JP3100760B2 - Scanning optical system with temperature compensation - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning optical system used for, for example, a laser printer and the like, and more particularly to a scanning optical system which suppresses a focus shift due to a temperature change.

[0002]

2. Description of the Related Art A scanning lens of a laser printer includes:
Plastic lenses are often used to reduce cost and weight. However, plastic has a greater degree of linear expansion and a change in refractive index with respect to a temperature change than glass, and the quality of drawing is liable to be degraded due to defocus.

A scanning optical system of a laser printer or the like generally adopts a configuration in which a light beam is once focused only in the sub-scanning direction near the polygon mirror in order to reduce the influence of a tilt error of the polygon mirror. In many cases, the anamorphic optical system has a stronger power in the sub-scanning direction than in the main scanning direction. Therefore, a focus shift due to a temperature change is also a problem mainly only in the sub-scanning direction.

[0004] Conventionally, there has been known a method of combining a plurality of plastic lenses in a scanning lens and arranging them so that the generated focal shifts cancel each other in order to reduce the influence of such a temperature change.

[0005]

However, in the above-mentioned conventional method, the number of scanning lenses must be increased for temperature compensation, which is against the trend of downsizing and cost reduction.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a scanning optical system capable of reducing the influence of a temperature change without increasing the number of scanning lenses. With the goal.

[0007]

SUMMARY OF THE INVENTION A temperature-compensated scanning optical system according to the present invention has the following objects.
A collimating lens that converts the divergent light emitted from the light source into a parallel light beam, a cylindrical lens that temporarily forms an image of the parallel light beam only in the sub-scanning direction, In a scanning optical system including a deflector for deflecting the light and a plastic anamorphic scanning lens for forming an image of the deflected light beam on an image plane, a rotationally symmetric negative plastic lens is provided in a collimating lens, and a cylindrical lens is provided. An anamorphic plastic lens having a positive power in the main scanning direction is provided inside, and the power of the plastic lens included in the collimator lens and the cylindrical lens is canceled out by the temperature change of both lenses in the main scanning plane. Running in the sub-scanning plane Characterized by being configured to cancel the effect of temperature changes occurring in the lens.

[0008]

Embodiments of the present invention will be described below. FIG.
1A and 1B are lens diagrams of a scanning optical system according to an embodiment of the present invention, wherein FIG. 1A shows a cross section in the main scanning direction and FIG.

This optical system includes a collimator lens 10 for converting divergent light emitted from a semiconductor laser 1 as a light source into a parallel light beam, and a cylindrical lens 20 for temporarily forming an image of the parallel light beam in the sub-scanning direction near a deflector P. And a plastic anamorphic scanning lens 30 for imaging the deflected light beam on the image plane.

Although FIG. 1 is an optical path development view, the deflector P is shown as a straight line, but it is well known that a polygon mirror or the like is specifically used. In addition, in order to shorten the optical path, a part of the optical path is omitted in a portion indicated by a broken line.

The collimating lens 10 comprises a first lens 11 which is a rotationally symmetric biconvex glass lens, and a second lens 12 which is a rotationally symmetric biconcave plastic lens, and has a positive power as a whole. .

The cylindrical lens 20 has a first lens 21 which is a cylindrical plastic lens having a positive power only in the main scanning direction, and has a negative power in the main scanning direction and a positive power in the sub-scanning direction. It has a second lens 22 which is a glass lens, has a positive power as a whole in the sub-scanning direction, and has almost no power in the main scanning direction.

The scanning lens 30 is a single fθ lens, and has a stronger positive power in the sub-scanning direction than in the main scanning direction. Since a plastic lens can be easily processed with an aspherical surface or the like, an fθ lens that satisfies the performance of a laser printer or the like can be created with a single lens configuration. However, since the influence of the temperature change cannot be canceled in the scanning lens with one sheet, the collimating lens 1
0, a plastic lens is also provided in the cylindrical lens 20 to compensate for the temperature.

Since the focus change due to the temperature change is caused by the second lens 12 of the collimating lens 10 and the first lens 21 of the cylindrical lens 20 in the main scanning direction, the influence of the temperature change generated by these is canceled out. The power of both lenses is determined.

On the other hand, the focus fluctuation in the sub-scanning direction is
Second lens 12 of collimating lens 10 and scanning lens 3
Therefore, the power is determined so that the influence of the temperature change generated by these lenses is canceled.

Here, the focal length and the change of the focal length of the second lens 12 of the collimating lens 10 are f ₁₂ and Δf ₁₂ , the focal length and the change of the focal length of the scanning lens 30 in the sub-scanning direction are f _30Z and Δf _30Z , and cylindrical. _Assuming that the focal length of the second lens 22 of the lens 20 is f _22Z and the magnification of the scanning lens 30 in the sub-scanning direction is m, the movement amount ΔP _IM of the sub-scanning image plane due to a temperature change is expressed by the following equation 1. .

[0017]

## EQU1 ## ΔP _IM = (Δf ₁₂ / f ₁₂ ² ) × f _20Z ² × m ² + (1−m) ² × Δf _30Z

When the same plastic is used for the second lens 12 and the scanning lens 30 of the collimating lens 10, when the following equation 2 is satisfied, the focal point shift due to the temperature change of the scanning lens is corrected.

[0019]

(Δf ₁₂ / f ₁₂ ² ) × f _22Z ² × m ² + (1-m) ² × Δf _30Z = 0

At the same time, it is necessary to satisfy f ₁₂ ≒ f _22Z .

The specific numerical configuration of the collimating lens 10 and the cylindrical lens 20 is shown in Table 1. In the table, the surface number is the number of the surface counted from the object side on the left side in the figure, ry is the radius of curvature in the main scanning direction, rz is the radius of curvature in the sub-scanning direction (surfaces not described are rotationally symmetric), d is the lens The thickness or air spacing, n, is the refractive index at a wavelength of 780 nm.

The first surface (surface number 3) of the second lens 12 of the collimating lens 10 is a rotationally symmetric aspheric surface. For the aspheric surface, the distance from the tangent plane to the aspherical vertex of the coordinate point on the aspheric surface whose height from the optical axis is Y is X, the curvature of the aspherical vertex (1 / r)
Is C, the conic coefficient is K, and the fourth, sixth, and eighth order aspheric coefficients are A ₄ , A ₆ , A
₈ is represented by the following equation. The radius of curvature of the aspherical surface in Table 1 is the radius of curvature of the apex of the aspherical surface.

[0023]

Equation 3] ^{X = CY 2 / (1 +} √ (1- (1 + K) C 2 Y 2)) + A 4 Y 4 + A 6 Y 6 + A 8 Y 8

[0024]

[Table 1] f _30Z = 70 mm m = -2.5 Surface number ry rz dn 1 67.152 3.00 1.88080 2 -4.980 0.52 3 -8.475 1.50 1.48617 4 -17.611 30.00 5 ∞ 3.00 1.48617 6 -17.201 ∞ 0.00 7 -18.075 ∞ 3.00 1.51072 8 ∞ -35.735

[0025]

[Table 2] K = 0.87366 A ₄ = -1.36 220 × 10 ^-3 A ₆ = 4.16374 × 10 ^-5 A ₈ = -4.68 250 × 10 ^-5

[0026]

As described above, according to the present invention, the number of scanning lenses having a relatively large diameter can be increased by providing the collimating lens and the cylindrical lens having a relatively small diameter with a temperature compensation function. The effect of temperature changes can be reduced, miniaturizing the device,
Cost can be reduced.

[Brief description of the drawings]

FIG. 1 is a development view of an optical system showing a configuration of an embodiment;
(a) shows the main scanning direction, and (b) shows the sub-scanning direction.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 ... Collimating lens 11 ... 1st lens 12 ... 2nd lens 20 ... Cylindrical lens 21 ... 1st lens 22 ... 2nd lens 30 ... Scanning lens

Continuation of the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G02B 26/10 G02B 13/00

Claims (2)

(57) [Claims] 1. A collimator lens having a positive power as a whole to convert divergent light emitted from a light source into a parallel light beam, a cylindrical lens for temporarily forming an image of the parallel light beam only in the sub-scanning direction, and a light beam generated by the cylindrical lens. In a scanning optical system that is provided near an image forming position and includes a deflector that deflects a light beam and a plastic anamorphic scanning lens that forms an image of the deflected light beam on an image plane, the collimating lens is rotationally symmetric. A negative plastic lens, wherein the cylindrical lens is in the main scanning direction.
Includes anamorphic plastic lens having a positive power, the power of the plastic lenses included in the said collimating lens the cylindrical lens cancel each other the influence of temperature change of the both lenses in the main scanning plane, the sub-scan plane Wherein a temperature-compensated scanning optical system is set so as to cancel the influence of a temperature change occurring in the scanning lens. 2. The temperature-compensated scanning optical system according to claim 1, wherein said scanning lens comprises a single lens.