JP5004455B2 - Multi-beam optical scanning device and image forming apparatus having the same - Google Patents

Description

  The present invention relates to a multi-beam optical scanning device used for a digital copying machine, a FAX, a laser printer, and the like, and an image forming apparatus including the device.

  In recent years, in image forming apparatuses such as digital copying machines and laser beam printers, the density and speed of image formation by optical scanning are increasing, and it is required to reduce the diameter of the beam spot on the photoconductor and to increase the number of beams. ing. Particularly in the case of multi-beam, high-speed writing can be performed without increasing the number of rotations of the polygon motor, which also saves energy.

  In addition, it is desired to promote the use of resin for constituent lenses in order to aim at low costs. By the way, in the glass lens used, the lens surface curvature, thickness, refractive index variation due to environmental temperature variation, focus position variation due to refractive index variation due to wavelength change of the semiconductor laser as the light source, and spot diameter Increases and causes image deterioration.

  Furthermore, in the lens made of resin, the surface curvature, thickness, and refractive index variation due to environmental temperature changes, and the refractive index variation due to the wavelength change of the semiconductor laser, which is the light source, are larger than the glass lens. large.

As means for solving the above problems, for example, Patent Document 1 discloses a method of correcting by combining at least three lenses in the optical system before the deflector. Patent Document 2 discloses a method of correcting by providing a diffractive surface on a scanning lens. In Patent Documents 2 to 4, a resin lens using a diffractive surface is used in front of the deflector to reduce beam spot fluctuation due to temperature change.
JP 2002-214556 A Japanese Patent Application Laid-Open No. 11-223783 JP 2004-126192 A JP 2003-337295 A

  However, as described above, Patent Document 1 discloses a technique for correcting by combining at least three lenses in the optical system before the deflector, but the problem is that the cost increases due to an increase in the number of lenses. was there. In this case, one glass lens is necessary, which also increases the cost. Further, Patent Document 2 discloses a technique for correcting a scanning lens by providing a diffractive surface. However, the scanning lens has a wide area through which a light beam passes, and it takes time to process the diffractive surface, which increases costs. There was a problem of becoming.

  As described above, in Patent Documents 2 to 4, a resin lens using a diffractive surface is used in front of the deflector to reduce beam spot fluctuation due to temperature change. However, Patent Document 3 does not consider the focus position fluctuation due to the temperature change of the optical system (scanning optical system) after the deflector. Therefore, when the scanning optical system has an optical element made of resin, the focus position fluctuation. However, there was a problem that it could not be reduced sufficiently. In Patent Documents 2 and 4, since the arrangement change of the optical elements of the first optical system is not taken into consideration, there is still a problem that the focus position fluctuation cannot be sufficiently reduced.

  In the multi-beam optical scanning device, the wavelength may be different at each light emitting point. Since the power of the diffractive surface is strongly dependent on the wavelength, in an optical system using a diffractive surface that is a conventional technology, the focus position is shifted between the beams unless the power arrangement of the diffractive surface and the refracting surface is appropriately selected. The beam spot diameter differs and image degradation occurs.

  In view of the above-described problems, the present invention provides a multi-beam optical scanning apparatus capable of obtaining a stable small beam spot regardless of low cost, energy saving, and temperature fluctuation, and image formation including the apparatus. An object is to provide an apparatus.

According to a first aspect of the present invention, a light source having a plurality of light emitting points made of a semiconductor laser, a deflecting unit for deflecting a plurality of light beams emitted from the plurality of light emitting points, and guiding the plurality of light beams to the deflecting unit. A multi-beam optical scanning device comprising: a first optical system; and a second optical system that guides the plurality of light beams deflected by the deflecting unit to a scanned surface, wherein the scanned surface is scanned with the plurality of light beams. The second optical system includes at least one resin optical element, and the first optical system includes at least a resin coupling lens having a diffraction surface for coupling a divergent light beam from the light source. And an aperture for limiting the light beam emitted from the coupling lens, and an anamorphic resin lens having a diffractive surface for condensing the light beam from the aperture at least in the sub-scanning direction. The Anamofukku diffraction surface of the resin lens grating spacing of the diffraction grating has become finer as the distance from the optical axis, a plurality of light beams to scan the same surface to be scanned, a common coupling lens, an aperture, diffraction surface And passing through an anamorphic resin lens having the following formula (G).
L2 <L / 2 Formula (G)
L: Distance between the surface near the aperture of the coupling lens and the surface closer to the aperture of the resinous lens in the first optical system closest to the aperture than the aperture L2: Image side from the aperture and the aperture The distance from the surface close to the aperture of the resinous lens in the first optical system closest to the aperture

According to a second aspect of the present invention, an image forming apparatus including the multi-beam optical scanning device according to the first aspect is provided.

  According to the present invention, it is possible to realize a multi-beam optical scanning apparatus capable of obtaining a stable small beam spot regardless of low cost, energy saving, and temperature fluctuation, and an image forming apparatus including the apparatus. .

The best mode for carrying out the present invention includes a light source having a plurality of light emitting points made of a semiconductor laser, a first optical system for guiding a plurality of light beams emitted from the plurality of light emitting points to deflecting means, and a deflection. And a plurality of second optical systems for guiding a plurality of light beams deflected by the means to the surface to be scanned, and in the multi-beam optical scanning device having at least one or more deflecting means, the first optical system has a diffraction surface. At least one resin lens is provided, at least one resin optical element is provided in the second optical system, and the plurality of light beams correspond to the first light system and the second optical system corresponding to the plurality of light beams. And a multi-beam optical scanning device satisfying the following expression (A).
| Δm′1 + Δm′2 + Δm′3 | <Wm / 2 Formula (A)
Δm′1: In the first optical system, when the light source wavelength increases by 1 nm, the power of the refracting part changes.
Main scanning beam waist position change Δm′2 due to the power change of the diffraction part when the light source wavelength is increased by 1 nm in the first optical system
Main scanning beam waist position change Δm′3: Main run due to power change when the light source wavelength is increased by 1 nm in the second optical system
Inspection beam waist position change Wm: Depth of main scanning beam diameter on scanned surface

  Preferred embodiments of the present invention will be described below in detail with reference to the accompanying drawings. The embodiments described below are preferable specific examples of the present invention, and thus various technically preferable limitations are given. However, the scope of the present invention is particularly limited in the following description. Unless otherwise described, the present invention is not limited to these embodiments.

