JP2016038524A - Optical scanner and image forming apparatus including the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical scanner in which a height in a sub-scanning direction is sufficiently decreased while reducing the number of components, and an image forming apparatus including the optical scanner.SOLUTION: The optical scanner includes: first and second light sources; a deflector 5 that deflects each light beam emitting from the first and second light sources on a first deflection surface 5a to scan first and second scanning target surfaces 8A, 8B in a main scanning direction; and first and second imaging optical systems SA, SB that condense each light beam deflected by the deflector 5 onto the first and second scanning target surfaces 8A, 8B. The first and second imaging optical systems SA, SB include a common multi-stage lens 7 that includes first and second optical surfaces arranged in a sub-scanning direction, to which the light beams are respectively incident. A second optical path length from the first deflection surface 5a to the second scanning target surface 8B is longer than a first optical path length from the first deflection surface 5a to the first scanning target surface 8A. The optical scanner satisfies the condition represented by 0.15<|m1-m2|<0.50.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical scanning device provided in an image forming apparatus such as a laser beam printer (LBP), a digital copying machine, or a multifunction printer (MFP).

  As an optical scanning device provided in an image forming apparatus, an optical scanning device capable of simultaneously optically scanning the photosensitive surfaces (scanned surfaces) of a plurality of photosensitive members with a single deflector is known. Patent Document 1 describes an optical scanning device in which each optical path is bent to reduce the size by disposing a mirror in each optical path from a deflector to each scanned surface. Further, Patent Document 2 describes an optical operation device in which the number of lenses is reduced by adopting a multistage lens in which a plurality of lens surfaces are integrated by overlapping in the sub-scanning direction. Further, Patent Document 3 describes an optical scanning device in which the number of mirrors is reduced by devising the arrangement of mirrors that fold light beams after passing through a multistage lens.

JP 2013-64857 A JP 2014-48563 A JP 2007-155838 A

  However, since the optical scanning devices described in Patent Documents 1 to 3 have the same optical path length with respect to each scanned surface, the degree of freedom of arrangement of each optical component is small, and the light beam and the image are formed while downsizing the entire device. It becomes difficult to avoid interference with the lens.

  In the optical scanning device described in Patent Document 1, since the imaging lens is disposed between the two mirrors in the optical path corresponding to the photoconductor on the side close to the deflector, each of the optical scanning devices avoids interference with the light flux. When the members are arranged, it becomes difficult to further reduce the thickness of the entire apparatus. In addition, in the optical scanning device described in Patent Document 2, three mirrors are arranged in the optical path corresponding to the photoconductor on the side close to the deflector to avoid interference between the multistage lens and the light beam. It is difficult to reduce the total number of parts. Furthermore, in the optical scanning device described in Patent Document 3, the reflection angle of the mirror is set large in order to avoid interference between the multistage lens and the light beam in the optical path corresponding to the photoconductor on the side close to the deflector. The distance between the two mirrors becomes large and the apparatus becomes large.

  An object of the present invention is to provide an optical scanning device in which the height in the sub-scanning direction is sufficiently reduced while reducing the number of components, and an image forming apparatus including the same.

  In order to achieve the above object, an optical scanning device according to one aspect of the present invention includes a first light source, a second light source, and first and second light beams emitted from the first and second light sources. A deflector that deflects on the first deflecting surface and scans the first and second scanned surfaces in the main scanning direction, and the first and second light beams deflected by the deflector are the first and second light beams, respectively. And a first and second imaging optical system for condensing on the second scanning surface, wherein the first and second imaging optical systems are the first and second imaging optical systems. And a common multi-stage lens including a first optical surface and a second optical surface arranged in the sub-scanning direction on which each of the two light beams is incident, and a first multi-layer lens extending from the first deflection surface to the first scanned surface. The second optical path length from the first deflection surface to the second scanned surface is longer than the first optical path length. The optical path length from the rear main plane of the imaging optical system to the first scanned surface is Sk1 (mm), the focal length of the first imaging optical system is f1 (mm), and the second connection The optical path length from the rear main plane of the image optical system to the second scanned surface is Sk2 (mm), and the focal length of the second imaging optical system is f2 (mm). When the first convergence degree of the image optical system is m1 = 1−Sk1 / f1, and the second convergence degree of the second imaging optical system is m2 = 1−Sk2 / f2, 0.15 <| It satisfies the condition of m1-m2 | <0.50.

  According to the present invention, it is possible to provide an optical scanning device in which the number of parts is reduced and the height in the sub-scanning direction is sufficiently reduced, and an image forming apparatus including the same.

Schematic view of a main part of the optical scanning device according to the first embodiment of the present invention (sub-scan sectional view). Schematic view of main parts of an optical scanning apparatus according to Embodiment 1 of the present invention (main scanning sectional view) Schematic view of a main part of an incident optical system according to Embodiment 1 of the present invention (sub-scan sectional view) FIG. 6 is a diagram illustrating field curvature in the main scanning direction and the sub-scanning direction according to the first embodiment of the invention. The figure which shows the f (theta) characteristic which concerns on Example 1 of this invention. The figure which shows the scanning line curve which concerns on Example 1 of this invention. The figure which shows the spot shape in each image height which concerns on Example 1 of this invention. The figure which shows the main scanning jitter which concerns on Example 1 of this invention. Schematic diagram of the multi-stage lens surface of the multi-stage lens according to Example 1 of the present invention The figure which shows the shape of the imaging lens 7 which concerns on Example 1 of this invention. Schematic view of essential parts of an optical scanning device according to Embodiment 2 of the present invention (sub-scanning sectional view) Schematic view of main parts of an optical scanning apparatus according to Embodiment 2 of the present invention (main scanning sectional view) Schematic view of essential parts of an incident optical system according to Embodiment 2 of the present invention (sub-scan sectional view) FIG. 10 is a diagram illustrating curvature of field in the main scanning direction and the sub-scanning direction according to the second embodiment of the present invention. The figure which shows the f (theta) characteristic which concerns on Example 2 of this invention. The figure which shows the scanning line curve which concerns on Example 2 of this invention. The figure which shows the spot shape in each image height which concerns on Example 2 of this invention. The figure which shows the main scanning jitter which concerns on Example 2 of this invention. The figure which shows the shape of the imaging lens 7 which concerns on Example 2 of this invention. Schematic view of essential parts of an optical scanning device according to Embodiment 3 of the present invention (sub-scan sectional view) 1 is a schematic view of a main part of an image forming apparatus according to an embodiment of the present invention (sub-scan sectional view).

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. Each drawing may be drawn on a different scale for convenience. Moreover, in each drawing, the same reference number is attached | subjected about the same member and the overlapping description is abbreviate | omitted. In the following description, the main scanning direction (Y direction) is a direction perpendicular to the rotation axis (or swing axis) of the deflector and the optical axis direction (X direction) (the surface to be scanned is scanned by the deflector). The sub-scanning direction (Z direction) is a direction parallel to the rotation axis (or swing axis) of the deflector. The main scanning section (XY section) is a section perpendicular to the sub-scanning direction (a section including the main scanning direction and the optical axis), and the sub-scanning section (ZX section) is perpendicular to the main scanning direction. It is a cross section (a cross section including a sub-scanning direction and an optical axis).

[Example 1]
1 to 3 are schematic views of a main part of an optical scanning device 100 according to a first embodiment of the present invention. FIG. 1 is a ZX sectional view (sub-scanning sectional view), and FIG. 2 is an XY sectional view (main scanning). 3 is a YZ cross-sectional view. In FIG. 2, the optical path folded by the mirror is developed, and the mirror is omitted. Moreover, in each figure, the principal part of the optical scanning device 100 is enlarged and shown, and other members are omitted.

  The optical scanning device 100 according to the present embodiment simultaneously records image information corresponding to four different hues (K, C, M, Y) on each of four different scanned surfaces (photosensitive surfaces) 8A to 8D. This is a tandem type optical scanning device. The optical scanning device 100 includes light sources 1A to 1D, incident optical systems LA to LD, a deflector 5, imaging optical systems SR and SL, and mirrors M1 to M′3. In the present embodiment, the members constituting the optical scanning device 100 are symmetrically arranged on the left and right with the deflector 5 in between, and the left and right configurations and optical actions are the same. The configuration (scanning unit) on the right side (+ X side) of FIG.

