JPH05341217A - Temperature compensated scanning optical system - Google Patents

Abstract

PURPOSE:To obtain a scanning optical system which can reduce the influence caused by a temperature variation without increasing the number of component pieces of a scanning lens. CONSTITUTION:The system is constituted so that a semiconductor laser 1 and a collimating lens 10 are attached to a lens barrel 2 made of plastic, a first lens 21 made of plastic, having positive power in the main scanning direction and being anamorphic is provided in a cylindrical lens 20, a luminous flux is deflected by a deflector P provided in the vicinity of an image formed position of the luminous flux by the cylindrical lens 20, and the deflected luminous flux is subjected to image formation on the image face by an anamorphic scanning lens 30 made of plastic, and the system is set so that the linear expansion of the lens barrel 2 and the focal fluctuation of a first lens 21 are mutually canceled in the main scanning face, and linear expansion and a focal fluctuation of the scanning lens 30 negate each other in the sub-scanning face.

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 used in, for example, a laser printer, and more particularly to a scanning optical system in which focus movement due to temperature change is suppressed.

[0002]

2. Description of the Related Art Laser scanning lenses include
Plastic lenses are often used to reduce cost and weight. However, compared with glass, plastic has a greater degree of linear expansion and refractive index change with respect to temperature change, and thus drawing quality is more likely to deteriorate due to defocus.

Further, a scanning optical system such as a laser printer generally employs a configuration in which a light beam is once focused in the sub-scanning direction in the vicinity of the polygon mirror in order to reduce the influence of the surface tilt error of the polygon mirror. In many cases, the anamorphic optical system has a power in the sub-scanning direction stronger than that in the main-scanning direction. Therefore, defocus due to temperature change is also a problem mainly only in the sub-scanning direction.

In order to reduce the influence of such temperature change, a method of combining a plurality of plastic lenses in a scanning lens and arranging them so as to cancel out the focal movements generated is known.

[0005]

However, in the above-mentioned conventional method, the number of scanning lenses must be increased for temperature compensation, which is against the trend of downsizing and cost reduction.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art, and provides a scanning optical system capable of reducing the influence of temperature change without increasing the number of scanning lenses. With the goal.

[0007]

The temperature-compensated scanning optical system according to the present invention achieves the above object.
A collimator lens having a positive power that converts divergent light emitted from a light source into a parallel light flux, a cylindrical lens that temporarily forms an image of the parallel light flux only in the sub-scanning direction, and a cylindrical lens that is provided near the image formation position of the light flux. In the scanning optical system including a deflector for deflecting a light beam and a plastic anamorphic scanning lens for forming the deflected light beam on the image plane, a collimator lens is formed of a plastic whose light source is fixed at one end. An anamorphic plastic lens fixed to the other end of the lens frame and having a cylindrical lens mainly having a positive power only in the main scanning direction and an anamorphic lens having a negative power in the main scanning direction and a positive power in the sub scanning direction. It is composed of a simple glass lens, and the linear expansion and Command helical focus variation and are canceled out of a plastic lens in the lens, within the sub-scan plane, characterized in that set so that the the focus variation in the linear expansion between the scanning lens cancel each other.

[0008]

Embodiments of the present invention will be described below. Figure 1
FIG. 3 is a lens diagram of a scanning optical system according to an embodiment of the present invention, in which (a) shows a cross section in the main scanning direction and (b) shows a cross section in the sub scanning direction.

This optical system comprises a collimator lens 10 for converting a divergent light emitted from a semiconductor laser 1 as a light source into a parallel light beam, and a cylindrical lens 20 for temporarily forming an image of the parallel light beam in the sub-scanning direction in the vicinity of the deflector P. And an anamorphic scanning lens 30 made of plastic for forming the deflected light beam on the image plane.

Since FIG. 1 is an expanded view of the optical path, the deflector P is shown by a straight line, but it is well known that specifically a polygon mirror or the like is used. In addition, in order to shorten the optical path, a part of the optical path is omitted in the part shown by the broken line.

The semiconductor laser 1 is fixed to one end of a lens frame 2 formed of PMMA (polymethylmethacrylate), and the collimator lens 10 is fixed to the other end. Since the lens frame 2 expands and contracts due to temperature change, when the temperature rises, the distance between the semiconductor laser 1 and the collimator lens 10 widens, and the light flux emitted from the collimator lens becomes convergent light. On the contrary, when the temperature drops,
The interval is narrowed, and the emitted light flux becomes divergent light.

The cylindrical lens 20 has a first lens 21 which is a cylindrical plastic lens having a positive power only in the main scanning direction, and a negative power in the main scanning direction and a positive power in the sub scanning direction. The second lens 22, which is a glass lens, has a positive power in the sub-scanning direction as a whole and almost no power in the main scanning direction.

The scanning lens 30 is a single-lens fθ lens and has a strong positive power in the sub-scanning direction as compared with the main scanning direction. Since the aspherical surface processing of the plastic lens is easy, it is possible to create an fθ lens that satisfies the performance of a laser printer or the like even with a single lens structure. However, since the influence of the temperature change cannot be canceled out in the scanning lens with one lens, the lens frame 2 is formed of plastic, and a plastic lens is also provided in the cylindrical lens 20 to perform temperature compensation. .

The focus variation due to temperature change is the expansion and contraction of the lens frame 2 and the cylindrical lens 2 in the main scanning direction.
It is generated by the first lens 21 of 0, and is generated by the expansion and contraction of the lens frame 2 and the scanning lens 30 in the sub scanning direction. For this reason, the length of the lens frame and the power of the lens are set so that the influence of the temperature change generated by them is canceled out.

Here, the expansion / contraction of the plastic lens frame 2 due to temperature change is ΔL, the focal length change of the fθ lens 30 in the sub scanning direction is Δf _30Z , the focal length of the collimator lens 10 is f ₁₀ , and the sub lens of the second lens 22. The focal length in the scanning direction is f
_If the magnification of _22Z and fθ lens 30 in the sub-scanning direction is m, the amount of movement ΔP _IM of the sub-scanning image plane due to temperature change is expressed by the following equation.

[0016]

[ _Formula 1] ΔP _IM = ΔL × (f _22Z ² / f ₁₀ ² ) × m ² + (1-m) ² × Δf _30Z

Therefore, when the following equation is satisfied, f
The focus movement due to the temperature change of the θ lens is corrected.

[0018]

[ _Formula 2] ΔL × (f _22Z ² / f ₁₀ ² ) × m ² = (1-m) ² × Δf _30Z

At the same time, ΔL / f ₁₀ ² _{≈Δf 22Z} ² / f
_Need to satisfy _22Z ² .

The PMMA used as the material of the lens barrel 2 in this example has a coefficient of linear expansion of 62 × 10 ^-6 mm per unit length due to a change in unit temperature. The distance between the semiconductor laser 1 and the collimator lens 10 is 8 mm, and the focal length of the collimator lens is 8 mm.

Table 1 shows specific numerical configurations of the collimator lens 10 and the cylindrical lens 20. In the table, the surface number is the surface number counted from the object side on the left side in the figure, ry is the radius of curvature in the main scanning direction, rz is the radius of curvature in the sub-scanning direction (the surface not described is rotationally symmetric), d is the lens Thickness or air gap, n is the refractive index at a wavelength of 780 nm.

[0022]

[Table 1] Surface number ry rz d n 1 ∞ 3.00 1.76622 2 6.130 30.00 3 ∞ 3.00 1.48617 4 -17.201 ∞ 0.00 5 -18.075 ∞ 3.00 1.51072 6 ∞ -35.735

The second surface r2 of the collimator lens 10 is a rotationally symmetric aspherical surface. This aspheric surface has a height from the optical axis
The distance from the tangent plane of the aspherical vertex of the coordinate point on the aspherical surface to be Y, the curvature (1 / r) of the aspherical vertex of C, and the conic coefficient of K are expressed by the following mathematical formula 3.

[0024]

[Formula 3] X = CY ² /(1+√(1-0.36×C ² Y ² ))

[0025]

As described above, according to the present invention, a collimating lens having a relatively small diameter and a lens barrel holding the collimating lens have a temperature compensating function, so that a scanning lens having a relatively large diameter can be used. The influence of temperature change can be reduced without increasing the number of sheets, and the size and cost of the device can be reduced.

[Brief description of drawings]

FIG. 1 is a development view of an optical system showing the configuration of an embodiment,
(a) shows the main scanning direction and (b) shows the sub scanning direction.

[Explanation of symbols]

 10 ... Collimating lens 20 ... Cylindrical lens 21 ... First lens 22 ... Second lens 30 ... Scanning lens

Claims (2)

[Claims] 1. A collimator lens having a positive power for converting a divergent light emitted from a light source into a parallel light flux, a cylindrical lens for temporarily focusing the parallel light flux only in the sub-scanning direction, and an image formation of the light flux by the cylindrical lens. In a scanning optical system provided with a deflector provided near the position for deflecting a light beam, and a plastic anamorphic scanning lens for forming an image of the deflected light beam on an image plane, the collimator lens is configured such that the light source has one end. The cylindrical lens, which is fixed to the other end of the plastic lens frame fixed to, is mainly composed of an anamorphic plastic lens having a positive power only in the main scanning direction and a negative power in the main scanning direction and the sub-scanning direction. Anamorphic glass lens with positive power in the direction The linear expansion of the lens frame and the focus variation of the plastic lens in the cylindrical lens cancel each other inside, and the linear expansion and the focus variation of the scanning lens cancel each other within the sub-scanning plane. A temperature-compensated scanning optical system characterized by the above. 2. The temperature-compensated scanning optical system according to claim 1, wherein the scanning lens is composed of a single lens.