JP2001350115A - Optical scanner - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make suppressible the sagittal image field curvature to be small in an optical scanner which uses a rotary polygon mirror. SOLUTION: In the case of making a scan by deflecting the laser light L emitted from a light source optical system 10 by a rotary polygon mirror 21 which is rotating and converging the deflected laser light on a scanning surface 1 by an image formation optical system 30, an asymmetrical cylindrical mirror 33 which has power at right angles of the plane P of deflection of the laser light and continuously varies in the power in the deflecting direction is so arranged that its cylindrical surface is asymmetrical about a plane which contains the optical axis of the image formation optical system and is perpendicular to the plane P of deflection.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical scanning device for converging a laser beam deflected by a rotary polygonal mirror on a plane, and more particularly to an optical scanning device having an optical system for suppressing curvature of field. It is.

[0002]

2. Description of the Related Art Conventionally, a laser beam deflected by a rotary polygon mirror is converged on a scanning surface so that a condensed spot of the laser beam is main-scanned on the scanning surface, and at the same time as the main scanning direction. There is known an optical scanning device which moves the scanning surface in a perpendicular sub-scanning direction and scans the scanning surface two-dimensionally by the main scanning and the sub-scanning.

Since the rotating polygon mirror described above is driven at a high speed, wobbling due to vibration occurs, and a main scanning line for scanning on a scanning surface is distorted in a sub-scanning direction. It is difficult to perform processing as being completely parallel to the rotation axis, resulting in surface tilt and unevenness in the pitch of the main scanning lines. Therefore, in such an optical scanning device, in order to correct the error in the sub-scanning direction of the scanning line as described above, the laser beam is scanned through a cylindrical lens or a cylindrical mirror having a cylindrical surface, so that the optical path of the laser beam is adjusted. The scanning optical system is configured so that the position of the condensed spot of the laser beam on the scanning surface returns to the predetermined position by the force for correcting the optical path of the cylindrical surface even if the laser beam departs from the predetermined optical path.

Further, in a scanning optical system for correcting an error in the sub-scanning direction due to wobbling of a rotary polygon mirror or surface tilt using a cylindrical lens or a cylindrical mirror, the curvature of a cylindrical surface is changed by an optical axis passing substantially at the center of a deflecting surface. An optical scanning device having an optical system for reducing sagittal curvature of field by continuously changing symmetrically with respect to is proposed (Japanese Patent Laid-Open No. 3-54513). It is disclosed that the maximum width (PP value) of sagittal field curvature becomes 1.5 mm in a range of 36 degrees.

[0005]

In such an optical scanning apparatus, there is a demand that the deflection angle of the scanning optical system should be further widened and the optical components should be arranged closer to each other in order to reduce the size. .

However, when the deflection angle of the scanning optical system is increased, the sagittal curvature of field of the condensed spot on the scanning surface increases, so that the laser beam does not converge on the scanning surface at a predetermined size, and the curvature of the cylindrical surface is reduced. In some cases, the sagittal curvature of field cannot be suppressed to a desired value even if a method of continuously changing is used.

The present invention has been made in view of the above circumstances, and has as its object to provide an optical scanning device capable of suppressing sagittal field curvature.

[0008]

An optical scanning device according to the present invention comprises:
A light source for emitting laser light, a rotary polygon mirror having a plurality of reflection surfaces parallel to the rotation axis and reflecting and deflecting the laser light by rotating about the rotation axis, and a deflecting surface of the laser light. A cylindrical lens or a cylindrical mirror having a cylindrical surface whose power continuously changes along the deflection direction and condenses laser light deflected by the rotary polygon mirror on a plane. In an optical scanning device including an imaging optical system, the cylindrical surface has an asymmetric shape with respect to a plane that includes an optical axis of the imaging optical system and is perpendicular to the deflection surface.

The image forming optical system includes two spherical lenses and one spherical lens.
It can be composed of one cylindrical mirror.

The cylindrical surface having the asymmetric shape is such that the cylindrical surface having a plane symmetric shape is arranged so as to be asymmetric with respect to a plane including the optical axis of the imaging optical system and perpendicular to the deflection surface. It can be.

[0011]

According to the optical scanning device of the present invention, when condensing the laser beam deflected by the rotary polygon mirror on the scanning surface through the imaging optical system, the cylindrical surface included in the imaging optical system Has an asymmetric shape with respect to a plane that includes the optical axis of the imaging optical system and is perpendicular to the deflecting surface. It is possible to more accurately correct asymmetric sagittal curvature of field caused by being converged on a plane.

