JP2856475B2 - Fθ lens for optical scanning device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an fθ lens of an optical scanning device.

2. Description of the Related Art An fθ lens is used for an optical scanning device using an optical deflecting device, such as a rotary polygon mirror or a pyramidal mirror, that deflects a light beam at a constant angular velocity, so as to perform optical scanning of a surface to be scanned at a constant speed. Known as a lens, various types have conventionally been proposed.

[Problems to be Solved by the Invention] Rotating polygon mirrors and pyramidal mirrors have a problem of so-called “surface tilt”. The problem of surface tilt can be solved by using a lens for correcting surface tilt as an auxiliary, but in the near future, various types of fθ lenses that have fθ lenses themselves with surface tilt correction functions by using fθ lenses as anamorphic lens systems have been proposed. Have been.

However, since an anamorphic fθ lens includes a lens surface that is not spherical, such as a toric surface, its manufacture is not always easy, and the cost tends to be high.

The present invention has been made in view of the above-described circumstances, and can be configured by cutting spherical lenses that are easy to manufacture.
Accordingly, it is an object of the present invention to provide a novel fθ lens that can be realized at low cost.

[Means for Solving the Problems] Hereinafter, the present invention will be described.

The fθ lens of the present invention is configured such that “a substantially parallel light beam from the light source device is made incident on the deflecting and reflecting surface of the light deflecting device via a cylinder lens for correcting the light spot diameter in the sub-scanning direction,
An optical scanning device that deflects the light beam reflected by the deflecting and reflecting surface by the above-described optical deflecting device and forms an image as a light spot on the surface to be scanned by the imaging lens and the surface tilt correction lens to perform optical scanning. " And has fθ characteristics in the main scanning direction.

As shown in FIG. 1, a first group 5 and a second group 6 are arranged from the light deflecting device side (the left side in the figure) to the surface to be scanned (the right side in the figure).
Are arranged in this order.

The first group 5 is a spherical single lens having a negative refractive power. The second group 6 includes two identical positive lenses. Second group 6
In the two positive lenses, the lens surface on the side to be scanned has a strong positive refractive power. Therefore, the overall configuration is a two-group, three-element configuration. The fact that the two positive lenses that constitute the second group 6 are the same means that “the shape and the material are the same”.

The composite focal length of the entire system is f, and the composite focal length of the second group is f
_g2, the radius of curvature of the i-th lens surface as counted from the light deflecting device side R _i, the surface distance D _i, a distance leading to the sixth lens surface from the origin of the deflection by the deflection reflecting surface and D _a Then, f, f _g2 , R ₃ , R ₄ , D ₂ , and _Da are as follows: (I) −0.4 <f / R ₃ <0.4 (II) 0.06 <D ₂ / f _g2 <1.2 (III) − 0.52 <D _a / R ₄ <−0.35 is satisfied.

Incidentally, the outside of the fθ lens above condition (I) ~ (III), the thickness D _i of each lens (i = 1,3,5) are the following conditions _{(IV) D i (i =} 1,3 , 5) It is desirable to satisfy <0.05f.

[Operation] FIG. 2 shows an optical scanning device 1 using the fθ lens of the present invention.
An example is shown. This figure shows a state in which the optical arrangement is viewed from a direction perpendicular to a plane formed by the deflection of the deflected light beam.

The light source device 1 includes, for example, a semiconductor laser and a collimating lens, and emits a substantially parallel light beam. This light beam is incident on the deflection reflection surface of the light deflector 3 (which is a rotating polygon mirror in the illustrated example) via the cylinder lens 2 and is reflected. The reflected light beam is deflected at a constant angular velocity by the light deflecting device 3, and is imaged as a light spot on the scanned surface 8 by the fθ lens composed of the first group 5 and the second group 6 and the surface tilt correcting lens 7. Then, the scanned surface 8 is optically scanned. In the figure, the scanned surface 8
Is the main scanning direction, and the direction perpendicular to the drawing is the sub-scanning direction.

The surface tilt correction lens 7 is a long cylinder lens in this example, and has a positive refractive power only in the sub-scanning direction.

The first group 5 of the fθ lenses is a spherical single lens having a negative refractive power.

The second group 6 of fθ lenses is composed of a combination of two positive lenses having the same material and the same shape. As described above, these two positive lenses have a strong positive refractive power on the lens surface on the scanning surface side.

The cylinder lens 2 is for correcting the spot diameter of the light spot formed on the surface to be scanned, and has a weak positive refractive power.

FIG. 3 shows an optical arrangement extending from the light source device 1 to the surface 8 to be scanned along the optical path, with the sub-scanning direction being the vertical direction.

As shown in the drawing, the surface tilt correcting lens 7 has the strongest influence on the image formation in the sub-scanning direction, and the deflecting / reflecting surface 4 of the optical deflecting device is tilted and reflected as indicated by reference numeral 4 '. Even if the light beam is swung in the sub-scanning direction as shown by the broken line, the image forming position of the light spot hardly moves in the sub-scanning direction, and the effect of surface tilt is corrected well.

In all of the conditions (I) to (III), the curvature of field in the main scanning direction and f
This is a condition for correcting the θ characteristic.

For the condition (I), the field curvature in the main scanning direction · fθ
Both of the characteristics are under when the upper limit is exceeded, and over when the lower limit is exceeded.

Regarding the condition (II), the curvature of field in the main scanning direction becomes over when the upper limit is exceeded, and becomes under when the lower limit is exceeded.
The fθ characteristic is under when the upper limit is exceeded, and over when the lower limit is exceeded.

With respect to the condition (III), the curvature of field in the main scanning direction becomes under when exceeding the upper limit, becomes over when exceeding the lower limit, and the fθ characteristic becomes over when exceeding the upper limit and under when exceeding the lower limit.

Condition (IV) is a condition for facilitating lens molding when all the lenses of the fθ lens of the present invention are formed of plastic lenses.

EXAMPLES Hereinafter, 15 specific examples will be given.

As described with reference to FIG. 1, the combined focal length of the entire system is f (normalized to 100), the radius of curvature of the i-th lens surface counted from the optical deflecting device side is R _i , and the surface spacing Is D _i , and the distance from the starting point of deflection by the deflecting reflection surface to the first lens surface is D ₀ . Further, the refractive index of the j-th lens is n _j as counted from the light deflection device. Further, each deflection (unit: degree) is represented by 2θ, and K ₁ = f / R ₃ , K ₂ = D ₂ / f _g2 , K ₃ = D _a / R ₄
And

Example 1 f = 100, 2θ = 60.0, K ₁ = −0.178, K ₂ = 0.132, K ₃ = −0.
_{471, D 0 = 5.704 i R} i D i j n j 1 - 30.108 2.182 1 1.48601 2 ∞ 5.883 3 -560.289 2.424 2 1.48601 4 - 39.772 0.097 5 -560.289 2.424 3 1.48601 6 - 39.772 Example 2 f = 100,2θ _{= 60.0, K 1 = -0.160,} K 2 = 0.174, K 3 = -0.
_{408, D 0 = 5.484 i R} i D i j n j 1 - 83.387 2.182 1 1.48601 2 ∞ 12.024 3 -623.320 2.424 2 1.48601 4 - 60.389 0.097 5 -623.320 2.424 3 1.48601 6 - 60.389 Example 3 f = 100,2θ _{= 64.0, K 1 = -0.147,} K 2 = 0.124, K 3 = -0.
_{485, D 0 = 6.190 i R} i D i j n j 1 - 33.640 2.327 1 1.48601 2 ∞ 5.850 3 -679.668 3.103 2 1.48601 4 - 42.617 0.103 5 -679.668 3.103 3 1.48601 6 - 42.617 Example 4 f = 100,2θ _{= 64.0, K 1 = -0.117,} K 2 = 0.156, K 3 = -0.
428, D ₀ = 6.348 i R _i D _i j n _j 1 − 70.877 2.327 1 1.48601 2 ∞ 10.201 3 −852.304 3.103 2 1.48601 4 − 58.821 0.103 5 −852.304 3.103 3 1.48601 6 − 58.821 Example 5 f = 100,2θ _{= 68.0, K 1 = -0.134,} K 2 = 0.121, K 3 = -0.
_{480, D 0 = 6.191 i R} i D i j n j 1 - 36.167 2.473 1 1.48601 2 ∞ 5.914 3 -747.225 3.297 2 1.48601 4 - 44.364 0.110 5 -747.225 3.297 3 1.48601 6 - 44.364 Example 6 f = 100,2θ _{= 68.0, K 1 = -0.110,} K 2 = 0.148, K 3 = -0.
430, D ₀ = 6.369 i R _i D _i j n _j 1 − 68.679 2.473 1 1.48601 2 ∞ 9.553 3 −910.817 3.297 2 1.48601 4 − 58.377 0.110 5 −910.817 3.297 3 1.48601 6 − 58.377 Example 7 f = 100, 2θ _{= 68.0, K 1 = 0.0,} K 2 = 0.093, K 3 = -0.485,
D ₀ = 6.044 i R _i D _i j n _j 1 −31.173 2.472 1 1.48601 2 217.026 3.863 3 ∞ 3.297 2 1.48601 4 −39.723 0.275 5 ∞ 3.297 3 1.48601 6 −39.723 Example 8 f = 100,2θ = 68.0, K _{1 = 0.0, K 2 = 0.098} , K 3 = -0.481,
D ₀ = 5.769 i R _i D _i j n _j 1 −31.156 2.198 1 1.48601 2 202.794 4.029 3 ∞ 3.297 2 1.48601 4 −39.224 0.275 5 ∞ 3.297 3 1.48601 6 −39.224 Example 9 f = 100,2θ = 68.0, K _{1 = 0.0, K 2 = 0.100} , K 3 = -0.462,
D ₀ = 5.769 i R _i D _i j n _j 1 −34.310 2.198 1 1.48601 2 208.775 4.340 3 ∞ 3.296 2 1.48601 4 −41.514 0.275 5 ∞ 3.296 3 1.48601 6 −41.514 Example 10 f = 100,2θ = 68.0, K _{1 = 0.091, K 2 = 0.093} , K 3 = -0.46
7, D ₀ = 5.769 i R _i D _i j n _j 1 −31.953 2.198 1 1.48601 2 138.061 3.748 3 1099.029 3.297 2 1.48601 4 −39.779 0.275 5 1099.029 3.297 3 1.48601 6 −39.779 Example 11 f = 100,2θ = 68.0 , K ₁ = 0.364, K ₂ = 0.074, K ₃ = -0.45
5, D ₀ = 5.769 i R _i D _i j n _j 1 −30.385 2.198 1 1.48601 2 74.954 2.625 3 274.728 3.297 2 1.48601 4 −38.413 0.275 5 274.728 3.297 3 1.48601 6 −38.413 Example 12 f = 100,2θ = 72.0 _{, K 1 = -0.127, K 2} = 0.114, K 3 = -0.
480, D ₀ = 6.417 i R _i D _i j n _j 1 − 38.108 2.618 1 1.48601 2 ∞ 5.721 3 −788.071 3.491 2 1.48601 4 − 45.511 0.116 5 −788.071 3.491 3 1.48601 6 − 45.511 Example 13 f = 100, 2θ _{= 60.0, K 1 = -0.060,} K 2 = 0.179, K 3 = -0.
371, D ₀ = 5.258 i R _i D _i j n _j 1 ∞ 2.182 1 1.48601 2 115.805 13.484 3 −1680.543 2.424 2 1.48601 4 −69.680 0.097 5 −1680.543 2.424 3 1.48601 6 −69.680 Example 14 f = 100,2θ = _{60.0, K 1 = -0.174, K} 2 = 0.198, K 3 = -0.
426, D ₀ = 5.737 i R _i D _i j n _j 1 ∞ 2.182 1 1.48601 2 205.066 16.738 3 −574.838 2.909 2 1.48601 4 −71.730 0.097 5 −574.838 2.909 3 1.48601 6 −71.730 Example 15 f = 100,2θ = _{64.0, K 1 = -0.129, K} 2 = 0.191, K 3 = -0.
415, D ₀ = 5.691 i R _i D _i j n _j 1 ∞ 2.327 1 1.48601 2 181.697 15.832 3 −772.918 3.103 2 1.48601 4 −72.673 0.103 5 −772.918 3.103 3 1.48601 6 −72.673 f F = 206.2 in 1,2,13,14
6, f = 193.37 in Examples 3, 4, and 15, f = 182.00 in Examples 5 to 11, and f = 171.89 in Example 12.

FIG. 4 to FIG. 18 sequentially show field curvature diagrams and fθ characteristic diagrams for Examples 1 to 15, respectively. In the field curvature, a solid curve represents an image forming position in the sub-scanning direction, and a broken curve represents an image forming position in the main scanning direction.

In all the embodiments, the use of the cylinder lens 2 and the surface tilt correction lens 7 is omitted in calculating the curvature of field and the fθ characteristic. Since these lenses have no refracting power in the main scanning direction, they do not affect the image formation in the main scanning direction at all. However, with the omission of the surface tilt correction lens 7, the curvature of field in the sub-scanning direction increases in each of the embodiments, but the field curvature in the sub-scanning direction can be eliminated by using the surface tilt correction lens. There is no.

[Effects of the Invention] As described above, according to the present invention, a novel fθ lens of an optical scanning device can be provided. As described above, the fθ lens has a small curvature of field in the main scanning direction and has good fθ characteristics, so that extremely excellent optical scanning can be realized. Further, since it is used together with the surface tilt correction lens, it can be constituted only by a spherical lens which is easy to manufacture. In particular, since a material having a low refractive index can be used, the whole system can be constituted by a plastic lens.

[Brief description of the drawings]

FIG. 1 is a view for explaining the lens configuration of the fθ lens of the present invention, FIGS. 2 and 3 are views for explaining an optical scanning device using the fθ lens of the present invention, and FIGS. FIG. 18 shows a field curvature diagram and an fθ characteristic diagram for each embodiment. 5 First group, 6 Second group

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G02B 13/00 G02B 26/10

Claims (1)

(57) [Claims] A light beam from a light source device is incident on a deflecting / reflecting surface of a light deflecting device via a cylinder lens for correcting a light spot diameter in a sub-scanning direction. A lens used as an imaging lens in an optical scanning device for performing optical scanning by deflecting by an optical deflector and forming an image as a light spot on a surface to be scanned by an imaging lens and a surface tilt correcting lens; It has fθ characteristics in the scanning direction, and is arranged in the order of first and second groups from the optical deflector to the surface to be scanned, the first group being a spherical single lens having a negative refractive power, The second group is composed of the same two positive lenses whose lens surfaces on the side to be scanned have strong positive refractive power. The combined focal length of the entire system is f, and the combined focal length of the second group is f.
f _g2, the radius of curvature of the i-th lens surface as counted from the light deflecting device side R _i, the surface distance D _i, a distance leading to the sixth lens surface from the origin of the deflection by the deflection reflecting surface D _a when _{a, f, f g2, R 3} , R 4, D 2, D a is, (I) -0.4 <f / R 3 <0.4 (II) 0.06 <D 2 / f g2 <1.2 (III) - Fθ of the optical scanning device characterized by satisfying the following condition: 0.52 <D _a / R ₄ <−0.35
lens.