Optical scanner and image formation apparatus

Abstract

To provide a small-sized optical scanner capable of preventing luminous intensity from becoming uneven on a surface to be scanned.SOLUTION: An optical scanner includes: a first optical element having a first microstructure configured to change a polarization state of a first light flux from a first light source, the first microstructure formed at least on one of an incidence plane and an emission plane; a deflector configured to deflect the first light flux having passed through the first optical element to scan a first surface to be scanned in a principal scanning direction; and a first imaging optical system configured to guide the first light flux deflected by the deflector to the first surface to be scanned. The first microstructure is structured with a plurality of protrusions each extending in a first direction and arranged in a second direction perpendicular to the first direction. If a phase difference is Δ1 between a first direction and a second direction of birefringence by the first microstructure, and a wavelength of the first light flux is λ1, a following condition is satisfied: 1 / 20≤Δ1 / λ1≤1 / 6.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] The present invention relates to an optical scanning device, and is particularly suitable for image forming apparatuses such as laser beam printers (LBPs), digital copiers, and multifunction printers (MFPs). [Background technology]

[0002] Conventionally, it has been known that in optical scanning devices, the reflectance of the deflecting surface or the reflecting surface for a given light beam differs depending on the angle of incidence between the light beams scanning each image height on the scanned surface, resulting in uneven illuminance of the light beam on the scanned surface due to the light beam.

[0003] Patent document 1 discloses an optical scanning device in which a λ / 2 plate is provided in the input optical system to tilt the polarization direction of the light beam when it enters the deflector by 45° from the sub-scanning direction in a cross section perpendicular to the optical axis, thereby reducing the change in the reflectance of the deflecting surface of the deflector for the light beam that corresponds to the angle of incidence, thereby preventing the illuminance caused by the light beam on the scanned surface from becoming uneven. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2000-267034 A Summary of the Invention [Problem to be solved by the invention]

[0005] In the optical scanning device disclosed in Patent Document 1, a λ / 2 plate is further provided in the incident optical system, so the device becomes large in size. SUMMARY OF THE PRESENT EMBODIMENT An object of the present invention is to provide a small-sized optical scanning device capable of suppressing non-uniformity in illuminance on a surface to be scanned. [Means for solving the problem]

[0006] The optical scanning device according to the present invention includes a first optical element having a first microstructure formed on at least one of an entrance surface and an exit surface, the first microstructure changing the polarization state of a first light beam from a first light source; a deflector that deflects the first light beam that has passed through the first optical element to scan a first scanned surface in a main scanning direction; and a first imaging optical system that guides the first light beam deflected by the deflector to the first scanned surface, the first microstructure being configured with a plurality of convex portions each extending in a first direction and arranged in a second direction perpendicular to the first direction, and a phase difference between the first direction and the second direction of birefringence due to the first microstructure being defined as Δ 1 , the wavelength of the first light beam is λ 1 When 1 / 20≦Δ 1 / λ 1 ≦1 / 6 The present invention is characterized in that it satisfies the following conditions. Effect of the Invention

[0007] According to the present invention, it is possible to provide a small-sized optical scanning device capable of suppressing non-uniformity in illuminance on a surface to be scanned. [Brief description of the drawings]

[0008] [Figure 1] 2A and 2B are developments in a main scanning section and a sub-scanning section of the optical scanning device according to the first embodiment. [Diagram 2] 5 is a diagram showing the incidence angle dependency of the reflectance of a deflecting surface of a deflector included in the optical scanning device according to the first embodiment. [Diagram 3] 2 is a sub-scanning sectional view of a scanning optical system included in the optical scanning device according to the first embodiment. FIG. [Figure 4] 5 is a diagram showing the incidence angle dependency of the reflectance of a folding mirror included in the optical scanning device according to the first embodiment. FIG. [Diagram 5] 3A and 3B are a schematic perspective view and a schematic partially enlarged cross-sectional view of a collimator lens included in the optical scanning device according to the first embodiment. [Figure 6] 3A and 3B are developments of the verification device in the main scanning section and the sub-scanning section. [Figure 7]6A and 6B are diagrams showing the results of measurement of the light quantity at each image height on the scanned surface by a verification device. [Figure 8] FIG. 11 is a schematic front view showing a method for setting the structural axis angle of the microstructure. [Figure 9] 13A and 13B are developments in a main scanning section and a sub-scanning section of an optical scanning device according to a second embodiment. [Figure 10] 11A and 11B are a schematic perspective view and a schematic partially enlarged cross-sectional view of an optical element included in an optical scanning device according to a second embodiment. [Figure 11] 2 is a sub-scanning sectional view of a main part of the color image forming apparatus according to the embodiment. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, an optical scanning device according to the present embodiment will be described in detail with reference to the accompanying drawings. Note that the drawings shown below may be drawn at a scale different from the actual scale in order to facilitate understanding of the present embodiment.

[0010] In the following description, the main scanning direction is the direction in which the light beam is deflected and scanned by the deflector. The sub-scanning direction is the direction parallel to the rotation axis of the deflector. The main scanning cross section is a cross section perpendicular to the sub-scanning direction. The sub-scanning cross section is a cross section perpendicular to the main scanning direction.

[0011] [First embodiment] Conventionally, it has been known that in optical scanning devices, the reflectance of the deflecting surface or the reflecting surface for a given light beam differs depending on the angle of incidence between the light beams scanning each image height on the scanned surface, resulting in uneven illuminance of the light beam on the scanned surface due to the light beam. An optical scanning device has been proposed that prevents the illuminance caused by the light beam on the scanned surface from becoming uneven by providing a λ / 2 plate in the incident optical system and tilting the polarization direction of the light beam entering the deflector by 45° from the sub-scanning direction in a cross section perpendicular to the optical axis, thereby reducing the change in the reflectance of the deflecting surface of the deflector for the light beam that varies with the angle of incidence.

[0012] However, in such an optical scanning device, since a λ / 2 plate is further provided in the incident optical system, the device becomes large and complicated, and the cost of the device becomes high due to the cost of manufacturing the λ / 2 plate. Therefore, an object of this embodiment is to provide a small and simple optical scanning device that can suppress non-uniformity in illuminance on the scanned surface while suppressing increases in cost.

[0013] FIG. 1(a) shows a development view in the main scanning section of an optical scanning device 10 according to the first embodiment. 1(b) and 1(c) are developments of an incident optical system and a scanning optical system provided in the optical scanning device 10 according to the first embodiment in the sub-scanning cross section, respectively.

[0014] As shown in FIG. 1(a), the optical scanning device 10 has a left scanning system 10L and a right scanning system 10R which are arranged symmetrically with respect to a deflector 11 and have the same configuration. The left scanning system 10L and the right scanning system 10R scan the first and second surfaces to be scanned 1008 and 1208, and the third and fourth surfaces to be scanned 1108 and 1308, respectively, in a facing manner. Therefore, the configuration described below can be applied to both the left scanning system 10L and the right scanning system 10R in the same manner, and therefore, for the sake of simplicity, only the left scanning system 10L will be described below.

[0015] The optical scanning device 10 according to this embodiment includes first and second light sources 1001 and 1201, and first and second collimator lenses 1002 and 1202 (first and fourth optical elements). The optical scanning device 10 according to this embodiment also includes first and second cylindrical lenses 1003 and 1203 (third optical elements), and first and second aperture stops 1004 and 1204. The optical scanning device 10 according to this embodiment also includes a deflector 11, first fθ lenses 1006 and 1206 (imaging optical elements), and second fθ lenses 1007 and 1207.

[0016] As the first and second light sources 1001 and 1201, edge-emitting laser diodes that emit a light beam having a wavelength λ=790 nm are used. The light beams LA and LB (first and second light beams) emitted from the first and second light sources 1001 and 1201 are linearly polarized light that is approximately parallel to the main scanning cross section. That is, when reflected by the deflecting surface 1005 of the deflector 11, the light beams LA and LB incident on the deflector 11 from the first and second light sources 1001 and 1201 have more P-polarized components than S-polarized components.

[0017] The first and second collimator lenses 1002 and 1202 convert the light beams LA and LB emitted from the first and second light sources 1001 and 1201 into parallel light beams. The parallel light beam mentioned here includes not only a strictly parallel light beam but also an approximately parallel light beam such as a weakly divergent light beam or a weakly convergent light beam.

[0018] The first and second cylindrical lenses 1003 and 1203 have finite power (refractive power) in the sub-scanning cross section, and collect the light beams LA and LB that have passed through the first and second collimator lenses 1002 and 1202 in the sub-scanning direction. The first and second aperture stops 1004 and 1204 limit the diameters of the light beams LA and LB that have passed through the first and second cylindrical lenses 1003 and 1203 . In this way, the light beams LA and LB emitted from the first and second light sources 1001 and 1201 are collected only in the sub-scanning direction near the deflecting surface 1005 of the deflector 11, and are formed as long line images in the main scanning direction.

[0019] The deflector 11 is rotated in the direction of the arrow A in the figure by a driving means such as a motor (not shown), thereby deflecting the light beams LA and LB incident on the deflector 11. The deflector 11 is composed of, for example, a polygon mirror.

[0020] The first fθ lens 1006 and the second fθ lens 1007 are made of a resin material and are anamorphic imaging lenses having different powers in the main scanning section and the sub-scanning section. The first fθ lens 1006 and the second fθ lens 1007 collect (guide) the light beam LA deflected by the deflector 11 onto a first surface 1008 to be scanned.

[0021] The first fθ lens 1206 and the second fθ lens 1207 are made of a resin material and are anamorphic imaging lenses having different powers in the main scanning section and the sub-scanning section. The first fθ lens 1206 and the second fθ lens 1207 focus (guiduce) the light beam LB deflected by the deflector 11 onto a second surface 1208 to be scanned. At this time, since the deflector 11 is rotating in the direction A in the figure, the deflected and scanned light beams LA and LB scan the first and second scanned surfaces 1008 and 1208 in the direction of the arrow B in the figure, respectively.

