JP4800897B2 - Optical scanning apparatus and image forming apparatus - Google Patents

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 from a light source and an image forming apparatus including the optical scanning apparatus.

  In recent years, further price reduction has been demanded for image forming apparatuses such as optical printers, digital copying machines, and optical plotters. These image forming apparatuses generally include an optical scanning device that scans a surface to be scanned with a light beam from a light source.

  As a typical method for reducing the price of an image forming apparatus, there is resinization of an optical element used in an optical scanning apparatus. However, in the lens made of resin, the variation in the curvature, thickness, and refractive index of the lens due to temperature change and the variation in refractive index due to the wavelength change of the semiconductor laser that is the light source are larger than those of the glass lens. There is a possibility that fluctuation occurs, the spot diameter increases, and the image quality deteriorates.

  Therefore, for example, in Patent Document 1, a resin lens having a diffractive surface is disposed on the optical path between the light source and the deflector to reduce the variation of the beam spot due to the temperature change.

JP 2006-154701 A

  However, when a so-called multi-beam light source is used in order to increase the image forming speed, the light beam from the light source is scattered by the diffraction surface, and this scattered light overlaps with the original beam spot, thereby reducing the beam spot diameter. There is a risk that the beam spot shape may be deteriorated by causing interference with a light beam from another light source.

  The present invention has been made under such circumstances, and a first object of the invention is to provide an optical scanning apparatus capable of stably forming a small beam spot on a surface to be scanned.

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

According to a first aspect of the present invention, there is provided an optical scanning device that scans a surface to be scanned with a light beam, comprising: at least one light source; a deflector that deflects the light beam from the at least one light source; A first optical system disposed on an optical path between one light source and the deflector and guiding a light beam from the at least one light source to the deflector; and a light beam deflected by the deflector on the surface to be scanned The first optical system has at least one diffractive surface, and the virtual light source when the virtual light source is disposed immediately before the scanned surface and the position of the diffractive surface distance on the optical axis and the main scanning direction of the imaging position of the light beam from the light source is a light scanning device being greater than the focal length in the main scanning direction of the second optical system.

This makes it possible to stably form a small beam spot on the surface to be scanned.

  In the present specification, the image formation position when forming an image with respect to the main scanning direction of the light beam from the virtual light source is a position where the spread in the main scanning direction of the light beam from the virtual light source is converged by the optical element. The image formation position when forming an image with respect to the sub-scanning direction of the light beam from is a position at which the spread in the sub-scanning direction of the light beam from the virtual light source is converged by the optical element. In addition, the focal length when condensing in the main scanning direction of the second optical system is the focal length of the second optical system obtained from the position where the spread in the main scanning direction of the light beam via the second optical system converges. The focal length when condensing in the sub-scanning direction of the second optical system is the focal length of the second optical system obtained from the position where the spread in the sub-scanning direction of the light flux through the second optical system converges.

  According to a second aspect of the present invention, there is provided at least one image carrier; and at least one optical scanning device according to the invention for scanning a plurality of light beams including image information on the at least one image carrier. An image forming apparatus.

  According to this, since at least one optical scanning device of the present invention that can stably form a small beam spot on the surface to be scanned is provided, a high-quality image can be formed.

<< First Embodiment >>
Hereinafter, a first embodiment of the present invention will be described with reference to FIGS. 1 to 6B. FIG. 1 shows a schematic configuration of a laser printer 500 according to the first embodiment of the present invention.

  A laser printer 500 shown in FIG. 1 includes an optical scanning device 900, a photosensitive drum 901, a charging charger 902, a developing roller 903, a toner cartridge 904, a cleaning blade 905, a paper feed tray 906, a paper feed roller 907, and 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. The surface of the photosensitive drum 901 is a scanned surface.

  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 longitudinal direction (direction along the rotation axis) of the photosensitive drum 901 is referred to as “main scanning direction”. 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. The amount of toner in the toner cartridge 904 is checked when the power is turned on or when printing is completed. When the remaining amount is low, a message prompting replacement is displayed on a display unit (not shown).

  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. A latent image to which toner is attached (hereinafter referred to as “toner image” for convenience) moves in the direction of the transfer charger 911 as the photosensitive drum 901 rotates.

  Recording paper 913 is stored in the paper feed tray 906. The 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 the drum 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 toner 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 FIG.

  The optical scanning device 900 includes two light sources (104a and 104b), two coupling lenses (105a and 105b), an aperture plate 106, an anamorphic lens 107, a reflection mirror 108, a polygon mirror 103, and the polygon mirror 103. A polygon motor (not shown) that rotates and a scanning lens 101 are provided. In FIG. 2, the vertical direction of the paper is the main scanning direction, and the direction perpendicular to the paper is the sub-scanning direction.

  Each of the light source 104a and the light source 104b includes a semiconductor laser that emits a single laser beam. A cover glass having a thickness of 0.3 mm is disposed on the front surface of each semiconductor laser.

  Each semiconductor laser is a semiconductor laser having an emission wavelength of 785 nm at 25 ° C. Each semiconductor laser has a characteristic that the emission wavelength changes to 790 nm when the temperature reaches 45 ° C.

  The coupling lens 105a is disposed on the optical path of the light beam emitted from the light source 104a, and makes the light beam emitted from the light source 104a substantially parallel light. The coupling lens 105b is disposed on the optical path of the light beam emitted from the light source 104b, and makes the light beam emitted from the light source 104b substantially parallel light.

Here, as an example, each coupling lens is a resin lens having a thickness (reference numeral d2 in FIG. 3) of 3.0 mm, and an optical path length from each light source (reference numeral d1 in FIG. 3) is 12. It is arranged at a position of 686 mm. The resin of the material of each coupling lens has a refractive index of 1.523867 and a linear expansion coefficient of 7.0 × 10 ⁻⁵ K ⁻¹ at 25 ° C. with a wavelength of 785 nm.

  The shape of the coupling lens 105a is shown in FIGS. 4A and 4B as an example. The shape of the coupling lens 105b is the same as the shape of the coupling lens 105a.

  The surface on the light source side (first surface) of each coupling lens is a diffraction surface in which concentric diffraction gratings are formed on the aspherical surface represented by the following equation (1). Here, x is a depth in the optical axis direction (mm), h is a distance (mm) from the optical axis, R is a paraxial radius of curvature, and K is a conic constant. In the first embodiment, R = −8.9239 mm and K = −1.0. As described above, an axially symmetric diffraction surface is formed on the first surface of each coupling lens.

The phase function φ (h) of the diffractive surface is expressed by the following equation (2). Here, C1 is a phase coefficient, and in the first embodiment, C1 = −2.935 × 10 ⁻² .

φ (h) = C1 · h ² (2)

The image-side surface (second surface) of each coupling lens has a positive power and is an aspherical refracting surface represented by the following equation (3). Here, x is a depth in the optical axis direction (mm), h is a distance (mm) from the optical axis, R is a paraxial radius of curvature, K is a conic constant, A ₄ , A ₆ , A ₈ , A ₁₀ ,. Are coefficients.

In the first embodiment, R = −7.668560 mm, K = −0.78146, A ₄ = −6.29096 × 10 ⁻⁶ , A ₆ = −7.666286 × 10 ⁻⁵ , A ₈ = 2. 44685 × 10 ⁻⁵ , A ₁₀ = −2.53351 × 10 ⁻⁶ .

  Further, each coupling lens is set such that the direction (trend) of the change in the refraction angle due to the change in the wavelength of the incident light is opposite to the direction (trend) between the first surface and the second surface.

Each light source and each coupling lens are fixed to the same holding member made of aluminum. The linear expansion coefficient of this holding member is 4.0 × 10 ⁻⁵ K ⁻¹ .

  The aperture plate 106 is disposed on the optical path between each coupling lens and the anamorphic lens 107, and defines the beam diameter of the light beam that passes through each coupling lens.

  The anamorphic lens 107 is disposed on the optical path between the aperture plate 106 and the reflection mirror 108, and the light beam that has passed through the aperture of the aperture plate 106 is sub-positioned near the deflection reflection surface of the polygon mirror 103 via the reflection mirror 108. An image is formed in the scanning direction.

Here, as an example, the anamorphic lens 107 is a glass lens having a thickness (symbol d4 in FIG. 3) of 3.0 mm, and the optical path length from the second surface of each coupling lens (symbol d3 in FIG. 3). ) Is arranged at a position of 18.0 mm. The glass of the material of the anamorphic lens 107 has a refractive index of 1.514371 at 25 ° C. and a refractive index of 1.514291 at 45 ° C. and a refractive index of 656 nm at 25 ° C. 1.511084, and the linear expansion coefficient is 7.5 × 10 ⁻⁶ K ⁻¹ .

  The first surface of the anamorphic lens 107 is a flat surface in the main scanning direction and has an arc shape with a radius of 19.25 mm in the sub scanning direction. The second surface of the anamorphic lens 107 is a flat surface.

  Further, a soundproof glass 201 having a thickness of 1.9 mm (25 ° C.) is disposed between the anamorphic lens 107 and the polygon mirror 103 and between the polygon mirror 103 and the scanning lens 101 (see FIG. 2). . The material of the soundproof glass 201 is the same glass as the material of the anamorphic lens 107.

  By the way, the optical system arranged on the optical path between each light source and the polygon mirror 103 is also called a coupling optical system. In the first embodiment, the coupling optical system includes a coupling lens 105a, a coupling lens 105b, an aperture plate 106, an anamorphic lens 107, and a reflection mirror 108.

  The polygon mirror 103 has four deflecting reflection surfaces and rotates at a constant speed around a rotation axis parallel to the sub-scanning direction. Here, as an example, the polygon mirror 103 is disposed at a position where the optical path length from the second surface of the anamorphic lens 107 to the rotation axis is 37.7 mm.

  The scanning lens 101 is disposed on the optical path of the light beam deflected by the polygon mirror 103 and condenses the light beam deflected by the polygon mirror 103 on the surface of the photosensitive drum 901.

  Here, as an example, the scanning lens 101 is a resin lens having a center (on the optical axis) thickness (symbol d7 in FIG. 3) of 13.5 mm, and the first of the scanning lens 101 from the rotation axis of the polygon mirror 13. The optical path length to the surface (symbol d6 in FIG. 3) is arranged at a position of 31.5 mm. The material of the scanning lens 101 is the same resin as that of each coupling lens.

  The focal length of the scanning lens 101 in the main scanning direction is 160.0 mm, and the focal length in the sub scanning direction is 31.6 mm.

The surface of the first surface of the scanning lens 101 in a virtual cross section (hereinafter referred to as “main scanning cross section”) parallel to both the main scanning direction and the sub scanning direction is represented by the following equation (4). It has an arc shape. Here, x is the depth in the optical axis direction (mm), y is the distance in the main scanning direction from the optical axis (mm), Rm is the paraxial radius of curvature in the main scanning direction, K is the conic constant, A ₄ , A ₆ , A ₈ , A ₁₀ ,... Are coefficients. Note that the radius of curvature in a virtual cross section (hereinafter referred to as “sub-scan cross section”) parallel to both the optical axis direction and the sub-scan direction is constant at 96.399 mm.

In the first embodiment, Rm = 179.0 mm, K = −5.35548 × 10, A ₄ = −1.38469 × 10 ⁻⁶ , A ₆ = −1.57916 × 10 ⁻⁹ , A ₈ = 3.65531 × 10 ⁻¹² , A ₁₀ = −8.330685 × 10 ⁻¹⁵ , A ₁₂ = 1.112844 × 10 ⁻¹⁷ , A ₁₄ = −5.998173 × 10 ⁻²¹ .

