JP3420439B2 - Optical scanning optical system and laser beam printer including the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical scanning optical system and a laser beam printer having the optical scanning optical system, and in particular, a light beam modulated and emitted from a light source means is deflected by an optical deflector such as a rotating polygon mirror, and then f.theta. A laser beam printer (LB) having, for example, an electrophotographic process, which records image information by optically scanning a surface to be scanned, which is a recording medium surface, through an optical element (fθ lens) having characteristics.
P) and digital copying machines.

[0002]

2. Description of the Related Art Conventionally, a light beam (light beam) deflected (reflected) by each deflection surface (reflection surface) of an optical deflector composed of a rotating polygon mirror is used to optically scan a surface to be scanned. An optical scanning optical system is proposed in, for example, Japanese Patent Publication No. 62-36210.

The optical scanning optical system proposed in the above publication is provided with an imaging optical system for optical scanning having an f-θ characteristic between an optical deflector and a surface to be scanned, and one of the imaging optical systems is provided. By properly setting the refracting power of the toric lens, the angular error when the deflecting surface of the optical deflector is not parallel to the rotation axis and is tilted, so-called surface tilt is corrected.

That is, a toric lens is used to make the deflection surface of the optical deflector and the surface to be scanned optically conjugate with each other to eliminate the adverse effect of the surface tilt. As a result, the traveling direction of the light beam deflected by the deflecting surface on the scanning surface is corrected to prevent unevenness in the pitch of the scanning lines.

As described above, various optical scanning optical systems for correcting the tilt of the deflecting surface have been proposed in the past, and various imaging optical systems having two or more lenses have been proposed and put to practical use. Has been done.

On the other hand, as a simpler optical scanning optical system, an optical scanning optical system in which an image forming optical system is composed of one lens (fθ lens) is disclosed in, for example, Japanese Patent Laid-Open No. 1-224721.
JP-A-62-138823 and JP-A-63-138823
Various proposals are made in JP-A-157122 and JP-A-2-87109.

Of these publications, Japanese Patent Laid-Open No. 1-24722
In Japanese Patent No. 1, the imaging optical system is composed of one toric lens, and the focused light flux from the condenser lens as the focusing optical system is incident on the toric lens.

JP-A-62-138823, JP-A-63-157122 and JP-A-2-87109.
In Japanese Patent Laid-Open Publication No. JP-A-2003-264, a scanning lens (aspherical lens) having a lens surface with a high-order aspherical surface is used as an imaging optical system, and a parallel light flux from a collimator lens is incident on the scanning lens.

[0009]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
In Japanese Patent No. 224721, a toric lens is used in an image forming optical system, and a focused light beam converted by a condenser lens (collimator lens) is made incident on the toric lens to correct aberration. There is a problem that it is difficult to achieve both curvature of field and constant velocity. Further, in the same publication, the curvature of field is focused and corrected by limiting the correction so that the constant velocity can be electrically corrected. Therefore, it is necessary to correct the distortion of the written image by continuously changing the timing of the image information when writing the image. However, in this case, the velocity of the spot on the surface to be scanned is constantly changed due to insufficient correction of the constant velocity, and the unit time and the amount of light per unit area received by the surface to be scanned are changed. There was a problem.

It is possible to correct this problem by continuously changing the laser light amount, but there is a problem that the number of correction circuits becomes too large and the merit of a single lens cannot be obtained. There is.

JP 62-138823 A, JP 63-157122 A and JP 2-87109 A.
The aspherical lens as an imaging optical system disclosed in Japanese Patent Laid-Open Publication No. 2003-242242 is formed such that the thickness of the lens is thicker than the width of the surface to be scanned. Since such aspherical lenses are more difficult to process and mold than ordinary spherical lenses, the problems in manufacturing can be solved by processing and molding using a material with high processability such as plastic. There is.

However, generally, a lens made of a plastic material is easily affected by environmental changes, and particularly, the refractive index is easily changed by temperature, humidity, etc. For example, when the lens is thick, the light flux passing through the lens is refracted. There is a problem in that the image forming position is largely changed because the image forming position is greatly changed. In addition, the fact that the thickness of the lens is large has been a cause of deteriorating processing conditions such as homogeneity and distortion inside the lens in processing and molding, and molding time.

According to the present invention, when the focused light flux from the collimator lens is imaged on the surface to be scanned by one scanning lens (fθ lens) made of a plastic material through the optical deflector, the lens shape of the scanning lens is used. It is an object of the present invention to provide a light scanning optical system suitable for plasticization by suppressing the thickness of the lens thinly by appropriately configuring the arrangement and the arrangement. A further object of the present invention is to provide an optical scanning optical system capable of obtaining a sufficiently small spot shape corresponding to high-definition image output over an effective scanning width of A3 size or more.

[0014]

The optical scanning optical system of the present invention comprises: (1) a light beam emitted from a light source unit is guided to a deflecting unit via an optical unit, and the light beam deflected by the deflecting unit is guided. In an optical scanning optical system in which an optical element guides light onto a surface to be scanned and optically scans, a light beam incident on the optical element is made into a substantially focused light beam, and at least one surface of both surfaces of the optical element is within a main scanning section. Is an aspherical surface, the shape of the optical element in the main scanning direction is asymmetrical with respect to the optical axis center, the focal length of the optical element in the sub scanning direction is fb, and the focused light flux is deflected by the deflecting means. When the distance from the point to the surface to be scanned is La, and the distance from the deflection point deflected by the deflecting means to the virtual focusing point when the focused light beam does not have the optical element is Lb, 0.13 <fb / La <0.25 (1) Lb / La <10 ‥ is characterized by satisfying the (2) The condition.

In particular, (1-1) the material of the optical element is a plastic material, and (1-2) the optical element is a single lens. The laser beam printer of the present invention is characterized by including the optical scanning optical system having the above-mentioned configuration (1) and the photosensitive drum as the surface to be scanned. The digital copying machine of the present invention is characterized by including the optical scanning optical system having the above-mentioned configuration (1) and the photosensitive drum as the surface to be scanned.

[0016]

1 is a sectional view (main scanning sectional view) of a main portion in a main scanning direction according to a first embodiment of the present invention. In the figure, reference numeral 1 denotes a light source means, which is composed of, for example, a semiconductor laser. A collimator lens 2 converts the divergent light flux emitted from the light source means 1 into a substantially focused light flux.
An aperture stop 3 adjusts the diameter of the passing light beam. Reference numeral 4 denotes a cylindrical lens, which has a predetermined refracting power only in the sub-scanning direction, and of a light beam (light beam) that has passed through the aperture stop 3 in a sub-scanning section of an optical deflector 5 as a deflecting means described later. An image is formed on the deflection surface (reflection surface) 5a as a substantially linear image. The optical deflector 5 is composed of, for example, a polygon mirror (rotary polygonal mirror) or the like, and a driving means (not shown) such as a motor.
Due to this, it rotates at a constant speed in the direction of arrow A in the figure. still,
Each element such as the collimator lens 2, the aperture stop 3, and the cylindrical lens 4 constitutes one element of optical means.

Reference numeral 6 is 1 having an fθ characteristic as an optical element.
It is a scanning lens (fθ lens) consisting of one lens,
A lens shape made of a plastic material, in which at least one of the lens surfaces of the scanning lens 6 is formed of an aspherical surface in the main scanning section, and the lens shape in the main scanning direction with respect to the optical axis center of the scanning lens 6. Is formed asymmetrically. The scanning lens 6 forms a light beam based on the image information deflected by the optical deflector 5 on the surface of the photosensitive drum 7 as a surface to be scanned, and corrects the tilt of the deflecting surface of the optical deflector 5.

In this embodiment, the divergent light beam (light beam) emitted from the semiconductor laser 1 is converted into a convergent light beam by the collimator lens 2, and the light beam (amount of light) is limited by the aperture stop 3 and is incident on the cylindrical lens 4. ing. The light flux that has entered the cylindrical lens 4 exits as it is in the main scanning cross section. Further, in the sub-scanning cross section, they are converged and imaged on the deflection surface 5a of the optical deflector 5 as a substantially linear image (a linear image longitudinal in the main scanning direction). Then, the light beam deflected by the deflecting surface 5a of the optical deflector 5 is guided to the surface of the photosensitive drum 7 via the scanning lens 6, and the optical deflector 5 is rotated in the direction of arrow A, whereby the photosensitive drum 7 is rotated. The surface is scanned in the direction of arrow B with a substantially uniform linear motion. Image recording is thus performed.

FIG. 2 is a sectional view (sub-scanning sectional view) of a main part of a main portion in a vertical direction (sub-scanning direction) in the main scanning section of FIG.

In the figure, P indicates the position of the reflection surface (deflection surface) of the optical deflector 5, and the light beam (light beam) is condensed at substantially this reflection surface position P in the sub-scan section as described above. I am trying. Here, the reflection surface position P and the surface of the photosensitive drum 7 are in an optically substantially conjugate positional relationship with respect to the scanning lens 6. As a result, even if the reflecting surface is tilted in the sub-scanning cross section, or if there is so-called surface tilt, the light flux is focused on the same scanning line on the surface of the photosensitive drum 7.
In this way, in this embodiment, the surface tilt of the optical deflector 5 is corrected.

In general, the light beam incident on the scanning lens (fθ lens) is generally set to be a substantially parallel light beam within the main scanning section. In the present embodiment, as described above, the light beam is made incident on the scanning lens 6 in a state of substantially a focused light beam. The scanning lens 6 is set so that the relationship of y = fθ is established between the focal length f in the main scanning section and the scanning angle θ and the image height y. When the light beam is incident on the scanning lens 6, the focal length of the scanning lens 6 is y = fθ because the focused light beam itself has power.
The focal length f can be set longer than the focal length f.

As a result, the thickness of the scanning lens 6 can be suppressed to be thin (small), and the lens shape can be made such that the thickness in the vicinity of the optical axis and the thickness at the lens effective end portion are not significantly different from each other. Therefore, it is possible to greatly improve the processing conditions such as homogeneity, distortion, and processing time when the lens material is molded with a plastic material.

Further, in the present embodiment, as described above, at least one of the lens surfaces of the scanning lens 6 in the main scanning section is formed of an aspherical surface, so that the field curvature can be improved over a wide angle of view. Correcting.

Generally, when a rotary polygon mirror is used as the deflecting means, the amount of field curvature generated by the scanning lens 6 is not the same when the deflection angle θ shown in FIG. 1 is positive and when it is negative. It is normal. FIG. 3 illustrates this state by taking the field curvature in the meridional direction as an example. The asymmetrical component of the field curvature as shown in the figure remains to some extent as long as the lens shape in the main scanning cross section of the scanning lens is symmetrical with respect to the optical axis center.

The configuration in which the focused light flux is incident on the scanning lens 6 as in the present embodiment has the merit that the conditions for processing and molding the lens can be greatly improved as described above, but the focal length f of the scanning lens 6 is large. Is set to be long, the amount of the asymmetric component of the field curvature tends to increase accordingly.

However, if the effective scanning width is up to about A4 size, the above-mentioned asymmetrical component is also a sufficiently permissible amount, and a sufficiently small spot shape corresponding to high-definition image output can be obtained over the entire effective scanning width. However, when the effective scanning width becomes A3 size or more, it tends to be difficult to obtain a desired spot shape.

Therefore, in the present embodiment, as described above, the aspherical shape of at least one of the lens surfaces of the scanning lens 6 is made asymmetric with respect to the optical axis center, so that the effective scanning width is A3 size or more. It is possible to obtain a sufficiently small spot shape over the entire area, whereby the asymmetrical component of the field curvature described above is corrected to a negligible extent over the entire effective scanning width of A3 size or more.

Further, in this embodiment, the focal length of the scanning lens 6 in the sub-scanning direction is fb, the distance from the deflection point P where the focused light beam is deflected by the optical deflector 5 to the scanned surface 7 is La, and FIG.
As shown in (1), when the distance from the deflection point P to the virtual focusing point P'when the focused light beam does not have the scanning lens 6 is Lb, 0.13 <fb / La <0.25 (1) Lb / La <10 (2) At least one of the following conditions is satisfied. As a result, an optical scanning optical system suitable for plasticization and high-definition printing is obtained.

The conditional expression (1) is the focal length fb of the scanning lens 6 in the sub-scanning direction and the distance L from the deflection point P to the surface 7 to be scanned.
When the value exceeds the lower limit of conditional expression (1), it becomes difficult to correct the field curvatures in the meridional direction and the sagittal direction in a well-balanced manner, and in particular, the scanning lens 6 is made of a plastic material. When molded with
It is not good because the movement of focus on the surface 7 to be scanned becomes unacceptable due to the influence of environmental changes. Conditional expression (1)
When the value exceeds the upper limit of 1, it is advantageous for aberration correction, but it is not good because the scanning lens 6 approaches the surface to be scanned 7 and the entire apparatus becomes large.

Conditional expression (2) is derived from the deflection point P to the virtual focusing point P.
Regarding the ratio of the distance Lb to ′ ′ and the distance La from the deflection point P to the surface 7 to be scanned, it is for making the focused light beam enter the scanning lens 6 efficiently, and if the conditional expression (2) is not satisfied, scanning is performed. The refracting power (power) of the lens 6 in the main scanning direction must be set strongly, and the wall thickness of the lens becomes large, and the wall thickness near the optical axis and the wall thickness at the lens end portion are significantly different. This is not desirable because the processing conditions such as homogeneity, distortion, and processing time when the lens material is molded from a plastic material are significantly deteriorated.

FIG. 5 and FIG. 6 are various aberration diagrams showing field curvature, distortion (f.theta. Characteristic) and the like in the first embodiment of the present invention. As is apparent from these aberration diagrams, it is understood that each aberration is corrected well in this embodiment.

FIG. 7 is a sectional view (main scanning sectional view) of a main portion in the main scanning direction according to the second embodiment of the present invention. In FIG.
The same elements as those shown in are given the same reference numerals.

The present embodiment is different from the first embodiment described above in that the scanning lens (fθ lens) 16 has a different lens shape, and other configurations and optical functions are substantially the same as those of the first embodiment. Therefore, a similar effect is obtained.

FIG. 8 and FIG. 9 are various aberration diagrams showing field curvature, distortion (f.theta. Characteristic) and the like in the second embodiment of the present invention. As is apparent from these aberration diagrams, it is understood that each aberration is corrected well in this embodiment.

Next, numerical examples 1 and 2 of the scanning lens (fθ lens) according to the present invention will be shown. Numerical Examples 1 and 2 are numerical examples after the optical deflector 5 of Embodiments 1 and 2 of the present invention, respectively. In Numerical Embodiments 1 and 2, the radius of curvature of each lens surface in the main scanning section is R1 and R2 in order from the optical deflector 5 side, and the radius of curvature in the sub scanning section is r1 and r2 in order, respectively. The distances are indicated by D1 and D2, respectively. The refractive index of the lens at a wavelength of 780 nm is represented by N1.

Further, K _{U to} D _U and K _{L to} D _L are X when the optical axis is the X axis as shown by the first lens surface and the axis orthogonal to it and existing in the main scanning cross section is the Y axis. A relational expression between the height y of the lens surface on the Y plane and the distance x from the vertex of the lens surface.

[0037]

[Equation 1] It is represented by the aspherical coefficient of each degree of.

Further, a _{U to} e _U and a _{L to} e _L are relational expressions y ≧ 0 between the height y of the lens surface on the XY plane and the radius of curvature r ′ in the sub-scan section. Then r ′ = r (1 + a _U y ² + b _U y ⁴ + c _U y ⁶ + d _U y
⁸ + e _U y ¹⁰ ) When y <0, r ′ = r (1 + a _L y ² + b _L y ⁴ + c _L y ⁶ + d _L y
⁸ + e _L y ¹⁰ ). Table 1 shows the relationship between the numerical examples 1 and 2 and the conditional expressions (1) and (2) described above.

[0039] [Numerical Example 1] focal length 212.2mm maximum scan angle 82.7 ° deflection point ~R1 surface 56.1mm R1 = 111.77 D1 = 16.0 N1 = 1.5242 of the total system _{K U = -15.3092 K L = -14.7594} A U = _{^{-3.74175 × 10 -7 A L = -4.05918}} × 10 -7 B U = 4.72734 × 10 -11 B L = 1.39652 × 10 -11 C U = -1.79928 × 10 -15 C L = -8.16509 × 10 -15 D ^{_{U = 2.16557 × 10 -19 D L}} = 1.40250 × 10 -18 r1 = -29.7564 a U = 9.34082 × 10 -4 a L = 8.43382 × 10 -4 b U = 2.10889 × 10 -8 b L = -3.1011 × 10 ^-7 c _U = -2.38825 x 10 ^-10 c _L = -2.99730 x 10 ^-11 d _U = 1.20716 x 10 ^-13 d _L = 8.14391 x 10 ^-14 e _U = -1.57294 x 10 ^-17 e _L = -1.24936 ^{× 10 -17 R2 = 294.01 D2 =} 174.07 K U = -110.227 K L = -104.3 A U = -6.56411 × 10 -7 A L = -6.6408 × 10 -7 B U = 8.70817 × 10 -11 B L = 6.54217 _{^{× 10 -11 C U = -1.27139 ×}} 10 -14 C L = -1.9152 × 10 -14 D U = 1.38190 × 10 -18 D L = 1.45205 × 10 -18 r _{= -16.775 a U = 2.91036 × 10} -4 a L = 2.75506 × 10 -4 b U = -8.98738 × 10 -8 b L = -1.23201 × 10 -7 c U = 2.13687 × 10 -12 c L = 1.45427 × 10 ^-11 d _U = 6.74772 × 10 ^-15 d _L = 7.76902 × 10 ^-15 e _U = -1.01846 × 10 ^-18 e _L = -1.45361 × 10 ^-18 La = 246.17 mm Lb = 495 mm fb = 51.5 mm [Numerical value example 2 focal length 212.4mm maximum scan angle 82.7 ° deflection point ~R1 surface 56.7mm R1 = 110.49 D1 = 15.4 N1 = 1.5242 of the total system _{K U = -17.0315 K L = -8.27899} a U = -3.53181 × 10 - ^{_{7 A L = -4.47142 × 10 -7}} B U = 2.61693 × 10 -11 B L = -1.32162 × 10 -11 C U = 3.13471 × 10 -15 C L = 1.83137 × 10 -14 D U = 3.88356 × 10 - ^{_{20 D L = -1.18971 × 10 -18}} r1 = -29.3783 a U = 1.05556 × 10 -3 a L = 1.04329 × 10 -3 b U = 4.48777 × 10 -7 b L = -2.08003 × 10 -7 c U = -2.81497 × 10 ^-10 c _L = -6.58331 × 10 ^-11 d _U = 9.79329 × 10 ^-14 d _L = 1.00037 × 10 ^-13 e _U = -1.43003 × 10 ^{-17 _{e L = -2.00591 × 10 -17 R2}} = 290.48 D2 = 174.07 K U = -99.642 K L = -67.6373 A U = -7.28050 × 10 -7 A L = -4.99539 × 10 -7 B U = 1.08505 × 10 - ^{_{10 B L = -9.24564 × 10 -12}} C U = -1.93659 × 10 -14 C L = 1.3113 × 10 -15 D U = 2.38781 × 10 -18 D L = 1.42004 × 10 -18 r2 = -16.52 a U = 3.40524 × 10 ^-4 a _L = 3.15986 × 10 ^-4 b _U = -5.78602 × 10 ^-8 b _L = -1.20982 × 10 ^-7 c _U = -1.02494 × 10 ^-11 c _L = 2.31786 × 10 ^-11 d _U = 7.18632 x 10 ^-15 d _L = 3.0093 x 10 ^-15 e _U = -9.78821 x 10 ^-19 e _L = -1.04963 x 10 ^-18 La = 246.17 mm Lb = 495.3 mm fb = 51.6 mm

[0040]

[Table 1]

[0041]

As described above, according to the present invention, when the focused light flux from the collimator lens is imaged on the surface to be scanned by one scanning lens (fθ lens) made of a plastic material via the optical deflector. , At least one of the lens surfaces of the scanning lens is an aspherical surface in the main scanning section, and the lens shape in the main scanning direction is asymmetric with respect to the optical axis center of the scanning lens. By satisfying the conditional expression, an optical scanning optical system suitable for plasticization can be achieved. Further, according to the present invention, as described above, it is possible to achieve an optical scanning optical system capable of obtaining a sufficiently small spot shape suitable for high-definition image output over an effective scanning width of A3 size or more, and a laser beam printer including the same. .

[Brief description of drawings]

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

FIG. 2 is a sectional view of a main part in a sub-scanning direction according to the first embodiment of the present invention.

FIG. 3 is an aberration diagram illustrating conventional field curvature.

FIG. 4 is an explanatory view showing how a focused light beam is incident on a scanning lens in the first embodiment of the present invention.

FIG. 5 is an aberration diagram illustrating field curvature according to the first embodiment of the present invention.

FIG. 6 is an aberration diagram illustrating an fθ characteristic according to the first embodiment of the present invention.

FIG. 7 is a sectional view of an essential part of a main part in a main scanning direction according to a second embodiment of the present invention.

FIG. 8 is an aberration diagram illustrating field curvature according to the second embodiment of the present invention.

FIG. 9 is an aberration diagram illustrating an fθ characteristic according to the second embodiment of the present invention.

[Explanation of symbols]

1 light source means 2 Collimator lens 3 aperture stop 4 Cylindrical lens 5 Deflection means 6,16 Optical element 7 Scanned surface (photosensitive drum)

Claims (5)

(57) [Claims] 1. Light for scanning a light flux emitted from a light source means to a deflecting means via an optical means, and guiding the light flux deflected by the deflecting means onto a surface to be scanned by an optical element for optical scanning. In a scanning optical system, a light beam incident on the optical element is made to be a substantially convergent light beam, at least one surface of both surfaces of the optical element is an aspherical surface in a main scanning section, and the optical axis center of the optical element is set. The shape in the main scanning direction is asymmetric, the focal length of the optical element in the sub-scanning direction is fb, the distance from the deflection point where the focused light beam is deflected by the deflecting means to the scanned surface is La, and the deflecting means is Satisfying the condition of 0.13 <fb / La <0.25 Lb / La <10, where Lb is the distance from the deflected deflection point to the virtual focusing point when the focused light beam does not have the optical element. Optical scanning optical system characterized by. 2. The optical scanning optical system according to claim 1, wherein the material of the optical element is a plastic material. 3. The optical scanning optical system according to claim 1, wherein the optical element is a single lens. 4. The light according to claim 1.
Equipped with a scanning optical system and a photosensitive drum as a surface to be scanned
A laser beam printer that is characterized. 5. The light according to any one of claims 1 to 3.
Equipped with a scanning optical system and a photosensitive drum as a surface to be scanned
A digital copying machine characterized by