JP3156052U - Two-piece fθ lens with short focal length suitable for laser beam scanning device - Google Patents

Abstract

A two-piece fθ lens having a short focal length suitable for miniaturization of a laser beam scanning device is provided. In a laser beam scanning apparatus having a rotating polygon mirror, a first lens is a positive refraction lens and a second lens is a negative refraction lens in the main scanning direction of the scanning center axis. The first lens 131 includes a first optical surface 131a and a second optical surface 131b, and the second lens 132 includes a third optical surface 132a and a fourth optical surface 132b. The concave surfaces of the first optical surface 131a, the second optical surface 131b, and the third optical surface 132a are provided on the rotary polygon mirror 10 side. The fourth optical surface 132b has an inflection point, and the convex surface is provided on the rotary polygon mirror 10 side in the scanning center axis, and satisfies the optical conditions. [Selection] Figure 1

Description

  The present invention relates to a short focal length two-piece fθ lens suitable for a laser beam scanning device, and in particular, has a short focal length applicable to a laser beam scanning device (Laser scanning unit) using a rotating polygon mirror. The present invention relates to a two-piece fθ lens capable of reducing the volume of a laser beam scanning device.

  A laser beam scanning device (Laser Scanning Unit, LSU) currently used in a laser beam printer (LBP) is a laser beam scanning by a polygon mirror that rotates at high speed. What controls the operation is, for example, US Pat. No. 7,079,171 (Patent Document 1), US Pat. No. 6,377,293, US Pat. No. 6,295,116, or Taiwan Patent No. 198966. As disclosed in (Patent Document 2), the principle will be briefly explained. A laser beam is emitted from a semiconductor laser, passed through a collimator, and passed through an aperture device. And parallel beam is formed. The parallel beam further passes through a cylindrical lens, is converged to form a line image, and then is projected onto a high-speed polygon mirror. Uniform and continuous polygon mirrors are provided on the rotating polygon mirror. These reflecting mirrors are provided on the focal position of the above-described line image or in the proximity thereof. The rotating polygon mirror controls the projection direction of the laser beam, and when a plurality of reflection mirrors provided in succession rotate at high speed, the laser beam projected on one of the reflection mirrors is scanned in the scanning direction (X axis). Are reflected obliquely to the fθ linear scanning lens at the same angular velocity (angular velocity). The fθ linear scanning lens is provided near the rotating polygon mirror, and may be a single-element scanning lens or a two-piece lens structure. As disclosed in U.S. Pat. No. 4,707,085, U.S. Pat. No. 6,757,088 or Japanese Patent Application Laid-Open No. 2004-294713, the function of this f.theta. Due to the reflection, the laser beam incident on the fθ lens converges the scanning light beam into a circular (or elliptical) light spot, and enters a light receiving surface formed by a photosensitive drum (photoreptor drum), that is, an imaging surface. Project and satisfy the requirements of scanning linearity.

US Pat. No. 7,079,171 Taiwan Patent No. 198966 Specification US Pat. No. 5,111,219 US Pat. No. 5,136,418 Japanese Patent No. 2756125 US Pat. No. 7,057,781 US Pat. No. 6,919,993 Japanese Patent Laid-Open No. 04-50908 US Pat. No. 7,130,096 US Pat. No. 6,324,015 US Pat. No. 6,933,961

However, the known fθ-line scanning lens still has the following problems.
(A) When the laser beam is reflected by the rotating polygon mirror, the scanning center axis of the laser beam projected on the reflecting mirror of each rotating polygon mirror is not aimed at the center axis of the rotating polygon mirror, so the fθ linear scanning lens In designing the lens, it is necessary to consider the problem of reflection deviation of the rotating polygon mirror. Among known techniques, means for correcting optical compensation in the main scanning direction by an optical compensation method in the sub-scanning direction are disclosed in US Pat. No. 5,111,219 (Patent Document 3), US Pat. No. 5,136, No. 418 (Patent Document 4), Japanese Patent No. 2756125 (Patent Document 5) and the like. However, since the reflection deviation is optically corrected in the sub-scanning direction and a longer focal length is required, there is a problem that the volume of the laser beam scanning device is increased.

  (B) As disclosed in U.S. Pat. No. 2002/0063939, the publicly known technique is such that the scanning light beam of the f.theta. Linear scanning lens allows the light spot diameter on the photosensitive drum to satisfy the requirements of the practical standard. In addition, in the known technology, a method of improving the imaging quality by using a lens having a long focal length or extending the imaging distance by a reflecting mirror is often used. Others include three-piece lenses disclosed in US 2002/0030158, US 5,086,350 and JP-A 63-172217, or US 2001/0009470 and US patents. Diffractive lenses that are difficult to manufacture as disclosed in US Pat. No. 5,838,480 or the like, or US Pat. No. 5,111,219 (Patent Document 3) US Pat. No. 7,057,781 Patent Document 6), US Pat. No. 6,919,993 (Patent Document 7) and the like, a two-piece lens having an inflection point, and Japanese Patent Application Laid-Open No. 04-50908 (Patent Document) And a single piece lens having an inflection point disclosed in 8).

(C) When a small printer is used, as one means for reducing the volume of the laser beam scanning device, on the photosensitive drum disclosed in US Pat. No. 7,130,096 (Patent Document 9) or the like. Means for shortening the image forming distance can be mentioned. Thereby, the image forming distance on the photosensitive drum is shortened by the means for limiting the ratio between the effective scanning distance and the image forming optical path length, and at the same time, the ghost image phenomenon occurs. can avoid. In US Pat. No. 6,324,015 (Patent Document 10), the ratio of the distance from the rotary polygon mirror to the photosensitive drum (hereinafter referred to as “focal distance”) and the focal length of the fθ lens ( By limiting d / f), the distance is shortened. However, as an example where the focal length is 100 mm, the focal length is about 200 mm. The two-piece fθ lens disclosed in US Pat. No. 6,933,961 (Patent Document 11) has an asymmetric optical surface so that the optical axis of the two-piece fθ lens is in the main scanning direction or the sub-scanning direction. The focal length can be shortened by generating a deviation from the scan direction in the scanning direction. In order to satisfy consumer demands for light and thin laser beam scanning devices, a short focal length (for example, an A4-compatible laser beam scanning device whose focal length is 150 mm or less) and main scanning Effective optical distortion can be corrected also in the sub-scanning direction, and there is an urgent demand from the user due to improvement in scanning quality and resolution.
An object of the present invention is to provide a two-piece fθ lens having a short focal length suitable for a laser beam scanning device.

  In the present invention, the two-piece fθ lens is applied to a laser beam scanning apparatus including a rotating polygon mirror, and a first lens and a second lens are provided in order from the rotating polygon mirror side. The first lens includes a first optical surface and a second optical surface, and the second lens includes a third optical surface and a fourth optical surface. The short focal length two-piece fθ lens is characterized in that each optical surface of the two-piece fθ lens is aspheric in the main scanning direction. In the main scanning direction of the scanning center axis, the first lens is a meniscus lens having a positive refractive index, while the second lens is a lens having a negative refractive index. The concave surfaces of the first and second optical surfaces of the first lens and the third optical surface of the second lens face the rotary polygon mirror side. The fourth optical surface of the second lens has an inflection point, and its convex surface faces the rotating polygon mirror side, and mainly uniformizes the scanning light beams in the main scanning direction and the sub-scanning direction. Due to the deviation, an imaging deviation on the photosensitive drum is formed. By correcting the scanning light beam and converging on the target, the scanning light beam reflected by the rotary polygon mirror is accurately imaged on the target, and the scanning effect necessary for the laser beam scanning device is realized.

  Another aspect of the present invention is to provide a short focal length two-piece fθ lens suitable for a kind of laser beam scanning device. In the present invention, by having a short focal length, the volume of the laser beam scanning device can be reduced and a good imaging effect can be obtained. Further, by satisfying the condition of 0.4557 ≦ tan (β) ≦ 0.7265, the maximum effective window angle of β is such that the laser beam reflected by the rotating polygon mirror is reflected by this short focal length. The objective of reducing the volume of the laser beam scanning device by satisfying the requirement of the projected light spot (spot) area on the target object while having a short focal length with a shorter scanning ray through the two-piece fθ lens Can be achieved.

  Yet another aspect of the present invention is to provide a short focal length two piece fθ lens for a type of laser beam scanning device. In the present invention, the deviation of the optical axis of the scanning light beam increases the shift in the main scanning direction and the sub-scanning direction, and the light spot imaged on the photosensitive drum is deformed. The point size is made uniform, and an improvement effect of resolution quality is achieved.

  Therefore, the short focal length two-piece fθ lens suitable for the laser beam scanning apparatus according to the present invention is applied to the laser beam scanning apparatus and is emitted from the light source by the reflecting mirror of the rotating polygon mirror in the laser beam scanning apparatus. The laser beam is reflected, formed into a scanning beam, and imaged on the target. For a laser beam printer, this target is usually a photoreceptor drum (ie, a drum), that is, the light spot waiting for image formation is emitted by a laser beam from a light source and scanned by a reflecting mirror of a rotating polygon mirror. Then, the scanning light beam passes through a short focal length two-piece fθ lens suitable for the laser beam scanning apparatus according to the present invention, corrects the angle and position, and then forms a light spot (on the photosensitive drum). spot) is formed. Further, since the photosensitive agent is applied on the photosensitive drum, the toner can be gathered on the paper and the data can be printed out.

From the description of the embodiments below, the present invention can realize at least the following effects.
(B) By providing a two-piece fθ lens with a short focal length suitable for the laser beam scanning apparatus according to the present invention, the rotating polygon mirror corrects the light spot distance on the imaging surface to non-constant speed scanning to constant speed scanning. Then, by projecting the laser beam onto the imaging surface by constant speed scanning, the distance between the two adjacent light spots formed on the target can be matched.

  (B) By installing a short focal length two-piece fθ lens suitable for the laser beam scanning apparatus according to the present invention, the distortion of the scanning light source in the main scanning direction and the sub-scanning direction can be corrected, and an image is formed on the target. The light spot can be reduced.

  (C) By installing a two-piece fθ lens with a short focal length suitable for the laser beam scanning apparatus according to the present invention, distortion of the scanning light source in the main scanning direction and the sub-scanning direction can be corrected and imaged on the target. The light spot size can be made uniform.

  (D) By installing a short focal length two-piece fθ lens suitable for the laser beam scanning device according to the present invention, the focal length can be reliably shortened, the volume of the laser beam device can be reduced, and the demand for miniaturization can be achieved. .

It is explanatory drawing which shows the optical path of the two piece type f (theta) lens which concerns on this invention. It is explanatory drawing which shows the optical path through which a scanning ray passes the 1st lens and the 2nd lens. It is explanatory drawing which expanded the part explaining the inflection point of a 4th optical surface. It is explanatory drawing explaining the geometric light spot area on a photoconductive drum. It is explanatory drawing which shows the maximum effective window. It is explanatory drawing explaining the optical path of each Example which concerns on this invention. It is explanatory drawing which shows the light spot size of each position on the target by Example 1. FIG. 2 is a light spot distribution diagram on a photosensitive drum according to Embodiment 1. FIG. It is explanatory drawing which shows the light spot size of each position on the target by Example 2. FIG. 6 is a light spot distribution diagram on a photosensitive drum according to Embodiment 2. FIG. It is explanatory drawing which shows the light spot size of each position on the target by Example 3. FIG. 6 is a light spot distribution diagram on a photosensitive drum according to Embodiment 3. FIG. It is explanatory drawing which shows the light spot size of each position on the target by Example 4. FIG. 7 is a light spot distribution diagram on a photosensitive drum according to Embodiment 4. FIG. It is explanatory drawing which shows the light spot size of each position on the target by Example 5. FIG. 10 is a light spot distribution diagram on a photosensitive drum according to Embodiment 5. FIG. It is explanatory drawing which shows the light spot size of each position on the target by Example 6. FIG. 10 is a light spot distribution diagram on a photosensitive drum according to Embodiment 6. FIG.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
(First embodiment)
FIG. 1 is a diagram showing an optical path of a short focal length two-piece fθ lens suitable for the laser beam scanning apparatus according to the first embodiment of the present invention. Referring to FIG. 1, a short focal length two-piece fθ lens suitable for the laser beam scanning apparatus according to the present embodiment includes a first lens 131 and a second lens 132. The first lens 131 has a first optical surface 131a and a second optical surface 131b, and the second lens 132 has a third optical surface 132a and a fourth optical surface 132b. In FIG. 1, the laser beam scanning apparatus mainly includes a laser light source 11, a rotary polygon mirror 10, a cylindrical lens 16, and a photosensitive target. As shown in FIG. 1, the target is a photosensitive drum (drum) 15 in this embodiment. The light beam 111 generated from the laser light source 11 passes through the cylindrical lens 16 and is then projected onto the rotary polygon mirror 10. A plurality of reflecting mirrors are provided on the rotating polygon mirror 10. In FIG. 1, there are five reflecting mirrors. Each reflection mirror rotates about the central rotation axis of the rotary polygon mirror 10 to reflect the light beam 111 to form scanning light beams 113a, 113b, and 113c. Therefore, the projection of the scanning light beams 113a, 113b, and 113c in the X direction is referred to as a sub scanning direction, and the projection in the Y direction is referred to as a main scanning direction. Further, after the scanning light beams 113c and 113b are emitted from the fourth optical surface 132b of the second lens 132, the distance between the left end (left end) and the right end (right end) on the photosensitive drum 15 is set to an effective window ( effective window) 3 distances are formed. As shown in FIG. 2, the light spot 2 within the range of the effective window 3 distance gathers the toner on the paper by heat, and prints out the document data.

  Please refer to FIG. 1 and FIG. FIG. 2 is an explanatory diagram showing the optical path of the scanning light beam that passes through the first lens 131 and the second lens 132. A laser beam 111 is emitted from the laser light source 11, is reflected through the rotary polygon mirror 10, and then becomes a scanning beam. When the scanning light beam passes through the first lens 131, the distance and time of the reflected light beam reflected from the rotary polygonal mirror 10 by the refraction of the first optical surface 131a and the second optical surface 131b of the first lens 131 are nonlinear. Replace the distance and time with a scanning ray in a linear relationship. After the scanning beam passes through the first lens 131 and the second lens 132, the scanning beam is sensitized by the optical properties of the first optical surface 131a, the second optical surface 131b, the third optical surface 132a, and the fourth optical surface 132b. The light beam converges on the photosensitive drum 15 and forms a row of light spots (Spot) 2 on the photosensitive drum 15. Of these, d0 is the shortest distance from the center of the laser beam to the reflecting mirror of the rotary polygon mirror 10 through the optical surface of the rotary polygon mirror 10 by the cylindrical lens 16, and d1 is the first optical surface 131a from the rotary polygon mirror 10. D2 is the distance from the first optical surface 131a to the second optical surface 131b, d3 is the distance from the second optical surface 131b to the third optical surface 132a, and d4 is the first 3 is a distance from the third optical surface 132a to the fourth optical surface 132b, and d5 is a distance from the fourth optical surface 132b to the photosensitive drum 15. R1 is the curvature radius of the first optical surface 131a, R2 is the curvature radius of the second optical surface 131b, R3 is the curvature radius of the third optical surface 132a, and R4 is the first radius of curvature. 4 is the radius of curvature of the optical surface 132b.

  The fourth optical surface 132b is an optical surface having an inflection point in the main scanning direction. As shown in FIG. 3, the convex surface of the scanning center axis faces the rotary polygon mirror 10 side, and after leaving the scanning center axis and passing through the inflection point P, the concave surface faces the rotary polygon mirror 10 side. It becomes.

  FIG. 4 is an explanatory diagram in which the spot area is changed depending on each projection position after the scanning light beam is projected onto the photosensitive drum. Referring to FIG. 4, when the scanning light beam 113 a passes through the first lens 131 and the second lens 132 along the main scanning direction and then is projected onto the photosensitive drum 15, the first lens 131 and the second lens 132. Since the incidence angle is zero, the deviation rate formed in the main scanning direction is the smallest. Therefore, the light spot 2a imaged on the photosensitive drum 15 has a circular outer shape. The scanning light beams 113b and 113c pass through the first lens 131 and the second lens 132, and when projected onto the photosensitive drum 15, the scanning light beams incident on the first lens 131 and the second lens 132 are scanned. Since the scissor angle with the central axis is not zero, a shift rate is produced in the main scanning direction, which is larger than the projection length in the main scanning direction and the light spot formed from the scanning light beam 113a. This phenomenon also occurs in the sub-scanning direction, and the light spot formed from the scanning light beams 113b and 113c separated from the scanning light beam 113a becomes larger. Therefore, the light spots 2b and 2c imaged on the photosensitive drum 15 are elliptical, and the areas of the light spots 2b and 2c are larger than the light spot 2a. In this figure, Sa0 and Sb0 are the root mean square radii (root mean square) in the main scanning direction (Y direction) and the sub-scanning direction (X direction), respectively. of spot size radius on target). Sa and Sb are the root mean square radii of the light spot size on the target in the X and Y directions, respectively. Smax and Y are the maximum radii in the main scanning direction at an arbitrary light spot.

FIG. 5 is an explanatory diagram showing an effective window and a maximum effective window angle β that the scanning light beam projects onto the photosensitive drum 15. Referring to FIG. 5, after the leftmost scanning beam 113c is emitted from the fourth optical surface 132b of the second lens 132, the scissor angle between the scanning beam 113c and a straight line parallel to the scanning center axis is an effective window angle. This is the maximum value of β. In order to achieve the purpose of reducing the volume of the laser beam scanning device, the focal length can be shortened by shortening the imaging distance from the rotary polygon mirror 10 to the photosensitive drum 15. As means for shortening the focal length, the optical characteristics of the four optical surfaces of the first lens 131 and the second lens 132, or the optical characteristics (refractive index) of the materials used for the first lens 131 and the second lens 132, respectively. , Abbe number, etc.) through the optical design, the focal length (d1 + d2 + d3 + d4 + d5) is shortened. In addition, since the value of the maximum effective window angle β can be increased, the widening angle of scanning can be increased. The maximum effective window angle β and the distance from the second lens 132 to the photosensitive drum 15 are expressed by the following equation (11). As shown in FIG. 5, if the β value is increased, the ya value can be effectively reduced in a fixed effective window.

In Equation (11), y _a is emitted from the main scanning direction outermost end of the scanning beam in the direction (Y direction) (leftmost 113c or rightmost 113b), a fourth optical surface 132b of the second lens 132 is scanned This is the distance from a straight line parallel to the central axis to the imaging surface of the photosensitive drum 15. y _b is emitted from the scanning light beam (the leftmost end 113c or the rightmost end 113b) at the most distal end in the sub-scanning direction (X direction), and forms an image on the photosensitive drum 15 from the fourth optical surface 132b of the second lens 132. The distance to the surface.

  As described above, the short focal length two-piece fθ lens suitable for the laser beam scanning apparatus according to the present embodiment performs distortion correction on the reflected scanning light beam by the rotating polygonal mirror 10, and each projection light spot. In the distance to the target, the relationship between time and angular velocity is replaced with the relationship between time and distance. In the main scanning direction and the sub-scanning direction, each angle at which the scanning light beam passes through the fθ lens in the sub-scanning direction (X direction) and the main scanning direction (Y direction) is uniform light on the imaging surface. Dots can be formed, the required resolution can be provided, and the focal length can be effectively shortened. As a result, the volume of the laser beam device is reduced.

  In order to achieve the above-described effect, a short focal length two-piece fθ lens suitable for the laser beam scanning apparatus according to this embodiment includes the first optical surface 131a or the second optical surface 131b of the first lens 131, and the second lens 132. The third optical surface 132a or the fourth optical surface 132b can be designed to have a spherical surface curved surface or an aspheric curved surface structure in the main scanning direction (Y direction) or the sub scanning direction (X direction). When an aspheric curved surface design is used, the aspheric curved surface is based on the following curved surface equation.

1: Anamorphic equation (Anamorphic equation)

In Equation (12), Z represents a switching plane distance from the optical axis direction of any point on the lens to the origin (SAG), C _x and C _y are respectively X and Y directions of the curvature (curvature) in and, K _x and K _y is the conical coefficient in the X and Y directions _{(conic coefficient), a R,} B R, C R, D r is 4 respectively rotationally symmetrical part (rotationally symmetric portion) , 6, 8 and 10 to the power of cone deformation, A _P , B _P , C _P , and D _P are 4, 6, 8 of the non-rotationally symmetrical components, respectively. And the deformation from the power of ten (deformation from the co) Is ic). When C _x = C _y , K _x = K _y , and A _P = B _P = C _P = D _P = 0, it is simplified to a single aspherical surface.

2: Toric equation

In Equation (13), Z is, switching plane distance from the optical axis direction of any point on the lens until origin (SAG), C _x and C _y are respectively the Y direction and X direction curvature (curvature) , K _y is the conic coefficient in the Y direction, and B ₄ , B ₆ , B ₈ , B ₁₀ are the conical deformation coefficients (deformation) of the _fourth , _sixth , _eighth and tenth powers (4th to 10th order coefficient). from the conic).

  As an example of maintaining a constant velocity scan on the imaging plane on the target, the scanning beam can be considered to maintain the two light spot distances equal in two identical time intervals. The short focal length two-piece fθ lens suitable for the laser beam scanning apparatus according to the present embodiment corrects the emission angle of the scanning light beam by the first lens 131 and the second lens 132 between the scanning light beam 113a and the scanning light beam 113b. The two light spot distances formed on the photosensitive drum 15 are made equal by correcting the emission angles of two scanning beams having the same time interval. That is, the light spot size imaged on the photosensitive drum 15 is made uniform (restricted to the range required by the resolution), and the optimum resolution effect is obtained.

主走査方向において、数式（１６）の条件を満足する。

副走査方向において、数式（１７）を満足する。

The short focal length two-piece fθ lens suitable for the laser beam scanning apparatus according to this embodiment includes a first lens 131 and a second lens 132 in order from the rotary polygon mirror 10. The first lens 131 has a first optical surface 131a and a second optical surface 131b, and the second lens 132 has a third optical surface 132a and a fourth optical surface 132b. In the main scanning direction of the scanning center axis, the first lens 131 is a meniscus lens having a positive refractive index, and the second lens 132 is a lens having a negative refractive index. The concave surfaces of the first optical surface 131a, the second optical surface 131b, and the third optical surface 132a face the rotary polygon mirror 10 side. The fourth optical surface 132b has an inflection point, and the convex surface of the fourth optical surface 132b faces the rotary polygon mirror 10 side. As a result, the angle and the time reflected by the rotating polygonal mirror 10 are replaced with the scanning light beam spot having a nonlinear relationship with the distance and the time, and the optical distortion is corrected and the target is placed on the target. Converge. Thus, the first optical surface 131a, the second optical surface 131b, the third optical surface 132a, and the fourth optical surface 132b are all composed of optical surfaces that are aspheric in the main scanning direction. Further, at least one of the first optical surface 131a, the second optical surface 131b, the third optical surface 132a, and the fourth optical surface 132b has an optical surface composed of an aspheric surface in the sub-scanning direction. Subsequently, regarding the optical design of the first lens 131 and the second lens 132, the air interval and the maximum effective window angle β of the two-piece fθ lens according to the present embodiment are further determined by the conditions of the equations (14) and (15). Satisfied.

In the main scanning direction, the condition of Expression (16) is satisfied.

Formula (17) is satisfied in the sub-scanning direction.

In Equations (14), (15), (16), and (17), d ₁ is the distance from the reflecting surface of the rotary polygon mirror 10 on the optical axis to the optical surface of the first lens 131 on the rotary polygon mirror side. , D ₃ is the distance from the target-side optical surface of the first lens 131 on the scanning center axis to the optical surface of the second lens 132 on the rotary polygon mirror 10 side, and d ₅ is the first on the scanning center axis. The distance from the optical surface of the two lenses 132 on the target side to the target, f _s indicates the composite focal length of the two-piece fθ lens, and β indicates the maximum effective window angle. f _{(1) y} is the focal length of the first lens 131 in the main scanning direction, and f _{(2) y} is the focal length of the second lens 132 in the main scanning direction. f _{(1) x} is the focal length of the first lens 131 in the sub-scanning direction, and f _{(2) x} is the focal length of the second lens 132 in the sub-scanning direction. n _d1 and n _d2 are the refractive indices of the first lens 131 and the second lens 132, respectively.

Furthermore, the light spot size formed by the two-piece fθ lens according to the present invention can be represented by a ratio value of η _max and η _min . η _max is a proportional value of the maximum value of the geometric light spot scanned on the photosensitive drum 15 by the geometric light spot of the scanning light beam on the reflecting surface of the rotating polygon mirror 10, and η _min is the rotation value. The geometric light spot of the scanning light beam on the reflecting surface of the polygon mirror 10 is a proportional value of the minimum value of the geometric light spot scanned on the photosensitive drum 15. η _max and η _min satisfy the conditions of the following formulas (18) and (19), respectively.

In Equations (18) and (19), S _a and S _b are the root mean square radii in the sub-scanning direction and the main scanning direction, respectively, for any light spot formed from the scanning light beam on the photosensitive drum 15. Show. δ is the proportional value of the minimum light spot and the maximum light spot on the photosensitive drum 15, and η is the light spot formed by the scanning light beam on the reflecting surface of the rotary polygon mirror 10 and the photosensitive drum 15. Proportional values with respect to the light spot, S _a0 and S _b0 indicate the root mean square radii in the sub-scanning direction and the main scanning direction, respectively, of the light spot formed by the scanning light beam on the reflecting surface of the rotary polygon mirror 10.

  In order to clarify the present embodiment, the structure and technical features according to the present embodiment will be described in detail with reference to the following drawings.

The embodiments disclosed below are for the purpose of explaining main components of a short focal length two-piece fθ lens suitable for the laser beam scanning apparatus according to the present embodiment. Therefore, the present invention is applied to a laser beam scanning apparatus using a general rotary polygon mirror in the embodiments disclosed below. In a general laser beam scanning device, structures other than the two-piece fθ lens disclosed in this embodiment are known techniques. Therefore, a person having ordinary knowledge in the field has a structure of a two-piece fθ lens suitable for a laser beam scanning apparatus using a rotating polygon mirror disclosed in this embodiment in the structure of the embodiment disclosed below. It does not add any restrictions. That is, each constituent element of the short focal length two-piece fθ lens suitable for the laser beam scanning device can be variously replaced, changed, or replaced by an equivalent. As an example, it is assumed that the design or material selection of the radius of curvature of the first lens 131 and the second lens 132, the distance adjustment, and the like are not limited. Further, for convenience of explanation and comparison, in all of the following examples, the light spot formed by the scanning beam on the rotary polygonal mirror 10 has a root mean square radius in the X direction and Y direction, respectively, S _a0 = 47.89 (μm). And S _b0 = 641.49 (μm), but this is not the case.

In the short focal length two-piece fθ lens in the present embodiment, the second optical surface 131b of the first lens 131 and the third optical surface 132a of the second lens 132 are both aspherical surfaces. ) To design the curved surface of the optical surface. The first optical surface 131a of the first lens 131 and the fourth optical surface 132b of the second lens 132 are all aspheric in the main scanning direction, and the curved surface of the optical surface is obtained by the aspheric formula of Equation (13). To design. The optical characteristics and aspheric parameters are shown in Tables 1 and 2, and the optical path diagram is shown in FIG. The inflection point of the fourth optical surface 132b is located at ψ = 2.33 °.

The optical surface of the short focal length two-piece fθ lens configured in this way is f _{(1) Y} = 88.111 (mm), f _{(2) Y} = −502.724 (mm), f _{(1) X} = −9.844 (mm), f _{(2) X} = 24.685 (mm), y _a = 85.00 (mm), y _b = 50.44 (mm), maximum window angle β = 30. For 68 °, the light spot on the rotating polygonal mirror 10 is scanned with a scanning light beam and converged on the photosensitive drum 15 to form a small light spot, and the formulas (14) to (17) shown in Table 31 and The conditions of Expression (18) to Expression (19) are satisfied. Table 4 shows the geometrical light spot diameter (μm) of the light spot at the distance (mm) from the photosensitive drum 15 to the scanning center axis Y in the Y direction in the scanning center axis (Z axis). Further, a light spot distribution diagram and a light spot size shape diagram according to this embodiment are shown in FIGS.

In the short focal length two-piece fθ lens in the present embodiment, the second optical surface 131b of the first lens 131 and the third optical surface 132a of the second lens 132 are both aspherical surfaces. ) To design the curved surface of the optical surface. The first optical surface 131a of the first lens 131 and the fourth optical surface 132b of the second lens 132 are all aspheric in the main scanning direction, and the curved surface of the optical surface is obtained by the aspheric formula of Equation (13). To design. The optical characteristics and aspheric parameters are shown in Tables 5 and 6, and the optical path diagram is shown in FIG. The inflection point of the fourth optical surface 132b is located at ψ = 8.21 °.

The optical surfaces of the short focal length two-piece fθ lens configured in this way are f _{(1) Y} = 84.264 (mm), f _{(2) Y} = -335.022 (mm), f _{(1 ) X} = −7.838 (mm), f _{(2) X} = 26.919 (mm), y _a = 82.0 (mm), y _b = 43.0 (mm), maximum window angle β = 27 .67 ° is obtained by scanning the light spot on the rotary polygon mirror 10 with a scanning light beam and converging it on the photosensitive drum 15 to form a small light spot, and formulas (14) to (17) shown in Table 7 are formed. ) And Expressions (18) to (19) are satisfied. Table 8 shows the geometrical light spot diameter (μm) of the light spot at the distance (mm) from the photosensitive drum 15 to the scanning center axis Y in the Y direction. Further, a light spot distribution diagram and a light spot size shape diagram according to this embodiment are shown in FIGS.

In the short focal length two-piece fθ lens in the present embodiment, the second optical surface 131b of the first lens 131 and the third optical surface 132a of the second lens 132 are both aspherical surfaces. ) To design the curved surface of the optical surface. The first optical surface 131a of the first lens 131 and the fourth optical surface 132b of the second lens 132 are all aspheric in the main scanning direction, and the curved surface of the optical surface is obtained by the aspheric formula of Equation (13). To design. The optical characteristics and aspheric parameters are shown in Table 9 and Table 10, and the optical path diagram is shown in FIG. The inflection point of the fourth optical surface 132b is located at ψ = 6.57 °.

The optical surfaces of the short focal length two-piece fθ lens configured in this way are f _{(1) Y} = 83.522 (mm), f _{(2) Y} = -357.438 (mm), f _{(1 ) X} = -12.477 (mm), f _{(2) X} = -357.434 (mm), y _a = 85.0 (mm), y _b = 42.474 (mm), maximum window angle β = For 28.74 °, the light spot on the rotary polygon mirror 10 is scanned with a scanning beam and converged on the photosensitive drum 15, and a small light spot is formed. 17) and the expressions (18) to (19) are satisfied. Table 12 shows the geometrical light spot diameter (μm) of the light spot at the distance (mm) from the photosensitive drum 15 to the scanning center axis Y in the Y direction in the scanning center axis (Z axis). Further, a light spot distribution diagram and a light spot size shape diagram according to this embodiment are shown in FIGS.

In the short focal length two-piece fθ lens in the present embodiment, the second optical surface 131b of the first lens 131 and the third optical surface 132a of the second lens 132 are both aspherical surfaces. ) To design the curved surface of the optical surface. The first optical surface 131a of the first lens 131 and the fourth optical surface 132b of the second lens 132 are all aspheric in the main scanning direction, and the curved surface of the optical surface is obtained by the aspheric formula of Equation (13). To design. The optical characteristics and aspheric parameters are shown in Tables 13 and 14, and the optical path diagram is shown in FIG. The inflection point of the fourth optical surface 132b is located at ψ = 6.569 °.

The optical surfaces of the short focal length two-piece fθ lens configured in this way are f _{(1) Y} = 133.630 (mm), f _{(2) Y} = −5116.737 (mm), f _{(1) X = -0.445 (mm), f} (2) X = 20.124 (mm), y a = 85.0 (mm), y b = 42.474 (mm), the maximum window angle beta = 26. For 55 °, the light spot on the rotary polygon mirror 10 is scanned with a scanning light beam and converged on the photosensitive drum 15 to form a small light spot, and Equations (14) to (17) shown in Table 15 are formed. The conditions of Expression (18) to Expression (19) are satisfied. Table 16 shows the geometrical light spot diameter (μm) of the light spot at the distance (mm) from the photosensitive drum 15 to the scanning center axis Y in the Y direction. Further, a light spot distribution diagram and a light spot size shape diagram according to this embodiment are shown in FIGS.

In the short focal length two-piece fθ lens in the present embodiment, the second optical surface 131b of the first lens 131 and the third optical surface 132a of the second lens 132 are both aspherical surfaces. ) To design the curved surface of the optical surface. The first optical surface 131a of the first lens 131 and the fourth optical surface 132b of the second lens 132 are all aspherical in the main scanning direction, and the curved surface of the optical surface is determined by the aspherical formula of Equation (13). design. The optical characteristics and aspheric parameters are shown in Table 17 and Table 18, and the optical path diagram is shown in FIG. The inflection point of the fourth optical surface 132b is located at ψ = 2.04 °.

The optical surface of the short focal length two-piece fθ lens configured in this way has f _{(1) Y} = 99.246 (mm), f _{(2) Y} = -1228.670 (mm), f _{(1 ) X} = −10.704 (mm), f _{(2) X} = 106.623 (mm), y _a = 85.0 (mm), y _b = 48.607 (mm), maximum effective window angle β = For 29.763 °, the light spot on the rotary polygon mirror 10 is scanned with the scanning light beam and converged on the photosensitive drum 15, and a small light spot is formed. The conditions of (17) and Expressions (18) to (19) are satisfied. Table 20 shows the geometrical light spot diameter (μm) of the light spot on the photosensitive drum 15 where the scanning center axis (Z axis) is a distance (mm) from the scanning center axis Y in the Y direction. Further, a light spot distribution diagram and a light spot size shape diagram according to this embodiment are shown in FIGS.

In the short focal length two-piece fθ lens in the present embodiment, the second optical surface 131b of the first lens 131 and the third optical surface 132a of the second lens 132 are both aspherical surfaces. ) To design the curved surface of the optical surface. The first optical surface 131a of the first lens 131 and the fourth optical surface 132b of the second lens 132 are all aspherical in the main scanning direction, and the curved surface of the optical surface is determined by the aspherical formula of Equation (13). design. The optical characteristics and aspheric parameters are shown in Table 21 and Table 22, and the optical path diagram is shown in FIG. The inflection point of the fourth optical surface 132b is located at ψ = 6.30 °.

