JP2008026661A - Beam spot shaping method, optical scanning apparatus, and image forming device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a beam spot shaping method used in a digital copying machine, a laser printer, a laser facsimile or the like, and to provide an optical scanning apparatus and an image forming device. <P>SOLUTION: This is the beam spot shaping method of the optical scanning apparatus having: a light source; an amplitude element for transmitting a part by shading a part of an optical beam emitted from the light source; a phase element for controlling the phase of the optical beam; and an imaging optical system for imaging the optical beam. The amplitude element has at least two opening regions, obtains a flat beam spot shape in intensity distribution of a center part more than a Gaussian beam by shaping the beam spot shape on an image face, and excellently reduces influence due to side lobe. In addition, position relationship of the amplitude element and the phase element can be maintained highly precisely by forming to integrate the amplitude element and the phase element and can avoid change of the beam spot shape due to installment errors. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a beam spot shaping method, an optical scanning apparatus, and an image forming apparatus, and more particularly to a beam spot shaping method, an optical scanning apparatus, and an image forming apparatus used for a digital copying machine, a laser printer, a laser facsimile, and the like.

  In general, an optical scanning device widely known in connection with a laser printer or the like generally deflects and scans a light beam from a light source side by an optical deflector and forms an image on a surface to be scanned by a scanning optical system such as an fθ lens. A beam spot is formed, and the surface to be scanned is optically scanned (main scan) with this beam spot. What constitutes the surface to be scanned is a photosensitive surface of a photosensitive medium such as a photoconductive photosensitive member.

  In general, the intensity distribution of the light emitted from the laser has a Gaussian distribution, and a beam spot having an intensity distribution of the Gaussian distribution can be obtained even on the scanning surface on which the beam is imaged. However, when a part of the beam is cut in the middle with an aperture or the like, a beam spot having a side lobe called a sinc function or an Airy pattern is obtained according to the shape of the aperture, but in the vicinity of the center of the beam, It has an intensity distribution almost equal to the Gaussian distribution. The Gaussian intensity distribution has the strongest intensity at the center and gradually attenuates as it goes to the periphery.

  In an electrophotographic process used for a laser printer or the like, an image is formed in the following flow. First, the photosensitive member is charged, the beam is irradiated onto the photosensitive member to expose the photosensitive member to form an image pattern, and the charged toner is adsorbed on the photosensitive member and developed to form a toner image on the photosensitive member. Then, the toner image is transferred to a medium such as paper, and finally the toner is melted by heat or the like and fixed on the medium such as paper.

  Here, as an important technique for continuously outputting a good and stable image over time, there is a toner charge control technique, and it is ideal that the toner is charged to the same potential as the initial value over time. . However, it is very difficult to keep the toner charge the same between the initial time and time, and the charge state of the toner fluctuates with time. When the charged state of the toner fluctuates, even if exposure is performed with the same beam spot diameter, the toner spot diameter after development with toner fluctuates, and the output image deteriorates.

  When the charging potential of the toner is high, development is performed to the bottom of the beam spot diameter, and when the charging potential of the toner is low, development is performed only near the center portion of the beam spot. Accordingly, the reason that the toner spot diameter after development changes when the charged state of the toner fluctuates is that the intensity distribution of the beam spot during exposure is Gaussian. Ideally, if the intensity distribution of the beam spot is rectangular instead of Gaussian, the toner spot diameter after development will not change even if the charge state of the toner changes, and a uniform dot image will always be obtained. Therefore, a good image can be provided stably over time.

  However, it is practically difficult to create a beam spot having a rectangular intensity distribution, and the size of the optical system is increased. In view of this, it is practically best to produce the above effect in a pseudo manner by flattening the intensity distribution as much as possible in the vicinity of the center of the beam spot.

  As a method for flattening the intensity distribution in the vicinity of the center portion of the beam spot, there is known a method of superimposing the beam on the surface to be scanned by swinging the beam as described in Patent Documents 1 and 2. ing.

  As another method for flattening the intensity distribution near the center of the beam spot, as described in Patent Documents 3 and 4, the intensity distribution near the center of the beam spot is flattened using an aspheric lens. There is a known method to make it.

Patent Documents 5 and 6 disclose a method of flattening the intensity distribution near the center of a beam spot using a DOE (Diffractive Optical Element).
JP 2004-284133 A JP-A-11-70694 JP 2004-252275 A JP-A-10-153750 JP 2004-230432 A JP 09-61610 A

  However, the above invention has the following problems.

  The method of superimposing the beams on the surface to be scanned by oscillating the beams described in Patent Documents 1 and 2 requires a movable part in order to oscillate the beams, and enables such movement. There are no devices (actuators, MEMS, etc.) that are fast enough to support high-speed image forming apparatuses at present (light scanning using MEMS is possible, but the beam is oscillated many times in one scan. That's why you need a very fast device). In addition, the movable part is a factor that degrades the quality over time, so it is desirable not to use it as much as possible.

  The method of flattening the intensity distribution in the vicinity of the center portion of the beam spot using the aspheric lens described in Patent Documents 3 and 4 is very effective when a part of the beam from the light source is not cut off. However, if the beam from the light source is used as it is, the NA will change due to changes in the relative position of the light source and the lens over time, changes in the divergence angle of the beam, etc., the beam spot diameter on the image plane will vary, and the image Causes quality degradation. Further, when a semiconductor laser is used as the light source, the semiconductor laser has a large variation in divergence angle between individuals, so that the beam spot diameter on the image plane differs between models and the quality varies. Therefore, in practice, it is essential to include an aperture that cuts off a part of the beam from the light source. Therefore, when part of the light source is cut off, it is difficult to flatten the intensity distribution near the center of the beam spot by the methods described in Patent Documents 3 and 4. Furthermore, since this method does not take into account the influence of laser beam diffraction, there is no problem in applications where the beam spot diameter on the image plane is large (for example, a laser processing machine), but a small beam spot diameter on the image plane is not possible. It does not work well in the required application (for example, an optical scanning device used in an image forming apparatus).

  Since the DOEs described in Patent Documents 5 and 6 basically have the same function as an aspheric lens, for the same reason as described above, when a part of the beam from the light source is cut off, the beam spot diameter on the image plane is small. If it is small, it will not work.

  Therefore, an object of the present invention is to propose a beam spot shaping method for flattening the intensity distribution near the center of the beam spot using an amplitude element having at least two aperture regions. It is another object of the present invention to propose an optical scanning apparatus that can provide a stable dot image even over time by applying the beam spot shaping method and can stably provide a good image over time. Another object of the present invention is to propose an image forming apparatus having the optical scanning device.

  The invention according to claim 1 is a light source, an amplitude element that blocks a part of a light beam emitted from the light source and transmits a part thereof, a phase element that controls a phase of the light beam, and the light beam A beam spot shaping method for an optical scanning device having an imaging optical system for imaging a light beam, wherein the amplitude element has at least two aperture regions and shapes the shape of the beam spot on the image plane It is characterized by.

  According to a second aspect of the present invention, in the beam spot shaping method according to the first aspect, the intensity of the beam spot in the vicinity of the central portion of the beam spot is flatter than that in the vicinity of the central portion of the Gaussian beam. It is characterized by shaping the shape.

  According to a third aspect of the present invention, in the beam spot shaping method according to the first or second aspect, the amplitude element has a second opening region around a central opening region.

  According to a fourth aspect of the present invention, in the beam spot shaping method according to any one of the first to third aspects, the phase element generates a phase difference in at least two aperture regions.

  According to a fifth aspect of the present invention, in the beam spot shaping method according to any one of the first to fourth aspects, the amplitude element has an elliptical opening region.

  According to a sixth aspect of the present invention, in the beam spot shaping method according to any one of the first to fifth aspects, the amplitude element and the phase element are integrally formed.

  According to a seventh aspect of the present invention, there is provided a light source, an amplitude element that blocks part of a light beam emitted from the light source and transmits part of the light beam, a phase element that controls a phase of the light beam, and the light source. 7. An optical scanning device comprising: deflection means for deflecting and scanning the light beam; and a scanning optical system for imaging the light beam scanned by the deflection means on an image carrier. A beam spot on the image carrier is shaped using the beam spot shaping method according to claim 1.

  According to an eighth aspect of the present invention, in the optical scanning device according to the seventh aspect, the shape of the beam spot diameter in the sub-scanning direction is shaped.

  According to a ninth aspect of the present invention, in the optical scanning device according to the seventh or eighth aspect, one pixel is formed by a plurality of dots in the main scanning direction.

  According to a tenth aspect of the present invention, in the optical scanning device according to any one of the seventh to ninth aspects, the amplitude element and the phase element are disposed between a light deflection unit and the light source. To do.

  According to an eleventh aspect of the present invention, a developing unit that visualizes an electrostatic image formed on an image carrier with toner, and an image visualized on the image carrier is transferred to a medium such as paper. 11. An image forming apparatus having a transfer unit and outputting an image, wherein an electrostatic image is formed on the image carrier by the optical scanning device according to any one of claims 7 to 10.

  The present invention relates to a light source, an amplitude element that blocks a part of a light beam emitted from the light source, passes the part, a phase element that controls the phase of the light beam, and imaging optics that forms an image of the light beam The amplitude element is configured to have at least two aperture areas, and by shaping the shape of the beam spot on the image plane, the beam spot having a flat intensity distribution in the center portion than the Gaussian beam Shape can be obtained.

