U.S. patent application number 11/132316 was filed with the patent office on 2005-12-01 for laser diode bar integrator/reimager.
Invention is credited to Brown, Daniel M..
Application Number | 20050264893 11/132316 |
Document ID | / |
Family ID | 32326564 |
Filed Date | 2005-12-01 |
United States Patent
Application |
20050264893 |
Kind Code |
A1 |
Brown, Daniel M. |
December 1, 2005 |
Laser diode bar integrator/reimager
Abstract
A beam integrator system is disclosed, which integrates multiple
beams into fewer beams of increased intensity by using a
combination of optical elements and lenses.
Inventors: |
Brown, Daniel M.; (Madison,
AL) |
Correspondence
Address: |
OLDS, MAIER & RICHARDSON, PLLC
PO BOX 20245
ALEXANDRIA
VA
22320-1245
US
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Family ID: |
32326564 |
Appl. No.: |
11/132316 |
Filed: |
May 19, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11132316 |
May 19, 2005 |
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PCT/US03/37153 |
Nov 20, 2003 |
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11132316 |
May 19, 2005 |
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10716864 |
Nov 20, 2003 |
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6888679 |
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60427571 |
Nov 20, 2002 |
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Current U.S.
Class: |
359/618 |
Current CPC
Class: |
H01S 5/4012 20130101;
H01S 5/005 20130101; G02B 19/0028 20130101; H01S 5/4031 20130101;
H01S 5/4025 20130101; G02B 19/0057 20130101 |
Class at
Publication: |
359/618 |
International
Class: |
G02B 027/10 |
Claims
What is claimed is:
1. An apparatus configured to integrate a plurality of beams to
form a beam with a near circular cross-section, comprising: means
for rotating beams by an angle to obtain associated rotated beams,
wherein the beams are generated by a plurality of emitters; means
for combining the associated rotated beams by passing the
associated rotated beams through a positive lens to form at least
one combined beam; and means for varying the cross section of the
combined beam, by passing the at least one combined beam through an
anamorphic lens.
2. The apparatus of claim 1, wherein the means for rotating beams
includes an asymmetric prism.
3. The apparatus of claim 1, wherein the means for rotating beams
includes a dove prism.
4. The apparatus of claim 3, further comprising: an integrator a
beam combining lens; and an anamorphic lens.
5. The apparatus of claim 1, wherein the means for rotating beams
includes at least one element having a positive optical power.
Description
CORRESPONDING APPLICATIONS
[0001] This application is a utility application, which is a
conversion of provisional 60/427,571, filed 20 Nov. 2002, the
entire contents of which are incorporated in their entirety.
FIELD OF THE INVENTION
[0002] The present invention relates to a method, apparatus and
system for combining and integrating the multiple laser beams of a
laser diode bar into fewer beam(s).
BACKGROUND OF THE INVENTION
[0003] Since the advent of low-cost laser diodes, their inherently
low power densities have limited the usefulness of these devices.
Various combinations of lenses and other optical devices have been
used to combine multiple laser diode beams into fewer beams (e.g.,
a single beam) to increase power density. More commonly, multiple
laser diodes or laser diode bar arrays simply serve as optical
pumps for solid state lasers, rather than using their light
directly in the laser machining or other high power laser
process.
[0004] Using the laser diode light directly in laser machining and
product marking could increase the efficiency and reduce the cost
for such systems, but has proven to be a challenge due to the low
average brightness or radiance of diode bar arrays. The low average
radiance is due to the geometry of laser diode bars. Since a single
emitting junction cannot provide the required high power, a stripe
or series of long narrow emitters stacked end-to-end is usually
fabricated into a high power laser diode bar. Each individual
emitter is approximately 500 microns or more. Thus, although the
radiance within each emitter aperture is quite high, the average
radiance over the entire length of the stripe is low due to the
dead space between the emitters.
[0005] A typical laser diode bar consists of a linear array of
rectangular emitters, with each emitter having a narrow width about
1 micron and a length of several microns up to more than one
hundred microns. Typically, the long dimension of the emitter is
coplanar with the long dimension of the array, effectively
producing a long thin line source as illustrated in FIG. 1. This is
the easiest way to manufacture a high power laser diode bar in a
single monolithic substrate. Good collimation of the light from the
line source can be achieved in a narrow direction using a simple
collimating lens, but not in the long direction. Due to the total
length of the source in the long dimension, the collimated beam
will have considerable divergence in this direction, as well as
multiple dips in the intensity profile due to the non-emitting
space between emitters.
[0006] In order to reduce this problem, some laser diode array
manufacturers solder multiple emitter bars side-by-side in a stack
as illustrated in FIG. 2. The amount of dead space is reduced
relative to emitted power (although not completely eliminated) and
the source size is more compact. Due to the soldering process, the
emitter spacing is not nearly as uniform as that of FIG. 1, and
thus is not conductive to coupling with microlens arrays for beam
integration.
SUMMARY OF THE INVENTION
[0007] An exemplary embodiment of the invention provides an optical
system for combining multiple laser beams of a laser diode bar
array into fewer beams (e.g. a single beam, two beams, . . . ) with
greater power density.
[0008] A further exemplary embodiment of the invention provides an
optical system that optically removes the non-emitting space
between the emitters so that the intensity profile of the
collimated beam is more uniform.
[0009] Other exemplary embodiments of the invention provide a means
of optically combining the individual emitters to increase the
average radiance using a multi-aperture beam integrator system.
[0010] Further exemplary embodiments of the invention provides a
lens (e.g. anamorphic lens) or array of lenses (e.g. an array of
anamorphic lenses) to reshape an incident laser beam profile(s) in
the fast and slow axis directions into desired laser beam
profile(s). The invention provides a multi-beam integrator system,
which is used along with a means of optically rotating the emitters
by an angle (e.g. 90 degrees). The angle rotation allows for better
balancing of the optical invariants in the slow and fast axis
directions, making it easier to "circularize" the image for
coupling into fibers or to increase the irradiance for laser
machining applications. Furthermore, the present invention includes
a lens (e.g. anamorphic lens) to achieve a nearly circular beam
along with providing a significant increase in irradiance.
[0011] The invention and the methods derived thereof effectively
reduce the source size so that one can achieve greater collimation
and energy density.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] Features of the invention can be more fully appreciated
through consideration of the detailed description of the invention
in conjunction with the several drawings in which:
[0013] FIG. 1 illustrates a conventional diode bar consisting of
multiple emitters stacked end-to-end in a linear array, with
non-emitting space between each emitter.
[0014] FIG. 2 illustrates another conventional stacking of the
emitters side-by-side in a more compact array;
[0015] FIG. 3 illustrates an exemplary embodiment of the invention
showing the microlens array and mirror array rotating the images of
the emitters by 90 degrees, the integrator lens superimposing all
eleven (11) far-field images of the emitters, and an anamorphic
lens circularizing the combined emitter image on the image plane,
where the source emitters are stacked end on end in the y-direction
and imaged to a single bar oriented in the x-direction;
[0016] FIG. 4 shows the image of the superimposed emitters produced
at the image plane of the embodiment in FIG. 3, where the length of
a bar image is shorter than original laser diode bar due to the
anamorphic lens, which also increases the irradiance of the
image;
[0017] FIG. 5 shows the image plane irradiance (image of
superimposed emitters) without the anamorphic lens. Emitter image
is rotated 90 degrees with respect to source emitter;
[0018] FIG. 6 shows the image plane irradiance (image of
superimposed emitters) with the dove prism array or image rotator
device removed, where the image is oriented in same direction as
source emitter;
[0019] FIG. 7 illustrates a cross-sectional view of the system of
FIG. 3, according to an exemplary embodiment of the invention;
and
[0020] FIG. 8 illustrates a cross-sectional view of a system in
accordance with an exemplary embodiment of the invention wherein an
optical array can be a prism array where the prism array element is
non symmetric, or the optical array is a diffractive array.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0021] Exemplary embodiments of the invention can be used to
increase the average source radiance and decrease the source size
of a laser diode bar, comprised of an array of emitters, by
optically reimaging and combining the emitters in such a way as to
remove or reduce the non-emitting space between adjacent emitters
and by optionally applying nonsymmetric (anamorphic) magnification
to the superimposed image.
