JP5842699B2 - Optical scanning apparatus and image forming apparatus - Google Patents

Description

  The present invention relates to an optical scanning apparatus and an image forming apparatus, and more particularly to an optical scanning apparatus that scans a surface to be scanned with light and an image forming apparatus that includes the optical scanning apparatus.

  In electrophotographic image recording, an image forming apparatus using a laser is widely used. The image forming apparatus includes a photosensitive drum (hereinafter also referred to as “photosensitive drum”), an optical scanning device that forms a latent image on the surface of the photosensitive drum, and the like. The optical scanning device includes a light source that emits laser light, an optical deflector that deflects the laser light emitted from the light source (for example, a polygon mirror), and laser light deflected by the optical deflector on the surface of the photosensitive drum. It has a scanning optical system that collects light.

  In recent years, colorization and speeding-up have progressed in image forming apparatuses, and tandem type image forming apparatuses having a plurality of (usually four) photosensitive drums have become widespread.

  Tandem-type image forming apparatuses tend to increase in size as the number of photosensitive drums increases, and downsizing is required including optical scanning apparatuses. In order to reduce the size of the optical scanning device, it is effective to partially overlap the optical paths of a plurality of laser beams from the optical deflector to each photosensitive drum (see, for example, Patent Document 1 and Patent Document 2).

  However, the conventional optical scanning device may generate ghost light during polarization separation.

  The present invention is an optical scanning device that individually scans a plurality of scanned surfaces along one direction with light, and includes a first light flux corresponding to two different scanned surfaces in the plurality of scanned surfaces, and A light source device that emits a second light beam, a pre-deflector optical system that emits the first light beam and the second light beam from the light source device as circularly polarized light having different rotation directions, and a rotation axis A plurality of reflective surfaces, and the first and second light beams through the pre-deflector optical system are directed to the same reflective surface from a direction inclined with respect to the surface orthogonal to the rotation axis. An optical deflector that enters and deflects the first and second light fluxes, and a polarization separation element that separates the first and second light fluxes deflected by the optical deflector, The first light flux and the second light flux are individually collected on the corresponding scanned surface. And 査光 science system, an optical scanning device comprising a.

  According to the optical scanning device of the present invention, it is possible to suppress the generation of ghost light during polarization separation.

1 is a diagram illustrating a schematic configuration of a color printer according to an embodiment of the present invention. FIG. 2 is a diagram (part 1) for describing the optical scanning device in FIG. 1; FIG. 3 is a second diagram for explaining the optical scanning device in FIG. 1; It is a figure for demonstrating light source unit LU1 in FIG. It is a figure for demonstrating the light source 10a in light source unit LU1. It is a figure for demonstrating the light source 10b in light source unit LU1. It is a figure for demonstrating the positional relationship of the light source 10a and the light source 10b. It is a figure for demonstrating the function of the quarter wave plate 12A. It is a figure for demonstrating light source unit LU2 in FIG. It is a figure for demonstrating the light source 10c in light source unit LU2. It is a figure for demonstrating the light source 10d in light source unit LU2. It is a figure for demonstrating the positional relationship of the light source 10c and the light source 10d. It is a figure for demonstrating the function of the quarter wavelength plate 12B. It is a figure for demonstrating the light beam LBa and the light beam LBb which inject into a polygon mirror. It is a figure for demonstrating the light beam LBc and the light beam LBd which inject into a polygon mirror. FIGS. 16A to 16C are diagrams for explaining the polarization state of the light beam LBa deflected by the polygon mirror. FIGS. 17A to 17C are diagrams for explaining the polarization states of the light beam LBb deflected by the polygon mirror. 18A to 18C are diagrams for explaining the polarization states of the light beam LBc deflected by the polygon mirror. FIGS. 19A to 19C are diagrams for explaining the polarization state of the light beam LBd deflected by the polygon mirror. It is a figure for demonstrating the optical path of the light beam LBa deflected by the polygon mirror, and the light beam LBb. It is a figure for demonstrating the optical path of the light beam LBc deflected by the polygon mirror, and the light beam LBd. It is a figure for demonstrating the structure of a polarization separation element. It is a figure for demonstrating the cholesteric liquid crystal layer in a polarization separation element. It is a figure for demonstrating the relationship between the incident angle and spiral pitch in a cholesteric liquid crystal layer. It is a figure for demonstrating an effect | action of the polarization splitting element 16A when clockwise circularly polarized light injects into the polarization splitting element 16A. It is a figure for demonstrating the effect | action of the polarization separation element 16A when counterclockwise circularly polarized light injects into the polarization separation element 16A. It is a figure for demonstrating the effect | action of the polarization separation element 16B when clockwise circularly polarized light injects into the polarization separation element 16B. It is a figure for demonstrating an effect | action of the polarization separation element 16B when counterclockwise circularly polarized light injects into the polarization separation element 16B. It is FIG. (1) for demonstrating the modification of the polarization separation element 16A. It is FIG. (2) for demonstrating the modification of the polarization splitting element 16A. It is a figure for demonstrating rotation of an entrance plane. It is a figure for demonstrating the principal axis of the quarter wavelength plate in a modification. It is FIG. (1) for demonstrating the modification of the polarization splitting element 16B. It is FIG. (2) for demonstrating the modification of the polarization splitting element 16B. It is a figure for demonstrating the modification of an optical scanning device. It is a figure for demonstrating light source unit LU3 in FIG. It is a figure for demonstrating light source unit LU4 in FIG. It is a figure for demonstrating the positional relationship of light source 10A and light source 10B. It is a figure for demonstrating the scanning optical system in the optical scanning device of a modification. It is FIG. (1) for demonstrating the optical path of the two light beams reflected by the different deflection | deviation reflective surface of a polygon mirror. It is FIG. (2) for demonstrating the optical path of the two light beams reflected by the different deflection | deviation reflective surface of a polygon mirror. It is FIG. (3) for demonstrating the optical path of the two light beams reflected by the different deflection | deviation reflective surface of a polygon mirror.

