JP3266350B2 - fθ lens and optical scanning device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an f.theta. Lens and an optical scanning device.

[0002]

2. Description of the Related Art An fθ lens focuses a deflecting light beam deflected at an equal angular velocity by an optical deflecting device such as a rotary polygon mirror toward a surface to be scanned, and moves the light spot on the surface to be scanned at a constant speed. Various types of lenses have been proposed.

In recent years, there has been a demand for an optical scanning device which is small in size, has a wide scanning area, and has excellent optical scanning characteristics. The fθ lens of which is small is required.

[0004]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and the curvature of field in the main and sub-scanning directions is satisfactorily corrected over the entire effective scanning area. It is an object of the present invention to provide a novel fθ lens which is close to a wide angle, has excellent fθ characteristics, is small in size and can be manufactured at low cost, and an optical scanning device using the fθ lens.

[0005]

As shown in FIG. 1, the fθ lens according to the present invention comprises a first group 5, a second group 6, and a third group from the optical deflector 3 toward the surface 8 to be scanned. 7 in a three-group, three-element configuration. The first group 5 has a concave surface on the optical deflector 3 side.
, But at least one surface is an aspheric surface. The second group 6 is a positive lens that has a “curvature on the surface to be scanned side stronger than a curvature on the surface on the optical deflector side”.
The third group is a single lens, and both the surface on the side of the optical deflector 3 and the surface on the side of the scanned surface 8 are “toric surfaces in which the curvature in the sub-scanning direction is stronger than the curvature in the main scanning direction”. The surface on the third side is a concave toric surface, and the surface on the scanned surface side is a convex toric surface. The “concave toric surface” includes the case where the radius of curvature in the main scanning corresponding direction is ∞, that is, the “concave cylinder surface” (Example 5).

The “main scanning corresponding direction” is a direction parallel to the main scanning direction on an imaginary optical path in which an optical path from the light source device to the surface to be scanned is linearly developed along the optical axis. The direction corresponding to the sub-scanning direction on the optical path in parallel to the sub-scanning direction is the “sub-scanning corresponding direction”.

The combined focal length in the main scanning direction of the entire lens system is f, the focal length of the first lens unit is f ₁ , and the radius of curvature of the first lens unit on the optical deflector side and the surface to be scanned is on the optical axis. R ₁ and R ₂ , the focal length in the sub-scanning corresponding direction of the third group are f _3y , and the radii of curvature of the third group on the optical deflector side and the surface to be scanned in the main scanning corresponding direction are R _5x and R _5x , respectively.
When the R _6x, these conditions _{(1) 0.7 <R 1 /} R 2 <1.1 (2) -0.4 <R 1 /f<-0.1 (3) -0.4 < f / f ₁ <0.3 (4) −0.6 <R _6x / R _5x <1.1 (5) 0.3 <f _3y /f<0.4 is satisfied. Since the third lens unit 7 is anamorphic, the fθ lens itself is also anamorphic and has a stronger positive refractive power in the sub-scanning corresponding direction.

The first group of aspherical surfaces may be used for the “surface on the side to be scanned” (claim 2) or may be used for the “surface on the optical deflector side” (claim). 3) The "surface on the optical deflector side and the surface to be scanned side" may both be aspherical. The lens configuration of the second group may be a “biconvex lens” (Claim 5) or a “positive meniscus lens having a concave surface facing the optical deflector” (Claim 6).

As shown in FIG. 1, the optical scanning device according to the present invention forms a substantially parallel light beam emitted from a light source device 1 as a line image long in a main scanning corresponding direction by a line image forming optical system 2. And a deflecting reflecting surface 4 near the image forming position of the line image.
An optical deflector 3 having an optical deflector 3 having a constant angular velocity, and converging a deflected light beam as a light spot on a surface 8 to be scanned by an image forming optical system to perform uniform optical scanning of the surface to be scanned. An apparatus, wherein an anamorphic fθ lens according to claim 1, 2, 3, 4, 5, or 6 is used as the imaging optical system (claim 7). For example, a cylinder lens can be used as the line image forming optical system 2, and a rotary two-sided mirror or a rotary single-sided mirror can be used as the optical deflector in addition to the rotary polygonal mirror.

[0010]

When the fθ lens of the present invention is used in an optical scanning device, as described in claim 7, an image is formed in the vicinity of the deflecting / reflecting surface 4 of the optical deflector 3 in the sub-scanning corresponding direction. Since the line image is formed on the surface 8 to be scanned, the fθ lens sets the position of the surface to be scanned and the position of the deflecting / reflecting surface in the sub-scanning corresponding direction in a “geometric optical conjugate relationship”. The optical scanning device has a function of correcting a so-called "surface tilt" in the optical deflector.

