JP2008209675A - Optical scanning apparatus and image forming apparatus - Google Patents

Abstract

An object of the present invention is to accurately scan a surface to be scanned with light without increasing costs.
A light source having 40 light emitting units arranged two-dimensionally, a polygon mirror for deflecting light from the light source, and scanning optics for condensing the light deflected by the polygon mirror onto a photosensitive drum. The plurality of light emitting units have an interval d between two light emitting units that are closest to each other in the S direction corresponding to the sub-scanning direction. As a result, even when an inexpensive scanning optical system is used, it is possible to reduce the variation in the spot diameter of the light spot formed on the photosensitive drum. Therefore, as a result, it is possible to accurately scan the surface to be scanned with light without increasing the cost.
[Selection] Figure 4

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 light and an image forming apparatus including the optical scanning apparatus.

  In electrophotographic image recording, an image forming apparatus using a laser is widely used. In this case, the image forming apparatus includes an optical scanning device, and forms a latent image by rotating the drum while scanning laser light using a polygon scanner (for example, a polygon mirror) in the axial direction of the photosensitive drum. Is common. In the field of electrophotography, an image forming apparatus is required to increase image density in order to improve image quality and to increase image output speed in order to improve operability.

  As a method for achieving both high density and high speed, there is a method of making multi-beams of light emitted from a light source.

  For example, Patent Document 1 discloses a photoelectric conversion element that includes a plurality of stacked semiconductor layers and includes a plurality of photoelectric conversion portions on the same substrate that have side surfaces at end portions in a direction perpendicular to the stacking direction. ing.

  In Patent Document 2, a plurality of substrate layers, a single substrate supported by the distribution substrate, and a plurality of semiconductor layers formed by stacking on the opposite side of the substrate from the distribution substrate are provided. A plurality of ohmic electrodes provided on the opposite side of the substrate with respect to the light emitting units, and electrically connected to the light emitting units in a one-to-one correspondence with the substrates; A semiconductor light emitting device is disclosed that includes at least one heat dissipation layer formed on the side opposite to the substrate of each light emitting unit by being stacked via each light emitting unit.

  However, in order to use the photoelectric conversion element disclosed in Patent Document 1 and the semiconductor light-emitting element disclosed in Patent Document 2 for a multi-beam light source, the power consumption is large and the heat generation amount is large. The beam or about 8 beams was the limit. In addition, since each element has a one-dimensional arrangement of a plurality of light emitting units, if the number of beams is extremely increased, the amount of deviation of the beam from the optical axis of the optical system increases, which may deteriorate the beam characteristics. It was.

  Therefore, it has been proposed to use a vertical cavity surface emitting laser (VCSEL) that can easily form a plurality of light emitting points on one element in a two-dimensional manner and consumes less power. .

  Japanese Patent Application Laid-Open No. 2004-228561 is equipped with a light-emitting element and its drive circuit, a first substrate attached to a housing fixed to the image forming apparatus main body, and a connector for connecting a harness from the image forming apparatus main body. The second substrate that is separated from the first substrate and is attached to the housing, and the terminals of the first substrate and the terminals of the second substrate that are electrically connected are elastically deformable. An optical scanning device having a connecting member is disclosed.

  In Patent Document 4, a plurality of combinations of a light source in which a plurality of light-emitting points that can be independently modulated are arranged two-dimensionally and a coupling lens that couples divergent light emitted from the light source are combined. A configured light source device is disclosed.

  Patent Document 5 discloses a light source that emits a light beam, a deflection device that deflects the light beam in a main scanning direction, an optical system that forms an image of the deflected light beam on a surface to be scanned, and the deflection device. An optical scanning device including a plurality of cylinder mirrors provided on an optical path between scanned surfaces is disclosed. The optical scanning device includes a skew adjusting unit that adjusts the inclination of the scanning line by rotating a cylinder mirror close to the surface to be scanned around the normal line of the reflection surface, and a bus mirror on the reflection surface of the cylinder mirror close to the deflection device. Angle adjusting means for adjusting the angle in the sub-scanning direction by rotating the shaft.

JP 11-340570 A JP 11-354888 A JP 2004-287292 A JP-A-2005-250319 JP 2005-309301 A

  Recently, image forming apparatuses have come to be used for simple printing as an on-demand printing system, and there is a demand for higher image quality without increasing the price.

  The present invention has been made under such circumstances, and a first object thereof is to provide an optical scanning device capable of accurately scanning a surface to be scanned with light without incurring an increase in cost. It is in.

  A second object of the present invention is to provide an image forming apparatus capable of forming a high quality image at high speed without incurring an increase in cost.

  From a first aspect, the present invention is an optical scanning device that scans a surface to be scanned with light, and includes a light source having a plurality of light emitting units arranged two-dimensionally; and deflects light from the light source And a scanning optical system that condenses the light deflected by the deflector onto the surface to be scanned, and the plurality of light emitting units are arranged in the closest two in the direction corresponding to the sub-scanning direction. The optical scanning device is characterized in that the interval between the light emitting portions is 1 μm or more and less than 5 μm.

  Note that in this specification, the interval between the light emitting units refers to the distance between the centers of the two light emitting units.

  According to this, in the plurality of light emitting units, the interval between the two closest light emitting units in the direction corresponding to the sub-scanning direction is set to 1 μm or more and less than 5 μm, so an inexpensive scanning optical system is used. In addition, it is possible to reduce the variation in the spot diameter of the light spot formed on the surface to be scanned. Therefore, as a result, it is possible to accurately scan the surface to be scanned with light without increasing the cost.

  According to a second aspect of the present invention, there is provided at least one image carrier; and at least one optical scanning device of the present invention that scans the at least one image carrier with light containing image information. An image forming apparatus provided.

  According to this, since at least one optical scanning device of the present invention is provided, as a result, it is possible to form a high-quality image at a high speed without increasing the cost.

  Hereinafter, an embodiment of the present invention will be described with reference to FIGS. FIG. 1 shows a schematic configuration of a laser printer 500 as an image forming apparatus according to an embodiment of the present invention.

