JP2014203011A - Optical element, scanning optical system, optical scanner, and image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical element that can suppress a change in a polarization state of light after passage without causing a price increase.SOLUTION: A scanning lens (optical element) in scanning optical systems has a main body part 101 and a microstructure part 102. The microstructure part 102 includes a plurality of cuboids in which a longitudinal direction coincides with the main axis azimuth of birefringence in the main body part 101. With this configuration, a polarization state of light hardly changes after passing through the scanning lens. The main body part 101 and microstructure part 102 are molded integrally with each other. Thus, the resin-made optical element can be achieved that can suppress a change in the polarization state of light after passage without causing a price increase.

Description

  The present invention relates to an optical element, a scanning optical system, an optical scanning apparatus, and an image forming apparatus, and more specifically, a resin optical element, a scanning optical system including the optical element, and an optical scanning apparatus including the scanning optical system, And an image forming apparatus including the optical scanning device.

  In electrophotographic image recording, an image forming apparatus using a laser beam is widely used. This image forming apparatus includes an optical scanning device that scans the surface of a photosensitive drum with laser light and forms a latent image (electrostatic latent image) on the surface of the drum.

  The optical scanning device includes an optical element, and a resin molded product is often used as the optical element in terms of surface accuracy of the optical surface and manufacturing cost. However, birefringence causes various problems in resin optical elements.

  Thus, for example, Patent Document 1 discloses a method for molding a plastic molded product for the purpose of reducing birefringence. In addition, even if stray light occurs due to the influence of birefringence of the optical element, a ghost image is not formed unless it is condensed on the surface to be scanned. Therefore, the two light beams propagate at different angles with respect to the main scanning plane. "Method" is known (for example, see Patent Document 2).

  However, with conventional resin optical elements, it has been difficult to suppress changes in the polarization state of light after transmission without incurring an increase in cost.

  The present invention is a resinous optical element having a main body portion and a fine structure portion provided on at least one of an incident-side optical surface and an emission-side optical surface in the main body portion, and the fine structure The portion includes a plurality of columnar bodies, and at least a part of the plurality of columnar bodies is an optical element whose longitudinal direction coincides with the principal axis direction of birefringence in the main body portion.

  According to the optical element of the present invention, it is possible to suppress a change in the polarization state of light after transmission without increasing the cost.

1 is a diagram illustrating a schematic configuration of a color printer according to an embodiment of the present invention. It is a figure for demonstrating the optical scanning device in FIG. It is a figure for demonstrating light source unit LU1. It is a figure for demonstrating light source unit LU2. It is a figure for demonstrating the effect | action of a half mirror prism. It is a figure for demonstrating the scanning optical system A and the scanning optical system B. FIG. FIG. 7A to FIG. 7C are diagrams for explaining an example of the polarization separation element. It is a figure for demonstrating the difference of circularly polarized light and elliptically polarized light with respect to the shading in a to-be-scanned surface. It is a figure for demonstrating the change of the polarization state of the light by passing through a resin scanning lens. It is a figure for demonstrating the scanning lens in this embodiment. It is a figure for demonstrating the relationship between main axis direction (theta) and the lens height H in a main-body part single-piece | unit. It is a figure for demonstrating the relationship between the retardation R and lens height H in a main-body part single-piece | unit. It is a figure for demonstrating the relationship between the stray-light ratio Pg and lens height H in a main-body part single-piece | unit. It is a figure for demonstrating the rectangular parallelepiped of a fine structure part. It is a figure for demonstrating the arrangement | sequence pitch of a rectangular parallelepiped. It is a figure for demonstrating the relationship between the stray-light ratio Pg and the lens height H in the scanning lens in this embodiment. It is a figure for demonstrating the several area | region divided | segmented in the scanning lens. It is a figure for demonstrating the longitudinal direction of the rectangular parallelepiped in each area | region. It is a figure for demonstrating +/- code | symbol of a longitudinal direction. It is a figure for demonstrating an example of the rectangular parallelepiped in each area | region. It is a figure for demonstrating the modification of the relationship between main axis direction (theta) and the lens height H in a main-body part single-piece | unit. It is a figure for demonstrating the fine structure part corresponding to FIG. It is a figure for demonstrating the principal axis direction in the case of a spherical lens. FIG. 24A and FIG. 24B are diagrams for explaining principal axis directions in the case of a long lens. It is a figure for demonstrating the modification 1 of a scanning lens. It is a figure for demonstrating the modification 2 of a scanning lens. It is a figure for demonstrating a triangular prism. It is a figure for demonstrating a rectangular parallelepiped row | line | column.

  Hereinafter, an embodiment of the present invention will be described with reference to FIGS. FIG. 1 shows a schematic configuration of a color printer 2000 according to an embodiment.

