JPH09146030A - Optical scanner - Google Patents

Abstract

PROBLEM TO BE SOLVED: To correct the positional deviation of scanning lines by curving a second image forming lens toward the end parts thereof in the longitudinal direction from the center thereof in the longitudinal direction so that it gets away from a surface to be scanned. SOLUTION: Plural beams are emitted from a semiconductor laser array 1 and the images thereof are formed near the reflection surface 6 of a rotary polygon mirror 5 being a deflector in a sub-scanning direction. Besides, they are reflected on the reflection surface 6 and deflected in accordance with the rotation of the mirror 5. The deflected beams receive a converging action by a first image forming lens 7 and the second image forming lens 8 and form plural beam spots on the surface to be scanned 9. Then, the lens 8 is curved toward the end parts from the center in the longitudinal direction so as to be kept at a distance from the surface 9. Since optical magnification in the sub-scanning direction between the reflection surface 6 and the surface 9 is uniformized in an effective scanning area, the plural scanning lines are made to be the straight lines without being curved.

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 which is used in a laser beam printer or the like and which scans a surface to be scanned with a plurality of beams.

[0002]

2. Description of the Related Art Conventionally, an optical scanning device for scanning a surface to be scanned with a plurality of beams has been used in a laser beam printer or the like. A beam passing through a position distant from the optical axis of the image optical system in the sub-scanning direction, that is, a beam scanning a spherical surface offset from the optical axis of the imaging optical system in the sub-scanning direction is formed on the surface to be scanned. There is a problem in that the scanning line for scanning is curved.

In order to solve this problem, JP-A-60-
If an ultrasonic light deflector is provided between the laser light source and the rotating polygon mirror as in Japanese Patent No. 21031, the scanning line curve can be corrected by adjusting the Bragg diffraction angle.

Alternatively, as in Japanese Patent Laid-Open No. 2-54211, by setting the absolute value of the lateral magnification of the optical system from the light emitting portion to the surface to be scanned to 2 or less, the scan line curve can be reduced. it can.

[0005]

However, the ultrasonic optical deflector disclosed in Japanese Unexamined Patent Publication No. 60-21031 has problems that it is expensive, complicated and large in size, and the driving circuit is also complicated.

Further, according to Japanese Patent Application Laid-Open No. 2-54211, since the lateral magnification is as low as 2 times or less, most of the light output of the semiconductor laser is lost due to "vignetting" and reaches the surface to be scanned. Is very small, and the efficiency of the optical system for light output is very low. Therefore, the optical output of the semiconductor laser is insufficient, which is not practical. Further, the curve amount of the scanning line is simply reduced in proportion to the reduction of the lateral magnification, and the scanning line curve cannot be completely corrected, which is not a fundamental solution.

SUMMARY OF THE INVENTION The present invention solves such a problem, and an object of the present invention is to provide a practical optical scanning device which has no scanning line curvature and has high light output utilization efficiency.

[0008]

An optical scanning device according to the present invention comprises:
A light source for generating a plurality of light beams from a plurality of light emitting parts, a deflector for deflecting the light beams, and an imaging optical system for forming an image of the light beams deflected by the deflector on a surface to be scanned, The image forming optical system is an optical scanning device including a first image forming lens and a second image forming lens, and the second image forming lens is a surface to be scanned from a longitudinal center toward a longitudinal end. It is characterized by being curved away from.

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

1) The surface of the second imaging lens on the deflector side is a saddle-shaped toric surface. 2) The main scanning cross section of the saddle-shaped toric surface has an aspherical shape. 3) The surface of the second imaging lens on the scanned surface side is a non-cylindrical surface. 4) The refractive power of the second imaging lens in the main scanning direction is negative at the scanning center and positive at the scanning end. 5) The second imaging lens is made of resin.

[0011]

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

FIG. 1 shows an optical scanning device which is an embodiment of the present invention. First, at any point on the optical axis of the optical system, the direction perpendicular to the optical axis at that point and the rotation axis of the deflector is defined as the main scanning direction, and the optical axis and the main scanning direction are defined as The direction perpendicular to is defined as the sub-scanning direction. Further, a surface including the optical axis and parallel to the main scanning direction is defined as a main scanning cross section, and a surface including the optical axis and parallel to the sub scanning direction is defined as a sub scanning cross section.

A plurality of beams are emitted from the semiconductor laser array 1 which is a light source. The emitted beam is a diverging beam and is converted into a parallel beam by the collimator lens 2. The parallel beam is focused by the aperture 3. The narrowed beam is focused only in the sub-scanning direction by the cylindrical lens 4 having a refractive power only in the sub-scanning direction. Further, the beam forms an image in the sub-scanning direction in the vicinity of the reflecting surface 6 of the rotary polygon mirror 5 which is a deflector, is reflected by the reflecting surface 6, and is deflected as the rotary polygon mirror 5 rotates. The deflected beam is focused by the first imaging lens 7 and the second imaging lens 8 to form a plurality of beam spots on the surface 9 to be scanned.

Next, the structure and function of the image forming optical system will be described. 2A is a main scanning sectional view of the imaging optical system, FIG.
(B) is a sub-scanning sectional view. The first imaging lens 7 is a single lens, has a positive refracting power in the main scanning direction, and has no refracting power in the sub-scanning direction. The second imaging lens 8 is also a single lens, and the main scanning section has an aspherical shape on both the incident surface and the exit surface, and the refracting power in the main scanning direction is negative at the scanning center and positive at the scanning end. However, their refractive powers are weak. It has a strong positive refractive power in the sub-scanning direction.

The incident surface of the second imaging lens 8 is a surface formed by rotating the curve of the main scanning section about the rotation axis 10. The main scanning section is concave, and the rotary shaft 10
Is closer to the surface 9 to be scanned than the second imaging lens 8, and therefore the sub-scan section is convex. The surface having such a shape is referred to as a saddle-shaped toric surface. The exit surface of the second imaging lens 8 has an aspherical shape with a convex main scanning section and a straight line with a sub-scanning section, and such a surface is called a non-cylindrical surface.

The beam 11 is parallel on the reflecting surface 6 in the main scanning direction, and is focused on the surface 9 to be scanned mainly by the refractive power of the first imaging lens 7. On the other hand, in the sub-scanning direction, the beam 11 is imaged on the reflecting surface 6 and is imaged on the surface 9 to be scanned by the refracting power of the second imaging lens 8. In the sub-scanning direction, the reflecting surface 6 and the surface to be scanned 9 are in an optically conjugate relationship, and have a function of correcting surface tilt of the reflecting surface 6 of the rotary polygon mirror 5.

The second imaging lens 8 is curved so as to move away from the surface 9 to be scanned from the center in the longitudinal direction toward the end in the longitudinal direction. Therefore, the optical magnification in the sub-scanning direction between the reflecting surface 6 and the surface 9 to be scanned becomes uniform in the effective scanning area, and the plurality of scanning lines are straight lines without being curved. This will be described below.

First, for comparison, a main scanning sectional view of an image forming optical system in a conventional optical scanning device is shown in FIG. The shape of the second imaging lens 108 of the related art is linear. FIG.
In the figure, a beam 112 passing on the optical axis and a beam 113 passing outside the optical axis are shown. The beam 113 receives the positive refracting power of the first imaging lens 107 and is refracted, but is hardly refracted by the second imaging lens 108 having a small refracting power in the main scanning direction. Along the beam 112 on the optical axis,
The distance from the reflecting surface 106 to the first imaging lens 107 is D, the distance from the first imaging lens 107 to the second imaging lens 108 is E, and the second imaging lens 108 to the surface to be scanned 10.
Let F be the distance to 9. Further, the distance from the reflecting surface 106 to the first imaging lens 107 along the beam 113 is d, and the first imaging lens 107 to the second imaging lens 108.
To the scanning surface 1 from the second imaging lens 108
The distance to 09 is f. Reflecting surface 106 and scanned surface 1
09, since only the second imaging lens 108 has the refractive power in the sub-scanning direction, the optical magnification in the sub-scanning direction between the reflecting surface 106 and the surface to be scanned 109 is the same for each beam. It depends on the position of the second imaging lens 108 above. That is, the optical magnification of the beam 112 in the sub-scanning direction is F / (D + E), and the optical magnification of the beam 113 in the sub-scanning direction is f / (d + e).

