JPH11271657A - Optical scanning device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the size of a light spot on a scanned surface by adding a small number of components and to minimize the increase of cost by providing the function of a beam expander in a horizontal scanning direction and a function for forming a line image nearby an optical deflection surface for the constitution of an optical system between a collimating lens and the optical deflection surface. SOLUTION: The lens constitution from a light source 6 to an optical deflection reflecting surface 6 includes the collimating lens 2, a negative cylindrical lens 2 having power only in the horizontal scanning direction, and a positive spherical lens 11. The luminous flux from the light source 1 is collimated by the collimating lens 2 into nearly parallel luminous flux, which is expanded in luminous flux width in the horizontal scanning direction by the said cylindrical lens 10 and positive spherical lens 11 and converged in the vertical scanning direction nearby the optical deflection reflecting surface 6 to form an image. Namely, the function of a beam expander and the function of an image forming lens are obtained only by the two lenses 10 and 11.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a function of correcting pitch unevenness due to surface tilt of a light deflecting means when a light beam emitted from a light source is reflected and deflected by a light deflecting means and scans a surface to be scanned. More particularly, the present invention relates to an optical scanning device having a beam expander optical system between a light source and a light deflecting unit.

[0002]

2. Description of the Related Art Conventionally, laser printers, laser plate making apparatuses, and the like, which scan a light beam such as a laser beam on a surface to be scanned such as a photosensitive material and form an image on the surface, have been conventionally used. Is known. Such an optical scanning device includes a light deflecting unit such as a rotary polygon mirror that reflects and deflects a light beam emitted from a light source and scans the surface to be scanned, and a scanning device that forms an image of the light beam on the surface to be scanned. And an image optical system.

In such an optical scanning device, when the light deflecting and reflecting surface of the light deflecting means is tilted, a direction (sub scanning direction) orthogonal to the main scanning direction on the surface to be scanned is used.
The imaging position of the light beam changes for each scanning line, causing pitch unevenness in the scanning line, and it is impossible to obtain a good image. For this reason, various optical scanning devices having a function of correcting surface tilt of the light deflecting unit are known.

For example, in a surface tilt correction optical scanning device in which a collimating lens and a cylindrical lens having power only in the sub-scanning direction are disposed between a light source and a light deflecting / reflecting surface in this order from the light source side, After the light beam from the light source is made substantially parallel by the focusing lens, a line image is formed near the light deflection reflection surface by the cylindrical lens. The light beam deflected and reflected by the light deflecting / reflecting surface of the light deflecting unit forms an image on a surface to be scanned via a cylindrical lens, a scanning image forming optical system, and the like, and is scanned.

In order to improve the accuracy of a formed image, it is important to reduce the size of a light spot on the surface to be scanned. Since the light spot size in the main scanning direction is substantially inversely proportional to the width of a linear light beam incident on the scanning image forming optical system in the main scanning direction, in order to reduce the light spot size, the focal length of the collimating lens is generally set to be smaller. A method has been adopted in which the aperture is increased to increase the aperture, or the width of the parallel light beam after the collimating lens is emitted is expanded by a beam expander optical system.

However, if the focal length or the diameter of the collimating lens is increased, the size of the collimating lens is increased and the cost is increased.

On the other hand, as a conventional technique for expanding the width of a parallel light beam by a beam expander optical system, for example, there is one shown in FIGS. 4 and 5 are a sectional view in the main scanning direction and a sectional view in the sub-scanning direction showing the configuration from the light source to the light deflecting reflection surface of the light deflecting means in each optical scanning device.

The optical scanning optical system shown in FIG. 4 has a collimating lens 2, a negative spherical lens 3, a positive spherical lens 4, and a positive spherical lens 4, between the light source 1 and the light deflecting / reflecting surface 6, in this order from the light source side. Are arranged, and two spherical lenses 3 and 4 among them function as a beam expander optical system.

When the beam expander optical system is constituted by a spherical lens as described above, the collimating lens 2
The parallel light beam emitted from the light source has a wider light beam width in both the main scanning direction and the sub-scanning direction. Therefore, in order to form a linear image on the light deflecting / reflecting surface 6, the focal length of the positive cylindrical lens 5 must be increased by the magnification of the beam expander optical system. The size of the device increases, and the size of the entire device also increases. Cylindrical lenses are expensive, and larger sizes lead to higher costs.

Next, the optical scanning optical system shown in FIG. 5 includes a collimating lens 2, a negative cylindrical lens 7, and a positive cylindrical lens 8 between the light source 1 and the light deflecting / reflecting surface 6 in order from the light source side. , And a positive cylindrical lens 9, of which two cylindrical lenses 7, 8 function as a beam expander optical system.

