JPH116954A - Line image forming lens and optical scanner - Google Patents

Abstract

PROBLEM TO BE SOLVED: To extremely excellently compensate spherical aberration in a subscanning corresponding direction in the entire system of the image forming optical system of an optical scanner. SOLUTION: In this optical scanner; a surface to be scanned is optically scanned by coupling luminous flux from a light source 1 by a coupling lens 2, focusing the coupled luminous flux in the subscanning corresponding direction and forming it as a line image long in a main scanning corresponding direction near the deflection and reflection surface of a light deflector 4, and condensing the luminous flux deflected by the deflector 4 as a light spot on the surface to be scanned by a scanning and image forming optical system. The scanner has one or more lenses hardly having power in the main scanning corresponding direction and having positive power in the subscanning corresponding direction as a lens 3 forming the coupled luminous flux as the line image, and at least one of the surfaces having the positive power in the subscanning corresponding direction of the lens 3 has a non circular arc shape in a subscanning cross section, and the non circular arc shape is fixed to the shape for compensating the synthetic spherical aberration in the subscanning corresponding direction with the scanning and image forming optical system.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a line image forming lens and an optical scanning device.

[0002]

2. Description of the Related Art A light beam from a light source is coupled by a coupling lens, and the coupled light beam is focused in a direction corresponding to the sub-scanning (a direction corresponding to the sub-scanning direction on the optical path from the light source to the surface to be scanned). Then, an image is formed as a long line image in the main scanning direction (direction corresponding to the main scanning direction on the optical path from the light source to the surface to be scanned) near the deflection reflecting surface of the optical deflector, and is deflected by the optical deflector. An optical scanning device that condenses the luminous flux as a light spot on a surface to be scanned by a scanning image forming optical system and optically scans the surface to be scanned is widely known in relation to an optical printer or the like. In order to achieve high recording density and high recording image quality using the optical scanning device as described above, it is indispensable to reduce the diameter of the light spot condensed on the surface to be scanned. A small-diameter light spot close to the image forming limit of the image optical system is required.

In order to "reduce the diameter of a light spot in a writing area by optical scanning", it is of course necessary to satisfactorily correct the curvature of field, but this is not sufficient, and the spherical surface of the imaging optical system is not sufficient. It is necessary that aberrations are sufficiently removed.

[0004] As an optical scanning device in which the spherical aberration of the image forming optical system is effectively removed, the one described in Japanese Patent Application Laid-Open No. 63-143523 is known. In this optical scanning apparatus, focusing on the point that a scanning imaging optical system that forms a deflected light beam as a light spot on a surface to be scanned has a negative spherical aberration, the light beam from the light source is reflected in the vicinity of the deflective reflection surface of the light deflector. In addition, by including a refracting surface having negative power in the line image forming optical system that forms a long line image in the main scanning corresponding direction, the spherical aberration of the entire image forming optical system is corrected. I have. Although the correction of spherical aberration in the present invention is effectively performed, it cannot be said that it is still sufficient for realizing a small-diameter light spot that has recently been required.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to correct satisfactorily spherical aberration in the sub-scanning corresponding direction of the entire image forming optical system of an optical scanning device.

[0006]

SUMMARY OF THE INVENTION A linear image forming lens according to the present invention comprises a light deflector that couples a light beam from a light source by a coupling lens and focuses the coupled light beam in a sub-scanning corresponding direction. And a light beam deflected by the optical deflector is condensed as a light spot on the surface to be scanned by the scanning image forming optical system, and the light is deflected by the optical deflector. Is a lens that forms an image of the coupled luminous flux as the above-mentioned line image in the optical scanning device that optically scans. That is, the line image forming lens is "a lens having almost no power in the main scanning corresponding direction and having positive power in the sub-scanning corresponding direction".
, And at least one of the surfaces having power in the sub-scanning corresponding direction has a “shape in the sub-scanning cross section is a non-arc shape”. The non-circular shape is defined as “a shape for correcting a combined spherical aberration of the line image forming lens and the scanning image forming optical system in the sub-scanning corresponding direction”. The “sub-scan section” is a virtual section of the line image forming lens, and refers to a plane parallel to the optical axis of the line image forming lens and the direction corresponding to the sub-scan. Therefore, the sub-scan section is orthogonal to the main scanning direction. The “scanning optical system” can be configured not only by a lens system such as an fθ lens, but also by a mirror optical element having an image forming function, or can be configured to include the mirror optical element.

