JP2010117588A - Optical scanner and image forming device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a highly stable optical scanner with improved robustness of optical characteristics for temperature fluctuation of an oblique-incidence optical system, in which the number of parts has been reduced in achieving high functionality and a large variety of optical scanners. <P>SOLUTION: The scanner comprises: an incidence optical system having one or a plurality of luminous points and including a light source 1, a coupling lens 2 for coupling luminous fluxes with a succeeding optical system, an aperture 3 for shaping the luminous fluxes from the coupling lens, an optical deflector 5 for deflecting the luminous fluxes in a main-scanning direction, and a single linear image forming lens 4 for condensing the luminous fluxes on an optical deflection surface as a linear image that is long in a main-scanning direction; and two lenses of a first scanning lens 6a and a second scanning lens 6b for causing the luminous fluxes deflected by the optical deflector to form an image on a surface to be scanned. The first scanning lens that is closer to the optical deflector is configured such that groups of luminous fluxes guided to the different surfaces to be scanned are incident in common. The linear image forming lens 4 has a diffraction surface and is formed by step-like level difference parallel to the sub-scanning direction. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an optical scanning device, an optical writing device, and an image forming apparatus suitable for a digital copying machine, a FAX, a laser printer, or the like.

  Conventionally, optical scanning devices are widely known in connection with image forming apparatuses such as optical printers, digital copying machines, and optical plotters. However, in recent years, image quality that is less susceptible to environmental fluctuations as well as downsizing and cost reduction. Therefore, there is a need for a good, high-stability, high-definition, and high-speed image forming apparatus, and an optical scanning apparatus corresponding to the image forming apparatus.

  An oblique incidence optical system has been proposed as an optical investigation device for realizing a reduction in size and cost of an image forming apparatus. By using an oblique incidence optical system, a single optical deflector is required for writing on a plurality of scanned surfaces, and a layout that allows a light beam directed to a plurality of scanned surfaces to be incident on a single scanning lens This is because it becomes possible.

  As a method to achieve high stability, low cost, and reduction in the number of parts of optical equipment, technologies related to optical elements with fine shapes (diffractive optical elements, diffractive lenses, phase shifters, SWS, etc.) are the latest high-precision processing technologies. Is widely known and used in various optical systems. In particular, mounting of a diffractive optical element (hereinafter referred to as “diffractive lens”) to an optical scanning device is widely used because a temperature compensation function utilizing the negative dispersion characteristic of the diffractive lens can be obtained ( For example, see Patent Document 1).

  Since the oblique incidence optical system is an optical system that scans with a skew beam with respect to the scanning lens, it ensures optical characteristics on the scanned surface compared to the conventional horizontal incidence method in which a light beam enters the optical axis of the scanning lens. Hard to do. Therefore, in order to realize high-accuracy scanning with the oblique incidence optical system, it is desirable to correct aberration using the central portion of the scanning lens in the main scanning section as much as possible. Based on such an optical design concept, it is necessary to make the angle of view narrow, and an optical system having a “narrow angle of view and a long optical path length” is finally preferable. Further, since the oblique incidence optical system generates a large amount of bending of the scanning line, a second scanning lens having a power in the sub-scanning direction for correcting each scanning line is required. The optical design guidelines for the oblique incidence optical system are summarized as follows: “The second scanning lens with a narrow angle of view, a long optical path length, and a power in the sub-scanning direction is placed near the surface to be scanned. It will be. In such an optical system, the main plane in the sub-scanning direction is generally close to the surface to be scanned, and the lateral magnification is extremely small. When the lateral magnification is small, the optical error due to disturbance is reduced, so that the optical system is inherently highly stable in the sub-scanning direction.

  However, in the oblique incidence optical system having two scanning lenses, the magnification in the main scanning direction is particularly larger than the magnification in the sub-scanning direction, and the robustness of the optical characteristics in the main scanning direction with respect to temperature fluctuations is reduced. In view of this, regarding the oblique incidence optical system, a proposal using a temperature compensating lens has been made (see Patent Document 2). Furthermore, regarding the temperature compensation of a diffractive lens using a temperature compensation lens, a form of a diffractive surface having a linear step has been proposed (see Patent Document 3). In addition, regarding an oblique incidence optical system, a conditional expression relating to a diffractive surface of a lens used for temperature compensation has been proposed. (See Patent Document 4).

JP 2005-031357 A JP 2007-293182 A JP 2002-287062 A JP 2004-126192 A

  However, in Patent Document 2, there is no statement regarding the form of the temperature compensating diffraction lens. The diffractive lens may not be able to perform temperature compensation due to the problem of shape change, and the invention described in Patent Document 2 is insufficient in terms of high stability. Furthermore, Patent Document 3 proposes a diffractive surface form having a linear step, but no proposal has been made regarding the temperature compensation concept unique to the oblique optical system and the accompanying diffractive lens form. Patent Document 4 describes a conditional expression related to a diffractive surface used for temperature compensation, but there is no description about the idea of temperature compensation peculiar to an oblique incidence optical system and the accompanying improvement of the form of a diffractive lens, which is insufficient. Met.

An object of the present invention is to solve the unsolved problems of the prior art.
Specifically, of the two scanning lenses that form an image on the scanned surface, the scanning lens closer to the optical deflector has a configuration in which a group of light beams guided to different scanned surfaces are incident in common. The line image forming lens has a diffractive surface, and the diffractive surface has a step shape that is strong against a stepwise temperature variation parallel to the sub-scanning direction. An object of the present invention is to improve the robustness of optical characteristics with respect to temperature fluctuation of an oblique incidence optical system.

The present invention includes a light source that emits one or a plurality of light beams, a coupling lens that couples the one or a plurality of light beams to a subsequent optical system, and a light beam emitted from the coupling lens. Aperture for shaping, light deflecting device for deflecting the light beam or beams in the main scanning direction, light beam or light beams emitted from the coupling optical system to the light deflecting device, and light as a long line image in the main scanning direction An incident optical system that includes a single line image forming lens that condenses on the deflecting surface, and is installed so that a light beam is incident at an angle in the sub-scanning direction with respect to the normal surface of the reflecting surface of the light deflector;
Two first scanning lenses (hereinafter referred to as “L1”) and a second scanning lens (hereinafter referred to as “L2”) that form an image of the single or plural light beams deflected by the optical deflecting device on the surface to be scanned. The scanning lens L1 on the side close to the optical deflector has a scanning lens system in which a group of light beams guided to different scanning surfaces are incident in common. In
The line image forming lens has a diffractive surface, and the diffractive surface is formed by a stepped step parallel to the sub-scanning direction.

  In the scanning optical system having the two scanning lenses L1 and L2, the power in the main scanning direction is concentrated from the optical deflector to the first scanning lens L1, and the power in the sub-scanning direction is applied to the second scanning lens L2. It is preferable to concentrate. With respect to the main scanning direction, the scanning lens system is required to “focus on the entire image height while ensuring the scanning characteristics on the surface to be scanned”. It is preferable to secure it at a position close to the vessel. Ensuring the scanning characteristics acts in the traveling direction of the light beam. Therefore, a strong refractive power in the main scanning direction is required to obtain performance at a position far from the optical deflector. Therefore, the main scanning shape of the scanning lens becomes curved or uneven, which is not preferable for production.

  On the other hand, regarding the sub-scanning direction, the light beam deflected by the optical deflector need only be condensed in the sub-scanning direction over the entire image height. Here, if the condensing power in the sub-scanning direction is provided at a position far from the optical deflector, the lateral magnification in the sub-scanning direction of the scanning lens system can be reduced, and robustness can be ensured.

  The oblique incidence optical system refers to one type of optical scanning device in which a plurality of light beams directed to different scanned surfaces are incident at angles with respect to the normal surface of the optical deflector reflecting surface. The oblique incidence optical system is an effective system for simplification and miniaturization of the apparatus because a plurality of light beams directed to different scanning surfaces can be deflected by a single optical deflector.

  In the case of a conventional non-oblique incidence method (hereinafter referred to as “horizontal incidence optical system”), it is necessary to separate a plurality of light beams directed to different scanning surfaces in the sub-scanning direction. The optical scanning device becomes larger and more expensive, such as requiring a plurality of optical deflectors.

  The oblique incidence optical system is an optical system that is decentered in the sub-scan section as a compensation for reducing the size, and therefore, it is difficult to correct aberrations compared to the horizontal incidence optical system. Therefore, aberration correction using a plurality of scanning lenses is preferable.

  In the oblique incidence optical system, light beams directed toward different scanning surfaces are close to each other in the vicinity of the optical deflector. Accordingly, a configuration in which the first scanning lens L1 of the scanning lens system is shared by a plurality of light beams directed to different scanning surfaces (hereinafter referred to as “L1 sharing configuration”) is preferable. If L1 is prepared for each surface to be scanned, L1 is installed in a region where the light flux corresponding to each surface to be scanned is sufficiently separated, and the optical scanning device becomes large. The L1 shared configuration is also effective in reducing the parts of the optical scanning device.

  Assuming a shared L1 configuration, it is preferable to arrange L1 as close as possible to the optical deflector. This is because the light beams corresponding to the scanned surfaces are close to the vicinity of the optical deflector as described above. If an L1 shared configuration at a location away from the optical deflector is to be realized, L1 must be enlarged in the sub-scanning direction.

At the same time, the second scanning lens L2 disposed corresponding to each scanned surface needs to be disposed in a region where the light beams directed to each scanned surface are separated from each other, and is disposed away from the optical deflector. Assuming the conventional “oblique incidence optical system”, “two scanning lens configuration”, and “L1 sharing configuration”, the following scanning lens system is preferable.
L1: Close to the optical deflector and has power mainly in the main scanning direction. Light beams traveling toward different scanned surfaces are incident on the same L1.
L2: A long lens far from the optical deflector, which has a power mainly in the sub-scanning direction and is arranged corresponding to each surface to be scanned.

  Therefore, the optical system of the present invention is an optical system in which “the lateral magnification in the main scanning direction> the lateral magnification in the sub-scanning direction” in the entire system, and the oblique incident optical system has an optical performance in the main scanning direction with respect to the sub-scanning direction. It is suggested to be particularly vulnerable. In general optical scanning devices, the numerical aperture NA in the main scanning direction is large and the focal depth is shallow, so that the robustness of the focus in the main scanning direction is improved in order to reduce the beam spot on the surface to be scanned. There is a need to.

  Diffractive lenses are generally known to have strong negative dispersion characteristics. When the temperature rises, a general refractive lens is affected by material dispersion due to expansion and a longer wavelength of the light source, and the focal length of the entire optical system becomes longer, which is observed as a focus shift. In particular, when a resin material is used, the amount of expansion is large and the focus shift is extremely large. On the other hand, the diffractive lens has a characteristic (negative dispersion characteristic) of shortening the focal length as the light source wavelength becomes longer. If a diffractive surface that compensates for a focus shift at the time of temperature rise is formed using this characteristic, an optical system having robustness against temperature fluctuations can be realized.

  In particular, the main cause of defocusing when the temperature rises is often the scanning lens which is an optical element made of resin and having the thickest finite focal length. In other words, in an optical scanning device in which temperature compensation is established, it can be said that the defocus due to expansion of the scanning lens and the correction power by the diffraction lens are balanced. In consideration of the above problems, when the correction power by the diffractive surface is provided in the main scanning direction, the lens form is a stepped step parallel to the sub-scanning direction as shown in FIGS.

  The diffractive surface of the diffractive lens is a shape obtained by folding the surface shape of the refracting surface with an appropriate step and pitch, and has the same power as the refracting surface as described above. The shape and power of the surface are a combination of that of the diffraction part and that of the refraction part. Usually, the pitch becomes fine toward the periphery of the lens. The surface shape of the diffractive surface of the present invention is preferably a multi-step shape. The multi-step shape refers to a step-like shape in which the angle of the folded portion is a right angle and symmetric with respect to the optical axis. The ease of molding is further improved as compared with the sawtooth diffractive surface. Optically equivalent to a non-power surface, optical performance is less likely to deteriorate with respect to decentration, and an optical scanning device having robustness against component / assembly errors can be realized. The multi-step shape can be understood as a combination of a concave refracting surface and a convex diffractive surface (for example, Fresnel lens type) as shown in FIG.

  Various photosensitive image carriers can be used in the image forming apparatus. For example, a silver salt film can be used as the image carrier. In this case, a latent image is formed by writing by optical scanning, and this latent image can be visualized by processing by a normal silver salt photographic process. Such an image forming apparatus can be implemented as an optical plate making apparatus or an optical drawing apparatus that draws a CT scan image or the like. As the photosensitive image carrier, it is also possible to use a color developing medium (positive printing paper) that develops color by the thermal energy of the beam spot during optical scanning. In this case, a visible image is directly formed by optical scanning. it can.

  As the photosensitive image bearing member, a photoconductive photosensitive member can also be used. As the photoconductive photoreceptor, a sheet-like one such as zinc oxide paper can be used, or a selenium photoreceptor, an organic optical semiconductor, or the like that is repeatedly used in the form of a drum or a belt can be used. .

  When a photoconductive photoreceptor is used as an image carrier, an image is formed by performing an electrophotographic process. That is, an electrostatic latent image is formed by uniform charging of the photosensitive member and optical scanning by the optical scanning device. The electrostatic latent image is visualized as a toner image by development. The toner image is directly fixed on the photoconductor when the photoconductor is in the form of a sheet such as zinc oxide paper, and transfer paper or an OHP sheet when the photoconductor can be used repeatedly. It is transferred and fixed on a sheet-like recording medium such as (plastic sheet for overhead projector).

The transfer of the toner image from the photoconductive photosensitive member to the sheet-like recording medium may be directly transferred from the photosensitive member to the sheet-like recording medium (direct transfer method), or may be temporarily transferred from the photosensitive member to an intermediate transfer belt or the like. After transfer to the intermediate transfer medium, transfer from the intermediate transfer medium to a sheet-like recording medium (intermediate transfer method) may be performed.

  Such an image forming apparatus can be implemented as an optical printer, an optical plotter, a digital copying apparatus, or the like. In the image forming apparatus of the present invention, a plurality of the photoconductors are arranged along the conveyance path of the sheet-like recording medium, and an electrostatic latent image is formed for each photoconductor using a plurality of optical scanning devices. The toner image obtained by visualizing these can be transferred and fixed to the same sheet-like recording medium, and can be implemented as a tandem type image forming apparatus that synthetically obtains a color image or a multicolor image.

  According to the optical scanning device of the present invention, among the two scanning lenses L1 and L2 that form an image on the surface to be scanned, the scanning lens L1 on the side close to the optical deflector is a light beam group that is guided to a different surface to be scanned. Have a diffractive surface, and a line image forming lens that forms a long line figure in the main scanning direction for light deflection, By having a step shape that is strong against a stepwise temperature variation parallel to the sub-scanning direction, it is possible to improve the robustness of the optical characteristics against the temperature variation of the oblique incidence optical system. Therefore, it is possible to realize a highly stable optical scanning device while simultaneously reducing the number of parts associated with increasing the functionality of the optical scanning device and realizing various types of optical scanning devices. In addition, the amount of material used in the production of the optical scanning device can be reduced, leading to a reduction in the environmental burden with respect to the amount of mined resources and the amount of plastic waste discharged.

  Embodiments of an optical scanning apparatus and an image forming apparatus according to the present invention will be described below with reference to the drawings. FIG. 1 corresponds to a main scanning section showing a main part of an optical scanning apparatus embodying the present invention. FIG. 1 shows the incident optical system on the light deflection surface folded for convenience.

"Injection optics"
The light beam emitted from the light source 1 which is a semiconductor laser is coupled to the subsequent optical system by the coupling lens 2. When the light beam transmitted through the coupling lens 2 passes through the opening of the aperture 3, the peripheral portion of the light beam is blocked and shaped, and enters the diffraction lens 4 that is a line image forming optical system. The diffractive lens 4 focuses the incident light beam in the sub-scanning direction and collects it as a long line image in the main scanning direction in the vicinity of the deflection reflection surface of the polygon mirror 5 that is an optical deflector. The diffractive lens 4 has a power for correcting fluctuations in the scanning optical system.

  The optical system incident on the optical deflector may be bent by a mirror or the like because of layout requirements. In this embodiment, a semiconductor laser element having a single light emitting point is taken as an example, but a semiconductor laser array having a plurality of light emitting points may be used, and an incident optical system is provided for one scanning optical system. There may be more than one.

  The incident optical system is provided so that the light beam enters at a specific angle with respect to the normal surface of the reflecting surface of the optical deflector. (Hereafter, this is referred to as “giving an oblique incidence angle”). There are several methods for giving the oblique incidence angle. As the simplest system, there is a system in which the entire incident optical system is inclined with respect to the optical deflector by an oblique incident angle.

  In this embodiment, as shown in FIG. 2, a method is assumed in which light beams 201, 202, 203, and 204 directed to four scanned surfaces are deflected by a common reflecting surface of the optical deflector 5. The light beams 201, 202, 203, and 204 are incident on the reflective surface normal line 51 of the optical deflector 5 at + 3.3 °, + 1.46 °, −1.46 °, and −3.3 °, respectively. Is provided with an incident optical system. As another method for providing an oblique incident angle, a method in which an incident angle is applied to the line image forming lens 4 while being decentered in the sub-scanning direction, and the oblique incident angle is provided using the refractive power of the line image forming lens 4 in the sub-scanning direction There is. Any mode may be used as an embodiment of the present invention.

"Scanning optical system-surface to be scanned"
In this embodiment, the optical deflector 5 is preferably a polygon mirror. The light beams 201, 202, 203, and 204 reflected by the reflecting surface pass through the scanning lens system 6 forming the scanning optical system while being deflected at a constant angular velocity as the polygon mirror 5 rotates at a constant speed, and the light beams are scanned. The optical path is bent by a bending mirror 7 for guiding light to the surface, and the light is condensed as a beam spot on the photoconductive photosensitive member 8 forming the substance of the surface to be scanned, and the surface to be scanned is optically scanned. The scanning lens system 6 has a two-lens configuration.

  As shown in FIG. 2, the light beams 201, 202, 203, and 204 are configured to pass through the first scanning lens 6 a out of the two scanning lenses. Further, as shown in FIG. 3, a method of scanning a plurality of scanned surfaces 8 using the opposing reflecting surfaces of the polygon mirror 5 may be used.

表１において「中央値」は、基準温度２５℃における使用波長に対する屈折率、「温度変動」は、温度が基準温度から２５℃上昇したときの屈折率である。 Example 1
The data of the glass material and resin material used in Example 1 are as shown in Table 1.

In Table 1, “median” is the refractive index with respect to the wavelength used at the reference temperature of 25 ° C., and “temperature fluctuation” is the refractive index when the temperature is increased by 25 ° C. from the reference temperature.

"light source"
The semiconductor laser 1 as the light source has a designed emission wavelength of 780 nm, and when the temperature rises by 1 ° C. with respect to the standard temperature of 25 ° C., the emission wavelength shifts to the longer wavelength side of 0.25 nm. This property is common for edge-emitting semiconductor lasers.

"Coupling lens"
The coupling lens 2 is a glass mold lens formed of glass, and has a focal length of about 12.5 mm and is arranged to have a function of converting into a substantially parallel light beam. The substantially parallel light beam may be weak divergence / weak convergent light. The coupling lens 2 is an aspherical surface. The aspheric coefficient (which is not disclosed) is preferably such that the wavefront aberration of the coupled light beam is sufficiently corrected by the aspheric surface.

"aperture"
The aperture 3 is an opening having a size corresponding to the beam spot diameter required on the scanned surface 8. The aperture 3 may be integrated with the housing or may be assembled as an element. As the shape of the aperture, a rectangle, an ellipse, a circle, or the like is selected as necessary.

"Diffraction lens"
The diffractive lens 4 as a line image forming lens is a resin diffractive lens with a thickness of 3 mm, having a cylindrical surface with a curvature radius of 69.5 mm and a multi-step diffractive surface on the exit surface. It is. The multi-step diffractive surface is composed of an infinite number of regions perpendicular to the optical axis divided by the stepped steps 22 shown in FIG. The diffractive lens 4 has a focal length of about 145 mm with respect to the sub-scanning direction.

  As shown in FIG. 6, the multi-step shape is realized as a shape combining a “diffractive portion” and a “diffractive portion” of a Fresnel lens type obtained by folding a refractive lens shape. At this time, the “diffractive part” and the “refractive part” have the same power, and in the multi-step shape, each region delimited by a step is a surface perpendicular to the optical axis. By adopting a multi-step shape, the ease of processing molding and the robustness against eccentricity are improved. The power of the “diffractive part” inherent in the multistep shape corresponds to the magnitude of power fluctuation due to negative dispersion at the time of wavelength fluctuation or the temperature compensation power described in this specification, and the shape corresponds to the number of steps. .

  In the first embodiment, the diffractive surface shape is realized by superimposing the “refractive portion” of the main concave cylinder surface on the “refractive portion” of the main concave cylinder surface and superimposing the “diffractive portion” of the main convex cylinder surface turned back at a specific step. . The curvature in the main scanning direction and the sub-scanning direction of the main concave cylinder surface that is the “refractive portion” is referred to as “substrate curvature” in this specification. Since the curvatures of the “refractive part” and “diffractive part” are equal to each other on the multi-step surface, the temperature compensation power may be considered to be proportional to the substrate curvature.

As shown in FIG. 6, the shape of the diffractive surface can take the following form.
・ Multi-step type Equivalent to an optical plane when there is no temperature variation and wavelength variation, but when the wavelength varies or varies due to temperature variation, it has a wavefront conversion function according to the shape of the diffractive surface. .
-Fresnel lens type The wavefront conversion function is exhibited at the design value level without considering disturbance. In either case, the effect aimed at by the present invention can be obtained without change, but it is more preferable to select the multi-step type for reasons such as ease of processing.

`` Arrangement of incident optical system ''
The arrangement of the incident optical system described above on the optical axis is as follows.
From light source to coupling lens entrance surface: 10.8mm
Coupling lens thickness: 2.5mm
Coupling lens From exit surface to aperture: 1.0 mm
From aperture to line image forming lens entrance surface: 81.5 mm
Line image forming lens thickness: 3mm

表２においてＲｍは「主走査方向の近軸曲率」、Ｒｓは「副走査方向の近軸曲率」であり、ｘ、ｙは「各光学素子の原点から次の光学素子の原点までの相対距離」を表している。単位はｍｍである。 "Optical deflector and scanning optical system"
FIG. 1 shows a schematic diagram of a scanning optical system. In the scanning optical system, the optical path is actually folded by other mirrors such as the folding mirror 7 shown in FIG. 2, but is omitted in FIG. 1 for simplification. Table 2 shows the layout of the scanning optical system. This optical system performs writing by optical scanning over an image height of ± 108 mm on the scanned surface 8.

In Table 2, Rm is “paraxial curvature in the main scanning direction”, Rs is “paraxial curvature in the sub-scanning direction”, and x and y are “relative distances from the origin of each optical element to the origin of the next optical element. ". The unit is mm.

  As for x and y with respect to the hexahedral polygon mirror 5 having an inscribed circle radius of 13 mm, the origin of the incident surface of the first scanning imaging lens 6a (the optical axis position of the incident side surface) is viewed from the rotational axis of the polygon mirror 5. Are 11.30 mm apart in the optical axis direction (x direction, left-right direction in FIG. 1) and 6.34 mm apart in the main scanning direction (y direction, vertical direction in FIG. 1). In addition, between the scanning lens L2 (6b) and the surface 8 to be scanned, a dust-proof glass 14 having a thickness of 1.9 mm made of glass is disposed.

  Each surface of the scanning lens system is aspheric, and all surfaces are non-circular in the main scanning direction, and other than the exit surface of L1 (6a), the sub-scanning cross section (virtual cross section parallel to the optical axis and the sub-scanning direction) ) Is a special surface whose curvature changes in accordance with the main scanning direction. The scanning lens system is designed so that the light beam deflected by the optical deflector 5 rotating at a constant angular velocity is converted to a substantially constant speed movement with respect to the scanned surface 8 and is always condensed on the scanned surface 8. Has been.

  When focusing on the power arrangement in the main scanning direction or the sub-scanning direction and reviewing Table 2, the condensing power is provided only on the incident surface of L2 in the sub-scanning direction. Therefore, it can be seen that the power in the main scanning direction is concentrated on L1 and the power in the sub-scanning direction is concentrated on L2 as the scanning lens system. By concentrating the power in the sub-scanning direction at a position far from the polygon mirror 5, the lateral magnification in the sub-scanning direction is lowered, which is advantageous in reducing the imaging position deviation in an oblique incident optical system in which the scanning line is originally bent. .

"Effect of diffraction lens"
In the first embodiment, the following diffractive lens is used as the shape of the diffractive surface.
"Example: temperature-compensated diffractive lens" (diffractive lens of the present invention)
Curvature radius: 69.5mm only in the first surface sub-scanning direction
Substrate curvature radius: main scanning direction -130mm
Step: 1.490μm
Second-order coefficient of phase function of diffraction part: main scanning direction: +0.002015235
The diffractive surface is provided on the exit surface of the line image forming lens 4 and has a form as shown in FIG. Depending on the optical system, the diffractive surface for performing temperature compensation may have a reverse power as shown in FIG.

"Comparative example: Resin cylindrical lens without diffractive surface"
Curvature radius: 74.9mm only in the first surface sub-scanning direction
The phase function φ (y) of the diffraction surface is expressed by the following equation. Y represents the main scanning direction.
φ (y) = C1 ・ Y ²
As a model of temperature fluctuation of the optical scanning device, a model in which all the optical elements in the optical scanning device have a temperature of the optical deflector of 50 ° C. is considered. This corresponds to a saturation state when an environmental temperature change is given to an optical scanning device that is not driven in a thermostatic chamber or the like.

本発明の回折レンズを用いない場合、主走査方向に３mmものピントずれが生じる。副走査方向に関しては、光学系のもつ副走査方向横倍率の小ささのために１mm以内のピントずれで収まっている。 The type 1 diffractive lens is set so that the out-of-focus due to temperature fluctuation is just canceled. Table 3 below shows the amount of defocus when Type 1 and 2 diffractive lenses and resin cylindrical lenses are applied. Here, for simplification, the value of the image height 0 mm of the light beam 201 in the embodiment of FIG. 2 is shown.

When the diffractive lens of the present invention is not used, a focus shift of 3 mm occurs in the main scanning direction. Regarding the sub-scanning direction, the optical system has a defocus within 1 mm due to the small lateral magnification in the sub-scanning direction of the optical system.

  In the present invention, the stability of the optical characteristics is improved by setting the temperature-compensating diffraction power in the main scanning direction based on the main / sub magnification ratio in the oblique incidence optical system. It has an effective form for solving the problems specific to the incident optical system.

"Image forming device"
FIG. 7 shows an embodiment of the image forming apparatus. This image forming apparatus forms an image by performing an electrophotographic process of charging, exposure, transfer, fixing, and cleaning on a photosensitive image carrier 1110 as a center.
This image forming apparatus is a laser printer. The laser printer 1000 has a “photoconductive photosensitive member formed in a cylindrical shape” as a photosensitive image carrier 1110. Around the image carrier 1110, a charging roller 1121, a developing device 1131, a transfer roller 1141, and a cleaning device 1151 as a charging unit are provided. A “corona charger” can also be used as the charging means.

  An optical scanning device 1171 that performs optical scanning with the laser beam LB is provided as a device that performs the above exposure process, and “exposure by optical writing” is performed between the charging roller 11211 1 and the developing device 1131. . In FIG. 7, reference numeral 1161 denotes a fixing device, reference numeral 1181 denotes a cassette, reference numeral 1191 denotes a registration roller pair, reference numeral 1201 denotes a paper feeding roller, reference numeral 1211 denotes a conveyance path, reference numeral 1221 denotes a discharge roller pair, reference numeral 1231 denotes a tray, reference numeral P Indicates transfer paper as a sheet-like recording medium.

  When image formation is performed, the image carrier 1110 that is a photoconductive photosensitive member is rotated at a constant speed in the clockwise direction, the surface thereof is uniformly charged by the charging roller 1121, and the optical beam of the laser beam LB of the optical scanning device 1171 is written. An electrostatic latent image is formed upon exposure to the image. The formed electrostatic latent image is a so-called “negative latent image”, and the image portion is exposed.

  This electrostatic latent image is reversely developed by the developing device 1131, and a toner image is formed on the image carrier 1110. The cassette 1181 storing the transfer paper P is detachable from the main body of the image forming apparatus 1000. When the transfer paper P is mounted as shown in FIG. The fed transfer paper P is fed to the registration roller pair 1191 at the leading end. The registration roller pair 1191 feeds the transfer paper P to the transfer unit in time with the toner image on the image carrier 1110 moving to the transfer position.

The transferred transfer paper P is superimposed on the toner image at the transfer portion, and the toner image is electrostatically transferred by the action of the transfer roller 1141. The transfer paper P to which the toner image has been transferred is sent to the fixing device 1161, where the toner image is fixed by the fixing device 1161, passes through the conveyance path 1211, and is discharged onto the tray 1231 by the discharge roller pair 1221. The surface of the image carrier 1110 after the toner image has been transferred is cleaned by a cleaning device 1151 to remove residual toner, paper dust, and the like.
By using the optical scanning device of the present invention as the optical scanning device 1171, good image formation is possible.

"Multicolor image forming device"
FIG. 8 shows an embodiment of a multicolor image forming apparatus.
In FIG. 8, photoconductors 1Y, 1M, 1C, and 1K rotate in the direction of the arrow, and in the order of rotation, chargers 2Y, 2M, 2C, and 2K, developing devices 4Y, 4M, 4C, and 4K, and transfer charging units 6Y and 6M. , 6C, 6K and cleaning means 5Y, 5M, 5C, 5K are provided. The charging members 2Y, 2M, 2C, and 2K are charging members that constitute a charging device for uniformly charging the surface of the photoreceptor. A beam is irradiated by a writing unit onto the surface of the photosensitive member between the charging member and the developing members 4Y, 4M, 4C, and 4K, so that an electrostatic latent image is formed on the photosensitive member. Then, based on the electrostatic latent image, a toner image is formed on the photoreceptor surface by the developing member. Further, each color sequential transfer toner image is transferred onto the recording paper 11 by the transfer charging means 6Y, 6M, 6C, 6K, and finally the image is fixed on the recording paper by the fixing means 30.

  By deploying the optical scanning device using the phase type optical element of the present invention to a multicolor image forming apparatus, it is possible to suppress fluctuations in the beam spot diameter on the photosensitive member. Therefore, since fluctuations in the dot diameter of the output image can be suppressed, a high-quality image with a uniform dot diameter can be provided. Further, stabilization of the beam spot diameter on the photoconductor means that one of a plurality of process control conditions is stabilized. Therefore, the process control frequency can be reduced, and environmental loads such as energy saving can be reduced. The present invention can also be applied to a monochrome image forming apparatus.

It is the main scanning sectional view showing the important section of the optical scanning device which implemented the present invention. It is the figure which showed the subscanning cross section of the optical scanning device which implemented this invention. It is an optical arrangement | positioning figure which shows the example of the diagonally-facing opposing scanning system optical writing apparatus. It is a perspective view of a lens with a level | step difference parallel to a subscanning direction. It is a perspective view of the other aspect of the lens which has a level | step difference parallel to a subscanning direction. It is a model figure for demonstrating the general shape of the refractive part and diffraction part in an optical element. 1 is a front view showing an embodiment of an image forming apparatus. 1 is a front view illustrating an embodiment of an image forming apparatus capable of multicolor image formation.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Light source 2 Coupling lens 3 Aperture 4 Diffraction lens as a line image formation lens 5 Polygon mirror 6 Scanning lens system which makes a scanning optical system 6a First scanning imaging lens 6b Second scanning coupling lens 7 Beam spot mirror 8 Photoconductor 14 Dust-proof glass 22 Back cut of diffractive lens 23 Outer surface of diffractive lens 51 Reflective surface normal of polygon mirror 5 201 First light beam 202 Second light beam 203 Third light beam 204 Fourth light beam

Claims (6)

A light source that emits one or a plurality of light beams, a coupling lens that couples the one or a plurality of light beams to a subsequent optical system, an aperture that shapes the light beam emitted from the coupling lens, The light deflector for deflecting the single or plural light beams in the main scanning direction and the single or plural light beams emitted from the coupling optical system are guided to the optical deflecting device and collected on the light deflecting surface as a long line image in the main scanning direction. An incident optical system that includes a single line-image-forming lens that illuminates, and is installed so that a light beam is incident at an angle in the sub-scanning direction with respect to the reflection surface normal of the light deflector;
Consists of two first scanning lenses and a second scanning lens that image one or more light beams deflected by the optical deflecting device on the surface to be scanned, and the first scanning on the side close to the optical deflector In the optical scanning device, the lens has a scanning lens system in which a group of light beams guided to different scanning surfaces are incident in common.
The line image forming lens has a diffractive surface, and the diffractive surface is formed by a stepped step parallel to the sub-scanning direction.   The optical scanning device according to claim 1, wherein the diffraction surface has a multi-step surface.   3. The optical scanning device according to claim 1, wherein the optical deflection device is a polygon mirror.   The optical scanning device according to claim 1, wherein the optical element provided with the diffractive surface is made of resin.   An image forming apparatus for forming an image by executing an electrophotographic process, wherein the optical scanning device according to claim 1 is used as an apparatus for performing an exposure process of an electrophotographic process. An image forming apparatus.   An image forming apparatus that includes a plurality of photoconductors and forms an image by performing an electrophotographic process on each photoconductor, and includes an optical scanning device that writes an image signal corresponding to each color on each photoconductor, The optical scanning device is a color-compatible image forming apparatus that is the optical scanning device according to claim 1.

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