JP2007171689A - Optical scanner and image forming apparatus using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To keep the stability of exposure intensity, particularly the stability of the exposure intensity of a half-tone image, by suppressing the dependency of a laser light source such as a surface light emitting laser on a driving current. <P>SOLUTION: The optical scanner comprises: a light source 1; a luminous flux focusing means 2 which focuses a plurality of luminous fluxes emitted from the light source 1; a luminous flux deflecting means 3 which deflects and scans the focused luminous flux; and an imaging optical means 4 which images the deflected and scanned luminous flux on a imaging face 5. The luminous flux focusing means 2 comprises: a parallel luminous flux converting member 2a which converts the luminous flux emitted from the light source 1 into a substantially parallel luminous flux; a luminous flux branching member 2d which branches the parallel luminous flux from the parallel luminous flux converting member 2a into a passing luminous flux and a reflected luminous flux, and is so disposed that the reflecting face of which member intersects substantially parallel to or at right angle with the reflecting face of the luminous flux deflecting means 3; and a 1/4-wavelength plate 2c which is disposed between the parallel luminous flux converting member 2a and the luminous flux branching member 2d, and so disposed that the reflectance of the luminous flux branching member 2d is kept constant even when the ratio of the polarized light direction component of the luminous flux varies. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical scanning device used in an image forming apparatus such as a copying machine or a printer, and in particular, a plurality of light emitting beams such as a surface emitting laser are used as an exposure light source for forming a latent image by electrophotography. The present invention relates to an optical scanning device and an image forming apparatus using the same.

  In general, in an electrophotographic image forming apparatus, a light beam from a light source using a semiconductor laser is deflected by a rotary polygon mirror, and a desired latent image is formed using an optical scanning device that performs exposure scanning on a charged photoreceptor. A method of forming is known. In such a system, one using a surface emitting laser (VCSEL) as a light source is known. Unlike the edge-emitting laser (edge-emitting laser), this surface-emitting laser is easy to simplify the optical system due to the perfect circular beam profile, easy to make the laser array two-dimensional, low power consumption / high-speed driving, etc. There is an advantage, and the use is increasing in each field.

  On the other hand, in order to configure an optical scanning device using a semiconductor laser, it is usually necessary to monitor the light emission amount of the semiconductor laser itself and perform laser power control (light amount control) based on this. In this case, the rear emission light can be used with the edge emitting laser, but the front emission light can only be used with the surface emitting laser. Therefore, for example, as shown in FIG. 6, a beam splitter 102 is inserted into an optical system through which outgoing light emitted forward from the surface emitting laser 101 passes, and a part of the outgoing light is branched by this beam splitter 102. It has been proposed to use the monitor light 103 (see Patent Document 1).

Further, since the basic shape of the surface emitting laser is isotropic, the polarization direction is basically random, and even in the same element, the polarization direction varies depending on the injection current (drive current). Therefore, proposals have been made to make the polarization direction of a plurality of beams uniform in a predetermined direction by making the cross-sectional shape of the resonator elliptical or applying stress by wiring.
However, the effect of aligning the polarization directions of a plurality of beams of a surface emitting laser differs depending on the structure and manufacturing method of the surface emitting laser, and is not effective for all surface emitting lasers. Therefore, it is difficult for the surface emitting laser to secure a polarization ratio as that of the edge emitting laser, and the ratio may change depending on the injection current (drive current).

  On the other hand, various coatings on optical components have a dependency on the polarization direction of the beam. Therefore, for example, depending on the coating on the half mirror, when the polarization ratio becomes worse or the polarization ratio fluctuates, the reflectivity of the folding mirror that folds the beam deflected by the rotating polygon mirror changes depending on the scanning angle. Therefore, it becomes difficult to expose the imaging surface (photosensitive member) with a uniform light amount.

  When a beam splitter (half mirror) is used for light quantity control, if the transmission / reflection ratio of the half mirror has a polarization direction dependency, a light receiving element that receives monitor light according to the polarization ratio and an imaging plane ( There is a problem that the correlation with the light quantity on the photosensitive member) changes, the light quantity detection accuracy deteriorates, and proper light quantity control cannot be performed.

  To solve such a problem, as shown in FIG. 7, a linearly polarizing element (polarizing beam splitter) 112 is arranged in a direction inclined 45 degrees from the A-axis direction to the B-axis direction in the crystal growth of the surface emitting laser 111. A technique is also known in which only the beam polarized at 45 degrees between the A axis and the B axis is taken out to stabilize the output (see Patent Document 2).