The second surface of the scanning lens 101 has a non-arc shape whose surface shape in the main scanning section is represented by the above equation (4). Further, the curvature Cs (mm ⁻¹ ) in the sub-scanning cross section changes in the main scanning direction according to the following equation (5). Here, y is the distance in the main scanning direction from the optical axis, R _s is the paraxial radius of curvature in the sub-scanning direction, and B ₁ , B ₂ , B ₃ ,.

In the first embodiment, Rm = −157.258 mm, Rs = −19.327 mm, K = 1.94524, A ₄ = −9.004035 × 10 ⁻⁷ , A ₆ = −1.03608 × 10 ^{− 9} , A ₈ = 1.32700 × 10 ⁻¹² , A ₁₀ = −3.07707 × 10 ⁻¹⁵ , A ₁₂ = 3.339516 × 10 ⁻¹⁸ , A ₁₄ = −1.38719 × 10 ⁻²¹ , B ₁ = −2.08484 × 10 ⁻⁵ , B ₂ = 1.67626 × 10 ⁻⁵ , B ₃ = −1.08187 × 10 ⁻⁸ , B ₄ = −1.01661 × 10 ⁻⁸ , B ₅ = 4. 95931 × 10 ⁻¹² and B ₆ = 9.76946 × 10 ⁻¹⁴ .

  Note that the scanning lens 101 is arranged such that its longitudinal direction is inclined by 0.68 ° counterclockwise with respect to the main scanning direction in a plane including the optical axis direction and the main scanning direction.

  Incidentally, the optical system disposed on the optical path between the polygon mirror 103 and the photosensitive drum 901 is also called a scanning optical system. In the first embodiment, the scanning optical system includes the scanning lens 101.

  In the first embodiment, the optical scanning device 900 is arranged so that the optical path length (the symbol d8 in FIG. 3) from the second surface of the scanning lens 101 to the photosensitive drum 901 is 176.0 mm. Yes. A dust-proof glass 202 having a thickness of 1.9 mm (25 ° C.) is disposed between the scanning lens 101 and the photosensitive drum 901. The material glass of the dustproof glass 202 is the same glass as the material of the anamorphic lens 107.

  By the way, light may be scattered on the diffraction surface due to manufacturing errors. In particular, in a relief type diffraction grating that obtains a diffraction effect due to unevenness on the surface, scattered light is likely to be generated due to a corner portion or a step portion inclination.

  In the first embodiment, as shown in FIG. 5 as an example, when a virtual light source is arranged immediately before the diffraction surface of the coupling lens 105a (position P in FIG. 5), the virtual light source emits the light. The distance between the image forming position when the focused light beam is focused in the main scanning direction and the surface to be scanned is set to be larger than the focal length when the scanning optical system is focused in the main scanning direction. . In addition, the distance between the imaging position when the light emitted from the virtual light source forms an image in the sub-scanning direction and the surface to be scanned is larger than the focal length when the light is condensed in the sub-scanning direction of the scanning optical system. It is set to be.

  Specifically, the image formation position when forming an image with respect to the main scanning direction of the light beam emitted from the virtual light source is a position of 406.0 mm from the scanned surface in the light source direction, and the light beam emitted from the virtual light source is The imaging position when imaging in the sub-scanning direction is a position of 133.6 mm from the surface to be scanned in the light source direction. That is, the distance between the imaging position in the main scanning direction and the surface to be scanned (symbol dm1 in FIG. 6A) is 406.0 mm, and the imaging position and the surface to be scanned when imaging is performed in the sub-scanning direction. Is 133.6 mm (symbol ds1 in FIG. 6B). The focal length of the scanning lens 101 when condensing in the main scanning direction is 160.0 mm, and the focal length when condensing in the sub-scanning direction is 31.6 mm.

  As a result, the scattered light greatly spreads on the scanned surface, so that the intensity per unit area of the scattered light on the scanned surface becomes negligibly small, and the beam spot diameter due to the overlap of the scattered light with the original beam spot is reduced. Thickening can be prevented and the beam spot diameter can be reduced.

  As described above, according to the optical scanning device 900 according to the first embodiment, the light beam emitted from the light source 104a and the light beam emitted from the light source 104b and the light beam emitted from the light source 104b are substantially parallel. The coupling lens 105b to be light has a diffractive surface, and forms an image with respect to the scanning surface and the sub-scanning direction of the light beam from the virtual light source when the virtual light source is disposed immediately before the position of the diffractive surface. Is set to be larger than the focal length when the light is condensed in the sub-scanning direction of the scanning optical system.

  In addition, an interval between the surface to be scanned and an image forming position when the light beam from the virtual light source is imaged in the main scanning direction is larger than a focal length when the light is condensed in the main scanning direction of the scanning optical system. It is set to be.

  Therefore, the beam spot diameter can be prevented from becoming thick due to the scattered light overlapping the original beam spot, and a small beam spot can be stably formed on the surface of the photosensitive drum 901.

  Further, according to the laser printer 500 according to the first embodiment, since the optical scanning device 900 that can stably form a small beam spot on the surface of the photosensitive drum 901 is provided, a high-quality image is provided. Can be formed.

<< Second Embodiment >>
Next, a second embodiment of the present invention will be described with reference to FIGS.

  As shown in FIG. 7, the second embodiment uses coupling lenses 115a and 115b in place of the coupling lenses 105a and 105b in the optical scanning apparatus 900 of the first embodiment described above. It is characterized in that an anamorphic lens 117 is used in place of the anamorphic lens 107 and an aperture plate 116 is added. Other configurations are the same as those of the first embodiment. Accordingly, the following description will be focused on the differences from the first embodiment, and the same reference numerals will be used for the same or equivalent components as in the first embodiment, and the description thereof will be simplified or omitted. Shall.

  The coupling lens 115a is disposed on the optical path of the light beam emitted from the light source 104a, and makes the light beam emitted from the light source 104a substantially parallel light. The coupling lens 115b is disposed on the optical path of the light beam emitted from the light source 104b, and makes the light beam emitted from the light source 104b substantially parallel light.

  Here, as an example, each coupling lens is a glass lens having a thickness (reference numeral d12 in FIG. 8) of 3.0 mm, and an optical path length from each light source (reference numeral d11 in FIG. 8) is 12. 669 mm. The glass of the material of each coupling lens is the same glass as the material of the anamorphic lens 107 in the first embodiment.

  The first surface of each coupling lens is a flat surface.

The second surface of each coupling lens is an aspherical refracting surface represented by the above equation (3). In the second embodiment, R = −7.499 mm, K = −0.78146, A ₄ = −6.29096 × 10 ⁻⁶ , A ₆ = −7.666286 × 10 ⁻⁵ , A ₈ = 2. 44685 × 10 ⁻⁵ , A ₁₀ = −2.53351 × 10 ⁻⁶ .

  The anamorphic lens 117 is disposed on the optical path between the aperture plate 106 and the reflection mirror 108, and the light beam that has passed through the aperture of the aperture plate 106 is sub-positioned near the deflection reflection surface of the polygon mirror 103 via the reflection mirror 108. An image is formed in the scanning direction.

  Here, as an example, the anamorphic lens 117 is a resin lens having a thickness (symbol d14 in FIG. 8) of 3.0 mm, and the optical path length from the second surface of each coupling lens (symbol d13 in FIG. 8). ) Is arranged at a position of 18.0 mm. The resin of the material of the anamorphic lens 117 is the same resin as the material of each coupling lens in the first embodiment.

  The shape of the first surface of the anamorphic lens 117 is a flat surface in the main scanning direction and an arc shape with a radius of 19.72 mm in the sub-scanning direction.

