U.S. patent application number 15/635909 was filed with the patent office on 2017-10-19 for single-emitter line beam system.
This patent application is currently assigned to nLIGHT, Inc.. The applicant listed for this patent is nLIGHT, Inc.. Invention is credited to Scott R. Karlsen, David C. Senders.
Application Number | 20170299875 15/635909 |
Document ID | / |
Family ID | 53730624 |
Filed Date | 2017-10-19 |
United States Patent
Application |
20170299875 |
Kind Code |
A1 |
Karlsen; Scott R. ; et
al. |
October 19, 2017 |
SINGLE-EMITTER LINE BEAM SYSTEM
Abstract
A line beam system includes a single-emitter light engine
including a plurality of separately spaced single-emitter diode
lasers, each emitter configured to emit a diode laser beam. Beam
spacing optics are optically coupled to the single-emitter light
engine and situated to provide propagation axes of the diode laser
beams in a close-packed parallel configuration. A light pipe having
a longitudinal axis is situated to provide an output beam with a
homogenized intensity profile across one or more axes by receiving
a close-packed, combined beam and reflecting the beam within the
light pipe. Coherence reduction is produced by diffraction of a
close-packed combined beam or by propagation in the light pipe.
Inventors: |
Karlsen; Scott R.; (Battle
Ground, WA) ; Senders; David C.; (Portland,
OR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
nLIGHT, Inc. |
Vancouver |
WA |
US |
|
|
Assignee: |
nLIGHT, Inc.
Vancouver
WA
|
Family ID: |
53730624 |
Appl. No.: |
15/635909 |
Filed: |
June 28, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
14614194 |
Feb 4, 2015 |
9709810 |
|
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15635909 |
|
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|
|
61935962 |
Feb 5, 2014 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B 27/48 20130101;
G02B 27/0994 20130101; G02B 5/04 20130101; G02B 6/4296 20130101;
G02B 27/0944 20130101; G02B 27/283 20130101; G02B 5/1861 20130101;
G02B 6/262 20130101; G02B 27/0927 20130101; G02B 5/08 20130101;
G02B 27/108 20130101; G02B 27/0905 20130101 |
International
Class: |
G02B 27/09 20060101
G02B027/09; G02B 27/09 20060101 G02B027/09; G02B 27/09 20060101
G02B027/09; G02B 27/28 20060101 G02B027/28; G02B 5/08 20060101
G02B005/08 |
Claims
1. A system, comprising: a plurality of spaced-apart single-emitter
diode lasers, each configured to emit a corresponding laser beam; a
coherence-reducing optical system situated to receive the laser
beams and to establish an optical path length difference among the
laser beams so as to produce a reduced coherence beam, the
coherence-reducing optical system including a diffraction grating
situated to receive the laser beams propagating along parallel beam
axes and form diffracted laser beams that propagate at a
diffraction angle along respective parallel diffracted beam axes so
as to provide an optical path length difference among the
diffracted laser beams; and a line beam optical system situated to
receive the reduced coherence beam and to direct a line beam to a
target.
2. The system of claim 1, wherein the coherence-reducing optical
system includes a light guide situated to receive the diffracted
laser beams at a light guide input aperture; wherein the light
guide is situated so as to provide an optical path length
difference among the laser beams based on multiple reflections of
the diffracted laser beams in the light guide.
3. The system of claim 2, wherein the light guide has a
longitudinal axis that is situated to provide selected respective
asymmetric incidence angles with respect to opposite marginal
propagation beam axes associated with the diffracted beams at the
light guide input aperture so as to asymmetrically receive the
diffracted beams; wherein the light guide is situated so as to
provide an optical path length difference among the laser beams
based on the asymmetric incidence angles and subsequent propagation
through the light guide.
4. The system of claim 3, wherein the light guide is situated so
that an angular beam diameter of the diffracted laser beams
received by the light guide is at most the larger of the asymmetric
incidence angles to the light guide.
5. The system of claim 3, wherein the diffraction angle and the
asymmetric incidence angles are selected in relation to each other
so as to increase a total optical path length difference among the
laser beams.
6. The system of claim 1, further comprising a beam spacing optical
system situated to receive the laser beams and to direct the laser
beams along parallel close-packed axes that are more closely spaced
than associated emitted beam axes so as to form a close-packed
combined beam, wherein the diffraction grating is situated to
receive the close-packed combined beam.
7. The system of claim 6, wherein the beam spacing optical system
includes at least one rhomboidal prism situated to direct at least
one of the laser beams along a corresponding close-packed axis.
8. The system of claim 7, wherein the beam spacing optical system
is situated to direct at least one of the laser beams along an axis
that is perpendicular to an associated close-packed axis.
9. The system of claim 2, further comprising a cylindrical mirror
situated to receive the diffracted beams and to convergently direct
the diffracted beams to the light guide.
10. The system of claim 9, further comprising one or more optical
elements situated to optically couple the convergently directed
diffracted beams into the light guide.
11. The system of claim 1, wherein the diffraction grating is a
reflective diffraction grating.
12. The system of claim 11, further comprising a beam stop situated
to receive a beam portion other than the diffracted laser
beams.
13. The system of claim 2, wherein the light guide has a
longitudinal axis that is symmetrically situated with respect to
marginal beam propagation axes associated with the diffracted laser
beams.
14. A method, comprising: directing a plurality of laser beams
having respective beam axes so as to form a combined beam;
diffracting the combined beam with a diffraction grating so as to
form a diffracted combined beam at a diffraction angle with the
beam axes of the laser beams parallel to each other as received at
the diffraction grating and parallel to each other as diffracted by
the diffraction grating to provide a first optical path length
difference among the laser beams associated with the diffraction
angle; coupling the diffracted combined beam into a light pipe with
respect to a longitudinal axis of the light pipe so as to
homogenize the intensity of the diffracted combined beam across at
least one axis that is orthogonal to the longitudinal axis and to
produce an output beam and to provide a second optical path length
difference among the laser beams based on propagation of the laser
beams in the light pipe, wherein a total optical path length
difference is greater than either the first optical path length
difference or the second optical path length difference; and
forming a line beam at a target based on the output beam of the
light pipe.
15. The method of claim 14, wherein the diffracted combined beam is
coupled into the light pipe so that opposite marginal beam axes of
the laser beams of the diffracted combined beam have asymmetrical
incidence angles with respect to the longitudinal axis of the light
pipe at an entrance aperture so as to provide a third optical path
length difference among the laser beams.
16. The method of claim 15, wherein the beam axes with a longest
path length delay associated with the diffraction grating are
directed into the light pipe at a largest angle of the asymmetric
incidence angles.
17. The method of claim 15, wherein the light guide is situated so
that an angular beam diameter of the received diffracted combined
beam is at most the larger of the asymmetric incidence angles to
the light guide.
18. The method of claim 14, further comprising directing the laser
beams through a beam spacing optical system so as to reduce a
spacing between at least two of the beam axes so as to define
close-packed axes that are more closely spaced than the beam axes
as emitted from the single-emitter diode lasers and so as to form a
close-packed combined beam, wherein the diffraction grating is
situated to receive the close-packed combined beam.
19. The method of claim 18, wherein the beam spacing optical system
includes at least one rhomboidal prism situated to direct at least
one of the laser beams along a corresponding one of the
close-packed axes.
20. The method of claim 19, wherein the beam spacing optical system
is situated to direct at least one of the laser beams along an axis
that is perpendicular to an associated close-packed axis.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of U.S. application Ser.
No. 14/614,194, filed Feb. 4, 2015, which claims the benefit of
U.S. Provisional Application No. 61/935,962, filed Feb. 5, 2014,
both of which are incorporated herein by reference in their
entirety.
FIELD
[0002] The disclosure pertains to high power laser line beam
systems.
BACKGROUND
[0003] Conventional line beam generators are typically based on
stacks of microchannel cooled laser diode bars and have numerous
reliability problems. Some problems include erosion and leaking of
the microchannel coolers, leakage around O-rings or other seals,
and movement or misalignment over time of fast-axis collimation
(FAC) optics. FIG. 1 shows a conventional bar-based line generator
100 that includes fourteen diode laser bars 112, each having
sixty-three emitters, and that emits a total of eight hundred
eighty-two diode laser beams. Additionally, fast-axis collimation
lenses, an interleaver, a polarizer and mirror monolithic assembly
116, and other line beam optics, including a homogenizing light
pipe 118 which receives incident beams directly aligned with a
longitudinal axis thereof, are required to form a laser line output
beam 119. While high reliability single emitter laser sources,
which are cooled with standard cold plates, can be used in high
power line beam systems instead of microchannel cooled bars, the
possibility of such use is typically dismissed as an alternative
since the reduction in the total number of laser sources increases
the probability of non-uniformity in the line beam due to
self-interference (or coherence) effects between sources.
