Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2000-04-20
2001-07-10
Phan, James (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S207110, C359S019000
Reexamination Certificate
active
06259547
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a scanning optical system which is used as an optical system for a scanning optical device such as a laser beam printer.
The scanning optical device deflects beams by means of, for example, a polygonal mirror emitted from a light source such as a laser diode, and converges the beam to form a spot on a surface to be scanned such as a surface of a photoconductive drum, through an f&thgr; lens (scanning lens). The beam spot formed on the surface to be scanned moves (i.e., scans) on the surface in a predetermined scanning direction as the polygonal mirror rotates. In this specification, the direction in which the beam spot scans is referred to as a main scanning direction. Further, a plane including the scanning beam scanning in the main scanning direction is referred to as a main scanning plane. Furthermore, a direction perpendicular to the main scanning plane is referred to as an auxiliary scanning direction.
The f&thgr; lens consists of a single lens element or a plurality of lens elements, and various types of aberrations are corrected such that the spot on the surface to be scanned scans linearly in the main scanning direction, at a constant speed on the surface to be scanned as the polygon mirror rotates at a constant rotation speed.
Recently, in order to increase an image forming speed, there has been developed a multi-beam scanning device which is provided with a plurality of light sources, such as laser diodes. The plurality of laser diodes emit a plurality of beams to form a plurality of scanning lines simultaneously. In a scanning optical system employed in such a multi-beam scanning device, positional relationship between the scanning lines formed by the plurality of scanning beams should be adjusted accurately such that a plurality of scanning lines are apart from each other by a predetermined distance.
In such a multi-beam scanning device, generally, wavelengths of the beams emitted by the plurality of laser diodes distribute, for example, within a range of (a standard designed value ±15) nm. Therefore the wavelengths of two laser diodes forming the adjacent scanning lines may be different by 30 nm at the maximum. If the f&thgr; lens has a lateral chromatic aberration, a write start position along the main scanning direction, from which the scanning beam spot contributes to image formation, and a write complete position, which is the end of the image portion on the scanning line, may differ between a plurality of lines, which may exceed an allowable range, and affects the quality of formed image.
Conventionally, the chromatic aberration of the f&thgr; lens is compensated by combining a positive lens and a negative lens having different dispersion. Alternatively, the effect of the chromatic aberration due to variation of wavelengths emitted by the respective laser diodes may be reduced by selecting laser diodes which emit laser beams having closer wavelengths, i.e., the wavelengths emitted by the laser diodes distribute within a smaller range.
In order to correct the chromatic aberration of the f&thgr; lens by selecting a lens materials (glass materials) having different dispersion as in the prior art described above, the number of lens elements of the f&thgr; lens increases when compared with a case where the chromatic aberration is not corrected. In addition, in order to compensate for the chromatic aberration, lens materials cannot be selected only by their refractive indexes, and types of available lens materials are limited, thereby degree of freedom in designing the lens is lowered. On the other hand, when light sources are used after selected by differences of their emission wavelengths, the selection operation itself takes much time, and measures cannot be taken when there arises difference in the emission wavelength due to variation in used periods of the light sources.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide an improved scanning optical system whose lateral chromatic aberration is compensated so that deterioration in image forming characteristics due to uneven wavelengths emitted by a plurality of laser diodes is avoidable without increasing the number of lens elements. For the above object, according to the invention, there is provided an improved scanning optical system, comprising: a light source; a deflector, which deflects a beam emitted from the light source; and a scanning lens for converging the beam deflected by the deflector onto a surface to be scanned, wherein the scanning lens has a positive power, the scanning lens consisting of a plurality of refraction lens elements, a diffraction lens structure being formed on at least one surface of one of the plurality of lens elements, the diffraction lens structure compensating a lateral chromatic aberration caused by the refraction lens elements.
As above, according to the present invention, the chromatic aberration caused by the refraction lens elements can be cancelled without increasing the number of lenses.
Optionally, the light source emits a plurality of beams which are to be converged on different positions on the surface to be scanned, the deflector deflect all of the plurality of beams. When a plurality of laser diodes are used to emit the plurality of beams simultaneously, wavelengths of the beams may be different from each other. Even in such a case, since the chromatic aberration is compensated, the optical system can be used for above-described multi-beam emitting device, and image forming operation can be performed at high accuracy.
It is preferable, that the diffraction lens structure may have circular zones concentric about an optical axis of the diffraction lens structure.
Preferably, the diffraction lens structure may be formed on a rotationally symmetrical lens surface of the scanning lens. In view of processing a mold, if the diffraction lens structure is formed on the rotation-symmetrical surface of the refraction lens, it is relatively easy to process.
It is further preferable that the diffraction lens structure may be formed on a surface of the plurality of refraction lens elements closest to the deflector.
According to another aspect of the invention, there is provided a scanning lens having a positive power, the scanning lens consisting of a plurality of refraction lens elements, a diffraction lens structure being formed on at least one surface of the plurality of refraction lens elements, the diffraction lens structure compensating a lateral chromatic aberration caused by the plurality of refraction lens elements.
REFERENCES:
patent: 5208701 (1993-05-01), Maeda
patent: 5646767 (1997-07-01), Iima et al.
patent: 5748354 (1998-05-01), Iizuka
patent: 5838480 (1998-11-01), McIntyre et al.
patent: 6124962 (2000-09-01), Kamikubo
Diffractive Optics at Eastman Kodak Company, SPIE, vol. 2689, pp. 228-254.
Asahi Kogaku Kogyo Kabushiki Kaisha
Greenblum & Bernstein P.L.C.
Phan James
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