JPH1090620A - Optical scanner - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize a high performance and inexpensive fθ lens by using a plastic lens having a low refractive index consisting of a single lens. SOLUTION: A light beam outgoing from a laser light source 1 is guided onto a rotary polygon mirror 5 through an optical system, and is guided onto a surface 7 to be scanned through the fθ lens 6 after it is reflected and deflected by the rotary polygon mirror 5 to be optical scanned on the surface 7 to be scanned. The fθ lens 6 is constituted of a sheet of plastic fθ lens 6. The fθlens 6 is constituted so that the optical axial vicinity of the main scan direction in a deflection surface is made a both side aspherical shape being a both convex shape, and the sub-scan direction in the direction vertical to the deflection surface is made a meniscus shape turning a concave surface in the deflective direction, and at least one surface in the curvature of the sub-scan direction is made so that the radius of curvature is changed asymmetrically continuously for the center of the fθ lens 6.

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 f.theta. Lens as an image forming means.

[0002]

2. Description of the Related Art A conventional optical scanning optical device of this type is disclosed in Japanese Patent Publication No. Sho 62-36210, Japanese Patent Publication No. Hei 21-21565, and Japanese Patent Laid-Open Publication No. Hei 4-50908. In general, an fθ lens that converts a light beam deflected and scanned at a constant angular speed by a mirror into a light spot that moves at a constant speed on a surface to be scanned is used as an image forming means.

[0003]

However, in the above conventional example, for example, in Japanese Patent Publication No. 62-36210,
The apparatus is expensive because it uses a two-lens structure and a rotationally asymmetric glass lens called a toric lens. Further, in Japanese Patent Publication No. 22565/1990, it is difficult to obtain a uniform spot diameter in a wide angle range because of a large curvature of field. Further, in Japanese Patent Application Laid-Open No. H4-50908, since the lens has a complicated shape, the difference in the emission F number on the image plane side due to the angle of view is large, and the spot diameter changes in the scanning area. There is a point.

An object of the present invention is to solve the above-mentioned problems and to provide an inexpensive optical scanning optical device which can be manufactured by molding while maintaining the optical performance of the imaging means optically high.

[0005]

An optical scanning optical apparatus according to the present invention for achieving the above object is to guide a light beam emitted from a light source means to a deflecting means via an optical system, and the deflecting means uses the light beam. In the optical scanning optical device which guides the light on the surface to be scanned through the image forming means after being reflected and deflected, and optically scans the surface to be scanned, the image forming means comprises one plastic lens. The lens has a bilaterally convex aspheric surface near the optical axis in the main scanning direction in the deflecting surface, a sub-scanning direction perpendicular to the deflecting surface has a meniscus shape with a concave surface facing the deflecting direction, and a sub-scanning direction. Is characterized in that at least one surface has a continuously changing radius of curvature asymmetrically with respect to the lens center.

[0006]

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

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail with reference to 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, and a collimator lens 2, a diaphragm 3, a cylindrical lens 4, and a rotary polygon mirror 5 composed of a polygon mirror driven and rotated by a motor are arranged in the emission direction of the light source 1. An fθ lens 6 having fθ characteristics and a scanned surface 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, the beam diameter thereof is adjusted, and passes through the cylindrical lens 4. The cylindrical lens 4 has no 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 that has passed through the cylindrical lens 4 is incident on the rotary polygon mirror 5 while remaining parallel light. The rotating polygon mirror 5 is rotated at a high angular speed by a motor at a high speed, and the incident light beam is repeatedly deflected and scanned at a high speed. The light beam that has been deflected and scanned is in the form of parallel light, and the light beam is condensed by the fθ lens 6 and corrected so that the scanning speed is constant. Is done.

FIG. 2 shows a sub-scanning surface for 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 changed.
It is almost imaged on. The imaged light beam is reflected by the deflection surface P and passes through the fθ
Leads to. The distance between the deflection point P and the scanned surface 7 is f
The θ lens 6 has an optically 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 has a refractive index n of 1.519 at a working wavelength of 780 nm. FIG.
In the main scanning section shown in FIG. 7, the vicinity of the optical axis is a biconvex double-sided aspheric surface. Further, with respect to the sub-scanning direction, it has a meniscus shape with the concave surface facing the rotating polygon mirror 5 side as shown in FIG. 2, and the curvature on the rotating polygon mirror 5 side in the sub-scanning direction is constant at R1. Curvature on the scanned surface 7 side
With respect to R2, the radius of curvature changes asymmetrically in the longitudinal direction with respect to the optical axis of the fθ lens 6.

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-scanning section is the rotating polygon mirror 5 side is R1b, the scanned surface 7 side is R2b, the type of the surface shape of the R1 surface is T, and the surface of the R2 surface is T. Shape expression type
When defined as ST2, the shape formula of each surface is shown in the following form.

The intersection of the fθ lens 6 and the optical axis is the origin, 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 is orthogonal in the sub-scanning plane. Let the axis be the Z axis.

Aspherical shape {in case of type T} (R1 surface in the case of the first embodiment)
It is symmetric. Generic line corresponding to the main scanning direction of the fθ lens 6
The aspheric function of the direction is given by the following equation. X = (Y^Two / R2a) / [1+ ｛1- (1 + Ky) · (Y / R2a)
^Two]^1/2 + B4Y^Four + B6Y⁶ + B8Y⁸ + B10 Y^Ten  Here, Ky, B4, B6, B8, and B10 are aspherical coefficients. Ma
The radius of curvature in the sagittal direction corresponding to the sub-scanning direction is R2b.
It is constant.

{Case of 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 of Y (+) and _XL is Y (−)
, R _u ′ is the radius of curvature of the sagittal line at Y (+), and r _L ′ is the radius of curvature of the sagittal line at Y (−). Also, Ky _u , B4 _u , B6 _u , B8 _u , B10 _u , Ky _L , B4 _L , B6
_L , B8 _L , B10 _L are aspheric coefficients, and D2 _u , D4 _u , D6
_{u, D8 u, D10 u,} D2 L, D4 L, D6 L, D8 L, D10 L is a coefficient that changes in radius of curvature.

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

[0017]

FIG. 5 shows the curvature of field 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 fθ lens 6 at low cost, if optical plastic is used as a material, it is difficult to correct optical aberrations because only those having a lower refractive index than glass materials are available. 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 on the optical axis of the fθ lens 6 a biconvex surface, the positive refractive power is shared on both sides, so that aberration correction is improved. Further, by making both sides aspherical, it is possible to satisfactorily correct the curvature of field in the main scanning direction within a range where the effective scanning angle is ± 35 ° or more with a single lens.

Further, with respect to the sub-scanning section (satellite line), the main plane position is shifted to the scanning surface 7 side by forming a meniscus-shaped lens having a positive refractive power with the concave surface facing the rotating polygon mirror. Since it can be pushed and the conjugate magnification can be set relatively small, the curvature of field in the sub-scanning direction can be reduced.

Further, in order to correct the curvature of field in the sub-scanning due to the asymmetrical change of the deflection point P with respect to the optical axis,
By changing the sagittal R of at least one of the surfaces asymmetrically with respect to the optical axis, it is possible to obtain good performance at a wide angle of view. If the absolute value of the lateral magnification between the deflection point P and the surface 7 to be scanned in the sub-scanning direction is too large, the residual aberration in the sub-scanning direction increases, the curvature changes due to the thermal expansion of plastic, and the refractive index increases. The influence of a focus shift or the like due to a temperature change becomes too large. On the other hand, if it is too small, the center thickness t of the fθ lens 6 becomes too thick, the curvature of the sagittal line becomes too tight and the spherical aberration becomes too large, so the lateral magnification β is set to −4 <β <−2. Is preferred.

In order to satisfactorily correct the fθ characteristics of the scanning linearity, it is necessary to set the distance D1 to a certain value. However, if the distance D1 is too large, the outer shape of the fθ lens 6 becomes too large. If the thickness of the fθ lens 6 is too large, there is a problem in the moldability of the plastic lens. This preferable range is preferably set to about 0.18 <D1 / f <0.33.

If the center thickness t is smaller than the outer diameter of the fθ lens 6, the fθ lens 6 will be warped during molding and a highly accurate one cannot be obtained. In addition, if the thickness is too large, sink occurs during molding, and a high-precision product cannot be obtained. Further, if the center thickness t is too small with respect to the distance D1 and the focal length f, there is a problem that the asymmetry of the image surface curvature in the main scanning direction increases. The preferable ranges are preferably 0.3 <t / D1 <0.45 and 0.06 <t / f <0.12. In particular, when this conditional expression is applied to a scanning angle of ± 35 ° or more, the effect is large.

Regarding the radii of curvature R1b and R2b, if the absolute value of the radius of curvature R2b becomes too small with respect to the focal length f and the radius of curvature R1b, the processing difficulty of the mold increases, and the spherical aberration increases. A problem arises. On the other hand, if the absolute value is too large, the rate of change of the sagittal line R becomes too large, the difficulty of mold processing increases, and the change in the sub-scanning magnification due to the change in image height increases. 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. This makes it possible to obtain a high-performance lens considering the basic optical setting values and manufacturing errors comprehensively.

In order to correct the asymmetry of the curvature of field in the main scanning direction, it is conceivable to set the scanning cross-sectional shape (generating line shape) to be asymmetric. Is slightly shifted to reduce the asymmetric component. The portion described as symmetric in the aspherical term in Table 1 means that the generatrix shape is symmetric with respect to the optical axis.

Regarding the radii of curvature R1a and R2a of the generatrix on the optical axis, if the ratio of the refractive powers of the surfaces is shifted, it becomes difficult to correct the off-axis fθ characteristics even if an aspherical surface is used. . If the correction is forcibly performed using the aspherical amount, the F number at the image height in the main scanning direction changes.
It is preferable to set a range of 0.2 <-R1a / R2a <3 as a condition for easily obtaining a uniform spot diameter and a good balance.

Table 2 shows the second to fourth embodiments, Table 3 shows the fifth to sixth embodiments, and Table 4 shows the numerical values regarding the lens shape and lens arrangement of the eighth embodiment.

Table 2 Example 2 34 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 surface Type ST2 ST2 ST2 R2 Aspherical surface type TTT Type ST2 kyu 0.000 -53.239 -58.486 Aspherical 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 Symmetry Symmetry Symmetry 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 Focal length f 149.99 149.97 150.01 β Lateral magnification in the sub scanning plane -3.66 -3.29 -3.15

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 aspheric Type ST2 ST2 ST2 R2 Aspherical surface type TTT Type ST2 kyu 0.000 0.000 -46.892 Aspherical 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 symmetry symmetry -5.615 ・ 10^-7 B6l Symmetry Symmetry 1.210 ・ 10^-11 B8l Symmetry Symmetry 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 Focal length f 149.98 150.00 150.00 β Lateral magnification in the sub scanning plane -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 aspheric surface type ST2 R2 aspheric surface type ST2 R1 side kyu 0.000 type ST2 B4u -1.190 ・ 10 ^-6 aspheric surface Coefficient B6u 3.185 ・ 10 ^-10 B8u -2.937 ・ 10 ^-14 B10u 3.243 ・ 10 ^-19 kyl Symmetry B4l Symmetry B6l Symmetry B8l Symmetry 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 Aspheric coefficient B6u -8.617 ・ 10 ^-11 B8u 1.843 ・ 10 ^-14 B10u 8.481 ・ 10 ^-18 kyl Symmetry B4l Symmetry B6l Symmetry B8l Symmetry B10l Symmetry 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 Focal length f 15 in the main scanning plane of the fθ lens 0.00 β Lateral magnification within the sub-scanning plane -3.12

The seventh embodiment is an example in which the generatrix is asymmetric with respect to the optical axis. The eighth embodiment is an example in which the radius of curvature of the sagittal section of both surfaces changes. In particular, by setting both surfaces such that the radius of curvature decreases as the distance from the optical axis increases, the radius of curvature of the lens peripheral portion decreases. The position of the main plane in the scanning direction can be set on the surface to be scanned, the emission F number in the sub-scanning direction can be constant, and further uniformity of the spot diameter can be obtained.

[0032]

As described above, the optical scanning optical apparatus according to the present invention is affected by design, manufacturing, and environmental fluctuations even if an imaging means having a low refractive index is formed by one plastic lens. High performance and low cost.

[Brief description of the drawings]

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

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

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

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

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

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

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Light source 2 Collimator lens 3 Aperture stop 4 Cylindrical lens 5 Rotating polygon mirror 6 fθ lens 7 Scanning surface

Claims (4)

[Claims] 1. A light beam emitted from a light source means is guided to a deflecting means via an optical system, reflected and deflected by the deflecting means, and then guided on a surface to be scanned via an image forming means. In an optical scanning optical apparatus that optically scans a surface to be scanned, the imaging means is formed of a single plastic lens, and the lens has a biconvex shape near the optical axis in the main scanning direction on the deflection surface. A spherical shape, a sub-scanning direction perpendicular to the deflecting surface is a meniscus shape with a concave surface directed in the deflecting direction, and the curvature in the sub-scanning direction has a curvature radius of at least one surface continuously and asymmetrically with respect to the lens center. An optical scanning optical device characterized by changing. 2. In the main scanning direction, a light beam incident from the deflecting means on the imaging means as substantially parallel light is imaged on the surface to be scanned.
2. The optical scanning optical apparatus according to claim 1, wherein a deflection point of the deflection unit and the surface to be scanned have an optically approximately conjugate relationship, and an 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 imaging means is D1, 0.3 <t / D1 <0.45 is satisfied. The optical scanning optical device according to claim 1, wherein: 4. The curvature of the lens in the sub-scanning direction is such that the curvature radius changes continuously with respect to the lens center on both surfaces, and the absolute value of the curvature radius decreases on both surfaces as the distance from the optical axis of the lens increases. Item 2. The optical scanning optical device according to item 1.