JPH07107582B2 - Optical scanning device - Google Patents

Description

The present invention relates to an optical scanning device used in a laser beam printer. More specifically, it relates to a scanning lens system.

[Conventional technology]

Laser beam printers, which record image information by deflecting and scanning a laser beam at high speed, have the outstanding features of high speed, high resolution, and low noise, and the demand for them rapidly increases as miniaturization and cost reduction proceed. Is coming. Therefore, as an optical writing head, which is an important component thereof, there is a great demand for downsizing and cost reduction of an optical scanning device. The optical scanning device is roughly divided into a light source and an image forming lens system of a deflector. Above all, simplification of the imaging lens system is effective for downsizing and cost reduction.

Conventionally, an optical scanning device of a laser beam printer has an imaging lens between a deflecting mirror 13 and a scanning plane S _l as shown in FIG.
Most of them are pre-obvious active scanning optical systems in which 11 is arranged. The pre-objective type scanning optical system has the advantages that it can correct field curvature aberration and that it can optically correct the scanning speed, but since it is necessary to cover a wide angle of view, the diameter of the laser beam is Despite being extremely small, the imaging lens 11 has a large aperture.

On the other hand, in the post-objective type scanning optical system in which the imaging lens 11 is arranged between the light source and the deflecting mirror 13 as shown in FIG. 5, the imaging lens 11 has a small aperture and the cost can be reduced. is there. In this case, the problem of field curvature aberration (the image plane S _G has an arc shape as shown in FIG. 5) is that the depth of focus is deep because the beam aperture angle on the image side is small. It can be solved by a trade-off between the depth of focus and the curvature of the image plane S _G. The scanning speed can be corrected by electrically correcting the lighting cycle of the light source. Further, since the light beam emitted from the laser diode which is the light source is a divergent light beam, in the pre-objective scanning optical system,
While the collimator lens 16 is required between the light source 12 and the deflecting mirror 13, in the post-objective type, the divergent light beam emitted from the light source 12 can be directly incident on the imaging lens 11, and the collimator lens can be used. Will be needed.

By the way, when such an imaging lens is used in a laser beam printer, the aperture angle on the image side, which is determined by the beam spot diameter on the image plane and the light wavelength, becomes very small, but the emission beam of the laser diode has a divergent angle. The aperture angle on the light source side, which is decided by a large number, is quite large. Therefore, an imaging lens having a large numerical aperture must be used, and it is necessary to satisfactorily correct spherical aberration. Therefore, conventionally, such a lens is composed of a plurality of lenses.

[Problems and Objectives to be Solved by the Invention]

As described above, in the pre-objective type scanning optical system, the imaging lens has a large aperture, and in the post-objective type scanning optical system, a plurality of lenses are used to form an image of the divergent light beam emitted from the laser diode. Requires an imaging lens. Since the laser optical system generally requires high surface accuracy, the optical scanning device based on the above-mentioned scanning optical system has a drawback that it must be expensive.

The present invention has been made in view of the above points, and an object thereof is to provide a small-sized, low-cost, high-performance optical scanning device for a laser printer.

[Means for solving problems]

The optical scanning device of the present invention includes a light source that emits a divergent light beam, a rotary polygonal mirror that deflects and scans the light beam, and a photoconductor that forms an image of the light beam that is deflected by the rotary polygonal mirror, and performs optical scanning. In the optical scanning device, an aspherical lens is arranged between the light source and the rotating mirror deflecting device, and an optical system having an aspherical lens at the location is f: focal length r ₁ of a single-lens aspherical lens: Radius of curvature of the apex of the first surface of the single-lens aspherical lens s ₁ : Distance between the light source and the first principal plane of the single-lens aspherical lens s ₂ : Between the image plane and the second principal plane of the single-lens aspherical lens Distance l ₁ : Distance between the deflecting mirror surface of the rotating polygon mirror of the rotating mirror deflecting device and the image plane l ₂ : When the effective scanning width is set, (i) 0.689 <r1 / f <2.377 (ii) l ₂ ² s ₁ It is characterized by satisfying the condition of ² / l ₁ s ₂ ² <1.0 × 10 ^-2 mm.

