WO2000074940A1

WO2000074940A1 - Laser imaging using selective beam deflection

Info

Publication number: WO2000074940A1
Application number: PCT/US2000/015214
Authority: WO
Inventors: John Gary Sousa; Steven Stoltz
Original assignee: Presstek, Inc.
Priority date: 1999-06-03
Filing date: 2000-06-01
Publication date: 2000-12-14
Also published as: AU5458900A

Abstract

A laser beam is switched between imaging and non-imaging states by means of beam steering. In one orientation, the beam is prevented from reaching a recording construction; in a second orientation, the beam travels through an optical path (typically through a focusing element) to the surface of the construction to effect imaging thereof. The laser output is itself unaffected; indeed, the laser may remain in an 'on' (lasing) state throughout imaging.

Description

LASER IMAGING USING SELECTIVE BEAM DEFLECTION

FIELD OF THE INVENTION

The present invention relates to imaging with laser devices, and in

particular to the construction and operation of emitter arrays for imaging

purposes.

BACKGROUND OF THE INVENTION

Laser devices are employed to impress a pattern upon, or "image," a

wide variety of graphic-arts recording constructions. The type of laser

employed (i.e., its λ_max, power output, etc.) depends on the nature of the

medium to be imaged. Exposure of a light-sensitive photographic emulsion

requires substantially less power than, for example, media requiring ablation

of a recording layer (in ablation-type constructions) or transfer of donor

material to an acceptor sheet (in transfer-type constructions). The cost of

a laser is directly— and not always linearly— dependent on its output power

capabilities. For this reason, in imaging processes necessitating relatively

high energy fluxes, measures are taken to maximize utilization of available

laser power. These measures have permitted the use of small, relatively inexpensive diode-type lasers in applications that once required more

sophisticated and powerful equipment.

One practical power-conservation measure is maintaining satisfactory

depth-of-focus— that is, a tolerable deviation from perfect focus on the

recording surface. Any such deviation reduces the energy flux density of

the beam on the recording surface. Accordingly, the smaller the working

depth-of-focus, the greater will be the performance impact of the alignment

shifts that can accompany normal use. Systems with large depths-of-focus

ensure that minor changes or imperfections in alignment will not

substantially reduce the useful laser flux density. Depth-of-focus is

maximized by keeping output beam divergence to a minimum.

Unfortunately, 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 can adversely effect the very property it is ultimately intended to

improve. U.S. Patent 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. A substantial impediment to the widespread use of laser crystals in

commercial imaging systems is the relatively gradual change in their output

intensities in response to corresponding changes in the intensity of the

pump laser. Practical imaging equipment requires lasers that respond

nearly instantaneously to high-frequency square-wave control pulses so

that imaging dots appear as sharp, discrete, and ordinarily round shapes. In

printing applications, dots must also be printed, or recording space left

blank, at very closely spaced distances to achieve typical print resolutions.

However, since the minimum duration and intensity of peak power level in

an imaging pulse is a function of available pump power and the carrier

dynamics of the lasing medium, it may be difficult to deliver sufficient

overall energy to produce an image dot during the short time intervals

realistically required in commercial imaging systems. Moreover, the time

required for the laser to undergo radiative transitions between lasing and

non-lasing states results in misshapen dots that will not be placed properly

on the recording medium: during the imaging process the recording

medium and the laser ordinarily undergo relative movement, so delays

between a pump pulse and laser output result in translational offsets

between target and actual dot locations.

U.S. Patent No. 5,822,345 discloses a technique for overcoming this

undesirable response characteristic. The pumping source is continuously

maintained at a bias power level, thereby keeping the crystal at a

continuous, baseline power level just short of lasing or imaging; and then selectively and intermittently increasing power to the pumping source to

cause the crystal to lase (or lase at higher power) in an imagewise pattern.

The result is a very sharp crystal response that closely follows the selective

pump power increases— that is, imaging pulses— vary closely in terms of

pump response profiles. The crystal and pumping source are chosen such

that the baseline power level is insufficient to cause imaging of the

recording medium, while the intermittent "imaging" power level does cause

imaging. (As used herein, the term "imaging" refers generally to

permanent alteration to a recording construction, e.g., changing the affinity

characteristics of a lithographic printing plate).

This technique, while well-suited to many applications, nonetheless

presumes a single device that can be individually controlled. In some

applications, it may prove desirable to split a single laser beam into multiple

components that are themselves separately controlled. A need exists,

therefore, for a switching technique that does not rely on adjustment or

modulation of the source-laser power yet offers very rapid, sharp

transitions between imaging and non-imaging states.

DESCRIPTION OF THE INVENTION

Brief Summary of the Invention

The present invention steers, rather than modifies, a laser beam to

switch between imaging and non-imaging states. In one orientation, the

beam is prevented from reaching a recording construction; in a second

orientation, the beam travels through an optical path (typically through a

focusing element) to the surface of the construction to effect imaging

thereof. The laser output is itself unaffected; indeed, the laser may remain

in an "on" (lasing) state throughout imaging.

Beam steering is preferably accomplished electrooptically, resulting in

very fast switching times between imaging and non-imaging states.

Although prior efforts have been made to utilize beam steering for imaging

purposes (see, e.g., U.S. Patent No. 4,61 4,408), these have tended to

involve graduated placement and activation of the beam rather than, as in

the present invention, selective binary switching of an unmodulated beam.

The approach of the invention is useful in connection with pumped laser

sources that emit with low beam divergence, thereby maximizing depth-of-

focus.

Accordingly, in a first aspect, the invention comprises a method of

imaging a recording construction in which one or more beams of imaging

radiation each pass through an optical path leading to the recording construction. Typical commercial applications will utilize plural beams, and

these (or, more accurately, the optical paths through which they pass in

the "on" state) are scanned over the construction, which is exposed to the

beams in a pattern representing an image. Imagewise exposure is

accomplished by selectively deflecting the beams from their optical paths

during the course of the scan, the undeflected beams being substantially

prevented from reaching the construction.

In another aspect, the invention comprises an imaging apparatus for

effecting the method. In addition to means defining the optical path and

means for effecting the scan, the apparatus may comprise a selectably

actuable beam steerer through which the beam passes, and which deflects

the beam into the optical path (and away from a beam stop disposed

adjacent thereto). A controller, responsive to signals representing the

pattern to be imaged, may selectably actuate one or more beam steerers

during the scan to cause the construction to be exposed to the beam or

beams in accordance with the pattern.

Brief Description of the Drawings

The foregoing discussion will be understood more readily from the

following detailed description of the invention, when taken in conjunction

with the accompanying drawings, in which: FIG. 1 is a plan schematic of a system embodying the present

invention;

FIG. 2 is a side elevation of an electrooptic element suitable for use

with the present invention; and

FIGS. 3A, 3B, 3C are isometric, plan and back end views,

respectively, of a single electrooptic element with multiple

independent channels; and

FIGS. 4A, 4B are back end and side elevational views, respectively,

of a beam stop useful in connection with the electrooptic element

shown in FIGS. 3A, 3B, 3C.

Detailed Description of the Preferred Embodiments

Refer first to FIG. 1 , which schematically illustrates the basic

components of the invention. 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. In the case of lithography,

cylinder 52 may be straightforwardly incorporated into the design of a

conventional lithographic press, serving as the plate cylinder of the press. (Numerous alternatives to this configuration are also possible. For example,

recording medium 50 can constitute the exterior surface of cylinder 52, or,

as described in U.S. Patent No. 5,385,092 (hereby incorporated by

reference), recording medium 50 can be supported on the interior of a

curved platen, or on a flatbed arrangement.)

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 of the invention, described hereinbelow, 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 pumping laser diode 60. The laser

diode is operated by a driver circuit 62, which is itself governed by a

system controller 64. This component is described in greater detail below.

The output of laser diode 60 is directed through a focusing lens 65 onto

the anterior face 70 of a laser crystal 70. A variety of laser crystals can

serve in this context so long as they lase efficiently at the desired imaging

wavelength and produce a collimated output. Preferred crystals are doped

with a rare earth element, generally neodymium (Nd), and include Nd:YVO₄, Nd:YLF and Nd:YAG crystals. It should be understood, however, that

advantageous results may be obtainable with other laser crystals.

