JP3341121B2 - Scanning optical system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning optical system for scanning a laser beam or the like, and more particularly to a scanning image forming optical system after a deflector.

[0002]

2. Description of the Related Art In a laser beam scanning optical system, a cylindrical lens or a toroidal surface is provided as an optical system for correcting a tilt angle of a light deflecting surface such as a polygon between an image forming lens composed of a rotation target surface and an image surface. Is widely used, in which a lens having the following configuration is provided, and a light deflecting surface and an image surface are geometrically conjugated in a plane in the sub-scanning direction. However, if the deflection angle is widened to reduce the size of the scanning device and the optical system is widened accordingly, insufficient correction of the curvature of field becomes a problem. On the contrary,
In Japanese Patent Application Laid-Open No. 61-112112, a deformed cylindrical surface is introduced into a scanning image forming optical system so that sagittal curvature of field can be well corrected, and a deflection angle w can be widened to sin (w / 2) = 0.7. Has been realized.

[0003] Here, the deformed cylindrical surface is shown in FIG.
Is a rotation curved surface of the curve 2 about the axis 1. The intersection of the optical axis and the curve 2 is the origin, the optical axis is the x axis, and the curve 2 on the optical axis
The tangent to is the y-axis, R ₀ is the radius of curvature in the sub-scanning direction plane on the optical axis, and the curve 2 is expressed as a function X = f (Y). In the sagittal direction at a position separated by Y from the optical axis, Radius of curvature R
s is represented by Rs = _R0- f (Y). An optical element having this deformed cylindrical surface can be easily manufactured at low cost as described in the above-mentioned publication.

[0004]

However, in the scanning lens disclosed in the above publication, when Σd is the total length of the lens and f is the focal length of the scanning image forming optical system, Σd /
f is about 0.9, which accounts for a large proportion of the lens length, and it is becoming inadequate at present every year that compactness and low cost are required. If the lens length of the scanning image forming optical system is shortened, the diameter of the lens constituting the scanning image forming optical system can of course be reduced, the cost for manufacturing the lens can be reduced, and the housing for fixing the lens is also provided. The size is small, and the entire scanning and imaging optical unit is compact, and it is possible to meet the demand for miniaturization and cost reduction. The present invention provides a deflection angle w
Is a wide angle sin (w / 2) = 0.6 to 0.7, and Δd /
While trying to obtain a compact scanning imaging optical system with f of about 0.35 to 0.55 ,
That the sagittal image plane can be corrected well
In addition, a scanning optical system that can reduce the inclination of the image plane
That is what we are trying to get .

[0005]

In Means for Solving the Problems] The present invention, scanning image forming optical system is constituted by two lenses, the sub-scan
The first that the positive power in the directional plane becomes weaker as it goes off-axis
The negative cylindrical surface in the sub-scanning direction and the deformed cylindrical surface
The second deformed cylindrical decays become weaker as the work goes off-axis
The above-mentioned object is achieved by providing a lubrication surface . That is, a semiconductor laser, a collimator lens for converting divergent light from the semiconductor laser into parallel light, an imaging optical system that focuses the parallel light only in the sub-scanning direction, and forms a linear image, a deflector having a linear imaging position near the deflecting and reflecting surface, run of the optical scanning apparatus comprising a scanning image forming optical system for forming the light beam deflected by the deflector on the photoreceptor
In the scanning optical system, the scanning image forming optical system is composed of two lenses , and the positive power in the sub-scanning direction plane is off-axis.
The first deformed cylindrical surface, which becomes weaker as
The negative power in the scanning direction plane becomes weaker as it goes off-axis
And a second deformed cylindrical surface .

[0006]

The scanning image forming optical system according to the present invention is composed of two lenses.
This is because using more than one lens is disadvantageous for cost reduction. From the deflector, light rays entering the scanning imaging optical system obliquely enter the lens surface as they move away from the axis,
Generally, the sagittal image plane is under-corrected.
As described in Japanese Patent Application Laid-Open No. 1-1120112, when a deformed cylindrical surface is used, in the main scanning direction, the power of the refraction surface can be reduced toward the off-axis. Works more effectively than a cylindrical surface or a toroidal surface.

