JPH02254410A - Laser scanning optical system - Google Patents

Abstract

PURPOSE:To obtain the system consisting of a compact constitution by setting a ratio of a radius of curvature which each edge of a polygon mirror of a convex mirror has in a plane in which a main scan is executed and a distance extending from the polygon mirror to a first surface of a scanning lens to a prescribed value. CONSTITUTION:A laser light from a laser light source 1 passes through a beam shaping optical system and is made incident three-dimensionally on a polygon mirror 2 as a convergent light. The mirror 2 executes an equal angular velocity rotation, and the incident laser light becomes a scanning beam in accordance with the rotation and made incident on an f-theta lens 3. The mirror 2 is a convex mirror in which each edge thereof has a radius r0 of curvature in a plane in which a main scan is executed, and also, the scanning lens is constituted of one piece of spherical lens having positive power in the main scanning surface, so that a relation of an inequality I is satisfied between the surface of this mirror 2 and a distance d0' to a first surface of the scanning lens. In such a way, the constitution is simplified, and a correction of a curvature of image and a uniform scan can be realized.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a laser scanning optical system used in LBPs (laser beam printers) and the like. In particular, it relates to a laser scanning optical system that uses the rotation of a rotating polygon mirror to scan a predetermined surface with a laser beam.

(Prior Art) Laser scanning optical systems have been widely used in computer output devices and the like due to their characteristics of high speed processing and high quality, and have established a firm position. The performance required for a laser scanning optical system to have such characteristics can be mainly summarized into the following two. That is, (a) the curvature of field is corrected, and (b) the scanning speed is constant. (a) is a matter related to high quality.

By keeping the field curvature value within the permissible depth over the entire scanning area, a uniform spot diameter can be obtained over the entire screen. On the other hand, (b) is 111 items related to high speed and high quality.

If uniformity is not maintained well, simple time-series signal transfer will result in image distortion and variations in light intensity depending on location.

Correcting distortion using signal processing is extremely complicated and costly, and the same is true for controlling the amount of light at high speed. Therefore, it can be said that uniformity of the system is an extremely important structural requirement for the system.

On the other hand, the laser scanning optical system as an output device has another aspect in that it is exposed to harsh conditions due to its compactness and cost, in order to differentiate it from other alternative technologies. (a) above.

In order to achieve consistency between the technical problem (b) and the commercial problem, various proposals have been made for the configuration of a laser scanning optical system. Typical examples are listed below while looking at a cross-sectional view within the plane where main scanning is performed.

(1) Parallel light is incident on a polygon mirror made of a plane mirror, and a scanning lens made of two or more spherical lenses is used. (Fig. 14) (2) Parallel light is incident on a polygon mirror made of a plane mirror, and a scanning lens made of one or more aspheric lenses is used. (Fig. 15) (3) Parallel light or convergent light is incident on a polygon mirror made of a plane mirror, and a scanning lens is constructed with one spherical lens.

(Fig. 16) (4) Convergent light is incident on a polygon mirror made of a plane mirror, and the distance between the mirror and the drum, which is the writing surface, is set large without using a scanning lens.

(Fig. 17) (5) Convergent light is incident on a polygon mirror having a mirror surface made of a convex mirror, and scanning is performed directly without using a scanning lens. (FIG. 18) The scanning lens described here is generally called an f-theta lens because of its distortion correction method. If we look at (1) to (5) above from the perspective of performance requirements (a) and (b) and product requirements of cost and compactness, each method has its own problems. I understand.

First, (1) and (2) have a big cost problem. f
Using two or more −θ lenses, even if they are spherical, poses a cost problem. Furthermore, if even one aspherical surface is used, it will be difficult to manufacture, resulting in higher costs.

When using an f-θ lens, it is preferable that it can be configured with one spherical lens.

