JP2010014768A - Scanning optical system, image forming apparatus having scanning optical system and image reading apparatus having scanning optical system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a compact and high performance scanning optical system, an image reading apparatus and an image forming apparatus having the scanning optical system. <P>SOLUTION: The scanning optical system focuses and scans the beam emitted from a light source as a spot onto a face to be scanned, and includes: a resonance type deflector which main-scans by reflecting and deflecting the beam from the light source; a first imaging optical system which focuses the beam from the light source in the form of a line extending in a main scanning direction in the vicinity of the resonance type deflector; and a second imaging optical system which focuses again the beam which is once focused in the form of a line extending in the main scanning direction in the vicinity of the resonance type deflector as a spot on the face to be scanned, wherein the second imaging optical system is composed of an arcsine lens system corresponded to the angle of the reflection face, the arcsine lens system includes, from the side of the resonance type deflector, a first lens element having a positive meniscus form and a second lens element having a negative meniscus form. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning optical system for scanning a laser beam, and more specifically, a scanning optical system used for an exposure optical system of an image forming apparatus such as a laser printer, a laser facsimile, a digital copying machine, or a reading optical system such as a scanner. About. The present invention also relates to an image forming apparatus and an image reading apparatus provided with a scanning optical system.

  The scanning optical system is incorporated in an apparatus that reads and writes an image mainly by scanning a light beam. The scanning optical system is used as an exposure optical system for forming an image, particularly in an image forming apparatus such as a laser printer, a laser facsimile, or a digital copying machine. The scanning optical system is also used as a reading optical system of an image reading apparatus such as a scanner.

  A conventional scanning optical system used in the image forming apparatus as described above includes a semiconductor laser, a polygon mirror, and a scanning lens. In recent years, high resolution and high speed of image forming devices and image reading devices have progressed, so there is a need for an optical system with small spot diameter variation for each scan, arc sine characteristics with a scanning lens, and high scanning speed. Has been. For this reason, in the scanning optical system, it has been a big problem to ensure both the optical performance and the high speed, but in general, there is a limit to the compatibility of both in the scanning optical system using the polygon mirror. Therefore, as a technique aiming to achieve both at a high level, a scanning optical system combining a resonant deflector and an arc sine lens corresponding to high speed has been proposed.

For example, Patent Document 1 discloses a laser scanning device including two scanning lenses. In the laser scanning device described in Patent Document 1, a technique has been proposed in which incident light is made convergent light in the main scanning section, and the size of the device is reduced without increasing the cost of the scanning lens.
JP 2006-227044 A

  However, in the laser scanning device disclosed in Patent Document 1, since the scanning lens is composed of a lens with a strong negative power, the degree of deviation of the lens is large, which makes it difficult to mold.

  An object of the present invention is to provide a scanning optical system that suppresses spot diameter variation for each scanning, maintains an arc sine characteristic, and suppresses the degree of deviation of the scanning lens as much as possible by using two scanning lenses.

  Another object of the present invention is to provide an image reading apparatus and an image forming apparatus provided with these scanning optical systems.

  The above object is achieved by the following scanning optical system. A scanning optical system that forms and scans a beam emitted from a light source as a spot on a surface to be scanned, and that performs a main scanning by reflecting and deflecting the beam from the light source by a reflecting surface; and A first imaging optical system that forms a beam from a light source in a linear shape extending in the main scanning direction in the vicinity of the reflecting surface; and a beam imaged in a linear shape extending in the main scanning direction in the vicinity of the reflecting surface. A second imaging optical system that forms an image again as a spot on the surface to be scanned. The first imaging optical system collimates a divergent beam from the light source, and the collimating means. An arc sine lens system in which the second imaging optical system corresponds to the angle of the reflecting surface. The arc sine 'S system comprises, in order from the reflective surface side, and a first lens element having a positive power in the at least the main scanning direction, a second lens element having negative optical power at least with respect to the main scanning direction.

  According to the present invention, a scanning lens that is easy to be molded with a small degree of thickness deviation while ensuring high optical performance can be provided, so that a high-quality and inexpensive scanning optical system can be provided. Further, according to the present invention, it is possible to provide an image reading apparatus and an image forming apparatus provided with a high-quality and inexpensive scanning optical system while ensuring high optical performance.