  A laser printer 500 shown in FIG. 1 includes an optical scanning device 900, a photosensitive drum 901 as an image carrier, a charging charger 902, a developing roller 903, a toner cartridge 904, a cleaning blade 905, a paper feed tray 906, and a paper feed roller 907. A registration roller pair 908, a transfer charger 911, a fixing roller 909, a paper discharge roller 912, a paper discharge tray 910, and the like.

  The charging charger 902, the developing roller 903, the transfer charger 911, and the cleaning blade 905 are disposed in the vicinity of the surface of the photosensitive drum 901, respectively. Then, the charging charger 902, the developing roller 903, the transfer charger 911, and the cleaning blade 905 are arranged in this order with respect to the rotation direction of the photosensitive drum 901 (the arrow direction in FIG. 1).

  A photosensitive layer is formed on the surface of the photosensitive drum 901. That is, the surface of the photoconductive drum 901 is the surface to be scanned.

  The charging charger 902 uniformly charges the surface of the photosensitive drum 901.

  The optical scanning device 900 irradiates the surface of the photosensitive drum 901 charged by the charging charger 902 with light modulated based on image information from a host device (for example, a personal computer). As a result, on the surface of the photosensitive drum 901, the charge is lost only in the portion irradiated with light, and a latent image corresponding to the image information is formed on the surface of the photosensitive drum 901. The latent image formed here moves in the direction of the developing roller 903 as the photosensitive drum 901 rotates. The configuration of the optical scanning device 900 will be described later.

  The toner cartridge 904 stores toner, and the toner is supplied to the developing roller 903.

  As the developing roller 903 rotates, the toner supplied from the toner cartridge 904 is charged and thinly and uniformly attached to the surface thereof. Further, the developing roller 903 has an electric field in the opposite direction between a charged portion (a portion not irradiated with light) and an uncharged portion (a portion irradiated with light) in the photosensitive drum 901. A voltage is generated to generate. By this voltage, the toner adhering to the surface of the developing roller 903 adheres only to the portion irradiated with light on the surface of the photosensitive drum 901. That is, the developing roller 903 causes the toner to adhere to the latent image formed on the surface of the photosensitive drum 901 and visualizes the image information. Here, the latent image to which the toner is attached moves in the direction of the transfer charger 911 as the photosensitive drum 901 rotates.

  A recording sheet 913 as a transfer object is stored in the sheet feeding tray 906. A paper feed roller 907 is disposed in the vicinity of the paper feed tray 906, and the paper feed roller 907 takes out the recording paper 913 one by one from the paper feed tray 906 and conveys it to the registration roller pair 908. The registration roller pair 908 is disposed in the vicinity of the transfer roller 911, temporarily holds the recording paper 913 taken out by the paper feed roller 907, and the recording paper 913 is synchronized with the rotation of the photosensitive drum 901. It is sent out toward the gap between 901 and the transfer charger 911.

  A voltage having a polarity opposite to that of the toner is applied to the transfer charger 911 in order to electrically attract the toner on the surface of the photosensitive drum 901 to the recording paper 913. With this voltage, the latent image on the surface of the photosensitive drum 901 is transferred to the recording paper 913. The recording sheet 913 transferred here is sent to the fixing roller 909.

  In the fixing roller 909, heat and pressure are applied to the recording paper 913, whereby the toner is fixed on the recording paper 913. The recording paper 913 fixed here is sent to the paper discharge tray 910 via the paper discharge roller 912 and sequentially stacked on the paper discharge tray 910.

  The cleaning blade 905 removes toner remaining on the surface of the photosensitive drum 901 (residual toner). The removed residual toner is used again. The surface of the photosensitive drum 901 from which the residual toner has been removed returns to the position of the charging charger 902 again.

  Next, the configuration of the optical scanning device 900 will be described with reference to FIGS.

  The optical scanning device 900 includes a light source 14, a coupling lens 15, an aperture plate 16, a line image forming lens 17, a polygon mirror 13, a polygon motor (not shown) that rotates the polygon mirror 13, and two scanning lenses (11a, 11a, 11b) and the like.

  As shown in FIG. 4 as an example, the light source 14 includes a two-dimensional array 100 in which 40 light emitting units 101 are formed on one substrate. Each light emitting unit 101 is a VCSEL having an oscillation wavelength of 780 nm. By the way, the VCSEL has a feature that the temperature variation of the oscillation wavelength is small, and in principle, a discontinuous change in wavelength (so-called wavelength jump) does not occur.

  The divergence angle (FWHM) of the light beam emitted from the light source 14 is a direction corresponding to the main scanning direction (hereinafter also referred to as “M direction” for convenience) and a direction corresponding to the sub-scanning direction (hereinafter referred to as “for convenience”. Both of them are 7 ± 1 degrees, and the near field pattern is a circle having a diameter of 4 μm.

  As shown in FIG. 4, the 40 light emitting units 101 are arranged along a direction (hereinafter referred to as “T direction” for convenience) and an S direction that form an inclination angle α from the M direction toward the S direction. Two-dimensionally arranged. Here, in the light source 14, ten light emitting units are arranged at equal intervals along the T direction, and four light emitting units are arranged at equal intervals along the S direction. The interval between the two light emitting units located at both ends with respect to the S direction is smaller than the interval between the two light emitting units located at both ends with respect to the M direction. That is, each light emitting part is arranged in a parallelogram having the long side in the T direction and the short side in the S direction. As a result, a larger number of light emitting portions can be arranged in the M direction than in the S direction.

  In a two-dimensional array of VCSELs, the interval between two adjacent light emitting units needs to be increased to some extent from the viewpoint of preventing heat generation and wiring. By setting the light emitting portion interval d in the S direction between two adjacent light emitting portions to 1 ≦ d <5 μm, the light emitting portion interval dm in the M direction can be made larger than d. In this embodiment, as an example, the writing density is 4800 dpi, dm = 30 μm, ds = 25 μm, and d = 2.5 μm. The interval between the two light emitting units located at both ends in the M direction is dm × 9 = 270 μm, and the interval between the two light emitting units located at both ends in the S direction is d × 39 = 97.5 μm.

