JP5754617B2 - Optical scanning apparatus and image forming apparatus - Google Patents

Description

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

  In electrophotographic image recording, an image forming apparatus using a laser is widely used. In this case, the image forming apparatus includes an optical scanning device, and forms a latent image by rotating the drum while scanning laser light using a polygon scanner (for example, a polygon mirror) in the axial direction of the photosensitive drum. Is common. In the field of electrophotography, an image forming apparatus is required to increase image density in order to improve image quality and to increase image output speed in order to improve operability. As one of the methods for achieving both high density and high speed, a so-called multi-beam method of simultaneously scanning with a plurality of lights has been considered.

  However, since the number of beams increases in the multi-beam method, the cost of the light source and the cost of the control board (including the ASIC) for driving the light source increase.

  For example, Patent Document 1 discloses an optical scanning device that optically scans n optical scanning positions with a light beam from one light source.

  Patent Document 2 discloses an optical scanning device that optically scans two scanned surfaces sandwiching an optical deflector with a light beam from one light source.

  In the optical scanning devices disclosed in Patent Document 1 and Patent Document 2, a semiconductor laser is used as a light source. When light emitted from the semiconductor laser and reflected by the scanning optical system returns to the semiconductor laser, There was a risk of adverse effects such as

(1) The return light is further reflected by the end face of the laser, and the reflected light reaches the surface to be scanned, so that the image density changes.
(2) The laser oscillation becomes unstable due to the return light, and the image density changes.
(3) Return light reaches the light receiving element for monitoring the light amount, and APC (Auto Power Control) for keeping the light emission power of the laser becomes unstable. As a result, the light emission power is changed and the image density is changed. Changes.

  Note that in a normal optical scanning device having the number of light sources corresponding to the number of scanned surfaces, even if the light reflected by the surface of the optical element returns to the light source, the return light is not collected on the light source. There is almost no adverse effect from the return light.

  However, as in the optical scanning devices disclosed in Patent Document 1 and Patent Document 2, in the method of scanning by switching a plurality of different scanned surfaces with a light beam from one light source in time, the angle of view of the scanning lens If the light is wide, the light reflected by the optical deflector such as a polygon mirror returns to the light source, and the return light is condensed on the light source, which may cause the above-described adverse effects. For this reason, in a method of scanning a plurality of different scanning surfaces by temporally switching with a light beam from one light source, the angle of view of the scanning lens cannot be widened, resulting in a long optical path length and an increase in the size of the optical scanning device. There was an inconvenience.

  The present invention has been made under such circumstances, and a first object of the present invention is to provide an optical scanning device capable of reducing the number of light sources without degrading the stability of optical scanning.

  A second object of the present invention is to provide an image forming apparatus that can be reduced in size and price without degrading image quality.

  From a first viewpoint, the present invention is an optical scanning device that individually scans at least two scanned surfaces with light in the main scanning direction, and includes a light source, a light beam from the light source, a first light beam, and a second light beam. A beam splitting unit having a split surface for splitting the beam into a beam and emitting the first beam and the second beam separated from each other in the sub-scanning direction orthogonal to the main scanning direction; An optical deflector that has one polygon mirror and a second polygon mirror, deflects the first light flux from the light beam splitting means with the first polygon mirror, and deflects the second light flux with the second polygon mirror; A scanning optical system that guides the first light flux and the second light flux deflected by the optical deflector to the corresponding scanned surface, respectively, and one of the first light flux and the second light flux. At one timing while scanning the corresponding scanned surface, the other Beam is incident perpendicularly on the corresponding polygon, at that timing, the conjugate point in the sub-scanning direction of the light source, an optical scanning device which is present at a position shifted from the surface of the multi surface mirror.

  According to this, the number of light sources can be reduced without reducing the stability of optical scanning.

  According to a second aspect of the present invention, there is provided an image comprising: at least two image carriers; and an optical scanning device according to the present invention that scans the at least two image carriers with a light beam modulated according to image data. Forming device.

  Accordingly, it is possible to reduce the size and the price without deteriorating the image quality.

1 is a diagram for describing a schematic configuration of a color printer according to an embodiment of the present invention. FIG. 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; FIG. 3 is a third diagram for explaining the optical scanning device in FIG. 1; FIG. 4 is a diagram (part 4) for explaining the optical scanning device in FIG. 1; It is a figure for demonstrating the surface emitting laser array contained in a light source. It is a figure for demonstrating the arrangement | sequence of the several light emission part in a surface emitting laser array. It is a figure for demonstrating a light beam splitting member. FIG. 9A and FIG. 9B are diagrams for explaining the difference in the formation site of the polarization separation plane. It is FIG. (1) for demonstrating the return light when the quadrilateral mirror of a 2 step | paragraph structure rotates with a phase difference of 45 degrees. It is FIG. (2) for demonstrating return light when the four-stage mirror of 2 steps | paragraphs rotates with a phase difference of 45 degrees. It is a figure for demonstrating the return light removal optical system 2303A. FIGS. 13A to 13F are diagrams for explaining the polarization states of the light beams in FIG. It is FIG. (1) for demonstrating the effect of the return light removal optical system 2303A. FIG. 11 is a second diagram for explaining the effect of the return light removing optical system 2303A; It is a figure for demonstrating the return light removal optical system 2303B. FIGS. 17A to 17F are diagrams for explaining the polarization states of the light beams in FIG. It is FIG. (1) for demonstrating the effect of the return light removal optical system 2303B. FIG. 11 is a second diagram for explaining the effect of the return light removing optical system 2303B; It is a figure for demonstrating the modification 1 of the return light removal optical system 2303A. FIGS. 21A and 21B are diagrams for explaining the angle of the slow axis of the half-wave plate in FIG. 20, respectively. It is a figure for demonstrating the modification 2 of the return light removal optical system 2303A. FIG. 11 is a diagram (No. 1) for describing an effect of a return light removal optical system 2303A of Modification 2; FIG. 14 is a diagram (No. 2) for explaining the effect of the return light removal optical system 2303A of Modification 2; It is a figure for demonstrating the modification 3 of the return light removal optical system 2303A. It is FIG. (1) for demonstrating a holding member. It is FIG. (2) for demonstrating a holding member.

