JP2011154118A - Light scanning optical device, and image forming apparatus using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To achieve a reduction in cost, size and weight of a light scanning optical device. <P>SOLUTION: The light scanning optical device includes: a light source that emits laser light; a scanning optical means that deflects the laser light to scan a surface to be scanned with the laser light in a main-scanning direction; a scanning optical system that equalizes the scanning speed of the laser light; a linear light-condensing element that changes the laser light into linear light and forms an image by the linear light on a reflective surface. The linear light-condensing element includes a diffraction part having a diffraction effect, provided on one of the laser light-incident surface and the laser light exit surface thereof. The diffraction part is formed so that the power variation of the diffraction part caused by the ambient temperature fluctuation may correct the focal position variation caused by the ambient temperature fluctuation in a sub-scanning direction. The diffraction part has a blazed grating shape including first and second diffraction surfaces. The first diffraction surface is formed to be a planar shape, and the second diffraction surface is formed to be a curved-surface shape. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an optical scanning optical device and an image forming apparatus using the optical scanning optical device (such as a copying machine, a printer, a facsimile, and a composite machine using these using an electrophotographic system).

  Image forming apparatuses such as copiers, printers, facsimiles, and complex machines using electrophotography use a light that is modulated according to the data of the image to be formed to expose a uniformly charged photoreceptor. Is required. In a high-speed image forming apparatus, light composed of a rotating polyhedral mirror (polygon mirror) that deflects light from a light source such as a laser diode, and an fθ lens that scans the deflected light at a constant speed on a photoconductor. A scanning optical device is used.

  Currently, there is a demand for personalization of such an image forming apparatus, and there is a demand for reduction in price, size, weight, and high definition of an image to be formed. For this reason, the optical scanning optical apparatus is also required to be capable of forming a high-definition image regardless of environmental changes such as cost reduction, size reduction, weight reduction, and temperature.

  Among these, regarding reduction of cost and weight, for example, Patent Document 1 discloses a method of making an aspherical lens or a single lens. In addition, as shown in Patent Document 2, a plastic lens is generally used.

  Patent Document 3 discloses an optical scanning optical system having an incident optical unit that causes a light beam emitted from a light source unit to enter a deflecting unit, and a scanning optical unit that forms an image of the light beam reflected and deflected by the deflecting unit on a surface to be scanned. An apparatus and an image forming apparatus using the apparatus are disclosed. The scanning optical means of the optical scanning optical device of Patent Document 3 has a first lens on the deflecting means side and a second lens on the scanned surface side. The first lens has a positive power in the main scanning direction and a negative power in the sub-scanning direction. Further, the power in the main scanning direction of the first lens is larger than the power in the main scanning direction of the second lens. The second lens has positive power in the sub-scanning direction. The optical scanning optical device of Patent Document 3 and an image forming apparatus using the same are designed such that the length from the deflection unit to the first lens is short and the length from the second lens to the imaging surface is long. The optical scanning optical device and the image forming apparatus contribute to further miniaturization.

  However, when compared with a glass lens, the plastic lens has a problem that the degree of linear expansion and refractive index change with respect to temperature change is large, and drawing quality is likely to deteriorate due to focus shift. Further, in order to reduce the influence of the surface deflection error of the optical deflector (polygon mirror), the optical scanning optical device generally forms a light beam (light beam) once in the vicinity of the polygon mirror only in the sub-scanning direction. Therefore, the power (refractive power) in the sub-scanning direction is often configured as an anamorphic optical system that is stronger than the power in the main scanning direction. Therefore, focus shift due to temperature change is also a problem mainly in the sub-scanning direction.

  In order to incorporate such a response to environmental temperature fluctuations into the design, Patent Document 2 discloses a first optical system that forms an image of a light-modulated beam emitted from the light beam generating means as linear light that is long in the main scanning direction. An optical deflector having a deflecting surface in the vicinity of the image forming position of the first optical system, deflecting and scanning the incident light beam in the main scanning direction, and the scanned light beam deflected by the optical deflector An optical scanning optical device having a second optical system that forms an image thereon is disclosed. The first optical system of this optical scanning optical apparatus has a cylindrical lens made of a glass material having a positive power in the sub-scanning direction and a cylindrical lens made of a plastic material having a negative power in the sub-scanning direction. The second optical system has a plurality of lenses. At least one lens of the plurality of lenses is formed of a lens made of a plastic material, and a plastic lens includes a cylindrical lens made of a negative power plastic material of the first optical system and a plastic lens of the second optical system. Cancels temperature fluctuations.

  Similarly, in the technique disclosed in Patent Document 3, the first lens of the scanning optical means has a positive power in the main scanning direction and a negative power in the sub-scanning direction. Further, the power of the first lens in the main scanning direction is made larger than the power of the second lens in the main scanning direction. By making the second lens have a positive power in the sub-scanning direction, the magnification in the sub-scanning direction is reduced to reduce the sensitivity to changes in the focal position in the sub-scanning direction.

  Further, as a countermeasure against environmental temperature fluctuations, Patent Document 4 discloses a technique in which light from a laser diode is transmitted through a condensing lens having a diffraction effect to form an image on a rotating polyhedral mirror (polygon mirror) for temperature compensation. Is disclosed. Patent Document 5 discloses an optical scanning optical device in which a light beam deflected by a deflecting element (polygon mirror) is imaged on a surface to be scanned by a scanning optical element having a refracting portion and a diffracting portion. ing. In Patent Document 6, a light beam from a light source is imaged on a deflecting element by an anamorphic optical element having a diffractive part, and then the deflected light beam is converted into a scanning optical element having a refracting part using this deflecting element. An optical scanning optical device that scans on a surface to be scanned is disclosed, and this optical scanning optical device detects a focus change in the main scanning direction due to a temperature change, and a power change of a diffraction unit due to a wavelength change of a light source due to a temperature change. Use to correct.