〔Example〕

Hereinafter, the scanning optical system which constitutes the optical scanning device of the present invention will be described in detail. Before starting the description of the scanning optical system, an overall image of the optical scanning device of the present invention will be described with reference to FIG. The laser beam emitted from the laser diode 2 is focused by the single-lens aspherical imaging lens 1 so that the entire surface to be scanned on the photosensitive drum 5 falls within the depth of focus range. The laser beam is deflected and scanned by the deflecting mirror 3 of the rotary polygon mirror deflector 4. At this time, the scanning speed of the laser spot on the plane to be scanned is not constant, but the light emitting timing of the laser diode 2 is corrected in accordance with the scanning speed to perform equal pitch scanning. The latent image is formed on the photosensitive drum by rotating the photosensitive drum by one pitch and repeating it for each scanning.

The optical system of the present invention has a function of forming a laser beam emitted from a laser diode with a considerably large divergence angle into a desired spot size, and a spot formed on the image plane by a laser beam deflected by a rotating polygon mirror. It can be considered by dividing it into the functions to make the spot size of the image uniform throughout the image plane.

FIG. 2 is a diagram for explaining the former function. The laser beam emitted from the emission point of the laser diode 2 at one point P _O on the optical axis is also on the optical axis by the single-lens imaging lens 1. An image is formed at one point P _I. Therefore, the single-lens imaging lens 1 only needs to take spherical aberration into consideration, and by making either the first surface S ₁ or the second surface S ₂ of the lens aspherical, almost no aberration is generated. The system can be configured. However, if the exit point is allowed to deviate from the optical axis due to assembly error, it is necessary to make both surfaces S ₁ and S ₂ aspherical in consideration of coma.

Now, with the above aspherical shape, the distance from the tangent plane S _P between the point on the surface of the incident height y and the surface vertex is x, Express. In this way, an imaging system with almost no aberration can be obtained by considering the term of y ¹⁰ . Such an aspherical shape can be obtained at low cost by using plastic molding or glass molding.

In FIG. 2, S _M represents the mirror surface of the deflecting mirror 3, and the laser beam is scanned while rotating in the direction away from the paper surface with the S _M on the paper surface as the rotation axis. FIG. 3 is a diagram for explaining the latter scanning function described above. Since the aperture angle of the laser beam after passing through the deflecting mirror 3 is very small, it is not necessary to consider spherical aberration and coma aberration, and the principal ray of the laser beam passes through the center of the imaging lens 1. Astigmatism need not be considered. Further, the distortion aberration can be corrected by the lighting period of the laser diode. Therefore, only the curvature of field should be considered. That is, as shown in FIG. 3, a circular spot having a width of 2δ, where the focal depth of the laser beam is δ, includes the entire plane S _I to be scanned, thereby obtaining a uniform spot size over the entire scanning direction. You can get it.

The optical system of the present invention satisfies the conditions (i) and (ii) described in the claims, which will be described. (I) is a condition for not deteriorating the utilization efficiency of the output of the laser diode, that is, for making the aperture diameter of the lens large. If r ₁ / f is smaller than (i), the curvature of the first surface is too strong, and if r ₁ / f is larger than (i), the curvature of the second surface is too strong to increase the lens aperture,
As a result of various studies, it was found that the utilization efficiency of the optical output cannot be 90% or more under the beam divergence angle of a normal laser diode.

(Ii) is a condition for obtaining a uniform spot size over the entire scanning direction. Spot uniformity is l ₂ ² · S
Being proportional to ₁ ² / l ₁ S ₁ ² can be derived as follows.
That is, it is known that the depth of focus δ of the laser beam is inversely proportional to the square of the beam-side opening angle θ ′ of the beam. That is, δ = A · 1 / θ ′ ² (2) Further, the condition 16δl ₁ <l ₂ ² (3) and the condition that the circular arc band of the central radius l ₁ covers the scanning width of l _{2 with} the width of 2δ The relationship between the side opening angle θ and the image side opening angle θ ′ S ₁ θ = S ₂ θ ′ (4) Becomes Experimental spot size measurements for various l ₁ , l ₂ , S ₁ , and S ₂ Within the range of, the change in spot size in the scanning direction is 10%.
It turned out to fit within.

Above, specific examples of the scanning optical system of the present invention will be shown by numerical values.