If the image forming optical system is composed of two spherical lenses and one cylindrical mirror, an optical scanning device can be provided without adding new optical parts or the like for correcting sagittal field curvature. Can be further miniaturized.

A cylindrical surface having an asymmetric shape, wherein a cylindrical surface having a plane symmetric shape is arranged so as to be asymmetric with respect to a plane which includes an optical axis of the imaging optical system and is perpendicular to the deflection surface. Then, the cylindrical surface can be easily manufactured, and the manufacturing cost of the optical scanning device can be reduced.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, specific embodiments of the optical scanning device of the present invention will be described with reference to the drawings. FIG.
FIG. 1 is a diagram illustrating a schematic configuration of an optical scanning device according to a first embodiment of the present invention. Optical scanning device 80 according to the present embodiment
Is composed of a scanning optical system 70 and a scanning surface conveying means (not shown). The scanning optical system 70 includes a semiconductor laser 11, a collimator lens 12, and a cylindrical lens 13, and transmits laser light L emitted from the semiconductor laser 11 in one direction. A light source optical system 10 for linearly condensing light, a laser having a plurality of reflecting surfaces parallel to the rotation axis, and being rotated in the direction of arrow ω by a motor (not shown) about a rotation axis G and emitted from the light source optical system 10 Rotating polygon mirror 21 for reflecting and deflecting light L
(Also called a polygon mirror), and the first spherical lens 3
1, a second spherical lens 32, and a first asymmetric having a cylindrical surface having power in a direction perpendicular to the deflecting surface P of the laser beam L and having this power continuously changing along the deflecting direction. The imaging optical system 30 includes a cylindrical mirror 33 and focuses the laser beam L deflected by the rotary polygon mirror 21 in the main scanning direction on the scanning surface 1 (the direction of arrow X in FIG. 1). The cylindrical surface includes the optical axis C of the image forming optical system 30 and the deflecting surface P of the laser light L.
It has an asymmetric shape with respect to a plane perpendicular to.

The scanning surface 1 is conveyed by the scanning surface conveying means (not shown) in the direction of arrow Y substantially perpendicular to the main scanning direction with respect to the scanning optical system 70, and is subjected to sub-scanning.

Next, the operation of the above embodiment will be described.

The laser light L emitted from the semiconductor laser 11 of the light source optical system 10 is converted into parallel light by a collimator lens 12, and the parallel light is directed in one direction by a cylindrical lens 13 (in the direction of the rotation axis G of the rotary polygon mirror 21). To)
Only the light is condensed and condensed linearly on the reflection surface 21a of the rotating polygon mirror 21.

The laser beam L condensed linearly on the reflecting surface 21a of the rotary polygon mirror is rotated by the rotary polygon mirror 21 rotating in the direction of arrow ω.
The light is deflected by the rotation of, and is converged on the scanning surface 1 by the imaging optical system 30. When the laser light condensed linearly on the reflecting surface 21a is condensed on the scanning surface 1, the light is condensed in the sub-scanning direction (sagittal plane direction in the arrow Y direction) by the first spherical lens 31 and the second spherical lens. This is performed by the power of the spherical lens 32 and the first asymmetric cylindrical mirror 33, while the main scanning direction (meridional plane direction in the direction of arrow X)
The light is focused by the power of only the first spherical lens 31 and the second spherical lens 32. The first asymmetric cylindrical mirror 33 has no power to converge or diverge the laser light in the main scanning direction.

The laser light L emitted from the light source optical system 10 is deflected by the rotary polygon mirror 21 rotating in the direction of the arrow ω, condensed into a spot through the imaging optical system 30, and is focused on the scanning surface 1 by an arrow. Main scanning is repeatedly performed in the X direction,
At the same time, the scanning surface 1 is conveyed in the arrow Y direction substantially perpendicular to the main scanning direction to perform sub-scanning, whereby the scanning surface 1 is two-dimensionally scanned by the laser light L.

Next, the details of the imaging optical system 30 will be described.

FIG. 2 is a cross-sectional view in which each optical element of the light source optical system 10 and the imaging optical system 30 is cut by a plane that includes the optical axis of these optical systems and is perpendicular to the deflection plane P of the laser light L. Yes, the design data for these optical elements is shown in FIG.

