Scanning optical system

Abstract

The scanning optical system includes: a deflector; and an imaging optical system having a first scanning lens closer to the deflector and a second scanning lens farther from the deflector. The scanning direction is a direction perpendicular to the rotation axis of the deflector and the optical axis of the imaging optical system, the value obtained by dividing the half value of the scanning width by the maximum value of the deflection angle [theta] is defined as a system focal length f, and the distance from the deflection surface of the deflector to the scanning surface on the optical axis is defined as L, when the distance from the deflection surface to the surface of the second scanning lens farther from the deflector is defined as d, 0.88 < = f / Ld / L < = 0.3 is satisfied, the scanning direction is defined as the Y-axis direction, the y-coordinate of the optical axis is defined as 0, and the y-coordinate of the position of the principal ray of the light flux on the scanning surface is represented by a function of a deflection angle [theta], the y-coordinate of the position of the principal ray of the light flux on the scanning surface is defined as a function of a deflection angle [theta]. The absolute value of the differential function dy / d [theta] of the function increases with the absolute value of the deflection angle [theta], and the differential function dy / d [theta] has a plurality of inflection points in the ranges of-1 < = r < =-0.4 and 0.4 < = r < = 1, respectively, where r is the ratio of the deflection angle to the maximum value of the absolute value of the deflection angle [theta].

Description

Technical Field

[0001] The present invention relates to scanning optical systems. Background Art

[0002] In the imaging optical system used in printers and multifunction machines, in addition to the function of converging the light beam on the scanning surface (convergence function), a function of scanning the scanning light spot on the scanning surface at a certain speed (constant speed scanning function) is also required. However, it is difficult to miniaturize the scanning optical system that realizes the converging function and the constant speed scanning function. Therefore, a scanning optical system that sacrifices the converging function and the constant speed scanning function to some extent to achieve miniaturization has been proposed (for example, Patent Document 1).

[0003] In such a scanning optical system, the function of scanning the scanning spot at a constant speed can be realized by electrically controlling the light emission timing of the light source, even if it cannot be realized by the scanning optical system. On the other hand, regarding the converging function of such an optical system, there is a problem that the scanning spot diameter of the light beam increases at the periphery of the scanning area.

[0004] Therefore, in a scanning optical system that has been miniaturized by sacrificing the convergence function and the constant-speed scanning function to some extent, there is a demand for a scanning optical system that suppresses an increase in the scanning spot diameter of the light beam at the periphery of the scanning area.

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent Application Publication No. 2015-31824 Summary of the invention

[0008] Problems to be solved by the invention

[0009] An object of the present invention is to provide a scanning optical system that achieves miniaturization by sacrificing a converging function and a constant-speed scanning function to some extent and suppresses an increase in a scanning spot diameter of a light beam at the periphery of a scanning area.

[0010] Means for solving problems

[0011] The scanning optical system of the present invention has: a deflector that deflects a light beam from a light source; and an imaging optical system that has a first scanning lens closer to the deflector and a second scanning lens farther from the deflector, and the imaging optical system guides the polarized light beam to a scanning surface. The scanning direction is a direction perpendicular to the rotation axis of the deflector and the optical axis of the imaging optical system, and when the angle formed by the straight line as the projection of the main light ray of the deflected light beam to the plane perpendicular to the rotation axis of the deflector and the straight line as the projection of the optical axis to the plane is taken as the deflection angle θ, the value obtained by dividing the half value W / 2 of the scanning width W by the maximum value of the deflection angle θ is taken as the system focal length f, the distance from the deflection surface of the deflector on the optical axis to the scanning surface is taken as L, and the distance from the deflection surface to the surface of the second scanning lens farther from the deflector is taken as d, it is satisfied

[0012] 0.88≤f / L,

[0013] d / L≤0.3,

[0014] When the scanning direction is taken as the Y-axis direction, the y-coordinate of the optical axis is taken as 0, and the y-coordinate of the position of the main light ray of the light beam on the scanning surface is expressed as a function of the deflection angle θ, the absolute value of the differential function dy / dθ of the function increases with the absolute value of the deflection angle θ, and the ratio of the deflection angle to the maximum value of the absolute value of the deflection angle θ is taken as r, then the differential function dy / dθ has multiple inflection points in the ranges of -1≤r≤-0.4 and 0.4≤r≤1.

[0015] In the scanning optical system of the present invention, since 0.88≤f / L is satisfied, f is relatively large and the length of the second scanning lens in the Y-axis direction is relatively small. In addition, in the scanning optical system of the present invention, since d / L≤0.3 is satisfied, the second scanning lens is relatively close to the deflector, and the scanning optical system becomes compact.

