JP3486508B2 - Optical scanning optical device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning optical device having an aspherical so-called fθ lens as an image forming means.

[0002]

2. Description of the Related Art A conventional optical scanning optical device of this type is a rotating polyhedral device as disclosed in Japanese Patent Publication No. 62-36210, Japanese Patent Publication No. 21565/1990, Japanese Patent Application Laid-Open No. 4-50908. Generally, an fθ lens that converts a light beam deflected and scanned by a mirror at a constant angular velocity into a light spot that moves at a constant velocity on a surface to be scanned is used as an image forming unit.

[0003]

However, in the above-mentioned conventional example, for example, in Japanese Patent Publication No. 62-36210,
The device is expensive because it has a two-lens structure and uses a rotationally asymmetric glass lens called a toric lens. Further, in Japanese Patent Publication No. 21565/1990, it is difficult to obtain a uniform spot diameter in a wide angle range because of a large curvature of field. Further, in JP-A-4-50908, since the lens has a complicated shape, there is a large difference in the emission F number on the image plane side depending on the angle of view, and the spot diameter changes within the scanning region. There is a point.

An object of the present invention is to solve the above problems and to provide an inexpensive optical scanning optical device which can be produced by molding while maintaining the optical performance of the image forming means optically.

[0005]

In order to achieve the above object, an optical scanning optical device according to the present invention guides a light beam emitted from a light source means to a deflecting means through an optical system, and the deflecting means In the optical scanning optical device which is reflected and deflected and then guided to the scanned surface through the image forming means and optically scans the scanned surface, the image forming means is composed of one plastic lens, The lens has a bi-convex aspherical shape in the deflection surface near the optical axis in the main scanning direction, and a sub-scanning direction perpendicular to the deflection surface has a meniscus shape with a concave surface in the deflection direction. The curvature radius of at least one surface is such that the radius of curvature continuously changes asymmetrically with respect to the lens center.

[0006]

In the optical scanning optical device having the above-mentioned structure, the lens shape of the image forming means is such that the curvature of the central portion of the scanning line of the generatrix is biconvex, the generatrix is both aspherical, and the line of sight is the condition that the performance is obtained even with a low refractive index. The cross-sectional shape is a meniscus shape with the concave surface facing the deflector side, and it is set so that the sagittal curvature of at least one of the two surfaces changes asymmetrically with respect to the optical axis, and it can be used even if the coefficient of linear expansion is large. To

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail based on the illustrated embodiments. FIG. 1 shows the configuration of each element in the deflection scanning plane of the scanning optical device of the embodiment. Reference numeral 1 denotes a semiconductor laser light source. A collimator lens 2, a diaphragm 3, a cylindrical lens 4, and a rotary polygon mirror 5 composed of a polygon mirror which is rotationally driven by a motor are arranged in the emitting direction of the light source 1. An fθ lens 6 having an fθ characteristic and a surface to be scanned 7 such as a photosensitive drum are provided in the deflection direction.

The light beam emitted from the semiconductor laser light source 1 passes through the collimator lens 2 so that its divergence is converted into substantially parallel light. The parallel light passes through the aperture stop 3 so that its beam diameter is adjusted and passes through the cylindrical lens 4. The cylindrical lens 4 does not have a refractive power in the scanning section, but has a refractive power only in the sub-scanning section.

Therefore, in the scanning cross section of FIG. 1, the light beam which has passed through the cylindrical lens 4 is incident on the rotary polygon mirror 5 as parallel light. The rotary polygon mirror 5 is rotated at high speed at a constant angular speed by a motor, and the incident light beam is repeatedly deflected and scanned at high speed. The deflected and scanned light beam is in the state of parallel light, and is corrected by the fθ lens 6 so that the light beam is condensed and the scanning speed becomes constant. To be done.