  In order to perform high-density scanning, it is desirable to narrow the interval between the light emitting portions in the S direction. In addition, in order to improve the performance of the two-dimensional array, the manufacturing yield, and the lifetime, it is desirable that the region where the plurality of light emitting units are arranged be longer in the M direction than in the S direction.

  If a plurality of light emitting units are arranged over a wide range with respect to the S direction, a position in the sub-scanning direction is formed on a light spot formed on the photosensitive drum due to a deformation of the optical system and a change in optical characteristics due to a temperature change. Deviation (hereinafter also referred to as “sub-scanning direction deviation” for convenience) occurs. This sub-scanning direction deviation increases as the light emitted from the light source moves away from the optical axis of the optical system in the sub-scanning direction.

  Further, the variation in the scanning line interval due to the temperature change is the maximum between the two light emitting units located at both ends in the S direction. Since the lateral magnification in the sub-scanning direction in the scanning optical system differs depending on the position in the main scanning direction, the scanning line by the light from the light emitting unit located on the + S side with respect to the optical axis of the scanning optical system, and the −S side This is because the scanning lines are bent in opposite directions with respect to the scanning line by the light from the light emitting unit located in the area. An error in the scanning line interval that occurs when scanning lines that are bent in opposite directions are adjacent to each other is shifted in the sub-scanning direction, and is perceived as banding that appears every scanning, resulting in a reduction in scanning accuracy (scanning quality).

  When the scanning lens is made of resin in order to reduce the cost of the apparatus, the temperature change of the thermal expansion coefficient and the refractive index is larger than that of glass, so that the deviation in the sub-scanning direction becomes more remarkable. FIG. 5 shows the relationship between the maximum fluctuation amount on the plus side and the minus side depending on the temperature of the scanning line interval in the sub scanning direction and the image height in the present embodiment. Here, the temperature variation of the scanning line interval is suppressed to 5 μm or less, and the influence on the image quality deterioration is in an allowable range. When the writing density is 4800 dpi, since the resolution is 5.3 μm, further fluctuations cannot be allowed. Therefore, it is desirable that the interval between the two light emitting units located at both ends in the direction corresponding to the sub-scanning direction is 100 μm or less.

  For example, as shown in FIG. 6, each light emitting unit includes a lower reflector 112, a spacer layer 113, an active layer 114, a spacer layer 115, an upper reflector 117, and a p-contact layer 118 on an n-GaAs substrate 111. Such semiconductor layers are sequentially stacked. Hereinafter, a structure in which a plurality of these semiconductor layers are stacked is also referred to as a “stacked body” for convenience. An enlarged view of the vicinity of the active layer 114 is shown in FIG.

The lower reflecting mirror 112 includes a low refractive index layer (referred to as a low refractive index layer 112a) made of n-Al _0.9 Ga _0.1 As and a high refractive index layer made of n-Al _0.3 Ga _0.7 As. 40.5 pairs are provided as a pair (with a high refractive index layer 112b). Each refractive index layer is set to have an optical thickness of λ / 4 when the oscillation wavelength is λ. Note that a composition gradient layer (not shown) in which the composition is gradually changed from one composition to the other composition between the low refractive index layer 112a and the high refractive index layer 112b in order to reduce electrical resistance. Is provided.

The spacer layer 113 is a layer made of Al _0.6 Ga _0.4 As.

The active layer 114 includes a quantum well layer 114a made of Al _0.12 Ga _0.88 As and a barrier layer 114b made of Al _0.3 Ga _0.7 As (see FIG. 5).

The spacer layer 115 is a layer made of Al _0.6 Ga _0.4 As.

  A portion composed of the spacer layer 113, the active layer 114, and the spacer layer 115 is also called a resonator structure, and the thickness thereof is set to be one wavelength optical thickness (see FIG. 7).

The upper reflecting mirror 117 includes a low refractive index layer (referred to as a low refractive index layer 117a) made of p-Al _0.9 Ga _0.1 As and a high refractive index layer made of p-Al _0.3 Ga _0.7 As. There are 24 pairs (with high refractive index layer 117b). Each refractive index layer is set to have an optical thickness of λ / 4. A composition gradient layer (not shown) in which the composition is gradually changed from one composition to the other composition in order to reduce electric resistance between the low refractive index layer 117a and the high refractive index layer 117b. Is provided.

  A selective oxidation layer 116 made of AlAs is provided at a position away from the resonator structure in the upper reflecting mirror 117 by λ / 4.

"Production method"
Next, a method for manufacturing the two-dimensional array 100 will be briefly described.

(1) The laminate is formed by crystal growth using metal organic chemical vapor deposition (MOCVD) or molecular beam crystal growth (MBE).

(2) Grooves are formed by dry etching around each of a plurality of regions, each of which serves as a light emitting portion, to form a so-called mesa portion. Here, the bottom surface of the etching is set so as to reach the lower reflecting mirror 112. Note that the bottom surface of the etching may be at least beyond the selective oxidation layer 116. As a result, the selectively oxidized layer 116 appears on the sidewall of the trench. In addition, the size (diameter) of the mesa portion is preferably 10 μm or more. This is because if it is too small, heat will be trapped during device operation, which may adversely affect the light emission characteristics.

(3) The laminated body in which the groove is formed is heat-treated in water vapor, and the periphery of the selective oxidation layer 116 in the mesa portion is selectively oxidized to be changed to an Al _x O _y insulator layer. Then, an unoxidized AlAs region in the selective oxidation layer 116 remains in the central portion of the mesa portion. As a result, a so-called current confinement structure is formed in which the drive current path of the light emitting part is limited to only the central part of the mesa part.

(4) Except for the region where the upper electrode 103 of each mesa part is formed and the light emitting part 102, for example, a 150 nm thick SiO ₂ protective layer 120 is provided, and polyimide 119 is buried in each groove and planarized.

(5) The upper electrode 103 is formed in each mesa portion on the p contact layer 118 except for the light emitting portion 102, and each bonding pad (not shown) is formed around the laminated body. And each wiring (not shown) which connects each upper electrode 103 and the bonding pad corresponding to each is formed.

(6) A lower electrode (n-side common electrode) 110 is formed on the back surface of the laminate.

(7) The laminate is cut into a plurality of chips.

  Returning to FIG. 2, the coupling lens 15 is a glass lens having a focal length (F 1) of 47.7 mm as an example, and the light emitted from the light source 14 is substantially parallel light.

  The focal length F1 is desirably as small as possible in order to prevent the apparatus from becoming large. However, if F1 is reduced, the lateral magnification of the optical system is increased, so that it is difficult to reduce the beam spot diameter. As described above, the value of F1 is largely limited in the range of selection. For example, it is desirable that the value is within a range of 25 to 48 mm for an optical system corresponding to A3, and 18 to 34 mm for an optical system compatible with A4.

  As an example, the aperture plate 16 has a rectangular or elliptical opening having a front width in the main scanning direction of 5.44 mm and a front width in the sub scanning direction of 2.10 mm, and is related to the main scanning direction and the sub scanning direction. The beam diameter of light through the coupling lens 15 is defined.

  The line image forming lens 17 is a glass lens having a focal length (F2) of 107.0 mm as an example, and the light that has passed through the opening of the aperture plate 16 is near the deflecting reflection surface of the polygon mirror 13 in the sub-scanning direction. Imaging with respect to.

  By the way, especially in the sub-scanning direction, the deviation of the beam waist position due to temperature change is large. Therefore, in order to stably reduce the beam spot diameter in the sub-scanning direction, it is necessary to increase the depth of focus. Therefore, it is necessary to reduce the numerical aperture of the optical system so that the beam is not focused too much. In order to reduce the numerical aperture, it is conceivable to (1) reduce the aperture diameter (aperture diameter) of the aperture plate 16 and (2) increase the focal length of the line image forming lens 17. Therefore, it is preferable that the aperture diameter is as large as possible. Therefore, the focal length of the line image forming lens 17 is preferably as long as possible.

  The optical system arranged on the optical path between the light source 14 and the polygon mirror 13 is also called a coupling optical system. In the present embodiment, the coupling optical system includes a coupling lens 15, an aperture plate 16, and a line image forming lens 17.

  For example, the polygon mirror 13 is a four-sided mirror having an inscribed circle with a radius of 7 mm, and rotates at a constant speed around an axis parallel to the sub-scanning direction.

  As an example, the scanning lens 11a is a resin lens having a center (on the optical axis) thickness of 13.5 mm.

  As an example, the scanning lens 11b is a resin lens having a center (on the optical axis) thickness of 3.5 mm.

Each surface (incident surface, exit surface) of the scanning lens 11a and the scanning lens 11b is an aspheric surface expressed by the following equation (1) and the following equation (2). Here, X is the coordinate in the optical axis direction (left and right direction in FIG. 2), and Y is the coordinate in the main scanning direction (up and down direction in FIG. 2). The center of the incident surface is Y = 0. C _m0 represents the curvature in the main scanning direction at Y = 0, and is the reciprocal of the curvature radius R _m . a ₀₀ , a ₀₁ , a ₀₂ ,... are main scanning aspherical coefficients. Further, Cs (Y) is a curvature in the sub-scanning direction with respect to Y, R _s0 is a radius of curvature on the optical axis in the sub-scanning direction, b ₀₀ , b ₀₁ , b ₀₂ ,. is there. The optical axis is an axis that passes through the center point in the sub-scanning direction when Y = 0.

Table 1 shows an example of values of R _m , R _s0, and each aspheric coefficient on each surface (incident surface, exit surface) of each scanning lens.

  FIG. 8 shows the shape of the scanning lens 11a obtained by substituting the values in Table 1 into the equation (1). FIG. 9 shows the shape of the scanning lens 11b obtained by substituting the values in Table 1 into the equation (1).

  FIG. 10 shows Cs (Y) on the entrance surface and exit surface of the scanning lens 11a obtained by substituting the values in Table 1 into the equation (2). FIG. 11 shows Cs (Y) on the entrance surface and exit surface of the scanning lens 11b obtained by substituting the values in Table 1 into the equation (2).

  An optical system disposed on the optical path between the polygon mirror 13 and the photosensitive drum 901 is also called a scanning optical system. In this embodiment, the scanning optical system includes a scanning lens 11a and a scanning lens 11b.

  The focal length in the main scanning direction of this scanning optical system is 237.8 mm as an example, and the focal length in the sub-scanning direction is 71.4 mm as an example.

  As an example, the lateral magnification (β2s) in the sub-scanning direction of the scanning optical system is 0.97 times. Further, the horizontal magnification (βs) in the sub-scanning direction of the entire optical system of the optical scanning device 900 is 2.18 times as an example.

  If the absolute value | β2s | of the lateral magnification in the sub-scanning direction of the scanning optical system is large, the beam waist position shifts due to a manufacturing error. Therefore, the same magnification to a reduction system is desirable. Another problem occurs. That is, the diffracted light generated when beam shaping is performed by the aperture plate 16 reaches the photosensitive drum 901, and the beam spot diameter becomes asymmetric with respect to the defocus direction. Variation in diameter increases. This is a phenomenon that occurs when the imaging position of the diffracted light is close to the photosensitive drum 901, and is particularly noticeable when the scanning optical system has the same magnification to a reduction magnification.

  In order to explain this specifically, FIGS. 12A to 12C show the imaging position of the diffracted light from the aperture stop in a general writing optical system having substantially the same configuration. An example of a change in the beam spot diameter in the sub-scanning direction with respect to the defocus amount due to the difference is shown. The horizontal axis is the defocus amount (mm), and the vertical axis is the beam spot diameter (μm) in the sub-scanning direction. 12A shows the case where the imaging position of the diffracted light is −30 mm from the scanned surface, and FIG. 12B shows the case where the imaging position of the diffracted light is −100 mm from the scanned surface. (C) is when the imaging position of the diffracted light is +30 mm from the scanned surface. 12A and 12C, the depth of focus is an asymmetric curve due to the influence of diffracted light, and the depth of focus is shallow. In FIG. 12B, since the imaging position of the diffracted light is farther from the scanned surface than in the case of FIGS. 12A and 12C, it is closer to a symmetrical shape with respect to the scanned surface. The depth of focus is deep.

  FIG. 13A shows the relationship between the beam spot diameter and the defocus amount in the main scanning direction in this embodiment, and FIG. 13B shows the relationship between the beam spot diameter and the defocus amount in the sub scanning direction. It is shown.

  In the present embodiment, since the imaging position of the diffracted light from the aperture plate 16 is −42.5 mm from the scanning surface, the depth of focus is an asymmetric curve, and the depth of focus is shallow. Reducing | β2s | requires an arrangement in which the scanning lens approaches the scanned surface, and the imaging position of the diffracted light also approaches the scanned surface, so reducing | β2s | It is not suitable for achieving high image quality because it is easily affected by variations in beam spot diameter due to manufacturing errors. In a multi-beam optical system for further speeding up, light from a plurality of light emitting portions separated in a direction corresponding to the main scanning direction passes through different positions of the scanning lens, so that all beam waist positions are It will not be aligned with the beam of. Therefore, since the depth of focus in all light becomes shallower, the problem for stabilizing the beam spot diameter is further increased. Therefore, | β2s | cannot be reduced. Also, if | β2s | is increased, the variation in refractive power of the optical system due to temperature variation increases, and the variation in beam waist position in the sub-scanning direction increases, leading to an increase in variation in beam spot diameter. Therefore, in this embodiment, | β2s | = 0.97.

  Due to the optical constraints described above, in this embodiment, the lateral magnification βs in the sub-scanning direction of the entire optical system is βs = F2 / F1 × | β2s | = 2.18. When βs is set to a value different from that of the present embodiment, F1 must be a smaller value, and F2 and | β2s | must be larger values for higher image quality and higher speed. Is equal to or greater than Therefore, βs is desirably 2.1 times or more.

  By the way, when the value of the light emitting portion interval d in the two-dimensional array is less than 1 μm (lower limit), the lateral magnification in the sub-scanning direction of the writing optical system becomes high, and the variation in the light emitting portion interval in the light source is enlarged as it is. This is reflected on the surface to be scanned, and the variation in the scanning line interval is increased. Therefore, in order to realize high-precision optical scanning, a two-dimensional array with a small variation in the interval between the light emitting sections is required. However, (1) since the interval between the light emitting parts cannot be made wide, wiring becomes very difficult and highly accurate wiring is required. (2) Because the heat radiation performance decreases as the distance between the light emitting parts decreases. Heat generation of the element at the time of driving cannot be ignored, and the deterioration of the element is accelerated, resulting in an increase in cost.

  Therefore, if the countermeasure for heat generation is taken by expanding the interval in the direction corresponding to the main scanning direction while maintaining the value of d smaller than the lower limit value, the light emitting part is too far away, the variation in the scanning line interval, the beam spot diameter The variation increases and the optical performance deteriorates. For these reasons, the value of d is desirably 1 μm or more.

  In the case of 300 dpi, 600 dpi, or 1200 dpi, which is a conventional writing density, an oblique line of an image, that is, a line that is not parallel to the main scanning direction and the sub-scanning direction is not smoothly connected and is perceived as jaggy. Therefore, in the case of 2400 dpi, which is more dense and difficult to perceive jaggies, the scanning line interval is 25.4 / 2400 = 10.58 μm. The light emitting portion interval d that satisfies this condition is 10.58 / 2.18 = 4.9 μm when the sub-scanning lateral magnification βs of the entire optical system is 2.18. Further, in the case of corresponding to a higher density of 4800 dpi, d = 25.4 / 4800 / 2.18 = 2.5 μm. Thus, by setting the light emitting portion interval d to be smaller than 5 μm, the beam spot diameter is stable with high density, the light use efficiency is excellent, and the optical scanning can be performed with a sufficient exposure amount. However, if the distance d between the light emitting portions is 5 μm or more, the exposure amount is insufficient and the beam spot diameter becomes unstable, which is not desirable. Furthermore, if the light emitting portion interval d is less than 1 μm, the life of the device may be shortened and the scanning line interval may become unstable.

  In this embodiment, the target beam spot diameter of the light spot formed on the surface of the photosensitive drum 901 is 52 μm in the main scanning direction and 55 μm in the sub-scanning direction.

  Table 2 shows an example of specific values of the symbols d1 to d11 in FIG.

  Further, the length of the effective scanning area (writing width in the main scanning direction) on the photosensitive drum 901 is 323 mm (± 161.5 mm). That is, the image height is −161.5 mm to +161.5 mm.

  An example of the measured value of the field curvature in the present embodiment is shown in FIG. 14A, and an example of the measured value of the constant velocity characteristic is shown in FIG. According to these, it can be seen that the image planes are very well aligned in the sub-scanning direction, and that the variation in the beam spot diameter is very small even though the thickness of the scanning lens is reduced as compared with the conventional case.

  FIG. 15 shows an example of a deviation (Δβ) in the lateral magnification in the sub-scanning direction in the present embodiment. According to this, the difference between the maximum value and the minimum value in the lateral magnification in the sub-scanning direction is suppressed to about 0.5%, and can be regarded as substantially constant. In FIG. 15, the horizontal magnification in the sub-scanning direction at the image height 0 (the center of the effective scanning area (see FIG. 3)) is set as a reference (Δβ = 0).

  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.

