U.S. patent application number 15/984656 was filed with the patent office on 2018-09-27 for layered infrared transmitting optical elements and method for making same.
The applicant listed for this patent is The GOV of the USA, as represented by the Secretary of the Navy, Naval Research Laboratory, The GOV of the USA, as represented by the Secretary of the Navy, Naval Research Laboratory. Invention is credited to Shyam S. Bayya, Geoff Chin, Daniel J. Gibson, Mikhail Kotov, Jasbinder S. Sanghera.
Application Number | 20180272683 15/984656 |
Document ID | / |
Family ID | 57601488 |
Filed Date | 2018-09-27 |
United States Patent
Application |
20180272683 |
Kind Code |
A1 |
Gibson; Daniel J. ; et
al. |
September 27, 2018 |
LAYERED INFRARED TRANSMITTING OPTICAL ELEMENTS AND METHOD FOR
MAKING SAME
Abstract
Infrared transmitting glasses bonded into an optical element
without interlayer voids by stacking at least two different
infrared transmitting glasses inside a vessel where each glass has
a different refractive index, a different dispersion, or both, and
where the glasses all have similar viscosities, thermal expansion
coefficients, and glass transition temperatures; placing a weight
on top of the stack; applying a vacuum to the vessel; applying an
isostatic pressure of at least 1500 psi; and after releasing the
isostatic pressure, annealing at a temperature within 10.degree. C.
of the glass transition temperature at a pressure between 0 and
1000 psi. Applying the vacuum, applying the isostatic pressure, and
annealing are done sequentially and with no intermediate
transitions to ambient temperature or pressure.
Inventors: |
Gibson; Daniel J.; (Falls
Church, VA) ; Kotov; Mikhail; (Silver Spring, MD)
; Chin; Geoff; (Arlington, VA) ; Bayya; Shyam
S.; (Ashburn, VA) ; Sanghera; Jasbinder S.;
(Ashburn, VA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The GOV of the USA, as represented by the Secretary of the Navy,
Naval Research Laboratory |
Arlington |
VA |
US |
|
|
Family ID: |
57601488 |
Appl. No.: |
15/984656 |
Filed: |
May 21, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
14210828 |
Mar 14, 2014 |
9981459 |
|
|
15984656 |
|
|
|
|
61787365 |
Mar 15, 2013 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B32B 2309/68 20130101;
B32B 37/144 20130101; B32B 2038/0048 20130101; B32B 2551/00
20130101; B32B 2307/418 20130101; B32B 2309/125 20130101; B32B
37/1018 20130101; B32B 2309/62 20130101; B32B 2315/08 20130101;
B32B 37/1009 20130101; B32B 17/06 20130101; B32B 33/00 20130101;
B32B 37/003 20130101; B32B 2037/268 20130101; B32B 2309/022
20130101; B32B 38/0036 20130101; B32B 2307/412 20130101; B32B
2037/0092 20130101; B32B 37/18 20130101; B32B 37/0007 20130101 |
International
Class: |
B32B 37/10 20060101
B32B037/10; B32B 33/00 20060101 B32B033/00; B32B 37/00 20060101
B32B037/00 |
Claims
1. A layered infrared transmitting optical element without
interlayer voids made by the method, comprising: stacking at least
two different infrared transmitting glasses inside a vessel to form
a stack of glasses, wherein each glass has a different refractive
index, a different dispersion, or both, wherein the at least two
different infrared transmitting glasses all have similar
viscosities, thermal expansion coefficients, and glass transition
temperatures, wherein the stack of glasses is placed between two
non-reactive plates, and wherein one or both non-reactive plates
have a flat center portion and a raised perimeter or wherein one or
both non-reactive plates are in the shape of a ring; placing a
weight on top of the stack of glasses; applying a vacuum to the
vessel; applying an isostatic pressure of at least 1500 psi; and
after releasing the isostatic pressure, annealing at a temperature
within 10.degree. C. of the glass transition temperature at a
pressure between 0 and 1000 psi, wherein applying the vacuum,
applying the isostatic pressure, and annealing are done
sequentially and with no intermediate transitions to ambient
temperature or pressure.