Since the off-optical beam 113 is hardly refracted by the second imaging lens 108, the ratio of E to e and the ratio of F to f are almost the same. However, since the first imaging lens 107 receives positive refracting power and is refracted, the ratio of D and d becomes smaller than those,
The optical magnification f / (d + e) off the optical axis is smaller than the optical magnification F / (D + E) on the optical axis. From this,
It can be seen that the optical magnification in the sub-scanning direction decreases from the scanning center toward the scanning end. Due to such a change in the optical magnification in the sub-scanning direction, as shown in FIG. 4, among the plurality of beams, a beam that scans a surface including the optical axis of the imaging optical system is formed on the surface to be scanned 109. The scanning line 122 is a straight line, but when scanning the scanning center, a beam that passes through a position distant from the optical axis of the imaging optical system in the sub-scanning direction, that is, the sub-scanning from the optical axis of the imaging optical system. Scan line 1 formed by a beam that scans a spherical surface that is offset in the direction
23 will bend.

FIG. 5 is a diagram showing a main scanning section of the imaging optical system of this embodiment. Since the second imaging lens 8 has a small refracting power in the main scanning direction, it is schematically drawn as a lens having no refracting power in the main scanning direction. FIG. 5 also shows a beam 12 passing on the optical axis and a beam 13 passing outside the optical axis. In the present embodiment, the second imaging lens 8 is curved so as to move away from the surface 9 to be scanned from the longitudinal center toward the longitudinal end. Therefore, the beam 13 off the optical axis
3, the distance e from the first imaging lens 7 to the second imaging lens 8 is shorter than that in the case where the second imaging lens 108 is not curved as shown in FIG. The distance f from the scanning surface 9 to the surface 9 to be scanned increases, and the optical magnification f / (d + e) in the sub-scanning direction increases. By utilizing this fact, if the bending amount of the second imaging lens 8 is set to an appropriate value, the optical magnification in the sub-scanning direction between the reflecting surface 6 and the surface 9 to be scanned is made uniform in the effective scanning area. And all of the plurality of scan lines can be straight.

The principle of aberration correction of the image forming optical system will be described with reference to FIG. The field curvature in the main scanning direction is mainly corrected by the first imaging lens 7. The correction of the fθ characteristic is insufficient with the first imaging lens 7. Therefore, the refracting power of the second imaging lens 8 in the main scanning direction is set to be negative at the scanning center and positive at the scanning end so that the scanning speed is high at the scanning center and slow at the scanning end. The imaging lens 8 also corrects the fθ characteristic.

The field curvature in the sub-scanning direction is corrected by the entrance surface of the second imaging lens 8. As described above, the incident surface of the second imaging lens 8 has the curve of the main scanning cross section as the rotation axis 10
Is a saddle-shaped toric surface formed by rotating about the center, and since the second imaging lens 8 is curved, the radius of curvature in the sub-scanning direction increases from the scanning center toward the scanning end, The focal length in the sub-scanning direction is longer from the scanning center toward the scanning end. On the other hand, the beam entering the second imaging lens 8 enters obliquely from the scanning center toward the scanning end, and the focal length in the sub-scanning direction becomes shorter than in the case where the beam enters from the front. .. Further, the angle between the beam and the optical axis increases from the scanning center toward the scanning end, and the second angle along the beam increases.
The distance between the imaging lens 8 and the scanned surface 9 becomes long. In the present embodiment, by making the incident surface of the second imaging lens 8 a saddle-shaped toric surface, these three conditions cancel each other out,
It is possible to correct the field curvature in the sub-scanning direction. Also,
Since the radius of curvature of the saddle-shaped toric surface in the sub-scanning direction depends on the main-scanning cross-sectional shape, the curvature of the saddle-shaped toric surface in the sub-scanning direction can be changed by making the main-scanning cross-section of the saddle-shaped toric surface an aspherical shape. The radius can be arbitrarily set in the effective scanning area, and the field curvature in the sub-scanning direction can be corrected particularly well.

