Scanning Optical Device

Abstract

To provide a scanning optical device having a coupling lens with a diffraction surface in which image forming state hardly changes due to temperature variation.SOLUTION: A scanning optical device 10 comprises: a semiconductor laser 1; a coupling lens 2 converting light from the semiconductor laser 1 into a beam; an optical deflector 50 deflecting the beam from the coupling lens 2; a scanning optical system 40 imaging the beam deflected by the optical deflector 50 on a scanned surface 9A; and a holder HL holding the semiconductor laser 1 and the coupling lens 2. The coupling lens 2 includes a refractive surface with positive refractive power and a diffraction surface with positive diffraction power. A ratio φn / φd of refractive power φn of the refractive surface in a main scanning direction to diffraction power φd of the diffraction surface satisfies 1.85≤φn / φd<6.0. Linear expansion coefficient of the holder HL is 8.0×10-5 to 20×10-5[ / K].SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a scanning optical device used in an image forming apparatus or the like. [Background technology]

[0002] In a scanning optical device used in an electrophotographic image forming apparatus, light emitted from a light source is converted into a beam by a coupling lens, and this beam is deflected in the main scanning direction by a deflector having a rotating reflecting surface. The beam deflected by the deflector is imaged on the surface of a photosensitive drum by a scanning lens. The incident optical system from the light source to the reflecting surface of the deflector focuses the beam in the sub-scanning direction to form an image on the reflecting surface, and the scanning optical system between the deflector and the photosensitive drum focuses the beam reflected by the reflecting surface in both the main scanning direction and the sub-scanning direction to form an image.

[0003] Conventionally, glass lenses have often been used as coupling lenses, and glass lenses are suitable for coupling lenses because their focal position does not fluctuate much with temperature changes.

[0004] However, glass lenses are difficult to manufacture and expensive. Therefore, it is conceivable to improve productivity and costs by using resin lenses. However, because resin has a high linear expansion coefficient, the focal length changes with temperature. Specifically, when a resin lens thermally expands, the radius of curvature of the lens surface increases, and the refractive index decreases due to a decrease in the density of the resin, both of which cause the focal length to increase. The change in focal length deteriorates the image formation on the surface of the photosensitive drum.

[0005] To address this issue, there is a technology that provides both a refractive surface and a diffractive surface on a resin lens to suppress changes in imaging state due to temperature changes (Patent Document 1). A semiconductor laser used as a light source has a characteristic that its wavelength increases as the temperature increases, while a diffractive surface has a characteristic that its diffractive power increases as the wavelength increases. Therefore, by compensating for the decrease in the refractive power of the refractive surface with an increase in the diffractive power of the diffractive surface as the temperature increases, it is possible to reduce changes in focal length due to temperature changes. The technology in Patent Document 1 achieves a balance between the refractive power and diffractive power by setting the ratio φn / φd of the refractive power φn of the refracting surface to the diffractive power φd of the diffractive surface to 0.6<φn / φd<0.9. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] U.S. Patent No. 7,750,933 Summary of the Invention [Problem to be solved by the invention]

[0007] However, semiconductor lasers not only undergo gradual wavelength changes with temperature changes, but also experience a phenomenon known as mode hopping. Specifically, within a certain temperature range, semiconductor lasers gradually increase their wavelength as the temperature rises, but once the temperature exceeds that range, the wavelength may increase suddenly and discontinuously. For this reason, if the diffractive power of the diffractive surface is made too high, the laser becomes susceptible to the effects of mode hopping.

[0008] SUMMARY OF THE INVENTION It is therefore an object of the present invention to suppress changes in the imaging state due to temperature changes in a scanning optical device equipped with a coupling lens having a diffractive surface. [Means for solving the problem]

[0009] In order to achieve the above-mentioned object, the present invention comprises a semiconductor laser, a coupling lens that converts light from the semiconductor laser into a beam, an optical deflector that deflects the beam from the coupling lens, a scanning optical system that forms an image of the beam deflected by the optical deflector on a surface to be scanned, and a holder that holds the semiconductor laser and the coupling lens. The coupling lens has a refractive surface having a positive refractive power and a diffractive surface having a positive diffractive power. The ratio φn / φd of the refractive power φn of the refractive surface to the diffractive power φd of the diffractive surface in the main scanning direction of the coupling lens is 1.85≦φn / φd<6.0 Meet the following. The linear expansion coefficient of the holder is 8.0 x 10 -5 ~20×10 -5 [ / K]

