JPH1073781A - Optical scanning device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical scanning device at high speed on which a light beam is made obliquely incident twice preventing the positional fluctuation of a scanning line caused by the deviation of the position of each reflection surface based on the eccentricity of the rotating axis of a rotating polygon mirror. SOLUTION: This optical scanning device is provided with a light source 1, a rotating polygon mirror 4 having plural reflection surfaces for reflecting/deflecting the light beam from the source, a transfer optical system 22 for transferring the light beam reflected/deflected by a first reflection surface 5 of the rotating polygon mirror 4 to and making incident on a second refection surface 6 and a scanning optical system 23 for scanning the light beam reflected/deflected by the second reflection surface 6 on a surface to be scanned 17. The first reflection surface 5 are the second reflection surface 6 are opposed to each other interposing the rotating axis 41 of the rotating polygon mirror 4, the light beam is arranged so as to make incident on the first reflection surface 5 and the second reflection surface 6 being inclined in the sub-scanning direction and the first reflection surface 5, the second reflection surface 6 and the surface to be scanned 17 have almost a conjugate relation in the sub-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 for a laser beam printer or the like, and more particularly, to an optical scanning device which rotates a light beam at an angle with respect to a scanning plane perpendicular to a rotation axis of a rotating polygon mirror. The present invention relates to an optical scanning device in which the position of a scanning line is prevented from fluctuating due to a displacement of a reflection surface with respect to a rotation axis of a rotary polygon mirror in an optical scanning device in which light is sequentially incident twice.

[0002]

2. Description of the Related Art In an image recording device such as a laser beam printer or an optical scanning device used for various image reading and measuring devices, a rotary polygon mirror is often used as a deflector for deflecting and scanning a light beam.

In these apparatuses, a light beam is repeatedly scanned on a surface to be scanned in a straight line or a curved line.
The two-dimensional scanning is performed by relatively moving the medium to be scanned located on the surface to be scanned in a direction substantially orthogonal to the above-described scanning direction. The scanning direction of the former optical scanning device is referred to as a main scanning direction, and the latter direction of relative movement of the medium to be scanned is referred to as a sub-scanning direction.

In recent years, a higher-speed optical scanning device has been demanded in the above-mentioned devices in order to improve resolution and processing speed. In an optical scanning device using a rotating polygon mirror to deflect a light beam, to increase the scanning speed (scanning frequency), (1) increase the number of revolutions of the rotating polygon mirror. (2) Increasing the number of rotating polygon mirrors. The following two methods can be considered.

[0005] In order to increase the rotation speed of the rotary polygon mirror, a bearing capable of high-speed rotation is required. However, the upper limit of the most commonly used ball bearing is about 20,000 rotations per minute. 30 minutes per minute with air bearings
Although it can be used at rotation speeds of 000 or more, the devices that can be used are limited because the bearing is expensive. In particular, it cannot be used for inexpensive laser beam printers for general consumers.

On the other hand, when the number of faces of the rotary polygon mirror is increased,
The rotation angle per reflection surface becomes small. Further, if it is attempted to secure the size of each reflecting surface to a certain value or more, the diameter of the rotary polygon mirror becomes large.

[0007] In an optical scanning device, a light beam is often imaged on a surface to be scanned, but when scanning with a laser beam, in order to form an image on a small spot, it is necessary to adjust the spread angle of the light beam. The reflecting surface of the rotary polygon mirror needs to have a certain size in the main scanning direction. However, when the number of surfaces of the rotating polygon mirror is increased, the scanning angle of the light beam becomes smaller because the rotation angle on one reflecting surface is small. If the scanning angle of the light beam is small, the focal length of the scanning optical system becomes longer in order to obtain a predetermined scanning width, and the optical path length from the rotary polygon mirror to the surface to be scanned also becomes longer. For this reason, the diameter of the light beam on the reflecting surface of the rotating polygon mirror in the main scanning direction also increases, and the reflecting surface becomes larger than when the number of surfaces is small, and the size of the rotating polygon mirror further increases. .

