JP3345234B2 - Scanning optical system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a scanning optical system used for an optical scanning unit such as a laser printer.

[0002]

2. Description of the Related Art In a conventional scanning optical system, generally, a light beam emitted from a laser light source is formed into a linear image by a cylindrical lens having power in a sub-scanning direction, and a polygon mirror provided near the image forming position is used. Deflected by reflection, f
An image is formed on the scanning target surface via the θ lens. This type of scanning optical system is disclosed in, for example, Japanese Patent Application Laid-Open No. 61-120112.

[0003] As disclosed in this publication, an fθ lens is composed of two lenses arranged close to a polygon mirror and a long lens arranged near the image plane and having power mainly in the sub-scanning direction. , The imaging magnification of the fθ lens in the sub-scanning direction tends to be low. When such an fθ lens having a low imaging magnification is used, if the divergence angle of the light beam incident on the fθ lens in the sub-scanning direction is large, the spot diameter in the sub-scanning direction is excessively small. It is necessary to set the F-number of the cylindrical lens large so as to keep the spread angle of the light beam incident on the fθ lens in the sub-scanning direction small.

In order to increase the F-number of the cylindrical lens, it is necessary to reduce the light beam diameter or to increase the focal length of the cylindrical lens in the sub-scanning direction. However, if the focal length is increased, the distance between the cylindrical lens and the fθ lens is increased and the optical system is enlarged. To avoid the enlargement and increase the F-number, the diameter of the luminous flux passing through the cylindrical lens is apertured. It is necessary to squeeze through small.

[0005]

In general, when a laser beam is focused by a lens, if the diameter of the aperture becomes smaller than a predetermined value, the beam waist position moves away from the Gaussian image plane due to the influence of diffraction, and the beam with respect to defocusing. It is known that the change in diameter becomes asymmetric before and after the beam waist position.

Such a phenomenon also appears in a scanning optical system. That is, when the diameter of the light beam transmitted through the cylindrical lens becomes smaller than a predetermined value, the position of the beam waist on the scanning target surface side formed via the fθ lens moves away from the Gaussian image plane and approaches the fθ lens side, and The change in beam diameter due to defocus is asymmetric with respect to the beam waist position.

Therefore, in the above configuration, when defocus occurs due to a change in the focal length due to a change in the field of view in the sub-scanning direction or a temperature change when a plastic lens is used, the sub-scanning direction on the surface to be scanned is reduced. However, there is a problem that the change of the spot diameter becomes large and the drawing performance deteriorates.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art, and a light beam emitted from a light source is transmitted in the sub-scanning direction near a deflector by a first imaging optical system. In a scanning optical system in which an image is formed once and a light beam deflected by a deflector is formed on a surface to be scanned by a second imaging optical system, a wavefront of the light beam formed on the surface to be scanned is scanned. The first and second imaging optical systems are configured so as to be delayed at a peripheral portion in the sub-scanning direction with respect to a reference spherical surface centered on the upper paraxial image point. The delay of the wavefront is regarded as a phase delay on the reference spherical surface.

By delaying the wavefront of the light beam at the peripheral portion in the sub-scanning direction from the reference spherical surface, the image forming point of the light beam at the peripheral portion in the sub-scanning direction moves in a direction away from the second imaging optical system. The imaging point where the aberration is minimized moves in the same manner. Thereby, the beam waist shifted in the direction approaching the second imaging optical system due to the influence of diffraction can be moved so as to approach the Gaussian image plane.

[0010] The delay of the wavefront can be generated by the first imaging optical system. In this case, the first imaging optical system causes the wavefront of the light beam to be imaged near the deflector at a peripheral portion in the sub-scanning direction with respect to the reference spherical surface centered on the image point near the deflector. Configured to delay.

As the first image forming optical system, a cylindrical lens having power in the sub-scanning direction can be used. The wavefront as described above can be given by the shape of the cylindrical lens or by a change in the refractive index.