  Hereinafter, a beam spot shaping method according to an embodiment of the present invention, an optical scanning device using the beam spot shaping method, and an image forming apparatus having the optical scanning device will be described.

  First, referring to FIG. 3, the shape of the beam spot is shaped by using an amplitude element that blocks a part of the light beam from the light source and transmits the part and a phase element that controls the phase of the light beam. A method will be described.

  In FIG. 3A, after diverging light from a light source is converted into parallel light by a coupling lens, a part of the beam is cut by an amplitude element (aperture) having an opening at the center and shielding the periphery. A beam spot shape having a side lobe called a sinc function or an Airy pattern is obtained on the image plane in accordance with the shape of the aperture. Here, the intensity distribution of the beam when collimated by the coupling lens is Gaussian as shown in the figure.

  FIG. 3B is a diagram when the intensity distribution of the beam after being collimated by the coupling lens is assumed to be a uniform intensity as shown in the figure instead of a Gaussian distribution. In FIG. 3B as well, a beam spot shape having side lobes called a sinc function or an Airy pattern is obtained on the image plane in accordance with the shape of the aperture, almost as in FIG. 3A and 3B on the image plane, the effective NA increases when the intensity is uniform as shown in FIG. 3B, so that the beam spot diameter is slightly narrower than that in FIG. The side lobe intensity is increased, but the shape of the beam spot is not significantly different.

  Thus, the shape of the beam spot is roughly determined by the shape of the aperture rather than the intensity distribution of the beam before focusing. In the following, in order to simplify the examination, the intensity distribution after the diverging light from the light source is converted into parallel light by the coupling lens is assumed to be a uniform intensity, not a Gaussian distribution.

  In FIG. 3C, after diverging light from a light source is converted into parallel light by a coupling lens, the intensity distribution and phase distribution of the beam are controlled by an amplitude element and a phase element having at least two openings. It is a figure showing that the shape of a beam profile can be produced | generated. Hereinafter, an amplitude element and a phase element that flatten the intensity distribution near the center of the beam spot will be described.

  FIG. 4 shows an example in which an amplitude element having a second circular opening area around a central circular opening area and a phase element for adjusting the phase difference between the two circular opening areas are used as the first embodiment. Will be described.

  4A and 4B show an amplitude element and a phase element, respectively. In FIG. 4A, a ring-shaped second circular opening region is provided around the first circular opening provided in the center. In the phase element shown in FIG. 4B, a groove is provided in the portion of the second circular opening region in FIG. 4A so that a phase difference is created between the first circular opening portion and the second circular opening region. I have to. FIG. 4C shows an example in which FIGS. 4A and 4B are integrated into one element.

  FIG. 5 shows the simulation results. FIG. 5A shows an optical system in which simulation is performed, and divergent light from a light source is collimated with a coupling lens (not shown) having a Gaussian intensity distribution, and then passed through an amplitude element and a phase element. Then, the light is condensed by the lens and formed on the image plane. Here, as the amplitude element and the phase element, those obtained by integrating the amplitude element and the phase element shown in FIG. 4C were used. A lens is installed immediately after the amplitude element and the phase element. Various parameters are shown below.

<Incoming light>
Plane wave with uniform intensity <Lens>
Ideal lens with a focal length of 50 mm <Amplitude element>
Central first circular opening: 600 μm in diameter, circular Second circular opening region: (inner) diameter 1.20 mm, circular, (outer) diameter 1.26 mm, circular <Phase element>
Phase difference between the first circular opening and the second circular opening region: 5.5π / 8

  Simulation results are shown in FIGS. 5B, 5C, and 5D. FIG. 5B is a cross-sectional view passing through the center of the beam profile on the image plane. When there is no second circular aperture region (only with the first circular aperture), the second circular aperture region is shown. 2 is shown (the phase difference between the first circular opening and the second circular opening region is 5.5π / 8). FIGS. 5C and 5D show 3D display of simulation results when there is no second circular opening region and when there is no second circular opening region. FIG. 5B is a cross-sectional view including the centers of FIGS. 5C and 5D.

  In FIG. 5B, the peak intensity of the beam spot when there is no second circular opening area is normalized, and in FIGS. 5C and 5D, there is a case where there is no second circular opening area. It is shown normalized by the peak intensity of the beam spot at each time. As can be seen from the simulation results of FIG. 5, it can be seen that the intensity distribution near the center of the beam spot is flattened by using an amplitude element and a phase element having at least two circular aperture regions. The 1 / e ^ 2 beam diameter is approximately equal to 86.7 μm when there is no second circular aperture region and 87.3 μm when there is a second circular aperture region.