When the beam expander optical system is constituted by a cylindrical lens, the width of the parallel light beam emitted from the collimating lens 2 can be expanded only in the main scanning direction. Therefore, the entire apparatus can be made compact as compared with the optical scanning optical system shown in FIG. 4, but the cost of the cylindrical lens is higher than that of the spherical lens, and using three of them increases the cost.

When a plurality of cylindrical lenses are used, their top line (TOP LINE) or bottom line (BO
If the direction of the TTOM LINE does not match, the image quality deteriorates. Therefore, it is necessary to precisely match this direction with the optical axis of the optical system as the rotation axis, and the burden on assembly adjustment is large.

As shown in the above example, the method of expanding the parallel light beam width using the beam expander optical system is effective for reducing the size of the light spot and improving the performance. However, since the number of lens components increases and the cost increases, it is demanded that the number of such components be reduced as much as possible and the cost increase be minimized.

The present invention has been made in view of such circumstances, and uses a beam expander optical system without lengthening the focal length of a collimating lens or increasing the diameter of a collimating lens. It is an object of the present invention to provide an optical scanning device capable of reducing the number of components to be added and minimizing cost increase when achieving reduction in light spot size.

[0015]

An optical scanning device according to the present invention comprises a light source, a collimating lens that emits a light beam from the light source as a substantially parallel light beam, and a light scanning surface that deflects the light beam by a light deflecting surface. A light deflecting means for scanning on a surface, a cylindrical lens arranged between the collimating lens and the light deflecting means and having power only in the main scanning direction of the light deflecting means, and a light beam from the cylindrical lens. A positive spherical lens that linearly forms an image on or near the light deflecting surface of the light deflecting means, a beam expander optical system disposed in this order from the light source side, and a light deflected by the light deflecting means. An imaging optical system for forming an image of the light beam on the surface to be scanned. Here, the “main scanning direction” means a direction parallel to the trajectory of the deflected light beam on the surface to be scanned.

[0016]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a configuration of an optical scanning device according to an embodiment of the present invention, and is a schematic diagram showing a cross section in the main scanning direction including an optical axis (hereinafter, referred to as a main scanning direction cross section).

As shown in FIG. 1, the optical scanning device includes a collimating lens 2 for converting a light beam emitted from a light source 1 into substantially parallel light, and a line functioning as a beam expander optical system. Lens 1 having negative power only in the main scanning direction for forming an image
A polygon which has a 0 and a positive spherical lens 11 and a light deflecting / reflecting surface 6 near an image forming position of the positive spherical lens 11 and is a light deflecting means for reflecting and deflecting the light beam to scan on the surface to be scanned 15 A mirror 12, scanning imaging lenses 13 a and 13 b for imaging the light beam reflected and deflected by the polygon mirror 12 on the surface to be scanned 15, and a light beam transmitted through these scanning imaging lenses 13 a and 13 b. Scanned surface 1
The negative cylindrical mirror 14 has a negative power that is reflected toward the scanning surface 5 and has only a negative power on the scanned surface 15 in a direction substantially orthogonal to the main scanning direction (hereinafter, referred to as a sub-scanning direction).

FIG. 2 shows a basic configuration of the optical system from the light source 1 to the light deflection reflection surface 6 according to the embodiment of the present invention in a main scanning direction section and a plane perpendicular to the main scanning direction section and including the optical axis. Hereinafter, this is referred to as a section in the sub-scanning direction). FIG. 3 is a sectional view in the sub-scanning direction showing a basic configuration of an optical system from the light deflection reflecting surface 6 to the surface to be scanned 15 according to the embodiment of the present invention.

1 and 3, the scanning imaging lenses 13a and 13b and the negative cylindrical mirror 14 constitute a scanning imaging optical system. The light beam emitted from the light source 1 is imaged on the surface to be scanned 15 and
As the polygon mirror 12 rotates in the direction of arrow A, the surface is scanned in the direction of arrow B on the surface 15 to be scanned. Further, the scanned surface 15 is moved in the sub-scanning direction (the direction of arrow C) to form an image. Light deflection reflecting surface 6 and scanned surface 15
Are scanning imaging lenses 13a,
13b and a negative cylindrical mirror 14 having power only in the sub-scanning direction is in a conjugate relationship with the optical system, so that a function of optically correcting the tilt of the light deflecting / reflecting surface 6 can be obtained.

