Coherent light generators – Particular pumping means – Pumping with optical or radiant energy
Reexamination Certificate
1999-06-16
2001-04-24
Scott, Jr., Leon (Department: 2881)
Coherent light generators
Particular pumping means
Pumping with optical or radiant energy
C372S070000, C372S034000
Reexamination Certificate
active
06222870
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to digital printing apparatus and methods, and more particularly to a system for imaging of recording media such as lithographic printing members.
BACKGROUND OF THE INVENTION
It is important, when focusing laser radiation onto many types of recording medium, to maintain satisfactory depth-of-focus—that is, a tolerable deviation from perfect focus on the recording surface. Adequate depth-of-focus is important to construction and use of the imaging apparatus; the smaller the working depth-of-focus, the greater will be the need for fine mechanical adjustments and vulnerability to performance degradation due to the alignment shifts that can accompany normal use. Depth-of-focus is maximized by keeping output beam divergence to a minimum.
Optical efforts to reduce beam divergence also diminish power density, since a lens cannot alter the brightness of the radiation it corrects; a lens can only change the optical path. Thus, optical correction presents an inherent tradeoff between depth-of-focus and power loss. U.S. Pat. No. 5,822,345 discloses an approach that utilizes the divergent output of a semiconductor or diode laser to optically pump a laser crystal, which itself emits laser radiation with substantially less beam divergence but comparable power density; the laser crystal converts divergent incoming radiation into a single-mode output with higher brightness. The output of the laser crystal is focused onto the surface of a recording medium to perform the imaging function.
The arrangements described in the '345 patent employ a separate crystal for each diode pumping source. This is ordinarily necessary due to the nature of laser crystals and their operation. In the absence of optical excitation, resonant cavities formed from these optical-gain crystals are flat-flat monoliths; when optical power is delivered to an end face of such a crystal, however, this and the opposed face bow—an effect called bulk thermal lensing. To obtain a single transverse mode of operation (preferably the lowest-order, fundamental TEM
00
mode), with the output divergence as close as possible to that of a diffraction-limited source, the crystal must be implemented in a design that accounts for bulk thermal lensing.
This phenomenon makes it even more difficult to obtain multiple, independent outputs from a single laser crystal. Even if the energy of each pumping source is confined to a discrete region on one of the crystal faces, the thermal lensing action required for lasing in one region will ordinarily affect the other regions, resulting in mutual interference. This condition is known as “thermal crosstalk.” Accordingly, the current state of the art prescribes the use of a separate crystal for each laser channel, resulting not only in added cost for the crystals and their mounts, but also for separate focusing assemblies.
U.S. Ser No. 09/245,102, filed on Jan. 25, 1999 (the entire disclosure of which is hereby incorporated by reference) describes configurations that permit a single laser crystal to be driven by multiple pumping sources to obtain discrete, collimated outputs without substantial thermal crosstalk.
FIG. 1
illustrates a generalized configuration as disclosed in this earlier-filed application. A recording medium
50
, such as a lithographic plate blank or other graphic-arts construction, is affixed to a support during the imaging process. In the depicted implementation, that support is a cylinder
52
around which recording medium
50
is wrapped, and which rotates as indicated by the arrow. If desired, cylinder
52
may be straightforwardly incorporated into the design of a conventional lithographic press, serving as the plate cylinder of the press. Cylinder
52
is supported in a frame and rotated by a standard electric motor or other conventional means. The angular position of cylinder
52
is monitored by a shaft encoder associated with a detector
55
. The optical components may be mounted in a writing head for movement on a lead screw and guide bar assembly that traverses recording medium
50
as cylinder
52
rotates. Axial movement of the writing head results from rotation of a stepper motor, which turns the lead screw and indexes the writing head after each pass over cylinder
52
.
Imaging radiation, which strikes recording medium
50
so as to effect an imagewise scan, originates with a series of pumping laser diodes
60
, four of which are representatively designated D
1
, D
2
, D
3
, D
4
. The optical components concentrate laser output onto recording medium
50
as small features, resulting in high effective power densities. A controller
65
operates a series of laser drivers collectively indicated at
67
to produce imaging bursts when the outputs of the lasers
60
are directed at appropriate points opposite recording medium
50
.
Controller
65
receives data from two sources. The angular position of cylinder
52
with respect to the laser output is constantly monitored by detector
55
, which provides signals indicative of that position to controller
65
. In addition, an image data source (e.g., a computer)
70
also provides data signals to controller
65
. The image data define points on recording medium
50
where image spots are to be written. Controller
65
, therefore, correlates the instantaneous relative positions of the focused outputs of lasers
60
and recording medium
50
(as reported by detector
55
) with the image data to actuate the appropriate laser drivers at the appropriate times during scan of recording medium
50
. The driver and control circuitry required to implement this scheme is well-known in the scanner and plotter art.
The output of each of the lasers
60
is conducted, by means of an optical fiber
72
1
,
72
2
,
72
3
,
72
4
to an alignment bench
75
that has a series of parallel grooves
77
for receiving the fibers. Bench
75
, which may be fabricated from materials such as metal or silicon, is aligned with a laser crystal to direct the outputs of lasers
60
at appropriate points on the anterior face
80
f
of laser crystal
80
.
It is the emissions of crystal
80
that actually reach the recording medium
50
. A first lenslet array
82
concentrates the outputs of lasers D
1
-D
4
onto crystal
80
, and a second lenslet array
84
concentrates the outputs from crystal
80
onto a focusing lens
85
. The latter lens, in turn, demagnifies the incident beams in order to concentrate them further and draw them closer together on the surface of recording medium
50
. The relationship between the initial pitch or spacing P between beams from crystal
80
and their final spacing on recording medium
50
is given by P
f
=P/D, where P
f
is the final spacing and D is the demagnification ratio of lens
85
. For example, the grooves
77
of bench
75
may be spaced 400 &mgr;m apart, which also determines the pitch P. If the demagnification ratio of lens
85
is 4:1, then the spots will be spaced 100 &mgr;m apart on the surface of recording medium
50
.
To avoid substantial thermal crosstalk, the anterior face of the laser crystal (i.e., the side facing the pumping sources) may be provided with a series of parallel grooves and a pair of spaced-apart metal strips extending across the anterior face of the crystal perpendicular to the grooves. The strips and grooves serve to isolate thermomechanically the regions they define, and are aligned with the pumping sources such that the pumping-source outputs strike the anterior crystal face in the centers of the regions bounded by the strips and the grooves. This arrangement is expensive to manufacture, requiring precision operations to be performed on a single crystal monolith. Furthermore, the crystal must be housed in an appropriate environment to allow contact with the metal strips so that heat may be continuously withdrawn from the crystal.
DESCRIPTION OF THE INVENTION
BRIEF SUMMARY OF THE INVENTION
In accordance with the present invention, the grooves on the laser crystal are eliminated. Instead, separate, individually addressable l
Foster Josh P.
Sousa John Gary
Jr. Leon Scott
Presstek Inc.
Testa Hurwitz & Thibeault LLP
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