JP2856491B2 - 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 to perform light scanning of a surface to be scanned at a constant speed in an optical scanning device using an optical deflector such as a rotary polygon mirror or a pyramidal mirror that deflects a light beam at a constant angular velocity. Various types of lenses have been known.

Rotating polygon mirrors and pyramidal mirrors have a problem of so-called "face 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. (For example, JP-A-62-105114).

[Problems to be Solved by the Invention] However, the known anamorphic fθ lenses are mainly
The field curvature in the sub-scanning direction is not sufficiently considered over the entire effective scanning area, and the light spot diameter may fluctuate when high-density optical scanning is performed.

The present invention has been made in view of the above-described circumstances, and provides a novel fθ lens having a surface tilt correction function, and in which the curvature of field in the main and sub scanning directions has been favorably corrected over the entire effective scanning area. Aim.

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

The fθ lens according to the present invention is configured such that “a substantially parallel light beam from the light source device is imaged as a long linear image in the main scanning corresponding direction in the vicinity of the deflecting reflection surface of the light deflecting device, and the light beam reflected by the deflecting reflection surface is reflected by the light deflecting device. A lens used as an image forming lens in an optical scanning device that performs optical scanning by deflecting at an angular velocity and forming an image as a light spot on a surface to be scanned by an image forming lens. And a function of making the position and the surface to be scanned substantially geometrically conjugate with each other in the main scanning direction.

As shown in FIG. 1, there is a three-group configuration in which a first group 5, a second group 6, and a third group 7 are arranged in this order from the light deflecting device side toward the surface to be scanned.

The first group 5 is a meniscus spherical single lens having a concave surface facing the light deflector.

The second group 6 is a single lens in which the lens surface on the side of the optical deflector has a curvature only in the sub-scanning direction or a flat surface and the lens surface on the scanned surface side is a convex toric surface having a strong curvature in the sub-scanning direction. It is.

The third group 7 is a single lens in which the lens surface on the optical deflector side is a concave cylinder surface having a curvature only in the sub-scanning direction and the lens surface on the scanned surface side is a convex toric surface having a strong curvature in the sub-scanning direction. It is.

 Therefore, the overall configuration is a three-group, three-element configuration.

The combined focal length of the entire system in the main scanning direction is f, and the focal lengths of the second and third groups in the sub-scanning direction are f _2Y and f _2Y , respectively.
f _3Y , the radius of curvature of the i-th lens surface counted from the optical deflector side is R ₁ , R ₂ for i = ₁ , ₂ and R _iY for the sub-scanning direction for i = 3 to 6, Is D _i , and the distance from the origin of deflection by the deflecting reflecting surface to the first lens surface is D ₀ , where (I) −0.3 <R ₁ /f<−0.05 (II) 0.7 <R ₁ / _{R 2 <1.3 (III) -0.7} <D 0 / R 1 <-0.2 (IV) 0.1 <D 2 / f <0.3 (V) -0.35 <f 2Y / f 3Y <0.15 (VI) 0.55 <R 5Y / The condition of R _4Y <1.2 or 0.55 <R _5Y / R _6Y <1.2 is satisfied.

In the fθ lens of the present invention, in addition to the above conditions (I) to (VI), the thickness D _i (i = 2n−1, n: 1 to 3) of each lens is set according to the following condition (VII) D _i / It is desirable to satisfy f (i = 2n-1, n: 1 to 3) <0.05.

[Operation] FIG. 2A shows an example of an optical scanning device using the fθ lens of the present invention. 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 extended by the cylinder lens 2 near the deflecting and reflecting surface of the light deflecting device 3 (which is a rotating polygon mirror in the example in the figure) in a main scanning direction (a direction parallel to the drawing of FIG. 2A). The image is formed on the image.

The light beam reflected by the deflecting / reflecting surface is deflected at a constant angular velocity by the light deflecting device 3, and is formed as a light spot on the surface 8 to be scanned 8 by an fθ lens composed of a first group 5, a second group 6, and a third group 7. An image is formed, and the scanned surface 8 is optically scanned. In the figure, the vertical direction of the scanned surface 8 is the main scanning direction, and the direction orthogonal to the drawing is the sub-scanning direction.

FIG. 2 (B) 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 figure, in the sub-scanning direction, since the fθ lens has a geometrically optically conjugate relationship between the position of the deflecting reflecting surface and the surface to be scanned, the deflecting reflecting surface 4 is tilted as indicated by reference numeral 4 '. Even if the resulting reflected 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 well corrected.

When the optical deflector is a rotary polygon mirror, the rotation axis does not coincide with the deflecting and reflecting surface, so the position of the deflecting and reflecting surface fluctuates in the optical axis direction of the fθ lens with the rotation of the rotating polygon mirror, and FIG. As shown in ()), the distance ΔX between the imaging position P of the line image and the deflecting reflection surface 4 fluctuates. The imaging position P 'of the light spot changes by ΔX' with respect to the surface 8 to be scanned in accordance with ΔX. As is well known, ΔX ′ is given by ΔX ′ = β ² ΔX, where β is the vertical magnification of the fθ lens in the sub-scanning direction.

With the rotation of the rotary polygon mirror, the position of the line image serving as a light source in the sub-scanning direction with respect to the fθ lens (reflection image of the line image by the deflecting reflection surface) changes two-dimensionally.
It is also necessary to correct the field curvature due to the entire scanning range, and f
The θ characteristic must also be corrected well.

 Each of the above conditions will be described.

The condition (I) is a condition for correcting the curvature of field in the main scanning direction. The curvature of field in the main scanning direction is under when the upper limit is exceeded, and over when the lower limit is exceeded.

The condition (II) is also a condition for correcting the field curvature in the main scanning direction. The field curvature in the main scanning direction is over when the upper limit is exceeded, and under when the lower limit is exceeded.

The condition (III) is a condition for correcting the field curvature in the main scanning direction and the fθ characteristic, that is, the linearity. The field curvature in the main scanning direction becomes over when the upper limit is exceeded, and becomes under when the lower limit is exceeded. Is over if the condition of this condition is not satisfied.

The condition (IV) is a condition for satisfactorily correcting the fθ characteristic. When the value exceeds the upper limit, the fθ characteristic becomes under, and when the value exceeds the lower limit, the value becomes over.

The conditions (V) and (VI) are both for correcting the field curvature in the sub-scanning direction. The field curvature in the sub-scanning direction becomes under when the upper limit of these conditions is exceeded, and becomes over when the lower limit is exceeded. become.

Condition (VII) is a condition for facilitating lens molding when all the lenses 5, 6, and 7 of the fθ lens are formed of plastic lenses.

EXAMPLES Hereinafter, eight specific examples will be given.

The combined focal length of the entire system in the main scanning direction is f (normalized to 100), and the radius of curvature of the i-th lens surface counted from the optical deflecting device side is R _{ix in} the main scanning direction. _Here , R _iY , the surface interval is D _i , and the distance from the starting point of deflection by the deflecting reflecting surface to the first lens surface is D ₀ . Further, the refractive index of the j-th lens counted from the optical deflector side is n _j, and the focal lengths of the second and third lenses in the sub-scanning direction are f _2Y and f _2Y .
_3Y . Further, the deflection angle (unit: degree) is represented by 2θ, and K ₁ = R ₁ / f, K ₂ = R ₁ / R ₂ , K ₃ = D ₀ / R ₁ , and K ₄ = D ₂ / f, K ₅ =
f _2Y / f _3Y , K ₆ = R _5Y / R _4Y (or R _6Y ). Since the first lens counted from the optical deflecting device is a spherical lens, R _iX = R _iY for i = 1,2.

A rotating polygon mirror is assumed as an optical deflecting device throughout the embodiments. The number of deflecting and reflecting surfaces of the rotating polygon mirror is 6, the radius of the inscribed circle is 15, the central ray of the light beam incident on the deflecting and reflecting surface, and the fθ lens light. The angle between the axes is 90 degrees.

FIGS. 3 to 11 show diagrams of the field curvature and the fθ characteristics for the first to ninth embodiments. In the field curvature diagram, a solid line indicates an image forming position in the sub-scanning direction, and a broken line indicates an image forming position in the main scanning direction.

As is well known, the fθ characteristic is obtained by using the focal length f in the main scanning direction, the deflection angle θ, and the actual image height H ′. fθ characteristic = {(H′−f · θ) / f · θ} · 100 (%) Is the quantity defined by

[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 the field curvature in the main and sub-scanning directions well corrected over the entire effective scanning area, and has a good fθ characteristic. Good optical scanning can be realized. Further, since a material having a low refractive index can be used, the entire 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, FIG. 2 is a view for explaining an optical scanning device using the fθ lens of the present invention, and FIGS. FIG. 4 is a field curvature diagram and an fθ characteristic diagram for each embodiment. 5 First group, 6 Second group, 7 Third group

Claims (1)

(57) [Claims] An optical system according to claim 1, wherein a substantially parallel light beam from the light source device is formed into a long linear image in the main scanning direction in the vicinity of the deflecting / reflecting surface of the light deflecting device. Is a lens used as an imaging lens in an optical scanning device that performs optical scanning by optically scanning by deflecting a deflected light beam onto a surface to be scanned by an imaging lens as a light spot, and deflecting and reflecting surface in the sub-scanning direction. The position and the surface to be scanned are substantially conjugated geometrically and optically, and have an fθ characteristic in the main scanning direction.
The first group is a spherical single lens of a meniscus lens having a concave surface facing the light deflector, and the second group is a cylinder surface or a plane having a lens surface on the light deflector side having a curvature only in the sub-scanning direction. A single lens whose lens surface on the surface to be scanned is a convex toric surface having a strong curvature in the sub-scanning direction, and a third group is a concave cylinder surface whose lens surface on the optical deflector side has a curvature only in the sub-scanning direction. The lens surface on the scanning surface side is a single lens having a convex toric surface having a strong curvature in the sub-scanning direction, and has a three-group, three-element configuration. The combined focal length of the entire system in the main scanning direction is f, The focal length of the group in the sub-scanning direction is f _2Y , the focal length of the third group in the sub-scanning direction is f _3Y , and the radius of curvature of the i-th lens surface counted from the optical deflector is i = 1,2. And R ₁ , R ₂ , i =
When R _{iY in} the sub-scanning direction, D _{i is} the surface distance, and D ₀ is the distance from the origin of deflection by the deflecting reflecting surface to the first lens surface with respect to 3 to 6, (I) −0.3 _{<R 1 / f <-0.05 (} II) 0.7 <R 1 / R 2 <1.3 (III) -0.7 <D 0 / R 1 <-0.2 (IV) 0.1 <D 2 / f <0.3 (V) -0.35 <F _2Y / f _3Y <0.15 (VI) 0.55 <R _5Y / R _4Y <1.2 or 0.55 <R _5Y / R _6Y <1.2
lens.