  The color printer 2000 is a tandem multi-color printer that forms a full-color image by superimposing four colors (black, cyan, magenta, and yellow), and includes an optical scanning device 2010, four photosensitive drums (2030a, 2030b, 2030c, 2030d), four cleaning units (2031a, 2031b, 2031c, 2031d), four charging devices (2032a, 2032b, 2032c, 2032d), four developing rollers (2033a, 2033b, 2033c, 2033d), transfer A belt 2040, a transfer roller 2042, a fixing roller 2050, a paper feed roller 2054, a paper discharge roller 2058, a paper feed tray 2060, a paper discharge tray 2070, a communication control device 2080, and a printer control device 20 that comprehensively controls the above-described units. It has a such as 0.

  The communication control device 2080 controls bidirectional communication with a host device (for example, a personal computer) via a network or the like.

  The printer control device 2090 includes a CPU, a program written in a code decipherable by the CPU, a ROM storing various data used when executing the program, a RAM as a working memory, an analog signal An A / D converter for converting the signal into a digital signal. The printer control device 2090 controls each unit in response to a request from the host device, and sends multicolor image information from the host device to the optical scanning device 2010.

  The photosensitive drum 2030a, the charging device 2032a, the developing roller 2033a, and the cleaning unit 2031a are used as a set, and constitute an image forming station (hereinafter also referred to as “K station” for convenience) that forms a black image.

  The photosensitive drum 2030b, the charging device 2032b, the developing roller 2033b, and the cleaning unit 2031b are used as a set, and constitute an image forming station (hereinafter also referred to as “C station” for convenience) that forms a cyan image.

  The photosensitive drum 2030c, the charging device 2032c, the developing roller 2033c, and the cleaning unit 2031c are used as a set, and constitute an image forming station (hereinafter also referred to as “M station” for convenience) that forms a magenta image.

  The photosensitive drum 2030d, the charging device 2032d, the developing roller 2033d, and the cleaning unit 2031d are used as a set, and constitute an image forming station (hereinafter also referred to as “Y station” for convenience) that forms a yellow image.

  Each photosensitive drum has a photosensitive layer formed on the surface thereof. Each photosensitive drum is rotated in the direction of the arrow in the plane of FIG. 1 by a rotation mechanism (not shown).

  Each charging device uniformly charges the surface of the corresponding photosensitive drum.

  The optical scanning device 2010 uses four-color image information (black image information, cyan image information, magenta image information, yellow image information) from the printer control device 2090 based on the light flux modulated for each color. The surface of the corresponding charged photosensitive drum is scanned. Thereby, a latent image corresponding to the image information is formed on the surface of each photosensitive drum. That is, here, each photosensitive drum is an image carrier, and the surface of each photosensitive drum is a surface to be scanned. The latent image formed here moves in the direction of the corresponding developing roller as the photosensitive drum rotates. The details of the optical scanning device 2010 will be described later.

  By the way, an area in which image information is written on each photosensitive drum is called an “image forming area” or an “effective image area”.

  As each developing roller rotates, toner from a corresponding toner cartridge (not shown) is thinly and uniformly applied to the surface thereof. Then, when the toner on the surface of each developing roller comes into contact with the surface of the corresponding photosensitive drum, the toner moves only to a portion irradiated with light on the surface and adheres to the surface. In other words, each developing roller causes toner to adhere to the latent image formed on the surface of the corresponding photosensitive drum so as to be visualized. Here, the toner-attached image (toner image) moves in the direction of the transfer belt 2040 as the photosensitive drum rotates.

  The yellow, magenta, cyan, and black toner images are sequentially transferred onto the transfer belt 2040 at a predetermined timing, and are superimposed to form a color image.

  Recording paper is stored in the paper feed tray 2060. A paper feed roller 2054 is disposed in the vicinity of the paper feed tray 2060. The paper feed roller 2054 takes out the recording paper one by one from the paper feed tray 2060. The recording paper is sent out toward the gap between the transfer belt 2040 and the transfer roller 2042 at a predetermined timing. As a result, the color image on the transfer belt 2040 is transferred to the recording paper. The recording paper on which the color image is transferred is sent to the fixing roller 2050.

  In the fixing roller 2050, heat and pressure are applied to the recording paper, whereby the toner is fixed on the recording paper. The recording paper on which the toner is fixed is sent to the paper discharge tray 2070 via the paper discharge roller 2058 and is sequentially stacked on the paper discharge tray 2070.

  Each cleaning unit removes toner (residual toner) remaining on the surface of the corresponding photosensitive drum. The surface of the photosensitive drum from which the residual toner has been removed returns to the position facing the corresponding charging device again.

  Next, details of the optical scanning device 2010 will be described.

  As shown in FIG. 2 as an example, the optical scanning device 2010 includes two light source units (LU1, LU2), a half mirror prism 2205, two cylindrical lenses (2204A, 2204B), and two reflecting mirrors (M1, M2). ), An optical deflector 2104, a scanning optical system A, a scanning optical system B, and a scanning control device (not shown). These are assembled at predetermined positions of the optical housing 2300 (not shown in FIG. 2, see FIG. 6).

  Here, in the XYZ three-dimensional orthogonal coordinate system, the direction along the longitudinal direction (rotation axis direction) of each photosensitive drum is described as the Y-axis direction, and the direction along the rotation axis direction of the optical deflector 2104 is described as the Z-axis direction. .

