Abstract:
An image forming apparatus includes: a plurality of light-emitting elements, each light-emitting element receiving a driving current from a power supply and converting the current into a beam; a plurality of driving elements, each driving element being interposed between the power supply and each light-emitting element and controlling the driving current based on an image signal; and a photoconductor on which the beams from the plurality of light-emitting elements form an image by scanning, wherein each light-emitting element is positioned so immediately adjacent to each driving element that a power leakage factor floating therebetween is reduced, whereby an intensity of the beam is substantially equivalent to the image signal in waveform.

Description:
BACKGROUND OF THE INVENTION 
     1.Field of the Invention 
     The present invention relates to an image forming apparatus, and in particular, to a multi-beam image forming apparatus such as a copier or a printer that forms an image with a plurality of laser beams for improvement in image forming speed. 
     2.Description of the Related Art 
     Due to a need for printing a high-quality image with high resolution and at high speed, a multi-beam image forming apparatus, which is made to scan in parallel the surface of a drum photoconductor by a plurality of laser beams, is being tested for realization in a latest image forming apparatus of electrophotographic type. Such a multi-beam image forming apparatus has a significant technical problem in practical use. That is, the problem is how the light-and-shade difference among image lines formed by the laser beams can be harmoniously adjusted. 
     More specifically, in the multi-beam image forming apparatus, a distortion occurs in an electric current due to a stray capacitance existing in a driving element supplying a driving electric current to a laser diode, in its electric current passageway and the like, rendering it difficult to adjust an image density. Moreover, since a fluctuation occurs in the image density for each dot and a moire occurs in the formed image, it is difficult to solve the problem of image quality by merely adjusting the driving electric current for each laser diode. 
     SUMMARY OF THE INVENTION 
     The present invention has been made under such circumstances and aims to provide an image forming apparatus, which can decrease the undesirable effect due to a stray capacitance existing in the environments of each driving element, electric current passageways, and the like, and in which the density of image formed by each beam can be easily adjusted by reforming the electric current waveform, and substantially no moiré is produced. 
     Accordingly, the present invention provides an image forming apparatus comprising: a plurality of light-emitting elements, each light-emitting element receiving a driving current from a power supply and converting the current into a beam; a plurality of driving elements, each driving element being interposed between the power supply and each light-emitting element and controlling the driving current based on an image signal; and a photoconductor on which the beams from the plurality of light-emitting elements form an image by scanning, wherein each light-emitting element is positioned so adjacent to each driving element that a power leakage factor floating between the light-emitting element and the driving element is reduced, whereby an intensity of the beam is substantially equivalent to the image signal in waveform. 
     The feature that the light-emitting element is positioned adjacent to the driving element means that the distance between the both elements is designed to be smaller than several centimeters to minimize a wiring from the light-emitting element to the driving element. This can be realized by mounting the light-emitting element and the driving element on a single printed circuit board, for example. Here, the power leakage factor includes at least one of a stray capacitance, a stray conductance, and a stray inductance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an explanatory view showing a basic construction of a laser printer according to an embodiment of the present invention; 
     FIG. 2 is a top view showing an optical system in FIG. 1; 
     FIG. 3 is a perspective view showing a construction of the optical system in FIG. 1; 
     FIG. 4 is a top view of a light source device according to the embodiment of the present invention; 
     FIG. 5 is a rear view of the light source device according to the embodiment of the present invention; 
     FIG. 6 is a view showing an electrical circuit diagram of the light source device according to the embodiment of the present invention; 
     FIG. 7 is a view corresponding to FIG. 6 according to a comparative example; 
     FIG.  8 ( a ) is a view showing a waveform of a video signal supplied to a terminal of a switching element according to the present invention. 
     FIG.  8 ( b ) is a view showing a light intensity waveform of the laser diode in response to the video signal according to a comparative example; 
     FIG.  8 ( c ) is a view showing a light intensity waveform of the laser diode in response to the video signal according to a comparative example; 
     FIG.  8 ( d ) is a view showing a light intensity waveform of the laser diode in response to the video signal according to a comparative example; and 
     FIG.  8 ( e ) is a view showing a light intensity waveform of the laser diode in response to the video signal according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferably, the light-emitting element of the present invention may include a laser diode. 
     The laser diode is, in general, mounted in a metallic package, either or its cathode and anode being connected to the package. Particularly in the present invention, it is desirable to use a laser diode in which the anode is electrically connected to the metallic package. 
     With reference to such a laser diode, the anode is connected to a positive electrode of the power supply, and the cathode is connected to a negative electrode of the power supply through the driving element, thus allowing the metallic package (anode) to be connected to the power supply. Accordingly, because any stray capacitance existing between the metallic package and the ground is always charged always by the power supply, the driving current is not affected by an electric current for charging the stray capacitance. 
     In this case, the driving element is connected to the cathode of the laser diode, and the driving current is switched by a video signal to perform on-off control of the beams. Thus, the video signal is directly reflected upon the laser diode driving current unless there is an extra electric current flowing passageway other than the driving element. 
     Also, in the case where the laser diode further includes a photodiode within the metallic package for monitoring a laser beam emitted by the laser diode, a cathode of the photodiode is preferably connected electrically to the metallic package corresponding to the anode of the laser diode so as to be kept in a reverse bias state. This allows a stray capacitance existing between the photodiode and the ground to be always charged by the power supply, so that this charging current no longer affects the laser diode driving current. 
     Furthermore, it is desirable that each laser diode and each driving element are mounted on a single printed circuit board. This allows a conductor wire connecting the laser diode and the driving element to be minimized, reducing the stray capacitance existing between the conductor wire and the ground and stabilizing a waveform of the driving current. In this case, it is further desirable that the driving element integrally includes a switching circuit equipped with a means for adjusting the driving current, and is arranged as close to the corresponding laser diode as possible. 
     It is also desirable that a snubber circuit is electrically connected parallel to the laser diode. This allows the waveform of the laser diode driving current to be reformed. 
     The present invention is now described with reference to an embodiment shown in the drawings. 
     1. Overall Construction and Operation of a Printer 
     An overall construction of a laser printer of multi-beam type according to the present invention is now described with reference to FIG. 1 to FIG.  3 . 
     Referring now to the drawings, FIG. 1 is a view showing a general construction of a laser printer (as seen from the side thereof), FIG. 2 is a top view showing an optical system in FIG. 1, and FIG. 3 is a perspective view showing a construction of the optical system in FIG.  1 . 
     As shown in these figures, the laser printer according to the present invention is equipped with a pair of light source devices  1   a,    1   b  (FIG. 2) for emitting beams L 1 ,L 2  corresponding to input video signals, respectively, and cylindrical lenses  2   a,   2   b  for adjusting cross-sectional forms of the two beams L 1 ,L 2  emitted by the light source devices  1   a,   1   b . The light source devices  1   a,   1   b  and the cylindrical lenses  2   a,   2   b  are disposed in an optical system housing  21 . As shown in FIG. 4, the light source devices,  1   a,   1   b  include laser diodes  30   a,   30   b for emitting the beams L 1 ,L 2  and collimator lenses  31   a,   31   b  for collimating the beams L 1 ,L 2 , respectively. 
     The laser printer is further equipped with a polygonal mirror  3  for reflecting the beams L 1 ,L 2  passing through the cylindrical lenses  2   a,   2   b  with its six mirror surfaces, a motor  3   a for rotating the polygonal mirror  3  at a constant speed in the direction of arrow A, an f·θ lens  4  for correcting the distortion aberration of beams reflected by the polygonal mirror  3 , cylindrical lens  22  (with f·θ function) for correcting surface troubles of the beams L 1 ,L 2 , and a plane mirror  6  for reflecting the beams passing through the f·θ lens  4  and the cylindrical lens  22  to form images at designated positions P 1 ,P 2  on a photoconductor drum  5  (FIG.  1 ). 