[0022] In the optical scanning device 10 according to this embodiment, the first collimator lens 1002, the first cylindrical lens 1003, and the first aperture stop 1004 form a first incident optical system 75a. The second collimator lens 1202, the second cylindrical lens 1203, and the second aperture stop 1204 constitute a second incident optical system 75b.

[0023] In the optical scanning device 10 according to this embodiment, the first fθ lens 1006 and the second fθ lens 1007 configure a first scanning optical system 85a (first imaging optical system). The first fθ lens 1206 and the second fθ lens 1207 constitute a second scanning optical system 85b (a second imaging optical system).

[0024] In the optical scanning device 10 according to this embodiment, the optical axes of the first and second incident optical systems 75a and 75b form angles of α=−3.0° and +3.0° with respect to the main scanning cross section in the sub-scanning cross section, respectively.

[0025] In this embodiment, the first and second photosensitive drums 1008 and 1208 are used as the first and second scanned surfaces 1008 and 1208 . The exposure distribution in the sub-scanning direction on the first and second photosensitive drums 1008 and 1208 is created by rotating the first and second photosensitive drums 1008 and 1208 in the sub-scanning direction for each main scanning exposure. As will be described later, a reflecting member for folding back the optical path is provided between the deflector 11 and the fθ lenses provided in the first and second scanning optical systems 85a and 85b.

[0026] Next, the characteristics of the first incident optical system 75a and the first scanning optical system 85a of the optical scanning device 10 according to this embodiment are shown in Tables 1 and 2 below. As for the characteristics of the second incident optical system 75b and the second scanning optical system 85b, it is only necessary to change the signs of the Z coordinate and the elevation angle relative to the first incident optical system 75a and the first scanning optical system 85a, respectively, so they will be omitted from Tables 1 and 2 and the following explanations related thereto.

[0027] [Table 1]

[0028] [Table 2]

[0029] In Tables 1 and 2, the intersection point G0 (hereinafter referred to as the deflection reference point G0) between the axial light beam LAon and the deflector 11 is taken as the origin, the main scanning direction is taken as the Y axis, the sub-scanning direction is taken as the Z axis, and the direction perpendicular to the Y axis and Z axis is taken as the X axis. In addition, in Table 2, "Ex" is "×10 -x" It means.

[0030] The aspheric shape of each lens surface of the first and second collimator lenses 1002 and 1202 provided in the optical scanning device 10 according to this embodiment is expressed by the following formula (1).

[0031]

number

[0032] Furthermore, the aspheric shape (meridian shape) in the main scanning cross section of each lens surface of the first fθ lenses 1006 and 1206 and the second fθ lenses 1007 and 1207 provided in the optical scanning device 10 according to this embodiment is expressed by the following formula (2).

[0033]

number

[0034] Note that equations (1) and (2) use a local coordinate system whose origin is the vertex of each lens surface (optical surface), which is the intersection point with the optical axis. The direction in which the light beam travels (i.e., the optical axis) is the x-axis, the axis perpendicular to the x-axis in the main-scanning cross section is the y-axis, and the axis perpendicular to the x-axis in the sub-scanning cross section is the z-axis. The local coordinate system is also used in the following equations (3) and (4).

[0035] In formula (2), Bi (i=4, 6, 8, 10, 12) is an aspheric coefficient. In addition, when the aspherical coefficients Bi are different between the positive and negative sides of y, as shown in Table 2, the coefficient on the positive side is given the subscript u (i.e., Biu), and the coefficient on the negative side is given the subscript l (i.e., Bil).

[0036] The aspheric shape (sagittal shape) of each lens surface of the first fθ lenses 1006 and 1206 and the second fθ lenses 1007 and 1207 in the sub-scanning cross section is expressed by the following formula (3).

[0037]

number

[0038] In formula (3), M jk (j=0 to 12, and k=1) are aspheric coefficients. In this embodiment, the sagittal tilt amount is M 01 Therefore, the sagittal tilt plane is M 01 The surface where M is not 0 is called the sagittal tilt change surface. j1 This refers to a face where at least one of (j=1 to 12) is not 0.

[0039] Moreover, the radius of curvature r' in the sub-scanning cross section changes continuously according to the y coordinate of the lens surface as shown in the following equation (4).

[0040]

number

[0041] In equation (4), r is the radius of curvature on the optical axis, E j (j=1 to 10) is the variation coefficient.

[0042] Next, the features and effects of the optical scanning device 10 according to this embodiment will be described.

[0043] As described above, in the optical scanning device 10 of this embodiment, the light beams LA and LB that pass through the first and second incident optical systems 75a and 75b respectively enter the deflector 11 and are deflected to scan the first and second scanned surfaces 1008 and 1208 in the direction of arrow B in the figure. In this case, the upstream end of the scanning area of ​​each of the first and second scanned surfaces 1008 and 1208 is referred to as a negative image height, and the downstream end is referred to as a positive image height. As will be described below, the light beam scanning the negative image height and the light beam scanning the positive image height have different angles of incidence on the deflector 11.

[0044] FIG. 2 shows the incidence angle dependency of the reflectance R of the deflecting surface 1005 of the deflector 11 provided in the optical scanning device 10 according to this embodiment. Specifically, FIG. 2 shows the incidence angle dependence of reflectances Rp and Rs of deflecting surface 1005 of deflector 11 for a light beam having P polarization and a light beam having S polarization, respectively.

[0045] As described above, in the optical scanning device 10 according to this embodiment, the light beams LA and LB pass through the first and second incident optical systems 75a and 75b, respectively, and are incident on the deflector 11 as light beams having almost only P-polarized light, and are deflected. Furthermore, in the optical scanning device 10 of this embodiment, the light beams LA and LB are incident on the deflector 11 at an incident angle of 22.6° when scanning the -image height of the first and second scanned surfaces 1008 and 1208, respectively, and are incident on the deflector 11 at an incident angle of 67.6° when scanning the +image height.

[0046] Therefore, the reflectance of the deflecting surface 1005 of the deflector 11 with respect to the light beams LA and LB scanning the negative image heights of the first and second scanned surfaces 1008 and 1208, respectively, is Rp=79.5%, as shown in FIG. The reflectance of the deflecting surface 1005 of the deflector 11 with respect to the light beams LA and LB that scan the positive image heights of the first and second scanned surfaces 1008 and 1208, respectively, is Rp=80.4%, as shown in FIG.

[0047] Therefore, due to reflection from the deflecting surface 1005 of the deflector 11, a light amount ratio difference between negative and positive image heights ΔR 1008 and 1208 is generated between the negative and positive image heights as expressed by the following formula (5): I It can be seen that occurs.

[0048]

number

[0049] Here, the image height light amount ratio difference ΔR I is a quantity defined as the difference between the light quantity ratio obtained by dividing the amount of light at a negative image height by the average light quantity at the negative image height and the positive image height, and the light quantity ratio obtained by dividing the amount of light at a positive image height by the average light quantity.

[0050] Although a discussion of specific numerical values ​​is omitted, when the light beams LA and LB are reflected by the deflection surface 1005 of the deflector 11, a phase is also added to the light beams LA and LB depending on the incident angle and polarization state of the light beams LA and LB. That is, when the light beams LA and LB are reflected by the deflecting surface 1005 of the deflector 11, not only the amount of light of the light beams LA and LB but also the polarization states of the light beams LA and LB change.

[0051] The light beams LA and LB, the polarization states of which have been changed by being deflected by the deflector 11, enter the first and second fθ lenses 1006 and 1206, respectively. In the case where the first and second fθ lenses 1006 and 1206 are made of a resin material and have birefringence, as in the optical scanning device 10 of this embodiment, the polarization states of the light beams LA and LB are further changed by passing through the first and second fθ lenses 1006 and 1206.

[0052] FIG. 3 shows a sub-scanning cross-sectional view of the first and second scanning optical systems 85a and 85b included in the optical scanning device 10 according to this embodiment. As shown in FIG. 3, the optical scanning device 10 according to this embodiment is provided with folding mirrors 1009, 1209, and 1210 (reflective members).

[0053] The light beam LA deflected by the deflector 11 passes through a first fθ lens 1006 and is then folded back (reflected) by a folding mirror 1009 so as to travel toward a first surface 1008 to be scanned. Moreover, the light beam LB deflected by the deflector 11 passes through a first fθ lens 1206 and is then folded (reflected) by folding mirrors 1209 and 1210 so as to travel toward a second surface 1208 to be scanned.

[0054] FIG. 4 shows the incidence angle dependency of the reflectance R of the return mirrors 1009, 1209, and 1210 provided in the optical scanning device 10 according to this embodiment. Specifically, FIG. 4 shows the incidence angle dependence of the reflectances Rp and Rs of the folding mirrors 1009, 1209, and 1210 for a light beam having P polarization and a light beam having S polarization, respectively.

[0055] First, let us consider the light beam LA. The incident angle θ of the light beam LA with respect to the return mirror 1009 is 1 is approximately 58.4° when scanning the negative image height and when scanning the positive image height. Therefore, the reflectance of the return mirror 1009 with respect to the light beam LA scanning the negative image height and the light beam LA scanning the positive image height is R p = 75.7% and R s =85.1%.

[0056] As described above, the polarization state of the light beam LA is changed by being deflected by the deflector 11, and therefore the reflectance R of the return mirror 1009 for the light beam LA differs depending on the polarization state of the light beam LA. Specifically, if the intensity ratio of P-polarized light and S-polarized light in the light beam LA when it enters the folding mirror 1009 is expressed as α and (1-α), respectively, the reflectance R of the folding mirror 1009 for the light beam LA is expressed by the following equation (6). R = α × R p +(1-α)×R s (6)

[0057] For example, when the intensity ratio α of the light beam LA scanning the negative image height when it is incident on the return mirror 1009 is set to 0.70, the reflectance R of the light beam LA scanning the negative image height by the return mirror 1009 is given by the following equation (6): R=0.70×75.7%+0.30×85.1%=78.6% It is required.

