Optical: systems and elements – Deflection using a moving element
Reexamination Certificate
2002-01-28
2004-02-24
Phan, James (Department: 2872)
Optical: systems and elements
Deflection using a moving element
C359S205100, C359S212100, C359S216100
Reexamination Certificate
active
06697181
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical scanning device which is used as an optical writing device of a digital image forming device using an electrophotographic method such as a laser printer, a digital copier, a facsimile machine or other such method. The present invention further relates to an optical scanning device and an image forming apparatus including the optical scanning device which has an intensity distribution transforming optical component for transforming the intensity distribution of a light flux, and the image forming apparatus can be used in a digital outputting apparatus, for example, a digital copier, a printer, a facsimile machine or other apparatus.
2. Description of the Related Art
A conventional optical scanning device includes a coupling lens which couples a light flux emitted by a light source to form a parallel light flux, a weakly convergent light flux, or a weakly divergent light flux. An optical deflector deflects the light flux received from the coupling lens at a uniform angular velocity. A scanning image-formation optical system converges the light flux deflected by the optical deflector to form a beam spot on a surface to be scanned, i.e. a photosensitive body, and, thus, the surface to be scanned is scanned with the beam spot. Such an optical scanning device is used as an optical writing device in a digital image forming apparatus using the electrophotographic method such as a laser printer, a digital copier, a facsimile machine or other apparatus.
In such an optical scanning device, in order to achieve high-density writing (more than 1200 dpi, for example), it is necessary to form a beam spot having a sufficiently small diameter.
In order to obtain a beam spot having such a small diameter, it is necessary to increase the NA of the optical system of the optical scanning device. Further, in order to obtain a stable small-diameter beam spot, it is necessary that the optical system provides a large depth of focus which tolerates possible component allowance (the curvature radiuses, thicknesses, refractive indexes) for deviations of optical components of the optical system, mounting errors, and environment variations (temperature, humidity).
Assuming that the intensity distribution on the exit pupil of the optical system is a Gaussian distribution, the allowable degree of depth of focus 2 d is in proportion to the second power of the beam spot diameter w, as shown in the following expression:
2
d∝w
2
/&lgr; (1)
In the above expression, &lgr; represents the used wavelength. Thus, the allowable degree of depth of focus decreases sharply as the beam spot diameter is reduced. Therefore, when reduction of the beam spot diameter is attempted, the allowable degree of depth of focus decreases, and, as a result, it is not possible to obtain a stable small-diameter beam spot when the above mentioned component allowance deviations or environmental variations occur.
One solution to this problem is to generate the zero-order Bessel beam of the first kind and obtain a beam spot having a large allowable degree of depth of focus.
For example, Japanese Laid-Open Patent Application No. 9-304714 discloses an optical system providing a large allowable degree of depth of focus by arranging a shading member having a shading portion which shades a portion of a light flux on an optical path between a light source and an optical deflector.
Further, Japanese Laid-Open Patent Application No. 10-227992 discloses generation of a Bessel beam having an intensity distribution which is approximately in proportion to the second power of the zero-order Bessel function of the first kind, in a system in which a laser beam is incident on a diffraction optical component consisting of a binary optical component having an optical performance approximately equivalent to a conical prism.
However, in each of these systems, the intensity distribution of the beam is axially symmetric. Therefore, when the system is used as an optical system of an optical scanning device, it is difficult to independently set a beam spot in a main scanning direction (in which scanning is performed with a light flux) and in a sub-scanning direction (perpendicular to the main scanning direction).
In the above-mentioned system, a Bessel beam is obtained as a result of transforming the distribution of the amplitude term u
1
(y
1
, z
1
) of the following equation (2) into an arbitrary amplitude distribution. The intensity distribution thereof is expressed by the second power of the amplitude distribution. In the following equation (2), the direction of the optical axis is coincident with the x direction, the main scanning direction perpendicular to the optical axis is coincident with the y direction and the sub-scanning direction also perpendicular to the optical axis is coincident with the z direction.
u
2
(
y
2
,
z
2
)
=
j
ⅇ
-
ik
(
x
+
y
2
2
+
z
2
2
2
x
)
λ
x
∫
∫
u
1
(
y
1
,
z
1
)
ⅇ
-
i
k
2
x
(
y
1
y
2
+
z
1
z
2
)
ⅆ
y
1
ⅆ
z
1
(
2
)
The above equation (2) is expressed assuming that the intensity distribution u
2
2 (y
2
, z
2
) of the beam spot on the image surface is approximately in accordance with the Fraunhofer diffraction.
In the above equation (2):
u
2
(y
2
, z
2
): the amplitude distribution of the beam spot on the image surface;
u
1
(y
1
, z
1
): amplitude distribution on the pupil;
−ik (y
1
y
2
+z
1
z
2
)/2x: phase difference on the pupil (k represents the wave number); and
j/&lgr;: Fresnel inclination coefficient (where X represents the used wavelength).
The expression of the Fraunhofer diffraction of the above equation (2) has the same meaning as that of Fourier transform expression, and the amplitude distribution u
2
(y
2
, z
2
) on the image surface is equal to that obtained from Fourier transform being performed on the amplitude distribution u
1
(y
1
, z
1
) on the pupil. Therefore, the expression of the Fraunhofer diffraction of the above equation (2) is referred to as a Fourier transformed image.
Further, in any method, when a Bessel beam is generated, side lobes develop. Therefore, when the sensitivity of the photosensitive body is high, image degradation such as resolution degradation and/or stain in background occurs.
FIGS. 1
,
2
A and
2
B show an example of an optical scanning device according to the related art.
FIG. 1
shows an optical arrangement of the optical scanning device. In
FIGS. 2A and 2B
, the optical scanning device is shown in a condition in which the optical scanning device is expanded along an optical path of a light flux extending from a light source to a surface to be scanned.
FIG. 2A
shows the sectional view of the optical scanning device taken along a deflection plane (including the plane formed as a result of the light flux scanning the surface to be scanned), and
FIG. 2B
shows the sectional view of the optical scanning device taken along the plane including the optical path of the light flux and perpendicular to the deflection plane.
As shown in
FIGS. 1
,
2
A and
2
B, the optical scanning device
30
includes a light source
1
which emits a laser light, a first optical system
2
for directing the laser light emitted by the light source
1
to an optical deflecting portion
3
, the optical deflecting portion
3
which deflects the light flux from the first optical system
2
, and a second optical system
4
for forming a beam spot on the surface
5
to be scanned using the thus-deflected light flux. The above-mentioned first optical system
2
includes a collimating lens
21
, an aperture
22
and a cylindrical lens
23
. The second optical system
4
includes a spherical lens
41
and an f&thgr; lens
42
.
A process of optical scanning will now be described more specifically. The light flux emitted by the semiconductor laser
1
, for example, is transformed into an approximately parallel light flux by the collimating lens
21
, and passe
Greenberg & Traurig, LLP
Manak Joseph M.
Phan James
Ricoh & Company, Ltd.
Rzucidlo Eugene C.
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