Japanese Patent No. 3567408 (Japanese Patent Laid-Open No. 10-148755) JP 2000-206432 A JP 2002-48993 A Japanese Patent No. 3317355 (Japanese Patent Laid-Open No. 4-328951) Japanese Patent No. 3432085 (Japanese Patent Laid-Open No. 10-68903) JP 2003-337295 A

  However, as described above, the plastic lens has a greater degree of linear expansion and refractive index change with respect to temperature change than the glass lens, and is liable to cause deterioration in drawing quality due to focus shift. Further, a first lens having a positive power in the main scanning direction and a negative power in the sub-scanning direction and a second lens having a positive power in the sub-scanning direction are used, and the first lens is brought close to the deflecting unit, If the length from the second lens to the imaging surface is increased, the optical scanning optical device can be made compact, but conversely, a slight change in the refractive index of the lens constituting the optical scanning optical device is affected by temperature fluctuations. There is a problem that the focal position changes greatly on the scanning plane.

  For this reason, in the technique disclosed in Patent Document 3, the first lens has a negative power in the sub-scanning direction, and the second lens has a positive power in the sub-scanning direction. The magnification in the scanning direction is reduced to reduce the sensitivity to changes in the focal position in the sub-scanning direction. However, in the current situation where high performance is required, there may be a case where the effect of reducing the sensitivity to changes in the focal position is insufficient even if the disclosed technique of Patent Document 3 is used. Further, in Patent Document 2, a glass lens is used for the optical set, and a concave cylinder lens is added only for reducing the focus variation in the sub-scanning direction, resulting in high costs. Furthermore, in the techniques disclosed in Patent Documents 2, 3, 5 and 6, the cylindrical lens used for adjusting the power distribution in the main scanning direction and the sub-scanning direction is after the light from the light source is converted into parallel light. Therefore, it is conceivable that the light amount control of the light source is adversely affected by the influence of the reflected light from the cylindrical lens entrance surface.

  Further, in the disclosed technologies of Patent Document 4, Patent Document 5, and Patent Document 6, a diffraction grating is introduced as temperature compensation, and the focus change in the main scanning direction is corrected by the power change of the diffraction unit due to the wavelength fluctuation of the light source due to the temperature fluctuation. However, no consideration is given to the sub-scanning direction. In addition, Patent Document 4, Patent Document 5, and Patent Document 6 do not mention any problems related to aberrations caused by the shape of the diffraction grating.

  An object of the present invention is to reduce the price, size, and weight of an image forming apparatus including an optical scanning optical device.

  An optical scanning optical apparatus according to an embodiment of the present invention includes a light source that emits laser light, scanning optical means that deflects the laser light from the light source and scans the surface to be scanned in the main scanning direction, and the scanning A scanning optical system that makes the scanning speed of the laser light deflected by the optical means constant on the surface to be scanned, and the laser light from the light source is made linear light that is long in the main scanning direction, and the scanning optical means A linear condensing element that forms an image on a reflecting surface, and the linear condensing element is one of a surface on which laser light from the light source is incident or a surface on which laser light from the light source is emitted. A diffractive portion having a diffractive effect, so that the power change of the diffractive portion due to ambient temperature fluctuations corrects the focal position change due to ambient temperature fluctuations in the sub-scanning direction orthogonal to the main scanning direction. The diffraction part is formed and the circuit The portion has a blazed grating shape including a first diffractive surface and a second diffractive surface, the first diffractive surface is formed in a planar shape, and the second diffractive surface is formed in a curved surface shape. (Claim 1).

  According to the above configuration, the scanning optical means deflects the laser light from the light source and scans the surface to be scanned in the main scanning direction. The scanning optical system makes the scanning speed of the deflected laser light on the surface to be scanned uniform. The laser light is converted into linear light that is long in the main scanning direction by the linear condensing element. A linear condensing element is provided with the diffraction part which has a diffraction effect in one surface among the surfaces which a laser beam injects or radiate | emits. The power change of the diffractive portion caused by the ambient temperature fluctuation corrects the focal position change caused by the ambient temperature fluctuation in the sub-scanning direction orthogonal to the main scanning direction. Therefore, it is possible to prevent the focal position change in the sub-scanning direction. Thus, the optical system can be configured without using a glass lens, and the manufacturing cost can be reduced. The diffractive portion includes a first diffractive surface and a second diffractive surface. Since the first diffractive surface is formed in a planar shape, it can be formed at a relatively low cost. Since the second diffractive surface is formed in a curved surface shape, it is possible to prevent a decrease in image quality due to an increase in aberration.

  In the above configuration, the second diffractive surface preferably forms a kinoform.

  According to the above configuration, since the second diffractive surface forms a kinoform, it is possible to suitably prevent a decrease in image quality due to an increase in aberration.

  In the above configuration, it is preferable that the second diffractive surface is formed at the center of the diffractive portion.

  According to the above configuration, since the second diffractive surface is formed at the center of the diffractive portion, it is possible to more suitably prevent a decrease in image quality due to an increase in aberration.