[0022] An exemplary embodiment of the invention, the multiple
emitters of a laser diode bar configuration are reimaged and
superimposed into a single emitter image with much smaller length
so that the irradiance is increased and the image dimensions are
more nearly equal in the orthogonal directions. If this image were
reimaged to infinity, good collimation could be achieved in both
the slow and fast axis directions of the emitter. A device produced
in accordance with exemplary embodiments of the invention optically
removes the non-emitting space between the emitters so that the
intensity profile of the collimated beam is more uniform. The
images of all the emitters are optically superimposed, effectively
producing a single emitter. Furthermore, the long dimension of the
single emitter image can be reduced with a lens (e.g., simple
cylindrical lens, anamorphic lens, . . . ) so that the numerical
apertures (NAs) of the emitter image are nearly identical in both
orthogonal directions.
[0023] Although this invention applies to and illustrates focused
beams (see FIG. 3), the same advantages also apply to collimated
beams with the emitter imaged to infinity. Applications for focused
beams include laser welding, marking and materials processing. A
smaller, more uniform source is more useful to these applications.
Collimated beam applications include targeting and illumination of
distant objects.
[0024] For the purposes of describing the present invention, a
typical high power laser diode bar in linear configuration,
comprising of an array of 11 emitters on a 708-micron pitch, with
each emitter having dimensions of 1 micron by 100 microns, will be
described in this disclosure. The wavelength is 980 nm and the beam
divergences in the slow and fast axis directions are 10 deg and 28
deg FWHM respectively. Each emitter produces 1 W of power. However,
it is clear that this invention can be used with numerous other
laser diode bars with different parameters (e.g. the laser diodes
can have different dimensions, pitches, and wavelengths). With the
above described laser diode bar, 608 microns of space between
emitters is non-radiant or non-emitting. It is desired to remove
this non-radiant space and superimpose all emitters into real or
virtual image of a single uniform emitter.
[0025] Examples according to exemplary embodiments of the invention
are illustrated in FIGS. 3, 7, and 8. In one exemplary embodiment
of the invention, due to its ease of fabrication, separates the
microlens array from the image rotator array. (It is clear that
these elements could be combined into a single monolithic element.)
The image rotator array described here consists of an optical array
( e.g., an array of micro-dove prisms). Each optical array element
(e.g. prism) consists of refractive wedges on each side of the
array with a reflective mirror between the wedges. The dove prism
array may be fabricated by polishing the edges of strips of glass
plates to the proper wedge angle, then coating one side of each
glass plate with silicon, and then anodically bonding the strips
into a stack. The wedge can be non-symmetric. Alternatively, the
plates can be optically cemented. In order to combine the microlens
array and dove prism array into one element, the wedge or tilt
would be fabricated into the microlens surface profile FIGS. 3 and
7 show a ray trace of an embodiment of the present invention with
the emitter array source 17 on the left side of the figures and the
superimposed image 90 on the right. The emitters 15 are stacked
end-to-end in the vertical or y-direction, with the fast axis
divergence in the x-direction. A magnified image of the emitter, is
formed on the image plane. All the individual images of the
emitters are superimposed on the image plane forming a single
magnified bar image of the emitter, as illustrated in FIGS. 4, 5,
and 6.
[0026] FIG. 4 shows the image with the anamorphic lens included to
reduce the image size. FIGS. 5 and 6 show the image irradiance
without the anamorphic lens. The image is proportional in shape to
source emitter. FIG. 5 includes the dove prism array showing
rotation of the image by 90 degrees; FIG. 6 results when the dove
prism array is removed. Light emitted from the laser diode bar is
first collimated by the microlens array into eleven collimated
beamlets. Each beamlet is directed onto the angled face of a
micro-dove prism. The tilted face refracts the beamlet toward the
reflective face of the dove prism, which then reflects the beamlet
toward the second face of the dove prism. The second angled face
bends the collimated beam back into the original direction of
travel from the microlens. The mirror face is parallel to the
optical axis (z-axis) but angled by 45 degrees in the x-y plane.
The integrator lens 60 combines all eleven beamlets to a single
image of the emitter at image plane 80.
[0027] FIG. 7 illustrates a beam integrator system 10 according to
an exemplary embodiment according to the invention. The laser diode
bar 1 7 includes a plurality of emitters 15 which are located in an
end-to-end position with respect to the neighboring emitter. In the
exemplary embodiment each individual emitter 15 aperture can be of
variable size (e.g. 1 micron wide by 100 microns ). The spacing
between emitters 15 call vary (e.g 400 microns).
[0028] Each of the emitters 15 can have a corresponding micro-lens
20 so as to collimate the light being emitted from the emitters 15.
Alternative embodiments do not have a corresponding micro-lens 20,
instead the optical array 30 can be designed to collimate the laser
diode light. Each micro-lens 20 has a first side 25 and a second
side 27, wherein the first side 25 and second side 27 generally
opposite to one another. The emitters 15 are located on the first
side 25 of the micro-lens 20 and a corresponding dove micro-prism
30 is located on the second side 27 of each micro-lens 20. As shown
in FIG. 7, the optical elements 30 (e.g. dove micro-prisms) are
stacked top to bottom upon one another. In alternative embodiments
the micro-prisms can be staggered (not shown).
[0029] In an exemplary embodiment the optical element is a dove
micro-prism 30 have a general three dimensional trapezoidal shape
having a top side 32 and bottom side 34 and two sloped sides 36,
38. The top side 32 of each dove micro-prism 30 can be connected to
the bottom side 34 of the neighboring dove micro-prism 30 to form
the stacked array as shown in FIG. 7. Alternatively there can be a
spacing between optical elements 90. Furthermore, each of the
corresponding emitters 15, micro-lens 20 and dove micro-prisms 30
all share an axis. The array of dove micro-prisms 30 can optically
rotate the individual emitter images by a chosen angle (e.g. 90
degrees). Such rotation can be used in conjunction with a lens to
reduce the long dimension of the long dimension of the emitter
image (e.g. a rotation of 90 degrees allows an anamorphic lens
(discussed later) to easily reduce the long dimension of the
emitter image).
[0030] The beam integrator system 10 also includes an integrator
lens 60. In the exemplary embodiment, the diameter of the
integrator lens 60 can be greater than or equal to the height of
the stacked dove micro-prisms 30. In the exemplary embodiment the
integrator lens is comprised of a normal plano-convex lens.
However, this could be a bi-convex lens, a meniscus lens, or
combination of elements to minimize aberrations. The specific shape
or number of elements is not germane to the invention only that it
has positive optical power and combines the beams in an
aberration-free maimer. The integrator lens 60 combines the light
beams exiting emitters and overlap the real images of the emitters
at the focal point of the integrator lens.
[0031] FIG. 8 illustrates an exemplary embodiment of the invention
having an anamorphic lens 70 and a asymmetric optical array 30.
[0032] Once the beams of light exit the integrator lens 60, the
beams of light can be passed through an anamorphic lens--to reduce
the long dimension of the emitter image. In exemplary embodiments
of the present invention spots or linear images formed in the image
plane 80 can have sizes between 1-1000 microns, with peek power
densities from 10-1000 kW/cm2. Other exemplary embodiments have
various peek powers and sizes and the discussion herein should not
be interpreted to limit the image formed on the image plane, for
example the image can be a few microns in width and millimeters in
length having a peek power of hundreds of kW/cm2.
[0033] Variations in the shape and type of optical arrays are
intended to fall within the scope of the invention. For example the
optical array can be a diffractive array as opposed to the
refractive array.
* * * * *