JP4404667B2 - Optical scanning device - Google Patents

Description

  The present invention relates to an optical scanning device. In particular, the present invention relates to an optical scanning device suitable for a device that scans a plurality of scanned media after deflecting a plurality of light beams by one light deflecting unit, for example, a tandem color image forming system.

2. Description of the Related Art Conventionally, in an image forming system such as a printer or a digital copying machine, an optical scanning device that scans a beam deflected by an optical deflecting unit with an imaging optical system using a single plastic fθ lens is used.
As such an optical scanning device, a so-called parallel light beam in which the incident light beam is parallel to the main scanning direction has been proposed in order to facilitate the arrangement of the light source in the front stage of the light deflecting means (for example, Patent Documents). 1).
In this optical scanning device, a light beam emitted from a semiconductor laser is converted into a parallel light beam by a collimator lens, and is converged only in the sub-scanning direction, which is a spherical missing direction, by a cylindrical lens and used as a light deflecting unit. An image is formed in the vicinity of the polygon mirror, and is scanned at a constant speed in the axial direction of the photosensitive drum by a single scanning lens.

In order to facilitate the production of a single plastic fθ lens, there has been proposed one in which the power required for the single plastic fθ lens is reduced by emitting a convergent light beam from a light source (see, for example, Patent Document 2). ).
This optical scanning device is composed of a first optical system that condenses a light beam emitted from a light source and emits it toward a deflection point provided on a deflection surface of an optical deflection unit, and an fθ lens composed of one lens. A scanning optical system having the configured second optical system is included. As a result, it is possible to adopt a plastic lens with a low mass production cost and a lens that can be manufactured easily by reducing the thickness of the lens.
Japanese Patent No. 2621838 (FIGS. 1 and 13) Japanese Patent Publication No.7-111501 (FIGS. 1, 3, and 4)

However, the following problems remain in the conventional optical scanning device. That is, in the former optical scanning device, the power required for image formation in the main scanning direction is realized by only a single scanning lens, and therefore it is necessary to increase the power of the scanning lens. As a result, the shape of the scanning lens is thicker at the center and thinner at the end, and there is a problem that internal distortion is likely to occur during lens molding.
In the latter optical scanning device, convergent light is incident on the fθ lens, and the moldability of the lens is better than that of the former optical scanning device described above. However, if the optical path from the light source to the photoconductor is lengthened, the magnification is very high. Therefore, there is a problem that the assembly adjustment accuracy of the light source section becomes strict and the productivity of assembly deteriorates. Further, since it is necessary to increase the outer diameter of the collimating lens, there is a problem that the cost of parts is increased, and further, the performance change with respect to environmental changes such as a temperature change is increased. In recent years, there has been a demand for a scanning optical system for tandem color printers, which is a low-cost scanning optical system that uses four common deflectors. However, if the deflectors are used in common, the optical paths of the four beams are combined. An optical path combining unit and an optical path dividing unit that again divides the four combined optical paths are required, and the total length of the optical path tends to be long, and it is difficult to employ the latter optical scanning device.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide an optical scanning device that can use an inexpensive fθ lens that can be easily manufactured and that can relax adjustment accuracy.

The present invention employs the following configuration in order to solve the above problems. That is, the optical scanning device of the present invention is an optical scanning device that repeatedly scans the convergent light on the scanning line, and includes a multi-beam light source that emits at least two parallel light beams, and each parallel light beam emitted from the light source. At least two first light condensing elements each condensing in the sub-scanning direction, at least two second light condensing elements condensing each of the parallel light beams in the main scanning direction, and the first and second light condensing elements. A light deflector for deflecting the light beam collected by the element in the main scanning direction, and the first light collecting element and the second light collecting element; Optical path changing means having first and second adjustment mirrors each having a biaxial tilt angle adjusting means for changing each path of light reflected and irradiated to the light deflecting means, and the optical deflection Scanning each light beam deflected by the means And a least two single lens plastic fθ lens to each converge above, the light source includes a semiconductor laser, is constituted by a collimating lens and a holding member for holding them, at the time of temperature change, thermal contraction of the holding member The amount of defocus due to the variation in the distance between the emitting surface of the generated semiconductor laser and the collimator lens is the amount of defocus due to the change in wavelength of the semiconductor laser, the amount of defocus due to the change in the focal length of the second condensing element, The amount of defocus due to the focal length change of the single plastic fθ lens is offset by the opposite signs, and when the temperature changes, the defocus amount due to the wavelength change of the semiconductor laser and the defocus due to thermal contraction of the holding member. The amount of defocus due to the change in the focal length of the second condensing element, and the single ball plastic fθ The amount obtained by adding all of the defocus amount by changing the focal length's, characterized in that is within ± 2 mm.

