JP2001004943A - Optical scanner - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical scanner excellent in the characteristic of the field curvature, especially, and capable of excellently reproducing an image at high speed. SOLUTION: This scanner is equipped with a light beam source 1, a deflector 4 having plural reflection surfaces reflecting and deflecting a light beam from the beam source 1, a transmission optical system 22 being an afocal optical system in a main scanning direction, which transmits light beams reflected and deflected by the 1st reflection surface 5 of the deflector 4 and being parallel at least in the main scanning direction and makes them incident on the 2nd reflection surface 6 of the deflector 4, and a scanning optical system 23 performing scanning with the light beam reflected and deflected by the 2nd reflection surface 6 of the deflector 4 and formed as a beam spot on a surface to be scanned 17. In such a case, the optical system 22 has diopter curved to be convex in a positive diopter direction at least near the optical axis in the main scanning direction, and the optical system 23 has the field curvature to be convex with respect to the advancing direction of the light at least near the optical axis in the main scanning direction.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical scanning device used in a laser beam printer or the like, and more particularly to an optical scanning device in which a light beam is sequentially incident twice on a deflector having a plurality of reflecting surfaces such as a rotary polygon mirror. The present invention relates to a configuration of a transmission optical system for transmitting light from a first reflection surface to a second reflection surface in an apparatus.

[0002]

2. Description of the Related Art Heretofore, the present applicant has proposed an optical scanning apparatus using a rotating polygonal mirror, in which light is incident twice on different reflecting surfaces in order, thereby enabling high-speed and good image reproduction. Proposed. In particular, JP-A-11-
In the apparatus of No. 64771, the transmitting optical system for transmitting and entering from the first reflecting surface to the second reflecting surface of the rotary polygon mirror is an afocal optical system in the main scanning direction. It consists of two spherical lenses (cylindrical lenses), and the diopter in the main scanning direction of the transmission optical system (the reciprocal of the distance expressed in m from the second reflecting surface to the focal point) is positive with respect to the diopter direction. It is concavely curved near the optical axis.

In US Pat. No. 5,392,149 and US Pat. No. 5,585,955, similarly, an afocal transmission optical system is used in the main scanning direction. Not specified.

[0004]

In the apparatus disclosed in Japanese Patent Application Laid-Open No. H11-64771 by the present applicant, the front group of the transmission optical system is composed of three elements, and the rear group is composed of four elements. Because the diopter in the main scanning direction of the optical system is concavely curved near the optical axis, a convex field curvature near the optical axis in the main scanning direction caused by the scanning optical system between the rotating polygon mirror and the surface to be scanned is generated. (An image surface having a convex shape with respect to the traveling direction of light), and the field curvature of the entire optical system in the main scanning direction is increased.

The present invention has been made in view of such problems of the prior art, and an object of the present invention is to provide an optical scanning device which has a particularly good curvature of field and is capable of high-speed and good image reproduction. It is.

[0006]

To achieve the above object, the present invention provides an optical scanning device comprising: a light source for generating a light beam; a deflector having a plurality of reflecting surfaces for reflecting and deflecting the light beam from the light source; A transmission optical system that is an afocal optical system in the main scanning direction in which a light beam reflected and deflected by the first reflection surface of the deflector is transmitted and incident on the second reflection surface of the deflector at least in a main scanning direction; A scanning optical system that forms a beam spot on the surface to be scanned with the light beam reflected and deflected by the second reflecting surface of the deflector, and scans the light beam. Having a diopter curved convexly to the positive diopter direction at least near the optical axis,
The scanning optical system is characterized in that it has a convex curvature of field at least in the vicinity of the optical axis in the main scanning direction with respect to the traveling direction of light.

In this case, it is desirable that the front group of the transmission optical system be composed of a single lens having at least one aspheric surface (non-arc surface) in the main scanning direction.

It is desirable that the surface on the incident side of the single lens be an aspherical surface (a non-circular surface) in the main scanning direction.