R1 to r8 shown in FIGS. 2 and 3 are the radii of curvature of the respective lens surfaces or mirror surfaces. The sign of the curvature radii indicates that the surface is directed in the direction of incidence of light on the lens surface. The case of a convex surface is positive, and the case of a concave surface is negative. Further, d1 to d8 represent the thickness of the lens or the air gap between the lenses. Here, d8 represents the distance from the reflecting surface of the first asymmetric cylindrical mirror 33 to the scanning surface 1, and the angle θ between the incident light of the laser beam L and the reflected light on the first asymmetric cylindrical mirror 33 is 88 degrees. is there. FIG. 3 also shows the names of glass materials used for each optical element.

Next, the shape of the cylindrical surface of the first asymmetric cylindrical mirror 33 and the condensing error of the condensed spot condensed and scanned on the scanning surface 1 in the sagittal plane direction, that is, the sagittal field curvature (FIG. (Refer to the broken line D of 1)
Will be described.

First, in order to make it easy to understand the relationship between the shape of the cylindrical surface and the sagittal curvature of field, the curvature of the cylindrical surface should be adjusted to include the optical axis of the imaging optical system and to the deflection surface P of the laser beam L. A description will be given of a case where a conventionally used symmetric cylindrical mirror 34, which is continuously changed symmetrically with respect to a vertical plane, is designed in place of the first asymmetric cylindrical mirror 33 (see FIG. 1).

FIG. 4A shows a symmetric cylindrical mirror 3.
4 is a design formula of the cylindrical surface, FIG. 4B is a coefficient value corresponding to the design formula of the symmetric cylindrical mirror 34, and FIG. 4C is a main scanning direction of the cylindrical surface of the symmetric cylindrical mirror 34 (arrow in FIG. 1). The value of the radius of curvature at each coordinate value in the (X direction) is shown. The origin of the X coordinate of the cylindrical surface (point of X = 0) is the position where the optical axis C and the cylindrical surface intersect, and the radius of curvature is In the same manner as described above, the positive and negative signs are positive when the surface is convex with respect to the incident direction of light to the lens surface, and negative when the surface is concave.

The design formula for the cylindrical surface shown in FIG. 4A represents the relationship between the coordinates of the cylindrical surface in the X direction and the radius of curvature R of the cylindrical surface at that position, and is shown in FIG. 4B. When the value of each coefficient is substituted into the design equation and expressed, R = −0.0019X ² −230.6, and the increase in the absolute value of the radius of curvature in the X direction is proportional to the square of the distance from the origin.

That is, as can be seen from the table of FIG. 4C, the radius of curvature of the cylindrical surface at the origin (point X = 0) of the X coordinate on the cylindrical surface is R = −230.6.
mm, the position X of both scanning ends on the cylindrical surface X = ± 10
The radius of curvature at 0 mm is R = -249.6, and the curvature of the cylindrical surface is symmetric with respect to the origin X and changes continuously along the main scanning direction.

The symmetric cylindrical mirror 3
FIG. 5 shows the values of the sagittal field curvature of the condensed spot on the scanning surface 1 when the laser beam L is scanned using the laser beam L4.
The origin of the beam deflection angle (beam deflection angle θ = 0) in FIG. 5 is the optical axis C, the range of the beam deflection angle θ is ± 48 degrees, and the light source side (see FIG. 1) is in the minus direction of the beam deflection angle. ing. On the other hand, the origin of the Z axis representing the sagittal curvature of field (Z =
The position of 0) is on the scanning surface 1 and the lower part of the scanning surface is in the plus direction.

As can be seen from the drawing, the beam deflection angle θ =
The value of the sagittal field curvature at the position of 0 is 0, and the converging position of the converging spot in the sagittal plane direction (sagittal field curvature) as the beam deflection angle θ goes in the positive direction.
Moves in the minus direction, that is, above the scanning plane 1, and the error due to the sagittal field curvature increases. On the other hand, the sagittal curvature of field increases in the positive direction as the beam deflection angle θ moves in the minus direction, that is, the error increases below the scanning plane 1, and the error decreases again when the beam deflection angle θ is near -45 degrees. The sagittal field curvature has an asymmetric value with respect to the origin.

The maximum width (PP value) of sagittal field curvature including the plus direction and the minus direction is 4.5.
4 mm, and the swing angle ±
The sagittal field curvature peak-to-peak value is 1.5 m in the range of 36 degrees.
m, the maximum width of sagittal curvature of field (PP value) when the range of the deflection angle is expanded to ± 48 degrees
Is about three times as large.

Next, the case where the first asymmetric cylindrical mirror 33 used in the first embodiment is used will be described.

FIG. 6A is a design equation of the cylindrical surface of the first asymmetric cylindrical mirror 33, FIG. 6B is a coefficient value of the design equation of the first asymmetric cylindrical mirror 33, and FIG. It shows the value of the radius of curvature of the cylindrical surface of the first asymmetric cylindrical mirror 33 at each X coordinate in the main scanning direction, and the X of the cylindrical surface is the same as in the case of the symmetric cylindrical mirror 34.
The origin of the coordinates (the point where X = 0) is the position where the optical axis C and the cylindrical surface intersect, and the direction on the light source side is the minus direction as described above.

By substituting the values of the coefficients shown in FIG. 6B into the design formula of the cylindrical surface shown in FIG. 6A, R = −0.0018 (X + 5) ² −230.6 Unlike the case of the symmetrical cylindrical mirror 34, the amount of increase in the absolute value of the radius of curvature becomes asymmetric with respect to the origin, and the curvature of the cylindrical surface is changed to include the optical axis of the imaging optical system and to deflect the laser light L. The shape is continuously changed asymmetrically with respect to a plane perpendicular to P.

That is, as can be seen from the table of FIG. 6C, the radius of curvature of the cylindrical surface at the origin of the X coordinate on the cylindrical surface is R = −230.65 mm, and the radius of curvature at the position X = + 100 mm of the cylindrical surface is R.
= −250.4, and the radius of curvature at the position X = −100 mm is R = −246.8.

As can be seen from the design equation, the shape of the asymmetric cylindrical surface is such that the plane-symmetric cylindrical surface represented by the following formula: R = −0.0018X ² -230.6 is shifted by 5 mm from the light source side (X-axis). (Minus direction), and the plane of the cylindrical surface having a plane symmetry is asymmetric with respect to the plane including the optical axis of the imaging optical system 30 and perpendicular to the plane of deflection P of the laser light. It is arranged in.
The cylindrical mirror having such a shape is easier to design and manufacture than a cylindrical mirror having an asymmetric shape with respect to a plane such as perpendicular to the laser light deflection plane P.

FIG. 7 shows the sagittal field curvature when the laser beam L is scanned within the range of the beam deflection angle ± 48 degrees using the first asymmetric cylindrical mirror 33. As can be seen from the figure, the value Z of the sagittal field curvature at the position of the beam deflection angle θ = 0 is 0, and the value Z of the sagittal field curvature is also substantially 0 in the positive direction of the beam deflection angle θ. Θ = −3 for the minus direction of the angle θ
In the vicinity of 0 degree, the sagittal field curvature becomes maximum in the plus direction, and the value Z of the sagittal field curvature becomes substantially zero again in the vicinity of the maximum deflection angle θ = -48 degrees. The maximum width (PP value) of the sagittal field curvature is 1.87 mm, and the sagittal field curvature is greatly improved in the range of the beam deflection angle ± 48 degrees as compared with the case where the symmetric cylindrical mirror 34 is used. You.

Next, as a second embodiment, a second asymmetric cylindrical mirror 35 (see FIG. 1) is used in place of the first asymmetric cylindrical mirror 33 used in the first embodiment. The case where sagittal field curvature is further reduced will be described.

FIG. 8A is a design equation of the cylindrical surface of the second asymmetric cylindrical mirror 35, FIG. 8B is a coefficient value of the design equation of the second asymmetric cylindrical mirror 35, and FIG. The figure shows the value of the radius of curvature at each X coordinate in the main scanning direction of the cylindrical surface of the second asymmetric cylindrical mirror 35.

[0039] When the cylindrical surface design equations that shown in FIG. 8 (a) represents by substituting the values of the coefficients of FIG. 8 (b), R = -0.000467 (| X + 5 |) 2.3 -23
0.6, as in the case of the asymmetric cylindrical mirror 33, the increase in the absolute value of the radius of curvature becomes asymmetric with respect to the origin.

That is, as can be read from the table of FIG. 8C, the radius of curvature of the cylindrical surface at the origin of the X coordinate set on the cylindrical surface is R = −230.62 m.
m, the radius of curvature at the position X = + 100 mm of the cylindrical surface is R = −251.4, and the radius of curvature at the position X = −100 mm is R = −247.1.

It should be noted, first the exemplary shape of the asymmetric cylindrical surface similar to the form the following formulas, only 5mm plane symmetrical cylindrical surface represented by R = -0.000467X ^2.3 -230.6 source Side (minus direction of the X-axis), and a cylindrical surface having a plane-symmetric shape is asymmetric with respect to a plane including the optical axis of the imaging optical system 30 and perpendicular to the deflection plane P of the laser light. They are arranged in a shape.
The cylindrical mirror having such a shape is easier to design and manufacture than a cylindrical mirror having an asymmetric shape with respect to a plane such as perpendicular to the laser light deflection plane P.

