Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2000-01-18
2001-02-06
Phan, James (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S205100, C359S216100, C359S217200
Reexamination Certificate
active
06185027
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a monitoring optical system, which is applicable to a scanning optical device such as a laser printer, for detecting a drawing-start timing or a drawing-end timing per scan. Particularly, the present invention relates to the monitoring optical system for the scanning optical device in which a laser beam deflected by a deflector is separated from a laser beam incident thereon in an auxiliary scanning direction.
U.S. Pat. No. 5,621,562 discloses the scanning optical device that has such an arrangement of the deflected laser beam separated from the incident laser beam.
FIG. 4
is a perspective view of the scanning optical device disclosed in the U.S. patent. A laser beam emitted by a semiconductor laser
10
passes through a collimator lens
11
and a cylindrical lens
12
and is reflected by a flat mirror
13
to be incident on a polygonal mirror (deflector)
14
. The deflected laser beam by a reflection surface
14
a
of the polygonal mirror
14
is reflected by a curved surface mirror
15
and forms a beam spot on a photoconductive drum
18
(shown by two-dot chain line) through a drawing anamorphic lens
16
and an optical path bending mirror
17
.
The beam spot scans on the photoconductive drum
18
as the polygonal mirror
14
rotates.
In this specification, a direction equivalent to the scanning direction of the beam spot on the photoconductive drum
18
is referred to as a main scanning direction, a direction perpendicular to the main scanning direction is referred to as an auxiliary scanning direction.
The laser beam at the surface of the photoconductive drum
18
is the reference point for defining the direction of the optical power of the optical elements. That is, the power in the main scanning direction means the power contributing to converge or diverge the laser beam in the main scanning direction at the drum
18
. The power in the auxiliary scanning direction means the power which contributes to converge or disperse the laser beam in the auxiliary scanning direction at the drum
18
.
The laser beam deflected by the polygonal mirror
14
is separated from the laser beam incident thereon in the auxiliary scanning direction, i.e., a direction of a rotation axis
14
b
of the polygonal mirror
14
. Further, the laser beam reflected by the curved surface mirror
15
is also separated from the laser beam incident thereon in the auxiliary scanning direction.
The cylindrical lens
12
has a positive refractive power only in the auxiliary scanning direction for forming a line-spread image near the polygonal mirror
14
. The curved surface mirror
15
primarily has a positive power in the main scanning direction and the drawing anamorphic lens
16
primarily has a positive power in the auxiliary scanning direction.
A monitoring flat mirror
40
is located between the curved surface mirror
15
and the drawing anamorphic lens
16
. The monitoring flat mirror
40
separates a monitor beam from the deflected laser beam at a separation point outside of the scanning range. When the laser beam reflected by the curved surface mirror
15
reaches the end of the scanning range, the laser beam is reflected by the monitoring flat mirror
40
and is converged onto a monitoring sensor
42
through a monitoring cylindrical lens
41
. The monitoring sensor
42
generates a synchronizing signal to indicate the drawing-start timing of each scan in response to each detection of the monitor beam.
The laser beam reflected by the curved surface mirror
15
is converged in the main scanning direction, but is diverged in the auxiliary scanning direction, so it is re-converged by the monitoring cylindrical lens
41
in the auxiliary scanning direction to form a spot on the monitoring sensor
42
.
In
FIG. 4
, a y-direction and a z-direction are defined in a plane P that is perpendicular to the monitor beam incident on the monitoring sensor
42
. The y-direction is a scanning direction of the monitor beam on the plane P and the z-direction is perpendicular to the y-direction on the plane P. When the main meridian of the monitoring cylindrical lens
41
is projected onto the plane P, the direction of the main meridian N is parallel to the z-direction. The main meridian is perpendicular to a generatrix of the monitoring cylindrical lens
41
and indicates a direction of main refractive power thereof.
The laser beam is incident on the polygonal mirror
14
with an inclination in the auxiliary scanning direction, which causes a skew distortion in the deflected laser beam when a scanning angle W is not zero. The skew distortion increases as the absolute value of the scanning angle w increases. It should be noted that the scanning angle w is defined as an angle formed between the center axis of the laser beam deflected by the polygonal mirror
14
and a reference light ray that points at the center of the scanning range on the photoconductive drum
18
. That is, when the deflected laser beam points at the center of the scanning range, the scanning angle W is zero. The scanning angle W has a positive value at the side of the monitoring flat mirror
40
with respect to the reference light ray and has a negative value at the other side. The deflected beam of the scanning angle W=47.0° is incident on the monitoring mirror
40
.
FIGS. 5A
,
5
B,
5
C and
5
D show the skews of the deflected laser beams at the scanning angle W=0.0°, W=20.0°, W=43.0° and W=47.0°, respectively. In each of the figures, a horizontal axis means the main scanning direction and a vertical axis means the auxiliary scanning direction.
Since the drawing anamorphic lens
16
acts as a correcting optical system for correcting effect of the skew of the deflected laser beam, the skew distortion has little effect on the beam spot formed on the photoconductive drum
18
.
However, the monitor beam travels to the monitoring sensor
42
through the monitoring cylindrical lens
41
without passing the drawing anamorphic lens
16
, which disturbs the wavefront of the monitor beam on the monitoring sensor
42
as shown in FIG.
6
. The disturbance of the wavefront distorts the shape of the monitor beam spot on the monitoring sensor
42
.
FIGS. 7A
,
7
B and
7
C are spot diagrams to show the shapes of the monitor beam spots at the point in front of the sensor by 2 mm (the side of the monitoring cylindrical leas
41
), on the sensor and at the point in the rear of the sensor by 2 mm, respectively. The monitor beam is not sufficiently converged on the monitoring sensor
42
as shown in
FIG. 7B
, which blunts the rising edge of the signal generated by the monitoring sensor
42
. Therefore, the scanning-start points determined by the signal from the monitoring sensor
42
may vary widely, which reduces printing quality.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a monitoring optical system for a scanning optical device, which is capable of preventing disturbance of the wavefront of the monitor beam due to the skew distortion thereof, in spite of that the deflected laser beam is separated from the light beam incident on the deflector in the auxiliary scanning direction and that the monitoring beam is separated from the deflected laser beam without passing the correcting optical system.
For the above object, according to the present invention, there is provided an improved monitoring optical system for a scanning optical device in which a laser beam emitted from a laser source is incident on a deflector through a first anamorphic optical system such that the deflected laser beam is separated from the incident laser beam in a auxiliary scanning direction, and that the deflected laser beam is converged onto a drawing surface through a second anamorphic optical system that includes a correcting optical system for correcting an effect of skew of the laser beam. The monitoring optical system includes monitoring anamorphic lens for converging a monitor beam that is separated from the deflected laser beam at a separation
Asahi Kogaku Kogyo Kabushiki Kaisha
Greenblum & Bernstein P.L.C.
Phan James
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