  Next, as a second embodiment, an example using an amplitude element having a second rectangular opening area around a central rectangular opening area and a phase element for adjusting the phase difference between the two opening areas will be described with reference to FIG. Will be described. FIG. 6A shows an integrated amplitude element and phase element. The amplitude element has a second rectangular opening area around a rectangular opening provided in the center. A groove is provided in the second rectangular opening region so that a phase difference is created between the central rectangular opening and the second rectangular opening region.

  Simulation results are shown in FIGS. 6B, 6C, and 6D. The simulated optical system is the same as that shown in FIG. Various parameters are shown below.

<Incoming light>
Plane wave with uniform intensity <Lens>
Ideal lens with a focal length of 50 mm <Amplitude element>
First rectangular opening at the center: width 600 μm, rectangular (square)
Second rectangular opening area: (inside) width 1.20 mm, rectangle (square), (outside) width 1.26 mm, rectangle (square)
<Phase element>
Phase difference between the first rectangular opening and the second rectangular opening area: 5.5π / 8

  Simulation results are shown in FIGS. 6B, 6C, and 6D. FIG. 6B is a cross-sectional view passing through the center of the beam profile on the image plane. When there is no second rectangular opening area (only with the first rectangular opening), the second rectangular opening area is There are two cases (the phase difference between the first rectangular opening and the second rectangular opening region is 5.5π / 8). FIGS. 6C and 6D show the simulation results in 3D when there is no second rectangular opening area and when there is no second rectangular opening area. A cross-sectional view including the centers of FIGS. 6C and 6D is FIG. 6B.

  In FIG. 6 (b), the peak intensity of the beam spot when there is no second rectangular opening area is normalized, and in FIGS. 6 (c) and (d), there is a case where there is no second rectangular opening area. Each time is shown normalized. As can be seen from the simulation results of FIG. 6, it can be seen that the intensity distribution near the center of the beam spot is flattened by using an amplitude element and a phase element having at least two aperture regions. The 1 / e ^ 2 beam diameter is 74.1 μm when there is no second rectangular opening region and 76.7 μm when there is a second rectangular opening region, and is approximately equal.

  In the above description, the phase element is considered to be a constant phase in each opening region. However, the phase element is not necessarily required to have a constant phase, and may have a two-dimensional phase distribution. Further, although the embodiment has been described in which the amplitude element has one ring-shaped opening region around the opening of the central portion, the present invention is not limited to this, and a plurality of rings may be provided or a part of the ring may be provided. May be used as a light shielding region, or a pattern in which openings and light shielding portions are two-dimensionally configured.

  Next, the phenomenon in which the intensity distribution near the center of the Gaussian beam is flattened by using an amplitude element and a phase element having at least two aperture regions will be physically considered.

  In a beam spot having a side lobe that is generated when an image is formed by shielding a part of the light beam with an amplitude element (aperture), the phase is approximately shifted by π between the central main lobe and the side lobe. This can be understood by considering diffraction by the slit. Theoretically, the amplitude distribution of the beam diffracted by the slit has a sinc function shape. The sinc function has a negative amplitude at the side lobe peak position when the amplitude at the center peak position is positive, which indicates that the phase of the side lobe is shifted by π with respect to the main lobe (amplitude). Since only takes a positive value, “negative amplitude” indicates that the phase is shifted by π with respect to the positive amplitude).

  When the beam passing through the aperture is imaged by the lens, the shape of the beam spot is proportional to the Fourier transform of the aperture shape, and if the aperture is at the position of the front focal point of the lens, the relationship of the Fourier transform is accurate. become. Here, as the diameter of the aperture is increased, the high-frequency component increases, so that the beam spot diameter becomes smaller. Here, when the aperture is formed in a ring shape, a part of the high-frequency components included in the aperture shape becomes stronger, so that a beam spot with a narrow beam spot diameter and a high sidelobe intensity can be obtained. Increasing the size of the ring-shaped aperture further reduces the beam spot diameter.

  Here, when considering the shape of the beam spot imaged through the amplitude element as shown in FIG. 4A, the beam imaged through only the central circular aperture and the ring-shaped circular aperture only. It can be considered by superimposing the beams formed in this way. FIG. 7 shows a beam spot imaged only through the first circular opening at the center and a beam spot imaged only through the ring-shaped second circular aperture region, respectively. The vertical axis in FIG. 7 represents the amplitude, which is normalized by the maximum value of the beam spot amplitude when there is no second circular aperture region.

  The beam spot imaged only through the second circular aperture region is substantially included up to the second side lobe in the main lobe of the beam imaged only through the central circular aperture. . Further, as described above, the main lobe and the first side lobe are approximately π out of phase, and the first side lobe and the second side lobe are also approximately π out of phase. Note that the phase in the main lobe and the side lobe can be considered to be substantially constant (it is not exactly constant, but is considered to be substantially constant for the sake of simplicity). Here, when the phase between these two beams is adjusted (by adjusting the phase between the first circular opening and the second circular opening region in the center) and superposed, the phase adjustment amount depends on the phase adjustment amount. "With reference to the beam profile without the second aperture, the amplitude is reduced at the position of the main lobe of the beam spot imaged only through the second circular aperture region, and the first side lobe It is possible to increase the amplitude at the position and decrease the amplitude at the position of the second side lobe. ”By adding the second circular opening region, as shown in FIG. A beam profile with “no two apertures” can be converted to a beam profile with “second aperture”.