  In the above configuration, it is preferable that the second diffractive surface is formed within a distance range of 0.5 mm or less from the center line of the diffractive portion.

  According to the above configuration, since the second diffractive surface is formed within a distance range of 0.5 mm or less from the center line of the diffractive portion, most of the diffractive surfaces of the diffractive portion are formed from the planar first diffractive surface. The diffraction part can be formed at a relatively low cost.

  In the above configuration, the linear condensing element includes a refracting unit that refracts laser light from the light source on a surface opposite to the diffractive unit, and the diffractive unit and the refracting unit that are caused by ambient temperature fluctuations. It is preferable that the diffractive part and the refracting part are formed so that the power change corrects the focal position change caused by the ambient temperature fluctuation in the sub-scanning direction.

  According to the above configuration, since the change in the focal position caused by the ambient temperature fluctuation is corrected by using the power change in the refracting portion, the degree of freedom in designing the diffraction grating can be improved.

  The said structure WHEREIN: While the said refractive part is formed in the surface into which the laser beam from the said light source injects, it is preferable to make concave surface shape (Claim 6).

  According to the above configuration, it is possible to prevent the reflected light from the refraction part interface from reaching the light source.

  In the above configuration, the scanning optical system includes a first lens disposed on the scanning optical means side and a second lens disposed on the scanned surface side, and the first lens is aligned in the main scanning direction. The second lens has a positive power in the sub-scanning direction, and the power of the first lens in the main scanning direction is the first power in the main scanning direction. It is preferable that one of the first and second lenses has an aspherical shape that is greater than the power of the second lens in the main scanning direction.

  According to the above configuration, the distance between the scanning optical means and the first lens can be shortened, and the distance between the surface to be scanned and the second lens can be lengthened, and a small scanning optical system can be used. Therefore, the optical scanning optical device can be configured to be small and light.

  The said structure WHEREIN: It is preferable that the said linear condensing element is formed with an olefin resin material (Claim 8).

  According to the said structure, the fluctuation | variation of a refractive index can be minimized by using the olefin resin material which has little moisture absorption in a high humidity place, and the refractive index change by moisture absorption does not occur.

  An image forming apparatus according to another embodiment of the present invention includes an optical scanning optical device having the above-described features.

  According to the above configuration, an image forming apparatus having various advantages shown with respect to the optical scanning optical device is provided.

  As described above, the optical scanning optical apparatus and the image forming apparatus according to the present invention can suitably prevent the focal position change in the sub-scanning direction due to the fluctuation of the environmental temperature.

1 is a schematic perspective view of a configuration of an image forming apparatus in which an optical scanning optical apparatus according to an embodiment of the present invention is incorporated as an exposure apparatus. 1 is a perspective view of a schematic configuration in which an optical scanning optical device according to an embodiment of the present invention is applied to a photoconductor exposure apparatus in an image forming apparatus. FIG. 3 is a cross-sectional view in the sub-scanning direction of the optical scanning optical device shown in FIG. 2. 4A is a perspective view of the cylindrical lens 4 used in the optical scanning optical apparatus according to the embodiment of the present invention as seen from the diffraction grating side, and FIG. 4B is a perspective view as seen from the cylindrical surface side. 4C is a cross-sectional view of the cylindrical lens, and an elliptical portion in FIG. 4C shows an enlarged portion of a Fresnel diffraction grating having no power in the main scanning direction. . It is a figure explaining the effect of the concave refracting part formed in the entrance plane of a cylindrical lens. It is a graph explaining the effect of the concave refracting part formed in the entrance plane of a cylindrical lens. It is a figure explaining the further improvement form of the diffraction part of a cylindrical lens. FIG. 4 is a graph comparing the light intensity distribution when using a cylindrical lens with a diffractive part (blazed grating shape) shown in FIG. 4 and the light intensity distribution when using a cylindrical lens with a diffractive part (kinoform) shown in FIG. It is. It is a figure explaining the further improvement form of the diffraction part of a cylindrical lens. The light intensity distribution when forming a protrusion when a curved diffractive part is formed in the central region of the diffractive part is shown. The light intensity distribution when a protrusion is formed when a planar diffractive part is formed in the central region of the diffractive part is shown.

  Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the drawings. However, the dimensions, materials, shapes, relative arrangements, and the like of the components described in this embodiment are not intended to limit the scope of the present invention unless otherwise specified, and are merely illustrative. Absent.

  First, the configuration of an optical scanning optical device according to an embodiment of the present invention will be briefly described. An optical scanning optical apparatus according to an embodiment of the present invention includes a light source including a semiconductor laser, a cylindrical lens, scanning optical means, and a scanning optical system. The cylindrical lens converts the light from the light source into linear light that is long in the main scanning direction, and forms an image on the reflecting surface of the scanning optical means. The scanning optical means deflects the light linearized by the cylindrical lens and scans in the main scanning direction on the surface to be scanned (such as a uniformly charged photoconductor in the image forming apparatus). An example of the scanning optical means is a polygon mirror. The scanning optical system makes the scanning speed of the deflected light on the surface to be scanned uniform. An fθ lens can be exemplified as the scanning optical system.

  Next, the cylindrical lens and the scanning optical system will be described in detail. The cylindrical lens includes a diffractive portion on one surface where light enters or exits, and includes a cylindrical surface (refractive portion) that is curved in an arc shape and forms a concave surface on the other surface. The diffractive part exhibits a diffractive effect having no power in the main scanning direction.

  A scanning optical system including an fθ lens and the like includes a first lens disposed on the scanning optical means (polygon mirror) side and a second lens disposed on the scanned surface side. The first lens has positive power in the main scanning direction and negative power in the sub-scanning direction orthogonal to the main scanning direction. The second lens has positive power in the sub-scanning direction. Note that the power of the first lens in the main scanning direction is larger than the power of the second lens in the main scanning direction.

  Either one of the first lens and the second lens has an aspherical shape. Thus, the first lens and the second lens are installed so that the distance between the first lens and the scanning optical means (polygon mirror) is long and the distance between the scanned surface and the second lens is short. Can do. By adopting such a configuration, a small scanning optical means can be used, and the optical scanning optical device can be reduced in size and weight.

  Temperature fluctuations in the optical scanning optical device cause wavelength fluctuations in the semiconductor laser, and cause power changes in the refractive and diffractive parts of the cylindrical lens. Therefore, the optical scanning optical device described above can correct the focal position change in the sub-scanning direction of the scanning optical system caused by the temperature variation by using the power change of the refractive part and the diffraction part of the cylindrical lens. Therefore, this embodiment has a very simple configuration, does not cause a change in the focal position in the sub-scanning direction even when the environmental temperature fluctuates, and is an optical scanning optical device and image formation that are small and light. An apparatus can be provided.

  FIG. 1 is a diagram schematically showing the internal structure of an image forming apparatus incorporating the above-described optical scanning optical device. The image forming apparatus shown in FIG. 1 is a printer, but the principle of this embodiment can also be applied to other image forming apparatuses such as a facsimile machine and a copier. In FIG. 1, the Y-Y direction is referred to as the front-rear direction, in particular, the -Y direction is referred to as the front, and the + Y direction is referred to as the rear. The above-described optical scanning optical device is incorporated as an exposure device 23 in the image forming apparatus shown in FIG.

  The printer 100 includes a device main body 11. A sheet feeding device 30 is disposed at the lower part of the apparatus main body 11. The sheet feeding device 30 feeds the sheets P1 one by one from a bundle of stacked sheets P1 (recording medium) (hereinafter referred to as a sheet bundle P). A partition plate 115 is disposed above the sheet feeding device 30. Above the partition plate 115, the image forming unit 20 that transfers the toner image onto the paper P1 supplied from the paper feeding device 30, and the toner image on the paper P1 transferred by the image forming unit 20 is fixed to the paper P1. A fixing unit 27 is provided. The sheet P1 on which the toner image is fixed by the fixing unit 27 is discharged to a discharge tray 117 that forms a part of the upper surface of the apparatus main body 11.

  The sheet feeding device 30 stacks the sheet bundle P, and the sheet cassette 39 that is detachably attached to the apparatus main body 11 and the sheet bundle P one by one at a corresponding position on the front side of the sheet cassette 39. A large-diameter paper feed roller 311 for feeding the uppermost paper P1 is provided. A transport roller 312 having a small diameter is provided immediately above the paper feed roller 311.

  The paper P1 fed out of the paper cassette 39 by driving the paper feed roller 311 passes through a transport roller 312 and passes through a pair of registration rollers 314 provided at the downstream end of the paper feed transport path 313 and the paper feed transport path 313 in the paper transport direction. The paper is sequentially fed to the image forming unit 20. The front wall 38 of the paper cassette 39 serves as a guide surface for guiding the paper P1 from the paper feed roller 311 to the image forming unit 20.

  The image forming unit 20 transfers the toner image onto the paper P1 based on image information transmitted from a computer or the like. The image forming unit 20 includes a photosensitive drum (image carrier) 21. The photosensitive drum 21 includes a rotation center axis extending in the left-right direction (a direction orthogonal to the paper surface of FIG. 1). A charger 22, an exposure device 23, a developing device 24, a transfer roller 25, and a cleaning device 26 are disposed in a clockwise direction along the peripheral surface of the photosensitive drum 21 from a position immediately above the photosensitive drum 21.

  An electrostatic latent image corresponding to image data transmitted from an external device such as a computer and a toner image along the electrostatic latent image are formed on the peripheral surface of the photosensitive drum 21. The charger 22 forms a uniform charge on the peripheral surface of the photosensitive drum 21 rotating clockwise. In the present embodiment, a corona discharge type that applies a charge to the peripheral surface of the photosensitive drum 21 by corona discharge from a wire is employed. Instead of this, a charging roller type charger 22 may be adopted. When the charging roller type charger 22 is used, the charging roller comes into contact with the peripheral surface of the photosensitive drum 21 and charges the peripheral surface of the photosensitive drum 21 while being driven to rotate.

  The exposure device 23 irradiates the peripheral surface of the rotating photosensitive drum 21 with laser light to which intensity is applied based on image data transmitted from an external device such as a computer. The electrostatic latent image is formed on the peripheral surface of the photosensitive drum 21 by erasing the charge of the portion irradiated with the laser light.

  The developing device 24 supplies toner from the toner container 241 that stores toner to the circumferential surface of the photosensitive drum 21, and attaches the toner to a portion of the circumferential surface of the photosensitive drum 21 where the electrostatic latent image is formed. As a result, a toner image is formed on the peripheral surface of the photosensitive drum 21.

  The transfer roller 25 transfers a positively charged toner image formed on the peripheral surface of the photosensitive drum 21 to the paper P1 with respect to the paper P1 sent to a position immediately below the photosensitive drum 21. The transfer roller 25 applies a negative charge having a polarity opposite to that of the toner image to the sheet P1.