(Embodiment 1)
FIG. 1 is a configuration diagram of a scanning optical system according to the first embodiment. The scanning optical system includes a collimating lens 2, a cylindrical lens 3, a resonant deflection mirror 4, and a scanning lens 6 (the first scanning lens is 6a and the second scanning lens is 6b). In the scanning optical system, the collimating lens 2 and the cylindrical lens 3 constitute a first imaging optical system, and the scanning lens 6 constitutes a second imaging optical system. The first imaging optical system and the second imaging optical system are both anamorphic optical systems having different optical powers in the main scanning direction and the sub-scanning direction orthogonal to the main scanning direction. In FIG. 1, the main scanning surface for performing the main scanning by the resonance type deflection mirror 4 and the plane including the optical axis of the scanning optical system are parallel to the paper surface.

  The collimating lens 2 converts a divergent laser beam emitted from the semiconductor laser 1 as a light source into a substantially parallel laser beam.

  The cylindrical lens 3 has a cylindrical surface having optical power only in the sub-scanning direction and formed on the light source side, and a plane on the resonant deflection mirror side. The cylindrical lens 3 acts only in the sub-scanning direction of the substantially parallel laser beam emitted from the collimating lens 2 and extends linearly in the main scanning direction in the vicinity of the reflecting surface (planar shape) of the resonant deflection mirror 4. Condensed to

  The resonant deflection mirror 4 deflects the incident laser beam by sine vibration about the rotation axis 5. Further, the resonant deflection mirror 4 is arranged so that the optical axis of the laser beam incident on the resonant deflection mirror 4 and the scanning center in the main scanning direction of the laser beam reflected by the reflecting surface form a predetermined finite angle. Is done.

  The scanning lens 6 forms an image of the laser beam deflected and scanned on the surface to be scanned 7 by being reflected by the reflecting surface of the resonant deflection mirror 4. The scanning lens 6 corrects the field curvature and linearity of the laser beam on the scanned surface 7.

  The first scanning lens 6a has positive power in the main scanning direction, and has cylindrical surfaces having optical power only in the main scanning direction on both sides of the resonant deflection mirror side and the scanned surface side. . Further, the first scanning lens 6a has a positive meniscus shape with a convex surface facing the surface to be scanned in the main scanning direction.

  The second scanning lens 6b has anamorphic surfaces having different optical powers in the main scanning direction and the sub-scanning direction on both the resonant deflection mirror side and the scanned surface side. Further, the second scanning lens 6b has a negative meniscus shape with the convex surface facing the surface to be scanned in the main scanning direction.

  FIG. 2 is a sectional view of the second imaging optical system of the scanning optical system according to Embodiment 1 in the sub-scanning direction. In the second imaging optical system of the scanning optical system, as shown in FIG. 2, the reflection surface of the resonant deflection mirror 4 and the surface to be scanned 7 are optically conjugate in the sub-scan section. Due to this conjugate relationship, even if the reflection surface of the resonant deflection mirror 4 is tilted with respect to the rotation axis 5, the imaging position in the sub-scanning direction on the surface to be scanned formed by the scanning lens 6 does not change. Furthermore, the required area on the reflecting surface of the resonant deflection mirror 4 can be reduced.

  In the above configuration, the divergent laser beam emitted from the semiconductor laser 1 enters the collimating lens 2 and is converted into a substantially parallel laser beam. The laser beam is then imaged in a linear shape extending in the main scanning direction in the vicinity of the reflection surface of the resonant deflection mirror 4 by the cylindrical lens 3. Thereafter, the laser beam is incident on the scanning lens 6 as a divergent laser beam in the sub-scanning direction, scanned by the scanning lens 6 in the main scanning direction, and imaged as a spot.

  In the first embodiment, the anamorphic surface of the scanning lens 6b is preferably a free-form surface, and in particular, a free-form surface such that a line connecting the centers of curvature in the sub-scanning direction is a curved non-arc. Is desirable. By configuring the scanning lens in this way, it is possible to satisfactorily correct the arc sine characteristic and the curvature of field.

  In the first embodiment, it is desirable that the anamorphic surface of the scanning lens 6 introduces odd-order terms into the surface shape. By introducing odd-order terms, the curve connecting the centers of curvature in the sub-scanning plane is asymmetric with respect to the optical axis, and the field curvature in the sub-scanning direction that occurs asymmetrically can be highly corrected.

  Next, numerical conditions that the scanning optical system according to the first embodiment should satisfy will be described. Each condition described below is preferably satisfied at the same time, but is not limited thereto. Each conditional expression can obtain a corresponding effect by satisfying only individual conditions.

In the scanning optical system according to Embodiment 1, when the amplitude angle of sinusoidal vibration in the main scanning surface of the reflecting surface is ω and the rotation angle corresponding to the effective writing width of the reflecting surface is θ,
0.29 <θ / ω <0.72 (1)
It is preferable to satisfy the following conditional expression.

  By satisfying conditional expression (1), the optical characteristics on the surface to be scanned can be maintained in a well-balanced manner. That is, if the lower limit of conditional expression (1) is exceeded, the usage ratio of the reflection surface with respect to the amplitude angle of the sinusoidal vibration decreases, and the focal length of the scanning lens increases, so that the spot diameter on the surface to be scanned is reduced. It becomes difficult and high-quality output images cannot be obtained. When the upper limit of conditional expression (1) is exceeded, the influence of sinusoidal vibration in the resonant deflection mirror becomes large, and it becomes difficult to correct linearity on the scanned surface.

  Conditional expression (1) can achieve the above effect more remarkably by setting the following range.

0.35 <θ / ω <0.52 (2)
In the scanning optical system according to Embodiment 1, when the magnification in the sub-scanning direction in the second imaging optical system is m,
0.5 <m <3.5 (3)
It is preferable to satisfy the following conditional expression.

  By satisfying conditional expression (3), it is possible to achieve a configuration in which the amount of jitter generated on the surface to be scanned is suppressed in a well-balanced manner due to the vibration of the scanning lens and the resonant deflection mirror. That is, when the lower limit of conditional expression (3) is exceeded, the amount of jitter generated due to the resonance-type deflection mirror increases, causing pitch unevenness between scanning lines in the sub-scanning direction on the surface to be scanned, resulting in a homogeneous image. I can't get it. If the upper limit of conditional expression (3) is exceeded, the amount of jitter generated due to lens vibration increases, resulting in uneven pitch between scanning lines in the sub-scanning direction on the surface to be scanned, and obtaining a homogeneous image. I can't.

  Conditional expression (3) can achieve the above effect more remarkably by setting the following range.

0.7 <m <3.0 (4)
(Embodiment 2)
FIG. 3 is an optical configuration diagram of a scanning optical system applied to the image reading apparatus according to the second embodiment. The image reading apparatus according to the second embodiment is equipped with the scanning optical system described in the first embodiment as an image reading optical system. In FIG. 3, the scanning optical system of the image reading apparatus according to the second embodiment has the same schematic configuration as the scanning optical system according to the first embodiment, so only the differences will be described. Note that in FIG. 3, configurations denoted by the same reference numerals are the same as the configurations described in the first embodiment.

  The scanning optical system applied to the image reading apparatus according to the second embodiment includes a half mirror 11, a detector 12, and a detection optical system 13 in addition to the scanning optical system according to the first embodiment. . The half mirror 11 transmits the laser beam from the semiconductor laser 1 to illuminate the reading surface 10 which is a two-dimensional image to be read, and reflects the return light from the reading surface 10 toward the detection optical system 13. The detection optical system 13 guides the return light to the detector 12 reflected by the half mirror. Since the image reading apparatus according to the second embodiment uses the scanning optical system according to the first embodiment, it can read a high-quality image.

(Embodiment 3)
FIG. 4 is a cross-sectional view showing the configuration of the image forming apparatus according to the third embodiment. This image forming apparatus is a printer apparatus that forms a monochrome image based on a data signal input from the outside. The image forming apparatus according to the third embodiment includes the scanning optical system according to the first embodiment as the exposure optical system 21.

  The image forming apparatus according to the third embodiment is a known electrophotographic printer, and includes a primary charger 20, an exposure optical system 21, a developing device 22, a transfer charger 23, a cleaner 24, and a fixing device 25. The sheet feeding cassette 26 and the photosensitive drum 27 (electrostatic latent image carrier) are included. The exposure optical system 21 forms an electrostatic latent image, and print information is written on the photosensitive drum 27 as an electrostatic latent image. The surface of the photosensitive drum 27 is covered with a photosensitive member whose charge changes when irradiated with light. By the primary charger 20, electrostatic ions adhere to the surface of the photosensitive drum 27 and are charged. The charged photosensitive drum 27 is developed by the developing unit 22 with charged toner attached to the print unit. The toner adhering to the photosensitive drum 27 is transferred to the paper supplied from the paper feed cassette 26 by the transfer charger 23. The transferred toner is fixed on the paper by the fixing device 25. The remaining toner is removed by the cleaner 24.

