JP2001350116A - Scanning optical system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a scanning optical system in which a correction element for correcting the chromatic difference of magnification can be arranged between a polygon mirror and a scanning lens without causing a ghost light or the curvature of a scanning line. SOLUTION: The laser beam emitted by a laser light source 10 is made incident on respective reflecting surfaces 14a of a polygon mirror 14 through a collimator lens 11 and a cylindrical lens 12 and then reflected. The laser beam reflected by a reflecting surface 14a of the polygon mirror 14 is transmitted through a diffracting element 16 to form a scanning line on a scanning object surface 40 through an fθ lens 20 and a correcting lens 30. A diffraction structure for chromatic difference of magnification correction is formed on the rear surface of the diffracting element 16. This diffracting element 16 is obliquely arranged so that reflected light on its front surface will not impinge on the polygon mirror 14 again. To correct the curvature o the scanning line resulting from the oblique arrangement, the base shapes of the front and rear surfaces of this diffracting element 16 are made to be special shapes.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning optical system used in an optical scanning unit such as a laser printer or a scanning drawing device for semiconductor element circuit patterns.

[0002]

2. Description of the Related Art Generally, an optical scanning unit deflects and scans a light beam from a laser light source using a deflector such as a polygon mirror, and scans a scanning target surface such as a photosensitive drum through a scanning lens such as an fθ lens. An image is formed as a spot on the top. The spot on the surface to be scanned is scanned in the main scanning direction with the rotation of the polygon mirror. At this time, the laser beam is turned on and off to form a part of the electrostatic latent image on the surface to be scanned. Form a line.

In a scanning optical system used in such an optical scanning unit, it is necessary to reduce lateral chromatic aberration of the optical system in order to suppress a change in drawing performance due to a variation in emission wavelength of a semiconductor laser. Generally, chromatic aberration is corrected by combining lenses of a plurality of materials having different dispersions.However, a scanning optical system that satisfies such a requirement is not a combination of a plurality of lenses having a complicated shape having a toric surface. Therefore, there is a problem that it is difficult to form the scanning lens with high accuracy and that the number of components of the scanning lens increases.

[0004] Therefore, in recent years, a technique for correcting lateral chromatic aberration by combining a refraction-type scanning lens and a diffraction element has been conventionally known. For example, the optical system disclosed in Japanese Patent Application Laid-Open No. H11-09514 has a correction element having a diffraction surface formed between a polygon mirror and a scanning lens, and thereby corrects the chromatic aberration of magnification of the fθ lens. .

In this publication, however, the correction element is described as being orthogonal to the optical axis of the scanning lens. However, as a practical matter, such a correction element having a substantially parallel flat plate shape performs scanning. If they are arranged orthogonal to the optical axis of the lens, there is a problem that the reflected light on the rear surface of the correction element is re-entered by the polygon mirror and easily reaches the scanning target surface as ghost light. Therefore, by arranging the correction element disposed between the polygon mirror and the scanning lens obliquely with respect to the optical axis of the scanning lens,
Various proposals have been made by the present applicant to prevent the light reflected on the reflection surface of the correction element from re-entering the polygon mirror. For example, Japanese Patent Application No. 2000-005448 proposes to prevent ghost light by tilting a correction element in the main scanning direction.

[0006]

As in the above-mentioned proposal, it is conceivable to incline the correction element in the sub-scanning direction. In this case, however, the curvature of the scanning line caused by tilting the correction element (see FIG. 1). Bow) must be corrected.
That is, when the correction element in the form of a substantially parallel flat plate is arranged so as to be inclined in the sub-scanning direction with respect to the optical axis of the scanning lens, the laser beam reflected by the polygon mirror is applied to both the front and back surfaces of the correction element. Since each is refracted, it is shifted in parallel in the sub-scanning direction. This shift amount changes depending on the sub-scanning direction component of the incident angle of the laser beam to the correction element. Therefore, the laser beam deflected by the polygon mirror changes the incident direction to the correction element every moment, and the shift amount also changes according to the change in the direction in the main scanning direction. As a result, the scanning line is curved due to a change in the focused position of the laser beam in the sub-scanning direction on the surface to be scanned. Further, when the diffraction surface structure formed on the correction element has a rotationally symmetric shape, the diffraction effect on the laser beam also changes depending on the incident angle of the laser beam, so that the scanning line on the scanning target surface is changed. Is further curved.

An object of the present invention is to provide a scanning optical system in which a correction element for correcting chromatic aberration of magnification can be arranged between a polygon mirror and a scanning lens without causing ghost light or bending of a scanning line. Is to provide.

