JP2743176B2 - Optical scanning device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an arrangement in which an fθ lens system is arranged so that a deflecting reflection surface and a surface to be scanned are in an optically conjugate relationship, BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical scanning device configured to form an image on a surface to be scanned while converting a deflected linear condensed light at a constant speed, and more particularly, to an optical scanning device in which the fθ lens system has a three-lens configuration. Related to the device.

[Prior Art] Conventionally, as shown in FIG. 1A, for example, a laser beam or other light beam 1 modulated according to input information is converted into a parallel linear focused light through a collimating lens 7 or another imaging lens system 2. Incident on the deflecting / reflecting surface 3,
After deflecting and reflecting the light beam 1 at a constant angular velocity by the deflecting motion, the luminous flux 1 is incident on an imaging lens system 4 having fθ characteristics (hereinafter referred to as an fθ lens system) and converted into a constant velocity motion.
An optical scanning device configured to repeatedly form an image on a surface to be scanned 5 such as a photosensitive drum and form an electrostatic latent image corresponding to image information on the surface to be scanned 5 is known. . (Note that the three lens shapes that constitute the imaging lens system 4 are all related to the present invention and are not prior art.) In general, this type of apparatus includes a deflecting unit in which the deflecting unit is generally orthogonal to a deflecting surface. Although a polygon mirror 30 that rotates at a constant speed about a rotation axis 3A that is supported in the direction of rotation is used, such a deflecting means uses a vertical error of the rotation axis 3A and the rotation axis 3A and the deflection reflection surface 3A.
Is slightly inclined in the direction perpendicular to the deflection surface due to a small error such as surface inclination between the scanning surface 5 and the scanning surface 5.
It appears as uneven beam pitch above.

Such uneven pitches can be eliminated by arranging the fθ lens system 4 so that the deflection reflecting surface 3 and the scanned surface 5 are in an optically conjugate relationship.

Therefore, in the fθ lens system 4, the light flux 1
Optical characteristics are different between a deflection surface (hereinafter referred to as an X surface) on which deflection scanning is performed and a cross section orthogonal to the deflection surface (hereinafter referred to as a Y surface), so that the pitch unevenness and the image plane of the beam on the surface 5 to be scanned. Although it is necessary to realize constant-speed scanning of the beam while preventing bending, it is generally difficult to constitute such a lens with a single lens. At least one lens in the group is constituted by using a cylindrical lens or a toric lens.

On the other hand, as a lens group constituting the fθ lens system, for example, a combination of a spherical lens and a toric lens (Japanese Patent Application Laid-Open No.
No. 36622) and a three-lens configuration to achieve high image quality. In recent years, a three-lens configuration has been used to ensure good image quality. By arranging the lens group closer to the deflecting / reflecting surface 3 side, it is proceeding in the direction of achieving compactness.

As an fθ lens system having such a three-lens configuration, for example, in JP-A-60-100118,
There is disclosed a technology capable of achieving high image quality and high resolution by being constituted by a spherical single lens having a negative refractive power, a spherical single lens having a positive refractive power, and a single lens having a toric surface.

In order to reduce the size (flattening) of the fθ lens system, it is preferable to form the fθ lens system in a flat shape so that the lens width (height) in the Y plane direction is small. It is extremely difficult to polish the lens material cut into a shape. In general, a configuration is adopted in which the polished spherical lens is cut into strips with the optical axis interposed, and only the central portion is used.

However, in such a configuration using only a part of the polished spherical lens, the manufacturing cost is inevitably increased, and it is difficult to reduce the cost. Cutting parallel to the optical axis involves difficulty and often causes processing errors.

Further, since the two spherical lenses have refractive power in both directions of the orthogonal XY plane, the optical scanning is performed by making the optical axes of the two spherical lenses coincide in either direction of the two XY planes. It must be arranged at a predetermined position in the apparatus, which makes assembly adjustment extremely complicated and causes an assembly error.

In order to eliminate such disadvantages, for example, Japanese Patent Application Laid-Open No. 62-65011
The spherical lens having a negative refractive power in which one surface of the lens surface is a flat surface, having a negative refractive power in the Y-plane direction, and one surface of the lens surface being flat in order from the optical deflector side. And a three-lens fθ lens system composed of a toric lens having a positive refractive power in the X-plane direction smaller than the positive refractive power in the Y-plane direction and a flat lens surface on one side. .

According to the second prior art, since all three lenses constituting the fθ lens system form a flat surface on one side of the lens, processing accuracy, positioning and other factors are higher than those of the first prior art. Although the assembling accuracy can be greatly improved, disadvantages due to the use of the spherical lens, such as an increase in manufacturing cost due to using only the central portion of the polished spherical lens, an increase in both the X and Y planes, Disadvantages such as the fact that the optical axes must be aligned are still present.

Furthermore, the problem of the second prior art is that emphasis is placed on workability.
This is a point that the lens system could not be made compact.

That is, in the second prior art, in order to maintain good fθ characteristics, the axial distance between the first spherical lens and the second cylindrical lens located on the deflecting / reflecting surface side as compared with the first prior art. the d ₂ large, specifically in that must be set to large 3 to 4 times. As a result, the second and third lenses move toward the surface to be scanned by that much, and the axial length in the X-plane direction must be set to be large inevitably, and the overall size of the lens system can be reduced. I can't.

Further, in any of the prior arts described above, in order to improve the lens characteristics, the refractive index of the lens system is increased, and the refractive indices of all three lenses are set to 1.6 or more. Among them, in particular, the refractive index of the lens located on the exit surface side is set to 1.75 or more, which requires the use of expensive lens materials capable of increasing the refractive index in the prior art, The manufacturing cost increases accordingly.

In view of the drawbacks of the prior art, the present invention provides an optical scanning device that facilitates processing and assembly, makes the fθ lens system compact, improves lens characteristics, and achieves high image quality and high resolution on the surface to be scanned. The purpose is to provide.

Another object of the present invention is to achieve the above object smoothly even if a low refractive index lens of 1.6 or less is used for all of the fθ lens systems, thereby making the lens plastic or inexpensive. An object of the present invention is to provide an optical scanning device which enables the use of a glass material, thereby further reducing the manufacturing cost.

"Means for Solving the Problems" In order to achieve the above technical objects, the present invention provides an fθ lens system composed of three single lenses,
A first feature of the single lens group is that two lenses located on the deflection / reflection surface side are cylindrical lenses, and a third lens located on the exit surface side is a toric lens. I do.

The second feature is that each of the two cylindrical lenses is not a cylindrical lens having one flat surface but two lenses positioned on the deflecting / reflecting surface side are deflected in order from the reflecting surface side. It is characterized in that it is constituted by a cylindrical lens that is concave and flat and plano-convex in a plane (hereinafter referred to as X plane), and has a plano-convex and convex plane in a plane perpendicular to the deflection plane (hereinafter referred to as Y plane). .

That is, in the two lenses, one of the X surface and the Y surface has a refractive power on the incident surface side, and one of the Y surface and the X surface on the exit surface side, contrary to the incident surface side. Is a cylindrical lens having a refractive power. In the opposite view, the incident surface and the exit surface each have a linear portion having no refractive power on the X surface and the Y surface. The toric lens is a single lens composed of a lens surface having a refractive power on only one of the X surface and the Y surface and a toric surface, and the deflecting surface is plano-convex and deflecting surface. The configuration requirement is that the lens is a single lens having an uneven shape in a plane perpendicular to the inside.

According to the technical means, each of the cylindrical lenses is not a cylindrical lens having one flat surface as in the second conventional technique, but is divided into an entrance surface and an exit surface, each having a Y surface and an X surface. Since it has a refractive power, it has a refractive power for each lens in both the X-plane direction and the Y-plane direction together with the toric lens located on the exit side, resulting in at least three or more refractive powers. The desired lens characteristics can be formed by the combination of.

As a result, the large refracting power combined means that the lens characteristics in the X-plane direction and the Y-plane direction are improved and the lens characteristics of the desired accuracy are maintained without being particularly limited by the refracting power of the lenses themselves and the distance between the lenses. Can be planned.

That is, specifically without the distance between the axes d ₂ between the first cylindrical lens and second cylindrical lens large, and even when suppressing the refractive index of each lens material 1.6, the design Without any difficulty, maintaining the conjugate relationship between the deflecting reflecting surface 3 and the surface 5 to be scanned in the Y plane direction, X
Within the plane, lens characteristics such as good fθ scanning and prevention of image field distortion can be realized with high accuracy.

However, both the cylindrical lens group and the toric lens are different from the spherical lens which is always restricted in the other axial direction, and can freely design any refractive power independently in the X-plane or the Y-plane. The effect is more certain.

That is, the present invention enables the use of inexpensive glass materials and plasticization of lens materials as well as the compactness of the lens system, and at the same time, realizes favorable lens characteristics with high precision.

Further, the first cylindrical lens and the second cylindrical lens do not have a flat surface on one side of the lens unlike the second prior art, but are divided into an entrance surface and an exit surface to be refracted on an X surface and a Y surface, respectively. Since there is a linear portion having no force, positioning can be performed using the linear portion, thereby enabling accurate assembly.

Also in the toric lens of the third lens, a lens surface having a refractive power only on one of the X surface and the Y surface, in other words, a linear portion must exist on either the Y surface or the X surface. Therefore, the positioning can be easily performed using the linear portion.

Further, since the present invention does not use a spherical lens at all as in the prior art, the use of the spherical lens results in an increase in manufacturing cost due to the presence of a cut-out portion, and the optical axes in both directions of the XY plane coincide. This eliminates the complexity of assembling, in which it is necessary to arrange them.

Furthermore, the fact that each of the lenses has a linear portion on either one of the X surface and the Y surface facilitates processing using the linear portion and improves processing accuracy.

Hereinafter, preferred embodiments of the present invention will be illustratively described in detail with reference to the drawings. However, unless otherwise specified, the dimensions, materials, shapes, relative arrangements, and the like of the components described in this embodiment are not intended to limit the scope of the present invention thereto, but are merely illustrative examples. It's just

1A to 1C are schematic configuration diagrams of an optical scanning device according to an embodiment of the present invention. Briefly describing the configuration, 6 oscillates a light beam 1 modulated according to input information. The semiconductor laser has an oscillation wavelength λ set to 785 nm, and incorporates a collimator lens (not shown) for shaping a light beam oscillated from the semiconductor laser into a parallel beam. Reference numeral 2 denotes a cylindrical lens that oscillates from the semiconductor laser 6 and forms a parallel light beam as convergent light only in the vertical direction and enters the deflection / reflection surface 3. The deflecting reflecting surface 3 formed in a hexagonal shape is deflected at a constant angular velocity.

Reference numeral 4 denotes an fθ lens system, and the first
, A second cylindrical lens 42, and a toric lens 43. Each of these lenses is positioned and fixed at a predetermined position by aligning the optical axes in the X-plane and the Y-plane directions. An optically conjugate relationship is maintained in the vertical direction with respect to the imaging point on the scanning surface 5 side. Reference numeral 5 denotes a photosensitive drum and other surfaces to be scanned for image-forming and scanning the constant velocity scanning beam after passing through the fθ lens system.

Next, the following table shows the shape and mounting position of each lens constituting the fθ lens system.

Tables 1 and 2 show the shape and mounting position of each lens according to the first and second embodiments of the optical system of the present invention.

FIG. 2 and FIG. 3 are lens characteristic diagrams of fθ characteristics, field curvature, and spherical aberration confirmed in each of the above embodiments. In this case, each lens characteristic is obtained by setting the radius of the inscribed circle of the deflecting / reflecting surface 3 to 8.66 and the angle of incidence on the deflecting / reflecting surface 3 to 65 °.

Incidentally, the radius of curvature of each lens in the following Table in r1~r6 the X plane (direction), r1'~r6 'is the radius of curvature of each lens in the direction perpendicular to the X plane (mm), d ₀ is the entrance pupil position , d ₁ ~d ₅
Is the thickness of each lens on the optical axis and the distance between the lenses (m
m), n ₁ , n ₃ and n ₅ are the refractive indexes of the respective lens materials. (1B
(See Figure and Figure 1C) As understood from Tables 1 and 2, each lens shape of the fθ lens system in both the first and second embodiments has the first cylindrical lens 41 and the incident surface having refractive power only on the X surface and the Y surface. The second cylindrical lens 42 is formed from an entrance surface having a refractive power only on the Y surface and an exit surface having a refractive power only on the X surface. More toric lenses
In 43, an entrance surface having refractive power only on the Y surface and an exit surface having a toric surface are formed, and both are included in the technical scope of the present invention. The distance between the first cylindrical lens 41 and the second cylindrical lens 42 is 0.7 to 0.8 mm.
And the second and third lenses can be made closer to the deflecting / reflecting surface 3 by that much, and the size can be further reduced.

Also, regarding the ease of positioning, the first cylindrical lens 41 has a straight portion on the Y surface on the incident surface side and a straight line portion on the X surface on the output surface side, and the second cylindrical lens 42 has
Contrary to the above, the incident surface side has a linear portion on the X surface, and the output surface side has a linear portion on the Y surface. Further, the toric lens 43 has a linear portion on the X surface on the incident surface side. is there.

Next, the lens characteristics of each of the above embodiments will be examined.

(Study on fθ Characteristics) FIGS. 2 (A) and 3 (A) show fθ of each of the above embodiments.
FIG. 4 is an aberration diagram for determining characteristics.

The fθ characteristic is such that the focal length in the X plane (direction) of the lens system 4 is f, the incident angle of the beam to the lens is θ, and the distance from the center of the scanned surface 5 to the spot position at this time is an image height h. [H−f · θ) / f · θ] × 100 (1) and corresponds to a percentage deviation from the ideal image height.

The permissible amount of this fθ characteristic is, for example, when recording on an A4 size sheet, the beam scanning width is 210 mm on the short side of A4 size.
It has been empirically known that distortion of the obtained image can hardly be recognized even when the image position is shifted about ± 0.7 mm. Therefore, the allowable value of the fθ characteristic can be regarded as ± 0.67%.

Therefore, according to FIG. 2 (A) and FIG. 3 (A),
All of the A4 size fθ characteristics are 0.2% or less, and it can be understood that the A4 size has extremely high fθ characteristics.

(Study on curvature of field) On the other hand, in order for this optical system to function as an fθ lens, the image plane must be flat. In general, this field curvature can be divided into sagittal field curvature and meridional field curvature and displayed.

By the way, the allowable amount of curvature of the image plane can be considered by replacing this with the allowable amount of the spot system.

In the case of a beam, the intensity distribution of the beam cross section is a Gaussian distribution, and an extremely small diameter portion when focused by a lens or the like is called a beam waist, and its radius is usually indicated by Wo.

Here, assuming that the beam radius at a position away from the beam waist by the distance Z is W (z), the following equation holds.

Where λ is the wavelength of the laser light.

Since the value of Z at which the beam increases by a% is a = {(W (z) / Wo) −1} × 100 Becomes

Here, assuming a resolution of 240 dpi, assuming that a change in beam diameter of about 120 μm is allowed to be 10%, a = 10. Therefore, when such a numerical value is inserted into the above equation (2), Z = ± 6.6 m
m.

Therefore, both the meridional field curvature ΔM and the sagittal field curvature Δs may be ± 6.6 mm or less, and ± 2 mm even when the beam diameter is divided in order to obtain gradation.
The following is sufficient. Therefore, according to FIGS. 2 (B) and 3 (B), both ΔM and ΔS are 0.5 mm or less in each embodiment, and it is possible to maintain extremely high precision and high resolution.

Further, it is understood from FIGS. 2C and 3C that there is no problem in spherical aberration on a line perpendicular to the scanning line which affects pitch unevenness.

[Effects of the Invention] As described above, according to the present invention, in parallel with facilitation of processing and assembly and downsizing of the fθ lens system, lens characteristics are significantly improved, and high image quality and high image quality on the surface to be scanned are achieved. The resolution was improved.

Further, in the present invention, even if a low refractive index lens having a refractive index of 1.6 or less is used for all of the fθ lens systems, the above-mentioned effect can be achieved smoothly, thereby making it possible to use a plastic lens or an inexpensive glass material. Thus, the manufacturing cost can be further reduced. And so on.

[Brief description of the drawings]

1A to 1C are schematic configuration diagrams of an optical scanning device according to an embodiment of the present invention. FIG. 1A is a schematic perspective view, FIG. 1B is an arrangement diagram of an fθ lens system in a scanning direction, and FIG. The figure shows an arrangement diagram of the fθ lens system in a direction orthogonal to the scanning direction. 2 and 3 are aberration diagrams showing characteristics of each embodiment of the fθ lens.

Claims (1)

(57) [Claims] 1. An fθ lens system is disposed between a deflecting reflecting surface and a surface to be scanned, and the linear converged light deflected at a constant angular velocity by the deflecting reflecting surface is converted at a constant speed onto the surface to be scanned. In the optical scanning device configured to form an image, the fθ lens system includes three single lenses, and two lenses located on the deflecting / reflecting surface side of the single lens group are connected to the reflecting surface side. A cylindrical lens having a concave-planar, plano-convex, planar-convex, convex-planar shape in a plane perpendicular to the deflecting surface, and a third lens positioned on the exit surface side. A single lens having a lens surface having a refractive power only on the one surface and a toric surface, wherein the deflection surface is a plano-convex and has a concave-convex shape in a plane perpendicular to the deflection surface. Optical scanning device characterized in that it is configured