U.S. patent application number 14/048001 was filed with the patent office on 2014-02-06 for selective repositioning and rotation wavelength beam combining system and method.
This patent application is currently assigned to TERADIODE, INC.. The applicant listed for this patent is Bien Chann, Robin Huang. Invention is credited to Bien Chann, Robin Huang.
Application Number | 20140036375 14/048001 |
Document ID | / |
Family ID | 44531130 |
Filed Date | 2014-02-06 |
United States Patent
Application |
20140036375 |
Kind Code |
A1 |
Chann; Bien ; et
al. |
February 6, 2014 |
Selective Repositioning and Rotation Wavelength Beam Combining
System and Method
Abstract
A system and method for reconfiguring a plurality of
electromagnetic beams to take advantage of various wavelength beam
combining techniques. The reconfiguring of beams includes
individual rotation and selective repositioning of one or more
beams with respect to beam's original input position.
Inventors: |
Chann; Bien; (Merrimack,
NH) ; Huang; Robin; (North Billerica, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Chann; Bien
Huang; Robin |
Merrimack
North Billerica |
NH
MA |
US
US |
|
|
Assignee: |
TERADIODE, INC.
Wilmington
MA
|
Family ID: |
44531130 |
Appl. No.: |
14/048001 |
Filed: |
October 7, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13042042 |
Mar 7, 2011 |
8553327 |
|
|
14048001 |
|
|
|
|
Current U.S.
Class: |
359/634 |
Current CPC
Class: |
H01S 3/094057 20130101;
H01S 3/09408 20130101; G02B 27/0905 20130101; H01S 3/175 20130101;
G02B 19/009 20130101; H01S 3/1618 20130101; G02B 23/04 20130101;
H01S 5/405 20130101; H01S 3/0675 20130101; H01S 5/4087 20130101;
G02B 19/0057 20130101; H01S 3/094053 20130101; G02B 27/1006
20130101; H01S 3/094096 20130101; H01S 5/0057 20130101; H01S
2301/03 20130101; G02B 27/14 20130101; H01S 3/06733 20130101; H01S
3/09415 20130101; H01S 5/4062 20130101; H01S 5/4012 20130101; G02B
19/0028 20130101; G02B 19/0014 20130101; H01S 3/176 20130101 |
Class at
Publication: |
359/634 |
International
Class: |
G02B 27/10 20060101
G02B027/10 |
Claims
1. A wavelength beam combiner comprising: an optical rotator
configured to selectively rotate beams emitted by a plurality of
beam emitters; a collecting optic configured to receive and deliver
the selectively rotated beams onto a dispersive element, wherein
the dispersive element transmits the selectively rotated beams as a
combined beam profile; and a partially-reflecting output coupler
arranged to receive the combined beams from the dispersive element,
to reflect a portion of the combined beams toward the dispersive
element, and to transmit the combined beams as a multi-wavelength
beam comprising optical radiation having a plurality of
wavelengths.
2. The wavelength beam combiner of claim 1, wherein at least two of
the beam emitters have a fixed-position relationship.
3. The wavelength beam combiner of claim 1, wherein the beam
emitters include a first reflective surface and an optical gain
medium.
4. The wavelength beam combiner of claim 1, further including a
collimation optic configured to receive beams from the beam
emitters and collimate one or more beams along a dimension of the
beam.
5. The multi-wavelength beam combiner of claim 1, wherein the diode
elements include a first reflective surface and an optical gain
medium.
6. The multi-wavelength beam combiner of claim 1, wherein the
partially-reflective output coupler has a curved surface.
7. The multi-wavelength beam combiner of claim 1, wherein the
emitted beams have an asymmetrical profile.
8. A wavelength beam combiner comprising: a spatial repositioning
element configured to spatially-reposition beams emitted by a
plurality of beam emitters; a collecting optic arranged to receive
the spatially-repositioned beams and deliver the beams onto a
dispersive element, wherein the dispersive element transmits the
spatially-repositioned beams as a combined beam profile; and a
partially-reflecting output coupler configured to reflect a portion
of the combined beams back into each of the beam emitters.
9. The wavelength beam combiner of claim 8, wherein the plurality
of beam emitters produces a two-dimensional profile and the
spatial-repositioning element reduces the number of beams along a
first dimension of the profile while increasing the number of beams
across a second dimension of the profile.
10. The wavelength beam combiner of claim 8, wherein at least two
of the beam emitters have a fixed-position relationship.
11. The wavelength beam combiner of claim 8, further including an
optical rotator configured to selectively rotate beams prior to
being received by the collecting optic.
12. A wavelength beam combining method including: selectively
rotating electromagnetic beams emitted by a plurality of beam
emitters; directing the selectively rotated beams onto a dispersive
element; transmitting a combined beam profile from the dispersive
element; redirecting a portion of the combined beams back into the
beam emitters; and transmitting the combined beams as a
multi-wavelength beam comprising optical radiation having a
plurality of wavelengths.
13. The method of claim 12, further including: individually
collimating the emitted beams along a dimension prior to
selectively rotating the beams.
14. The method of claim 12, wherein at least two of the beam
emitters have a fixed-position relationship.
15. A method for wavelength beam combining including:
selectively-repositioning electromagnetic beams emitted by a
plurality of beam emitters; directing the selectively-repositioned
beams onto a dispersive element; and dispersing the
selectively-repositioned beams as a combined beam profile; and
redirecting a portion of the dispersed beams back into the beam
emitters.
16. The method of claim 15, wherein the plurality of beam emitters
produces a two-dimensional profile and the
selectively-repositioning step reduces the number of emitted beams
along a first dimension while increasing the number of emitted
beams across a second dimension.
17. The method of claim 15, wherein an array of periscopes is used
in the selectively-repositioning step.
18. The method of claim 15, further including the step of
selectively rotating the electromagnetic beams emitted by a
plurality of beam emitters after the selectively-repositioning
step.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. Ser. No.
13/042,042 filed Mar. 7, 2011.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present embodiments relate generally to laser systems
and more particularly to wavelength beam combining systems and
methods.
[0004] 2. Description of the Prior Art
[0005] Wavelength beam combining (WBC) is a method for scaling the
output power and brightness from laser diode bars, stacks of diode
bars, as well as other lasers arranged in one or two-dimensional
array.
[0006] WBC methods have been developed to combine beams along the
slow dimension of each emitter as well as the fast dimension of
each emitter. As such, the system is more sensitive to
imperfections in the optical gain elements. Furthermore, when
broad-area optical gain elements are used the spectral utilization
is poor. In some cases beam combining is performed along the
stacking dimension. In such implementations the WBC stabilizing
system is much less sensitive to imperfections in optical gain
elements. Furthermore, since beam combining is performed along the
stacking dimension or near diffraction-limited dimension spectral
utilization is high. However, one of the main drawbacks of this
implementation is the output beam quality is limited to the beam
quality of a single beam combining element or a single diode bar.
Within the prior art these individual emitters are pre-aligned or
have a fixed in position and as such, the output beam profile
generated from combining across one of these dimensions is a result
of this pre-alignment or fixed positioning of the array of
emitters. This application addresses manipulating individual,
one-dimensional, two-dimensional, as well as randomly placed
emitters into a preferred alignment conducive to generating a
preferred output beam profile. The result is more robust, and much
higher spatial brightness can be obtained using commercially
available diode laser bars and stacks with a large number of
optical gain elements. Additional benefits will become apparent in
the detailed description of the application.
[0007] The following application seeks to solve the problems
stated.
SUMMARY OF THE INVENTION
[0008] Optical and mechanical means have been developed to
selectively rotate and/or selectively reposition emitted
electromagnetic beams into a desired orientation and/or pattern in
a one-dimensional or two-dimensional array for use with various
wavelength beam combining systems and methods.
[0009] In particular, these systems and methods are applicable to
emitters that have a fixed-position relative to other emitters.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1A is a schematic of a wavelength beam combining (WBC)
method along the array dimension of a single row of emitters.
[0011] FIG. 1B is a schematic of a WBC method along the array
dimension of a two-dimensional array of emitters.
[0012] FIG. 1C is a schematic of a WBC method along the stack
dimension of a two-dimensional array of emitters.
[0013] FIG. 2 is a schematic showing the effects of smile in a WBC
method along the stack dimension of a two-dimensional array of
diode laser emitters.
[0014] FIG. 3A is a schematic of a WBC system including an optical
rotator selectively rotating a one-dimensional array of beams.
[0015] FIG. 3B is a schematic of a WBC system including an optical
rotator selectively rotating a two-dimensional array of beams
[0016] FIG. 3C is a schematic of a WBC system including an optical
rotator selectively reorienting a two-dimensional array of
beams.
[0017] FIG. 3D illustrates output profile views of the system of
FIG. 3C with and without an optical rotator.
[0018] FIGS. 4A-C illustrate examples of optical rotators.
[0019] FIGS. 5A-C illustrate related methods for placing combining
elements to generate one-dimensional or two-dimensional optical
gain elements
[0020] FIG. 6 illustrates a WBC embodiment having a spatial
repositioning element.
[0021] FIG. 7 illustrates an embodiment of a two-dimensional array
of emitters being reconfigured before a WBC step and individual
beam rotation after the WBC step.
[0022] FIG. 8 illustrates the difference between slow and fast
WBC.
[0023] FIG. 9A illustrates embodiments using an optical rotator
before WBC in both a single and stacked array configurations.
[0024] FIG. 9B illustrates additional embodiments using an optical
rotator before WBC.
[0025] FIG. 10 is illustrative of a single semiconductor chip
emitter.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0026] Aspects and embodiments relate generally to the field of
scaling laser sources to high-power and high-brightness using
wavelength beam combining techniques. More particularly, methods
for increasing brightness, stability, and effectiveness of
wavelength beam combining systems.
[0027] Embodiments described herein include addressing: 1)
manipulating beam profiles through rotation and spatial
repositioning techniques in a WBC system, which allows for
increasing output power and brightness through combining multiple
emitters in a common system. Through the various embodiments and
techniques described herein a stabilized, high brightness
multi-wavelength output laser system may be achieved.
[0028] The approaches and embodiments described herein may apply to
one and two-dimensional beam combining systems along the slow-axis,
fast-axis, or other beam combining dimension. For purposes of this
application optical elements may refer to any of lenses, mirrors,
prisms and the like which redirect, reflect, bend, collect or in
any other manner optically manipulate electromagnetic radiation.
Additionally, the term beam includes electromagnetic radiation.
Beam emitters include any electromagnetic beam-generating device
such as semiconductor elements, which generate an electromagnetic
beam, but may or may not be self-resonating. These also include
fiber lasers, disk lasers, non-solid state lasers, diode lasers and
so forth. Generally each emitter is comprised of a back reflective
surface, at least one optical gain medium, and a front reflective
surface. The optical gain medium refers to increasing the gain of
electromagnetic radiation and is not limited to the visual, IR or
ultraviolet portions of the electromagnetic spectrum. An emitter,
may be comprised of multiple beam emitters such as a diode bar
configured to emit multiple beams. Many of the examples and
embodiments used herein describe use a diode bar; however, it is
contemplated that any emitter and in particular emitters having
optical gain elements and particularly those with broad gain
bandwidth may be used.
[0029] Additionally, some prior art defines the term "stack or
stacking dimension" referred to as two or more arrays stacked
together, where the beams' fast dimension is the same as the
stacking dimension. These stacks were pre-arranged mechanically or
optically. However, for purposes of this application a stack refers
to a column of beams or optical gain elements and may or may not be
along the fast dimension. Particularly, as discussed above,
individual beams or elements may be rotated within a stack or
column.
[0030] The individual slow or fast dimension of the emitters of the
array may also be aligned along the array dimension, but this
alignment is not to be assumed. This is important because some
embodiments described herein individually rotate the slow dimension
of each beam aligned along an array or row. Additionally, the slow
axis of a beam may refer to the wider dimension of the beam exiting
the optical gain medium and is typically also the slowest diverging
dimension, while the fast axis usually refers to the narrower
dimension of the beam and is typically the fastest diverging
dimension. The slow axis may also refer to single mode beams
[0031] In some embodiments it is useful to note that the array
dimension and the slow dimension of each emitted beam are initially
oriented across the same axis; however, those dimensions, as
described in this application, may become oriented at an offset
angle with respect to each other. In other embodiments, the array
dimension and only a portion of the emitters arranged along the
array or perfectly aligned the same axis at a certain position in a
WBC system. For example, the array dimension of a diode bar may
have emitters arranged along the array dimension, but because of
smile (often a deformation or bowing of the bar) individual
emitters' slow emitting dimension is slightly skewed or offset from
the array dimension.
[0032] An example of a single semiconductor chip emitter 1000 is
shown in FIG. 10. The aperture 1050 is also indicative of the
initial beam profile. Here, the height 1010 at 1050 is measured
along the stack dimension. Width 1020 at 1050 is measured along the
array dimension. Height 1010 is the shorter dimension at 1050 than
width 1020. However, height 1010 expands faster or diverges to beam
profile 1052, which is placed at a distance away from the initial
aperture 1050. Thus, the fast axis is along the stack dimension.
Width 1020 which expands or diverges at a slower rate as indicated
by width 1040 being a smaller dimension than height 1030. Thus, the
slow axis of the beam profile is along the array dimension. Though
not shown, multiple single emitters such as 1000 may be arranged in
a bar side by side along the array dimension.
[0033] Aspects and embodiments herein relate to high-power and/or
high-brightness multi-wavelength stabilized systems that generate a
combined or coaxial beam from very low output power to hundreds and
even to megawatts of output power. The combined beam may have a
varying beam product parameters as a result of intentional
placement of collecting optics and dispersive elements used in the
WBC systems described herein.
[0034] Wavelength beam combining methods have been developed to
combine asymmetrical beam elements across their respective slow or
fast axis dimension. One advantage this invention seeks to provide
is the ability to selectively-reconfigure input beams either
spatially or by orientation to be used in slow and fast axis WBC
methods, as well as a hybrid of the two. Another advantage is to
selectively-reconfigure input beams when there is a fixed-position
relationship to other input beams.
[0035] FIG. 1A illustrates a basic WBC architecture. In this
particular illustration, WBC is performed along the array dimension
or slow dimension for broad-area emitters. Individual beams 104 are
illustrated in the figures by a dash or single line, where the
length or longer dimension of the beam (dash) represents the array
dimension or slow diverging dimension for broad-area emitters and
the height or shorter dimension represents the fast diverging
dimension. (See also left side of FIG. 8). The emitters of diode
bar 102 are aligned in a manner such that the slow dimension ends
of each emitted beam 104 are aligned to one another side by side
along a single row--sometimes referred to as an array. In some
configurations a collimation lens 106 is used to collimate each
beam along the fast diverging dimension. The collimation optics may
be composed of separate fast axis collimation lenses and slow axis
collimation lenses.
[0036] An optical element 108 is used to combine each beam along
the WBC dimension 110 as shown by the input front view 112. Optical
element 108 may be a cylindrical or spherical lens or mirror. The
optical element 108 then overlaps the combined beam onto a
dispersive element 114 (here shown as a reflecting diffraction
grating). The first-order diffracted beams are incident onto a
partially reflecting mirror. A resonator is formed between the back
facet of the optical gain elements and the partially reflecting
mirror. As such, the combined beam is then transmitted as a single
output profile onto an output coupler 116. This output coupler then
transmits the combined beams 120, as shown by the output front view
118. The output coupler 116 may be a partially reflective mirror or
surface or optical coating and act as a common front facet
providing wavelength stabilized feedback for all the optical gain
elements in diode array 102. The feedback is directed toward
dispersive element 114, which filters it into unique wavelengths
where it is redirected back into each emitter.
[0037] Similarly, FIG. 1B illustrates a stack of laser diode bars
each having four emitters where those bars are stacked three high.
(See also left side of FIG. 8). Like FIG. 1A, the input front view
112 of FIG. 1B, which in this embodiment is a two-dimensional array
of beams, is combined to produce the output front view 118 or a
single column of beams 120. The emitted beams in WBC system 100b
were combined along the array dimension. Here optical element 108
is a cylindrical lens used to combine the beams along the array.
However, a combination of optical elements or optical system can be
used as such that the optical elements arrange for all the beams to
overlap onto the dispersive element and make sure all the beams
along the non-beam-combining dimension are propagating normal to
the output coupler. A simple example of such an optical system
would be a single cylindrical lens with the appropriate focal
length along the beam-combining dimension and two cylindrical
lenses that form an afocal telescope along the non beam-combining
dimension wherein the optical system projects images onto the
partially reflecting mirrors. Many variations of this optical
system can be designed to accomplish the same functions.
[0038] The array dimension FIG. 1B is also the same axis as the
slow dimension of each emitted beam in the case of multimode diode
laser emitters. Thus, this WBC system may also be called slow axis
combining, where the combining dimension is the same dimension of
the beams.
[0039] By contrast, FIG. 1C illustrates a stack 150 of laser diode
arrays 102 forming a two-dimensional array of beams, as shown by
120, where instead of combining along the array dimension as in
FIGS. 1A-B, the WBC dimension now follows along the stack dimension
of the emitters. Here, the stacking dimension is also aligned with
the fast axis dimension of each of the emitted beams. The input
front view 112 is now combined to produce the output front view 118
wherein a single column 120 of beams is shown.
[0040] There are various drawbacks to all three configurations. One
of the main drawbacks of configuration shown in FIGS. 1A and 1B is
that beam combining is performed along the array dimension. As such
wavelength stabilizing operation is highly dependent on
imperfections of the diode array. A disadvantage of configuration
1C is that the output beam quality is limited to that of a single
laser bar and external beam shaping for beam symmetrization may be
required for efficient coupling into a fiber.
[0041] As illustrated in FIG. 2, a diode array with smile or
pointing errors, may prevent feedback from the WBC system's optical
elements, which consist of the collecting lens, grating, and output
coupler, to couple back to the diode optical gain elements. Some
negative effects of this mis-coupling are that the WBC laser breaks
wavelength lock and the diode laser or related packaging may be
damaged from mis-coupled or misaligned feedback not re-entering the
optical gain medium. For instance the feedback may hit some epoxy
or solder in contact or in close proximity to a diode bar and cause
the diode bar to fail catastrophically.
[0042] Row 1 of FIG. 2 shows a single laser diode bar 202 without
any errors. The embodiments illustrated are exemplary of a diode
bar mounted on a heat sink and collimated by a fast-axis
collimation optic 206. Column A shows a perspective or 3-D view of
the trajectory of the output beams 211 going through the
collimation optic 206. Column D shows a side view of the trajectory
of the emitted beams 211 passing through the collimation optic 206.
Column B shows the front view of the laser facet with each
individual optical gain element 213 with respect to the collimation
optic 206. As illustrated in row 1, the optical gain elements 213
are perfectly straight. Additionally, the collimation optic 206 is
centered with respect to all the optical gain elements 213. Column
C shows the expected output beam from a system with this kind of
input. Row 2 illustrates a diode laser array with pointing error.
Shown by column B of row 2 the optical gain elements and
collimation optic are slightly offset from each other. The result,
as illustrated, is the emitted beams having an undesired trajectory
that may result in reduced lasing efficiency for a multi-wavelength
stabilizing system. Additionally, the output profile may be offset
to render the system ineffective or cause additional modifications.
Row 3 shows an array with packaging error. The optical gain
elements no longer sit on a straight line, and there is curvature
of the bar. This is sometimes referred to as `smile.` As shown on
row 3, smile can introduce even more trajectory problems as there
is no uniform path or direction common to the system. Column D of
row 3 further illustrates beams 211 exiting at various angles. Row
4 illustrates a collimation lens unaligned with the optical gain
elements in a twisted or angled manner. The result is probably the
worst of all as the output beams generally have the most
collimation or twisting errors. In most systems, the actual error
in diode arrays and stacks is a combination of the errors in rows
2, 3, and 4. In both methods 2 and 3, using VBG's and diffraction
gratings, optical gain elements with imperfections result in output
beams no longer pointing parallel to the optical axis. These off
optical axis beams result in each of the optical gain elements
lasing at different wavelengths. The plurality of different
wavelengths increases the output spectrum of the system to become
broad as mentioned above.
[0043] One of the advantages of performing WBC along the stacking
dimension (here also primarily the fast dimension) of a stack of
diode laser bars is that it compensates for smile as shown in FIG.
2. Pointing and other alignment errors are not compensated by
performing WBC along the array dimension (also primarily slow
dimension). A diode bar array may have a range of emitters
typically from 19 to 49 or more. As noted, diode bar arrays are
typically formed such that the array dimension is where each
emitter's slow dimension is aligned side by side with the other
emitters. As a result, when using WBC along the array dimension,
whether a diode bar array has 19 or 49 emitters (or any other
number of emitters), the result is that of a single emitter. By
contrast, when performing WBC along the orthogonal or fast
dimension of the same single diode bar array, the result is each
emitted beam increases in spectral brightness, or narrowed spectral
bandwidth, but there is not a reduction in the number of beams
(equivalently, there is not an increase in spatial brightness).
[0044] One embodiment that addresses this issue is illustrated in
FIG. 3A, which shows a schematic of WBC system 300a with an optical
rotator 305 placed after collimation lenses 306 and before the
optical element 308. It should be noted the optical element 308 may
be comprised of a number of lenses or mirrors or other optical
components. The optical rotator 305 individually rotates the fast
and slow dimension of each emitted beam shown in the input front
view 312 to produce the re-oriented front view 307. It should be
noted that the optical rotators can selectively rotate each beam
individually irrespective of the other beams or in some instances
it is possible to rotate all the beams through the same angle
simultaneously. It should also be noted that a cluster of two or
more beams can be rotated simultaneously. The resulting output
after WBC is performed along the array dimension is shown in output
front view 318 as a single emitter. Dispersive element 314 is shown
as a reflection diffraction grating, but may also be a dispersive
prism, a grism (prism+grating), transmission grating, and Echelle
grating.
[0045] This particular embodiment illustrated in FIG. 3A shows only
four laser emitters; however, as discussed above this system could
take advantage of a laser diode array that included many more
elements, e.g., 49. Additionally, this embodiment shows a single
bar having a particular wavelength band (example at 976 nm) but in
actual practice it can be composed of multiple bars, all at the
same particular wavelength band, arranged side-by-side.
Furthermore, multiple wavelength bands (example 976 nm, 915 nm, and
808 nm), with each band composing of multiple bars, can we combined
in a single cavity. Because WBC was performed across the fast
dimension of each beam it easier to design a system with a higher
brightness (higher efficiency due to insensitivity due to bar
imperfections); additionally, narrower bandwidth and higher power
output are all achieved. As previously discussed it should noted
that some versions of WBC system 300a may not include output
coupler 316 and/or collimation lens(es) 306. Furthermore, pointing
errors and smile errors are compensated for by combining along the
stack dimension (here shown as the fast dimension).
[0046] FIG. 3B, shows an implementation similar to FIG. 3A except
that a stack 350 of laser arrays 302 forms a 2-D input profile 312.
WBC system 300b similarly consists of collimation lens(es) 306,
optical rotator 305, optical element 308, dispersive element 308
(here a diffraction grating), and an output coupler 316 with a
partially reflecting surface. Each of the beams is individually
rotated by optical rotator 305 to form an after rotator profile
307. The WBC dimension is along the array dimension, but with the
rotation each of the beams will be combined across their fast axis.
Fast axis WBC produces outputs with very narrow line widths and
high spectral brightness. These are usually ideal for industrial
applications such as welding. After optical element 308 overlaps
the rotated beams onto dispersive element 314 an single output
profile is produced and partially reflected back through the cavity
into the optical gain elements. The output profile 318 is now
comprised of a line of three (3) beams that is quite
asymmetric.
[0047] FIG. 3C shows the same implementation when applied to 2-D
optical gain elements. The system consists of 2-D optical gain
elements 302, optical rotator 305, optical system (308 and 309a-b)
a dispersive element 314, and a partially reflecting mirror 316.
FIG. 3C illustrates a stack 350 of laser diode bars 302 with each
bar having an optical rotator 305. Each of the diode bars 302
(three total) as shown in WBC system 300c includes four emitters.
After input front view 312 is reoriented by optical rotator 305,
reoriented front view 307 now the slow dimension of each beam
aligned along the stack dimension. WBC is performed along the
dimension, which is now the slow axis of each beam and the output
front view 318 now comprises single column of beams with each
beam's slow dimension oriented along the stack dimension.
[0048] Optic 309a and 309b provide a cylindrical telescope to image
along the array dimension. The function of the three cylindrical
lenses are to provide two main functions. The middle cylindrical
lens is the transform lens and its main function is to overlap all
the beams onto the dispersive element. The two other cylindrical
lenses 309a and 309b form an afocal cylindrical telescope along the
non-beam combining dimension. Its main function is to make sure all
optical gain elements along the non-beam combining are propagation
normal to the partially reflecting mirror. As such the
implementation as shown in FIG. 3C has the same advantages as the
one shown in FIG. 1C.
[0049] However, unlike the implementation as shown in FIG. 1C the
output beam is not the same as the input beam. The number of
emitters in the output beam 318 in FIG. 3C is the same as the
number of bars in the stack. For example, if the 2-D laser source
consists of a 3-bar stack with each bar composed of 49 emitters,
then the output beam in FIG. 1C is a single bar with 49 emitters.
However, in FIG. 3C the output beam is a single bar with only 3
emitters. Thus, the output beam quality or brightness is more than
one order of magnitude higher. This brightness improvement is very
significant for fiber-coupling. For higher power and brightness
scaling multiple stacks can be arranged side-by-side.
[0050] To illustrate this configuration further, for example,
assume WBC is to be performed of a 3-bar stack, with each bar
comprising of 19 emitters. So far, there are three options. First,
wavelength beam combining can be performed along the array
dimension to generate 3 beams as shown in FIG. 1B. Second,
wavelength beam combining can be performed along the stack
dimension to generate 19 beams a shown FIG. 1C. Third, wavelength
beam combining can be performed along the array dimension using
beam rotator to generate 19 beams as shown FIG. 3C. There are
various trade-offs for all three configuration. The first case
gives the highest spatial brightness but the lowest spectral
brightness. The second case gives the lowest spatial brightness
with moderate spectral brightness and beam symetrization is not
required to couple into a fiber. The third case gives the lowest
spatial brightness but the highest spectral brightness and beam
symmetrization is required to couple into an optical fiber. In some
applications this more desirable.
[0051] To illustrate the reduction in asymmetry FIG. 3D has been
drawn showing the final output profile 319a where the system of
300b did not have an optical rotator and output profile 319b where
the system includes an optical rotator. Though these figures are
not drawn to scale, they illustrate an advantage achieved by
utilizing an optical rotator, in a system with this configuration
where WBC is performed across the slow dimension of each beam. The
shorter and wider 319b is more suitable for fiber coupling than the
taller and slimmer 319a.
[0052] An example of various optical rotators are shown in FIG.
4A-C. FIG. 4A illustrates an array of cylindrical lenses (419a and
419b) that cause input beam 411a to be rotated to a new orientation
at 411b. FIG. 4B similarly shows input 411a coming into the prism
at an angle, which results in a new orientation or rotation beam
411b. FIG. 4C illustrates an embodiment using a set of step mirrors
417 to cause input 411a to rotate at an 80-90 degree angle with the
other input beams resulting in a new alignment of the beams 411b
where they are side by side along their respective fast axis. These
devices and others may cause rotation through both non-polarization
sensitive as well as polarization sensitive means. Many of these
devices become more effective if the incoming beams are collimated
in at least the fast dimension. It is also understand that the
optical rotators can selectively rotate the beams at various
including less than 90 degrees, 90 degrees and greater than 90
degrees.
[0053] The optical rotators in the previous embodiments may
selectively rotate individual, rows or columns, and groups of
beams. In some embodiments a set angle of rotation, such as a range
of 80-90 degrees is applied to the entire profile or subset of the
profile. In other instances, varying angles of rotation are applied
uniquely to each beam, row, column or subset of the profile. (see
FIGS. 9A-B) For instance, one beam may be rotated by 45 degrees in
a clockwise direction while an adjacent beam is rotated 45 degrees
in a counterclockwise direction. It is also contemplated one beam
is rotated 10 degrees and another is rotated 70 degrees. The
flexibility the system provides can be applied to a variety of
input profiles, which in turn helps determine how the output
profile is to be formed. For instance, performing WBC along an
intermediate angle between the slow and fast dimension of the
emitted beams is also well within the scope of the invention (See
for example 6 on FIG. 9B).
[0054] The previous illustrations, FIGS. 1A-C, showed pre-arranged
or fixed position arrays and stacks of laser emitters. Generally,
arrays or stacks are arranged mechanically or optically to produce
a particular one-dimensional or two-dimensional profile. Thus,
fixed-position is used to describe a preset condition of optical
gain elements where the optical gain elements are mechanically
fixed with respect to each other as in the case of semiconductor or
diode laser bars having multiple emitters or fiber lasers
mechanically spaced apart in V-grooves, as well as other laser
emitters that come packaged with the emitters in a fixed position.
Alternatively, fixed position may refer to the secured placement of
a laser emitter in a WBC system where the laser emitter is
immobile. Pre-arranged refers to an optical array or profile that
is used as the input profile of a WBC system. Often times the
pre-arranged position is a result of emitters configured in a
mechanically fixed position. Pre-arranged and fixed position may
also be used interchangeably. Examples of fixed-position or
pre-arranged optical systems are shown in FIGS. 5A-C.
[0055] FIGS. 5A-5C refer to prior art illustrated examples of
optically arranged one and two-dimensional arrays. FIG. 5A
illustrates an optically arranged stack of individual optical
elements 510. Mirrors 520 are used to arrange the optical beams
from optical elements 530, each optical element 530 having a near
field image 540, to produce an image 550 (which includes optical
beams from each optical element 530) corresponding to a stack 560
(in the horizontal dimension) of the individual optical elements
510. Although the optical elements 500 may not be arranged in a
stack, the mirrors 520 arrange the optical beams such that the
image 550 appears to correspond to the stack 560 of optical
elements 510. Similarly, in FIG. 5B, the mirrors 520 are used to
arrange optical beams from diode bars or arrays 570 to create an
image 550 corresponding to a stack 560 of diode bars or arrays 575.
In this example, each diode bar or array 570 has a near field image
540 that includes optical beams 545 from each individual element in
the bar or array. Similarly, the mirrors 520 may also be used to
optically arrange laser stacks 580 into an apparent larger overall
stack 560 of individual stacks 585 corresponding to image 550, as
shown in FIG. 5C.
[0056] Another method for manipulating beams and configurations to
take advantage of the various WBC methods includes using a spatial
repositioning element. This spatial repositioning element may be
placed in a WBC system at a similar location as to that of an
optical rotator. For example, FIG. 6 shows a spatial repositioning
element 603 placed in the WBC system 600 after the collimating
lenses 606 and before the optical element(s) 608. The purpose of a
spatial repositioning element is to reconfigure an array of
elements into a new configuration. FIG. 6 shows a three-bar stack
with N elements reconfigured to a six-bar stack with N/2 elements.
Spatial repositioning is particularly useful in embodiments such as
600, where stack 650 is a mechanical stack or one where diode bar
arrays 602 and their output beams were placed on top of each other
either mechanically or optically. With this kind of configuration
the optical gain elements have a fixed-position to one another.
Using a spatial repositioning element can form a new configuration
that is more ideal for WBC along the fast dimension. The new
configuration makes the output profile more suitable for fiber
coupling.
[0057] For example, FIG. 7 illustrates an embodiment wherein a
two-dimensional array of emitters 712 is reconfigured during a
spatial repositioning step 703 by a spatial repositioning optical
element such as an array of periscope mirrors. The reconfigured
array shown by reconfigured front view 707 is now ready for a WBC
step 710 to be performed across the WBC dimension, which here is
the fast dimension of each element. The original two-dimensional
profile in this example embodiment 700 is an array of 12 emitters
tall and 5 emitters wide. After the array is transmitted or
reflected by the spatial repositioning element a new array of 4
elements tall and 15 elements wide is produced. In both arrays the
emitters are arranged such that the slow dimension of each is
vertical while the fast dimension is horizontal. WBC is performed
along the fast dimension which collapses the 15 columns of emitters
in the second array into 1 column that is 4 emitters tall. This
output is already more symmetrical than if WBC had been performed
on the original array, which would have resulted in a single column
15 emitters tall. As shown, this new output may be further
symmetrized by an individually rotating step 705 rotating each
emitter by 90 degrees. In turn, a post WBC front view 721 is
produced being the width of a single beam along the slow dimension
and stacked 4 elements high, which is a more suitable for coupling
into a fiber.
[0058] One way of reconfiguring the elements in a one-dimensional
or two-dimensional profile is to make `cuts` or break the profile
into sections and realign each section accordingly. For example, in
FIG. 7 two cuts 715 were made in 713. Each section was placed side
by side to form 707. These optical cuts can be appreciated if we
note the elements of 713 had a pre-arranged or fixed-position
relationship. It is also well within the scope to imagine any
number of cuts being made to reposition the initial input beam
profile. Each of these sections may in addition to being placed
side by side, but on top and even randomized if so desired.
[0059] Spatial repositioning elements may be comprised of a variety
of optical elements including periscope optics that are both
polarized and non-polarized as well as other repositioning optics.
Step mirrors as shown in FIG. 4a may also be reconfigured to become
a spatial repositioning element.
[0060] It is contemplated spatial repositioning elements and
optical rotators may be used in the same WBC system or a
combination of inside and outside of the multi-wavelength
stabilizer system. The order of which element appears first is not
as important and is generally determined by the desired output
profile.
[0061] This point is illustrated in FIG. 8. On the left of FIG. 8
is shown a front view of an array of emitters 1 and 2 where WBC
along the slow dimension is being performed. Along the right side
using the same arrays 1 and 2, WBC along the fast dimension is
being performed. When comparing array 1, WBC along the slow
dimension reduces the output profile to that of a single beam,
while WBC along the fast dimension narrows the spectral bandwidth,
as shown along the right side array 1, but does not reduce the
output profile size to that of a single beam.
[0062] Using COTS diode bars and stacks the output beam from beam
combining along the stack dimension is usually highly asymmetric.
Symmetrization, or reducing the beam profile ratio closer to
equaling one, of the beam profile is important when trying to
couple the resultant output beam profile into an optical fiber.
Many of the applications of combining a plurality of laser emitters
require fiber coupling at some point in an expanded system. Thus,
having greater control over the output profile is another advantage
of the application.
[0063] Further analyzing array 2 in FIG. 8 shows the limitation of
the number of emitters per laser diode array that is practical for
performing WBC along the fast dimension if very high brightness
symmetrization of the output profile is desired. As discussed
above, typically the emitters in a laser diode bar are aligned side
by side along their slow dimension. Each additional optical gain
element in a diode bar is going to increase the asymmetry in the
output beam profile. When performing WBC along the fast dimension,
even if a number of laser diode bars are stacked on each other, the
resultant output profile will still be that of a single laser diode
bar. For example if one uses a COTS 19-emitter diode laser bar, the
best that one can expect is to couple the output into a 100
.mu.m/0.22 NA fiber. Thus, to couple into a smaller core fiber
lower number of emitters per bar is required. One could simply fix
the number of emitters in the laser diode array to 5 emitters in
order to help with the symmetrization ratio; however, fewer
emitters per laser diode bar array generally results in an increase
of cost of per bar or cost per Watt of output power. For instance,
the cost of diode bar having 5 emitters may be around $2,000
whereas the cost of diode bar having 49 emitters may be around
roughly the same price. However, the 49 emitter bar may have a
total power output of up to an order-of-magnitude greater than that
of the 5 emitter bar. Thus, it would be advantageous for a WBC
system to be able to achieve a very high brightness output beams
using COTS diode bars and stacks with larger number of emitters. An
additional advantage of bars with larger number of emitters is the
ability to de-rate the power per emitter to achieve a certain power
level per bar for a given fiber-coupled power level, thereby
increasing the diode laser bar lifetime or bar reliability.
[0064] Additional embodiments encompassing, but not limiting the
scope of the invention, are illustrated in FIGS. 9A-B. The system
shown in 1 of FIG. 9A shows a single array of 4 beams aligned side
to side along the slow dimension. An optical rotator individually
rotates each beam. The beams are then combined along the fast
dimension and are reduced to a single beam by WBC. In this
arrangement it is important to note that the 4 beams could easily
be 49 or more beams. It may also be noted that if some of the
emitters are physically detached from the other emitters, the
individual emitter may be mechanically rotated to be configured in
an ideal profile. A mechanical rotator may be comprised of a
variety of elements including friction sliders, locking bearings,
tubes, and other mechanisms configured to rotate the optical gain
element. Once a desired position is achieved the optical gain
elements may then be fixed into place. It is also conceived that an
automated rotating system that can adjust the beam profile
depending on the desired profile may be implemented. This automated
system may either mechanically reposition a laser or optical
element or a new optical element may be inserted in and out of the
system to change the output profile as desired.
[0065] System 2 shown in FIG. 9A, shows a two-dimensional array
having 3 stacked arrays with 4 beams each aligned along the slow
dimension. (Similar to FIG. 3C) As this stacked array passes
through an optical rotator and WBC along the fast dimension a
single column of 3 beams tall aligned top to bottom along the slow
dimension is created. Again it is appreciated that if the three
stacked arrays shown in this system had 50 elements, the same
output profile would be created, albeit one that is brighter and
has a higher output power.
[0066] System 3 in FIG. 9B, shows a diamond pattern of 4 beams
wherein the beams are all substantially parallel to one another.
This pattern may also be indicative of a random pattern. The beams
are rotated and combined along the fast dimension, which results in
a column of three beams aligned along the slow dimension from top
to bottom. Missing elements of diode laser bars and stacks due to
emitter failure or other reasons, is an example of System 3. System
4, illustrates a system where the beams are not aligned, but that
one beam is rotated to be aligned with a second beam such that both
beams are combined along the fast dimension forming a single beam.
System 4, demonstrates a number of possibilities that expands WBC
methods beyond using laser diode arrays. For instance, the input
beams in System 4 could be from carbon dioxide (CO.sub.2) lasers,
semiconductor or diode lasers, diode pumped fiber lasers,
lamp-pumped or diode-pumped Nd:YAG lasers, Disk Lasers, and so
forth. The ability to mix and match the type of lasers and
wavelengths of lasers to be combined is another advantage
encompassed within the scope of this invention.
[0067] System 5, illustrates a system where the beams are not
rotated to be fully aligned with WBC dimension. The result is a
hybrid output that maintains many of the advantages of WBC along
the fast dimension. In several embodiments the beams are rotated a
full 90 degrees to become aligned with WBC dimension, which has
often been the same direction or dimension as the fast dimension.
However, System 5 and again System 6 show that optical rotation of
the beams as a whole (System 6) or individually (System 5) may be
such that the fast dimension of one or more beams is at an angle
theta or offset by a number of degrees with respect to the WBC
dimension. A full 90 degree offset would align the WBC dimension
with the slow dimension while a 45 degree offset would orient the
WBC dimension at an angle halfway between the slow and fast
dimension of a beam as these dimension are orthogonal to each
other. In one embodiment, the WBC dimension has an angle theta at
approximately 3 degrees off the fast dimension of a beam.
[0068] The above description is merely illustrative. Having thus
described several aspects of at least one embodiment of this
invention including the preferred embodiments, it is to be
appreciated that various alterations, modifications, and
improvements will readily occur to those skilled in the art. Such
alterations, modifications, and improvements are intended to be
part of this disclosure, and are intended to be within the spirit
and scope of the invention. Accordingly, the foregoing description
and drawings are by way of example only.
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