When the wavefront delay is caused by the shape of the cylindrical lens, the cylindrical lens is
What is necessary is just to add a lens thickness at the peripheral portion in the sub-scanning direction to the base shape defined by the cylindrical surface. The amount of addition of the lens thickness to the base shape can be set so as to increase continuously or stepwise according to the height in the sub-scanning direction.

When the amount of addition is continuously changed, a lens surface whose curvature radius gradually increases in accordance with the height in the sub-scanning direction may be formed. In this case, since the power changes according to the height in the sub-scanning direction, the wavefront lags behind the reference spherical surface in the peripheral portion in the sub-scanning direction. When the amount of addition is changed stepwise, a step is added to the line where the height in the sub-scanning direction is a fixed value, and a certain amount of lens thickness is added to the base shape in the area on the peripheral side of this step. do it. In this case, the wavefront of the light beam transmitted on the peripheral side from the step is later than the wavefront of the light beam transmitted on the center side of the step.

When the base shape of both lens surfaces of the cylindrical lens is formed by a flat surface and a cylindrical surface, the amount added to the base shape may be given to any lens surface.

When the delay of the wavefront is given by a change in the refractive index while maintaining the shape of the cylindrical lens, the peripheral portion in the sub-scanning direction may be set to have a higher refractive index than the central portion. Further, the cylindrical lens may be a distributed refractive index lens in which both lens surfaces are flat. A distributed refractive index lens having a positive power in the sub-scanning direction can be obtained by setting the refractive index distribution in the sub-scanning direction to be continuously smaller from the center to the periphery. This can be obtained by setting the difference in the refractive index between the central part and the peripheral part in the sub-scanning direction smaller than in the case.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a scanning optical system according to the present invention will be described below. FIG. 1 is a plan view of a scanning optical system according to an embodiment of the present invention in a main scanning direction.

The parallel laser light emitted from the light source 1 is
An image is formed once in the sub-scanning direction near a polygon mirror 3 serving as a deflector via a cylindrical lens 2 serving as a first image forming optical system. The laser light reflected and deflected by the polygon mirror 3 forms an image on a scanning target surface 5 by an fθ lens 4 which is a second imaging optical system composed of three lenses 4a, 4b, and 4c. A spot that scans in the scanning direction is formed.

The light source unit 1 includes, for example, a semiconductor laser that emits divergent light and a collimator lens that converts the divergent light into parallel light.

The first and second lenses 4a and 4b on the polygon mirror 3 side of the fθ lens 4 are rotationally symmetric lenses,
The third lens 4c on the scanning target surface 5 side is a toric lens having power mainly in the sub-scanning direction.

In this example, the cylindrical lens 2 serving as the first image forming optical system shifts the wavefront of the light beam imaged near the polygon mirror 3 to the reference spherical surface centered on the paraxial image point. It is configured to delay at the peripheral portion in the scanning direction. Thereby, the fθ lens 4 as the second imaging optical system
, The wavefront of the light beam imaged on the scanning target surface 5 can be delayed at the peripheral portion in the sub-scanning direction with respect to the reference spherical surface centered on the paraxial image point on the scanning target surface 5.

As shown in FIG. 2, one lens surface of the cylindrical lens 2 is a flat surface, and the other lens surface is a curved surface having power only in the sub-scanning direction. The curved surface of the cylindrical lens 2 is formed as a non-cylindrical surface in which the amount of addition to the base shape continuously increases toward the peripheral portion with respect to the cylindrical surface having the base shape indicated by the broken line in the figure. That is, the radius of curvature of this curved surface in the sub-scanning direction increases as the height from the optical axis in the sub-scanning direction increases.

With such a shape, the plane wave incident on the cylindrical lens 2 has a shape delayed from the reference spherical surface (dashed line) centered on the paraxial image point at the peripheral portion as shown by the solid line after the emission. Becomes

As shown in FIG. 3, the cylindrical lens 2 has a discontinuous surface in which the lens thickness gradually increases in accordance with the height in the sub-scanning direction with respect to the base cylindrical surface. It may be configured. In the example of FIG. 3, a step is provided at a position where the height in the sub-scanning direction has a constant value, and a fixed amount is added to the base shape on the peripheral side of the step. According to such a configuration, the wavefront after exiting the cylindrical lens 2 has a shape that is delayed at the peripheral portion with respect to the reference spherical surface.

FIGS. 4A and 4B are views in which the scanning optical system of FIG. 1 is developed along the optical axis. FIG. 4A shows the main scanning direction, and FIG. 4B shows the sub-scanning direction. The laser beam enters the fθ lens 4 in a parallel state in the main scanning direction as shown in FIG. 4A, where it is focused and reaches the scanning target surface.
On the other hand, in the sub-scanning direction, as shown in FIG. 4B, an image is once formed near the polygon mirror 3 by the cylindrical lens 2 and is incident on the fθ lens 4 as divergent light. lens 4 is set to have a higher power in the sub-scanning direction than in the main scanning direction, and re-images the laser beam diverging in the sub-scanning direction on the scanning target surface 5.

As described above, the cylindrical lens 2 is
By forming the wavefront of the light beam to be delayed from the reference spherical surface in the peripheral portion in the sub-scanning direction, the image forming point of the light beam in the peripheral portion in the sub-scanning direction moves in a direction away from the cylindrical lens 2, so that the wavefront aberration is reduced. The minimum imaging point also moves. This allows fθ due to diffraction effects.
The position of the beam waist on the scanning target surface 5 side shifted toward the lens 4 can be made closer to the Gaussian image plane, and the change in beam diameter with respect to defocus can be made symmetrical with respect to the beam waist.

[0026]

Next, the scanning optical system according to the embodiment will be described using specific numerical values. As shown in FIG. 4, the laser beam directed to the cylindrical lens 2 is a Gaussian beam having a diameter of 5.6 mm in the main scanning direction and 2.0 mm in the sub-scanning direction, and is provided before the cylindrical lens 2. 2.5 in main scanning direction by aperture Ap
mm in the sub-scanning direction.

The light emission wavelength of the light source 1 is 780 nm, and the focal length in the sub-scanning direction (focal length due to the paraxial portion) due to the base shape of the cylindrical lens 2 is 120 m.
The combined focal length of the first and second lenses 4a and 4b of the m, fθ lens 4 is 180 mm, and the focal length of the third lens 4c in the sub-scanning direction is 57 mm. The base shape of the cylindrical lens having a focal length of 120 mm is 1.5072.
Is used, the radius of curvature Rz in the sub-scanning direction is Rz = 61.
286 mm.

Subsequently, when the cylindrical lens 2 has a base shape, the aperture diameter a in the sub-scanning direction is
How the beam diameter on the axis on the scanning target surface 5 side changes due to the change in the scan direction will be described. FIG. 5 is a graph showing the degree of change of the beam diameter with respect to defocus when the beam diameter a in the sub-scanning direction is 1.8 mm, 1.2 mm, and 0.8 mm. The defocus corresponds to a distance in the optical axis direction from the Gaussian image plane, and the sign indicates a direction in which a minus approaches the polygon mirror 3 side.

As shown in FIG. 5, when the aperture is sufficiently large (a = 1.8 mm), the beam waist almost coincides with the Gaussian image plane indicated by the defocus of 0 mm, and the beam for the defocus is detected. The change in diameter is also substantially symmetric with respect to the beam waist. On the other hand, when the aperture becomes relatively narrow (a = 1.2, 0.
8) Due to the effect of diffraction, the beam waist position is separated from the Gaussian image plane to the near side, and the change in beam diameter with respect to defocus is asymmetric with respect to the beam waist.

Since the position of the scanning target surface 5 is set with reference to the Gaussian image plane, it is desirable that the change in the beam diameter near the Gaussian image plane be small in order to reduce the change in spot diameter due to defocus. . If the beam waist almost matches the Gaussian image plane, the defocus will be on the plus side,
Since the beam diameter changes only in the direction in which the beam diameter increases in any direction on the minus side, the fluctuation width of the beam diameter within a defocus range of, for example, ± 4 mm is relatively small.