(Embodiment 1)
First, before describing the multi-beam optical scanning apparatus of the present embodiment, a conventional multi-beam optical scanning apparatus will be described below as a comparative example with reference to FIG. FIG. 8 is a diagram for explaining a conventional multi-beam optical scanning device. In the conventional multi-beam optical scanning device described below, a scanning optical system without a diffractive optical surface is used.

  As shown in FIG. 8, the conventional multi-beam optical scanning device includes a light source (semiconductor laser) 11, a coupling lens 12, an aperture 13, an anamorphic lens 14, a polygon mirror 15, a deflector side scanning lens 16, an image plane. A side scanning lens 17, a dustproof glass 18, an image surface 19, and a soundproof glass 20 are provided.

  A light source 11 shown in FIG. 8 is a semiconductor laser with a cover glass having a thickness of 0.3 mm. As shown in FIG. 8, the light beam emitted from the light source 11 becomes weakly divergent light by the coupling lens 12, passes through the aperture 13, and is converted into parallel light in the main scanning direction by the anamorphic lens 14 forming the first optical system. In the sub-scanning direction, the light beam is focused near the polygon mirror 15.

  Then, the light is further deflected by the polygon mirror 15, and is imaged on the image surface by the deflector side scanning lens 16 and the image surface side scanning lens 17 through the dust-proof glass 18. A soundproof glass 20 is provided between the polygon mirror (deflector) 15 and the deflector side scanning lens (deflector side lens) 16. The light source 11 and the coupling lens 12 are fixed to the same member made of aluminum.

The optical system data of the conventional multi-beam optical scanning device is shown below. The light source wavelength was 780.1 nm at 25 ° C. and 786.5 nm at 45 ° C.
The coupling lens 12 is as follows. The light source side surface shape is a coaxial aspheric surface represented by the formula (1).
x = (h ^ 2 / R) / [1 + √ [1- (1 + K) (h / R) ^ 2]] + A4 ・ h ^ 4 + A6 ・ h ^ 6 + A8 ・ h ^ 8 + A10 ・ h ^ 10 ・ ・ Formula (1)

Here, the distance from the optical axis is h, the paraxial radius of curvature is R, the conic constant is K, the higher order coefficients are A4, A6, A8, A10, and the depth in the optical axis direction is x. The coefficients are as follows:
R = 86.09118
K = 361.987634
A4 = -.827025E-04
A6 = -.413360E-05
A8 = 0.942600E-06
A10 = -.936986E-07

The image side surface shape is an aspherical surface represented by Expression (1), and the coefficients are as follows.
R = -8.71000
K = -0.310240
A4 = 0.592273E-04
A6 = 0.250465E-06
A8 = 0.119847E-06
A10 = -.563217E-08

The anamorphic lens 14 is as follows. The light source side surface shape is an anamorphic surface represented by the formula (2).
x = [(1 / Rm) ・ y ^ 2 + (1 / Rs) ・ z ^ 2] / [1 + √ [1- (y / Rm) ^ 2- (z / Rs) ^ 2]] ・ ・・ Formula (2)
Here, the main scanning direction distance from the optical axis is y, the sub scanning direction distance is z, the main scanning direction radius of curvature is Rm, the sub scanning direction radius of curvature is Rs, and the depth in the optical axis direction is x. The coefficients are as follows: The side surface shape of the image plane is a plane.
Rm = 500
Rs = 35.83

The deflector side scanning lens 16 is as follows. The light source side surface shape is a coaxial aspheric surface represented by the formula (1). The coefficients are as follows:
R = -312.6
K = 2.667
A4 = 1.79E-07
A6 = -1.08E-12
A8 = -3.18E-14
A10 = 3.74E-18

The image side surface shape is a coaxial aspheric surface represented by the formula (1). The coefficients are as follows:
R = -83.0
K = 0.02
A4 = 2.50E-07
A6 = 9.61E-12
A8 = 4.54E-15
A10 = -3.03E-18
Further, the vertices on both sides are shifted by 1.16 mm upward in the figure with respect to the principal ray in FIG.

The image plane side scanning lens 17 is as follows. The light source side surface shape is a non-circular arc represented by Expression (3) with respect to the main scanning direction, and the sub-scanning radius of curvature continuously changes as represented by Expression (4) with respect to the sub-scanning direction.
x = (y ^ 2 / Rm) / [1 + √ [1- (1 + K) (y / Rm) ^ 2]] + A4 ・ y ^ 4 + A6 ・ y ^ 6 + A8 ・ y ^ 8 + A10 ・ y ^ 10 ・ Formula (3)
Here, the main scanning direction distance from the optical axis is y, the main scanning paraxial radius of curvature is Rm, the conic constant is K, the higher order coefficients are A4, A6, A8, A10, and the depth in the optical axis direction is x. .
Rs (y) = Rs + Σbj · y ^ j (j = 1, 2, 3,...) (4)

Here, the main scanning direction distance from the optical axis is y, the sub scanning radius at the main scanning direction distance y from the optical axis is Rs (y), the sub scanning radius on the optical axis is Rs, and the higher order coefficient is Let bj (j = 1, 2, 3,...). The coefficients are as follows:
Rm = -500K = -71.73
A4 = 4.33E-08
A6 = -5.97E-13
A8 = -1.28E-16
A10 = 5.73E-21
Rs = -47.7
b2 = 1.60E-03
b4 = -2.32E-07
b6 = 1.60E-11
b8 = -5.61E-16
b10 = 2.18E-20
b12 = -1.25E-24

The image surface side surface shape is a toroidal surface, and the sub-scanning shape is an arc represented by the formula (5), which is rotated around an axis parallel to the sub-scanning direction that is Rm away from the vertex of the arc in the optical axis direction. Shape.
x = (z ^ 2 / Rs) / [1 + √ [1- (z / Rs) ^ 2]] ... Formula (5)
Here, the distance in the main scanning direction from the optical axis is y, the sub-scanning paraxial radius of curvature is Rs, and the depth in the optical axis direction is x. The coefficients are as follows:
Rm = -1000
Rs = -23.38
Further, the vertices on both sides are shifted by 1.21 mm upward from the principal ray in FIG.