[0010] In the scanning optical device having such a configuration, since φn / φd is greater than 1.85, the diffraction power φd is appropriately smaller than the refraction power φn, and therefore, even if there is a change in wavelength due to mode hopping, the influence of the diffraction power φd can be suppressed. The holder, which defines the distance between the semiconductor laser and the coupling lens by holding the semiconductor laser and the coupling lens, has a linear expansion coefficient of 8.0×10 -5 [ / K] or more, when the temperature rises, the distance between the semiconductor laser and the coupling lens widens relatively significantly. Therefore, when the temperature rises, the increase in the distance between the semiconductor laser and the coupling lens has the effect of shortening the focal length of the entire optical system. The refractive surface has the property of lengthening the focal length of the coupling lens when the temperature rises, but in addition to the effect of the diffractive surface shortening the focal length when the temperature rises, the effect of shortening the focal length due to thermal expansion of the holder acts in the opposite direction to the property of lengthening the focal length of the refractive surface, so that even if the diffraction power φd is small, changes in the image position can be appropriately suppressed.

[0011] The scanning optical device may further include a housing that holds the holder, the optical deflector, and the scanning optical system. In this case, it is desirable that the linear expansion coefficient of the holder is greater than the linear expansion coefficient of the housing.

[0012] The housing that holds the holder, the optical deflector, and the scanning optical system has a small linear expansion coefficient, which suppresses changes in the positional relationship of each component and suppresses changes in the imaging state when the temperature changes. On the other hand, the linear expansion coefficient of the holder is larger than that of the housing, so that when the temperature changes, the change in the focal length of the entire optical system due to the change in the distance between the semiconductor laser and the coupling lens cancels out the change in the refractive power of the refractive surface, thereby suppressing changes in the imaging position.

[0013] The holder may be made of, for example, resin.

[0014] The refractive surface may have an axisymmetric shape, and the diffractive surface may have an axisymmetric shape.

[0015] The longitudinal magnification in the main scanning direction of the entire optical system from the semiconductor laser including the coupling lens and the scanning optical system to the surface to be scanned may be 70 to 164 times.

[0016] When the longitudinal magnification is large, the change in the image position during mode hopping becomes large, but by setting the longitudinal magnification to 70 to 164 times, the change in the image position during mode hopping can be suppressed.

[0017] The scanning optical system may be an fθ scanning optical system, and the focal length of the scanning optical system in the main scanning direction may be 200 to 260 mm.

[0018] The scanning optics can consist of one or more lenses. [Effects of the Invention]

[0019] According to the scanning optical device of the present invention, when the temperature rises, the effect of shortening the focal length due to thermal expansion of the holder cancels out the property of lengthening the focal length of the refractive surface, so that even if the diffraction power φd is reduced, the change in the focal length of the coupling lens can be appropriately suppressed, thereby suppressing changes in the imaging state due to temperature changes. [Brief explanation of the drawings]

[0020] [Figure 1] 1 is a main scanning cross-sectional view of a scanning optical device according to an embodiment. [Figure 2] 10 is a table comparing the image plane shift of the embodiment with that of other scanning optical devices. [Figure 3] 3 is a table showing specifications of optical systems other than those shown in FIG. 2 of the embodiment and other scanning optical devices. DETAILED DESCRIPTION OF THE INVENTION

[0021] Next, an embodiment of the present invention will be described in detail with reference to the accompanying drawings. 1, a scanning optical device 10 according to one embodiment includes a housing 20, a semiconductor laser 1 as a light source, a coupling lens 2, an aperture stop 3, and a cylindrical lens 4 as an incident optical system 30, an optical deflector 50, and an fθ lens 6 and a plane tilt correction lens 7 as a scanning optical system 40. The scanning optical device 10 is configured to focus the laser light emitted from the semiconductor laser 1 into a point on a scanned surface 9A of a photosensitive drum 9 to perform scanning.