JP-A-8-330661 (Example, FIG. 11) Japanese Utility Model Laid-Open No. 4-121771 (Example, FIG. 1) JP-A-8-264873 (Example, FIG. 1) JP-A-2002-40350 (Embodiment of the Invention, FIG. 9)

However, this method has a problem that even if most of the beam has a desired polarization direction, the amount of light is attenuated by about half, and the efficiency of use of the amount of light is always reduced.
Since the surface emitting laser has a shorter cavity length than the edge emitting laser, the output is smaller than that of the edge emitting laser. Further, as described above, if the beam other than the desired polarization direction is removed, the effective power becomes smaller. For this reason, the power of each beam becomes too smaller than the sensitivity of the light receiving element for detecting the light amount (for detecting the monitor light), and the light amount cannot be detected by the light receiving element, or the detection accuracy is lowered.

  In response to such a problem, the applicant of the present invention collects a plurality of beams, rather than one beam at a time, and enters the light receiving element to increase the amount of light detected, and measures and stores in advance the relationship between the drive current and emission intensity of each beam. Thus, a method has been proposed in which the light amount control is performed with the total light amount of all the beams while taking into account the variation (see Patent Document 3). However, since the light emission intensity with respect to the drive current of each beam changes depending on the use temperature and time, the method of change differs for each beam, and there is a concern that the dispersion itself between the beams becomes large. Furthermore, since the polarization direction varies depending on the driving current, the variation in exposure intensity when a solid image is exposed is absorbed, but the variation in transmittance at the rise of laser emission that occurs depending on the driving current cannot be corrected, and half There is also a concern that exposure fluctuations when a tone image is exposed cannot be suppressed.

  The present invention has been made to solve the above technical problems, and suppresses the drive current dependency of a laser light source that emits a plurality of light beams having different polarization directions, such as a surface emitting laser, and exposure intensity. In particular, the present invention provides an optical scanning apparatus and an image forming apparatus using the same, which maintain the stability of the exposure, particularly the stability of the exposure intensity during halftone image exposure.

  For the above problems, the applicant of the present application collimates a plurality of beams and then stops the aperture with an aperture and uses a quarter-wave plate to make all the polarization to the beam splitter circular. Thus, a method has been proposed in which the influence of the reflectance at the beam splitter and the folding mirror is reduced (see Patent Document 4). However, since the polarization direction of the beam by the surface emitting laser is not uniform, it is difficult to obtain all the light beams that have been transmitted through the quarter-wave plate, and to obtain desired characteristics. Therefore, the present inventors have found the present invention by paying more attention to the characteristics of the wave plate in order to stabilize the amount of light.

  That is, as shown in FIG. 1, the present invention includes a light source 1 that emits a plurality of light fluxes having different polarization direction component ratios p / (p + s) and varying polarization component ratios due to drive currents, and a light source 1. A light beam condensing unit 2 that condenses a plurality of light beams emitted from the light beam, a light beam deflecting unit 3 that deflects and scans the collected light beam, and an imaging optical that forms an image on the imaging surface 5 with the deflected and scanned light beam The light beam condensing means 2 includes a parallel light beam conversion member 2a that converts the light beam emitted from the light source 1 into a substantially parallel light beam, and the parallel light beam from the parallel light beam conversion member 2a into a transmitted light beam and a reflected light beam. The light beam branching member 2d is arranged between the parallel light beam conversion member 2a and the light beam branching member 2d. The light beam branching member 2d is arranged so as to be branched and its reflection surface is substantially parallel or substantially orthogonal to the reflection surface of the light beam deflecting means 3. In addition, the reflectance at the light beam branching member 2d is light. It is characterized in that even when the variation ratio of the polarization direction component and a quarter-wave plate 2c disposed so as to be constant.

In such technical means, the light source 1 may be any one that emits a plurality of light beams, and the number of the light sources 1 is not particularly limited. For example, a type in which a plurality of light beams are emitted by one surface emitting laser (VCSEL), or a type in which a plurality of surface emitting lasers and end surface emitting lasers are provided may be used. Note that the case where the polarization directions of a plurality of edge-emitting lasers are aligned is not included in this case.
In general, when a surface emitting laser is used, the ratio p / (p + s) between the p component in the polarization direction (a component parallel to the reflection surface) and the s component (a component perpendicular to the reflection surface) is different. The ratio of the polarization direction component varies. Therefore, this stabilization is required.

Further, the light beam deflecting means 3 may be any device that can deflect and scan an incident light beam, and a typical embodiment includes a rotating polygon mirror. Further, the imaging optical means 4 is only required to form an image of the light beam deflected and scanned by the light beam deflecting means 3 on the imaging surface 5, and is composed of, for example, a reflecting mirror or an fθ lens.
The light beam condensing means 2 in the present invention only needs to collect the light beam emitted from the light source 1 and irradiate the light beam polarizing means 3. The light beam concentrating means 2 disposed on the light source 1 side and substantially emitting the light beam emitted from the light source 1. A parallel light beam conversion member 2a that converts the light beam into a parallel light beam and the light beam deflecting means 3 side, and splits the parallel light beam into an equivalent light beam and a reflected light beam, and its reflecting surface is parallel or orthogonal to the reflecting surface of the light beam deflecting means 3. And a quarter-wave plate 2c provided between them.
Here, as the parallel light beam conversion member 2a, the light beam from the light source 1 is converted into a parallel light beam, and a typical mode includes a collimator lens. Further, as the light beam branching member 2d, a light beam is branched into a transmitted light beam and a reflected light beam, and a mode using a half mirror or a mode using a beam splitter can be mentioned. The “reflecting surface” in this case means a plane formed by incident light and outgoing light (reflected light) with respect to a certain boundary surface.

  In the present invention, it is preferable that the quarter wavelength plate 2c is disposed with the optical axis inclined by ± 45 degrees with respect to the reflecting surface of the light beam branching member 2d. By arranging in this way, the light beam incident on the wave plate Even if there are various polarization directions, the light beam that has passed through the quarter-wave plate 2c is either linearly polarized light, circularly polarized light, or elliptically polarized light, and each polarized light is different, but the time-averaged amplitude reflectivity is all It becomes equal and can be regarded as a stable light beam regardless of the polarization of the light beam on the light source 1 side. Therefore, it is possible to stabilize both the transmitted light beam and the reflected light beam in the light beam branching member 2d. In addition, ± 45 degrees includes a tolerance when the wave plate is actually installed, and means approximately ± 45 degrees.

  In addition, the present invention preferably includes light intensity correction means 6 that corrects the emission intensity of the light source 1 based on the intensity of either the transmitted light beam or the reflected light beam by the light beam branching member 2d. The light flux emitted from the light source 1 can be stabilized.

  Further, the light beam condensing means 2 in the present invention preferably includes a light beam regulating member 2b for regulating the shape of the parallel light beam between the parallel light beam converting member 2a and the light beam deflecting means 3, and according to this, the light beam deflecting means. The light beam incident on 3 can be effectively reduced, and the beam spot on the image plane 5 can be easily reduced. The quarter-wave plate 2c described above may be disposed between the light beam restricting member 2b and the light beam branching member 2d, or may be disposed between the parallel light beam converting member 2a and the light beam restricting member 2b. However, it is preferable to arrange the light beam after the light beam shape restriction by the light beam restriction member 2b from the viewpoint of improving the stability of the characteristics.

The imaging optical means 4 in the present invention has a reflecting mirror, and the reflecting surface of the light beam reflected at the central portion in the main scanning direction of the reflecting mirror is substantially the same as the reflecting surface of the light beam branching member 2d. It is preferable that they are arranged so as to be orthogonal or substantially parallel, and by providing such a reflecting mirror in the imaging optical means 4, freedom of layout and miniaturization are promoted.
Further, the reflecting mirror preferably includes an antireflection layer on the surface of the reflecting mirror so that the difference between the p component and the s component in the polarization direction of the reflected light beam is 2% or less of each other. According to this, since the difference between the p component and the s component of the reflected light beam is as small as 2% or less, the influence of the polarization due to the reflection by the reflecting mirror can be ignored. Further, by providing the antireflection layer, unnecessary reflection on the surface of the reflecting mirror can be prevented, and reflection on the reflection boundary surface can be reliably performed.
The antireflection layer is preferably a laminated film in which a plurality of thin film layers having different refractive indexes are laminated, and more preferably a laminated film comprising 4 or more thin film layers. According to this, it becomes relatively easy to make the difference between the p component and the s component of the reflected light beam 2% or less.

The light source 1 for emitting a plurality of light beams according to the present invention is preferably a single surface emitting laser, and in this case, the degree of freedom in design can be greatly improved.
Furthermore, the present invention is not limited to an optical scanning device, and is also intended for an image forming apparatus. In this case, an image carrier on which an electrostatic latent image is carried and an image can be formed on the image carrier. What is necessary is just to make it provide the above-mentioned optical scanning device as an optical scanning device.

According to the present invention, a light source that emits a plurality of light beams, a light beam condensing unit, a light beam deflecting unit, and an imaging optical unit are provided. The light beam condensing unit includes a parallel light beam conversion member and a parallel light beam conversion unit. A light beam branching member that branches a parallel light beam from the member into a transmitted light beam and a reflected light beam and whose reflection surface is substantially parallel or substantially orthogonal to the reflection surface of the light beam deflecting means; a parallel light beam conversion member; And a quarter-wave plate provided between the branching member and the optical axis inclined at +45 degrees or -45 degrees with respect to the reflecting surface of the beam branching member. Even if the polarization directions are different, stable light quantity control is possible, and the exposure intensity can be stabilized. Therefore, in particular, it is possible to provide an optical scanning device that can stabilize the exposure intensity at the time of halftone image exposure and can form a high-quality latent image.
Further, by using such an optical scanning device, it is possible to provide an image forming apparatus capable of improving the image quality, particularly the halftone image.

Hereinafter, the present invention will be described in detail based on embodiments shown in the accompanying drawings.
FIG. 2 shows an embodiment of an image forming apparatus to which the present invention is applied.
In the figure, the image forming apparatus according to the present embodiment includes a photoconductor drum 20 and an intermediate transfer disposed opposite to the photoconductor drum 20 in order to transfer a toner image from the photoconductor drum 20 in the apparatus main body 50. This is a so-called four-cycle type intermediate transfer type color image forming apparatus that includes a belt 30 and performs four times of multiple transfers on the intermediate transfer belt 30 to obtain four color images.
In the present embodiment, the photosensitive drum 20 includes a photosensitive layer whose resistance value is reduced by light irradiation. Around the photosensitive drum 20, a charging device 21 for charging the photosensitive drum 20 and a charging device 21 are provided. An optical scanning device (exposure device) for writing an electrostatic latent image of each color component (in this example, black (K), yellow (Y), magenta (M), cyan (C)) on the charged photosensitive drum 20. ) 60, a rotary developing device 23 that visualizes each color component latent image formed on the photosensitive drum 20 with each color component toner, an intermediate transfer belt 30, and residual toner on the photosensitive drum 20. A cleaning device 27 for cleaning is disposed.

  Here, as the charging device 21, for example, a charging roll is used, but a charger such as a corotron may be used. Further, the rotary developing device 23 is rotatably mounted with developing units 23a to 23d containing respective color component toners. For example, the respective color component toners are attached to a portion of the photosensitive drum 20 where the potential is lowered by exposure. The toner to be used is not particularly limited in shape and particle size, and any toner may be used as long as it is accurately placed on the electrostatic latent image on the photosensitive drum 20. In this example, the rotary developing device 23 is used, but four developing devices may be used. Furthermore, as long as the cleaning device 27 cleans the residual toner on the photosensitive drum 20, a cleaning device employing a blade cleaning method may be appropriately selected. However, there may be a mode in which the cleaning device 27 is not used when toner having a high transfer rate is used.

Further, the intermediate transfer belt 30 is stretched around three stretching rolls 31 to 33, and circulates and moves, for example, using the stretching roll 31 as a driving roll.
Here, the intermediate transfer belt 30 may be appropriately selected from a resin material such as polyimide resin. However, in order to effectively suppress image quality defects such as a holocharacter, the contact surface pressure with the photosensitive drum 20 may be lowered. In consideration of the viewpoint of walkless and tensionerless, it is preferable to use a rubber belt material such as urethane rubber having an elastic rubber as a base (elastic layer).

Further, in the present embodiment, a primary transfer roll 25 as a primary transfer member is disposed in contact with the back side of the intermediate transfer belt 30 at a portion of the intermediate transfer belt 30 facing the photosensitive drum 20, and a predetermined primary A transfer bias is applied.
Furthermore, a secondary transfer roll 35 as a secondary transfer member is disposed opposite to the tension roll 32 of the intermediate transfer belt 30 with the tension roll 32 as a backup roll. For example, the secondary transfer roll A predetermined secondary transfer bias is applied to 35, and a stretching roll 32 that also serves as a backup roll is grounded.
Note that a belt cleaning device 36 is disposed at a portion of the intermediate transfer belt 30 facing the stretching roll 31 so as to clean the residual toner on the intermediate transfer belt 30.
Further, the recording material 40 such as paper is accommodated in a supply tray (not shown), and is supplied by a feed roll 41 and then guided to a secondary transfer portion through a resist roll 42 and a secondary transfer roll 35. As a result, the toner images that have been multiple-transferred onto the intermediate transfer belt 30 are collectively transferred onto the recording material. The recording material on which the toner images are collectively transferred is conveyed to the fixing device 45 via the conveyance belt 43, and then accommodated in the discharge tray 48 formed on the upper portion of the apparatus main body 50 via the conveyance roll 46 and the discharge roll 47. It has become so.

  In particular, the optical scanning device 60 according to the present embodiment, as shown in FIG. 3, is a surface emitting laser (VCSEL) 61 that emits a plurality of emission beams, and a condensing beam that collects the emission beams emitted from the VCSEL 61. A device 70, a polygon mirror 80 that deflects and scans the light beam condensed by the light collector 70, and an imaging optical device 90 that forms an image on the photosensitive drum 20 of the light beam deflected and scanned by the polygon mirror 80. Yes. Note that the VCSEL 61 of the present embodiment has a property that the polarization direction tends to be irregular because the emitted light beam is emitted perpendicularly to the active layer.

The condensing device 70 also collimates the emitted light beam emitted from the VCSEL 61 into a parallel light beam, and a diaphragm 72 as a light beam regulating member that shapes the cross-sectional shape of the parallel light beam by the collimator lens 71. The quarter-wave plate 73 through which the shaped light beam is transmitted, the half mirror 74 that branches the light beam that has passed through the quarter-wave plate into the transmitted light beam and the reflected light beam, and the light beam that has passed through the half mirror 74 along the sub-scanning direction And a cylindrical lens 75 that collects light.
Further, the diaphragm 72 is disposed at the image-side focal position of the collimator lens 71, and a plurality of emission beams emitted from the VCSEL 61 intersect at the position of an opening (aperture) 72 a provided in the diaphragm 72. . For this reason, a plurality of emitted light beams can be equivalently shaped by one aperture 72a. Since the aperture width of the aperture 72a depends on the lateral magnification of the optical system and the spot size of the photosensitive drum 20, the shape of the aperture 72a is determined so that a desired optical system lateral magnification and spot size are selected. Yes.
Furthermore, the quarter-wave plate 73 is a birefringent element in which a phase difference occurs between the p component and the s component in the polarization direction when the light beam shaped by the aperture 72a is transmitted, and the phase difference is 90 degrees. Has become.

Here, the layout centering on the condensing device 70 is demonstrated in detail using FIG.
The quarter wavelength plate (λ / 4 plate) 73 has an axis tilted by +45 degrees or −45 degrees with respect to the reflecting surface of the half mirror 74 (the surface formed by the incident light and the reflected light on the half mirror 74). Has been provided. Further, the reflecting surface of the half mirror 74 is arranged to be parallel to the reflecting surface of the polygon mirror 80. In the present embodiment, the reflecting surface of the half mirror 74 and the reflecting surface of the polygon mirror 80 are arranged so as to be parallel to each other.

In this embodiment, since the emission intensity of the VCSEL 61 is corrected by the light beam reflected by the half mirror 74, the light receiving element 62 that receives the reflected light beam by the half mirror 74 and the signal from the light receiving element 62 are used. A control device 63 for controlling the VCSEL 61 is provided. Accordingly, an output signal photoelectrically converted by the light beam incident on the light receiving element 62 is input to the control device 63, and the VCSEL 61 is controlled to have a predetermined output by controlling the drive current of the VCSEL 61. Therefore, so-called APC control (Automatic Power Control) is performed.
Since the plurality of light beams intersecting at the position of the aperture 72a are gradually separated thereafter, the light beam reflected by the half mirror 74 and incident on the light receiving element 62 tends to spread, and the light receiving surface of the light receiving element 62 is The one wider than the incident light beam is preferable. Further, if a condensing lens is separately provided between the half mirror 74 and the light receiving element 62, the area of the light receiving surface can be reduced.

  In the present embodiment, as shown in FIG. 3, the polygon mirror 80 has a regular polygonal shape (in this example, a regular hexagon) having a plurality of reflection boundary surfaces on its side surface. A shaft is attached to the motor, and is rotated at a predetermined rotational speed in the direction of arrow R in the figure by the driving force of the polygon motor. Therefore, the light beam incident on the polygon mirror 80 deflects and scans the imaging optical device 90 along the main scanning direction, that is, the photosensitive drum 20 along the axial direction in accordance with the rotation.

On the other hand, the imaging optical device 90 has, for example, a cylindrical lens 91, a toric lens 92, a coupling lens as an imaging lens system for keeping the scanning speed of the light beam deflected and scanned by the polygon mirror 80 on the surface of the photosensitive drum 20 constant. An image lens 93 is included. In the present embodiment, such a configuration is shown as the imaging lens system. However, the present invention is not limited to this, and any other imaging lens system may be used as long as the scanning speed on the surface of the photosensitive drum 20 is constant. There is no problem depending on the configuration.
In the imaging optical device 90 according to the present embodiment, the light beam transmitted through the imaging lens system passes through two reflecting mirrors, that is, a cylindrical mirror 94 having a paraboloid and a folding mirror 95 of a plane mirror. The image is formed as a spot image on the surface of the photosensitive drum 20, and an electrostatic latent image corresponding to the image information is formed on the photosensitive drum 20.
Further, the imaging optical device 90 includes a mirror 96 that reflects a part of the light beam that has passed through the imaging lens system (the light beam corresponding to the scanning start position by each reflection boundary surface of the polygon mirror 80). An SOS (Start of Scan) light receiving unit 97 including a photodetector or the like that detects the reflected light is provided. For this reason, the SOS light receiving unit 97 detects the scanning start timing for each line (main scanning direction) to the photosensitive drum 20. Needless to say, the SOS light receiving unit 97 is provided outside the region of the light beam deflected and scanned by the polygon mirror 80.

In the present embodiment, the cylindrical mirror 94 and the folding mirror 95 are provided so that the reflection surface at the center in the main scanning direction is orthogonal to the reflection surface of the half mirror 74 of the light collector 70. Furthermore, an antireflection layer in which four or more layers having different refractive indexes are laminated is provided on the surfaces of these reflecting mirrors so that the difference between the p component and the s component of the reflected light beam is 2% or less. Yes. Therefore, reflection depending on the deflection direction by the polygon mirror 80 having a reflection surface parallel to the reflection surface of the half mirror 74 can be performed by the cylindrical mirror 94 and the folding mirror 95, and the drive current in the VCSEL 61 and the interval between the emission beams. Thus, it is possible to minimize the reflectance variation due to the variation of the.
The reflective surfaces of the cylindrical mirror 94 and the folding mirror 95 may be arranged so as to be orthogonal to the above-described surface.

Here, the antireflection layer will be described in detail. As the antireflection layer of this embodiment, a film having a high refractive index (for example, ZnSe: n = 2.55, ZnTe: n = 2.92, ZnS: n = 2.37, TiO ₂ : n = 2.45). Etc.) and a film having a low refractive index (for example, MgF ₂ : n = 1.39, NaF: n = 1.32, LiF: n = 1.39, etc.) are applied to form a multilayer coating. The p / s difference in rate can be reduced.
At this time, the thickness of each layer is equivalent to λ / 4, and necessary layers are laminated in order from the substrate side so as to become a low refractive index layer, a high refractive index layer, and an air layer. However, the thickness of the lowermost layer in contact with the metal surface (for example, Al or the like) such as a metal mirror is not a dielectric, and it is necessary to make the thickness about 10% thinner than λ / 4.

Next, the operation of the image forming apparatus according to the present embodiment will be described focusing on the optical scanning device 60. In FIG. 3, a plurality of light-emitting beams emitted from the VCSEL 61 become parallel light beams by the collimator lens 71, a part of the parallel light beams are shielded by the aperture 72 a of the diaphragm 72, and only the shaped light beam passes through the diaphragm 72. To Penetrate.
The shaped light beam passes through the λ / 4 plate 73. At this time, the polarization directions (ratio between the p component and the s component) of the plurality of emitted beams emitted from the VCSEL 61 vary depending on the drive current and the variation between the emitted beams, but the λ / 4 plate 73 has a slow axis. Since it is provided with an inclination of +45 degrees or −45 degrees with respect to the reflection surface of the subsequent half mirror 74, the incident light on the λ / 4 plate 73 is transmitted through the λ / 4 plate 73 in any state. The luminous flux can be narrowed down to the following three types.
That is, circularly polarized light, elliptically polarized light inclined +45 degrees or −45 degrees with respect to the reflecting surface of the half mirror 74, and linearly polarized light inclined +45 degrees or −45 degrees with respect to the reflecting surface of the half mirror 74.
Therefore, the reflectance at the half mirror 74 (the transmittance through the half mirror 74 is also equal) is equal, and changes due to the drive current of the VCSEL 61 (that is, polarization direction dependence) and variations among the emitted beams are eliminated. Will be able to.

Here, the operation of the λ / 4 plate in the present embodiment will be described in detail. FIG. 5 shows the state when the λ / 4 plate is set in the electric field of monochromatic light. The electric field vector at each position is drawn with the center line of the light beam as the base point, and the tips of the vectors are connected. The curve is shown.
In the same figure, the λ / 4 plate is installed so that its slow axis is inclined by 45 degrees. Therefore, when linearly polarized light (shown in the figure) inclined by 45 degrees as incident light passes through the λ / 4 plate, The linearly polarized light is tilted 45 degrees as it is. Further, if the incident light of the λ / 4 plate is vertically polarized light (y-axis direction), the transmitted light is circularly polarized light. Further, if the incident light is linearly polarized light at an angle different from these, the transmitted light becomes elliptically polarized light.

Next, the effect of inserting a phase plate (wavelength plate) into linearly polarized light will be verified.
Now, linearly polarized light with a polarization plane tilted Ω from the x-axis
Ex = A cosΩ cos 2πνt, Ey = A sinΩ cos 2πνt (1)
Is rotated by an angle ω between the slow axis of the phase plate and the x axis,
Ex '＝ Ex cosω ＋ Ey sinω
= A cos 2πνt (cosΩ cosω + sinΩ sinω) = A cos2πνt cos (Ω−ω)
Ey '＝ − Ex sinω ＋ Ey cosω
= A cos 2πνt (−cosΩ sinω + sinΩ cosω) = A cos2πνt sin (Ω−ω) (2)
Can be written.
When (2) passes through the phase plate, it changes as follows due to the phase difference (δ) due to the phase plate.
Ex '＝ A cos 2πνt cos (Ω−ω)
Ey '＝ A cos (2πνt ＋ δ) sin (Ω−ω) (3)
If you rotate -ω to rewrite (3) about the original axis,
Ex = Ex 'cosω−Ey' sinω
= A {cos 2πνt cos (Ω−ω) cosω−cos (2πνt + δ) sin (Ω−ω) sinω}
= A {cos 2πνt cos (Ω−ω) cosω−cos 2πνt cosδsin (Ω−ω) sinω
+ Sin 2πνt sinδsin (Ω−ω) sinω}
= A [cos 2πνt {cos (Ω−ω) cosω−cosδsin (Ω−ω) sinω}
+ Sin 2πνt sinδsin (Ω−ω) sinω]
Ey = Ex 'sinω + Ey' cosω
= A {cos 2πνt cos (Ω−ω) sinω + cos (2πνt + δ) sin (Ω−ω) cosω}
= A {cos 2πνt cos (Ω−ω) sinω + cos 2πνt cosδsin (Ω−ω) cosω
−sin 2πνt sinδsin (Ω−ω) cosω}
= A [cos 2πνt {cos (Ω−ω) sinω + cosδsin (Ω−ω) cosω}
−sin 2πνt sinδsin (Ω−ω) cosω] (4)
It becomes.
(4) is elliptically polarized,
Ex = Ap cos (2πνt + δp) = Ap (cos2πνt cosδp−sin2πνt sinδp)
Ey = As cos (2πνt + δs) = As (cos2πνt cosδs−sin2πνt sinδs) (5)
Can be written,
Ex = P cos 2πνt + Q sin 2πνt
Ey = S cos 2πνt + T sin 2πνt (6)
Compared with Toki (5)
P = Ap cosδp, Q = -Ap sinδp, S = As cosδs, T = -As sinδs (7)
Compared to (4)
P = A {cos (Ω−ω) cosω−cosδsin (Ω−ω) sinω}
Q = A sinδsin (Ω−ω) sinω
S = A {cos (Ω−ω) sinω + cosδsin (Ω−ω) cosω}
T = −A sinδsin (Ω−ω) cosω (8)
(7)
Ap ² = P ² + Q ² , As ² = S ² + T ² , tanδp = − Q / P, tanδs = − T / S (9)
However, if (8) is substituted,
^{Ap 2 = A 2 [{cos} (Ω-ω) cosω-cosδsin (Ω-ω) sinω} 2
+ Sin ² δsin ² (Ω−ω) sin ² ω]
＝ A ² (cos ² (Ω−ω) cos ² ω−2cosδsin (Ω−ω) cos (Ω−ω) sinω cosω
+ Cos ² δsin ² (Ω−ω) sin ² ω + sin ² δsin ² (Ω−ω) sin ² ω}
＝ A ² (cos ² (Ω−ω) cos ² ω + sin ² (Ω−ω) sin ² ω
−2cosδsin (Ω−ω) cos (Ω−ω) sinω cosω}
As ² = A ² [{cos (Ω−ω) sinω + cosδsin (Ω−ω) cosω} ²
+ Sin ² δsin ² (Ω−ω) cos ² ω]
＝ A ² [{cos ² (Ω−ω) sin ² ω + 2cosδsin (Ω−ω) cos (Ω−ω) sinω cosω
^{+ Cos 2 δsin 2 (Ω-} ω) cos 2 ω + sin 2 δsin 2 (Ω-ω) cos 2 ω]
＝ A ² [{cos ² (Ω−ω) sin ² ω + sin ² (Ω−ω) cos ² ω
+ 2cosδsin (Ω−ω) cos (Ω−ω) sinω cosω] (10)
It becomes. This sum (light intensity)
Ap ² + As ² = A ² (11)
Next, although the degree of polarization by Ω and ω are changed, since the loss of energy is not the light intensity (Ap ² + As ²⁾ it is constant.

Therefore, when the λ / 4 plate is inserted with an inclination of λ / 4, ω = π / 4 and δ = π / 2 may be substituted into the equation (10).
Ap ² = A ² {cos ² (Ω−π / 4) cos ² π / 4 + sin ² (Ω−π / 4) sin ² π / 4
−2cosπ / 2sin (Ω−π / 4) cos (Ω−π / 4) sinπ / 4 cosπ / 4}
= A ² / √2
Similarly,
As ² = A ² / √2
It becomes.
Therefore, regardless of the polarization direction Ω of incident light,
Ap ² = As ²
Comes to hold.

  That is, regardless of the angle of the linearly polarized light of the incident light, the intensities of the p component and the s component of the transmitted light transmitted through the λ / 4 plate are equal, so that the reflectances of the p component and s component are also equal. .

As described above, the quarter-wave plate (λ / 4 plate) 73 is installed with an inclination of ± 45 degrees with respect to the reflecting surface of the half mirror 74, so that the λ / 4 plate 73 is transmitted regardless of the incident light beam. Since the reflectance of the light beam becomes equal, fluctuations in reflectance and transmittance at the half mirror 74 due to the polarization direction of the light beam incident on the λ / 4 plate 73 can be canceled, and changes due to the drive current in the VCSEL 61 (Depending on the polarization direction) and variations between emitted light beams can be eliminated. Therefore, it is possible to reduce the exposure amount fluctuation of the halftone exposure image.
If the λ / 4 plate 73 is arranged at a position other than 45 degrees, the reflectance and transmittance at the half mirror 74 vary due to the deviation of the polarization direction of the emitted beam emitted from the VCSEL 61, and in particular, the halftone The exposure amount in the image also varies, and the image quality deteriorates.

In the present embodiment, a configuration in which the cylindrical mirror 94 and the folding mirror 95 are provided in the imaging optical device 90 is shown. However, the configuration of the reflecting mirror is not limited to this. You may make it prepare a sheet.
The image forming apparatus according to the present embodiment is a four-cycle image forming apparatus. However, for example, the image forming apparatus can be applied to a so-called tandem image forming apparatus including four photosensitive drums 20 in parallel. Nor.

It is explanatory drawing which shows the outline | summary of the optical scanning device based on this invention. 1 is an explanatory diagram illustrating an image forming apparatus according to an embodiment to which the present invention is applied. It is explanatory drawing which shows the outline | summary of the optical scanning device of embodiment. It is a top view which shows the condensing device of the optical scanning device of embodiment. It is explanatory drawing which shows an example of an effect | action of a quarter wavelength plate. It is explanatory drawing which shows the example using the conventional beam splitter. It is explanatory drawing which shows the example using the conventional polarizing beam splitter.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Light source, 2 ... Light beam condensing means, 2a ... Parallel light beam conversion member, 2b ... Light beam control member, 2c ... 1/4 wavelength plate, 2d ... Light beam branching member, 3 ... Light beam deflection means, 4 ... Imaging optical means 5 ... imaging plane, 6 ... light intensity correction means

Claims (10)

A light source that emits a plurality of light fluxes each having a different ratio p / (p + s) of polarization direction components and a change in the ratio of polarization components due to a drive current;
Luminous flux condensing means for collecting a plurality of luminous fluxes emitted from the light source;
Luminous flux deflecting means for deflecting and scanning the condensed luminous flux;
Imaging optical means for imaging the deflected and scanned light beam on the imaging surface;
The light beam condensing means includes a parallel light beam conversion member that converts the light beam emitted from the light source into a substantially parallel light beam,
A light beam branching member that branches the parallel light beam from the parallel light beam conversion member into a transmitted light beam and a reflected light beam, and whose reflection surface is substantially parallel or substantially orthogonal to the reflection surface of the light beam deflecting means;
A quarter-wave plate provided between the parallel light beam conversion member and the light beam branching member, and disposed so that the reflectance at the light beam branching member is constant even if the ratio of the polarization direction component of the light beam varies. An optical scanning device comprising: The optical scanning device according to claim 1,
The optical scanning device according to claim 1, wherein the quarter-wave plate is disposed with an optical axis inclined at +45 degrees or -45 degrees with respect to the reflecting surface of the light beam branching member. The optical scanning device according to claim 1,
An optical scanning device further comprising: a light intensity correction unit that corrects the emission intensity of the light source based on the intensity of either the transmitted light beam or the reflected light beam by the light beam branching member. The optical scanning device according to claim 1,
The light beam condensing unit includes a light beam regulating member for regulating the shape of the parallel light beam between the parallel light beam converting member and the light beam deflecting unit. The optical scanning device according to claim 1,
The imaging optical means has a reflecting mirror, and is arranged so that the reflecting surface of the light beam reflected at the central portion in the main scanning direction of the reflecting mirror is substantially orthogonal or substantially parallel to the reflecting surface of the light beam branching member. An optical scanning device characterized by that. The optical scanning device according to claim 5.
The reflection mirror is provided with an antireflection layer on the surface of the reflection mirror so that the difference between the p component and the s component in the polarization direction of the reflected light beam is 2% or less of each other. Scanning device. The optical scanning device according to claim 6.
2. The optical scanning device according to claim 1, wherein the antireflection layer is a laminated film in which a plurality of thin film layers having different refractive indexes are laminated. The optical scanning device according to claim 7.
2. The optical scanning device according to claim 1, wherein the laminated film is composed of four or more thin film layers. The optical scanning device according to claim 1,
An optical scanning device characterized in that a light source for emitting a plurality of light beams is one surface emitting laser. An image carrier on which an electrostatic latent image is carried;
An image forming apparatus comprising: the optical scanning device according to claim 1, which is capable of forming an image on the image carrier.

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