[0016] In the scanning optical system of the present invention, when the y coordinate of the position of the main light ray of the light beam on the scanning surface is represented by a function of the deflection angle θ, the increase in the scanning spot diameter of the light beam at the periphery of the scanning area can be suppressed by appropriately determining the shape of the curve of the differential function dy / dθ of the function.

[0017] In the scanning optical system of the first embodiment of the present invention, the direction of the optical axis is defined as the Z-axis direction.

[0018] The refractive power of the first scanning lens in the yz cross section is positive for light passing through the vicinity of the optical axis, decreases with the absolute value of the y coordinate of the position where the light passes, and is negative for light passing through the peripheral portion in the Y-axis direction.

[0019] The refractive power of the second scanning lens in the yz cross section is negative for light passing near the optical axis, increases with the absolute value of the y coordinate of the position where the light passes, and is positive for light passing the peripheral portion in the Y-axis direction.

[0020] In the scanning optical system of the present embodiment, by adjusting the absolute value of the y-coordinate of the position where the light passes and the relationship between the refractive powers of the first scanning lens and the second scanning lens associated with the light, the shape of the curve of the differential function dy / dθ is appropriately specified, thereby suppressing the increase in the scanning spot diameter of the light beam at the periphery of the scanning area.

[0021] In the scanning optical system of the second embodiment of the present invention, when the deflection angle θ is 0, the value of (|dy / dθ|-f) / f is less than -3%, and when the absolute value of the deflection angle θ is the maximum value, the value of (|dy / dθ|-f) / f is greater than 5%.

[0022] The present invention is particularly effective when the value of (|dy / dθ|-f) / f satisfies the above-mentioned condition. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a perspective view showing an example of an optical system including the scanning optical system of the present invention.

[0024] Figure 2 It is a plan view showing an example of an optical system including the scanning optical system of the present invention.

[0025] Figure 3 This is a diagram showing an embodiment of a scanning optical system of the present invention described later.

[0026] Figure 4 It is a diagram for explaining the deflection angle of the scanning optical system.

[0027] Figure 5A This is a diagram showing the relationship between the deflection angle and the image height of the scanning optical system that satisfies the relationship of Expression (1) for various values ​​of B.

[0028] Figure 5B This is a diagram showing the difference between the image height of a scanning optical system that satisfies the relationship of Expression (1) for various values ​​of B and the image height of a scanning optical system when B is 0.

[0029] Figure 6 This is a diagram showing the relationship between the deflection angle of the scanning optical system and the local magnification of the light beam at the deflection angle when the relationship of Expression (1) is satisfied for various values ​​of B.

[0030] Figure 7The diagram shows the deflection angle of the scanning optical system that satisfies the relationship of Expression (1) for various values ​​of B and the diameter of the beam spot in the main scanning direction on the scanning surface of the light beam at the deflection angle.

[0031] Figure 8 It is a diagram showing a scanning optical system according to a comparative example of the present invention.

[0032] Fig. 9 It is a diagram showing the local magnification of the scanning optical system of the comparative example.

[0033] Fig.10 It is a diagram showing the diameter of a beam spot in the main scanning direction on a scanning surface of a scanning optical system according to a comparative example.

[0034] Fig.11 It is a diagram for explaining the refractive power of the yz cross section of the scanning lens.

[0035] Fig.12 : is a graph showing the refractive power of the first scanning lens in the yz cross section of the comparative example.

[0036] Fig.13 : is a graph showing the refractive power of the second scanning lens in the yz cross section of the comparative example.

[0037] Fig.14 : is a diagram showing the refractive power of the first scanning lens in the yz cross section according to the embodiment.

[0038] Fig.15 2 is a diagram showing the refractive power of the second scanning lens in the yz cross section according to the embodiment.

[0039] Fig.16 It is a diagram for explaining the functions of the first scanning lens 301 and the second scanning lens 302 of the embodiment.

[0040] Fig.17 It is a diagram showing a local magnification of the scanning optical system of the embodiment.

[0041] Fig.18 It is a diagram showing the differential value of the local magnification with respect to the deflection angle of the scanning optical system of the embodiment.

[0042] Fig.19 It is a diagram showing the diameter of the beam spot in the main scanning direction on the scanning surface of the scanning optical system of the embodiment. DETAILED DESCRIPTION

[0043] Figure 1 This is a perspective view showing an example of an optical system including the scanning optical system of the present invention.

