JPS63218916A - Optical scanner - Google Patents

Abstract

PURPOSE:To easily realize an image forming spot of a roughly round shape without shaping a luminous flux cross section shape, and to obtain an optical scanner being satisfactory in an aberration, as well, by providing a specific image forming lens and a convex cylinder lens. CONSTITUTION:An image forming lens 14 is single lens, and the lens surface of its deflecting surface side has a different radius of curvature in each main and sub-scanning direction, and also, it is an annular surface 14a having negative power in the sub-scanning direction, and the lens surface of the scanning image forming surface side is an aspheric surface 14b in an expression I. Also, a convex cylinder lens 15 is arranged so as to shift a focal position in the optical axis direction from on the image forming scanning surface, so that the diameter of a beam in the sub-scanning direction becomes almost the same as a beam wait in the main scanning direction. Moreover, as for the aspheric surface of the image forming lens, a column coefficient K satisfies a condition of -1.0<K>0, and a radius of curvature R3 in the vicinity of an optical axis, a focal distance (f) of the image forming lens, and a refractive index (n) satisfy a condition of 0.7<R3/(n-1)f<1.5.

Description

[Detailed description of the invention] (Industrial application field) The present invention relates to an optical scanning device.

(Prior Art) An optical scanning device deflects a light beam to scan a photosensitive recording medium to write optical information.

Alternatively, it has been put into practical use as an optical printer, etc., as a device that scans a document and converts the document information into an optical signal.

There are various methods for deflecting a light beam in an optical scanning device, but the most practical method is a method that utilizes rotation or swing of a reflective deflection surface such as a rotating polygon mirror or a galvano mirror.

However, when the light beam is deflected by rotating or swinging the deflection surface, it is necessary to correct the tilt of one surface, as is well known.

A conventionally known method for correcting surface tilt is to focus the light beam incident on the deflection surface in the sub-scanning direction and form an image on the deflection surface as a line image parallel to the main-scanning direction. It was also installed between the optical deflector and the scanning imaging plane. The starting point of deflection is determined by the imaging lens and cylinder lens.

A method is known in which the scanning and imaging planes are connected in a conjugate relationship in the sub-scanning direction (Japanese Patent Publication No. 52-28666).

In addition, as a modification of this method, it acts as an imaging lens with 10 functions in the main scanning direction, and in the sub-scanning direction,
It has the effect of connecting the starting point of the deflection and the scanning imaging plane to a conjugate relationship. A method using a lens of a special shape is known (Japanese Patent Application Laid-open No. 147316/1983).

These surface tilt correction methods will be hereinafter referred to as conjugate correction methods for convenience.

Another method for correcting surface tilt is that the amount of variation in the imaging spot due to surface tilt is proportional to the composite focal length in the sub-scanning direction of a lens system placed between the deflector and the scanning imaging surface. Focusing on this point, a method has been proposed in which the combined focal length of the lens system in the main scanning direction is made smaller than that in the sub-scanning direction (Japanese Patent Publication No. 15767/1983). This method will hereinafter be referred to as a short focal length method.

(Problems to be Solved by the Invention) The correction using the above-mentioned conjugate correction method has the following problems. In other words, in this method, the light incident on the deflection surface forms a line image on the deflection surface, so optical scanning is easily affected by scratches or dirt on the deflection surface, and the energy of the light beam is Since the deflection surface is concentrated in a specific area, seizure of the deflection surface is likely to occur. Also, the incident side N in the sub-scanning direction
Since A is extremely small, the utilization efficiency of light energy is poor.

On the other hand, the surface tilt correction using the short focal length method described above has the following problems.

In other words, it is desirable that the shape of the imaging spot that main scans the scanning imaging plane be circular, but in the short focal length method, in order to obtain a circular imaging spot, it is necessary to use an optical deflector. A special optical system is required in order to make the cross section of the light beam deflected by the beam into an elliptical shape that is long in the main scanning direction.

As is well known, the size of the imaging spot formed on the scanning imaging plane is determined by the beam waist Wd.

This beam waist Wd is the rough focal length of the lens system, f,
Let the diameter of the incident light beam be d and the wavelength be λ.

is given by However, in the short focal length method, the composite focal length fp in the main scanning direction of the lens disposed between the optical deflector and the scanning imaging plane is not equal to the composite focal length fs in the sub-scanning direction. fs, in order to equalize the diameter Wdp in the main scanning direction and the diameter Wds in the sub-scanning direction of the imaging spoiler 1 to The diameter dp must be made larger than the diameter ds in the sub-scanning direction so that the following equation holds true, and a special optical system is required to realize the curved beam cross section.

Therefore, an object of the present invention is to solve various problems in the above-mentioned conjugate correction method, and to easily form a substantially perfect circular imaging spot using a special optical system without having to shape the cross-sectional shape of the light beam. It is possible to realize this.

An object of the present invention is to provide an optical scanning device that is also good in terms of aberrations.

(Means for Solving the Problems) The optical scanning device of the present invention includes an irradiation means, an optical deflector, an imaging lens, and a convex cylinder lens.

The irradiation means irradiates the light from the light source as a substantially parallel light beam having a predetermined diameter. Since the emitted light beam has a predetermined diameter,
The cross section of the light beam is approximately circular.

The optical deflector deflects the light beam from the irradiation means.

This deflection is performed by a deflection surface.

The imaging lens is a single lens, and forms an image of the light beam deflected by the optical deflector.

The convex cylinder lens is arranged between the imaging lens and the scanning imaging surface so that its direction in which it has no power is parallel to the main scanning direction.

The main features of the present invention are as follows.

First, the arrangement of the convex cylinder lens is determined as follows. In other words, the focal point position is moved in the optical axis direction with respect to the scanning imaging plane. This shift amount is determined so that the beam diameter in the sub-scanning direction is approximately the same as the beam waist in the main-scanning direction on the imaging scanning surface.

Second, the imaging lens has a lens surface on the entrance side, that is, the deflection surface side, and a lens surface on the exit side, that is, the scanning imaging surface side, each having a special shape.

The lens surface on the deflection surface side has different lens surface curvature radii in the main scanning direction and the sub-scanning direction, and this lens surface has a toric surface and has negative power in the sub-scanning direction.

Further, the lens surface on the scanning imaging surface side is an aspherical surface.

This aspheric surface is analytically.

It is expressed by the general formula: Here, H is the height from the optical axis,
K is the cylinder coefficient, A is the curvature near the optical axis, and B.

C is a constant, and X represents the distance between the apex tangent plane, that is, the tangent plane to the lens surface at the intersection of the aspherical lens surface and the optical axis, and the lens surface.

This aspherical surface satisfies the following conditions.

That is, the cylinder coefficient K in the above general formula is (1)-1
,0<K<0. Also, the focal ratio of the imaging lens @
f, the refractive index n, the radius of curvature R near the optical axis of the aspherical surface satisfies the following conditions. In addition, H in the general formula of an aspherical surface
The terms of the fourth power or higher can be ignored in practice, and therefore, the present invention defines conditions for obtaining the necessary characteristics for the coefficient k in the above general formula.

(Function) Since the optical scanning device of the present invention is configured as described above, it has the following functions.

That is, since the irradiation means emits a substantially parallel light beam having a predetermined diameter, this light beam is not focused on the deflection surface of the optical deflector.

Furthermore, the combination of the toric surface of the imaging lens and the convex cylinder lens reduces the focal length in the sub-scanning direction. In the sub-scanning direction, it is a combination of a concave toric surface and a convex convex cylinder lens.

It becomes easy to correct various aberrations related to imaging performance.

Furthermore, by making the lens surface of the imaging lens on the scanning imaging surface side an aspherical surface, it is possible to obtain good imaging characteristics Φ on the scanning imaging surface.

The above-mentioned condition (1) is -1, OI (<O is the condition for maintaining the flatness of the image plane. K is the lower limit of -1,0
When the value exceeds 0, the meridional direction of astigmatism increases in the positive direction, and exceeds the upper limit of 0.

It increases in the negative direction, and in any case, the flatness of the image surface cannot be maintained.

defines the power distribution and shape of the imaging lens, and when the lower limit of 0.7 is exceeded, the linearity and field curvature of the characteristics of the imaging lens as an fO lens increase in the positive direction. However, the function as an fθ lens is significantly deteriorated.

If the upper limit of 1.5 is exceeded, the curvature of the field increases in the negative direction and the distortion increases in the negative direction.

The function as an fθ lens cannot be satisfied.

Next, the effect of shifting the focal position of the convex cylinder lens from the scanning imaging plane in the optical axis direction will be explained.

In the case of the present invention, since the cross-sectional shape of the light beam from the irradiation means is approximately circular, the beam diameter dP+ds of the light beam in the main and sub-scanning directions is dpyuds. On the other hand, if the combined focal length of the lens is fP in the main scanning direction and fs in the sub-scanning direction, fs is determined by the action of the toric surface of the imaging lens and the convex cylinder lens, as described above. te, fp
It has been shortened to .

Therefore, fp>fs. Therefore, referring to equation (1), the beam waist of the light beam converging toward the scanning imaging plane is expressed as WdP in the main scanning force direction and Wds in the sub-scanning direction, and the combined focal length is 1ttlfp.
, fs, WdP>Wds.

Therefore, if the position of the composite focus is made to coincide with the scanning imaging plane in both the main scanning direction and the WA scanning direction, the shape of the imaging spot will become an ellipse that is elongated in the main scanning direction.

On the other hand, since the beam waist is the minimum value of the beam diameter, the beam diameter increases as the beam moves away from the beam waist.

FIG. 2(1) shows an exaggerated state in which the deflected light beam is imaged as a beam waist on the scanning imaging plane 16 in the main scanning direction. The diameter of this beam waist is W
It is dp.

FIG. 2 (II) shows that the deflected light beam is in the sub-scanning direction.
A state in which the image is formed as a beam waist at a position ΔL behind the scanning imaging plane 16 is shown. The beam waist diameter at this time is Wds.

At this time, if the beam waist position is shifted by ΔL, and the beam diameter in the sub-scanning direction becomes equal to the beam waist diameter W d s in the main scanning direction, then in the scanning direction, the proportional relationship ΔL: Wdp =fs:
d'' (3) holds, and d is the beam diameter of the parallel beam with a circular beam cross section emitted from the irradiation means. From this, can be obtained.

Since Wdp is the beam waist diameter with respect to the focal length fρ, from equation (1).

is given. Using this.

becomes.

The beam waist diameter Wdp in the main scanning direction is larger than Wds, and the beam diameter in the main scanning direction is approximately given by Wdp near the beam waist. By shifting the focal point position relative to the scanning imaging plane in the optical axis direction by ΔL given in (6) above, the beam diameter on the scanning imaging plane is approximately equal in each of the main and sub-scanning directions. A substantially circular imaging spot can be obtained.

As shown in FIG. 2(m), even if the focal point position in the WII scanning direction is shifted by ΔL toward the front of the scanning imaging plane 16, a circular imaging spot can be realized in the same manner as described above.

(Example) A detailed description will be given below with reference to the drawings.

FIG. 1 (1) shows only the main parts of a specific example of the optical scanning device of the present invention. Reference numeral 11 indicates a substantially parallel light beam irradiated by the irradiation means.The light beam 11 has a substantially circular cross-sectional shape and a substantially constant diameter.

The light beam 11 enters the deflection surface of a rotating polygon fi 12 serving as a light deflector, is reflected once, and is deflected by the rotation of the rotating polygon mirror 12.

The deflected light beam enters the imaging lens 14A.

The lens surface 14a on the deflecting surface side of the imaging lens HA is formed into a surface having a constant radius of curvature that is different in the main scanning direction and the sub-scanning direction, and has a negative power in the sub-scanning direction. formed on the surface. Here, one toric surface refers to a cylindrical surface that is generated when the axis of the cylinder is bent along a circle.

Further, the lens surface 14b on the side of the scanning imaging plane 16 is an aspherical surface and satisfies the conditions (1) and (II) described above.

The light beam transmitted through the imaging lens 14A enters the scanning imaging plane 16 via the convex cylinder lens 15.

Figures 1(n) and 1(m) are in the main scanning direction, respectively.

It shows the lens action in the sub-scanning direction. By displacing and disposing the convex cylinder lens 15 in the optical axis direction, the focal position in the sub-scanning direction is shifted to the rear of the scanning imaging plane 16, and as described in FIG. A circular imaging spot is achieved.

Two specific examples of the configuration shown in FIG. 1 will be given below.

In each example, the surface number is the lens surface of each lens counted from the deflection surface side, with the deflection surface as the first surface. Therefore, the 1st page number 1, 2, 3°4.5 are respectively,
It means a deflection surface, a toric surface of an imaging lens, an aspheric surface of an imaging lens, an entrance surface of a convex cylinder lens, and an exit surface of a convex cylinder lens.

In addition, the distance between surfaces indicates the distance between adjacent surfaces, the refractive index and material indicate the refractive index of each lens, and the material fB indicates the distance between the imaging lens and the convex cylinder lens in the main scanning and sub-scanning directions. The wavelength of the light beam is 780
It is nm.

(Example 1) fA=200.0. fB=76.0454, and the cylinder coefficient in the general formula representing an aspherical surface is =-4.5×10''. In this example, terms higher than H4 are ignored.

Surface number RA RB Surface spacing Refractive index
Material 1 ω (X) 80.02
co-182,518,01,486 acrylic 3
-97.2 -97.2 150.04 C
x330.0 6.0 1.486 Acrylic 5 cooo 45.962 In this Example 1, the shift ΔL between the focal point of the convex cylinder lens and the scanning imaging plane is about 2.5+am, and the focal point is It is shifted to the rear of the imaging plane, and the diameter of the scanning spot is approximately 80 μm in both the main and sub-scanning directions.

(Example 2) fA=200. ．． 5, fB=50.0. ni knee 8.47
674X10-1, B=111.89511X10-”
, C=1.89187X10-12 surface number RA
RB spacing Refractive index Material 1
ω ■ 85.02 400.0
-115.7 22.0 1.486 Acrylic 3 -126.512 -126.512 142
．． 9274 cx) 27.424 6
．． 0 1.5118 8 to 5 clo=X
The deviation ΔL between the focal point of the l50.0 convex cylinder lens and the scanning imaging plane is ΔL42.5m+m, and the focal point is shifted to the rear of the scanning imaging plane. What is the diameter of the scanning spot?

The width is approximately 80 μm in both the main and sub-scanning directions.

(Effects of the Invention) According to the present invention as described above, a novel optical scanning device can be provided.

In this optical scanning device, the deflection surface of the optical deflector is

Since a nearly parallel beam of light is incident and the optical energy is not concentrated on a part of the deflection surface, optical scanning is less susceptible to scratches and dirt on the deflection surface, and the deflection surface is less likely to become burnt. Since the imaging lens is a single lens, the structure of the optical scanning device is simplified, and the combination of the toric surface of the imaging lens and the convex cylinder lens effectively shortens and shortens the combined focal length in the secondary scanning direction. It is possible to perform surface inclination correction satisfactorily.

Furthermore, because the focal position of the convex cylinder lens is shifted from the scanning imaging plane in the optical axis direction, the shape of the imaging spot can be made approximately circular despite the difference in focal length in the main and sub-scanning directions. can.

In addition, by making the lens surface of the imaging lens aspheric and satisfying certain conditions, it is possible to maintain good fθ while maintaining the flatness of the image surface.
characteristics can be realized.

Note that it is also possible to change the order of the toric surface and the aspheric surface in the imaging lens.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing only the essential parts of an example of an optical scanning device according to the present invention, and FIG. 2 is a diagram for explaining the present invention. 11... Luminous flux by irradiation means, 12... Rotating polygon mirror as a light deflector, 14... Imaging lens, 14
a... Toric lens surface, 14b... Aspherical lens surface, 15... Convex cylinder lens, 16... Scanning imaging surface. Most Z exit (■) (K) (I)

Claims (1)

[Scope of Claims] An irradiation means for irradiating a light beam from a light source as a substantially parallel light beam with a predetermined diameter; an optical deflector provided with a deflection surface for deflecting the light beam from the irradiation means; An imaging lens for forming an image of the deflected light beam, and a convex cylindrical lens placed between the imaging lens and the scanning imaging surface with its direction of no power parallel to the main scanning direction. The imaging lens is a single lens, and the lens surface on the deflection surface side has a different radius of curvature in the main and sub scanning directions,
In addition, it is a toric surface with negative power in the sub-scanning direction, and the lens surface on the scanning imaging plane side is X=AH^2/[1+√{1
-(K+1)}(A^2H^2)]+BH^4+CH^
6+...However, H: Height from the optical axis K: Cylindrical coefficient A: Curvature near the axis B, C: Constant X: Distance from the vertex tangent plane to the lens surface at the height H from the optical axis The convex cylinder lens has an aspherical surface, and the beam diameter in the sub-scanning direction is
The aspheric surface of the imaging lens is arranged such that the focal position is shifted from the imaging scanning surface in the optical axis direction so that it is approximately the same as the beam waist in the main scanning direction on the imaging scanning surface. , the cylinder coefficient K satisfies the condition (I)-1.0<K<0, the radius of curvature R_3 near the optical axis, the focal length f of the imaging lens, and the refractive index n, (III)0.7< An optical scanning device characterized by satisfying the following condition: R_3/(n-1)f<1.5.