Accordingly, a need remains for innovation directed to solving the
latent reliability problems as well as other problems in the high
power line beam systems.
SUMMARY
[0004] In some examples, line beam systems comprise a plurality of
spaced-apart single-emitter diode lasers, each configured to emit a
corresponding laser beam along a respective emitted beam axis. A
coherence-reducing optical system is situated to receive the laser
beams and establish an optical path length difference among the
optical beams so as to produce a reduced coherence beam. A line
beam optical system receives the reduced coherence beam and directs
a line beam towards a target. According to some examples, the
coherence-reducing optical system includes a light guide having a
longitudinal axis, the light guide producing the coherence-reduced
output beam based on path length differences in the light guide,
wherein the light guide is situated with respect to the light guide
so as to asymmetrically receive the emitted beams. In some
examples, the light guide is situated so that an angular beam
diameter of the received emitted beams is about 1/2 an angle of
incidence to the light guide. According to other embodiments, the
coherence-reducing optical system includes a diffraction grating
situated to receive the emitted beams and produce the reduced
coherence beam as a diffracted beam have a beam path difference
associated with diffraction angle. In some other embodiments, a
beam spacing optical system receives the emitted beams and directs
the emitted beams along close-packed axes that are more closely
spaced that the emitted beam axes as a close-packed, combined beam,
wherein the diffraction grating receives the close-packed combined
beam and produces the reduced coherence beam based on the
close-packed, combined beam. According to additional examples, the
beam spacing optical system includes at least one rhomboidal prism
that directs at least one emitted beam along a close-packed axis.
In typical examples, the beam spacing optical system includes as
least one beam splitter situated to produce at least two beams from
at least one emitted beam, and direct the two beams along
respective close-packed axes. In some embodiments, the at least one
beam splitter is a polarizing beam splitter that produces the at
least two beams in orthogonal states of polarization. According to
other examples, a cylindrical mirror is situated to receive the
diffracted beam and direct the diffracted beam into the light
guide.
[0005] Methods comprise collimating a plurality of single-emitter
diode laser beams and directing the collimated single-emitter diode
laser beams to produce a close-packed, combined beam. The
close-packed, combined beam is directed into a light pipe so as to
reduce beam spatial coherence and produce a coherence-reduced
output beam. The intensity of the coherence-reduced output beam is
homogenized across at least one axis that is orthogonal to a
direction of propagation of the coherence-reduced output beam and a
line beam is formed based on the coherence-reduced output beam.
Typically, the close-packed, combined beam is directed
asymmetrically into the light pipe. In other examples, the
close-packed, combined beam is diffracted so as to form a
diffracted beam so as to reduce beam coherence and the diffracted
beam is directed into the light pipe. In still further examples,
emitted beams with a longest path length delay associated with the
diffraction grating are directed into the light pipe at a largest
angle with respect to a longitudinal axis of the light pipe.
[0006] Line beam systems comprise at least two single emitter diode
laser modules that include respective pluralities of diode lasers
situated along a first axis so that the diode lasers emit beams
parallel to a second axis, wherein the at least two single emitter
laser diode modules are displaced with respect to each other along
a third axis, wherein the first, second, and the third axes are
substantially mutually orthogonal. A beam-spacing optical system
receives the emitted beams and forms a close-packed combined beam,
the beam-spacing optical system including at least one rhomboid
prism that establishes a close-packed beam propagation axis and a
beam splitter that receives at least one of the emitted beams and
produces at least two associated close-packed beams. A diffraction
grating and a light pipe are situated to receive the close-packed,
combined beam and produce a coherence-reduced beam. A beam steering
optical includes a first cylindrical lens situated to receive and
converge the coherence-reduced beam. A fold mirror is situated to
receive the converged, coherence-reduced diode beam and a second
cylindrical lens is situated to receive the converged,
coherence-reduced diode beam from the fold mirror. A polarizing
mirror receives the coherence-reduced, combined beam and reflects
the coherence-reduced, combined beam in first state of
polarization. A focus optical system is situated to receive the
reflected, coherence-reduced, combined beam in the first state of
polarization line and direct a line beam toward a target. In some
examples, the polarizing mirror is situated to transmit portions of
the line beam from the target to a beam dump. According to other
embodiments, the beam splitter of the beam-spacing optical system
is a polarizing beam splitter that produces output beams in
orthogonal states of polarization.