It is desirable that the conic coefficient K of the aspherical surface (aspherical surface) of the single lens of the transmission optical system be K <−1.

It is desirable that the front group of the scanning optical system be composed of a single lens.

In the present invention, the transmission optical system has a diopter which is convexly curved with respect to the positive diopter direction at least near the optical axis in the main scanning direction, and the scanning optical system has a diopter at least near the optical axis in the main scanning direction. The transmission optical system and the scanning optical system have characteristics opposite to each other with respect to the curvature of field because the curvature of field is convex with respect to the traveling direction of light. By canceling each other out, the curvature of field of the entire optical system in the main scanning direction is improved. Therefore, good image reproduction can be performed at high speed.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an optical scanning device according to the present invention will be described in detail with reference to the drawings.

First, an embodiment of the optical scanning device according to the present invention will be described. FIG. 1 is a plan view showing the configuration of the optical scanning device of the present embodiment, FIG. 2 is a side view thereof, FIG. 3 is a perspective view of a main part thereof, and FIG. 4 is a side view of the main part thereof. Hereinafter, in the present invention, at any position of the optical system, the rotation axis 4 of the rotary polygon mirror 4 which is a deflector and includes the optical axis of the optical system at that position.
A plane parallel to 1 is defined as a sub-scanning plane, and a plane including the optical axis and perpendicular to the sub-scanning plane is defined as a main scanning plane. Further, a direction perpendicular to the optical axis in the main scanning plane is defined as a main scanning direction, and a direction perpendicular to the optical axis in the sub-scanning plane is defined as a sub-scanning direction.

A light beam emitted from a semiconductor laser 1 as a light source is supplied to a first shaping lens 2 and a first aperture 61.
(FIG. 5), the second shaping lens 3, and the rotating polygon mirror 4 which is shaped by passing through the second aperture 62 (FIG. 5) as a deflector.
Incident on the first reflection surface 5, and is deflected for the first time. At this time, since the light beam is incident on the first reflection surface 5 at an angle with respect to a plane perpendicular to the rotation axis 41 of the rotary polygon mirror 4, the incident light beam and the reflected light beam do not interfere with each other. The light beam reflected by the first reflection surface 5 passes through the first transmission lens 7 and is reflected by the first transmission mirror 10,
The light passes through the transmission lens 11 and the third transmission lens 12, is reflected by the second transmission mirror 13, reenters the second reflection surface 6 of the rotary polygon mirror 4, and is deflected a second time. Again,
Since the light beam enters the second reflection surface 6 at an angle with respect to a plane perpendicular to the rotation axis 41 of the rotary polygon mirror 4, the incident light beam and the reflected light beam do not interfere with each other.

The light beam reflected by the second reflecting surface 6 is formed as a light beam spot on the surface 17 to be scanned by the first scanning lens 14, the second scanning lens 15, and the third scanning lens 16 and is scanned. You. The number of surfaces of the rotary polygon mirror 4 is 12 (even number). The third scanning lens 16 is decentered in the sub-scanning direction, which is the direction of the arrow in FIG.
The reason why the third scanning lens 16 is decentered in this manner is that the light beam reflected and deflected by the second reflecting surface 6 of the rotary polygon mirror 4 draws a conical trajectory, and the coordinate system of the cross section of the light beam is deflected. This is because the image formation spot on the scanned surface 17 is distorted due to the rotation depending on the angle, but the decentering of the third scanning lens 16 can prevent the dislocation.