The surface spacing is as follows.
d1 = 12.843
d2 = 3.8
d3 = 102.8
d4 = 3.0
d5 = 69.3
d6 = 51.7
d7 = 31.4
d8 = 78.0
d9 = 3.5
d10 = 143.62

In the present optical system, calculation is performed by inserting a dust-proof glass 18 having a thickness of 1.9 mm (25 ° C.). This glass had a refractive index of 1.511161 at a light wavelength of 780.1 nm and a temperature of 25 ° C., 1.511161 at a light wavelength of 786.5 nm and a temperature of 45 ° C., and a linear expansion coefficient of 7.5E-06K-1.
The lenses were all made of the same resin material, and the refractive index was 1.523946 at a light wavelength of 780.1 nm and a temperature of 25 ° C., 1.522105 at a light wavelength of 786.5 nm and a temperature of 45 ° C., and the linear expansion coefficient was 7.0E-05K-1.

  Under the above conditions, when the image plane position relative to the image plane position is calculated in consideration of changes in the light wavelength, refractive index, surface shape, and thickness due to temperature, the following results shown in FIG. 9 are obtained. From this result, it can be seen that when the environmental temperature changes from 25 ° C. to 45 ° C., the image plane position changes significantly.

  Next, the multi-beam optical scanning device of this embodiment will be described below with reference to FIGS. FIG. 1A is a cross-sectional view in the main scanning direction of the multi-beam optical scanning device of the present embodiment. FIG. 1B is a cross-sectional view in the sub-scanning direction of the multi-beam optical scanning device of the present embodiment. The multi-beam optical scanning device of this embodiment is characterized in that a scanning optical system provided with a diffractive optical surface is used, whereas a conventional device uses a scanning optical system provided with no diffractive optical surface. .

  As shown in FIGS. 1A and 1B, the multi-beam optical scanning device of this embodiment includes a light source (semiconductor laser) 1, a coupling lens 2, an aperture 3, a first lens 4, a polygon mirror 5, and a deflection. A device side scanning lens 6, an image surface side scanning lens 7, a dustproof glass 8, an image surface 9, and a soundproof glass 10 are provided.

  A light source 1 shown in FIGS. 1A and 1B is a semiconductor laser array having a cover glass having a thickness of 0.3 mm and having two light emitting points. The light source 1 emits a plurality of semiconductor laser elements (including a semiconductor laser array) having one or more light emission points, even if a plurality of semiconductor laser elements (including a semiconductor laser array) having one or more light emission points are arranged. Even if the light beams are collected by a prism or the like, the number of light emitting points of the semiconductor laser array is not limited to two and may be more. The larger the total number of light emitting points, the faster writing is possible, which is desirable.

  The light beam emitted from the light source 1 becomes substantially parallel light by the coupling lens 2 that forms the coupling optical system, passes through the aperture 3, and is focused near the polygon mirror 5 only in the sub-scanning direction by the first lens 4 that forms the first optical system. It becomes a luminous flux. Further, the light is deflected by the polygon mirror 5 and formed on the image plane through the dust-proof glass 8 by the deflector side scanning lens 6 and the image plane side scanning lens 7. Here, all the lenses are made of resin. In particular, it is desirable to use a resin having a diffractive surface made of a resin, because a negative image of the diffractive surface can be engraved in advance on a mold and a lattice shape can be formed by injection molding or thermal transfer, enabling mass production.

The coupling lens 2 is a resin lens in which a diffractive surface in which a concentric grating is engraved on the surface on the light source side and a refractive surface having a positive power with an aspherical shape on the image side surface are provided. . The diffractive surface is opposite to the normal refracting surface in the direction of change of the refraction angle due to wavelength change. Therefore, by assigning a part of the positive power of the entire optical system to the diffractive surface, not only the focus position shift due to the temperature change that occurs because the coupling lens 2 itself is a resin, but also another resin of this optical system. The focus position shift due to the temperature change by the lens made can also be corrected.
The first lens 4 is a resin lens in which a refracting surface having a negative power only in the sub-scanning direction is provided on the light source side surface and a diffractive surface having a positive power only in the sub-scanning direction is provided on the image side surface. It is.

  The image plane position shift due to the temperature change in the main scanning direction can be corrected by the diffraction surface provided with the coupling lens 2, but the positive power of the entire optical system is stronger in the sub scanning direction than in the main scanning direction. In order to correct the image plane position shift due to the temperature change in the sub-scanning direction, further positive power of the diffractive optical element is required in the sub-scanning direction, and this is the sub-scanning direction provided in the first lens 4. It is a diffractive optical element having power only.

By the way, the power of the entire first lens 4 is determined from the target value of the beam spot diameter, and this value is based on the positive power of the diffractive optical element necessary for correcting the image plane position shift due to the temperature change in the sub-scanning direction. small. Therefore, the refractive power of the refracting surface is negative, and the power of the entire first lens 4 can be set to an appropriate value while ensuring the positive power of the diffractive optical element.
Furthermore, while the focal length of the entire system extends in the positive direction due to the temperature rise, the concave surface extends in the negative direction due to thermal expansion, and this also has an effect of reducing image plane deviation due to temperature change. The grating of the diffractive optical element of the first lens 4 is a linear grating in the sub-scanning direction.

  Next, there is a wavelength variation of about ± 1 nm between the light emitting points in the semiconductor laser element as the light source and between the semiconductor laser elements. The light refraction angle at the diffractive surface is strongly wavelength-dependent, and unless the power of the diffractive surface is selected appropriately, the focus position will shift due to the wavelength difference between the light emitting points, so the main scanning beam spot diameter will differ and the image will be remarkably different. Deteriorate.

  The object of the present invention is to use a resin lens for both the first and second optical systems, while ensuring cost reduction and improving the initial optical characteristics by expanding the degree of freedom of the shape, and further, there is no wavelength difference between the light emitting points. It is an object of the present invention to provide a multi-beam optical scanning device that can ensure optical characteristics even in such a case.

As described above, since the wavelength may vary by ± 1 nm, a wavelength change of 2 nm in width must be considered. The amount of deviation of the main scanning beam waist position between the light emitting points having different wavelengths by 2 nm is expressed by the following equation (6).
| 2 (Δm′1 + Δm′2 + Δm′3) | Equation (6)

  The allowable amount of misalignment of the main scanning beam waist is defined by defining the main scanning beam spot diameter as the maximum intensity e-2 in the light quantity distribution of the main scanning beam spot, and the main scanning beam spot diameter is the main scanning beam waist diameter. If the thickness from the main scanning beam spot diameter at the position does not fall within 10%, unacceptable image deterioration (deterioration of gradation and sharpness) occurs.

