JPH01185514A - Scanning optical system - Google Patents

Abstract

PURPOSE:To effectively use a luminous flux of a laser light source and to execute a scan by a beam waist diameter required on the surface to be scanned by using a retrofocus type as a collimator lens for obtaining a parallel luminous flux from a luminous flux of the laser light source, etc. CONSTITUTION:A collimator lens 3 is constituted of a retrofocus type. That is, a light beam I which is emitted from a light source 2 passes through the collimator lens 3, is reflected by the reflecting surface 1a of a rotary mirror 3, and next, by the reflecting surface 1b, becomes a reflected light beam R and passes through an (f)theta lens 4, is reflected by a mirror 7 and reaches on the surface to be scanned A. Subsequently, when the rotary mirror 5 rotates centering around a rotation axis 6, a scanning line is drawn in the direction vertical to the paper surface on the surface to be scanned A. In such a way, a luminous flux from a laser is used effectively without using a light source of a laser of a high output, etc., and also, a scan can be executed by obtaining the beam diameter required on the scanning surface (image forming surface).

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to a scanning optical system that scans a predetermined surface using a beam from a light source such as a laser through a collimator lens, a rotating mirror, and an fθ lens. It is related to the system.

(Prior Art) Conventionally, a beam from a light source such as a laser is incident on a so-called rotating mirror having at least one reflective surface on each side of a polygonal column, and the rotating mirror is rotated to scan a predetermined surface. However, for example, it is used to display television images, computer characters, etc., and to measure the surface shape of an object to be inspected. However, scanning systems operated in this manner have two main technical problems.

One of these is the inclination of the rotating shaft caused by the abalone (micro distance) between the rotating shaft and the bearing of the rotating mirror, and the tilting of the rotating mirror body of each reflecting surface, which is composed of a single planar reflecting surface. This is because an error in the angle at which the scanning beam is disposed on the rotating member causes a deviation in the angle of reflection of the scanning beam, resulting in disturbance of the scanning beam.

The other is that the beam diameter at the imaging plane (scanning plane) is determined by the beam waist diameter (beam cross-sectional diameter) wavelength of the light flux from a light source such as a laser used for the surface scanning beam, and the focal length of the optical system. Therefore, the beam diameter at the imaging plane is determined by these factors, and it is difficult to obtain a desired, for example, large beam diameter.

In order to solve the former of the above two problems,
Conventionally, there have been the following methods, for example. This is to support the rotating shaft of the rotating mirror with an air bearing in order to maintain the rotating shaft in a precisely fixed position.

With this method, the rotating shaft is supported at a fixed position, and the above-mentioned problem of the disturbance of the scanning line caused by the tilting of the rotating shaft is solved.

However, with this method, it is not possible to eliminate the problem of disturbance of the scanning line caused by an error in the angle at which the reflecting surface of the rotating mirror is arranged with respect to the rotating member. Moreover, in order to operate the air bearing device normally, complicated configurations and procedures are required, resulting in an expensive device.

Another method for solving the former of the above two problems is to use two cylindrical lenses and one imaging lens, as proposed in U.S. Pat. No. 3,995,110. There is.

If this method is used, it is possible to solve not only the problem of tilting of the rotating shaft as described above, but also the disturbance of the scanning line caused by the error in the angle at which the reflecting surface is arranged on the rotating member. You can, but you can't ignore that there are other drawbacks. This is due to problems with the components, and it is generally very difficult to manufacture a cylindrical lens that meets the requirements to solve the above problems.Also, when incorporating a cylindrical lens into an optical system, it is easy to position it. etc.

Next, as a method of solving the latter of the above two technical problems, conventional methods have been to use a collimator lens and an aperture with a predetermined opening. This is the light beam from the laser.

If the beam cross-sectional diameter at the scanning plane becomes larger than the desired beam diameter even if the beam diameter is within the diameter, the beam diameter at the scanning plane can be reduced by limiting the beam diameter using the collimator lens and the aperture diameter. This method involves adjusting the cross-sectional diameter to a desired size.

However, in this method, a part of the light beam from the laser light source is used, but in order for the beam light beam expanded at the scanning plane to exert the desired effect, a light source with considerably high output is required. It becomes. Generally, high-power lasers and the like are expensive, so the conventional method of using the collimator lens and aperture has the drawback of increasing costs.

(Problems to be Solved by the Invention) The present invention effectively uses the light beam from the laser without using a light source such as a high-power laser, and can obtain the necessary beam diameter on the scanning plane (imaging plane). The object of the present invention is to provide a scanning optical system that can perform scanning using the following methods.

(Means for solving the problem) A light beam from a light source such as a laser is made incident on a rotating mirror composed of a plurality of reflecting surfaces through a collimator lens,
In a scanning optical system that guides the reflected light beam from the rotating mirror onto a scanning surface using an fθ lens and performs optical scanning by rotating the rotating mirror, the collimator lens is configured of a retrofocus type. That's true.

(Embodiment) FIG. 1 is a schematic diagram of an optical system according to an embodiment of the present invention. In the figure, 5 is a rotating mirror, 6 is the rotation axis of the rotating mirror 5, and l
Reference characters a and lb are planar reflecting surfaces constituting the reflecting portion of the rotating mirror 5, and both reflecting surfaces are fixed at an angle θ with respect to each other. 2 is a light source such as a laser, and 3 is a collimator lens of a retrofocus type. 4 is an fθ lens, and 7 is a mirror.

In this embodiment, the light ray I emitted from the light source 2 passes through the collimator lens 3, is reflected by the reflective surface 1a of the rotating mirror 5, and then by the reflective surface 1b, becomes a reflected light ray R, and passes through the fθ lens 4. The light is reflected by the mirror 7 and reaches the scanned surface A. In this case and in the following description, no problem will arise if the incident ray I first strikes the reflective surface 1b or other reflective surface, provided that all the elements are properly arranged. When the rotating mirror 5 rotates around the rotating shaft 6, a scanning line is drawn on the scanned surface A in a direction perpendicular to the plane of the paper.

FIG. 2 is an explanatory diagram of a light beam incident on the rotating mirror 5 shown in FIG. 1. This figure and FIG. 1 show a reflective surface 1a,
lb, the incident ray I, and the reflected ray R are shown as orthogonal projections onto a plane orthogonal to both the reflecting surface 1a and the reflecting surface 1b. In FIG. 2(A), if the included angle between the reflective surface 1a and the reflective surface 1b is θ, then the reflective surface 1a and the reflective surface 1
Incident ray I and reflected ray R on the plane perpendicular to both b
If the angle formed by each orthogonal projection is α, then α = 18
It can be expressed as 0-20 (1). Therefore, if the incident light ray I is in the form of a thin line and its direction is kept constant, the reflected light ray R is in the form of a thin line, and as the rotating mirror 5 rotates, the reflecting surface 1b
From the reflection point, the light is directed only in each direction within a plane parallel to the line of intersection between the reflection surfaces 1a and 1b.

FIG. 2(B) is an explanatory diagram regarding the state of reflection of the reflected light beam due to the tilting of the rotating shaft 6. FIG. As mentioned above, this figure is shown as an orthogonal projection onto a plane orthogonal to each reflecting surface. As shown in the figure, if the rotating shaft 6 is tilted by an angle γ from the normal position direction shown by the solid line and becomes the rotating shaft 61 shown by the dotted line, the reflecting surfaces 1a and 1b are each tilted by the angle γ. , reflecting surfaces 1al and lbl. When the rotation axis 6 is in the normal position direction, the incident angle of the incident ray I to the reflecting surface 1a is i, and the reflection angle at which the incident ray is reflected by the reflecting surface 1b on its optical path is j,
The final reflected ray after that is R, and as shown in the figure, when the rotation axis 6 is tilted by an angle γ and the rotation axis becomes 61, the incident angle of the incident ray I with respect to the reflection surface 1al is 11, If the reflection angle at which the incident ray is reflected on its optical path is jl, and the final reflected ray after that is R1, then i, il1 . The relationship between the angles i and jl is expressed by the following equation.

il = i + γ (2) j
l = j −γ・・・・・・(3)(2
), the following equation is derived from equation (3).

it + jl = i + j...(
4) As shown in equation (4), the sum of the incident angle 11 and the reflection angle j1 when the rotating shaft 6 is tilted and becomes the rotating shaft 61, and when the rotating shaft 6 is in the normal position. Since the sum of the angle of incidence i and the angle of reflection j is equal, the reflected ray R when normal and the reflected ray R1 when tilting occurs are parallel to each other as shown.

Therefore, after the reflected rays R and R1 pass through the fθ lens 4,
Due to the optical properties of the fθ lens, it converges to the same position on the imaging plane A. Therefore, the scanning line on a predetermined plane formed in the optical scanning optical system using the rotating mirror 5 having the shape shown in FIG. Since the fixed position direction can be maintained, stable optical scanning becomes possible.

FIG. 3 is a schematic diagram of one embodiment of the rotating mirror 5. In this embodiment, the rotating mirror 5 has a shape in which the corresponding sides of the upper bases of two congruent regular hexagonal truncated pyramids are joined, and the reflecting surface 1a is formed by each side surface of the hexagonal truncated pyramid. The included angle θ of the reflective surface tb, etc. is constant over each reflective portion around the rotating mirror 5. Further, the rotating shaft 6 passes through the center of the bottom surface of the double-hole truncated pyramid.

FIGS. 4(A) and 4(B) are explanatory diagrams of an embodiment of the present invention regarding effective use of the luminous flux of the light source, and FIG.
A) is a schematic diagram showing the relationship between the beam waist (beam cross-sectional diameter) of a light source 2 such as a laser and the surface to be scanned. The focal length of the collimator lens 3 is F, the focal length of the fθ lens 4 is F2, and the beam waist diameter of the light source is W. , the width of the light beam that has passed through the collimator lens 3 and become almost parallel light is D,
When the beam waist diameter on the scanned surface A is Wl and the excavation wavelength of the light source is λ, the beam waist diameter wl is expressed by the following equation.

Here, for example, if the wavelength value is 780 nm and you want to obtain a relatively large beam waist diameter W of 0.4 mm on the scanned surface A, then from equation (5), the focal length! IF2
It is conceivable to make the width longer or to make the width smaller. However, since the focal length F2 is a value that affects the size of the entire device, it is desirable to make it as short as possible, for example, 300 mm or less. Then, the width of the luminous flux is 0.
This is a fairly small value of 7 mm.

Further, the beam waist diameter W on the scanned surface is also expressed by the following equation.

Beam waist diameter W of the light source. is about 1 μm, so
The focal length F is approximately 0.7 mm from equation (6). However, with − lenses, the focal length of the collimator lens is ta
When F l is set to 0.7 mm as described above, the beam waist of the laser light source is inside the laser light source, etc.
The collimator lens 3 and the laser light source 2 are too close to each other and cannot be properly arranged.

Therefore, in this embodiment, the collimator lens 3 is constructed of a retrofocus type as shown in FIG. 4(B), and both lenses are properly arranged.

Next, the principle will be explained using FIG. 4(B). When the laser light source and the convex lens L are separated to a position where they can be easily arranged, the laser light source will be located at a location farther away than the focal length 111f1 of the convex lens Ll. Therefore, the luminous flux of the laser light source that has passed through the convex lens L1 is narrowed down. Therefore, if a concave lens L2 is provided on the exit side of the lens L, the concave lens L2 can convert the narrowed light beam into a parallel light beam.

In this way, the collimator lens 3 is constructed of a retrofocus type having two lens groups having negative and positive refractive powers in order from the rotary mirror 5 side, and the arrangement of both lenses is maintained in a good manner.

In Fig. 4(B), Hl and H2 are collimator lenses 3
The main plane of , the part circled by the dotted line is the laser light source 2 .

As shown in the figure, since there is a beam waist of light [2] outside the focal length f of the convex lens L1, the luminous flux is squeezed by the convex lens L1, but becomes a desired parallel luminous flux by the concave lens L2, and the convex lens L1 is connected to the laser light source 2. without hitting the
For example, the focal length F1 of the collimator lens 3 is set to 0,
Since the desired length can be set to about 7 mm, it is possible to obtain the required beam waist diameter on the surface to be scanned.

FIG. 5 is a schematic diagram of another embodiment of the present invention. In the figure,
Reference numeral 8 designates a document table, reference numerals 71 to 74 reflect mirrors, reference numeral 9 a photosensitive drum, and reference numeral 10 a lens.

This embodiment uses reflective mirrors 71 and 72 to capture an image of a document placed on a document table 8, which is illuminated by an inscription scanning optical system using a rotating mirror using a laser as a light source and a broadband illumination light source such as an incandescent bulb. This shows an electrophotographic apparatus having a so-called slit exposure optical system, which performs slit exposure by moving parallel to the surface of the A document table 8 at a ratio of 2=1 onto a photosensitive tram 9 through a lens 1o.

Here, a light source 2, a collimator lens 3, a rotating mirror 5,
The scanning optical system consisting of the fθ lens 4 has a configuration similar to that illustrated in FIG. 1, and is characterized in that the writing stage of the scanning optical system and the writing stage of the slit exposure optical system are configured close to each other. are doing.

(Effects of the Invention) According to the present invention, as described above, by making the collimator lens for obtaining a parallel light flux from a light flux of a laser light source, etc., a retrofocus type, the light flux of the laser light source can be used effectively. A scanning optical system that can perform scanning with a required beam waist diameter on the scanning plane can be achieved.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram of an embodiment of the present invention, Fig. 2 (A), (
B) is an explanatory diagram of the rotating mirror 5 according to the present invention, FIG. 3 is a schematic diagram showing an embodiment of the rotating mirror 5 according to the present invention, and FIGS. 4 (A) and (B) are a laser light source according to the present invention. FIG. 5 is a schematic diagram showing the relationship between the beam waist and the surface to be scanned. FIG. 5 is a schematic diagram of an embodiment in which the present invention is applied to an electrophotographic apparatus. In the figure, 1 (including those with subscripts a, b and 1 added) is a reflective surface, 2 is a light source such as a laser, 3 is a collimator lens,
4 is an fθ lens, 5 is a rotating mirror, 6 is a rotation axis, A is a surface to be scanned, ■ is an incident light beam, R is a reflected light beam, 8 is a document table, 9
is a photosensitive drum. Patent applicant Canon Co., Ltd. Figure 1 Figure 2 B 3 Mimoe 4 Cause (A) En 0 (B)

Claims (2)

[Claims] (1) A light beam from a light source such as a laser is incident on a rotating mirror composed of a plurality of reflective surfaces via a collimator lens, and the reflected light beam from the rotating mirror is guided onto a scanning surface using an fθ lens, A scanning optical system that performs optical scanning by rotating the rotating mirror, wherein the collimator lens is of a retrofocus type. (2) The scanning optical system according to claim 1, wherein the collimator lens is a retrofocus type consisting of two lens groups having negative and positive refractive powers in order from the rotating mirror side.