  In the following, for convenience, in each optical member, a direction corresponding to the main scanning direction is abbreviated as “main scanning corresponding direction”, and a direction corresponding to the sub scanning direction is abbreviated as “sub scanning corresponding direction”.

  The optical housing 2300 is provided with four slit-shaped exit windows (2111a, 2111b, 2111c, and 2111d) through which a light beam directed to each photosensitive drum passes (see FIG. 6). Each exit window is covered with dust-proof glass.

  As an example, the light source unit LU1 includes a light source 2200A, a coupling lens 2201A, an aperture plate 2203A, and the like, as shown in FIG.

  The light source 2200A is installed such that linearly polarized light whose polarization direction (vibration plane of the electric field vector) is parallel to the Z-axis direction is emitted. Hereinafter, for convenience, linearly polarized light whose polarization direction is parallel to the Z-axis direction is also referred to as “longitudinal polarization”. Specifically, the posture of the light source 2200A with respect to the circumference of the optical axis of the coupling lens 2201A may be adjusted so that the light beam emitted from the light source 2200A becomes longitudinally polarized light, or the light beam emitted from the light source 2200A is longitudinal. If the polarization is orthogonal to the polarization, a half-wave plate may be provided on the optical path of the light beam to convert it into longitudinal polarization.

  As an example, the light source unit LU2 includes a light source 2200B, a coupling lens 2201B, an aperture plate 2203B, and the like, as shown in FIG.

  The light source 2200B is installed such that linearly polarized light whose polarization direction (vibration plane of the electric field vector) is orthogonal to the Z-axis direction is emitted. Hereinafter, for the sake of convenience, linearly polarized light whose polarization direction is orthogonal to the Z-axis direction is also referred to as “lateral polarization”. Specifically, the attitude of the light source 2200B with respect to the optical axis of the coupling lens 2201B may be adjusted so that the light beam emitted from the light source 2200B is laterally polarized, or the light beam emitted from the light source 2200B is longitudinal. In the case of polarized light, a half-wave plate may be provided on the optical path of the light beam to convert it into laterally polarized light.

  Each coupling lens makes a light beam emitted from a corresponding light source a substantially parallel light beam.

  The aperture plate 2203A has an aperture and shapes the light beam that has passed through the coupling lens 2201A. A light beam that has passed through the opening of the aperture plate 2203A is a light beam emitted from the light source unit LU1.

  The aperture plate 2203B has an aperture and shapes the light beam that has passed through the coupling lens 2201B. A light beam that has passed through the opening of the aperture plate 2203B is a light beam emitted from the light source unit LU2.

  Hereinafter, the light beam emitted from the light source unit LU1 is also referred to as “light beam LB1”, and the light beam emitted from the light source unit LU2 is also referred to as “light beam LB2”.

  Returning to FIG. 2, the half mirror prism 2205 is arranged on the optical path of the light beam LB1 emitted from the light source unit LU1 and the light beam LB2 emitted from the light source unit LU2.

  The half mirror prism 2205 has a beam splitting surface with the same transmittance and reflectance for the incident light beam LB1 and light beam LB2. Here, the beam splitting surface is a so-called half mirror surface.

  Therefore, the light beam LB1 and the light beam LB2 are respectively divided by the half mirror prism 2205 with the same light intensity as the reflected light and the transmitted light. Hereinafter, the reflected light of the light beam LB1 divided by the half mirror prism 2205 is referred to as “light beam LBa”, the transmitted light of the light beam LB1 is referred to as “light beam LBd”, the transmitted light of the light beam LB2 is referred to as “light beam LBb”, and the reflected light of the light beam LB2. The light is referred to as “beam LBc” (see FIG. 5). Here, the optical paths of the light beam LBa and the light beam LBb are the same. The optical paths of the light beam LBc and the light beam LBd are also the same.

  The cylindrical lens 2204A is disposed on the optical path of the light beams LBa and LBb emitted from the half mirror prism 2205, and condenses each light beam in the sub-scanning corresponding direction (here, the same as the Z-axis direction). That is, the cylindrical lens 2204A is shared by the light beam LBa and the light beam LBb.

  The cylindrical lens 2204B is disposed on the optical path of the light beams LBc and LBd emitted from the half mirror prism 2205, and condenses each light beam in the sub-scanning corresponding direction (here, the same as the Z-axis direction). That is, the cylindrical lens 2204B is shared by the light beam LBc and the light beam LBd.

  The reflection mirror M1 bends the optical paths of the light beam LBa and the light beam LBb via the cylindrical lens 2204A in a direction toward the optical deflector 2104.

  The reflection mirror M2 bends the optical paths of the light beam LBc and the light beam LBd via the cylindrical lens 2204B in a direction toward the optical deflector 2104.

  In this case, line images of the light beam LBa and the light beam LBb are formed on the same deflection reflection surface of the optical deflector 2104. In addition, line images of the light beam LBc and the light beam LBd are formed on the same deflection reflection surface of the optical deflector 2104.

  The optical system disposed between each light source and the optical deflector 2104 is also called a pre-deflector optical system.