  In the example of FIG. 7, the beam spot imaged only through the second circular aperture region is included in the main lobe of the beam imaged only through the central circular aperture up to the second side lobe. However, it is not always necessary to do so, as long as at least a part of the first side lobe is included in the main lobe.

  Next, FIG. 8 shows simulation results when the phase difference between the first circular opening and the second circular opening region is changed to 5π / 8, 5.5π / 8 (described above), and 6π / 8. Shown in As shown in FIG. 8, the degree of flatness of the intensity distribution near the center of the Gaussian beam is changed by changing the phase difference between the first circular opening and the second circular opening region. I understand. In any of the results shown in FIG. 8, the intensity distribution near the center is flattened compared to the Gaussian beam (“No second aperture” in FIG. 8 can be approximately regarded as a Gaussian beam). It is desirable to be flattened to the extent shown in FIG.

  In the above, the method of flattening the intensity distribution near the central part of the Gaussian beam using the amplitude element and the phase element has been described. However, by controlling the aperture distribution of the amplitude element and the phase distribution of the phase element, various methods can be used. A beam spot shape can be generated. Therefore, the present invention is not limited to the use for flattening the intensity distribution near the center of the Gaussian beam, but can be applied to various uses.

  As described above, FIG. 5 shows a simulation result when the shape of the opening is circular, and FIG. 6 shows a simulation result when the shape of the opening is rectangular. Comparing these two simulation results, the side lobe intensity of the beam spot on the image plane is larger when the aperture shape is rectangular. For reference, the peak intensity of the side lobe (the peak intensity of the beam spot when there is no second circular (rectangular) opening region is 100%) is as follows.

<Circular opening>
Without second circular opening area: 1.7%
When there is a second circular opening area: 3.3%
<Rectangular opening>
When there is no second rectangular opening area: 4.7%
When there is a second rectangular opening area: 11.4%

  From the above, it can be seen that the peak intensity of the side lobe is increased by providing the second circular (rectangular) opening region. Further, in the rectangular opening, the peak intensity of the side lobe is increased even when the second rectangular opening region is not provided. In this way, the influence of diffraction becomes strong at the rectangular opening, and the peak intensity of the side lobe increases. Since the sidelobe light is noise light and may cause image quality deterioration, the sidelobe light is preferably as small as possible. A rectangular opening can be applied to the present invention, but a circular or elliptical shape is preferable.

  In the above simulation (shown in FIGS. 5 and 6), an example in which the amplitude element and the phase element are integrated is shown, but it is not always necessary to integrate, and the same thing as in FIGS. It is possible to realize. When the amplitude element and the phase element are placed apart from each other, the phase difference between the first circular (rectangular) opening and the second circular (rectangular) opening area is not simply adjusted as described above. The phase distribution of the element becomes more complicated. Since the light emitted from the amplitude element (phase element) is affected by diffraction before reaching the phase element (amplitude element), the phase distribution of the phase element has a complicated shape.

  In order to control the shape of the beam spot on the image plane using the amplitude element and the phase element as described above, the relative position between the amplitude element and the phase element is important. The shape of will also change. Therefore, it is preferable for the present invention to integrate the amplitude element and the phase element. By using the present invention, when shaping the beam spot shape, it is possible to strengthen against the installation error of the amplitude element and the phase element. Further, the phase element has a merit that the simple phase distribution is easy to manufacture.

  Next, an optical scanning device used in the image forming apparatus will be described.

  FIG. 1 shows an optical scanning device developed in a full-color image forming apparatus. In the embodiment of FIG. 1, scanning is performed for two stations in a direction facing the polygon mirror 213. Further, in FIG. 1, only one station is shown for the sake of simplicity. FIG. 2 is a sectional view thereof.

  The image forming apparatus shown in FIG. 1 forms four color photosensitive drums 101, 102, 103, and 104 along the moving direction of the transfer belt 105 and sequentially transfers toner images of different colors to form a color image. Each optical scanning device is integrally configured, and all light beams are scanned by a single polygon mirror 213.

  Further, here, a pair of semiconductor lasers are provided for each photoconductor, and scanning is performed by shifting one line pitch in the sub-scanning direction in accordance with the recording density so that two lines are simultaneously scanned. The cylinder lens 209 has one surface having a common curvature in the plane and the other in the sub-scanning direction, and each light beam is converged so as to be linear in the sub-scanning direction on the deflection surface. Are made conjugate with each other in the sub-scanning direction, thereby forming a polygon mirror surface tilt correction optical system.