Here, the lens structure from the light source 1 to the light deflecting / reflecting surface 6 is, as described above, a collimating lens 2, a negative cylindrical lens 10 having power only in the main scanning direction, and a positive spherical lens 11. The operation and effect of the above will be described. As shown in FIG. 2, in the present embodiment, a light beam from a light source 1 is converted into a substantially parallel light beam by a collimating lens 2, and a negative cylindrical lens 10 and a positive spherical lens 11 having power only in the main scanning direction are used. The light beam width in the main scanning direction is increased, and in the sub-scanning direction, light is condensed near the light deflection reflection surface 6 to form a line image.

That is, the negative cylindrical lens 10
The positive spherical lens 11 has a function of a beam expander and a function of an image forming lens for forming a linear image on the light deflecting / reflecting surface 6 with only two lenses. This is because the position of the rear focal point of the negative cylindrical lens 10 and the position of the front focal point of the positive spherical lens 11 are substantially coincident with each other, so that a function of a beam expander is provided in the main scanning direction. This is because a line image can be formed by making the focal point substantially coincide with the light deflection reflection surface 6 or its vicinity.

As a result, the beam expander function can be achieved by adding a smaller number of components, and the light spot size on the scanned surface 15 can be reduced without changing the collimating lens. Here, the negative cylindrical lens 10 is a cylindrical lens having power only in the main scanning direction. However, in the present embodiment, a negative cylindrical lens is used, but the light beam width in the main scanning direction of the light beam incident on the subsequent positive spherical lens 11 is changed in the main scanning direction of the parallel light beam incident on the cylindrical lens. A positive cylindrical lens can be used if the lens arrangement is set to be larger than the light beam width.

The direction of the sub-scan indicated by the arrow C is a relative moving direction between the surface to be scanned 15 and the scanning light beam. The surface to be scanned 15 is fixed and the scanning light beam is moved in the direction of the arrow C. You may do so.

Hereinafter, embodiments of such an optical scanning device will be described using specific numerical values. In this embodiment, the radii of curvature R _M and R _S of each lens surface (R _M is the radius of curvature in the cross section in the main scanning direction, R _S is the radius of curvature in the cross section in the sub scanning direction), the air spacing D of each lens, and the wavelength of each lens 78
Table 1 shows the refractive index N at 0 nm. Furthermore, the curvature radius R _M, R _S, and air space D is standardized composite focal length of the beam expander optical system 100.
However, in Table 1 and the tables described later, each symbol R
_M, R _S, D, the numbers referring to N successively increase from the light source side.

The lower part of Table 1 shows the values of the focal lengths f _M and f _S in the main scanning section and the sub-scanning section of the beam expander optical system of this embodiment, and the back focus Bf _S in the sub-scanning section.

[0026]

[Table 1]

Next, Comparative Examples 1 and 2 will be described in order to clarify the effects of the present invention. Comparative Example 1
FIG. 4 shows an example in which the above-described beam expander optical system is constituted by a spherical lens, and FIG. 4 is a cross section in the main scanning direction showing a structure from the light source 1 to the light deflecting reflection surface 6 of the light deflecting means in the optical scanning device. FIG. 3 is a diagram and a sectional view in the sub scanning direction. The configuration of the optical system shown in FIG. 4 is as described above, and the negative spherical lens 3 and the positive spherical lens 4 function as a beam expander optical system.

Hereinafter, specific numerical values of the optical scanning device of Comparative Example 1 will be shown. The radii of curvature R _M and R _S of each lens surface in Comparative Example 1 (R _M is the radius of curvature in the cross section in the main scanning direction, R _S is the radius of curvature in the cross section in the sub scanning direction,
Table 2 shows the air gap D of each lens and the refractive index N of each lens at a wavelength of 780 nm. Note that the radii of curvature R _M , R _S , and the air gap D are standardized with the combined focal length of the negative spherical lens 3, the positive spherical lens 4, and the positive cylindrical lens 5 being 100. The lower part of Table 2 shows the composite focal lengths f _M , f _S , and sub-scanning distances of the negative spherical lens 3, the positive spherical lens 4, and the positive cylindrical lens 5 in the main scanning section and the sub-scanning section of Comparative Example 1. Each value of the back focus Bf _S in the scanning section is shown.

[0029]

[Table 2]

As is apparent from Table 2, according to Comparative Example 1 in which the beam expander optical system is constituted by a spherical lens, Bf _S is twice that of the embodiment according to the present invention.
The whole device becomes large. Comparative Example 2 is as described above,
FIG. 5 is an example in which the beam expander optical system is constituted by a cylindrical lens. FIG. 5 is a sectional view in the main scanning direction and a sub-scanning direction showing a configuration from the light source 1 to the light deflecting reflection surface 6 of the light deflecting means in the optical scanning device. It is sectional drawing. The configuration of the optical system shown in FIG. 5 is as described above, and the negative cylindrical lens 7 and the positive cylindrical lens 8 function as a beam expander optical system.

Hereinafter, specific numerical values of the optical scanning device of Comparative Example 2 will be shown. Table 3 shows the radii of curvature R _M and R _S of each lens surface, the air gap D of each lens, and the refractive index N of each lens at a wavelength of 780 nm in Comparative Example 2. The radii of curvature R _M , R _S , and the air gap D are determined by setting the composite focal length of the negative cylindrical lens 7, the positive cylindrical lens 8, and the positive cylindrical lens 9 to 10
It is standardized as 0. Comparative Example 2 is shown in the lower part of Table 3.
The values of the focal lengths f _M and f _S of the negative cylindrical lens 7, the positive cylindrical lens 8, and the positive cylindrical lens 9 in the main scanning section and the sub scanning section, and the back focus Bf _S in the sub scanning section are Show.

[0032]

[Table 3]

As is clear from Tables 1 and 3, according to the present invention, the same function as that of Comparative Example 2 using three cylindrical lenses between the light source 1 and the light deflecting / reflecting surface 6 is provided. This can be achieved by one cylindrical lens and one spherical lens.

The lens system constituting the optical scanning device according to the present invention is not limited to the above-described embodiment, but various modifications can be made. For example, the radius of curvature R _{M of} each lens can be changed. , R _S and the distance (or lens thickness) D between the lenses can be changed as appropriate.

[0035]

As described above, according to the optical scanning device of the present invention, the configuration of the optical system between the collimating lens and the light deflecting surface is changed in order from the light source to the power only in the main scanning direction. By having a cylindrical lens and a positive spherical lens that have the function of a beam expander in the main scanning direction and the function of forming a line image near the light deflection surface, the light spot size on the surface to be scanned Reduction can be achieved with a smaller number of parts without changing the collimating lens. In addition, since the spherical lens has a rotationally symmetric shape, the high-precision alignment required for the cylindrical lens is not required. As a result, the number of expensive cylindrical lenses can be reduced, the assembly process can be simplified, and the cost increase due to the addition of the beam expander optical system can be minimized.

[Brief description of the drawings]

FIG. 1 is a sectional view in the main scanning direction showing a configuration of an optical scanning device according to an embodiment of the present invention.

FIG. 2 is a sectional view in a main scanning direction and a sectional view in a sub-scanning direction showing a basic configuration of an optical system from a light source to a light deflection reflection surface according to an embodiment of the present invention.

FIG. 3 is a sectional view in the sub-scanning direction showing a basic configuration of an optical system from a light deflection reflecting surface to a surface to be scanned according to the embodiment of the present invention.

FIG. 4 is a sectional view in the main scanning direction and a sectional view in the sub-scanning direction showing a basic configuration of an optical system from a light source to a light deflecting / reflecting surface in Comparative Example 1.

FIG. 5 is a sectional view in the main scanning direction and a sectional view in the sub-scanning direction showing a basic configuration of an optical system from a light source to a light deflecting / reflecting surface in Comparative Example 2.

[Explanation of symbols]

R ₁ -R ₆ Radius of curvature of lens surface D ₁ -D ₅ Lens surface spacing (lens thickness) X Optical axis 1 Light source 2 Collimating lens 3 Negative spherical lens 4,11 Positive spherical lens 5,8,9 Positive 6 Cylindrical lens 6 Light deflection / reflection surface 7, 10 Negative cylindrical lens 12 Polygon mirror 13a, 13b Scanning imaging lens 14 Negative cylindrical mirror 15 Scanned surface

Claims (1)

[Claims] A light source; a collimating lens for emitting a light beam from the light source as a substantially parallel light beam; a light deflecting means for deflecting the light beam by a light deflecting surface to scan on a surface to be scanned; A cylindrical lens disposed between the mating lens and the light deflecting means and having power only in the main scanning direction of the light deflecting means; and a light beam from the cylindrical lens is provided on the light deflecting surface of the light deflecting means or on the light deflecting surface. A beam expander optical system arranged in this order from a light source side to a positive spherical lens for linearly forming an image in the vicinity thereof; and a light beam deflected by the light deflecting means to form an image on the surface to be scanned. An optical scanning device comprising an image optical system.