FIG. 9 shows the sagittal curvature of field when the laser beam L is scanned using the second asymmetric cylindrical mirror 35 in a range of a beam deflection angle of ± 48 degrees. As can be seen from the drawing, the value Z of the sagittal field curvature at the position of the beam deflection angle θ = 0 is 0, and the sagittal field curvature is negative in the vicinity of θ = + 30 degrees with respect to the plus direction of the beam deflection angle θ. Maximum, then θ = +
In the vicinity of 45 degrees, the sagittal curvature of field Z again becomes substantially zero. On the other hand, in the minus direction of the beam deflection angle θ, the value Z of the sagittal field curvature becomes substantially zero from θ = 0 to −48 degrees. The maximum width (PP value) of the sagittal field curvature including the plus direction and the minus direction is 1.42 mm, which is smaller than that of the first embodiment using the first asymmetric cylindrical mirror 33. The sagittal field curvature can be further improved in the range of the shake angle ± 48 degrees.

As described above, according to the present invention, sagittal field curvature can be suppressed by using an asymmetric cylindrical mirror even if the beam deflection angle of the optical scanning device 80 is increased.

As the cylindrical surface in the first and second embodiments, a cylindrical surface having an asymmetric shape is a cylindrical surface having a plane symmetrical shape including the optical axis of the imaging optical system and a deflecting surface of laser light. An example is shown in which an asymmetric cylindrical mirror is arranged so as to have an asymmetric shape with respect to a plane perpendicular to the surface, that is, a mirror having a function of an asymmetric cylindrical mirror by arranging a plane symmetric cylindrical mirror by moving it in the main scanning direction. The cylindrical surface is not limited to such a shape, and a cylindrical surface having an asymmetric shape with respect to a surface perpendicular to the laser light deflection surface may be used.

In the first and second embodiments, the imaging optical system has been described as having two spherical lenses and one cylindrical mirror. Is not limited to such a configuration, and may be configured using, for example, a cylindrical lens instead of a cylindrical mirror.

[Brief description of the drawings]

FIG. 1 is a diagram showing a schematic configuration of an optical scanning device according to an embodiment of the present invention;

FIG. 2 is a diagram showing details of an optical path of an optical axis of the optical scanning device.

FIG. 3 is a diagram showing design data of an optical system of the optical scanning device.

FIG. 4 is a diagram showing design data of a conventional symmetric cylindrical mirror surface;

FIG. 5 is a diagram showing sagittal field curvature of a conventional optical scanning device.

FIG. 6 is a diagram showing design data of an asymmetric cylindrical mirror surface according to the first embodiment;

FIG. 7 is a diagram illustrating sagittal field curvature of the optical scanning device according to the first embodiment;

FIG. 8 is a diagram showing design data of an asymmetric cylindrical mirror surface according to the second embodiment;

FIG. 9 is a diagram illustrating sagittal field curvature of the optical scanning device according to the first embodiment;

[Explanation of symbols]

 Reference Signs List 1 scanning surface 10 light source optical system 11 semiconductor laser 12 collimator lens 13 cylindrical lens 21 rotating polygon mirror 30 imaging optical system 31 first spherical lens 32 second spherical lens 33 asymmetric cylindrical mirror 70 scanning optical system 80 optical scanning device

Claims (3)

[Claims] A light source for emitting laser light; a rotating polygon mirror having a plurality of reflecting surfaces parallel to a rotation axis and reflecting and deflecting the laser light by rotating about the rotation axis;
A laser which has a power in a direction perpendicular to the deflection surface of the laser beam and includes a cylindrical lens or a cylindrical mirror having a cylindrical surface in which the power continuously changes along the deflection direction; and a laser deflected by the rotary polygon mirror. An optical scanning device comprising: an imaging optical system that condenses light on a plane; wherein the cylindrical surface has an asymmetric shape with respect to a plane that includes an optical axis of the imaging optical system and is perpendicular to the deflection surface. An optical scanning device, comprising: 2. An imaging optical system comprising two spherical lenses and one spherical lens.
2. The optical scanning device according to claim 1, comprising a plurality of cylindrical mirrors. 3. The cylindrical surface having an asymmetric shape is arranged such that a cylindrical surface having a plane-symmetric shape is asymmetric with respect to a plane including an optical axis of the imaging optical system and perpendicular to the deflection surface. An optical scanning device, comprising: