JPS63204225A - Optical mechanism for light beam scanner - Google Patents

Abstract

PURPOSE:To obtain an optical mechanism which has a large angle of deflection and is easily compacted and formed while its curvature of field can be corrected excellently by setting the focal lengths of a 1st and a 2nd lenses constituting an ftheta lens properly and forming a concave cylindrical surface on the 2nd lens. CONSTITUTION:A concave cylindrical mirror 6 which has curvature only at right angles to a scanning surface is arranged between the ftheta lens and a scanned surface 5, and this concave cylindrical mirror 6 and ftheta lens 1 constitute a surface tilt correction optical system. The surface of a plano-convex lens 3 on the side of a deflection point is composed of a concave cylindrical surface 7 having curvature only at right angles to the scanning plane. Here, the concave cylindrical surface 7 that the plano-convex lens 3 has is for reducing the curvature of field perpendicular to the scanning direction which can not be removed only by the concave cylindrical mirror 6. Consequently, even when the concave cylindrical mirror 6 is arranged at a distance from the scanned surface 5, excellent curvature of field is obtained and the whole optical system can be made compact.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical mechanism for a light beam scanning device used in a laser printer or the like.

[Prior Art] Conventionally, in laser printers and the like, a laser beam generated from a light source is modulated by an information signal in order to record the beam, and the modulated laser beam is then deflected by a rotating polygonal reflector or the like through a shaping optical system. The beam is deflected at a constant angular velocity by a means, and is focused onto a photoreceptor by an imaging lens system. In this case, an fθ lens is widely used as the imaging lens system. This f
A θ lens is used to correct the deflection of a laser beam at a constant angular velocity so that the light spot moves at a constant speed on the scanning surface. is f× from the optical axis of the imaging lens with focal length f
This name means that the light is focused at the image height position of θ.

Surface tilt correction is to correct the positional deviation of the scanning spot in the direction perpendicular to the scanning direction, which is caused by the tilt error of the mirror surface of the rotating polygon mirror used in the deflection means.
By configuring the optical system so that the deflection point position and the scanned position have a substantially conjugate relationship in a plane perpendicular to the scanning plane, surface tilt correction becomes possible.

Conventionally, a cylindrical lens has been used as the surface tilt correction optical system, or an f-theta lens and a toroidal lens have an aspherical surface, as disclosed in Japanese Patent Laid-Open No. 61-175616, No. 61-170715, and No. 56-36622. A combination of lenses, or JP-A-59
As disclosed in Japanese Patent Application No. 84218, there is one constructed by combining two fθ lenses with negative and positive powers and a cylindrical mirror.

[Problems to be Solved by the Invention] However, in a lens that uses a cylindrical lens, as the polarization angle increases, the field curvature in the direction perpendicular to the scanning direction increases, so in order to reduce this, the arrangement position must be changed. There is a problem in that it has to be placed close to the scanning surface, making it difficult to make it compact.

Further, there is a problem in that it is difficult to manufacture a device using an fθ lens and a toroidal lens. Two more fθ
A device using a lens and a cylindrical mirror has a problem in that it has a strong negative power and a long back focus, making it difficult to make it compact.

The present invention has been made in view of the above-mentioned circumstances, and provides an optical mechanism for a light beam scanning device that has a large deflection angle, is compact and easy to manufacture, and can satisfactorily correct field curvature. The purpose is to

Means for Solving Problem E] The present invention includes an fθ lens and a correction optical member that constitutes a surface tilt correction optical system together with the fθ lens, and the fθ lens has a power on the deflection point side within the scanning plane. It consists of a first lens with a meniscus shape with a weak power, and a second lens with a plano-convex shape with a convex surface with a strong power facing the image side, and the surface of the second lens on the deflection point side is perpendicular to the scanning plane. It is composed of a concave cylindrical surface with curvature only in the direction, and furthermore,
The correction optical member is arranged between the fθ lens and the scanned surface, and is composed of a concave circular mirror having curvature only in a direction perpendicular to the scanning plane.

[Function] By appropriately setting the focal lengths of the first and second lenses that make up the fθ lens, distortion can be reduced.
In addition, good fθ characteristics are obtained, and field curvature during scanning is also reduced. Furthermore, the concave cylindrical surface provided on the second lens can also reduce field curvature in the direction perpendicular to the scanning direction, which cannot be removed by using a concave circular mirror alone. Due to the presence of this concave cylindrical surface, a good edge surface curvature can be obtained even if the concave circular mirror is placed away from the surface to be scanned, and the entire optical system can be made compact.

[Example] FIG. 1 is a block diagram showing an example of the present invention, and FIG.
Figure a) is a view seen from the scanning plane of the laser beam, and figure (b) is a view seen from a plane perpendicular to the scanning direction. In these figures, 1 is an fθ lens, which includes a meniscus lens 2 with low power placed on the deflection point P side in the scanning plane, and a plano-convex lens 3 with high power with the convex surface 4 facing the distorted surface 5 side. It consists of And this fθ
As shown in FIG. 1(b), a concave circular mirror 6 having a curvature only in the direction orthogonal to the scanning plane is arranged between the lens 1 and the scanned surface 5. The fθ lens 1 constitutes a surface tilt correction optical system.

Note that the surface of the plano-convex lens 3 on the deflection point side is constituted by a concave cylindrical surface 7 having curvature only in the direction orthogonal to the scanning plane.

In the above configuration, assuming that the focal lengths of each lens when viewed within the scanning plane are fl and f2 in order from the deflection point P side, and the combined focal length is f, it is desirable that these satisfy the following conditions.

(a) −0,3≦f/f1≦0.2 (b) −0,1/f1 +0.85/f≦1/f2≦−
0.6/f1 +0.95/f Conditions (a) and (b) are used to limit the distortion of fθ lens 1, provide good fθ characteristics, and reduce field curvature in the scanning direction. This is the condition. Further, the concave cylindrical surface 7 provided on the plano-convex lens 3 is for reducing field curvature in the direction orthogonal to the scanning direction, which cannot be removed by the concave circular mirror 6 alone. Due to the presence of this concave cylindrical surface 7, a good field curvature can be obtained even if the concave circular mirror 6 is placed away from the scanned surface 5, and the entire optical system can be made compact.

Here, as shown in FIG. 1, dl, d2. d3 is the distance between the lens surfaces, d4 is the distance from the lens 3 to the concave mirror 6,
d5 is the distance from the concave circular mirror 6 to the scanned surface 5, dO is the distance from the deflection point P to the lens 2, R1, R2, R3, R
4 is the radius of curvature of each lens surface in the scanning plane, R1
', R2', R3', R4' are the radii of curvature in a plane perpendicular to the scanning plane, R5' is the radius of curvature of the concave circular mirror 6, N
1, N2 is lens 2. Refractive index of the glass material of lens 3, fl
, f2 in the scanning plane. When the focal length of the lens 3, f, is the composite focal length, and is normalized as f=100, the field curvature and fθ characteristics with respect to changes in the distance between lens surfaces, etc., are as follows.

Specific example 1 f = 100,1/f1-0,000001,1/f2
-0.009dO=8.57 R1=-26,074R1°=-26,074d1=8
．． 57 N1=1.51 R2=-28,967R2'=-28,967d2=4
．． 29 R3=oo R3°=-20d3=10.
71 N2=1.71 R4=-78,889R4'=-78,889d4=3
0 R5°=-63,355 d5=81.1 Figures 2 (a) and (b) show the field curvature and fθ of this specific example 1.
This is a diagram showing the characteristics. In the diagram (a), the broken line shows the main scanning direction, the solid line shows the field curvature in the sagittal plane, and in the diagram (b), h = image height (h −fθ
)/fθ×100% shows the fθ characteristic. Note that in all the specific examples following this specific example, the wavelength of the laser beam is 780 nn+, and the angle of view θ is 44°.

Furthermore, although the example is shown in which the optical axes of the concave circular mirrors 6 are aligned with the optical axes of the fθ lens 1, even if the optical axes of the concave circular mirrors 6 are tilted, the radius of curvature of R5' and the radius of curvature of R3' Equivalent performance can be obtained by appropriately changing the radius of curvature.

1 ship ■Yu f = 100.1/f1 = o, ooooool, 1/f2
=0.009ゝdo=12.8f3 R1=-32,585R1°=-32,585d1=
10.71 N1=1.51 R2=-36,200R2°=-36,200d2=
5.00 R3=ooR3'=-20 d3=10.71 N2=1.61 84=-67,778R4°=-67,778d4=
30 R5°=-67,213 d5=81.1 FIG. 3 shows the field curvature and fθ characteristics of this specific example 2.

As is clear from the above specific example, the above-mentioned condition (a)
, (b), the deflection angle can be made large, and the entire lens system can be made more compact. Furthermore, it has the advantage of being easy to manufacture.

[Effects of the Invention] As explained above, according to the present invention, a large deflection angle can be obtained, compactness and IJ construction are easy, and curvature of field caused by tilting of the surface of the deflection means can be well corrected. I can do it.

[Brief explanation of the drawing]

FIGS. 1(a) and (b) are configuration diagrams of an fθ lens and a cylindrical mirror showing one embodiment of the present invention, and FIGS. 2 and 3 are diagrams showing the field curvature and fθ characteristics of FIG. 1. It is. 1...fθ lens', 2...meniscus lens, 3
...Plano-convex lens, 4...Convex surface, 5...Scanned surface,
6...Concave circular mirror, 7...Concave cylindrical surface. Izomenwa L song fθ word "匪 Figure 1

Claims (1)

[Claims] An optical mechanism disposed between a deflection point of a light beam deflected by a light beam polarizer and a scanned surface, which includes an fθ lens and a correction optical member that constitutes a surface tilt correction optical system together with the fθ lens. The fθ lens is composed of a first lens having a meniscus shape with a weak power toward the deflection point side in the scanning plane, and a second lens having a plano-convex shape with a convex surface having a strong power facing the image side, and The surface of the second lens on the deflection point side is constituted by a concave cylindrical surface having curvature only in the direction orthogonal to the scanning plane, and further, the correction optical member is disposed between the fθ lens and the scanned surface, An optical mechanism for a light beam scanning device, comprising a concave cylindrical mirror having curvature only in a direction perpendicular to a scanning plane.