[0022] The housing 20 is a member that holds the incident optical system 30, the optical deflector 50, and the scanning optical system 40. Of the incident optical system, the semiconductor laser 1, the coupling lens 2, and the aperture stop 3 are held by a holder HL. That is, the housing 20 holds the holder HL, the aperture stop 3, the cylindrical lens 4, the optical deflector 50, the fθ lens 6, and the plane tilt correction lens 7. The housing 20 has an opening 21. The beam that has passed through the scanning optical system 40 passes through the opening 21 and reaches the scanned surface 9A. Note that the housing 20 and the optical system in FIG. 1 are shown schematically; in an actual device, mirrors are appropriately placed to reflect the beam, and the various components are arranged to fit within the housing 20.

[0023] The linear expansion coefficient of the housing 20 is smaller than that of the holder HL. The linear expansion coefficient of the housing 20 is 5.0×10 -5 [ / K] or above, 8.0×10 -5 [ / K]. The housing 20 is made of a resin such as a PC / AS polymer alloy. When the housing 20 is made up of multiple parts such as a case body and a lid, the relationship of the linear expansion coefficients described above means that the linear expansion coefficients of the parts of the housing 20 that hold the incident optical system 30, the optical deflector 50, and the scanning optical system 40 are smaller than the linear expansion coefficient of the holder HL.

[0024] The holder HL is made of, for example, resin. The linear expansion coefficient of the holder HL is greater than the linear expansion coefficient of the housing 20. The linear expansion coefficient of the holder HL is 8.0×10 -5 [ / K] or more. The linear expansion coefficient of the holder HL is 8.0×10 -5 [ / K] or more, when the temperature of the scanning optical device 10 rises, the distance between the semiconductor laser 1 and the coupling lens 2 increases relatively greatly. Therefore, when the temperature rises, the increase in the distance between the semiconductor laser 1 and the coupling lens 2 shortens the focal length of the entire optical system. The linear expansion coefficient of the holder HL is 20×10 -5[ / K] or less. With such a linear expansion coefficient, the material of the holder HL can be selected from common resins. The holder HL may hold the semiconductor laser 1, coupling lens 2, aperture stop 3, and cylindrical lens 4. The holder HL does not have to be made of a single material, but may be made of a combination of multiple materials. For example, the holder HL may be made of a combination of metal and resin. In this case, the composite linear expansion coefficient Z of each material that maintains the distance between the light source and the incident optical system is the linear expansion coefficient Z.

[0025] The semiconductor laser 1 is a device that emits a slightly divergent laser beam. The light emitting element of the semiconductor laser 1 is turned on and off by a control device (not shown) in accordance with the image to be exposed on the scanned surface 9A of the photosensitive drum 9.

[0026] The coupling lens 2 is disposed between the semiconductor laser 1 and the optical deflector 50 and converts the light emitted from the semiconductor laser 1 into a parallel or slightly convergent beam in the main scanning direction (the direction deflected by the optical deflector 50 relative to the beam's direction of travel within the plane of FIG. 1) and the sub-scanning direction (the direction perpendicular to the main scanning direction, extending into the plane of FIG. 1). The cylindrical lens 4 has no refractive power in the main scanning direction but has refractive power in the sub-scanning direction, and focuses the light emitted from the coupling lens 2 in the sub-scanning direction near the mirror surface 5A of the optical deflector 50. The coupling lens 2 has a refractive surface with positive refractive power and a diffractive surface with positive diffractive power. The refractive surface has an axially symmetric shape. The diffractive surface also has an axially symmetric shape. In other words, the refractive surface and the diffractive surface have rotationally symmetric shapes at any angle around the optical axis of the coupling lens 2. The coupling lens 2 is formed, for example, as a diffractive surface on the beam incident side and as a refractive surface on the beam exit side.

[0027] In the coupling lens 2 of this embodiment, the ratio φn / φd of the refractive power φn of the refractive surface to the diffractive power φd of the diffractive surface in the main scanning direction is: 1.85≦φn / φd<6.0 Meet the following.

[0028] The aperture stop 3 is a member having an opening that defines the size of the beam that has passed through the coupling lens 2 in the sub-scanning direction.

[0029] The optical deflector 50 has a polygon mirror 5 with multiple mirror surfaces 5A arranged at equal distances from a rotation axis 5B. While Fig. 1 shows an example of the polygon mirror 5 having six mirror surfaces 5A, the number of mirror surfaces is not particularly limited. The polygon mirror 5 rotates at a constant speed around the rotation axis 5B, and deflects the beam from the coupling lens 2 that has passed through the aperture stop 3 in the main scanning direction.

[0030] The scanning optical system 40 focuses the beam deflected by the optical deflector 50 onto the scanned surface 9A. In this embodiment, the scanning optical system 40 is an fθ scanning optical system. The scanning optical system 40 has fθ characteristics such that the beam deflected by the polygon mirror 5 at a constant angular velocity is scanned onto the scanned surface 9A at a constant velocity. In this embodiment, the scanning optical system 40 is composed of multiple lenses, and includes an fθ lens 6 and a wobble correction lens 7. The fθ lens 6 focuses the beam deflected by reflection from the polygon mirror 5 onto the scanned surface 9A in a point-like shape. The wobble correction lens 7 corrects the wobble of the mirror surface 5A of the polygon mirror 5. In this embodiment, the fθ lens 6 and the wobble correction lens 7, which are lenses constituting the scanning optical system 40, only have refractive surfaces and do not have diffractive surfaces. The focal length of the scanning optical system 40 in the main scanning direction is 200 to 260 mm.

[0031] The longitudinal magnification in the main scanning direction of the entire optical system from the semiconductor laser 1 including the incident optical system 30 and the scanning optical system 40 to the scanned surface 9A is preferably 70 to 164 times.

[0032] According to the scanning optical device 10 of the present embodiment described above, since φn / φd is greater than 1.85, the diffraction power φd is appropriately smaller than the refraction power φn, and therefore, even if there is a change in wavelength due to mode hopping, the influence of the diffraction power φd can be suppressed. Furthermore, the linear expansion coefficient of the holder HL that holds the semiconductor laser 1 and the coupling lens 2 is set to 8.0×10 -5 [ / K] or more, when the temperature rises, the distance between the semiconductor laser 1 and the coupling lens 2 increases, shortening the focal length of the entire optical system. This effect cancels out the property of the refractive surface of the coupling lens 2 that lengthens the focal length when the temperature rises, so that even if the diffraction power φd is reduced, it is possible to appropriately suppress changes in the imaging position in the main scanning direction.

[0033] Furthermore, the housing 20 that holds the holder HL, the optical deflector 50, and the scanning optical system 40 has a linear expansion coefficient smaller than that of the holder HL, which suppresses changes in the positional relationship of the various components and suppresses changes in the imaging state when the temperature changes. On the other hand, the linear expansion coefficient of the holder HL is larger than that of the housing 20, so that when the temperature changes, the change in the focal length of the entire optical system due to a change in the distance between the semiconductor laser 1 and the coupling lens 2 cancels out the change in the refractive power of the refractive surface, thereby suppressing changes in the imaging position in the main scanning direction.

[0034] Furthermore, the scanning optical device 10 of this embodiment has a longitudinal magnification in the main scanning direction of the entire optical system from the semiconductor laser 1 to the scanned surface 9A of 70 to 164 times, which makes it possible to suppress changes in the imaging position in the main scanning direction during mode hopping.

[0035] Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments. The specific configurations can be appropriately changed without departing from the spirit of the present invention.

[0036] For example, in the above embodiment, the incident side of the coupling lens 2 is a diffractive surface and the exit side is a refractive surface, but this may be reversed, with the incident side being a refractive surface and the exit side being a diffractive surface.

[0037] Furthermore, in the above embodiment, the scanning optical system 40 is made up of a plurality of lenses, but it may be made up of a single lens. [Example]

[0038] The inventors calculated the change in the imaging position in the main scanning direction (paraxial principal image plane shift [mm]) when the linear expansion coefficient of the holder of the scanning optical device is changed. The principal image plane shift was calculated for three cases: -5°C, 55°C, and 25°C where the wavelength is extended by 1 nm due to mode hopping, with the imaging position in the main scanning direction at 25°C as the reference temperature. φn / φd shows the optimized value that minimizes the maximum value of the principal image plane shift while other conditions are kept constant.

[0039] As shown in Figures 2 and 3, when a glass lens is used for the coupling lens (Reference Examples 1 and 2), the magnitude of the main image plane shift due to mode hopping is small, but the image plane shift is large when the temperature changes (-5°C, 55°C). Note that in Figures 2 and 3, the wavelength of the light source is the standard wavelength of the semiconductor laser, and the focal length is the value at a wavelength of 780 nm.

[0040] On the other hand, when the coupling lens is made of resin (cycloolefin polymer) (Comparative Examples 1 to 5, Examples 1 to 3), the combination of a refractive surface and a diffractive surface reduces the shift of the main image plane when the temperature changes. However, because of the diffractive surface, the amount of image plane shift when the wavelength of the laser light changes due to mode hopping is larger than when the coupling lens is made of glass (N-SF8, SCHOTT) (Reference Examples 1 and 2).

[0041] Therefore, calculations were performed by changing the linear expansion coefficient of the holder. As in Examples 1 to 3 and Comparative Examples 3 and 4, the linear expansion coefficient was set to 8 × 10 -5 ~40×10-5 [ / K], the absolute value of the main image plane shift during temperature change and mode hopping was suppressed to be smaller than those of Comparative Examples 1 and 2. However, when the linear expansion coefficient was 30×10 -5 [ / K] or more, the diffractive power of the diffractive surface becomes negative, and in this case, the change in diffractive power does not cancel out the change in refractive power when the temperature changes, which is therefore deemed undesirable.

[0042] Therefore, the linear expansion coefficient of the holder is 8×10 -5 ~20×10 -5 [ / K], and it was confirmed that when φn / φd of the coupling lens is 1.85≦φn / φd<6, the main image plane shift is suppressed due to temperature change and wavelength change caused by mode hopping. [Explanation of symbols]

[0043] 1. Semiconductor laser 2 Coupling Lens 5 Polygon Mirror 6 fθ lenses 7. Corrective lenses 9 Photosensitive drum 9A Scanned surface 10. Scanning optical device 20 Case 30 Input optical system 40 Scanning optical system 50 optical deflector HL Holder

Claims

1. a semiconductor laser; a coupling lens that converts light from the semiconductor laser into a beam; an optical deflector that deflects the beam from the coupling lens; a scanning optical system that forms an image of the beam deflected by the optical deflector on a surface to be scanned; a holder for holding the semiconductor laser and the coupling lens; a housing that holds the holder, the optical deflector, and the scanning optical system, the coupling lens has a refractive surface having a positive refractive power and a diffractive surface having a positive diffractive power, The ratio φn / φd of the refractive power φn of the refractive surface to the diffractive power φd of the diffractive surface in the main scanning direction is 1.85≦φn / φd<6.0 Fulfilling The linear expansion coefficient of the holder is 8.0×10 －５ ~20 x 10 －５ [ / K], the linear expansion coefficient of the housing is 5.0×10 −5 [ / K] or more and less than 8.0×10 −5 [ / K]; The main image plane shift is positive when the temperature rises from 25°C to 55°C, 10. A scanning optical device, comprising: a scanning lens for scanning a light beam from a semiconductor laser, the scanning lens being irradiated with light from the semiconductor laser;

2. A scanning optical device as described in claim 1, characterized in that the main image plane shift is positive when the temperature drops from 25°C to -5°C.

3. 3. The scanning optical device according to claim 1, wherein the holder is made of resin.

4. the refractive surface has an axisymmetric shape, 4. The scanning optical device according to claim 1, wherein the diffractive surface has an axially symmetric shape.

5. A scanning optical device according to any one of claims 1 to 4, characterized in that the longitudinal magnification in the main scanning direction of the entire optical system from the semiconductor laser to the scanned surface, including the coupling lens and the scanning optical system, is 70 to 164 times.

6. 6. The scanning optical device according to claim 1, wherein the scanning optical system is an fθ scanning optical system, and the focal length in the main scanning direction is 200 to 260 mm.

7. 7. The scanning optical device according to claim 1, wherein the scanning optical system is made up of one or more lenses.

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