating manner, the imaging position of the light beam on the light receiving surface  50 A of the photosensor  50  is adjusted, however, according to this exemplary embodiment, after plural light beams emitted from plural VCSELs  16 A configuring the SOS detection group pass through the anamorphic lens  80 , the inclined angle to the sub scanning direction of the row of the light beams is decreased, and further, the row of the light beams is adjusted to be made to be parallel to the longitudinal direction of the light receiving surface  50 A of the photosensor  50 . 
   Therefore, as compared to a case that the light beams are incident to the light receiving surface  50 A of the photosensor  50  in a state in which the row of the light beams emitted from the plural VCSELs  16 A configuring the SOS detection group is inclined to the sub scanning direction, a difference in timings that the plural light beams enter the light receiving surface  50 A of the photosensor  50  is smaller. In the meantime, each member configuring the lens adjusting mechanism  81  is arranged so as not to intercept the light beams progressing to the photosensor  50  or the photoreceptor  46 . 
   Thereby, when lighting all of the VCSELs  16 A configuring the SOS detection group, as shown in the graph of  FIG. 3 , a difference in timings of rising and falling of the respective light beams is smaller and the received light energy profile of which maximum value is larger than the light emission energy of each VCSEL  16 A is formed on the light receiving surface  50 A. As a result, it is possible to have two cross points between the received light energy profile and the energy level corresponding to the threshold for generating the SOS signal without raising the amplification gain of the photodiode PD or lowering the threshold voltage. 
   Particularly, since the edge at the side of the upstream in the main scanning direction of the light receiving surface  50 A is made to be parallel to the row of the light beams scaning the light receiving surface  50 A, plural light beams emitted from the plural VCSELs  16 A enter the light receiving surface  50 A of the photosensor  50  at the same time. Thereby, a received light energy profile having rising and falling once, of which maximum value is plural times (for example, three times as shown in the drawing) as the light emission energy amount of each VCSEL is formed. 
   Accordingly, the stable SOS signal that is not easily affected by the noise can be generated by the photodetector for SOS signal detection  52 , so that the accuracy for control of the scanning start position in the main scanning direction at the photoreceptor  46  can be improved. 
   The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The exemplary embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.