Multiple wobble correction optical elements to reduce height...

Details Multiple wobble correction optical elements to reduce height... Multiple wobble correction optical elements to reduce height...

1999-12-20

2001-06-26

Schuberg, Darren (Department: 2872)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S208100, C347S259000

Reexamination Certificate

active

06252695

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to reducing the height of a raster output scanning (ROS) system and, more particularly, to using multiple, shorter focal length, wobble correction optical elements in the raster output scanning (ROS) system to reduce the ROS height.
Printing systems utilizing lasers to reproduce information are well known in the art. The printer typically uses a Raster Output Scanner (ROS) to expose the charged portions of the photoreceptor to record an electrostatic latent image thereon. Generally, a ROS has a laser for generating a collimated beam of monochromatic light. This laser beam is modulated in conformance with an image information data stream by either an external acousto-optic modulator or by internal laser diode driver electronics. The modulated beam is transmitted through a lens onto a scanning element, typically a rotating polygon having mirrored facets.
The light beam is reflected from a facet and thereafter focused to a “spot” on the photosensitive medium. The rotation of the polygon causes the spot to scan across the photoreceptor in a scan (i.e., line scan) direction. Meanwhile, the photoreceptor is advanced relatively more slowly than the rate of the scan in a slow cross-scan direction which is orthogonal to the scan direction. In this way, the beam scans the photoreceptor recording medium in a raster scanning pattern. The light beam is intensity-modulated in accordance with the input image information serial data stream so that individual picture elements (“pixels”) of the image represented by the data stream are exposed on the photoreceptor to form a latent image, which is then transferred to an appropriate image receiving medium such as paper.
While raster output scanner based printing systems are well known, implementing such printing systems that fit into a small space or on a desk is difficult. One reason is the optical cross-sectional area of the raster output scanner. This optical area must remain obstruction free so that the charged photoreceptor can be properly illuminated which limits how small the printing systems can be. Raster output scanner designs which reduce the optical cross-sectional area are exceedingly useful.
A compact design for the scanning optics of these prior art type of ROS systems is desirable to make the machine itself as compact as possible and to enable extension of the same ROS design into many machine architectures.
One well known technique to reduce the size of a ROS system is to introduce folding mirrors to fold the optical path and allow the optical components to be positioned in a more compact area.
Prior art raster output scanner based printing systems often use mirrors to fold the laser beam onto the photoreceptor. Folding is beneficial since the optical path length can remain relatively large while the physical length of the path is reduced. Reflecting the laser beam with folding mirrors prior to sweeping the laser beam with the rotating polygon mirror is relatively straightforward. Using folding mirrors after the laser beam is sweeping after reflection from the rotating polygon mirror becomes more difficult since the resulting scan line must have a direction substantially perpendicular to the motion of the photoreceptor surface.
It would be desirable to improve the efficiency, shorten the optical path lengths, and use as few optical elements as possible to decrease hardware, assembly and alignment costs in a ROS system.
A typical prior art raster output scanning system
10
of
FIG. 1
consists of a pre-polygon mirror optical section
12
, a rotating polygon mirror scanning element
14
comprising a plurality of reflective facets
16
, and a post-polygon mirror optical section
18
to correct for wobble of the rotating polygon mirror and to focus the beam along a scan line on the photoreceptor
20
.
A light source,
22
, such as a laser diode, emits a modulated coherent light beam
24
of a single wavelength. The light beam
24
is modulated in conformance with the image information data stream contained in the video signal sent from image output control circuit
26
to the light source
22
.
The modulated light beam
24
is collimated by a collimating lens
28
in both the scan and cross-scan planes.
The collimated light beam
24
is focused by a cross-scan cylindrical lens
30
. The lens
30
is cylindrical in the cross-scan plane and piano in the scan plane. Thus, the lens converges the cross-scan portion of the beam
24
focusing it on a reflective facet
16
of the rotating polygon mirror
14
but allows the scan portion of the beam
24
to remain collimated when the beam
24
strikes the reflective facet
14
.
The collimating lens
28
and the cross-scan cylinder lens
30
are usually the only optical elements in the pre-polygon mirror optical section
12
.
The polygon mirror
14
is rotated around its axis of rotation by a conventional motor (not shown), known to those of ordinary skill in the art.
The beam
24
reflected from the facet
16
is still collimated in the scan plane and is now diverging in the cross-scan plane. After reflection from the reflective facet
16
, the beam then passes through post-polygon optical section
18
, consisting of the f-theta scan lenses
32
and the anamorphic wobble correction lens
40
.
The f-theta scan lens
32
consists of a negative plano-spherical lens
34
, a positive piano-spherical lens
36
, and the cross-scan cylinder lens
38
. This configuration of f-theta scan lenses has sufficient negative distortion to produce a linear scan beam. The light beam will be deflected at a constant angular velocity from the rotating mirror which the f-theta scan lens optically modifies to scan the surface at a constant velocity.
The f-theta scan lens
32
will focus the light beam
24
in the scan plane onto the scan line
42
on the photoreceptor
20
. The f-theta scan lens
32
only has optical power in the scan plane so the f-theta scan lens
32
will not effect the divergence of the light beam
24
in the cross-scan plane.
After passing through the f-theta scan lens
32
, the light beam
24
then passes through a wobble correction anamorphic lens element
40
. The wobble correction optical element can be a lens or a mirror and is sometimes referred to as the “motion compensating optics”. The purpose of optical element
40
is to correct wobble along the scan line generated by inaccuracies in the polygon mirror/motor assembly.
The wobble correction lens
40
focuses the light beam in the cross-scan plane onto the scan line
42
on the photoreceptor
20
. The wobble correction lens
40
only has optical power in the cross-scan plane so the wobble correction lens
40
will not effect the convergence of the light beam
24
in the scan plane from the f-theta scan lens
32
.
The optical path length, and consequently the overall size of a rotating polygon ROS, is largely determined by the focal lengths of the lenses used to focus the beam onto the polygon and thence onto the scan line.
As shown in
FIG. 2
, in the side view in the cross-scan plane, the light beam
24
is reflected from the facet
16
of the polygon mirror
14
as a point
44
. The light beam
24
will then diverge at a divergence angle
46
along the optical path
48
through the f-theta scan lens
32
. The f-theta scan lens
32
only has optical power in the scan plane so the f-theta scan lens
32
will not effect the divergence of the light beam
24
in the cross-scan plane. The light beam
24
will diverge until the wobble correction lens
40
which then focuses the light beam
24
at a convergence angle
50
in the cross-scan plane to a point
52
on the scan line
42
on the photoreceptor
20
. The point
52
at the photoreceptor
20
is at the focal length
54
from the wobble correction optical element
40
, i.e., the distance from the optical element
40
to the point
52
. The light beam
24
is at its maximum height
56
in the post-polygon optics
18
at its maximum divergence along the optical path
48
at the wobble correction optical element
40
.
The overall h

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