Although the deformed cylindrical surface is effective as described above, while properly correcting the image surface in the main scanning direction,
For the purpose of compactness, as the lens constituting the scanning imaging optical system is moved closer to the deflector side,
The curvature of the deformed cylindrical surface in the optical system in the main scanning plane generally tends to be large, so that the curvature in the sub-scanning direction when the height of a ray passing through the deformed cylindrical surface in the main scanning plane is displaced up and down. The amount of change also increases.
This poses a problem in correcting the inclination of the image plane due to the entrance and exit of the deflecting surface. In other words, since the deflecting surface is a plane, the distance from the rotation axis to the deflecting surface of the deflector such as a polygon becomes larger from the center to the peripheral portion, and the deflecting surface position with respect to a light beam incident at a certain position is a polygon or the like. Will change with the rotation of. Rays having the same image height in both directions (positive and negative) on the scanning plane about the optical axis are
Due to the entrance and exit of the surface due to the rotation of the light deflection surface,
They do not necessarily follow the same path and are displaced in either direction, resulting in a difference in the height of light rays passing through the deformed cylindrical surface. The image passes through a portion having a large curvature or a small curvature, and the image plane is inclined with respect to the scanning plane. Such an inclination of the image plane naturally increases if the amount of change in the curvature of the deformed cylindrical surface in the sub-scanning direction is large.

Therefore, if cylindrical surfaces acting in opposite directions in the sub-scanning surface are provided in the scanning image forming optical system,
The above problems can be solved by offsetting each other. For example,
Of the two light beams having the same positive and negative image heights, the one closer to the height from the optical axis when passing through the deformed cylindrical surface that acts as a convex has a smaller area in the sub-scanning plane than the one farther from the optical axis. Is large, so that the light is focused on the front side of the scanning surface, that is, on the side of the scanning image forming optical system, while when passing through the deformed cylindrical surface acting as a concave, Since the light tends to condense, the inclination of the image plane can be reduced by canceling the light.

Furthermore, if at least one of the deformed cylindrical surfaces has an aspherical shape in the main scanning direction (the meridional direction), it is effective for correcting the fθ characteristic and the field curvature in the main scanning direction. Here, the aspherical surface shape has an origin at the vertex of the surface, an x-axis in the optical axis direction, a radius of curvature r, a conic constant κ, an aspherical coefficient Ai, and an exponent of the aspherical surface Pi. Is represented by Equation 1.

(Equation 1) h ² = y ² + z ²

[0010]

EXAMPLES Examples and comparative examples of the scanning optical system according to the present invention will be described below. In the table, f is the focal length of the scanning imaging optical system in the main scanning direction plane w is the scanning angle R in the main scanning direction plane R is the radius of the inscribed circle of the rotating polygon mirror d ₀ is the entrance pupil and the The distance r from one surface is the radius of curvature r 'in the plane in the main scanning direction, the radius of curvature d in the plane in the sub-scanning direction on the optical axis, d is the on-axis thickness n of the lens, and the thickness of the lens material with respect to light having a wavelength of 780 nm. Refractive index * indicates a deformed cylindrical surface. The comparative example is a case where only one modified cylindrical surface is used.

Example 1 f = 184.0 w / 2 = 38 ° R = 20.0 d ₀ = 35.6 Surface Number rr′dn 1 −200.000 10.0 1.84595 * 2 −58 .410 36.800 56.5 * 3 -260.000 25.350 4.0 1.48595 4 -479.580 122.0 Aspherical surface coefficient Second surface K = -7.38230 × 10 ^-2 A1 = 6.1325 × 10 ⁻⁷ P1 = 4 A2 = 7.665730 × 10 ⁻¹² P2 = 6 A3 = −2.445140 × 10 ⁻¹⁵ P3 = 8 A4 = 4.99310 × 10 ⁻²¹ P4 = 10 Four sides K = −8.999690 × 10 A1 = −4.1950 × 10 ⁻⁷ P1 = 4 A2 = 4.15420 × 10 ⁻¹¹ P2 = 6 A3 = −4.632230 × 10 ⁻¹⁵ P3 = 8 A4 = 2.65130 × 10 ^-19 P4 = 10 Σd / f = 0.383