The condition (1) is a condition for correcting the fθ characteristic. When the value exceeds the upper limit of the condition (1), the fθ characteristic becomes under, and when the value exceeds the lower limit, the value becomes over.

Condition (2) is a condition for correcting the fθ characteristic, similarly to condition (1). When the value exceeds the upper limit of the condition (2), the negative refractive power on the side surface of the optical deflector of the first group becomes weak, and the fθ characteristic becomes over. Conversely, when the value goes below the lower limit of the condition (2), the negative refractive power of the surface becomes strong, and the fθ characteristic becomes under.

Condition (3) is that the curvature of field in the main scanning direction and f
This is a condition for correcting the θ characteristic. If the upper limit is exceeded, the curvature of field in the main scanning direction becomes over, and the fθ characteristic becomes under. Below the lower limit, the curvature of field in the main scanning direction is under, and the fθ characteristic is over.

Condition (4) is that the curvature of field in the sub-scanning direction and f
This is a condition for correcting the θ characteristic. When the value exceeds the upper limit of the condition (4), the refractive power of the third lens unit in the main scanning corresponding direction becomes strong to the negative side, the fθ characteristic becomes over, and the field curvature in the sub-scanning direction has a large remaining field curvature. Become. Condition (4)
If the lower limit is exceeded, the refracting power of the second group in the main scanning corresponding direction becomes positive, and the fθ characteristic becomes under.

Condition (5) is a condition for correcting the curvature of field in the sub-scanning direction. When the value exceeds the upper limit of the condition (5), the refractive power of the third lens unit in the sub-scanning corresponding direction becomes too strong, and when the value goes below the lower limit, the refractive power becomes relatively weak, which is outside the range of the condition (5). In this case, the remaining field curvature becomes large.

[0016]

[Embodiments] Hereinafter, eight specific embodiments will be described.

As shown in FIG. 1, in each embodiment, the radius of curvature of the i-th lens surface counted from the side of the optical deflector 3 (the radius of curvature on the optical axis for an aspheric surface) corresponds to the main scanning. R _{ix in} the direction and R _iy (i = 1 to
6, provided that R _ix = R _iy = R _{i for} i = 1 to 4,
The distance between the i-th lens surface and the (i + 1) -th lens surface on the optical axis is D _i (i = 1 to 5). The distance on the optical axis from the optical axis and the deflecting / reflecting surface 4 at the time of coincidence with the lens optical axis to the first lens surface is D ₀ (i =
0). The refractive index of the j-th lens counted from the optical deflector 3 with respect to light having a wavelength of 780 nm is represented by N _j (j
= 1 to 3). Further, f is the focal length of the entire system and is 1
Standardize to 00. 2θ represents the effective deflection angle. Also represent conditions (1) to the value of each expression (5) J ₁ through J ₅ respectively.

As is well known, the aspherical surface is made to coincide with the
When taking the coordinates and setting the Y coordinate perpendicular to the optical axis,
When the radius of curvature on the optical axis is R, the conic constant is K, and the fourth-order aspheric coefficient is A, Z = (1 / R) · Y ² / {1 +} [1- (1 + K) · (Y
/ R) ² ]｝ + A · Y ⁴ is a curved surface obtained by rotating the curve around the optical axis, and has a radius of curvature on the optical axis: R, a conical constant: K, an aspheric coefficient: A To specify the shape.

Throughout the embodiments, the optical deflector is a rotary polygon mirror having six deflecting and reflecting surfaces as shown in FIG. 1, and its inscribed circle radius is 18 (when the focal length of the entire system is standardized to 100). Value), the angle between the light beam incident on the deflecting / reflecting surface 4 and the optical axis of the fθ lens is 80 degrees, and the transmission wavelength of the semiconductor laser as the light source in the light source device 1 is 780 nm.

[0020] Example 1 f = 100,2θ = 88.798 degrees _{i R ix R iy D i j} N j 0 12.546 1 -24.428 4.258 1 1.48601 2 -25.798 6.916 3 2214.022 8.856 2 1.48601 4-79.705 4.1125 5-191.882-17.269 8.856 3 1.51920 6-85.609-10.140.

Aspherical surface: first surface K = 0.6770513, A = 0 Aspherical surface: second surface K = 0.38876636, A = 0.12761313 × 1
Parameter values of 0 to ⁶ conditional expressions J ₁ = 0.947, J ₂ = −0.244, J ₃ = 0.17
8 × 10 to ² , J ₄ = 0.446, J ₅ = 0.332
.