The end faces of crystal 70 have mirror coatings that limit the entry

of radiation other than that originating from the pumping source, and trap

the output radiation. In this way, the two coatings facilitate the internal

reflections characteristic of laser amplification while preventing the entry of

spurious radiation. For example, a laser crystal 70 may receive input

radiation from a laser diode 60 at 808 nm, and produce imaging output

radiation at 1 064 nm. In that case, the pump face of crystal 70 may be

provided with an HR/HT coating that produces > 99.8% reflection of 1 064

nm (output) radiation and 95% transmission of 808 nm (input) radiation

from laser diode 60. On the exit face, a PR coating that produces 95%

(±0.5%) reflection of 1064 nm radiation and > 95% transmission of 808

nm radiation may be provided.

The face and exit plane of crystal 70 may be metallized and bonded

to a heat sink, preferably one having a coefficient of thermal expansion

close to that of the crystal itself. For this purpose, a heat sink fabricated

from a metal oxide (e.g., AI₂O₃ or BeO), aluminum nitride, diamond,

sapphire, or a YAG crystal.

The emission from crystal 70 strikes a diffractive beam splitter optic

75 that divides the beam into a plurality of parallel beams having reduced

(but equal) intensities. Diffractive beam splitters, which utilize precisely etched microscopic structures to create a pattern of output beams, are

typically fabricated from fused silica; devices suitable for use herein are

available commercially (e.g., from Digital Optics Corp., Charlotte, NC) . For

purposes of presentation, five outputs are illustrated, but it should be

recognized that more or fewer outputs can be obtained depending on the

application; of course, the more outputs that are obtained from a single

pattern generator, the greater will be the necessary input power.

One of the outputs from beam splitter 75 is received by a

photodetector 77, which generates a signal indicative of the intensity of

the received beam. A feedback circuit 80 is responsive to the signal

generated by detector 77, and modulates the power applied to laser diode

60 to maintain output stability thereof. This arrangement facilitates

compensation for power output variations from laser diode 60 that arise,

for example, due to current fluctuations, temperature and performance

variations affecting the laser diode or crystal 70.

The remaining outputs from beam splitter 75 each encounter an

electrooptic element 81 ., ... 81₄. As detailed below, each of the

electrooptic elements (which may be fabricated separately or as

independently addressable regions of a single monolith) can allow the

incident beam to continue undisturbed away from the optical path

terminating at cylinder 52, or can instead deflect the beam into that path to

image the recording medium. The electrooptic elements do not significantly alter the numerical aperture (NA) or beam divergence of the diffraction-

limited beam emitted from laser crystal 70.

A focusing lens 85 demagnifies the beams from electrooptic

elements 82 in order to concentrate them 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. Preferably, lens 85

is a bi-aspheric lens (see, e.g., U.S. Patent No. 5,764,274).

As noted above, controller 64 operates laser driver 62 and thereby

governs the operation of laser diode 60. In addition, controller 64 operates

electrooptic elements 82 to direct laser energy onto recording medium 50

when the optical paths leading to the medium reach appropriate points

thereon. To effect this function, controller 64 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 64. In addition, an image data

source (e.g., a computer) 90 also provides data signals to controller 64.

The image data define points on recording medium 50 where image spots

are to be written. Controller 64, therefore, correlates the instantaneous

relative positions of the focused outputs of beam splitter 75 and recording

medium 50 (as reported by detector 55) with the image data to actuate the appropriate electrooptic elements 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;

suitable designs are described in the '092 patent and in U.S. Patent No.

5, 1 74,205, both commonly owned with the present application and hereby

incorporated by reference.

The operation of electrooptic elements 82 and the mechanics of

beam deflection are best understood from FIG. 2. The beam from crystal

70 enters the anterior or input face 1 00, of the depicted electrooptic

element 82, and exits from the posterior or output face 1 00₀. Output face

1 00₀ is beveled at an angle α, which is determined as set forth below.

When the beam passes through element 82 without deflection, it exits at

an angle due both to the beveled output face and to the difference in

refractive index between air and the material of element 82. The

undeflected beam strikes a beam stop 1 05, which prevents it from reaching

recording medium 50 (see FIG. 1 ) . A suitable beam stop will fully absorb

or harmlessly deflect all incident beam energy.

When the beam is deflected upward by an angle β, however, its exit

from output face 100_o is substantially parallel to its entry into face 1 00,.

Accordingly, the angle α is selected in accordance with the deflection β

(itself determined as set forth below) and Snell's Law, which dictates that

the angle of refraction θ₂ (from the dashed normal to face 1 00₀) is related to the angle of incidence ^ by the formula n, sinθ_ϊ = n₂ sinθ₂, where n,

and n₂ are the refractive indices of element 82 and air, respectively. The

angle α is equal to 90°- θ₂.

The deflection angle β must be sufficiently large that the undeflected

beam is substantially prevented, by beam stop 105, from reaching

recording medium 50. By "substantially prevented" is meant that enough

of the beam energy is diverted that any portion of the beam actually

reaching recording medium 50 is insufficient to cause an imaging transition

(e.g., in the case of an ablation construction, to cause ablation of the

imaging layer). In general, it is preferred to divert the beam so that its

entire working diameter, plus a safety margin (of, for example, 4-8 μm),

strikes beam stop 1 05. Accordingly, the required angular deflection

depends on the working diameter of the beam and the available distance

between output face 1 00₀ and beam stop 1 05 (since the greater this

distance, the more gradual the rise angle of the exiting beam may be); in

general, to minimize power losses and to conserve working space, that

distance is kept as short as possible. In a typical working environment,

using a beam having a 1 60 μm diameter, a deflection angle β equal to

about 2° is found to be adequate.

Beam deflection is caused by the response of electrooptic element

82 to an electric field, which causes its refractive index to change in the

region bounded by the field. Consequently, in a region sandwiched between congruent electrodes disposed on opposite faces of element 82,

the refractive index of the defined region (relative to the surrounding

unaffected region of electrooptic element 82) will change by an amount

dictated by the potential difference between the electrodes. In this way,

optical elements with controllable characteristics can be introduced

electrically into element 82. A field created between triangular electrodes

disposed on opposite faces of element 82 is prismatic in spatial extent and,

in fact, acts as a prism, deflecting an incident beam by an angle dictated by

the shape of the electrodes and the magnitude of the electric field.

Although a single pair of oppositely disposed triangular electrodes

would suffice to cause some deflection, the more substantial deflections

required herein require relatively long interactions through element 82,

resulting in a minimum element length L; for example, to achieve a 2 °

deflection of a 1 60 μm beam, an interaction length L of about 1 5 mm is

needed. As a result, more than a single pair of electrodes is ordinarily

required for the present application. FIG. 2 illustrates a series of six

electrodes 1 1 5, 1 20, arranged in an alternating fashion along the depicted

side wall 1 25. An identical series of electrodes, not shown in the figure, is

disposed on the opposite side wall at equivalent opposed locations. To

achieve beam deflection, electrodes 1 20 are grounded, while a voltage V is

applied to electrodes 1 1 5; on the opposite wall, the electrodes facing

electrodes 1 20 receive the voltage V, while those facing electrodes 1 1 5 are

grounded. As a result of the applied voltage pattern, the electric field in element 82 changes polarity between regions defined by electrodes 1 1 5

and regions defined by electrodes 1 20. The change in the electric field

causes a corresponding change to the refractive index of the element 82,

and the beam from crystal 75 experiences a refraction at each interface

between regions.

During the course of a scan, controller 64 (see FIG. 1 ) applies the

potential difference V to the various electrodes 1 1 5, 1 20 (and their

opposed complements) as described above each time the imaging path I

comes into adjacency with a point on recording medium 50 to be exposed.

It is preferred to deflect the beam into the imaging path I rather than away

from it because of the tendency for unimaged regions (background) to

exceed, in area, the imaged regions in most graphic-arts applications;

consequently, this arrangement minimizes the overall time the beam is

deflected, which results in more reliable operation.

In an alternative arrangement, described in U.S. Patent No.

5,31 7,446, a single pair of rectangular electrodes is used and the prismatic

regions created by permanent modification to the electrooptic material

itself; in particular, the refractive index of the bulk material is electrically

polarized in opposite directions (i.e., "reverse poled") within the prismatic

regions defined, in FIG. 2, by the oppositely disposed electrodes 1 1 5, 1 20.

As a result, each set of triangular electrodes is replaced by a rectangular

electrode covering the reverse-poled regions. When a voltage is applied to one such electrode and the other electrode is grounded, the result is the

same as applying the alternating voltages as shown in FIG. 2.

As is well-known, numerous materials can serve as electrooptic

elements (see, e.g., U.S. Patent No. 5,714,240, the disclosure of which is

hereby incorporated by reference); for the present application, lithium

niobate (LiNbO₃) or lithium tantalate (LiTaO₃) is preferred.

FIG. 1 shows a series of independent electrooptic elements 82, each

addressed independently by controller 64. It is possible, however, to utilize

a single electrooptic monolith with multiple, independently addressable

channels. Such a device, indicated generally at 1 50, is shown in FIGS. 3A-

3C. The top and bottom surfaces 1 55f, 1 55b, respectively, are provided

with four opposed series of triangular electrodes arranged in an alternating

fashion, each pair of opposed series defining a separate beam channel

1 60,, 1 60₂, 1 60₃, 1 60₄. Shown in FIGS. 3A, 3B are the top-surface

electrode series 1 65_t1, 1 65_t2, 1 65_t3, 1 65_t4; as indicated in FIG. 3C,

congruent, opposed series of electrodes 1 65_bl, 1 65_b2, 1 65_b3, 1 65_b4 are

disposed on the bottom surface 1 556. A separately controlled voltage is

applied to alternating electrodes in each electrode series. Thus, a voltage

source V is connected to alternate electrodes of series 1 65_tl , while the

remaining electrodes are grounded; and the voltage source V. is likewise

connected to electrodes on the bottom surface opposite the grounded

electrodes of series 1 65_tl . Actuation of any of the voltage sources V_η , V₂, V₃, V₄ causes deflection of the associated beam in channel 1 60, , 1 60₂,

1 60₃, 1 60₄. Element 1 50 is shown configured for lateral deflection (rather

than the vertical action shown in FIG. 2); accordingly, the front face 1 55

of element 1 50 is beveled laterally.

As shown by the dashed lines in FIG. 3C, actuating opposed

electrode series creates an electric field between the surfaces 1 55f, 1 55b.

It is important that these fields do not interact with each other in order to

permit each beam to be addressed independently. Accordingly, the series

of electrodes must be spaced adequately; an interseries spacing d of 400

μm or greater is sufficient.

Once again, each series of alternating triangular electrodes may be

replaced by a single rectangular electrode by reverse poling the electrooptic

material within the prismatic regions defined by the opposed triangular

electrodes illustrated in the figures.

With reference to FIG. 3A, the field created by actuating a voltage

source deflects the associated beam. When deflected, the beam exits face

1 55/" along the optical path of the associated channel 1 60, striking the

recording medium. The undeflected beam exits at a lateral angle β to the

optical path, encountering a beam stop such as that illustrated in FIGS. 4A,

4B. The depicted beam stop 200 comprises a series of four tilted, raised

fingers 205^ 205₂, 205₃, 205₄. When undeflected, a beam from channel

1 60, , 1 60₂, 1 60₃, 1 60₄ (see FIG. 3A) strikes the associated finger 205 and is thereby substantially prevented from reaching the recording medium;

otherwise, the beam passes unobstructed to the side of the associated

finger 205. With particular reference to FIG. 4B, a beam striking one of the

fingers 205 is reflected to a back wall 21 0 of structure 200 due to the tilt

of fingers 205; a projecting lip 21 5 prevents any component of the

reflection from back wall 21 0 from reaching the incoming beam above.

The beam energy is substantially dissipated when it reaches the interior

floor 220 of beam stop 200. All interior surfaces of beam stop 200 are

blackened for maximum energy absorption. Moreover, a pair of coolant

conduits 230, 232 conduct a flow of coolant fluid through the structure

200, thereby removing absorbed energy on a continuous basis. Beam stop

200 is preferably fabricated from metal such as steel or aluminum.

It will therefore be seen that we have developed new and useful

approaches to the design and operation of multiple-beam, diode-pumped

laser systems applicable to a variety of digital-imaging environments. The

terms and expressions employed herein are used as terms of description

and not of limitation, and there is no intention, in the use of such terms and

expressions, of excluding any equivalents of the features shown and

described or portions thereof, but it is recognized that various modifications

are possible within the scope of the invention claimed.

What is claimed is:

Claims

1 . A method of imaging a recording construction, the method comprising

the steps of:

a. conducting at least one beam of imaging radiation through an

optical path leading to the recording construction; and

b. scanning the optical path over the construction and exposing the

construction to the beam in a pattern representing an image by

selectively steering the beam into and away from the optical path

during the course of the scan, the beam, when oriented away

from the optical path, being substantially prevented from reaching

the construction.

2. The method of claim 1 wherein the steering is accomplished

electrooptically, the beam being deflected into the optical path and, when

undeflected, being oriented away from the optical path.

3. The method of claim 2 wherein the beam passes through a selectably

actuable beam steerer, a beam stop for substantially preventing the beam

from reaching the construction being disposed adjacent the optical path,

the undeflected beam encountering the beam stop.

4. The method of claim 1 wherein multiple beams are simultaneously

conducted through optical paths leading to the recording construction, the

beams being collectively scanned over the construction and individually

selectively steered into their respective optical paths during the course of

the scan.

5. The method of claim 4 wherein each of the beams passes through a

separate beam steerer.

6. The method of claim 4 wherein a plurality of beams pass through a

single beam steerer providing an independently addressable channel for

each beam.

7. Imaging apparatus comprising:

a. means for supporting a recording construction responsive to

imaging radiation;

b. means defining an optical path from a source of imaging radiation

to the recording construction;

c. means for effecting a scan of the optical path over the recording

construction;

d. a beam stop, located off the optical path, for substantially

preventing the beam from reaching the construction; e. a beam steerer for steering the beam to the beam stop or into the

optical path; and

f. a controller for selectably actuating the beam steerer during the

scan to cause the construction to be exposed to the beam in

accordance with a pattern.

8. The apparatus of claim 7 wherein the beam steerer comprises:

a. an electrooptic element through which the beam passes;

b. at least one pair of oppositely disposed electrodes associated with

the electrooptic element; and

c. a power source for selectably applying a voltage between the

electrodes to cause the beam to deflect as it passes through the

element.

9. The apparatus of claim 8 wherein each pair of electrodes defines a prism

through the electrooptic element.

1 0. The apparatus of claim 8 wherein a series of oppositely polarized

prismatic regions are defined through the element between the electrodes.

1 1 . The apparatus of claim 8 wherein the beam is deflected into the optical

path and, when undeflected, encounters the beam stop.

1 2. The apparatus of claim 8 wherein the electrooptic element has a

beveled output face.

1 3. The apparatus of claim 7 wherein the source of imaging radiation

comprises a series of beams, the optical-path defining means defining

separate optical paths from the beams to discrete spaced-apart locations on

the construction, the scanning means causing the beams to be collectively

scanned over the construction, and further comprising a separate beam

steerer for each beam, the controller operating each of the beam steerers

individually to selectively deflect the associated beam into its respective

optical path during the course of the scan.

1 4. The apparatus of claim 7 wherein the source of imaging radiation

comprises a series of beams, the optical-path defining means defining

separate optical paths from the beams to discrete spaced-apart locations on

the construction, the scanning means causing the beams to be collectively

scanned over the construction, a single beam steerer being operable to

selectively and independently steer each of the beams into its respective

optical path during the course of the scan.

1 5. The apparatus of claim 1 2 wherein the beam steerer comprises:

a. an electrooptic element having a rear face through which the

beams enter; b. a congruent series of electrodes disposed on opposed faces of the

electrooptic element, the series defining linear channels through

the electrooptic element, each of the beams passing through a

channel; and

c. a power source for selectably applying a voltage between various

of the electrodes to cause the beams to deflect as they pass

through the element.

1 6. The apparatus of claim 1 5 wherein the congruent series of electrodes

comprises a plurality of linear arrangements of triangular electrodes

disposed congruently on the opposed faces and defining prisms through the

electrooptic element.

1 7. The apparatus of claim 1 5 wherein the congruent series of electrodes

comprises a plurality of rectangular electrodes disposed congruently on the

opposed faces, the electrooptic element being reverse poled to define

oppositely polarized prismatic regions between the electrodes.

1 8. The apparatus of claim 1 4 wherein the electrooptic element has a front

face through which the beams emerge, the front face being beveled.

1 9. The apparatus of claim 7 wherein the source of imaging radiation

comprises: a. a laser; and

b. a diffractive beam splitter.

20. The apparatus of claim 1 9 wherein (i) the laser has an output and (ii)

the source of imaging radiation further comprises a feedback beam in

addition to the series of beams, the apparatus further comprising:

a. a detector for receiving the feedback beam; and

b. a feedback controller responsive to the detector for adjusting the

output of the laser.