(Example 1) f = 4.207 mm S ₁ = 4.246 mm S ₂ = 450 mm l ₁ = 400 mm l ₂ = 210 mm The shape of the imaging lens is r ₁ = 3.5 mm r ₂ = −3.686 mm d = 3.5 mm The surface is an aspherical surface, and the aspherical surface coefficient expressed by the equation (1) is A = −2.274 × 10 ⁻⁵ B = −3.014 × 10 ⁻² C = 4.971 × 10 ⁻³ D = −9.717 × 10 ^{− 4} E = 9.807 × 10 ⁻⁵ . In the above numerical values, r ₁ and r ₂ are radii of curvature at the vertices of the lens surfaces S ₁ and S ₂ , and d is the distance between the first surface and the second surface measured on the optical axis. The refractive index of the lens medium is 1.511.

At this time, the conditions (i) and (ii) are r ₁ /f=0.832, l ₂ ² S ₁ ² / l ₁ S ₂ ² = 9.82 × 10 ⁻³ mm.

(Example 2) f = 4.207 mm S ₁ = 4.246 mm S ₂ = 450 mm l ₁ = 400 mm l ₂ = 210 mm r ₁ = 4.5 mm r ₂ = −3.033 mm d = 3.5 mm A = −1.535 × 10 ⁻⁵ B ^{= -3.788 × 10 -2 C = 6.432} × 10 -3 D = -1.652 × 10 -3 E = 1.852 × 10 -4 r 1 /f=1.07 l 2 2 S 1 2/1 1 S 2 2 = 9.82 × 10 ⁻³ mm FIG. 6 is a graph showing the spot size on the plane to be scanned in Examples 1 and 2. Spot diameter is 120 μm
It is ± 10%.

7 and 8 show spherical aberration (wavefront aberration) of the imaging lenses of Examples 1 and 2, respectively. It can be said that the lens has almost no aberration.

〔The invention's effect〕

As described above, according to the present invention, the imaging lens for focusing the light source emitted from the laser diode on the image plane to a desired spot size is arranged between the rotary polygon mirror deflector and the image plane. Since it is a single-lens aspherical lens that is designed to meet the specified conditions, a uniform spot diameter with little aberration can be obtained with only a small-aperture single-lens lens. It is possible to provide a simple optical scanning device. If a single-lens aspherical lens is installed between the rotary polygon mirror deflector and the photoconductor, the optical system will not be completely different from the optical system of the present invention, and the entire scanning width will be covered. The large size of the optical scanning device is required. Therefore, the size of the entire optical scanning device becomes large, and it is difficult to obtain a uniform spot diameter with little aberration. It is extremely difficult to produce an effect.

[Brief description of drawings]

FIG. 1 is a perspective view showing an overall image of the optical scanning device of the present invention, FIGS. 2 and 3 are principle diagrams for explaining a scanning optical system in the optical scanning device of the present invention, FIGS. FIG. 7 is a diagram showing a typical example of a conventional scanning optical system, FIG. 6 is a diagram showing spot size changes in an embodiment of the optical scanning device of the present invention, and FIG.
FIG. 8 and FIG. 8 are diagrams showing spherical aberration in the first and second examples of the present invention, respectively. 1 ... Imaging lens, 2 ... Laser diode, 3 Deflection mirror, 5 ... Photosensitive drum

Claims (1)

[Claims] 1. An optical scanning device for performing optical scanning, comprising a light source for emitting a divergent light beam, a rotary polygonal mirror for deflecting and scanning the light beam, and a photoconductor for forming an image of the light beam deflected by the rotary polygonal mirror. In, an optical system in which an aspherical lens is arranged between the light source and the rotating mirror deflecting device and an aspherical lens is provided at the position is f: focal length of single-lens aspherical lens r ₁ : single-lens non-lens Radius of curvature of the apex of the first surface of the spherical lens s ₁ : Distance between the light source and the first principal plane of the single-lens aspherical lens s ₂ : Distance between the image plane and the second principal plane of the single-lens aspherical lens l ₁ : Distance between the deflecting mirror surface of the rotating polygon mirror of the rotating mirror deflecting device and the image plane l ₂ : When the effective scanning width is set, (i) 0.689 <r1 / f <2.377 (ii) l ₂ ² s ₁ ² / l An optical scanning device characterized by satisfying the condition of ₁ s ₂ ² <1.0 × 10 ^-2 mm.