JP3381333B2 - Optical scanning device - Google Patents

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.

[0002]

2. Description of the Related Art A conventional optical scanning device used in a laser beam printer or the like emits light from a light source such as a semiconductor laser, deflects and scans a beam collimated by a collimator lens by a rotary polygon mirror, and fθ lens. Thus, a beam spot is formed on the surface to be scanned. Since the angular velocity of the rotating polygon mirror is constant, in order to keep the scanning speed on the surface to be scanned constant, the fθ lens is given a negative distortion aberration characteristic,
Realized uniform scanning.

Incidentally, the aberration characteristics required for the fθ lens are the following two points. One is to give a specific negative distortion aberration in order to obtain uniform velocity scanning, and the other is to make the spot diameter close to the diffraction limit and to obtain the flatness of the image plane. To reduce the surface curvature. In order to generate the negative distortion, geometrically optics, a positive lens may be arranged behind the entrance pupil, or a negative lens may be arranged in front of the entrance pupil. In the conventional optical scanning device,
The fθ lens having a positive power is arranged rearward from the entrance pupil, that is, the deflection point to generate negative distortion. To obtain better lens performance, it is desirable that the number of fθ lenses is large. However, if the number of sheets increases, the cost increases, the adjustment becomes complicated, and the beam intensity decreases. Also, if an aspherical lens is used, the degree of freedom in aberration correction is increased, and it is possible to reduce the number of lenses.
Even if a plastic is formed by molding using a mold, it requires high-level technology to form an aspherical shape, which is expensive and not very practical.

As an fθ lens used in a conventional optical scanning device, there is, for example, a single-lens fθ lens disclosed in Japanese Patent Laid-Open No. 58-5706. Also, f of the two-sheet configuration
As the θ lens, there is one disclosed in JP-A-53-137631.

[0005]

However, in the above-described conventional optical scanning device, the fθ lens is arranged far away from the entrance pupil, that is, the deflection point, in order to impart a negative distortion to the fθ lens. However, the fθ lens has a large aperture and is expensive. Therefore, there is a problem that the device becomes large and the cost becomes high.

Further, particularly in a single-lens fθ lens, the aberration characteristics are poor because the degree of freedom of the lens surface for aberration correction is small, and in particular, the Petzval sum, which represents field curvature aberration, inevitably becomes a positive value. The curvature aberration could not be removed. Therefore, there is also a problem that the scanning angle cannot be made so large that the optical path length becomes large and the apparatus becomes large in size.

In contrast to a single-lens fθ lens, when the number of constituent fθ lenses is plural, the aberration characteristic becomes good. Particularly, by combining a positive lens and a negative lens, the Petzval sum is set to 0, and the field curvature is increased. Although the characteristics could be improved, there was also a problem in that a plurality of expensive lenses having a large aperture are required.

The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide an optical scanning device which simultaneously realizes downsizing and cost reduction.

[0009]

The optical scanning device of the present invention comprises:
An optical scanning device having a light source for generating a light beam, a deflecting means for deflecting the light beam, and an imaging optical system for forming an image of the light beam deflected by the deflecting means on a surface to be scanned, the deflecting means. It is characterized in that it has an optical system which rotates together with it, the optical system has a negative power at least in the main scanning direction, and the optical system is at a position where an incident beam to the deflecting means passes.

Further, the optical scanning device of the present invention is characterized by being provided with any one of the following in addition to the above configuration.

1) The optical system is composed of a single lens.

2) The deflecting means has one or two reflecting surfaces, and has the same number of optical systems as the reflecting surfaces.

3) The image forming optical system has a second optical system which rotates together with the deflecting means, and the second optical system is at a position where the beam reflected by the deflecting means passes.

4) The optical system has different powers in the main scanning direction and the sub scanning direction.

5) The optical system has no power in the sub-scanning direction.

6) The optical system has a cylindrical surface.

7) The deflecting means has a plurality of reflecting surfaces and has the same number of optical systems as the reflecting surfaces, and the optical systems are arranged at equal angular intervals with respect to the center of rotation of the deflecting means.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to the drawings.

1 to 4 show an optical scanning device as a typical embodiment of the optical scanning device of the present invention. 1 and 2, the beam emitted from the semiconductor laser 1 which is the light source is collimated by the collimator lens 2. A rotating lens 3 as an optical system that rotates,
The deflecting mirror 4 as a deflecting means is arranged in the rotating portion of the motor 5, and both of them rotate at a constant speed as the motor 5 rotates. The parallel beam emitted from the collimator lens 2 is made into a divergent beam by the rotating lens 3 having negative power, reflected by the deflecting mirror 4, and deflected by the rotation of the deflecting mirror 4. The deflected beam is focused by the image forming lens 6 which is an image forming optical system to form a beam spot on the surface 7 to be scanned. The direction orthogonal to the optical axis and perpendicular to the rotation axis of the deflecting mirror 4 is called the main scanning direction, and the direction orthogonal to the optical axis and perpendicular to the main scanning direction is called the sub-scanning direction.