  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 described in a code decodable by the CPU, a ROM storing various data used when executing the program, a RAM that is a working memory, an amplification circuit And an A / D converter for converting analog data into digital data. Then, the printer control device 2090 sends 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. That is, the surface of each photoconductive drum is a surface to be scanned. 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 is charged correspondingly by a light beam modulated for each color based on multi-color image information (black image information, cyan image information, magenta image information, yellow image information) from the printer control device 2090. Each surface of the photosensitive drum is scanned. Thereby, a latent image corresponding to the image information is formed on the surface of each photosensitive drum. The latent image formed here moves in the direction of the corresponding developing roller as the photosensitive drum rotates. The configuration of the optical scanning device 2010 will be described later.

  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 toner image on the transfer belt 2040 is transferred to the recording paper. The recording paper on which the toner 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, the configuration of the optical scanning device 2010 will be described.

  As shown in FIG. 2 and FIG. 3 as an example, the optical scanning device 2010 includes two light source units (LU1, LU2), two cylindrical lenses (22A, 22B), a polygon mirror 14, and two scanning lenses (15A). 15B), two polarization separation elements (16A, 16B), two reflection mirrors (17A, 17B), four folding mirrors (18a, 18b, 18c, 18d), a scanning control device (not shown), etc. ing.

  Here, in the XYZ three-dimensional orthogonal coordinate system, the direction along the longitudinal direction (rotational axis direction) of each photosensitive drum is defined as the Y-axis direction, and the direction along the rotational axis of the polygon mirror 14 is defined as the Z-axis direction. . In the following, for convenience, the direction corresponding to the main scanning direction of each optical member is abbreviated as “main scanning corresponding direction”, and the direction corresponding to the sub scanning direction is abbreviated as “sub scanning corresponding direction”.

  As shown in FIG. 4 as an example, the light source unit LU1 includes two light sources (10a, 10b), two coupling lenses (11a, 11b), a light combining element 13A, a quarter-wave plate 12A, and the like. Yes. The light source 10a and the light source 10b are each mounted on a circuit board.

  The light source 10a includes one semiconductor laser 101a as shown in FIG. 5 as an example. The installation angle of the semiconductor laser 101a is adjusted so that linearly polarized light whose polarization direction 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 referred to as “longitudinal polarization”. The vertically polarized light emitted from the semiconductor laser 101a is denoted as a light beam LBa.

  The light source 10b includes one semiconductor laser 101b as shown in FIG. 6 as an example. The semiconductor laser 101b has an installation angle adjusted so that linearly polarized light whose polarization direction is orthogonal to the Z axis is emitted. Hereinafter, linearly polarized light whose polarization direction is orthogonal to the Z axis is referred to as “laterally polarized light” for convenience. The laterally polarized light emitted from the semiconductor laser 101b is denoted as a light beam LBb.

  Instead of adjusting the installation angle of the semiconductor laser 101a, a ½ wavelength for changing the polarization direction of the light emitted from the semiconductor laser 101a to the direction of longitudinal polarization between the semiconductor laser 101a and the light combining element 13A. An optical element such as a plate may be arranged. Similarly, instead of adjusting the installation angle of the semiconductor laser 101b, a 1/2 for changing the polarization direction of the light emitted from the semiconductor laser 101b between the semiconductor laser 101b and the light combining element 13A to the direction of lateral polarization. An optical element such as a wave plate may be arranged.

  Returning to FIG. 4, the coupling lens 11a is disposed on the optical path of the light beam LBa from the light source 10a, and makes the light beam LBa substantially parallel light.

  The coupling lens 11b is disposed on the optical path of the light beam LBb from the light source 10b, and makes the light beam LBb substantially parallel light.

  The light combining element 13A is disposed on the optical path of the light beam LBa via the coupling lens 11a and the light beam LBb via the coupling lens 11b. The light combining element 13A has a combining surface that reflects vertically polarized light and transmits horizontally polarized light. Therefore, the light beam LBa is reflected by the composite surface, and the light beam LBb passes through the composite surface. That is, the light combining element 13A combines the light beam LBa and the light beam LBb.

  Here, as shown in FIG. 7 as an example, the light sources and the coupling lenses are arranged so that the light beams LBa and LBb emitted from the light combining element 13A are separated from each other by a predetermined distance at least in the Z-axis direction. Has been.

  The quarter wavelength plate 12A is disposed on the optical path of the light beams LBa and LBb emitted from the light combining element 13A, and converts the polarization state of each light beam from linearly polarized light to circularly polarized light. The direction of the main axis (fast axis) of the quarter-wave plate 12A is set to a direction rotated 45 ° counterclockwise with respect to the + Z direction in a plane orthogonal to the traveling direction of the light beam. In this case, the vertically polarized light beam LBa is converted into clockwise circularly polarized light, and the horizontally polarized light beam LBb is converted into counterclockwise circularly polarized light (see FIG. 8).

  A light beam LBa and a light beam LBb emitted from the quarter-wave plate 12A are emitted from the light source unit LU1.

  As shown in FIG. 9 as an example, the light source unit LU2 includes two light sources (10c, 10d), two coupling lenses (11c, 11d), a light combining element 13B, a quarter wavelength plate 12B, and the like. Yes. The light source 10c and the light source 10d are each mounted on a circuit board.

  As an example, the light source 10c includes one semiconductor laser 101c as shown in FIG. The installation angle of the semiconductor laser 101c is adjusted so that linearly polarized light (transversely polarized light) whose polarization direction is orthogonal to the Z axis is emitted. Hereinafter, the laterally polarized light emitted from the semiconductor laser 101c is referred to as a light beam LBc.

  As an example, the light source 10d includes one semiconductor laser 101d as shown in FIG. The installation angle of the semiconductor laser 101d is adjusted so that linearly polarized light (longitudinal polarized light) whose polarization direction is parallel to the Z-axis direction is emitted. Hereinafter, the longitudinally polarized light emitted from the semiconductor laser 101d is referred to as a light beam LBd.

