U.S. patent application number 12/423738 was filed with the patent office on 2009-08-13 for low stress optics mount using thermally conductive liquid metal or gel.
This patent application is currently assigned to LOCKHEED MARTIN COHERENT TECHNOLOGIES, INC.. Invention is credited to Glenn Bennett, Michael Robert Browning, Mark Jon Kukla, Steven Craig Palomino, Lawrence Francis Rubin, Dane Lewis Schnal.
Application Number | 20090199389 12/423738 |
Document ID | / |
Family ID | 38575822 |
Filed Date | 2009-08-13 |
United States Patent
Application |
20090199389 |
Kind Code |
A1 |
Bennett; Glenn ; et
al. |
August 13, 2009 |
LOW STRESS OPTICS MOUNT USING THERMALLY CONDUCTIVE LIQUID METAL OR
GEL
Abstract
An optical assembly with mounting provided to effectively
transfer heat away from an optic, such as a slab or waveguide
amplifier or laser disk, while limiting internal stresses. The
assembly includes an optic with a planar surface. A heat sink is
positioned in the assembly with an upper surface next to the planar
surface of the optic. The upper surface of the heat sink comprises
a recessed surface defining a reservoir for containing a compliant
heat transfer material. The assembly may further include a volume
of the heat transfer material, such as a liquid metal or thermally
conductive gel, in the reservoir of the heat sink. In one
embodiment, the optic is a slab amplifier with a reflective coating
or layer that directly contacts the heat transfer material in the
heat sink reservoir or a foil or membrane is provided between the
heat transfer material and the slab.
Inventors: |
Bennett; Glenn; (Boulder,
CO) ; Browning; Michael Robert; (Littleton, CO)
; Kukla; Mark Jon; (Brookline, NH) ; Palomino;
Steven Craig; (Longmont, CO) ; Rubin; Lawrence
Francis; (Lafayette, CO) ; Schnal; Dane Lewis;
(Boulder, CO) |
Correspondence
Address: |
MCDERMOTT WILL & EMERY LLP
18191 VON KARMAN AVE., SUITE 500
IRVINE
CA
92612-7108
US
|
Assignee: |
LOCKHEED MARTIN COHERENT
TECHNOLOGIES, INC.
Louisville
CO
|
Family ID: |
38575822 |
Appl. No.: |
12/423738 |
Filed: |
April 14, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11392151 |
Mar 29, 2006 |
7551656 |
|
|
12423738 |
|
|
|
|
Current U.S.
Class: |
29/434 ;
29/460 |
Current CPC
Class: |
Y10T 29/4984 20150115;
H01S 3/0941 20130101; H01S 3/025 20130101; H01S 3/0407 20130101;
H01S 3/042 20130101; Y10T 29/49888 20150115 |
Class at
Publication: |
29/434 ;
29/460 |
International
Class: |
B23P 11/00 20060101
B23P011/00; B23P 19/04 20060101 B23P019/04 |
Claims
1. A method of mounting an optic to a heat sink, comprising:
providing a metallic heat sink assembly comprising a surface with a
recessed portion having a predefined volume; mounting an optic onto
the heat sink assembly such that at least one surface of the optic
is positioned proximate to the recessed portion of the surface of
the heat sink assembly; and filling the recessed portion of the
heat sink assembly with liquid metal or thermally conductive gel,
wherein the liquid metal or thermally conductive gel contacts the
at least one surface of the optic.
2. The method of claim 1, further comprising prior to the mounting
of the optic, attaching a metallic foil to the surface of the heat
sink assembly, the attaching being performed to create a
liquid-resistant seal about the periphery of the recessed portion,
whereby the liquid metal or thermally conductive gel contacts the
metallic foil which in turn contacts the at least one surface of
the optic.
3. The method of claim 1, wherein the metallic heat sink assembly
further comprises an expansion reservoir of variable volume that is
fluidically linked to the recessed portion and wherein the filling
of the recessed portion of the heat sink assembly comprises filling
the expansion reservoir with a volume of the liquid metal or
thermally conductive gel.
4. The method of claim 1, wherein the heat sink assembly comprises
a frame holding the optic and wherein the method further comprises
after the filling of the recessed portion, detaching a portion of
the heat sink from the frame.
5. The method of claim 1, further comprising attaching onto the
heat sink assembly means for controlling movement of the mounted
optic relative to the heat sink assembly.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 11/392,151 entitled "Low Stress Optics Mount Using
Thermally Conductive Liquid Metal or Gel," filed on Mar. 29, 2006,
which is hereby incorporated by reference in its entirety for all
purposes.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention is generally related to mechanical mounts and
methods of mechanical mounting, and in particular, to methods and
apparatus for mounting optical components, such as lasers, in a
manner that controls stress development and that also permits high
heat flow from the components.
[0004] 2. Relevant Background
[0005] High power lasers and laser amplifiers having very high beam
quality are difficult to make because a number of factors degrade
performance. One such factor is heat deposition within the laser
material resulting from imperfect conversion of pump radiation
power into laser output power. Most frequently it is not absolute
temperature rise that is of concern but, instead, spatial gradients
within the material. It is well known that parameters, such as the
refractive index, are temperature dependent, and consequently
temperature gradients lead to refractive index gradients, which in
turn degrade the performance of the laser or amplifier. Heat loads
and poor mechanical mounting techniques also cause stresses in the
materials that degrade performance by introducing wavefront
distortions.
[0006] In an attempt to address problems associated with stresses
generated during mounting, considerable efforts have been expended
within the laser industry in finding ways to mount laser rods and
slabs that optimize optical performance. Compounding the problem is
the fact that a relatively large amount of heat must be removed
from the material during mounting, and as a result, implemented
mounting techniques typically provide a good heat conduction path
to a heat sink or cold plate where the heat is removed through
convection, conduction, or radiation. A further complication is
that laser radiation must be extracted from the rod or slab through
apertures and at least some of the laser material must be exposed
to permit pump light to enter the material for absorption.
[0007] A number of techniques have been conceived to mount round
laser rods, and these techniques are aimed at permitting good heat
flow and at addressing stress production. One technique is
disclosed by Guch in U.S. Pat. Nos. 4,594,716 and 4,601,038.
Another technique is described by Rapoport et al. in U.S. Pat. No.
5,331,652. A third technique that is more specifically aimed at
mounting round laser rods for low stress is disclosed by Sumida in
U.S. Pat. No. 5,272,710 and involves mounting the rod in a
transparent sleeve with an elastomer providing a mechanically
compliant conduction path between the rod and sleeve. Common to all
these techniques is the assumption of round laser rods. Such
circular geometries are attractive in part because of the ease of
fabrication, but, unfortunately, these mounting techniques are not
particularly well suited for slabs having a rectangular shape (or
not being round in cross section).
[0008] Slab geometries are attractive for several reasons and are
particularly useful in the generation of high optical output
powers. First, they provide at least one rectangular flat surface
through which pump light can enter. Second, with uniform pumping,
slab geometries promote one-dimensional temperature gradients.
Third, they provide a method to "zig-zag" a laser beam within the
medium. The latter provides for a way to extract energy efficiently
with good beam quality. In order to maximize the advantages
inherent in such slab geometry, it is, however, important to design
the mounting and cooling arrangements very carefully.
[0009] Some efforts have been made in the laser industry to address
some of the challenges associated with mounting of slab geometries.
For example, a number of issued patents describe methods of
mounting slabs that incorporate a gas or liquid flow channel
between the source of pump radiation and the slab, e.g., U.S. Pat.
Nos. 4,378,601; 4,468,774; 4,881,233; and 4,563,763. These methods
or approaches have several drawbacks and do not adequately meet the
needs of the laser industry. First, these methods assume the
presence of a flowing cooling liquid and, therefore, preclude
operating with a passive heat disposing mechanism. Second, these
methods are often susceptible to the depositing of unwanted
contaminants on the surface of the slab that may degrade the
performance of the laser.
[0010] A somewhat more attractive solution may involve an
arrangement in which passive heat spreaders and radiators are used
without flowing coolants, such as cooling gas or liquid, being
provided as an intrinsic part of the construction. For example,
U.S. Pat. No. 4,949,346 discloses a method to sandwich a slab
between transparent conductive heat sinks which also act as guides
to transport pump light to the slab. However, the described method
fails to solve the problem of removing large amounts of heat from
the assembly. Also, the method includes bonding the slab to the
conductive heat sinks, which does not work well at high thermal
loads since it promotes stresses within the laser slab. An
alternative method applicable to end-pumped slabs is disclosed in
U.S. Pat. No. 6,014,391. In this alternative method, curved
surfaces are used to concentrate pump light, and absorbing
materials are attached to the ends of the slab. However, this
approach is very complex and requires fabrication of curved
surfaces, which is more difficult and expensive than the
fabrication of flat surfaces.
[0011] None of these methods effectively provides for heat
conduction directly to a high thermal conductivity material.
Generally, heat conduction takes place through an intermediate
material, such as elastomers, glasses, or crystals. These materials
have thermal conductivities that are generally 10 to 1000 times or
more lower than metals and consequently provide a far higher
thermal resistance than metals.
[0012] Some attempts have been made utilizing amalgams of mercury.
Unfortunately, the use of toxic mercury carries with it potential
health hazards, and additionally, these efforts have applied only
to round geometries. More significantly, these efforts do not
effectively reduce stresses on the laser material because the
mounting techniques teach providing a fixed volume of amalgam,
which typically will result in transferring stresses to the laser
rod when parts of the assembly undergoes thermal expansion as is
explained below with reference to FIGS. 1A to 1D.
[0013] Of course, removing heat from confined spaces is a problem
faced in industries other than the optics or laser industry. For
example, removing heat is often a concern in the operation of
integrated circuits. Numerous methods have been devised to conduct
heat from integrated circuits, and several of these methods involve
the use of liquid metals. Such heat removal methods are disclosed,
for example, in U.S. Pat. Nos. 6,665,186; 6,748,350; 6,281,573;
6,656,770; 5,658,831; 5,572,404; and 5,561,590. The general purpose
of these heat removal techniques differs in fundamental ways from
the purpose in optics in that the desire is to permit the
conducting medium to expand and contract rather than the heat
source. The integrated circuit chip itself (i.e., the heat
generator) is typically rigidly bonded on one side to a mount, and
the compliant conducting material fills the space between the other
side of the chip and a heat sink. As a result of such construction,
the conducting material may deform as a result of relative motion
between the chip and heat sink generating stresses internal to the
chip. For at least this reason, bonding techniques used for
integrated circuits do not address the needs for "stress-free"
mounting of optics such as lasers.
SUMMARY OF THE INVENTION
[0014] The present invention addresses the problems discussed above
by providing a method for mounting slabs in a substantially
stress-free state while also permitting pump light to enter the
slab without passing through a liquid flow and also not assuming
the presence of a flowing gas or liquid for cooling. Furthermore,
the thermal resistance is very low in assemblies created by
mounting techniques of the invention as a conduction path between
slab and heat removal is in many embodiments constructed entirely
from high thermal conductivity materials. Furthermore reservoirs
with variable volume may be created, which enable stresses on the
laser slab to be minimized as the slab expands and contracts during
heat loading.
[0015] According to one aspect of the invention, a semi-rigid slab
mount is provided that holds a slab firmly in place while also
permitting the slab to expand and contract under a heat load. This
may be accomplished by contacting the slab with a layer of a high
thermal conductivity medium, preferably a liquid metal or a
thermally conductive gel or slurry confined to a hollowed-out space
in a heat sink. To prevent leakage of the liquid metal or gel, the
slab is preferably sealed to the heat sink such as by using, for
example, but not as a sealing limitation, an elastomeric insulating
adhesive gasket. The gasket is preferably thermally insulating and,
in some embodiments of the invention, is made from a cured
adhesive.
[0016] In another embodiment of the invention, the mounting of a
slab firmly while also allowing expansion/contraction is
accomplished by bonding the slab to a flexible membrane, e.g., a
thin metal foil, on one side. This would preferably be soldered if
the membrane were a metal foil. The other side of the foil is in
contact with a high thermal conductivity medium, preferably a
liquid metal or a thermally conductive gel or slurry. In both of
the preceding embodiments, the liquid or gel fills a reservoir in
the heat sink, which is preferably outfitted with bellows to permit
the volume of the reservoir to change as the slab deforms due to
the thermal gradient present and as the thermally conductive medium
expands or contracts. This arrangement has been demonstrated to
permit stress-free mounting of slabs and is compatible with
multiple heat removal mechanisms including liquid flow channels or
mechanical coupling to radiators or heat pipes.
[0017] In one preferred embodiment, the liquid or gel is confined
to a thin layer between the optic (or other object to be mounted)
and the heat sink to promote substantially one-dimensional
conductive heat flow through the liquid. In an alternative
embodiment, this thin layer is made thicker to permit convective
flow, as well as conduction, in order to increase the thermal
conduction rate.
[0018] In addition to providing the functions of stress-free
mounting and high thermal conductance, the presence of the liquid
layer as provided by the mounting methods of the invention has
other benefits. One such benefit is damping of mechanically
introduced acoustic noise. The invention is highly suited to
fabricating operating slab lasers and amplifiers and is also suited
to a range of other applications including, but not limited to,
stress-free mounting and cooling of thin-disk lasers or optics that
experience heating due to partial absorption of incident laser
light. Bidirectional heat flow through a liquid metal interface is
also suitable in applications where optical elements must be
temperature controlled, such as, for example, optical parametric
oscillator/amplifier or laser waveguide crystals.
[0019] More particularly, an optic assembly is provided that has
enhanced heat removal. The assembly includes an optic with a heat
transfer surface (such as one or more of the sides of a slab) and a
heat sink that is configured for conductive heat transfer. The heat
sink is positioned in the assembly with an upper surface adjacent
to the heat transfer surface of the optic. The upper surface of the
heat sink includes a recessed surface that defines a reservoir or
internal (but open) cavity, and the assembly further includes a
volume of compliant or non-rigid heat transfer material, e.g.,
liquid metal or gel, provided to fill the reservoir. In some
embodiments, the reservoir is sized to have a depth (measured from
the upper surface) to provide a thickness of the compliant heat
transfer material of less than about 0.005 inches. In some cases, a
liquid-resistant sheet (e.g., a metallic foil or elastomeric
material) is provided in the assembly and is interposed between the
upper surface of the heat sink (or bonded to this surface about the
periphery of the recessed portion or reservoir) and the heat
transfer surface of the optic, whereby the heat transfer material
is sealed into the reservoir. In other cases, the heat transfer
material directly contacts or wets to the optic, which may have a
layer of reflective material or other coating applied to form all
or a part of the heat transfer surface.
[0020] Further, a method is provided for mounting an optic to a
heat sink to form an optic assembly with improved heat transfer.
The method includes providing a metallic heat sink assembly with an
upper surface with a recessed portion having a predefined volume.
An optic is then mounted onto the heat sink assembly such that at
least one surface of the optic is positioned adjacent or proximate
to the recessed portion of the upper surface of the heat sink
assembly. The method continues with filling the recessed portion of
the heat sink assembly with liquid metal or thermally conductive
gel such that the liquid metal or gel contacts the surface of the
optic that was positioned adjacent or proximate to the heat sink
assembly. This may be a direct contact (e.g., with the metal or gel
wetting to the surface) or may be indirect, with the method further
including attaching a metallic foil to the upper surface of the
heat sink assembly about the periphery of the recessed surface to
provide a seal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIGS. 1A to 1D show a slab under varying conditions of
thermal loading and mounting.
[0022] FIGS. 2A and 2B show cross-sectional views of a liquid metal
mount or mount assembly according to the present invention, using
an elastomeric "gasket."
[0023] FIGS. 3A and 3B show the liquid metal mount, such as the
mount or assembly of FIGS. 2A and 2B, further incorporating heat
exchangers.
[0024] FIGS. 4A and 4B show a 3-dimensional or perspective,
exploded view and cross-sectional view of a liquid metal mount
according to the present invention, using an elastomeric
"gasket."
[0025] FIGS. 5A and 5B show an alternative construction or
embodiment of a liquid metal mount according to the present
invention, using a flexible membrane.
[0026] FIGS. 6A and 6B show a 3-dimensional or perspective view of
a liquid metal mount formed according to the alternative
construction shown in FIGS. 5A and 5B.
[0027] FIGS. 7A to 7C show the construction of a liquid metal mount
according to the present invention as applied to a thin-disk
laser.
[0028] FIGS. 8A and 8B illustrate representative heat flow in thin
and thick layers of liquid.
[0029] FIG. 9 illustrates a pair of assemblies configured to
operate in conjunction according to some embodiments of the
invention.
[0030] FIGS. 10A to 10D shown another alternative construction or
embodiment of a liquid metal mount according to the present
invention configured to control vertical movements of a mounted
element such as a slab.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0031] FIG. 1A illustrates a rectangular optical amplifier slab 101
having a top surface 102 through which pump light 105 enters the
material and is absorbed. The slab 101 may be formed of a host
crystal doped with an ion, such as, for example, ytterbium doped
yttrium aluminum garnet (Yb:YAG) or other similar material such as
materials useful for slabs in solid state lasers, including
crystalline materials, glasses, and ceramics. Left end surface 103
acts as the signal input surface where an incident beam 104 enters
the amplifying material, traverses the slab 101, and exits as
amplified beam 106 through the other end surface. When the slab 101
is pumped in this manner, side surfaces (e.g., surface 107) do not
play a vital part in the operation of the slab 101. As pump light
105 is absorbed in the slab 101, a fraction of the optical power is
converted to heat that preferably is removed. For single-sided
pumping, this is typically done through conduction through the
bottom surface opposite the top surface 102. A problem that arises
during fabricating a slab-based optic assembly or during mounting
methods is how to mechanically mount the slab 101 within such an
assembly in a manner that heat is efficiently removed while at the
same time the slab 101 is maintained in a stress-free or relatively
stress free state.
[0032] If the slab 101 were initially resting on a conducting heat
sink 108 and heat was being removed through the bottom surface, the
slab 101 would tend to deform as illustrated in highly exaggerated
form in FIG. 1B. This results from differing expansion in the slab
101 as the top portion of the slab 101 has a higher temperature
than the bottom of the slab 101 adjacent the heat sink 108. For a
number of applications, this deformed state is actually desired
because permitting the slab 101 to bow minimizes the internal
stresses. However, in other conventional mountings of a slab 101,
techniques such as that illustrated in FIG. 1C are frequently used.
In this arrangement, the slab 101 is essentially forced to sit flat
against the heat sink 108 through the use of a bonding layer 109
that bonds the slab 101 to the heat sink 108. One problem with this
method is that by forcing the slab 101 to be flat the top part of
the slab 101 is internally in compression while the bottom part of
the slab 101 near the bonding layer 109 is in tension. The
resulting stress gradients can easily be large enough to cause
stress-optic effects that severely distort the quality of the laser
beam undergoing amplification in the slab 101. As one skilled in
the art may note, one way to mount the slab would be to create heat
sink 108 such that its interface surface is fabricated in the
operationally deformed shape of the slab, but the stresses would
then be present in the non-operational state, which is again
generally undesirable.
[0033] For the sake of completeness, it is also noted that slabs
are frequently not constructed with perpendicular end faces, as
shown, where the incident beam 104 makes a straight pass through
the slab 101 as indicated in FIG. 1A. More commonly, the
construction is as illustrated in FIG. 1D. Here, the end faces are
cut at an angle such that an incident beam 110 refracts at the
input face and traverses the slab in a zig-zag fashion as a result
of total internal reflection before exiting the slab as beam 111.
In the following discussion, slabs are illustrated with a
rectangular construction (or cross section), but it is stressed
that this is for illustrative purposes only with "rectangular"
slabs often having the form shown in FIG. 1D. The manner in which
optical beams are propagated through the material is not essential
to operation of the invention. Similarly, the side faces need not
be perpendicular for this invention to apply.
[0034] Additionally, as shown in FIG. 1D, coatings may be applied
to the top and bottom surfaces of slabs. The top coating 112 is
generally an anti-reflection dielectric coating designed to
minimize the reflective loss for pump light entering the slab. The
bottom coating 113 is generally designed to be reflective at the
pump wavelength such that pump light that is not absorbed in one
pass through the slab is further absorbed in a second pass
following reflection from coating 113. The top coating 112 is not
essential to the present invention and will not be discussed
further (although it may, of course, be utilized in laser
assemblies or devices formed according to the present invention).
In contrast, the bottom coating 113 is one useful feature of the
invention, and how it is included and utilized in laser assemblies
will be discussed further below.
[0035] One parameter of interest in building high quality
amplifiers is the transmitted wavefront, i.e., the degree to which
an optically flat wavefront is distorted in propagation through the
slab, such as in the slab between or from 110 to 111 as shown in
FIG. 1D. Spatially uneven cooling or stresses imparted to the slab
from the mechanical mounting leads directly to such distortions.
Extensive testing of mounting techniques by the inventors have
shown that bonded interfaces, such as the one illustrated in FIG.
1C, generally yield unsatisfactory results (i.e., uneven and/or
poor cooling) and present a great limitation on optical performance
of slabs under high heat loads. For example, thermally conductive
silicone, exemplified by NuSil CV-2946 from NuSil Corporation,
provides a good heat conduction path (e.g., when provided as a
layer of material contacting the slab such as bonding layer 109 in
FIG. 1C, but the post-curing stiffness of the material still
imparts sufficient stresses in slabs to degrade performance when
heat loads are applied. Similar or worse results have been obtained
with a variety of materials that include epoxies, RTV-type
adhesives, thermal greases, and thermal greases filled with diamond
particles and copper meshes.
[0036] To address these disappointing results obtained with
conventional materials, the current invention uses liquid metal to
provide stress-free mounting and a heat transfer or cooling path
with high thermal conductivity for efficient heat removal. The
general idea is that a liquid provides an excellent interface to a
solid without producing local pressure variations. Preferably,
mounting techniques (and assemblies produced by such methods) are
adapted to allow the liquid to expand and contract. As a result,
the interface is also highly compliant in the sense that it permits
the heat source dimensions to vary without increasing the internal
stresses. This is in contrast to the electronic chip applications
discussed in the background in which the heat source dimensions
vary little but the liquid interface is compliant against relative
movement between a heat sink and the chip. Further, liquid metals
have very high thermal conductivity and are consequently very
efficient at transferring heat. Materials like thermal greases and
silicone have typical thermal conductivities less than
approximately 4 W/m.degree. K. whereas liquid metals employed in
the invention can have thermal conductivities>15 W/m.degree. K.,
which represents a considerable improvement in heat transfer
capabilities. As the field of materials science introduces higher
conductive gels or slurries in the future, these materials would
likely be acceptable for use with this invention as well, and the
invention is not limited to presently available materials.
[0037] FIG. 2A shows a cross-sectional view of one embodiment or
laser assembly (or mount) of the invention, and as shown, liquid
metal is provided in direct contact with the slab of the assembly.
The slab 101 preferably has a bottom coating (not shown due to its
relative thinness). The slab 101 rests on a heat sink 202 with the
bottom coating contacting or being adjacent to the heat sink. Small
areas 205 and 206 (e.g., "mechanical reference contacts") provided
in opposite ends or edges of the heat sink 202 near the ends of the
slab 101 are in contact with the slab 101 to provide a mechanical
reference. A portion (e.g., a "liquid reservoir") of the heat sink
202 is hollowed out and filled with a liquid metal 203. This
filling is done, for example, by providing an access port 208 where
the liquid can be injected as shown by arrow 209. The hollowed out
portion or liquid reservoir of the heat sink 202 is substantially
completely filled with liquid metal after which the filling port
208 is sealed off. This may be done in a number of different ways.
One example is to use a thin walled tube as the filling port 208
and crimping the tube as illustrated at point 210. In a preferred
embodiment, an expansion reservoir is also provided to allow the
volume of the liquid metal 203 to change. This may be achieved, as
an example, with bellows 211, which are also filled with the liquid
metal 203. Another element of the complete unit (mount or laser
assembly) 200 is a seal or "gasket" 207 provided on a recessed
surface of the heat sink 202, created for example using an
elastomeric thermally insulating adhesive, which reduces the risk
of the liquid metal 203 escaping the assembly 200.
[0038] FIG. 2B shows the assembly in operation under a thermal
load. In operation, pump light 212 enters the slab as discussed
above and differentially heats it. As the slab distorts, the center
portion of the slab moves in the upwards direction (e.g.,
attempting to create a somewhat arcuate cross section). This
movement slightly increases the volume of the liquid filled cavity
in the heat sink. The required extra volume is drawn from the
reservoir or bellows 211, which consequently contracts as indicated
at 213. It is also clear that if the liquid metal is heated such
that its expansion is greater than the volume increase due to slab
deformation, the bellows would expand rather than contract. As can
be seen, the assembly 200 operates so as to permit the slab 101 to:
a) be mechanically registered to the heat sink 202; b) deform
without creating stresses (or, at least, with reduced stresses
being created) in the slab material; and c) conduct heat very
efficiently to the heat sink 202.
[0039] Uniform heat conduction and stress free mounting are two
desired operating conditions of slab amplifiers or slab laser
assemblies. A third desired operating condition or parameter is the
effective removal of heat through the use of a heat exchange or
transfer mechanism, which may be provided as an additional
component or feature of assemblies of the invention and may be
accomplished in numerous ways. Two examples of heat exchange or
transfer mechanisms are illustrated in FIGS. 3A and 3B. In FIG. 3A,
the assembly 200 is connected (e.g., heat sink 202 or the like) to
a passive radiator having cooling fins 301. In the alternative
embodiment shown in FIG. 3B, the heat sink of assembly 200 contains
one or more internal channel 302 having an input port 303 and an
output port 305 such that a cooling medium, for example water, can
be pumped through the channel from 304 to 306 and carry heat away
from the heat sink. In some cases, the heat exchanger element or
feature does not need to be a permanent part of the heat sink
assembly but may be attached for easy removal and mounting in any
number of conventional ways, such as bolted on with an interface
having a high heat transfer coefficient.
[0040] The inventors have fabricated an assembly 400 (with
components similar to that as described for the assembly of FIGS.
2A and 2B), which is illustrated in an exploded view in FIG. 4A.
The fabricated assembly 400 comprises five primary parts: the slab
101 as has been described above, an injection-molded elastomeric
seal 410, a "picture frame" element 420, a cold plate 430, and a
heat exchanger 440 (e.g., in this assembly 400, a heat sink can be
thought of as being provided by the elements 410, 420, 430, and
440). The seal 410 has several features. The seal 410 is
preferably, but not necessarily, formed in-place using tooling and
includes a thermally insulating elastomeric adhesive that, when
cured, acts to both seal and hold the slab 101 in place in the
picture frame element 420. When the assembly 400 is assembled, the
slab 101 seats into the seal 410, which in turn seats in recess 421
in the picture frame 420. This forms a picture frame subassembly
indicated by bracket 470. In this assembled state, the slab 101 is
registered against picture frame features (e.g., end walls or slab
registration members) 404 and 405. Side ribs 401 of the seal 410
seals the sides of slab 101 against the elongate sides of the
recess 421, and end ribs 402 and 403 seal the ends of the slab 101
against the picture frame 420. An advantage of making this
subassembly 470 made up of the slab 101, the seal 410, and the
picture frame 420 is that the subassembly 470 can be removed from
the rest of the assembly 400, for example to examine or clean the
underside of the slab 101, without having to break the elastomeric
seal 410.
[0041] Cold plate 430 generally includes a flat part that seats
against the underside of picture frame 420. The cold plate 430 may
have a protrusion 408 that fits into the underside of recess 421 in
the picture frame 420 leaving a small gap between the protrusion
408, the picture frame 420, and the slab 101 (or a heat transfer
surface of the slab 101 positioned proximate or adjacent to the
picture frame element 420) to be filled with liquid metal or gel
(not shown). While the protrusion 408 is not necessary for
functioning of the invention, it is sometimes convenient to have in
an assembly 400. For example, by altering the height of the
protrusion 408, the thickness of the liquid metal layer may be
easily altered. Holes 406 and 407 are provided in order to allow
the assembly to be filled with liquid metal (i.e., are
inlets/outlets for a reservoir formed in the heat sink of assembly
400). The holes 406 and 407 may be sealable or preferably, are
connected to an expansion reservoir as discussed above with these
additional components are not shown for clarity. During assembly,
the picture frame subassembly 470 is mated to cold plate 430 in
order to form cold plate assembly 480. Generally, there is an
o-ring gland cut into the cold plate 430 indicated generally by
dashed outline 409, so that the picture frame 420 and the cold
plate 430 may be sealed against leaks of the liquid metal. Screws
and other hardware (not shown) may also be included to mate the two
parts.
[0042] The complete assembly process for the full assembly 400
includes attaching the cold plate assembly 480 to a heat exchanger
440 that removes heat that is transferred from the slab 101 to the
cold plate 430. In cases where water cooling is used, the heat
exchanger has a water inlet 413 and a water outlet 414 and an
internal flow channel 415, such that water can flow from direction
411 into the flow channel, and out in direction 412. The heat
exchanger 440 also generally has a machine o-ring gland indicated
generally by dashed outline 416. Screws and other hardware (not
shown) may be used to mate the cold plate subassembly to the heat
exchanger 440.
[0043] The fully assembled assembly 400 as described is further
illustrated by a functional (not to scale) cross-sectional drawing
in FIG. 4B. In FIG. 4B it can be seen that slab 101 seats against
the picture frame 420 via seal 410. Cold plate 430 seats against
the bottom of picture frame 420 leaving a small gap that is filled
with the liquid metal or gel 450. The cold plate 430 in turn seats
against heat exchanger 440 leaving an internal space which gets
filled with flowing water 460. Approximate locations of o-rings to
seal the subassemblies as discussed above, are also indicated by
exemplary black dots, such as o-ring 417.
[0044] The mounting techniques described with respect to FIGS. 2A
to 4B have several advantages as discussed above. In particular,
the assemblies provide for a highly compliant interface directly to
the liquid metal and consequently, substantially no stresses are
transmitted to the slab from the "bonding layer" or heat sink. From
this description, it should be apparent that it is desirable in the
construction of the assembly to provide an effective method or way
to mount the slab and to also seal it but often the exact manner in
which these are achieved is not limiting of the invention. The
in-place formed gasket is convenient and has been demonstrated to
work well but other methods to accomplish the same sealing result
will likely be evident to those skilled in the art after reading
this description and are considered within the breadth of this
disclosure.
[0045] FIGS. 5A to 6B illustrate a variation on the disclosed
mounting technique that uses a flexible membrane (such as for
example a thin metal foil) through which heat flows to the liquid
metal or other highly conductive liquid, gel, or slurry and that is
also compatible with harsh environments. The general construction
is very similar to the already described technique, with at least
one significant difference. Rather than contacting the liquid metal
directly to the slab, the heat sink assembly 501 (see FIGS. 5A and
5B) is fabricated as a sealed unit using a thin membrane that also
acts to transfer heat from the slab to the liquid metal.
[0046] A cross-sectional view of such an assembly 500 is shown in
FIG. 5A. The assembly 500 has a number of elements in common with
FIGS. 2A and 2B, and these will not be described in detail again.
These include the heat sink 502 and the liquid metal 503 in a
liquid reservoir of heat sink 502. In the assembly 500, a thin
membrane 504, such as a 0.001 inch thick sheet of a metal such as
nickel or the like, is stretched across the heat sink frame and
welded (illustrated as bead 505) or otherwise sealably attached to
the heat sink 502. In this manner, a completely sealed unit 501 is
formed having an internal cavity or reservoir that is filled with
liquid metal 503, where the thin foil membrane 504 will still allow
the slab 101 to deform under the heat load.
[0047] Optical operation and attachment of the slab 101 is achieved
with a multi-layer structure as illustrated by layers 506-508 in
FIG. 5A. For clarity in illustration, the layers 506-508 are shown
with highly exaggerated thicknesses. Layer 506 is generally a
highly reflective coating, such as a reflective at the pump
wavelength, and serves to confine light to the interior of the slab
101. Coating 507 is one or more coatings that protect layer 506 as
well as acting to provide a solderable surface (when required).
Layer 508 is a low melting point and relatively ductile solder and
may have a thickness of approximately 0.002'' (0.05 mm) or the
like. The thickness determination is driven by two principal
factors. On one hand, it is desirable to have as thin a layer as
possible to minimize heat transfer resistance while on the other
hand, it is desirable to use a thick enough layer that internal
solder stresses can be "relaxed out." As an example, indium-based
solders used in thicknesses of approximately 0.002-0.005 inches
meet these specifications or factors. Lines 509 indicate that the
slab 101 may be located such that solid metal support is present
under the slab 101. It is also noted that the slab 101 may be of
substantially the same length as the heat sink assembly 502 or the
ends of the slab 101 may protrude as illustrated in FIG. 5A. In
such cases, it is generally desired that the pumped region of the
slab 101 does not extend very far past the unsupported portions of
the slab 101. However, it may be desired in some applications to
have the supports 509 located at the neutral points of the slab 101
such that the deformation at the ends would go down while the
center portion goes up (or vice versa). This configuration would be
covered by the present invention as well as any modifications
thereof. The disclosed method is not limited to slabs consisting of
a single piece of laser active material. On the contrary it is
compatible with hybrid structures as well, such as slabs that
consist of a laser active portion and undoped endcaps attached to
the ends of slab 101. Such constructions are well known in the art
and are in some cases used to ensure that heat deposition and
consequent surface deformation is minimized at surfaces where the
laser beam enters and exits the amplifying medium.
[0048] Heat removal from the heat sink 502 may be carried out in a
number of ways, including the use of fins or cooling channels as
previously discussed with reference to FIGS. 3A and 3B.
Alternatively, multiple cooling channels 514 may be provided within
the heat sink 502 as shown. These may also constitute heat pipes.
Alternatively, as illustrated in FIG. 5B, the foil 504 can be
mounted to a frame or first heat sink 502 with an opening into
which a separate or second heat sink 515 can be inserted. This
would enable the heat sink 515 to be made such that it could be
removed, which, in turn, would allow the same slab mount design to
be used for different methods of heat removal (i.e. liquid, heat
pipe, or convective cooling) by use of differing heat sink assembly
515 designs. ill cases where such a removable heat sink 515 is
used, it is generally necessary to seal the insert 515 to the frame
502 to prevent leakage of liquid metal. This may be accomplished
for example through the use of o-rings as illustrated by 520.
[0049] FIGS. 6A and 6B illustrate the construction of a suitable or
exemplary sealed assembly 600 according to the invention. FIG. 6A
illustrates an exploded view showing the main elements: a slab 601,
a solder layer 602, a membrane 603 that is stretched over a heat
sink 604, and an internal cavity or reservoir 605 for containing
liquid metal (not shown). The width of the foil membrane 603 is
such that any edge effects from the welded or other seal will not
have an appreciable effect on the deformed shape of the slab 601.
Not shown in FIGS. 6A and 6B are the filling port and the bellows,
which would also typically be incorporated into the design of
assembly 600. FIG. 6B shows the assembled state of assembly 600 and
also illustrates with line 606 the typical location of a welding
seam that seals the membrane 603 against the heat sink or frame
604. In a prototype assembly similar to the illustrated assembly
600 fabricated by the inventors, the weld was produced using
electron-beam welding after stretching a nickel foil across the
heat sink in a jig built for that purpose.
Materials Selection
[0050] Selecting proper materials for use in the invention is
important to adequate operation. One of the more important
materials is the compliant heat transfer medium (e.g., liquid metal
203 of FIG. 2A). Several liquid metals are available that can
advantageously be used. Mercury (Hg) has excellent thermal and
mechanical properties but has the disadvantages of generally being
considered unsafe. NaK has a desirably low melting point of
-12.degree. C. but reacts strongly with air and water and
therefore, also has some undesirable properties. Gallium (Ga) is
liquid at room temperature and this property is frequently used
when liquid metals are required. However, the 29.8.degree. C. the
melting point is sometimes too high to be useful in laser
applications. Pure Ga is also highly corrosive and/or absorptive on
most metals, including aluminum (AI), which is frequently used to
fabricate lightweight heat sinks and other mechanical mounts.
However, this material can advantageously be used if contact
surfaces are first appropriately coated to prevent corrosion. This
may be done with, for example, a nickel, platinum, or chromium
coating.
[0051] In some embodiments, the heat transfer materials employed
are alloys of Ga. In particular, the material known as Galinstan
(available from Geratherm Medical AG, Germany), which is an alloy
of gallium, indium (In), and tin (Sn), is used and has excellent
thermal and mechanical properties and is less corrosive than pure
gallium. The low toxicity in comparison with mercury and low
melting point (-20.degree. C.) of this gallium alloy makes it easy
to handle and highly suitable for laser applications. It should be
noted that, if a Ga alloy is used, the side of the foil and heat
sink/frame can be plated with, for example, gold to aid wetting of
the Ga alloy to the foil and heat sink/frame. Another suitable
material that has been used by the inventors is Indalloy 46L.
[0052] The above discussion has specifically stated liquid metal as
the compliant thermally conductive medium. However, it is noted
that what is important is not the specific material, but rather the
physical properties of the material in maintaining a good thermal
interface without requiring pressure between surfaces. The term
"liquid metal" should therefore be interpreted as any material that
meets desired operating parameters of compliance and thermal
conductivity. The compliance parameter can also be met with: fluids
of low to high viscosity; non-Newtonian visco-elastic fluids;
flowing or thixotropic gels; or materials that are similar in
nature. The main properties of concern are that the material remain
compliant and continuous and present low thermal impedance (for
example, ensuring that the material wets the surfaces) over the
operating temperature regime while the heated surface distorts. The
thermal conductivity parameter is generally met by materials having
a thermal conductivity in excess of approximately 5 to 10
W/m.degree. K. It is further noted that while the liquid metal
layer can be formed simply by the liquid, it is quite possible, and
in certain circumstances highly desirable, to produce hybrid
interfaces. One example would be to incorporate a metal mesh, such
as one made from copper, into the liquid metal layer. One advantage
of this approach is that it may increase wicking, i.e., the liquid
is drawn more easily into the layer through the presence of the
mesh. The presence of a high thermal conductivity mesh in the
reservoir or internal cavity of the heat sink would also increase
the effective thermal conductivity of the liquid metal layer and
thereby, reduce the temperature rise across the interface.
Alternative Embodiments
[0053] The above discussion has indicated a primary use of the
disclosed slab assemblies with improved heat transfer as being
laser amplifiers. However, it is stressed that this is only an
example of a use. The disclosed technique can be used in a wide
variety of situations where an object must be mounted in a
stress-free manner while providing a high thermal conductivity path
for heat removal. In the field of photonics, another exemplary use
is mounting of thin disks used for disk lasers. Such lasers are
typically constructed from a thin circular disk of laser active
material. Typical materials include doped crystals, such as YAG,
YLF, or others were known in the art, or doped glasses. Common
dopants include rare-earth materials, including Nd, Yb, Er, Tm, Pr,
and Ho. Highly doped crystals are preferred as they permit pump
light incident on the circular face of the crystal to be absorbed
in a short distance. The disk thickness may range from on the order
of 1 mm to only several hundred micrometers. Use of very thin disks
is desired to minimize temperature rise in the material as the
disks are generally cooled from the backside. The thin nature of
the disk, in conjunction with a diameter that may be for example 10
mm or much greater, makes mounting critical in order that stresses
and deformations are well controlled. In conventional construction,
the disk is generally bonded to a heat sink using a thin layer of
permanent bonding material.
[0054] FIGS. 7A to 7C illustrate the use of liquid metal in
mounting thin laser disks (e.g., where the "object" to be mounted
is a disk rather than a rectangular slab). FIG. 7C is a front view
of a typical thin disk laser assembly 701 comprising the laser disk
702 and a gasket 705 to provide a seal between the disk 702 and
heat sink 703. FIG. 7A is a cross-sectional view of the assembly
701, wherein is also illustrated a cavity in the heat sink that is
filled with liquid metal 704 in contact with the back side of the
laser disk 702. The purpose of the liquid metal 704 is again to
provide a stress-free interface to the thin disk 702 while
efficiently transferring heat to the heat sink 703. The assembly
701 as described may also incorporate heat exchange mechanisms as
discussed in conjunction with FIGS. 3A and 3B, and one exemplary
arrangement is shown in FIG. 7A by finned radiator 709 that may be
attached to heat sink 703 or may be an integral part of the heat
sink 703. Filling ports and bellows mechanisms are not explicitly
shown in FIGS. 7A to 7C but may use the same construction
principles as those discussed above.
[0055] It is also possible to bond the disk 702 to a sealed
assembly 710 as illustrated in FIG. 7B, where a membrane 711 is
attached by, for example, welding 712 the membrane 711 to heat sink
703. The method of bonding the disk 702 to the sealed assembly 710
follows the same principles as those used to discuss FIGS. 5A and
5B including layers 708 and 707 (corresponding to previously
discussed layers 506 and 507) and solder layer 706.
[0056] In the preceding discussion of the liquid, the layer
thickness has not been illustrated to scale and has been shown in
FIGS. 5A and 5B as a relatively thick cavity or reservoir in the
illustrated heat sinks. The thickness of the liquid layer is
generally not critical but should be optimized for the particular
application. Too thick a layer may lead to a larger than acceptable
temperature rise across the liquid metal layer as the thermal
conductivity of the liquid metal is typically substantially less
than the thermal conductivity of the heat sink. For example, the
thermal conductivity of the liquid metal may be in the range of
8-20 W/m.degree. K., while that of aluminum, as frequently used for
heat sinks, is greater than 200 W/m.degree. K. Too thin a layer may
impede the flow of the liquid metal (e.g., when filling the heat
sink) or may create problems when freezing. It has been found that
layer thicknesses in the range of, for example, 0.005 to 0.020
inches work well, although other thicknesses outside this range may
also work well for specific applications.
[0057] In general the thickness can be designed or selected to
promote two types of heat flow, as illustrated in FIGS. 8A and 8B.
In FIG. 8A, the interface between an optic 801 (e.g., a slab, disk,
or other "object") and a heat sink 802 is illustrated as being
filled with a liquid metal layer 803. In the case where the liquid
metal layer 803 is thin, for example, having a thickness in the
range from less than 0.001'' to several thousandths of one inch,
the heat transfer is primarily one-dimensionally conductive as
illustrated by arrows 804 that indicate the direction of heat flow.
FIG. 8B illustrates another useful configuration of the invention
in which optic 801 and heat sink 802 are spaced apart by a thicker
layer 805 of the liquid metal. With a thicker layer 805, conduction
still takes place as indicated by straight arrows 806 but at the
same time, convective currents as indicated by arrows 807 may form
in the liquid 805. This liquid circulation 807 is sometimes
beneficial as it increases the heat transfer rate from optic 801 to
heat sink 802. It is noted with reference to FIGS. 8A and 8B that
for simplicity only the operating principle is illustrated.
Consequently membranes and/or coating layers that would normally be
present at the liquid metal/optic interface have been omitted.
[0058] The preceding discussion has further assumed that the
desired heat transfer is from an optic or mounted object to a heat
sink. However, the process is entirely reversible so that heat can
equally well flow from a heat sink or more accurately a heating
element to an optic. This may be desirable for example where
thermal control of an optic or other stress-sensitive items is
required. One example of such a case of practical interest is the
temperature control of optical parametric oscillator (OPO) crystals
(e.g., these crystals would be the mounted objects in an assembly
built according to the invention). When temperature tuning of an
OPO is used, it is desired to control the temperature of the
crystal using external heaters/coolers. Such crystals are also
susceptible to stresses, not only because of thermally induced
distortions, but also because some such materials fracture easily.
Mounting such a crystal with a highly compliant and thermally
conductive interface permits heat to flow both to and from the
crystal in an efficient manner. In such cases, the heat sink
discussed above should be interpreted as a heat conduction
interface that thermally connects the optic to a heater/cooler that
may include for example thermo-electric coolers capable of
bi-directional heat transfer.
[0059] The above discussion has noted use of an optical amplifier.
It is understood that the invention is not limited to one-pass or
multiple-pass amplifiers, but that the term incorporates more
general use, such as the use of an amplifying medium contained
within a laser resonator.
[0060] It was noted previously that operation of a slab or similar
device to a degree involves a choice between letting the slab
deform but be stress-free on the one hand or forcing the slab not
to deform but to accept stresses. The discussion in this disclosure
assumed that in some cases the benefits of deformation outweigh the
negative aspects of deformation, and in many cases, this trade-off
is even optimal. However, in other situations, it may be that
deformations cause an incident laser beam to undergo a change in
pointing direction, or translation, or both, as a result of the
deformation. In situations where this is a concern, it may be
remedied in several ways.
[0061] As one example, in a case where the primary effect is
mispointing of a laser beam, two assemblies may be operated in
opposition such that the mispointing in one slab is compensated by
the reverse mispointing in the second assembly. An example of such
a configuration is shown in FIG. 9 where a slab assembly 901 that
includes generally a slab 902, a liquid metal interfaced heat sink
903, and pump light 904 is followed by a second substantially
identical assembly 905 (e.g., the two assemblies 901, 905 are
placed adjacent to each other or generally "in-line"). The latter
assembly 906 is, however, flipped in the vertical plane as shown.
The benefit of this arrangement is that if an incident light beam
906 propagating in the horizontal direction exits the first slab at
an angle .beta. to the horizontal, the angular misalignment .beta.
is reversed in the second assembly 905. The result is that the
output beam 908 emerging from the second assembly 905 is now
aligned with the incident beam 906. It is stressed that
misalignments .beta., when they exist to any significant degree,
are generally very small but is illustrated in FIG. 9 as large for
clarity. It is also noted that the above is meant to describe the
type of method useful in remedying a specific issue. In many
realistic cases, particularly for slabs operated in zig-zag mode,
great care must generally be used in designing both the assemblies
themselves and compensation methods as, for example, the number of
bounces within a slab may undergo a fractional change, which may
not translate into a simple angular deviation of the laser
beam.
[0062] It is finally noted that the discussion herein has centered
around optics where one surface acts as the heat transfer surface.
From the disclosure it is, however, clear the technique can be
extended to multiple surfaces, including non-flat surfaces. For
example, it is possible to end pump a slab and use the disclosed
heat transfer technique to remove heat through both top and bottom
surfaces using liquid metal heat transfer, e.g., by providing a
heat sink assembly on the top as well as the bottom surface (as is
shown in the supporting figures). Another specific example of such
utility is to use top and bottom cooling of waveguide lasers or
amplifiers. These devices have an active region that is generally
thin, such as 100 micrometers, sandwiched between upper and lower
layers that promote wave guiding of light in the central active
region parallel to the active layer. Such devices may be end pumped
or side pumped (or in some cases face pumped perpendicular to the
guiding layer) and can benefit from top and bottom cooling with a
method that does not strain the device. The double-sided cooling
approach can then equally well be extended to cool all four sides
of the slab while still permitting end-pumping and entry/exit of
the laser beams. Further extensions of the approach then also
permits round rods and other non-flat surfaces to be mounted and
cooled using the disclosed liquid metal technique.
[0063] In the preceding discussion, it has been noted that the
liquid metal is contained within a closed volume that may vary in
magnitude (e.g., amount of liquid metal or gel may vary as well as
the shape and depth of the recessed surface receiving the liquid
metal). At the same time, some of the embodiments described above
are constructed in such a manner that they may allow the slab to
move vertically in the presence of differential pressure variations
between the liquid and the outside region of the slab. Similar
motion may take place in the presence of mechanical vibrations or
shocks. Of particular concern is that operation in a low-pressure
environment or a vacuum (for example, in space) may cause the slab
to not be as rigidly fixed to the mounting frame as desired for
particular implementations. Such situations may be remedied in a
number of ways, a few examples of embodiments that are useful for
better limiting movement of the slab or optical element are shown
in FIGS. 10A to 10D.
[0064] In FIG. 10A, the bellows attached to tube 1001 of other
illustrated embodiments has been replaced by a piston 1002 that can
move to the right or left (or toward and away from the recessed
surface/heat sink). By attaching the left end 1003 of the piston
tube or cylinder 1001 to a vacuum pump (or a low pressure P2), a
pressure difference is created between P2 and the slab surroundings
at another pressure P1. The low pressure may be created and
maintained more effectively by providing a valve 1004 that may be
closed after the vacuum or low pressure has been applied. The
piston 1001, which is normally outfitted with piston rings or other
seals (not shown) to prevent liquid from escaping past the piston,
then pulls on the liquid which creates a positive pressure
difference P1-P2 when P1>P2 which pushes the slab against the
mount (with elements shown in FIGS. 2A and 2B and other figures not
being numbered or described in detail with reference to FIGS. 10A
to 10D). In some embodiments, one or more supports 1005 are
provided internal to the liquid metal, i.e., within the recessed
surface of the mount, in order to control excessive excursions
(e.g., bending or other movements) of the slab due to the pressure
differential. Such internal stops 1005 are in some cases provided
primarily as a safety feature, to prevent slab fracture in the
event of an accidental overload in the pressure difference P1-P2.
For this reason, it is possible to make the stop or stops flexible
or of a flexible or non-rigid material so as not to exert localized
pressure on the slab.
[0065] A variation on the embodiment of FIG. 10A is illustrated in
FIG. 10B. As shown, two pistons 1006 and 1007 are used to create an
internal sealed space 1008 in the piston cylinder or tube 1001
which is filled with a gas. The left or outer piston 1007 may also
work against a spring or elastic member 1010 whose motion is
constrained by a neck-down 1009 (or other stop element) in the tube
1001. If a vacuum or low pressure is created to exert a force on
the piston 1007, the pressure internal to the liquid metal in the
mount may be varied by attaching a heating (or cooling) element
1011, which can be used to control the temperature of the gas in
sealed space or volume 1008. Since the gas pressure is dependent on
temperature, this provides a way to tune the pressure difference
between the liquid metal and P1 (e.g., pressure in surrounding
environment or external to the mount and slab). It is also noted
that incorporation of a controllable pressure differential by the
indicated method, or other methods with the shown or other
components, also enables one to tune the curvature of the mounted
optic or slab while retaining the advantages of uniform pressure
and high thermal conductivity.
[0066] Other ways of applying a positive pressure on the slab to
control vertical movement (i.e., movement toward and away from the
mount or heat sink) are shown in FIGS. 10C and 10D. FIG. 10C
illustrates in a partial view of a liquid metal mount assembly the
use of a strap 1013 which is fastened on top of the slab with a
fastener or connector 1015 and secured to a fixed position 1014
using a spring or elastic member 1012. FIG. 10D shows yet another
liquid metal mount assembly with a channel 1016 cut in the slab
mount (or heat sink) and sealed against the slab using an o-ring
1017. The channel 1016 is typically connected to a vacuum or low
pressure source and, once the low pressure is established, sealed
with valve 1018. This method or assembly also is useful for
establishing a pressure differential between the outside (or
environment adjacent the slab exterior to the mount assembly) and
the low pressure region which then exerts a downward force on the
slab.
[0067] The embodiments of the liquid metal mount assemblies
illustrated in FIGS. 10A to 10D are advantageous for several
reasons. The assemblies provide a pressure differential between the
exterior of the assemblies and exterior surfaces of the mounted
optic, e.g., slab element or the like, and the liquid metal
contained in the mount or heat sink. This pressure differential can
be selected or controlled statically or dynamically to positively
seat the optic against the liquid metal and the mount/heat sink
adjacent the liquid metal reservoir (e.g., portions of the heat
sink forming the reservoir or recessed surface). The pressure
differential at least in some of the embodiments shown in FIGS. 10A
to 10B may be generated in a manner that allows the magnitude of
the pressure differential to be controlled in a dynamic or periodic
fashion, e.g., to lower the pressure at P2 when the external or
surrounding pressure P1 decreases and vice versa to maintain a
differential setting, which may utilize one or more pressure or
differential sensors and a control device (not shown but understood
by those skilled in the arts). In some implementations, the
differential pressure magnitude is selected so as not only to
provide ongoing seating/sealing of the optic against the heat sink
and liquid metal but also to control or even select the shape of
the mounted optic. For example, a particular pressure differential
may be chosen to obtain a desired or predetermined curvature of the
mounted optic. In some implementations, the pressure differential
is not dynamically adjustable but is instead selected for a
particular or anticipated external pressure or range of pressures,
such as if the optic if being designed for use in the low pressure
environments of space. In these cases, the predetermined curvature
of the optic is obtained when the external pressure is within a
particular range (e.g., the anticipated external operating pressure
for the optic assembly), such as when the assembly is deployed in
space or other planned operating environment.
[0068] Although the invention has been described and illustrated
with a certain degree of particularity, it is understood that the
present disclosure has been made only by way of example, and that
numerous changes in the combination and arrangement of parts can be
resorted to by those skilled in the art without departing from the
spirit and scope of the invention, as hereinafter claimed.
* * * * *