The line image forming lens can be constructed by combining two or more lenses having little power in the main scanning direction and having positive power in the sub-scanning direction.
This can be "configured with a single lens" (claim 2).

Since the lens constituting the line image forming lens has a positive power in the direction corresponding to the sub-scanning and has almost no power in the direction corresponding to the main scanning, the shape of the lens is generally a "cylinder lens". However, in some cases, weak power may be added in the main scanning corresponding direction as long as the performance in the sub scanning corresponding direction is not affected. Means

According to the line image forming lens of the second aspect,
When constituted by a single lens, the shape in the sub-scanning section is a non-arc shape having power on at least one surface,
At the outermost periphery of the effective diameter, the amount of aspherical displacement on the side of the light source: Δ ₁ and the amount of aspherical displacement on the side of the optical deflector: Δ ₂ can satisfy the following condition: (1) Δ ₁ −Δ ₂ <0 (Claim 3). Aspheric displacement:
Δ ₁ and Δ ₂ are the amounts of deviation (in the direction parallel to the optical axis) between the paraxial curvature spherical surface and the actual lens surface at “the outermost periphery of the effective diameter (in the direction corresponding to sub-scanning)” in each lens surface, The direction in which the light beam from the light source is transmitted is defined as “positive”. Note that the “non-arc shape” in the sub-scan section includes the “straight line”, and the “aspheric displacement amount” when the non-arc shape is a straight line is 0.

The scanning imaging optical system needs to have a conjugate relationship between the position near the deflecting reflection surface of the optical deflector and the position of the surface to be scanned in the sub-scanning corresponding direction. Is required, and the spherical aberration in the sub-scanning direction generated by the scanning image forming optical system is almost under. It is difficult to correct all of such “under-spherical aberration” even if “a component having negative power in the sub-scanning corresponding direction” is included in the scanning image forming optical system. By satisfying the condition (1), the spherical aberration in the sub-scanning direction of the scanning imaging optical system, which is the optical system after the optical deflector, can be brought to the over side, and the spherical aberration of the entire system can be corrected. Becomes easier.

[0011] The line image forming lens according to the second aspect of the present invention may also be formed by molding a plastic.
Both surfaces are non-planar, the shape of the lens in the sub-scanning section is at least one surface non-circular, the amount of aspherical displacement of the side of the light source near the effective diameter: Δ ₁ , aspherical displacement amount: delta ₂ is, the condition (1) can be configured to satisfy that "as the (claim 4).

When a line image forming lens is manufactured by plastic molding as a plastic lens, if one of the lens surfaces is a flat surface, the molded planar shape becomes “accurate” due to so-called “sinking or shrinkage” during molding. Surface that deviates from a simple planar shape ". When the lens surfaces are "non-planar" on both surfaces as in the invention described in claim 4, it is easy to correct a mold or the like, and it is easy to realize a highly accurate surface shape. Also in this case, by satisfying the condition (1), it becomes easy to correct spherical aberration of the entire system.

According to the optical scanning apparatus of the present invention, a light beam from a light source is coupled by a coupling lens, the coupled light beam is focused in a sub-scanning corresponding direction, and the main scanning is performed near a deflecting reflection surface of an optical deflector. An optical scanning device that forms an image as a long linear image in the corresponding direction, condenses the light beam deflected by the optical deflector as a light spot on the surface to be scanned by the scanning image forming optical system, and optically scans the surface to be scanned. A line image forming lens according to claim 1 or 2 or 3 or 4 is used as a line image forming lens for forming a coupled light beam as the line image. 5).
As the optical deflector, a rotating two-sided mirror or a rotating single-sided mirror can be used in addition to the “rotating polygon mirror”. The coupling lens may convert the light beam from the light source into a parallel light beam or a light beam having a weak divergence or a weak converging property. The “scanning image forming optical system” can be formed not only by a lens system such as an fθ lens, but also by a “mirror optical element” having an image forming function, or by a “configuration including the above mirror optical element”.
It can also be.