Here, when the depth width at which the main scanning beam spot diameter is 10% or less from the main scanning beam spot diameter at the main scanning beam waist position is Wm, the following equation (7) is obtained.
| 2 (Δm′1 + Δm′2 + Δm′3) | <Wm Expression (7)
Then, when formula (7) is modified, formula (A) shown below is obtained.
| Δm′1 + Δm′2 + Δm′3 | <Wm / 2 Formula (A)

  Next, an object of the present invention is to use a resin lens for both the first and second optical systems, to reduce the cost and to improve the initial optical characteristics by expanding the degree of freedom of the shape, and between the light emitting points. It is an object of the present invention to provide a multi-beam optical scanning device that can ensure optical characteristics even when there is a wavelength difference, and also taking into account refractive index changes and shape changes due to temperature fluctuations of the refractive part.

In the LD light source, the cause of the main scanning beam waist position deviation other than the oscillation wavelength shifting to the longer wavelength side due to temperature rise is as follows.
1) Due to the similar deformation due to thermal expansion, the radius of curvature of the refractive surface increases and the focal length increases.
2) Due to a decrease in refractive index due to temperature increase, the focal length is further increased on the refractive surface.
3) Due to the similar deformation due to thermal expansion, the grating spacing of the diffractive surface widens and the focal length becomes longer.
The above 1) to 3) are directions in which the focus position is shifted to the image plane side.
Therefore, since these must be offset by the increase in the light source wavelength, it is necessary to satisfy the following formula (B) .
Δm′1 + Δm′2 + Δm′3 <0 Formula (B)

  Next, as a method for reducing the beam spot position variation due to temperature variation using a resin lens having a diffractive surface in front of the deflector, there is the following. In Patent Document 3, since the focus position fluctuation due to the temperature of the optical system (scanning optical system) after the deflector is not taken into consideration, when the scanning optical system includes a resin optical element, the focus position fluctuation is actually changed. Cannot be reduced sufficiently. In Patent Documents 2 and 4, since the arrangement change of the optical elements of the first optical system is not taken into consideration, the focus position fluctuation cannot be sufficiently reduced in practice.

  The object of the present invention is to use a resin lens for both the first and second optical systems, while ensuring cost reduction and improving the initial optical characteristics by expanding the degree of freedom of the shape, and further, there is no wavelength difference between the light emitting points. Even if it exists, it is providing the multi-beam optical scanning device which can ensure the optical characteristic in consideration of the refractive index change by the temperature fluctuation of a refractive part, and a shape change.

  Since the second optical system uses a resin refractive lens, the main scanning beam waist position is generated in the direction toward the image plane (plus direction) due to shape expansion due to temperature rise, light source wavelength change, and refractive index change (Δm3). ). Further, the diffractive portion of the first optical system also generates the main scanning beam waist position in the direction toward the image plane (plus direction) due to shape expansion due to temperature rise, light source wavelength change, and refractive index change (Δm1).

  Therefore, by making the power of the diffractive portion of the first optical system positive, the main scanning beam waist position change (Δm2) due to temperature rise is made negative, and further, between the main scanning front main point of the light source 1 and the first optical system. The amount of change (Δd1) in the distance is positive, and the change in main scanning beam waist position ((−Δd1 × (f2 / f1) ^ 2)... The main scanning beam waist position fluctuation (left side of the formula (1)) in the optical scanning device is reduced.

Usually, since the temperature change in the multi-beam optical scanning device needs to be expected to be about ± 20 ° C. with respect to normal temperature, the upper limit is set to Wm / 40. (3) of the left side is more than the right side, the main scanning beam spot径太Ri is image deterioration exceeds an allowable level (gradation, deterioration of sharpness) raw sly.

  Next, there is a wavelength variation of about ± 1 nm between the light emitting points in the semiconductor laser element as the light source and between the semiconductor laser elements. The light refraction angle at the diffractive surface is strongly wavelength-dependent, and unless the power of the diffractive surface is selected appropriately, the focus position shifts due to the wavelength difference between each light emitting point, and the sub-scanning beam spot diameter varies accordingly, resulting in a marked image. Deteriorate.

The object of the present invention is to use a resin lens for both the first and second optical systems, while ensuring cost reduction and improving the initial optical characteristics by expanding the degree of freedom of the shape, and further, there is no wavelength difference between the light emitting points. It is an object of the present invention to provide a multi-beam optical scanning device that can ensure optical characteristics even in such a case. Since the wavelength may vary ± 1 nm, a wavelength change of 2 nm in width must be considered. The amount of sub-scanning beam waist position shift between the light emitting points having different wavelengths by 2 nm is expressed by the following equation (9).
| 2 (Δs′1 + Δs′2 + Δs′3) | Expression (9)

  Also, the allowable amount of sub-scanning beam waist position deviation is defined by defining the sub-scanning beam spot diameter as the maximum intensity e-2 in the light quantity distribution of the sub-scanning beam spot, and this sub-scanning beam spot diameter is the sub-scanning beam waist position. If the thickness from the sub-scanning beam spot diameter does not fall within 10%, unacceptable image deterioration (deterioration of gradation and sharpness) occurs.

Here, when the width of the depth at which the sub-scanning beam spot diameter is 10% or less from the sub-scanning beam spot diameter at the sub-scanning beam waist position is Ws, the following equation (10) is obtained.
| 2 (Δs′1 + Δs′2 + Δs′3) | <Ws Expression (10)
And if Formula (10) is changed, it will become Formula (E) shown below .
| Δs′1 + Δs′2 + Δs′3 | <Ws / 2 Formula (E)

  Next, an object of the present invention is to use a resin lens for both the first and second optical systems, to reduce the cost and to improve the initial optical characteristics by expanding the degree of freedom of the shape, and between the light emitting points. It is an object of the present invention to provide a multi-beam optical scanning device that can ensure optical characteristics even when there is a wavelength difference, and also taking into account refractive index changes and shape changes due to temperature fluctuations of the refractive part.

In the LD light source, the cause of the sub-scanning beam waist position deviation other than the oscillation wavelength shifting to the longer wavelength side due to temperature rise is as follows.
1) Due to the similar deformation due to thermal expansion, the radius of curvature of the refractive surface increases and the focal length increases.
2) Due to a decrease in refractive index due to temperature increase, the focal length is further increased on the refractive surface.
3) Due to the similar deformation due to thermal expansion, the grating spacing of the diffractive surface widens and the focal length becomes longer.
The above 1) to 3) are directions in which the focus position is shifted to the image plane side. Therefore, since these must be offset by the increase in the light source wavelength, it is necessary to satisfy the following formula (F) .
Δs′1 + Δs′2 + Δs′3 <0 Expression (F)

  Next, as a method for reducing the beam spot position variation due to temperature variation using a resin lens having a diffractive surface in front of the deflector, there is the following. In Patent Document 3, since the focus position fluctuation due to the temperature of the optical system (scanning optical system) after the deflector is not taken into consideration, when the scanning optical system includes a resin optical element, the focus position fluctuation is actually changed. Cannot be reduced sufficiently. In Patent Documents 2 and 4, since the arrangement change of the optical elements of the first optical system is not taken into consideration, the focus position fluctuation cannot be sufficiently reduced in practice.

  The object of the present invention is to use a resin lens for both the first and second optical systems, while ensuring cost reduction and improving the initial optical characteristics by expanding the degree of freedom of the shape, and further, there is no wavelength difference between the light emitting points. Even if it exists, it is providing the multi-beam optical scanning device which can ensure the optical characteristic in consideration of the refractive index change by the temperature fluctuation of a refractive part, and a shape change.

  Since the second optical system uses a resin-made refractive lens, the sub-scanning beam waist position is generated in the direction toward the image plane (plus direction) due to shape expansion due to temperature rise, light source wavelength change, and refractive index change (Δs3). ). Further, also in the diffraction part of the first optical system, the sub-scanning beam waist position is generated in the direction toward the image plane (plus direction) due to the shape expansion due to the temperature rise, the light source wavelength change, and the refractive index change (Δs1).

  Therefore, by making the power of the diffractive portion of the first optical system positive, the change in the sub-scanning beam waist position (Δs2) due to temperature rise is made negative, and further, between the light source 1 and the principal point before the sub-scanning of the first optical system. The amount of change in the distance (Δd1) is positive, and the sub-scanning beam waist position change (−Δd1 × (β2 × β1) ^ 2 Equation (11)) due to this distance change is negative, and the multi-beam due to temperature rise Sub-scanning beam waist position fluctuation (left side of equation (D)) in the optical scanning device is reduced.

Usually, since the temperature change in the multi-beam optical scanning device needs to be expected to be about ± 20 ° C. with respect to normal temperature, the upper limit is set to Ws / 40. When the left-hand side of formula (D) is more than the right side, sub scanning beam spot径太Ri is image deterioration exceeds an allowable level (gradation, deterioration of sharpness) raw sly.

  Next, an object of the present invention is to use a resin lens for both the first and second optical systems, to reduce the cost and to improve the initial optical characteristics by expanding the degree of freedom of the shape, and between the multiple beams. It is an object of the present invention to provide a multi-beam optical scanning device that can reduce pitch variation and obtain a good scanning image.

  In the multi-beam, the difference in the beam spot position in the main scanning direction disappears due to scanning, so there is no problem. However, the difference in the beam spot position in the sub scanning direction has a great influence on the optical performance as the sub scanning beam pitch. Therefore, by setting the absolute value of the horizontal magnification in the main scanning direction of the entire system to be larger than the absolute value of the horizontal magnification in the sub scanning direction, it is possible to reduce the sub scanning beam pitch error caused by the fluctuation of the light source section and obtain a good scanning image. .

In addition, since the main scanning direction is scanned at a higher speed, the dynamic main scanning beam spot diameter is formed larger than the dynamic sub-scanning beam spot. Therefore, in order to obtain the same dynamic beam spot diameter in both the main scanning and the sub scanning, it is desirable that the (static) sub scanning beam spot diameter is larger than the (static) main scanning beam spot diameter. Since the sub-scanning beam spot diameter is smaller than the main-scanning beam spot diameter, the sub-scanning beam is more susceptible to temperature fluctuations and fluctuations due to the wavelength difference between the light emitting points. large it is desirable arbitrariness.

  Next, an object of the present invention is to use a resin lens for both the first and second optical systems, to reduce the cost and to improve the initial optical characteristics by expanding the degree of freedom of the shape, and between the multiple beams. It is an object of the present invention to provide a multi-beam optical scanning device that can reduce pitch variation and obtain a good scanning image.

In the multi-beam scanning optical system, a method of rotating the light emitting point group is used to finely adjust the sub-scanning beam pitch. However, if this is done, the position will change in the main scanning direction on the lens in the first optical system as the sub-scanning beam pitch is adjusted. In order to minimize the change in the optical characteristics due to the position change in the main scanning direction, it is desirable to have optical elements whose sub-scanning sections are the same regardless of the position in the main scanning direction. Further, even when the light emitting point group is not rotated, there is an effect of minimizing the change in the optical characteristics even if the position changes in the main scanning direction on the lens in the first optical system due to attachment tolerance or the like .

  Next, an object of the present invention is to use a resin lens for both the first and second optical systems, to reduce the cost and to improve the initial optical characteristics by expanding the degree of freedom of the shape, and between the multiple beams. An object of the present invention is to provide a multi-beam optical scanning device that can reduce pitch variation, obtain a good scanning image, and save energy.

  The luminous flux from the light emitting point whose position is shifted in the sub-scanning direction passes through the coupling lens 2 to become substantially parallel light that is different in the traveling direction, and the luminous flux width is limited by the aperture 3 and has an diffractive surface. The light enters the first lens 4. Here, the diffractive surface of the anamorphic lens (the first lens 4) has a finer grating interval as the distance from the optical axis increases. In such a place where the lattice spacing is small, it is difficult to maintain optical performance in manufacturing.

Therefore, in order to maintain optical performance, it is desirable that the light beam be incident in the vicinity of the optical axis of the anamorphic lens (first lens 4). As shown in FIGS. 2A and 2B, the distance between the aperture 3 and the optical element on the image side of the aperture in order for the light beam to enter the vicinity of the optical axis of the anamorphic lens (first lens 4). Must be short. If L2 exceeds L / 2, the optical performance deteriorates beyond an allowable level, so it is necessary to satisfy the following formula (G). (Claim 1)

By configuring the image forming apparatus shown in FIG. 7 using the multi-beam optical scanning apparatus of this embodiment as described above, it is possible to output a high-quality image with high stability at low cost . Further, according to the multi-beam optical scanning device of the present embodiment, the multi-beam optical scanning device can obtain a constant beam spot even if there is a wavelength difference between the light emitting points, and has little change in the beam waist position due to temperature change. Can be provided. According to the multi-beam optical scanning device of the present embodiment, it is possible to provide a multi-beam optical scanning device that does not deteriorate in performance even when the sub-scanning beam pitch is adjusted. According to the multi-beam optical scanning device of this embodiment, a multi-beam optical scanning device with less aberration can be provided. According to the invention described in the multi-beam optical scanning device of the present embodiment , it is possible to output a good image.

Below, the optical system data of Examples 1-3 in this embodiment are shown.
<Example 1>
The light source 1 is a semiconductor laser array having two light emitting points that are 14 μm apart from each other. The light source wavelength was 655 nm at 25 ° C. and 659 nm at 45 ° C.
The coupling lens 2 is as follows.
The light source side shape is a diffractive optical element having a concentric lattice. The phase function φ (h) of the diffractive optical element is represented by the following formula (11).
φ (h) = C1 · h ^ 2 Formula (11)
Here, the distance from the optical axis is h, and the phase coefficient is C1. The coefficients are as follows:
C1 = -1.127e-02
The image side surface shape is an aspherical surface represented by Expression (1), and the coefficients are as follows.
R = -34.32865
K = -71.517137
A4 = -0.208422E-03
A6 = 0.651475E-05
A8 = -0.238199E-05
A10 = 0.770435E-08

The first lens (anamorphic lens) 4 is as follows. The light source side surface shape is a flat surface in the main scanning direction and a non-circular shape represented by Expression (7) in the sub scanning direction.
x = (z ^ 2 / Rs) / [1 + √ [1- (1 + K) (z / Rs) ^ 2]] + B4 ・ z ^ 4 + B6 ・ z ^ 6 + B8 ・ z ^ 8 + B10 ・ z ^ 10 ・ ・ ・
... Formula (12)
Here, the distance in the sub-scanning direction from the optical axis is z, the paraxial radius of curvature in the sub-scanning direction is Rs, the conic constant is K, the higher order coefficients are A4, A6, A8, A10, and the depth in the optical axis direction is x. To do.
The coefficients are as follows:
Rs = -54.46507
K = -0.072542
B4 = 0.577350E-07
B6 = 0.474038E-07
B8 = -0.190253E-07
B10 = 0.247352E-08

The image side surface shape is a diffractive optical element having a grating in the sub-scanning direction.
The phase function φ (z) of the diffractive optical surface is expressed by the following equation (13).
φ (z) = C1 · z ^ 2 Formula (13)
Here, the sub-scanning direction distance from the optical axis is z, and the phase coefficient is C1.
The coefficients are as follows:
C1 = -8.8148E-03

The deflector-side scanning lens 6 is as follows.
The light source side surface has a non-circular arc shape represented by Expression (14) in the main scanning plane.
x = (y ^ 2 / Rs) / [1 + √ [1- (1 + K) (y / Rs) ^ 2]] + A4 ・ y ^ 4 + A6 ・ y ^ 6 + A8 ・ y ^ 8 + A10 ・ y ^ 10
... Formula (14)

Here, the main scanning direction distance from the optical axis is y, the paraxial radius of curvature in the main scanning direction is Rm, the conic constant is K, the higher order coefficients are A4, A6, A8, and A10, and the depth in the optical axis direction is x. To do. The sub-scanning curvature changes as represented by Expression (15) according to the distance in the main scanning direction from the optical axis.
Cs (Y) = 1 / Rs (0) + B1 ・ Y + B2 ・ Y ^ 2 + B3 ・ Y ^ 3 + B4 ・ Y ^ 4 + B5 ・ Y ^ 5 + ・ ・ ・ Formula (15)

Here, the main scanning direction distance from the optical axis is Y, the sub-scanning direction curvature is Cs, the conic constant is K, and the higher order coefficients are B1, B2, B3, B4, and B5. The coefficients are as follows:
Rm = -279.9, Rs = -61.
K = -2.900000E + 01
A4 = 1.755765E-07
A6 = -5.491789E-11
A8 = 1.087700E-14
A10 = -3.183245E-19
A12 = -2.635276E-24

B1 = -2.066347E-06
B2 = 5.727737E-06
B3 = 3.152201E-08
B4 = 2.280241E-09
B5 = -3.729852E-11
B6 = -3.283274E-12
B7 = 1.765590E-14
B8 = 1.372995E-15
B9 = -2.889722E-18
B10 = -1.984531E-19

The image side surface shape is an aspherical surface represented by Expression (1), and the coefficients are as follows.
R = -83.6
K = -0.549157
A4 = 2.748446E-07
A6 = -4.502346E-12
A8 = -7.366455E-15
A10 = 1.803003E-18
A12 = 2.727900E-23

The image side scanning lens 7 is as follows. The light source side surface has a non-circular arc shape represented by Expression (14) in the main scanning plane. The sub-scanning curvature changes as represented by Expression (15) according to the distance in the main scanning direction from the optical axis. The coefficients are as follows:
Rm = 6950, Rs = 110.9
K = 0.000000E + 00
A4 = 1.549648E-08
A6 = 1.292741E-14
A8 = -8.811446E-18
A10 = -9.182312E-22

B1 = -9.593510E-07
B2 = -2.135322E-07
B3 = -8.079549E-12
B4 = 2.390609E-12
B5 = 2.881396E-14
B6 = 3.693775E-15
B7 = -3.258754E-18
B8 = 1.814487E-20
B9 = 8.722085E-23
B10 = -1.340807E-23

As for the image side surface shape, the surface shape in the main scanning plane is a non-arc shape represented by the equation (14). The sub-scanning curvature changes as represented by Expression (15) according to the distance in the main scanning direction from the optical axis. The coefficients are as follows:
Rm = 766, Rs = -68.22
K = 0.000000E + 00
A4 = -1.150396E-07
A6 = 1.096926E-11
A8 = -6.542135E-16
A10 = 1.984381E-20
A12 = -2.411512E-25
B2 = 3.644079E-07
B4 = -4.847051E-13
B6 = -1.666159E-16
B8 = 4.534859E-19
B10 = -2.819319E-23

The surface spacing is as follows.
d1 = 26.07144
d2 = 3.8
d3 = 92.8
d'3 = 10.0
d4 = 3.0
d5 = 121.7448
d6 = 64.007
d7 = 22.6
d8 = 75.85
d9 = 4.9
d10 = 158.71

In this optical system, calculation is performed by inserting a soundproof glass 10 and a dustproof glass 8 having a thickness of 1.9 mm (25 ° C.).
The refractive index of this glass is 1.514371 at a light wavelength of 655 nm, a temperature of 25 ° C., a light wavelength of 659 nm, a temperature of 4514 ° C., 1.514291, a light wavelength of 656 nm, a temperature of 25 ° C. of 1.514327, and a linear expansion coefficient of 7.5E-06K-1. did.
The lenses are all made of the same resin material, and the refractive index is 1.527257 at a light wavelength of 655 nm and a temperature of 25 ° C., 1.525368 at a light wavelength of 659 nm and a temperature of 45 ° C., a light wavelength of 656 nm, 1.527222 at a temperature of 25 ° C., and the linear expansion coefficient is 7.0E. -05K-1. The linear expansion coefficient of the holding member for the light source 1 and the coupling lens 2 was 4.0E-05K-1.

<Example 2>
The light source 1 is a semiconductor laser array having two light emitting points that are 14 μm apart from each other. The light source wavelength was 655 nm at 25 ° C. and 659 nm at 45 ° C.
The coupling lens 2 is made of glass and is as follows. The light source side shape is a flat surface. The image surface side shape is an aspherical surface expressed by Expression (1), and the coefficient is optimized so as to correct the wavefront aberration.
R = -18.49

The first lens 4 is as follows. The side shape of the light source is a toroidal surface, and the main scanning radius of curvature is -246.5 and the sub-scanning radius of curvature is -52.2. The image side surface shape is a diffractive optical element having an elliptical grating on a flat substrate.
The phase function φ (z) of the diffractive surface is expressed by the following equation (16).
φ (y, z) = C3 · y ^ 2 + C4 · z ^ 2 Equation (16)

Here, the main scanning direction and sub-scanning direction distances from the optical axis are y and z, and the phase coefficients are C3 and C4. The coefficients are as follows:
C3 = -0.009027
C4 = -0.001065
The surface spacing is as follows.
d1 = 24.25
d2 = 4.5
d3 = 51.71
d'3 = 10.0
d4 = 3.0
d5 = 121.7448
d6 = 64.007
d7 = 22.6
d8 = 75.85
d9 = 4.9
d10 = 158.71

  The lens data after the deflector are all the same as in the first embodiment. In this optical system, calculation is performed by inserting a soundproof glass 10 and a dustproof glass 8 having a thickness of 1.9 mm (25 ° C.). The refractive index of this glass is 1.514371 at a light wavelength of 655 nm, a temperature of 25 ° C., a light wavelength of 659 nm, a temperature of 4514 ° C., 1.514291, a light wavelength of 656 nm, a temperature of 25 ° C. of 1.514327, and a linear expansion coefficient of 7.5E-06K-1. did.

  All the lenses other than the coupling lens 2 are made of the same resin material, and the refractive index is 1.527257 at a light wavelength of 655 nm and a temperature of 25 ° C., 1.525368 at a light wavelength of 659 nm and a temperature of 45 ° C., a light wavelength of 656 nm, and a temperature of 25272 ° C. The linear expansion coefficient was 7.0E-05K-1. The coupling lens 2 is made of glass. The refractive index of the glass is 655 nm at a light wavelength of 1.689631 at a temperature of 25 ° C., 659 nm at a light wavelength, 1.689528 at a temperature of 45 ° C., 1.689581 at a light wavelength of 656 nm, and a temperature of 25 ° C. Was 7.5E-06K-1. The linear expansion coefficient of the holding member for the light source 1 and the coupling lens 2 was 2.3E-05K-1.

<Example 3>
The light source 1 is a semiconductor laser array having two light emitting points that are 14 μm apart from each other. The light source wavelength was 655 nm at 25 ° C. and 659 nm at 45 ° C.
The coupling lens 2 is made of glass and is as follows. The light source side shape is a flat surface. The image surface side shape is an aspherical surface expressed by Expression (1), and the coefficient is optimized so as to correct the wavefront aberration.
R = -18.49

The first lens 4 is as follows. The side surface shape of the light source is a shape in which a concentric diffraction grating is attached on a spherical substrate, and the substrate surface shape is a spherical surface having a curvature radius of -246.5. The phase function φ (h) of the diffractive surface is expressed by Expression (11). The coefficients are as follows:
C1 = -0.00107
The image side surface shape is a shape in which a diffraction grating having a grating in the sub-scanning direction is attached on a cylindrical surface substrate having power only in the sub-scanning direction, and the curvature radius of the substrate in the sub-scanning direction is 69.16. The phase function φ (z) of the diffractive surface is expressed by the following equation (17).
φ (z) = C2 · z ^ 2 Formula (17)

Here, the sub-scanning direction distance from the optical axis is z, and the phase coefficient is C4. The main scanning radius of curvature is -246.5 and the sub-scanning radius of curvature is -52.2. It is a diffractive optical element having an elliptical grating on a flat substrate. The coefficients are as follows:
C4 = -0.001069

The surface spacing is as follows.
d1 = 24.25
d2 = 4.5
d3 = 51.71
d'3 = 10.0
d4 = 3.0
d5 = 121.7448
d6 = 64.007
d7 = 22.6
d8 = 75.85
d9 = 4.9
d10 = 158.71

  The lens data after the deflector are all the same as in the first embodiment. In this optical system, calculation is performed by inserting a soundproof glass 10 and a dustproof glass 8 having a thickness of 1.9 mm (25 ° C.). The refractive index of this glass is 1.514371 at a light wavelength of 655 nm, a temperature of 25 ° C., a light wavelength of 659 nm, a temperature of 4514 ° C., 1.514291, a light wavelength of 656 nm, a temperature of 25 ° C. of 1.514327, and a linear expansion coefficient of 7.5E-06K-1. did.

All the lenses other than the coupling lens 2 are made of the same resin material, and the refractive index is 1.527257 at a light wavelength of 655 nm and a temperature of 25 ° C., 1.525368 at a light wavelength of 659 nm and a temperature of 45 ° C., a light wavelength of 656 nm, and a temperature of 25272 ° C. The linear expansion coefficient was 7.0E-05K-1.
The coupling lens 2 is made of glass. The refractive index of the glass is 655 nm at a light wavelength of 1.689631 at a temperature of 25 ° C., 659 nm at a light wavelength, 1.689528 at a temperature of 45 ° C., 1.689581 at a light wavelength of 656 nm, and a temperature of 25 ° C. Was 7.5E-06K-1. The linear expansion coefficient of the holding member for the light source 1 and the coupling lens 2 was 2.3E-05K-1.

  The beam spot diameters of the multi-beam optical scanning devices in Examples 1 to 3 described above are shown in FIGS. 3 (a), 3 (b) to 5 (a) and 5 (b), respectively. 3A shows the beam spot diameter in the main scanning direction of the multi-beam optical scanning apparatus according to the first embodiment, and FIG. 3B shows the beam spot diameter in the sub-scanning direction of the multi-beam optical scanning apparatus according to the first embodiment. It is a figure. 4A shows the beam spot diameter in the main scanning direction of the multi-beam optical scanning apparatus according to the second embodiment, and FIG. 4B shows the beam spot diameter in the sub-scanning direction of the multi-beam optical scanning apparatus according to the second embodiment. It is a figure. FIG. 5A shows the beam spot diameter in the main scanning direction of the multi-beam optical scanning apparatus of the third embodiment, and FIG. 5B shows the beam spot diameter in the sub-scanning direction of the multi-beam optical scanning apparatus of the third embodiment. It is a figure.

  FIG. 6 shows the characteristics of the multi-beam optical scanning devices obtained in Examples 1 to 3. The characteristics of the multi-beam optical scanning device obtained in Examples 1 to 3 shown in FIG. 6 satisfied the above-described conditional expression, and good optical performance including temperature change could be obtained.

  Next, an image forming apparatus provided with the multi-beam optical scanning device of this embodiment will be described. FIG. 7 is a cross-sectional view of an image forming apparatus provided with the multi-beam optical scanning device of this embodiment. As shown in FIG. 7, this image forming apparatus is an optical printer, and has a photoconductive photosensitive member 111 formed in a cylindrical shape as a photosensitive medium, and a charging means 112 (contact type by a charging roller) around the photosensitive member. A corona charger or a charging brush can be used), a developing device 113, a transfer means 114 (a transfer roller is shown, but a corona charger may be used), and a cleaning device 115. Have.

  Reference numeral 116 denotes a fixing device. In addition, a multi-beam optical scanning device 117 is provided, and image writing by optical scanning is performed between the charging unit 112 and the developing device 113. As the optical scanning device, the multi-beam optical scanning device of the present embodiment can be used. To perform image formation, the photosensitive member 111 is rotated at a constant speed in the direction of the arrow, and the surface thereof is uniformly charged by the charging unit 112. Then, the image is written and written by optical scanning by the multi-beam optical scanning device 117. An electrostatic latent image corresponding to the image is formed. The formed electrostatic latent image is a so-called “negative latent image” and the image portion is exposed.

  The electrostatic latent image is reversely developed by the developing device 113 and visualized as a toner image. The toner image is transferred by a transfer unit 114 onto a sheet-like recording medium S such as transfer paper or an OHP sheet, and is fixed by a fixing device 116. The sheet-like recording medium S on which the toner image has been fixed is discharged out of the apparatus, and the photoreceptor 111 after the toner image is transferred is cleaned by a cleaning device 115 to remove residual toner and paper dust.

(A) is sectional drawing of the main scanning direction of the multi-beam optical scanning device of this embodiment. FIG. 2B is a cross-sectional view in the sub-scanning direction of the multi-beam optical scanning device according to the present embodiment. (A) is sectional drawing for demonstrating the multi-beam optical scanning apparatus of this embodiment. (B) is sectional drawing for demonstrating the multi-beam optical scanning apparatus of this embodiment. (A) is a beam spot diameter in the main scanning direction of the multi-beam optical scanning apparatus of Example 1, and (b) is a diagram showing a beam spot diameter in the sub-scanning direction of the multi-beam optical scanning apparatus of Example 1. It is. (A) is the figure which showed the beam spot diameter of the multi-beam optical scanning apparatus of Example 2 in the main scanning direction, (b) is the figure which showed the beam spot diameter of the sub-scanning direction of the multi-beam optical scanning apparatus of Example 2. . (A) is the figure which shows the beam spot diameter of the multi-beam optical scanning apparatus of Example 3 in the main scanning direction, (b) is the figure which showed the beam spot diameter of the sub-scanning direction of the multi-beam optical scanning apparatus of Example 3. . It is a figure which shows the characteristic of the multi-beam optical scanning apparatus of Examples 1-3. 1 is a cross-sectional view of an image forming apparatus including a multi-beam optical scanning device according to an embodiment. It is sectional drawing for demonstrating the conventional multi-beam optical scanning apparatus. It is a figure which shows the characteristic of the conventional multi-beam optical scanning apparatus.

Explanation of symbols

1 Light source (semiconductor laser)
DESCRIPTION OF SYMBOLS 2 Coupling lens 3 Aperture 4 1st lens 5 Polygon mirror 6 Deflector side scanning lens 7 Image surface side scanning lens 8 Dustproof glass 9 Image surface 10 Soundproof glass

Claims (2)

A light source having a plurality of light emitting points made of a semiconductor laser;
Deflecting means for deflecting a plurality of light beams emitted from the plurality of light emitting points;
A first optical system for guiding the plurality of light beams to the deflecting means;
And a second optical system for guiding the scanned surface the plurality of light beams deflected by the pre-Symbol deflection means,
A multi-beam optical scanning apparatus before Symbol scanned surface is scanned by a plurality of light beams,
The second optical system has at least one resin optical element,
The first optical system includes:
At least one of the plastic of the coupling lens having a diffractive surface for coupling the divergent light beam from the previous SL source,
An aperture for limiting the luminous flux emitted from the coupling lens;
An anamorphic resin lens having a diffractive surface that condenses the light beam from the aperture in at least the sub-scanning direction;
The diffractive surface of the anamorphic resin lens has a finer grating spacing as it moves away from the optical axis, and a plurality of light beams that scan the same surface to be scanned have a common coupling lens, aperture, and diffractive surface. Pass through the anamorphic resin lens you have,
A multi-beam optical scanning device satisfying the following formula (G):

L2 <L / 2 Formula (G)
L: Distance between the surface near the aperture of the coupling lens and the surface near the aperture of the resinous lens in the first optical system closest to the aperture closer to the image than the aperture L2: the aperture And the surface of the resinous lens in the first optical system closest to the aperture on the image side of the aperture and the surface closer to the aperture   An image forming apparatus comprising the multi-beam optical scanning device according to claim 1.

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