  As shown in FIG. 3, the first light beam Ra emitted from the first light source 1A and the second light beam Rb emitted from the second light source 1B are guided to the deflector 5 by the incident optical systems LA and LB. The At this time, the light beams Ra and Rb are deflected by one deflection surface (first deflection surface) 5a among the plurality of deflection surfaces of the deflector 5. Here, C0 in the drawing is a deflection point (reference point) when the principal ray of the axial light beam is deflected, and P0 is a plane (reference) that passes through the deflection point C0 and is perpendicular to the rotation axis of the deflector 5. Surface). The light beams Ra and Rb incident on the deflection surface 5a are deflected so as to intersect at a deflection point C0 in the sub-scan section. Hereinafter, the length of the optical path from the deflection point C0 to each scanned surface is defined as the optical path length from the deflection surface 5a to each scanned surface.

  As shown in FIG. 1, the imaging optical system SR has imaging lenses 6 and 7, which are shared by the light beams Ra and Rb. In the present embodiment, the imaging lens 6 closest to the deflector 5 in the optical path has a symmetrical shape (the same shape with respect to the light beams Ra and Rb) with respect to the reference plane P0 in the sub-scanning direction. On the other hand, the imaging lens 7 closest to the scanning surface in the optical path has an asymmetric shape in the sub-scanning direction with respect to the reference surface P0, and the upper shape and the lower shape with respect to the reference surface P0 are main-scanned. It is different both in the cross section and in the sub-scan section.

  Specifically, the imaging lens 7 is a multistage lens in which a first imaging unit 7A including a first optical surface and a second imaging unit including a second optical surface are arranged in the sub-scanning direction. It is. By employing such a multistage lens, the number of imaging lenses constituting the imaging optical system SR is reduced, and the optical scanning device 100 is reduced in size and cost. Hereinafter, a portion corresponding to the light beam Ra in the image forming optical system SR is referred to as a first image forming optical system SA, and a portion corresponding to the light beam Rb is referred to as a second image forming optical system SB.

  The light beam Ra deflected by the deflector 5 is collected by the imaging lens 6 and the first imaging unit 7A, is folded by the mirror M3, and is guided to the first scanned surface 8A (K). The light beam Rb deflected by the deflector 5 is collected by the imaging lens 6 and the second imaging unit 7B, and is folded by the mirror M1 (first mirror) and the mirror M2 (second mirror). To the second surface to be scanned 8B (C). In the present embodiment, no mirror other than the first and second mirrors (other than the mirror M1 and the mirror M2) is disposed in the optical path from the deflection surface 5a to the scanned surface 8B.

  In this embodiment, in the optical path of the imaging optical system SR, the imaging lens 7 closest to the surface to be scanned is arranged closer to the deflector 5 than all the mirrors. In other words, no other imaging lens is disposed on the scanning surface side of the imaging lens 7 in the optical path, and the imaging lens is located on the scanning surface side of the mirror M1 closest to the deflector 5. The structure which is not arranged is adopted. Thereby, the length of the imaging lens 7 in the main scanning direction can be shortened, and the optical scanning device 100 can be downsized.

  As in this embodiment, when all mirrors are arranged behind all imaging lenses and the light beam is folded after passing through all the imaging lenses, each folded light beam, each imaging lens, It is necessary to avoid interference. However, in the above-described prior art documents, since the optical path lengths for the respective scanned surfaces are equal to each other, there is a problem in that the entire apparatus is enlarged and the number of mirrors is increased.

  Therefore, in this embodiment, the second optical path from the deflector 5 to the scanned surface 8B spatially closest to the first optical path length from the deflector 5 to the scanned surface 8A that is spatially farthest. The optical scanning device 100 is configured so that the longer one is longer. Thereby, compared with the case where each optical path length is the same, the reflection angle of the mirror M1 (the light flux Rb reflected by the mirror M1 and the reference plane) necessary for avoiding interference between each imaging lens and each light flux. The angle formed with P0 can be reduced. Therefore, in this embodiment, the light beam Rb from the imaging lens 7 can be guided to the scanned surface 8B only by the mirror M1 and the mirror M2 while avoiding interference with each imaging lens. It is possible to reduce the number of parts constituting the.

  Further, as shown in FIG. 1, the mirror M1 according to the present embodiment is disposed above the center position of the outer shape of the imaging lens 7 in the sub-scanning direction, and connects the light beam Rb from the imaging lens 7. The light is reflected upward of the image lens 7. Furthermore, the mirror M2 according to the present embodiment is disposed above the center position of the outer shape of the imaging lens 7 in the sub-scanning direction, and the light beam Rb from the mirror M1 is converted into the imaging lens 6 and the imaging lens 7. The light is reflected so as to pass through and is guided to the scanned surface 8B. Thus, by making the second optical path length longer than the first optical path length, the first reflection point in the mirror M1 of the light beam Rb and the second reflection point in the mirror M2 are connected in the sub-scanning direction. It can be configured to be located on the same side with respect to the outer shape center position of the image lens 7. Therefore, the mirror M2 can be disposed at a position closer to the reference plane P0, and the height of the optical scanning device 100 in the sub-scanning direction can be sufficiently reduced.

  In the present embodiment, the light beam Rb reflected by the mirror M2 is configured to pass between the imaging lens 6 and the imaging lens 7. However, the present invention is not limited to this. That is, the optical path from the deflector 5 to the imaging lens 7 and the optical path from the mirror M2 to the scanned surface 8B may be configured to intersect each other in the sub-scanning section. According to this configuration, there is no light beam passing below each imaging lens, so that the configuration of the optical box that holds each optical component can be simplified.

2A is a main scanning sectional view in which the optical path from the light source 1A to the scanned surface 8A is developed, and FIG. 2B is a main scanning sectional view in which the optical path from the light source 1B to the scanned surface 8B is developed. As shown in these drawings, in this embodiment, the optical path lengths corresponding to the imaging optical systems SA and SB are made different from each other, thereby increasing the degree of freedom of arrangement of the optical components. Here, when the shorter optical path length is T1 and the longer optical path length is T2, it is desirable that the following conditional expression (1) is satisfied.
25 ≦ T2-T1 ≦ 65 (1)

If the lower limit of conditional expression (1) is not reached, it will be difficult to suppress the height of the optical scanning device 100 while avoiding interference between the light beam Rb and the imaging lens 7. If the upper limit value of the conditional expression (1) is exceeded, it is necessary to increase the shape difference between the first imaging portion 7A and the second imaging portion 7B in the imaging lens 7, and each is integrally molded. It becomes difficult to do. Furthermore, in order to suppress the height of the optical scanning device 100 while reducing the number of parts, it is more preferable to satisfy the following conditional expression (1a).
30 <T2-T1 <50 (1a)

  In this embodiment, the optical path length Ta = T1 = 155.733 mm corresponding to the imaging optical system SA, the optical path length corresponding to the imaging optical system SB is Tb = T2 = 197.000 mm, and T2−T1 = 41. Therefore, conditional expressions (1) and (1a) are satisfied.

  As described above, in order to reduce the shape difference between the first image forming unit 7A and the second image forming unit 7B in the image forming lens 7 while making the optical path lengths different from each other, the light enters the deflection surface 5a. It is desirable to appropriately set the convergence of each light beam in the main scanning section (the convergence of the imaging optical system). In the main scanning section, the optical distance (optical path length) from the rear main plane of the imaging optical system to the surface to be scanned is Sk (mm), and the focal length of the imaging optical system is f (mm). At this time, the degree of convergence of the imaging optical system is expressed as m = 1−Sk / f. The state of the light beam incident on the deflecting surface 5a in the main scanning section differs depending on the degree of convergence: parallel light beam when m = 0, divergent light beam when m <0, and convergence when m> 0. Luminous flux.

Here, when the convergence of the imaging optical system corresponding to the shorter optical path is m1, and the convergence of the imaging optical system corresponding to the longer optical path is m2, the following conditional expression (2) is satisfied. It is desirable to make it so.
0.15 <m1-m2 <0.50 (2)

If the lower limit value of conditional expression (2) is not reached, it is necessary to increase the shape difference between the first image forming unit 7A and the second image forming unit 7B in order to generate an optical path length difference. It becomes difficult to mold. If the upper limit value of conditional expression (2) is exceeded, the absolute value of either one of the convergence degrees m1 and m2 increases, which is mainly caused by the surface eccentricity (shift eccentricity error) of each deflection surface of the deflector 5. A large amount of jitter in the scanning direction (main scanning jitter) occurs. Furthermore, in order to suppress the height of the optical scanning device 100 while reducing the number of parts, it is more preferable to satisfy the following conditional expression (2a).
0.20 <m1-m2 <0.40 (2a)

Here, in the main scanning section, the optical path length from the rear main plane of the imaging optical system SA to the scanned surface 8A is Sk1 (mm), and the focal length of the imaging optical system SA is f1 (mm). . In the main scanning section, the optical path length from the rear main plane of the imaging optical system SB to the scanned surface 8B is Sk2 (mm), and the focal length of the imaging optical system SB is f2 (mm). The first convergence degree of the imaging optical system SA is defined as m1 = 1−Sk1 / f1, and the second convergence degree of the imaging optical system SB is defined as m2 = 1−Sk2 / f2. At this time, in this embodiment, the convergence degree ma = m1 = 0.047 of the imaging optical system SA, the convergence degree mb = m2 = −0.254 of the imaging optical system SB, and m1−m2 = 0. .30, the conditional expressions (2) and (2a) are satisfied. Further, it is desirable that the larger one of | m1 | and | m2 | be m so that the following conditional expression (3) is satisfied.
0.2 <m <0.5 (3)

If the lower limit value of conditional expression (3) is not reached, it is necessary to increase the shape difference between the first image forming unit 7A and the second image forming unit 7B in order to generate an optical path length difference. It becomes difficult to mold. On the other hand, if the upper limit of conditional expression (3) is exceeded, a large amount of main scanning jitter will occur due to the shift eccentricity error of each deflecting surface of the deflector 5. Furthermore, it is more preferable that the following conditional expression (3a) is satisfied.
0.22 <m <0.4 (3a)

  In this embodiment, since m = | m2 | = 0.254, conditional expressions (3) and (3a) are satisfied.

In addition, when the scanning image height is Y (mm) and the scanning field angle corresponding to the scanning image height Y is θ (rad), the scanning characteristic (Kθ characteristic) of the imaging optical system SR is an equation Y = Kθ. It is represented by At this time, a coefficient K which is a ratio of the scanning angle of view θ to the scanning image height Y is set as a Kθ coefficient. When the Kθ coefficient of the imaging optical system corresponding to the shorter optical path is K1, and the Kθ coefficient of the imaging optical system corresponding to the longer optical path is K2, the following conditional expression (4) is satisfied. It is desirable to make it so.
0.65 <K1 / K2 <0.85 (4)

If the lower limit of conditional expression (4) is not reached, it will be difficult to achieve both Kθ characteristics and correction of curvature of field in the main scanning direction. If the upper limit value of conditional expression (4) is exceeded, it is necessary to increase the shape difference between the first image forming unit 7A and the second image forming unit 7B in order to generate an optical path length difference. It becomes difficult to integrally mold. In order to obtain good imaging performance while reducing the shape difference between the first imaging unit 7A and the second imaging unit 7B, it is more preferable to satisfy the following conditional expression (4a).
0.70 <K1 / K2 <0.83 (4a)

  In this embodiment, the Kθ coefficient Ka = K1 = 132.0 (mm / rad) of the imaging optical system SA, and the Kθ coefficient Kb = K2 = 167.0 (mm / rad) of the imaging optical system SB. Since K1 / K2 = 0.79, the conditional expressions (4) and (4a) are satisfied.

  As shown in FIG. 3, each of the incident optical systems LA and LB according to the present embodiment includes aperture stops 2A and 2B, coupling lenses 3A and 3B, and cylindrical lenses (cylinder lenses) 4A and 4B. ing. The incident optical systems LA and LB are sub-scanning oblique incidence optical systems that allow the light beams Ra and Rb to enter the deflecting surface 5a from an oblique direction within the sub-scan section. By adopting such a sub-scanning oblique incidence optical system. Each light beam can be separated and deflected while suppressing an increase in size of each deflection surface in the sub-scanning direction.

  At this time, if the oblique incident angle with respect to the reference plane P0 is too large, it is difficult to correct the spot collapse due to the twist of the wavefront aberration, and if the oblique incident angle is too small, it is difficult to separate the optical paths. Therefore, in this embodiment, the oblique incident angles of the incident optical systems LA and LB are αsA = −3.0 ° and αsB = 3.0 °, respectively, and the absolute values of the oblique incident angles are both 3.0 °. By setting as described above, correction of spot collapse and separation of each optical path are facilitated.

  In this embodiment, since a semiconductor laser is used as the light source, the light beams Ra and Rb emitted from the light sources 1A and 1B are divergent light beams, and the incident optical systems LA and LB are light beams in the sub-scan section. Ra and Rb are converted into substantially parallel light. In the main scanning section, the light beam Ra is converted into a weakly convergent light beam (m = 0.047) by the coupling lens 3A, and the light beam Rb is converted into a weakly divergent light beam (m = −0. To 254). At this time, the exit surfaces of the coupling lenses 3A and 3B are anamorphic surfaces, and the radii of curvature in the respective main scanning sections are made different from each other, so that each convergence is set to a desired value.

The aperture stops 2A and 2B have different aperture diameters in the sub-scanning direction so that the spot diameters (1 / e ² slice diameters of the spot peak light quantity) on the corresponding scanned surfaces 8A and 8B are equal. Have. The cylindrical lenses 4A and 4B are formed by the aperture stops 2A and 2B (the amount of light is limited), and the light beams Ra and Rb collected by the coupling lenses 3A and 3B are deflected on the deflection surface 5a only in the sub-scan section. And a long line image in the main scanning direction is formed. At this time, in the main scanning section, an angle formed by the optical axes of the incident optical systems LA and LB (or principal rays of the light beams Ra and Rb incident on the deflecting surface 5a) and the optical axes of the imaging optical systems SA and SB. Is α = 90 °.

  The incident optical systems LA and LB according to the present embodiment have the same configuration except for the shape of the aperture provided in the aperture stops 2A and 2B and the shape of the exit surface of the coupling lenses 3A and 3B. The optical path lengths from the light sources 1A and 1B to the deflecting surface 5a are also the same. Thus, by sharing the arrangement of the optical components constituting the incident optical systems LA and LB, it is possible to reduce the types of holding parts and the types of assembly tools that hold the components and improve productivity. it can.

  The deflector 5 according to the present embodiment is a rotating polygon mirror (polygon mirror) having four deflecting surfaces with a circumscribed circle radius of 20 mm. The deflector 5 is shown in each drawing by a driving force generated by a driving unit (motor) (not shown). It rotates at a constant speed in the direction of arrow A. As shown in FIG. 2, the scanning surfaces 8A and 8B can be optically scanned in the direction of arrow B by the rotation of the deflector 5. As shown in FIG. 1, when the light beams Ra and Rb are deflected by the deflecting surface 5a of the deflector 5, the light sources 1C and 1D of the scanning unit on the left side (−X side) with respect to the deflector 5 The emitted light beams R′a and R′b are deflected by a deflecting surface different from the deflecting surface 5a. Then, the light is guided to the scanned surfaces 8C (M) and 8D (Y) by the imaging optical system SL having the same optical action as the imaging optical system SR described above. With this configuration, it is possible to simultaneously form images of four colors of yellow (Y), magenta (M), cyan (C), and black (Bk).

  Next, the configuration of the imaging optical system will be described in detail. Since the configuration of the imaging optical system SB other than the imaging lens 7 is the same as that of the imaging optical system SA, the following description will focus on the imaging optical system SA. The imaging optical system SA collects the light beam Ra deflected by the deflector 5 on the scanned surface 8A to form a spot image. Further, the imaging optical system SA is configured such that the deflection surface 5a and the surface to be scanned 8A are optically conjugate in the sub-scan section. In other words, the imaging optical system SA forms a surface tilt correction optical system that corrects the influence (surface tilt correction) due to the difference in tilt angle (surface tilt) of each deflection surface in the sub-scan section.

