U.S. patent application number 09/733065 was filed with the patent office on 2001-06-21 for multi-beam scanning optical system and image forming apparatus using the same.
Invention is credited to Kimura, Kazumi.
Application Number | 20010004292 09/733065 |
Document ID | / |
Family ID | 18435165 |
Filed Date | 2001-06-21 |
United States Patent
Application |
20010004292 |
Kind Code |
A1 |
Kimura, Kazumi |
June 21, 2001 |
Multi-beam scanning optical system and image forming apparatus
using the same
Abstract
A multi-beam scanning optical system includes a light source
having a plurality of light-emission points, an incident optical
system for guiding a plurality of light beams emitted from the
light source to a deflector, a scanning optical system for forming
the plurality of light beams reflected/deflected by the deflector
into images on a scanned surface, and a synchronization detection
device in which a part of the plurality of light beams from the
deflector are reflected at a predetermined angle in a sub-scanning
cross-section by using a mirror to be scanned on a surface of a
slit member and to be guided to a surface of a synchronization
detection element via the slit member, and a timing at a scanning
start position on the scanned surface is controlled by using a
signal from the synchronization detection element. The slit member
is positioned such that a plurality of light beams scanned on a
surface of a slit opening are substantially vertically scanned on
the slit opening.
Inventors: |
Kimura, Kazumi; (Toda-shi,
JP) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
30 ROCKEFELLER PLAZA
NEW YORK
NY
10112
US
|
Family ID: |
18435165 |
Appl. No.: |
09/733065 |
Filed: |
December 11, 2000 |
Current U.S.
Class: |
359/204.1 ;
359/235 |
Current CPC
Class: |
G02B 26/123
20130101 |
Class at
Publication: |
359/204 ;
359/235 |
International
Class: |
G02B 026/08; G02B
026/02 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 14, 1999 |
JP |
11-354080 |
Claims
What is claimed is:
1. A multi-beam scanning optical system comprising: light source
means having a plurality of light-emission points; incident optical
means for guiding a plurality of light beams emitted from said
light source means to deflection means; scanning optical means for
forming the plurality of light beams reflected/deflected by the
deflection means into images on a scanned surface; and
synchronization detection means in which a part of the plurality of
light beams from the deflection means are reflected at a
predetermined angle in a sub-scanning cross-section by reflection
means to be scanned on a surface of a slit member and to be guided
to a surface of a synchronization detection element via the slit
member, and a timing at a scanning start position on the scanned
surface is controlled by using a signal from the synchronization
detection element, wherein the slit member is positioned such that
a plurality of light beams scanned on a surface of a slit opening
are substantially vertically scanned on the slit opening.
2. A system according to claim 1, wherein said synchronization
detection means controls a timing at a scanning start position on
the scanned surface in a cycle of the plurality of light beams
emitted from said light source means.
3. A system according to claim 1, wherein a longitudinal direction
of the slit opening of the slit member is nonparallel to a
rotational axis direction of the deflection means.
4. A system according to claim 1, wherein the plurality of
light-emission points are spaced apart from each other in at least
the main scanning direction.
5. An image forming apparatus comprising: the multi-beam scanning
optical system defined in claim 1; a photosensitive member disposed
on the scanned surface; a developing unit for developing an
electrostatic latent image formed on said photosensitive member by
a light beam scanned by said multi-beam scanning optical system
into a toner image; a transfer unit for transferring the developed
toner image onto a transfer medium; and a fixing unit for fixing
the transferred toner image on the transfer medium.
6. An image forming apparatus comprising: the multi-beam scanning
optical system defined in claim 1; and a printer controller for
converting code data input from an external device into an image
signal and inputting the signal to said multi-beam scanning optical
system.
7. A multi-beam scanning optical system comprising: light source
means having a plurality of light-emission points; incident optical
means for guiding a plurality of light beams emitted from said
light source means to deflection means; scanning optical means for
forming the plurality of light beams reflected/deflected by the
deflection means into images on a scanned surface; and
synchronization detection means in which a part of the plurality of
light beams from the deflection means are reflected at a
predetermined angle in a sub-scanning cross-section by reflection
means to be scanned on a surface of a slit member and to be guided
to a surface of a synchronization detection element via the slit
member, and a timing at a scanning start position on the scanned
surface is controlled by using a signal from the synchronization
detection element, wherein the slit member is positioned such that
a plurality of light beams scanned on a surface of a slit opening
vertically cross one of edge portions of the slit opening.
8. A system according to claim 7, wherein said synchronization
detection means controls a timing at a scanning start position on
the scanned surface in a cycle of the plurality of light beams
emitted from said light source means.
9. A system according to claim 7, wherein a longitudinal direction
of the slit opening of the slit member is nonparallel to a
rotational axis direction of the deflection means.
10. A system according to claim 7, wherein the plurality of
light-emission points are spaced apart from each other in at least
the main scanning direction.
11. An image forming apparatus comprising: the multi-beam scanning
optical system defined in claim 7; a photosensitive member disposed
on the scanned surface; a developing unit for developing an
electrostatic latent image formed on said photosensitive member by
a light beam scanned by said multi-beam scanning optical system
into a toner image; a transfer unit for transferring the developed
toner image onto a transfer medium; and a fixing unit for fixing
the transferred toner image on the transfer medium.
12. An image forming apparatus comprising: the multi-beam scanning
optical system defined in claim 7; and a printer controller for
converting code data input from an external device into an image
signal and inputting the signal to said multi-beam scanning optical
system.
13. A multi-beam scanning optical system comprising: light source
means having a plurality of light-emission points; incident optical
means for guiding a plurality of light beams emitted from said
light source means to deflection means; scanning optical means for
forming the plurality of light beams reflected/deflected by the
deflection means into images on a scanned surface; and
synchronization detection means in which a part of the plurality of
light beams from the deflection means are reflected at a
predetermined angle in a sub-scanning cross-section by reflection
means to be scanned on a surface of a slit member and to be guided
to a surface of a synchronization detection element via the slit
member, and a timing at a scanning start position on the scanned
surface is controlled by using a signal from the synchronization
detection element, wherein a longitudinal direction of the slit
opening of the slit member is nonparallel to a rotational axis
direction of the deflection means, and the slit member is
positioned such that a plurality of light beams scanned on a
surface of a slit opening are substantially vertically scanned in
the longitudinal direction of the slit opening.
14. A system according to claim 13, wherein said synchronization
detection means controls a timing at a scanning start position on
the scanned surface in a cycle of the plurality of light beams
emitted from said light source means.
15. A system according to claim 13, wherein the slit member is
positioned to be vertical or substantially vertical to a plane
formed by a plurality of light beams scanned on the surface of the
slit member.
16. A system according to claim 13, wherein the plurality of
light-emission points are spaced apart from each other in at least
the main scanning direction.
17. An image forming apparatus comprising: the multi-beam scanning
optical system defined in claim 13; a photosensitive member
disposed on the scanned surface; a developing unit for developing
an electrostatic latent image formed on said photosensitive member
by a light beam scanned by said multi-beam scanning optical system
into a toner image; a transfer unit for transferring the developed
toner image onto a transfer medium; and a fixing unit for fixing
the transferred toner image on the transfer medium.
18. An image forming apparatus comprising: the multi-beam scanning
optical system defined in claim 13; and a printer controller for
converting code data input from an external device into an image
signal and inputting the signal to said multi-beam scanning optical
system.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] 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.
[0003] 2. Related Background Art
[0004] 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:
[0005] (1) a light source having a plurality of emission points
(light-emitting portions) on one chip;
[0006] (2) a light source in which a plurality of laser emission
elements are used to combine optical paths by a beam splitter;
and
[0007] (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.
[0008] 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.
[0009] 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)
54a of an optical deflector 54. The plurality of light beams
reflected/deflected by the deflecting surface 54a of the optical
deflector 54 are formed into spots on a scanned surface 56 by an
imaging optical system (f-.theta. lens system) 55 having first and
second f-.theta. lenses 55a and 55b 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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 83a and 83b. The first and second edge portions 83a and
83b 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 A3 and A4 in FIG. 6.
[0014] 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.
[0015] A scanning start point 61 (image writing start position) of
a plurality of light beams A1 for image formation on the scanned
surface 56 is determined as follows.
[0016] Assume that BD detection corresponds to the timing at which
a BD light beam B3 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.
[0017] To more accurately detect the timing at which the BD light
beam B3 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 83a and 83b. A distance L
between the first and second edge portions 83a and 83b 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.
[0018] Since the first and second edge portions 83a and 83b 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.
[0019] 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.
[0020] 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.
[0021] The conventional multi-beam scanning optical system
described above has the following problems.
[0022] (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
by a light beam scanned on the BD slit 83 for synchronization
detection tilts with respect to the first and second edge portions
83a and 83b. For this reason, each light beam is obliquely scanned
on a slit opening 83c. As a consequence, the time intervals at
which a plurality of light beams are scanned on the slit opening
83c differ from the time intervals at which a plurality of light
beams are scanned on the scanned surface, resulting in a failure to
obtain a correct sync signal.
[0023] In addition, when each light beam is obliquely scanned on
the slit opening 83c, the respective light beams travel different
distances from the BD detection positions to the image writing
start positions. If this apparatus is driven while the delay times
between BD detection and image writing start positions remain the
same, the image writing start positions shift from each other in a
cycle of the number of light beams. As a result, an image is
observed as jitter in the main scanning direction with a straight
line in the sub-scanning direction becoming jagged.
[0024] To prevent this, the apparatus may be driven with different
delay times being set between BD detection and image writing start
positions for the respective light beams. This method, however,
requires an independent delay circuit for each light beam,
resulting in an increase in complexity of the overall apparatus and
an increase in cost.
SUMMARY OF THE INVENTION
[0025] It is an object of the present invention to provide a
multi-beam scanning optical system capable of high-speed printing
operation, in which a slit member forming one element of a
synchronization detection means for controlling the timing at a
scanning start position on a scanned surface is set in a proper
direction to prevent jitter in the main scanning direction and
attain an improvement in image quality without requiring any
complicated optical path arrangement, and an image forming
apparatus using the multi-beam scanning optical system.
[0026] According to one aspect of the invention a multi-beam
scanning optical system comprises light source means having a
plurality of light-emission points incident optical means for
guiding a plurality of light beams emitted from said light source
means to deflection means scanning optical means for forming the
plurality of light beams reflected/deflected by the deflection
means into images on a scanned surface, and synchronization
detection means in which a part of the plurality of light beams
from the deflection means are reflected at a predetermined angle in
a sub-scanning cross-section by reflection means to be scanned on a
surface of a slit member and to be guided to a surface of a
synchronization detection element via the slit member, and a timing
at a scanning start position on the scanned surface is controlled
by using a signal from the synchronization detection element,
[0027] wherein the slit member is positioned such that a plurality
of light beams scanned on a surface of a slit opening are
substantially vertically scanned on the slit opening.
[0028] According to further aspect of the invention, the
synchronization detection means controls a timing at a scanning
start position on the scanned surface in a cycle of the plurality
of light beams emitted from said light source means.
[0029] According to further aspect of the invention, a longitudinal
direction of the slit opening of the slit member is nonparallel to
a rotational axis direction of the deflection means.
[0030] According to further aspect of the invention, a plurality of
light-emission points are spaced apart from each other in at least
the main scanning direction.
[0031] According to further aspect of the invention, an image
forming apparatus comprises the multi-beam scanning optical system
set out in the foregoing, a photosensitive member disposed on the
scanned surface, a developing unit for developing an electrostatic
latent image formed on said photosensitive member by a light beam
scanned by said multi-beam scanning optical system into a toner
image, a transfer unit for transferring the developed toner image
onto a transfer medium, and a fixing unit for fixing the
transferred toner image on the transfer medium.
[0032] According to further aspect of the invention, an image
forming apparatus comprises the multi-beam scanning optical system
set out in the foregoing, and a printer controller for converting
code data input from an external device into an image signal and
inputting the signal to said multi-beam scanning optical
system.
[0033] According to further aspect of the invention, a multi-beam
scanning optical system comprises light source means having a
plurality of light-emission points, incident optical means for
guiding a plurality of light beams emitted from said light source
means to deflection means, scanning optical means for forming the
plurality of light beams reflected/deflected by the deflection
means into images on a scanned surface, and synchronization
detection means in which a part of the plurality of light beams
from the deflection means are reflected at a predetermined angle in
a sub-scanning cross-section by reflection means to be scanned on a
surface of a slit member and to be guided to a surface of a
synchronization detection element via the slit member, and a timing
at a scanning start position on the scanned surface is controlled
by using a signal from the synchronization detection element,
[0034] wherein the slit member is positioned such that a plurality
of light beams scanned on a surface of a slit opening vertically
cross one of edge portions of the slit opening.
[0035] According to further aspect of the invention, the
synchronization detection means controls a timing at a scanning
start position on the scanned surface in a cycle of the plurality
of light beams emitted from said light source means.
[0036] According to further aspect of the invention, a longitudinal
direction of the slit opening of the slit member is nonparallel to
a rotational axis direction of the deflection means.
[0037] According to further aspect of the invention, the plurality
of light-emission points are spaced apart from each other in at
least the main scanning direction.
[0038] According to further aspect of the invention, an image
forming apparatus comprises the multi-beam scanning optical system
set out in the foregoing, a photosensitive member disposed on the
scanned surface, a developing unit for developing an electrostatic
latent image formed on said photosensitive member by a light beam
scanned by said multi-beam scanning optical system into a toner
image, a transfer unit for transferring the developed toner image
onto a transfer medium, and a fixing unit for fixing the
transferred toner image on the transfer medium.
[0039] According to further aspect of the invention, an image
forming apparatus comprises the multi-beam scanning optical system
set out in the foregoing, and a printer controller for converting
code data input from an external device into an image signal and
inputting the signal to said multi-beam scanning optical
system.
[0040] According to further aspect of the invention, a multi-beam
scanning optical system comprises light source means having a
plurality of light-emission points, incident optical means for
guiding a plurality of light beams emitted from said light source
means to deflection means, scanning optical means for forming the
plurality of light beams reflected/deflected by the deflection
means into images on a scanned surface, and synchronization
detection means in which a part of the plurality of light beams
from the deflection means are reflected at a predetermined angle in
a sub-scanning cross-section by reflection means to be scanned on a
surface of a slit member and to be guided to a surface of a
synchronization detection element via the slit member, and a timing
at a scanning start position on the scanned surface is controlled
by using a signal from the synchronization detection element,
[0041] wherein a longitudinal direction of the slit opening of the
slit member is nonparallel to a rotational axis direction of the
deflection means, and the slit member is positioned such that a
plurality of light beams scanned on a surface of a slit opening are
substantially vertically scanned in the longitudinal direction of
the slit opening.
[0042] According to further aspect of the invention, the
synchronization detection means controls a timing at a scanning
start position on the scanned surface in a cycle of the plurality
of light beams emitted from said light source means.
[0043] According to further aspect of the invention, the slit
member is positioned to be vertical or substantially vertical to a
plane formed by a plurality of light beams scanned on the surface
of the slit member.
[0044] According to further aspect of the invention, the plurality
of light-emission points are spaced apart from each other in at
least the main scanning direction.
[0045] According to further aspect of the invention, an image
forming apparatus comprises the multi-beam scanning optical system
set out in the foregoing, a photosensitive member disposed on the
scanned surface, a developing unit for developing an electrostatic
latent image formed on said photosensitive member by a light beam
scanned by said multi-beam scanning optical system into a toner
image, a transfer unit for transferring the developed toner image
onto a transfer medium, a fixing unit for fixing the transferred
toner image on the transfer medium.
[0046] According to further aspect of the invention, an image
forming apparatus comprises the multi-beam scanning optical system
set out in the foregoing, and a printer controller for converting
code data input from an external device into an image signal and
inputting the signal to said multi-beam scanning optical
system.
BRIEF DESCRIPTION OF THE DRAWINGS
[0047] FIG. 1 is a main scanning sectional view of the first
embodiment of the present invention;
[0048] FIG. 2 is a main scanning sectional view of a portion around
a BD sensor in the first embodiment of the present invention;
[0049] FIG. 3 is a sub-scanning sectional view of the portion
around the BD sensor in the first embodiment of the present
invention;
[0050] FIG. 4 is an enlarge view of a portion around a BD slit
portion in the first embodiment of the present invention;
[0051] FIG. 5 is a main scanning sectional view of a conventional
multi-beam scanning optical system;
[0052] FIG. 6 is an enlarged view of a portion near a BD slit
portion in the prior art;
[0053] FIG. 7 is a view showing an example of the arrangement of a
multi-beam light source in the present invention;
[0054] FIG. 8 is a view showing another example of the arrangement
of a multi-beam light source in the present invention; and
[0055] FIG. 9 is a view showing an image forming apparatus
according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
First Embodiment
[0056] FIG. 1 is a sectional view (main scanning cross-section) of
the first embodiment in the main scanning direction, in which a
multi-beam scanning optical system of the present invention is
applied to an image forming apparatus such as a laser beam printer
or digital copying machine. FIG. 2 is a main scanning sectional
view of a portion around a BD sensor in FIG. 1. FIG. 3 is a
sub-scanning sectional view of the portion around the BD sensor in
FIG. 1.
[0057] Note that in this specification, a direction in which a
light beam is reflected/deflected (deflection scanning) by an
optical deflector is defined as a main scanning direction, and a
direction perpendicular to the optical axis of a scanning optical
system and the main scanning direction is defined as a sub-scanning
direction.
[0058] Referring to FIG. 1, a light source means 1 is formed by a
monolithic multi-beam semiconductor laser having two light sources
A and B arranged on one chip to be spaced apart from each other in
the main scanning direction and sub-scanning direction, as shown in
FIG. 7. A collimator lens 2 converts light beams emitted from the
multi-beam semiconductor laser 1 into substantially parallel light
beams or convergent light beams. A cylindrical lens (cylinder lens)
3 has a predetermined refracting power in only a sub-scanning
cross-section. Note that each of the collimator lens 2, cylindrical
lens 3, and the like forms one element of an incident optical means
14. Note that an aperture stop may be placed in an incident optical
means 14.
[0059] An optical deflector 4 serving as a deflection means is
formed by, for example, a polygon mirror (rotary polyhedral
mirror), which is rotated at a uniform speed in the direction
indicated by an arrow A in FIG. 1 by a driving means (not shown)
such as a polygon motor.
[0060] A scanning optical means (imaging optical system) 5 has
f-.theta. characteristics and a function to correct a tilt of the
deflection surface. The scanning optical means 5 has first and
second f-.theta. lenses 5a and 5b and serves to form a light beam
based on image information and reflected/deflected by the optical
deflector 4 into an image on a photosensitive drum surface 6
serving as a scanned surface. The first and second f-.theta. lenses
5a and 5b are formed by anamorphic lenses having different powers
in a sub-scanning cross-section.
[0061] The photosensitive drum surface (recording medium surface) 6
serve as a scanned surface.
[0062] A reflection means 7 is formed by a return mirror (to be
also referred to as a "BD mirror" hereinafter), and reflects a
synchronization detection light beam (to be also referred to as a
"BD light beam") B1 for adjusting the timing at a scanning start
position on the photosensitive drum surface 6 toward a
synchronization detection element 10 (to be described later) at
predetermined angles in a main scanning cross-section and
sub-scanning cross-section.
[0063] A slit member (to be also referred to as a "BD slit"
hereinafter) 8 has a slit opening 8c and first and second edge
portions 8a and 8b in a linear form which a BD light beam B2
crosses via the BD mirror 7. The slit member 8 is placed at a
position equivalent to the photosensitive drum surface 6 such that
a plurality of BD light beams 2 scanned on the slit opening 8c
plane are scanned on the slit opening 8c almost vertically.
[0064] An imaging lens 9 (to be also referred to as a "BD lens"
hereinafter) serving as an imaging means is used to make the BD
mirror 7 and synchronization detection element 10 substantially
optically conjugate to each other. The imaging lens 9 corrects an
optical face tangle error in the BD mirror 7 to reduce variations
in light beams incident on the BD sensor 10.
[0065] In this embodiment, the synchronization detection element 10
(to be also referred to as a "BD sensor" hereinafter) adjusts the
timing at a scanning start position of image recording on the
photosensitive drum surface 6 by using a write position sync signal
(BD signal) obtained by detecting an output signal from the BD
sensor 10.
[0066] Each of the BD mirror 7, slit member 8, BD lens 9, BD sensor
10, and the like forms one element of a synchronization detection
means (detection means) 21. The synchronization detection means 21
in this embodiment controls the timing at a scanning start position
on the photosensitive drum surface 6 for each of a plurality of
light beams emitted from the multi-beam semiconductor laser 1
(starting image writing a predetermined period of time after
detection by the detection means). Note that the synchronization
detection means 21 may control the timing at a scanning start
position for at least one of a plurality of light beams.
