Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2000-07-27
2002-07-02
Phan, James (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S212100, C359S216100, C347S238000, C347S243000, C250S234000
Reexamination Certificate
active
06414778
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a multi-beam scanning device, and particularly to a synchronizing signal detecting method and device for the multi-beam scanning device.
In recent imaging apparatuses, such as a laser beam printer, there are ones employing a scanning device, which is provided with a plurality of laser diodes. In such a printer, by emitting a plurality of laser beam, which are modulated respectively, a plurality of line images are formed on a surface to be scanned, such as a photoconductive surface, simultaneously, thereby image forming speed becomes faster. Such a scanning device using a plurality of beams will be referred to as a multi-beam scanning device. In order to adjust the timing of drawing start position from which a scanning line is formed by each laser beam, a horizontal synchronizing signal should be detected for each laser beam.
In a conventional scanning device using a single laser beam, a beam detector as shown in
FIG. 9A
is employed.
FIG. 9A
schematically shows a structure of the conventional scanning device using a single layer beam.
In
FIG. 9A
, a laser diode
1
emits a diverging laser beam, which is collimated by a collimating lens
2
. The collimated laser beam is incident on a polygonal mirror
3
, which is rotated at a high speed. The laser beam incident on reflection surface of the polygonal mirror
3
is deflected thereby to scan within a predetermined angular range. The deflected (i.e., scanning) beam passes through an f&thgr; lens
4
and is converged, in a direction perpendicular to the scanning direction, thereby so that a scanning beam spot is formed on a photoconductive drum
5
. It should be noted that the photoconductive drum
5
is rotatable about an axis AX (see
FIG. 9A
) thereof, and the beam spot formed on the photoconductive drum moves, at a constant speed due to the function of the f&thgr; lens on the circumferential surface thereof in a direction parallel to the axis thereof. The direction in which the beam spot moves will be reflected to as an main scanning direction hereinafter. While the main scanning is performed, the photoconductive drum is rotated. The direction in which the circumferential surface moves with respect to the main scanning line (i.e., the direction perpendicular to the main scanning direction) will be referred to as an auxiliary scanning direction.
Within a scanning range of the laser beam, but outside a range contributing to the image formation, a beam detector
6
is provided. The beam detector
6
outputs a detection signal when the scanning beam is incident on the beam detector
6
. The output of the beam detector
6
is used for generating a horizontal synchronizing signal.
FIG. 9B
illustrates a light receiving surface of the beam detector
6
, and a beam spot at various positions.
If the diameter of the beam spot in the main scanning direction on a plane including the light receiving surface of the beam detector
6
is smaller than the width of the light receiving surface of the beam detector
6
as shown in
FIG. 9B
, then when the laser beam traverses the light receiving surface of the beam detector
6
, the output voltage VO varies as shown in FIG.
9
C. By comparing the output voltage VO with a predetermined threshold value Vth, a horizontal synchronizing signal as shown in
FIG. 9D
is obtained.
If the beam detector as described above is used for a multi-beam scanning device, a plurality of beam spots traverse the light receiving surface of the beam detector
6
as shown in FIG.
10
A. In
FIG. 10A
, two beam spots LB
11
and LB
12
scan on the beam detector
6
. The beam detector
6
outputs the voltage VO which represents synthesized signals corresponding to the beam spots LB
11
and LB
12
. Generally, in a multi-beam scanning device, the beam spots are aligned along a line that is inclined with respect to the main scanning direction.
If the distance between the two beams LB
11
and LB
12
is sufficiently larger than width, in the main scanning direction, of the beam detector
6
, there is a period during which none of the two beams LB
11
and LB
12
are incident on the beam detector
6
. In such a case, as shown in
FIG. 10B
, the beam detector
6
outputs a signal VO having two distinct peaks respectively corresponds to the two beams LB
11
and LB
12
. Thus, the output signal VO shown in
FIG. 10B
can be used for generating the horizontal synchronizing signals for the laser beams LB
11
and LB
12
, respectively, as shown in FIG.
10
C. It should be noted, however, the two peaks of the output signal VO have similar waveforms, and therefore, it is impossible to determine which peak corresponds to which beam. In order to generate the horizontal synchronizing signals Hsync
11
and Hsync
12
, a circuit for distinguish the two peaks should be added.
Further, if the scanning speed is relatively fast, i.e., the scanning period is shortened, and therefore the interval between the two beams, in the main scanning direction, relative to the scanning period becomes greater, and therefore, a period of time during which the beam detector
6
detects the laser beams LB
11
and LB
12
becomes shorter, a response of the beam detector
6
(i.e., a photo-electric conversion speed) may not follow the scanning speed. In such a case, the peaks of the signal VO cannot be identified. That is, if the waveform of the output signal of the beam detector
6
is likened to ridge and valley portions, the valley portion becomes shallower if the scanning speed becomes faster. Thus, also in such a case, a circuit for distinguish the peaks, which results in a large size of the scanning device.
If a distance between the two beams LB
11
and LB
12
, in the main scanning direction, is small relative to the width of the beam detector
6
, as shown in
FIG. 11A
, the output signal VO may have waveform as shown in FIG.
11
B. The output signal VO includes components corresponding to the two beams LB
11
and LB
12
, which are overlapped (see Hsync
11
and Hsync
12
shown in FIGS.
11
C and
11
D). It is impossible to detect the horizontal synchronizing signals for the two beams LB
11
and LB
12
from the signal VO alone. In order to distinguish the two beams, the output signal VO is compared with two threshold values Vth
1
and Vth
2
as shown in FIG.
11
B. In
FIG. 11B
, positions A-E corresponds to the beam positions shown in FIG.
11
A. At position B, the beam LB
11
is detected to be incident on the beam detector but the beam LB
12
has not yet been incident on the beam detector
6
. At position C, the second beam LB
12
is detected to be incident on the beam detector
6
, beam LB
11
being also incident on the beam detector
6
. At position D, the beam LB
11
has passed the beam detector
6
, and at position E, the beam LB
12
has passed the beam detector
6
. Thus, by comparing the output signal VO with the threshold values Vth
1
and Vth
2
, the components can be distinguished.
However, if the saturation level of the photodiode included in the beam detector is not set appropriately, the output VO of the beam detector
6
may have a waveform as shown in FIG.
11
E. In such a case, it is difficult to distinguish the two components by comparing the output signal VO with the threshold values Vth
1
and Vth
2
. Further, if the number of the beams is three or more, the same number of threshold values should be set, which make it more difficult to distinguish the components corresponding to the beams.
Alternatively, by providing a plurality of beam detectors respectively corresponding to the plurality of laser beams, the horizontal synchronizing signals can be detected accurately for respective beams. However, it is particularly very difficult to align a plurality of beam detectors at equivalent positions with respect to the corresponding beams. In particular, for three of more laser beams, such an alignment is practically impossible. Further, the beam detector is generally expensive, and thus, employing a plurality of beam detectors increases a manufacturing cost.
SUMMARY OF THE INVENTION
It
Asahi Kogaku Kogyo Kabushiki Kaisha
Greenblum & Bernstein P.L.C.
Phan James
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