  A light beam emitted from a semiconductor laser (not shown) is converted into a parallel light beam by a coupling lens (201), shaped by an aperture (not shown), and then focused only in the sub-scanning direction by a cylindrical lens 209. A line image that is long in the main scanning direction is formed at the position of the deflecting reflecting surface.

  The beams from the respective stations are emitted from the semiconductor laser in parallel with the sub-scanning direction, and are incident on the polygon mirror reflecting surface perpendicular to the reflecting surface while maintaining the equilibrium state.

  The polygon mirror 213 is a six-sided mirror and has a shape in which a groove is provided in a portion between the beams not used for deflection so that the diameter is slightly smaller than the inscribed circle of the polygon mirror to reduce the windage loss. Is about 2 mm.

  The fθ lenses 2181, 2182, 2183, and 2184 are all lenses having the same shape, and each photoconductor corresponds to one fθ lens. Note that the fθ lenses 2181 and 2182 (2183 and 2184) are held overlapping in the sub-scanning direction.

  The fθ lens 2181 has a non-arc surface shape in which power is given so that the beam moves at a constant speed on each photoconductor surface in accordance with the rotation of the polygon mirror in the main scanning direction. After having passed through the fθ lens 2181, it is reflected by the folding mirror 224, enters the toroidal lens 220, is further reflected by the folding mirror 227, and forms a spot image on the photosensitive drum 102. For example, a yellow image is formed as one optical scanning unit. In this embodiment, four photoconductors can be scanned simultaneously to form four color images simultaneously.

  In each optical scanning means, a plurality of sheets are folded so that the optical path lengths from the polygon mirror 213 to the surface of the photoconductor coincide with each other, and the incident positions and incident angles with respect to the photoconductor drums arranged at equal intervals are equal. A mirror is placed.

  In the optical scanning device used in the image forming apparatus described above, the following describes the merit of shaping the beam spot shape so that the beam spot shape is flatter than the intensity distribution near the center of the Gaussian beam.

  In general, the intensity distribution of the light emitted from the laser has a Gaussian distribution, and a beam spot having an intensity distribution of the Gaussian distribution can be obtained even on the scanning surface on which the beam is imaged. However, when a part of the beam is cut in the middle with an aperture or the like, a beam spot having a side lobe called a sinc function or an Airy pattern is obtained according to the shape of the aperture, but in the vicinity of the center of the beam, It has an intensity distribution almost equal to the Gaussian distribution. The Gaussian intensity distribution has the strongest intensity at the center and gradually attenuates as it goes to the periphery.

  In an electrophotographic process used for a laser printer or the like, an image is formed in the following flow. First, the photosensitive member is charged, and the photosensitive member is exposed by irradiating a beam onto the photosensitive member to form an image pattern, and the charged toner is adhered to the photosensitive member and developed to form a toner image on the photosensitive member. Then, the toner image is transferred to a medium such as paper, and finally the toner is melted by heat or the like and fixed on the medium such as paper.

  Here, as an important technique for continuously outputting a good and stable image over time, there is a toner charge control technique, and it is ideal that the toner is charged to the same potential as the initial value over time. . However, it is very difficult to keep the toner charge the same between the initial time and time, and the charge state of the toner fluctuates with time. When the charged state of the toner fluctuates, even if exposure is performed with the same beam spot diameter, the toner spot diameter after development with toner fluctuates, and the output image deteriorates.

  When the charging potential of the toner is high, development is performed to the bottom of the beam spot diameter, and when the charging potential of the toner is low, development is performed only near the center portion of the beam spot. Accordingly, the reason that the toner spot diameter after development changes when the charged state of the toner fluctuates is that the intensity distribution of the beam spot during exposure is Gaussian. Ideally, if the intensity distribution of the beam spot is rectangular instead of Gaussian, the toner spot diameter after development will not change even if the charge state of the toner changes, and a uniform dot image will always be obtained. Therefore, a good image can be provided stably over time.

  However, it is practically difficult to create a beam spot having a rectangular intensity distribution, and the size of the optical system is increased. In view of this, it is practically best to produce the above effect in a pseudo manner by flattening the intensity distribution as much as possible in the vicinity of the center of the beam spot.

  As a method for flattening the intensity distribution as much as possible near the center of the beam spot, as described above, there is known a method in which the beam is superimposed on the surface to be scanned by swinging the beam. However, in order to swing the beam, a movable part is indispensable, and among devices (actuators, MEMS, etc.) that enable such movement, those that are fast enough to handle high-speed image forming apparatuses are currently available. No (optical scanning using MEMS is possible, but since the beam is oscillated many times in one scanning, a very high-speed device is required). In addition, the movable part is a factor that degrades the quality over time, so it is desirable not to use it as much as possible. Therefore, as described above, it is preferable to create a beam spot that is as flat as possible in the optical intensity distribution near the center using an amplitude element having at least two aperture regions and a phase element.