  Since the image forming apparatus according to the third embodiment uses the scanning optical system according to the first embodiment, a high-quality image can be formed.

  In each of the above embodiments, the laser beam is deflected by a resonant deflection mirror. However, the present invention is not limited to this. For example, a galvanometer mirror or MEMS may be used. That is, a configuration that functions as a deflector having a reflecting surface that deflects an incident beam can be used instead of the resonant deflection mirror.

  The wavelength of the laser beam is not particularly limited, and a beam having any wavelength such as an infrared region, a visible red region, or a visible blue region may be used.

  Hereinafter, specific numerical examples of the scanning optical system according to the first embodiment will be described. Example 1 is a numerical example embodying the first embodiment shown in FIGS.

  Regarding the shape of the lens surface of the scanning lens, the sub-scanning direction coordinate with the vertex of the surface as the origin is x (mm), the main scanning direction coordinate is y (mm), and the sag amount z (mm) from the vertex is the incident beam. It is defined by the following formula, with the direction of heading being positive.

  Where P (y) is the shape in the main scanning plane including the optical axis, RDy (mm) is the radius of curvature in the main scanning plane, K is the conic constant contributing to the main scanning direction, AD, AE, AF and AG are Even-order constants contributing to the main scanning direction, AOD, AOE, AOF, and AOG are odd-order constants, RDx is a function representing the radius of curvature in the sub-scanning plane at each y coordinate, and RDs (mm) is the center sub-scanning. In-plane curvature radii, BC, BD, BE, BF, and BG are even-order constants, and BOC, BOD, BOE, BOF, and BOG are odd-order constants.

  In Example 1, the shape of the reflecting mirror is a plane. Further, in each table, Si represents the surface number, RDyi represents the radius of curvature in the main scanning plane, RDsi represents the radius of curvature of the cross section in the sub-scanning direction, di represents the axial top surface spacing, and ni represents the refractive index of the optical material. The design wavelength is 780 nm, the units of RDyi, RDsi, and di are (mm), and the units of ω and θ are (deg). S0 corresponds to the reflection surface of the resonant deflection mirror. In Table 3, ω represents the amplitude angle of the resonant deflection mirror, θ represents the rotation angle corresponding to the effective writing width of the resonant deflection mirror, and m represents the magnification in the sub-scanning direction in the second imaging optical system. ing.

Example 1
Table 1 shows parameters related to the second imaging optical system, and Table 2 shows parameters related to the shape of the scanning lens for specific numerical values of Example 1.

  5 and 6 are aberration diagrams on the scanned surface of Example 1. FIG. In each figure, the vertical axis represents the scanning width corresponding to the deflection angle on the surface to be scanned. FIG. 5 shows the arc sine characteristic on the surface to be scanned, where the side on which the arc sine characteristic is compressed is negative and the side on which it is expanded is positive. FIG. 6 shows the field curvature in the main scanning direction on the surface to be scanned. The case where the imaging position is in front of the ideal image plane position is positive, and the case where the imaging position is in the rear side of the ideal image plane position. Negative.

  It should be noted that the expression format of the expression representing the lens shape used in the embodiment is an example, and other expression expressions may be used as long as a similar shape can be expressed.

(Example 2)
Example 2 is a numerical example embodying the first embodiment shown in FIGS. 7 to 8.

  Table 3 shows parameters related to the second imaging optical system, and Table 4 shows parameters related to the shape of the scanning lens with respect to specific numerical values of Example 2.

  FIGS. 9 and 10 are aberration diagrams on the scanned surface of Example 2. FIGS.

  The present invention is suitable for image forming apparatuses such as laser printers, laser facsimiles, and digital copying machines, and image reading apparatuses such as image scanners and copying machines.

Configuration diagram of scanning optical system according to Embodiment 1 Sectional drawing of the 2nd image formation optical system of the scanning optical system concerning Embodiment 1 in the subscanning direction Optical configuration diagram of a scanning optical system applied to an image reading apparatus according to Embodiment 2 Sectional drawing which shows the structure of the image forming apparatus which concerns on Embodiment 3. FIG. Aberration diagram showing linearity characteristics on the scanned surface of Example 1 Aberration diagram showing curvature of field on the surface to be scanned of Example 1 Configuration diagram of a scanning optical system according to another example of Embodiment 1 Sectional drawing of the 2nd imaging optical system of the scanning optical system which concerns on another example of Embodiment 1 in the subscanning direction Aberration diagram showing linearity characteristics on the scanned surface of Example 2 Aberration diagram showing curvature of field on the surface to be scanned in Example 2

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Semiconductor laser 2 Collimating lens 3 Cylindrical lens 4 Resonance type deflection mirror 5 Rotating shaft 6 Scanning lens 6a 1st scanning lens 6b 2nd scanning lens 7 Scanned surface 10 Reading surface 11 Half mirror 12 Detector 13 Detection optical system 20 Primary charging 21 Exposure optical system
22 Developing Device 23 Transfer Charging Device 24 Cleaner 25 Fixing Device 26 Paper Feed Cassette 27 Photosensitive Drum

Claims (14)

A scanning optical system that forms and scans a beam emitted from a light source as a spot on a surface to be scanned,
A deflector that performs main scanning by reflecting and deflecting the beam from the light source by a reflecting surface that changes in angle at a predetermined period;
A second imaging optical system that re-images the beam from the polarizer as a spot on the scanned surface;
The first imaging optical system includes collimating means for collimating a divergent beam from the light source,
The second imaging optical system is a scanning optical system including an arc sine lens system corresponding to an angle change of the reflecting surface. A scanning optical system that forms and scans a beam emitted from a light source as a spot on a surface to be scanned,
A deflector that performs main scanning by reflecting and deflecting the beam from the light source by a reflecting surface that changes in angle at a predetermined period;
A second imaging optical system that re-images the beam from the polarizer as a spot on the scanned surface;
The first imaging optical system includes a collimator that collimates a divergent beam from the light source, and a beam extending from the collimator in a linear shape extending in the main scanning direction in the vicinity of the reflecting surface of the deflector. Imaging means for imaging,
The second imaging optical system is a scanning optical system including an arc sine lens system corresponding to an angle change of the reflecting surface. The arc sine lens system includes, in order from the reflective surface side, a first lens element having a positive power at least in the main scanning direction and a second lens element having a negative power in at least the main scanning direction. The scanning optical system according to claim 1. 4. The scanning optical system according to claim 3, wherein the first lens element has a positive meniscus shape with a convex surface facing the surface to be scanned in the main scanning direction. 4. The scanning optical system according to claim 3, wherein the second lens element has a negative meniscus shape with a convex surface facing the surface to be scanned in the main scanning direction. The scanning optical system according to claim 1, wherein the reflection surface changes its angle by rotating sinusoidally. 6. The scanning optical system according to claim 1, wherein the reflecting surface reciprocally rotates sinusoidally and writing is performed in the reciprocating rotation. The scanning optical system according to claim 1, wherein the shape of the reflecting surface is a substantially flat surface. When the amplitude angle of sinusoidal vibration in the main scanning plane of the reflection surface is ω, and the rotation angle corresponding to the effective writing width of the reflection surface is θ,
0.29 <θ / ω <0.72 (1)
The scanning optical system according to claim 1, wherein the conditional expression is satisfied. When the amplitude angle of sinusoidal vibration in the main scanning plane of the reflection surface is ω, and the rotation angle corresponding to the effective writing width of the reflection surface is θ,
0.35 <θ / ω <0.52 (2)
The scanning optical system according to claim 1, wherein the conditional expression is satisfied. When the magnification in the sub-scanning direction in the second imaging optical system is m,
0.5 <m <3.5 (3)
The scanning optical system according to claim 1, wherein the conditional expression is satisfied. When the magnification in the sub-scanning direction in the second imaging optical system is m,
0.7 <m <3.0 (4)
The scanning optical system according to claim 1, wherein the conditional expression is satisfied. An image forming apparatus that forms an image based on an image signal,
A light source in which a beam to be emitted is controlled based on an input image signal;
An exposure optical system for forming an electrostatic latent image on an electrostatic latent image carrier based on a beam emitted from the light source;
Development means for developing the electrostatic latent image formed by the exposure optical system,
The image forming apparatus according to claim 1, wherein the exposure optical system includes the scanning optical system. An image reading apparatus that reads a two-dimensional image and forms an electrical image signal,
A light source in which a beam to be emitted is controlled based on an input image signal;
A reading optical system for illuminating a two-dimensional image to be read by a beam emitted from the light source;
A detector for detecting a return beam from the two-dimensional image illuminated by a reading optical system;
The image reading apparatus according to claim 1, wherein the reading optical system includes the scanning optical system.

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