[0044] Figure 2 It is a plan view showing an example of an optical system including the scanning optical system of the present invention.

[0045] exist Figure 1 as well as Figure 2 In the example shown, multiple light beams are incident on a deflector that is a polygon mirror to perform scanning on multiple scanning planes. Figure 1 and Figure 2 In the example shown, four light beams are incident on one polygonal mirror through four light sources. The first scanning optical system includes a first light source 1, a first lens 1 of the incident optical system, a polygonal mirror, a first scanning lens 1, and a third scanning lens 3. The second scanning optical system includes a second light source 2, a second lens 2 of the incident optical system, a polygonal mirror, a second scanning lens 2, and a fourth scanning lens 4. The third scanning optical system includes a third light source 3, a third lens 3 of the incident optical system, a polygonal mirror, a first scanning lens 1, and a third scanning lens 3. The fourth scanning optical system includes a fourth light source 4, a fourth lens 4 of the incident optical system, a polygonal mirror, a second scanning lens 2, and a fourth scanning lens 4. That is, the polygonal mirror is shared by the first to fourth scanning optical systems, the first scanning lens 1 and the third scanning lens 3 are shared by the first and third scanning optical systems, and the second scanning lens 2 and the fourth scanning lens 4 are shared by the second and fourth scanning optical systems.

[0046] Figure 3 1 is a diagram showing an embodiment of the scanning optical system of the present invention described later. A light beam emitted from a light source 101 passes through an aperture and a lens 103, is deflected by a deflector 200 as a polygonal mirror, and then is converged on a scanning surface 400 by an imaging optical system including a first scanning lens 301 and a second scanning lens 303. The x-axis in the direction of the rotation axis of the deflector, the z-axis in the direction of the optical axis of the imaging optical system, and the y-axis perpendicular to the x-axis and the z-axis are determined. The scanning direction is the direction of the y-axis. The direction of the y-axis is also referred to as the main scanning direction, and the direction of the x-axis is referred to as the sub-scanning direction.

[0047] Figure 4 θ is a diagram for explaining the deflection angle of the scanning optical system. The deflection angle θ is the angle between the straight line that is the projection of the principal ray of the light beam deflected by the deflector 200 onto the yz plane and the straight line that is the projection of the optical axis of the imaging optical system onto the yz plane. Figure 4 In , P represents the reflection point of the main light on the deflection surface of the deflector 200. Figure 4 In FIG. 1 , the solid line indicates the path of the principal ray of the first scanning optical system having a relatively small maximum value of the deflection angle θ, and the solid line indicates the path of the principal ray of the second scanning optical system having a relatively large maximum value of the deflection angle θ. If the effective scanning width of both scanning optical systems is set to W, the passing range of the light of the first scanning lens 301 and the second scanning lens 303 of the first scanning optical system is smaller than the passing range of the light of the first scanning lens 301 and the second scanning lens 303 of the second scanning optical system.

[0048] On the other hand, in the scanning optical system, in order to realize the uniform speed scanning function of the scanning spot on the scanning surface 400, the relationship y=f·θ is required. This relationship is called the fθ characteristic. Here, y is the position of the main light on the scanning surface 400 when the coordinate of the optical axis is set to 0, that is, the y coordinate of the imaging position, also called the image height. When the above relationship is established, when the deflector rotates at a constant speed, the scanning spot on the scanning surface moves at a constant speed. The effective scanning width of the scanning optical system is set to W, and f is the value obtained by dividing the half value of the effective scanning width W / 2 by the maximum value of the absolute value of the deflection angle θ, which is called the system focal length.

[0049] In order to realize the converging function and the constant-speed scanning function of the scanning optical system, the scanning lens needs to be arranged sufficiently away from the deflector, resulting in that the scanning lens and the scanning optical system cannot be sufficiently miniaturized.

[0050] Patent Document 1 proposes a scanning optical system in which the y coordinate of a scanning spot and the deflection angle θ have the following relationship in order to realize a compact scanning optical system.

[0051] y=(K / B)·tan(B·θ) (1)

[0052] B is a parameter with a value ranging from 0 to 1, and K is a constant. When B is 0,

[0053] y=K·θ

[0054] When B is 1,

[0055] y = K·tanθ.

[0056] Next, the properties of the scanning optical system that satisfies the relationship of formula (1) are examined for various values ​​of B. As an example, if the scanning width W is 220 mm and the maximum value of the absolute value of the deflection angle is 45 degrees, the values ​​of K corresponding to various values ​​of B are shown in the following Table 1. The unit of K is millimeter.