FIG. 2 shows a sub-scanning surface of deflection scanning including the optical axis. The cylindrical lens 4 has a refracting power in this cross section, and the light beam passes through the cylindrical lens 4 so that the deflection point P of the rotary polygon mirror 5 is increased.
Is almost imaged. The imaged light beam is reflected by the deflecting surface P and passes through the fθ lens 6 so that the surface to be scanned 7 is scanned.
Leading to. Between the deflection point P and the surface to be scanned 7 is f with respect to the sub-scanning surface.
The θ lens 6 has an optical conjugate relationship, and constitutes a so-called tilt correction optical system.

The fθ lens 6 is a rotationally asymmetric single lens made of plastic, and its refractive index n is 1.519 at a wavelength of 780 nm. Also, FIG.
In the main scanning cross section shown in (2), the vicinity of the optical axis is a biconvex double-sided aspherical shape. Further, with respect to the sub-scanning direction, it has a meniscus shape with a concave surface facing the rotary polygonal mirror 5 side as shown in FIG. 2, and the curvature on the rotary polygonal mirror 5 side in the sub-scanning direction is constant at R1, Curvature on the scanned surface 7 side
Regarding R2, the radius of curvature changes asymmetrically with respect to the optical axis of the fθ lens 6 in the longitudinal direction.

Further, the distance between the deflection point P of the rotary polygon mirror 5 and the fθ lens 6 is D1, the center thickness of the fθ lens 6 is t, the radius of curvature near the optical axis of the scanning section is R1a on the rotary polygon mirror 5 side, and The scanning surface 7 side is R2a, the radius of curvature in the optical axis of the sub-scan section is R1b on the rotating polygonal mirror 5 side, the scanned surface 7 side is R2b, and the surface shape formula type of the R1 surface is T, the surface of the R2 surface. The type of shape expression
When defined as ST2, the shape formula of each surface is shown as follows.

The origin is the intersection of the fθ lens 6 and the optical axis, the optical axis direction is the X axis, the axis orthogonal to the optical axis in the main scanning plane is the Y axis, and the optical axis in the sub scanning plane is orthogonal to the optical axis. Let the axis be the Z axis.

Aspherical shape << Type T >> (R1 surface in the case of the first embodiment) The shape of the generatrix (scan section) is relative to the optical axis X as shown in FIG.
It is symmetrical. Bus line corresponding to the main scanning direction of the fθ lens 6
Let the aspherical function of the direction be X = (Y² / R2a) / [1+ {1- (1 + Ky) ・ (Y / R2a)
²]^1/2 ＋ B4Y^Four ＋ B6Y⁶ ＋ B8Y⁸ ＋ B10 Y^Ten However, Ky, B4, B6, B8 and B10 are aspherical coefficients. Well
Also, the radius of curvature in the sagittal direction corresponding to the sub-scanning direction is R2b.
It is constant.

<< Type ST2 >> (R2 surface in the case of the second embodiment) The generatrix shape is asymmetric with respect to the optical axis X as shown in FIG. ^{_{X u = (Y 2 / R1a}} ) / [1+ {1- (1 + Ky u) · (Y /
^{R1a) 2} 1/2] + B4} u Y 4 + B6 u Y 6 + B8 u Y 8 + B10 u Y 10 X L = (Y 2 / R1a) / [1+ {1- (1 + Ky L) · (Y /
^{R1a) 2} 1/2] + B4} L Y 4 + B6 L Y 6 + B8 L Y 8 + B10 L Y 10 children vertical radius of curvature varies asymmetrically with respect to the optical axis. _{r u '= R1b (1 +} D2 u Y 2 + D4 u Y 4 + D6 u Y 6 + D8 u
_{^{Y 8 + D10 u Y 10)}} r L '= R1b (1 + D2 L Y 2 + D4 L Y 4 + D6 L Y 6 + D8 L
Y ⁸ + D10 _L Y ¹⁰ ), where X _u is the X coordinate in Y (+), and X _L is Y (−).
Is the X coordinate, r _u 'is the radius of curvature of the sagittal line in Y (+), and r _L ' is the radius of curvature of the sagittal line in Y (-). Also, Ky _u , B4 _u , B6 _u , B8 _u , B10 _u , Ky _L , B4 _L , B6
_L , B8 _L , B10 _L are aspherical coefficients, and D2 _u , D4 _u , D6
_u , D8 _u , D10 _u , D2 _L , D4 _L , D6 _L , D8 _L , and D10 _L are coefficients that change the radius of curvature.

Next, Table 1 shows the actual numerical values of the first embodiment.

[0017]

Further, FIG. 5 shows the image plane curvature in the main scanning direction and the sub-scanning direction, and FIG. 6 shows the distortion characteristic (fθ characteristic). As can be seen from FIG. 6, the aberration is well corrected.

In order to manufacture the f.theta. Lens 6 at a low cost, when considering the material as an optical plastic, it is difficult to correct the optical aberration because there is only one having a lower refractive index than the glass material of glass. Even if the refractive index of the optical plastic material used for the fθ lens is in the range of about 1.48 to 1.6,
By making the scanning cross-sectional shape of the fθ lens 6 on the optical axis biconvex, the positive refracting power is shared by both sides, so that aberration correction becomes good. Further, by making both sides aspherical, it becomes possible to satisfactorily correct the image plane curvature in the main scanning direction with a single lens in the range where the effective scanning angle is ± 35 ° or more.

With respect to the sub-scanning section (sagittal line), a meniscus lens having a positive refracting power with a concave surface facing the rotary polygonal mirror is used, so that the position of the main plane is on the scanned surface 7 side. Since it can be pushed and the conjugate magnification can be set to a relatively small value, the curvature of field in the sub-scanning direction can be reduced.

Further, in order to correct the sub-scanning image plane curvature due to the deflection point P changing asymmetrically with respect to the optical axis,
By changing the sagittal line R on at least one of the surfaces asymmetrically with respect to the optical axis, it becomes possible to obtain good performance in a wide angle of view. If the absolute value of the lateral magnification in the sub-scanning direction between the deflection point P and the surface to be scanned 7 becomes too large, the residual aberration in the sub-scanning direction becomes large, and the curvature change and the refractive index of the plastic due to the thermal expansion of the plastic are increased. The effect of focus shift due to temperature change becomes too large. On the contrary, if it is made too small, the center thickness t of the fθ lens 6 becomes too thick, or the curvature of the sagittal line becomes too tight, and the spherical aberration becomes too large. Therefore, the lateral magnification β is set to -4 <β <-2. Preferably.

In order to satisfactorily correct the fθ characteristic of the scanning linearity, it is necessary to set the distance D1 to a certain extent, but if it is set too large, the outer shape of the fθ lens 6 becomes too large. Further, if the thickness of the fθ lens 6 becomes too large, a problem occurs in the moldability of the plastic lens. This good range is preferably set to about 0.18 <D1 / f <0.33.

If the central thickness t becomes smaller than the outer diameter of the fθ lens 6, warpage occurs during the molding of the fθ lens 6, and a high precision lens cannot be obtained. Further, even if it is too thick, sink marks will occur during molding, and it will not be possible to obtain a highly accurate product. Further, if the center thickness t becomes too small with respect to the distance D1 and the focal length f, there is a problem that the asymmetry of the image plane curvature in the main scanning direction becomes large. As this favorable range, a range of 0.3 <t / D1 <0.45 and 0.06 <t / f <0.12 is preferable. In particular, if this conditional expression is applied to a scanning angle of ± 35 ° or more, its effect is great.

Regarding the radii of curvature R1b and R2b, if the absolute value of the radius of curvature R2b is too small with respect to the focal length f and the radius of curvature R1b, the difficulty in machining the mold increases and the spherical aberration becomes large. The problem arises that On the other hand, if the absolute value becomes too large, the rate of change of the sagittal line R becomes too large, the difficulty of die machining increases, and the change of the sub-scanning magnification due to the change of the image height becomes large. It becomes difficult to obtain a uniform spot on the surface to be scanned. Therefore, it is preferable to set the range of 0.06 <−R2b / f <0.125 1.2 <R1b / R2b <7.0. As a result, a high-performance lens can be obtained by comprehensively considering basic optical setting values and manufacturing errors.

Further, in order to correct the asymmetry of the image plane curvature in the main scanning direction, it is conceivable to set the scanning cross sectional shape (generic shape) asymmetrically, but in this embodiment, the lens is arranged in the longitudinal direction of the lens. By a little shifting, the asymmetric component is reduced. The point described as symmetric in the item of aspherical surface in Table 1 means that the generatrix shape is symmetric with respect to the optical axis.

Regarding the curvature radii R1a and R2a of the generatrix on the optical axis, if the ratios of the refracting powers of the surfaces deviate, it becomes difficult to correct the off-axis fθ characteristics even if an aspherical surface is used. . If an attempt is made to correct the aspherical amount, the F number at the image height in the main scanning direction will change.
It is preferable to set the range of 0.2 <−R1a / R2a <3 as a condition for obtaining a uniform spot diameter and a good balance.

Table 2 shows numerical values relating to lens shapes and lens arrangements of the second to fourth examples, Table 3 of the fifth to sixth examples, and Table 4 of the eighth example.

[0028]   Table 2           Example 2 3 4             D1 30 35.844 39             t 10 15 15             n 1.519 1.519 1.519             R1a 95.192 182.046 195.732             R2a -411.888 -132.139 -125.891             R1b -12.164 -51.885 -83.468             R2b -9.149 -15.378 -17.061             R1 Aspherical type ST2 ST2 ST2             R2 aspherical type T T T   Type ST2 kyu 0.000 -53.239 -58.486   Aspheric coefficient B4u -5.242 ・ 10^-Five   -6.242 / 10^-7     -5.908 / 10^-7              B6u 2.283 ・ 10^-9    6.034 / 10^-11     5.586 / 10^-11             B8u -3.670 / 10^-13   2.559 / 10^-Five      3.503 / 10^-Five              B10u 2.002 / 10^-18  -1.341 / 10^-18    -2.300 ・ 10^-18             kyl symmetry symmetry symmetry             B4l symmetry symmetry symmetry             B6l symmetry symmetry symmetry             B8l symmetry symmetry symmetry             B10l Symmetric Symmetric Symmetric             D2u 1.137 ・ 10^-Four     -4.035 / 10^-Four     -5.564 / 10^-Four              D4u 7.364 ・ 10^-7      6.079 / 10^-7      5.871 / 10^-7              D6u -4.509 / 10^-Ten    -2.343 / 10^-Ten    -1.928 / 10^-Ten             D8u 2.382 ・ 10^-14     0.000 / 10⁰       0.000 / 10⁰             D2l 1.194 ・ 10^-Four     -3.629 / 10^-Four     -5.136 / 10^-Four              D4l 6.168 / 10^-7      6.083 / 10^-7      5.929 / 10^-7              D6l -4.397 / 10^-Ten    -2.398 / 10^-Ten    -2.035 / 10^-Ten             D8l 8.665 / 10^-14     0.000 / 10⁰                       Type T ky 0.000 3.123 3.192   Aspheric coefficient B4 -2.700 ・ 10^-6     -5.191 / 10^-7     -4.987 / 10^-7              B6 -1.554 ・ 10^-Ten    -2.030 / 10^-11     1.566 / 10^-14             B8 3.065 / 10^-13    -1.517 / 10^-14    -1.637 / 10^-14             B10 1.770 / 10^-17     0.000 / 10⁰       0.000 / 10⁰             D1 / f 0.200 0.239 0.260             t / D1 0.333 0.418 0.385             (-R1a / R2a) 0.231 1.378 1.555             R1b / R2b 1.330 3.374 4.892             t / f 0.067 0.100 0.100             (-R2b / f) 0.061 0.103 0.114   in the main scanning plane of the fθ lens             Focal length f 149.99 149.97 150.01   β Sub-scanning lateral magnification -3.66 -3.29 -3.15

[0029]   Table 3           Example 5 6 7             D1 45 40 36.38             t 15 15 15             n 1.519 1.519 1.519             R1a 299.537 220 185.599             R2a -103.372 -117.68 -130.391             R1b -40.042 -31.179 -58.736             R2b -16.047 -14.493 -15.787             R1 Aspherical type ST2 ST2 ST2             R2 aspherical type T T T   Type ST2 kyu 0.000 0.000 -46.892   Aspheric coefficient B4u -5.120 ・ 10^-7   -1.190 / 10^-6   -6.897 / 10^-7              B6u 9.112 / 10^-11     3.185 / 10^-Ten     5.973 / 10^-11             B8u -7.060 / 10^-15    -2.737 / 10^-14     4.617 / 10^-15             B10u 3.228 ・ 10^-20     3.243 / 10^-19    -2.046 / 10^-18             kyl symmetry symmetry -5.952E + 01             B4l Symmetric Symmetric -5.615 ・ 10^-7              B6l Symmetric Symmetric 1.210 ・ 10^-11             B8l Symmetric Symmetric 4.081 ・ 10^-Five              B10l Symmetry Symmetry 3.843 ・ 10^-19             D2u 1.633 ・ 10^-Four      8.820 / 10^-Five     -4.487 / 10^-Four              D4u 3.476 ・ 10^-9      2.311 / 10^-7      6.070 / 10^-7              D6u 0.000 ・ 10⁰      -1.636 / 10^-Ten    -2.222 / 10^-Ten             D8u 0.000 ・ 10⁰       4.198 / 10^-14     0.000 / 10⁰             D2l 1.633 / 10^-Four      1.696 / 10^-Four     -3.286 / 10^-Four              D4l 3.476 ・ 10^-9      1.042 / 10^-14     4.430 / 10^-7              D6l 0.000 ・ 10⁰      -7.248 / 10^-11    -1.515 ・ 10^-Ten             D8l 0.000 ・ 10⁰       1.790 / 10^-14     0.000 / 10⁰   Type T ky 0.000 0.000 3.339   Aspheric coefficient B4 -1.814 ・ 10^-7     -5.235 / 10^-7     -5.022 / 10^-7              B6 -4.171 / 10^-11    -8.617 / 10^-11    -1.054 ・ 10^-11             B8 3.073 / 10^-15     1.843 / 10^-14    -2.573 / 10^-14             B10 1.349 / 10^-18     8.481 / 10^-18     0.000 / 10⁰             D1 / f 0.300 0.267 0.243             t / D1 0.333 0.375 0.412             (-R1a / R2a) 2.898 1.869 1.423             R1b / R2b 2.495 2.151 3.721             t / f 0.100 0.100 0.100             (-R2b / f) 0.107 0.097 0.105   in the main scanning plane of the fθ lens             Focal length f 149.98 150.00 150.00   β Sub-scanning lateral magnification -2.65 -2.81 -3.29

Table 4 Example 8 D1 40 t 15 n 1.519 R1a 220 R2a -117.68 R1b -113.12 R2b -17.832 R1 aspherical type ST2 R2 aspherical type ST2 R1 side kyu 0.000 type ST2 B4u -1.190 ・ 10 ^-6 aspherical type Coefficient B6u 3.185 ・ 10 ^-10 B8u -2.937 ・ 10 ^-14 B10u 3.243 ・ 10 ^-19 kyl Symmetric B4l Symmetric B6l Symmetric B8l Symmetric B10l Symmetry D2u -4.830 ・ 10 ^-4 D4u 1.821 ・ 10 ^-7 D6u -1.023 ・ 10 ^-10 D8u 7.237 / 10 ^-14 D2l -7.016 / 10 ^-4 D4l 3.641 / 10 ^-7 D6l -1.035 / 10 ^-11 D8l -7.659 / 10 ^-14 D10l 2.035 / 10 ^-17 R2 side kyu 0.000 Type ST2 B4u -5.235 / 10 ^-7 Aspherical coefficient B6u -8.617 ・ 10 ^-11 B8u 1.843 ・ 10 ^-14 B10u 8.481 ・ 10 ^-18 kyl Symmetric B4l Symmetric B6l Symmetric B8l Symmetric B10l Symmetric D2u 4.596 ・ 10 ^-5 D4u -7.121 ・ 10 ^-8 D6u 1.739 ・10 ^-11 D8u -4.303 ・ 10 ^-5 D2l 1.199 ・ 10 ^-5 D4l -5.997 ・ 10 ^-8 D6l -1.770 ・ 10 ^-12 D8l 2.185 ・ 10 ^-14 D10l -9.255 ・ 10 ^-18 D1 / f 0.267 t / D1 0.375 (-R1a / R2a) 1.869 R1b / R2b 6.344 t / f 0.100 (-R2b / f) 0.119 fθ Lens focal length f 15 in the main scanning plane 0.00 β Sub-scanning lateral magnification -3.12