[0058] In addition, when the intensity ratio α of the light beam LA scanning the positive image height when it is incident on the return mirror 1009 is set to 0.60, the reflectance R of the light beam LA scanning the positive image height by the return mirror 1009 is given by the following equation (6): R=0.60×75.7%+0.40×85.1%=79.5% It is required.

[0059] Therefore, due to reflection by the folding mirror 1009, a light amount ratio difference between negative and positive image heights occurs on the first scanned surface 1008, as expressed by the following formula (7): I It can be seen that occurs.

[0060]

number

[0061] Then, a light amount ratio difference ΔR between the negative image height and the positive image height on the first scanned surface 1008 due to reflections from the deflecting surface 1005 of the deflector 11 and the return mirror 1009 is generated. I can be approximately calculated using the calculation results of equations (5) and (7) as shown in equation (8) below. ΔR I =(-1.1%)+(-1.1%)=-2.2% (8)

[0062] Next, the light beam LB is considered. The incident angle θ of the light beam LB to the return mirror 1209 is 2 is approximately 37.4° when scanning the negative image height and when scanning the positive image height. Therefore, the reflectances of the folding mirror 1209 for the light beam LB scanning the negative image height and the light beam LB scanning the positive image height are Rp=80.3% and Rs=86.2%, as shown in FIG.

[0063] For example, when the intensity ratio α of the light beam LB scanning the negative image height when it is incident on the return mirror 1209 is set to 0.95, the reflectance R of the light beam LB scanning the negative image height by the return mirror 1209 is given by the following equation (6): R=0.95×80.3%+0.05×86.2%=80.6% It is required.

[0064] In addition, when the intensity ratio α of the light beam LB scanning the positive image height when it is incident on the return mirror 1209 is set to 0.90, the reflectance R of the light beam LB scanning the positive image height by the return mirror 1209 is given by the following equation (6): R=0.90×80.3%+0.10×86.2%=80.9% It is required.

[0065] Therefore, due to reflection by the folding mirror 1209, a light amount ratio difference between negative and positive image heights occurs on the second scanned surface 1208, as expressed by the following formula (9): I It can be seen that occurs.

[0066]

number

[0067] Next, the incident angle θ of the light beam LB to the return mirror 1210 3 is approximately 63.5° when scanning the negative image height and when scanning the positive image height. Therefore, the reflectances of the folding mirror 1210 for the light beam LB scanning the negative image height and the light beam LB scanning the positive image height are Rp=76.1% and Rs=85.1%, as shown in FIG.

[0068] For example, when the intensity ratio α of the light beam LB scanning the negative image height when it is incident on the return mirror 1210 is set to 0.95, the reflectance R of the light beam LB scanning the negative image height by the return mirror 1210 is given by the following equation (6): R=0.95×76.1%+0.05×85.1%=76.5% It is required.

[0069] In addition, when the intensity ratio α of the light beam LB scanning the positive image height when it is incident on the return mirror 1210 is set to 0.90, the reflectance R of the light beam LB scanning the positive image height by the return mirror 1210 is given by the following equation (6): R=0.90×76.1%+0.10×85.1%=77.0% It is required.

[0070] Therefore, due to reflection by the folding mirror 1210, a light amount ratio difference between negative and positive image heights occurs on the second scanned surface 1208, as expressed by the following formula (10): I It can be seen that occurs.

[0071]

number

[0072] Then, a light amount ratio difference between image heights ΔR occurs between the negative image height and the positive image height on the second scanned surface 1208 due to reflections from the deflecting surface 1005 of the deflector 11, the return mirror 1209, and the return mirror 1210. I can be approximately calculated using the calculation results of equations (5), (9), and (10) as shown in the following equation (11). ΔR I =(-1.1%)+(-0.4%)+(-0.7%)=-2.2% (11)

[0073] As described above, depending on the incidence angle and polarization state of the light beams LA and LB with respect to the deflection surface 1005 and the reflecting mirrors 1009, 1209, and 1210, a difference in light amount occurs between the negative image height and the positive image height on the first and second scanned surfaces 1008 and 1208, respectively. If such a difference in light amount between image heights occurs, when the optical scanning device 10 according to this embodiment is mounted on an image forming apparatus, uneven density occurs in the printed image, resulting in a decrease in printing performance. It should be noted that the difference in light intensity between image heights here refers to the difference in light intensity between -image heights and +image heights, as described above, i.e., between the image heights at the upstream and downstream ends of the scanning area (the two most off-axis image heights), and does not refer to the amount of variation in light intensity across the entire scanning area.

[0074] FIG. 5(a) shows a schematic perspective view of first and second collimator lenses 1002 and 1202 provided in the optical scanning device 10 according to this embodiment. FIG. 5(b) is a schematic partially enlarged cross-sectional view taken along line 5B-5B in FIG. 5(a).

[0075] As shown in Figures 5(a) and (b), in the optical scanning device 10 of this embodiment, a one-dimensional comb-tooth shaped microstructure is integrally formed on the exit surface of each of the first and second collimator lenses 1002 and 1202. In FIG. 5(a), the oblique lines on the exit surfaces of the first and second collimator lenses 1002 and 1202 indicate the direction in which the one-dimensional comb-shaped convex portions of the microstructure extend, that is, the structural axis direction. That is, the structural axis direction is rotated by the structural axis angle Φ with respect to the X direction in a cross section perpendicular to the optical axis.

[0076] As shown in FIG. 5(b), a one-dimensional comb-shaped microstructure having a height H, a width W, a period P, and a filling width a is formed on the exit surface of each of the first and second collimator lenses 1002 and 1202. Here, the relationship between the width W, the period P, and the filling width a is P=W+a. That is, as shown in Figures 5(a) and (b), on the exit surface of each of the first and second collimator lenses 1002 and 1202, a plurality of convex portions extending in the structural axis direction (first direction, third direction) in a cross section perpendicular to the optical axis are arranged at a distance from each other in a direction perpendicular to the structural axis direction (second direction, fourth direction). The plurality of protrusions are preferably aligned and spaced apart from one another in a direction perpendicular to the structural axis direction.

[0077] In the optical scanning device 10 of this embodiment, the refractive indexes of the exit surfaces of the first and second collimator lenses 1002 and 1202 are different for a light beam polarized parallel to the x-direction perpendicular to the structural axis direction (called TE waves) and a light beam polarized parallel to the y-direction parallel to the structural axis direction (called TM waves). That is, at the exit surface of each of the first and second collimator lenses 1002 and 1202, structural birefringence occurs.

[0078] Specifically, the refractive indexes n x and n y are expressed by the following equations (12) and (13), where n is the refractive index of the first and second collimator lenses 1002 and 1202, respectively.

[0079]

number

number

[0080] The phase delay Δ (phase difference) of the TM wave relative to the TE wave caused by passing through the exit surfaces of the first and second collimator lenses 1002 and 1202 on which the above-mentioned microstructures are formed is expressed by the following equation (14). Δ=(n x -n y )×H ···(14)

[0081] In the first and second collimator lenses 1002 and 1202 provided in the optical scanning device 10 according to this embodiment, a one-dimensional comb-shaped microstructure is formed with a height H, a period P and a filling width a of 500 nm, 688 nm and 300 nm. Moreover, the first and second collimator lenses 1002 and 1202 are made of ZEONEX (registered trademark) K22R material, and therefore have a refractive index n of 1.52819 (wavelength λ=790 nm).

[0082] By substituting these values ​​into equations (12) to (14), n x , n y and Δ are found to be 1.32404, 1.21489 and 54.6 nm, respectively. Furthermore, since the phase delay Δ divided by the wavelength λ=790 nm of the light beams LA and LB is Δ / λ=54.6 nm / 790 nm ≒ 1 / 14.5, the microstructure formed on the exit surfaces of the first and second collimator lenses 1002 and 1202 has the same effect as a λ / 14.5 plate.

[0083] It is known that when linearly polarized light is incident on a λ / 4 plate placed at an appropriate angle, the λ / 4 plate emits circularly polarized light, but when linearly polarized light is incident on a λ / 14.5 plate, the λ / 14.5 plate emits elliptically polarized light. By changing the structural axis angle Φ of the λ / 14.5 plate, it is possible to adjust to some extent the orientation and flatness of the elliptically polarized light of the light emerging from the λ / 14.5 plate.

[0084] Here, we consider the i direction parallel to the x direction and the j direction perpendicular to the x direction in a plane perpendicular to the optical axis, and use the Jones matrix in the i direction and the j direction to describe the change in the polarization state of the light beams LA and LB after passing through the first and second collimator lenses 1002 and 1202. Specifically, when the polarization angle, which is the rotation angle from the i direction in the ij cross section of the polarized light of the light beams LA and LB emitted from the first and second light sources 1001 and 1201, is represented as ψ, the polarization vector E of the light beams LA and LB when they are incident on the first and second collimator lenses 1002 and 1202 is represented as in is expressed as the following equation (15).

[0085]

number

[0086] In equation (15), A is a predetermined amplitude. Next, a matrix T representing the effect on the polarization state of the light beams LA and LB by the first and second collimator lenses 1002 and 1202 having a microstructure formed on the exit surface is expressed as the following equation (16).

[0087]

number

[0088] Therefore, the polarization vectors E of the light beams LA and LB after passing through the first and second collimator lenses 1002 and 1202 are out is expressed as the following equation (17).

[0089]

number

[0090] In equation (17), R(Φ) is the rotation matrix of the structural axis angle Φ. As shown in equation (17), it can be seen that the polarization states of the light beams LA and LB after passing through the first and second collimator lenses 1002 and 1202 change depending on the polarization angle ψ, the structural axis angle Φ, and the phase delay Δ.