According to a fifth aspect of the present invention, in the optical scanning apparatus, the scanning image forming optical system does not include an optical element having a negative power in the sub-scanning corresponding direction, and the surface shape in the sub-scanning cross section is an arc shape or a straight line. An optical element "(claim 6). The “sub-scan section” in this case refers to a plane section parallel to the optical axis and the direction corresponding to the sub-scan in the scanning image forming optical system.

[0015] In the above optical scanning device,
The scanning image forming optical system can be constituted by "single lens" (claim 7).

[0016]

FIG. 1 is a diagram for explaining an embodiment of an optical scanning device according to the present invention. In FIG. 1A, a radiated light beam emitted from a semiconductor laser 1 which is a "light source" is coupled to a subsequent optical system by a coupling lens 2, and becomes a parallel light beam or a light beam having weak focusing or weak divergence. The light is focused by the line image forming lens 3 in the direction corresponding to the sub-scanning (the direction orthogonal to the drawing in FIG. 1A), and is located in the vicinity of the deflecting reflection surface of the rotary polygon mirror 4 as the "optical deflector". To a long line image.
The deflected light beam deflected at a constant angular velocity by the rotary polygon mirror 4 is spotted on a scanned surface 6 (substantially a photoconductive photoconductor) by a scanning imaging lens 5 forming a "scanning optical system". The light is condensed and imaged, and the scanned surface 6 is optically scanned at a constant speed.

FIG. 1B shows the optical arrangement of FIG. 1A virtually linearly developed and viewed from the main scanning corresponding direction. The vertical direction of the drawing is the sub-scanning corresponding direction. The drawing is parallel to the sub-scan section. In the sub-scanning corresponding direction, the light beam from the semiconductor laser 1 is focused on a line image near the deflecting / reflecting surface 4A, and the scanning imaging lens 5 uses the image having the line image as an object point as a light spot on the surface 6 to be scanned. Image on top.

That is, in the embodiment of FIG. 1, the light beam from the light source 1 is coupled by the coupling lens 2, and the coupled light beam is focused in the sub-scanning corresponding direction.
A long line image is formed in the vicinity of the deflecting reflection surface 4A of the optical deflector 4 in the main scanning direction, and the light beam deflected by the optical deflector 4 is formed as a light spot on the surface 6 to be scanned by the scanning image forming optical system 5. An optical scanning device for condensing and optically scanning a surface to be scanned, comprising a line image forming lens 3 for forming a coupled light beam as the line image.

The scanning image forming lens 5 which is a "scanning image forming optical system" is constituted by a "single lens" (claim 7).
Excluding the optical element whose power in the sub-scanning corresponding direction is negative,
The surface shape in the sub-scan section is an arc shape on both surfaces (claim 6). The line image forming lens 3 is constituted by a "single lens" (claim 2). As will be described later, the shape of the lens in the sub-scanning section is a non-circular shape on both sides, and the condition (1) is satisfied. Satisfies (claims 3 and 4). Note that a “beam shaping lens or aperture” can be added between the coupling lens 2 and the optical deflector 4.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the embodiment shown in FIG. 1, the coupling action by the coupling lens 2 is referred to as "collimation action", and the coupled light flux becomes "parallel light flux". Radius of curvature in the sub-scanning section (paraxial radius of curvature if the lens surface shape is non-circular), and surface spacing,
Data on the refractive index (value at the operating wavelength) of the material will be given. The unit of the quantity having the dimension of length is “mm”.

Example 1 Curvature Radius Surface Interval Refractive Index Linear Imaging Lens First Surface 52.1 3.0 1.51118 Second Surface １ 129.9 Scanning Imaging Lens First Surface −60.0 12.36 1.53664 2nd surface -13.72 145.25 The last surface interval “145.25” is the second value of the scanning imaging lens.
This is the surface distance between the surface and the surface to be scanned. The deflecting / reflecting surface 4A of the optical deflector is provided between the second surface of the line image forming lens and the first surface of the scanning image forming lens, but since it is a reflecting surface, it does not affect spherical aberration. The first and second surfaces of the scanning imaging lens are both arc-shaped within the sub-scanning section.

The first surface of the line image forming lens has a non-arcuate shape in the sub-scan section. This non-circular shape can be represented by, for example, a well-known formula: X = Z ² / [R + R {1- (1 + K) (Z / R) ² }] + AZ ⁴ + BZ ⁶ + CZ ⁸ +. . . According to (2), X can be expressed as coordinates that match the optical axis, and Z can be expressed as coordinates in a direction orthogonal to the optical axis in the sub-scan section (direction corresponding to the sub-scan).

The first example of the line image forming lens in the first embodiment
The non-arcuate shape of the surface is given by R = 52.1,
K = -1.47, A = B = C =. . = 0. The effective diameter of the line image forming lens in the sub-scanning corresponding direction is 6 mm, the outermost periphery of the effective diameter (± 3 mm from the optical axis), the aspherical displacement of the first surface: Δ ₁ = −0.00011, the second surface Aspherical displacement: Δ ₂ = 0, Δ ₁ −Δ ₂ = −0.
Since 00001 <0, the condition (1) is satisfied. At this time, "combined spherical aberration in the sub-scanning direction with respect to the entire system of the line image forming lens and the scanning image forming lens" is corrected very well as shown in FIG.

Comparative Example As a comparative example with respect to the first embodiment, the first line image forming lens
When K = 0 on the surface (radius of curvature: R = 52.1)
FIG. 4 (a) shows the spherical aberration of the whole system (arc shape). With such large spherical aberration, it is extremely difficult to reduce the light spot diameter in the sub-scanning direction. From comparison with FIG. 2, it will be understood that the spherical aberration of the entire system is improved by making the first surface of the line image forming lens non-circular in Example 1. FIG. 4B shows the spherical aberration of the line image forming lens alone in the sub-scanning corresponding direction in the comparative example. If the line image forming lens alone has such a spherical aberration on the under side, the spherical aberration near the under side of the scanning image forming lens is further increased on the under side.

Example 2 Only the line image forming lens in Example 1 was changed as follows. Curvature radius Surface interval Refractive index Linear image forming lens First surface 3.0 1.51118 Second surface −51.085 The second surface of the linear image forming lens in the second embodiment has a non-circular shape, and is expressed by formula (2). , R = −51.085, K = −
2.5, A = 6.5 × 10 ~ 7, B = C =. . = 0. The effective diameter of the line image forming lens in the sub-scanning corresponding direction is 6 mm, and the outermost periphery of the effective diameter (± 3 m from the optical axis)
m), the aspherical displacement of the first surface: Δ ₁ = 0, the aspherical displacement of the second surface: Δ ₂ = 0.00019, and Δ ₁ −Δ
_{Since 2} = −0.00019 <0, the condition (1) is satisfied. The “spherical aberration in the sub-scanning direction with respect to the entire system of the linear image forming lens and the scanning image forming lens” according to the second embodiment is corrected very well as shown in FIG.

Example 3 Only the line image forming lens in Example 1 was changed as follows. Curvature radius Surface distance Refractive index Linear image forming lens First surface 47.36 3.0 1.53664 Second surface 339.6 The first surface of the linear image forming lens in the third embodiment has a non-circular shape, and (2) In the formula, R = 47.36, K = −1.1
1, A = B = C =. . = 0. The effective diameter of the line image forming lens in the sub-scanning corresponding direction is 6 mm, and the amount of aspherical displacement of the first surface at the outermost periphery of the effective diameter (± 3 mm from the optical axis): Δ ₁ = −0.00011. Aspherical displacement of two surfaces: Δ ₂ = 0, and Δ ₁ −Δ ₂ = −0.000
Since 11 <0, the condition (1) is satisfied.

The "spherical aberration in the sub-scanning direction with respect to the entire system of the line image forming lens and the scanning image forming lens" relating to the third embodiment is corrected very well as shown in FIG. Example 3
Since the line image forming lens has power in both directions corresponding to the sub-scanning direction, it is easy to correct a mold and the like, and it is easy to realize a highly accurate surface shape.

The "line image forming lens" as described above is
It can be easily manufactured at low cost by plastic molding or glass molding.

[0029]

As described above, according to the present invention, a novel line image forming lens and optical scanning device can be realized. The line image forming lens of the present invention has a non-circular shape in the sub-scanning section as described above, and corrects "synthetic spherical aberration in the direction corresponding to sub-scanning with the scanning image forming optical system" by the non-circular shape. Therefore, the spherical aberration in the sub-scanning direction can be corrected extremely well, and the diameter of the light spot in the sub-scanning direction can be easily reduced. Further, the optical scanning device of the present invention uses such a line image forming lens, so that high density recording is possible.

Further, the line image forming lens of the present invention may be formed by forming the lens surface of the existing line image forming lens into a non-circular shape in the sub-scanning cross section, as disclosed in Japanese Patent Application Laid-Open No. 63-143523. No increase in the number of parts is required as compared with the "method of adding a lens having a concave cylinder surface".

Further, the linear image forming lens can add a weak power in the main scanning direction, and the addition of the power can "contribute to the correction of the combined spherical aberration in the main scanning direction".

[Brief description of the drawings]

FIG. 1 is a diagram for explaining one embodiment of an optical scanning device of the present invention.

FIG. 2 is a diagram illustrating a combined spherical aberration in the sub-scanning direction of the line image forming lens and the scanning image forming lens according to the first exemplary embodiment.

FIG. 3 is a diagram illustrating a combined spherical aberration in a sub-scanning direction of a line image forming lens and a scanning image forming lens according to a second embodiment.

FIGS. 4A and 4B are a spherical aberration diagram (a) of a combined image in a sub-scanning direction of a line image forming lens and a scanning image forming lens in a comparative example, and a spherical aberration diagram (b) of a line image forming lens in a sub-scanning corresponding direction. is there.

FIG. 5 is a diagram illustrating a combined spherical aberration in the sub-scanning direction of the line image forming lens and the scanning image forming lens according to the third exemplary embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Semiconductor laser which is a light source 2 Coupling lens 3 Linear image forming lens 4 Rotating polygon mirror which is an optical deflector 5 Scanning image forming lens which is a scanning image forming optical system

Claims (7)

[Claims] A light beam from a light source is coupled by a coupling lens, and the coupled light beam is focused in a sub-scanning corresponding direction, and a linear image long in a main scanning corresponding direction near a deflection reflecting surface of an optical deflector. The light beam deflected by the optical deflector is condensed as a light spot on the surface to be scanned by the scanning image forming optical system, and the coupled light beam is scanned by the optical scanning device that optically scans the surface to be scanned. A lens having substantially no power in the main scanning corresponding direction and having at least one lens having a positive power in the sub-scanning corresponding direction. At least one of the surfaces having positive power has a shape in the sub-scanning cross section having a non-arc shape, and the non-arc shape is a combined spherical surface in the sub-scanning corresponding direction with the scanning imaging optical system. Make up for aberrations Linear image forming lens characterized by defined in shape. 2. The line image forming lens according to claim 1, wherein the line image forming lens is constituted by a single lens. 3. The line image forming lens according to claim 2, wherein at least one surface of the lens in the sub-scanning section is a non-circular shape having power, A linear image forming lens characterized in that the aspherical displacement: Δ ₁ and the aspherical displacement on the side surface of the optical deflector: Δ ₂ satisfy the following condition: (1) Δ ₁ −Δ ₂ <0. 4. The line image forming lens according to claim 2, wherein the lens is molded from plastic, both surfaces are non-planar, and the shape of the lens in the sub-scan section is at least one surface non-circular. At the periphery of the effective diameter, the amount of aspherical displacement on the side of the light source: Δ ₁ , and the amount of aspherical displacement on the side of the optical deflector: Δ ₂ satisfies the condition: (1) Δ ₁ −Δ ₂ <0. A line image forming lens. 5. A light beam from a light source is coupled by a coupling lens, and the coupled light beam is focused in a direction corresponding to the sub-scanning direction. An optical scanning device that forms an image, converges a light beam deflected by the optical deflector as a light spot on a surface to be scanned by a scanning image forming optical system, and optically scans the surface to be scanned. An optical scanning device, comprising: the line image forming lens according to claim 1, 2, 3, or 4 as a line image forming lens that forms the light beam as the line image. 6. The optical scanning device according to claim 5, wherein the scanning image forming optical system does not include an optical element having a negative power in the sub-scanning corresponding direction, and the surface shape in the sub-scanning cross section is an arc shape or a straight line. An optical scanning device, comprising: an optical element. 7. The optical scanning device according to claim 6, wherein the scanning image forming optical system is constituted by a single lens.