[0067] In this embodiment, a plurality of light beams optically
modulated in accordance with image information and emitted from the
multi-beam semiconductor laser 1 are converted into substantially
parallel light beams or convergent beams by the collimator lens 2
and strike the cylindrical lens 3. Of the light beams incident on
the cylindrical lens 3, the light beams emerge in a main scanning
cross-section without any change. The light beams 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 4a of the optical deflector 4. The plurality
of light beams reflected/deflected by the deflecting surface 4a of
the optical deflector 4 are formed into spots on the photosensitive
drum surface 6 by the scanning optical means 5. By rotating the
optical deflector 4 in the direction indicated by an arrow A, the
light beams are scanned on the photosensitive drum surface 6 in the
direction indicated by an arrow B (main scanning direction) at a
constant speed. With this operation, an image is recorded on the
photosensitive drum surface 6 serving as a recording medium.
[0068] In this case, before the photosensitive drum surface 6 is
optically scanned, in order to adjust the timing at a scanning
start position on the photosensitive drum surface 6, some (BE light
beams) of the plurality of light beams reflected/deflected by the
optical deflector 4 are reflected by the BD mirror 7 to be scanned
on the BD slit 8 surface, and are guided to the BD sensor 10 by the
BD lens 9 via the BD slit 8. By using a BD signal obtained by
detecting an output signal from the BD sensor 10, the timing at a
scanning start position of image recording on the photosensitive
drum surface 6 is adjusted. Note that FIG. 1 shows only one light
beam.
[0069] In this embodiment, the BD light beam B1 reflected/deflected
by the optical deflector 4 is deflected by the BD mirror 7 at
predetermined angles in a main scanning cross-section (X-Y
cross-section) and sub-scanning cross-section (X-Z cross-section),
and propagates as the BD light beam B2 toward the BD sensor 10, as
shown in FIGS. 1 to 3. At this time, if the optical path is bent in
the main scanning cross-section and sub-scanning cross-section,
jitter occurs in the main scanning direction, as described
above.
[0070] In this embodiment, therefore, as shown in FIG. 4, the BD
slit 8 is positioned such that the plurality of BD light beams B2
scanned on the slit opening 8c plane are substantially vertically
scanned on the rectangular slit opening 8c, thereby preventing
jitter that occurs in the main scanning direction in a cycle of the
number of a plurality of light beams. This makes it possible to
attain an increase in speed and an improvement in image
quality.
[0071] FIG. 4 is a sectional view showing the main part of the BD
slit 8 when viewed from the incident side of the BD light beams B2.
Referring to FIG. 4, a coordinate system Y'Z' is a local coordinate
system on the BD slit 8, the Z'-axis obtained by shifting the
Z-axis in FIG. 1 parallelly, and the coordinate system Y' is
parallel to an X-Y plane in FIG. 1.
[0072] In this embodiment, as described above, the BD slit 8 is
positioned such that the plurality of BD light beams B2 scanned on
the slit opening 8c plane with the first edge portion 8a being
parallel to the second edge portion 8b are substantially vertically
scanned on the slit opening 8c. In addition, the BD slit 8 is
positioned to be nonparallel to the Z'-axis in the coordinate
system in FIG. 4, and the longitudinal direction (the longitudinal
direction of the slit opening 8c) of the slit opening 8c of the BD
slit 8 becomes nonparallel to the direction of a rotational axis 13
of the optical deflector 4. That is, the BD slit 8 is positioned
such that the plurality of BD light beams B2 scanned on the slit
opening 8c plane vertically cross the first and second edge
portions 8a and 8b of the slit opening 8c.
[0073] First and second laser spots 11 and 12 of the plurality of
BD light beams B2 are formed on the slit opening 8c plane. When the
optical deflector 4 rotates in the direction indicated by the arrow
A in FIG. 1, the first and second laser spots 11 and 12 are scanned
in the directions indicated by arrows A1 and A2 in FIG. 4,
respectively. A plane including the directions indicated by the
arrows A1 and A2 coincides with the plane formed by the plurality
of BD light beams B2 scanned on the slit opening 8c plane.
[0074] As shown in FIG. 4, the first and second laser spots 11 and
12 are spaced apart from each other by predetermined distances in
the scanning direction (the direction indicated by arrows A1 and
A2) and a direction perpendicular thereto, respectively. If the
distance between the first and second laser spots 11 and 12 in the
scanning direction is represented by L', the first and second laser
spots 11 and 12 are scanned on the photosensitive drum surface 6
while always being spaced apart from each other by the distance L'
in the scanning direction.
[0075] In this embodiment, a scanning start point 22 (image writing
start position) of a plurality of image forming light beams A1 on
the photosensitive drum surface 6 is determined as follows.
[0076] Assume that BD detection corresponds to the timing at which
the BD light beam B2 strikes the BD sensor 10. This BD detection is
independently performed for each light beam, and image writing
starts a predetermined time delay after the BD detection.
[0077] In this embodiment, to more accurately detect the timing at
which the BD light beam B2 strikes the BD sensor 10, the BD slit 8
is placed in front of the BD sensor 10. As shown in FIG. 4, the BD
slit 8 is made up of the first and second edge portions 8a and 8b.
A distance L between the first and second edge portions 8a and 8b
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 10. 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 10. 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.
[0078] As described above, since the BD slit 8 having the first and
second edge portions 8a and 8b is positioned such that the
plurality of BD light beams B2 scanned on the slit opening 8c plane
are substantially vertically scanned on the slit opening 8c, the
respective BD light beams B2 travel the same distance from the BD
detection position 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
addition, in this embodiment, there is no need to prepare delay
circuits with different delay times for the respective light beams,
and hence the circuit arrangement can be simplified, leading to a
reduction in cost.
[0079] 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, as described
above.
[0080] In this embodiment, plastic lenses are used for some or all
of the f-.theta. lens system 5, collimator lens 2, cylindrical lens
3, and the like constituting the multi-beam scanning optical
system. This makes it possible to attain an improvement in
performance owing to the formation of aspherical surfaces and a
reduction in cost.
[0081] In this embodiment, the first and second edge portions 8a
and 8b are formed such that the plurality of BD light beams B2
scanned on the slit opening 8c plane vertically cross the two edge
portions. However, the BD light beams B2 may vertically cross only
one of the edge portions. That is, effects similar to those of the
first embodiment described above can be obtained even if the first
and second edge portions 8a and 8b are formed to be nonparallel to
each other.
[0082] The reason why the plurality of BD light beams B2 scanned on
the slit opening 8c plane may vertically cross only one edge
portion is that BD detection may be performed either at the first
edge portion 8a on the BD light beam B2 incident side of the slit
opening 8c or at the second edge portion 8b on the BD light beam B2
exit side of the slit opening 8c.
[0083] In the first embodiment, the number of emission points
(light sources) is two. However, the present invention is not
limited to two, and can be applied to a case wherein three or more
beams are used.
[0084] In the first embodiment, the monolithic multi-beam
semiconductor laser is used. However, the present invention is not
limited to a monolithic laser. The present invention can also be
applied to a hybrid scheme of forming a multi-laser light source by
preparing a plurality of laser elements each designed to emit a
single beam or a plurality of beams, and combining optical paths
using a beam combining optical system such as a polarization beam
splitter.
[0085] In the present invention, a plurality of BD light beams are
reflected by the BD mirror 7 at a predetermined angle in a
sub-scanning cross-section so as to be scanned on the BD slit 8
surface. The "predetermined angle" indicates the angle through
which each light beam is rotated clockwise or counterclockwise with
respect to a main scanning cross-section including a linear image
elongated in the main scanning direction which is formed on the
deflecting surface 4a of the optical deflector 4, as shown in FIG.
3, and a tilt is defined by the BD light beams B1 and B2 in a
sub-scanning cross-section owing to the BD mirror 7. The
synchronization detection means 21 is therefore located above or
below the main scanning cross-section including the linear image
elongated in the main scanning direction.
[0086] In this embodiment, the BD mirror 7 is used for a compact
structure. This effect is noticeable especially in a form having
the synchronization detection means 21 located above or below the
f-.theta. lens system 5.
[0087] In the first embodiment, the two emission points (light
sources) are spaced apart from each other in the main scanning
direction and sub-scanning direction. However, the present
invention can be applied to a form in which two emission points
(light sources) are spaced apart from each other in only the
sub-scanning direction, as shown in FIG. 8. In this case, BD
sensors equal in number to BD light beams are preferably arranged.
For example, one BD sensor is preferably provided for each of two
BD light beams, and a total of two BD sensors are preferably
arranged in a multi-beam scanning optical system. In addition, the
present invention can be applied to a form in which two emission
points (light sources) are spaced apart from each other in only the
main scanning direction.
[0088] FIG. 9 is a sectional view of the main part of an image
forming apparatus according to an embodiment of the present
invention. Referring to FIG. 9, an image forming apparatus 104
receives code data Dc from an external device 117 such as a
personal computer. This code data Dc is converted into image data
(dot data) Di by a printer controller 111 in the apparatus. This
image data Di is input to an optical scanning unit 100 having an
arrangement like the one described in each of the first to third
embodiments. A light beam 103 modulated in accordance with the
image data Di emerges from the optical scanning unit 100, and the
photosensitive surface of a photosensitive drum 101 is scanned in
the main scanning direction with the light beam 103.
[0089] The photosensitive drum 101 serving as an electrostatic
latent image carrier (photosensitive member) is rotated clockwise
by a motor 115. Upon this rotation, the photosensitive surface of
the photosensitive drum 101 moves with respect to the light beam
103 in the sub-scanning direction perpendicular to the main
scanning direction. A charging roller 102 for uniformly charging
the surface of the photosensitive drum 101 is placed above the
photosensitive drum 101 such that the surface of the charging
roller 102 is in contact with the photosensitive drum 101. The
surface of the photosensitive drum 101 charged by the charging
roller 102 is irradiated with the light beam 103 scanned by the
optical scanning unit 100.
[0090] As described above, the light beam 103 is modulated on the
basis of the image data Di. By irradiating the surface of the
photosensitive drum 101 with the light beam 103, an electrostatic
latent image is formed on the surface of the photosensitive drum
101. This electrostatic latent image is developed as a toner image
by a developing unit 107 which is placed downstream from the
radiation position of the light beam 103 in the rotational
direction of the photosensitive drum 101 so as to be in contact
with the photosensitive drum 101.
[0091] The toner image developed by the developing unit 107 is
transferred onto a paper sheet 112 as a transfer medium by a
transfer roller 108 placed below the photosensitive drum 101 to
oppose the photosensitive drum 101. The paper sheet 112 is stored
in a paper cassette 109 in front of the photosensitive drum 101 (on
the right side in FIG. 9). However, a paper sheet can also be
manually fed. A feed roller 110 is placed at an end portion of the
paper cassette 109 to feed the paper sheet 112, stored in the paper
cassette 109, onto a convey path.
[0092] The paper sheet 112 on which the unfixed toner image is
transferred in the above manner is further conveyed to a fixing
unit behind (the left side in FIG. 9) the photosensitive drum 101.
The fixing unit is made up of a fixing roller 113 incorporating a
fixing heater (not shown) and a press roller 114 which is pressed
against the fixing roller 113. The fixing unit fixes the unfixed
toner image on the paper sheet 112 conveyed from the transfer unit
by heating the paper sheet 112 while pressing it between the fixing
roller 113 and the pressing portion of the press roller 114. In
addition, a paper discharge roller 116 is placed behind the fixing
roller 113 to discharge the image-fixed paper sheet 112 outside the
image forming apparatus.
[0093] Although not shown in FIG. 9, the printer controller 111
controls the respective components in the image forming apparatus,
including the motor 115, and the polygon motor in the optical
scanning unit (to be described later) as well as data conversion
described above.
[0094] The present invention can also be applied a tandem type
color image forming apparatus having a plurality of photosensitive
drums.
[0095] According to the present invention, as described above,
there are provided a multi-beam scanning optical system capable of
high-speed printing operation, in which a slit member forming one
element of a synchronization detection means for controlling the
timing at a scanning start position on a scanned surface is set in
a proper direction to make the respective light beams travel the
same distance from the BD detection positions to the writing start
positions with the same delay time, thereby preventing jitter in
the main scanning direction, which occurs in a cycle of the number
of light beams, and attaining an increase in operation speed and an
improvement in image quality, and an image forming apparatus using
the multi-beam scanning optical system.
* * * * *