[0007] The foregoing and other objects, features, and advantages of
the disclosed technology will become more apparent from the
following detailed description, which proceeds with reference to
the accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 shows a side-view of a conventional, laser bar-based
diode laser line generator.
[0009] FIG. 2 shows a side-view of a representative laser line
generator based on a plurality of single-laser diode emitters.
[0010] FIG. 3 shows a side-view of another representative
single-emitter based laser line generator.
[0011] FIG. 4 shows an expanded view of a portion of the embodiment
shown in FIG. 2.
[0012] FIG. 5 shows an expanded view of a portion of the embodiment
shown in FIG. 3.
[0013] FIGS. 6-8 show diagrams depicting path-length
variations.
[0014] FIGS. 9 and 10 depict example coherence reduction
methods.
[0015] FIGS. 11A-11B illustrate prism assemblies for combining
beams from individual laser diodes.
[0016] FIG. 12 illustrates asymmetric incidence of a combined laser
beam containing a plurality of individual laser beams to a light
pipe.
DETAILED DESCRIPTION
[0017] As used in this application and in the claims, the singular
forms "a," "an," and "the" include the plural forms unless the
context clearly dictates otherwise. Additionally, the term
"includes" means "comprises." Further, the term "coupled" does not
exclude the presence of intermediate elements between the coupled
items.
[0018] The systems, apparatus, and methods described herein should
not be construed as limiting in any way. Instead, the present
disclosure is directed toward all novel and non-obvious features
and aspects of the various disclosed embodiments, alone and in
various combinations and sub-combinations with one another. The
disclosed systems, methods, and apparatus are not limited to any
specific aspect or feature or combinations thereof, nor do the
disclosed systems, methods, and apparatus require that any one or
more specific advantages be present or problems be solved. Any
theories of operation are to facilitate explanation, but the
disclosed systems, methods, and apparatus are not limited to such
theories of operation.
[0019] Although the operations of some of the disclosed methods are
described in a particular, sequential order for convenient
presentation, it should be understood that this manner of
description encompasses rearrangement, unless a particular ordering
is required by specific language set forth below. For example,
operations described sequentially may in some cases be rearranged
or performed concurrently. Moreover, for the sake of simplicity,
the attached figures may not show the various ways in which the
disclosed systems, methods, and apparatus can be used in
conjunction with other systems, methods, and apparatus.
Additionally, the description sometimes uses terms like "produce"
and "provide" to describe the disclosed methods. These terms are
high-level abstractions of the actual operations that are
performed. The actual operations that correspond to these terms
will vary depending on the particular implementation and are
readily discernible by one of ordinary skill in the art.
[0020] In some examples, values, procedures, or apparatus' are
referred to as "lowest", "best", "minimum," or the like. It will be
appreciated that such descriptions are intended to indicate that a
selection among many used functional alternatives can be made, and
such selections need not be better, smaller, or otherwise
preferable to other selections.
[0021] Examples are described with reference to directions
indicated as "above," "below," "upper," "lower," and the like.
These terms are used for convenient description, but do not imply
any particular spatial orientation.
[0022] As used herein, optical radiation refers to electromagnetic
radiation at wavelengths of between about 100 nm and 10 .mu.m, and
typically between about 500 nm and 2 .mu.m. Examples based on
available laser diode sources generally are associated with
wavelengths of between about 800 nm and 1700 nm. In some examples,
propagating optical radiation is referred to as one or more beams
having diameters, beam cross-sectional areas, and beam divergences
that can depend on beam wavelength and the optical systems used for
beam shaping. For convenience, optical radiation is referred to as
light in some examples, and need not be at visible wavelengths.
[0023] Optical beams and optical elements are described in some
examples with respect to one or more axes. Typically, an axis
includes one or more straight line segments along which an optical
beam propagates or along which one or more optical elements are
situated. Such axes can be bent or folded with reflective surfaces,
so that axes need not be single straight line segments. In some
examples, reflective surfaces defined by internal reflection in one
or more prisms are used, but such reflective surfaces can be
provided as reflective surfaces such as dielectric or metallic
coatings. In addition, rhomboidal prisms are used in the examples
for convenient illustration. As used herein, a rhomboidal prism is
a solid having two sets of parallel optical surfaces, with the
optical surfaces of each set at an angle of 45 degrees with respect
to the surfaces of the other set. In some cases, polarization
dependent coatings are used to separate s- and p-polarization
components of optical beams, typically as part of a polarizing beam
splitter. Light guides are used to reduce beam coherence. As used
herein, light guides include light pipes of circular, rectangular
or other cross-section. Light guides can be light pipes having a
cavity in which beams propagate, but other types of light guides
such as rod integrators or other beam homogenizers can be used. To
from line beams, laser beams in a common plane or forming a narrow
sheet are directed into a rectangular or square light pipe so as to
be reflected by the light pipe while remaining substantially in the
common plane or within the narrow sheet.
[0024] In some examples, a plurality of laser beams propagating
along a respective beam axes are directed to a beam-spacing optical
system that redirects the beams along more closely spaced axes.
Such beams are referred to as close-packed, and the combined beams
are referred to as a close-packed, combined beam. In some examples
such beam spacing optical systems also include beam splitters so as
to increase beam number.
[0025] In one embodiment, a line generator includes a plurality of
single emitter diode lasers that are free-space coupled to line
generator optics. The free-space coupling can have the added
benefit of maintained beam quality. Moreover, the interleaver
required by conventional systems can be eliminated. In some
disclosed examples, a single light engine module housing a
plurality of single emitters is used. The plurality of diode
emitters can generate 200 W to 1,000 W of continuous wave output
power. In one such example, seventy-two single-emitters are
separately arranged to generate 500 W of continuous wave power. To
mitigate spatial and temporal coherence problems associated with
single-emitter outputs, phase delays of many temporal coherence
lengths across the width of each beam are introduced. The phase
delays and associated coherence reduction are created by one or
more of a diffraction grating, a light pipe used for homogenization
of single emitter diode laser beams, and a selection of launch
angle between the laser beams and the light pipe.
[0026] FIG. 2 depicts a line beam system 220 that includes four
rows 222 of eighteen single-emitter diode lasers 224 that produce
corresponding beams 226 in two parallel plane groupings with one
plane grouping being shown and the other plane grouping being
situated below the first. The diode lasers 224 of reach of the rows
222 are arranged so as to stack in a direction perpendicular to the
plane of FIG. 2, i.e., in an X-direction with respect to a
coordinate system 10. The diode lasers in a selected row or all
rows can also be offset along a Z-axis. The beams 226 are directed
to beam spacing control optics 228, such as one or more prisms
which translate the beams 226 so as to propagate more closely
together and in the same plane grouping. In other examples, beam
spacing control optics can be configured to adjust beam spacings so
as to be larger or smaller, and beam spacings between the beams
need not be the same. In one example, each of the single-emitter
diode lasers 224 operates at approximately fourteen times higher
power than each emitter in a corresponding diode bar, though it
will be appreciated that a range of single-emitter diode laser
output powers is possible.
[0027] Since interference effects tend to be proportional to the
square root of the number of optical beams that are used to produce
a combined beam, interference effects attributed to the use of
fourteen times fewer single-emitter diodes increase by a factor of
about 3.7. To decrease coherence in systems using reduced numbers
of single emitters such as shown in FIG. 2, the beams 226 are
directed with beam forming optics 236 into a light pipe 230 at an
angle with respect to a longitudinal axis 232 of the light pipe
230. As shown in FIG. 2, the beam forming optics can include on or
more lenses that converge the combined beams for coupling to the
light pipe 230 as well as one or more reflective surfaces to direct
the combined beams along a preferred direction. The light pipe 230
can be a solid or hollow light pipe as may be convenient. The beams
226 are incident to the light pipe 230 at a range of angles with
respect to an axis 232 of the light pipe 230. As shown in FIG. 2,
the beams 226 are incident with a single sided half angle, and
typically the combined beam does not fill the full available
numerical aperture of the light guide 230. An output beam from the
light pipe 230 is directed by a lens system 249 as an output beam
248 to a target surface 272.
[0028] FIG. 3 illustrates a representative line beam system 300
that can provide additional coherence reduction. Single-emitter
diode lasers 324 arranged in rows 322 produce beams 326 that
propagate along parallel axes. A beam spacing prism system 328
receives the beams 326 and outputs the beams 326 along parallel,
but differently spaced axes, typically more closely spaced axes, as
a spacing-adjusted combined beam. A diffraction grating 342 is
disposed to receive the spacing-adjusted combined beam 326 and
diffract a substantial portion of the beam power (e.g., 80%, 90%,
95% or more of the beam power) at an angle as a combined redirected
beam 343. An undiffracted beam portion is directed to a beam stop
344. The redirected combined beam 343 is then focused and directed
into a light pipe 330 with a concave mirror 360, lenses 362, 364,
and a reflector 363. The redirected combined beam is directed into
the light pipe 330 at an angle with respect to a light pipe axis
332. A homogenized, coherence-reduced beam from the light pipe 300
is incident to a lens system 350 and directed to a target surface
372 as a working beam 348.
[0029] Some portions of the representative embodiments of FIGS. 2-3
such as beam spacing prism systems, the focusing systems that
direct combined beams into light pipes, and light pipes or light
guides can be used with one or more laser diode bars as well. In
some cases, one or more such portions can be incorporated into a
previously deployed line beam system so that much of the existing
line beam installation, including housing portions and optics can
remain intact.
[0030] FIG. 4 shows an expanded view of a portion of a
representative line beam system 400. Single-emitter diode lasers
(not shown) associated with a diode assembly 402 are arranged in a
staircase that extends vertically with respect to the plane of FIG.
4 (i.e., in an X-direction) and emit diode laser beams which are
collimated individually by fast-axis collimation optics (not shown)
and slow-axis collimation optics such as representative slow axis
collimation optics 434. In a specific example, the diode assembly
402 includes seventy-two single-emitter chiplet diode lasers
arranged in a staircase fashion on a conductively cooled plate 450.
In this example, each emitter has a 350 .mu.m stripe width and can
emit up to 15 W at a wavelength of about 808 nm. It will be
appreciated that other laser diodes can be used and such laser
diodes can have other features, such as different stripe widths,
output powers, wavelengths, etc., and can be selected based on
particular application requirements. In some cases, the diode
assembly 402 includes different types of laser diodes. A staircase
configuration of beams can permit close packing of optical beams
with respect to beam slow axes.
[0031] The chiplet emitters are oriented such that propagation axes
of respective output beams are parallel to an axis 447 of a light
engine output beam 446 which is parallel to, but propagating in the
opposite direction of, an output beam 148 of the line beam system
20 as shown in FIG. 2. The chiplets are typically evenly-spaced in
a direction perpendicular to the axis 447 within the diode assembly
402. In some examples, the chiplets are grouped into two groups
spaced apart from each other in the X direction, i.e., into or out
of the plane of FIG. 4. In some examples, the diode assembly 402 is
secured so as to be removable without opening any housings
associated with a workpiece or downstream optics, limiting the risk
of particulate contamination. In one representative line generator
system, chiplets are mounted to the cooled base plate 450 that is
thermally coupled to a housing surface. The cooled base plate 450
is preferably made of aluminum or copper, but other metals or
suitable heat conductive materials can be used. The cooled base
plate 450 can be formed as part of the diode assembly housing or
can be separately formed and bolted to a housing.
[0032] In the example of FIG. 4, four columns of eighteen beams are
shown in two separate groups associated with parallel planes, one
plane being below the other with respect to the plane of FIG. 4. A
beam spacing prism assembly 428 is coupled to the beams and
translates the beams so that the beams propagate in a close-packed
configuration. As shown in FIG. 4, the close-packed beams propagate
parallel to the axis 447, but in other examples, the beam spacing
prism assembly 428 can direct the beams so as to propagate at one
or more different angles with respect to the axis 447. In one
example, thirty-six collimated beams in first and second columns
452, 458 are directed to propagate adjacent each other and into the
same plane grouping of the two plane groupings of beams 426. Other
columns of beams are similarly combined into a close-packed
relationship.
[0033] A cylinder lens 462 is situated to receive the close-packed
beams and to converge the closed-packed beams. A fold mirror 464
receives the converged beams to a cylinder lens 466 and are then
reflected at another fold mirror 468, which is polarizing, at
another approximately ninety degree angle. An optical system 470
receives the beams reflected by a polarizer mirror 468 and directs
the beams into a light pipe 430 so that the converged, combined
beams are directed into the light pipe 430 at a non-zero angle with
respect to an axis 432 of the light pipe 430. Portions of a beam
reflected at a target surface or that is otherwise back-coupled
into the light pipe 430 can propagate through the polarizer mirror
468 to a mirror 474 so as to be directed away from other elements
of the line beam system 400 instead of being reflected back into
the line beam system 400 so as to prevent component damage.
Back-reflected beam portions reflected by the mirror 474 can be
directed to a beam dump.
[0034] FIG. 5 shows a portion of a representative line beam system
500. A plurality of single-emitter laser diodes in a common package
504 direct laser beams to beam spacing prisms 506 so that a
combined, close-packed beam is delivered to a diffraction grating
508. The diffracted, combined beam is coupled into a light guide
530 with a cylindrical mirror 510, a cylindrical lens 512, a beam
shaping optical system 514, and a polarizing mirror 516. The
diffraction grating 508 is disposed in the path of the diode laser
beams so as to receive the close-packed, combined beams from the
beam spacing prisms 506. The diffraction grating 508 preferably
diffracts most of the power of the close-packed, combined beams at
an angle between zero and ninety degrees with respect to the
propagation direction upon exiting the beam spacing prisms 506.
Undiffracted beam portions and beam portions returned from a work
piece surface are captured by beam dumps 520, 522,
respectively.
[0035] In order to reduce coherence in continuous wave laser beams,
a variable path length delay can be introduced such that a path
length is different for different locations across a beam width. In
some disclosed examples, a light pipe or light guide that receives
an off-axis combined beam provides suitable path length variation
as well as providing a more uniform beam intensity. Beam portions
associated with higher incidence angles have longer path lengths
than on-axis portions or portions at smaller incidence angles. If a
combined beam is launched straight into a light pipe, beam portions
at symmetric angles of incidence have identical path lengths. To
reduce coherence and beam interference, a combined beam can be
asymmetrically launched into a light guide.
[0036] Referring now to FIG. 6, a hollow light pipe 602 having a
central axis 604 is situated to receive an optical beam along an
axis 606 that is at an angle .theta. with respect to the central
axis 604. An optical path length L for such a beam is given by
D/cos .theta., wherein D is a light pipe length. An optical path
length difference with respect to an optical beam propagating
parallel to the axis 604 is D/cos .theta.-D. This path length
difference may be small unless the angle .theta. is sufficiently
large because cos .theta. varies as .theta..sup.2 for small angles.
For a solid light guide of refractive index n, refraction of the
input beam results in an angle of propagation .theta.' in the light
guide so that the optical path difference is n(D/cos
.theta.'-D).
[0037] Path difference in a light pipe is further illustrated in
FIG. 12. A light pipe 1200 having an axis 1202 is situated to
receive a combined laser beam 1204 propagating along an axis 1206
that is at an angle of incidence .theta. with respect to the light
pipe axis 1202. The combined beam has an angular diameter of
.beta.. Setting the angle of incidence .theta. to one-half the
angular diameter, i.e., .beta./2, the maximum path difference for
beam edge 1209 is D/cos(.beta./2)-D with respect to beam edge 1210
for a light pipe of length D as discussed above. Maximum path
differences for other angles can be similarly determined.
[0038] In some examples, sufficient optical path difference for
coherence reduction may not be provided by asymmetric optical beam
launch into a light pipe or an asymmetric launch may be
impractical. Referring to FIG. 7, a transmissive diffraction
grating 702 is situated to receive an optical beam 704 propagating
parallel to an axis 706. The transmissive grating produces a
diffracted beam 708 that propagates parallel to an axis 710 at an
angle 2.phi. with respect to the axis 706. A total path length
difference introduced is a sum of path segments A and B. This path
length difference varies across the width of the optical beam 704
and can provide sufficient coherence reduction. A transmissive
grating is shown for convenience, but a reflective grating can be
used as well.
[0039] Both a transmission grating and an asymmetric launch angle
in a light pipe can be used to increase path length difference and
decrease coherence. In one example, depicted in FIG. 8, an input
optical beam 802 is diffracted by a grating 804, and a diffracted
beam 806 is focused into a light guide 808 with a lens 810. A
portion 812 of the input optical beam 802 that experiences a
longest path length as a result of diffraction at the grating 804
is directed at a largest angle of incidence .theta. into the light
guide 808. This portion 812 of the diffracted beam 806 accumulates
a path length that is a sum of the path lengths associated with the
grating 804 and the light guide 808. This arrangement produces a
largest path difference for the beam portion 812 and a beam portion
814.
[0040] FIG. 9 illustrates a representative method 900 of forming
laser line beams using single-emitters instead of more conventional
diode laser bars. At 902, a plurality of single-emitter diode
lasers are situated to emit laser beams along respective axes.
These beams are typically collimated to preserve beam
characteristics as the beams are directed to and manipulated by
subsequent optical components in the propagation paths. The beams
are combined at 904 using prisms, mirrors, or other optical
elements so as to adjust beam spacings, typically to produce more
closely packed beams. The combined beams are focused and folded at
906 to be received by a light guide such as a light pipe. At 908,
the combined beams are directed into the light pipe at a selected
angle (typically asymmetrically) with respect to a longitudinal
axis of the light pipe such that beam coherence is reduced,
typically based on propagation and multiple reflection in the light
pipe. At 910, an intensity profile of the beams is homogenized, if
needed, but typically, propagation in the light pipe produces
sufficient beam homogenization. At 912, a laser line beam based on
an output beam from the light pipe is directed to a target. The
line beam can have a uniform intensity along a line axis which is
generally perpendicular to the principal direction of beam
propagation. Intensity variations can be less than 5%, 2%, 1%,
0.5%, or 0.1% over at least 75%, 80%, 90%, 95%, or more of the line
beam width. Optical powers of greater than 200 W and up to 1 kW or
more can be produced in such line beams.
[0041] In an alternative shown in FIG. 10, a method 1000 included
combining a plurality of single emitter laser beams at 1002 and
reducing beam coherence at 1004 using a diffraction grating. At
1006, the combined reduced coherence beams are reflected,
refracted, and/or focused as desired to form a suitable line beam
for a selected application. In some examples, additional path
length is difference is introduced with a light pipe.
[0042] Referring to FIG. 11A, a laser diode module 1172 includes a
plurality of laser diodes and associated collimation optics so as
to produce a plurality of beams that are directed to a prism
assembly 1174 as a combined beam 1176. As shown in FIG. 11A, the
lasers are stacked along an X-direction, and have different
propagation distances to the prism assembly. In some examples, the
lasers are situated so as to have a common propagation distance.
One or more laser diode modules such as the laser diode module 1172
can be used to produce a line beam. In an example shown in FIG.
11B, four such modules are used. The modules are situated so that
the laser diodes are arranged in an X-direction (perpendicular to
the plane of FIG. 11B). Representative laser diodes 1102a-1105a and
associated collimation lenses 1106-1109 direct respective beams
along axes 1122-1125. Laser diodes 1102b-1105b are situated below
the laser diodes 1102a-1102b (i.e., along the X-axis) are have
associated collimation lenses as well, and direct respective beams
along the axes 1122-1125. For convenient illustration, only a
single collimation lens of each of the modules is shown in FIG.
11B. Rhomboid prisms 1132, 1134 are situated to directed beams
propagating along the axes 1122, 1125 so as to propagate on axes
closer to a central axis 1101. Rhomboid prisms 1145, 1149 are
situated to receive and beams from laser diodes 1102b, 1105b
leaving beams from laser diodes 1102a, 1105a unaffected. Lower
beams associated with laser diodes 1103b, 1104b are jogged by
respective prisms 1145, 1149 leaving upper beams 1103a, 1104a
unaffected. The lower beams 1102b, 1103b, 1104b, 1105b jogged by
the rhomboid prisms 1145, 1149 are then jogged up with prisms 1116,
1118 to be closer to, or approximately in the same plane as, the
upper beams associated with laser diodes 1102a-1105a. The output
beams 1160, 1162 form a close-packed combined beam 1170 that is
directed on a lens 1172 or other optical elements so as to be
shaped and focused for delivery to a target. Particular prism
configurations are shown in FIG. 11B, but other arrangements can be
used, and reflective surfaces can be provided with or without solid
prisms. In addition, similar prism assemblies are used with other
laser diodes in the laser diode modules, or the prism assemblies
shown in FIG. 11B can extend along the X-axis so as to be suitable
for some or all laser diodes of a diode laser module.
[0043] In view of the many possible embodiments to which the
principles of the disclosed technology may be applied, it should be
recognized that the illustrated embodiments are only preferred
examples and should not be taken as limiting the scope of the
disclosure. We claim all that comes within the scope and spirit of
the appended claims.
* * * * *