JP7581103B2 - Optical scanning device and image forming device - Google Patents

Description

The present invention relates to an optical scanning device that is suitable for image forming devices such as laser beam printers (LBPs), digital copiers, and multifunction printers (MFPs).

Conventionally, it is known that in optical scanning devices, reflected light from the optical surface of an imaging optical element enters the scanned surface as unwanted light (ghost), thereby degrading image quality, or that the reflected light returns to the light source via a deflector as unwanted light (return light), thereby destabilizing the output of the light source.
Japanese Patent Laid-Open No. 2003-233693 discloses an optical scanning device using an imaging optical element having an optical surface (sagittal tilt surface) whose sagittal line is tilted with respect to the optical axis in order to suppress the generation of such unnecessary light.

JP 2004-157205 A

In Patent Document 1, an aspheric surface is provided for correcting scanning line curvature caused by a sagittal tilt surface in a configuration (sub-scanning oblique incidence system) in which a light beam is obliquely incident on a deflector in a sub-scanning cross section.
However, in a configuration in which a light beam is perpendicularly incident on a deflector in the sub-scanning cross section, it is difficult to sufficiently correct the degradation of optical performance caused by the sagittal tilt surface by simply providing one aspheric surface as in Patent Document 1.
Furthermore, since the definition equation of the aspheric surface in the sub-scanning cross section according to Patent Document 1 includes a third-order aspheric coefficient, the sensitivity of the optical performance to deviations of light rays on the imaging optical element becomes high.
SUMMARY OF THE PRESENT EMBODIMENT An object of the present invention is to provide an optical scanning device that can ensure sufficient optical performance while suppressing the generation of unnecessary light.

第１の結像光学素子の入射面及び出射面の少なくとも一方と第２の結像光学素子の入射面及び出射面とにおいて、ｍが０ではないＭ_ｍｎの少なくとも一つが０ではなく、
第２の結像光学素子の入射面及び出射面において、Ｍ_０１は互いに同符号であることを特徴とする。 The optical scanning device according to the present invention includes a first deflector that deflects a light beam from a first light source to scan a first scanned surface in a main scanning direction, and a first imaging optical system that includes first and second imaging optical elements and guides the light beam deflected by the first deflector to the first scanned surface, and when the intersection point with the optical axis of each of the entrance surfaces and exit surfaces of the first and second imaging optical elements is defined as the origin, the axis parallel to the optical axis is defined as the x-axis, the axis perpendicular to the optical axis in the main scanning cross section is defined as the y-axis, the axis perpendicular to the optical axis in the sub-scanning cross section is defined as the z-axis, the aspheric coefficients are M _mn , the radius of curvature in the sub-scanning cross section including the optical axis is defined as r, and the change coefficient is defined as E _i , and the shapes of the entrance surfaces and exit surfaces of the first and second imaging optical elements in the sub-scanning cross section are expressed by the following equations:

At least one of the entrance surface and the exit surface of the first imaging optical element and the entrance surface and the exit surface of the second imaging optical element is not 0, and at least one of M _mn , where m is not 0, is not 0,
The second imaging optical element is characterized in that M ₀₁ has the same sign on the entrance surface and the exit surface.

The present invention provides an optical scanning device that can ensure sufficient optical performance while suppressing the generation of unnecessary light.

3A and 3B are a main-scanning sectional development view and a sub-scanning sectional view of the optical scanning device according to the first embodiment. 5A and 5B are diagrams showing a state of reflection by an imaging optical element of the optical scanning device according to the first embodiment. 5A to 5C are sub-scanning sectional views of imaging optical elements in a conventional optical scanning device and the optical scanning device according to the first embodiment. 13 is a diagram showing the main scanning direction position dependency of the sagittal tilt angle of the optical surface of the imaging optical element in the conventional optical scanning device. 11A and 11B are diagrams showing image height dependence of optical performance in a conventional optical scanning device. 5 is a diagram showing the main-scanning direction position dependency of a sagittal tilt angle of an optical surface of an imaging optical element in the optical scanning device according to the first embodiment. FIG. 5A and 5B are diagrams showing image height dependence of optical performance in the optical scanning device according to the first embodiment. 11A to 11C are a main-scanning sectional view, a sub-scanning sectional view, and a sub-scanning sectional view of an optical scanning device according to a second embodiment. 13 is a diagram showing the main scanning direction position dependency of the sagittal tilt angle of the optical surface of the imaging optical element in the optical scanning device according to the second embodiment. FIG. 13A to 13C are diagrams showing image height dependence of optical performance in an optical scanning device according to a second embodiment. 13A and 13B are development views in a main scanning section and a sub-scanning section of an optical scanning device according to a third embodiment. FIG. 11 is a diagram showing an arrangement of light emission points of a light source in an optical scanning device according to a third embodiment. 1A and 1B are diagrams showing how main scanning jitter occurs in a conventional optical scanning device. 13A and 13B are diagrams showing image height dependence of the amount of main scanning jitter in an optical scanning device according to a third embodiment and an optical scanning device according to a comparative example. 13A to 13C are a development view in a main scanning section, a development view in a sub-scanning section, and a sub-scanning sectional view of an optical scanning device according to a fourth embodiment. FIG. 13 is a diagram showing an arrangement of light emission points of a light source in an optical scanning device according to a fourth embodiment. 13A and 13B are diagrams showing the lens shape of an imaging optical element in an optical scanning device according to a fourth embodiment. 13A and 13B are diagrams showing the main-scanning-direction position dependency of the sagittal tilt angle of the optical surface of the imaging optical element in the optical scanning device according to the fourth embodiment. 13A and 13B are diagrams showing image height dependence of the amount of main scanning jitter in an optical scanning device according to a fourth embodiment. 2 is a sub-scanning sectional view of a main part of the color image forming apparatus according to the embodiment. FIG.

The optical scanning device according to this embodiment will be described below with reference to the drawings. Note that the drawings shown below may be drawn at a scale different from the actual scale in order to make this embodiment easier to understand.

2. Description of the Related Art Conventionally, optical scanning devices have been widely used in laser beam printers (LBPs), digital copiers, and the like.
In an optical scanning device, a light beam emitted from a light source such as a laser after being optically modulated in response to an image signal is periodically deflected by a deflector formed of, for example, a rotating polygon mirror.
The deflected light beam is then focused into a spot on the photosensitive surface (surface to be scanned) of a photoconductor (image carrier) by an imaging optical system having an fθ characteristic, thereby optically scanning the photosensitive surface to record an image.

In addition, there is a demand for miniaturization and high image quality in image forming apparatuses such as laser beam printers, digital copiers, and multifunction printers.
It is also known that in an attempt to improve the image quality of an image forming apparatus, reflected light from an optical surface of an imaging optical element provided in an imaging optical system of an optical scanning device can be one factor that deteriorates image quality.

Specifically, there is a possibility that light reflected from the optical surface of the imaging optical element may re-enter the deflector and then return to the light source, causing return light that destabilizes the output of the light source.
In addition, there is a possibility that a ghost image will occur in which light reflected from the optical surface of an imaging optical element provided in the imaging optical system re-enters the deflector, then re-enters the imaging optical system, and reaches the surface to be scanned.
Furthermore, there is a possibility that reflected light from the optical surface of an imaging optical element provided in the imaging optical system may enter the optical path of another imaging optical system arranged on the opposite side, resulting in a ghost image reaching the scanned surface on the opposite side.
Furthermore, when miniaturization is attempted, optical elements are arranged in close proximity to each other, making such return light and ghosts more likely to occur.

In order to reduce such returning light and ghosts, a configuration is known in which the optical surface of the imaging optical element is tilted to suppress intrusion of reflected light from the imaging optical element into the optical path.
However, when the optical surface of the imaging optical element is tilted in this way, there is a possibility that the optical performance may be deteriorated accordingly, and in such a case, a configuration for correcting the deteriorated optical performance becomes necessary.
The optical scanning device of this embodiment aims to provide an optical scanning device and an image forming device using the same that can achieve high image quality by suppressing the occurrence of return light and ghosts while ensuring sufficient optical performance.