2. The optical element of claim 1, wherein each glass comprises S,
Se, Te, Ga, Ge As, Sn, Sb, Ag, or any combination thereof.
3. The optical element of claim 1, wherein at least one glass
comprises F, Cl, Br, I, or any combination thereof.
4. The optical element of claim 1, wherein a guide sleeve is used
to constrain lateral movement of the stack of glasses.
5. The optical element of claim 1, wherein the vacuum is between
0.1 and 100 mTorr.
6. The optical element of claim 1, where the vacuum is held for
between 10 seconds and 60 minutes.
7. The optical element of claim 1, wherein the isostatic pressure
is between 1500 and 3000 psi.
8. The optical element of claim 1, wherein the isostatic pressure
is held for between 5 and 60 minutes.
9. The optical element of claim 1, wherein the annealing is for a
time between 20 minutes and 2 hours.
10. The optical element of claim 1, wherein the at least two
different infrared transmitting glasses are substantially flat
prior to bonding.
11. The optical element of claim 1, wherein after bonding the at
least two different infrared transmitting glasses are slumped or
molded.
12. A layered infrared transmitting optical element without
interlayer voids made by the method, comprising: stacking at least
two different infrared transmitting glasses inside a vessel to form
a stack of glasses, wherein each glass has a different refractive
index, a different dispersion, or both, wherein the at least two
different infrared transmitting glasses all have similar
viscosities, thermal expansion coefficients, and glass transition
temperatures, wherein the at least two different infrared
transmitting glasses have a convex curvature on at least one face
prior to bonding; placing a weight on top of the stack of glasses;
applying a vacuum to the vessel; applying an isostatic pressure of
at least 1500 psi; and after releasing the isostatic pressure,
annealing at a temperature within 10.degree. C. of the glass
transition temperature at a pressure between 0 and 1000 psi,
wherein applying the vacuum, applying the isostatic pressure, and
annealing are done sequentially and with no intermediate
transitions to ambient temperature or pressure.
13. The optical element of claim 12, wherein each glass comprises
S, Se, Te, Ga, Ge As, Sn, Sb, Ag, or any combination thereof.
14. The optical element of claim 12, wherein at least one glass
comprises F, Cl, Br, I, or any combination thereof.
15. The optical element of claim 12, wherein a guide sleeve is used
to constrain lateral movement of the stack of glasses.
16. The optical element of claim 12, wherein the vacuum is between
0.1 and 100 mTorr.
17. The optical element of claim 12, where the vacuum is held for
between 10 seconds and 60 minutes.
18. The optical element of claim 12, wherein the isostatic pressure
is between 1500 and 3000 psi.
19. The optical element of claim 12, wherein the isostatic pressure
is held for between 5 and 60 minutes.
20. The optical element of claim 12, wherein the annealing is for a
time between 20 minutes and 2 hours.
Description
PRIORITY CLAIM
[0001] The present application is a divisional application of U.S.
application Ser. No. 14/210,828, filed on Mar. 14, 2014 by Dan J.
Gibson et al., entitled "LAYERED INFRARED TRANSMITTING OPTICAL
ELEMENTS AND METHOD FOR MAKING SAME," which claimed the benefit of
U.S. Provisional Application No. 61/787,365, filed on Mar. 15, 2013
by Dan J. Gibson et al., entitled "Layered Infrared Transmitting
Optical Elements and Method for Making Same," the entire contents
of both are incorporated herein by reference.
BACKGROUND OF THE INVENTION
Field of the Invention
[0002] This disclosure pertains to the field of infrared optics
and, more specifically, to infrared lens elements and multi-element
infrared imaging lens systems.
