JP2013015611A - Optical scanner and image forming apparatus having the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical scanning apparatus capable of reducing pitch unevenness caused by a face tangle of a deflection face and forming a high definition image, and to provide an image forming apparatus having the same.SOLUTION: An optical scanner includes: light source means; deflection means for deflecting a light flux from the light source means; an incident optical system for guiding the light flux from the light source means to the deflection means; and an imaging optical system for imaging the light flux reflected and deflected by the deflection means as a spot on a surface to be scanned. The incident optical system makes the light flux from the light source means obliquely incident to a plane perpendicular to the rotational axis of a deflection surface of the deflection means at a predetermined angle in a sub-scanning cross section. The imaging optical system has a non-arcuate shaped optical element in which the slave line shape of at least one optical surface includes an aspherical coefficient of a 4-th or higher even-numbered order. Among the aspherical coefficients of a 4-th or higher even-numbered order of the slave line shapes of the optical surfaces, the aspherical coefficients of any order change according to the position in a main scanning direction, and an original point of the non-arcuate shape of the slave line in the position in each main scanning direction of the optical surface is positioned within the light flux in an entire effective area.

Description

  The present invention relates to an optical scanning apparatus and an image forming apparatus using the same, and is suitable for an image forming apparatus such as a laser beam printer, a digital copying machine, or a multifunction printer having an electrophotographic process.

  Conventionally, optical scanning devices have been widely used in laser beam printers, digital copying machines, and the like. In the conventional optical scanning device, the deflection reflection surface of the rotary polygon mirror, which is a deflection means, causes the surface to enter and exit (so-called sag) depending on the rotation angle, so that the positional relationship between the focal line and the deflection reflection surface changes. For this reason, even if the tilt correction optical system is configured, if surface tilt occurs, the light beam arrival position in the sub-scanning direction on the surface to be scanned is shifted, and pitch unevenness due to so-called surface tilt occurs.

  Depending on the sag of the deflection reflection surface, if the deflection reflection surface and the surface to be scanned are conjugated in the entire effective scanning range, pitch unevenness due to surface tilt can be reduced, but the field curvature in the sub-scanning direction deteriorates. Variation in spot diameter will increase. Recently, for the purpose of increasing the printing speed or increasing the recording density, a monolithic multi-beam laser having a plurality of light emitting units or a surface emitting laser (VCSEL) has been used as a light source means. As the number of light emitting portions increases, the pitch unevenness period increases and the unevenness becomes more conspicuous. Therefore, improvement of pitch unevenness is an important issue for high-definition image formation.

  In such an optical scanning device, a technique has been proposed for improving surface tilt performance, reducing pitch unevenness, and reducing field curvature in the sub-scanning direction (Patent Document 1).

JP 2009-14953 A

  However, in the prior art disclosed in Patent Document 1, the geometrical optical spot image position shift due to surface tilt is suppressed to reduce geometric optical pitch unevenness, and the sub-wires are made non-circular. Although a technique for correcting the curvature of field in the scanning direction has been disclosed, in terms of wave optics, the intensity distribution center of gravity of the light spot is shifted in the sub-scanning direction on the surface to be scanned, and the wave caused by the tilting of the deflecting reflection surface There arises a problem that optical pitch unevenness is not reduced and image quality deteriorates.

  Also, in Patent Document 1, the origin of the child non-arc coincides with the principal ray passing position on the axis and off the axis, but a specific method is not disclosed. In the case of the sub-scanning oblique incidence system in which the light beam is incident obliquely with respect to the deflecting / reflecting surface in the sub-scanning direction, the locus of the principal ray passing position on the lens surface constituting the imaging optical system is curved in the sub-scanning direction. Even if the origin of the non-arc arc and the principal ray passing position are matched on the axis and off-axis, the positional relationship between the origin of the non-arc arc and the principal ray passing position is not fixed at other image heights. If is large, the principal ray passing position and the origin of the child non-arc are greatly shifted, and the effect of the child non-arc may not be obtained effectively.

  Therefore, in the present invention, even when the locus of the principal ray passing position is curved on the lens surface as in the sub-scanning oblique incidence system, high-definition image formation is achieved by reducing wave optical pitch unevenness at the entire image height. An object of the present invention is to provide an optical scanning device and an image forming apparatus that can be used.

  In the optical scanning device of the present invention, the sub-line shape of at least one optical surface of the imaging optical system is a non-arc shape, and the positional relationship between the origin of the non-arc shape of the sub-line and the principal ray passing position is appropriately set. By setting, even when the locus of the principal ray passing position is curved on the lens surface as in the sub-scanning oblique incidence system, it is possible to reduce wave optical pitch unevenness at the entire image height.

  Specifically, the optical scanning device according to the present invention includes a light source unit having one or more light emitting points, a deflecting unit for deflecting a light beam from the light source, and an incident optical for guiding the light beam from the light source to the deflecting unit. And an imaging optical system that forms an image of the light beam reflected and deflected by the deflecting means as a spot on the surface to be scanned. The incident optical system deflects the light beam from the light source by the deflecting means. The imaging optical system is obliquely incident at a predetermined angle in a sub-scanning direction with respect to a surface perpendicular to the rotation axis of the surface, and the imaging optical system has an even-order aspherical surface in which at least one optical surface has a quadratic or higher order. An optical element having a non-arc shape including a coefficient, and the aspheric coefficient of any order among the fourth-order and higher-order aspheric coefficients of the sub-line shape of the optical surface is a position in the main scanning direction. A non-arc of a child line at each position in the main scanning direction of the optical surface Jo of origin, characterized in that located in the light beam in the effective region throughout.

  According to the present invention, the sub-line shape of at least one surface of the imaging optical system is a non-arc shape, and the positional relationship between the origin of the non-arc shape and the principal ray passing position is appropriately set. Even when the locus of the principal ray passing position is curved on a lens surface like a scanning oblique incidence system, the fluctuation of the center of gravity of the intensity distribution of the light spot on the scanned surface due to the surface tilt is suppressed, and the surface tilt It is possible to achieve an optical scanning device and an image forming apparatus capable of reducing wave optical pitch unevenness and forming a high-definition image.