[0091] In view of the above, in the optical scanning device 10 according to this embodiment, the change in the polarization state of the light beam LA by the first collimator lens 1002 is appropriately set so that the intensity ratio α of the light beam LA scanning the negative image height and the light beam LA scanning the positive image height when they are incident on the return mirror 1009 in the above discussion is opposite to each other. Similarly, the change in the polarization state of the light beam LB by the second collimator lens 1202 is appropriately set so that the intensity ratio α of the light beam LB scanning the negative image height and the light beam LB scanning the positive image height when incident on the return mirrors 1209 and 1210 in the above discussion is opposite to each other.

[0092] Specifically, for the light beam LA, when the intensity ratio α of the light beam LA scanning the negative image height when it is incident on the return mirror 1009 is set to 0.60, the reflectance R of the light beam LA scanning the negative image height by the return mirror 1009 is given by the following equation (6): R=0.60×75.7%+0.40×85.1%=79.5% It is required.

[0093] In addition, when the intensity ratio α of the light beam LA scanning the positive image height when it is incident on the return mirror 1009 is set to 0.70, the reflectance R of the light beam LA scanning the positive image height by the return mirror 1009 is given by the following equation (6): R=0.70×75.7%+0.30×85.1%=78.6% It is required.

[0094] Therefore, due to reflection by the folding mirror 1009, between the negative image height and the positive image height on the first scanned surface 1008, a light amount ratio difference ΔR between image heights is expressed by the following formula (7A) instead of the above formula (7). I occurs.

[0095]

number

[0096] Then, a light amount ratio difference ΔR between the negative image height and the positive image height on the first scanned surface 1008 due to reflections from the deflecting surface 1005 of the deflector 11 and the return mirror 1009 is generated. I can be obtained from equations (5) and (7A) as shown in the following equation (8A). ΔR I =(-1.1%)+1.1%=0.0% (8A)

[0097] Regarding the light beam LB, when the intensity ratio α of the light beam LB scanning the negative image height when it is incident on the return mirror 1209 is set to 0.90, the reflectance R of the light beam LB scanning the negative image height by the return mirror 1209 is given by the following equation (6): R=0.90×80.3%+0.10×86.2%=80.9% It is required.

[0098] In addition, when the intensity ratio α of the light beam LB scanning the positive image height when it is incident on the return mirror 1209 is set to 0.95, the reflectance R of the light beam LB scanning the positive image height by the return mirror 1209 is given by the following equation (6): R=0.95×80.3%+0.05×86.2%=80.6% It is required.

[0099] Therefore, due to reflection by the folding mirror 1209, between the negative image height and the positive image height on the second scanned surface 1208, a light amount ratio difference between image heights ΔR I occurs.

[0100]

number

[0101] In addition, when the intensity ratio α of the light beam LB scanning the negative image height when it is incident on the return mirror 1210 is set to 0.90, the reflectance R of the light beam LB scanning the negative image height by the return mirror 1210 is given by the following equation (6): R=0.90×76.1%+0.10×85.1%=77.0% It is required.

[0102] In addition, when the intensity ratio α of the light beam LB scanning the positive image height when it is incident on the return mirror 1210 is set to 0.95, the reflectance R of the light beam LB scanning the positive image height by the return mirror 1210 is given by the following equation (6): R=0.95×76.1%+0.05×85.1%=76.5% It is required.

[0103] Therefore, due to reflection by the folding mirror 1210, between the negative image height and the positive image height on the second scanned surface 1208, a light amount ratio difference ΔR between image heights is expressed by the following formula (10A) instead of the above formula (10). I occurs.

[0104]

number

[0105] Then, a light amount ratio difference between image heights ΔR occurs between the negative image height and the positive image height on the second scanned surface 1208 due to reflections from the deflecting surface 1005 of the deflector 11, the return mirror 1209, and the return mirror 1210. I can be obtained from equations (5), (9A), and (10A) as shown in the following equation (11A). ΔR I =(-1.1%)+0.4%+0.7%=0.0% (11A)

[0106] As described above, in the optical scanning device 10 of this embodiment, the polarization state of the light beams LA and LB is changed by forming a microstructure on the exit surface of the first and second collimator lenses 1002 and 1202, thereby reducing the light intensity difference between the image heights on the first and second scanned surfaces 1008 and 1208.

[0107] In addition, when the polarization state of the light beams LA and LB is changed by using the fine structure as described above, the image height light amount ratio difference ΔR I Strictly speaking, it also changes. However, even if fine structures are formed on the exit surfaces of the first and second collimator lenses 1002 and 1202, the light beams LA and LB still have almost P polarization when they enter the deflector 11. Then, the image height light amount ratio difference ΔR Iis mainly due to the difference in the angle of incidence between the light beams LA and LB that scan the negative image height and the light beams LA and LB that scan the positive image height, and is therefore assumed not to change in the above discussion.

[0108] Typically, the difference in light intensity between image heights is at most about 10% of the amount of light in the light beam, and in order to improve such a difference in light intensity between image heights, it is sufficient for the formed microstructure to have the function of a λ / 6 to λ / 20 plate. In other words, in the optical scanning device 10 according to this embodiment, when the phase difference between the structural axis direction of the birefringence due to the formed microstructure and the direction perpendicular to the structural axis direction is Δ and the wavelength of the light beam is λ, it is sufficient if the following conditional formula (18) is satisfied.

[0109]

number

[0110] If the formed microstructure has the function of a wave plate larger than λ / 6, it is possible to reduce the difference in light amount between image heights. However, this is not preferable because it increases the sensitivity of the structure axis angle Φ to placement errors. On the other hand, if the formed fine structure has the function of a wavelength plate smaller than λ / 20, the reduction in the difference in light amount between image heights becomes insufficient, which is not preferable.

[0111] In the optical scanning device 10 according to this embodiment, it is preferable that the following conditional formula (18a) be satisfied instead of the conditional formula (18).

[0112]

number

[0113] In the optical scanning device 10 according to this embodiment, it is more preferable that the following conditional formula (18b) be fulfilled instead of the conditional formula (18a).

[0114]

number

[0115] Next, the results of measurements performed by the verification device 10A to verify the effects of the optical scanning device 10 according to the present embodiment will be described. FIG. 6(a) shows a development view of the verification device 10A in the main scanning section. 6(b) and 6(c) are developments of the incident optical system and the scanning optical system provided in the verification apparatus 10A in the sub-scanning cross section, respectively.

[0116] The verification device 10A has the same configuration as the optical scanning device 10 of this embodiment, except that first and second collimator lenses 1002A and 1202A are provided instead of the first and second collimator lenses 1002 and 1202, and first and second optical elements X1 and X2 are newly provided. Therefore, the same members of the verification device 10A as those of the optical scanning device 10 according to this embodiment are denoted by the same reference numerals, and the description thereof will be omitted.

[0117] Specifically, the configuration of the verification device 10A differs from the optical scanning device 10 according to this embodiment in the following points. First, the above-mentioned fine structure is not formed in the first and second collimator lenses 1002A and 1202A, respectively.

[0118] A first optical element X1, which is a λ / 13.2 plate, is disposed between the first collimator lens 1002A and the first cylindrical lens 1003. In addition, a second optical element X2, which is a λ / 13.2 plate, is disposed between the second collimator lens 1202A and the second cylindrical lens 1203.

[0119] As a result, in the verification device 10A, the light beam LA passes through the first collimator lens 1002A and the first optical element X1, and this has an effect on the light beam LA that is approximately equivalent to the phase delay Δ=λ / 14.5 in the optical scanning device 10 of this embodiment. Furthermore, by passing the light beam LB through the second collimator lens 1202A and the second optical element X2, an effect substantially equivalent to the phase delay Δ=λ / 14.5 in the optical scanning device 10 according to this embodiment can be exerted on the light beam LB.

[0120] Then, in the verification device 10A, the fast axes of the first and second optical elements X1 and X2 are rotated in a cross section perpendicular to the optical axis so as to correspond to the rotation of the structural axis angle Φ of each of the first and second collimator lenses 1002 and 1202 in the optical scanning device 10 of this embodiment, and the light amounts on the first and second scanned surfaces 1008 and 1208 are measured.

[0121] In the verification device 10A, the incident angle θ of the light beam LA with respect to the folding mirror 1009 is 1 However, this differs from the optical scanning device 10 according to this embodiment. Specifically, in the optical scanning device 10 according to this embodiment, the incident angle θ of the light beam LA that scans the axial image height of the first scanned surface 1008 with respect to the return mirror 1009 is 1 is 50.4°, whereas the incident angle θ 1 is 33.4°.

[0122] In the verification device 10A, the incident angle θ of the light beam LB with respect to the return mirrors 1209 and 1210 is 2 and θ 3 However, this differs from the optical scanning device 10 according to this embodiment. Specifically, in the optical scanning device 10 according to this embodiment, the incident angle θ of the light beam LB that scans the axial image height of the second scanned surface 1208 with respect to the return mirrors 1209 and 1210 is 2 and θ 3 are 11.8° and 56.2°, respectively, whereas the incidence angle θ 2 and θ 3 are 12.3° and 50.2°, respectively.

[0123] Furthermore, in the verification device 10A, the shapes of the first fθ lens 1006 and the second fθ lens 1007 are slightly different from those of the optical scanning device 10 according to this embodiment. In the optical scanning device 10 of this embodiment, the second fθ lens 1007 is positioned so that the distance d16 on the optical path of the light beam LA between the exit surface of the second fθ lens 1007 and the first scanned surface 1008 is 68.50 mm, while in the verification device 10A, the second fθ lens 1007 is positioned so that the distance d16 is 84.50 mm. However, it has been found that such differences in shape and arrangement of the first fθ lens 1006 and the second fθ lens 1007 in the verification apparatus 10A have almost no effect on the difference in light amount between image heights. On the other hand, the first fθ lens 1206 and the second fθ lens 1207 in the verification apparatus 10A are the same as those in the optical scanning device 10 according to this embodiment.

[0124] 7A and 7B respectively show the results of measuring the light amount at each image height Y on the first and second scanned surfaces 1008 and 1208 by the verification apparatus 10A. In addition, in FIGS. 7(a) and (b), the light amount at each image height Y is normalized with the light amount at image height Y=0 mm.

[0125] Specifically, FIG. 7(a) shows the results when the angle Φ (corresponding to the structural axis angle Φ of the microstructure) that the fast axis of the first optical element X1 makes with respect to the X direction in a cross section perpendicular to the optical axis is set to 0°, 40°, and 130°. Note that the angles Φ=0°, 40°, and 130° here correspond to an angle equivalent to that of a configuration without a fine structure, an angle at which a small difference in light intensity between image heights is measured, and an angle at which a large difference in light intensity between image heights is measured, respectively.

[0126] When image heights Y = -100 mm and 100 mm are defined as -image height and +image height, respectively, it can be seen from Figure 7(a) that the light intensity difference between image heights on the first scanned surface 1008 by the verification device 10A is 3.8%, 1.0%, and 7.2% at angles Φ = 0°, 40°, and 130°, respectively. Therefore, in the verification device 10A, by rotating the first optical element X1, the light intensity difference between image heights on the first scanned surface 1008 can be changed between 1.0% and 7.2%, and it can be seen that the light intensity difference between image heights on the first scanned surface 1008 can be sufficiently reduced especially at an angle Φ=40°.

[0127] FIG. 7(b) also shows the results when the angle Φ (corresponding to the structural axis angle Φ of the microstructure) that the fast axis of the second optical element X2 makes with respect to the X direction in a cross section perpendicular to the optical axis is set to 0°, 50°, and 140°. Note that the angles Φ=0°, 50°, and 140° here correspond to an angle equivalent to that of a configuration without a fine structure, an angle at which a large difference in light intensity between image heights is measured, and an angle at which a small difference in light intensity between image heights is measured, respectively.

[0128] When image heights Y = -100 mm and 100 mm are defined as -image height and +image height, respectively, it can be seen from Figure 7(b) that the light intensity difference between image heights on the second scanned surface 1208 by the verification device 10A is 3.2%, 7.2%, and -0.5% at angles Φ = 0°, 50°, and 140°, respectively. Therefore, in the verification device 10A, by rotating the second optical element X2, the light intensity difference between image heights on the second scanned surface 1208 can be changed between -0.5% and 7.2%, and it can be seen that the light intensity difference between image heights on the second scanned surface 1208 can be sufficiently reduced especially at an angle Φ=140°.

[0129] It should be understood that the above results are actual measurements taken by the verification device 10A and therefore contain measurement errors such as product variations and noise. In the verification apparatus 10A, the fluctuation amounts of the inter-image-height light amount difference on the first and second scanned surfaces 1008 and 1208 are approximately the same.

[0130] Furthermore, the difference in configuration between the verification device 10A and the optical scanning device 10 of this embodiment is smaller than the difference between the configuration of the first incident optical system 75a and the first scanning optical system 85a for the light beam LA in the verification device 10A and the configuration of the second incident optical system 75b and the second scanning optical system 85b for the light beam LB. From the above, it can be interpreted that the above measurement results obtained by the verification apparatus 10A fully demonstrate the effects of the optical scanning device 10 according to this embodiment.

[0131] That is, in the optical scanning device 10 according to this embodiment, by setting the structural axis angle Φ of the microstructure formed on the first and second collimator lenses 1002 and 1202 to 40° and 140°, respectively, it is possible to obtain the effect of sufficiently reducing the difference in light intensity between image heights on the first and second scanned surfaces 1008 and 1208.

[0132] As with the optical scanning device 10 according to this embodiment, even if a microstructure having a simpler composition and a smaller height H than conventional microstructures is used, it is possible to obtain the effect of sufficiently reducing the difference in light amount between image heights on the scanned surface. This allows the microstructure to be molded integrally with the first and second collimator lenses 1002 and 1202, respectively, using the same material, which also has the effect of enabling the first and second collimator lenses 1002 and 1202, respectively, having the microstructure formed thereon, to be manufactured at low cost.

[0133] When scanning multiple scanned surfaces, such as the first and second scanned surfaces 1008 and 1208, as in the optical scanning device 10 of this embodiment, the structural axis angles Φ of the microstructures in the first and second collimator lenses 1002 and 1202, which are set to reduce the difference in light intensity between image heights, are typically different from each other. In this case, considering the measurement results using the above-mentioned verification device 10A, if the difference between the structural axis angles Φ of each microstructure set to reduce the light intensity difference between image heights is approximately less than 10°, the structural axis angles Φ of each microstructure can be set to be identical to each other. On the other hand, if the difference is 10° or more, it is preferable to set the structural axis angle Φ of each fine structure to an appropriate value in order to reduce the difference in light amount between image heights.

[0134] There are several possible methods for setting the structural axis angle Φ of each of the multiple microstructures. Figures 8(a), (b), and (c) are schematic front views viewed from a direction parallel to the optical axis, showing a method of setting the structural axis angle Φ1 of the microstructure (first microstructure) formed on the first collimator lens 1002 and the structural axis angle Φ2 of the microstructure (second microstructure) formed on the second collimator lens 1202.

[0135] For example, as shown in FIG. 8(a), a conceivable method is to use optical elements having the same shape as the first and second collimator lenses 1002 and 1202, and arrange the first and second collimator lenses 1002 and 1202 so that they have different structural axis angles. In this case, since the same optical element can be used for the first and second collimator lenses 1002 and 1202, there is an advantage in that the first and second collimator lenses 1002 and 1202 can be manufactured easily. On the other hand, it becomes necessary to form the optical surfaces of the first and second collimator lenses 1002 and 1202, respectively, into axially symmetric shapes and to set position references G for the first and second collimator lenses 1002 and 1202, respectively, in the optical scanning device 10 according to this embodiment. It should be noted that optical elements having the same shape as mentioned here may have base surfaces with the same shape, but the microstructures formed on the base surfaces may be different from each other. In addition, optical elements having the same shape as mentioned here may have the same shape in the effective area that contributes to image formation, while having different shapes in the non-effective area that does not contribute to image formation.

[0136] Also, as shown in FIG. 8(b), a method of arranging the first and second collimator lenses 1002 and 1202 using different optical elements having microstructures formed thereon so that the structural axis directions relative to the position reference G are different can be considered. In this case, since different optical elements are used for the first and second collimator lenses 1002 and 1202, there is a disadvantage that the manufacture of the first and second collimator lenses 1002 and 1202 becomes complicated. On the other hand, the optical surfaces of the first and second collimator lenses 1002 and 1202 do not need to be formed into axially symmetric shapes, which has the advantage that it is easier to set a common position reference G for the first and second collimator lenses 1002 and 1202 in the optical scanning device 10 of this embodiment. The optical scanning device 10 according to this embodiment employs the method shown in FIG. 8(b).

[0137] As a method for forming a microstructure such that the structural axis directions relative to the position reference G in the first and second collimator lenses 1002 and 1202 are different from each other, a method of manufacturing the first and second collimator lenses 1002 and 1202 as an integrated unit as shown in Figure 8(c) can also be considered. When the first and second collimator lenses 1002 and 1202 are integrally formed using such a compound lens, there is an advantage in that the first and second collimator lenses 1002 and 1202 can be manufactured at low cost. On the other hand, there is a disadvantage in that the relative positions between the first and second collimator lenses 1002 and 1202 in the compound eye lens are fixed.

[0138] In the optical scanning device 10 according to this embodiment, the light beams LA and LB emitted from the first and second light sources 1001 and 1201 respectively have linear polarization. The angle between the polarization direction of the light beams LA and LB when they enter the first and second collimator lenses 1002 and 1202 and the structural axis direction of the microstructure formed on the first and second collimator lenses 1002 and 1202 is 10° or more. This provides the effect of making it easier for the polarization states of the light beams LA and LB to change due to structural birefringence, even if the height H of the fine structure is small.

[0139] However, the present embodiment can be achieved by emitting light beams LA and LB from the first and second light sources 1001 and 1201, which have a polarization state that is not linearly polarized, that is, have a small amount of polarization component perpendicular to the predetermined direction in addition to the polarization component in a predetermined direction. Furthermore, the effect of this embodiment can be obtained even if the angle between the polarization direction of the light beams LA and LB when they are incident on the first and second collimator lenses 1002 and 1202 and the structural axis direction of the microstructure formed on the first and second collimator lenses 1002 and 1202 is less than 10°.

[0140] In the optical scanning device 10 of this embodiment, as described above, a folding mirror 1009 is provided on the optical path of the light beam LA between the deflector 11 and the first scanned surface 1008, and folding mirrors 1209 and 1210 are provided on the optical path of the light beam LB between the deflector 11 and the second scanned surface 1208. This makes full use of the difference between the reflectance of the return mirrors 1009, 1209 and 1210 for P-polarized light and S-polarized light, thereby reducing the difference in light amount between image heights on the first and second scanned surfaces 1008 and 1208. However, this is not limited to the above, and the difference in light intensity between image heights can be reduced by utilizing the difference in reflectance for P-polarized light and S-polarized light in the deflector 11 and the difference in transmittance for P-polarized light and S-polarized light in the first fθ lenses 1006 and 1206 and the second fθ lenses 1007 and 1207, respectively, even in a configuration that does not include a folding mirror.

[0141] In the optical scanning device 10 according to this embodiment, the wavelengths of the light beams LA and LB emitted from the first and second light sources 1001 and 1201 are longer than 700 nm. Therefore, in order to generate the same phase delay Δ as that of visible light with a wavelength of 550 nm, the height H of the fine structure must be increased. In other words, the optical scanning device 10 according to this embodiment proposes a method for reducing the difference in light intensity between image heights on the first and second scanned surfaces 1008 and 1208 using a microstructure that is useful for such long wavelengths and easy to manufacture.

