JP2008209803A - Optical scanner and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical scanner capable of forming a high quality image by scanning in a multibeam system using a plurality of light beams, with its cost reduced. <P>SOLUTION: A rotating polygon mirror is miniaturized by setting respective elements so that, for an example, the size of the rotating polygon mirror is minimized within an extent that a predetermined condition is satisfied while preventing the vignetting of the light beams in a main scanning direction. Thus the cost reduction of apparatus is made possible. Further, the energy consumption and the heat emission are reduced in the driving system of the rotating polygon mirror by miniaturizing the rotating polygon mirror. Accordingly, it is made possible to suppress the deterioration in various optical characteristics such as the increase in the light beam spot diameter, the nonuniformity in the scanning pitch and the variation in the beam pitch in a subscanning direction or the like, due to the variation in temperature. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an optical scanning apparatus and an image forming apparatus, and more particularly, to an optical scanning apparatus that scans a surface to be scanned with a light beam, and an image forming apparatus including the optical scanning apparatus.

  In the current optical scanning device and an image forming apparatus using the same, there are increasing demands for high image quality and full color, high image forming speed, low cost of the apparatus, and the like. Challenges to achieve higher image quality include reduction in scan line curvature, reduction in beam spot diameter variation on the surface to be scanned, reduction in beam spot misalignment by improving constant velocity characteristics, etc. It is.

  The challenge for achieving higher image forming speed is the realization of an optical scanning device that can simultaneously scan the surface to be scanned with a plurality of light beams. In recent years, scanning of a plurality of scanning lines has been performed at once. Possible multi-beam image forming apparatuses have appeared. In this main image forming apparatus, since the light source that emits the light beam has a certain distance in the sub-scanning direction, the curved shape differs between the scanning lines, and as a result, the image quality is reduced. is there.

  In particular, in a full-color image forming apparatus that scans the scanned surfaces corresponding to a plurality of colors with different scanning optical systems and superimposes toner images corresponding to the respective colors, for example, a tandem image forming apparatus, the sub-scanning direction The problem is that the image quality deteriorates due to the occurrence of color misregistration and the difference in image density.

  The use of a vertical cavity surface emitting laser (hereinafter abbreviated as VCSEL) that can easily form a large number of light emitting points for emitting a light beam on one element is particularly effective for multi-beams. As the number increases, the region where the light emitting points are arranged becomes wider around the optical axis, and the above problem becomes very large. Specifically, since there are many light emitting points, the position of the optical element through which the light beam passes differs from channel to channel, and the difference in optical characteristics between the channels increases, so that the beam spot diameter in the main scanning direction or sub-scanning direction is increased. Various problems such as an increase, unevenness in scanning pitch, and an increase in beam pitch in the sub-scanning direction arise.

  In addition, even if a good image can be obtained during normal use, the arrangement and shape of the optical elements in the device change slightly due to changes in the room temperature and environmental changes such as temperature rise in the device due to continuous printing. As the path of the light beam fluctuates, the interval in the sub-scanning direction of the light beam fluctuates, causing a color shift in the output image. This color shift due to environmental fluctuations is a serious problem in image forming apparatuses, in particular color image forming apparatuses, which are increasing in density and image quality, and countermeasures are essential. Therefore, for this problem, for example, techniques described in Patent Document 1, Patent Document 2, and Patent Document 3 have been proposed.

  However, in the image forming apparatus using the VCSEL described in Patent Document 1, the distance between the light sources that are the most separated in the main scanning direction is shorter than the distance between the light sources that are the most separated in the sub-scanning direction. It is difficult to satisfy all the conditions such as cost reduction, ultra-high image density, and securing of the light quantity of the light beam.

  In addition, the image forming apparatus described in Patent Document 2 performs skew adjustment and angle adjustment in the sub-scanning direction with respect to a plurality of cylinder mirrors in a scanning optical system of a plurality of light beams, thereby curving a scanning line. It is an apparatus that can adjust and improve the image quality. However, since the cylinder mirror is expensive, there is a limit to reducing the cost of the apparatus.

  Patent Document 3 discloses an image forming apparatus capable of reducing a focus position variation due to a temperature variation by combining a diffractive surface and a refracting surface in an optical system on the incident side of a deflector. . However, this patent document does not mention a problem in a method of scanning the same photosensitive member with a plurality of light beams.

  In Patent Documents 1 to 3 described above, an appropriate distance between light emitting points of a surface emitting laser light source that generates a plurality of light beams from two-dimensionally arranged light emitting points, and scanning optics corresponding to a surface emitting laser. There is no description of system conditions. Also, there is no disclosure of solutions for problems such as a decrease in scanning speed due to insufficient light quantity during scanning, an increase in cost of the apparatus by using expensive optical elements, an increase in beam spot diameter, etc. At present, the problems such as high image quality, high image forming speed, and low cost of the apparatus have not been cleared.

JP 2004-287292 A JP 2005-309301 A

  The present invention has been made under such circumstances. A first object of the present invention is to form a high-quality image by multi-beam scanning using a plurality of light beams while reducing the cost of the apparatus. It is an object of the present invention to provide an optical scanning device capable of performing the above.

  A second object of the present invention is to provide an image forming apparatus capable of forming a high-quality image while reducing the cost of the apparatus.

