JPH05107495A - Scanning optical device - Google Patents

Abstract

PURPOSE:To enable design of a thin ftheta lens by forming a deflection surface of a light deflecting device comprising polygon mirrors in recessed form with a specified refracting power in main scanning direction only. CONSTITUTION:In a scanning optical device in which a beam radiated from a light source means 1 is reflected and deflected by a light deflecting device 5, introduced on a surface to be scanned 7 by an image-form optical system (ftheta lens 6), and the surface to be scanned is scanned by light, a deflection surface of a light deflecting device 5 comprises multiple reflection surfaces in recessed form with refracting power in main scanning direction only. Namely, the light deflecting device 5 is provided with multiple reflection surfaces (polygon surfaces) in recessed form with positive refracting power in main scanning direction only. Then, by distributing a part of the refracting power of an ftheta lens 6 on its reflection surface, the refracting power of the ftheta lens 6 is reduced to form the lens with one lens only. Thus the refracting power of the image- form optical system (ftheta lens 6) can be reduced, and the ftheta lens 6 can be constructed with a thin one lens only.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning optical device, and in particular, a light beam emitted from a light source means is reflected and deflected by an optical deflector (polygon mirror) having a concave reflecting surface as a deflecting surface.
The present invention relates to a scanning optical device suitable for a device such as a laser beam printer (LBP) or a digital copying machine, which records image information by optically scanning a surface to be scanned through an imaging optical system (fθ lens). It is a thing.

[0002]

2. Description of the Related Art Conventionally, a laser beam printer (L
In a scanning optical device such as BP), the light flux emitted from the light source means is optically modulated according to the image signal. Then, the light-modulated light beam is periodically deflected by an optical deflector composed of, for example, a polygon mirror, focused on a photosensitive recording medium surface in a spot shape by an imaging optical system having an fθ characteristic, and optically scanned. Image recording.

FIG. 4 is a sectional view of a main part of a conventional scanning optical device in the main scanning direction. In the figure, the luminous flux (divergent luminous flux) emitted from the light source means 41 is converted into a parallel luminous flux by the collimator lens 42, and the luminous flux (amount of light) is limited by the diaphragm 43 so that the cylindrical lens 44 has a refracting power only in the sub-scanning direction. Is incident on.

Of the parallel light beam incident on the cylindrical lens 44, the parallel light beam is emitted as it is in the main scanning plane. Further, in the sub-scanning plane, the reflection surface (polygon surface) of the optical deflector 45 which is converged and formed of a polygon mirror.
The image is formed on the line 45a as a substantially linear image. Then, the light beam reflected and deflected by the reflecting surface 45a of the optical deflector 45 is guided to the surface 47 to be scanned through an imaging optical system (fθ lens system) 46 including two lenses 46a and 46b having fθ characteristics. is doing.

By rotating the optical deflector 45 in the direction of arrow A, the surface 47 to be scanned is optically scanned.

[0006]

Conventionally, in this type of scanning optical apparatus, a collimator lens is generally used in order to convert a light beam (divergent light beam) emitted from a light source means into a parallel light beam.

When this collimator lens is used, for example, the relative alignment between the collimator lens and the light source means, that is, the light emission of the light source means in the main scanning plane with respect to the optical axis of the collimator lens. It becomes necessary to adjust the collimator such as positioning the surface with high accuracy. Generally, this collimator adjustment is very troublesome, and this is one of the causes that hinders cost reduction.

Therefore, various scanning optical devices have been proposed in which the divergent light beam from the light source means is directly incident on the reflecting surface of the optical deflector composed of a polygon mirror without using a collimator lens.

When a polygon mirror having a plane reflecting surface is used as a light deflector in such a scanning optical device, a light beam reflected by the reflecting surface and incident on the imaging optical system (fθ lens) is diverged. It becomes a luminous flux.

Therefore, in order to focus the light flux on the surface to be scanned, it is necessary to change the light flux from the divergent light flux to the focused light flux, and therefore, the refractive power (power) of the fθ lens is in the case of using a collimator lens. I had to make it quite large.

Further, in order to follow the low cost, it is preferable that the fθ lens is composed of one lens. fθ lens 1
Various scanning optical devices composed of a single lens have been proposed, for example, in JP-A-55-7727 and JP-A-58-5706.

Generally, when the fθ lens is composed of one lens, it is necessary to make the lens surface aspherical in order to maintain the optical performance. When the fθ lens is composed of an aspherical lens, it is desirable to process and mold it with a material such as plastic in terms of aspherical surface processing. Further, from the point of view of processing the aspherical surface, the aspherical lens is required to be a thin lens.

However, if the fθ lens is to be given a predetermined refractive power as described above, the lens thickness cannot be made too thin in view of the curvature.

The present invention does not use a collimator lens, but instead uses an optical deflector having a concave reflecting surface having a refracting power only in the main scanning direction as a deflecting surface. The reflecting surface has a positive refracting power. By making the reflecting surface share a part of the refracting power of the imaging optical system (fθ lens), the refracting power of the imaging optical system (fθ lens) can be reduced, and one thin sheet can be used. It is an object of the present invention to provide a scanning optical device in which the fθ lens is configured by the above lens and the cost is reduced and the assembly adjustment is simplified.

[0015]

In a scanning optical apparatus of the present invention, a light beam emitted from a light source means is reflected and deflected by an optical deflector and guided by an imaging optical system onto a surface to be scanned, and the surface to be scanned is scanned. In the scanning optical device for optical scanning, the deflecting surface of the optical deflector is composed of a plurality of reflecting surfaces each having a concave shape having a refractive power only in the main scanning direction.

[0016]

Embodiments FIGS. 1A and 1B show Embodiment 1 of the present invention.
FIG. 3 is a cross-sectional view of the main parts of the scanning optical device in the main scanning direction and the sub scanning direction.

In FIG. 1, reference numeral 1 denotes a light source means, which is composed of, for example, a semiconductor laser. A diaphragm 3 limits the divergent light flux (light quantity) emitted from the light source means 1. Four
Is a cylindrical lens (cylinder) and has a predetermined refractive power only in the sub-scanning direction. An optical deflector 5 is composed of a polygon mirror and is rotated in the direction of arrow A by driving means (not shown) such as a motor.

The optical deflector 5 in this embodiment has a plurality of reflecting surfaces (polygonal surfaces) each having a concave shape having a positive refractive power only in the main scanning direction. Then, as will be described later, a part of the refracting power of the fθ lens 6 is shared by the reflecting surface to reduce the refracting power of the fθ lens 6 to form one lens.

Reference numeral 6 denotes an image forming optical system (f.theta. Lens) having f.theta. Characteristics, which is composed of a single lens having a positive refractive power and having different curvatures in the main scanning direction and the sub scanning direction. The fθ lens 6 forms a light beam based on the image information reflected and deflected by the optical deflector 5 on the surface to be scanned (image surface) 7.

In this embodiment, the divergent light flux emitted from the light source means 1 is limited in quantity by the diaphragm 3 and is incident on the reflecting surface 5a of the optical deflector 5 through the cylindrical lens 4.

At this time, since the reflection surface 5a has a curvature, the light beam deflected and reflected by the reflecting surface 5a has a different deflection angle (incident angle at which the deflected light beam enters the fθ lens 6). The light collection positions are different. This phenomenon remarkably appears as the deflection angle increases.

Therefore, in this embodiment, the divergent light beam emitted from the light source means 1 is incident on the reflecting surface 5a at a predetermined angle from the sub-scanning direction. As a result, the deflection angle of the light flux is minimized and the above-mentioned influence is reduced.

The reflecting surface 5a of the optical deflector 5 can be easily processed by cutting it by inclining the rotary shaft 12 with the diamond cutting tool 11 by a predetermined amount as shown in FIG.

At this time, the shape of the reflecting surface 5a in the main scanning direction is not a perfect arc due to processing, and an axis connecting the rotation center O of the optical deflector 5 and the center of the reflecting surface 5a in the main scanning surface. X
Axis, and an axis orthogonal to the x-axis is the y-axis,

[0025]

[Equation 2] R ₁ is composed of a quadric surface which can be represented by the distance from the rotation center O of the optical deflector 5 to the diamond bite 11.

Since the reflecting surface 5a formed by this quadric surface has a positive refractive power, the light beam reflected and deflected by the reflecting surface 5a becomes a convergent light beam and enters the fθ lens 6.

The fθ lens 6 is composed of one lens, and its lens surface shape is an aspherical surface in order to reduce cost and maintain good optical performance. The aspherical shape has, for example, the origin at the intersection of the fθ lens 6 and the optical axis, the optical axis direction is the x-axis, and the axis orthogonal to the optical axis in the main scanning plane is y.
When the axis perpendicular to the optical axis in the sub-scanning plane is the z-axis, the generatrix direction corresponding to the main scanning direction is

[0028]

[Equation 3] Where R is the paraxial radius of curvature, K, B ₄ , B ₆ , B ₈ , B ₁₀
Are each expressed by an aspherical coefficient.

The sagittal direction corresponding to the sub-scanning direction is

[0030]

[Equation 4] Where r ′ = r (1 + D ₂ y ² + D ₄ y ⁴ + D ₆ y ⁶ +
D ₈ y ⁸ + D ₁₀ y ¹⁰ ) S is a lens shape in the sub-scanning plane, and r is a paraxial radius of curvature.

The above equation represents that the generatrix direction is an aspherical surface which can be expressed by a function up to the tenth order, and the sagittal direction is a toric surface having a different curvature depending on the value of y.

As described above, the shape of the fθ lens in this embodiment is formed with different curvatures in the main scanning direction and the sub scanning direction.

Further, in the fθ lens 6, the reflecting surface 5a and the surface to be scanned 7 are substantially in a conjugate relationship within the sub-scanning surface. Thus, for example, even if the reflecting surface is tilted in the sub-scanning plane, the light flux is focused on the same scanning line on the surface to be scanned. Then, the focused light flux incident on the fθ lens 6 is imaged on the scanned surface 7 by the fθ lens 6 and optically scans the scanned surface 7 with the light flux.

On the other hand, in the sub-scanning direction, surface tilt correction is performed as in the conventional scanning optical device. Therefore, after the light flux emitted from the light source means is once condensed on the reflecting surface 5a by the cylindrical lens 4, it is covered by the fθ lens 6. An image is formed on the scanning surface 7 and the surface to be scanned 7 is optically scanned with the light beam.

As described above, in this embodiment, the reflecting surface of the optical deflector is given a positive refracting power as described above, and a part of the refracting power of the fθ lens is shared by the reflecting surface. The refracting power of the lens can be made small, which enables the design of a thin fθ lens, and makes it possible to easily perform processing and molding with an inexpensive material such as plastic.

Next, Table-1 shows a design example in the first embodiment of the present invention.

In Table 1, it is designed to sufficiently satisfy the field curvature and the fθ characteristic. As shown in Table 1, most of the refracting power of the fθ lens used in the conventional scanning optical device is shared by the reflecting surface (polygon surface) of the optical deflector (R polygon), so that the refracting power of the fθ lens is increased. Can be quite small.

Therefore, by using a polygon mirror having a concave reflecting surface, it is possible to reduce the refracting power of the fθ lens, which makes it possible to design a thin lens.

Further, in this embodiment, since the divergent light beam emitted from the light source means 1 is made incident from the sub-scanning direction, it is possible to design an fθ lens which is bilaterally symmetrical in the main-scanning direction. It has the feature that it can be manufactured easily and at low cost.

[0040]

[Table 1] FIG. 3 is a sectional view of a main part of a scanning optical device according to a second embodiment of the present invention in the main scanning direction. In the figure, the same elements as those shown in FIG. 1 are designated by the same reference numerals.

The difference of this embodiment from the first embodiment is that the divergent light beam emitted from the light source means 1 is made incident on the reflecting surface of the optical deflector 5 in the sub-scanning direction, but at a predetermined angle from the main scanning direction. It was made to enter. Other configurations are similar to those of the first embodiment.

Also in this embodiment, as in the case of the first embodiment, there occurs a phenomenon that the converging points (converging positions) of the light beams differ depending on the deflection angles of the light beams. Therefore, the light beam is incident on the reflecting surface 5a at an angle from the main scanning direction so that the incident angle of the light beam with respect to the optical axis becomes small. In this way, the divergent light beam incident on the reflecting surface 5a is reflected and deflected as a convergent light beam by the reflecting surface 5a having a positive refracting power, and fθ formed by the lens surface shown in the above equations (2) and (3)
An image is formed on the surface 7 to be scanned by the lens 6.

In this way, in the present embodiment, as in the case of the first embodiment, most of the refracting power of the fθ lens 6 is shared by the reflecting surface 5a of the optical deflector 5, so that the fθ lens has a refracting power. The size of the lens can be reduced, which enables the design of a lens having a thin thickness as in the first embodiment.

Further, in this embodiment, since the divergent light beam from the light source means 1 is made incident from the main scanning direction, there is a feature that the scanning line in the sub-scanning direction is not bent.

[0045]

According to the present invention, by forming the reflecting surface of the optical deflector composed of a polygon mirror into a concave shape having a predetermined refractive power only in the main scanning direction as described above, the positive refraction on the reflecting surface is achieved. By giving a force, it is possible to reduce the refracting power of the fθ lens as an image forming optical system, which makes it possible to design an fθ lens having a small thickness, and easily use an inexpensive material such as plastic, for example. It is possible to achieve a scanning optical device capable of performing aspherical processing.

[Brief description of drawings]

FIG. 1 is a sectional view of a main part in a main scanning direction and a sub scanning direction according to a first embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a state of cutting a deflection surface of an optical deflector.

FIG. 3 is a sectional view of a main part in a main scanning direction according to a second embodiment of the present invention.

FIG. 4 is a sectional view of a main part of a conventional scanning optical device in a main scanning direction.

[Explanation of symbols]

 1 Light Source Means 3 Aperture 4 Cylindrical Lens 5 Optical Deflector (Polygon Mirror) 6 Imaging Optical System (fθ Lens) 7 Scanning Surface 11 Diamond Bit 12 12 Rotation Axis

Claims (4)

[Claims] 1. A scanning optical device in which a light beam emitted from a light source means is reflected and deflected by an optical deflector, guided onto a surface to be scanned by an imaging optical system, and optically scanned on the surface to be scanned.
A scanning optical device, wherein a deflecting surface of the optical deflector comprises a plurality of reflecting surfaces each having a concave shape having a refracting power only in a main scanning direction. 2. The shape of the reflecting surface of the optical deflector in the main scanning direction is defined by an x-axis, and an x-axis, which is an axis connecting the rotation center of the optical deflector and the center of the reflecting surface in the main scanning surface. The orthogonal axis is y
When the axis is used, 2. A quadric surface that is
Scanning optics. 3. The scanning optical device according to claim 1, wherein the light beam emitted from the light source means is incident on the reflection surface of the optical deflector from within the sub-scanning surface or from within the main scanning surface. 4. The scanning optical device according to claim 1, wherein the imaging optical system has at least one lens having different curvatures in the main scanning direction and the sub scanning direction.