Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2000-06-16
2002-04-02
Phan, James (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S207110, C359S216100, C359S571000, C347S259000, C347S261000
Reexamination Certificate
active
06366386
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a scanning optical device. More particularly, it relates to a scanning optical device realized by using an optical system comprising a diffraction optical element for focussing one or more than one light beams that is deflected by a deflection element on a surface to be scanned. A scanning optical system according to the invention can suitably be used for an image forming apparatus utilizing an electrophotographic process such as a laser beam printer or a digital copying machine that is adapted to record image information by optically scanning a surface by means of one or more than one light beams.
2. Related Background Art
Optical scanners to be used for image-forming apparatus including laser beam printers (LBPs) and digital copying machines are adapted to cyclically deflect a light beam that is optically modulated according to an image signal and emitted from a light source by means of an optical deflector such as a rotary polygon mirror, converge the deflected light beam to a spot of light on the surface to be scanned of a photosensitive drum by means of an imaging optical system having an f&thgr; feature and cause the light beam to scan the surface in order to record image information thereon.
FIG. 1
of the accompanying drawings is a schematic illustration of a known scanning optical system of the type under consideration, showing only principal portions thereof. Referring to
FIG. 1
, a divergent light beam emitted from a light source
91
is substantially collimated by a collimator lens
92
, limited for its width by an aperture
93
and then made to enter a cylindrical lens
94
having a predetermined refractive power only in the sub-scanning direction. The substantially collimated light beam entering the cylindrical lens
94
leaves the latter, keeping the substantially collimated state in the main-scanning plane. It is, however, converged in the sub-scanning plane and focussed on a deflecting plane (reflecting plane)
95
a
of an optical deflector
95
, which is a rotary polygon mirror, to produce a substantially linear image extending in the main-scanning direction.
Then, the light beam deflected/reflected by the deflecting plane
95
a
of the optical deflector
95
is led to the surface (to be scanned) of a photosensitive drum
98
by way of a scanning optical system (f&thgr; lenses) having an f&thgr; feature to optically scan the surface of the photosensitive drum
98
in the direction of arrow B (main-scanning direction) in
FIG. 1
as the optical deflector
95
is driven to rotate in the sense of arrow A in FIG.
1
.
A number of scanning optical devices of the above described type have been proposed and many of them use plastic resin for the lenses of scanning optical system of the device because it is possible to accurately correct the aberration of a plastic resin lens and such a lens can be manufactured at low cost by injection molding.
However, a plastic lens shows large fluctuations in the aberration thereof (particularly in terms of off-focus and variance of magnification) when the environment changes and this problem is serious particularly when the scanning optical device is made to produce a spot of light having a very small diameter.
Recently, scanning optical devices using a diffraction optical element for the scanning optical system have been proposed to compensate the fluctuations of aberration that are specific to plastic lenses. Japanese Patent Application Laid-Open No. 10-68903 describes such an arrangement. According to the patent document, a diffraction optical element is used to generate chromatic aberration in order to compensate the change in the aberration due to a lowered refractive index of a plastic lens with the change in the aberration due to the fluctuations of the wavelength of a semiconductor laser operating as light source. Additionally, a diffraction optical element provide an advantage of showing a highly uniform thickness when formed by injection molding if it is used by itself.
While a diffraction optical element is very effective when used for the optical system of a scanning optical device, it is accompanied by a problem that the efficiency of use (as defined by the quantity of light output/quantity of light input for the designed order of diffraction=&eegr;, which is referred to as “diffraction efficiency &eegr;” hereinafter) varies depending on various conditions unlike a refraction optical element. This will be discussed below by using a diffraction grating model.
FIG. 2
is a schematic illustration of a diffraction grating model that can be used for a diffraction optical element. The diffraction optical element of
FIG. 2
comprises a continuous grating showing a pitch p (&mgr;m) and a depth h (&mgr;m). The ratio of the pitch p to the depth h of the grating is referred to as aspect ratio AR. In other words, AR=grating pitch p/grating depth h.
The light beam striking the diffraction grating model with an angle of incidence of &thgr;i is diffracted in the direction of the designed order of diffraction. However, when the grating pitch p is particularly small, the diffraction efficiency is theoretically aggravated to reduce the quantity of light for the designed order of diffraction on the surface to be scanned to make diffracted light of orders other than the designed order of diffraction (hereinafter referred to as “diffracted light of adjunctive orders of diffraction”) noticeable and consequently give rise to undesired phenomena including those of flare and ghost.
FIG. 3
is a graph showing the aspect ratio dependency of the diffraction efficiency of the diffraction grating model of
FIG. 2
when the angle of incidence &thgr;i of light striking the grating (diffraction grating) is equal to zero, or &thgr;i=0. In
FIG. 3
, the aspect ratio AR is made to vary by changing the grating pitch p while holding the grating depth h to a constant value. From
FIG. 3
it will be seen that the diffraction efficiency falls dramatically when the aspect ratio is made smaller than 4.
FIG. 4
is a graph showing the diffraction efficiency for the operational order of diffraction and those for the adjunctive orders of diffraction of the diffraction grating model of
FIG. 2
when the aspect ratio=3.4 (pitch=10.2 &mgr;m and depth=3.0 &mgr;m) and the angle of incidence of light &thgr;i relative to the grating=23°. Note that the diffraction efficiency is computed by using a technique of close-coupled wave analysis. The operational order of diffraction refers to the designed order of diffraction. Thus, a diffracted beam of light of the order is used and focussed to form a spot of light on the surface to be scanned.
Conventionally, the profile of the grating is determined only from the viewpoint of improving the diffraction efficiency of the diffraction grating for the operational order of diffraction. This will be discussed below by referring to FIG.
5
.
FIG. 5
is a graph illustrating the change in the ratio of the quantity of diffracted light of the adjunctive orders of diffraction used for exposure (relative to the quantity of diffracted light of the operational order of diffraction used for exposure) that varies as a function of the blaze angle of diffraction grating under the above condition. It will be seen from
FIG. 5
that the quantity of diffracted light of the adjunctive orders of diffraction of the negative side used for exposure increases when the blaze angle is smaller than the one that maximizes the diffraction efficiency of diffracted light of the operational order of diffraction. On the other hand, the quantity of diffracted light of the adjunctive orders of diffraction of the positive side used for exposure increases when the blaze angle is greater than the one that maximizes the diffraction efficiency of diffracted light of the operational order of diffraction. Then, the quantity of diffracted light of the operational order of diffraction is maximized at or near the blaze angle that equalizes the ab
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Phan James
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