  The optical deflector 2104 has a rotating polygon mirror, and each mirror surface becomes a deflection reflection surface. The rotating polygon mirror rotates at a constant speed around an axis parallel to the Z-axis direction, and deflects the light beam LBa and the light beam LBb via the reflection mirror M1, and the light beam LBc and the light beam LBd via the reflection mirror M2 at an equal angular velocity. To do. Here, the rotating polygon mirror is a rotating four-sided mirror.

  The light beam LBa and the light beam LBb are incident on the deflection reflection surface located on the −X side with respect to the rotation axis of the optical deflector 2104, and the light beam LBc and the light beam LBd are deflected and reflected on the + X side with respect to the rotation axis. Incident on the surface.

  When orthogonally projected onto a plane orthogonal to the Z-axis direction, the optical paths of the light beams LBa and LBb and the optical paths of the light beams LBc and LBd are orthogonal to each other (see FIG. 2). Therefore, the light beam LBa and the light beam LBd do not simultaneously scan the image forming areas on the corresponding photosensitive drums. Similarly, the light beam LBb and the light beam LBc do not simultaneously scan the image forming areas on the corresponding photosensitive drums.

  As shown in FIG. 6 as an example, the scanning optical system A includes a scanning lens 2105A, a polarization separation element 2110A, and three folding mirrors (2106a, 2106b, 2108b).

  As shown in FIG. 6 as an example, the scanning optical system B includes a scanning lens 2105B, a polarization separation element 2110B, and three folding mirrors (2106c, 2106d, 2108c).

  The scanning lens 2105A is disposed on the optical path of the light beam LBa and the light beam LBb deflected by the optical deflector 2104. That is, the scanning lens 2105A is shared by the light beam LBa and the light beam LBb.

  The scanning lens 2105B is disposed on the optical path of the light beams LBc and LBd deflected by the optical deflector 2104. That is, the scanning lens 2105B is shared by the light beam LBc and the light beam LBd.

  Here, each scanning lens is a resin lens having an elongated optical surface. The resin is a resin that is transparent to incident light. Each scanning lens is formed by injecting the molten resin into a mold cavity and molding the resin into a predetermined shape. The effective scanning range of each scanning lens in the main scanning corresponding direction (here, the same as the Y-axis direction) is ± 50 mm.

  Hereinafter, in each scanning lens, a surface on which the light beam deflected by the optical deflector 2104 enters is also referred to as an “incident optical surface”, and a surface on which the light beam incident on the incident optical surface is emitted toward the polarization separation element is expressed as “ Also referred to as “emission optical surface”.

  Each polarization separation element has a polarization separation surface (for example, refer to JP 2010-134411 A) formed by a wire grid. This polarization separation surface reflects a polarization component parallel to the direction in which the wire is stretched and transmits a perpendicular polarization component. Here, the direction in which the wire is stretched is set to be parallel to the Y-axis direction. Therefore, in each polarization separation element, the horizontally polarized light is reflected and the vertically polarized light is transmitted. That is, laterally polarized light and longitudinally polarized light are separated into two directions orthogonal to each other in the XZ plane by each polarization separation element.

  An example of the polarization separation element is shown in FIGS. 7 (A) to 7 (C). The wire grid is a fine structure grating formed on a plate-like substrate and having a grating pitch smaller than the wavelength of incident light. FIG. 7C is a cross-sectional view taken along line AA in FIG.

  As an example, the grid pitch of the wire grid is selected to be 0.15 μm, the duty ratio (lattice width / lattice pitch) is 50%, and the depth of the grid is 0.05 μm. The wire material is selected from a highly conductive material such as aluminum, silver, or platinum. Further, a transparent material such as glass or hard plastic is selected as the substrate.

  The polarization separation element 2110A is disposed on the −X side of the scanning lens 2105A, transmits the light beam LBa via the scanning lens 2105A, and reflects the light beam LBb in the −Z direction.

  The polarization separation element 2110B is disposed on the + X side of the scanning lens 2105B, reflects the light beam LBc via the scanning lens 2105B in the −Z direction, and transmits the light beam LBd.

  The light beam LBa transmitted through the polarization separation element 2110A is guided to the surface of the photosensitive drum 2030a through the folding mirror 2106a and the exit window 2111a.

  The light beam LBb reflected in the −Z direction by the polarization separation element 2110A is guided to the surface of the photosensitive drum 2030b through the folding mirror 2106b, the folding mirror 2108b, and the exit window 2111b.

  The light beam LBc reflected in the −Z direction by the polarization separation element 2110B is guided to the surface of the photosensitive drum 2030c through the folding mirror 2106c, the folding mirror 2108c, and the exit window 2111c.

  The light beam LBd transmitted through the polarization separation element 2110B is guided to the surface of the photosensitive drum 2030d through the folding mirror 2106d and the exit window 2111d.

  By the way, as each scanning lens, a resin scanning lens (hereinafter abbreviated as “resin scanning lens”) is generally used. The resin scanning lens can be easily processed into an aspherical shape, can easily obtain desired optical performance, and can be manufactured at a lower cost than a glass scanning lens.

  However, the resin scanning lens has a disadvantage that birefringence is likely to occur.

  For example, when the incident light is circularly polarized light, it may be converted to elliptically polarized light after passing through the resin scanning lens. In this case, the reflectance characteristic of a folding mirror or the like disposed at the rear stage of the resin scanning lens changes, and “shading” in which the amount of light differs for each image height occurs on the surface to be scanned. As a specific example, as shown in FIG. 8, shading increases due to elliptical polarization instead of small shading when circularly polarized. Such an increase in shading is undesirable because it results in uneven density in the output image.

  Also, two light beams (first light beam and second light beam) in which the directions of linearly polarized light are orthogonal to each other are mixed and made incident on one resin scanning lens, and then the optical path of the two light beams is changed by a polarization separation element. In addition to the above shading, the optical scanning device that separates and collects the first light flux on the scanned surface A and collects the second light flux on the scanned surface B has the following disadvantages.

  For example, a first light flux whose linear polarization direction is parallel to the longitudinal direction of the resin scanning lens and a second light flux whose linear polarization direction is parallel to the short direction of the resin scanning lens are incident on the resin scanning lens. Consider the case. At this time, if birefringence is present in the resin scanning lens, the first light flux generates a polarization component in the short direction after passing through the resin scanning lens. If it is originally, all of the first light flux is transmitted through the polarization separation element. As expected, a part of the light is reflected by the polarization separation element, and the reflected first light beam is condensed as stray light on the surface to be scanned B to form an undesired ghost image. Similarly, in the second light beam, a polarization component in the longitudinal direction is generated. Originally, all of the second light beam is supposed to be reflected by the polarization separation element, but a part thereof is transmitted through the polarization separation element. The second light flux thus formed becomes stray light and is condensed on the surface A to be scanned, thereby forming an undesirable ghost image.

  In the resin scanning lens, four balances of lens volume, molding time, cost, and remaining birefringence must be considered. The method for molding a plastic molded article disclosed in Patent Document 1 is intended to reduce the birefringence of a resin scanning lens. However, if a resin scanning lens having a large volume is to be molded, the residual birefringence is reduced. In order to reduce it, it is necessary to lengthen the molding time. However, if the molding time is lengthened, the number of molded resin scanning lenses per unit time is reduced, so an increase in cost cannot be avoided.

  Further, in order to adjust the reflectance characteristics of the folding mirror, it is necessary to vary the film thickness of the dielectric multilayer film according to the incident position of light in the folding mirror. For this purpose, a complicated process has to be included in the vapor deposition process, which greatly increases the working time and increases the cost.

  Further, the optical scanning device disclosed in Patent Document 2 is a thin optical scanning device using polarization separation, and even if ghost light (stray light) is generated due to birefringence, the ghost is obtained by using oblique incidence. Light (stray light) is prevented from reaching the photosensitive drum (scanned surface). In this optical scanning device, it is necessary to increase the oblique incident angle in order to reliably remove stray light from the image forming area on the surface to be scanned. However, when a light beam having a large oblique incident angle enters the scanning lens, the scanning line is inevitably generated on the surface to be scanned. Further, the increase in wavefront aberration is increased, and there arises a disadvantage that the writing light beam (signal light beam) is not collected on the scanned surface. In addition, the oblique incidence increases the height of the optical housing in which the scanning optical system is accommodated, resulting in a disadvantage that the optical housing is greatly hindered in size reduction.

  As an example, FIG. 9 shows a case where longitudinally polarized light deflected by an optical deflector becomes elliptically polarized light when emitted from a resin scanning lens. Here, for easy understanding, the direction along the rotation axis of the optical deflector is the z-axis direction, the main scanning corresponding direction is the y-axis direction, and the direction orthogonal to any of the z-axis direction and the y-axis direction. A local coordinate system with x in the x-axis direction is used.

  The degree of ellipse in the elliptically polarized light varies depending on the material and shape of the resin scanning lens and the angle φ. Here, the angle φ is an angle formed between the traveling direction of the light emitted from the resin scanning lens and the x-axis direction.

  Birefringence in a birefringent material is often expressed by a refractive index difference, a phase difference δ, and a principal axis direction θ. Here, the principal axis direction θ of birefringence in the resin scanning lens is positive in the clockwise angle with respect to the + z direction. However, −180 ° ≦ θ ≦ 180 °. The phase difference δ is also set to −180 ° ≦ δ ≦ 180 °. When this range is exceeded, 360 × m (m is an arbitrary integer) is added or subtracted and adjusted so as to be within the above range.

  As shown in FIG. 9, when a polarization separation element is provided in the subsequent stage of the resin scanning lens, if the polarization state of light changes by passing through the resin scanning lens, desired polarization separation is performed in the polarization separation element. It becomes difficult to do. For example, in the case of FIG. 9, the y-direction component of elliptically polarized light is a polarization separation failure. This component is hereinafter referred to as “stray light”. The light intensity of stray light with respect to the light intensity incident on the polarization separation element is referred to as “stray light ratio”.

  Next, each scanning lens in this embodiment will be described. Note that since the scanning lens 2105A and the scanning lens 2105B have the same shape, the scanning lens 2105B will be described as a representative here.

  By the way, the position of the scanning lens in the main scanning corresponding direction (here, the same as the Y-axis direction) is called “lens height”, and the center of the scanning lens is set to zero.

  As an example, the scanning lens 2105B has a main body 101 and a fine structure 102 as shown in FIG.

  First, the main axis direction θ and the retardation R were measured using the analyzer rotation method for only the main body 101 (without the fine structure 102), that is, for the main body 101 alone. Note that the wavelength (hereinafter abbreviated as “measurement wavelength”) λ of light used for measurement is 655 nm.

  FIG. 11 shows the measurement result of the relationship between the principal axis direction θ and the lens height H. The main axis direction θ is within ± 4 °. Here, as an example, using a scanning lens birefringence measuring apparatus (see Norihiro Morita, “Development of a scanning lens birefringence measuring apparatus”, Ricoh Technical Report, NOVEMBER, 2000, No. 26, p. 115-120), the main axis direction θ Was measured. In addition, the measuring method of principal axis direction (theta) is not limited to this.

FIG. 12 shows the measurement result of the relationship between the retardation R and the lens height H. The retardation R tends to increase near the center of the main body 101, but is within 110 nm. The relationship between the retardation R and the phase difference δ _r in the main body 101 can be expressed by the following equation (1) using the measurement wavelength λ.

FIG. 13 shows the measurement result of the relationship between the stray light ratio Pg and the lens height H when linearly polarized light is incident on the main body 101 alone. Incidentally, the stray light ratio Pg is dependent on the size of the principal axis directions θ and the phase difference [delta] _r.

  The fine structure portion 102 is formed by arranging a plurality of rectangular parallelepipeds at intervals shorter than the wavelength of incident light, and is formed integrally with the main body portion 101. Here, as an example, as shown in FIG. 14, the height of the rectangular parallelepiped is D, the thickness of the rectangular parallelepiped is W, and the length of the rectangular parallelepiped is L. L> W. That is, the longitudinal direction of the rectangular parallelepiped is the longitudinal direction of the rectangular parallelepiped. The refractive index of the rectangular parallelepiped material is n.

  The plurality of rectangular parallelepipeds are formed such that their length directions (longitudinal directions) are the same as the main axis direction θ of the main body 101 based on the measurement result of FIG. 11. In addition, the length direction of the rectangular parallelepiped at this time is also referred to as “TE direction”, and the thickness direction is also referred to as “TM direction”. As an example, as shown in FIG. 15, P is an arrangement pitch of a plurality of rectangular parallelepipeds in the TM direction. Therefore, the interval between two adjacent rectangular parallelepipeds is PW.

Incidentally, the effective refractive index n _TE in the TE direction in the fine structure portion 102 can be estimated by the following equation (2).

In addition, the effective refractive index n _TM in the TM direction in the microstructure 102 can be estimated by the following equation (3).

  Note that f in the above equations (2) and (3) is called a filling factor, and is obtained by the following equation (4).

Further, the phase difference δ _s in the fine structure 102 with respect to the wavelength λ of the incident light can be obtained by the following equation (5).

Accordingly, based on the retardation R of the main body 101 measured as shown in FIG. 11, the phase difference δ _r is calculated for each lens height H using the above equation (1), and the following ( By determining the fine structure 102 so that the expression 6) is satisfied, it is possible to suppress the change in the polarization state of light when passing through the scanning lens 2105B that is a resin scanning lens.

FIG 16, L = 100μm, P = 200μm, n = 1.52583 and then, the relationship between the stray light ratio _{P g} and the lens height H when changing according the cuboid height D to the lens height H The measurement results are shown. Compared with FIG. 13, it can be seen that the stray light ratio is effectively reduced.

Since the phase difference δ _r can be adjusted by changing the filling factor f, for example, the thickness W of the rectangular parallelepiped may be changed according to the lens height H, and the arrangement pitch P of the rectangular parallelepiped may be set. It may be changed according to the lens height H. However, adjusting the height D of the rectangular parallelepiped (1) makes it easier to process the mold for molding. (2) Since the sensitivity is high, the above equation (6) should be satisfied with a small adjustment amount. There is an advantage that you can.

  Further, the scanning lens may be divided into a plurality of regions with respect to the Y-axis direction, and the longitudinal directions of the plurality of rectangular parallelepipeds may be aligned within the region. FIG. 17 shows a case where the area is divided into 16 areas (area A to area P) as an example. In this case, the length of each region in the Y-axis direction is 6.25 mm. And based on FIG. 11, the average value of the principal axis direction was calculated | required for every area | region, and it was set as the longitudinal direction of the rectangular parallelepiped in each area | region (refer FIG. 18). Here, the longitudinal direction is defined as + (plus) when inclined to the right side with respect to the −Z direction, and − (minus) when inclined in the left direction (see FIG. 19). FIG. 20 is relatively illustrated for easy understanding of an example of a plurality of rectangular parallelepipeds in each region, and the longitudinal direction is not strict.

Further, the phase difference depending on the [delta] _r levels in the body portion 101, rectangular arrangement pitch, of rectangular thickness may not vary continuously in accordance with the cuboid height lens height H. For example, when the birefringence of the main body 101 is suppressed as much as possible by molding under appropriate molding conditions and the remainder is suppressed by the fine structure 102, the optical surface of the main body 101 is divided into several areas. In the first area, the arrangement pitch of the rectangular parallelepiped is P ₁ , the thickness of the rectangular parallelepiped is W ₁ , and the height of the rectangular parallelepiped is D _{1. In the} second area, the arrangement pitch of the rectangular parallelepiped is P ₂ , and the thickness of the rectangular parallelepiped. W ₂ , and the height of the rectangular parallelepiped is D _{2. In} each area, the arrangement pitch of the rectangular parallelepiped, the thickness of the rectangular parallelepiped, and the height of the rectangular parallelepiped are the same, and the arrangement pitch of the rectangular parallelepiped for each area, the thickness of the rectangular parallelepiped Even if the height of the rectangular parallelepiped is changed, a desired effect can be obtained. In this case, any one of the arrangement pitch of the rectangular parallelepiped, the thickness of the rectangular parallelepiped, and the height of the rectangular parallelepiped may be changed for each area.

  If a plurality of rectangular parallelepipeds of the fine structure portion 102 are formed in advance on the cavity surface of a mold used for injection molding of the scanning lens, they are transferred to the resin molding in the injection molding process. This is simpler than applying a film or the like in a separate process. Moreover, there are fewer types of dielectric multilayer materials suitable for resins compared to dielectric multilayer materials suitable for glass, and because of the difference in linear expansion coefficient, the change in environmental temperature after forming the dielectric multilayer film There is a concern that the dielectric multilayer film may crack due to the influence. Therefore, the present embodiment in which the fine structure portion 102 is integrally formed with the main body portion 101 can provide a stable scanning lens.

  Furthermore, compared with the case where the phase compensation plate is provided separately from the scanning lens, in this embodiment, the alignment process at the time of assembly is not required.

  By the way, as shown in FIG. 21 as an example, when the main axis azimuth θ of the main body 101 alone is substantially constant regardless of the lens height H, as shown in FIG. The longitudinal directions of all the rectangular parallelepipeds may be aligned.

  As a method for confirming the presence or absence of the fine structure portion, there is the following method. For example, in a scanning lens in which the fine structure 102 is provided in the main body 101, the polarization state is ideally unchanged for incident light and emitted light. However, when the scanning lens is immersed in a liquid (matching oil) having the same refractive index as that of the scanning lens, the effect of the fine structure portion 102 is relatively lost, so that it is affected by the birefringence existing in the main body portion 101. Thus, the polarization state of the emitted light is different from the polarization state of the incident light. That is, it can be confirmed that the fine structure 102 is applied to the scanning lens if the polarization state of the incident light and the emitted light changes with or without matching oil.

  In the case of a spherical lens, birefringence occurs as shown in FIG. 23 as an example because of the molding die. In this case, it is difficult to suppress the change in the polarization state of the light after transmission in the fine structure portion. On the other hand, in the case of a long lens (long lens), the principal axis directions are easily aligned as shown in FIGS. 24A and 24B as an example. In this case, it is possible to suppress a change in the polarization state of the light after transmission through the fine structure portion.

  As described above, the optical scanning device 2010 according to the present embodiment includes the two light source units (LU1, LU2), the half mirror prism 2205, the two cylindrical lenses (2204A, 2204B), and the two reflection mirrors (M1, M2). ), An optical deflector 2104, a scanning optical system A, a scanning optical system B, and a scanning control device.

  Each scanning optical system includes a resin scanning lens, a polarization separation element, and a plurality of folding mirrors.

  Resin-made scanning lenses tend to generate birefringence. In this case, when light enters the scanning lens, light having a polarization state different from the polarization state of the incident light may be emitted from the scanning lens due to birefringence. That is, the polarization state of light may change by passing through the scanning lens. For example, a phenomenon occurs in which linearly polarized light changes to elliptically polarized light or the polarization direction rotates.

  In the present embodiment, the scanning lens of each scanning optical system has a main body 101 and a fine structure 102. This fine structure portion 102 includes a plurality of rectangular parallelepipeds whose longitudinal direction coincides with the principal axis direction of birefringence in the main body portion 101. In this case, the polarization state of the light hardly changes even when passing through the scanning lens. The main body 101 and the fine structure 102 are integrally formed. Therefore, it is possible to realize a resin scanning lens that can suppress a change in the polarization state of light after transmission without increasing the cost.

  Then, since the polarization state of the light hardly changes even after passing through the scanning lens, the desired light can be separated by the polarization separation element arranged at the subsequent stage of the scanning lens. As a result, the optical scanning device 2010 can suppress the generation of stray light and can form a desired latent image on each photosensitive drum.

  In each scanning optical system, since the scanning lens and the polarization separation element are shared by the two image forming stations, the number of parts can be reduced and the optical scanning device can be downsized. Therefore, generation of stray light can be stably suppressed without increasing the cost and size.