[0008]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and a light source and light emitted from the light source are reflected by a reflection surface, and at the same time, this reflection surface is formed. In a scanning optical system having a deflector that deflects by swinging in the main scanning direction and a scanning lens that draws a scanning line by converging light deflected by the deflector on a surface to be scanned, the deflector and the deflector A diffractive element having a diffractive surface for correcting chromatic aberration of the scanning lens is arranged in the sub-scanning direction with respect to the optical axis of the scanning lens in an optical path between the scanning lens and the scanning lens. One of the base shape of the diffractive surface and the shape of the opposite side surface of the diffractive element in the diffractive element has an anamorphic surface shape in order to cancel the curvature of the scanning line caused by the inclination. That it is formed, characterized.

With this configuration, the diffractive element having the above-described configuration is inserted obliquely with respect to the optical axis of the scanning lens into a scanning optical system having a scanning lens in which aberrations other than chromatic aberration are corrected to a practical level or less. Then, the chromatic aberration of the scanning lens is corrected by the diffraction surface. At this time, the reflected light on the front surface or the rear surface of the diffraction element deviates from the optical path of the laser beam because the surface is arranged obliquely with respect to the optical axis of the scanning lens, and is scanned as ghost light. Never reach. Moreover, the curvature of the scanning line, which would be caused by the insertion of the diffraction element obliquely with respect to the optical axis of the scanning lens, is such that the base shape of the scanning surface of the diffraction element or the shape of the opposite surface thereof has an anamorphic surface shape. It is negated by being formed.

The anamorphic shape may be one that can correct the curvature of the scanning line by itself, or one that corrects the curvature of the scanning line in correlation with the shape of the opposite side.

The anamorphic shape may be a shape in which the radius of curvature in the sub-scanning direction at a position distant from the center position is set independently of the cross-sectional shape in the main scanning direction. In this case, the cross-sectional shape in the main scanning direction may be a circular arc or a non-circular curve.

The shape of the opposite side surface may be, of course, an anamorphic surface or a curved shape obtained by rotating a cross-sectional shape in the main scanning direction about an axis oriented in the main scanning direction. It may be formed.
In the latter case, the cross-sectional shape in the main scanning direction may be a straight line, an arc or a non-arc. Note that the distance between the cross-sectional shape in the main scanning direction and the axis for rotating the same is desirably finite, but may be infinite if the cross-sectional shape in the main scanning direction is a non-circular arc. .

It is desirable that the diffraction structure of the diffractive surface has a rotationally symmetrical shape centered on the normal line at the pole of the anamorphic surface shape or a stripe shape oriented in the sub-scanning direction, since cutting can be performed. In the case where the diffraction structure has a rotationally symmetric shape, if the base shape of the diffraction surface is a rotationally symmetric aspherical shape, the diffraction structure can be processed simultaneously with the diffraction structure. When the diffraction structure has a stripe shape,
If the base shape of the diffractive surface is a curved surface obtained by rotating the cross-sectional shape in the main scanning direction about an axis oriented in the main scanning direction, it becomes possible to simultaneously process the diffractive structure by cutting.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the reflection type scanning optical system according to the present invention will be described. The reflective scanning optical system according to the present embodiment is used in a laser scanning unit of a laser printer, and deflects a plurality of laser beams, each of which is ON / OFF-modulated according to a drawing signal for a plurality of lines, which are simultaneously input, by a polygon mirror. This is a multi-beam scanning optical system that forms an electrostatic latent image by scanning on a scanning target surface such as a photosensitive drum.
In this specification, the direction in which the laser beam scans on the surface to be scanned is defined as the main scanning direction (y direction), and the direction orthogonal thereto is defined as the sub scanning direction (z direction), and the shape and power of each optical element are defined. The directionality is described based on the direction on the scanning target surface.

FIG. 1 is a plan view of the scanning optical system according to the present embodiment (a diagram in which the main scanning direction is directed up and down and the sub-scanning direction is orthogonal to the paper). FIG. 2 is an arrow II in FIG. FIG. 5 is a side view showing the state viewed from the direction (a view in which the sub-scanning direction is directed up and down and the main scanning direction is orthogonal to the paper surface). As shown in these figures, the reflection-type scanning optical system according to the present embodiment converts a divergent light emitted from a semiconductor laser 10 as a light source into a parallel light beam by a collimating lens 11, and has a cylindrical power having a positive power in the sub-scanning direction. The light enters a polygon mirror 14 as a deflector via a lens 12. The laser light reflected and deflected by the reflection surface 14a of the polygon mirror 14 is transmitted to the fθ lens 2
The light is focused through the zero and correction lens 30 to form a spot on the scanning target surface 40 that scans in the main scanning direction.

The cylindrical lens 12 has a lens surface having a positive power only in the sub-scanning direction on the side of the collimator lens 11 and a flat surface on the side of the polygon mirror 14. Cylindrical lens 1
The power of 2 is determined so that the line image formed by the cylindrical lens 12 is located near the reflection surface 14a of the polygon mirror 14.