[0022] Example 2 f = 100,2θ = 88.798 degrees _{i R ix R iy D i j} N j 0 11.808 1 -24.428 4.258 1 1.48601 2 -25.904 8.118 3-3690.037 8.856 2 1.48601 4 -75.277 7.011 5 -191.882 -20.295 8.856 3 1.51920 6 -84.133-10.923.

Aspherical surface: first surface K = 0.7031411, A = 0 Aspherical surface: second surface K = 0.4735150, A = 0.1806106 × 1
^0-6 Condition parameter values _{J 1 = 0.943, J 2 =} -0.244, J 3 = -0.6
40 × 10 ~ ² , J ₄ = 0.438, J ₅ = 0.344
.

[0024] Example 3 f = 100,2θ = 88.798 degrees _{i R ix R iy D i j} N j 0 12.546 1 -29.812 4.796 1 1.48601 2 -30.888 7.010 3-295.203 8.856 2 1.48601 4 -66.421 1.6685 3690.037 -17.638 8.118 3 1.51920 6 147.601 -9.934.

Aspherical surface: first surface K = 0.357533, A = 0 Aspherical surface: second surface K = 0.747923, A = 0.5146346 × 1
^0-6 Condition parameter values _{J 1 = 0.965, J 2 =} -0.298, J 3 = 0.26
0 × 10 to ¹ , J ₄ = −0.400 × 10 to ¹ , J ₅ = 0.3
22.

[0026] Example 4 f = 100,2θ = 88.798 degrees _{i R ix R iy D i j} N j 0 8.587 1 -30.406 4.258 1 1.51920 2 -32.327 11.552 3-3690.037 8.856 2 1.48601 4-75.277 4.8875 5-118.081-19.557 8.856 3 1.51920 6-61.945-10.635.

Aspherical surface: first surface K = 0.2302611, A = 0 Aspherical surface: second surface K = 0.46000602, A = 0.2070036 × 1
Parameter values of 0 to ⁶ conditional expressions J ₁ = 0.941, J ₂ = −0.304, J ₃ = 0.24
6 × 10 ^-1 , J ₄ = 0.550, J ₅ = 0.335
.

[0028] Example 5 f = 100,2θ = 88.798 degrees _{i R ix R iy D i j} N j 0 11.070 1 -22.135 4.797 1 1.48601 2 -22.890 8.856 3-221.402 9.594 2 1.48601 4 -66.421 8.1185} -26.199 9.594 3 1.48601 6 -129.151 -11.661.

Aspherical surface: first surface K = 1.304490, A = −0.7181326 ×
10 to ⁶ aspherical surface: second surface K = 0.9203333, A = 0 Parameter values of conditional expressions J ₁ = 0.967, J ₂ = −0.221, J ₃ = 0.78
1 × 10 to ¹ , J ₄ = 0, J ₅ = 0.356
.

[0030] Example 6 f = 100,2θ = 88.798 degrees _{i R ix R iy D i j} N j 0 11.808 1 -24.428 4.258 1 1.48601 2 -25.951 8.118 3-199.262 7.380 2 1.48601 4-75.277 7.011 5-738.007-36.531 11.070 3 1.51920 6-81.181-13.048.

Aspherical surface: first surface K = 0.64993995, A = 0 Aspherical surface: second surface K = 0.580850, A = 0.2835771 × 1
Parameter values of conditional expressions 0 to ⁶ J ₁ = 0.941, J ₂ = −0.244, J ₃ = −0.1
_{^{00 × 10 ~ 1, J 4}} = 0.110, J 5 = 0.337
.

[0032] Example 7 f = 100,2θ = 88.798 degrees _{i R ix R iy D i j} N j 0 11.808 1 -22.878 4.428 1 1.48601 2 -24.707 11.070 3-701.107 8.118 2 1.51920 4 -81.181 7.011 5-214.022 -26.937 11.070 3 1.51920 6-77.491-12.686.

Aspherical surface: second surface K = -0.0180904, A = -0.185660
0 × 10 to ⁶ Parameter values of conditional expressions J ₁ = 0.926, J ₂ = −0.229, J ₃ = −0.3
27 × 10 ^-1 , J ₄ = 0.362, J ₅ = 0.365
.

[0034] Example 8 f = 100,2θ = 88.798 degrees _{i R ix R iy D i j} N j 0 11.808 1 -26.568 4.428 1 1.51920 2 -30.901 11.070 3-590.406 10.332 2 1.51920 4-59.041 4.059 5 -147.601 -19.188 7.380 3 1.51920 6 -79.705-10.923.

Aspherical surface: first surface K = 0.3539556, A = 0.79852545 ×
^10-6 condition parameter values _{J 1 = 0.860, J 2 =} -0.266, J 3 = -0.1
78, J ₄ = 0.540, J ₅ = 0.374
.

FIGS. 2 to 9 show field curvature diagrams and fθ characteristic diagrams for Examples 1 to 8, respectively. In the curvature of field diagram, a solid line indicates an image forming position in the sub-scanning direction, and a broken line indicates an image forming position in the main scanning direction. In FIG. 1, these Examples 1 to 3 are used as fθ lenses.
The embodiment using the optical scanning device according to claim 7 employs each of the embodiments.

[0037]

As described above, according to the present invention, a novel fθ lens and optical scanning device can be provided. Since the fθ lens has a good correction of the curvature of field in the main and sub-scanning directions as described above, the fluctuation of the light spot diameter on the surface to be scanned is small, the fθ characteristics are good, and the lens has a tilt correction function. Therefore, the optical scanning device of the present invention using such an fθ lens can realize extremely good optical scanning.

Further, the fθ lens of the present invention can be realized at low cost because the whole system has three lenses and the number of components is small, and the cost and size of the optical scanning device can be reduced.

[Brief description of the drawings]

FIG. 1 shows a lens configuration of an fθ lens according to the present invention,
FIG. 3 is a diagram illustrating an optical arrangement of an optical scanning device using a θ lens.

FIG. 2 is a diagram illustrating field curvature and fθ characteristics according to the first embodiment.

FIG. 3 is a diagram illustrating field curvature and fθ characteristics according to a second embodiment.

FIG. 4 is a diagram illustrating field curvature and fθ characteristics according to a third embodiment.

FIG. 5 is a diagram illustrating field curvature and fθ characteristics according to a fourth embodiment.

FIG. 6 is a diagram illustrating field curvature and fθ characteristics according to a fifth embodiment.

FIG. 7 is a diagram illustrating field curvature and fθ characteristics according to a sixth embodiment.

FIG. 8 is a diagram illustrating field curvature and fθ characteristics according to a seventh embodiment.

FIG. 9 is a diagram illustrating field curvature and fθ characteristics according to an eighth embodiment.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 light source device 2 line image forming optical system 3 optical deflector 5 first group 6 second group 7 third group

Claims (7)

(57) [Claims] An fθ lens for converging a light beam deflected at an equal angular velocity by an optical deflector as a light spot on a surface to be scanned and optically scanning the surface to be scanned at a constant speed, From the optical deflector side toward the surface to be scanned, the first to third
The first group is a meniscus lens having a concave surface facing the optical deflector , and at least one surface is an aspheric surface. The second group is a surface on the surface to be scanned. Is a positive lens whose curvature is stronger than the curvature of the surface on the optical deflector side. The third group is a toric surface in which the curvature in the sub-scanning corresponding direction is stronger than the curvature in the main scanning corresponding direction. A single lens having a concave toric surface on the side and a convex toric surface on the surface to be scanned, the combined focal length of the entire lens system in the main scanning corresponding direction is f, the focal length of the first group is f ₁ , The radii of curvature of the first group on the optical axis on the optical deflector side and on the surface to be scanned are R ₁ and R, respectively.
₂ , the focal length of the third group in the sub-scanning corresponding direction is f _3y , and the radii of curvature of the third group on the optical deflector side and the surface to be scanned in the main scanning corresponding direction are R _5x and R ₅ .
When the _6x, they condition _{(1) 0.7 <R 1 /} R 2 <1.1 (2) -0.4 <R 1 /f<-0.1 (3) -0.4 <f / F ₁ <0.3 (4) -0.6 <R _6x / R _5x <1.1 (5) The third lens group 3 characterized by satisfying 0.3 <f _3y /f<0.4. An anamorphic fθ lens with a single lens configuration. 2. An anamorphic fθ lens according to claim 1, wherein the surface of the first group on the surface to be scanned is an aspherical surface. 3. An anamorphic fθ lens according to claim 1, wherein the surface of the first group on the optical deflector side is an aspheric surface. 4. The anamorphic fθ lens according to claim 1, wherein both the first group of optical deflectors and the surface to be scanned have aspheric surfaces. 5. An anamorphic fθ lens according to claim 1, wherein the second group is a biconvex lens. 6. An anamorphic fθ lens according to claim 1, wherein the second group is a positive meniscus lens having a concave surface facing the optical deflector. 7. A substantially parallel light beam emitted from a light source device is formed as a long line image in a main scanning corresponding direction by a line image forming optical system, and a deflecting reflection surface is provided near an image forming position of the line image. An optical deflector having an optical deflector having a uniform angular velocity, and a deflected light beam condensed as a light spot on a surface to be scanned by an imaging optical system to perform uniform optical scanning of the surface to be scanned. Claim 1 or 2 or 3 or 4 as an imaging optical system
Or an optical scanning device using the anamorphic fθ lens described in 5 or 6.