  As in the present embodiment, when an optical system having 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 2.2 times) is used. FIG. 16A to FIG. 19B show simulation results of the relationship between the beam spot diameter and the defocus amount. FIG. 16A 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. 16B 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. 17A shows the beam spot diameter in the main scanning direction when Am is 2 μm, and FIG. 17B shows the beam spot diameter in the sub scanning direction when As is 2 μm. 18A shows the beam spot diameter in the main scanning direction when Am is 4 μm, and FIG. 18B shows the beam spot diameter in the sub-scanning direction when As is 4 μm. FIG. 19A shows the beam spot diameter in the main scanning direction when Am is 6 μm, and FIG. 19B 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) 20A to 23B show simulation results of the relationship between the distance and the defocus amount. FIG. 20A shows the beam spot diameter in the main scanning direction when Am is infinitesimal (= 0), and FIG. 20B shows the sub-scanning direction when As is infinitesimal (= 0). The beam spot diameter is shown. FIG. 21A shows the beam spot diameter in the main scanning direction when Am is 2 μm, and FIG. 21B shows the beam spot diameter in the sub scanning direction when As is 2 μm. 22A shows the beam spot diameter in the main scanning direction when Am is 4 μm, and FIG. 22B shows the beam spot diameter in the sub-scanning direction when As is 4 μm. FIG. 23A shows the beam spot diameter in the main scanning direction when Am is 6 μm, and FIG. 23B 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 set to 52 ± 5 μm, and the target value of the beam spot diameter in the sub-scanning direction is set to 55 ± 5 μm, and each of FIGS. 16 (A) to 23 (B). 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.

  This shows 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 (3) 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 (3)

This will be described with reference to FIG. FIG. 24 shows the difference between the beam spot diameter when the defocus amount is 0 and (β × A) = 0 (ie, A = 0) and the beam spot diameter when (β × A)> 0. FIG. 16 (A) to FIG. 23 (B) are obtained and plotted. The curve in FIG. 24 is a curve showing the following equation (4). 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 (5).

From FIG. 24, 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 (6). However, k = βA / ω ₀ .

In the present embodiment, k = 5.0 × 4 ÷ 50 = 0.4 in 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 = 0.16 in the sub-scanning direction, the change is further small, and hardly changes with respect to ω ₀ = 55 μm and remains 55 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 (7) from the above equation (6).

  By the way, it is known that the VCSEL is advantageous for output when the near field pattern is larger. Therefore, by setting so that the above expression (3) is satisfied, it is possible to reduce the beam spot diameter while using a VCSEL advantageous in light quantity.

  As described above, according to the optical scanning device 900 according to the present embodiment, the light source 14 having 40 light emitting units arranged two-dimensionally, the polygon mirror 13 that deflects the light from the light source 14, and the polygon A scanning optical system for condensing the light deflected by the mirror 13 onto the photosensitive drum 901, and the plurality of light emitting units are spaced apart from the two closest light emitting units in the S direction corresponding to the sub-scanning direction. Is 2.5 μm. Thereby, even if an inexpensive scanning optical system is used, it is possible to reduce the variation in the spot diameter of the light spot formed on the photosensitive drum 901. Therefore, as a result, it is possible to accurately scan the surface to be scanned with light without increasing the cost.

  By the way, the method for increasing the writing density in the sub-scanning direction using the multi-beam light source includes (1) a method for reducing the lateral magnification of the optical system in the sub-scanning direction, and (2) a light emitting unit interval in the sub-scanning direction. There is a way to make it smaller. However, in the method (1), it is necessary to reduce the width of the opening in the sub-scanning direction in the aperture plate that defines the beam diameter on the surface to be scanned, resulting in insufficient light quantity. On the other hand, in the method (2), it is difficult to secure the space necessary for the influence of thermal interference between the light emitting units and the wiring from each light emitting unit.

  In the present embodiment, the plurality of light emitting units are two-dimensionally arranged, and the interval between the two light emitting units located at both ends with respect to the direction corresponding to the main scanning direction is 2 located at both ends with respect to the direction corresponding to the sub scanning direction. Since it is larger than the interval between the two light emitting units, the light emitting unit interval in the sub-scanning direction should be reduced while reducing the influence of thermal interference between the respective light emitting units and ensuring the space necessary to pass the wiring of each light emitting unit. Can do.

  In addition, the laser printer 500 according to the present embodiment includes the optical scanning device 900 that can accurately scan the photosensitive drum 901 with light without incurring an increase in cost. High-quality images can be formed at high speed without incurring costs.

  In the above-described embodiment, the case where the light source 14 includes the 40 light emitting units 101 formed on the same substrate has been described. However, the present invention is not limited to this. For example, as shown in FIG. 25, the light source 14 may have 20 light emitting units 101 formed on the same substrate. Here, in the light source 14, ten light emitting units are arranged at equal intervals along the T direction, and two light emitting units are arranged along the S direction. The interval between the two light emitting units located at both ends with respect to the S direction is smaller than the interval between the two light emitting units located at both ends with respect to the M direction.

  Here, the writing density is 2400 dpi, dm = 25 μm, ds = 49 μm, and d = 4.9 μm. The interval between the two light emitting units located at both ends in the M direction is dm × 9 = 225 μm, and the interval between the two light emitting units located at both ends in the S direction is d × 19 = 93.1 μm.

  Moreover, in the said embodiment, as shown in FIGS. 26-28 as an example, it replaces with the said two-dimensional array 100, and the material of the one part semiconductor layer of the said some semiconductor layers of the two-dimensional array 100 is used. A modified two-dimensional array (referred to as a two-dimensional array 200) may be used. In the two-dimensional array 200, the spacer layer 113 in the two-dimensional array 100 is changed to a spacer layer 213, the active layer 114 is changed to an active layer 214, and the spacer layer 115 is changed to a spacer layer 215. is there.

The spacer layer 213 is a layer made of (Al _0.7 Ga _0.3 ) _0.5 In _0.5 P having a wide band gap.

As shown in FIG. 27, the active layer 214 has a composition in which compressive strain remains and has a Ga ₀ having a tensile strain of four layers lattice-matched with the three GaInPAs quantum well layers 214a having a band gap wavelength of 780 nm. _.6 In _0.4 P barrier layer 214b.

