JP2015014639A - Optical scanner, and image forming apparatus using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inexpensive, compact optical scanner that has high definition and reduces the curvature on the image surface of an incident optical system, and an image forming apparatus using the same.SOLUTION: An optical scanner includes: a light source that has a plurality of light emitting units; an incident optical system that has a collimator optical system for converting light beams from the light source, and a diaphragm for regulating the width of the light beams from the collimator optical system in one of the main scanning direction and sub scanning direction with a wider arrangement interval of the light emitting units; a deflector that is rotated around a rotation shaft parallel to the sub scanning direction and deflects the light beams from the incident optical system on a main scanning plane; and an imaging optical system that guides the light beams from the deflector to a scanning target surface. The collimator optical system includes a first optical element group and second optical element group composed of optical elements not including an aspherical surface and arranged from the light source side in order; when the distance from the incident surface of the second optical element group to the diaphragm is L, and the focal point distance of the incident optical system in one of the main and sub scanning directions with the wider arrangement interval of the light emitting units is f, the condition: 0.5<|L|/f<2 is satisfied.

Description

  The present invention relates to an optical scanning apparatus and an image forming apparatus using the same, and is suitable for an image forming apparatus such as a laser beam printer having an electrophotographic process, a digital copying machine, or a multi-function printer (multi-function printer).

  2. Description of the Related Art Conventionally, an optical scanning device used in an image forming apparatus such as a laser beam printer or a digital copying machine guides a light beam emitted from a light source means to an optical deflector as a deflecting means by an incident optical system. The light beam deflected and scanned by the optical deflector is imaged in a spot shape on the surface of the photosensitive drum, which is the surface to be scanned, by the imaging optical system, and the light beam is optically scanned on the surface of the photosensitive drum.

  In such an optical scanning device, the light beam emitted from the light source means is converted into a parallel light beam by a collimator lens, and is condensed as a line image on the deflection surface of the optical deflector by a cylindrical lens. The light beam deflected by the deflecting surface of the optical deflector is condensed as a spot image on the photosensitive drum surface by the imaging lens system, and is scanned at a constant speed on the photosensitive drum surface.

  In such an optical scanning device, speeding up and high resolution can be achieved by increasing the number of light emitting points (light emitting portions) of the light source means.

  Various methods for reducing the spot diameter difference between beams caused by the curvature of field of the incident optical system when the number of light emitting units of the light source means is increased have been proposed (see Patent Documents 1 and 2). .

  Patent Document 1 discloses a technique for reducing the curvature of field of an incident optical system using a collimator lens having a non-arc surface in an optical scanning device having a multi-beam light source. In Patent Document 2, in an image forming apparatus having a multi-beam light source, a lens having a non-arc optical surface in the main scanning direction is used as the incident optical system in order to reduce curvature of field in the main scanning direction that occurs in the incident optical system. The technique used is disclosed.

Japanese Patent No. 3767167 Japanese Patent Application Laid-Open No. 2010-134430

Optical Technology Series 1 Lens Design Method by Yoshiya Matsui Kyoritsu Shuppan 1972

  In the conventional optical scanning device, when a multi-beam laser having a light emitting point arranged at a position far from the optical axis of the incident optical system is used as the light source means, the focus of each beam emitted from the multi-beam laser on the surface to be scanned is focused. There is a problem that the position is shifted and a spot diameter difference between the beams is generated, and the image is deteriorated.

  In a conventional optical scanning device, an aspherical lens is used to correct aberrations (curvature of the image plane) generated in the incident optical system, and a focus difference (spot diameter difference) between the light emitting points of the multi-beam on the scanned surface. It was reduced.

  However, when an aspherical lens is manufactured by molding, it is necessary to use an expensive mold, so manufacturing cost becomes a problem especially in the case of small lots. The problem is that the productivity is low and the productivity is low. (First issue)

  If a light source means having a plurality of light emitting points that are separated from the optical axis of the collimator lens is used, if the incident optical system including the collimator lens has a field curvature, the light beams from the two light emitting points are scanned. The condensing position (focus position) on the surface or the deflection surface is different. Then, since the spot diameters of the two light beams on the surface to be scanned are different from each other, a difference occurs in the image quality based on the two light emitting points. When an aspherical collimator lens is used to correct this, a manufacturing error tends to occur in the manufacturing process, which is particularly noticeable in molding. Such a manufacturing error with respect to the design shape causes a focus difference between the light emitting points, which causes a problem of deteriorating image quality. (Second problem)

  SUMMARY OF THE INVENTION An object of the present invention is to provide an optical scanning device that reduces the curvature of field of an incident optical system, is high-definition, is inexpensive, and is compact, and an image forming apparatus using the same.

In order to achieve the above object, an optical scanning device according to one aspect of the present invention includes a light source having a plurality of light emitting units, a collimator optical system that converts a light beam from the light source, and a main scanning direction and a sub scanning direction. An incident optical system having a diaphragm for limiting a light beam width of the light beam from the collimator optical system in a wider arrangement interval of the light emitting units, and rotating about a rotation axis parallel to the sub-scanning direction, the incident optical A deflector that deflects the light beam from the system in a main scanning plane, and an imaging optical system that guides the light beam from the deflector to the scanned surface, and the collimator optical system includes an aspherical surface Comprising a first optical element group and a second optical element group, which are arranged in order from the light source side, and the distance from the incident surface of the second optical element group to the stop is L, Among the main scanning direction and sub-scanning direction, When the focal length of the incident optical system location spacing in the wide direction f, and, 0.5 <| L | / f <2, satisfies the following condition, and wherein the.
Further objects and other features of the present invention will become apparent from the preferred embodiments described below with reference to the accompanying drawings.

  According to the present invention, it is possible to provide a high-definition, inexpensive, and compact optical scanning device that reduces the curvature of field of the incident optical system and an image forming apparatus using the same.

FIG. 3 is a main scanning sectional view of the optical scanning device according to the first embodiment. FIG. 3 is a sub-scan sectional view of the optical scanning device according to the first embodiment. FIG. 3 is an enlarged view of a main scanning section of an incident optical system of the optical scanning device according to the first embodiment. Schematic diagram of the light source means of the optical scanning device according to the first embodiment. The figure which shows the distance from the surface by the side of the deflector of the 2nd collimator lens group of the optical scanning device of Example 1 to the stop with respect to the main scanning direction, and a field curvature aberration Schematic diagram for explaining curvature of field of the incident optical system The figure which shows the main scanning direction field curvature of the incident optical system of the optical scanning device of Example 1. The figure which shows the comparative example of the main scanning direction field curvature of the incident optical system of an optical scanning device The figure which shows the distance from the surface by the side of the deflector of the 2nd collimator lens group of the optical scanner of Example 2, and the stop with respect to the main scanning direction, and a field curvature aberration The figure which shows the main scanning direction field curvature of the incident optical system of the optical scanning device of Example 2. Schematic diagram of main parts of a monochrome image forming apparatus to which the optical scanning device of the present invention is applied. Schematic diagram of essential parts of a color image forming apparatus to which an optical scanning device of the present invention is applied.

  Hereinafter, embodiments of the optical scanning device of the present invention will be described with reference to the drawings.

<Configuration of the whole optical scanning device>
FIG. 1 shows a cross section in the main scanning direction (main scanning cross section) of the main part of the optical scanning apparatus according to Embodiment 1 of the present invention. FIG. 2 is a cross-sectional view (sub-scanning cross-sectional view) of the main part in the sub-scanning direction. As shown in FIGS. 1 and 2, the optical scanning device of the present invention includes, in order from the light source side, a light source means 1, an incident optical system LA (2-6) shown in an enlarged view in FIG. 3, and an optical deflector as a deflection means. Rotating polygon mirror 10 and imaging optical system 20.

  In the following description, the main scanning direction (Y-axis direction) is a direction perpendicular to the rotation axis of the rotating polygon mirror and the optical axis of the imaging optical system (the light beam is reflected and deflected (deflected and scanned) by the rotating polygon mirror). Direction). The sub-scanning direction (Z-axis direction) is a direction parallel to the rotation axis of the rotary polygon mirror. The main scanning section is a plane (cross section perpendicular to the sub scanning direction) including the main scanning direction and the optical axis of the imaging optical system. The sub-scan section is a section that includes the optical axis of the imaging optical system 20 and is perpendicular to the main scan section.

<Light source means>
The light source means 1 is composed of a semiconductor laser (multi-beam light source) having a plurality of light emitting points (light emitting portions). In the semiconductor laser 1, at least one of the light emitting points has a distance from the optical axis of the first collimator lens group 3 to be described later that is different from the distance from the optical axis of the other one light emitting point. In this embodiment, as shown in FIG. 4, the light emitting points are arranged in a one-dimensional manner, and are composed of a surface emitting laser composed of 32 light emitting points. The surface emitting laser can be easily arrayed, and the light emitting points including the light emitting point LD16 on or near the axis and the light emitting points LD1 and LD32 off the most axis in the main scanning direction have different positions in the main scanning direction 32. A one-dimensional array structure in which beams are arranged. This is because high speed and high definition are achieved by using a 32-beam laser. Each light emitting point is configured to be independently modulated, and the light emission intensity and timing are controlled by a laser driver (not shown). Further, in order to suppress a beam interval error caused by an attachment error at the time of assembling the laser light source, the laser light source is held rotatably about an axis parallel to the optical axis of the incident optical system LA.
In the embodiment, the light source means is exemplified as having a one-dimensionally arranged structure, but the present invention is not limited to this, and the light source means having a two-dimensionally arranged structure is also included. The same can be applied.

<Incoming optical system>
The incident optical system LA of this embodiment includes, in order from the light source side, a first diaphragm 2, collimator optical systems 3 and 4, a cylindrical lens 5 having power only in the sub-scanning direction, a second diaphragm 6, and a part of the light beam. And a prism 7 for reflecting the light.

  The first aperture stop 2 shapes the beam shape by limiting the beam width of the passing beam in the sub-scanning direction. The first aperture stop 2 is disposed in the vicinity of the collimator lens 3, the exit pupil position in the sub-scanning direction is positioned in the vicinity of the fθ lens 20b of the imaging optical system 20 to be described later, and the chief rays of 32 beams are in the vicinity of the fθ lens 20b. Pass through the same position in the sub-scanning direction.

  The collimator optical system includes, in order from the light source side, a first optical element group (first collimator lens group) 3 having a convex power and a second optical element group (second collimator) whose convex power is weaker than that of the first collimator lens group. And an optical system that converts a divergent light beam emitted from the light source means 1 into a parallel light beam. The collimator optical system of the present invention does not include an aspherical surface. In this specification, the plane is a spherical surface having an infinite radius and is not included in the aspherical surface. The first collimator lens group 3 is composed of a cemented lens (glass condensing lens) of a concave lens and a convex lens in order from the light source side. As a result, the condensing positions (focus positions) of the light beams from the plurality of light emitting points on the scanning surface or the deflection surface can be made substantially the same, and the spot diameters of the plurality of light beams on the scanning surface can be made substantially the same. That is, as in Patent Document 2, when the incident optical system is configured by only a convex lens, the axial chromatic aberration cannot be corrected, and the focus difference due to the initial wavelength difference between a plurality of beams cannot be corrected. Deterioration can be avoided. The second collimator lens 4 is a glass convex spherical lens, and is a lens for adjusting the spot diameter on the surface to be scanned. The first collimator lens 3 and the second collimator lens 4 convert the divergent light beam from the light source means 1 into parallel light and reduce the spot diameter difference between the light emitting points.

The combined focal length of the first collimator lens group and the second collimator lens group in the collimator optical system is determined by the amount of laser light taken in. The focal length f _col1 of the first collimator lens group and the second collimator lens group in either the main scanning section or the sub-scanning section including the main scanning direction and the sub-scanning direction including the direction in which the light emitting units are arranged widely. The focal length f _col2 is
0.1 <f _col1 / f _col2 <1/3 (1)
It is desirable to satisfy. Thereby, the focus change sensitivity due to the movement in the optical axis direction within the main body of the second collimator lens group is appropriately set, and the second collimator lens group is caused by a manufacturing error by moving in the optical axis direction within the main body. The focus position in the main scanning direction can be adjusted appropriately.

  The cylindrical lens 5 has power only in the sub-scan section (sub-scan direction). The cylindrical lens 5 focuses the light beam that has passed through the collimator optical system on the deflection surface (reflection surface) 10a of the optical deflector 10 as a substantially line image in the main scanning plane. The second diaphragm 6 shapes the beam shape by limiting the beam width of the passing beam in the main scanning direction. The second diaphragm 6 in the present invention is disposed closer to the optical deflector 10 than the second collimator lens 4.

In the optical scanning device of the present invention, L is an incident surface of the second optical element group (second collimator lens group) (“lens surface (incident surface) on the deflector side)”, and the second collimator lens group includes a plurality of lenses. Is the distance from the second stop to the second stop, and f _col is the focal length in the main scanning section of the incident optical system. And when
0.5 <| L | / f _col <2 (2)
Satisfied. With this configuration, field curvature is reduced. Furthermore, when using an optical system in which the distance from the optical axis to the off-axis light emitting point is 0.5 mm or more, more preferably,
0.5 <| L | / f _col <1.5 (2a)
If the condition is satisfied, the curvature of field is reduced more favorably. If Expression (2) is not satisfied, the amount of field curvature increases and the spot diameter difference between the beams increases, so that it is difficult to achieve high resolution.

<APC optical system>
Unlike the edge-emitting laser, the surface emitting laser has an APC (Auto Power Control) sensor in the element, and therefore has a light amount detection sensor (APC sensor) 9 outside the laser. The APC sensor 9 is a light amount detection sensor for detecting the light amount, and feeds back the light amount in order to cause the surface emitting laser to emit light so that each beam of the surface emitting laser as the light source means 1 has a desired light amount. The prism 7, the condensing lens 8, and the light quantity detection sensor 9 constitute an APC optical system. The light beam emitted from the surface emitting laser that is the light source means 1 passes through the second diaphragm 6, is partially reflected by the incident surface of the prism 7, and the reflected light is condensed by the condenser lens 8 to detect the light amount. The sensor 9 detects the amount of light. Based on the detected light quantity, the light emission quantity of the surface emitting laser of the light source means 1 is controlled.

  The prism 7 (wedge prism) having a wedge shape in the main scanning direction (main scanning cross section) is disposed so that the incident surfaces of the second diaphragm 6 and the prism 7 coincide with each other. In order to prevent the reflected light from the exit surface from entering the light quantity detection sensor 9 described later, the entrance surface and the exit surface of the prism 7 have an angle of 4 ° in the main scanning section. The condensing lens 8 is a lens for condensing the light beam reflected by the incident surface of the prism 7 on the light amount detection sensor 9.

  Further, the second diaphragm 6 arranged in the vicinity of the deflector 10 can limit the light beam width in the main scanning direction and can make the chief ray of the light beam from each light emitting point on the deflection surface approach. The vertical line fluctuation that occurs during multi-beam can be reduced.

<Optical deflector>
An optical deflector 10 as a deflecting unit is composed of a polygon mirror (rotating polygonal mirror) having a five-surface configuration, and is driven around a rotation axis parallel to the sub-scanning direction by a driving unit (not shown) configured by a motor or the like. It rotates at a constant speed in the direction of arrow A. The light beam incident on the reflecting surface from the incident optical system is deflected in one direction within the main scanning surface by the rotation of the polygon mirror.

<Imaging optical system>
The imaging optical system (fθ lens system) 20 includes a first imaging lens 20a and a second imaging lens (fθ lens) 20b, and has a condensing function and fθ characteristics. The first imaging lens 20a is a glass plano-convex spherical lens, and the second imaging lens 20b is an aspherical anamorphic lens in the main scanning plane. The imaging optical system 20 guides a light beam based on the image information reflected and deflected by the optical deflector 10 onto a photosensitive drum surface 30 as a scanning surface, and forms an image in a spot shape. Further, the surface tilt correction optical system is configured by providing a conjugate relationship between the reflecting surface 10a of the optical deflector 10 and the photosensitive drum surface 7 in the sub-scan section.

  The exit surface of the second imaging lens (fθ lens) 20b of the present embodiment has a non-arc shape in the sub-scan section, and the non-arc amount is determined in the longitudinal direction of the second imaging lens (fθ lens) 20b ( By changing in the main scanning direction, the amount of wavefront aberration in the sub-scanning direction is changed, and wave optical field curvature in the sub-scanning direction is reduced. Further, by appropriately generating the sub-scanning paraxial field curvature of the imaging optical system (fθ lens system) 20, the sub-scanning spot position deviation on the surface to be scanned when the surface tilt occurs is reduced, and the pitch unevenness is reduced. Reduced.

  The first imaging lens 20a is made of a plano-convex spherical lens made of glass, thereby improving the stability against temperature change. Further, the second imaging lens (fθ lens) 20b is formed of a plastic lens having light-transmitting power, and has a high degree of freedom with respect to the aspherical design. Note that both of the imaging optical elements 20a and 20b of the imaging optical system 20 may be made of plastic, or may be an imaging optical element having a diffraction power. When the imaging optical system 20 is configured with a diffractive surface, it is possible to provide an optical scanning device with more excellent characteristics with respect to temperature changes.

  In this embodiment, the imaging optical system is composed of two imaging optical elements. However, the present invention is not limited to this, and the above-described implementation is possible even if it is composed of one or three or more. The same effect as the example can be obtained.

  In the optical scanning device of the present embodiment, the light beam width in the sub-scanning direction is limited by the first diaphragm 2 for a plurality of (32 in this embodiment) light beams that are light-modulated according to image information and emitted from the light source means 1. They are converted into parallel light beams by the collimator optical systems 3 and 4 and enter the cylindrical lens 5. The light beam incident on the cylindrical lens 5 is emitted as it is in the main scanning section, and the second diaphragm 6 limits the light beam width in the main scanning direction. The light beam incident on the cylindrical lens 5 converges in the sub-scan section and passes through the second diaphragm 6 (the light beam width in the main scanning direction is limited) in the vicinity of the deflection surface 10a of the optical deflector 10. The image is formed as (line image elongated in the main scanning direction).

  The plurality of light beams reflected and deflected by the deflecting surface 10 a of the optical deflector 10 enter the imaging optical system 20 having a convex power, and form an image on the photosensitive drum surface 30 in a spot shape. By rotating the optical deflector 10 in the direction of arrow A, optical scanning is performed on the photosensitive drum surface 30 in the direction of arrow B (main scanning direction) at a constant speed. As a result, a plurality of scanning lines are simultaneously formed on the photosensitive drum surface, which is a recording medium, and image recording is performed.

  Numerical data of the optical scanning device in this embodiment are shown in Tables 1 to 4. In this embodiment, the shape of the imaging optical system (fθ lens system) 20 is based on the intersection of the imaging lens and the optical axis as the origin, and as shown in FIG. 1, the scanning start side and the scanning end side with respect to the optical axis. For each, the optical axis is the X axis, the direction perpendicular to the optical axis in the main scanning section is the Y axis, and the direction orthogonal to the optical axis in the sub-scanning section is the Z axis.

走査終了側

但し、Ｒは曲率半径、Ｋ、Ｂ₄、Ｂ₆、Ｂ₈、Ｂ₁₀は非球面係数である。添字ｓは走査開始側、添字ｅは走査終了側を表す。 Scan start side

End of scanning

Here, R is the radius of _{curvature, K, B 4, B 6} , B 8, B 10 are aspherical coefficients. The subscript s represents the scanning start side, and the subscript e represents the scanning end side.

  In the present embodiment, the first imaging lens 20a is configured such that the shapes of the entrance surface (R1 surface) and the exit surface (R2 surface) in the main scanning direction are symmetric with respect to the optical axis. That is, the aspherical coefficients on the scanning start side and the scanning end side are matched (Table 4).

  Further, in the second imaging lens (fθ lens) 20b, the sub-scanning cross-sectional shape of the incident surface (R3 surface in Table 4) is an arc shape, and the exit surface (R4 surface in Table 4) is the sub-scanning direction. The cross-sectional shape is a non-arc shape having a fourth-order term of Z. On the exit surface, the non-arc amount changes in the longitudinal direction. Further, the shape of the second imaging lens 20b in the sub-scanning direction is within the sub-scanning section of the incident surface (a plane that includes the optical axis and is orthogonal to the main scanning surface) on the scanning start side and the scanning end side with respect to the optical axis. The curvature 1 / r and the fourth-order aspherical coefficient are functions of Y, and are continuously changed in the effective portion of the lens.

  The shape in the sub scanning direction is the X axis on the scanning start side and the scanning end side with respect to the optical axis, the Y axis in the direction perpendicular to the optical axis in the main scanning plane, and the optical axis in the sub scanning plane. The shape in the sub-scanning direction of the incident surface (R3 surface) and the exit surface (R4 surface) of the second imaging lens 20b can be expressed by the following continuous function with the direction as the Z axis.

走査終了側

ここで、ｒは光軸位置（ｙ＝０）における副走査断面における曲率半径、ｒ’は主走査方向の位置ｙにおける副走査断面での曲率半径、Ｄｊは曲率変化係数、Ｍｉは子線非球面係数を表す。Ｙのプラス側とマイナス側で係数が異なる場合は、添字ｓは走査開始側、添字ｅは走査終了側を表している。また、副走査断面の曲率半径とは主走査方向の形状（母線）に直交する断面内における曲率半径である。 Scan start side

End of scanning

Here, r is the radius of curvature in the sub-scanning section at the optical axis position (y = 0), r ′ is the radius of curvature in the sub-scanning section at the position y in the main scanning direction, Dj is the curvature change coefficient, and Mi is the non-wire line. Represents the spherical coefficient. When the coefficients are different on the positive side and the negative side of Y, the subscript s represents the scanning start side, and the subscript e represents the scanning end side. Further, the radius of curvature of the sub-scanning section is the radius of curvature in the section orthogonal to the shape (bus line) in the main scanning direction.

Table 1: Numerical data of the optical scanning device of Example 1

Table 2: Rotation center coordinates of deflecting means of Example 1

Table 3: Numerical data of the incident optical system of Example 1

Table 4: Numerical data of the imaging optical system in Example 1

(About field curvature reduction)
FIG. 3 shows a development view of the incident optical system LA of the present embodiment.
The collimator optical system of this embodiment has a combined focal length f = 75 mm. Further, the focal length f _col1 of the first collimator lens group is 88.6 mm, and the focal length of the second collimator lens group is 335.1 mm, so that the power ratio is 1 / 88.6: 1 / 335.1≈4: 1 and satisfies Expression (1). Also, by configuring the first collimator lens group with a bonded lens, the first collimator lens group can be easily held and adjusted when compared to the case where the installation positions of a plurality of independent lenses need to be adjusted. To be able to.

  When the power and refractive index of the optical system constituting the incident system are determined, the Petzval sum P serving as an index representing the amount of field curvature can be calculated. The Petzval sum in the main scanning direction of the incident optical system of the present embodiment is 0.68. In general, the field curvature of the meridional section is represented by 3 × III + P from the aberration coefficient III and the Petzval sum P. Therefore, it is necessary to cancel the Petzval sum P (= 0.68). When the aberration coefficient III (= −0.23) is given, field curvature can be suppressed. However, on the assumption that the position of the second diaphragm is fixed to the deflector side surface of the second collimator lens group, only the curvature, arrangement, and refractive index of the two groups of three spherical lenses constituting the collimator lens group are included. It is difficult to find a solution that balances other aberrations (wavefront aberration, chromatic aberration) and curvature of field with the above as a parameter. Therefore, in the present invention, the optimal solution of the aberration coefficient III and appropriate suppression of other aberration fluctuations are compatible by appropriately controlling the position of the second diaphragm 6.

According to the paraxial theory, the curvature of field is proportional to the aberration coefficient III (Non-Patent Document 1). Further, when h is the ray height of the object paraxial ray, ha is the ray height of the pupil paraxial ray, φ is the power of the collimator lens, and A and B are eigencoefficients, the aberration coefficient III is
III = h ² ha ² A + 2h ² ha ² B + φ (3)
It is expressed. Accordingly, the amount of field curvature can be reduced by appropriately setting the power arrangement of the optical system and the aperture position in the optical system. As described above, the power arrangement of the incident optical system is determined by requirements other than aberration correction (light quantity, focus adjustment sensitivity), so the aperture position in the incident optical system can be adjusted within the constraints of the size of the device and manufacturing costs. The field curvature can be corrected.

FIG. 5 shows the relationship between the aperture position and the paraxial field curvature (3III + P) in this example. In the figure, the horizontal axis indicates the distance L from the deflector side surface of the second collimator lens group 4 to the second diaphragm 6, and the vertical axis indicates the paraxial field curvature. In this embodiment, the position where the paraxial field curvature becomes the smallest is the position where L = 100 mm, and the paraxial field curvature at that position is zero. In this embodiment, considering the balance with other aberrations, the distance L from the deflector side surface of the second collimator lens group 4 to the second diaphragm 6 is set to 87 mm to balance with other aberrations. ing. Since the focal length f _col in the main scanning section of the incident optical system is 75 mm,
0.5 <87/75 = 1.16 <2
0.5 <87/75 = 1.16 <1.5
Thus, the expressions (2) and (2a) are satisfied, and the field curvature is appropriately suppressed.

  FIG. 6 is a schematic diagram for explaining the curvature of field of the incident optical system. Δmcol in FIG. 6A represents the curvature of field of the incident optical system, that is, the focus difference between the light emitting points (focus difference converted to the light source position difference of the incident optical system). FIG. 6B is a diagram showing the spot diameter difference on the surface to be scanned when a focus difference occurs between the light emitting points. The horizontal axis indicates the defocus amount indicating the drum position, and the vertical axis indicates the spot. The position from the optical axis is shown. FIG. 6B shows that the spot diameter α1 of LD1 and the spot diameter α16 of LD16 are different by 5 μm at the drum position (horizontal axis = 0).

  In the optical scanning device of the present embodiment, the light source means is arranged so that the curvature of field of the incident optical system becomes zero at the position between the point light sources LD1 and LD16 in consideration of the balance of curvature of field of all point light sources. Has been placed. Therefore, the sign of the field curvature differs between the light emitting point LD16 on or near the axis (similar to LD16 in FIG. 4) and the most off-axis light emitting point LD1 in the main scanning direction (similar to LD1 in FIG. 4). That is, both the light beams from the light emitting points LD16 and LD1 are imaged at positions shifted in the focus direction (X-axis direction, that is, the optical axis direction) on the surface to be scanned.

In this embodiment, the imaging position difference (focus difference) ΔM between the light emitting points LD16 and LD1 in the main scanning direction on the scanned surface 7 is equal to | Δmcol |, the light emitting points LD1 (on-axis light emitting portion) and LD16 for the same wavelength. The field curvature difference of the light source closest to the optical axis and the light source farthest away from the optical axis, off-axis light emitting section, f _fθ is the focal length in the main scanning section of the imaging optical system, and f _col is the main scanning of the incident optical system When the focal length in the cross section,
ΔM = | Δmcol | × (f _fθ / f _col ) ² (4)
It can be calculated by The focus difference ΔM in this embodiment is
ΔM = 0.015 × (250/75) ² = 0.017 mm
It is. Usually, there is no problem if the focus difference ΔM in the main scanning direction is 0.5 mm or less. That is,
ΔM = | Δmcol | × (f _fθ / f _col ) ² ≦ 0.5 mm (5)
If it meets, there is no problem. Furthermore, in consideration of manufacturing errors and mounting errors of optical elements in a small spot high resolution compatible optical system,
ΔM = | Δmcol | × (f _fθ / f _col ) ² ≦ 0.2 mm (5a)
More preferably,
ΔM = | Δmcol | × (f _fθ / f _col ) ² ≦ 0.05 mm (5b)
It is better to satisfy. In this example,
| Δmcol | = 0.015
(F _fθ / f _col ) ² = 11.1
So,
| Δmcol | × (f _fθ / f _col ) ² = 0.0167
Thus, a high-resolution imaging optical system that satisfies the conditions (5), (5a), and (5b) and has a small variation in spot diameter between light emitting points is realized.

  FIG. 7 shows the curvature of field in the main scanning direction of the incident optical system LA of the present embodiment. In FIG. 7, the horizontal axis indicates the amount of focus shift (unit: mm) in terms of the light source position, and the vertical axis indicates the light emitting point position (unit: mm). FIG. 8 shows, as a comparative example, curvature of field when the incident optical system does not satisfy the conditional expression (2). As can be seen from FIG. 7, the curvature of field generated in the incident optical system of the present embodiment is 1 / compared to the curvature of field of the optical system configured by the incident optical system that does not satisfy the conditional expression shown in FIG. It can be reduced to 6 or less.

(About chromatic aberration reduction)
In this embodiment, chromatic aberration is also corrected by using a convex lens and a concave lens. When the incident optical system has axial chromatic aberration, there arises a problem that the spot diameter (focus) varies on the surface to be scanned due to variations in the wavelength of the laser light source. In this embodiment, the first collimator lens group is a cemented lens of a convex lens and a concave lens, and is configured such that the dispersion of the concave lens is larger (the Abbe number is smaller) than the dispersion of the other convex lenses. Specifically, s-tih11 (manufactured by OHARA) having an Abbe number of 25.68 is used as the glass material of the concave lens.

In the present embodiment, the focus change ΔT (1.5) in the main scanning direction and the sub-scanning direction due to the longitudinal chromatic aberration of the incident optical system when the laser oscillation wavelength differs by 1.5 nm between the plurality of light sources is 0.0005 mm. And
Δmcol> ΔT (1.5) (6)
It is configured to satisfy. As a result, the axial chromatic aberration is corrected to a level at which the focus difference can be ignored compared to the case where no concave lens is used.

(About the adjustment method of the incident optical system)
In the incident optical system of the present invention, the bonded lens of the light source and the first collimator lens group is held by the same member, and the distance between the light source and the first collimator lens group is adjusted in the manufacturing process. After the adjustment, the first collimator lens group is bonded and fixed, and the distance from the light source is fixed. In this state, a light source unit that integrally holds the light source and the first collimator lens group is attached to the scanning apparatus main body. In the scanning device main body, the focus position in the main scanning direction is adjusted by moving the convex lens of the second collimator lens group in the optical axis direction. In the present embodiment, the first collimator lens group and the light source, which have a large focus change due to the positional deviation in the optical axis direction, are integrated into a light source unit, thereby enabling more accurate position adjustment. In addition, since the second collimator lens group can be configured to be independently movable within the optical scanning device, it is possible to correct a focus shift caused by manufacturing variations other than the light source unit. That is, the first collimator lens group and the second collimator lens group are held independently so that their positions in the optical axis direction can be adjusted independently. The second collimator lens, which is the lens having the weakest power in the main scanning direction among the lenses of the incident optical system, is held so as to be movable in the optical axis direction in the optical scanning device. Therefore, there is an advantage that the focus position on the surface to be scanned can be adjusted with high accuracy compared to the case where the light source and the second collimator lens group are all held integrally in the operating device main body. In this embodiment, the focus position in the sub-scanning direction is adjusted by moving the cylindrical lens 5 having power only in the sub-scanning direction in the optical axis direction.

  As described above, the collimator optical system is composed of two groups like the incident optical system of the present embodiment, and the power arrangement and the aperture position of the second collimator lens group (spherical lens) are moved in the optical axis direction for optimization. By doing so, the focus difference (spot diameter difference) between multiple beams can be reduced, and a high-definition and high-speed optical scanning device can be provided.

In the present embodiment described above, the second collimator lens group has a diaphragm for limiting the beam width in the main scanning direction, which is the wider in the main scanning direction and the sub-scanning direction, among the arrangement intervals of the plurality of light emitting units of the light source means. The effect of the present invention was obtained by installing between the optical deflector and the optical deflector so as to satisfy the formula (2). However, the present invention is not limited to the configuration of the illustrated first embodiment. That is, a diaphragm that limits the light beam width in a wide direction in the main scanning direction and the sub-scanning direction among the arrangement intervals of the plurality of light emitting units of the light source means is installed between the second collimator lens group and the optical deflector, The distance L from the lens surface on the deflector side of the second collimator lens group to the stop, the focal length f of the incident optical system in the direction in which the arrangement interval of the light emitting units is wide among the main scanning direction and the sub-scanning direction,
0.5 <| L | / f <2
The effect of the present invention can be obtained by configuring so as to satisfy the above.

  In the optical scanning apparatus of the present embodiment, the differences from the first embodiment are the light emitting point interval of the light source laser and the configuration of the incident optical system. Other configurations such as the imaging optical system are the same as those of the optical scanning apparatus of the first embodiment.

  The schematic configuration of the incident optical system of the present embodiment is the same as that of the first embodiment shown in FIG. The light source of this example is the same as the light source of Example 1 shown in FIG. 4, but the pitch of the point light source is 50 μm in Example 1, whereas it is 25 μm in this example. Is different. For this reason, since many lasers can be produced from the same wafer from Example 1, it has the merit that the manufacturing cost of a light source can be reduced. In addition, since the distance between the light emitting points is narrower than that of the first embodiment, the distance range from the optical axis of the point light source is narrow, so that it is difficult to be affected by the curvature of field of the optical system. In the incident optical system of this embodiment, the distance from the second diaphragm 6 to the lens surface on the deflector 10 side of the second collimator lens group 4 is set to 40 mm, and the optical path length of the incident optical system is set to that of the first embodiment. Further shortening and further miniaturization are possible.

FIG. 9 shows paraxial field curvature. In the figure, the vertical axis indicates the absolute value of the distance L from the lens surface on the deflector 10 side of the second collimator lens group 4 to the second stop 6 and the aberration coefficient indicating the amount of field curvature. In the figure, there is a point where the paraxial field curvature becomes zero in the vicinity of L = 40 mm, and before and after that, the field curvature coefficient increases and decreases linearly. In this embodiment, L = 36 mm. In this example,
| Δmcol | = 0.00015 (mm)
(F _fθ / f _col ) ² = 11.1
So,
| Δmcol | × (f _fθ / f _col ) ² = 0.0167
Therefore, conditions (5), (5a), and (5b) are satisfied, and a high resolution imaging optical system can be realized satisfactorily.

  FIG. 10 shows the curvature of field in the main scanning direction of the incident optical system LA of the present embodiment. In FIG. 10, the horizontal axis indicates the amount of focus shift (unit: mm) in terms of light source position, and the vertical axis indicates the light emitting point position (unit: mm). The incident optical system of the present embodiment has a configuration in which a difference in spot diameter due to field curvature is unlikely to occur, and realizes a high-resolution imaging optical system.

Also in this embodiment, the focus change in the main scanning direction and the sub-scanning direction due to the axial chromatic aberration of the incident optical system when the transmission wavelength of the laser differs between the plurality of light sources by 1.5 nm is 0.0005 mm,
ΔM> ΔT (1.5) (6)
It is configured to satisfy. Therefore, the sum of the focus difference due to field curvature and the focus change due to wavelength difference is
0.15 μm + 0.5 μm = 0.65 μm
Since the surface to be scanned is 7.2 μm, the difference in spot diameter does not cause a problem.

  Table 5 shows numerical data of the incident optical system of this example. Since the numerical data of the imaging optical system and the like are the same as those in the first embodiment, a description thereof will be omitted.

Table 5: Numerical data of the incident optical system of Example 2

The focal length in the main scanning section of the synthesis of the first collimator lens group and the second collimator lens group (focal length in the main scanning section of the incident optical system) is determined by the amount of laser light taken in, etc. In the present embodiment, as in the first embodiment, it is set to = 75 mm. Further, since the focal length of the second collimator lens group is 335.1 mm and the focal length of the first collimator lens group is 88.5 mm, the power ratio is 1 / 88.5: 1 / 335.1≈4: 1. It is configured. This is to adjust the focus position in the main scanning direction caused by a manufacturing error by moving the second collimator lens group in the optical axis direction within the main body. In addition, the first collimator lens group is formed of a bonded lens, so that it can be easily held during adjustment and fixing. In particular,
0.5 <87/75 <2
And the expression (1) is satisfied.

  As described above, by optimizing the power arrangement and aperture position of the two groups of spherical lenses as in the incident optical system of the present embodiment, the focus difference (spot diameter difference) between multiple beams can be reduced and more compact. Therefore, it is possible to provide a high-definition and luminous flux optical scanning device.

[Image forming apparatus]
FIG. 11 is a cross-sectional view of a main part in the sub-scanning direction showing an embodiment of an image forming apparatus including the optical scanning device shown in the first or second embodiment of the present invention. Code data Dc is input to the image forming apparatus 104 from an external device 117 of the personal computer. The code data Dc is converted into image data (dot data) Di by a printer controller 111 in the image forming apparatus. The image data Di is input to the optical scanning unit 100 having the configuration shown in the first or second embodiment. From the optical scanning unit 100, a light beam 103 modulated in accordance with the image data Di is emitted, and the photosensitive surface of the photosensitive drum 101 is scanned in the main scanning direction by the light beam 103.

  The photosensitive drum 101 serving as an electrostatic latent image carrier (photoconductor) is rotated clockwise by a motor 115. With this rotation, the photosensitive surface of the photosensitive drum 101 moves in the sub-scanning direction perpendicular to the main scanning direction with respect to the light beam 103. Above the photosensitive drum 101, a charging roller 102 for uniformly charging the surface of the photosensitive drum 101 is provided so as to contact the surface. The surface of the photosensitive drum 101 charged by the charging roller 102 is irradiated with the light beam 103 scanned by the optical scanning unit 100.

  As described above, the light beam 103 is modulated based on the image data Di, and by irradiating the light beam 103, an electrostatic latent image is formed on the surface of the photosensitive drum 101 (on the photosensitive member). . The electrostatic latent image is developed as a toner image by a developing unit 107 disposed so as to contact the photosensitive drum 101 further downstream in the rotation direction of the photosensitive drum 101 than the irradiation position of the light beam 103.

  The toner image developed by the developing unit 107 is transferred onto a sheet 112 as a transfer material by a transfer roller 108 disposed below the photosensitive drum 101 so as to face the photosensitive drum 101. The paper 112 is stored in the paper cassette 109 in front of the photosensitive drum 101 (on the right side in FIG. 8), but can be fed manually. A paper feed roller 110 is provided at the end of the paper cassette 109, and feeds the paper 112 in the paper cassette 109 into the transport path.

  As described above, the sheet 112 on which the unfixed toner image has been transferred is further conveyed to the fixing device behind the photosensitive drum 101 (left side in FIG. 11). The fixing device includes a fixing roller 113 having a fixing heater (not shown) therein and a pressure roller 114 disposed so as to be in pressure contact with the fixing roller 113. Then, the sheet 112 conveyed from the transfer unit is heated while being pressed by the pressure contact portion between the fixing roller 113 and the pressure roller 114 to fix the unfixed toner image on the sheet 112. Further, a paper discharge roller 116 is disposed behind the fixing roller 113, and the fixed paper 112 is discharged out of the image forming apparatus.

  Although not shown in FIG. 8, the print controller 111 controls not only the data conversion described above but also the motor 115 and other components in the image forming apparatus, a drive motor in the optical scanning unit described later, and the like. I do.

  The recording density of the image forming apparatus used in the present invention is not limited. However, considering that the higher the recording density, the higher the image quality is required, the configuration of the first or second embodiment of the present invention is more effective in an image forming apparatus of 1200 dpi or more.

[Color image forming apparatus]
FIG. 12 is a schematic view of the essential portions of an embodiment of a color image forming apparatus including the optical scanning device shown in the first or second embodiment of the present invention. This embodiment is a tandem type color image forming apparatus in which four optical scanning devices (optical imaging optical systems) are arranged in parallel and image information is recorded on a photosensitive drum surface as an image carrier. In FIG. 12, 60 is a color image forming apparatus, 11, 12, 13, and 14 are optical scanning devices each having the configuration shown in the first or second embodiment, and 21, 22, 23, and 24 are photosensitive members as image carriers. Drums 31, 32, 33, and 34 are developing units, and 51 is a conveyor belt. In FIG. 12, there are a transfer device (not shown) for transferring the toner image developed by the developing device to the transfer material, and a fixing device (not shown) for fixing the transferred toner image to the transfer material. doing.

  In FIG. 12, the color image forming apparatus 60 receives R (red), G (green), and B (blue) color signals from an external device 52 of a personal computer. These color signals are converted into C (cyan), M (magenta), Y (yellow), and B (black) image data (dot data) by a printer controller 53 in the apparatus. These image data are input to the optical scanning devices 11, 12, 13, and 14, respectively. From these optical scanning devices, light beams 41, 42, 43, and 44 modulated according to each image data are emitted, and the photosensitive surfaces of the photosensitive drums 21, 22, 23, and 24 are caused by these light beams. Scanned in the main scanning direction.

  The color image forming apparatus in this embodiment has four optical scanning devices (11, 12, 13, and 14) arranged in each color of C (cyan), M (magenta), Y (yellow), and B (black). It corresponds. In parallel, image signals (image information) are recorded on the surfaces of the photosensitive drums 21, 22, 23, and 24, and color images are printed at high speed.

  As described above, the color image forming apparatus in this embodiment uses the light beams based on the respective image data by the four optical scanning devices 11, 12, 13, and 14, and the photosensitive drums 21 and 22 respectively corresponding the latent images of the respective colors. , 23, 24 on the surface. Thereafter, a single full color image is formed by multiple transfer onto a recording material.

  As the external device 52, a color image reading device including a CCD sensor may be used. In this case, the color image reading apparatus and the color image forming apparatus 60 constitute a color digital copying machine.

1 Light source means (surface emitting laser)
3 Collimator lens 4 Cylindrical lens 5 Second aperture 10 Deflection means (polygon mirror)
10a Deflection surface 20 Imaging optical system (scanning optical system)
20a First imaging lens 20b Second imaging lens 11, 12, 13, 14 Optical scanning device 100 Optical scanning device

Claims (11)

A light source having a plurality of light emitting portions;
A collimator optical system for converting a light beam from the light source, and a diaphragm for limiting a light beam width of the light beam from the collimator optical system in the main scanning direction and the sub-scanning direction in which the arrangement interval of the light emitting units is wide. An incident optical system;
A deflector that rotates about a rotation axis parallel to the sub-scanning direction and deflects the light beam from the incident optical system in a main scanning plane;
An imaging optical system for guiding the light beam from the deflector to the surface to be scanned;
Have
The collimator optical system includes a first optical element group and a second optical element group arranged in order from the light source side, which are configured by optical elements not including an aspherical surface,
When the distance from the incident surface of the second optical element group to the stop is L, and the focal length of the incident optical system in the direction in which the light emitting unit is arranged widely in the main scanning direction and the sub-scanning direction is f. ,
0.5 <| L | / f <2
An optical scanning device characterized by satisfying the following conditions. The arrangement interval of the plurality of light emitting units in the light source is wider in the main scanning direction than in the sub scanning direction,
The diaphragm restricts the beam width in the main scanning direction,
When the focal length in the main scanning section of the incident optical system is f _col ,
0.5 <| L | / f _col <2
The optical scanning device according to claim 1, wherein the following condition is satisfied. The first optical element group includes at least one convex lens and at least one concave lens,
When the focal length of the first optical element group is f _{col1 and} the focal length of the second optical element group is f _col2 ,
0.1 <f _col1 / f _col2 <1/3
The optical scanning device according to claim 1, wherein the following condition is satisfied. In either one of the main scanning section and the sub-scanning section including the direction in which the light emitting units are arranged widely in the main scanning direction and the sub-scanning direction, the focal length of the first optical element group is f _col1, the second When the focal length of the optical element group is f _col2 ,
0.1 <f _col1 / f _col2 <1/3
The optical scanning device according to claim 3, wherein the following condition is satisfied.   5. The optical scanning device according to claim 3, wherein the first optical element group is a bonded lens including one concave lens and one convex lens arranged in order from the light source side. The focal length in the main scanning section of the imaging optical system is f _fθ , the focal length in the main scanning section of the incident optical system is f _col , and the focus and axis for the on-axis light emitting unit at the same wavelength of the incident optical system The focus difference Δmcol with respect to the outer light emitting unit is
| Δmcol | × (f _fθ / f _col ) ² ≦ 0.5 mm
The optical scanning device according to claim 1, wherein:   2. The optical scanning device according to claim 1, wherein the first optical element group and the second optical element group are independently held so that their positions in the optical axis direction can be adjusted. 3. The optical scanning device according to claim 7, wherein the first optical element group and the light source are integrally held. 9. The optical element having the smallest power in the main scanning section among the lenses of the incident optical system is held so as to be movable in the optical axis direction. The optical scanning device described. An optical scanning device according to any one of claims 1 to 9,
A developing device for developing, as a toner image, an electrostatic latent image formed on the photoconductor disposed on the scanned surface by the optical scanning device;
A transfer device for transferring the developed toner image to a transfer material;
A fixing device for fixing the transferred toner image to the transfer material;
An image forming apparatus comprising:   The image forming apparatus according to claim 10, further comprising: a printer controller that converts code data input from an external device into an image signal and inputs the image signal to the optical scanning device.

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