[0057] Table 1

[0058] B K 0 140.06 0.2 138.9 0.4 135.4 0.6 129.5 0.8 121.1

[0059] Figure 5A 1 is a diagram showing the relationship between the deflection angle and the image height of the scanning optical system that satisfies the relationship of Expression (1) for various values ​​of B. Figure 5A The horizontal axis represents the ratio of the deflection angle to the maximum value of the absolute value of the deflection angle. Figure 5A The vertical axis represents the y coordinate of the light ray of the horizontal axis deflection angle in the scanning plane, that is, the image height. The unit of the vertical axis is millimeter.

[0060] Figure 5B1 is a diagram showing the difference between the image height of the scanning optical system that satisfies the relationship of Expression (1) for various values ​​of B and the image height of the scanning optical system when B is 0. Figure 5B The horizontal axis represents the ratio of the deflection angle to the maximum value of the absolute value of the deflection angle. Figure 5B The vertical axis represents the difference between the y coordinate of the light of the scanning optical system at the deflection angle of the horizontal axis in the scanning plane and the y coordinate of the light of the scanning optical system at the same deflection angle in the scanning plane when B is 0. The unit of the vertical axis is millimeter.

[0061] Figure 6 1 is a diagram showing the relationship between the deflection angle of the scanning optical system and the local magnification of the light beam at the deflection angle when the relationship of Expression (1) is satisfied for various values ​​of B. Figure 6 The horizontal axis represents the ratio of the deflection angle to the maximum value of the absolute value of the deflection angle. Figure 6 The vertical axis represents the local magnification. The unit of the vertical axis is percentage. The local magnification is defined by the following formula.

[0062]

[0063] Here, yr in formula (2) represents the y coordinate on the scanning surface of the light ray at the deflection angle θ of the scanning optical system having the relationship of formula (1) and B is not 0, and y in formula (2) represents the y coordinate on the scanning surface of the light ray at the deflection angle θ of the scanning optical system having the relationship of y=fθ.

[0064] Figure 6 The local magnification shown indicates the deviation of the scanning spot from the constant speed characteristic of the scanning optical system. When the local magnification is 0, the scanning spot is at a constant speed. When the absolute value of the local magnification increases, the deviation of the scanning spot from the constant speed characteristic increases.

[0065] Figure 7 The diagram shows the deflection angle of the scanning optical system that satisfies the relationship of Expression (1) for various values ​​of B and the diameter of the beam spot in the main scanning direction on the scanning surface of the light beam at the deflection angle. Figure 7 The horizontal axis represents the ratio of the deflection angle to the maximum value of the absolute value of the deflection angle. The unit of the horizontal axis is millimeter. Figure 7 The vertical axis represents the diameter of the beam spot in the main scanning direction. Figure 7 In the figure, the diameter of the beam spot in the main scanning direction is recorded as the main spot diameter. The unit of the vertical axis is micrometer. The main spot diameter when the deflection angle is 0 is 60 micrometers.

[0066] Figure 7 The diameter of the beam spot shown represents the focusing function of the scanning optics.

[0067] As a result, when B is close to 1, the imaging optical system can be made compact, but the local magnification and the diameter of the beam spot in the main scanning direction on the scanning surface increase with the absolute value of the deflection angle θ, and become the maximum at the maximum value of the absolute value of the deflection angle θ. Here, even if the constant speed scanning function of the scanning spot cannot be achieved by the scanning optical system, it can be achieved by electrically controlling the light emission timing of the light source. Therefore, a scanning optical system in which B is not 0 is required, especially a scanning optical system in which the diameter of the beam spot in the main scanning direction is relatively small near the maximum value of the absolute value of the deflection angle θ.

[0068] Next, comparative examples and examples of the present invention will be described.

[0069] The shapes of the incident and exit surfaces of the first scanning lenses of the comparative example and the embodiment are represented by the following formula A. In the following description, the main scanning direction cross section refers to the yz cross section perpendicular to the x-axis, and the sub-scanning direction cross section refers to the xz cross section perpendicular to the y-axis.

[0070]

[0071] in,

[0072]

[0073] y: Main scanning direction coordinate

[0074] x: coordinate in the secondary scanning direction

[0075] z: droop

[0076] k: cone coefficient

[0077] Ry: Radius of curvature of the cross section in the main scanning direction

[0078] rx(y): The radius of curvature of the sub-scanning direction section at the main scanning direction coordinate y

[0079] rx(0): radius of curvature on the optical axis of the sub-scanning direction section

[0080] Ai: Aspheric coefficient of the cross section in the main scanning direction (i = 1, 2, 3, 4...)

[0081] Bi: coefficient that determines the radius of curvature of the sub-scanning direction cross section (i = 1, 2, 3, 4, ...)

[0082] The shapes of the incident surface and the exit surface of the second scanning lens in the comparative example and the embodiment are expressed by the following formula B.

[0083] z=z m +z s B

[0084] in,

[0085]

[0086]

[0087] in,

[0088]

[0089] y: Main scanning direction coordinate

[0090] x: coordinate in the secondary scanning direction

[0091] z: droop

[0092] zs: Main scanning direction droop

[0093] zm: Sub-scanning direction droop

[0094] ky: cone coefficient in the main scanning direction

[0095] Ry: Radius of curvature of the cross section in the main scanning direction

[0096] h: busbar bending function

[0097] rx(y): The radius of curvature of the sub-scanning direction section at the main scanning direction coordinate y

[0098] rx(0): radius of curvature on the optical axis of the sub-scanning direction section

[0099] Ai: Aspheric coefficient of the cross section in the main scanning direction (i = 1, 2, 3, 4...)

[0100] Bi: coefficient that determines the radius of curvature of the sub-scanning direction cross section (i = 1, 2, 3, 4, ...)

[0101] Ci: Busbar bending coefficient (i=1, 2, 3, 4...)

[0102] Di: Aspheric coefficient of the sub-scanning direction cross section (i = 1, 2, 3, 4, etc.)

[0103] The coefficients Ai and Bi take individual values ​​according to the sign of the main scanning direction coordinate + / -. In the +y region, they take Api and Bpi, and in the -y region, they take Ami and Bmi.

[0104] Comparative Example

[0105] Figure 81 is a diagram showing a scanning optical system of a comparative example of the present invention. A light beam emitted from a light source 101 passes through an aperture and a lens 103', is deflected by a deflector 200 as a polygon mirror, and then is converged on a scanning surface 400 by an imaging optical system including a first scanning lens 301' and a second scanning lens 303'.

[0106] The scanning optical system of the comparative example is designed so that the light beams at various deflection angles satisfy the relationship of equation (1), aberrations are appropriately corrected, and the spot diameter of the light beam can be used.

[0107] In the scanning optical system of the comparative example, equation (1)

[0108] y=(K / B)·tan(B·θ) (1)

[0109] B is 0.62, the effective scanning width is 216 mm, and the maximum absolute value of the deflection angle θ is 44 degrees.

[0110] Table 2 is a table showing numerical data of the scanning optical system of the comparative example. In Table 2 and the following table, the first scanning lens is represented as Lens A, and the second scanning lens is represented as Lens B. In addition, the "deflection reference point" is the reflection point on the surface of the deflector of the main light beam after deflection when the straight line projected onto the yz plane is in the Z-axis direction. In addition, the "primary incident angle" and the "secondary incident angle" are the incident angles of the yz plane and the xz plane, respectively.

[0111] On the optical axis of the imaging optical system, assuming that the distance from the deflection surface of the deflector to the scanning surface is L, and the distance from the deflection surface to the surface of the second scanning lens farther from the deflector is d, the following relationship holds.

[0112] f / L =139.4 / 158.32=0.88

[0113] d / L=(38.4+4.5) / 158.32=0.270

[0114] [Table 2]

[0115]

[0116] Table 3A and Table 3B are tables showing coefficients of equations representing the surface shapes of the first and second scanning lenses of the comparative example.

[0117] [Table 3A]

[0118]

[0119] [Table 3B]

[0120]

[0121] Fig. 9 It is a diagram showing the local magnification of the scanning optical system of the comparative example. Fig. 9 The horizontal axis represents the ratio of the deflection angle to the maximum value of the absolute value of the deflection angle. Fig. 9 The vertical axis represents the local magnification of the scanning optical system with respect to the deflection angle shown on the horizontal axis. Fig. 9 The curve representing the local magnification is Figure 6 The curve representing the local magnification when B is 0.6 is approximated.

[0122] Fig.10 It is a diagram showing the diameter of a beam spot in the main scanning direction on a scanning surface of a scanning optical system according to a comparative example. Fig.10 The horizontal axis represents the coordinate of the actual imaging position of the light beam. The unit of the horizontal axis is millimeter. Fig.10 The vertical axis represents the diameter of the light beam spot in the main scanning direction on the scanning surface of the light beam whose coordinates are the actual imaging position represented by the horizontal axis. Fig.10 The curve representing the diameter of the beam spot in the main scanning direction is Figure 7 The curve representing the diameter of the beam spot in the main scanning direction when B is 0.6 is approximated.

