JPS61185717A - Optical beam scanner - Google Patents

Abstract

PURPOSE:To make the spot shape of an optical beam on a scan surface uniform by providing a mask having a slit, which is extended in the main scanning direction and has the width increased gradually from the center toward both ends, on the optical path of the optical beam between a rotary polygonal mirror and the scan surface. CONSTITUTION:A mask 9 having a slit 9a which is extended in the main scanning direction and has the width increased gradually from the center toward both ends is provided on the optical path of an optical beam 2 between a rotary polygonal mirror 5 and a scan surface 8. The breadthwise direction of this slit 9a is the subscanning direction of a device, and the optical beam made incident on the narrow center part of the slit 9a has the beam diameter expanded in the subscanning direction, and the optical beam 2 made incident on both end parts of the wide slit 9a passes the slit 9a without changing the beam diameter by the slit 9a. Consequently, the beam diameter in the subscanning direction of the optical beam is changed properly in accordance with the scan position by the width of the slit 9a. Thus, the optical beam 2 is always focused on a scan surface 8 as a uniform spot shape, and high-precision scanning is possible.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of the Invention) The present invention relates to a light beam scanning device that deflects a light beam to scan a scanning surface, and particularly relates to a mechanism for removing pitch unevenness of scanning lines on the scanning surface. The present invention relates to uniformizing the spot shape of a light beam on a scanning surface in a light beam scanning device equipped with a light beam scanning device.

(Technical Background of the Invention and Prior Art) In recent years, various systems have been developed that read and/or record images using light beams. In such a system, a light beam emitted from a light source is
It is reflected and deflected by a deflector such as a rotating polygon mirror, and sent at a constant speed in a direction perpendicular to the direction of deflection (sub-scanning).
It is designed to scan on a scanning surface. however,
In conventional scanning devices, the deflector that reflects and deflects the light beam to perform main scanning is prone to axial wobbling, and as a result, the deflected scanning line that scans on the scanning surface is distorted in the sub-scanning direction. There is a risk that it will become a thing. In addition, especially when using a rotating polygon mirror as a deflector, it is technically difficult to make each surface of the rotating polygon mirror completely parallel to the rotation axis. There is a problem in that the pitch of the scanning lines becomes uneven due to the surface tilt. Therefore, various scanning devices have been proposed in which an anamorphic optical system is installed between the deflector and the scanning surface in order to correct this surface tilt, etc. For examples, see FIGS. 3 and 4. and explain.

A light beam 102 emitted from a laser light source 101 is made into an appropriate thickness by a beam expander 103, passes through a cylindrical lens 104, and is directed to a rotating polygon 12105, which is a deflector, perpendicular to the rotation axis of the rotating polygon mirror. The light enters as a line image and is reflected and deflected as the rotating polygon mirror 105 rotates in the direction indicated by the arrow. FIG. 3 is a schematic diagram of the optical path of the reflected and deflected light beam 102 viewed from a direction parallel to the rotation axis, and FIG. 4 is a schematic diagram of the optical path viewed from a direction perpendicular to the rotation axis. be. First, to explain the main scanning of the light beam 102 deflected as shown in FIG. 3, the light beam 10 reflected and deflected by the rotating polygon mirror 05
2 is a spherical lens 10 such as an fθ lens provided on the optical path.
7 as parallel light, and after passing through this spherical lens 107, it is focused on the scanning surface 108, which is provided apart from the spherical lens 107 by the focal pressure of the spherical lens 107, 11 ft07, and the arrow 6 in the range from a to a2 Main scanning is performed repeatedly in the direction. Further, between the spherical lens 107 and the rotating polygon mirror 105, a cylindrical lens 106 extending in the main scanning direction is provided at a distance of its own focal length f lo from the rotating polygon 1105. It is a lens that refracts light only in a direction perpendicular to the main scanning direction (i?lI scanning direction), and in FIG. 3, it only transmits the light beam 102. The spherical lens 107 and the cylindrical lens 106 constitute an anamorphic optical system. Although the spherical lens 107 is a so-called scanning lens, an axially symmetrical aspherical lens may be used instead of the spherical lens 107 as the scanning lens.

As mentioned above, the rotating polygon mirror 105 often falls down, and a system for correcting this by the anamorphic optical system will be described with reference to FIG. 4.