  Instead of adjusting the installation angle of the semiconductor laser 101c, a ½ wavelength for changing the polarization direction of the light emitted from the semiconductor laser 101c between the semiconductor laser 101c and the light combining element 13B to the direction of lateral polarization is used. An optical element such as a plate may be arranged. Similarly, instead of adjusting the installation angle of the semiconductor laser 101d, 1/2 is applied between the semiconductor laser 101d and the light combining element 13B to change the polarization direction of the light emitted from the semiconductor laser 101d to the direction of longitudinal polarization. An optical element such as a wave plate may be arranged.

  Returning to FIG. 9, the coupling lens 11c is disposed on the optical path of the light beam LBc from the light source 10c, and makes the light beam LBc substantially parallel light.

  The coupling lens 11d is disposed on the optical path of the light beam LBd from the light source 10d, and makes the light beam LBd substantially parallel light.

  The light combining element 13B is disposed on the optical path of the light beam LBc via the coupling lens 11c and the light beam LBd via the coupling lens 11d. The light combining element 13B has a combining surface that reflects vertically polarized light and transmits horizontally polarized light. Therefore, the light beam LBc is transmitted through the combined surface, and the light beam LBd is reflected by the combined surface. That is, the light combining element 13B combines the light beam LBc and the light beam LBd.

  Here, as shown in FIG. 12 as an example, the light sources and the coupling lenses are arranged so that the light beams LBc and LBd emitted from the light combining element 13B are separated from each other by a predetermined distance at least in the Z-axis direction. Has been.

  The quarter wave plate 12B is disposed on the optical path of the light beams LBc and LBd emitted from the light combining element 13B, and converts the polarization state of each light beam from linearly polarized light to circularly polarized light. The direction of the principal axis (fast axis) of the quarter-wave plate 12B is set to a direction rotated 45 ° counterclockwise with respect to the + Z direction in a plane orthogonal to the traveling direction of the light beam. In this case, the light beam LBc that is laterally polarized light is converted into counterclockwise circularly polarized light, and the light beam LBd that is vertically polarized light is converted into clockwise circularly polarized light (see FIG. 13).

  A light beam LBc and a light beam LBd emitted from the quarter-wave plate 12B are emitted from the light source unit LU2.

  Returning to FIG. 2, the cylindrical lens 22 </ b> A focuses the light beam LBa and the light beam LBb emitted from the light source unit LU <b> 1 in the vicinity of the deflection reflection surface of the polygon mirror 14 in the Z-axis direction. Here, the light beam LBa and the light beam LBb are separated from each other at least in the Z-axis direction and enter the cylindrical lens 22A. The optical path of the light beam LBa emitted from the cylindrical lens 22A and the optical path of the light beam LBb are non-parallel with respect to the Z-axis direction (see FIG. 14).

  The cylindrical lens 22B forms an image of the light beam LBc and the light beam LBd emitted from the light source unit LU2 in the vicinity of the deflection reflection surface of the polygon mirror 14 in the Z-axis direction. Here, the light beam LBc and the light beam LBd are separated from each other at least in the Z-axis direction and enter the cylindrical lens 22B. The optical path of the light beam LBc emitted from the cylindrical lens 22B is not parallel to the optical path of the light beam LBd (see FIG. 15).

  In the following, when the light beam is incident on the deflecting / reflecting surface, it is referred to as “oblique incidence” to be incident from a direction inclined with respect to the surface orthogonal to the rotation axis of the polygon mirror, and orthogonal to the rotation axis of the polygon mirror. Incident from a direction parallel to the surface to be projected is called “horizontal incidence”. The incident angle at the time of oblique incidence is referred to as “oblique incidence angle”. That is, in this embodiment, each light beam is incident obliquely on the deflecting / reflecting surface.

  By the way, an optical system arranged between each light source and the polygon mirror 14 is called a “pre-deflector optical system”.

  The polygon mirror 14 has a four-sided mirror as an example, and each mirror serves as a deflection reflection surface. The polygon mirror 14 rotates at a constant speed around an axis parallel to the Z-axis direction, and deflects the light beam from each cylindrical lens at a constant angular velocity. The angle (acute angle side) formed by the traveling direction of the light beam deflected by the deflecting reflecting surface and the X-axis direction is called a “deflection angle”.

  The light beam LBa and the light beam LBb are separated from each other at least in the Z-axis direction and enter the same deflecting / reflecting surface, and both are deflected to the −X side of the polygon mirror 14. The light beam LBc and the light beam LBd are separated from each other at least in the Z-axis direction and are incident on the same deflection reflection surface, and both are deflected to the + X side of the polygon mirror 14.

  Each light beam incident on the polygon mirror 14 changes its direction of emission from the polygon mirror 14 as the polygon mirror 14 rotates, but the difference in reflectance between the p-polarized light and the s-polarized light on the deflecting reflecting surface, and the deflecting reflecting surface. When the phase shift between the p-polarized light and the s-polarized light at the time of reflection at can be ignored, the polarization direction of each light beam is the direction reversed with respect to the incident light regardless of the exit direction.

  Therefore, the light beam LBa deflected by the polygon mirror 14 becomes counterclockwise circularly polarized light as shown in FIGS. 16A to 16C, and the light beam LBb is converted into FIGS. 17A to 17C. ) As shown in FIG.

  Further, the light beam LBc deflected by the polygon mirror 14 becomes clockwise circularly polarized light as shown in FIGS. 18A to 18C, and the light beam LBd is converted into FIGS. 19A to 19C. ) Becomes counterclockwise circularly polarized light.

  The emission positions of the light beam LBa and the light beam LBb at the polygon mirror 14 are different from each other in the sub-scanning corresponding direction (here, the same as the Z-axis direction). Further, the traveling directions of the light beams LBa and LBb deflected by the polygon mirror 14 are not parallel to each other.

  Returning to FIG. 2, the scanning lens 15 </ b> A is arranged on the −X side of the polygon mirror 14 and on the optical paths of the light beams LBa and LBb deflected by the polygon mirror 14.

  The polarization separation element 16A is disposed on the optical path of the light beam via the scanning lens 15A. The polarization separation element 16A transmits or reflects incident light according to the rotation direction of the circularly polarized light. Here, the polarization separation element 16A is set to transmit counterclockwise circularly polarized light and to transmit counterclockwise circularly polarized light.

  Here, the light beam LBa and the light beam LBb are separated from each other at least in the Z-axis direction, and the principal rays thereof are incident on the polarization separation element 16A in a non-parallel state (see FIG. 20).

  Therefore, most of the light beam LBa (referred to as “light beam LBa1”) is transmitted to the polarization separation element 16A, but a part (referred to as “light beam LBa2”) is reflected. Further, most of the light beam LBb (referred to as “light beam LBb1”) is reflected, but a part (referred to as “light beam LBb2”) is transmitted to the polarization separation element 16A.

  The light beam (light beam LBa1) transmitted through the polarization separation element 16A is irradiated onto the surface of the photosensitive drum 2030a through the folding mirror 18a and the exit window 19a.

  On the other hand, the light beam (light beam LBb1) reflected in the −Z direction by the polarization separation element 16A is reflected in the + X direction by the reflection mirror 17A, and then is reflected on the surface of the photosensitive drum 2030b via the folding mirror 18b and the exit window 19b. Irradiated.

  By the way, since the optical path of the light beam LBa and the optical path of the light beam LBb are separated from each other, and the beam diameter is reduced as the distance from the photosensitive drum approaches, the light beam LBa1 and the light beam LBb2, and the light beam LBb1 and the light beam LBa2 are Spatially spaced from each other. In particular, when an image in the sub-scanning corresponding direction on the polygon mirror 14 is formed as an enlarged image on the photosensitive drum by the scanning lens 15A, the separation distance between the light beam LBa1 and the light beam LBb2, and the light beam LBb1 The separation distance from the light beam LBa2 further increases.

  The size and position of the exit window 19a are set so that the light beam LBa1 can be transmitted but the light beam LBb2 cannot be transmitted. The size and position of the exit window 19b are set so that the light beam LBb1 can be transmitted but the light beam LBa2 cannot be transmitted. Therefore, LBa2 and LBb2 are shielded from light at the bottom surface of the optical housing 2300.

  Returning to FIG. 2, the scanning lens 15 </ b> B is arranged on the + X side of the polygon mirror 14 and on the optical path of the light beams LBc and LBd deflected by the polygon mirror 14.

  The polarization separation element 16B is disposed on the optical path of the light beam via the scanning lens 15B. The polarization separation element 16B transmits or reflects incident light according to the rotation direction of the circularly polarized light. Here, the polarization separation element 16B is set to transmit counterclockwise circularly polarized light and reflect clockwise circularly polarized light.

  Here, the light beam LBc and the light beam LBd are separated from each other at least in the Z-axis direction, and the principal rays thereof are incident on the polarization separation element 16B in a non-parallel state (see FIG. 21).

  Therefore, most of the light beam LBc (referred to as “light beam LBc1”) is reflected but partially transmitted (referred to as “light beam LBc2”) to the polarization separation element 16B. Further, most of the light beam LBd (referred to as “light beam LBd1”) is transmitted to the polarization separation element 16B, but a part (referred to as “light beam LBd2”) is reflected.

  The light beam (light beam LBc1) reflected in the −Z direction by the polarization separation element 16B is reflected in the −X direction by the reflection mirror 17B, and then irradiates the surface of the photosensitive drum 2030c through the folding mirror 18c and the exit window 19c. Is done.

  On the other hand, the light beam (light beam LBd1) transmitted through the polarization separation element 16B is irradiated onto the surface of the photosensitive drum 2030d through the folding mirror 18d and the exit window 19d.

  By the way, since the optical path of the light beam LBc and the optical path of the light beam LBd are separated from each other and the beam diameter is reduced as the distance from the photosensitive drum is approached, the light beams LBc1 and LBd2 and the light beams LBd1 and LBc2 are Spatially spaced from each other. In particular, when an image in the sub-scanning corresponding direction on the polygon mirror 14 is formed as an enlarged image on the photosensitive drum by the scanning lens 15B, the separation distance between the light beam LBc1 and the light beam LBd2, and the light beam LBd1 The separation distance from the light beam LBc2 further increases.

  The size and position of the exit window 19c are set so that the light beam LBc1 can be transmitted but the light beam LBd2 cannot be transmitted. The size and position of the exit window 19d are set so that the light beam LBd1 can be transmitted but the light beam LBc2 cannot be transmitted. Therefore, LBc2 and LBd2 are shielded from light at the bottom surface of the optical housing 2300.

  The light spot on the surface of each photosensitive drum moves in the longitudinal direction of the photosensitive drum as the polygon mirror 14 rotates. The moving direction of the light spot at this time is the “main scanning direction”, and the rotation direction of the photosensitive drum is the “sub scanning direction”.

  The scanning control device drives the light source 10a according to the black image information, drives the light source 10b according to the cyan image information, drives the light source 10c according to the magenta image information, and turns the light source 10d according to the yellow image information. To drive.

  Therefore, the light beam LBa1 that scans the surface of the photoconductor drum 2030a, the light beam LBb1 that scans the surface of the photoconductor drum 2030b, the light beam LBc1 that scans the surface of the photoconductor drum 2030c, and the light beam LBd that scans the surface of the photoconductor drum 2030d are , Each is a writing light beam. Each writing light beam is also called signal light.

  On the other hand, the light beam LBa2, the light beam LBb2, the light beam LBc2, and the light beam LBd2 that are shielded by the bottom surface of the optical housing 2300 are all ghost light. In this embodiment, each ghost light is not irradiated onto the photosensitive drum.

  A scanning area in which image information is written on each photosensitive drum is called an “effective scanning area”, an “image forming area”, or an “effective image area”. The optical system disposed between the polygon mirror 14 and the photosensitive drum is called a “scanning optical system”.

  Here, the scanning lens 15A, the scanning lens 15B, the polarization separation element 16A, and the polarization separation element 16B are shared by the two image forming stations. Each folding mirror is provided so that the optical path lengths at the respective image forming stations are equal to each other.

  Next, the configuration of each polarization separation element will be described. Each polarization separation element is a so-called liquid crystal element having a pair of transparent substrates (2101a, 2101b) and a cholesteric liquid crystal layer 2104 sandwiched between the transparent substrates as shown in FIG. 22 as an example.

  The cholesteric liquid crystal layer 2104 has a layered structure in which rod-like liquid crystal molecules overlap each other in a direction perpendicular to the surface of the transparent substrate (here, the X-axis direction). In the layer, a plane in which liquid crystal molecules are aligned in a certain direction (a plane parallel to the YZ plane) has a spiral structure that is stacked while twisting the alignment direction little by little. The twist axis is called a helical axis (helical axis), and the twist pitch is called a helical pitch (helical pitch) (see FIG. 23). Here, the spiral axis is orthogonal to the surface of the transparent substrate and exhibits a so-called planar state.

  In this case, when light is incident, the cholesteric liquid crystal layer 2104 selectively reflects circularly polarized light having a wavelength corresponding to the helical pitch and in the same direction as the helical direction. The spiral direction and the initial spiral pitch can be adjusted by the type and concentration of the chiral compound that is an optically active substance added to the liquid crystal.

  In the optical scanning device 2010, since the light beam deflected by the polygon mirror 14 is incident on the polarization separation element, the incident angle of the light beam with respect to the cholesteric liquid crystal layer 2104 differs depending on the incident position. Therefore, in order for the polarization separation element to have a good polarization separation characteristic regardless of the incident position of the light beam, it is preferable that the spiral pitch is varied according to the incident position of the light beam (see FIG. 24). Specifically, when the optimum helical pitch at the position where the light beam is incident from a direction parallel to the X-axis direction is p and the incident angle of the light beam with respect to the cholesteric liquid crystal at each incident position is θ, The optimum helical pitch is p · cos θ.

  Since the spiral pitch of the cholesteric liquid crystal layer has a property that changes depending on the temperature and applied voltage, a voltage is applied or heated in advance so that the spiral pitch becomes p · cos θ for each incident position of the light beam. If the liquid crystal is cured by a technique such as thermal curing or ultraviolet curing, the helical pitch can be varied according to the incident position of the light beam.

  Here, the polarization separating element 16A reflects the light beam LBb that is clockwise circularly polarized light (see FIG. 25) and transmits the light beam LBa that is counterclockwise circularly polarized light (see FIG. 26). The direction of rotation of the spiral in 2104 is clockwise.

  Similarly, the polarization separation element 16B reflects the light beam LBc that is clockwise circularly polarized light (see FIG. 27) and transmits the light beam LBd that is counterclockwise circularly polarized light (see FIG. 28). The direction of rotation of the spiral in 2104 is clockwise.

  By the way, in a conventional optical scanning device having a polarization separation element, linearly polarized light (vertically polarized light) whose polarization direction is parallel to the rotation axis and linearly polarized light (transversely polarized light) whose polarization direction is orthogonal to the rotation axis are incident on the polygon mirror. The longitudinally polarized light and the laterally polarized light deflected by the polygon mirror are separated by a polarization separation element. At this time, when the longitudinally polarized light and the laterally polarized light are obliquely incident on the deflecting reflection surface of the polygon mirror, the incident surfaces of the longitudinally polarized light and the laterally polarized light on the deflecting reflecting surface correspond to the rotation angle of the deflecting reflecting surface (that is, the deflection angle). Tilt from the horizontal). Then, the light beam reflected by the deflecting and reflecting surface includes a p-polarized component whose polarization direction is parallel to the incident surface and an s-polarized component whose polarization direction is orthogonal to the incident surface. The intensity ratio of the p-polarized component and the s-polarized component at this time depends on the rotation angle of the deflecting reflecting surface. Further, when reflected by the deflecting reflecting surface, the p-polarized component has no phase change, and the s-polarized component undergoes a 180 ° phase change. Therefore, even if longitudinally polarized light and laterally polarized light are incident on the polygon mirror, they are not reflected as longitudinally polarized light and laterally polarized light, and rotation of the polarization direction according to the rotation angle of the deflection reflection surface occurs. This leads to poor polarization separation in the polarization separation element. In order to prevent the polarization direction of the reflected light from rotating, it is conceivable to make the oblique incident angle as small as possible. However, the oblique incident angle cannot always be kept small due to restrictions on the layout of optical members.

  On the other hand, in the present embodiment, two circularly polarized lights having different rotation directions are obliquely incident on the polygon mirror. In this case, the rotational directions of the two circularly polarized light are different from each other even after being deflected by the polygon mirror. Therefore, the polarization separation failure in the polarization separation element can be made smaller than in the past.

  As described above, according to the optical scanning device 2010 according to the present embodiment, the two light source units (LU1, LU2), the two cylindrical lenses (22A, 22B), the polygon mirror 14, the scanning optical system, and the scanning control device. Etc.

  Each light source unit includes two light sources, two coupling lenses, a light combining element, and a quarter wavelength plate, and emits clockwise circularly polarized light and counterclockwise circularly polarized light separated from each other at least in the Z-axis direction.

  The clockwise circularly polarized light and the counterclockwise circularly polarized light emitted from the light source unit are obliquely incident on the same deflecting and reflecting surface of the polygon mirror 14 through the cylindrical lens, respectively. Is deflected as

  The scanning optical system includes two scanning lenses (15A, 15B), two polarization separation elements (16A, 16B), two reflecting mirrors (17A, 17B), and four folding mirrors (18a, 18b, 18c, 18d). have. Each polarization separation element transmits counterclockwise circularly polarized light and reflects clockwise circularly polarized light.

  In this case, it is possible to suppress the generation of ghost light during polarization separation in each polarization separation element. Further, the cost and size (thinning) of the optical scanning device can be reduced.

  Since the color printer 2000 includes the optical scanning device 2010, as a result, the cost and size can be reduced without degrading the image quality.

  In the above embodiment, instead of the polarization separation element 16A, as shown in FIG. 29 and FIG. 30 as an example, a quarter wavelength plate 161A and a polarization beam splitter 162A may be used. In this case, since an inexpensive polarizing beam splitter can be used, further cost reduction can be achieved.

  The quarter-wave plate 161A is disposed on the optical path of the light beam that passes through the scanning lens 15A, and converts the light beam LBa into vertically polarized light and converts the light beam LBb into horizontally polarized light. Here, the quarter-wave plate 161A is sandwiched between a pair of transparent substrates.

  The polarization beam splitter 162A is disposed on the optical path of the light beam via the quarter-wave plate 161A, and has a polarization separation surface that transmits vertically polarized light and reflects horizontally polarized light. Therefore, the polarization beam splitter 162A transmits the light beam LBa and reflects the light beam LBb. Here, the polarization separation surface is provided on one surface of the transparent substrate. An antireflection film is provided on the other surface of the transparent substrate.

  The polarization separation surface can be composed of a dielectric multilayer film, a wire grid, or the like. On the polarization separation surface, the incident surface is inclined with respect to the Z-axis direction as the absolute value of the deflection angle increases (see FIG. 31). Therefore, the linearly polarized light emitted from the quarter-wave plate 161A. However, it is preferable not to use simple vertical polarization or horizontal polarization, but to incline with respect to vertical polarization or horizontal polarization in accordance with the deflection angle.

  That is, it is preferable that the orientation of the principal axis of the quarter-wave plate 161A is set to an angle at which the polarization beam splitter 162A at the subsequent stage can be most easily polarized and separated. As an example, as shown in FIG. 32, the quarter-wave plate 161A has a simple circular polarization at the center in the Y-axis direction where the light beam with a deflection angle of 0 ° is incident within the effective scanning range. As the main axis is inclined 45 ° with respect to the Y-axis direction so that it is converted into longitudinally polarized light or laterally polarized light, and the incident position of the light beam approaches the end in the + Y direction, the inclination angle of the main axis is continuously or stepwise. It is preferable to set the angle of inclination of the main axis to be greater than 45 ° continuously or stepwise as the incident position of the light beam approaches the end in the −Y direction.

  At a position where the inclination angle of the main axis of the quarter-wave plate with respect to the Y-axis direction is 45 °, the light beam emitted from the quarter-wave plate becomes horizontal or vertical linearly polarized light. Further, at a position where the inclination of the main axis of the quarter-wave plate with respect to the Y-axis direction is different from 45 °, the light beam emitted from the quarter-wave plate is linearly polarized light whose polarization direction has changed.

  Differentiating the inclination angle of the main axis as described above, for example, uses nematic liquid crystal as the material of the quarter-wave plate 161A, and controls the alignment of the alignment film so that the alignment direction of the liquid crystal molecules changes continuously. This is possible by setting the rubbing conditions for imparting the characteristics and the photo-alignment conditions by the photo-alignment method.

Here, a method for specifying the orientation (tilt angle) of the main axis will be described below.
(1) A quarter wavelength plate is set in the orthogonal coordinate system shown in FIG.
(2) A light source is disposed on the −X side of the quarter-wave plate.
(3) A polarizer having a transmission axis in the Z-axis direction is set between the light source and the quarter-wave plate.
(4) A photodetector such as an optical power meter is disposed on the + X side of the quarter wavelength plate.
(5) An analyzer having a transmission axis in the Y-axis direction is disposed between the quarter-wave plate and the photodetector.
(6) The polarizer, the quarter-wave plate, and the analyzer are adjusted so that the light from the light source is incident substantially perpendicularly.
(7) The quarter wavelength plate is rotated around the X axis, and the rotation angle at which the light intensity detected by the photodetector is lowest is obtained. This rotation angle is the orientation (tilt angle) of the main axis.

  Similarly, in the above embodiment, instead of the polarization separation element 16B, as shown in FIG. 33 and FIG. 34 as an example, a quarter wavelength plate 161B and a polarization beam splitter 162B may be used.

  In the above embodiment, the optical paths of the two light beams emitted from each light source unit may be separated when viewed from the Z-axis direction.

  Next, a modification of the optical scanning device will be described. In the following description, differences from the above embodiment will be mainly described, and the same or equivalent components as those in the above embodiment will be denoted by the same reference numerals, and the description thereof will be simplified or omitted. .

  The optical scanning device 2010B of this modification is characterized in that the pre-deflector optical system includes a light beam splitting element 30, as shown in FIG.

  The optical scanning device 2010B includes two light source units (LU3, LU4), a beam splitting element 30, two cylindrical lenses (22A, 22B), two incident mirrors (M1, M2), a polygon mirror 14, a scanning optical system, and A scanning control device (not shown) is included.

  As an example, the light source unit LU3 includes a light source 10A, a coupling lens 11A, and a quarter-wave plate 12A as shown in FIG. The light source 10A emits vertically polarized light. Hereinafter, for convenience, a light beam emitted from the light source 10A is referred to as “light beam LB1”. The quarter-wave plate 12A converts the light beam LB1 into clockwise circularly polarized light.

  As an example, the light source unit LU4 includes a light source 10B, a coupling lens 11B, and a quarter-wave plate 12B as shown in FIG. The light source 10B emits vertically polarized light. Hereinafter, for convenience, the light beam emitted from the light source 10B is referred to as “light beam LB2”. The quarter-wave plate 12B converts the light beam LB2 into clockwise circularly polarized light.

  Here, each light source and each coupling lens are arranged so that the light beams LB1 and LB2 incident on the light beam splitting element 30 are separated from each other by a predetermined distance at least in the Z-axis direction (see FIG. 38).

  Referring back to FIG. 35, the light beam splitting element 30 is a non-polarization half mirror, in which the light beam LB1 is reflected light (referred to as light beam LB1a) and transmitted light (referred to as light beam LB1b), and the light beam LB2 is reflected light (referred to as light beam LB2a). And transmitted light (referred to as light beam LB2b). Here, the light beam LB1b and the light beam LB2b are both clockwise circularly polarized light, but the light beam LB1a and the light beam LB2a are both converted to counterclockwise circularly polarized light.

  The cylindrical lens 22A forms an image of the light beam LB1a and the light beam LB2b emitted from the light beam splitting element 30 in the vicinity of the deflection reflection surface of the polygon mirror 14 with respect to the Z-axis direction via the incident mirror M1.

  The cylindrical lens 22B forms an image of the light beam LB1b and the light beam LB2a emitted from the light beam splitting element 30 in the vicinity of the deflection reflection surface of the polygon mirror 14 with respect to the Z-axis direction via the incident mirror M2.

  Both the light beam LB1b and the light beam LB2b deflected by the polygon mirror 14 are counterclockwise circularly polarized light. Further, the light beam LB1a and the light beam LB2a deflected by the polygon mirror 14 are both clockwise circularly polarized light.

  In the scanning optical system, as shown in FIG. 39 as an example, a polarization separation element 16C is used instead of the polarization separation element 16A. In this polarization separation element 16C, the direction of spiral rotation in the cholesteric liquid crystal layer is counterclockwise, clockwise circular polarized light is transmitted, and counterclockwise circular polarized light is reflected.

  The light beams LB1a and LB2b deflected by the polygon mirror 14 pass through the scanning lens 15A and enter the polarization separation element 16C. Most of the light beam LB1a passes through the polarization separation element 16C and is irradiated onto the surface of the photosensitive drum 2030a through the folding mirror 18a and the exit window 19a. On the other hand, most of the light beam LB2b is reflected in the −Z direction by the polarization separation element 16C, and is irradiated onto the surface of the photosensitive drum 2030b through the reflection mirror 17A, the folding mirror 18b, and the exit window 19b.

  The light beam LB1b and the light beam LB2a deflected by the polygon mirror 14 pass through the scanning lens 15B and enter the polarization separation element 16B. Most of the light beam LB2a is reflected in the −Z direction by the polarization separation element 16B, and is irradiated onto the surface of the photosensitive drum 2030c through the reflection mirror 17B, the folding mirror 18c, and the exit window 19c. On the other hand, most of the light beam LB1b passes through the polarization separation element 16B and is irradiated onto the surface of the photosensitive drum 2030d through the folding mirror 18d and the exit window 19d.

  That is, the light beam LB1a corresponds to the light beam LBa of the above embodiment, and the light beam LB2b corresponds to the light beam LBb of the above embodiment. The light beam LB2a corresponds to the light beam LBc in the above embodiment, and the light beam LB1b corresponds to the light beam LBd in the above embodiment. Therefore, hereinafter, the light beam LB1a is also referred to as a light beam LBa, and the light beam LB2b is also referred to as a light beam LBb. The light beam LB2a is also referred to as a light beam LBc, and the light beam LB1b is also referred to as a light beam LBd.

  Here, the number of deflecting and reflecting surfaces in the polygon mirror 14 is four, and the light beam via the incident mirror M1 and the light beam via the incident mirror M2 are incident on different deflecting and reflecting surfaces. The angle formed by the light beam that has entered the polygon mirror 14 via the incident mirror M1 and the light beam that has passed through the incident mirror M2 is set to be approximately 90 ° in plan view.

  Therefore, the light beam LBa and the light beam LBd, and the light beam LBb and the light beam LBc do not simultaneously scan the effective scanning area on the corresponding photosensitive drum.

  For example, as shown in FIG. 40, when the light beam LBa reflected by the deflection reflection surface of the polygon mirror 14 goes to the writing start position on the photosensitive drum 2030a, the light beam LBd reflected by the deflection reflection surface of the polygon mirror 14. Is directed to a position on the + Y side of the writing end position on the photosensitive drum 2030d.

  As shown in FIG. 41, when the light beam LBa reflected by the deflection reflection surface of the polygon mirror 14 is directed to the center (image height 0) position of the effective scanning area of the photosensitive drum 2030a, the deflection of the polygon mirror 14 is performed. The light beam LBd reflected by the reflecting surface is directed in the + Y direction.

  When the light beam LBa reflected by the deflecting / reflecting surface of the polygon mirror 14 exceeds the center (image height 0) position of the effective scanning region on the photosensitive drum 2030a, the deflecting / reflecting surface of the polygon mirror 14 that reflects the light beam LBd is formed. The direction in which the light beam LBd reflected by the deflecting reflection surface of the polygon mirror 14 is switched from the + Y direction to the −Y direction.

  Then, as shown in FIG. 42, when the light beam LBa reflected by the deflecting / reflecting surface of the polygon mirror 14 goes to the writing end position of the effective scanning area of the photosensitive drum 2030a, it is reflected by the deflecting / reflecting surface of the polygon mirror 14. The emitted light beam LBd is directed to a position on the −Y side of the writing start position on the photosensitive drum 2030d.

  As described above, when the light beam LBa reflected by the deflecting / reflecting surface of the polygon mirror 14 scans the effective scanning area of the photosensitive drum 2030a, the light beam LBd reflected by the deflecting / reflecting surface of the polygon mirror 14 is photosensitive. It does not go into the effective scanning area of the body drum 2030d.

  On the other hand, when the light beam LBd reflected by the deflection reflection surface of the polygon mirror 14 scans the effective scanning area of the photosensitive drum 2030d, the light beam LBa reflected by the deflection reflection surface of the polygon mirror 14 is It does not go into the effective scanning area of the drum 2030a.

  Similarly, when the light beam LBb reflected by the deflecting / reflecting surface of the polygon mirror 14 scans the effective scanning area of the photosensitive drum 2030b, the light beam LBc reflected by the deflecting / reflecting surface of the polygon mirror 14 is It does not go into the effective scanning area of the drum 2030c.

  Further, when the light beam LBc reflected by the deflecting / reflecting surface of the polygon mirror 14 scans the effective scanning area of the photosensitive drum 2030c, the light beam LBb reflected by the deflecting / reflecting surface of the polygon mirror 14 is changed to the photosensitive drum. It does not go into the effective scanning area in 2030b.

  Therefore, at the timing when the light beam LBa scans the effective scanning region on the photosensitive drum 2030a, the light beam LB1 is modulated according to the black image information, and at the timing when the light beam LBd scans the effective scanning region on the photosensitive drum 2030d, The light beam LB1 is modulated in accordance with yellow image information.

  Similarly, at the timing when the light beam LBb scans the effective scanning area on the photosensitive drum 2030b, the light beam LB2 is modulated according to the cyan image information, and at the timing when the light beam LBc scans the effective scanning area on the photosensitive drum 2030c. The light beam LB2 is modulated in accordance with magenta image information.

  In this case, the same effect as that of the above embodiment can be obtained, and the cost of the optical scanning device can be further reduced.

  By the way, if longitudinally polarized light and laterally polarized light are incident on the beam splitter 30 from a direction inclined with respect to the XY plane, the polarization direction of the transmitted light does not change, but the polarization direction of the reflected light is inclined in the incident direction. Rotates according to the corner. Therefore, the relationship between the polarization direction of the transmitted light and the polarization direction of the reflected light is not an orthogonal relationship. In this case, ghost light is generated during polarization separation in each polarization separation element. On the other hand, when circularly polarized light enters the light beam splitting element 30, such inconvenience does not occur.

  It should be noted that the angle formed between the light beam that has entered the polygon mirror 14 via the incident mirror M1 and the light beam that has passed through the incident mirror M2 may be slightly shifted from 90 ° in plan view.

  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 directly transferred to the recording paper. Also good.

  In the above embodiment, the case where the optical scanning device 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.

  10A, 10B: Light source (part of the light source device), 10a to 10d ... Light source (part of the light source device), 11A, 11B ... Coupling lens, 11a-11d ... Coupling lens, 12A, 12B ... 1/4 wavelength Plate (conversion optical element), 13A, 13B ... photosynthetic element, 14 ... polygon mirror (optical deflector), 15A, 15B ... scanning lens (part of scanning optical system), 16A, 16B, 16C ... polarization separation element, 17A , 17B: Reflection mirror (part of the scanning optical system), 18a to 18d ... Folding mirror (part of the scanning optical system), 19a to 19d ... Ejection window, 22A, 22B ... Cylindrical lens, 30 ... Beam splitting element, 161A , 161B: 1/4 wavelength plate, 162A, 162B: Polarizing beam splitter, 2000: Color printer (image forming apparatus), 2030a to 2030 ... photosensitive drum (image bearing member), 2010 ... optical scanning device, 2010B ... optical scanning device, 2104 ... cholesteric liquid crystal layer, LU1～LU4 ... light source unit, M1, M2 ... incidence mirror (light-guiding optical system).

JP-A-60-32019 JP-A-7-144434

Claims (9)

An optical scanning device that individually scans a plurality of scanned surfaces along one direction with light,
A light source device that emits a first light beam and a second light beam corresponding to two different scanned surfaces of the plurality of scanned surfaces;
A pre-deflector optical system that emits the first light flux and the second light flux from the light source device as circularly polarized light having different rotation directions;
A plurality of reflecting surfaces parallel to the rotation axis, wherein the first light flux and the second light flux through the pre-deflector optical system are the same from a direction inclined with respect to a plane perpendicular to the rotation axis; An optical deflector that is incident on the reflecting surface and deflects the first light flux and the second light flux;
A polarization separation element that separates the first light beam and the second light beam deflected by the optical deflector, and individually collects the first light beam and the second light beam on a corresponding scanned surface; An optical scanning device comprising: a scanning optical system that emits light. The pre-deflector optical system is
A conversion optical element that converts the first light flux and the second light flux into circularly polarized light having the same rotation direction;
It is disposed on the optical path of the first light flux and the second light flux through the conversion optical element, transmits a part of the first light flux, reflects the rest, and one of the second light fluxes. A beam splitting element that transmits the part and reflects the rest,
The transmitted light component of the first light beam and the reflected light component of the second light beam emitted from the light beam dividing element are guided to the same reflecting surface of the optical deflector, and emitted from the light beam dividing element. And a light guiding optical system for guiding the reflected light component of the first light flux and the transmitted light component of the second light flux to the same reflecting surface of the optical deflector. 2. An optical scanning device according to 1.   2. The optical scanning device according to claim 1, wherein the pre-deflector optical system includes a conversion optical element that converts the first light beam and the second light beam into circularly polarized light having different rotation directions. .   4. The optical scanning device according to claim 1, wherein the polarization separation element transmits one of two circularly polarized lights having different rotation directions and reflects the other. 5.   The optical scanning device according to claim 4, wherein the polarization separation element is a polarization separation element including a cholesteric liquid crystal.   6. The optical scanning device according to claim 5, wherein the spiral pitch of the cholesteric liquid crystal is set so as to decrease as the absolute value of the incident angle of the light beam on the cholesteric liquid crystal increases. The scanning optical system includes a ¼ wavelength plate disposed on an optical path between the optical deflector and the polarization separation element,
The optical scanning device according to claim 1, wherein the polarization separation element is a polarization beam splitter that separates two linearly polarized light having different polarization directions.   In the main axis of the quarter-wave plate, the inclination angle with respect to the direction corresponding to the one direction increases monotonously from one end of the corresponding direction to the other end within the effective scanning range. The optical scanning device according to claim 7, wherein the tilt angle is 45 ° at a position where a light beam that monotonously decreases and has a minimum incident angle at the polarization separation element is incident. A plurality of image carriers;
An image forming apparatus comprising: the optical scanning device according to claim 1, wherein the plurality of image carriers are scanned with a light beam modulated in accordance with image information.

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