The polygon mirror 14 reflects the laser beam transmitted through the cylindrical lens 12 as a parallel beam in the main scanning direction and as a divergent beam in the sub-scanning direction by any one of the reflecting surfaces 14a. While reflecting the laser beam in this manner, the polygon mirror 14 rotates around its central axis. Thereby, the polygon mirror 14 deflects the incident laser beam within a predetermined angle range in the main scanning direction.

The diffraction element 16 is an optical element having a rectangular front shape whose longitudinal direction is oriented in the main scanning direction. A curved surface corresponding to a lens surface is formed on the front surface or the rear surface of the diffraction element 16 with an axis passing through the center of each surface as a center. That is, the normals at the poles of each surface coincide with each other. Therefore, the axis (normal line) is hereinafter referred to as “optical axis”. A diffractive structure for correcting the chromatic aberration of magnification of the scanning lens including the fθ lens 20 and the correcting lens 30 is formed on the rear surface (the surface on the fθ lens 20 side) of the diffraction element 16. Further, as shown in FIG. 2, the diffraction element 16 has an optical axis with respect to the optical axis of the fθ lens 20 in the sub-scanning direction so that the reflected light on the rear surface does not re-enter the reflection surface of the polygon mirror 14. Are inclined so as to form an angle of 5 degrees. Further, the base shape of the diffractive structure on the rear surface of the diffractive element 16 and the shape of the front surface of the diffractive element 16 have bows caused by the inclination of the diffractive element 16 with respect to the optical axis of the fθ lens 20 as described above. In order to cancel, a special combination of shapes is used as described later.

The fθ lens 20 converges the laser beam transmitted through the diffraction element 16 in the main scanning direction and the sub-scanning direction, and changes the direction of the beam axis in the main scanning direction on the scanning target surface 40. The light is refracted so that the height is proportional to the rotation angle of the polygon mirror 14.
Lens 20 includes a first
The lens 21 and the second lens 22 are arranged. Each of the first lens 21 and the second lens 22 is a lens including only a lens surface that is rotationally symmetric about the optical axis, and the fθ lens 20 has a positive power as a whole.

The correcting lens 30 is a long lens for correcting the curvature of field which is arranged close to the scanning target surface 40, and its lens surface on the fθ lens 20 side is effective in the sub-scanning direction. An anamorphic surface in which the refractive power gradually decreases from the center toward the periphery, and has a strong positive power in the sub-scanning direction. The light beam transmitted through the correction lens 30 becomes convergent light in both the main scanning direction and the sub-scanning direction, and forms a beam spot on the scanning target surface 40.

The scanning optical system according to the present embodiment is designed such that a predetermined performance can be obtained without the diffraction element 15 when the emission wavelength of the semiconductor laser 10 matches the design wavelength. . That is, the scanning optical system according to the present embodiment is configured by inserting the diffraction element 16 into an existing scanning optical system in which the chromatic aberration of magnification is not corrected. Therefore, the diffraction element 16 has only an action for correcting chromatic aberration, and does not affect the performance of the scanning optical system in other respects. As described above, since chromatic aberration of magnification can be corrected only by adding the diffraction element 16,
A scanning optical system in which chromatic aberration of magnification is corrected can be provided without changing the design of an existing scanning optical system.

Next, the shape of the diffraction element 16 will be described with reference to FIGS. In the present embodiment, the diffraction structure that can be formed on the rear surface of the diffraction element 16 is, as shown in FIG.
A rotationally symmetrical shape (hereinafter, referred to as a “first type shape”) centered on a position coincident with the optical axis of the θ lens 20, and a plurality of stripes oriented in the sub-scanning direction as shown in FIG. Are arranged roughly in a grid shape (hereinafter, referred to as “second type shape”) arranged in the main scanning direction.