Description of the Prior Art
[0003] It is common to refer to an optical glass as having a
refractive index at a certain wavelength and to describe the shape
of the dispersion function using the Abbe number, V (or
v)=(n.sub.d-1)/(n.sub.F-n.sub.C), and various partial dispersion
values, P.sub.x,y=(n.sub.x-n.sub.y)/(n.sub.F-n.sub.C), as dictated
by the precision of the optical design. Since infrared transmitting
glasses often have poor transmission for visible wavelengths, a
`modified` Abbe number is used where the visible wavelengths,
.lamda..sub.F, .lamda..sub.d, and .lamda..sub.C, are replaced with
more suitable infrared wavelengths. Two common examples are the
mid-wave infrared (MWIR), where the wavelengths 3, 4 and 5 .mu.m
are used and the long-wave infrared (LWIR) where the wavelengths 8,
10 and 12 .mu.m are used to define the MWIR dispersion, V.sub.(3-5)
(or V.sub.MWIR)=(n.sub.4-1)/(n.sub.3-n.sub.5) and LWIR dispersion,
V.sub.(8-12) or (V.sub.LWIR)=(n.sub.10-1)/(n.sub.8-n.sub.12)
respectively. While these dispersion parameters describe the
wavelength dependent refractive index of IR-transmitting materials
sufficiently to aid the selection of materials for a lens design,
they lack the precision required for modern high performance
optical design software. As a result, the refractive index is also
represented in either tabular form (a list of indices at specific
wavelengths) or more precisely by Sellmeier coefficients that
permit interpolation and extrapolation of refractive index
values.
[0004] Refractive optical imaging systems typically utilize
multiple refractive optical elements to manipulate light and create
an image. Commonly, these individual optical elements are comprised
of different optical materials with different optical properties,
including refractive indices, dispersions, or thermo-optic
coefficients, in such combinations that attempt to reduce or
eliminate problems associated with using a single material,
including for example chromatic dispersion and thermal drift. For
various reasons, including reducing system size, weight and
complexity or improving performance and reliability, optical
designers can use specialized optical elements, for example
achromatic doublets or gradient index (GRIN) optics. Achromatic
doublets and triplets are comprised of separate optical elements of
dissimilar materials, with different optical properties that have
been bonded to each other using transparent adhesives. This is a
common practice for visible imaging systems, but the lack of
suitable IR-transparent optical cements limits application of this
technology to IR optical elements. GRIN optics are single optical
elements wherein the optical properties vary in a controlled way
within the bulk of the optical element. GRIN optics are also
limited to primarily visible wavelengths as the methods used in
their fabrication are not well-suited to IR transparent materials.
The majority of GRIN optics are fabricated using an ion-exchange
process wherein the optical element is submerged in a hot salt bath
for an extended time such that the ions in the element diffuse
through the element into the bath and ions from the bath diffuse
into the element, imparting a compositional concentration gradient
and thereby a gradient in the optical properties of the optical
element. This process is typically not possible with IR transparent
materials, especially those used beyond 2 .mu.m. Furthermore, the
thermodynamics of diffusion limit the size of optical elements
fabricated via the method under reasonable times to about 1 inch in
diameter.
[0005] Layered optical elements have been fabricated with gradient
index using polymer sheets (U.S. Pat. No. 7,002,754 to Baer et al.
(2006)) which are bonded to one another using pressure (Beadie et
al., "Optical properties of a bio-inspired gradient refractive
index polymer lens," Optics Express, 16, 15, 11540-47 (2008)) and
later molded and/or machined. Hagerty et al. describe a method of
stacking an assemblage of glass sheets and heating to bond them,
using a vacuum pressure of 20-25 inches of mercury (508-635 Torr)
to remove trapped air between the sheets and recommend selecting
glasses with similar melting and annealing temperatures (U.S. Pat.