[0142] In the optical scanning device 10 according to this embodiment, the first and second collimator lenses 1002 and 1202 on which the microstructure is formed are manufactured by injection molding, thereby reducing the manufacturing cost. However, the present invention is not limited to this, and the effects of this embodiment can be obtained even if the first and second collimator lenses 1002 and 1202 on which the fine structure is formed are manufactured by other molding methods such as press molding.

[0143] In the optical scanning device 10 according to this embodiment, a fine structure is formed on the exit surface of the first and second collimator lenses 1002 and 1202, but the present invention is not limited to this. That is, the fine structure may be formed on both the entrance surface and the exit surface of the first and second collimator lenses 1002 and 1202 . Furthermore, a fine structure may be formed on the exit surfaces of the first and second collimator lenses 1002 and 1202, and a fine structure may be formed on one surface of the first and second cylindrical lenses 1003 and 1203. That is, the effect of this embodiment can be obtained by forming a fine structure on at least one optical surface of at least one optical element provided in the first and second incident optical systems 75a and 75b.

[0144] In the optical scanning device 10 of this embodiment, a microstructure is formed on the exit surfaces of the first and second collimator lenses 1002 and 1202, which have axially symmetric shapes, but this is not limited to the above, and a microstructure may also be formed on the optical surface of a specified optical element that does not have an axially symmetric shape.

[0145] In the optical scanning device 10 according to this embodiment, the microstructure is provided on both the first and second collimator lenses 1002 and 1202, but the present invention is not limited to this. In other words, when the difference in light amount between image heights is sufficiently reduced on a specified scanned surface, the configuration of the optical scanning device 10 of this embodiment can be simplified by not providing a fine structure in the incident optical system corresponding to the light beam guided to the specified scanned surface.

[0146] In the optical scanning device 10 of this embodiment, the first and second collimator lenses 1002 and 1202 having a microstructure formed thereon are manufactured using a resin material, but this is not limited thereto and they may be manufactured using other materials such as glass.

[0147] In the optical scanning device 10 according to this embodiment, the polarization directions of the light beams LA and LB when they enter the deflector 11 and the deflection surface 1005 of the deflector 11 are parallel to each other, but this is not limited to the above. In the optical scanning device 10 according to this embodiment, the light beams LA and LB are obliquely incident on the deflecting surface 1005 of the deflector 11 in the main scanning section and the sub-scanning section, respectively, but this is not limiting.

[0148] The optical scanning device 10 of this embodiment uses a deflector 11 having the reflectance characteristics shown in FIG. 2 and folding mirrors 1009, 1209 and 1210 having the reflectance characteristics shown in FIG. 4, but is not limited to this. Generally speaking, in the reflectance characteristics of the deflector 11 and the folding mirrors 1009, 1209, and 1210, respectively, the greater the change in reflectance with respect to the incident angle and the greater the difference in reflectance between P-polarized light and S-polarized light, the more likely it is that a difference in light intensity between image heights will occur on the first and second scanned surfaces 1008 and 1208, respectively, and the easier it will be to improve this by the above-mentioned fine structure. However, if the change in reflectance with respect to the angle of incidence or the difference in reflectance between P-polarized light and S-polarized light is too large, it may be difficult to sufficiently reduce the difference in light intensity between image heights on the first and second scanned surfaces 1008 and 1208, respectively, using only the above-mentioned microstructure.

[0149] [Second embodiment] FIG. 9(a) shows a development view in the main scanning section of the optical scanning device 20 according to the second embodiment. 9(b) and (c) are developments of the incident optical system and the scanning optical system included in the optical scanning device 20 according to the second embodiment in the sub-scanning cross section, respectively.

[0150] In the optical scanning device 20 of this embodiment, first and second light sources 2001 and 2201 are provided instead of the first and second light sources 1001 and 1201, and first and second collimator lenses 2002 and 2202 are provided instead of the first and second collimator lenses 1002 and 1202. Furthermore, in the optical scanning device 20 of this embodiment, first and second optical elements X3 and X4 are newly provided between the first and second collimator lenses 2002 and 2202 and the first and second cylindrical lenses 1003 and 1203. Since the other configurations are the same as those of the optical scanning device 10 of the first embodiment, the same components are given the same reference numbers and descriptions thereof are omitted.

[0151] As the first and second light sources 2001 and 2201, a vertical cavity surface emitting laser (VCSEL) that emits a light beam having a wavelength λ=790 nm is used. The light beams LA and LB emitted from the first and second light sources 2001 and 2201 are linearly polarized light that is approximately parallel to the main scanning cross section. That is, when reflected by the deflecting surface 1005 of the deflector 11, the light beams LA and LB incident on the deflector 11 from the first and second light sources 2001 and 2201 have more P-polarized components than S-polarized components.

[0152] The first and second collimator lenses 2002 and 2202 convert the light beams LA and LB emitted from the first and second light sources 2001 and 2201 into parallel light beams. The first and second collimator lenses 2002 and 2202 do not have the fine structure as shown in the first embodiment.

[0153] The first and second optical elements X3 and X4 are respectively optical elements in which a parallel plate element having a microstructure formed on both the entrance surface and the exit surface, and a polarizer are bonded together.

[0154] The optical scanning device 20 of this embodiment uses VCSELs, which have a large number of light-emitting points, as the first and second light sources 2001 and 2201 in order to further reduce costs and improve printing speed compared to the optical scanning device 10 of the first embodiment. Here, if a VCSEL is used as the first and second light sources 2001 and 2201, the printing speed can be improved, but the illuminance on the first and second scanned surfaces 1008 and 1208 increases as the number of light-emitting points increases. Therefore, it may become impossible to use the first and second photosensitive drums 1008 and 1208 that are used as the first and second scanned surfaces 1008 and 1208 in the optical scanning device 10 according to the first embodiment.

[0155] In this case, in order to suppress such an increase in illuminance, it is possible to consider a method of providing a new polarizer in each of the first and second incident optical systems 75a and 75b. However, in that case, the polarization state of the light beams LA and LB when they enter the deflector 11 will change. Therefore, there is a possibility that the difference in light amount between the image heights on the first and second scanned surfaces 1008 and 1208 becomes large. Therefore, in the optical scanning device 20 of this embodiment, a polarizer is newly provided in each of the first and second incident optical systems 75a and 75b, and an optical element having a fine structure is newly provided downstream of the polarizer, thereby reducing the difference in light amount between the image heights.

[0156] FIG. 10(a) shows a schematic perspective view of first and second optical elements X3 and X4 provided in the optical scanning device 20 according to this embodiment. FIG. 10(b) shows a schematic partially enlarged cross-sectional view of the first and second parallel plate elements X3b and X4b included in the first and second optical elements X3 and X4, taken along line 10B-10B in FIG. 10(a).

[0157] As shown in FIG. 10(a), the first and second optical elements X3 and X4 respectively have first and second polarizers X3a and X4a (second optical elements) that convert the polarization of the light beams LA and LB into linear polarization in a predetermined direction. The first and second optical elements X3 and X4 each have a planar shape in which the entrance surface and exit surface are parallel to each other, and each have a first and second parallel plate element X3b and X4b (first optical element) on which a microstructure is formed. The incidence surface and the emission surface of the first and second parallel plate elements X3b and X4b herein refer to the base surfaces on which the fine structure is formed.

[0158] In FIG. 10(a), in order to make it easier to understand the configuration of the first and second optical elements X3 and X4, the first and second polarizers X3a and X4a and the first and second parallel plate elements X3b and X4b are shown separated from each other, but in reality they are joined to each other. In the optical scanning device 20 according to this embodiment, as shown in FIG. 10(a), the first and second optical elements X3 and X4 are arranged such that the first and second polarizers X3a and X4a are respectively disposed between the first and second light sources 2001 and 2201 and the first and second parallel plate elements X3b and X4b.

[0159] In FIG. 10(a), the diagonal lines in the first and second polarizers X3a and X4a indicate the direction of the transmission axis, which is rotated by a transmission axis angle θ with respect to the X direction in a cross section perpendicular to the optical axis. By changing the transmission axis angle θ, the light amounts of the light beams LA and LB emitted from the first and second polarizers X3a and X4a can be adjusted.

[0160] In FIG. 10(a), the oblique lines on the emission surfaces of the first and second parallel plate elements X3b and X4b indicate the direction in which the one-dimensional comb-shaped convex portions of the formed microstructure extend, that is, the structural axis direction. Although not shown in FIG. 10(a), the same fine structure is also formed on the entrance surfaces of the first and second parallel plate elements X3b and X4b. The structural axis direction is rotated by a structural axis angle Φ with respect to a direction parallel to the transmission axes of the first and second polarizers X3a and X4a in a cross section perpendicular to the optical axis. The angle between the direction of linear polarization of the light beams LA and LB emitted from the first and second polarizers X3a and X4a, respectively, and the structural axis direction of the microstructure formed in the first and second parallel plate elements X3b and X4b, i.e., the structural axis angle Φ, is a parameter that changes the polarization state of the light beams LA and LB when they enter the deflector 11.

[0161] As shown in FIG. 10(b), a one-dimensional comb-shaped microstructure having height H, width W, period P and filling width a is formed on the entrance surface and exit surface of each of the first and second parallel plate elements X3b and X4b. Here, the relationship between the width W, the period P, and the filling width a is P=W+a.

[0162] Specifically, the height H, period P and filling width a of the fine structures formed on the first and second parallel plate elements X3b and X4b are 900 nm, 688 nm and 300 nm, respectively. Moreover, the first and second parallel plate elements X3b and X4b are made of ZEONEX (registered trademark) K22R material, and therefore have a refractive index n of 1.52819 (λ=790 nm).

[0163] Therefore, by substituting these values ​​into equations (12) to (14), the phase delay Δ at the entrance surface and exit surface of each of the first and second parallel plate elements X3b and X4b is found to be 98.2 nm. Furthermore, since the phase delay Δ divided by the wavelength λ=790 nm of the light beams LA and LB is Δ / λ=98.2 nm / 790 nm ≒ 1 / 8.0, the microstructures formed on the entrance and exit surfaces of the first and second parallel plate elements X3b and X4b, respectively, have an effect equivalent to that of a λ / 8 plate. That is, the first and second parallel plate elements X3b and X4b as a whole have an effect equivalent to that of a λ / 4 plate.

[0164] For example, by setting the structural axis angle Φ of the microstructure formed in the first and second parallel plate elements X3b and X4b to 45°, the polarization states of the light beams LA and LB emitted from the first and second parallel plate elements X3b and X4b become circularly polarized. However, changing the polarization state of the light beams LA and LB emitted from the first and second parallel plate elements X3b and X4b to circular polarization does not necessarily sufficiently reduce the difference in light intensity between image heights on the first and second scanned surfaces 1008 and 1208. That is, by setting the structural axis angle Φ in accordance with the transmission axis angle θ of each of the first and second polarizers X3a and X4a so that the difference in light intensity between the image heights is sufficiently reduced, the polarization states of the light beams LA and LB emerging from the first and second parallel plate elements X3b and X4b are set to appropriate elliptical polarization.

[0165] In the optical scanning device 20 according to this embodiment, the phase delay Δ of each of the microstructures formed on the entrance and exit surfaces of the first and second parallel plate elements X3b and X4b is increased, thereby making the polarization state of the light beams LA and LB emitted from the first and second parallel plate elements X3b and X4b closer to circular polarization. As a result, even if the transmission axis angle θ of each of the first and second polarizers X3a and X4a changes slightly, the effect of reducing the difference in light amount between image heights on the first and second scanned surfaces 1008 and 1208 does not change significantly.

[0166] Moreover, by increasing the height H of the fine structures formed on the first and second parallel plate elements X3b and X4b, the phase delay Δ of the fine structures can be increased. On the other hand, it is difficult to manufacture the first and second parallel plate elements X3b and X4b so as to have the microstructure with a height H of 1000 nm or more.

[0167] Therefore, in the optical scanning device 20 according to this embodiment, a fine structure is formed on both the entrance surface and the exit surface of each of the first and second parallel plate elements X3b and X4b. This makes it possible to ensure a large phase delay Δ for the first and second parallel plate elements X3b and X4b as a whole, while setting the height H of the microstructures on each of the entrance surface and the exit surface to less than 1000 nm.

[0168] In the optical scanning device 20 according to this embodiment, the first and second parallel plate elements X3b and X4b have a microstructure formed equivalent to that of a λ / 4 plate as described above, but the present invention is not limited to this. That is, in order to set the image height difference in light amount on the first and second scanned surfaces 1008 and 1208 to a desired value, the phase delay Δ of the fine structure may be set to an appropriate value.

[0169] In the optical scanning device 20 of this embodiment, the first and second polarizers X3a and X4a and the first and second parallel plate elements X3b and X4b are integrated with each other so that the light amounts of the light beams LA and LB can be slightly adjusted by simply changing the transmission axis angle θ while keeping the polarization state of each of the light beams LA and LB when they enter the deflector 11 set to approximately circular polarization. However, by forming the first and second polarizers X3a and X4a and the first and second parallel plate elements X3b and X4b integrally with each other, the difference in light amount between the image heights on the first and second scanned surfaces 1008 and 1208 changes according to the transmission axis angle θ. Therefore, when it is difficult to allow such a variation in the difference in light amount between image heights according to the transmission axis angle θ, the first and second polarizers X3a and X4a and the first and second parallel plate elements X3b and X4b may be formed separately from each other.

[0170] In the optical scanning device 20 of this embodiment, the incident surfaces of the first and second collimator lenses 2002 and 2202 have curved shapes, making it difficult in manufacturing to provide a fine structure on the incident surfaces of the first and second collimator lenses 2002 and 2202. Therefore, in the optical scanning device 20 of this embodiment, instead of forming a microstructure on the entrance and exit surfaces of the first and second collimator lenses 2002 and 2202, first and second parallel plate elements X3b and X4b on which the microstructure is formed are provided. However, the present invention is not limited to this, and a fine structure may be further provided on the curved entrance surfaces of the first and second collimator lenses 2002 and 2202 . In order to ensure a large phase delay Δ of the microstructure, in addition to providing the first and second parallel plate elements X3b and X4b on which the microstructure is formed, the microstructure may also be formed on the entrance surface or exit surface of the first and second collimator lenses 2002 and 2202.

[0171] The configuration of the optical scanning device 20 of this embodiment, in which first and second parallel plate elements X3b and X4b having a fine structure are newly provided, has the advantage that the impact on imaging performance is small even when the environmental temperature changes or manufacturing errors occur, and therefore it can be easily applied to existing products. However, it also has the disadvantage of increased component costs. In this case, if it is possible to provide first and second parallel plate elements X3b and X4b that are common to multiple existing products, there is a possibility that this will have a greater cost advantage than a configuration in which a microstructure is formed on an optical element provided in each of the multiple existing products.

[0172] Furthermore, if the optical scanning device 20 according to this embodiment is configured such that the first and second parallel plate elements X3b and X4b are provided downstream of the first and second collimator lenses 2002 and 2202, the effect on the imaging performance is significantly reduced even if the environmental temperature changes or manufacturing errors occur, which is even more preferable. The entrance and exit surfaces of the first and second parallel plate elements X3b and X4b do not need to be completely flat, and may have a gentle curvature.

[0173] The extension directions of each one-dimensional comb-tooth-shaped convex portion formed on the entrance surface and exit surface of the first and second parallel plate elements X3b and X4b provided in the optical scanning device 20 in this embodiment are parallel to each other, but are not limited to this. That is, the extending direction of each of the convex portions of the one-dimensional comb shape may be set so as to appropriately reduce the difference in light amount between image heights on the first and second scanned surfaces 1008 and 1208 .

[0174] In the optical scanning device 20 of this embodiment, by adopting the above configuration when using VCSELs as the first and second light sources 2001 and 2201, the increasing light amount of the light beams LA and LB is reduced, and the light amount difference between the image heights on the first and second scanned surfaces 1008 and 1208 is reduced. However, the present invention is not limited to this, and the above configuration can also be used in the case where, for example, photosensitive drums having high sensitivity are used as the first and second scanned surfaces 1008 and 1208 . In other words, when using a polarizer to reduce the amount of light in the light beams LA and LB in such a case, by providing an optical element having a fine structure, the difference in light amount between the image heights on the first and second scanned surfaces 1008 and 1208 can be reduced.

[0175] Although the preferred embodiments have been described above, the present invention is not limited to these embodiments and various modifications and changes are possible within the scope of the gist thereof.

[0176] [Image forming equipment] FIG. 11 is a sub-scanning sectional view of a main part of a color image forming apparatus 90 equipped with the optical scanning device 30 according to the first or second embodiment.

[0177] The image forming apparatus 90 is a tandem type color image forming apparatus that records image information on the surfaces of the photosensitive drums serving as image carriers, using the optical scanning device 30 according to the first or second embodiment. The image forming apparatus 90 includes the optical scanning device 30 according to the first or second embodiment, photosensitive drums (photoconductors) 23, 24, 25, and 26 as image carriers, and developing units 15, 16, 17, and 18. The image forming apparatus 90 also includes a conveyor belt 91, a printer controller 93, and a fixing unit 94.

[0178] To the image forming apparatus 90, R (red), G (green), and B (blue) color signals (code data) output from an external device 92 such as a personal computer are input. The input color signals are converted by a printer controller 93 in the image forming apparatus 90 into image data (dot data) for each of C (cyan), M (magenta), Y (yellow), and K (black). Each converted image data is input to each optical scanning device 30. Then, light beams 33, 34, 35, and 36 modulated according to each image data are emitted from the optical scanning device 30, and the photosensitive surfaces of the photosensitive drums 23, 24, 25, and 26 are exposed to these light beams.

[0179] Charging rollers (not shown) that uniformly charge the surfaces of the photosensitive drums 23, 24, 25, and 26 are provided in contact with the surfaces. Then, the surfaces of the photosensitive drums 23, 24, 25, and 26 charged by the charging rollers are irradiated with light beams 33, 34, 35, and 36 by the optical scanning device 30. As described above, the light beams 33, 34, 35, and 36 are modulated based on image data for each color, and electrostatic latent images are formed on the surfaces of the photosensitive drums 23, 24, 25, and 26 by irradiating the surfaces with the light beams 33, 34, 35, and 36. The formed electrostatic latent images are developed into toner images by the developing devices 15, 16, 17, and 18 arranged so as to come into contact with the photosensitive drums 23, 24, 25, and 26.

[0180] The toner images developed by the developing units 15 to 18 are transferred in multiple layers onto a sheet of paper (transfer material) (not shown) transported on a transport belt 91 by a transfer roller (transfer unit) (not shown) arranged opposite the photosensitive drums 23 to 26, forming a single full-color image. The paper onto which the unfixed toner image has been transferred in the above manner is further transported to a fixing device 94 provided behind the photosensitive drums 23, 24, 25, and 26 (on the left side in FIG. 11). The fixing device 94 is composed of a fixing roller having an internal fixing heater (not shown) and a pressure roller arranged to be in pressure contact with the fixing roller. The paper transported from the transfer section is heated and pressurized by the pressure contact portion between the fixing roller and the pressure roller, thereby fixing the unfixed toner image on the paper. Furthermore, a paper discharge roller (not shown) is provided behind the fixing roller, and the paper discharge roller discharges the fixed paper out of the image forming device 90.