In other words, since the required size of the reflecting surface becomes larger as the number of surfaces of the rotary polygon mirror increases as compared with the case where the number of surfaces is small, the size of the rotary polygon mirror (the Once the size of the tangent cylinder is determined, the upper limit of the number of faces is determined. For example, in an optical scanning device used for a laser beam printer, the required scanning width is 350 mm, the wavelength is 780 nm, the radius of the inscribed cylinder of the rotating polygon mirror is 25 mm,
When the spot diameter in the main scanning direction on the surface to be scanned is set to 50 μm or less, the upper limit of the number of surfaces is approximately seven.

Therefore, when the diameter of the rotary polygon mirror is increased to increase the number of surfaces, the weight and the second moment of inertia of the rotary polygon mirror increase, and the air resistance (windage) accompanying rotation increases. The number of revolutions is limited low.

As described above, since the number of surfaces of the rotating polygon mirror and the number of rotations have upper limits, various optical scanning devices have been devised in order to obtain a scanning speed exceeding the upper limit.

For example, in the technique described in Japanese Patent Application Laid-Open No. 51-100742, a scanning speed is improved by using a semiconductor laser array as a light source and simultaneously scanning the surface to be scanned with a plurality of laser beams. According to this method,
The scanning speed can be increased by the number of lasers integrated in the element without increasing the number of rotations of the rotating polygon mirror.

On the other hand, in Japanese Patent Application Laid-Open No. 51-32340, a light beam emitted from a light source is incident on a rotary polygon mirror with a very small diameter in a main scanning direction, and a deflected light beam is transmitted to a transmission optical system. A method of re-entering the rotating polygon mirror via the system is disclosed. That is, the light beam is made to enter the rotating polygon mirror twice.

In the latter method, when the light beam first enters the rotary polygon mirror, the diameter of the light beam in the main scanning direction is extremely small as compared with the case of entering the second time, and the second time the light beam is rotated. The transmission optical system is configured so that the light beam incident on the polygon mirror follows the center point of the rotating reflecting surface in the main scanning direction.

With this configuration, when the light beam first enters the rotary polygon mirror, the diameter of the light beam can be extremely reduced, so that scanning can be performed to the full division angle of the rotary polygon mirror. When the light beam deflected by the first reflecting surface enters the rotary polygon mirror for the second time via the transmission optical system, the diameter of the light beam is necessary to obtain a predetermined spot on the surface to be scanned. Although the size of the light beam is enlarged, the size of the light beam can be set irrespective of the rotation angle of the rotary polygon mirror because it follows the rotation of the reflection surface.

On the other hand, in an optical scanning apparatus, a light beam is incident at an angle with respect to a scanning plane perpendicular to the rotation axis of a rotary polygon mirror to deflect the light beam.
No. 2 and the like.

In the present specification, the two times incidence on the different reflecting surfaces of the rotary polygon mirror as described above will be referred to as "double incidence". Further, letting the light beam enter the scanning plane of the rotating polygon mirror at an angle will be referred to as “oblique incidence”.

[0017]

In the case of deflecting a rotating polygonal mirror by making such an oblique incidence, the position of the reflecting surface with respect to the rotation axis of the rotating polygonal mirror is shifted. The position of the scanning line fluctuates in the sub-scanning direction perpendicular to the direction of the scanning line, causing a problem that the image becomes uneven and it is difficult to reproduce an accurate and good image.

The present invention has been made in view of such a point of the prior art, and an object of the present invention is to eliminate the positional fluctuation of a scanning line caused by the positional deviation of each reflecting surface due to the eccentricity of the rotation axis of a rotary polygon mirror. An object of the present invention is to provide a high-speed optical scanning device that prevents oblique incidence at twice incidence.

[0019]

According to the present invention, there is provided an optical scanning apparatus comprising: a light source for generating a light beam; a rotary polygon mirror having a plurality of reflecting surfaces for reflecting and deflecting the light beam from the light source; A transmission optical system for transmitting the light beam reflected and deflected by the first reflection surface of the rotary polygon mirror to a second reflection surface of the rotary polygon mirror; and a transmission optical system reflected and deflected by the second reflection surface of the rotary polygon mirror. A scanning optical system that scans the scanned light beam by forming a beam spot on the surface to be scanned, wherein the rotating polygon mirrors are parallel to each other across a rotation axis and form an angle of 180 ° with each other. A plurality of pairs of reflecting surfaces facing each other, wherein the first reflecting surface and the second reflecting surface are parallel to each other with a rotation axis of the rotary polygon mirror facing each other at an angle of 180 °; Set on a reflective surface, said The light beam from the light source is incident on the first reflection surface at an angle in the sub-scanning direction, and the light beam transmitted by the transmission optical system is incident on the second reflection surface at an angle in the sub-scanning direction. And the transmission optical system has a substantially conjugate relationship between the first reflection surface and the second reflection surface in the sub-scanning direction, and the scanning optical system has the second reflection surface in the sub-scanning direction. The surface and the surface to be scanned are substantially conjugated.

In this case, the angles of incidence of the light beam on the first and second reflecting surfaces in the sub-scanning direction are α ₁ and α ₂ , respectively, the magnification of the transmission optical system in the sub-scanning direction is β _t , and the scanning optical system. The magnification in the sub-scanning direction is β _s , the maximum value of the positional deviation of the first reflecting surface and the second reflecting surface with respect to the rotation axis of the rotary polygon mirror is δ, and the interval between the scanning lines in the sub-scanning direction on the scanned surface is when the p, .delta..beta _s | it is desirable to satisfy the / p ≦ 1/8 ··· ( 2) | α 1 β t -α 2.

Alternatively, when the incident angles of the light beam on the first reflecting surface and the second reflecting surface in the sub-scanning direction are α ₁ and α ₂ , and the magnification of the transmission optical system in the sub-scanning direction is β _t , β It is desirable that _t = α ₂ / α ₁ (1) is satisfied.

Also, the optical axis of the optical system that causes the light beam from the light source to enter the first reflecting surface, the optical axis of the transmission optical system, and the optical axis of the scanning optical system are common sub-axes including the rotation axis of the rotary polygon mirror. It may be arranged in the scanning plane.

According to the present invention, in the optical scanning device which is twice incident on the rotary polygon mirror, the first reflection surface and the second reflection surface are parallel to each other with the rotation axis interposed therebetween and face each other at an angle of 180 °. Since the first reflecting surface, the second reflecting surface, and the surface to be scanned are substantially conjugated with each other, the light beams are offset from each other by the reflecting surfaces. The position of the scanning line in the sub-scanning direction due to the position deviation of each reflecting surface due to the eccentricity of the rotation axis of the rotating polygon mirror is corrected, the position of the scanning line becomes constant, and the image becomes uneven. It is possible to reproduce an accurate and good image without the need.

[0024]

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 of 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,
A plane that includes the optical axis of the optical system at that position and that is parallel to the rotation axis 41 of the rotary polygon mirror 4 that is a deflector is defined as a sub-scanning plane,
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 passes through a first shaping lens 2 and a second shaping lens 3 and is shaped, and a first beam of a rotary polygon mirror 4 as a deflector is formed.
The light enters the 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, the second transmission lens 8, and the third transmission lens 9 and is reflected by the first transmission mirror 10, and is reflected by the fourth transmission lens 11, The light passes through the fifth transmission lens 12 and is reflected by the second transmission mirror 13, re-enters the second reflection surface 6 of the rotary polygon mirror 4, and is deflected a second time. Also at this time, since the light beam is incident on the second reflecting surface 6 at an angle with respect to the 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, as will be described in detail later, it is possible to prevent the scanning line from fluctuating in the sub-scanning direction due to the eccentricity of the rotary shaft 41 of the rotary 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. A light beam emitted from a semiconductor laser 1 having a cover glass is converted into a parallel light beam by a first shaping lens 2 constituting an aspherical collimator lens. The second shaping lens 2 is a positive cylindrical lens having a positive refractive power only in the sub-scanning direction. Therefore, the light beam transmitted through the second shaping lens 2 is incident on the first reflection surface 5 as a parallel light beam on the main scanning surface, and forms an image (convergence) near the first reflection 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. Each of the first transmission lens 7, the second transmission lens 8, and the third transmission lens 9 is a cylindrical lens having a refractive power only in the main scanning direction, and the first transmission lens 7 and the second transmission lens 8 are positive cylindrical lenses. The third transmission lens 9 is a negative cylindrical lens, and these three lenses form a main scanning direction positive refractive power transmission lens group 24. The fourth transmission lens 11 is a positive cylindrical lens having a positive refractive power only in the sub-scanning direction, and the fifth transmission lens 12 is a spherical lens having a positive refractive power. In these operations, the light beam reflected by the first reflection surface 5 is once formed on the main scanning surface by the main scanning direction positive refractive power transmission lens group 24. The image-side focal point of the transmission lens group 24 and the object-side focal point of the fifth transmission lens 12 coincide, and constitute an afocal optical system on the main scanning plane. Therefore, the light beam is converted again into a parallel light beam by the fifth transmission lens 12 and enters the second reflection surface 6. On the sub-scanning surface, the first reflection surface 5 and the second reflection surface 6 are in a conjugate relationship due to the combined positive refracting power of the fourth transmission lens 11 and the fifth transmission lens 12, and the vicinity of the first reflection surface 5 Is formed again in the vicinity of 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 entrance surface of the third scanning lens 16 has a concave shape with a large radius of curvature in the main scanning direction and 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 has a convex non-arc shape having a large radius of curvature in the main scanning direction, and has a linear cross section in the sub-scanning direction and has no refractive power. Scanning optical system 23 having such a configuration
Makes the second reflection surface 6 and the surface to be scanned 17 conjugate in the sub-scanning surface, and forms a convergence point near the second reflection surface 6 on the surface to be scanned 17. On the main scanning surface, the parallel light beam reflected from the second reflecting surface 6 forms an image on the surface 17 to be scanned.

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. The main scanning direction positive refractive power transmitting lens group 24 composed of the first transmitting lens 7, the second transmitting lens 8, and the third transmitting lens 9 is simplified and shown as a single lens. The fourth transmission lens 11 has no refractive power in the main scanning direction, and 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 FIGS. 1 to 4 and the like, the optical path of the transmission optical system 22 is
The light 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 transmission lens group 24 is f ₁ and the focal length of the fifth 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 divided by f ₁ . Is equal to

As shown in FIG. 8B, when the rotary 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 transmission lens group 24 and the fifth 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 is deflected to the side where the incident angle is increased by an angle θ _{2 with} respect to the second reflection surface 6, so that the light beam reflected by the second reflection surface 6 is scanned. The angle θ _s is represented as θ _s = 2θ ₁ + θ ₂ .

Since the transmission optical system 22 of this embodiment is an afocal optical system on the main scanning plane, its 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 In} this embodiment, the optical magnification β is set to 1 <β <20.

The optical scanning device in which the light beam is deflected twice by the rotary polygon mirror 4 as in this embodiment can increase the scanning speed compared to the conventional optical scanning device in which the light beam is deflected only once. it can. This will be described below.

In a conventional optical scanning device that deflects only once, the reflection surface moves when the rotary polygon mirror rotates, so
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 that deflects only once, the optical scanning device that deflects twice in the present embodiment has the 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 specific examples of numerical values of the optical scanning device thus configured. 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. In addition,
The unit of the length is mm.

[0042] 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

Second shaping lens 2 and third scanning lens 16
The expression representing the aspherical surface of is given by z _i = (y ² / r _i ) / [1+ ｛1− (K _i +1) (y
/ R _i) ^{2} 1/2]} a ^{_{+ A i y 4 + B i}} y 6 + C i y 8, showing the aspherical coefficients in the following Table 2.

[0044] Note) S ₄ : Aspherical surface coefficient of surface number 4, S _26y : Aspherical surface coefficient of surface number 26 in the main scanning direction.

In this specific example, the third scanning lens 16
Of the incident surface S ₂₅ is a toric surface formed by an arc of r _25y = -1475.39378 rotated at r _25x = 37.95675. The second scanning lens 15 and the third
When the optical path is refracted, such as when passing through the scanning lens 16, the optical axis is refracted in the same manner as the principal ray. , Always coincides with the principal ray of the beam that scans the scanning center.

The number of faces of the rotary polygon mirror 4 is 12, and the diameter of the inscribed circle is 38.64 mm.
The incident angle of the light beam on the reflecting surface 5 and the second reflecting surface 6 in the sub-scanning direction is 6 °, and the incident angle of the light beam on the first transmitting mirror 10 and the second transmitting mirror 13 in the sub-scanning direction. Is 3 °. Also, the exit surface S ₂₄ of the second scanning lens 15
Is inclined by 13 ° in the sub-scan section, and the incident surface S ₂₅ of the third scanning lens 16 is 8.7503 in the sub-scan section.
The exit surface S ₂₆ of the third scanning lens 16 is inclined by 2.875374 ° in the sub-scan section. See FIGS. 2 and 4 for the directions of these inclination angles.

Further, in accordance with the first shaping lens incident surface S ₃ , the main scanning direction is 0.7154 mm, and the sub-scanning direction is 1.05 mm.
A 26 mm rectangular aperture 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, the second reflecting surface 6, and the scanned surface 17 of the rotating polygon mirror 4 are all in a conjugate relationship with each other.
The tilting of the rotary polygon mirror 4 is corrected. Therefore, it can be said that the light emitting point 1 and the scanned surface 17 are out of the conjugate relationship. However, the position where the light beam is minimized is shifted from the geometrical optical imaging point due to the influence of diffraction, and the scanned surface 17 is arranged at the position where the light beam is substantially minimized.

The optical magnification β in the main scanning direction of the transmission optical system 22 in the above specific example is 8.24, the optical magnification β _t in the sub scanning direction is 1.12, and the optical magnification of the scanning optical system 23 in the sub scanning direction. β _s is 0.406.

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 of 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 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 section 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 is incident on a portion where the shape error of the third scanning lens 16 is convex, the imaging position in the sub-scanning direction is shifted forward from the designed position, but the entire scanning region , 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, here, the rotating shaft 41 of the rotating polygon mirror 4 will be described.
Consider the displacement of the reflecting surface with respect to. The following two points can be cited as causes of the displacement of the reflecting surfaces 5 and 6 with respect to the rotation axis 41 of the rotating polygon mirror 4. One is the eccentricity between the center axis 41 of the rotary polygon mirror 4 and the rotation axis of the motor.
One is due to a manufacturing error when cutting each reflecting surface of the rotary polygon mirror 4.

First, the former will be described. As shown in FIG. 12, the rotary polygon mirror deflector 25 is composed of a rotary polygon mirror 4 attached to a rotating shaft of a motor 26. As shown schematically in FIG. The axis 41 ′ and the center 28 of the rotating shaft 27 of the motor 26 are eccentric by δ due to a manufacturing error, and the position of each reflecting surface of the rotary polygon mirror 4 with respect to the rotating shaft 27 is, as shown in FIG.
It fluctuates sinusoidally in one cycle per rotation. The number of surfaces of the rotary polygon mirror 4 in the embodiment is 12 and the center axis 4 of the rotary polygon mirror 4
The difference between 1 ′ and the rotating shaft 27 of the motor 26 is usually several tens μm.
m.

In the latter case, the displacement of each reflecting surface of the rotary polygon mirror 4 has no regularity and occurs randomly. However, the latter position shift is smaller than the former position shift,
The displacement due to the former cause is dominant.

Here, the influence on the scanning line due to the displacement of the reflection surface of the rotary polygon mirror 4 will be described. Rotating polygon mirror 4
Reflection surface 6 when incident on the second reflection surface 6 only once
15, when the light is incident on the reflecting surface 6 of the rotating polygon mirror 4 at an incident angle α in the sub-scanning direction (oblique incidence) as shown in the optical path in the sub-scanning direction in FIG. If the displacement of the reflection surface 6 with respect to the rotation axis 41 is δ, the displacement d of the reflected beam is d = 2αδ. However, since α is small, it is close to tan α ≒ α. When the sub-scanning direction scaling ratio of the scanning optical system 23 and beta _s, positional deviation e in the sub-scanning direction of the beam on the scanned surface 17 becomes e = 2αδβ _s.

Therefore, in the present invention, as in the above embodiment, the light is made to enter the two reflecting surfaces 5 and 6 of the rotary polygon mirror 4 twice, and the first reflecting surface 5 and the second reflecting surface are used. 6
Are arranged so as to be parallel to each other and to face each other at an angle of 180 ° so as to eliminate the displacement of the scanning line in the sub-scanning direction due to the eccentricity of the rotating shaft 41 of the rotary polygon mirror 4. . The principle will be described with reference to FIG.
FIG. 16 shows the optical path of the principal ray in the sub scanning direction near the rotary polygon mirror 4 of the optical scanning device according to the present invention. As shown in FIG.
The light beam from the light source first enters the first reflecting surface 5 of the rotary polygon mirror 4 for the first time, and the light beam deflected for the first time is transmitted by the transmission optical system 22 to be reflected by the first reflection surface. The second reflection is made on the second reflection surface 6 of the rotary polygon mirror 4 facing the surface 5 to be deflected a second time, and the deflected beam is imaged on the surface 17 to be scanned by the scanning optical system 23 and scanned. You. Each of the light beams incident on the first reflection surface 5 and the second reflection surface 6 is set so as to enter at an angle in the sub-scanning direction. With such an arrangement, FIG.
As is clear from FIG. 6, when the rotation axis 41 of the rotary polygon mirror 4 is eccentric, the shift direction δ between the first reflection surface 5 and the second reflection surface 6 is the same direction, and the optical path changes from a solid line to a broken line in the figure. I do. When the first reflecting surface 5 and the second reflecting surface 6 are shifted to the left as shown in the figure, the optical path immediately after the reflection on the first reflecting surface 5 is shifted downward, but due to the image forming action of the transmission optical system 22. Immediately before the second reflection surface 6, the optical path is shifted upward. However, since the light is obliquely incident on the second reflection surface 6 from above, the principal ray after reflection passes through substantially the same position as the reflection point when there is no displacement (solid line). Although the angle of the beam reflected by the second reflecting surface 6 is different, the second reflecting surface 6 and the scanned surface 17 are different.
Are in a conjugate relationship by the scanning optical system 23, and therefore have substantially the same position on the scanned surface 17 as in the case where there is no displacement. This effect is clear when FIG. 16 and FIG. 15 are compared.
The principle of the present invention is that, even if the rotation axis 41 of the rotary polygon mirror 4 is eccentric, the shift direction of the first reflecting surface 5 and the second reflecting surface 6 is the same direction. 5,6
It can be said that the arrangements cancel each other out.

Although the above is a qualitative explanation, a quantitative explanation will be given below. As shown in FIG. 17, the first reflection surface 5
Assuming that the angle of incidence on the _first reflecting surface 6 is α ₁ , the angle of incidence on the second reflecting surface 6 is α ₂ , and the magnification of the transmission optical system 22 in the sub-scanning direction is β _t ,
When the displacement of the reflecting surface 5 and the second reflecting surface 6 due to the eccentricity of the rotation axis 41 of the rotary polygon mirror 4 is δ, the displacement d ₁ of the light beam reflected by the first reflecting surface 5 is d ₁ = 2α ₁ δ. However, since α ₁ is small, tan α ₁ ≒ α ₁
Is approximated. α ₁ of radians. The position shift d ₁ is equal to 2 on the second reflection surface 6 by the transmission optical system 22.
α ₁ δβ _t . Further, the displacement d ₂ of the light beam reflected by the second reflection surface 6 caused by the displacement of the second reflection surface 6 is d ₂ = 2α ₂ δ. However, since α ₂ is small, tan α ₂ ≒ α ₂
Is approximated. The unit of α ₂ is radian. In consideration of the displacement of both surfaces of the first reflecting surface 5 and the second reflecting surface 6, the second
Positional deviation d of a light beam reflected by the reflecting surface _{6, d = | 2α 1 δβ t} -2α 2 δ | = 2δ | α 1 β t -α 2 | become. Assuming that the magnification in the sub-scanning direction of the scanning optical system 23 is β _s , the displacement e of the light beam on the surface 17 to be scanned is e = 2δβ _s | α ₁ β _t −α ₂ |. Therefore, if β _t = α ₂ / α ₁ (1), the position of the scanning line in the sub-scanning direction due to the displacement of the reflecting surfaces 5 and 6 based on the eccentricity of the rotating shaft 41 of the rotary polygon mirror 4. The displacement is corrected, and the position of the scanning line becomes constant.

Here, apart from the expression (1), the allowable amount of e is １ ／ of the interval p between the scanning lines in the sub-scanning direction on the surface 17 to be scanned. The uneven exposure amount on the scanning surface 17 poses a problem. _{Therefore, 2δβ s | α 1 β t} -α 2 | ≦ p / 4 That _{is, δβ s | α 1 β t} -α 2 | / p ≦ 1/8 satisfy the conditions is required of (2) is there.

In the above specific numerical example, β _t =
1.12, β _s = 0.406, α ₁ = α ₂ = 0.105
Radian (= 6 °), p = 0.0423 mm (600d
a pi equivalent), the maximum value of δ is a _{0.03mm, δβ s | α 1 β} t -α 2 | /p=0.0036 next, which satisfies the expression (2). FIG. 18 shows the displacement of the scanning line in this case. The Y direction in this figure is the main scanning direction,
The X direction indicates the sub-scanning direction. The position shift of the scanning line due to the position shift of only the first reflecting surface 5 is indicated by a broken line, and the second reflecting surface 6
The position shift of the scanning line due to only the position shift is indicated by a chain line, and the position shift of the scanning line due to the position shift of both surfaces of the first reflection surface 5 and the second reflection surface 6 is indicated by a solid line. It can be seen that the present invention cancels out the influence of the displacement of the first reflecting surface 5 and the second reflecting surface 6 and satisfactorily corrects the displacement of the scanning line.

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 can be made.

[0061]

As is clear from the above description, according to the optical scanning device of the present invention, in the optical scanning device which is twice incident on the rotary polygon mirror, the first reflecting surface and the second reflecting surface are rotated by the rotation axis. Are set parallel to each other with a 180 ° angle to each other, and are obliquely incident on both reflecting surfaces, so that the first reflecting surface, the second reflecting surface, and the surface to be scanned are substantially conjugated. Therefore, the displacement of the light beam in the sub-scanning direction due to the displacement of each reflection surface due to the eccentricity of the rotation axis of the rotary polygon mirror is offset by offsetting the light beam between the two reflection surfaces. Corrected, the position of the scanning line becomes constant, and an accurate and good image can be reproduced without causing unevenness in the image.

[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 field curvature does not occur in one specific example of the present invention.

FIG. 12 is a perspective view showing an example of a rotary polygon mirror deflector used in the optical scanning device of the present invention.

FIG. 13 is a diagram schematically showing a state in which the center axis of the rotating polygon mirror and the center of the rotating shaft of the motor are eccentric due to a manufacturing error.

FIG. 14 is a diagram illustrating a state in which the position of each reflection surface of the rotary polygon mirror with respect to the rotation axis varies sinusoidally in one cycle of one rotation.

FIG. 15 is an optical path diagram in the sub-scanning direction showing a displacement of a beam when the beam is incident on the rotary polygon mirror only once.

FIG. 16 is an optical path diagram in the sub-scanning direction showing that beam misalignment does not occur in the optical scanning device of the present invention.

FIG. 17 is a diagram showing parameters in the optical scanning device of the present invention.

FIG. 18 is a diagram showing a displacement of a scanning line according to one embodiment of the present invention.

[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 8 Second transmission lens 9 Third transmission lens 10 First transmission mirror 11 Fourth transmission lens 12 Fifth transmission lens 13 Second transmission mirror 14 First scanning lens 15 Second scanning lens (prism) 16: Third scanning lens (long lens) 17: Scanned surface 21: Shaping optical system 22: Transmission optical system 23: Scanning optical system 24: Main scanning direction positive refractive power transmission lens group 25: Rotating polygon mirror deflector 26 ... motor 27 ... motor rotation axis 28 ... motor rotation axis center 41 ... rotation polygon mirror rotation axis 41 '... rotation polygon mirror center axis

Continuation of the front page (72) Inventor Yujiro Nomura 3-5-5 Yamato, Suwa-shi, Nagano Seiko Epson Corporation

Claims (4)

[Claims] 1. A light source for generating a light beam, a rotating polygon mirror having a plurality of reflecting surfaces for reflecting and deflecting the light beam from the light source, and a light beam reflected and deflected by a first reflecting surface of the rotating polygon mirror Optical system for transmitting light to the second reflecting surface of the rotating polygon mirror, and scanning the light beam reflected and deflected by the second reflecting surface of the rotating polygon mirror by forming a beam spot on the surface to be scanned. An optical scanning device comprising a scanning optical system, wherein the rotary polygon mirror is parallel to each other with a rotation axis interposed therebetween, and
A plurality of pairs of reflecting surfaces facing each other at an angle of 80 °, wherein the first reflecting surface and the second reflecting surface are parallel to each other with a rotation axis of the rotating polygon mirror being 180 ° from each other; The light beam from the light source is incident on the first reflection surface at an angle in the sub-scanning direction, and the light beam transmitted by the transmission optical system is The two reflection surfaces are arranged so as to be incident on the reflection surface at an angle in the sub-scanning direction, and the transmission optical system has the first reflection surface and the second reflection surface in a substantially conjugate relationship in the sub-scanning direction. An optical scanning device, wherein the scanning optical system has a substantially conjugate relationship between the second reflection surface and the surface to be scanned in the sub-scanning direction. 2. The angle of incidence of the light beam on the first reflecting surface and the second reflecting surface in the sub-scanning direction is α ₁ and α ₂ , respectively, the magnification of the transmission optical system in the sub-scanning direction is β _t , The magnification in the sub-scanning direction of the optical system is β _s , the maximum value of the positional deviation of the first reflecting surface and the second reflecting surface with respect to the rotation axis of the rotary polygon mirror is δ, Scan line spacing is p
When _{the, δβ s | α 1 β t} -α 2 | / p ≦ 1/8 ··· (2) The optical scanning apparatus according to claim 1, wherein satisfying the. 3. The angle of incidence of the light beam on the first reflecting surface and the second reflecting surface in the sub-scanning direction is α ₁ and α ₂ , respectively, and the magnification of the transmission optical system in the sub-scanning direction is β _t. 3. The optical scanning device according to claim 2, wherein the following condition is satisfied: β _t = α ₂ / α ₁ (1) 4. An optical axis of an optical system for causing a light beam from the light source to enter the first reflection surface, an optical axis of the transmission optical system,
The optical scanning device according to claim 1, wherein an optical axis of the scanning optical system is arranged in a common sub-scanning plane including a rotation axis of the rotary polygon mirror.