JP2010276861A - Scanning optical system in image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce focal point shift on an image face in main scanning direction due to the variation in environmental temperature and moisture even when an fθ lens and a coupling lens are composed of an optical transparent resin. <P>SOLUTION: The scanning optical system satisfies the relation (4), where t<SB>0</SB>stands for a designed temperature, t stands for the temperature in environment variation, α<SB>h</SB>stands for the linear expansion coefficient of a housing, f<SB>col</SB>(t) stands for the focal distance of the coupling lens at the temperature t, f<SB>fθ</SB>(t) stands for the main scanning focal distance of a scanning optical system at the temperature t, L stands for a desired depth of focus, β stands for the lateral magnification of the whole optical system, n<SB>fθ</SB>(t) stands for the refractive index at the temperature t when the scanning optical system lenses are assumed to be a synthesized single lens, S<SB>fθ</SB>(t) stands for the thickness of the center of the lens, R<SB>fθ1</SB>(t) stands for the incident side radius of curvature in main scanning direction, R<SB>fθ2</SB>(t) stands for the emission side radius of curvature in main scanning direction, n<SB>col</SB>(t) stands for the refractive index of the coupling lens at the temperature t, S<SB>col</SB>(t) stands for the thickness of the center of the lens, R<SB>coll</SB>(t) stands for the incident side radius of curvature, and R<SB>col2</SB>(t) stands for the emission side radius of curvature. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning optical system in an image forming apparatus such as a copying machine, a printer, a facsimile, or a composite machine using an electrophotographic method, and more particularly, a coupling lens (collimator lens) that uses light from a light source as a parallel light beam. ) And an f.theta. Lens that makes the scanning speed of light deflected by the polygon mirror constant on the surface to be scanned, and nevertheless on the image surface in the main scanning direction due to environmental temperature changes. The present invention relates to a scanning optical system in an image forming apparatus that can keep the focus shift at an allowable range.

  In image forming apparatuses such as copiers, printers, facsimiles, and composite machines using electrophotography, the photosensitive drum is uniformly charged by a charging device, and then is exposed to light modulated by a signal of an image to be formed. The photosensitive drum is exposed to form an electrostatic latent image, the electrostatic latent image formed on the photosensitive drum is developed by the developing device, and the toner image on the photosensitive drum formed by the development is transferred to a sheet by the transferring device. Image formation is performed by a process of transferring and fixing a toner image transferred onto a sheet by a fixing device.

  Among these, as a scanning optical system for exposing a uniformly charged photosensitive drum, in a high-speed machine, it is modulated by a signal of an image forming a laser diode or the like, and is scanned in a main scanning direction (of the photosensitive drum by a polygon mirror). A scanning optical system that scans the photosensitive drum by converting the constant angular velocity scanning into the constant velocity scanning by the scanning optical system using the fθ lens is used.

  In such a scanning optical system, generally, a laser beam emitted from a light source device such as a laser diode is converted into a substantially parallel light beam by a coupling lens such as a collimator lens, and then scanned on a photosensitive drum. In order to obtain a desired beam spot diameter above, the optical beam having a refractive power only in the sub-scanning direction (direction orthogonal to the main scanning direction), such as a cylindrical lens, by narrowing the light beam to a certain size by an aperture (aperture). The element is transmitted and imaged linearly on the polygon mirror in the main scanning direction, and imaged as a light beam spot on the scanning surface through an fθ lens that converts constant angular velocity scanning on the scanning surface to constant velocity scanning. It is configured as follows.

  Of these, the fθ lens is often composed of two elements in two groups. The first lens corrects aberrations in the main scanning direction and the conversion function from constant angular velocity scanning to constant velocity scanning, and the second lens tilts the polygon mirror. In general, the correction function is designed with the optical function separated.

  As the material of the fθ lens, optical glass (such as BK7) that is not easily affected by a change in refractive index, a change in refractive index distribution, thermal deformation, or the like due to environmental fluctuations has been used. In view of demands for reducing the cost of materials and parts, optical transparent resins such as optical acrylic resins (PMMA) and cycloolefin-based resins (COP) are increasingly used.

  However, these optical transparent resins such as optical acrylic resin (PMMA) and cycloolefin resin (COP) are more susceptible to changes in temperature and humidity than glass, and optical acrylic resin (PMMA) is hygroscopic. Therefore, the refractive index distribution in the lens is changed and the optical performance is likely to be deteriorated. On the other hand, cycloolefin resin (COP) has low hygroscopicity with respect to changes in humidity and is not easily affected by changes in the refractive index distribution. Cause deterioration.