  The shape of the second surface of the anamorphic lens 117 is, for example, as shown in FIGS. 9A and 9B, in the sub-scanning direction on the non-arc in the sub-scanning direction expressed by the following equation (6). A diffraction surface on which a plurality of linear diffraction gratings extending in the direction is formed. Here, x is a depth (mm) in the optical axis direction, z is a distance (mm) in the sub-scanning direction from the optical axis, R is a paraxial radius of curvature, and K is a conic constant. In the second embodiment, R = 16.774 mm and K = −1.0. Thus, on the second surface of the anamorphic lens 117, a diffractive surface whose cross-sectional shape in the sub-scanning direction is the same regardless of the position in the main scanning direction is formed.

Further, the phase function φ (z) of the diffraction surface is expressed by the following equation (7). Here, C1 is a phase coefficient, and in the second embodiment, C1 = −1.562 × 10 ⁻² .

φ (z) = C1 · z ² (7)

  Here, as an example, the polygon mirror 103 is disposed at a position where the optical path length from the second surface of the anamorphic lens 117 to the rotation axis is 37.7 mm. Further, in FIG. 8, d16 = 31.5 mm, d17 = 13.5 mm, and d18 = 176.0 mm.

  In the second embodiment, as shown in FIG. 10 as an example, when a virtual light source is arranged immediately before the diffractive surface of the anamorphic lens 117 (position P ′ in FIG. 10), the virtual light source emits light. The distance between the image forming position when the focused light beam is focused in the main scanning direction and the surface to be scanned is set to be larger than the focal length when the scanning optical system is focused in the main scanning direction. . In addition, the distance between the imaging position when the light emitted from the virtual light source forms an image in the sub-scanning direction and the surface to be scanned is larger than the focal length when the light is condensed in the sub-scanning direction of the scanning optical system. It is set to be.

  Specifically, the image formation position when forming an image with respect to the main scanning direction of the light beam emitted from the virtual light source is a position of 271.4 mm in the light source direction from the scanned surface, and the light beam emitted from the virtual light source. The imaging position when imaging in the sub-scanning direction is a position of 111.0 mm from the surface to be scanned in the light source direction. That is, the distance between the imaging position and the surface to be scanned when imaging is performed in the main scanning direction (symbol dm2 in FIG. 11A) is 271.4 mm, and the imaging position when imaging is performed in the sub-scanning direction. And the scanned surface (reference numeral ds2 in FIG. 11B) is 111.0 mm. The focal length of the scanning lens 101 when condensing in the main scanning direction is 160.0 mm, and the focal length when condensing in the sub-scanning direction is 31.6 mm.

  Thus, similar to the first embodiment, the scattered light greatly spreads on the surface to be scanned, so that it is possible to prevent the beam spot diameter from becoming thick due to the overlap of the scattered light with the original beam spot, and to reduce the beam spot diameter. Can do.

  In addition, an aperture plate 116 that blocks scattered light from the diffraction surface is disposed on the optical path between the anamorphic lens 117 and the reflection mirror 108. This further prevents scattered light from affecting the beam spot from the diffraction surface.

  As described above, according to the optical scanning device 900 according to the second embodiment, the anamorphic lens 117 that is emitted from each light source and guides the light flux through each coupling lens and the aperture plate 106 to the polygon mirror 103 is as follows. The distance between the surface to be scanned having a diffractive surface and the image forming position when forming an image in the sub-scanning direction of the light beam from the virtual light source when the virtual light source is arranged immediately before the position of the diffractive surface It is set to be larger than the focal length when the light is condensed in the sub scanning direction of the optical system.

  In addition, an interval between the surface to be scanned and an image forming position when the light beam from the virtual light source is imaged in the main scanning direction is larger than a focal length when the light is condensed in the main scanning direction of the scanning optical system. It is set to be.

  Therefore, as in the first embodiment, the beam spot diameter can be prevented from being increased due to the scattered light overlapping the original beam spot, and a small beam spot can be stably formed on the surface of the photosensitive drum 901. It becomes possible.

  Further, according to the laser printer 500 according to the second embodiment, since the optical scanning device 900 that can stably form a small beam spot on the surface of the photosensitive drum 901 is provided, a high-quality image is provided. Can be formed.

  In each of the embodiments described above, the light source 104a and the light source 104b each have a semiconductor laser that emits a single laser beam. However, the present invention is not limited to this. At least one of the light sources 104b may include a semiconductor laser array that can emit a plurality of laser beams. In this case, the beam spot diameter can be prevented from becoming thicker due to the scattered light further overlapping the portion where the skirts of the plurality of beam spots overlap, and deterioration of the beam spot shape can be suppressed. Thereby, the image forming speed can be increased.

  In each of the above-described embodiments, the case where the wavelength of the light beam emitted from each semiconductor laser is 785 nm (25 ° C.) has been described. However, the present invention is not limited thereto, and any wavelength corresponding to the sensitivity characteristic of the photosensitive drum 901 may be used. good.

  In each of the above embodiments, the distance between the surface to be scanned and the image forming position when forming an image in the sub-scanning direction of the light beam from the virtual light source is the focal length when the light is condensed in the sub-scanning direction of the scanning optical system. And the distance between the scanning surface and the image forming position when the light beam from the virtual light source forms an image in the main scanning direction is focused in the main scanning direction of the scanning optical system. In the above description, the distance between the scanning surface and the image forming position when forming an image in the sub-scanning direction of the light beam from the virtual light source is the same as that of the scanning optical system. It is set to be larger than the focal length at the time of condensing in the sub-scanning direction, or the interval between the scanning surface and the imaging position when imaging in the main scanning direction of the light beam from the virtual light source is Scanning optical system It may be set to be larger than the focal distance when collecting light in the main scanning direction.

  In each of the above embodiments, 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 as a result.

  Further, even an image forming apparatus that forms a color image can form a high-quality image by using an optical scanning device corresponding to the color image.

  As an example, as shown in FIG. 12, the image forming apparatus may be a tandem color machine corresponding to a color image and including a plurality of photosensitive drums. The tandem color machine shown in FIG. 12 includes a black (K) photosensitive drum K1, a charger K2, a developing unit K4, a cleaning unit K5, a transfer charging unit K6, and a cyan (C) photosensitive unit. Drum C1, charging device C2, developing device C4, cleaning device C5, transfer charging device C6, magenta (M) photosensitive drum M1, charging device M2, developing device M4, cleaning device M5, and transfer charging Means M6, yellow (Y) photosensitive drum Y1, charger Y2, developing unit Y4, cleaning means Y5, transfer charging means Y6, optical scanning device 900, transfer belt 80, fixing means 30, etc. It has.

  In this case, the optical scanning device 900 includes a light source for black, a light source for cyan, a light source for magenta, and a light source for yellow. The light beam from the black light source is applied to the photosensitive drum K1, the light beam from the cyan light source is applied to the photoconductive drum C1, and the light beam from the magenta light source is applied to the photoconductive drum M1, The light beam from the light source for yellow is applied to the photosensitive drum Y1. An optical scanning device 900 may be provided for each color.

  Each photosensitive drum rotates in the direction of the arrow in FIG. 12, 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 a light beam 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.

  Further, an image forming apparatus using a silver salt film as the image carrier may be used. In this case, a latent image is formed on the silver salt film by optical scanning, and this latent image can be visualized by a process equivalent to a developing process in a normal silver salt photographic process. Then, it can be transferred to photographic paper by a process equivalent to a printing process in a normal silver salt photographic process. Such an image forming apparatus can be implemented as an optical plate making apparatus or an optical drawing apparatus that draws a CT scan image or the like.

  Further, an image forming apparatus using a color developing medium (positive photographic paper) that develops color by the heat energy of a beam spot as an image carrier may be used. In this case, a visible image can be directly formed on the image carrier by optical scanning.

  As described above, the optical scanning device of the present invention is suitable for stably forming a small beam spot on the surface to be scanned. The image forming apparatus of the present invention is suitable for forming a high quality image.

It is a figure for demonstrating schematic structure of the laser printer which concerns on the 1st Embodiment of this invention. It is the schematic which shows the optical scanning device in FIG. It is a figure for demonstrating the position of each optical element in the optical scanning device of FIG. 4A and 4B are diagrams for explaining the shape of the coupling lens in the optical scanning device of FIG. It is a figure for demonstrating the position of a virtual light source. FIG. 6A is a diagram for explaining the imaging position when imaging is performed in the main scanning direction of the light beam emitted from the virtual light source, and FIG. It is a figure for demonstrating the image formation position when image-forming regarding a scanning direction. It is the schematic which shows the optical scanning device in the 2nd Embodiment of this invention. It is a figure for demonstrating the position of each optical element in the optical scanning device of FIG. FIGS. 9A and 9B are diagrams for explaining the shape of the anamorphic lens in the optical scanning device of FIG. It is a figure for demonstrating the position of a virtual light source. FIG. 11A is a diagram for explaining the imaging position when imaging is performed with respect to the main scanning direction of the light beam emitted from the virtual light source, and FIG. 11B is a diagram illustrating the sub-portion of the light beam emitted from the virtual light source. It is a figure for demonstrating the image formation position when image-forming regarding a scanning direction. It is a figure for demonstrating schematic structure of a tandem color machine.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 101 ... Scan lens (2nd optical system) 103 ... Polygon mirror (deflector), 104a ... Light source, 104b ... Light source, 105a ... Coupling lens (a part of 1st optical system), 105b ... Coupling lens (1st 1 part of one optical system), 106 ... an aperture plate (part of the first optical system), 107 ... anamorphic lens (part of the first optical system), 108 ... reflection mirror (part of the first optical system), 115a, coupling lens (part of the first optical system), 115b, coupling lens (part of the first optical system), 116, aperture plate (part of the first optical system), 117, anamorphic lens (first part) 1 part of an optical system), 500... Laser printer (image forming apparatus), 900... Optical scanning apparatus, 901.

Claims (7)

An optical scanning device that scans a surface to be scanned with a light beam,
At least one light source;
A deflector for deflecting a light beam from the at least one light source;
A first optical system disposed on an optical path between the at least one light source and the deflector to guide a light beam from the at least one light source to the deflector;
A second optical system for guiding the light beam deflected by the deflector to the surface to be scanned;
The first optical system has at least one diffractive surface;
The distance on the optical axis between the surface to be scanned and the imaging position in the main scanning direction of the light beam from the virtual light source when the virtual light source is disposed immediately before the position of the diffraction surface is the second optical system. An optical scanning device characterized by being larger than the focal length in the main scanning direction . The second optical system includes at least one optical element made of resin,
The first optical system includes at least one lens having a spherical surface or an aspheric surface,
The optical scanning device according to claim 1, wherein the at least one lens includes the axially symmetric diffraction surface. The second optical system includes at least one optical element made of resin,
The first optical system includes at least one lens having an anamorphic surface;
2. The optical scanning device according to claim 1, wherein the at least one lens has the diffractive surface whose sectional shape in the sub-scanning direction is the same regardless of a position in the main scanning direction. An optical system consisting of the second optical system and the first optical system is arranged behind the diffraction surface, according to claim 1 to 3, characterized in that it comprises an aperture plate for blocking scattered light by the diffraction surface The optical scanning device according to any one of the above. The at least one light source, an optical scanning apparatus according to any one of claims 1 to 4, characterized in that a plurality of light sources. At least one image carrier;
An image forming apparatus comprising: at least one optical scanning device according to any one of claims 1 to 5 that scans a plurality of light beams including image information on the at least one image carrier. The image forming apparatus according to claim 6 , wherein the image information is color image information.

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