The seventh embodiment is an example in which the generatrix shape is asymmetric with respect to the optical axis. The eighth embodiment is an example in which the radii of curvature of the sagittal cross sections on both sides change, and in particular, by setting the radii of curvature on both sides to decrease with increasing distance from the optical axis, the sub-region of the lens peripheral portion The main plane position in the scanning direction can be set on the scanned surface side, the emission F number in the sub-scanning direction can be made constant, and further uniformity of the spot diameter can be obtained.

[0032]

As described above, the optical scanning optical device according to the present invention is affected by environmental fluctuations in design, manufacturing and environment even if the image forming means having a low refractive index by one plastic lens is used. In addition, high performance and low cost can be achieved.

[Brief description of drawings]

FIG. 1 is an optical layout diagram of a main scanning section of a first embodiment.

FIG. 2 is an optical layout diagram of a sub-scan section.

FIG. 3 is an explanatory diagram of a busbar shape in the case of type T.

FIG. 4 is an explanatory diagram of a busbar shape in the case of type ST2.

FIG. 5 is a characteristic diagram of the amount of curvature of field.

FIG. 6 is a characteristic diagram of an fθ characteristic (distortion aberration).

[Explanation of symbols]

1 light source 2 Collimator lens 3 aperture stop 4 Cylindrical lens 5 rotating polygon mirror 6 fθ lens 7 Scanned surface

Claims (5)

(57) [Claims] 1. A light beam emitted from a light source means is guided to a deflecting means through an optical system, reflected and deflected by the deflecting means, and then guided onto a surface to be scanned through an image forming means, In an optical scanning optical device that optically scans a surface to be scanned, the image forming means is composed of one plastic lens, and the lens has a biconvex shape in the vicinity of the optical axis in the main scanning direction in the deflection surface. The lens has a spherical shape, and the sub-scanning direction perpendicular to the deflecting surface has a meniscus shape with a concave surface in the deflecting direction, and the curvature in the sub-scanning direction is such that at least one surface is asymmetric with respect to the lens center and has a radius of curvature continuously. An optical scanning optical device characterized by changing. 2. With respect to the main scanning direction, a light beam which is incident on the image forming means from the deflecting means as substantially parallel light is imaged on the surface to be scanned, and with respect to the sub scanning direction.
The optical scanning optical device according to claim 1, wherein the deflection point of the deflecting unit and the surface to be scanned are optically in a substantially conjugate relationship, and the optical lateral magnification β is -4 <β <-2. 3. When the center thickness of the lens is t and the distance between the deflection point of the deflection means and the deflection side of the image formation means is D1, 0.3 <t / D1 <0.45 is satisfied. The optical scanning optical device according to claim 1, configured as described above. 4. The curvature of the lens in the sub-scanning direction is such that the radius of curvature continuously changes with respect to the center of the lens on both sides, and the absolute value of the radius of curvature on both sides decreases with increasing distance from the optical axis of the lens. Item 2. The optical scanning optical device according to Item 1. 5. The surface to be scanned is a photosensitive drum.
The optical scanning optical device according to any one of claims 1 to 4.