JP2023032556A - Scanning optical device - Google Patents

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

The present invention relates to a scanning optical device used in image forming apparatuses and the like.

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 the beam is deflected in the main scanning direction by a deflector having a rotating reflecting surface. be done. The beam deflected by the deflector is imaged on the surface of the photosensitive drum by the scanning lens. The incident optical system from the light source to the reflecting surface of the deflector converges the beam in the sub-scanning direction and forms an image on the reflecting surface, and the scanning optical system between the deflector and the photosensitive drum reflects the beam from the reflecting surface. The light beams are focused in the main scanning direction and the sub-scanning direction to form an image.

By the way, conventionally, a glass lens was often used as the coupling lens. A glass lens is suitable for a coupling lens because the focal position does not fluctuate due to temperature changes.

However, the glass lens has poor productivity and high cost. Therefore, it is conceivable to improve productivity and cost by making the lens made of resin. However, since resin has a large coefficient of linear expansion, there arises a problem that the focal length changes due to temperature rise. Specifically, when the resin lens thermally expands, both the increase in the radius of curvature of the lens surface and the decrease in the refractive index due to the decrease in density of the resin become factors for increasing the focal length. A change in focal length deteriorates the imaging condition on the surface of the photosensitive drum.

As a solution to this problem, there is a technique in which a resin lens is provided with both a refractive surface and a diffractive surface to suppress changes in imaging state due to temperature changes (Patent Document 1). The wavelength of a semiconductor laser used as a light source increases as the temperature rises, while the diffraction power increases as the wavelength increases. By compensating for the increase in the diffraction power of the surface, it becomes possible to reduce the change in the focal length when the temperature changes. In the technique of Patent Document 1, the ratio φn/φd between the refractive power φn of the refracting surface and the diffraction power φd of the diffractive surface is set to 0.6<φn/φd<0.9 to balance the refractive power and the diffraction power. I'm taking

U.S. Pat. No. 7,750,933

However, a semiconductor laser causes a phenomenon called mode hopping as well as a gradual change in wavelength due to a change in temperature. Specifically, the wavelength of a semiconductor laser gradually increases as the temperature rises within a certain temperature range, but beyond that temperature range, the wavelength may increase discontinuously and abruptly. For this reason, if the diffraction power of the diffraction surface is increased too much, there is a problem that the effect of mode hopping is likely to occur.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to suppress changes in imaging state due to temperature changes in a scanning optical apparatus having a coupling lens having a diffractive surface.

The present invention for achieving the above object comprises a semiconductor laser, a coupling lens for converting the light from the semiconductor laser into a beam, an optical deflector for deflecting the beam from the coupling lens, and a deflection by the optical deflector. It comprises a scanning optical system that forms an image of the emitted beam on a surface to be scanned, and a holder that holds a semiconductor laser and a coupling lens.
The coupling lens has a refractive surface with positive refractive power and a diffractive surface with positive diffraction power. The ratio φn/φd between the refractive power φn of the refracting surface and the diffraction power φd of the diffractive surface in the main scanning direction of the coupling lens is
1.85≦φn/φd<6.0
meet.
Also, the coefficient of linear expansion of the holder is 8.0×10 ⁻⁵ to 20×10 ⁻⁵ [/K].

According to the scanning optical apparatus having such a configuration, since φn/φd is larger than 1.85, the diffraction power φd is moderately smaller than the refraction power φn. can also suppress the influence of the diffraction power φd. The linear expansion coefficient of the holder that defines the distance between the semiconductor laser and the coupling lens by holding the semiconductor laser and the coupling lens is set to be greater than 8.0×10 ⁻⁵ [/K]. Therefore, when the temperature rises, the distance between the semiconductor laser and the coupling lens increases relatively greatly. As a result, when the temperature rises, the distance between the semiconductor laser and the coupling lens increases, thereby shortening the focal length of the entire optical system. The refracting surface has the property of lengthening the focal length of the coupling lens when the temperature rises, but in addition to the action of the diffractive surface shortening the focal length when the temperature rises, the thermal expansion of the holder also shortens the focal length. , the change in the imaging position can be appropriately suppressed even if the diffraction power φd is reduced by acting opposite to the property of lengthening the focal length of the refracting surface.

The scanning optics can further comprise a housing that holds the holder, the optical deflector and the scanning optics. In this case, it is desirable that the linear expansion coefficient of the holder is larger than that of the housing.

The case that holds the holder, the optical deflector, and the scanning optical system has a small coefficient of linear expansion, so that it is possible to suppress changes in the positional relationship of each member and to suppress changes in the imaging state when the temperature changes. can. On the other hand, since the coefficient of linear expansion of the holder is larger than that of the case, when the temperature changes, the change in the focal length of the entire optical system caused by the change in the distance between the semiconductor laser and the coupling lens is reflected by the refraction of the refracting surface. The change in the imaging position can be suppressed by canceling out the change in power.

The holder may be made of resin, for example.

The refractive surface may have an axisymmetric shape. Also, the diffractive surface may have an axially symmetrical shape.

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

When the longitudinal magnification is large, the change in the imaging position during mode hopping becomes large. However, a longitudinal magnification of 70 to 164 times can suppress the change in the imaging position during mode hopping.

The scanning optical system is 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].

The scanning optics can consist of one or more lenses.

According to the scanning optical apparatus of the present invention, the action of shortening the focal length due to the thermal expansion of the holder when the temperature rises cancels out the effect of lengthening the focal length of the refracting surface. , the change in the focal length of the coupling lens can be suppressed appropriately. As a result, it is possible to suppress changes in imaging state due to temperature changes.

1 is a main scanning cross-sectional view of a scanning optical device according to an embodiment; FIG. 4 is a table comparing image plane shifts of the example and other scanning optical devices. FIG. 3 is a table showing specifications of optical systems other than those shown in FIG. 2 of an example and another scanning optical device; FIG.

Next, one 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 one embodiment includes a housing 20, a semiconductor laser 1 as a light source, a coupling lens 2 as an incident optical system 30, an aperture stop 3 and a cylindrical lens 4. , an optical deflector 50, an f.theta. The scanning optical device 10 is configured to converge the laser light emitted from the semiconductor laser 1 on the surface 9A to be scanned of the photosensitive drum 9 in the form of dots and scan the surface.

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, coupling lens 2 and aperture stop 3 are held by a holder HL. That is, the housing 20 holds the holder HL, the aperture diaphragm 3, the cylindrical lens 4, the optical deflector 50, the f.theta. The housing 20 has an opening 21 . The beam that has passed through the scanning optical system 40 passes through the aperture 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 apparatus, mirrors are appropriately arranged so that the beam is folded back so that each component can be accommodated within the housing 20. placed in

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 more and less than 8.0×10 ⁻⁵ [/K]. The housing 20 is made of, for example, a resin such as a PC/AS-based polymer alloy. In addition, when the housing 20 is formed by combining a plurality of parts such as a case body and a lid, the relationship between the above linear expansion coefficients is determined by the incident optical system 30, the optical deflector 50, and the scanning It means that the coefficient of linear expansion of the parts holding the optical system 40 is smaller than the coefficient of linear expansion of the holder HL.

The holder HL is made of resin, for example. The coefficient of linear expansion of the holder HL is greater than that of the housing 20 . The linear expansion coefficient of the holder HL is 8.0×10 ⁻⁵ [/K] or more. Since the linear expansion coefficient of the holder HL is set to a value greater than 8.0×10 ⁻⁵ [/K], when the temperature of the scanning optical device 10 rises, the distance between the semiconductor laser 1 and the coupling lens 2 increases. spread relatively large. As a result, when the temperature rises, the distance between the semiconductor laser 1 and the coupling lens 2 increases, thereby shortening the focal length of the optical system as a whole. The linear expansion coefficient of the holder HL is 20×10 ⁻⁵ [/K] or less. With such a coefficient of linear expansion, the material of the holder HL can be selected from general resins. The holder HL may hold the cylindrical lens 4 in addition to the semiconductor laser 1, the coupling lens 2 and the aperture stop 3. The holder HL need not consist of one member, but may consist of a combination of a plurality of members. For example, the holder HL may be composed of a combination of metal and resin. In this case, the linear expansion coefficient Z is the sum of the linear expansion coefficients of the members that maintain the distance between the light source and the incident optical system.

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

The coupling lens 2 is provided between the semiconductor laser 1 and the optical deflector 50, and deflects the light emitted from the semiconductor laser 1 in the main scanning direction (in the plane of FIG. , and are converted into parallel or slightly converging beams in the direction deflected by the optical deflector 50) and the sub-scanning direction (the direction perpendicular to the main scanning direction, the depth direction of the paper surface 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. is imaged in the vicinity of The coupling lens 2 has a refractive surface with positive refractive power and a diffraction surface with positive diffraction power. The refracting surface has an axisymmetric shape. Also, the diffractive surface has an axially symmetrical shape. That is, the refracting surface and the diffractive surface have rotationally symmetrical shapes at arbitrary angles around the optical axis of the coupling lens 2 . The coupling lens 2 has, for example, a diffractive surface on the beam incident side and a refractive surface on the exit side.

In the coupling lens 2 of this embodiment, the ratio φn/φd between the refractive power φn of the refracting surface and the diffraction power φd of the diffractive surface in the main scanning direction is
1.85≦φn/φd<6.0
meet.

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

The optical deflector 50 has a polygon mirror 5 in which a plurality of mirror surfaces 5A are arranged equidistantly from a rotation axis 5B. In FIG. 1, the polygon mirror 5 has six mirror surfaces 5A, but the number of mirror surfaces is not particularly limited. The polygon mirror 5 is rotated at a constant speed around a 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.

The scanning optical system 40 forms an image of the beam deflected by the optical deflector 50 on 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 at a constant angular velocity by the polygon mirror 5 scans the surface 9A to be scanned at a constant velocity. In this embodiment, the scanning optical system 40 is composed of a plurality of lenses, and has an fθ lens 6 and a tilt correction lens 7 . The fθ lens 6 forms an image of the beam deflected by being reflected by the polygon mirror 5 into a point on the surface to be scanned 9A. The tilt correction lens 7 corrects the tilt of the mirror surface 5A of the polygon mirror 5. FIG. In the present embodiment, the fθ lens 6 and the tilt correction lens 7, which are lenses constituting the scanning optical system 40, have only refracting surfaces and do not have diffractive surfaces. The focal length of the scanning optical system 40 in the main scanning direction is 200-260 [mm].

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.

According to the scanning optical apparatus 10 of the present embodiment described above, since φn/φd is larger than 1.85, the diffraction power φd is moderately smaller than the refraction power φn. , the influence of the diffraction power φd can be suppressed. By setting the linear expansion coefficient of the holder HL that holds the semiconductor laser 1 and the coupling lens 2 to 8.0×10 ⁻⁵ [/K] or more, the semiconductor laser 1 and the coupling lens 2 are The effect of widening the distance and shortening the focal length of the entire optical system is obtained. This effect cancels out the property of the refracting surface of the coupling lens 2 that increases the focal length when the temperature rises, so that even if the diffraction power φd is reduced, the change in the imaging position in the main scanning direction can be properly controlled. can be suppressed.

Since the housing 20 that holds the holder HL, the optical deflector 50 and the scanning optical system 40 has a coefficient of linear expansion smaller than that of the holder HL, changes in the positional relationship of each member are suppressed, and the result when the temperature changes is reduced. A change in the image state can be suppressed. On the other hand, since the coefficient of linear expansion of the holder HL is larger than that of the housing 20, 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 1 and the coupling lens 2 is Since it cancels out the change in the refractive power of the refracting surface, it is possible to suppress the change in the imaging position in the main scanning direction.

Further, the scanning optical device 10 of the present embodiment has a longitudinal magnification of 70 to 164 times in the main scanning direction of the entire optical system from the semiconductor laser 1 to the surface to be scanned 9A. It is possible to suppress the change of the image forming position in .

Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments. The specific configuration can be changed as appropriate without departing from the gist of the present invention.

For example, in the above embodiment, the incident side of the coupling lens 2 is the diffractive surface and the output side is the refracting surface. .

Further, although the scanning optical system 40 is composed of a plurality of lenses in the above embodiment, it may be composed of a single lens.

The inventor calculated the change in the imaging position in the main scanning direction (paraxial main image plane shift [mm]) when the linear expansion coefficient of the holder of the scanning optical device was changed. The main image plane shift is based on the imaging position in the main scanning direction at 25°C, and is calculated for three cases: -5°C, 55°C, and 1 nm wavelength lengthening due to mode hopping at 25°C. bottom. For φn/φd, values optimized to minimize the maximum value of the main image plane shift are shown with other conditions determined.

As shown in FIGS. 2 and 3, when a glass lens is used as the coupling lens (reference examples 1 and 2), the magnitude of the main image plane shift due to mode hopping is small, but when the temperature changes (-5 °C, 55°C), the image plane shift is large. 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 the wavelength of 780 nm.

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 the refracting surface and the diffractive surface suppresses the shift of the main image plane when the temperature changes. be done. However, since it has a diffractive surface, the amount of image plane shift when the wavelength of the laser light changes due to mode hopping is reduced when the coupling lens is made of glass (SCHOTT, N-SF8) (reference example) 1, 2).

Therefore, when calculating 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 8 × 10 ^-5 to 40 × 10 ^-5 [/K]. In this case, the absolute value of the main image plane shift at the time of temperature change and mode hopping was kept smaller than in Comparative Examples 1 and 2. However, when the coefficient of linear expansion is set to 30×10 ⁻⁵ [/K] or more as in Comparative Examples 3 to 5, the diffraction power of the diffraction surface becomes negative. It was determined that this is not desirable because the change in power has a non-cancelling relationship with the change in refraction power.

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

REFERENCE SIGNS LIST 1 semiconductor laser 2 coupling lens 5 polygon mirror 6 fθ lens 7 correction lens 9 photoreceptor drum 9A surface to be scanned 10 scanning optical device 20 housing 30 incident optical system 40 scanning optical system 50 optical deflector HL holder

Claims (7)

a semiconductor laser;
a coupling lens for converting 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 that holds the semiconductor laser and the coupling lens,
The coupling lens has a refractive surface with positive refractive power and a diffraction surface with positive diffraction power,
The ratio φn/φd between the refracting power φn of the refracting surface and the diffraction power φd of the diffractive surface in the main scanning direction is
1.85≦φn/φd<6.0
The filling,
A scanning optical device, wherein the linear expansion coefficient of the holder is 8.0×10 ⁻⁵ to 20×10 ⁻⁵ [/K]. further comprising a housing that holds the holder, the optical deflector, and the scanning optical system;
2. A scanning optical device according to claim 1, wherein the coefficient of linear expansion of said holder is larger than that of said housing. 3. A scanning optical device according to claim 1, wherein said holder is made of resin. the refracting surface has an axisymmetric shape,
4. A scanning optical apparatus according to claim 1, wherein said diffraction surface has an axially symmetrical shape. 1 to 164, wherein the longitudinal magnification in the main scanning direction of the entire optical system, including the coupling lens and the scanning optical system, from the semiconductor laser to the surface to be scanned is 70 to 164 times. 5. The scanning optical device according to any one of 4. 6. The scanning optical system according to any one of claims 1 to 5, wherein the scanning optical system is an fθ scanning optical system and has a focal length of 200 to 260 [mm] in the main scanning direction. scanning optics. 7. A scanning optical apparatus according to claim 1, wherein said scanning optical system comprises one or more lenses.

Images