No. 4,929,065 to Hagerty et al. (1990)). Hagerty et al. acknowledge
that "[i]f the plate surfaces are very smooth, however, the use of
vacuum seems to have a minimal effect." One skilled in the art
would appreciate that the application of their method to infrared
glasses would result in the formation of voids at the interfaces,
not from trapped air, but rather due to sublimation of volatile
species at the softening temperature. Hagerty et al. recommend long
heating schedules (1.5 to 8 hours) and sequential zone heating to
allow bubbles to settle out of the glasses, which will not work for
IR glasses where volatile components outgas further increasing
bubble formation.
[0006] Sanghera et al. teach a method for fabricating a core/clad
fiber optic preform using two infrared transmitting glasses in a
nested rod/tube configuration wherein one seals the rod and tube
together under vacuum and subsequently applies high isostatic
pressure to the assembly to remove trapped porosity (U.S. Pat. No.
5,735,927 to Sanghera et al. (1998)). Sanghera et al. mention that
their method could also be applied to "other glass bodies such as
lenses, windows, and planar wave guides which are a composite of
two glass bodies having a common interface" but (i) do not teach
the necessary steps to apply their method successfully to the
application of non-concentric geometry, (ii) do not teach the
application of uniaxial load to a linear stack of sheets, (iii) do
not teach a method with more than two glass bodies, (iv) do not
teach a method with more than one interface and (v) teach softening
of linearly separated portions of the glass body separately and
sequentially to form hermetic sealing of the interfacial area under
a vacuum, and a separate hot isostatic pressing step, which require
that one skilled in the art would necessarily return the partially
bonded body to ambient temperature and pressure prior to
transferring it to the hot isostatic pressing vessel.
BRIEF SUMMARY OF THE INVENTION
[0007] The aforementioned problems are overcome in the present
invention which provides a method for bonding infrared transmitting
glasses into an optical element without interlayer voids by
stacking at least two different infrared transmitting glasses
inside a vessel where each glass has a different refractive index,
a different dispersion, or both, and where the glasses all have
similar viscosities, thermal expansion coefficients, and glass
transition temperatures; placing a weight on top of the stack;
applying a vacuum to the vessel; applying an isostatic pressure of
at least 1500 psi; and after releasing the isostatic pressure,
annealing at a temperature within 10.degree. C. of the glass
transition temperature at a pressure between 0 and 1000 psi.
Applying the vacuum, applying the isostatic pressure, and annealing
are done sequentially and with no intermediate transitions to
ambient temperature or pressure. Also disclosed is the related
optical element made by this method.
[0008] The invention provides a method for bonding infrared
transmitting glasses into optical elements with controlled varying
internal optical properties (refractive index, dispersion,
thermo-optic coefficient, and others) and shaped internal
interfaces, without internal voids and without the use of epoxy or
glue. The optical elements of this invention will enable infrared
optical system engineers to design and build improved imagers with
advantages and features previously only available to designers of
visible systems. The elements could be used in broad-band infrared
imagers, for example achromatic dual-band IR imaging systems, which
will reduce the system size, weight and complexity by reducing the
number of separate optical elements in imaging systems. System
tolerances and costs can be reduced by eliminating the air space
between closely spaced elements in lens systems. System performance
can be improved as needed by next generation focal plane
arrays.
[0009] The glasses described herein can be used to make lenses for
applications in the SWIR to LWIR regions, represented by 1 to 14
microns wavelength. They can be used for specific wavelength
applications such as SWIR (1-2 microns), MWIR (2-5 microns), or
LWIR (7-14 microns) or their combinations, and wavelengths in
between.
[0010] The optical elements of the current invention will replace
existing elements in infrared optical system designs with a size,
weight and performance advantage. The alternatives to the current
invention are already in use in the form of many-element IR imaging
lenses, but they are becoming larger, heavier and more complex as
IR imaging sensor arrays become smaller and more sensitive.
[0011] These and other features and advantages of the invention, as
well as the invention itself, will become better understood by
reference to the following detailed description, appended claims,
and accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a schematic of an apparatus for performing the
method of the present invention.
[0013] FIG. 2 is a schematic of one embodiment wherein the
non-stick plates are shaped to concentrate the uniaxial force from
the weight to the perimeter of the glass sheets.