[0181] The color image forming apparatus 90 uses an optical scanning device 30 to record image signals (image information) on the photosensitive surfaces of photosensitive drums 23, 24, 25, and 26 corresponding to each of the colors C, M, Y, and K, and prints a color image at high speed. For example, a color image reading device equipped with a CCD sensor may be used as the external device 92. In this case, the color image reading device and the color image forming device 90 constitute a color digital copying machine.

[0182] The disclosure of this embodiment includes the following configuration. (Configuration 1) A first optical element having a first microstructure formed on at least one of an entrance surface and an exit surface for changing a polarization state of a first light beam from a first light source, a deflector for deflecting the first light beam that has passed through the first optical element to scan a first scanned surface in a main scanning direction, and a first imaging optical system for guiding the first light beam deflected by the deflector to the first scanned surface, wherein the first microstructure is configured with a plurality of convex portions each extending in a first direction and arranged in a second direction perpendicular to the first direction, and a phase difference between the first direction and the second direction of birefringence due to the first microstructure is defined as Δ 1 , the wavelength of the first light beam is λ 1 Then, 1 / 20≦Δ 1 / λ 1 An optical scanning device characterized by satisfying the condition: ≦1 / 6. (Configuration 2) The optical scanning device according to configuration 1, wherein the first light beam emitted from the first light source is linearly polarized light. (Configuration 3) The optical scanning device according to configuration 2, wherein the angle between the polarization direction of the linearly polarized light when it is incident on the first optical element and the first direction is 10° or more. (Configuration 4) In the optical scanning device according to any one of configurations 1 to 3, the first light beam is elliptically polarized light when it enters the deflector. (Configuration 5) In the optical scanning device according to any one of configurations 1 to 4, the first optical element converts the first light beam into a parallel light beam. (Configuration 6) The optical scanning device according to any one of configurations 1 to 5, wherein the surface on which the first microstructure is provided of the entrance surface or the exit surface is a flat surface. (Configuration 7) The optical scanning device according to configuration 1, further comprising a second optical element that converts the first light beam into linearly polarized light before it enters the deflector. (Configuration 8) The optical scanning device according to configuration 7, wherein the second optical element is disposed between the first light source and the first optical element. (Configuration 9) The optical scanning device according to configuration 7 or 8, wherein the entrance surface and the exit surface are flat surfaces parallel to each other. (Configuration 10) The optical scanning device according to any one of configurations 7 to 9, wherein the first and second optical elements are cemented to each other. (Configuration 11) The optical scanning device according to any one of configurations 1 to 10, wherein the first microstructure is formed on both the entrance surface and the exit surface. (Configuration 12) In the optical scanning device according to configuration 1, the first optical element focuses the first light flux in the sub-scanning direction. (Configuration 13) The optical scanning device according to any one of configurations 1 to 11, further comprising a third optical element having a first microstructure formed on at least one of the entrance surface and the exit surface. (Configuration 14) In the optical scanning device according to configuration 13, the third optical element focuses the first light flux in the sub-scanning direction. (Configuration 15)λ 1 15. The optical scanning device according to any one of configurations 1 to 14, wherein the condition of ≧700 nm is satisfied. (Configuration 16) A fourth optical element having a second microstructure formed on at least one of an entrance surface and an exit surface for changing a polarization state of a second light beam from a second light source, and a second imaging optical system for guiding the second light beam deflected by a deflector to a second scanned surface, the deflector deflects the second light beam that has passed through the fourth optical element to scan the second scanned surface in a main scanning direction, the second microstructure is configured with a plurality of convex portions each extending in a third direction and arranged in a fourth direction perpendicular to the third direction, and a phase difference between the third direction and the fourth direction of birefringence due to the second microstructure is defined as Δ 2 , the wavelength of the second light beam is λ 2 Then, 1 / 20≦Δ 2 / λ 216. The optical scanning device according to any one of configurations 1 to 15, wherein the condition of ≦1 / 6 is satisfied. (Configuration 17) The optical scanning device according to configuration 16, wherein the angle between the first direction and the third direction is 10° or more. (Configuration 18) An optical scanning device described in Configuration 16 or 17, characterized in that the surface on which the first microstructure is provided in the first optical element and the surface on which the second microstructure is provided in the fourth optical element have the same shape as each other. (Configuration 19) In the optical scanning device according to any one of configurations 16 to 18, the first and fourth optical elements are integrally formed with each other. (Configuration 20) An image forming apparatus comprising an optical scanning device according to any one of configurations 1 to 19, a developing unit which develops an electrostatic latent image formed on a first scanned surface by the optical scanning device into a toner image, a transfer unit which transfers the developed toner image to a transfer material, and a fixing unit which fixes the transferred toner image to the transfer material. (Configuration 21) An image forming apparatus comprising: an optical scanning device according to any one of configurations 1 to 19; and a printer controller that converts a signal output from an external device into image data and inputs the image data to the optical scanning device. [Explanation of symbols]

[0183] 10 Optical scanning device 11 Deflector 85a First imaging optical system 1001 First light source (First light source) 1002 First collimator lens (first optical element) 1008 First scanned surface LA Luminous flux (first luminous flux)

Claims

1. A first optical element having a first microstructure formed on at least one of its incident surface and exit surface that changes the polarization state of a first luminous beam from a first light source, A deflector that deflects the first light beam that has passed through the first optical element to scan the first surface to be scanned in the main scanning direction, The system comprises a first imaging optical system that guides the first light beam deflected by the deflector to the first scanning surface, The first microstructure on at least one of the incident surface and the exit surface of the first optical element is configured such that a plurality of protrusions extending in a first direction are arranged in a second direction perpendicular to the first direction. The phase difference between the first direction and the second direction of birefringence due to the first microstructure at at least one of the incident surface and the exit surface of the first optical element is Δ １ , the wavelength of the first luminous beam is λ １ In that case, 1 / 20≦D １ / l １ ≦1 / 6 An optical scanning device characterized by satisfying the following conditions.

2. The optical scanning apparatus according to claim 1, characterized in that the first luminous beam emitted from the first light source is linearly polarized.

3. The optical scanning apparatus according to claim 2, characterized in that the angle between the polarization direction of the linearly polarized light incident on the first optical element and the first direction is 10° or more.

4. The optical scanning apparatus according to claim 1, characterized in that the first light beam incident on the deflector is elliptically polarized.

5. The optical scanning apparatus according to claim 1, characterized in that the first optical element converts the first luminous beam into a parallel luminous beam.

6. The optical scanning apparatus according to claim 1, characterized in that the surface on which the first microstructure is formed among the incident surface and the exit surface is a flat surface.

7. The optical scanning apparatus according to claim 1, further comprising a second optical element that converts the first light beam, before it is incident on the deflector, into linearly polarized light.

8. The optical scanning apparatus according to claim 7, characterized in that the second optical element is disposed between the first light source and the first optical element.

9. The optical scanning apparatus according to claim 7, characterized in that the incident surface and the exit surface are planes parallel to each other.

10. The optical scanning apparatus according to claim 7, characterized in that the first and second optical elements are joined to each other.

11. The optical scanning apparatus according to claim 1, characterized in that the first microstructure is formed on both the incident surface and the exit surface.

12. The optical scanning apparatus according to claim 1, characterized in that the first optical element focuses the first light beam in the sub-scanning cross-section.

13. The optical scanning device according to claim 1, further comprising a third optical element having a microstructure that changes the polarization state of the first light beam formed on at least one of the incident surface and the exit surface.

14. The optical scanning apparatus according to claim 13, characterized in that the third optical element focuses the first light beam in the sub-scanning cross-section.

15. l １ ≧700nm The optical scanning apparatus according to claim 1, characterized in that it satisfies the following conditions.

16. The optical scanning apparatus according to claim 1, characterized in that at least one of the width of each protrusion, the height of each protrusion, and the spacing between adjacent protrusions is the same as one another.

17. The optical scanning apparatus according to claim 1, characterized in that the surface on which the first microstructure is formed among the incident surface and the exit surface has optical power in at least one of the main scanning cross section and the sub-scanning cross section.

18. A fourth optical element having a second microstructure formed on at least one of its incident surface and exit surface that changes the polarization state of a second luminous beam from a second light source, The system comprises a second imaging optical system that guides the second light beam deflected by the deflector to a second scanning surface, The deflector deflects the second light beam that has passed through the fourth optical element to scan the second surface to be scanned in the main scanning direction. The second microstructure on at least one of the incident surface and the exit surface of the fourth optical element is configured such that a plurality of protrusions extending in the third direction are arranged in a fourth direction perpendicular to the third direction. The phase difference between the third direction and the fourth direction of birefringence due to the second microstructure at at least one of the incident surface and the exit surface of the fourth optical element is Δ ２ , the wavelength of the second luminous beam is λ ２ In that case, 1 / 20≦D ２ / l ２ ≦1 / 6 The optical scanning apparatus according to claim 1, characterized in that it satisfies the following conditions.

19. The optical scanning apparatus according to claim 18, characterized in that the angle between the first direction and the third direction is 10° or more.

20. The optical scanning apparatus according to claim 18, characterized in that the surface on which the first microstructure is formed in the first optical element and the surface on which the second microstructure is formed in the fourth optical element have the same shape as each other.

21. The optical scanning apparatus according to claim 18, characterized in that the first and fourth optical elements are integrally formed with respect to each other.

22. An image forming apparatus comprising: an optical scanning apparatus according to any one of claims 1 to 21; a developer for developing an electrostatic latent image formed on the first surface to be scanned by the optical scanning apparatus as a toner image; a transfer unit for transferring the developed toner image to a transfer material; and a fuser for fixing the transferred toner image to the transfer material.

23. An image forming apparatus comprising an optical scanning device according to any one of claims 1 to 21, and a printer controller that converts a signal output from an external device into image data and inputs it to the optical scanning device.

Images