  In the present embodiment, since a special incident method such as a so-called oblique incidence method is not used, so-called beam thickening on the surface to be scanned and shading characteristics can be improved. That is, it is possible to suppress degradation of the image quality.

  The color printer 2000 according to the present embodiment includes the optical scanning device 2010. As a result, it is possible to form a small and high quality image without increasing the cost.

  In the above embodiment, the case where the fine structure portion 102 is provided on the incident optical surface of the scanning lens has been described. However, the present invention is not limited to this. For example, as shown in FIG. 25, the fine structure 102 may be provided on the exit optical surface of the scanning lens. Further, as shown in FIG. 26, the fine structure 102 may be provided on both the incident optical surface and the exit optical surface of the scanning lens.

  In the above embodiment, each scanning optical system may include a plurality of scanning lenses.

  Moreover, although the said embodiment demonstrated the case where the fine structure part 102 was provided in a scanning lens, it is not limited to this. For example, when each scanning optical system includes a resin optical element having no optical power, the fine structure portion 102 may be provided in the optical element.

  Moreover, in the said embodiment, it replaces with a rectangular parallelepiped and a triangular prism may be used as shown in FIG. 27 as an example. In short, it may be a columnar body having a longitudinal direction.

  In the above embodiment, instead of one rectangular parallelepiped, a rectangular parallelepiped row in which a plurality of small rectangular parallelepipeds are arranged along the TE direction may be used as shown in FIG. 28 as an example. In this case, the plurality of rectangular parallelepipeds constituting the rectangular parallelepiped row may have the same longitudinal direction or may have different longitudinal directions.

  Moreover, in the said embodiment, each light source may have a some light emission part. In this case, it may be realized by a plurality of semiconductor lasers, or may be realized by one semiconductor laser array.

  In the above embodiment, the toner image is transferred from the photosensitive drum to the recording paper via the transfer belt. However, the present invention is not limited to this, and the toner image is transferred from the photosensitive drum to the recording paper. It may be directly transferred.

  Further, an image forming apparatus using a color developing medium (positive photographic paper) that develops color by the thermal energy of a light spot as an image carrier may be used. In this case, a visible image can be directly formed on the image carrier by optical scanning.

  In the above embodiment, the case where the optical scanning device 2010 is used in a printer has been described. However, the present invention is also suitable for an image forming apparatus other than a printer, for example, a copier, a facsimile, or a multifunction machine in which these are integrated. .

  DESCRIPTION OF SYMBOLS 101 ... Main-body part, 102 ... Fine structure part, 2000 ... Color printer (image forming apparatus), 2010 ... Optical scanning apparatus, 2030a-2030d ... Photosensitive drum (image carrier), 2104 ... Optical deflector, 2105A, 2105B ... Scanning lenses (resin-based optical elements), 2106a to 2106d ... Folding mirrors (part of the scanning optical system), 2108b, 2108c ... Folding mirrors (part of the scanning optical system), 2110A, 2110B ... Polarization separation elements (polarization separation) Means), 2111a to 2111d ... exit window, 2200A, 2200B ... light source, 2201A, 2201B ... coupling lens, 2203A, 2203B ... aperture plate, 2204A, 2204B ... cylindrical lens, 2205 ... half mirror prism, 2300 ... optical housing, LU1 , LU2 ... Light source unit Doo (light source device), M1, M2 ... reflecting mirror.

JP 2011-140148 A JP 2011-221493 A

Claims (10)

A resinous optical element,
A main body portion, and a fine structure portion provided on at least one of an incident-side optical surface and an emission-side optical surface in the main body portion,
The fine structure portion includes a plurality of columnar bodies, and at least a part of the plurality of columnar bodies has a longitudinal direction coinciding with a principal axis direction of birefringence in the main body portion.   2. The optical element according to claim 1, wherein the at least some of the plurality of columnar bodies are arranged at an interval shorter than a wavelength of incident light. In at least a part of the plurality of columnar bodies, δ _{s +} δ _r = 2mπ is satisfied using the phase difference δ _r in the main body portion, the phase difference δ _s in the fine structure portion, and an integer m. The optical element according to claim 1 or 2.   The optical element according to any one of claims 1 to 3, wherein the at least some of the plurality of columnar bodies have different heights depending on a phase difference in the main body portion.   The optical element according to any one of claims 1 to 4, wherein the plurality of columnar bodies are aligned in a longitudinal direction.   The optical element according to any one of claims 1 to 5, wherein the columnar body is a rectangular parallelepiped. A scanning optical system including an optical element disposed on an optical path of light deflected by an optical deflector,
A scanning optical system, wherein the optical element is the optical element according to claim 1. Two light beams having different polarization directions are incident on the optical element through the same optical path,
The scanning optical system according to claim 7, further comprising polarization separation means for separating an optical path of the two lights via the optical element. A light source device;
An optical deflector for deflecting light from the light source device;
9. An optical scanning apparatus comprising: the scanning optical system according to claim 7 or 8, wherein the light deflected by the optical deflector is guided to a surface to be scanned. An image carrier;
An image forming apparatus comprising: the optical scanning device according to claim 9, wherein the image carrier is scanned with light modulated in accordance with image information.

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