[First embodiment]
1A and 1B are a development view in a main scanning section of an optical scanning device 100 according to a first embodiment and a sub-scanning section of an imaging optical system, respectively.
In the following description, the main scanning direction is the direction perpendicular to the rotation axis of the deflector (rotating polygon mirror) and the optical axis of the imaging optical system (the direction in which the scanned surface is optically scanned by the deflector), and the sub-scanning direction is the direction parallel to the rotation axis of the deflector.
Further, the main scanning section is a section perpendicular to the sub-scanning direction (a section parallel to the main scanning direction and the optical axis of the imaging optical system), and the sub-scanning section is a section perpendicular to the main scanning direction (a section parallel to the sub-scanning direction and the optical axis of the imaging optical system).

As shown in Figures 1(a) and (b), the optical scanning device 100 of this embodiment includes light sources 1a, 1b, 1c and 1d (first, second, third and fourth light sources), anamorphic lenses 2a, 2b, 2c and 2d, and sub-scanning apertures 31a, 31b, 31c and 31d.
The optical scanning device 100 according to this embodiment also includes main scanning apertures 32a, 32b, 32c, and 32d, deflectors 41 and 42, and first imaging optical elements 71a, 71b, 71c, and 71d.
The optical scanning device 100 according to this embodiment also includes second imaging optical elements 72a, 72b, 72c, and 72d, and folding mirrors 7a, 7b, 7c, and 7d.

The light sources 1a to 1d are, for example, semiconductor lasers having light emitting points.
The anamorphic lenses 2a to 2d have different positive powers (refractive powers) in the main scanning cross section and the sub-scanning cross section. The anamorphic lenses convert the incident light beam into a substantially parallel light beam in the main scanning cross section and condense the light in the sub-scanning direction. Note that the substantially parallel light beam here includes a weakly divergent light beam, a weakly convergent light beam, and a parallel light beam.

The sub-scanning diaphragms 31a to 31d regulate the shape of the incident light beam in the sub-scanning direction (the width and diameter of the light beam in the sub-scanning direction).
The main scanning diaphragms 32a to 32d regulate the shape of the incident light beam in the main scanning direction (the width and diameter of the light beam in the main scanning direction).
Therefore, the incident light beam is shaped into a desired shape by the sub-scanning apertures 31a to 31d and the main-scanning apertures 32a to 32d.

The deflectors 41 and 42 are rotating polygon mirrors serving as deflection means, and rotate at a constant speed in the direction of the arrow A in the drawing. In the optical scanning device 100 according to this embodiment, the deflectors 41 and 42 are aligned in the sub-scanning direction so as to be independently rotatable.
The first imaging optical elements 71a to 71d and the second imaging optical elements 72a to 72d are, for example, imaging lenses that guide (concentrate) incident light beams onto the surfaces to be scanned 9a to 9d.
The folding mirrors 7a to 7d folding (reflect) the incident light beams toward the surfaces to be scanned 9a to 9d.

In the optical scanning device 100 according to this embodiment, the sub-scanning aperture 31a, the anamorphic lens 2a, and the main-scanning aperture 32a form a first incident optical system 75a.
Further, the sub-scanning aperture 31b, the anamorphic lens 2b, and the main-scanning aperture 32b constitute a second incident optical system 75b.
Further, the sub-scanning aperture 31c, the anamorphic lens 2c, and the main-scanning aperture 32c form a third incident optical system 75c.
Furthermore, the sub-scanning aperture 31d, the anamorphic lens 2d, and the main-scanning aperture 32d constitute a fourth incident optical system 75d.

In addition, in the optical scanning device 100 according to this embodiment, a first imaging optical system 85a is formed by the first imaging optical element 71a and the second imaging optical element 72a, and a second imaging optical system 85b is formed by the first imaging optical element 71b and the second imaging optical element 72b.
The first imaging optical element 71c and the second imaging optical element 72c constitute a third imaging optical system 85c, and the first imaging optical element 71d and the second imaging optical element 72d constitute a fourth imaging optical system 85d.

As shown in FIGS. 1A and 1B, the shapes of the light beams Ra, Rb, Rc and Rd emitted from the light sources 1a to 1d, respectively, in the sub-scanning direction are restricted by the sub-scanning stops 31a to 31d.
The light beams Ra to Rd passing through the sub-scanning stops 31a to 31d are converted into approximately parallel light beams in the main-scanning cross section by the anamorphic lenses 2a to 2d, respectively, and are condensed in the sub-scanning direction.

Next, the shapes of the light beams Ra to Rd that have passed through the anamorphic lenses 2a to 2d in the main scanning direction are restricted by the main scanning diaphragms 32a to 32d, respectively.
The light beams Ra and Rb that have passed through the main scanning stops 32a and 32b are perpendicularly incident on a first deflection surface 41a and a second deflection surface 41b of a deflector 41 (first deflector), respectively.
Moreover, the light beams Rc and Rd that have passed through the main scanning stops 32c and 32d are incident perpendicularly on the first deflecting surface 42a and the second deflecting surface 42b of the deflector 42 (second deflector), respectively.

That is, the first to fourth incident optical systems 75a and 75d are arranged so that their optical axes are parallel to the main scanning cross section perpendicular to the rotation axes of the deflectors 41 and .
The light beams Ra to Rd emitted from the light sources 1a to 1d pass through the first to fourth incident optical systems 75a and 75d, respectively, and are incident on the deflection surfaces (deflection reflection surfaces) 41a, 41b, 42a and 42b of the deflectors 41 and 42 in the main scanning cross section.
As a result, the light beams Ra to Rd are each condensed only within the sub-scanning cross section, and formed as a linear image that is long in the main scanning direction in the vicinity of the deflecting surfaces 41a, 41b, 42a, and 42b.

Then, the light beam Ra reflected and deflected by the first deflection surface 41a of the deflector 41 is focused (imaged as a light spot) on the scanned surface 9a (first scanned surface) via the first imaging optical element 71a, the folding mirror 7a, and the second imaging optical element 72a.
In addition, the light beam Rb reflected and deflected by the second deflection surface 41b of the deflector 41 is focused (imaged as a light spot) on the scanned surface 9b (second scanned surface) via the first imaging optical element 71b, the folding mirror 7b and the second imaging optical element 72b.
In addition, the light beam Rc reflected and deflected by the first deflection surface 42a of the deflector 42 is focused (imaged as a light spot) on the scanned surface 9c (third scanned surface) via the first imaging optical element 71c, the folding mirror 7c, and the second imaging optical element 72c.
In addition, the light beam Rd reflected and deflected by the second deflection surface 42b of the deflector 42 is focused (imaged as a light spot) on the scanned surface 9d (fourth scanned surface) via the first imaging optical element 71d, the folding mirror 7d, and the second imaging optical element 72d.

The deflectors 41 and 42 rotate in the direction of the arrow A in the figure, and the light spots scan the surfaces to be scanned 9a to 9d in the directions of the arrows Ba, Bb, Bc and Bd, respectively, to form electrostatic latent images.
The surfaces to be scanned 9a to 9d include, for example, photosensitive drum surfaces.

In addition, instead of the anamorphic lenses 2a to 2d used in the optical scanning device 100 according to this embodiment, a collimator lens that converts the incident light beam into a substantially parallel light beam and a cylindrical lens that focuses the light in the sub-scanning direction may be used.

Next, the characteristics of the first to fourth incident optical systems 75a to 75d and the first to fourth imaging optical systems 85a to 85d of the optical scanning device 100 according to this embodiment are shown in Table 1 below.
In Table 1, "E±x" indicates "10 ^±x ", and all coefficients not specifically indicated are zero.

ここで、Ｒは曲率半径、ｋ_ｙは離心率、Ｂ_ｉ（ｉ＝１，２，３，・・・，１６）は非球面係数である。 Here, the intersection point between each lens surface and the optical axis (the vertex of the lens surface) is taken as the origin, the axis parallel to the optical axis is taken as the x-axis, the axis perpendicular to the optical axis in the main-scanning section is taken as the y-axis, and the axis perpendicular to the optical axis in the sub-scanning section is taken as the z-axis.
In this case, the aspheric shapes (meridian shapes) of the entrance and exit surfaces of the first imaging optical elements 71a to 71d and the second imaging optical elements 72a to 72d in the main scanning cross section are expressed by the following formula (1).

Here, R is the radius of curvature, k _y is the eccentricity, and B _i (i=1, 2, 3, . . . , 16) are aspheric coefficients.

ここで、Ｍ_ｍｎ（ｍ＝０～１６及びｎ＝１～８）は非球面係数である。 The aspheric shapes (sagittal shapes) of the entrance and exit surfaces of the first imaging optical elements 71a to 71d and the second imaging optical elements 72a to 72d in the sub-scanning cross section are expressed by the following formula (2).

Here, M _mn (m=0 to 16 and n=1 to 8) are aspheric coefficients.

Moreover, the radius of curvature r' in the sub-scanning cross section changes continuously according to the y coordinate of the lens surface as expressed by the following equation (3).

Here, r is the radius of curvature in the sub-scanning cross section including the optical axis, and E _i (i=1 to 16) is a change coefficient.

は、ｎ次の子線における非球面係数と称することができる。 In addition, the following coefficients in formula (2)

can be referred to as the aspheric coefficient in the nth sagittal.

と表され、これを副走査断面内におけるチルト角（子線チルト角）と称することができる。
また、光軸上ではｙ＝０であるため、光軸を含む副走査断面内におけるチルト角はＭ_０１で表される。 In particular, the aspheric coefficients for the first sagittal are

This can be called the tilt angle in the sub-scanning cross section (sagittal tilt angle).
Moreover, since y=0 on the optical axis, the tilt angle in the sub-scanning cross section including the optical axis is represented by M ₀₁ .

As shown in Table 1, in the optical scanning device 100 according to this embodiment, the exit surfaces of the first imaging optical elements 71a to 71d, the entrance surfaces of the second imaging optical elements 72a to 72d, and the exit surfaces of the second imaging optical elements 72a to 72d have first-order aspheric surfaces in z.
That is, each optical surface is a sagittal tilt varying surface in which the sagittal tilt angle changes according to the position y in the main scanning direction.
In other words, at least one of M mn where m is not 0 is not 0 at the exit surfaces of the first imaging optical elements 71a to 71d, the entrance surfaces of the second imaging optical elements 72a to 72d, and the exit surfaces of the second imaging optical _elements 72a to 72d.

As shown in Table 1, M ₀₁ is not 0 at the entrance and exit surfaces of the second imaging optical elements 72a to 72d.
That is, the entrance and exit surfaces of the second imaging optical elements 72a to 72d have a sagittal tilt angle even on the optical axis.
Therefore, on the entrance and exit surfaces of the second imaging optical elements 72a to 72d, the origin of the shape definition and the surface vertex (the point most protruding in the optical axis direction) do not coincide with each other.

In addition, in the optical scanning device 100 according to this embodiment, the first imaging optical elements 71a to 71d have the same shape, and the first imaging optical elements 71b and 71d are arranged inverted in the sub-scanning direction relative to the first imaging optical elements 71a and 71c, respectively.
Similarly, the second imaging optical elements 72a to 72d have the same shape, and the second imaging optical elements 72b and 72d are disposed inverted in the sub-scanning direction with respect to the second imaging optical elements 72a and 72c, respectively.
In the optical scanning device 100 according to this embodiment, the first imaging optical elements 71a to 71d and the second imaging optical elements 72a to 72d are formed of plastic molded lenses.

Next, the effects of the optical scanning device 100 according to this embodiment will be described.
As shown in Figures 1(a) and (b), in the optical scanning device 100 of this embodiment, the first and third imaging optical systems 85a and 85c, and the second and fourth imaging optical systems 85b and 85d are arranged on either side of each other, sandwiching the deflectors 41 and 42.
The light beams Ra to Rd are deflected by the different deflection surfaces 41a, 41b, 42a and 42b of the deflectors 41 and 42, thereby scanning the multiple surfaces to be scanned 9a to 9d.

In such a double-sided scanning system, there is a possibility that light reflected by an optical surface of an imaging optical element provided in one imaging optical system may enter the other imaging optical system.
This results in the generation of a ghost image, in which light reflected by an imaging optical element provided on one side reaches a surface to be scanned that should not actually be scanned, i.e., a surface to be scanned on the other side.

Furthermore, there is a possibility that light reflected by the optical surfaces of the imaging optical element may re-enter the deflectors 41 and 42 and then return to the light source, resulting in return light that may cause the output of the light source to become unstable.
In addition, there is a possibility that the light reflected by the optical surface of the imaging optical element re-enters the deflectors 41 and 42 and is deflected again, resulting in a ghost image reaching the surface to be scanned via the imaging optical element.
Therefore, in a double-sided scanning system such as that used in the optical scanning device 100 according to this embodiment, there is a risk that image defects will be caused by the ghosts and return light described above.

Therefore, in the optical scanning device 100 according to this embodiment, in the second imaging optical elements 72a to 72d, where reflected light from the optical surfaces is particularly problematic, each optical surface (i.e., the entrance surface and exit surface) is designed to have a sagittal tilt angle on the optical axis.
As a result, as shown in Figures 2(a) and (b), respectively, the reflected light reflected by the incident and exit surfaces of the second imaging optical elements 72a to 72d passes above or below the deflectors 41 and 42 in the sub-scanning direction.
Therefore, in the optical scanning device 100 of this embodiment, it is possible to prevent the reflected light reflected by the entrance and exit surfaces of the second imaging optical elements 72a to 72d from entering the deflectors 41 and 42 or the imaging optical system located on the other side of the deflectors 41 and 42.

FIG. 3A shows a sub-scanning sectional view of a second imaging optical element in a conventional optical scanning device using an oblique incidence system.
FIG. 3B shows a sub-scanning sectional view of the second imaging optical elements 72a to 72d in the optical scanning device 100 according to this embodiment using a normal incidence system.

As shown in FIG. 3(a), the second imaging optical element in a conventional optical scanning device using an oblique incidence system is designed so that the sagittal tilt angle T1a of the entrance surface and the sagittal tilt angle T2a of the exit surface are usually opposite in sign or have a large difference between them. This corrects the scanning line curvature and 45-degree astigmatism (astigmatism in the 45-degree direction).

On the other hand, if such a configuration is used in the optical scanning device 100 according to this embodiment using a perpendicular incidence system, the sagittal coma aberration on the optical axis will become large, which is not preferable from the viewpoint of optical performance.
Therefore, in the optical scanning device 100 according to this embodiment, as shown in FIG. 3(b), the tilt angles T1b and T2b in the sub-scanning section including the optical axes of the entrance and exit surfaces of each of the second imaging optical elements 72a to 72d are set in the same direction.
In other words, in the optical scanning device 100 according to this embodiment, M ₀₁ has the same sign on the entrance surface and exit surface of each of the second imaging optical elements 72 a to 72 d .

FIG. 4A shows the main scanning direction position dependency of the sagittal tilt angles of the entrance surface and exit surface of the first imaging optical element in the optical scanning device of the comparative example.
FIG. 4B shows the main scanning direction position dependency of the sagittal tilt angles of the entrance surface and exit surface of the second imaging optical element in the optical scanning device of the comparative example.

5A, 5B and 5C respectively show the image height dependence of the irradiation position, sagittal coma aberration and 45-degree astigmatism, which are the optical performance of the optical scanning device of the comparative example.
The characteristics of the optical scanning device of the comparative example are shown in the following Table 2. In the optical scanning device of the comparative example, the sagittal tilt angle is set so that only the entrance surfaces and exit surfaces of the second imaging optical elements 72a to 72d become sagittal tilt changing surfaces.

As shown in FIG. 4B, in the optical scanning device of the comparative example, when the sagittal tilt angles at the entrance and exit surfaces of the second imaging optical elements 72a to 72d are set so as to correct the scanning line curvature and 45-degree astigmatism, the sagittal tilt angles at both surfaces become large near the most off-axis image height.
This causes the sagittal coma aberration to worsen in the vicinity of the most off-axis image height, as shown in FIG.

FIG. 6A shows the main scanning direction position dependency of the sagittal tilt angles of the entrance and exit surfaces of the first imaging optical elements 71a to 71d in the optical scanning device 100 according to this embodiment.
FIG. 6B shows the main scanning direction position dependency of the sagittal tilt angles of the entrance and exit surfaces of the second imaging optical elements 72a to 72d in the optical scanning device 100 according to this embodiment.
7A, 7B, and 7C respectively show the image height dependence of the irradiation position, sagittal coma aberration, and 45-degree astigmatism, which are the optical performance of the optical scanning device 100 according to this embodiment.