  Note that the amplitude element in the above-described embodiment (for example, FIG. 4A and FIG. 6A) is provided with the second circular (rectangular) opening region in the peripheral portion of the conventionally used aperture. It is the composition. Therefore, a beam spot shape having a flattened intensity distribution is created from a Gaussian shape using a light that has not been used in the past. Almost unchanged, an optical scanning device with no light loss can be obtained even if the beam spot shape is shaped.

  In the above, an amplitude element having at least two aperture regions and a phase element are used to flatten the intensity distribution near the center of the beam spot, and the amplitude element and the phase element are used to constitute an optical scanning device. However, as a method for flattening the intensity distribution in the vicinity of the center portion of the beam spot, methods other than those described above are not excluded. A method of optical scanning using a beam spot having a flat intensity distribution near the center of the beam spot is included in the scope of the present invention. For example, even when an amplitude element and a phase element having one aperture region are used, it is possible to flatten the intensity distribution near the center of the beam spot, which is included in the scope of the present invention.

  When an optical scanning device is configured using the beam spot shaping method described above, it is most desirable to flatten the vicinity of the central portion of the beam spot shape in both the main scanning direction and the sub-scanning direction. However, if flattening is performed for both main scanning and sub-scanning, the following problems occur when applied to an optical scanning device.

  In an optical scanning device, a polygon scanner or the like is usually used as an optical deflection unit. When optical scanning is performed using a polygon scanner or the like, if the beam diameter in the main scanning direction is wide, the angle of view that can be optically scanned becomes narrow, so the polygon scanner becomes larger in order to obtain a desired scanning width on the image plane. And a problem that the optical path length becomes long and the optical scanning device becomes large. Therefore, it is necessary to avoid the increase in the beam diameter in the main scanning direction as much as possible. When the intensity distribution of the beam near the center is flattened using the method as described above, the width of the beam emitted from the amplitude element and the phase element of the present invention is equal to the aperture having a single opening at the center as in the prior art. Becomes wider (assuming that the beam spot diameter on the image plane is the same).

  The beam incident on the reflection surface of the polygon scanner is parallel in the main scanning direction, but is incident so as to be condensed once on the reflection surface of the polygon scanner in the sub-scanning direction. Therefore, it is preferable to flatten the intensity distribution of the beam near the center by the above method only in the sub-scanning direction, so that the polygon scanner and the optical scanning device can be prevented from becoming large. However, it is possible to realize an optical scanning device in which dots are uniform and stable over time.

  For the main scanning direction, a technique called PWM (Purse Width Modulation) is preferably used. PWM is a technique for forming one pixel with a plurality of beam spots (shifted in the main scanning direction) (for example, when 32-value PWM is used at 600 dpi, the main scanning direction is 42.3 μm / 32 = 1.3 μm). The intensity distribution of beam spots is superimposed at intervals), and various gradations can be expressed by PWM. When an image is formed using this PWM, since one pixel is composed of a plurality of beam spots, the intensity distribution of the beam in the vicinity of the center of one pixel compared to when one pixel is formed with one beam spot. Approaches flat. However, it is impossible to superimpose beams in the sub-scanning direction at high density (high density in the sub-scanning direction and printing speed are inversely proportional). Therefore, it is preferable to use both PWM and flattening near the center of the intensity distribution of the beam spot in the sub-scanning direction.

  Since the beam spot shaping as described above is the same shaping for all the beam spots in the image forming area, both the amplitude element and the phase element are provided between the light deflection means and the light source. It is good to install in.

  FIG. 9 shows a basic configuration of the multicolor image forming apparatus.

  In FIG. 9, photoconductors 1Y, 1M, 1C, 1K rotate in the direction of the arrow, and in the order of rotation, chargers 2Y, 2M, 2C, 2K, developing devices 4Y, 4M, 4C, 4K, transfer charging means 6Y, 6M. , 6C, 6K and cleaning means 5Y, 5M, 5C, 5K are provided. The writing unit 20 is formed by using a plurality of optical scanning devices of the present invention.

  The charging members 2Y, 2M, 2C, and 2K are charging members that constitute a charging device for uniformly charging the surface of the photoreceptor. The writing unit irradiates the photosensitive member surface between the charging member and the developing members 4Y, 4M, 4C, and 4K with a writing unit so that an electrostatic latent image is formed on the photosensitive member. Then, based on the electrostatic latent image, a toner image is formed on the photoreceptor surface by the developing member. Further, the respective transfer toner images are sequentially transferred to the recording paper 11 by the transfer charging means 6Y, 6M, 6C, and 6K, and finally the image is fixed on the recording test by the fixing means 30.