FIG. 3 schematically shows how the beam is deflected as the rotary lens 3 and the deflecting mirror 4 rotate. The rotating lens 3 and the deflecting mirror 4 rotate about a rotation center O in the reflecting surface R of the deflecting mirror 4 and are displaced as indicated by I, II, and III. The incident beam L passes through different positions of the rotary lens 3 as the rotary lens 3 rotates, and thus receives the different refracting power and is deflected. The beam is further deflecting mirror 4
Is reflected by the reflection surface R of the laser beam, the deflection angle thereof is further increased, and the beams are deflected like the emission beams M ₁ , M ₂ , and M ₃ . The deflected beam is focused by the imaging lens 6.

FIG. 4 shows a main scanning section of the optical system of this embodiment.
FIG. 6 is a schematic diagram developed by showing a reflecting surface R of a deflecting mirror 4. 4A is a diagram when the beam spot starts scanning the surface 7 to be scanned, and FIG. 4B is a diagram when the beam spot scans the center of the surface 7 to be scanned. The entrance pupil P exists near the reflecting surface. The principle of aberration correction will be described with reference to these drawings.

As can be seen from FIG. 4, the optical system of this embodiment differs from the optical system of the conventional optical scanning device in that the entrance pupil P
There is an optical system (rotating lens 3) on the front side, and according to the incident angle of the beam B, the rotating lens 3 has a half of the angle.
It is equivalent to rotating only. Rotating lens 3
Is in front of the entrance pupil P, so that negative distortion can be generated by providing the rotating lens 3 with negative power in the main scanning direction. Further, since the imaging lens 6 having a positive power is behind the entrance pupil P, it also produces a negative distortion. As described above, since the rotation lens 3 and the imaging lens 6 share the amount of negative distortion generated, the amount of negative distortion generated in the imaging lens 6 may be small. Therefore, the imaging lens 6 can be brought close to the entrance pupil P, and the diameter can be reduced. The fact that the amount of negative distortion aberration of the imaging lens 6 can be reduced leads to a reduction in the refractive index of the imaging lens 6.

Further, the rotating lens 3 and the imaging lens 6 are
Since each has a negative power and a positive power, the field curvature characteristics are improved by combining these.

Further, the rotating lens 3 rotates in the same direction as the rotating direction of the beam deflected by the deflecting mirror 4,
Since the rotation angle is only 1/2 of the deflection angle formed by the deflected beam and the optical axis, the incident angle is small and the effective diameter is small, and the rotating lens 3 can be made extremely small. .

As described above, by using the rotating lens 3, the diameter of the imaging lens 6 can be reduced, and the diameter is smaller than that of the fθ lens used in the conventional optical scanning device. Further, when comparing the optical system of the present embodiment, which is composed of the two lenses of the rotating lens 3 and the imaging lens 6, with the fθ lens of the conventional two-element structure, the aberration characteristics are almost the same. However, in the present embodiment, the rotary lens 3 has an extremely small diameter as compared with the conventional technique using two large-diameter lenses.

In general, as the diameter of the lens increases, the amount of glass material used increases in proportion to the square to the cube of the glass material, and the processing time also increases, resulting in a dramatic increase in cost and the lens surface. The surface accuracy of is deteriorated. In the present invention, since the diameter of the lens used can be reduced as described above, the cost reduction and the surface accuracy improvement are remarkable. Therefore, a compact, high-precision and low-cost optical scanning device can be realized.

Further, since the rotary lens 3 has a small diameter and a thin lens thickness, the moment of inertia of the rotary lens 3 is small and the load on the motor 5 is also small. Since the rotating lens 3 is rotated at a high speed, it is desirable to make the ridge line arc or chamfer in order to reduce wind noise and wind loss.

The optical specifications of a typical design example of this embodiment are shown below. However, the rotation angle of the deflection mirror from the start of one scan to the end of the scan is 2ω. The entrance surface of the rotating lens,
The exit surfaces are S ₁ and S ₂ , the reflecting surface of the deflecting mirror is S ₃ , and the entrance and exit surfaces of the imaging lens are S ₄ and S ₅ , respectively.
Regarding the symbols of the respective optical specifications, the radius of curvature of the _i- th surface S _i is r _i , the axial distance from the i-th surface to the next surface is d _i, and the refractive indices of the rotating lens and the imaging lens are respectively. Let n ₁ and n ₂ .

[0029]

[Table 1]

FIG. 5 is a main-scan sectional view of this design example, and FIG. 6 is an aberration diagram of this design example. Regarding the aberration diagram, the broken line shows the aberration in the main scanning direction, and the solid line shows the aberration in the sub scanning direction. The scanning linearity is represented by% of the deviation of the image height from the ideal image height y = fθ in the usual case of the fθ lens, but in the present invention, since the rotating lens rotates, the ideal image height is f.
It does not become θ. Therefore, as an equivalent display method, for a light beam near the optical axis, the rate of change of the image height with respect to the rotation angle of the deflecting mirror is ζ, and the deviation from the ideal image height Y = ζθ is displayed in%. ω is the rotation angle of the deflecting mirror while the beam spot scans the surface to be scanned from the scanning center to the scanning end.
As shown in FIG. 6, the field curvature is 2.5 mm at the maximum, and the scanning linearity is 1.1% at the maximum, which are well corrected.

In the present design example, each optical surface is a spherical surface, but if it is an aspherical surface or a toric surface, the degree of freedom in aberration correction is further increased and the optical characteristics are further improved.

Although the deflecting mirror 4 has been described as a single-sided mirror in the present embodiment, the same effect can be obtained even if it is a multi-sided mirror.
In this case, for example, as shown in FIG. 7, the rotating lenses 3 may be arranged in the rotating portion of the motor 5 in the same number as the reflecting surfaces R of the deflecting mirror 4. If the deflecting mirror is a polygonal mirror, scanning is performed a plurality of times every one rotation of the motor, so that the scanning speed as the number of times of scanning per unit time is further increased. Further, if the reflecting surface R and the rotating lens 3 are provided in plural, the deflecting mirror 4 can be simply arranged by arranging them at the same angle around the rotation center O.
The center of gravity of the plurality of rotating lenses can be made to coincide with the center of rotation O, balancing is facilitated, unbalance is reduced, the life of the motor is extended, and reliability is improved.

In order to reduce the load on the motor, it is desirable that the deflecting mirror be a single-sided mirror or a two-sided mirror. If it is a single-sided mirror or a two-sided mirror, the deflecting mirror can be made into a flat plate shape, and even if the deflecting mirror is rotated, the point where the beam is reflected is hardly displaced, so that the area of the reflecting surface can be narrowed. Therefore, the moment of inertia of the deflecting mirror becomes extremely small. As described above, the moment of inertia of the rotating lens is small, but in order to take advantage of this advantage, it is particularly effective to use a deflecting mirror of a single-sided mirror or a two-sided mirror.

Next, the case where the imaging optical system is composed of a plurality of lenses will be described. In FIG. 8, the imaging optical system includes a second rotation lens 8 as a second optical system that rotates,
The imaging lens 9 is composed of two lenses. The second rotating lens 8 is arranged in the rotating portion of the motor 5,
With the rotation of the motor 5, the rotary lens 3 and the deflecting mirror 4 rotate at a constant speed. The beam collimated by the collimator lens 2 is diverged by the rotating lens 3 having a negative power, reflected by the deflecting mirror 4, and has a positive power.
One lens, that is, the second rotation lens 8 and the imaging lens 9
And a beam spot is formed on the surface 7 to be scanned.

If the image forming optical system is composed of a plurality of lenses in this way, the degree of freedom in aberration correction is further increased, and an optical scanning device having better optical characteristics can be obtained. In particular, as shown in FIG. 8, if a rotating lens (second rotating lens 8) is also provided on the side where the beam deflected by the deflecting mirror 4 passes, the aperture of the second rotating lens 8 also rotates. Since it is extremely small like the lens 3, the optical characteristics can be improved without increasing the cost so much and without increasing the load on the motor.

The optical specifications of a typical design example in the configuration shown in FIG. 8 are shown below. However, the entrance and exit surfaces of the turning lens are S ₁ and S ₂ , the reflection surface of the deflecting mirror is S ₃ , the entrance and exit surfaces of the second turning lens are S ₄ and S ₄ , respectively.
S ₅ , the entrance and exit surfaces of the imaging lens are S ₆ and S ₇ , respectively.
And the refractive indices of the rotary lens, the second rotary lens, and the imaging lens are n ₁ , n ₂ , and n ₃ , respectively.