As shown in FIG. 6A, in the optical scanning device 100 according to this embodiment, the sagittal tilt angle is set so that the entrance surfaces and exit surfaces of the second imaging optical elements 72a to 72d, as well as the exit surfaces of the first imaging optical elements 71a to 71d, become sagittal tilt changing surfaces.
Also, as shown in Figures 6(a) and (b), in the optical scanning device 100 according to this embodiment, the sagittal tilt angles of the exit surfaces of the first imaging optical elements 71a to 71d and the sagittal tilt angles of the entrance surfaces of the second imaging optical elements 72a to 72d are set in the same direction.

In other words, in the optical scanning device 100 according to this embodiment, when the sagittal tilt angles of the exit surfaces of the first imaging optical elements 71 a to 71 d and the sagittal tilt angles of the entrance surfaces of the second imaging optical elements 72 a to 72 d are _T1 and _T2 , respectively, the following formula (4) is satisfied at each y coordinate.

As a result, the optical scanning device 100 according to this embodiment can effectively correct each optical performance, i.e., the irradiation position, sagittal coma aberration, and 45-degree astigmatism, as shown in Figures 7(a), (b), and (c).

Therefore, by adopting the configuration described above, the optical scanning device 100 according to this embodiment can effectively correct optical performance while suppressing the occurrence of ghosts and return light caused by the reflected light from the second imaging optical elements 72a to 72d.

As described above, the optical scanning device 100 according to this embodiment can reduce ghosts and return light while correcting optical performance, and can easily form good images when used in an image forming device.

[Second embodiment]
8A, 8B and 8C are a main-scanning sectional view, a sub-scanning sectional view and a sub-scanning sectional view, respectively, of an optical scanning device 200 according to the second embodiment.
In addition, since the optical scanning device 200 of the second embodiment has the same configuration as the optical scanning device 100 of the first embodiment except for the provision of dustproof glass 8a to 8d, the same components are given the same numbers and their descriptions are omitted.

The characteristics of the first to fourth incident optical systems 75a to 75d and the first to fourth imaging optical systems 85a to 85d of the optical scanning device 200 according to this embodiment are shown in Table 3 below.
In addition, the aspheric shapes of the entrance and exit surfaces of the first imaging optical elements 71a to 71d and the second imaging optical elements 72a to 72d provided in the optical scanning device 200 according to the second embodiment are expressed by the above equations (1) to (3), similarly to the optical scanning device 100 according to the first embodiment.

As shown in Figures 8(a) to (c), in the optical scanning device 200 of this embodiment, dust-proof glass 8a, 8b, 8c, and 8d (transparent members) that do not have the power to suppress the intrusion of dust, toner, etc. are provided between the second imaging optical elements 72a to 72d and the scanned surfaces 9a to 9d, respectively.
As shown in Table 3, the dust-proof glasses 8a to 8d are disposed so as to be inclined by 9.6 degrees with respect to the optical axes of the first to fourth imaging optical systems 85a to 85d, respectively.
As a result, as shown in FIG. 8C, the dust-proof glasses 8a to 8d are inclined in the same direction as the sagittal tilt on the optical axis of the second imaging optical elements 72a to 72d, respectively.
In other words, the normal to the optical axis in the sub-scanning cross section of the optical surfaces of the dustproof glasses 8a to 8d is inclined in the same direction as the normal to the optical axis in the sub-scanning cross section of the entrance surfaces and exit surfaces of the second imaging optical elements 72a to 72d.

FIG. 9A shows the main scanning direction position dependency of the sagittal tilt angles of the entrance surface and exit surface of the first imaging optical element in the optical scanning device 200 according to this embodiment.
FIG. 9B shows the main scanning direction position dependency of the sagittal tilt angles of the entrance surface and exit surface of the second imaging optical element in the optical scanning device 200 according to this embodiment.
10A, 10B, and 10C respectively show the image height dependence of the irradiation position, sagittal coma aberration, and 45-degree astigmatism, which are the optical performance of the optical scanning device 200 according to this embodiment.

As shown in FIG. 9(b), it can be seen that the sagittal tilt angle near the most off-axis image height on the entrance and exit surfaces of the second imaging optical elements 72a to 72d in the optical scanning device 200 of this embodiment is reduced compared to the optical scanning device 100 of the first embodiment.
Furthermore, as shown in FIGS. 10A to 10C, the optical scanning device 200 according to this embodiment also allows excellent correction of the optical performance, that is, the irradiation position, sagittal coma aberration, and 45-degree astigmatism.

As described above, the optical scanning device 200 according to this embodiment is provided with the dustproof glasses 8a to 8d. The dustproof glasses 8a to 8d are arranged at an inclination of 9.6 degrees with respect to the optical path so as to be inclined in the same direction as the sagittal tilt direction on the optical axis of the second imaging optical elements 72a to 72d, respectively.
This makes it possible to reduce the sagittal tilt angle near the most off-axis image height on the entrance and exit surfaces of the second imaging optical elements 72a to 72d, while suppressing the occurrence of ghosts and return light caused by reflected light from the second imaging optical elements 72a to 72d, and to satisfactorily correct the optical performance.

As described above, the optical scanning device 200 according to this embodiment can reduce ghosts and return light while correcting optical performance, making it easy to form good images when used in an image forming device.

[Third embodiment]
Consider a case where a multi-beam light source having a plurality of light emitting points is used in an optical scanning device such as in the first and second embodiments in which the optical surface of the imaging optical element is formed as a sagittal tilt changing surface.
In this case, since the multiple light-emitting points are arranged at intervals from each other in the sub-scanning direction, the positions at which the chief rays of each light beam emitted from the multiple light-emitting points enter the imaging optical element are spaced apart from each other in the sub-scanning direction.

This causes the magnifications of the individual light beams in the main scanning cross section to differ from one another, which causes the distances between the light focusing points at the on-axis image height and the off-axis image height on the scanned surface to which the light is guided to differ from one another for the individual light beams, resulting in main scanning jitter.
Therefore, an object of the present invention is to provide an optical scanning device capable of suppressing such main scanning jitter.

Figures 11(a), (b), and (c) respectively show a development view in the main scanning section of the optical scanning device 300 according to the third embodiment, a development view in the sub-scanning section of the incident optical system 75, and a development view in the sub-scanning section of the imaging optical system 85.

As shown in Figures 11(a) to (c), the optical scanning device 300 according to this embodiment includes a light source 1, an anamorphic lens 2, a sub-scanning aperture 31, a main-scanning aperture 32, a deflector 6, a first imaging optical element 71, a second imaging optical element 72, and a dustproof glass 8.

The light source 1 has a plurality of light emitting points, and may be, for example, a semiconductor laser. In the optical scanning device 300 according to this embodiment, the light source 1 has four light emitting points as described below.
The anamorphic lens 2 is a lens having different positive powers (refractive powers) in the main scanning cross section and the sub-scanning cross section, and converts the incident light beam into a substantially parallel light beam in the main scanning cross section and condenses the light beam in the sub-scanning cross section. Note that the substantially parallel light beam here includes a weakly divergent light beam, a weakly convergent light beam, and a parallel light beam.

The sub-scanning diaphragm 31 regulates the shape of the incident light beam in the sub-scanning direction (the width and diameter of the light beam in the sub-scanning direction).
The main scanning diaphragm 32 regulates the shape of the incident light beam in the main scanning direction (the width and diameter of the light beam in the main scanning direction).
Therefore, the incident light beam is shaped into a desired shape by the sub-scanning diaphragm 31 and the main-scanning diaphragm 32 .

The deflector 6 is a rotating polygon mirror serving as a deflection means, and rotates at a constant speed in the direction of the arrow A in the figure.
The first imaging optical element 71 and the second imaging optical element 72 are, for example, imaging lenses that guide (concentrate) an incident light beam onto the surface to be scanned 9 .
The dust-proof glass 8 is a flat glass that does not have the power to prevent dust, toner, and the like from entering the optical scanning device 300 .

In the optical scanning device 300 according to this embodiment, the sub-scanning aperture 31 , the anamorphic lens 2 , and the main-scanning aperture 32 form an incident optical system 75 .
In the optical scanning device 300 according to this embodiment, the first imaging optical element 71 and the second imaging optical element 72 form an imaging optical system 85 .

As shown in FIGS. 11A and 11B, the shape of the light beam R emitted from the light source 1 in the sub-scanning direction is restricted by the sub-scanning diaphragm 31.
The light beam R passing through the sub-scanning diaphragm 31 is converted by the anamorphic lens 2 into a substantially parallel light beam in the main-scanning cross section, and is condensed in the sub-scanning direction.

Next, the shape of the light beam R that has passed through the anamorphic lens 2 in the main scanning direction is restricted by the main scanning diaphragm 32 .
Then, the light beam R having passed through the main scanning diaphragm 32 is incident perpendicularly on the deflecting surface (deflecting reflecting surface) 6 a of the deflector 6 .

That is, the incident optical system 75 is disposed so that its optical axis is parallel to the main scanning cross section perpendicular to the rotation axis of the deflector 6 .
Then, the light beam R emitted from the light source 1 passes through an incident optical system 75 and is incident on the deflecting surface 6a of the deflector 6 in the main scanning cross section.
As a result, the light beam R is condensed only within the sub-scanning cross section, and is focused as a line image that is long in the main scanning direction in the vicinity of the deflecting surface 6a.

The light beam R reflected and deflected by the deflection surface 6a of the deflector 6 is then focused (imaged as a light spot) on the scanned surface 9 via the first imaging optical element 71, the second imaging optical element 72, and the dustproof glass 8.

The deflector 6 rotates in the direction of the arrow A in the figure, and the light spot scans the surface 9 to be scanned in the direction of the arrow B, thereby forming an electrostatic latent image.
The surface to be scanned 9 may be, for example, a photosensitive drum surface.

In addition, instead of the anamorphic lens 2 used in the optical scanning device 300 according to this embodiment, a collimator lens that converts the incident light beam into a substantially parallel light beam and a cylindrical lens that focuses the light in the sub-scanning direction may be used.
In the optical scanning device 300 according to this embodiment, the anamorphic lens 2, the first imaging optical element 71, and the second imaging optical element 72 are formed of plastic molded lenses.
In addition, in the optical scanning device 300 according to this embodiment, the imaging optical system 85 is composed of two imaging optical elements, but this is not limited thereto, and the same effect can be obtained even if it is composed of three or more imaging optical elements.

Figure 12 shows the arrangement of light-emitting points LD1, LD2, LD3, and LD4 in the light source 1 of the optical scanning device 300 according to this embodiment.

As shown in FIG. 12, adjacent light-emitting points among the light-emitting points LD1 to LD4 are disposed at equal intervals in both the main-scanning section and the sub-scanning section.
The light emitting points LD1 to LD4 are linearly arranged at intervals of 30 μm along a direction that forms an angle γ with respect to the main scanning direction in a cross section perpendicular to the optical axis direction, which includes the main scanning direction and the sub-scanning direction.

Here, in the optical scanning device 300 according to this embodiment, the rotation angle around the optical axis, i.e., angle γ, can be changed so that the spacing in the sub-scanning direction between each light spot imaged on the scanned surface 9 becomes the desired size depending on the resolution in the sub-scanning direction and the manufacturing error of the incident optical system 75 and the imaging optical system 85, etc.

Next, the characteristics of the incident optical system 75 and the imaging optical system 85 of the optical scanning device 300 according to this embodiment are shown in Table 4 below.

In the optical scanning device 300 according to this embodiment, a diffractive surface is formed on the entrance surface of the anamorphic lens 2, thereby suppressing fluctuations in spot diameter due to environmental changes.
Here, the phase coefficient of the diffractive surface formed on the entrance surface of the anamorphic lens 2 is expressed by the following formula (5).

Here, m is the diffraction order and _Cij is the phase coefficient. In the optical scanning device 300 according to the present embodiment, the diffraction order m is 1, i.e., the first-order diffracted light is used, which is advantageous in manufacturing, specifically, the variation in the refractive index and the variation in the wavelength during temperature rise are offset each other.

Furthermore, the aspheric shapes of the entrance and exit surfaces of the first imaging optical element 71 and the second imaging optical element 72 are expressed by the above formulas (1) to (3), similarly to the optical scanning device 100 according to the first embodiment.

Also, as shown in Table 4, in the optical scanning device 300 according to this embodiment, the entrance surface of the first imaging optical element 71, the exit surface of the first imaging optical element 71, the entrance surface of the second imaging optical element 72, and the exit surface of the second imaging optical element 72 have first-order aspheric surfaces in z.
That is, each optical surface is a sagittal tilt varying surface in which the sagittal tilt angle changes according to the position y in the main scanning direction.
In other words, at the entrance surface of the first imaging optical element 71, the exit surface of the first imaging optical element 71, the entrance surface of the second imaging optical element 72, and the exit surface of the second imaging optical element 72, at least one of M _mn , where m is not 0, is not 0.

As shown in Table 4, M ₀₁ is not 0 on the entrance surface and exit surface of the second imaging optical element 72 .
That is, the entrance surface and exit surface of the second imaging optical element 72 have a sagittal tilt angle even on the optical axis.
Therefore, on the entrance surface and exit surface of the second imaging optical element 72, the origin of the shape definition and the surface vertex (the point most protruding in the optical axis direction) do not coincide with each other.

Next, we will explain the causes of main scanning jitter in conventional optical scanning devices.

13A and 13B are developments in the sub-scanning section and the main-scanning section, respectively, showing trajectories of a plurality of light beams emitted from a light source 1 in a conventional optical scanning device.
The conventional optical scanning device shown here has the same configuration as the optical scanning device 300 according to this embodiment except for the different characteristics, so the same components are given the same numbers and the description is omitted.
13A and 13B, only the trajectories of the light beams R1 and R4 emitted from the light-emitting points LD1 and LD4, which are the farthest apart from each other among the four light-emitting points of the light source 1, are shown.

As shown in FIG. 13A, the light beams R1 and R4 emitted from the light emitting points LD1 and LD4, respectively, pass through different heights in the first imaging optical element 71 and the second imaging optical element 72 included in the imaging optical system 85.
At this time, the entrance surface and exit surface of the first imaging optical element 71 and the second imaging optical element 72 each have a radius of curvature r' and a sagittal tilt angle in the sub-scanning cross section that change according to the position y in the main scanning direction.

Therefore, when the light beams are incident on the entrance and exit surfaces of the first and second imaging optical elements 71 and 72 at different heights, the magnifications of the light beams in the main scanning cross section differ from each other.
As a result, as shown in FIG. 13B, the distance between the focal point (i.e., light spot) at the on-axis image height on the scanned surface 9 and the focal point at the most off-axis image height differs for each light beam, resulting in a main scanning jitter amount ΔY.

Therefore, in the optical scanning device 300 according to this embodiment, the amount of main scanning jitter ΔY is reduced by adopting the configuration described below.
First, the lateral magnification of the incident optical system 75 in the sub-scanning cross section is defined as βs, and the distance on the optical axis from the light source 1 to the sub-scanning stop 31 is defined as Ls.
Furthermore, the distance from the deflection point C0 (hereinafter referred to as the on-axis deflection point) on the deflection surface 6a when the deflector 6 deflects the light beam (hereinafter referred to as the on-axis light beam) that reaches the axial image height on the scanned surface 9 to the scanned surface 9 is defined as Tc.

At this time, in the optical scanning device 300 according to this embodiment, the sub-scanning diaphragm 31 is disposed so as to satisfy the following formula (6). This makes it possible to reduce the amount of separation in the sub-scanning direction of the respective light beams when they are incident on the entrance surface and the exit surface of the first imaging optical element 71 and the second imaging optical element 72.

In the optical scanning device 300 according to this embodiment, βs = 2.24, Ls = 14.4, and Tc = 153.85, which satisfies formula (6).

In the optical scanning device 300 according to this embodiment, the entrance surface and the exit surface of the first imaging optical element 71 closest to the deflector 6 are sagittal tilt changing surfaces.
Here, "the imaging optical element closest to the deflector 6" means the imaging optical element that is optically closest, i.e., the imaging optical element that is located at the position closest to the deflector 6 on the optical path from the deflector 6 to the scanned surface 9.

In addition, the refractive power of the exit surface of the first imaging optical element 71 in the main scanning cross section is the largest among the entrance surfaces and exit surfaces of the first imaging optical element 71 and the second imaging optical element 72, and the first imaging optical element 71 is suitable for correcting magnification in the main scanning cross section.
In the optical scanning device 300 according to this embodiment, by adopting the configuration described above, it is possible to reduce the magnification deviation in the main scanning cross section that occurs depending on the difference in the incident position in the sub-scanning direction of each light beam on the second imaging optical element 72.

FIG. 14 shows the image height dependency of the main scanning jitter amount ΔY in each of the optical scanning device 300 according to the present embodiment and the optical scanning device of the comparative example.
The characteristics of the optical scanning device of the comparative example are shown in Table 5 below. In the optical scanning device of the comparative example, only the exit surface of the first imaging optical element 71, the entrance surface of the second imaging optical element 72, and the exit surface of the second imaging optical element 72 are sagittal tilt changing surfaces.

As shown in FIG. 14, the optical scanning device 300 according to this embodiment is able to reduce the amount of main scanning jitter ΔY that occurs depending on the difference in the incident position in the sub-scanning direction of each light beam on the first imaging optical element 71 and the second imaging optical element 72.

As described above, the optical scanning device 300 according to this embodiment can simultaneously reduce ghosts and return light and correct optical performance including main scanning jitter, making it easy to form good images when used in an image forming device.

[Fourth embodiment]
15A and 15B are development views in the main scanning section of an optical scanning device 400 according to the fourth embodiment and in the sub-scanning section of an incident optical system, respectively.
15C and 15D are a development view and a sub-scanning sectional view, respectively, of an imaging optical system of an optical scanning device 400 according to the fourth embodiment.

As shown in FIGS. 15A to 15D, an optical scanning device 400 according to this embodiment includes light sources 1a and 1b, collimator lenses 3a and 3b, cylindrical lenses 4a and 4b, and sub-scanning apertures 31a and 31b.
The optical scanning device 400 according to this embodiment also includes main scanning apertures 32a and 32b, a deflector 6, a first imaging optical element 71, a second imaging optical element 72a and 72b, folding mirrors 81a, 81b and 82a, and dustproof glasses 8a and 8b.

The light sources 1a and 1b each have a plurality of light emitting points, and may be, for example, semiconductor lasers. In the optical scanning device 400 according to this embodiment, the light sources 1a and 1b each have four light emitting points, as shown below.
The collimator lenses 3a and 3b convert the incident light beam into a substantially parallel light beam in the main scanning cross section, where the substantially parallel light beam includes a weakly divergent light beam, a weakly convergent light beam, and a parallel light beam.
The cylindrical lenses 4a and 4b have a finite power (refractive power) in the sub-scanning cross section, and condense the incident light beam in the sub-scanning cross section.

The sub-scanning stops 31a and 31b regulate the shape of the incident light beam in the sub-scanning direction (the width and diameter of the light beam in the sub-scanning direction).
The main scanning diaphragms 32a and 32b regulate the shape of the incident light beam in the main scanning direction (the width and diameter of the light beam in the main scanning direction).

The deflector 6 is a rotating polygon mirror serving as a deflection means, and rotates at a constant speed in the direction of the arrow A in the figure.
The first imaging optical element 71a and the second imaging optical element 72a are, for example, imaging lenses that guide (focus) the incident light beam to the scanned surface 9a. Similarly, the first imaging optical element 71b and the second imaging optical element 72b are, for example, imaging lenses that guide (focus) the incident light beam to the scanned surface 9b.
The folding mirrors 81a, 81b, and 82a folding (reflecting) the incident light beams toward the surfaces to be scanned 9a and 9b.
The dust-proof glasses 8 a and 8 b are flat glasses that do not have the power to prevent dust, toner, and the like from entering the optical scanning device 400 .

In the optical scanning device 400 according to this embodiment, a first incident optical system 75a is configured by the sub-scanning aperture 31a, the collimator lens 3a, the cylindrical lens 4a, and the main-scanning aperture 32a.
Further, the sub-scanning aperture 31b, the collimator lens 3b, the cylindrical lens 4b, and the main-scanning aperture 32b form a second incident optical system 75b.
The first imaging optical element 71 and the second imaging optical element 72a constitute a first imaging optical system 85a.
Furthermore, the first imaging optical element 71 and the second imaging optical element 72b constitute a second imaging optical system 85b.

As shown in FIGS. 15A and 15B, the shapes of the light beams Ra and Rb emitted from the light sources 1a and 1b, respectively, in the sub-scanning direction are restricted by the sub-scanning stops 31a and 31b.
The light beams Ra and Rb that have passed through the sub-scanning stops 31a and 31b, respectively, are converted into substantially parallel light beams in the main-scanning cross section by the collimator lenses 3a and 3b.
The light beams Ra and Rb that have passed through the collimator lenses 3a and 3b, respectively, are condensed in the sub-scanning cross section by cylindrical lenses 4a and 4b.

Next, the shapes of the light beams Ra and Rb that have passed through the cylindrical lenses 4a and 4b, respectively, in the main scanning direction are restricted by the main scanning diaphragms 32a and 32b.
Then, the light beam Ra that passes through the main scanning aperture 32a is obliquely incident on the deflection surface (deflection reflection surface) 6a of the deflector 6 from above in the sub-scanning direction, and the light beam Rb that passes through the main scanning aperture 32b is obliquely incident on the deflection surface (deflection reflection surface) 6a of the deflector 6 from below in the sub-scanning direction.

That is, in the optical scanning device 400 according to this embodiment, the first and second incident optical systems 75a and 75b are each arranged so that their optical axes form an angle in the sub-scanning section with respect to the main-scanning section perpendicular to the rotation axis of the deflector 6.
The light beams Ra and Rb emitted from the light sources 1a and 1b, respectively, pass through the first and second incident optical systems 75a and 75b and are obliquely incident on the deflecting surface 6a of the deflector 6 in the sub-scanning section.
As a result, the light beams Ra and Rb are each condensed only within the sub-scanning cross section, and are focused as elongated line images in the main scanning direction in the vicinity of the deflecting surface 6a.
In the optical scanning device 400 according to this embodiment, the angles that the optical axes of the first and second incident optical systems 75a and 75b make in the sub-scanning cross section with respect to the main-scanning cross section are +3.0° and −3.0°, respectively.

Then, the light beam Ra reflected and deflected by the deflection surface 6a of the deflector 6 is focused (imaged as a light spot) on the scanned surface 9a via the first imaging optical element 71, the folding mirrors 81a and 82a, the second imaging optical element 72a and the dustproof glass 8a.
Similarly, the light beam Rb reflected and deflected by the deflection surface 6a of the deflector 6 is focused (imaged as a light spot) on the scanned surface 9b via the first imaging optical element 71, the second imaging optical element 72b, the folding mirror 81b and the dustproof glass 8b.

The deflector 6 then rotates in the direction of the arrow A in the figure, and the light spot scans the surfaces 9a and 9b to be scanned in the direction of the arrow B, thereby forming an electrostatic latent image.
The surfaces to be scanned 9a and 9b include, for example, photosensitive drum surfaces.

FIG. 16A shows an arrangement of light-emitting points LD1a, LD2a, LD3a, and LD4a in a light source 1a (first light source) of an optical scanning device 400 according to this embodiment.
FIG. 16B shows the arrangement of light-emitting points LD1b, LD2b, LD3b, and LD4b in a light source 1b (second light source) of an optical scanning device 400 according to this embodiment.

As shown in FIG. 16A, adjacent light-emitting points among the light-emitting points LD1a to LD4a are disposed at equal intervals in both the main-scanning section and the sub-scanning section.
Similarly, as shown in FIG. 16B, adjacent light-emitting points LD1b to LD4b are disposed at equal intervals in both the main-scanning cross section and the sub-scanning cross section.

The light emitting points LD1a to LD4a are linearly arranged at intervals of 30 μm along a direction that forms an angle γa with the main scanning direction in a cross section perpendicular to the optical axis direction, including the main scanning direction and the sub-scanning direction.
The light emitting points LD1b to LD4b are linearly arranged at intervals of 30 μm along a direction that forms an angle γb with the main scanning direction in a cross section perpendicular to the optical axis direction, including the main scanning direction and the sub-scanning direction.
In the optical scanning device 400 according to this embodiment, the angles γa and γb have opposite signs.

Figure 17 shows the lens shape of the first imaging optical element 71 in the optical scanning device 400 according to this embodiment.

As shown in Figure 17, on the incident side of the first imaging optical element 71, a first incident surface 711b on which the light beam Rb is incident and a second incident surface 711a on which the light beam Ra is incident are arranged side by side in the sub-scanning direction.
Further, on the exit side of the first imaging optical element 71, a first exit surface 712b from which the light beam Rb exits and a second exit surface 712a from which the light beam Ra exits are arranged side by side in the sub-scanning direction.
That is, in the optical scanning device 400 according to this embodiment, the first imaging optical element 71 is formed as a multi-stage lens through which the light beams Ra and Rb pass, respectively, on the lower and upper sides in the sub-scanning direction.

Next, the characteristics of the first and second incident optical systems 75a and 75b and the first and second imaging optical systems 85a and 85b of the optical scanning device 400 according to this embodiment are shown in Table 6 below.
All coefficients not specifically indicated are zero.

In the optical scanning device 400 according to this embodiment, a diffractive surface is formed on the exit surface of each of the cylindrical lenses 4a and 4b, thereby suppressing fluctuations in spot diameter due to environmental changes.
The phase coefficient of the diffractive surface formed on the exit surface of each of the cylindrical lenses 4a and 4b is expressed by the above formula (5).

The aspheric shapes of the entrance and exit surfaces of the first imaging optical element 71 and the second imaging optical elements 72a and 72b are expressed by the above formulas (1) to (3), similar to the optical scanning device 100 according to the first embodiment.

In addition, in the optical scanning device 400 of this embodiment, similar to the optical scanning device 300 of the third embodiment, in order to reduce the main scanning jitter amount ΔY, the sub-scanning apertures 31a and 31b are arranged so as to satisfy the above formula (6).
In the optical scanning device 400 according to this embodiment, βs=3.1, Ls=9.9, and Tc=240, which satisfies the formula (6).
This makes it possible to reduce the amount of separation in the sub-scanning direction of the light beams when they are incident on the entrance and exit surfaces of the first imaging optical element 71 and the second imaging optical elements 72a and 72b.

Also, as shown in Table 6, in the optical scanning device 400 according to this embodiment, the first entrance surface 711b, the first exit surface 712b, the second entrance surface 711a, and the second exit surface 712a of the first imaging optical element 71 have first-order aspheric surfaces in z.
That is, each optical surface is a sagittal tilt varying surface in which the sagittal tilt angle changes according to the position y in the main scanning direction.
In other words, at the first entrance surface 711b, the first exit surface 712b, the second entrance surface 711a, and the second exit surface 712a of the first imaging optical element 71, at least one of M _mn where m is not 0 is not 0.

Similarly, the entrance and exit surfaces of the second imaging optical element 72a and the entrance and exit surfaces of the second imaging optical element 72b have first-order aspheric surfaces in z.
That is, each optical surface is a sagittal tilt varying surface in which the sagittal tilt angle changes according to the position y in the main scanning direction.
In other words, at least one of M _mn where m is not 0 is not zero on the entrance surface and exit surface of the second imaging optical element 72a and the entrance surface and exit surface of the second imaging optical element 72b.

Furthermore, as shown in Table 6, in the optical scanning device 400 according to this embodiment, M01 is not ₀ at the first entrance surface 711b, the first exit surface 712b, the second entrance surface 711a, and the second exit surface 712a of the first imaging optical element 71.
That is, the first entrance surface 711b, the first exit surface 712b, the second entrance surface 711a, and the second exit surface 712a of the first imaging optical element 71 have a sagittal tilt angle even on the optical axis.
Therefore, at the first entrance surface 711b, the first exit surface 712b, the second entrance surface 711a, and the second exit surface 712a of the first imaging optical element 71, the origin of the shape definition and the surface vertex (the point most protruding in the optical axis direction) do not coincide with each other.

Similarly, M ₀₁ is not 0 at the entrance and exit surfaces of the second imaging optical element 72a and at the entrance and exit surfaces of the second imaging optical element 72b.
That is, the entrance surface and exit surface of the second imaging optical element 72a and the entrance surface and exit surface of the second imaging optical element 72b have a sagittal tilt angle even on the optical axis.
Therefore, at the entrance surface and exit surface of the second imaging optical element 72a, and the entrance surface and exit surface of the second imaging optical element 72b, the origin of the shape definition and the surface vertex (the point most protruding in the optical axis direction) do not coincide with each other.

FIG. 18A shows the main scanning direction position dependency of the sagittal tilt angle of the first incidence surface 711b and the second incidence surface 711a of the first imaging optical element 71 in the optical scanning device 400 according to this embodiment.
FIG. 18B shows the main scanning direction position dependency of the sagittal tilt angle of the first exit surface 712b and the second exit surface 712a of the first imaging optical element 71 in the optical scanning device 400 according to this embodiment.

As shown in FIG. 18A, the shapes of the first entrance surface 711b and the second entrance surface 711a of the first imaging optical element 71 are different from each other.
As shown in FIG. 18B, the shapes of the first exit surface 712b and the second exit surface 712a of the first imaging optical element 71 are different from each other.

Here, the sagittal tilt angle of the first incident surface 711b of the first imaging optical element 71 is denoted as T _i1 , and the sagittal tilt angle of the second incident surface 711a of the first imaging optical element 71 is denoted as T _i2 .
At this time, the optical scanning device 400 according to this embodiment satisfies the following formula (7) at each of the main scanning direction positions y.

By satisfying equation (7), the optical scanning device 400 of this embodiment can ensure a sufficient distance in the sub-scanning direction of the light beam Rb from the folding mirror 81a at the position of the folding mirror 81a so that the light beam Rb does not enter the folding mirror 81a.
This makes it possible to suppress interference caused by the arrangement of optical elements in the optical scanning device 400 according to this embodiment.

In the optical scanning device 400 according to this embodiment, the angles γ a and γ b that the arrangement direction of the light-emitting points LD1 a to LD4 a of the light source 1 a and the arrangement direction of the light-emitting points LD1 b to LD4 _{b of the light source 1 b make} with respect to the main scanning direction in a cross section perpendicular to the optical axis satisfy the following formula (8).

The optical scanning device 400 according to this embodiment satisfies equation (8), thereby reducing the difference between the amount of main scanning jitter ΔY that occurs on each of the scanned surfaces 9a and 9b.

Figure 19 shows the image height dependence of the main scanning jitter amount ΔY in the optical scanning device 400 according to this embodiment.

As shown in FIG. 19, the optical scanning device 400 according to this embodiment also reduces the amount of main scanning jitter ΔY that occurs depending on the difference in the incident position in the sub-scanning direction of each light beam on the first imaging optical element 71 and the second imaging optical element 72.

In this way, in the optical scanning device 400 according to this embodiment, by adopting the configuration described above, it is possible to achieve high image quality by reducing main scanning jitter.
Moreover, by suppressing interference caused by the arrangement of optical elements, compactness can be achieved.

As described above, the optical scanning device 400 according to this embodiment can reduce ghosts and return light while correcting optical performance including main scanning jitter even when using an oblique incidence system, and can easily form good images when used in an image forming device.

[Image forming apparatus]
FIG. 20 is a schematic cross-sectional view of a main part of a color image forming apparatus 360 including an optical scanning device 311 according to the first embodiment.

The color image forming apparatus 360 is a tandem type color image forming apparatus in which an optical scanning device 311 records image information on the surface of a photosensitive drum, which is an image carrier.
The color image forming apparatus 360 includes the optical scanning device 311 according to the first embodiment, photosensitive drums 341 , 342 , 343 and 344 as image carriers, developing units 321 , 322 , 323 and 324 , a conveyor belt 351 , a printer controller 353 and a fixing unit 354 .

To the color image forming apparatus 360, R (red), G (green), and B (blue) color signals are input from an external device 352 such as a personal computer.
These color signals are then converted by a printer controller 353 within the device into image data (dot data) of C (cyan), M (magenta), Y (yellow), and K (black).

The acquired image data is input to the optical scanning device 311, and the optical scanning device 311 emits light beams 331, 332, 333, and 334 modulated in accordance with the image data.
These light beams scan the photosensitive surfaces of the photosensitive drums 341, 342, 343 and 344 in the main scanning direction.

Then, light beams 331, 332, 333 and 334 are emitted by the optical scanning device 311 based on the image data, and latent images of each color are formed on the photosensitive surfaces of the corresponding photosensitive drums 341, 342, 343 and 344, respectively.
Thereafter, the latent images of the respective colors are developed into toner images of the respective colors by developers 321, 322, 323 and 324.
Then, the toner images of each color developed on the recording material (transfer material) transported by the transport belt 351 are multi-transferred by a transfer device (not shown), and the transferred toner images are fixed by a fixing device 354 to form a single full-color image.

The color image forming apparatus 360 according to this embodiment records image signals (image information) in parallel on the photosensitive surfaces of photosensitive drums 341, 342, 343, and 344, each of which corresponds to one of the colors C, M, Y, and K, using an optical scanning device 311, thereby printing a color image at high speed.
That is, in the color image forming apparatus 360 according to this embodiment, as described above, latent images of each color are formed on the corresponding photosensitive drum surfaces using light beams based on each image data by the optical scanning device 311. After that, the latent images are transferred onto a recording material in a multi-layered manner to form one full-color image.

In addition, in the color image forming device 360 of this embodiment, the optical scanning device of the second embodiment may be used instead of the optical scanning device 311 of the first embodiment, or four optical scanning devices of the third embodiment or two optical scanning devices of the fourth embodiment may be used.
Also, for example, a color image reading device equipped with a CCD sensor can be used as the external device 352. In this case, the color image reading device and the color image forming device 360 constitute a color digital copying machine.

Although the above describes preferred embodiments, the present invention is not limited to these embodiments and various modifications and variations are possible within the scope of the invention.

1a, 1b, 1c, 1d Light source (first light source)
41, 42 Deflector (first deflector)
9a, 9b, 9c, 9d: Scanned surface (first scanned surface)
71a, 71b, 71c, 71d First imaging optical elements 72a, 72b, 72c, 72d Second imaging optical elements 85a, 85b, 85c, 85d Imaging optical system (first imaging optical system)
100 Optical scanning device Ra, Rb, Rc, Rd Light beam

Claims (17)

a first deflector that deflects a light beam from a first light source to scan a first scanned surface in a main scanning direction;
a first imaging optical system including first and second imaging optical elements and guiding the light beam deflected by the first deflector to the first scanned surface;
With regard to each of the entrance surfaces and exit surfaces of the first and second imaging optical elements, when the point of intersection with the optical axis is the origin, the axis parallel to the optical axis is the x-axis, the axis perpendicular to the optical axis in the main scanning section is the y-axis, the axis perpendicular to the optical axis in the sub-scanning section is the z-axis, the aspheric coefficient is M _mn , the radius of curvature in the sub-scanning section including the optical axis is r, and the change coefficient is E _i , and the shapes of each of the entrance surfaces and exit surfaces of the first and second imaging optical elements in the sub-scanning section are expressed by the following equations:

At least one of the entrance surface and the exit surface of the first imaging optical element and the entrance surface and the exit surface of the second imaging optical element is not 0, and at least one of M _mn , where m is not 0, is not 0,
2. The optical scanning device according to claim 1, wherein _M01 has the same sign on the entrance surface and the exit surface of said second imaging optical element. At least one of the entrance surface and the exit surface of the first imaging optical element and the entrance surface of the second imaging optical element

When the values are _T1 and _T2 , at each coordinate of the y-axis,
_T1 × _T2 ≧0
2. The optical scanning device according to claim 1, wherein the following conditions are satisfied: 3. The optical scanning device according to claim 1, wherein _M01 is 0 on at least one of the entrance surface and the exit surface of the first imaging optical element. a transmitting member that transmits a light beam from the first imaging optical system,
An optical scanning device as described in any one of claims 1 to 3, characterized in that the normal on the optical axis of the optical surface of the transparent member in the sub-scanning section is inclined in the same direction as the normal on the optical axis of the entrance surface and exit surface of the second imaging optical element in the sub-scanning section. The optical scanning device according to any one of claims 1 to 4, characterized in that it is provided with a first incident optical system that causes a light beam from the first light source to be perpendicularly incident on a first deflection surface of the first deflector in the sub-scanning cross section. a second incidence optical system that causes a light beam from a second light source to be perpendicularly incident on a second deflection surface of the first deflector in a sub-scanning cross section;
a second imaging optical system that guides the light beam deflected by the second deflection surface to a second scanned surface,
6. The optical scanning device according to claim 1, wherein the first deflector deflects a light beam emitted from the second light source to scan the second surface to be scanned in a main scanning direction. a second deflector that deflects the light beams from the third and fourth light sources to scan the third and fourth scanned surfaces in the main scanning direction;
third and fourth incident optical systems that make the light beams from the third and fourth light sources perpendicularly incident on the first and second deflection surfaces of the second deflector in the sub-scanning cross section;
third and fourth imaging optical systems that guide the light beams deflected by the first and second deflecting surfaces of the second deflector to the third and fourth scanned surfaces;
7. The optical scanning device according to claim 1, further comprising: a first incidence optical system having a sub-scanning diaphragm that restricts a beam diameter in a sub-scanning direction of a plurality of beams emitted from the first light source including a plurality of light emitting points, and that causes the plurality of beams to be incident on a first deflection surface of the first deflector;
At least one of M _mn , where m is not 0, is not 0 on the entrance surface and the exit surface of the first imaging optical element,
When the lateral magnification of the first incident optical system in the sub-scanning cross section is βs, the distance on the optical axis from the first light source to the sub-scanning stop is Ls, and the distance from the axial deflection point of the first deflector to the first scanned surface is Tc,
Ls≦Tc/(βs) ²
2. The optical scanning device according to claim 1, wherein the following conditions are satisfied: The optical scanning device according to claim 8, characterized in that the first imaging optical element has an optical surface that has the largest positive refractive power in the main scanning cross section in the first imaging optical system. The optical scanning device according to claim 8 or 9, characterized in that the first imaging optical element has an optical surface that has the smallest positive refractive power in the sub-scanning cross section in the first imaging optical system. The optical scanning device according to any one of claims 8 to 10, characterized in that the first incident optical system makes the plurality of light beams obliquely incident on the first deflector in the sub-scanning cross section. a second incidence optical system that makes a plurality of light beams from a second light source including a plurality of light emitting points obliquely incident on the first deflection surface of the first deflector;
a second imaging optical system that guides the plurality of light beams from the second light source deflected by the first deflection surface to a second scanned surface;
Equipped with
12. The optical scanning device according to claim 11, wherein the first deflector deflects a plurality of light beams emitted from the second light source to scan the second scanned surface in a main scanning direction. The optical scanning device according to claim 12, characterized in that the second imaging optical system includes the first imaging optical element. a plurality of light beams from the first and second light sources are incident on first and second entrance surfaces of the first imaging optical element and then exit from first and second exit surfaces;
At each of the first and second entrance surfaces

When the values are T _i1 and T _i2 , at each coordinate of the y-axis,
T _i1 ×T _i2 ≦0
14. The optical scanning device according to claim 13, wherein the following condition is satisfied: In a cross section perpendicular to the optical axis, when the angle that the arrangement direction of the plurality of light emitting points of the first light source makes with respect to the main scanning direction is γ _a and the angle that the arrangement direction of the plurality of light emitting points of the second light source makes with respect to the main scanning direction is γ _b ,
γ _a / γ _b < 0
15. The optical scanning device according to claim 12, wherein the following condition is satisfied: An image forming apparatus comprising the optical scanning device according to any one of claims 1 to 15, a developing unit that develops an electrostatic latent image formed on the first scanned surface by the optical scanning device into a toner image, a transfer unit that transfers the developed toner image to a transfer material, and a fixing unit that fixes the transferred toner image to the transfer material. An image forming apparatus comprising the optical scanning device according to any one of claims 1 to 16, and a printer controller that converts a signal output from an external device into image data and inputs the image data to the optical scanning device.

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