     The polygonal mirror  3  is arranged such that it rotates in the direction of arrow A to scan the photoconductor drum  5  in the direction of arrow C in such a manner that the beams L 1 ,L 2  are received through a mirror  61  by a beam sensor  53  made of a photodiode and the sensor  53  detects the beginning of each scanning. 
     As shown in FIG. 1, the printer further includes a charging corona-discharge device  7  for uniformly charging the surface of the photoconductor drum  5  which rotates in the direction of arrow B, a development unit  9  for supplying a developing agent to the surface of the photoconductor drum  5  by a development roller  8 , a cassette  11  for housing recording sheets  10  therein, a feeding roller  12  for feeding the recording sheet  10  in the cassette  11 , a pair of conveying rollers  13 , 13  for conveying the recording sheet  10 , a resist roller  14  for intermittently transmitting the recording sheet  10  toward the photoconductor drum  5  in synchronization with the prescribed timing, a transferring corona-discharge device  9  for charging the recording sheet  10  conveyed from the resist roller  14  by corona discharge and transferring the developed image from the photoconductor drum  5  onto the surface of the recording sheet  10 , a pair of separation rollers  15  for separating the image-transferred recording sheet from the photoconductor drum  5 , a pair of fixing rollers  17  for fixing the image on the separated recording sheet by heating, a discharging roller  18  for discharging the recording sheet  10  which has finished the fixing process, a tray  19  for receiving the discharged recording sheet  10 , and a cleaning unit  20  for cleaning the surface of photoconductor drum  5  which has finished the transferring process. 
     Next, overall operation of the printer constructed as described above is now described. 
     As shown in FIG.  2  and FIG. 3, when the beams L 1 ,L 2  are emitted by the light source devices  1   a,   1   b,  the beams L 1 ,L 2  are reflected by the polygonal mirror  3  rotating in the direction of arrow A and received, at first, by the beam sensor  53  through the f·θ lens  4 , the cylindrical lens  22 , and the plane mirror  6 . Next, the beams are focused to form images at the positions P 1 ,P 2  on the surface of the photoconductor drum  5  and are allowed to scan the photoconductor drum  5  in the direction of arrow C by means of rotation of the polygonal mirror  3 . In each scanning period, when a detection signal from the beam sensor  53  receiving the beams L 1 ,L 2  is inputted into a control section (not shown), the control section modulates the beams L 1 ,L 2  in synchronization therewith for a prescribed printing period of time according to the video signal. 
     On the other hand, the surface of the photoconductor drum  5  previously charged uniformly by the charging corona-discharge device  7  and rotating in the direction of arrow B is scanned by the beams L 1 ,L 2  during the printing period, whereby an electrostatic latent image is formed thereon. The electrostatic latent image is visualized (revealed) as the developing agent is applied by the development roller  8 . 
     The recording sheet  10  housed in the cassette  11  is drawn out by the feeding roller  12 . When the recording sheet  10  is conveyed by the conveying roller  13  and the top thereof reaches the resist roller  14 , it stops for a while. When the resist roller  14  is actuated in synchronization with the progress of forming a visualized image on the photoconductor drum  5 , the recording sheet  10  is conveyed to the lower side of the photoconductor drum  5  by the resist roller  14  so that it comes in contact with the visualized image on the photoconductor drum  5 . 
     Then, the transferring corona-discharge device  9  is discharged, and the developing agent which forms the visualized image on the surface of the photoconductor drum  5  is moved (transferred) to the recording sheet. The image-transferred recording sheet is detached from the photoconductor drum  5  by the separation roller  15  to be conveyed to the fixing roller  17 . 
     Next, when the recording sheet which has finished the fixation of image through heating by the fixing roller  17  is discharged to the tray  19  by the discharging roller  18 , a printing cycle for one recording sheet is finished. The surface of the photoconductor drum  5 , which has just finished transferring the image, is cleaned by the cleaning unit  20  and kept ready for the next printing cycle. 
     2. Detailed Construction and Operation of the Principal Elements 
     Next, the construction and operation of the light source device characterizing this invention is now described in detail. 
     FIGS. 4 and 5 are a top view and a rear view showing the light source devices  1   a,   1   b,  respectively. As shown in these figures, the laser diodes  30   a,   30   b,  which serve as light-emitting elements, and collimator lenses  31   a,   31   b  are mounted in a common plastic holder  32  and fixed by an adhesive. 
     Lead wires of the laser diodes  30   a,   30   b  are mounted on two printed circuit boards  33   a,   33   b,  respectively, and laser diode driving IC modules  34   a,   34   b  for driving the laser diodes  30   a,   30   b  are also mounted on the printed circuit boards  33   a,   33   b,  respectively. 
     FIG. 6 is an electric circuit diagram showing the laser diode driving IC module  34   a mounted on the printed circuit board  33   a.  As shown in FIG. 6, the laser diode  30   a  includes a laser diode element LD 1 , a photodiode element PD 1  for monitoring an intensity of the light emitted from the laser diode element LD 1 , and a metallic package (a can package) P 1  for housing them therein. The anode Al of the laser diode element LD 1  and the cathode of the photodiode element PD 1  are electrically connected to the package P 1  The laser diode  30   b  has the same construction as the laser diode  30   a  and also includes a laser diode element LD 1 . 
     Here, the laser diode elements LD 1  of the laser diodes  30   a,    30   b  may be included as a pair in a common package. Generally, each laser diode element is individually constituted. Alternatively, however, a plurality of laser elements can be formed by device isolation on a common semiconductor substrate to obtain parallel beams. 
     The laser diode driving IC module  34   a  is connected through a terminal T 1  to the cathode K 1  of the laser diode element LD 1 . The laser diode driving IC module  34   a  includes a switching element  35   a  and a current adjustable circuit  36   a.  The switching element  35   a  switches the electric current IL for driving the laser diode element LD 1  according to a video signal VD supplied through an input terminal T 3  from outside. The current adjustable circuit  36   a  adjusts a magnitude of the driving electric current IL according to a control signal CS supplied through an input terminal T 4  from outside. The magnitude of the current IL determines the maximum dot density. 
     A snubber circuit, which is a series circuit including a variable resistor R 1  and a capacitor C 1 , and a variable resistor R 2  for discharging the capacitor C 1  are connected between the anode and the cathode of the laser diode LD 1 . 
     The laser diode driving element on the printed circuit board  33   b  is equivalent to the circuit shown in FIG.  6 . In this embodiment, RLD 78 NP-D type made by Rohm Co., Ltd. is used as the laser diodes  30   a ,  30   b  and SN 65ALS542 type made by Texas Instruments Co., Ltd. is used as the laser diode driving IC, respectively. 
     In such a construction, the anode A 1  is connected to a positive electrode of a d.c. power supply  100  and the terminal T 2  of the IC  34   a  is connected to a negative electrode of the power supply  100 , wherein a direct current voltage is applied therebetween. When the video signal VD having a pulse waveform shown in FIG.  8 ( a ) (one pulse corresponding to one dot in the image) is thus applied to the terminal T 3 , the intensity of light emitted by the laser diode  30   a  changes in an approximately rectangular pulse shape in correspondence with the video signal VD, as shown in FIG.  8 ( e ). The intensity of light emitted by the laser diode  30   b  also changes in an approximately rectangular shape, as shown in FIG.  8 ( e ). 
     Next, the characteristics, function and effect of the circuit structure of this embodiment shown in FIG. 6 are now described with reference to a comparative example shown in FIG.  7 . 
     In FIG. 7, the laser diodes  30   a,   30   b  consist of a laser diode element LD 2 , a photodiode PD 2  for monitoring, and a metallic package P 2  for housing these therein. The cathode K 2  of the laser diode element LD 2  and the photodiode PD 2  are electrically connected to the package P 2 . 
     The terminal T 5  of the switching element  35 C is connected through a lead cable  37  of about 200 mm to the cathode K 2 , the switching element  35 C being connected through a terminal T 6  to a current adjustable circuit  36 C. Thus, a magnitude of the laser diode driving current IL is to be adjusted by supplying a video signal to the input terminal T 8  of the switching element to switch the laser diode driving current IL and by supplying a control signal CS to the input terminal T 9  of the current adjustable circuit  36 C when a direct current voltage is applied between the anode A 2  and the terminal T 7  of the current adjustable circuit  36 C. 
     In such a construction of the comparative example, when a direct current voltage is applied between the anode A 2  and the terminal T 7  of the current adjustable circuit  36 C in the same manner as in FIG.  6  and the video signal VD having a pulse waveform shown in FIG.  8 ( a ) is supplied to the input terminal T 8  of the switching element  35 C, the intensity of light emitted by the laser diode  30   a  vibrates greatly with large fluctuations as shown in FIG.  8 ( b ), so that it is impossible to control the magnitude of the intensity by means of the control signal CS supplied to the input terminal T 9  of the current adjustable circuit  36 C. 
     This is caused by the fact that the electric current IL for driving the laser diode  30   a  is affected by the stray capacitance of the circuit. Therefore, the lead cable  37  is removed and the switching element  35 C is integrated with the current adjustable circuit  36 C. That is, instead of the switching element  35 C and the current adjustable circuit  36 C, the same laser diode driving IC module as that used in the embodiment is mounted on a single printed circuit board together with the laser diode  30   a.  Thus, the waveform of the intensity of light emitted by the laser diode  30   a  is reformed in shape as shown in FIG.  8 ( c ). 
     However, the waveform of FIG.  8 ( c ) has a large rising edge, showing an overshoot. Accordingly, a series circuit of a variable resistor R 1  and capacitor C 1 , and a variable resistor R 2  are connected in parallel between the anode A 2  and the cathode K 2  in a manner similar to the embodiment of the present invention. Thus, the waveform of the intensity of emitted light becomes as shown in FIG.  8 ( d ) and the overshoot is eliminated by adjustment of the variable resistors R 1 ,R 2 . 
     It is however obvious that a moiré occurs when an image is formed by such a laser light, because the height of the intensity waveform of emitted light corresponding to the first dot is high compared with the height of the other waveforms succeeding thereto. 
     Thus, the intensity waveform of the emitted light corresponding to the first dot only is high. This seems to be due to the following reason. Since the cathode K 2  for the laser diode  30   a  and the photodiode PD 2  are electrically connected to the metallic package P 2  as shown in FIG. 7, a relatively large stray capacitance exists among the package P 2 , the photodiode PD 2 , and the ground, so that the electric current for charging the stray capacitance is added to the electric current flowing into the laser diode element LD 2  for the first dot, thereby increasing the laser diode driving current. 
     The laser diode  30   a  is accordingly replaced with the same one as in the embodiment. That is, it is so constructed that the anode of the laser diode element and the cathode of the photodiode element are connected to the package, and the positive electrode of the power supply is always connected to the package to charge the stray capacitance beforehand. At this time, the intensity waveform of emitted light is approximately rectangular and uniform for every dot, as shown in FIG.  8 ( e ), allowing the height of the waveform to be controlled by the control signal CS. As a result, it has been accordingly ascertained that the comparative example should have the same circuit structure as the embodiment of this invention. 
     According to the light source device of this invention, the density of each dot forming the image becomes uniform and phenomena like moiré does not occur, because a stable intensity waveform of emitted light can be obtained in correspondence with the video signal, thus providing high-quality images. 
     According to this invention, the intensity waveform of light emitted by the laser diode is approximately rectangular and uniform for every dot in the image, so that dot by dot fluctuations in the image density can be prevented, thereby providing a high-quality image without moiré. 
     Although the present invention has fully been described by way of example with reference to the accompanying drawings, it is to be understood that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the spirit and scope of the invention, they should be construed as being included therein.