  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 as an image forming apparatus 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), 4 Toner cartridges (2034a, 2034b, 2034c, 2034d), transfer belt 2040, transfer roller 2042, fixing device 2050, paper feed roller 2054, registration roller pair 2056, paper discharge roller 2058, paper feed tray 2 60, the discharge tray 2070 includes a communication control unit 2080, and a printer controller 2090 for totally controlling the above elements.

  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 ROM described in a program written in code readable by the CPU, various data used when executing the program, a RAM as a working memory, an analog data An AD conversion circuit for converting the signal into digital data. The printer control device 2090 controls each unit in response to a request from the host device, and 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, the toner cartridge 2034a, and the cleaning unit 2031a are used as a set and form an image forming station (hereinafter also referred to as “K station” for convenience) that forms a black image. Configure.

  The photosensitive drum 2030b, the charging device 2032b, the developing roller 2033b, the toner cartridge 2034b, and the cleaning unit 2031b are used as a set and form an image forming station (hereinafter also referred to as “C station” for convenience) that forms a cyan image. Configure.

  The photosensitive drum 2030c, the charging device 2032c, the developing roller 2033c, the toner cartridge 2034c, and the cleaning unit 2031c are used as a set, and form an image forming station (hereinafter also referred to as “M station” for convenience) that forms a magenta image. Configure.

  The photosensitive drum 2030d, the charging device 2032d, the developing roller 2033d, the toner cartridge 2034d, and the cleaning unit 2031d are used as a set, and form an image forming station (hereinafter also referred to as “Y station” for convenience) that forms a yellow image. Configure.

  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.

  Based on the multicolor image information (black image information, cyan image information, magenta image information, yellow image information) from the higher-level device, the optical scanning device 2010 charges the light flux modulated for each color correspondingly. Irradiate each surface of the photosensitive drum. As a result, on the surface of each photoconductive drum, the charge disappears only in the portion irradiated with light, and a latent image corresponding to the image information is formed on the surface of each photoconductive drum. The latent image formed here moves in the direction of the corresponding developing roller as the photosensitive drum rotates.

  The toner cartridge 2034a stores black toner, and the toner is supplied to the developing roller 2033a. The toner cartridge 2034b stores cyan toner, and the toner is supplied to the developing roller 2033b. The toner cartridge 2034c stores magenta toner, and the toner is supplied to the developing roller 2033c. The toner cartridge 2034d stores yellow toner, and the toner is supplied to the developing roller 2033d.

  As each developing roller rotates, the toner from the corresponding toner cartridge 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, and the paper feed roller 2054 takes out the recording paper one by one from the paper feed tray 2060 and conveys it to the registration roller pair 2056. The registration roller pair 2056 feeds the recording paper 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 sheet transferred here is sent to the fixing device 2050.

  In the fixing device 2050, heat and pressure are applied to the recording paper, thereby fixing the toner on the recording paper. The recording paper fixed here is sent to a paper discharge tray 2070 via a 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 FIGS. 2 to 5 as an example, the optical scanning device 2010 includes two light sources (2200A, 2200B), six quarter-wave plates (QvA, QvB, Qa, Qb, Qc, Qd), 2 Two coupling lenses (2201A, 2201B), two aperture plates (2202A, 2202B), two light beam splitting members (2203A, 2203B), four cylindrical lenses (2204a, 2204b, 2204c, 2204d), polygon mirrors 2104, 4 Two first scanning lenses (2105a, 2105b, 2105c, 2105d), eight folding mirrors (2106a, 2106b, 2106c, 2106d, 2108a, 2108b, 2108c, 2108d) and four second scanning lenses (2107a, 2107b, 2107c) 2107d), 2 Synchronization lens (2301A, 2301B), 2 two synchronization detection sensor (2302A, 2302B), and a like scan control device (not shown). These are assembled at predetermined positions of an optical housing (not shown).

  In the following, for convenience, the direction corresponding to the main scanning direction is abbreviated as “main scanning corresponding direction”, and the direction corresponding to the sub scanning direction is abbreviated as “sub scanning corresponding direction”.

  The light source 2200A and the light source 2200B are arranged at positions separated from each other in the X-axis direction.

  As shown in FIG. 6 as an example, each light source has a surface emitting laser array 100 in which 32 light emitting units are two-dimensionally arranged on the same substrate.

  As shown in FIG. 7, the 32 light emitting units are arranged so that the intervals between the light emitting units are equal to each other when the light emitting units are orthogonally projected onto a virtual line extending in the sub-scanning corresponding direction. . In this specification, the “light emitting portion interval” refers to the distance between the centers of two light emitting portions.

  Each light emitting section has an oscillation wavelength in the 780 nm band and emits linearly polarized light.

  Each light source has a branching optical system for branching a part of the light beam emitted from the surface emitting laser array 100, and a light receiving element for monitoring the light amount for receiving the light beam branched by the branching optical system. . The scanning control device performs APC (Auto Power Control) for keeping the light emission power of each light emitting unit constant based on the output signal of the light receiving element.

  Hereinafter, the light beam emitted from the light source 2200A is referred to as “light beam L0A”, and the light beam emitted from the light source 2200A is referred to as “light beam L0B”.

  Returning to FIG. 2, the quarter-wave plate QvA is arranged on the optical path of the light beam L0A emitted from the light source 2200A, and gives a quarter-wave phase difference between the linearly polarized light components orthogonal to each other in the light beam L0A. To do. Hereinafter, the light beam that has passed through the quarter-wave plate QvA is referred to as “light beam LvA”.

  The quarter-wave plate QvB is disposed on the optical path of the light beam emitted from the light source 2200B, and gives a quarter-wave phase difference between the linearly polarized light components orthogonal to each other in the light beam. Hereinafter, the light beam that has passed through the quarter-wave plate QvB is referred to as “light beam LvB”.

  The coupling lens 2201A is disposed on the optical path of the light beam LvA that has passed through the quarter-wave plate QvA, and makes the light beam LvA a substantially parallel light beam.

  The coupling lens 2201B is disposed on the optical path of the light beam LvB that has passed through the quarter-wave plate QvB, and makes the light beam LvB a substantially parallel light beam.

  The aperture plate 2202A has an aperture and shapes the light beam that has passed through the coupling lens 2201A.

  The aperture plate 2202B has an aperture and shapes the light beam that has passed through the coupling lens 2201B.

  The light beam splitting member 2203A is disposed on the optical path of the light beam that has passed through the opening of the aperture plate 2202A, and splits the light beam into two light beams. The light beam dividing member 2203B is disposed on the optical path of the light beam that has passed through the opening of the aperture plate 2202B, and divides the light beam into two light beams.

  The beam splitting member 2203A and the beam splitting member 2203B are beam splitting members having the same configuration. Therefore, when it is not necessary to distinguish between the light beam dividing member 2203A and the light beam dividing member 2203B, they are collectively referred to as “light beam dividing member 2203”.

  As an example, as shown in FIG. 8, the light beam splitting member 2203 transmits linearly polarized light in the first polarization direction included in the incident light beam, and converts linearly polarized light in the second polarization direction orthogonal to the first polarization direction. A polarization separation surface that reflects in the -Z direction; and a reflection mirror surface that is disposed in parallel to the polarization separation surface on the optical path of the light beam reflected by the polarization separation surface. The light beam incident on the reflection mirror surface is reflected in the same direction as the traveling direction of the light beam transmitted through the polarization separation surface. That is, the light beam splitting member 2203 splits the incident light beam into two parallel light beams in the sub-scanning corresponding direction (here, the same as the Z-axis direction). Hereinafter, the light beam transmitted through the polarization separation surface is referred to as “light beam L1”, and the light beam reflected by the polarization separation surface is referred to as “light beam L2”.

  Further, the traveling direction of the light beam is defined as “x direction”, two directions orthogonal to each other in a plane orthogonal to the x direction, the main scanning corresponding direction in the light beam dividing member 2203 is “y direction”, and the sub scanning corresponding direction. Is “z direction”. Each polarization direction is represented by a counterclockwise angle with respect to the + y direction in the yz plane.

  Here, with respect to the polarization separation plane, the first polarization direction is p-polarization, and the second polarization direction is q-polarization. Note that p-polarized light is light in which an electric field is oscillating in a direction parallel to a plane including both incident light and reflected light with respect to a polarization separation surface. It is light with an oscillating electric field. Therefore, the light beam L1 is p-polarized light, and the light beam L2 is q-polarized light.

  As shown in FIGS. 9A and 9B, the light beam splitting member 2203 is formed by adhering a prism having a prism shape and a prism having a quadrangular prism shape with a parallelogram in cross section. The separation surface may be formed on the adhesion surface of a triangular prism, or may be formed on the adhesion surface of a quadrangular prism.

  Thus, since the light beam splitting member 2203 is integrated, it becomes easy to accurately maintain the parallelism of the light beam L1 and the light beam L2 reflected by the reflecting surface, and the parallelism is less likely to change with time.

  The light beam LvA is split by the light beam splitting member 2203A into the light beam L1 and the light beam L2 at a ratio of approximately 1: 1.

  The light beam LvB is split by the light beam splitting member 2203B into the light beam L1 and the light beam L2 at a ratio of approximately 1: 1.

  The quarter-wave plate Qa is disposed on the optical path of the −Z side light beam (light beam L2) of the two light beams from the light beam splitting member 2203A, and a quarter between the linearly polarized light components orthogonal to each other in the light beam. A wavelength phase difference is given. Hereinafter, the light beam that has passed through the quarter-wave plate Qa is referred to as “light beam La”.

  The quarter-wave plate Qb is disposed on the optical path of the + Z side light beam (light beam L1) of the two light beams from the light beam splitting member 2203A, and a quarter wavelength between the linearly polarized light components orthogonal to each other in the light beam. Is given. Hereinafter, the light beam that has passed through the quarter-wave plate Qb is referred to as “light beam Lb”.

  The quarter-wave plate Qc is disposed on the optical path of the + Z side light beam (light beam L1) of the two light beams from the light beam splitting member 2203B, and a quarter wavelength between the linearly polarized light components orthogonal to each other in the light beam. Is given. Hereinafter, the light beam that has passed through the quarter-wave plate Qc is referred to as “light beam Lc”.

  The quarter-wave plate Qd is disposed on the optical path of the −Z side light beam (light beam L2) of the two light beams from the light beam splitting member 2203B, and a quarter between the linearly polarized light components orthogonal to each other in the light beam. A wavelength phase difference is given. Hereinafter, the light beam that has passed through the quarter-wave plate Qd is referred to as “light beam Ld”.

  The cylindrical lens 2204a is disposed on the optical path of the light beam that has passed through the quarter-wave plate Qa, and forms an image of the light beam in the vicinity of the deflection reflection surface of the polygon mirror 2104 in the Z-axis direction.

  The cylindrical lens 2204b is disposed on the optical path of the light beam that has passed through the quarter-wave plate Qb, and forms an image of the light beam in the vicinity of the deflection reflection surface of the polygon mirror 2104 in the Z-axis direction.

  The cylindrical lens 2204c is disposed on the optical path of the light beam that has passed through the quarter-wave plate Qc, and forms an image of the light beam in the vicinity of the deflection reflection surface of the polygon mirror 2104 in the Z-axis direction.

  The cylindrical lens 2204d is disposed on the optical path of the light beam that has passed through the quarter-wave plate Qd, and forms an image of the light beam in the vicinity of the deflection reflection surface of the polygon mirror 2104 in the Z-axis direction.

  The polygon mirror 2104 has a four-stage mirror having a two-stage structure, and each mirror serves as a deflection reflection surface. The light beam from the cylindrical lens 2204a and the light beam from the cylindrical lens 2204d are respectively deflected by the first-stage (lower) tetrahedral mirror, and the light beam from the cylindrical lens 2204b and the cylindrical light are deflected by the second-stage (upper) tetrahedral mirror. It arrange | positions so that the light beam from the lens 2204c may be deflected, respectively. Note that the first-stage tetrahedral mirror and the second-stage tetrahedral mirror are rotated with a phase shift of 46 °, and writing scanning is alternately performed in the first and second stages. The radius of the circle circumscribing each quadrilateral mirror is 7 mm. The upper and lower four-sided mirrors may be formed integrally, or may be formed individually and then assembled.

  Here, the light beams from the cylindrical lens 2204 a and the cylindrical lens 2204 b are deflected to the −X side of the polygon mirror 2104, and the light beams from the cylindrical lens 2204 c and the cylindrical lens 2204 d are deflected to the + X side of the polygon mirror 2104.

  The first scanning lens 2105 a and the first scanning lens 2105 b are disposed on the −X side of the polygon mirror 2104, and the first scanning lens 2105 c and the first scanning lens 2105 d are disposed on the + X side of the polygon mirror 2104.

  The first scanning lens 2105a and the first scanning lens 2105b are stacked in the Z-axis direction, the first scanning lens 2105a is opposed to the first-stage four-sided mirror, and the first scanning lens 2105b is the second-stage four-surface. Opposite the mirror. Further, the first scanning lens 2105c and the first scanning lens 2105d are stacked in the Z-axis direction, the first scanning lens 2105c is opposed to the second-stage four-sided mirror, and the first scanning lens 2105d is the first-stage four-surface. Opposite the mirror.

  Each of the first scanning lenses has a non-circular surface shape having such a power that the light spot moves at a constant speed in the main scanning direction on the corresponding photosensitive drum surface as the polygon mirror 2104 rotates. Yes.

  Therefore, the light beam from the cylindrical lens 2204a deflected by the polygon mirror 2104 is applied to the photosensitive drum 2030a via the first scanning lens 2105a, the folding mirror 2106a, the second scanning lens 2107a, and the folding mirror 2108a, and the light. A spot is formed. This light spot moves in the longitudinal direction of the photosensitive drum 2030a as the polygon mirror 2104 rotates. That is, the photosensitive drum 2030a is scanned. The moving direction of the light spot at this time is the “main scanning direction” on the photosensitive drum 2030a, and the rotational direction of the photosensitive drum 2030a is the “sub-scanning direction” on the photosensitive drum 2030a.

  The light beam from the cylindrical lens 2204b deflected by the polygon mirror 2104 is applied to the photosensitive drum 2030b through the first scanning lens 2105b, the folding mirror 2106b, the second scanning lens 2107b, and the folding mirror 2108b, and the light. A spot is formed. This light spot moves in the longitudinal direction of the photosensitive drum 2030b as the polygon mirror 2104 rotates. That is, the photosensitive drum 2030b is scanned. The moving direction of the light spot at this time is the “main scanning direction” on the photosensitive drum 2030b, and the rotational direction of the photosensitive drum 2030b is the “sub-scanning direction” on the photosensitive drum 2030b.

  The light beam from the cylindrical lens 2204c deflected by the polygon mirror 2104 is applied to the photosensitive drum 2030c via the first scanning lens 2105c, the folding mirror 2106c, the second scanning lens 2107c, and the folding mirror 2108c, and the light. A spot is formed. This light spot moves in the longitudinal direction of the photosensitive drum 2030c as the polygon mirror 2104 rotates. That is, the photosensitive drum 2030c is scanned. The moving direction of the light spot at this time is the “main scanning direction” on the photosensitive drum 2030c, and the rotational direction of the photosensitive drum 2030c is the “sub-scanning direction” on the photosensitive drum 2030c.

  The light beam from the cylindrical lens 2204d deflected by the polygon mirror 2104 is applied to the photosensitive drum 2030d via the first scanning lens 2105d, the folding mirror 2106d, the second scanning lens 2107d, and the folding mirror 2108d, and the light is emitted. A spot is formed. This light spot moves in the longitudinal direction of the photosensitive drum 2030d as the polygon mirror 2104 rotates. That is, the photosensitive drum 2030d is scanned. The moving direction of the light spot at this time is the “main scanning direction” on the photosensitive drum 2030d, and the rotational direction of the photosensitive drum 2030d is the “sub-scanning direction” on the photosensitive drum 2030d.

  Each folding mirror is arranged so that the optical path lengths from the polygon mirror 2104 to each photosensitive drum coincide with each other, and the incident position and the incident angle of the light flux on each photosensitive drum are equal to each other. .

  Further, the cylindrical lens and the corresponding second scanning lens constitute a surface tilt correction optical system in which the deflection point and the corresponding photosensitive drum surface are conjugated in the sub-scanning direction.

  An optical system disposed on the optical path between the polygon mirror 2104 and each photosensitive drum is also called a scanning optical system. In this embodiment, the first scanning lens 2105a, the second scanning lens 2107a, and the folding mirrors (2106a, 2108a) constitute a scanning optical system for the K station. The first scanning lens 2105b, the second scanning lens 2107b, and the folding mirrors (2106b, 2108b) constitute a scanning optical system for the C station. The first scanning lens 2105c, the second scanning lens 2107c, and the folding mirrors (2106c, 2108c) constitute a scanning optical system for the M station. Further, the Y scanning optical system is composed of the first scanning lens 2105d, the second scanning lens 2107d, and the folding mirrors (2106d, 2108d).

  Incidentally, a scanning area in the main scanning direction 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 synchronization lens 2301A is arranged on the optical path of the light beam that has been deflected by the polygon mirror 2104 and passed through a portion (non-power portion) having no power in the main scanning direction at the −Y side end of the first scanning lens 2105b. Condenses the luminous flux.

  The synchronization detection sensor 2302A is disposed on the optical path of the light beam via the synchronization lens 2301A, and outputs a signal corresponding to the light amount of the light beam to the scanning control device.

  The scanning control device determines the writing start timing on the photosensitive drum 2030a and the photosensitive drum 2030b based on the output signal of the synchronization detection sensor 2302A.

  The synchronizing lens 2301B is disposed on the optical path of the light beam that has been deflected by the polygon mirror 2104 and passed through a portion (non-power portion) having no power in the main scanning direction at the + Y side end of the first scanning lens 2105c. Condensing.

  The synchronization detection sensor 2302B is disposed on the optical path of the light beam via the synchronization lens 2301B, and outputs a signal corresponding to the light amount of the light beam to the scanning control device.

  The scanning control device determines the timing of starting writing on the photosensitive drum 2030c and the photosensitive drum 2030d based on the output signal of the synchronization detection sensor 2302B.

  Note that the configuration including the synchronization lens and the synchronization detection sensor is also called a “synchronization detection system”. The light beam deflected by the polygon mirror 2104 and received by the synchronization detection sensor is also referred to as “synchronization detection light beam”.

  Here, since the synchronization detection light beam passes through the non-power portion of the first scanning lens, even if the first scanning lens expands / contracts due to a change in the environmental temperature, the period detection light beam toward the synchronization detection sensor. The optical path of is unchanged.

  In this embodiment, out of two light beams from a common light source, when the light beam deflected by the upper four-sided mirror scans the corresponding photosensitive drum, the light beam deflected by the lower four-sided mirror. Does not reach the photosensitive drum. In addition, when the light beam deflected by the lower four-sided mirror among the two light beams from the common light source is scanning the corresponding photosensitive drum, the light beam deflected by the upper four-sided mirror is Does not reach the drum.

  Therefore, when scanning the photosensitive drum corresponding to the light beam deflected by the upper four-sided mirror, the scanning control device is based on image information of a color corresponding to the photosensitive drum (here, cyan or magenta). When the light source is modulated and scanned and the photosensitive drum corresponding to the light beam deflected by the lower four-sided mirror is scanned, the image information of the color corresponding to the photosensitive drum (here, black or yellow) is used. The light source is modulated and driven.

  By the way, when a light beam emitted from a single light source is divided into two and a different surface to be scanned is scanned using a polygon mirror having a two-stage configuration and relatively out of phase, the reflected light is reflected by the polygon mirror. If the light does not return to the light source, the angle of view of the scanning lens is narrowed.

  For example, if the phase difference between the upper and lower four-sided mirrors is 45 °, as shown in FIG. 10, the angle difference between the light beam reflected by the upper four-sided mirror and the light beam reflected by the lower four-sided mirror. Is 90 °. The surface to be scanned is scanned with this angular difference maintained. Assuming that the angle of incident light is 60 °, as shown in FIG. 11, when the angle of the light beam reflected by the upper four-way mirror is −30 °, the light beam reflected by the lower four-way mirror is exactly Return to the light source.

  Considering the symmetry of the angle of view and application to the counter scanning method, it is desirable that the angle of view is symmetric. Therefore, when scanning the surface to be scanned with the upper four-sided mirror, in order to prevent the light beam reflected by the lower four-sided mirror from returning to the light source, the angle of view of the scanning light needs to be ± 30 ° or less. There is.

  Assuming that the position on the image plane is y, the focal length of the scanning lens is f, and the angle of view is θ, there is a relationship of y = fθ. Y = ± 163.5 mm), and when the angle of view θ becomes narrow, the focal length of the scanning lens becomes long. When the focal length of the scanning lens is increased, the optical scanning device is increased in size.

  In order to reduce the size of the optical scanning device, it is necessary to shorten the focal length of the scanning lens. For this purpose, it is necessary to increase the angle of view.

  Therefore, if it is acceptable that at least part of the light beam incident on the polygon mirror is reflected to the light source side by the polygon mirror and the reflected light returns to the light source, the optical scanning device can be downsized and the number of light sources and light source drive boards can be reduced. It is possible to achieve both cost reduction by reduction.

  However, when the return light enters the light source, the following adverse effects may occur.

(1) The return light is further reflected by the end face of the laser, and the reflected light reaches the surface to be scanned, so that the image density changes.
(2) The laser oscillation becomes unstable due to the return light, and the image density changes.
(3) Return light reaches the light receiving element for monitoring the light amount, and APC (Auto Power Control) for keeping the light emission power of the laser becomes unstable. As a result, the light emission power is changed and the image density is changed. Changes.

  Therefore, in the present embodiment, three quarter-wave plates (QvA, Qa, Qb) are set as the return light removal optical system (return light removal optical system 2303A) on the optical path between the light source 2200A and the polygon mirror 2104. ) And three quarter-wave plates (QvB, Qc, Qd) are provided as a return light removal optical system (referred to as return light removal optical system 2303B) on the optical path between the light source 2200B and the polygon mirror 2104. ing.

  First, the return light removal optical system 2303A will be described with reference to FIG.

  The light beam L0A emitted from the light source 2200A is linearly polarized light, and becomes a circularly polarized light beam LvA by the quarter-wave plate QvA.

  This light beam LvA is split by a light beam splitting member 2203A into a light beam L1 that is p-polarized light and a light beam L2 that is s-polarized light at a ratio of approximately 1: 1.

  The light beam L2 emitted from the light beam splitting member 2203A becomes a circularly polarized light beam La on the quarter-wave plate Qa.

  The light beam L1 emitted from the light beam splitting member 2203A becomes a circularly polarized light beam Lb by the quarter wavelength plate Qb.

  As an example, the polarization state of the light beam L0A is shown in FIG. 13A, and the polarization state of the light beam LvA is shown in FIG. 13B. The polarization state of the light beam L1 is shown in FIG. 13C, and the polarization state of the light beam L2 is shown in FIG. 13D. Further, the polarization state of the light beam Lb is shown in FIG. 13 (E), and the polarization state of the light beam La is shown in FIG. 13 (F).

  Therefore, even if the light beam Lb that has passed through the quarter-wave plate Qb is reflected by the polygon mirror 2104 and returned, the return light is linearly polarized light (s-polarized light) in the second polarization direction by the quarter-wave plate Qb. ) Lbr, and is reflected in the + z direction by the polarization separation surface of the light beam splitting member 2203A as shown in FIG. 14 as an example. That is, the return light does not return to the light source.

  Even if the light beam La that has passed through the quarter-wave plate Qa is reflected by the polygon mirror 2104 and returned, the return light is linearly polarized (p-polarized light) in the first polarization direction by the quarter-wave plate Qa. ) Lar, and as an example, as shown in FIG. 15, it is reflected in the + z direction by the reflecting mirror surface of the light beam splitting member 2203A and passes through the polarization splitting surface. That is, the return light does not return to the light source.

  Next, the return light removal optical system 2303B will be described with reference to FIG.

  The light beam L0B emitted from the light source 2200B is linearly polarized light, and becomes a circularly polarized light beam LvA by the quarter wavelength plate QvB.

  The light beam LvB is split by a light beam splitting member 2203B into a light beam L1 that is p-polarized light and a light beam L2 that is s-polarized light at a ratio of about 1: 1.

  The light beam L1 emitted from the light beam splitting member 2203B becomes a circularly polarized light beam Lc by the quarter wavelength plate Qc.

  The light beam L2 emitted from the light beam splitting member 2203B becomes a circularly polarized light beam Ld by the quarter-wave plate Qd.

  As an example, the polarization state of the light beam L0B is shown in FIG. 17A, and the polarization state of the light beam LvB is shown in FIG. 17B. The polarization state of the light beam L1 is shown in FIG. 17C, and the polarization state of the light beam L2 is shown in FIG. The polarization state of the light beam Lc is shown in FIG. 17E, and the polarization state of the light beam Ld is shown in FIG.

  Therefore, even if the light beam Lc that has passed through the quarter-wave plate Qc is reflected by the polygon mirror 2104 and returned, the return light is linearly polarized light (s-polarized light) in the second polarization direction by the quarter-wave plate Qc. ) Lcr, and is reflected in the + z direction on the polarization separation surface of the light beam splitting member 2203B as shown in FIG. 18 as an example. That is, the return light does not return to the light source.

  Even if the light beam Ld that has passed through the quarter-wave plate Qd is reflected by the polygon mirror 2104 and returned, the return light is linearly polarized (p-polarized light) in the first polarization direction by the quarter-wave plate Qd. ) Ldr, and as an example, as shown in FIG. 19, it is reflected in the + z direction by the reflection mirror surface of the light beam splitting member 2203B and passes through the polarization separation surface. That is, the return light does not return to the light source.

  As described above, the return light removal optical system 2303A and the return light removal optical system 2303B suppress the light beam reflected by the polygon mirror 2104 from returning to each light source. This makes it possible to achieve both downsizing and cost reduction of the optical scanning device while avoiding adverse effects on the image quality due to the return light.

  Even if a return light removing optical system is provided, return light to the light source is generated slightly due to manufacturing errors of the optical components. In addition, if the return light to the light source is completely blocked, the required specification of the return light removal optical system becomes very strict, so that mass productivity is lost and the cost is increased.

  Therefore, in this embodiment, even if there is a little return light to the light source, the return light is not focused by the light source in order to reduce the effect. Specifically, in the condition where the light reflected by the deflecting / reflecting surface of the polygon mirror 2104 returns to the light source, that is, when the incident angle to the deflecting / reflecting surface of the polygon mirror 2104 becomes 0 °, the direction corresponding to the sub-scanning of the light source The focal length of the optical element, its position, etc. are set so that the conjugate point at is closer to the rotational axis than the deflecting / reflecting surface of the polygon mirror 2104. Thereby, the return light to each light source does not focus on the light source, and even if there is a little return light, the adverse effect of the return light can be reduced.

  As described above, according to the optical scanning device 2010 according to the present embodiment, the two light sources (2200A, 2200B), the two light beam splitting members (2203A, 2203B), the optical deflector 2104, and the two return light removal optical systems. (2303A, 2303B) includes four scanning optical systems, a scanning control device, and the like.

  Each light beam splitting member transmits linearly polarized light in the first polarization direction included in the incident light beam and reflects linearly polarized light in the second polarization direction orthogonal to the first polarization direction in the −Z direction; And a reflecting mirror surface disposed in parallel to the polarization separation surface on the optical path of the light beam reflected by the polarization separation surface.

  The return light removing optical system 2303A is a quarter wavelength plate QvA disposed between the light source 2200A and the light beam dividing member 2203A, and a quarter disposed on the optical path of the light beam L1 emitted from the light beam dividing member 2203A. It has a wave plate Qb and a quarter wave plate Qa disposed on the optical path of the light beam L2 emitted from the light beam splitting member 2203A.

  In this case, even if the light beam Lb reflected by the polygon mirror 2104 returns, the return light becomes linearly polarized light in the second polarization direction by the quarter wavelength plate Qb, and the polarization separation surface of the light beam dividing member 2203A. Is reflected in the + z direction. Even if the light beam La reflected by the polygon mirror 2104 returns, the return light becomes linearly polarized light in the first polarization direction by the quarter wavelength plate Qa, and + z on the reflection mirror surface of the light beam dividing member 2203A. Reflected in the direction and transmitted through the polarization separation surface. That is, each return light does not return to the light source 2200A.

  The return light removing optical system 2303B is a quarter-wave plate QvB disposed between the light source 2200B and the light beam dividing member 2203B, and a quarter disposed on the optical path of the light beam L1 emitted from the light beam dividing member 2203B. It includes a wave plate Qc and a quarter wave plate Qd disposed on the optical path of the light beam L2 emitted from the light beam splitting member 2203B.

  In this case, even if the light beam Lc reflected by the polygon mirror 2104 returns, the return light becomes linearly polarized light in the second polarization direction by the quarter wavelength plate Qc, and the polarization separation surface of the light beam dividing member 2203B. Is reflected in the + z direction. Even if the light beam Ld reflected by the polygon mirror 2104 returns, the return light becomes linearly polarized light in the first polarization direction by the quarter-wave plate Qd, and + z on the reflection mirror surface of the light beam dividing member 2203B. Reflected in the direction and transmitted through the polarization separation surface. That is, each return light does not return to the light source 2200B.

  Therefore, the number of light sources can be reduced without reducing the stability of optical scanning.

  Then, it is possible to achieve both reduction in size and cost of the optical scanning device while avoiding adverse effects on the image quality due to the return light.

  The color printer 2000 according to the present embodiment includes the optical scanning device 2010. As a result, it is possible to reduce the size and the price without degrading the image quality.

  In the above embodiment, as shown in FIG. 20 as an example, a ½ wavelength plate H may be used instead of the ¼ wavelength plate QvA of the return light removal optical system 2303A.

  This half-wave plate H is a linearly polarized light L0A from a light source, a linearly polarized light having a deflection direction θ of 45 ° (see FIG. 21A), or a linearly polarized light having a deflection direction θ of 135 ° (see FIG. 21B). The angle of the slow axis is set as shown in ()). The circularly polarized light is referred to as “beam Lh”. In this case, the ratio of transmitted light and reflected light at the light beam splitting member 2203A is approximately 1: 1.

  Similarly, the half-wave plate H may be used in place of the quarter-wave plate QvB of the return light removal optical system 2303B.

  Further, in the above embodiment, as shown in FIG. 22 as an example, a polarizer T may be used instead of the ¼ wavelength plate Qa and the ¼ wavelength plate Qb of the return light removing optical system 2303A. .

  The polarizer T is disposed on the optical path between the light source 2200A and the quarter-wave plate QvA. Further, the transmission direction of the polarizer T coincides with the polarization direction of the light beam L0A.

  In this case, as shown in FIG. 23, when the light beam L1 emitted from the light beam dividing member 2203A is reflected by the polygon mirror 2104 and returned, the return light L1r is reflected by the quarter-wave plate QvA. Since it becomes linearly polarized light in a direction orthogonal to the transmission direction of T, it is shielded by the polarizer T and does not reach the light source 2200A.

  Also, as shown in FIG. 24, when the light beam L2 emitted from the light beam splitting member 2203A is reflected by the polygon mirror 2104 and returned, the return light L2r is converted into a polarizer T by the quarter-wave plate QvA. Accordingly, the light is shielded by the polarizer T and does not reach the light source 2200A.

  In this case, as shown in FIG. 25, the light beam splitting member 2203A may be a half mirror surface instead of the polarization separation surface.

  Then, the half mirror surface so that the polarization state of the incident light, the polarization state of the light beam transmitted through the half mirror surface, and the polarization state of the light beam reflected by the half mirror surface and reflected by the reflection mirror surface substantially coincide with each other. It is preferable to set the multilayer coating conditions. The polarization state is generally elliptically polarized, and there are linearly polarized light and circularly polarized light as special cases of elliptically polarized light. Here, “the polarization state is coincident” means that the amplitude ratio and the phase difference of the light of the p-polarized component and the s-polarized component are coincident.

  Also, when the polarization state of the light beam transmitted through the half mirror surface is different from the polarization state of the incident light to the half mirror surface, or the polarization state of the light beam reflected by the half mirror surface and reflected by the reflection mirror surface When the polarization state of the light incident on the half mirror surface is different, a part of the return light includes a linearly polarized component in a direction that transmits the polarizer T. In such a case, a wave plate is provided on the optical path of the light beam transmitted through the half mirror surface, or on the optical path of the light beam reflected by the half mirror surface and reflected by the reflection mirror surface, and the light beam that has passed through the wave plate is transmitted. The polarization state may be substantially matched with the polarization state of the incident light on the half mirror surface.

  Similarly, the polarizer T may be used instead of the quarter-wave plate Qc and the quarter-wave plate Qd of the return light removing optical system 2303B. In this case, the light beam splitting member 2203B may be a half mirror surface instead of the polarization separation surface.

  In the above embodiment, each light beam splitting member may be a total reflection surface that totally reflects the light beam L2 instead of the reflection mirror surface.

  Moreover, although the case where each light beam splitting member is an integrated member has been described in the above embodiment, the present invention is not limited to this, and as shown in FIG. 26 as an example, each light beam splitting member is a polarization beam splitter 2203a. And a reflection member 2203b and a holding member 2203c for holding them.

  At this time, by allowing the polarization separation surface of the polarization beam splitter 2203a and the reflection surface of the reflection member 2203b to face each other, parallelism between the polarization separation surface and the reflection surface can be easily ensured.

  The polarizing beam splitter 2203a and the reflecting member 2203b may be fixed to the holding member 2203c with an adhesive, or the polarizing beam splitter 2203a and the reflecting member 2203b are pressed against the holding member 2203c with a spring member (not shown). It may be fixed by pressing. Further, at least one of the polarizing beam splitter 2203a and the reflecting member 2203b may penetrate the holding member 2203c.

  As an example, as shown in FIG. 27, the polarization beam splitter 2203a and the reflecting member 2203b may be held by holding members 2203c from both sides.

  In this case, the polarizing beam splitter 2203a and the reflecting member 2203b are preferably plate-like members because the manufacturing process becomes simpler than that of, for example, a prism-like member.

  The holding member 2203c is preferably made of glass. Thereby, even if there is a temperature change, the interval between the polarizing beam splitter 2203a and the reflecting member 2203b is unlikely to change. In this case, the side surfaces of the polarizing beam splitter 2203a and the reflecting member 2203b can be brought into contact with the holding member 2203c and fixed with an ultraviolet curing adhesive.

  Further, the holding member 2203c may be configured such that the relative angle between the polarizing beam splitter 2203a and the reflecting member 2203b can be adjusted. In this case, the polarizing beam splitter 2203a and the reflecting member 2203b are bonded and fixed to the holding member 2203c after their relative angles are adjusted.

  In the above embodiment, the case where the ratio of transmitted light and reflected light at each light beam splitting member is approximately 1: 1 has been described. However, the scanning optical system through which the light beam L1 passes and the scanning optical system through which the light beam L2 passes are described. When the light use efficiency is different, the ratio of transmitted light and reflected light may not be approximately 1: 1. In this case, the ratio of transmitted light and reflected light is set according to the light use efficiency of each scanning optical system.

  In the above-described embodiment, the case where one synchronization detection system is shared by two stations has been described. However, the present invention is not limited to this, and each station may have a synchronization detection system.

  In the above embodiment, instead of the surface emitting laser array 100, an LD (Laser Diode) having a single light emitting unit or an LD array having a plurality of light emitting units may be used. Note that the cost reduction effect is greater with the multi-beam light source.

  In the above embodiment, the case where the synchronization detection light beam passes through the first scanning lens has been described. However, the present invention is not limited to this, and the synchronization detection light beam may not pass through the first scanning lens. good.

  In the above-described embodiment, the case where only the light beam before the start of writing is received by the synchronization detection sensor has been described. However, a synchronization detection sensor that receives the light beam after the writing may be provided.

  In the above embodiment, the case where the polarization separation surface of each light beam splitting member has a function of transmitting p-polarized light and reflecting s-polarized light has been described. However, the present invention is not limited to this. The polarization separation surface may have a function of transmitting s-polarized light and reflecting p-polarized light.

  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 may be directly transferred to the recording paper.

  In the above embodiment, the color printer 2000 is described as the image forming apparatus. However, the present invention is not limited to this, and may be, for example, an optical plotter or a digital copying apparatus.

  Further, an image forming apparatus using a silver salt film as the image carrier may be used. In this case, a latent image is formed on the silver salt film by optical scanning, and this latent image can be visualized by a process equivalent to a developing process in a normal silver salt photographic process. Then, it can be transferred to a photographic paper as a transfer object by a process equivalent to a printing process in a normal silver salt photographic process. Such an image forming apparatus can be implemented as an optical plate making apparatus or an optical drawing apparatus that draws a CT scan image or the like.

  Further, an image forming apparatus using a color developing medium (positive photographic paper) that develops color by the heat energy of a beam 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 short, if it is an image forming apparatus provided with the optical scanning device 2010, as a result, it is possible to reduce the size and the price without degrading the image quality.

  As described above, the optical scanning device of the present invention is suitable for reducing the number of light sources without reducing the stability of optical scanning. The image forming apparatus of the present invention is suitable for reducing the size and the price without degrading the image quality.

  2000 ... Color printer (image forming apparatus), 2010 ... Optical scanning device, 2030a-2030d ... Photosensitive drum (image carrier), 2104 ... Polygon mirror (optical deflector), 2105a-2105d ... First scanning lens, 2106a- 2106d ... Folding mirror, 2107a to 2107d ... Second scanning lens, 2108a to 2108d ... Folding mirror, 2200A, 2200B ... Light source, 2201A, 2201B ... Coupling lens, 2203A, 2203B ... Flux splitting member, 2203a ... Polarizing beam splitter, 2203b ... reflective member, 2203c ... holding member, 2303A, 2303B ... return light removal optical system, H ... 1/2 wavelength plate, Qa, Qb, Qc, Qd, QvA, QvB ... ¼ wavelength plate, T ... polarizer .

Japanese Patent No. 4445234 Japanese Patent Laid-Open No. 2002-23085

Claims (12)

An optical scanning device that individually scans at least two scanned surfaces with light in the main scanning direction,
A light source;
A light beam having a split surface that divides the light beam from the light source into a first light beam and a second light beam, and emits the first light beam and the second light beam separated from each other in the sub-scanning direction orthogonal to the main scanning direction. Dividing means;
The first polygon mirror and the second polygon mirror that rotate out of phase with each other are provided, the first beam from the beam splitting means is deflected by the first polygon mirror, and the second beam is converted into the second polygon mirror. An optical deflector that deflects with
A scanning optical system for guiding the first light flux and the second light flux deflected by the optical deflector to the corresponding scanned surface, respectively.
At one timing while one of the first luminous flux and the second luminous flux scans the corresponding scanned surface, the other luminous flux is perpendicularly incident on the corresponding polygon mirror, and at that timing, An optical scanning device in which a conjugate point of the light source in the sub-scanning direction exists at a position shifted from the surface of the polygon mirror.   2. The optical scanning device according to claim 1, wherein the conjugate point of the light source in the sub-scanning direction is present at a position closer to the rotation axis of the polygon mirror than the surface of the polygon mirror. A return light removing optical system disposed between the light source and the optical deflector;
The split surface is a polarization separation surface that transmits one of two linearly polarized lights whose polarization directions are orthogonal to each other and reflects the other,
The return light removing optical system includes a quarter-wave plate or a half-wave plate disposed on an optical path between the light source and the light beam dividing unit, and the light of the first light beam from the light beam dividing unit. A quarter-wave plate disposed on the path, and a quarter-wave plate disposed on the optical path of the second light flux from the light beam splitting means,
The angle of the optical axis of the quarter-wave plate arranged on the optical path of the first light flux and the quarter-wave plate arranged on the optical path of the second light flux is 45 with respect to the polarization direction of the incident light. The optical scanning device according to claim 1, wherein the optical scanning device is inclined at an angle. A return light removing optical system disposed between the light source and the optical deflector;
The dividing surface is a half mirror that divides incident light into transmitted light and reflected light at a predetermined ratio,
The return light removal optical system is disposed on a polarizer and an optical path between the polarizer and the optical deflector, and the direction of the optical axis is inclined by 45 ° with respect to the transmission axis direction of the polarizer. The optical scanning device according to claim 1, further comprising a quarter-wave plate.   5. The polarization state of the light beam incident on the light beam splitting means, the polarization state of the light beam transmitted through the half mirror, and the polarization state of the light beam reflected by the half mirror are substantially the same. The optical scanning device according to 1. The polarization state of the light beam incident on the light beam splitting means, the polarization state of the light beam transmitted through the half mirror, and the polarization state of the light beam reflected by the half mirror are different from each other,
The return light removing optical system splits the polarization state of the two light beams toward the optical deflector on at least one of the light beam transmitted through the half mirror and the light beam reflected by the half mirror. 5. The optical scanning device according to claim 4, further comprising a wave plate for substantially matching the polarization state of the light beam incident on the means.   The optical beam scanning unit according to claim 1, wherein the light beam splitting unit includes a prism, and the prism includes the split surface and a reflective surface parallel to the split surface. apparatus.   8. The optical scanning device according to claim 7, wherein one of two opposing surfaces of the prism is the dividing surface and the other is the reflecting surface.   The optical scanning device according to claim 1, wherein the light beam splitting unit includes a first member having the split surface and a second member having the reflective surface.   The optical scanning device according to claim 9, wherein the light beam splitting unit includes a holding member that integrally holds the first member and the second member.   The optical scanning device according to claim 10, wherein the holding member is a glass member, and the first member and the second member are bonded and fixed to the holding member. At least two image carriers;
An image forming apparatus comprising: the optical scanning device according to claim 1, wherein the at least two image carriers are scanned with a light beam modulated according to image data.