According to the present invention, the power required for the single plastic fθ lens is reduced by converging the parallel light flux in the main scanning direction. Therefore, the shape of the single plastic lens can be a uniform thickness type, and the moldability is improved as compared with the uneven thickness type.
Further, by reducing the magnification of the optical system, the adjustment accuracy on the object side can be relaxed, and the assembly productivity is improved.
Further, by using the first and second light condensing elements that condense independently in the sub-scanning direction and the main scanning direction, it is easy to adjust the focal position with respect to the scanning direction in the sub-scanning direction or the main scanning direction alone. Become.

In the present invention, in order to show the relative directions in a concise manner, the sub-scanning direction and the main-scanning direction are used in a broad sense by common usage when there is no possibility of misunderstanding.
That is, the sub-scanning direction means a direction orthogonal to the scanning line on the surface to be scanned (recording medium) and a direction orthogonal to the optical path direction and reaches the scanning line along the optical path. In some cases, a direction perpendicular to the scanning line is meant.
The main scanning direction means the direction of the scanning line, the direction perpendicular to the optical path direction, the direction perpendicular to the sub-scanning direction, and the light of the optical system in the plane where the light beam is deflected. It may mean a direction perpendicular to the axis.

In the optical scanning device according to the aspect of the invention, it is preferable that the first light collecting element is a glass lens and the second light collecting element is a plastic lens.
According to the present invention, the cost can be reduced by using the plastic lens for the second condensing element.

In the optical scanning device according to the present invention, the light source includes a semiconductor laser, a collimating lens, and a holding member that holds these, and when the temperature changes, the amount of defocus due to the wavelength change of the semiconductor laser and the holding An amount obtained by adding all of the defocus amount due to thermal contraction of the member, the defocus amount due to the change in the focal length of the second condensing element, and the defocus amount due to the change in the focal length of the single lens plastic fθ lens is ± It is preferably within 2 mm.
According to the present invention, the spot diameter of the optical scanning device used in the printer is usually about 60 to 70 μm. However, if the amount of defocus is within ± 2 mm, the spot diameter change on the photosensitive member is within the depth of focus. It does not become a big problem in performance.

Further, in the optical scanning device according to the present invention, a plurality of light sources and a plurality of first and second light condensing elements are provided for each of the light deflecting means, so that a plurality of light beams are placed on different scanning lines. It is preferable to scan simultaneously.
According to the present invention, a plurality of light sources and a plurality of first and second light condensing elements are provided for one light deflection unit, so that the so-called tandem scanning optical system can be reduced in size and cost.

According to the optical scanning device of the present invention, the power required for a single plastic fθ lens is reduced by converging parallel light beams independently in the main scanning direction. Thereby, the curvature radius of a single piece plastic lens can be enlarged, the shape of a single piece plastic lens can be made into a uniform thickness type, and a moldability improves compared with an uneven thickness type.
Further, by reducing the magnification of the optical system, the tolerance for the adjustment error on the object side can be increased, and the assembly of the optical system is facilitated and the productivity is improved.
Furthermore, since the first and second light condensing elements condense independently in the sub-scanning direction or the main scanning direction, the focal position in the scanning direction can be adjusted in the sub-scanning direction or the main scanning direction alone.

An embodiment of an optical scanning device according to the present invention will be described below with reference to FIGS. Here, FIG. 1 is a plan view explanatory diagram for explaining a schematic configuration of an optical scanning device according to an embodiment of the present invention. Moreover, Fig.2 (a) is the front view explanatory drawing similarly. FIG. 2B is a schematic explanatory diagram for explaining a part of the optical path in the cross section in the sub-scanning direction.
The laser scanning unit 1 (optical scanning device) according to the present embodiment is an optical scanning optical system used in a so-called tandem color printer, and forms a plurality of laser beams on predetermined scanning diameters on different scanning lines, and is constant. It repeatedly scans in the direction.

  The laser scanning unit 1 includes a multi-beam light source 2 that emits parallel light, first cylindrical lenses (first condensing elements) 3A and 3B that image incident light from the multi-beam light source 2 in the sub-scanning direction, Between the second cylindrical lenses (second condensing elements) 5A and 5B that image the emitted light of the first cylindrical lenses 3A and 3B in the main scanning direction, and the first cylindrical lenses 3A and 3B and the second cylindrical lenses 5A and 5B. The optical path changing means 7 that folds the light emitted from the first cylindrical lenses 3A and 3B and changes the light path applied to the polygon mirror (light deflecting means) 6 and the optical path separation optics that separates the light emitted from the polygon mirror 6. System 8, single plastic fθ lens (hereinafter abbreviated as fθ lens) 10a to 10d, folding mirror 11, and synchronous sensor unit 12, and these are arranged in the housing 13.

  In addition, laser beams 15a to 15d emitted from LD arrays (semiconductor laser arrays) 21A and 21B, which will be described later, are emitted from a protective glass 16 provided in the casing, and are respectively yellow and full color image forming systems (not shown). The photosensitive drums 17a to 17d on which electrostatic latent images for development with magenta, cyan and black toners are formed are irradiated.

  The multi-beam light source 2 includes LD arrays 21A and 21B, an LD drive circuit board 22 that drives the LD arrays 21A and 21B, two collimating lenses 23A and 23B, apertures 25A and 25B, and a holding member 27 that holds these. And.

The LD arrays 21A and 21B are two-beam LD array elements, each having two light emitting portions separated by a distance h in the sub-scanning direction. Then, laser beams 15a and 15b and laser beams 15c and 15d are emitted from the respective light emitting units. The LD arrays 21A and 21B are mounted on the base 26 with a distance W in the main scanning direction.
This distance W is a distance that secures an arrangement space for the LD array 21A21B and the like and is hardly affected by heat.
In addition, when the distance h is used in a full-color image system as in the present embodiment, the distance h may be determined as close as possible in consideration of the arrangement space of each member, the size of the reflecting surface of the polygon mirror 6, and the like. . On the other hand, for example, when used for multi-beam scanning, it is necessary to set the sub-scanning pitch of the scanning line to an accurate value, so that the sub-scanning pitch of the scanning line matches the desired print density. The distance is determined based on the magnification in the scanning direction.

  The base 26 is made of an appropriate metal plate having a high thermal conductivity, and has an advantage that the heat dissipation of the LD arrays 21A and 21B is promoted and the life of the LD array element can be extended. Also, since temperature uniformity is measured, even if the amount of light emission differs between LD array elements, a shift in wavelength change due to temperature characteristics occurs, and a magnification shift due to chromatic aberration that varies the scanning area for each laser beam occurs. First, image quality deterioration when used in an image forming system can be prevented.

  The LD drive circuit board 22 is electrically connected to the LD arrays 21A and 21B, and a drive circuit for performing modulation drive based on a modulation signal input from the outside to each light emitting portion of the LD arrays 21A and 21B. Is formed.

The collimating lenses 23A and 23B are lenses or lens groups that are arranged in front of the light emitting portions of the LD arrays 21A and 21B and have the laser beams 15a to 15d emitted from the LD arrays 21A and 21B as parallel light beams.
Apertures 25A and 25B are light restricting members for shaping the parallel light beams emitted from the collimating lenses 23A and 23B into a predetermined light beam diameter, and are metal having a substantially elliptical shape whose major axis is extended in the main scanning direction. It consists of a plate.

The holding member 27 includes the LD arrays 21A and 21B, the LD drive circuit board 22, the collimating lenses 23A and 23B, and the apertures 25A and 25B, and is detachably attached to the housing 13. The LD arrays 21A and 21B, the LD drive circuit board 22, the collimating lenses 23A and 23B, and the apertures 25A and 25B are preferably adjusted in position between the members as necessary. As a result, four parallel light beams shaped to a predetermined light beam diameter in a predetermined position and direction can be emitted in a state of being attached to an attachment surface (not shown) provided with high precision on the housing 13. . And it can replace | exchange with other light source units adjusted similarly, without adjustment.
Here, the defocus when the temperature of the holding member 27 is changed is configured such that the defocus direction and the shift direction of the second cylindrical lenses 5A, 5B and the fθ lenses 10a to 10d are different from each other.
The holding member 27 may be formed of the same material as that of the base 26. By doing so, the number of parts can be reduced.

The first cylindrical lenses 3A and 3B are glass lenses having power only in the sub-scanning direction, and form images of the laser beams 15a to 15d emitted from the multi-beam light source 2 in the sub-scanning direction, respectively in the main scanning direction. It is an optical element that forms a substantially linear light beam that extends.
The second cylindrical lenses 5A and 5B are plastic lenses having power only in the main scanning direction, and are optical elements that converge the laser beams 15a to 15d in the main scanning direction to form spot-like light beams.

  The polygon mirror 6 is for deflecting the laser beams 15a to 15d in the main scanning direction in the vicinity of the image forming position. For example, the polygon mirror 6 is in a plane orthogonal to the sub-scanning direction (hereinafter also referred to as a beam scanning surface). It is a rotating polygonal mirror such as an octagon. A motor (not shown) that receives a drive signal from the outside of the laser scanning unit 1 and rotates at a predetermined speed rotates in a direction indicated by an arrow Z1 shown in FIG. Although the laser beams 15a to 15d are exaggerated in the sub-scanning direction in FIG. 2, they are substantially linear spots extending in the main scanning direction. The pitch in the sub-scanning direction is distances a, b, and a (here, a + b = h).

The optical path changing unit 7 includes first adjustment mirrors 31A and 31B and second adjustment mirrors 32A and 32B.
The first adjustment mirrors 31A and 31B and the second adjustment mirrors 32A and 32B reflect the laser beams 15a to 15d that have passed through the first cylindrical lenses 3A and 3B on the mirror surface, and are almost in a plane mainly along the main scanning direction. It is a mirror for folding in a Z shape.
The first adjustment mirrors 31A and 31B and the second adjustment mirrors 32A and 32B are each, for example, a biaxial tilt angle adjustment unit (illustrated) composed of a combination of an urging unit and an adjustment screw that presses the mirror body. Abbreviation). Thereby, the posture of each mirror surface becomes variable.
In this embodiment, the two axial directions are a rotation about the sub-scanning direction axis and a rotation about the main scanning direction axis orthogonal thereto, and the respective rotation centers are on the axial principal ray of the optical system. Provided.
The first adjustment mirror 31A and the second adjustment mirror 32A, and the first adjustment mirror 31B and the second adjustment mirror 32B are axes extending from the first cylindrical lenses 3A and 3B toward the imaging position in the neutral positions of the respective adjustments. The upper chief rays are arranged on the plane along the sub-scanning direction between the second adjustment mirrors 32A and 32B and the imaging position, and are arranged at positions where they are parallel to each other.

  The optical path separation optical system 8 is an optical system that separates the optical paths of the laser beams 15a to 15d changed by the polygon mirror 6 so that the scanning lines are finally scanned in parallel at the pitch d between the drums. It is comprised by the separation reflective mirrors 40-49 provided in the location. Each of the separation reflection mirrors 40 to 49 is a surface reflection mirror that extends to a predetermined length in the main scanning direction and has an inclination angle with respect to the sub scanning direction.

The separation / reflection mirror 40 folds the laser beam 15d substantially in the sub-scanning direction, and guides the laser beam 15d to the separation / reflection mirror 41 disposed substantially parallel to the separation / reflection mirror 40. The separation reflection mirror 41 is arranged so that the laser beam 15d is scanned in a plane orthogonal to the sub-scanning direction.
The separation / reflection mirror 42 folds the laser beam 15c substantially in the sub-scanning direction and guides the optical path to separation / reflection mirrors 43 and 44 arranged so as to be folded in a substantially triangular shape in the cross-section in the sub-scanning direction. The mirrors 43 and 44 are arranged so that the laser beam 15c is scanned in a plane parallel to the laser beam 15d.
The separating / reflecting mirror 45 folds the laser beam 15b substantially in the sub-scanning direction and guides the optical path to the separating / reflecting mirrors 46 and 47 arranged so as to be folded in a substantially triangular shape in the sub-scanning direction. The mirrors 46 and 47 are arranged so that the laser beam 15b is scanned in a plane parallel to the laser beams 15c and 15d.
The separation / reflection mirror 48 folds the laser beam 15a substantially in the sub-scanning direction and guides the laser beam 15a to the separation / reflection mirror 49 disposed substantially parallel to the separation / reflection mirror 48. The separation reflection mirror 49 is arranged so that the laser beam 15a is scanned in a plane parallel to the laser beams 15b to 15d.

  The separating / reflecting mirrors 40, 42, 45, and 48 are arranged with their positions shifted in the optical axis direction and the sub-scanning direction so that they can be separated without leaving the optical paths of the laser beams 15a to 15d emitted simultaneously. When the pitches of the laser beams 15a to 15d in the sub-scanning direction are too close to be separated, the LD arrays 21A and 21B are rotationally adjusted to obtain a predetermined pitch.

The fθ lenses 10a to 10d are single plastic lenses, and laser beams 15a to 15d scanned in planes parallel to each other by the optical path separation optical system 8 at scanning line positions on the photosensitive drums 17a to 17d, respectively. This is a lens or lens group that forms an image so as to have a predetermined light beam diameter and has fθ characteristics for making the scanning speed in the main scanning direction substantially constant.
In the sub-scanning direction, the imaging position of the fθ lens 10 and the imaging positions of the first cylindrical lenses 3A and 3B and the second cylindrical lenses 5A and 5B are optically conjugate. Accordingly, a surface tilt correction optical system is configured in which the deviation of the scanning line in the sub-scanning direction due to the surface tilt of the polygon mirror 6 is remarkably reduced.

The folding mirror 11 is an optical element that bends the light beam on the scanning start side of the non-image area in the direction intersecting the optical axis among the laser beams 15 a to 15 d changed by the polygon mirror 6 and guides it to the synchronous sensor unit 12. is there.
The synchronization sensor unit 12 condenses and shapes the light flux into an appropriate shape to improve the S / N ratio of the light detection, and the light flux collected through the synchronization detection lens 51. It consists of a horizontal synchronization sensor 52 to detect. The horizontal synchronization sensor 52 is composed of, for example, a light detection sensor having a light beam response such as a PIN photodiode. The horizontal synchronization sensor 52 is connected to an electric circuit that signals the generation timing of the light detection output, and is configured to output a horizontal synchronization signal to the outside of the laser scanning unit 1. Such an electric circuit may be integrated into an IC and formed integrally in the vicinity of the horizontal synchronization sensor 52.

Next, the operation of the optical scanning device of the present embodiment having the above configuration will be described.
First, a drive signal for rotationally driving the polygon mirror 6 is input from the outside of the laser scanning unit 1, and the polygon mirror 6 is rotated at a constant speed. The laser beam 15d is DC-lit by at least the LD array 21B.

The laser beam 15d is emitted as divergent light from the LD array 21B, then passes through the collimator lens 23B and the aperture 25B, becomes a parallel light beam having a predetermined light beam diameter, and enters the first cylindrical lens 3B.
The laser beam 15d incident on the first cylindrical lens 3B is condensed in the sub-scanning direction by the first cylindrical lens 3B, and the optical path is folded back into a Z shape by the first adjustment mirror 31B and the second adjustment mirror 32B. The light enters the two cylindrical lens 5B.
The laser beam 15d incident on the second cylindrical lens 5B is condensed in the main scanning direction by the second cylindrical lens 5B, imaged in the sub-scanning direction in the vicinity of the reflecting surface of the polygon mirror 6, and substantially extends in the main scanning direction. It becomes a linear luminous flux. Then, it is deflected in the main scanning direction by the polygon mirror 6.

  The laser beam 15d deflected by the polygon mirror 6 is incident on the fθ lens 10d after the optical path is folded by the separation and reflection mirrors 40 and 41. When incident on the fθ lens 10d, an image is formed on the scanning line of the photosensitive drum 17d by the image forming action. Then, the deflection angle increases at a constant speed, and scanning is performed at a constant speed on the scanning line in the direction of the arrow Z2 in FIG. 1 by the fθ characteristic of the fθ lens 10d.

  Here, the laser beam 15d is folded back by the folding mirror 11 on the scanning start side and enters the synchronous sensor unit 12. When the laser beam 15d reaches a predetermined position, the synchronization sensor unit 12 outputs a horizontal synchronization signal and temporarily turns off the laser beam 15d. After a predetermined delay, a modulation signal for modulating the LD array 21B is input from this horizontal synchronization signal, and the laser beam 15d is modulated in accordance with this modulation signal.

Thus, when the deflection scanning by one reflecting surface of the polygon mirror 6 is completed, the same scanning is performed on the next reflecting surface, and the laser beam 15d modulated according to the image signal is repeatedly scanned on the scanning line. . On the other hand, the photosensitive drum 17d is rotated at a constant linear velocity in the sub-scanning direction. For this reason, the laser beam 15d scans two-dimensionally on the photosensitive drum 17d, and a latent image is formed on the photosensitive drum 17d.
If the image forming system is in a mode for black printing, image formation proceeds as described above, and a monochrome image is formed.

On the other hand, similarly to the above, the laser beams 15a to 15c can scan the photosensitive drums 17a to 17c in substantially the same manner except that the optical path in the optical path separation optical diameter 8 is different.
The laser beams 15a to 15c are parallel on a plane along the sub-scanning direction by the action of the first adjustment mirrors 31A and 31B and the second adjustment mirrors 32A and 32B in the optical path immediately before entering the reflection surface of the polygon mirror 6. Therefore, the incident angles with respect to the polygon mirror 6 are the same. The scanning angle of view for the fθ lenses 10a to 10d is also the same. As a result, the laser beams 15a to 15c have substantially the same scanning characteristics and effective scanning areas except for differences due to manufacturing errors and arrangement errors of the optical path separation optical system 8 and the fθ lenses 10a to 10d.

  That is, since the scanning of the laser beams 15a to 15c is substantially synchronized, for example, when printing a full-color image, the writing timing of the laser beams 15a to 15c can be accurately determined with reference to the horizontal synchronization signal by the laser beam 15d. it can. That is, even if the delay time of the writing start signal is the same, the error inherent to the reflecting surface due to the division accuracy of the polygon mirror 6 is common to the laser beams 15a to 15d, so that the optical path separation optical system 8 and the fθ lens 10a to Only the positional deviation unique to the apparatus depending on 10d or the like occurs. Therefore, according to the present embodiment, this can be easily corrected by finely adjusting the delay time of the write start signal for each of the laser beams 15a to 15d.

  According to the laser scanning unit 1 configured in this way, the second cylindrical lens 5A has power in the main scanning direction and converges the laser beams 15a to 15d emitted from the LD arrays 21A and 21B in the main scanning direction. By providing 5B between the multi-beam light source 2 and the polygon mirror 6, the power of the fθ lenses 10a to 10d can be reduced. Therefore, the shape of the fθ lenses 10a to 10d can be made uniform, so that the lens can be easily molded and the cost can be reduced.

  Further, by providing the second cylindrical lenses 5A and 5B between the multi-beam light source 2 and the polygon mirror 6, the main scanning magnification of the optical system can be kept small. Accordingly, it is possible to increase the tolerance of adjustment errors and manufacturing errors of the scanning device.

In addition, the first cylindrical lenses 3A and 3B independently form a light beam in the sub-scanning direction and the second cylindrical lenses 5A and 5B independently form a light beam in the main scanning direction, respectively. Can be independently adjusted.
Further, since the defocus amount of the holding member 27 with respect to the temperature change and the defocus amounts of the second cylindrical lenses 5A and 5B and the fθ lenses 10a to 10d cancel each other, the performance change when the environment changes Can be suppressed.

In addition, this invention is not limited to the said embodiment, A various change can be added in the range which does not deviate from the meaning of this invention.
For example, in a full-color image forming system, each color may be used for multi-beam scanning, or two light beams for each color may be scanned using eight light sources.

Next, a defocus amount with respect to a temperature change of the laser scanning unit 1 according to the present invention will be specifically described with reference to a numerical example using FIG. Here, FIG. 3 is an explanatory diagram for explaining an optical path in the main scanning direction of the laser scanning unit 1 according to the present embodiment.
The configuration parameters of the optical system in the numerical examples are shown below. The r _i and n _i (i is an integer) shown in FIG. 3 correspond to the configuration parameters r _i and n _i of the optical system shown below, respectively. Further, the surface spacing is shown at 25 ° C., and the refractive index is shown at a wavelength of 783 nm. Further, α which is the sum of the incident angle and the reflection angle in the polygon mirror 6 is set to 65 °.

Surface number Curvature radius [mm] Surface spacing [mm] Refractive index
1 r ₁ = ∞ (light source)
2 r ₂ = 265 d ₁ = 18.87
3 Aspherical surfaces [1] d ₂ = 2 n ₂₃ = 1.576
4 Main scanning direction r _3a = ∞ (plane) d ₃ = 22
Sub scanning direction r _3b = 103
5 r ₅ = ∞ (plane) d ₄ = 3 n ₃ = 1.511
6 Main scan direction r _6a = 211 d ₅ = 157
Sub-scan direction r _6b = ∞ (plane)
7 r ₇ = ∞ (plane) d ₆ = 3 n ₅ = 1.503
8 r ₈ = ∞ (plane) d ₇ = 40.48
9 Aspherical surface [2] d ₈ = 144.52
10 Aspherical surface [3] d ₉ = 15 n ₁₀ = 1.503
11 r ₁₁ = ∞ (plane) d ₁₀ = 8
12 r ₁₂ = ∞ (plane) d ₁₁ = 2 n ₁₆ = 1.511
13 ∞ (image plane) d ₁₂ = 156

Here, a coordinate system for expressing a rotationally symmetric aspherical surface in this embodiment and a curved surface formula will be described.
The coordinate system is the X-axis in the sub-scanning direction, the Y-axis in the main scanning direction, and the Z-axis in the optical axis (axial principal ray) with the light traveling direction being positive, and is a right-handed orthogonal coordinate system.
And the aspherical surface definition formula of the surface number 3 which is an output surface of the collimating lens 23B is as shown in Formula 1.

In the above equation 1, C is the reciprocal of the radius of curvature, k is the conic constant (conical constant), and r = √ (x ² + y ² ). Further, a, b, c, and d are fourth-order, sixth-order, eighth-order, and tenth-order aspherical coefficients, respectively.
Further, the aspherical definition formulas of the surface numbers 9 and 10 which are the entrance surface and the exit surface of the fθ lens 10d are as shown in Equation 2.

In Equation 2, c ₁ is the reciprocal of the radius of curvature in the main scanning direction, c ₂ is the reciprocal of the radius of curvature in the sub-scanning direction, and Y = √ (y ² ). Further, ΣA _m Y ^m is as shown in Equation 3, and ΣB _m Y ^m is as shown in Equation 4.

Aspherical [1]
Radius of curvature r ₃ = 12.075
k = -7.6722x10 ^-1
a = 7.8967x10 ^-6 b = 1.8077x10 ^-8 c = -1.2414x10 ^-9 d = 1.6923x10 ^-11

Aspherical [2]
Main scanning direction radius of curvature r _9a = 160.449
When y> 0
k = 0
A ₄ = -1.6004x10 ^-6 A ₆ = 1.6687x10 ^-9 A ₈ = -2.9158x10 ^-13 A ₁₀ = -7.9890x10 ^-17
A ₃ = -6.9062x10 ^-7 A ₅ = 1.5387x10 ^-8 A ₇ = -3.5950x10 ^-11 A ₉ = 1.2314x10 ^-15
B ₄ = 0 B ₆ = 0 B ₈ = 0 B ₁₀ = 0
B ₃ = 0 B ₅ = 0 B ₇ = 0 B ₉ = 0
When y ≦ 0
k = 0
A ₄ = 3.4929x10 ^-7 A ₆ = 7.1298x10 ^-10 A ₈ = 2.1438x10 ^-13 A ₁₀ = 3.0461x10 ^-18
A ₃ = -9.6570x10 ^-6 A ₅ = -2.6498x10 ^-8 A ₇ = -1.5641x10 ^-11 A ₉ = -1.4091x10 ^-15
B ₄ = 0 B ₆ = 0 B ₈ = 0 B ₁₀ = 0
B ₃ = 0 B ₅ = 0 B ₇ = 0 B ₉ = 0

Sub-scanning direction radius of curvature r _9b = -135.335

Aspherical [3]
Main scanning direction radius of curvature r _10a = 257
When y> 0
k = 0
A ₄ = -1.5010x10 ^-6 A ₆ = 2.0157x10 ^-9 A ₈ = -2.0990x10 ^-13 A ₁₀ = -6.8089x10 ^-17
A ₃ = -9.1236x10 ^-7 A ₅ = 4.4772x10 ^-9 A ₇ = -4.0528x10 ^-11 A ₉ = 1.0677x10 ^-14
B ₄ = 8.7154x10 ^-8 B ₆ = -3.0748x10 ^-10 B ₈ = 7.6960x10 ^-14 B ₁₀ = 1.0350x10 ^-17
B ₃ = -3.8363x10 ^-6 B ₅ = 6.2140x10 ^-9 B ₇ = 2.8536x10 ^-12 B ₉ = -1.7861x10 ^-15
When y ≦ 0
k = 0
A ₄ = 8.7154x10 ^-8 A ₆ = -3.0748x10 ^-10 A ₈ = 7.6960x10 ^-14 A ₁₀ = 1.0350x10 ^-17
A ₃ = -3.8363x10 ^-5 A ₅ = 6.2140x10 ^-9 A ₇ = 2.8536x10 ^-12 A ₉ = -1.7861x10 ^-15
B ₄ = 8.6452x10 ^-8 B ₆ = -1.5609x10 ^-10 B ₈ = 8.6804x10 ^-14 B ₁₀ = 7.3001x10 ^-18
B ₃ = -3.6210x10 ^-6 B ₅ = 3.7587x10 ^-9 B ₇ = -2.2218x10 ^-13 B ₉ = -1.4269x10 ^-15

Curvature radius r _10b = -32.021

  FIG. 4 shows the spot diameter with respect to the image height on the photosensitive drum 17d in this embodiment, FIG. 5 shows the spot diameter with respect to the image height at a position shifted by +2 mm from the photosensitive drum 17d, and −2 mm from the photosensitive drum 17d. FIG. 6 shows the spot diameter with respect to the image height at the shifted position. From FIG. 4 to FIG. 6, it can be seen that the change of the spot diameter when the defocus of ± 2 mm occurs is 5 μm or less, which is a sufficiently small value.

Here, the amount of defocus with respect to the temperature change of the laser scanning unit 1 is four factors, the wavelength change amount with respect to the temperature change of the LD array 21B, the exit surface of the LD array 21B due to the temperature change of the holding member 27, and the collimating lens. The amount of change in the distance 23B, the amount of change in the focal length with respect to the temperature change of the second cylindrical lens 5B, and the amount of change in the focal length with respect to the temperature change of the fθ lens 10d are considered. The amount of change of each element is as shown in Table 1 below.
Since the first cylindrical lens 3B is a glass lens, the amount of change in focal length with respect to temperature change can be ignored.

In Table 1, the distance between the exit surface and the collimator lens 23B of the LD array 21B which _{d 1} is described above, f is the focal length, n represents a refractive index, dn / dt is the temperature coefficient of the refractive index, Delta] t is the temperature The amount of change, α, indicates the thermal expansion coefficient. The first term showing the amount of change in the focal length of the second cylindrical lens 5B and the fθ lens 10d shows the amount of deviation of the focal length with respect to the refractive index change, and the second term shows the amount of deviation of the focal length with respect to the shape change. Yes.
Here, if the thermal expansion coefficient of the holding member 27 is 3.7 × 10 ⁻⁵ , the amount of change in the distance between the exit surface of the LD array 21B and the collimating lens 23B is 2.4 × 10 ⁻² [mm]. Become. The numerical data in the main scanning direction of the second cylindrical lens 5B, the main scanning direction of the fθ lens 10d, and the sub-scanning direction are as shown in Table 2 below.

Here, f (25 ° C.) represents the focal length at 25 ° C., and f (55 ° C.) represents the focal length at 55 ° C.
As described above, the defocus amount of the entire optical system due to the wavelength change of the LD array 21B, the defocus amount of the entire optical system due to the variation in the distance between the exit surface of the LD array 21B and the collimator lens 23B, and the defocus amount of the second cylindrical lens 5B. The amount of defocus of the entire optical system due to the above and the amount of defocus of the entire optical system due to the focus variation of the fθ lens 10d are as shown in Table 3 below in the main scanning direction and the sub-scanning direction, respectively.
In each numerical value, + is the same as the positive direction of the Z axis that is the above-described axial principal ray.

  Accordingly, the defocus amount when the fθ lens 10d changes in temperature is configured such that the defocus amount when the temperature of the multi-beam light source 2 changes and the shift direction thereof are different from each other, so that the temperature change from 25 ° C. to 55 ° C. It was confirmed that the amount of defocus in the main scanning direction and the sub-scanning direction of the optical system was within the range of ± 2.0 mm. Therefore, the performance change at the time of environmental fluctuation can be suppressed more.

1 is a schematic plan view illustrating an optical scanning device according to an embodiment of the present invention. 1 shows an optical scanning device according to an embodiment of the present invention, where (a) is a front view explanatory view and (b) is a schematic explanatory view for explaining a part of an optical path in a cross section in the sub-scanning direction. It is explanatory drawing explaining the optical path in the main scanning direction of the optical scanning device in the Example of this invention. It is a graph which shows the spot diameter with respect to the image height on the photosensitive drum of the optical scanning device in the Example of this invention. 6 is a graph showing a spot diameter with respect to an image height at a position shifted by +2 mm from the photosensitive drum of the optical scanning device in an example of the present invention. It is a graph which shows the spot diameter with respect to the image height in the position which shifted -2 mm from the photosensitive drum of the optical scanning device in the Example of this invention.

Explanation of symbols

1 Laser scanning unit (optical scanning device)
2 Multi-beam light source 3A, 3B 1st cylindrical lens (1st condensing element)
5A, 5B Second cylindrical lens (second condensing element)
6 Polygon mirror (light deflection means)
10a to 10d fθ lens (single plastic fθ lens)
21A, 21B LD array (semiconductor laser array)
23A, 23B Collimating lens 27 Holding member

Claims (2)

An optical scanning device that repeatedly scans converged light on a scanning line,
A multi-beam light source that emits at least two parallel luminous fluxes;
At least two first light condensing elements for condensing each parallel light beam emitted from the light source in the sub-scanning direction;
At least two second condensing elements for condensing each of the parallel light fluxes in the main scanning direction;
Light deflecting means for deflecting the light beam collected by the first and second light condensing elements in the main scanning direction;
It arrange | positions between the said 1st condensing element and the said 2nd condensing element, reflects each emitted light of each 1st condensing element, and changes each path | route of the light irradiated to the said optical deflection | deviation means Therefore, an optical path changing unit having first and second adjustment mirrors each including a biaxial tilt angle adjusting unit;
And at least two single plastic fθ lenses for converging each light beam deflected by the light deflecting means on the scanning line ,
The light source is constituted by a semiconductor laser, a collimating lens, and a holding member that holds these,
When the temperature changes, the amount of defocus due to the variation in the distance between the emission surface of the semiconductor laser and the collimating lens caused by the thermal contraction of the holding member is the amount of defocus due to the wavelength change of the semiconductor laser and the second condensing. The amount of defocus due to the change in the focal length of the element and the amount of defocus due to the change in the focal length of the single plastic fθ lens are offset by opposite values.
At the time of temperature change, the amount of defocus due to the wavelength change of the semiconductor laser, the amount of defocus due to thermal contraction of the holding member, the amount of defocus due to the change in the focal length of the second condensing element, and the single plastic fθ An optical scanning device characterized in that an amount obtained by adding all the defocus amounts due to a change in the focal length of the lens is within ± 2 mm. The first condensing element is a glass lens;
The optical scanning device according to claim 1, wherein the second light collecting element is a plastic lens.

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