Example 2 f = 150.0 w / 2 = 46 ° R = 20.0 d ₀ = 29.0 Surface number rr′dn 1 −116.038 12.0 1.48595 * 2 −43 .332 100.000 64.0 * 3 -300.000 21.750 4.0 1.48595 4 -1572.632 81.4 Aspheric coefficient second surface K = −3.36845 × 10 ⁻¹ A1 = 2.53255 × 10 ⁻⁷ P1 = 4 A2 = −1.47911 × 10 ⁻¹⁰ P2 = 6 A3 = −9.48930 × 10 ⁻¹⁵ P3 = 8 A4 = 2.454508 × 10 ⁻²⁰ P4 = 10 Fourth surface K = −1.65111 × 10 A1 = −4.000891 × 10 ⁻⁷ P1 = 4 A2 = 4.2360 × 10 ⁻¹¹ P2 = 6 A3 = −4.474437 × 10 ⁻¹⁵ P3 = 8 A4 = 1.85986 × 10 ^-19 P4 = 10 Σd / f = 0.533

Comparative Example f = 184.0 w / 2 = 38 ° R = 20.0 d ₀ = 35.6 Surface Number rr′dn 1 −200.000 10.0 1.84595 2 −58.352 56.5 * 3 -230.000 31.370 4.0 1.48595 4 -389.342 122.0 Aspherical surface second surface K = -0.11301 A1 = 0.58164 x ^10-6 P1 = 4 A2 = 0.80584 × 10 ⁻¹¹ P2 = 6 A3 = −0.19920 × 10 ⁻¹⁴ P3 = 8 A4 = 0.56919 × 10 ⁻²⁰ P4 = 10 Fourth surface K = −0.49818 × 10 ² A1 = −0.41406 × 10 ⁻⁶ P1 = 4 A2 = 0.38304 × 10 ⁻¹⁰ P2 = 6 A3 = −0.42823 × 10 ⁻¹⁴ P3 = 8 A4 = 0.23978 × 10 ⁻¹⁸ P4 = 10 Σd / f = 0.383

[0014]

According to the scanning optical system of the present invention, as shown in the embodiment and the aberration diagram showing the curvature of field, the deflection angle w is sin
(W / 2) = 0.6-0.7 with a wide angle and Σd / f of 0.
Although the scanning optical system is a compact scanning optical system of about 35 to 0.55, it is possible to realize a scanning optical system in which the image plane is corrected, in particular, the sagittal image plane is satisfactorily corrected. That is, as shown in FIG. 5, the sagittal image plane is not completely corrected in the comparative example, whereas in the first embodiment, the deformed cylindrical surface is used for the other lens, as shown in FIG. In addition, the inclination of the image plane can be reduced, and the sagittal image plane is favorably corrected.

[Brief description of the drawings]

FIG. 1A is a sectional view in a main scanning direction of a first embodiment of a scanning image forming optical system according to the present invention, and FIG.

FIG. 2A is a cross-sectional view in the main scanning direction, and FIG. 2B is a cross-sectional view in the sub-scanning direction, of a comparative example of the scanning image forming optical system.

FIG. 3 is an aberration diagram of the first embodiment.

FIG. 4 is an aberration diagram of the second embodiment.

FIG. 5 is an aberration diagram of a comparative example.

FIG. 6 is an explanatory diagram of a deformed cylindrical surface.

Claims (2)

(57) [Claims] 1. A semiconductor laser, a collimator lens for converting divergent light from the semiconductor laser into parallel light, and an imaging optical system for condensing the parallel light only in the sub-scanning direction and forming an image in a linear shape. When, a deflector having the linear imaging position deflecting reflection surface near the scan of the optical scanning device comprising a scanning image forming optical system for forming the light beam deflected by the deflector on the photoreceptor In the optical system , the scanning image forming optical system is composed of two lenses , and is positive in the sub-scanning direction plane.
First modified cylinder whose power becomes weaker as it goes off axis
The negative power in the vertical plane and the vertical plane in the sub-scanning direction
And a second deformed cylindrical surface that becomes weaker . 2. The method according to claim 1, wherein one of the two lenses is
Providing the first deformed cylindrical surface and the other lens
Characterized in that the second deformed cylindrical surface is provided on
The scanning optical system according to claim 1, wherein: