U.S. patent application number 13/210786 was filed with the patent office on 2013-02-21 for method for tailoring the dopant profile in a laser crystal using zone processing.
This patent application is currently assigned to RAYTHEON COMPANY. The applicant listed for this patent is Robert W. BYREN. Invention is credited to Robert W. BYREN.
Application Number | 20130044779 13/210786 |
Document ID | / |
Family ID | 47712638 |
Filed Date | 2013-02-21 |
United States Patent
Application |
20130044779 |
Kind Code |
A1 |
BYREN; Robert W. |
February 21, 2013 |
METHOD FOR TAILORING THE DOPANT PROFILE IN A LASER CRYSTAL USING
ZONE PROCESSING
Abstract
A lasing medium having a tailored dopant concentration and a
method of fabrication thereof is disclosed. The lasing medium has a
single crystal having a continuous body having a selected length,
wherein the crystal comprises dopant distributed along the length
of the body to define a dopant concentration profile. In one
embodiment, the dopant concentration profile results in a uniform
heating profile. A method of fabricating a laser crystal having a
tailored dopant concentration profile includes arranging a
plurality of polycrystalline segments together to form an ingot,
the polycrystalline segments each having dopant distributed,
providing a crystal seed at a first end of the ingot, and moving a
heating element along the ingot starting from the first end to a
second end of the ingot, the moving heating element creating a
moving molten region within the ingot while passing therealong.
Inventors: |
BYREN; Robert W.; (Manhattan
Beach, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BYREN; Robert W. |
Manhattan Beach |
CA |
US |
|
|
Assignee: |
RAYTHEON COMPANY
Waltham
MA
|
Family ID: |
47712638 |
Appl. No.: |
13/210786 |
Filed: |
August 16, 2011 |
Current U.S.
Class: |
372/41 ; 117/37;
252/584 |
Current CPC
Class: |
C30B 13/20 20130101;
C30B 29/28 20130101; C30B 13/24 20130101; C30B 29/12 20130101; H01S
3/1643 20130101; H01S 3/1611 20130101; C30B 13/10 20130101; H01S
3/0617 20130101 |
Class at
Publication: |
372/41 ; 117/37;
252/584 |
International
Class: |
H01S 3/16 20060101
H01S003/16; C30B 13/24 20060101 C30B013/24 |
Claims
1. A method of fabricating a single, continuous laser crystal
having a tailored dopant concentration profile, the method
comprising: arranging a plurality of polycrystalline segments
together to form an ingot, the polycrystalline segments each having
dopant distributed therein; providing a crystal seed at a first end
of the ingot; moving a heating element along the ingot starting
from the first end to a second end of the ingot, the moving heating
element creating a moving molten region within the ingot while
passing therealong.
2. The method of claim 1, wherein the dopant comprises neodymium,
ytterbium, erbium, holmium, or a combination thereof.
3. The method of claim 1, wherein the heating element uses RF
induction.
4. The method of claim 1, wherein each of the segments has a
different dopant concentration from other segments.
5. The method of claim 1, wherein each of the segments has a
different length.
6. The method of claim 1, wherein the seed crystal comprises a
dopant concentration equal to a target concentration at the first
end of the ingot.
7. The method of claim 1, wherein a segment at the first end of the
ingot has a higher dopant concentration than a segment at the
second end of the ingot.
8. The method of claim 1, wherein moving the heating element along
the ingot comprises the heating element surrounding a portion of a
length of the ingot.
9. The method of claim 1, wherein interfaces between the moving
molten region and the segments of the ingot are substantially
normal to the axis.
10. The method of claim 1, wherein the crystal comprises YAG
(Y.sub.3Al.sub.5O.sub.12), YLF (YLiF.sub.4), GGG
(Gd.sub.3Ga.sub.5O.sub.12).
11. A lasing medium comprising: a single crystal having a
continuous body having a selected length, wherein the crystal
comprises dopant distributed along the length of the body to define
a dopant concentration profile.
12. The lasing medium of claim 11, wherein the dopant concentration
profile results in a substantially uniform heating profile.
13. The lasing medium of claim 11, wherein the laser crystal is
fabricated with the method of claim 1, and wherein the laser
crystal is machined to a final size.
14. The lasing medium of claim 13, wherein the laser crystal is
machined using saw cutting, core drilling, grinding, polishing,
coating, or a combination thereof.
15. The lasing medium of claim 11, wherein the dopant comprises
neodymium, ytterbium, erbium, holmium, or a combination
thereof.
16. The lasing medium of claim 11, wherein the crystal is YAG
(Y.sub.3Al.sub.5O.sub.12), YLF (YLiF.sub.4), or GGG
(Gd.sub.3Ga.sub.5O.sub.12).
17. The lasing medium of claim 11, wherein the lasing medium is in
the shape of a rod or slab or configured as a planar waveguide.
18. The lasing medium of claim 11, wherein the dopant concentration
is a function of distance along the length of the crystal.
19. A single crystal having a tailored dopant concentration
profile, produced by a process comprising the steps of: arranging a
plurality of polycrystalline segments together to form an ingot,
the polycrystalline segments each having dopant distributed
therein; providing a crystal seed at a first end of the ingot;
moving a heating element along the ingot starting from the first
end to a second end of the ingot, the moving heating element
creating a moving molten region within the ingot while passing
therealong.
Description
BACKGROUND
[0001] The present disclosure relates to solid-state lasers. More
specifically, the present disclosure relates to a laser crystal
having a tailored dopant profile, the method of fabricating
thereof, and a lasing medium fabricated from said laser
crystals.
[0002] Solid-state lasers are currently being developed and used
for a variety of military and industrial applications, including
range finding, target designation/marking, illumination,
three-dimensional imaging, vibration sensing, profilometry,
cutting, drilling, welding, heat treating and other material
processing, electro-optical and infrared countermeasures, and
directed energy weapons. A solid-state laser typically includes a
laser amplifier medium or lasing medium disposed within an optical
resonant cavity. The resonant cavity or resonator provides the
feedback necessary to build oscillation of electromagnetic
radiation within the laser. The bulk lasing medium is typically in
the shape of a slab, rod, or disk. When pumped, the medium provides
amplification by a process of stimulated emission. The provision of
reflective surfaces or gratings at the ends of the lasing medium
provides a resonator.
[0003] In a typical laser, an incoherent light source imparts
energy to the lasing medium, which produces light in which the
waves are in phase through particular electron transitions. Where
the lasing medium is properly designed, this "coherent light" is
emitted as a beam.
[0004] Commercial laser gain media typically comprise single
crystals having substantially uniform dopant concentration, such as
Nd:YAG (neodymium doped yttrium-aluminum-garnet). Developmental
lasers are being designed with optical-quality poly-crystalline
ceramic lasing media which offer size and cost advantages over
conventional single-crystal media. Solid-state lasing media, doped
with an active ion, often use one or more flash lamps or laser
diodes to provide "pump light." The diode pump light excites the
active ions in the doped crystalline or ceramic lasing medium to a
higher energy state. This process is known as "absorption." A "pump
cavity" typically contains a uniformly doped lasing medium, which
may be a crystal or glass or polycrystalline ceramic element
fabricated in the shape of a rod, slab, or disk, and other
elements, such as a pump light reflector or relay optics. Pump
light is coupled into the cavity, typically with one or more flash
lamps or laser diodes, either from the side of the cavity (i.e.,
side pumping) or the end of the cavity (i.e., end pumping).
[0005] Efficient absorption, in which nearly all of the pump light
is absorbed by the doped medium, is a primary goal of laser
designers. One method of attaining efficient absorption is by using
high-absorption (highly doped) laser materials. A ray of pump light
going through a doped crystal one time is known as a "pass." With
most existing designs, a pump light ray makes only one or two
passes through the doped crystal before escaping, necessitating the
use of high-absorption materials to achieve efficient absorption.
Absorption is governed by an exponential function. Thus, when such
a crystal is side-pumped, non-uniform absorption and thus
non-uniform gain often result, with the highest gain being near the
edge of the lasing medium. The concentration of gain near the edge
of the medium leads to problems with parasitic oscillation and
amplified spontaneous emission (ASE), extraction, efficiency, and
beam quality (mode control). This is particularly problematic with
respect to rod shaped media.
[0006] Another approach to the goal of high efficiency absorption
uses end pumping, in which pump light comes into a pump cavity
along its longitudinal axis. End pumping requires high-brightness
pump diodes and durable dichroic coatings, since the pumping and
laser light extraction take place through the same optical surfaces
(i.e. the ends of the rod) while requiring quite different
reflectivity characteristics. In the case of quasi-four level
(e.g., ytterbium doped yttrium aluminum garnet, Yb:YAG) or
three-level systems (e.g., ruby) where the high threshold requires
greater pumping rate, pump "bleaching" can occur, in which a large
fraction of the active ions have been excited and correspondingly
fewer ions are in the ground state available for pump light
absorption, resulting in reduced absorption for both side- and
end-pumping geometries.
[0007] A laser crystal having a tailored concentration profile may
be especially useful for laser applications which may use a
high-aspect-ration slab geometry for the lasing medium. One special
case of the slab geometry is the planar waveguide (PWG) which is
advantageous for applications requiring high gain, high average
power, and high efficiency. As is known in the art, the PWG has a
planar geometry, which guides light only in the thin dimension of
the slab. For an end-pumped laser, the dopant absorbs the pump
energy along the length of the medium and releases it radiatively
as photons and non-radiatively as heat. Thus, heat is a function of
both the energy pumped into the laser material and the dopant level
of the laser material. Accordingly, as the pump energy and/or the
dopant concentration are increased, both laser emission and heat
generation are increased. Uniform dopant concentration that is
typically used in lasing media results in localized heating. This
is because the pump energy is absorbed by the dopant and thus
decreases as it travels through the lasing medium. Accordingly, the
material near the pump light end receives the most energy and
produces the most heat, resulting in localized heating. Heat
effects may have a negative impact on the laser efficiency and beam
quality of high-average-power solid state lasers, and thus
composite structure materials such as bonded crystals and composite
ceramics have been used to mitigate the heat effects.
[0008] To achieve constant heating throughout the lasing medium, a
tailored longitudinal concentration profile (of active ion dopant)
is needed. Lasing media with different concentrations along the
length thereof have been used. For example, multiple single crystal
segments each having a different concentration are bonded together
to form a lasing medium with a stepped dopant concentration
profile. However, these lasing media are bonded at interfaces that
cross the laser beam axis, resulting in media that are more
expensive to fabricate and prone to damage. Alternatively, there
are lasing media having a concentration profile that is created by
mixing ultra-fine powders of different concentrations along the
laser axis of the body. The structure is then sintered to form a
dense, optically clear ceramic. However, these media may not
exhibit the same superior lasing performance as a pristine single
crystal.
[0009] What is needed is a method and apparatus that addresses one
or more of the deficiencies noted above in fabricating lasing media
having a tailored dopant concentration profile.
SUMMARY
[0010] One embodiment of this disclosure provides a method of
fabricating a single, contiguous laser crystal having a tailored
dopant concentration profile. The method includes arranging a
plurality of polycrystalline segments together to form an ingot.
The polycrystalline segments each have dopant distributed therein.
The method further includes providing a crystal seed at a first end
of the ingot and moving a heating element along the ingot starting
from the first end to a second end of the ingot. The moving heating
element creates a moving molten region within the ingot while
passing therealong.
[0011] Another embodiment provides a single crystal having a
tailored dopant concentration profile, produced by a process that
includes the steps of arranging a plurality of polycrystalline
segments together to form an ingot. The polycrystalline segments
each have dopant distributed therein. The steps also include
providing a crystal seed at a first end of the ingot and moving a
heating element along the ingot starting from the first end to a
second end of the ingot. The moving heating element creates a
moving molten region within the ingot while passing therealong.
[0012] Another embodiment provides a lasing medium that includes a
single crystal having a continuous body having a selected length,
wherein the crystal comprises dopant distributed along the length
of the body to define a dopant concentration profile that results
in a uniform heating profile. The lasing medium may be produced by
machining the single crystal using processes known in the art such
as core drilling, saw cutting, grinding, polishing, and coating to
produce a final lasing medium with a desired shape and optical
characteristics.
[0013] These and other features and characteristics of the present
disclosure, as well as the methods of operation and functions of
the related elements of structure and the combination of parts and
economies of manufacture, will become more apparent upon
consideration of the following description and the appended claims
with reference to the accompanying drawings, all of which form a
part of this specification, wherein like reference numerals
designate corresponding parts in the various figures. It is to be
expressly understood, however, that the drawings are for the
purpose of illustration and description only and are not intended
as a definition of the limits of the inventive concept. As used in
the specification and in the claims, the singular form of "a",
"an", and "the" include plural referents unless the context clearly
dictates otherwise.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] In the accompanying drawings:
[0015] FIG. 1a illustrates a starting ingot formed from a plurality
of polycrystalline segments;
[0016] FIG. 1b illustrates a heating element passing through the
ingot and creating a liquid zone therein during float zone
processing;
[0017] FIG. 2 illustrates a liquid zone and post-melt and pre-melt
regions of the ingot during float zone processing;
[0018] FIG. 3 schematically depicts the liquid zone and the
post-melt and pre-melt regions of the ingot during float zone
processing and identifies the parameters used in corresponding
mathematical equations;
[0019] FIG. 4 is a plot of a final concentration profile for a
laser crystal;
[0020] FIG. 5 is a plot of comparisons between a target
concentration, a concentration profile resulting from float zone
processing performed on a uniform ingot, and a concentration
profile resulting from float zone processing performed on multiple
segments having different dopant concentrations.
DETAILED DESCRIPTION
[0021] Lasing media can be fabricated to have a tailored dopant
concentration profile. In some embodiments, the lasing media
includes an elongated, single crystal having a continuous body
having a selected length. The crystal may include dopant
distributed along the length of the body and may have a dopant
concentration profile in accordance with a target dopant
concentration profile.
[0022] The lasing medium may be fabricated using float zone
processing or zone melting. Float zone processing has been used in
the semiconductor industry to purify crystals by melting a narrow
region of the crystal. This molten zone is then moved along an
ingot by moving a heating element along the longitudinal axis of
the crystal. As the molten region moves through the ingot, this
molten region melts impure solids and leaves behind a single
crystal region of purer materials as it solidifies. As a result,
the impurities concentrate in the melt, and are moved to one end of
the ingot. The purifying process works on the principle that, since
the segregation coefficient k, which is the ratio of an impurity in
the solid phase to that in the liquid phase, is usually less than
one, the impurity atoms will diffuse to the liquid region at the
solid/liquid boundary. Thus, by passing a crystal boule through a
thin section of furnace very slowly, such that only a small region
of the boule is molten at any time, the impurities may be
segregated at the end of the crystal.
[0023] FIGS. 1a-1b illustrates using float zone processing to
tailor the concentration of active lasing species (or dopants)
within a laser crystal. As known in the art, dopants are typically
inserted into a substance in order to alter the electrical
properties or the optical properties of the substance. In the case
of crystalline substances, the atoms of the dopant commonly take
the place of elements that were in the crystal lattice of the
material. For example, YAG, which is also known as yttrium aluminum
garnet (Y.sub.3Al.sub.5O.sub.12), is a popular synthetic crystal
material that is usually doped with some element to form a laser
crystal. The yttrium ions in YAG can be replaced with laser-active
rare earth ions (e.g., neodymium) up to some concentration limit
without strongly affecting the lattice structure. The concentration
limit is determined by size of the dopant ion (e.g., neodymium)
relative to that of the substituted ion (e.g., yttrium). These
dopant ions may essentially carry out the lasing process in the
crystal. The other atoms in the crystal (i.e., the yttrium,
aluminum, and oxygen atoms) support the dopant atoms and provide a
crystal field that influences the energy band structure of the
laser. A variety of crystal materials may be used, for example,
Y.sub.3Al.sub.5O.sub.12, YLiF.sub.4, or Gd.sub.3Ga.sub.5O.sub.12. A
variety of dopants may also be used, just for example, ytterbium,
erbium, thulium, or holmium.
[0024] As shown in FIG. 1a, starting ingot 10 is formed from
plurality of polycrystalline segments 12. The starting ingot may be
oriented such that its longitudinal axis is vertical. In one
embodiment, each polycrystalline segment 12 has a different dopant
concentration from the other segments. However, it is contemplated
that segments 12 may have the same dopant concentration or
alternatively may be a single polycrystalline segment. It should be
appreciated that the number of segments 12 may vary in other
embodiments. The length of each segment 12 and the concentration in
each segment 12 may also vary to achieve the target concentration
profile. Segments 12 may be vertically stacked without bonding or
sintering and held in place only by gravity. Single seed crystal 14
may be provided at first end 16 of ingot 10 and arranged with
segments 12 to form ingot 10. Seed crystal 10 may be substantially
pure or doped with a concentration of dopant. Seed crystal 14
lattice orientation is the same as the desired orientation of the
resulting lasing crystal. Ingot 10 may also include a second end 18
opposite first end 16. Heating element 20 may be used to form
liquid zone 22. In one embodiment, ingot 10 is oriented such that
its longitudinal axis is vertical with seed crystal at the top, the
end of seed crystal not adjacent to a polycrystalline segment is
clamped or bonded to a holding fixture to offset the force of
gravity, and heating element 20 is moved vertically from top to
bottom while ingot 10 is held stationary. The starting location of
heating element 20 is near the interface between the seed crystal
14 and the adjacent polycrystalline segment 12' such that end 16 of
seed crystal 14 remains a crystallized solid and defines the
crystal structure and lattice orientation of the resulting lasing
crystal. Heating element 20 may be provided by RF induction or any
other methods or apparatuses. Just for example, in some
embodiments, heating element 20 may be induction coils, ring-wound
resistance heaters, or gas flames. In one embodiment, ingot 10 may
be heated radiatively using an induction-heated tungsten ring. In
some embodiments wherein ingot 10 is electrically conductive an
electric current may be passed through the ingot while it is
suspended in a magnetic field with the current controlled such that
the material is magnetically levitated to minimize gravity sag in
the liquid zone 22.
[0025] The liquid zone 22 formed by heating element 20 may similar
to the "molten zone" described above with respect to purification
of crystals. Liquid zone 22 moves through ingot 10 and disperses
the dopants through ingot 10 to form the dopant concentration
profile. FIG. 1b shows heating element 20 moving through ingot 10
in the direction of A, and thus moving liquid zone 22 through ingot
10. As heating element 20 moves liquid zone 22 through ingot 10, a
resulting crystal portion 24 having the desired concentration
profile is formed.
[0026] Seed crystal 14 and each segment 12 of ingot 10 may be doped
with a selected active lasing species, which behaves as the
"impurity" in the float zone purification process described above.
However, rather than refining the ingot, the process of FIG. 1
produces a single crystal having a desired or target
one-dimensional dopant profile. The resulting profile may be
achieved by selecting the proper dopant concentration within each
segment 12 such that the natural diffusion of active lasing species
within the liquid zone and the difference in solubility of active
lasing species between solid and liquid phases results in the
desired profile. The difference in solubility of active lasing
species between solid and liquid phases which gives rise to the
lowering of concentration in the single crystal region is
characterized by the segregation coefficient for the particular
dopant within the particular crystal. In some embodiments, seed
crystal 14 should be doped with the same concentration as desired
at first end 16 of the resulting crystal and segments 12 near seed
crystal 14 that will be melted first should have a higher
concentration of dopant than the target concentration. In such
embodiments, segments 12 closer to second end 18 of ingot 10 may
have less concentration of dopant. Thus, in the embodiment shown in
FIG. 1a, the concentrations of segments 12 are decreasing from
first end 16 to second end 18. The smoothness of the doping profile
may depend on the steepness of the desired concentration gradient
and the number of polycrystalline segments 12 in ingot 10.
[0027] FIG. 2 shows an expanded view of the region around liquid
zone 22 where the dopant species are mixed within the liquid during
melting. Region 21 represents a pre-melt region or condition and
has a concentration of C.sub.I. C.sub.I represents the initial
concentration of dopant species by weight. C.sub.I can be a
constant if a single uniformly-doped segment is used.
Alternatively, if multiple segments 12 having different dopant
concentrations are used, C.sub.I may be a function of distance
along the length of ingot 10 in the direction of A. Region 23
represents the post-melt region or condition having a concentration
of C.sub.F. C.sub.F represents the final concentration, which is a
function of distance along the length of the resulting crystal. It
should be appreciated that multiple segments 12 may be used and
their lengths and concentrations tailored to give a final
concentration profile. For example, the final concentration of the
resulting crystal may be tailored by varying the length of each
polycrystalline segment, varying the concentration of the dopant in
each polycrystalline segment, varying the length of the liquid
zone, varying the number of passes that the heating element is
moved along the ingot, and varying other factors that will be
described below. Accordingly, C.sub.I represents the pre-melt
condition and C.sub.F represents the post-melt condition.
[0028] Float zone processing that is performed on polycrystalline
segments 12 can convert polycrystalline lasing material to a single
crystal where a standard growth process (e.g., Czochralski growth
process) is impossible, impractical from a size standpoint, and/or
results in unwanted stress regions within the crystal. For example,
neodymium-doped YAG formed by the Cazochralski growth process has a
stressed region formed along the center of the crystal that is not
useable for lasing media. In contrast, the resulting crystal formed
by the float zone processing of multiple polycrystalline segments
12 has a continuous body, a tailored dopant concentration profile
along the length of the body, and no substantially stressed
regions. The resulting crystal may be a single crystal with the
identical crystal structure and lattice orientation as crystal seed
14 and a concentration profile that can be arbitrarily tailored
with precision by varying any of the factors or parameters
described below.
[0029] FIG. 3 shows the same region as FIG. 2 and shows the
parameters used to analyze the doping profile. The parameters are
defined as below:
[0030] L=length of liquid zone
[0031] x=distance along ingot
[0032] C.sub.I(x)=concentration (by weight) of the starting
ingot
[0033] C.sub.F(x)=concentration (by weight) of final laser
crystal
[0034] s=amount of dopant present in liquid zone at a given
location
[0035] A=cross section area of ingot
[0036] k=segregation coefficient (ratio of dopant concentration in
solid to that in liquid across solidus/liquidus interface)
[0037] .rho.=specific gravity of solid crystal
[0038] The molten region (liquid zone 22 shown in FIGS. 1a, 1b)
propagates from left to right in the direction of A as heater 20 is
moved accordingly. As liquid zone 22 advances by an infinitesimal
distance, dx, the amount of dopant added to liquid zone 22 from the
ingot is C.sub.I(x)A.rho.dx. The amount of dopant removed from
liquid zone 22 at the retreating crystal interface is (ks/L)dx.
Therefore, the net addition of dopant to liquid zone 22 when zone
22 advances by dx is ds=[C.sub.I(x)A.rho.-(ks/L)]dx.
[0039] The boundary condition at the seed crystal end is
s(0)=C.sub.I(0)AL.rho.. The concentration of the final crystal
boule is given by C.sub.F(x)=ks/(AL.rho.). If the starting ingot
has uniform doping (C.sub.I=constant), then the differential
equations can be solved explicitly, yielding an exponentially
increasing value of C.sub.F(x) given by C.sub.F(x)=C.sub.I[1-(1-k)
exp (-kx/L)].
[0040] The above equations can be solved for any given
concentration profile for the polycrystalline ingot. That is, to
tailor the concentration profile, the above equations may be used
to determine the value of the parameters. Alternatively, the input
values of the parameters may be used to determine the resulting
concentration profile.
[0041] In one embodiment, the resulting lasing medium is
neodymium-doped yttrium aluminum garnet (Nd:YAG). Nd:YAG offers
substantial laser gain even for moderate excitation levels and pump
intensities. The gain bandwidth may be relatively small, but this
allows for a high gain efficiency and thus low threshold pump
power. The segregation coefficient for neodymium in YAG (k=0.18) is
very low due to the poor fit of the neodymium ion as a substitute
impurity in the yttrium lattice site. This low value, however,
produces a substantial concentration gradient in the float zone
process, which may be desirable for certain end-pumping
applications. The area of the ingot might not be a factor in the
analysis, but the interfaces between the solid and the liquid
phases should be relatively flat and normal to the direction of A.
This may prevent or minimize a lateral component to the
concentration gradient, which may not be desirable. However, the
resulting crystal may have features or performance characteristics
that vary based on the float zone processing apparatuses, the
physical and thermal design of the laser pump head, and the
handling and thermal robustness of laser crystal 12.
[0042] In one embodiment, the crystal may have the following
parameters:
[0043] L=0.5 cm, 1 cm, 1.5 cm
[0044] .rho.=4.56 g/cm.sup.3
[0045] C.sub.I(0)=1 atomic percent=1.36.times.10.sup.20 Nd
atoms/cm.sup.3=2.98.times.10.sup.19 Nd atoms/g
[0046] k=0.18
[0047] FIG. 4 plots the final concentration profile for a laser
crystal with the above parameters. That is, FIG. 4 plots the final
concentration profile for laser crystal 12 for several liquid zone
lengths after a single pass of heating element 20 along the length
of the starting ingot that was doped at 1 atomic percent neodymium.
In particular, FIG. 4 shows the concentration profiles for crystals
having liquid zone lengths of 0.5 cm, 1 cm, and 1.5 cm. Plot A
shows the concentration profile for crystals having liquid zone
length of 0.5 cm, plot B shows the concentration profile for
crystals having liquid zone length of 1 cm, and plot C shows the
concentration profile for crystals having liquid zone length of 1.5
cm. The mass density is 4.56 g/cm.sup.3. The starting amount of
neodymium dopant (C.sub.I(0)) may be one atomic percent
(1.36.times.10.sup.20 Nd atoms/cm.sup.3=2.98.times.10.sup.19 Nd
atoms/g). The segregation coefficient of neodymium in YAG is
0.18.
[0048] FIG. 5 shows a comparison of a resulting crystal having a
final concentration produced by using multiple segments of
different concentrations versus the resulting crystal having a
final concentration produced by a simple float zone process with a
uniformly doped starting ingot 10. The target concentration shown
in this Figure represents a near-optimal concentration profile for
a small 5 cm long laser crystal designed to be the active layer of
a high aspect ratio PWG slab structure. The resultant concentration
profile for the uniformly-doped ingot is also shown in FIG. 5,
where the starting concentration of the ingot (1.95.times.10.sup.19
atoms/g) is tailored to give the same final concentration as the
target profile at the lean end (3.32.times.10.sup.18 atoms/g). The
resultant concentration for the segmented ingot is also shown where
each segment is 0.5 cm long and has the following
concentration:
[0049] Segment 1: 1.8.times.10.sup.19 atoms/g
[0050] Segment 2: 6.0.times.10.sup.18 atoms/g
[0051] Segment 3: 8.0.times.10.sup.18 atoms/g
[0052] Segment 4: 9.5.times.10.sup.18 atoms/g
[0053] Segment 5: 1.2.times.10.sup.19 atoms/g
[0054] Segment 6: 1.6.times.10.sup.19 atoms/g
[0055] Segment 7: 1.8.times.10.sup.19 atoms/g
[0056] Segment 8: 2.3.times.10.sup.19 atoms/g
[0057] Segment 9: 3.1.times.10.sup.19 atoms/g
[0058] Segment 10: 3.7.times.10.sup.19 atoms/g
[0059] Segment 11: 4.8.times.10.sup.19 atoms/g
[0060] In some embodiments, the extra segment at the end may be
sacrificed to allow the float zone to pass through the entire
useful region of the slab without discontinuity. As shown in FIG.
5, the concentration profile produced by float zone processing on
multiple segments of different dopant concentrations as described
above is closer to the target concentration than the concentration
produced by a simple float zone process on a uniformly doped
starting ingot 10. Accordingly, the dopant concentration profile of
a single crystal may be tailored by performing float zone
processing on a plurality of polycrystalline segments.
[0061] As mentioned above, uniformly doped lasing media may result
in the material in the pump end receiving the most energy and
producing the most heat, thus resulting in localized heating.
However, the tailored dopant levels within the single crystal
produced by the float zone processing described above may result in
uniform heating and uniform laser emission throughout the crystal.
That is, the tailored dopant profile of the single crystal may
result in a strong, robust lasing medium having a uniform heating
profile that can produce higher output power.
[0062] The above description has been provided for the purpose of
illustration based on what are currently considered to be the most
practical implementations, but it is to be understood that such
detail is solely for that purpose, and that the inventive concept
is not limited to the disclosed embodiments, but, on the contrary,
is intended to cover modifications and equivalent arrangements that
are encompassed by the appended claims. For example, it is to be
understood that the present disclosure contemplates that, to the
extent possible, one or more features of any embodiment can be
combined with one or more features of any other embodiment.
[0063] Furthermore, since numerous modifications and changes will
readily occur to those with skill in the art, it is not desired to
limit the inventive concept to the exact construction and operation
described herein. Those with skill in the art may discover other
advantages of and applications for the inventive concept in the
manufacture of solid-state lasers and other fields without
departing from the spirit and scope of this invention.
* * * * *