Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2000-12-11
2002-04-02
Phan, James (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C347S243000, C347S235000, C359S216100
Reexamination Certificate
active
06366385
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a multi-beam scanning optical system and an image forming apparatus using the same and, more particularly, to a multi-beam scanning optical system suitably used for an image forming apparatus, e.g., a laser beam printer (LBP) or digital copying machine, which can obtain a high-resolution, high-quality image without any print position error in the main scanning direction by properly placing a slit member as a constituent element of a synchronization detection means.
2. Related Background Art
Conventionally, as a method of enabling high-speed optical scanning, a method (multi-beam scanning optical system) of simultaneously scanning a plurality of beams of light (light beams) on a scanned surface and forming a plurality of scanning lines at predetermined intervals on the scanned surface by using a multi-beam light source (multi-laser light source) as a laser light source is known. As multi-laser sources that can be used in such a multi-beam scanning optical system, for example, the following light sources are available:
(1) a light source having a plurality of emission points (light-emitting portions) on one chip;
(2) a light source in which a plurality of laser emission elements are used to combine optical paths by a beam splitter; and
(3) a light source for splitting one light beam into a plurality of light beams by using a beam splitter and independently driving modulators provided for the respective split light beams.
FIG. 5
is a schematic view showing the main part of a conventional multi-beam scanning optical system having two light emission points on one chip.
Referring to
FIG. 5
, a plurality of light beams optically modulated in accordance with image information and emitted from a multi-beam semiconductor laser
51
serving as a multi-laser source are converted into substantially parallel light beams or convergent beams by a collimator lens
52
and strike a cylindrical lens
53
. Of the light beams incident on the cylindrical lens
53
, the light beams emerge without any change in a main scanning cross-section but converge in a sub-scanning cross-section to be formed into substantially linear images (linear images elongated in the main scanning direction) on a deflecting surface (reflecting surface)
54
a
of an optical deflector
54
. The plurality of light beams reflected/deflected by the deflecting surface
54
a
of the optical deflector
54
are formed into spots on a scanned surface
56
by an imaging optical system (f-&thgr; lens system)
55
having first and second f-&thgr; lenses
55
a
and
55
b
exhibiting different powers in a sub-scanning cross-section. By rotating the optical deflector
54
in the direction indicated by an arrow A, the light beams are scanned on the scanned surface
56
in the direction indicated by an arrow B (main scanning direction) at a constant speed. Note that
FIG. 5
shows only one light beam.
In this multi-beam scanning optical system, to accurately control the write position of an image, a synchronization detection means is generally placed immediately before a position where an image signal is written.
Referring to
FIG. 5
, a slit member (BD slit)
83
is placed at a position equivalent to the photosensitive drum surface
56
. An optical sensor (BD sensor)
84
serves as a synchronization detection element. Note that each of the BD slit
83
, BD sensor
84
, and the like forms one element of a synchronization detection means
91
.
Referring to
FIG. 5
, the timing at the scanning start position of image recording on the photosensitive drum surface
56
is adjusted by using an output signal from the BD sensor
84
.
FIG. 6
is a sectional view showing the main part of the BD slit
83
in
FIG. 5
when viewed from the light beam incident side. Referring to
FIG. 6
, the BD slit
83
has first and second edge portions
83
a
and
83
b
. The first and second edge portions
83
a
and
83
b
are arranged parallel to the Z-axis in the coordinate system in FIG.
6
. First and second laser spots
11
and
12
of a plurality of light beams (BD light beams) for synchronization detection are formed on the BD slit
83
surface. When the optical deflector
54
rotates in the direction indicated by the arrow A in
FIG. 5
, the first and second laser spots
11
and
12
are respectively scanned in the directions indicated by arrows A
3
and A
4
in FIG.
6
.
As shown in
FIG. 6
, the first and second laser spots
11
and
12
are spaced apart from each other by predetermined distances in the main scanning direction (Y-axis direction) and sub-scanning direction (Z-axis direction). If the distance in the main scanning direction is represented by L′, the first and second laser spots
11
and
12
are scanned on the scanned surface
56
while always being spaced apart from each other by the distance L′ in the main scanning direction at the same time.
A scanning start point
61
(image writing start position) of a plurality of light beams A
1
for image formation on the scanned surface
56
is determined as follows.
Assume that BD detection corresponds to the timing at which a BD light beam B
3
strikes the BD sensor
84
placed above the scanned surface
56
on the upstream side in the main scanning direction. This BD detection is independently performed for each light beam, and image writing starts a predetermined time delay after the BD detection.
To more accurately detect the timing at which the BD light beam B
3
strikes the BD sensor
84
, the BD slit
83
is placed in front of the BD sensor
84
. As described above, the BD slit
83
is made up of the first and second edge portions
83
a
and
83
b
. A distance L between the first and second edge portions
83
a
and
83
b
in the main scanning direction is set to be smaller than the distance L′ between the first and second laser spots
11
and
12
in the main scanning direction. This setting prevents the first and second laser spots
11
and
12
from simultaneously striking the BD sensor
84
. By scanning the first and second laser spots
11
and
12
, therefore, first and second detection signals can be independently obtained from the BD sensor
84
. The timing of BD detection is then specified by the time when a predetermined slice level is attained at the leading edge or trailing edge of a detection signal.
Since the first and second edge portions
83
a
and
83
b
are arranged parallel to the Z-axis in the coordinate system in
FIG. 6
, the respective light beams travel the same distance from the BD detection positions to the image writing start positions with the same delay time. This makes it possible to reduce variations in image writing start positions for the respective light beams.
In this multi-beam scanning optical system, a photosensitive device (not shown) serving as a recording medium is placed on the scanned surface
56
and is exposed by laser modulation driving based on image information. The resultant image is then visualized by a known electrophotographic process. In this manner, an image forming apparatus such as a laser printer or digital copying machine can be implemented.
If the distance from the BD sensor to an image writing start position changes depending on the dimensional precision of components and the focal length of an optical component, the delay time from BD detection to an image writing start position may be adjusted by a known method, e.g., shifting at least some of the elements constituting the synchronization detection means in a direction perpendicular to the optical axis.
The conventional multi-beam scanning optical system described above has the following problems.
(1) If a return mirror is inserted in an optical path for synchronization detection to bend the optical path in a main scanning cross-section and sub-scanning cross-section so as to make the multi-beam scanning optical system compact, jitter occurs in the main scanning direction. More specifically, if the optical path in the multi-beam scanning optical system is bent, the plane formed
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Phan James
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