  The paper P1 that has reached the position directly below the photosensitive drum 21 is pressed and sandwiched between the transfer roller 25 and the photosensitive drum 21, and a transfer process is performed on the paper P1. This transfer process is a process of peeling off the toner image on the circumferential surface of the photosensitive drum 21 that is positively charged toward the surface of the negatively charged sheet P1.

  The cleaning device 26 removes the toner remaining on the peripheral surface of the photosensitive drum 21 after the transfer process to the paper P1 and cleans it. The peripheral surface of the photoconductive drum 21 cleaned by the cleaning device 26 goes to the charger 22 again for the next image forming process.

  The fixing unit 27 fixes the toner image on the paper P1 transferred by the image forming unit 20 to the paper P1. The fixing unit 27 includes a heating source, a heating assembly, and a pressure assembly. The heating assembly includes a heating roller 271 for fixing the toner image by heat. The heating source is accommodated in a heating roller 271 formed in a substantially cylindrical shape. In this embodiment, a halogen heater 273 is used as a heating source. The heating assembly includes a pressure roller 272 and a suitable biasing mechanism for pressing the pressure roller 272 against the heat roller 271. The heating roller 271 rotates clockwise around the rotation center axis, the pressure roller 272 rotates following the heating roller 272, and rotates counterclockwise around the rotation center axis. A nip portion is formed between the heating roller 271 and the pressure roller 272. The sheet P1 after the transfer process passes through the nip portion. The toner image on the paper P <b> 1 is fixed by the heat from the heating roller 271 and the pressure from the pressure roller 272. The sheet P1 on which the toner image is fixed is discharged to the discharge tray 117 through the discharge conveyance path 315.

  2 is a schematic perspective view of the configuration of the optical scanning optical apparatus incorporated as the exposure apparatus 23 in FIG. 1, and FIG. 3 is a cross-sectional view in the sub-scanning direction of the optical scanning optical apparatus shown in FIG. In the following description, the same number is attached | subjected to the same component.

  The optical scanning optical device according to the present embodiment includes, for example, a light source 1 made of a 780 nm band semiconductor laser, a glass aspheric collimating lens 2 used as a coupling lens that makes light from the light source 1 parallel light, and the parallel light. An aperture stop (aperture) 3 having an aperture 31 having a predetermined size and parallel light from the coupling lens are converted into linear light that is long in the main scanning direction 10 and is connected to the reflecting surface 5a of the rotary reflecting mirror (polygon mirror) 5. A linear condensing element such as a cylindrical lens 4 to be imaged, a rotating reflecting mirror (polygon mirror) 5 that scans the light imaged by the linear condensing element on the surface to be scanned, and a rotating reflecting mirror (polygon mirror) 5 is provided with scanning optical systems 6 and 7 such as an fθ lens for making the scanning speed of the light deflected by 5 constant on the surface to be scanned. In the exposure apparatus shown in FIG. 2, the reflecting mirror 8 reflects the light that has passed through the scanning optical systems 6 and 7 to the surface 9 to be scanned, but the present invention is not limited to this. A configuration without the reflecting mirror 8 can also be employed. In the exposure apparatus shown in FIG. 2, the cylindrical lens 4 includes a cylindrical surface (concave surface: a refracting portion) curved in an arc shape on an incident surface on which light from the light source 1 is incident, and has a diffraction surface on an output surface. It is also possible to form a diffractive surface on the incident surface side and a cylindrical surface (concave surface: refracting portion) on the output surface side. Examples of the material of the cylindrical lens 4 include olefin-based resins such as ZEONEXE48R (registered trademark of Nippon Zeon Co., Ltd.).

  A diffractive surface provided in a linear condensing element such as the cylindrical lens 4 is a Fresnel diffraction grating and has a shape having no power in the main scanning direction 10. Thereby, it is possible to prevent the change in the diffraction effect due to the temperature fluctuation from affecting the main scanning direction.

  The scanning optical system such as the fθ lenses 6 and 7 has a positive power in the main scanning direction, a first lens 6 having a negative power in the sub scanning direction orthogonal to the main scanning direction, and a positive power in the sub scanning direction. And a second lens 7 having power. In the optical scanning optical apparatus shown in FIG. 2, the first lens 6 has an aspherical shape and the second lens 7 has a spherical shape. However, the first lens 6 has a spherical shape and the second lens 7 has an aspherical shape. It is also possible to have a shape.

  In the present embodiment, the focal position change in the sub-scanning direction accompanying the temperature fluctuation in the scanning optical system such as the fθ lenses 6 and 7 is caused by the wavelength fluctuation accompanying the temperature fluctuation of the light source 1 such as a semiconductor laser. And correction using the power change of the diffraction part.

  By configuring the optical scanning optical device in this way, it is possible to prevent a change in the focal position in the sub-scanning direction due to a change in the environmental temperature with a very simple configuration. Further, the distribution between the positive and negative powers of the fθ lenses 6 and 7 in the main scanning direction shortens the distance between the first lens 6 and the polygon mirror 5, and the surface to be scanned (photosensitive member) 9 and the second lens 7. The optical scanning optical device can be made small and light. In addition, an image forming apparatus using this optical scanning optical device can be reduced in size and weight, and in addition, a high-definition image can be formed.

  4A is a perspective view of the cylindrical lens 4 used in the optical scanning optical apparatus according to the present embodiment as seen from the diffraction grating side, and FIG. 4B is a perspective view of the cylindrical lens 4 as seen from the cylindrical surface (refractive portion) side. FIG. 4C is a cross-sectional view of the cylindrical lens in the sub-scanning direction. In the portion surrounded by an ellipse in FIG. 4C, a portion of a Fresnel diffraction grating having no power in the main scanning direction 10 is shown enlarged.