The spacer layer 215 is a layer made of (Al _0.7 Ga _0.3 ) _0.5 In _0.5 P having a wide band gap.

  A portion composed of the spacer layer 213, the active layer 214, and the spacer layer 215 is called a resonator structure, and the thickness thereof is set to be one wavelength optical thickness (see FIG. 27).

  Since the two-dimensional array 200 uses an AlGaInP-based material for the spacer layer, the band gap difference between the spacer layer and the active layer can be extremely large compared to the two-dimensional array 100 in the above embodiment. it can.

  In FIG. 28, the spacer layer / quantum well layer material is an AlGaAs / AlGaAs-based VCSEL having a wavelength of 780 nm band (hereinafter referred to as “VCSEL_A” for convenience), and the spacer layer / quantum well layer material is AlGaInP / GaInPAs. In this system, a VCSEL having a wavelength of 780 nm band (hereinafter referred to as “VCSEL_B” for convenience) and a spacer layer / quantum well layer material is an AlGaAs / GaAs system and a VCSEL having a wavelength of 850 nm band (hereinafter referred to as “ For VCSEL_C "), the band gap difference between the spacer layer and the quantum well layer and the band gap difference between the barrier layer and the quantum well layer in a typical material composition are shown. Note that VCSEL_A corresponds to the VCSEL 101 in the two-dimensional array 100, and VCSEL_B with x = 0.7 corresponds to the VCSEL (referred to as VCSEL 201) in the two-dimensional array 200.

  According to this, it can be seen that VCSEL_B can take a larger band gap difference than VCSEL_C as well as VCSEL_A. Specifically, the band gap difference between the spacer layer and the quantum well layer in VCSEL_B is 767.3 meV, which is extremely larger than 465.9 meV in VCSEL_A. Similarly, the difference in the band gap between the barrier layer and the quantum well layer is superior to VCSEL_B, and better carrier confinement is possible.

  Further, in the VCSEL 201, since the quantum well layer has a compressive strain, the gain increase is large due to the band separation of the heavy hole and the light hole, and the gain becomes high, so that high output is possible with a low threshold. For this reason, the reflectance of the reflecting mirror on the light extraction side (here, the upper reflecting mirror 117) can be reduced, and a further increase in output can be achieved. Furthermore, since the gain can be increased, it is possible to suppress a decrease in light output due to a temperature rise, and it is possible to narrow the interval between the VCSELs in the two-dimensional array.

  In the VCSEL 201, since both the quantum well layer 214a and the barrier layer 214b are made of a material not containing aluminum (Al), the incorporation of oxygen into the active layer 214 is reduced. As a result, formation of a non-radiative recombination center can be suppressed, and the life can be further extended.

  By the way, for example, when a VCSEL two-dimensional array is used for a so-called writing optical unit, the writing optical unit is disposable when the life of the VCSEL is short. However, since the VCSEL 201 has a long life as described above, the writing optical unit using the two-dimensional array 200 can be reused. Therefore, promotion of resource protection and reduction of environmental load can be achieved. This also applies to other devices using a two-dimensional array of VCSELs.

  In the above-described embodiment, the case where the wavelength of the laser light emitted from each light emitting unit is in the 780 nm band has been described. However, the wavelength is not limited to this and may be any wavelength according to the sensitivity characteristic of the photosensitive drum 901. In this case, at least a part of the material constituting each light emitting part or at least a part of the structure of each light emitting part is changed according to the oscillation wavelength.

  In the above embodiment, when a large temperature change is expected, a diffraction grating for correcting the influence of the temperature change may be formed on at least one surface of the scanning optical system.

  In the above embodiment, the case where both the coupling lens 15 and the line image forming lens 17 are made of glass has been described. However, at least one of them may be made of a resin lens for cost reduction. However, if a large temperature change is expected at this time, a diffractive optical element may be used instead of the resin lens.

  In the above embodiment, the case where the scanning optical system includes two scanning lenses has been described. However, the present invention is not limited thereto, and the scanning optical system may include three or more scanning lenses.

  In the above embodiment, the case of the laser printer 500 as the image forming apparatus has been described. However, the present invention is not limited to this. In short, an image forming apparatus provided with the optical scanning device 900 can form a high-quality image at a high speed without increasing the cost.

  Even in an image forming apparatus that forms a multicolor image, a high-quality image can be formed at high speed without increasing the cost by using an optical scanning device that supports color images. It becomes possible.

  For example, as shown in FIG. 29, a tandem color machine that corresponds to a color image and includes a plurality of photosensitive drums may be used. The tandem color machine includes a black (K) photosensitive drum K1, a charger K2, a developing device K4, a cleaning unit K5, a transfer charging unit K6, a cyan (C) photosensitive drum C1, and a charger. C2, developing unit C4, cleaning unit C5, transfer charging unit C6, magenta (M) photosensitive drum M1, charging unit M2, developing unit M4, cleaning unit M5, transfer charging unit M6, yellow (Y) photosensitive drum Y1, charging unit Y2, developing unit Y4, cleaning unit Y5, transfer charging unit Y6, optical scanning device 900, transfer belt 80, fixing unit 30 and the like.

  In this case, in the optical scanning device 900, the plurality of light emitting units in the light source 14 are divided into black, cyan, magenta, and yellow. The light from each black light emitting unit is irradiated to the photosensitive drum K1 through the black scanning optical system, and the light from each cyan light emitting unit is passed through the cyan scanning optical system. The light emitted from each light emitting unit for magenta is irradiated to the photosensitive drum M1 through the scanning optical system for magenta, and the light from each light emitting unit for yellow passes through the scanning optical system for yellow. Through the photosensitive drum Y1.

  Each photosensitive drum rotates in the direction of the arrow in FIG. 29, and a charger, a developer, a transfer charging unit, and a cleaning unit are arranged in the order of rotation. Each charger uniformly charges the surface of the corresponding photosensitive drum. The surface of the photosensitive drum charged by the charger is irradiated with light by the optical scanning device 900, and an electrostatic latent image is formed on the photosensitive drum. Then, a toner image is formed on the surface of the photosensitive drum by the corresponding developing device. Further, the toner images of the respective colors are transferred onto the recording paper by the corresponding transfer charging means, and finally the image is fixed on the recording paper by the fixing means 30.

  In this tandem color machine, instead of the optical scanning device 900, as shown in FIG. 30, an optical scanning device 900K for black, an optical scanning device 900C for cyan, an optical scanning device 900M for magenta, and a yellow scanning device. Alternatively, the optical scanning device 900Y may be used.

  As described above, the optical scanning device of the present invention is suitable for accurately scanning the surface to be scanned with light without increasing the cost. The image forming apparatus according to the present invention is suitable for forming a high-quality image at high speed without incurring an increase in cost.

It is a figure for demonstrating schematic structure of the laser printer which concerns on one Embodiment of this invention. FIG. 2 is a schematic diagram (part 1) of the optical scanning device in FIG. FIG. 3 is a schematic diagram (part 2) of the optical scanning device in FIG. 1. It is a figure for demonstrating the two-dimensional array of VCSEL contained in a light source. It is a figure for demonstrating the relationship between the maximum variation | change_quantity of the plus side by the temperature of the scanning line space | interval regarding a subscanning direction, and the minus side, and image height. FIG. 5 is a diagram for explaining the structure of each VCSEL in the two-dimensional array of VCSELs of FIG. 4. It is the figure which expanded a part of VCSEL of FIG. It is a figure for demonstrating the shape of the scanning lens 11a in FIG. It is a figure for demonstrating the shape of the scanning lens 11b in FIG. It is a figure for demonstrating the curvature of the subscanning direction of the scanning lens 11a in FIG. It is a figure for demonstrating the curvature of the subscanning direction of the scanning lens 11b in FIG. FIGS. 12A to 12C are diagrams for explaining the change of the beam spot diameter with respect to the sub-scanning direction with respect to the defocus amount due to the difference in the imaging position of the diffracted light from the aperture stop. FIGS. 13A and 13B are diagrams for explaining the relationship between the beam spot diameter and the defocus amount in the main scanning direction and the sub-scanning direction, respectively. FIG. 14A and FIG. 14B are diagrams for explaining field curvature and linearity, respectively. It is a figure for demonstrating the deviation of the horizontal magnification of a subscanning direction. FIGS. 16A and 16B 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. 17A and 17B 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. 18A and 18B 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. 19A and 19B are views (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. 20A and 20B 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. 21A and 21B are views (No. 2) for explaining the relationship between the beam spot diameter and the defocus amount when a conventional optical system is used, respectively. FIGS. 22A and 22B are views (No. 3) for explaining the relationship between the beam spot diameter and the defocus amount when a conventional optical system is used, respectively. FIGS. 23A and 23B are diagrams (No. 4) for explaining the relationship between the beam spot diameter and the defocus amount when the conventional optical system is used. It is a figure for demonstrating the difference of the beam spot diameter in case of ((beta) * A) = 0 and the beam spot diameter in case of ((beta) * A)> 0 in the case of 0 defocus amount. It is a figure for demonstrating the modification of a two-dimensional array. It is a figure for demonstrating the modification of VCSEL. It is the figure which expanded a part of VCSEL of FIG. It is a figure for demonstrating the characteristic of VCSEL of FIG. It is a figure for demonstrating the schematic structure 1 of a tandem color machine. It is a figure for demonstrating the schematic structure 2 of a tandem color machine.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 11a ... Scanning lens (part of scanning optical system), 11b ... Scanning lens (part of scanning optical system), 13 ... Polygon mirror (deflector), 14 ... Light source, 101 ... Light emission part, 201 ... Light emission part, 500 DESCRIPTION OF SYMBOLS: Laser printer (image forming apparatus), 900 ... Optical scanning apparatus, 900K ... Optical scanning apparatus, 900C ... Optical scanning apparatus, 900M ... Optical scanning apparatus, 900Y ... Optical scanning apparatus, 901 ... Photosensitive drum (image carrier), K1, C1, M1, Y1... Photosensitive drum (image carrier).

Claims (8)

An optical scanning device that scans a surface to be scanned with light,
A light source having a plurality of light emitting portions arranged two-dimensionally;
A deflector for deflecting light from the light source;
A scanning optical system for condensing the light deflected by the deflector onto the surface to be scanned,
In the plurality of light emitting units, an interval between two closest light emitting units in a direction corresponding to the sub-scanning direction is 1 μm or more and less than 5 μm.   In the plurality of light emitting units, an interval between two light emitting units located at both ends in a direction corresponding to the sub-scanning direction is smaller than an interval between two light emitting units located at both ends in the direction corresponding to the main scanning direction. The optical scanning device according to claim 1, wherein: The optical system includes at least one resin lens,
3. The optical scanning device according to claim 1, wherein in the plurality of light emitting units, an interval between two light emitting units located at both ends in a direction corresponding to the sub-scanning direction is 100 μm or less. The plurality of light emitting units are two-dimensional arrays of vertical cavity surface emitting lasers,
The plurality of light emitting units include a near field pattern A of light emitted from the light emitting unit in the direction corresponding to the sub-scanning direction and the direction corresponding to the main scanning direction, and the scanning optical system. lateral magnification of the entire optical system beta, using said beam spot diameter omega on the scanning ^{plane, {(ω / β · a} ) 2 -1/2} -2 < the 0.7 relation is satisfied The optical scanning device according to any one of claims 1 to 3.   5. The optical scanning device according to claim 1, wherein a lateral magnification in the sub-scanning direction of the entire optical system including the scanning optical system is 2.1 times or more.   6. The optical scanning device according to claim 1, wherein an absolute value of the lateral magnification in the sub-scanning direction of the scanning optical system is approximately one time. At least one image carrier;
An image forming apparatus comprising: at least one optical scanning device according to claim 1 that scans the at least one image carrier with light including image information.   The image forming apparatus according to claim 7, wherein the image information is multicolor image information.

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