JP5780093B2 - Scanning optical device - Google Patents

Description

  The present invention relates to a scanning optical device used in an electrophotographic image forming apparatus.

  In a scanning optical device used in an electrophotographic image forming apparatus, a light beam from a light source is formed in a dot shape on a surface to be scanned such as a photosensitive drum, and this image is formed in the axial direction of the photosensitive drum (mainly Scanning in the scanning direction). This scanning optical apparatus has a deflector that deflects a light beam in the main scanning direction, and an incident optical system is provided in the front stage of the deflector, and a scanning optical system is provided in the subsequent stage. The incident optical system forms an image of the light beam in the sub-scanning direction in the vicinity of the deflector and makes the light beam substantially parallel light in the main scanning direction. On the other hand, the scanning optical system has a function of forming a light beam from the deflector in a dot shape on the surface to be scanned.

  By the way, when the environmental temperature at which the scanning optical device is used changes, there is a problem that the imaging point is shifted back and forth from the surface to be scanned due to a change in the dimensions of each part, a change in the performance of the optical element, and the like. This problem becomes particularly prominent when a resin lens is used in the incident optical system for cost reduction. In order to solve this problem, conventionally, a refracting surface and a diffractive surface are provided in the incident optical system as in Patent Document 1 to suppress (temperature compensation) an image point shift (image surface shift) due to a temperature change. Has been made. In Patent Document 1, it is preferable that the ratio φr / φd between the refractive power φr of the refractive unit and the diffraction power φd of the diffraction unit satisfies a condition such as 0.6 <φr / φd <0.9.

US Pat. No. 7,750,933

  However, since the scanning optical device of Patent Document 1 has a large lateral magnification of the scanning lens, as disclosed in Japanese Patent No. 3303558, there is a problem that the jitter of the polygon surface period due to the deflection of the deflecting surface of the deflector becomes large. There is. On the other hand, if the lateral magnification of the scanning lens is too small, the distance between the scanning lens and the surface to be scanned is required, and there is a problem that the size of the scanning optical device cannot be made compact. According to the researches by the inventors, it has been found that the scanning optical device of Patent Document 1 cannot achieve both jitter reduction and temperature compensation.

  The present invention was devised in view of the above background, and provides a scanning optical device that achieves good temperature compensation while realizing compactness of the device and suppression of jitter due to deflection of the deflecting surface of the deflector. The purpose is to do.

The present invention that solves the problem includes a light source, a deflecting unit that deflects a light beam from the light source in the main scanning direction, and a light beam that is provided between the light source and the deflecting unit and is substantially parallel light in the main scanning direction. And a scanning optical device having an incident optical system that forms an image in the vicinity of the deflecting unit in the sub-scanning direction, and a scanning lens that forms a light beam deflected by the deflecting unit in a dot shape on the surface to be scanned. .
The scanning lens satisfies 0.2 ≦ 1-s ′ / fm ≦ 0.5, where s ′ is the distance from the image side main point in the main scanning direction to the image point, and fm is the focal length in the main scanning direction. .
Further, the incident optical system, a rotationally symmetrical diffraction surface on the incident surface, made from a single resin lens with an anamorphic refractive surface on the exit surface, the focal length in the main scanning direction as fi [mm], 10 ≦ It satisfies fi ≦ 22.
Further, assuming that the lateral magnification in the main scanning direction of the entire optical system is mM and the lateral magnification in the sub-scanning direction is mS, the ratio of mM to mS, mM / mS, satisfies mM / mS ≧ 1.38.
Further, the refractive power in the main scanning direction of the incident optical system is φn, the diffraction power is φd, and the ratio φn / φd of φn and φd is
g2 (fi) ≦ φn / φd ≦ g1 (fi)
g1 (fi) = 0.015fi + 1.073
g2 (fi) =-0.01fi + 1.184
Meet.

According to such a configuration, when the lateral magnification (β = 1−s ′ / fm) of the scanning lens is 0.5 or less, jitter can be suppressed, and when it is 0.2 or more, the apparatus is compact. Can be
Further, when the ratio mM / mS of the lateral magnification mM in the main scanning direction and the lateral magnification mS in the sub-scanning direction satisfies mM / mS ≧ 1.38, the image plane shift can be suppressed.

The ratio φn / φd between the refractive power φn and the diffraction power φd is
g2 (fi) ≦ φn / φd ≦ g1 (fi)
g1 (fi) = 0.015fi + 1.073
g2 (fi) =-0.01fi + 1.184
By satisfying the above, curvature of field due to temperature change can be suppressed in the main scanning direction and the sub-scanning direction.

  In such a scanning optical apparatus, it is desirable that the incident optical system is composed of a single resin lens. By making the incident optical system one resin lens, the manufacturing can be facilitated, and when such a lens is adopted, the effect of temperature compensation can be effectively utilized.

According to the present invention, it is possible to achieve good temperature compensation of the scanning optical device while realizing compactness of the device and suppression of jitter due to deflection of the deflecting surface of the deflector.
it can.

It is a main scanning sectional view of a scanning optical device concerning one embodiment. It is a figure explaining the lateral magnification of a scanning lens. FIG. 5 is a graph showing a relationship between a temperature change and an image plane shift for explaining a method of creating the graph of FIG. 4. 6 is a graph showing the relationship between φn / φd and the maximum absolute value of image plane shift. It is a graph which shows the range where temperature compensation is made favorable. It is the data of the optical system of Example 1,2.

Next, an embodiment of the present invention will be described in detail with reference to the drawings as appropriate.
As shown in FIG. 1, a scanning optical device 10 according to an embodiment includes a semiconductor laser 1 as an example of a light source, a diffractive lens 2 as an example of an incident optical system, an aperture stop 3, and a polygon mirror as an example of a deflecting unit. 5. An fθ lens 6 as an example of a scanning lens is provided, and by these, the laser beam emitted from the semiconductor laser 1 is condensed in a dot shape on the scanned surface 9A of the photosensitive drum 9 and scanned. Has been. The semiconductor laser 1, the diffraction lens 2, the aperture stop 3, the polygon mirror 5 and the fθ lens 6 are fixedly disposed on a resin or metal casing (not shown).

  The semiconductor laser 1 is a device that emits a slightly divergent laser beam (light beam). The light emitting element of the semiconductor laser 1 is blinked corresponding to an image to be exposed on the scanned surface 9A of the photosensitive drum 9 by a control device (not shown).

  The diffractive lens 2 is provided between the semiconductor laser 1 and the polygon mirror 5, and the light beam emitted from the semiconductor laser 1 is swung to the left and right with respect to the traveling direction of the light beam in the paper plane of FIG. Near the mirror surface 5A of the polygon mirror 5 in the sub-scanning direction (the direction perpendicular to the main scanning direction and the depth direction of the drawing in FIG. 1). It is a lens that forms an image with. The diffractive lens 2 is formed such that one surface, for example, the incident side of the light beam is a diffractive surface and the exit side is a refracting surface. The diffractive lens 2 is preferably made of a single resin lens from the viewpoint of cost reduction. However, the incident optical system referred to in the present invention is not limited to a single resin lens, but may be a lens made of glass, or as long as there is at least one refracting surface and at least one diffractive surface. Any number is acceptable.

  The diffractive lens 2 is in the range of 10 ≦ fi ≦ 22 with the focal length in the main scanning direction as fi [mm]. When the focal length fi is 10 mm or more, the lateral magnification can be suppressed from becoming too large, and when the focal length fi is 22 mm or less, the apparatus can be made compact, and the decrease in the light utilization efficiency of the laser diode can be suppressed.

  The diffractive lens 2 has a magnification ratio mM / mS, which is the ratio of mM to mS, where the horizontal magnification in the main scanning direction of the entire optical system (diffraction lens 2 to fθ lens 6) is mM, and the horizontal magnification in the sub-scanning direction is mS. mS satisfies mM / mS ≧ 1.38. When the magnification ratio mM / mS in the main scanning direction is 1.38 or more, the amount of image plane shift due to a change in environmental temperature can be reduced.

Further, in this embodiment, the refractive power φn of the diffraction lens 2 in the main scanning direction, the diffraction power is φd, and the ratio φn / φd of φn and φd is
g2 (fi) ≦ φn / φd ≦ g1 (fi)
g1 (fi) = 0.015fi + 1.073
g2 (fi) =-0.01fi + 1.184
Meet. As will be described later, by satisfying this condition, the amount of image plane shift can be suppressed even if there is a change in the environmental temperature in both the main scanning direction and the sub-scanning direction. Accurate exposure is possible.

  The aperture stop 3 is a member having an aperture that defines the size of the light beam that has passed through the diffraction lens 2 in the sub-scanning direction.

  The polygon mirror 5 is a member in which a plurality of mirror surfaces 5A are arranged at an equal distance from the rotation shaft 5B, and FIG. 1 illustrates an example having six mirror surfaces 5A. The polygon mirror 5 is rotated at a constant speed around the rotation axis 5B, and deflects the light beam that has passed through the aperture stop 3 in the main scanning direction.

  In the present embodiment, only one fθ lens 6 is provided in the scanning optical device 10. The fθ lens 6 focuses the light beam deflected by being reflected by the polygon mirror 5 on the surface to be scanned 9 </ b> A, and corrects the tilt of the mirror surface 5 </ b> A of the polygon mirror 5. . Further, the fθ lens 6 has an fθ characteristic such that the light beam deflected at a constant angular velocity by the polygon mirror 5 is scanned on the scanned surface 9A at a constant velocity.

As shown in FIG. 2, the fθ lens 6 has a distance s from the object point OB to the object side principal point H in the main scanning direction, and a distance from the image side principal point H ′ to the image point IM in the main scanning direction. ′, Where fm is the focal length of the scanning lens in the main scanning direction,
1 / fm = 1 / s′−1 / s
It is. At this time, the lateral magnification β of the fθ lens 6 is
β = s ′ / s = 1−s ′ / fm
It is represented by

In the present embodiment, the lateral magnification β (= 1−s ′ / fm) in the main scanning direction is
0.2 ≦ 1-s ′ / fm ≦ 0.5
It is. When the lateral magnification β is 0.2 or more, the scanning optical device 10 can be made compact, and when it is 0.5 or less, jitter due to shake of the mirror surface 5A of the polygon mirror 5 can be suppressed to a small value. it can.

The inventors of the present application adjust the ratio φn / φd (hereinafter referred to as “power ratio”) of the refractive power φn and the diffraction power φd in the main scanning direction of the diffractive lens 2 (incident optical system), thereby changing the environmental temperature. The effect of the image plane shift due to the was investigated.
Specifically, using an optical system such as Examples 1 and 2 below, (1) magnification ratio mM / mS, (2) power ratio φn / φd, and (3) focal length fi of the diffractive lens in the main scanning direction. The amount of image plane shift was calculated by changing the size of. In the following Examples 1 and 2, (1) magnification ratio mM / mS = 1.38, (2) power ratio φn / φd = 1.15, and (3) focal length fi The cases of 22 mm and 10 mm are shown, respectively. The data of the optical system is shown in FIG.

[Example 1]
Wavelength of semiconductor laser 788 [nm]
Temperature range -5 to 55 [° C]
Wavelength change rate of semiconductor laser 0.25 [nm / ° C]
Focal length fi 22 [mm] in the main scanning direction of the diffractive lens
Linear expansion coefficient of the member that maintains the distance between the semiconductor laser and the diffractive lens
6.50 × 10 ⁻⁵ [1 / K]
Horizontal magnification mM in main scanning direction of entire optical system 6.70
Horizontal magnification mS in the sub-scanning direction of the entire optical system 4.85
Magnification ratio mM / mS 1.38
Refractive power φn 0.02468 in the main scanning direction of the diffractive lens
Diffraction power in the main scanning direction of the diffraction lens φd 0.02146
Power ratio φn / φd 1.15

Ｃ_１＝−０．０１０７３２ Phase function of diffraction surface

C ₁ = −0.010732

[Example 2]
Wavelength of semiconductor laser 788 [nm]
Temperature range -5 to 55 [° C]
Wavelength change rate of semiconductor laser 0.25 [nm / ° C]
Focal length fi 10 [mm] in the main scanning direction of the diffractive lens
Linear expansion coefficient of the member that maintains the distance between the semiconductor laser and the diffractive lens
6.50 × 10 ⁻⁵ [1 / K]
Horizontal magnification in the main scanning direction of the entire optical system mM 15.11
Horizontal magnification in the sub scanning direction of the entire optical system mS 10.95
Magnification ratio mM / mS 1.38
Refractive power in the main scanning direction of the diffractive lens φn 0.05536
Diffraction power in the main scanning direction of the diffractive lens φd 0.04813
Power ratio φn / φd 1.15

Ｃ_１＝−０．０２４０６７ Phase function of diffraction surface

C ₁ = −0.024067

  As an example, when the power ratio φn / φd = 0.8, the magnification ratio mM / mS = 1.625 (a different value from the first and second embodiments), and fi = 10 mm, the image plane shift and the sub-scanning in the main scanning direction are performed. The temperature dependence of the image plane shift in the scanning direction is shown in FIG. In FIG. 3, the focal positions in the main scanning direction at −5 ° C. and 55 ° C. are plotted with reference to the image plane position at room temperature (25 ° C.). Here, the exposure position on the scanned surface 9A is the center of the scanning range, as shown in FIG. 1, when the laser light is incident on the scanned surface 9A from the front (orthogonally).

  A value having a large absolute value is selected from the values of the image plane shift due to the temperature shown in FIG. 3, and the relationship between the power ratio φn / φd in the main scanning direction and the maximum absolute value of the image plane shift is shown as shown in FIG. Created a graph. For example, in FIG. 3, an image plane shift of 3.4 mm at −5 ° C. is the maximum absolute value in the main scanning direction and an image plane shift at 55 ° C. in the sub-scanning direction is 2. 1 mm is the maximum absolute value. In the graph of FIG. 4, this 3.4 mm (▲ mark) and 2.1 mm (△ mark) are plotted at the position of φn / φd = 0.80 where fi = 10 mm and mM / mS = 1.625. Yes.

  In FIG. 4, the filled marks are image plane shifts in the main scanning direction, and the white marks are image plane shifts in the sub-scanning direction. Further, when fi = 10 mm, it is displayed with a solid line, and when fi = 22 mm, it is displayed with a broken line.

As shown in FIG. 4, when looking at the magnitude of the image plane shift due to the difference in magnification ratio mM / mS, the smaller the magnification ratio mM / mS, the smaller the image plane shift in both the main scanning direction and the sub-scanning direction. You can see that it ’s big. Therefore, paying attention to mM / mS = 1.38 having the smallest magnification ratio mM / mS, when the magnification ratio mM / mS = 1.38, the power ratio φn / φd at which the image plane shift becomes equal to or less than a predetermined reference is set. investigated.
The predetermined reference for this image plane shift was an upper limit of 1 mm in the main scanning direction and an upper limit of 4 mm in the sub-scanning direction. When the magnification ratio is mM / mS = 1.38, the power ratio φn / φd is in the range of 1.09 to 1.23 when fi = 10 mm. It was found that the power ratio was in the range of φn / φd 0.97 to 1.41 when = 22 mm. FIG. 5 schematically illustrates the range of conditions that satisfy the above-described criteria in both the main scanning direction and the sub-scanning direction. In the range of fi = 10 to 22 mm,
g1 (fi) = 0.015fi + 1.073
g2 (fi) =-0.01fi + 1.184
Φn / φd is
g2 (fi) ≦ φn / φd ≦ g1 (fi)
It was found that it should satisfy.

The range indicated by g1 (fi) and g2 (fi) is different from the range disclosed in Patent Document 1. In Patent Document 1, since the lateral magnification (β = 1−s ′ / fm) of the scanning lens is not taken into consideration, a range different from the present embodiment is set as a favorable range, but in the example in Patent Document 1, Since the lateral magnification β of the scanning lens is as large as 0.674 (value when the lateral magnification described in Patent Document 1 is represented by β = 1−s ′ / fm), the mirror surface 5A of the polygon mirror 5 There has been a problem that the jitter of the polygonal surface period due to shake is large. However, in the present embodiment, since the lateral magnification β is set to 0.5 or less as in Patent Document 2, jitter can be reduced, and the power ratio φn / φd is different from that in Patent Document 1. The temperature-compensated scanning optical device 10 can be realized in a completely different range.
Further, since the scanning optical device 10 of the present embodiment has a lateral magnification β of 0.2 or more, the distance from the fθ lens 6 to the surface to be scanned 9A can be reduced, and the device can be made compact.

  Although the embodiment of the present invention has been described above, the present invention is not limited to the above-described embodiment. About a concrete structure, it can change suitably in the range which does not deviate from the meaning of this invention.

  For example, in the above-described embodiment, the incident side of the diffractive lens 2 is a diffractive surface and the exit side is a refracting surface, but this may be reversed, and the incident side may be a refracting surface and the exit side may be a diffractive surface.

DESCRIPTION OF SYMBOLS 1 Semiconductor laser 2 Diffraction lens 5 Polygon mirror 5A Mirror surface 5B Rotating shaft 6 f (theta) lens 9 Photosensitive drum 9A Scanned surface 10 Scanning optical apparatus

Claims (1)

A light source, deflecting means for deflecting a light beam from the light source in the main scanning direction, and a light beam provided between the light source and the deflecting means to be substantially parallel light in the main scanning direction; A scanning optical apparatus comprising: an incident optical system that forms an image in the vicinity of a deflecting unit; and a scanning lens that forms a light beam deflected by the deflecting unit in a dot shape on a surface to be scanned;
The scanning lens has s ′ as the distance from the image side principal point to the image point in the main scanning direction, and fm as the focal length in the main scanning direction.
0.2 ≦ 1-s ′ / fm ≦ 0.5
The filling,
The incident optical system is rotationally symmetric diffractive surface on the incident surface, made from a single resin lens with an anamorphic refractive surface on the exit surface, the focal length in the main scanning direction as fi [mm],
10 ≦ fi ≦ 22
The filling,
Assuming that the lateral magnification in the main scanning direction of the entire optical system is mM and the lateral magnification in the sub-scanning direction is mS, the ratio of mM to mS, mM / mS,
mM / mS ≧ 1.38
The filling,
The ratio φn / φd between φn and φd, where the refractive power in the main scanning direction of the incident optical system is φn and the diffraction power is φd,
g2 (fi) ≦ φn / φd ≦ g1 (fi)
g1 (fi) = 0.015fi + 1.073
g2 (fi) =-0.01fi + 1.184
A scanning optical device characterized by satisfying the above.

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