  Tables 1 to 4 below show specification values, optical arrangements, and surface shapes of the respective imaging lenses of the optical scanning device 100 according to the present example. Table 1 shows specification values and lens arrangements of the incident optical system LA and the imaging optical system SA, and Table 2 shows lens shapes of the incident optical system LA and the imaging optical system SA. Table 3 shows specification values and lens arrangements of the incident optical system LB and the imaging optical system SB, and Table 4 shows lens shapes of the incident optical system LB and the imaging optical system SB. The columns of optical arrangement in Tables 1 and 3 show the coordinates of the reflection points at the mirrors of the light beams Ra and Rb going to the image center (axial image height) in the main scanning direction on the surface to be scanned. .

  Here, in the optical axis direction of the imaging optical system SR, the distance from the scanned surface 8B to the reflection point of the first mirror M1 is LD1, and the distance from the scanned surface 8A to the reflection point of the first mirror M1 is Let LD2. At this time, in this embodiment, each member is arranged so as to satisfy the condition of LD1> LD2, specifically, LD1 = 46.886 (mm) and LD2 = 20.114 (mm). .

  In the optical scanning devices described in Patent Documents 1 to 3, since it is necessary to arrange the members such that LD1 <LD2 in order to make the optical path lengths for the scanned surfaces equal to each other, the light flux Rb and the imaging lens It was difficult to reduce the size of the entire apparatus while avoiding interference with. On the other hand, in this embodiment, the optical path length corresponding to the imaging optical system SB can be made longer than the optical path length corresponding to the imaging optical system SA, so that LD1> LD2. That is, since the reflection point of the light beam Rb at the first mirror M1 can be made closer to the scanned surface 8A than the scanned surface 8B, the above problem can be solved.

  Further, in the optical axis direction of the imaging optical system SR, the distance from the exit surface of the imaging lens 7 to the reflection point of the first mirror M1 is LM1, and the distance from the reflection point of the first mirror M1 to the scanned surface 8A. Let the distance be LM2. At this time, in this embodiment, each member is arranged so as to satisfy the condition of LM1> LM2, specifically, LM1 = 30.503 (mm) and LM2 = 20.114 (mm). . As a result, since the reflection point of the first mirror can be brought closer to the scanned surface 8A than the exit surface of the imaging lens 7, the entire apparatus can be reduced in size while avoiding interference between the imaging lens 7 and the light beam Rb. It becomes possible to do.

The exit surfaces of the cylindrical lenses 4A and 4B according to the present embodiment are diffractive surfaces on which diffraction gratings are formed. The cylindrical lenses 4 and 4B are formed by injection molding using a plastic material, and a temperature compensation optical system that compensates for changes in refractive power caused by environmental fluctuations by changes in diffraction power caused by wavelength changes in the semiconductor laser. It has become. Here, when the diffraction order is M and the design wavelength is λ, the diffraction surfaces of the cylindrical lenses 4A and 4B are defined by a phase function φ = 2πM / λ (C3Z ² ). Since the first-order diffracted light is used in this embodiment, the diffraction order M = 1, the design wavelength λ = 790 nm, Z is the position in the Z direction in FIG. 3, and C3 is a coefficient.

  The generatrix shapes (shapes in the main scanning section) of the entrance and exit surfaces of the imaging lenses 6, 7 </ b> A, 7 </ b> B according to the present embodiment are both aspherical shapes represented by functions up to the 12th order. Yes. Here, when the intersection of each lens surface (optical surface) and each optical axis is the origin, the axis in the optical axis direction is the X axis, and the axis orthogonal to the X axis in the main scanning section is the Y axis, the generating line The shape X is represented by the following formula.

  However, R is a curvature radius (bus curvature radius) in the main scanning section, and K, B4, B6, B8, B10, and B12 are aspherical coefficients. The aspheric coefficients B4, B6, B8, and B12 are on the side opposite to the light source (+ Y side in FIG. 2) and the light source side (−Y side in FIG. 2) with respect to the optical axis in the optical scanning device 100. The numerical values may be different from each other. As a result, the bus shape can be asymmetric in the main scanning direction with respect to the optical axis. In Tables 2 and 4, the numerical values on the side opposite to the light source with respect to the optical axis are B4U, B6U, B8U, B10U, B12U, and the numerical values on the light source side with respect to the optical axis are B4L, B6L, B8L, B10L, B12L is set to the same value.

  In addition, the sub-line shapes (shapes in the sub-scan section) of the incident surfaces and the exit surfaces of the imaging lenses 6, 7A, 7B according to the present embodiment are expressed by the following equations.

  The sub-line shape S indicates a surface shape in a section perpendicular to the main scanning section including the surface normal on the generatrix at each position (each image height) in the main scanning direction, and Mj_k in the equation is Aspheric coefficient. R ′ represents a radius of curvature (sub-wire curvature radius) in the sub-scan section at a position separated from the optical axis by Y in the main scanning direction, and is represented by the following equation.

  Here, r is the radius of curvature of the strand on the optical axis, and E2, E4, E6, E8, E10 are the strand change coefficients. Note that the aspherical amount of the child-wire shape can be set asymmetric in the main scanning direction by making the aspheric coefficients E2 to E10 different between the light source side and the opposite side with respect to the optical axis. In addition, the first-order term of Z in the formula of the sub-line shape S is a term that contributes to the tilt amount (sub-line tilt amount) of the lens surface in the sub-scan section. Therefore, by making the aspheric coefficients M0_1 to M16_1 different between the numerical values M0_1U to M16_1U on the side opposite to the light source with respect to the optical axis and the numerical values M0_1L to M16_1L on the light source side with respect to the optical axis, The amount can be changed asymmetrically in the main scanning direction.

  For the imaging lens 7 according to this example, as shown in Tables 2 and 4, the bus line shape and the child wire shape (the child wire curvature and the child wire tilt amount) are set to the first image forming unit 7A and the second image forming unit 7A. It differs from the image forming part. As described above, the aspheric coefficients relating to the first optical surface and the second optical surface constituting the multi-stage lens surface of the imaging lens 7 are made different from each other, and the respective surface shapes are optimized, so that the optical path Each of the optical characteristics of the imaging optical systems SA and SB having different lengths can be corrected.

  In this embodiment, the first optical surface included in the first imaging unit 7A and the second optical surface included in the second imaging unit 7B are different from each other as shown in Tables 2 and 4. By using the surface shape expressed by the following formula, good imaging performance is obtained in each of the imaging optical systems SA and SB. However, if the shape difference between the first image forming unit 7A and the second image forming unit is too large in order to correspond to different optical path lengths, it becomes difficult to integrally mold each of them. .

  Therefore, in the present embodiment, the values of the bus curvature radius R and the aspherical coefficient K are made equal for the first imaging unit 7A and the second imaging unit 7B, so that the vicinity of the optical axis (on the optical axis and on the optical axis). The shape in the main scanning direction in the vicinity thereof is the same. Thereby, it can be configured to satisfy the conditional expressions (2) and (2a) described above while reducing the shape difference between the first image forming unit 7A and the second image forming unit 7B. Image performance can be obtained. Strictly speaking, the shapes in the main scanning direction in the vicinity of the optical axis need not be the same, and the same effect can be obtained if they are configured to be substantially the same. In this embodiment, the shape of each lens surface is defined by the above expression (function). However, the present invention is not limited to this and may be defined by another expression.

  FIG. 4 is a graph showing the field curvature (defocus characteristics) in the main scanning direction and the sub-scanning direction according to the present embodiment. FIG. 4A corresponds to the light beam Ra, and FIG. It corresponds to b. In this embodiment, the effective width of the image (the width of the effective scanning area on the scanned surface) is W = 210 mm. As shown in FIG. 4A, the field curvature in the main scanning direction of the imaging optical system SA is dm = 1.8 mm, and the field curvature in the sub-scanning direction is ds = 1.1 mm. As shown in FIG. 4B, the field curvature in the main scanning direction of the imaging optical system SB is dm = 2.0 mm, and the field curvature in the sub-scanning direction is ds = 1.6 mm. From this, it can be seen that the curvature of field is satisfactorily corrected for both the imaging optical systems SA and SB.

  FIG. 5 is a graph showing the fθ characteristic dy1 according to the present embodiment. FIG. 5A corresponds to the light beam Ra, and FIG. 5B corresponds to the light beam b. The fθ characteristic dy1 here indicates the difference between the position (image height) where the light beam actually reaches on the surface to be scanned and the design value (ideal image height). As shown in FIG. 5A, a maximum deviation of 0.23 mm occurs in the imaging optical system SA, and as shown in FIG. A deviation of 26 mm has occurred. Therefore, in this embodiment, by changing the image clock (light emission timing of the light source) in accordance with each image height, the shift of the fθ characteristic dy1 is reduced and the color shift in the main scanning direction is suppressed. Insufficient correction of the fθ characteristic can be electrically corrected by changing the image clock, but if the deviation of the fθ characteristic becomes too large, the spot diameter itself in the main scanning direction changes. However, as shown in FIG. 5, in this embodiment, the deviation of the fθ characteristic that causes a large change in the spot diameter itself does not occur, so that the occurrence of density unevenness in the image can be suppressed. .

  FIG. 6 is a graph showing the scanning line curve dz according to the present embodiment. FIG. 6A corresponds to the light beam Ra, and FIG. 6B corresponds to the light beam b. Here, the scanning line curve dz indicates the difference between the imaging position in the sub-scanning direction at each image height and the imaging position in the sub-scanning direction at the image center (axial image height). As shown in FIG. 6A, a maximum deviation of 7 μm occurs in the imaging optical system SA, and as shown in FIG. 6B, a maximum deviation of 6 μm occurs in the imaging optical system SB. However, none of them has a level that affects the formed image.

  FIG. 7 is a diagram illustrating the cross-sectional shape of the spot at each image height according to the present embodiment. FIG. 7A corresponds to the light beam Ra, and FIG. 7B corresponds to the light beam b. FIG. 7 shows a spot shape in a cross section sliced at 2%, 5%, 10%, 13.5%, 36.8%, and 50% of the peak light quantity of the spot at each image height Y. . In general, in an optical scanning apparatus employing a sub-scanning oblique incidence optical system, a phenomenon that a spot collapses due to twist of wavefront aberration is observed. However, in this embodiment, by optimizing the power arrangement of each lens surface and the amount of sub-line tilt on the entrance surface and exit surface of the imaging lens 7, both scanning line bending and wavefront aberration twist correction can be achieved. doing.

  Thereby, as shown in FIG. 7, a favorable spot shape with little collapse over the entire image height can be obtained. However, in this embodiment, in order to suppress the difference in surface shape between the image forming unit 7A and the image forming unit 7B, the occurrence of coma aberration at the off-axis image height is allowed within a range that does not affect the image. . Therefore, as shown in FIG. 7B, the side lobes in the main scanning direction are large at the image heights Y = −105, −50, 50, and 105. However, since the hue of the surface to be scanned (photosensitive drum) corresponding to the imaging optical system SB is cyan, the influence on the image is smaller compared to the imaging optical system SA corresponding to the black photosensitive drum. .

  FIG. 8 is a diagram showing the main scanning jitter dy2 when the shift decentering error of the deflecting surface is 10 μm. FIG. 8A corresponds to the light beam Ra, and FIG. 8B corresponds to the light beam b. It corresponds. As shown in FIG. 8A, the main scanning jitter for the imaging optical system SA is 0.8 μm at the maximum, and as shown in FIG. 8B, the main scanning jitter for the imaging optical system SB is Since it is 5.1 μm at the maximum, it can be suppressed to a level where there is no problem.

  The imaging lens 7 according to the present embodiment is arranged so that the outer shape center position in the sub-scanning direction coincides with the reference plane P0, and the outer shape center position is the first imaging unit 7A and the second imaging image. It is comprised so that it may become a boundary part with the part 7B. Further, the optical axis of the first imaging unit 7A is set to a position shifted by 2.24 mm downward in the sub-scanning direction with respect to the reference plane P0, and the optical axis of the second imaging unit 7B is set to the reference plane P0. Is set to a position shifted by 2.24 mm upward in the sub-scanning direction. With this configuration, the position of each optical axis that serves as a reference for the expression defining the first and second optical surfaces can be arranged in the vicinity of the incident position of the light beams Ra and Rb. It becomes easy to cope with performance, and it is possible to easily evaluate the surface shape at the time of molding.

  Here, in the first imaging unit 7A and the second imaging unit 7B, the intersection with each optical axis is defined as a surface vertex. At this time, as shown in Table 1 and Table 3, the position of each surface vertex of the entrance surface and the exit surface of the first imaging unit 7A and each surface of the entrance surface and the exit surface of the second image formation unit 7B The vertex positions coincide with each other in the main scanning direction and the optical axis direction. With this configuration, it is possible to minimize the shape difference in the vicinity of the optical axis between the first imaging unit 7A and the second imaging unit 7B. The coincidence here includes substantially coincidence, and the same effect can be obtained even if it is not exact coincidence.

  FIG. 9 is a schematic diagram (sub-scan sectional view) for explaining a boundary portion on the exit surface (multi-stage lens surface) of the imaging lens 7. In FIG. 9, in order to make the explanation easy to understand, an actual scale and shape that are different (enlarged and emphasized) are shown. In the present embodiment, the shape of the first optical surface included in the first imaging unit 7A and the second optical surface included in the second imaging unit 7B in the sub-scanning section (in the sub-scanning section). (Curvature and sub-line tilt amount) are different from each other in the entire effective area in the main scanning direction. Therefore, in the sub-scan section at each image height in the main scanning direction, the first optical surface and the second optical surface in the boundary part have different amounts of entry and exit, and each boundary part becomes a discontinuous point. That is, as shown in FIG. 9, the first optical surface and the second optical surface are shifted from each other in the optical axis direction (X direction) at the boundary portion, and a step is generated.

  FIG. 10A is a diagram showing the difference between the generatrix shape of the first optical surface and the generatrix shape of the second optical surface of the imaging lens 7. In FIG. 10A, the vertical axis represents the bus shape difference, and when the sign of this shape difference is positive, the second imaging portion 7B is more in the optical axis direction than the first imaging portion 7A. Is located farther from the deflector. Further, when the sign of the shape difference is negative, it indicates that the second imaging unit 7B is closer to the deflector than the first imaging unit 7A. In the present embodiment, the entrance surface and the exit surface of the imaging lens 7 are arranged in such a manner that the difference in bus shape between the imaging unit 7A and the imaging unit 7B is zero on and near the optical axis. The surface shape is set. In addition, the bus shape of the second optical surface with the longer optical path length is separated from the deflector 5 with respect to the first optical surface with the shorter optical path length, as it goes from the optical axis to the end in the main scanning direction. It is changing to go.

  Specifically, when the shape difference on the optical axis of the first image forming unit 7A and the second image forming unit 7B is X (0), the shape difference X (0) = 0 (mm ), The shape difference X (0) = 0 (mm) of the emission surface. Next, considering, for example, −40, −30, +30, and 40 (mm) as off-axis coordinates away from the optical axis in the main scanning direction, the first imaging unit 7A and the second second coordinate at each coordinate are used. The shape difference of the image forming unit 7B is as follows. That is, the shape difference at each coordinate of the incident surface is X (−40) = 900 (mm), X (−30) = 245 (mm), X (+30) = 245 (mm), X (+40) = 825 (mm). Further, the shape difference at each coordinate of the exit surface is X (−40) = 1189 (mm), X (−30) = 362 (mm), X (+30) = 362 (mm), X (+40) = 1110 (mm).

  As described above, in the present embodiment, the shapes of the first imaging unit 7A and the second imaging unit 7B are such that the shape difference on the off-axis is larger than the shape difference on the optical axis. Is set. By changing the difference in the bus shape in this way, the bus shape difference for obtaining good optical performance for both the imaging optical systems SA and SB having different optical path lengths can be kept to a minimum. 7 is easy to mold. Strictly speaking, the difference in bus shape in the vicinity of the optical axis does not have to be zero, and the same effect can be obtained if it is configured to be substantially zero.

Here, when the maximum value of the surface shape difference between the first optical surface and the second optical surface is Xmax (mm) in the entire light beam use region (effective region) of the imaging lens 7, the following condition is satisfied. It is desirable to configure so as to satisfy Expression (5).
0.1 ≦ | Xmax | ≦ 5.0 (5)

If the upper limit value of conditional expression (5) is exceeded, the imaging lens 7 is warped in the sub-scanning section during injection molding, and the scanning line curvature and wavefront aberration are deteriorated. If the lower limit of conditional expression (5) is not reached, it is difficult to achieve both the imaging performance such as the main scanning image plane and the fθ characteristic of the imaging optical system SA and the imaging optical system SB. Furthermore, it is more preferable that the following conditional expression (5a) is satisfied.
0.2 ≦ | Xmax | ≦ 3.0 (5a)

  In this embodiment, since Xmax = 0.90 (mm) on the incident surface and Xmax = 1.19 (mm) on the exit surface, both satisfy the conditional expressions (5) and (5a). As described above, the imaging optical system SA can be focused on the surface to be scanned even when the shapes of the imaging portions 7A and 7B in the vicinity of the optical axis are substantially matched in the main scanning section. Is different from the convergence degree mb of the imaging optical system SB.

  FIG. 10B is a diagram showing the difference between the thickness of the first imaging unit 7A and the thickness of the second imaging unit 7B. The thickness here indicates the distance from the incident surface to the exit surface at each position (each image height) in the main scanning direction of the imaging lens 7. In this embodiment, as shown in FIG. 10B, the thickness difference near the optical axis is zero, and from the thickness of the imaging unit 7A toward the end portion from the optical axis in the main scanning direction. Also, the thickness of the imaging portion 7B is thicker. By changing the thickness difference in this way, the negative power of the imaging unit 7B is relatively increased with respect to the imaging unit 7A as it goes from the optical axis toward the end in the main scanning direction. ing. With this effect, it is possible to satisfactorily correct the curvature of field in the main scanning direction for both of the imaging optical systems SA and SB having different optical path lengths with only a minimum shape difference. Strictly speaking, the thickness difference in the vicinity of the optical axis need not be zero, and the same effect can be obtained if it is configured to be substantially zero.

Further, when the maximum value of the difference in thickness between the imaging portion 7A and the imaging portion 7B is dmax (mm) in the entire effective area of the imaging lens 7, the following conditional expression (6) is satisfied. It is desirable to configure.
0.05 ≦ | dmax | ≦ 5.0 (6)

If the upper limit value of conditional expression (6) is exceeded, the imaging lens 7 warps in the sub-scan section during injection molding, and the scanning line curvature and wavefront aberration are deteriorated. If the lower limit of conditional expression (6) is not reached, it is difficult to achieve both the imaging performance such as the main scanning image plane and fθ characteristics of the imaging optical system SA and the imaging optical system SB. Furthermore, it is more preferable that the following conditional expression (6a) is satisfied.
0.1 ≦ | dmax | ≦ 4.0 (6a)

  In this example, since dmax = 0.29 (mm), both satisfy conditional expressions (6) and (6a).

  FIG. 10C is a diagram illustrating a step at the boundary between the image forming unit 7A and the image forming unit 7B. In FIG. 10C, the vertical axis indicates the step at the boundary, that is, the shift (difference in and out) in the optical axis direction between the first optical surface and the second optical surface at the boundary. When the sign of this step is positive, it indicates that the second image forming unit 7B is located farther from the deflector than the first image forming unit 7A in the optical axis direction. Further, when the sign of the step is negative, it indicates that the second imaging unit 7B is closer to the deflector than the first imaging unit 7A. As shown in FIG. 10C, the step is 1400 μm or less in the entire light beam use region of the imaging lens 7. Therefore, it is possible to suppress the deformation of the lens surface due to the thermal deformation stress in the vicinity of the step and the occurrence of habits caused by the step during the injection molding of the imaging lens 7 to a level with no problem.

Furthermore, in this embodiment, when the imaging magnification in the sub-scan section of the imaging optical system corresponding to the longer optical path is βs, the following conditional expression (7) can be satisfied. desirable.
2.5 <| βs | <5.0 (7)

If the upper limit value of conditional expression (7) is exceeded, deterioration of pitch unevenness due to surface tilt and insufficient correction of wavefront aberration will occur. If the lower limit of conditional expression (7) is not reached, the imaging lens 7 closest to the scanned surface in the optical path will be too close to the scanned surface. Therefore, even if the optical path lengths corresponding to the imaging optical system SA and the imaging optical system SB are made different from each other, it becomes difficult to avoid interference between the light beam Rb and the imaging lens 7. Furthermore, it is more preferable that the following conditional expression (7a) is satisfied.
2.7 <| βs | <4.0 (7a)

  In this embodiment, the imaging magnification βa = −2.29 (times) in the sub-scanning section of the imaging optical system SA, and the imaging magnification βb = −3.2 in the sub-scanning section of the imaging optical system SB. Since 07 (times) and βs = βb, both satisfy the conditional expressions (7) and (7a).

  As described above, according to the optical scanning device 100 of the present embodiment, the height in the sub-scanning direction can be sufficiently reduced while reducing the number of components.

[Example 2]
11 to 13 are schematic views of the main part of an optical scanning device 200 according to Embodiment 2 of the present invention. FIG. 11 is a ZX sectional view (sub-scanning sectional view), and FIG. 12 is an XY sectional view (main scanning). FIG. 13 shows a YZ sectional view. In the optical scanning device 100 according to the first embodiment described above, the light beam Rb deflected and reflected by the deflector 5 above the reference surface P0 (+ Z side) is guided to the scanned surface 8B. On the other hand, in the optical scanning device 200 according to the present embodiment, the light beam Rb deflected and reflected by the deflector 5 below (−Z side) the reference plane P0 is guided to the scanned surface 8B. This is different from the optical scanning device 100 according to the first embodiment.

  That is, in this embodiment, as shown in FIG. 11, the light beam Rb from the imaging lens 7 is converted into the imaging lens by the mirror M1 disposed below the center position of the imaging lens 7 in the sub-scanning direction. 7 is reflected downward. Further, the light beam Rb from the mirror M1 is reflected and guided to the scanned surface 8B by the mirror M2 disposed below the center of the outer shape of the imaging lens 7 in the sub-scanning direction. With this configuration, in this embodiment, the difference in optical path length between the imaging optical system SA and the imaging optical system SB is made smaller than that in the first embodiment, while avoiding interference between the light beam Rb and the imaging lens 7. It is possible to simultaneously reduce the size of the apparatus.

  In this embodiment, the optical path length Ta = T1 = 161.114 mm corresponding to the imaging optical system SA, the optical path length corresponding to the imaging optical system SB is Tb = T2 = 197.000 mm, and T2−T1 = 35. Therefore, conditional expressions (1) and (1a) are satisfied. Further, since the convergence degree ma = m1 = 0.008 of the imaging optical system SA and the convergence degree mb = m2 = −0.254 of the imaging optical system SB, and m1−m2 = 0.26, the condition Expressions (2) and (2a) are satisfied.

  As described above, in this embodiment, since the difference in optical path length is smaller than that in the first embodiment, the difference in convergence can be reduced as compared with the first embodiment. As a result, even when the convergence degree of the imaging optical system SA is set to a small value close to 0 (ma = 0.008≈0), the light beam incident on the deflector 5 becomes a substantially parallel light beam in the main scanning section. be able to. Therefore, according to the imaging optical system SA according to the present embodiment, it is possible to suppress main scanning jitter caused by having a large degree of convergence.

  In this embodiment, since m = | m2 | = | mb | = 0.254, the conditional expressions (3) and (3a) are satisfied. Furthermore, Kθ coefficient Ka = K1 = 136.5 (mm / rad) of the imaging optical system SA, Kθ coefficient Kb = K2 = 167.0 (mm / rad) of the imaging optical system SB, and K1 / K2 = 0.82, so conditional expressions (4) and (4a) are satisfied.

  Similarly to Example 1, Tables 5 to 8 below show the specification values, optical arrangement, and surface shape of each imaging lens of the optical scanning device 200 according to this example.

  Also in the present embodiment, as in the first embodiment, in order to satisfy the condition of LD1> LD2, specifically, LD1 = 58.873 (mm) and LD2 = 8.127 (mm). The member is arranged. Further, each member is arranged so that LM1 = 42.490 (mm) and LM2 = 8.127 (mm), specifically, so as to satisfy the condition of LM1> LM2. Further, as shown in Tables 6 and 8, in this example as well as Example 1, the aspheric coefficients corresponding to the surface shapes of the first imaging unit 7A and the second imaging unit 7B are set. They are different from each other.

  Here, in this embodiment, since the optical path length difference is smaller than that in the first embodiment, the shape difference between the first imaging section 7A and the second imaging section 7B in the imaging lens 7 is implemented. This can be reduced as compared with Example 1. In the first embodiment, the shapes in the main scanning direction in the vicinity of the optical axis of the first imaging unit 7A and the second imaging unit 7B are substantially the same, whereas in the present example, the optical axis The shape in the sub-scanning direction in the vicinity is also substantially symmetrical in the vertical direction across the boundary. Specifically, the values of the generating radius R, the aspherical coefficient K, and the child curvature radius r of the incident surface are made equal for the first imaging unit 7A and the second imaging unit 7B. Yes. Furthermore, the aspherical coefficient m0_1 indicating the amount of sub-axis tilt on the optical axis is set to a value that is the same in absolute value and inverted in sign for the first imaging unit 7A and the second imaging unit 7B.

  FIG. 14A is a graph showing field curvature in the main scanning direction and the sub-scanning direction according to the present embodiment. As shown in FIG. 14A, the field curvature in the main scanning direction of the imaging optical system SA is dm = 1.9 mm, and the field curvature in the sub-scanning direction is ds = 1.6 mm. As shown in FIG. 14B, the field curvature in the main scanning direction of the imaging optical system SB is dm = 2.0 mm, and the field curvature in the sub-scanning direction is ds = 1.6 mm. From this, it can be seen that the curvature of field is satisfactorily corrected for both the imaging optical systems SA and SB.

  FIG. 15 is a graph showing the fθ characteristic dy1 according to the present embodiment. As shown in FIG. 15A, the image forming optical system SA has a maximum deviation of 0.16 mm, and as shown in FIG. 15B, the image forming optical system SB has a maximum of 0.2 mm. A deviation of 26 mm has occurred. Also in this embodiment, since there is no deviation in the fθ characteristic that greatly changes the spot diameter itself, the occurrence of uneven density in the image can be suppressed.

  FIG. 16 is a graph showing the scanning line curve dz according to the present embodiment. As shown in FIG. 16A, a maximum deviation of 7 μm occurs in the imaging optical system SA, and as shown in FIG. 16B, a maximum deviation of 6 μm occurs in the imaging optical system SB. However, none of them has a level that affects the formed image.

  FIG. 17 is a diagram illustrating a cross-sectional shape of a spot at each image height according to the present embodiment. Also in this embodiment, by optimizing the power arrangement of each lens surface and the amount of child line tilt of the entrance surface and the exit surface of the imaging lens 7, both the correction of the scanning line bending and the twist of the wavefront aberration can be achieved. Yes. Thereby, it is possible to obtain a favorable spot shape with little collapse over the entire image height.

  FIG. 18 is a diagram showing the main scanning jitter dy2 when the shift eccentricity error of the deflection surface is 10 μm. As shown in FIG. 18A, the main scanning jitter for the imaging optical system SA is 0.8 μm at the maximum, and as shown in FIG. 18B, the main scanning jitter for the imaging optical system SB is Since it is 5.1 μm at the maximum, it can be suppressed to a level where there is no problem.

  FIG. 19A is a diagram showing the difference between the generatrix shape of the first optical surface and the generatrix shape of the second optical surface of the imaging lens 7. Also in the present embodiment, the entrance surface and the exit surface of the imaging lens 7 are in a state in which there is substantially no difference in bus shape between the imaging unit 7A and the imaging unit 7B on and around the optical axis. And the surface shape are set. In addition, the bus shape of the second optical surface with the longer optical path length is separated from the deflector 5 with respect to the first optical surface with the shorter optical path length, as it goes from the optical axis to the end in the main scanning direction. It is changing to go. In this embodiment, since Xmax = 0.28 (mm) on the incident surface and Xmax = 0.44 (mm) on the exit surface, both satisfy the conditional expressions (5) and (5a).

  FIG. 19B is a diagram showing a difference between the thickness of the first imaging unit 7A and the thickness of the second imaging unit 7B. Also in the present embodiment, the thickness difference in the vicinity of the optical axis is substantially zero, and the thickness of the imaging portion 7B is larger than the thickness of the imaging portion 7A as it goes from the optical axis to the end in the main scanning direction. Is thicker. In this example, since dmax = 0.17 (mm), both satisfy the conditional expressions (6) and (6a).

  FIG. 19C is a diagram showing a step at the boundary between the image forming unit 7A and the image forming unit 7B, and the step is 600 μm or less in the entire region where the image forming lens 7 uses the light beam. In this embodiment, the imaging magnification βa in the sub-scanning section of the imaging optical system SA = −2.36 (times), and the imaging magnification βb in the sub-scanning section of the imaging optical system SB = −. Since 3.07 (times) and βs = βb, both satisfy the conditional expressions (7) and (7a).

  As described above, according to the optical scanning device 200 according to the present embodiment, the optical path length difference corresponding to the imaging optical system SA and the imaging optical system SB is set smaller than that in the first embodiment, so that good optical performance is achieved. The shape difference between the image forming unit 7A and the image forming unit 7B necessary for obtaining the image can be further reduced. As shown in FIG. 19, since the shape difference in the main scanning direction, the thickness difference, and the step difference at the boundary can be made smaller than in the first embodiment, the shape of the imaging lens 7 is asymmetric in the vertical direction. Problems at the time of injection molding such as lens warpage due to the property can be further improved.

[Example 3]
FIG. 20 is a ZX sectional view (sub-scanning sectional view) of the optical scanning device 300 according to the third embodiment of the present invention. The optical scanning device 300 according to the present embodiment is different from the optical scanning device 100 according to the first embodiment in that a multi-stage polygon mirror in which each deflection surface is divided into two upper and lower parts is adopted as the deflector 5. By adopting a multi-stage polygon mirror as the deflector 5, the light beam emitted from each light source can be incident on the deflecting surface 5 a without making an angle in the sub-scan section, and the sub-scan oblique incidence system is adopted. Compared to 1, the surface shape of the imaging lens 7 can be configured simply. Specifically, it is possible to design the imaging lens 7 without using the sub-line tilt for correcting the scan line curve and the twist of the wavefront aberration caused by the sub-scanning oblique incidence system.

[Image forming apparatus]
FIG. 21 is a schematic view (ZX cross-sectional view) of a main part of the image forming apparatus 60 according to the embodiment of the present invention. The image forming apparatus 60 is a tandem type color image forming apparatus that includes the optical scanning device 100 according to any of the above-described embodiments and records image information in parallel on the photosensitive surfaces of four photosensitive drums. .

  As shown in FIG. 21, R (red), G (green), and B (blue) color signals are output from an external device 52 such as a personal computer. These color signals are converted into image data (dot data) of Y (yellow), M (magenta), C (cyan), and K (black) by the printer controller 53 in the apparatus, and are transmitted to the optical scanning apparatus 100. Entered. The printer controller 53 controls not only the data conversion described above but also each part in the image forming apparatus 60 such as a motor described later.

  Then, the optical scanning device 100 causes the photosensitive surfaces (scanned surfaces) of the photosensitive drums (photosensitive members) 21 to 24 as image carriers to move in the main scanning direction (by the light beams 41 to 44 modulated according to each image data. Scan in the Y direction). Each of the photosensitive drums 21 to 24 is rotated clockwise by a motor (not shown), and along with this rotation, the photosensitive surface of each photosensitive drum moves in the sub-scanning direction (Z direction) with respect to the light beams 41 to 44. To do. Each of the light beams 41 to 44 exposes each photosensitive surface of each photosensitive drum charged by a charging roller (not shown), thereby forming an electrostatic latent image on each photosensitive surface.

  Thereafter, the electrostatic latent images of the respective colors formed on the photosensitive surfaces of the photosensitive drums 21 to 24 are developed as toner images of the respective colors by the developing devices 31 to 34, respectively. The toner images of the respective colors are multiplexed and transferred onto the transfer material conveyed by the conveyance belt 51 by a transfer device (not shown) and then fixed by the fixing device 70. One full color image is formed by the above process.

  For example, a color digital copying machine may be configured by connecting a color image reading apparatus including a line sensor such as a CCD sensor or a CMOS sensor to the image forming apparatus 60 as the external device 52.

[Modification]
The preferred embodiments and examples of the present invention have been described above, but the present invention is not limited to these embodiments and examples, and various combinations, modifications, and changes can be made within the scope of the gist.

  For example, in each of the embodiments described above, a polygon mirror having a plurality of deflecting surfaces is used as the deflector. However, the present invention is not limited to this, and the light beam is obtained by reciprocally oscillating one deflecting surface around the swing axis. A resonance type deflector that deflects the light may be employed. By using this resonance type deflector, it is possible to suppress the occurrence of the above-described pitch unevenness due to surface tilt and main scanning jitter due to surface eccentricity. The deflector 5 in the optical scanning device 200 according to the second embodiment may be a multistage polygon mirror as shown in the third embodiment.

  In each embodiment, the multistage lens surface of the imaging lens 7 is formed such that the first optical surface and the second optical surface are discontinuous at the boundary and have a step. However, it is not limited to this. For example, a boundary portion having continuity may be formed by expressing a region near the boundary portion in a spline shape (the first optical surface and the second optical surface are connected by a spline function), or a multistage lens. The entire surface may be expressed by a power polynomial in the main scanning direction and the sub-scanning direction.

  Further, in each embodiment, the imaging optical system is configured by two imaging lenses, but is not limited thereto, and may be configured by three or more imaging lenses or a single imaging lens. Each light source may be a monolithic multi-beam laser having a plurality of light emitting points.

5 Deflector 7 Imaging lens (multistage lens)
8A First scanned surface 8B Second scanned surface SA First imaging optical system SB Second imaging optical system M1 First mirror M2 Second mirror 100 Optical scanning device

Claims (26)

The first and second light sources and the first and second light beams emitted from the first and second light sources are deflected by the first deflecting surface, and the first and second scanned surfaces are mainly used. A deflector that scans in the scanning direction, and first and second imaging optics that condense each of the first and second light beams deflected by the deflector onto the first and second scanned surfaces. An optical scanning device comprising: a system;
The first and second imaging optical systems have a common multi-stage lens including first and second optical surfaces arranged in a sub-scanning direction on which the first and second light beams are incident. ,
The second optical path length from the first deflection surface to the second scanned surface is longer than the first optical path length from the first deflection surface to the first scanned surface,
In the main scanning section, the optical path length from the rear main plane of the first imaging optical system to the first scanned surface is Sk1 (mm), and the focal length of the first imaging optical system is f1. (Mm), Sk2 (mm) as the optical path length from the rear main plane of the second imaging optical system to the second scanned surface, and f2 (mm) as the focal length of the second imaging optical system. ), And the first convergence degree of the first imaging optical system is m1 = 1−Sk1 / f1, and the second convergence degree of the second imaging optical system is m2 = 1−Sk2 / f2, And when
0.15 <| m1-m2 | <0.50
An optical scanning device characterized by satisfying the following conditions.   The optical scanning device according to claim 1, wherein the shapes of the first and second optical surfaces are asymmetric with each other in the sub-scanning direction.   A first mirror that reflects the second light flux that has passed through the second optical surface, and a second mirror that reflects the second light flux reflected by the first mirror, The first reflection point of the first mirror and the second reflection point of the second mirror of the second light beam are located on the same side with respect to the outer shape center position of the multistage lens in the sub-scanning direction. The optical scanning device according to claim 1 or 2.   The distance from the first reflection point to the second scanned surface in the optical axis direction is longer than the distance from the first reflective point to the first scanned surface. Item 4. The optical scanning device according to Item 3.   The distance from the exit surface of the multistage lens to the first reflection point in the optical axis direction is longer than the distance from the first reflection point to the first scanned surface. 5. The optical scanning device according to 3 or 4.   6. The optical scanning according to claim 3, wherein in the sub-scanning direction, the second reflection point is located farther from the second scanning surface than the multistage lens. apparatus.   The optical path from the deflector to the multistage lens and the optical path from the second mirror to the second scanned surface intersect each other in a sub-scanning section. The optical scanning device according to any one of the above.   8. The mirror according to claim 1, wherein no mirror other than the first and second mirrors is disposed in an optical path from the first deflection surface to the second scanned surface. The optical scanning device according to Item.   9. The optical scanning device according to claim 1, wherein a lens is not disposed in an optical path from the first mirror to the second mirror. 10.   10. The optical scanning device according to claim 1, wherein a lens is not disposed in an optical path from the multistage lens to the second mirror. 10.   The first and second imaging optical systems include a common imaging lens on which each of the first and second light beams deflected by the deflector enters, and the first that has passed through the imaging lens. The optical scanning device according to claim 1, further comprising: the multistage lens on which each of the second light beams is incident. When the first optical path length is T1 (mm) and the second optical path length is T2 (mm),
25 ≦ T2-T1 ≦ 65
The optical scanning device according to claim 1, wherein the following condition is satisfied.   The optical scanning device according to claim 1, wherein the second scanned surface is disposed closer to the deflector than the first scanned surface. . In the main scanning section, the optical path length from the rear main plane of the first imaging optical system to the first scanned surface is Sk1 (mm), and the focal length of the first imaging optical system is f1. (Mm), Sk2 (mm) as the optical path length from the rear main plane of the second imaging optical system to the second scanned surface, and f2 (mm) as the focal length of the second imaging optical system. ), And the first convergence degree of the first imaging optical system is m1 = 1−Sk1 / f1, and the second convergence degree of the second imaging optical system is m2 = 1−Sk2 / f2, And the larger of | m1 | and | m2 |
0.2 <m <0.5
The optical scanning device according to claim 1, wherein the following condition is satisfied. When the Kθ coefficient of the first imaging optical system is K1, and the Kθ coefficient of the second imaging optical system is K2,
0.65 ≦ K1 / K2 ≦ 0.85
The optical scanning device according to claim 1, wherein the following condition is satisfied.   16. The optical scanning device according to claim 1, wherein the first and second optical surfaces have the same shape on the optical axis in the main scanning direction.   The relative surface shape of the second optical surface with respect to the first optical surface is a shape in which the distance from the deflector increases from the optical axis toward the end in the main scanning direction. The optical scanning device according to any one of claims 1 to 16. When the maximum value of the surface shape difference between the first optical surface and the second optical surface is Xmax (mm),
0.1 ≦ | Xmax | ≦ 5.0
The optical scanning device according to claim 1, wherein the following condition is satisfied.   The optical scanning device according to claim 1, wherein the first and second optical surfaces are arranged so as to be shifted from each other in the optical axis direction at a boundary portion.   The relative surface shape of the second optical surface with respect to the first optical surface at the boundary is such that the distance from the deflector increases from the optical axis toward the end in the main scanning direction. The optical scanning device according to claim 19.   The difference between the first thickness corresponding to the first optical surface and the second thickness corresponding to the second optical surface in the multistage lens is from the optical axis toward the end in the main scanning direction. The optical scanning device according to claim 1, wherein the optical scanning device increases in accordance with the above. When the maximum value of the difference between the first thickness corresponding to the first optical surface and the second thickness corresponding to the second optical surface in the multistage lens is dmax (mm),
0.05 ≦ | dmax | ≦ 5.0
The optical scanning device according to claim 1, wherein the following condition is satisfied. When the magnification in the sub-scan section of the second imaging optical system is βs,
2.5 ≦ | βs | <5.0
The optical scanning device according to claim 1, wherein the following condition is satisfied.   The optical scanning device according to any one of claims 1 to 23, wherein positions of surface vertices of the first and second optical surfaces coincide with each other in a main scanning direction and an optical axis direction.   25. The optical scanning device according to claim 1, and a developing device for developing, as a toner image, electrostatic latent images formed on the first and second scanned surfaces by the optical scanning device. An image forming apparatus comprising: a transfer device that transfers the developed toner image onto a transfer material; and a fixing device that fixes the transferred toner image onto the transfer material.   26. The image forming apparatus according to claim 25, further comprising a printer controller that converts color signals output from an external device into image data of different colors and inputs the image data to the optical scanning device.

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