[0014] FIG. 3 shows an IR-transmitting two-layered optical element
fabricated by uniaxial hot pressing, with a return to ambient
temperature before the isostatic pressure step at elevated
temperature. Regions A and B show voids at the interface between
the two layers and highlight the need for the method of the present
invention.
[0015] FIG. 4 shows a two-layered IR transparent optical element
(a) face view and (b) edge view.
[0016] FIG. 5 shows a side view of a four-layered IR transparent
optical element.
[0017] FIG. 6 is an infrared image of a four-layered IR transparent
optical element.
[0018] FIG. 7 shows an IR-transparent four-layer optical element
with curved internal interfaces and surfaces: (a) side-view photo
and (b) sketch of internal interfaces.
[0019] FIG. 8 shows an IR-transparent two-layer optical element
with curved surfaces and a curved internal interface. The total
center thickness is 1.54 mm, the total height is 4.45 mm, and the
total diameter is 24.4 mm.
DETAILED DESCRIPTION OF THE INVENTION
[0020] The present invention provides layered optical elements for
the transmission and manipulation of infrared light, and more
specifically infrared lens elements and multi-element infrared
imaging lens systems. The described invention is a class of optical
elements having a refractive index profile and a dispersion profile
and a method for the fabrication of the elements without internal
voids. Specifically, the optical elements comprise multiple layers
of non-silica, infrared transmitting specialty glasses with
different refractive indices and/or dispersions, bonded together
without optical cements. The method for making the optical elements
avoids the need for optical cements and prevents the formation of
inter-layer voids. The optical elements enable lenses that function
over a broad wavelength range in the infrared, 800 nm-18 .mu.m, or
a portion thereof, depending on the glasses used.
[0021] The infrared transmitting optical elements of this invention
are comprised of 2 or more layers, wherein each layer comprises
infrared transmitting glass, and the layers are bonded together
without interlayer voids. The glasses are typically, but not
exclusively, chosen from a set of chalcogenide glasses that may
comprise one or more of the following elements: S, Se, Te, Ga, Ge,
As, Sn, Sb and Ag. Halides such as F, Cl, Br and I can also be
added to the glass compositions to make chalcohalide glasses. The
glasses comprising each layer have a set of optical properties
including a nominal refractive index, a mid-wave dispersion, a
long-wave dispersion and a thermo-optic coefficient. The interfaces
between the layers, as well as the outer surfaces, may have
positive, negative or infinite curvature. The glasses, layer
thicknesses and interface profiles are chosen or designed such that
the optical properties of the layers are different and
complimentary. For example the layers may have different refractive
indices to refract light within the optic as in a gradient index
(GRIN) lens, or the layers may have different dispersions to
correct for chromatic aberrations as in an achromatic lens.
Furthermore, the glasses are chosen or designed such that their
viscosities and thermal expansion coefficients are similar and they
have similar glass transition temperatures (Tg).
[0022] The individual layers within the optical elements comprise
sheets of infrared glass and are bonded to each other using both
vacuum-pressure and high-pressure sequentially in a single method
(see FIG. 1). The individual sheets 50, 60, 70, 80 are cleaned of
debris and stacked in an order prescribed by the optical design and
the resulting stack is placed between a pair of non-reactive,
non-stick plates 40 with a high quality surface finish. The
individual sheets comprising the layers are between about 100 .mu.m
and 20 mm thick, but could be thicker or thinner, and may have a
positive, negative, infinite or some compound curvature, and may
substantially resemble a flat disc. The plates can be made from
vitreous or glassy carbon, silicon carbide, tungsten carbide,
silica glass or another suitable material which is non-reactive to
the glass in contact with it. The stack is then placed inside a
vessel 10 and a suitable weight 20 typically between 100 and 1000
grams (for a 25 mm diameter, 10 mm thick optical element, but could
be more or less as needed) is placed on top of the stack, prior to
closing the vessel. In another embodiment of the invention, shown
in FIG. 2, one or both of the plates 45 may be shaped to resemble a
shallow cup, wherein the center portion is flat and the perimeter
is raised, to concentrate the uniaxial load from the weight 20 to
the perimeter of the stack of glass sheets. In another embodiment,
not shown, one or both of the plates are replaced by rings. A guide
sleeve 30 may be used to constrain the lateral movement of the
weight, plates and sheets. The vessel is purged with an inert gas
and then subjected to vacuum via suitable valves 110 and supply
plumbing 120 and heated to a temperature near the glass transition
temperature. The vacuum level supplied by the vacuum pump 100 is
between about 0.1 and 100 mTorr (but could be higher or lower); the
temperature is between Tg-10.degree. C. and Tg+70.degree. C. (but
could be higher or lower) and in some embodiments is provided by a
furnace 130 that surrounds the vessel. The vacuum is held for a
time typically between 10 sec and 60 minutes but could be longer
depending on the size of the vessel and parts inside it. After that
time, the vessel is slowly filled with an inert gas. The inert gas
is typically argon, helium, nitrogen or other suitable gas and may
be pre-heated to some temperature at or near the temperature of the
glass and vessel prior to entering the vessel. The inert gas inside
the vessel is pressurized to a high-pressure between about 1500 psi
and 3000 psi using a pump 90. A higher pressure could also be used.
The high-pressure is held for a time typically between 5 and 60
minutes but could be longer if needed. The high-pressure is
released prior to cooling the vessel below Tg. The temperature is
typically held between Tg-10.degree. C. and Tg+10.degree. C. for a
time between 20 minutes and 2 hours, with a pressure between 0 psi
and 1000 psi to anneal and relieve stress in the bonded glass
optical element. The vessel is cooled to room temperature at a rate
typically between 0.1 and 20.degree. C./minute.
[0023] The optical elements of the present invention and the method
to make them are novel and have unique features. The layers in the
optical element comprise infrared transmitting glasses, which may
be prone to sublimation, decomposition, devitrification or phase
separation and therefore require special handling and processing
considerations, atypical of glasses used in optics for visible
light including silica, silica-based and oxide glasses. The unique
features of this method are needed for producing layered infrared
optical elements using infrared transparent glasses, and are not
typically needed for other materials. Specifically, the method
consists of three stages: (i) uniaxial pressure under vacuum, (ii)
hot isostatic pressure and (iii) anneal, which are applied
sequentially in the same vessel. In the first stage, uniaxial
pressure is exerted on the stack of sheets by gravity resulting
from the weight above the stack, and the glass stack is exposed to
temperatures and vacuum pressure sufficient to cause some
sublimation of some chemical elements (for example sulfur) from the
glasses. While the first stage is sufficient to mechanically bond
the sheets, partial pressure may cause tiny gaps to form between
even perfectly flat sheets resulting in trapped porosity at the
interfaces. In the second stage, a very high pressure (greater than
1000 psi) is applied isostatically to all sides of the optical
element forcing the closure of internal porosity. In the third
stage, the optical element is cooled to an annealing temperature
and isostatic pressure is reduced to remove internal stress that
may have been imposed by the earlier stages.
[0024] The execution of the three stages, sequentially and without
intermediate transitions to ambient temperature or pressure, is a
unique requirement to the infrared glasses of the invention and is
a unique feature of the method. For example, if one were to perform
the stages of the method separately, returning to ambient
temperature and pressure between each stage, reheating of the
optical element at the start of the second stage could cause
trapped porosity to grow beyond a size that can be removed using
isostatic pressure as is demonstrated by the IR-transmitting
two-layer optical element in FIG. 3, where 2 large interstitial
voids are visible and marked as A and B.
[0025] In some embodiments, the sheets are substantially flat,
infinite curvature, prior to bonding. The surface figures of the
sheets are very important, as any imperfections are opportunities
for trapped interstitial voids. For example, the sheets should have
surface flatness <0.23 wave, surface roughness <17 Angstrom
and parallelism <1.9 arc sec or thereabouts. In other
embodiments, the sheets have a slight convex curvature on at least
one face, such that adjacent sheets contact each other only at the
center. This arrangement allows any potential interstitial voids to
be eliminated as the interfacial gap closes during the bonding
process and results in a layered optical element with substantially
flat interfaces. In other embodiments, the sheets have substantial
curvature prior to bonding and the curvature between adjacent
sheets is matched to permit nesting during the method, and the
resultant optical elements will have curved internal and or
external interfaces.
[0026] In some embodiments, the optical elements are slumped or
molded after the method to impart curvature to the internal
interfaces. In some embodiments the faces of the optical elements
are machined using, for example, single-point diamond turning or
other grinding and polishing methods, to impart positive, negative,
infinite or compound curvature to one or two faces.
EXAMPLES
Example 1
[0027] An IR-transparent optical element with two layers was
fabricated using the method of the present invention and is shown
in FIG. 4. The element comprises two IR-transmitting glasses and
has a diameter of 25 mm and a total thickness of 2.0 mm. One layer
(top layer in the top view image and left layer in the side-view
image) comprises As.sub.39 S.sub.61 glass and is 1.0 mm thick; the
other layer (bottom layer in the top view image and right layer in
the side-view image) comprises As.sub.39 S.sub.46 Se.sub.15 glass
and is 1.0 mm thick.
Example 2
[0028] An IR-transparent optical element with four layers was
fabricated using the method of the present invention, and its
profile is shown in FIG. 5. The element comprises o four
IR-transmitting glasses and has a diameter of 25 mm and a total
thickness of 6.1 mm. The top layer is 2.0 mm thick and comprises an
As--S based glass; the second layer is 1.0 mm thick and comprises a
Ge--As--S--Te based glass; the third layer is 1.0 mm thick and
comprises a Ge--As--Se based glass; and the bottom layer is 2.1 mm
thick and comprises a Ge--As--Se--Te based glass. Since the element
is opaque to visible light, a long-wave infrared (LWIR) camera was
used to look through the optical element. As shown in FIG. 6, the
infrared image demonstrates a lack of interstitial voids within the
optical element. The bright spot in the center of the image is the
hot filament of the illuminating light behind the optic.
Example 3
[0029] An IR-transparent optical element with four layers
fabricated using the method of the present invention was then
subsequently molded into a lens shape as shown in FIG. 7. The
optical element comprises four IR transparent glasses: (i) the top
layer comprises an As--S based glass; (ii) the second layer
comprises a Ge--As--S--Te based glass; (iii) the third layer
comprises Ge--As--Se based glass; and (iv) the bottom layer
comprises a Ge--As--Se--Te based glass. FIG. 7(a) shows a side view
of the curved IR-transparent four-layer optical element with curved
internal interfaces and surfaces. A sketch of the internal
interfaces is shown in FIG. 7(b).
Example 4
[0030] An IR-transparent optical element with two layers fabricated
using the method of the present invention was then subsequently
slumped into a lens shape as shown in FIG. 8. The optical element
comprises two As--S based IR transparent glasses wherein the layer
with the convex surface (top layer in FIG. 8) has a larger As/S
ratio than the layer with the concave surface (bottom layer in FIG.
8). Each layer has a center thickness of 0.77 mm. The total center
thickness of the element is 1.54 mm, the total height is 4.45 mm,
and the total diameter is 24.4 mm. The optical element could also
be slumped where the thickness of each layer changes after the
slumping process.
[0031] The above descriptions are those of the preferred
embodiments of the invention. Various modifications and variations
are possible in light of the above teachings without departing from
the spirit and broader aspects of the invention. It is therefore to
be understood that the claimed invention may be practiced otherwise
than as specifically described. Any references to claim elements in
the singular, for example, using the articles "a," "an," "the," or
"said," is not to be construed as limiting the element to the
singular.
* * * * *