By the way, the optical system between the semiconductor laser 1 and the first reflecting surface 5 is formed by the shaping optical system 21 and the first reflecting surface 5.
The optical system between the second reflecting surface 6 and the transmission optical system 22,
If the optical system between the reflecting surface 6 and the surface to be scanned 17 is called a scanning optical system 23, the first reflecting surface 5 of the rotary polygon mirror 4
And the second reflective surface 6 are mutually parallel reflective surfaces facing each other across the rotation axis 41, and the optical axes of the shaping optical system 21, the transmission optical system 22, and the scanning optical system 23 include the rotation axis 41. Are arranged in the sub-scanning plane. Therefore, this optical scanning device has a symmetrical configuration with respect to the sub-scanning surface, while being incident twice and obliquely incident. With such an arrangement, the optical axes of the shaping optical system 21, the transmission optical system 22, and the scanning optical system 23 are arranged in a straight line when viewed in the main scanning plane. Each element constituting the optical system can be arranged with high accuracy on a plane. In addition, since the optical axis of the transmission optical system 22 partially overlaps the optical axes of the shaping optical system 21 and the scanning optical system 23 when viewed on the main scanning plane, the optical axis can be arranged in a small space, and the installation area of the optical scanning device can be reduced. The size of the device can be reduced. With such an arrangement, it is possible to prevent a position change of the scanning line in the sub-scanning direction due to the eccentricity of the rotating shaft 41 of the rotating polygon mirror 4.

FIG. 5 shows an optical path diagram (a) in the main scanning direction of the shaping optical system 21 and an optical path diagram (b) in the sub-scanning direction. The light beam b emitted from the semiconductor laser 1 having a bonding surface that is perpendicular to the main scanning surface and parallel to the sub-scanning surface and has a cover glass so as to spread in the main scanning direction as compared with the sub-scanning direction is emitted by the aspherical lens. The light beam is converted into a focused light beam by the first shaping lens 2, and only the peripheral portion in the sub-scanning direction is shielded by the first aperture 61 having a rectangular opening arranged at the exit surface position of the first shaping lens 2. The second shaping lens 3 is a negative cylindrical lens having a negative refractive power only in the main scanning direction. Therefore, the light beam transmitted through the second shaping lens 3 is converted into a parallel light beam on the main scanning plane, and
Only the peripheral portion in the main scanning direction is blocked by the aperture 62 and enters the first reflecting surface 5, and forms an image (convergence) near the first reflecting surface 5 on the sub-scanning surface.

FIG. 6 shows an optical path diagram (a) of the transmission optical system 22 in the main scanning direction and an optical path diagram (b) of the transmission optical system 22 in the sub-scanning direction. The first transmission lens 7 is a non-cylindrical cylindrical lens having a refractive power only in the main scanning direction. Also, the second transmission lens 1
Reference numeral 1 denotes a positive cylindrical lens having a positive refractive power only in the sub-scanning direction, and the third transmission lens 12 is a spherical lens having a positive refractive power. Then, in these operations, the light beam reflected by the first reflection surface 5 is once formed into an image by the first transmission lens 7 on the main scanning surface. First transmission lens 7
The image-side focal point 71 and the object-side focal point of the third transmission lens 12 coincide, and constitute an afocal optical system on the main scanning plane. Therefore, on the main scanning surface, the first transmission lens 7 forms a front group of the afocal optical system, and the third transmission lens 12 forms a rear group of the afocal optical system. Therefore, the light beam is converted into a parallel light beam again in the main scanning plane by the third transmission lens 12 and is incident on the second reflecting surface 6. On the sub-scanning surface, the second transmission lens 11 and the third
Due to the combined positive refractive power of the transmission lens 12, the first reflecting surface 5 and the second reflecting surface 6 are in a conjugate relationship, and the first reflecting surface 5
An image of a nearby focal point is formed again near the second reflecting surface 6.

FIG. 7 shows an optical path diagram (a) of the scanning optical system 23 in the main scanning direction and an optical path diagram (b) of the scanning optical system 23 in the sub-scanning direction. The first scanning lens 14 is a spherical lens having a positive refractive power. The second scanning lens 15 is a prism having a refracting action only in the sub-scanning direction, and the third scanning lens 16 is a long lens made of resin and long in the main scanning direction. The incident surface of the third scanning lens 16 has a concave shape with a large radius of curvature in the main scanning direction, and has a convex shape with a small radius of curvature in the sub-scanning surface direction. This is a surface formed by rotating around an axis parallel to the main scanning direction, which is located closer to the scanned surface 17 than the incident surface. Such a surface is also called a saddle-shaped toric surface. The exit surface of the third scanning lens 16 is an aspherical surface that is rotationally symmetric about the optical axis. The scanning optical system 23 having such a configuration makes the second reflecting surface 6 and the scanned surface 17 conjugate in the sub-scanning surface,
The focal point near the second reflecting surface 6 is imaged near the scanned surface 17. On the main scanning surface, the parallel light beam reflected from the second reflecting surface 6 forms an image near the surface to be scanned 17.