The configuration (3) satisfies the requirements of -. However, a single simple spherical lens cannot provide a large angle of view in terms of aberration correction. Therefore, the necessary performance requirements,
In order to satisfy (a) and (b), it is necessary to increase the distance between the mirror and the drum, which increases the size of the device and does not satisfy the requirement for compactness.

In configuration (4), there is no f-theta lens, so the restriction on the planning angle becomes severe, and the distance between the mirror and drum is reduced to (3).
It is necessary to make the device even larger, which leads to an increase in the size of the device.

In configuration (5), out of the required performance due to the action of the convex mirror, (a
) is satisfied, but property (b) is not satisfied because there is no optical correction mechanism after reflection at the polygon. Performing this correction on the electrical circuit side requires a complicated circuit to control the signal transfer rate, which poses a problem in terms of cost. Furthermore, it must be taken into consideration that the problem (b) also causes a change in the amount of light due to scanning, as described above.

(Problems to be Solved by the Invention) As seen above, the conventionally known embodiments satisfy conditions (a) and ('b), which are performance requirements for optical systems, while at the same time achieving low cost. It has been difficult to obtain an optical system that is compact.

The present invention has been made based on this background, and is intended to realize the above four conditions with a simple optical system configuration.

(Means for Solving the Problem) For this reason, in the present invention, from the requirements of cost and compactness, the f-theta lens is constituted by one spherical lens with positive power, and at the same time, the mirror surface of the polygon mirror is scanned at least in the main scanning direction. It is characterized by attempting to satisfy performance requirements (a) and (b) by adopting a simple arrangement such as a convex mirror in the plane.

According to the analysis of the present invention, in order to satisfy the above four conditions, the radius of curvature r of the mirror surface. It has been found that the following relationship preferably holds between and the distance do' between the mirror surface and the first surface of the f-theta lens. The present invention will be explained below using specific examples.

(Example) FIG. 1 is a drawing that best represents the features of the present invention. In the figure, l is a laser light source that is a light source, 2 is a polygon mirror that is a deflector, and 3 is a spherical lens f.
-θ lens, 4 a photosensitive drum, and 5 a scanning line exposed by scanning on the photosensitive drum.

The laser light emitted from the laser light source 1 passes through a beam shaping optical system such as a collimator lens or an imaging lens (not shown), and then enters the polygon mirror 2 three-dimensionally as convergent light. The incident direction of the laser beam at this time is the same as the optical axis direction of the f-theta lens when observed from the direction perpendicular to the plane where main scanning is performed, and is also at a position symmetrical to the f-theta lens with respect to the polygon mirror. It is set to be related. As described above, the mirror surface of the polygon mirror 2 is not a plane mirror, but a convex mirror with curvature at least in the main scanning plane. The polygon mirror 2 rotates at a constant angular velocity, and the incident laser light becomes a scanning beam according to the rotation and enters the f-theta lens 3. The state of the beam at this time is determined by the convergence of the beam incident on the polygon and the curvature of the polygon mirror, and it may be a convergent light, a parallel light, or a diverging light, but after passing through the f-theta lens it becomes a convergent light. As a result, a spot of a predetermined size is formed on the photosensitive drum 4.

FIG. 2 depicts the optical system after the polygon mirror, which is a feature of the present invention, in the main scanning plane. The number in the diagram is the first
Corresponding to the figure, 2 is a polygon mirror, 3 is an f-θ lens, and 5 is a scanning line scanned on the photosensitive drum. The scanning line 5 corresponds to the image plane drawn by the f-θ lens.

The alphabets in the figure are for more detailed explanation of each component, and some of them indicate correspondence with the numerical examples shown in Table 1.

First, O indicates the rotation center of the polygon mirror, and the polygon mirror 2 rotates at a constant angular velocity about this point. Rp is the radius of the circumscribed circle of the polygon mirror, and ro is the radius of curvature that the mirror surface has in the main scanning plane. rl and r
2 indicates the radius of curvature of the fe lens, and r
, is the polygon mirror side (first surface), and r2 is the image surface side (second surface).
). do' is the mirror surface and the first lens
This is the distance from the surface. Since the distance between the mirror surface and the first surface of the lens changes according to the rotation of the polygon, do' occurs when the polygon is in the reference state, that is, when the laser beam travels along the optical axis of the f-theta lens and the scanning spot light Position is image plane 5
The value at the center of . dl' is the thickness of the f-theta lens, and Xo is the distance from the rotation center O of the voribon to the image plane.

After considering the range that satisfies the above four conditions when using this configuration, we found that in order to obtain suitable results, there are two parameters:
That is, the radius of curvature ro of the mirror surface of the polygon and the polygon f-
It has been found that it is sufficient to focus on the relationship of the distance 1lIdo' between the θ lenses. This relationship can be expressed by the inequality. If this is compared with, for example, the conventionally known example (3) using a plane mirror polygon mirror and one spherical f-theta lens, the meaning of the relationship in conditional expression (A) will become clear. r for a plane mirror. Since is infinite, it goes without saying that equation (A) cannot be satisfied. Therefore (A
) can be said to substantiate the comment regarding (3), which states that it is difficult to correct aberrations when using a plane mirror.

If the lower limit of conditional expression (A) is exceeded, the thickness of the lens will increase significantly, and if the upper limit is exceeded, the outer shape of the lens will become large and the curvature will become severe, making it difficult to manufacture the lens.

Table 1 shows specific numerical examples of the present invention. R.P.
The value of indicates the case where the polygon mirror has a hexahedral structure, and N and indicates the refractive index of the f-theta lens. Table 1
Graphs of field curvature and scanning speed unevenness for each numerical example are shown in FIGS. 3 to 10.

(Other Embodiments) FIG. 11 shows a second embodiment in which a cylinder lens 6 is added to the basic scanning optical system shown in FIG. As is well known, problems with scanning optical systems include blurring due to polygon rotation and uneven scanning in a direction perpendicular to the scanning direction due to surface tilt due to the degree of processing of the polygon itself. The introduction of the cylinder lens has two meanings: correction of this surface tilt effect and correction of field curvature in the sub-scanning direction. For this reason, in this embodiment, a cylinder lens 6 for this purpose is arranged between the f-theta lens and the drum, and a cylinder lens (not shown) is arranged on the incident side of the polygon mirror corresponding to the cylinder lens 6. Another system has been added. The two cylinder lenses both include the lens optical axis and are arranged so as to have power in a plane perpendicular to the main scanning plane. How to use this cylinder lens is widely known in conventional scanning optical systems, so it will not be described in detail. The method of correction can be easily applied.

FIG. 12 shows a third embodiment in which a toric lens 7 is used as the lens corresponding to the cylinder lens 6 in the second embodiment shown in FIG. The toric lens 7 has a stronger power on the side of the main scanning plane than on the side perpendicular to the main scanning plane, and is shaped like a tire tube. While a cylinder lens only acts as a parallel plane plate with respect to the main scanning plane, a toric lens has a curvature also with respect to the main scanning plane, so this is corrected for aberrations on the main scanning plane. Since it can be used for

FIG. 13 shows the fourth embodiment of FIG. 12, which combines the functions of the spherical f-θ lens and the toric lens 7.
This is an example in which a spherical lens is replaced with a toric lens 8.

As shown in Figure 1, which is the basic system, aberrations on the main scanning plane can be sufficiently corrected with a spherical system, so there is no need to use two lenses after the polygon mirror as in the third embodiment. If a toric lens is used as in this embodiment, sufficient performance can be achieved in the optical system. Even a toric lens has a configuration like that shown in Figure 2 when considered within the main scanning plane.
Needless to say, inequality (A) is satisfied within the main scanning plane. Furthermore, as described in the second embodiment, a cylinder lens (not shown) is included on the incident side in accordance with the toric lens 8.

Note that the second and subsequent embodiments include aspherical lenses such as cylinder lenses and toric lenses, but these are in a different range from the systems (1) to (5) listed in the conventional example. For the sake of clarity, I would like to add this as an example. The second and subsequent embodiments are examples in the area where a new consideration item of tilt correction has been added, and the conventional example (
1) to (5) are examples in the area where there is no concept of optical surface tilt correction. Therefore, it is only the first embodiment that corresponds to (1) to (5), and it can be said that it is significant that it was constructed only with a spherical system.

The second to fourth embodiments show system configurations in which a surface tilt correction optical system is applied to this system. It can be said that the embodiment of the present invention has an advantage over the case where a tilt correction optical system is added to a conventionally known system to the extent that the configuration in the main scanning plane is simple. For example, consider a case in which a toric surface tilt correction system is used as in the fourth embodiment in contrast to the conventional example (2). At this time f-
The difference from the fourth embodiment is obvious because the θ lens is aspherical in the main scanning plane and has a complex lens shape that also has power in a direction perpendicular to the main scanning plane.

Table 1 (m excluding Nl) (Effects of the invention) As explained above, according to the configuration of the present invention, it is possible to correct field curvature and uniform velocity scanning, which are required for optical performance, while having a simple configuration. It can be realized. In addition, as a result of the simple configuration, the cost is low, and a compact laser scanning system can be realized by improving the configuration, making it possible to simultaneously satisfy performance and product requirements.

The basic configuration of the present invention also allows easy introduction of a surface tilt correction system, and can be said to be extremely practical.

[Brief explanation of drawings]

FIG. 1 is a first embodiment of the present invention, FIG. 2 is a sectional view of the main scanning plane of the scanning section of the first embodiment of the present invention, and FIGS. 3 to 10 are numerical examples shown in Table 1. 11 is a second embodiment of the present invention, FIG. 12 is a third embodiment of the present invention, FIG. 13 is a fourth embodiment of the present invention, and FIGS. 14 to 18 are conventionally known aberration diagrams. FIG. 2 is a diagram showing the principle of the scanning system of FIG. In the figure, 1 is a laser light source, 2 is a polygon mirror (curvature bone), 3 is an f-theta lens, 4 is a photosensitive drum, 5 is an image plane which is a scanning line, 6 is a cylinder lens, 7 is a toric lens, 8 is a toric lens, 9 is a polygon mirror (no curvature),! O is f-θ lens, 11 is aspherical surface f
-θ lens.

Claims (6)

[Claims] (1) In a laser scanning optical system that scans light from a laser light source over an object plane via a beam shaping optical system, a deflector, and a scanning lens, the deflector is a polygon mirror that rotates at a constant speed, and the polygon Each edge of the mirror is a convex mirror with a radius of curvature r_o in the plane where main scanning is performed, and at the same time, the scanning lens is composed of one spherical lens with positive power in the main scanning plane, and the polygon mirror A laser scanning optical system characterized in that, where d_o' is the distance from the surface to the first surface of the scanning lens, 0.1<d_o'/r_o<0.7 holds true. (2) The laser scanning optical system according to claim 1, wherein the laser light incident on the deflector is not included in the main scanning plane but is arranged three-dimensionally. (3) The laser scanning optical system according to claim 2, wherein the projection of the laser light incident on the deflector onto a plane on which main scanning is performed coincides with the optical axis of the scanning lens. (4) A cylinder lens having power only in the sub-scanning direction orthogonal to the plane on which main scanning is performed is added on the incident side of the deflector and between the scanning lens and the object surface. laser scanning optics. (5) The laser scanning optical system according to claim 3, characterized in that a cylinder lens having power only in the sub-scanning direction is added to the incident side of the deflector, and a toric lens is added between the scanning lens and the object surface. . (6) A cylinder lens having power only in the sub-scanning direction is used on the incident side of the deflector as the scanning lens, and a toric lens having a power different from that in the main scanning direction in the sub-scanning direction is used. The laser scanning optical system according to claim 3.