Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2000-01-18
2001-04-24
Schuberg, Darren (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S570000, C359S571000
Reexamination Certificate
active
06222661
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a scanning optical system that is used as an optical system for a scanning optical device such as a laser beam printer.
The scanning optical device deflects a beam emitted from a light source such as a semiconductor laser 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 (i.e., a 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 such a scanning optical system, since an optical path length and an incident angle for each of the lens surfaces are different according to a scanning angle, transmitting light quantity, i.e., the quantity of light transmitted through the f&thgr; lens varies as the laser beam is deflected. In general, the quantity of light transmitted through the optical elements between the polygonal mirror nd the photoconductive drum along the optical axis is larger than that through the peripheral portion of the elements. The variation of the quantity of the transmitted light due to change of the scanning angle is referred to as a power variation.
If the power variation exceeds the tolerance level, an exposure on the photoconductive dram varies widely, which reduces printing quality.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide an improved scanning optical system that is capable of reducing the power variation due to the change of the transmitted light quantity.
For the above object, according to the invention, there is provided a scanning optical system, including a light source; a deflector, which deflects a beam emitted from the light source; a scanning lens having positive refractive power for converging the beam deflected by the deflector onto a surface to be scanned; and a diffractive surface, which is located between the deflector and the surface to be scanned. The diffractive surface is designed for compensating chromatic aberration caused by the refractive power of the scanning lens, and the diffraction efficiency of the diffractive surface on an optical axis of the scanning lens is different from that of a peripheral portion to cancel a power variation due to a variation of quantity of light transmitted through the scanning lens.
The diffractive surface for compensating the chromatic aberration has been employed, particularly, in a multi-beam scanning optical system to reduce the variation of the scanning lines due to the variation of the wavelengths of the light sources. The diffractive surface is, in general, designed such that the maximum and constant diffraction efficiency is obtained in the entire area of the scanning range. On the contrary, the diffractive surface of the present invention varies the diffractive efficiency according to the scanning angle to cancel the power variation. That is, the diffraction efficiency for the axial ray is different from that for the marginal ray.
The diffractive surface may be formed on a plane parallel plate that is independent from the scanning lens, or may be formed on one surface of the scanning lens. When the diffractive surface is formed on the independent plate, it is preferable that the plate is located between the deflector and the scanning lens to minimize a size of the plate.
When the product of the ratio T of the transmitted light quantities and the ratio N of the diffractive efficiencies is the order of 1.0, the light quantity on the surface to be scanned is substantially constant in the entire area of the scanning range. The ratio T is defined as T=Ty
1
/Ty
0
, where Ty
0
is transmitting light quantity through the optical elements between the deflector and the surface to be scanned along the optical axis, and Ty
1
is transmitting light quantity through the peripheral portion of the optical elements. The ratio N is defined as N=&eegr;
1
/&eegr;
0
, where &eegr;
0
is a diffractive efficiency of the diffractive surface on the optical axis, and &eegr;
1
is a diffractive efficiency of the diffractive surface on the peripheral portion.
Preferably, the product of the ratios satisfies the following condition (1);
0.8
<N×T<
1.1. (1)
When the transmitted light quantity along the optical axis Ty
0
is larger than that of the peripheral portion Ty
1
, the diffraction efficiency of the peripheral portion &eegr;
1
may be larger than that on the optical axis &eegr;
0
.
The diffractive efficiency can be varied by shifting a blazed wavelength with respect to an actual wavelength in use. The larger the difference of the blazed wavelength from the actual wavelength is, the smaller the diffractive efficiency becomes. When the transmitted light quantity along the optical axis Ty
0
is larger than that of the peripheral portion Ty
1
, the blazed wavelength &lgr;
1
of the diffractive surface on the optical axis may be different from the actual wavelength &lgr;
0
in use. When &lgr;
1
is smaller than &lgr;
0
, the ratio &Lgr; of the wavelengths (&Lgr;=&lgr;
1
/&lgr;
0
) and the ratio T of the transmitted light quantities (T=Ty
1
/Ty
0
) may satisfy the condition (2). On the other hand, when &lgr;
1
is larger than &lgr;
0
, the ratio &Lgr; and the ratio T may satisfy the condition (3).
0.8
<&Lgr;/T<
1.2 (2)
0.8
<&Lgr;×T<
1.2 (3)
According to another aspect of the invention, there is provided a scanning optical system, including a light source; a deflector, which deflects a beam emitted from the light source and is incident from outside of an effective scanning range; a scanning lens having positive refractive power for converging the beam deflected by the deflector onto a surface to be scanned; and a diffractive surface whose diffraction efficiency varies along a main scanning direction, and the variation of the diffractive efficiency being asymmetrical with respect to an optical axis of the scanning lens. The diffractive surface is located between the deflector and the surface to be scanned, for compensating chromatic aberration caused by the refractive power of the scanning lens.
When the light beam from the light source is incident on the deflector from outside of the effective scanning range, the power variation becomes asymmetrical with respect to the optical axis. Such an asymmetrical power variation can be compensated by the asymmetrical variation of the diffractive efficiency as described above.
In this case, when the light beam from the light source is incident on the deflector as S-polarized light, the diffraction efficiency of the diffractive surface at the same side of the incident beam on the deflector with respect to the optical axis may be higher than that at the other side. On the contrary, when the incident light on the deflector is P-polarized light, the diffraction efficiency of the diffractive surface at the same side of the incident beam on the deflector with respect to the optical axis may be lower than that at the other side.
It is preferable that the light beam travels to the surface to be scanned is a first order diffractive light diffracted by the diffractive surface. Still further, the light source may be a multi-beam light source for emitting a plurality of light beams, and the plurality of light beams forming a plurality of scanning lines per scan.
REFERENCES:
patent: 5422753 (1995-06-01), Harris
patent: 6067106 (2000-05-01), Ishibe et al.
patent: 6094286 (2000-07-01), Kato
Kamikubo Junji
Takeuchi Shuichi
Asahi Kogaku Kogyo Kabushiki Kaisha
Greenblum & Bernstein P.L.C.
Schuberg Darren
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