Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2001-07-17
2004-08-10
Phan, James (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C250S234000, C359S216100
Reexamination Certificate
active
06775040
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a laser scanner for which the light source is comprised of a semiconductor laser. In particular, it relates to a circuit for detecting a scanning sync signal that is used for controlling the timing of a laser beam scanning.
2. Description of the Related Art
In a laser scanner which utilizes a semiconductor laser and so forth for a light source, a required graphic pattern is transferred to a photosensitive member by driving the photosensitive member in a sub scanning (vertical scanning) direction while a main scanning (horizontal scanning) is carried out for the photosensitive member by the laser beam. In this type of the laser scanner, the horizontal scanning of the laser beam should be synchronized with the vertical scanning motion of the photosensitive member. Therefore, a detection of a horizontal sync signal is required.
For example,
FIG. 7
illustrates a general construction of a laser scanner
1
. A laser beam LB, emitted from a semiconductor laser
11
, is collimated by a collimator lens
13
and shaped through a cylindrical lens
14
, then projected to a polygon mirror
15
which is rotating at high speed. The laser beam LB is then reflected toward a direction of the main scanning by a reflecting mirror of the polygon mirror
15
, and the main scanning at a photosensitive surface of a photosensitive drum is carried out via an f&thgr; lens
16
. The photosensitive drum
17
has the rotating axis
17
a
which is parallel with the main scanning direction. The drum is rotated around the rotating axis
17
a
, so that the photosensitive drum is sub scanned by the laser beam LB and a required pattern is formed on the drum surface.
Further, a laser detector
12
, comprised of a photodiode, is disposed at a side of the photosensitive drum
17
. The detector
12
receives the laser beam LB just before the beginning of each main scanning and sends detecting signals to a sync-detecting circuit
2
. From the detecting signals, the sync-detecting circuit
2
generates a horizontal synchronizing signal HSYNC which standardizes the timing of the main scanning. Proper horizontal scanning with the laser beam LB is achieved by controlling the speed and timing of the polygon mirror's
15
rotation, so that the horizontal synchronizing signals are generated in a predetermined timing.
In general, as for the above-mentioned laser detector
12
, a package-made detector is used. As shown in
FIG. 8
, a photodetector
32
is provided in the package. The photodetector
32
is disposed on a photodetector circuit board
31
and is electrically connected to the circuit patterns
34
of the circuit board
31
with bonding wires (jumper wires)
33
, and entirely enclosed with a transparent resin
35
.
Laser beam scans over the laser detector
12
are in the order A, B, C, and D shown in the figure. However, in the case of the type of laser detector
12
, laser beams, such as B, C, or D, which penetrate the transparent resin
35
before and after the proper beam to be detected for generating the horizontal synchronizing signal, which incidents straight on the photodetector
32
, are reflected by the circuit patterns
34
of the photodetector circuit board
31
and bonding wires
33
and the like, or by the inner surface of the transparent resin
35
. The reflected laser beams are then made incident to the photodetector
32
as stray light. Therefore, in some cases, the laser detector
12
misidentifies these stray lights as a horizontal synchronizing signal. As a result, when the misdetection of a horizontal synchronizing signal occurs, the laser scanner
1
fails in transferring a proper image.
In the case of the laser detector shown in
FIG. 8
, in order to avoid the above misdetection of the horizontal synchronizing signals, a stray-beam-preventing slit board
37
may be provided to reduce the stray beams made incident on the photodetector
32
by allowing only the proper beam to penetrate the slit. However, it is difficult to prevent the generation of the stray beams originating from the proper beam, the beams which penetrate the slit board
37
, and light reflected inside the laser detector as described above.
Further, although the antireflection coating (not shown) for preventing the reflection inside laser detector may be applied for structural improvement, it is still difficult to completely prevent the reflection.
Furthermore, from the aspect of an electrical circuit, a sync-detecting circuit can be designed to reject a detecting signal arising during a period when no synchronizing signal should possibly be detected. However, it is hard to distinguish and reject a detecting signal generated by the stray beams from the proper signal when the detecting signal arises at a time close to the time of the proper beam to be detected. As a result, the misdetection of the synchronizing signal induced by the stray beams remains.
Since the stray beams are reflected several times before they reach the photodetector, the power of each stray beam is attenuated and is lower than that of the proper beam. Thus the stray beams may be distinguished and rejected by setting a slice or boundary level of the slice circuit for output of the laser detector.
FIG. 9
illustrates an example of the slice circuit of the sync-detecting circuit to which the boundary level is applied. Waveforms illustrated in
FIG. 10A
represent normal behavior of the sync-detecting circuit. A photocurrent produced in the laser detector
12
by the effect of incident laser beam LB is transferred to a voltage output Va at an I-V converter
301
, and then applied to the noninverting input terminal of a comparator
302
. A standard voltage V1 that is higher than the voltage normally produced by the stray beams is applied to the inverting input terminal of the comparator
302
from a criterion power source
303
. The output voltage Va and the standard voltage V1 are compared at the comparator
302
. When the output Va is higher than the standard voltage V1, a high level output Vb is output from the comparator
302
and the horizontal synchronizing signal HSYNC is output from a driver
304
in accordance with the output Vb. Namely, outputs of the horizontal synchronizing signals can be synchronized with the laser beam LB reception at the laser detector
12
by classifying the output signal Va as to the boundary level which is set to the standard voltage V1.
However, in the case of a laser scanner for which the power of the proper beams are varyingly controlled, such as a laser scanner that controls the power of the laser in accordance with the sensitivity of the photosensitive member or the depth of the image to be formed, a stray beam may be detected as the synchronizing signal.
For example, as shown in
FIG. 10A
, when the powers of the stray beams are sufficiently small compared to the power of the proper beam, the proper beam can be discriminated from the stray beams by setting the boundary level or the standard voltage appropriately, so that the synchronizing signals are detected properly, as described above. However, as shown in
FIG. 10B
, when the power of the laser beam is increased and the output Va of the proper beam exceeds a saturating level of the photodetector, the power of the stray beam is increased as well. So that, in this case, the output Va of the stray beam may exceed the boundary level V1. As a result, the comparator
302
may output Vb
2
by the stray beam as well as output Vb
1
by the proper beam. Thus, the driver
304
may output the synchronizing signal HSYNC in error, in accordance with the output Vb
2
by the stray beam.
Further, the power level of the stray laser beam, in the high-power mode, may become higher than the level of the proper laser beam in the low power mode depending on the setting of the laser power controls. In this case, if the standard voltage of the boundary level is set at a certain constant voltage, the stray beam may be detected as the synchronizing signal.
SUMMARY OF THE INVENTION
Therefore, an object
Greenblum & Bernstein P.L.C.
PENTAX Corporation
Phan James
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