  The cylindrical lens 4 illustrated in FIG. 4 is formed in a substantially rectangular parallelepiped shape, includes a cylindrical surface (refractive portion) 42 having a negative power at the center, and has a Fresnel surface on the opposite side of the cylindrical surface (refractive portion) 42. A diffraction grating 41 is provided. The cylindrical surface (refractive portion) 42 is a surface having a concave shape curved in an arc shape in the sub-scanning direction. Although not shown in FIG. 4, the cylindrical lens of FIG. 4 is used in combination with a cylindrical lens having a positive power.

  The Fresnel diffraction grating 41 of the cylindrical lens 4 is provided substantially parallel to the main scanning direction 10 (perpendicular to the sub-scanning direction) as is apparent in the figure, and has no power in the main scanning direction 10. It is a diffraction grating. As clearly shown in FIG. 4 (c), the diffraction grating 41 is formed by connecting protrusions 411 having a substantially right-angled triangular cross section extending in the main scanning direction 10 in the sub-scanning direction. It has a blade shape (generally called a blazed lattice shape). Further, the protrusions 411 are symmetrical with respect to the center of the diffraction grating 41. The slope of each protrusion becomes the diffraction surface 412.

  The cylindrical lens 4 is formed of an olefin resin material as described above. Since the olefin-based resin material has little moisture absorption even in a place with high humidity, the refractive index change of the cylindrical lens 4 due to moisture absorption can be minimized. In the above description, the case where the diffraction grating is formed on the cylindrical lens 4 has been described as an example. However, it is needless to say that the diffraction grating member may be provided independently of the cylindrical lens 4.

  Next, the correction for the focal position change accompanying the temperature fluctuation in the optical scanning optical apparatus will be described. In the above description, the cylindrical lens 4 used as the linear condensing element is formed of an olefin resin such as ZEONEEXE 48R, and a system in which a diffractive surface is formed on one surface of the cylindrical lens 4 is used. Consider a system using a cylindrical lens without a diffractive surface. In a system using a cylindrical lens having no diffractive surface, the sensitivity to temperature fluctuations in the entire system is, for example, 0.2 mm / 1 ° C.

  Here, consider a case where the temperature of the optical scanning optical apparatus is increased by, for example, dt. As the temperature rises, the refractive index n of the scanning optical element changes by dn / dt, and the accompanying focal length change dφI can be expressed as a function of the refractive index of the following equation (1). Note that L (n) is a coefficient for converting the refractive index change rate with respect to the temperature rise to the focal length change, and is a function of the refractive index n.

dφI = L (n) dn / dt (1)
On the other hand, the oscillation wavelength λ of the light source 1 such as a semiconductor laser also changes by dλ / dt due to the temperature rise dt. Refractive and diffractive focal length changes dφL and dφD accompanying the temperature rise dt can be expressed as a function of wavelength by using the following equations (2) and (3), respectively. Here, the coefficients Lw (λ) and Dw (λ) for converting the rate of change of the oscillation wavelength λ with respect to the temperature rise dt into the focal length change are functions of the wavelength.

dφL = Lw (λ) dλ / dt (2)
dφD = Dw (λ) dλ / dt (3)
Here, a system in which the diffraction grating 41 is formed is considered. The diffraction grating 41 is designed to satisfy the following expression (4). By using the diffraction grating 41 that satisfies the following equation (4), the magnification change and focus change in the sub-scanning direction due to environmental fluctuations (temperature rise dt) are offset by the focal length change due to refraction and diffraction of the diffraction grating 41.

dφI + dφL + dφD ≈ 0 (4)
That is, the scanning optics required to form the diffraction grating 41 so as to have dφL and dφD satisfying the expression (4) and correct the wavelength change of the light source 1 such as a semiconductor laser corresponding to the design of the diffraction grating. The power distribution of refraction and diffraction of the system can be determined.

  Although it is possible to improve the focus shift only by the power of the diffraction section, in this case, there is a possibility that the distance between the diffraction gratings becomes a size that cannot be manufactured (depending on the manufacturer). In order to avoid this, it is easier to correct the focus shift by using the power of the refracting portion. In designing a diffraction grating, the refractive index can be treated as a function of wavelength by using a Cauchy dispersion formula represented by the following formula (5). In the following formula (5), n is a refractive index, λ is a wavelength, and A, B, C,... Are constants. As a result, the design of the diffraction grating can be simplified.

n = A + (B / λ2) + (C / λ4) + (5)
As described above, when the focal variation amount due to the temperature of the optical scanning optical device is 0.2 mm / 1 ° C., the temperature change with respect to the combined focal length f of the optical system is represented by the following equation (6). Can be seen as a function.

f (T) = f + 0.2T (6)
For this reason, the focus fluctuation dφI due to refraction accompanying the temperature fluctuation expressed by the above equation (1) is almost the same value as f (T) in the above equation (6). It can be canceled by setting dφL + dφD to −1 / f (T).

  In this way, by adding a diffraction grating to the cylindrical lens 4 which is a linear condensing element, the characteristics of the optical elements having different powers in the main scanning direction and the sub-scanning direction, which are the characteristics of the anamorphic optical system, are not lost. Thus, an optical scanning optical device that does not cause a change in focus due to temperature fluctuation can be obtained.

  FIG. 5 is a schematic diagram showing the effect of the refracting portion 42 of the cylindrical lens 4. FIG. 5A is a schematic diagram of an optical system from the light source 1 to the cylindrical lens 4 when the refraction part 42 is formed, and FIG. 5B is a view from the light source 1 when the refraction part 42 is not formed. 2 is a schematic diagram of an optical system up to a cylindrical lens 4. FIG.

  In the optical system shown in FIG. 5A, the laser light emitted from the light source 1 is made into substantially parallel light through the collimating lens 2 and then reaches the refractive part 42 of the cylindrical lens 4 through the aperture stop 3. . Most of the parallel light that reaches the refracting section 42 travels to the downstream optical system through the diffraction section 41, but a part of the parallel light becomes reflected light RF at the interface of the cylindrical lens 4. The light reflected through the refracting portion 42 having a concave surface curved in an arc shape in the sub-scanning direction forms a focal point F before reaching the light source 1, and the reflected light RF reaching the light source 1 is small.

  On the other hand, when the cylindrical lens 4 having a flat incident surface not provided with the refracting portion 42 as shown in FIG. 5B is used, a part of the parallel light passing through the collimating lens 2 does not form a focal point. , Reflected as parallel light and easily reach the light source 1.

  In general, the light amount of light from the light source 1 is monitored using a photodiode 50 disposed on the opposite side of the light source 1 from the light emission side, and feedback control is performed based on the monitored light amount, whereby the light source 1 However, when the reflected light RF from the cylindrical lens 4 reaches the light source 1 as shown in FIG. 5B, the feedback control system for the light source 1 is adversely affected.

  FIG. 6 is a graph in which two cylindrical lenses 4 having different concave curvatures of the refracting portion 42 are prepared and the influence of the refracting portion 42 on the feedback control system is compared. FIG. 6A is a graph showing the laser power fluctuation of the light source 1 when the refracting portion 42 is formed so that 2% of the laser light emitted from the light source 1 returns to the light source 1 as reflected light. FIG. 6B is a graph showing the laser power fluctuation of the light source 1 when the refracting portion 42 is formed so that 0.9% of the laser light emitted from the light source 1 returns to the light source 1 as reflected light. is there. In both graphs shown in FIGS. 6A and 6B, the horizontal axis represents time (seconds), and the vertical axis represents laser power fluctuation (%).

  As is apparent from the graph shown in FIG. 6, it can be seen that the condition of 0.9% reflected light is almost constant with little variation in laser power compared to the condition of 2% reflected light. As a result of verification using other reflected light conditions other than the conditions shown in FIG. 6, the laser power fluctuation may be substantially constant under the condition that less than 1% of the laser light emitted from the light source 1 is reflected light. It turns out.

  When an optical scanning optical device having laser power fluctuation characteristics as shown in FIG. 6A is used in an image forming apparatus, the laser power fluctuation results in image spots created by the image forming apparatus. On the other hand, when an optical scanning optical device having the characteristic that the laser power is constant as shown in FIG. 6B is mounted on the image forming apparatus, it is possible to create a spotless image. Therefore, it is preferable to determine the concave curvature of the refraction part 42 so that less than 1% of the laser light emitted from the light source 1 becomes reflected light.

  FIG. 7 is a view showing a further improved form of the diffractive part 41, and shows a cross-sectional form of the improved protrusion 411 constituting the diffractive part 41.

  Although the diffractive surface 412 of the ridge 411 of the diffractive portion 41 shown in FIG. 4 has a linear shape, each of the diffractive surfaces 412 of the ridge 411 of the diffractive portion 41 shown in FIG. The curved surface shape (generally referred to as kinoform) is formed.

  FIG. 8 shows the light intensity distribution (FIG. 8A) when the cylindrical lens 4 having the flat diffractive surface 412 having the diffractive portion 41 having the blazed grating shape shown in FIG. 4 and the kinoform shape shown in FIG. The light intensity distribution (FIG. 8B) when the cylindrical lens 4 having the diffractive portion 41 is used is shown.

  As shown in FIG. 8A, when the blazed grating-shaped cylindrical lens 4 is used, the light intensity distribution in the main scanning direction is substantially normal, but the light intensity distribution in the sub-scanning direction is as shown in FIG. It shows that a phenomenon similar to the side lobe caused by the aberration occurs in the portion surrounded by the middle dotted line portion. The phenomenon similar to the side lobe causes degradation of image quality when the optical scanning optical device is used in an image forming apparatus.

  On the other hand, as shown in FIG. 8B, when the cylindrical lens 4 having the kinoform-shaped diffraction portion 41 is used, the light intensity distribution has a substantially normal distribution in both the main scanning direction and the sub-scanning direction. I understand that. Therefore, it can be seen that the image quality can be further improved by using the cylindrical lens 4 having the kinoform-shaped diffraction portion 41.

  FIG. 9 shows a further improved diffraction part 41. The diffractive portion 41 shown in FIG. 9 has a blazed grating shape substantially the same as that of the diffractive portion 41 shown in FIG. 7, but the shape of the diffractive surface 412 of the protrusion 411 is different. In FIG. 9, a center line CL is shown at the center of the diffraction part 41. A ridge 411 having a planar first diffractive surface 412a is formed in the left and right regions of the diffractive portion 41, and a second diffractive surface 412b having a curved surface shape (preferably a kinoform shape) is formed in the central region of the diffractive portion 41. The protruding ridge 411 is formed. In the diffractive portion 41 shown in FIG. 9, only the central ridge 411 has the second diffractive surface 412 b and the other ridge 411 has the first diffractive surface 412 a, but the present invention is limited to this. Instead, a protrusion 411 having a second diffractive surface 412b is formed within a predetermined distance range (indicated by ± X in FIG. 9) from the center of the diffractive portion 41, and the first ridge is outside the distance range. A protrusion 411 having a diffractive surface 412a may be formed.

  FIG. 10 shows a light intensity distribution when the diffractive portion 41 shown in FIG. 9 is used. In the diffractive portion 41 used for the light intensity distribution, the protrusion 411 having the distance X from the center line CL shown in FIG. 9 in the range of 0.5 mm has the second diffractive surface 412b.

  FIG. 11 shows a light intensity distribution when using the diffractive portion 41 in which the protrusion 411 having the first diffractive surface 412a is formed in the range of the predetermined distance X from the center line CL shown in FIG. FIG. 11A shows the light intensity distribution when using the diffractive portion 41 in which the protrusion 411 having the first diffractive surface 412a is formed in the range of X = 0.5 mm, and FIG. , X = 0.7 mm shows the light intensity distribution when using the diffractive portion 41 in which the protrusion 411 having the first diffractive surface 412a is formed, and FIG. 11C shows X = 1.0 mm. The light intensity distribution when the diffraction part 41 in which the protrusion 411 having the first diffractive surface 412a is formed is used.

  As shown in FIG. 10, the light intensity distribution in which the protrusion 411 on which the second diffractive surface 412b is formed is formed in the central region draws a smooth normal distribution curve. Therefore, by forming the protrusion 411 on which the second diffractive surface 412b is formed in the central region of the diffractive portion 41 (preferably at least within a range of ± 0.5 mm from the center of the diffractive portion 41), The image quality can be improved.

  On the other hand, as shown in FIG. 11, the light intensity distribution in which the protrusion 411 having the first diffractive surface 412a is formed in the central region is not smooth compared to the curve shown in FIG. A phenomenon similar to the side lobe that occurs. The phenomenon similar to the side lobe causes degradation of image quality when the optical scanning optical device is used in an image forming apparatus.

  From the comparison of the light intensity distribution data shown in FIG. 10 and FIG. 11, the formation of the second diffractive surface 412b in the region of the predetermined range in the central portion of the diffractive portion 41 can greatly affect the improvement of the image quality. I understand. Therefore, since the planar first diffractive surface 411a can be formed on many protrusions 41 excluding the central region of the diffractive part 41, the manufacturing cost of the diffractive part 41 can be reduced. It becomes possible.

  This embodiment is preferably applied to an image forming apparatus.

1. Light source (semiconductor laser)
2. Collimating lens 3. Aperture (aperture stop)
31... Aperture 4... Cylindrical lens 41... Fresnel diffraction grating 411 a... First diffraction surface 411 b. Negative power cylindrical surface 5... Polygon mirror 6, 7... Fθ lens 8.
10... Scanning direction 100... Image forming apparatus

Claims (9)

A light source that emits laser light;
Scanning optical means for deflecting the laser light from the light source and scanning the surface to be scanned in the main scanning direction;
A scanning optical system for making the scanning speed of the laser beam deflected by the scanning optical means constant on the surface to be scanned;
A linear condensing element that forms laser light from the light source into linear light that is long in the main scanning direction and forms an image on the reflection surface of the scanning optical means,
The linear condensing element includes a diffractive portion having a diffraction effect on one of a surface on which laser light from the light source is incident or a surface on which laser light from the light source is emitted,
The diffractive part is formed so that the power change of the diffractive part due to the ambient temperature fluctuation corrects the focal position change caused by the ambient temperature fluctuation in the sub-scanning direction orthogonal to the main scanning direction,
The diffractive portion has a blazed grating shape including a first diffractive surface and a second diffractive surface,
The first diffractive surface is formed in a planar shape,
The optical scanning optical device, wherein the second diffractive surface is formed in a curved surface shape.   The optical scanning optical apparatus according to claim 1, wherein the second diffractive surface forms a kinoform.   3. The optical scanning optical device according to claim 1, wherein the second diffractive surface is formed at a center of the diffractive portion.   4. The optical scanning optical device according to claim 3, wherein the second diffractive surface is formed within a distance range of 0.5 mm or less from a center line of the diffractive portion. The linear condensing element includes a refracting unit that refracts laser light from the light source on a surface opposite to the diffraction unit,
The diffractive part and the refracting part are formed so that a power change of the diffractive part and the refracting part due to an ambient temperature fluctuation corrects a focal position change caused by the ambient temperature fluctuation in the sub-scanning direction. The optical scanning optical apparatus according to claim 1.   6. The optical scanning optical device according to claim 5, wherein the refracting portion is formed on a surface on which laser light from the light source is incident and has a concave shape. The scanning optical system includes a first lens disposed on the scanning optical means side and a second lens disposed on the scanned surface side,
The first lens has a positive power in the main scanning direction and a negative power in the sub-scanning direction;
The second lens has a positive power in the sub-scanning direction;
The power in the main scanning direction of the first lens is greater than the power in the main scanning direction of the second lens,
7. The optical scanning optical device according to claim 1, wherein one of the first and second lenses has an aspherical shape.   The optical scanning optical device according to claim 1, wherein the linear condensing element is formed of an olefin resin material.   An image forming apparatus comprising the optical scanning optical device according to claim 1.

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