The base shape of these lattice structures plays a part in the function of canceling the bow described above.
It is formed as a curved surface. However, the specific shape is limited to a predetermined shape because it is processed simultaneously with the lattice structure. That is, when the diffractive structure is the first type shape, the grating structure is formed by cutting while rotating the material of the diffractive element 16, so that the base shape is also a rotationally symmetric shape (spherical or rotationally symmetric aspherical surface). ). As shown in FIG. 4, when the diffractive structure is of the second kind, the material of the diffractive element 16 is placed on the surface of the drum 50 that rotates about a rotation axis 50a that is oriented in the main scanning direction (y direction). If the cutting is performed in a state where is fixed, the diffraction structure can be easily formed.
It is desirable that the center of curvature (direction) be on a straight line (on the rotation axis 50a) (that is, a curved surface shape obtained by rotating the cross-sectional shape in the main scanning direction about an axis oriented in the main scanning direction). . Specifically, a cylinder surface shape having an infinite radius of curvature in the main scanning direction (that is, a curved surface shape having a straight cross-sectional shape in the main scanning direction) or the rotating shaft 50
a toric surface shape (hereinafter, referred to as “y”) formed by rotating a non-circular curve (cross-sectional shape in the main scanning direction) in a plane including “a” around a rotation axis 50 a (axis in the main scanning direction). Toric aspherical shape ”). Although it takes some processing time, if the diffraction structure is formed by grinding, the base shape is set so that the radius of curvature in the sub-scanning direction is set independently of the cross-sectional shape in the main scanning direction, and the rotation axis is A non-aspherical shape (hereinafter referred to as a “progressive toric aspherical shape”) can be used. The degree of freedom of design in these various base shapes decreases in the order of progressive toric aspherical shape, y toric aspherical shape, cylinder surface shape, and rotationally symmetrical shape.

The shape of the front surface of the diffraction element 16 is designed so as to be able to cancel the above-mentioned bow in correlation with the base shape of the diffraction structure on the rear surface. However, the shape depends on the degree of freedom in the design of the base shape of the diffractive structure on the rear surface, and is limited to complement the degree of freedom. Specifically, when the base shape of the diffractive structure on the rear surface is a rotationally symmetric shape, a cylinder surface shape, or a y-toric aspherical shape, the degree of freedom in their design is low. It must be a progressive toric aspherical shape with a high degree of design freedom. On the other hand, when the base shape of the diffractive structure on the rear surface is a progressive toric surface shape, the degree of freedom of design is high, so the front surface shape is a y toric aspherical shape or a cylinder surface shape. be able to.

The above is summarized as follows. That is, when the diffraction structure has the first type, the diffraction element 1
The base shape on the diffraction surface of No. 6 is a rotationally symmetric shape,
The back surface has a progressive toric aspherical shape. When the diffraction structure has the second type, the diffraction element 1
Either one of the base shape and the back surface of the diffraction surface of No. 6 has an anamorphic surface shape (cylinder surface shape or y-toric aspherical surface shape) in which only the sub-scanning direction has an arc shape, and the other is a progressive toric non-spherical surface shape. It has a spherical shape. In the present invention, the y-toric aspherical shape includes a surface having an infinite radius of curvature in the sub-scanning direction.

Next, four specific numerical examples of the above-described scanning optical system will be described, corresponding to the combinations of the shapes of the front surface and the rear surface of the diffraction element 16 described above, respectively. In each of the embodiments, since the numerical values other than the respective surface shapes of the diffraction element 16 are common, before describing the respective surface shapes of the diffraction element 16 according to the respective embodiments individually, a description of the parts common to the respective embodiments will be given. I do.

Table 1 shows the configuration of the scanning optical system of the embodiment on the scanning target surface 40 side of the cylindrical lens 12. The symbol f in the table is the focal length of the entire scanning optical system in the main scanning direction, and Ry is the paraxial radius of curvature [unit: mm] of each optical element in the main scanning direction (the code when the center of curvature is behind the surface). Is assumed to be +, and the case where it exists ahead is assumed to be-), Rz
Is the paraxial radius of curvature in the sub-scanning direction [unit: mm] (the sign when the center of curvature is behind the surface is +, and when it is ahead is-), d is on the optical axis between the surfaces. And n is the refractive index at the design wavelength. The angle of view indicates the angle of view when a scanning line formed on the scanning target surface 40 is viewed from the polygon mirror 14. In the table, the first surface and the second surface are the respective surfaces of the cylindrical lens 12, the third surface is the reflecting surface 14a of the polygon mirror 14, the fourth surface and the fifth surface are the respective surfaces of the diffraction element, and the sixth surface. And the seventh surface is each surface of the first lens 21 of the fθ lens 20, and the eighth and ninth surfaces are the second lens 2.
2, the tenth surface and the eleventh surface indicate the respective surfaces of the correcting lens 30.

[0028]

[Table 1] f = 200.0mm Scanning width 300mm Field of view 43.0deg Design wavelength 780nm Surface number Ry Rz dn1 ∞ 50.0 4.000 1.51072 2 ∞ ∞ 94.500 3 ∞ ∞ 15.000 4 Depends on each embodiment 2.000 1.48617 5 For each embodiment Depends on 33.500 6 -175.567-7.000 1.48617 7 -116.265-2.000 8 ∞ 000 15.000 1.76591 9 -212.490-115.000 10 -1712.510 29.348 5.000 1.48617 11 -3498.000-79.500 The sixth and seventh surfaces are rotationally symmetric aspheric surfaces, The tenth surface is an aspheric surface having a radius of curvature in the sub-scanning direction at a position away from the optical axis irrespective of the cross-sectional shape in the main scanning direction and having no rotation axis, and the other surface is a spherical surface. Or it is a plane.