  By flattening the intensity distribution near the center of the beam spot shape as described above as much as possible, the toner spot diameter after development does not fluctuate even if the charge state of the toner fluctuates. And can provide a stable and good image over time. In addition, since the toner spot diameter hardly changes over time, the number of process controls for controlling the storage conditions of the electrophotographic process can be reduced, and the toner consumption can be reduced.

  In the above description, the multicolor image forming apparatus is described as an example. However, the present invention can be applied to a single color image forming apparatus.

  The beam shaping method of the present invention can also be applied to a laser processing apparatus.

  When drilling or the like using laser light, if the beam spot has a Gaussian shape, a through hole is formed in the center, but there is an inconvenience that the hole does not penetrate due to insufficient light in the periphery. However, the above problem can be solved by flattening the intensity distribution at the center of the Gaussian beam spot using the beam spot shaping method of the present invention.

It is a figure which shows the outline of an optical scanning device. It is sectional drawing of an optical scanning device. (A), (b), (c) It is a figure which shows a mode that a beam spot is shaped using an amplitude element and a phase element. (A) is an amplitude element having two circular openings. (B) is a phase element which provides a phase difference between two circular openings. (C) is an example in which the amplitude element shown in (a) and the phase element shown in (b) are integrated into one sheet. (A) is the optical system which performed the simulation. (B) is sectional drawing which cut off the simulation result with the line which passes along the center of a 1st circular opening part. (C) is a simulation result when there is no 2nd circular opening area | region. (D) is a simulation result when there exists a 2nd circular opening area | region. (A) is a simulated optical system (an example in which an amplitude element and a phase element are integrated into one sheet). (B) is sectional drawing which cut off the simulation result by the line which passes along the center of a 1st rectangular opening part. (C) is a simulation result when there is no second rectangular opening region. (D) is a simulation result when there exists a 2nd rectangular opening area | region. It is a figure explaining the principle which flattens the intensity distribution near the center part of a Gaussian beam. It is the figure which flattened intensity distribution near the central part of a Gaussian beam by changing the phase difference of the 1st circular opening part and the 2nd circular opening field. 1 is a diagram illustrating an outline of an image forming apparatus.

Explanation of symbols

101, 102, 103, 104 Photosensitive drum 105 Transfer belt 209 Cylindrical lens 213 Polygon mirror 220 Toroidal lens 224, 227 Folding mirror 2181, 2182, 2183, 2384 fθ lens Y Yellow M Magenta C Cyan K Black 1Y, 1M, 1C, 1K photoconductor 2Y, 2M, 2C, 2K charger 20 writing unit 4Y, 4M, 4C, 4K developing device 5Y, 5M, 5C, 5K cleaning means 6Y, 6M, 6C, 6K transfer charging means

Claims (11)

A light source, an amplitude element that blocks and transmits part of the light beam emitted from the light source, a phase element that controls the phase of the light beam, and an imaging optical system that forms an image of the light beam A beam spot shaping method for an optical scanning device comprising:
The amplitude element has at least two aperture regions, and shapes the shape of the beam spot on the image plane.   2. The beam spot shaping method according to claim 1, wherein the shape of the beam spot is shaped so that the intensity distribution near the center of the beam spot is flatter than the intensity distribution near the center of the Gaussian beam.   The beam spot shaping method according to claim 1, wherein the amplitude element has a second opening region around a central opening region.   The beam spot shaping method according to any one of claims 1 to 3, wherein the phase element generates a phase difference in at least two aperture regions.   5. The beam spot shaping method according to claim 1, wherein the amplitude element has an elliptical opening region. 6.   The beam spot shaping method according to any one of claims 1 to 5, wherein the amplitude element and the phase element are integrally formed. A light source, an amplitude element that blocks a part of the light beam emitted from the light source and transmits the light part, a phase element that controls the phase of the light beam, and deflects and scans the light beam from the light source And a scanning optical system that forms an image on the image carrier with the light beam scanned by the deflection unit.
An optical scanning device characterized by shaping a beam spot on the image carrier using the beam spot shaping method according to any one of claims 1 to 6.   8. The optical scanning device according to claim 7, wherein the shape of the beam spot diameter in the sub-scanning direction is shaped.   9. The optical scanning device according to claim 7, wherein one pixel is formed by a plurality of dots in the main scanning direction.   10. The optical scanning device according to claim 7, wherein the amplitude element and the phase element are disposed between an optical deflecting unit and the light source. A developing unit that visualizes the electrostatic image formed on the image carrier with toner, and a transfer unit that transfers the image visualized on the image carrier to a medium such as paper. In an image forming apparatus that outputs
An image forming apparatus, wherein an electrostatic image is formed on the image carrier by the optical scanning device according to claim 7.

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