JP3206947B2 - Scanning optical device - 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 such as a laser printer for scanning a scanning surface with a light beam using a scanning deflector, and more particularly to a countermeasure against ghost light in the scanning optical system.

[0002]

2. Description of the Related Art In a conventional scanning optical system using a polygon mirror as a deflector, a light beam is once imaged in a sub-scanning plane and an anamorphic fθ lens is used in order to correct the influence of a surface tilt error of the polygon mirror. To form a spot on the scanning surface.

With such a configuration, the image forming position in the sub-scanning plane tends to be farther from the fθ lens at the periphery of the scanning plane. Therefore, by setting the diameter of the polygon mirror to be relatively large, the above-described change in the imaging position, that is, the scanning surface curvature, is corrected by the change in the deflection point due to the rotation.

[0004]

However, in the conventional scanning optical system, the angle between the light beam incident on the polygon mirror and the optical axis of the fθ lens is generally about 50 ° to 90 °. There is a problem that the point change becomes asymmetric with respect to the optical axis, the scanning surface curvature appears asymmetrically, and correction cannot be performed when a lens symmetrical with respect to the optical axis is used.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and provides a scanning optical system capable of suppressing the occurrence of a curved scanning surface and suppressing the influence of ghost light on the scanning surface. The purpose is to:

[0006]

According to the present invention, there is provided a scanning optical system comprising: a laser light source;
A scanning deflector for deflecting laser light emitted from the laser light source in the main scanning plane, a scanning lens for converging the deflected laser light on an image plane, and a light beam from the laser light source.
Before entering the scanning deflector, a sub-scan perpendicular to the main scanning section is used.
A focusing lens that forms an image once in the scanning plane and a scanning deflector
Of light flux by the condenser lens in the optical path of reflected light
Light beam from a laser light source
A narrow slit mirror for reflecting light to the scanning deflector
And a parallel plane plate provided between the scanning lens and the scanning lens.
The parallel flat plate is characterized in that a side surface thereof is subjected to an anti-reflection treatment .

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS.

FIG. 1 is a perspective view showing an arrangement of optical elements of a scanning optical system according to an embodiment.

The optical system shown in FIG. 1 forms a semiconductor laser 10 as a light source, a collimating lens 11 that converts divergent light emitted from the semiconductor laser 10 into a parallel light beam, a mirror 12, and a linearly formed image of the collimated light beam. A prism block 20 as a parallel plane plate having a cylindrical lens 13 and a slit mirror 21 provided at the position corresponding to the line image position
A polygon mirror 30 as a scanning deflector for reflecting and deflecting the light beam reflected by the slit mirror 21, and an anamorphic fθ as a scanning lens for condensing the light beam reflected by the polygon mirror 30 and forming a spot on a scanning surface.
The lens 40 is provided.

In the following description, a surface on which a light beam is scanned by the polygon mirror 30 is defined as a main scanning surface, and a surface perpendicular to the main scanning surface and including the optical axis of the scanning lens is defined as a sub-scanning surface.

The prism block 20 includes a triangular prism 22
Is a rectangular parallelepiped shape bonded to a trapezoidal prism 23 serving as a base, and a slit mirror 21 which is a total reflection mirror on the bonding surface.
Has been deposited. The angle of the slit mirror 21 with respect to the main scanning plane is approximately 45 °, and the line of intersection between the slit mirror 21 and the main scanning plane is perpendicular to the sub-scanning plane.

The divergent light emitted from the semiconductor laser 10 is collimated and then imaged by a cylindrical lens 13 in a line perpendicular to the sub-scanning surface. Since the slit mirror 21 is provided so as to coincide with this line image, the light beam from the light source forms an image on the slit mirror 21 and the total amount of light is reflected by this reflecting surface, and passes through the optical axis of the fθ lens 40. To the polygon mirror 30.

The light beam reflected and deflected by the polygon mirror 30 reaches the prism block 20 again with a predetermined spread. Here, most of the light flux passes through a portion around the slit mirror 21 and enters the fθ lens 40 to form a spot on a scanning plane (not shown). According to such a configuration,
Since the angle between the light beam incident on the polygon mirror and the optical axis of the fθ lens is 0, the curvature of field is completely symmetric,
Correction becomes easy.

In the optical system of the above-described embodiment, the positive power in the sub-scanning direction is set to be large in order to temporarily form the light beam on the slit mirror. Tends to be under. Therefore,
In this embodiment, a prism block 20 having a function of causing the light condensing position in the sub-scanning surface in the peripheral portion of the scanning surface to be substantially closer to the fθ lens side than the light condensing position in the central portion is arranged. The correction of the scanning surface curvature is performed by using the change of the deflection point and the movement of the condensing point by the prism block.

The light beam traveling from the polygon mirror to the fθ lens is a parallel light in the main scanning plane and a divergent light beam in the sub-scanning plane. For this reason, the prism block 20 provided in the optical path does not act on the light beam on the main scanning surface, but produces a function of moving the focal point on the sub-scanning surface depending on the incident angle. That is, the object distance of the off-axis light is shorter than that of the on-axis light, the position of the image forming point is closer to the fθ lens side, and the peripheral field curvature is improved.

Next, generation of ghost light due to internal reflection on the side surface of the prism block will be described with reference to FIGS.

In this type of optical system, the semiconductor laser is turned on even when a light beam strikes the end face of the prism block 20 in order to obtain a horizontal synchronization signal. The light beam incident on the slit mirror 21 has a spread between the points P1 and P6, and the light beams F1 and F2 indicate the width of the light beam. Figures 2 and 3 show
In both cases, the light beam reflected by the polygon mirror impinges on the end face of the prism block, that is, the light beam is outside the drawing range.

In FIG. 2, a point P on the slit mirror 21
1. The drawing light F1 indicated by a solid line reflected at the point P2 on the polygon mirror 30 reaches a point P3 on the end surface 20b of the prism block 20. The light beam transmitted through the end face 20b at the point P3 is out of the drawing range and is removed by the lens frame or the like.

On the other hand, the light beam reflected at the point P3 is reflected at the point P4 on the side surface 20d, and reaches the point P5 of the polygon mirror 30 as ghost light G1 indicated by a broken line.

The end faces 20a, 20b and side faces 20 of the prism block
Drawing light F that reaches P3 from point P1 because it is perpendicular to c and 20d
1 and the ghost light G1 reaching the point P5 from the point P4 are parallel to each other regardless of the angle of the polygon mirror. Therefore, the light beam G1 reflected by the polygon mirror is incident on the fθ lens 40 in parallel with the light incident on the polygon mirror 30, that is, in parallel with the optical axis Ax of the fθ lens, regardless of the rotation angle of the polygon mirror. It is converged toward the center of the scanning plane.

The drawing light F2 reflected on the point P6 on the slit mirror 21 and the point P7 on the polygon mirror 30 and incident on the prism block 20 is reflected on the point P8 on the side surface 20d, reaches the end surface 20b, and partially. Is the point P1 of the polygon mirror 30 as the ghost light G2
Returning to 0, the light passes through the prism block 20 and the fθ lens 40 and reaches the scanning surface.

FIG. 3 shows an optical path when the polygon mirror 30 is slightly rotated counterclockwise from the state shown in FIG. The ghost light beams G1 and G2 indicated by broken lines are shown in FIG.
Incident on the fθ lens 40 in parallel to the optical axis Ax as in the case of
The incident position has moved downward in the figure from the case of FIG. However, the luminous flux incident parallel to the optical axis Ax is converged toward the center of the scanning surface by the fθ lens, so that the position of the ghost on the scanning surface is substantially constant even if the incident position changes.

FIG. 4 shows the path of ghost light generated when the angle between the end face and the side face of the prism block is not 90 °. In FIG. 4, for simplicity of description, refraction at the end face 20a which is not related to reflection at the side face is not considered.

The angle formed between the side surface 20d and the end surface 20b of the prism block 20 is (90-α) °, the angle formed between the side surface 20d and the reflection surface of the polygon mirror 30 is β, and the angle of incidence and reflection between the light beam and each surface. Are θ0, θ1, θ2, and θ3, respectively, the relationship between these angles is as shown in Equation 1 below.

[0025]

[Equation 1] θ1 = 90 ° + θ0 + α-β θ2 = 90 °-θ1-α θ3 = β-θ2 θ3 = θ0 + 2α

That is, the direction of the light beam that first enters the polygon mirror and the direction of the light beam that is internally reflected at the corner of the prism, reflected twice by the polygon mirror, and enters the fθ lens have a fixed relationship. The relationship does not depend on the rotational position of the polygon mirror 30. Therefore, the ghost light always enters the fθ lens at a fixed angle, and forms a ghost at a fixed position on the scanning surface. In the state shown in FIG. 2, α = 0 in FIG. 4, that is, θ3 = θ0 from the above equation.
This is a unique case.

Since the optical path lengths of the ghost lights G1 and G2 reaching the scanning surface are longer than the normal drawing light F1 transmitted through the end face 20b at one time, the ghost light must be imaged before the scanning surface. Thus, the energy density per unit time on the scanning surface is low. However, while the drawing light scans on the scanning surface, the ghost light always stops at the center of the scanning surface and the energy is concentrated at one point, so that the energy per unit time is smaller than that of the drawing light. However, the energy applied to one point on the scanning surface tends to be large.

The drawing light power is Ii, and the ghost light power is I
g, number of scans per second r, scan efficiency η, scan width L,
Assuming that the scanning pitch is p and the main scanning spot diameter is s, the drawing light energy Ji and the ghost light energy Jg are respectively
It is expressed by the equations (1) and (2).

[0029]

[Equation 2] Ji = Ii · (η / rLp) (1) Jg = Ig / rsp (2)

The equation (3) is obtained from the equations (1) and (2).

[0031]

[Equation 3] Jg / Ji = (Ig / Ii) L / (η · s) (3) Here, for example, when values of L = 600 mm, s = 30 μ, η = 0.5 are set, the equation (4) is obtained. Can be

[0032]

[Equation 4] Jg / Ji = 40000Ig / Ii (4)

That is, the ghost light intensity is 1/40 of the drawing light.
Even if it is 000, the photosensitive pair drum to be exposed is exposed with the same energy as the drawing light.

In the embodiment, in order to prevent the ghost light from reaching the scanning surface, the side surfaces 20c,
20d is formed as a rough surface. For this reason, for example, side 2
The light rays incident on the points P4 and P4 on 0d are scattered by the rough surface, do not return to the polygon mirror, and can prevent the occurrence of ghost due to reflection on the side surface.

In the embodiment, the case where the rough surface is formed on the side surface of the prism block has been described. However, the present invention is not limited to the embodiment, and for example, an antireflection film may be formed on the side surface.

[0036]

As described in the foregoing, by arranging the parallel flat plate between the scanning deflector and the scanning lens in the present invention, it is possible to suppress the occurrence of field curvature, also flat <br / > By providing antireflection means on the side surface of the row plane plate, it is possible to prevent the occurrence of stationary ghost due to reflection on the side surface.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an arrangement of optical elements of a scanning optical system according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing generation of ghost light due to internal reflection on a side surface of a prism block.

FIG. 3 is an explanatory diagram showing generation of ghost light due to internal reflection on the side surface of a prism block, and shows a case where the rotational position of a polygon mirror is different from that in FIG. 2;

FIG. 4 is an explanatory diagram showing generation of ghost light due to internal reflection of the side surface of the prism block, and shows a case where the end surface and the side surface of the prism block are not perpendicular.

[Explanation of symbols]

 10 ... Semiconductor laser 20 ... Prism block 21 ... Slit mirror 30 ... Polygon mirror 40 ... Scanning lens

Claims (2)

(57) [Claims] A laser light source [1 claim], a scanning deflector for deflecting the laser beam emitted from the laser light source in the main scanning plane, a scanning lens for the deflected laser beam is converged on the image plane, from a laser light source Before the light beam enters the scanning deflector
A collection that forms an image once in a sub-scanning plane perpendicular to the main scanning section
An optical lens and the condenser lens in the optical path of the light reflected by the scanning deflector.
Beam from the laser light source
Having a narrow slit mirror for reflecting the light to the scanning deflector, and a parallel flat plate provided between the scanning deflector and the scanning lens, and a side surface of the parallel flat plate is subjected to antireflection processing. A scanning optical device. 2. The apparatus according to claim 1 , wherein said slit mirror is provided in said plane parallel plate.
The scanning optical device according to claim 1 , wherein the scanning optical device is provided.