According to a first aspect of the present invention, a first optical system includes a light source having a plurality of light emitting points arranged two-dimensionally and a coupling lens for coupling a plurality of light beams emitted from the light emitting points. A rotating polygon mirror having a system, an aperture member disposed at or near the focal position of the coupling lens, a plurality of deflection surfaces, and deflecting a plurality of light beams that have passed through the aperture member; A second optical system for condensing the plurality of light beams deflected by the rotary polygon mirror onto the writing area of the surface to be scanned, and main scanning the writing area on the surface to be scanned with the plurality of light beams In the optical scanning device that scans in the direction, among the plurality of light emitting points, the distance between the light emitting points farthest in the main scanning direction is W, and the dimension in the main scanning direction of the opening provided in the aperture member is E. The inscribed circle radius of the rotary polygon mirror is A, the number of the deflection surfaces of the rotary polygon mirror is M, the incident angle at which the light beam is incident on the deflection surface is α, and from the start of scanning of the writing area to the end of scanning. When the angle of view of the second optical system is φ, the distance from the aperture member to the rotary polygon mirror is x, and the focal length of the coupling lens is f ₁ , the following equation is satisfied: An optical scanning device characterized by the above.

  According to this, within the range that satisfies the above conditions, for example, if each element is set so that the size of the rotary polygon mirror is minimized, the light beam in the main scanning direction is avoided from being kicked. Therefore, it is possible to reduce the size of the rotary polygon mirror. As a result, the cost of the apparatus can be reduced. In addition, the downsizing of the rotary polygon mirror can reduce energy consumption and heat generation in the drive system of the rotary polygon mirror. Therefore, it is possible to suppress deterioration of various optical characteristics such as an increase in the spot diameter of the light beam due to temperature fluctuation, unevenness in scanning pitch, and fluctuation in beam pitch in the sub-scanning direction. Further, even when the rotary polygon mirror has a conventional size, the angle of view for scanning the surface to be scanned can be increased. Thereby, since the path of the light beam after the rotary polygon mirror can be shortened, it is possible to reduce the size and cost of the optical scanning device.

  According to a second aspect of the present invention, there is provided an image forming apparatus for forming an image by fixing a toner image formed based on a latent image obtained from information relating to an image to a recording medium. An optical scanning device; a photosensitive member on which a latent image is formed by the optical scanning device; a developing unit that visualizes the latent image formed on the surface to be scanned of the photosensitive member; And a transfer unit that fixes the toner image to the recording medium.

  According to this, the image forming apparatus includes the optical scanning device of the present invention. Therefore, it is possible to accurately form an image on the recording medium while reducing the cost of the apparatus.

  Hereinafter, an embodiment of the present invention will be described with reference to FIGS. FIG. 1 shows a schematic configuration of an image forming apparatus 200 according to the present embodiment.

  The image forming apparatus 200 is a printer that prints an image by transferring a toner image onto plain paper (paper) using a Carlson process. As shown in FIG. 1, the image forming apparatus 200 includes an optical scanning device 100, a photosensitive drum 201, a charging charger 202, a toner cartridge 204, a cleaning case 205, a paper feeding tray 206, a paper feeding roller 207, and a registration roller pair 208. A transfer charger 211, a fixing roller 209, a paper discharge roller 212, a paper discharge tray 210, and a housing 215 for housing them.

  The housing 215 has a substantially rectangular parallelepiped shape, and openings that communicate with the internal space are formed on the side walls on the + X side and the −X side.

  The optical scanning device 100 is disposed above the housing 215 and scans the surface of the photosensitive drum 201 by deflecting a light beam modulated based on image information in the main scanning direction (Y-axis direction in FIG. 1). To do. The configuration of the optical scanning device 100 will be described later.

  The photosensitive drum 201 is a cylindrical member in which a photosensitive layer having a property that becomes conductive when irradiated with a light beam on the surface thereof. The direction is arranged as a longitudinal direction, and is rotated clockwise in FIG. 1 (direction indicated by an arrow in FIG. 1) by a rotation mechanism (not shown). In the vicinity thereof, the charging charger 202 is disposed at the 12 o'clock (upper) position in FIG. 1, the toner cartridge 204 is disposed at the 2 o'clock position, and the transfer charger 211 is disposed at the 6 o'clock position. A cleaning case 205 is disposed at the hour position.

  The charging charger 202 is disposed with a predetermined clearance with respect to the surface of the photosensitive drum 201, and charges the surface of the photosensitive drum 201 with a predetermined voltage.

  The toner cartridge 204 includes a cartridge main body filled with toner, a developing roller charged with a voltage having a polarity opposite to that of the photosensitive drum 201, and the toner charged in the cartridge main body via the developing roller. Supply to the surface.

  The cleaning case 205 includes a rectangular cleaning blade whose longitudinal direction is the Y-axis direction, and is disposed so that one end of the cleaning blade is in contact with the surface of the photosensitive drum 201. The toner adsorbed on the surface of the photosensitive drum 201 is peeled off by the cleaning blade as the photosensitive drum 201 rotates, and is collected in the cleaning case 205.

  The transfer charger 211 is arranged with a predetermined clearance with respect to the surface of the photosensitive drum 201, and a voltage having a polarity opposite to that of the charging charger 202 is applied.

  The paper feed tray 206 is disposed in a state where the + X side end protrudes from an opening formed in the side wall on the + X side of the housing 215 and can accommodate a plurality of sheets 213 supplied from the outside. .

  The sheet feeding roller 207 takes out the sheets 213 one by one from the sheet feeding tray 206, and enters a gap formed by the photosensitive drum 201 and the transfer charger 211 via a registration roller pair 208 including a pair of rotating rollers. To derive.

  The fixing roller 209 is composed of a pair of rotating rollers, overheats and pressurizes the paper 61, and guides it to the paper discharge roller 212.

  The paper discharge roller 212 is composed of a pair of rotating rollers and the like, and with respect to the paper discharge tray 210 disposed with the −X side end protruding from the opening formed in the −X side side wall of the housing 215, The sheets 213 sent from the fixing roller 209 are sequentially stacked.

  Next, the configuration of the optical scanning device 100 will be described. FIG. 2 is a diagram showing a schematic configuration of the optical scanning device 100. As shown in FIG. 2, the optical scanning device 100 includes a light source 10, a coupling lens 11, an aperture member 12, a line image forming lens 13, The polygon mirror 15, the first scanning lens 16 and the second scanning lens 17 that are sequentially arranged on the + X side of the polygon mirror 15, and the light beam that has passed through the second scanning lens 17 is reflected and guided to the photosensitive drum 201. And a mirror 18. Hereinafter, a coordinate system in which the arrangement direction of the coupling lens 11, the aperture member 12, the line image forming lens 13, and the polygon mirror 15 is defined as the x-axis direction is defined, and an explanation based on this coordinate system will be given as appropriate.

  The light source 10 is a surface-emitting type semiconductor laser array in which, for example, VCSELs are two-dimensionally arranged as light emitting points. As shown in FIG. 3, 40 VCSELs on the light emitting surface (the surface on the −x side) They are arranged in a matrix of 4 rows and 10 columns with the direction parallel to the straight line L1 forming an angle θ1 with the y axis as the row direction and the direction parallel to the z axis as the column direction. Each VCSEL has a near-field pattern diameter of 4 μm, and a light beam having a wavelength of 780 nm is emitted with a divergence angle of 7 ± 1 degrees in the main scanning direction and the sub-scanning direction. In this embodiment, the row interval Dz is 25.0 μm, the column interval Dy is 30.0 μm, and the interval dz between adjacent VCSELs in the Z-axis direction (sub-scanning direction) of each VCSEL is 2.5 μm. (At 4800 dpi).

  The coupling lens 11 is a lens having a focal length of 47.7 mm, and couples the light beam from the light source 10 at the focal position on the emission side.

  The aperture member 12 has a rectangular or elliptical opening having a size in the y-axis direction (main scanning direction) of 5.44 mm and a size in the z-axis direction (sub-scanning direction) of 2.10 mm. It arrange | positions so that an aperture center may be located in the focus position vicinity of the coupling lens 11. FIG.

  The line image forming lens 13 is a cylindrical lens having a focal length of 107.0 mm, and forms an image of the light beam that has passed through the aperture member 12 in the vicinity of the reflecting surface of the polygon mirror 15 in the sub-scanning direction.

  The polygon mirror 15 is a quadrangular prism-shaped member whose upper surface is a square inscribed in a circle having a radius of 7 mm. Deflection surfaces are formed on the four side surfaces of the polygon mirror 15, and are rotated at a constant angular velocity around an axis parallel to the Z axis by a rotation mechanism (not shown). As a result, the light beam incident on the polygon mirror 15 is deflected (scanned) in the Y-axis direction.

The first scanning lens 16 and the second scanning lens 17 are, for example, resin-made scanning lenses having thicknesses of 13.5 mm and 3.5 mm at the center (on the optical axis), and their optical surfaces. The shape is represented by a function represented by the following formula (1) and the following formula (2). Where Y is the coordinate in the main scanning direction with the optical axis position as the origin, Rm is the radius of curvature of the lens, a ₀₀ , a ₀₁ , a ₀₂ ,... Are the aspherical coefficients of the main scanning shape, and R _S0 is the curvature on the optical axis in the sub-scanning direction, and b ₀₀ , b ₀₁ , b ₀₂ ,... Are aspherical coefficients of the sub-scanning shape. The values of the coefficients are as shown in Table 1 below.

  FIG. 4A shows a function Cs (Y) for each lens height of the curvature in the sub-scanning direction of the first scanning lens 16 represented by Expression (2), and is shown by a dotted line in FIG. 4A. The curved line indicates the optical surface shape of the entrance surface L1R1, and the curve indicated by the solid line indicates the optical surface shape of the exit surface L1R2. FIG. 4B shows a function Cs (Y) for each lens height of the curvature in the sub-scanning direction of the second scanning lens 17 shown by the equation (2). The dotted line in FIG. The curve indicated by indicates the optical surface shape of the entrance surface L2R1, and the curve indicated by the solid line indicates the optical surface shape of the exit surface L2R2.

  FIG. 5A shows the shape of the first scanning lens 16 in the main scanning section shown by the equation (2), and the curve shown by the dotted line in FIG. 5A is the incident surface L1R1. The optical surface shape is shown, and the curve indicated by the solid line shows the optical surface shape of the exit surface L1R2. FIG. 5B shows the shape of the second scanning lens 17 in the main scanning section, and the curve shown by the dotted line in FIG. 5B shows the optical surface shape of the incident surface L2R1, and is a solid line. The curve shown shows the optical surface shape of the exit surface L2R2. The vertical axis indicates the coordinate X in the optical axis direction, the horizontal axis indicates the lens height, and the center in the sub-scanning direction when Y = 0 in Equation (2) unless otherwise specified. The axis that passes through the point.

  The optical distances d1, d3, d4, d6, d7, d9, d11 between the elements shown in FIG. 2 and the sizes d2, d5, d8, d10 of each element in the optical axis direction are shown in Table 2 below. The horizontal magnification in the sub-scanning direction of the entire optical system included in the optical scanning device 100 is 2.18 times, and the sub-scanning magnification of only the optical system on the scanning surface side from the polygon mirror 15 is -0.97. It has doubled. In the optical scanning device 100, the aim of the beam spot diameter on the surface to be scanned is set to 52 μm in the main scanning direction and 55 μm in the sub-scanning direction. The writing area has a range of ± 161.5 mm in the main scanning direction (Y-axis direction) with the point O shown in FIG. 2 as the center, and the rotation angle of the polygon mirror 15 when scanning this range ( The angle of view is 36 degrees and the angle of view is 72 degrees. Note that point O is a point where a straight line passing through the rotation center of the polygon mirror 15 and parallel to the X axis in FIG. 2 intersects the surface to be scanned of the photosensitive drum 201.

  Next, the operation of the image forming apparatus 200 configured as described above will be described. When image information is received from the host device, the optical scanning device 100 is driven by the modulation data based on the image information, and 40 light beams modulated based on the image information are emitted from the light source unit 70. These light beams are formed into substantially parallel light by the coupling lens 11 and then pass through the aperture member 12.

  When each light beam that has passed through the aperture member 12 is condensed on the deflection surface of the polygon mirror 15 by the line image forming lens 13, it is deflected in the Y-axis direction by the polygon mirror 15, and the first scanning lens 16 and the second scanning are performed. After the moving speed of the light beam spot in the main scanning direction is adjusted by the lens 17, the light is condensed on the surface of the photosensitive drum 201 via the reflection mirror 18.

  The surface of the photosensitive drum 201 is charged with a predetermined voltage by the charging charger 202, so that charges are distributed at a constant charge density. Then, when the photosensitive drum 201 is scanned by the light beam deflected by the polygon mirror 15, carriers (charges) are generated in the photosensitive layer irradiated with the light beam, and charge movement occurs in that portion and the potential is lowered. To do. Accordingly, the photosensitive drum 201 rotating in the direction of the arrow in FIG. 1 is scanned with a light beam modulated based on image information, thereby forming an electrostatic latent image defined by the charge distribution on the surface. Is done.

  When an electrostatic latent image is formed on the surface of the photosensitive drum 201, toner is supplied to the surface of each photosensitive drum 201 by the developing roller of the toner cartridge 203. At this time, since the developing roller of the toner cartridge 203 is charged with a voltage having a polarity opposite to that of the photosensitive drum 201, the toner attached to the developing roller is charged with the same polarity as that of the photosensitive drum 201. Therefore, no toner adheres to the portion of the surface of the photosensitive drum 201 where the electric charges are distributed, and the toner adheres only to the scanned portion, so that the electrostatic latent image is visualized on the surface of the photosensitive drum 201. A toner image is formed. The toner image is attached to the sheet 213 by the transfer charger and then fixed by the fixing roller 209 to form an image on the sheet. The paper 213 on which the image is formed in this manner is discharged by the paper discharge roller 212 and sequentially stacked on the paper discharge tray 210.

In the optical scanning device 100 according to the present embodiment, the margin width δ with respect to the polygon mirror 15 when viewed from the light beam incident on the polygon mirror 15 is the distance between the light emitting points most distant in the main scanning direction of the light source 10. W (= 270 μm), the size of the aperture provided in the aperture member 12 in the main scanning direction is E (= 5.44 mm), the inscribed circle radius of the polygon mirror 15 is A (= 7 mm), and the polarization of the polygon mirror 15 The number of surfaces is M (= 4), the incident angle at which the light beam enters the polygon mirror 15 is α (= 70 degrees), the angle of view when scanning the writing area is φ (= 72 degrees), and the aperture member 12 When the distance to the polarization plane of the polygon mirror 15 is x (= 61 mm) and the focal length of the coupling lens 11 is f ₁ (= 47.7 mm), it can be expressed by the following equation (3). In this embodiment, the margin width δ is 0.93, which is larger than zero.

  For example, if each element is set so that the size of the polygon mirror 15 is minimized within the range satisfying the condition expressed by the expression (3), the light beam in the main scanning direction is avoided from being kicked. Therefore, the polygon mirror 15 can be downsized. As a result, the cost of the apparatus can be reduced. Further, by reducing the size of the polygon mirror 15, it is possible to reduce energy consumption and heat generation in the drive system of the polygon mirror 15. Therefore, it is possible to suppress deterioration of various optical characteristics such as an increase in the spot diameter of the light beam due to temperature variation, uneven scanning pitch, and variation in beam pitch in the sub-scanning direction, and high-quality images can be obtained even in continuous printing. be able to. Further, even when the rotary polygon mirror has a conventional size, the angle of view for scanning the surface to be scanned can be increased. Thereby, since the path of the light beam after the rotary polygon mirror can be shortened, it is possible to reduce the size and cost of the optical scanning device.

  In the case of using the same optical system except for the shape of the polygon mirror 15 and using, for example, a polygon mirror having six polarization planes and an inscribed circle radius of 13 mm, the margin width δ is −1.23, and the formula ( The condition shown in 3) is not satisfied, and in this case, the light beam incident on the polygon mirror is kicked. When a polygon mirror having 6 polarization planes is used, if the inscribed circle radius is 18 mm, the margin width δ is 0.37, which satisfies the condition expressed by the expression (3), and the light beam Thus, an optical scanning device with little deterioration of various optical characteristics can be obtained.

  In the first term of Equation (3), 2A · tan (π / M) represents the length of the reflecting surface in the main scanning direction and is multiplied by COS ((α + (φ / 4) / 2). This represents the length of the projection of the light beam on the polarization plane of the polygon mirror 15. The third term represents the width of the light beam in the main scanning direction incident on the rotary polygon mirror, and the fourth term represents the main length of the reflecting surface. The center in the scanning direction represents the length of the moving distance when scanning one line as viewed from the incident direction of the light beam.From the above, the value obtained by subtracting the second, third, and fourth terms from the first term. By being larger than 0, it is possible to obtain an optical scanning device that is not kicked by a rotating polygon mirror.

  In the present embodiment, the light emitting points of the light source 10 are arranged in a direction parallel to the straight line L1 with the interval between the light emitting points adjacent in the sub-scanning direction being equal to 2.5 μm (at 4800 dpi). The number of light emitting points is ten, and the number of light emitting points arranged in the sub-scanning direction is four. The distance between the light emitting points farthest in the sub-scanning direction is smaller than the distance between the light emitting points farthest in the main scanning direction. As a result, more light emission points are arranged in the main scanning direction than in the sub-scanning direction, and the following advantages are obtained.

  In the light source 10, ten light emitting points are arranged in the main scanning direction, the distance between the light emitting points farthest in the main scanning direction is 270 μm (Dm × 9), and the total number of light emitting points is 40. The distance between the light emitting points farthest from each other is 97.5 μm (= d × 39). In order to perform high-density scanning, light emission points are concentrated in the sub-scanning direction, and in order to improve the performance, yield, and life of the element, the light-emitting point array region is longer in the main scanning direction than in the sub-scanning direction. It is desirable to have a form. If the light emitting points are arranged over a wide range in the sub-scanning direction, the deterioration of the optical performance due to the temperature change is remarkably increased. The dot position shift in the sub-scanning direction occurs due to the expansion of the scanning lens accompanying a change in temperature and the change in optical magnification due to the change in refractive index, but this increases as the distance from the optical axis of the optical system increases in the sub-scanning direction. Further, the influence of the variation in the scanning line interval due to the temperature becomes the maximum among the light emitting points farthest in the sub scanning direction. The reason for this is that due to the difference in sub-scanning lateral magnification in the scanning optical system, the scanning line by the + side emission point and the scanning line by the − side emission point in the sub-scanning direction with respect to the optical axis are bent. This is because the direction is reversed. A large error in the scanning line interval that occurs when scanning lines that are bent in the opposite direction are adjacent to each other is a dot position shift in the sub-scanning direction, which is perceived as banding that appears in each scanning, causing the scanning quality to deteriorate. Become. In the case where the scanning lens is made of a resin-made product in order to reduce the cost of the apparatus, this problem becomes more prominent because the thermal expansion and the temperature change of the refractive index are larger than those of glass. In the present invention, these problems are effectively improved.

  FIG. 6 shows a modification of the light source 10. Here, Dy is 40 μm, Dz is 45 μm, dz is 4.5 μm (at 2400 dpi), the distance between light emitting points farthest in the main scanning direction is 360 μm, and between light emitting points farthest in the sub scanning direction. The distance is 85.5 mm. Therefore, even at 2400 dpi, in order to realize low-cost and high-quality optical scanning of the apparatus, it is desirable that the width in the sub-scanning direction of the light emitting point array region is 100 μm or less. When this light source is used, the margin width δ is 0.81, and there is little deterioration or fluctuation in optical characteristics between channels.

  Next, the relationship among the magnification of the optical system, the near field pattern of the light emitting portion, and the beam spot diameter will be described.

  Beam spot diameter and defocus when using an optical system with a coupling lens focal length of 47.7 mm (horizontal magnification in the main scanning direction is approximately 5.0 times and lateral magnification in the sub-scanning direction is 2.0 times). The simulation result of the relationship with the quantity is shown in FIG. 7 (A) to FIG. 10 (B). FIG. 7A shows the beam spot diameter in the main scanning direction when the near field pattern (Am) in the main scanning direction is infinitesimal (= 0), and FIG. 7B shows the sub-scanning. The beam spot diameter in the sub-scanning direction when the near field pattern (As) in the direction is infinitesimal (= 0) is shown. FIG. 8A shows the beam spot diameter in the main scanning direction when Am is 2 μm, and FIG. 8B shows the beam spot diameter in the sub-scanning direction when As is 2 μm. 9A shows the beam spot diameter in the main scanning direction when Am is 4 μm, and FIG. 9B shows the beam spot diameter in the sub-scanning direction when As is 4 μm. FIG. 10A shows the beam spot diameter in the main scanning direction when Am is 6 μm, and FIG. 10B shows the beam spot diameter in the sub-scanning direction when As is 6 μm.

  Beam spot diameter when using a conventional optical system with a coupling lens focal length of 26.8 mm (horizontal magnification in the main scanning direction is about 8.9 times and lateral magnification in the sub-scanning direction is 4.5 times) 11A to 14B show simulation results of the relationship between the distance and the defocus amount. FIG. 11A shows the beam spot diameter in the main scanning direction when Am is infinitesimal (= 0), and FIG. 11B shows the sub-scanning direction when As is infinitesimal (= 0). The beam spot diameter is shown. 12A shows the beam spot diameter in the main scanning direction when Am is 2 μm, and FIG. 12B shows the beam spot diameter in the sub-scanning direction when As is 2 μm. 13A shows the beam spot diameter in the main scanning direction when Am is 4 μm, and FIG. 13B shows the beam spot diameter in the sub-scanning direction when As is 4 μm. 14A shows the beam spot diameter in the main scanning direction when Am is 6 μm, and FIG. 14B shows the beam spot diameter in the sub-scanning direction when As is 6 μm.

  In these optical systems, the target value of the beam spot diameter in the main scanning direction is 52 ± 5 μm, the target value of the beam spot diameter in the sub-scanning direction is 55 ± 5 μm, and each of FIGS. The boundary line is indicated by a broken line in the figure.

  In the conventional optical system, the depth of focus is sufficiently secured until Am is 2 μm, and the variation of the beam spot diameter is not so large. However, if Am = 4 μm or more, the depth of focus is drastically decreased in the main scanning direction, resulting in a device in which variation tends to increase. On the other hand, in the optical system similar to the present embodiment, the depth of focus is ensured up to Am = 6, and the increase of the beam spot diameter is allowed.

Thus, it is understood that a low lateral magnification optical system is required for a light source having a near field pattern of several μm or more. Since the VCSEL is such a light source, the lateral magnification of the entire optical system including the near field pattern A and the scanning optical system in at least one of the direction corresponding to the sub-scanning direction and the direction corresponding to the main scanning direction. It is desirable that the following expression (4) is satisfied using β and the beam spot diameter ω on the surface to be scanned.
{(Ω / β · A) ² −1/2} ⁻² <0.7 (4)

This will be described with reference to FIG. FIG. 15 shows the difference between the beam spot diameter when (β × A) = 0 (ie, A = 0) and the beam spot diameter when (β × A)> 0 when the defocus amount is 0. FIG. 7 (A) to FIG. 14 (B) are obtained and plotted. The curve in FIG. 15 is a curve showing the following equation (5). Here, ω ₀ is the beam spot diameter when A = 0, and it is assumed that the relationship with the beam spot diameter ω when A ≠ 0 is expressed by the following equation (6).

From FIG. 15, it can be seen that the increase in the beam spot diameter in the actual writing optical system agrees well with the above assumption. An increase rate δ of the beam spot diameter generated because A has a magnitude is expressed by the following equation (7). However, k = βA / ω ₀ .

In the present embodiment, k = 5.0 × 4 ÷ 50 = 0.40 with respect to the main scanning direction. At this time, δ is suppressed to about 4%, and the beam spot diameter in the main scanning direction is almost 52 μm. It does not change. Since k = 0.14 in the sub-scanning direction, the change is further small, and substantially no change with respect to ω ₀ = 55 μm, and becomes 55.5 μm.

In the conventional example, since the lateral magnification is 8.9 times in the main scanning direction, k = 0.712, and ω = 56 μm, which is an increase of 10% or more. When the near field pattern is increased, for example, when A = 7 μm, when the lateral magnification is 5 and the target beam spot diameter is 50 μm, k = 0.7 and ω = 56 μm. Therefore, this is also not desirable for improving the image quality. If k = 0.65 (for example, β = 5, A = 6.5, ω ₀ = 50), the change in image quality is within an allowable range because ω = 55 μm and the change is about 10%.

Therefore, it is desirable that at least k = βA / ω ₀ <0.7. At this time, the relationship with the actual beam spot diameter ω is expressed by the following equation (8) from the above equation (7).

  The VCSEL has only about 1 to 2 mW, and there is an improvement in light output as a problem. However, it is known that a larger near field pattern is advantageous for this. Even if the diameter can be increased and the light output can be increased in a region where no multimode oscillation occurs, the image quality cannot be improved unless the beam spot diameter can be reduced. Therefore, by setting so that the above expression (4) is satisfied, the beam spot diameter can be reduced while using a VCSEL advantageous in light quantity.

  In the light source 10 of the present embodiment, the distance between the light emitting points arranged farthest in the main scanning direction is 270 μm, and the distance between the light emitting points arranged farthest in the sub scanning direction is 97.5 μm. The lateral magnification in the main scanning direction of the optical scanning device 100 is larger than the lateral magnification in the sub-scanning direction of the optical system. Therefore, when performing high-density writing, the beam pitch in the sub-scanning direction can be set to a desired size. Hereinafter, a description will be given based on FIGS. 16A and 16B.

FIG. 16A shows a conventional surface emitting laser arranged two-dimensionally. As shown in FIG. 16A, in the conventional surface emitting laser, when the main scanning direction is substantially the column direction and the sub-scanning direction is the row direction, the VCSELs as the light emitting points are arranged in a matrix of 8 rows and 4 columns. Has been placed. When the interval between Nn light emitting points arranged substantially in the main scanning direction is Dn and the light emitting point interval arranged Nm in the sub-scanning direction is Dm, the distance between the light emitting points most distant in the main scanning direction is (Nn−1). ) × Dn = 3Dn (Nn = 4), and the distance between the light emitting points farthest in the sub-scanning direction is (Nm−1) × Dm = 7Dm (Nm = 8), and 7Dm> 3Dn. At this time, the pitch of the scanning line interval on the surface to be scanned is | β _m | × Dn / 4, and it is difficult to obtain a desired pitch in the case of high density correspondence. For example, in the case of 2400 dpi, 25.4 / 2400 = 10.6 μm, and in the case of 4800 dpi, 25.4 mm / 4800 = 5.3 μm, but | β _m | × Dn / Nn is made equal to this value. There is a need. Β _m is a lateral magnification of the optical scanning apparatus 100 in the main scanning direction.

One means for solving this will be described with reference to FIG. In the light source shown in FIG. 16B, the light emitting points are arranged in a matrix of 4 rows and 8 columns. Therefore, the distance between the light emitting points farthest in the main scanning direction is (Nn−1) × Dn = 7Dn (Nn = 8), and the distance between the light emitting points farthest in the sub scanning direction is (Nm−1) ×. Dm = 3Dm (Nm = 4), and 7Dn> 3Dm. Therefore, the distance between the light emitting points farthest in the main scanning direction is longer than the distance between the light emitting points farthest in the sub scanning direction. At this time, the pitch of the scanning line interval on the surface to be scanned is | β _m | × Dn / 7, which makes it easier to obtain a desired pitch in the case of high density support than in the conventional example. That is, if the distance between the light emitting points farthest in the main scanning direction is set longer than the distance between the light emitting points farthest in the sub scanning direction, it is easy to cope with high density.

Further, by making the absolute value | β _m | of the lateral magnification in the sub-scanning direction of the optical scanning device 100 smaller than the lateral magnification | β _n | in the main scanning direction, the pitch on the surface to be scanned can be reduced and the density is high. Correspondence to scanning becomes easy. Although a method is also conceivable in which the distance between the light emitting points farthest in the main scanning direction is made shorter than the distance between the light emitting points farthest in the sub scanning direction and | β _m | Various problems occur, such as a decrease in

  Further, in the present embodiment, the light emitting points of the light source 10 are arranged at equal intervals in the sub scanning direction, and 10 light emitting points are arranged in a direction parallel to the straight line L1, and four light emitting points are arranged in the sub scanning direction. Are arranged. In this way, the number of light emitting points arranged in the direction along the straight line L1 that forms a predetermined angle with the main scanning direction so that the light emitting points are equally spaced in the sub scanning direction is parallel to the sub scanning direction. By increasing the number of light emitting points arranged in any direction, the beam pitch in the sub-scanning direction can be set to a desired size when performing high-density writing.

  In this embodiment, the distance Dz (= 25 μm) between the light emitting points in the sub-scanning direction is smaller than the distance Dy (= 30 μm) between the light emitting points adjacent in the main scanning direction. As described above, how to reduce the scanning line interval on the surface to be scanned is a big problem. By making Dz smaller than Dy and widening the element spacing in the main scanning direction that does not affect the density increase in the sub-scanning direction, the influence of thermal interference between the elements is reduced, and wiring to each element is performed. The space necessary for passing through can be secured.

  In the present embodiment, the optical element of the first optical system is an optical element having a diffractive optical surface, thereby reducing the occurrence of curvature of field due to temperature fluctuations and reducing the cost of the optical element. Is possible.

  The combination of the diffractive optical effect and the refraction effect can reduce the beam waist position (focus position) change accompanying the temperature change, but there are the following problems. For example, when a resin lens having an aspheric surface is used as the optical element of the first optical system, if the temperature rises in the optical scanning device due to heat generated by rotation of the polygon scanner or fixing means of the image forming apparatus, the wavelength increases. Refractive index reduction, element shape expansion, and the like occur.

  When a normal refracting surface is used, the beam waist position changes in a direction away from the deflecting means (here, a plus direction) due to an increase in wavelength or the like. However, the diffractive surface changes so that the focal length becomes shorter as the wavelength becomes longer. When the temperature rises, the diffractive surface changes in the direction in which the focal length becomes shorter (in this case, the negative direction). This cancels out the fluctuation caused by the refracting surface, and as a total, the focal length at the time of temperature fluctuation can be reduced.

By the way, when a plurality of beams pass through the diffraction surface, there are the following problems.
1) Difference in diffraction efficiency due to wavelength difference between multiple beams
2) Keep the scanning line spacing between multiple beams uniform
3) Reduction in beam spot diameter with multiple beams Here, the difference in diffraction efficiency due to the wavelength difference is better when the diffraction surface is designed with a first order, the smaller the diffraction surface, the more stable the beam waist position described above. In consideration of the optimization, in the case of a plurality of beams, it is preferable to set only one diffraction surface in the first optical system. For example, considering the case where a plurality of semiconductor lasers having a center value of 780 nm are used, the specification of the semiconductor laser is normally 780-10 / + 15 nm, and a difference of about 25 nm at maximum may occur. At this time, if there is only one diffractive surface, the difference in diffraction efficiency is 0.35%, which is within the allowable level for an optical scanning device compatible with high image quality. The difference is 0.7%, which causes a problem as an optical scanning device compatible with high image quality. Further, in the case of a plurality of beams, the first optical system has only one diffractive surface in order to obtain fluctuations in scanning line intervals between the plurality of beams due to wavelength differences and a reduction in beam spot diameter in the plurality of beams. Is good. Here, the case where a resin lens is used as the scanning lens has been described, but the present invention can also be applied to a case where a glass lens is used as the optical element of the first optical system.

  In the above embodiment, the case where the scanning device 100 is used in the single-color image forming apparatus 200 has been described. However, the image forming apparatus corresponds to a color image and may be a tandem color machine including a plurality of photosensitive drums. good.

  Hereinafter, a multicolor image forming apparatus 1000A that corresponds to a color image and includes a plurality of photosensitive drums will be described with reference to FIGS. 17A and 17B. The image forming apparatus 1000 </ b> A is an apparatus that visualizes a plurality of color images by scanning and exposing a plurality of scanned surfaces and superimposing the plurality of images to form a color image.

  In image forming apparatus 1000A, photoconductive photoreceptors 1302a to 1302d rotate at a constant speed in a clockwise direction as indicated by arrows in the drawing. The surfaces of the photoreceptors 1302a to 1302d are uniformly charged by the charging device 1305, and the surface of the photoreceptor is scanned by the optical scanning device 1301 similarly to the image forming apparatus 200 described above. The electrostatic latent images formed on the photoreceptors 1302a to 1302d by this scanning are visualized as toner images by the developing device 1303, respectively. Each toner image is transferred in a state of being superimposed on the intermediate transfer belt 1308 by the transfer unit 1306, and the toner image is fixed on the paper S by the transfer unit 1309, thereby forming a full color image.

  Note that the corresponding colors of the photoreceptors 1302a to 1302d can be cyan C, magenta M, yellow Y, and black K, and the correspondence with 1302a to d can be freely selected and optimized. For example, in this embodiment, it is configured to correspond to the colors of YMCK in the order of abcd. For optical scanning devices corresponding to positions where it is difficult to improve scanning quality, and optical scanning devices corresponding to photoreceptors that require reduction of adjustment processes, the corresponding colors can be selected according to the level of brightness and visibility. It is desirable to choose. For example, it is desirable to use yellow Y having low visibility or high brightness for a photoreceptor corresponding to an optical system having the worst optical characteristics (large curvature of field, large scanning line curvature, etc.). Also, the stability of the entire apparatus can be improved by setting the durability of the photoconductor that is frequently used, for example, by increasing the size of the photoconductor corresponding to black K, for example. Further, as a modification of the multicolor image forming apparatus 1000A, a system using a plurality of optical scanning devices can be adopted as in the multicolor image forming apparatus 1000B shown in FIG.

  In each of the above embodiments, the case where the optical scanning device of the present invention is used in a printer has been described. However, the image forming device other than the printer, for example, a copier, a facsimile, or a multifunction machine in which these are integrated. Is preferred.

1 is a diagram illustrating a schematic configuration of a printer 200 according to a first embodiment of the present invention. 1 is a diagram illustrating a schematic configuration of an optical scanning device 100. FIG. It is a figure which shows the light source. 4A is a diagram illustrating a function Cs (Y) of the first scanning lens 16 in the sub-scanning direction for each lens height, and FIG. 4B is a diagram illustrating the second scanning lens 17 in the sub-scanning direction. It is a figure showing function Cs (Y) for every lens height of curvature. FIG. 5A is a diagram showing the shape of the first scanning lens 16 in the main scanning section, and FIG. 5B is a diagram showing the shape of the second scanning lens 17 in the main scanning section. It is a figure which shows the modification of the light source. FIGS. 7A and 7B are views (No. 1) for explaining the relationship between the beam spot diameter and the defocus amount when the same optical system as that of the present embodiment is used. FIGS. 8A and 8B are views (No. 2) for explaining the relationship between the beam spot diameter and the defocus amount when the same optical system as that of the present embodiment is used. FIGS. 9A and 9B are views (No. 3) for explaining the relationship between the beam spot diameter and the defocus amount when the same optical system as that of the present embodiment is used. FIGS. 10A and 10B are diagrams (No. 4) for explaining the relationship between the beam spot diameter and the defocus amount when the same optical system as that of the present embodiment is used. FIG. 11A and FIG. 11B are views (No. 1) for explaining the relationship between the beam spot diameter and the defocus amount when a conventional optical system is used, respectively. FIGS. 12A and 12B are views (No. 2) for explaining the relationship between the beam spot diameter and the defocus amount when the conventional optical system is used, respectively. FIGS. 13A and 13B are views (No. 3) for explaining the relationship between the beam spot diameter and the defocus amount when the conventional optical system is used. FIGS. 14A and 14B are views (No. 4) for explaining the relationship between the beam spot diameter and the defocus amount when a conventional optical system is used, respectively. It is a figure for demonstrating the difference of the beam spot diameter when a defocus amount is 0, and the beam spot diameter when ((beta) * A) = 0 and (beta * A)> 0. 16A and 16B, the distance between the light emitting points arranged farthest in the main scanning direction is longer than the distance between the light emitting points arranged farthest in the sub scanning direction. FIG. 10 is a diagram for explaining an effect obtained when the lateral magnification in the main scanning direction of the optical scanning device 100 is made larger than the lateral magnification in the sub-scanning direction of the optical system. FIG. 17A is a diagram illustrating a schematic configuration of the image forming apparatus 1000A, and FIG. 17B is a diagram illustrating a schematic configuration of the image forming apparatus 1000B.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Light source, 11 ... Coupling lens, 12 ... Aperture member, 13 ... Line image formation lens, 15 ... Polygon mirror, 16 ... 1st scanning lens, 17 ... 2nd scanning lens, 18 ... Reflection mirror, 200 ... Image formation Device 201: Photosensitive drum 202: Charger charger 204 ... Toner cartridge 205 ... Cleaning case 206 ... Paper feed tray 207 ... Paper feed roller 208 ... Registration roller pair 209 ... Fixing roller 210 ... Paper discharge tray 211, transfer charger, 212, paper discharge roller, 213, paper, 215, housing.

Claims (7)

A light source having a plurality of light emitting points arranged two-dimensionally, a first optical system including a coupling lens for coupling a plurality of light beams emitted from the light emitting points, and a focal position of the coupling lens or An aperture member disposed in the vicinity of the focal position, a rotating polygon mirror having a plurality of deflecting surfaces and deflecting a plurality of light beams that have passed through the aperture member, and the plurality of deflected beams by the rotating polygon mirror A second optical system that focuses the light beam on the writing area of the surface to be scanned; and an optical scanning device that scans the writing area on the surface to be scanned in the main scanning direction with the plurality of light beams.
Among the plurality of light emitting points, the distance between the light emitting points farthest in the main scanning direction is W, the dimension of the opening provided in the aperture member is E, and the inscribed circle of the rotary polygon mirror The radius of A, the number of deflection surfaces of the rotary polygon mirror M, the incident angle of incidence of the light beam on the deflection surface α, and the second optical system from the start of scanning to the end of scanning of the writing area. An optical scanning device characterized by satisfying the following expression, where φ is the angle of view, x is the distance from the aperture member to the rotary polygon mirror, and f ₁ is the focal length of the coupling lens: .

The light source is a vertical cavity surface emitting laser;
A size of the near-field pattern of the light emitting point in the main scanning direction or the sub-scanning direction orthogonal to the main scanning direction is A, and the lateral magnification of the optical system composed of the first optical system and the second optical system The optical scanning device according to claim 1, wherein β satisfies a condition represented by the following expression, where β is a spot diameter of the light beam and ω is a spot diameter of the light beam.

In the light source, the distance between the light emitting points arranged farthest in the main scanning direction is longer than the distance between the light emitting points arranged farthest in the sub scanning direction,
The lateral magnification in the main scanning direction of the optical system including the first optical system and the second optical system is larger than the lateral magnification in the sub-scanning direction of the optical system. Optical scanning device. The light emitting points are two-dimensionally arranged in a first arrangement direction that forms a predetermined angle with the main scanning direction and a second arrangement direction that is parallel to the sub-scanning direction so that the light emitting points are equally spaced with respect to the sub-scanning direction. Arranged,
4. The optical scanning device according to claim 3, wherein the number of light emitting points arranged in the first arrangement direction is larger than the number of light emission points arranged in the second arrangement direction. 5.   The plurality of light emitting points are arranged at a first interval in the first arrangement direction, and are arranged at a second interval smaller than the first interval in the second arrangement direction. The optical scanning device according to claim 4.   The optical scanning device according to claim 1, wherein the first optical system includes an optical element having a diffractive optical surface. An image forming apparatus that forms an image by fixing a toner image formed based on a latent image obtained from information about an image to a recording medium,
An optical scanning device according to any one of claims 1 to 6;
A photoreceptor on which a latent image is formed by the optical scanning device;
Developing means for visualizing a latent image formed on the surface to be scanned of the photoreceptor;
An image forming apparatus comprising: a transfer unit that fixes the toner image visualized by the developing unit to the recording medium.

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