[0037]

[Table 2]

FIG. 9 is a main scanning sectional view of this design example, and FIG. 10 is an aberration diagram of this design example.

In the conventional optical scanning device, when a polygon mirror is used as the deflecting mirror, the optical system is provided with a tilt correction function in order to eliminate the pitch unevenness of the scanning lines due to the surface tilt of each reflecting surface. Things have been done well. The optical system of the optical scanning device of the present invention can also have a tilt correction function.

A configuration having a tilt correction function will be described with reference to FIG. The beam collimated by the collimator lens 2 is focused by the cylindrical lens 10 only in the sub-scanning direction, passes through the rotary lens 3, and is reflected by the deflecting mirror 4 to form a linear image long in the main scanning direction. To form. The deflecting mirror 4 is a two-sided mirror, and two rotating lenses 3 are also arranged symmetrically with respect to the rotation center of the motor 5. Both surfaces of the imaging lens 6 are toric surfaces, and the reflecting surface of the deflecting mirror 4 and the surface to be scanned 7 are optically conjugate with each other in the sub-scanning direction. Therefore, even if each reflecting surface of the deflecting mirror 4 is tilted, the position of the beam spot on the surface to be scanned 7 in the sub-scanning direction does not change, and the pitch unevenness of the scanning lines does not occur.

The optical specifications of a typical design example in the configuration shown in FIG. 11 are shown below. However, the entrance and exit surfaces of the cylindrical lens are S ₁ and S ₂ , respectively, the entrance and exit surfaces of the rotating lens are S ₃ and S ₄ , respectively, the reflecting surface of the deflecting mirror is S ₅ , and the entrance surface of the imaging lens. , The exit surface is S ₆ , S
₇ , the refractive indices of the cylindrical lens, the rotating lens, and the imaging lens are n ₁ , n ₂ , and n ₃ , respectively. On the anamorphic optical surface, the radii of curvature in the sub-scanning direction and the radii of curvature in the main-scanning direction are r _ix and r _iy , respectively.

[0042]

[Table 3]

12A and 12B are views showing this design example. FIG. 12A is a main scanning sectional view and FIG. 12B is a reflection surface S.
A sub-scan sectional view of the developed respect _5. In addition, FIG.
[Fig. 3] is an aberration diagram of this design example.

In this design example, the entrance surface and the exit surface of the rotary lens are spherical surfaces, but they may be anamorphic optical surfaces. If the rotary lens is a lens having different powers in the main scanning direction and the sub-scanning direction, it is considered in combination with the cylindrical lens 10 in FIG. 11, and if the focal position in the sub-scanning direction by these two lenses does not change, A line image can always be formed on the reflecting surface, and the power distribution of the two lenses can be set arbitrarily. If the power of the rotary lens in the sub-scanning direction can be arbitrarily set, the degree of freedom of aberration correction in the sub-scanning direction is expanded, and the field curvature aberration characteristic in the sub-scanning direction is improved. Incidentally, as can be seen from the field curvature aberration diagram of FIG. 13 described above, the characteristics of the optical system of this design example in the sub-scanning direction limit the characteristics of the optical scanning device. Therefore, if the rotating lens is a lens having different powers in the main scanning direction and the sub-scanning direction, it is possible to obtain an optical scanning device having better optical characteristics.

When a plurality of rotating lenses are used, not only the reflecting surface but also the rotating lens may collapse. However, if the rotating lens is a lens having no power in the sub-scanning direction. Even if the two rotating lenses tilt relative to each other, the beam is not deflected in the sub-scanning direction.
The pitch unevenness of the scanning line due to the tilt of the rotary lens does not occur at all. Therefore, it is possible to configure a highly accurate surface tilt correction optical system. In this case, if the rotary lens is a cylindrical lens having a straight sub-scanning cross section, its manufacture will be easy.

In this embodiment, the description has been given only to the case where the deflecting mirror which is the deflecting means and the rotating lens which is the rotating optical system are separate bodies. Is composed of one member, that is,
For example, as shown in FIG. 14, in the case where the rotating lens 3 is provided with a reflecting surface R, the beam L enters the entrance surface I, is internally reflected by the reflecting surface R, and exits from the exit surface J. But it has the same effect.

In the present embodiment, the case where the deflecting mirror rotates at a constant speed has been described. However, for example, a galvano mirror that performs sinusoidal vibration around the rotation axis can be easily realized, and the same effect can be obtained. .

The present invention is applicable not only to a laser beam printer, but also to an image forming apparatus such as a digital copying machine, a facsimile and a laser scanning display, and an image input apparatus such as a scanner.
Alternatively, it can be applied to a laser scanning device for reading an optical mark, a laser scanning device for surface inspection, etc., and the above-described effects can be obtained.

[0049]

As described above, according to the present invention,
It is possible to correct aberrations with a rotating lens having an extremely small aperture, and also to reduce the aperture of the imaging lens, so that not only the surface accuracy of the optical system is improved,
It also enables downsizing and cost reduction. Therefore, an optical scanning device with high precision and good optical characteristics can be realized at a small size and a low price.

If the deflecting mirror is a polygonal mirror and the same number of rotating lenses as the reflecting surface are provided, not only the scanning speed can be increased, but also the balancing can be facilitated to improve the assemblability and the motor. Life is extended and reliability is improved. Alternatively, even if scanning is performed by a single-sided mirror or a two-sided mirror, the load on the motor is reduced, the life of the motor is extended, and the reliability is improved.

If the second rotating lens is provided at a position where the beam deflected by the deflecting mirror passes, the second rotating lens has an extremely small diameter like the rotating lens, which increases the cost too much. Optical characteristics can be improved without increasing the load on the motor.

If the rotary lens is an anamorphic lens having different powers in the main scanning direction and the sub-scanning direction, the aberration characteristic in the sub-scanning direction will be further improved. A rotating lens,
If the lens has no power in the sub-scanning direction, the effect of the tilt correction function becomes large. At that time, if the rotating lens is a cylindrical lens, the manufacturing is easy.

[Brief description of drawings]

FIG. 1 is a perspective view of an optical scanning device according to a first exemplary embodiment of the present invention.

FIG. 2 is a conceptual diagram of an optical system of an optical scanning device according to a first exemplary embodiment of the present invention.

FIG. 3 is a diagram showing how the beam is deflected as the rotary lens and the deflection mirror rotate in the present invention.

FIG. 4 is an optical path diagram of an optical system of an optical scanning device according to a typical first embodiment of the present invention.

FIG. 5 is a sectional view of an optical system according to a first embodiment of the present invention.

FIG. 6 is an aberration diagram of an optical system according to a representative example 1 of the present invention.

FIG. 7 is a cross-sectional view in which a large number of rotating lenses are arranged in the present invention.

FIG. 8 is a perspective view of an optical scanning device according to a second embodiment of the present invention.

FIG. 9 is a sectional view of an optical system according to a second embodiment of the present invention.

FIG. 10 is an aberration diagram of an optical system according to Example 2 of the present invention.

FIG. 11 is a perspective view of an optical scanning device according to a third embodiment of the invention.

FIG. 12 is a sectional view of an optical system according to Example 3 of the present invention.

FIG. 13 is an aberration diagram of an optical system according to Example 3 of the present invention.

FIG. 14 is a cross-sectional view in which the rotating lens and the deflecting mirror are formed by one member in the present invention.

[Explanation of symbols]

1 Semiconductor laser 2 Collimator lens 3 rotating lens 4 Deflection mirror 5 motor 6 Imaging lens 7 Scanned surface

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Nozomu Inoue No. 3-5 Yamato, Suwa City, Nagano Seiko Epson Corporation (58) Fields investigated (Int.Cl. ⁷ , DB name) G02B 26 / Ten

Claims (8)

(57) [Claims] 1. A light source for generating a light beam, a deflection means for deflecting the light beam, and an imaging optical system for forming an image of the light beam deflected by the deflection means on a surface to be scanned. In the optical scanning device, an optical system that rotates together with the deflecting unit is provided, the optical system has a negative power at least in the main scanning direction, and the optical system has a position where an incident beam to the deflecting unit passes. The optical scanning device according to claim 1. 2. The optical scanning device according to claim 1, wherein the optical system is composed of a single lens. 3. The optical scanning device according to claim 1, wherein the deflecting unit has one or two reflecting surfaces and has the same number of the optical systems as the reflecting surfaces. 4. The image forming optical system includes a second optical system that rotates together with the deflecting unit, and the second optical system is located at a position where the beam reflected by the deflecting unit passes. The optical scanning device according to claim 1. 5. The optical scanning device according to claim 1, wherein the optical system has different powers in the main scanning direction and the sub scanning direction. 6. The optical scanning device according to claim 5, wherein the optical system has no power in the sub-scanning direction. 7. The optical scanning device according to claim 6, wherein the optical system has a cylindrical surface. 8. The deflecting means has a plurality of reflecting surfaces, and has the same number of the optical systems as the reflecting surfaces, and the optical systems are arranged at equal angular intervals with respect to a rotation center of the deflecting means. The optical scanning device according to claim 1, wherein: