Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-06-23
2001-03-13
Schuberg, Darren (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S196100, C359S570000
Reexamination Certificate
active
06201626
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. Particularly, the present invention relates to the achromatic scanning optical system.
The scanning optical device deflects beams emitted from a light source such as a laser diode by means of, for example, a polygonal mirror, 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 lane is referred to as an auxiliary scanning direction.
The f&thgr; lens is provided with a single lens element or 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. It the f&thgr; lens has a lateral chromatic aberration, a writing start position along the main scanning direction, from which the scanning beam spot contributes to image formation, and a writing 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 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., by selecting laser diodes such that the emission wavelengths thereof 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, the selection of the laser diodes takes much time, and difference in the emission wavelengths due to variation in used periods o0 the light sources cannot be overcome.
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, including a light source; a deflector, which deflects a beam emitted from the light source; and a scanning lens having passive refractive power for converging the beam deflected by the deflector onto a surface to be scanned. The scanning lens includes at least one refractive lens element and a diffractive lens structure which is formed on at least one surface of the refractive lens element. The diffractive lens structure compensates a lateral chromatic aberration caused by the refractive power of the scanning lens and satisfies the following condition (1);
0.3≦m/W≦4.0 (1)
where m is a number of periodic patterns of the diffractive lens structure covered by the incident light beam having the maximum scan angle W (unit: degrees).
With this construction, the chromatic aberration caused by the refractive lens element can be cancelled without increasing the number of lenses.
In particular case, to minimize the lateral chromatic aberration, the range of the condition (1) may be narrowed as the following condition (2);
0.7≦m/W≦3.0. (2)
Optionally, the light source emits a plurality of beams which are to be converged on different positions on the surface to be scanned, and the deflector deflects 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 the above-described multi-beam emitting device, and image forming operation can be performed at high accuracy.
It is preferable that the diffractive lens structure may have rings concentric about an optical axis of the diffractive lens structure.
Preferably, the diffractive lens structure may be formed on a rotationally symmetrical lens surface of the refractive lens element. in view of processing a mold, if the diffractive lens structure is formed on the rotationally symmetrical surface of the refractive lens element, it is relatively easy to process.
It is further preferable that the diffractive lens structure may be formed on a surface, which is closest to the defector, of the plurality of surfaces of the refractive lens elements.
According to another aspect of the invention, the diffractive lens structure is Formed on one surface of the refractive lens element where the width of the incident light beam having the maximum scan angle in a main scanning direction is maximized.
According to still another aspect of the invention, the diffractive lens structure is formed on the surface that is closest to the deflector in the surfaces having positive refractive power or the surface having non-positive power (i.e., no power or negative power) that is closer to the deflector than the closest surface having positive power.
According to yet anther aspect of the invention, the diffractive lens structure is formed on one surface of the refractive lens element where the incident angle of the incident light beam having the maximum scan angle in a main scanning direction is maximized.
REFERENCES:
patent: 5208701 (1993-05-01), Maeda
patent: 5294945 (1994-03-01), Omura et al.
patent: 5422753 (1995-06-01), Harris
patent: 5486694 (1996-01-01), Harris
patent: 5541760 (1996-07-01), Iizuka
patent: 5739940 (1998-04-01), Kondo
patent: 5796520 (1998-08-01), Maruyama
patent: 5838480 (1998-11-01), McIntyre et al.
patent: 5940214 (1999-08-01), Suzuki et al.
patent: 827004 (1998-03-01), None
Asahi Kogaku Kogyo Kabushiki Kaisha
Greenblum & Bernstein P.L.C.
Schuberg Darren
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