  In addition, a coupling lens that converts diffused light from a light source device such as a semiconductor laser into a substantially parallel light beam has a large change in optical performance when a focus shift occurs due to a change in temperature and humidity. Conventionally, this coupling lens is optical. Glass was used. However, when optical glass is used, a cycloolefin-based (COP) resin may be used in order to increase the cost as compared with the case where an optical transparent resin is used. Affected, causing the focal length of the coupling lens to vary. Therefore, there is a method of solving this problem by providing a diffraction grating in the coupling lens, or having two coupling lenses, each having positive and negative refractive powers, and canceling the focal length variation. It was taken.

  As described above, for example, in Patent Document 1, a cylindrical lens is made of an acrylic resin and an fθ lens is made of glass in order to prevent a focal position shift due to an environmental temperature change by using an optical transparent resin in the scanning optical system. In this case, a laser beam scanning optical system is shown in which the focal position deviation due to temperature change is suppressed within a practically allowable range. In the laser beam scanning optical system disclosed in Patent Document 1, the light source unit holding the cylindrical lens is deformed by a temperature change, and the imaging position in the vicinity of the polygon mirror fluctuates from the original position, and near the photoconductor. The image forming position moves from the original position, but as the temperature rises, the plastic cylindrical lens also deforms, and the fact that the image forming position changes in a direction to cancel the deformation of the light source unit is used to reduce the defocus. It is kept within a practically acceptable range.

Japanese Patent Laid-Open No. 5-19189

  When an fθ lens or a coupling lens is configured with an optical transparent resin, since the refractive index variation with respect to the temperature change is larger than that of the optical glass as described above, the change in the focal length by each lens becomes large. . Also, a coupling lens that converts diffused light from a light source device such as a semiconductor laser into a parallel light beam has a distance in the optical axis direction between the semiconductor laser and the coupling lens when the temperature changes. It expands and contracts by an amount proportional to the linear expansion coefficient of the common fixing member installed.

When optically transparent resin is used as the lens material, the focal length variation due to temperature changes is larger than the variation in the distance between the semiconductor laser and the coupling lens due to thermal expansion, and the distance between the coupling lens and the semiconductor laser. Will be in the same state as is shortened. At this time, the surface to be scanned is proportional to the square of the ratio (lateral magnification) of f _col (t) representing the focal length of the coupling lens as a function of temperature t and the focal length f _fθ (t) of the fθ lens. There is a problem that the focal position shift in the optical axis direction occurs.

  In order to deal with these problems, the method of providing a diffraction grating in the coupling lens requires a separate design of the diffraction grating, and the production of the diffraction grating mold is special compared to ordinary lens molds. More man-hours are required. Since the method of canceling the configuration with two lenses uses two lenses, the number of members increases and the cost increases.

  The laser beam scanning optical system disclosed in Patent Document 1 is a case where the cylindrical lens is made of acrylic resin and the fθ lens is made of glass. As described above, the apparatus is reduced in size and weight, and the member cost is reduced. In view of such demands, it cannot be applied when an optical transparent resin such as optical acrylic resin (PMMA) or cycloolefin resin (COP) is used for the fθ lens and the coupling lens.

  Therefore, in the present invention, even when an optical transparent resin such as an optical acrylic resin (PMMA) or a cycloolefin resin (COP) is used for the fθ lens and the coupling lens, the main scanning direction due to a change in environmental temperature and humidity. An object of the present invention is to provide a scanning optical system in an image forming apparatus capable of reducing the focus shift on the image plane.

であり、使用環境温度内で、

を満たすことを特徴とする。 In order to solve the above problems, the scanning optical system in the image forming apparatus according to the present invention is:
A light source device, a coupling lens that converts the diffused light from the light source device into a parallel light beam, and deflects light from the light source device that has been converted into a parallel light beam by the coupling lens, and scans the surface to be scanned in the main scanning direction. An image forming apparatus comprising: a scanning optical unit that causes the light deflected by the scanning optical unit to be condensed as a light spot on the surface to be scanned, and a scanning speed on the surface to be scanned is constant. In the scanning optical system in
The light source device and the coupling lens (collimator lens) are held in a casing made of a composite material made of resin and glass fiber, and the fθ lens and the coupling lens in the scanning optical system are made of an optical transparent resin. The coupling lens (collimator lens) when the design temperature of the optical system is t ₀ , the temperature when the environmental temperature varies is t, the linear expansion coefficient of the housing holding the optical system is α _h , and the environmental temperature is t _Is the main scanning focal length of the scanning optical system at the environmental temperature t, f _fθ (t), the desired focal depth is L, and the lateral magnification of the entire optical system in the main scanning direction is When the scanning optical system lens at β and the environmental temperature t is regarded as a synthetic single lens, the refractive index is n _fθ (t), the lens center thickness is S _fθ (t), and the main scanning direction incident side radius of curvature is R _fθ1. (T) Main scanning method R _fθ2 (t) for the exit-side radius of curvature, refractive index n _col (t) of the coupling lens at temperature t, lens center thickness S _col (t), entrance-side radius of curvature R _col1 (t), exit side When the curvature radius is R _col2 (t),

And within the operating environment temperature,

It is characterized by satisfying.

  As described above, when the scanning optical system lens is regarded as a synthetic single lens, it is designed so as to satisfy the expression (4), so that the coupling lens composed of the light source device held in the housing and the optical transparent resin can be used. The focal shift amount of the lens itself and the combined shift amount of the focal shift amount due to the deformation of the housing and the focal shift amount of the fθ lens formed of the optical transparent resin cancel each other, thereby minimizing the focal shift. If the focus shift amount when the environment changes is small, the design depth of focus can be reduced, and the design difficulty of the scanning optical system can be lowered. In addition, the coupling lens can use a conventional transparent optical resin without using expensive glass. Further, since it is not necessary to provide a diffraction grating, the cost can be reduced and the diffraction grating can be designed. Optical design can be simplified because it becomes unnecessary.

  Further, when the casing is made of a composite material made of polycarbonate, glass fiber, and ABS resin, it is possible to provide a casing capable of minimizing the focus shift by the formula (1) while having strength. .

  As described above, the scanning optical system in the image forming apparatus according to the present invention can minimize the focus shift amount due to the environmental temperature of the scanning optical system, and can suppress deterioration of the optical performance when the environmental temperature changes. At the same time, since the focus shift amount when the environment changes is small, the design depth of focus can be made shallow, so that the difficulty level in designing the scanning optical system can be lowered.

1 is a perspective view of a schematic configuration in which an optical scanning optical device according to the present invention is applied to a photoconductor exposure apparatus in an image forming apparatus. FIG. 2 is a cross-sectional view in the sub-scanning direction of the optical scanning optical device shown in FIG. 1.

  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, but are merely illustrative examples. Absent.

  FIG. 1 is a perspective view of a schematic configuration of a scanning optical system in an image forming apparatus according to the present invention, and FIG. 2 is a sectional view in the sub-scanning direction of the optical scanning optical apparatus shown in FIG. The same constituent elements in FIGS. 1 and 2 are denoted by the same reference numerals.

  The optical scanning optical device according to the present invention includes, for example, a light source 1 made of a semiconductor laser having a wavelength of 670 nm, an aspherical collimating lens 2 as a coupling lens that makes parallel light from the light source 1 made of the semiconductor laser, An aperture stop (aperture) 3 having an opening 31 having a predetermined size of parallel light and light from the light source 1 that has been made parallel light are scanned in a line that is long in the main scanning direction 10 to scan the surface 9 to be scanned. The linear condensing element such as the cylindrical lens 4 that forms an image on the reflecting surface 5a of the rotating mirror (polygon mirror) 5 and the scanning speed of the light deflected by the polygon mirror 5 on the surface to be scanned are made constant. A scanning optical system such as fθ lenses 6 and 7, a reflecting mirror 8, and the like are included. In the present invention, S-BSL7 (manufactured by OHARA INC.), Which is an optical glass, was used as the cylindrical lens. The reflecting mirror 8 may be provided as necessary, and may be omitted depending on the arrangement configuration (layout) of each optical member.

  The fθ lenses 6 and 7 have 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 first lens 6 having a positive power in the sub scanning direction. 2 lenses 7 and one of the lenses has an aspherical shape. In the present invention, the collimating lens 2 and the fθ lenses 6 and 7 are made of an optical transparent resin such as ZEONEXE 330R (manufactured by Nippon Zeon Co., Ltd.), which is a cycloolefin resin (COP). The lens 2 is held in a housing made of a material such as RN7740D (manufactured by Teijin Ltd.) made of a composite material of polycarbonate, glass fiber, and ABS.

The refractive index of the scanning optical system in the image forming apparatus of the present invention thus configured, the focal length f of the main scanning direction of the fθ lens 6 and 7, when the n _L were considered fθ lens and synthetic single lens, S _{When L} is the lens center thickness, R ₁ is the curvature radius on the incident side in the main scanning direction of the fθ lens, and R ₂ is the curvature radius on the exit side in the main scanning direction of the fθ lens, the following equation (2) is given.

When the environmental temperature changes, these four parameters n _L, S _L, R _{1, and} R ₂ are functions of the environmental temperature t, and although details are omitted, the lens center thickness S _L is S _L (t), main scanning. The direction-incident-side radius of curvature R ₁ is R ₁ (t) and the main-scanning direction exit-side radius of curvature R ₂ is R ₂ (t), which is approximated by a linear expression of temperature t, and combines all lenses of the scanning optical system. The refractive index n _L when regarded as a single lens is sufficiently well approximated by a quadratic expression of the temperature t as n _L (t). Therefore, when the lens thickness at the design temperature t ₀ is S ₀ , the main scanning direction incident side radius of curvature is R ₁ , the main scanning direction exit side radius of curvature is R ₂ , and the linear expansion coefficient of the lens is α _L , the environmental temperature t These four parameters can be expressed by the following equations (6) to (9).

Then, like _{S 0,} R 1, _{R 2} of _{S 0,} R 1, _{R 2,} and fθ lenses 6 and 7 in the coupling lens 2, and is defined as follows.

また、コリメートレンズ２の温度tのときの焦点距離は（５）式より以下のようになる。

Accordingly, the focal length of the fθ lenses 6 and 7 at the temperature t is as follows from the equation (5).

Further, the focal length at the temperature t of the collimating lens 2 is as follows from the equation (5).

Further, the lateral magnification β of the entire optical system in the main scanning direction is expressed by the following expression (12) using the expressions (10) and (11), and the vertical magnification of the entire optical system in the main scanning direction is expressed by the following expression (13). .

Further, around the coupling lens 2, the housing holding the semiconductor laser 1 and the coupling lens 2 expands and contracts when the environmental temperature changes, and the distance between the semiconductor laser 1 and the coupling lens 2 expands and contracts. Accordingly, the positional deviation amount of the semiconductor laser with respect to the coupling lens in the optical axis direction is expressed by the following equation (14), where α _h is the linear expansion coefficient of the housing, t _{0 is} the design temperature, and t is the environmental temperature.

この時、半導体レーザ１とカップリングレンズ２の距離が伸びる方向が負となる。これは、被走査面１０上ではシフトする方向が逆転するためである。 Further, the focal length f _col (t) of the coupling lens 2 at the environmental temperature t is calculated by the equation (11), and the focal length changes due to the temperature change, so that the semiconductor laser 1 and the coupling lens 2 The substantial distance variation ΔL _col is expressed by the following equation (15).

At this time, the direction in which the distance between the semiconductor laser 1 and the coupling lens 2 increases is negative. This is because the shifting direction is reversed on the scanned surface 10.

fθレンズの焦点シフト量ΔＬ_{ｉｍａｇｅ_ｆθ}は、下記（１７）式になる。

従って、両者の効果を合わせた焦点シフトは（１５）式、（１６）式、（１７）式から、下記（１８）式になる。

The relationship between the shift amount ΔL _col in the optical axis direction on the object point and the shift amount ΔL _{image_col} at the image point is _{expressed by} the following equation (16).

The focal shift amount ΔL _{image_fθ} of the fθ lens is _expressed by the following equation (17).

Therefore, the focus shift combining both effects is expressed by the following equation (18) from the equations (15), (16), and (17).

Here, assuming that the focal depth at which a desired optical performance can be obtained is L, deterioration of the optical performance due to a focus shift can be avoided if the following expression (18) is satisfied.

  That is, when the focal depth is L, it means that there is no problem even if a focus shift occurs in the range of the focal depth L on the optical axis. Therefore, the equation (19) is first set to a position of 0 in the optical axis direction. If the focus is adjusted, it indicates that there is no problem if the deviation is within the range of ± L / 2 from the focal position 0.

Next, an actual optical design example will be described. First, each parameter of the linear expansion coefficient α _h of the casing, the design temperature t ₀ , and the focal depth L was set as follows.
α _h = 3.01 × 10 ⁻⁴ (/ ° C.)
t ₀ = 20 (/ ° C.)
L = 5 (mm)
The operating temperature t is 10 ° C. to 35 ° C., the wavelength of the semiconductor laser is 670 (nm), and the focal depth L is the beam spot diameter on the scanned surface 10 depending on the optical design of the fθ lenses 6 and 7. Although it depends on the setting, it is considered good if about 5 mm is obtained as described above. As described above, ZEONEEXE 330R (registered trademark of Nippon Zeon Co., Ltd.), which is a cycloolefin resin, was used as the optical transparent resin used for the fθ lenses 6 and 7. In the expression (10), the radius of curvature of the synthetic lens is used. However, in the examples, the calculation is made with a single lens for simplicity.

Linear expansion coefficient alpha _L of the lens coupling lens 2, the design temperature coupling lens thickness _{S 0Col} at _{t 0,} design temperature _{t 0} the coupling lens in the main scanning direction incident-side radius of curvature _{R 0Col1} in the cup at the design temperature _{t 0} Specifications of the ring lens in the main scanning direction exit side radius of curvature R _{0col2 and the} like are as follows.
α _L = 9.0 × 10 ⁻⁵ (/ ° C.)
S _0col = 8 (mm)
R _0col1 = 25.349 (mm)
R _0col2 = ∞ (mm)
Further, also the linear expansion coefficient alpha _L in f [theta] lens 6, 7, f [theta] lens thickness _{S 0Efushita} at design temperature _{t 0,} the main scanning direction incident-side radius of curvature _{R 0Efushitaeru} of f [theta] lens at the design temperature _{t 0,} f [theta] at the design temperature _{t 0 Specifications} such as the main scanning direction exit side radius of curvature R _{0fθ2 of the} lens are as follows.
α _L = 9.0 × 10 ⁻⁵ (/ ° C.)
S _0fθ = 8 (mm)
R _0fθ1 = −48.695 (mm)
R _0fθ2 = −73.614 (mm)

As a result, the focus shift amount at each temperature of 10 ° C., 15 ° C., 20 ° C., 25 ° C., 30 ° C., and 35 ° C. is as follows, and the shift amount is within the range of 1.178 mm to −0.656 mm. Thus, the equation (19) is satisfied.
10 ° C -0.656 (mm)
15 ° C -0.341 (mm)
20 ° C 0.000 (mm)
25 ° C 0.366 (mm)
30 ° C 0.759 (mm)
35 ° C 1.178 (mm)

  As described above, according to the present invention, it is possible to minimize the focus shift amount due to the environmental temperature of the scanning optical system, and it is possible to suppress the deterioration of the optical performance when the environmental temperature changes. In addition, if the focus shift amount when the environment changes is small, the design depth of focus can be made shallow, so that the design difficulty of the scanning optical system can be reduced. Further, the coupling lens 2 can use a conventional transparent optical resin without using expensive glass, and it is not necessary to provide a diffraction grating. The design becomes unnecessary and the optical design can be simplified.

  According to the present invention, even if the fθ lens and the coupling lens in the scanning optical system are made of an optical transparent resin, the focus shift can be within an allowable range even if there is a change in environmental temperature. It can be manufactured at a low cost so that the image quality does not deteriorate even when the environmental temperature changes.

1 Light source (semiconductor laser)
2 Collimating lens 3 Aperture (aperture stop)
31 Aperture 4 Cylindrical lens 5 Polygon mirror 6, 7 fθ lens 8 Reflecting mirror 9 Photosensitive member (scanned surface)
10 Scanning direction

Claims (2)

A light source device, a coupling lens that converts the diffused light from the light source device into a parallel light beam, and deflects light from the light source device that has been converted into a parallel light beam by the coupling lens, and scans the surface to be scanned in the main scanning direction. An image forming apparatus comprising: a scanning optical unit that causes the light deflected by the scanning optical unit to be condensed as a light spot on the surface to be scanned, and a scanning speed on the surface to be scanned is constant. In the scanning optical system in
The light source device and the coupling lens (collimator lens) are held in a casing made of a composite material made of resin and glass fiber, and the fθ lens and the coupling lens in the scanning optical system are made of an optical transparent resin. The coupling lens (collimator lens) when the design temperature of the optical system is t ₀ , the temperature when the environmental temperature varies is t, the linear expansion coefficient of the housing holding the optical system is α _h , and the environmental temperature is t _Is the main scanning focal length of the scanning optical system at the environmental temperature t, f _fθ (t), the desired focal depth is L, and the lateral magnification of the entire optical system in the main scanning direction is When the scanning optical system lens at β and the environmental temperature t is regarded as a synthetic single lens, the refractive index is n _fθ (t), the lens center thickness is S _fθ (t), and the main scanning direction incident side radius of curvature is R _fθ1. (T) Main scanning method R _fθ2 (t) for the exit-side radius of curvature, refractive index n _col (t) of the coupling lens at temperature t, lens center thickness S _col (t), entrance-side radius of curvature R _col1 (t), exit side When the curvature radius is R _col2 (t),

And within the operating environment temperature,

And a scanning optical system in an image forming apparatus.   2. A scanning optical system in an image forming apparatus according to claim 1, wherein the casing is made of a composite material made of polycarbonate, glass fiber, and ABS resin.

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