JPS63189822A - Optical scanning device - Google Patents

Abstract

PURPOSE:To obtain a laser beam scanning system having high accuracy and high resolution by arranging a scanning lens between a laser beam deflecting device and a plane to be scanned and arranging a spherical aberration correcting lens between a laser beam source and the deflecting device. CONSTITUTION:Diffused beams radiated from a semiconductor laser 5 are converted into parallel beams by a collimater lens 4, and while passing through planar convex lens and planar concaved lens which curved surfaces are bent to a laser side 5 a reverse code having the same quantity as that of spherical aberration included in an ftheta lens 3 is applied to the parallel beams and a linear image is formed on the face 11 of a rotary polygon mirror. Further, the image is scanned by the face 11 and an image is formed on the surface 61 of a photosensitive body through the ftheta lens 3. Since the spherical aberration is suppressed to a small value and other aberrations also are effectively corrected, the shape of an image-formed optical point is excellent and a required diameter f the optical point can be obtained. The correction lens 3 is constituted of two cylindrical lenses having at least positive and negative refractive forces.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to the optical system of an image signal optical writing device used in a device that records an image signal on a recording medium such as a photographic film or an optically addressed liquid crystal element, and then enlarges and projects the image signal. Relating to a scanning device.

[Conventional technology]

Conventionally, the scanning optical system of a laser beam printer is a typical technique for directly forming an image on a photoreceptor by focusing a laser beam on a minute spot, scanning it onto a photoreceptor, and modulating the intensity of the laser beam. be done.

BACKGROUND ART A rotating polygonal mirror deflection system is widely used as a light beam deflector of an optical scanning device used in a laser beam printer or the like because it is relatively inexpensive, enables high-speed, and stable optical scanning. FIG. 5 shows an example of an optical scanning device using a rotating polygonal mirror deflection device. A diffused beam emitted from a semiconductor laser 5 is made into a parallel beam by a collimator lens 4, and is then turned into a parallel beam by a rotating polygonal mirror 1 rotated by a motor 2. 6 is scanned. The fθ lens 3 forms an image of the collimated beam onto the photosensitive drum 6 to a desired spot diameter, and so that the imaged spot moves at a constant speed on the photosensitive drum 6 with respect to the beam being scanned at a constant angular speed. It has the function of giving a certain amount of distortion.

However, when performing high-precision, high-resolution optical scanning using the rotating polygon mirror deflection method, a problem is the pitch unevenness of the scanning line caused by the error in parallelism between each surface of the polygon mirror. In order to suppress the surface tilt error to such an extent that this pitch unevenness can be ignored, extremely high precision is required for the polygon mirror and the rotating shaft, which inevitably leads to an increase in price. Therefore, by adding some kind of surface tilt correction mechanism, scanning pitch unevenness can be reduced.
It is necessary to suppress it. This surface tilt correction is generally performed optically. Fig. 6 shows the principle of this correction method. Fig. 7 (α) is a diagram showing a cross section C (meridian ray) in the direction of the deflection plane of the light beam. The fθ lens 3 is made up of three lenses: a spherical lens 31 with positive refractive power, a cylindrical lens 32 with positive refractive power, and 5303 spherical lenses with negative refractive power such as a concave lens. imaged. On the other hand, the cross section (spherical beam) in the direction perpendicular to the deflection plane shown in FIG. Then, the image is again formed on the photosensitive drum surface 61. That is, the polygonal mirror surface 11 and the photosensitive drum surface 61 are in a conjugate image point relationship. Therefore, as shown by the dotted line in the figure, even if the emission direction changes somewhat due to the tilting of the polygonal mirror surface 11, the image point hardly changes. In this way, it is possible to correct the surface tilt error.

[Problem that the invention seeks to solve]

The conventional scanning optical system for laser beam printers mentioned above is
Since the aperture angle of the laser beam is extremely small, spherical aberration and comatic aberration hardly occur, and the optical system is corrected for field curvature, astigmatism, and distortion. This means that the resolution required for a laser beam printer is SO
O (lot/1 nch), so the diameter of the imaging spit (however, the diameter of the peak intensity) is 80 μm ~
It may be about 100μrn, and is based on the well-known glass beam imaging formula that expresses the relationship between the laser beam imaging spot diameter and the aperture angle (where Wo is the imaging spot radius, λ is the laser beam wavelength, and θ is the aperture angle). ), the aperture angle θ is [Lo 05~no O6 (rad
) (λ=780 n rn ) 19 degrees. Therefore, for example, in FIG. 6, the fθ lens 30 is replaced with a biconvex spherical lens 31 to correct field curvature, astigmatism, and distortion. Plano-convex cylindrical lens 32. It is composed of 3303 meniscus spherical lenses, but the cylindrical lens 7 before deflection is a plano-convex cylindrical lens, since the laser beam does not move on the optical axis, the above three aberrations do not occur, and only paraxial imaging needs to be considered. Although only one piece is required, the recording medium targeted by the present invention is a photographic film or an optically addressable liquid crystal element,
Unlike laser beam printers, images are recorded in a smaller size than the actual screen size when viewed by humans. Specifically, the present invention is directed to images to be recorded with an imaging spot diameter of approximately 10 μm to 20 μm. That is, for example, the number of resolution points is 2000 points x 2000 points, and the imaging spot diameter is 15μ.
In the case of a door, the screen would be as small as 5 cm x 5 cm. At this time, according to the Gaussian beam imaging formula described above, when a laser beam with a wavelength of 780 nm is focused on a spot diameter of 15 μm, the aperture angle θ is 0.0.
53, the influence of spherical aberration and comatic aberration appears and the shape of the imaged spot is disturbed, causing a problem that a desired spot diameter cannot be obtained.

The inventor of the present invention has discovered that this disturbance in the spot shape mainly occurs in the spherical beam and is caused by the above-mentioned surface tilt correction optical system. This is due to the following reasons. That is, as shown in Figure 6, for meridional rays, a parallel beam is once converged to form an image, whereas for spherical rays, the spherical beam is once converged on a polygonal mirror, then made into a parallel beam again, and then converged again. This is because spherical aberration several times as large as the meridional light beam is generated to form an image. As described above, the scanning optical system used in conventional laser beam printers has the disadvantage that a good imaging spot diameter cannot be obtained.

Therefore, a method can be considered to correct the spherical aberration over the entire scanning width by increasing the number of fθ lenses. However, fθ
The lens must also correct aberrations such as curvature of field, astigmatism, and distortion simultaneously over the entire scanning width, making the lens complex and having a large aperture, resulting in an expensive lens.

The present invention has been made in view of the above points, and its purpose is to provide a high-precision, high-resolution laser beam scanning optical system that can form an image down to an extremely small spot diameter without increasing the number of scanning lenses. It's about doing.

[Means for solving problems]

The present invention comprises: (1) a laser light source that emits a laser beam;
an optical deflection device that deflects and scans the laser beam, a scanning lens that is located between the optical deflection device and the scanned plane and forms an image on the scanned plane, which is the laser beam, and the laser light source and the optical deflection device. (2) a correction lens configured to form a linear image in a direction parallel to the deflection direction on the deflection curve of the light deflection device, which is a laser beam, and to correct the spherical aberration of the deflection lens; The correction lens is characterized in that it is composed of at least two lenses, a cylindrical lens having a positive refractive power and a cylindrical lens having a negative refractive power.

[Effect]

The principle of operation of the present invention will be explained below. Figure 7 (α) (b)
(C) and Figures 8(a), (J), and (a) are composed of one cylindrical lens for correcting the surface tilt between the laser light source and the rotating polygon mirror, that is, the spherical aberration can be corrected. This shows how the beam spot is formed when the beam spot is not formed.
Figures (α), (b), and (C) show the meridional light flux, in which the X-axis represents the traveling direction of the laser beam, the y-axis represents the spread of the beam in the calf direction, and the intensity of the beam is represented by contour lines. Note that the intensity value is a relative value with the peak value of the imaging point in the case of no aberration being 1. This lens is designed so that the diameter of the imaging spot is 15 μm when spherical aberration is not considered. (α>, Cb) and (C) are beams focused on one end, the center, and the other end of the scanning width, respectively. Figure 8 (α
)(b)(C) is a spherical beam, and the two axes in the figure represent the spread of the beam in the spherical direction.
) Same as (C). From Figures 7 and 8, it can be seen that it is the spherical beam that does not form the desired spot shape due to the disturbance of the spot shape, and that the shape of the disturbance is constant regardless of the scanning position, that is, the deflection angle. This means doing the following. This means that the spherical aberration of the spherical beam to be corrected has a nearly constant value over the entire deflection angle. If the spherical aberration of is corrected for a light beam having an arbitrary deflection angle, it will be corrected over the entire deflection angle, and a good imaging spot will be obtained.

By the way, as mentioned above, the spherical beam is a lens system that converges at 2@, so lenses that add positive refractive power are often used, so in order to correct these spherical aberrations, lenses that add negative refractive power are used. Requires a face. Therefore, it is sufficient that the cylindrical lens for correcting the inclination is composed of at least a cylindrical lens having a positive refractive power and a cylindrical lens having a negative refractive power.

〔Example〕

FIG. 1(α)(b) shows an embodiment of the scanning optical system of the present invention. (α) is a developed diagram showing a meridional luminous flux, and (b) is a developed diagram showing a spherical luminous flux. The diffused beam emitted from the conductor laser 5 is converted into a parallel beam by the collimator lens 4, and is then turned into a parallel beam by the plano-convex cylindrical lens 71, which is directed toward the laser 5 side. The light passes sequentially through the plano-concave cylindrical lens 72 and is given an aberration of the same amount and opposite sign to the spherical aberration of the fθ lens 3 on the polygonal mirror surface 11 of the rotating polygonal mirror, forming a linear image. The image is further scanned by the rotating polygonal mirror surface 11 and imaged by the fθ lens 5 onto the photoreceptor surface 61 so as to be scanned at a constant speed. This temporal spherical aberration is suppressed to a small value, and other aberrations are also well corrected, so that the imaged spot shape is good and a desired spot diameter is obtained.

(Numerical example) 8g Tables 1 to 3 show the optical parameters of an embodiment of the optical system of the present invention. The symbols of the surfaces in Table 1 are as shown in Figure 1 (b).
Table 1 shows the lens parameters of the two-piece face tilt corrected cylindrical lens after collimation, and Table 2 shows the lens parameters of the fθ lens. In addition, Table 3 shows the parameters related to the arrangement of the rotating polygon mirror. Table 1 shows the characteristics of the optical system shown in Table 1.
(b) is an aberration diagram showing field curvature aberration and astigmatism, and (b) is distortion aberration (scanning linearity). Also, Figure 5 (α) (A) (
C) are respectively (α〉 is the deflection angle 0 = -10c', (b)
is a diagram showing the imaging state of the meridional ray when θ = 00 and (c) is θ = 10', and Figure 4 (a), (A), and (C) similarly show the imaging state of the spherical ray. The figure is drawn in the same format as FIGS. 7 and 8.

In this way, the optical system of this embodiment not only has the flatness of the image plane and the linearity of scanning which the fθ lens of the scanning optical system has, but also the disturbance of the spot shape due to the spherical aberration caused by the surface tilt correction optical system. A scanning optical system capable of scanning an extremely small spot of 15 .mu.rIL x 15 .mu.m has been realized.

〔Effect of the invention〕

As described above, according to the present invention, the scanning optical system that corrects the surface tilt error of the polygonal mirror by linearly forming an image on the rotating polygonal mirror is a cylindrical lens that forms a linear image on the rotating polygonal mirror. In the system, the cylindrical lens system is composed of at least two lenses, one with positive refractive power and the other with negative refractive power, so that the spherical aberration of the spherical beam of the fθ lens system is corrected. This has the effect that an imaging spot with an extremely small diameter can be obtained without making the fθ lens complicated or expensive, and that a simple and low-cost optical scanning device with high precision and high resolution can be provided.

[Brief explanation of the drawing]

FIG. 1 is an optical path diagram showing the structure of the optical system of the present invention, and is a perspective view showing the structure of the optical system, and FIG. 6 is a diagram showing the conventional surface tilt shape. 1...Rotating polygon mirror 11...Rotating polygon mirror 3...Fθ lens 4...Collimator lens 5...・・・・・・
... Semiconductor laser 6 ... Photosensitive drum 7 ... Cylindrical lens for surface tilt correction 71 ...
...Correction cylindrical lens 72 having positive refractive power...
...Corrective cylindrical lens with negative refractive power or more Applicant Seiko Epson Co., Ltd. Agent Patent Attorney Mogami (and 1 other person) 3fθ lens 4 Premeter lens (see Figure 1) Thoughts & collections
L4 length 1L line drawing 1 crop point ahead <a+ Ibl Fig. 2 Fig. 3 Fig. 4 Fig. 6

Claims (2)

[Claims] (1) A laser light source that emits a laser beam, an optical deflection device that deflects and scans the laser beam, and a scanning lens that is located between the optical deflection device and the scanned plane and forms an image of the laser beam on the scanned plane. , located between the laser light source and the optical deflection device, configured to form a linear image of the laser beam on the deflection surface of the optical deflection device in a direction parallel to the deflection direction, and to correct the spherical aberration of the deflection lens. An optical scanning device characterized by comprising a correction lens. (2) The optical scanning device according to claim 1, wherein the correction lens is composed of at least two lenses, a cylindrical lens having a positive refractive power and a cylindrical lens having a negative refractive power. .