Sectional drawing of the principal part of the optical scanning apparatus of Example 1 of this invention of the main scanning direction The figure explaining the subwire shape of Example 1 of this invention The figure which shows the scanning line curve by the surface fall of Example 1 of this invention The figure which shows the field curvature of the subscanning direction of Example 1 of this invention. The figure which shows the origin position and principal ray passage position of a child non-arc shape of Example 1 of the present invention The figure which shows the change of the 4th-order aspherical coefficient of the subwire of Example 1 of this invention The figure which shows the power change of the subscanning direction of Example 1 of this invention. Sectional drawing of the principal part of the optical scanning apparatus of Example 2 of this invention of the main scanning direction The figure explaining the subwire shape of Example 2 of this invention The figure which shows the scanning line curve by the surface fall of Example 2 of this invention The figure which shows the field curvature of the subscanning direction of Example 2 of this invention. The figure which shows the origin position and principal ray passage position of child non-arc shape of Example 2 of this invention The figure which shows the change of the 4th-order aspherical coefficient of the child wire of Example 2 of this invention. Sectional drawing of the principal part of the optical scanning apparatus of Example 3 of this invention of the main scanning direction The figure which shows the scanning line curve by the surface fall of Example 3 of this invention The figure which shows the field curvature of the subscanning direction of Example 3 of this invention. The figure which shows the origin position and principal ray passage position of child non-arc shape of Example 3 of this invention The figure which shows the change of the 4th-order aspherical coefficient of the child wire of Example 3 of this invention. Sectional drawing of the principal part of the image forming apparatus of Example 5 of this invention Schematic view of essential parts of a color image forming apparatus according to Embodiment 6 of the present invention.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 is a sectional view (main scanning sectional view) of a main part in the main scanning direction of the optical scanning apparatus according to the first embodiment of the present invention.

  In the following description, the main scanning direction is a direction perpendicular to the rotation axis of the deflecting unit and the optical axis of the imaging optical system (the direction in which the light beam is deflected and reflected (deflected and scanned) by the deflecting unit). The direction is a direction parallel to the rotation axis of the deflecting means. The main scanning section is a plane including the optical axis of the imaging optical system and the main scanning direction, and the sub-scanning section is a section including the optical axis of the imaging optical system and perpendicular to the main scanning section. It is.

  In FIG. 1, the light source means 10 comprises a semiconductor laser having one light emitting point. The light beam emitted from the light source means 10 is converted into a substantially parallel light beam by the collimator lens 20, and then enters the cylindrical lens 30 and is refracted only in the sub-scanning direction. After that, the beam width is limited by the aperture stop 40, and then condensed in the vicinity of the deflection reflection surface 51 of the rotary polygon mirror 50, which is a deflecting means, only in the sub-scanning direction, and is formed as a long line image in the main scanning direction. Imaged.

  Each element of the collimator lens 20, the cylindrical lens 30, and the stop 40 constitutes one element of the incident optical system LA.

  In this embodiment, the collimator lens 20 converts the light beam into a substantially parallel light beam. However, instead of the collimator lens 20, an optical element that converts the light beam into a divergent light beam and a convergent light beam may be used. Moreover, it is not limited to two lenses, and may be constituted by one anamorphic optical element having different powers in the main scanning direction and the sub-scanning direction.

  The incident optical system LA has its optical axis tilted in the sub-scanning direction with respect to the plane perpendicular to the rotation axis of the rotary polygon mirror 50, and the light beam emitted from the light source means 10 is perpendicular to the rotation axis of the rotary polygon mirror 50. Is incident obliquely at a predetermined angle (3 °) in the sub-scanning direction (sub-scanning oblique incidence system).

  In the drawing, the rotary polygon mirror 50 shows only the deflection reflection surface 51 thereof.

  The light beam reflected and deflected by the rotary polygon mirror 50 is imaged as a light spot on the scanning surface 70 by the imaging optical system 60 including the first fθ lens 61 and the second fθ lens 62. As the rotary polygon mirror 50 rotates in the direction of the arrow in the figure, the light spot scans the surface to be scanned 70 to form an electrostatic latent image. Examples of the scanned surface 70 include a photosensitive drum surface.

  A dustproof glass 80 is provided between the second fθ lens 62 and the scanned surface 70 to prevent dust, toner, and the like from entering the optical scanning device.

  In this case, the imaging optical system 60 defines an imaging position (focal line position) imaged near the deflection reflection surface 51 (near the deflection surface) by the cylindrical lens 30 and the scanned surface 70 in the sub-scan section. A so-called tilt correction optical system having a conjugate relationship is formed.

  Table 1 shows the characteristics of the optical system in Example 1. Here, “E-x” indicates “10-x”. In addition, all the coefficients not particularly described are 0.

で表わされる。Ｒは曲率半径、Ｋは離心率、Ｂ₄〜Ｂ₁₂は４次〜１２次の母線の非球面係数である。 The lens surfaces 61a to 62b of the first fθ lens 61 and the second fθ lens 62 constituting the imaging optical system 60 have a generatrix shape, with the intersection (lens surface vertex) between each lens surface and the optical axis as the origin, and the optical axis direction. Is the X axis, the direction perpendicular to the optical axis in the main scanning section is the Y axis, and the direction orthogonal to the optical axis in the sub scanning section is the Z axis,

It is represented by R is a radius of curvature, K is an eccentricity, and B _{4 to} B ₁₂ are aspherical coefficients of the fourth to _twelfth buses.

  Here, when the coefficients on the laser side (the lower side of the figure) and the anti-laser side (the upper side of the figure) are different, the subscript u is added to the coefficient on the laser side, and the subscript l is added to the coefficient on the anti-laser side. doing.

で表される。子線の曲率半径ｒ´は、主走査方向中央部における子線曲率半径ｒに対して主走査方向周辺部に向かい変化しており、D₂〜D₁₀は子線曲率半径の変化係数である。ここで、レーザ側（図の下側）と反レーザ側（図の上側）で係数が異なる場合は、レーザ側の係数には添字ｕを附し、反レーザ側の係数には添字ｌを附している。 The sub-line shapes of the lens surfaces 61a and 61b of the first fθ lens 61 and the lens surface 62a of the second fθ lens 62 on the polygon mirror 50 side are:

It is represented by The radius of curvature r ′ of the child line changes toward the peripheral part in the main scanning direction with respect to the radius of curvature r in the central part in the main scanning direction, and D _{2 to} D ₁₀ are change coefficients of the radius of curvature of the child line. . Here, when the coefficients on the laser side (the lower side of the figure) and the anti-laser side (the upper side of the figure) are different, the subscript u is added to the coefficient on the laser side, and the subscript l is added to the coefficient on the anti-laser side. doing.

で表される。子線の曲率半径ｒ´は、主走査方向中央部における子線曲率半径ｒに対して主走査方向周辺部に向かい変化しており、D₂〜D₁₀は子線曲率半径の変化係数である。ΣG_jkY^j、は４次、６次、…の子線の非球面係数である。子線の非球面係数は主走査方向に沿って変化しており、G_0k〜G_16kは各2k次の子線の非球面係数の変化係数である。表１に示した子線の非球面係数G₀〜G₆は、それぞれ、G_0k〜G_6kであって、添え字のｋ＝２の場合（Ｚに対する４次の場合）の子線の非球面係数ΣG_jkY^j中のG_jkの数値を示している。表１より明らかなように本実施例においては、４次以外の次数の非球面係数は0としているが、これに限らず６次以上の非球面係数を有限の値としても良い。 Next, the sub-wire shape of the lens surface (optical surface) 62b on the scanned surface 70 side of the second fθ lens 62 is a non-arc shape (aspheric shape) including a fourth-order term,

It is represented by The radius of curvature r ′ of the child line changes toward the peripheral part in the main scanning direction with respect to the radius of curvature r in the central part in the main scanning direction, and D _{2 to} D ₁₀ are change coefficients of the radius of curvature of the child line. . ΣG _jk Y ^j is the aspheric coefficient of the fourth, sixth,. The aspheric coefficients of the child lines change along the main scanning direction, and G _{0k to} G _16k are change coefficients of the aspheric coefficients of the 2k-th order child lines. The aspheric coefficients G _{0 to} G ₆ of the child lines shown in Table 1 are G _{0k to} G _6k , respectively, and the non-element of the child line when the subscript k = 2 (fourth order with respect to Z) is shown. The numerical value of G _{jk in} the spherical coefficient ΣG _jk Y ^j is shown. As is apparent from Table 1, in this embodiment, the aspherical coefficients of orders other than the fourth order are set to 0. However, the present invention is not limited to this, and the aspherical coefficients of the sixth or higher order may be set to finite values.

FIG. 2A shows a cross-sectional view (sub-scanning cross-sectional view) of the main part on the plane including the sub-scanning direction and the optical axis in the present embodiment. 2B is a sub-scan sectional view of the lens surface 62b at the center in the main scanning direction and a sub-scan sectional view of the lens surface 62b at the end portion in the main scanning direction (lower side in FIG. 1). In the figure, the vertex of the child line means the intersection of the bus and the child line. As can be seen from FIG. 2B, the origin of the non-arc shape of the child line is eccentric with respect to the bus in the sub-scanning direction, and the amount of eccentricity in the sub-scanning direction differs depending on the position in the main scanning direction. . Further, the amount of eccentricity continuously changes from the center in the main scanning direction toward the end (FIG. 2C). A _{0 to} A ₁₆ are the change coefficients. Therefore, the origin of the non-arc shape of the child line is curved in the sub-scanning direction and does not necessarily exist on the bus line. Here, when the coefficients on the laser side (the lower side of the figure) and the anti-laser side (the upper side of the figure) are different, the subscript u is added to the coefficient on the laser side, and the subscript l is added to the coefficient on the anti-laser side. doing. The radius of curvature of the child line is the radius of curvature in the cross section perpendicular to the Y axis at each main scanning position.

  In the present embodiment, the second fθ lens 62 is arranged with an eccentricity of 2.15 mm in the sub-scanning direction.

  FIG. 3 shows scanning line curvature (pitch unevenness) due to surface tilt when the deflecting / reflecting surface 51 is tilted 3 'upward in the sub-scanning direction due to a processing error in this embodiment. In FIG. 3, the broken line indicates the shift amount of the principal ray arrival position on the scanned surface 70, and the solid line indicates the shift amount of the spot centroid position on the scanned surface 70.

  FIG. 4 shows field curvature in the sub-scanning direction in this embodiment. In FIG. 4, “Slice-” is a negative defocus position where the spot diameter in the sub-scanning direction is 90 μm, “Slice +” is a positive defocus position where the spot diameter in the sub-scanning direction is 90 μm, and “Center "Indicates a defocus position between" Slice- "and" Slice + ", and" Center "indicates field curvature in the sub-scanning direction.

Hereinafter, the designed wave optical pitch unevenness reducing method in the present embodiment will be described in detail.
As a comparative example, the optical scanning device of Example 1 disclosed in Patent Document 1 is cited.

  In Patent Document 1, since the geometrical optical pitch unevenness is reduced by conjugating the deflecting / reflecting surface 51 and the surface to be scanned 70 with the total image height, the principal ray arrival position is 0.3 over the entire region even when the surface is tilted. It is well corrected to μm or less. However, it is considered that the center of gravity of the spot on the surface to be scanned is deviated from the arrival position of the principal ray by making the sub-line shape non-circular in order to correct the field curvature of the sub-scanning. Absent. Therefore, the wave optical pitch unevenness is not sufficiently corrected, and the pitch unevenness occurs.

  On the other hand, in this embodiment, the geometrical optical pitch unevenness is left without conjugating the deflecting / reflecting surface 51 and the surface to be scanned 70 with the total image height. In FIG. 3, the principal ray arrival position is shifted, and geometrical optical pitch unevenness occurs at Peak to Peak of 1.7 μm.

  At this time, the principal ray arrival position on the lens surface 62b is shifted downward in the sub-scanning direction from the design position due to the downward surface tilt in the sub-scanning direction. Therefore, by making the vicinity of the designed principal ray arrival position on the lens surface 62b the origin and making the sub-wire shape a non-circular shape, the light flux on the lens surface 62b is a secondary light across the principal ray when surface tilt occurs. Asymmetrical refractive power is applied to the upper part and the lower part in the scanning direction, and the position of the center of gravity of the spot when a surface tilt occurs is corrected to reduce wave optical pitch unevenness. In the sub-scanning oblique incidence system, since the locus of the principal ray arrival position on the lens surface 62b is curved in the sub-scanning direction, the amount of eccentricity with respect to the generatrix of the origin of the non-arc-shaped child line is set along the main scanning direction. By changing it, the origin of the non-circular shape of the child line is made near the principal ray arrival position at the entire image height. Further, since the correction amount of the center of gravity of the spot differs for each image height, the aspheric coefficient of the child line is also changed along the main scanning direction. As a result, in this embodiment, when the deflecting / reflecting surface 51 is tilted 3 'downward in the sub-scanning direction due to a processing error, the shift amount of the spot centroid position due to the surface tilt is corrected by the total image height, and Peak to Peak A good performance of 0.4 μm or less is achieved. In addition, by substituting the geometric optical field curvature for the sub-scanning field curvature, we succeeded in simultaneously achieving good performance with wave optical field curvature of 0.60 mm or less in Peak to Peak. doing.

  In the present embodiment, the sub-wire shape of the lens surface 62b on the scanned surface 70 side of the second fθ lens 62 is a non-arc shape including a quaternary aspheric term as described above, and the sub-arc non-arc shape is included. It is characterized in that the origin of the shape is decentered in the sub-scanning direction with respect to the generatrix, and that the aspheric coefficient and the eccentric amount of the origin of the non-arc shape are changed according to the Y coordinate.

  FIG. 5 shows the origin position and the principal ray passing position of the non-arc shape of the child wire in the present embodiment.

  As shown in FIG. 5, the curve connecting the origins of the non-arc shapes of the child lines has a shape along the locus of the principal ray passing position. In addition, the amount of deviation in the sub-scanning direction between the origin of the non-arc shape of the child line and the principal ray passing position at the same Y coordinate is 0.027 mm at the maximum, and the light flux width in the sub-scanning direction on the lens surface 62b is 2.2 mm. The origin of the non-arc shape of the child line is within the light beam in the entire effective area.

  The tilting error of the deflecting / reflecting surface 51 due to processing errors or assembly errors may occur upward in the sub-scanning direction or downward in the sub-scanning direction. When a tilt error occurs upward in the sub-scanning direction, the light beam passage position on the lens surface 62b is shifted upward in the sub-scanning direction with respect to the case where there is no tilt error. On the other hand, when a tilt error occurs downward in the sub-scanning direction, the light beam passage position on the lens surface 62b is shifted downward in the sub-scanning direction with respect to the case where there is no tilt error.

  Non-arc shape of the child line so that when the principal ray arrival position shifts up and down on the lens surface due to surface tilt, it receives asymmetrical refractive power at the upper and lower parts in the sub-scanning direction across the principal ray The origin of is preferably within the luminous flux throughout the effective area.

  FIG. 6 shows how the fourth-order aspheric coefficient of the child line changes. As can be seen from FIG. 6, the fourth-order aspheric coefficient of the child line is changed to have an extreme value. Further, the position in the main scanning direction indicating the extreme value is 0 to the position of the effective end on the side opposite to the side where the light source is disposed, with reference to the center in the main scanning direction (Y = 0). It is between 50%. That is, the position H from the central portion (Y = 0) in the main scanning direction indicating the extreme value is 0 <with respect to the position W of the effective end on the side opposite to the side where the light source is disposed. H <0.5W is satisfied.

  Since the sag amount of the deflecting surface accompanying the rotation of the rotary polygon mirror has an extreme value, the amount of deviation (geometrical optical pitch unevenness) of the principal ray arrival position also has an extreme value in FIG. For this reason, in this embodiment, in order to reduce the deviation of the spot center of gravity position, the local power in the sub-scanning direction increases as the distance from the origin of the non-circular arc shape in the sub-scanning direction increases in the central part in the main scanning direction. Thus, at the periphery in the main scanning direction, the fourth-order aspheric coefficient of the child line is set to Y so that the local power in the sub-scanning direction decreases as the distance from the origin of the child non-arc-shaped shape increases in the sub-scanning direction. It is changed according to the coordinates.

  FIG. 7A shows a state of local power change in the sub-scanning direction at the central portion (Y = 0) in the main scanning direction.

  In FIG. 7A, the origin is a non-arc-shaped origin of a child line (a point of 1.97 mm in the sub-scanning direction from the generatrix). As shown in FIG. 7A, the local power in the sub-scanning direction increases in the central part in the main scanning direction as the distance from the non-arc-shaped origin of the child line increases.

  FIG. 7B shows a state of local power change in the sub-scanning direction at the periphery in the main scanning direction (Y = 46.7, upper optical path in FIG. 1).

  In FIG. 7B, the origin is a non-arc-shaped origin of the child wire (a point of 2.57 mm in the sub-scanning direction from the bus). As shown in FIG. 7B, the local power in the sub-scanning direction decreases as the distance from the origin of the non-arc shape of the child line increases in the periphery in the main scanning direction.

  In the present embodiment, in the central portion in the main scanning direction, the local power in the sub-scanning direction is increased in the peripheral portion in the main scanning direction so that the local power in the sub-scanning direction increases as the distance from the origin of the non-arc shape in the sub-scanning direction increases. Although the fourth-order aspherical coefficient of the child line is changed according to the Y coordinate (position in the main scanning direction) so that the local power in the sub scanning direction decreases as it moves away from the arcuate origin in the sub scanning direction, The way of change is not limited to this.

  For example, in an overfilled scanning optical system in which a light beam from an incident optical system is incident with a light beam width wider than the width of the deflection surface, geometrical optical pitch unevenness is different from that of this embodiment when the deflection surface is tilted downward. Protrusions occur in the opposite direction. In this case, in order to correct the position of the center of gravity of the spot, the main scanning is performed so that the local power in the sub-scanning direction becomes smaller as the distance from the origin of the non-arc shape of the child line increases in the sub-scanning direction. In the periphery of the direction, the fourth-order aspheric coefficient of the child line is set to the Y coordinate (in the main scanning direction) so that the local power in the sub-scanning direction increases as the distance from the origin of the non-arc shape of the child line increases in the sub-scanning direction. Need to be changed according to position).

  In general, since the sag amount of the deflection surface accompanying the rotation of the rotary polygon mirror has an extreme value, pitch unevenness due to surface tilt also has an extreme value. Therefore, in order to correct wave optical pitch unevenness at the entire image height, it is preferable to change the aspheric coefficient of the child line so as to have an extreme value.

  As described above, the fourth-order aspheric term of the child line is introduced into the lens surface 62b, the aspheric coefficient is changed according to the Y coordinate (position in the main scanning direction), and the origin of the non-arc shape of the child line is changed. Is decentered in the sub-scanning direction, and the amount of decentering is changed according to the Y coordinate so that the origin of the non-arc shape of the child line is located in the light beam in the entire effective region, thereby allowing the field curvature in the sub-scanning direction. It has succeeded in reducing the shift of the center of gravity position of the spot due to the tilting of the surface without deteriorating.

  Thereby, the wave optical pitch unevenness due to the shift of the center of gravity of the spot is suppressed to a very small amount of 0.4 μm or less without deteriorating the variation of the spot diameter, so that a higher definition image can be formed.

  In the present embodiment, a fourth-order aspherical surface is introduced as the non-arc shape of the child wire, but a sixth-order or higher term may be added if it is an even-order term of the fourth or higher order. Further, it has only an even-order aspherical coefficient of 6th order or higher, and the aspherical coefficient has an extreme value in the effective area of the optical surface, and the extreme value in the main scanning direction of the extreme value with respect to the central part in the main scanning direction. The position H satisfies 0 <H <0.5 W with respect to the position W of the effective end of the optical surface on the side opposite to the side where the light source is disposed with respect to the optical axis in the main scanning direction. The sign of the aspheric coefficient is inverted between the center of the effective area of the optical surface and the effective end of the optical surface on at least one side opposite to the side where the light source is disposed with respect to the optical axis in the main scanning direction. The same effect can be obtained by configuring as described above.

  In the present embodiment, in order to obtain a greater effect of the non-arc shape of the child line, the child line shape of the lens surface 62b having the largest beam width in the sub-scanning direction on the lens surface is a non-arc shape. It is not limited to.

  In this embodiment, the imaging optical system 60 is composed of two imaging optical elements. However, the imaging optical system 60 is not limited to this, and may be composed of one or more imaging optical elements.

  In this embodiment, the light source means 10 is composed of a semiconductor laser having one light emitting point. However, the present invention is not limited to this, and it may be composed of a monolithic multilaser having two or more light emitting points. good.

  FIG. 8 is a sectional view (main scanning sectional view) of the main part in the section including the main scanning direction and the optical axis of the optical scanning apparatus according to the second embodiment of the present invention.

  The expressions representing the shapes of the lens surfaces 61 a to 62 b of the first fθ lens 61 and the second fθ lens 62 constituting the imaging optical system 60 are different between the second embodiment and the first embodiment. Other configurations are the same as those in the first embodiment.

  Table 2 shows the characteristics of the optical system in Example 2.

で表わされる。第２ｆθレンズ６２の被走査面７０側のレンズ面６２ｂの母線形状は、

で表される。Ｒは曲率半径、Ｋは離心率、Ｂ₄〜Ｂ₁₂は４次〜１２次の母線の非球面係数である。実施例２が実施例１と異なる点は、ΣA_iYⁱにより母線を副走査方向に湾曲させている点である（図９（ｃ））。 The generatrix shape of the lens surfaces 61a and 61b of the first fθ lens 61 and the lens surface 62a of the second fθ lens 62 on the polygon mirror 50 side constituting the imaging optical system 60 is the intersection of each lens surface and the optical axis (lens surface). When the optical axis direction is the X axis, the direction orthogonal to the optical axis in the main scanning section is the Y axis, and the direction orthogonal to the optical axis in the sub scanning section is the Z axis,

It is represented by The generatrix shape of the lens surface 62b on the scanned surface 70 side of the second fθ lens 62 is

It is represented by R is a radius of curvature, K is an eccentricity, and B _{4 to} B ₁₂ are aspherical coefficients of the fourth to _twelfth buses. Example 2 differs from Example 1 in that the bus is curved in the sub-scanning direction by ΣA _i Y ⁱ (FIG. 9C).

  Here, when the coefficients on the laser side (the lower side of the figure) and the anti-laser side (the upper side of the figure) are different, the subscript u is added to the coefficient on the laser side, and the subscript l is added to the coefficient on the anti-laser side. doing.

で表される。子線の曲率半径ｒ´は、主走査方向中央部における子線曲率半径ｒに対して主走査方向周辺部に向かい変化しており、D₂〜D₁₀は子線曲率半径の変化係数である。ここで、レーザ側（図の下側）と反レーザ側（図の上側）で係数が異なる場合は、レーザ側の係数には添字ｕを附し、反レーザ側の係数には添字ｌを附している。 The sub-line shapes of the lens surfaces 61a and 61b of the first fθ lens 61 and the lens surface 62a of the second fθ lens 62 on the polygon mirror 50 side are:

It is represented by The radius of curvature r ′ of the child line changes toward the peripheral part in the main scanning direction with respect to the radius of curvature r in the central part in the main scanning direction, and D _{2 to} D ₁₀ are change coefficients of the radius of curvature of the child line. . Here, when the coefficients on the laser side (the lower side of the figure) and the anti-laser side (the upper side of the figure) are different, the subscript u is added to the coefficient on the laser side, and the subscript l is added to the coefficient on the anti-laser side. doing.

  The difference from the first embodiment is that the radius of curvature of the child line is the radius of curvature in the cross section perpendicular to the bus line at each main scanning position.

で表される。ここでZ´は各主走査方向の位置における母線からみた副走査方向の座標である。子線の曲率半径ｒ´は、主走査方向中央部における子線曲率半径ｒに対して主走査方向周辺部に向かい変化しており、D₂〜D₁₀は子線曲率半径の変化係数である。ΣG_jkY^jは４次、６次、…の子線の非球面係数である。子線の非球面係数は主走査方向に沿って変化しており、G_0k〜G_16kは各２ｋ次の子線の非球面係数の変化係数である。表２に示した子線の非球面係数G₀〜G₆は、それぞれ、G_0k〜G_6kであって、添え字のｋ＝２の場合（Ｚに対する４次の場合）の子線の非球面係数ΣG_jkY^j中のG_jkの数値を示している。表２より明らかなように本実施例においては、４次以外の次数の非球面係数は０としているが、これに限らず６次以上の非球面係数を有限の値としても良い。ここで、レーザ側（図の下側）と反レーザ側（図の上側）で係数が異なる場合は、レーザ側の係数には添字ｕを附し、反レーザ側の係数には添字ｌを附している。 Next, the sub-wire shape of the lens surface 62b on the scanning surface 70 side of the second fθ lens 62 is a non-arc shape (aspheric shape) including a quartic term,

It is represented by Here, Z ′ is a coordinate in the sub-scanning direction as viewed from the bus at each position in the main scanning direction. The radius of curvature r ′ of the child line changes toward the peripheral part in the main scanning direction with respect to the radius of curvature r in the central part in the main scanning direction, and D _{2 to} D ₁₀ are change coefficients of the radius of curvature of the child line. . ΣG _jk Y ^j is the aspheric coefficient of the fourth, sixth,. The aspheric coefficients of the child lines change along the main scanning direction, and G _{0k to} G _16k are change coefficients of the aspheric coefficients of the 2k-th order child lines. The aspheric coefficients G _{0 to} G ₆ of the child lines shown in Table 2 are G _{0k to} G _6k , respectively, and the non-element of the child line when the subscript k = 2 (fourth order with respect to Z) is shown. The numerical value of G _{jk in} the spherical coefficient ΣG _jk Y ^j is shown. As is apparent from Table 2, in this embodiment, the aspheric coefficients of orders other than the fourth order are set to 0. However, the present invention is not limited to this, and the aspheric coefficients of the sixth order or higher may be set to finite values. Here, when the coefficients on the laser side (the lower side of the figure) and the anti-laser side (the upper side of the figure) are different, the subscript u is added to the coefficient on the laser side, and the subscript l is added to the coefficient on the anti-laser side. doing.

  The radius of curvature of the child line is the radius of curvature in the cross section perpendicular to the bus line at each main scanning position.

Further, it has a sub-line tilt change term composed of ΣF _j Y ^j . Accordingly, the lens surface 62b is a sub-line tilt changing surface in which the sub-line tilt amount is different at each position in the main scanning direction, and the sub-line shape is a non-arc shape.

  Furthermore, the origin of the non-arc shape of the child line is always on the bus. In the second embodiment, the origin of the non-arc shape of the child line is curved in the sub-scanning direction by curving the bus shape itself in the sub-scanning direction (FIGS. 9B and 9C).

  FIG. 9A is a cross-sectional view of the main part in the sub-scanning direction of the optical scanning device of this embodiment. As can be seen from FIG. 9A, in the present embodiment, the second fθ lens 62 is arranged eccentrically in the sub-scanning direction, and the amount of eccentricity is 4.074 mm.

  FIG. 10 shows scanning line curvature (pitch unevenness) due to surface tilt when the deflecting / reflecting surface 51 is tilted 3 'upward in the sub-scanning direction due to a processing error in this embodiment. In FIG. 10, the broken line indicates the shift amount of the principal ray arrival position on the scanned surface 70, and the solid line indicates the shift amount of the spot centroid position on the scanned surface 70.

  FIG. 11 shows field curvature in the sub-scanning direction in the present embodiment. In FIG. 11, “Slice-” is a negative defocus position where the spot diameter in the sub-scanning direction is 90 μm, “Slice +” is a positive defocus position where the spot diameter in the sub-scanning direction is 90 μm, and “Center "Indicates a defocus position between" Slice- "and" Slice + ", and" Center "indicates field curvature in the sub-scanning direction.

  In this embodiment, as can be seen from FIGS. 10 and 11, the deviation amount of the spot center of gravity due to the surface tilt when the deflecting / reflecting surface 51 is tilted 3 'downward in the sub-scanning direction due to the processing error is Peak to Peak. As a result, good performance of 0.56 μm or less is achieved. Further, the field curvature of the sub-scanning has been achieved at the same time with a peak-to-peak of 0.31 mm or less and good performance.

  In the present embodiment, the sub-wire shape of the lens surface 62b on the scanned surface 70 side of the second fθ lens 62 is a non-arc shape including a quaternary aspheric term as described above, and the bus bar is in the sub-scanning direction. Is characterized in that the origin of the non-arc shape of the child line is curved in the sub-scanning direction and the aspheric coefficient is changed according to the Y coordinate.

  FIG. 12 shows the origin position and the principal ray passing position of the non-arc shape of the child wire in this embodiment. In the second embodiment, the curved line connecting the non-arc-shaped origins of the child lines coincides with the curved bus bar.

  As shown in FIG. 12, the curve connecting the origins of the non-arc shapes of the child lines has a shape along the locus of the principal ray passing position. Further, the maximum amount of deviation in the sub-scanning direction between the origin of the non-arc shape of the child line and the principal ray passing position at the same Y coordinate is 0.24 mm at the maximum, and the light flux width in the sub-scanning direction on the lens surface 62b is 2.2 mm. The origin of the non-arc shape of the child line is located in the light beam throughout the effective area.

  FIG. 13 shows how the fourth-order aspheric coefficient of the child line changes. As can be seen from FIG. 13, the fourth-order aspheric coefficient of the child line is changed to have an extreme value. Further, the position in the main scanning direction indicating the extreme value is 0 to the position of the effective end on the side opposite to the side where the light source is disposed, with reference to the center in the main scanning direction (Y = 0). It is between 50%. That is, the position H from the central portion (Y = 0) in the main scanning direction indicating the extreme value is 0 <with respect to the position W of the effective end on the side opposite to the side where the light source is disposed. H <0.5W is satisfied.

  As described above, the fourth-order aspherical term of the child line is introduced into the lens surface 62b, the aspheric coefficient is changed according to the Y coordinate, and the non-arc-shaped origin of the child line is the luminous flux in the entire effective area. By being positioned within, the deviation of the position of the center of gravity of the spot due to surface tilt has been successfully reduced without deteriorating the curvature of field in the sub-scanning direction.

  Thereby, the wave optical pitch unevenness due to the shift of the center of gravity of the spot is suppressed to a very small amount of 0.56 μm or less without deteriorating the variation of the spot diameter, so that a higher definition image can be formed.

  In the present embodiment, a fourth-order aspherical surface is introduced as the non-arc shape of the child wire, but a sixth-order or higher term may be added if it is an even-order term of the fourth or higher order. Further, it has only an even-order aspherical coefficient of 6th order or higher, and the aspherical coefficient has an extreme value in the effective area of the optical surface, and the extreme value in the main scanning direction of the extreme value with respect to the central part in the main scanning direction. The position H satisfies 0 <H <0.5 W with respect to the position W of the effective end of the optical surface on the side opposite to the side where the light source is disposed with respect to the optical axis in the main scanning direction. The sign of the aspheric coefficient is inverted between the center of the effective area of the optical surface and the effective end of the optical surface on at least one side opposite to the side where the light source is disposed with respect to the optical axis in the main scanning direction. The same effect can be obtained by configuring as described above.

  In the present embodiment, in order to obtain a greater effect of the non-arc shape of the child line, the child line shape of the lens surface 62b having the largest beam width in the sub-scanning direction on the lens surface is a non-arc shape. It is not limited to.

  In this embodiment, the imaging optical system 60 is composed of two imaging optical elements. However, the imaging optical system 60 is not limited to this, and may be composed of one or more imaging optical elements.

FIG. 14 is a sectional view (main scanning sectional view) of the main part in the section including the main scanning direction and the optical axis of the optical scanning apparatus according to the third embodiment of the present invention.
In Example 3 and Example 2, the expressions representing the shapes of the lens surfaces 61 a to 62 b of the second fθ lens 62 constituting the imaging optical system 60 are different. Other configurations are the same as those in the second embodiment.

  Table 3 shows various characteristics of the optical system in Example 3.

で表わされる。実施例２と異なる点は、レンズ面６２ｂの母線を湾曲させていない点である。
ここで、レーザ側（図の下側）と反レーザ側（図の上側）で係数が異なる場合は、レーザ側の係数には添字ｕを附し、反レーザ側の係数には添字ｌを附している。 The generatrix shape of the lens surfaces 61a to 62b has an intersection (lens surface vertex) between each lens surface and the optical axis as the origin, the optical axis direction as the X axis, and the direction orthogonal to the optical axis in the main scanning section as the Y axis, When the direction perpendicular to the optical axis in the sub-scan section is the Z axis,

It is represented by The difference from the second embodiment is that the generatrix of the lens surface 62b is not curved.
Here, when the coefficients on the laser side (the lower side of the figure) and the anti-laser side (the upper side of the figure) are different, the subscript u is added to the coefficient on the laser side, and the subscript l is added to the coefficient on the anti-laser side. doing.

で表される。子線の曲率半径ｒ´は、主走査方向中央部における子線曲率半径ｒに対して主走査方向周辺部に向かい変化しており、D₂〜D₁₀は子線曲率半径の変化係数である。ここで、レーザ側（図の下側）と反レーザ側（図の上側）で係数が異なる場合は、レーザ側の係数には添字ｕを附し、反レーザ側の係数には添字ｌを附している。 The sub-wire shape of both lens surfaces 61a and 61b of the first fθ lens 61 is

It is represented by The radius of curvature r ′ of the child line changes toward the peripheral part in the main scanning direction with respect to the radius of curvature r in the central part in the main scanning direction, and D _{2 to} D ₁₀ are change coefficients of the radius of curvature of the child line. . Here, when the coefficients on the laser side (the lower side of the figure) and the anti-laser side (the upper side of the figure) are different, the subscript u is added to the coefficient on the laser side, and the subscript l is added to the coefficient on the anti-laser side. doing.

で表される。ここでZ´は各主走査方向の位置における母線からみた副走査方向の座標である。子線の曲率半径ｒ´は、主走査方向中央部における子線曲率半径ｒに対して主走査方向周辺部に向かい変化しており、D₂〜D₁₀は子線曲率半径の変化係数である。ΣG_jkY^jは４次、６次、…の子線の非球面係数である。子線の非球面係数は主走査方向に沿って変化しており、G_0k〜G_16kは各２ｋ次の子線の非球面係数の変化係数である。表３に示した子線の非球面係数G₀〜G₆は、それぞれ、G_0k〜G_6kであって、添え字のｋ＝２の場合（Ｚに対する４次の場合）の子線の非球面係数ΣG_jkY^j中のG_jkの数値を示している。表３より明らかなように本実施例においては、４次以外の次数の非球面係数は０としているが、これに限らず６次以上の非球面係数を有限の値としても良い。ここで、レーザ側（図の下側）と反レーザ側（図の上側）で係数が異なる場合は、レーザ側の係数には添字ｕを附し、反レーザ側の係数には添字ｌを附している。 Next, the sub-wire shape of the lens surfaces 62a and 62b of the second fθ lens 62 is an aspherical shape (aspherical shape) including a quartic term,

It is represented by Here, Z ′ is a coordinate in the sub-scanning direction as viewed from the bus at each position in the main scanning direction. The radius of curvature r ′ of the child line changes toward the peripheral part in the main scanning direction with respect to the radius of curvature r in the central part in the main scanning direction, and D _{2 to} D ₁₀ are change coefficients of the radius of curvature of the child line. . ΣG _jk Y ^j is the aspheric coefficient of the fourth, sixth,. The aspheric coefficients of the child lines change along the main scanning direction, and G _{0k to} G _16k are change coefficients of the aspheric coefficients of the 2k-th order child lines. The aspherical coefficients G _{0 to} G ₆ of the child lines shown in Table 3 are G _{0k to} G _6k , respectively, and the non-element of the child line when the subscript k = 2 (fourth order with respect to Z) is shown. The numerical value of G _{jk in} the spherical coefficient ΣG _jk Y ^j is shown. As is apparent from Table 3, in this embodiment, the aspherical coefficients of orders other than the fourth order are set to 0. However, the present invention is not limited to this, and the aspherical coefficients of the sixth or higher order may be finite values. Here, when the coefficients on the laser side (the lower side of the figure) and the anti-laser side (the upper side of the figure) are different, the subscript u is added to the coefficient on the laser side, and the subscript l is added to the coefficient on the anti-laser side. doing.

  The lens surface 62b is a sub-line tilt changing surface in which the sub-line tilt amount at each position in the main scanning direction is different, and the sub-line shape is a non-arc shape.

  FIG. 15 shows scanning line curvature (pitch unevenness) due to surface tilt when the deflecting / reflecting surface 51 is tilted 3 'upward in the sub-scanning direction due to a processing error in this embodiment. In FIG. 15, the broken line indicates the amount of deviation of the principal ray arrival position on the scanned surface 70, and the solid line indicates the amount of deviation of the spot centroid position on the scanned surface 70.

  FIG. 16 shows field curvature in the sub-scanning direction in the present embodiment. In FIG. 16, “Slice−” is a negative defocus position where the spot diameter in the sub-scanning direction is 90 μm, “Slice +” is a positive defocus position where the spot diameter in the sub-scanning direction is 90 μm, and “Center "Indicates a defocus position between" Slice- "and" Slice + ", and" Center "indicates field curvature in the sub-scanning direction.

  In this embodiment, as can be seen from FIG. 15 and FIG. 16, the deviation amount of the center of gravity position of the spot due to the surface tilt when the deflecting / reflecting surface 51 is tilted 3 'downward in the sub-scanning direction due to the processing error is Peak to Peak. As a result, good performance of 0.64 μm or less has been achieved. In addition, the sub-scan field curvature is 0.50 mm or less in Peak to Peak, and it has succeeded in achieving good performance at the same time.

In this embodiment, the sub-wire shape of the lens surface 62b on the scanned surface 70 side of the second fθ lens 62 is an aspherical shape including a fourth-order aspherical term as described above, and the aspherical coefficient is expressed as a Y coordinate. It is changed according to. Further, the second fθ lens 62 is decentered by 4.074 mm in the sub-scanning direction so that the non-arc-shaped origin of the child line is located in the light beam in the entire effective area without bending the bus.
FIG. 17 shows the origin position and the principal ray passing position of the non-arc shape of the child line in the present embodiment.

  In Example 3, since the bus bar is not curved, the origin of the non-arc shape of the child wire is at the position of Z = 0 in the entire effective area. The maximum amount of deviation in the sub-scanning direction between the origin of the non-arc shape of the child line at the same Y coordinate and the principal ray passing position is 0.54 mm, which is larger than the light flux width of 2.2 mm in the sub-scanning direction on the lens surface 62b. The origin of the non-arc shape of the child wire is located in the light beam in the entire effective area.

  FIG. 18 shows how the fourth-order aspheric coefficient of the child line changes. As can be seen from FIG. 18, the fourth-order aspheric coefficient of the child line is changed to have an extreme value. Further, the position in the main scanning direction indicating the extreme value is 0 to the position of the effective end on the side opposite to the side where the light source is disposed, with reference to the center in the main scanning direction (Y = 0). It is between 50%. That is, the position H from the central portion (Y = 0) in the main scanning direction indicating the extreme value is 0 <with respect to the position W of the effective end on the side opposite to the side where the light source is disposed. H <0.5W is satisfied.

  As described above, the fourth-order aspherical term of the child line is introduced into the lens surface 62b, the aspheric coefficient is changed according to the Y coordinate, and the non-arc-shaped origin of the child line is the luminous flux in the entire effective area. By being positioned within, the deviation of the position of the center of gravity of the spot due to surface tilt has been successfully reduced without deteriorating the curvature of field in the sub-scanning direction.

  In the present embodiment, a fourth-order aspherical surface is introduced as the non-arc shape of the child wire, but a sixth-order or higher term may be added if it is an even-order term of the fourth or higher order. Further, it has only an even-order aspherical coefficient of 6th order or higher, and the aspherical coefficient has an extreme value in the effective area of the optical surface, and the extreme value in the main scanning direction of the extreme value with respect to the central part in the main scanning direction. The position H satisfies 0 <H <0.5 W with respect to the position W of the effective end of the optical surface on the side opposite to the side where the light source is disposed with respect to the optical axis in the main scanning direction. The sign of the aspheric coefficient is inverted between the center of the effective area of the optical surface and the effective end of the optical surface on at least one side opposite to the side where the light source is disposed with respect to the optical axis in the main scanning direction. The same effect can be obtained by configuring as described above.

In the present embodiment, in order to obtain a greater effect of the non-arc shape of the child line, the child line shape of the lens surface 62b having the largest beam width in the sub-scanning direction on the lens surface is a non-arc shape. It is not limited to.
In this embodiment, the multiplication optical system 60 is composed of two imaging optical elements. However, the present invention is not limited to this, and it may be composed of one or more imaging optical elements.

  Next, a fourth embodiment of the present invention will be described. The fourth embodiment is different from the first embodiment in that a monolithic multi-beam laser having four light emitting units is used as the light source means 10. Other configurations and optical functions are the same as those in the first embodiment, and the same effects are obtained.

  Since the displacement of the center of gravity of the spot due to surface tilt is suppressed to a very small value of 0.4 μm or less by Peak to Peak, high-definition image formation is possible even when the number of light emitting portions increases and the pitch unevenness period increases.

  Next, a fifth embodiment of the present invention will be described. The fifth embodiment differs from the fourth embodiment in that a surface emitting laser having 32 light emitting portions arranged two-dimensionally is used as the light source means 10. Other configurations and optical actions are the same as those in the fourth embodiment, and the same effects are obtained.

  The deviation of the center of gravity of the spot due to surface tilt is suppressed to a very small value of 0.4 μm by Peak to Peak, so even if the number of light emitting parts increases, uneven pitch can be reduced, enabling higher speed and higher definition image formation. .

[Image forming apparatus]
FIG. 19 is a cross-sectional view of the main part in the sub-scanning direction showing the embodiment of the image forming apparatus 104 of the present invention.

  Code data Dc is input to the image forming apparatus 104 from an external device 117 such as a personal computer, and converted into image data (dot data) Di by the printer controller 111. The image data Di is input to the optical scanning unit 100 having the configuration shown in any of Embodiments 1 to 4, and the optical beam 103 modulated in accordance with the image data Di is emitted from the optical scanning unit 100. The photosensitive surface of the photosensitive drum 101 is scanned in the main scanning direction. The photosensitive drum as the electrostatic latent image carrier is rotated clockwise by the motor 115 and the photosensitive surface thereof moves in the sub-scanning direction perpendicular to the main scanning direction with respect to the light beam 103. The charging roller 102 contacts the surface of the photosensitive drum above the photosensitive drum, uniformly charges the photosensitive drum surface, and the surface of the charged photosensitive drum is irradiated with a light beam scanned by the optical scanning unit. A latent image is formed.

  The electrostatic latent image is developed as a toner image by a developing device 107 disposed so as to abut on the photosensitive drum downstream of the light beam irradiation position, and a transfer roller disposed so as to face the photosensitive drum. The image is transferred onto a sheet 112 as a transfer material stored in a sheet cassette 109 in front of the photosensitive drum. A paper feed roller 110 is disposed at the end of the paper cassette, and feeds the paper 112 in the paper cassette into the transport path.

  As described above, the sheet 112 on which the unfixed toner image is transferred is further conveyed to a fixing device behind the photosensitive drum. The fixing device includes a fixing roller 113 having a fixing heater (not shown) inside and a pressure roller 114 disposed so as to be in pressure contact with the fixing roller 113, and the sheet 112 from the transfer portion is pressed against the pressure contact portion between the rollers 113 and 114. The unfixed toner image is fixed by pressurizing and heating at. Further, a paper discharge roller 116 is disposed behind the fixing roller 113, and discharges the paper 112 out of the image forming apparatus.

[Color image forming apparatus]
FIG. 20 is a schematic view of a main part of a color image forming apparatus according to an embodiment of the present invention.

  The present embodiment is a tandem type color image forming apparatus in which a plurality of (four) optical scanning devices are arranged in parallel and image information is recorded on a photosensitive drum surface as an image carrier. Color image forming apparatus 360, optical scanning apparatuses 311, 312, 313, and 314 having any one of Embodiments 1 to 4, photosensitive drums 341, 342, 343, and 344 as image carriers, and developing devices 321 and 322, respectively. , 323, 324 and the conveyor belt 351.

  The color image forming apparatus 360 receives R (red), G (green), and B (blue) color signals from an external device 352 such as a personal computer. These color signals are converted into C, M, Y, and B image data (dot data) by a printer controller 353 in the apparatus. The image data is respectively input to the optical scanning device, and light beams 331, 332, 333, and 334 modulated according to the respective image data are emitted from the optical scanning device, and the photosensitive surface of the photosensitive drum is mainly used by these light beams. Scanned in the scanning direction.

  In this embodiment, the color image forming apparatus includes two optical scanning devices (311, 312), (313, 314), each corresponding to each color of C, M, Y, and B, and parallel to the photosensitive drum surface. An image signal (image information) is recorded thereon, and a color image is printed at high speed.

  In the color image forming apparatus according to this embodiment, the latent images of the respective colors are formed on the corresponding photosensitive drum surfaces by using the light beams based on the respective image data by the four optical scanning devices as described above. Thereafter, a single full color image is formed by multiple transfer onto a recording material.

  As the external device 352, for example, a color image reading apparatus including a CCD sensor may be used. In this case, the color image reading apparatus and the color image forming apparatus 360 constitute a color digital copying machine.

DESCRIPTION OF SYMBOLS 10 Semiconductor laser 20 Collimator lens 30 Cylindrical lens 40 Aperture stop 50 Rotating polygon mirror 51 Deflection reflecting surface 60 of rotating polygon mirror Imaging optical system 61 The first imaging lens (61a: entrance surface, 61b: exit surface)
62 Second imaging lens (62a: entrance surface, 62b: exit surface)
70 Photosensitive drum surface

Claims (9)

Light source means;
Deflecting means for deflecting a light beam from the light source means;
An incident optical system for guiding the light beam from the light source means to the deflecting means;
An imaging optical system that forms an image of the light beam reflected and deflected by the deflecting means as a spot on the surface to be scanned;
Have
The incident optical system obliquely enters a light beam from the light source means at a predetermined angle within a sub-scan section with respect to a plane perpendicular to a rotation axis of a deflection surface of the deflection means,
The imaging optical system includes an optical element having a non-arc shape in which a sub-wire shape of at least one optical surface includes a fourth-order or higher-order aspheric coefficient,
Among the even-order aspherical coefficients of the fourth or higher order of the sub-line shape of the optical surface, the aspherical coefficient of any order changes according to the position in the main scanning direction,
The origin of the non-arc shape of the child line at the position in each main scanning direction of the optical surface is located in the light flux in the entire effective area,
An optical scanning device. The aspherical coefficient of any order among the fourth-order and higher-order aspherical coefficients of the sub-line shape of the optical surface changes in the main scanning direction so as to have an extreme value within the effective area of the optical surface. The optical scanning device according to claim 1, wherein:   The extreme value of any order of the aspherical coefficients of the fourth and higher order aspherical coefficients of the sub-line shape of the optical surface is the position of the extreme value in the main scanning direction with respect to the central part in the main scanning direction. H satisfies 0 <H <0.5W with respect to the position W of the effective end of the optical surface on the side opposite to the side where the light source means is disposed with respect to the optical axis in the main scanning direction. The optical scanning device according to claim 2.   Among the fourth-order or higher-order even-numbered aspheric coefficients of the sub-line shape of the optical surface, the aspheric coefficient of any order is the center of the effective area in the main scanning direction of the optical surface and light in the main scanning direction. The sign is inverted between the side where the light source means is arranged with respect to the axis or the effective end of the optical surface on at least one side opposite to the axis. 2. An optical scanning device according to item 1.   5. The optical scanning device according to claim 1, wherein the light source unit is a monolithic multi-laser having two or more light emitting points. 6.   5. The optical scanning device according to claim 1, wherein the light source means is a surface emitting laser.   The imaging optical system constitutes a surface tilt correction optical system having a conjugate relation between a vicinity of a deflection surface of the deflection unit and the surface to be scanned in a sub-scan section. The optical scanning device according to any one of 1 to 6. An optical scanning device according to any one of claims 1 to 7,
A printer controller that converts code data input from an external device into an image signal and inputs the image signal to the scanning optical device;
An image forming apparatus. A plurality of optical scanning devices each comprising the optical scanning device according to any one of claims 1 to 8,
A color image forming apparatus having a plurality of image carriers that are arranged at positions on the surface to be scanned of the plurality of optical scanning devices and form images of different colors.

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