However, when the beam waist position is
When the defocus occurs on the minus side, the beam diameter decreases when the defocus occurs on the minus side, and conversely, when the defocus occurs on the plus side, as indicated by the dashed line or the dashed line, the beam diameter decreases. For example, ± 4m
The change in the beam diameter within the defocus range of m becomes monotonous, and consequently the fluctuation width becomes larger than the case of the solid line.

Next, a description will be given of a change in beam diameter with respect to defocusing when a shape is adopted in which the wavefront lags behind the reference spherical surface in the peripheral portion in the sub-scanning direction in the cylindrical lens 2. FIG. 6 is a graph showing how the beam diameter changes depending on the amount of delay of the wavefront (the amount of phase difference on the reference spherical surface) given by the cylindrical lens, in the case where a = 1.2 mm is set as an example. It is. The graph of the base shape shown by the solid line in FIG. 6 is the same as the graph of a = 1.2 shown by the dotted line in FIG.

Short dashed lines, dash-dot lines, and long pitch dashed lines in FIG. 6 indicate the case where the amount of addition of the lens thickness to the base shape is continuously changed as shown in FIG. 1 / wavelength λ
This shows characteristics when a delay (phase difference) of 16, 2/16 and 3/16 is given. The two-dot chain line shows the characteristics in the case where a step of 0.19 μm is provided in the optical axis direction at a portion having a height of 0.45 mm in the sub-scanning direction in the shape shown in FIG.

In each case, the beam waist is closer to the Gaussian image plane and the change in the beam diameter is more symmetrical with respect to the beam waist as compared with the characteristics of the base shape shown by the solid line.

For example, the variation range of the beam diameter with respect to the defocus of ± 4 mm is 81 μm in the case of the base shape.
１０４104 μm, the fluctuation width is 23 μm, whereas 3λ /
96 μm to 104 μm when 16 wavefront delays are given
Thus, the fluctuation width becomes 8 μm, and the fluctuation width can be suppressed to about 1/3.

The shape of the cylindrical lens in the sub-scanning direction when the above-described phase difference is given is expressed by the following equation (1). Where Xz is the SAG at a height Z from the optical axis.
The quantity (distance in the optical axis direction from the tangent plane to the lens surface), K is a conic coefficient, and B4 is a fourth-order aspheric coefficient. Λ / 16, 2λ at the periphery with a beam diameter of 1.2 mm in the sub-scanning direction
In order to provide a phase difference of / 16, 3λ / 16, the aspherical coefficient may be set to a value represented by the following equation (2). The radius of curvature (vertical radius of curvature) Rz of the base curve in the sub-scanning direction is 61.286 mm as described above.

[0037]

(Equation 1)

FIG. 7 is a graph showing the lens thickness added to the base shape in order to delay the wavefront. The solid line shows 3λ by continuously changing the addition amount to the base shape as shown in FIG. When giving a wavefront delay of / 16,
The broken line indicates a case where the additional amount is changed stepwise as shown in FIG. In addition, these additional amounts can be attached to either the curved surface side or the plane side.

[0039]

As described above, according to the present invention, even when the beam diameter of the incident light beam in the sub-scanning direction is small enough to be affected by diffraction, the beam waist can be brought close to the Gaussian image plane. In addition, it is possible to suppress a change in the beam diameter due to the defocus to be small, and to suppress the deterioration of the drawing performance to be small. Therefore, even when an fθ lens having a low imaging magnification in the sub-scanning direction is used, the F-number of the light beam incident on the fθ lens can be increased by reducing the diameter of the light beam in the sub-scanning direction. A desired spot diameter can be maintained without increasing the size.

[Brief description of the drawings]

FIG. 1 is a plan view in the main scanning direction of a scanning optical system to which the present invention is applied.

FIG. 2 is a side view in the sub-scanning direction showing an example of a cylindrical lens used in the scanning optical system according to the present invention.

FIG. 3 is a side view in the sub-scanning direction showing another example of the cylindrical lens used in the scanning optical system according to the present invention.

FIGS. 4A and 4B are explanatory diagrams showing the operation of each lens in the scanning optical system of FIG. 1, wherein FIG. 4A shows a main scanning direction and FIG. 4B shows a sub-scanning direction.

FIG. 5 is a graph showing a relationship between a beam diameter of a beam incident on a cylindrical lens in a sub-scanning direction and a change in spot diameter with respect to defocus on a scanning target surface side.

FIG. 6 is a graph showing a relationship between a wavefront delay amount provided by a cylindrical lens and a change in spot diameter with respect to defocus on a scanning target surface side.

FIG. 7 is a graph showing a lens thickness added to a base shape to delay a wavefront.

[Explanation of symbols]

 Reference Signs List 1 light source section 2 cylindrical lens 3 polygon mirror 4 fθ lens 4a first lens 4b second lens 4c third lens 5 scan target surface

Claims (11)

(57) [Claims] 1. A light beam emitted from a light source is once imaged in the sub-scanning direction by a first imaging optical system near a deflector, and the light beam deflected by the deflector is scanned by a second imaging optical system. In a scanning optical system that forms an image on a target surface, the beam is narrowed down to an optical path between the light source and the first imaging optical system so that a beam waist position on the scanning target surface changes due to diffraction. An aperture is provided, and the first and second imaging optical systems convert the wavefront of the light beam imaged on the scanning target surface to a reference spherical surface centered on a paraxial image point on the scanning target surface. A scanning optical system characterized in that the scanning optical system is configured to be delayed at a peripheral portion in the sub-scanning direction. 2. The image forming optical system according to claim 1, wherein the first imaging optical system is configured to sub-scan the wavefront of the light beam imaged near the deflector with respect to a reference spherical surface centered on a paraxial image point near the deflector. 2. The scanning optical system according to claim 1, wherein the scanning optical system is configured to delay at a peripheral portion in the direction. 3. The first imaging optical system is a cylindrical lens having power in a sub-scanning direction, and a lens thickness is added at a peripheral portion in the sub-scanning direction to a base shape defined by a cylindrical surface. 3. The method according to claim 2, wherein
The scanning optical system according to 1. 4. The scanning optical system according to claim 3, wherein the amount of addition to the base shape continuously increases according to the height in the sub-scanning direction. 5. The scanning optical system according to claim 3, wherein the amount of addition to the base shape increases stepwise according to the height in the sub-scanning direction. 6. The base shape of the cylindrical lens is such that one lens surface is a flat surface and the other lens surface is a cylindrical surface, and one of the lens surfaces has a lens thickness added to the base shape. 4. The scanning optical system according to claim 3, wherein: 7. The scanning optical system according to claim 1, wherein the first and second imaging optical systems are configured to generate an excessive spherical aberration in a light beam in a sub-scanning direction. . 8. The first imaging optical system is a cylindrical lens having power in a sub-scanning direction, and the radius of curvature of the cylindrical lens in the sub-scanning direction is larger at a peripheral portion than at a central portion in the sub-scanning direction. The scanning optical system according to claim 1, wherein the scanning optical system is configured to: 9. A light beam emitted from a light source is once imaged in a sub-scanning direction near a deflector by a first imaging optical system, and the light beam deflected by the deflector is scanned by a second imaging optical system. In a scanning optical system that forms an image on a target surface, the beam is narrowed down to an optical path between the light source and the first imaging optical system so that a beam waist position on the scanning target surface changes due to diffraction. An aperture is provided; the first and second imaging optical systems move a beam waist position of a light beam formed on the scanning target surface closer to a Gaussian image surface, and adjust a beam diameter before and after the beam waist position. A scanning optical system characterized in that the change is made close to symmetric. 10. The first and second imaging optical systems are arranged so that, in the sub-scanning direction, a wavefront of a light beam formed on the surface to be scanned is centered on a paraxial image point on the surface to be scanned. The scanning optical system according to claim 9, wherein the scanning optical system is configured to be delayed at a peripheral portion with respect to the reference spherical surface. 11. An optical path between the light source and the first imaging optical system is provided with an aperture for reducing the beam waist position on the scanning target surface side by diffraction so as to change. The scanning optical system according to claim 1, wherein: