JPS6366531A - Laser beam scanning optical system - Google Patents

Abstract

PURPOSE:To reduce the cost of an inclination correcting optical system by linearly condensing a laser beam on the mirror surface of a deflector by a distributed index lens which has refractive index distribution only in one direction. CONSTITUTION:The distributed index lens which has refractive index distribution only in one direction is used instead of a conventional cylindrical lens. Parallel rays of the luminous flux from a laser unit 1 as the light source pass a distributed index lens 7 having refractive index distribution only in one direction, and the image of the section in the subscanning direction is formed no a rotating polygonal mirror but that in the main scanning direction is not formed and rays are parallel as they are. Thus, the distributed index lens 7 is used in an inclination correcting scanning optical system to reduce the cost of the inclination correction optical system.

Description

[Detailed Description of the Invention] [Technical Field] The present invention relates to a scanning optical system used in a laser beam printer or the like, and particularly to a scanning optical system using a deflector having a mirror surface such as a rotating polygon mirror or a galvano mirror. be.

C. Prior Art] In a device that scans a laser beam with a rotating polygon mirror or galvanometer mirror to form an image on a photoreceptor, each surface of the rotating polygon mirror is at a different angle with respect to a reference angle, or the axis of the galvanometer mirror is If there is axial vibration, the position at which the imaging beam scans the photoreceptor differs from scan to scan, resulting in poor quality images.

The tilt correction optical system shown in FIG. 5 solves this drawback. This optical system is designed between the rotating polygon mirror 3 and the laser unit in order to image a parallel beam of a certain diameter from the laser unit 1, which is the light source, on the rotating polygon mirror 3 (the same applies in the case of a galvanometer mirror) in the cross-section in the sub-scanning direction. A cylindrical lens is arranged at , and the reflected light from the rotating polygon mirror 3 is guided to a spherical lens 4 and a toric lens 5 having different refractive powers in the sagittal direction and the generatrix direction. The cross-sectional image formation in the main scanning direction is shown in Figure 6 (
As shown in a), the parallel light reflected by the rotating polygon mirror 3 is imaged on the photoreceptor 6 by the refractive power of the spherical lens 4 and the toric lens 5 in the generatrix direction. In the cross section in the sub-scanning direction, as shown in FIG. 6(b), the parallel light beam from the laser unit is imaged on the rotating polygon mirror 3 by the cylindrical lens 2, and the light reflected by the rotating polygon mirror becomes diverging light. This is imaged on the photoreceptor by the refractive power of the spherical lens 4 and the sagittal direction of the toric lens 5. In this way, the parallel light beam from the laser unit is imaged onto the rotating polygon mirror 3 by the cylindrical lens. Furthermore, the image is formed on a photoreceptor 6, and the rotating polygon mirror and the photoreceptor are in a conjugate relationship.

Therefore, even if the rotating polygon mirror falls down, the imaging position on the photoreceptor 6 does not change.

Conventionally, this correction optical system uses a cylindrical lens to image the parallel light beam from the laser unit on the rotating polygon mirror 6 in a cross section in the sub-scanning direction, but the processing of the cylindrical lens is similar to the processing method of a spherical lens. Differently, special jigs and tools and machining methods are required, which increases the machining cost, and it is also difficult to determine the position of the optical axis and the direction of the generatrix with good viscosity.

[Purpose of the Invention] The purpose of the present invention is to improve the conventional scanning optical system described above, and for this purpose, in the present invention, instead of the conventional cylindrical lens described above, a lens having a refractive index distribution in only one direction is used. This uses a graded refractive index lens. Hereinafter, the present invention will be explained in detail using the drawings.

Figures 1 and 2 (a) and (b) are diagrams showing an embodiment of the present invention, in which 1 is a laser unit as a light source, and 7 is a gradient index lens that has a gradient index in only one direction. , 3 is a rotating polygon mirror, 4 is a spherical lens, 5 is a toric lens, 6
is a photoreceptor.

The parallel light beam from the laser unit 1, which is a light source, passes through the refractive index distribution type lens 7 which has a refractive index distribution in only one direction, so that the cross section in the sub-scanning direction is imaged on the rotating polygon mirror, and the cross-section in the main scanning direction is formed as an image on the rotating polygon mirror. remain parallel without being imaged. That is,
As shown in FIG. 1, a gradient index lens 7 is used in place of the cylindrical lens 2 in the conventional example.

In this case, the imaging optical path in the cross section in the main scanning direction is shown in Figure 2(a).
FIG. 2(b) shows a cross-sectional imaging optical path in the sub-scanning direction. In the gradient index lens, ion exchange occurs when a flat glass containing ions of Tλ0, Cs, or Li is placed in a molten salt of potassium nitrate (KNO3) at 500 to 600°C.

The degree of ion exchange differs depending on the distance from the surface, and a refractive index distribution occurs only in the thickness direction. By cutting this flat glass to the required size and polishing both cut ends, it can function in the same way as a cylindrical lens. The image formation state is shown in FIG. 3, and the distribution of the gradient index lens is shown in FIG. 4.

In FIG. 4, the vertical axis indicates the refractive index n, the horizontal axis indicates the X direction, the X direction indicates the main scanning direction, and the X direction indicates the sub scanning direction. With gradient index lenses, many lenses can be cut out from a single sheet of ion-exchanged glass, and the cost per lens is low. Furthermore, in manufacturing, the optical axis is located at the center of the glass plate, and the generatrix is also parallel to both surfaces of the glass, so there is no need for a process to align the generatrix direction or to bring out the optical axis.

As described above, the cost of the tilt correction optical system can be reduced by using a gradient index lens having a refractive index distribution in only one direction as the cylindrical lens in the tilt correction scanning optical system.

4 (2! Business and Industry Explanation) Figures 1 and 2 (a) and (b) are diagrams showing an embodiment of the scanning optical system according to the present invention, and Figures 3 and 4 are diagrams showing the scanning optical system of the present invention. FIGS. 5 and 6 (a) and (b) are diagrams for explaining a gradient index lens used in an optical system, and are diagrams showing an example of a conventional scanning optical system.

1-111 laser unit, 3-m = one rotation polygon mirror,
4-111 spherical lens, 5-111 toric lens,
6---Photoreceptor, 7---Gradient refractive index lens.

Claims (1)

[Claims] (1) In a scanning optical system in which a laser beam is scanned by a deflector such as a rotating polygon mirror or a galvano mirror, and this deflected laser beam scans a scanned surface such as a photoreceptor, the scanning optical system has a refractive index distribution in only one direction. A laser beam scanning optical system characterized in that a laser beam is focused linearly onto a mirror surface of the deflector using a gradient index lens.