Since the field curvature in the sub-scanning direction is corrected on the entrance surface of the second imaging lens 8, the refracting power of the exit surface in the sub-scanning direction may be eliminated. Therefore, in this embodiment, the sub-scanning cross section of the exit surface is a straight line. However, in the main scanning direction, since the fθ characteristic is corrected, the main scanning sectional shape is an aspherical shape. Therefore, the exit surface is a non-cylindrical surface.
In order to correct the fθ characteristic, it is necessary to change the scanning speed by the refracting power of the surface. In this embodiment, the aspherical surface displacement amount indicating the displacement from the spherical surface is determined by the non-cylindrical surface as compared with the saddle type toric surface. By making it larger, good fθ characteristics are realized.

The saddle-shaped toric surface is a surface formed by rotating a curved line around the rotating shaft 10. Since the saddle-shaped toric surface can be processed only by rotary motion,
Easy to manufacture. Further, since the non-cylindrical surface has a straight line in the sub-scanning section, it can be easily processed and manufactured.

However, the structure of the surface of the second imaging lens 8 is as follows.
This is not the case. The entrance surface and the exit surface may be configured by a spherical surface, a cylindrical surface, or a toric surface.
It may be an aspherical surface in which the radius of curvature in the main scanning direction and the radius of curvature in the sub scanning direction change arbitrarily in the effective area of the lens. They may be combined so that each aberration is corrected. In any case, if the second imaging lens 8 is curved from the center in the longitudinal direction toward the end in the longitudinal direction and away from the surface 9 to be scanned, the optical magnification in the sub-scanning direction is in the effective scanning area. It has the effect of becoming uniform and making a plurality of scanning lines linear.

In the present embodiment, the second imaging lens 8 is made of resin. It is not practical to manufacture a lens having an aspherical surface from glass because the cost is high, but molding with a resin allows easy mass production and lower cost.

In this embodiment, by curving the second imaging lens 8, the optical magnification in the sub-scanning direction is made uniform and the field curvature in the sub-scanning direction is corrected. However, the amount of curvature of the second imaging lens 8 for completely equalizing the optical magnification in the sub-scanning direction and the amount of curvature of the second imaging lens 8 for completely correcting the field curvature in the sub-scanning direction. The amount of bending does not always match. If they do not match, if the positional deviation of the scanning lines is important, then the amount of curvature may be determined so that the optical magnification is uniform, and if the uniformity of the shape and size of the beam spot is important. For example, the amount of curvature may be determined so as to correct the field curvature.
In order to make the optical magnification in the sub-scanning direction completely uniform,
Alternatively, the amount of curvature of the second imaging lens 8 for completely correcting the field curvature in the sub-scanning direction depends on the positions of the first imaging lens 7 and the second imaging lens 8.

In this embodiment, the case where the deflector is a rotary polygon mirror has been described, but in addition to this, a sine vibration is performed about a rotary single-sided mirror, a rotary two-sided mirror, or a rotation axis. A galvanometer mirror or the like can be easily realized, and the same effect can be obtained.

In the present embodiment, the case where the first imaging lens is a single lens has been described, but the effect of the present invention can be obtained in the same manner even if the first imaging lens is composed of a plurality of lenses. By doing so, aberration correction becomes even better.

As described above, the present invention is particularly effective when used in a laser beam printer, but it is an image forming apparatus such as a digital copying machine, a facsimile, a laser scanning display, an image input apparatus such as a scanner, or an optical mark. It can also be applied to a laser scanning device for reading, a laser scanning device for surface inspection, and the like.

[0031]

[Examples] Table 1 shows optical specifications of typical examples of the present embodiment. However, the rotation angle of the rotary polygon mirror from the start of one scan to the end of the scan is 2ω. The emission point of the semiconductor laser array is S ₁ , the entrance and exit surfaces of the collimator lens are S ₂ and S ₃ , the entrance and exit surfaces of the cylindrical lens are S ₄ and S ₅ , respectively, and the reflection surface of the rotating polygon mirror is S 1. _6. Let S ₇ and S _{8 be} the entrance and exit surfaces of the first imaging lens, and let S ₉ and S ₁₀ be the entrance and exit surfaces of the second imaging lens, 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 upper surface distance from the i-th surface to the next surface is d _i , and the collimator lens, the cylindrical lens, the first imaging lens, The refractive index of the second imaging lens is n ₂ , respectively.
Let n ₄ , n ₇ , and n ₉ . On the anamorphic lens surface, the radii of curvature in the sub-scanning direction and the main-scanning direction are r _ix and r _iy , respectively, and the radius of curvature of the aspherical surface indicates a value on the optical axis.

[0032]

[Table 1]

The main scanning section of the second imaging lens has an aspherical shape,

[0034]

(Equation 1)

It is represented by. However, the coordinates have the origin at the point where the lens surface intersects the optical axis, the z axis in the optical axis direction, and the y axis in the main scanning direction. K _i and A _i are aspherical coefficients. The values of these coefficients are shown in Table 2.

[0036]

[Table 2]

[0037]

As described above, according to the present invention,
It has the following effects.

First, according to the invention of claim 1, the second
By curving the imaging lens from the center in the longitudinal direction toward the end in the longitudinal direction and away from the surface to be scanned, the optical magnification in the sub-scanning direction becomes uniform in the effective scanning area in a practical configuration, All of the scanning lines are straight lines, and there is an effect that the positional deviation of the scanning lines does not occur.

According to the second aspect of the present invention, the deflector-side surface of the second imaging lens is a saddle-shaped toric surface, so that the field curvature in the sub-scanning direction is favorably corrected. Have. Further, since the processing can be performed only by the rotational movement, there is an effect that the lens can be easily manufactured.

According to the third aspect of the present invention, by making the main scanning section of the saddle-shaped toric surface an aspherical shape,
The radius of curvature in the sub-scanning direction can be arbitrarily set in the effective scanning area, and the field curvature in the sub-scanning direction can be corrected even better.

According to the fourth aspect of the present invention, by making the surface of the second imaging lens on the scanned surface side a non-cylindrical surface,
This has the effect that the fθ characteristic is well corrected. Further, since the sub-scanning cross section is a straight line, there is an effect that the lens can be easily manufactured.

According to the fifth aspect of the invention, by making the refracting power of the second imaging lens in the main scanning direction negative at the scanning center and positive at the scanning end, the fθ characteristic can be corrected even better. Has the effect.

According to the sixth aspect of the invention, by forming the second imaging lens with resin, there is an effect that processing is easy and mass production is possible, and the cost is reduced.

[Brief description of the drawings]

FIG. 1 is a perspective view of an embodiment of the present invention.

FIG. 2 is a sectional view of the image forming optical system according to the embodiment of the present invention.

FIG. 3 is a main scanning sectional view of an image forming optical system of a conventional optical scanning device.

FIG. 4 is a view showing a curve of a scanning line of a conventional optical scanning device.

FIG. 5 is a main scanning sectional view of the imaging optical system according to the embodiment of the present invention.

[Explanation of symbols]

 1 Semiconductor Laser Array 2 Collimator Lens 3 Aperture 4 Cylindrical Lens 5 Rotating Polygonal Mirror 6 Reflective Surface 7 First Imaging Lens 8 Second Imaging Lens 9 Scanned Surface

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Internal reference number FI Technical indication location G02B 13/18 B41J 3/00 D (72) Inventor Yujiro Nomura 3-3 Yamato, Suwa City, Nagano Prefecture No. 5 within Seiko Epson Corporation

Claims (6)

[Claims] 1. A light source for generating a plurality of light beams from a plurality of light emitting parts, a deflector for deflecting the light beams, and a light beam imaged on the surface to be scanned, which is deflected by the deflector. And an image optical system, wherein the image forming optical system is an optical scanning device including a first image forming lens and a second image forming lens, and the second image forming lens extends from a center in a longitudinal direction. An optical scanning device characterized in that the optical scanning device is curved so as to move away from the surface to be scanned toward the end portion in the direction. 2. The surface of the second imaging lens on the side of the deflector is a saddle-shaped toric surface.
The optical scanning device according to claim 1. 3. The main scanning cross section of the saddle-shaped toric surface is
The optical scanning device according to claim 2, wherein the optical scanning device has an aspherical shape. 4. The surface of the second imaging lens on the side of the surface to be scanned is a non-cylindrical surface.
The optical scanning device according to claim 1. 5. The optical scanning device according to claim 1, wherein the second imaging lens has a negative refractive power in the main scanning direction at the scanning center and a positive refractive power at the scanning end. 6. The optical scanning device according to claim 1, wherein the second imaging lens is made of resin.