The light beam 102 reflected by the rotating polygon mirror 105 is made incident on the cylindrical lens 106, and since this cylindrical lens 106 is provided at a distance of its own focal length from the rotating polygon 1 ft 105, the incident light beam 102 is parallelized. Make it light. The parallel light beam 102 then passes through the spherical lens 107 and reaches the scanning surface 1 which is separated by the focal length of the spherical lens 107.
Focus on 08. At this time, if the rotating polygon mirror 105 is driven without surface tilt etc., the light beam will pass through the optical path shown by the solid line in the figure, but if the rotating polygon mirror 105 has a surface tilt etc. If it deviates to the position 1058-, the optical path will move to the position shown by the dashed line in the figure. However, both the light beam in the optical path shown by the solid line and the light beam in the optical path shown by the dashed-dot line are reflected on the reflective surface 105a.
The light is emitted from the same point above, is made into parallel light by the cylindrical lens 106, and is converted into parallel light by the spherical lens 107.
Therefore, both the light beam shown by the solid line and the light beam shown by the dashed-dotted line are incident on the same point a3 on the scanning surface 108.
Even if the optical path of the light beam shifts in the vertical direction in FIG. 4 due to surface tilt, etc., the shift can be corrected.

However, when using a rotating polygon mirror as described above as a deflector, when a light beam is incident from a certain direction, the reflecting surface of the rotating polygon mirror moves in and out as the rotating polygon mirror rotates, and the light beam is reflected. Since the position changes, the optical path length of the light beam changes, which causes the problem that the beam diameter becomes non-uniform depending on the scanning position. This problem will be explained with reference to FIGS. 3, 4, and 5.

In FIG. 5, when the light beam 102 is made incident on the rotating polygon mirror 105 from a certain direction, when the main scanning direction is the six directions of arrows, when the rotating polygon lIO3 is at the position shown by the solid line in the figure, When the mirror is in the position shown by the dashed line in the figure, the optical path length of the light beam until it enters and exits the reflecting surface of the rotating polygon mirror and the optical path length from the rotating polygon mirror to the scanning surface. (For example, the difference in optical path length from the rotating polygon mirror to the scanning surface is indicated by S). Therefore, if the light beam is accurately focused on the scanning plane when the rotating polygon mirror is at the position shown by the solid line, the light beam will not be focused correctly on the scanning plane when the rotating polygon mirror is at a position other than the position indicated by the solid line. However, the problem arises that the beam becomes blurred. The difference in the optical path length of this light beam is that when the rotating polygon mirror is at the center of deflection, as in the example shown in Figure 3, the optical path of the incident light beam and the optical path of the reflected light beam are perpendicular. By arranging the optical system in this way, it is possible to almost eliminate the difference in the optical path length after reflection by the reflecting surface, but it is not possible to eliminate the change in the incident position of the light beam on the rotating polygon mirror. The blur of the light beam on the scanning plane remains. Furthermore, as is clear from Figures 3 and 4, the light beam becomes parallel light in the main scanning direction in the optical path before and after reflection from the rotating polygon mirror, so the change in the reflection position in the main scanning direction The blurring of the light beam is mostly in the sub-scanning direction without being affected. Therefore, as shown in Fig. 3, the focusing position of the light beam in the sub-scanning direction depending on the scanning position follows the trajectory shown by curve B, and the spot shape of the light beam on the scanning surface is a perfect circle in the center with both ends in the sub-scanning direction. It becomes an ellipse with the longitudinal direction. Also, if the optical system is adjusted so that both ends are perfect circles, the spot shape will become narrower in the sub-scanning direction at all scanning positions, and the spot shape in the center will become an ellipse with the longitudinal direction in the main scanning direction.
The beam diameter becomes smaller than the required diameter.

The influence of the change in the reflection position of the light beam due to the rotation of the rotating polygon mirror described above on the spot shape of the light beam on the scanning surface can be determined by scanning the cylindrical lens placed between the rotating polygon mirror and the scanning lens. It can be made smaller by placing the cylindrical lens between the lens and the scanning surface and bringing it as close to the scanning surface as possible, but this method does not fundamentally solve the above problem, and in this case This requires a long cylindrical lens close to the scanning width, and there is a problem in that long cylindrical lenses are expensive and impractical.

(Object of the Invention) The present invention has been made in view of the above-mentioned problems, and uses a rotating polygonal mirror as a light beam deflector and an anamorphic optical system to solve problems such as pitch irregularities in scanning lines. A light beam scanning device that can make the spot shape of a light beam uniform on a scanning surface even if the reflection position of the light beam changes due to the rotation of a rotating polygon mirror. The purpose is to provide the following.

(Structure of the Invention) A light beam scanning device of the present invention includes a mask having a slit extending in the main scanning direction and gradually increasing in width from the center to both ends on the optical path of the light beam between the rotating polygon mirror and the scanning surface. It is characterized by having the following.

The width direction of the slit is the same as the sub-scanning direction of the device, and the beam diameter of the light beam incident on the narrow center slit increases in the sub-scanning direction, and the light beam enters the wide end portions of the slit. The beam passes through the slit without the beam diameter being changed by the slit. Therefore, depending on the width of the slit, the beam diameter of the light beam in the sub-scanning direction can be changed as appropriate depending on the scanning position, and the light beam can always be imaged into a uniform spot shape on the scanning surface.

(Actual M Mode) Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a schematic diagram of an apparatus according to an embodiment of the present invention and an optical path of a light beam, viewed from a direction perpendicular to the rotation axis of a rotating polygon mirror.

A light beam 2 emitted from a beam light source 1 passes through a beam expander 3 and a cylindrical lens 4 provided on the optical path, and is incident on a rotating polygon mirror 5 as a line image perpendicular to the rotation axis of the rotating polygon mirror. , are reflected and deflected by the rotating polygon mirror 5. The direction of incidence of the light beam 2 on the rotating polygon mirror 5 is set such that the direction of incidence and the direction of reflection of the light beam are perpendicular to each other when the polygon mirror 5 is at the center of deflection. That is, on the optical path of the reflected and deflected light beam, there is a focal length of "6" from the rotating polygon mirror 5.
The optical axis of the fθ lens 7, which is a spherical lens (that is, the optical axis of the anamorphic optical system), is provided at a distance of f7 from the cylindrical lens 6 and the scanning surface 8. The direction of incidence of the beam is vertical. Therefore, even if the reflection position of the light beam incident on the rotating polygon mirror changes with the rotation of the rotating polygon mirror 5, as long as the light beam 2 is always focused on the reflecting surface of the rotating polygon mirror, the reflection The deflected light beam is always focused on the scanning surface 8, and the spot diameter of the light beam becomes uniform at all scanning positions. However, in reality, due to changes in the reflection position of the rotating polygon mirror 5, the incident light beam cannot be focused at -41 on the reflection surface, and for this reason, the spot shape of the light beam on the scanning surface 8 is limited to the central part of the scan. The diameter of the beam in the sub-scanning direction at the central portion is smaller than that at both ends, as shown by the broken line in FIG. Therefore, in this embodiment, a mask 9 having a slit that extends in the main scanning direction and whose width gradually increases from the center to both ends is provided on the optical path between the cylindrical lens 6 and the fθ lens 7 to spot the light beam. Adjusting the shape. The shape of the mask 9 is shown in the plan view of FIG.

Since the width of the slit 9a is small, the light passing through the center of the mask 9 is changed so that the beam diameter in the width direction of the slit 9a, that is, the beam diameter in the sub-scanning direction, becomes larger, and therefore the spot shape on the scanning surface is changed. is similarly adjusted. Since the width of the slit 9a gradually increases as it approaches both ends, the incident light beam gradually becomes unaffected by the deformation of the slit 9a, and at both ends the light beam passes through the slit 9a without being affected at all. . The slit 9a is arranged so that the beam diameter in the aj scanning direction is aligned at both ends according to the spot shape at each scanning position on the scanning surface when the mask 9 is not disposed, and a spot shape close to a perfect circle is always obtained. The width of the spot is adjusted, and by disposing the mask 9, the spot shape on the scanning surface is always uniform. Therefore, it is possible to correct the pitch unevenness using the anamorphic optical system consisting of the cylindrical lens 6 and the fθ lens 7, and it is also possible to make the spot shape of the light beam on the scanning surface uniform using the mask 9. This makes it possible to perform highly accurate scanning.

Note that in the above embodiment, the mask 9 is placed between the cylindrical lens 6 and the fθ lens 7;
The mask 9 may be placed anywhere between the rotating polygon mirror 5 and the scanning surface 8. Also, the rotating polygon mirror 5 of the light beam 2
The direction of incidence of the beam does not necessarily have to be perpendicular to the optical axes of the cylindrical lens 6 and the fθ lens 7; instead, the beam may be incident from other directions, and the beam diameter can be adjusted by designing the slit 9a of the mask 9 and adjusting the fθ lens 7. You may try to make it uniform. Furthermore, the cylindrical lens 6 may be arranged closer to the scanning surface 8 than the fθ lens 7.

In the embodiments of the present invention described above, a combination of a spherical lens such as an fθ lens and a cylindrical lens is used as the anamorphic optical system provided between the rotating polygon mirror and the scanning surface. The anamorphic optical system that can be used in the present invention is not limited to this, and even if the rotating polygon mirror falls down and axial wobbling occurs, the rotating polygon mirror can be used as a line image perpendicular to the rotation axis. Any optical system that functions as a scanning lens and corrects pitch unevenness of scanning lines by constantly focusing a light beam incident on the scanning surface as a point image at a predetermined position on the scanning surface. Good too. Specific examples of anamorphic optical systems that can be used in the present invention other than the above-mentioned combination of a spherical lens and a cylindrical lens include the following.

i) Combination of an axially symmetrical aspherical lens and a cylindrical lens 11) Combination of a spherical lens or an axially symmetrical aspherical lens and a toric lens 1ii) Combination of a spherical lens or an axially symmetrical aspherical lens and a cylindrical mirror Combination M Combination of a spherical lens or an axially symmetrical aspherical lens and a toric mirror■) Toric lens Vl with toric surfaces on both sides
) A combination of a cylindrical lens and a toric lens ■ A combination of a spherical lens or an axially symmetrical aspheric lens and a cylindrical lens and a toric lens viii) A combination of two cylindrical lenses arranged so that their optical axes are orthogonal ( Effects of the Invention) As explained in detail above, according to the light beam scanning device of the present invention, by providing a mask having a slit whose width gradually increases from the center toward both ends, rotation of the rotating polygon mirror is Even if the reflection position changes as a result, the spot shape of the light beam on the scanning surface can always be made uniform, making it possible to perform highly accurate scanning.

[Brief explanation of the drawing]

1 is a schematic diagram of a light beam scanning device according to an embodiment of the present invention and the optical path of the light beam viewed from a direction perpendicular to the rotation axis of a rotating polygon mirror; FIG. 2 is a plan view showing the shape of a mask; The figure is a schematic diagram of the optical path of a light beam in a conventional device viewed from a direction parallel to the rotation axis of the rotating polygon mirror, and Figure 4 is a schematic diagram of the optical path of a light beam in a conventional device viewed from a direction perpendicular to the rotation axis of the rotating polygon mirror. FIG. 5 is a schematic diagram for explaining changes in the reflection position of the light beam as the rotating polygon mirror rotates. 2... Light beam 4, 6... Cylindrical lens 5... Rotating polygon mirror 7... fθ lens 8...
・Scanning surface 9...Mask 9a...Slit

Claims (1)

[Claims] On the scanning surface that is continuously sent in the sub-scanning direction at a constant speed,
In a light beam scanning device that scans a light beam reflected and deflected by a rotating polygon mirror in the main scanning direction, there is a beam light source that emits a light beam, and a light beam provided on the optical path of the light beam emitted from the beam light source, and a light beam that emits the light beam. an input optical system that makes the light enter the rotating polygon mirror as a line image perpendicular to the rotation axis of the rotating polygon mirror; an optical path of the light beam reflected and deflected by the rotating polygon mirror between the rotating polygon mirror and the scanning surface; and a mask having a slit extending in the main scanning direction on an optical path between the rotating polygon mirror and the scanning surface, the width of which gradually increases from the center toward both ends. , a light beam scanning characterized in that, by the anamorphic optical system and the mask, the light beam incident on the rotating polygon mirror as a line image is imaged on the scanning surface as a point image with a uniform spot shape. Device.