Next, the operation of the transmission optical system 22 will be described. FIG. 8 is an expanded sectional view of the main scanning surface of the transmission optical system 22. Since the second transmission lens 11 has no refracting power in the main scanning direction, it is not shown. FIGS. 8A and 8B show the state of the light beam when the rotary polygon mirror 4 rotates. By the way, as shown in FIG. 1 to FIG. 4, the optical path of the transmission optical system 22 is reflected twice by the transmission mirrors 10 and 13. That is, the light is reflected an even number of times. In FIG. 8, since the reflections are developed for these even-numbered reflections, the rotation directions of the first reflection surface 5 and the second reflection surface 6 are the same as in FIG. 8B.

The diameter of the parallel light beam incident on the first reflecting surface 5 is w _i . Since the transmission optical system 22 forms an afocal optical system in the main scanning plane, the light beam incident on the second reflection surface 6 is also parallel, and the diameter of the light beam is w _o.
It is. Assuming that the focal length of the first transmission lens 7 is f ₁ and the focal length of the third transmission lens 12 is f ₂ , the ratio of the diameter of the light beam obtained by dividing w _o by w _i is f ₂ by f ₁ . It is equal to the divided value.

As shown in FIG. 8B, when the rotating polygon mirror 4 rotates by the angle θ ₁ , the light beam is deflected by the first reflection surface 5 by the angle 2θ ₁ . The deflected light beam passes through the first transmission lens 7 and the third transmission lens 12 and has an angle θ ₂
Is only deflected. This light beam intersects the optical axis at point Q. On the second reflecting surface 6, the distance between the deflected light beam and the optical axis is d, the rotary polygon mirror 4 is rotated by an angle theta _1, so that also the second reflecting surface 6 moves by the same distance d Is set to a proper positional relationship. Therefore, the movement amount of the light beam and the movement amount of the second reflection surface 6 match, and the light beam does not protrude from the second reflection surface 6.

At this time, the deflected light beam forms the second beam
Angle θ to launch surface 6_TwoDeflection to the side where the incident angle increases
Scanning of the light beam reflected by the second reflecting surface 6
Angle θ _sIs θ_s= 2θ₁+ Θ_TwoIt is expressed as

Since the transmission optical system 22 of this embodiment is an afocal optical system on the main scanning plane, the optical magnification β is a value obtained by dividing the focal length f ₂ by the focal length f ₁ . It is also equal to the light beam diameter ratio w _o / w _i . Further, since the deflection angle changes the light beam transmitted through the transmission optical system 22 from the angle 2 [Theta] ₁ in the angle theta _2, the optical magnification β is 2 [Theta]
It can also be expressed as ₁ / θ ₂ . Therefore, the optical magnification β
Is represented by the following equation.

Β = w _o / w _i = f ₂ / f ₁ = 2θ ₁ / θ ₂ The optical scanning device in which the light beam is deflected twice by the rotating polygon mirror 4 as in the present embodiment is a conventional one. The scanning speed can be increased as compared with an optical scanning device that is deflected only once.
This will be described below.

In a conventional optical scanning device that deflects only once, when the rotating polygon mirror rotates, the reflection surface moves.
In order to always put the entire light beam on the same reflecting surface in each scanning, the size of the reflecting surface must be larger than the size of the light beam incident on the rotary polygon mirror in the main scanning direction. Therefore, the number of reflecting surfaces of the rotary polygon mirror cannot be increased so much.

In this embodiment, a light beam parallel to the first reflecting surface 5 is incident on the main scanning surface. Since β> 1, the diameter w _i of the light beam on the first reflection surface 5 in the main scanning direction is smaller than the diameter w _o of the light beam on the second reflection surface 6 in the main scanning direction. Therefore, even if the size of the first reflecting surface 5 is smaller than that of the conventional optical scanning device,
In each scan, the entire light beam can always enter the same reflecting surface. The smaller the value of w _i , the smaller the size of the first reflecting surface 5 can be. Also,
In the second deflection, the amount of movement of the light beam when the rotary polygon mirror 4 rotates and the amount of movement of the second reflecting surface 6 match.
The size of the second reflection surface 6 in the main scanning direction only needs to be at least as large as the size of the incident light beam.

Therefore, in comparison with the conventional optical scanning device which deflects only once, the optical scanning device which deflects twice in this embodiment has a diameter w in the main scanning direction of the light beam on the second reflecting surface 6. _{On the} other hand, by reducing the diameter w _i of the light beam on the first reflecting surface 5 in the main scanning direction, the reflecting surface of the rotary polygon mirror 4 can be reduced, so that the number of reflecting surfaces is increased. And the scanning speed can be increased accordingly.

Table 1 shows a numerical example of the first embodiment of the optical scanning device having such a configuration. In Table 1, the radius of curvature of the cylindrical surface and the toric surface in the sub-scanning direction and the main scanning direction are _rix and _riy (i indicates the surface number from the light source 1 to the surface 17 to be scanned). For the aspherical surface, the radius of curvature indicates a value on the optical axis. The unit of the length is mm. Note) S _i: the surface of the surface number i, r _i: curvature of the surface number i radius, d _i: surface distance between the surface of number i and i + 1, n _i: wavelength 780nm of the medium between the surface number i and i + 1 Is the refractive index of

The expression representing the aspherical surface of the first shaping lens 2, the first transmission lens 7, and the third scanning lens 16 is as follows: z _i = (y ² / r _i ) / [1+ ｛1- (K _i +1) ( y
^{_{/ R i) 2} 1/2]}} + A i y 4 + B i y 6 + C i y 8 +
A D _i y ^10, showing the aspherical coefficients in the following Table 2. However,
K is a conic coefficient defined by the K _i in the above formula. Note) S ₄ : Aspherical surface coefficient of surface number 4, S _8y : Aspherical surface coefficient of surface number 8 in the main scanning direction, S ₂₂ : Aspherical surface coefficient of surface number 22

In this example, the third scanning lens 16
Incident surface S ₂₁ of a toric surface formed by an arc of r _21y = -1038.19726 rotated at r _21x = 37.59661. Note that, although the optical path is refracted by the second scanning lens 15, the optical axis serving as a reference for the parameters in Table 1 is also refracted.

The number of faces of the rotary polygon mirror 4 is 12, of which
The tangent diameter is 38.64 mm, and the first polygon
The light beam enters the reflecting surface 5 and the second reflecting surface 6 in the sub-scanning direction.
The angle of incidence is 6 °, and the first transmission mirror 10 and the second transmission
The incident angle of the light beam in the sub-scanning direction
Is also 3 °. Also, the exit surface S of the second scanning lens 15 ₂₀
Is inclined at 12.076526 ° in the sub-scan section.
The incident surface S of the third scanning lens 16_{twenty one}Is in the sub-scan section.
8.609610 degrees. Of these tilt angles
Regarding the orientation, in FIG.
Clockwise. The exit surface of the third scanning lens 16 is shown in FIG.
Is eccentric downward by 0.2 mm.

Immediately after the exit surface of the first shaping lens 2,
A rectangular first aperture 61 having a main scanning direction of 4 mm and a sub-scanning direction of 1.64 mm is arranged.
1m in the main scanning direction at a position 53.3mm behind the exit surface of
A second aperture 62 having a rectangular shape of m and 4 mm in the sub-scanning direction is arranged. Then, in the sub-scanning direction, the light emitting point 1
And the first reflecting surface 5 of the rotary polygon mirror 4 are out of the geometrical conjugate relationship. However, the first reflecting surface 5 of the rotating polygon mirror 4,
Since the three surfaces of the second reflection surface 6 and the surface to be scanned 17 are all in a conjugate relationship with each other, the tilting of the rotary polygon mirror 4 is corrected. Therefore, the light emitting point 1 and the scanned surface 17 are out of the conjugate relationship. However, due to the effects of diffraction,
The position (beam waist) where the light beam is minimum is shifted from the geometrical optical imaging point, and the scanned surface 17 is arranged at the position (beam waist) where the light beam is substantially minimum.

Here, as described above, the second scanning lens 15 is a prism having a refraction function only in the sub-scanning direction. The operation of the prism will be described. The light beam reflected and deflected by the reflecting surface 6 of the rotary polygon mirror 4 draws a conical trajectory, and when the prism of the second scanning lens 15 is not disposed, the beam is curved on the long lens of the third scanning lens 16. It becomes a trajectory. As schematically shown in FIG. 9, the prism 16 forms a trajectory of the conical light beam a on the incident surface of the third scanning lens 16 in a linear beam trajectory A.
Has the effect of converting to

FIG. 10 is a diagram showing a beam locus on the incident surface of the third scanning lens 16 in the above specific example, and the beam locus is shown by a solid line. In the drawing, the Y direction indicates the main scanning direction, and the X direction indicates the sub scanning direction. For comparison, the third reflection surface 6 to the third reflection surface 6
The trajectory of the beam on the incident surface of the third scanning lens 16 when only the second scanning lens 15 is removed without changing the distance to the scanning lens 16 is indicated by a broken line. From FIG.
It can be seen that the prism function of the second scanning lens 15 has the function of correcting the beam trajectory into a straight line.

FIG. 11 shows the sub-scanning cross section of the third scanning lens 16 at several (5) positions in the main scanning direction, and shows the error of the measured value with respect to the design value of the cross-sectional shape. Things. In the drawing, X, Y, and Z are the sub-scanning direction, the main scanning direction, and the optical axis direction, respectively. As shown in FIG. 11, the shape error of a lens having a saddle-shaped toric surface such as the third scanning lens 16 shows substantially the same state regardless of the position in the main scanning direction, but changes periodically in the sub-scanning direction. There are features. As described above, by the prism action of the second scanning lens 15, the beam trajectories on the third scanning lens 16 linearly A, and the beam is always shape error of point regardless of the position in the main scanning direction B ₁ .about.B ₅ is It is incident on the convex part. At any position in the main scanning direction, when the light beam enters a portion where the shape error of the third scanning lens 16 is convex, the image forming position in the sub-scanning direction is shifted forward from the designed position. , The third scanning lens 1
Correction can be made by adjusting the optical system such as by adjusting the position of 6, and such adjustment does not cause curvature of field.

Now, the characteristics required for the transmission optical system 22 will be described. Two characteristics are the surface tracking characteristic and the parallelism in the main scanning direction, and one of the field curvature characteristics in the sub-scanning direction.

The surface following characteristic in the main scanning direction is, as described with reference to FIG. 8, the second characteristic when the rotary polygon mirror 4 rotates.
That is, the moving amount of the reflecting surface 6 and the moving amount of the light beam incident on the second reflecting surface 6 match. The parallelism in the main scanning direction means that the parallelism of the light beam emitted from the transmission optical system 22 is always maintained during scanning. The curvature of field in the sub-scanning direction refers to the curvature of field at the second reflection surface 6, that is, the curvature of field due to an image point near the second reflection surface 6 in the sub-scanning direction.

The parallelism of the transmission optical system 22 in the main scanning direction can be evaluated with a diopter. The diopter is the reciprocal of the distance from the second reflection surface 6 to the image forming point in the main scanning direction of the light beam incident on the second reflection surface 6 in the unit of m, and is the light emitted from the transmission optical system 22. The value is positive when the beam is converged in the main scanning direction, and negative when the beam is divergent.

As shown in FIG. 12, the diopter in the main scanning direction of the transmission optical system 22 in the above embodiment decreases in the negative direction as the distance from the optical axis increases. The light beam tends to diverge. In order to make the diopter bend convexly in the vicinity of the optical axis, the conical coefficient K of the convex aspheric surface of the first transmission lens 7 (the surface on the incident side in the above embodiment) may be K <−1. If K <-1, the convex aspheric surface becomes a convex hyperboloid, and the positive refracting power decreases as the distance from the optical axis increases, and the diopter of the transmission optical system 22 in the main scanning direction curves convexly near the optical axis. Shape.

The scanning optical system 23 is divided into a front group which is the first scanning lens 14 and a rear group which is the third scanning lens 16. Here, the second scanning lens 15 as a prism is omitted because it has no function as a lens. The criterion for dividing the scanning optical system 23 into a front group and a rear group is the second reflection surface 6.
The lens group on the side of the second reflecting surface 6 is defined as a front group, and the lens group on the side of the scanned surface 17 is defined as a rear group. Since the rear group of the scanning optical system 23 is close to the surface 17 to be scanned, the lens becomes a large-diameter lens. Therefore, if strong power is provided in the main scanning direction, the lens becomes too thick, so that strong power cannot be provided. Therefore, the positive power of the scanning optical system 23 in the main scanning direction is mainly borne by the front group. As in the above embodiment, if the front group is a single lens (first scanning lens 14) having positive power,
The curvature of field in the main scanning direction becomes convex in the vicinity of the optical axis (because the Petzval sum is negative), and the scanning optical system 23 of the above embodiment is used.
13A (dashed line) in FIG. The curvature of field is such that the position and angle of the principal ray of the light beam incident on the scanning optical system 23 from the transmission optical system 22 are the same as in the actual case, and the perfectly parallel light beam is
Is the field curvature determined to be incident on.

As shown in FIG. 12, the diopter in the main scanning direction of the transmission optical system 22 is convexly curved in the vicinity of the optical axis, and as shown in FIG. The fact that the curvature is convex in the vicinity of the optical axis is a property opposite to each other, so that the field curvatures of all the optical systems cancel each other out, resulting in the field curvature shown by B (solid line) in FIG. , A (broken line). The variation width of B is as small as 1.6 mm, while the variation width of A is 1.7 mm. It should be noted that, when the scanned surface 17 is geometrically arranged within this fluctuation range, a good scanned image can be formed. However, in actuality, as described above, the position of the beam waist at which the light beam is substantially minimized as described above. The surface 17 to be scanned is arranged at the same time.

As described above, in the present invention, in the main scanning direction, the transmission optical system 22 has a diopter which is convexly curved in the positive diopter direction at least near the optical axis, and the scanning optical system 23 has at least the light In the vicinity of the axis, the transmission optical system 22 and the scanning optical system 23 have characteristics opposite to each other with respect to the curvature of field so that the transmission optical system 22 and the scanning optical system 23 have characteristics opposite to each other. The curvature of field cancels each other, so that the curvature of field in the main scanning direction of all optical systems is improved.

In the above embodiment, the case where a rotary polygon mirror is used as the deflector has been described. However, similar effects can be achieved in the case of a rotary dihedral mirror as well as a rotary polygon mirror. be able to.

Although the optical scanning device of the present invention has been described based on the embodiments, the present invention is not limited to these, and various modifications are possible.

[0046]

As is apparent from the above description, according to the optical scanning apparatus of the present invention, the transmission optical system is provided with a diopter that is convexly curved with respect to the positive diopter direction at least near the optical axis in the main scanning direction. Since the scanning optical system has a convex curvature of field in the light traveling direction at least in the vicinity of the optical axis in the main scanning direction, the transmission optical system and the scanning optical system are opposite to each other with respect to the curvature of field. And the field curvature of the entire optical system cancels each other out,
The curvature of field in the main scanning direction of all optical systems is improved. Therefore, good image reproduction can be performed at high speed.

[Brief description of the drawings]

FIG. 1 is a plan view illustrating a configuration of an optical scanning device according to an embodiment of the present invention.

FIG. 2 is a side view of the optical scanning device of FIG.

FIG. 3 is a perspective view of a main part of the optical scanning device of FIG.

FIG. 4 is a side view of a main part of the optical scanning device of FIG.

5 is an optical path diagram of a shaping optical system of the optical scanning device of FIG. 1 in a main scanning direction and a sub scanning direction.

6 is an optical path diagram of a transmission optical system of the optical scanning device of FIG. 1 in a main scanning direction and a sub scanning direction.

7 is an optical path diagram of a scanning optical system of the optical scanning device in FIG. 1 in a main scanning direction and a sub-scanning direction.

FIG. 8 is a sectional development view of a main scanning plane for explaining the operation of the transmission optical system.

FIG. 9 is a diagram for explaining a correcting operation of the refractive prism.

FIG. 10 is a diagram showing a beam trajectory on an incident surface of a third scanning lens according to one embodiment of the present invention.

FIG. 11 is a diagram for explaining the reason why curvature of field does not occur due to a shape error of the third scanning lens in one specific example of the present invention.

FIG. 12 is a graph showing a diopter in a main scanning direction by a transmission optical system according to one embodiment of the present invention.

FIG. 13 is a graph showing the field curvature of only the scanning optical system according to one embodiment of the present invention, and the field curvature caused by the combination of the transmission optical system and the scanning optical system.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Semiconductor laser (light source) 2 ... 1st shaping lens 3 ... 2nd shaping lens 4 ... Rotation polygon mirror 5 ... 1st reflection surface of rotation polygon mirror 6 ... 2nd reflection surface of rotation polygon mirror 7 ... 1st transmission lens Reference Signs List 10: first transmission mirror (front group of transmission optical system) 11: second transmission lens 12: third transmission lens (rear group of transmission optical system) 13: second transmission mirror 14: first scanning lens (scanning optical system) 15) Second scanning lens (prism) 16 ... Third scanning lens (back group of scanning optical system) 17 ... Scanned surface 21 ... Shaping optical system 22 ... Transmission optical system 23 ... Scanning optical system 41 ... Rotation Axis 61: first aperture 62: second aperture 71: image-side focal point of first transmission lens b: light beam

Continuation of front page (72) Inventor Takeshi Kenwa 3-3-5 Yamato, Suwa-shi, Nagano F-term in Seiko Epson Corporation (reference) 2C362 BA83 BA86 BB14 2H045 AA01 BA41 CB14 DA15 5C072 AA03 CA02 CA06 DA02 DA21 DA23 HA02 HA13 HB10 XA05

Claims (5)

[Claims] 1. A light source for generating a light beam, a deflector having a plurality of reflection surfaces for reflecting and deflecting the light beam from the light source, and at least a main scanning direction reflected and deflected by a first reflection surface of the deflector. A transmission optical system which is an afocal optical system in the main scanning direction in which a parallel light beam is transmitted to and incident on the second reflection surface of the deflector; and a light beam reflected and deflected by the second reflection surface of the deflector. A scanning optical system for forming and scanning a beam spot on the surface to be scanned, wherein the transmission optical system is convexly curved with respect to the positive diopter direction at least near the optical axis in the main scanning direction. An optical scanning system having a diopter, wherein the scanning optical system has a convex curvature of field with respect to a traveling direction of light at least in the vicinity of an optical axis in a main scanning direction. Location. 2. The optical scanning device according to claim 1, wherein the front group of the transmission optical system comprises a single lens having at least one aspheric surface (a non-arc surface) in the main scanning direction. 3. The optical scanning device according to claim 2, wherein a surface on the incident side of the single lens is an aspheric surface (a non-arc surface) in the main scanning direction. 4. The optical scanning device according to claim 2, wherein a conic coefficient K of an aspheric surface (a non-arc surface) of the single lens of the transmission optical system is K <−1. 5. The optical scanning device according to claim 1, wherein a front group of the scanning optical system includes a single lens.