The shapes of the sixth surface and the seventh surface are as follows: h is the distance from the optical axis in the radial direction, and X (h) is the sag amount from the position of h on the tangent plane to the respective surfaces in the optical axis. It is represented by the following equation (1).

X (h) = y ² / (r _h0 (1 + √ (1- (κ + 1) c ² h ² ))) + A ₄ h ⁴ + A ₆ h ⁶ + A ₈ h ⁸ ...... (1) where r _h0 = Ry, k is a cone coefficient, and A ₄ , A ₆ ,
A ₈ is the fourth and sixth order respectively. This is an eighth-order aspheric coefficient. The values of these coefficients on the sixth and seventh surfaces are as shown in Table 2 below.

[0031]

[Table 2] Surface number κ A ₄ A ₆ A ₈ 6 2.80 -7.488 × 10 ^-7 3.283 × 10 ^-10 -2.570 × 10 ^-15 7 0.80 -5.112 × 10 ^-7 1.319 × 10 ^-10 3.760 × 10 ^-14 The cross-sectional shape of the tenth surface in the main scanning direction is as follows, where y is the distance from the optical axis in the main scanning direction, and x (y) is the sag amount from the position of y in the tangent plane on the optical axis to each surface. It is represented by equation (2).

X (y) = y ² / (r _y0 (1 + √ (1- (κ + 1) c ² y ² ))) + A ₄ y ⁴ + A ₆ y ⁶ +... (2) Where r _y0 = Ry, k is a cone coefficient, and A ₄ , A ₆ ,
.. Are fourth-order, sixth-order,... Aspherical coefficients, respectively. The radius of curvature R _z (y) in the sub-scanning direction at each position y in the main scanning direction on the tenth surface is represented by the following equation (3).

1 / R _z (y) = 1 / r _z0 + B ₁ y + B ₂ y ² + B ₃ y ³ + B ₄ y ⁴ +... (3) where r _z0 = Rz , B ₁ , B ₂ ,...
The following are the aspheric coefficients. The values of these coefficients on the tenth surface are as shown in Table 3 below.

[0034]

[Table 3] The surface shape of the fourth surface is a progressive toric aspherical shape in Examples 1 to 3, and a y-toric aspherical shape in Example 4. The sectional shape of the fourth surface in the main scanning direction is represented by the above equation (2), where y is the distance from the optical axis in the main scanning direction, and X (y) is the sag amount from the tangent plane to each surface in the optical axis. expressed. In the first to third embodiments (in the case of a progressive toric aspherical shape), the radius of curvature R _z (y) in the sub-scanning direction at each position y in the main scanning direction on the fourth surface is calculated by the above equation (3). Represented by On the other hand, in the case of Example 4 (in the case of a y-toric aspherical shape), the shape of the fourth surface in the sub-scanning direction is _represented by the curvature r _z0 in the sub-scanning direction at the optical axis position (y = 0). Is done. Specific numerical values in each embodiment will be described later.

The fifth surface is a diffractive surface, and the shape of the diffractive structure is the first type in the first embodiment.
Is a second type shape.

When the distance from the optical axis is h, the sag amount SAG (h) from the position of h on the tangent plane with respect to the optical axis is defined as the cross-sectional shape in the radial direction of the diffraction surface having the first type diffraction structure. h) can be represented by the following equation (4).

SAG (h) = X (h) + S (h) (4) where X (h) is the base shape of the diffraction surface, and is represented by the above equation (1). S (h) is the base shape X
The sag amount added to (h) is represented by the following equation (5).

S (h) = [| MOD (Δφ (h) + C, −1) | −C] λ / [n−1] (5) where MOD (a, b) is a C is a constant that sets the phase of the boundary position of the annular zone,
Takes an arbitrary value from 0 to 1 (in the following embodiment, c =
0.5). Δφ (h) is an optical path length addition amount that the diffraction surface should have, and is represented by the following equation (6).

Δφ (h) = P ₂ h ² + P ₄ h ⁴ + P ₆ h ⁶ + P ₈ h ⁸ + P ₁₀ h ¹⁰ (6) where P _n is an n-th (even-order) optical path difference function. It is a coefficient.
The number N of each ring zone on the diffractive lens surface is represented by the following equation (7), with the area on the optical axis being 0.

N = INT (| Δφ (h) / λ + C |) (7) Here, INT (i) is a function that gives an integer part of i.

On the other hand, the sectional shape in the main scanning direction of the diffractive surface having the second kind of diffractive structure has a distance from the optical axis of y.
, The sag amount from the position of y on the tangent plane on the optical axis
SAG (y) can be represented by the following equation (4 ′).

SAG (y) = X (y) + S (y) (4 ′) Here, X (y) is the base shape of the diffraction surface, and is represented by the above equation (2). S (y) is the base shape X
The sag amount added to (y) is represented by the following equation (5 ′).

S (y) = [| MOD (Δφ (y) + C, −1) | −C] λ / [n−1] (5 ′) Here, Δφ (y) is expressed by This is an optical path length addition amount to be possessed, and is represented by the following equation (6 ′).

Δφ (y) = P ₂ y ² + P ₄ y ⁴ + P ₆ y ⁶ + P ₈ y ⁸ + P ₁₀ y ¹⁰ (6 ′) Further, the number N of each orbicular zone on the diffraction lens surface is represented by the above formula. (7)
Is represented by

The sectional shape in the sub-scanning direction of the diffractive surface having the second kind of diffractive structure is such that the optical axis position (y = 0) when the base shape is a cylinder shape or a y toric shape. is represented by the sub-scanning direction of curvature r _z0 in the case base shape is progressive toric surface shape is represented by the above formula (3).

[0046]

Embodiment 1 As described above, in Embodiment 1, the surface shape of the fourth surface is a progressive toric aspherical surface,
The base shape of the surface is a rotationally symmetric aspherical shape, and the diffractive structure is a first type shape. The focal length at the design wavelength (780 nm) based on the diffraction component itself on the fifth surface is 1150.95 mm.

Table 4 shows coefficients applied to the above equations (2) and (3) for the fourth surface of the first embodiment.

[0048]

[Table 4] r _y0 = -1026.890 r _z0 = -33.192 κ = 0.000 B ₁ = 0.000 A ₄ = -4.029 × 10 ^-6 B ₂ = -1.166 × 10 ^-4 A ₆ = 7.377 × 10 ^-9 B ₃ = 0.000 A ₈ = -6.417 × 10 ^-12 B ₄ = 2.546 × 10 ^-7 A ₁₀ = 0.000 B ₅ = 0.000 Table 5 shows the above expressions (4), (1), and (5) for the fifth surface of the first embodiment. The coefficients applied to (5) and equation (6) are shown.

[0049]

[Table 5] r _h0 = 962.208 κ = 0.0 P ₂ = -6.25266 × 10 ^-1 A ₄ = -3.748 × 10 ^-6 P ₄ = 9.27623 × 10 ^-5 A ₆ = 5.535 × 10 ^-9 P ₆ = 8.75272 × 10 ^-8 A ₈ = -4.162 × 10 ^-12 P ₈ = 0.000 A ₁₀ = 0.000 P ₁₀ = 0.000 FIGS. 5 to 7 show various aberrations of the scanning optical system according to the configuration of the first embodiment, and FIG. Chromatic aberration, FIG. 6 shows a bow, and FIG. 7 shows a differential bow. In each of these figures, the unit of each axis is [mm], the vertical axis represents the image height (the distance from the optical axis in the main scanning direction on the scanning target surface 40), and the horizontal axis represents the amount of occurrence of each aberration. Show. That is, the horizontal axis in FIG.
The shift amount in the main scanning direction of the spot formed on the scanning target surface 40 by the 770 nm laser beam is shown with reference to the position of the spot formed on the scanning target surface 40 by the 780 nm laser beam. Also, the horizontal axis in FIG.
The amount of shift in the sub-scanning direction of the scanning line passing through the optical axis on the scanning target surface 40 is shown. The horizontal axis in FIG. 7 is a unit distance: 0.0423 in the sub-scanning direction at y = 0 with respect to the reference scanning line.
The difference between the unit distance and the distance from another scanning line passing through the position shifted by mm is shown. As is clear from these figures, according to the first embodiment, each aberration is corrected well.

[0050]

Embodiment 2 As described above, in Embodiment 2, the surface shape of the fourth surface is a progressive toric aspherical surface,
The base shape of the surface is a cylinder surface shape, and the diffractive structure is a second type shape. The focal length at the design wavelength (780 nm) based on the diffraction component itself on the fifth surface is 1150.95 mm.

Table 6 shows various coefficients applied to the above equations (2) and (3) for the fourth surface of the second embodiment.

[0052]

[Table 6] Table 7 shows various coefficients applied to the above equations (4 ′), (2), (5 ′) and (6 ′) with respect to the fifth surface of the second embodiment.
And a radius of curvature of the optical axis in the sub-scanning direction.

[0053]

[Table 7] r _y0 = ∞ r _z0 = -100.000 κ = 0.0 P ₂ = -6.27352 × 10 ^-1 A ₄ = 0.000 P ₄ = 1.03767 × 10 ^-4 A ₆ = 0.000 P ₆ = 1.13559 × 10 ^-7 A ₈ = 0.000 P ₈ = 0.000 A ₁₀ = 0.000 P ₁₀ = 0.000 FIGS. 8 to 10 show various aberrations of the scanning optical system according to the configuration of Example 2, FIG. 8 shows chromatic aberration of magnification, FIG.
Indicates a differential bow. As is apparent from these drawings, according to the second embodiment, each aberration is corrected well.

[0054]

Embodiment 3 As described above, in Embodiment 3, the surface shape of the fourth surface is a progressive toric aspherical surface,
The base shape of the surface is a y-toric aspherical shape, and the diffractive structure is a second type shape. The focal length at the design wavelength (780 nm) based on the diffraction component itself on the fifth surface is 1150.95.
mm.

Table 8 shows various coefficients applied to the above equations (2) and (3) for the fourth surface of the third embodiment.

[0056]

[Table 8] Table 9 shows various coefficients applied to the above equations (4 ′), (2), (5 ′) and (6 ′) for the fifth surface of the third embodiment.
And a radius of curvature of the optical axis in the sub-scanning direction.

[0057]

Table 9 _{r y0 = ∞ r z0 = -100.000} κ = 0.000 P 2 = -6.25778 × 10 -1 A 4 = -7.882 × 10 -7 P 4 = 9.45345 × 10 -5 A 6 = -1.537 × 10 - ¹⁰ P ₆ = 9.75887 × 10 ^-8 A ₈ = 9.642 × 10 ^-14 P ₈ = 0.000 A ₁₀ = 0.000 P ₁₀ = 0.000 FIGS. 11 to 13 show various aberrations of the scanning optical system according to the configuration of the third embodiment. , FIG. 11 shows lateral chromatic aberration, FIG. 12 shows bow,
FIG. 13 shows a differential bow. As is clear from these drawings, according to the third embodiment, each aberration is corrected well.

[0058]

Embodiment 4 As described above, in Embodiment 4, the surface shape of the fourth surface is a y-toric aspherical shape, the base shape of the fifth surface is a progressive toric aspherical shape, and the diffraction structure is There are two types of shapes. The focal length at the design wavelength (780 nm) based on the diffraction component itself on the fifth surface is 1150.95.
mm.

Table 10 shows the coefficients applied to the above equation (2) for the fourth surface of the fourth embodiment, and the radius of curvature of the optical axis in the sub-scanning direction.

[0060]

[Table 10] r _y0 = -681.487 r _z0 = κ κ = 0.000 A ₄ = -3.366 × 10 ^-6 A ₆ = 5.846 × 10 ^-9 A ₈ = -4.889 × 10 ^-12 A ₁₀ = 0.000 With respect to the fifth surface of the fourth embodiment, the above equation (4 ′),
The coefficients applied to the equations (2), (3), (5 ') and (6') and the radius of curvature in the sub-scanning direction on the optical axis are shown.

[0061]

[Table 11]_y0= 1813.351 r_z0= -110.835 κ = 0.000 B₁= 0.000 P_Two= -6.26043 × 10^-1 A_Four= -3.165 × 10^-6 B_Two= 2.383 × 10^-Four P_Four= 8.65439 × 10^{- Five} A₆= 4.381 × 10^-9 B_Three= 0.000 P₆= 9.51369 × 10^-8 A₈= -3.185 × 10^-12 B_Four= -5.551 × 10^-7 P₈= 0.000 A_Ten= 0.000 B_Five= 0.000 P_Ten= 0.000 B_Four= 2.415 × 10^-Ten 14 to 16 show a scanning optical system according to the configuration of the fourth embodiment.
14 shows magnification chromatic aberration, FIG. 15 shows bow,
FIG. 16 shows a differential bow. Each of these figures
As is apparent from Example 4, according to the fourth embodiment, each aberration is favorably reduced.
Will be corrected.

[0062]

As described above, according to the present invention, ghost light can be prevented even if a correction element for correcting chromatic aberration of magnification of a scanning optical system is arranged between a polygon mirror and a scanning lens. In addition to this, the curvature of the scanning line can be prevented.

[Brief description of the drawings]

FIG. 1 is an optical configuration diagram in a main scanning direction of a scanning optical system according to an embodiment of the present invention.

FIG. 2 is an optical configuration diagram of the scanning optical system in a sub-scanning direction.

FIG. 3 is an explanatory view of a first-type diffraction structure of a diffraction element.

FIG. 4 is an explanatory view of a diffraction structure of a second type of diffraction element.

FIG. 5 is a graph showing chromatic aberration of magnification in the first embodiment.

FIG. 6 is a graph showing a bow of Example 1.

FIG. 7 is a graph showing a differential bow of the first embodiment.

FIG. 8 is a graph showing chromatic aberration of magnification in the second embodiment.

FIG. 9 is a graph showing a bow of Example 2.

FIG. 10 is a graph showing a differential bow according to the second embodiment.

FIG. 11 is a graph showing chromatic aberration of magnification in the third embodiment.

FIG. 12 is a graph showing a bow of Example 3.

FIG. 13 is a graph showing a differential bow of Example 3;

FIG. 14 is a graph showing chromatic aberration of magnification in the fourth embodiment.

FIG. 15 is a graph showing a bow of Example 4.

FIG. 16 is a graph showing a differential bow according to the fourth embodiment.

[Explanation of symbols]

 Reference Signs List 10 laser light source 14 polygon mirror 16 diffractive element 20 fθ lens 30 correction lens 40 scan target surface

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (reference) G02B 13/18 B41J 3/00 DF term (reference) 2C362 BA83 BA86 BA90 2H045 AA01 BA02 CA04 CA34 CA55 CA68 CB63 2H049 AA03 AA04 AA17 AA18 AA23 AA32 AA42 AA55 AA64 2H087 KA19 LA22 NA14 PA04 PB04 RA05 RA07 RA08 RA13 RA34 RA46

Claims (14)

[Claims] A light source, a deflector for reflecting light emitted from the light source on a reflecting surface and simultaneously deflecting the reflecting surface by swinging the reflecting surface in a main scanning direction, and a light deflected by the deflector. In a scanning optical system having a scanning lens that draws a scanning line by converging on a scanning target surface, a diffraction for correcting chromatic aberration of the scanning lens is provided in an optical path between the deflector and the scanning target surface. A diffractive element having a surface is disposed obliquely in the sub-scanning direction with respect to the optical axis of the scanning lens, and one of a base shape of the diffractive surface and a shape of an opposite side surface of the diffractive element in the diffractive element is A scanning optical system formed to have an anamorphic surface shape in order to cancel a curvature of the scanning line caused by the inclination. 2. The anamorphic surface shape is a shape in which a radius of curvature in a sub-scanning direction at a position distant from a center position is set independently of a cross-sectional shape in a main scanning direction. The scanning optical system as described in the above. 3. The scanning optical system according to claim 2, wherein the cross-sectional shape in the main scanning direction in the anamorphic surface shape is a non-circular curve. 4. A scanning optical system according to claim 2, wherein a normal line at a pole of said anamorphic surface shape is inclined in a sub-scanning direction with respect to an optical axis of said scanning lens. 5. The other of the base shape of the diffraction surface and the shape of the opposite side surface of the diffraction element in the diffraction element correlates with the anamorphic surface shape of the one of the diffraction lines. 5. A curved surface shape for canceling the curvature.
The scanning optical system according to any one of the above. 6. The other of the base shape of the diffraction surface and the shape of the opposite side surface of the diffraction element in the diffraction element is a curved surface shape obtained by rotating a cross-sectional shape in the main scanning direction about an axis oriented in the main scanning direction. 6. The scanning optical system according to claim 5, wherein the scanning optical system is formed as: 7. The other of the base shape of the diffraction surface and the shape of the opposite side surface of the diffraction element in the diffraction element is formed as a curved surface shape in which the cross-sectional shape in the main scanning direction is a straight line. The scanning optical system according to claim 6. 8. The diffractive element, wherein the other of the base shape of the diffractive surface and the shape of the opposite side surface is formed as a curved surface shape in which the cross-sectional shape in the main scanning direction is a non-circular curve. 7. The scanning optical system according to claim 6, wherein: 9. The scanning optical system according to claim 6, wherein the other of the base shape of the diffractive surface and the shape of the opposite side surface of the diffractive element is formed as a rotationally symmetric aspherical shape. . 10. The scanning optical system according to claim 2, wherein the diffractive structure of the diffractive surface has a shape rotationally symmetric about a normal to a pole of the anamorphic surface. 11. The scanning optical system according to claim 1, wherein the diffraction structure on the diffraction surface has a stripe shape oriented in the sub-scanning direction. 12. The scanning optical system according to claim 10, wherein a base shape of the diffraction surface is the rotationally symmetric shape, and a shape of an opposite side surface is the anamorphic surface shape. 13. The base shape of the diffraction surface is a curved shape obtained by rotating a cross-sectional shape in the main scanning direction about an axis facing the main scanning direction, and the shape of the opposite side surface is the anamorphic surface shape. The scanning optical system according to claim 11, wherein 14. The diffractive surface has a base shape of the anamorphic surface shape, and a shape of an opposite side surface is a curved surface shape obtained by rotating a sectional shape in the main scanning direction about an axis oriented in the main scanning direction. The scanning optical system according to claim 11, wherein the scanning optical system is provided.