The optical surface of the short focal length two-piece fθ lens configured in this way is f _{(1) Y} = 82.522 (mm), f _{(2) Y} = −300.994 (mm), f _{(1 ) X} = −8.349 (mm), f _{(2) X} = 21.894 (mm), y _a = 82.3 (mm), y _b = 49.273 (mm), maximum window angle β = 30 .908 ° scans the light spot on the rotary polygonal mirror 10 with a scanning beam, converges it on the photosensitive drum 15, and forms a small light spot. 17) and the expressions (18) to (19) are satisfied. Table 24 shows the geometrical light spot diameter (μm) of the light spot at the distance (mm) from the scanning center axis (Z axis) to the scanning center axis Y on the photosensitive drum 15 in the Y direction. Further, a light spot distribution diagram and a light spot size shape diagram according to this embodiment are shown in FIGS.

  The above is a preferred embodiment of the present invention and does not limit the scope of rights of the present invention. Therefore, changes and modifications such as the effects of the scope of rights of the present invention and the contents of the description are still included in the scope of the present invention.

  10: Rotating polyhedral reflection mirror, 11: Laser light source, 111: Light flux, 113a, 113b, 113c: Scanning light beam, 131: First lens, 131a: First optical surface, 131b: Second optical surface, 132: Second lens 132a: third optical surface, 132b: fourth optical surface, 14a, 14b: photoelectric sensor, 15: photosensitive drum, 16: cylindrical lens, 2, 2a, 2b, 2c: light spot, 3: effective scanning window

Claims (4)

In a short focal length two-piece fθ lens suitable for a laser beam scanning device,
The laser beam scanning device includes:
A light source from which a laser beam is emitted;
A rotating polygon mirror for scanning the laser beam with a scanning beam;
A target to be exposed,
Including
The two-piece fθ lens is
A meniscus lens, a first lens having a first optical surface and a second optical surface, and a second lens having a third optical surface and a fourth optical surface;
Including
In the two-piece fθ lens having a short focal length suitable for the laser beam scanning device which is the first optical surface, the second optical surface, the third optical surface, and the fourth optical surface in order from the rotary polygon mirror,
In the two-piece fθ lens, in the main scanning direction of the scanning center axis, the first lens has a positive refractive index, and the second lens has a negative refractive index,
The concave surfaces of the first optical surface, the second optical surface, and the third optical surface face the rotating polygon mirror side,
The fourth optical surface has an inflection point, and the convex surface of the fourth optical surface faces the rotary polygon mirror side in the scanning center axis;
The first optical surface, the second optical surface, the third optical surface, and the fourth optical surface are all aspheric in the main scanning direction, and the conditions of the formulas (1) and (2) A two-piece fθ lens with a short focal length suitable for a laser beam scanning device, wherein:

(Where d ₁ is the distance from the reflecting surface of the rotary polygon mirror to the optical surface of the first lens on the rotary polygon mirror side in the scanning center axis, and d ₃ is the first axis in the scanning center axis. the distance from the optical surface of the target side of the first lens to the optical surface of the rotary polygonal mirror side of said second lens, d _5, in the scanning central axis, an optical surface of the target side of the second lens To the target, f _s is the compound focal length of the two-piece fθ lens, and β is the maximum effective window angle.) In a short focal length two-piece fθ lens suitable for a laser beam scanning device,
2. A two-piece fθ lens with a short focal length suitable for the laser beam scanning device according to claim 1, wherein the condition of the formula (3) is satisfied in the main scanning direction.

(Where f _{(1) Y} is the focal length of the first lens in the main scanning direction, f _{(2) Y} is the focal length of the second lens in the main scanning direction, and f _s is 2 (The compound focal length of the single-type fθ lens, and n _d1 and n _d2 are the refractive indexes of the first lens and the second lens, respectively.) In a short focal length two-piece fθ lens suitable for a laser beam scanning device,
2. A two-piece f.theta. Lens with a short focal length suitable for the laser beam scanning device according to claim 1, wherein the condition of Formula (4) is satisfied in the sub-scanning direction.

(Where f _{(1) X} is the focal length of the first lens in the sub-scanning direction, f _{(2) X} is the focal length of the second lens in the sub-scanning direction, and f _s is (The composite focal length of the two-piece fθ lens, and n _d1 and n _d2 are the refractive indexes of the first lens and the second lens, respectively.) In a short focal length two-piece fθ lens suitable for a laser beam scanning device,
The ratio value of the maximum light spot on the target and the ratio value of the minimum light spot on the target satisfy the conditions of the equations (5) and (6), respectively. A short focal length two-piece fθ lens suitable for a laser beam scanning device.

(However, S _a0 and S _b0 are the root mean square radii in the sub-scanning direction and main scanning direction, respectively, where the light spot formed by the scanning light beam on the reflecting surface of the rotating polygon mirror is S _a and S _b Each light spot formed from the scanning beam on the target is a root mean square radius in the sub-scanning direction and the main scanning direction, and η _max is the scanning on the reflecting surface of the rotating polygon mirror. A ratio between the light spot formed by the light beam and the maximum light spot scanned on the target, and η _min is the light spot formed by the scanning light beam on the reflecting surface of the rotary polygon mirror and the target light. (It is the ratio with the minimum light spot scanned in

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