[0123] Fig.11 It is a diagram for explaining the refractive power of the yz cross section of the scanning lens. Fig.11 The yz cross section including the points where the principal ray of each deflection angle intersects the optical surface is shown. The radius of curvature at the incident position Ym1 of the light incident surface is set to Rm1, and the radius of curvature at the light incident surface position Ym2 of the exit surface is set to Rm2. The angle formed by the principal ray of the light beam incident on the incident surface and the normal to the incident surface is set to θo1, and the angle formed by the principal ray of the light beam passing through the incident surface and the normal to the incident surface is set to θi1. The angle formed by the principal ray of the light beam reaching the exit position of the exit surface and the normal to the exit surface is set to θo2, the angle formed by the principal ray of the light beam passing through the exit surface and the normal to the exit surface is set to θi2, and the optical path length from the incident surface to the exit surface is set to dm. The refractive index of the scanning lens material is set to N. The refractive power of the yz cross section of the scanning lens at this time is It is defined by the following formula.

[0124]

[0125] Fig.12 2 is a diagram showing the refractive power of the first scanning lens 301 ′ in the yz cross section of the comparative example. Fig.12 The horizontal axis represents the coordinate of the actual imaging position of the light beam. The unit of the horizontal axis is millimeter. Fig.12The vertical axis represents the refractive power of the yz cross section of the first scanning lens 301 ′ of the comparative example for the light reaching the coordinates of the actual imaging position represented by the horizontal axis.

[0126] Fig.13 2 is a diagram showing the refractive power of the second scanning lens 302 ′ in the yz cross section of the comparative example. Fig.13 The horizontal axis represents the coordinate of the actual imaging position of the light beam. The unit of the horizontal axis is millimeter. Fig.13 The vertical axis represents the refractive power of the yz cross section of the second scanning lens 302 ′ of the comparative example for the light reaching the coordinates of the actual imaging position represented by the horizontal axis.

[0127] Example

[0128] The scanning optical system of the embodiment is designed by adjusting the surface shapes of the first and second scanning lenses through the steps described later, based on the scanning optical system of the comparative example.

[0129] In the scanning optical system of the embodiment, the effective scanning width is 216 mm, and the maximum value of the absolute value of the deflection angle θ is 42 degrees.

[0130] Table 4 is a table showing numerical data of the scanning optical system of the embodiment.

[0131] On the optical axis of the imaging optical system, assuming that the distance from the deflection surface of the deflector to the scanning surface is L, and the distance from the deflection surface to the surface of the second scanning lens farther from the deflector is d, the following relationship holds.

[0132] 0.88≤f / L=147.3 / 158.07=0.93

[0133] d / L=(37.93+4.5) / 158.07=0.268≤0.3

[0134] Since f / L in the scanning optical system of the embodiment is greater than f / L in the scanning optical system of the comparative example, the length of the second scanning lens in the Y-axis direction of the embodiment can be shorter than that of the comparative example. In addition, since d / L in the scanning optical system of the embodiment is less than d / L in the scanning optical system of the comparative example, the scanning optical system of the embodiment is more compact than the scanning optical system of the comparative example.

[0135] [Table 4]

[0136]

[0137] Table 5A and Table 5B are tables showing coefficients of equations representing the surface shapes of the first and second scanning lenses of the example.

[0138] [Table 5A]

[0139]

[0140] [Table 5B]

[0141]

[0142] Fig.14 2 is a diagram showing the refractive power of the first scanning lens 301 in the yz cross section according to the embodiment. Fig.14 The horizontal axis represents the coordinate of the actual imaging position of the light beam. The unit of the horizontal axis is millimeter. Fig.14 The vertical axis of represents the refractive power in the yz cross section of the first scanning lens 301 of the embodiment for the light reaching the coordinates of the actual imaging position shown on the horizontal axis. Fig.14 In the embodiment, the solid line represents the refractive power in the yz cross section of the first scanning lens 301, and the dotted line represents Fig.12 The refractive power of the first scanning lens 301' in the yz cross section of the comparative example shown in FIG. Fig.14 The refractive power in the yz section of the first scanning lens 301' of the comparative example is independent of the coordinates of the actual imaging position and is a positive, approximately constant value. However, the refractive power in the yz section of the first scanning lens 301 of the embodiment is positive in an area where the absolute value of the coordinates of the actual imaging position is relatively small, decreases as the absolute value of the coordinates of the actual imaging position increases, and is negative in an area where the absolute value of the coordinates of the actual imaging position is relatively large.

[0143] In other words, the refractive power of the first scanning lens 301 in the yz cross section is positive for light passing near the optical axis, decreases with the absolute value of the y coordinate of the position where the light passes, and is negative for light passing the peripheral portion in the Y-axis direction.

[0144] Fig.15 2 is a diagram showing the refractive power of the second scanning lens 302 in the yz cross section according to the embodiment. Fig.15 The horizontal axis represents the coordinate of the actual imaging position of the light beam. The unit of the horizontal axis is millimeter. Fig.15 The vertical axis represents the refractive power of the second scanning lens 302 in the yz cross section of the embodiment for the light reaching the coordinates of the actual imaging position shown on the horizontal axis. Fig.15 In the embodiment, the solid line represents the refractive power in the yz cross section of the second scanning lens 302, and the dotted line represents Fig.13 The refractive power of the second scanning lens 302' in the yz cross section of the comparative example shown in FIG. Fig.15, the refractive power on the yz section of the second scanning lens of the comparative example and the embodiment increases with the increase of the absolute value of the coordinates of the actual imaging position. The refractive power in the yz section of the second scanning lens 302' of the comparative example is positive in the entire area of ​​the coordinates of the actual imaging position. The refractive power in the yz section of the second scanning lens 302 of the embodiment is negative in the area where the absolute value of the coordinates of the actual imaging position is relatively small, increases with the increase of the absolute value of the coordinates of the actual imaging position, is positive in the area where the absolute value of the coordinates of the actual imaging position is relatively large, and is greater at the periphery of the scanning area than the refractive power in the yz section of the second scanning lens 302' of the comparative example.

[0145] In other words, the refractive power of the second scanning lens 302 in the yz cross section is negative for light passing near the optical axis, increases with the absolute value of the y coordinate of the position where the light passes, and is positive for light passing the peripheral portion in the Y-axis direction.

[0146] Fig.16 301 and 302 of the embodiment. In the region where the absolute value of the y coordinate of the first scanning lens 301 is relatively small, the refractive power in the yz cross section is positive, so the light beam is converged. In the region where the absolute value of the y coordinate of the first scanning lens 301 is relatively large, the refractive power in the yz cross section is negative, so the light beam diverges. In the region where the absolute value of the y coordinate of the second scanning lens 302 is relatively small, the refractive power in the yz cross section is negative, so the light beam diverges. In the region where the absolute value of the y coordinate of the second scanning lens 302 is relatively large, the refractive power in the yz cross section is positive, so the light beam is converged.

[0147] Fig.17 It is a diagram showing a local magnification of the scanning optical system of the embodiment. Fig.17 The horizontal axis represents the ratio of the deflection angle to the maximum value of the absolute value of the deflection angle. Fig.17 The vertical axis represents the local magnification of the scanning optical system with respect to the deflection angle shown on the horizontal axis. The unit of the vertical axis is percentage. Fig.17 In the figure, the solid line represents the local magnification of the embodiment, and the dotted line represents Fig. 9 The local magnification of the comparative example shown. The local magnification of the embodiment and the comparative example is negative and minimum when the deflection angle is 0, increases with the absolute value of the deflection angle, and becomes maximum at the maximum absolute value of the deflection angle. The local magnification of the scanning optical system of the embodiment is smaller than the local magnification of the comparative example, especially near the maximum absolute value of the deflection angle, that is, near the ratio of the deflection angle to the maximum absolute value of the deflection angle is 1 and -1.

[0148] In the embodiment, the local magnification when the deflection angle is 0 is about -8%, and the local magnification when the absolute value of the deflection angle is the maximum is about 10%.

[0149] Fig.18 It is a diagram showing the differential value of the local magnification with respect to the deflection angle of the scanning optical system of the embodiment. Fig.18 The horizontal axis represents the ratio of the deflection angle to the maximum value of the absolute value of the deflection angle. Fig.18 The vertical axis represents the differential value of the local magnification of the scanning optical system with respect to the deflection angle of the light beam shown on the horizontal axis. The unit of the vertical axis is percentage. Fig.18 In the figure, the solid line represents the differential value of the local magnification of the embodiment with respect to the deflection angle, and the dotted line represents Fig. 9 The differential value of the local magnification with respect to the deflection angle of the comparative example shown. Fig.18 , regarding the differential value of the local magnification of the scanning optical system of the embodiment with respect to the deflection angle, the ratio of the deflection angle to the maximum value of the absolute value of the deflection angle is set to r, and has three extreme values ​​in the ranges of -1≤r≤-0.4 and 0.4≤r≤1. Therefore, the function of the deflection angle of the local magnification of the scanning optical system of the embodiment has three inflection points in the ranges of -1≤r≤-0.4 and 0.4≤r≤1.

[0150] According to formula (2), when the y coordinate of the position of the main ray of the light beam on the scanning surface is expressed by a function of the deflection angle θ, the absolute value of the differential of the function with respect to the deflection angle θ is

[0151] As the absolute value of the deflection angle θ increases, the ratio of the deflection angle to the maximum absolute value of the deflection angle θ is set to r, and the differential of the function with respect to the deflection angle θ is

[0152] There are three inflection points in the ranges of -1≤r≤-0.4 and 0.4≤r≤1 respectively.

[0153] By adjusting the relationship between the coordinates of the actual imaging position of the light beam and the refractive powers of the first and second scanning lenses for the light beam reaching the coordinates based on the scanning optical system of the comparative example, the scanning optical system of the embodiment can be adjusted. Fig.17 The shape of the curve of the local magnification is shown.

[0154] Fig.19 It is a diagram showing the diameter of the beam spot in the main scanning direction on the scanning surface of the scanning optical system of the embodiment. Fig.19 The horizontal axis represents the y coordinate of the actual imaging position of the light beam. The unit of the horizontal axis is millimeter. Fig.19 The vertical axis represents the diameter of the beam spot in the main scanning direction on the scanning surface of the beam having the principal ray passing through the coordinates of the actual imaging position shown on the horizontal axis. Fig.19In the figure, the solid line represents the diameter of the beam spot on the scanning surface of the scanning optical system of the embodiment in the main scanning direction, and the dotted line represents Fig.10 The diameter of the beam spot in the main scanning direction on the scanning surface of the scanning optical system of the comparative example shown in FIG. Fig.19 In the embodiment, the maximum value of the diameter of the beam spot in the main scanning direction on the scanning surface of the scanning optical system is smaller than the maximum value of the diameter of the beam spot in the main scanning direction on the scanning surface of the scanning optical system of the comparative example, and the difference is greater than 3 microns.

Claims

1. A scanning optical system, comprising: a deflector that deflects a light beam from a light source; and an imaging optical system having a first scanning lens closer to the deflector and a second scanning lens farther from the deflector, the imaging optical system directing the polarized light beam to a scanning surface, in, The scanning direction is a direction perpendicular to the rotation axis of the deflector and the optical axis of the imaging optical system. When the angle between the straight line as the projection of the main light ray of the deflected light beam onto the plane perpendicular to the rotation axis of the deflector and the straight line as the projection of the optical axis onto the plane is taken as the deflection angle θ, the value obtained by dividing the half value W / 2 of the scanning width W by the maximum value of the deflection angle θ is taken as the system focal length f, the distance from the deflection surface of the deflector to the scanning surface on the optical axis is taken as L, and the distance from the deflection surface to the surface of the second scanning lens further away from the deflector is taken as d, it is satisfied 0.88≤f / L, d / L≤0.3, When the scanning direction is taken as the Y-axis direction, the y-coordinate of the optical axis is taken as 0, and the y-coordinate of the position of the main ray of the light beam on the scanning surface is expressed as a function of the deflection angle θ, the absolute value of the differential function dy / dθ of the function increases with the absolute value of the deflection angle θ, and the ratio of the deflection angle to the maximum value of the absolute value of the deflection angle θ is taken as r, then the differential function dy / dθ is -1≤r≤-0.4 and There are a plurality of inflection points in the range of 0.4≤r≤1.

2. The scanning optical system according to claim 1, wherein: The direction of the optical axis is defined as the Z-axis direction. The refractive power of the first scanning lens in the yz cross section is positive for light passing through the vicinity of the optical axis, decreases with the absolute value of the y coordinate of the position where the light passes, and is negative for light passing through the peripheral portion in the Y-axis direction. The refractive power of the second scanning lens in the yz cross section is negative for light passing near the optical axis, increases with the absolute value of the y coordinate of the position where the light passes, and is positive for light passing the peripheral portion in the Y-axis direction.

3. The scanning optical system according to claim 1, wherein: When the deflection angle θ is 0, the value of (|dy / dθ|-f) / f is less than -3%. When the absolute value of the deflection angle θ is maximum, the value of (|dy / dθ|-f) / f is 5% or more.

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